Post Exposure Bake Research Articles

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.

Alkaline‐developable and negative‐type photosensitive polyimide with high sensitivity and excellent mechanical properties using photo‐base generator

AbstractAn alkaline developable and negative‐type PSPI with a high sensitivity and excellent mechanical properties based on a poly(amic acid) (PAA) and a photo‐base generator has been developed. The PAA was prepared by the polycondensation of p‐phenylenediamine (PDA) with an equimolar of 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA) and 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6‐FDA) and converted thermally to the corresponding polyimide, PI(PDA‐BPDA/6‐FDA). PI(PDA‐BPDA/6‐FDA) showed the high thermal and mechanical properties and the dimensional stability such as the thermal decomposition temperature of 530°C, glass transition temperature of 369°C, linear coefficient of thermal expansion of 28 ppm/K, ultimate tensile strength of 148 MPa, elongation at break of 25% and dielectric constant of 2.8. The PSPI was formulated directly from PAA(PDA‐BPDA/6‐FDA) with a photo‐base generator (PBG), (E)‐3‐(2‐hydroxy‐4‐methoxyphenyl)‐1‐(piperidin‐1‐yl)prop‐2‐en‐1‐one (HMPP) (10 wt% to PAA) and the optimized parameters for photolithographic process were investigated including the PBG content, post‐exposure bake (PEB) temperature, and PEB time. The PSPI based on PAA(PDA‐BPDA/6‐FDA) and HMPP (10 wt% to PAA) showed a sensitivity of 114 mJ/cm2 and contrast of 1.29 when exposed to 365‐nm light (i‐line), post‐exposure baked at 160°C for 5 min, and developed with an aqueous solution of 2.38 wt% tetramethylammonium hydroxide and iso‐propanol. A clear negative 8‐μm features pattern was obtained by contact‐printing and converted into the PI pattern upon heating at 250°C, confirming by scanning electron microscopy and infrared spectroscopy.

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.

Thermal design aspects for improving temperature homogeneity of silicon wafer during thermal processing in microlithography

Stringent temperature control is of great importance in the post-exposure baking of the lithography process, and this is usually maintained through the lamp heating and multizone heating method, which demands a precise control system. There is a vast scope for the wafer temperature homogeneity improvement by considering the thermal aspects in bake station design. In this study, both an experimental and a transient numerical analysis are carried out to examine the factors that could seriously affect the temperature uniformity of the wafer during the baking process. It is found that the guide plate arrangement, guide plate slope, and the air gap between the lid and hot plate play a critical role in controlling the wafer temperature uniformity. On this basis, some further improved bake stations are proposed to attain a better temperature homogeneity across the wafer. The performance of the improved designs is analyzed numerically. Yet, a better design based on the simulation is developed and tested experimentally. The new design features a 2.5° guide plate with the funnel arrangement subject to the air gap condition, offering a temperature homogeneity within 0.2–0.3 °C on a 6″ wafer.

Grayscale e-beam lithography: Effects of a delayed development for well-controlled 3D patterning

Grayscale electron beam lithography (g-EBL) is a fabrication technique that allows for tunable control of resist topography. In most cases, the height of the structures is in the submicron regime. Here, we present an extensive experimental characterization of the post electron beam exposure behavior of poly(methyl methacrylate) (PMMA) 950 K for grayscale structuring with several micrometers in height. The obtained results show that the development depth for the same electron dose is dependent on the time between exposure and development. This dependence becomes more prominent at higher exposure doses. Additionally, it was found that a post-exposure bake influences the dose-response behavior of the resist material and, therefore, also the obtained three-dimensional (3D) structure. This work paves the way for well-controlled 3D micrometer structuring via g-EBL.

Alkaline‐developable positive‐type photosensitive polyimide with high mechanical strength and high resolution based on chain extendable poly(amic acid), thermally degradable cross‐linker and photoacid generator

AbstractA positive‐type photosensitive polyimide (PSPI) based on a chain extendable poly(amic acid) (PAA), a thermally degradable cross‐linker 1,3,5‐tris[(2‐vinyloxy)ethoxy]benzene (TVEB), and a photoacid generator (PAG) (5‐propylsulfonyloxyimino‐5H‐thiophene‐2‐ylidene)‐(2‐methylphenyl)acetonitrile (PTMA) has 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 (TMAH aq) and undergoes a chain extending reaction during curing stage. The photosensitive resist solution was formulated with the polymerization solution (30 wt% in NMP), TVEB (15 wt% for the polymer), and PAG (4.5 wt% for the polymer). The PSPI showed a sensitivity of 47 mJ cm−2 and a contrast of 5.8 when exposed to 365‐nm light, followed by postexposure baking at 90 °C for 10 min and development with the 2.38 wt% TMAH aq at room temperature. A fine‐positive image with 3‐μm line‐and‐space patterns was obtained on a 3‐μm thick film exposed to UV light at 365 nm in the contact‐printed mode. After thermal curing at 350 °C for 1 hr, the resulting PSPI features excellent mechanical strength and elongation.

