A general approach to characterize compositional heterogeneity in polymer thin films using Fourier transform
infrared (FTIR) spectroscopy has been demonstrated Polymer films with varying degrees of heterogeneity were
prepared using a model chemically amplified photoresist where a photoacid catalyzed reaction-diffusion process results
in the formation of methacrylic acid (MAA)-rich domains. Within these domains, the carboxylic acid groups dimerize
through hydrogen bonding. FTIR measurements of the relative fraction of hydrogen-bonded versus free carboxylic
groups are used to quantify the degree of compositional heterogeneity. It was shown that the degree of the spatial
heterogeneity varies with changes in the deprotection level and initial copolymer composition. The degree of
heterogeneity is small at very low and very high deprotection level and maximize when the deprotection level is around
0.25. Increased non-reactive comonomer content decreases the degree of heterogeneity by reducing the hydrogen
bonding efficiency.
The spatial distribution of polymer photoresist and deuterium labeled developer highlights a fraction of material at a
model line edge that swells, but does not dissolve. This residual swelling fraction remains swollen during both the in
situ development and rinse steps uncovering that the final lithographic feature is resolved by a collapse mechanism
during the drying step. We demonstrate that contrast variant neutron reflectivity provides a general method to probe the
nanometer resolved in situ development and rinse process step.
The controlling factors in the formation of the compositional heterogeneity at the deprotection front were
investigated using 3D computer simulation. The results illustrate that the chemical composition fluctuation (CCF)
formed by the photoresist deprotection reaction is an important factor contributing to the line-edge-roughness (LER) in
addition to the deprotection gradient (DG) of the reaction front. The magnitude of the chemical composition fluctuation
and the deprotection gradient are found to depend on the ratio of the deprotection reaction rate constant to diffusion
coefficient (kP/D) and the number of hoping step (n) With this new finding, the influence on LER from various
process/material parameters such as dose/contrast, diffusivity, and reactivity can all be understood through their effects
on kP/D and n.
An understanding of acid diffusion-reaction in chemically amplified photoresists during the post-exposure bake (PEB) is critical for both critical dimension (CD) and line edge roughness (LER) control. Despite its importance, there remains insufficient understanding of the diffusion-reaction process. This is due in part to the complex interplay between diffusion and reaction where the deprotection of the resin modifies the local acid diffusivity which in turn changes the rate of deprotection. Here, we report the direct measurement of the reaction diffusion front at a model line edge from neutron reflectivity and Fourier transform infrared spectroscopy measurements. The photoacid generator size influences the reaction extent and breath of the deprotection profile. A larger photoacid results in a sharper deprotection profile and a shorter reaction length. Under the same post-exposure bake time and temperature, the smaller photoacid leads to a much broader deprotection profile. These measurements illustrate the complexity of the reaction-diffusion process.
The dissolution of partially deprotected chemically amplified photoresists is the final step in printing lithographic features. Since this process step can be tuned independently from the design of the photoresist chemistry, measurements of the dissolution behavior may provide needed insights towards improving line-edge roughness. We have studied the dissolution behavior of a model 193-nm photoresist, poly (methyladamantyl methacrylate), as a function of deprotection extent and developer strength. The kinetics of the dissolution process is followed using the quartz crystal microbalance technique. These photoresist films exhibit strong swelling without dissolution over a significant range of deprotection levels. At larger extents of deprotection, we observe a combination of swelling with dissolution. Additionally, we find that the degree of film swelling decreases with tetramethylammonium hydroxide developer concentration. These studies provide the insight needed to better understand the fundamentals of the dissolution of the photoresist - a key step in lithographic process.
More demanding requirements are being made of photoresist materials for fabrication of nanostructures as the feature critical dimensions (CD) decrease. For extreme ultraviolet (EUV) resists, control of line width roughness (LWR) and high resist sensitivity are key requirements for their success. The observed LWR and CD values result from many factors in interdependent processing steps. One of these factors is the deprotection interface formed during the post-exposure bake (PEB) step. We use model EUV photoresist polymers to systematically address the influence of exposure-dose on the spatial evolution of the deprotection reaction at a model line edge for fixed PEB time using neutron reflectivity. The bilayer consists of an acid feeder layer containing photoacid generator (PAG) and a model photoresist polymer, poly(hydroxystyrene-co-tert-butylacrylate) with perdeuterated t-butyl protecting group. The deuterium labeling allows the protection profile to be measured with nanometer resolution. The evolution of two length scales that contribute to the compositional profile is discussed.
A correlation between polymer molecular structure and acid catalyzed reaction kinetics is demonstrated by a photoresist copolymer with an acid-labile and a non-reactive monomer. The acid catalyzed deprotection kinetics depend significantly on the composition of the non-reactive comonomer in the polymer chain. The apparent reaction rate constant decreases monotonically with increasing non-reactive comonomer composition. The phenomena are interpreted as the reduction of diffusivity of photoacid in the polymer matrix from a hydrogen-bonding interaction with the polar group in the inert comonomer. In addition, hydrogen-bonding interactions between the photoacid and the reaction product, primarily methacrylic acid, can account for the acid loss or trapping effect observed by various researchers.
The emergence of immersion lithography for the extension of current lithography tools requires a fundamental understanding of the interactions between the photoresist and the immersion liquid such as water. Neutron reflectometry was used to measure the water concentration depth profile within immersed photoresist films. The bulk of the films swelled to the equilibrium water concentration. However a gradient in water concentration was observed near the polymer/substrate interface. Dependent on the relative hydrophilicity of the polymer and the substrate, either a depletion or excess of water was observed at the interface. Using HMDS treated silicon wafers as the substrate results in approximately 17% water by volume at the interface. The interfacial concentration decreases (or increases) to the bulk water solubility limit approximately 40 Å from the substrate. As the total film thickness approaches this length scale, the substrate induced concentration gradients lead to a film thickness dependent swelling; enhanced or suppressed swelling is witnessed for the excess or depleted interfacial concentrations, respectively. Variation of the substrate surface energy allows for tuning of the interfacial water concentration, ranging from 30% to less than 1% water by volume.
