TECH FOCUS—300mm

Evaluating 300-mm fab automation technology options and selection criteria

Mitchell Weiss, PRI Automation

Although many people in the semiconductor industry know that 300-mm fabs present new opportunities to increase overall fab effectiveness, few understand how to take advantage of those improvements. Recently, a consensus has begun to take shape on the design of these new facilities, and one can start to determine which options should be applied to which situation. This article will describe some of the consensus items relating to 300-mm fab design and then discuss three alternative methods for moving material around the fab. A method will be provided that readers can use to analyze their particular situations and help determine which 300-mm material-handling method might be best suited to their circumstances. But first a brief overview of how to measure fab performance is in order.

Measuring Fab Performance

Before beginning any discussion of the potential benefits of the various types of automated layouts in a 300-mm fab, a way to measure fab performance must be established so that the effects of the various options can be determined. Two widely used models are cost of ownership (COO) and overall equipment effectiveness (OEE), both of which were developed to analyze the performance of individual machines, not overall fab productivity. Two methods that can be used to measure fabwide performance are fab productivity (revenues per dollar investment) and manufacturing lead time (MLT), which describes fab cycle time. Factory productivity is primarily affected by the cost of the equipment and materials as well as the facility. Assuming a fixed yield, one can have an impact on the amount of equipment required by reducing downtime caused when no operator or material is present at the tool. The selection of automation techniques can also affect the ultimate size of the facility, since some automation approaches require less footprint than others.

MLT is defined as the total time required to process a given product (or work part) through the plant. MLT is made up of a series of production steps, some of which add value to the product, some of which do not, and some of which are a result of overhead functions. We can define these as T^_vi, time adding value at production step i; T^_hi, time handling material for production step i; T^_ti, time the tool requires to prepare for the step; and T^_ai, administrative time for production step i. There is also the time T^_sb, which represents the overall setup time for starting the batch into the production process. Total MLT can be given by:

Over the past few years, all segments of the industry have been working hard to ensure that 300-mm processing enhances factory performance. The effects of material handling on MLT have been analyzed as a part of reaching factory design consensus. Selections of cassette type, orientation, and batch size have all been intended to reduce handling time and setup time. This article will look at the possible effects that options to reduce the cost, size, and MLT effects can have on the operating expenses of a 300-mm fab.

The 300-mm Consensus

An impressive international effort is under way to develop 300-mm manufacturing standards before the facilities themselves are designed and built. These efforts are being led by two industry consortia—the International 300-mm Initiative (I300I) and J300. I300I is made up of chipmakers from the United States, Europe, Taiwan, and South Korea, while J300 is made up of Japanese chipmakers and suppliers. Both groups have been active in initiating discussions and developing standards related to 300-mm manufacturing. The consortia have agreed to let SEMI administer the international standards that come out of these discussions, including guidelines on wafers, materials, carriers, and factory interfaces. I300I has also published a list of guidelines to be used in the design of new facilities, the existence of which will speed development of the tools required for handling 300-mm wafers and carriers and provide the semiconductor manufacturers with multiple supplier options. The guidelines were developed with an eye toward increasing overall fab effectiveness as well as the capital productivity of each facility. In the following sections, some of these guidelines and their implications for facility design, layout, and operation will be examined.

Horizontal Wafer and Transport Loading. Traditionally, wafers have been transported in cassettes of 25, with near-vertical alignment. In most cases, as a cassette is loaded in the process tools, it has to be rotated to orient the wafers horizontally. Wafers were transported vertically to protect them from shock, vibration, and contamination. Unfortunately, the rotation of the carrier could itself result in damage to the wafers or injury to the operators. The use of SMIF pods demonstrates that wafers can be handled horizontally in an enclosure and not suffer damage or contamination, in addition to reducing operator difficulties. Since approximately 85% of the process tools handle wafers horizontally, the elimination of carrier rotation at these tools can contribute to increased factory throughput.

