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Orientation of silicon wafers can now be calibrated

A new procedure – developed at PTB – makes it now possible to reference the crystal orientation of silicon single crystals to industry. This will facilitate the alignment of wafers and reduce production costs.

The new transfer standard with polished mirror (Flat) inside PTB's Crystal-Orientation Apparatus

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.

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