Safe usage of C1F3: Supply, vacuum service, and exhaust gas management
09/01/1997
Safe usage of ClF3: Supply, vacuum service, and exhaust gas management
A.M. Pierce, Edwards High Vacuum International, Milpitas, California
M. Taylor, Edwards High Vacuum International, Nailsea, United Kingdom
J. Sauer, D. Ruppert, BOC Gases, Murray Hill, New Jersey
Chlorine trifluoride (ClF3) is a highly reactive process gas used in chemical vapor deposition (CVD) and diffusion furnace applications for nonplasma cleans. Relatively new to US wafer manufacturers, ClF3 is an aggressive oxidizer. There is growing concern about safety, materials compatibility, and gas handling as ClF3 is increasingly used in semiconductor processing. Recent work in the laboratory and the field demonstrates an effective, safe system for ClF3 supply, vacuum service, and abatement.
In CVD processes, gases such as silane (SiH4), dichlorosilane (SiH2Cl2), and tungsten hexafluoride (WF6) leave a solid residual deposit on the internal surface of the process chambers. These residues must be periodically removed to prevent wafer contamination. Previous cleaning methods were performed by maintenance staff donning full breathing apparatus. Chamber parts are removed for wet cleaning, and the chamber walls are manually scrubbed. Traditional plasma cleaning, which uses a plasma to generate a reactive species from a gaseous source, is limited by the mean free path of the reactive species. ClF3 provides a dry, nonplasma alternative and for a number of years has been used successfully as a cleaning gas in the Pacific Rim.
ClF3 cleans process tool chambers by reacting with nonvolatile solid materials such as Si and W, forming volatile fluorinated and chlorinated reaction products that are pumped away for subsequent abatement. Studies show that ClF3 effectively cleans the internal surfaces of CVD chambers, penetrating remote corners where plasma-generated reactive species cannot easily reach. The in-situ chamber clean does not require a vacuum break, leading to longer process tool up-time and safer working conditions for operators. ClF3 cleans result in lower particulate levels and less scheduled maintenance [1], increased tool utilization, and lower overall operating costs.
ClF3 system design and safety issues
ClF3 is a toxic, corrosive, low vapor pressure (LVP) gas. It is a powerful oxidizing agent, undergoing vigorous reaction with oxidizable substances, e.g., organic materials, hydrides, water. ClF3 has a short term exposure limit or Threshold Limit Value (TLV) of 0.1 ppm (see table). Exposure to higher levels of ClF3 will lead to extremely severe chemical and thermal burns. Clearly, gas delivery, materials compatibility, and point-of-use abatement are key requirements for the safe use of ClF3.
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ClF3 is supplied to users in cylinders under its own vapor pressure of 22 psia @ 70?F. Special design considerations are needed to maintain a LVP gas such as ClF3 in its vapor phase after it leaves the cylinder and enters the distribution system. Thermodynamic conditions, e.g., a sharp drop or sudden increase in pressure, temperature or volume, leading to liquefaction, must be avoided because liquid ClF3 can degrade distribution system components. A systems approach and a thorough understanding of ClF3 properties are necessary to ensure a safe and reliable design for each particular application and site. This is best done through careful fluid and thermodynamic calculations in order to avoid environmental extremes.
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Figure 1. Typical ClF3 distribution system. The number of chambers/exhaust gas management can vary with gas flow rates and process.
The ClF3 gas supply system
Figure 1 shows the basic system diagram for a typical ClF3 supply system. It is based on a single cabinet with auto-switch-over capabilities supplying a number of process chambers through manifold piping. Other configurations are possible based on fab layout and require considerations (cost, reliability, operational simplicity, tool use rates, equipment location, etc.) for maintaining ideal gas flow conditions and personnel safety at all points.
BOC has characterized the temperature-dependent reactions between the gas phase of ClF3 and the various metal alloys used in gas distribution piping [2]. Although there is a clear difference in temperature dependence between the alloys tested, the differences at room temperatures are negligible and therefore provide options for material selection based on other considerations (cost, particle shedding, expected life, etc.). All of the alloys tested (e.g., nickel, Hastelloy, Elgiloy, and stainless steels) developed a fluorinated passivation layer that protects the base metal from subsequent corrosion at ambient temperatures. High temperatures (100-200?C or higher) in some alloys will destroy the protective layer, leading to further corrosive attack. Temperature control in the distribution system is an important design consideration. Any intrusion of atmospheric air or moisture through a leak will destroy the passivation layer. Repassivation would be required.
