Thick Photoresist Films Research Articles

Relationship between the surface free energy of underlayers and the dissolution kinetics of poly(4-hydroxystyrene) partially protected by t-butoxycarbonyl groups in tetramethylammonium hydroxide aqueous developer

In the lithography used for the high-volume production of semiconductor devices, the photoresist film becomes thin with the reduction in pattern size to prevent the pattern collapse due to the surface tension of rinsing liquids. The interfacial effect becomes strong with the reduction in photoresist film thickness. In the development process, it is of importance to clarify the relationship between the photoresist and the underlayer for fine patterning. In this study, the dissolution kinetics of poly(4-hydroxystyrene) (PHS) partially protected by t-butoxycarbonyl (t-Boc) groups in tetramethylammonium hydroxide (TMAH) aqueous solution was found to be related to the surface free energy of the underlayer. The attenuation rate of developer viscosity first decreased and then increased with the polar-to-dispersion component ratio. An inflection point with the lowest rate existed. The TMAH concentration affected not only the attenuation rate but also the ratio of polar to dispersion components at the minimum attenuation rate.

Fabrication of pyrolytic carbon interdigitated microelectrodes by maskless UV photolithography with epoxy-based photoresists SU-8 and mr-DWL

Maskless UV photolithography is increasingly used, especially in research environments where low turn-around time for new designs improves productivity. Here, we fabricate pyrolytic carbon interdigitated microelectrodes with small interelectrode gaps, good adhesion to the carrier substrate, high surface area and excellent electrochemical properties using maskless UV photolithography with two negative epoxy-based photoresists, namely the commonly used SU-8 and the recently developed mr-DWL. The minimum realizable trench width in 15 μm thick photoresist films is 2.4 ± 0.15 μm for mr-DWL 5 and 3.1 ± 0.10 μm for SU-8 2035. After pyrolysis, the two resulting pyrolytic carbon materials show similar electrochemical properties. However, shrinkage during pyrolysis is significantly lower for mr-DWL compared to SU-8, which is beneficial for the fabrication of interdigitated microelectrodes. Furthermore, delamination of the electrodes during processing and operation is prevented due to the introduction of poly silicon adhesion structures. This work provides valuable insights into maskless UV lithography as well as into the pyrolytic carbon process to increase the yield, performance and productivity for fabrication of microelectrodes.

Self-organized submicron structures in photoresist films by UV-laser irradiation at water-confined conditions

A new kind of self-organized pattern formation process has been found during laser irradiation of polymer films in water confinement just below the laser ablation threshold, resulting in a randomly oriented pattern with a period of about 475 nm. The morphology, orientation, period, and amplitude of these patterns are inconsistent with both laser-induced periodic surface structures that typically consist of linear grooves with periods smaller the laser wavelength and wrinkling patterns that feature a much larger period and appear at layered systems. Excimer laser (λ = 248 nm, tp = 25 ns) exposure of 650 nm thick photoresist films on silicon wafers cause the growth of irregular submicron patterns. The pattern morphology that is examined with imaging techniques is correlated to processing parameters. The amplitude of these laser-induced self-organized (LISE) submicron structures are strengthened with pulse number and laser fluence. The experimental results are discussed together with simulations of laser heating the photoresist film in water confinement. The proposed pattern formation mechanism of such laser-induced self-organized submicron structures at temporal excitation of a confined polymer surface comprises the formation of an oriented roughness based on LIPSS that are developed to wrinkled structures due to the transient formation of a soft subsurface area that provides conditions for wrinkling of the water cooled, stiff polymer surface by laser-induced stress fields. Size, amplitude, and morphology of the LISE pattern provide good properties for applications in such fields as wetting, friction, optics, and bioactivity.

Evaluation of quality of thick photoresist film by acoustic resonant imaging technique

In this paper, the effects of coating conditions on the thickness and quality, especially the hardness and density, of a photoresist film are reported. The photoresist film was deposited on a Si wafer by a spin coater under various conditions, including baking temperature, rotation speed, and the number of coats. The thickness of the film was measured by a surface profilometer. The sound velocity, which is closely related to hardness, and the density of the film were obtained by acoustic resonant imaging technique. The thickness and sound velocity of the film increased with increasing the number of coats and decreased with increasing the baking temperature and the rotation speed. Furthermore, the density of the film reached its maximum value for each condition. From multiple regression analysis, it was found that, among the three parameters of coating conditions, the rotation speed affects the quality of the film the most. It was shown that a dense photoresist film is obtained by deciding the baking temperature and number of coats in advance, then choosing a suitable rotation speed.

3D OPC method for controlling the morphology of micro structures in laser direct writing.

