How immersion lithography saved Moore’s Law – Stories | ASML
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How immersion lithography saved Moore’s Law<br>Find out how a eureka moment during an industry conference triggered a paradigm shift in chip manufacturing.
7-minute read - by Sander Hofman, August 2, 2023
After more than 70 years of relentless innovation to produce smaller, better and cheaper chips, the world’s most advanced chipmakers are currently getting ready to mass produce chips on the 3-nanometer process node. But did you know that the industry’s ability to make ever-finer circuitry on silicon almost got stopped short at the 65-nanometer threshold – if it weren’t for a puddle of water? Read on to discover the inspiring history behind our immersion DUV lithography systems.
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Table of contents
The risk of shrink
A puddle of water, a twist of fate
Three advantages that sped up ASML’s immersion program
Controlling the pesky puddle
Multi-patterning and the ultimate immersion platform
Immersion lithography continues to drive innovation
The risk of shrink
In the early 2000s, the chip industry had been working on a transition from lithography with argon fluoride (ArF) light sources at 193 nanometer to lithography with fluorine (F2) light sources at 157 nanometer. Like an artist wishing to draw a more precise, detailed picture by replacing a marker with a fineliner pen, this significant shift to a smaller wavelength was the industry’s hope to continue shrinking transistors and enable more computing power as well as memory function on a chip. However, in an unexpected turn of events demonstrating the risk of pushing engineering to its limits, the laws of physics disagreed.
As the engineering came together for the first 157-nanometer lithography systems, adopting calcium fluoride lenses as the new type of optics in these systems was considered challenging yet feasible. However, imaging experiments on actual prototype lithography systems revealed significant double refraction effects. What’s worse, the effect was inherent to calcium fluoride and far exceeded the imaging specifications. The chip industry hit a wall as the obvious roadmap for lithography seemed to have come to a grinding halt.
A puddle of water, a twist of fate
In December 2001, ASML researcher Jan Mulkens (now an ASML Fellow) attended an industry conference on 157-nanometer lithography in the United States where industry professionals came together to identify potential next steps. Their discussion honed in on adding a layer of purified water under the lens to sharpen the resolution, an optical phenomenon first discovered and harnessed by microscope pioneers Robert Hooke and Antoni van Leeuwenhoek  , and first described for use in lithography by IBM in the 1980s. Jan and his colleagues realized that this optical technique could extend 193-nanometer lithography further, bypassing the industry's burning challenge of trying to fix 157-nanometer lithography. Furthermore, by using water as the optical fluid, all of the existing optics, masks and photoresists could continue to be used. This was the best chance to keep Moore’s Law going.
Viewing an object through water makes the object appear larger.
“Projecting light through highly purified water would allow significantly smaller chip features to be printed, because the liquid allows the design of an optical lens that more accurately images the fine patterns on the wafer,” explains Jan. “But when we first started thinking about using this principle in a lithography machine, people found it odd. Water was associated with splashes, droplets and bubbles – would that really work in a complex and highly accurate imaging system?” Introducing water into the system that might not flow safely and securely through a hose appeared to be an impossible task.
Upon returning to the ASML headquarters in Veldhoven, the Netherlands, Jan gathered a small team, convinced that competitors would soon follow suit. If they wished to seize this opportunity, they would have to act quickly. With a heightened sense of urgency, Jan’s team started by defining and testing some of the basic immersion concepts. “When we showed the basic feasibility of immersion lithography through a series of experiments and conceptual ideas for the system architecture, we got the green light to scale our concept to a full-size prototype system with a team of 20 colleagues,” Jan recalls.
Three advantages that sped up ASML’s immersion program<br>The team made quick progress thanks to the TWINSCAN platform. Our systems were able to do two things at once thanks to the proprietary dual-stage architecture: one stage measured the wafer position for accurate alignment and focus, while the other stage moved precisely under the lens to image the pattern on the wafer. Jan's team realized as they sketched out the dual-stage architecture of an immersion system that chipmakers could leverage the resolution enhancements of...