Enhancing Retarding Potential Analyzer Energy
Measurements with Micro-Aligned Electrodes
by
Eric Vincent Heubel
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Mechanical Engineering
MASSACHUSETTS INSTITUTE,
OF TECHNOLOGY
at the
AUG 15 2014
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
LI BRARiE2
June 2014
@ Massachusetts Institute of Technology 2014. All rights reserved.
Signature redacted
A u thor ..............................................................
Department of Mechanical Engineering
May 2, 2014
Certified by .....
Signature redacted
......
Luis Fernando Velaisquez-Garcia
Principal Research Scientist of the Microsystems Technology
Laboratories
-Thesis Supervisor
Certified by.......
Signature
.................
redacted..
Anastasios John Hart
Ass te Professor of Mechanical Engineering
Signature redacted e~ts Committee Chair
A ccepted by .............
. ..........................................
David E. Hardt
Chairman, Department Committee on Graduate Theses
2
Enhancing Retarding Potential Analyzer Energy
Measurements with Micro-Aligned Electrodes
by
Eric Vincent Heubel
Submitted to the Department of Mechanical Engineering
on May 2, 2014, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy in Mechanical Engineering
Abstract
Plasmas are ionized gases, and constitute a large fraction of the known universe.
For example, solar wind is a plasma that emanates from the sun reaching the Earth's
magnetosphere. At times these ionized species cause beautiful auroral displays over
the planet's magnetic poles. Moreover, when a hypersonic object enters the atmosphere, the shock wave that is generated induces a plasma sheath that surrounds the
object. The resulting plasma is hot and dense and may cause material ablation from
the surface of the object. Other plasmas of similar or greater density exist in fusion
reactors, and in silicon processing chambers.
A Retarding Potential Analyzer (RPA) is a sensor that measures the ion energy
distribution of a plasma. The ion energy influences the ablation of surfaces, or plasma
etching, as well as the deposition processes. Integrated circuit foundries could greatly
benefit from a diagnostic tool such as an RPA in plasma chambers. Measuring particle
energy during reactive ion etching, ion implantation, ion milling, plasma enhanced
chemical vapor deposition, etc, in situ would close the control loop to improve the
uniformity and repeatability of numerous processes.
In order to measure the ion energy of a plasma, an RPA utilizes a system of grids
with holes smaller than a few Debye length - a characteristic length proportional to
the square root of the electron temperature divided by the electron number density.
Thus, cold, dense plasmas have the smallest associated Debye lengths and require
smaller grid openings than can be achieved using conventional machining.
In this thesis an improved RPA design is proposed that utilizes the following
three key concepts: (i) the aperture size and inter-electrode spacing required by
dense plasmas are defined using micro electromechanical systems (MEMS) processing
techniques; (ii) aperture alignment across successive grids is mechanically enforced
to improve the transmission of ion species through the device; (iii) densely packing
apertures in each RPA grid multiplexes the signal onto the collector.
A MEMS RPA is built with apertures as small as 100 pm in diameter having
an inter-grid spacing of only 200 pm. These are the narrowest aperture and gap
dimensions for an RPA with enforced electrode alignment to date. The new RPA
3
design is benchmarked against the present state of the art downstream of an ion
source from a mass spectrometry (MS) system. An ion source is chosen because of
the fine control it offers over the ion energies, as a low energy with little variability
increases mass resolution in MS systems. Through enforced alignment, the MEMS
RPA shows an order of magnitude increase in signal strength over a conventional RPA.
In improving the transmission of ions through the sensor, the artificial broadening
of RPA ion energy distribution measurements is mitigated, resulting in a threefold
improvement in sensor energy resolution. This is characterized by a reduction in the
full width half maximum (FWHM) value from 2.5 V for the conventional device down
to 0.85 V for the MEMS RPA.
The various RPAs are then tested in a helicon plasma, capable of replicating
many dense plasmas in the range of 1 x 1016 m-1 to 1 x 1018 m-3. Langmuir probe
measurements provide estimates of the electron temperature and plasma density, from
which the Debye length is derived. In these experiments, only the new RPA designs
were able to effectively trap the plasma down to a Debye length of approximately
50 pm and obtain ion energy distributions. The range of application for RPAs is thus
expanded through the use of microfabrication techniques.
Thesis Supervisor: Luis Fernando Velasquez-Garcfa
Title: Principal Research Scientist of the Microsystems Technology Laboratories
4
Acknowledgments
When I set out to earn this degree I was following in the footsteps of my father
and grandfather, albeit in a slightly different field. My grandfather taught Chemistry
at the Universit6 des Sciences et Techniques de Lille in France, my father obtained his
PhD in Chemistry from Michigan State University, and I was led toward Mechanical
Engineering by a passion for "tinkering." Over the past eight years in the Boston
area, I have come to know many people throughout the Mechanical Engineering
department, across MIT, and outside. Since moving into Ashdown in 2006, I have had
the pleasure of making friends from around the world and have greatly appreciated
the time we spent sharing meals, working out, walking around the river, getting
coffee, and partaking in this journey together. I am grateful to so many people from
numerous aspects of life on and off campus here that there are simply too many to
list by name. Please forgive me if this short section fails to express how important
you all are to me.
I first wish to thank my family, my mother Nancy and father Pierre-Henri for their
love, encouragement, and prayerful support. Along with my siblings, Caroline, Alex,
and Mariette, they have been an example of what goals can be achieved through
perseverance, and I am very proud of them. And to by brother-in-law Steve, and
niece Margaux, for their encouraging words and songs.
To my many friends from MIT's Tech Catholic Community, thank you for your
support, for helping me keep sight of what is important. I have been blessed to be a
part of such a wonderfully loving and supportive group of people. With my friends
on campus, you have truly been my family away from home. Thank you Fr. Clancy
for keeping such a strong faith present in our chapel and across MIT. I am glad to
have spent these years with you.
I would like to thank Bill Butera, my boss during my internship, for encouraging
me to follow my desire to pursue this degree. My experience working in an Electrical
Engineering research lab led me to the Microsystems Technology Laboratories (MTL)
at MIT.
5
Throughout these last four years, I have learned a lot about microfabrication, and
would like to thank the MTL technical staff for training me on the various machines in
the cleanrooms here. I thank my advisor, Luis Velisquez-Garcia, for the opportunity
to work on this project and for imparting his microfabrication expertise. I would like
to extend a special thanks as well to the other members of this research group for
their helpful feedback and numerous conversations.
I would like to express my gratitude to NASA, for funding part of this research under Award No. NNC08CA58C with program managers Robert Manning and Thomas
Wallett. As well as to Professor Tayo Akinwande for our discussions regarding the
work and experiments performed in his laboratory. I thank MIT's Plasma Science
and Fusion Center for allowing me to use its facility to carry out measurements, and
specifically to Regina, Graham, and Professor Dennis Whyte for meeting with me to
discuss my results. I am especially thankful to my committee, Professor John Hart,
Professor Sangbae Kim, and Professor Jeff Lang, for keeping me on track with my
work. I greatly appreciated your feedback, and it was a real pleasure working with
you.
I am grateful for having had the opportunity to work alongside Professor Dave
Gossard and the rest of the 2.003 teaching staff for these past two years. I enjoyed
helping teach the course and working with the MITx team to offer 2.03x worldwide.
Thank you for the advice and insight on pursuing a career in academia.
I would also like to express a special thanks to the administrative staff. Thank
you for all you do to help keep students apprized of deadlines and progressing in their
studies, but most especially for lending a welcoming ear when panic strikes. Thanks
Leslie, Joan, Una, Carolyn and Debb.
6
Contents
List of Symbols
15
1 Introduction
17
2 Plasma Diagnostics
23
2.1
2.2
Langmuir Probe ..............................
23
2.1.1
Single Langmuir Probe Theory
2.1.2
Double Langmuir Probe Theory .....................
Retarding Potential Analyzer
.................
24
27
. . . . . . . . . . . . . . . . . . . . . .
31
2.2.1
Theory of Operation . . . . . . . . . . . . . . . . . . . . . . .
31
2.2.2
Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . .
34
3 Hybrid RPA
37
3.1
Hybrid RPA design ............................
37
3.2
Fabrication
39
3.3
................................
3.2.1
RPA Housing ..........................
3.2.2
Packaging and Electrical Connections ..............
39
3.2.3
G rids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
3.2.4
Assembly
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
Hybrid RPA Ion Source Characterization . . . . . . . . . . . . . . . .
43
4 MEMS RPA
. 39
59
4.1
MEMS RPA Design ............................
60
4.2
Fabrication
64
................................
7
4.2.1
Housing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
4.2.2
Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
67
4.2.3
Assembler . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
69
4.2.4
Assembly Procedure . . . . . . . . . . . . . . . . . . . . . . .
70
4.2.5
MEMS RPA Ion Source Characterization . . . . . . . . . . . .
72
5 Device Characterization Using a High-Density Plasma
77
5.1
Helicon Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
5.2
Langmuir Probe Data . . . . . . . . . . . . . . . . . . . . . . . . . .
80
5.3
Conventional RPA DIONISOS Plasma Characterization ........
87
5.4
Hybrid RPA DIONISOS Plasma Characterization ..............
90
5.5
MEMS RPA DIONISOS Plasma Characterization ..............
92
5.6
Conventional, Hybrid, and MEMS RPA Comparison ..........
94
6 Future Work
97
7 Conclusion
101
A Detailed Microfabrication Process Flow
105
B Mask Detail
107
B.1 Hybrid RPA Grids ............................
107
B.2 MEMS RPA Housing ................................
108
B.3 MEMS RPA grids ..................................
108
C Engineering Drawings
121
8
List of Figures
1-1
Electron number density from reentry flight experiments
. . . . . . .
19
1-2 Electron temperature measurements from reentry flight experiments .
19
1-3 Debye length estimates from reentry flight experiments . . . . . . . .
20
1-4
Debye lengths for various plasmas as a function of electron temperature
and density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
2-1 Typical single Langmuir probe trace . . . . . . . . . . . . . . . . . . .
24
2-2 Double Langmuir probe plasma measurement
. . . . . . . . . . . . .
27
2-3
RPA schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
3-1
Hybrid RPA schematic . . . . . . . . . . . . . . . . . . . . . . . . . .
38
3-2
Hybrid RPA microfabricated electrodes . . . . . . . . . . . . . . . . .
40
3-3
Hybrid RPA assembly
. . . . . . . . . . . . . . . . . . . . . . . . . .
42
3-4 Mass spectrometry ionizer energy measurement experiment . . . . . .
44
3-5
Conventional RPA construction . . . . . . . . . . . . . . . . . . . . .
45
3-6
Conventional RPA ion energy sweep . . . . . . . . . . . . . . . . . . .
46
3-7 Modified conventional RPA energy sweep . . . . . . . . . . . . . . . .
47
3-8
Conventional RPA electron emission sweep at 10V ion energy and
3 x 10- 5 Torr ......
3-9
...............................
48
Conventional RPA total collected current versus electron emission current 49
3-10 Conventional RPA electron emission sweep at 10 V ion energy and
6 x 10-7 Torr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
3-11 Simulation of ion source using CPO 2D . . . . . . . . . . . . . . . . .
51
9
3-12 Hybrid RPA ion energy sweep with 0.2 mA electron emission current
and 3 x 10~5 Torr . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
53
3-13 Comparison of the hybrid energy distribution and conventional data
for an ion source set to 10 V at 3 x 10~ Torr with 0.2mA electron
emission current.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
54
3-14 Hybrid RPA current trace for 10 V ion energy region and 0.2 mA electron emission current at 3 x 10-5 Torr . . . . . . . . . . . . . . . . . .
55
3-15 Simulation of single RPA aperture using CPO 2D . . . . . . . . . . .
56
3-16 Simulation of modified RPA aperture stack using CPO 2D . . . . . .
57
4-1
Stress analysis of a MEMS RPA retaining spring . . . . . . . . . . . .
62
4-2 MEMS RPA Concept . . . . . . . . . . . . . . . . . . . . . . . . . . .
63
4-3 Misalignment simulation of a single 100 pm RPA aperture stack using
CPO 3D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
4-4 MEMS RPA housing fabrication . . . . . . . . . . . . . . . . . . . . .
65
4-5 MEMS RPA electrode fabrication . . . . . . . . . . . . . . . . . . . .
68
4-6 MEMS RPA assembly tool . . . . . . . . . . . . . . . . . . . . . . . .
69
4-7 MEMS RPA assembly . . . . . . . . . . . . . . . . . . . . . . . . . .
71
4-8 Backlit MEMS RPA assembly . . . . . . . . . . . . . . . . . . . . . .
72
4-9 MEMS RPA testplate . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
4-10 150 pm, 300 pm, 300 pm grid stack MEMS RPA ion energy sweep test
at 3 x 10-1 Torr and 0.2 mA emission . . . . . . . . . . . . . . . . . .
73
4-11 100pm, 250pm, 300pm grid stack MEMS RPA ion energy sweep test
at 3 x 10-' Torr and 0.2 mA emission . . . . . . . . . . . . . . . . . .
74
4-12 Comparison of 150 pm grid apertures to 100 pm in the MEMS RPA .
75
4-13 MEMS RPA comparison to hybrid and conventional probes at 10 V ion
energy region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
4-14 MEMS RPA normalized distribution (normalized height) comparison
5-1
with hybrid and conventional probes at 10V ion energy region . . . .
76
DIONISOS Helicon plasma chamber . . . . . . . . . . . . . . . . . . .
78
10
5-2
Plasma modes for helium plasma excited with an RF-powered helicon
antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
Single Langmuir probe trace for 1000W plasma . . . . . . . . . . . .
80
5-4 Semilog plot of Langmuir probe current for 1000 W plasma . . . . . .
81
5-3
5-5
Hybrid RPA distribution measurement for a helicon plasma at 1000 W
RF power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-6
82
Double Langmuir probe traces for a helicon plasma at varying RF power 83
5-7 Double probe temperature estimates for different helicon plasma powers during conventional RPA testing . . . . . . . . . . . . . . . . . . .
85
5-8 Double probe density estimates for different helicon plasma powers
during conventional RPA testing . . . . . . . . . . . . . . . . . . . . .
86
5-9 Double probe Debye length estimates for different helicon plasma powers during conventional RPA testing . . . . . . . . . . . . . . . . . . .
87
5-10 Conventional RPA current measurements and distributions for a helicon plasma of varying RF power . . . . . . . . . . . . . . . . . . . . .
88
5-11 Hybrid RPA current measurements and distributions for a helicon
plasma of varying RF power . . . . . . . . . . . . . . . . . . . . . . .
91
5-12 MEMS RPA current measurements and distributions for a helicon
plasma of varying RF power . . . . . . . . . . . . . . . . . . . . . . .
93
5-13 RPA plasma data comparison . . . . . . . . . . . . . . . . . . . . . .
95
B-1 First mask layer for the hybrid grids
B-2 Second mask layer for hybrid grids
B-3 MEMS housing align mark mask
. . . . . . . . . . . . . . . . . . 109
. . . . . . . . . . . . . . . . . . . 110
. . . . . . . . . . . . . . . . . . . . 111
B-4 MEMS housing recess mask . . . . . . . . . . . . . . . . . . . . . . . 112
B-5 MEMS housing aperture mask . . . . . . . . . . . . . . . . . . . . . . 113
B-6 MEMS housing spring layer 1 . . . . . . . . . . . . . . . . . . . . . . 114
B-7 MEMS housing spring layer 2 . . . . . . . . . . . . . . . . . . . . . . 115
B-8 MEMS housing spring layer 3 . . . . . . . . . . . . . . . . . . . . . . 116
B-9 MEMS housing spring layer 4 . . . . . . . . . . . . . . . . . . . . . . 117
11
B-10 MEMS housing spring layer 5 ......................
118
B-11 MEMS RPA grid apertures and recesses . . . . . . . . . . . . . . . . 119
B-12 MEMS RPA grid cutouts . . . . . . . . . . . . . . . . . . . . . . . . . 120
12
List of Tables
35
A.1 Microfabricated grid process flow . . . . . . . . . . . . . . . . . . .
105
A.2 MEMS RPA housing process flow . . . . . . . . . . . . . . . . . . .
106
.
.
.
RPA review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1
13
14
List of Symbols
A, Nominal probe surface area.
Xf Non-dimensional floating potential, normalized to the electron temperature in eV.
3 Deflection.
dG Infinitesimal arc angle.
E Young's modulus of elasticity.
e The elementary charge.
c0 The permittivity of free space.
F Force.
I Probe current.
IA Moment of area.
Id
Differential current flowing in a double Langmuir probe circuit from probe 1 to 2.
I, Electron current.
Ie,sat Electron saturation current.
Ii Ion current.
'isat
Ion saturation current.
15
kE Boltzmann's constant.
keff Effective spring constant.
L Ionization length.
AD
The plasma Debye length.
M Moment.
me The mass of an electron.
mi The ion mass.
n, The electron number density.
ni The ion number density.
#
Non-dimensional probe biasing potential, normalized to the electron temperature.
r Neutral axis radius of curvature.
- Ionization cross-section.
Te The electron temperature.
E Arc angle.
U Virtual work
UB
Bohm velocity.
V Probe potential.
v Particle velocity.
Vd
Differential voltage, applied to the double Langmuir probe.
V Floating potential.
V Plasma potential.
16
Chapter 1
Introduction
Plasma diagnostics is a speciality devoted to the study of the highest energy state
of matter. Sensors developed in this field are of interest to many disciplines; from
nuclear engineering, to aeronautical and aerospace engineering, as well as electrical
and mechanical engineering. The focus of this thesis is to improve the performance of
Retarding Potential Analyzers (RPAs) - also known as retarding field energy analyzers or gridded energy analyzers - to increase their sensitivity and extend their range
of application to denser plasmas.
Plasma sensors that are able to withstand harsh environments are desirable for
space missions in monitoring reentry as well as for hypersonic flight. Shock waves
around high speed vehicles compress and heat the air; resulting in high temperature
that dissociate air molecules and form a plasma sheath around the craft [1]. The heat
generated between the hypersonic shock wave and vehicle is sufficient to ionize and
ablate its surface. Monitoring conditions of the reentry plasma at the vehicle surface
could provide valuable information to enact protective maneuvers that would maintain
safe heat levels.
