Semiconductor Exhaust Gas Treatment

The process exhaust downstream from the cleaning, deposition and metal etch processes employed in a semiconductor fab often experiences problems due to fouling. This is most often caused by the condensation and build-up of solid or liquid process by-products on the internal surfaces of vacuum components and exhaust lines. Such build-up can be controlled by maintaining the exposed surfaces at elevated temperatures; therefore fabs require technologies that accurately and precisely control the surface temperatures of the different components in the vacuum train and exhaust gas handling systems. Exhaust streams in a fab frequently contain very corrosive and/or toxic gases that must be removed by chemical scrubbing prior to release to the atmosphere. This entails the use of equipment in most fabs that is dedicated to managing and treating the process exhaust gas stream. Maintenance of this equipment constitutes an important cost factor in all fabs and there is a keen interest in equipment that can reduce or prevent build-up and/or corrosion and prolong maintenance cycles.

CVD processes, for example, convert significantly less than 100% of the process precursor within the processes chamber, and unreacted precursor gases are sent through the vacuum conductance lines to the vacuum pumping and exhaust systems. In many CVD processes, precursor compounds undergo gas phase reactions that produce highly reactive gas-phase intermediate compounds that live long enough to react and produce solids and liquids as they pass through the downstream conductance and exhaust system. The best understood example of this phenomenon is the pyrolysis of TEOS to produce silicon dioxide. When TEOS is introduced into the process chamber it initially undergoes the following gas-phase decomposition reaction:

Si(OC2H5)4 → Si(OC2H5)3OH + C2H4

Since this reaction is endothermic by only 10 kcal/mol, almost all of the TEOS going into the deposition chamber (at ~720°C in low pressure CVD TEOS) is rapidly converted to Si(OC2H5)3OH in the gas phase. The OH group in this molecule makes the intermediate much more reactive than pure TEOS. Residual gases containing this intermediate are pumped out of the deposition chamber and into the exhaust lines of the system where they continue to react even up to and through the vacuum pumps and into the exhaust gas collection system. These reactions produce loosely adhering solid deposits on the walls and other surfaces in the vacuum/exhaust train. These deposits build up and eventually create flow constrictions that impact the conductance characteristics of the system and, ultimately, the yield of the process. The deposits are also a particle source within the system. The stainless steel or aluminum surfaces within the process system and other system components have thermal expansion coefficients that are very different from those of any solid that coats the surface, resulting in high levels of stress between the surface and the coating. This stress causes the coating to flake off as the surface expands and contracts from temperature and pressure changes, which generates particles that remain loosely adhered to the surface. Additionally, particulates produced by gas phase nucleation can also lodge in the uneven surfaces of the stressed film. These loosely adhering particles can be transported back into the process chamber during gas cycling, causing unwanted and unpredictable particle excursions in the process. Effective traps for particles and condensation products, when coupled with heating of the exhaust lines and other downstream components, have been found to resolve this issue since they keep much of the precursors or byproducts in the vapor phase until they reach the waste treatment unit, and the constant temperature of the components reduces the flaking of any deposited material due to film stress and thermal cycling. Figure 1 shows a CVD installation having a particle trap and vacuum conductance lines and vacuum control components that are encased in conformal heaters to control solid deposition and condensation on the internal surfaces.

CVD system vacuum lines equipped with MKS S49 thermal management system process line heaters that reduce solid build-up and particle generation
Figure 1. CVD system vacuum lines equipped with Series 49 process line heaters that reduce solid build-up and particle generation.

