Semiconductor Technology from A to Z

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.

Sawing

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.

Lapping

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.

Etching

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.

Polishing

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.

The manufacture of integrated circuits on silicon wafers started in the mid 1960s on wafers with a diameter of 25 mm. Nowadays, in modern semiconductor manufacturing wafers with a diameter of 150-300 mm are used. By 2012 the mass production of microchips on wafers with a diameter of 450 mm is expected; prototypes have already been produced for research purposes. The wafer surface is then increased by more than 300-fold of the tiny 1-inch wafer 50 years ago, whereas the disk diameter was only increased by a factor of 18.

With larger wafers, the throughput rate increases significantly in the manufacture of microchips, whereby the cost is reduced accordingly in the production. Thus, with identical structure sizese more than twice as many chips can be produced on a 300 mm wafer as on a 200 mm wafer. In addition, with increasing diameter the wafer's edge is less curved and thus the cut-off minimized (since chips are off rectangular shape).

Typical data of wafers:

Different sizes of wafers: 25, 38, 51, 75, 100, 125, 150, 200, 300, 450 [mm] (scaled)

Wafers are fabricated in a round shape although the final microchips are rectangular. Therefore there is always some blend on the wafer - some area where no entire chips can be placed and which has to be discarded at the end of the semiconductor manufacturing.

After the description of the two manufacturing processes - the Czochralski process and the Float-zone process - this procedure is understandable.

A silicon wafer for microchip fabrication needs to be in single crystalline shape. This is only possible by using the two mentioned techniques which deliver a round wafer.

Even if it is possible to cut the round single crystal into rectangular shape afterwards (e. g. by sawing) the round wafers have several advantages over an angular shape.

• Straightening the round silicon causes additional stress to the crystal, leading to defects and dislocation and finally a worse quality of the silicon
• Round wafers are more stable, angular wafers could hardly be transported and processed without damage
• A homogen processing during microchip manufacturing with radially symmetrical processes (CPM, etching, spin on) is much easier
• Also on angular wafers a small area on the extreme edge would have to be discarded since the processing can't be done to the outermost edge. The wafers need to be clamped during transport and layers would spalling off if reaching to the edge

With increasing wafer surface the blend can also be reduced.

Rectangular wafers, however, can be found in solar cell manufacturing. In general polycrystaline wafers are used which can be poured in a rectangular form. The manufacturing is relatively easy compared to microchip fabrication, so that angular wafers can be used; the edges can also be chamfered.