Cleaning the slate: 300-mm transition can improve factory effectiveness
07/01/1997
Cleaning the slate:300-mm transition can improve factory effectiveness
Jon Alexanderson, PRI Automation, Billerica, Massachusetts
The transition to 300-mm wafer processing has forced the semiconductor industry to re-engineer its process tools, rethink plans for current capacity expansion, and push the limits on existing technology. It also presents an opportunity to begin with a clean slate in the manufacturing process. Assumptions made and validated for 150- and 200-mm wafers may not hold true for 300 mm.
As processes are re-engineered and costs soar, semiconductor manufacturers are looking for ways to reduce new factory cost while increasing factory efficiency. Fortunately, with a "clean-slate" mentality, the industry can ignore past limitations and focus on process improvements. To help achieve this goal, the International 300mm Initiative (I300I), a consortium of semiconductor manufacturers, is attempting to unify manufacturers` approach to the conversion, in the hope of reducing the cost and learning curve of this latest technology. I300I immediately recognized the importance of developing factory automation and wafer handling standards, and developed 14 guidelines to manage the process. These guidelines, as well as customer-driven requirements, are changing the way manufacturers approach automated material handling.
Several material handling requirements have emerged during standards discussions:
Process tools are required to provide local material buffering in an effort to increase capital asset utilization and increase overall equipment effectiveness (OEE).
Process tools will be required to interface with the standard, side-door oriented pods. These require door-opening mechanisms mounted to the tool front-end to enable access to the wafers. In many cases, two door openers will be required/tool.
Process tools will be required to comply with the new 300-mm SEMI E-15.1 specifications for tool load ports, which standardize tool loading heights and configurations. This standard simplifies and reduces the intrabay automation complexity and significantly improves its viability.
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Figure 1. Overall equipment effectiveness - the measure of a tool`s productivity or the percentage of time that equipment is being used to produce wafers that will be shipped to a customer.
I300I established recommendations for wafer lot delivery, buffering, and loading (see "I300I guidelines on 300-mm process tool mechanical interfaces for wafer lot delivery, buffering, and loading"). This article examines several of those guidelines in detail and explains the relationship between the guidelines, OEE, tool buffer systems, and the changing face of fab automation.
Overall equipment effectivenes
OEE is a measure of a tool`s productivity or the percentage of time that equipment is being used to produce wafers that will be shipped to a customer. OEE accounts for losses due to speed, set up, unscheduled down time, unscheduled use, no operator, no product, test wafer use, and quality. Obviously, any improvement in OEE is directly related to tool throughput; and throughput improvements affect overall factory effectiveness (OFE) and ultimately the semiconductor manufacturer`s bottom line.
SEMATECH estimates that a tool is effective only 30% of the time (Fig. 1). SEMATECH studies also indicate that process tools are idle as much as 20% of the time due to scheduling, lack of a human operator, or lack of work in process (WIP) materials. An enhancement in any one of these areas, through the use of intrabay factory automation, can increase production and enable increased tool utilization, leading to a reduction in manufacturing cost and an increase in profits. An improvement of even 1% can result in millions of dollars in savings.
I300I guideline review
The I300I guidelines, formed by supporting consortium members, were created to help semiconductor equipment suppliers better understand the direction of future 300-mm factories, which will require new process tools, minienvironments, and automation. The guidelines detail the expectations for wafer carriers, wafer quantity/carrier, carrier mechanical interfaces, carrier orientation and configuration, automation compatibility, and carrier doors.
The most significant component of these guidelines involve process tool material handling interfaces. When adhered to, these guidelines will benefit semiconductor factory operations by increasing the productivity of process tools and reducing factory cycle time, as well as provide for standard tool interfaces compatible with complete fab-wide automation systems.
Process tools shall be equipped with lot buffers. Lot buffers store WIP within the process tool to provide a constant supply of wafers to the process module of the tool. The buffer size is determined by the throughput and loading method of the tool. Some I300I member companies are suggesting buffers capable of supporting as much as one hour`s worth of WIP; others recommend as little as one lot.
Compliance to SEMI E-15.1 loadports. E-15.1 defines the human and automation access and envelope zones of the tool`s loading and unloading area (material input/output or I/O). Compliance with this standard will provide common process tool I/O designs, allowing for ergonomic tool loading by humans and less complex automated tool loading. Tools must be designed to accommodate various automated delivery systems, such as autonomous guided vehi cles, rail guided vehicles, manual carts, overhead monorail delivery systems, and hoist vehicle monorail systems. Communication to the process tools and the factory automation systems is handled through SEMI-specified protocol outlined in SECS/GEM and E-23, respectively.
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Figure 2. 300-mm high capacity random access buffer.
