IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 6, JUNE 2011
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Microfabricated Flexible Electrodes for Multiaxis
Sensing in the Large Plasma Device at UCLA
Franklin C. Chiang, Patrick Pribyl, Walter Gekelman, Bertrand Lefebvre,
Li-Jen Chen, and Jack W. Judy, Senior Member, IEEE
Abstract—As conventional sensors are scaled down in size for
proper usage in high-density laboratory plasmas, they become
harder to construct reliably by hand. Devices fabricated utilizing
microelectromechanical systems (MEMS) techniques are superior
to hand-made devices in terms of size scale, process control, and
precision. Microprobes give experimentalists the ability to take
direct measurements under controlled conditions. This paper discusses flexible MEMS multiaxis probes that have been developed
for use in the Large Plasma Device, a cathode-discharge plasma, at
UCLA. The probes are custom built and tailored to fit the unique
specifications of individual experiments. Postfabrication assembly
also allows for simultaneous sensing in multiple axis. MEMS
electric-field probes have been successfully used to detect electron
solitary structures in a high-density plasma that are predicted in
theory but never seen before except in low-density space plasmas.
Index Terms—B-dot microcoil, electric-field (E-field) measurements, microelectromechanical systems (MEMS) devices, plasma
diagnostics.
I. I NTRODUCTION
T
HE ABILITY TO take plasma measurements without
disturbing the plasma itself is extremely important when
diagnosing terrestrial plasmas. In particular, high-density plasmas, such as those used in fusion research or semiconductor
manufacturing, have hitherto been extremely hard to accurately
diagnose because conventionally made sensors are simply too
large to take measurements without adversely disturbing the
plasma. Fortunately, advances in micromachining technology
now enable us to build devices that are smaller than the characteristic size scales of laboratory plasmas (i.e., Debye length,
ion gyroradius, etc.).
MEMS, or microelectromechanical systems, technology offers many of the same advantages as semiconductor manufacturing. For example, the nature of batch fabrication allows for
Manuscript received October 10, 2010; revised February 8, 2011; accepted
February 24, 2011. Date of publication April 21, 2011; date of current version
June 10, 2011. The LAPD Laboratory is part of the Basic Plasma Science User
Facility, which is funded by an NSF/DOE cooperative agreement (NSF-PHY05316121). This work was also supported by a Multidisciplinary University
Research Initiative (MURI) Z882801 from the Office of Naval Research.
F. C. Chiang and J. W. Judy are with the Electrical Engineering Department, University of California, Los Angeles, CA 90095-1594 USA (e-mail:
franklin.c.chiang@ucla.edu; jack.judy@ee.ucla.edu).
P. Pribyl and W. Gekelman are with the Physics Department, University
of California, Los Angeles, CA 90095-1594 USA (e-mail: pribyl@ucla.edu;
gekelman@physics.ucla.edu).
B. Lefebvre and L.-J. Chen are with the Space Science Center and the
Physics Department, University of New Hampshire, Durham, NH 03824 USA
(e-mail: bertrand.lefebvre@unh.edu; lijen@mailaps.org).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPS.2011.2129601
dozens of devices to be built simultaneously, with very high
device reliability and uniform electrical and mechanical characteristics. This opens up the possibility of using a large number
of sensors for real-time monitoring of plasma parameters over
a large volume of plasma, eventually limited only by the backend data acquisition systems.
Previously, a first-attempt MEMS probe successfully sampled electric-field (E-field) perturbations during an experiment
to observe electron solitary structures in the Large Plasma
Device (LAPD) at UCLA [1]. Although successful, the use of a
5 cm × 7 cm × 0.5 cm copper box to house amplifiers and associated circuitry introduced a great deal of uncertainty regarding
possible plasma disturbance from the housing itself. This was
in part due to the fact that the MEMS probes extruded only
a short distance (∼500 λD ) from the box edge. Fortunately,
the results from the experiments, coupled with refinement of
the fabrication process, enabled probes with smaller overall
dimensions to be made.
In addition to E-fields, plasma phenomena often have a
magnetic-field component that can be measured. In particular,
waves, such as Alfvén waves, give off unique signatures as
they propagate through plasma [2], [3]. A commonly utilized
diagnostic for measuring magnetic fields is a B-dot or inductive
loop probe [4]. For outer-space experiments, these loops can be
quite large, while the size limit for conventional probes made
by hand under a microscope is about 1 mm.
Ideally, experimentalists would like to use probes that are
less than 1 mm in length, perhaps even as small as 500 or
250 μm if attempting to measure very fast high-density discharges. Probes of this size would give unprecedented resolution and possibly reveal signals previously undetectable by
conventional probes.
Once again, MEMS technology holds much promise in
this regard because it enables wire traces to have widths on
the order of micrometers. Already, the ability to pattern and
fabricate such narrow metal traces has proven invaluable for
some experiments [1], [5]. In fact, loops of micrometer-wide
wire have already been made for magnetometer applications
[6]–[9], and adapting them for use in a plasma environment is
a logical next step.