Figure 1: Example of a standardized 300-mm wafer carrier. Photo Courtesy of Empak

Strict Dimensional Standards for Wafer Carriers. SEMI standards have been developed for 300-mm cassettes and carriers, such as the unit shown in Figure 1. While these standards cover a large number of items, the following features are the most important. The carriers will transport either 13 or 25 wafers at a 10-mm pitch (wafer-to-wafer spacing). The 10-mm pitch was a hotly debated item, since a smaller pitch results in greater storage densities for work in process (WIP), but a larger pitch potentially allows for faster operation of the wafer-handling robots in the process tools. It was also decided that cassettes and carriers will make use of a kinematic coupling to register the carrier at the tool load port. This will allow for interchangeability between carriers and should let the carriers be placed more precisely, again speeding the operation of the tool automation system. Another benefit of this precise location will be a reduction in contamination sometimes caused by wafer interactions with the carriers during removal and insertion.

Carrier Type. I300I and J300 have both determined that they will use horizontally oriented carriers with a kinematic coupling loading feature. While I300I has decided to advocate a closed carrier, J300 members are recommending the use of open cassettes. That may seem at first glance to be a significant difference between the two groups, but the fact that the cassettes interface in the same fashion and in the same orientation to the tool allows one to migrate from open cassettes to closed carriers without much difficulty. The real difference will be in the overall factory architecture.

Tool Interfaces. All tools will be have to meet the pending SEMI E15.1 standard. This standard requires all tools to be loaded at a height of 900 mm from the floor and within certain horizontal constraints. It also attempts to address the need for clearances for the tool-loading systems (referred to as easements). By combining these interface features with the standard carriers and with the horizontal transport, the task of automating the loading and unloading of the tools is simplified.

Automation Options for the 300-mm Fab

The term factory automation in the semiconductor industry is generally used to describe the four components utilized in automating and mechanizing the movement of wafers and cassettes throughout the fab: cassette-to-cassette automation, intrabay automation, interbay automation, and a material control system (see Figure 2).

Figure 2: Four components of fab automation—cassette-to-cassette (or tool) automation, intra- and interbay automation, and material control system (software products and automation services).

Cassette-to-Cassette Automation. Also known as tool automation, this component provides automated handling of the cassettes or wafers throughout the process in an individual tool. For example, a cassette is placed on the load port of a process tool, and the individual wafers are then removed from the cassette and transferred through the process steps in the tool. As the wafers complete the processing in the tool, they are returned to a cassette for transfer. Cassette-to-cassette automation replaces the manual handling of individual wafers with tweezers or vacuum wands. It produces a significant improvement in terms of yield and process consistency. Most if not all process tools employ some form of cassette-to-cassette automation. The guidelines for 300-mm carriers will also maintain slot-to-slot integrity of the wafers. Almost all 300-mm process tools will include wafer-handling systems operating in a minienvironment. Assuming that the material can be delivered effectively to the tool, OEE and process yield will be the sole responsibility of the tools.

Intrabay Automation. This component involves the transport and loading of wafer carriers (cassettes, boxes, or pods) from tool to tool. The carriers may be transported directly between tools or to and from intermediate storage locations. The intrabay automation system must be capable of loading and unloading tools as well as providing transport capabilities. In order to make the most effective use of intrabay automation, most 300-mm tools will include some type of cassette buffer. This buffer will store supplemental material so that, even in the case of delivery perturbations, there will be enough material to allow the tool to continue operating at full speed. The development of the new standards in time for 300-mm deployment will cause an explosion in the use of intrabay automation, therefore this area is seen as having the most new options for automation.

Interbay Automation. Most modern fabs are built using a variation of the bay-and-chase layout. Transport of the carriers from bay to bay is the responsibility of the interbay automation system, which usually consists of some type of transport technology and storage units located within each bay. The transport system transfers carriers from storage unit to storage unit as required by the process flow. Typical interbay systems are composed of overhead monorails and distributed stockers. This traditional layout will be used for at least the first generation of 300-mm fabs.

