New high fill-factor triangular microlens array fabrication method using UV proximity printing

A simple and effective method for fabricating a high fill-factor triangular microlens array using the proximity printing in lithography process is reported. The technology utilizes the UV proximity printing by controlling the printing gap between the mask and substrate. The designed approximate triangle microlens array pattern can be fabricated in photoresist. This is because to the UV light diffraction deflects away from the aperture edges and produces a certain exposure in photoresist material outside the aperture edges. This method can precisely control the geometric profile of a high fill-factor triangular microlens array. The experimental results showed that the triangular photoresist microlens array could be formed automatically when the printing gap ranged from 240 to 840 μm. The gapless triangular microlens array will be used to increase the luminance for the backlight module of liquid crystal displays. An erratum to this article can be found at     

A novel fabrication method of micro-needle mold by using the micro-lens mask through contact printing

This study presents a novel and precision process for fabricating a microneedle mold. The process includes a microlens array mask with contact printing in ultraviolet lithography. This method can precisely control the geometric profile of a microneedle array without the use of an etching process. The micro tapered cone microneedle mold utilizes the microlens array mask with geometrical optics. The light passes through the microlens and a hole in a Cr film of a mask, and then radiates onto the photoresist film. The light transmitted through the microlens has an aligned focal point on the photoresist film. An optical system is set up to characterize the optical performance of the machined microneedle, and then compared with theoretical data. The results show that the length of the microneedle from the experiment is close to the derived results. Moreover, the length of the microneedle is significantly influenced by the height and diameter of the microlens. Therefore, this method could also simplify the process and reduce the time needed for the fabrication of the microneedle. The micro cone microneedle has have great potential in the area of the drug delivery applications.     

New high fill-factor dual-curvature microlens array fabrication using UV proximity printing

A new high fill-factor dual-curvature microlens array fabrication method using lithographic proximity printing process is reported. The proposed technology utilizes UV proximity printing by controlling a printing gap between the mask and substrate. The designed microlens array pattern with high density can produce a high fill-factor dual-curvature microlens array in photoresist. Because the UV light diffraction deflects away from the aperture edges and produces exposure in photoresist material outside the aperture edges, this method can precisely control the geometric profile of a high fill factor dual-curvature microlens array. The experimental results showed that the dual-curvature micro-lens array can be formed automatically in photoresist when the printing gap ranged from 360 to 600 μm. The gapless dual-curvature microlens array will be used to enhance the luminance uniformity for light-emitting diodes (LEDs).     

Microlens array fabrication by backside exposure using Fraunhofer diffraction

In UV-lithography, a gap between photoresist and UV-mask results in diffraction. Fresnel or near-field diffraction in thick positive and negative resists for microstructures resulting from a small gap in contact or proximity printing has been previously investigated. In this work, Fraunhofer or far-field diffraction is utilized to form microlens arrays. Backside-exposure of SU-8 resist through Pyrex 7740 transparent glass substrate is conducted. The exposure intensity profile on the interface between Pyrex 7740 glass wafer and negative SU-8 resist is modeled taking into account Fraunhofer diffraction for a circular aperture opening. The effects of varying applied UV-doses and aperture diameters on the formation of microlens arrays are described. The simulated surface profile shows a good agreement with the experimentally observed surface profiles of the microstructures. The paper demonstrates the ease with which a microlens array can be fabricated by backside exposure technique using Fraunhofer diffraction.     

Fabrication of microlens array with graduated sags using UV proximity printing method

A graduated microlens array is presented in this paper. The proposed device has the same aperture microlens with a gradually increasing sag in the substrate. The design produces gradual decrease in the focal length and intensity when the light passes through the graduated microlens array. This paper presents a new graduated microlens array fabrication method that uses a variable printing gap in the UV lithography process. This method can precisely control the geometric profile of each microlens array without using the thermal reflow process. The angles between the mask and photoresist were placed at 5°, 8°, 10°, 15°, and 20° using a fixture designed in this study. The mask patterns were ellipses with an isosceles triangle arrangement to compensate for the partial geometry.     

Concave microlens array mold fabrication in photoresist using UV proximity printing

This paper presents a simple and effective method for fabricating a polydimethyl-siloxane (PDMS) microlens array with a high fill factor. The proposed method utilizes the UV proximity printing and photoresist replication methods. A concave microlens array mold is made using a printing gap in a lithography process. Optical UV light diffraction of UV light is used to deflect light away from the aperture edges to produce a certain exposure in the photo-resist material outside the aperture edges. This method can precisely control the geometric profile of a concave microlens array. The experimental results show that a concave micro-lens array can be formed automatically in photo-resist when the printing gap ranges from 240 to 720 μm. A high fill factor microlens array can be produced when the control pitch distance between the adjacent apertures of the concave microlens array is decreased to the aperture size.     

