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Monday, 12 December 2011

UV Lithography: Taking Extreme Measures

National Institute of Standards and Technology (NIST)
Dec 9, 2011

Contamination forms on a clean multi-layer surface (top) when EUV photons react (middle) with gases, resulting in carbonaceous deposits (bottom). Photos: SEMATECH

Sometime soon, microchip fabricators will take the next major step in the relentless reduction of feature size, from the current minimum of 22 nm down to 10 nm and perhaps even smaller. Getting there, however, will entail much more than incremental progress. It will require adopting entirely new technology and surmounting a formidable roster of technological problems. One of most daunting of those – identifying and characterizing the factors that cause contamination of key lithographic components – has begun to yield to investigators in PML's Sensor Science Division, who have made some surprising and counterintuitive discoveries of use to industry.

In general, feature size is proportional to the wavelength of the light aimed at masks and photoresists in the lithography process. Today's super-small features are typically made with "deep" ultraviolet light at 193 nm. "But now we're trying to make a dramatic shift by dropping more than an order of magnitude, down to extreme ultraviolet (EUV) at 13.5 nm," says physicist Shannon Hill of the Ultraviolet Radiation Group. "That's going to be a big change."

In fact, it complicates nearly every aspect of lithography. It will necessitate new kinds of plasmas to generate around 100 watts of EUV photons. It demands a high-quality vacuum for the entire photon pathway because EUV light is absorbed by air. And of course it requires the elimination of chemical contaminants on the Bragg-reflector focusing mirrors and elsewhere in the system – contaminants that result from outgassing of materials in the vacuum chamber.

As a rule, the focusing mirrors are expected to last five years and decrease in reflectivity no more than 1 percent in that period. Innovative in-situ cleaning techniques have made that longevity possible for the present deep UV environments. But the EUV regime raises new questions. "How can we gauge how long they're going to last or how often they will have to be cleaned?" says Ultraviolet Group leader Thomas Lucatorto. "Cleaning is done at the expense of productivity, so the industry needs some kind of accelerated testing."

Unfortunately, Hill adds, "You can't even test one of these mirrors until you know how everything outgasses. Ambient hydrocarbon molecules outgassing from all the components will adsorb on the mirror's surface, and then one of these high-energy photons comes along and, through various reactions, the hydrogen goes away and you're left with this amorphous, baked-on carbonaceous deposit."

But what, exactly, is its composition? How long does it take to form, and what conditions make it form faster or slower? To answer those questions, the researchers have been using 13.5 nm photons from the NIST synchrotron in a beam about 1 mm in diameter to irradiate a 12-by-18 mm target in multiple places.

"We built a chamber where we can take a sample, admit one of these contaminant gases at some controlled partial pressure, and then expose it to EUV and see how much carbon is deposited," Hill says. The chamber is kept at 10-10 torr before admission of contaminant gases, and the inside surface plated in gold. "Gold is very inert," Hill explains, "and we want to be able to pump the gases out of the chamber with no traces remaining."
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