X-ray topography is the general term for a family of X-ray diffraction imaging techniques capable of providing information on the nature and distribution of imperfections such as dislocations, inclusions/precipitates, stacking faults, growth sector boundaries, twins and low angle grain boundaries as well as other lattice distortions in crystalline materials of a wide range of chemical compositions and physical properties, such as semiconductors, oxides, metals, and organic materials. This technique is usually non-destructive and suitable for imaging single crystals of large cross-section with thickness ranging from hundreds of microns to several millimeters. The capability of in situ characterization during crystal growth, heat treatment, stress application, device operation, etc. is to study the generation, interaction and propagation of defects, making it a versatile technique to study many materials processes.

Synchrotron White Beam X-ray Topography

Synchrotron white-beam x-ray topography can be used to image the surfaces of the as-grown boules or large-sized crystal plates in reflection geometry. It can reveal the overall distribution of defects and distortion around the cylindrical surface of these crystals. Investigating as-grown boules enables observation of the true microstructures and striations developed during growth, and it can substantially reduce the time and process costs in cutting and polishing. Topographs could be recorded covering the entire length of the boule in strips by using the synchrotron beam.

Synchrotron x-ray topography in the reflection geometry also can be used to examine substrate/epilayer systems that have devices fabricated on them. The features that make up the device topology typically provide contrast on x-ray topographs. The contrast usually originates from the strain experienced by the crystal at the edges of growth mesas, or metallization layers, although some absorption contrast also may superimpose on this. Topographs recorded from such structures provide an image of the defect microstructure that may also be superimposed on the backdrop of the device topology. Direct comparisons can be drawn between the performance of specific devices and the distribution of defects within their active regions. This has made it possible to determine the influence of threading screw dislocations (closed and hollow core) on device performance. Back-reflection geometry is particularly useful here, because it gives a clear image of the distribution of screw dislocations on the background device topology that is imaged with sufficient clarity to unambiguously identify the device.

Synchrotron Monochromatic Beam X-ray Topography

When the synchrotron white beam is passed through a monochromator, an x-ray topograph is obtained when the crystal is set to the Bragg angle for a specific set of lattice planes for the selected x-ray energy. Images from different atomic planes are acquired by orienting the sample to satisfy the Bragg condition for those planes and orienting the detector to the new scattering angle to record the image. With monochromatic radiation, only one topograph is recorded at a time, but the experimenter controls the energy or wavelength of the x-ray beam, the x-ray collimation, the energy or wavelength spread of the x-ray beam, and the size of the incident beam on the sample crystal. Monochromators used at synchrotron radiation facilities can be either single-crystal or multiple-crystal designs, which can condition the x-ray beam to achieve optimal spatial and angular resolutions. X-ray topographs recorded in the transmission and grazing incidence geometry from a SiC single crystal show different types of dislocations. The presence of misorientations greater than a few arcseconds leads to the situation in which only part of the crystal fulfills the diffracting condition at a given time (i.e., Bragg contours are produced). These contours delineate those regions of the crystal that are in the diffracting condition from those that are not. Such contours can be used to obtain quantitative information on the distribution of lattice tilt and lattice strain across the wafer.

X-Ray Topographic Contour Mapping

When a single crystal is illuminated by monochromatic x-ray radiation of certain divergence, only a limited region will diffract. This is due to the existence of lattice deformation (effective misorientation) that deviates from the rest of the crystal from perfect Bragg condition by Δω, and thus, only a small region is accepted for diffraction. With a single exposure, a so-called equimisorientation contour can be obtained on the recording plate. Therefore, a contour map can be generated by rocking the crystal through the perfect Bragg condition with small steps of angular rotation and taking exposure at each step. According to Bragg’s law, for an arbitrary location in the crystal referenced to perfect lattice, the deviation from perfect Bragg condition locally is due to the convoluted effect of lattice dilation/compression. A set of + and – g contour maps can be used to deconvolute the lattice strain and tilt effect.

Synchrotron X-Ray Plane-Wave Topography

Rocking curve imaging can be considered to be a quantitative version of monochromatic beam x-ray diffraction topography that combines the advantages of x-ray topography and x-ray diffractometry (Ref 48–50). In this technique, a two-dimensional detector (charged coupled device, or CCD, camera) records the diffracted spot where each pixel of the camera records its own “local” rocking curve, so that several images (or maps) can be reconstructed by extracting data from these local rocking curves. In particular, maps of three parameters of interest for each rocking curve can be obtained:

• Angular peak position, which indicates macroscopic curvature and local lattice tilts
• Peak intensity
• Peak full width at half maximum, which indicates the local crystalline perfection

These maps provide a visual impression of the defect distribution and quantitative measures of lattice homogeneity.