Post Exposure Bake Research Articles

Electron-Beam Sensitivity and Patterning of an Aqueous-Develop, Epoxy-Based Polynorbornene Dielectric Material

The lithographic characteristics of electron-beam (e-beam)-induced crosslinking of an epoxy–polynorbornene (PNB) mixture were studied in an effort to explore the feasibility of using this material as a high-contrast, high-sensitivity e-beam-imageable dielectric material. High resolution (for isolated lines) and e-beam sensitivity were obtained for specific mixtures of PNB– epoxy materials. The interaction of an electron beam with the PNB mixture, which includes epoxy cross-linkers, a photoacid generator (PAG), and a sensitizer, was investigated. The contribution of each of the components to the contrast, resolution for isolated lines, and e-beam sensitivity was evaluated. Cross-linking of the irradiated PNB polymer, by itself, was found to occur at relatively high e-beam doses. The primary route to high sensitivity was through epoxy ring-opening and PNB cross-linking by e-beam activation of the PAG. The effects of the post-exposure bake and aqueous-develop conditions on the lithographic characteristics were investigated. Physical structures with critical dimension of 100 nm to 500 nm were fabricated with line edge roughness of 13.5 nm. Contrast values as high as approximately 8 were obtained at doses as low as 0.38 lC/cm 2 .

Micro-Image with Photo-Acid Generator Polymer Nano Sheet

A new application of polymer Langmuir-Blodgett (LB) films which contain a photoacid generator introduced into polymer main chain as a co-monomer to draw photo patterns as a photolithographic is described here. The chemically positive-tone resist system terpolymers p (DDMA-DNMMA-AOPTs) s used here contains N-dodecylmeth acrylamide (DDMA), 1,4-dioxaspiro [4.4] nonane-2-methylmeth acrylate and 4-(allyloxy) phenyl-4-toluenesulfonate (AOPTs), which was prepared by free radical polymerization. In the terpolymers system, the acid generated by the PAG catalyzed the ketal moiety liberates cyclopentanone and regenerating derivative of glyceryl alcohol in an exposed region during the postexposure baking process. The rending moiety is dissolved into alkaline aqueous to form a positive tone in the expose region. The decompose mechanism of [p (DDMA-DNMMA-AOPTs) s] LB films irradiated by deep UV light was investigated. The photopatterning properties were also investigated. Results showed that the resolution of patterns with p (DDMA-DNMMA-AOPTs) s b LB films with 40 layers were 0.75μm which is the limit resolution of the photo-mask employed in the experiment. The effect of photogenerated acid from the photo-acid generator after irradiated by deep UV light on a typical chemically amplified resist system was studied by thermogravimetric analysis, which showed that the existence of the photoacid generator of AOPTs in terpolymer can improve the sensitivity of ketal-protected polymers without decreasing resolution.

Sealing SU-8 microfluidic channels using PDMS

A simple method of irreversibly sealing SU-8 microfluidic channels using PDMS is reported in this paper. The method is based on inducing a chemical reaction between PDMS and SU-8 by first generating amino groups on PDMS surface using N(2) plasma treatment, then allowing the amino groups to react with the residual epoxy groups on SU-8 surface at an elevated temperature. The N(2) plasma treatment of PDMS can be conducted using an ordinary plasma chamber and high purity N(2), while the residual epoxy groups on SU-8 surface can be preserved by post-exposure baking SU-8 at a temperature no higher than 95 °C. The resultant chemical bonding between PDMS and SU-8 using the method create an interface that can withstand a stress that is greater than the bulk strength of PDMS. The bond is permanent and is long-term resistant to water. The method was applied in fabricating SU-8 microfluidi-photonic integrated devices, and the obtained devices were tested to show desirable performance.

Fabrication of a seamless roll mold using inorganic electron beam resist with postexposure bake

The fabrication of next-generation devices via roll-to-roll (RTR) nanoimprinting has received considerable attention due to the high throughput and resolution associated with the process. In particular, RTR nanoimprinting with ultraviolet (UV)-curable resin is expected to be useful for the fabrication of large-area devices such as nonreflective films, transparent conductive sheets, and organic solar cells. In order to improve the resolution of such devices, the roll mold must be able to produce seamless fine patterns. Generally, a roll mold for nanoimprinting is made by attaching a planar mold replicated from the master mold via a nickel electroforming technique. However, this method produces seams, which reduces the product yield. In order to resolve these issues, the authors have developed a direct writing method using electron beam (EB) lithography on a rotating cylindrical substrate. In this study, the authors examined the potential for fabricating a seamless roll mold using high-resolution inorganic EB resist and the pattern shrink effect from postexposure bake (PEB). The authors used the dip-and-pull method to form the EB resist layer on the roll mold substrate (32 mm diameter, made of brass). The cleaned roll mold was dipped in the inorganic EB resist and then pulled out at a constant speed. The resulting sample was cured at 300 °C for 1 h. Next, the sample was mounted on rotating equipment and placed in a scanning electron microscope (SEM) equipped with an EB writing system, and the roll substrate was exposed to 10 kV EB while rotating at a constant speed. The revolution speed was set to ten turns per line pattern. Subsequently, PEB was carried out at 200 °C for 10 min in air. After the sample was cooled to room temperature, the EB-exposed area of the EB resist layer was developed using buffered HF. The obtained pattern on the roll mold was examined with the same SEM used for writing. Finally, RTR nanoimprinting using the obtained roll mold was carried out using UV-photocurable resin. At that time, the sample was coated with an antisticking layer of fluorinated silane coupling agent. As a result, a seamless line pattern 190 nm wide was obtained on a polyethylene terephthalate film over a length of 10 cm.