Ultrasonic vibration used for improving interfacial adhesion strength between metal substrate and high-aspect-ratio thick SU-8 photoresist mould

The fabrication of high-aspect-ratio metal micro device on metal substrate is largely limited by its poor interfacial adhesion strength between metal substrate and thick SU-8 photoresist mould. In this paper, ultrasonic treatment is introduced to improve the interfacial adhesion strength between metal substrate and a high-aspect-ratio inertial switch SU-8 mould. Firstly, a device for ultrasonic treatment was developed, ultrasonic vibration is applied to SU-8 film after post exposure baking in order to improve the interfacial adhesion strength. Compared with the traditional one, SU-8 photoresist mould treated by ultrasonic vibration can effectively improve the interfacial adhesion strength. After 90 min cavitation erosion test, SU-8 film treated by ultrasonic vibration remains 34.4% relative to nothing left of the SU-8 film without ultrasonic treatment. Besides, the mechanisms of ultrasonic treatment on improving interfacial adhesion strength are investigated. Finally, an inertial switch is successfully fabricated on metallic substrate with the ultrasonic treated SU-8 photoresist mould.

Theoretical study on protected unit fluctuation of chemically amplified resists used for photomask production by electron beam lithography

Stochasticity in the sensitization processes of resist materials is a significant concern in fine patterning. Such stochasticity results in protected unit fluctuation in the case of a chemically amplified resist, which is a standard resist for photomask production. After development, protected unit fluctuation comes to the surface as line edge roughness, stochastic pinching, and stochastic bridging. In this study, protected unit fluctuation after postexposure baking was investigated, assuming the fabrication of line-and-space patterns. The standard deviation of the number of protected units connected to a polymer molecule was calculated using a Monte Carlo method on the basis of the sensitization mechanism of chemically amplified electron beam resists. The dependences of the standard deviation on half-pitch, sensitivity, sensitizer concentration, and exposure pattern width were clarified.

Study of SU‐8 photoresist cross‐linking process by atomic force acoustic microscopy

In this paper, a method is presented to detect the different phases of epoxy cross-linking process and the subsurface structures of SU-8 thin films by atomic force acoustic microscopy (AFAM). The AFAM imaging of SU-8 thin films was investigated under different exposure and bake conditions. Optimized conditions were obtained for the cross-linking of SU-8 thin film at the exposure does of eight laser pulses with the laser fluence 10mJcm-2 per pulse and the post exposure bake (PEB) time at 90 s. The subsurface structures of undeveloped SU-8 thin films were visible in the AFAM images. This method provides an effective and low-cost way for the determination of different phases of epoxy cross-linking process in nanostructured compounds, for the non-destructive testing of subsurface defects, and for the evaluation of the quality of patterned structures.

Chemical reaction and diffusion kinetics during laser-induced submillisecond heating for lithographic applications

Reactions in solid-state chemistry are controlled by both underlying chemical reaction rates and temperature-dependent diffusion of reactants and products. Due to distinctly different activation energies, the relative rates of reaction and diffusion may shift dramatically with temperature. In this work, the acid catalyzed deprotection of acid labile groups of model chemically amplified photopolymers was studied to understand the effects of temperature on diffusion-coupled reactions. In these systems, strong acids form during exposure of photoacid generators (PAGs) to ultraviolet radiation and then diffuse and catalyze deprotection reactions. By using very short duration laser-induced heating for the postexposure bake step, the potential reaction temperature range is extended by several hundred degrees compared to traditional thermal processing temperatures. Thermal degradation at these temperatures is avoided by simultaneously reducing the heating time from minutes to submilliseconds. Both diffusion and reaction rates were measured in this high temperature region for three different photoresists combined with two different PAGs, with activation energies of the key processes determined. The interplay of diffusion and reaction rates was also examined by comparing results of high-resolution pattern formation. The best image formation at high temperature was achieved using photoresists with high deprotection rates combined with PAGs exhibiting low diffusivity.

