Investigation of the Electrochemical Stop Etching of Silicon Upon the Fabrication of Cantilevers

Anisotropic etching in potassium hydroxide (KOH) solution with electrochemical stop is commonly used in the formation of thin silicon membranes of sensitive elements in various microelectromechanical devices. However, there are almost no data on investigating the process of the electrochemical stop etching of elements for p-n junctions formed on a relief silicon surface. Here, the process of electrochemical stop etching in KOH solution for structures using n-silicon layers formed by diffusion on a p-silicon substrate with smooth and relief surfaces is studied. It is found that etching stops at the p-n-junction boundary when using a two-electrode circuit with a positive (relative to the solution) voltage applied to the n-type layer for a structure with an n-type layer formed on a smooth surface. As a result, a membrane, which is reasonably uniform in thickness over the entire area of wafer, manages to form. When using a two-electrode circuit for a structure with a relief surface, the current density is 70-100 times higher during etching than in the case of a smooth surface; etching is nonuniform over the wafer and stops long before achieving a p-n junction. It is found that etching stops at the p-n junction for a structure with a relief surface when using a circuit with two voltage sources and an additional negative (relative to the solution) voltage of -2.0 V applied to the p-type substrate. In this case, it is possible to obtain a silicon membrane, which is fairly uniform in thickness over the entire area of the wafer. The dependences of the density of current between the n-type layer of silicon and the solution on the etching time are obtained for structures with smooth and relief surfaces, which make it possible to determine the end of the etching process.