Vivek M. Prabhu

Structure and Properties of Small Molecule−Polymer Blend Semiconductors for Organic Thin Film Transistors

A comprehensive structural and electrical characterization of solution-processed blend films of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) semiconductor and poly(alpha-methylstyrene) (PalphaMS) insulator was performed to understand and optimize the blend semiconductor films, which are very attractive as the active layer in solution-processed organic thin-film transistors (OTFTs). Our study, based on careful measurements of specular neutron reflectivity and grazing-incidence X-ray diffraction, showed that the blends with a low molecular-mass PalphaMS exhibited a strong segregation of TIPS-pentacene only at the air interface, but surprisingly the blends with a high molecular-mass PalphaMS showed a strong segregation of TIPS-pentacene at both air and bottom substrate interfaces with high crystallinity and desired orientation. This finding led to the preparation of a TIPS-pentacene/PalphaMS blend active layer with superior performance characteristics (field-effect mobility, on/off ratio, and threshold voltage) over those of neat TIPS-pentacene, as well as the solution-processability of technologically attractive bottom-gate/bottom-contact OTFT devices.

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.

Dynamic light scattering investigations of nanoparticle aggregation following a light-induced pH jump

There are many important processes where the stability of nanoparticles can change due to changes in solution environment. These processes are often difficult to study under controlled changes to the solution conditions. Dynamic light scattering was used to measure the initial kinetics of aggregation of carboxylated polystyrene nanoparticles after well-defined pH jumps using aqueous solutions of photoacid generator (PAG). With this approach, the pH of the solution was controlled by exposure to ultraviolet (UV) light without the delays from mixing or stirring. The aggregation kinetics of the nanoparticles was extremely sensitive to the solution pH. The UV exposure dose is inversely correlated with the resulting surface charge of the nanoparticles. Decreasing pH decreases the electrostatic repulsion force between particles and leads to aggregation. The reaction-limited or diffusion-limited aggregation kinetics was sensitive to the pH quench depth, relative to the acid-equilibrium constant (pK(a)) of the surface carboxylic acid groups on the nanoparticles. Since numerous PAGs are commercially available, this approach provides a flexible method to study the aggregation of a variety of solvent-dispersed nanoparticle systems.

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...

Influence of base additives on the reaction-diffusion front of model chemically amplified photoresists

The effects of amine base quencher on the photoacid catalyzed deprotection reaction-diffusion front in model photoresists were measured by combination of neutron reflectivity and Fourier transform infrared spectroscopy. Modulation in the location of the base with respect to the diffusing photoacid catalyst changes the spatial reaction extent and illuminates the complex role of the base on the shape of the reaction-diffusion front. Despite similar total extents of reaction, a comparison between uniform base and model photodegradable base distributions demonstrates distinct reaction time and base concentration effects on the deprotection profile shape. These differences arise from the modification of the initial deprotection extent due to both the neutralization of the photoacid and the influence of the changing photoresist composition on the reaction-diffusion process. The use of the model photodegradable base results in a sharper front due to these effects. Lastly, aqueous hydroxide development of these l...

Counterion associative behavior with flexible polyelectrolytes

At low ionic strength, organic counterions dress a flexible charged polymer as measured directly by small-angle neutron scattering and neutron spin-echo spectroscopy. This dressed state, quantified by the concentration dependence of the static correlation length, illustrates the polymer-counterion coupled nature on the nanometer length scale. The counterions, made visible by selective hydrogen and deuterium labeling, undress from the polymeric template by addition of sodium chloride. The addition of this electrolyte leads to two effects: increased Debye electrostatic screening and decoupled organic counterion-polymer correlations. Neutron spin-echo spectroscopy measures a slowing down of the effective diffusion coefficient of the labeled counterions at the length scale of 8 nm, the static correlation length, indicating the nanosecond counterion dynamics mimics the polymer. These experiments, performed with semidilute solutions of tetramethylammonium poly(styrene sulfonate) [(h-TMA(+)) d-PSS], apply to relevant biopolymers including single and double stranded DNA and unfolded proteins, which undergo orchestrated dynamics of counterions and chain segments to fold, unfold, and assemble.

Gel phase formation in dilute triblock copolyelectrolyte complexes

Assembly of oppositely charged triblock copolyelectrolytes into phase-separated gels at low polymer concentrations (<1% by mass) has been observed in scattering experiments and molecular dynamics simulations. Here we show that in contrast to uncharged, amphiphilic block copolymers that form discrete micelles at low concentrations and enter a phase of strongly interacting micelles in a gradual manner with increasing concentration, the formation of a dilute phase of individual micelles is prevented in polyelectrolyte complexation-driven assembly of triblock copolyelectrolytes. Gel phases form and phase separate almost instantaneously on solvation of the copolymers. Furthermore, molecular models of self-assembly demonstrate the presence of oligo-chain aggregates in early stages of copolyelectrolyte assembly, at experimentally unobservable polymer concentrations. Our discoveries contribute to the fundamental understanding of the structure and pathways of complexation-driven assemblies, and raise intriguing prospects for gel formation at extraordinarily low concentrations, with applications in tissue engineering, agriculture, water purification and theranostics.

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.

Photoresist latent and developer images as probed by neutron reflectivity methods.

Photoresist materials enable the fabrication of advanced integrated circuits with ever-decreasing feature sizes. As next-generation light sources are developed, using extreme ultraviolet light of wavelength 13.5 nm, these highly tuned formulations must meet strict image-fidelity criteria to maintain the expected performance gains from decreases in feature size. However, polymer photoresists appear to be reaching resolution limits and advancements in measurements of the in situ formed solid/solid and solid/liquid interface is necessary. This Review focuses on the chemical and physical structure of chemically amplified photoresists at the lithographic feature edge at length scales between 1 nm and 100 nm. Neutron reflectivity measurements provide insight into the nanometer-scale composition profiling of the chemical latent image at an ideal lithographic line-edge that separates optical resolution effects from materials processing effects. Four generations of advanced photoresist formulations were examined over the course of seven years to quantify photoresist/photoacid and photoresist/developer interactions on the fidelity of lithographic features. The outcome of these measurements complement traditional resist design criteria by providing the effects of the impacts of the photoresist and processing on the feature fidelity. These physical relations are also described in the context of novel resist architectures under consideration for next-generation photolithography with extreme-ultraviolet radiation.

Nanoparticle assembly: DNA provides control

Solution-based syntheses of nanoclusters typically produce a broad range of species. A step-by-step process using DNA-encoded nanoparticles assembled on a solid support aids in the design and production of specific self-assembled nanoclusters in high yields.