Patent Application
20090002796
1. Field of the Invention
This invention relates generally to the field of adaptive optics, and in particular to an electrostatic deformable mirror suitable for use in a wide range of adaptive optics applications.
2. Background of the Invention
Adaptive optics is a technique for controlling the spatial phase of light that has been under development for several decades. In a general adaptive optics system, light is reflected from a deformable mirror and a small fraction is split off to illuminate a sensor. The sensor provides feedback to a control computer that adjusts the deformable mirror to change some property of the beam of light. Astronomers have used adaptive optics systems to remove the distortions induced by the atmosphere and achieve higher quality images from large telescopes. Adaptive optics systems have also been used on lasers to improve the beam quality and to shape the intensity profile.
Membrane deformable mirrors have been developed by several groups as a low-cost alternative to conventional piezoelectric or electrostrictive actuator plate-type deformable mirrors. Grosso published results of a metal membrane deformable mirror in 1977. Takami and Zamkotsian published work on nitrocellulose membrane deformable mirrors using a polished Macor electrostatic pad array in 1994 and 2005 respectively. Vdovin and Mansell published work on silicon and silicon nitride membrane deformable mirrors made using MEMS technology in the late 1990's. Finally, Mansell published work in 2006 on a polymer membrane deformable mirror with a low-cost electrostatic pad array made using printed circuit board technology. These membrane deformable mirrors have been shown to be scalable in size, low-cost, high optical quality, and capable of handling significant amounts of laser power.
Although membrane deformable mirrors are ideal for many applications, they do have some potential problems. One of the most significant of these is the potential for damage during operation due to electrostatic snap-down. In membrane mirrors, electrostatic snap-down occurs when the electrostatic force applied to the membrane exceeds the mechanical restoring force and the membrane collapses into the electrostatic pad arrays. In most electrostatic mirrors, snap-down results in damage due to an electrostatic discharge (spark) striking between the membrane and the electrostatic pad array.
Mansell and Vdovin have offered several techniques to address the issue of electrostatic snap-down. Vdovin created a membrane that is resistant to damage by electrostatic snap-down by fabricating a thick membrane of polycrystalline silicon surrounded by layers of silicon nitride. Mansell demonstrated a similar technique with a much thinner polymer (nitrocellulose) membrane. These techniques demonstrate a step forward, but do not solve the problem entirely.
Mansell also demonstrated that a layer of Teflon could be applied over electrostatic pads on a silicon wafer by spin coating it and annealing it to prevent damage due to an electrostatic spark. This technique is better because it prevents the electrostatic spark, but involves an expensive added step and is only effective for work on polished wafers. Furthermore, electrons were often injected into the Teflon coating during electrostatic snap-down and trapped there permanently making the Teflon coating into electret. The permanent charge on the Teflon caused the deformable mirror to deform permanently.
The most recent designs of electrostatically actuated membrane deformable mirrors (Mansell and Vdovin), use low-cost printed circuit board technology for electrostatic pad arrays. We present here techniques for achieving electrostatic snap-down protection that are more conducive to use on membrane deformable mirrors using a printed-circuit board as an electrostatic pad array.
The primary object of this invention is to minimize or eliminate damage due to electrostatic snap-down of a membrane deformable mirror.
The above objectives are obtained by creating a mechanical stop for the membrane before it gets close to the printed circuit board electrostatic pad arrays. In the preferred embodiment, a membrane mirror is actuated electrostatically by an electrostatic pad array fabricated using a printed circuit board below the membrane. In normal fabrication, a polymer dielectric coating is applied to the surface of the printed circuit board after fabrication. This coating can be used as protection from electrostatic snap-down. In our experience, this coating is thick enough that it does not breakdown during an electrostatic snap-down event. Furthermore, it appears not to charge like the Teflon coating that Mansell used earlier.
In another embodiment described here, pillars of material are created as electrostatic stand-offs. One version of this technique involves not coating the electrostatic pad arrays or their associated vias with solder and creating separate stand-alone vias that are coated with solder that makes them significantly taller than the surrounding electrostatic pads. The vias with solder then create electrostatic stand-offs that will intercept the membrane during snap-down and prevent it from reaching the electrostatic pads. The stand-offs can be biased at the same voltage as the mirror, or be entirely unbiased. The natural smoothing nature of the plating process that can be used to apply create the solder tends to eliminate any sharp edges which could damage the surface. Another advantage of this technique is that when metal is used properly for the stand-off, it cannot be permanently electrostatically charged.
| Number | Date | Country | |
|---|---|---|---|
| 60861475 | Nov 2006 | US |
