Automation in a Semiconductor Fab

Automation is commonly defined as the hardware and software systems that are integrated into the manufacturing process to reduce the level of manual operation. Automation can be subdivided into two parts — information automation and material automation (Fig. 1 ).¹ Information automation is associated with the execution of process steps by a manufacturing execution system (MES), communication connections and monitoring of the process equipment for recipe and process management, and material identification and tracking. Material automation is generally associated with moving work-in-progress (WIP) containers (FOUPs, SMIF pods, WIP boxes) to the process tool or to and from a storage location, and movement of wafers within the process tool that includes internal tool buffers, equipment load ports, equipment front-end assemblies, and wafer handling robots.

1. Factory automation is typically divided into two parts: information automation and transport automation (shown here as material automation).

From a factory perspective, many of the wafer-handling automation components contained within the tool are considered part of the tool control and management functions of the information automation system. In addition to wafer automation, lithography reticle automation is also a component of the transport automation system. The automation systems focused on the transport of WIP and reticle containers are known as automated material handling systems (AMHS), and are the largest single automation investment in today’s 300 mm factories.

Over the past several years, information automation has driven yield improvements in factories through a number of factors including improved equipment management, data collection and statistical process control (SPC) tools, more consistent material flow, and reductions in misprocessing and rework through better recipe management and material identification. Throughput gains have been made through improved WIP visibility and advanced real-time dispatching, and by using a push-type manufacturing flow. Information automation has been proven as a value-added investment that is fully aligned with the factory’s business objectives.

Historically, material automation has had the task of keeping material on the process tool, maintaining a sufficient queue of WIP next to the process tool, and moving the WIP to the next process tool. In general, 200 mm factories use manual transport and storage of WIP or a combination of manual handling augmented with automated interbay transport and storage. The more complex intrabay transport, storage and delivery of the “right WIP to the right tool at the right time” are controlled by the fab operator as directed by the factory information system. In this case, interbay automation replaces the non-value-added transport of WIP over long distances by fab operators.

For 300mm factories, manual WIP handling has been replaced by the transport automation system, including all transport, storage and direct delivery of the carriers (FOUPs) to the process tools. For early 300 mm factories, the MES drove the transport and storage moves and the AMHS was primarily reactive to the MES “next process” material demands. Additionally, it was the AMHS control software that handled material storage management and routing optimization.

Today’s 300 mm AMHS systems are pre-integrated with the factory’s higher-level information system components, use predictive WIP transport job requests based on estimated process completion times, and respond to factory anomalies or changes in the production environment (e.g., unplanned tool down situations). As a predictive and push-type material process flow system, today’s AMHS has advanced from the earlier-generation “next process” response system to a system that is focused on minimizing tool idle time, increasing throughput, reducing cycle time and achieving the fab’s business objectives.

200 mm transport automation

For early 200 mm fabs, the majority of transport, storage and delivery of WIP to the tool was performed manually. Later-generation 200 mm fabs combined manual handling and storage with automated interbay transport and storage; however, delivery of WIP to the tool was still not automated. 200 mm fabs migrated to automated transport because of the additional pressures to achieve higher production volumes with physically larger factories. To achieve higher efficiencies or because of changes in product mixes, many 200 mm fabs used AMHS to push their manufacturing flow and to better manage a larger inventory. Finally, fab layouts were developed to take advantage of automated interbay transport efficiencies.

Because AMHS allows for larger 200 mm factories, the driver for fab throughput and “delivery to promise” is the fab operator working closely with the fab’s information automation systems and production management. In general, fab operators and area managers apply local knowledge to meet production schedules and to drive throughput. Human intelligence enables the operator to rapidly respond to changing conditions, such as pulling forward lots for batch process, pre-positioning WIP, tool down situations, maintenance schedules, and other planned and unplanned events.

The role of the 200 mm AMHS is to provide controlled and visible storage of a large number of WIP containers and to replace the non-value-added activities of manual interbay transport. AMHS storage capacity and transport rates need to keep up with production demands and to provide the operator with timely access to WIP. In a 200 mm fab, the AMHS plays an important role in supporting larger fab production volumes and further enabling the fab operator to drive throughput and minimize cycle time.

Early 300 mm transport automation

Early 300 mm AMHS physically extended the 200 mm interbay transport system by adding intrabay transport loops to the existing interbay-stocker layouts. The stocker also acquired a new role as the interbay-to-intrabay transfer mechanism, and the stocker’s storage capacity was increased to replace the operator intrabay WIP storage. These systems also added a new vehicle type — the intrabay overhead transport (OHT) vehicle to deliver material directly to the tool. An automated lot move would now be “picked” from a process load port by an intrabay OHT vehicle and delivered to the local stocker, the stocker robot would then move the carrier to the interbay interface to be loaded onto an overhead shuttle (OHS) interbay transport vehicle, and then the OHS vehicle would move the carrier to the stocker closest to the next process step for storage (Fig. 2). When the carrier material was needed for processing, an available intrabay OHT vehicle would go to the local stocker to retrieve the carrier, transport the carrier inside the bay to the tool location, and “place” the carrier onto the tool loadport. The move sequence is shown in Figure 3 .

