Integrate gas, chemical, vacuum, and exhaust design
10/01/1999
The integrated fab approach to gas management is now being extended to include vacuum and abatement solutions through sophisticated software modeling. The benefits of full integration of principal fab facilities include lower costs in fab construction and operations; improved environmental, safety, and handling results; faster execution of new fab construction; and the ability to address unique tool process needs that are enhanced with guarantees of total gas, chemical, and vacuum system performance.
A new fab's core gas, chemical, vacuum, and exhaust treatment facilities package might cost $50-60 million. Adding distribution and interconnecting piping, utility, control, and monitoring systems often brings the sum nearer $100 million. Increasingly, we are seeing "total" and "integrated" facilities solutions promoted by today's ever-bigger equipment and systems suppliers to better manage these costs. BOC Edwards, for example, has brought together under one company the supply of gas, chemical, vacuum, and exhaust treatment solutions. Multiskilled teams were established for turnkey design, project execution, site services, and specialist services across all product areas.
 Figure 1. A typical inlet and outlet gas, chemical, vacuum, and exhaust treatment facilities system for wafer fabrication. |
Clearly, an integrated approach spanning conventional wafer fab facilities can have cost, safety, environmental, schedule, and performance benefits. But the type of integrated approach, and hence the available benefits, can vary significantly. Typically, more than six companies can be involved in supplying or managing the input and output systems for process tools as outlined in Fig. 1.
Current trends
Cost efficiencies from larger-scope projects and services vary with savings from shared management, better use of subcontractors, and reduced infrastructures. In a truly integrated solution, the benefits of improved design, safety, reliability, and operability can provide the greatest cost-of-ownership (COO) competitive advantage.
Most facilities design options result in a trade-off between system COO and one of the following criteria: safety; quality control; reliability and redundancy; purity; flexibility (i.e., supplying future needs with minimal additional expenditure); schedule (i.e., faster response or project completion); environment and noise; space; operability and ease of maintenance; technology; and capacity (i.e., single or phased projects).
Decisions to optimize a wafer fab facility design are complex. Having accurate information to quantify the performance of systems in terms of reliability, purity, or risk assessments is becoming increasingly more valuable in achieving the optimal, lowest COO solution along with appropriate and known design or safety margins. We believe that the best route to quantifying the performance of systems is to link construction-level design tools with engineering and process calculations. The former includes 2- and 3-D CAD software tools. Engineering and process performance calculations can include pressure drops, velocities, contamination performance, safety margins, and system reliabilities.
Linking construction designs with performance guarantees is particularly important for the semiconductor industry, where fast start-up is critical and scope changes are the norm. An integrated construction and engineering design package can result in lower COO, known design safety margins, improved designs, and faster response and start-up.
Vacuum example
Consider conventional wisdom where, typically, vacuum and gas-exhaust equipment requirements are not fixed until the process-tool set is fixed. By this point, many facilities systems are designed and individual process-tool solutions for vacuum and exhaust treatment are often a compromise with established space or other constraints. The earlier the process-tool solution can be planned, the greater chance there is to optimize the overall facilities design and impact installation costs positively.
Progress toward integrated wafer fab solutions is evident even within conventional commercial packages from facilities suppliers. With gas supply, for example, recent trends have included bulk process gas solutions, combined nitrogen and compressed dry air systems, and integrated utility supplies. Semiconductor manufacturers increasingly want a "process-tool solution" for vacuum, point-of-use exhaust treatment (abatement), and related utilities. This innovation has extended to equipment design. New products combining vacuum and exhaust treatment functions have been developed with lower cost, faster installation, and simpler construction benefits.
Reviewing integration
There are two ways of reviewing integration across gas, chemical, vacuum, and gas-exhaust systems:
- Horizontal integration considers the full system from source material to fab outlet and accounts for the fact that some systems service multiple process tools.
- Vertical integration or process-tool solutions focus on a process-tool support system as a smaller integrated solution. For example, a process-tool solution may require dedicated vacuum and exhaust treatment equipment, but take gas or chemical input from shared systems into local manifolds dedicated to the tool. Vertical integration would only consider the local manifolds and their input rather than the full supply chain.
The best approach is to consider both horizontal and vertical integration approaches and their benefits.
Horizontal solutions
 Figure 2. Typical process gas flow from supply cylinder to exhaust with supporting vacuum. |
Figure 2 illustrates process gas flow from cylinder supply through the process tool with its vacuum requirements, and on to exhaust ducting. Conventionally, the industry has used inlet gas system management separate from vacuum and exhaust systems. Today, however, if we consider the inlet, vacuum, and outlet systems to process tools as a complete solution, we gain definitive improvements.
One benefit is reduced risk of fire in exhaust ducting. Many materials used in semiconductor processing are flammable, toxic, or pyrophoric. Gases enter process tools where they react with other gases often at high temperatures to produce even more hazardous mixtures. The process exhausts generally pass through vacuum equipment before entering local, point-of-use exhaust treatment equipment. Optimal design of vacuum forelines or connections between vacuum and gas exhaust equipment depends on process and operating needs. For example, it may be appropriate to control temperature on exhaust lines, include water-cooled traps, or build in other measures to protect against the most harsh corrosive and abrasive mixtures.
