Post Exposure Bake Temperature Research Articles

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.

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.

An ultra heat-resistant polyimide formulated with photo-base generator for alkaline-developable, negative-type photoresist

An alkaline-developable and negative-type photosensitive polyimide (PSPI) based on poly(amic acid) (PAA) and a photo-base generator (PBG) has been developed by using chemical amplification method. The PAA(PDA-BPDA) is composed of p-phenylenediamine (PDA) and 4,4′-biphtalic dianhydride (BPDA), and the derived PI possesses excellent thermal and thermo mechanical stability due to the highly rigid and linear backbone. The photolithographic parameters for the PSPI including the PBG content, post-exposure baking (PEB) temperature, PEB time and the molecular weight of the PAA was investigated with the adoption of (E)-3-(2-hydroxy-4-methoxyphenyl)-1-(piperidin-1-yl)prop-2-en-1-one (HMPP) as the PBG. The PSPI based on PAA(PDA-BPDA) with a molar ratio of 0.9/1.0 between PDA and BPDA, and HMPP (15 wt% to PAA) showed a sensitivity of 142 mJ/cm2 and contrast of 0.58 when exposed to 365-nm light (i-line), PEB at 170 °C for 5 min, and developed with an aqueous solution of 2.38 wt% tetramethylammonium hydroxide and isopropanol. A clear negative 4-μm feature pattern was obtained and imidized into the PI pattern upon heating at 250 °C, confirming by scanning electron microscopy. This is the first work to utilize a highly rigid backbone with high thermal stability for PSPIs. The outstanding thermal and thermo mechanical stability manifests that developed PSPIs is highly potential in the microelectronics.

Direct polymer microcantilever fabrication from free-standing dry film photoresists

Traditionally, polymeric microcantilevers are assembled by a multitude of process steps comprising liquid spin-coated photoresists and rigid substrate materials. Polymer microcantilevers presented in this work rely instead on commercially available dry film photoresists and allowed an omittance of multiple fabrication steps. Thin, 5 μm thick dry film photoresists are thermally laminated onto prepatterned silicon substrates that contain AFM compatible probe bodies. Partially suspended dry film resists are formed between these probe bodies, which are patterned to yield microcantilevers using conventional photolithography protocols. A limited amount of thermal cycling is required, and sacrificial probe-release layers are omitted as microcantilevers form directly through resist development. Even 1 mm long polymeric cantilevers were fabricated this way with superior in-plane alignment. The general effects of post-exposure bake (PEB) and hardbake protocols on cantilever deflection are discussed. Generally, higher PEB temperatures limit out-of-plane cantilever bending. Hardbake improved vertical alignment only of high-PEB temperature cantilevers, while surprisingly worsening the alignment of low-PEB temperature cantilevers. The mechanism behind the latter is likely explained by complex interactions between the resist and the substrate related to differences in thermal expansion, heat conduction, as well as resist cross-linking gradients. We present furthermore multilayer structures of dry film resists, specifically cylindrical dry film resist pillars on the polymer cantilever, as well as the integration of metal structures onto the polymer cantilever, which should enable in future integrated piezoresistive deflection readout for various sensing applications. Finally, cantilever spring constants were determined by measuring force–displacement curves with an advanced cantilever calibration device, allowing also the determination of both, dry film resist cantilever density and Young’s modulus.

Design, fabrication, and characterization of a multidimensional prism.

In this research, a novel multidimensional prism with three distinct 45°, 135°, and 225° inclined optical surfaces were realized by using inclined exposure technology and SU-8 polymer. To obtain a smooth surface, the solvent loss percentage and temperature of post-exposure bake (PEB) are two key factors that need to be well controlled during fabrication. The experimental results showed that surface roughness can achieve 42.9±7.6 nm, which is one-tenth of high-precision machining or molding processes, under the processes parameter combinations of PEB temperature, and solvent loss percentage is 95°C and 82.86%, respectively. The surface reflectivity of the prism was measured by using a He-Ne laser, and reflectivity of the prism surface without and with aluminum metal film was 90.8% and 91.8%, respectively. The slight difference of reflectivity means that a prism with a high-quality inclined surface can be realized. The functionality of the prism for three lasers was also verified with RGB lasers, and it successfully demonstrated the feasibility of application of a multidimensional prism on the optical system. Finally, the utilization of inclined exposure technology not only monolithically integrates three 45° inclined surfaces into one prism without precision assembly but also greatly simplifies the fabrication processes to further reduce the cost. This component and technology can also be applied to medical endoscope systems if the SU-8 is replaced by PDMS or other biocompatible materials using a molding process. These results provide the potential for mass manufacturing, which is of considerable value to the optical markets.

