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Technical Insight

The challenges of running a dual-use foundry for military and commercial users (GaAs Device Manufacturing)

In a GaAs wafer fab, the vastly different requirements of military and commercial customers can be managed using a flexible approach and adherence to fundamental manufacturing basics, write Lisa Aucoin and colleagues from Raytheon RF Components.
Most GaAs wafer fabrication facilities in existence today focus their product offerings on the commercial marketplace for applications ranging from discrete FETs to cellular telephone amplifiers. As the demand for volume increases along with market-driven price pressures, most GaAs manufacturers have followed the trend of high-volume silicon fabrication facilities; a dedicated fab for each process. For foundries producing modest volumes and serving multiple customers, including military users, a dedicated facility for each process is not feasible. Through the application and adaptation of some basic operational principles, there exists a business model that effectively allows multiple processes without sacrificing the introduction of crucial technology innovations. The first GaAs wafer fabrication facilities were built mainly to provide the GaAs MMICs required to meet performance specifications for radar applications. Out of this military heritage grew a host of commercial applications needing the frequency response and bandwidth provided by GaAs. These applications included satellite telecommunications and cellular telephones and base stations. The rapid rise in the mobile handset industry drove the demand for GaAs MMICs to the highest levels in history, thus changing the paradigm for GaAs from engineering to full-scale manufacturing. Following the trend of the silicon industry, GaAs manufacturers were dedicating fabrication facilities to one or two major processes and adopting the operational principles of the silicon manufacturers in order to maximize productivity and reduce operational costs. The military customer s need for highly engineered components at a competitive cost appears to be diametrically opposed to the requirements of a high-volume commercial wafer fab. This paper addresses a business model and an operational philosophy that balances the military and commercial product requirements with the manufacturing tools required to run a cost-effective semiconductor fabrication facility. Dual-use business model The methodology of running an operation requires an understanding of the underlying business model. Raytheon RF Components business model, shown in , is implemented with the requirement to provide Raytheon s defense segments with leading-edge technology to enable orders to be won for next-generation systems. The technologies pursued by Raytheon RF Components are intended to provide the next "order of magnitude" improvements in either power density, gain, efficiency, linearity, noise figure or bandwidth, as shown by the technology roadmap in . It is not sufficient to merely prove that a technology is achievable but rather it is necessary to make the technology manufacturable. Due to the capital-intensive nature of semiconductor processing, asset utilization is critical to reducing overall cost. The business model is realized by utilizing commercial high-volume production to reduce MMIC costs to military system customers. Understanding demand Executing the business model is where the dual-use challenge begins. outlines some of the major differences that cause disparity between the high-volume operations and military requirements. The key focus of any fabrication facility is to maximize productivity. In high-volume facilities, manufacturing controls all variables that could potentially affect productivity. Strict adherence to design rules is an absolute must and the number of process variations is kept to a minimum. With this approach, statistics and sample probing can be used to eliminate defec-tive product. Further, in the commercial world, product life cycles are short and old products are continuously being replaced with higher performance products or products with more functionality. In the mobile handset market, for example, the market has moved from single-mode handsets to dual-mode phones with Internet capability in less than 18 months. Contrast this with the high performance, high reliability requirements for military components. Due to the nature of the business, it is necessary to show traceability to the component level. The next level of assembly is also highly engineered, requiring a need for known good die. Also, military systems have long life cycles, many in excess of 10 years. The longevity of these systems means that it must be possible to obtain parts throughout the product life. To the wafer fabrication facility, this means sustaining processes for which the volumes are low and often unpredictable. Some of these requirements are also true for highly engineered commercial products. The high level of engineering impacts cycle time, and the necessity to carry processes for long periods of time is contradictory to the rapid response and simplification of processes common in high-volume manufacturing facilities. A further dichotomy arises when considering the number of wafers; this ranges from only a few per month to several thousand per month. A quantitative view of processes or product types within a manufacturing plant can be obtained using a process called demand variability analysis. In this analysis, the variation of occurrence is measured over a fixed time period. shows such an analysis by process type, which is carried out on a weekly basis at Raytheon RF Components. Using this technique, the operations management can understand what processes and products are being run and with what consistency they can be predicted. Referring back to , Raytheon typically runs and optimizes its yield improvement, tool SPC (statistical process control) and shop floor control system to those products which are located in "B" cell on the diagram. Those products in the "C" cell are research processes or candidates for obsolescence. As volume and predictability for a given process increases towards the "A" cell, a mixed-mode wafer fabrication facility would be forced to either dedicate a set of resources (equipment and