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An integrated laboratory-reactor MOCVD safety system
Journal of Crystal Growth 75 (1986) 421—428 North-Holland, Amsterdam
421
AN INTEGRATED LABORATORY-REACTOR MOCVD SAFETY SYSTEM R.M. LUM, J.K. KLINGERT and B.V. DUTF * AT&T Bell Laboratories, Holmdel, New Jersey 07733, USA Received 10 November 1985; manuscript received in final form 10 March 1986
A comprehensive MOCVD safety interlock system has been designed and implemented which provides continuous monitoring of critical reactor and laboratory systems. Operator response to system alarms is limited to a single rule — immediate evacuation of the laboratory. The safety interlock system automatically aborts reactor and laboratory operation to predetermined latched states, provides audible and visual indication of the alarm condition, and identifies the detector and location giving rise to the alarm. Computer control of all operating systems enables additional safety to be programmed through interactive software design that eliminates most operator-based errors and provides for safe and reliable reactor operation. A full description is presented of the safety concepts incorporated into the design and construction of the laboratory and reactor system. Since its installation during the past year, the safety interlock system has operated successfully through several power failures and has demonstrated its effectiveness for ensuring a dependable fail-safe environment.
1. Introduction Safety considerations in a metalorganic chemical vapor depositon (MOCVD) laboratory center on the highly toxic and flammable properties of the chemicals used. Hydride concentrations at the 100 ppm level vary in effect from acting as a severe irritant (HC1) to being lethal [1] (AsH3, PH3, and H2Se). Since the hydride sources are usually mixtures in hydrogen, all pose a flammability hazard. In general, the Group-Il and GroupVI organometallic compounds are toxic, especially (CH3 ) ~ which has a TLV value of 1 ppb [2]. Although not as well characterized, the Group-Ill organometallics appear to be nontoxic. However, most of the organometallic compounds are pyrophoric and can be dangerous even in the limited quantities generally used, since a small leak could act as an ignition source for the hydrogen carrier gas. This problem can be avoided by using an inert gas (e.g. helium [3] or nitrogen [4]) as the carrier. Although procedures for the save handling, storage and disposal of the hydride and organo*
Present address: Codenol Technology Inc., Yonkers, New York 10701, USA
metallic compounds used in MOCVD growth are well established, the design of a more general fail-safe system has received far less attention due mainly to the research environment of most MOCVD laboratories. However, as reactor designs have become more complex and the number of laboratories involved with MOCVD growth has —
increased, the necessity for a more comprehensive safety system has become apparent. Separate safety sessions have been held at two recent conferences [5,6]devoted to MOCVD growth, due to increased concern with the topic. Although several papers [7,8] have described the incorporation of safety interlocks into the reactor design, little information has been published on the characteristics of a more complete system in which both reactor and laboratory safety designs are interlocked to form a common alarm system. This paper describes an automated laboratory and reactor safety interlock system which provides multiple-zone status monitoring and alarm warning signals for critical reactor and laboratory subsystems. Abort alarms result in automatic shutdown of the reactor and laboratory to predetermined latched states. Recovery from the latched abort state is accomplished via an interactive operator—computer procedure. Warning alarms are
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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Integrated laboratory-reactor MOCVD safety system
logged by the reactor computer system which supplies a printout of time and alarm identity. Also described are safety concepts and features built-in to the design and construction of the laboratory and reactor system.
(4) Construction of U/L approved 2-hour minimum fire-rated walls for laboratory and gas supply rooms. (5) Installation of U/L approved fire-rated doors at opposite ends of the laboratory, each opening outward. (6) Installation of two large-area viewing windows equipped with fuse-linked metal shutters to maintain fire-wall integrity. A schematic of the laboratory plan indicating the above features is shown in fig. 1. In the event of fire, building alarms would sound and the room sprinkler system would be activated. The laboratory fire detection system consists of a network of temperature and ionization (smoke) detectors in addition to the sprinkler heads. Hydrogen detectors sensitive to the ppm level are also included since hydrogen, which has a lower explosive limit of 4% (by volume), is used as the main carrier gas. Finally, a fire emergency pull-box is located just
2. Laboratory design The overall objective of the MOCVD laboratory design was to prevent the spread of fire or accidental gas release from the controlled laboratory area. Thus, the following safety specifications were required for the laboratory as a whole. (1) Separate gas supply room for isolation of the high pressure gas cylinders, (2) Dedicated ventilation system for laboratory and gas supply room. (3) Negative room pressure with respect to outside halls.
