Why In Situ , Real-Time Characterization of Thin-Film Growth Processes?
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key factors which must be considered when choosing in situ probes of thinfilm growth phenomena. In most cases, the sampling depth depends on the mean range of the exit species (ion, photon, or electron) in the sample. Techniques such as low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and ultraviolet (UPS) and x-ray photoelectron spectroscopies (XPS) detect 100-2,000 eV electrons which have a typical range of —5-40 A in solids. However, electrons also undergo significant gas-phase scattering which degrades the energy information and limits their analytical usefulness to high and ultrahigh-vacuum environments. Reflection high energy electron diffraction (RHEED) employs higher energy electrons (~20 keV) and may be used at pressures up to 10 6 -10' 5 Torr. Low-energy (several keV) ion-beam techniques such as ion scattering spectroscopy (ISS) and direct recoil spectroscopies (DRS) provide perhaps the most surface-specific information of any analysis method, but because they are relatively insensitive to multiple-scattering effects, the quality of the information is not seriously degraded by passage through a region of modest ambient-gas pressure. Methods which detect higher energy (MeV) ions such as Rutherford backscattering spectroscopy (RBS) and elastic recoil detection (ERD) are even less subject to gas-phase scattering, and may be used at pressures up to 1 atm.2 However the sampling depth increases to
0.5-2 /xm. A similar sampling depth is obtained for methods which detect x-ray photons, unless they employ a grazing exit angle to limit the depth of signal origin or grazing incidence to limit the probe depth. Finally, methods which employ visible light such as ellipsometry and interference spectroscopy are not surface-specific, but are useful probes of thin-film growth processes because they determine macroscopic properties such as film thickness, roughness, index of refraction, and growth rate. Methods which detect x-ray or visible photons can typically be used at almost any pressure. The techniques that are discussed in this issue of the MRS Bulletin (see schematics in Figure 1) have been chosen because they may be used for in situ, real-time analysis of film-growth phenomena in vacuum and in the presence of ambient gases resulting either from the deposition process or as a requirement for the production of the desired chemical phase. A second criterion for inclusion is that the instrumentation be sufficiently compact and inexpensive to permit use as a dedicated tool in a thinfilm deposition system. The article by A.R. Krauss, O. Auciello, and J.A. Schultz describes the development and application of low-energy (5-15 keV) time-of-flight ion scattering and recoil spectroscopy (TOF-ISARS) methods,3 which can provide a remarkably wide range of information on surface composition, atomic structure of the first few monolayers, lattice-defect density, trace-element analysis, phonon characteristics, and in some cases, the chemical phase of the growing film in thin-film deposition environment
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