Structural effects on the performance of epoxide-based negative-tone molecular resists

Several 0.26N tetramethylammonium hydroxide (TMAH)-soluble epoxide molecular resists have been synthesized and are reported here. Previously, the patterning performance of 1,1,2,2-tetrakis(p-hydroxyphenyl)ethane-3 epoxide (TPOE-3Ep) was reported and resolved 26 nm lines using extreme ultraviolet lithography. Here, a deeper study is performed to determine the effects of various structural features on the lithographic performance of 0.26N TMAH-soluble molecular resists. Increasing the number of phenols resulted in a notable increase in the glass transition temperature (Tg) of these materials, which required high postexposure bake (PEB) temperatures to achieve normalized remaining thickness (NRT) values of 1 in methyl isobutyl ketone development. Such high PEB temperatures resulted in insoluble material (high NRT values) in unexposed regions in the 248 nm contrast curves. Methyl groups were introduced adjacent to the hydroxyl group of phenol in an attempt to lower the Tg of the resists to allow the use of lower PEB temperatures. The methyl groups only slightly lowered the Tg of the resists, while detrimentally reducing the final NRT. Thus, instead of using the TPOE core or its methylated analog, a smaller core was used, and the resist trihydroxyphenyl ethane (THPE)-2Ep was designed and synthesized to be a low-Tg base-soluble resist. THPE-2Ep has a Tg of 41 °C and showed promising early performance using e-beam lithography and resolves 30 nm lines in 0.26N TMAH developer at a dose of 72 μC/cm2 at a PEB of 50 °C.

Fabricating method of SU-8 photoresist conical nozzle for inkjet printhead

ABSTRACTA novel conical nozzle fabrication method for piezoelectric inkjet printhead is proposed based on the microelectromechanical system technology. By optimizing the exposure gap, exposure dose, postexposure bake (PEB), and development time, the SU-8 conical nozzle was fabricated. After thermal bonding with an open chamber, the piezoelectric inkjet printhead chamber was formed. The results show that the optimum SU-8 conical nozzle can be fabricated by applying exposure dose of 105 mJ/cm2, exposure gap of 50 µm, PEB temperature of 65°C, PEB time of 10 min, and development time of 210 s. The final size of optimum conical nozzle is the large aperture of 30.46 ± 1.17 µm, small aperture of 20.34 ± 1.42 µm, and nozzle height of 45.38 ± 1.02 µm.

Characterization for the photomask fabrication based on a high-resolution technique with a non-chemically amplified resist and a post-exposure bake

A high-resolution technique has been developed for the fabrication of photomasks for 10nm logic nodes and beyond. Current mask manufacturing techniques use a chemically amplified resist (CAR) material that has a complex mechanism of acid generation; this complicates the criteria for selecting the polymer and the quencher for industrial purposes. It is therefore important to validate non-CAR materials as alternative solutions for mask fabrication. In this research, we used diluted ZEP520A as a non-CAR material in conjunction with a JBX9000 variable-shaped electron-beam (VSB) lithography tool. Additionally, a post-exposure bake (PEB), routinely used in mask fabrication, was also applied. We investigated the effects of the PEB temperature on the fabrication of masks from non-CAR resists, and we demonstrated the feasibility of using a PEB as a high-resolution technique. The critical dimensions (CDs) for 1:1 line-and-space, isolated space, and isolated line patterns on a diluted ZEP520A resist were measured and showed a shrinkage, an extension effect, and retention of the integrity of the shape after the PEB process, respectively. Furthermore, the line-edge roughness (LER) of the 1:1 line-and-space and isolated space CDs was improved by approximately 40% by optimizing the PEB temperature. We investigated the process characteristics of this PEB annealing effect by examining the hardness of the cured resist with and without exposure to PEB at various temperatures, with the aim of elucidating the underlying mechanism. Optimizing the PEB temperature of the non-CAR increased the resist contrast, annealing the resist and improving the LER. This permitted us to demonstrate an advanced fabrication technique capable of high resolutions of the order of 20nm. The insights gained from the optimization of the PEB process might be useful in advanced methods of fabricating masks of the next generation.

