In situ surface topography measurement of MOEMS structures under laser exposure

Spatial light modulators (SLM) developed at the Fraunhofer Institute for Photonic Microsystems (Fraunhofer IPMS) are based on arrays of tiltable micro mirrors on a semiconductor chip. Development and optimization of such complex micro-opto-electro-mechanical systems (MOEMS) require detailed knowledge of the device behaviour under application specific operating conditions. In this context, the need for a high resolution surface topography measurement under laser exposure (in situ) was identified, complementing ex situ characterizations where laser exposure and micro-mirror topography measurements are carried out sequentially. For this purpose an interferometric setup using the phase-shift principle was designed and is presented in this paper. For setup verification SLMs were irradiated at 248 nm (KrF) with energy densities of up to 10 mJ/cm2. In general, the setup is neither limited to a specific illumination wavelength nor to micromirrors as structures under test. Influences of different illumination parameters such as energy density, laser repetition rate etc. on the mirror topography can be studied in detail. Results obtained so far reveal valuable feedback for further technological optimization of mirror array devices.