The Case for Continuous Flow Transport of Wafer Carriers

		At a Glance
	Higher transport capacity, shorter and more predictable delivery times, and lower CoO are the major advantages of Continuous Flow Transport (CFT) conveyor systems.

Conveyor-type Continuous Flow Transport (CFT) has seen only limited use in the semiconductor industry to date. The CFT concept, however, offers several distinctive advantages over traditional car-based monorails and manual delivery systems. Higher transport capacity, shorter and more predictable delivery times and lower cost-of-ownership (CoO) offered by CFT are well suited for the automation requirements of new fabs.

1. Continuous flow transport (CFT) includes an overhead or floor-mounted conveyor for high-capacity interbay and long-distance, point-to-point pod transport.

New 200 mm and 300 mm fabs will place stringent requirements on their automated material handling systems (AMHS). Traffic density and intensity will increase dramatically. Equipment scheduling will demand more predictability for when lots will be available for processing. Long-distance, point-to-point transport between distributed fab areas will be required to handle CMP or copper processing areas. The predicted trend toward smaller-batch or single-wafer processing will generate a much higher volume of moves from tool to tool.

Another distinct requirement for new fabs is to provide local buffering of material for process tools. A feature of the CFT is that short stubs or loops linked to the main track installed above the process tool can serve as local zero-footprint pod buffers. The conventional approach to solve this problem is to have larger stockers located at the head of the bay, or to build in local buffering capability, which increases the process tool's footprint. The CFT solution for local buffering increases overall equipment effectiveness (OEE) and reduces the need for large end-of-bay stockers or larger process tool footprints.

This article discusses different types of automated material handling systems technologies and where they best meet emerging fab requirements. The CFT concept and its components are described in detail, and performance data for two CFT systems is presented.

2. Hybrid automated material handling systems include interbay CFT loop and intrabay OHT (overhead hoist transport), linked via end-of-bay stockers.

The evolution of AMHS

In a traditional fab, wafer process steps are performed in discrete areas (bays or cells). Wafers must be transported between bays (interbay) and within the bay (intrabay). Due to the highly asynchronous and reentrant flow, interbay transport usually originates and terminates in stockers, where the wafers wait for the availability of the correct process tool for the next step.

3. A comparison of CFT and OHS (overhead shuttle), based on an AutoMOD simulation of a 25,000 wafer/month ASIC fab with a 300 step process flow.

An intrabay move involves removing wafers from the end-of-bay stocker and moving them to the first process tool, and then from tool to tool within the bay, often requiring intermediate returns to the stocker to wait for the next process tool's availability.

Historically, interbay and intrabay transport of wafers in cassettes or containers was performed by operators. With the introduction of the SMIF (Standard Mechanical InterFace), wafer carriers (pods) and tool load ports became standardized (Table 1), allowing transfer of wafers from the pod into the tool and vice versa without exposure to the fab environment. While the motivation for SMIF was the isolation of wafers from particles created in the fab environment, the standardized dimensions and coupling features of the pod and tool load port made SMIF a critical "enabler" for fully automated material handling.

Table 1. SEMI AMHS Hardware & Safety Standards

•E15.1-0298: Provisional Specification for 300 mm Tool Load Port •E19.4-94: 200 mm Standard Mechanical Interface (SMIF). •E47.1-0298: Provisional Mechanical Specification for Boxes and Pods used to Transport and Store 300 mm Wafers. •E57-0298: Provisional Mechanical Specification for Kinematic Couplings Used to Align and Support 300 mm Wafers Carriers. •E62-0298: Provisional Specification for 300 mm Front-Opening Interface Mechanical Standard (FIMS). •E84-0200A: Specification for Enhanced Carrier Handoff Parallel I/O Interface. •E85-0999: Provisional Specification for Physical AMHS Stocker to Interbay Transport System Interoperability. •S2-93A: Safety Guidelines for Semiconductor Manufacturing Equipment. •S8-95: Safety Guidelines for Ergonomics/Human Factors Engineering of Semiconductor Manufacturing Equipment.

