Photoacid Generator Concentration Research Articles

Dissociative photoionization of phenyl triflate, a photoacid generator for photolithography, at 92 eV.

The dissociative photoionization of phenyl triflate (C6H5OSO2CF3), a neutral photoacid generator used in photolithography, was investigated in a gas phase experiment employing extreme ultraviolet (EUV) synchrotron radiation at 92eV and photoelectron-photoion coincidence detection. The interaction of EUV photons with the molecule leads almost exclusively to dissociation, which is dominated by a sequential fragmentation mechanism, in which SO2, CF3, CO, and C2H2 are lost. For lithographic purposes, the lack of the observation of a fragment that could serve as a precursor for the formation of triflic acid means that the effective photoacid generator concentration in a photoresist is reduced, impacting its patterning performance in EUV lithography. A better understanding of the dissociative photoionization of photoresist components and proxies thereof can provide a crucial handle that guides the design of photoresists for the upcoming technology nodes with ever decreasing feature sizes for more powerful computer chips.

Regression analysis of photodecomposable quencher concentration effects on chemical gradient in chemically amplified extreme ultraviolet resist processes

Chemically amplified resists have been used in the state-of-the-art extreme ultraviolet (EUV) lithography. A basic additive has been added to the resist formula as a quencher to control acid diffusion. In this study, the effects of photodecomposable quencher (PDQ) concentration on the chemical gradient (an indicator of line edge roughness) in chemically amplified EUV resist processes were investigated. The chemical gradient was calculated on the basis of the sensitization and reaction mechanisms for different half-pitches, sensitivities, photoacid generator (PAG) concentrations, PDQ concentrations, and effective reaction radii for deprotection. The obtained 130 000 data were analyzed by least squares, lasso, ridge, and elastic net regressions. Regarding the contribution to the chemical gradient, PDQ concentration was apparently symmetric to PAG concentration. Optimum PDQ concentrations were well predicted (accuracy of approximately 10%) within the examined variable ranges even with 495 training data using the fitted parameters obtained by lasso, ridge, and elastic net regressions.

Tunable Wrinkle and Crease Surface Morphologies from Photoinitiated Polymerization of Furfuryl Alcohol

Addition of a small fraction of hydrophobic photoacid generator (PAG) to furfuryl alcohol provides a facile route to generate wrinkle topology by acid-catalyzed polymerization that is induced by ultraviolet (UV) light. Here, we describe how the primary characteristic parameters, wavelength and amplitude, of these periodic wrinkles can be tuned through control of the thickness of this furfuryl alcohol-PAG solution prior to UV exposure and the environmental humidity. As the initial coating thickness is increased, the wavelength remains unchanged at fixed temperature and PAG concentration, but the amplitude of the wrinkles increases exponentially with increased coating thickness. A wrinkle to crease transition is observed in some cases as the thickness of the solution coating is increased; this behavior is dependent on the PAG selection. Conversely, variation in relative humidity does not significantly impact the amplitude of the wrinkles, but there is a step change in the wavelength of the wrinkles near approximately 45% relative humidity with a factor of 3 decrease in the wavelength at high humidity. Through this knowledge, we have been able to fabrication wrinkles with an aspect ratio greater than 0.7 in a single step by UV exposure. These simple processing parameters to independently control wavelength and amplitude provide a facile route to systematically examine the role of aspect ratio of wrinkles on physical properties.

Critical dimension uniformity and contact edge roughness in extreme ultraviolet lithography: effect of photoacid generator, sensitizer and quencher

One of the main challenges for developing extreme ultraviolet resists is to satisfy critical dimension uniformity (CDU) and sidewall roughness of contacts to the allowable limit. To this end, further understanding of the effects of resist ingredients on CDU and contact edge roughness (CER) is required. We investigate the effects of a photoacid generator (PAG), sensitizer and quencher concentrations on the CDU and CER. We find that the dependencies of CDU on sensitizer and quencher are dominated by photon shot noise (PSN) effects whereas a more complicated interplay between PSN and PAG distribution statistics should be considered in the dependence of CDU on PAG concentration. The estimated CER parameters [root mean square (RMS) value and correlation length ξ ] exhibit a merging trend when plotted against the final critical dimension (CD). In addition, RMS value increases with exposure dose and PAG loading contrary to shot noise expectations. Power spectrum analysis reveals the dominant contribution of low-frequency undulations to CER, which is attributed to the enhanced interaction along specific directions between the aerial image and/or acid kinetics of nearby contacts. This inter-contact effect is further intensified with CD for fixed pitch and may explain the observed CER behavior.

