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A new electrolyte is presented for the electrochemical formation of porous silicon. The electrolyte, an ammonium fluoride etch mixture (AFEM), is used instead of hydrofluoric acid (HF) which is normally used for porous silicon (PS) formation. The main advantage of AFEM is that it attacks aluminium at a significantly lower rate than HF thus allowing porous formation to be performed as a post-processing step, without the need to protect aluminium. This method has been applied to an epi-micromachining process where the epitaxial layer is used as the structural layer and a buried region of the p-type substrate is made porous (the sacrificial layer). Initial results are presented, with comparisons made to HF-formed porous layers, illustrated by SEM images of epi-micromachined structures.

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