Hazardous Gas Monitoring
June 1, 2008
Semiconductor manufacturing facilities employ a number of hazardous gases in their production processes. Whenever these gases are stored, distributed or used in manufacturing processes, the potential for a hazardous condition exists. The primary hazards associated with these gases include fire, explosion, and contamination resulting in product loss or unscheduled preventative maintenance. These gases must be continuously monitored to ensure the health and safety of employees, to protect property, and to maintain regulatory compliance.
Continuous gas monitoring in semiconductor facilities is a requirement of local and state regulations, which are typically based on information from the Occupational Health & Safety Administration (OSHA), the Code of Federal Regulations, FM Approvals, the NFPA Fire Protection standards, the Uniform Building Code and the Uniform Fire Code. These regulations and standards offer guidelines concerning the proper design, installation and operation of hazardous gas detection systems.
Reliable gas detection and monitoring systems are an essential element of a semiconductor plant’s safety system. A variety of systems are available for different monitoring applications, and using the correct system will result in managing gas hazards in the most effective and efficient way possible.
Hazardous gas monitoring systems offer the semiconductor industry parts per million (ppm) detection in ventilated gas cabinets, enclosures, process equipment chases and clean rooms; flammability monitoring of lower flammable limit/lower explosive limit (LFL/LEL) levels in and around process tools; and area monitoring for toxics and combustibles in storage areas, distribution, delivery piping and equipment cases.
Parts per Million DetectionContainers of hazardous gases, both flammable and toxic, are often isolated from their surrounding environment by safety enclosures (gas cabinets). Some process tools have a gas control enclosure section that serves the same purpose as a gas cabinet. Common gases used in semiconductor manufacturing include hydrogen, hydrogen chloride, ETO, chlorine, ammonia and oxygen.
It is important to monitor these enclosures for leaking gas to save product and prevent a toxic condition. The cabinets and enclosures are ventilated to prevent the buildup of any leaking gas. The ventilation dilutes the vapors, which quickly mix with air flowing through the cabinet to prevent buildup.
Detecting the diluted leak requires a gas sensor in the low ppm range (see Figure 1). Placement of parts per million sensors in the exhaust duct enables the continuous sampling of all air moving through the enclosure. When a leak is detected, the detection system sounds an alarm to notify personnel, and/or close a delivery valve to stop the flow of gas from the cylinder.
Some process tools have gas enclosures that contain the piping jungle that carries hydrogen and toxic gases like arsine, phosphine, diborane or silane. These gases are introduced into processes in metered quantities to obtain product specifications. Any type of leak could jeopardize the integrity of the lot and result in loss of product. In addition, many of these gases are highly toxic and require precautions to protect personnel. If a technician opens the enclosure section when a leak is present, they expose themselves to the toxic gases. Continuous monitoring and early warning are the best means of preventing both of these accidents.
A clean room is an enclosed, contamination-free environment where state-of-the-art manufacturing and assembly take place. Clean rooms range from very small chambers to large-scale rooms. The piping that carries process gases is connected to deposition, etching and other process tools, and all of the connections are sources of possible leaks and contamination. Therefore, the air must be continuously monitored for ppm levels of hazardous leaking gas.
Some ppm sensors employ electrochemical sensing technology, which employs a micro-sized fuel cell. The sample diffuses into the fuel cell, where it chemically reacts to produce an electrical current proportional to the concentration of gas present. The signal is then amplified into a mA output. The output signal is linear, and readings are displayed in parts per million concentrations.
The sensors are rugged, highly stable and excellent for detecting low ppm concentrations of selected gases in gas cabinets, enclosures and clean rooms. They offer immunity to cross-interference, low maintenance, excellent repeatability and long-term stability.
These ppm sensors can be calibrated to read a variety of gases, including hydrogen, hydrogen chloride, carbon monoxide and ammonia, among others. Hydrogen is also often employed as a carrier for other gases like arsine, phosphine and silane. The ppm sensors can be calibrated to hydrogen in a variety of ranges, making them useful for the detection of either pure hydrogen or toxic process gases being carried in hydrogen.
Monitoring LFL/LEL LevelsProcess tools are enclosed areas in which specific wafer processing functions occur. Any process tools handling flammable gases require a hazardous gas detection system to ensure safety and be in compliance to codes. The FM standard states that “ventilation shall be provided for all tools handling flammable and combustible liquids. Ventilation shall be provided to ensure the atmosphere does not exceed 25% of the LEL (LFL) in the event of the largest possible leak.”
The equipment contains exhaust ducts to remove hazardous gases, and detectors are located in each process control cabinet and exhaust plenum to monitor the atmosphere for % LFL/LEL. When an alarm occurs, the process is shut down and the gas supply to the tool is turned off. Some of the process tools that need to monitor for flammable gases include furnaces, reactors, alcohol vapor dryers and ion implanters.
Catalytic sensors are typically used to monitor flammable gases and vapors in the 0-100% LFL/LEL range. These sensors feature a high-performance design that offers fast response, high accuracy and long life. They are stable and have superior tolerance to catalytic poisoning agents.
Area MonitoringHazardous gases are stored and distributed in semiconductor plants, and the possibility that hazardous gas could accidentally leak or spill into the surrounding area is ever-present. Pumps, control valves, manifolds, piping junctions, fittings and connections are some of the potential sources for leaks or spills. With so many opportunities for leakage, continuous monitoring of such hazards is an essential part of keeping the plant safe.
The importance of early warnings should be considered when determining detector placement. Effective early warnings can be accomplished by placing the sensors to favor the probable gas release point while maintaining the ability to protect the total area selected. Sensor selection (ppm or LFL/LEL) depends on the gas hazard present.
System SelectionMonitoring within a semiconductor plant requires gas detection systems that can accommodate a variety of combustible and toxic gas applications with both single- and multi-sensor network solutions. A new system has been designed to work with catalytic and electrochemical sensors for LFL/LEL and ppm monitoring.* Each system can continuously monitor and control the readings from as many as four same-type sensors. It is fully equipped with the alarm, display and output features needed, including on-board relays for interlocking to alarms, 4-20 mA output and RS-485 serial port.
The system can also be combined with powerful operator interfaces that allow users to view, access and control multiple remote sensors from a convenient central location. Operators and management can remotely request on-line, detailed information regarding the status of the sensors’ operation, including diagnostics and historical records.
*SmartMaxII from Control Instruments Corp.
For additional information regarding continuous gas monitoring, contact Control Instruments Corp., 25 Law Dr., Fairfield, NJ 07004; (973) 575-9114; fax (973) 575-0013; e-mail the author at firstname.lastname@example.org; or visit www.controlinstruments.com.