Post Exposure Bake Research Articles

Photoactivated Cyclic Polyphthalaldehyde Microcapsules for Payload Delivery.

Microcapsules with a cyclic polyphthalaldehyde (cPPA) shell and oil core were fabricated by an emulsification process. The low ceiling temperature cPPA shell was made phototriggerable by incorporating a photoacid generator (PAG). Photoactivation of the PAG created a strong acid which catalyzed cPPA depolymerization, resulting in the release of the core payload, as quantified by 1H NMR. The high molecular weight cPPA (197 kDa) yielded uniform spherical microcapsules. The core diameter was 24.8 times greater than the cPPA shell thickness (2.4 to 21.6 μm). Nonionic bis(cyclohexylsulfonyl)diazomethane (BCSD) and N-hydroxynaphthalimide triflate (HNT) PAGs were used as the PAG in the microcapsule shells. BCSD required dual stimuli of UV radiation and post-exposure baking at 60 °C to activate cPPA depolymerization while room temperature irradiation of HNT resulted in instantaneous core release. A 300 s UV exposure (365 nm, 10.8 J/cm2) of the cPPA/HNT microcapsules resulted in 66.5 ± 9.4% core release. Faster core release was achieved by replacing cPPA with a phthalaldehyde/propanal copolymer. A 30 s UV exposure (365 nm, 1.08 J/cm2) resulted in 82 ± 13% core release for the 75 mol % phthalaldehyde/25 mol % propanal copolymer microcapsules. The photoresponsive shell provides a versatile polymer microcapsule technology for on-demand, controlled release of hydrophobic core payloads.

7.4 nm linewidth Pt nanowires by electron-beam lithography using non-chemically amplified positive-tone resist and post-exposure bake

Abstract 7.4 nm linewidth Pt nanowires were demonstrated on SiO2/Si substrates via electron-beam lithography using a non-chemically amplified positive resist ZEP520A and post-exposure bake (PEB) treatment. The effect of the PEB treatment conditions on the nanowires’ characteristics was investigated. As the PEB temperature and time increased, a decrease in the mean linewidth and an improvement of the line-width (line-edge) roughness of the nanowires were observed. Pt nanowires with an ultrafine linewidth of 7.4 nm were successfully fabricated using the optimal condition of 100 °C for 2 min, verifying the effectiveness of PEB for fabricating sub-10 nm linewidth robust metal nanowires.

Dissolution dynamics of zirconia nanocluster resist

The metal oxide resists are promising materials for the high numerical aperture EUV lithography. In the metal oxide resists, the bridging ligands are generated upon exposure to radiations, which results in the formation of a pattern insoluble in developers. In this study, the dissolution dynamics of zirconia nanocluster resist was investigated using a quartz crystal microbalance method. The ligand was methacrylate. The developers used were ethyl, butyl, amyl, hexyl, 2-methylbutyl, and 3-methylbutyl acetates. The zirconia nanocluster resist showed characteristic dissolution dynamics. After the immersion into developers, the frequency slowly decreased with approximately constant impedance (no viscosity change of the film) for a while and then rapidly dissolved with the softening of resist film. The dependences of dissolution dynamics on the molecular structures of acetates and post exposure baking were clarified. The effect of branched structure at the third position of butyl was, in particular, remarkable.

Effects of photoacid generator decomposition on dissolution kinetics of poly(4-hydroxystyrene) in tetraalkylammonium hydroxide aqueous solutions

For the advancement of lithography, the resist materials and processes are the most critical factors. In particular, the process of developing resist materials is important in reducing the line width roughness and stochastic defects. In this study, a quartz crystal microbalance method was used to investigate the dissolution dynamics of poly(4-hydroxystyrene) (PHS) films containing triphenylsulfonium nonaflate (TPS-nf) in tetraalkylammonium hydroxide (TAAH) aqueous solutions. The time required for dissolution increased with the alkyl chain length of TAAH. The dissolution rate of PHS films increased with UV irradiation dose for tetramethylammonium hydroxide and tetraethylammonium hydroxide developers, whereas the dissolution time increased for the tetrabutylammonium hydroxide developer. For the tetrapropylammonium hydroxide developer, the dissolution time of PHS films without postexposure baking (PEB) slightly decreased with UV irradiation, whereas that with PEB increased. Both the surface free energy of PHS films and the molecular size of TAAH are considered to affect dissolution kinetics.