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Fig. 1. Schematic laboratory plan outlining safety construction design.
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/ Integrated laboratory-reactor MOCVD safety system
outside the laboratory exit door. An accidental gas release would be contained within the laboratory by the negative room pressure and exhausted by the high-flow (3500 cfm) ventilation system, which exchanges the room air once a minute. An air flow switch monitors operation of the ventilation system. An activatedcharcoal filter drum, located in a separately vented enclosure, is used to remove arsine, phosphine and other toxic waste gases from the reactor exhaust. Rather than regenerating the filter cartridge, the entire drum assembly is sealed and replaced on a periodic basis. This procedure eliminates system maintenance requiring direct handling of exhaust waste products, as is necessary with other techniques (e.g. wet-scrubbing processes). Toxic gas detectors, sensitive to the 5 ppb range, monitor the laboratory, gas supply room, reactor tube cabinet and filter-drum exhaust for trace amounts of metal hydrides,
•VENTILONMONITOR
3. Gas supply room design All high pressure gas cylinders (metal hydride and hydrogen) are kept in a separate gas supply room adjacent to the MOCVD laboratory. This room was constructed with the same fire-rating and negative room pressure specifications as the main laboratory. The room contains sprinkler heads and toxic gas, hydrogen and fire detectors. The gas sources are classified according to four categories: (1) toxic (arsine, hydrogen sulfide and hydrogen selenide); (2) pyrophoric (phosphine); (3) flammable (hydrogen); (4) corrosive (hydrogen chloride). Gas cylinders from different categories are installed in separate exhausted gas cabinets as shown in figs. 1 and 2. Sprinkler heads are mounted in each cabinet. Access openings to the gas cabinets have a minimum face velocity of 150 FPM.
~~LUIE~Y
ICO1T PACK FOR CYLINDER CHANGES
SEPARATE PURGE CYLINDERS FOR EACH GAS BOTTLE GAS ROOM Fig. 2. Photograph of gas-supply room.
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Integrated laboratory-reactor MOCVD safety system
A schematic of the cylinder manifold system is shown in fig. 3. The cylinder valve assemblies are fitted with flow restrictors and an air-operated shut-off valve is installed immediately downstream of the CGA connector on each tank. These valves are operated remotely from a control panel mounted in the MOCVD laboratory, which also serves as a cylinder status indicator. An excess pressure switch is located in the outlet line from each gas cylinder manifold system so that only low pressure (< 15 psi) gas delivery lines enter the MOCVD laboratory. Each cylinder manifold system is purged with a dedicated nitrogen tank to eliminate the possibility of cross contamination of the gas lines. Purgeevacuate cycles are accomplished with a Venturi educator and the purged gases are directed to the activated-charcoal filtration system prior to discharge. All lines are constructed of stainless steel tubing (seamless 316, 0.035 inch minimum wall thickness) and all interconnections are located in
exhausted enclosures and made with metal gasket fittings. Since the greatest danger of exposure to toxic gases occurs during installation or removal of a gas cylinder, the following safety procedures are followed. (1) A minimum of two people are present during delivery and installation of any toxic gas. (2) Delivery of high toxicity gas cylinders is scheduled only during low occupancy periods in the building. (3) Use of pressure-demand self-contained breathing apparatus by both people during cylinder changes. (4) Shut-off of cylinder valve and evacuation of cylinder manifold system after each run (i.e. gas lines are not kept pressurized with toxic process gases when the reactor is not in use). (5) All cylinder changes are performed according to a posted procedure. (6) A panic button is located just outside the gas
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Fig. 3. Schematic of compressed gas cylinder tank—connection and manifold system.
