We report the observation of a very large piezoresistive effect in both polycrystalline and homoepitaxial chemical-vapor-deposited diamond films. The gauge factor for polycrystalline p-type diamond at 500 microstrains was found to be only 6 at room ambient, but increased rapidly with temperature, exceeding that of polycrystalline silicon (30) at 35 'C, and that of single-crystal Si (120) at 50 "C!. For strain and current flow in the [loo] direction, the gauge factor of a (lOO)-oriented homoepitaxial diamond film was found to be at least 550 at room temperature. Although the origins and unexpected temperature dependence of piezoresistive effect in diamond are not yet understood, these findings may suggest diamond-based sensors with performance significantly superior to that of their Si counterparts.

Three techniques for patterning of CVD diamond films are developed. First, predeposition patterning is performed by standard lithography using photoresist mixed with fine diamond particles to act as seed crystals. Second, substrates are masked either before ultrasonic treatment with diamond powder (which promotes nucleation), and the mask removed before diamond growth, or masked after treatment but before diamond deposition. Third, patterning of continuous diamond thin films by selective etching in oxygen at 700~ has been performed in a rapid thermal processor using SiO2 or Si3N4 as masking layer. The selectivity and resolution was found to be good in all cases.Patterning chemical vapor deposited (CVD) semiconducting films to suitable dimensions is necessary for their application in microelectronic and micromechanical devices. Direct patterning of diamond is difficult due to its extreme resistance to chemical attack. An alternative approach is to grow a prepatterned film through selective nucleation. Previous workers have described various techniques which involve pretreatment of the entire substrate (usually Si) followed by patterned suppression of nucleation by etching off or simply burying nucleation sites (1-3). Techniques to selectively enhance nucleation by ion bombardment have also been described (4). In this letter we describe three simple techniques for the patterning of diamond films which involve both selective nucleation and patterned etching which are all compatible with conventional integrated circuit fabrication technology.Diamond films ranging from 1 to 3 Fm in thickness were synthesized by two methods: hot filament chemical vapor deposition (HFCVD) and microwave plasma-assisted chemical vapor deposition (MPCVD) (5). Films were characterized by scanning electron microscopy (SEM) and Raman spectroscopy to ascertain the diamond morphology, purity, nucleation density, and growth rate. All the films displayed typical Raman spectra of diamond with a strong peak at 1332 cm -~ due to diamond bonding and little or no intensity near 1550 cm -~ indicative of the near-absence of nondiamond phases in the film.The first technique achieves selective nucleation by using photoresist mixed with fine diamond powder patterned by standard photolithographic process (DPR patterning). The diamond powder with particle size of 0.1-0.2 i~m was mixed in positive photoresist (Shipley 1470J) and suspended by ultrasonic agitation for 10 min to achieve a homogeneous mixture. Fig. 1. The SEM micrograph of a MPCVD diamond film on SiO2 patterned by DPR method.The nucleation density, uniformity, and smoothness of the resulting diamond films were optimized by controlling the photoresist thickness through the careful choice of spinning time and speed. The SEM micrograph of a DPR patterned diamond film on SiO2 is presented in Fig. 1. It was observed that after resist development, scattered diamond particles remained on the substrate resulting in growth of diamond in undesired locations. To remove these residues, the substrates were de...

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Electron transport mechanisms in thin boron-doped diamond filmsMaterial and electrical characterization of polycrystalline borondoped diamond films grown by microwave plasma chemical vapor deposition