Fabrication of 120° silicon double mirrors robust against misalignment for use in micro optical gyroscopes

• A novel concept to miniaturized ring laser gyroscope is presented.
• In this concept continuously controllable mirrors aren’t necessary.
• The key elements of this concept are 120° silicon double mirrors with fix angle.
• Silicon mirrors are fabricated and characterized.

This paper presents the fabrication of silicon double mirrors which are indispensable to realize an innovative concept for the miniaturization of an optical ring laser gyroscope (RLG). The 120° double mirrors provide a reduction of misalignment errors. Starting with the device layout, all major processing and manufacturing steps of the mirror elements are discussed. For the fabrication of these mirrors (1 0 0) silicon wafers are used, which are tilted by 5.3° towards the (1 1 0) plane, thereby ensuring that an etch flank of 60° is achieved by KOH wet chemical etching. A 36% KOH solution with addition of isopropanol is used to obtain uniform and smooth etched sidewalls for good reflectivity. For generating double mirror elements two structured wafers are connected by silicon direct bonding and then cut into small mirror elements to be mounted onto the RLGs micro optic platform. Roughness measurements on the mirror facets yielded values in the range of 20 nm for Ra and 60 nm for Rz. These roughness values are below what is considered as critical for the mirrors when used with laser radiation at a wavelength of λ = 630 nm. Optionally, the mirrors can be coated with a metal film to further improve the reflection properties. This novel micro mirror technology is extremely useful for the miniaturized optical gyroscope but may find other MOEMS applications where alignment inaccuracies can hardly be tolerated.

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