Green and Clean

Investigating an integrated approach
to etch emissions management

Steve Whitten, Lam Research; Mat Waltrip, formerly of Lam Research; and Joseph Van Gompel and Peter Mawle, BOC Edwards

An exhaust management system developed to meet the industry's GWP emissions targets provides cost and time savings and can lower 300-mm tool footprint requirements below those of 200-mm fabs.

Reducing their fabs' emission of global-warming perfluorocarbons (GWPs) continues to challenge semiconductor manufacturers. While NF^₃ has largely replaced GWPs for chemical vapor deposition chamber cleans, identifying suitable substitute gases for use in etch processes has proven more difficult. Therefore, any attempt to reduce etch-related emissions requires understanding the chemistries being emitted and then adopting effective exhaust management strategies, including the use of sophisticated abatement.¹

Perfluorocarbon (PFC) compounds were adopted for use in etch processes because they achieve the critical balance of carbon and fluorine that is needed for anisotropic (directional) etching, which, in turn, is required to create high-aspect-ratio structures. The advantages of these compounds include their low cost and safe-handling properties. However, their stability and strong infrared absorption make them potent contributors to global warming (see Table I).² Though not a concern for global warming, by-products generated during etch and present in the exhaust stream, along with unutilized feedstock gases, are toxic and therefore also require treatment. Examples of such by-products include hydrofluoric and hydrochloric acids, carbon monoxide and dioxide, COF_², OF_², F_², and Br_².

Gas Global Warming Potential CF4 6,300 CH2F2 650 CHF3 11,700 Cl2 NA HBr NA SF6 23,900 C4F8 8,700 CO NA C2F6 9,200 C3F8 7,000

Table I: The 100-year global warming potential of typical plasma etch feedstock gases.2

Historically, approaches to reducing plasma-etch GWP emissions have involved process and hardware optimization, gas substitution, and the use of abatement equipment. Process optimization also has been important for improving throughput, performance, and consumables costs. Optimization strategies have included designing more-efficient plasma reactors, reducing recipe stabilization times, and achieving precision etch stops by employing automated end-point detection. Etch systems suppliers have sought to improve carbon-equivalent (CE) emissions per wafer pass for each tool generation.³ For example, the data in Figure 1 reflect via-etch emissions for 200-mm dielectric etch systems supplied during the 1990s, along with those for a new 300-mm tool, which was linked to an integrated abatement system.

Figure 1: GWP-emissions-per-wafer-pass comparison of dielectric etch systems that became commercially available during the past 10 years.

Substitution of more environmentally benign gases for CF^₄, CHF^₃, and C^₂F^₆ has been employed whenever possible in developing etch processes; for example, multiple low GWP C^_xF^_y gases are in mainstream use for low-k dielectric etch processes. Use of complex fluorocarbons and smaller amounts of fluoromethane additives also help reduce the global warming impact of PFCs.

Although these strategies have already led to improvements, the World Semiconductor Council has voluntarily committed to further reductions in overall industry GWP emissions, setting the goal at 90% of the 1995 baseline levels by 2010.⁴ Achieving this target will require more-aggressive approaches, such as abatement technologies and the development of more-sophisticated exhaust management strategies to minimize the costs of implementation. To address these challenges, etch-tool supplier Lam Research (Fremont, CA) and BOC Edwards (Wilmington, MA), which develops environmental, health, and safety systems, began a joint study and development project in September 2000. This article describes the goals, methods, and results of that study.

Developing and Testing an Integrated System

The primary goals of the joint project were to characterize 300-mm etch process equipment emissions and develop cost-effective point-of-use (POU) abatement options. To that end, BOC Edwards developed a unit that incorporated multiple abatement technologies and added that unit to an integrated exhaust management system architecture that was already in development. The combined system includes an integral vacuum pump, two POU abatement systems, a piping temperature management system, automatic bypass valves, and control system intelligence capabilities, all of which are housed in a single compact enclosure.

System Design. In concept, any mix of several abatement technologies could have been combined with exhaust components to achieve an integrated system. The options for POU abatement technology on plasma etch processes fall into five broad categories: wet scrubbing systems, oxidation systems, cold bed systems (e.g., adsorbers, chemisorbers), hot-chemical-bed systems, and reactors (e.g., plasma, microwave).¹ The system designed in this study integrates roughing pump, plasma reactor, and hot chemical bed. The reactor destroys PFCs upstream of the roughing pump before the gases are diluted, using plasma and oxygen as a forming gas. By-products from the plasma reactor are pumped to the hot bed, which removes hazardous gases from the vacuum pump exhaust. The hot-bed system consists of two cartridges containing an inorganic granule mixture that chemically reacts with the hazardous gases at elevated temperatures to form inert, stable, inorganic salts. An onboard programmable logic controller automatically switches the gas flow between the cartridges, based on a high-pressure signal, end-point monitor signal, or elapsed time. When one cartridge is depleted, it can be replaced without interrupting operations. This design addresses all of the issues involved in plasma etch exhaust management, including PFC abatement, abatement of hazardous air pollutants (HAPs), and pressure control.