Polyvinyl-alcohol-based color resist for low-temperature color-filter production

A color resist comprising partially hydrolyzed polyvinyl alcohol, a crosslinker, a photoacid generator, and dyes was designed for color-filter production. The resist was photoexposed, followed by post-exposure baking, development, and curing at 150 °C to yield a solvent-resistant film. The degree of hydrolysis of polyvinyl alcohol affected the preparation of the homogeneous resist solution and the solvent resistance of the cured film. The cured film was insoluble in the resist solvent when the degree of hydrolysis was high, whereas the film displayed insufficient solvent resistance when degree of hydrolysis was low. Nanoindentation experiments of the polyvinyl alcohol film revealed good mechanical properties that were comparable to those of poly(methyl methacrylate). The crosslinking reaction of the polyvinyl alcohol film was analyzed by monitoring the amount of methanol, a volatile compound emitted during the curing of the film; the results suggest that the reaction was likely completed below the process temperature.

Evaluating nanoscale molecular homogeneity in extreme ultraviolet resists with nanoprojectile secondary ion mass spectrometry

Characterizing chemical changes in photoresists during fabrication processes is critical to understanding how nanometric defects contribute to film stochastics. We used nanoprojectile secondary ion mass spectrometry (NP-SIMS) to evaluate the nanoscale homogeneity of components in positive-tone extreme ultraviolet resists. NP-SIMS was operated in the event-by-event bombardment/detection mode, where a suite of individual gold nanoprojectiles separated in time and space stochastically bombard the surface. Each impact ejects secondary ions from a volume 10 to 15 nm in diameter and up to 10 nm in depth allowing for analysis of colocalized moieties with high spatial resolution. Individual partially exposed extreme ultraviolet resists were analyzed after light exposure, postexposure bake, and development. Results showed an expected increase in protonated quencher versus exposure dose, while after development, we observed increased abundance in the remaining film. The latter, we attribute to poor solubility in the developing solvent. Examining the photoacid generator (PAG), we found decreased PAG cation abundance versus exposure dose in the exposed films, likely due to photodecomposition of the PAG cation. Moreover, after the development, we observed decreased homogeneity of PAG ions, which we attribute to preferential extraction caused by ion-exchange interactions with the developer. We found that the insoluble moieties persisting on the surface after the development were relatively rich in the protecting group, likely due to uneven deprotection of the polymer. Overall, NP-SIMS allows to characterize the resist at the nanoscale and identify conditions that lead to defect formation.

Fabrication of Multilayer Molds by Dry Film Photoresist.

Dry film photoresists are widely employed to fabricate high-aspect-ratio microstructures, such as molds for microfluidic devices. Unlike liquid resists, such as SU-8, dry films do not require a cleanroom facility, and it is straightforward to prepare uniform and reproducible films as thick as 500 µm. Multilayer patterning, however, can be problematic with dry film resists even though it is critical for a number of microfluidic devices. Layer-to-layer mask alignment typically requires the first layer to be fully developed, making the pattern visible, before applying and patterning the second layer. While a liquid resist can flow over the topography of previous layers, this is not the case with dry film lamination. We found that post-exposure baking of dry film photoresists can preserve a flat topography while revealing an image of the patterned features that is suitable for alignment to the next layer. We demonstrate the use of this technique with two different types of dry film resist to fabricate master molds for a hydrophoresis size-sorting device and a cell chemotaxis device.

Water developable non-chemically amplified photoresist for electron beam and extreme ultraviolet lithography

Background: Non-chemically amplified resist (n-CAR) shows great potential as a unique lithographic material because it avoids some disadvantages of CAR, such as post-exposure instability and acid diffusion. Furthermore, since toxic and flammable developers are widely used in semiconductor manufacturing, the implementation of innovative environmentally friendly water-developable photoresists is of interest. Aim: A unique n-CAR, which could be developed with an environmentally friendly developer, was prepared for electron beam (e-beam) lithography (EBL) and extreme ultraviolet lithography (EUVL). Approach: A polymer containing radiation/photosensitive sulfonium triflate group (PSSF) was synthesized and characterized by infrared, H1 NMR, and gel permeation chromatography. The lithography performance of the PSSF photoresist was evaluated by EBL and EUVL. The patterns were analyzed with scanning electron microscope and atomic force microscope. Results: The PSSF photoresist can be used in EBL and EUVL. Post-exposure bake had no significant effect on the resolution of photoresist. Development in water should be kept at an appropriate time of 30 s to obtain the repeatable and high-resolution patterns. It shows a similar sensitivity to polymethyl methacrylate but higher contrast. Conclusions: The PSSF acts as n-CAR, and 20 nm line patterns and 35 nm 1:1 line/space patterns were achieved in EBL and EUVL, respectively. It can be developed in pure water with high contrast (γ = 5.49).

