SI ACOUSTIC DELAYACCOUSTIC

Lingy*Department of E

Univers ABSTRACT We studied the effect of excess carrierand current on acoustic waves in silicon deand excess carriers reduce the acousticrespectively reducing the Young’s moduluthe deformation potential. Electrical currenhand, can increase the acoustic velocity their drift velocity. In addition to current, acan also be controlled through field effect aof depletion width in periodic p-n junctionthe delay line. INTRODUCTION It is well-known that the existing suwave (SAW) devices almost exclusively using piezoelectric materials. Very hacoustic waves in silicon play an importantransfer and as the device size shrinks belolength. Therefore, it is interesting to decurrent or other signals (electric field-effectof embedded p-n junction depletion regionto modify wave velocity and amplitude of ato improve heat conduction and programmable SAW devices. SAW devicevery sophisticated signal processing tamatched filtering with very low power. DESIGN AND EXPERIMENTS We designed and fabricated a silicon dperiodic electrostatic transducers as shown acoustic waves in these devices weelectrostatically using a periodic electrocoupled through a 1-20 nm air-gap to a sregion as shown in Fig. 1. The propagatindetected using a dc-biased identical elsituated 0.7 cm away from the first array aslab. The λ/2 sections in our devices were 1 Depending on the biasing conditionresonated at 301 MHz or at 904 MHz (the thThe 904 MHz oscillation is shown in Figports were impedance matched (at 904 periodic excitation/detection electrodes uinductors. When we passed a dc current (2 boron-doped polysilicon slab (poly2 in velocity increased by a small amount (0.1%the zero bias case and the signal magnitude by a small amount (~0.1%). Conversely, negative current, affected the wave ampopposite direction (Fig. 1.e). As can be seen

Y LINES AND THE EFFECT OF CUR WAVE ATTENUATION AND SPEED

yao Chen* and Massood Tabib-Azar*#

Electrical and Computer Eng., *# Biomedical Engsity of Utah, Salt Lake City, Utah, USA

rs, temperature elay lines. Heat c velocity by us and through nt, on the other

depending on acoustic waves and modulation ns embedded in

urface acoustic are fabricated

high-frequency nt role in heat ow 10 nm gate etermine if dc t or modulation ns) can be used acoustic waves

also enable es can perform asks such as

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978-1-4244-9634-1/11/$26.00 ©2011 IEEE 633 MEMS 2011, Cancun, MEXICO, January 23-27, 2011

RESULTS AND DISCUSSIONS To investigate the origin of the dc curreof the wave characteristics (velocity and atcoupled two 13 MHz (longitudinal) transducers as schematically shown in Fig. mm long highly doped p-type (100) siliconthe two faces of the silicon sample were thecurrent parallel or against the acoustic wave Fig. 2.b shows the oscilloscope traces ofpulse, the reflected wave pulses and the tranunder different biasing conditions. The cur15 V was around 0.1 mA/cm2. The deladifferent biasing conditions were studied the time corresponding to the peak rms transmitted acoustic waveforms as shown in Figure 3 shows the scatter in the valuepulse arrival times and attenuation/amplifiexperiments. The average change in arrivaV/cm was -0.1 μs and pulse height amplificaAt -115 V/cm, the average change in arriv0.05 μs while the attenuation was 18%. It inote that the increase in sound velocity iswith an amplification factor.

Fig. 2: a) Excitation and reflection/transmrecorded from two 13 MHz piezoelectricoupled to 1.3 mm p-type silicon slab. b) Tcurrent on the transmitted pulses througsample. The wave RMS values are superimtransmitted RF pulses.

ent dependence ttenuation), we

piezoelectric 2 inset to a 1.3

n. Electrodes at en used to pass e. f the excitation nsmitted pulses rrent density at ay time under by comparing values for the

n Fig. 2.b. s of change in

fication from 8 al time at +115 ation was 12%. val time was -s interesting to s accompanied

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mission pulses ic transducers

The effect of dc gh the silicon mposed on the

It is well known in separatethat dc current in the coupled sembe used to amplify the SAW Current flow through the semiconacoustic wave through thermal efpossible charge injection in Schmetal electrodes and the semicothrough coupling between theacoustic wave through the deform Passage of current can heat Young’s modulus that in turn redCharge injection effectively modeaverage carrier density also reducsilicon through the deformationWe studied the thermal and excseparately using photo-generatiobelow.

