Fabrication Of Bimetallic Cantilevers For Chemical Sensors

Abstract

There have been great interests in developing micromachined cantilever stylus, scanning probe based chemical sensor, including thermal probe Sensor and nano-resolution mass and molecule detectors. The microfabricated cantilevers offer various possibilities as chemical sensors such as nanocalorimeters on high resolution mass detectors based on resonance frequency shift. The microcantilever coated with a thin metal layer was recently reported as a high sensitive thermal Sensor when heat was generated by reaction of hydrogen and oxygen on the cantilever in vacuum. Nanoscale mass measurement was reported in particulate mass deposited on microcantilevers using resonance frequency shifl techniques. Bing et al. also proposed a micro cantilever sensor with MHz resonance frequency and a mass resolution of IO-'* g. More recently, vapor adsorption on the micro cantilever surface was also found to create a shift of resonance frequency and angular bending of the bimetallic cantilevefll,2,3,4,5]. In this work, we fabricated a Si3N4 cantilever using micromachining techniques. Initially a Si,N, layer was deposited using low pressure chemical vapor deposition techniques. The cantilever was defined and patterned by photolithography on the front side and etched into the silicon. Finally, the backside etching was performed until both etch fronts meet and the cantilever becomes released. -100nm AJ and 20nm Pt thin film layers were deposited on the backside of the fabricated S13N, cantilever using electron beam evaporator. The temperature change and heat flow across the fabricated bimetallic lever would create angular bending of the bimetallic cantilever. The heat was supplied through a stainless steel block attached to a cantilever-supporting beam. The block was wrapped with a nichrome wire in order to supply heat to the lever. The thenal couple was also attached to the stainless steel block. The hysteris curve of the lever upon heating and cooling was measured without a chemical substance. The chemical substance, tetra decand CH3(CH2),,0H, was used and its theoretical temperature for phase change from solid phase to liquid phase is known to be -31 3K. Very tiny amounts of tetra decanol were placed on top of the bimetallic lever and its thermal response was examined during an endothermic chemical reaction using optical deflection method. The abrupt change of the angular bending of the bimetallic lever due to endothermic chemical reactions was observed at -315K. This introductory experiment presents bimetallic cantilevers as excellent candidates for chemical Sensor (sensitive in mass resolution) in an atmospheric environment. Depending upon chemical reaction, whether it is exothermic or endothermic, specific micro-scale or nano-scale cantilever sensors can be tailored to the specific chemical reaction of the interest.