Soft X-ray Characterization of Directed Self Assembly Block Copolymers

Daniel Sunday, Wen-li Wu and R. Joseph Kline


Significant challenges face the semiconductor industry in their pursuit of increasingly smaller features sizes for integrated circuits. The currently implemented lithographic technologies are facing fundamental and economic limitations that have seen the industry begin to explore alternative and supplemental technologies. Augmenting traditional lithography with the directed self assembly (DSA) of block copolymers (BCPs) is one approach being considered. In DSA BCPs are patterned on top of a chemical template prepared by a lithographic technique, amplifying the template pitch. This leads to a wealth of competing stresses on the assembled BCP structure, creating complex interfacial profiles between the BCP components. In order for this technique to be industrially feasible the factors which lead to efficient assembly must be studied in detail, which requires measurement techniques which are capable of resolving the interfacial profile to a high level of precision.

We have developed a variable angle transmission x-ray technique (Critical Dimension SAXS, CD-SAXS) which is capable of evaluation the three dimensional profile of periodic nanostructures. By using soft x-rays near the absorption edge the contrast between BCP components may be sufficiently enhanced to allow this technique to evaluate the interfacial profile to a high degree of precision. PS-b-PMMA with a native pitch of 25 nm was patterned on a chemical template having a pitch of 100 nm, resulting in a 4:1 pitch amplification. CD-SAXS measurements were conducted on these samples near the carbon absorption edge, and the BCP structure was reconstructed from the scattering pattern using an inverse process, including the determination of parameter uncertainty with a Monte-Carlo Markov Chain method. These results show that the lamella assembled on top of the neutral portion of chemical pattern have a different structure compared to the lamella which reside on the bare substrate. The CD-SAXS results were compared to soft x-ray reflectivity measurements conducted on the same samples. The vertical scattering length density profile determined from the reflectivity measurement was found to be in good agreement with the vertical composition profile obtained from the CD-SAXS measurement.