A 3D optical proximity correction (OPC) method for controlling the morphology of micro-structures in laser direct writing is proposed, considering both the optical proximity effect and nonlinear response of a thick-film photoresist. This method can improve the manufacturability and optical performance of devices, and can be used for most 3D micro\nano structures. Its application in the fabrication of a quadratic curvature microlens array shows that the shape of the lens is well controlled; that is, when the height of the lens is 5.25 µm, the average height error of the lens shape is less than 5.22%.

Hierarchically porous GaN thin films fabricated using high fluence Ar ion implantation of epitaxial GaN on sapphire

• We show the fabrication of hierarchical pores in GaN by means of Ar ion implantation. • This approach is free of the limitations of anodic etching of porous GaN. • There is no need for uniform current spreading to obtain uniform porosity. • The submission focuses on the material properties not on the implantation process. • We demonstrate different damage behavior for single and double ionized Ar. We show a method to fabricate porous gallium nitride (GaN) films without the need for wet chemical etching nor high donor concentrations in the starting material. The pores in epitaxial GaN films were formed using 200 keV Ar + ion implantation at fluences ranging from 2.5∙10 16 Ar + ∙cm −2 to 10 17 Ar + ∙cm −2 . The properties of the created nanostructures are discussed by means of complementary structural, optical and electrical studies. Electrochemical capacitance of the samples in an aqueous electrolyte is studied to deconvolute information about the surface area available after the implantation. Single and double ionized implantation is compared in the discussion and different damage in the two cases is shown. The results show information complementary to the current models of damage buildup in gallium nitride films upon high fluence irradiation as well as present a means of fabricating on-wafer porous areas for future GaN-based devices. The method overcomes the drawbacks of the commonly applied anodic etching: it does not require a high GaN donor concentrations nor difficult setups for achieving uniform etching. Furthermore, it can be applied locally using appropriate masking of the selected areas with a thick photoresist or other strippable inorganic ion stopping film.

Investigating the optimum parameters of a negative photoresist to prepare a V-grooved diffraction grating on Si using photolithography and reactive ion etching techniques

In this study, the deposition parameters of the SU8 2000.5 negative photoresists have been investigated and optimized for the photolithographic technique. Then, applied the inductive coupled plasma reactive ion etching (ICP RIE) to produce V-shaped groove diffraction grating on a silicon substrate. Spin coater (speed: 1000 rpm) was used to coat the photoresist over the Si substrate. The observed photoresist film thickness was measured by ellipsometry and found to be 800 nm. Interestingly, the film exhibited some stability with increasing the spin speed. The thermogravimetric analysis was used to optimize the baking temperature which was found to be ~105 °C. Contrast curves were obtained experimentally and used to optimize the exposure energy along with the images obtained from the field emission scanning electron microscopy (FESEM). The optimized energy fluence was found to be 17 mJ/cm2. It was interesting to observe that the thickness of the photoresists film was increasing with the elevation of the exposure energy fluence. The FESEM images were used to optimize the ICP RIE etching process and the best etching conditions for the Si substrate were ICP power: 150 W, bias power: 100 W, and SF6 gas flow rate: 32 SCCM (standard cubic centimeters per minute), O2 gas flow: 8 SCCM, and Ar gas flow of 8 SCCM. It is worth to mention that well-defined V-shaped grooves were observed with a depth of 2 μm under the same experimental conditions.

Methodology for the formation of photoresist films with uniform thicknesses of several hundred micrometers

To enhance the capabilities of photolithography in the field of microfluidic technology, this study establishes a method to produce a photoresist film with a uniform thickness spanning several hundred micrometers. Herein, the target thickness of the trial production films is 400 μm. The SU-8 3000 series and 4-inch silicon wafers were adopted as the photoresist and substrates, respectively. It was found that when a large amount of SU-8 is formed using the normal procedure, and the film thickness is biased in a certain direction and becomes non-uniform. The reason for this film thickness bias has not been clarified, but it is confirmed that the bias is induced during the soft baking of the resist and not during the coating process. Accordingly, this study proposes a new method to remediate and equalize the biased films. Before film remediation, the photoresist should be adequately soft-baked. However, it is difficult to do this inside of the thick film by simple heating. Thus, some techniques used to facilitate baking thick resist layers are also introduced. Trial production films that are appropriately remediated are obtained. It is shown that in the center region of a remedied film with a 40-mm diameter, the deviation of the local thickness is within 1%, i.e., the thickness bias is completely eliminated. The thickness can also be controlled by remediation with a reproducibility of 1%. The results indicate that the proposed method used to form thick photoresist films is effective for practical applications.