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supply room which closes the air-operated cylinder shut-off valves,
4. Reactor design The MOCVD reactor consists of four major sections that are contained in a single vented cabinet which is kept at a negative pressure with respect to the laboratory. These sections are mdicated schematically in fig. I and shown in the photograph of the reactor presented in fig. 4. Brief descriptions of each section are given below, (1) Wafer load area. This section consists of a nitrogen atmosphere glovebox which enables access to the reactor tube load-chamber for wafer loading and unloading in a contaminant-free environment. The load-chamber door is interlocked to the reactor alarm system and prevents
the hydrogen or toxic gas valve electrical control circuitry from being enabled when the door is open. A pressure switch is used to signal the alarm system if the reactor tube becomes over pressurized. If this occurs the reactor inlet and toxic gas valves are automatically closed and the excess pressure is bled-off through a check valve located in the reactor outlet line. (2) Reactor-lube chamber. This is a separately vented compartment which contains the reactor tube, RF coils, etch furnace and reactor pumping system. Hydrogen, toxic gas, fire and ventilationfailure detectors are located within the compartment. The compartment access door is interlocked with the RF generator to prevent accidental contact with live RF coils. A chemical-resistant vacuum pump is used to evacuate the reaction tube and gas handling systern, as well as to enable low-pressure reactor
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laboratory-reactor MOCVD safety system
operation. The pump is equipped with an oil filtration system and has a filter flow sensor which is interlocked to the reactor alarm system. Reaction effluents are evacuated through the vacuum pump and suspended in the oil which is then removed from the pump, filtered and recycled, The reactor exhaust gases are directed though a mineral oil trap located downstream of the vacuum pump and from there pass through a 1-inch diameter stainless steel line to the activated-charcoal filter drum. (3) Gas-handling system. This section consists of the hydride inlet valves, organometallic sources, mass flow controllers and gas-switching valves. The gas system is built in a separately sealed and
at one-second intervals. On receipt of an alarm signal the computer logs the time and alarm identity and transfers control to subroutines that either enable automatic system recovery from the alarm condition or place the reactor into a controlled abort state.
vented enclosure or “gas-box” which is installed as a modular unit within the main cabinet. It contains its own hydrogen, toxic gas, fire and ventilation detectors and the outer cabinet doors are interlocked to the reactor alarm system. The gas-box thus acts as a separate containment area in the event of a gas leak. The gas handling system employs normallyclosed packless valves in all sections except the nitrogen inlet, hydrogen purifier, vent and main reactor inlet lines. These use normally open valves to enable a nitrogen purge of the reactor and inlet/outlet sections of the hydrogen purifier during a power failure. Pressure sensors mounted in the nitrogen and hydrogen gas inlet lines are set to provide low-pressure warning or abort signals. All pneumatically operated valves are controlled by digital output signals from the computer. After each I/O cycle the computer checks the status of the valve relay coil drive circuits. Although this verifies transmission of a relay control signal, it does not provide an unambiguous indication of valve operation. Installation and monitoring of pressure switches in the valve airsupply line would provide additional information regarding relay operation. (4) Computer control system. This section contains the computer electronics, gas system display panel and safety interlock circuits. The computer provides complete control of all reactor sub-systems (air-operated valves, mass flow controllers, furnaces, organometallic source-baths, tube pressure, etc.) and monitors all of the safety interlocks
(3) (4)
5. Reactor safety-interlock system The reactor safety-interlock circuit is interfaced to the system computer and provides status information on: (1) air pressure to the solenoid valves; (2) N2-purge inlet pressure; H2-carrier gas inlet pressure; reactor tube end-cap seal; (5) reactor tube over-pressure; (6) H2-leak detectors; (7) cabinet ventilation; (8) reactor exhaust/vent line flow; (9) RF coolant flow; (10) furnace over-temperature; (11) cabinet doors; (12) vacuum pump filter/trap flow; (13) computer operation; (14) electrical power. A dual-level warning/abort alarm system is employed for monitor points 1—4 of the above list. The remaining status signals are set to give only warning or abort alarms as shown in the diagram of fig. 5. Warning and abort alarms are indicated by LEDs on the reactor status panel, displayed on the computer CRT screen, and listed on a printer. After a warning alarm has been indicated and logged, computer control continues in a normal sequence. On an abort alarm the computer either places the reactor into a nitrogen-purge state or, on an over-pressure signal, turns off all gas and reaction tube inlet valves. Beyond the hardware-based safety measures listed above, computer control enables additional safety to be programmed through interactive software design. All reactor valve sequence and control settings are totally transparent to the operator. For example, if arsine flow is desired in a particular growth step, the operator simple enters the flow rate required and the computer program
R.M. Lum et a!. WARNINGS
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Integrated laboratory-reactor MOCVD safety system
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Fig. 5. Diagram of reactor safety-interlock alarm system.
