5.8 Chip Size and Yield

As with any manufacturing process, a number of things can go wrong during chip production. The ratio of the number of packaged chips that function correctly to the total number of chips one started with on the wafers is called the final yield. This can vary widely (from 0 to nearly 100 percent) and is affected by defects on the wafer, problems during fabrication, unreliable wire bonds, improper handling, etc. Here we will concentrate on the intermediate yield at the end of the wafer fabrication process. This quantity is intimately related to chip size, as illustrated in Fig. 5.28. We assume for the sake of this example that in both cases shown in the figure the same defects (indicated by dots) occur at the same locations, and that one defect per chip is enough to render that chip nonfunctional. The defects are supposed to be due to fabrication, independent of the nature of the circuits on the chips. In Fig. 5.28a, where a small chip size is used, the yield is 87/94 ? 93 percent. In Fig. 5.28b, where the chip size is large, the yield is zero.

Figure 5.28: The effect of chip size on yield. The two wafers are assumed to have identical defects. ( a) Small chip size; ( b) large chip size.

The above simple example illustrates why chips cannot be made too large in a given technology. Given that the fabrication cost per (good) chip equals...