Silicon nitride CVD provides another example in which reaction by-products must be maintained in the vapor state until they can be effectively trapped or chemically scrubbed from the exhaust stream. The process employs a large excess of ammonia to ensure that the corrosive HCl reaction product is rendered less corrosive through reaction to produce ammonium chloride:

3H2SiCl2 + 10NH3 → Si3N4 + 6NH4Cl

The ammonium chloride, NH4Cl, is pumped through the downstream vacuum lines and these lines must be heated to a temperature that keeps this abrasive solid in the vapor phase until it can be condensed in an effective particle trap. If allowed to condense in unheated downstream vacuum lines, the ammonium chloride produces a loose, powdery deposit that will backstream into the process chamber during pressure cycling. Heating the vacuum lines to 130 - 150°C in an arrangement such as that shown in Figure 1 keeps the ammonium chloride in the gas phase until it reaches the unheated particle trap.

Etch processes have similar issues with the accumulation of deleterious build-up in the conductance and exhaust lines. Source gases for etching are usually fluorine- or chlorine-containing molecules such as CF4, NF3, SF6, CCl4, Cl2, BCl3 or CCl2F2. The products of etch reactions can form deposits on downstream surfaces. For example, the plasma etching of silicon using CF4 proceeds according to:

CF4 + e-.CF3 + F.

Si + 4F. → SiF4

Here the dot by a chemical formula denotes a radical fragment. In any etch process that uses CF4 or other fluoro- or chlorofluoro-carbons, .CF3 and other radical fragments can sequentially react to produce polymers for a CF4-based etch as illustrated below:

For example, Teflon-like polymer build-up, due to this or a similar reaction sequence, is observed in MEMS manufacturing using the Bosch etch process.

Chlorine-based metal etching processes such as aluminum etch can also produce downstream deposits, in this case crystalline aluminum trichloride, AlCl3. The vapor pressure curve in Figure 2 shows that AlCl3 is a solid at room temperature (25°C) at pressures as low as sub-millitorr. Thus, in an aluminum etch process, with unheated vacuum lines, most of the downstream surface will be coated with aluminum trichloride when the process pressure is in the millitorr range unless the byproduct is trapped close to the source. Heating the vacuum lines to temperatures that maintain AlCl3 in the vapor state at the process pressure resolves this problem. Many other etch processes that use fluorocarbon and inorganic halide gasses produce either polymeric fluorocarbon or solid halide deposits on downstream surfaces.

Downstream of the vacuum pumping system, the process exhaust is fed to a centralized exhaust treatment facility in most semiconductor fabs. These facilities are generally described as exhaust "scrubbers". Typically, fabs employ more than one such unit, differentiated by the type of exhaust stream that the unit must treat. "Acid" gases such as AlCl3 emanate from etch processes and include hydrogen fluoride, hydrogen chloride, chlorine, fluorine, silicon tetrafluoride, Perfluorocarbons (PFCs), nitric and sulphuric acids, as well as with other acidic compounds. Wet scrubbers are used to react acid gas residues with aqueous neutralizing solutions prior to discharge to atmosphere. "Basic" effluents such as ammonia are normally treated separately from acid gases, both because they react with the acid gases and because they require different scrubbing chemistries. Wet scrubbers are normally used for neutralization of basic gases as well. Furthermore, many CVD processes employ flammable, pyrophoric, and corrosive gases that are best removed or rendered harmless by combustion in a dedicated chamber. Finally, dry, chemisorption scrubbers are commonly employed for the removal of toxic gas residues from the exhaust stream.

In addition to acid and ammonia process effluent abatement, the U.S. Environmental Protection Agency has amended its Green House Gas (GHG) Mandatory Reporting Rule 40 Part 98 to include semiconductor fabs in the requirement to report GHG emissions from their facilities. GHG's are emitted from many semiconductor processes including CVD chamber cleans, etch, and wafer cleaning.

AlCl3 vapor pressure curve
Figure 2. AlCl3 vapor pressure curve.

Series 49 Thermal Management System

MKS offers a series of advanced heaters and heater control systems. The MKS Series 49 thermal management system is well-suited for vacuum line heating applications in ALD, CVD, and etch processes, as well as in many other deposition applications. These heaters are conformal, use less energy than conventional heat tracing/insulation installations and have superior control of temperature uniformity. The Series 49 simplifies maintaining accurate and precise temperature control while simultaneously providing improved safety functions and more comprehensive communications options.

