UV-VUV Absorption Spectroscopy for Radical Density Measurement in Processing Plasmas
During plasma processes such as etching and deposition, the fundamental plasma parameters play the key role determining process yield and quality via controlling etching/depositing rate, uniformity, selectivity and anisotropy. Electron temperature and electron density are well known as fundamental plasma parameters which can be precisely determined by various diagnostic methods such as electrostatic probe. However, recently, radicals of small reactive atomic or molecular forms have been strongly attracted in processing plasmas because the most of plasma processes are influenced by chemical reactions between plasma and substrates. Especially, the measurement of radical density directly linked to etch or deposition rate is very important. Therefore, various non-invasive methods to measure radical density in the plasma have been studied. In this presentation, a spectroscopic method based on Beer-Lambert law calculating absorption intensity of external light is suggested to measure absolute density of atomic and molecular species generated in low-pressure inductively coupled plasmas. For the measurement of molecular species such as CF2, a broadband UV absorption spectroscopic methods is employed. Especially, in this work, a probe-type absorption spectroscopic system is suggested to investigate a spatial profile of CF2 density. On the other hand, a self-absorption VUV absorption spectroscopic method is evaluated to measure absolute density of atomic species such as hydrogen in the plasma. In this case, the effect of self-absorption by plasma lamp emitting VUV light is theoretically evaluated to improve the measurement precision. Based on UV and VUV absorption spectroscopic method, the absolute density of atoms and molecules for various plasma conditions are measured in low pressure plasmas and discussed the relation with plasma temperature and density.