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The focus of this paper is on the use of SUEX Thick Dry Film Sheet (TDFS) laminates which DJ DevCorp is developing as a thick resist material in optical and X-ray lithography. Preliminary thick dry film sheets up to 1mm thickness were successfully prepared and patterned at the CAMD X-ray beamlines and presented at HARMST 2007. Recently, new results have been published using SUEX resist sheets in UV lithography showing great market potential including plating molds for metal microparts, polymer MEMS, multilayer microfluidics structures, BioMEMS, medical devices, wafer level packaging processes, and displays. The SUEX TDFS are available in a range of thicknesses from 100μm to 1mm or more and are pre-cut into a number of standard wafer sizes. This new material is a modified epoxy formulation containing an antimony-free photo acid generator (PAG) prepared under a highly controlled solvent-less process which provides uniform coatings between two throw-away layers of protective polyester film. As part of our initial studies resist layers of 250, 500 and 1000μm were laminated onto regular silicon wafers using a hot roll laminator at a speed of 1ft/min at 75°C. The entire substrate preparation takes about 1 hour and with practice users can prepare up to 10 substrates in this time which are typically ready to use within 2 hours. In our efforts to develop a commercially viable product we have conducted experiments using standard equipment available at CAMD (Quintel UV aligner and CAMD XRLM 1 and 4 beamline). Initial X-ray exposure tests were done with a bottom dose ranging between 100 and 400 J/cm3 and a top/bottom dose ratio of less than 3 for sheets up to 2mm in thickness. Exposure time for typical conditions of the CAMD storage ring (ring current ranging between 100 and 160mA, beam lifetime of about 10hrs at 100mA ring current) is about 10-15min for a 4' wafer. After exposure the samples were immediately post exposure baked between 70°C and 110°C using a convection oven, taken out and cooled to RT then relaxed up to 3 days before development to reduce stress. Development was done in PGMEA for up to 3 hours for the 1000μm thick samples followed by a short IPA rinse and drying in air. Very high aspect ratios of 100 or more have been routinely patterned with nearly perfectly straight sidewalls (~1-1.5μm deviation for a 1mm tall structure) and excellent image fidelity.

The objective of this effort was to fabricate a waveguide integrated in a polymer microfluidic chip in order to deliver excitation light to fluorescent probes contained in a fluidic channel. Instead of exciting the volume at a certain point along a fluidic channel, the goal herein was to excite all the probes contained along the length of the fluidic channel. An air-waveguide structure was designed and integrated into a polymer microfluidic chip. Fabrication of the microfluidic chip was carried out by double-sided hot embossing of poly methyl methacrylate (PMMA) in sheet form. The efficacy of the waveguide was evaluated by coupling light from a laser into it and testing the fluorescence intensity from dye contained in the microfluidic channel. The results demonstrate illumination of the entire length of the microfluidic channel with excitation wavelength light from the waveguide. Details of the design, fabrication process and initial experimental results are presented in the course of this paper.

In this research, first a modular polymer-based (PMMA) injection micromixer prototype has been designed, fabricated and tested. This micromixer is easy to be integrated into biochemical microfluidic systems under development for BioMagnetICs DARPA funded project at CAMD. To improve the mixing efficiency, layout of micronozzles of the mixer was optimized according to the simulation results. Also because SU-8, an epoxy-based negative photoresist, has high chemical resistance, an SU-8 injection mixer was designed and fabricated to run some biochemical sample liquids. Internal stress in patterned SU-8 structures has been reduced and multi-layer SU-8 processing has been successfully developed to fabricate SU-8 injection mixer.

Fluorescence calibration is usually done by preparing bio-samples with a series of concentrations and measuring their corresponding fluorescence intensities. A simplified approach is studied by using a microfluidic chip and microspheres. The fluorescence calibration can be carried out on the chip with only one concentration of the microspheres. Microspheres with the diameter of 1~5μm are very useful in bio-detection research. These microspheres are manufactured using high-quality, ultraclean polystyrene microspheres and loaded with a variety of proprietary dyes. They can be labeled with biotin-, NeutrAvdin-, streptavidin- and protein, which can be used as tracers for bio-detections. A microfluidic chip was successfully fabricated for the experiment, and preliminary experimental results have proved the feasibility of the approach for fluorescence calibration.

