SU-8 Photopolymer Research Articles

Taking internally wetted capillary electrospray emitters to the sub-ten-micrometer scale with 3D microlithography

Electrospray emitters are an obvious choice for miniaturized thrusters for a variety of applications on small satellites (e. g., CubeSats), as well as for other micropropulsion purposes. They are inherently small and require a relatively low electric power for operation, and therefore, they fulfill the requirements imposed due to the small volume of CubeSats. Electrospray emitters of the internally wetted capillary type were fabricated by 3D microlithography in the SU-8 photopolymer down to capillary diameters of about 10 μm. Thus, the emitters are an order of magnitude smaller in lateral dimensions than those fabricated by advanced 3D printing methods and still half an order of magnitude smaller than those made by planar photolithography in SU-8. Fabrication methods and process parameters are presented. Furthermore, the preliminary results of the electric characterization of the emission behavior are shown. The experiments show promising results regarding the fabrication quality and extraction behavior.

Capillary folding of patterned polymer polyhedra

This paper presents a method to fabricate SU8 photopolymer and carbon polyhedral structures. First, patterned origami films with fewer cross-linked compliant folds and more cross-linked stiff faces are produced through differential UV exposure. The obtained patterned films are heated to increase their compliance by increasing polymer chain mobility. These softened films with designated compliant fold regions are then folded towards their target polyhedral shape using capillary forces induced by droplets deposited on top of them. The polyhedral shape of the folded films is locked in position by cooling them to room temperature and subsequently completing their cross-linking by an additional UV exposure step. The obtained polymer polyhedral structures can be converted into isometrically shrunken carbon structures through pyrolysis. This method for fabricating three-dimensional carbon polyhedra opens new ways to potentially fabricate complex structures of different material and surface properties through techniques such as electroplating and electroless deposition.

Fabrication of Micro- and Nanopillars from Pyrolytic Carbon and Tetrahedral Amorphous Carbon.

Pattern formation of pyrolyzed carbon (PyC) and tetrahedral amorphous carbon (ta-C) thin films were investigated at micro- and nanoscale. Micro- and nanopillars were fabricated from both materials, and their biocompatibility was studied with cell viability tests. Carbon materials are known to be very challenging to pattern. Here we demonstrate two approaches to create biocompatible carbon features. The microtopographies were 2 m or 20 m pillars (1:1 aspect ratio) with three different pillar layouts (square-grid, hexa-grid, or random-grid orientation). The nanoscale topography consisted of random nanopillars fabricated by maskless anisotropic etching. The PyC structures were fabricated with photolithography and embossing techniques in SU-8 photopolymer which was pyrolyzed in an inert atmosphere. The ta-C is a thin film coating, and the structures for it were fabricated on silicon substrates. Despite different fabrication methods, both materials were formed into comparable micro- and nanostructures. Mouse neural stem cells were cultured on the samples (without any coatings) and their viability was evaluated with colorimetric viability assay. All samples expressed good biocompatibility, but the topography has only a minor effect on viability. Two m pillars in ta-C shows increased cell count and aggregation compared to planar ta-C reference sample. The presented materials and fabrication techniques are well suited for applications that require carbon chemistry and benefit from large surface area and topography, such as electrophysiological and -chemical sensors for in vivo and in vitro measurements.

Fabrication and testing of polymer-based capacitive micromachined ultrasound transducers for medical imaging

The ultrasonic transducer industry is dominated by piezoelectric materials. As an emerging alternative, capacitive micromachined ultrasound transducers (CMUTs) offer wider bandwidth, better integration with electronics, and ease of fabricating large arrays. CMUTs have a sealed cavity between a fixed electrode and a suspended metalized membrane. Manufacturing cost and sensitivity are limiting factors in current CMUTs that depend on the fabrication equipment and, especially, on the materials used. For widespread use of CMUTs, a much lower fabrication cost that uses inexpensive materials, which maintain or improve upon existing sensitivity, is needed. Herein, a new fabrication process is described for polymer-based CMUTs (polyCMUTs) using the photopolymer SU-8 and Omnicoat. The first ultrasound B-mode image of a wire phantom created with a 64-element linear array using synthetic aperture beamforming techniques is presented. A 12 VAC signal superimposed on a 10 VDC signal was used on the transmission side, and only a bias-tee, with no amplifiers, was used on the receiving side. The low operational voltage and high sensitivity of this device can be partially attributed to a pre-biasing condition on the membrane. By using a novel sacrificial layer combined with a top electrode embedded inside the membrane, we demonstrated that SU-8 can be used to manufacture CMUTs inexpensively. Moreover, the fabrication used relatively simple equipment, and the number of fabrication steps was reduced compared to traditional CMUT fabrication. This new fabrication process has the potential to increase the use of CMUTs in the ultrasound market, including the market for wearable transducers.