Most 200-mm facilities use some type of automated interbay delivery system although it is still possible to handle the payloads manually. In 300-mm fabs, manual handling will most likely not be an option, but existing interbay transport technology can easily be modified to handle 300-mm carriers. Because batch sizes will be similar to those used in 200-mm and earlier-generation fabs, transaction rates will not increase for the interbay system. Payload weight will be about twice the load for a 200-mm carrier, and cleanliness requirements will not change much and, because of the use of minienvironments, may even be reduced.

Material Control System. The final component of the factory automation system is the material control system (MCS), which is responsible for coordinating the efforts of the various automation systems to move the materials to the appropriate bay or tool according to the process requirements. Typically the MCS acts as an interface between the manufacturing execution system (MES) and the automation equipment as well as providing an interface for the material dispatching and scheduling systems. Since 300-mm fabs are expected to make wide use of intrabay automation, the transaction rate (number of moves controlled per unit time) of the MCS should increase by a factor of 3 to 5. The control system will also have to manage more complex moves (such as tool loads), which require more data per transaction. Product developments are taking place which will meet these needs in time for the rollout of the first 300-mm production fabs.

Intrabay Automation Technologies

The intrabay transport system is used to move material (cassettes or boxes of wafers) to and from tools, and if possible to load the tool directly. The bay or cell controller that oversees the transport and delivery then directs the tool to automatically begin processing the wafers. There are a number of technologies available for automating the movement of materials within a bay, such as robots, rail robots, track vehicles, automated guide vehicles (AGVs), and monorails.

Robots. Robots are typically used to load and unload cassettes from and to process tools. They can also be used to transport materials from tool to tool if the equipment is arranged radially around the base of the robot. In other cases, the robots and tools may be arranged to allow material transport by robot-to-robot transfer. Because of the fixed nature of this design, along with its expense and inefficiency when applied on a large scale, it is more appropriate for small cells than for a full intrabay application. This type of system is not expected to be used extensively in any 300-mm facilities.

Rail Robots. Robots may also be coupled with rails that allow the robots to travel from tool to tool. Typical rail robots can carry one or two payloads at a time, have travel distances of up to 10 m, and run at speeds from 10 to 30 m/min. Rail robots can be used for applications where the throughput is limited and where the dexterity of the robot is important for performing complex loading tasks. Rail robots have seen action automating wet benches, chemical mechanical polishing (CMP) cells, and diffusion furnaces. Typically, these tools are capable of handling cassettes of bare wafers in Class 1 environments, while contributing little contamination (see Figure 3). Because of their speed and length limitations, rail robots will probably see limited use in 300-mm fabs. Since 300-mm carriers are reasonably large, the footprint of many tools will grow as will the distance between tools.

Figure 3: Rail robots are used when throughput is limited and dexterity is important. Examples include automation of wet benches, CMP cells, and diffusion furnaces.

Track Vehicles. In most fabs, each bay is more than 20 m long, a distance which cannot be serviced efficiently by rail robots. Track vehicles can then be used. Track vehicles come in a variety of configurations but all have the following characteristics:

• There is a track mounted at floor level, which provides a stable platform for movement of the vehicle.

• There is the transport vehicle (also called a rail-guided vehicle, or RGV) capable of carrying multiple payloads (usually up to four cassettes).

• There is some type of loader or robot mounted to the vehicle which transfers the payloads to the process tools.

• There is a control interface available for communication between the vehicle and the process tools.

