Design, fabrication and characterization of surface plasmon resonance based MEMS displacement sensors
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Strong dependence of surface plasmon resonance (SPR) on coupling parameters offers new varieties of sensing mechanisms in nano and micro-scale engineering fields. In this study, design, fabrication and characterization of MEMS displacement sensors that utilize angular dependence of grating coupled SPR condition are explored. Several surface plasmon polariton (SPP) excitation mechanisms are reported in the academic literature. One of them which is quite adaptable to microelectromechanical systems is grating coupling scheme. In this scheme, thin metallized grating structures are particularly designed depending on the desired wavelength and the angle of incidence of the SPP excitation light. Geometric parameters like periodicity, surface profile, depth and duty cycle of the grooves and material parameters like dispersion and thickness of the top metal layer have to be chosen with care in order to reach sharp SPR curves in the reflected intensity spectra with respect to either wavelength or angle of incidence. As the first step, geometric and material parameters of SPR gratings are numerically optimized using rigorous coupled-wave analysis (RCWA). Angular quality factors on the order of tens are shown to be achievable. Various lithographic techniques (nanoimprint, electron beam and optical lithography) are used to nanofabricate those certainly defined gratings. It is observed that p-polarized reflected intensity measurements using spectroscopic ellipsometry are in quite good agreement with those numerically calculated. Spectroscopic scan measurements are also provided to show the polarization dependence of SPP excitation. All effort to obtain high angular Q-factor grating structures is aimed at enhancing the sensitivity of angular displacement detection scheme. In this scheme, angular position of the grating structure in the polarization plane is detected through the reflected intensity response of the photodetector. Dependence of sensitivity on excitation light source wavefront parameters and photodetector noise are analyzed. MEMS displacement sensor designs relying on the principle of angular displacement detection scheme are developed. Simply, SPR grating structures are transferred on conventional micromembranes. Two types of such particular designed micromembranes are introduced: corrugated microcantilevers (singly clamped) and corrugated microbridges (doubly clamped). They are fabricated through well-known surface micromachining processes in addition to SPR grating nanofabrication procedures. Mechanical resonance frequencies, flexural mode shapes and effective spring constants are analytically, numerically and experimentally obtained. In addition, a MEMS accelerometer design with plasmonic readout with nano-G noise floor is presented. An experimental configuration for micromechanical displacement sensing is investigated. According to the results of this work, novel arrayed sensors combining the sensitivities of plasmon resonance and micromembrane type sensors may provide unprecedented performance.
Grating Coupled Surface Plasmon Resonance
Micromechanical Displacement Detection