Development of crystallized thick, highly doped a-Si:H layers for surface micromachining of MEMS inertial sensors

Abstract This study evaluates the viability of crystallized thick hydrogenated amorphous silicon (a-Si:H) films as structural materials for MEMS devices. The research aims to assess the electrical resistivity, mechanical stress, crystallinity, and surface roughness of a-Si:H layers deposited with varying hydronated phosphine (PH 2 : H2) flow rates and annealed at different crystallization temperatures. Methodology involves the deposition of 15–21 µ m a-Si:H layers on p-type silicon (100) wafers with different SiO x coatings using plasma-enhanced chemical vapor deposition (PECVD) processes with PH 2 : H2 flow rates of 100 sccm, 200 sccm, and 500 sccm. Post-deposition, the layers undergo controlled crystallization through annealing at temperatures ranging from 650°C to 900°C. Characterization techniques include wafer bow measurements, x-ray diffraction, electron backscatter diffraction, atomic force microscopy, and electrical sheet resistance measurements. The results show that the use of PECVD-SiO x layers instead of oxidation-generated SiO x under the silicon layers prevents the formation of blisters. Compared to epitaxial grown polycrystalline silicon (epi-poly Si), the crystallized a-Si layers exhibit significantly reduced surface roughness and lower resistivity. In addition, it was found that a well-chosen PH 2 : H2 flow rate supports the deposition rate, the growth of the Si crystallites and reduces the mechanical stresses within the layers. However, excessive PH 2 : H2 doping, leads to an increase in the resistivity. In conclusion, it can be stated that tailored deposition and annealing processes can optimize the a-Si:H properties, so that they become suitable for advanced MEMS applications.