Design and fabrication of a micro Alvarez lens array with a variable focal length

A 5 × 5 micro Alvarez lens array mold was fabricated using a 5-axis ultraprecision diamond machine and an Alvarez lens array was manufactured by injection molding process. Unlike conventional processes for asymmetrical element fabrication such as small tool grinding, this research demonstrates slow tool servo broaching process that allows the entire Alvarez lens array to be accurately machined on a metal mold in a single operation. To further reduce manufacturing cost, injection molding was used to fabricate the Alvarez lens arrays. The mold and molded lenses were both measured using an optical profiler. All measured profiles showed a good agreement with design and surface roughness also indicated an optical surface finish. The functionality of the molded polymeric lens arrays was achieved when the focal lengths were varied by laterally translating the molded Alvarez lens array pair. This research is a demonstration of the capability of fabricating complex optics using the same approach.     

Fabrication and analysis of the reflowed microlens arrays using JSR THB-130 N photoresist with different heat treatments

This paper reports that the fabrication of the reflowed microlens by the negative tone JSR THB-130 N photoresist can be treated with different thermal treatments using hotplate and oven. The different disk or thin cylinder arrays with diameters of 40–70 μm and thickness of about 7.4 μm were patterned using photolithography technology, and baked at 220°C by two kinds of thermal treatments using hotplate and oven to form reflowed microlens arrays. The spot size of the refractive microlens was then measured by optical microscopy and the total focal length of refractive microlens was simulated by curve fitting the lens profiles. The resolution of the microlens arrays approaches to 400 dpi as coated with Hexamethyldisilizane material. The smallest spot size of about 2.72 μm at the nominal 40 μm microlensis is obtained by the oven heat treatment, and the shortest total focal length of about 150 μm at the nominal 40 μm microlens is achieved by the hotplate heat treatment. The reduced spot size and total focal length of the microlens could improve the density and performance of optical devices and imaging systems.     

Replication of microlens arrays by injection molding

Injection molding could be used as a mass production technology for microlens arrays. It is of importance, and thus of our concern in the present study, to understand the injection molding processing condition effects on the replicability of microlens array profile. Extensive experiments were performed by varying processing conditions such as flow rate, packing pressure and packing time for three different polymeric materials (PS, PMMA and PC). The nickel mold insert of microlens arrays was made by electroplating a microstructure master fabricated by a modified LIGA process. Effects of processing conditions on the replicability were investigated with the help of the surface profile measurements. Experimental results showed that a packing pressure and a flow rate significantly affects a final surface profile of the injection molded product. Atomic force microscope measurement indicated that the averaged surface roughness value of injection molded microlens arrays is smaller than that of mold insert and is comparable with that of fine optical components in practical use.This paper was presented at the Fifth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2003 in June 2003.The authors would like to thank Korean Ministry of Science and Technology for the financial supports via the National Research Laboratory Program (2000-N-NL-01-C-148) and RAYGEN Co., Ltd. for the technical help in using the 3D profile measuring system.     

Deep lithography with a low energy synchrotron source: fabrication of X-ray refractive single lenses

 Lithography as deep as 400 μm has been carried out to fabricate X-rays refractive lenses using a low energy synchrotron source (AURORA-2 S, 0.7 GeV). The lens made of PMMA has two parabolic curvatures with radii R=4 μm and apertures A=2(2Rz)^1/2=179 μm, thus the aspect ratio z/R=250 for its curvatures, which is too great for traditional techniques to achieve. Upon fabrication of the lenses, precision of the curvatures has been evaluated by digital imaging analysis. The lens can singly focus a beam of hard X-rays into several microns at a reasonable focal length F=1.5 m. Advantages of using a low energy source for the LIGA process will be discussed regarding problems such as thick absorbers demanded by the LIGA mask and heat-load occurring in thick resist layers. Received: 10 August 2001/Accepted: 24 September 2001     

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.     

Fabrication and testing of embedded microvalves within PMMA microfluidic devices

The integration of a PDMS membrane within orthogonally placed PMMA microfluidic channels enables the pneumatic actuation of valves within bonded PMMA–PDMS–PMMA multilayer devices. Here, surface functionalization of PMMA substrates via acid catalyzed hydrolysis and air plasma corona treatment were investigated as possible techniques to permanently bond PMMA microfluidic channels to PDMS surfaces. FTIR and water contact angle analysis of functionalized PMMA substrates showed that air plasma corona treatment was most effective in inducing PMMA hydrophilicity. Subsequent fluidic tests showed that air plasma modified and bonded PMMA multilayer devices could withstand fluid leakage at an operational flow rate of 9 μl/min. The pneumatic actuation of the embedded PDMS membrane was observed through optical microscopy and an electrical resistance based technique. PDMS membrane actuation occurred at pneumatic pressures of as low as 10 kPa and complete valving occurred at 14 kPa for ~100 μm by 100 μm channel cross-sections.     

