Versatile integrable reflective coatings on micro-mirror arrays for VIS and NIR spectral range

Inhalt:
Fraunhofer IPMS has a long-term expertise in spatial light modulator (SLM) consisting of micro-mirror array (MMA) operating in various spectral ranges from DUV to NIR. The device presented in this work consists of 512 x 320 pixel of 48micrometer pitch using a mirror based on a Titanium-Aluminium core and an Al-alloy reflector, both deposited by DC magnetron sputtering. While the pure reflector by itself is feasible to provide reflectivity close to 90% in VIS spectral range, it suffers from a characteristic drop at 800nm wavelength. Furthermore, specific applications might demand higher reflectivity values at certain wavelengths. To provide high reflectivity in VIS and NIR up to 1000nm, we developed Bragg stacks consisting of Al2O3/Ta2O5 and Al2O3/TiO2 multilayers, respectively. The process flow for the MMA is based on SiO2 sacrificial layers later released by hydrogen fluoride vapor on chip level. While Al2O3 is a good barrier against HF, Ta2O5 and TiO2 is attacked. Thus, the process integration of the Bragg stack requires full sidewall coverage of the stack to keep its mechanical stability and avoid delamination. In this work we describe the single process for all three materials and show their optical properties. Further reflectivity data, experimental and simulation, are shown for both stacks and compared to pure Al-alloy. Additionally, we describe the process flow for the integration of the stack into the standard process flow.