Organic–inorganic tandem route to polymer nanocomposites: kinetic products versus thermodynamic products

We report a facile single-step photochemical methodology to afford alkylsiloxane-polymer hybrid films which relies on the tandem photoacid-catalyzed polymerization of n-alkyltrimethoxysilane precursors and a diglycidyl ether organic monomer. Photoacids liberated by the UV decomposition of iodonium salt triggers simultaneously the sol–gel process and epoxy cationic polymerization. Such conditions are intended to favour the formation of kinetic products trapped by cross-linking reactions instead of the thermodynamically most stable structures, whose preference would be for macrophase segregation. Organosilane precursor exhibiting different structures and chain lengths (n-butyl, n-octyl, n-dodecyl, n-hexadecyl, isobutyl and isooctyl) were systematically investigated to afford a range of transparent alkylsiloxane-polyether hybrids. The competitive organic–inorganic reaction kinetics were investigated using in situ real-time Fourier transform infrared spectroscopy. A main emphasis has been on discussing the effect of the alkyl substituent structure on the photoinduced polymerization kinetics and the silicate networks characterized by 29Si solid-state NMR. To avoid phase separation, the rate of formation of the two phases was tailored to favour concomitancy upon modulating several experimental parameters: film thickness, alkyl structure, photoacid generator concentration. Finally, the viscoelastic and surface properties were also assessed by dynamic mechanical analysis and water contact angle measurements, respectively.

Insights into Photoinduced Sol–Gel Polymerization: An in Situ Infrared Spectroscopy Study

Photoacid-catalyzed sol-gel polymerization is now recognized as a powerful single-step synthetic approach to the synthesis of hybrid films, which can be distinguished from conventional sol-gel methods by higher reactivity and a solvent-free process. Despite its utility, the mechanism is not yet understood, in particular what chemical, physical, and photochemical parameters determine the precise sequence, kinetics, and advancement of this UV inorganic photopolymerization. Here, using mainly transmission real-time Fourier transformed infrared (RT-FTIR) spectroscopy, we characterize in situ the hydrolysis-condensation reactions of oligomeric silicon alkoxides and the formation of byproducts. Systematic review and assessment of numerous processing variables (relative humidity, film thickness, precursor structure, nature, and the concentration of photoacid generator) prove that the reaction kinetics are controlled by the two independent phenomena: the intrinsic chemical reaction rates and the water vapor permeation into the film.

Integrating process and novel sacrificial layer fabricating technique based on diluted SU-8 resist

Although there are many types of commercially available SU-8 photoresist, all of them contain the same concentration of photo acid generator (PAG). This directly produces the restriction that the upper layer of the resist structure has to be smaller than or of the same size as the lower one when we fabricate multilayer SU-8 resist structure by an integrating process. In this paper we introduce a method to fabricate SU-8 resist with varying concentration of PAG – diluted SU-8 resist. The PAG concentration of this diluted resist can easily be changed. On the basis of the research of optical performance and proper utilization of the diluted SU-8 resist series, the existing restriction is successfully removed and the application scope of integrating process is expanded. A new sacrificial layer fabricating technique based on the diluted SU-8 resist is also introduced in this paper.

Photoinitator surface segregation induced instabilities from polymerization of a liquid coating on a rigid substrate

Periodic wrinkled surfaces have generated significant interest over the past decade as these structures can be easily fabricated over large areas with minimal fabrication cost, but these structures have generally been limited to thin films on soft elastomeric substrates, which limits their general applicability and utility. Here we present a simple methodology to generate wrinkled surfaces on rigid substrates by surface segregation of a photocatalyst. Upon ultraviolet light (UV) induced photopolymerization, increased catalyst concentration yields a cross-linked layer at the free surface that is supported on top of a more liquid-like bulk film due to differences in polymerization rate. Further polymerization of the underlayer provides the requisite mechanical stress (contraction due to polymerization) to create a wrinkled pattern. A system based upon the renewable monomer, furfuryl alcohol, that is cross-linked with a photoacid generator, triphenyl sulfonium triflate, is utilized to illustrate this concept. Moreover, the polymerized furfuryl alcohol can be transformed into amorphous carbon by heating at elevated temperatures in an inert environment. The role of photoacid generator concentration and substrate temperature on the wrinkle formation and morphology is presented. Finally, exposure through a simple mask can generate hierarchical structures with the wrinkled structure conforming to the geometric constraints of the photopatterned area, including curvilinear structures. This photocatalyzed surface segregation-based methodology provides a promising route to the facile fabrication of microstructured surfaces based upon the wrinkling instability.

