High-aspect-ratio MEMS deformable mirrors for next-generation telescopes

The next generation of extremely large telescopes (ELTs) that will be built on a scale of 30–50 meters in diameter necessarily require an increased ability to correct wavefront errors. Currently, wavefront correction is achieved through the use of a deformable mirror (DM). The degree and amount of correction depends on a mechanical displacement of segments of the mirror (stroke), and the number of actuators in the mirror surface (order). An effective ELT will require a high-stroke (10 microns) and high-order (100X100 actuators) mirror, a costly prospect using current technology.¹ Lowering the cost while improving the performance of DMs is possible using microelectromechanical systems (MEMS) technology. However, current surface machining processes used to fabricate MEMS deformable mirrors, such as the Sandia ultra-planar multilevel MEMS technology (SUMMiT) and MEMSCAP's PolyMUMPS process, limit a DM's stroke due to the use of thin-film sacrificial layers (which are dissolved away after the etching process to free up micromechanical components in the actuators).^{2, 3} Commercially available surface micromachined MEMS mirrors are limited in stroke to approximately 5.5 microns, about half of the stroke required by ELTs. A high aspect ratio fabrication process capable of depositing thick layers will allow the DM's to provide both high-stroke and high-order corrections, thus bypassing the need for a complex dual woofer (high stroke, low order)/tweeter (high order, low stroke) DM configuration.⁴

In our work, we tested different actuator designs with a bonded faceplate constructed using the LIGA (German acronym for lithography, electroplating, and molding) process, enabling multilayer fabrication of MEMS devices.⁵ Various types of high-stroke gold actuators consisting of folded springs with rectangular and circular membranes as well as x-beam actuators supported diagonally by fixed-guided springs were designed, simulated, and fabricated individually and as part of a continuous-face-sheet DM system. Our high-aspect ratio monolithic electro-deposition process uses gold structural and copper sacrificial layers. The fabrication process is performed on optically flat glass-ceramic substrates that have a root-mean-square roughness of <1nm and a coefficient of thermal expansion (CTE) of 11.4×10⁻⁶K⁻¹ that is close to the CTE of gold at 14.2×10⁻⁶K⁻¹. The final device is obtained when the copper sacrificial material is etched, leaving behind a gold continuous faceplate attached to gold actuators (see Figure 1). Since we are able to deposit thick sacrificial layers (20–30 microns), we are able to fabricate deformable mirrors with larger stroke than is possible with polysilicon surface micromachining processes. In addition, gold is a more robust material than polysilicon for the telescope environment.

Figure 1. Diagram showing the final release of the monolithic fabricated structure created using our technique. The gap for the X-beam electrostatic actuators is 20 microns, enabling a mirror stroke of 10 microns. WMS-15: specific glass-ceramic substrate. Not to scale.

We tested an X-beam actuator for ten trials with five voltage points. In each case, the displacement versus voltage for each individual actuator was highly repeatable (see Figure 2). We fabricated continuous faceplate deformable mirrors with a 1mm pitch ranging in array sizes of 3×3, 10×10, and 16×16 (see Figure 3).

Figure 2. (A) An X-beam actuator (B) Graph of displacement vs. voltage of an X-beam actuator for ten trials (T1–T10).

Figure 3. Top view of a gold deformable mirror consisting of a 3×3 array of X-beam actuators with a continuous facesheet. Four etch-holes have been patterned into the surface to facilitate release of the deformable mirror.

Our work has shown that high-aspect ratio MEMS devices can be fabricated monolithically using electro-deposition of gold structural layers and copper sacrificial layers on a WMS-15 glass-ceramic substrate. These devices hold promise for the manufacture of DMs for next-generation ELTs. We have now fabricated continuous facesheet DMs, and our future work will include characterizing the properties of these mirrors.

The authors would like to acknowledge Yanting Zhang and Fardad Chamran from Innovative Micro Technology for their help in modifying their fabrication process to meet our needs. This material is based upon work supported by the National Science Foundation under grant No. AST-1032362