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Integrated laboratory-reactor MOCVD safety system
automatically selects the proper valves that need to be energized to accomplish the task. Or an entire evacuation-backfill purge sequence of the reaction tube (or any selected section of the gashandling system) can be specified with one keystroke which selects a menu option. Another menu selection enables automatic evacuation and nitrogen backfill of the metalorganic bubbler input lines prior to a source cylinder change. These software enhancements eliminate most operatorbased errors and provide for safe and reliable reactor operation.
6. Laboratory-reactor safety-interlock system Complete alarm coverage of the MOCVD laboratory and reactor is accomplished by a safety interlock network formed from the fire, hydrogen, toxic gas and ventilation flow detectors located in the laboratory, gas-supply room and reactor. Output signals from these detectors are brought to a central alarm panel mounted outside the laboratory. The safety interlock system is subdivided into sixteen separately defined alarm monitoring zones as shown in fig. 6. This permits exact identification of the specific detector and location giving rise to an alarm condition, which enables immediate determination of the most effective emergency response. Alarm signals from the central safety interlock system result in automatic shutdown of the laboratory and reactor to a latched power-off, nitrogen-purge state. Recovery from the latched abort state is accomplished by restoring power first to the system computer, which then automatically re-boots the operating program to an interactive computer-aided power recovery subroutine. This subroutine guides the operator through the various steps required to safely power-up the remaining laboratory and reactor subsystems.
7. Conclusion A comprehensive MOCVD safety interlock systern has been designed and implemented which provides multiple-zone status monitoring and
alarm signals for critical reactor and laboratory subsystems. Operator response to system alarms is limited to a single rule immediate evacuation of the area. The safety interlock system automatically aborts reactor and laboratory operation to predetermined latched states, provides audible and visual indication of the alarm condition, and identifies the detector and location giving rise to the alarm. Computer control of all operating systems enables additional safety to be programmed through advanced interactive software design that eliminates most operator-based errors and provides for safe and reliable reactor operation. Since its installation during the past year, the safety interlock system has operated successfully through several power failures, both in- and out-of-hours, and has demonstrated its effectiveness for ensuring a dependable fail-safe environment. —
Acknowledgements The authors would like to acknowledge E.G. Burkhardt for his many helpful discussions on the design and implementation of laboratory safety systems, and members of the Environmental Health, Environmental Management and Safety Center for their active participation in the safety design aspects of this laboratory.
References [1] W. Braker, AL. Mossman and D. Siegel, Effects of Exposure to Toxic Gases — First Aid and Medical Treatment, 2nd ed. (Matheson Gas Company, Lyndhurst, NJ, 1979) PP 60, 76, 88. [2] Threshold Limit Values for Chemical Substances in the Work Environment, American Conference of Governmental Industrial Hygienists, Cincinnati, OH, 1984, p. 22. [3] V. Swaminathan, J.L. Zilko and S.F. Nygren, Mater. Letters 2 (1984) 308. [4] A. Mircea, R. Azonlay, L. Dugrand, R. Mellet; K. Rao and M. Sacilotti, J. Electron. Mater. 13 (1984) 603. [5] First Biennial OMVPE Workshop, Cornell University, Ithaca, NY, Aug. 1983. [6] 2nd Intern. Conf. on Metalorganic Vapour Phase Epitaxy, Sheffield, April 1984 [Proceedings published in J. Crystal Growth 68 (1984) 1—502]. [7] M.J. Bevan and K.T. Woodhouse, J. Crystal Growth 68 (1984) 254. [8] E. Johnson, R. Tsui, D. Convey, N. Mellen and J. Curless, J. Crystal Growth 68 (1984) 497.