Tool buffer systems
One method of reducing tool idle time and improving OEE while meeting I300I guidelines is through in-tool lot buffers. Intra tool buffers (Fig. 2) provide a constant supply of local WIP material, independent of the operator and factory automation system. These buffers must also provide an interface for the wafer carrier. Current options include an open cassette or a side opening SMIF pod, for 13 or 25 wafers. In the case of an open cassette, the interface is a simple loading position that complies with SEMI standard E-15.1. Side opening SMIF carriers require the use of a door open and close system (DOCS) (Fig. 3). The DOCS is also required to be fully integrated into the tool and buffer system and must comply with the standard.
Lot buffering systems buffer WIP within process tools, and load/unload wafer carriers to and from load ports to ensure continuous wafer availability. Such solutions offer the end user increased tool utilization and throughput, lot tracking, and empty carrier storage. Lot buffering systems are capable of buffering 1-24 lots. The buffer size is determined based on the tool`s throughput/hr, the duration of the buffer required, the wafer lot size (13 or 25), and the operating mode of the tool (for example, batch or serial). Lot buffers are typically built into the process tool the same way as a single wafer handler or a vacuum loadlock.
Factory automation
In-tool lot buffering is one way to increase efficiency in 300-mm wafer fabs. It is just one component of a true fab-wide automated material handling system. Indicative of the "out-of-the-box" thinking occurring in the industry today, semiconductor manufacturers can now envision a fab in which every process tool is linked to an intrabay automation system, which in turn links to an interbay system, which is managed and tracked via integrated material control software - wafer handling can be entirely automated, right down to the individual tool.
Several factors are driving the industry toward this level of true fab-wide integration for 300-mm wafer fabs:
Economics. Device manufacturers are moving toward 300-mm wafers for increased productivity and improved device yield/wafer. R&D costs have made 300-mm process tools extremely expensive. To ensure adequate return on their investments, semiconductor manufacturers need to achieve high utilization rates for those process tools. Factory automation will help accomplish this goal by reducing the overall cycle time and maximizing tool uptime. Automation also ensures smooth transport and less breakage of valuable WIP.
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Figure 3. 300-mm side opening door open/close system, DOCS.
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Figure 4. Overhead material transport systems save footprint space.
Ergonomics. Ergonomics has been one of the most significant drivers in the establishment of SEMI standard E-15.1. When packaged for transport within the fab in lot sizes of 13 or 25, a fully loaded 300-mm wafer carrier could weigh as much as 20 pounds, 150% heavier and larger than current 200-mm SMIF pods. Automated material handling will be a necessity. Fab managers and 300-mm equipment planners project that ergonomics and wafer market value will drive the need for more factory automation.
Footprint. The industry cost of cleanroom floor space averages $1000-$2000/ft2 and runs as high as $5000/ft2. A comparison between pod sizes for 200- and 300-mm wafers demonstrates that 300-mm automated storage and retrieval systems (AS/RS) or stockers will occupy twice the current 200-mm volume for the same number of lots. Conventional 200-mm approaches would simply force the expansion of floor space to store the same number of lots. As further evidence of the "clean-slate" approach, automation suppliers are instead looking for new areas within the fab to place these lots while matching and even reducing floor space consumption from 200 mm.
Novel approaches include using space both below the cleanroom floor and above the cleanroom air handling systems, including noncleanroom space. Other options being evaluated include the use of "tall" stockers up to 20 ft high instead of the current 12.
A common term used today in discussing 300-mm storage is zero footprint storage (ZFS). ZFS is a design philosophy that dictates that WIP storage will not occupy any cleanroom floor space. ZFS requires that all storage be distributed within the fab using solutions that place lots within process tools, in localized buffers near the tools, in stockers suspended from the ceiling (in the main aisle as well as in the process bays), along the fab walls, and above and below the fab`s cleanroom areas. These techniques are often conceived in the context of an integrated interbay and intrabay system. Although footprint is minimized, other problems arise in facility constraints, delivery times, and maintenance.
To facilitate this application, an overhead WIP transport and tool loading system, such as a hoist-vehicle monorail system, would be required. These designs indicate that this system is capable of reducing bay widths by as much as 4 ft when compared to floor-based vehicle delivery systems, which are less complex, but require additional footprint. Figure 4 illustrates the footprint saving that overhead material delivery systems can offer.
Conclusion
The I300I guidelines provide incentives for the development of standardized tool material handling interfaces. If these guidelines are followed, they will act as a catalyst for the increased implementation of factory-wide automation systems. Creative application of these systems, in compliance with the I300I guidelines, will directly result in increased factory efficiency, productivity, and yields for 300-mm manufacturing.
Jon Alexanderson is product manager for 300-mm development at PRI Automation, 805 Middlesex Turnpike, Billerica, MA 01821; ph 508/663-8555, fax 508/671-9430.