Mems 3-D scan mirror for an endoscopic confocal microscope

Abstract

Optical MEMS is at a very exciting stage and has become an enabling technology for a variety of applications in telecommunications and high-resolution display and imaging. Among many novel MEMS devices, MEMS scanners and deformable membrane mirrors are especially useful for scanned-beam imaging systems. MEMS scanners are usually used for laser beam scanning. Deformable mirrors provide an approach to modify the optical wavefront adaptively. In optical microsystems, packaging is often a critical part of the system design. Combining functionalities at a single device can reduce complexity of the system. Biaxial beam scanning and focus control are combined together in the MEMS 3-D scan mirror. The mirror surface is made of a goldcoated silicon nitride deformable membrane and works as a positive lens of variable focal length. The membrane sits on a torsional plate that can scan about two orthogonal axes. This architecture is able to move the focus of a laser beam throughout a threedimensional space with a single optical surface. The overall size of the 3-D mirror is 1.5 mm with a usable optical aperture of 0.7 mm. Both the inner and outer scanning axes achieved more than 5o zero-to-peak mechanical scan angles; the deformable membrane achieved a maximum center displacement of more than 3.7 æm, corresponding to an adjustable focal length from infinity to approximately 8 mm. For a confocal laser scanning microscope with illumination wavelength at 500 nm, they provide Nx = Ny = 488 resolvable spots for lateral resolution, and Nz = 32 depth-of-focus distances for depth resolution. To drive the 3-D mirror, the inner axis is operated near resonance (~ 650 Hz); and the outer axis quasi-statically. Operating the outer axis at 2 Hz provides a line resolution of 325 lines/frame at a refresh rate of 2 frames/second. The performance of the 3-D mirror is well matched to the intended application of endoscopic confocal microscopy, and similar devices could prove useful in a variety of optical microsystems needing beam scanning and focus control. This dissertation describes the design, fabrication, characterization, imaging experiment and target applications of the MEMS 3- D scan mirror.