421
AN INTEGRATED LABORATORY-REACTOR MOCVD SAFETY SYSTEM R.M. LUM, J.K. KLINGERT and B.V. DUTF * AT&T Bell Laboratories, Holmdel, New Jersey 07733, USA Received 10 November 1985; manuscript received in final form 10 March 1986
A comprehensive MOCVD safety interlock system has been designed and implemented which provides continuous monitoring of critical reactor and laboratory systems. Operator response to system alarms is limited to a single rule — immediate evacuation of the laboratory. The safety interlock system automatically aborts reactor and laboratory operation to predetermined latched states, provides audible and visual indication of the alarm condition, and identifies the detector and location giving rise to the alarm. Computer control of all operating systems enables additional safety to be programmed through interactive software design that eliminates most operator-based errors and provides for safe and reliable reactor operation. A full description is presented of the safety concepts incorporated into the design and construction of the laboratory and reactor system. Since its installation during the past year, the safety interlock system has operated successfully through several power failures and has demonstrated its effectiveness for ensuring a dependable fail-safe environment.
1. Introduction Safety considerations in a metalorganic chemical vapor depositon (MOCVD) laboratory center on the highly toxic and flammable properties of the chemicals used. Hydride concentrations at the 100 ppm level vary in effect from acting as a severe irritant (HC1) to being lethal [1] (AsH3, PH3, and H2Se). Since the hydride sources are usually mixtures in hydrogen, all pose a flammability hazard. In general, the Group-Il and GroupVI organometallic compounds are toxic, especially (CH3 ) ~ which has a TLV value of 1 ppb [2]. Although not as well characterized, the Group-Ill organometallics appear to be nontoxic. However, most of the organometallic compounds are pyrophoric and can be dangerous even in the limited quantities generally used, since a small leak could act as an ignition source for the hydrogen carrier gas. This problem can be avoided by using an inert gas (e.g. helium [3] or nitrogen [4]) as the carrier. Although procedures for the save handling, storage and disposal of the hydride and organo*
Present address: Codenol Technology Inc., Yonkers, New York 10701, USA
metallic compounds used in MOCVD growth are well established, the design of a more general fail-safe system has received far less attention due mainly to the research environment of most MOCVD laboratories. However, as reactor designs have become more complex and the number of laboratories involved with MOCVD growth has —
increased, the necessity for a more comprehensive safety system has become apparent. Separate safety sessions have been held at two recent conferences [5,6]devoted to MOCVD growth, due to increased concern with the topic. Although several papers [7,8] have described the incorporation of safety interlocks into the reactor design, little information has been published on the characteristics of a more complete system in which both reactor and laboratory safety designs are interlocked to form a common alarm system. This paper describes an automated laboratory and reactor safety interlock system which provides multiple-zone status monitoring and alarm warning signals for critical reactor and laboratory subsystems. Abort alarms result in automatic shutdown of the reactor and laboratory to predetermined latched states. Recovery from the latched abort state is accomplished via an interactive operator—computer procedure. Warning alarms are
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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R.M. Lum et al.
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Integrated laboratory-reactor MOCVD safety system
logged by the reactor computer system which supplies a printout of time and alarm identity. Also described are safety concepts and features built-in to the design and construction of the laboratory and reactor system.
(4) Construction of U/L approved 2-hour minimum fire-rated walls for laboratory and gas supply rooms. (5) Installation of U/L approved fire-rated doors at opposite ends of the laboratory, each opening outward. (6) Installation of two large-area viewing windows equipped with fuse-linked metal shutters to maintain fire-wall integrity. A schematic of the laboratory plan indicating the above features is shown in fig. 1. In the event of fire, building alarms would sound and the room sprinkler system would be activated. The laboratory fire detection system consists of a network of temperature and ionization (smoke) detectors in addition to the sprinkler heads. Hydrogen detectors sensitive to the ppm level are also included since hydrogen, which has a lower explosive limit of 4% (by volume), is used as the main carrier gas. Finally, a fire emergency pull-box is located just
2. Laboratory design The overall objective of the MOCVD laboratory design was to prevent the spread of fire or accidental gas release from the controlled laboratory area. Thus, the following safety specifications were required for the laboratory as a whole. (1) Separate gas supply room for isolation of the high pressure gas cylinders, (2) Dedicated ventilation system for laboratory and gas supply room. (3) Negative room pressure with respect to outside halls.
