Automated Reticle Delivery in a 300 mm Fab
Carl Johnson PRI Automation Billerica, Mass. -- Semiconductor International, 6/1/2001
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Emerging capabilities in automated material handling systems (AMHS) are making manufacturers look at how these systems can be applied to automating the storage and movement of reticles in the lithography bay (Fig. 1). This article explores the requirements for reticle automation and the potential productivity increases that can be realized.
Lithography automation drivers
Wafer manufacturing facilities are designed to maximize wafer output per dollar of investment. The lithography process is by far the most expensive process, representing up to 40% of the total cost of a wafer.1 This is due, in part, to the high cost of exposure tools. Exposure tools to support 193 nm technology are approaching $11M, and prices are expected to increase some 20% per technology node into 2002.2 It is not uncommon for the lithography tools to be the limiting factor in the wafer capacity of a fab.
Data from the SEMATECH Cost of Ownership report shows that exposure tools are in a standby mode — defined as able to operate, but waiting for an operator or material — 16% of the time (Fig. 2).3 Although a detailed analysis of this lost time is not available, it is accepted that a significant portion of this lost time is due to the correct reticle not being available.
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| 1. PRI Automation's Guardian lithography family is designed to improve reticle handling and management. |
Elements of automated system
Reticle automation can be implemented in varying degrees, ranging from reticle storage in stand-alone stockers to fully automated reticle delivery directly to the exposure tools. Many high-volume production fabs are already benefiting from storing reticles in reticle stockers because they provide a safe, clean and efficient method of managing expensive reticle inventory.
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| 2. According to a report from International SEMATECH, exposure tools are in standby mode about 16% of the time. |
Again, this type of system may be useful in most fabs, but it provides the greatest benefit to fabs with high product mix where there is likely to be a large number of reticles and frequent reticle changes. The most extensive phase of reticle automation includes direct transport of reticles to the exposure tools. This approach is most applicable to fabs with high volume and high product mix where reticle changes are frequently required.
The effective implementation of fully automated reticle delivery requires coordination between many subsystems, including:
- Reticle storage systems.
- Automated transport system.
- Automation-compatible process tools.
- Automation-compatible reticle carriers.
- Software systems (including reticle management, scheduling and transport control).
These subsystems must work together as an integrated system. To enable this, standardized interfaces between these components will be required. The standards are similar in scope to those used for 300 mm wafer carrier automation. Developing standards will help ease implementation issues, decrease fab start-up time, lower costs and improve system reliability.
Reticle stockers
Automated reticle delivery begins with the ability to access and control the reticle inventory. Specific reticles must be accessible by both the transport system and authorized personnel. But access to reticles should be controlled so their location and status is known at all times. Reticle stockers serve this purpose by providing:
- Automatic ID tracking.
- Direct access by operators.
- A safe, secure and clean storage area.
- A means of controlling access.
- A connection to the transport system.
Currently, many fabs store some or all of their reticles in manual racks and keep an off-line inventory of the reticles. In cases where reticle stockers are used, it is not uncommon for only a portion of the fab's reticle inventory to be stored in the stockers. Storing reticles in stockers enables safe and secure storage along with the ability to keep an inventory of what reticles are available and their status. This information, accessible by components of the CIM system, gives the automation system complete control and provides access to reticles so they can be moved to the appropriate tool as needed.
There are two methods of storing reticles in a stocker. The first is reticle box storage, where the reticle stays in the carrier and the carrier is stored in the stocker. The second is bare reticle storage, where the reticle is removed from the carrier and only the reticle is stored in the stocker.
It is not unusual for high-mix, high-volume fabs to have reticle counts approaching 10,000. The amount of space required to store this number of reticles in carriers is not practical given the value of fab floor space. Bare reticle stockers provide twice the storage density of box stockers and fives times greater storage density than manual racks. In addition, the number of reticle carriers required is greatly reduced because the fab only needs carriers for the reticles that are actually in production use. For these reasons, bare reticle stockers are the method of choice for storing large numbers of reticles within the fab. However, there is still a need to have a small number of reticle carrier stockers to store empty carriers as well as for short-term storage of reticles within carriers (kitted).
