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SMART InspEction system for High Speed and multifunctional testing of MEMS and MOEMS

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Micro optical processing: LI

The fabrication of the first version of the Laser Interferometer array comprised high resolution electron beam lithography, preparation of adapted micro-lenses, and sophisticated assembly technologies.

The fabrication of the first version of the Laser Interferometer array comprised high resolution electron beam lithography, preparation of adapted micro-lenses, and sophisticated assembly technologies.

 

The optical set-up of the single laser interferometer channels are made of a sequence of three diffraction gratings forming a Twymann-Green configuration. The first grating having a period of 1┬Ám is coupling the laser light into the main interferometer path and directing it towards a second beam splitting grating. Both gratings do have a specially designed binary surface profile. The incoupling grating however, is composed of a sub-structure within one period with lateral feature sizes well below the wavelength of the used laser light. This special structure improves the coupling efficiency into the desired diffraction order considerably. The second grating spits the light equally into the measurement arm of the interferometer which contains the MEMS-surface to be characterized, and the reference arm. This reference arm is terminated by the third grating of 500nm period and covered by a silver coating acting as a retro-reflecting mirror.

The re-combined light of the two interferometer arms is then imaged onto the smart-pixel camera chip by a dedicated micro-lens replicated on top of the probing wafer.

The complete wafer consists of an array of 25 interferometer channels. It has been installed in the final instrument in September 2010.

Photograph of first LI probing wafer.

 

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