A new x-ray diffractometer design for thin-film texture, strain, and phase characterization

Abstract

Powder diffraction techniques are potentially extremely useful for the characterization of a variety of metallic thin films which are used as interconnection materials in very large scale integrated (VLSI) devices. Phase identification, texture determination, elastic strain measurement, and grain size distribution can, in principle, be obtained nondestructively. Although x-ray techniques have long been applied to bulk materials for these purposes, conventional x-ray equipment, particularly the widely used Bragg–Brentano powder diffractometer, is often unsuitable for use on these thin films. High-angle reflections are extremely weak, strong texture renders many reflections inaccessible, and reflections from the silicon single crystal substrate can be a serious interference. The Seemann–Bohlin focusing geometry with a fixed low angle of incidence provides improved intensities and reduced substrate interference, but is unsuitable for texture determinations or strain measurements. We have designed a unique instrument, called a generalized focusing diffractometer (GFD), which combines the intensity advantages of a focusing geometry with the flexibility necessary for texture and strain measurements. The key capability is the arbitrary setting of the incidence beam angle α, independent of the Bragg angle 2θ, which allows accessing of practically any desired set of Bragg diffraction planes in the sample. The focusing condition is achieved in such a geometry by computer control of the sample to detector (receiving) slit distance. Four distinct modes of operation are possible with the GFD: Bragg–Brentano (BB), Seemann–Bohlin (SB), texture analysis (TA), and strain analysis (SA). The BB and SB modes are conventional, except that the incident beam angle α, can be varied arbitrarily in the SB mode, allowing small 2θ values to be explored. In the TA mode, the sample is rocked through a range of incidence angles while the detector is fixed in 2θ, but continuously positioned relative to the sample for optimal focus. In the SA mode, profile scans of particular Bragg reflections are obtained at varying beam incidence angles while the focusing conditions are continuously maintained by detector positioning. Several examples illustrate the application of the GFD. Untextured powdered Si provides a comparison of BB and SB modes. A film of 5 nm of Au on a glass substrate with well-developed (111) texture further illustrates the differences between these modes, and indicates the sensitivity of the GFD focusing geometry. A sample consisting of alternating layers of 50-nm sputtered amorphous TiSi2 and 500 nm of polycrystalline Al–Si on a Si substrate is examined in the BB and SB mode both as synthesized and after two thermal cycles at 450 °C. The scans indicate well-developed texture in the Al–Si film and the thermally induced growth of silicide crystallites. A sample of highly textured 1000-nm Al sputtered on Si is examined in TA mode to demonstrate this capability. Finally, SA scans on a sample of highly textured 750-nm Al-1% Sm sputtered on Si have been used to determine the strain in the thin film. The results are compared with those obtained by utilizing the wafer curvature method of strain analysis.