BSTJ Briefs: Electrochemically Controlled Thinning of Silicon

Electrochemically Controlled Thinning of Silicon By H. A. WAGGENER (Manuscript received December 31, 1969) A method for precision thinning silicon integrated circuit slices has been developed whereby either n or p type regions may be selectively removed from material of opposite conductivity. The existence of a simple and economical means to attain precise thickness control permits more complete advantage to be taken of many silicon IC structures. For example, precise thickness control, together with anisotropic1 etching of isolation/separation slots, is expected to permit economical fabrication of high component density, air-isolated monolithic2 integrated circuits. This method differs from previous electrochemical techniques'1 in that unwanted silicon is removed chemically, while the regions to be retained are passivated electrochemically. Accordingly, etchants are used for which silicon to be retained is passive when biased above some critical voltage, VpnsB, while regions to be removed are at a potential below VPOBB. Hot aqueous alkaline solutions form a useful class of etchants for this application, for orientations other than (111). These etchants are characterized by a relatively sharp active/passive transition (Voen = VpaBB 0.5 volt) and by a large ratio of silicon etch rates between the active and passive states. Ratios of greater than 200 : 1 are readily obtained. The ratio of active etch rate/passive etch rate is very important, because this quantity in part determines the thickness uncertainty.