[sic-en-2013-21] silicon carbide (sic) antennas for high-temperature and high-power applications

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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013 409

Silicon Carbide (SiC) Antennas for High-Temperatureand High-Power Applications

Tutku Karacolak, Member, IEEE, Rooban Venkatesh K. G. Thirumalai, Member, IEEE, J. Neil Merrett,Yaroslav Koshka, and Erdem Topsakal, Senior Member, IEEE

AbstractThe main objective of this letter is to evaluate semi-in-

sulating silicon carbide (SiC) material as a candidate for dielectricsubstrate for patch antennas, with a long-term potential for mono-lithic antenna integration on a SiC semiconductor chip, operationin extreme environments, and other applications. First, computer-

aideddesign of microstrip patch antennas operating at 10 GHz wasconducted. The antenna designs were implemented using semi-in-sulating SiC substrates and gold ground planes and patches. Agood agreement between the experimental results and simulationwas obtained at the band of operation. Return loss and radiationpatterns were investigated. As future work, a possibility of uti-

lizing highly conductive (heavily doped) SiC epitaxial layers as the

ground planes and radiating patches were investigated using com-puter simulations.

Index TermsHigh-temperature and high-power applications,

microstrip patch antenna, silicon carbide (SiC).

I. INTRODUCTION

S ILICON carbide (SiC) materials have attracted greatinterest recently for high-temperature and high-powerelectronics as well as for biomedical, gas-sensing, and other

applications in harsh environments due to its wide energy

band-gap, high thermal conductivity, excellent physical sta-bility, and maximum operating temperature [1], [2]. High

breakdown electric field of SiC offers important advantages

for high-voltage and high-power diodes and transistors. High

saturated drift velocity enables high-frequency operation in

the RF and microwave range. In more traditional semicon-

ductor technologies (e.g., Si and GaAs), RF and monolithic

microwave (MM) integrated circuits are well known for their

applications in transmit/receive modules. In recent years, there

has been a great deal of research undertaken to extend the

level of integration to include the antenna subsystem. The need

Manuscript received December 28, 2012; accepted February 06, 2013. Dateof publication March 06, 2013; date of current version April 04, 2013.

T. Karacolak was with the Department of Electrical and Computer Engi-neering, Mississippi State University, Starkville, MS 39759 USA. He is nowwith the School of Engineering and Computer Science, Washington State Uni-versity, Vancouver, WA 98696 USA.

R. V. K. G. Thirumalai and Y. Koshka are with the Department of Electricaland Computer Engineering, Mississippi State University, Starkville, MS 39759USA.

J. N. Merrett is with the Air Force ResearchLaboratory, Wright-Patterson AirForce Base, OH 45433 USA.

E. Topsakal is with the Department of Electrical and Computer Engi-neering, Mississippi State University, University, MS 39762 USA (e-mail:[email protected]).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LAWP.2013.2251599

TABLE IELECTRICAL PROPERTIES OF SEMI-INSULATING SiC SUBSTRATE

in antenna integration is dictated by applications in mobile

communications, automotive sensors, and nongeostationarysatellites.

Monolithic integration of a SiC power transistor and control

circuitry in a single module (so-called Smart Power integra-

tion) offers huge improvements in efficiency and reliability of

the power-electronic systems, which justifies aggressive efforts

in this direction [3]. However, virtually no efforts have been

demonstrated in developing SiC RF ICs. Moreover, no efforts

to utilize SiC as the dielectric substrate (which is the required

first step toward including the antenna into the monolithically

integrated system) have been reported. Printed microstrip patch

antennas are low-profile and low-weight and can easily be in-

tegrated on the same SiC chip with SiC circuitry. These an-

tennas can also be miniaturized for wideband or multiband op-erations. The dielectric properties of semi-insulating SiC sub-

strates allow for antenna miniaturization and higher antenna ef-

ficiency due to its high dielectric constant , low loss

tangent , and low conductivity S/m

values. Table I shows electrical properties of semi-insulating

SiC.

In this letter, our aim is to explore the microstrip patch an-

tenna fabrication technology utilizing semi-insulating SiC ma-

terials, which would be the first step for antenna integration on

a SiC wafer.

II. ANTENNA DESIGN

Two sample microstrip patch antennas were designed for

operation at 10 GHz to test the proposed idea. We have chosen

10 GHz for the operation frequency to design a small-size

low-profile antenna that would be easier to integrate on a SiC

chip. The antenna dimensions were determined using particle

swarm optimization with an objective function satisfying

return-loss value less than 10 dB. The geometry and the

dimensions of the optimized antennas are given in Fig. 1 and

Table II. Simulations were conducted using ANSYS HFSS

software combined with MATLAB. During the simulations,

the dielectric constant and conductivity for the SiC substrate

were taken as 10 and 10 S/m, respectively. These values are

1536-1225/$31.00 2013 IEEE


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410 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Fig. 1. Geometry of the sample patch antennas utilizing SiC as the dielectricsubstrate.