2 HOUR FIRE RATED WALLS ALL AROUND LABORATORY AND GAS ROOM
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Fig. 1. Schematic laboratory plan outlining safety construction design.
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/ Integrated laboratory-reactor MOCVD safety system
outside the laboratory exit door. An accidental gas release would be contained within the laboratory by the negative room pressure and exhausted by the high-flow (3500 cfm) ventilation system, which exchanges the room air once a minute. An air flow switch monitors operation of the ventilation system. An activatedcharcoal filter drum, located in a separately vented enclosure, is used to remove arsine, phosphine and other toxic waste gases from the reactor exhaust. Rather than regenerating the filter cartridge, the entire drum assembly is sealed and replaced on a periodic basis. This procedure eliminates system maintenance requiring direct handling of exhaust waste products, as is necessary with other techniques (e.g. wet-scrubbing processes). Toxic gas detectors, sensitive to the 5 ppb range, monitor the laboratory, gas supply room, reactor tube cabinet and filter-drum exhaust for trace amounts of metal hydrides,
•VENTILONMONITOR
3. Gas supply room design All high pressure gas cylinders (metal hydride and hydrogen) are kept in a separate gas supply room adjacent to the MOCVD laboratory. This room was constructed with the same fire-rating and negative room pressure specifications as the main laboratory. The room contains sprinkler heads and toxic gas, hydrogen and fire detectors. The gas sources are classified according to four categories: (1) toxic (arsine, hydrogen sulfide and hydrogen selenide); (2) pyrophoric (phosphine); (3) flammable (hydrogen); (4) corrosive (hydrogen chloride). Gas cylinders from different categories are installed in separate exhausted gas cabinets as shown in figs. 1 and 2. Sprinkler heads are mounted in each cabinet. Access openings to the gas cabinets have a minimum face velocity of 150 FPM.
~~LUIE~Y
ICO1T PACK FOR CYLINDER CHANGES
SEPARATE PURGE CYLINDERS FOR EACH GAS BOTTLE GAS ROOM Fig. 2. Photograph of gas-supply room.
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Integrated laboratory-reactor MOCVD safety system
A schematic of the cylinder manifold system is shown in fig. 3. The cylinder valve assemblies are fitted with flow restrictors and an air-operated shut-off valve is installed immediately downstream of the CGA connector on each tank. These valves are operated remotely from a control panel mounted in the MOCVD laboratory, which also serves as a cylinder status indicator. An excess pressure switch is located in the outlet line from each gas cylinder manifold system so that only low pressure (< 15 psi) gas delivery lines enter the MOCVD laboratory. Each cylinder manifold system is purged with a dedicated nitrogen tank to eliminate the possibility of cross contamination of the gas lines. Purgeevacuate cycles are accomplished with a Venturi educator and the purged gases are directed to the activated-charcoal filtration system prior to discharge. All lines are constructed of stainless steel tubing (seamless 316, 0.035 inch minimum wall thickness) and all interconnections are located in
exhausted enclosures and made with metal gasket fittings. Since the greatest danger of exposure to toxic gases occurs during installation or removal of a gas cylinder, the following safety procedures are followed. (1) A minimum of two people are present during delivery and installation of any toxic gas. (2) Delivery of high toxicity gas cylinders is scheduled only during low occupancy periods in the building. (3) Use of pressure-demand self-contained breathing apparatus by both people during cylinder changes. (4) Shut-off of cylinder valve and evacuation of cylinder manifold system after each run (i.e. gas lines are not kept pressurized with toxic process gases when the reactor is not in use). (5) All cylinder changes are performed according to a posted procedure. (6) A panic button is located just outside the gas
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Fig. 3. Schematic of compressed gas cylinder tank—connection and manifold system.
