Byungki Jung

Transient laser heating induced hierarchical porous structures from block copolymer–directed self-assembly

Laser patterning polymer membranes Porous materials are useful for membranes, filters, energy conversion, and catalysis. Their utility often depends on the ability to finely control both the pore sizes and their connectivity. Tan et al. prepared porous thin films of block copolymers mixed with phenol-formaldehyde resins (resols) on silicon substrates using a simple laser process. On exposure to ultraviolet light, rapid heating of the substrate causes polymerization of the resols and decomposition of the block copolymer. This method allows direct patterning of the films on a local scale, with tunable pore sizes and size distributions. Science, this issue p. 54 Laser heating drives block copolymers to self-assemble into patterned hierarchical porous structures. Development of rapid processes combining hierarchical self-assembly with mesoscopic shape control has remained a challenge. This is particularly true for high-surface-area porous materials essential for applications including separation and detection, catalysis, and energy conversion and storage. We introduce a simple and rapid laser writing method compatible with semiconductor processing technology to control three-dimensionally continuous hierarchically porous polymer network structures and shapes. Combining self-assembly of mixtures of block copolymers and resols with spatially localized transient laser heating enables pore size and pore size distribution control in all-organic and highly conducting inorganic carbon films with variable thickness. The method provides all-laser-controlled pathways to complex high-surface-area structures, including fabrication of microfluidic devices with high-surface-area channels and complex porous crystalline semiconductor nanostructures.

Laser-induced sub-millisecond heating reveals distinct tertiary ester cleavage reaction pathways in a photolithographic resist polymer.

Acid-catalyzed, thermally activated ester cleavage reactions are critical for lithographic patterning processes used in the semiconductor industry. The rates of these high-temperature reactions within polymer thin films are difficult to characterize because of the thermal instability of many polymers and a lack of temperature-resolved measurement techniques. Here we introduce the use of transient laser irradiation to heat a methyladamantane-protected acrylate copolymer to 600 °C in less than a millisecond. These conditions mediate the removal of the protecting groups and enable accurate kinetic measurements. At sub-millisecond exposure to high temperatures (∼600 °C), the rate of the ester cleavage reaction exhibits the expected first-order dependence on acid concentration. In contrast, the reaction exhibits more complex kinetics when the polymer film is heated to lower temperatures (115 °C) on a conventional hot-plate. We identify distinct methyladamantane-derived deprotection products under the high- and low-temperature conditions that are consistent with the observed rate differences. The acid-catalyzed dimerization of 1-methyleneadamantane occurs at low temperature, which reduces the acid concentration available for the ester cleavage. This dimerization reaction is minimized during transient laser-induced heating because bimolecular reactions are disfavored under these conditions. We constructed a mathematical model based on these observations that accounts for the competition for the catalyst between the dimerization and ester cleavage processes. This laser-induced, sub-millisecond heating technique provides a means to probe and model temperature and time regimes of thermally activated reactions in polymer films, and these regimes exhibit distinct and advantageous reaction pathways that will inform future advances in high-performance photolithography.

Experimental determination of thermal profiles during laser spike annealing with quantitative comparison to 3-dimensional simulations

Thin film platinum resistors were used to directly measure temperature profiles during laser spike annealing (LSA) with high spatial and temporal resolution. Observed resistance changes were calibrated to absolute temperatures using the melting points of the substrate silicon and thin gold films. Both the time-dependent temperature experienced by the sample during passage of the focussed laser beam and profiles across the spatially dependent laser intensity were obtained with sub-millisecond time resolution and 50 µm spatial resolution. Full 3-dimensional simulations incorporating both optical and thermal variations of material parameters were compared with these results. Accounting properly for the specific material parameters, good agreement between experiments and simulations was achieved. Future temperature measurements in complex environments will permit critical evaluation of LSA simulations methodologies.

Kinetic rates of thermal transformations and diffusion in polymer systems measured during sub-millisecond laser-induced heating.

Probing chemical reaction kinetics in the near-solid state (small molecules and polymers) is extremely challenging because of the restricted mobility of reactant species, the absence of suitable analytical probes, and most critically the limited temperature stability of the materials. By limiting temperature exposure to extremely short time frames (sub-millisecond), temperatures in excess of 800 °C can be accessed extending kinetic rate measurements many orders of magnitude. Here we demonstrate measurements on a model system, exploiting the advantages of thin-films, laser heating, and chemically amplified resists as an exquisite probe of chemical kinetic rates. Chemical reaction and acid diffusion rates were measured over 10 orders of magnitude, exposing unexpected and large changes in dynamics linked to critical mechanism shifts across temperature regimes. This new approach to the study of kinetics in near-solid state materials promises to substantially improve our understanding of processes active in a broad range of temperature-sensitive, low-mobility materials.

LWR reduction and flow of chemically amplified resist patterns during sub-millisecond heating

Chemically amplified resists are critical for sub-30 nm photolithography. As feature sizes decrease, challenges continue to arise in controlling the aerial image during exposure, acid diffusion during post exposure bakes, and swelling during development. Ultimately these processes limit the line width roughness (LWR). While there exists substantial research to modify resists and exposure protocols, post-development treatment of resist patterns to improve the LWR has received only modest attention. In this work, we use a scanned laser spike annealing system to anneal fully developed resist patterns at temperatures of 300-420oC for sub-millisecond time frames. When heated above its glass transition temperature for a controlled time, patterned resist flows to minimize the surface energy resulting in reduced roughness. While LWR and critical dimension (CD) of the resist is very sensitive to the hardbake temperature, SEM and AFM analysis show a >30% reduction in LWR with <1 nm change in CD at 26W (385oC) hardbake power compared to that of features without hardbake. Quantitative determination of surface roughness, resist trench profiles, LWR, and CD is presented and discussed.

