Porous silicon is emerging in micromachining technology as an excellent material for use as a sacrificial layer. This is largely due to the ease of fabrication and freedom of design it allows. The rate of pore formation is heavily dependent upon the doping type and concentration of the silicon, allowing patterned porous silicon formation through selective doping of the substrate. Etch-rates above 10 mm min −1 have been reported for highly doped material. Silicon that has been made porous can be quickly and easily removed in a dilute hydroxide solution, as low as 1%. Porous silicon technology offers the unique ability to fabricate free-standing structures in single-crystal silicon with separation distances from the substrate ranging from a few microns to over one hundred microns. A review of the development of porous silicon for micromachining applications is given.

This paper presents a new wet etching technique for micromachining called single-step electrochemical etching for micro structures (SEEMS), which is based on electrochemical etching in hydrofluoric acid. During the etching, the shape of the etched structure can be controlled by changing the light intensity. Several problems involved in the SEEMS process had to be solved. The two main problems are, firstly, that over-etching can be seen at the clamping point of the free standing beam and, secondly, it is difficult to remove large areas. These problems can be considerably reduced by an improved mask layout and a perforated mass supported by a single cantilever can be achieved. In addition, a new initial pit formation is demonstrated to make structures which are free from crystal orientation of silicon substrate.

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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