Integration risks in 300-mm wafer fab automation
07/01/1998
Integration risks in 300-mm wafer fab automation
John Chrisos, Jeff Nestel-Patt, PRI Automation Inc., Billerica, Massachusetts
A crucial requirement for emerging 300-mm wafer fabrication facilities is recognizing and addressing the many risks associated with full factory automation. While pieces of such automation have been successfully applied for 200-mm wafer production, 300 mm dramatically increases the need for automated material-handling systems throughout the fab, the expansive software control for work-in-process (WIP) planning and scheduling, and service and support. It is also important to know that while automation standards are key, they do not guarantee successful 300-mm wafer fab automation installation.
As the industry prepares for 300-mm wafer processing, semiconductor manufacturers recognize that automated material-handling systems (AMHS) are an enabling technology needed to optimize factory productivity. They understand that the economics of equipment utilization and the ergonomics of larger wafers are forcing them to automate the entire manufacturing process (Fig. 1). During the 1990s and the life of 200-mm wafers, the increasing costs of process tools and the importance of their efficient utilization were significant drivers that continually forced more interbay automation (see definitions in table). For the 2000s and emergence of 300-mm wafers, the complementary need for intrabay automation will be driven by ergonomics; a fully loaded 25-wafer carrier or front-opening unified pod (FOUP) is simply too heavy to be safely handled by fab workers.
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Figure 1. The 1980s saw automated wafer handling put inside process tools for yield improvements and the 1990s interbay automation to improve equipment utilization. In the future, intrabay automation will be driven by ergonomics associated with 300-mm wafers.
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But automating the entire semiconductor manufacturing process poses a number of potential integration risks as work in progress (WIP) moves through the fab. A 300-mm wafer will travel approximately 8-10 miles within the factory, see 250 process tools, and receive up to 400 individual process steps. Automation of this complex sequence involves many layers of hardware and software integration. The engineering team responsible for integration must address many issues to ensure wafers are handled safely and efficiently. While standards are playing an increasing role in preparing for automation, they are not a panacea.
Preparing for 300-mm automation
Semiconductor manufacturers introduced 200-mm tool automation to increase product yield and interbay AMHS to improve tool utilization in the 1990s, as Fig. 1 outlines. Interbay AMHS systems became the standard form of factory automation. Today, these are deployed in virtually every fab throughout the world, providing an effective means of logistically controlling wafer flow. In the 1990s, intrabay automation, albeit in limited installations, was particularly effective in high-throughput bays where overall bay efficiency and tool utilization produced a significant return on investment.
The future scenario is that as manufacturers move to 300-mm wafers, the integration of interbay-to-intrabay automation and intrabay-to-tool automation will require a higher level of support from both process tool and automation suppliers. These two suppliers must work closely together to reduce the risks of integration and to help semiconductor manufacturers make a smooth transition to full 300-mm production.
Today, for most semiconductor manufacturers, factory automation is focused along the central bay of a wafer fab. Even in so-called automated facilities, often an operator still removes a wafer lot from a process bay WIP stocker and loads it directly to a process tool (see "A `systems` approach to 300-mm intratool transport" on page 194). After processing, wafer lots are manually moved back to the bay stocker and transported to the next scheduled process bay where the manual process is again repeated. For the majority of manufacturers, there is little, if any, automated material handling done within the process bay itself; integration risks are minimized through manual loading and unloading. Only a few manufacturers have deployed some form of intrabay automation within the process bay, such as rail-guided vehicles.
A "systems" approach to 300-mm intratool transport
Dave Calhoun, Equipe Division of PRI Automation Inc., Sunnyvale, California
Single-wafer handling within a wafer process system or cluster tool is an integral part of total automation for 300-mm production facilities. How well this is done has a direct impact on processing speed and yield, and impacts overall equipment effectiveness. Thus, it is crucial that process engineers know what factors are changing in intratool wafer handling and, specifically, how a "systems approach" can benefit tool vendors, automation suppliers, and fabs. It is instructive to think of chamber-to-chamber wafer handling as a "system" where the main subsystems are interbay and intrabay cassette transport and intratool wafer transport.
