Residual strain effects on large aspect ratio micro-diaphragms [capacitance transducer]

The authors describe techniques that result in low-strain materials and that reduce the effects of residual strain in microdiaphragms, which are used in capacitive-readout sensors. Square polysilicon grilles and perforated diaphragms made from both single and double polysilicon layers and from single-crystal silicon, with aspect ratios (side/thickness) of up to 1000 and very low compressive strain (about 6×10^-5), have been fabricated. Strain reduction is achieved by combining thermal annealing with one of two mechanical design techniques. The first technique makes use of a series of cantilever beams to support the diaphragms. In a second procedure, corrugated surfaces, in thinned membranes of single-crystal silicon are formed. The corrugations result from the use of boron doping and anisotropic silicon etching. In both of these techniques to produce low-strain diaphragms, an etched cavity is purposely formed in the substrate crystal below them. Only one-sided processing of wafers is used, thus aiding reproducibility and providing ease of compatibility with a metal-oxide-semiconductor process. A fast etching sacrificial-support layer (phosphorus-doped chemical-vapor-deposited oxide) is used