Silicon nitride deformable mirrors for focus and spherical aberration correction in micro-optical systems

Abstract

Laser beam scanning systems benefit from dynamic focus control and aberration correction using a deformable mirror, enabling 3D real-time scanning. Designed for focus and spherical aberration control in optical beam scanning systems, these mirrors are capable of video rate bandwidths allowing real-time 3D imaging in micro-optical scanning systems. Field curvature aberration can also be corrected with the same mirror. Previous work with polysilicon deformable mirrors validated the concept of using specialized deformable optics in beam scanning systems. Fabrication of micro-optics in this dissertation was achieved using a modified surface micromachining technique with silicon nitride replacing polysilicon as the structural material. A bulk anisotropic silicon etch following a PSG release etch allows the creation of a variable cavity depth, overcoming the typical deformation limitations of standard surface micromachining. To perform as desired, parabolic curvature of the mirror surface is required with the ability to introduce positive and negative quartic curvature. Analytical theory showed that deformed simply supported plates have curvature closer to parabolic than deformed clamped plates. Segmentation and thinning of the perimeter was done to in order to emulate this simply supported boundary. Two annular actuation zones were implemented to give independent control over the second and fourth order curvatures. It was found that residual stress present in the silicon nitride structural plate improved the surface curvature and mechanical resonance at the expense of a larger actuation pressure. Mirrors with diameters of 1500, 1000, 750, and 300 microns were built and tested. Zonal actuation with annular electrodes proved successful in providing sufficient correction of the mirror surface curvature, allowing spherical aberration free focus control. Perimeter segmentation greatly reduced required actuation loads allowing improved focal ranges. It was found that the sacrificial layer thickness has a significant impact on the initial curvature of the mirror. Sacrificial layers 0.2 microns thick proved sufficient for release, improved device yield, and resulted in an initially flat mirror.