TO CHARCOAL FILTER DRUM
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Integrated lahorator~-reactor MOCVD safety
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supply room which closes the air-operated cylinder shut-off valves,
4. Reactor design The MOCVD reactor consists of four major sections that are contained in a single vented cabinet which is kept at a negative pressure with respect to the laboratory. These sections are mdicated schematically in fig. I and shown in the photograph of the reactor presented in fig. 4. Brief descriptions of each section are given below, (1) Wafer load area. This section consists of a nitrogen atmosphere glovebox which enables access to the reactor tube load-chamber for wafer loading and unloading in a contaminant-free environment. The load-chamber door is interlocked to the reactor alarm system and prevents
the hydrogen or toxic gas valve electrical control circuitry from being enabled when the door is open. A pressure switch is used to signal the alarm system if the reactor tube becomes over pressurized. If this occurs the reactor inlet and toxic gas valves are automatically closed and the excess pressure is bled-off through a check valve located in the reactor outlet line. (2) Reactor-lube chamber. This is a separately vented compartment which contains the reactor tube, RF coils, etch furnace and reactor pumping system. Hydrogen, toxic gas, fire and ventilationfailure detectors are located within the compartment. The compartment access door is interlocked with the RF generator to prevent accidental contact with live RF coils. A chemical-resistant vacuum pump is used to evacuate the reaction tube and gas handling systern, as well as to enable low-pressure reactor
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laboratory-reactor MOCVD safety system
operation. The pump is equipped with an oil filtration system and has a filter flow sensor which is interlocked to the reactor alarm system. Reaction effluents are evacuated through the vacuum pump and suspended in the oil which is then removed from the pump, filtered and recycled, The reactor exhaust gases are directed though a mineral oil trap located downstream of the vacuum pump and from there pass through a 1-inch diameter stainless steel line to the activated-charcoal filter drum. (3) Gas-handling system. This section consists of the hydride inlet valves, organometallic sources, mass flow controllers and gas-switching valves. The gas system is built in a separately sealed and
at one-second intervals. On receipt of an alarm signal the computer logs the time and alarm identity and transfers control to subroutines that either enable automatic system recovery from the alarm condition or place the reactor into a controlled abort state.
vented enclosure or “gas-box” which is installed as a modular unit within the main cabinet. It contains its own hydrogen, toxic gas, fire and ventilation detectors and the outer cabinet doors are interlocked to the reactor alarm system. The gas-box thus acts as a separate containment area in the event of a gas leak. The gas handling system employs normallyclosed packless valves in all sections except the nitrogen inlet, hydrogen purifier, vent and main reactor inlet lines. These use normally open valves to enable a nitrogen purge of the reactor and inlet/outlet sections of the hydrogen purifier during a power failure. Pressure sensors mounted in the nitrogen and hydrogen gas inlet lines are set to provide low-pressure warning or abort signals. All pneumatically operated valves are controlled by digital output signals from the computer. After each I/O cycle the computer checks the status of the valve relay coil drive circuits. Although this verifies transmission of a relay control signal, it does not provide an unambiguous indication of valve operation. Installation and monitoring of pressure switches in the valve airsupply line would provide additional information regarding relay operation. (4) Computer control system. This section contains the computer electronics, gas system display panel and safety interlock circuits. The computer provides complete control of all reactor sub-systems (air-operated valves, mass flow controllers, furnaces, organometallic source-baths, tube pressure, etc.) and monitors all of the safety interlocks
(3) (4)
5. Reactor safety-interlock system The reactor safety-interlock circuit is interfaced to the system computer and provides status information on: (1) air pressure to the solenoid valves; (2) N2-purge inlet pressure; H2-carrier gas inlet pressure; reactor tube end-cap seal; (5) reactor tube over-pressure; (6) H2-leak detectors; (7) cabinet ventilation; (8) reactor exhaust/vent line flow; (9) RF coolant flow; (10) furnace over-temperature; (11) cabinet doors; (12) vacuum pump filter/trap flow; (13) computer operation; (14) electrical power. A dual-level warning/abort alarm system is employed for monitor points 1—4 of the above list. The remaining status signals are set to give only warning or abort alarms as shown in the diagram of fig. 5. Warning and abort alarms are indicated by LEDs on the reactor status panel, displayed on the computer CRT screen, and listed on a printer. After a warning alarm has been indicated and logged, computer control continues in a normal sequence. On an abort alarm the computer either places the reactor into a nitrogen-purge state or, on an over-pressure signal, turns off all gas and reaction tube inlet valves. Beyond the hardware-based safety measures listed above, computer control enables additional safety to be programmed through interactive software design. All reactor valve sequence and control settings are totally transparent to the operator. For example, if arsine flow is desired in a particular growth step, the operator simple enters the flow rate required and the computer program
R.M. Lum et a!. WARNINGS
/
Integrated laboratory-reactor MOCVD safety system
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__________________________
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Fig. 5. Diagram of reactor safety-interlock alarm system.