Solid state NMR investigation of photoresist molecular glasses including blend behavior with a photoacid generator

We have examined four molecular glass (MG) materials that show promise as photoresists for extreme-ultraviolet (EUV) lithography. These glass-forming materials were investigated by proton and 13C solid state nuclear magnetic resonance (NMR) techniques in the bulk state as pure materials and as mixtures with (5 or 10) % by mass of the photoacid generator (PAG), triphenylsulfonium perfluorobutanesulfonate. The 13C techniques gave information about crystallinity, purity, and the qualitative existence of multiple phases. Proton studies focused on using spin diffusion to characterize the intimacy of mixing of the PAG and MG blends. The four MGs were largely aromatic materials containing several hydroxyl groups that were partially protected by t-butoxycarbonyl (t-BOC) groups. In two cases, this fraction was varied and the impact on mixing noted. Phase separation of the PAG into PAG-rich larger domains was never seen; the PAG was always finely distributed and the maximum size for any PAG clustering was estimated; however, in some cases, the average local concentration of PAG appeared to vary. Crystallinity was only seen associated with the underivatized materials implying that the mixing of the PAG with any derivatized MG was not restricted by crystallization. It was also noted that some very strong hydrogen bonds exist in three of the four underivatized materials and were eliminated or weakened upon partial derivatization with t-BOC.

Resist deconstruction as a probe for innate material roughness

We developed an atomic force microscopy (AFM)-based technique to measure intrinsic material roughness after base development. This method involves performing an interrupted development of the resist film and measuring the resulting film roughness after a certain fixed film loss. Employing this technique, we have deconstructed the resist into component materials and established that the photoacid generator (PAG) is a major material contributor of film roughness and that PAG segregation in the resist is likely responsible for nanoscale dissolution inhomogeneities. Small differences in PAG concentration as a result of standing waves in the resist can lead to large changes in surface roughness due to PAG or PAG-photoproduct segregation and the resultant nonlinear change in nanoscale dissolution rates. The temperature dependence of the PAG segregation suggests that increased mobility of the PAG that occurs may be due to a lowering of the film Tg during the deprotection process.

In situ fabrication of SU-8 movable parts by using PAG-diluted SU-8 as the sacrificial layer

In this paper we proposed a novel technology to make SU-8 movable parts in situ by using PAG-diluted SU-8 or SU-8R, a SU-8 solution with no photoacid generator (PAG) content, as the sacrificial layer. Since they are not or less sensitive to UV light the unexposed bottom layer in UV lithography will be dissolved to release the movable components during the final develop in PGMEA (Propylene Glycol Monomethyl Ether Acetate). The use of PAG-diluted SU-8 or SU-8R as a sacrificial layer offers several advantages including significant reduction in processing steps. It becomes the simplest technique currently available for in situ fabrication of SU-8 movable parts. In this paper the lithographic sensitivities of SU-8 as a function of the PAG concentration, detailed fabrication process and examples of fabricated SU-8 movable parts will be presented.

Material and process effects on line-edge-roughness of photoresists probed with a fast stochastic lithography simulator

Stochastic simulation of photoresist line-edge roughness (LER) is attempted using a three dimensional (3D) lithography simulator incorporating a fast dissolution algorithm based on a modified critical ionization model. The fast 3D simulation permits detailed evaluation of the material and process effects on LER. In this article the effects of deprotection fraction, critical ionization fraction, photoacid generator concentration, acid diffusion range, and polymerization length on LER are investigated through simulation. It is found that the relation of LER to photoresist polymerization length is greatly affected by the photoacid concentration and diffusion range.

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

Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) is utilized to provide insight into surface chemical effects in model photoresist films. First, NEXAFS was used to examine the resist/air interface including surface segregation of a photoacid generator (PAG) and the extent of surface deprotection in the film. The concentration of PAG at the resist-air interface was higher than the bulk concentration, which led to a faster deprotection rate at that interface. Second, a NEXAFS depth profiling technique was utilized to probe for compositional gradients in model resist line edge regions. In the model line edge region, the surface composition profile for the developed line edge was dependent on the post exposure bake time.

