Litho cells: a dream working: track speed improvements make stepper/coater-developer combo possible

Electronic News, June 8, 1992 by Peter Dunn

Track Speed Improvements Make Stepper/Coater-Developer Combo Possible

A few years back, combining wafer steppers and coater-developer units into "lithography cells" was a pipe dream, the province of a few development labs and the hardy pioneers who explore the limits of integrated circuit (IC) fabrication technology.

Today, however, litho cells are in operation on new lines at most volume semiconductor producers and about three-quarters of new steppers are destined for cell installations.

At its simplest, a litho cell needs only a passthrough from coater-developer to stepper and a single-cable connection between the two systems' host computers. A cassette of wafers is placed into the coater-developer; the wafers are processed and passed over to the stepper for exposure, then back to the track system for developing.

Traditionally, differing throughput rates for track units and steppers have made the linkage difficult. But track speeds have improved to the point where they can keep up, say all stepper makers. Uptime is less of a problem than before, and software links have proven to be relatively simple to implement, as they require a fairly limited set of messages and commands--ready to send, ready to receive, wafer received, last wafer, etc.

Ultimately, in the ongoing quest for precise process controll, cells will be controlled by a single personal computer or workstation that also shares information with the wafer fab's central host computer. This will allow one-stop operation and recipe management for the cell, and permit statistical data and information on work in progress to be passed back to the host.

While this sounds relatively straightforward, it brings up a number of management issues.

"Sometimes customers want more management responsibility by the stepper vendor for the whole cluster," said Richard LaFrance, director of marketing for Nikon Precision Inc. "They will ask us to oversee the installation, set-up and operation." In some cases, that can create problems--Mr. LaFrance notes that track suppliers are occasionally loathe to give up control over their installed base, and that stepper suppliers are not necessarily experts with coater-developers.

"Sometimes a customer will want to own the project, but we've also had programs where we are the main contractor and the track company is a subcontractor," added Alan Levine, a technical support staff member at General Signal Corp.'s Ultratech Stepper unit. "The big issue is that everyone has to believe in the program, and work within the same framework. You have to have a lot more preparation up front."

Most stepper makers are working, to one degree or another, on some sort of unified cell control system that will help obviate these problems. General Signal's GCA unit is working with Fastech Integration, a producer of industrial cell control software, on a special customizable user interface that will link stepper and track using the Semiconductor Equipment Communication Standard (SECS), said Peter DiSessa, GCA's vice president of marketing.

Meanwhile, Silicon Valley Group Inc., the only lithography equipment supplier that also has a wafer track business, will show at Semicon/West one of its Micrascan 92 step-and-scan tools combined with an SVG 90S stacked-module track system. "And we,re working on development of a cluster controller product to manage the whole system," added Tom St. Dennis, vice president of technology and strategic planning for SVG's Track Systems division. "This is the next level--it will help with factory automation. We'll probably announce it at Semicon/Japan."

Another issue associated with stepper-track integration sounds minor, until one delves into it: the physical arrangement of the stepper and track units. As IC producers strive to get the most out of every square foot of clean room space, they demand litho cells that can be assembled in different ways.

"It used to be that if you were looking at a Nikon stepper, the wafer input/output would always be on the left--Canon, too," said SVG's Mr. St. Dennis. "Now a hot topic is having right-hand and left-hand versions, so that you can integrate them into a photobay in a way that utilizes the space most efficiently."

"Our new models have a very flexible robot handler, to make them complementary to track vendors," added Larry Wise, director of development engineering at Ultratech Stepper. "We can give a number of physical options--front connection, left connection, right connection. "We will see more of that, since everyone has their own idea of how to save floor space."

Looking to the future, there are a number of trends that will continue to drive IC producers toward lithography cells. First of all, a cell configuration minimizes operator interaction with the wafers being processed, thus removing some opportunities for error. Over-all system utilization and productivity also increase in a cell, despite the fact that a combination of two machines will almost certainly be less reliable than stand-alone units. This lowers the cost of ownership.