Anisotropic etching of silicon in hydrazine

This paper contains a detailed discussion of the practical issues related to the anisotropic etching of single crystal silicon using a 5050 hydrazinewater solution. Characteristics of the etchant, etching reactor design, etch procedures, safety precautions, etch rate data for typical samples and appropriate etch-masks are among the topic discussed. The etching process is carried out in a atmospheric reflux reactor, continuously purged with nitrogen. The etch rate of (100) silicon at 115°C in this hydrazine solution is nearly 3 μm/min, which is much higher than that of ethylenediaminepyrocatecholwater (EDP) solutions. Silicon dioxide, silicon nitride and most metallic thin films, except aluminium, can be used to mask the etching process. The etch rate is reduced significantly in highly-boron-doped silicon; a boron concentration of 1.5 × 10²⁰ cm⁻³ practically stops the etch. The use of the hydrazine solution for micromachining thin silicon diaphragms, cantilevers and fibers is demonstrated.