Enhanced signal coupling in wide-field fiber-coupled imagers.

Some high-performance imaging systems, including wide angle "monocentric" lenses made of concentric spherical shells, form a deeply curved image surface coupled to focal plane sensors by optical fiber bundles with a curved input and flat output face. However, refraction at the angled input facet limits the range of input angles, even for fiber bundles with numerical aperture 1. Here we investigate using a curved beam deflector near the focal surface to increase the field of view and improve spatial resolution at the edges of the field of view. We show the field of view of such an imager can be increased from approximately 60° (full width at half maximum intensity) to over 90° using an embossed refractive microprism array, where the prism angle varies across the aperture to maintain coupling. We describe a proof-of-principle experiment using a f = 17.8mm fiber-coupled monocentric singlet lens, and show that a local region of microprisms embossed into a thin layer of SU-8 photopolymer can increase the field of view by 50%.

Tight control of light beams in photonic crystals with spatially-variant lattice orientation.

Spatially-variant photonic crystals (SVPCs), in which the orientation of the unit cell changes as a function of position, are shown to be capable of abruptly controlling light beams using just low index materials and can be made to have high polarization selectivity. Multi-photon direct laser writing in the photo-polymer SU-8 was used to fabricate three-dimensional SVPCs that direct the flow of light around a 90 degree bend. The lattice spacing and fill factor were maintained nearly constant throughout the structure. The SVPCs were characterized at a wavelength of 2.94 μm by scanning the faces with optical fibers and the results were compared to electromagnetic simulations. The lattices were shown to direct infrared light of one polarization through sharp bends while the other polarization propagated straight through the SVPC. This work introduces a new scheme for controlling light that should be useful for integrated photonics.

Influence of SU8 Photopolymer for Silicon RFMEMS Packaging Applications

Epoxy based photopolymer–SU8 is evaluated as microwave packaging material for packaging of Radio Frequency Micro Electro Mechanical Systems (RFMEMS). Standard and Slotted Coplanar Wave guide (CPW) transmission lines are chosen as test vehicles. Transmission lines of lengths up to 10mm are formed on HRS substrates. The fabricated transmission lines are subjected to wafer level packaging. The measured S-parameter data shows a significant difference prior to and after packaging. Further, the effects due to the packaging could be model educing lumped components and the measured data are presented in this work.

Fabrication of Micro-Sized Specimen for Axial Compressive Strength Evaluation of Single Fiber Using UV Lithography and Its Testing Method

In this paper, a newly developed process to fabricate micro-sized specimen for axial compressive strength evaluation of single fiber using ultraviolet (UV) lithography was proposed. Technique of UV lithography, which was used in semiconductor production, was adopted to fabricate a test specimen that fiber was partially embedded in a photo-polymer. The photo-polymer SU-8 was used as a resist for high aspect ratio test specimen. A PBO fiber, which has excellent characteristics in tensile strength, tensile modulus and chemical resistance, was used as a test fiber material. Direct axial compression test was performed using the micro-compression testing machine with a flat indenter of 50 μm in diameter and two camera units. Finite element analysis was carried out to compute the internal stress distribution in the fiber under compression and to determine the optimum fiber length. The results showed that the proposed specimen fabrication method using UV lithography was demonstrated to be effective for direct measurement of the compressive strengths of PBO fibers. Finite element analysis revealed that the PBO fiber length in the compression test should be at least 20 μm to avoid the influence of fiber rooting in the resin. The experimental values obtained for the compressive strengths of PBO fibers exhibited Weibull distribution with a mean of 217 MPa, which is lower than the values generally obtained by other test methods such as the loop test or resin embedment test.

