Facility Automation

Improving work-in-progress visibility with active product tags

The installation of a product-tracking system facilitated a fab's changeover to application-specific integrated circuit manufacturing.

Ken Van Antwerp, Brooks Automation/Infab; and Ulrich Rohrer, Philips SMST

Changing global manufacturing capacities and customer demands have caused some segments of the semiconductor manufacturing industry to shift focus from commodity devices to specialized products. To accommodate the change in product mix while maintaining productivity, cost-efficiency, and product quality, many fabs have had to revamp their logistics systems. For example, the Philips SMST facility in Böblingen, Germany, made the transition from an all-memory fab using a "push" product-flow model to a large, mixed-product application-specific integrated circuit (ASIC) fab using a "pull" product-flow model. This transition necessitated an improvement in work-in-progress (WIP) visibility—the ability to locate specific lots for processing to meet promised delivery dates.

Designed with a traditional bay-and-chase configuration, the SMST fab has a complex layout as a result of having undergone several technology upgrades since its initial construction in 1978. Product was stored in a variety of locations on multilevel shelves, racks, and carts. Operators selected the next lot for processing from a list displayed on a computer-integrated manufacturing (CIM) terminal. They then had to spend several minutes—known as lot search time—to physically locate the selected lot. In the complex fab environment with its many storage opportunities, a lot's registered location according to the manufacturing execution system (MES) and its actual location sometimes did not match, forcing operators and supervisors to spend considerable time searching for lost lots. Such a situation is unacceptable for ASIC environments, in which short cycle times are a strategic advantage and predictable delivery dates critical. Thus, it was necessary to include a physical product-tracking system as part of the fab changeover.

Another complicating factor in the fab arose after a yield study indicated that significant line-yield improvements could be achieved by converting certain critical tools to enclosed standard mechanical interface (SMIF) environments. As a result of this study, some open cassette tools were replaced with new SMIF tools. Because of the new equipment mix, wafer cassettes are transferred to and from both WIP boxes and SMIF pods as part of the normal process flow.

In deciding which tracking technology to install, SMST needed to compare systems using active product tags with displays and those using passive tag technologies such as bar code labels and radio-frequency (RF) tags. Systems with tag displays can communicate information on cart, rack, and WIP storage locations without requiring that operators use terminals. This capability was especially important to the company because the fab lacks an automated materials-handling system and operators transport all material manually. Using active tags also allows operators to reference the tag lot information to receive routing instructions anywhere in the fab. Another benefit of active tags is the ability to reroute product flow quickly in response to changes in processing sequence without the need to change paper travel cards. The biggest drawback to display-based product tags is the system's high cost.

Another key decision involved the use of short- or long-range tracking technology. Short-range technologies, such as bar code, RF, and some infrared (IR) systems, require tag-reader stations (also called docking stations or read points) to physically track material. For automated material identification at tool interfaces, single reader stations work well to associate the material located there with the tool's I/O port. For physical product tracking throughout a fab, however, the large number of stations that would be required to cover all product storage areas would make such a system prohibitively expensive. It also could not track material on carts.

Because SMST uses racks as the primary means of WIP storage, it was important to choose a material-tracking system that tracks WIP on racks efficiently. In the lot-dispatch area, shown in Figure 1, a cluster of six-shelf-high, single-deep racks placed side by side against a wall has the capacity to store approximately 200 WIP boxes. Lots are created in this area and then await dispatch to the appropriate process tool. To track this material using long-range IR technology requires only three ceiling-mounted sensors compared to 200 reader stations for a short-range tracking system. Similarly, at stand-alone racks one sensor can do the work of 10 to 12 reader stations. The resulting cost-efficiencies made installation of a long-range tracking system practical at SMST.

Figure 1: Rack cluster with the capacity to store approximately 200 WIP boxes containing product lots that are tracked by three ceiling-mounted sensors. Photos Courtesy of Brooks Automation/INFAB.

Based on its consideration of these factors, SMST chose the IridNet system from Brooks Automation/Infab (Jena, Germany/Chelmsford, MA), which offered the additional advantages of having a flexible wiring system and a client-and-server—based tracking software engine that could be integrated into the fab's existing CIM system. As illustrated in Figure 2, the IridNet hardware consists of wireless IR tags that can be mounted to WIP boxes and SMIF pods, ceiling- and tool-mounted IR transceivers (sensors), and network interface devices that distribute data and power to the IR sensors. Each product tag is affixed to an attachment plate on the product container using a single screw. Made of the same material as the carrier (polypropylene for boxes and polycarbonate for SMIF pods), these attachment plates are welded to SMST's existing WIP boxes and SMIF pods.

Figure 2: Components of the active tag—based product-tracking system.

