Investigation of Nanometric Thin-Film Bismuth Piezoresistors Deposited on Silicon Substrates
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Investigation of Nanometric Thin-Film Bismuth Piezoresistors Deposited on Silicon Substrates Horacio V. Estrada1 1 Centro Nacional de Metrología (CENAM), Querétaro, QRO 76246, MEXICO. Keywords: Bi thin films, piezoresistance, longitudinal and transverse strain sensitivity. ABSTRACT Thin film bismuth piezoresistors, defined on oxidized silicon wafers, are investigated as a function of their orientation for their eventual integration on micro-electro-mechanical (MEMS) microsensors. Bismuth’s piezoresistance (or elasto-resistance) is experimentally investigated to accurately determine its longitudinal and transverse strain sensitivities. Whisker-shaped resistive elements defined on different orientations (from 0o, the beam’s main strain axis, to 90o, perpendicular to that axis) undergo changes of resistance (R), associated with the induced strains on silicon cantilevers beam’s surface when these are mechanically loaded under pure bending stress conditions. For Bi-resistors, the traditional gage factor concept, (R/Ro)/l, is found to be equal to +16 and +33, for elements oriented along 0 and 90o, respectively, considerably larger than those for metals or metal alloys. These high sensitivity values and the “unusual” positive, higher value for the 90o (perpendicular) resistors can be of considerable interest for microsensors applications. The results of this study enable us to precisely determine the bismuth’s longitudinal and transverse strain sensitivities that are calculated to be equal to +26 and +40.5 respectively. This experimental study is extended to explore the Bi-films’ response to bi-axial strain fields. INTRODUCTION Bismuth is of considerable interest for various applications due to its distinct properties, including its magnetoresistance [1] and its near-ideal thermoelectric properties [2]. Bismuth and its alloys have also been considered as a transducer element for mechanical applications due to its piezoresistive (elastoresistive) properties [3]. However, in most of these applications it appears that some degree of crystalline structure of the bismuth films is necessary; thus the synthesis method implemented for these films may be crucial to get best responses. In this study, thermally evaporated nanometric thin films (~50 nm) were used; these films exhibit a columnar structure of 100-400 nm in diameter and considerably aligned along the [003]-crystallographic direction. The films have been investigated for their piezoresistive properties when deposited on silicon wafers by different synthesis methods, although this work focuses on the e-beam thermal evaporation methods. For this purpose, silicon cantilever beams have been obtained from oxidized silicon (100)-wafers, on which the bismuth films have been synthesized and patterned using photo-lithographic methods, to explore its strain sensitivity (both, longitudinal and
transverse sensitivities). By placing the silicon beams under pure bending conditions, a simple and calculable strain field along the x- and z-directions (related by the Poisson’s ratio)
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