This is an extended version of the interview that appeared in oemagazine.
C. Grant Willson, of The University of Texas at Austin, is this year's recipient of the Frits Zernike Award for Microlithography primarily for his development of chemical amplification. This photochemical route to high sensitivity was able to produce resists with 1000 times greater sensitivity than conventional materials with no loss in resolution.
"The impact of Dr. Willson's work on chemically amplified resists is hard to overestimate," says SPIE member Chris Mack, KLA-Tencor (Austin, TX). "The ability to tailor high resolution with acceptable sensitivity at DUV wavelengths would not have been possible without this significant and novel innovation in resist chemistry. This is one of just a handful of absolutely essential technological breakthroughs in lithography over the last 20 years that has enabled the continuation of Moore's Law."
In 1978 Willson joined the IBM Almaden Research Center, where he reached the level of IBM fellow. In 1993 he began teaching in the Department of Chemistry and the Department of Chemical Engineering at The University of Texas at Austin. He has received two teaching awards since that time: the Semiconductor Research Corporation Aristotle Award in 1999 and a Student Engineering Council Teacher Excellence Award in 2003.
Willson has received numerous other awards including the American Chemical Society Carothers Award in 1992, the National Academy of Sciences Award for Chemistry in Service to Society in 1999, the Photopolymer Science and Technology Award in 2003, six IBM Innovation Awards, eight IBM Patent Awards, and two IBM Corporate Awards.
Willson was interviewed for oemagazine by Erin M. Schadt.
SPIE: Can you explain how and when you developed the method of chemical amplification to create high sensitivity resist materials?
Willson: The original work was done in the early 80s at the IBM San Jose Research Center. It was done in collaboration with Professor Jean Fréchet, now at UC Berkeley, who was there on sabbatical leave and Dr. Hiroshi Ito, who I had hired as a post doctoral research fellow. Dr. Ito is still doing great work at IBM. We realized that the resolution advantages associated with moving from I-line to deep-UV [ultraviolet] lithography were accompanied by a huge reduction in productivity because the mercury discharge light sources available at that time had limited brightness in the deep UV. The only way to deal with this problem was to find a very much faster resist system or a very much brighter light bulb. We pursued both pathways. The bright light bulb was the excimer laser, and Dr. Kanti Jain and I are co-inventors on the IBM patent claiming excimer lasers as light sources for lithography. Kanti designed and built the first excimer laser scanner tool at IBM. In the meantime, we explored the use of photogenerated catalysts as a means of increasing resist sensitivity. That work quickly led to the "Chemical Amplification" patents. The term was chosen as a means to communicate the concept of chemical catalysis to our higher management, who were largely electrical engineers who clearly understood the concept of amplification but were not as familiar with chemical catalysis.
SPIE: What are the greatest impacts chemical amplification has had on lithography so far?
Willson: This is difficult to answer. I suppose it is best answered by saying that every advanced semiconductor device that is manufactured today is made with chemically amplified resists that are based on the early work done at IBM. It makes me very, very proud to have been a part of that work.
SPIE: Do you think chemical amplification and the further advancement of polymer resists will continue to help engineers pursue Moore's Law, or will new technologies be needed as transistor density increases?
Willson: I believe that quite soon, we need to find a new family of resist materials because the chemical amplification scheme as currently practiced suffers from an intrinsic bias or blur that is the result of mass transport of the catalyst. This problem can be minimized, but so far only by greatly reducing the speed of the resists. Efficient manufacturing of feature sizes in the 20- to 30-nm regime and below may require a very different resist design or a printing technology that is radically different from that in use today.
SPIE: Do you think 157 nm lithography has a promising future or do you think EUV or 193-nm immersion lithography holds more potential?
Willson: I am afraid that 157-nm lithography has lost all momentum and is moribund without a doubt. It is destined to join some of the other "NGL's" in a very expensive grave yard. The program produced some exceptional PhDs though and a great deal of knowledge. I am not optimistic about EUV lithography. There seem to be a number of fundamental problems that are not resolving. Chief among these are source power and the limitations of chemically amplified resists. The future of EUV is not bright (pun intended). Immersion lithography will be implemented at some level because there is no viable alternative but hyper-NA tools will not be cheap and the process window will be very small.
SPIE: Can you tell me about some of the research projects you are currently leading at UT Austin?
Willson: My group is working on a wide range of projects. We have been working toward the development of a fundamental, mesoscale simulator for lithography that can be used to explore the stochastic nature of the process and accurately simulate line edge roughness and the effect on the final image of changes in, for example, polymer molecular weight, bake temperatures and times, etc., without having to rely on response parameters as we do now. SRC has been a great supporter of this interesting work. We are also heavily involved in step and flash imprint lithography. We believe that this low-cost, high-resolution printing technology holds great promise for the future. Our current efforts are heavily focused on implementation of SFIL in the back end of the line where it offers the potential for removing over 100 unit process steps from the metallization flow. We also have projects related to biosensor array technology in which we are exploiting lithographic techniques to fabricate biosensors in a new way. We are heavily involved in a number of joint projects with SEMATECH under the auspices of the Texas Advanced Materials Research Center and are proud of our long and fruitful association with SEMATECH. The group continues to have a balance of fundamental and applied projects that exploit our ability to make new materials and study them in their end use.
SPIE: You've won a number of awards throughout your careerwhat one(s) are most significant or special to you?
Willson: I have been very fortunate in this regard and I am deeply touched and honored by the recognition I have received. It is very difficult to pick out one or two such honors. The SPIE award is a huge honor. I am very proud to have received this award and to see that I have followed my friend Burn Lin in this way. I suppose the Aristotle award from SRC and the teaching award from the College of Engineering have to be among the most rewarding I have received as they recognize efforts to produce good students and that is my job. I am intensely proud of my students who I see in responsible and productive activities all over the semiconductor industry. I cannot leave this question without mentioning the IBM Fellow appointment as well. I suppose that only if you work for IBM is it possible to understand what a high honor this is. At the time I received that recognition, there were about 400,000 employees at IBM and only about 50 Fellows. It was and is a high honor.
SPIE: Can you tell me a little about your life outside work?
Willson: I have been heavily involved in racing sail boats and ocean racing yachts in the past having participated five times in the Los Angeles to Honolulu Race, the Los Angeles to Tahiti Race and many others. I raced motorcycles for many years, and I love to ski but of late, my commitments at the university have limited my ability to pursue these activities. I have a wonderful wife and two sons that I am very proud of. The boys are in college at this time. One is studying psychology and the other communications. I don't think I have any budding engineers or chemists, but I am very proud of them both.
SPIE: Is there anything else that you would like to add?
Willson: [That] I have the best students in the world . . . they are the very best at what they do. and I am extremely fortunate to have had the opportunity to work with them.