Fabricating a high-resolution mask with improved line-edge roughness by using a nonchemically amplified resist and a postexposure bake.

The authors have developed a high-resolution technique for fabricating photomasks at the 10-nm half-pitch logic nodes and beyond. Current mask-manufacturing techniques use a chemically amplified resist (CAR) that has a complex mechanism of acid generation, complicating the criteria for selecting the polymer and the quencher for industrial purposes. Thus, it is important to study fabricating masks with non-CARs. The authors exposed a non-CAR, diluted ZEP520A, to variable-shaped electron-beam lithography and used a postexposure bake (PEB) to modify the resist. Studying how the PEB temperature affected the non-CAR and resultant masks, the authors demonstrate that their technique can produce high-resolution structures. By measuring the critical dimensions (CDs), the authors show that the PEB shrunk, enlarged, and retained the size of 1:1 line-and-space, isolated space, and isolated line patterns, respectively. By optimizing the PEB temperature, the authors improved the line-edge roughness (LER) of the 1:1 line-and-space and isolated space CDs by ∼40%. To understand how the PEB affected the resultant structures, the authors measured the hardness of cured resists with and without a PEB at various temperatures. Optimizing the PEB temperature of the non-CAR increased the resist contrast, annealing the resist and improving the LER. As such, their technique is capable of high resolutions on the order of 20 nm. The insights the authors gained from optimizing the PEB might be useful when fabricating next-generation masks.

High resolution spin-on electron beam lithography resist with exceptional dry etching resistance

Presently, no resist material has suitable properties to satisfy simultaneously all requirements on lateral resolution and plasma etching resistance for pattern transfer of high aspect ratio nano-structures.Herein, a recent innovative alumina spin-on material for use as both positive and negative high resolution resist for electron beam lithography (EBL) is presented. This system turns into an almost completely inorganic alumina system when exposed to the electron beam, showing exceptional selectivity (100:1), compared to even the most robust polymeric resist, when used as mask for dry etching of silicon in fluorinated plasmas, obviating the need for a hardmask layer deposition. A full characterization of the resist selective removal under different processing conditions, such as electrons acceleration voltage and post-exposure bake temperature and time, is presented. Owing to the high selectivity and high resolution, silicon nano-structures of size down to 11nm and high aspect ratio (up to 20) have been obtained for the first time in a one-step process.

Characterizing acid diffusion lengths in chemically amplified resists from measurements of deprotection kinetics

The acid-catalyzed deprotection of glassy poly(4-hydroxystyrene-co-tertbutyl acrylate) films was studied with infrared absorbance spectroscopy and stochastic simulations. Experimental data were interpreted with a simple description of subdiffusive acid transport coupled to second-order acid loss. This model predicts key attributes of observed deprotection rates, such as fast reaction at short times, slow reaction at long times, and a nonlinear dependence on acid loading. Fickian diffusion is approached by increasing the postexposure bake temperature or adding plasticizing agents to the polymer resin. These findings demonstrate that acid mobility and overall deprotection kinetics are coupled to glassy matrix dynamics. To complement the analysis of bulk kinetics, acid diffusion lengths were calculated from the anomalous transport model and compared with nanopattern line widths. The consistent scaling between experiments and simulations suggests that the anomalous diffusion model could be further developed into a predictive lithography tool.

Guided wrinkling with nanoimprinted SU-8 surfaces

The wrinkling of SU-8 may be useful for microelectromechanical systems and optical applications, but highly regular and periodic wrinkles are the most convenient for such applications. This paper reports wrinkles generated on SU-8 by exposure to 172 nm-wavelength ultraviolet under ozone. To identify the relevant processing parameters, the impact of the SU-8 layer thickness and the effects of the postexposure bake temperature and UV-ozone treatment time are investigated. With 5 μm-thick SU-8 layers, distinct wrinkles are obtained with an ultraviolet-ozone treatment of 3 min and at a postexposure bake temperature of at least 140 °C. To guide the wrinkling, topography is induced on the SU-8 surface via capillary force lithography. By using a stamp with equidistant cavities possessing different widths, the structure size dependence is investigated. Owing to local stress relaxation, single or double wrinkles form along the elevated lines depending upon the line width. Within the gaps between the lines, wrinkles oriented perpendicular to the lines develop induced by the nonrelaxed compressive stress. Wrinkling represents a simple alternative for covering the surface of SU-8 with largely self-forming regular structures that may act as transmissive optical elements.