CHARACTERIZATION AND APPLICATION OF PAG DILUTED SU-8

The concentration of photoacidgenerator (PAG) in SU-8 determines the UV absorption and, therefore, is a limiting factor of the applicable SU-8 thickness. The motivation of this work is to improve SU-8 process control and expand the application scope of SU-8 by modifying its PAG concentration. In this paper we present the experimental results on lithographic properties of SU-8 as the function of PAG concentration (varying up to 2 orders of magnitude). It includes determining the minimum bottom dose and minimum effective energy density for x-ray and UV lithography of SU-8, respectively, and the UV absorption spectra of SU-8. It is fund that as the PAG concentration reduced, SU-8 has lower sensitivities in UV and x-ray lithography, and lower UV absorption coefficients. By using PAG diluted SU-8 taller microstructures with vertical sidewall can be obtained with UV lithography. Based on the large difference in lithographic sensitivities between normal and PAG diluted SU-8, we have developed a new process concept for making 3D microstructures. The 3D microstructures fabricated based on this new process, including microcantilever and microchannel, will be presented.

Monte Carlo simulation of gel formation and surface and line-edge roughness in negative tone chemically amplified resists

A molecular-based representation of a negative tone chemically amplified resist was developed and studied using stochastic simulations. The gel formation mechanism resulting from the reaction–diffusion phenomena in the polymer matrix during the postexposure bake step, as well as the surface and line-edge roughness formation were investigated in two dimensions. Graph theory computational techniques were employed to determine the gel cluster size and its relation to the fraction of the total number of chemically and physically cross-linked chains. Results of the simulation include the maximum acid diffusion range, the cross-link density, the surface and line edge roughness, versus exposure dose and photoacid generator concentration.

Effect of photoacid generator concentration on sensitivity, photoacid generation, and deprotection of chemically amplified resists

The dependence of resist processes (sensitivity, efficiency of photoacid generation, polymer deprotection) on the concentration of the photoacid generator (PAG) in formulations of chemically amplified photoresist was investigated as a function of the concentration of photoacid that was generated upon exposure. Although the sensitivity of a photoresist formulation improved by increasing the concentration of PAG in the resist formulation, the concentration of photoacid required to achieve an appropriate extent of deprotection to render development also increased. Using a standard addition technique, the efficiency of photoacid generation was quantified for resist systems consisting of triphenylsulfonium triflate as the PAG in an environmentally stable chemically amplified photoresist-type polymer primarily composed of 4-hydroxystyrene and tert-butyl acrylate monomers. Determination of the apparent efficiency of photoacid generation (the resist C parameter) allowed the concentration of photoacid within the resist film to be calculated for any exposure dose. Deprotection of the polymer was measured using infrared spectroscopy and analyzed as a function of the photoacid concentration within the resist. We determined from these results that the increase in the concentration of photoacid required for development of the resist was caused by a change in the deprotection process during the postexposure bake and was not due to dissolution inhibition by the unexposed PAG.

Characterization and simulation of surface and line-edge roughness in photoresists

The problem of surface and line-edge roughness characterization and prediction is discussed. Different roughness parameters, such as the root mean square deviation (rms or σ), the fractal dimension, and the Fourier spectrum, are presented and compared. These roughness parameters for three negative tone resists (wet and plasma developed) are analyzed versus exposure dose, photoacid generator concentration, and plasma development conditions. Finally, a molecular type simulator is used to predict the experimental roughness behavior.

Optimization of DUV Negative Resists for 0.15 .MU.m Lithography.

Recent performance improvements of a negative tone chemically amplified DUV resist consisting of a hydroxystyrene based copolymer, a melamine crosslinker, a photoacid generator (PAG) and certain amine additives are described. The general trends of formulation and process changes on the lithographic performance were investigated using a response surface method. PAG concentration variations and optimization of the prebake conditions allowed for the elimination of microbridging in the subquarter micron region. The pattern profiles were improved by the optimization of the polymer properties and the selection of a specific amine/ammonium hydroxide combination. The effects of certain polymer properties and of individual amine components are discussed in more detail and a correlation with their structures is given based on dissolution rate measurements. The optimized resist material has a resolution potential below 0.15μm (NA=0.55) combined with a large depth of focus (>1.0μm @ 0.15μm iso lines), acceptable iso-dense bias, and small sensitivity towards changes in the post exposure bake conditions.

Effect of photo acid generator concentration on the process latitude of a chemically amplified resist

A positive tone chemically amplified photoresist was evaluated for use on a 0.44 NA 248 nm excimer laser stepper. The effects of various formulation changes were examined with respect to exposure latitude, depth of focus, resolution, and bias between isolated and grouped features. Of particular interest was the relationship between the percent of photo acid generator (PAG) in the resist and the process latitude. It was found that several aspects of the process window increased as the PAG content of the resist decreased. An increase in dose was expected and observed with the decrease in PAG concentration. This would reduce excimer stepper throughput by approximately 25%.