Combining these elements into a single system required the designers to address several areas of concern:

• Ensuring POU abatement systems are in close proximity to the roughing pump.
• Retaining the volatility of by-products during exhaust stream removal.
• Providing bypass valves to keep the system operating in case of blockage.
• Providing an exhausted enclosure to ensure compliance with SEMI S2 and F15.

System Testing. As the next step in the project, the system was tested on 300-mm lab systems for dielectric and silicon etch at the Lam facility. A block diagram of the test setup is presented in Figure 2. The International Sematech protocol was adopted as a guideline for conducting the quantitative emissions analysis.⁵ This protocol specifies the use of quadrupole mass spectroscopy (QMS) and Fourier transform infrared (FTIR) analytic measurement equipment that has been calibrated to ensure that the results are repeatable and linked to a known standard. The protocol also requires a halide volume balance closure to ensure that all compounds containing halides (chlorine, fluorine, and bromine) are accounted for in the analysis. A diagram of the analytic equipment layout is shown in Figure 3.

Figure 2: Schematic of the equipment layout during testing of the integrated exhaust management system, including sampling points. All components within dashed rules are within a single enclosure.

Figure 3: Schematic of the analytic equipment layout.

All measurements were performed in a simulated production environment, and quantitative measurements of by-products from the 300-mm etch equipment were taken first to establish an emissions baseline for use in evaluating abatement effectiveness. QMS and FTIR spectrometry were used to characterize the emissions both upstream and downstream of the abatement system for four 300-mm processes: dual-damascene via oxide etch, dual-damascene trench oxide etch, shallow trench isolation silicon etch, and polysilicon gate etch.

HAPs and reactive by-product emissions following abatement were below the detection limits of the analytical instrumentation, indicating that the exhaust was being effectively scrubbed. The results for GWP emissions are summarized in Table II. These results represent the dual-damascene etch process flow for a seven-layer 100-nm logic device, including shallow trench isolation and polysilicon gate etches. The overall percent destruction removal efficiency (%DRE) for the process flow was 88%, which included gas usage inside the plasma etch reactor and any remaining exhaust gas in the abatement module. On an absolute emissions basis, the total GWP emissions per 300-mm wafer start was 32,188 kg of carbon-equivalent emissions per 10,000 wafer starts, or 3.2 kgCE per wafer start.

GWP Emissions Metric. An industry consensus on an absolute environmental impact metric for process tools, such as mass of carbon-equivalent emissions per wafer pass or per micron processed, has not been achieved. Yet such a quantitative tool is needed if researchers are to examine projected emissions and establish design targets for emissions reduction. It is also a prerequisite to comparative evaluations of results such as those presented in Table II.

GWP emissions are typically accounted for by converting all gases to million metric tons of carbon equivalents (MMTCE).⁴ According to U.S. EPA estimates, 1.5 MMTCE of high-GWP gases were emitted by the semiconductor industry in the United States in 1995 and 4.7 MMTCE were emitted globally.² Adopting a budgetary approach toward future emissions requirements, an optimistic worldwide target for 2005 could be 90% of 4.7 MMTCE, or 4.23 MMTCE (although the World Semiconductor Council target is to reach this level by 2010). VLSI Research projects that 71.5 x 10⁹ cm² of silicon will be processed worldwide in that year, based on a 17% compound annual growth rate. Taking the ratio of the target high-GWP gas emission level to the total silicon processed, the 2005 target metric for global warming impact can be calculated as 59.2 x 10^­3 kgCE/cm² of silicon. Translating this result to high-GWP emissions per 300-mm wafer starts, the target metric becomes 41.8 kgCE per 300-mm wafer start. Finally, if it is assumed that etch emissions account for 40% of the total carbon equivalent, the etch emissions budget would be 16.7 kgCE per 300-mm wafer start for 2005.

The 3.2 kgCE per 300-mm wafer start achieved with the integrated exhaust management system is well below this emissions target. Therefore, the study's focus shifted to assessing the system's implementation costs. Several factory integration­related concepts were addressed, including equipment interfaces, footprint, and installation and start-up.

Exhaust Management and Factory Integration

The 2001 update of The International Technology Roadmap for Semiconductors lists several needed improvements in production equipment technology in its section on factory integration.⁶ These goals include reductions in relative equipment footprints, production equipment lead times, and production equipment installation costs (see Table III). The study evaluated whether taking an integrated approach to exhaust management could meet all of those related requirements.

