MEMC launches 300-mm wafer line
Wafer manufacturer models 300-mm pilot line on device manufacturers` facilities.
By Sheila Galatowitsch
MEMC Electronic Materials Inc. modeled its 300-mm wafer pilot line in St. Peters, MO, after some of the best cleanrooms in the world — those run by its own customers.
“We put ourselves in our customers` shoes and designed the final stages of our 300-mm wafer manufacturing environment to be as good or better than our customers` facilities,” says Steve Brunkhorst, director of MEMC`s 300-mm wafer program. Built like device manufacturers` cleanrooms, “our cleanroom becomes an extension of their factories,” he says.
The Class 1 cleanroom`s configuration, with utilities below and air handling above the core process floor, provides device manufacturers a sense of familiarity and confidence, Brunkhorst says.
Online controls rigorously monitor everything from airborne particles to whether the ionization bars are properly balanced. Robotic handling has replaced manual wafer handling. And even as MEMC customers inspect the surface of the silicon wafers for particles as small as 0.09 micron, the company is looking for equipment that will help it monitor particles as small as 0.065 micron.
Cameras have been installed in the cleanroom so device manufacturers can observe the most critical stage in wafer manufacturing, the final wafer cleaning, as it happens. Visiting customers can view real-time wafer cleaning from TV monitors in the hall or through the cleanroom`s large viewing windows.
These efforts to meet or exceed their customers` expectations are fueled by challenges facing the entire industry: the transition to 300-mm wafers and the introduction of 0.25 micron process technology. During the next few years, wafer and device manufacturing environments must meet — and maintain — the most stringent cleanliness requirements ever devised. That is, until the next set of even more rigid specifications is developed.
“It is an industry certainty that our customers are going to make their requirements more stringent in the future. The roadmap is headed for 0.18 and 0.13 micron technology — in just a few years from now,” Brunkhorst says. “This cleanroom was designed to give us the capability to satisfy our product requirements three years out. In every case, we have used the best known design practice and state-of-the-art materials, not to meet today`s specs, but specs three years down the road.”
$250 million investment
MEMC has recently invested $250 million in two 300-mm wafer facilities — the St. Peters pilot line and a developmental line in Utsunomiya, Japan (see sidebar). The St. Peters pilot line was completed last month in a fast-track project that owes much of its success to a previous project: construction of a 200-mm wafer expansion facility that houses the 300-mm wafer pilot line.
The 200-mm expansion facility was constructed by McCarthy (St. Louis, MO) as part of a 16-month project that concluded in December 1996. The two-level, 200,000-square-foot facility features mostly Class 1 cleanroom space for the wafer cleaning processes and also includes a Class 10,000 crystal puller facility and a Class 10,000 cleanroom for material handling. The 300-mm pilot line has three cleanrooms, ranging from Class 10,000 to Class 1.
Experience on the 200-mm construction helped ease the way for the 300-mm project, says Lloyd Hansen, project director for McCarthy, which worked on both projects. Most all the 300-mm construction and renovation took place in existing 200-mm cleanrooms, which meant construction workers had to wear cleanroom gowns, booties and headwear, and follow cleanroom protocols as they completed electrical and pipe fitting work for new machine installation. Equipment was wiped down and cleaned in a separate area prior to being moved into the cleanrooms.
Most of the construction workers had worked on the 200-mm facility, but old and new workers alike were trained and certified in cleanroom protocol by an MEMC department set up for that purpose. Cleanroom “sheriffs” supervised the workers to enforce compliance. “The rule was: three strikes and you`re out. You`re not allowed to enter the cleanroom again,” says Rao Yalamanchili, project leader of 300-mm cleaning.
In addition, all materials of construction had to be certified to be cleanroom-compatible before entering the cleanroom construction area. But much of that work had been performed for the 200-mm facility, so an approved materials list was already available.
The design and construction team was the same on both projects, as well. The design engineers were from Fru-Con Engineering (St. Louis, MO). PC&E (St. Louis, MO) supplied electrical engineering and in strumentation, and Daw Technologies Inc. (Salt Lake City, UT) provided mod ular cleanrooms for the project.
“The 300-mm project went well. We were running substantially under budget, and there was an excellent team effort by MEMC, the engineers and McCarthy,” Hansen says.
