An Evaluation of Silicon Carbide Based Bimorph Actuators for Optical Coherence Tomography Applications

Abstract

Optical coherence tomography (OCT) probes rely on MEMS micro-scanners to provide fast and reproducible scans covering as much area as possible. A popular choice of actuators employed in this role are electrothermal bimorph actuators. They comprise of two layers of different materials which are rigidly joined together. Differential expansion under heating causes the bimorph to deflect. Polycrystalline silicon carbide has been proposed to replace polysilicon as a multi-purpose material in MEMS but it has not been yet explored as a bimorph material. Under this premise, an in-plane bimorph actuator based on doped polycrystalline silicon carbide and silicon was proposed to fulfill the requirements of an OCT probe. This actuator suffered from yield and reliability issues, which were, however, linked primarily to the design and not the material itself. Therefore, in order to evaluate silicon carbide as a bimorph “hot” layer, an optimization and characterization procedure was carried out to determine its relevant mechanical, thermal and electrical properties. Specifically, the residual stress, Young modulus, coefficient of thermal expansion, thermal coefficient of resistance and electrical resistivity were determined. The layer residual stress and resistivity could be reduced by adjusting the SiH2Cl¬2, C2H2 and NH3 gas ratio¬, achieving 0.03 ? cm and 300 MPa respectively. The resistivity of the layer is adequate for it to act as a heater. The electrostatic pull-in instability technique was used to extract the Young modulus for different layers and it was found to decrease with increasing Si/C composition, with a value of 250 GPa for our selected optimum layer. The coefficient of thermal expansion was determined through fabricated v-beam actuators to be 4.3 ppm/K which indicates that SiC must be heated more to produce strain comparable to other bimorph ”hot” materials. As a final step in poly-SiC evaluation, we constructed rudimentary bimorph poly-SiC/SiO¬2 cantilevers which were subsequently used to test the static and dynamic performance of this material combination. Our conclusion is that poly-SiC is a good bimorph “hot” material that operates optimally at higher temperatures, can be used in more diverse bimorph designs and is more suited to applications that require high force. Throughout the material characterization process, we used numerical and analytical models extensively. This helped us identify good modelling practices and led to the refinement of the models, allowing us to obtain better matching between theoretical predictions and experimental data. Based on these findings of the poly-SiC properties and better models, we propose a novel in-plane, spring form actuator for use in an OCT micro-scanner. It employs a poly-SiC/SiO2 material combination in a mechanical amplifying geometry and in simulation, it achieves 80.9 mN of force and 250 ?m of displacement at 43 mW power consumption and a 0.3 mm2 footprint. All these parameters are an improvement over the initial design.