This paper presents an improved MEMS E-field probe, as
well as a MEMS “B-dot” probe that can be used to measure changing magnetic fields. Both devices utilize polyimide
as a flexible dielectric encapsulating a metal conductor. This
flexible nature allows for rough handling without damaging
the devices, enabling postprocessing assembly into true 3-D
detectors.
0093-3813/$26.00 © 2011 IEEE
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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 6, JUNE 2011
II. D ESIGN
A. E-Field Probes
The first-generation MEMS E-field probe fabricated in [1]
showed great promise in observing a previously undetectable
phenomenon in high-density plasmas. However, being a first
attempt, there was much room for improvement. For instance,
the copper housing protecting the preamplifier circuitry was
very large with respect to the Debye length. As a result, it
likely created a large disturbance in the plasma. In addition, the
MEMS probe tips were located ≤ 5 mm away from the housing.
Although this distance is many times larger than the Debye
length, there is a legitimate concern of measuring disturbances
caused by the housing. Thus, even though measurements were
obtained and signals not seen with conventional macroscopic
hand-made probes were observed, it was felt that a newer
design with a longer shaft and narrower profile was needed.
Unless data collected by the probes are transmitted out of the
plasma chamber wirelessly, a metal shaft and housing for the
probes is required. Fortunately, the signals observed from
the early experiment were strong (i.e., ≥ 0.1 mV), which means
that the onboard discrete preamplifiers kept in the copper housing are not necessary to detect a useable signal. Eliminating the
preamplifiers also greatly simplifies the overall design of the
bond-pad areas of the MEMS E-field probe tips. In addition,
by reducing the number of sensing elements on the probe tip to
only two differential pairs, the overall probe profile was reduced
to minimize possible disturbance in the plasma.
Instead of using wire bonds to electrically connect the probes
to the conductive traces inside the shaft, we decided to make
the bond-pad areas on the probe much larger so that conductive
epoxy could be used to form a direct electrical connection to
the probe. This approach not only eliminates any inductance
introduced by wire bonds but also simplifies the fabrication
process as the gold layer no longer needs to be thick enough
to support wire bonding. Thus, the electroplating step used in
the previous design is no longer necessary and is replaced by an
evaporated layer of gold.
One benefit of using only evaporated gold to form the metal
traces is improved thickness uniformity across the device, allowing us to extend the MEMS probe tip shaft length to 4 cm,
further isolating the tips from the large metal housing. Gold was
still the conductor of choice, as the metal tips were going to
be exposed to air and thus needed to be resistant to corrosion
and oxidation effects. A schematic diagram of the probe-tip
dimensions can be seen in Fig. 1, where g is the gap between
two signal traces, d is the gap between two differential pairs,
w is the width of a signal trace, and s is the width of a shield
trace.
Each probe tip contained four signal traces, making up two
differential pairs. In addition, three traces, one on each side of
the differential pairs and one in between, were also incorporated
to help shield the signals from each other as much as possible. A
variety of probe tips with different dimensions were fabricated.
The tips had a w of either 10 or 15 μm, with a g of 20 μm, s
of 20 μm, and d of either 60 or 120 μm, and all values were
individually selected to correspond with particular experiment
parameters.
Fig. 1.
Schematic illustration of a new MEMS E-field probe tip.
B. B-Dot Probes
The simplest way to detect time-varying magnetic fields is
with the use of a coil of wire. Magnetic flux passing through a
coil of wire introduces a current to flow in the wire, an effect
described by Faraday’s law of induction ε = −N · (∂ΦB /∂t ),
where ε is the resulting electromagnetic force in volts, ΦB is
the magnetic flux through the circuit in webers, and N is the
number of identical loops in the coil of wire. ΦB is equal to
the magnetic flux density B in teslas multiplied by the enclosed
area A of a single loop in square meters. This same principle
is widely used in a variety of applications, from magnetometers
to inductive coupling for wireless energy transfer.
The design of the MEMS B-dot probes was a matter of
balancing various tradeoffs with one another. Since we are
measuring the induced current and solving for the magnetic
field, our goal is to maximize the number of loops in the device,
as well as the area of each individual loop. At the same time,
we wanted the total area of the probe face to be less than
1 mm2 so that it would be considered an improvement over
conventional probes built by hand. Also, the entire sensor must
be encapsulated within a dielectric so that it is electrically
shielded from the plasma environment.
An additional consideration was the physical impedance of
the loop of wire. In order to minimize the size of the probe
housing, we decided to forego the use of onboard amplifiers.
Thus, the resistance of the probes needed to be as close as possible to 50 Ω to match the input impedance of the oscilloscope
and coaxial transmission line. This became a severe constraint
in determining the total length of the trace, as well as the trace
dimensions.