Additionally, the vehicle may include a minienvironment and other features. Track arrangements may be straight line, bidirectional systems, or closed loops. Usually one vehicle is used per straight-line track, and one or two are used on oval configurations. The track is aligned with the load ports on the process tools, with a stocker or similar storage system at some point along the track. Payloads are retrieved from the stocker and delivered to the appropriate process tools. Speeds of available RGVs range from 30 to 60 m/min. Because the vehicle is constrained to a rail, the time spent aligning or docking the vehicle to the tool is minimal, so the load/unload cycle takes about 30 seconds. Track vehicles have been used in furnace, lithography, and other high-throughput bays. Because the vehicles can be designed with an onboard minienvironment, cassettes of wafers can be handled in sub-Class 1 conditions (see Figure 4).

Figure 4: Track, also known as rail-guided, vehicles efficiently cover long bay distances.

Automated Guided Vehicles. AGVs can be used for delivery only or, when equipped with automated loaders, for delivery and tool loading (Figure 5). They have been employed for inter- and intrabay applications, but in the front-end areas of semiconductor fabs they are mostly used for intrabay transport. AGVs perform the same function as RGVs but offer some added benefits as well as constraints. Unlike an RGV, an AGV does not need a rail to travel on—it travels on the cleanroom floor directly. Some AGVs follow a guidepath made of wire or tape applied to the floor, while others can navigate without a guidepath. Because of this ability to operate on a floor instead of on a track, the AGV path and the system layout can be modified as needed without any disruption to the factory operation, allowing greater flexibility.

Figure 5: Automated guided vehicles can be used for both inter- and intrabay applications.

These benefits do not come without drawbacks, however. Because the motion of an AGV is unconstrained (compared to an RGV), practical travel speeds are lower and docking times at the process tools are longer. Practical top speeds for an AGV are 40 m/min, and load/unload and docking sequences take 1—2 minutes. In a typical bay, because of bay width and the vehicle's ability to pass, two to three vehicles, usually carrying four payloads each, are the practical maximum number per bay. AGVs are used in fabs throughout Asia, typically transporting bare cassettes in Class 1 environments.

AGVs for 300-mm fabs are being designed to carry two payloads at a time, not because of any thorough analysis of transport efficiency, but due to the fact that a practical-sized AGV only has enough room for two loads. Remember, a 300-mm carrier is almost 16-in. wide.

Monorails. Overhead monorails are used mainly for interbay transport from stocker to stocker. With the continuing standardization of tool load ports, however, it is becoming practical to consider monorails for intrabay transport and tool loading. A monorail system can be configured in any shape or length to conform to the tool layout. Monorails mounted overhead can use payload hoists to load and unload the tools, can use a transfer device at tool level. In the case of overhead mounting, a system could be retrofitted to an existing application. The single-payload monorail vehicles can operate at speeds of 30 to 60 m/min. In order to use a monorail for direct tool loading, the following features must be available for the system:

• Extremely clean operation. Since the monorail is over the process tools, it should be able to operate at better than Class 1.

• Passing/rerouting capability. Since each vehicle carries a unit load, multiple vehicles are required, which means the monorail system must have some type of rerouting or bypass capability.

• Intelligent positioning. The monorail vehicles must align exactly with the tool load ports.

Overhead hoist monorails have only been used in situations where the payload is enclosed, such as in SMIF pods. This makes the technology acceptable to the I300I group because of its advocacy of 300-mm pods. J300 prefers the use of AGVs, because hoists that operate cleanly enough to work with open cassettes have not yet been demonstrated. One can assume, however, that the potential of the upcoming market will lead to the development of ultraclean hoist systems, such as the one illustrated in Figure 6.

Figure 6: Hoist monorails have so far been used only where the payload is enclosed in SMIF pods or similar carriers.

Intrabay Automation Summary. Of the five types of intrabay automation discussed above, only RGVs, AGVs, and hoists are likely to be seriously considered for use in 300-mm fabs. Therefore, the rest of the discussion will be limited to these three intrabay delivery options.

Performance Measures

There are several performance measures used to analyze the requirements of material-handling systems.

• Transport work. A system's transport work capability is measured as the number of required moves multiplied by the length of each move per unit time. Typical units are move-feet per hour or move-meters per hour. Each vehicle is capable of a certain transport work quantity.

