OPERATION PRINCIPLE

Yuying Zhang, PradeeDepartment of Electrical and Co

ABSTRACT We present design and theory micro-plasma field effect transistors operate with atmospheric plasma. Both were generated internally inside the MOseparate device that generates the plasmaThe voltage applied to the MOPFET ionization rates of the gases in the chadrain and source. When the drain-sourcethan a few micrometers, the standardictates the breakdown voltage while fmodified Paschen behavior based on field emission predicts a breakdown linearly with the gap distance becoming sgaps. MOPFETs terminal characteristihybrid pi model parameters are described KEYWORDS

Electric field Effect in plasmamodified Paschen curve, micro-plasmmetal-oxide field-effect plasma transisto

INTRODUCTION

Micro-plasmas are encountered iranging from displays to electrostatic btheir larger-scale version in neon lamps. Tin medical sanitizing devices, inprocessing, and other industrial applicutilized micro-plasmas generated in thforce microscope (AFM) probes nano-manufacturing [1] where nano-scaas quantum dots of different materials etched with AFM precision.

One of the important characteristicsthey exhibit very small dc resistance thain power applications where large curover a very short period of time such aand switches. Our main objective in thiscontrol the plasma resistance using elsimilar to MOSFET. A simple way oelectric field effect in plasmas is thatpositive or negative charges in the insularesults in “repulsion” or “attraction” ochannel region contacting drain-soHowever, electron density directly afferates and a more accurate model than the is needed. Moreover, the mechaniinteraction with gas molecules leadinchanges when the electrode spacing becoin large gap devices field emission wodevices relatively larger mean free pathreduces the ionization efficiency. In smhowever, the ion-enhanced field emisbreakdown field that nearly linearly scdistance leading to the modified Paschen

ES OF MICRO-PLASMA FIELD EFFECT

ep Pai, Faisal K. Chowdhury and Massood Tomputer Engineering, University of Utah, Salt

of 3-dimensional (MOPFETs) that rf and dc plasmas

OPFET or using a a for the MOPFET. gate modifies the

annel between the e distance is larger rd Paschen curve for smaller gaps a the ion-enhanced field that scales

smaller for smaller ics along with its d.

as, micro-plasmas, ma characteristics, ors

in many devices body discharge to They are also used n semiconductor cations. We have

he apex of atomic for tip-based

ale materials such are deposited and

s of plasmas is that at can be exploited rrent is conducted as in flashes, fuses s work has been to lectric field effect of looking at the t the presence of ated gate electrode of charges in the ource electrodes. ects the ionization simple field effect ism of electron ng to ionization, omes small. While orks, in small gap h of gas molecules mall gap devices, ssion results in a cales with the gap n curve [2].

In addition to devices thatdensities, MOPFETs are inhionizing radiation and high operate in extremely harsh envi

In this paper, we present3-dimensional MOPFETs thathelium plasma. Atmospheric pcurrent densities and are morelow-pressure plasmas and pcost-effective since they do packaging.

In the following sectiofabrication and testing of Mterminal characteristics. Both simulated transistor characteris

DESIGN

The schematic diagram ofshown in Figure 1. The devicsubstrates including silicon, gla

The device structure conelectrodes that are separated bydesigned to overlap the drain cover the gap region. The stangate electrode and the gap ismicrometers. Figure 2 shows devices. Polysilicon was used athe stand-off distance and wTungsten-titanium alloy that halow sputtering rate was chosendrain and gate electrodes. Gateto 12µm were used. The drapatterned to form tips that defintips provide parallel conducteffective channel width and prover-hanging gate electrode substrate. Tip dimensions rangito 9µm x 9µm were used. Dewere incorporated into the maMOPFETs is described elsewhe

Figure 1: Schematic diagram ofand source (right) regions arregion “hugged” by the gate elis 3-D and stands at 0.5-2µmplane.

T TRANSISTOR

Tabib-Azar t Lake City, UT, USA

t can conduct large current herently immune to high temperatures [3] and can ironment. t design and principles of t operate with atmospheric plasmas can support larger e stability [4] compared to potentially can be more not require high vacuum

ons we describe design, MOPFETs along with their

analytical and COMSOL stics are also discussed.

f a typical 3D MOPFET is ce is fabricated on various ass or quartz substrates. nsists of drain and source y a gap. The gate electrode is

and source electrodes and nd-off distance between the s varied between 0.5 to 2

SEMs of microfabricated s a sacrificial layer to define

was removed using XeF2. as a high melting point and n as the material for source e widths ranging from 3µm, ain-source electrodes were ne the gap regions. Multiple tion paths to increase the rovide landing spots for the

to be supported by the ng in size from 3µm x 3µm, evices with 1,2,5 & 10 tips ask set. The fabrication of ere [4].

