Europe Developing Fab Techniques for the Next Millenium

Before the reunification of Germany, the economy of the East German Bundesländer was very weak. The support of the former West Germany has helped lift the economy of this region that now has one of the most advanced semiconductor fabs in the world plus leading associated suppliers. Although the Asian crisis has caused some companies to delay or "push-out" 300 mm efforts, it is not really a question of "if" but "when." In a talk at Semicon Europa earlier this year, George Lee, director of SEMI's 300 mm initiative, said that Siemens will be the first company to produce devices on 300 mm wafers (SEMI's 300 mm effort came to a conclusion at the end of July).

CNET (France Telecom, Grenoble, France) is processing 300 mm wafers in its R&D facility. Wacker (Burghausen, Germany) was one of the first wafer suppliers to grow 300 mm wafers. Soitec (Grenoble), which specializes in silicon-on-insulator (SOI) wafer production, has already supplied its first 300 mm SOI wafer samples to customers. Siemens/Motorola (Dresden, Saxony, Gemany) is one of five companies constructing 300 mm pilot lines, the others being IBM (Hopewell Junction, N.Y.), Intel (Santa Clara, Calif.) and Texas Instruments (TI, Dallas, Texas) in the United States and TSMC (Hsinchu, Taiwan).

Semiconductor 300, a completely independent company formed last January, is a joint venture between Siemens (51%) and Motorola (Austin, Texas) (49%). Its 300 mm pilot wafer line occupies an area of 1800 m² in a Class 1 part of module 2 of the Siemens fab in Dresden. It represents a $250 million investment. Tool installation is currently in progress, and fully integrated lot processing is due to start late this year or early in 1999. The manufacturing technology will be developed in the following two years, with transfer to volume production in 2001. This line will use 0.25 mm and later 0.18 mm technology to produce 64 Mbit and 256 Mbit DRAM devices with about 450 employees.

"This facility will be amongst the first in the world to demonstrate integrated 300 mm wafer processing," said Dan Tull, deputy general manager of Semiconductor 300, at a press conference hosted by DAS GmbH Dresden. "The pilot line using 0.25 mm technology will provide a direct comparison with production from existing 200 mm 0.25 mm lines." Through use of 300 mm wafers with 500 wafer starts per week, Semiconductor 300 aims to reduce chip manufacturing costs by 20% to 40%. In comparison with 200 mm wafer production, the 300 mm wafers provide a factor of 2.25 increase in wafer area, increasing the number of 64 Mbit chips per wafer by a factor of 2.3 and the number of 256 Mbit chips by a factor of 2.5. Fabrication with 300 mm wafers will involve 112 shots per wafer, as opposed to 44 shots with 200 mm wafer fabrication.

Fig. 1. This graph shows a conservative estimate of 300 mm fab ramp up. (Source: Rose Associates)

Fig. 2. This roadmap is for a superfast ramp up to 300 mm wafer use. (Source: Rose Asociates)

Minienvironments

The reason for moving to 300 mm wafers is mainly economic, as each wafer provides some 2.5 times as many die as can be obtained from a 200 mm wafer, for an increase of some 30% in the cost of the equipment. A change to finer dimensions made at the same time will bring still greater benefits but also more problems. Figures 1 and 2 show projected 300 mm adoption rates. The 300 mm fabs will be based on a minienvironment system throughout the entire fab, including wafer pods, tool enclosures and automated interfaces. Wafers are moved from one process to the next within these sealed pods that connect with the process tools only through an automated load port.

When filled with 25 wafers, a new front opening unified pod (FOUP, Fig. 3), developed by Jenoptik Infab (Jena, Germany) and Empak (Colorado Springs, Colo.), is too heavy to be carried comfortably by a person, showing the need for robotic handling. The wafers contained in such a full pod have a total value of about $500,000. Great care must be taken to minimize wafer loss with a wafer of this size, as the cost per wafer is high. The need to prevent particulate contamination from causing loss of wafers with very fine features points to the use of minienvironment manufacturing systems. The change to minienvironments and the use of efficient fan-filter units save much power formerly used to circulate air (Table 1).

According to studies of Meissner & Wurst (Stuttgart, Germany) and Jenoptik Infab, this has reduced the power required per square meter of process area from 1000 W/m² in 1989 to 200 W/m² in 1997. Siemens carried out benchmarking investigations and found that fabs with the highest productivity were using minienvironments.

Fig. 3. This 300 mm load port opens a 25 wafer capacity FOUP. (Source: Jenoptik Infab)

A major challenge in the change to 300 mm wafers is the higher level of automation required to transport large and fairly heavy wafer-filled pods efficiently. The pods can be moved by automatic guided vehicles (AGVs), but these tend to be expensive and cause problems by interfering with operators. An overhead or p-track system is used widely for 200 mm wafer systems but can carry the pods only where there are tracks. Stockers to store pods for 300 mm wafers take up much valuable cleanroom space. Siemens is planning to employ a novel system in which the wafer pods are kept above the fab ceiling and are fed down via doors in the ceiling.