Stochastic simulation of pattern formation for chemically amplified resist in electron beam lithography

A molecular scale simulation of the pattern formation process for chemically amplified resist in electron beam lithography based on the stochastic approach is proposed. The initial resist structures are formed by the sequential bonding of the randomly selected monomers. The effects of electron exposure are introduced by the activation of the acid generator which randomly determined according to the absorbed energy distribution in the resist. The absorbed energy distribution is calculated using the Monte Carlo simulation of electron scattering. The effects of the post exposure bake are introduced by the acid diffusion and the polymer deprotection reactions. The fundamental properties, such as quencher concentration dependence, molecular weight dependence, and exposure condition effects on pattern profiles, are well reproduced by the simulation.

Micro-patterning of functional coatings guided by a photosensitive degradable template

A photosensitive disposable template material for patterning a functional coating, which is generally considerably challenging to perform by photolithography, was formulated. The template material is composed primarily of a multi-acrylate resin containing of a tertiary ester structure, a photo radical initiator, photo-acid generators (PAGs), and a fluorine-based surface repellent polymer. The photo initiator induces crosslinking of the resin and the acid generated from the PAGs by the additional exposure catalyzes the decomposition of the crosslinked resin on the exposed area to yield the liquid-repellent patterned template after alkaline development. Organic droplets of a carbon black dispersion (CB) and a high refractive index (RI) material on the patterns slide down and spread only on the area without the patterns. Light irradiation is performed on the entire surface of the templates after the application of the CB and the high-RI material, and subsequent post exposure bake and alkaline development can remove the templates to yield their micro-scale patterns along the template respectively.

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.

Fabricating chambers of inkjet printhead by bonding SU-8 nozzle plate with suitable crosslinked degree

The process of fabricating chambers is becoming increasingly important for inkjet printhead. However, the majority of present fabrication methods suffer from some problems, such as chamber collapsed, chamber blocked, and the deformation of nozzle plate. This paper develops a novel method for fabricating inkjet chambers by bonding the nozzle plate of SU-8 photoresist with suitable crosslinked degree onto the open microchamber. The softbake temperature of the nozzle plates was optimized for the high bonding strength and the low chamber deformation. The crosslinked degree of the SU-8 photoresist with different exposure dose and post exposure bake (PEB) parameters were calculated based on Fourier transform infrared spectroscopy experiments. The effects of exposure dose and PEB parameters on crosslinked degree of the SU-8 photoresist were analyzed. The influence of crosslinked degree of the SU-8 photoresist on the chamber deformation rate and bonding strength were investigated. The optimized bonding strength of 0.57 MPa and chamber deformation rate of 4.35% were obtained in the final inkjet printhead. Compared with most of the current processing methods, this fabrication technique has great potential due to the decreased chamber deformation rate and the increased bonding strength.

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.

Analysis of dissolution factor of line edge roughness formation in chemically amplified electron beam resist

Line edge roughness (LER) is a serious issue for the fine patterning in electron beam (EB) lithography. LER is formed as a consequence of the accumulation of stochastic events induced in resist materials. In this study, the dissolution factor a of LER formation was investigated. The line-and-space patterns of a highly resolving chemically amplified EB resist were analyzed using a Monte Carlo simulation. From the dependence of the relationship between the dissolution factor, exposure dose, and pitch on the initial standard deviation of the number of protected units per polymer molecule, the initial distribution of protected units was estimated to be random for the resist analyzed in this study. a/2 was estimated to be roughly 1.0–1.6. The fluctuation of the number of protected units ±aσp/2 was considered to contribute to the LER formation, where σp is the standard deviation of the number of protected units per polymer molecule after postexposure baking. The resist performance is considered to be improved by decreasing the initial standard deviation of the number of protected units and/or the dissolution factor.

Spatial Control of Photoacid Diffusion in Chemically Amplified Resist (CAR) via External Electric Field.

We investigated the effect of an electric field-based post exposure bake (EF-PEB) process on photoacid diffusion and pattern formation. To investigate the control of photoacid diffusion experimentally, the EF-PEB processes was performed at various temperatures. Cross sectional images of various EF-PEB processed samples were obtained by scanning electron microscopy (SEM) after ion beam milling. In addition, we conducted a numerical analysis of photoacid distribution and diffusion with following Fick's second law and compared the experimental results with our theoretical model. The drift distance was theoretically predicted by multiplying drift velocity and EF-PEB time, and the experimental values were obtained by finding the difference in pattern depths of PEB/EFPEB samples. Finally, an EF-PEB temperature of 85 °C was confirmed as the optimum condition to maximize photoacid drift distance using the electric field.