Control of PS-b-PMMA directed self-assembly registration by laser induced millisecond thermal annealing

Directed self-assembly of PS-b-PMMA during laser spike annealing at peak temperatures of 300-800°C for dwells of 1- 10 ms has been explored. The enhanced mobility of polymer chains at these temperatures improves registration compared to conventional thermal anneals. PS-b-PMMA films (forming 15 nm line/space standing lamellae) were cast on chemically patterned substrates with a copolymer neutral layer and annealed by laser and hot-plate (2 minutes 250°C). Annealing by hot plate or multiple laser scans resulted in well-aligned features over micron length scales. By laser annealing multiple times, defectivity can be reduced by ~60%. However, laser annealing for only 10 ms before performing a hot plate anneal reduced defectivity by ~80%. Additionally, defects are more often localized as dislocation pairs rather than regions perpendicular to the underlying directing pattern resulting in far greater total alignment.

Sub-millisecond post exposure bake of chemically amplified resists by CO2 laser heat treatment

Chemically amplified photoresists require a post exposure bake (PEB), typically on a hot plate at 90-150°C for 30-120 seconds, to catalytically deprotect the polymer backbone. During PEB, excessive diffusion of the photo-generated acid results in loss of line edge definition, blurring of latent images and changes in the line edge roughness. Both acid diffusion and deprotection are thermally activated processes, with the relative rates affected by the time/temperature profile of the PEB. In this work, we introduce an alternate PEB method involving 500 μs time scale heating over a temperature range of 130°C to 450°C using a continuous wave CO2 laser. A methodology is developed for characterizing this laser PEB and comparing the behavior with conventional hot plate PEB. The thermal stability of several polymer and photoacid generator (PAG) resist systems were studied and shown to be stable at these high temperatures due to the short heating duration. Sensitivity of resists under hot plate and laser PEB were measured. Under moderate temperatures, the laser PEB sensitivity can exceed that of hot plate PEB by an order of magnitude. Quantitative determination of the acid diffusion was obtained using resist bilayers (PAG loaded / PAG free). Despite the five orders of magnitude difference in PEB time, systems with l-PEB and hot-plate PEB exhibit comparable imaging quality under deep ultraviolet exposure.

Control of polystyrene- block -poly(methyl methacrylate) directed self-assembly by laser-induced millisecond thermal annealing

Abstract. Directed self-assembly of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) during laser thermal annealing at peak temperatures of 300°C–800°C for dwells of 1–10 ms has been explored. The enhanced mobility of polymer chains at these temperatures improves registration compared with conventional thermal anneals. PS-b-PMMA films (forming 15-nm line/space standing lamellae) were cast on chemically patterned substrates with a copolymer neutral layer and annealed by laser and hot plate. Annealing by hot plate or multiple laser scans resulted in well-aligned features over micron length scales. By laser annealing multiple times, defectivity was reduced by ∼60%. However, laser annealing for only 10 ms before performing a hot plate anneal reduced defectivity by >80%. We believe that this reduction arises from improved interfacial alignment of the film to the template during laser annealing near the order–disorder transition.

Addressing challenges in lithography using sub-millisecond post exposure bake of chemically amplified resists

Chemically amplified photoresists require a post exposure bake (PEB), typically on a hot plate at 90-150°C for 30-120 seconds, to catalytically deprotect the polymer backbone. During PEB, excessive diffusion of the photo-generated acid results in loss of line edge definition, blurring of latent images and changes in the line edge roughness. Both acid diffusion and deprotection are thermally activated processes, with the relative rates affected by the time/temperature profile of the PEB. In this work, we introduce an alternate PEB method involving millisecond time frame heating over a temperature range of 300°C to 420°C using a continuous wave CO2 laser. A methodology is developed for characterizing the resist deprotection and acid diffusion kinetics under laser PEB (l-PEB) and comparing the behavior with conventional hot plate PEB. Results show that the deprotection rate sensitivity with temperature is smaller for l-PEB compared to that of hot plate PEB, suggesting a possible change in the deprotection mechanism. Acid diffusivity under l-PEB is reduced by a factor of 102-103 compared to values extrapolated from the hot plate PEB data. Under EUV exposure, patterns formed using l-PEB show significantly smoother surface roughness while requiring less than half the dose required for hot plate PEB - a direct consequence of enhanced deprotection at high PEB temperature and reduced acid diffusion in the millisecond time frame.

Submillisecond post-exposure bake of chemically amplified resists by CO2 laser spike annealing

Pattern formation in a chemically amplified photoresist requires a post-exposure bake (PEB) to catalytically deprotect the polymer. Excessive diffusion of the photogenerated acid results in the loss of line edge definition, blurring of latent images, and changes in the line edge roughness. To optimize the process, the authors have explored submillisecond PEB using a CO2 laser-based scanned annealing system [M. Chandhok (private communication)]. Several polymer and photoacid generator resist systems were studied under 800μs laser spike annealing at estimated temperatures between 200 and 400°C. All the resist systems exhibit remarkable stability in this time/temperature regime, with the maximum useful temperature limited by thermal deprotection and/or decomposition of the polymer backbone. At lower temperatures, high resolution patterns with sub-100-nm features are formed, comparable to hotplate reference samples. Resist sensitivity is improved significantly for several resist systems (dose to clear is lowe...