An electrothermally actuated bi-axial scanning micromirror for medical imaging applications

Until recently there has not been a system available to physicians that can allow them to perform real-time, non-invasive, in vivo imaging with micrometer resolution and millimeter penetration depth without the effect of ionizing radiation or special preparation of the sample area. With the development of optical coherent tomography (OCT), physicians have a bio imaging technique with the aforementioned characteristics. The ultimate goal in this area is to produce miniaturized OCT system on a chip (SOC) devices. In MEMS, the focus has been on constructing suitable micro mirrors for integration into these OCT SOC devices. With this in mind, time-domain OCT with a single photodetector seems the most practical technology for a prototype.;In this dissertation, a novel micromirror design that requires low power and operating temperature for use with other optical devices is presented. This work also outlines our envisioned integration scheme with embedded waveguides, on-chip laser source and photodetectors on GaAs/Si chips. A detailed account of design, fabrication and characterization results are presented here. The micromirror is composed of a single crystal silicon base with reflective aluminum coating. The mirror is connected to four bimorph electrothermal actuators through four polysilicon flexural connectors. Each electrothermal actuator consists of a polysilicon lower layer and an aluminum upper layer with embedded platinum heaters in the lower layer. Materials selection and thickness optimization (Timoshenko theory) for the bimorph actuators are carried out to maximize the out of plane displacement. The results show that this mirror can undergo large angular displacements of up to +/-32 degrees and vertical displacements of up to 131mum at a low power of 12mW with a temperature increase of 63 degree C in the actuator. Moreover, the temperature increase in the device die stays at 5 degree C, which makes this micromirror device a feasible candidate for an integrated OCT on chip system. Comparison of performance parameters using figure of merit shows that our device performs better than its counterparts. Although device characteristics like cut-off frequency and mirror flatness needs to be improved, this device has all the potential to be utilized with an actual OCT system.