of MOPFET. The drain (left) re separated by a channel lectrode. The gate electrode m above the drain-source

M3P.143

978-1-4673-5983-2/13/$31.00 ©2013 IEEE 578 Transducers 2013, Barcelona, SPAIN, 16-20 June 2013

Figure 2: SEM Images of 3-Dimensional MOPFET. a) View of entire device with Source, Drain and Gate as indicated, b device with 3µm D-S tip. OPERATIONAL PRINCIPLES

The setup used for electrical testing is schematically shown in Figure 3 [4]. Helium plasma was generated by a separate torch placed over the device. In this configuration, the drain-source electrodes last much longer compared to internal generation of plasmas. In the latter case, the drain-source electrodes are directly involved in generating the plasma and become aggressively sputtered and deteriorate much faster.

In the separate medium structure shown in Figure 3 the plasma ions and electrons diffuse to the MOPFETs active region and increase the drain-source conductance with typical I-V characteristics shown in Figure 4.

Figure 3: Separate Medium structure where a plasma torch is used to generate charge carriers for the MOPFET.

In the separate-medium configuration, the MOPFET’s

behavior is very similar to the well-known Langmuir probe as can be clearly seen in Figure 4. Langmuir probes consist of two electrodes that when immersed in the plasma can be used to measure plasma conductance. There are three regimes in the Langmuir I-V characteristics: depletion, accumulation, and saturation as referred to electron density. The slope in the accumulation region is steep, indicating that a small change in the voltage across the device ( ∆ ) results in a relatively large change in its current ( ∆ ). This is in contrast to the depletion and saturation regimes where the slope is much smaller.

The equation for terminal characteristics of a MOPFET operating in the accumulation region is given by: 1.6 1 1 , (1)

where , Ii is the current in the depletion

region, is the mass of ions, is the mass of electrons, K is Boltzmann constant, is the electron temperature,

the drain-source voltage, q is the magnitude of the electrical charge of the electron, is the intrinsic

plasma potential (see Figure 4), is the gate voltage, and α and are fitting parameters. This equation is modified to take into account the gate field effect through Vg. For helium plasma m=137.

Figure 4: I-V 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.

When a positive MOPFET gate voltage is applied with respect to the source, the ions are repelled and the electrons are attracted to the gate region. Electrons have a large thermal energy and our simulations using the plasma module of the COMSOL have shown that gate dielectric is quite leaky for these energetic electrons. Thus, the gate electrode along with the drain and source electrodes constitute a voltage divider in this case and shift the Langmuir characteristics horizontally (Figure 4).

When a negative gate voltage is applied, the ions are attracted to the gate region while the electrons are repelled. Ions cannot flow into the gate electrode and their accumulation over the gate region results in the formation of a pronounced double layer. Since the plasma is neutral, the increased density of the ions near the gate region increases concentration of electrons in the channel that in turn increases the drain-source current. In this case, the gate field effect causes the slope of the - curves to change (Figure 4) setting it apart from a simple voltage divider effect.

ELECTRICAL CHARACTERISTICS OF MOPFET

Figure 5 shows the - characteristic of a 5-pair drain-source device with 3µm gate width, 9µmx9um drain/source area per pair and 1 μm gate-drain/source stand-off distance. The first curve at the left with the rapid rise of was obtained when the gate voltage was negative. When the gate voltage was positive there is a large horizontal shift indicating voltage divider effect discussed above.

Drain Source

Gate

RF Signal

Helium

MOPFET

Tuning Coil

Copper Tape

Helium Plasma

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Figure 5: Typical - characteristic of MOPFET in the separate medium configuration.

We calculated the in the accumulation region using the equation 1 and compared it with the typical experimental values. We used 20V, 97 10 K, 5.68 10 A, α=0.021V-1, β=0.005V-1 to obtain the curves shown in Figure 6. For 0V, the value of at the transition point from depletion to accumulation was around 0V. Therefore at 0V was chosen as I (the ion saturation current). When V was greater than 20V, the device entered the electron saturation region. Form Figure 6, it can be seen that the curves for 0V are similar. The absolute values for =30V are different, but the trends are almost the same. For 30V transition point from accumulation to saturation regions is located at around 10Vand the values form numerical calculation do not match the experimental values very well. For 30V, lower is require for device to enter into the electron saturation region, which is due to the formation of double layer under the gate that increases the electron density in the channel region, pushing the device towards the electron saturation region.

Figure 6: Experimental and calculated I-Vs. The solid lines are experimental data from Figure 5 and the broken lines are calculated using equation 1 and values discussed above.