Transport system
There are several choices for the type of delivery system that can be used to move the reticles from the storage area to the lithography tools. Available systems include automatic guided vehicles (AGVs), overhead transport (OHT) vehicles, and conveyor systems. The system choice will be driven by several factors:
- Ability to meet the required move rate of reticles.
- Flexibility in implementation.
- Ability to work in combination with human operators.
- Cost.
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Additionally, mature OHT systems will be able to transport both wafers and reticles on the same track network. This will help to reduce the investment cost of the required transport system.
Many standards have already been created that define the implementation of hoists for wafers. It is reasonable to invest the incremental effort to further extend these standards for reticle applications. Areas that require further standards development are outlined below.
Exposure tools
The goal of an automated reticle delivery system is to deliver reticles to the exposure tools for processing. Therefore, these tools need to have features and functions that support automated delivery of reticles. Key features include:
- Automation-compatible load ports and easements.
- Ability to communicate with the automation system (SEMI standard E84).
- Set of commands for moving reticles in and out of the tool, including the internal reticle buffer.
The reticle load ports on lithography tools were not originally designed for automated delivery of carriers. When the applicable SEMI standards (SEMI E19.3 for 150 mm load ports) were written, overhead transport systems were not seriously considered. Features of the load ports were not defined to enable automated delivery in an effective and reliable manner. Therefore, further work needs to be done to define reticle load port features and easements around the load ports to enable automated delivery by hoist vehicles.
Carriers
A substantial amount of work has been done in the past few years to standardize reticle carriers for 150 and 230 mm reticles. SEMI standard E100 was developed for a new SMIF carrier capable of holding both sizes of reticles. This standard was adopted in 1999. However, the industry has delayed the adoption of these larger carriers, preferring to continue to use the current 150 mm SMIF pods.4
New SEMI standards (SEMI 3141 and 3142) are being created to address the current SMIF monopods and six-pack pods for 150 mm reticles. These standards specify the same top robotic handling flange that is used on 300 mm FOUPs. This flange is the handling feature used by all OHT suppliers and has been proven in early 300 mm testing and implementation. Using this flange on reticle carriers will lower risks and reduce support and component costs.
Control software
The most important and difficult aspect of automated reticle delivery is the control system that must be put in place to support and manage the reticles. In a truly automated factory, reticle delivery needs to be made in concert with the delivery of the wafers. The challenge is in knowing what specific lot of wafers is to be placed on which specific exposure tool and getting the right reticle to the same tool before processing is started. This task seems simple, but requires a great deal of information and planning. The goals of the reticle management system are:
- Manage systemwide reticle storage and status.
- Manage reticle move logistics.
- Efficiently meet the production schedules.
The basic tasks and information to be managed are:
- Determine the reticle to be used with the lot for that process step.
- Check reticle status (inspected?, qualified?, etc.).
- Schedule the tool, lot and reticle.
- Locate the reticle.
- Check for available space on the tool.
- Manage and coordinate reticle moves.
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To accomplish the required reticle management, four major software systems must be integrated. These systems are described below.
Scheduling System: Provides information to the reticle management system on the specific reticle required, the tool at which it is required, and when it is required to be there.
Manufacturing Execution System (MES): Provides information to the scheduler on the reticle required for a specific lot and process step.
Reticle Management System (RMS): This system maintains a database of all reticles in the fab universe, including all the reticles in exposure tools, stockers and inspection areas, as well as those that are at mask shops for cleaning. Data maintained in this database includes reticle identification, location, number of times a reticle has been used, status, cleaning and inspection intervals. The RMS also handles the kitting of reticles into carriers in preparation for transport. In the case of six-pack SMIF pods, the RMS determines which reticles should be placed in the pod. It instructs the MCS to make the necessary transport moves.
Material Control System (MCS): The MCS is responsible for coordinating the AMHS to move reticles from point to point. It communicates to the stockers to move the reticle carriers to the I/O ports and requests a transport vehicle to retrieve the carrier and move it to the destination.
