by M. David Levenson, Senior Editor
The International Technology Roadmap for Semiconductors (ITRS) plots a route to 45nm half-pitch through water immersion lithography for critical levels, but customers of ASML have requested an alternative — one that employs familiar dry DUV exposure and the double pattern method being considered for 32nm. The company has responded with the XT:1450G, a version of its familiar 0.93NA Twinscan system speced to print 143 wafers/hour at 40nm double-patterning resolution.
Of course, there is a trick — the wafers must be sent through the exposure tool twice, each time printing one of two >80nm half-pitch patterns. Getting the 4nm overlay required for the summed geometry means making sure that the wafer sits in the same chuck during both exposures. If the chuck selection is random, the overlay spec goes to 6nm, reports Bill Arnold, chief scientist of ASML. If only one chuck is being used, the throughput drops to 65 wph/pass.
This double patterning scheme requires that the wafer be processed between exposures to develop the resist and transfer the first image into a hard mask layer. Thus the actual throughput of finished wafer layers will be less than 143 wph, but probably not much less than ~70 wph for optimized processes.
Such optimization would be appropriate for chips made in long production runs, such as flash memories, and the geometries used in flash may be less sensitive to the kind of errors that might occur in double patterning. The 4nm chuck integrity spec, for example, is quite adequate for layer-to-layer overlay, but consumes all of the 10% CD uniformity spec for a 40nm design. If double patterning is to be successful, gates and other structures where a 4nm CD spec applies will have to be printed entirely in a single exposure. While 70 wph throughput (for finished wafers) may seem slow compared with immersion lithography tools designed to print 45nm chips in a single exposure, the tool cost and operating expenses are likely to be lower as well.
The critical difference in cost-of-ownership may come down to mask costs. While one might expect that the two reticles needed to print a layer by double patterning would cost more than a single mask used for immersion, the total number of printable features will be the same. That means that the immersion-capable reticle will take about as long to write, inspect, and repair as both the double patterning reticles together. Since the k1 factor for immersion at 45nm half-pitch will be ~0.31 for immersion (at NA=1.35), vs. the XT:1450G operating at k1~0.43 with double patterning, the yield of the immersion-capable reticles is likely to be lower, and requiring more challenging OPC schemes. The net effect is that reticle costs may actually be lower for the double patterning method.
The ASML XT:1450G is expected to be available 2H07, at which point there may be a real horse race to reach 45nm. Will the dark horse on the dry lane pay off better than the favorite splashing through the water? Now’s the time to place your bets! — M.D.L.