feature size, made possible by lithography equipment that can produce ever-finer lines. Size reduction has accounted for nearly half of productivity growth. Yield improvement, too, has contributed a sizeable fraction, growing at 4 to 5 percent annually for a long time. Yields on integrated circuits are now above 90 percent, however, and these are achieved six to nine months after opening a new factory. Therefore, yield improvement will not be able to contribute much more to productivity growth. Wafer size will improve further, he said. Wafer diameter has risen by a factor of 10 in the last 30 years. Engineers in Japan are experimenting with 450-mm wafers, “which is the size of a large pizza.” It is a challenge to hold 1-micron uniformity on a wafer that large. The rest of the productivity gains result from other equipment changes and better processes.
The interesting questions, said Dr. Spencer, are what will happen if that productivity growth begins to taper off and what can be done to perpetuate the growth. He showed a graph, used by the Dell Company, of cumulative DRAM production versus cost—DRAM prices per bit—in real dollars (Figure 6). It