Bryan D. Vogt

Effects of humidity on unencapsulated poly(thiophene) thin-film transistors

The effects of humidity on unencapsulated polymeric thin-film transistors (TFTs) of poly[5,5’-bis(3-dodecyl-2-thienyl)-2,2’-bithiophene] (PQT-12) were investigated. The field effect mobility of PQT-12 TFTs decreases and the rate of trapping of charge carriers increases under increasing humidity. The amount of water absorbed by the PQT-12 films was measured using a quartz crystal microbalance. Thin films of PQT-12 absorb comparable amounts of water to the carrier concentration in TFTs under routine operating conditions (humidity of 30% relative humidity and gate voltage of −30V); the changes in electrical characteristics under humid atmospheres are attributed to the interaction of absorbed water with the carriers in the film.

Wrinkling of ultrathin polymer films

This paper presents a bilayer model to account for surface effects on the wrinkling of ultrathin polymer films. Assuming a surface layer of finite thickness, effects of surface properties on the critical strain, the equilibrium wavelength, and the wrinkle amplitude are discussed in comparison with conventional analysis. Experimental measurements of wrinkling in polymer films with thickness ranging from 200 nm to 5 nm are conducted. The bilayer model provides a consistent understanding of the experiments that deviate from conventional analysis for thickness less than 30 nm. A set of empirical surface properties is deduced from the experimental data.

Resolution limitations in chemically amplified photoresist systems

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.

X-ray and neutron reflectivity measurements of moisture transport through model multilayered barrier films for flexible displays

One encapsulation approach to extend the lifetime of flexible organic light-emitting diode (OLED) devices uses inorganic Al2O3-polymer multilayer barrier films. However, a recent theoretical examination of multilayer barriers indicated that the barriers should not be effective for OLED applications, despite empirical evidence of success. It was suggested that a long-lived transient process in the transport of water molecules through multilayer films is responsible for its practical success, but has not been directly observed experimentally. X-ray reflectivity (XR) and neutron reflectivity measurements are used to measure permeation rates and structural changes in model barrier films upon exposure to water vapor. A film consisting of a stack of an undercured organic and the typical inorganic phases was found to barely swell [(7±5)A] after an 11-d exposure to moisture [60 °C, 100% relative humidity (RH)]. Current measurements of ultralow moisture permeation assume that 10 d is sufficient for the equilibrium...

The deprotection reaction front profile in model 193 nm methacrylate-based chemically amplified photoresists

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.

Water immersion of model photoresists: interfacial influences on water concentration and surface morphology

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 A 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 A 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.

Direct measurement of the in-situ developed latent image: the residual swelling fraction

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.

Dissolution fundamentals of 193-nm methacrylate based photoresists

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.

Fundamentals of the reaction-diffusion process in model EUV photoresists

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.

Copolymer fraction effect on acid catalyzed deprotection reaction kinetics in model 193 nm photoresists

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.