Diagnostics and Experiments on LAPPS*
D. Leonhardt, D. P. Murphy, S. G. Walton, R. A. Meger,
R. F. Fernsler, R. E. Pechacek
Plasma Physics Division, U.S. Naval Research Laboratory, Washington, DC
20375-5346
presented at ICOPS99, Monterey, CA
ABSTRACT
NRL is developing a new plasma processing reactor called the ‘Large Area Plasma
Processing System’ with applications to semiconductor processing and other forms of
surface modification. The system consists of a planar plasma distribution generated by a
magnetically collimated sheet of 2-5kV, 10 mA/cm2 electrons injected into a neutral gas
background. This beam ionization process is both efficient at plasma production and readily
scalable to large (square meters) area. The use of a beam ionization source largely
decouples the plasma production from the reactor chamber. Ion densities (oxygen, nitrogen,
argon, helium) of up to 5x1012 cm-3 in a volume of 2 cm x 60 cm x 60 cm have been
produced in the laboratory. Typical operating pressures range from 20–200 mtorr with beam
collimating magnetic fields strengths of 10–300 Gauss. Thus far the system has been
operated with a pulsed (10-2000 s pulse length, <10 kHz pulse repetition frequency) hollow
cathode. Temporally resolved measurements of the plasma sheet using Langmuir probes,
spectrally resolved optical emission, microwave interferometry, and cyclotron harmonic
microwave emission will be presented. Results of initial processing tests using an oxygen
plasma showing isotropic ashing of a photoresist will be shown. Progress in the
development of a dc hot filament cathode will be presented along with the status of the 1 m2
UHV chamber for future processing tests. An overview of the LAPPS process along with
theoretical treatments and issues will also be presented by co-authors1.
1 Presented
in 5A01-02 by R. F. Fernsler.
Work supported by the Office of Naval Research
Plasma Production
HOLLOW CATHODE
BEAM SOURCE
THIS PRODUCTION PROCESS
THUS SCALES WITH THE
ELECTRON BEAM SOURCE
emits
KILOVOLT ELECTRON BEAM
which efficiently
IONIZES THE BACKGROUND GAS
resulting in
A COLD PLASMA DISTRIBUTION
ANODE
1-2 CM
Lapps Diagnostics
A variety of diagnostics are necessary to determine the critical parameters in the
plasma environment and surface interactions:
• ELECTRON BEAM
– Current and voltage monitors
– Electron energy analyzer - beam energy loss, distribution
• PLASMA
–
–
–
–
–
Langmuir probes - time resolved determination of floating potential, Te, ne
Microwave transmission - highly accurate but global measurement of ne
Charge collectors/photodetachment experiments - to study negative ion production
Optical spectroscopy - non-intrusive determination of ionic species, temperature
Laser induced fluorescence - non-intrusive determination of ion/neutral species with
high spatial resolution
• SURFACE
– Quadrupole mass spectrometer - fluxes of charged and neutral particles to surfaces
being studied as well as ion/neutral energy distributions
– Topological diagnostics post processing - SEM/AFM
Acrylic Test Chamber
Excellent for diagnostics
• Linear hollow cathode beam source
• 500 s, 2.4 kV pulse
• base pressure ~ 10 mTorr
OPTICAL EMISSION
SPECTROMETER
low resolution, 350-1100nm,
minimum integration time of
2ms. Quickly gives entire
emission spectrum of plasma
30 cm wide
plasma layer
Coils
SIDE VIEW in operation
TOP VIEW
MICROWAVE
TRANSMISSION AND
NOISE MEASUREMENTS
X band system operating
8.5-12.5 GHz. Attenuation
of microwaves can be
directly related to ne
LANGMUIR PROBE
Th-W probe to temporally
resolve plasma’s Te, ne,
floating potential, saturation
currents...
PHOTOMULTIPLIER TUBE to
determine temporal response of light
emission. Can be coupled to 1/4 m
monochrometer to temporally resolve
specific lines when applicable
Oxygen Discharge: Temporal
data 1
75 mTorr O , 210 Gauss
2
75mTorr/210Gauss
0
0
-500
-2
-1000
-4
-1500
-6
PULSE END
-2000
-8
-2500
Vfl1M
6
VOLTAGE SENSOR (V)
BEAM CURRENT (amps)
O75mTorr/225Gauss
: 75mTorr/225Gauss
2
4
2
0
Vfloat (probe into 1M)
-2
PULSE END
-4
PMT SIGNAL (arb)
0.3
-6
VOLTS
0.2
Iisat (Vprobe= -30VdC)
0.1
W RECEIVER SIGNAL
0.0
-100
8.5 GHz
0
-5
12.0 GHz
0
100 200 300 400 500 600 700 800 900
MICROSECONDS
Iesat (Vprobe= +20VdC)
-10
-15
-100
0
100 200 300 400 500 600 700 800 900
MICROSECONDS
Oxygen Discharge: Temporal data 2
2
0
0
-500
-2
-1000
-4
-1500
-6
PULSE END
-8
-2000
-2500
PMT SIGNAL (arb)
-10
Basically, the O2 discharge shows two
preferred operating modes:
(1) a short lived (~150s) high density mode
at lower pressures and high magnetic
fields
(2) a long lived high density /low impedance
mode at higher pressures
W RECEIVER SIGNAL
THE LANGMUIR PROBE QUICKLY BECOMES
CONTAMINATED, so only Iesat, Iisat and Vfloat are
shown. Presently we are looking at heated and
emissive probes to circumvent this problem.
