Self-Aligned Formation of Porous Silicon Membranes Using Si Diaphragm Structures.

We report a new method of forming porous silicon membranes that is useful for micromachining applications. Using a p+ layer on an n-type silicon substrate, silicon diaphragms were formed by anisotropic etching with p+ etch stop characteristics. By choosing an appropriate polarity of current, local porosification of the membranes was achieved by anodic oxidation in hydrofluoric acid. Optical transmission was measured in the wavelength range from 400 nm to 900 nm. Transmittance of the membrane was calculated from reflectance and absorption coefficients using data from the literature. Quite reasonable agreement was obtained between the measured and calculated values, which confirms that the membrane had been converted uniformly to porous silicon.