Fabrication of microgratings on PMMA plate and curved surface by using copper mesh as X-ray lithography mask

We demonstrate experimentally the X-ray lithography technique to fabricate microgratings on a PMMA plate and on curved surfaces such as PMMA cylinder lens surfaces with X-ray lithography by copper mesh as mask. Some gratings with 12.7 μm pitches on the plate and on PMMA curved surface with large area (10 mm × 10 mm) by vertically moving or rotating the resist stage exposure are realized.     

Relationships between process, microstructure and properties of molded zirconia micro specimens

Due to size effects the mechanical behavior of micro-components with dimensions in the range of some 100 μm and structure details of about 10 μm differs markedly from those of larger components. This is a crucial aspect for the design of micro-components for applications where demands for high strength are critical. The present study, which was performed in the frame of the Collaborative Research Centre 499 (SFB 499), approaches this issue by investigating the relationship between production process, microstructure and the mechanical properties of micro-specimens made from zirconia using two different feedstocks. The specimens were produced by a sintering process. The sintering temperature was varied between 1,300 and 1,500°C. Mechanical and tribological behavior of the specimens was determined by three-point bending tests as well as static and sliding friction tests, respectively. Properties derived from these tests were then correlated to the surface states in the specimens such as porosity, edge radius and roughness. The strength of the micro-specimens was found to be significantly influenced by these surface features. Whilst low porosity alone is not sufficient for high strength, notch effects resulting from pores as well as surface roughness can lower the strength. With increasing edge radius the strength of the material also increases. The porosity, edge radius and surface roughness were mathematically correlated with the strength to allow for a forecast. Within the SFB 499 feedstocks with specific properties were designed and reliable processes were developed to guarantee desirable surface roughness and porosity in the specimens. A characteristic bending strength of about 2,000 MPa is realizable in the micro-specimens within a good statistical reliability. The tribological tests revealed that the wear properties of the zirconia micro-components are strongly dependent on the quality of the feedstock.     

Fabrication of metallic micromolds by laser and electro-discharge micromachining

A method of creating metallic micromolds with features that have high-aspect ratios is described in this paper. The proposed manufacturing process utilizes laser micromachining to cut the negative two-dimensional profiles of the desired microfeatures and fluidic network patterns on a 100 μm thick brass sheet. The positive relief of the cut pattern is then created by using electro-discharge micromachining (micro-EDM) die-sinking the metallic mask onto a brass substrate. The final substrate with the desired relief pattern becomes the molding tool used for either elastomer casting or thermoplastic hot embossing. To validate the proposed fabrication methodology and evaluate the quality of surface finishes, a brass mold master of a T-channel micromixer (50 μm width, 25 μm height) is developed and multiple replicated devices are cast on this mold using poly-di-methyl-siloxane (PDMS). The surface finish of both the original micromold master and final molded channels on PDMS are measured using an optical profiler and found to have a roughness of approximately 400 nm Ra. The ability of the proposed fabrication technique to create high-aspect ratio features is illustrated by manufacturing a Y-channel micromixer with an aspect ratio of 4. Experimental results are discussed and suggestions for improvement are presented.     

Polydimethylsiloxane Microlens Arrays Fabricated Through Liquid-Phase Photopolymerization and Molding

We report on polydimethylsiloxane (PDMS) microlens arrays fabricated through liquid-phase photopolymerization and molding. The gist of this fabrication process is to form liquid menisci of variable radii of curvature at an array of apertures through pneumatic control, followed by photopolymerization under ultraviolet radiance. The resultant polymerized structures are then transferred to PDMS utilizing two molding steps. By adjusting the pneumatic pressure during the process, a single aperture array can be used to fabricate PDMS microlens arrays with variant focal lengths. The liquid menisci are formed by liquid-air interfaces that are pinned at the top edges of the apertures along hydrophobic-hydrophilic boundaries generated through surface chemical treatments. The microlens arrays are optically characterized. Variant focal lengths from 2.35 to 5.54 mm and f-numbers from 1.27 to 5.88, dependent on the diameter of apertures and the applied pressure to form the liquid menisci, are achieved with this relatively simple process and match well with the physical model. Owing to the formation from the liquid-air interfaces, the surface roughness of microlenses is measured to be around 25 nm.     