There is an industry-wide consensus that, in 300-mm fabs, the physical interface between the intrabay automated material-handling system and a specific wafer-processing tool will be the Pod Door Opener (PDO). These are well defined by SEMI standards E63 (the BOLTS or Box-Opener/Loader-to-Tool Standard), E15.1 (the Tool Loadport Standard), and E47.1 (the Box/Pod Shell standard).
The PDO is the logical interface point for several reasons: It is the only place where single wafers are accessible by the intratool wafer-handling robot. The PDO requires real-time interaction with the process tool`s wafer-handling system. The tool`s host computer logically controls buffers, but this interaction occurs at a high level and does not require enormous processing power or communications bandwidth.
Software interface
Wafer handling within the tool has two aspects of software control: robotic motion control and wafer tracking. Typically, the tool`s host computer supervises wafer location, and a separate robot controller is responsible for robotic motion control. These two systems operate independently except for exchange of high-level commands and status and error reporting. For example, the robot controller handles all robot motion control and senses wafer location, but the host computer tracks and schedules wafers through the process. The tool`s host computer still needs to be the main interface to the fab computer network because the fab`s network not only schedules wafers through a particular piece of equipment, but loads recipes and collects process data. These functions have little to do with automation. Future tools, especially tools designed for complete factory automation, may have wafer-tracking functions as well as motion control under the supervision of an automation computer. It makes sense to then integrate all wafer tracking into this system so all wafer and pod tracking is integrated within the factory`s material control software (MCS) computer. This has the advantage of reducing host control overhead while maintaining good system-level design.
Special challenges
Once the PDO is open, the wafer-handling robot gets, aligns, and places the wafer in a chamber. While wafer handling within the tool is understood by equipment vendors and fab operators, 300-mm wafer handling presents some special challenges:
Backside contamination will become an increasingly important factor in yield management. Handling wafers with vacuum-end effectors might not be clean enough for prime wafer manufacture and photolithography. Edge handling will become popular in these steps.
Identification of individual wafers will be needed. Optical character recognition and barcode readers will become standard on most 300-mm process tools. Wafer ID readers are available now and good standards exist for readable fonts and barcodes. They simply haven`t been used in 200 mm because MCS computers are not in common use in fabs.
300-mm wafers weigh twice as much as 200-mm wafers. Therefore, wafer-handling systems will need to be appropriately sized to maintain throughput. This means larger, more powerful automation components. Thanks to improvements in motion control systems and components - such as DSP-based motion controllers, rare-earth bushless motors, and high modulus materials - robotic wafer-handling systems for 300-mm wafers are available. These advances allow faster accelerations and stiffer structures resulting in higher system throughputs.
Molecular contamination issues will be increasingly important as IC features shrink. It may be prohibitively expensive to scrub fab air for molecular contaminants. Performing wafer handling in vacuum and inert atmospheres will help keep molecular contaminants away. Vacuum cluster systems for 300-mm wafer handling are now available. In future fabs, and to facilitate workflow through the fab, it may be desirable to group process and inspection tools in logical clusters.
All these factors point to several big changes in the relationship between tool suppliers, OEM automation vendors, and factory automation suppliers. Lack of standards adoption often results in needless duplication of effort and unnecessary customization, resulting in higher costs and delays. The various 300-mm consortia have assisted all suppliers by promoting a standardized, systems engineering approach to the problem of fab-wide automation. These standards will help us develop systems in time for the anticipated demand and to keep costs down. These standards will also allow us to concentrate on core competencies.
To make 300-mm wafer handling work, we need to look beyond robotics hardware and think of chamber-to-chamber automation as a service. Solid, universally agreed-upon standards and a systems approach to material handing will present an entirely new perspective on factory automation. For the first time, fabs will be assured that all material handling will work with different vendors` automation solutions. Instead of delivering to customers a collection of robots and software with "some assembly required," automation companies will provide turnkey solutions, on-site service, and one-stop shopping.
