Tailored wafer holder for a reliable deposition of sputtered aluminium nitride thin films at low temperatures

Active actuated resonant micro-electro-mechanical-systems (MEMS) are used for sensing purpose like topography analysis and viscosity sensors. Those applications require straight beams and they rely on controlled film stress of the involved thin films, e.g. the active piezoelectric aluminium nitride (AlN) layer. The AlN consists of aluminium and nitrogen and is deposited with a reactive sputter process. The deposition process heats up the substrate and therefore the wafer bow of the substrate causes a variation of the thermal connection between wafer and sample holder. This goes along with undefined film stress of the AlN layer. In order to minimize the derivation of film stress, the reduction of substrate temperature and the enhancement of thermal connection between substrate and substrate holder is targeted. Therefore a novel clamped substrate holder is designed. High thermal connection to the ambient equipment, equal heat distribution and clamping of wafer stabilize the deposited AlN layer. By examining the layer stress and applying an acid structuring method, an improvement of deposited film is observed. A long term study with AlN deposition with thicknesses of 0.5 µm, 1.0 µm and 2.0 µm on silicon wafers was made to confirm the enhancement.