The first AMHS applications in semiconductor fabs handled only interbay moves. Interbay transport was done all on one level, typically 18-24 in. below the ceiling. The robots in stockers then moved the material vertically down to an operator I/O port, where an operator picked up the lot for delivery to a process tool.

4. Stockers with an elevated load port allow a lateral transfer from track to stocker, without the need for an external elevator.

In contrast to interbay moves, intrabay pod transport requires the placement of the pod to the tool load ports, typically located 900 mm above the floor. While this 900 mm height allows for manual pod transfer, it complicates loading and unloading of pods from and to overhead tracks. Some AMHS manufacturers offer overhead cars with a cable hoist for the direct placement of pods to the tool load port. A robotic elevator also can achieve direct pod transfer between the tool and the overhead AMHS. A semi-automated approach is to use an elevator to lower the pod to a manual port near the tool, with an operator moving the pod from the elevator to the process tool load port. Because of the complexities of the vertical motion and the scheduling for intrabay moves, AMHS solutions for interbay applications are much more factory-proven than intrabay applications.

Emerging fab trends

The following trends for the design of new and up-graded fabs will affect the wafers transported within the fab:

• Wafers will be transported and stored in SMIF Pods (200 mm) or FOUPs (300 mm, "Front Opening Universal Pods"). This allows the relaxation of cleanroom cleanliness requirements.
• 300 mm process tools will be fitted with FOUP load ports (Table 1).
• To accommodate overhead transport systems and larger stockers, ceiling heights in excess of 14 feet are being considered, especially along the central corridor of fabs.
• The weight of a loaded FOUP no longer allows ergonomically safe handling by operators.
• The SMIF isolation technology enables certain process steps to be performed in distributed (remote) areas with a less stringent clean environment. Examples are CMP, implant, copper processes and wafer test.
• Smaller lot sizes and single-wafer processing¹ will increase the transport volume of pods.
• To increase overall tool effectiveness, pods will be buffered local to the tool.²

AMHS for wafer fabs

5. A fully automated intrabay AMHS transfers pods between stocker and tool or between two tools without operator interference.

The following automated material handling systems are either available or in use:

Overhead Shuttle (OHS): Overhead monorail with cars, carrying wafer cassettes, boxes or pods supported at the base. Suitable for low to moderate interbay stocker-to-stocker traffic. Stacking multiple tracks increases capacity but requires extra ceiling height. Adding extra loops on the same level also can increase capacity, but at the expense of system flexibility and performance.

Overhead Hoist Transport (OHT): Overhead monorail and cars with built-in hoists, holding the pod at the top flange. This is the current accepted solution for intrabay transport with direct pod delivery to the tool port³, but it remains unproven in even pilot plant environments.

Continuous Flow Transport (CFT): Overhead or floor-mounted conveyor for high-capacity interbay and long-distance, point-to-point pod transport (Fig. 1). The CFT concept also is well suited for small, zero-footprint buffers local to process tools.

Automated Guided Vehicles (AGV): Floor-based tape or wire-guided vehicles, for low-volume intrabay cassette or pod transport with direct transfer to the tool port. The AGV typically has a six-axis robot mounted on it to deliver cassettes to process tools. It can be effective for fabs with non-uniform or frequently changing tool layout, but has significant safety implications.

Automated Rail Guided Vehicles (RGV): Similar to the AGV, these travel on a fixed rail embedded in the floor. They typically have higher horizontal speed than an AGV, but less layout flexibility. Frequent applications are in assembly and packaging areas for transport of lead frames and flat-panel displays. There have been limited applications for intrabay transport.

