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FABRICATION OF A PIEZORESISTIVE PRESSURE SENSOR

FOR ENHANCING SENSITIVITY USING SILICON NANOWIRE

J. H. Kim 1 , K. T. Park 1 , H. C. Kim 2 , and K. Chun 1 1SoEE, Seoul National University, Seoul, KOREA

2SoEE, University of Ulsan, Ulsan, KOREA

ABSTRACT

This paper presents a piezoresistive pressure sensor toenhance sensitivity using silicon nanowire. According to

published paper, silicon nanowire under 340 nm has good piezoresistive effect. Silicon nanowire of 140 x 200 nm 2 size has seven times more piezoresistive effect than bulk silicon. This paper proposes the piezoresistive pressuresensor using the high piezoresistive effect of the siliconnanowire. The nanowire is fabricated to be connected like a

bridge between the bossed silicon diaphragm and the edgeof the silicon substrate. The fabricated piezoresistive

pressure sensor has high sensitivity of 337.5 mV/VMPaand dynamic range of 150 kPa ~ 300 kPa. The pressuresensor size is less than 1mm 2 using diaphragm of 200 x 200m2.

KEYWORDS

Silicon nanowire, Piezoresistive, Pressure sensor

INTRODUCTION

There are many applications for the MEMS pressuresensor in automobile, military, biomedicine, and aerospaceareas [1]. The MEMS pressure sensors are categorized into

piezoresistive and capacitive type according tomeasurement way [2]. Piezoresistive type pressure sensors

perceive resistance change and capacitive pressure sensorssense capacitance variation under the applied pressure.Especially, piezoresistive pressure sensors have mainly

been studied and commercialized because of high yield andwide dynamic range. However, the piezoresistive pressuresensor suffers from the low sensitivity and large size ascompare with the capacitive pressure sensor. Many of the

piezoresistive pressure sensors studies are trying to improvetheir low sensitivity using various materials such as highdoped silicon and porous silicon [3]. In this paper, the

piezoresistive pressure sensor with silicon nanowire is proposed to enhance the sensitivity and decrease the sizedue to its high piezoresistive effect.

DESIGN

According to the published paper, silicon nanowireunder width or thickness of 340 nm has good piezoresistiveeffect [4]. Particularly, silicon nanowire of 140 x 200 nm 2 size has seven times more piezoresistive effect than bulk silicon. To enhance the sensitivity and decrease the chipsize, the proposed piezoresistive pressure sensor uses thesilicon nanowires of high piezoresistive effect.

Figure 1: Schematic of piezoresistive pressure sensor using silicon nanowires

Fig. 1 shows the schematic of the piezoresistive pressure senor using silicon nanowire. The four silicon

nanowires of high piezoresistive effect are respectivelyconnected like a bridge between the bossed silicondiaphragm and the edge of the silicon substrate. When

pressure applies on the diaphragm, these silicon nanowiresreceive stress to change resistance of the silicon nanowire.The changed resistance perceives using fixed current signal

passed through the nanowire and diaphragm. Themeasurement principle of the pressure sensor is shown inFig. 2

Figure 2: Measurement principle of Silicon nanowire pressure sensor

978-1-4244-4193-8/09/$25.00 2009 IEEE Transducers 2009, Denver, CO, USA, June 21-25, 20091936

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To get the high sensitivity, nanowires position and size of the proposed pressure sensor should be determined. Todecide structure, we used ANSYS simulation. According tosimulation result, the silicon nanowires are located toreceive the maximum stress when pressure applies on thediaphragm. After arrangement of nanowires, structures of the Fig. 1 are simulated. Fig. 3 shows maximum stress asfunction of piezoresistive pressure sensor designs such asdiaphragm size, membrane thickness and release height.

