Integrating Self Assembly and Lithography

Since the late 1970s the vast majority of lithographic processes have used a process called chemical amplification. In this process a molecule called a photoacid generator (PAG), generates a hydrogen ion, and corresponding counter ion, on absorption of a photon of light. The H+ ion is then capable of diffusing through the matrix, acting to catalyse a cascade of reactions to deprotect multiple ester or carbonate groups, thus changing the solubility of the resist exposed to light and allowing removal of the exposed regions. This has been a hugely successful method for achieving high volume fabrication of integrated circuits, and since its inception has been applied to all lithographic nodes up to this time. The advantage of chemical amplification lies in the fact that a single photoreaction can result in hundreds or thousands of deprotection events. However, this advantage also directly contributes to so-called line-edge roughness (LER), a major limitation of chemically-amplified resists when printing features less than 50 nm.

LER is the variation in the regularity of features developed in the resist compared to the projected image and it arises from a number of sources,including shot noise, non-uniform distribution of PAG in the resist, but most notably from the random nature of the diffusion processes occurring during development of the photoresists. The diffusive pathlength of a triflate-based PAG at 130 ºC in a model resist is on the order of 50 nm, which is large compared with the dimensions of the patterns being projected onto the photoresist. This level of LER results in a greater number of defective devices on a chip and consequently reduced chip performance. A number of workers have embarked on programs to limit LER through, for example, limiting the diffusive pathlength, by the use of larger PAG molecules. While reductions in LER have been achieved, this has come at the expense of significantly-reduced sensitivity of the resist formulations, and hence lower throughput in the fabrication facility. Thus the problem of how to reduce LER while maintaining high sensitivity has not yet been solved.

It is the objective of this project to develop methods of “healing” LER post-exposure, using a novel approach involving carefully prepared block copolymers.


  1. Cheng, H.H.; Keen, I.; Yu, A.; Chuang, Y.M.; Blakey, I.; Jack, K.S.; Leeson, M.J.; Younkin, T.R.; Whittaker, A.K., Electron beam induced freezing of positive tone, EUV resists for directed self-assembly applications, Proceedings of SPIE 2011, 7970(Alternative Lithographic Technologies III), 79701V/1-79701V/9.

  2. Keen, I.; Yu, A.; Cheng, H.H.; Jack, K.S.; Nicholson, T.M.; Whittaker, A.K.; Blakey, I., Control of the Orientation of Symmetric Poly(styrene)-block-poly(d,l-lactide) Block Copolymers Using Statistical Copolymers of Dissimilar Composition, Langmuir 2012, 28, 15876.

  3. Keen, I.; Yu, A.; Cheng, H.H.; Jack, K.S.; Younkin, T.R.; Leeson, M.J.; Whittaker, A.K.; Blakey, I., Behavior of lamellar forming block copolymers under nanoconfinement: implications for topography directed self-assembly of sub-10 nm structures, Macromolecules 2013, DOI: 10.1021/ma4019735.


Drs. Todd Younkin and Michael Leeson, Intel;
Drs. Pete Trefonas and Jim Thackeray, Dow Chemical Company

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