Additionally, determining parameters such as plasma frequency
would shed light on ways in which plasma induced communication blackout might be
alleviated. Reentry flight experiments of conically shaped vehicles were performed in
the 1960s to acquire data regarding the electron temperature and number density in
the sheath surrounding crew return vehicles [2]. From these values, one may calculate
17
the Debye length,
AD,
of the plasma
(1.1
AD
EokTe
nlee
where fo is the permittivity of free space, kB is Boltzmann's constant, T the electron
temperature, ne the electron number density, and e the elementary charge.
This
formula stems from the characteristic length derived for an idealized plasma where
ions are stationary and thermal electrons assume a Boltzmann relation [3]. It governs
the distance over which an electric field penetrates a plasma. A plasma sheath is
usually on the order of a few Debye lengths and forms around confining surfaces.
Beyond this sheath, a plasma may be assumed not to sense the presence of the walls.
In the bulk of a plasma, far from contacting surface, ions and electrons exist in nearly
equal amounts, such that ne a ni, the ion density, since plasmas are quasi-neutral.
It stands to follow that a critical dimension in sensor design is proportional to the
Debye length. For gridded electrostatic sensors, a typical rule of thumb requires that
apertures be no larger than two Debye lengths [3]. This permits effective electrical
shielding of plasma constituents.
It may be observed that given the nature of the
previous expression, cold dense plasmas are the hardest to measure.
Data from eight Langmuir probes on the RAM C-II flight experiments have been
analyzed to obtain electron density and temperature measurements at various altitudes, Figures 1-1 and 1-2 [2]. Note that the temperature here is reported in units
of Kelvin, but a frequently used alternative is to specify the electron temperature as
kBTe in units of electron-Volts (where 1 eV e 11600 K). From the temperature and
density, a Debye length is calculated to determine the most stringent design requirements (Figure 1-3). The Debye length during reentry decreases from 1 mm at the
start of descent to approximately 4 lim near the end of the ballistic trajectory.
In addition to space plasma, many laboratory plasmas require sensors suited for
the harsh environments generated by large quantities of high velocity charged species.
An entire field of study devoted to plasma surface interactions seeks to improve the
materials used in fusion reaction chambers to prevent plasma quenching, material
18
10 18
10 1
o
*
x
3
1
+
*
10
Probe
Probe
Probe
Probe
Probe
Probe
Probe
1
s-
2
3
4
5
6
7
-
-
-
-
a
A
10105 I
1
-
16
-
10
-
S
I
t* Probe 8
55
60
65
70
75
80
85
90
Altitude (kin)
Figure 1-1: Electron number density from reentry flight experiments: RAM C-II data
collected by NASA in 1960s provided a measure of the electron number density, as
one might expect, the density increases as altitude decreases [2].
105
a
Probe 1
A Probe 2
o Probe 3
0 Probe 4
x Probe 5
+ Probe 6
* Probe 7
10
4
Probe 8
aa
1031
55f
-
~1aaaa
60
65
70
75
Altitude (km)
80
85
90
Figure 1-2: Electron temperature measurements from reentry flight experiments:
RAM C-II measurements of the electron temperature [2]. The decrease in temperature with decreasing altitude may be associated with lower craft velocity.
19
10
a#
102
13 Probe 1
S 10
-...
.. ...........
-.-...
A Probe 2
0 Probe 3
not34
Probe 4
x
Probe 5
Probe 6
* Probe 7
t* Probe 8
+
100
55
65
60
70
75
80
85
90
Altitude (km)
Figure 1-3: Debye length estimates from reentry flight experiments: RAM C-II electron temperate and density data for plasmas surrounding blunt bodies at hypersonic
speeds was used to obtain an estimate of the Debye length of these plasmas.
ablation, and to safely confine the fuel in the system. One typical fusion reactor is
known as a Tokamak. It utilizes a toroidal magnetic field to confine the generated
plasma.
The parameters characteristic of a plasma at the edge of a Tokamak is
reported in Figure 1-4 along with other magnetized and non-magnetized plasmas [4].
Surface mounted sensors could find application in monitoring the interior walls of
these large torus-shaped reactors. Alternatively, probes suited for these high-plasma
densities could be used in conjunction with helicon-wave plasmas designed to simulate
fusion reactor wall conditions.
On a smaller scale, microchip foundries utilize plasma tools pervasively throughout
their manufacturing processes to define features at sizes and uniformity not attainable by wet processing.
Ion implantation tools are utilized to dope semiconductor
substrates, reactive ion chambers use high energy plasmas to chemical etch semiconductor and dielectric material, and ion milling tools use the mechanical impact
of ions to ablate surfaces.
All of these reactors could greatly benefit from in situ
monitoring of the plasmas being generated to both better tune processing recipes
20
104
Earth
Magnetosphere
2
-- 103 K
--
Ionosphere
10
-10
10 4 K
10 K
106 K
K
10-2
Solar WindTokamak
10
Edge
10
Mic ro f'a bricati on R"''uied....
M
Pa
10-6Helicon.
Lasef-Produced,
Plasmas
10-8
10
10
10
10
Electron Density (m 3
Figure 1-4: Debye lengths for various plasmas as a function of electron temperature
and density: The square of the Debye length is inversely proportional to the density
of a plasma, and proportional to the electron temperature. From [4].
and increase uniformity and yield. Dry etching, or plasma etching, chambers utilize
plasmas with densities in the range of 1 x 1017 m
3
to 1 x 1018 m- 3 [5].
This ion
density is comparable to that of helicon plasmas. Large ion and radical densities in a
low energy plasmas help increase process etch rates while maintaining material selectivity in semiconductor fabrication. Typically an inductively coupled plasma (ICP),
or transformer coupled plasma (TCP), is used in these machines, but new plasma
sources are currently being investigated, including a helicon plasma [6].
Due to the large densities characteristic of most terrestrial plasmas, e.g., fusion reactors, semiconductor processing facilities, and plasmas generated during hypersonic
flight, more capable sensors are required. Of the numerous plasma parameters,
e.g.,
the ion and electron density, electron temperature, plasma frequency, etc., the ion
energy distribution plays a vital role in plasma etching and deposition processes [7].
This thesis reports a novel microfabricated retarding potential analyzer (RPA) that
incorporates the required small scales necessary to measure the ion energy distribution
in dense plasmas. Through three key concepts, the signal strength and resolution of
typical RPAs is improved. First, the required grid aperture sizes are defined using mi21
cro electromechanical systems (MEMS) processing techniques to enable the creation
of high aspect ratio holes with small diameters. These sturdier silicon electrodes can
be held in close proximity to one another without risk of buckling and shorting, thus
achieving an unprecedented inter-grid spacing of 200 pm or less in order to prevent
space-charge effects. Second, the alignment of successive electrode apertures is mechanically enforced to improve the transmission of ion species through the device.
The third and final advantage of the proposed RPA is the multiplexing of the signal
achieved by densely packing a plurality of these aligned aperture stacks in electrodes
all culminating onto a single detector.
22
Chapter 2
Plasma Diagnostics
The study of plasmas is of great importance to many disciplines, hence instrumentation is needed to accurately measure the properties of a plasma. In this chapter,
we will describe two of these probes, namely Langmuir's probe and the retarding
potential analyzer (RPA).
2.1
Langmuir Probe
The Langmuir probe is named after the scientist Irving Langmuir, who used it
to investigate glow discharge while at G.E. labs. The concept is simple, a wire is
introduced into the plasma to be analyzed and its potential is varied with respect to
the walls confining the discharge. The measure of the resulting current-voltage (I-V)
trace yields some insight into the plasma.
Since its introduction, much theory has contributed to the extraction of plasma
parameters from the resulting I-V characteristic, such as electron temperature, electron density, and plasma frequency, to name a few. Additionally, variations on the
single probe approach have led to the adoption of a double probe, triple probe, and
even quadruple probe as different means of gathering plasma data. All of these use
slightly different circuitry, and some probes are better suited than others to measure
certain plasma properties. In this thesis, focus will be centered on single and double
probe theory, both of which make use of only one swept voltage.
23
2.1.1
Single Langmuir Probe Theory
Single Langmuir probes are often custom built and used in laboratory experiments
because of their simplicity of implementation and operation. Many factors influence
the location of the transitions in the I-V trace, including whether the probe collects
current from a collisional or collision-less plasma, depending on the density, as well
as thin-sheath or orbital motion limited regimes, depending on the probe dimensions
relative to the Debye length. Additionally, the generally accepted assumption of
a Maxwellian electron energy distribution governs many empirical Langmuir probe
equations.
Vf
'-V
p
V
Ion saturation
region
Transition
region
e- saturation
region
Figure 2-1: Typical single Langmuir probe trace: The I-V characteristic for a Langmuir probe in a typical plasma is characterized by three distinct region, ion saturation,
transition and electron saturation.
For a Langmuir probe biased at a potential V near the plasma potential, V, there
is no plasma sheath, and the probe surface collects predominantly electrons such that
the current is essentially
I ~
1
e = eAp-ne
8kBTe
8kB7e
(2.1)
lrme
where I is the collected current, e the electron current, A, the probe surface area, e
the elementary charge, kB is Boltzmann's constant, and n, me and Te are the electron
24
number density, mass, and temperature, respectively.
In the transition region (in the center of Figure 2-1), with a voltage lower than
the plasma potential, electrons start being repelled. Only electrons with sufficient
kinetic energy will impact the probe, such that the minimum approach velocity, vmin,
is determined via
eVp =
!mevin -
(2.2)
-eV
1mevn = e(V - V)
The resulting electron current is then
Ie = eApn,
2kETe
Vrme
(
e(V - V)
kBTe
(2.3)
At the floating potential, Vf, the electron flux is equal to the ion flux, for zero net
collected current.
I(Vf) = 0
(2.4)
The ion current is always present in the transition region but is usually neglected.
In the ion saturation region, the voltage is sufficiently negative to repel incoming
electrons, and the ion saturation current I,.t
is measured.
Is,,.t ~- - 1eApniUB
(2-5)
where UB is the Bohm velocity, defined as
UB
=
kBTe
(2.6)
with the interesting combination of mi, the ion mass, and Te, electron temperature.
The Druyvesteyn criterion states that the plasma potential is the point where the
second derivative of the collected current is zero [8]
d21
=0
(2.7)
As indicated by Figure 2-1, this value is often approximated as the point where a
25
linear fit to the electron saturation region intersects a fitted line to the transition
region. Using the sum of the ion and electron currents, the following expression for
probe current is obtained
1
I-'
I =--
A
(V
2kBTe
eApniuB+ -eA~n,
2
2
exp -
7rme
-____
V)
(2.8)
kB Te
which assumes a constant collection area A,, Maxwell-Boltzmann electron energy
distribution, and equal number densities of ions and electrons. Subtracting i,.t from
the total current yields
1
I - iisat = -eApne
2 pe
2kTe
exp
mex
-
e(V -V)
V)
kETe
(2.9)
the logarithm can be taken to yield
(2.10)
In (I - Ii,at) = In
1
-eApne
2
2kTe
rm,
e(V - V)
-
kBTe
A linear fit with respect to V provides an estimate for the electron temperature
(2.11)
(In (I - Ij sat))]
Te = $ [
kB IdV
With an estimate of T, the number density can either be inferred from
kBTe
2irm,
exp
e(V - V
kBTe
(2.12)
(
Ie = eApne
)
7B"e
or
1
Ik
Te
I,sat ~ -- eApn VEme
2
m
(2.13)
the latter of which is preferred, as these two evaluations may differ by up to a factor
of five, due to the assumptions in the calculations [8].
The typical means of analyzing the single probe characteristic is to subtract the
ion saturation from the curve to make the trace entirely positive. The difficulty arises
in that for sufficiently negative and sufficiently positive potentials - i.e., in the satu-
26
ration regions - the effective collection area grows for ions or electrons, respectively.
Extracting exact plasma parameters by the preceding method may result in errors
from a failure to effectively separate the contribution to the current of ions from that
of electrons.
2.1.2
Double Langmuir Probe Theory
A double Langmuir probe is often preferred over a single probe as it does not draw
any net current from the plasma. Furthermore, the applied voltage is between the
two probes, and does not require a reference voltage, usually chamber ground in the
case of a single Langmuir probe (Figure 2-2) [9].
VC
VO ~
V
O
Idl A
Figure 2-2: Double Langmuir probe plasma measurement: The floating double probe
setup draws no net current from the plasma it measures. For a symmetric double
probe characteristic, each electrode should have the same dimension, and no potential
gradient should exist in the plasma.
With no contact potential or probe-to-probe variation in the plasma potential, if
a 0 V difference is maintained between the two Langmuir probes, both would have
the same voltage and no current will be flowing through the circuit. Now, if a bias is
applied, one probe (say probe 2) will become more positive than the other and will
attract more electrons, while the other will move further away from plasma potential
gathering fewer electrons to maintain no net current draw from the system. Current
27
will be flowing through the circuit (from probe 1 to probe 2) and between the probes
within the plasma. With a large bias, the difference between the probes may be so
large that all of the electrons are collected by a single of the two probes, the other
probe being so negative that no electrons can reach it. In this case, half of the
electrons will flow from the relatively positive probe (probe 2) to the other in order
to match the incoming ion current.
Typically, however, there is an increase in sheath thickness associated with decreasing voltage. As the voltage difference becomes larger, the sheath surrounding
the negatively biased probe expands gathering more ions. Consequently, more current
must flow through the circuit. Symmetry in the double probe setup (when the probe
surface areas are identical) would state that reversing the bias should only reverse
the sign in the measured current.
Neglecting sheath expansion, then, the total ion current flowing to the probes is
simply the sum of the ion saturation currents, or double the maximum current,
I,sat = I',satj - I'sat2 = Ii,saaij I+ |Isat 2 = 2 1Ii,sati 1
(2.14)
and electron current from the plasma to a probe is the total current reaching the
probe less its ion saturation current, or the measured circuit current minus the ion
saturation current.
Ie2
=
(2.15)
Id - 'i,st 2
Because the net current to the system must be zero, as the double probe is floating
4Iat = Iah Il + Is,=t2 I= IIell + Ie21
(2.16)
Substituting the Boltzmann relation, and assuming equal probe areas, Ap,
hIsat =
Where ji and
j2
Apji exp (-kTe ) + Aj 2 exp
are the electron current density near probe 1 and 2 (i.e.,
28
(2.17)
-kBTe
ji =
ene),
respectively, and V is the difference between the plasma and probe surface potential
(Vo) at probe 1.
(2.18)
Vi = V,1 - Vo
With the differential voltage between the probes defined as Vd = Vo 2 - Vo0 and assuming some plasma potential variation between the probes V = V, 2 - Vp, (Figure 2-2)
yields
V1 = V, 2-V
(2.19)
-V%+Vd
V1 = V2 -Vc+V
Entering this relation into the total current equation
I,eat =
Aji exp
=
Aji exp
i,sat
I!,
|Ie2|
= Apj1
=
A~j2
(
(e(V
exp-
1 =
1e21
In
'lat
1
j2
=In
|Ied|
2-V
+ Aj 2 exp -
2 -V+d))
+ Ie2)
(220)
e(V2 -Vc+Vd-V 2))+
kBTe
k~
(2.21)
e
(2.22)
(
e(V-
ii exp
(j2
Plotting the logarithm of the ratio of 2
kBTe
kBTe
-
~
kBTe
(2.23)
against the biasing voltage, and fitting a
line to the region between ion saturation current will yield an estimate of the electron
temperature. The slope of this line is unaffected by probe areas, contact potential of
the probes, and differences in plasma potential between probes [9].
In the case of the thin-sheath limit for the double probe, the probe current has
the form
I = Apenm
2ire tanh
eaiyid
w
eV
2kBTe
(2.24)
t
The second derivative of this equation yields two extrema that may be used to deter29
mine the electron temperature by the separation between these peaks, AV as
kBTe =
e
A(2.25)
In (22
Although meant for the thin-sheath limit, applying this formula to the orbital motion
limit, or even the transition between the two regimes for probe collection, yields
adequate results with only 5 % error in the worst case [10]. Corrections to account
for the expansion of the plasma sheath with changing probe potential have been
suggested by making use of the Laframboise graphs [10,11]. However the correction
factors require iteratively solving for one of the dimensionless probe potentials, '1
through the following implicit relationship
01 = -In
(1+ /
')+
(1+
)]
+ ln (1+ exp,)
(2.26)
Where q is the dimensionless biasing potential between the probes (normalized to
the electron temperature in eV), Xf is the dimensionless floating potential, and
#
and
a are parameters governing the probe sheath determined by the following empirical
formulae [10]:
a =
2.9
#3=1.5+
+ 0.07
+ 2.3
In
0.85 +0.135
Xf = 1 In
2
( Mi )+
[In (
-i
Te
-0.34
(2.27)
r)]}
(2.28)
aln ( - Xf)
(2.29)
Not only does the floating potential require the iterative solving of the above equation,
but also every data point requires solving for 01 implicitly. Furthermore, this process
assumes an ion temperature, and after curve fitting, the ion and electron densities
should be used to check for self-consistency of the a and
8 terms, which may also
require iteration.
Experimental measurements may contain certain asymmetries or offsets. A differ-
30
ing slope between the two ion saturation regions might suggest unequal probe areas,
and a shifted crossing from the origin may indicate different contact potential, or a
variation in floating potential due to a gradient in the plasma [12].
2.2
Retarding Potential Analyzer
In the 1950s, as a means of measuring electron velocity from electron guns for
scanning electron microscopy, Boersch made use of a gridded energy analyzer [13].
Since then such sensors have been used onboard satellites, to measure ion drift velocity and ionospheric chemistries [14-19]; to measure plasma propulsion systems,
for efficiency calculations [20-23]; as well as in commercial plasma sources, such as
plasma etchers [24]. Still, these devices rely on the same basic structure: a stack of
metallic meshes and insulators backed by a collector plate.
2.2.1
Theory of Operation
In its simplest form, a Retarding Potential Analyzer (RPA) consists of mesh grids
held at nearly static biasing voltages. Figure 2-3 shows the potential hill that positive
biasing establishes on electron-deficient ion flux. If the kinetic energy of these ions is
insufficient to overcome the potential, ions cannot penetrate this central grid. The two
negatively biased grids screen electrons from the flow, so that only ions are analyzed.
The purpose of the first grid is to minimize the sensor's impact on the surround
plasma, it is kept floating such that only the plasma's self shielding potential appears
on this electrode, which for quasi-neutral plasma's is near zero.