Asymmetric focusing microlens array fabricated using off-axis lithography

An asymmetric microlens with a given inclination angle was fabricated. Two circular pattern masks with different diameters were used to form a metal pattern and photoresist column on the substrate using the photolithography process. The metal pattern on the substrate was used to control the asymmetric microlens profile using thermal reflow. A lift-off process was applied to the first lithography to precisely define the metal pattern. A second lithography used deviation counterpoint exposure to pattern the photoresist column. The photoresist column was converted into a rubbery state when its temperature was increased to its glass transition temperature (Tg) during the thermal reflow. The asymmetric microlens structure was formed by shifting the arc vertex of the microlens toward one direction taking into account the fact that the copper coating surface has superior hydrophobicity to the silicon substrate surface. A 55° asymmetric microlens array was fabricated in this research by properly controlling the copper pattern size and the offset of two centers.     

A roller embossing process for rapid fabrication of microlens arrays on glass substrates

This paper reports an innovative technique for rapid fabrication of polymeric microlens arrays based on UV roller embossing process. In this method, a thin flat mold is fabricated by electroforming of nickel against a microlens master. The thin Ni mold with microlens cavities is then wrapped onto cylinder to form the roller. During rolling operation, the roller pressing and dragging the UV-curable photopolymer layer on the glass substrate through the rolling zone, the microlens array is formed. At the same time, the microlens array is cured by the UV light radiation while traveling through the rolling zone. The technique can be developed to an effective roll-to-roll process at room temperature and with low pressure. In this study, a roller embossing facility with UV exposure capacity has been designed, constructed and tested. Under the proper processing conditions, the 100×100 arrays of polymeric microlens, with a diameter of 100 μm, a pitch of 200 μm and a sag height of 21 μm can be successfully fabricated.     

Design and fabrication of a freeform microlens array for uniform beam shaping

Freeform optics has become a practical solution to solving number of problems in modern optical design. In this paper, we proposed a fabrication method using the combination of ultraprecision diamond machining and microinjection molding to achieve high volume and low cost freeform microlens manufacturing. The freeform microlens array discussed in this research is capable of redistributing a collimated light into a pre determined, in this case, a uniform pattern. The optical design, slow tool servo diamond machining, microinjection molding process and optical measurement were discussed. The simple optical design provided a platform for freeform microlens calculation. Slow tool servo diamond broaching was selected to fabricate the mold insert. After the mold insert was fabricated, microinjection molding machine was utilized to replicate the optical geometry into plastic substrates. The freeform microlens array that was fabricated in this research could achieve light re-distribution at the target with approximately 80% uniformity. The research conducted in this paper can be readily implemented in optical industry.     

Ferrofluid-molding method for polymeric microlens arrays fabrication

This study proposes a method named as ferrofluid-molding method for polymer microlens array fabrication. In this method, the master of the mother mold for microlens molding is an array of ferrofluid droplets. We generated droplet arrays by inducing the droplet’s magnetic hydrodynamic instability under different magnetic fields, and used the field-dependent droplet dimensions to fabricate numerous mold cavities. By this we could fabricate arrays of microlens with different bottom area, height, radius of curvature, and focal length. From our analysis, all the fabricated microlens arrays possessed good uniformity, and the largest numerical aperture of our microlens array was found as 0.54. In addition, we also designed a light uniformity experiment to demonstrate a potential application of our microlens arrays.     

Modelling and simulation of forming process of the lithographically fabricated out-of-plane microlens using a cellular automata method

Three dimensional (3D) cellular automata (CA) model has been successfully applied in photoresist etching simulation in recent years. In this paper, a simplified 3D CA model is used to simulate the etching process of out-of-plane microlens fabricated on thick SU-8 photoresist. The simulation results are compared with experimental results. This CA model can be developed as a computer-aided design tool to predict the optimum process parameters during the forming of lithographically fabricated microlens.     

