The phase angle between surface and lattice planes of wafer disks is a crucial quantity for the semiconductor industry. It must be traced back to the SI units. To use silicon in metrology, for instance, to re-determine the Avogadro constant, precise determination of the phase angle is an important issue. For this purpose a new apparatus for high precision crystal orientation measurements has been designed and constructed at PTB. It has now gone into service.
The measurement method exploits X-ray beam interferences on single crystal samples to determine the orientation of a certain lattice plane normal in relation to defined geometrical directions. The main components of the apparatus are an X-ray goniometer with rotary table, goniometer head, X-ray tube, X-ray detector and an optical measuring system with an autocollimator and a reference mirror. The sample holder with the sample is rotated around the goniometer axis until, after rotating through 90°, a maximum intensity is found and the lattice plane normal is adjusted parallel to the goniometer axis. The angular deviation of geometrical directions from this crystallographic direction are then determined by optical methods.
The apparatus is set up in an air-conditioned laboratory. It is used to check the so-called off-angle between the wafer surface and the lattice planes of selected Si, Ge and GaAs reference samples. The accuracy achieved by this method depends on the specific meaurement task and ranges from 1'' to 25'' (angular seconds). It is thus more than an order of magnitude higher than the measurement accuracies achieved so far with comparable measuring methods (DIN 50 433).
For industrial applications such as semiconductor processing knowing the precise orientation of wafers is fundamental, i. e.: for the growth of epitaxial layers. It is of great economic relevance in this field.