Pavlos C. Tsiartas

Design of Reversible Cross-Linkers for Step and Flash Imprint Lithography Imprint Resists

Progress in the semiconductor manufacturing industry depends upon continuous improvements in the resolution of lithographic patterning through innovative materials development and frequent retooling with expensive optics and radiation sources. Step and Flash Imprint Lithography is a low-cost, nanoimprint lithography process that generates nanopatterned polymeric films via the photopolymerization of low-viscosity solutions containing cross-linking monomers in a transparent template (mold). The highly cross-linked imprint materials are completely insoluble in all inert solvents, which poses a problem for reworking wafers with faulty imprints and cleaning templates contaminated with cured imprint resist. Degradable cross-linkers provide a means of stripping cross-linked polymer networks. The controlled degradation of polymers containing acetal- and tertiary ester-based cross-linkers is demonstrated herein. The viscosity and dose to cure are presented for several prepolymer formulations, along with imprint resolution and tensile modulus results for the cured polymers. Optimum conditions for de-cross-linking and stripping of the cross-linked polymers are presented, including demonstrations of their utility.

Effect of molecular weight distribution on the dissolution properties of novolac blends

The dissolution properties of nearly monodisperse fractions of metacresol novolac resin in aqueous potassium hydroxide solutions has been investigated. A relationship between the molecular weight of the fractions and dissolution rate was examined for a range of developer concentrations. Deliberate and calculated blending of the various fractions afforded an opportunity to investigate the fundamental relationship between molecular weight and dissolution characteristics over a range of developer concentrations.

Advancements to the critical ionization dissolution model

The microlithographic process is dependent upon the dissolution of acidic polymers in aqueous base. The fundamental mechanism that governs the dissolution of these polymers has been the subject of considerable discussion, and a number of theories have been proposed to explain this behavior. Our research group has presented the critical ionization (CI) dissolution model to explain the dissolution of phenolic polymers in aqueous base. Specifically, the model proposes that a minimum or critical fraction of ionized sites, fcrit, on a given polymer chain must be ionized in order for that chain to dissolve. The main input parameters to this model are the critical fraction of ionized sites, fcrit, and the fraction of ionized surface sites, α. In this work methods are established for measuring these parameters. A quantitative link between the CI model and experiment has been demonstrated for the dissolution rate and surface roughness dependence on polymer molecular weight. Methods for calculating α are discussed,...

Understanding the photoresist surface-liquid interface for ArF immersion lithography

Extraction of small molecule components into water from photoresist materials designed for 193 nm immersion lithography has been observed. Leaching of photoacid generator (PAG) has been monitored using three techniques: liquid scintillation counting (LSC); liquid chromatography mass spectrometry (LCMS); and scanning electrochemical microscopy (SECM). LSC was also used to detect leaching of residual casting solvent (RCS) and base. The amount of PAG leaching from the resist films, 30 - 50 ng/cm2, was quantified using LSC. Both LSC and LCMS results suggest that PAG and photoacid leach from the film only upon initial contact with water (within 10 seconds) and minimal leaching occurs thereafter for immersion times up to 30 minutes. Exposed films show an increase in the amount of photoacid anion leaching by upwards of 20% relative to unexposed films. Films pre-rinsed with water for 30 seconds showed no further PAG leaching as determined by LSC. No statistically significant amount of residual casting solvent was extracted after 30 minutes of immersion. Base extraction was quantified at 2 ng/cm2 after 30 seconds. The leaching process is qualitatively described by a model based on the stratigraphy of resist films.

Electrostatic effects during dissolution of positive tone photoresists

Positive tone photoresist function is based upon modulation of the dissolution rate of an acidic polymer in aqueous base developer. During dissolution, a negative surface charge accumulates at the surface of the photoresist film as acidic sites on the polymer are ionized by the basic developer. This negative surface charge causes a depletion of hydroxide ions in the developer solution immediately adjacent to the film. One effect of this phenomenon is to reduce the dissolution rate relative to rates predicted from the bulk base concentration. The depletion of base at the resist/developer interface has been studied through the application of the Poisson–Boltzmann equation and through Monte Carlo simulations. It is shown that the extent of depletion of hydroxide at the surface of the film is related to the bulk developer concentration and the local geometry of the interface. Interfacial geometry is found to be most influential on the length scale of 1–10 nm. When electrostatic effects are considered in conju...