2. 200 mm interbay automation using overhead shuttle (OHS) vehicles and stockers.

3. This diagram shows the move sequence for a 300 mm AMHS segregated transport.

This highly mechanized and segregated transport system requires two stocker transfers to complete the move job, OHT vehicles consigned to their respective intrabay loop, and a linking of various control systems for each participating member (intrabay, stocker, interbay). Traffic management is confined to limited areas of responsibility within the transport job, and overall system throughput is established by mechanization. The initial AMHS design, which involves the allocation of the number of vehicles to specific intrabay high- or low-throughput loops, is based upon the initial factory process flow. The rigidity of the segregated layout limits the flexibility of the AMHS to respond to changes in process flow and other local anomalies.

Segregated 300 mm systems are reactive to move requests driven by MES “where next” logic and achieve the overall goal of mechanizing the complete manufacturing system. Segregated systems have a fixed number of OHS vehicles in the interbay subsystem and OHT vehicles in the intrabay systems. The reallocation of vehicles is a manual process and thus limits the AMHS responsiveness to changes in the fab.

Today’s 300 mm transport automation

300 mm AMHS intrabay transport and direct tool delivery provided the first opportunity and challenge for AMHS to add value similar to the flexible and adaptive 200 mm fab operator. Using the experience gained from early 300 mm segregated systems, AMHS suppliers are now putting into place the changes for flexible and adaptive AMHS automation.

First, the OHT track system was extended into the interbay area allowing the OHT vehicles to roam anywhere in the fab. This unified approach to layout has a number of distinct advantages over segregated fixed intrabay layouts:

• Eliminates the need for stocker transfers to route carriers between bays.
• Allows direct tool-to-tool delivery for faster delivery times between bays.
• Enables dynamic load balancing where the transport system can “anticipate” throughput needs and assign vehicle populations to support area loading.
• Enables dynamic job assignments to the nearest vehicles — whenever a vehicle completes a job it can be assigned the next available job (even if another vehicle is farther away and has already been assigned the job).

Like the 200 mm fab operator that pre-stages carriers close to the process tool, FOUP storage was moved from the stocker to the process bay closer to the tool using track-based storage, known commonly as under-track storage (UTS) or zero-footprint storage (ZFS). UTS acts like a linear distributed stocker with many pick-and-place opportunities, and uses an area of the cleanroom previously unoccupied, while making available space on the manufacturing floor for additional process tools (Fig. 4 ). UTS reinforces the decreased role of the stocker in the modern AMHS and provides the following advantages:

• It positions the WIP closer to the tool for decreased storage-to-tool delivery times.
• It reduces storage footprint that can be used for value-added processing equipment.
• It is a passive shelf with a lower cost of ownership due to reduced capital cost, reduced footprint, increased reliability, and elimination of scheduled maintenance.

4. 300 mm unified AMHS with under-track storage (UTS).

To support a unified AMHS, dynamic routing, and UTS intrabay storage, the AMHS transport control and material control systems must have built-in intelligence to make the right move job assignments, efficiently use resources, dynamically balance the factory load, and continuously monitor and react to hundreds of parameters that are related to the status of WIP in a 300 mm fab. By unifying the intrabay and interbay systems, the control system can have complete visibility and control of the coordination of all transport moves. This allows for full optimization of transport resources by dynamic positioning vehicles and carriers within the fab. This dynamic decision-making changes the philosophy of attempting to minimize cycle time through ultrafast mechanization, to one in which intelligent pre-positioning and dispatching is used.

Additionally, the information and transport automation systems are linked together using fabwide activity management software that uses MES data, real-time dispatching information, equipment status monitoring, and AMHS performance to drive throughput and dynamically assign transport jobs. Additional benefits are achieved when these systems are pre-integrated and take full advantage of the product’s feature set without the added cost of customization. With this level of integration, the AMHS can predict material demands and dynamically react to a wider set of manufacturing environment variables. In this way, today’s 300 mm systems add value by enabling fab business performance beyond what is capable using fab operators and earlier-generation AMHS.

To evaluate today’s AMHS, the fab must rely upon increasing sophisticated system simulations that link AMHS layouts to throughput and cycle time. The end user must evaluate which AMHS best supports this level of functionality and best integrates with the fab’s information system tools. The end user must also consider the evolution of the fab’s information systems and what is required to maintain compliance and compatibility with these ongoing improvements.

The move to unified systems and UTS has decreased the role of the stocker; however, experience has shown that non-UTS-based storage is still required when the fab storage requirements exceed the available track space for UTS or there is a need for immediate adhoc access to large quantities of carriers. For these applications, large storage clusters and distributed OHT-compliant operator I/O ports inside the bays are a solution.

Future challenges for 300 mm transport automation

Today’s 300 mm AMHS must continue to focus on the business objectives of increasing throughput, reducing cycle time, and accommodating rapid changes within the fab’s manufacturing environment. In addition, 300 mm AMHS systems must ramp and scale quickly from first silicon to mature manufacturing flows while accommodating the changing needs of the fab through its complete lifecycle. Lastly, to support the requirement to optimize and take advantage of today’s value-added systems, the factories must extend their AMHS simulation studies to comprehend the impacts of a fully integrated software system on enabling the AMHS to meet their business objectives.