High-aspect-ratio photolithography for MEMS applications

High-aspect-ratio photolithography using a commercially available positive photoresist and a conventional contact mask aligner with standard UV light source is described. A multiple coating process is developed to obtain a photoresist thickness up to 23 μm while maintaining a smooth photoresist surface. Intimate contact between the mask and wafer is found to be most critical for high-resolution photolithography. Vacuum contact is found to be well-suited for this purpose. Additionally, edge bead removal is found to be of significant importance for intimate contact between the mask and the substrate. Prebake, exposure, and development conditions are optimized for resolution and aspect ratio. Maximum prebake temperature still allowing the photoresist to be developed is found to be the optimal temperature for obtaining high resolution. Prebake time distribution is optimized for avoiding residual stress in the photoresist, as well as maintaining high resolution, when multiple coating is applied. Minimum linewidth and spacing of 3.5 μm and 2.5 μm, respectively, and a maximum aspect ratio of 7.7 have been achieved in a photoresist thickness of 23 μm. Postbake improves the chemical resistance to subsequent processes, for example, electroless nickel plating using the photoresist as a mold. However, postbake also causes pattern distortion, which can be severe at times. Therefore, optimal process and design conditions for minimizing the pattern distortion have been studied     

Design and fabrication of microlens arrays as beam relay for free-space optical interconnection

In this paper we report the design and fabrication of a beam relay for free space optical interconnection using microlens arrays. Multiple microlens arrays with same focal lengths were designed and fabricated in an out-of-plane layout. This design can be easily integrated with silicon-based optical interconnection devices. The beam relay was fabricated using direct lithography of SU-8 photoresist, and then replicated using UV curable polymer molded with a PDMS intermediate mold. The optical performance was tested and the experimental results show that the optical performances are mainly limited by the aberration of microlenses. Further study needs to be conducted to improve the surface quality of the lenses to reduce the aberrations.     

Novel direct‐view LED backlight unit with an aspheric microlens array and a rough‐texture sheet

Abstract— This study proposes a novel direct‐view light‐emitting‐diode (LED) backlight unit with a high‐fill‐factor aspheric microlens array and a rough‐texture sheet. An aspheric microlens array and a rough‐texture sheet made from polysiloxine were used as the grating element and optical diffuser, respectively, which increases light‐extraction efficiency and improves luminance uniformity. The specific aspheric microlens‐array mold was fabricated by using a heating encapsulated air process based on a glass wafer. This microlens array has the features of high fill factor and square‐foot boundary with a continuous surface‐relief profile. This unique out‐of‐plane surface profile creates a square light pattern with uniform luminance, and thus composes a uniform large‐sized light pattern exactly in accordance with the layout of the LED array. The rough‐texture sheet, which can scatter light uniformly, was formed by fine‐grit‐sandpaper molding. Experimental results show that by using an aspheric microlens array and rough‐texture sheet reported here as the backlight diffusing components is highly effective in improving light uniformity at a wide viewing angle. An increase in illuminance by more than 10% was achieved in comparison with commercial backlight modules. Low cost in fabricating the aspheric microlens array and rough‐texture sheet is anticipated due to the simplicity of the process.     

Integration of polystyrene microlenses with both convex and concave profiles in a polymer-based microfluidic system

This paper reports a new technique of fabricating polystyrene microlenses with both convex and concave profiles that are integrated in polymer-based microfluidic system. The polystyrene microlenses, or microlens array, are fabricated using the free-surface thermal compression molding method. The laser fabricated poly(methyl methacrylate) (PMMA) sheet is used as the mold for the thermal compression molding process. With different surface treatment methods of the PMMA mold, microlenses with either convex or concave profiles could be achieved during the thermal molding process. By integrating the microlenses in the microfluidic systems, observing the flow inside the microchannels is easier. This new technique is rapid, low cost, and it does not require cleanroom facilities. Microlenses with both convex and concave profiles can be easily fabricated and integrated in microfluidic system with this technique.     

New production method of convex microlens arrays for integrated fluorescence microfluidic detection systems

A new method for producing microlens array with large sag heights is proposed for integrated fluorescence microfluidic detection systems. Three steps in this production technique are included for concave microlens array formations to be integrated into microfluidic systems. First, using the photoresist SU-8 to produce hexagonal microchannel array is required. Second, UV curable glue is injected into the hexagonal microchannel array. Third, the surplus glue is rotated by a spinner at high velocity and exposed to a UV lamp to harden the glue. The micro concave lens molds are then finished and ready to produce convex microlens in poly methsiloxane (PDMS) material. This convex microlens in PDMS can be used for detecting fluorescence in microfluidic channels because a convex microlens plays the light convergence role for optical fiber detection.     

