Transparent Photoresist Research Articles

A Transparent Photoresist Made of Titanium Dioxide Nanoparticle-Embedded Acrylic Resin with a Tunable Refractive Index for UV-Imprint Lithography

Transparent photoresists with a high refractive index (RI) and high transmittance in visible wavelengths have promising functionalities in optical fields. This work reports a kind of tunable optical material composed of titanium dioxide nanoparticles embedded in acrylic resin with a high RI for ultraviolet (UV)-imprint lithography. The hybrid film exhibits a tunable RI of up to 1.67 (589 nm) after being cured by UV light, while maintaining both a high transparency of over 98% in the visible light range and a low haze of less than 0.05%. The precision machining of optical microstructures can be imprinted easily and efficiently using the hybrid resin, which acts as a light guide plate (LGP) to guide the light from the side to the top in order to conserve the energy of the display device. These preliminary studies based on both laboratory and commercial experiments pave the way for exploiting the unparalleled optical properties of nanocomposite resins and promoting their industrial application.

Diffuser optimization for enhancing light extraction from light-emitting electrochemical cells

Light-emitting electrochemical cells (LECs) have enabled many novel solution-processable, low-cost, and flexible emission applications. Nevertheless, relatively inferior device efficiencies render LECs less competitive with other emission technologies. Light extraction would be a promising approach to enhance the device efficiencies of LECs. The diffuser film composed of a transparent photoresist layer doped with large and small TiO2 nanoparticles (NPs) embedded between the indium-tin-oxide layer and the glass substrate has shown superior light extraction performance (>2.5X). However, the physical insight of optimization, e.g., adjusting the concentration of small TiO2 NPs in diffuser film, has not been explored yet. In this work, increasing the concentration of small TiO2 NPs is found to increase the thickness, roughness, and refractive index of the diffuser film. The optical simulation indicates that the thickness of the diffuser film has little effect on the light extraction. However, a higher roughness but a lower refractive index is preferred for the diffuser film to achieve better light extraction. As such, it is rational to expect that a proper concentration of small TiO2 NPs to reach a high enough roughness but still keep a relatively low refractive index is required for diffuser optimization. This study clarifies the physical mechanism responsible for diffuser optimization and thus shows a high potential for lighting applications of LECs.

Direct laser writing of birefringent photonic crystals for the infrared spectral range

Infrared optical photonic crystals fabricated using direct laser writing, which is based on the two-photon polymerization of suitable monomers, have received substantial interest since the emergence of this process. Two-photon polymerization could be a disruptive technology for the fabrication of all-dielectric photonic crystals in the infrared spectral range, as it allows the synthesis of large scale arrays of uniform structures with arbitrary geometries and arrangements. However, all-dielectric photonic crystals that provide birefringent optical responses in the infrared spectral range have not yet been demonstrated using direct laser writing techniques. Here, the authors explore the form birefringence observed in photonic crystals composed of arrays of subwavelength-sized slanted polymer microwires. The photonic crystals investigated here were fabricated in a single fabrication step using direct laser writing of an infrared transparent photoresist (IP-Dip). A strong contrast of the cross-polarized reflectance of photonic crystals as a function of the in-plane orientation is observed in the mid-infrared spectral range at λ≈6.5μm. This observation is indicative of an anisotropic optical behavior. Finite element based techniques corroborate the experimentally observed responses qualitatively.

High-resolution and nondestructive profile measurement by spectral-domain optical coherence tomography with a visible broadband light source for optical-device fabrication

We developed a spectral-domain optical coherence tomography (OCT) using a visible broadband light source (vis-OCT) for application to high-resolution and nondestructive profile measurement and imaging in semiconductor optical-device fabrication. By using visible broadband light centered at 625 nm and with spectral bandwidth of 260 nm, an axial resolution of 0.69 µm in air was obtained. This was effective for inspection of a transparent photoresist film with thickness of 1–2 µm coated on a semiconductor wafer; the interface between the photoresist film and its substrate and the interface between the photoresist and air were resolved, and the film thickness was measured. In addition, the interface between an opaque epitaxially grown semiconductor layer (Al0.35Ga0.65As) and a GaAs substrate was also detected by vis-OCT. Here we propose a thickness-measurement technique that combines finite-difference time-domain simulation with vis-OCT. This method enables us to determine the thickness of even an optically absorbent epitaxial layer and offers a profile-measurement method that is particularly suitable for the fabrication of semiconductor optical devices.

Specific features of single-pulse femtosecond laser micron and submicron ablation of a thin silver film coated with a micron-thick photoresist layer

Specific features of ablation of a thin silver film with a layer of a highly transparent photoresist and the same film without a photoresist layer under single tightly focused femtosecond laser pulses in the visible range are experimentally investigated. Interference effects of internal modification of the photoresist layer, its spallation ablation from the film surface and formation of through hollow submicron channels in the resist without its spallation but with ablation of the silver film lying under the resist are found and discussed.

