Thin Film Silicon Microbridges for DNA Detection
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J11.2.1
Thin film silicon microbridges for DNA detection T.Adrega1, J.Gaspar1, F.Fixe1,2, V.Chu1, D.M.F.Prazeres2,3 and J.P.Conde1,3 1 INESC Microsistemas e Nanotecnologias (INESC MN), Lisbon, Portugal 2 Center of Biological & Chemical Engineering, Instituto Superior Técnico, Lisbon, Portugal 3 Department of Chemical Engineering, Instituto Superior Técnico, Lisbon, Portugal ABSTRACT Thin-film MEMS molecular sensors are fabricated at temperatures below 110ºC on glass substrates. The microelectromechanical structure consists of a surface micromachined bilayer bridge of phosphorous-doped hydrogenated amorphous silicon and aluminum with a patterned SiO2 layer on the top. Specific binding of DNA to functionalized SiO2 on the bridge is confirmed using fluorescence microscopy. Microbridges are electrostatically actuated and the resonance frequency measurements are performed in vacuum in the initial state after fabrication, after the chemical functionalization of the SiO2 surface and after DNA immobilization. The sensor is able to detect the functionalization molecular layer, the cross-linker molecular layer, and the DNA molecules attached to the surface through a shift in its resonance frequency. The binding of molecules to the surface results in a shift of the resonance frequency due to contributions from surface stresses and mass loading. INTRODUCTION Recently, there has been growing interest in using microelectromechanical systems (MEMS) as biological sensors. Microresonators (microbridges and cantilevers) have been proposed as sensors to detect the presence and to quantify specific compounds [1-5]. These MEMS-based transducers can operate as stress sensors or as mass sensors. In the former case, the microresonator undergoes bending due to asymmetric stresses caused by the binding of the molecules on one side of the microstructure. The presence of residual stresses can also change the resonator effective spring constant, thus causing a shift in the resonance frequency. In the latter case, the resonator acts as a microbalance where the addition of mass results in a shift in the resonance frequency. MEMS sensors are usually made using bulk micromachining of a silicon wafer or surface micromachining of low stress poly-Si, which requires an annealing at 900ºC [6,7]. In both cases, the substrate of choice is crystalline silicon. Recently, thin-film silicon MEMS fabricated using surface micromachining were developed. Thin-film silicon technology, with its low temperature processing (< 350ºC), allows the use of substrates such as glass, plastic and stainless steel, among others [8-11]. In addition, thin-film MEMS are CMOS compatible enabling the monolithic integration of MEMS with its control electronics. This work presents a thin-film doped hydrogenated amorphous silicon (n+-a-Si:H) MEMS DNA sensor fabricated on a glass substrate. The sensing method is based on the detection of the resonance frequency shift induced by the specific DNA immobilization on the microresonator. The top layer of the resonator is silicon dioxide (Si
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