Formation of wide and deep pores in silicon by electrochemical etching

Electrochemical etching of n-type silicon in hydrofluoric acid electrolyte is now well known as a technique for micro- or macroporous silicon formation. It is commonly admitted that the width of pores can extend over four orders of magnitude, from 2 nm to 20 μm. In this study the feasibility of using this technique to form larger pores is demonstrated. The use of a water–ethanol solvent mixture (1:1) is shown to modify the electrochemistry of silicon dissolution and pore formation. The formation of stable wide pores requires adjustment of the etching current during the pore formation as a function of the evolution of the current–voltage curve with etching time. An array of 42-μm wide pores with 2-μm wall thickness and 200-μm depth were etched using this method. The feasibility to etch pores up to 100 μm in width is also presented. The results enable to conclude that the electrochemical etching of n-type silicon could be used to form vertical structures, without restrictions concerning the wall spacing. This provides a useful tool for micro-machining.