“In the selection of the project team for the 300-mm project at St. Peters, MEMC had three performance criteria: quality, cost and safety,” says MEMC operations manager for services Neal Schaeffer. “The McCarthy/Fru-Con/PC&E team has consistently delivered outstanding results on all of the 200-mm and 300-mm projects for MEMC, which has translated to very successful startups.”
Four-phase project
Work began on the 300-mm wafer line in May 1997. The first of four project phases centered on the Class 10,000 crystal pulling cleanroom. Crystal pulling is the first stage of wafer manufacturing. Seed crystals are dipped in raw, heated silicon and formed into cylinders of silicon, which are then sliced into wafers. New 300-mm crystal puller machines were installed in an existing Class 10,000 cleanroom, along with an additional cooling tower and electrical switch gear. “Each crystal puller took two months once it was brought in, installed and started up,” Hansen says.
The crystal pullers are complicated pieces of machinery, according to Hansen. They arrive in numerous pieces (3 to 4 tractor trailers of material) and have to be assembled onsite. They are heavy: Each crystal puller has two magnets, each of which weigh 15,000 pounds. And they are tall, approaching 30 feet tall when assembled. The installation is further complicated by the physical constraints of the building and the complex electrical and pipe fitting connections that are required.
The second phase of the project, the longest and most extensive phase according to Hansen, was the Class 10,000 wafer processing cleanroom, where the cut wafers are put through polishing, grinding and shaping processes. This phase took longer because the cleanroom was a new expansion onto an existing building. It also required significant expansion of utilities, including the boiler, cooling capacity and deionized water.
The third and fourth portions of the project, the Class 1 final cleaning and epitaxy areas, were constructed within the 200-mm Class 1 cleanroom, which continued its daily 24-hour operation. A wall was built to separate the operational area from the construction area, then a disassembled air handler was hoisted into place in the interstitial space, re-assembled and started up. “We barricaded our air from their air to keep them in operation,” Hansen says.
After initial construction and piping was completed, the new 300-mm bench assembly was installed. The temporary wall was taken down, and now both 200-mm and 300-mm Class 1 operations are in one room.
With the exception of the Class 10,000 crystal pulling cleanroom, the cleanrooms on the 300-mm pilot line have perforated raised floors. All the facility lines and utilities are underneath the raised floors so they don`t interfere with the activities taking place above the floor. The utilities, including electrical, deionized and chilled water lines, have also been expanded in each phase. The cleanrooms` standard ceilings are equipped with diffuser screens to protect the ULPA filters in use.
Equipment proved to be a major challenge in setting up 300-mm wafer production lines.
“None of the wafer manufacturing equipment for 200-mm can be used for 300-mm,” Brunkhorst says. “Either the 200-mm equipment will not accommodate the larger size, or it can accommodate the larger size, but it`s not cost-effective. In addition, the 200-mm equipment will not produce the 300-mm wafer characteristics that are required.”
MEMC is using a combination of modified 200-mm tools and some of the 300-mm specialty tools, such as inspection and analytical tools, which have recently become available. All the tools in the Class 1 cleanroom are housed in Class 1 minienvironments with ULPA filters. Combined with the Class 1 ballroom-style cleanroom itself, the resulting environment inside the tools is well below Class 1, Brunkhorst says.
Wafer sampling in the cleanroom
The Class 1 cleanroom also houses all the analytical facilities, a novel feature that has impressed visiting customers, according to Yalamanchili. In some older facilities, most analytical equipment is located outside of prime cleanroom space in a Class 10,000 or 1,000 cleanroom, he says. “All of MEMC`s 300-mm analytical equipment is present inside the Class 1 cleanroom, next to the tools. This eliminates potential contamination problems with carrying samples to the analytical lab, and it reduces the turnaround from a few days to a few hours. If we have a problem in the process, we know that right away.”
The size of a 300-mm wafer requires that a production line be designed with an emphasis on monitoring contaminants and as little human interaction with the wafers as possible, Yalamanchili says. Airborne particulates, organics, ions and surface metals at sizes as small as 0.09 micron are constantly monitored by a central monitoring station with remote sensors. “In a Class 1 cleanroom, nobody really monitors airborne particles at those levels, but we do,” Yalamanchili says. “Every aspect of our cleanroom has been designed to minimize the surface migration of those kinds of contamination onto the wafer surface,” he says.