A method to fabricate a high precision X-ray mask for Ultra Deep X-ray Lithography (UDXRL) is presented in this paper by use of a single substrate. Firstly, an 8-μm layer of positive photoresist is patterned on a 500 μm thick beryllium substrate by use of UV lithography and 5 μm gold is electroplated out of a sulfite based commercial plating solution. Secondly, the photoresist is removed and 15 μm of SU-8 is spincoated and baked. The layer of SU-8 is patterned by use of an exposure from the backside of the substrate with a soft X-ray source, followed by post-exposure bake and development. An additional 5 μm layer of gold is electroplated on top of the first gold pattern thereby increasing the total thickness of the absorber on the X-ray mask to 10 μm. After the removal of the SU-8 resist, the second step of the process is repeated by use of a thicker layer of SU-8 (up to 100 μm) to obtain the high-precision and high-aspect ratio absorber pattern. Using this method, the maximum dimensional error of the fabricated gold pattern remains under 1 μm, while the smallest absorber feature size is 10 μm.

Challenging requirements in optical and BioMEMS application with high aspect ratios of microstructures in access of 20 and smallest structure details of less than 1 μm have motivated this work to irmpove x-ray mask fabrication. Several approaches to pattern an intermediate x-ray mask with the gold absorber thickness of 1.6-2.2μm using a 1μm thick silicon nitride membrane have been explored. E-beam lithography is employed for primary patterning and experimental results show that high energy (100keV) e-beam lithography is a very promising approach. So-called working x-ray mask can be fabricated from intermediate x-ray mask through x-ray lithography. More than 10μm thick PMMA x-ray resist has been coated on the silicon nitride membrane by multi-coating process without crack. First exposure results indicate that adhesion and stability of sub-micrometer structures wiht these heights is critical. In order to overcome these problems a novel approach has been proposed by coating resist on both sides of the silicon nitride membrane and simultaneous patterning of both sides using x-rays. First successful experimental results have been achieved for proving the feasibility.

The production of X-ray masks is one of the key techniques for X-ray lithography and the LIGA process. Different ways for the fabrication of X-ray masks has been established. Very sophisticated, difficult and expensive procedures are required to produce high precision and high quality X-ray masks. In order to minimize the cost of an X-ray mask, the mask blank must be inexpensive and readily available. The steps involved in the fabrication process must also be minimal. In the past, thin membranes made of titanium, silicon carbide, silicon nitride (2-5μm) or thick beryllium substrates (500μm) have been used as mask blanks. Thin titanium and silicon compounds have very high transparency for X-rays; therefore, these materials are predestined for use as mask membrane material. However, the handling and fabrication of thin membranes is very difficult, thus expensive. Beryllium is highly transparent to X-rays, but the processing and use of beryllium is risky due to potential toxicity. During the past few years graphite based X-ray masks have been in use at various research centers, but the sidewall quality of the generated resist patterns is in the range of 200-300 nm Ra. We used polished graphite to improve the sidewall roughness, but polished graphite causes other problems in the fabrication of X-ray masks. This paper describes the advantages associated with the use of polished graphite as mask blank as well as the fabrication process for this low cost X-ray mask. Alternative membrane materials will also be discussed.

Poly-methylmethacrylate (PMMA), a positive resist, is the most commonly used resist for deep X-ray lithography (DXRL)/LIGA technology. Although PMMA offers superior quality with respect to accuracy and sidewall roughness but it is also extremely insensitive. In this paper, we present our research results on SU-8 as negative resist for deep X-ray lithography. The results show that SU-8 is over two order of magnitude more sensitive to X-ray radiation than PMMA and the accuracy of the SU-8 microstructures fabricated by deep X-ray lithography is superior to UV-lithography and comparable to PMMA structures. The good pattern quality together with the high sensitivity offers rapid prototyping and direct LIGA capability. Moreover, the combinational use of UV and X-ray lithography as well as the use of positive and negative resists made it possible to fabricate complex multi-level 3D microstructures. The new process can be used to fabricate complex multi-level 3D structures for MEMS, MOEMS, Bio-MEMS or other micro-devices.

