Thin films are typically layered materials with a thickness ranging from a few fractions of nanometers to several microns. They have the capability to emphasize and even modify the physical characteristics of the corresponding bulk material has spurred significant technological interest.
The desired film properties can be obtained by producing the preferred atomic structure and morphology using various materials and deposition methods, including physical and chemical methods. For instance, it is possible to apply a tungsten coating in a polycrystalline form to prevent corrosion or deposit a SiGe film as a single crystal subsequent to the crystalline structure of the substrate (epitaxially) for use in semiconductor applications.
Reciprocal Space Mapping (RSM) coupled to X-ray diffraction (XRD) provides a non-destructive method to characterize the aforementioned parameters. RSMs typically require a series of several scans to map the substrate and film diffraction intensity in angular space and to optionally transform to reciprocal space for subsequent analysis.
A 2D detector can be used to perform these types of measurements easily and rapidly. The D8 family of diffraction solutions, in conjunction with the DECTRIS PILATUS3 R 100K-A hybrid photon counting (HPC) pixel detector, provides a unique 2D X-ray diffraction (XRD2) system that is suitable for multipurpose modern materials research characterization. This article presents the examples of RSMs obtained with this detector for thin film characterization.
Here, this technique was demonstrated by performing measurements on the following four samples:
- A randomly oriented polycrystalline tungsten film on Si substrate
- A preferentially oriented (textured) copper film on Si substrate
- An epitaxial GaAs film on a Si substrate
- An epitaxial 30% SiGe on Si substrate
Richard Matyi at Florida Polytechnic University contributed the GaAs on Si and SiGe on Si samples utilized in this experiment. A D8 DISCOVER coupled to a Cu target IµS micro-focus source, PILATUS3 detector, 300 µm collimator, and a parallel beam Montel optic was used to measure the samples. Two frames were acquired by setting the detector at a distance of 2.3 cm.
The coverage of Frame 1 was from 36° to 102° 2θ with the sample being rocked between 20° and 70°, whereas the coverage of Frame 2 was from 66° to 132° 2θ with the sample being rocked between 26° and 104°. It took 5 minutes for each frame collection, and the total data acquisition time was 10 minutes. The PILATUS detector’s high count rate capability eliminates saturation effects, even in the case of strong single crystal peak intensities. The ensuing 2D scattering was applied to generate an RSM and to assess the thin films.
The resulting XRD2 images were then imported into DIFFRAC.EVA, where a projection was used to merge the images together onto a cylindrical coordinate system. Many reflections are captured in a single frame covering a large fraction of the reciprocal space with one rocking motion.
The point-like high intensity reflections can be observed in all four images corresponding to the single crystal Si substrate. Figures 1 to 4 show the 2D RSM of all the four samples. Figure 1 shows the continuous conic sections at the expected locations for the tungsten film, indicating a random crystallographic orientation of the film.
Figure 1. 2D RSM of randomly oriented W film on Si
Figure 2 illustrates the Cu film diffraction rings with intensity differences along each ring indicating texture. Further analysis revealed a fiber texture that was along the  crystallographic direction toward the plane of the film.
Figure 2. 2D RSM of fiber textured Cu film on Si
Epitaxial films of GaAs on Si and SiGe on Si are illustrated in Figures 3 and 4, respectively. The horizontal alignment of the film reflections corresponding to the Si substrate reflections reveal the degree of relaxation of the film. This is a measure of how much the film’s lateral lattice parameters have been strained to match the substrate. Figure 3 shows the 2D RSM of the GaAs film, which is fully relaxed (no strain). Figure 4 shows the fully strained SiGe film. The horizontal broadening of the GaAs reflections implies a large degree of mosaic spread, whereas the sharp SiGe reflections are an indicator of the high crystalline quality of the films.
Figure 3. 2D RSM of relaxed epitaxial GaAs film on Si
Figure 4. 2D RSM of strained epitaxial SiGe film on Si
This information has been sourced, reviewed and adapted from materials provided by Bruker AXS Inc.
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