Improving wafer fab productivity with efficient floor layouts
02/01/1998
Improving wafer fab productivity with efficient floor layouts
Jose M. Padillo, Doron Meyersdorf, TEFEN USA, Foster City, California
As the cost of building a new fab approaches $2 billion, semiconductor manufacturers are analyzing all factors that influence profitability. Production floor layout design can significantly increase product fabrication and material-handling efficiency. Recent studies have estimated that a 10-30% reduction in manufacturing operating expenses can be realized through redesigning floor layouts [1]. In fact, if optimal layout designs were implemented throughout all industries, the annual manufacturing productivity in the US alone would increase approximately three times more than it has in any year over the last decade [2]. The semiconductor industry is no exception.
Proper fab floor layout affects wafer output, cycle times, labor productivity and overall equipment effectiveness (OEE). The majority of fabs in production today, however, were designed and built by architectural or engineering firms with the emphasis on building construction and facilities installation, and only minimal consideration of manufacturing goals [3]. Due to this general lack of operational and manufacturing perspective, many newly constructed fabs encounter unforeseen manufacturing limitations, which only become apparent after the fabs have become fully operational. In some cases, these problems can prevent fabs from achieving their originally forecast production objectives. For example, inappropriate floor layout may cause: poor communication among operators, WIP flow difficulties, excessive transaction distances, unbalanced operator workloads due to inappropriate placement of machinery, traffic congestion in critical work areas, inadequate WIP storage space, inflexibility due to improper placement of machinery and automation systems, and excessive yield loss due to improper placement of high-contamination areas.
Although building/facilities constraints are still important, more and more companies are including an operational perspective at the beginning of the planning process. Today, fab design places increased emphasis on optimizing product flows, improving work methods, and generally enhancing the production environment. Research institutions such as SEMATECH have recently advocated the use of advanced software tools to increase the productivity and effectiveness of layout designs [4].
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Figure 1. Micro layout design methodology [6]
Unfortunately, software applications and an operational perspective are not enough to ensure effectiveness. Before the facility is planned or a blueprint drawn, designers must define criteria that match the manufacturing objectives of the organization. Prioritization of each element in the criteria must occur early in the project because - if neglected - manufacturers risk dire consequences. For example, a facility layout designed to maximize equipment utilization will look quite different from one designed to minimize cycle time.
While every facility must follow a unique path to optimal productivity, each must have a vehicle for selecting and ranking layout design criteria. Communication and cooperative decision-making are critical between the layout design team and all of the layout`s stakeholders - engineering, maintenance, manufacturing, and other divisions of the company. In the final phase of a layout design project, these criteria are also used to evaluate alternative layouts.
The facility layout design process
Semiconductor fabs present unique challenges to any design team [4, 5]. A few of the issues specific to wafer fabs are: very high construction and space costs requiring compact factories; multiple, long process flows share the same equipment sets; re-entrant flows - product returns to the same process mode multiple times; capacity mismatching - sequential processes with batch and process time variables; expensive equipment - constant search for lower-cost units; highly dynamic market and technology spur layout changes during life cycle of facility; increasingly constrained facilities due to cross-contamination concerns, complex utility/chemical requirements, and safety/ergonomic requirements.
Obviously, semiconductor facilities are too complex to design in one phase. Instead, we recommend dividing the design task into three main stages: 1) macro layout design, 2) micro layout design, and 3) detailed layout design. Each stage should be focused at a different level of aggregation, ideally establishing design constraints for the following stage.
Macro layout design analysis focuses on the interaction among the large functional areas of the facility (e.g., photo, diffusion, etch, etc.). The macro layout design describes the optimal relative location and space requirements for the functional areas of the fab. Important factors include the process flows between areas, overall fab capacity, tool libraries and footprints, cleanroom and subfab space, facility/process constraints (e.g., particle contamination) and safety (e.g., building emergency exits).
The next stage, the micro layout design, analyzes the facility at the module level (for example, photobays). As illustrated in Fig. 1, the macro layout can be used as a general constraint for the micro layout. This level considers detailed analyses by clusters or bays, safety and maintenance clearances, and general WIPstorage and staging requirements.
The detailed layout follows directly from the micro design. At this third stage, the design team should perform a detailed WIP storage analysis to determine operational methods specific to each cluster. This stage also develops optimal workstation designs with detailed work methods for each tool and wall elevation drawings. Finally, the detailed layout should consider labor requirements and material-handling methods.
Layout design criteria
Definition and ranking of the layout design criteria are critical stages in the facility layout design. These criteria give the design team the direction necessary to meet the manufacturing objectives of the organization. The design and evaluation of floor layout alternatives is an example of multiple criteria decision-making (MCDM), the general class of problems involving multiple attributes, objectives and goals. Industrial engineers have been applying MCDM techniques to layout design for years [7]. This perspective provides a framework for objectively evaluating alternative layouts. The remainder of this paper illustrates how MCDM works.
How does the design team define and prioritize the set of design criteria? Basically, this process has three steps: 1) defining the plant layout stakeholders; 2) defining the layout design criteria; and, 3) ranking the design criteria.
Defining the plant layout stakeholders. Composed of individuals who dedicate the bulk of their time to the design effort, the plant layout team generally consists of facilities engineers, industrial engineers, and/or CAD designers. This team usually reports to a layout project "owner" - for example, the fab/modulemanager or operations director - with key budgetary responsibilities over the entire project.