An RPA selectively repels ions with a retarding potential bias applied to a grid
that acts as a high-pass filter, only allowing ions with a sufficient energy-to-charge
ratio to overcome the potential bias and reach the collector electrode. As a result of
this effect, the negative collector current plotted against the retarding voltage gives
a measure of the cumulative voltage distribution function. Its derivative is related to
31
V
Vf
Ve
Vion Ve
l
Collector
|7
Figure 2-3: RPA schematic: An RPA operates by "trapping" ions that penetrate
through a floating grid, Vf, and negatively biased electron shield, the leftmost Ve_.
Given sufficient energy, these particles surpass the swept voltage on the central electrode, Vm,, to be gathered as current on the rightmost electrode, the collector.
the voltage distribution function via the following equation [25].
dIIc
_
d~=on
Zie2 n A,
-f(V)
(2.30)
Where Zi is the ion charge-state, e the elementary electron charge, ni the ion number
density, AP the probe's effective collection area, and mi the mass of the ions.
Hofer defines the spread in ion energy as the dispersion efficiency,
rid
=
(Vo)
TId
[21]
(2.31)
This metric is characterized by the full width at half maximum (FWHM) of the ion
voltage distribution.
In most cases a three-grid RPA may be used, removing the last electron repelling
grid as secondary emission from ion bombardment of a stainless steel surface at energies below 1 keV is not a concern [23]. However, four-grid RPAs are still the most
common. A more important metric is the grid aperture size. These have to be chosen
such that they are smaller than the sheath thickness of the plasma to which they
32
are exposed in order to ensure effective filtering by the bias potential. The sheath
thickness varies depending on the type of plasma, and may be as large as 5 to 10
Debye lengths [23], but again, the typical design rule is to make apertures smaller
than approximately 2 Debye lengths [3]. Space-charge limitations beyond the electron repelling grid may result from the presence of a large charge density in the space
between this electrode and the ion retarding grid. These charges make their fields felt
in the space between grids resulting in a larger ion retarding bias than that applied
by external power supplies, or a virtual anode (in the case of ions). To avoid spacecharge, the maximum permissible spacing between grids corresponds to the thickness
of a sheath, t, for a specified potential difference. Equating the Bohm flux to the
Child-Langmuir flux yields
t, = 1.02Ad
eV
(kT2
(2.32)
where V is the potential applied across the sheath [23]. For an electrode at the floating
jln ()
potential, Hutchinson states that ev oc
For hydrogen, the smallest ion,
this is roughly 3.75. The resulting sheath thickness of approximately four Debye
lengths is the recommended maximum spacing between RPA electrodes [3].
Azziz relates the collected current to the ion velocity distribution via
I(V) = AZienj
J
uif(ui)du
JuMv
(2.33)
with A, the probe collection area, ni the ion number density, Zi the ion charge-state,
e the elementary charge, V the ion retarding potential, ui the ion velocity, and f(ui)
the ion velocity distribution normalized to unity [23]. Relating the velocity to the
energy in eV
u=
2Z~eV
VMi
(2.34)
a change in variables yields
du=
2Zie 1
dV
mi t A
(2.35)
and
f(u)du = f(V)dV
33
(2.36)
After substitution into the equation for collected current,
I(V) = AZjen
2Z
/
IVf
YeV
(V)dV
(2.37)
differentiation yields the ion energy (or voltage) distribution function [23]
dI
-- = -AZieni
dV
2ZieV
f(V)
Mi
(2.38)
The coefficients in front of this expression differ slightly from the previously suggested
distribution equation [25]. In fact an estimate of the RPA collection area is also
necessary, and makes use of a few assumptions as well, most notably that particle
flux through a single grid is proportional to its optical transparency. Simulations
have shown that this later assumption may not be well suited for RPAs [26-28].
Consequently, a simple proportionality is often assumed, and the distribution function
is often normalized to unity [20].
f(V) c
2.2.2
dI
dV
(2.39)
Design Criteria
The greatest challenge in designing an RPA is its compatibility with the plasma in
question. The Debye length drives the critical dimensions of any plasma sensor. This
characteristic length related to how far an electric field penetrates a plasma decreases
with increasing particle density and diminishing temperatures. As a result, dense
cold plasmas are the most restrictive in terms of RPA design. An ideal sensor should
be capable of spanning a wide range of plasmas. By designing for the worst-case
scenario (i.e., smallest Debye length), without sacrificing signal-to-noise ratio, a more
versatile plasma sensor can be achieved.
Several RPA designs have been suggested to accommodate for very dense plasmas
typically encountered in the analysis of pulsed plasma thrusters. Marrese designed
such an RPA using a single 200 jim channel [29]. This sensor was then modified
34
Table 2.1: RPA review: A comparison of this work (hybrid and MEMS RPAs, starred)
to others found in the literature. The hybrid and MEMS sensors demonstrate the
smallest grid aperture for aligned electrodes, and narrowest inter-grid spacing.
Author
Sense Ap. (mm) Grid Ap. (pm) Grid Space (pm)
T
[ans.
Mat.
Ref.
Semion''M
0.83
15
235
34%
Ni
[7]
Beal
Azziz
Hofer
18.54
6.35
18.6
279
140
300
1727
500
1700
38 %
72 %
38 %
Cu
Mo
SS
[22]
[23]
[25
200
100
100
457
300
200
N/A
Mo
40 % W on Si
5.7 % Au on Si
Aligned electrode apertures
Marrese
Hybrid*
MEMS*
0.2
6.35
7.5
[29}
[31]
[32]
by Partridge who introduced a collimator and micro channel plate [30]. However,
although these tactics increase the range over which the sensor may be applicable, all
reduce the sensed signal strength.
With the advent of micromachining technology, finer dimensions and more stringent tolerances can be rendered to improve RPA performance over the current stateof-the-art. Using complimentary metal oxide semiconductor (CMOS) processing, silicon devices can be made with incredible accuracy. Critical RPA dimensions and tight
tolerances in assembly can be precisely controlled to achieve exceptional alignment in
electrode stacks made from a silicon substrate. Other advantages, such as batch fabrication of sensors would alleviate manufacturing costs and increase throughput when
compared to custom sensor design and construction while simultaneously improving
dimensional accuracy.
In this thesis, a novel RPA design is proposed that harnesses batch microfabrication technology to: (1) create the smallest electrode apertures and inter-grid spacing
in order to sense plasmas with the smallest Debye lengths, (2) incorporate the most
electrode openings feasible to increase the signal strength through multiplexing, and
(3) enforce inter-electrode aperture alignment to maximize signal transmission to the
collector plate. A hybrid RPA is first built that utilized microfabricated grids into
a conventional RPA. This sensor demonstrates the smallest aperture diameter for
35
an RPA with aligned electrodes (compared to Marrese's single-channel RPA [29],
see Table 2.1), and utilizes a plurality of these channels. The MEMS RPA achieves
the smallest inter-electrode spacing, and despite a reduced open area fraction, will be
shown to have greatly increased effective transmission as a result of precise grid alignment. Table 2.1 summarizes the size of the sensor aperture (Sense Ap.), the smallest
grid opening (Grid Ap.), inter-electrode spacing (Grid Space), optical transparency
of a single grid (Trans.), and construction material (Mat.) for various RPAs. For
unaligned grids the overall device transparent may be estimated by multiplying the
open area fraction of all grids together, for identical grids this translates to raising
the transparency reported in the table to a power corresponding to the number of
grids in the RPA, usually three or four. The MEMS and hybrid RPAs, since they
have enforced grid alignment, maintain the transparency of the grid throughout the
device and do not suffer the same attenuation.
36
Chapter 3
Hybrid RPA
3.1
Hybrid RPA design
The first-generation RPA incorporating microfabricated electrodes had overall exterior dimensions similar to previous designs [23,25,29,30]. The innovation in this
sensor lies in the precise grid dimensions and enforced mechanical alignment of successive electrode apertures across the grid stack. The only other RPA to date to
suggest aperture alignment consists of a single channel [29, 30]. Other sensors attempt to apply RPAs to dense plasmas either through attenuating the density with
low transparency grids, or through extensive collimation, thus degrading the signal
strength.
To maintain design flexibility, the sensor was built with modularity in mind. Contacts made through spring-loaded pogo pins remove the need to weld wires to electrodes. The travel of these springs allows for various thickness electrodes and insulating spacers. Additionally, the ability to position the entire set of contacts at various
locations by use of a set screw permits the addition or removal of grids. A housing
was machined out of 316 stainless steel for its resistance to secondary electron emission, durability in plasma applications, and applicability to vacuum environments.
Rails that guide assembly make light work of replacing electrodes and maintain electrical insulation. The use of rails as sidewall standoffs, instead of an insulating sleeve,
mitigates shorting through surface breakdown by relying on vacuum gap electrical
37
1 CM
alumina rails
and spacers
housing
pogo pins
collector
grids
Figure 3-1: Hybrid RPA schematic: The first sensor with microfabricated silicon electrodes includes alumina rails to enforce alignment to the order of the manufacturing
tolerances of the conventional parts, i.e. one thousandth of an inch or about twentyfive microns.
insulation.
Unlike previous RPAs, this sensor utilizes silicon-based electrodes manufactured
using Complimentary Metal Oxide Semiconductor (CMOS) batch microfabrication
technology. With the dimensional resolution micromanufacturing techniques enable,
grid apertures and alignment features can be incorporated to enforce stringent alignment of electrode apertures.
Furthermore, etch depths are well defined in CMOS
manufacturing methods with the use of etch stop layers, or simply through timed
etches.
This is the first RPA that makes use of these fabrication techniques. Due to its
incorporation of conventional Computer Numerically Controlled (CNC) machining
techniques in the RPA's steel housing as well as CMOS processed internal components, namely the grids and collector electrode, this sensor is dubbed the hybrid
RPA.
38
3.2
3.2.1
Fabrication
RPA Housing
The housing was turned from a piece of steel stock to define the sensor's outer
diameter as well as the threads for convenient mounting in the test chamber. Internal
features were then CNC machined using a square end mill. These include a recess for
an alumina washer to isolate the first electrode from the housing, the inner diameter
was chosen to keep alumina rods in close proximity to the electrodes to ensure their
alignment, and three lobes were cut into the interior sensor wall to provide space for
alumina spacers that set the gap between successive grids. The part was machined
with a thousandth of an inch manufacturing tolerance, or 25.4 pm.
3.2.2
Packaging and Electrical Connections
Instead of using a compression spring to maintain all of these components in
close contact, pogo pins from the Everett Charles Technologies company were used
to make electrical contact and compress the entire assembly. These were held in
a Vespel@ (polyimide-based, DuPontTM high-temperature plastic), which was also
CNC machined. A tapped hole was later added to the threaded portion of the housing
to allow a set screw to hold the assembled components in place.
3.2.3
Grids
The grids themselves originated from a 700 pm-thick n-type doped 150 mm silicon
wafer with a resistivity of 0.1 0-cm to 0.2 0-cm (Figure 3-2 (a)). These double-side
polished (DSP) wafers were ordered from Ultrasil with 0.5 pm of protective thermal
oxide. The wafers are first stripped of their oxide using hydrofluoric acid (HF). They
are then coated with 1 pm of photoresist (OCG825, Fujifilm) which is exposed for
2.5 s with an alignment mark pattern. After developing, a timed etch in hydrobromic
acid (HBr) and chlorine (C12) chemistries removes 250 nm of silicon using an Applied
Materials Precision 5000 reactive ion etcher (Figure 3-2 (b)). The photoresist is then
39
(b)W
(c)
-
-
(a)
-
(g)J
(h)
(d)
(e)
Silicon
vinoiimiii
()
amm
(k)
amm
U Silicon Oxide U Photoresist U Quartz
Figure 3-2: Hybrid RPA microfabricated electrodes: (a) Machined from a 700 pmthick silicon wafer, (b) alignment marks are etched for front to back alignment,
(c)
a hard oxide mask is deposited and (d) etched for (e) DRIE of the apertures and
recesses. After (f) mounting the wafer to a quartz substrate, (g) the remaining depth
of the apertures is etched with the cutouts. (h) Pieces are cleaned and stripped
of oxide before (i) growing a thermal oxide to smooth the etched surfaces. (j) A
final oxide strip is followed by (k) a metalization step where the grids are coated in
tungsten.
stripped and the wafers are cleaned using the RCA standard. The top side of each
wafer is then coated with 4 pm of Plasma-Enhanced Chemical Vapor Deposited oxide
using an Applied Materials Centura system 5200 (Figure 3-2 (c)). This layer is then
densified through annealing in a nitrogen environment for an hour at 900 C.
Next,
10 pm-thick resist (AZ4620, AZ Electronic Material) is applied to both sides. The
top side is exposed with a contact mask consisting of the grid apertures as well as
grid outlines/borders which will eventually cut out each electrode with three notches
as alignment features. Using the crosshair overlay feature of the Electronic Visions
EV620, the backside is exposed with a mask containing the same grid apertures as
well as circular recess for ceramic spacers. After developing, the top oxide layer is
removed using a dry plasma etch of trifluoromethane (CHF 3 ),
tetrafluoromethane
(CF 4 ) and argon (Ar). The etch is stopped intermittently and Ar is flowed to help
40
cool the substrate and prevent the photoresist from burning (Figure 3-2 (d)). The
backside of the wafer is then etched using a Deep Reactive Ion Etching (DRIE)
recipe in an ST Systems Multiplex ICP tool. Using sulfurhexafluoride (SF6 ) as the
etching gas, and octafluorocyclobutane/Halocarbon C318 (C 4 F8 ) for the passivation
cycle, 350 jm of silicon are removed (Figure 3-2 (e)). The wafer is then mounted
to a quartz wafer using photoresist and the same DRIE recipe etches the remaining
silicon in the apertures and releases the grids (Figure 3-2 (f) and (g)). Following the
dismount of these pieces in an acetone bath, the grids are cleaned using an oxygen
plasma asher to remove residual passivation polymer and photoresist. The pieces are
then cleaned and stripped of their oxide using HF (Figure 3-2 (h)). To smooth the
sidewall surfaces, 1 im of thermal oxide is grown in a wet environment (i.e. hydrogen
and oxygen pyrolytic), Figure 3-2 (i). Again the oxide is removed, and finally, the grids
are coated with sputtered tungsten (W) or another metal using an AJA international
Orion 5 sputter system (Figure 3-2 (j) and (k)).
3.2.4
Assembly
With the grids complete, device assembly is rather straightforward (Figure 3-3).
First an alumina washer (ALW00283, LSP Ceramics) is fitted to the steel housing.
The first grid is inserted with spacer recesses facing the back of the sensor, and three
alumina spacers (AL203-SP-B-025, Kimball Physics) are inserted into the electrode
recesses. Next three alumina rails (AL203-TU-B-500, Kimball Physics) are placed
into the alignment features. Two or three more grids are inserted followed by their
inter-grid spacers, depending on the sensor application. And lastly, the collector is
placed behind the last grid, followed by the Vespel@ holder and pogo pins (HPA-0,
Everett Charles Technologies). A set screw is tightened to hold the sensor together,
and wires can be attached to the pogo pin receptacles (SPR-OW-1, Everett Charles
Technologies). The sensor's overall grid alignment is within a few thousandths of an
inch, or several tens of microns.
One of the advantages of the hybrid RPA design is its capacity to be retrofitted
with other mesh grids. Typical RPAs use ceramic washers to separate stainless steel,
41
Figure 3-3: Hybrid RPA assembly: Construction sequence of the hybrid RPA starts
with the insertion of an alumina washer to stand off the grid stack from the housing.
The pogo pins are soldered into their Vespel@ holder. Each grid is inserted followed
by three alumina spacers that serve to enforce a 300 pm gap between them, and,
along with the alumina rails, prevent shorting with the housing sidewalls. The stack
is finished with a collector, and the Vespel@ holder is held in place with a setscrew
before closing the back of the RPA.
copper, or molybdenum meshes with spot welded wires. In their design, however,
inter-electrode hole alignment is not enforced. The hybrid housing may be utilized
in this fashion to create a benchmark sensor with which to compare the performance
of microfabricated electrodes.
In this work, an RPA made using the housing and
alumina spacers of the hybrid RPA along with electrodes machined from commercially
available meshes is called a conventional RPA.
42
3.3
Hybrid RPA Ion Source Characterization*
In order to determine the performance of the sensor, a reliable plasma or ion source
with control over the ion energy is necessary. Since mass spectrometry requirements
demand that ion sources have low energy with little energy spread to obtain high
resolution spectra, they present an adequate choice for a benchmark test. An Ardara Technologies SlimlineTM ionizer from a lab-built mass spectrometry system was
used to generate and accelerate ions with air as the analyte. The commercial ion
source consists of a gold wire cage, which sets the ion energy, surrounded by four
tungsten-iridium filaments powered by a current-controlled supply with variable bias.
With the bias set 50 V below the ion region, thermionic electrons are emitted from
the heated filament and reach the ion region with approximately 50 eV of energy.
Through electron impact, air is ionized within the cage and subsequently accelerated
toward a grounded (or slightly negatively biased) extraction lens. After one or more
electrostatic lenses, these positive ions travel a short distance toward the RPA under test. All electrical connections previously mentioned are made through a ten-pin
feedthrough to an Ardara filament power supply and an optics supply. It is impor-
tant to note that in these experiments, the first RPA grid - the floating electrode
is removed from the device. Were it not, ions accelerated toward the sensor would
positively charge the floating grid, thus resulting in self-shielding of the ion beam
from the rest of the sensor, and most importantly, the collector plate. A positively
charged grid, with no conductive path aside from impinging ions continues to amass
charge until no further ions may reach the electrode due to insufficient kinetic energy.
In the aforementioned test, the sampled medium consists of an ion beam, rather than
a quasi-neutral plasma.
The experimental procedure involves first installing the RPA onto the flange and
making electrical connections via micro high voltage (MHV) connectors. The chamber
is then pumped to a pressure of approximately 1 Torr before the turbo molecular pump
*Other works refer to "calibration" of RPAs using a known or commercial plasma source for the
sake of measuring a new thruster or laboratory plasma [331.
43
electrical
feedthroughs
10" flange
SlimLine TM
ion source
hybrid RPA
Figure 3-4: Mass spectrometry ionizer energy measurement experiment: The RPA is
installed in a custom testplate downstream of an Ardara Technologies SlimLineTM
ionizer. In this setup, the floating grid is removed to prevent self-shielding of the ion
beam under analysis.
is turned on. Typically, the chamber is pumped for eight hours or over night to reach
a base pressure around 1 x 10- 7 Torr. Air is then allowed into the system through a
leak valve, and typically stabilizes to about 3 x 10- 5 Torr within half an hour. The
base operating conditions for the filament supply are setting an ion region of 10 V
and electron energy of -50 eV with an emission current of 0.2 mA. The extractor and
optic lens are typically kept at 0 V relative to ground. Figure 3-4 shows the ionizer
at the leftmost end of the image, the RPA is installed near the center in a custom
testplate. The right most end of the figure shows the ten-inch flange, with holes where
several BNC and MHV connectors are welded. During testing, this flange rests atop
a cylindrical chamber, with the ionizer at the bottom.
