Piezoresistive Sensors on Plastic Substrates Using Doped Microcrystalline Silicon
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Piezoresistive Sensors on Plastic Substrates Using Doped Microcrystalline Silicon P. Alpuim1,2, V. Chu1, J. P. Conde2 Instituto de Engenharia de Sistemas e Computadores (INESC), Rua Alves Redol, 9, 1000-029 Lisbon, Portugal 2 Department of Materials Engineering, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisbon, Portugal 1
ABSTRACT The piezoresistive behavior of optimized n-type and p-type microcrystalline silicon films deposited on polyethylene terephthalate plastic substrate by hot-wire and radio-frequency plasma-enhanced chemical vapor deposition, at a substrate temperature of 100 ºC, is studied. A 4-point bending jig allowed the application of positive and negative strains in the films. Repeated measurements of the relative changes in the resistance of the samples during the strained condition showed reversible behavior, with p-type microcrystalline films having positive gauge factor in the range from 25 to 30 and n-type µc-Si:H films having negative values of gauge factor from -40 to -10. The induced strain in the films was in the range between 0 and ±0.3%. A sensor utilizing the piezoresistive property of doped µc-Si:H was used to map a contour with the shape of an Archimedes’ spiral. INTRODUCTION The piezoresistive properties of crystalline silicon – the change of resistance due to the application of strain or stress – have long been known [1,2] and used in the fabrication of strain gauges and other electromechanical transducers [3,4]. On the other hand, deposition of intrinsic and doped microcrystalline (µc-Si:H) silicon thin films at very low substrate temperatures (Tsub-40 were obtained for p-type and for n-type films, respectively. In the present work the piezoresistive behavior of p-type and n-type µc-Si:H films deposited on plastic (polyethylene terephthalate, PET) at Tsub=100ºC by radio-frequency plasma enhanced chemical vapor deposition, RF, and by hot-wire chemical vapor deposition, HW, is studied both under tensile and compressive stress and the GFs are evaluated. Some samples were then used as large area piezoresistive sensors and two acrylic models, one convex and the other concave, with the edge in the shape of an arc of an Archimedes’ spiral (r∝θ), were used to test the ability of the sensors to map their contours. An array of microsensors was also deposited and patterned on a PET substrate and tested on the same Arquimedes’ spirals. EXPERIMENTAL PROCEDURES The films were deposited by HW and RF in a UHV-quality system. PET substrates 0.125 mm thick were cut in a rectangular shape, with app. 5 cm length and 2 cm width, and clamped to the grounded upper electrode of the chamber, which was heated to the substrate temperature, Tsub. Detailed studies and optimization of the doping of µc-Si:H by HW and RF deposited on PET at Tsub=100ºC were published elsewhere [10, 11]. The film thicknesses were ∼2000 Å. The structural and electrical properties of the films are summarized in table I. Table I. Properties of µc-Si:H films deposited on PET Tsub CVD Sample H2dil ( ºC) (%) 100 100 100 100 †
H
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