Effects of post exposure bake temperature and exposure time on SU-8 nanopattern obtained by electron beam lithography

SU-8 is a photoresist imaged using UV rays. However, we investigated the characteristics of an SU-8 nanopattern obtained by electron beam lithography (EBL). In particular, we studied the relationship between post-exposure bake (PEB) temperature and exposure time on an SU-8 nanopattern with a focus on phase transition temperature. SU-8 residue was formed by increasing both PEB temperature and exposure time. To prevent the formation of this, Monte Carlo simulation was performed; the results of such simulation showed that decreasing the thickness of SU-8 can reduce the amount of residue from the SU-8 nanopattern. We confirmed that decreasing the thickness of SU-8 can also prevent the formation of residue from the SU-8 nanopattern with EBL.

Model-based breakdown of resist and mask contributions to local CDU for sub-30-nm contact holes in EUV lithography

The resist and mask contributions to the local critical dimension uniformity (L.CDU) of 30-nm contact holes (CHs) printed using extreme ultra-violet lithography (EUVL) are decomposed quantitatively by three independent budget breakdown methods. An asymptotic behavior model is proposed for the prediction of the resist-only contribution via a shifted and averaged multiple exposure (SAME) method that statistically averages out the collective mask local contributions. The other two budget breakdown methods of assessing, the variance of site-to-site difference for the derivation of resist-only contribution and the global mask error enhancement factor (G.MEEF) for the mask-only contribution, are performed and compared to the resist and mask contributions from the SAME method. The three budget breakdown methods consistently indicate that approximately 75–80% of L.CDU is attributed to the resist and process regardless of the process conditions, such as post exposure bake (PEB) temperature, PEB time, and exposure dose. L.CDU exhibits a linear correlation with 1/sqrt(dose), which is a potential indication that photon shot noise (PSN) could be a key contributor to the L.CDU on a wafer. Although a significant reduction of the total L.CDU is observed in the low PEB temperature process, i.e., high exposure dose, a relatively small change in the ratio of the resist-to-mask contribution may be observed because the chemical contrast enhancement by acid diffusion reduction is relatively small compared to PSN reduction.

FTIR SPECTROSCOPIC STUDIES ON CROSS LINKING OF SU-8 PHOTORESIST

The usage of chemically-amplified, negative tone SU-8 photoresist is numerous, spanning industrial, scientific and medical fields. Hence, in this study, some preliminary studies were conducted to understand the dosage and heat treatment requirements of the SU-8 photoresist essential for pattern generation using X-ray lithography. In this work, using Synchrotron as the X-ray source, SU-8 photoresist was characterized for X-ray lithography in terms of its process parameters such as X-ray exposure dose, post exposure bake (PEB) time and temperature for various photoresist thicknesses which is considered worthwhile in view of applications of SU-8 for the fabrication of very high aspect ratio micro structures. The process parameters were varied and the resultant cross linking of the molecular chains of the photoresist was accurately monitored using a Fourier Transform Infra-Red (FTIR) spectrometer and the results are discussed. The infrared absorption peak at 914 cm-1 in the spectrum of the SU-8 photoresist was found to be a useful indicator for the completion of cross linking in the SU-8 photoresist. Results show that the cross linking of the SU-8 photoresist is at a higher rate from 0 J/cm3 to 30 J/cm3 after which the peak almost saturates regardless of the PEB time. It is a good evidence for the validation of dosage requirement of SU-8 photoresist for effective completion of cross linking, which in turn is a requirement for efficient fabrication of micro and nano structures. An analogous behavior was also observed between the extent of cross linking and the PEB time and temperature. The rate of cross linking declines after a certain period of PEB time regardless of PEB temperature. The obtained results also show a definite relation between variation of the absorbance area of the peak at 914 cm-1 and the X-ray exposure dose.

Impact of Temperature on Thick Photoresist Lithography Process of Glass Microfluidic Chip

The positive photoresist AZ4620 was used for thick photoresist lithography of glass microfluidic chip. Different test temperature of soft baking, post exposure baking, hard baking, impacted the surface quality of photoresist layer, the adhesion of photoresist and glass matrix, the photoresist tolerance time, the etching rate and the maximum etching depth. The results show that the soft-bake temperature of 80°C improves the adhesion of the photoresist and the glass substrate well; PEB (post exposure bake) temperature of 60~70°Chelps to the quality of graphics after development; hard bake temperature of 80~90°Ccan conducive to a larger etching depth; through the controlling of heating temperature and mixing rate, it effectively improves the etching rate, increases the etching depth as well. It has been applied to the fast and high-volume production of a certain microfluidic detection chips.

