An inside job
To meet the fast changing needs of semiconductor manufacturing, Texas Instruments is moving forward with an in-house cleanroom construction team.
Since the 1960s, changes in Texas Instrument`s (TI; Dallas) manufacturing processes have required modifications and additions to existing cleanroom space. In the fast moving world of semiconductors, those changes must be implemented fast and with minimal impact to existing operations. TI`s response to this need has been to develop a core group of employees who design and build cleanrooms in-house. The latest challenge undertaken by this Cleanroom Projects (CRP) team was the CMP expansion of TI`s DMOS5 fab.
The purpose of the project was to add a Class 1 cleanroom to the main fab — a classic three-level ballroom; to consolidate all CMP processes; and to provide cleanroom space for future CMP growth.
The new room accommodates existing CMP tools located in the original fab, CMP tools that supported TI`s memory business and new tungsten polishers. The project also included wrecking out the building`s original compliment of comfort suit changing rooms and smock room. A 2,300-square-foot, Class 100 smock room was built and connected to the CMP cleanroom via a new 300-square-foot Class 1 link. The CMP expansion cleanroom design called for 6,100 square feet at Class 1 and Class 10,000, mixed with a common plenum ceiling filtered to seven-9s efficiency; a 24-inch raised floor return; and forty 12,000 cfm vertical laminar flow (VLF) modules. A utility matrix was put together showing the need for:
40,000 cfm of 100 percent make-up air;
two slurry systems — a tungsten and an oxide;
a CMP waste collection system and industrial waste collection system;
a 108-gpm, high-pressure deionized (DI) water system;
13,200 cfm of acid exhaust and 11,400 cfm of caustic exhaust;
14 gpm hot and 140 gpm cold DI water;
55 gpm process cooling water, process nitrogen, compressed air, plant vacuum, industrial waste, hydrogen peroxide, 0.49 percent hydrofluoric acid, ammonia hydroxide; and
350 amps of 480V power.
Teamwork tactics
“Once we know what`s required for the cleanroom space, all efforts are made to quickly assemble a cross-functional group that can work with the [internal] manufacturing customer to develop layouts, schedules and estimates for the proposed facility,” explains CRP manager Dale Litton.
Keeping the adjacent fab running and ensuring that equipment installed will be able to be maintained without future operations was a very high customer priority for this project. To make sure these requirements were met, the CRP team reviewed all stages of design and construction with the DMOS5 facilities operations team. Says Steve Russo, operations engineer, “it is important that operations and maintenance are involved in the construction design to help improve on previous designs, help the team learn building concerns, and understand how to operate the equipment for continuous operation.” A strict work authorization process was put in place to avoid potential interruptions, and clean protocols were enforced.
Integrity maintenance
In this particular installation, just as important as maintaining the room cleanliness of the construction site, was maintaining the area around the site. The entrance to the operating Class 1 fab was only 10 feet away, so the CRP team took immediate steps to keep any construction dust from being carried out and tracked into the adjoining hallway, which could have possibly affected the operating fab. Early on, booties were required, and as soon was possible, the VLFs were turned on to purify the air in the cleanroom.
Additionally, all items were wiped down and cleaned during installation, and in the final stages, a thorough IPA wipedown was done from slab to slab in the cleanroom space. Lastly, full smock protocol was established and the ceiling filters were installed.
The CRP team used 12,000 cfm VLF modules, manufactured by Oklahoma-based Temtrol, to achieve the required 65 to 80 fpm through the HEPA filters. Instead of placing units overhead or in a separate mechanical room, units were placed along two sides of the cleanroom — a common CRP team practice proven to save manufacturing space. Variations of the VLF modules have been in use at TI for 25 years.
Room specifications
“We were prevented from placing the VLF units above the room because of height restrictions, and because of floor space restrictions, we ended up using several modules strung together,” says Patrick Hubertus, project mechanical technician on the CRP team. “The VLF units have a very large cfm-to-floor space ratio and allowed us to utilize the given spaces to the maximum.”
The VLF units are supplied with 56-degree Fahrenheit water that circulates through A-shaped coils to remove sensible heat generated from equipment and VLF fan motors. Fans total static pressure is 2″ w.c. (water column) with 7.5 hp motors; sound attenuation was attached to the fan discharge above the ceiling.