Understanding Acid Reaction and Diffusion in Chemically Amplified Photoresists: An Approach at the Molecular Level

Acid diffusion in chemically amplified resist might limit the ultimate minimum half-pitch that can be achieved with high sensitivity resists unless diffusion length is reduced until new methods of sensitizing resists are found. Precise knowledge of molecular dynamics of resist materials and advanced techniques need to be developed actively for this issue. In this sense, computer simulations have become a valuable tool in terms of reducing time and costs. However, simulations are generally based on continuum or mesoscale models, which are unable to accurately predict variations at the molecular level. Deeper understanding and investigation of the coupled reaction-diffusion kinetics at the molecular scale during the postexposure bake (PEB) become crucial to achieve nanoscale features with good critical dimension control and good line-edge roughness. In this work we have developed a molecular level approach for understanding of the coupled acid-catalyzed diffusion process in chemically amplified resist syste...

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.

Thermal design of hot plate for 300-mm wafer heating in post-exposure bake

Temperature uniformity of a wafer during post-exposure bake (PEB) in lithography is an important factor in controlling critical dimension (CD) uniformity. In this study, a new hot plate system for the PEB of a 300-mm wafer was analyzed and designed. First, temperature deviation on the wafer caused by warpage was investigated, and the heater pattern of the multi-zone hot plate in the bake system was numerically analyzed. Then, a new heater pattern to enhance the temperature uniformity was proposed and tested experimentally. As a result, temperature uniformity within 0.087 °C on a 300-mm wafer was achieved.

Simulation of 3D inclined/rotated UV lithography and its application to microneedles

A 3D model is set up to simulate the exposure process of inclined/rotated UV lithography for negative SU-8 resists. The formation of inclined resist pillars and microstructures with truncated cone shapes is simulated based on a 3D exposure model in combination with a post exposure bake model for chemically amplified resists and the Mack development model. As one of the interesting applications employing this promising lithography technique for MEMS fabrication, a solid microneedle for drug delivery is simulated.

High-resolution permanent photoresist laminate TMMF for sealed microfluidic structures in biological applications

We demonstrate the use of photosensitive epoxy laminate TMMF S2045 for the fabrication and sealing of tapered microfluidic channels. The 45 µm thick resist enables the fabrication of shallow sealed cavities featuring extreme aspect ratios of less than 1:40 (h = 45 µm, w = 2000 µm). It also provides high resolution and enables minimum feature sizes of 10 µm. For the fabrication of free-standing structures, an aspect ratio of up to 7:1 was achieved. The dry-film photoresist can be applied easily by lamination onto structured substrates. The total thickness variation of the resist across a 100 mm wafer was determined to be less than ±0.6 µm. Process parameters for the fabrication and sealing of various micro-channels are discussed and optimized in this paper. The main focus was to minimize thermal impact during lamination, soft-bake, exposure and post–exposure bake, which could lead to lid sagging or channel clogging due to liquefaction of uncured resist. We tested TMMF according to ISO 10995-5 and found it to be non-cytotoxic, enabling its use for biological applications. Swelling of less than 5% for incubation of the dry-film resist in several biologically relevant solvents, buffers and cleaning solutions was observed.

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.

Fabrication of Less than 20-nm-Diameter Nanodot Arrays Using Inorganic Electron Beam Resist and Post Exposure Bake

Next-generation NanoImprint Lithography (NIL) includes photolithography and high-voltage-acceleration (>50 kV) Electron-Beam Lithography (EBL) both of which are expensive, which is why Low-Acceleration-Voltage (LAV) EBL has attracted attention. We fabricated nanodot arrays with a diameter of 20 nm or less combining inorganic resist and LAV EBL. Using Post-Exposure Bake (PEB), we reduced dot-array pattern size and conducted Ultra-Violet (UV) NIL using these nanodot molds. The results we obtained using nanodot arrays 10 nm in diameter fabricated by EBL at an acceleration voltage of 4 kV with PEB yielded a checkerboard design 10 nm in diameter spaced at 20 nm intervals using 10 kV EB. This density corresponds to nanodot-arrays of 1 Tb/in2.