DAVE CALHOUN has 22 years of experience designing semiconductor equipment and is the technical marketing manager for the Equipe Division of PRI Automation Inc., 733 N. Pastoria Ave., Sunnyvale, CA 94086; ph 408/616-5001, fax 408/522-0358.
Today, hundreds of interbay automation systems have been installed in major semiconductor manufacturing facilities worldwide. Though these systems have made improvements to overall tool utilization, there are many areas of potential automation that will further improve utilization. Interbay automation has been treated as a necessity, but has not been considered a "mission critical" component of a fab`s overall operation.
300-mm automation requirements
300-mm manufacturing requirements place a higher level of importance on the role that AMHS play in the overall manufacturing process. The new challenge for AMHS in 300-mm fabs is "process-chamber-to-process-chamber automation" across bays. This is a significant change in both the amount of automated material-handling content found in a fab, as well as the contribution it makes to the fab`s overall effectiveness.
In 300-mm fabs, AMHS will play an important role in overall fab operations. Rather than islands of automation that affect only a portion of the entire manufacturing process, automated material handling must integrate all of the complex wafer-handling moves into a seamless system that manages the flow of wafers during miles of travel through the fab (Fig. 2).
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Figure 2. A wafer`s journey from one process chamber to another requires many tightly integrated wafer-handling steps. This graphic depicts wafer flow from a process chamber, into the process bay, across the fab to the next process bay, and into the chamber of the next tool in the process sequence.
The risks of integrating the various hardware and software components into a tightly integrated system pose many challenges to both semiconductor manufacturers and their automation and equipment suppliers (Fig. 2). Understanding what is driving the need to automate the manufacturing process will be helpful in understanding where the integration risks are and what can be done to help eliminate them long before a new 300-mm wafer fab facility is equipped.
Consider that there are four key productivity drivers that require semiconductor manufacturers to automate their entire manufacturing process:
1. Smaller geometries are forcing manufacturers to find new ways of maintaining yields during production. A yield improvement of just 1-2% can mean millions of dollars to a manufacturer`s bottom line. 300-mm production was originally forecast to coincide with 0.25-?m geometries. Today, the forecast is for 300-mm production to begin at 0.18 ?m or even 0.13 ?m. Contamination will be a big concern at these levels and automation has been proven to enhance yield by helping to reduce contamination.
2. Process tools are becoming more expensive. A fully populated 300-mm fab will have 70% of its $2 billion cost invested in process tools. Tool utilization will be a critical factor in determining overall operational efficiency and fab profitability. At these levels of capital equipment investment, manufacturers must find ways to keep tools operating at peak performance. Automation is necessary to manage the flow of wafers from one process chamber to another, ensuring WIP is available at the tool when it is needed for processing.
3. Ergonomics will play an important role in how wafers are handled during the manufacturing process. Human operators cannot safely handle 300-mm wafers. A fully loaded pod of 300-mm wafers will weigh about 20 pounds and approach $1 million in value. Automation is required to move the wafers safely from one process step to another.
4. As semiconductors continue to increase in functionality, the manufacturing process increases in complexity. Optimizing total fab operations will require a new generation of software to schedule and plan each wafer-handling and FOUP-handling step. Automation in a 300-mm fab will be a completely integrated system capable of managing thousands of individual wafer moves safely and reliably. Planning-and-scheduling software will allow operators to schedule wafer starts based on an overall capacity plan capable of being changed in real time to address business needs.
The associated risks
The level of integration required for 300-mm wafer fabrication has three types of associated risks: the increased amount of AMHS throughout the fab; the expansive software control needed for WIP planning and scheduling; and the need for service and support. All semiconductor manufacturers must face these risks in their efforts to optimize future fab operations.