Sensor Placement Design Considerations

Once the tracking system had been selected, the project was given the name FiLS (find lot system) and a joint SMST-Infab project team was formed to address such issues as sensor placement and deployment. The team faced a number of challenges, including bay layouts with multilayer WIP storage shelves, the possibility that tags might sometimes be placed outside the tracking system viewing range, and the installation of a large number of sensors and the associated wiring in an active manufacturing facility.

Because the long-range IR system requires line-of-sight paths between the optical tracking sensors and the product tags for all tag-sensor communications, the project team needed to define specific tracking zones based on the sensors' fields of view (FOVs). Capable of being directed and shaped as needed, each sensor's FOV is determined by the transmit and receive characteristics of the sensor and the tag, optical apertures, and IR-opaque obstacles, such as walls and barriers. The team developed innovative coverage patterns (tracking zones) to accommodate the fab's complex bay layout and successfully tested this design at both the supplier's Colorado Springs facility and in a test bay at SMST prior to fabwide deployment. Figure 3 shows a schematic view of one bay, with WIP boxes stored on equipment tabletops and stacked on carts. Figure 4 is a photograph of another bay following installation of the sensors.

Figure 3: System components and sensor coverage patterns in a bay with WIP boxes on carts and equipment tabletops.

To ensure that all tabletops, WIP racks, and cart placements would be viewable by a sensor, the sensor coverage patterns were overlaid on an electronic version of the existing fab layout. This allowed the FiLS team to visualize the fabwide coverage and quickly identify areas that needed additional sensors or special sensor placement configurations. The team then used this master plan to assign physical location names to the sensors and placed each sensor's name in its nonvolatile random access memory. This sensor location name is reported to the fab's CIM system as the product location for the tagged WIP containers that are located within the sensor's tracking zone.

Figure 4: Vertical furnace process bay with operators at tool input ports and lot-tracking components integrated into the cleanroom ceiling.

When a tag is in view of multiple sensors, the tracking system uses sensor priorities to resolve zone preferences. The optical assembly and tracking features of the tags used at SMST were customized to quickly resolve severe zone-overlap conditions, while their optical arrays were adjusted to support both side-to-side and overhead zone coverage patterns. The tracking algorithm was also optimized to enable fast tracking in fringe and heavy zone-overlap areas. In addition to setting priorities, the algorithm measures the received sensor's signal strength to rapidly qualify the tag-to-sensor communications link to the preferred sensor.

The FiLS team anticipated that operators would sometimes place objects such as carts between sensors and tags, thus inadvertently obstructing the sensors' line of sight. The team solved this problem by creating a tag out-of-view (OOV) mode, in which the tag automatically beeps, displays an OOV message on its four-line display, and blinks its LED. The default time to trigger the OOV mode was set at several minutes to accommodate temporary situations, but it can be reprogrammed through the fab's automation system interface or manually using the system administration tools.

System Installation and Administration

The next challenge facing the FiLS project team was to install more than 840 sensors, including the wiring infrastructure, in an operating fab. The tracking system uses low-bandwidth wiring that distributes data and power to the sensors from network interface devices. As shown in Figure 2, these devices connect to the tracking server through a fab's Ethernet backbone and to the sensors using a separate dedicated wire network. Although centrally located network devices can be easily managed, the project team decided to distribute the devices throughout the facility because of SMST's complex fab layout. In addition to minimizing the wiring infrastructure, this approach offered several advantages: the installation was segmented and affected only small areas of the fab at any given time, all sensor wiring could be installed in the existing ceiling channels, and the installation did not have a significant impact on ongoing manufacturing processes.

The flexible wiring system for connecting the sensors to the network interface devices enabled the FiLS team to install the system in the existing fab infrastructure. Because these devices distribute both data and power to the sensors, there was no need to distribute sensor power supplies. In addition, the low sensor bandwidth requirements facilitated the use of star and stub wiring configurations between the sensors and network interface devices. This was especially important because the ceiling channels were already populated with sprinkler and lighting elements, requiring the sensor wiring installers to find opportunistic wiring paths.

The large numbers of components that were installed throughout the fab require system administration and maintenance tools. To provide these tools, the tracking system includes both a system administration client and a system monitoring client. The administration client provides graphic windows for configuring all the tracking system components, such as tag operating characteristics, sensor characteristics and names, network interface device parameters, and software server parameters. This client allows for the configuration of the automatic file backup features and level of reporting details. The monitoring client alerts the information technology system administrator to the health of the tracking system and reports when its components are no longer communicating. In addition, the tracking system components automatically generate self-monitoring messages to allow the tracking server to monitor the usability of the available communication channels. Any anomalies are displayed and reported in the server anomaly file. In this way, the system administrator can monitor the system and quickly react to any system-related problems. The tracking server anomaly files also provide a history of system transactions to help identify the origin or root cause of problems or conditions associated with an anomaly.