Novel U-shape gold nanoparticles-modified optical fiber for localized plasmon resonance chemical sensing

A novel fiber-optic localized plasma resonance (FO-LPR) sensor composed of a U-shape optical fiber was proposed and demonstrated in this study. The U-shape optical fiber was fabricated by a femtosecond laser micromachining system. The dimensions of the U-shape zone were 100 μm in depth measured from the surface of the polymer jacket layer, 80 μm in width in the jacket layer, 60 μm in width in the cladding layer. The total length is 5 mm. After laser annealing treatment, the average surface roughness was 205.8 nm as determined by Atom Force Microscope (AFM). The exposed surface of the U-shape fiber was modified with self-assembled gold nanoparticles to produce the FO-LPR sensor. The response of the sensor shows that the signal increases linearly with increasing refractive index. The sensor resolution of the sensor was determined to be 1.06 × 10⁻³ RIU.     

Effect of wall roughness on performance of microchannel applied in microfluidic device

The influence of wall roughness on flow and heat transfer performance in microchannels at low Reynolds number is investigated in this paper. Two sizes of PMMA microchannels are fabricated by microinjection molding and the width is 20 μm and 800 μm respectively. The surface profile of bottom wall is described by the two-dimensional fractal geometry method and it is found there is error within 5% between surface roughness obtained by the fractal geometry method and actual roughness. Then, the effects of dimensionless relative roughness (5–7.5%), fractal dimension (1.5–1.8), aspect ratio (0.025–4) on the flow resistance and heat transfer performance are analyzed by numerical and experimental method respectively. Reynolds number considered here are 10–60. The results show that the better flow performance and heat transfer performance can be obtained with high aspect ratio of rectangular microchannel. However, increasing surface roughness not only increases the heat transfer performance, but also introduces a large flow resistance, which makes the friction coefficient rise sharply. As a result, surface roughness has great influence on the flow and heat transfer performance, and the most suitable surface morphology should be obtained according to the specific application.

     

Roll manufacturing of flexible microfluidic devices in thin PMMA and COC foils by embossing and lamination

Microfluidics on foil is gaining momentum due to a number of advantages of employing thin films combined with the capability of cost-effective high-volume manufacturing of devices. In this work, ultra-thin, flexible Y-microreactors with microchannels of 100 μm width and 30 μm depth were fabricated in thermoplastic polymer foils. The fluidic pattern was hot roll embossed in 125 μm thick poly-methyl-methacrylate (PMMA) and 130 μm thick cyclic-olefin-copolymer (COC) films using a dry-etched microstructured silicon wafer as a flat embossing tool in a laminator. The sealing of the channels was performed with two different techniques, one based on lamination of SU8 dry film resist (DFR) and the other one based on spin-coated poly-dimethylsiloxane (PDMS). Testing of the interconnected microreactor was carried out using two dye colorant solutions to demonstrate mixing.     

A contact lens with integrated micro solar cells

We report the design, construction, characterization and in vivo testing of contact lenses incorporating solar cells. A fabrication process is outlined yielding free-standing 500 × 500 × 10 μm³ single crystal silicon solar cells which are subsequently integrated into a contact lens. Collections of micrometer-scale solar cells are interconnected on the contact lens in order to maintain flexibility, cover the proper area, and take advantage of crystalline materials. The solar cells show maximum efficiency at wavelength 725 nm with conversion efficiency of 1.24% at 310 mV. The contact lenses were tested on live rabbits and no adverse effects were detected. Contact lenses equipped with solar cells can harvest usable power from the environment and pave the way for the deployment of stand-alone contact lens systems that can be used for health-status monitoring.     

Miniaturized plastic micro plates for applications in HTS

This paper is focused on the development of plastic nano titer plates for applications in high throughput screening (HTS). For screening systems with integrated confocal microscopes plastic chips have been fabricated by injection molding and injection compression molding which contain micro wells with volumes of 0.9 μl and 1.4 μl and bottom plates with thicknesses of 120 μm and 200 μm. In addition, plastic chips with through holes have been joined with 160 μm glass plates by an adhesive printing process. First fluorescence correlation spectroscopy (FCS) measurements show that the plastic plates with glass bottoms are qualified as screening grade FCS nano titer plates. Received: 15 May 1999 / Accepted: 19 May 1999     

Fabrication of three-dimensional hemispherical structures using photolithography

A photolithography technique using SU-8 and PDMS was developed to fabricate three-dimensional hemispherical structures. This technique utilized a mask-aligner and normal binary coded photomasks to generate hemispherical pits on SU-8, followed by PDMS molding to obtain an array of dome-shaped structures. Using this technique, a microfluidic device was fabricated with a patterning area that consisted of an array of 5 μm wells and dome-shaped structures with 10 μm diameter and 6 μm height. Encoded microbeads, 6 μm in size, were immobilized and patterned in the microfluidic device under flow conditions and a DNA hybridization experiment was performed to demonstrate the incorporation of encoded beads that would enable a high level of multiplexing in bioassays.     

