X-ray diffraction imaging (XRDI), which is also known as X-ray topography, is a technique that images crystalline defects in single crystal materials, such as dislocation, slip, precipitates and micro-cracks. The diffracted intensity is imaged with micron resolution; defects that introduce strain or tilt into the crystal lattice will diffract more strongly than the surrounding perfect material leading to changes in intensity in the image. In a transmission geometry, XRDI can image buried defects that are not imaged by traditional optical techniques that only image the surface of the wafer. Defects in thin epilayers can also be imaged using a reflection geometry.
Within the Si industry, a large issue is the breakage of wafers during processing. BedeScan can image wafers even through to metal deposition to monitor edge defects, chipping and other damage which may cause wafer failure during further processing.
It is possible to scan a full wafers or smaller areas to identify regions of interest. If the likely location of defects are known, then smaller regions can be identified and scanned (for example, edge only).
Once the full wafer is surveyed then detailed investigation at ultra-high resolution can be performed, as shown below.
The technique can be applied to any single crystal substrate, and is often used to identify substrate defect issues.
A typical BedeScan application is the characterization of SiC substrate quality and subsequent GaN growth. Shown below are SiC images in reflection mode, with the same sample after GaN deposition. Notice how the defects replicate from the substrate to the layer.
The D1 topography attachment allows reflection mode XRDI images to be collected on a D1 system. This is an ideal attachment to the standard D1 tool to enhance the versatility of the system.