Design, fabrication, and test of an on-chip micro flow cytometer with integrated out-of-plane microlenses

A micro flow cytometer with an integrated three-dimensional hydro-focusing unit and out-of-plane microlenses was successfully fabricated and tested. The entire system was fabricated with SU-8 ultra-violet lithography process. In the hydro-focusing unit, sheath flows pass through a trapezoid-shaped chamber with three 30° slopes to focus the sample flow in both horizontal and vertical directions. As an essential component in the on-chip optical detection system, integrated out-of-plane microlens was embedded in the sidewall of the fluid outlet channel in the detection area. A pre-aligned optical fiber holder was fabricated on chip to fix the output optical fiber in a position aligned to the microlens. Optical simulation and analysis were also conducted using commercial software Zemax. Numerical simulation results confirmed that the use of microlens substantially improved the detection efficiency by focusing the fluorescent light from the sample cells into the output optical fiber. Preliminary cell counting experiment was performed using the fabricated micro flow cytometer system and the experimental results proved the feasibility of the integrated micro flow cytometer design.     

SU-8 3D microoptic components fabricated by inclined UV lithography in water

Out-of-plane microlenses and microoptical fiber holder are two of the most important components for building an integrated microoptic system with a precise alignment accuracy. In this paper, a simple and convenient method to fabricate these components from SU-8 by using inclined UV lithography in water is proposed. It consists of two perpendicular exposures in SU-8 at ±45°. DI water possesses a low absorption coefficient and a moderate and stable value of refractive index in near UV. Using water, the exposure angle in SU-8 can be increased to 50° from 35° in air necessary to pattern the desired 45° slope of the sidewalls. The principle of the proposed technique and the detailed fabrication process of the microoptic components will be presented. The integratability of the fabricated components was demonstrated by the fabrication of the microoptical fiber holder with a pre-aligned out-of-plane microlens.     

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.     

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.     

Modeling and characterization of surface generation in fast tool servo machining of microlens arrays

A microlens array is composed of a series of microlens distributed in a regular pattern and has been used in a wide range of photonic products. Fast Tool Servo (FTS) machining is an enabling and efficient technology for fabricating high quality microlens arrays with submicrometer form accuracy and nanometric surface finish. Although there have been a number of studies on modeling and characterization of surface generation in Single Point Diamond Turning (SPTD), there is relatively little research on the modeling and characterization of surface generation in FTS machining of microlens arrays, which is radically different from SPTD and has additional process parameters. This paper therefore establishes a theoretical model for the prediction of surface generation in FTS machining of microlens arrays based on the cutting mechanism of FTS, cutting tool geometry, machining parameters, and the workpiece surface contour. A surface matching based method has been developed to characterize the surface quality of the microlens array as a whole instead of a single lens evaluation. A series of cutting experiments have been conducted, the actual results of which were found to largely agree with the predicted results. The successful development of the deterministic models and methods not only make the surface generation in FTS machining of microlens array more predictable, but also allow a better evaluation of the surface quality of the machined microlens array. It also helps to minimize or eliminate the need for conducting trial-and-error cutting experiments to optimize the machining process.     

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.     

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.

     

Replication of precise polymeric microlens arrays combining ultra-precision diamond ball-end milling and micro injection molding

A simple and novel combination of ultra-precision diamond ball-end milling and micro injection molding technique is described to produce precise microlens arrays out of polycarbonate (PC), polymethylmethacrylate (PMMA) as well as polystyrene (PS). The microlens arrays consist of 100 lenses in a 10 × 10 array with a lens radius of 273 μm, a lens diameter of 300 μm and a lens depth of 45 μm. Pitch between the lenses is fixed at 800 μm. The injection molding parameters were optimized to get precise microlens geometries with low surface roughness. The results show a precise diamond milled mold insert and injection molded microlens arrays with minor deviations in radius and surface roughness of the microlenses, particularly for microlens arrays out of PMMA.     

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.     

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.     

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.     