An Efficient Simulation System for Inclined UV Lithography Processes of Thick SU-8 Photoresists

A 2-D simulation system based on a 2-D dynamic cellular automata method, integrating aerial image simulation, exposure simulation, post-exposure bake simulation, and development simulation modules is presented for inclined ultraviolet (UV) lithography processes of thick photoresists such as SU-8 photoresists. To verify the simulation system, a series of experiments have been performed for SU-8 2000 series photoresists under UV source with 365 nm (2.6 mW/cm2) radiation. The simulation results demonstrate to be in agreement with the experimental results. This is useful to optimize the inclined UV lithography processes of SU-8 photoresists, and to accurately design and control the dimensions of some SU-8 microstructures.

Large refractive index changes of a chemically amplified photoresist in femtosecond laser nonlinear lithography

We found that marked increases in refractive index of chemically amplified photoresists induced by highly repetitive femtosecond laser irradiation without post-exposure baking treatment. For laser writing speed less than 30 μm/s, the refractive index change of the nonlinear absorption region was as large as 8 × 10(-3). Moreover, cross-linking reactions of the resists were induced. The refractive index changes can generate optical confinement and subsequent channel propagations of femtosecond laser pulses. The coupling efficiency was estimated as high as 87% using a low numerical aperture objective lens. The peak intensities of the guiding modes exceeded the polymerization threshold of the resist.

Fabrication of a GHz band surface acoustic wave filter by UV-nanoimprint with an HSQ stamp

A GHz band surface acoustic wave (SAW) filter was fabricated by UV-nanoimprint lithography (UV-NIL). The key techniques to produce a SAW filter include stamp and interdigital transducer (IDT) fabrication. For the stamp, high-aspect-ratio (AR) hydrogen silsesquioxane (HSQ)/ITO/glass stamps were first exposed by low e-beam dose. Adequate post-exposure bake, tetramethylammoniumhydroxide concentration, and etch time were utilized to pattern the HSQ stamps with a perfect vertical sidewall. HSQ/ITO/glass IDT stamps with widths in the range 46–168 nm were fabricated. The stamps were then cured with a novel step-like heating cycle as hard-bake. The modified HSQ stamps have a high hardness of 19 GPa. The HSQ/ITO/glass layered structure shows 80% transmittance at 365 nm UV light. Afterward, the IDT patterns were transferred on UV-curable resist/lithium niobate (LiNbO3) by UV-NIL at room temperature and low pressure. After removing the residual layer, Al/Ti films were deposited on UV-curable resist/LiNbO3. IDTs with widths in the range 62–219 nm and thickness of 20 nm could be obtained by following a lift-off process. The center frequency of 4.3–8.6 GHz and insertion loss of −14.1 to −61.7 dB of the SAW filters were measured by a network analyzer HP 8510C. The reactive ion etching step for residual layer elimination has significant implication for the electrical characterization of a SAW filter. The thorough removal of the residual layer not only ensures a clean interface between IDTs and LiNbO3 for SAW filters with minimum insertion loss, but also keeps intact IDT patterns for small center frequency shift.

The Control of Shrinkage and Thermal Instability in SU‐8 Photoresists for Holographic Lithography

AbstractThe negative‐tone epoxy photoresist, SU‐8, expands ≈1% by volume after postexposure baking. However, if the maximum optical fluence is comparable to that at the insolubility threshold, as in a holographic exposure, the developed resist shrinks (≈35% by volume) due to the removal of light oligomers not incorporated into the polymeric network. IR spectroscopy shows that, at this level of exposure, only 15% of the epoxy groups in the insoluble polymer have reacted; consequently microstructural elements soften and collapse at >100 °C. When the light oligomers are removed, the sensitivity of the resist is unchanged, provided that 5% (w/w) of a high‐molecular‐weight reactive plasticizer (glycidoxy‐terminated polyethylene glycol) is added, but it shrinks less on development and, when used as a photonic crystal template, shows improved uniformity with less cracking and buckling. Reinforcing the polymer network by reaction with the polyfunctional amine (bis‐N,N′‐(3‐aminopropyl)ethylenediamine) increases the extent of cross‐linking and the thermal stability, allowing inverse replicas of photonic crystal templates to be fabricated from both Al:ZnO and Zr3N4 using atomic layer deposition at temperatures up to 200 °C.

