Piezoresistor Sensor Fabrication by Direct Laser Writing on Hydrogenated Amorphous Silicon

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Piezoresistor Sensor Fabrication by Direct Laser Writing on Hydrogenated Amorphous Silicon P. Alpuim1,2, M.F. Cerqueira1, G. Junior2, J. Gaspar2, and J. Borme2 1 Department of Physics, Universidade do Minho, Campus de Gualtar, 4715-057 Braga, Portugal. 2 INL – International Iberian Nanotechnology Laboratory, Av. Mestre José Veiga, 4715-330 Braga, Portugal. ABSTRACT In this paper we report on the 532 nm Nd:YAG laser-induced crystallization of 10 nm thick boron-doped hydrogenated amorphous silicon thin films deposited on flexible polyimide and on rigid oxidized silicon wafers by hot-wire or by plasma-enhanced chemical vapor deposition. The dark conductivity increased from ~10-7 -1cm-1, in the as-deposited films, to ~10 and 50 -1cm-1 after laser irradiation, on rigid and flexible substrates, respectively. Depending on type of substrate, laser power and fluence, a Raman crystalline fraction between 55 and 90 % was measured in HWCVD films, which was higher than observed in rf-PECVD films (35-55 %). Crystallite size remained small in all cases, in the range 6-8 nm. Due to a very high conductivity contrast (>7 orders of magnitude) between amorphous and crystallized regions, it was possible to define conductive paths in the a-Si:H matrix, by mounting the sample on a X-Y softwarecontrolled movable stage under the laser beam, with no need for the usual lithography steps. The resistors scribed by direct laser writing had piezoresistive properties, with positive gauge factor ~1. The details of the laser interaction process with the Si film were revealed by scanning electron microscopy imaging. INTRODUCTION The possibility of direct writing thin semiconductive channels or structures on an insulating substrate, in a photoresist-free process, is attractive for its simplicity, cost effectiveness and wide choice of substrates allowed. A broad range of applications, such as touch screens, flexible displays, various types of sensors and many other large-area electronic devices could benefit from such a process [1]. In this paper, we propose a route to attain that goal, using amorphous silicon (a-Si:H) thin-film technology. The fabricated devices are piezoresistive strain sensors made of B-doped nanocrystalline Si (nc-Si:H). When a-Si:H thin films with less than ~50 nm are deposited on an insulating substrate their electrical conductivity is normally very low (≤10-7 Ω-1cm-1) [2]. Adding P or B precursors to the reactive gas mixture during deposition does not lead to an increase in the conductivity, due to a very low doping efficiency in a-Si:H and to a large defect concentration caused by lattice mismatch between film and substrate [3,4]. Upon laser exposure, the film electrical conductivity increases many orders of magnitude, up to ~100 Ω-1cm-1. This increase is a consequence of laserinduced crystallization of the amorphous tissue accompanied by dopant activation of the impurity atoms [5,6]. The dark conductivity contrast between amorphous and crystallized regions is so high (>7 orders of magnitude) that it is possible to define cry

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