usage. Wafer cost depends on wafer shaping, especially wafer thickness that increases for the larger diameter. 450 mm Silicon: These are inevitable factors that push up wafer cost/area. Historically, silicon suppliers have An Opportunity and Wafer Scaling developed new growth technologies to meet industry demand and to maintain competitiveness. However, by Masaharu Watanabe and Scott Kramer the 450 mm case will be different. The expected R&D cost to develop a 450 mm wafer pilot production fab will be so large that some silicon oday, we take our favorite music challenging as complexity increases, suppliers will hesitate to start 450 mm and movies anywhere. It is business models diverge, technology R&D. Recent poly silicon shortage Ta result of the never-ending advances, and economic and market by strong solar cell demand and reduction in bit cost brought about dynamics come into play. For the the resulting increase in price9 will by the accelerating development of next wafer size transition, it will be certainly push up the R&D cost. semiconductor devices and their particularly important for the factory Cooperation among industry and production technology. The well- architecture to be adaptable across all research institutes, or any kind of known Moore’s law is a core of the business models in IC manufacturing: joint work in the pre-competitive International Technology Roadmap DRAM, high volume microprocessors, area, will help to reduce the industry’s of Semiconductors (ITRS)1 that foundry, and very high mix logic. R&D cost. Some discussions are serves as a technology target for the Based on historical production already occurring in the industry.6 semiconductor industry. An increase wafer development cycles, research in silicon wafer size is embedded in and collaboration must start early Technical Opportunities it for more efficient and economical enough for the industry to achieve production of devices. Silicon as consensus on a wafer strategy to 450 mm crystal growth.—450 mm a key material2 of the electronics ensure availability of supply when crystals are estimated to be so big (215 industry undergoes progress in needed. Industry dialogue, data cm) and heavy (800 kg) that their defect engineering and production collection, and analysis of timing, growth will have many challenging technology. The ITRS indicates that risk, and time to ROI, as well as issues,3 as shown in Fig. 1. The SSi the next generation wafer size, of technical challenges and initial wafer, project was pioneering work, making 450 mm, will be in production in have been initiated. This dialog will the issues clear and showing the 2012. In the 450 mm era, silicon provide the information needed to feasibility of a 400 mm crystal.5 wafer suppliers will grow a larger continuously evaluate and adjust Learning involves a so-called Dash and consequently heavier single plans across the industry. In addition, neck growth that will be discussed crystal ingot than ever before, 215 cm economic analysis and factory in detail in the following section. long and weighing 800 kg.3 At the simulation are even more important. The other is the design of a crystal beginning of the 300 mm era, there Fortunately, in the years since the 300 puller and growth process. Practical were intense discussions4 related to mm transition, modeling software simulation is inevitable to develop inexperienced 300 mm crystal and has increased dramatically in terms of 450 mm crystal technology since wafer technology. These discussions function and speed. That software has experimental works are quite limited also provided useful information the capability to model very complex due to a weeklong growth cycle and beyond the 300 mm technology. interactions in a short period of time. high cost. Sophisticated experimental Super Silicon Crystal Research design, based on intensive simulation, Initiative Corp. (SSi) pioneered Economics of 450 mm Wafers will be extremely useful and is the growth of a 400 mm crystal.5 really key for the 450 mm R&D. For How the 450 mm crystal will come The economic situation is the point defects, such as vacancies and to production, and whether it is most important concern in the interstitials, their control is a most economically feasible, is now a hot semiconductor industry. Moore’s law, competitive part of crystal growth topic in the silicon industry.6,7 The in the form of “more function by technology. The expected narrowing first 450 mm wafer was displayed same cost,” drives our technology. of choice of growth conditions10 to at SEMICON West in 2006, which One scenario to reduce bit cost, balance vacancies and interstitials in sparked industrial interest. This by employing the next generation the 450 mm crystal growth will be article presents a brief description of large diameter wafer, is that wafer overcome by the improved thermal emerging 450 mm wafer technology and device