A novel spectroscopic ellipsometer in the infrared

Abstract

Ellipsometry is a popular technique for surface and near surface analysis. Its main advantage is its non-intrusive nature and its high accuracy. Furthermore, with a suitable model applied, one can determine various parameters of the material such as the refractive index, the thickness of layered systems or the roughness. Infrared spectroscopic ellipsometry provides even more information, specially on the chemical composition of oxides and organic elements. However, application of this technique in the infrared became of interest only later, because of the lack of efficient sources or optical devices like polarizers or retarders. In this thesis, the development of a rotating compensator ellipsometer (RCE) working in the mid-infrared (2.5 to 12 ¹m) is described. The ellipsometer is associated to a fourier transform spectrometer. Compared to other ellipsometers, RCE offers several advantages that are highlighted in this study. The instrumentation, particularly the design of high-performance and broad band linear polarizers and retarders is detailed. Because of its high sensitivity, the calibration of the ellipsometer requires a lot of attention. The different sources of imperfection have been identified and a calibration procedure is proposed. We have demonstrated that the main cause of misalignment and imperfection is the source, a globar. Its weak intensity prevents it from being used as a point source and an intrinsic divergence of the beam can strongly alter the performance of the other optical devices. The ellipsometer has been used first for the analysis of silicon oxides and sub-oxides. In particular, the variation of the apparent transverse and longitudinal vibrational modes of the Si- O chemical bond groups with the thickness of the oxide has been monitored. The behavior of the dielectric function and the refractive index of the silicon sub-oxide as a function of the oxygen content has been investigated. We were able to demonstrate the transition from a semiconductor to a dielectric material. A model involving non-oxygen-saturated silicon tetrahedra has been successfully applied to explain the frequency shift of the main absorption peak of silicon suboxide towards higher frequencies. In parallel, a study has been carried out on the effect of non-thermal plasma on biological tissues. We exposed bone to a low pressure inductively coupled plasma. The modification and the etching have been observed with infrared spectroscopic ellipsometry as well as with ion beam analysis techniques (Rutherford backscattering spectrometry and elastic recoil detection analysis). Ellipsometry proved to be a suitable technique for biological tissues, by identifying mineral (hydroxyapatite) and organic (collagen amide groups) constituents. It has been shown that the selectivity of the plasma etching and modification processes can be monitored. The most important plasma parameters are the gas mixture and the discharge power. two different models of etching have been described involving either ion bombardment (argon plasma) or chemical etching (oxygen). These results could lead to further applications such as the treatment of dental cavities or the removal of tumorous cells on tissues.