12.0 GHz
-100
VOLTAGE SENSOR (V)
BEAM CURRENT (amps)
O2: 100mTorr/90Gauss
100mTorr/90Gauss
0
100 200 300 400 500 600 700 800 900
MICROSECONDS
USING MICROWAVE (W) TRANSMISSION TO
DETERMINE PLASMA DENSITY:
Microwaves penetrate a finite distance into plasma
even when below the critical frequency. Assuming
a uniform plasma profile with thickness < W
wavelength, attenuation of microwaves is (to first
order) given by ne(cm-3)  1.2x1012[f(GHz)/10]2.
Thus for complete attenuation of 8.5 GHz Ws,
ne9x1011cm-3. For 12 GHz, ne1.9x1012cm-3.
Oxygen Plasma Emission
EMISSION SPECTRUM OF O2 DISCHARGE
+
INTENSITY (arb)
O2 & O2 MOLECULAR CURVES
O emissions:
+
O2 first negative series
5
3
600
4
O2
3
1 negative system
845/777nm
2
700
750
800
850
900
WAVELENGTH (nm)
• High-lying excited states are seen in visible
regime with atomic emissions apparently
dominant.
• Excited atomic states have possible
channels from molecular parentage or
purely
atomic precursors after
dissociation of
ground state
molecule.
Time resolved line emissions should assist in
this determination (in progress...)
X g
10
3
3
O( P) + O( P)
5
O2
3
0
5
O( P/ P)
3 5
O( S/ S)
st
15
(0,2)
(0,1)
650
+ 4 0
a u
+
ENERGY (eV)
550
3
O( P) + O ( S )
3
& P to S
(0,0)
500
+ 4 0
(3p) P to (3s) S
(1,0)
450
-
5
(2,0)
400
4
b g
20
1
O( D) + O ( S )
-
X g
INTRANUCLEAR SEPARATION
Neon Discharge: TemporalLANGMUIR
DataPROBE
1 DATA:
9595
mTorr
Ne, 270 Gauss
mTorr/270Gauss
0.5
0
0.0
-500
-1000
-0.5
-1500
PULSE END
-1.0
-2000
-2500
-1.5
0.5
VOLTAGE SENSOR (V)
BEAM CURRENT (amps)
NEON: 95mTorr/300Gauss
95 mTorr/300Gauss
0.0
Vfloat (probe into 1M)
-0.5
-1.0
PULSE END
-1.5
PMT SIGNAL (arb)
-2.0
0.20
VOLTS
0.15
0.10
0.05
Isat (Vprobe= -30VdC)
W RECEIVER SIGNAL
0.00
-200
8.5 GHz
12.0 GHz
0
200 400 600 800 1000 1200 1400 1600 1800
MICROSECONDS
-0.05
0.4
0.0
-0.4
-0.8
-1.2
-1.6
-2.0
-2.4
-2.8
-3.2
-200
Esat (Vprobe= +20VdC)
0
200 400 600 800 1000 1200 1400 1600 1800
MICROSECONDS
Neon Plasma Emission
85 mTorr/300Gauss
3p manifold
(9 states)
3s manifold
(4 states)
~17 eV
Ne ground state
All observed emissions are from neutral atoms,
specifically from the 3p manifold of states to the
3s manifold. The 3s manifold is the lowest in
energy, ~ 17eV above the ground state and
consists of two metastable and two resonant
states.
Differences in O2 and Ne discharges
• O2 plasma destruction is recombination dominated (~n2), specifically by e + O2+  2O (or O +
O*) while the Ne discharge is diffusion limited (~Dd2n/dx2), since there are no strong
neutralization reactions in the 100mTorr regime. Gas mixtures can be very interesting...
• Neon discharges readily form high density (~1012cm-3) plasmas with or without large electron
beam currents. O2 discharges were less forgiving. For materials processing applications, all
possibilities should be explored; fluxes to the surface are to be measured via in situ mass
spectrometry as well basic materials’ test exposures.
• Ne plasma shows significant charged particle densities well after (500s) the electron beam
has been turned off. In sharp contrast to the O2 plasma whose charged particles densities
rapidly diminish after the pulse (40-60s). Conclusive measurements of specific species
(charged and neutral) along with their time dependencies will also be studied via mass
spectrometry.
• Argon shows very similar behavior as Neon, but Ar+ emission lines are also seen in the visible
spectrum. The analogous behavior is reassuring; Ne+ emission may merely be out of the
spectral region we have access to.
• Hollow cathode operation also varies, although this dependence is difficult to pinpoint at the
present time. Hence, we are intending to measure the electron beam energy/distribution at the
anode with a hemispherical energy analyzer. Additional work with different cathode shapes
show a variety of plasma operating conditions.