50x50 75x75 100x100125x125150x150175x175200x200

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0.5

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14 16 18 20 22 24 26 28 30 32

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u m s t r e s s

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Figure 3: Ansys simulation results (a) Max. stress vs. membrane size (b) Max. stress vs. membrane thickness (c) Max. stress vs.release height

However, to obtain high maximum stress should takeaccount of a yield issue. So, considering the stresssimulation, fabrication safety limit and published

piezoresistive effect of nanowire, the proposed piezoresistive pressure sensor was designed to have thenanowire size under 300 x 300 nm 2, the diaphragm size of 200 x 200 m2, the membrane thickness of 20 m, and therelease height of 12 m.

FABRICATION

The proposed piezoresistive pressure sensor wasfabricated using N-type (100) silicon wafer. Boron ions of 1019 Dose/cm 3 were implanted on the front of the silicon for the piezoresistance of the silicon nanowire. This is followed

by the silicon nitride (SiN) of 200 nm deposition using low pressure chemical vapor deposition (LPCVD). For the

silicon wire pattern and electrical isolation, SiN was patterned and the silicon substrate was etched by deepreactive ion etching (DRIE). To fabricate metal pad, after etching the SiN, 1.2 m thick Aluminum was depositedonto the silicon contact. TEOS (Tetra Ethyl Ortho silicate)of 200 nm deposited on the silicon sidewalls using TEOSdeposition and etch method. Silicon wires were releasedusing follow TMAH wet etch after DRIE. Backside of silicon substrate was etched by 363 K TMAH to getdiaphragm less than 20 m thickness. TEOS of front wasremoved using 49 % HF. Silicon wires were etched by 348K TMAH to make the silicon nanowires under 300 x 300nm2. Finally, SiN was removed using CF4 gas.

Figure 4: Illustration of the piezoresistive pressure sensor (a)Silicon nitride patterning & Silicon dry etch (b) Al metal deposition and patterning (c) TEOS deposition and etch

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Figure 7: Absolute relative change in resistance as a function of the pressure

As a result, sensor of the released silicon bridgestructure has 4.5 times more sensitivity than one of the bulk silicon. And pressure sensor using silicon nanowire has 8.8times more sensitivity than silicon bridge pressure sensor.

In other words, fabricated piezoresistive pressure sensor enhances the sensitivity due to released piezoresistors andsilicon nanowire of high piezoresistive effect.

CONCLUSION

The use of the silicon nanowire enables the fabricated pressure sensor to have the enhanced sensitivity and thereduced sensor size. For the pressure sensor using thesilicon nanowire of 250x250nm 2 cross area, the highsensitivity of 337.5 mV/V MPa was obtained. Pressuresensor of the silicon bridge has sensitivity of 38.1mV/V MPa and the bulk silicon pressure of theconventional flat diaphragm sensor has sensitivity of 8.5mV/V MPa.

For further researches, piezoresistive pressure sensor using the nanowire under 50x50 nm 2 size is under fabricated to be much more sensitivity compare to theconventional silicon piezoresistive pressure sensor despiteless size than bulk silicon pressure sensor.

REFERENCES:

[1] Y. H. Zhang, C. Yang, Z. H. Zhang, H. W. Lin, L. T.Liu, T. L. Ren, A Novel Pressure Microsensor With30- m-Thick Diaphragm and Meander-ShapedPiezoresistors Partially Distributed on High-Stress bulk Silicon Region, J. Micromech. Susc., vol. 14,

pp.1272-1282,2005.[2] M. X. Zhou, Q. A. Huang, M. Qin, W. Zhou, A novel

capacitive pressure sensor based on sandwichstructures, Journal of microelectromechanicalsystems, vol. 14, pp. 1272-1282, 2005.

[3] T. Toriyama, Y. Tanimoto, S. Sugiyama, Single crystalsilicon nano-wire piezoresistors for mechanicalsensors, Journal of microelectromechanical systems,vol. 11, pp. 605-611, 2002.

[4] K. Reck, J. Richter, O. Hansen and E.V. Thomsen,Piezoresistive Effect in Top-Sown Fabricated Siliconnanowires, Proc. MEMS 2008

CONTACT

* K. Chun , tel: +82-2-880-1811; kchun@mintlab.snu.ac.kr

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