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The design team interacts with a group of three to 15 individuals who have a clear stake in the success of the project and/or have expertise in some areas of interest. The size of this group varies depending upon the scope of the project and the size of the organization. The group usually includes individuals from process, product and/or manufacturing engineering, quality assurance, production planning, production management, operations management, facilities, information technology, and maintenance. These stakeholders act as clients and supply critical input to the design team. Generally, the project owner defines and heads the group.
The stakeholder group should have a balanced membership, equitably including individuals from all fab departments with a concern in the floor layout. The participants should have roughly the same level of responsibility and stature within the organization to eliminate any risk of intimidation during decision-making.
Defining the layout design criteria. A streamlined relationship between the design team and the stakeholders requires a clear set of design objectives to which everyone - the stakeholders and the project owner - will agree. This step can be the most important one in the entire process because of the conflicting interests of the stakeholders. The layout design team should use initial interviews with each stakeholder to generate measures, rules, and guidelines.
Layout design objectives often begin with broad mission statements, such as "select best layout for new Fab X." The design team then decomposes this global mission into specific manufacturing and operational objectives, such as minimizing cost, minimizing cycle time, and maximizing process quality. These objectives, in turn, are ranked and further broken down into elements that are easier to measure and evaluate. For example, an expansive goal such as maximizing process quality should be broken down into measurable objectives such as minimizing misprocessing and scrap, or supporting contamination control. A given layout`s ability to minimize misprocessing, control contamination, and minimize scrap can be evaluated by examining physical characteristics such as equipment accessibility, placement of inspection equipment, accessibility of production control information systems, and required WIP travel distances (especially where physical handling by operators is required). As an example, Table 1 ranks eight objectives and their supporting attributes for a specific layout project. Another layout project would probably have a different set of requirements.
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Figure 2. Example design criteria ranking: micro layout of etch area.
Ranking the design criteria. After defining the design objectives, the team must determine each item`s relative importance. The design team sets up a matrix allowing all the layout stakeholders to make pairwise comparisons of the relative importance of each design objective with respect to the overall project goal [8, 9].
The final matrix contains the combined judgment of all relevant stakeholders. Calculating the principal eigenvector of the matrix yields weights for the different design objectives. The normalized form of this vector quantitatively expresses the ranking of each layout design objective (Fig. 2).
A creative process, drawing upon business objectives, the experience of the engineering team, and established layout algorithms, generates layout alternatives. The candidate layouts usually include one or more representatives from the several types (e.g., farm, work cell, cross-corridor, etc.). Comparing each candidate layout to the design objectives gives a quantitative performance score in each area. These scores are then multiplied by the weights, and the weighted scores are summed up for each alternative.
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Micro layout project in the etch area
Figures 3-6 depict the alternatives generated by the layout team for the etch area. Each alternative receives a quantitative performance score for each design criterion. Finally, through the use of a decision matrix, all the scores are weighted by the ranking of the criteria to obtain a total score for each alternative (Table 2). The design team recommends the alternative with the highest overall score: the "cross-corridor" design, in this case (Fig. 5).
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Figure 3. Micro layout of etch area: "work-cell" alternative.
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Figure 4. Micro layout of etch area: "farm" alternative.
< Figure 5. Micro layout of etch area: "cross-corridor" alternative. Figure 6. Micro layout of etch area: "mini-work cell alternative." Conclusion The layout design process is more effective if it is linked to tangible manufacturing goals. This article presents a structured method that facilitates the interface between a layout design team and all of the layout`s stakeholders. We provide a vehicle for selecting and ranking layout design objectives that will have an impact on manufacturing performance. References 1. J.A. Tompkins, P. White, Facilities Planning, New York, Wiley, 1994. 2. G. Salvendy (Ed.), Handbook of Industrial Engineering, New York, Wiley, 1992. 3. A. Maslaton, "Excellence in fab layout," Vision, Foster City, CA: TEFENUSA, pp. 2-3, Fall 1995. 4. J.J. Plata, "Enhancing the semiconductor fab layout process," Proc. of the 1994 IEEE/SEMI Advanced Semi. Manufac. Conf., pp. 11-15, 1994. 5. J.M. Padillo, D. Meyersdorf, "Facility layout design in the semiconductor industry," SEMICON West 97, San Francisco, CA, July 14, 1997. 6. TEFENUSA R&D, Facility Layout Design, Foster City, CA: TEFENUSA, 1996. 7. R.L. Keeney, H. Raiffa, Decisions with Multiple Objectives, New York, Wiley, 1976. 8. F.Y. Partovi, et al., "Applications of analytical hierarchy process in operations management," International Journal of Operations & Production Management, Vol. 10, No. 3, pp. 5-19, 1990. 9. T.L. Saaty, The Analytic Hierarchy Process, New York, Pergamon Press, 1980. JOSE M. PADILLO is project manager for the central region in TEFEN USA`s Phoenix office. TEFEN USA, 1065 E. Hillsdale Blvd., Suite 400, Foster City, CA 94404; ph 800/983-3369, fax 650/577-9166. DORON MEYERSDORF is vice president of research and development for TEFEN Ltd., the parent company of TEFEN USA. He is based in the company`s office in Foster City, CA.