Simply stated, increasing automation means many more mechanical handoffs between different types of automation systems. Consider that WIP will travel from bay stockers and between bays using overhead monorail wafer transport systems, similar to those currently found in 200-mm fabs. Overhead intrabay delivery vehicles or ground-based vehicles will transport FOUPs directly to industry standard "E15.1" tool loadports from bay stockers. FOUPs will be stored in front-end buffers or ministockers at each tool -WIP waiting to be processed. Wafer-handling systems at the tool will move wafers to and from FOUPs through a pod door opener. Then, the entire automation process is reversed.
While some elements are similar to what has been used with 200-mm wafer processing, the fully automated 300-mm process bay outlined above departs in many ways from proven production methods. Manufacturers will need to redesign their processes in ways that take advantage of AMHS.
For example, manufacturers will have to decide which process tools need WIP buffering and how big those buffers need to be to ensure continuous tool utilization and optimum manufacturing cost. As noted above, in 200-mm fabs today, managing the flow of wafers in the process bay is a manual process where operators move WIP to and from the tool front end. In the automated process bay of tomorrow, WIP buffering requirements must be sized to process tool throughput times to ensure that a continuous supply of wafers is available for uninterrupted wafer processing. Modeling is being done today to match process tool buffering requirements with process bay AMHS delivery times to achieve optimum throughput.
Another example of how manufacturers can reduce risks in a fully automated 300-mm fab involves looking at alternative concepts of fab layout. Remember that tool productivity is a function of both WIP buffering capacity at the tool front end and the delivery times of the AMHS. Since the processing time of a tool is a fixed element, the variables affecting overall throughput are the distance the WIP must travel to reach a given tool and the time it takes to get there. This suggests that alternative bay designs that minimize transport times and distances from bay stockers will help to achieve optimal throughput for a given bay. Manufacturers should work closely with their automation supplier to explore alternatives to the traditional bay-and-chase designs.
Scheduling WIP production must address the fab-wide wafer transport capability that links one process tool to another whether those tools are across an isle from each other or at opposite ends of the fab. In a 300-mm fab, there can be no single point of failure that brings production to a standstill when a problem occurs. In addition, industry-wide cost and footprint constraints associated with 300-mm wafer processing (i.e., the controversial 1.3? for the former, 1.0? for the latter) dictate new automation concepts, such as tall stockers, low footprint storage, and robots re-engineered to cycle time requirements, which are all currently being modeled and tested. (PRI has a 300-mm test line in Billerica, MA, where customers model and test these new concepts prior to pilot production needs.)
The importance of software control and the reliability of software code come into play when you consider that all wafer-handling steps involve communications with the manufacturing execution system (MES) to orchestrate tracking, movement, and management of WIP. Material control software (MCS) must link intrabay and interbay wafer-handling movements, feeding data back to the MES for WIP scheduling. WIP must be linked between bays, within a bay, and into and out of a process tool. WIP is directed from tool to tool based on a dispatch schedule created by scheduling software. Redirecting WIP from tools that are down due to either planned or unplanned maintenance is crucial to overall productivity. Once again, to address these risks, semiconductor manufacturers must work with their suppliers to ensure the required software is robust enough to plan and optimize their production.
Finally, but crucially important, supporting all the additional automation content of a 300-mm fab will require a higher level of service and support from automation suppliers. This service must include the design, installation, integration, and testing of the complete system. As a mission-critical component of a fab, factory automation systems must be kept operational. Manufacturers are placing more reliance on the automation system as an enabling technology that delivers the return on investment necessary to meet the demanding price models of future semiconductor manufacturing.
Standards are not guarantees
Over the past few years, semiconductor manufacturers and many semiconductor equipment suppliers, under the umbrellas I300I, Selete, J300, and SEMI, have worked together to develop standards that address some of the most challenging aspects of equipment integration. For example, standardized E15.1 loadports and wafer pods mean there is one common interface for all tool loadports and wafer carriers for "plug-and-play" use throughout the fab. SEMI standard E47.1 defines the specification for boxes and pods used to transport and store 300-mm wafers. SEMI document 2708 proposes guidelines for 300-mm equipment footprint, height, and weight. There are dozens of 300-mm-based standards that will guide and help manufacturers reduce the integration risks associated with their capital equipment (Fig. 3).