Automation System Improvements

After the tracking system was installed, the fab's automation system was modified, including the addition of features to take advantage of improved WIP visibility. While working on the FiLS project, SMST had also developed a new graphical user interface for the operators. Known as the integrated operator interface (IOI), this enhancement was connected to the existing MES/dispatch system. Both the IOI and the tracking software engine had to interface with the fab's existing CIM system, which was built on a centralized AIX mainframe architecture, with mainframe terminals distributed throughout the fab. Since the centralized CIM system could accommodate a client-and-server—based system without affecting its existing infrastructure, the tracking server could be introduced into the CIM system and mainframe terminals could be replaced with distributed IOI clients.

When a tool becomes available, the operator uses a local IOI terminal to identify the lots scheduled for the tool in the order of their priority. Along with other manufacturing and logistics information, the IOI provides a lot-location data field containing the actual physical location of each lot on the list as communicated by the tracking system. Thus, the operator has easy access to the priority and physical location of each lot. After the operator selects a lot to be run, the IOI sends a message to the lot container's tracking tag, which causes the front-panel LED display to blink so that the operator can locate the selected lot rapidly.

The IOI also provides a set of tools that allow fab operators to locate lots anywhere in the fab independent of the schedule list. This feature not only provides a convenient and efficient way to move high-priority lots through the manufacturing environment but also enables operators to rapidly locate lots that require reworking or attention. These IOI tools also enable the "chaotic" storage of lots anywhere in the fab without loss of information about the physical lot location. This capability is particularly important in tool-down situations, in which WIP can easily build up in the affected area.

Coupled with the WIP visibility provided by the tracking system, the improved level of automation enables fab operators to better manage workflow and has resulted in predictable cycle times and improved adherence to promised delivery dates. The ability to adhere to the schedule has also permitted a one-time, cost-saving inventory reduction.

Other Benefits and Future Plans

One of the unexpected benefits of the tracking system was a reevaluation of SMST's material storage practices. The fab had been using a number of two-box-deep racks and stacking multiple WIP boxes for storage. The sensor-to-tag line-of-sight requirements made it necessary to replace the two-box-deep racks with narrower one-box racks, and the stacking of boxes on the new racks was restricted to two high. The general storage clusters also were rearranged to optimize storage capacity and WIP visibility. The general impact of these changes further improved the fab's overall WIP visibility, reduced clutter, and made the operators' jobs easier.

One of the expected benefits of the new system involved the reconfiguration of the fab. The ceiling-mounted sensors that track material on WIP racks at SMST were given names consisting of location prefixes, WIP rack names, and suffixes of physical fab coordinates. If a cluster of racks is moved under a different set of ceiling-mounted sensors, the affected sensors can be renamed quickly using the graphic system administration tools. In this way, a group of WIP racks has already been relocated in response to the changing fab environment with a minimum of cost.

SMST and Brooks Automation/Infab are analyzing a potential extension of the tracking system to include automated lot identification in the open cassette tools. A variety of approaches, as well as the cost-effectiveness of other related fab changes, are in the planning and analysis stages. Many of the requirements for this second autoidentification project phase were identified when the tracking system was originally installed, and solutions addressing these needs were incorporated into the FiLS project to maximize system installation and product efficiencies.

Conclusion

A tracking system based on long-range IR technology and using active product tags has been integrated into the complex SMST fab environment without disrupting ongoing production. The tracking system, along with other automation system enhancements, has allowed the fab to realize a considerable reduction in lot search time per operation and has significantly improved operator efficiency. Physical lot locations provided by the system's sensors are shown on the operator user interface as part of the lot scheduler information, enabling the operators to select and locate the highest-priority lots rapidly. The FiLS project team believes that the logistics improvements provided by global tracking will help SMST remain highly competitive as it makes the transition to ASIC manufacturing.

Ken Van Antwerp is the product marketing manager at Brooks Automation/Infab in Colorado Springs, CO. He has five years of experience in the semiconductor industry and has spent more than 20 years in the electronics industry, including 15 years in the defense electronics sector working in the fields of microelectronic device testing, evaluation, and failure analysis. He has also particpated in R&D programs and provided system-level support for fiber-optic systems. He received a BSE in electrical engineering from California State University, Northridge. (Van Antwerp can be reached at 719/532-7203 or ken.vanantwerp@infab.com.)

Ulrich Rohrer is a project manager at Philips SMST in Böblingen, Germany, where he is responsible for fab automation and contamination control. He has worked in the semiconductor industry for 22 years. In 1978 he joined IBM Germany and has been involved in semiconductor operations in process engineering, device characterization, manufacturing operations, quality control, facilities operations, and general project management. He received his BA in communications from the technical college in Esslingen, Germany. (Rohrer can be reached at +49 7031 185485 or u.rohrer@smst.philips.com.)

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