CFT for interbay transport

6. A “zero footprint” CFT has a buffer above the process tool, with an elevator for transfer to the tool.

Automated interbay transport usually consists of looped overhead track along the main corridor of the fab, moving pods between end-of-bay stockers (Fig. 2). A fab with 25,000 wafer starts per month might perform an average of 200-300 interbay moves per hour, with peak loads up to 500 moves per hour. The high transport capacity of the conveyor-type CFT is well suited to handle such loads. In contrast, the transport capacity of a car-based system depends on the number of alternate paths; the number, availability and speed of the cars; and the control algorithms used to ration cars.

Another advantage of the CFT is the immediate availability for transport, eliminating the time to wait for a car. This shortens delivery time, eliminates the standard deviation caused by irregular car availability and leads to more predictable delivery times.

A comparison of CFT vs. OHS systems is shown in Figure 3. This simulation, done using AutoMOD, shows a 25,000 wafer starts/month ASIC fab with a 300-step process flow. The

CFT systems show not only a faster delivery time, but also much less variation in the time (lower standard deviation). This result is consistent across various levels of production, CFT and OHS performance parameters, and routings.

Stockers with an elevated load port (Fig. 4) allow a lateral transfer from track to stocker, without the need for an external elevator. Instead, a simple lift mechanism raises the pod above the track to present it to the stocker's port robot (SEMI E85-0999, Option D Stocker port¹). Vertical pod moves are performed by the internal stocker robot.

The scheduling of interbay moves is relatively simple. Because the destination is a stocker with a high storage capacity, the move can be initiated without scheduling concerns like tool availability, priority and the formation of process batches.

Intrabay CFT

7. For many applications, the transport capacity of a single, bi-directional track is sufficient.

A fully automated intrabay AMHS (Fig. 5) transfers pods between stocker and tool or between two tools without operator interference. This requires some mechanism to lower and raise the pod from the tool load port to the overhead car or CFT conveyor.

Direct vertical pod delivery will be one of the prime methods in 300 mm fabs. To accomplish this on an industrywide basis, SEMI E15.1-0298 contains guidelines for the tool load port design and travel space for the pod (Table 1).

Overhead cars without a hoist and CFT conveyor systems will require an elevator or robotic mechanism to deliver pods to the tool load port. As a semi-automated alternative, an elevator delivers the pod to a manual port near the tool, followed by an operator move to the tool load port as illustrated in Fig. 5. Intrabay CFT loops are linked to the Interbay CFT via turntables.

"Zero footprint" local buffering

One major advantage of CFT is that it can serve as a local buffer to the process tool. Storing the next pod(s) to be processed near the tool for immediate loading increases tool effectiveness.² A small CFT loop — connected to the main intrabay system via turntables above the tool — stores incoming pods for immediate delivery upon tool availability and provides space for immediate pod removal at the completion of the process. A robotic hoist or elevator transfers pods between the CFT buffer and tool load port. Figure 6 illustrates a looped CFT buffer above the tool. An elevator lowers the pod to the operator level.

8a. Pod delivery times for bi-directional single track CFT for different pod volumes and distances. 8b. Transport pod batch size for bi-directional single track CFT for different pod volumes and distances.

Local CFT buffers are "zero footprint" devices, reducing the need or size of floor-based, end-of-bay stockers or within-the-tool buffering storage.⁴ Further, local buffering simplifies the scheduling software because pods can be delivered to the buffer independent of the tool's status. The communication between the tool and buffer can be held locally without burdening the higher-level material control system (MCS) software and the manufacturing execution system (MES).

Fabs with hybrid AMHS

For certain applications, it might be desirable to install an OHT system for intrabay transport and a conveyor-type CFT system for interbay transport. End-of-bay stockers with dual ports serve as the pod transfer point between the two types of systems (Fig. 2). SEMI's interoperability standard (E82-0999) provides the interface allowing such systems to work together.

Distributed fab areas

One new trend in the design of new or upgraded fabs is the concept of distributed processing, where certain steps are performed in remote areas either in the same or in a different building.¹ This can happen as part of upgrading two or more older fabs into a single "virtual fab," a fab expansion into a nearby building, remote processing for process steps that require different cleanliness levels than the main fab, or for the sharing of process resources between two adjacent fabs.