3D-structures with arbitrary shapes created in negative resists by grayscale proton beam writing

The direct and maskless technique of proton beam writing (PBW) was used for grayscale lithography which allows to create 3D microstructures with arbitrary surface topographies. For this purpose, several micrometer thick layers of the negative resists ma-N and SU-8 were irradiated with 2.25MeV H+ and 1.125MeV H2+ in arbitrary shapes using different fluences on different areas with values smaller than the threshold fluence for complete exposure. These irradiations result in multilevel microstructures, whose heights increase with increasing fluence. However, the comparison of the measured structure height with calculated predictions from SRIM simulations disproves the assumption that the structure height is proportional to the linear energy transfer (LET). In fact, the fluence reduction below the threshold for sufficient exposure is responsible for grayscale structuring due to reduced etching of the insufficiently exposed regions. The artifacts obtained with the first grayscale structures created by PBW in ma-N and strongly affecting the structure quality could be reduced by optimizing the scanning procedure, e.g. reducing the pixel distance. Therewith, a micro-Fresnel-lens could be fabricated in ma-N. The first PBW grayscale structures in SU-8 exhibited very strong mechanical instabilities which could be reduced by the use of a post exposure bake step, normally omitted for PBW with SU-8.

Time Dependent Behavior of Chemically Amplified Resist Characterized under Sub-millisecond Post Exposure Bake

Chemically amplified photoresists require a post exposure bake (PEB) to catalytically deprotect the polymer backbone. However, excessive diffusion of the photogenerated acid during PEB results in resolution loss and line edge roughness. As both deprotection and diffusion are thermally activated processes, understanding their reaction kinetics as a function of temperature and time is potentially key to optimizing these two competing processes. In this work, we characterize the resist deprotection kinetics under laser PEB (lPEB), utilizing continuous wave CO2 laser thermal processing to shift the duration into sub-millisecond time frames at temperatures up to 500°C. At these temperatures and times, the extent of deprotection under l-PEB is comparable to that observed under conventional hot plate PEB conditions in the second time frames. Results show that, under isothermal conditions, deprotection has a power law dependence as a function of PEB duration, exhibiting secondorder or higher order kinetics under second time frames but a first-order kinetics under lPEB in sub-millisecond times.

Material Development for ArF Immersion Resist Extension

ArF immersion process is being widely applied for advanced device manufacturing. ArF immersion lithography can cover 4x nm hp lithography. In addition, double patterning process allows extendibility towards 2x nm hp lithography. From the point of view of photoresist, it is utterly important for materials to bring out the maximum performance of ArF immersion process. In this study, we focused on photoresist development and substrate optimization, demonstrating good lithographic performance of sub-40nm hp resolution with well controlled line width roughness (LWR).First of all, we studied photo acid generator (PAG) behavior focusing on shortening acid diffusion length during Post Exposure Bake (PEB). Secondly, high dissolution contrast polymers in which dissolution properties are changed by acid catalyzed reaction through exposure and PEB are being studied. Thirdly, reflectivity control from substrate through optimization of light intensity over resist film thickness has been carried out. In this report, we have achieved 36.5nm line and space (LS) resolution with LWR of less than 10% of patterned critical dimension (CD). This achievement is very useful especially when utilized in combination with process tricks such as multiple patterning for extending existing ArF immersion lithography towards sub-20nm hp lithography.

Effects of SU-8 Cross-linking on Flip-Chip Bond Strength when Assembling and Packaging MEMS

New methods to assemble, integrate, and package micro devices are always needed in attempts to simplify and expedite fabrication methods to maximize throughput. Our paper focuses on assessing SU-8 as a viable material for packaging and flip chip bonding processes for MEMS and micro devices. In this paper, we vary the level of cross- linking through post exposure bake (PEB) times and assess rectangular ring test structures bonding strength following flip chip bonding through applied tensile loads. In addition, we performed initial assessments on the etching resiliency of varied cross-linking of SU-8. From initial results, the bonding strength is maximized following a 3-min PEB. Cross-linking appears to have minimal effects on SU-8's etch resiliency as all tested samples etched approximately 1.25μm. From our initial results, SU-8 appears to be a viable and inexpensive material for wafer bonding, assembling and packaging MEMS devices.

微細加工用レジスト向け新素材・材料の開発

We have recently developed novel resist materials for micro-lithographic patterning. One is a hydroxystyrene derivative, which is a key compound for KrF excimer laser resists. We found that microwave-assisted decarboxylation of hydroxycinnamic acids smoothly proceeded under the presence of a catalytic amount of amine base, to afford hydroxystyrene derivatives in good yield.The other is positive and negative-tone molecular resists utilizing the unique character of a furan ring. The synthesized compounds showed relatively high glass transition temperature and readily formed uniform amorphous films on a silicon wafer. The sensitivity as an EB resist of both positive and negative-tone resists was below 10 µC/cm2 and line and space patterns of 200 nm could be fabricated. The promising feature of the positive-tone resist is that no outgassed products from base matrixes are theoretically produced under the exposure and post-exposure bake procedure.