Quantitative analysis of composition and temperature of semiconductor processing plasmas via terahertz spectroscopy

Abstract

The application of terahertz (THz) absorption spectroscopy was developed for chemical characterization in inductively coupled plasmas. Plasma processing is a complex and important tool of the semiconductor manufacturing industry, which makes use of several diagnostic methods for precise process control. Electronically based THz spectroscopy is a technique with favorable attributes for the characterization of plasmas and process control in semiconductor reactors. These attributes include (1) plasmas are transparent and noise-free for THz transmission/detection, (2) concentration and temperatures of molecules can be calculated from first principles without adjustable variables, and (3) the technique has very high resolution and has absolute specificity. However, rotational spectroscopy requires that the molecule have a permanent dipole moment, precluding direct observation of atomic and symmetric species such as fluorine or CF4. In this work, an electronically based 500–750 GHz absorption spectrometer and a method to accurately and simultaneously determine number densities and temperatures were developed. Density and temperature measurements of molecular species in Ar/CF4/CHF3 and N2/CF4/CHF3 plasmas as a function of flow ratio, power, and pressure will be discussed. In addition, a quantitative survey of spectroscopically measurable molecules and radicals was conducted for plasma mixtures using varying quantities of CF4, CHF3, N2, and O2 feedstock gases.