Latent image characterization by spectroscopic reflectometry in the extreme ultraviolet

Background: In the extreme ultaviolet (EUV) lithography process the performance of the photoresist is a crucial factor regarding the quality and critical dimensions of the fabricated structures. Aim: The characterization of the latent image structures in photoresists during the process steps before the development of the resist is key to understand the relation between the material of the resists, the selection of process parameters, and the resulting quality of fabricated structures. Approach: Spectroscopic EUV reflectometry is a nondestructive metrology technique that measures the broadband reflectance of samples in the EUV spectral range and under grazing incidence angles. The technique offers a combination of high sensitivity to nanoscale structural parameters of periodic structures as well as a high sensitivity to the material composition samples, enabling the characterization of latent images of periodic structures. Results: Measurements of the reflectance of an EUV-exposed and unexposed photoresist reveal the contrast in optical constants after the resists are treated with a post-exposure bake as well as shrinkage of the resist layer thickness. Based on this data, simulative studies on latent images of periodic grating structures are conducted showing the possibility to extract information on the structure parameters including the latent image profile and surface topography. Conclusion: Spectroscopic EUV reflectometry shows to be sensitive to the contrast of exposed and unexposed photoresist which commends the technique to be adequate for the characterization of latent images in photoresists.

Development of optical‐driven nanoneedles using SU‐8 nanofabrication

AbstractWe developed an optical‐driven nanoneedle that is operated in aqueous solution by two focused laser beams. A nanoneedle tool consists of 2 μm‐sized structures for optical trapping and a nanoneedle having 200 nm width and 20 μm length. Nanoneedle tools are fabricated with single ultraviolet (UV) lithography of SU‐8 photoresist with highly scalable manner. We evaluated the conditions of the lithography processes: rinse, hard bake and post exposure bake (PEB) that determine the shape of the nanoneedles and recovery rate. We demonstrated the trapping of a nanoneedle tool to verify the manipulation of the position and the orientation. Nanoneedle tools have an advantage in the use of closed microchanells, which can lead to the applications including delivery and extraction of substances inside single cells and pinpoint molecular processing of biomolecules. In this paper, we report on the fabrication process of the nanoneedle tools and the demonstration of the manipulation.

Optimized ultraviolet grayscale process for high vertical resolution applied to spectral imagers

NanoCarb is a miniature Fourier transform imaging spectrometer dedicated to the measurement of atmospheric CO2 and CH4. The key element of NanoCarb is an array of Fabry–Perot microinterferometers having a stepcase shape. Lateral dimensions and height of each step depend on the used material, the focusing lenses, and the targeted optical path difference to be measured. In this paper, we developed a grayscale lithography process for a large surface patterning with high vertical resolution. This process is combined with plasma etching to transfer the as-obtained resist patterns into the silicon substrate. This method is an efficient and quick way for the realization of such arrays into silicon. A low contrast resist (ma-P1225G) was used for better control of the step height, and we investigated the effect of two annealing processes on the contrast curve slope: the soft bake and the postexposure bake. Therefore, combining the two processes leads to a 20 nm step height resolution in resist and 50 nm in silicon.

Effects of acid diffusion and resist molecular size on line edge roughness for chemically amplified resists in EUV lithography: computational study

Stochastic simulations are performed to analyze the effects of acid diffusion and resist molecular size on pattern profiles for chemically amplified resists in EUV lithography. In the simulation, polymer chains are formed by random connection of monomer units. The acid diffusion and the acid reaction with polymers in the post exposure bake are introduced by the random-walk model. Simulation results show that the inhomogeneous acid generation and diffusion more largely affects the pattern roughness than polymer molecular size for both deprotection and cross-linking types of resists. At high doses, the line edge roughness (LER) increases with increasing polymerization degree of resist polymer. In such condition, the acid distribution becomes uniform and molecular size affects the LER. On the other hand, at low doses, the LER decreases with increasing polymerization degree. That is because the larger polymer molecule is less sensitive to inhomogeneous distribution of acid accompanying low doses.