production equipment design crystal puller.11 Published data issues. prices do not increase as wafer of SSi’s 400 mm crystal12 show that In the last 300 mm transition, the area increases; and production both vacancy rich and interstitial rich industry, for the first time, held global throughput does not significantly crystals could be grown by a choice discussions to synchronize efforts to decrease while keeping the same of growth rate. This encourages 450 reduce cost and facilitate a smooth device yield. It worked well in the 300 mm crystal engineers to optimize hot transition. For the coming 450 mm mm transition; but when and what zone (crucible, heater, heat shield, transition, more collaboration and device production will be converted etc.) design and growth rate. deep discussion on the transition from 300 mm to 450 mm, and the Neck issues.—All silicon crystals for scenario will be indispensable. investment needed, are not yet clear. LSI are dislocation-free, that is, the The production of 450 mm wafers whole of the grown crystal ingot of a Lessons Learned from 300 mm is economics driven and fab cost is few 100 kg weight is completely free the largest economic factor. Single of dislocations. At the beginning, the The 300 mm transition clearly crystal cost depends on crystal Dash neck, which is a few millimeter exposed the need for industry growth conditions such as the slower in diameter thin single crystal, is collaboration in all aspects of a growth rate of a 450 mm crystal, grown to eliminate dislocations. The transition to the next wafer size. Each compared to current 300 mm crystal whole of the crystal is sustained by wafer size transition becomes more growth, and lower poly silicon the Dash neck, no matter how large 28 The Electrochemical Society Interface • Winter 2006 impose intense work for the production of 450 mm wafers. Wafer shape near the edge, especially flatness, will be a key issue. Scaling Along with Moore’s law, many scaling factors are compiled in ITRS. The current wafer diameter trend is 450 mm production for 2012 and beyond. A wafer property that is particular to the wafer diameter is the wafer thickness. Thickness and others factors related to 450 mm are considered here from the scaling point of view.6 Crystal Growth Scaling Major silicon suppliers participated in the SSi project, which pointed out issues associated with a large crystal growth. Based on their experience, thermal and mechanical design of a FIG. 1. 450 mm silicon single crystal growth issue. Poly silicon (1000 kg) is melted in quartz crystal puller can be scaled up from crucible and a Dash neck of several mm is grown to start dislocation-free crystal growth. Growth the current 300 mm puller to a 450 of a more than 900 kg crystal will take 3-5 days. A new mechanism is required to sustain such a heavy crystal. About 800 kg can be used for wafer production. mm puller, using the latest simulator running on a powerful computer. Many things still need to be solved to and heavy the ingot finally comes. of 3 mm neck is not enough even start real production. SSi grew a 400 Dash neck strength is roughly 144 for the current 300 mm crystals. An mm crystal at the rate of 0.45 and 0.25 kg/3 mm-diameter-neck.13 In the 450 alternative to the two-step growth mm/min.12 A 450 mm crystal will take mm case, and even the 400 mm case is a thick Dash neck. Industry grows 5 to 8 days to grow, including poly of SSi, the neck is not expected to be current dislocation-free crystals with silicon melting, growing the crystal, able to sustain a heavy crystal. Thus, a thicker neck. Developing a thicker cooling, and preparing the puller for some breakthrough was required. (~9 mm) neck is, though it is not easy, the next growth. SSi developed a two-step growth a possible alternative solution for the technique.12 A Dash neck was first 450 mm crystal. Wafer Thickness Scaling grown to eliminate dislocations. Then 450 mm wafer shaping.—Wafer a second suspending cone was grown shaping is an extension of the current Wafer thickness should be to support the heavy crystal weight technology. Wafer specification, considered carefully. Once it is by the other mechanics. A special like flatness, links to lithography decided, it is difficult to change tool, called a clamp or tray, held the technology and becomes tighter and afterwards. Many device production cone and sustained the weight of the tighter. So, both large diameter and technologies and much equipment crystal body afterward. The strength tight control, as shown in Fig. 2, are wafer thickness dependent and a change of wafer thickness has a major impact. Thickness factors to be taken into consideration include: empirical extrapolation without clear reason; slip generation due to thermal stress; sag due to wafer weight and any elastic deformation due to film stress; and wafer breakage due to wafer handling, thermal stress, and others.