Stress investigation of PECVD dielectric layers for advanced optical MEMS

Detailed stress investigations of silicon nitride and silicon dioxide defined by PECVD are presented. The spatial variation of the stress of the dielectric material is evaluated by both, laser induced diffraction imaging method (macroscopically averaged stress) and by a large number of laterally distributed MEMS structures on the wafer (microscopically detected stress). By changing the duty cycle of two different plasma excitation frequencies during the deposition, the stress of silicon nitride (deposited at 300 °C) is controlled in a wide range from +850 MPa (compressive) to −300 MPa (tensile). A similar dependence is also observed for silicon nitride deposited at 60 °C. In contrast, silicon dioxide shows in both cases no strong frequency dependence. The microscopically detecting involves a low cost MEMS technology based on photoresist as sacrificial layer. Applying this technology, differently shaped Fabry–Pérot filter membranes with various stress values are implemented. The cavity length of the filters and the radii of curvatures of the upper DBR membranes are also varied. Concave, convex and flat membranes with radii of curvature of −0.31 mm, 0.19 mm and −184.71 mm, respectively, are produced.