Evaluation of Growth and Cleaning Rates of Chamber-Wall Deposition during Silicon Gate Etching

Abstract

To assure the repeatability of the gate etching process, in situ plasma cleaning is used to remove the depositions on the chamber wall. We studied the mechanism of deposition and cleaning of silicon containing films on the chamber wall formed during silicon etching using the attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). During the silicon etching with a Cl₂/O₂ plasma, a silicon-oxychloride film grows on the chamber wall due to the oxidation of the etch products. In the growth of the deposition films, the oxygen gas plays a key role. In a pure Cl₂ plasma the deposition is removed even while the silicon etching is proceeding on the wafer. It was also found that the deposition rate is square proportional to the O₂ gas flow rate. A simple kinetic model that attributes the square dependence to the consecutive oxidations of the two adjacent dangling bonds of a silicon atom has been described. Then the in situ cleaning of silicon-oxychloride with SF₆ plasma was investigated with regard to the cleaning rate. Although oxygen gas is often added to the cleaning gas so as to remove commonly seen residues such as carbon or sulfur, there is a concern that oxidation may reduce the cleaning rate. It was confirmed that an oxygen addition of 30 sccm to the SF₆ gas of 150 sccm causes insignificant effect. Further study to increase the cleaning rate revealed that lower pressure and larger total gas flow rate are effective. The effect of pressure is understood combined with the change in the plasma density. On the other hand, the total gas flow rate changes the density of etch products near the chamber wall. It was confirmed that the total gas flow rate does not change the etching rate of silicon-oxychloride on the wafer as it lies near the stagnation point of the gas flow. [DOI: 10.1380/ejssnt.2013.1]