Micro-Plasma Field-Effect Transistors

Micro-Plasma Field-Effect Transistors
Mingming Caia, Faisal K. Chowdhurya and M. Tabib-Azara,b
a
Department of Electrical and Computer Engineering, bBioengineering
University of Utah, Salt Lake City, UT, USA
[email protected]
Abstract— We designed, fabricated and tested new microplasma FET (MOPFET) devices that operate inside RF helium
plasma that generated at atmospheric pressure. Unlike normal
FETs, micro-plasma FETs uses electrons and ions as carriers. It
has unique advantages over normal FETs in extreme conditions
at high temperature and ionizing radiation in space and in a
nuclear event. It also has potential applications in combustion
engine sensors and diagnostic circuits. MOPFET can potentially
operate with very few ions and have the additional potential of
producing nano-scale switches and amplifiers. The plasma for
our MOPFET was separately generated and sustained using an
RF plasma source. Thus, for the first time we achieved small
voltage (5-10V) plasma switches and amplifiers. We have
developed concentrated and distributed plasma sources suitable
for different sizes of integrated MOPFET circuits.
I.
INTRODUCTION
There is a need to develop devices that are inherently
immune to ionizing radiations and can operate at very high
temperatures for critical tasks in nuclear power stations, outer
space explorations, engines etc [1-2]. Semiconductor devices
are fundamentally unable to fill this role due to thermal and
radiation-induced generation of electron-hole pairs in their
channel. Devices that utilize carriers produced by ionization of
gases have the potential of operating at very high temperatures
in excess of 1000 degree Centigrade and in very high flux
ionizing radiation due to small interaction cross-section
between gas molecules and radiation.
Microplasma devices (MPD) [3-10] have found interesting
uses in a multitude of applications ranging from displays [6],
medical [7], tip-based nano-manufacturing [8-10], materials
processing [6, 8-10] and others. The mode of operation that
MPDs use is inherently immune to ionizing radiation - despite
their very large current carrying density. This is seen,
interestingly, in the increased efficiency of operation as the
external ionizing radiation is amplified. Nevertheless, this
phenomenon has not received extensive research effort for the
use in switches and amplifiers.
In this paper we discuss the use of atmospheric MPDs as
plasma sources for tip-based nanofabrication, switches and
amplifiers for computation and control electronics. Our choice
of atmospheric plasma devices ensures large current densities
and stability that is lacking in low-pressure plasma. We
This work was supported by DARPA grants N66001-08-1-2042
and N000141110932
978-1-4577-1767-3/12/$26.00 ©2012 IEEE
present results using MPDs with a distributed plasma source,
micro fabricated 2-dimensional (coplanar) MOPFET utilizing
crossed electrodes making direct contact to the plasma, a
second set of MPDs with capacitively coupled electrodes to
generate RF plasmas and, additionally, the fabrication and
characterization of 3-dimensional MOPFETs.
II.
THEORETICAL CONSIDERATIONS
The operation principle of MOPFET is very similar to
MOSFETs where the current between two electrodes (drain
and source) is controlled by a gate voltage through the field
effect. The charge carriers in the case of MOPFET are
components of ionized gases such as electrons and ions. The
ionization process can be accomplished by using large fields
between the drain and source electrodes or by using a separate
device nearby. Both approaches are possible and have been
shown to design three terminal switches. Here we use the
second approach where a separate device is used near the
MOPFET to ionize gases to provide charge carriers as
schematically shown in Fig. 1. One of the benefits of this
approach is that the MOPFET electrodes operate at low fields
and voltages and potentially can last longer than the first
approach where the ionization is maintained by these
electrodes.
Fig. 1: Separate medium metal-oxide-plasma field-effect transistor
(MOPFET). The device on the top is used to generate plasma and provide
charge carriers for the MOPFET.
In the separate medium device shown above, the
MOPFET can be viewed as a Langmuir probe that is
extensively used and analyzed in the plasma community. The
main difference between the MOPFET and the Langmuir
probes reported in the past is the size of the device. Langmuir
probes are usually mannually assembled with large parts and
electrodes in the milimeter range while the MOPFET is a
micro-scale device. Another main difference is the presence
of the gate electrode in the MOPFET that is absent in
traditional Langmuir probes. Nevertheless, we can start with
the well accepted Langmuir probe analysis to model the
operation of the MOPFET. Fig. 2 shows a typical current
versus voltage characteristics of a Langmuir probe. The three
different operation regimes are clearly identified as depletion,
accumulation and saturation regions while these terminologies
refer to electrons. The plasma potential is also shown.
made as this would result in roughening the glass substrate
and would be detrimental to all the processes that follow.
