Application of an external electric field across thin polymeric films is known to induce the formation of arrays of cylindrical structures commonly referred to as pillars. Low viscosity photocurable materials have been shown to possess significant advantages over their polymeric counterparts both in terms of a significant reduction in the time of formation, as well as in the elimination of additional processing steps, such as heating and cooling cycles. With proper design, pillar arrays with predetermined diameter, period, location and orientation can theoretically be formed over an entire substrate surface, thus enabling their use in applications such as optics-less patterning, micro-electro-mechanical systems, and micro fluidic devices. In this paper, we present our recent work in using a low viscosity thiolene system to produce pillar arrays and attempts at inducing long range order in the resulting pillar arrays by (1) varying the nature of the external electric field in the form of a sinusoidal AC voltage (instead of the conventional DC voltage) and (2) the use of ultrasonic piezo transducers to generate regular waves in the film with simultaneous application of a potential across the film sample. Our initial results show that although both the AC voltage and the ultrasonic piezo methods are successful in producing pillar arrays, their ordering is short-ranged and does not extend throughout the entire sample area.
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.
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.
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).
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 model's 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.
The differences in the dissolution behavior of novolac and poly(4-hydroxystyrene) (PHS) can be explained in terms of differences in the pKa of the two polymers. The proximity of the hydroxyl groups in ortho-ortho bonded novolac causes a large change in the pKa of the polymer as a function of the degree of ionization. The increased distance between phenolic sites in PHS attenuates this effect. Novolac and PHS samples have been titrated from basic to acidic conditions to determine the `apparent pKa' of each polymer as a function of degree of ionization, (alpha) . The turbidimetric endpoints show that novolac precipitates earlier in the titration than PHS. This differences provides an explanation for the `ortho-ortho effect' and the different inhibition efficiencies of these polymers.
KEYWORDS: Refractive index, Data modeling, Photoresist developing, Photoresist materials, Reflectivity, Software development, Silicon, Lithography, Absorbance, Picture Archiving and Communication System
Optical lithography with special thick film DNQ-novolac photoresists have been practiced for many years to fabricate microstructures that require feature heights ranging from several to hundreds of microns such as thin film magnetic heads. It is common in these thick film photoresist systems to observe interesting non-uniform profiles with narrow regions near the top surface of the film that transition into broader and more concave shapes near the bottom of the resist profile. A number of explanations have been proposed for these various observations including the formation of `dry skins' at the resist surface and the presence of solvent gradients in the film which serve to modify the local development rate of the photoresist. There have been few detailed experimental studies of the development behavior of thick films resists. This has been due to part to the difficulty in studying these films with conventional dissolution rate monitors (DRMs). In general, this lack of experimental data along with other factors has made simulation and modeling of thick film resist performance difficult. As applications such as thin film head manufacturing drive to smaller features with higher aspect ratios, the need for accurate thick film simulation capability continues to grow. A new multi-wavelength DRM tool has been constructed and used in conjunction with a resist bleaching tool and rigorous parameter extraction techniques to establish exposure and development parameters for two thick film resists, AZTM 4330-RS and AZTM 9200. Simulations based on these parameters show good agreement to resist profiles for these two resists.
In the past, resist parameters (exposure and development parameters) were typically only available for a single set of processing conditions. Therefore, it has been impossible to explore the effect of processing conditions on resist performance using simulation. In this work, a statistical experimental design and response surface analysis technique was used in conjunction with our improved parameter extraction techniques to investigate the effect of processing conditions on the exposure and development parameters for a commercial i- line resist. The effect of soft bake time and temperature on the exposure parameters and the effect of soft bake temperature, soft bake time, post-exposure bake temperature, and post-exposure bake time on development parameters is discussed. Using this information, it is possible for the first time to consider optimizing resist processing conditions using lithographic simulations.
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.
Resist function depends upon photoinduced changes in the dissolution rate of phenolic polymer films in aqueous base. The events that occur as the molecules in these films move from the glassy state into solution are not well understood. This paper presents an extensive study of the influence of developer composition and concentration on the dissolution rate of novolac films in aqueous base. The dissolution rate is shown to be a function of both the cation and anion composition and concentration in the developer at a constant pH. When salts are added to the base, the rate first increases, reaches a maximum, and then decreases with increasing salt concentration. In the concentration regime below the maximum, the rate increases linearly with cation concentration and is independent of anion composition. The dissolution of novolac is dependent on cation concentration and not ionic strength in this regime. In the concentration regime above the maximum, the composition of the anion has a marked influence on the rate.
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.
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