Ideally, we would also want to use a differential measuring
technique that will allow any detected dc offsets to be immediately subtracted out as long as the coils are identical to each
other but of reverse polarities. This requirement actually plays
to the strength of MEMS fabrication techniques, as devices
are built layer by layer with repeatable processes and accurate
alignment. It is for this exact reason that differential on-chip
inductors, which are basically identical in structure to differential magnetometers, have been so successful. Fig. 2 shows the
various design parameters of a single coil, where l is the size of
the sensor head, b is the outer border between the outer edge of
the dielectric and the first wire loop, w is the width of the wire,
and g is the gap between each loop.
We wanted to minimize the number of metal layers used
in the fabrication of the device to reduce the complexity of
CHIANG et al.: MICROFABRICATED ELECTRODES FOR MULTIAXIS SENSING IN THE LAPD
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Fig. 3. Plot of B-dot probe head area A as a function of wire width w for
R = 50 Ω and g = 3 μm.
Fig. 2.
Illustration of planar spiral coil with three loops (n = 3).
the fabrication process. Therefore, while conventionally wound
coils can have multiple loops of the exact same size, our
B-dot coils were designed as planar spirals. This negatively
impacts the total area of the sensing element, so the total area
is no longer simply equal to the number of coils multiplied by
the loop size. Instead, we approximate by summing areas of
progressively smaller coils, taking into consideration the width
of the wire and the gap size between them to get
A = n · (l − 2 · b + 2 · g)2 − 2 · n · (n + 1) · (l − 2 · b + 2 · g)
2
· (g + w) + · n · (n + 1) · 2 · n + 1) · g + w)2 (1)
3
where n is the number of loops in the planar coil. Similarly, the
total length of the coil can be calculated to be
4 · (n + 1) · (1 − b) − 2 · n · (n + 1) · (g + w).
(2)
Since the thickness of the deposited metal will be strictly
dependent on the final fabrication process, we can calculate the
expected resistance R of the metal loop in ohms and determine
suitable values for the width w and gap g.
Fig. 3 shows a plot of the enclosed area A as a function of
the wire width w for various lengths l for R = 50 Ω. As we can
see, the most effective way of increasing the enclosed area is
to increase the size of the probe head, followed by gradually
increasing the wire width in order to achieve more turns in
the spiral. Note that there is a point where increasing the wire
width no longer gives a larger enclosed area. Intuitively, this
makes sense because, as the spiral moves closer and closer to
the center point, the gain in area no longer keeps pace with the
increase in resistance, and hence, for a constant R, the area
begins to decrease. The only way to increase the area further
without changing l is to decrease g. However, g is dictated by
the lithography capabilities during fabrication.
Differential inductors require a center ground tap that serves
as a reference. Since our two coils are stacked on top of each
other, we decided to make a ground plane both above and below
the coils to help prevent direct coupling from the plasma to the
coils. In addition, the traces leading back to the bond pads for
each of the coils were stacked on top of each other. Stacking the
traces is important because the traces are long enough that they
could form an unintended loop, with a resulting magnetic flux
that could overpower the desired signal from the sensor head.
The presence of the ground planes and the long wire traces
introduces a large capacitance into the system, thereby slowing
down the frequency response of the probes. The capacitances
can be approximated as parallel plate capacitors, and the probes
were designed such that the frequency at which the imaginary
impedance 1/(2 · π · RC) is equal to the real impedance of the
trace R was greater than 50 MHz, far beyond our operating
region of interest.
Likewise, loops of wire have an inductance value that becomes very important when determining the resonating conditions of the system. The self-inductance of a planar coil is a
well-established area of research within the integrated circuits
community, and a good approximation is given by the modified
Wheeler formula as
L = 2.34 · μ0 ·
n2 · davg
1 + 2.75 · ρ
(3)
where μ0 is the permeability of free space, n is the number
of turns in the coil, davg = ((dout + din )/2), and ρ = (dout −
din )/(dout + din ) [10]. Once again, the probes were designed
such that the frequency at which the imaginary impedance
(R/2 · π · L) is equal to the real impedance of the trace R was
greater than 50 MHz.
Since the entire device was to be encapsulated in polyimide,
the oxidation and corrosion of the metal was not a concern.
Therefore, we decided against using gold due to the difficulty
with depositing thick layers. Although aluminum is 60% more
resistive than copper, it is much easier to handle. Thus, we
decided to use aluminum as our metal layer and deposited at
a thickness of 1.1 μm. Aluminum has a skin depth of 85 μm at
1 MHz, much greater than the expected thickness of our probe
head.
The gap g is dependent on the lithography accuracies and
etching capabilities. Given that we were performing contact UV
lithography, we decided to use a g of 3 μm in order to ensure
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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 6, JUNE 2011
Fig. 4. SolidWorks illustration of planar coils for 1 000-μm-wide B-dot probe
(ground planes that lie above and below the coils and are connected to the via
are not shown) with n = 5, w = 14 μm, g = 6 μm, and b = 10 μm.
reliable wire traces. Likewise, b was also conservatively set at
10 μm.
