SynopsisFundamental models have been developed to describe swelling and dissolution of glassy polymer thin films. The models account for solvent penetration by either Fickian or Case I1 diffusion mechanisms. The convective flux due to local swelling as the solvent penetrates is included. Chain disentanglement at the polymer-developer solution interface is scaled with the local solvent concentration and polymer molecular weight using reptation theory. The effective surface concentration during dissolution is estimated by applying thermodynamics of swollen networks to the entangled polymer. Swelling and dissolution of thin polymer films have direct application to microlithography. Various molecular and processing parameters affect the outcome of resist development. The utility of the models for selecting appropriate developer solvents, minimizing resist swelling, and providing a better understanding of the swelling and dissolution of resists is demonstrated.
SynopsisThe effects of solvent size, temperature, and polymer molecular weight on the swelling of poly(methy1 methacrylate) (PMMA) thin films in low molecular weight alcohols were investigated using an in situ ellipsometer. Apparent activation energies were indicative of non-Fickian diffusion, although optical data showed substantial Fickian character for swelling in methanol and moderate Fickian character in ethanol. Penetration rates were strongly dependent on the solvent molar volume for methanol, ethanol, and isopropanol, but 1-butanol and 2-pentanol had rates similar to isopropanol. The effective cross sections of these longer molecules are similar to isopropanol, and this apparently explains the similar penetration rates. The effect of polymer molecular weight (MW) on methanol penetration rates (21-27°C) was investigated with monodisperse PMMA ( M , = 6.4-40.0 X lo4 g/mol). A minimum at intermediate MW was observed.Isopropanol swelling rates (45-52OC) were insensitive to MW. The swelling data were also used to determine parameters for transport models that describe the swelling of thin polymer films.
We characterize the chemical constitutents of high dose implanted deep ultraviolet photoresist before and after dual-mode oxygen plasma processing, where a remote rf-plasma source is operated simultaneously with rf bias. Raman spectroscopy indicates that the organic composition of the crust comprises a mixture of sp 2 graphite and sp 3 diamondlike carbon structures. High dose ion implantation reduces the hydrogen content by about 50 at. % as measured by hydrogen forward scattering and explains the reduced optical emission signal intensity observed during crust removal. The crust thicknesses extracted from the secondary-ion-mass spectroscopy profile correspond well to prior scanning electron microscopy characterization ͓Kawaguchi et al., J. Vac. Sci. Technol. B ͑submitted͔͒ and support the existence of a transitional layer between the hardened crust and the underlying photoresist. Angle-resolved x-ray photoelectron spectroscopy analysis of arsenic implanted photoresist shows that dual-mode plasma processing causes substantial oxidation deep into the bulk. This result contrasts with downstream plasma processing, which proceeds by a near-surface mechanism. In addition, surface arsenic levels increase by an order of magnitude, which suggests that ion bombardment does not significantly sputter the dopant.
A single-element rotating-polarizer ellipsometer (psi-meter) was used for in situ characterization of the thermodynamic and kinetic behavior of poly-(methyl methacrylate), PMMA, thin films (1.2 μm) in solvent/nonsolvent binary mixtures of methyl ethyl ketone/isopropanol (MEK/IPA) and methyl isobutyl ketone/methanol (MIBK/MeOH). Thermodynamic effects were inferred from equilibrium behavior by the degree of swelling and polymer-solvent solubility. A sharp transition between complete solubility and almost total insolubility was observed in a narrow concentration range near 50:50 (by volume) solvent/nonsolvent for both MEK/IPA and MIBK/MeOH. In the insoluble regime, the polymer was found to swell up to three times the initial thickness. At 50:50 MEK/IPA, a temperature decrease from 24.8 to 18.4 °C caused a change from complete dissolution to combined swelling/dissolution behavior and rendered the PMMA film only 68% soluble. Kinetic effects were determined by dissolution and penetration rate measurements. A constant penetration velocity was observed for almost all compositions for both binary solvent mixtures with Case II transport assumptions providing good agreement with experimental results. For MEK/IPA, penetration rates increased with increasing MEK concentration. For MIBK/MeOH, however, a maximum was observed at 60:40 MIBK/MeOH.
Dissolution of poly(methyl methacrylate) thin films in ketones, alcohol/ketone mixtures, and hydroxy ketones was investigated using an in situ ellipsometer. The dissolution rate in pure ketones was limited by the penetration of solvent molecules into the polymer, as indicated by minimal swelling of the film during dissolution and significant solvent size effects. Activation energies for ketone dissolution were indicative of Case II penetration. Dissolution rate decreased with polymer molecular weight up to 10 ~ g/mol, above which the rate was constant. The thickness of the swollen surface layer was estimated from shifts in the ellipsometric data. No surface layer was detected at a polymer molecular weight of 7.2 x 104 g/mol, but above this value, the surface layer thickness increased with increasing molecular weight. The effects of polymer molecular weight on dissolution rate and thickness of the swollen layer indicated that at lower molecular weights, dissolution occurred by stress cracking. Case II transport model parameters were determined from the ketone dissolution data. Acetone/isopropanol mixtures at 20.4~ showed a transition from swelling to dissolution near 50% by volume acetone. Acetol, a hydroxy ketone, caused only swelling (34~176 whereas diacetone alcohol dissolved the films (20.0~176 at approximately one-fourth the rate of methyl isobutyl ketone.
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