For a benchmark comparison to the hybrid RPA with microfabricated grids, a
conventional RPA is first tested in this chamber. This more typical sensor is assembled in the same steel housing as the hybrid device.
Electrodes are first cut from
photochemically perforated stainless steel using a waterjet. Two assembly methods
were considered. Figure 3-5 (a) utilizes ceramic washers having an inner diameter of
0.094" (2.38 mm), an outer diameter of 0.25" (6.35 mm), and a thickness of 0.032"
44
(813 llm).
To avoid misalignment of the three washers, the modified assembly in
Figure 3-5 (b) was used. In this fashion, the spacing is reduced to the thickness of
the alumina spacers from the hybrid sensor 0.025" (635 pm), and the full 6.35 mm
sensor aperture can be utilized. The alumina rails prevent shorting of the grids to
the housing hold the spacers in place. The conventional RPA grids are 127 pm-thick
(a)
alumina washers
or spacers on rails
(b)
steel grids
and collector
Figure 3-5: Conventional RPA construction: The hybrid RPA design was made modular such that the micromachined electrodes could be replaced easily, which permit the
construction of a conventional sensor for benchmarking purposes: (a) shows the typical assembly with ceramic washers, in this work (b) was used to maintain the largest
transparency possible. Note that here the alumina rails do not serve the purpose of
alignment, but rather keep the grids electrically isolated from the housing.
with hexagonally packed apertures of 152 jim having a pitch of 280 jim. The resulting transparency of each grid is 26.73 %.
Note that in this setup, the rails serve no
alignment purpose, and no precaution was taken to attempt to increase overall device
transmission. Data collected with the standard RPA are reported in Figure 3-6 for
various ion energy values. In this experiment the ion energy region was varied from
10 V to 20 V in 2V increments. The reason for performing this sweep was due to the
observed shift between the reported most likely energy of 5.5 eV, and set ion energy
of 10 eV, or a difference of 4.5 eV. The fact that through the sweeping of the ion
energy the shift remains a fairly constant value of 4.5 eV to 5.5 eV detracts from the
45
8
20 V
6
-*-
0
A
i
184V
16 V.
....
-- 14 Vio
4
12 Vion
-.-
-
-10
10V
-5
0
5
10
15
20
Retarding Potential (V)
Figure 3-6: Conventional RPA ion energy sweep: As the ion energy region is increased
in increments of 2 V, the sensor effectively tracks the energy increase, but a constant
offset of approximately 5 V persists which further tests attributes to limitations of
the ionizer. The 0.5 pA V
resolution result from limitations of the Keithley SMUs
used to set the biasing voltages
likelihood of charge exchange collisions as the source of the anomaly.
The signal from the conventional sensor is rather weak, such that the resolution
limit of the Keithley 237 SMU power supply may be seen (jumps of roughly 0.5 pA/V).
Since secondary electron emission is unlikely for steel at energies below 1 keV, the
second electron repelling grid can be added to the collector current to amplify the
signal (Figure 3-7).The increase in signal strength by a factor of roughly three is
consistent with the grid transparency (slightly less than a third). Through this process
the sensor is effectively transformed into a two-grid RPA (with an outer floating grid,
this sensor is equivalent to a three-grid RPA).
Other causes for the shift in signal were considered such as material workfunction, space charge limitations, and, as mentioned previously, charge exchange and ion
neutral collisions.
Tungsten and gold coatings were used for the hybrid sensor, in
addition to the steel grids of the conventional RPA. However, these materials have
workfunctions in the range of 4 eV to 5.5 eV, making a clear distinction difficult to
46
16 . 14
-
- -
-
182
Vion
-18
o16nV
12
10 -..-
14V
..
4--o
-2
-10
-5
0
5
10
Retarding Potential (V)
15
20
Figure 3-7: Modified conventional RPA energy sweep: Adding the secondary electron
repelling grid current to that of the collector increases the signal strength proportionally to the inverse of the grid transparency while maintaining the same 5 V offset.
observe.
The RPAs were designed with high plasma densities in mind, and the observed
traces do not seem to indicate a space-charge limited operation. A few parameters
may be changed to investigate whether space charge within the ion source or in the
experimental setup plays an effect. These are namely changing the working distance or
ion density. Altering the chamber pressure or emission current would be two means of
modifying the density, as ion current is proportional to pressure and electron current
in impact ionization systems.
The effect of a changing emission current was investigated, and the results for a
10 V ion region and operating pressure of 3 x 10-5 Torr are shown in Figure 3-8. As
the stray filament current increases, the ion current is also expected to increase, with
more ionized species being generated through more frequent collisions. The primary
effect of this amplified current is an increase in detected signal strength which is
clearly visible as an increase from about 4 pA V
to nearly 14 pA V-
1 at
0.2 mA electron emission current
at 0.5 mA emission. Additionally, the higher peaks are shifted by
47
--
-- -
10
-0--0.5 mA
8
--e-0.45 mA
-0.4 mA
> 6 -
~w-0.35 mA
-A0.3 miA
-
12 - - - -
--
--
-
--
-.
-
-
-
141-
....
....
.....
A
S 4 - +-0.25 mA ...........
2
-+-0.2 miA
...
-...
-..
-.
.-.-
0
-10
-5
0
5
Retarding Potential (V)
10
Figure 3-8: Conventional RPA electron emission sweep at 10 V ion energy and
3 x 10- Torr: Increasing filament emission current while maintaining a fixed ion energy region results in the expected increase in collector current, but is associated with
an increasing offset from the prescribed ion energy.
a greater amount from the prescribed 10 V ion energy region, with peaks moving from
5 V to 3 V, or a shift increasing from 5 V to 7 V when the current is increased from
0.2 mA to 0.5 mA. If space-charge were limiting the flow of ions out of the ionizer, the
current would be expected to reach a maximum and plateau with increasing electron
emission current. Instead, a near linear relationship exists between the RPA total
collected current and electron emission current (Figure 3-9). This is consistent with
the electron impact ionization model from [34]
I, = IenLo-
where I. is the ion current, Ie the electron current, n the gas number density, L the
ionization length, and
1 x 10-16 cm 2 ).
- the ionization cross-sectional area (usually on the order of
For a given pressure, then, the ion current is linearly related to the
electron current. The slight shift with increasing current might then be due to some
negative charge buildup on the ion cage, or more ions being generated closer to the
48
extractor plate where the potential is lower.
600-R2= 94.2%
U
400 ........
.
Q500--
- .-.-.-
300
U
200 -
1 00
-.
--..
..
.--..-
-..
-..-..945.5
0
0.1
0.2
0.3
0.4
Electron Emission Current (mA)
0.5
Figure 3-9: Conventional RPA total collected current versus electron emission current: Increasing filament emission current while maintaining a fixed ion energy region
results in increased current to the sensor, thus out of the ionizer. If space-charge were
present, the current should plateau to a maximum. The slope of the linear fit through
the origin, and associated R-squared, are reported on the plot.
The other parameter altered to explore its impact on the data was the pressure. This last change consisted of shutting off the leak valve. The chamber shortly
reached a new pressure near 6 x 10-
7
Torr, and the emission current was once more
swept while keeping the ion energy region constant.
Again the data is more easily
visualized if the secondary electron repelling current is added to the collector trace,
so as to increase the signal strength (Figure 3-10). At the reduced pressure, the conventional RPA seems to indicated a most probable ion energy of approximately 7eV
at 0.2mA electron emission current, decreasing to about 5.5V at 0.5mA emission.
Interestingly enough, the current signal does not show a monotonically increasing
value with increasing electron emission, rather, it peaks at about 0.25 mA to 0.3 mA.
In addition to these experiments, axisymmetric simulations of the ionizer were
performed with Charged Particle Optics (CPO programs, free versions available at
www.electronoptics.com).
Ray tracing data indicate that ions that originate from
locations closer to the extractor are more likely to exit the lens aperture. These ions
49
3.
-+-0.5 mA
2.5
0.45 mA
2
-+-0.4 mA
--- 0.35 mA
-
...
- A
0.3
mnA
1
--
-
1.5
--
0.25 mA
-- +-0.2 mA
0.5
-0.5
-A
10
-5
0
5
Retarding Potential (V)
10
Figure 3-10: Conventional RPA electron emission sweep at 10 V ion energy and
6 x 10-' Torr: Increasing filament emission at a lower operating pressure seems to
have little effect on the collector current. This may be due to a reduced ionization
cross section, and could point to the electrons as the main reason for the energy shift.
are born in regions where the potential is reduced compared to the cage voltage (Figure 3-11).
It may be important to emphasize the general operation of an electron
impact ion source. The cage potential is responsible for setting the voltage at which
ions are born, however, an electric field must penetrate this region in order to preferentially accelerate ions out of the cage toward the optical components. The presence
of this field will influence the potential at which these charged particles are born and
will in fact contribute to a shift in the ion energy. In addition to the extraction lens
(maintained at ground potential in these experiments), the other end of the cage is
also open to allow the sampled gas to flow. This open end is influenced by the negative filament voltage, which also penetrates the cage. The two ends of each filament
are connected to respective discs surrounding the ion cage. The negative end (Fil. -)
is held 50 V below the ion region, setting the electron energy to -50 eV. The positive
end (Fil.
+) is slightly positive and closed-loop controlled to maintain the prescribed
electron current emitting from the filament surface.
Still the majority of ions exiting the ionizer should reflect the potential set by the
50
Grounded Extractor
and Aperture
Fi-
Fil.
U
+
Ion Region
(a)
(b)
(c)
(d)
I
I
im
__j
(e)
(f)
Figure 3-11: Simulation of ion source using CPO 2D: Ray tracing of ions born with
zero energy at various locations within the ionizer cage at (a) 10 V (b) 12 V (c) 14 V
(d) 16 V (e) 18 V (f) 20 V.
51
cage voltage. There should actually be a rather sharp cutoff at the maximum energy
the ions can have, which the hybrid and the MEMS detect at 4 V below the ion
energy region. This may be attributed to a buildup of charge on the gold wires that
make up the ion region cage. As a result of years of testing MEMS quadrupole mass
analyzers, operating at pressures above recommended, with no cleaning of the ionizer,
it is possible that a buildup of the calibration mass perfluorotributylamine (FC-43),
through plasma polymerization, now coats the cage with a thin layer of fluorinated
polymer insulator. Negative charge buildup could explain the shift in energy.
In reality, the difference between the prescribed and detected energy may be a
combination of a number of these factors. Still, the calculated distribution and collector current trace, show that no ions exist with energies in excess of the ion energy
region, as should be expected by the experimental setup. The conventional RPA data
shows a distribution that is not gaussian, but rather drops abruptly to zero at 4.0 V
below the assigned ion energy region, nearly exactly. This behavior is to be expected
for this type of electron impact ionizer, where the maximum accelerating potential
is that of the cage, and progressively decreases due to field lines penetrating the ion
region from the extractor plate
[351.
The ion density within the ionizer may be esti-
mated as the electron emission current, e, multiplied by the ionization efficiency, x,
divided by the ion velocity, vi, and ionizer cage cross-sectional area, A.
I
eX
vi A
(3.1)
Where the ion velocity resulting from the accelerating potential, as set by the ion
energy region, Vin and for a singly charged ion of mass mi, is
Vi=
[2Voe
V
Mi
(3.2)
Since the analyte is air, the ion mass is approximated as 28.8 g mol- 1 . With a ion cage
diameter of about 9.7 mm, an emission current of 0.2 mA, and an ionization efficiency
of 0.1 %, the estimated ion density within the ionizer is ni ~ 3.3 x 10-4 m- 3 . This
estimated effective ion density falls well below values where space-charge would have
52
an effect. In fact, the approximate distance between neighboring ions would be on the
order of several meters, so no interaction should take effect from one ion to the next.
Therefore, the shift in the ion energy measurements is most likely due to a shifted
ion energy region, either from ions being preferentially born near the extractor grid
where the accelerating potential is lower, or due to negative charge buildup on the
cage itself. The latter may result from an insulating polymer coating the cage, which
may have built up as a result of excessive use of the mass spectrometry calibration
15
-
mass, perfluortributylamine (FC-43), from prior laboratory work.
20 V.
18 V.
10
16 V.
14 Vio
5 .- 12V.
10V.
0
-5
-10
-5
0
5
10
Retarding Potential (V)
15
20
Figure 3-12: Hybrid RPA ion energy sweep with 0.2 mA electron emission current
and 3 x 10- Torr
Now with a baseline behavior from standard RPA downstream of this commercial
ion source, a comparison can be made with measurements from the hybrid RPA.
The
key differences between the conventional sensor and this new RPA is the incorporation
of microfabricated electrodes and inter-electrode hole alignment. The result is a device
with hexagonally packed apertures as small as 100 pm in diameter with a 150 pm
pitch.
These grids are made from a 700 pm-thick silicon substrate and coated with a 0.5
pmthick layer of tungsten. Recesses defined in these electrodes permit the alignment
of
successive apertures to a few tens of microns, and a reduction of the inter-grid spacing
53
to 300 pm enforced by the previously mentioned alumina spacers. The transparency
of each grid is 40.3 %.
By comparing Figure 3-12 with Figure 3-6, one may observe
that the peak location remains identical for both sensors. The amplitude is doubled
with the use of the hybrid RPA, but a new anomaly is present, i.e., the hybrid sensor
reports a negative distribution for the low ion energy tail end of the curve (Figure 3-13
shows the comparison for a 10 V ion energy region).
8
6
4.
2
0
-2.
-10
Hybrid RPA
o Conventional RPA
A~~
r~*):G ZAOLA
-5
0
5
Retarding Potential (V)
10
Figure 3-13: Comparison of the hybrid energy distribution and conventional data for
an ion source set to 10 V at 3 x 10' Torr with 0.2 mA electron emission current.
In order to investigate this non-physical effect, the hybrid RPA is reassembled
with a Keithley 237 source measure unit (SMU) per electrode. Looking at the current
intercepted by each of the grids sheds some light on the anomaly. Figure 3-14 shows
that the current collected when grid apertures are aligned represents only a fraction
of the signal theoretically attainable.
In fact, about 70 % of the current that is
transmitted through the first grid impinges on the ion retarding grid. Adding the
secondary electron current to the collector current would somewhat mitigate the
non-physical dip in the ion energy distribution, but not annihilate it entirely. The
reason for this large amount of interception in aligned grids, and resulting negative
distribution, is attributed to ion focusing. Each aligned electrode aperture acts as a
54
50
0
-50
-100
-300
2 -350
Z -400
-450
-1 0
-- Collector
-Secondary
e-Ion
retarding
e- repelling
-5
0
5
10
Retarding Potential (V)
15
Figure 3-14: Hybrid RPA current trace for 10 V ion energy region and 0.2 mA electron
emission current at 3 x 10- Torr
symmetric Einzel lens. When modeled using CPO, this effect is made clear (Figure 315) as the ion beam comes into focus on the collector plate around 0 V to 1 V.
By means of enforcing the electrode alignment, a larger peak signal was obtained
using the hybrid RPA with microfabricated electrodes. With this proof of concept,
a fully MEMS fabricated sensor was developed while incorporating changes to the
electrode aperture design to mitigate the effects of beam focusing. To this effect a
two-dimensional axisymmetric simulation was performed using CPO with grid apertures of 100 pm (first electron repelling grid), 260 pm (ion-filtering grid), and 160 pm
(secondary electron repelling grid). The idea behind this configuration is to allow for
expansion of the ion beam while mitigating ray interception at the electrode walls.
Figure 3-16 demonstrates that the beam still comes into focus, yet the ray interception is reduced by the various aperture dimensions utilized.
The MEMS RPA will
not only increase the alignment precision and further reduce the inter-grid spacing,
but by utilizing electrodes with a fixed pitch and differing apertures, will improve ion
transmission to increase signal strength.
55
Electron repellig W-d
loll retarding grid
Secondary
(-- repelling grid
Colletor
(a)
___
-I
(b)
(c)
(d)
(e)
(f)
(g)
mm
(hi)
Figure 3-15: Simulation of single RPA aperture using CPO 2D: Ray tracing of 10 eV
ions approaching RPA with axial velocity at various radial locations for ion retarding
potentials of (a) -4V (b) -2V (c) 0 V (d) 2V (e) 4 V (f) 6 V (g) 8 V (h) 10 V.
56
Electron repelling
Ion
retarding grid gridSecondary e- repelihng grid
Collector
(a)
(b)
(c)
(d)
(e)
(f)
(g)
II
II
(h)
Figure 3-16: Simulation of modified RPA aperture stack using CPO 2D: Ray tracing
of 10 eV ions approaching an RPA with altered aperture diameters demonstrate a
reduction in ray interception at various retarding voltages: (a) -4 V (b) -2 V (c) 0 V
(d) 2V (e) 4V (f) 6V (g) 8V (h) 10V.5
58
Chapter 4
MEMS RPA
The MEMS RPA seeks to improve sensor performance through three key modifications. First, using a fully microfabricated housing and grids, the alignment precision is
improved by an order of magnitude over the hybrid RPA. Second, the inter-electrode
spacing is reduced to 200 pm to prevent space-charge effects when measuring dense
plasmas. Lastly, the signal is multiplexed by having a plurality of aligned apertures
all leading to a single collector plate. Additionally, this new design makes use of
electrodes with holes that share a common packing density, or pitch, with various
aperture sizes, to mitigate the effects of ion ray interception previously simulated.
Minimizing the play between mating parts coupled with the possibility of exposure
to large heat loads gives rise to thermal expansion concerns. If not properly accounted
for, changing dimensions could result in thermally induced stresses that may lead to
cracking.
Resulting mechanical defects could provide a conducting path along the
surface of the device housing. Since the sensor may require large biasing voltages,
electrical breakdown could precede mechanical failure. By gripping the electrodes
only from a few points and using a high-performance dielectric coating, the opportunity for shorting via surface breakdown is mitigated. The MEMS RPA relies mainly
on the vacuum between grids to achieve electrical isolation. Therefore, no dielectric
is present near the flow of plasma species, preventing the inadvertent charging of
non-conductive surfaces in RPA channels. Springs not only provide a robust means
of aligning that is insensitive to temperature differences or manufacturing process
59
variations, but also maintains sensor modularity while reducing surface contact area.