We input all the design parameters into a spreadsheet to
find the optimal w value that would satisfy our bandwidth
requirements and found that, for l = 1000 μm, w should be
equal to 17 and 8 μm for the 500-μm probe heads (see Fig. 4).
III. FABRICATION
Fig. 5. Illustration of process flow for MEMS E-field probes. In (a), a wafer
is deposited with polyimide and PR in preparation for metal evaporation.
(b) After the metal is patterned, (c) the second layer of polyimide is deposited.
A thin layer of aluminum (d) is then deposited and patterned (e) to act as a mask
during the oxygen plasma etch and (f) is removed after the devices are released.
A. E-Field Probes
Fabrication remains a two-mask process, as discussed previously in [1], and is shown in Fig. 5. As before, since the only
role of the silicon wafer is to provide a flat and rigid mechanical
substrate upon which the probes are micromachined, its crystallographic orientation is not important. Polyimide, which was
used as the insulating layer for the probes, was deposited with
two conditions in mind. First, the polyimide must adhere well
to the wafer for the duration of the microfabrication process.
Second, the probes must be easily released from the substrate
without any damage in the final oxygen plasma etch step. In
order to achieve these two conditions, the polyimide adhesion
promoter (VM-652, HD MicroSystems, Santa Clara, USA) had
to be applied in a specific pattern, as shown in Fig. 6.
If no adhesion promoter was used, the layer of polyimide
would not adhere well to the wafer throughout the whole
process. If the adhesion promoter was used over too great of an
area, the probes were impossible to remove without damaging
them. Therefore, the solution that we found was to apply the
adhesion promoter to only the edges of the wafer before the
first layer of polyimide was spun on. We did this by spinning
the wafer at 300 r/min and gently touching the wafer edge with
a clean wipe moistened with the adhesion promoter, resulting
in a ring of adhesion promoter on only the outer 5 mm or so of
the wafer. As long as no air bubbles got trapped underneath
the polyimide layer after it was cured, the entire layer of
polyimide remained well attached to the wafer during the whole
fabrication process up to the release etch. No adhesion promoter
would be present to hold down the micromachined probes after
the release etch, allowing for damage-free release.
Fig. 6.
Illustration of the pattern of applied adhesion promoter.
After applying the adhesion promoter, the polyimide (HD
MicroSystems PI-2611LX, Parlin, NJ, USA) was deposited
onto the wafer and spun at 1600 r/min for 45 s. The polyimide
was cured with a two-step bake in a nitrogen oven: 200 ◦ C for
60 min and, then, 350 ◦ C for 30 min, ramping up at 4 ◦ C/min.
This type of bake was found to allow any trapped bubbles
sufficient time to escape and resulted in a very smooth and
uniform polyimide layer (i.e., 11 ± 0.15 μm). After curing, an
11-μm-thick film is formed that will serve as the lower insulator
and the mechanical base of support for the probe tips.
Next, the first photolithography step was performed using
AZ5214E photoresist (Clariant Ltd., Muttenz, Switzerland).
Usually a positive-tone photoresist, by following the imagereversal process provided by the resist manufacturer [11],
AZ5214E can be used as a negative-tone photoresist for liftoff
processes that call for a negative-sidewall profile. A retrograde
sidewall profile is desired because it prevents directionally
evaporated metals from being deposited onto the sidewall. The
CHIANG et al.: MICROFABRICATED ELECTRODES FOR MULTIAXIS SENSING IN THE LAPD
image-reversal process was used in this photolithography step
to obtain sidewalls that prevented metal wings from forming
during the liftoff process. As a result, the edges of the patterned
metal layer are very crisp and smooth. The reliable formation
of smooth metal patterns is critical to ensure that there are no
breaks in the extremely long (i.e., ≥ 3 mm) metal features. A
1.4-μm-thick layer of photoresist was patterned to reveal the
areas where the gold will be deposited [see Fig. 5(a)].
Then, a 25-nm-thick layer of chrome and a 700-nm-thick
layer of gold were deposited onto the wafer by electron-beam
evaporation (CHA Mark 40, CHA Industries, Fremont, CA,
USA). The wafers were then submerged in an acetone bath for
1 h. The metal adhered to areas on the wafer that were not
protected by photoresist, while the metal that was deposited
onto the photoresist cleanly floated away as the underlying
photoresist was dissolved [see Fig. 5(b)]. After cleaning the
wafers with methanol, IPA, and, then, DI water, the second
layer of polyimide was spun on at 1600 r/min and cured in the
same process as before [see Fig. 5(c)].
