Silicon Wafer Backgrinding

Wafer Thinning Options

There are various methods presently being used for thinning wafers, with the most popular being mechanical backgrinding (BG) and polishing technique.

This process is preferred because it is faster and less costly than the newer chemical or plasma etching processes. However, it does have the disadvantages of applying mechanical stress and heat during the grinding process, and of causing scratches on the backside of the wafer. These scratch patterns and the depth of the scratches on the surface of the wafer are directly proportional to the size of the grit and the pressure exerted on the wafer during the grinding process.

The depth of the scratches and the backside surface roughness of the semiconductor die have a direct correlation to the strength of the die, so it is critical that the finished backside surface of the wafer be as smooth (or polished) as possible.

The Backgrinding Process

Figure 1. a) The BG process leaves a characteristic scratch pattern on the back of the wafer. b) The back of the die from certain locations on the wafer have a primarily vertical pattern of scratches.

Scratches and Wafer Strength

After BG, the wafer will exhibit a scratch pattern on the backside (Figure 1a). The depth of these scratches will depend on the size of grit of the wheel and the amount of vertical pressure applied during grinding. (finer grit results in smaller and shallower scratches.) Because the strength of the silicon is inversely proportional to the depth of the scratches, it is important to minimize the roughness of the wafer surface.

To improve the productivity, a multi-step grinding operation is generally performed. Step one uses a large grit to coarsely grind the wafer and remove the bulk of the excess wafer thickness. Step two uses a finer grit to polish the wafer and to accurately grind the wafer to the required thickness.

For wafers with diameters of 200 mm, it is typical to start with a wafer thickness of roughly 720 µm and grind it to a thickness of 150 µm or less. The coarse grinding typically removes approximately 90 percent of the excess material. A typical two-step backgrinding operation will use dual spindles with grinding wheels mounted on each spindle.