Such distributed processing requires bi-directional, point-to-point transport of pods between the main and remote fab. The route typically leads through non-clean areas with passages through clean-tunnels and firewalls, often involving vertical translations between different floors or elevations.

9. Elevator moves pod between upper and lower CFT tracks.

Point-to-point transport of pods between remote fab areas is a natural application for CFT systems. The immediate availability for transport eliminates the need for pod storage at the sending and receiving terminals, as would be required for car-based transport. At the receiving end, pods form a queue on the CFT track until they are removed by an operator or a mechanical device. The high transport capacity of CFT handles uneven pod flow and peak loads without the necessity of adding cars.

Point-to-point CFT can be implemented either with two parallel, unidirectional, dual tracks or with a single, bi-directional track.

The point-to-point pod delivery time of the unidirectional dual-track CFT is determined by the length and speed of the track and the number and cycle times of turntables and other components. Transport capacity is determined by the cycle time of the slowest component. Assuming a system with a 100 m track at a speed of 1 m/sec and one turntable with a 6-sec cycle time, the pod delivery time is 100+[1×6] = 106 sec = 1.8 minutes, independent of the pod traffic volume up to the capacity of 60/6 sec = 10 pods per min = 600 pods per hour in each direction.

For many applications, the transport capacity of a single, bi-directional track (Fig. 7) is sufficient. Because the track has to be shared by pods to be moved in both directions, outbound pods might have to wait while the track is busy moving incoming pods. This requires small dual-buffers at both terminals to queue outgoing pods until the track is available. Pods that accumulate during this wait period will travel as a batch of varying sizes depending on pod volume and travel distance. The traffic control is similar to the flow of cars through a one-lane road construction site, where cars alternately pass by from both directions.

Figure 8a shows the average pod delivery time (buffer to buffer) of a typical bi-directional, single-track, point-to-point CFT layout, including four turntables, for various pod traffic volumes and travel distances. Track speed of 1 m/sec and turntable cycle time of 6 sec are assumed. Figure 8b shows the corresponding average number of pods in the transport batch.

10. For full inter/intrabay fab automation, the MES supplies the information lot routing and tool status.

Assuming the Figure 9 layout is 100 m in length and the average pod volume is 60 pods per hour in each direction, the average pod delivery time is 4.2 min (the time between when the pod is placed in the input buffer and its arrival at the destination buffer). The average transport batch size is 5 pods.

While the delivery time of the unidirectional, dual track for the same distance is faster (1.8 min), the advantages of the bi-directional, single track are in lower hardware and installation costs, and smaller clean tunnels and/or firewall doors.

Smaller-lot and single-wafer processing

With increasing wafer diameters, the future size of some ASIC lots might be smaller than the conventional 25 wafers. This will result in pods carrying multiple lot types, or, more likely, only partially filled pods or smaller pods with one wafer type. Another trend predicted in the ITRS¹ is the replacement of today's batch processing by single-wafer processing throughout the fab. Such occurrences will substantially increase the volume of interbay and intrabay pod traffic, for which the high capacity of CFT will be ideally suited.

The components of CFT

Track— The unit-length track segment is the fundamental building block for customized CFT configurations. The track shown in Figure 1 consists of a drive wheel rail and idler wheel rail. A local controller in each of the 50 cm rail segments activates the drive wheels as a pod transits through this segment. Built-in sensors monitor pod movement to prevent collisions and to adjust the speed as the pod approaches each rail segment, turntable, elevator or lift, or to stop the pod as it moves into a buffer zone. Pods move at 1m per second without compromising the wafers or the cleanroom due to vibration and particle generation.

