The evolution of AMHS rings in new cleaning challenges
10/01/2001
Cleaning & Maintenance
by Larry Hennessy
NOW THAT AUTOMATION SYSTEMS ARE EXTENDING INTO NEW AREAS OF THE FAB, THE FOCUS MUST SHIFT TO THE EQUIPMENT'S IMPACT ON YOUR CONTAMINATION CONTROL PROGRAM
 Figure 1. Classic ballroom cleanroom schematic. |
The automation tools used in the support of the manufacturing of wafers have evolved greatly over the past 10 years. What was once a simple loop of track connecting stockers at the head of the bay to manage the work in place (WIP) storage and lot delivery has quickly become one of the key enablers for manufacturing in the 300 mm environment. Automation systems now reach into all areas of the fab as we continue to migrate to lot movement directly from tool to tool.
This extension to new areas of the fab requires that we consider how these developing material handling systems impact the operation of the cleanroom. Hoist systems, used to deliver from stockers to the process tool, now extend down every bay with track networks over each process tool. Systems are under development for the automation of reticle delivery from central stocker storage to lithography tools. These types of extensions change how we need to manage the environment local to the process tool, and ultimately to the wafer level.
 Figure 2. Minienvironment cleanroom. |
Further considerations are needed for both planned and unplanned maintenance of these systems. Such close proximity of the tools changes our procedures for support and maintenance.
First, it is important to understand the evolution in cleanroom air management systems. Significant changes have occurred in this technology during the transition from 200 mm to 300 mm manufacturing. Figure 1 represents the traditional ballroom approach. Air dedicated to the clean manufacturing areas (bay) was filtered to ISO Class 3 (Class 1) quality. This was necessary because wafers were in open cassettes during handling.
As standard mechanical interface (SMIF) technology gained greater acceptance and minienvironments became more consistent, the cleanroom environment became more relaxed. The tool manufacturers design all 300 mm tools with integral minienvironments. The 300 mm facilities being built and operated now are specified at ISO Class 5 to ISO Class 6 (Class 100 to 1000). Air recycling is employed for heat management and removal of particles generated from the maintenance areas of the process tools. Figure 2 illustrates an example of this configuration. Note that the minienvironments are responsible for the maintenance of the local environment of the wafer.
 Figure 3. Key stocker areas. |
Current automated material handling systems (AMHSs) operate within a cleanliness range of ISO Class 3 to ISO Class 5 (Class 1 to Class 100). Stocker systems consistently achieve an ISO Class 3 (Class 1) rating with inter- and intrabay systems achieving an ISO Class 4 to ISO Class 5 (Class 10 to 100) range, depending upon the vendor and defined configuration set. Some vendors provide the option of additional filter systems to help manage any particles generated through the use of their systems. Thus, the clean operation of today's systems, coupled with the use of front-opening unified pod (FOUP) technology and minienvironments, has minimized the risk of direct contamination by the automation system.
However, application and support of the material handling system must be conducted in an appropriate manner to maintain acceptable levels of clean operation while maintaining adequate throughput. The approaches are application dependent. For example, the constraints for an ISO Class 3 (Class 1) clean facility will differ greatly for those of an ISO Class 4-5 (Class 100-1000).
The road to proper support
One key to proper support configurations is the importance of laying out the equipment consistently with the expected operation of the cleanroom. Examples include locating maintenance access for stockers in the gray areas for airflow, positioning interbay-stocker interfaces away from the main processing areas, and dedicating vehicle maintenance areas and offload points in areas outside of the main processing area.
 Figure 4. Open central aisle. |
Figure 3 illustrates the key areas of interest when considering stocker maintenance. Primary accesses to internal stocker systems are traditionally located in a fixed rear door position. While not mandatory, door location should be oriented in the fab area dedicated for support areas. The interbay interface point is also an area prone to maintenance issues, as certain stocker designs utilize dedicated robot system routing mechanisms to manipulate lot entry into the stocker. Again, location of these areas should consider proximity to the manufacturing environment.
Stocker placement usually takes one of two configurations. First is a traditional center aisle approach. Stockers are coupled more to the bay. There is usually a minimum of two stockers per bay. Stocker maintenance is usually provided along the chase-defined area of the local bay. Figures 4 and 5 illustrate this concept.
 Figure 5. Split central aisle. |
It is also important to recognize in these illustrations that the relationship between column placement and available stocker space must be evaluated. Final bay locations relative to the column placement will affect where stockers can be placed and where available maintenance space can be defined.
It is also possible that in Figure 4, the space along the central aisle is also utilized as part of the air-return system. Further space study would be required to understand the impact to available space for maintenance personnel during both installation and operational support.
As mentioned earlier, certain cleanroom configurations will feature tall central aisles. Figure 6 shows a version of this configuration. Noting the extreme elevation of the interbay system, maintenance technicians need to utilize special elevation equipment to reach these areas. Use of these types of equipment increases the risk of external contaminates being introduced to the environment. Interbay-stocker interfaces will be the focus of this type of activity.
 Figure 6. Tall stocker configuration elevation. |
Tall stocker configurations are not limited to the application of central aisles. Figure 7 illustrates additional configurations that will challenge cleanroom maintenance staff. Access to alternate floor storage or processing areas and extensions above the cleanroom ceiling constitute challenges in maintaining these systems. Consider gaining access to an interbay track system that operates above the cleanroom ceiling. Such a configuration requires that personnel can gain access to this area quickly to resolve any stoppage along the track line. Adequate access to the stocker-track interface is necessary, as well as special maintenance procedures to minimize contamination within the stocker due to service activity in a non-clean space.
 Figure 7. Stocker Elevation Versions. |
It is important to recognize that the impact of AMHS systems on cleanliness in the fab has been reduced with the advent of minienvironment technology. This relaxation provides for greater flexibility when routing track over process and measurement tools and developing alternate storage locations and track routing opportunities external to the manufacturing floor of the cleanroom.
However, it is still necessary to locate these equipment sets so that maintenance is performed in acceptable areas of the cleanroom, and not in the proximity of the load ports or air intakes of the minienvironment systems. It is mandatory to consider these situations when designing the AMHS layout. This need is further amplified in systems that utilize multiple floor manufacturing or tall stocker systems both inside and outside the cleanroom.
Larry Hennessy manages IDC's AMHS Technology Team. He is a specialist in the use of advanced simulation models to test and validate AMHS system designs before they are implemented. He is based in Phoenix, AZ.
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Key maintenance issues
- Stocker maintenance areas located in appropriate zones
- Vehicle maintenance area located away from main cleanroom area
- Vehicle removal using recommended methods
- Appropriate access to track systems located in tall central aisles
- Manual test wafer protocol relative to material handling equipment layout