Design safety considerations. During the design phase, the ClF3 system must be evaluated under maximum process flow conditions to determine the pressure at which the gas leaves the gas cabinet and enters the distribution lines. This pressure is controlled by a number of factors (flow rate, cylinder temperature, length of time flow, etc.). If a pressure regulator is used (an absolute pressure regulator is typically required), the set point becomes the pressure at the starting point of the distribution lines. Fluid dynamic principles (pressure drop, velocity, and flow equations for gases at variable temperatures) can be used to size the lines at this pressure for the lengths and flow ranges required. If a system is designed with cabinet regulation and for specific maximum flows, cylinder or line heating can be avoided.
Operational safety is achieved by eliminating the potential for ClF3 liquefaction, which corrodes system components. A proper design considers the range of temperature fluctuations possible throughout the system. Designers must also incorporate features to ensure personnel safety, including the use of coaxial tubing, leak detection, auto shut-down, fail-safe purge operations, and adequate design margins. The final safety asset is a seamless set of hazardous incident response team (HIRT) procedures and training of on-site personnel.
Vacuum pumping and exhaust management
To assure reliable and safe process equipment operation, vacuum pump and exhaust gas management systems are essential considerations. As with gas supply, materials compatibility (e.g., nickel, Hastelloy, Elgiloy and stainless steels) [3] is considered in system design and operation to forewarn of materials issues, to specify vacuum pump operating temperatures, and to establish exhaust line temperature control. Some materials may not be suitable for ClF3 handling under certain conditions and temperatures because of volatile chlorides. Air leaks that can introduce water vapor tend to oxidize ClF3 to HF and HCl, which aggressively corrodes many of the metals tested. Leak-tight exhaust lines and appropriate nitrogen dilution are therefore key to successful vacuum management.
Studies conducted under standard deposition process conditions using a ClF3 chamber clean revealed no problems with the pumping system after 33 days of continuous operation. Both laboratory and field trials have emphasized the importance of a leak tight system to protect fittings, mass flow controllers, and pipe work. In the field, vacuum systems pumping ClF3 have been operating in the US for more than 18 months without failure. In fact, ClF3 (as a chamber clean gas) also keeps the vacuum system free of particulate buildup.
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Figure 2. Experimental setup for ClF3 abatement verification in a GRC. Positions 1 and 2 correspond to locations where gases were analyzed, and 3 corresponds to sample lines to positions 1 and 2.
Exhaust gas management
The highly reactive and corrosive nature of ClF3 demands its point-of-use abatement whenever used. Exhaust gas management technology must be designed to suit the types of process gases as well as their by-products. Total exhaust gas flow rates frequently include additional pages specified for reliable vacuum pump operation or by the OEM for safety reasons.
Water scrubbers by themselves should be avoided, due to the high risk of explosive reactions; this risk increases with high ClF3 flow rates. Hydroxides in solid bed adsorbers risk the excess generation of thermal energy and formation of hazardous by-products such as chlorine oxides. Thermal treatment of ClF3 would require an efficient wet scrubber to remove acid gases from the exhaust stream. Dry exhaust conditioning with hot bed or controlled thermal treatment with effective wet scrubbing has proven successful and safe for ClF3 abatement.
Hot chemical reactor columns. Traditionally, the Edwards gas reactor column (GRC) has been used to abate ClF3 for flows up to 700 sccm. The GRC (Fig. 2) is a hot, dry cartridge-based system that converts hydride and halogen gases into stable solids and benign salts. The cartridge contains two stages. The first is a mixture of inert metals that initiates heat transfer and thermal decomposition of hydrides (e.g., WF6, SiH4); the second stage is predominantly lime, which decomposes halogen gases into stable salts (e.g., CaCl2, CaF2).
Prior to using the GRC in an actual fab, laboratory tests verified the safe and effective destruction of ClF3. A mass spectrometer provided the base line of the inlet concentration of ClF3 and monitored its destruction. Figure 3 is a mass spectrum of ClF3 at the GRC inlet. Position 1 in the heated cartridge illustrates the thermal decomposition of ClF3 into halogenated species. No halogen gases were detected by position 2 and the subsequent sample points within the heated cartridge also showed the absence of halogen gases (Fig. 4).