In order to enforce alignment while permitting expansion, compliant structures
are used. Retaining springs were designed such that the maximum flexural stress
did not exceed 150 MPa, which is about an order of magnitude less than the fracture
strength reported for DRIE silicon [36]. The springs are designed to work with an
assembly tool. In this fashion, there is no risk of added axial stress to the springs
when the device is assembled in a manner similar to [37], nor is there a risk of buckling
the spring during disassembly. The tool holds the housing in an open configuration
to accept RPA electrodes, allowing for ease of access during assembly and greatly
reducing the amount of time and effort required to construct a new MEMS RPA.
Gentle and controlled actuation of the retaining springs reduces the risk of inducing
high stresses, and promotes greater sensor fabrication yield.
4.1
MEMS RPA Design
Springs dimensions for the MEMS RPA housing were finely tuned to generate
sufficient clamping force whilst avoiding stress concentrations. An equivalent spring
stiffness for these curved structures was derived from Castigliano's theorem. The
virtual work U done by a moment M applied to a curved beam may be written
as [38]
e M2r
j
2EIA
where r is the radius of the beam's neutral axis, E is Young's modulus of elasticity,
IA
is the moment of area for the beam cross-section, dO is an infinitesimal arc angle,
and
E is the total beam's arc angle.
If the moment is the result of a force F acting on
the end of the beam and originating from the beams center of curvature, then looking
at elemental components of the beam having an arc length dG. The moment can be
expressed as
M = rF(cosE sin0 - sin 8 cos 0)
fe r3 F2 (cos E sin 0 - sin Ecos
2EIA
0
60
0)
2
U F
U
( Cos2
- 1sin (2) -cos 9 sin 9 sin2 9 + sin 2 9
Cos2 e
=
2 cos 0 sin OdO + sin2 E1
91 sin2 OdO - cos 9 sin 9 j
e + (sin2 9-
U=
3 2
F
U = r F
2EI
I
U=
- -
r
r3F2
E
(sin 2E) +
3F2
r32
E+
sin (2e)
sin (29) - cos E sin3 e
2
iei)E cos e - cos esin e
s_2 9e) -sin
2
(e+(i9-o
1F + (sin(2 e
COS2 e)
sin e cos E)
inco9
2
2EIA
U=
cos 2 9)
cos 2 Ode)
r3F2
(9 - sin 9 cos 9) = 8E1 (29- sin (29))
IAA
The beam deflection J is then obtained by differentiating the virtual work by the
force.
=
OU
_r
3
F
4EIA (29 - sin (29))
And the effective spring constant, key, is simply obtained from Hooke's law.
=
keff
ke
2EIA
r3
0
F
Jy
1
- 1 sin (20)
(4.2)
With the equivalent spring stiffness for a curved beam, the design parameters
may now be adjusted for the particular dimensions and desired grid retaining force.
Tapered springs were explored as an option [37], but did not serve to increase the
strength of these members as the maximum stresses moved to the narrow part of the
spring. With a thickness of 700 pm and 300 pm width, each of our curved springs
would apply a force of roughly 0.1N at a final deflection of 100 pm. Each spring
is set 105 pm from center, to account for the foreshortening of the spring when it is
actuated. At maximum deflection (limit stop at 200 pm) the principal stresses remain
61
below 150 MPa (Figure 4-1).
8
A 128.5
7
120
6
100
5
80
4-
60
3-
40
2
20
1
0
MPa
0
-8
-6
-4
-2
0
V -23.56
Figure 4-1: Stress analysis of a MEMS RPA retaining spring: A COMSOL simulation
was performed after the calculated behavior of a spring was determined and a design
chosen. Stresses agreed with those calculated, and are kept to an order of magnitude
less than maximum for the maximum allowable spring deflection of 200 pm.
Figure 4-3 depicts the effects of electrode misalignment for a single 100 pm diameter RPA channel.
When the ion retarding electrode is shifted by 5 pm or less
from the channel's center axis, ion transmission is slightly more than 90 % compared
to the aligned case (Figure 4-3 (a)). At 10 pm misalignment transmission drops to
approximately 78 %, at 20 pm to 38 %, and for 50 pm misalignment, the transmitted
ions drop to less than 3 %.
The idea behind the MEMS RPA is to have compli-
ant alignment, with an order of magnitude improvement in precision over the hybrid
RPA. The overall size of the RPA is further reduced by replacing the steel housing
with a micromachined one. With inspiration from the spring design by Gassend for
in-plane high voltage silicon devices [37,39,40], a new kind of assembly was conceived.
Due to size constraints and standoff features, twist engagement of grids is not feasible
(Figure 4-2). However, by moving the assembly actuation to an external tool, we can
remove additional stresses induced in the spring due to axial loading, and maintain
the same orientation for all springs such that all will expand or contract in the same
62
1 cm
Figure 4-2: MEMS RPA Concept: Left: RPA housing layers each 700 Prn-thick, upper
right: grids etched from 500 pm-thick wafers, lower right: the assembled RPA and
cross-section where a 200 pm inter-electrode gap results from the different substrate
thicknesses.
manner. In curving the retaining springs, the unused sensor area can be greatly reduced compared to the use of linear springs.
With a compliant architecture, grid
alignment can be maintained over a wide range of temperatures as the springs permit
either the housing or electrodes to expand relative to one another, all the while the
three point contacts effectively center each successive grid. Lastly, in using CMOS
processing instead of CNC machining, manufacturing tolerances improve by an order
of magnitude, moving from several tens of microns tolerance to a few microns.
63
e-
repellinig
21e- repefllng
(a)
(b)
(c)
(d)
(e)
(f)
Figure 4-3: Misalignment simulation of a single 100 pm RPA aperture stack using
CPO 3D: Ray tracing information for the case of (a) no misalignment, and (b) 1 pm,
(c) 5 pm, (d) 10 pm, (e) 20 pm, and (f) 50 pm ion grid offsets.
4.2
Fabrication
The new device is composed of a microfabricated housing built by bonding a
stack of six 700 pm-thick silicon wafers and micromachined electrodes etched from
a 500 pm-thick substrate.
By selecting different thicknesses, the inter-grid spacing
(currently 200 pm) may be adjusted. Additionally, an assembly tool was designed to
aid in sensor construction.
4.2.1
Housing
The MEMS RPA electrode housing consists of a stack of six silicon wafers. Each
layer is patterned with a slightly different layout to hold specific grids a set distance
apart, but all follow a similar processing, beginning with a 700 pm-thick, n-doped silicon, 150 mm-diameter wafers with 0.4 pm protective thermal oxide from Ultrasil. The
thermal oxide maintains the pristine silicon surface of the double-side polished wafers
to insure adequate adhesion in the final wafer bonding step that completes this device
component. The wafers are first coated with 1 pm-thick photoresist (OCG825), which
64
-
(a)
(b)
(g
(c)
(h)
(d)
0U
-
MW
(1)
(j)
(e)
V Silicon
(k)
Silicon Oxide
Photoresist
Silicon Nitride
Quartz
Figure 4-4: MEMS RPA housing fabrication: (a) Six DSP Si wafers are necessary
and arrive with 0.4 urm protective oxide. (b) Alignment marks are defined on both
sides, and (c) the wafers are coated with PECVD oxide and annealed for use as a
hard mask. (d) Double side photo is performed to minimize defects from handling
the wafers, and the oxide is removed with RIE. (e) Next, recesses are defined on the
backside of the wafer and (f) the spring pattern is through-etched using DRIE on a
quartz handle wafer - nearly all of the resist is etched due to this step. (g) Individual
wafers are dismounted with acetone, ashed, and cleaned in preparation for (h) silicon
direct bonding of the six wafers. (i) After annealing, 1 lim thermal oxide is grown and
(j) low-stress nitride is deposited. (k) Finally, the package is diesawed into 30 MEMS
RPA housings.
is then exposed with alignment features for 2.5 s. After developing, the underlying
thermal oxide and 250 nm of silicon are dry etched using Reactive Ion Etching (RIE)
in the Applied Materials Precision 5000 tool. The remaining resist is stripped in an
oxygen plasma, and the process is repeated for the backside of the wafer using the
overlay feature of the Electronic Visions EV620 mask aligner. After another oxygen
plasma ash, the wafers are then cleaned in a piranha bath (sulfuric acid, H 2 SO 4 , and
hydrogen peroxide, H 2 0 2 ) and treated to a standard RCA clean.
These are then
transferred to the Applied Materials Centura system 5200 where they receive 4 pm of
PECVD oxide deposited in two steps. This oxide is annealed at 950 C in a nitrogen
environment for an hour. Following the anneal, the wafers are coated with a layer
of 10 urm-thick photoresist (AZ4620) on both sides with a half-hour prebake at 95 'C
65
between coatings. After an hour in the prebake oven, a recess pattern is exposed
for 25s on the backside using topside alignment. Exposure of the frontside features
- consisting of the springs, 0-80 close-fit clearance screw holes, and specific support
features for each wafer layer - are also exposed for 25s with topside optical alignment
to the existing silicon features. The entire wafer stack is developed in AZ 440 MIF
using a teflon carrier to prevent damage to the photoresist. This polymer is dried
in a 30 min postbake step and transferred to the RIE plasma reactor. The oxide is
removed from both sides of each wafer 1 pm at a time with a CHF 3 , CF 4 , and Ar
chemistry, with intermittent cooling in an Ar environment. The backside recesses are
etched 15 jim in a Surface Technology Systems Multiplex ICP tool using an isotropic
RIE SF6 etch. Each wafer is then mounted to a quartz handle wafer using thick resist
to continue topside processing. The springs are defined by a wafer through etch using
the standard Bosch process with SF6 followed by C4 F8 passivation. When a wafer is
dismounted using acetone, the byproduct of the cutout pattern falls from the wafer.
An oxide etch and piranha clean removes any polymer and remaining photoresist.
In the most delicate step of the process, the wafers are stripped of their PECVD
and protective thermal oxide using HF. A final RCA clean prepares the surfaces for
bonding of the six wafers. Each wafer is placed into the Electronic Visions EV620
aligner one after the other. Using crosshair alignment, wafers are added to the stack
one at a time with 500 N of force for three minutes. The entire 4.2mm stack is then
transferred to the EV501 bonder where it is pressed for at least eight hours in a 10 mT
vacuum with 1000 N of force. The stack is then annealed for an hour at 1025 *C in
nitrogen, before being moved to a pyrolysis tube. Following the growth of 2 Pm of
wet thermal oxide, 1 pm of low-stress silicon nitride is deposited using PECVD in an
SVG/Thermco VTR 7000. This last coating protects the springs and housing from
abrasions and provides another layer of electrical isolation. The 30 device housings
are then separated from the wafer stack using a diesaw. Only two of the 450 springs
were lost during wafer etching and bonding, and one device was lost to a diesawing
mishap, for a total yield of 90 % (27 of 30 RPA housings).
66
4.2.2
Electrodes
The MEMS RPA grids are made in a similar fashion to the hybrid RPA electrodes,
aside from a few minor differences. First, the starting substrate for these electrodes is
a 500 pm-thick, n-doped DSP silicon wafer with 0.5 pm thermal oxide. The thickness
of the wafer becomes important in this design as the difference between the grid wafer
and housing layer wafer thicknesses determines the inter-electrode gap. On the other
hand, in the case of the hybrid RPA, this spacing is set by the difference between the
alumina spacer thickness of 25 thousandths of an inch, or 625 pm, and the depth of the
recess etch. The second design change for the MEMS RPA grids lies in the dimension
of the apertures. It was found that due to the consistent/constant aperture sizes in
the hybrid-sensor, much of the current signal was intercepted by intermediate grids.
For the new design, a pitch of 400 pm was chosen for the aperture packing, while
the diameter of the openings was varied in steps of 50 pm. Lastly, the grid cutout
boundaries do not fully release the electrodes; three 200 pm tabs hold each structure
to the wafer to ease processing complexity caused by the gathering and finishing of
90 individual pieces. The summarized fabrication process follows. Alignment marks
are defined in 1 pm thin resist, next a dry etch removes the thermal oxide and 250 nm
of silicon. After ashing the remaining resist with an oxygen plasma, a piranha and
RCA clean, 4pm of PECVD oxide are deposited on either side of the wafer. This
layer is then annealed for an hour at 950*C in nitrogen. Using thick resist, electrode
identification markings and some spacer recesses are defined on one side of the wafer,
and the apertures and grid cutouts are exposed on the other side. Again a cycled
dry etch removes the hard oxide mask in the areas defined by the resist. The recesses
are etched using DRIE before mounting the wafer to a quartz substrate in order to
through etch of apertures and cutouts. The wafer is cleaned of any remaining polymer
and resist. A standard clean follows and all the oxide (deposited and thermal) is
removed in HF. A smoothing step helps mitigate sidewall roughness and scalloping.
This involves growing 1 pm of wet thermal oxide, and removing it in an HF step prior
to the final metallization. An AJA sputtering tool deposits the desired tungsten or
67
gold coating.
(b)
-
(a)
(g)
(c)
(h) mimma
(d)
(e)
(k)
Silicon
Silicon Oxide
ammm
Photoresist
U Quartz
Figure 4-5: MEMS RPA electrode fabrication: (a) Starting with a 500 pim DSP Si
wafer a 200 pm gap will be established between electrodes. (b) Alignment marks are
etched on one side through the 0.5 pm protective thermal oxide, (c) next the wafer is
coated with PECVD oxide and annealed for use as a hard mask. (d) photolithography
is carried out on both sides simultaneously to reduce defects from excessive wafer
handling; the oxide is removed with RIE. (e) DRIE defines the identification marks
and recesses. (f) The wafer is then mounted to a quartz handle wafer, (g) and the
cutout pattern is through-etched along with the apertures using DRIE. (h) The wafer
is dismounted using acetone, ashed, cleaned, and stripped of oxide. (i) 1 pm thermal
oxide is grown and (j) stripped in HF to smooth the scalloped surfaces, tethered grids
are broken out from carrier wafer. (k) Lastly, the electrodes are coated
with tungsten,
or other metals.
On this same wafer, manipulating pins are etched to have a 500 pm square crosssection and an overall length of about 3.5 mm. These pins have two clearance holes for
0-80 screws which will be used in the construction of a tool to aid in sensor assembly
(see Section 4.2.3). They are broken out of the wafer and are not subject to the final
etch and metallization.
68
4.2.3
Assembler
Due to the intricate design of the MEMS RPA and its minimal footprint, assembly
methods such as twist locking components were not feasible. Furthermore, individually actuating the spring tips with tweezers or other manipulating tools for each
grid would prove tedious, unreliable, and time consuming. Instead, an assembly tool
was designed in conjunction with the MEMS RPA housing to arrange the springs
in a formation where they could accept the sensor grids, and then be released to
reach their latching configuration. A cam-like actuation was used to allow the spring
tips to slowly be pulled back over a rotation of about 1000 to their limit stop. The
MEMS RPA grids are then inserted one at a time using tweezer, and a rotation in
the opposite direction closes the springs onto each electrode, latching them in perfect
alignment.
spring-loaded
manipulator
inicromachined pin
rail
MEMS RPA
alignment features
angle guide
rotating insert
cam
base
Figure 4-6: MEMS RPA assembly tool: A central circular Delrin@ part holds a
hexagonal recess with protrusions for aligning the RPA housing to the center of the
tool, this part rotates relative to the cam base on which the top spring manipulators
ride to slowly pull back the clamping curved springs.
The assembler is constructed out of Delrin@ (a DuPontTM acetal homopolymer
resin) for its strength, dimensional stability, wear resistance, and low friction. First
69
a base is built with a recessed circular opening to accept the platform that will hold
the RPA housing during assembly. This base has three gently slopping sides on
which another assembly will ride to slowly retract the sensor's retaining springs. The
circular insert has a large clearance hexagonal hole to fit the MEM-RPA, and three
raised bumps, approximately the diameter of a 0-80 screw to align the housing to the
assembly tool's center. This circular insert freely rotates in the base, and holds three
raised angles to allow for the alignment of the second portion of the assembler.
The more complex of the two parts for this construction aid is the actual spring
manipulator. This piece consists of three arms with a central opening. Each arm
receives a spring per rail. A manipular pin, previously mentioned, is affixed at the
end of each rail. The springs push the rails to a limit stop closer to the opening's
center. This position is that which will allow the pins to meet the eyelets in the
RPA's springs. When inserted, this three-armed actuator is rotated to move the rails
along the base's outwardly sloping radius, thus opening all of the device's springs
simultaneously. The central opening in the assembler is necessary to allow the grids
to be placed within the MEMS RPA while the spring tips are pulled back.
4.2.4
Assembly Procedure
With the housing in the assembly tool, and the cam actuated such that the entire
spring stack is pulled to the open position (Figure 4-7 (a)), the grids can be inserted,
starting with the floating electrode. Each electrode is inserted with their alignment
notches in line with the spring tips. There are two MEMS RPA designs, one with
a contact tab for each grid, and another where the unnecessary contact tab on the
floating electrode has been removed. The latter design provides larger electrical clearances, and utilizes synthetic ruby spheres held in recesses in the first grid to prevent
this electrode from rising up from the housing surface and contacting the first electron
repelling grid (Figure 4-7 (c)). After the floating grid is inserted, and ruby spheres
installed if necessary, assembly continues in the direction of sensor aperture to sensor
collector. The first electron repelling grid is followed by the ion retarding electrode,
and a secondary electron repelling grid before being effectively closed by the collector
70
Figure 4-7: MEMS RPA assembly: (a) The housing is inserted into the assembly tool
and springs are opened, (b) a floating grid is inserted, (c) in this case the first grid
receives ruby spacers. The (d) electron repelling, (e) ion retarding, (f) secondary
electron repelling grid, and (g) collector are inserted. Assembly is complete after (h)
the springs are released.
electrode (Figure 4-7 (b), (d), (e), (f), and (g)). A counter rotation of the assembly
tool locks all of these in place, thus completing the microfabricated portion of the
MEMS RPA (Figure 4-8).
Unlike the case of the hybrid RPA, the pogo pins are not fastened to the MEMS
RPA housing.
Instead, the MEMS RPA is fastened to the test setup where the
electrical contacts protrude from a polyether ether ketone (PEEK) pogo pin holder.