TABLE IIDIMENSIONS OF THE SAMPLE PATCH ANTENNAS

measured using Agilents dielectric probe kit in our laboratory.

The thickness of the substrate was taken equal to the typical

thickness of commercial 4H-SiC wafers of 0.3673 mm. Notethat the patch and the ground plane were assigned as perfect

electric conductors (PECs) for these initial simulations.

The simulated return loss of the antennas is shown in Fig. 2.

As seen, both antennas resonate around 10 GHz. The band is

narrow due to the thickness of the available substrate [4].

III. FABRICATION PROCESS

Vanadium-doped semi-insulating SiC substrates were used as

the dielectric substrate for the antenna. Blanket metal deposition

was conducted on the front and back of the SiC substrate. The

metal stack consisted of 1 k Ti for adhesion, 1 k Pt for ametal diffusion barrier and an oxygen barrier, and 3 m Au for

a low-resistance wire-bondable layer, as shown in Fig. 3. The

fabrication steps for patterning front and backside metal layers

are as follows.

1) The metal layer was e-beam deposited over the front and

backside of the sample.

2) The antenna pattern was patterned with photoresist (PR)

on the front side, and the entire backside was coated with

PR.

3) The gold layer was etched with 3 HCl: 1 HNO : 2 H O at

30 .

4) The platinum layer was etched with an Ar plasma.

5) The Ti layer was etched with 20 H O: 1 HF: 1 H O at

room temperature. The photoresist was stripped in acetone.

Fig. 2. Simulated return loss (dB) with respect to frequency (GHz) of the de-signed antennas. (a) Antenna 1 (b) Antenna 2.

Fig. 3. Schematic representation of the cross section of the SiC antenna. Thedrawing shows (not in scale) the semi-insulating SiC substrate used as the di-electric medium and metal layers on the top and the back.

IV. RESULTS AND DISCUSSION

The fabricated SiC antennas are shown in Fig. 4. Simula-

tions including the entire SiC substrate and all three antennas

on the same substrate confirmed that the coupling is negligible,

which is why the three antennas in Fig. 4 have not been sep-

arated and have been measured while remaining on the same

substrate. Return-loss measurements were performed using an


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KARACOLAKet al.: SiC ANTENNAS FOR HIGH-TEMPERATURE AND HIGH-POWER APPLICATIONS 411

Fig. 4. Fabricated SiC microstrip patch antennas.

Fig. 5. Comparison of measured and simulated return loss (dB) of the first an-tenna with respect to frequency (GHz).

E8362B PNA network analyzer. The measured and simulated

return-loss responses of both antennas are shown in Figs. 5 and

6, respectively. Since the antennas consist of gold ground planes

and patches, simulations were carried out by assigning gold

S/m instead of perfectly conducting materialfor a more realistic and reasonable comparison. A good agree-

ment is seen for both antennas at the band of operation (10GHz),

confirming that sufficient antenna functionality can be achieved

when using SiC as the dielectric substrate. The differences be-

tween simulations and the experiment could be explained by

the fabrication imperfections. Specifically, the imperfect con-

nection of the SMA connector with the feeding line should af-

fect the measured value. Even small fabrication imperfections

of this kind at 10 GHz will result in discrepancy between simu-

lations and measurements. In addition, due to losses in SiC, the

entire curve shifts down, manifesting itself as an improved

bandwidth.

Fig. 7(a) and (b) shows the simulated co- and cross-polarized

radiated fields of the second antenna for and

Fig. 6. Comparison of measured and simulated return loss (dB) of the secondantenna with respect to frequency (GHz).

Fig. 7. Co- and cross-polarized radiated fields of the second antenna at 10 GHz

for (a) and (b) .

cuts at 10 GHz. It is apparent from the plots that the antenna

shows omnidirectional radiation characteristics for both cuts

in the band of interest. Radiation efficiency of the antenna is

also shown in Fig. 8. As seen, radiation efficiency is over 96%

though the entire band. These show that SiC-based patch an-

tennas radiate as well as the antennas made out of conventional

RF materials such as FR4 and Rogers. In addition, the antenna

has a gain around 2 dBi at 10 GHz, which is consistent with an-

tennas in similar size from the literature [5][7].

