Absolute fluorine atom concentrations in RIE and ECR CF4 plasmas measured by actinometry

Abstract

We have measured the absolute fluorine atom concentrations in electron cyclotron resonance (ECR) and reactive ion etching (RIE) plasmas by optimizing the actinometry technique. The major difference between this work and conventional actinometry is that the Ar concentration measurements were performed by a residual gas analyser (RGA). The emission intensities of F (7037 AA) and Ar (7504 AA) were simultaneously measured by an optical multichannel analyser (OMA) and the Ar concentration by a RGA. The F atom concentration at the wafer stage in the CF4 ECR plasma was measured to be (0.4 to 4)*1012 cm-3, the microwave power from 500 to 900 W, pressure from 0.5 (3.7 sccm) to 3.5 mTorr (58 sccm), and a fixed RF bias voltage of -50 V. The F atom concentration of the CF4 ECR plasma was four times larger in the source region than in the downstream region. The F atom concentration of the CF4 RIE plasma for pressures from 13 to 82 mTorr was measured as (0.8 to 4.2)*1013 cm-3, for a CF4 flow rate of 20 sccm, and power inputs from 250 to 1500 W. The F atom concentration was larger in the RIE etcher than in the ECR etcher, but the F atom production efficiency was eight times larger in the ECR etcher than in the RIE etcher for the same power level. In spite of the possibility of a factor two discrepancy in the measurements from absolute values because of the uncertainty in the absolute values of the cross sections, this technique provides a relatively simple and consistent reproducible measurement of F atom concentration to compare operation of the two types of etchers, which is reproducible to +or-10%. We further explored the actinometry for vacuum ultraviolet (VUV) emission region by using F (955 AA) and Ar (1048 AA). A similar trend of F atom concentration was found as for the visible actinometry, but the absolute value of fluorine atom concentration was typically 15% larger for the VUV actinometry.