In a typical GRC installation processing ClF3, a single system serves each chamber to prevent the mixture of incompatible gases as deposition and chamber clean gases are treated sequentially.
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Figure 3. The mass spectra of ClF3 and N2 at the inlet of the GRC (100 sccm of ClF3 + 25 slpm of N2).
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Figure 4. The mass spectra of the gases at the GRC outlet (100 sccm of ClF3 + 25 slpm of N2).
Thermal treatment. New process tools demand much higher rates of ClF3 delivery during their clean cycles. For flows in excess of 700 sccm, the thermal processing unit (TPU) provides safe and efficient abatement of deposition gases and ClF3. The TPU is an inward-fired combustor coupled with a highly efficient water scrubber where ClF3 is completely oxidized to HCl and HF (Fig. 5). The resulting acid gases are subsequently removed by the three-stage wet scrubber. There is no direct contact of ClF3 with water and no conversion into unwanted chlorinated species or other by-products.
The TPU can handle up to four exhaust gas streams in one system. Because the gases enter the combustion chamber through separate inlets, gases do not mix prior to oxidation. The TPU can therefore provide effective ClF3 abatement for a four-chambered tool where the deposition and clean gases are treated safely in one system.
TPU abatement efficiency of ClF3 gas (>1200 litres, 4.5 kg total) efficiency was monitored in situ using mass spectroscopy and gas detection analysis of the TPU off-gases. The exhaust gas line was monitored to ensure that toxic gases were below their respective TLVs. The delivery of ClF3 to the TPU was interrupted with periodic deliveries of silane (200 sccm - SiH4) through the same TPU inlet. The tests verified safe destruction of ClF3, deposition gases, and their reactants without the formation of any harmful by-products (i.e., chlorinated organics, Fig. 6).
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Figure 5. Edwards High Vacuum`s thermal processor unit (TPU).
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Figure 6. Experiment set-up.
Measurements of the Cl-ion content of the TPU waste water using a selective electrode (ISE) indicated that within the accuracy of the experiment (?3 ppm), all the chlorine from ClF3 was released from the TPU as aqueous HCl. The minimum measurable limit for Cl-ion detection by this method was 1 ppm. Lower flow rates of ClF3 gave very similar results, indicating that the conversion of chlorine from ClF3 to HCl is efficient inside the TPU (100% within experimental detection limits).
Gas chromatography-mass spectroscopy (GC-MS) analysis of the exhaust gases from the TPU and of the waste water to below ppb levels indicated that there was no formation of unwanted polychlorinated biphenyls (PCB), organochloride, or dioxin by-products. All SiH4 admitted into the TPU during the trials was destroyed to <1 ppm detection limit on a silane paper tape detector, to below the measurable resolution and hence below TLV. No SiH4 was observed by the mass spectrometer during these trials.
Chemical detection tube readings of the TPU gaseous exhaust at 2000 sccm ClF3 indicated the following:
Cl2 Draeger - 0 ppm
HF Draeger - 0 ppm
HCl Draeger - 0 ppm
NOx Draeger - 0 ppm
F2 Draeger - 0 ppm
ClF3 destruction efficiency - mass spectroscopy data. The mass spectra (Figs. 7, 8) show the effluent exhaust to extract gases from the area above the packed tower of the TPU before and after processing 2000 sccm ClF3. The spectra illustrate that there is no gaseous output from the TPU attributable to ClF3 or its destruction. The mass peaks present (Fig. 7) correspond to products of the combustion process occurring inside the TPU burner. There is no evidence of any gaseous ClF3, Cl2, HCl, PCB, dioxins, or organochlorides formed during the destruction of ClF3 by the TPU, suggesting that the TPU destruction efficiency for ClF3 is 100%.
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Figure 7. Background mass spectra of TPU exhaust gases: without ClF3.
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Figure 8. Mass spectra of TPU exhaust gases where inlet gases include 4.2 slpm of CH4 + 2000 sccm ClF3.
Figure 9 shows the mass spectrum obtained by sampling 200 sccm ClF3 in 50 slpm N2 before entering the TPU. There are no visible ClF3 peaks at an atomic mass of 89-90. There are, however, many peaks corresponding to the reaction products of ClF3 and the various materials inside the mass spectrometer housing and sample inlet. For example, the peak at 85 mass units represents the reaction product, SiF3, which is formed as ClF3 reacts with the quartz capillary inlet tube of the mass spectrometer. Formations of other reaction products (corresponding to remaining peaks) are similar. The SiF3 peaks are quite high and are easily distinguished by the mass spectrometer in the presence of ClF3.
