The automated semiconductor fabricator

05/01/1997

The automated semiconductor fabricator, circa 2020

Mitchell Weiss, PRI Automation Inc., Billerica, Massachusetts

The year is 2020. The place is Bethlehem, PA, where silicon fabrication has finally supplanted the steel industry. (Locally, semiconductor fabs are call "chip mills.") We are going to take a tour of two vastly different companies in the semiconductor industry. One company, Rabbit`s Foot Technology, specializes in the design and development of custom, complex circuit systems for embedded applications. The other company, Megavolume Computing, was formed by the merger of two former giants in the computers and telecommunications industry. Megavolume Computing circuits are used almost every minute of every day by every person on the face of the earth. Let`s start our tour at Rabbit`s Foot headquarters.

High on the hill, overlooking the town of Bethlehem, Rabbit`s Foot is one of the new companies made possible by the availability of the "fab-in-a-box," a unifab (Fig. 1). The unifab is a single-wafer process tool that performs every process step required to produce a multilevel integrated circuit. It is the logical next step after the super-cluster tool, introduced in 2002. Circuit designers at Rabbit`s Foot design their circuits on advanced, cell-based design systems. After successfully simulating the operation of their circuits, the engineers download the design to the fabrication preprocessing computer (FPC). The FPC analyzes the circuit design, then uses production rules specified by the engineer to develop a process recipe and mask patterns. These are sent to the production control computer, which downloads the recipe information to the appropriate process chamber at each step. Exposure uses a direct-write e-beam process to avoid the need for maskmaking. (Multiple chips are exposed by performing a "raster" scan of the entire wafer surface. Raster scan time for each wafer is identical, regardless of circuit complexity.) Although slower than photolithography, the flexibility outweighs the drawbacks.

Figure 1. The fab-in-a-box: each chamber processes single 300-mm wafer batches, with exposure by direct-write e-beam.

A number of developments made the unifab possible. First was the availability of every process step as a single chamber process. Second was the linear-array wafer-handling system. Finally, the ready availability of 300-mm wafers made single-wafer batches large enough for low-volume and R&D applications. The engineers at Rabbit`s Foot can design a circuit and then order time on the unifab. A single wafer can be completely processed in under 100 hours.

Unfortunately, only five wafers can be processed at a time, for a total throughput of only one wafer every 20 hours. The cost of the unifab, however, is under $50 million. If a single, proprietary Rabbit`s Foot chip is worth $1000, and 500 chips/wafer can be produced, Rabbit`s Foot can pay for the unifab after just 2000 hours of operation. If high volumes are required, Rabbit`s Foot transfers the design data to one of its many foundry partners.

Down the hill, on the banks of the Lehigh River, sits the Megavolume Computing Corporation`s (MCC) Fab 126 (Fig. 2). From the outside, the facility is nondescript, a 100-yard-diameter white concrete hockey puck. The only clue to what is going on inside is the MCC sign, and the small fusion power plant used to generate electricity for the factory.

Figure 2. The megafab: a network of radial and circumferential tunnels minimizes wafer transport time.

Inside the facility, transparent, 3-foot-square "tubes" connect a vast number of process tools. The tubes look like a giant hamster-trail system, but instead of hamsters, they are navigated by silent vehicles, riding on monorail tracks, transporting 50 wafer lots of 450-mm wafers. The tubes (or tunnels, as MCC engineers call them) are purged with nitrogen to protect the wafers. The tunnels connect the process tools radially and circumferentially, in a tool layout developed to minimize material-transport distances and decrease manufacturing cycle times. Each process tool is isolated in its own enclosure; small robotic systems on each monorail vehicle gently load and unload the wafer lots onto each tool. Operators and engineers at MCC are dressed in their shirtsleeves, and "bunny suits" are only needed on the rare occasions when chamber integrity must be breached to perform repairs. The MCC fab is capable of producing 40,000 good wafers/month, while occupying an area of just 70,000 square feet.

Advances in material control, automation, and computing have made the fabs of 2020 much more productive than the fabs of 1997. At the same time, flexibility is available to developers of the new technologies, which continue to feed the demand for semiconductors. Will all of this really come to fruition in the future? We shall just have to wait and see.

MITCHELL WEISS received his BS in mechanical engineering from the Massachusetts Institute of Technology. He worked at Unimation Inc. as an applications engineer, focusing on robotic and simulation systems, and in 1980 he co-founded United States Robots Inc. Later, he co-founded ProgramMation Inc. and served as its president until its acquisition by PRI Automation in 1993. Weiss is currently VP Strategy/Technology at PRI, responsible for developing new products, services, and technologies. PRI Automation Inc., 805 Middlesex Turnpike, Billerica, MA 01821; ph 508/670-4270, fax 508/663-9755, e-mail mweiss@exchange 1.pria.com.