Rudolf Simon, president of Jenoptik Infab, pointed out that the trend to use minienvironment systems will merge with the trend to highly automated fabs. He said that the minienvironment systems will be adopted globallyfor 300 mm wafers, except perhaps in a few Japanese fabs. He expects 80% to 90% of 300 mm fabs to use FOUP minienvironment systems, whereas only some 8% to 10% of 200 mm fabs use standard mechanical interface (SMIF) systems (Fig. 4). In 1990 only about 5% of new fabs were minienvironment-based, rising to about 35% in 1997 and forecast for 70% in 2005. Class 1 (and now Class 0.1 too) is vital at the wafer level rather than at the fab ceiling level. These and even better classes of cleanroom can be achieved within minienvironments (and the corresponding automated equipment load ports), while conventional cleanrooms are limited by the activities of operators in the wafer vicinity.

Simon said the main reasons for using the minienvironment concept in future fabs are reduced energy consumption, better contamination control, increased safety and better control over each process step (including deep UV lithography, chemical mechanical polishing and copper interconnect technology).

SMIF systems were introduced after 200 mm wafers, so there has been some resistance to the use of minienvironments due to conservative policies. However, companies that have adopted minienvironment-based automation for 200 mm facilities have gained experience with minenvironment automation that will ease the move to 300 mm systems.

Fig. 4. Jenoptik Infab ceiling suspended type of minienvironment and SMIF loading systems have been installed at the GEC Plessey 200 mm facility in England. (Source: Jenoptik Infab)

Simon said material tracking on wafer level will become even more important with 300 mm wafers. As wafers hold more die, future operators will ask "Where is my wafer?" rather than "Where is my lot?" Thus, some robot systems must be able to select individual wafers, not just complete pods. When asked if a transponder could be placed on each wafer to communicate with external systems, Simon suggested this may come after some 20 years. At present, bar code and optical character recognition systems are used and might be developed further to achieve higher productivity. He forecast that the high cost of fabs will result in fewer but larger fabs worldwide in the future, just as has occurred with modern car factories.

Standardization of fab design reduced fab construction time first from 30 to 24 months, and now to just 16 months, with very important interest savings on the huge capital expenditure. In Europe few new buildings are planned for 300 mm fabs in the next three years.

Waste gas abatement

A waste gas abatement system developed by Dünnschicht Anlagen Systeme GmbH (DAS, Dresden) has been adopted for the Semiconductor 300 line (Fig. 5) and also for the Wacker-Siltronic 300 mm project. DAS, as its name implies, has an interest in thin film deposition, but from its founding in 1991 it has specialized in the abatement of hazardous process gases. It has developed a modular system known as ESCAPE (Environmental Safety Cleaning And Protection Equipment) for the abatement of hazardous gases from semiconductor plants, but it is also applicable to other industries. The Siemens facility has installed 120 ESCAPE systems that also are used by the local Zentrum Mikroelektronik Dresden (ZMD), among 300 systems worldwide, including 100 in Taiwan.

Fig. 5. These ESCAPE systems are installed in a fab basement at Siemens, Dresden. (Source: DAS)

In the burner module, the thermal fragmentation takes place followed by oxidation, reduction or pyrolysis, depending on the gas composition. The resulting gas mixture is passed from the combustion unit upward through a scrubber section where it passes through an absorption column and comes into contact with an alkaline scrubber and quenching liquid. This removes acid gases formed in the burner and any acid gases present in the original waste gas mixture that are unaffected by passage through the burner, as well as suspended solids. A scrubber alone is only useful for removing water soluble gases, so the global warming gases must be decomposed in the combustion unit before the gases are passed through the scrubber.

Fig. 6. Waste gases enter through the four holes in the base of this burner chamber of ESCAPE MK II. (Source: DAS)

Waste gases are fed through up to four inlets into the combustion box developed by DAS (Fig. 6). This allows the gases to be kept separate if they interact with one another, as would happen with silane and oxygen, which can ignite spontaneously forming silicon dioxide. Natural gas, propane, hydrogen or a similar fuel gas enters the box through a ring-shaped burner. The latter is located in the center of a rotationally symmetric combustion chamber that is made of a special corrosion resistant ceramic material that needs periodic cleaning. A flame about 150 mm high is used at a temperature from 10008C to 12008C. The waste gases are fed into the center of this flame and undergo thermal fragmentation. Sensors monitor the flame and the pressure in the reactor. Ignition is affected without the use of a permanently burning pilot flame. The scrubbing lye consists of a solution of compounds such as potassium hydroxide or sodium hydroxide. It removes acid gases with the formation of salts in solution, while solids are absorbed or suspended in this aqueous lye. The lye is automatically renewed when its pH value falls. An ESCAPE system placed just after an etcher pump will prevent corrosion in the long stainless tube that carries waste gases into the atmosphere. It thus saves the frequent high costs for replacing this tube.