Top to bottom
The cleanroom ceiling was designed to provide 100-percent filter coverage in the Class 1 areas and greater than 10-percent coverage in the Class 10,000 areas. The ceiling is an M+W extruded anodized aluminum u-channel fluid seal grid. A silicone o-ring/gasket is used to seal the face of the joint between the cross-pieces and the u-channel profiles. This is a deviation from the main DMOS5 fab, which uses caulk to make the seal; the use of the o-ring/gasket makes for a much easier installation. The fluid seal is a silicone non-hardening gel manufactured by Dow Corning. Sprinklers were also located within the grid, eliminating the need for sprinkler pans. The filters, which are manufactured by American Air Filter (Columbia, MO), are 99.99999 [efficient at] maximum particle penetration size (mpps) and four inches deep, with 0.35 inches of pressure drop at 70 fpm.
A non-progressive aluminum and Lexan cleanroom wall, manufactured by Lasco Services, was selected. The wall provides an open and aesthetically pleasing appearance and means for pass-throughs with minimum disturbance to the fab.
After being shotpeened and primered, the floor was coated with 60 mils of a self-leveling Novolak epoxy. It consists of full-bolted grid, epoxy-coated aluminum panels with a coated aluminum stringer system manufactured by Tate Floor Systems. The raised floor panels have slide dampers attached for room air balancing. Fiberglass pedestals and bases were specified for their chemical resistance.
Making waves
Make-up air to the cleanroom is provided by three 20,000 cfm fans manufactured by Temtrol. The units are designed with energy efficiency in mind, taking advantage of free reheating in the summer and free cooling in the winter with run-around coils. The unit humidifiers are fed from a central plant DI water steam system, and cooled/heated using the central plant`s chiller/boiler capacity. The units` wetted surfaces are all stainless steel. Drain pans are sloped to prevent standing water. Access panels and interior lights are provided to assist operations. There are 30-percent prefilters, 85-percent bag filters and 99.95-percent HEPA filters. The fan is a 40-hp centrifugal controlled by a variable frequency drive (VFD).
With nearly 25,000 cfm of air to be removed from the cleanroom, design and construction of the process exhaust system provided many challenges. Due to the chemical process involved, the exhaust mains were separated into acid, caustic and general. The 30-inch diameter mains were routed around existing utilities that could not be shut down. Since several mains were routed throughout areas sensitive to water exposure, Kem-Tuff polymer-coated stainless steel duct was chosen because it doesn`t require fire protection. All duct was sized for a velocity not to exceed 2,000 fpm and to maintain 1″ of static pressure at the tool where required. Jeff Miller, project mechanical designer, says, “routing the exhaust mains throughout areas that weren`t part of the fab was the greatest challenge. A coordinated team effort between all the design team members and the DMOS5 facilities team was required to make it possible.”
Old vs. new
Differences between the new and existing cleanrooms also posed obstacles for the CRP team. According to David Meyers, CRP manager of the DMOS5 CMP expansion, “historically, the CRP team has constructed semiconductor wafer fabrication cleanrooms in areas that were originally designed for some other purpose, such as office/administration space, restrooms, storage, and even hallways. Our projects typically require design compromises not found in green-field site construction. For this particular project, the slab-to-slab height was 14 feet, compared to the 25 feet of the adjoining fab, yet we still had to maintain the same cleanliness requirements. The mechanical support space was extremely limited, which made this location favorable for the CMP process tools that don`t require vacuum pumps, chillers or support equipment associated with other semiconductor processes.”
Typically one of the last parts of the cleanroom puzzle, and sometimes one of the most frustrating, are the control systems. “From a controls standpoint, we were able to achieve our goals of completing this portion of the project on time and on budget, while simultaneously achieving a bonus goal of implementing new technology into the design. Our installation introduced the use of Profibus communication networks to connect our plc`s to the motor control centers [which controlled and monitored over 40 motors] and reduced the number of cable pulls from approximately 160 to three. This is what teamwork and dedication to common goals can accomplish,” reiterates Project Controls Technician Dave Marr.
Process controls
The entire controls portion of the project was done in-house from design to control cabinet building to testing and documentation. The control system integrates three programmable logic controllers into a distributive network, which utilizes proportional- integral-derivative control loops to maintain process variables — temperature, humidity and pressure — in the cleanroom at predetermined set-point values. Also provided is control and monitoring of 40 individual VLFs and three make-up air motor starters for on-off operation, manual switch position, and running status. Primary concern was given to providing a control system that was flexible, adaptable and reliable, and offered a large measure of redundancy, so that maintainability and serviceability can be achieved without cleanroom downtime. All process variables are monitored and displayed on supervisory control and data acquisition (SCADA) screens where CMP HVAC process descriptions are clearly indicated, with point-and-click tuning and trending available on board.
