National Semiconductor`s new 200 mm fab rises despite record winter
By Susan English
In April — just a record 23 months from groundbreaking, National Semiconductor`s $820 million 200-mm wafer fab will come on-line, proving that a harsh New England winter in South Portland, ME, is no match for advanced technology design/build techniques. In spite of one of the worst winters on record, while 120 inches of snow fell to the ground, anywhere from 800 to 1,000 contractors working `round-the-clock shifts seven days a week successfully constructed three levels of the fab`s cleanroom envelope under an inflated, climate-controlled polyester tent covering about an acre and a half.
With first production projected for October, National`s 200 mm Project marks the company`s first commercial venture into 8-in. wafer manufacturing and adds a core competency: sub-micron fab capability. It also marks the company`s first experiment with minienvironments: instead of making the entire fab Class 1, processing will take place in a work-cell environment in the new one-acre Class 100 ballroom-style cleanroom, utilizing minienvironments and SMIF technology. When completed — at a construction cost of $170 million — the facility will add 1,000 employees and 40,000 ft2 of cleanroom manufacturing area to the present 540,000-ft2 site.
The purpose of the 200 mm Project team was to build a cost-effective 200 mm, 0.35-micron wafer fab/sort facility capable of producing 30,000 CMOS7 starts per period in the manufacture of chips for laptops, desktop computers, and flat panel displays. The expanded facility will use 8-in. wafers in submicron CMOS (Complementary Metal Oxide Silicon) technology, which produces chips able to process information at higher speeds and lower power consumption than older technologies. The company plans to go to 0.25 micron during 1997, when it will incorporate flip chip technology, a CMOS8 technology currently under development, with the capability of going to 0.18 micron long range. “With this expansion and our BiCMOS expansion in 1994, South Portland becomes an important factor in National`s ability to grow at or beyond industry rates,” says Laurenz Schmidt, National`s managing director of the South Portland facility.
The project team
Because most CMOS7 development and processing takes place at the company`s Advanced Technology Group (ATG; Santa Clara, CA), 200 mm Project employees on both coasts worked together daily via phone, video conference, E-mail, or face-to-face on technology transfer. Bi-coastal training meant that South Portland fab employees — process engineers, equipment engineers and technicians — did six- to nine-month training stints with the ATG in California beginning in June 1996, while West Coast ATG engineers came to Maine to help install equipment and begin characterization of the CMOS7 process. On December 16, the first tool was installed. Just to make it interesting, the fab layout itself was changed midstream to accommodate an increased wafer start capacity of 30,000 starts per month up from the original 15,000 starts.
In March 1995, a 12-member corporate team was formed to plan and direct the project. In October, J.E. Merit Constructors Inc. (JEMCI; Portland, OR), a subsidiary of Jacobs-Sirrine, was awarded the construction management contract, and a construction team of over 100 contractors was hired. Groundbreaking took place on November 15, 1995. Besides using a system of five levels of clean/build protocol, from construction protocol to cleanroom standard protocol, the company ran a training program of four to six classes a week. Every contractor was required to complete at least one level of protocol training. The 12-month construction cycle for the project was short for transforming a 40,000 cubic yard hole in the ground into a four-story, state-of-the-art fab complete with one-acre process floor. Also, space was severely limited on the 540,000-ft2, 21-building site. The new fab had to “fit in” 20 feet from an operating chemical building. So the plan was to build the subfab first. But before construction could proceed, a newly closed strip mall had to be demolished, displaced parking replaced, and a retention pond relocated, all during Maine`s worst winter in 30 years.
Getting it right from the start
Another “first” for the company is the fabrication of wafers in a work cell environment, characterized by a Class 100 vs. a Class 1 environment, using Class 1 minienvironments which are actually far cleaner than Class 1. SMIF technology will be used to load and unload wafers into and out of tools. Since the installation of minienvironments was something of an innovation and an experiment, to shorten learning cycles, a simulation lab, or systems development lab was set up in an existing building to allow the team to study such things as the characteristics of airflow, the effects of tying directly into the grid. The simulation — a 10 ft x 12 ft Class 100 mini-cleanroom — complete with a mockup of the ceiling grid, filter system and everything included in the ballroom — provided an opportunity to learn about and work with the minienvironments ahead of time, before installing them in the fab. According to Paul Edmonds, National Semiconductor`s 200 mm Project director, “The lab allowed us to simulate the factory and experiment with the manufacturing line long before the actual manufacturing floor was ready. This allowed us to work out the bugs long before we ran silicon.”
Another key to getting the cleanroom up and running quickly, was the cleaning of the interstitial level, the fab level and the return air level — sometimes two or three times — before the recirculation units were turned on, according to Staff Facilities Engineer Jim Verrill, “We spent a lot of time researching the type of gel we were going to use to set the ceiling grid. We also 100 percent bench-tested every filter that went into the grid, so I know that what`s up there is good.”
Verrill says that in his experience, the number one reason for a new fab coming online late, are issues that arise during certification of the fab — leakage from the grid, the gels, etc. “Literally, I had counts in the thousands before we started the recirc, and within eight hours of turning the units on, I was down below 0.5 micron size and literally reading zeroes,” he says. Verrill also worked on the 43,500 ft2 of cleanrooms that function as support for the ballroom itself, and wrote the specifications and “Scope of Responsibility” for the cleanroom cleaning services who maintain the cleanrooms. Right now, he says, there are 12 cleaners cleaning the fab cleanroom on a daily basis. Cleaning of the waffle slab, pedestals, floors, walls, grid surface, return air level and all horizontal surfaces receives scheduled maintenance daily, weekly, quarterly and semi-annually on an ongoing basis.
