Orientation of silicon wafers can now be calibrated

In electronic device production it is indispensable to know the precise orientation of the crystal planes of a silicon wafer. Chip producers therefore require certified wafers with such a declaration. To this end producers mark the wafers with so-called “flats” or V-grooves. The angular position of the marks is to be given in reference to a crystal plane with a measurement uncertainty of less than a few angular minutes.

As a result of its research work with silicon single crystals PTB has developed a procedure and a transfer standard to calibrate these off-angles. The transfer standard is a cylindrically polished silicon single-crystal of roughly 20 cm length. A flat mirror (flat) parallel to the (100) crystal plane was polished on the standard’s shell surface and a V-groove was milled on the opposite side. In the future the groove will serve to align the wafers for the process of chip production. The cost-intensive “flat”-method would no longer be necessary. Mea-suring the V-groove in reference to the crystal plane occurs in two steps. First, the position of the groove is determined with respect to the flat mirror by a coordinate measuring machine. Then, the angular position of the mirror with respect to the crystal plane is measured by x-ray diffraction in the PTB crystal-orientation apparatus (see PTBnews 01.3). The achieved measurement uncertainty lies in range of a few angular seconds and meets the technical requirements specified for the transfer standard.