Piezoresistivity of Polycrystalline Diamond Films
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PIEZORESISTIVITY OF POLYCRYSTALLINE DIAMOND FILMS
Der-Rern Wur and Jim L. Davidson Department of Electrical Engineering, Vanderbilt University, Nashville,TN 37235
ABSTRACT Polycrystalline diamond film (PDF) is known for its high power, high temperature, and radiation hard potential. The interest in piezoresistivity of PDF is that it is a candidate for high temperature sensing (e.g., pressure sensor). Piezoresistivity measurements were taken of boron-doped PDF grown by microwave-plasma chemical vapor deposition(CVD). Three substrates, silicon, aluminum nitride and tungsten were used. Films were detached from these substrates, then attached to a ceramic substrate. The piezoresistivity varies, dependent on the original host substrate. For example, at room temperature, the PDF film from tungsten has a greater gauge factor, around 75. The carrier activation energy of this film, determined from log R(I/T), was nominally 0.25eV. Combining thick film technology and CVD processes, patterned B-doped PDF has been achieved monolithically on AIN substrates. The characteristics of this configuration is being investigated and will be presented. INTRODUCTION Chemically vapor deposited (CVD) polycrystalline diamond films (PDF) have the diamond property advantages of wide bandgap (5.4 eV), low thermal coefficient of expansion ( -2 ppm/C)[1 ], and high thermal conductivity (20 W/cmK)[2] compared with other semiconductor materials. These properties and the ability to fabricate PDF with semiconductor processing (doping, photolithographical patterning and interconnect)[3,4] indicate this potentially superior electronic material can provide advanced devices and sensors for high temperature, high power and harsh environments where Si and GaAs can not perform. Piezoresistance (PZR) is the phenomenon whereby a resistor changes resistance in response to an applied strain. If diamond films are to be piezoresistors and utilized for strain gauge applications, they must exhibit sufficient sensitivity to externally applied strain. Because of PZR, a resistance change will result when there is an increment change in piezoresistor length, &L/L, produced by reversible (elastic) applied strain. Gauge factor, an index of the sensitivity of a piezoresistive sensor device, is defined as the ratio of relative change in resistance to the relative applied strain: GF=AR/R
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Present technology sensors employing strain sensing elements often use semiconductor, thick-film or metal-film resistors. However, these devices have limitations. Diffused or ionimplanted semiconductor (e.g., silicon) piezoresistive sensors are limited to an operating temperature of < 150 °C, due to p-n junction leakage. Thin film piezoresistors, metal alloy or polysilicon, have been fabricated for high temperature operation up to 250 'C, but these devices offer a relatively small output signal. Metal thin film strain gauges have small gauge Mat. Res. Soc. Symp. Proc. Vol. 283. @1993 Materials Research Society
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factors, - 5. Diffused or ion-implanted p-n junction devices may
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