Study of the influence of ozone-air mixture supply conditions on the process of the photoresist removal from the silicon wafer surface

The study is devoted to the research of the dependence of the processing results of photoresistive films on the silicon wafers surface in an ozone environment on the conditions and parameters of the process. The high oxidizing potential of ozone justifies the possibility of its use for removing organic films under atmospheric pressure. The experiments were carried out using the developed research bench, in which the mode and method of heating, as well as the method of supplying gas to the surface of the photoresist, were varied. Silicon wafers with a formed 1,35-μm thick masking photoresist film were used as experimental samples. It was found expedient that uniform heating of the plate over its entire surface can be achieved using a ceramic IR heater. When the ozone-air mixture was introduced into the center of the heated sample, the presence of the removed photoresist residues was observed, which was associated with a temperature drop in its surface area. To solve this problem, the computer models of the temperature regimes of the reaction volume elements were calculated. They showed that the scattering of the working gas flow over the surface of the silicon wafer would significantly increase the efficiency of photoresist removal, and with a good selection of the treatment regime it would ensure complete removal of the photoresist. The data obtained were experimentally confirmed by using an ozone-air mixture flow separator. Experiments were carried out to study the effect of the distance from the wafer surface to the working gas inlet on the photoresist removal rate. They showed that a decrease in the distance reduces the ozone loss due to thermal decomposition and, consequently, increases the material removal rate.

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.

Evaluation of Thick-Film Photoresist for Grayscale Lithography Utilizing Direct Laser Writing

Evaluation of Thick-Film Photoresist for Grayscale Lithography Utilizing Direct Laser Writing

LED ULTRAVIOLET EXPOSURE UNIT WITH ADJUSTABLE EXPOSURE TIME

The possibilities of improving the quality of the topological pattern in the exposure of thick-film photoresists due to uniform irradiation with ultraviolet light-emitting diodes are investigated. The results of an experimental study of the developed LED ultraviolet photolithographic irradiator with a controlled exposure time are presented.

Patterning of thick electroplated CoPt magnets using SU‐8 micromoulds

Recently, electroplated cobalt platinum (CoPt) magnetic films have been developed that afford thick (up to ∼100 μm), high-energy-density (up to ∼100 kJ/m3) permanent-magnet microstructures. However, the aggressive electroplating conditions have shown to be incompatible with most thick-film photoresist mould materials, making patterning these films an unanswered technical challenge. In this work, it is reported a complete process flow of fabricating and removing SU-8 photoresist moulds to enable the ultra-thick patterned CoPt magnets on silicon wafers. The implementation of thick SU-8 2050 electroplating moulds makes it feasible to deposit 100 µm thick CoPt magnets in 4 h. A 25 nm thick sputtered titanium layer is used as a sacrificial layer to completely release the SU-8 mould in hydrofluoric acid after electrodeposition. Compared to prior research, significantly improved pattern definition and constant magnetic performance are observed and characterised by optical profilometer, scanning electron microscope, and vibrating sample magnetometer.

Atmospheric plasma etching of polymers: A palette of applications in cleaning/ashing, pattern formation, nanotexturing and superhydrophobic surface fabrication

Atmospheric Pressure Plasmas (APPs) are promising alternatives to their low-pressure counterparts for material surface treatment due to their potential for cost-effective continuous processing. Here we present for the first time to our knowledge a palette of applications of polymer etching showing the potential of APPs for cleaning/ashing, nanopattern formation, nanotexturing, and hierarchical superhydrophobic surface fabrication using the same plasma source configuration. First, we used a dielectric barrier discharge (DBD) plasma source to study etching/cleaning/ashing of different polymers such as PMMA films and AZ photoresist with lithographic patterns using He/O2 mixtures. We achieved high-rate, uniform etching, indicating the source potential for cleaning/ashing even in ambient air conditions. We have noticed that thick photoresist films are damaged after prolonged etching in air plasmas, a phenomenon resembling photoresist damage observed during low-pressure plasma etching. Second, we applied colloidal lithography with polystyrene (PS) nanoparticles followed by isotropic APP etching in order to form nanopatterns on both Silicon and Polymers. We observed that APP induces “cauliflower-like” nanotexturing of the PS spheres. This nanotexturing in combination with plasma etching of the underlying polymer and the spheres allow the fabrication of hierarchical surfaces. Optimization of plasma etching duration (i.e. hierarchical topography) followed by low-pressure plasma deposition of a low surface energy fluorocarbon (hydrophobic) coating led to the preparation of superhydrophobic surfaces with water contact angle 158° and hysteresis 9° (roll-off behavior).