personnel) to that process or alternatively, outsource the excess volume. Operational challenges Operating an effective dual-use semiconductor manufacturing facility requires strict adherence to fundamental manufacturing basics with some optimization around the differences described in the previous section. The basics include yield improvement, work-flow optimization (kanban) and SPC. While these criteria appear obvious, there is temptation to disregard the basics or to sub-optimize around a single process as the number of processes or complexity of a process increases. This section focuses on three approaches to meeting the operational challenges of a high-mix semiconductor wafer manufacturing facility: composite yield, a Web-based pull (kanban) system and the appropriate implementation of SPC. Composite yield Composite yield consists of measuring all yield points relative to a process or product (sometimes referred to as end-to-end). The composite yield consists of the following four yield points: line yield, dc yield, RF yield (where applicable) and die visual yield. The main difference between the dc and RF yield points in this model compared to most foundries is that they are product specific. The dc and RF test coupon data are part of the line yield and are reflective of the overall health of the process as it runs on the manufacturing floor. The line yield is process-specific and includes both mechanical and electrical criteria. Likewise, die visual is typically a measure of the process pertaining to cleanliness and mechanical robustness. The dc yield monitors the process stability and can also be specification driven to some extent. In the case of RF yield, most foundries consider this an MMIC design issue. Keeping in mind that "good die delivered to the customer" is the only thing that counts, all functions need to be held responsible for yield improvements, be it the design engineer or the manufacturing engineer. The composite yield charts drive design for manufacturability and promote a dialogue between processing, design and applications engineers. All personnel understand the yearly yield requirements and the facility s position against them. These charts drill down into the yield drivers with pareto charts and action plans to improve. Attention is immediately drawn to the drivers with the biggest impact. Web-based pull system A significant challenge in a high-mix semiconductor facility is ensuring that all wafers move and that they move according to a predetermined time and route. In setting the Takt time for the facility, the wafers must be at the right place at the right time. In a mixed-mode configuration, this challenge is exacerbated by the requirement to continuously change or calibrate tools or processes. Our Workstream shop floor control system gives real time process instructions for each wafer lot. To ensure that the right wafers are being pulled, we have supplemented Workstream with a simple Web-based tracking system by critical work cell. The six work cells are photolithography, etch, metal deposition, nitride deposition, inspection and backside. shows an example of the actual screen that is shown in the wafer fab. There are large wall mounted screens in every bay which scroll through each work cell and are updated every hour. Using this system, the operator works to the daily target for each of the major steps, and the chart is updated as a wafer is completed. The active work in process (WIP) is also shown so that all personnel can assess what work needs to be completed to feed the remaining steps down the line. The system also keeps track of the weekly progress by major operation. Using this system, all wafer lots are treated the same, and provisions can be made at the manufacturing management level to process the occasional "hot" lot. The pass-down of information during shift changes has become a real-time procedure, rather than dedicating supervisory time on the floor to update lot status. Further, the daily floor meetings are focused on why targets were not met. The operators now understand that exceeding a target is just as bad as under performing to the target. Statistical process control Running multiple processes requires strict adherence to SPC, particularly as it pertains to tool performance. With large numbers of tool set-ups, a tool set and its associated qualification must be optimized for throughput and simplicity of change-over. The tool set is one of the few constants that can be relied upon in diagnosing root cause and corrective action for any scrap generated on the manufacturing floor. Measurement tools must be kept at their peak performance. Driving the tool set to a process capability index (Cpk) greater than 2.0 significantly reduces the qualification time required to run low-volume and low-demand wafer lots, allowing personnel resources to solve fab-wide yield issues or bring on new processes, rather than attempt to steer a process. In a high-mix facility, manufacturing often has the requirement to run lots sporadically and not with optimal balance. The key to doing this successfully is to drive variation to its lowest level and to monitor the ongoing effect of that change long term, as shown by the example in . Conclusion By definition, the dual-use foundry is a low-volume, high-mix wafer manufacturing facility. However, the dual-use foundry is further defined by the continuous investment in leading-edge semiconductor technology. The combination of high technology requirements with the necessity of reducing these technologies to practice is the discriminator that separates the dual-use foundry from a conventional job shop or a development laboratory. The ability to provide high-performance MMICs for military applications at reasonable costs, requires a focus of fundamental manufacturing basics including SPC, yield improvement, preventative maintenance and efficient WIP flow management. Upon this foundation, the dual-use foundry requires flexibility, an extension of the yield improvement initiatives to include composite yield, and the application of the kanban across all products and processes regardless of volume. Acknowledgments The authors would like to acknowledge the invaluable help of R Beaudoin, R Cantuloupo, B Lauterwasser, E Newton and J Scott.
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