Viewpoint

Meeting the wafer-handling challenges of the brave new world of fully automated fabs

James Cameron

With every increase in wafer size and decrease in device geometry has come a change in how wafers are handled. Production cleanrooms are now Class 1, and standard mechanical interface (SMIF) technology and vacuum cluster tools have gained critical acceptance. Similarly, factory automation is a reality in state-of-the-art 200-mm fabs, replacing many human operators and cleanroom workers.

All of these trends will continue with the jump to 300-mm wafers. The drive for increased yields at smaller geometries with higher throughputs leaves no road untraveled. Sophisticated computer modeling will help lay out the fabs to improve efficiency, and cost of ownership (COO) models make equipment selection a more refined process. A $1.5 billion investment in fabs is becoming commonplace, with $2 billion and $3 billion price tags already in the works.

Future fabs will be fully automated, from stockers and automated guided vehicles (AGVs) or other types of pod movers, to automatic tools that rarely require operator intervention. Computers and network systems will track productivity and yields on a wafer-by-wafer basis, offering real-time critical analyses of a tool's performance. In addition, there will be an increasing burden on tool manufacturers to meet higher regulatory demands, imposed by CE marking and similar initiatives at SEMI and other organizations, to improve and standardize tool safety and electrical requirements.

New tools will be required to have mechanical and electrical interfaces compatible with inter- and intrabay transport systems and to have software that can be integrated with the fab's host systems, including single-wafer lot tracking. SMIF and AGV interfaces are no longer "add-ons" that the fab will install; instead, toolmakers will need to provide them. A partnership between automation providers and equipment makers will be necessary for success.

Because wafer handling is one of the most significant contributors to COO equations, OEM suppliers of robots are being held to much higher standards. Toolmakers will no longer take full responsibility for integrating the hardware and software of robots, prealigners, pod openers, optical character recognition (OCR) systems, and other wafer-handling components. They will demand from their automation partners full wafer-handling subsystems that are certified to meet the new standards.

Such front-end systems will link each tool to the fab. They will provide wafer buffering, identification tracking, and minienvironment and pod management, as well as handle wafer movement from pods to loadlocks or metrology stages. Reliability, throughput, and cleanliness will be the operative terms for these systems.

Reliability is fundamental on a production line costing hundreds of millions, even billions of dollars and creating a product--fully processed 300-mm wafers--that is more valuable than ever. Suppliers of front-end wafer-handling systems must demonstrate high reliability through sophisticated analysis (FRACAS, Weibull curves, and the like) and provide on-site support to minimize downtime in the event of a system failure. Handling systems must never drop wafers, so parallel cassettes of 300-mm tools must be addressed by mechanically derived straight-line motions to extract the wafers. System designers need to resist the temptation to compensate for improper mechanics with complicated software. Fail-safe mechanics will be necessary, in combination with smart software and electronics capable of self-diagnosis.

The robot system's throughput greatly affects the selling price and profitability of a tool, because the machine's wafer output is the divisor in COO equations. Hence, faster and smoother motions will be required. This is difficult with the larger, heavier payload of 300-mm wafers, so more sophisticated motion controls, such as sine-curve velocity profiling and time-optimized wafer-path algorithms, must be used.

Fab profitability demands high yields, so molecular and other airborne contaminants have to be analyzed and filtered more carefully, and wafer backside contamination must be measured and minimized. Just as ultraclean minienvironments will become standard for all atmospheric front-end wafer-handling systems, smaller linewidths will require purer environments during critical processing. Processes that have been done in the cleanroom ambient will be performed in vacuum or controlled inert gas ambient cluster tools.

Cluster tools will also undergo significant change. Since the 300-mm wafer size allows a greater number of expensive chips to be produced on each substrate, the stakes will be driven too high to accept the risks inherent in the old equipment designs. In situ process monitoring will take place through metrology stations in the cluster tools themselves, alongside the process modules. Fab hosts will monitor and verify process uniformity from wafer to wafer in real time, limiting the damage an out-of-specification machine can cause.

Handling also will have to move away from the batch approaches of the past. Wafers have become too valuable to be moved by an archaic batch arm. One solution could be a combination of the new front-end designs with the traditional cluster hubs.

Single-wafer transfer with alignment and identification verification will be required, both to minimize risk during atmospheric handling and to improve vacuum handling performance. Performing alignment and identification reading in atmosphere removes two bottlenecks for the vacuum handler, increasing overall system throughput. Single-wafer loadlocks can already be used to eliminate failure-prone vacuum indexer mechanisms, which are a major source of particle contamination.

Equipment makers must understand that this brave new world offers huge rewards to those who meet its challenges, but that the risks are greater than ever. Many designs that were acceptable for 200-mm wafers will become outdated. Process requirements for the 300-mm transition constitute nothing less than a paradigm shift, presenting clear opportunities for equipment makers to distinguish themselves with next-generation tools. Since the demands on process uniformity and metrology escalate geometrically with wafer size, current automation solutions can help OEMs concentrate resources on their core competencies. A true partnership must be forged between automation suppliers and process and metrology tool manufacturers, creating high value and improved COO for the fab end-users.

James Cameron is CEO of Equipe Technologies (Sunnyvale, CA), which designs and manufactures high-precision robotic systems for the semiconductor and flat-panel display industries.

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