In order to expose the bond pads and tips on each probe as
well as physically separate all the probes on the wafer from one
another, the two polyimide layers needed to be patterned and
etched. To accomplish this, a second photolithography step was
performed using AZ5214E photoresist and the same imagereversal process. A 50-nm-thick layer of aluminum was then
deposited by electron-beam evaporation. The deposited metal
is patterned with the same liftoff process (i.e., immersing the
wafer in sequential baths of acetone, methanol, IPA, and, then,
DI water). The aluminum deposited directly on the second layer
of polyimide was left untouched [see Fig. 5(d)].
Next, all unprotected polyimide was etched away using an
Oxford RIE plasma etcher (Plasmaline 515, Tegal, Petaluma,
CA, USA) with an O2 flow rate of 100 sccm, an RF power of
200 W, and a pressure of 27 Pa (0.2 torr). Notice that, once the
gold bond pads were exposed, they acted as a mask to protect
the layer of polyimide beneath it [see Fig. 5(e)]. Although
some undercut of the probe tips was observed (i.e., ∼1.25 μm),
the majority of the polyimide beneath the exposed probe tips
remained. The presence of the polyimide underneath the one
side of the probe tips did not affect the functionality of the probe
tips.
After the oxygen plasma etch, each micromachined structure
was gently peeled off of the silicon wafer and submerged in
aluminum etchant (Aluminum Etchant D, Transene Company
Inc., Danvers, USA) to remove the aluminum from the top
of the second polyimide layer [see Fig. 5(f)]. The final steps
of the fabrication process involved gluing the micromachined
E-field probes onto a PCB and making electrical connections to
the metal pads.
B. B-Dot Probes
The process flow for the MEMS B-dot probes is shown in
Fig. 7. Similar to the E-field probes, we began the fabrication
process by applying a VM-652 polyimide adhesion promoter
to the outer edge of a bare silicon wafer, followed by spinning
on a 6-μm-thick film of PI-2611LX polyimide at 3500 r/min
for 45 s. The polyimide was cured in a nitrogen oven by
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Fig. 7. Illustration of process flow for MEMS B-dot probes.
ramping up from 150 ◦ C to 350 ◦ C at 4 ◦ C/min and holding the
temperature at 350 ◦ C for 30 min. We found this shorter curing
step sufficient for this particular polyimide layer thickness.
Next, a 1.1-μm-thick blanket layer of aluminum was sputtered (CVC-601, Control Process Apparatus Inc., Fremont, CA,
USA) on top of the polyimide, followed by the first photolithography step using AZ5214E photoresist. We patterned a
1.4-μm-thick layer of resist to mask the subsequent aluminum
metal etching in a Unaxis SLR770 ICP etch tool (OC Oerlikon,
Pfäffikon, Schwyz, Switzerland) with 10 sccm of BCL3 , 5 sccm
of Ar, 40 sccm of Cl2 , 175 W of RF-coupled dc bias power,
500 W of ICP power, and at a pressure of 12 mtorr. We observed
an aluminum etch rate of ∼800 nm/min with this recipe.
The photoresist was then stripped in a 75 ◦ C ALEG-355
photoresist stripper bath (Mallinckrodt Baker Inc., Phillipsburg,
NJ, USA) for 30 s, and the final result is shown in Fig. 7(a).
We chose a photopatternable polyimide (HD MicroSystems
PI-2761, Parlin, NJ, USA) as the sandwiching dielectric layers
because it most closely matched the low-stress polyimide in
terms of physical and electrical characteristics. Both polyimides
have breakdown temperatures of over 500 ◦ C, making them
suitable for use in hot plasma environments. In addition, the
dielectric constants of both polyimides are the same, and the
ability to pattern PI-2761 in a fashion similar to photoresist
simplifies the overall process. Finally, both polyimides can be
controllably etched in oxygen plasma, making the final release
step of this process possible.
PI-2761 was spun onto the wafer in a two-step process, first
at 500 r/min for 5 s to spread the polyimide out over the wafer
and, then, at 4500 r/min for 45 s to achieve the desire thickness
of 5 μm. Another advantage of using the spin-on polyimide is
that it smoothes out the surface topology much more effectively
than vapor-deposited dielectrics would, which is very helpful
given the number of metal layers that are required in this
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Fig. 8. Top-view optical photograph of 1000-μm B-dot probe head (still
adhered to wafer).
process without the use of specialized planarization steps seen
in standard industry processes.
The polyimide was exposed using a Karl Suss MA4 aligner
(Suss Microtec, Garching, Germany) for 20 s at an 8-mW energy output (160 mJ), and the unexposed polyimide areas were
developed away to reveal the 15 μm × 15 μm via that connects
the stacked layers of aluminum. Developing the polyimide
required immersion and agitation in DE-9 040 solution (HD
MicroSystems, Parlin, NJ, USA) for 1 min and 30 s, followed
by a 30-s immersion in RI-9180 (HD MicroSystems, Parlin, NJ,
USA) to rinse. Using a photopatternable polyimide eliminates
the need for a hardmask, which saves one metal evaporation
and one plasma etching step for each layer of dielectric. For
our process with four layers of metal, this became a significant
saving in time and effort.
