Real-Time Monitoring of Epitaxial Processes by Parallel-Polarized Reflectance Spectroscopy
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Monitoring of Epitaxial Processes by Parallel-Polarized Reflectance Spectroscopy
Nikolaus Dietz and Klaus J. Bachmann Introduction The engineering of advanced microelectronic circuits, optoelectronic devices, and integrated optical circuits requires precise control of the lateral dimensions and thicknesses of device features and of the stoichiometry and doping of epitaxial semiconductor regions. This is preferably achieved by real-time monitoring and control of the individual deposition and etching processes that constitute the processing sequence. The use of optical probe techniques for the real-time monitoring of etching and/or growth processes is favored because of their nondestructive character and their potential use in realtime feedback control. Some of these methods are ideal in monitoring the overall growth process and/or substrate temperature in industrial applications, requiring low cost and maintenance. For example, in situ reflectance-spectroscopy methods, such as dynamic optical reflectivity (DOS),1 spectral-resolved normal incidence reflectance spectroscopy (MRS),2 or pyrometric interferometry (PI),3 are successfully applied to various deposition processes and provide information on both the growth rate and the composition of the deposits. However, small changes in the reflectance (be-
MRS BULLETIN/MAY 1995
cause of chemical interactions at the surface of the films with the reactants supplied from the vapor phase) are of the order of 10~3 to 10~4 and are hardly observable with normal-incidence reflectance techniques because of the high reflectivity of substrate/film interface, which is typically of the order of 40%60% for many semiconductors. In order to increase the sensitivity to surface- and interface-related growth properties, alternative optical-observation methods such as reflectance difference spectroscopy (RDS),4,5 surface
photoabsorption (SPA),6,7 and spectral ellipsometry (SE)8,9 have been developed. Recently, we added to these methods a new optical probe technique, parallelpolarized reflectance spectroscopy (PRS), which achieves both high sensitivity to surface-chemistry processes and control of film thickness with submonolayer resolution. For example, we have shown that the real-time monitoring (by PRS) of the epitaxial overgrowth of Si by GaP, under the conditions of pulsed chemical beam epitaxy (PCBE), provides valuable insights into the mechanism of nucleation and follows the growth process with submonolayer resolution over thousands of angstroms of film growth.10-13
Model Considerations A schematic representation of PRS is shown in Figure 1. When a monochromatic beam of parallel-polarized (p-polarized) light (e.g., light which is polarized parallel to the plane of incidence) impinges on an isotropic substrate surface at the Brewster angle cpis, the intensity of the reflected light is zero for a dielectric medium and a minimum for an absorbing medium. In a simplified description, an isotropic dielectric substrate may be viewed as a second polarization element, crossed to the polari
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