Heated Vacuum Valves

MKS's Jalapeño Series vacuum valves are internally heated valves that prevent condensation of reactive vapors and volatile solids. These include regular and low-profile heated bellows isolation valves. MKS also offers manual and pneumatic heated vacuum ball valves.

Process Traps

MKS offers a variety of traps designed for specific process applications. These range from particle and condensation traps for conventional deposition and etch processes to heated traps appropriate for ALD applications.

The Metal Etch Trap is an efficient, high capacity trap specifically designed to capture the condensable chloride vapors generated in aluminum etch processes. This trap is especially useful in preventing condensable, corrosive vapors such as AlCl3 from entering dry chemisorption scrubbers where they can rapidly clog the unit and destroy the scrubbing capacity. It is specifically designed to capture large quantities of condensable chlorides without reducing the vacuum pumping speed in the etch system.

MKS also supplies the TEOS Trap as one element of a TEOS Effluent Management Subsystem. The TEOS trap collects TEOS byproducts, preventing them from backstreaming into the process chamber and/or causing pump oil contamination and abrasive damage of internal pump components. The use of TEOS traps have been demonstrated to produce a reduction in particulates of greater than 20% in process. System maintenance is simplified with a TEOS trap in place, replacing the cleaning of many feet of piping with cleaning a single component. The large trapping capacity leads to longer preventative maintenance cycles. High trapping efficiency provides better protection to the pump, valves, and other downstream instrumentation.

CVD silicon nitride processes probably have the longest history of maintenance issues arising from pump wear due to abrasion by ammonium chloride particles in the exhaust stream. Newer material deposition processes such as CVD titanium nitride exhibit similar issues that require preventive action. MKS offers the Vapor Sublimation Trap for use in silicon nitride and titanium nitride processes. In combination with heated lines, valves, and other vacuum components, the vapor sublimation trap effectively eliminates ammonium chloride build up in vacuum forelines of these CVD systems.

Nitrogen Barriers and Scrubber Inlet Transition Devices

MKS offers a unique Virtual Wallª nitrogen barrier device that controls particulate and deposits using a nitrogen gas boundary layer between the inner surface of vacuum forelines and the exhaust gas stream. The Virtual Wall, originally developed as part of the TEOS Effluent Management Subsystem, is finding application in a number of semiconductor applications, including tungsten deposition and metal etch. The MKS Application Note titled: The Versatile MKS Virtual Wallª Gas Barrier Device includes a discussion of the different applications of the Virtual Wall.

MKS also offers custom scrubber inlet transition kits for wet scrubbers that prevent water back-migration into exhaust lines. Kits are customized to process chemistries to keep precursors and byproducts moving into the scrubber system.

Exhaust Monitoring

MKS provides a variety of analytical solutions for monitoring semiconductor fab exhaust streams. Data from these tools can be used for both process optimization and regulatory compliance. MKS's analytical tools are based on either optical spectrometry or mass spectrometry.

MKS's MultiGas 2030 continuous gas analyzer is based on Forier Transform Infrared (FTIR) Technology. It provides accurate and precise analyses of gas streams that can be configured for applications in both vacuum forelines and process exhausts. The 2030 has been certified EPA-compliant for measuring the destruction efficiencies of abatement systems for F-GHGs. MKS has publications available that describe the use of the MultiGasª 2030 for both process optimization (e.g., 1, 2, and 3) and exhaust monitoring for regulatory compliance.

The MultiGas 2030 continuous gas analyzer and Precisive 5 Trace Hydrocarbon/VOC analyzer have been proven as effective monitors in processes where monitoring the efficiency of volatile organic compound (VOC) abatement systems is necessary.

The Cirrus 3 benchtop atmospheric pressure gas analysis system has also been proven as an analytical system for exhaust monitoring, especially for greenhouse gas (GHG) emissions.

MKS Semiconductor Handbook Cover

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