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Figure 9. Mass spectra of TPU inlet of 200 sccm of ClF3 + 50 slpm of N2.
Data for tests run at 2000 sccm ClF3 was chosen here because it is the highest flow of ClF3 run in these tests and thus represents the greatest challenge in terms of destruction efficiency to the TPU. ClF3 at lower flow rates was equally well destroyed and gave similar mass spectra under identical experimental conditions.
Visual inspection of the TPU internals during testing revealed no evidence of damage to the combustor lining, quench zone, or head assembly. There was slight oxidation of the Inconel 600 inlet nozzles in the head assembly (inlet 2), indicated by a pale orange discoloration, but there was no evidence of measurable corrosion to any part of the head, including the Viton gasket seal, aluminosilicate insulator block, thermocouple, etc. Measurements of the combustor pad thickness before and after the testing indicated that there was no loss of the pad material. The quench zone was undamaged, with no visible evidence of corrosion to the flux force condensator, cyclone, or packed tower. There was no significant silica build-up on any of the TPU internals from the passage of the 0.2 kg of SiH4.
The materials compatibility, performance and environmental impact data collected from rigorous tests of the effects of ClF3 and SiH4 on the TPU indicate that it is suitable as an abatement tool for all ClF3 clean gas applications up to flows of 2000 sccm. The TPU completely reduces ClF3 in the combustor section to inorganic reaction products that are subsequently removed by the integral wet scrubber. The materials contained within the TPU exhibited no evidence of corrosion after passage and subsequent abatement of more than 1200 l of ClF3 and 200 l of SiH4. Independent GC-MS measurements of the exhaust gases and waste water from the TPU indicate that ClF3 destruction in the combustor and subsequent wet-scrubbing does not lead to the formation of any harmful organochlorine/PCB/dioxin based by-products [3].
Conclusion
ClF3 can be supplied, handled and managed safely. It is a highly reactive - but clean - gas and is an alternative for chamber cleaning. It reduces the atmospheric exposure of the deposition chamber and reduces particle-related maintenance issues with the vacuum system. Controlled laboratory tests and field applications are the foundation for designing a safe, reliable ClF3 gas train that incorporates temperature and structural variations for specific sites. A hot bed reactor or a combustor-water scrubber provide effective point-of-use abatement over a wide range of ClF3 gas flows (0-2000 sccm/chamber). An integrated ClF3 delivery, vacuum pumping, and exhaust management system minimizes safety risks while maximizing tool availability for CVD processes.
References
1. Varian MB2 CVD System`s Unique Plasma-Free Chamber Clean Favorably Impacts Cost Of Ownership, Varian Associates, Inc., April 10, 1995.
2. B. Fruhberger, A.P. Taylor, R. Hogle, and L.F. Link, Interaction of Chlorine Trifluoride (ClF3) with Metal Alloys, The BOC Group, April 1997.
3. M. Taylor. Life-Test and Performance Evaluation of the Edwards TPU for Destruction of ClF3. Edwards High Vacuum International, April 2, 1997.
ADRIENNE PIERCE received her degree in chemical engineering from Lafayette College and her master`s degree in international business from the American Graduate School of International Management. She is US product manager for Exhaust Management Systems at Edwards High Vacuum, where her responsibilities include project management of Beta sites and new product applications, and coordination of US product support and infrastructure. Edwards High Vacuum International, 550 Sycamore Drive, Milpitas, CA 95035; ph 408/428-1037, fax 408/428-1065.
MARK TAYLOR received his degree in chemistry, his MSc in surface chemistry and colliods, and a PhD in inorganic chemistry-all from Bristol University in the United Kingdom. He is a development engineer in the Exhaust Gas Treatment Division of Edwards High Vacuum International.
JOHN SAUER received his BS degree in mechanical engineering from New Jersey Institute of Technology. He is assistant product manager for Electronic Special Gases, with BOC Gases, Murray Hill, NJ.
DAVID RUPPERT received his BSdegree in electrical engineering from New Jersey Institute of Technology. He is manager of the Systems Engineering Dept. in the Electronic Gases Section of the BOC Group, Murray Hill, NJ.