Turntable— Unlike tracks of car-based systems, most CFT tracks cannot be curved. Instead, turns at various angles (typically multiples of 45deg) are obtained with turntables. In addition, turntables provide the ability to branch, tee and cross tracks. The unit shown in Figure 1 moves pods straight through at full speed, or decelerates the pod to a stop for rotation and accelerates the pod at the conclusion of the turn. Turntables also are used to attach small buffer loops or stubs to the main track (Fig. 6).

Lift— The lift raises the pod above the CFT track for access to another device, like a stocker, elevator, storage shelf or manual load port. A lift raises a pod to where it is accessible to the robotic arm of a storage shelf. Kinematic pins on the lift and robotic arm couple with the pod for precise positioning (reference SEMI E57-0298, Table 1).

Elevator— Because CFT tracks in fabs are customarily installed at the ceiling, vertical elevators are required to lower and raise the pods to and from the tool or operator's load port. Because typical elevators handle only one pod at a time, they usually are the slowest component in the system. Thus, for high pod traffic, elevator speed is of the essence. The elevator illustrated in Figures 6 and 10 moves a pod between the upper and lower CFT tracks. The platform of the elevator consists of a short section of rail, which mates with the CFT tracks in the upper and lower positions.

Control Architecture— The complexity of the control architecture depends on the application of the CFT. For full inter/intrabay fab automation, the MES supplies the information lot routing and tool status (Fig. 10). The MCS interprets MES data into AMHS instructions to the lower-level software layers executing "From-To" commands, real-time subsystem coordination and individual device control. MES and MCS are available commercially from other suppliers, while the lower layers typically are developed by the AMHS supplier. The communication protocol between the different layers must conform to SEMI-SECS, GEM and SEM standards (Table 2). •

Table 2. SEMI AMHS Communications Standards

•E4-0298: Equipment Communications Standard 1 Message Transfer (SECS-I). •E5-0298: Equipment Communications Standard 2 Message Content (SECS-II). •E30-0298: Generic Model for Communications and Control of SEMI Equipment (GEM). •E32-0997: Material Movement Management (MMM). •E37-0298: High-Speed SECS Message Services (HSMS) Generic Services. •E37.1-96: High-Speed SECS Message Services Single-Session Mode (HSMS-SS). •E810699: CIM Framework Domain Architecture •E82-0999: Specification for Interbay/Intrabay AMHS SEM (IBSEM). •E19.4-94: 200 mm Standard Mechanical Interface (SMIF).

David Feindel is Asyst Technologies’ vice president, Transport Products Group. He has been involved in the semiconductor equipment industry for 18 years. In the past eight years, he has been involved in fab automation in various roles, including engineering, sales, marketing and general management. He holds a B.S. in management science from Case Institute of Technology and an MBA from Harvard.

Before retiring from Hewlett-Packard in 1998, Ulrich Kaempf managed automated material handling projects at HP’s Inkjet wafer fab in Corvallis, Ore. From 1993 to 1996, he was assigned to SEMATECH in Austin, Texas. He has a diploma in electrical engineering from Bern State Institute of Engineering at Burgdorf, Switzerland, and is a co-inventor of the SMIF wafer isolation technology. While working on this paper as principle author, he has been employed by Asyst Technologies as an independent contractor.

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REFERENCES

1. 1999 International Technology Roadmap for Semiconductors, 1999 Edition, section Factory Integration.
   
2. Aaron Slettenhaugh et al, "Impact of Lot Buffering on Overall Equipment Effectiveness," Semiconductor International, July 1998, p. 153.
   
3. U. Kaempf, "Automated Transport in the Wafer Fab," SEMI/IEEE 1997 Advanced Semiconductor Manufacturing Conference.
   
4. 4. Peter Csatary et al, "300 mm Fab Layout and Automation Concepts," Future Fab, Issue 5.
   
5. Semiconductor Equipment and Materials International, 805 East Middlefield Rd., Mountain View, Calif. 94043, USA.
   

Acknowledgment

The authors want to thank Dave Adams and Brian Wehrung of Asyst Technologies for their review and valuable comments to this article.

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