4.1: Distinguished Paper: Cavity Shape Control of the Roll‐to‐Roll Fabricated Novel Microstructure Film for Improving the Viewing‐Angle Characteristics of LCDs

AbstractA microstructure film with many micro cavities positioned behind small black dots has been developed for improving LCD viewing‐angle characteristics. The micro cavities are fabricated by the self‐alignment photolithography of the transparent photoresist using the black dots as photo masks. The cavity shapes are optimized by controlling the photolithography conditions.

Fourfold power efficiency improvement in organic light-emitting devices using an embedded nanocomposite scattering layer

It has been demonstrated that internal extraction structures (IES) can be introduced in OLEDs to decrease the ratio of the waveguide mode and simultaneously increase the ratios of the substrate and radiation modes. In this study, titanium oxide (TiO2) nanoparticles (NPs) combined with transparent photoresist (TPR) were utilized to form an embedded nanocomposite scattering layer between the indium-tin-oxide (ITO) and glass substrate, leading to a significant boost in the out-coupling efficiency of the OLEDs. Inside the nanocomposite scattering layer, NPs of different sizes served distinct functions. The 250nm-TiO2 particles were used to induce scattering and diminish the light reflection back to the ITO layer. On the other hand, the refractive index of the TPR can be increased by increasing the concentration of the 25nm-TiO2, which reduced the difference in the refractive index between the ITO and TPR and thus multiplied the amount of light entering into the scattering layer. By employing nanocomposite substrate with mixed dual-sized NPs, we obtained power efficiencies of the blue phosphorescent OLEDs that were about 4.3 times higher than that of the control device at the high luminance of 5×103cd/m2.

Improving the outcoupling efficiency of indium-tin-oxide-free organic light-emitting diodes via rough internal interfaces

We present low-cost texturing methods to produce different surface roughnesses on glass substrates. Using sand blasting, abrasion and wet etching we achieve roughnesses of about 50 nm to 250 nm (root mean squared roughness Rq). These textured substrates are used as extraction elements for guided modes and substrate modes in organic light-emitting diodes (OLEDs). We evaporate 50 nm of the high index material Ta₂O₅ on the textured substrate, which acts as waveguide layer, and flatten it with the transparent photoresist SU-8. On top of that, we fabricate indium tin oxide (ITO)-free OLEDs, which are characterized by electroluminescence and photoluminescence measurements. The devices with rough interfaces obtain an up to 37.4% and 15.5% (at 20 mA/cm²) enhanced emission and it is shown that the enhancement is due to an increased outcoupling efficiency.

Laser fabrication of high-aspect-ratio holes and grooves in photoresist by time constant manipulation

We report on the cw-laser fabrication of sharp-edged holes and grooves in organic-dye-sensitized photoresists for g-line or i-line by manipulating its intensity and scanning rate. The laser fabrication is performed to locally control the reaction time constant of the photoresist. By scanning a tightly focused laser focal spot inside the transparent photoresist, various micron/submicron meter structures have been produced based on the reaction time constant depending on the beam intensity. The proposed method is considered to be a cost-effective technique for high-aspect-ratio holes and grooves fabrication.

Bis(trifluoromethyl)carbinol-Substituted Polynorbornenes: Dissolution Behavior

Alicyclic polymers, such as substituted polynorbornene (PNB), are expected to be one potential material solution for providing transparent photoresist polymer resins for photolithography at 193 and 157 nm wavelengths. In this work, the dissolution behavior of bis(trifluoromethyl)carbinol-substituted polynorbornene (HFAPNB) in aqueous alkaline solutions as a function of polymer molecular weight is explored. Surprisingly, it was found that the dissolution rate of bis(trifluoromethyl)carbinol-substituted polynorbornene increased with increasing molecular weight over the range from approximately 10,000 to 100,000 Mw. Experimental results show that this polymer displays strong hydrogen bonding in the solid film form and that such hydrogen bonding is disrupted as the polymer molecular weight is increased. On the basis of this evidence, an explanation for this unusual dissolution behavior based upon disruption of hydrogen bonding is presented.

Material Design for Application to High Performance LCDs.

Liquid Crystal Display(LCD) is a high performance tool for displaying many kinds of information. Application of LCD is diversifying to notebook PC, monitor display, TV and reflective type of PDA and personal handy phone. To achieve these performance for several applications of LCD, development of new materials has been investigated. Color photoresist is used for color filter production. Negative and positive type of transparent photoresist are applied for leveling and overcoating layer of LCD process, and further used for spacer materials. For the next generation of alignment layer, photo-alignment material is also investigated. Dimerization of chalcone moiety showed good alignment characteristics for liquid crystals and good LCD performance.

Effect of Microlens Array for Mos Color Imager

A high-photosensitivity and well-balanced color reproduction color imager with a monolithic microlens array was developed. The microlens array was formed on a 2/3-inch MOS color imager using a photolithographic technique with a transparent photoresist. The microlens array was arranged on the top of only the green portions of the mosaic filter (G-lens array) of the color imager. For an imager with a 2.0-um thick G-lens, the photosensitivity was raised 25% over the relative response of the green signal for an imager without a G-lens array, and the color image reproduced by the G-lens imager was better balanced than that of the original device.