• Langmuir probe data closely mirrors the dependencies of the optical emission and electron
beam current (somewhat) although the probe has a much smaller dynamic range (changes in
factors or 2-4) vs. the non-intrusive techniques (10-100’s). It is unclear at this time whether
this phenomena is a technical issue of probe applications.
LAPPS for Materials Processing
PLASMA
MAGNETIC FIELD
BEAM DUMP
CATHODE
T ~ cm
KV ELECTRONS
L (~ meters)
MATERI AL
TO BE
PROCESSED
STAGE
RF & TEMP
CONTROL
BEAM ELECTRONS
PLASMA ELECTRONS
IONS
BACKGROUND GAS
FREE RADICALS
Initial Material Processing Test: Setup
BEAM
ELECTRONS
MAGNETIC
FIELD
Ti FOIL
LIMITER
PLASMA
DISTRIBUTION
METAL PLATE
A
B
BIAS
CONNECTOR
Ti FOIL
WAFER
A
10 mm
B
6 mm
Collector
Current
1.9 mA/cm2-div
Discharge
Current
10 A/div
Collector current from 40 cm2 plates located
10 mm and 6 mm from oxygen plasma edge
for -20 V bias and total discharge current
ANODE SURFACE
Actual Material Modification:
Aluminum Mask on Photoresist
Silicon Wafer
2 cm
Aluminum
Photo-resist
20 micron
Etched Photoresist
0.1% duty, 20 sec total
50 mTorr Oxygen gas
LAPPS Prototype Processing Chamber
• Aluminum body construction
• Base pressure ~10-7 torr
• fine control over gas flow
– residence time
– gas mixture
Ground Plane, Diagnostics
Plasma Layer
Anode
Shielded Cathode
Beam Energy
Analyzer
B Field
Coils
SIDE VIEW of empty chamber in lab
RF Bias, Diagnostics
TOP VIEW
LAPPS Parameters to be Investigated
Neutral Gas
Plasma Density
Plasma Potential
Plasma Temp
Electron Temp Control
Free Radical Production
Free Radical Species
Plasma Duty Cycle
Bias and Temp Control
Plasma Processing Area
Uniformity
10-1000 mTorr process gas
N+, N-, Ne up to ~ 1012 cm-3
Vp low during pulse, Vp  0 in afterglow
Tion < Te < 1 eV
Auxiliary heating could raise Te to several eV
Direct beam and dissociative recombination
Species control via Te and pulse length (and gas)
DC or arbitrary pulse with 10 microsecond on/off time
Independent stage for RF or DC bias and temp control
Square meters, arbitrary location relative to surface
Better than 1% desired, adjustable in one dimension
LAPPS UHV Compatible Chamber
•
•
•
•
Scheduled for delivery 8/99
stainless steel construction
can accommodate 1m2 stage
separable cathode and processing chamber for cathode development
Field Coils
Processing Chamber
Beam Production
Chamber
Pumps
End View
Side View
1m
LAPPS UHV Compatible Chamber:
Internal Arrangement
Auxilary Grounding
Plane
Beam Dump
Electron Beam
Aperture
Aperture and
Thin Foil
Electron
Emitting
Filament
Thermal
Control
Stage
Adjustment
RF Bias
Beam
Optics
Processin g Stage
Material
Beam Sources: Hollow Cathode
Pulsed linear hollow cathode
used extensively to date
• Beam electrons produced
by secondary emission
from ion bombardment
– eff < 0.2
– cathode mat., ion species,
energy dependent
– resonance with B
Magnetic Field
Anode
Hollow Cathode
Plasma
• 60 cm long, 50 mA/cm2
beams produced
– 1-5 kV, 10-5000 s pulse,
10 kHz prf
• Significant plasma current
HV Electron
Grounded
Shield
Anode
Grid
Insulator
Reflexing
Electrons
Beam Sources: Hot Filament Cathode
Beam Collector (not in photo):
grounded through
a 5.4 Ohm resistor
• LAPPS beam requirements
–
–
–
–
CW or modulated pulse
<50 mA/cm2
15-20 keV beam energy
linear cathode with 1 cm x 10-100
cm width
– ~1% uniformity
Second Acceleration Stage:
+ 2-5kV wrt the Filament (grounded)
First Acceleration Stage:
+300V wrt the Filament
• Initial experiments with thoriated
tungsten filament
–
–
–
–
1 cm x 10 cm beam aperture
20 Gauss, 240 V extraction
3 cm FWHM, 50 mA beam
space charged limited beam
• LaB6 cathode in preparation
Focusing Element:
-45V wrt the Filament
Heated
Filament
WORKING PROTOTYPE ASSEMBLY
1st
Stage
10 cm
– Pierce design extraction cathode
– post accelerate beam to 15-20 kV
Filament
Heater
Contacts
Focusing Element
2nd Stage
Acknowledgements
We greatly appreciate the assistance of Dr. W. E. Amatucci with the
Langmuir probe measurements.
SGW is a National Research Council Postdoctoral Research Associate
and REP is a member of SFA, Inc., (Landover, MD).
This work is supported by the Office of Naval Research