Silane gas use poses challenge for cleanroom design team
Cooperation between engineers and local authorities ensures safe gas handling
by David Novak
The process for constructing cleanrooms is rarely cut-and-dried. Often design teams are faced with an unusual configuration of a new building or the architectural restrictions of old ones. But when Erland Construction (Burlington, MA) advanced technology division engineers were hired to build a silicon fabrication facility, their challenge was in addressing the gases used to manufacture the silicon wafers.
The facility was one with which Erland engineers were familiar. For nearly two decades, Erland has been providing construction management and design/build services to this New England-based gallium arsenide (GaAs) chip manufacturer.
About a year ago, in anticipation of projected growth in the silicon chip industry, the chip maker planned a pilot facility to determine the feasibility of entering this market. The facility, an upgrade to an existing one, was completed in December 1998.
Erland oversaw construction of the 2,900-square-foot Class 100 silicon fab, with 1,700 square feet of cleanroom and 1,200 square feet of utility and equipment space. The space is under positive airflow provided by a re-circulating air-handling unit with fan-powered HEPA filters. A unique feature of this facility was the use of a 55-mm grid system, an integrated ceiling system incorporating sprinkler heads and lightframes. The equipment for this room was set up in a bulkhead layout, which included two Bruce Technologies International furnaces, and a Novellus nitride deposition and a Genus chemical vapor deposition (CVD). Each shift, eight fully gowned technicians run this process; there are two shifts per day. The goal for chip production in this pilot facility would be 120 per week, with 9,000 to 10,000 die per wafer.
The facility was budgeted at $1.2 million, based on Erland`s experience with another gallium arsenide facility, but as the team developed the program and gained a better understanding of the scope of the project, the cost was recalculated to $1.75 million. One of the reasons for this increase was the use of 100-percent silane gas, as well as other hazardous gases used to process silicon wafers. This subjected the facility, user groups and the community to increased codes, regulations and safety issues from the federal, state and local governing organizations. The chip manufacturer`s local fire department had no experience with the installation of a silane gas storage facility; therefore it needed help and guidance throughout the permitting and approval process. To expedite the process, Erland`s engineers, scientists, and project managers met with city officials to discuss the facility. Several items were reviewed, which included the following:
Location — The silicon fab was to be located in an isolated leg of the existing facility. According to the National Fire Protection Association Article 318, the storage facility needed to be located a minimum of 12 feet away from the main building. Although a fenced in storage area would be acceptable, it was agreed that a bunker facility would be built 30 feet from the main facility for better protection for the chemicals and the users.
Chemicals — The chemical gases to be stored in the bunker included silane, dichlorosilane, germanium, phenyltrichlorosilane and diborene. The majority of these gases are pyrophoric (they ignite and burn on contact with air). The fire chief approached this issue with caution and asked for profiles of the chemicals and guidelines for safety and control, available, through the semiconductor industry and gas vendors.
Safety guidelines — The chip manufacturer instituted several safety guides and regulations to be observed by the fab`s users. Another level of guides was developed for the move to 100-percent silane at the facility.
Once the fire chief was made aware of the manufacturing process, the preliminary construction schedule, the time line for the gas implementation and an overview of the safety precautions already instituted at the facility, Erland received temporary approval to proceed with the design. To make sure everything was in order, the chief requested a second review of the final design before he would grant a permit.
Several safety precautions were instituted in the design of the gas piping distribution system. They included the following:
All gas distribution piping was installed in a coaxial stainless steel pipe and located in an at-grade accessible trench that runs from the main building to the bunker. Also, the pipe for the dichlorosilane gas was heat traced and insulated to maintain the gas at a specific temperature of 72 degrees Fahrenheit. Any moisture would cause a reaction and form the compound hydrogen chloride.
Silane and other hazardous gases required an automated purge panel or individual vent lines so a continuous flow of nitrogen would prevent back migration of air into the vent line. Exhaust ventilation was provided where there was a possibility for gas leakage in areas containing valves, fittings, connections, etc. A burnbox was installed so excess silane would burn off prior to entering the process exhaust ductwork.
The next step involved the approval of the city council.
