EUV glass still less than half full, but level is rising

EUV first drew the semiconductor industry’s attention in the late 1990s, as lithographers began to consider the “post-optical” future. At that time, the future was expected to arrive with the 100-nm technology node, by 2004. ArF lithography turned out to be far more extensible than anticipated, though, and is still going strong fifteen years later. Which is fortunate given that, as we now know, EUV lithography was not ready in 2004. Indeed the technology still is not ready for production in 2012.

Even at the time, many people felt the 2004 target was overly optimistic. Few would have expected EUV to still be “just around the corner” in 2012, however. Most would have guessed either that the technology would have reached production some years ago, or that it would have been abandoned as unworkable by now. EUV, as Winston Churchill famously said about democracy, appears to be the worst form of post-optical lithography except for all the others that have been tried. From 157-nm F₂ lasers to directed self-assembly, alternatives to EUV are proposed every few years, then drop back to the background as their limitations become evident. Meanwhile, EUV soldiers on, making slow but consistent progress.

For the short term, the EUV outlook remains grim. As my colleague Mark LePedus reported, Cymer was forced to delay deliveries of the company’s 20 watt EUV source. Originally expected in late 2011 or early 2012, this upgrade has been pushed out by at least a quarter. Such delays are especially worrisome because EUV source power remains far short of the 100 watts or more that even small-scale production would require. In the face of such delays, it’s easy to lose sight of the forest and miss the very substantial progress the technology has made. According to Mike Lercel, Cymer’s Senior Director of EUV product marketing, the company has demonstrated 50 watt average power in a continuously operating, production-style source. That result, and the imminent 20 watt upgrade to sources already in the field, represent dramatic improvements within the last year.

Similarly, Lercel said, Cymer has achieved a collector lifetime of approximately ten weeks. In a laser-produced plasma (LPP) source like Cymer’s, a laser pulse ionizes a droplet of tin. The resulting plasma emits EUV photons in all directions. To produce the directional beam needed for lithography, a mirror surrounding the plasma reflects these photons through an aperture and into the lithography system. Debris from the plasma gradually erodes the surface of this so-called “collector,” requiring its replacement. While a ten week lifetime is well short of the one year replacement interval that is Cymer’s ultimate goal, it still represents a 10-fold improvement over earlier designs.

Thanks to the collector lifetime increase and other improvements, Lercel reports that Cymer’s overall source availability now reaches 70%. Though not impressive by the standards of a production fab, 70% availability at 10 or more watts of source power is enough for device development. It’s enough to allow fabs to compare EUV and ArF-based process flows and imaging results. Based on these early results, Lercel said, process engineers like what they see.

At the 20-nm node and beyond, ArF-based lithography will be extremely difficult. Multiple patterning will be ubiquitous, with all the additional process steps and yield risk that implies. These challenges are why EUV has not yet been abandoned, and why the imaging results that are now becoming available are very exciting for lithographers. As Lercel explained, a big departure in 2012 relative to previous years is that people are starting to believe that EUV will really happen. “They’re more willing to invest; they want this technology in their fabs.”

It’s still not possible to say when, or even if, EUV-printed chips will start to make their way through production fabs. It’s not even clear that the glass is half full. But, finally, the level of liquid in the glass seems to be rising at a perceptible and reassuringly steady rate.