High Resolution X-ray Diffraction (HRXRD)

High resolution X-ray diffraction has long been used in the compound semiconductor industry for the characterization of epitaxial layers. Traditionally it has been used in the determination of thickness and composition of the epi-layers, but more recently the technique has advanced to enable the determination of strain and relaxation within a given layer of a multilayer structure. The example data below shows a measured HRXRD scan – symmetric 004 reflection from a single layer on a bulk substrate.

Scan is typically done by scanning sample and detector in 1:2 ratio. The substrate peak is normally the sharpest and most intense feature in the scan. The position of the Bragg peak is determined by Bragg's law, i.e. the lattice parameter of the substrate, the wavelength of the X-rays. The layer peak that in this example is on the left hand side of the substrate peak is visible. The lattice parameter of layer is larger than that of substrate in this example, as from Bragg's law it diffracts at smaller angles than the substrate. The difference in peak position relates to the difference in lattice parameter. These can be related to composition, strain or relaxation of the layer. On either side of the layer peak there are interference fringes that result from interference of the wavefields in the layer. These features can be used to accurately determine the layer thickness.

Once the data have been collected, the analysis of the data is required to extract the parameters of interest. The most common method used by Jordan Valley customers, and widely used by other HRXRD experts, is using the JV-HRXRD analysis software (formerly Bede RADS). This software can either be used as off-line software by the user, or automatically as part of the programmed measurement cycle on the tool itself. An example of a GaAs HEMT structure is shown below.