The one-acre Class 100 ballroom comprising the process floor is built on four levels and supported by 220-ft trusses weighing 32 tons. These trusses support the 44,000 ft2 expanse without pillars or posts. “We did that so we could get a very high packing density of equipment,” says Edmonds. “Also, four levels gives us much better cost per square foot than if we went with a larger footprint building.” Because there are no dividing walls on the process floor, tools are clustered in work cells within nine areas on the process floor: diffusion, photo, front-end etch, PVD, CVD, metal etch, via etch, integrated yield management, and metrology. Implant and CMP (chemical mechanical polishing) are located outside the ballroom. “A lot of fabs have a wall of HEPAs between them. When you put a wall up, you lose manufacturing space and you lose the openness,” says Edmonds, who is also proud of the fact that windows on the west side of the ballroom allow fab workers a view of the outside world. Another advantage was the decision to prepurchase $29 million worth of equipment, which enabled the project to go forward without waiting for equipment to arrive from vendors and suppliers. n
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Front view of the new 200-mm submicron semiconductor wafer fabrication facility under construction at National Semiconductor in South Portland, ME.
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Inside the dome con struction workers enjoy 50 degrees Farenheit to 60 degrees Farenheit temperatures, while exhaust scrubbers and ducted exhaust allow dozens of cranes, forklifts and trucks to operate inside and maintain clean air quality.
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National Semiconductor`s one-acre, Class 100 ballroom-style cleanroom, comprising the process floor, is built on four levels and contains no internal pillars or posts, which allows for greater packing density for equipment and savings in cost per square foot. Tools are clustered in work cells within nine areas on the process floor.
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An aerial view of the site.
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Above the subfab, which contains the fab`s “dirty” equipment, the second level, or return air plenum floor, contains tool process piping. The third floor is the HEPA-ceilinged process area, while the fourth floor is the interstitial or “hi vac” area, which number among its fittings 125 HVAC units and five boilers. Seven gigantic return air chases are located on the fab`s north and south walls.
Working in the dome
Perhaps the most crucial part of the “200 mm Project,” was an air-supported, cable-restrained tent, called “the Dome,” devised by construction management firm J.E. Merit Constructors Inc. (JEMCI; Portland, OR), a subsidiary of Jacobs-Sirrine. Measuring 240 ft x 220 ft x 100 feet high, the dome — a heavy-gauge, vinyl-coated polyester tent — was constructed to house the fab`s 35-foot-high, football-field-sized waffle table, because the construction schedule required the fab`s concrete manufacturing table to be completed during the winter of 1995. The climate-controlled tent enabled the project to proceed through the winter months, while protecting both project and workers from the harsh vicissitudes of Mother Nature. Maintaining internal temperatures of between 50 degrees and 60 degrees Farenheit, the tent allowed the pouring of concrete and completion of the subfab on three levels — waffle table, return air level and fab floor. According to Doug Briggs, JEMCI`s site manager, “We considered several structures and construction sequences, but the dome was the only one that could meet all of the parameters the weather and site logistics presented and still maintain the schedule.”
Staff Facilities Engineer Jim Verrill was entrusted with contamination control responsibilities for the fab, as well as responsibility for architectural design of the project. “Actually, what we did was build our subfab — the return air level and the fab floor itself — all underneath the tent through the course of the winter. When springtime rolled around, the tent came down, and we completed construction above that for the interstitial level and the roof. What we did was build a building around the fab from that point on.”
Paul Edmonds, National Semiconductor`s 200 mm Project director, notes: “In many ways, the air structure paid for itself. We were able to pour as much concrete in the first eight days the structure was in place as we had in the previous month without the tent. When you look at the envelope from the subfab on up to the interstitial, that represents 23,000 cubic yards of concrete under eight miles of ceiling grid.” In retrospect, Edmonds says, the tent saved three months of actual construction time. Also, adds Edmonds, working inside the tent`s comparatively balmy environment turned out to be a great morale booster for construction workers, who were able to work in T-shirts.
Working between intermittent snow flurries, JEMCI installed the table`s 19,000-cubic-yard foundation, nearly half of which would be needed during inflation of the dome, since an inflating air pressure of 2 psi would create an uplift of 2,700 lbs. per lineal foot around the dome`s perimeter. A three-day break in the weather around Christmas allowed the dome to be pumped full of 6 million ft3 of air.
A distinctive landmark on the Arctic-like landscape, the dome actually consisted of 13,000 yards of 28-oz. of heavy gauge, non-flammable, vinyl-coated polyester weighing 68,000 lbs and enclosed by over 15 miles of 0.375-in. diameter, galvanized steel aircraft cable. When inflated, it reached over 11 stories high. In fact, because of the dome`s height, its use required approval from the Federal Aviation Administration (FAA). Pressurized airlocks formed a “people interlock,” and two very large, long pass-thrus were constructed to allow two concrete-pumping trucks and three cranes to back up to the fab floor and deliver their contents. Exhaust scrubber and ducted exhaust allowed dozens of cranes, forklifts and trucks to operate inside the dome while maintaining clean air quality. The mat slab (the concrete floor beneath the first floor) was made vibration proof down to 0.25 micron. When the outdoor temperature was zero, the indoor temperature was about 50 degrees Farenheit at ground level and 80 degrees Farenheit at the top. In the spring, removal of the dome had to be completed under ideal weather conditions, since any wind could send the huge fabric billowing like a sail. Over a three-day period, the dome was deflated, carefully folded back with the use of cranes and stored. — SE