RESEARCH OF MASKING PROPERTIES OF SILICON OXIDE FILMS FOR SILICON MEMBRANE FABRICATION BY WET ETCHING
Abstract
Microelectromechanical sensors of the membrane type are fabricated by surface and bulk micromachining. In the latter case, the membranes are obtained by deep anisotropic etching of a single-crystal silicon layer or substrate to a thickness of 20–50 μm. In this case, both dry and wet etching methods are used. The advantage of wet etching is easy control of the lateral dimensions of the membranes and high selectivity. High selectivity of etching can be achieved due to the choice of the appropriate composition of the etching solution, the material of the protective coating and fabrication techniques. The paper presents an experimental study of the protective properties of silicon oxide films obtained by thermal oxidation, plasma-chemical deposition, and combined coating of these films under wet etching of single-crystal silicon in a 30% aqueous solution of potassium hydroxide at a temperature of 80°C. The etching selectivity, residual thickness, roughness, and surface concentration of local defects were calculated using data of stylus profilometry, optical interferometry, and microscopy. It was found that the rates and selectivity of etching of thermal oxide and plasma chemical oxide after rapid thermal annealing are quite close – 6,7 nm/min, 1:338 and 7 nm/min, 1:372, respectively. The surface roughness of the oxide films increased more when etching the thermal oxide films, as well as the plasma oxide of composite coating. The root-mean-square values of the residual roughness were 1–2 nm. Local defects of the etched alike with a concentration of 0,1–0,2 mm-2 were found in the films. It was found that the use of a 1 μm plasma oxide layer in a combined coating prevents etching of the thermal oxide, but to avoid local defects, its thickness should be increased to 1,5–2,0 μm; an annealed film of plasma oxide, with a thickness of 2,0 μm, can also be considered as an effective protective coating for deep wet etching of silicon.