X-ray lithography is commonly used to build high aspect ratio microstructures (HARMS) in a 1:1 proximity printing process. HARMS fabrication requires high energy X-rays to pattern thick resist layers; therefore the absorber thickness of the working X-ray mask needs to be 10-50 μm in order to provide high contrast. To realize high resolution working X ray masks, it is necessary to use intermediate X-ray masks which have been fabricated using e beam or laser lithographic techniques. The intermediate masks are characterized by submicron resolution critical dimensions (CD) but comparatively lower structural heights (~2 μm). This paper mainly focuses on the fabrication of high resolution X-ray intermediate masks. A three-step approach is used to build the high resolution X-ray masks. First, a so called initial mask with sub-micron absorber thickness is fabricated on a 1 μm thick silicon nitride membrane using a 50KeV e beam writer and gold electroplating. The initial X-ray mask has a gold thickness of 0.56 μm and a maximum aspect ratio of 4:1. Soft X-ray lithography and gold electroplating processes are used to copy the initial mask to form an intermediate mask with 1 μm of gold. The intermediate mask can be used to fabricate a working X-ray mask by following a similar set of procedures outlined above.

Two types of Microfluidic bioanalytical systems were designed and fabricated in polymer substrates using the LIGA process. A continuous flow polymerase chain reaction (CFPCR) Microfluidic device was fabricated in polycarbonate (PC), which utilized isothermal zone and shuttling the sample through each zone to achieve amplification. A 20-cycle PCR amplification of a fragment of a plasmid DNA template was achieved in 5.3 min. The results were comparable to those obtained in commercial laboratory-scale PCR system. The second system consisted of a microchip contating a low-density array assembled into the Microfluidic channel, which was hot-embossed in poly(methyl methacrylate) (PMMA). The detection of low-abundant mutations in gene fragments (K-ras) that carry point mutations with high diagnostic value for colorectal cancer was successfully performed. The array accessed microfluidics in order to enhance the kinetic associated with hybridization.

In this paper, we present a quasi-three-dimensional micro-fluidic device that has been constructed using the LIGA technology at CAMD. The idea is centered in the modular construction of molded plastic devices. A primary master template was patterned into SU-8 and PMMA, from which we made a reversed insertion mold by electroplating Nickel on it. Chips were patterned by hot embossing and the complex structure was obtained by stacking one layer on top of the other. Alignment marks were placed in each different layer to allow the accurate positioning of the structures. Each layer is a 2-dimensional micro-fluidic system and liquids can go from one level to another level, back and forth, producing this almost three-dimensional behavior. This work aims to introduce concepts and features that will be a step towards a complete modularization of micro-fluidic devices.

During the past few years, graphite based X-ray masks have been in use at CAMD and BESSY to build a variety of high aspect ratio microstructures and devices where low side wall surface roughness is not needed In order to obtain lower sidewall surface roughness while maintaining the ease of fabrication of the graphite based X-ray masks, the use of borosilicate glass was explored. A borosilicate glass manufactured by Schott Glas (Mainz, Germany) was selected due to its high purity and availability in ultra-thin sheets (30 μm). The fabrication process of the X-ray masks involves the mounting of a 30 μm glass sheet to either a stainless steel ring at room temperature or an invar ring at an elevated temperature followed by resist application, lithography, and gold electroplating. A stress free membrane is obtained by mounting the thin glass sheet to a stainless steel ring, while mounting on an invar ring at an elevated temperature produces a pre-stressed membrane ensuring that the membrane will remain taut during X-ray exposure. X-ray masks have been produced by using both thick negative- and positive-tone photoresists. The membrane mounting, resist application, lithography, and gold electroplating processes have been optimized to yield X-ray masks with absorber thicknesses ranging from 10 μm to 25 μm. Poly(methyl methacrylate) layers of 100 μm to 400 μm have been successfully patterned using the glass membrane masks.

The Center for Advanced Microstructures and Devices (CAMD) at Louisiana State University (LSU) supports one of the strongest programs in synchrotron radiation based microfabrication in particular, in deep X-ray lithography (DXRL) in the USA. For taller microstructures above 500 micrometers height, a harder source has been made available at CAMD using a 5-pole 7T super-conducting wiggler that has been installed in one of the straight sections of the synchrotron ring. A beamline and exposure station designed for ultra deep X-ray lithography (UDXRL) has been constructed and connected to the wiggler. An in-air scanner system has been built and installed at the beamline in approximately 10m distance to the source point. The scanner system features a fully water-cooled mask and substrate assembly to allow accurate patterning of high aspect ratio microstructures. The performance of the entire exposure system has been qualified and being proved compatible to standard exposure tools. Simultaneous exposure of a stack of four graphite substrates with 500 micrometers thick PMMA resist layers illustrate the potential for a cost-effective mass production of LIGA microstructures at hard UDXLR sources. The samples have been exposed using a 600 micrometers thick beryllium mask with 50 micrometers gold absorber. Dose calculations for the stacked exposures and preliminary exposure results as well as measurements of patterning accuracy over structure height and structure quality are presented.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

