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RADIATIVE PROCESSES AND DIAGNOSTICS 23 3 Radiative Processes and Diagnostics INTRODUCTION This chapter discusses gas phase and surface spectroscopic diagnostic techniques that can be used in plasma processing tools. As noted previously, techniques for measuring gas phase and surface quantities (species and energies) are essential for identifying the key chemical species in the plasma and for characterizing the chemical mechanisms that link these key species. Diagnostics are often best used in a complementary fashion with modeling: measurements help to test and validate models, and in turn, validated models help to augment the limited information provided by individual diagnostic techniques. For proper understanding and quantification, each diagnostic technique requires data on one or more physical quantities. Examples are discussed of sources of information on the physical principles of the various techniques, of sources on applications to specific systems, and of critically reviewed compilations of data on physical properties. It is shown that much can be done even when these sources do not provide all the information needed to plan or interpret a diagnostic experiment. TECHNIQUES FOR MEASUREMENTS OF GAS PHASE SPECIES There are various well-established optical techniques for the measurement of gas phase species in plasmas. Table 3.1 is a list derived from several recent review articles.1 These techniques are usually easier to apply at low pressure since lines and bands overlap less and so are easier to identify and analyze. Species identities are provided by spectral signatures (positions and shapes of spectral features), while absolute intensities of the spectral features can provide absolute concentrations. Often, linewidths (for atoms and small molecules) or band structure (for molecular species) can be related to translational and rotational energies or temperatures. TABLE 3.1 Optical Diagnostic Techniques for Plasma Processing Systems Gas Phase Surface Infrared absorption Reflection/absorption Ultraviolet/visible absorption Multiple internal reflection Electronic emission Emission Actinometry Ellipsometry Laser-induced fluorescence Reflectance difference Multiphoton ionization Photoluminescence Optogalvanic spectroscopy Optogalvanic spectroscopy Raman scattering Surface electromagnetic waves Stimulated Raman scattering Second harmonic generation Stimulated emission Photoacoustic absorption Laser-induced photofragment emission Photothermal displacement Third harmonic generation Photothermal deflection Particle scattering Laser desorption Mass spectrometry is also important, though not included in this chapter's discussion of optical spectroscopic techniques. The database needs for this diagnostic are associated primarily with interpretation of mass-resolved ion spectra that result from electron-impact dissociative ionization. The need for data regarding cross sections for electron-impact dissociative ionization, and especially the sensitivity of those cross sections to molecular internal energy near threshold, is addressed in Chapter 5, "Electron Collision Processes."