Two different designs were created, one for planar assembly in a testing chamber used
for mass spectroscopy, and another right-angle mount for testing in a helicon plasma.
These will later be discussed in the experimental setup.
71
Figure 4-8: Backlit MEMS RPA assembly: Optical examination shows good electrode
alignment.
4.2.5
MEMS RPA Ion Source Characterization
The testplate used for mounting the hybrid RPA in the mass analyzer chamber
(described in Section 3.3) was specifically designed with a hexagonal recess to accept
the MEMS RPA in a flush-mounted fashion (Figure 4-9). The same battery of tests
was performed with this new device to compare it to the conventional probe and hybrid sensor (Figures 3-6 and 3-12). The MEMS RPA used to obtain the following data
was equipped with grids of varying apertures but all with a pitch of 400 Prn between
centers. The first repelling grid has apertures of 150 pm followed by an ion retarding
grid and secondary electron repelling grid, both with 300 um apertures. Figure 4-10
shows peaks that are 1 V to 2 V closer to the expected energy when compared to the
hybrid and conventional RPA data. Additionally, the artificial negative distribution
is somewhat mitigated, and the expected sharp drop on the high-energy side of the
distribution is more pronounced.
At the expense of RPA transparency, another aperture sequence consisting of:
a 100
urm first electron repelling grid, a 250 pm ion retarding grid, and a 300 urm
secondary electron repelling grid was tested. The resulting ion source measurements
are shown in Figure 4-11. The effective transparency of the first grid for the MEMS
%
RPA drops from 12.8 % in the case of the 150 pm apertures (Figure 4-10) to only 5.7
72
testplate
large pan-head
screws
pogo pins
PEEK pin holder
MEMS RPA
Figure 4-9: MEMS RPA testplate: The custom testplate in which the hybrid and
conventional RPA can be threaded has a hexagonal recess on one side to accept the
MEMS RPA.
60
- --20 V.
50
18V
40 -
....
16 V.
14 V.
3 0 -20-
10
10
-10
........-
12V.
10 V.-
... ...- -.
.
-5
0
5
10
Retarding Potential (V)
15
20
Figure 4-10: 150 pm, 300 urm, 300 pm grid stack MEMS RPA ion energy sweep test at
3 x 10-5 Torr and 0.2 mA emission: The modifications greatly improve signal strength
when compared to the hybrid RPA, despite a reduced transparency. A slight negative
distribution is still apparent.
73
- -
-
20
20 V
15
18 V.
A
16 V
1014 V
-io
-io
-5
-10
12V.
5
10V,
-5
0
5
10
15
20
Retarding Potential (V)
Figure 4-11: 100 pm, 250 pm, 300 jim grid stack MEMS RPA ion energy sweep test
at 3 x 105 Torr and 0.2 mA emission: The narrower apertures reduce current interception and sharpen the energy peaks as is apparent in the reduced FWHM. The
negative low energy distribution is also mitigated.
for the 100 pm openings (Figure 4-11). The data reflect this drop in signal strength
by an approximate 60 % decrease in peak amplitude. Key advantages of the latter
grid sequence is the narrowed distribution for a sharper resolution, and mitigation of
the negative distribution (Figure 4-12). Furthermore, this sensor was then utilized
in helicon plasma measurements due to its application to denser plasmas, or plasmas
characterized by a smaller Debye length.
Comparisons may then be made with the conventional and hybrid RPAs (Figure 413), which shows a drastic increase in signal strength with enforced alignment, despite
reduction in grid transparency. It is not sufficient, however, to simply show a large
signal. A testament to improved sensing resolution is a narrower peak distribution for
a mono-energetic ion beam, as is expected from the present experimental setup. If the
measured distributions were of a known shape, normalizing to the area might be an
option, however, with different shapes across the different sensors, a simpler approach
is normalizing to the peak (Figure 4-14). The FWHM of the measured distribution is
reduced from 2.5V with the conventional probe to 1.6V with the hybrid, and 1.2V
74
-
1 .2
-
- -150Rm
0.4 0.48
--
-
--
--.-
-.
o 0.2
0
-0.2
-10
-5
0
5
Retarding Potential (V)
10
Figure 4-12: Comparison of 150 pim grid apertures to 100 urn in the MEMS RPA: The
normalized (to a height of unity) energy distribution of an ion source (set to 10 V ion
10 is increased. This is
region) shows that with the narrowed first aperture, resolution
characterized by a narrowed peak and mitigated negative distribution (attributed to
ion beam focusing).
30
-A
0
20
15
~10
SS
-
conventional
--
-
-
-
--
0
-
.
Au-MEMS
W -hybrid
-
0
25
05
-10
-5
0
5
Retarding Potential (V)
10
Figure 4-13: MEMS RPA comparison to hybrid and conventional probes at 10 V ion
energy region: The MEMS device shows more than an order of magnitude increase
over the conventional RPA.
75
with the MEMS RPA. The misalignment of grids leads to stochastic motion within
the conventional sensor that acts to artificially broaden the distribution function.
Again, as was mentioned previously, finer apertures may be used in combination with
I
Au - MEMS
A W -hybrid
0.8
0 SS - conventional
0
0.6
0
-
-3
-o 0.4
N
00
0
0
z
-
0.2
0
-0.2
-10
-5
5
0
Retarding Potential (V)
10
Figure 4-14: MEMS RPA normalized distribution (normalized height) comparison
with hybrid and conventional probes at 10 V ion energy region: The microfabricated
RPAs show narrowed distributions, while also shifting closer to the anticipated peak
energy. Between the hybrid and MEMS RPAs, the artificially negative distribution
is slightly mitigated.
larger ones within the MEMS device to minimize interception on subsequent grids.
With a grid sequence of 100 um, 250 pm, 300 pm, though the signal is reduced, the
negative dip in the distribution is alleviated, and the FWHM is further reduced from
1.2 V to 0.85 V (Figure 4-12).
76
Chapter 5
Device Characterization Using a
High-Density Plasma
A helicon-wave plasma source was selected as a means of verifying device capabilities because of its ability to generate a dense plasma of varying temperatures. The
facility utilized in the following experiments has been used to simulate fusion edgelike plasmas [41]. This versatile source has also been suggested as an alternative to
inductively coupled plasmas (ICPs) in microfabrication tools [6]. Additionally, helicon plasmas are capable of replicating conditions similar to those experienced during
atmospheric reentry [42]. As such, the testing carried out in the following sections
provides a good representation of the high-density plasmas that stand to benefit from
the proposed sensor.
5.1
Helicon Plasma
The plasma source utilized in these experiments was the Dynamics of IONic Implantation and Sputtering on Surfaces (DIONISOS) chamber at MIT's Plasma Science
and Fusion Center (PSFC). DIONISOS consists of a plasma chamber connected to
an ion accelerator for the purpose of ion beam analysis of exposed sample surfaces.
Through the implantation of atomic tracers, and subsequent ion beam analysis, the
physics of plasma surface interactions are explored for various samples. In this work,
77
the focus is geared toward monitoring plasma parameters of the helicon plasma to
which samples are subjected. The plasma chamber is pumped to a base pressure of
Magnetic Coil
Pump
Antenna
Gas Flow
Figure 5-1: DIONISOS Helicon plasma chamber: A Helmholtz coil is used to generate
an axial magnetic field for plasma confinement; gas flows from the left of the quartz
tube and is ionized by a helicon at the same axial distance and equivalent radial
distances.
about 2 x 10-
Torr before helium is flowed into the system. The operating pressure
for the tests carried out was approximately 5 Pa (about 4 x 102 Torr). The confining
magnetic field was set by 150 A of current flowing through the electromagnetic coils
resulting in 500 G confinement. Data was collected over a wide range of antenna RF
power (at a frequency of 13.56 MHz) using the various RPAs as well as Langmuir
probes already installed in the chamber. A large jump in plasma density is observed
in a helicon source when the regime shifts from a capacitively coupled mode (E mode,
Figure 5-2 (a)) to an inductively coupled mode (H mode, Figure 5-2 (b)). The transition between these regimes is governed not only by the RF power applied to the
antenna, but by the chamber pressure as well. An interesting effect associated with
this transition is a drop in plasma potential between the E and H modes [43]. An
even greater jump in density occurs between the inductive and helicon (W) modes,
Figure 5-2 (c). Other studies show that the capacitively coupled mode is characterized by a hollow plasma, with larger number density in a halo roughly the size of
the antenna diameter from the perspective of a poloidal plane cross-section; the inductively coupled mode is more of a flattop with roughly uniform distribution within
78
(a) Low power (capacitive mode) current
(b) Mid power (inductive mode) current
(c) High power (helicon mode) current
Figure 5-2: Plasma modes for helium plasma excited with an RF-powered helicon
antenna: (a) the low power capacitively coupled mode in the range of 100 W to 300 W
is characterized by a hollow, or haloed plasma, (b) the inductive mode in the midrange
power level of 400 W to 600 W is distinguished by a filled plasma column, and larger
luminescence, (c) the high power helicon mode around 700 W to 1000 W shows almost
a conical shape, and is the brightest of the three.
79
the antenna diameter; and finally, the helicon mode is distinguished by a conical
shape [44]. Each jump in plasma density is associated with an increased brightness
of the discharge [43].
Measurements were made with the RPAs and Langmuir probes at the same axial
distance downstream from the antenna exit at equivalent radial distance. The following measurements were taken approximately 25 mm radially from the central axis.
Data was not collected on axis due to the mechanical interference of the sensors at
that location.
5.2
Langmuir Probe Data
In the first set of experiments, single Langmuir probe measurements were taken
along with hybrid RPA data (Figure 5-3).
For a helium plasma with RF power of
1000 W, the floating potential is of roughly 8.8V, and ion saturation current around
-2.6 mA was measured.
Subtracting the latter from the Langmuir probe current, the
30-
-
20
10
-
-
-
15
-
-
-+- LP
X V~8.8V
25-
-5
-40
-30
-20
-10
0
10
Bias (V)
20
30
40
Figure 5-3: Single Langmuir probe trace for 1000 W plasma: The zero current crossing
corresponds to the plasma floating potential. The ion saturation current is the value
reached to the leftmost of the plot, in this case about -2.6 mA
80
102
L+sat
I/T e ,T e ~4.0 eV
-10
X VP
.......
....
....
16.6 V
P
..........
;J I.
........I
100
-
10
-4 0
-30
-20
-10
0
10
Bias (V)
20
30
40
Figure 5-4: Semilog plot of Langmuir probe current for 1000 W plasma: The slope
of the transition region when electrons are attracted and ions progressively repelled
corresponds to the reciprocal of the approximate electron temperature.
curve may then be plotted on a semi-logarithmic scale (Figure 5-4) to extract more
information. Assuming the electron energy distribution is Maxwellian, the electron
temperature may be taken as the reciprocal of the slope for the fitted line of the
transition region, in this case T a 4 eV. The plasma potential occurs at the location
of the knee between the transition and electron saturation regions and is determined
as the intersection of the fitted lines to these two regions, or Vp 1 16.6 V. A good rule
of thumb for this particular helicon plasma at MIT's PSFC states that the ion energy
is approximately four times the electron temperature.
The location of the plasma
potential seems to agree with this statement.
The hybrid RPA distribution collected with this Langmuir probe data, displayed in
Figure 5-5, exhibits a distribution centered around a peak of approximately 15 V. The
primary parameters of importance affecting the applicability of RPAs are the electron
temperature and ion density, which set the Debye length. Since a single probe is easily
plagued by parasitic current leaking to a ground path, and influences the plasma
around it, a better suited design for measuring these values is the double Langmuir
81
....
-..
.-........
.
.
-.
150
I OOF ....
-
-
-
-
-
-
-
--
50F-
-50F-30
-
0
-
-
-20
-10
0
10
Retarding Potential (V)
20
30
Figure 5-5: Hybrid RPA distribution measurement for a helicon plasma at 1000 W
RF power: The hybrid RPA equipped with 100 pm grid apertures with a 200 lrm pitch
reports a peak distribution of about 15 V, consistent with the plasma potential. The
characteristic negative distribution attributed to this sensor still persists.
probe configuration. In a floating double probe configuration, no net current leaves
the plasma, it simply flows from one probe to the next depending on the voltage
bias between them. For the remainder of the experiments, a double probe is used in
conjunction with the RPAs.
The typical double probe traces obtained in the helicon plasma experiment are
displayed in Figure 5-6. As discussed previously, the ideal double probe would have
the shape of a hyperbolic tangent [10, 45]
I =
Apeni
e
tanh
Bs
2,7mi
2kBie
(5.1)
The electron temperature may be estimated by fitting a line to a logarithmic expression of the data in the region about the origin (or zero crossing due to the offset)
equal to [9,46]
In
'1sti+
+zisat2
k Id + Ii,-at2
eV
_
-
(5.2)
kBTe
However this expression is valid only if the slope of the ion saturation region is
82
0.3
0.2
-
0.1
S0
-0.1
-50
-40
-20
-30
-10
0
10
20
30
40
50
Bias (V)
(a) Capacitive (E) mode
-
-
0.4
E
-0.2
-0.
- 400 W
0-
4
-+-500 W
-0.6
-%0
-40
-30
-600 W
-+ 700 W
---
-20
-10
0
10
20
30
40
50
40
50
Bias (V)
-4.
....
-2 ......
-90
-0-20
-..
.. ....
... ...
....
.
2 - -
(b) Inductive (H) mode
0
-10
Bias (V)
10
20
~..
30
(c) Helicon (W) mode
Figure 5-6: Double Langmuir probe traces for a helicon plasma at varying RF power:
The Langmuir double probe characteristics are slightly shifted from the origin and
saturation regions differ in slope for the two polarities, possibly indicating slightly
unequal collection areas.
83
sufficiently small [9]. Alternatively, the second derivative of the current trace may be
used as an estimate of the electron temperature [10,45]. The second derivative of the
hyperbolic tangent function is
C12
(tanh x) = -2 tanhx sech2 x
(5.3)
To obtain the location of the points of inflection, another derivative is required
d
~-(-2 tanh x sech 2 x)
=
2 sech4 x (cosh (2x)
-
2)
=
0
(5.4)
The solution of which (since sinh x is always positive) is derived from
cosh (2x) - 2
exp (2x) + exp (-2x) _ 2 = 0
(5.5)
or
exp (2x) + exp (-2x) - 4 = 0
(5.6)
for which the roots are
x = 2In (2 - v'3), 2In (2 + vr3) ~
Since x =
2',
O-6585
(5.7)
then we may write for the symmetric case
kBTe
e
_
V2
(5.8)
In (2+V a)
Where V2 corresponds to the positive point of inflection. By using both the high and
low inflection points, the method is made impervious to offsets in the current trace
:
kBTe
kBTe
=
=In 2+/
kBTe
-In
2-
=In
2
2 - A/3
(5-9)
which simplifies to
kBTe
AV
e
In [(2 + v ) / (2 - v 3)]
84
(5.10)
This method is reported to be adequate for probes operating in the thin-sheath
limit,
but is even applicable to the orbital motion limited regime with a worst case error of
five percent [10].
Using both of the above methods on the double probe data from Figure 5-6 yields
significantly different estimates of the electron temperature, as shown in Figure 5-7.
The double Langmuir probe data is rather noisy, and as a consequence, the second
-
10
and Malter
et al
-Brockhaus
Amemiya EVH est.
-Johnson
-8
H4
--
0
01
0
100
200
300 400 500 600 700
Helicon RF Power (W)
800
900 1000
Figure 5-7: Double probe temperature estimates for different helicon plasma powers
during conventional RPA testing: The method from [9, 46] tends to underestimate
the electron temperature, while that of [10] is noisy as a consequence of the data. A
proposed upper bound estimate to [45] is shown (Amemiya EVH est.).
derivative approach suggested by [10, 45] may yield erroneous results, especially for
the low energy plasma where the current is much smaller. Filtering of the data to
smooth the first and second derivatives is possible, but artificially broadens the shape
of the trace. Instead, we propose using the intersection of linear fits to the transition
and the two ion saturation regions to estimate the location of the points of inflection
in the hyperbolic tangent characteristic. In this fashion, the slope of the hyperbolic
tangent function at the origin would yield the worst case estimate of the location of
the points.
d (tanhx)Kxo = sech 2 x _ = 1
85
(5.11)
The points where this line intercepts the asymptotes of tanh x are simply x =
Thus, the worst case error in estimating AV is
2-In (2+V3)
1.
or an excess of 52 %.
This is used as an absolute upper bound for the temperature estimate represented
by the red line in Figure 5-7.
Yet this quick estimate shows good agreement with
the higher power plasma where the Langmuir probe current is large, and a second
derivative may be well defined. The temperature for the low power plasmas 25 mm
from axis is in the 2.5 eV to 6 eV range. At mid-level RF power (600 W to 700 W), the
electron temperature is around 3.5 eV to 9 eV, and at higher powers, the temperatures
surprisingly drop to 1 eV to 4 eV. This drastic change may be linked to the conical
shape of helicon mode plasmas [43,44,47].
10 1
T Johnson and Malter
T Amemiya EVH est.
Ee
18
10
16
.....
CapaCitive
0100
200
Inductive
300
400
500
600
Helicon
700
Helicon RF Power (W)
800
900
1000
Figure 5-8: Double probe density estimates for different helicon plasma powers during conventional RPA testing: The density is estimated from the saturation current
equation [101, the lower temperatures will overestimate the density, while higher Te
values underestimate it.
The ion number density is estimated from the total collected current, probe area,
ion mass, and electron temperature [10]
Ape
kBTe
Using the upper and lower bounds for Te, the calculated ion number densities are
86
10
......
Johnson and Malter
-- Amemiya EVH est
E
2
..
4
10 0
.
10..
Inductive
Capacitive
100
200
300 400 500 600 700
Helicon RF Power (W)
Helicon
800 900 10 00
Figure 5-9: Double probe Debye length estimates for different helicon plasma powers
during conventional RPA testing: Again, the lower temperature estimates provide a
lower bound for the Debye length, while higher temperatures result in a larger Debye
length.
shown in Figure 5-8.
For low power, densities fall in the range of 2 x
1016
rn-
3
to
3
8 x 1016 m- , at mid-level power, the density increases slightly to 7 x 1016 m- 3 to
2 x
1017
m-3, and at high power, values jump to 7 x 1017 m- 3 to 2 x 1018 m-3. This
translates to Debye lengths that span a range of 40 pm to 100 pm for low- to midpower levels, and dropping drastically to 5 pm to 15 pm at higher powers, above
approximately 700 W (Figure 5-9).