• Average delivery time. Given the transport work requirements, one can model or estimate the average delivery time for a load. Delivery time is usually measured from the time the move is requested by the MCS to the time the transport equipment signals successful completion of the move.

• Delivery distribution. Equally important to estimating the delivery time is the distribution of delivery times. In the case of an AGV carrying multiple loads, the delivery of the first load is a function of the speed of the vehicle and the distance it must travel. The last load, however, takes considerably longer to deliver. Thus we would have a fairly broad and flat distribution profile. In the case of multiple unit-load vehicles, the distribution would be narrower. Delivery distribution can have an impact on the processing of the wafers. Once the process is developed, consistent delivery times between steps should be maintained to permit management of oxidization and molecular contamination. The value of each 300-mm wafer makes this especially important.

• Machine interference. When servicing multiple tools with a single loader or robot, a number of tools will probably require servicing at the same time. The percentage of time that tools are waiting to be serviced by the limited resource, relative to the process time, is called the machine interference. If only one vehicle is used to service multiple machines so that the machines are often waiting for delivery, both OEE of the tool and factory productivity are adversely affected.

• Footprint. Since the cost of facilities is escalating, the choice of the material-handling system can have a significant impact on the total facility footprint requirements.

Relative Performance Capabilities

This section compares the relative performance capabilities in terms of the above outlined measures for RGVs, AGVs, and monorails. Table I summarizes and compares the performance specifications for the different technologies for typical intrabay applications. The following parameters are given for each technology in the table:

• Travel speed. The speed the vehicle can move while carrying loads.

• Number of loads. The typical number of loads transported per vehicle trip.

• Vehicles per bay. The typical number of vehicles that may be used per bay.

• Load/unload time. The time required to dock and perform a tool load/unload.

• Base cost. The cost for the base intrabay system, including rail and one vehicle and controls.

• Incremental cost. The cost for each additional vehicle.

• Footprint Factor. The amount of footprint added to the bay to allow the use of the material-handling equipment. This factor is used to describe the additional width of the bay versus a traditional manual bay.

Type of Automation	Travel Speed	No. of Loads	Vehicles per BAy	Load/ Unload Time	Base Cost	Incremental Cost	Footprint Factor
Track vehicles (RGVs)	50 m/min	4	2	30 sec	$300K	$250K	2x
Automated guided vehicles (AGVs)	30 m/min	4	2	90 sec	$250K	$200K	2x
Overhead monorails	40 m/min	1	10	30 sec	$200K	$30K	0.8x

Table I: Relative performance capabilities and costs of RGVs, AGVs, and monorails.

Using the information in Table I, a graph can be developed that shows the relative transport performed per dollar for each of the systems. This is solely a function of the vehicle speed and the number of vehicles used. It does not take into account the affects of batch handling or load/unload time on delivery time, distribution, or machine interference (see Figure 7).

Figure 7: Cost/performance comparison of different intrabay automation technologies.

Cost, Footprint, MLT Trade-offs

Our company has studied the use of the various technologies by simulating actual fab operations. The studies sought to determine which technology offered the lowest overall cost, footprint, and cycle time. In most cases, hoist vehicles were found to provide the lowest cost and the highest throughput. In some bays, AGVs or RGVs do the job less expensively, but chipmakers often want to limit a fab to a single type of technology. Because the J300 group favors the use of open cassettes, their studies indicate that AGVs are the best choice. This decision is based on the technical constraints of handling the cassettes, and not on an analysis of cost or MLT. Since Asian safety regulations also allow narrower aisles for AGVs and RGVs than those required elsewhere, overall footprints in Asia can be less than in other regions.

Automation Technology Selection Technique

• In order to select the appropriate intrabay technology the following steps are helpful:

• Determine the requirement for flexibility of layout. This is based on the process requirements, facility design, and fab operation requirements.

• Determine the required transport work of the bay using number of moves and distance data.