Comparative study of soft thermal printing and lamination of dry thick photoresist films for the uniform fabrication of polymer MOEMS on small-sized samples

A method called ‘soft thermal printing’ (STP) was developed to ensure the optimal transfer of 50 µm-thick dry epoxy resist films (DF-1050) on small-sized samples. The aim was the uniform fabrication of high aspect ratio polymer-based MOEMS (micro-optical-electrical-mechanical system) on small and/or fragile samples, such as GaAs. The printing conditions were optimized, and the resulting thickness uniformity profiles were compared to those obtained via lamination and SU-8 standard spin-coating. Under the best conditions tested, STP and lamination produced similar results, with a maximum deviation to the central thickness of 3% along the sample surface, compared to greater than 40% for SU-8 spin-coating. Both methods were successfully applied to the collective fabrication of DF1050-based MOEMS designed for the dynamic focusing of VCSELs (vertical-cavity surface-emitting lasers). Similar, efficient electro-thermo-mechanical behaviour was obtained in both cases.

Influence of the confinement on laser-induced dry etching at the rear side of fused silica

Laser-induced etching at the rear side of transparent material enables high-quality machining results. However, the mechanism is still not completely recognized which would allow further optimization. Therefore, multi-pulsed laser-induced backside dry etching with different thick photoresist films was studied experimentally for air (MP-LIBDE) and water confinements (cMP-LIBDE). The water confinement causes differences in photoresist ablation morphology and etching rate in dependence on laser fluence, film thickness and pulse number. Owing to the water confinement, the extent of photoresist film spallation and the etching rate slope difference in low and high fluence ranges are reduced. In particular, the etching rate of cMP-LIBDE keeps constant with different film thicknesses in contrast to MP-LIBDE. Two effects that are related to the water confinement, mechanical confinement and heat transfer alterations, are analysed and discussed in relation to the differences between MP-LIBDE and cMP-LIBDE.

Performance enhancement on a micro-column structure reformer via thick-film photoresist pre-protection

In this study, a reformer stack was made by incorporating silicon technology into catalyst preparation. The volumes of an individual micro-channel chip and a whole reformer stack were 0.4 cm3 and 16 cm3, respectively. Different weight ratios (B/C = 35, 15, 5, and 0 wt%) of binder (a mixture of boehmite, bentonite, and deionized water) and catalyst were mixed to find out the optimal adhesion between the catalyst and silicon substrate. The results from this study show that the percentage of weight loss of the catalyst on the silicon substrate increases as the concentration of inorganic binder decreases. To further increase the exposed surface area of the catalyst deposited on the micro-channels, micro-column structures were integrated into the channels; however, a blockage of the catalysts among the columns during deposition was encountered. To resolve this issue, a method of pre-protecting the micro-channel with thick-film photoresist was utilized for the catalyst deposition, and the performance of the fabricated micro-column reformer was able to reach a 95% methanol conversion rate, 90% hydrogen selectivity, and 1.6 × 10−5 (mol min−1) hydrogen yield at 225 °C in the partial oxidation of methanol reaction.

Numerical Simulation of Double Peaked Edge-Bead in Thick Photoresist Films(Industrial Materials)

Numerical Simulation of Double Peaked Edge-Bead in Thick Photoresist Films(Industrial Materials)

Direct fabrication of hexagonally ordered ridged nanoarchitectures via dual interference lithography for efficient sensing applications.

Novel, large-area uniform hexagonally-ordered ridged nanoarchitectures (HORNs) can be fabricated using dual interference lithography derived from a novel prism and substrate beneath a spin-coated photoresist (PR) film. The metallic HORN arrays provide tunable SERS effects with large-scale sample homogeneity that depends on the number of stacks present along the z-direction, which is determined by the PR film thickness. Furthermore, the HORN structures show significant potential for use in particle-based or fluorescence-based sensing platforms by forming a free suspension of ridged nanoparticles or achieving highly intensified fluorescence signals, respectively.

Investigations on a Multiple Mask Technique to Depress Processing-Induced Damage of ICP-Etched HgCdTe Trenches

A multiple mask technique, integrating patterned silicon dioxide (SiO2) film over patterned thick photoresist (PR) film, has been investigated as a method to perform mesa etching for device delineation and electrical isolation of mercury cadmium telluride (HgCdTe) third-generation infrared focal-plane arrays. The multiple mask technique was achieved by standard thick PR photolithography, SiO2 film deposition to cover the thick PR patterned film, and etching the SiO2 film at the bottom region after another photolithography process. The dynamic resistance in the zero-bias and low-reverse-bias regions of HgCdTe photodiode arrays isolated by inductively coupled plasma (ICP) etching with the multiple mask of patterned SiO2 and patterned thick PR film underneath was improved one- to twofold compared with a simple mask of patterned SiO2. It is suggested that the multiple mask technique is capable of maintaining high etching selectivity while reducing the side-wall processing-induced damage of ICP-etched HgCdTe trenches. The results show that the multiple mask technique is readily available and shows great promise for etching HgCdTe mesa arrays.