5.3
Conventional RPA DIONISOS Plasma Characterization
The conventional RPA tested was created with commercially available photochemically etched steel meshes having apertures of 152 pm, and a thickness of 127 pm.
These grids have a transparency of 26.7%, and four of them are inserted in the RPA
housing with ceramic spacers maintaining a 635 pm inter-electrode gap. As is clearly
visible in Figure 5-10 (a) (b) and (c), the expected monotonically decreasing current is
87
00
cc
-60
-40
-20
0
20
40
Retarding Potential (V)
60
-1 00
-50
C
501
-40
-20
0
20
Retarding Potential (V)
40
(d) Low power distribution
-60
1
-300
-+-
60
W
200 W
100 W
(a) Low power (capacitive mode) current
U'
100[
200-
300-
--- 100 W
+-200W
200W
-'-300 W
U
U
-60
-40
-20
0
20
Retarding Potential (V)
-400 W
-- 500 W
- 600 W
-+700 W
40
60
-200-
-100
0-
100-
200r
-40
-20
0
20
Retarding Potential (V)
40
(e) Mid power distribution
-60
-
60
50 W
-200600 W
-700 W
400 W
(b) Mid power (inductive mode) current
-500
0[
500-
1000-
1500-
2000-
E
U
U
-60
-40
-20
0
20
40
Retarding Potential (V)
60
-+-800 W
-- 900 W
---1000 W
-400
-200
0
200
400
-60
-60
-40
-40
(f) High power distribution
40
20
0
-20
-20
0
20
40
Retarding Potential (V)
60
1000 W
600
--
800 W
-900 W
800
1000
(c) High power (helicon mode) current
2000
4000
6000
8000
10000
Figure 5-10: Conventional RPA current measurements and distributions for a helicon plasma of varying RF power: The
conventional RPA measurements show a jump in the density, distinguished by an increased current, around 300 W to
400 W and
700 W to 800 W as depicted from the transition from (a) to (b) and (b) to (c) or (d) to (e) and (e) to (f), respectively.
C
C
C
a
C
C
U
U
00
400
absent from the reported curves at all power levels. Space-charge would limit the current to the collector plate, and since the measured current seems to be proportional to
that collected by the double probe for the various power levels, it seems unlikely that
this would be the reason for the observed behavior. Instead, the experimental data
suggest that the aperture size is too large compared to the Debye length such that a
planar sheath does not form in front of each electrode aperture to trap the plasma. A
break in the sheath at the various grid apertures permits the penetration of multiple
plasma species, thus invalidating assumptions for proper RPA operation. Electrons
flowing to the collector plate would in part negate the ion current. In Figures 5-10 (b)
and (c), there appears to be a saturation current for biasing voltages below -50 V.
This effect may be attributed to the decreased number density of plasma species with
every grid crossing. Since the transparency is roughly 25 %, the Debye length would
approximately double at each grid. Looking specifically at Figure 5-10 (c), where
the sharpest transition is visible at a -50V retarding potential, the first electrode
faces a Debye length of 5 pm to 15 pm, the second 10 pm to 30 pm, the third 20 pm
to 60 pm, and finally the fourth 40 pm to 120 pm. In this range, a 152 pm aperture
is suitable to establish a sheath by the approximate two-Debye length criteria, and
the second through fourth grids essentially act as one thick electrode biased at -50 V
which attenuate the density. This is a technique suggested by [26] to mitigate the
effects of small Debye lengths in dense plasmas, and increase the range of application
of RPAs. By stacking multiple electrodes biased at the same potential, the plasma
density can be artificially reduced to promote plasma trapping at the expense of RPA
signal strength and resolution of the energy distribution function.
Beyond -50 V, a positive electric field exists that could impart additional kinetic
energy to the electrons providing them with sufficient velocity to cross the third
electrode. Simultaneously, this same field would repel ions, and could be the cause
for the negative current seen in Figure 5-10 (b).
As the ion retarding potential
becomes positive, the plasma sheath vanishes around this grid, and electrons may be
collected, while ions are repelled, much like a single planar Langmuir probe. Further
increasing the bias decreases the ion flux; however, due to insufficient shielding as a
89
consequence of large apertures, the collector current never decays to zero. If electrons
still penetrate the third and fourth grids, the slight increase in collector current at
large biasing potentials (Figure 5-10 (c)) may be explained as increased electron
interception by attraction to the ion retarding grid (the third electrode).
Unfortunately, a plasma with a small enough density could not be struck at DIONISOS for the conventional RPA to adequately measure the ion energy distribution.
In order to enable this sensor to collect meaningful data, new grids would have to be
cut and installed in the sensor as additional floating grids, however, this would also
decrease its signal amplitude. Alternatively, one of the hybrid RPA electrodes with
smaller apertures could be used instead of the steel mesh as a first grid, to effectively
trap the plasma.
5.4
Hybrid RPA DIONISOS Plasma Characterization
The hybrid RPA replaces the stainless steel mesh grids with DRIE silicon grids
coated with tungsten.
These are machined from a 700 pm-thick substrate to have
100 pm apertures, and alignment features.
As a result, most of the 22.7 % grid
transmission is expected to be maintained for the entire sensor. Though the grid
thickness is increased more than five times over the conventional sensor, the intergrid spacing is reduced to roughly 300 pm. Results (Figure 5-11) show that nearly
all current traces decay to zero at large enough biasing potentials, indicating better
plasma screening. Figure 5-11 (a) and (d) exhibit the slowly increasing current and
associated artificial negative energy distribution attributed to the focusing of ions in
the hybrid design. Aberration of the ion beam and interception by intermediate grids
(namely the ion retarding grid) detract from the collector current.
A new anomaly presents itself in the case of the hybrid RPA for mid-range, to
high helicon power settings. A bimodal energy distribution is obtained for RF power
from 400 W to 1000 W. One possibility is that beam focusing still takes place within
90
0
20
60
-40
-20
0
20
Retarding Potential (V)
40
60
0
5
-60
-60
-20
0
20
-20
0
20
Retarding Potential (V)
40
40
(d) Low power distribution
-40
-40
-a-300 W
-=200 W
-4- 100 W
60
0
5
-60
-60
20
0
-20
-20
0
20
Retarding Potential (V)
40
40
(e) Mid power distribution
-40
-40
60
-400
W
~*500 W
-o-600 W
-+700 W
0
0
C
0
a
0
-60
-40
-20
0
20
40
Retarding Potential (V)
60
-1000 W
-.- 800 W
-'-900 W
-50
0
50
100-
150-
200-
-40
-4
0
20
-20
0
20
Retarding Potential (V)
-2
40
4
(f) High power distribution
-60
-60
00,
-980
W
-'-00 W
--
60
(c) High power (helicon mode) current
1000'
1000'
0
0
-60
1000
2000
3000
50
10
40
0
0
4000
5000r
- ----------
. ...
..
Figure 5-11: Hybrid RPA current measurements and distributions for a helicon plasma of varying RF power: The hybrid RPA
was equipped with 100 pm grid apertures, and shows a reduced signal when compared to the conventional probe, possibly due
to the collimation induced by grids nearly six times thicker.
0
C
0
a
0
-20
Retarding Potential (V)
400 W
500 W
-e- 600 W
-+-700 W
10
-40
0
-
U 100-
-
(b) Mid power (inductive mode) current
-60
-e 300 W
-- 200 W
-A- 100W
150-
200
250
(a) Low power (capacitive mode) current
0
20
U 40
60
80
100
120[
the RPA; however, impact with the ion retarding grid may be sufficiently energetic
to dislocate surface species. This grid sidewall surface ionization within the RPA
could yield an energy peak centered at a negative voltage where collector current
interception is the highest, roughly -40V, as seen from comparing Figure 5-11 (a)
to Figure 5-11 (e) and (f).
5.5
MEMS RPA DIONISOS Plasma Characterization
The MEMS RPA, similar to the hybrid, uses microfabricated electrodes with enforced inter-electrode hole alignment. These grids are etched from a 500 pm-thick
substrate, and all apertures lie on a pitch of 400 pm and are hexagonally packed. The
tested device used a four grid stack with the following aperture sequence: 150 pm
floating grid, 100 pm electron repelling, 250 pm ion retarding, and 300 pm secondary
electron repelling grids. The collected current and derived distributions (Figure 512) show little to no effect of interception due to focusing. At low power, a gradual
increase in ion energy may be seen with reasonable peak ion energies. In the 400 W
to 600 W range, the large spread in the distributions is more likely attributed to a
broad sampling of ion energies due to the sensor's off-axis location in the inductively
coupled mode rather than thermalized ions. As observed in Figure 5-2 (b), this mode
is characterized by a broad, nearly uniform cylinder, except at the edges. Finally at
high power levels, the current drastically increases, the plasma becomes more conical
(Figure 5-2 (c)), and the ion energy distribution narrows. The decreasing current
beyond 800W RF power in Figure 5-12 may be attributed to a progressive loss of
electrical contact with the MEMS RPA electrodes. As a result of the higher density,
and consequently larger current and heat flux to the probe, the PEEK pogo pin holder
reached its melting point and the spring force of the gold pins pushed them away from
the electrodes through the softened plastic. The silicon nitride coated housing and
gold coated grids remained intact. Future testing will require higher temperature
92
-60
-40
-20
0
20
Retarding Potential (V)
40
60
_+-300W
-50
0
50
00|-
150r
-60
-20
0
20
Retarding Potential (V)
40
(d) Low power distribution
-40
a&
100 W
60
-- 200 W
-u-300 W
-+-
(a) Low power (capacitive mode) current
0
500F
-+-100 W
-u--200 W
500r
C
0
C
0
C
-60
-40
-20
0
20
Retarding Potential (V)
40
60
0
50
1001
-60
-20
0
20
Retarding Potential (V)
I
40
(e) Mid power distribution
-40
a
600 W
400 W
60
(b) Mid power (inductive mode) current
0[
500-
O 1000
1500-
2000-
2
0
C
U
-60
-40
-20
0
20
Retarding Potential (V)
W
800 W
40
60
-100w
--- 900 W
.-
0
1000
-200
01
K
2000-
3000-
4000-
W
-40
-20
0
20
Retarding Potential (V)
40
(f) High power distribution
-60
-.- 800 W
A900 W
1000 W
-+-700
60
(c) High power (helicon mode) current
0'
0
0.5
-1000[
U
U
1.5
2,--700
4
2.5 X 10
..
..
.. .. .....
Figure 5-12: MEMS RPA current measurements and distributions for a helicon plasma of varying
RF power: The MEMS RPA
measurements are consistent with those of the conventional sensor, and actually show the monotonically
decreasing current
expected for electrostatic probes, at least until around 700 W to 800 W. Beyond this power level,
the helicon mode may
generate too large a density (and too small a Debye length) for the sensor to effectively shield the plasma.
C
0
C
0
C
U
0
U
1000-
1500-
2000-
material for the spring receptacle to extend the lifetime of the MEMS RPA.
Prior to packaging failure, the onset of a dip in current was observed at around
700W to 800W RF power. This behavior may be explained by the reduced Debye
length at these high power levels, and inadequate grid aperture sizes or inter-electrode
spacing. In creating a two-gridded energy analyzer with movable ion repelling grid,
Honzawa studied the effect of changing the inter-grid distance between the repelling
electron and retarding ion electrodes [48]. In the low density plasma he generated
(approximately 5 x 1014 m-3 ), an ion cloud formed about 29 Debye lengths behind
the electron repelling grid. The optimal spacing for the tested conditions appeared
to be on the order of 5mm or less. This gap would be greatly reduced for plasmas
approaching 1 x 1018 m-3 , typical of fusion experiments. It is interesting to note that
in the collected current nearing a grid separation of 10 mm, a dip begins to form while
still in ion saturation [48]. This peculiarity associated with the presence of an ion
cloud behind the first repelling grid resembles the dip that appears in the MEMS
RPA at a power in excess of 700 W. This might well be the point at which the
device reaches the limit of its range of application, due to the drastically increased
ion density of the helicon plasma.
5.6
Conventional, Hybrid, and MEMS RPA Comparison
Unlike the experiments carried out with a commercial ion source, the signal amplitudes for the three different RPAs show the MEMS RPA with the highest peak,
followed by the conventional probe, and finally the hybrid sensor (Figure 5-13). However, the analysis of the data from the conventional RPA in Section 5.3 revealed that
the sensor failed to adequately trap the plasma, and ion energy measurements with
this device should essentially be disregarded. The effective collection area for the
conventional probe is expected to be about 0.2 mm2 , approximately 7.0 mm2 for the
hybrid, and 2.4 mm 2 for the MEMS RPA. Solely looking at probe surface area, the
94
C0
-60
0
0
4
~nc
0
500-
1000
15004
20001
*
-60
-60
-40
-40
40
(d) 700 W
-20
0
20
-20
0
20
40
Retarding Potential (V)
V
(a) 100 W
-20
0
20
Retarding Potential (V)
-'-MEMS
-+-Hybrid
-4-Conventional
-40
60
60
C
C
C
0
C
0
1500
2000
2500
IM I
-1000 L
-500,
0
500
(e) 800 W
-1
500
-1000
-500
0
500
60
60
1000
-20
0
20
40
Retarding Potential (V)
40
-100-
-50-
0
1000
-40
-4-Conventional
(b) 300 W
-20
0
20
Retarding Potential (V)
MEMS
-40
-a-Hybrid
--
-60
-60
C
0
C
C
C
50
l00r
1500
2000
2500
3000
-60
-2
-40-
0-
-101
0
-20
-60 -40
0
10-
20-
20-
MEMS
-a-Hybrid
-Conventional
-40
-0
MEMS
2
(f) 1000 W
0
20
40
-20
Retarding Potential (V)
-0
0
(c) 500 W
-+-Conventional
-60
-60
-40
-40
--
60
- Hybrid
-4-Conventional
40
20
0
-20
0
20
40
-20
60
Retarding Potential (V)
7ww
-4-MEMS
-0-Hybrid
-60
-60
0
.............................................
......................
......
....
.....................
...
...
.....
Figure 5-13: RPA plasma data comparison: The MEMS RPA shows the largest signal strength in all cases, and along with the
hybrid RPA was able to measure a distribution for most power levels.
C
0
C
0
C
C
8
40
60-
80-
30-
-'-MEMS
-a-Hybrid
-4-Conventional
100-
40-
50-
60-
70
.
conventional sensor should have a peak current at least 16 times smaller than the
Langmuir probe (with 3.3mm2 area). In reality, the current is more than 100 times
smaller, possibly due to a sheath around the Langmuir probe increasing its effective
collection area, or more likely, the fact that the RPA is directed to measure axial
plasma energy, while the Langmuir probes may collect ions and electrons from all
sides.
Similarly, geometrically speaking, the hybrid RPA should have a signal that is 14
times stronger than the conventional RPA, yet it is in fact weaker. This is attributed
to the greater degree of collimation in the hybrid device, as the collection area in
the thickness of the grids is increased by a factor of 5.5. More importantly, the
conventional probe was unable to effectively shield the plasma, thus allowing a larger
flux of ions and electrons through the device. The hybrid RPA was able to trap
the plasma, and a first set of ion energy distributions was obtained, albeit showing
a negative distribution in the low energy tail associated with the internal sensor
dynamics.
Finally, the MEMS RPA demonstrated an improvement in peak signal strength
over the state of the art. The results in Figures 5-10 and 5-12 show that in fact,
the MEMS RPA peak current is more than double the signal from the conventional
RPA, despite the larger ion flux due to plasma leaking through the conventional
sensor. In fact, the MEMS RPA was the only sensor that demonstrated the expected
monotonically decreasing current traces up to a helicon power of 700W. Thus this
microfabricated sensor not only demonstrated the advantages of enforced alignment,
but expanded the measurement capabilities of RPAs. Even though the MEMS RPA
apertures were 100 pm at their narrowest (like the hybrid RPA), and nearing, if not
surpassing the suggested two Debye length design guideline, this sensor was able to
trap and measure dense laboratory plasmas. The fact that the sensor did not start to
show signs of ineffective trapping until a Debye length in the range of 5 pm to 15 pm
might be attributed to the thicker electrodes. A thick grid acts much like a sandwich
of two thinner grids which have been shown to mitigate cusping of the electric field,
thus improving sheath formation and the sensor's plasma trapping capabilities [29].
96
Chapter 6
Future Work
The experiments carried out in this thesis have shed light on the behavior of
helicon-wave plasmas. With the direct measurement of the ion energy distribution,
new questions present themselves to better understand the different helicon modes and
to explain the high energy observed in the inductively coupled mode. It was stipulated
that the reason for the large energy spread might be due to the location where RPA
measurements were taken. At the outside edge of the plasma boundary, a large range
of axial ion energies may be detected, and the width of the energy distribution is not
expected to be caused by thermalized ions. Additional RPA measurements should be
made with varying axial and radial locations to further investigate the data reported
here.
Other tests to explore RPA and plasma behavior include pointing the sensor in
the opposing axial direction and utilizing more power supplies to simultaneously monitor the current collected by all intermediate RPA electrodes. Varying the electron
repelling grid potentials to determine their effect on the collected RPA current is another worthwhile undertaking to establish device sensitivity and further investigate
internal sensor dynamics, particularly in the case of the conventional RPA. To enable
a comparison of the new designs with a conventional probe, one could add additional
floating grids to the standard RPA or replace the first electrode with a micromachined
grid having the required aperture dimension for the helicon plasma's Debye length.
A new pogo pin receptacle for the MEMS RPA has already been machined out of
97
Macor@ to better endure high power plasmas.
Though the advantage of aligning grids has been demonstrated, numerous improvements can be made to the sensor. Smaller grid apertures may be machined
using the method presented in this thesis. By etching grid openings from both sides
of the silicon wafer, the aspect ratio can be further increased to make holes smaller
than 50 pm in diameter in a 500 pm-thick silicon substrate. The inter-electrode gap
may also be reduced; for example, by utilizing 600 pm-thick layers for the MEMS
housing, the inter-grid spacing becomes 100 pm. In order to minimize the interception of ions on the sidewalls of channels formed by the RPA's aligned grids, a thinner
substrate can be used for these electrodes. Handling and processing wafers with a
thickness under about 300 pm becomes difficult as the flexibility of thin silicon combined with its brittle nature make it easily prone to shattering. To achieve thinner
electrodes, the substrate may be etched locally in selected areas; however, using this
type of local thinning may translated in larger inter-grid spacing depending on how
electrodes are oriented and on their final bulk thickness. The throughput ions in the
MEMS RPA may also be increased by packing apertures more closely. The choice of
a 400 pm pitch between hole centers was made with the largest aperture of 300 pm
in mind. A greater packing density can be achieved to further increase the signal
strength. A few concerns that may limit increasing the grid transparency in this way
are the overall stiffness of the grids, governed by the dimensions of the remaining
material between each hole, and the overall flow of ions and neutrals through the
RPA. If the transparency is so great that sufficient plasma constituents can enter
the RPA and not escape save through the plasma-facing apertures, this may result
in a pressure buildup that would cause a resistance to the flow of ions to the RPA
collector.