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428
R.M. Lum et a!.
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Integrated laboratory-reactor MOCVD safety system
automatically selects the proper valves that need to be energized to accomplish the task. Or an entire evacuation-backfill purge sequence of the reaction tube (or any selected section of the gashandling system) can be specified with one keystroke which selects a menu option. Another menu selection enables automatic evacuation and nitrogen backfill of the metalorganic bubbler input lines prior to a source cylinder change. These software enhancements eliminate most operatorbased errors and provide for safe and reliable reactor operation.
6. Laboratory-reactor safety-interlock system Complete alarm coverage of the MOCVD laboratory and reactor is accomplished by a safety interlock network formed from the fire, hydrogen, toxic gas and ventilation flow detectors located in the laboratory, gas-supply room and reactor. Output signals from these detectors are brought to a central alarm panel mounted outside the laboratory. The safety interlock system is subdivided into sixteen separately defined alarm monitoring zones as shown in fig. 6. This permits exact identification of the specific detector and location giving rise to an alarm condition, which enables immediate determination of the most effective emergency response. Alarm signals from the central safety interlock system result in automatic shutdown of the laboratory and reactor to a latched power-off, nitrogen-purge state. Recovery from the latched abort state is accomplished by restoring power first to the system computer, which then automatically re-boots the operating program to an interactive computer-aided power recovery subroutine. This subroutine guides the operator through the various steps required to safely power-up the remaining laboratory and reactor subsystems.
7. Conclusion A comprehensive MOCVD safety interlock systern has been designed and implemented which provides multiple-zone status monitoring and
alarm signals for critical reactor and laboratory subsystems. Operator response to system alarms is limited to a single rule immediate evacuation of the area. The safety interlock system automatically aborts reactor and laboratory operation to predetermined latched states, provides audible and visual indication of the alarm condition, and identifies the detector and location giving rise to the alarm. Computer control of all operating systems enables additional safety to be programmed through advanced interactive software design that eliminates most operator-based errors and provides for safe and reliable reactor operation. Since its installation during the past year, the safety interlock system has operated successfully through several power failures, both in- and out-of-hours, and has demonstrated its effectiveness for ensuring a dependable fail-safe environment. —
Acknowledgements The authors would like to acknowledge E.G. Burkhardt for his many helpful discussions on the design and implementation of laboratory safety systems, and members of the Environmental Health, Environmental Management and Safety Center for their active participation in the safety design aspects of this laboratory.
References [1] W. Braker, AL. Mossman and D. Siegel, Effects of Exposure to Toxic Gases — First Aid and Medical Treatment, 2nd ed. (Matheson Gas Company, Lyndhurst, NJ, 1979) PP 60, 76, 88. [2] Threshold Limit Values for Chemical Substances in the Work Environment, American Conference of Governmental Industrial Hygienists, Cincinnati, OH, 1984, p. 22. [3] V. Swaminathan, J.L. Zilko and S.F. Nygren, Mater. Letters 2 (1984) 308. [4] A. Mircea, R. Azonlay, L. Dugrand, R. Mellet; K. Rao and M. Sacilotti, J. Electron. Mater. 13 (1984) 603. [5] First Biennial OMVPE Workshop, Cornell University, Ithaca, NY, Aug. 1983. [6] 2nd Intern. Conf. on Metalorganic Vapour Phase Epitaxy, Sheffield, April 1984 [Proceedings published in J. Crystal Growth 68 (1984) 1—502]. [7] M.J. Bevan and K.T. Woodhouse, J. Crystal Growth 68 (1984) 254. [8] E. Johnson, R. Tsui, D. Convey, N. Mellen and J. Curless, J. Crystal Growth 68 (1984) 497.