Fast replication of out-of-plane microlens with polydimethylsiloxane and curable polymer (NOA73)

Out-of-plane microlens, as its in-plane counterpart, is an important micro optics component that can be used in building integrated micro-optic systems for many applications. In earlier publications from our group, an ultra violet (UV) lithography based technique for out-of-plane microlens fabrication was reported. In this paper, we report a replication technology for time-efficient fabrication of out-of-plane microlens made of a curable polymer, NOA73. Microlens of cured SU-8 polymer was fabricated using a unique tilted UV lithography process, polydimethylsiloxane (PDMS) was molded using the resulting SU-8 master to form a negative mold, curable polymer NOA73 was then casted in the PDMS mold and out-of-plane microlens replica made of NOA73 was finally obtained after curing. The entire replication process took less than 5 h. Since PDMS negative mold was reusable, multiple replications of the microlens could be done with the same mold and each replication only took about 30 min. Scanning electron microscopic (SEM) images showed that NOA73 microlens replica had almost identical shape as the SU-8 master. In Comparison to the SU-8 microlens, microlens replica of UV curable polymer had slightly longer focal length and smaller numerical aperture due to the lower refractive index of NOA73. In addition, NOA73 microlens replica also had improved spectral transmission. Because of its compatibility with soft lithography technique, the reported replication process may also be used to integrate out-of-plane microlens into micro-opto-electro-mechanical-systems (MOEMS) and BioMEMS chips.     

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.     

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 multilayer systems combining microfluidic andmicrooptical elements for fluorescence detection

This paper presents the fabrication of a microchemical chip for the detection of fluorescence species in microfluidics. The microfluidic network is wet-etched in a Borofloat 33 (Pyrex) glass wafer and sealed by means of a second wafer. Unlike other similar chemical systems, the detection system is realized with the help of microfabrication techniques and directly deposited on both sides of the microchemical chip. The detection system is composed of the combination of refractive microlens arrays and chromium aperture arrays. The microfluidic channels are 60 μm wide and 25 μm deep. The utilization of elliptical microlens arrays to reduce aberration effects and the integration of an intermediate (between the two bonded wafers) aluminum aperture array are also presented. The elliptical microlenses have a major axis of 400 μm and a minor axis of 350 μm. The circular microlens diameters range from 280 to 300 μm. The apertures deposited on the outer chip surfaces are etched in a 3000-Å-thick chromium layer, whereas the intermediate aperture layer is etched in a 1000-Å-thick aluminum layer. The overall thickness of this microchemical system is less than 1.6 mm. The wet-etching process and new bonding procedures are discussed. Moreover, we present the successful detection of a 10-nM Cy5 solution with a signal-to-noise ratio (SNR) of 21 dB by means of this system     

Semi-ellipsoid microlens simulation and fabrication for enhancing optical fiber coupling efficiency

This paper demonstrates the effective coupling between a single-mode fiber (SMF) and an edge emitting laser diode (LD) using a semi-ellipsoid microlens on the SMF endface. The semi-ellipsoid microlens was fabricated using photolithography, thermal reflow, mold electroforming and polydimethylsiloxane (PDMS) injection. UV-curing glue was smeared and exposed to fasten the lens onto the fiber. This microlens allows increasing the fiber spot size and numerical aperture. Low loss, wide misalignment tolerance and low manufacturing cost could be achieved. The optical simulation-software “TracePro” was used to predict the coupling efficiency parameters for the LD to SMF. “Taguchi method” was used for the parameter design concept to improve product quality searching for the optimal lens design. The proposed method facilitates mass production to achieve a high-yield and high-coupling method that is suitable to be used in the commercial fiber transmission industry.     

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.     

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.     

一种强度调制型频率编码光纤环传感器阵列

提出一种基于强度调制技术的频率编码光纤环传感器。给出光纤传感阵列结构并阐述了多传感器准频分复用原理,分析了阵列中传感器的反射信号特征,并重点讨论了不同长度光纤环的谐振频率特性。结果表明,通过改变光纤耦合系数及选用合适的光纤环长度,可以提高传感系统的复用能力和分辨能力。系统采用频率跟踪解调技术以提高信噪比和探测灵敏度。此系统可用于准分布应变和温度的测量。