Fig. 3: a) Change in arrival timea function of electric field (point zero and point 3 is -115 V/cm). bacoustic pulses’ RMS voltage afield. The average change in arrwas -0.1 μs and pulse height amm-115 V/cm, the average change iμs while the attenuation was 18% In a different experiment showabove delay line in the feedbacresulting in an oscillation frequen

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e-medium SAW devices miconductor medium can propagating in LiNbO3. nductor can influence the ffects (heat~ VI), through hottky junctions between onductor waveguide, and e carrier drift and the mation potential.

the silicon lowering its duces the wave velocity. eled as an increase in the ces the sound velocity in

n potential (drag effect). cess charge effects in Si on process as discussed

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e of the acoustic pulse as 1 is 115 V/cm, point 2 is

b) Change in transmitted as a function of electric rival time at +115 V/cm mplification was 12%. At in arrival time was -0.05

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wn in Fig. 4, we used the ck of a 60 dB amplifier ncy . was then

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down-converted by mixing with a local oscillation to of around 100 kHz.

Fig. 4: Schematic of the experiment set-up. The condition for oscillations in feedback loop is that the acoustic wavelength (λ) must be related to the sample thickness (d) through /2 , where “ " is an integer. Velocity of acoustic wave in the semiconductor sample is therefore (1) with fixed. Hence, by monitoring the change of system frequency , the change of acoustic velocity inside the semiconductor could be obtained. We monitored fs to study the effect of illumination on the acoustic wave as discussed below. Figure 5 shows the change in fs using a Si sample (<100> P-doped, 50Ω · and 0.50mm thick). It can be seen that when light is turned on or off, experiences a sharp fall or rise of around 50Hz for a period of time of less than 0.5 seconds. Same results can be observed for an InP sample.

Fig. 5: versus time for light pulses shown in the bottom. Illumination of Si generates excess carriers in a very short time. Through deformation potential, these excess carriers will drag the acoustic waves and slow down the

acoustic wave. Using equation (1), and let be 2, velocity of acoustic wave inside Si therefore is 13.6 13.6 0.5013.6 6800 / , considering that . The velocity change due to light effect is then Δ S Δ0.50 50 0.025 / , and the relative velocity change is ∆ . 3.68 10 . It is noticed fs changes more gradually after the initial “fast” response. The “slow” part of the response is very likely due to the thermal effects. When light is turned on, the strong visible light source heats the sample lowering its Young’s modulus and reducing the acoustic wave. Thermal effects being much slower than the photo-generation effects result in the slow part of the response. CONCLUSION We designed and fabricated silicon delay lines and examined the effect of illumination and current on the travel time of acoustic pulses and oscillation frequency of delay line when inserted the feedback of an amplifier. Results so far indicate that it is possible to affect the wave velocity using optical illumination, heating and current. To enable better control of acoustic waves, however, a more optimized structure is needed. We are in the process of designing silicon acoustic delay lines with embedded periodic p-n junctions and field effect to fully optimize the effects that we have observed so far in controlling acoustic waves in Si. ACKNOWLEDGEMENT This work is supported by USTAR Program at Utah. This work was partially accomplished while the author was with the EECS department at Case Western Reserve University, Cleveland, OH. REFERENCES [1] J. Bardeen and W. Shockley, “Deformation Potentials

and Mobilities in Non-Polar Crystals”, Phys. Rev. 80, 72–80, 1950.

[2] M. V. Fischetti and S. E. Laux, “Band structure, deformation potentials, and carrier mobility in strained Si, Ge, and SiGe alloys”, J. Appl. Phys. 80 (4), 15 August 1996

[3] B. C. Daly, K. Kang, D. G. Cahill, “Attenuation of Picosecond Ultrasonic Pulses in a thin Silicon Wafer”, 1st International Symposium on Laser Ultrasonics: Science, Technology and Applications July 16-18 2008, Montreal, Canada

Light

Fast response

Slow response

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