Spatially Resolved X-ray Diffraction Technique for Crystallographic Quality Inspection of Semiconductor Microstructures

Abstract

There is an increasing demand for techniques that allow detection and visualization of crystalline lattice microdefects. From the many techniques available the X-ray Rocking Curve Imaging (RCI), based on synchrotron X-ray diffraction, has gained much attention recently as a method of wafer defect analysis [1]. However, for most crystal growers laboratory techniques are preferred. Therefore, instead of applying RCI we have developed a microimaging technique based on spatially resolved X-ray diffraction (SRXRD) that makes use of conventional high resolution X-ray diffractometer. Briefly, the size of incident X-ray beam is reduced by a set of slits. Then the sample is moved in small steps while the rest of the system stays fixed. At each sample position X-ray diffraction curve is measured from precisely defined area. Finally, all diffraction curves are collected to construct the X-ray diffraction image of the sample. Both the ω and 2θ/ω scans are performed allowing independent mapping of crystallographic misorientation and lattice parameter distributions across the sample. Spatial resolution offered by the SRXRD technique is smaller than that obtainable with the use of synchrotron radiation based methods. This is due to a limited radiation intensity of conventional X-ray sources. We have checked that in our experimental setup measurable signals can be obtained with the X-ray beam as small as 10/sinθ × 100 μm 2 , where θ is the Bragg angle for the reflection studied. This corresponds to the beam spot size of ~18 × 100 μm 2 for 004 reflections of GaAs and Si. The aim of this report is to present advantages of SRXRD over a standard X-ray diffraction when applied for analysis of crystalline microstructures. First, we focus on SRXRD studies of epitaxial laterally overgrown (ELO) GaAs layers grown by liquid phase epitaxy on SiO2-masked GaAs substrates. ELO layer consists of a few hundreds micrometers wide monocrystalline stripes regularly arranged on a substrate. Since each stripe is strained such samples contain a periodic distribution of strain and/or defects. Thus, they are very suitable to demonstrate potential of the technique. We show that high spatial and angular resolutions of SRXRD allow studying a strong effect of ELO wings tilt towards the mask as well as very fine residual strain caused by dopant concentration inhomogeneity. Direction of the tilt and the distribution of tilt magnitude across width of each wing can be easily determined. In fully overgrown GaAs ELO layers additional strain was observed at a coalescence front where ELO wings grown from adjacent seeds merged. With the use of SRXRD a local structure of the crystal lattice around this coalescence front was studied. In heteroepitaxial GaSb/GaAs ELO layers local mosaicity in the wing area was found. By SRXRD the size of microblocks and their relative misorientation were analyzed. As the final example, we report on strain distribution in thermally oxidized Si wafers. Microscopic curvature of lattice planes confined between two neighboring slip bands was measured allowing its correlation with the dislocation structure. It is noting worthy that all the phenomena presented are difficult to study by standard X-ray diffraction. On the contrary, due to its high spatial resolution the SRXRD technique allows deeper insight into a localized strain fields present in semiconductor microstructures.