For patterning thick photoresist films, x-ray lithography is superior to optical lithography because of the use of a shorter wavelength and a very large depth of focus. SU-8 negative resist is well suited to pattern tall, high-aspect ratio microstructures in UV optical and x-ray lithography with rapid prototyping capability due to its high sensitivity. The negative tone of the SU-8 resist offers advantages in fabricating multi-level and non-planar microstructures using x-ray lithography or a combination of x-ray and UV optical lithography. In this paper, we present a fabrication process for multi-level metallic mold insert by a combination of multi-layer SU-8 patterning, poly-dimethylsiloxane (PDMS) molding, and nickel electroplating to make final nickel mold inserts that are suitable for injection molding and hot embossing of plastics and ceramics.

Many MEMS applications require multi-level microstructures in which two or more levels have to be aligned to each other in the processing. In this paper a passive alignment system based on a mechanical registration method utilizing reference posts is described. A detailed analysis of the test results was conducted to reveal main error sources and estimate the accuracy of this alignment method. An alignment accuracy of +/- 5(mu) m between 2 layers has been achieved. The further work on improving the alignment accuracy and expending in this alignment method to graphite masks for multi-level X-ray or combined optical/x-ray lithography is proposed.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The LIGA process is used to fabricate micro-optical benches which allow to mount hybridically active and passive optical components with very high precision and without active alignment. Moreover, also micro mechanical structures like electro-mechanical actuators are fabricated on the same substrate. To avoid any lateral misalignment al fixing structures in the optical bench are produced in the lithography step. Due to the high precision of x-ray lithography lateral tolerances are in the range of 0.1 to 0.2 micrometers depending on thermal distortions. Thus, optical losses for these components are rather small. The potential of the free space concept based on LIGA technology for the fabrication of devices for optical telecommunication has been demonstrated by a bi-directional transceiver module as well as an optical bypass. In the case of the optical bypass element, a movable mirror is fabricated on the substrate together with the fixing elements. This movable mirror is the end face of an electro-static actuator which allows to move the mirror into the collimated light beam between two fibers and thus, change the direction of the light. For the first prototypes the losses in the beam without mirror are about 1.7 dB, whereas the losses in the deflected beam are about 4.5 dB.

Microtechnology offers the possibility to fabricate photonic devices. Especially the LIGA technique allows the fabrication of micro-optical benches for free-space optical set-ups, with mounts for inserting hybrid components structured to micrometer accuracy well aligned to each other. These optical benches are not only used to build up passive optical devices but also opto-mechanical systems have been realized. Also concepts to integrate active optical components like laser and photo diodes have been tested. In the paper the concept and the technology to fabricate hybrid micro-optical systems is described. Two systems, a bi-directional transceiver module and an opto- mechanical by pass switch have been fabricated and evaluated. Their performance data prove the ability of the concept of micro-optical functioning modules based on free- space micro-optical systems fabricated by LIGA technique and assembly.

Deep etch lithography of PMMA by the LIGA-technique or H+ lithography is a powerful tool for the fabrication of very high refractive microprisms. We have realized a microintegrated optical imaging system using microprisms fabricated by the LIGA-process and microlenses with a diameter of 250 micrometers . Thus the size of the imaging system is less than 2 mm+.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Deep-etch x-ray lithography allows the fabrication of micro-optical components with critical lateral dimensions of a few micrometers and structural heights up to several hundred micrometers. Using interferometric methods, the structural tolerances of the side walls of fabricated microstructures are shown to be about 0.3 micrometers per 300 micrometers of structural height. A parallel HeNe laser beam is easily passed through several microprisms, which is evidence for the high precision achievable in positioning micro-optical components on a baseplate. Focussing a light beam by cylinder lenses is also possible. The free two- dimensional shape and the high precision achievable in positioning allow fabrication of micro- optical components with integrated fiber fixing grooves in which multi-mode fibers are precisely positioned. Simple fiber connectors, e.g., fiber forks or Y-couplers, can be achieved. By using a light-guiding resist system, divergence losses can be diminished.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.