A two axes scanning SOI MEMS micromirror for endoscopic bioimaging

A novel silicon on insulator (SOI) MEMS process has been designed and developed to realize a two axes thermally actuated single crystal silicon micromirror device, which consists of a mirror plate, four flexural springs and four thermal actuators. The mirror plate has the same thickness as a SOI device layer i.e. 4 mu m. The SOI layer is selectively thinned down to 2 mu m for fabricating flexural springs and thermal actuators. The thinning of the SOI layer is essential to lower ( control) the flexural rigidity of the springs and the actuators and thus to achieve a higher tilt angle at low thermal power. The developed single wafer process is based on dry reactive ion etching CMOS compatible chemistries. The minimum chip size design of 1 mm x 1 mm has a 400 mu m diameter mirror plate. Other chip designs include the mirror diameters in the range from 200 to 500 mu m. This paper also presents a study on the mirror plate curvature, thermal actuation mechanism and the experimental results. The measured maximum angular deflection achieved was 17 degrees at an operating applied voltage of less than 2 V, and the radius of curvature of the mirror plate was in the range from 20 to 50 mm. The micromirror was developed for a miniature catheter optical probe for optical coherence tomography in vivo imaging. A low cross-sectional size of the probe and higher resolution are essential for investigating inaccessible pathologies in vivo. This required a compact micromirror chip and yet sufficiently large mirror plate ( typically similar to 500 mu m or more), this trade-off was the key motivation for the research presented in this paper.