Plasma etching of poly-Si/SiO2/Si structures: Langmuir-probe and optical-emission-spectroscopy monitoring

An in-depth experimental evaluation is presented of the Langmuir probe as a tool for monitoring the etching of polysilicon-gate MOS structures with an inductively coupled high-density plasma excited in a mixture of SF 6 , O 2 , and Ar. It is shown that Langmuir-probe measurements made in parallel with optical actinometry offer ways of accurate end-point detection and etch-rate monitoring. It is established that the surface reactions involved lead to considerable changes in major parameters of the charged plasma species in the course of the process, affecting both the densities of electrons and ions and the electron energy distribution function (EEDF). It is found that the effective electron temperature and EEDF of the working plasma differ greatly from those of an Ar plasma under the same excitation conditions. It is concluded that wafer-surface charging could be minimized by varying the electron temperature and energy distribution. On the basis of optical-actinometry measurements, relationships are identified between the fluorine-atom density in the bulk plasma and the etch rate of the polysilicon or the oxide, taking account of the influence of ion bombardment. A conception is put forward of how the etch selectivity could be controlled.