DC PLASMA

The setup for DC plasma consisted of two Keithley 237 SMUs where one SMU controlled the drain-source

voltage and measured the drain current where the second SMU controlled the gate voltage. Figure 7 shows DC plasma generated internally in the drain-source gap region.

Figure 7: Image of DC plasma internally generated inside the MOPFET.

Figure 8 shows typical DC plasma I-V characteristics

of a 5 finger source-drain pairs with 10μm source drain width, 2μm source-drain gap distance (channel length), 1μm gate gap height, and 2μm gate width.

Double mode scan was used to sweep source-drain voltage, and hysteresis can be observed in the I-V curve. The mechanism of this phenomenon is similar to DC glow discharge under low pressure condition. For DC glow discharge, plasma is maintained by secondary electron emission near cathode (source) region, in which most voltage drop occurs. Before breakdown, the entire region between cathode (source) and anode (cathode) is involved in the process [5]. In other words, higher voltage is required to generate plasma, and the discharge can be maintained with less voltage after ionization happens. The gate voltage shifts the I-V curve horizontally. When Vg<0, ions are attracted to gate region and more electrons participates in the ionization process. When Vg>0, the electrons readily flow into the gate electrode and larger drain-source voltage is needed for plasma generation.

Figure 8: I-V characteristics of DC plasma.

COMSOL SIMULATION

In order to show the mechanism of electronic versus ionic control of the channel conductance of MOPFETs, we have carried out simulation using COMSOL Multiphysics. Figure 9 shows electron and ion densities simulated for

Source Gate Drain

Quartz substrate

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Vg=0 V and 50 V (applied on electrode on the right) in relatively large MOPFET-like structures with three electrodes. COMSOL’s Capacitively Coupled Plasma (CCP) module is used to emulate plasma capacitively between the drain and source region.

Figure 9: COMSOL simulation of charge carrier concentration for MOPFET-like structure.

In the above simulations, the pressure was set to 1 torr, low pressure is preferred in the simulation in order to clearly show the gate field effect. An RF signal (300V, 13.56MHz) was applied at the drain (upper electrode) while the source (lower electrode) was grounded.

Ion density near the gate for Vg=0V was about 9x1012/m3, for Vg=0V was about 1x1013/m3, however, the one for Vg=50 V was about 5x1010/m3

, which was three

orders of magnitude lower than Vg=0V case. The DC gate voltage changes the ion concentration near the gate region that in turn increases the electron density in the channel. When the gate voltage was positive, it reduced the electron density in the channel. HYBRID PI-MODEL

It appears that to realize very fast MOPFETs, we need to increase the ion density to enhance the channel conductance but at the same time modulate the electronic conduction directly without going through the ionic control. The hybrid pi-model of Figure 10 shows the relation between Vg and Ids by introducing a transconductance gm, where gm depends on the device parameters and the plasma characteristics. The transconductance gm can be calculated using equation 1 and is given by: 1.6 1 (3)

Figure 10: The hybrid-pi model for MOPFETs CONCLUSION

We discussed MOPFETs and their operations with external and self-generated plasmas. The electrical characteristics of MOPFETs with external plasma were analyzed in detail and an equation was provided to describe it. The electrical characteristic of internally generated DC plasma was also presented. For those two types of plasmas the gate field effect was clearly shown. We also used COMSOL plasma simulation to investigate the gate field effect in our MOPFETs.

ACKNOWLEDGEMENTS

The authors would like to acknowledge support from DARPA MPD grant.

REFERENCES

[1] Y. Wen, K. N. Chappanda and M. Tabib-Azar, "Fabrication Of Plasma Probe For Chemical Vapor Deposition," In Solid-State Sensors, Actuators And Microsystems Conference (Transducers), 2011 16th International, 2011, pp. 1622-1625. [2] D. B. Go and D. A. Pohlman, "A mathematical model of the modified Paschen's curve for breakdown in microscale gaps," Journal of Applied Physics, vol. 107, 2010, pp. 103303. [3] Y. Wen, F. K. Chowdhury and M. Tabib-Azar, "Microplasma Field Effect Transistors," In Micro Electro Mechanical Systems (Mems), 2012 IEEE 25th International Conference On, 2012, pp. 293-296. [4] C. Mingming, F. K. Chowdhury and M. Tabib-Azar, "Micro-Plasma Field-Effect Transistors," In Sensors, 2012 IEEE, 2012, pp. 1-4. [5] M.Lieberman And A. Lichtenberg, Principles Of Plasma Discharges And Materials Processing: John Wiley & Sons, Inc., 2005. CONTACT

*azar.m@utah.edu

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