One component photo‐curing agent for epoxy resins based on multifunctional photobase generators containing oxime–urethane groups

In order to develop a one‐component photo‐curing system for epoxy resin, the photo‐crosslinking reactions of the diglycidyl ether of bisphenol A (DGEBA) in the presence of a multifunctional photobase generator (PBG) containing oxime–urethane groups were studied. The cross‐linking of DGEBA films and adhesion properties of DGEBA formulations containing the PBG and benzophenone increased with irradiation dose, post‐exposure baking (PEB) time, PEB temperature, and the number of oxime–urethane groups in the PBG. A synergistic effect was observed between the PBG and a base amplifier on the film cross‐linking of DGEBA. A trifunctional PBG containing oxime–urethane groups was found to be the most efficient PBG in terms of the photo‐crosslinking and adhesion properties of the DGEBA‐based formulations. Moreover, the devised formulations, including the PBG and benzophenone, were stable for at least 1 month at room temperature. The photocuring system developed in this study appears to offer a one‐pack epoxy resin curing system with practical useful properties. Copyright © 2009 John Wiley & Sons, Ltd.

Extreme-ultraviolet secondary electron blur at the 22-nm half pitch node

In this paper the Arrhenius behavior of blur upon extreme ultraviolet (EUV) exposure is investigated through variation of the post-exposure bake (PEB) temperature. In this way, thermally activated parameters that contribute to blur (such as acid/base diffusion) can be separated from nonthermally activated parameters (such as secondary electron blur). The experimental results are analyzed in detail using multiwavelength resist modeling based on the continuum approach and through fitting of the EUV data using stochastic resist models. The extracted blur kinetics display perfectly linear Arrhenius behavior, indicating that there is no sign for secondary electron blur at 22-nm half pitch. At the lowest PEB setting the total blur length is ∼4 nm, indicating that secondary electron blur should be well below that. The stochastic resist model gives a best fit to the current data set with parameters that result in a maximum probability of acid generation at 2.4 nm from the photon absorption site. Extrapolation of the model predicts that towards the 16-nm half pitch the impact on sizing dose is minimal and an acceptable exposure latitude is achievable. In order to limit the impact on linewidth roughness at these dimensions it will be required to control acid diffusion to ∼5 nm.

Modification of Chemically Amplified Resists by Radical Copolymerization

AbstractModification of a phenolic resist has been carried out by copolymerization with methacrylic monomers. Influence of irradiation source and its intensity has been studied by analyzing sensitive curves of a various resist composition. Inversion of imaging type at varying post exposure bake temperature has been investigated for resists based on copolymers containing methacrylic acid units, which has been related with crosslinking of macromolecules.

Characterization of the Photoacid Diffusion Length and Reaction Kinetics in EUV Photoresists with IR Spectroscopy

A soft-contact film transfer method was developed to prepare multilayer photoresist thin films that enable high-resolution spectroscopic and reflectivity measurements for determining the reaction−diffusion kinetic parameters and photoacid diffusion length. Infrared reflectance absorption spectroscopy was applied to follow, quantitatively, the reaction−diffusion kinetics during the post-exposure bake (PEB) step; the time evolution of the average deprotection level across a bilayer film with model photoresists is described by a kinetics model with three parameters: a reaction rate constant (kP), the phenomenological photoacid trapping constant (kT), and the photoacid diffusion constant (DH). A polymeric and molecular resist for next-generation extreme ultraviolet (EUV) lithography with chemically analogous structure was studied with this methodology. The three kinetic parameters follow an Arrhenius dependence but show quantitative differences between these two photoresists at a given PEB temperature. Further it was demonstrated that the photoacid diffusion length is not a simple function of diffusion coefficient; instead, it is dictated by all three kinetics parameters jointly in addition to the deprotection level at which the resist becomes soluble in an aqueous developer solution. These observations qualitatively explain the experimentally observed shorter photoacid diffusion length of the molecular resist in comparison to its polymeric counterpart.