RCA Clean Glass substrate
Deposit 50nm HfO2 etch stop layer
Deposit 1um Sputtered SiO2
Sputter Cr Drain/Source metal and
pattern
Dry etch SiO2 to form “ledges” for
D/S metal
Deposit and pattern 50nm ALD
Al2O3 insulation layer
Fig. 2: Current versus voltage characteristics of the Langmuir probe is shown
in black. The additional gate electrode is used to apply a gate voltage and shift
the Langmuir I-V horizontally.
The presence of a gate voltage applied through the gate
electrode enables the above I-V curve to be shifted
horizontally and its slope in the accumulation region to be
modified. It can be shown that for small gate voltages, the
drain-source current is given by:
,
I
1 1
where V0 is the plasma potential, I0 is the effective depletion
(ion) current, kTe/e is the electron’s thermal voltage and α and
β are two parameters that depend on gate capacitance and
electrode geometry and effective area (nano-textured, glassy,
planar gate versus 3-D gate, etc.) The above equation clearly
shows that the gate voltage controls the drain current and its
effectiveness depends on the MOPFET’s structure and gate
capacitance.
III.
FABRICATION OF 3-DIMENSIONAL MOPFETS
We fabricated 2-dimensional as well as 3-dimensional
devices with different drain/source and gate dimensions. Fig. 3
shows the fabrication steps we used for a 3-dimensional gate
MOPFET. The substrate used for this process was a 4" x 4",
2.5mm thick glass that was thoroughly cleaned in RCA 1 and
RCA 2. It was ensured that no exposure to BOE or HF was
Deposit sacrificial sputtered
polysilicon. (1um+1um)
Pattern sacrificial polysilicon using
Cr or Al hard mask
Deposit gate insulator 20nm ALD
Al2O3 followed by Gate metal Cr
(100nm) followed by appropriate
patterning.
Sacrificial PolySilicon etch in XeF2
Fig. 3: Fabrication flow for 3D Microplasma Devices
The next step was to deposit 50nm ALD HfO2 at 200
degree Centigrade which was used as an etch stop layer for
processing SiO2 in the RIE. Silicon Nitride was avoided here
as the etch rates are comparable to SiO2. Following this, 1µm
SiO2 was deposited as the layer used to define the "ledge" on
the actual device. This layer of SiO2 could be either sputtered
or deposited via PECVD. In this case, PECVD was employed
using Silane at a chuck temperature of 300 degree Centigrade
resulting in a deposition rate of about 61nm/min. The next
step was to deposit the first metallization (Drain/Source
electrodes) layer. This was done by sputtering 100nm of Cr,
patterned using photolithography and etched in Cr-14 etchant.
Post etching, the patterned PR and Cr were used as a mask to
dry etch the underlying SiO2 to form the ledges. A CF4/O2
recipe (35/3.5 sccm) at 200W and 75mTorr was used for the
RIE step. The ALD HfO2 layer deposited underneath the
PECVD SiO2 acted as a very reliable etch stop. Following this
step, the photoresist was stripped off and a layer of ALD
AL2O3 (50nm thickness) was deposited at 200 degree
Centigrade. This was patterned using BOE to form the
insulating regions above the Source/Drain electrodes. The next
step was to deposit 2µm thick polysilicon via sputtering to
form the sacrificial layer. The first 1um was required to fill-in
the depth of the ledge up to the drain/source electrodes and the
final 1µm was used to define the gap between the drain/source
electrode and gate electrode (yet to be discussed).
Photolithography followed by RIE in a SF6/O2 plasma
(26sccm/3.5sccm at 100W and 100mTorr) was then used to
pattern the polysilicon into stubs just above the active device
region. This was followed by sputtering the final metal (gate
metal), Cr 100nm, and patterned using Cr-14 etchant at room
temperature. Before the final sacrificial release the a 2um
layer of photoresist was spin-coated and the glass substrate
diced into chip size pieces on a dicing saw. Following PR
strip, each chip was sacrificially etched in XeF2 requiring 100
cycles. SEM images of the final devices are provided in Fig. 4.
