At first the single crystal is turned to a desired diameter and then bedight with one or two flats. The larger, first flat allows an precise alignment of the wafer during manufacturing. The second flat is used to detect the type of the wafer (crystal orientation, p-/n-type doped), but is not always used. Wafers with a diameter of 200 mm or above use a notch instead. This tiny notches on the edge of the disk also provide an alignment of the wafer, but take up much less costly wafer surface.
- With an annular saw, whose cutting edge is filled with diamond splinters, the single crystal is sawn into thin discs = wafer. The saw provides a high accuracy during sawing without bumps. Up to 20 % of the crystal rod is lost due to the width of the saw blade. However, nowadays more often wire saws are used, in which multiple wafers can be cut at once from the staff. Therefore a long wire, which is wetted with a suspension of silicon carbide grains and a carrier (glycol or oil), is lead through rotating rollers. The silicon crystal is drained into the wire grid and thus cut into single wafers. The wire moves in counterstep with about 10 m/s and has a typically thickness of 0.1-0.2 mm.
Annular saw and wire saw
After sawing, the slices have a rough surface, and due to mechanical stress damages in the crystal lattice. For finishing the surface, the wafers pass several process steps.
- Using granular abrasives (e.g. aluminum) 50 microns (0.05 mm) of the wafer surface are removed on a rotating steel disc. The grain size is reduced in stages, but the surface is re-injured due to the mechanical treatment. The flatness after lapping is about 2 microns.
- Beveling of the edge
- In subsequent processes, the discs must have no sharp edges, as deposited layers may flake off otherwise. Therfore the bevel of the wafers are rounded with a diamond cutter.
- In an additional wet etch process, with a mixture of hydrofluoric, acetic, and nitric acid, 50 microns are removed. Because this is a chemical process, the surface is not damaged. Crystal defects are permanently resolved.
- This is the final step of surface refinement. At the end of the polishing step, the wafers do not have a bump of more than 3 nm (0.000003 mm). The wafers are treated with a mixture of sodium hydroxide NaOH, water, and silicon oxide grains. The oxide removes additional 5 microns from the surface, the hydroxide removes machining marks caused by the oxide grain.