Our next objective is to use computer simulations to evaluatea possibility of an all-SiC antenna. To achieve this, the gold ra-

diating patch and ground planes are replaced with SiC epitaxial

layers having very high electrical conductivity [8].

To show the effect of the SiC patches on the antenna per-

formance, we have also performed return-loss simulations for

the same SiC antenna designs. The conductivity value for SiC

patches is taken as 10 S/m on the basis of the best exper-

imental results obtained for the heavily doped SiC epitaxial

layers grown by low-temperature epitaxial growth methods [9].

The simulated values of the return loss of the full SiC antenna

are compared to the ones with gold-patch and PEC-patch SiC

antennas, and the results are shown in Figs. 9 and 10, respec-

tively. As expected, the replacement of metal patches with SiC

significantly affects the antenna performance, and in both cases,


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412 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Fig. 8. Radiation efficiency of the second antenna.

Fig. 9. Comparison of simulated return loss of SiC, gold, and PEC patch casesfor the first antenna.

Fig. 10. Comparison of simulatedreturnlossof SiC, gold, andPEC patch casesfor the second antenna.

the antenna operating frequency shifts to lower frequencies with

a very signifi

cant improvement in the bandwidth.

As a future work, we are aiming to fabricate a full SiC

microstrip patch antenna. The substrate material will be made

of semi-insulating SiC, and the patches and ground plane will

be made of very highly dopped SiC. The simulation results

show that such antennas can perform as good as their metal

counterparts.

V. CONCLUSION

In this letter, the semi-insulating SiC material was success-

fully used as the dielectric substrate in the fabrication of mi-

crostrip patch antennas potentially suitable for monolithic in-

tegration and operation in harsh environments. As for the radi-

ating element and ground plane, gold deposition was used on the

front and backside of the SiC substrate. Two sample microstrip

patch antennas are designed to operate at 10 GHz, and an agree-

ment is observed at the band of operation for the measured and

simulated reflection coefficients of the antennas. In addition, it

is shown that antenna bandwidth significantly increases with the

replacement of the gold layers with SiC material. The next stepof this study will be to design, fabricate, and measure patch an-

tennas that would consist of both SiC substrate materials and

highly conductive SiC patch layers. Such a design will allow

better interface between the substrate and the metal layers be-

cause of the use of same SiC material. The design and realiza-

tion of antennas and electronics made of SiC materials will be

the next step in realizing wireless systems that can withstand

very high temperatures and that can be used in harsh environ-

ments such as in deep space.

ACKNOWLEDGMENT

The authors acknowledge the help of SemiSouth Laborato-

ries, Inc., with metal deposition on the SiC substrate.

REFERENCES

[1] R. Kirschman, High Temperature Electronics. New York, NY, USA:WileyIEEE Press, 1998.

[2] V. B. Shields, Applications of silicon carbide for high temperatureelectronics and sensors, NASA Jet Propulsion Laboratory, TechBriefs, 0145-319X, Mar. 1996.

[3] J. A. Cooper, Jr., Silicon carbide electronic devices and integratedcircuits for extreme environments, inIEEE Aerosp. Conf. Proc., 2004,vol. 4, pp. 25072514.

[4] D. M. Pozar, Microstrip antennas, Proc. IEEE, vol. 80, no. 1, pp.

7991, Jan. 1992.[5] C. Yoon, W. Lee, W. Kim, H. Lee, and H. Park, Compact band-

notched ultra-wideband printed antenna using inverted l-slit, Microw.Opt. Technol. Lett., vol. 54, no. 1, pp. 143144, Jan. 2012.

[6] K. Kiminami, A. Hirata, and T. Shiozawa, Double-sided printedbow-tie antenna for UWB communications, IEEE Antennas WirelessPropag. Lett., vol. 3, pp. 152153, 2004.

[7] C.Yoon, W.Kim,S. Kang, H.Lee,and H.Park, Printedmonopole an-tenna on a thin substratefor UWBapplications,Microw. Opt. Technol.

Lett., vol. 53, no. 6, pp. 12621264, Jun. 2011.[8] B. Krishnan, S. P. Kotamraju, G. Melnychuk, H. Das, J. N. Merrett,

and Y. Koshka, Heavily aluminum-doped epitaxial layers for ohmiccontact formation to p-type 4H-SiC Produced by low-temperature ho-moepitaxial growth, J. Electron. Mater., vol. 39, no. 1, pp. 3438,2010.

[9] Y. Koshka, Method for epitaxial growth of silicon carbide at reduced

temperatures, U.S. 7404858, Jul. 29, 2008.

[sic-en-2013-21] silicon carbide (sic) antennas for high-temperature and high-power applications

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