Modern semiconductor wafer Fabrication facilities producing sub-micron IC’s are rapidly becoming prohibitively expensive to build and operate. The escalation in costs is driven by the industries ongoing need to reduce cost per bit in a competitive market. This demands smaller device geometry fabricated on larger wafers in factories producing greater quantities. The prevalent method of Fab design employs a bay and chase or ballroom clean room layout in a large rectangular building under a single roof.

This paper will describe an improved design approach offering significant advantages.

• Initial lower capital cost.

• Construction time is reduced which significantly improves return on capital investment.

• The Fab is incrementally scaleable as demand dictates with little to no impact on existing operations.

• Process equipment installation and change out is easily accomplished.

• The risk of catastrophic total loss in the event of a fire, such as those that occurred recently in two new Fabs in Taiwan, is eliminated.

• The Fab offers manufacturers the choice of bay and chase layout, a ballroom layout or the use of minienvironments.

The trend data of Table 1 illustrate the escalation of Fab capital cost over time. The Fab cost includes factory construction and processing equipment, of which construction cost is typically 30 - 35% [2].

The current state-of-the-art Fabs utilize a 3 story building with a structural system covering a very wide span (Figure 1). The Fab clean room occupies the middle story. All equipment in the Fab is installed on a raised floor system which is required to facilitate air return to the recirculating air handlers. In a bay and chase layout air is returned via the chases, in a ballroom layout a return air plenum under the raised floor is employed. As the ballroom is wide and long, the perforated raised floor serves to keep laminarity of clean room air flow.

The ground level functions as an area for locating support pumps and utilities, chemicals, and gases needed to operate the process equipment. The upper level of the building houses air recirculation fans. Scrubbers and exhaust treatment systems are also located on the upper level together with the stacks that expel treated exhaust to the atmosphere. Make up air handlers are located in a rooftop penthouse. Safety codes require emergency corridors on all levels of the Fab as well as a separation of the corridors used for chemical delivery and emergency exiting in case of alarm conditions. A central utilities building (CUB) for the process and clean room HVAC support systems is located along one side of the rectangular shaped Fab.

There are a number of drawbacks associated with this approach.

• The heavy steel structure is time consuming to erect and no other fit up work can be performed in the building until the shell is enclosed.

• Competition during construction for the limited space within the building requires extensive coordination of the various trades.

• The coordination and phasing of work is a constant problem. The Fab layout requires long and costly distribution runs for the delivery of support services and utilities to the clean room area.

• The process equipment must be maneuvered through corridors and over raised floors and set in place using specialized rigging, a difficult and risky operation.

• Area separation fire walls are difficult to construct as they interfere with the process flow, therefore, large single areas housing much expensive equipment can easily be damaged in a catastrophic event from fire and smoke.

• Expansion of the Fab is extremely difficult as the building is bounded by the structural limitations of a rectangular geometry.

• Emergency exit corridors are required which increase the building footprint as well as constrain the location of the H6 (Uniform Building Code Occupancy Classification For Fabs) processing area.

The FlexFabä departs from the conventional approach. It incorporates a multiple building structure in a hub and spoke or finger arrangement (Figure 2), where each spoke contains the clean processing area (Figure 3). Air recirculation fans are located in the attic space above the clean room. Makeup air handlers and where needed, gas or chemical vaults and scrubbers are located alongside an exterior wall of the spoke (Figure 4). Support facilities which are common to all spokes (electrical, bulk gas, chillers, process cooling water, etc.) are centrally located within the first floor of the hub or proximate thereto. Processing equipment of similar types are grouped within the spokes and those support services predominantly used by those equipment types (ultra pure water, specialty gases, bulk chemicals, liquid waste, exhaust, etc.) are located adjacent to the appropriate spokes. Exterior walls of the spokes are demountable for direct installation of process equipment.

The advantages of this approach are immediately apparent.

• The width of the process areas is reduced from 150 feet or more to about 60 feet (Figure 3). This greatly simplifies structural members which can now be prefabricated off site.

• Ready to assemble and low cost building structures manufactured by Star, Butler or other manufacturers are available.

• Site delivery of prefabricated components and their erection can be phased to match the Fab fitup schedule.