A class of azocarbazole‐based carboxylates: High‐efficiency ionic unimolecular photobase generators for thiol‐epoxy click polymerization under blue light

AbstractTwo new unimolecular azocarbazole‐based visible photobase generators (Ac‐C‐Base) with different strong bases such as 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD) and 1,1,3,3‐tetramethylguanidine (TMG) as the potential active species were prepared. Their structures were confirmed by 1H NMR, 13C NMR, and HR‐MS. According to the photolysis and EPR results, Ac‐C‐Base could generate free radicals and strong bases simultaneously through the photodecarboxylation mechanism under 455 nm LED irradiation. Thus they can effectively catalyze the thiol‐epoxy polymerization without post exposure baking under light exposure. In addition, the Ac‐C‐TBD containing a stronger base shows a faster polymerization rate and higher epoxy conversion.

Feasibility of unzipping polymer polyphthalaldehyde for extreme ultraviolet lithography

Background: Polyphthalaldehyde (PPA)-based systems can be interesting candidates for extreme ultraviolet (EUV) lithography as dry development resists with simple chemistry. Aim: We present EUV-induced mechanistic and contrast curve studies for the end-capped PPA. Approach: Fourier transform infrared (FTIR) spectroscopy and desorption studies were conducted to understand the EUV-induced mechanistic pathway. Although, contrast curve analysis was used to check its feasibility for EUV-patterning. Results: First, FTIR and desorption studies confirmed that the EUV-radiation is capable to remove the end-capping group and induce unzipping (direct depolymerization) reaction in the PPA polymer chain. Second, contrast curve analysis showed a gradual decrease in the film thickness with respect to the EUV dose, which is due to the low EUV sensitivity of the PPA polymer. A post exposure bake step is important to improve the contrast curve, as it helps to depolymerize any residual polymer remaining after exposure. Further, even though the polymer sublimates when exposed to the EUV radiation, eliminating the need to apply a wet development step, crosslinking at high doses causes deposition of residual film onto the wafer surface. Therefore, a thin film (below 20 nm) and a wet development process might be important to get clean patterns. Conclusion: This study confirms that the end-capped PPA, with a simple patterning mechanism, can be a useful system for the development of new dry development resists for EUV lithography.

A Method to Improve IR Drop issue of Large-Size Top-Emission AMOLED

Top-emitting active-matrix organic light-emitting diode (AMOLED) displays usually use a thin cathode layer to obtain high transmittance. However, the thin surface cathode inevitably raises an IR drop issue or even a Mura effect that is visible to the naked eyes, especially in large-size displays. Therefore, to reduce the IR-drop effect and improve the display quality, a plurality of cathode separators are provided to separate the cathode of AMOLEDs to electrically connect the cathode to the auxiliary electrodes under the cathode separators. In this work, we report how to control the process of organic photoresist to obtain a good anti-taper (cathode separator). The organic photoresist is formed through the steps of coating, pre-bake, exposure, post exposure bake (PEB) and then developing, but the exposure and PEB processes are the key factors, which can improve the anti-taper below 53° (The EL evaporation angle is 53°).

Modeling the impact of shrinkage effects on photoresist development

Background: Negative-tone development (NTD) photoresists are prone to shrinkage effects during lithographic processing. Along with deformation seen during the postexposure bake (PEB), there are additional effects during the development that cannot be fully explained by a conventional PEB shrinkage model alone. Aim: Understand the impact of PEB shrinkage on the development rate. Develop a model that can help predict resist profiles after chemical development. Approach: A PEB shrinkage model for NTD resists is introduced, which uses the thermal properties of the resist material to help simulate shrinkage. The deformed state of the resist is used as an input to the development rate equation to predict the final feature dimensions observed in experiments. Results: The strain concentration within the resist bulk can have an influence on the stability of the resist during the development. The strain influences the development rate depending on the resist feature shape and contours. Conclusions: The results from this study can help improve optical proximity correction (OPC) modeling performance and help better understand the deformation characteristics of NTD resist materials. The model also shows that the development shrinkage has an influence on the edge placement error.

