End-point detection of polymer etching using Langmuir probes

The adequate determination of the end point of a plasma-etching process is very important for integrated circuit fabrication. In this paper, the authors propose a new method, making use of the floating potential, as determined by a single Langmuir probe with a radio frequency (RF) choke. For the etching of a polymer film with an oxygen plasma using a reactive ion-etching system, this method yields a reproducible and reliable signal, which was successfully used to detect the end point for several wafers, it is better than the method using the DC self-bias voltage as the end-point detection signal, and approximately as good as when using emission spectrometry-at least when the resist area is larger than 4.4 cm²-whereas it uses a much cheaper equipment set. Langmuir probe measurements indicate that the floating potential changes are caused by several mechanisms: the average mass change, the plasma density, the average electron temperature, and the electron energy distribution all change after the end point of the etching.