In general, care must be taken to ensure that tasks are not duplicated between the components of the CIM system. For instance, decisions about what reticles are to be used should only be made at one point to avoid conflicting decisions.
Reticle management system
The RMS tracks all information that is specific to the state of the reticles that are in the system. Data such as reticle ID, location, usage data, time of next inspection, and time to next cleaning are maintained by the RMS.
The RMS maintains reticle location data by communicating with all stockers in the system to get the inventory from each stocker. The RMS may also communicate with the exposure tools, through the equipment controllers, to get an inventory of reticles currently in the buffers.
With this data, the RMS can determine if there is sufficient space in the exposure tools reticle buffer to allow the delivery of more reticles to support future processing. If there is not sufficient room, then the reticle manager can determine which reticles should be removed, based on schedule information, and commands the MCS to send a transport vehicle to remove reticles.
The above actions imply that a component of the CIM system is able to maintain control of all carriers. To pick up a reticle from a tool, an empty carrier must be sent. If there are no empty carriers available, some system must decide which reticles to remove from carriers.
This decision will be based on future scheduling information or the length of time a reticle has not been used. It is, therefore, proposed that the reticle management system provide the logistics for empty carrier management because the system has all the relevant information and can direct the MCS to initiate the required moves.
Scheduling system
To increase the utilization of exposure tools, both the wafers and the corresponding reticle must be in place on the exposure tool before the previous lot has completed its processing. There is a time interval required for the automation system to accomplish this objective. Typically, delivery times are on the order of several minutes.
Therefore, a component of the CIM system must be cognizant of the lots to be processed and determine the tools on which to process the lots. Ideally, a time-tagged list of lots to be processed and corresponding reticles could be generated covering a period of time into the future. This list would include the lot/reticle ID, tool ID, start time and stop time. This list could be furnished to the RMS, which would coordinate activity and command the MCS to move the reticle to the tool. Likewise, the WIP management system initiates the wafer lot move through the MCS.
Scheduling is a critical function in obtaining tool utilization gains. As we stated earlier, the reticle management system must know, in advance, what reticles are required so that the transport system has sufficient time to deliver the reticle to the exposure tool. Furthermore, if the RMS determines that a specific reticle is not available when required, for instance because it is not qualified for production, this must be reported back to the scheduling system so the lot can be rescheduled for another time.
There would be no point in delivering the lot to the coater/developer and start processing if the reticle is not available.
Material control system
The MCS for automated reticle delivery behaves in a similar way as for wafer delivery, responding to commands to move reticles given to it by the RMS. This is analogous to getting wafer lot move commands from the WIP management system as is done today.
Conclusions
Full reticle automation in a 300 mm production fab is an attainable goal, but will require additional work by tool, automation and software vendors as well as the standards community. Automated reticle delivery is the next logical step for fabs with high product mixes and high production volumes to further increase exposure tool utilization so that the productivity of these costly tools can be maximized.
Further work is needed to get the software systems in place and working with hardware such as the stockers, transport systems and exposure tools to enable complete automation, but much of this work is already underway.
Carl Johnson is director of product marketing at PRI Automation, responsible for intrabay automation products for semiconductor facilities. Prior to joining PRI, he was with QC Optics and involved in developing reticle inspection systems. He has a B.S. in applied physics from the University of Massachusetts (Lowell).REFERENCES
- R. Wright, a presentation on Equipment Market Segmentation, International SEMATECH, July 2000.
- R. Wright, Global Economic Workshop, Nov. 9, 2000. Equipment Market, 2000, p. 107.
- J. Ferrell, M. Pratt, I300I Factory Guidelines: Version 5.0, International SEMATECH, April 28, 2000.
- S. Sumner, W. Fosnight, "Analyzing Trends in Automated Reticle Manufacturing, Transport and Handling," Micro, May 2000.
The author wishes to thank the Texas Instruments DMOS VI automation team for its help in addressing the issues raised in reticle automation discussed in this article.