A depth profile of the base developer counterion concentration within thin photoresist films was measured in-situ using contrast variant specular neutron reflectivity to characterize the initial swelling stage of the film dissolution. We find a substantial counterion depletion near the substrate and an enrichment near the periphery of the film extending into the solution. These observations challenge our understanding of the charge distribution in photoresist and polyelectrolyte films and are important for understanding film dissolution in medical and technological applications.
The emergence of immersion lithography as a potential alternative for the extension of current lithography tools requires a fundamental understanding of the interactions between the photoresist and an immersion liquid such as water. The water concentration depth profile within the immersed photoresist films is measured with neutron reflectometry. The polymer/substrate interface affects both the water concentration near the interface and the surface morphology of the film. Immersed films are not stable (adhesive failure) over the course of hours when supported on a silicon wafer with a native oxide surface, but are stable when the substrate is first treated with hexamethyldisilazane (HMDS). The bulk of the polymer films swells to the equilibrium water concentration, however, a gradient in water concentration is observed near the polymer/HMDS substrate interface with a concentration of approximately 17% by volume fraction and extending up to 50 Å into the film. Thus, polymers that absorb more than this amount exhibit depletion near the interface, whereas polymers that absorb less exhibit a water excess layer. These concentration gradients extend approximately 50 Å away from the interface into the film. As the total film thickness approaches this length scale, the substrate-induced concentration gradients lead to a film-thickness-dependent swelling; enhanced or suppressed swelling is witnessed for the excess or depleted interfacial concentrations, respectively. The substrate also influences the surface morphology of immersed thin films. The film surface is smooth for the HMDS-treated substrate, but pin-hole defects with an average radius of 19±9 nm are formed in the films supported on the native oxide substrates.
Organic polar solvent (1-butanol) versus aqueous base (tetramethylammonium hydroxide, (TMAH)) development quality are distinguished by neutral versus charged polymer (polyelectrolyte) dissolution behavior of photoresist bilayers on silicon substrates comprising poly(4-hydroxystyrene) and poly(4-tert-butoxycarbonyloxystyrene), PHOSt and PBOCSt, respectively. This model line-edge was broadened by photoacid catalyzed deprotection to a final interfacial width of 35.7 Å and subjected to different developers. 1-butanol develops with an increased penetration depth than aqueous base development consistent with an increased solubility of the protected containing component in the organic solvent. This enhanced dissolution with the polar solvent results in an increased surface roughness of 73 Å, whereas the development with TMAH at concentrations between (0.1 to 1.1) M1 leads to surface roughness between (4.5 to 14.4) Å, as measured by atomic force microscopy. These measurements suggest that the elimination of resist swelling, in the presence of a protection gradient, is a viable strategy to reduce roughness and control critical dimensions. The influence of added salt to developer solutions was also examined by developing the model bilayer. A decrease in surface roughness from (10 to 6.5) Å was observed between (0 to 0.70) M KCl in 0.26 M TMAH.
Neutron and x-ray reflectivity measurements quantify the non-uniform distribution of water within poly(4-tert-butoxycarbonyloxystyrene) (PBOCSt) and poly(4-hydroxystyrene) (PHOSt) thin films on silicon wafer substrates. Two contrasting surface treatments were examined, silicon oxide, representing a hydrophilic interface and a trimethylsilane primed surface, representing a hydrophobic interface. The distribution of water in the films was sensitive to the surface preparation and photoresist relative hydrophilicity. Depending upon the water contact angle of the substrate in comparison to the polymer film, an excess of water near the interface occurs when the substrate is more hydrophilic than the photoresist. Likewise, interfacial depletion results when the photoresist is more hydrophilic than the substrate. These non-uniform water distributions occurs within (50 ± 10)Å of the photoresist/substrate interface. The water concentration in this interfacial region appears to be independent of the photoresist properties, but is strongly dependent upon the substrate surface energy.
A variety of experimental evidence suggests that positive-tone chemically amplified photoresists have an intrinsic bias that might limit resolution during high-volume lithographic processing. If this is true, the implications for the semiconductor industry require careful consideration. The design concept of chemical amplification is based on generation of a chemically stable catalytic species in exposed regions of the photoresist film. The catalytic action of the photoproducts on the photoresist polymer causes a change in the dissolution rate in the irradiated regions of the film. Formation of a stable catalyst species is required for chemical amplification, but it has long been recognized that catalyst migration can produce a difference between the initial distribution of exposure energy and the final distribution of photoproducts. This difference, known as diffusion bias, depends on the photoresist chemistry and processing conditions. Diffusion bias is insensitive to exposure conditions, but it is possible to reduce catalyst migration through changes to resist formulation such as increasing the size of the catalyst molecule or processing conditions such as reducing the post exposure bake temperature. Another common approach to limiting diffusion bias is to incorporate base additives into the photoresist formulation to scavenge diffusing acid catalyst. All of these approaches to reducing catalyst migration generally reduce the catalytic efficiency of each photoproduct and therefore increase the total exposure dose required to pattern the film. Increases in required exposure dosage reduce the throughput of the exposure tools and can reduce the profitability of the manufacturing process. In this paper we present experimental results that are suggestive of an intrinsic photoresist bias. This diffusion bias sets a minimum resolution limit for chemically amplified resist systems that can be improved at the cost of reduced throughput and productivity.
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