From the analytical point of view, with known alignment, more thorough analysis
of the output signal could yield better estimates of the ion energy distribution. Additionally, it may be possible to tune the dimensions of the sensor and the analysis
method to permit identification of individual charged species. The gridded energy
analyzer, without the collector grid, could be used in conjunction with other backend
98
analyzers to perform mass spectrometry, as well as the analysis of energetic neutrals.
In addition to the incorporation of the filtering method with other forms of analyzers,
the grid sequence could be used for focusing of parallel charged particle beams. Such
a tool could enable parallel writing using electron beam lithography.
Aside from the potential uses of the MEMS RPA and its assembly method, optimizing the materials used in its construction remains a significant task. Because
of the modular structure of the sensor, various fabrication methods may be utilized.
Already, DRIE of silicon carbide has been demonstrated as a possible alternative to
the silicon substrate [49,50]. And in terms of resilient metals, laser machining techniques have successfully achieved feature sizes on the order of the thickness of the
stock material. Finally a novel approach to machining steel using a silicon negative
in sink electrical discharge machining has been reported [51] and could be utilized to
make stainless steel electrode inserts for the MEMS RPA.
99
100
Chapter 7
Conclusion
In the course of this thesis, we set out to improve upon the state of the art concerning RPAs proposing three ideas: (i) reducing RPA aperture dimensions through
microfabrication, (ii) aligning successive grid openings with precise and compliant
features, and (iii) multiplexing the signal strength by using densely packed aperture
channels in a small grid area. Through the incorporation of MEMS manufacturing
techniques, apertures as small as 100 pm in diameter and an inter-electrode spacing
of 200 pm were achieved. These are the smallest dimensions yet obtained in an RPA
with precise aperture alignment. Through the proposed innovations, our alignmentenforced RPA demonstrated an order of magnitude improvement in signal strength
over the conventional design. Additionally, where previous sensors failed to measure
the ion energy distribution of a dense helicon plasma, our device was able to resolve a
distribution function. In using an optimized stack design with thick and robust electrodes, and enforcing inter-grid aperture alignment, signal strength does not suffer
from a reduced optical transparency. Instead, each grid aperture acts as an Einzel
lens, and sufficiently large biases eventually reflect incoming ions. Multiplexing this
effect, the flux of each of hundreds of such apertures are added as they impinge on a
single collector plate.
Experiments with a commercial ion source demonstrated an increased energy resolution and accompanied signal strength by using enforced alignment. The hybrid
sensor achieved a two to threefold improvement in peak signal over the conventional
101
RPA and an associated near twofold increase in energy resolution reducing the FWHM
to 1.6V compared to 2.5V for the conventional sensor (Section 3.3). The artificial
broadening of RPA ion energy distribution measurements in conventional RPAs with
unaligned grids was thus mitigated. In enforcing the alignment between successive
apertures, however, it was discovered that ions could be intercepted by the sidewalls
of intermediate grids. This effect is further aggravated by the degree of collimation
of the particular RPA channel dimensions. As a consequence, long narrow channels
are more prone to signal loss by this form of current interception. Simulations using
software from Charged Particle Optics, along with measurements of the currents collected by each grid of the hybrid RPA, confirmed this form of signal attenuation in
our first device.
Further improvement to the distribution were achieved by taking advantage of
the modified internal sensor dynamics and increasing the hole diameter of the ion
retarding and secondary electron repelling grids for the MEMS RPA (Figure 3-16).
This alteration mitigated the interception of ions on their path to the collector plate.
The focusing effect, which causes these aberrations and consequently the loss of ions
to the aperture sidewalls, is easily recognized in the hybrid RPA as an artificial
negative distribution.
The improved inter-electrode alignment accuracy increases
ion transmission through the MEMS RPA compared to the conventional and hybrid
RPAs. More importantly, mitigating the signal loss to the electrode sidewalls by
modifying the aperture diameters in the grid sequence helped prevent the artificial
broadening of the ion energy distribution function. With these latter improvements,
the MEMS RPA showed up to an order of magnitude increase in signal amplitude and
additional narrowing of the energy resolution to a FWHM value of 0.85 V (Chapter 4).
The MEMS design is modular, and permits rapid interchange of different grids
to tailor the sensor to the specific plasma of interest; a desirable feature for experimental research. This allows for the use of larger apertures to increase the transmitted current when operating in low density plasmas, or changing to narrow openings
for high-density, small Debye length applications. The compliant alignment method
using curvilinear springs maintains precise positioning of grids relative to one an102
other, as the housing will experience the same degree of thermal expansion, and the
electrode stack moves as one ensemble. High-density plasma measurements (up to
1 x 1017 m- 3 ) show that the hybrid and MEMS RPAs were able to measure plasmas
with Debye lengths as small as 50 1m (Chapter 5). The conventional sensor could
not effectively trap even the smallest density plasma generated in the helicon plasma
facility due to its large grid apertures. Results confirm that with enforced alignment,
and optimized aperture sizes to account for focussing effects with the sensor, the best
signal strength is achieved with the highest resolution. Although the MEMS RPA
sensor was compromised during testing in high power helicon plasmas, the failure
resulted from the choice of PEEK as electrode packaging material, while the microfabricated device itself withstood the harsh conditions. The techniques demonstrated
in this thesis prove to be a viable means to ameliorate RPA performance. Future
improvements to the sensor's packaging will extend its lifetime, while ever finer manufacturing dimensions and tolerances can enhance its range of application plasmas
with smaller Debye lengths.
103
104
Appendix A
Detailed Microfabrication Process
Flow
Table A.1: Microfabricated grid process flow: Detailed list of steps and tool names
used for manufacturing the hybrid and MEMS grids.
Step no. Process
Coat wafer with 1pm resist
1
Expose alignment marks
2
Develop
3
Etch axide
4
S____Etch silicon approx. 0.25 pm (Chlorine
Strip photoresist
6
Strip photoresist
7
RCA clean
8
Deposit 4 pm to 5 pm PECVD oxide
9
Anneal 1 hr in Nitrogen
10
HMDS (approx. 30 min)
11
Coat with 10 pm resist
12
13
Prebake 30 min
Coat backside with 10 pm resist
14
Prebake 60 min
15
Expose backside recess pattern
16
Expose boundary and hole pattern (25s hard contact)
17
Develop in teflon carrier so films don't scratch
18
Postbake 30 min
19
Etch oxide masks
20
21
22
23
24
25
26
27
28
29
30
31
AME5000
Location
ICL
TRL
ICL
ICL
ICL
asher-ICL
ICL
Machine
Coater6
EVI
Coater6
AME5000
Detail
(tlhmds)
2.5. hard contact)
puddle3
BaselineOX
B linePoly)
min 15 ac)
1H2 0 2:3H 2SO4)
3
premetal-Piranha
ICL
(1NH 40H,2HC)rTa-ICL
I__C
(950*C
TRL
i4)
(R
AZ P4620)
DCVD
B3-DryOx
TRL
HMDS-TRL
(95C)
(95*C)
(25s hard contact)
(95-C)
Timed etch 1pm at a time with
intermittent cooling etch both sides
Etch backside recess (350 pm)
Mount wafer to quartz wafer
Etch through
Dismount wafer and collect grids (acetone)
Ash resist
Piranha clean
Strip oaxide in HF dip
RCA clean
Grow 1pm thermal oxide
Remove oxide HF dip
Coat grids (both sides) with SiC or sputter metal
coater
prebakeoven
coater
prebakeoven
EVI
EVI
photo-wet-l
prebakeoven
AME5000
TRL
TML
T
TRL
TRL
TRL
TRL
TRL
ICL
sts2
TRL
coater, prebakeoven
TEL
sts2
photo-wet-I
asher-TL
acid-hod(2)
T
TRL
TEL
TRL
TRL
TRL
acd-hood(2)
rca-TRL
A2-WetOxBond
acid-hood(2)
sts-CVD or SputtererAJA
105
_
ICL
TEL
TRL or EML
Table A.2: MEMS RPA housing process flow: Detailed steps for the fabrication of
the MEMS RPA housing stack.
19
20
21
22
Procss
Coast wafer with 1pm resist
Expose alignment marks
Develop
1
Etch oxide (approx. 120 s at 240 As- )
1
Etch silicon approx. 0.25 pm (35 As- )
Strip photoresist
Coat wafer with 1 pm resist
Expose backside alignment marks(2.5 s hard contact)
Develop
Etch oxide
Etch silicon approx. 0.25 pm
Strip photoresist
Piranha wafer clean (about 1 hr)
RCA clean (about 1.5 hra)
Deposit 4 pm to 5 pm PECVD oxide both sides
Anneal 1 hr in Nitrogen (950*C)
HMDS (approx. 30 mi)
Coat with 10 pm resist
Prebake 30 min
Coat with 10 pm resist
Prebake 60 min
Expose backside recess pattern
23
24
25
26
Expose frontside cutout pattern
Develop in teflon carrier
Postbake 30 min
Etch oxide (PegasusOxide: timed 1pm
27
Etch recesses SF6-14 (10 pm to 15 pm)
28
29
30
31
32
33
Target mount to quartz wafer with blue tape (30 min)
DRIE through front side
Dismount wafer (acetone)
Ash resist 60 min
Piranha clean, rinse
Strip oxide in HF, rinse, spin-dry
34
35
36
37
38
39
RCA clean, no HF dip
Silicon direct bond wafers
Press overnight
Anneal bond 1 hr (1025C)
Grow 2pm oxide
Deposit 1 pm low stress nitride
40
Diesaw
Step
I
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Detail
Machine
Loc.
(tlhmds)
coater6
ICL
2.5 s hard contact)
(puddle3)
(BaselineOX)
(BaselinePoly)
(3 min 15 sec)
(tlhmda)
EVI
coater6
AME5000
AMEISOO
TRL
(puddle3)
(BaselineOX)
(BaselinePoly)
(3 mmi 15 sec)
(1H 20 2 :3H2 SO4)
(1NH4 0H,2HCl)
(15 min/side)
(reserve 3.5 hours)
(Recipe 4)
(AZ P4020)
(95*C)
backside
(95*C)
(25 a hard contact)
(30pm separation)
(25 a hard contact)
(AZ MIF 405)
(95*C)
etch both sides
with intermittent cooling, recipes contains breaks)
backside only
(3 mirk 15 sec/wafer)
(3x about 1 hr 48 min 13 sec)
pieces fall out
(1H 2 0g
2 3H 2YO4)
(49 %HF approx. 5 min)
(2pm SiO2/min)
(1NH4 OH,2HCI)
(use Fusion Bond)
(2 to 3x
1pm recipe)
I Airbrush OCG825 resist and prebake (30 min)
(ICL packaging)
106
CL
coater6
ICL
ICL
ICL
ICL
EV1
TR
coater6
AME5000
AMEOOO
ICL
aSher-ICL
ICL
premetal-Piranha
rca-ICL
DCVD
B3-DryOx
HMDS-TRL
coaster
preakeoven
coater
prebakeoven
EVI
ICL
ICL
ICL
ICL
ICL
TRL
TRL
TRL
TRL
TRL
TRL
TRL
EV1
photo-wet-I
prebakeoven
TRL
TRL
TRL
asher-ICL
AEICL
sts2
TRL
coater, prebakeoven
9ts2
photo-wet-I
asher-TRL
acid-hood()
acid-hood(2)
TRL
TRL
TRL
TRL
TRL
TRL
rca-TRL
EV620
EV501
B3-DryOx
A2-WetOxBond
VTR
Solvent-noAu
diesaw
TRL
TRL
TRL
TRL
TRL
ICL
TRL
ICL
Appendix B
Mask Detail
The hybrid RPA grids, the MEMS RPA housing, and MEMS RPA electrodes
are all machined from 6-inch silicon wafers with features defined through contact
photolithography. The required masks for each of these components are shown in
this appendix. All masks are dark field masks and their negative images are displayed
here. The black portion of the figures correspond to open ares of the contact mask
through which ultra-violet light penetrates to expose positive photoresist.
B.1
Hybrid RPA Grids
The hybrid RPA grids consist of a two-level mask. The first mask defines alignment marks, apertures, and recesses on the grids for the alumina spacers. This level
also contains identification marks specifying the aperture size and pitch in microns,
Figure B-1.
The second mask is aligned to the backside of the wafer and defines the same grid
apertures along with cutouts for each electrode. These cutouts have a notch that
accepts an alumina rail to help enforce alignment and provide electrical insulation
from the steel housing, Figure B-2.
107
B.2
MEMS RPA Housing
The MEMS RPA consists of eight mask layers. The alignment mark mask (Figure B-3) that is exposed on both sides of every housing layer. Next the recess mask
defines a shallow clearance on one side of every wafer to prevent springs from fusing
to other housing layers during bonding (Figure B-4). The first housing wafer defines
the overall RPA aperture exposed to the plasma as well as three clearance holes for
mounting screws, Figure B-5. Each of the remaining five wafers that make up the
MEMS RPA housing has its own standoff pattern. The springs remain identical for
these masks, but different supporting plateaus help hold the matching grid at the
proper inter-electrode spacing (Figures B-6-B-10).
B.3
MEMS RPA grids
The MEMS RPA grids are once again made using two mask layers. This time,
tethers are incorporated into the design for ease of processing (Figures B-11 and B12). Additional hybrid RPA electrodes are etched with the new constant 400 Pm pitch
along with micro-manipulator pins for the assembly tool.
108
A
0
Figure B-i: First mask layer for the hybrid grids: 80
hybrid RPA electrodes are etched with various aperture
sizes and pitch
for a wide range of transparencies, along with 6 square
test samples.
I.N
Cutouts
Mask 1
06-03-2010
Recesses
C.
*
*o+e
0
0
0
0
0O
0
LI0
4x
Figure B-2: Second mask layer for hybrid grids: Each electrode outline has three notches meant
to align subsequent electrodes
using alumina rails.
Grids
Cutouts
Contact Cuts
Mask 2
06-03-2010
I.
I.
X
-\
)(
/~
X
)(
7L
K
X
... ..7L
X
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A.........7L
)
X
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\-
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)(
X
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..
K
/-
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K7
7L
X
X
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-N
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.....
\-
*
*
X
)(
7L
-\
)(
7L
)
)
)
4x
.........
..............
Figure B-3: MEMS housing align mark mask: The outline of 30 RPA housings and alignment marks are
defined on both sides
of each housing layer.
(K
I
(K
Eric Heubel
02-16-2011
pRPA v1.0
housing
alignment
ND3
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( "**&
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Figure B-4: MEMS housing recess mask: The topside of every RPA housing layer requires a recess etch to provide clearance for
spring actuation.
housing
recess
pRPAv1 .0
02-16-2011
Eric Heubel
0
o
Qo
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0
0
Qo
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0
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oQoQ
0
oQo
Q
oQ
0
0
0
0
0
0
0
4x
Q
Q
Figure B-5: MEMS housing aperture mask: The first housing layer defines the overall sensor aperture and the location of the
clearance holes for mounting to a testplate or RPA feedthrough.
W0
C3aa0
o
o
hO.aperture
housing
Eric Heubel
02-16-2011
pRPA v1.0
I.
000
0
0
0
~0
0
0
0
0
0
0
0
000
0
0
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0
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000
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00
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0 00
,00
0
0
0
000000000
0
)
0
0O
4x
Figure B-6: MEMS housing spring layer 1: The first spring layer will accept the floating electrode, and defines a first set of
standoffs for the height of the second grid.
00
0
Eric He ubel
01-13-2 012
pRPA_ VI .1
housing
hlspri ngs
o
000
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0
0
0
0
00
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0
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0
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00
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o
0
0
0
0
0
0
oQ__
o
0
:00__
0
____0
40
_
Figure B-7: MEMS housing spring layer 2: The second spring layer accepts the first electron repelling grid, which rests on the
standoffs of spring layer 1.
0
Eric Heubel
01-13-2012
pRPA vI.1
housing
h2 _springs
I'
0
h3_springs
housing
Eric Heubel
01-13-2012
pRPA vI.1
0
0a
Oa
0
0
0
0
0
0
0
0
,O0
o
0
0
a
0
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aoa
0
o
0 0
0
0
00
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0
0
0
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0
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Figure B-8: MEMS housing spring layer 3: This housing layer will hold the crucial ion retarding electrode.
o
0
D
0
0
o
o
o
o
o
a
a
a
00
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a
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a
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o
0
o
o
0
a
0
a
0
ooQ
a
oO
0o
0
0
4x
Figure B-9: MEMS housing spring layer 4: This housing layer is designed to accept the fourth and final grid, the secondary
electron repelling electrode.
Eric Heubel
01-13-2012
pRPA v1.1
housing
h4springs
00
00
0
0C
0
o00
Qo
0
00.
0
0
0
0
0
0
0
0
0
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00
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00
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00
0
0
0
0
0Q
0
4x
Figure B-10: MEMS housing spring layer 5: The last spring layer has no standoffs, it will hold the collector plate, which is made
to rest on the standoffs of spring layer 4.
00
0
0
Eric Heubel
01-13-2012
pRPA v1.1
housing
h5springs
~
L~
L,
+
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Figure B-11: MEMS RPA grid apertures and recesses: This wafer defines the identification marks for the 35 MEMS RPA grids
along with those for 30 new hybrid grids. The dashed lines in the magnified image of two MEMS grids correspond to their
matching cutouts.
~5~4
4
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grids,"hybrid
recesses and
alignment
pRPA vI.0
09-29-2011
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Appendix C
Engineering Drawings
The following engineering drawings depict the components of the hybrid RPA
sensor on pages 122 through 124. The testplate designed to position the hybrid and
MEMS RPAs downstream of the ion source is found on page 125. The different pieces
that make up the assembly tool for the MEMS RPA are found on pages 126 through
131. Finally, drawings of the vacuum feedthrough used for testing of the RPAs in the
helicon plasma are included on pages 132 through 136.
121
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Bibliography
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