NEXAFS Measurements of Chemically Amplified Photoresists:
Surface versus Bulk Chemistry

Erin L. Jablonski*, Sharadha Sambasivan, Ronald L. Jones, Joseph L. Lenhart2, Daniel A. Fischer1, Christopher L. Soles, Vivek M. Prabhu, Wen-li Wu, Dario L. Goldfarb3, Marie Angelopoulos3

Polymers and 1Ceramics Divisions, National Institute of Standards and Technology
Gaithersburg, MD 20899-8541
2Sandia National Laboratories, Alburquerque, NM
3IBM T. J. Watson Research Center, Yorktown Heights, NY

Near Edge X-ray Absorption Fine Structure (NEXAFS) measurements are used to quantify the surface composition profile (top 1 nm to 10 nm) and bulk composition in various model chemically amplified photoresists. In general, photoresist formulations are complex multi-component materials including polymers, photoacid generators (PAGs), and other additives. The photoresist is effectively patterned optically through the photo-induced generation of an acid species that then catalyzes a deprotection reaction rendering the reacted areas soluble to a developer solution. These materials are extremely prone to interfacial and surface phenomena that cause deviations in the desired lithographic pattern. If interfacial excess or depletion of the photogenerated acid occurs, either from atmospheric contamination, evaporation, or segregation within the film, the resulting compositional heterogeneity will affect the interfacial resist structure, composition, and de-protection kinetics. Examples include T-topping, closure, footing, and undercutting. A significant technical challenge lies in measuring the surface composition and extent of reaction with high resolution at interfaces. NEXAFS allows simultaneous measurement of both the surface (top 10 nm) and bulk chemical composition, particularly for carbon, fluorine, oxygen, and nitrogen. When the polymer film is excited by the incident X-ray radiation, the entire region of the film that absorbs photons also emits electrons. The electron yield signal is surface sensitive; the top surface of the material releases Auger electrons that can be measured with an electron yield detector, while the bulk of the material emits photons, which can be measured with a fluorescence yield detector. By varying the negative voltage bias on the electron yield detector, it is possible to further differentiate Auger electrons escaping from depths up to 10 nm into the film. This measurement capability becomes increasingly important with the drive towards sub-100 nm lithography. The resist film thickness continually decreases and the interfacial regions dominate the behavior of the material. It is crucial to understand both their physical and chemical nature.

Keywords: NEXAFS, photoresist, surface chemistry
Category: Chemistry
Acknowledgements: The authors wish to acknowledge the support from both the NIST Office of Microelectronics Program and the Advanced Lithography Project of DARPA under contract N66001-00-C-8083.

*Contact Author information:
100 Bureau Dr., Stop 8541
Gaithersburg, MD 20899-8541
(301) 975-4769 (phone)
(301) 975-3928 (fax)
e-mail: erin.jablonski@nist.gov