X-ray Absorption Spectroscopy To Probe Surface Composition and Surface Deprotection in Photoresist Films

NEXAFS spectroscopy was used to probe the surface composition and under-water surface reconstruction of thin films of comb-like diblock copolymers with cylindrical and spherical microphases. The polymers consisted of a polystyrene block, and a second block prepared from a styrenic monomer grafted with fluoroalkyl-tagged poly(ethylene glycol) side chains. Compositional depth profiling of the microphase separated block copolymer films, in the top 1-3 nm of the film, was performed to understand the role of block copolymer microstructure and self-assembly on surface composition. Using experimentally determined concentration profiles, the surface concentration of phenyl ring carbon atoms was quantified and compared with those of homopolymer and random copolymer controls. The carbon atoms from the relatively high surface energy phenyl groups were depleted or excluded from the surface, in favor of the low surface-energy fluoroalkyl groups. While it is expected that block copolymer surfaces will be completely covered by a wetting lamellar layer of the lower surface energy block, a significant amount of the higher surface energy polystyrene block was found to be present in the surface region of the cylinderforming block copolymer. Evidently, the spontaneous formation of the cylindrical polystyrene microdomains in the near-surface region compensated for the lowering of the free energy that could have been achieved by completely covering the surfaces with a lamellar layer of the lower surface energy fluorinated block. All surfaces underwent molecular reconstruction after immersion in water. The experimental concentration depth profiles indicated an increased surface depletion of phenyl ring carbon atoms in the water-immersed thin films, due to the tendency of hydrophilic PEG side groups to be present at the polymer-water interface. Such a detailed characterization of the outermost layers of the block copolymer surfaces was possible because of the exceptional depth resolution of the NEXAFS depth profiling technique.

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“…[38][39][40] In the present study, however, we varied the X-ray incidence angle at a constant value of the entrance grid bias.…”

Section: Methodsmentioning

confidence: 99%

NEXAFS Depth Profiling of Surface Segregation in Block Copolymer Thin Films

et al. 2010

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“…29 However, in all these cases, the reactions involve multiple component reactive systems whereby the reactants and catalysts can be inhomogeneously distributed due to interface effects. 24 Recently, the reversible cross-linking of a polymer coating has been demonstrated using established mechanisms associated with coumarin moieties in the polymer. 30 In this context of reversephotodimerization of coumarin derivative units, Sato et al 30 illustrated that mobility of the main chain of the polymer determines the film shrinkage and recovery.…”

Section: ■ Introductionmentioning

confidence: 99%

Kinetics of UV Irradiation Induced Chain Scission and Cross-Linking of Coumarin-Containing Polyester Ultrathin Films

et al. 2014

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Photoresponsive thin films are commonly encountered as high performance coatings as well as critical component, photoresists, for microelectronics manufacture. Despite intensive investigations into the dynamics of thin glassy polymer films, studies involving reactions of thin films have typically been limited by difficulties in decoupling segregation of reacting components or catalysts due to the interfaces. Here, thin films of coumarin polyesters overcome this limitation where the polyester undergoes predominately cross-linking upon irradiation at 350 nm, while chain scission occurs with exposure to 254 nm light. Spectroscopic ellipsometry is utilized to track these reactions as a function of exposure time to elucidate the associated reaction kinetics for films as thin as 15 nm. The cross-linking appears to follow a second order kinetic rate law, while oxidation of the coumarin that accompanies the chain scission and enables this reaction to be tracked spectroscopically appears to be a first order reaction in coumarin concentration. Because of the asymmetry in the coumarin diol monomer and the associated differences in local structure that result during formation of the polyester, two populations of coumarin are required to fit the reaction kinetics; 10−20% of the coumarin is significantly more reactive, but these groups appear to undergo chain scission/oxidation at both wavelengths. These reaction rate constants are nearly independent (within 1 order of magnitude) of film thickness down to 15 nm. There is maximum rate at a finite thickness for the 254 nm exposure, which we attribute to constructive interference of the UV radiation within the polymer film, rather than typical confinement effects; no thickness dependence in reaction rates is observed for the 350 nm exposure. The utilization of a single polymer with two distinct reactions enables unambiguous investigation of thickness effects on reactions. ■ INTRODUCTIONFor nearly two decades, 1 the physical properties of polymer thin films have been investigated with an emphasis predominately on the glass transition temperature 2−4 as a function of film thickness. In many cases, a thickness dependent glass transition temperature (T g ) is found with the deviations from the bulk T g generally associated with the interfaces, 5 where T g is depressed 6,7 at a free interface (surface) and increased 8,9 at an attractive hard interface. The interactions of the polymer film with the underlying substrate or capping layer as well as the processing history 10−12 can impact the observed changes in T g . However, other physical properties of polymers under confinement can vary inconsistently with intuitive expectations based on the observed T g changes, 13−17 where typical relationships for polymers appear to breakdown for thin films, in particular for mechanical properties. This inconsistency leads to challenges in understanding a priori how polymers will behave when confined to thin films. For example, the rubbery compliance of ultrathin polymer films appears to decrease s...

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“…[34] Surface segregation of PAG was observed in polymer resists by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. [35][36][37] Time-of-flight secondary-ion mass spectrometry was used to show uniform depth profiles in optimized resist formulations under controlled-atmosphere clean-room conditions. [38] In addition to the non-uniform distribution of PAG, amine moieties in the ambient conditions quench the reaction at the free surface.…”

Section: Introductionmentioning

confidence: 99%

“…The surface chemistry, characterized by NEXAFS, verifies the presence of photoacid as well as the change in surface reaction with reaction time and temperature within the top 1 to 6 nm. Although PAG segregation and surface-versus-bulk reactions are known in polymer resists, [35][36][37]41] such effects are less understood for well-defined molecular resists with low polydispersity. [42,43] Further, the alternative topology found with molecular resists may aid in the design of resists with order-disorder transitions.…”

Section: Introductionmentioning

confidence: 99%

Characterization of the Non-uniform Reaction in Chemically Amplified Calix[4]resorcinarene Molecular Resist Thin Films

et al. 2011

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The ccc stereoisomer-purified tert-butoxycarbonyloxy-protected calix[4]resorcinarene molecular resists blended with photoacid generator exhibit a non-uniform photoacid-catalyzed reaction in thin films. The surface displays a reduced reaction extent, compared with the bulk, with average surface-layer thickness 7.0 ± 1.8 nm determined by neutron reflectivity with deuterium-labelled tert-butoxycarbonyloxy groups. Ambient impurities (amines and organic bases) are known to quench surface reactions and contribute, but grazing-incidence X-ray diffraction shows an additional effect that the protected molecular resists are preferentially oriented at the surface, whereas the bulk of the film displays diffuse scattering representative of amorphous packing. The surface deprotection reaction and presence of photoacid were quantified by near-edge X-ray absorption fine-structure measurements.

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X-ray Absorption Spectroscopy To Probe Surface Composition and Surface Deprotection in Photoresist Films

Cited by 29 publications

References 42 publications

NEXAFS Depth Profiling of Surface Segregation in Block Copolymer Thin Films

NEXAFS Depth Profiling of Surface Segregation in Block Copolymer Thin Films

Kinetics of UV Irradiation Induced Chain Scission and Cross-Linking of Coumarin-Containing Polyester Ultrathin Films

Characterization of the Non-uniform Reaction in Chemically Amplified Calix[4]resorcinarene Molecular Resist Thin Films

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