• Select a delivery technology that is compatible with the carrier design, safety requirements, and cleanliness requirements.

• Select the least expensive technology that can perform the transport work.

• Model or analyze the system to ensure that delivery times are adequate using techniques described in the second step or using a simulation tool.

Equipment	AGV	RGV	Hoist
A) Transport work requirement (mv-ft/hr)	Req't	Req't	Req't
B) Base unit transport capacity (mv-ft/hr)	Vendor	Vendor	Vendor
C) Incremental unit capacity (mv-ft/hr)	Vendor	Vendor	Vendor
D) Base unit cost	Vendor	Vendor	Vendor
E) Incremental unit cost	Vendor	Vendor	Vendor
F) Number of units required (integer)	(A-B) + (A-B)/C	(A-B) + (A-B)/C	(A-B) + (A-B)/C
G) Equipment cost	D + (F-1)•E	D + (F-1)•E	D + (F-1)•E
Space
H) Baseline footprint cost	Design	Design	Design
I) Footprint factor	2	2	0.8
J) Footprint cost (Benefit)	(I-1)•H	(I-1)•H	(I-1)•H
Total bay automation cost	G + J	G + J	G + J

Table II: Spreadsheet format for selection of automation technology.

The format shown in Table II may be used to develop a spreadsheet that can assist in the selection of automation technology. The spreadsheet determines the total cost of the different options as a function of the requirements. Item A in the table, transport work requirement, is determined by the fab designer based on the process requirements and tool layout as indicated earlier. Similarly, Item H, baseline footprint cost, is given by taking the area of the bay multiplied by the cost per unit area. Items labeled "Vendor" in the table are determined from vendor performance data.

Conclusion

The introduction of 300-mm wafers offers an opportunity to rationalize the interfaces between machines and the transport and delivery systems. This redesign also provides an opportunity to increase the level of automation in the fabs. Along with this increase of automation will come the expected benefits from its use, such as improved tool utilization, increased yield, lower cycle time, and decreased WIP. The development of new standards and new technologies and their concurrent implementation will permit efficient factory operation to help continue down the path described by Moore's Law.

Bibliography

Groover M, Automation, Production Systems and Computer Integrated Manufacturing, New York, Prentice-Hall, 1987.

"IC Factory Design for 300-mm Wafer Line Standardizing Study," second lecture at the Japan 300-mm Semiconductor Technology Conference (J300), Waseda University, Tokyo, December 1996.

"I300I Guidelines on 300-mm Process Tool Mechanical Interfaces for Wafer Lot Delivery, Buffering and Loading", I300I, Austin, TX, September³, rev. D, 1996.

VanLeeuwen C, "Implications of 300-mm for Fab Design and Automation," Semiconductor International, 19(4):91, 1996.

Weiss M, "Overall Factory Effectiveness (OFE) and Its Implications for 300-mm Tool Automation," Semiconductor Fabtech, fifth ed, pp 57­62, 1997.

Weiss M, "Using Analytical Methods to Accelerate Material Handling System Design Optimization," presented at the SEMI Taiwan Technical Symposium, National Technical University, Hsinchu, Taiwan, September 1995.

Mitchell Weiss is vice president of strategy/technology for PRI Automation, Billerica, MA. He is responsible for formulation of the company's long-term direction and technical development. Prior to taking this position, he was president of ProgramMation, Inc., which was acquired by PRI Automation in 1993. Weiss has more than 20 years of experience in robotics and automation, including the design of robots, teleoperator systems, and material-handling systems. He is coauthor of the textbook Industrial Robotics: Technology, Programming and Applications, has taught automation and robotics courses at the University of Pennsylvania and Pennsylvania State University, holds three patents, and has authored numerous papers and presentations. He received a BS in mechanical engineering at the Massachusetts Institute of Technology. (Weiss can be reached at 508/670-4270, ext. 3003; E-mail, mweiss@pria.com)

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