Reactive ion etching in CF4/H2 of silicon nitride deposited by either plasma-enhanced chemical vapor deposition (PECVD) or low pressure chemical vapor deposition (LPCVD) has been studied. The steady-state etch behavior of the two kinds of silicon nitride is very similar, e.g. the dependence of the nitride etch rate on the H2 fraction in the gas mixture. However, in situ ellipsometry shows that silicon nitride formed by PECVD exhibits a much greater and longer initial etch transient (---110 s) than LPCVD Si3N4(~20 s) if CF4/H2 gas mixtures with a high proportion of H2 are used. The strong etch rate transient seen for PECVD Si3N4 limits the ability to stop abruptly on this material relative to that of LPCVD Si3N4. © 1991, The Electrochemical Society, Inc. All rights reserved.