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.
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