Fabrication of sub-micron structures for MEMS using deep X-ray lithography

Fabrication techniques of microstructures with high resolution and high aspect ratio are necessary for practical microelectromechanical systems (MEMS) that have high performance and integration. In order to fabricate microstructures with sub-micron resolution and high aspect ratio, deep X-ray lithography has been investigated using the compact synchrotron radiation (SR) light source called “AURORA”. An X-ray mask for sub-micron deep X-ray lithography, which is composed of 1 μm thick Au as absorbers, 2 μm thick SiC as a membrane and 625 μm thick Si as a frame, was designed. In preliminary experiments, the following results were achieved: EB resist microstructures with an aspect ratio of 22 corresponding with 0.07 μm width and 1.3 μm height were formed; a 10 μm thick PMMA resist containing no warp was formed by direct polymerization, enabling more precise gap control.     

Aliquoting on the centrifugal microfluidic platform based on centrifugo-pneumatic valves

We present a new method for aliquoting liquids on the centrifugal microfluidic platform. Aliquoting is an essential unit operation to perform multiple parallel assays (“geometric multiplexing”) from one individual sample, such as genotyping by real-time polymerase chain reactions (PCR), or homogeneous immunoassay panels. Our method is a two-stage process with an initial metering phase and a subsequent transport phase initiated by switching a centrifugo-pneumatic valve. The method enables aliquoting liquids into completely separated reaction cavities. It includes precise metering that is independent on the volume of pre-stored reagents in the receiving cavities. It further excludes any cross-contamination between the receiving cavities. We characterized the performance for prototypes fabricated by three different technologies: micro-milling, thermoforming of foils, and injection molding. An initial volume of ~90 μl was split into 8 aliquots of 10 μl volume each plus a waste reservoir on a thermoformed foil disk resulting in a coefficient of variation (CV) of the metered volumes of 3.6%. A similar volume of ~105 μl was split into 16 aliquots of 6 μl volume each on micro-milled and injection-molded disks and the corresponding CVs were 2.8 and 2.2%, respectively. Thus, the compatibility of the novel aliquoting structure to the aforementioned prototyping and production technologies is demonstrated. Additionally, the important question of achievable volume precision of the aliquoting structure with respect to the production tolerances inherent to each of these production technologies is addressed experimentally and theoretically. The new method is amenable to low cost mass production, since it does not require any post-replication surface modifications like hydrophobic patches.     

Direct, high throughput LIGA for commercial applications: a progress report

Direct LIGA; LIGA without injection molding; has the potential to become a cost effective, high throughput form of LIGA. The process requires high energy photons; near 20,000 eV; which are best produced in facilities such as the X-ray ring at Brookhaven National Laboratory. The increased absorption lengths over lower energy photons eliminates the need for a membrane type X-ray mask. This in turn facilitates very large area X-ray masks fabricated from standard silicon wafers with 20 μm gold absorbers. The absorption length increase in PMMA to 2 cm is used to implement stacked PMMA exposures in which 1 mm thick PMMA layers are used to produce exposed PMMA sheets. These sheets are eventually solvent bonded to working substrates with plating bases. New high energy X-ray masks have been developed. Two exposure stations at Brookhaven are operational. The recently commissioned manufacturing exposure station which uses a 22 inch scanner which can expose four separate PMMA-mask combination is in the testing phase. Received: 7 July 1999/Accepted: 30 July 1999     

Micromachining of passive planar micromixer on poly (methyl methacrylate) substrate with excimer laser ablation

Excimer laser ablation technique was introduced into this work to fabricate a passive planar micromixer on the PMMA substrate. T-junction shaped and width-changed S-shaped microchannels were both designed in this micromixer to enhance mixing effect. The mixing experiment of distilled water and Rhodamine B with injection flow rate of 500 and 1,500 μm/s validates the mixing effectivity of this micromixer, and indicates the feasibility of excimer laser ablation in the microfabrication of μ-TAS device.