PI-2761 has a tendency to swell at the edges of patterned
structures, which is a function of the solvent development
and thus difficult to eliminate completely when developing.
Therefore, after development, the polyimide is placed in an
oxygen plasma etcher (Plasmaline 515, Tegal, Petaluma, CA,
USA) for 2 min at 200 W and 0.5 torr (67 Pa) in order to smooth
out any edges. The polyimide is subsequently cured with a twostep bake in a nitrogen oven: 220 ◦ C for 30 min and, then,
350 ◦ C for 60 min [see Fig. 7(b)], ramping up at 4 ◦ C from
150 ◦ C.
To ensure good electrical conductivity between the metal
layers through the via, a 2-μm-thick layer of aluminum is
sputtered onto the polyimide, partially filling the vias. We chose
this thickness in order to ensure adequate sidewall coverage in
the 4-μm-thick polyimide. Photoresist is then used to cover
the vias, and the bulk of the aluminum is etched away with
aluminum etchant type D [see Fig. 7(c)].
At this point, the aforementioned metal, dielectric, and aluminum plug deposition steps are repeated three times to deposit
the second [see Fig. 7(d)], third, and fourth metal layers, with
the final result after the fourth layer of metal shown in Fig. 7(e).
PI-2611 is used as the final dielectric capping layer, and the devices are released using the same thin aluminum mask process
as with the E-field probes [see Fig. 7(f)]. During the release
of the probes, the aluminum bond pads are also gently etched,
cleaning off any RIE polyimide residue that may have been
IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 6, JUNE 2011
Fig. 9. Zoom on corner of 1000-μm B-dot probe head, showing topology of
underlying metal layers.
Fig. 10. Optical top-view photograph of a third-generation microfabricated
MEMS E-field probe with w = 10 μm, g = 40 μm, and d = 70 μm.
Fig. 11. Optical photograph of third-generation MEMS E-field probe
mounted in stainless steel probe shaft.
inadvertently deposited during the oxygen plasma release etch.
Figs. 8 and 9 show top-view photographs of a 1000-μm probe
after the oxygen plasma release etch.
IV. R ESULTS
A. E-Field Probes
Fig. 10 shows an optical photograph of the fabricated device,
and Fig. 11 shows the probe after assembly into the larger probe
shaft. Note the lack of a copper box and the large separation
between the probe tips and the stainless steel tubing.
CHIANG et al.: MICROFABRICATED ELECTRODES FOR MULTIAXIS SENSING IN THE LAPD
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Fig. 13. Data plots showing (top) the measured tip potential of an electron
solitary structure and (bottom) the calculated E-field between the probe tips.
Fig. 14. Data plots showing (top) the measured tip potential of a Langmuir
wave packet and (bottom) the calculated E-field between the probe tips.
Fig. 12. Data plots taken using the third-generation MEMS E-field probe,
showing (top) the measured tip potential and calculated E-field and (bottom)
the frequency of measured activity.
The MEMS E-field probes were used in the LAPD in an
experiment to look for electron solitary structures. The probes
were placed next to an electron beam source, which was pulsed
at a variety of powers and frequencies. Fig. 12 shows a plot of
the potentials at the probe tips as well as the calculated E-field
and frequencies of the observed activity. The experiment lasted
for one week, yielding gigabytes worth of data. In addition
to solitary structures (see Fig. 13), other activities such as
Langmuir wave packets (see Fig. 14) and electron cyclotron
wave packets (see Fig. 15) were also recorded. Additional
details of the results can be found in [12].
B. B-Dot Probes
The 3-D B-dot probe tips are hand assembled by attaching three MEMS probe tips onto a custom-made support bar,
as shown in Fig. 16. The support bar for the 500-μm 3-D
B-dot probes was made by dicing a 500-μm-thick wafer into
individual beams, and the 1000-μm support bars were machined from ceramic.
The expected impedance from signal to ground for each
MEMS probe tip was 44 Ω for the 1000-μm tips and 48 Ω
for the 500-μm tips. The measured resistance was 69 Ω for the
1000-μm tip and 75 Ω for the 500-μm tips, which corresponds
to a resistivity of 4.95 × 10−8 Ω · m. This measured resistivity
is almost twice as high as the published value for bulk aluminum (2.7 × 10−8 Ω · m). Our higher sheet resistance value
was confirmed with a four-point-probe measurement (FPP
5 000, Veeco Instruments, Plainview, NY, USA). As expected,
the resistance from one signal trace to the other was the sum
of the two, confirming that the connecting via was functioning
properly.
The enclosed area for the 1000-μm probe tip was estimated
from (1) to be 3.77 × 10−6 m2 . We used a Helmholtz coil and
a network analyzer to calibrate our probe and determine the
true area. For a Helmholtz coil at low frequencies, the spiral
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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 6, JUNE 2011
Fig. 15. Data plots showing (top) the measured tip potential of a wave packet
at the electron cyclotron frequency and (bottom) the calculated E-field between
the probe tips.