The electrical equipment installed included a three-source static switch; four motor control centers; one 480-volt 800 I-line panel; two 225 KVA transformers; two 208-volt I-line panels; four 208/120-volt branch circuit panels; lighting; raised floor grounding; six VFDs; and miscellaneous outlets. “The design of the mechanical air systems permitted the installation of four motor control centers, which allowed one to be turned off at any time to complete preventative maintenance without requiring the room to go out of specification,” adds Ron Jolly, project electrical engineering technician.
Practice makes perfect
In addition to teamwork, practice seems to have perfected TI`s in-house building concept. CRP members know what`s required to complete a project, and they`ve established a planning schedule that takes a lot of variables out of the equation. For example, at the beginning of a project, a pre-plan review meeting is held with City of Dallas code administrators to inform them of pending construction and to ensure that all applicable codes are being followed. Because of the multiple reviews previously held between TI project teams and the City of Dallas, the task is mostly perfunctory, to maintain the trust and the cooperative spirit that`s been fostered between the company and the city. Litton says, “the CRP team is well versed on the city`s code requirements and works closely with code experts and city code administrators when there are any questions. We know that our approach to having pre-plan reviews with the city pays off because of our quick turn-around. When the CMP cleanroom design was completed, it was submitted to the city and a permit on this project was issued in three days.”
Timely endings
That kind of know-how allows construction to start amazingly quick. It eliminates time normally spent ensuring that contractors are aware of all site-specific requirements, such as hotwork permits, digging permits, odorous work permits, welding permits, confined space permits, work authorization procedures, where to bring in material, where to dispose of material, where to use restroom and cafeteria facilities, etc. The construction team can hit the ground running, with 17 TI craftsmen working along side hourly contractors (most of whom have worked with the CRP team before), and there are very few surprises. According to Kevin Hansard, project construction supervisor, “during the construction phases of the project, we make the most efficient use of our available resources by encouraging one-on-one communication between the mechanics, technicians and engineers. This allows us to complete our cleanrooms in an extremely short period of time.”
By following this tried course of action, the building schedule for the CMP cleanroom addition was completed on time and under budget — and the adjacent DMOS5 fab never experienced one interruption. The process runs so smoothly, in fact, that TI now has its CRP team working on three new cleanroom additions and modifications.
This article was contributed by TI`s Cleanroom Project (CRP) construction team, which includes Dale Litton, P.E., CRP manager; David Meyers, project manager; Steve Russo, operations engineer; Patrick Hubertus, project mechanical technician; Jeff Miller, project mechanical designer; Dave Marr, project controls technician; Ron Jolly, project electrical engineering technician; Kevin Hansard, project construction supervisor; Keith Knutson, project construction supervisor.
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TI`s cleanroom project team members (above) were responsible for the complete design and construction of the Class 1 (cmp) facility (left) that was added on to the company`s existing Dmos5 fab. (Photo courtesy of TI)-x14
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(Right) This high-pressure DI water station was one of the needs identified in the CRP team`s initial utility matrix.
(Far right) TI`s CMP cleanroom in operation.
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This piping leads to make-up air units, which are designed to be energy efficient.
Critical considerations
For Texas Instruments` Cleanroom Project (CRP) construction team to be successful, it is critical that a fab meets all of its design parameters, including airborne particle counts, lighting levels, temperature and humidity uniformity, sound levels, airflow parallelism, filter airflow velocity and uniformity, room pressurization and floor conductivity. All tests are conducted according to standards outlined in Federal Standard 209E and the Institute of Environmental Science and Technology`s (IEST) Recommended Practice IES-RP-CC006.2. Certification of the cleanroom is performed by Rhode Island-based Certification & Calibration Services.
Keith Knutson, project construction supervisor, says, “to meet our aggressive construction schedule, we typically allow one week for particle certification. The key to getting a room to meet specifications in this time frame is eliminating all potential particle sources before the room is turned on. We spend a lot of time above the ceiling, before filters are installed, closing potential contamination sources in walls and pans. Once the room is turned on, every square inch of the ceiling must be scanned for holes before actual particle counts are started. It`s tedious work, but critical. If you try to take short cuts and start counting without scanning, you`ll end up spending a lot more time chasing particle sources. By scanning before particle counting, potential problems are found and fixed, and particle certification goes smoother.”