Immersion lithography has taken off like a mushroom in spring over the past two years. Lest you assume immersion is a simple, "just add water" proposition, however, consider the breadth and caliber of engineering effort being channeled into its development.
Bruce Smith, an EE professor at the Rochester Institute of Technology, a hotbed of immersion research, recently put together an immersion microscanner with a field size of 2 mm on a side. Alex Raub at the University of New Mexico is creating beautiful 65-nanometer patterns, testing immersion with an interferometric exposure system made from a YAG laser with a beam splitter.
At RIT, International Sematech and elsewhere, chemists are adding phosphates and alkalines to boost the refractive index of water, the drink of choice for 193-nm lithography. Phosphoric acid could be the additive that takes 193 immersion to the 32-nm node and beyond, said Burn Lin, an early immersion researcher now at foundry TSMC.
Mechanical engineers schooled in fluid dynamics are working on novel ways to dispense and retrieve the thin film of water that lies between lens and wafer. And, please add to your vocabulary "meniscus," which for immersion researchers involves keeping the edge of the film of water exactly where it is supposed to be.
Alex Bard, Bob LeSuer, Bob Allen, Chris Taylor and others at the University of Texas at Austin are studying the photochemistry of water, peering in with scanning electrochemical microscopy and radiochemical analysis of carbon-14-labeled resist components.
The diversity of the research is astounding. The effort started at MIT Lincoln Labs, with Mordy Rothschild and others exploring immersion for 157-nm lithography. Then Lin suggested that, instead of thinking ahead to 157, immersion could extend the 193-nm scanners. That proposal started the boll rolling.
If Lin has his way, it won't be long before we know whether 193 immersion really works: TSMC plans to use it for two critical layers of the 65-nm node starting in mid-2005. This cooperative effort among EEs, materials scientists, mechanical engineers, metrologists and lens designers promises a huge payoff for the semiconductor industry.