(a)
(c)
G
D
S
G
D
G
Fig. 6: a) The schematic and b) image of the distributed plasma source
and MOPFET device and integrated circuits.
G
D
S
Fig. 4: SEM Image of 3-Dimensional Microplasma devices. a) View of
entire device with Source, Drain and Gate as indicated, b) close up of
active device area, c) device with 3µm D-S tip and d) device with 9µm
D-S tip.
IV.
(a)
(b)
(d)
S
Fig. 5: Images of MOPFET showing its different regions.
S
D
(b)
MHz. The MOPFET (Fig. 5) uses a gate to modify the
effective areas of the drain/source and/or the Debye sheath to
modulate the drain-source current (Ids)
ELECTRICAL CHARACTERIZATION
The coplanar MOPFET devices were also fabricated on
glass substrates (Fig. 5) in three layers using sputtered
tungsten as the metallization. The gate electrode was coated
with ALD Al2O3 while the source and drain electrodes were
left exposed to plasma for direct contact.
Fig. 6 shows the schematic and optical image of the
packaged MOPFETs using a distributed plasma source. The
plasma source was excited with 2W RF signal at 500 – 900
Figure 7 shows typical Ids-Vds curves of MOPFETs. We
also estimated plasma carrier mobility by measuring time-offlight with the Shockley-Haynes experiment (Fig. 8) where the
pulse travels from Electrode 1 to Electrode 3 and its delay
time is determined by the carrier mobility.
We used two different devices for the Hall measurement
shown in Fig. 9. The measured plasma ion mobility for the
first device ranged from 0.1 to 10 cm2/V·s as shown in Fig. 10,
depending on the plasma density that varied from 1011 up to
1014 cm-3. The second Hall device was smaller and had a 10
μm gap between the vertical electrodes, while the gap between
the horizontal electrodes was 100µm. The calculated plasma
ion mobility from the I-V data ranged from 0.1 to 5 cm2/V·s,
depending on the plasma density that varied from 1010 up to
1013 cm-3.
3-Dimensional MOPFET transistor characteristics were
tested with 650 MHz plasma torch placed above them. Fig. 11
shows typical Ids-Vds curves, which indicate consistent gate
control of current passing through drain and source (Ids).
Transconductance (
10
) for Vds=10 V is found to be 2.9
for this device.
4.00E-07
VS
2.00E-07
Ve
V
G
=
-50-2.00E-07
0
0=
5
V
-4.00E-07
V
0.00E+00
G
50
Fig. 11: Ids-Vds Results with 4th generation MPD device, with 6 um gate
width, 5 pairs of 9 um x 9 um drain/source tips, and 0.5 um gate to
drain/source gap.
-6.00E-07
Fig. 7: Ids-Vds Results with Distributed Source.
Fig. 12: (a) MPD switching characteristics inside a reactor: switch-on voltage
vs. radiation time, (b) MPD switching characteristics inside a heater( switch on
voltage vs. environmental temperature).
Fig. 8: Ids Shockley-Haynes experimental set-up and result.
Fig. 9: Hall structures to measure carrier mobility.
V.
CONCLUSIONS
The design, fabrication and electrical characterization of 3dimensional Microplasma devices are presented here.
Discussion on electrical performance of coplanar microplasma
devices are also included. In addition, quantitative data
demonstrating gate-controlled source-drain current based on
gaseous ions and electrons is provided. Furthermore, harsh
environment performance of these devices (in high ionizing
radiation and high temperature) was tested, and the results are
given here.
VI.
ACKNOWLEDGEMENT
Assistance of Mr. L. Chen and W. Yuan is appreciated.
VII. REFERNCES
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Fig. 10: Hall voltage as a function of magnetic field.
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Fig. 12(a). The switch-on voltage varied by less than 5%
throughout the 120 minutes of exposure. In addition, their
high temperature (100 degree Centigrade) switch-on
characteristics decreased by less than 1% (Fig. 12(b)), and 4%
when temperature was increased to 200 degree Centigrade.
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