Fig. 17. Plot of imaginary component from network analyzer during calibration of B-dot probe tips with Helmholtz coil.
that can take local measurements within a high-density magnetoplasma. In addition, the use of flexible materials allow for
postprocessing assembly to true 3-D structures, and the use of
MEMS fabrication has allowed us to break through the previous
size barrier that limited experiments in high-density plasmas.
With a well-established process, we have shown that new
generations of probes can be designed and implemented very
rapidly at a relatively low cost. It is our hope that one day we
will be able to provide experimentalists a toolkit of MEMS
plasma probes. This would allow them to conduct a much
broader range of experiments, possibly leading to a large number of plasma science breakthroughs.
ACKNOWLEDGMENT
Fig. 16. Photograph of fully assembled three-axis B-dot probe head.
behaves as a dc source, and the imaginary component of the
measured signal with respect to the reference signal of the
network analyzer can be approximated as
3
Vmeas (ω) ∼
4 2
μ0
·
·ω
(4)
Im
= ±A ·
Vref (ω)
5
r · Rp
where A is the enclosed area of the probe head, r is the radius
of the Helmholtz coil, and Rp is the parallel resistance of
the reference circuit [13]. In our test setup, r = 5.73 cm, and
Rp = 0.061 Ω.
Fig. 17 shows the resulting test plot from the network analyzer, and using the slope of fit line, we calculate the actual
area of our probe heads to be 3.6 × 10−6 m2 , which is in very
good agreement to our theoretical value.
V. C ONCLUSION
We believe that there is great value in combining the
controlled environment of laboratory-created plasma with the
sophistication and precision of microprobes to further our
understanding of fundamental plasma physics. We have demonstrated a reliable process to fabricate E-field and B-dot probes
The experiment was conducted at the Basic Plasma Science
Facility, a national user facility supported by the Department
of Energy and the National Science Foundation. The authors
would like to thank M. Nakamoto for her help in assembling
the probes for testing.
R EFERENCES
[1] P. Pribyl, W. Gekelman, M. Nakamoto, E. Lawrence, F. Chiang,
J. Stillman, and J. Judy, “Debye size microprobe for electric field measurements in laboratory plasmas,” Rev. Sci. Instrum., vol. 77, no. 073504,
pp. 1–8, Jul. 2006.
[2] M. VanZeeland, W. Gekelman, S. Vincena, and G. Dimonte, “Production
of Alfvén waves by a rapidly expanding dense plasma,” Phys. Rev. Lett.,
vol. 87, no. 10, pp. 1–4, 2001.
[3] B. V. Compernolle, W. Gekelman, P. Pribyl, and T. A. Carter, “Generation
of Alfvén waves by high power pulse at the electron plasma frequency,”
Geophys. Res. Lett., vol. 32, no. L08101, pp. 1–4, 2005.
[4] R. Peijak, V. Godyak, and B. Alexandrovich, “The electric field and current density in a low-pressure inductive discharge measured with different
B-dot probes,” J. Appl. Phys., vol. 81, no. 8, pp. 3416–3421, Apr. 1997.
[5] F. C. Chiang, “Voltage and electric-field probes for laboratory plasmas,”
M.S. thesis, Univ. California, Los Angeles, 2006.
[6] T. Hirota, T. Siraiwa, K. Hiramoto, and M. Ishihara, “Development of
micro-coil sensor for measuring magnetic field leakage,” Jpn. J. Appl.
Phys. 2, Lett., vol. 32, no. 7, pp. 3328–3329, 1993.
[7] A. Sprotte, K. Buckhorst, W. Brockherde, B. Hosticka, and D. Bosch,
“CMOS magnetic-field sensor system,” IEEE J. Solid-State Circuits,
vol. 29, no. 8, pp. 1002–1005, Aug. 1994.
CHIANG et al.: MICROFABRICATED ELECTRODES FOR MULTIAXIS SENSING IN THE LAPD
[8] B. Eyre, K. S. J. Pister, and W. Gekelman, “Multiaxis microcoil sensors
in standard CMOS,” Micromachined Devices Compon., vol. 2642, no. 1,
pp. 183–191, 1995.
[9] B. Eyre and K. Pister, “Micromechanical resonant magnetic sensor in
standard CMOS,” in Proc. Int. Conf. TRANSDUCERS, Chicago, IL,
Jun. 1997, vol. 1, pp. 405–408.
[10] W.-K. Chen, Ed., The Circuits and Filters Handbook, 2nd ed. Boca
Raton, FL: CRC Press, 2002.
[11] Clariant Ltd., Data Sheet: AZ5200 Series, Mutteuz, Switzerland,
Apr. 2000.
