Acid diffusion during postexposure baking is viewed to be a limiting factor in the extension of lithography using chemically amplified resists to formation of nanoscale features. Quantification of thermally activated reaction-diffusion kinetics in these materials is therefore an important step in understanding the extendability of this class of resist systems. Previous investigations have addressed this issue, however there is poor agreement among them, and too few data exist in the literature to allow the systematics of the effect of polymer, photoacid generator, added base or other resist components on the diffusion process to be understood. We describe in this article a combined experimental and modeling protocol that is designed to elucidate the chemistry and physics of the reaction-diffusion process. Because it is physically based, not phenomenological, it provides a means of developing a set of predictive, mutually comparable data that will allow new insights to be developed into the nanoscale behavior of chemically amplified resist materials. We apply the protocol to a p-t-butyloxycarbonyloxystyrene/bis͑t-butylphenyl͒iodonium perfluorobutanesulfonate positive-tone photoresist system. The resulting kinetics measurements show that diffusion is environment sensistive and describable with two limiting diffusion coefficients. The Arrhenius parameters for the coefficients in p-t-butyloxycarbonyloxystyrene are D 0 ϭ1.9ϫ10 8 cm 2 /s and E a ϭ36.5 kcal/mol; those for diffusion in the deprotected polymer product p-hydroxystyrene are D 0 ϭ9ϫ10 Ϫ3 cm 2 /s and E a ϭ22.1 kcal/mol. The coefficients are much smaller than previously reported, resulting in a very slow diffusion rate. The model indicates that the considerable image spreading observed during the postexposure bake process is attributable primarily to the efficiency of the catalytic chemistry. Our results suggest that numerical models currently used for prediction of imaging in chemically amplified resists may require refinement in order to be useful for feature sizes below 100 nm and for new classes of resist systems.

Chemically amplified (CA) resists are in widespread use for the fabrication of leadingedge microelectronic devices, and it is anticipated that they will see use well into the future. The refinement and optimization of these materials to allow routine imaging at dimensions that will ultimately approach the molecular scale will depend on an improved in-depth understanding of the materials and their processing. We provide here an overview of recent work in our laboratory on the chemical and physical processes that occur during post-exposure baking (PEB) of positivetone CA resists. Our results provide a clearer understanding of how this critical step in the lithographic imaging process will affect extendibility of the CA resist concept to nanoscale feature sizes.

Porous, low dielectric constant polyimide films have been made by a “nanofoam” approach. The pore sizes generated in the polymer films are in the tens of nanometers range, hence the term “nanofoams”. The nanoporous foams are generated by preparing triblock copolymers with the majority phase comprising polyimide and the minor phase consisting of a thermally labile block. Films of the copolymers are cast and then heated to effect solvent removal and annealing, resulting in microphase separation of the two dissimilar blocks. The labile blocks are selectively removed via thermal treatments, leaving pores the size and shape of the original copolymer morphology. The polyimide derived from 2,2-bis(4-aminophenyl)hexafluoropropane (6FDAm) and 9,9-bis(trifluoromethyl)xanthenetetracarboxylic dianhydride (6FXDA) was used as the matrix material for the generation of nanofoams, and specially functionalized poly(propylene oxide) oligomers were used as the thermally labile constituent. The synthesis and characterization of the copolymers were performed and the process for obtaining nanofoams was optimized. The foams were characterized by a variety of techniques including thermal gravimetric analysis (TGA), transmission electron microscopy (TEM), dynamic mechanical thermal analysis (DMTA), density, small-angle X-ray scattering (SAXS), refractive index, and dielectric constant measurements. Thin-film, high-modulus nanoporous films with good mechanical properties and dielectric constants ∼2.3 have been synthesized by the copolymer/nanofoam approach.

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A fast method to estimate the effects of line edge roughness is proposed. This method is based upon the use of multiple 2D device "slices" sandwiched together to form an MOS transistor of a given width. This method was verified to yield an accurate representation of rough edge MOS transistors through comparisons to full three dimensional simulations. A subsequent statistical study shows how the variation in line edge roughness affects the values and variances of several key device parameters.