MEMC also monitors the airflow inside the cleanroom. “Laminarity is very important to control particulate contamination,” Yalamanchili says. “We monitor the airflow inside the cleanroom to check whether it is laminar or cross-flow, which is bad for wafer quality. There is always a risk of contamination between different processes. In the past, once a cleanroom was certified, people did not care to monitor these flows, but because of the stringent requirements, we felt this was very important to monitor on a regular basis,” he says.
In addition, human interference is “one thing that we are focusing on” to ensure contaminant-free manufacturing, Yalamanchili says. “It is well known that handling of these wafers by humans contributes to particulate contamination,” he says. “In a past 300-mm pilot line, we would manually load these wafers. Machine loading and unloading has been fully automated on the new 300-mm line. Right now in the new line, the operator interaction with most of the tools is minimal, once they are loaded into the cassette.” Cleaning inspection is also robotically handled.
However, unlike interbay and intrabay transport in a device manufacturer`s facility, movement of the wafers between machines, inside of the cleanroom, is still done manually with carts or people, Brunkhorst points out. This manual interaction is common during pilot operations, until line flows are optimized and equipment design matures.
The 300-mm wafers are also more prone to static charge contamination. The new Class 1 cleanroom is “100 percent static-protected,” Yalamanchili says. The ionization bars placed “everywhere” around the room are monitored by the central monitoring station and re-balanced about once a month.
All the tools are in place to begin wafer manufacture, but MEMC will continue to evaluate new equipment and suppliers, and install new equipment through the end of this year, Brunkhorst says.
The careful design and operation of the 300-mm pilot line places MEMC in an excellent position to supply the wafers needed today for tool development, according to Brunkhorst. It will also prepare the company for production of the circuit-quality wafers that will be needed in the year 2000.
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Following chemical immersion cleaning, wafers are placed in shipping packages to maintain product cleanliness during transport to tool developers and device makers. The wafers are processed in MEMC`s pilot line. Photo courtesy of MEMC.
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Prior to shipment, 300-mm wafer surfaces are inspected for defects as small as 0.09 micron using reflected laser light. The inspection equipment is in MEMC`s Class 1 cleanroom. Photo courtesy of MEMC.
MEMC`s 300-mm wafer plant in Japan
MEMC has designed and constructed a 300-mm wafer development facility at its 200-mm manufacturing site in Utsunomiya, Japan. The 100,000-square foot facility is scheduled to begin production in mid-1998. “That line is fully integrated from crystal through epitaxy under one roof. It was designed specifically for 300-mm research and development and small-scale production,” says Steve Brunkhorst, director of MEMC`s 300-mm wafer program.
MEMC is the leading worldwide supplier of silicon wafers outside of Japan and is the only non-Japanese silicon wafer manufacturer with manufacturing and research facilities in Japan, according to the Silicon Investor. It is also the only wafer manufacturer that has a 300-mm pilot line in both the U.S. and Japan, according to Brunkhorst.
“We felt it was important to have a presence in the Japanese market because of intense development activity there by customers, equipment makers and our competitors. It will give MEMC two points of service to serve our customers, plus a window on worldwide technology, so when we build our mass production facility, we can choose the best of the world from our tool sets and process,” he says.
Second production line
The Utsunomiya line is MEMC`s second 300-mm wafer production line. The company first produced a small number of 300-mm wafers in 1991 for one of its customers. When the market for 300-mm test wafers began to materialize five years later, MEMC began construction on its first pilot line for 300-mm wafer production in St. Peters, MO, using existing 200-mm crystal pullers and cleaning line, and new 300-mm tools.
Phase one of that line was completed in March 1997 and has been producing wafers for more than a year. The phase two portion of the 300-mm wafer line in St. Peters, including new 300-mm crystal pullers and cleaning line, will become fully operational this year. Semiconductor consortia and equipment companies are the primary customers for the test wafers, which are used to develop 300-mm device processes and tools. In recent months, however, MEMC has been receiving orders for prime circuit-quality wafers, Brunkhorst says.
As the pilot lines begin to produce significant volumes of wafers, they will also be used to evaluate the behavior of equipment, materials and facilities, such as cleanrooms, Brunkhorst adds. “A successful production process must be `robust,` that is, the process must be insensitive to expected variation from all process conditions including tool wear, supply variability and measurement uncertainties,” he says. “The best way to confirm the process is to produce significant volumes of product in facilities that are very much like those which will be installed in the eventual mass production factory.” Today, MEMC is already producing thousands of wafers per month, which is a sufficient quantity to exercise the pilot line and ensure process robustness, Brunkhorst says.–SG