[12] B. Lefebvre, L. J. Chen, W. Gekelman, P. Kintner, J. Pickett, P. Pribyl,
S. Vincena, F. Chiang, and J. Judy, “Laboratory measurements of electrostatic solitary structures generated by beam injection,” Phys. Rev. Lett.,
vol. 105, no. 11, p. 115001, Sep. 2010.
[13] E. Everson, P. Pribyl, C. Constantin, A. Zylstra, D. Schaeffer, N. Kugland,
and C. Niemann, “Design, construction, and calibration of a three-axis,
high-frequency magnetic probe (B-dot probe) as a diagnostic for exploding plasmas,” Rev. Sci. Instrum., vol. 80, no. 11, p. 113505, Nov. 2009.
Franklin C. Chiang received the B.S, M.S., and Ph.D. degrees in electrical
engineering from the University of California, Los Angeles, in 2004, 2006, and
2009, respectively.
His research interests are on plasma systems used in the semiconductor
and MEMS industries, and he is currently working at Taiwan Semiconductor
Manufacturing Company (TSMC).
Patrick Pribyl received the Ph.D. degree from the
Massachusetts Institute of Technology, Cambridge,
in 1986, graduating from the Electrical Engineering
and Computer Science Department.
He is an expert on plasma diagnostics, plasma
sources, power systems engineering, and RF technology and currently works at the Basic Plasma Science
Facility, University of California, Los Angeles.
Walter Gekelman received the B.S. degree in
physics from Brooklyn College, Brooklyn, NY, in
1966, and the Ph.D. degree in experimental plasma
physics from the Stevens Institute of Technology,
Hoboken, NJ, in 1972.
At the University of California, Los Angeles, he
led the construction of the Large Plasma Device
(LAPD). This is widely perceived as the premier
machine for basic plasma studies and is presently
yielding important insight into basic processes observed in space by rockets and spacecraft. The
LAPD has an 18-m-long, highly magnetized, and quiescent plasma column
(http://plasma.physics.ucla.edu/bapsf). The LAPD is part of the nation’s first
user facility for basic plasma research, the BaPSF. The user facility is funded
by the National Science Foundation and the Department of Energy. He has over
150 publications in referred journals and has given numerous invited talks and
seminars around the world.
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Bertrand Lefebvre received the Ph.D. degree in plasma physics from the
University of Orleans, Orleans, France, in 2000.
He is currently with the Space Science Center, University of New Hampshire,
Durham. His primary research interests concern kinetic phenomena in space
plasmas.
Li-Jen Chen received the M.S. degree in physics
from the National Taiwan University, Taipei, Taiwan,
in 1993, and the M.S. and Ph.D. degrees in physics
from the University of Washington, Seattle, in 1997
and 2002, respectively.
In her doctoral research, she constructed analytical
solutions for electrostatic solitary waves in collisionless plasmas. She was at the University of Iowa
conducting research on magnetospheric substorms,
electrostatic solitary waves, and interaction of dispersive Alfven waves with auroral electrons. Since
2006, she has been with the Space Science Center and the Physics Department
at the University of New Hampshire, Durham, where she is currently a Research Faculty Member. Her research interests include magnetic reconnection,
particle acceleration, and electrostatic solitary structures in current layers. She
integrates theories, plasma simulations, and laboratory experiments with space
observations to address open questions in space physics.
Jack W. Judy (S’87–M’96–SM’02) received the B.S.E.E. degree (summa cum
laude) from the University of Minnesota, Minneapolis, in 1989, and the M.S.
and Ph.D. degrees from the University of California, Berkeley, in 1994 and
1996, respectively.
In his doctoral research, he developed a novel ferromagnetic microactuator
technology that is useful for a variety of applications, including optical, RF, and
biomedical MEMS. After graduation, he was with Silicon Light Machines, Inc.,
Sunnyvale, CA, an optical-MEMS startup company, from 1996 to 1997. Since
1997, he has been with the Faculty of the Electrical Engineering Department,
University of California, Los Angeles (UCLA), where he is currently an
Associate Professor. At UCLA, he is the Chair of the MEMS and Nanotechnology major field of the Electrical Engineering Department, the Director
of the Nanoelectronics Research Facility, and the Director of the UCLA
NeuroEngineering Training Program, which is an IGERT program supported
by the National Science Foundation and sponsored jointly by the Biomedical
Engineering Interdepartmental Program and the Brain Research Institute. His
present research interests include ferromagnetic MEMS magnetometers, magnetically reconfigurable frequency-selective surfaces, nanomagnetomechanical
devices, chemical sensors, and a variety of neuroengineering projects, such
as micromachined patch-clamp systems with integrated microfluidics, microprobes for Parkinson’s disease research, microactuator-imbedded ventricular
catheters for hydrocephalus, inexpensive and robust -D cortical microelectrode
arrays, electrode arrays for retinal prosthetics, simulating prosthetic vision,
wireless neural transceivers for basic neuroscience research, and neural control
systems for spinal cord injury, ocular motility, and deep brain stimulation.