New production method of convex microlens arrays for integrated fluorescence microfluidic detection systems

A new method for producing microlens array with large sag heights is proposed for integrated fluorescence microfluidic detection systems. Three steps in this production technique are included for concave microlens array formations to be integrated into microfluidic systems. First, using the photoresist SU-8 to produce hexagonal microchannel array is required. Second, UV curable glue is injected into the hexagonal microchannel array. Third, the surplus glue is rotated by a spinner at high velocity and exposed to a UV lamp to harden the glue. The micro concave lens molds are then finished and ready to produce convex microlens in poly methsiloxane (PDMS) material. This convex microlens in PDMS can be used for detecting fluorescence in microfluidic channels because a convex microlens plays the light convergence role for optical fiber detection.

     

Influence of residual layer on cross-sectional shape of thermal-reflowed photoresist structures

When photoresist structures are formed by employing various lithography technologies and followed by thermal reflow treatments, the mechanism that transforms the cross-section of a photoresist structure from a rectangular-shape into a circular-shape is seen as an integral constituent of the manufacturing method of microlens arrays. However, in the case, where a residual layer is absent, a photoresist film is completely exposed to the oncoming radiation down to the interface between the photoresist film and substrate. Even in the presence of a residual layer, it has been uncertain to the author if a photoresist structure with a circular cross-sectional shape could be obtained, and be made applicable for the fabrication of a microlens array. The author then executed a set of thermal reflow treatments under various conditions using a positive-tone photoresist AZP4903 known for its capability of forming relatively thick films. As a result, it became clear that the existence of a photoresist’s residual layer has large influence on the transformation of the cross-sectional shapes of photoresist structures. These observations can be attributed to whether the bottom surface of a photoresist structure is firmly fixed on a hard substrate, or if it happens to be in contact with a soft photoresist layer which can flow comparatively freely.     

Fabrication of microneedle array using LIGA and hot embossing process

We demonstrate a novel fabrication technology of the microneedle array applied to painless drug delivery and minimal invasive blood extraction. The fabrication technology consists of a vertical deep X-ray exposure and a successive inclined deep X-ray exposure with a deep X-ray mask whose pattern has a hollow triangular array. The vertical exposure makes triangular column array with a needle conduit. With the successive inclined exposure, the column array shapes into the microneedle array without deep X-ray mask alignment. Changing the inclined angle and the gap between the mask and PMMA (PolyMethylMetaAcrylic) substrate, different types of microneedle array are fabricated in 750–1000 m shafts length, 15^o–20^o tapered tips angle, and 190–300 m bases area. The masks are designed to 400–600 m triangles length, 70–100 m conduits diameter, 25–60EA/5 mm² arrays density, and various tip shapes such as triangular, rounded, or arrow-like features. In the medical application, the fabricated PMMA microneedle array fulfills the structural requirements such as three-dimensional sharp tapered tip, HAR (High-Aspect-Ratio) shafts, small invasive surface area, and out-of-plane structure. In the skin test, the microneedle array penetrates back of the hand skin with minimum pain and without tip break and blood is drawn after puncturing the skin. Hot embossing process and mold fabrication process are also investigated with silicon and PDMS mold. The processed tetrahedral PMMA structures are fabricated into the microneedle array by the additional deep X-ray exposure. With these processes, the microneedle array can be utilized as the mold base for electroplating process.The author thanks the staff in 9 C LIGA beamline, Pohang Light Source (PLS), Korea for their assistance on the fabrication process.     

Fabrication of cone-like microstructure using UV LIGA-like for light guide plate application

The microlens array is usually formed by thermal reflow of polymer disks and can be one microstructure of the light guide plate (LGP). Here, we propose an ultraviolet (UV) backside exposure technology to fabricate the photoresist cone-like microstructure on the PMMA substrate at room temperature and then use UV LIGA-like process to transfer the microstructure for the application of 3.6 in. (72 mm × 57.5 mm) LGP. The electroforming was used to transfer UV master mold to the inverse cone-like microstructure of nickel metal mold and then hot embossing was used for one more pattern transfer to the same cone-like microstructure on PMMA substrate. The optical microscope and alpha-stepper profiler were used to examine the morphology and profile of LGP microstructure. The optical luminance and uniformity of LGP were measured using BM9 luminance meter in comparison with commercial product. The light uniformity and luminance of the cone-like LGP microstructure reach 75–80% and 2,800–3,000 cd/cm², respectively which meet the requirements of commercial LGP.