The Effect of an Interfacial P‐N Junction on the Electrochemical Passivation of Silicon in Aqueous Ethylenediamine‐Pyrocatechol

Abstract

The electrochemical passivation of silicon may be utilized as an etch stop to control the thickness of thin diaphragms and beams in silicon sensors fabricated by wet‐etch micromachining. A potential is applied during etching to an n‐type epitaxial layer on a p‐type (100) oriented single‐crystal silicon wafer substrate. The potential is sufficient to passivate the epi layer but is insufficient to passivate the substrate across the potential barrier presented by the interfacial n‐p junction. Etching can then proceed through the substrate to the interface, but is stopped at the voltage‐biased epi layer, leaving a structure whose thickness is that of the epi layer. To define situations where this may be employed, the electrochemical potentials required to passivate (100) single‐crystal silicon in aqeous ethylenediamine‐pyrocatechol etchant were measured both directly and across an interfacial junction for a range of silicon resistivities of both dopant types and for a number of n‐p epi/substrate combinations. There is a 2V range of potentials which may be applied in this way to preserve an n‐type epi layer without stopping the etching of the p‐type substrate. However, the method was not similarly applicable to preserve a p‐type layer on an n‐type substrate. In addition, etch rates were measured as a function of applied potential less than passivation potential.