Grazing-incidence X-ray Technique for Surface, Interface, and Thin-film Analysis

Two applications of grazing-incidence X-rays, namely the grazing-incidence diffraction for characterization of epitaxial films and depth profiling of residual stresses are described. In a grazing-incidence experiment, the detector is placed in a horizontal plane parallel to the film surface to record diffraction from lattice planes which are perpendicular to the surface. An example on the analysis of molecular-beam epitaxial CoPt, and CoPt films are given. In a stress/strain depth profiling experiment, values of strains and residual stresses averaged over different penetration depths beneath the film surface (or z-profiles) are obtained by asymmetric diffraction from crystal planes which are inclined to the film surface. The actual stresses at different depths under the surface (z-profiles) are extracted from the average stresses (z profiles) using inverse Laplace and numerical inversion methods. Examples of such profiles in sputtered MO films and in the Al203 matrix of an Al203/SiC composite are given. In conventional X-ray diffraction and fluorescence with large incident angles, an incident x-ray beam penetrates deep into a material. To limit the incident X-ray beam to the surface, grazing-incidence X-rays are needed. At an incident angle near or below the critical angle for total reflection, the incident beam is evanescent and penetrates only the top 100 A or less into the surface. l-2 There is also an enhancement in the intensities at the surface. At incident angles near to the critical angle, X-rays are enhanced by 2-4 times at the surface over the intensities in the bulk.2 Small penetration depths and intensity enhancement make possible the use of X-rays for the characterization of surfaces, buried interfaces and thin films. Applications of grazing-incidence X-rays for the determination of structural depth profiling,3 for X-ray reflectivity analysis of surfaces and buried interfaces,1-2 for X-ray ’ spectrochemical analysis 4-7 etc. have been reported in the literature. In this paper we shall describe two other applications: namely, grazing-incidence diffraction analysis (GID) of epitaxial films278-g and depth profiling of residual stresses. IO-13 Copyright 0 JCPDS-International Centre for Diffraction Data 1997 Copyright (C) JCPDS-International Centre for Diffraction Data 1997 GRAZING-INCIDENCE DIFFRACTION GID was originally developed by Marra, Eisenberger and Cho in 1979.’ In a GID experiment, the incident x-ray beam impinges onto the surface of a film at an incident angle of 1” or less, and the detector is placed in a horizontal plane parallel to the film surface to collect diffraction from lattice planes which are perpendicular to the surface (Fig. 1). GID data can be collected using a W20 (or radial) scan and/or an o scan.g In a 8/20 scan, both the film and the detector are scanned at a speed of 112 to record diffraction along a fixed direction on the substrate. In an o scan the detector is stationary at a fixed 28 angle, while the film rotates about its surface normal diffraction from lattice planes which have the fixed value of d-spacing. to record in-plane rtical lattice planes Fig. 1. Schematic of the Symmetric GID geometry. In symmetric Bragg diffraction, one-dimensional crystallographic information along the surface normal is obtained. In GID, two-dimensional information parallel to the surface is measured. Using both methods, three-dimensional crystallographic information can be obtained. An example of the application of GID to analyze CoPt3 and CoPt films on sapphire substrates is described below.14 CoPt3 and CoPt films of 1400-A thicknesses were grown by the molecular-beam epitaxial technique (MBE) from pure Co and Pt sources on sapphire (0001) single-crystal substrates. The deposition conditions were 600’ C temperature and lo-” torr vacuum. A previously analysis of the CoPt3 film from symmetric Bragg diffraction data showed that the (111) plane of the film was parallel to the (0001) surface of the substrate. In other words, CoPts [ 11 l] 11 Al203 [OOOl]. In this study, the o scan method was used to determine the in-plane epitaxial relationships between the films and their substrates and to detect single or multiple Copyright 0 JCPDS-International Centre for Diffraction Data 1997 Copyright (C) JCPDS-International Centre for Diffraction Data 1997 domains in the films. The Q/28 scan technique was used mainly to determine the longrange order parameter and the average domain size for COP@. Analvsis of MBE CoPt, Film on Sapphire The LO scan pattern for the CoPt, (220) reflections from the film is plotted in the middle of Fig. 2. As a reference, the pattern for the Al203 (2110) reflections from the sapphire substrate is also shown at the top of Fig. 2. The (o scan patterns are plotted in an angular scale of 10-8 so that the orientation of the film and the substrate can be compared directly. The Al203 (2110) peaks were sharp and were separated by a 60’ interval showing a threefold symmetry along the c axis of Al203. The CoPtj (220) reflections also had narrow and intense peaks with a 60’ separation. However, the CoPt3 pattern was displaced from the Al203 pattern by 30’. This indicates that the CoPt, axes were rotated by 30’ from the Al203 <2l_lO>axes or were parallel to the Al203 axes.