An interesting parallel story involved the installation, start-up, and testing of two BTI furnaces in the silicon fab during the mid-point of the silane permit process. Erland worked with the fire chief to obtain a three-month temporary permit to allow silane gas to be stored in an unused corridor adjacent to the fab. All ventilation and life safety guidelines were implemented. This allowed the start-up and testing to occur without any delay in the schedule.
To gain approval from the city council, Erland engineers worked closely with the fire department to implement several safety alarms and systems, as well as an emergency procedure should a leak occur. The control/alarm system piping was composed of high purity Teflon tubing. It was connected to a hazardous gas monitoring system and ran parallel to the gas distribution piping. A PVC filter was located at each connection point to monitor each gas (it seeks trace elements of the hazardous gas). Cabinets for the controls and monitors are explosion proof. Annunciation panels and pull stations were denoted with a yellow panel. The controls monitor approximately 190 points on the tools, piping and valve boxes.
A continuously monitoring alarm system was installed, which activates a local alarm and shuts down the gas supply in the event of a leak. Exhaust ventilation and detection and shutdown systems have an emergency source of back-up power.
If an alarm occurs, a signal from the Simplex hazardous gas monitor will be sent to the MDA monitor at three levels:
Level one – trouble (gas will be shut off);
Level two – low-level alarm (leak enacted by triggers);
Level three – high-level alarm (too much pressure). This alarm calls for the fire department and on-site hazardous materials safety team.
Special concentration of silane
After reviewing the finalized design and emergency procedures with the fire chief and city council, the chief required a “true test” of the monitoring and alarm system. He wanted the system to be tested with silane gas. Several members of the Erland team experienced only “dry run testing” where the signals from the monitors were manually activated, but no one had experience with active silane gas, nor could they locate a facility/equipment manager who had. The next step was to contact several hazardous gas suppliers, but again, the search did not produce anyone with experience in this type of testing procedure. As a final effort to meet the needs of the fire chief, the team made arrangements with the gas supplier to provide a diluted silane/nitrogen mixture with enough parts per million to activate the monitoring system but with a low enough concentration to prevent exceeding regulated exposure limits. The testing procedure was approved by the facility`s health and safety team, and the staged test was a success: officials at the chip manufacturer were granted a permit to occupy the new facility.
David Novak is a program manager responsible for client development/relations for Erland Construction`s advanced technology division (Burlington, MA), which provides construction management and design services to industries requiring customized controlled environments. He spent over 15 years in facilities design, engineering, and construction, and he is now developing Erland`s specialized environments projects.
Key facts
Size of corporate facility: 110,000 sq. ft.
Size of new pilot plant: 2,900 sq. ft., with a 350-sq-ft. bunker facility
Cost of facility: $1.75 million
Purpose of facility: Pilot plant to manufacture silicon chips
The design/build team for this project consisted of:
Construction manager/ programmer: Erland Construction Inc.
Architect: Lloyd Architects
Mechanical engineer: Am-Tech
Gas monitoring controls: Simplex
Modular cleanroom system:
Ceiling and HEPA filters: Clestra
Wall: Plasscore
Sprinkler: Soloman Mechanical
Installation: Longdon
Process piping:
Distribution: RenTec
Equipment: Donovan Engineering and Construction Co.
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The Class 100 cleanroom bulkhead installation of the Novellus, Genus UHCVD (chemical vapor deposition) equipment (foreground left) which uses the silane gas in the production process. A BTI furnace (rear center) with sinks, spin dryers and quality control stations is located along the opposite wall.
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Clockwise from above:
The supporting utility room includes the rear chamber of the BTI furnace and utility piping systems (water, gases, waste) and ductwork for the adjacent production room.
The Class 100 modular cleanroom 55-mm ceiling grid system includes lights and sprinkler heads combined with modular wall panel system.
In the supporting utility room, a ventilated valve manifold box isolates the control valves for hazardous gases.
The utility room includes the production chambers supporting the Novellus, Genus UHCVD. Note the ceiling installation of the blue “burn box” (left rear) used as part of the exhaust purge system for hazardous gas leaks.
The hazardous gas bunker is located 30 feet form the main production building. The enclosure provides storage for silane, dichlorosilane, germanium and diborene. The majority of the equipment located on the roof supports the environmental controls for temperature-sensitive gases, an exhaust fan for gas evacuation, and explosion relief panels.