EUV Research Helps Solve 193 nm Resist Problems

Alexander E. Braun, Senior Editor -- Semiconductor International, 2/6/2008 7:21:00 AM

Leading-edge research into the requirements for extreme ultraviolet (EUV) photoresists is helping to solve some of the problems encountered by 193 nm lithography technology as it ventures into increasingly smaller CDs.

Robert Brainard, an associate professor at the College of Nanoscale Science and Engineering (CNSE) at the University at Albany in New York, is investigating new materials for use in EUV and 193 nm lithography. While the bulk of the work that he and his group are conducting is focused on EUV photoresists, it has also dealt with some of the problems encountered at 193, which are becoming very similar to those for EUV.

“The EUV resist challenges can be summed up succinctly,” Brainard said. “They are the three basic properties — resolution, line-edge roughness and sensitivity. My colleagues and I call it the RLS trade-off. You need all three properties for EUV [and EUV resists] to be successful, and when you improve one, you make another one worse.”

According to Brainard, one can take a simple resist and measure its line-edge roughness (LER) and sensitivity, and then perhaps add more base to it. However, when this is done, the resist slows down. “These are chemically amplified resists, so the light generates acid. If you add more base, it kills the acid and then you require more light,” Brainard said. The result is, however, that by adding the base, LER is reduced, resulting in smoother lines. Conversely, if the base is reduced, the resist is more sensitive, but LER worsens.

Large jumps in resist performance require the pursuit of discontinuous variables, such as synthesis of new materials. Resist performance must then be optimized within the new multidimensional performance surface. (Source: CNSE, University at Albany)

“In both cases you want a lower number — you want lower LER and a lower dose [sensitivity is measured in millijoules per square centimeter]. This means that the resist is sensitive,” Brainard said. “There are trade-offs for all three different properties. This, in my thinking, is EUV resist’s principal challenge — beating the trade-offs. I look at the problem much like a curved surface, where you can move around on it and trade one performance factor for another, but you do not get off the surface. What we want to do is invent something that will enable us to leave that surface.” Both 193 nm and EUV resists now face similar fundamental problems. “The industry has attributed those problems to EUV, simply because EUV reached those dimensions earlier — it uses shorter wavelengths,” Brainard said. “Meanwhile, 193 has continued advancing toward smaller dimensions, and it is reaching the CD size regime where the technology is beginning to face, or will soon face, the same problems.”

One of these hurdles is controlling acid diffusion. There may be ways to solve this that have not yet been tried, which are being developed at CNSE; however, because of intellectual property (IP) concerns, Brainard declined to be specific. “In terms of the RLS trade-off, there is need to make more acid. The more efficient you are in producing the acid, the better the performance gets. We’re focused on quantum yield. For me, that is the amount of acid generated in a film divided by the number of photons absorbed in the film. In traditional quantum yield, you ignore the solvent. Before, you’d use a transparent solvent; there would be a molecule floating there and would be absorbed only by that molecule. The problem is that this is not a good model for EUV resist because, in it, the polymer absorbs the light, the PAG absorbs the light, everything absorbs the light. I don’t think it is correct to just concentrate on the light absorbed by the PAG, because all of the light that is absorbed can be converted into photoelectrons and all of these can react to make acid.” Brainard, therefore, looks at all of the light absorbed by all of the acid created in the film — the moles of acid generated in the film divided by the moles of photons absorbed in the film. In this case, higher is better.

“We’re learning from EUV technology things that are directly applicable to 193. At the 22 nm node, whether it is 193 that gets there or EUV, there are fundamental problems in the understanding of things like acid diffusion that have to be solved by whichever resist technology is used.”