Logo of the Physikalisch-Technische Bundesanstalt

X-ray Optics

Working Group 4.25

Crystal Orientation Device

This device, which has been developed and manufactured at PTB, serves to determine the orientation of large-volume single crystals. In the metrological application of silicon, the knowledge of this measurand represents the basis for the production of X-ray interferometers and wavelength monochromators. In the semiconductor industry, the indication of the orientation of wafers is necessary for building up epitaxic layers - besides, it is also of economic importance.

This procedure serves to determine the orientation of a certain lattice plane normal of the crystal lattice in relation to pre-defined geometrical directions on single-crystal samples by means of X-ray interferences. The orientation device is shown schematically. Its main components are an X-ray goniometer with a rotary table, a goniometer head, an X-ray tube and an X-ray detector, as well as an optical measuring system with an autocollimation telescope and a reference mirror. By swivelling the bracket holding the sample about the goniometer axis, the maximum intensity after each 90° rotation is searched until the lattice plane normal is oriented parallel to the goniometer axis. By means of optical methods, it is possible to determine the angular deviation of geometrical directions from this crystallographic direction.

In this way, the device, which is set up in an air-conditioned laboratory room (temperature stability: 0.5 °C), is used for the testing of the so-called "off-angle" between the wafer surface and the lattice plane on selected reference samples made of Si, Ge and GaAs. Depending on the application, the required precision of the method lies within a range between 1 and 25 angular seconds and is thus by more than one order of magnitude lower than the measurement uncertainties achieved so far with comparable measurement methods (see DIN-50 433).