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Figure 3. Relevant 300-mm hardware and software standards that help define integration of wafer fabrication process tools with intrabay automation systems. (Source: I300I)
With standardization, as indicated above for software, integration is probably the most critical component in the fully automated 300-mm fab. Developing a standardized architecture for integrating embedded process tool controllers with other material-handling components is critical to overall factory effectiveness. Here, SEMATECH is developing a CIM Framework to address this challenge. This is a factory-wide environment that takes data from tools and the MES system, which contain the information about AMHS WIP location, to manage and control the fab-wide flow of wafers. This is a key requirement in optimizing overall fab operations in the fully automated, 300-mm fab.
Based on common object request broker architecture, CIM Framework defines the environment by which all equipment communicates together. It will provide process tool data directly to the MES and other control software to help manufacturers optimize WIP in the process bay.
However helpful standards are in solving integration and compatibility issues, they are not guarantees. Semiconductor manufacturers need to ask their tool and automation suppliers a series of questions as they prepare for their first 300-mm fabs. For instance:
Has the wafer-handling system been tested for overall system integration?
Are there sufficient test data to support the integration testing?
Is there agreement about who owns the integration of the tool to wafer handling and AMHS?
Have all software components been tested with all automation systems?
Do the selected components optimize their particular operation?
An automation supplier`s true value-add comes from taking responsibility for reliability and quality of a wafer fab system`s overall performance. But the need to standardize key equipment interfaces and the question of ownership for system reliability and integration may conflict with each other. On the one hand, standards level the playing field for manufacturers by taking the guesswork out of integrating similar hardware systems from different suppliers. In a truly standardized world, one vendor`s system should plug-and-play as well as the next. Plug-and-play turns key enabling technology into commodity items and, for semiconductor manufacturers, reduces vendor dependency.
But in the mix-and-match world of plug-and-play, who owns the integration responsibility for ensuring each component in the fully automated fab is functioning properly? The manufacturer? The tool supplier? The automation supplier? Automation solutions are not commodity items. Off-the-shelf standardized compatibility addresses first-level interoperability, but it does not guarantee a system will be truly optimized across the enterprise. The automation supplier is key in ensuring that the overall system integrity is maintained and that a higher level of integration service is available to the manufacturer to keep the fab operating at peak efficiency.
Conclusion
Reducing integration risks and helping manufacturers successfully implement seamless wafer handling throughout the fab is requiring automation suppliers to invest large amounts of time and resources to develop the next generation of production-ready 300-mm automation systems. The 300-mm transition will result in a completely new automation product set developed with today`s state-of-the-art engineering technology. The automation supplier must provide a multilevel approach to component and system development. Manufacturers are requiring the data that prove the concept of the truly integrated fab, and suppliers are testing solutions in anticipation of winning early pilot projects with the early 300-mm adopters. The level of integration testing is unlike anything the industry has undertaken before. Because the stakes are so high and the potential returns so great, the automation supplier that can address the fab-wide integration and optimization issues, in addition to product reliability and conformance to standards, will be positioned to be the vendor of choice for integrated 300-mm factory automation.
JOHN CHRISOS received his BS degree in mechanical engineering from Northeastern University and his MBA from Salem State College. Chrisos is VP of marketing at PRI Automation Inc., 805 Middlesex Turnpike, Billerica, MA 01821-3986; ph 978/663-8555, fax 978/663-9755.
JEFF NESTEL-PATT received his BA degree in English literature and communications from the University of Wisconsin. He has more than 10 years of experience in product marketing and corporate communications for high technology companies. Nestel-Patt is marketing communications manager at PRI Automation Inc.