SU8 / modified MWNT composite for piezoresistive sensor application
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SU8 / modified MWNT composite for piezoresistive sensor application Prasenjit Ray1, V. Seena1, Rupesh A. Khare2, Arup R. Bhattacharyya2, Prakash R. Apte1, Ramgopal Rao1 1. Centre of Excellence in Nanoelectronics, Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India. 2. Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India. ABSTRACT SU-8 is being increasingly used as a compliant structural material for MEMS applications due to its interesting properties such as lower Young’s modulus and higher mechanical and thermal stability. One of the popular classes of MEMS devices is a piezoresitive microcantilever. Ultra-sensitive polymer composite cantilevers made up of SU-8 as a structural layer and 10% carbon Black in SU8 as a piezoresistive layer with lower Young’s modulus and higher gauge factor have been reported recently by our group. Higher electrical conductivity at lower concentration of conductive filler is of increased interest. Here we report a novel composite with purified multiwall carbon nanotubes (MWNT) in SU8 as a piezoresistor. MWNT were modified with octadecyl triphenyl phosphonium bromide (OTPB) in order to achieve debundled MWNT. A microcantilever device with integrated MWNT/SU-8 composite has been fabricated and characterized. INTRODUCTION Microcantilevers are normally used in Atomic Force Microscopy (AFM) imaging, for biomolecular sensing and for applications in chemical sensing. For these applications optical detection (using laser) is used for sensing the deflection of the cantilever beam. However, optical detection scheme is rather complex and laser alignment makes this sensor inconvenient to use in the field. Due to this drawback, the integrated piezoresistor scheme looks very attractive. Several integrated piezoresistive cantilevers have been developed for AFM imaging [1-3], environmental sensors [4], bio-sensors [5-6] and mass-sensors [7]. Though most of the microcantilevers have been silicon based, only recently polymer based sensors based on materials such as SU8, because of their lower Young’s modulus and ease of fabricating high aspect ratio structures, have become more attractive alternatives. Earlier, SU8 based polymer cantilevers with gold (Au) and polysilicon have been tried as piezoresistive layers [8-10]. Due to the lower gauge factor of gold, of the order of 2, the gold strain gauge based cantilevers are not suitable for practical applications such as bio-sensors. Carbon black/SU8 composite is reported recently as another option for piezoresistive polymer cantilever with a very high gauge factor of 15-20 with 10 wt% CB in SU8 [11]. These cantilevers have been used successfully for bio applications and explosive detection application [12, 13]. Instead of carbon black, the use of multiwall carbon nano tubes (MWNT) is expected to show improvement in dispersion and hence resulting in higher gauge factor for MWNT/SU8 composite compared to CB/SU8 composite. In this paper
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