Comprehensive Simulations for Ultraviolet Lithography Process of Thick SU-8 Photoresist.

Thick SU-8 photoresist has been a popular photoresist material to fabricate various mechanical, biological, and chemical devices for many years. The accuracy and precision of the ultraviolet (UV) lithography process of thick SU-8 depend on key parameters in the set-up, the material properties of the SU-8 resist, and the thickness of the resist structure. As feature sizes get smaller and pattern complexity increases, accurate control and efficient optimization of the lithography process are significantly expected. Numerical simulations can be employed to improve understanding and process design of the SU-8 lithography, thereby allowing rapid related product and process development. A typical comprehensive lithography of UV lithography of thick SU-8 includes aerial image simulation, exposure simulation, post-exposure bake (PEB) simulation, and development simulation, and this article presents an overview of the essential aspects in the comprehensive simulation. At first, models for the lithography process of the SU-8 are discussed. Then, main algorithms for etching surface evolvement, including the string, ray tracing, cellular automaton, and fast marching algorithms, are introduced and compared with each other in terms of performance. After that, some simulation results of the UV lithography process of the SU-8 are presented, demonstrating the promising potential and efficiency of the simulation technology. Finally, a prospect is discussed for some open questions in three-dimensional (3D) comprehensive simulation of the UV lithography of the SU-8.

Dual-tone Application of a Tin-Oxo Cage Photoresist Under E-beam and EUV Exposure

We report on the dual-tone property of the tin-oxo cage (BuSn)l20I4(0H)6](0H)2 photoresist. After exposing the resist film to a low dose extreme ultraviolet radiation or electron beam, applying a post exposure bake step and development with isopropanol/H20 (2:1), a positive tone image is observed. The previously observed negative tone is found at higher doses. Atomic force microscopy and scanning electron microscopy were used to characterize the topography of the patterns. X-ray photoelectron spectroscopy was used to elucidate the chemical changes of the tin-oxo cages under different conditions. The photoresist, which has dual-tone property, paves the way to fabricate sophisticated structures in a single photoresist layer or may lead to metal-containing resists with improved sensitivity. Keywords: Dual-tone photoresist, EUV photoresist, Inorganic photoresist, Tin-oxo cages, Tone conversion, Mechanism

A new type of sulfonium salt copolymers generating polymeric photoacid: Preparation, properties and application

Photoacid generators (PAGs) are critical components in chemically amplified (CA) photoresists system, among which small molecular sulfonium salts are commonly used but possess inherent incompatibility with polymer matrixes and acid migration during the post exposure baking (PEB) processes. PAG-bound polymers would be better choices for CA photoresists to acquire excellent lithographic performance. In this work, a new and convenient way is designed to prepare polymeric PAGs which generate polymeric photoacids and with this method the existing small molecular sulfonium salts can be transformed into polymeric compounds with convenience. Poly (sodium p-styrenesulfonate-co-tertiary-butyl methacrylate) (p(SSNa-co-t-BMA)) was prepared through free radical polymerization and then was further reacted with various sulfonium halides to give sulfonium salt copolymers. Different molecular weights of the copolymers can be obtained from Mn 5.5 × 103–2.5 × 104 with distribution coefficient between 1.18‐1.88. The ionic copolymers display good solubilities in common resist solvents. The thermal decomposition temperatures (Td) are around 150–155 °C and the glass transition temperatures (Tg) are above 130 °C. After exposed to light, the copolymers can generate polymeric photoacids. The photoacid generation efficiencies are determined around 0.31–0.34. UV spectra show big difference between the polymeric PAGs and the mixture of the corresponding small molecular sulfonium salts and framework polymers and demonstrate much better transparency of polymeric PAGs. Thick film 248-nm CA photoresist can be formed by one of the polymeric PAGs and partly protected poly(4-hydroxystyrene-co- tertiary-butyl methacrylate) (p(4-HS-co-t-BMA)), and display excellent performance with high height/width (H/W) ratio of 5:1 and resolution of 0.35 μm. These polymeric PAGs also have potentiality to be used in CA photoresists for other photolithography technologies such as 193-nm immersion technology.