Equipment Interfaces. To maximize the benefits of integrated hardware, intelligent communication and control is needed. Specifically, effective exhaust management requires understanding the types of emissions events that occur and ensuring the system has the flexibility to handle them.

Wafer fab process emissions can be categorized as planned and unplanned. Planned emissions of HAPs and PFCs consist of the treated exhaust from all equipment in the facility. Unplanned emissions occur if the abatement unit installed to treat the exhaust of a particular process system trips off-line as the result of a fault condition and bypass valves divert the flow to an exhaust duct downstream. If the process equipment and its abatement unit do not have an intelligent communication interface, the system may continue to process wafer lots until an operator notices the problem and stops processing. In contrast, when there is such an interface, the system will automatically take action to prevent any unplanned emissions. This action could take the form of a "hard" or "soft" shutdown. A hard shutdown would immediately abort wafer processing and presumably scrap a wafer; a soft shutdown would allow the tool to finish processing the work in progress and then abort further processing. Preferably, the interface should be flexible enough to allow the end-user to balance wafer protection and yield requirements against environmental protection.

To link its 200/300-mm etchers with the integrated exhaust management system developed during this study, Lam provided an optional end-user-configurable LonWorks software interface from Echelon (San Jose). This interface provides two-way communication and controls the exhaust system's roughing pumps and abatement units. Specifically, its capabilities include an abatement request function to signal a POU abatement on-off command; user-configurable POU abatement error handling; and three fault-condition options, which are: to continue operating, to complete the process on the wafer in progress but not start another until the fault is cleared, or to abort wafer processing immediately. This type of interface allows the exhaust management system supplier to change its product as required to meet specific end-users' needs. For example, BOC Edwards could replace the plasma reactor and hot-bed abatement modules in the system discussed here with a thermal-processing-unit module.

Footprint Reduction. As the semiconductor industry switches to 300-mm wafers, overall equipment footprint is among the important facility-related concerns. To minimize footprint requirements, an early evaluation must be made of the floor space needed for abatement equipment. In this study, it was determined that use of 200/300-mm etch process equipment and the integrated abatement system reduced the relative subfab footprint by 18% compared with pre-300-mm layouts using similar auxiliary equipment, including vacuum pump and abatement modules (see Figure 4). This reduction resulted in part from the reincorporation of some etch tool auxiliary equipment into a more-compact process module design and in part from combining the exhaust management system's vacuum pumps and abatement reactors into a single unit.

Figure 4: Comparative subfab layouts: (a) a 200-mm fab with separate etch and abatement equipment and components, and (b) a 300-mm fab with the integrated exhaust management system. Both include service access areas. If the footprint in (a) equals 1, that of (b) equals 0.82, an 18% reduction.

Installation Costs and Scheduling. To evaluate relative costs, data were collected on actual installation expenditures at major U.S. and European fabs for individual exhaust system components and for the integrated exhaust management system. Overall cost for both approaches was based on a four-chamber process system using dry pumps and thermal-processing-unit technology, and included pumps, abatement units, connecting pipeworks, utility hookups, exhaust extraction for enclosures, interfacing and monitoring, and pipe heating and traps, among other components. Calculations based on the minimum actual expenditures for individual components demonstrated that end-users could reduce installation costs by more than 80% by using an integrated system.

Integrating exhaust and abatement components into a single unit reduces start-up time as well. A modular design helps minimize manufacturing lead times, greatly simplifies installation, and enables a single facility hookup connection. Built-in connecting pipework and interface connections can be tested prior to shipment, and installation can be accomplished quickly. The system discussed here, for example, can be installed in approximately three days, compared with 30 days or more for the installation of discrete items. These advantages significantly reduce the scheduling risk involved in starting up and qualifying process systems in new 300-mm fabs.

Conclusion

A thorough understanding of plasma etch process emissions and the adoption of new abatement technologies will be critical to achieving the semiconductor industry's GWP emissions reduction targets. By modeling the emissions produced by a typical process flow, environmental, health, and safety specialists can plan ahead and budget for facility emissions. The abatement technology evaluated in this study, which combines plasma and hot-bed reactors, met the established performance requirements when used with an etch system featuring intelligent interface software. Providing such an interface between the process equipment and the abatement system automates fab environmental controls and prevents unplanned emissions. In addition to its emissions capabilities, the integrated exhaust management system simplifies installation compared with individual components and contributes to shorter lead times for start-up and qualification of process equipment, which will help fabs meet the factory integration requirements of the industry's technology roadmap.

Acknowledgments

The authors wish to thank Albert Ho, formerly of Lam; Derek Baker and Farhad Fereydouni of BOC Edwards; and John Coles, formerly of BOC Edwards, for their contributions to the collection and analysis of emissions data described in this article.