Medusa 82—Hydrogen silsesquioxane based high sensitivity negative-tone resist with long shelf-life and grayscale lithography capability

The high suitability of hydrogen silsesquioxane (HSQ) as e-beam resist has long been known. Despite its undoubtedly good and reliable properties, HSQ nevertheless proves to be problematic in certain aspects due to its relatively short shelf-life and the small processing window between coating preparation and exposure. We thus intended to optimize the silsesquioxane with respect to a prolonged shelf-life and larger processing window while retaining all advantages like the high silicon content for high etch resistance and high pattern resolution. Our combined knowledge resulted in the development of the hydrogen silsesquioxane-based e-beam resist Medusa 82 with improved characteristics. Medusa 82 can be processed with HSQ standard procedures but allows for a delay of several weeks between layer preparation and exposure under standard conditions. Medusa 82 resist compositions tolerate storage periods of several weeks at room temperature. In addition, we generated and investigated variants of Medusa 82, which offer the possibility for exposure with less energy to cross-link the resist. Furthermore, weaker alkaline developers can be applied. A postexposure bake of these new Medusa 82 variants provides a significant enhancement of sensitivity and contrast. In this context, applications of Medusa 82 in deep to extreme ultraviolet and grayscale lithography are described. The use of glasslike resists with moderate electron beam sensitivity has the potential to reduce the effort and to simplify the manufacturing process of micro-optical devices that traditionally have to be structured in glass surfaces. The transformation process of Medusa 82 into a glasslike material involves an e-beam exposure, a thermal treatment, or a combination of both. Moreover, the adjustable contrast and sensitivity enable grayscale lithography. Different e-beam exposures trigger a different cross-linking degree within the layer, resulting in height variations after development. A postexposure bake step induces further cross-linking and a complete conversion into silicon oxide.

Adhesive wafer bonding of micro-actuators with SU-8 photoresist

Adhesive wafer bonding of micro-actuators using SU-8 photoresist is reported. It was investigated in this study that the bonding temperature, bonding pressure, baking time and thickness of the cross-linked SU-8 photoresist layer affects the bonding strength of the assembled micro-actuators. The adhesive wafer bonding was performed by applying different pressures while baking the wafers at different temperatures. In this work, we have demonstrated that decreasing the exposure dose and post-exposure baking temperature play significant roles in affecting the bonding strength of assembled micro-actuators. We believe that lowering the exposure dose and post-exposure temperature will inhibit hardening of the intermediate negative photo-resist layer, and it enhances softening of SU-8 thin film, which can facilitate the bonding strength between the micro-chips.

Multi-acrylate-based UV-curable dismantlable adhesives

Herein, a dismantlable, photo-curable adhesive is reported. The adhesive is composed of a multi-acrylate oligomer with a tertiary ester, a photo radical initiator, a photo acid generator, and a phosphate-substituted acrylate as an adhesion promoter. Photo-exposure to the adhesive sandwiched between two glass slides induces crosslinking and causes the slides to bond to each other. Post-exposure baking promotes the acid-catalyzed decomposition of the hardened adhesive, which results in significantly decreased adhesion strength. An alkaline developer can easily remove the residual adhesive on the debonded glass plates, thus recovering glass transparency. Photo-bonding and debonding mechanisms were elucidated by spectroscopic analyses. By using the dismantlable adhesive, a small glass plate attached to a glass slide can be successfully transferred to another glass slide.

Low‐temperature curable, alkaline‐developable, and negative‐type photosensitive polyimide with high resolution and mechanical properties based on chain extendable poly(amic acid) and photo‐base generator

A negative‐type photosensitive polyimide (PSPI) based on a chain extendable poly(amic acid) (PAA) and a photo‐base generator (PBG) (E)‐1‐piperidino‐3‐(2‐hydroxyphenyl)‐2‐propen‐1‐one (PHPP) have been developed. The chain extendable PAA was prepared from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA) and 4,4′‐oxydianiline (ODA) and end‐capped with di‐tert‐butyl dicarbonate (DIBOC) in N‐methyl‐2‐pyrrolidone (NMP), which has a controlled molecular weight for developing in a 2.38 wt% tetramethyl ammonium hydroxide aqueous solution (TMAHaq) and underwent a chain‐extending reaction during a curing stage. The PSPI was formulated with the polymerization solution (20 wt% in NMP) and PHPP (15 wt% for the polymer). The PSPI showed a sensitivity of 152 mJ cm−2 and a contrast of 5.9 when exposed to the i‐line, followed by post exposure baking at 160°C for 2 minutes and development with the 2.38 wt% TMAHaq containing 10 wt% iPrOH at room temperature. A fine negative image with 2‐μm pattern was obtained on a 3‐μm thick film in the contact‐printed mode. After thermal curing at 200°C for 1 hour, the resulting PSPI features low curable temperature, high resolution and excellent mechanical properties.