Litho Tools Find New Life in Special Applications

Aaron Hand, Managing Editor -- Semiconductor International, 9/1/2006

Given how we're always looking ahead at the coming generations of IC manufacturing, it can be easy to forget that there is still plenty of use out there for older technologies. But indeed, older platforms, including 248 nm systems, are being re-engineered to take on new challenges in growing markets.

ASML (Veldhoven, Netherlands) has had a Special Applications division for more than eight years, established to find new markets for tools in the company's existing product portfolio. Many of the markets are small, but nonetheless make up a significant source of revenue for technologies that have already seen sizable returns on initial development investments from the mainstream IC industry.

The market for thin-film heads, for example, accounts for only ~1% of the wafer exposure equipment sold today, according to Boudewijn Sluijk of ASML. Nonetheless, this is a key market for ASML Special Applications. With NAND flash growing in popularity, a bit of a war has ensued between flash and hard disk drives, where thin-film heads are used. As flash pushes further into the market for higher-capacity storage, so do hard disk drives push ahead with developments of their own. Moving to reduction lithography to produce thin-film heads was one way to get a leg up.

It was a couple of years ago that the move was made, printing thin-film heads on AlTiC wafers, Sluijk noted. To do that, current lithography platforms had to be adapted to accommodate requirements particular to the new wafers and the devices themselves. For example, AlTiC wafers are much thicker and heavier than silicon wafers — 1.2-2 mm thick vs. the 0.5 mm typical for silicon. The weight of the wafers required changes in the handling system, while the increased thickness changed focusing parameters. The wafers also have different thermal characteristics than silicon wafers, and have more stringent electrostatic discharge (ESD) prevention requirements. Thin-film heads use long of strips of material that are larger than the exposure field, calling for optimized stitching behavior as well.

A suitable holder can be made to adapt a 200 mm lithography platform to handle smaller square substrates. (Source: ASML)

Both 248 and 193 nm tools are being used these days to produce thin-film heads, according to Peter ten Berge, product marketing manager for ASML Special Applications. In fact, the roadmap for thin-film heads is not far behind the International Technology Roadmap for Semiconductors (ITRS), and getting closer all the time. The required resolution for thin-film heads is more or less equal to the tightest half-pitch on the ITRS, ten Berge said. However, the dense half-pitch for flash is much harder to do than the isolated lines and trenches of thin-film heads.

Like the IC industry, thin-film head specialists are updated on the latest scanner technologies, including the latest advances in immersion lithography, ten Berge noted, because requirements are accelerating so fast. “An immersion tool probably wouldn't have been developed only for thin-film heads, but we see where we can adjust the latest tools to suit TFH,” he said, adding that immersion technology will likely be adopted within three years by thin-film head producers.

Of course, there are other applications as well, including MEMS, compound semiconductors and image sensors. Several memory makers are making CMOS image sensors because it is technically not that different from making memory, according to ten Berge. Because the technology for CMOS image sensors lags slightly behind memory, the manufacturers can use older equipment to get the job done. One modification made, however, is to incorporate an extraction hood into the lithography system to remove contaminants because the thick color filter layers on the sensors give off a larger exhaust.

MEMS have traditionally used contact printers or 1× lithography. But when considering the alignment and overlay requirements, it's justified to look at reduction lithography, ten Berge said. There are some commonalities between thin-film heads and MEMS, such as thick layers. “Our systems offer a larger depth of focus for alignment, so the topography can easily be handled,” he added.

Customers continue to come up with new types of devices that require new lithography solutions. There are lots of surprises, and certainly no consolidated roadmap. “There's not something like the ITRS for MEMS,” ten Berge said. “There's an enormous diversity of device types, and also manufacturing processes.”

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