Probabilistic model for the mechanism of phenolic polymer dissolution

A probabilistic model for polymer dissolution was recently presented that aims to provide a fully molecular explanation for the complex dissolution behavior of phenolic polymers such as novolac in aqueous developers. It is based on the hypothesis that a phenolic polymer, which is below the entanglement molecular weight, becomes appreciably soluble only when a certain fraction of its phenol groups are deprotonated. If the rate of dissolution of the polymer is limited by this solubility criterion rather than by mass transfer, then the dissolution rate of the polymer may be predicted from the probability of deprotonation. This hypothesis has been supported by laboratory measurements that tested the models predictions for the effect of polymer molecular weight on the minimum base concentration for development and by combinatory potentiometric and turbidimetric titrations. The model can adequately account for the observed effects of residual casting solvent and novolac/inhibitor interactions and the differential dissolution behavior between novolac and poly(hydroxystyrene). No other model for phenolic polymer dissolution predicts all of these behaviors. This evidence suggests that even in a primitive form, the probabilistic model captures the important physical elements affecting the dissolution process that are absent from models based solely on diffusion theory.

Photoresist characterization for lithography simulation: III. Development parameter measurements

Accurate photoresist modeling parameters are required for correct lithographic simulations. In particular, three sets of data are required to model a typical non-chemically amplified resist: the refractive index as a function of wavelength and exposure, the exposure or ABC parameters (the Dill parameters), and the development parameters. This work focuses on an improved technique for the accurate extraction of development rate parameters for non-chemically amplified resists. Traditionally, the refractive index for photoresists is considered constant in simulations and other work. However, the refractive index of a photoresist varies as the chemical composition of the photoresist changes during exposure. This work presents a rigorous analysis technique for extracting development parameters from resist development rate data which accounts for this refractive index change. Development parameter measurements for a commercial resist are discussed. Comparisons of the various development rate models are made using this experimental data.

Using scanning electrochemical microscopy to probe chemistry at the solid-liquid interface in chemically amplified immersion lithography

Three modes of scanning electrochemical microscopy (SECM) - voltammetry, pH, and conductivity - have been used to better understand the chemistry at, and diffusion through, the solid/liquid interface formed between a resist film and water in 193 nm immersion lithography. Emphasis has been placed on investigating the photoacid generator (PAG), triphenylsulfonium perfluorobutanesulfonate, and the corresponding photoacid. The reduction of triphenylsulfonium at a hemispherical Hg microelectrode was monitored using square wave voltammetry to detect trace amounts of the PAG leaching from the surface. pH measurements at a 100 μm diameter Sb microelectrode show the formation of acid in the water layer above a resist upon exposure with UV irradiation. Bipolar conductance measurements at a 100 μm Pt tip positioned 100 μm from the surface indicate that the conductivity of the solution during illumination is dependent upon the percentage of PAG in the film. Liquid chromatography mass spectrometric analysis of water samples in contact with resist films has been used to quantify the amounts (< 10 ng/cm2) of PAG leaching from the film in the dark which occurs within the first 30 seconds of contact time. Washing the film removes approximately 80% of the total leachable PAG.

Fundamental study of photoresist dissolution with real-time spectroscopic ellipsometry and interferometry

The use of in situ spectroscopic ellipsometry (SE) is demonstrated as a technique for studying photoresist dissolution. Experiments carried out using a J.A.Woollam M-2000 ellipsometer and a custom built cell designed for in situ film measurements show that bulk dissolution rate measurements using the SE technique agree with dissolution rate data obtained using multiwavelength interferometry. SE is also demonstrated as a method for measuring thin film dissolution rates, water sorption, and films that swell. An additional focus of this work was the topic of interfacial “gel” layer formation during photoresist dissolution. Ellipsometry and interferometry were used to test several photoresist resins, with an emphasis on phenolic polymers. Single and multiple layer models were used to analyze the data, and were compared to model calculations predicting formation of a gel layer. For the materials studied, interfacial gel layer formation in low molecular weight phenolic polymers was not detected, within the resolution of the experimental techniques (< 15 nm).

Quantifying acid generation efficiency for photoresist applications

A direct analytical technique for measuring the solution quantum efficiencies of photoacid generators (PAGs) is presented. The technique is based on the nonaqueous potentiometric titration of the photogenerated acid and does not require separate calibrations, or the addition of sensing materials such as dyes or bases. Solutions of PAGs in acetonitrile were irradiated at 248 nm and subsequently titrated with known concentrations of triethanolamine base. The only quantities required for an accurate determination of the quantum efficiency are the incident radiation energy and the titration equivalence point. A mathematical analysis that enables this calculation is derived, and a table with calculated quantum efficiency values for three classes of PAGs having a variety of counterions is presented. The method is simple, accurate, and applicable to any class of PAG molecules.