Exhaust systems are often located where there is greatest risk of fire. The root causes of such events are often due to the interfaces between inlet systems, outlet systems, or modes of operation. For example, a flammable gas mixture can occur in a main exhaust duct if the exhaust treatment unit is bypassed or the exhaust flow rate exceeds design criteria. Purge nitrogen supplies to vacuum and exhaust treatment equipment need to be controlled to eliminate flammable mixture potentials. If flow meters or excess flow devices are installed in the cylinder or gas-supply systems, these can help identify what measures should be included in the exhaust system. Also, interfacing the inlet, tool, and exhaust equipment monitoring systems can provide a means to ensure that singe failures or abnormal operating conditions do not result in major incidents.
A complete solution can also provide improved reliability with, for example, ClF3 systems. Chlorine trifluoride (ClF3) is widely used in the semiconductor industry and its highly flammable, corrosive, and low-vapor-pressure properties provide quite a challenge. An integrated ClF3 installation from the inlet gas cabinet, to the tool vacuum pump, to exhaust treatment is clearly required (Fig. 3). The main issues for ClF3 systems are safety and reliability, as well as environmental considerations. As with all systems, there is a trade-off between these criteria and the lowest-cost solution.
For safety reasons, the ClF3 cabinet is normally located in a zoned bunker room some distance from the process tools it supplies. With input data on local temperatures and process-tool demand patterns, we have developed thermodynamic modeling capabilities to quantify the state of the gas through the distribution system relative to the saturation line (Fig. 4). Provided the system can be designed with suitable temperature and pressure margins, it may then be possible to install a system without heat tracing that would remove a source of potential failure and additional cost.
The caveat here is that extensive and accurate gas property data are needed to model the entire fluid dynamic and thermodynamic process to arrive at a design capable of operating with adequate safety margins under the full range of conditions that may occur. Full simulation of the operational sequences must account for complex flow patterns (duty cycles) and changing ambient conditions in the gas storage and transmission spaces [1].
With our ClF3 example, it is important to ensure liquefaction does not occur in the exhaust lines, as this could result in corrosion and a resultant operational or safety incident.
Our example with ClF3 shows that, in general, knowledge of inlet flow; demand at process tools; vacuum flow rates, including nitrogen purge; and the reaction within the exhaust treatment equipment collectively provide the knowledge to reduce the possibilities of incidents and improve reliability. In addition, knowledge of exhaust treatment reactions backed by analytical service data can complement a better understanding of the mass balance for ClF3 and other systems.
Vertical solutions
Increasingly, the demand for wafer fabs is to provide integrated vertical "process-tool" solutions. In a vertical solution for facilities management, each process tool requires a mixture of dedicated systems or equipment as well as shared services from other systems. For example, vacuum and gas exhaust equipment tend to be dedicated and supplied as discrete products.
Even though we are dealing with individual tools, the solutions can be integrated to improve vacuum and exhaust treatment system design. A relatively straightforward development is to arrange vacuum pumps in stacking frames to reduce footprint. Also, new pump developments (such as the IPX model from BOC Edwards) can be used for both loadlock and transfer chamber applications. This requires pump designs that can be located closer to the process tool with benefits of reduced hook-up cost, reduced space requirements, and faster pump-down speed. Products have been developed to provide modular vacuum, exhaust treatment, and related control and utility systems. This further step has similar advantages, reducing construction cost and space requirements, but it can also reduce overall equipment infrastructure costs.
 Figure 5. Broader process-tool integrated solution opportunities. |
Figure 5 illustrates some of the main input and output gas, chemical, vacuum, and exhaust treatment needs for better process-tool solutions. The opportunity exists for increased integration of process-tool solutions.
Conclusion
In the last few years, we have seen significant changes where, either directly in-house or by alliance, major semiconductor suppliers can now, to varying degrees, supply broader-scoped facilities. Suppliers have integrated core areas to create multidisciplined system design, turnkey project, and service teams. For example, single project execution, safety, and quality plans cover all facilities packages, making working with a main contractor on overall fab plans easier. The same management systems for site service teams are being implemented, so that permits-to-work, risk assessment, or maintenance systems can be common to gas, chemical, vacuum, or other site teams.
With these approaches, people expertise and capabilities are vital to achieving integration benefits, in addition to management efficiencies, through improved designs, projects, and services. While upgrading the multidisciplined skills in-house, the ability to integrate further with other company packages is also enhanced. These areas of improvement also provide the incentive for customers to partner with fewer key suppliers.
Reference
- P. Espitalier-Noel et al., "Gas System Design Requirements," Semicon/West Gas System Workshop, 1999.
Paul Espitalier-Noel has a BSc from Loughborough University, UK, and a PhD from the University of Surrey, UK, both in chemical engineering. In his 14 years with The BOC Group, he has held several positions covering responsibilities for design, development, proposals, projects, and services for electronic gases. Today, Espitalier-Noel is the global product manager for integrated gas, chemical, vacuum, and exhaust treatment systems at BOC Edwards, Priestley Centre, Surrey Research Park, Guildford, Surrey GU2 5XY, UK; ph 44/148-324-4802, fax 44/148-324-4399, e-mail [email protected].
Mike Brown is an economics graduate from Manchester University and qualified as a chartered accountant. He has been with BOC for 15 years and has held general management positions in Europe and Asia. Brown is managing director, semiconductor, for BOC Edwards. ph 44/129-360-3319, fax 44/129-356-5704, e-mail [email protected].
Paul Espitalier-Noel, Mike Brown, BOC Edwards, United Kingdom