Simulation and fabrication of a mechano-optical sensor for nano-displacements

We present the simulation and fabrication of a novel and highly sensitive mechano-optical sensor for nano-displacements, based on microcantilevers suspended above a Si3N4 grated waveguide (GWG). The presence of a dielectric object, in this case a suspended cantilever, in the evanescent field region of the GWG may lead to the occurrence of propagating modes for wavelengths inside the stop band of the grating, and so to resonances (defect modes) inside the stop band. The 2D bidirectional eigenmode propagation (BEP) method has been applied to analyze the effect of cantilever displacement on the optical transmission spectrum of the GWG. The simulation results show that as the cantilever approaches the grating, the first near band-edge resonance peak is pulled inside the stop band and its spectral width decreases. The resolution of displacement measurement is estimated to be 0.2 nm for a 200 nm thick cantilever at a 200 nm initial gap, assuming a signal-to-noise ratio (SNR) of 20 dB. Integrated microcantilever-GWG devices have been fabricated successfully using MEMS techniques. Uniform gratings have been defined with laser interference lithography. SiO2 cantilevers with low initial bending (i.e., low stress) have been fabricated by combining the tetra-ethyl-ortho-silicate chemical vapor deposition (TEOS-CVD) and plasma-enhanced chemical vapor deposition (PE-CVD) oxides, and by releasing them using a tetramethylammonium hydroxide (TMAH) wet-etching solution, followed by a freeze-drying process. High-resolution-SEM and AFM measurements revealed that the initial bending of 30 um long cantilevers is as low as 250 nm. Additionally it was found that TMAH etching improved the quality of the waveguides by reducing surface roughness from 1.41 nm down to 0.46 nm. The measured stop band of the GWG agrees well with the calculated result. The simulation, successful fabrication, and initial optical characterization results demonstrate the potential of the integrated microcantilever-GWG as a novel and compact mechano-optical sensor for nano-displacements.