• The cost for insuring against a major catastrophe is minimized as each spoke is separated form the remainder of the building. The risk of a total loss, as recently occurred in Taiwan during a fire in a new state of the art Fab , is eliminated.

• Plan review by local authorities will be expedited as the building is inherently more simple and safe when addressing structural, fire, toxic gas, and other life safety concerns.

• Work within the spokes can be performed by separate teams and can be sequenced to minimize interference, allowing faster fitup of process support services.

• The proximity match-up of process support services to the processing equipment allows shorter and smaller piping runs.

• The use of distributed package scrubbers avoids having to route 60 to 70 inch scrubber ducts.

• The ability to install process equipment directly in place from the outside through demountable exterior walls obviates the need to roll the equipment over large distances over the raised floor.

• Emergency exit corridors are replaced by direct exit doors from each spoke which saves on expensive floor area and improves operator safety.

• Tanker or truck delivery of bulk gases, specialty gases, and bulk chemicals is simplified as these can now be delivered directly into their storage vaults or tanks.

• Finally, vibration isolation systems are simplified since the need for entire building vibration isolation no longer exists.

As a result of these advantages, and despite the tradeoff of extra land and more exterior walls, the FlexFabä offers construction and equipment fitup savings in the 15 to 20% range and will be ready for wafer production three to five months earlier than conventional Fabs.

Perhaps the most significant advantage of the FlexFabä design is that it is incrementally expandable. Fab processing capacity is easily expandable by radial extension of the spokes (Figure 5) which can be accomplished with little to no impact on existing operations. Industrial Engineers attempt to forecast Fab equipment throughput. This can never be done exactly but is merely a simulation of an expected outcome. In the design of a traditional Fab, the tool layout is fundamental to the overall design. During design and construction, changes are often made to the equipment set and layout which causes significant changes and delays in the Fab design and construction in the traditional approach. However, changes to the anticipated tool set do not adversely affect the FlexFabä design. If for example it is determined that additional plasma etchers are required, the plasma etch spoke can simply be expanded without changing any other process tool layout..

This scalability feature offers an attractive option to hold off on incrementing building size until process yields are maximized and market demand is better determined. Phasing capital expenditure in this way, coupled with the overall savings and rapid time to production delivered by the FlexFabä leads to a much improved return on capital employed.

A Fab is analogous to an airport in that, just like an airport, construction of a Fab is never fully complete. When aviation was in its infancy, major city airports terminals were laid out in simple rectangles. Now that air travel is popular, out of necessity the layout of airport terminals mimics the FlexFabä approach. The analogy of the arrival and departure of planes, travelers and their luggage in an airport to that of microelectronic circuit processing is strong. However, unlike an airport, the FlexFabä can be expanded with negligible impact on existing operations.

The FlexFabä approach comprehends and addresses the inherent flexibility that is needed in component fabrication, if firms are to continue to prosper. The evolving FlexFabä will be constructed in several phases, with radial and lateral expansions appended to the original Fab (Figure 6). The ultimate area coverage ratio (building to land) in the FlexFabä approach will be slightly less (5% to 8%) than in a conventional layout, since much of the open space between the processing spokes will be used to house process support systems.

There is a trend toward the use of minienvironments and SMIF pods in modern wafer Fabs [5]. Asyst Technology claims to have sold their systems into as many as 43 Fabs. Air cleanliness requirements of Class 0.1 or better at the wafer surface (Table 1) are difficult to achieve without minienvironments, and the operational savings resulting from improved wafer yield and energy savings from the simplified clean room air handling system are becoming more attractive. Clean room personnel are more productive by eliminating the need for overly restrictive clean room attire.

Prior to starting SLS Partners, two and a half years ago, with Paul Reneau and Len Slater, Leveen spent six years as a senior project manager with, Industrial Design Corporation, the leading Fab design and construction firm in the USA where he managed the design and construction of several world class Fabs. Leveen, who graduated from Iowa State University as a chemical engineer, has considerable expertise in bulk and specialty gas applications for microelectronics fabrication. Prior to working in Fab design, he was the Vice President of Corporate Planning for L’Air Liquide in Paris, France.