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Fabrication Methods for Adaptive Deformable MirrorsPreviously, it was difficult to fabricate deformable mirrors made by piezoelectric actuators. This is because numerous actuators need to be precisely assembled to control the surface shape of the mirror. Two approaches have been developed. Both approaches begin by depositing a stack of piezoelectric films and electrodes over a silicon wafer substrate. In the first approach, the silicon wafer is removed initially by plasmabased reactive ion etching (RIE), and non-plasma dry etching with xenon difluoride (XeF2). In the second approach, the actuator film stack is immersed in a liquid such as deionized water. The adhesion between the actuator film stack and the substrate is relatively weak. Simply by seeping liquid between the film and the substrate, the actuator film stack is gently released from the substrate. The deformable mirror contains multiple piezoelectric membrane layers as well as multiple electrode layers (some are patterned and some are unpatterned). At the piezolectric layer, polyvinylidene fluoride (PVDF), or its co-polymer, poly(vinylidene fluoride trifluoroethylene P(VDF-TrFE) is used. The surface of the mirror is coated with a reflective coating. The actuator film stack is fabricated on silicon, or silicon on insulator (SOI) substrate, by repeatedly spin-coating the PVDF or P(VDFTrFE) solution and patterned metal (electrode) deposition. In the first approach, the actuator film stack is prepared on SOI substrate. Then, the thick silicon (typically 500-micron thick and called handle silicon) of the SOI wafer is etched by a deep reactive ion etching process tool (SF6-based plasma etching). This deep RIE stops at the middle SiO2 layer. The middle SiO2 layer is etched by either HF-based wet etching or dry plasma etch. The thin silicon layer (generally called a device layer) of SOI is removed by XeF2 dry etch. This XeF2 etch is very gentle and extremely selective, so the released mirror membrane is not damaged. It is possible to replace SOI with silicon substrate, but this will require tighter DRIE process control as well as generally longer and less efficient XeF2 etch. In the second approach, the actuator film stack is first constructed on a silicon wafer. It helps to use a polyimide intermediate layer such as Kapton because the adhesion between the polyimide and silicon is generally weak. A mirror mount ring is attached by using adhesive. Then, the assembly is partially submerged in liquid water. The water tends to seep between the actuator film stack and silicon substrate. As a result, the actuator membrane can be gently released from the silicon substrate. The actuator membrane is very flat because it is fixed to the mirror mount prior to the release. Deformable mirrors require extremely good surface optical quality. In the technology described here, the deformable mirror is fabricated on pristine substrates such as prime-grade silicon wafers. The deformable mirror is released by selectively removing the substrate. Therefore, the released deformable mirror surface replicates the optical quality of the underlying pristine substrate.
Document ID
20140002301
Acquisition Source
Headquarters
Document Type
Other - NASA Tech Brief
Authors
Toda, Risaku
(California Inst. of Tech. Pasadena, CA, United States)
White, Victor E.
(California Inst. of Tech. Pasadena, CA, United States)
Manohara, Harish
(California Inst. of Tech. Pasadena, CA, United States)
Patterson, Keith D.
(California Inst. of Tech. Pasadena, CA, United States)
Yamamoto, Namiko
(California Inst. of Tech. Pasadena, CA, United States)
Gdoutos, Eleftherios
(California Inst. of Tech. Pasadena, CA, United States)
Steeves, John B.
(California Inst. of Tech. Pasadena, CA, United States)
Daraio, Chiara
(California Inst. of Tech. Pasadena, CA, United States)
Pellegrino, Sergio
(California Inst. of Tech. Pasadena, CA, United States)
Date Acquired
March 24, 2014
Publication Date
November 1, 2013
Publication Information
Publication: NASA Tech Briefs, November 2013
Subject Category
Optics
Solid-State Physics
Report/Patent Number
NPO-48665
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.
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