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 µm. The SOI layer is selectively thinned down to 2 µm 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 × 1 mm has a 400 µm diameter mirror plate. Other chip designs include the mirror diameters in the range from 200 to 500 µm. 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 • 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 ∼500 µm or more), this trade-off was the key motivation for the research presented in this paper.

A MEMS optical coherence tomography (OCT) probe prototype was developed using a unique assembly based on silicon optical bench (SiOB) methodology. The probe is formed by integrating a three-dimensional (3D) scanning micromirror, gradient refractive index (GRIN) lens and optical fiber on SiOB substrates having prefabricated self-aligned slots. The two-axis scanning micromirror is based on electrothermal actuation with required voltage less than 2 V for mechanical deflections up to 17°. The optical probe was enclosed within a biocompatible, transparent and waterproof polycarbonate tube with a view of in vivo diagnostic applications. The diameter of the miniature probe is less than 4 mm and the length of its rigid part is about 25 mm. The probe engineering and proof of concept of the probe were demonstrated by obtaining en face and three-dimensional OCT images of an IR card used as a standard sample.

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A vacuum package has been developed for 128x128 array 1R bolometer device with Ge window having anti reflection (AR) coating. For a good vacuum package hermeticity and low out gassing are the two critical elements. A good hermetic sealing has been achieved with Ge window attachment using solder bonding. Different metallization structures have been tried and metallization of oxide/Ti/Ni/Au with additional annealing process was found to have good adhesion and solder wetting. Getters have been activated before final vacuum sealing of the package to absorb the outgassing gases from the packaging materials. Residual Gas analysis (RGA) showed that Thermo electric cooler used inside the package outgassed more compared to other materials. Vacuum inside the package was measured by using a single element IR bolometer device and found to have vacuum of 50milli torr. The developed vacuum package has been tested hnctionally and found to be no degradation in image before and after packaging.

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A miniaturized optical bioprobe package is developed using a 3D micro mirror and is tested for bio-imaging application. A silicon optical bench is designed and micro machined to assemble the fiber, lens and the 3D micro mirror device. A 45 degree angle trench is used to place the micro mirror to achieve larger scanning range. Trace lines are formed on the optical bench and are connected to silicon micro mirror using solder. A GRIN lens with lower numerical aperture has been used to focus the optical beam onto the micro mirror. The bio probe is packaged and is tested in a Time domain OCT (Optical Coherence Tomography) setup and optical image is obtained for plant tissue.

Design and development of a 3D scanning MEMS micromirror integrated miniaturized optical probe has been presented in this article. The probe is designed to be less than 2 mm in diameter and has dynamic scanning modality for larger field of view. Scanning is achieved using 3D micromirror device, which has 16º out of plane and 360º beam rotation capability. Initial target of 45º out of plane deflection is yet to be achieved. The probe being developed currently would have scanning capability in one quarter of 360º full rotation. The field of view would still be very large and multiple optical biopsies would be possible for planned cancer model diagnostics. The feasibility of using scanning mirror into an optical probe was demonstrated using scanning repeatability and OCT imaging tests. Geometrical optics and package design using silicon optical bench have been established. Miniaturized 3D scanning micromirror have been designed and developed with 16º out of plane deflection demonstrated. Probe package integration and optical testing are carried out.

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