Large-Area, Near-Infrared (IR) Photonic Crystals with Colloidal Gold Nanoparticles Embedding

A polymeric composite material composed of colloidal gold nanoparticles (<10 nm) and SU8 has been utilized for the fabrication of large-area, high-definition photonic crystal. We have successfully fabricated near-infrared photonic crystal slabs from composite materials using a combination of multiple beam interference lithography and reactive ion etching processes. Doping of colloidal gold nanoparticles into the SU8 photopolymer results in a better definition of structural features and hence in the enhancement of the optical properties of the fabricated photonic crystals. A 2D air hole array of triangular symmetry with a hole-to-hole pitch of approximately 500 nm has been successfully fabricated in a large circular area of 1 cm diameter. Resonant features observed in reflectance spectra of our slabs are found to depend on the exposure time, and can be tuned over a range of near-infrared frequencies.

Applications of SU-8 in the development of a Single Ion Hit Facility

The Lund Nuclear Microprobe (LNM) has been adapted to be used as a Single Ion Hit Facility (SIHF) for proton cell irradiation experiments at low dose. In order to test the capabilities of the system, Ni dot arrays and artificial cells have been fabricated with the photopolymer SU-8 and common lithographic techniques. The primary purpose of the Ni dot arrays was to determine the targeting accuracy of the beam in vacuum and in air. Additionally, this sample was employed to evaluate the system performance during cell target irradiation experiments. The Ni dot arrays were also used for beam characterization. The artificial cells were originally fabricated to test the software for cell recognition and localization, developed and implemented at the LNM. However, this sample became very functional to make small adjustments at the irradiation chamber for cell irradiation experiments. A description of the samples, fabrication procedure and applications are presented in this paper.

Microchamber arrays for the identification of individual cells exposed to an X-ray microbeam

To identify individual cells exposed to a X-ray microbeam in a cell population, we developed a biocompatible microchamber-array chip using UV lithography of photopolymer SU-8. The center-to-center distance between microchambers is 50 mum including a wall of 15 mum height. Using the microchamber-array chip, we performed tracking of individual exposed cells. Sample cells loaded in a microchamber array were selectively irradiated with the X-ray microbeam under microscopic observation. All the irradiated cells were indexed by the array arrangement of the microchambers. For about 24 h of post-irradiation incubation, the irradiated cells were identified successfully by time-lapse observation. In addition, the induction of radiation effects was observed in identified cells using immunofluorescence.

Enclosed SU-8 and PDMS microchannels with integrated interconnects for chip-to-chip and world-to-chip connections

This paper presents the simulation, fabrication and testing of fluid-tight enclosed microchannels with integrated interconnects for modular snap-together chip-to-chip and world-to-chip connections. Components consisting of microchannels with integrated interconnect were fabricated in SU-8 photopolymer and polydimethylsiloxane (PDMS). Simulations were performed in ANSYS for component microchannels with chip-to-chip and world-to-chip interconnects in order to determine the expected flow profile and pressure drop. Components with world-to-chip structures were then experimentally pressurized to 252 kPa and 148 kPa for SU-8 and PDMS respectively, without leakage. Components with chip-to-chip interconnects were assembled in a hybrid configuration utilizing SU-8 and PDMS for individual components assembled in a chain, and pressurized to 124 kPa before leakage. The duality of these structures as both world-to-chip and chip-to-chip connections increases their usability since they can provide micro- to macro-connections, as well as chip-to-chip connections without needing to redesign the interconnection scheme.

Polymer mechanically interlocking structures as interconnects for microfluidic systems

This paper describes peg-in-hole type mechanical interconnects made of SU-8 photopolymer, polydimethylsiloxane (PDMS) and silicon for joining microfluidic components of disparate materials to create complex microinstruments. Interconnects assemble as compliant interlocking cylinders that deform when assembled with holes that are smaller by several to several tens of microns, thus providing mechanical attachment as well as fluid interconnects between substrates. Example structures have nominal outer radii of 100–200 µm, inner radii of 50–100 µm and lengths of 200–400 µm. ANSYS mechanical simulations are employed to analyze the optimal interconnect sizing for a mechanical attachment, as well as predict disassembly forces ranging from zero to 232 mN. The interconnects are fabricated and experimentally tested with a controlled-force linear actuator to yield disassembly forces ranging from 20 to 130 mN. Pressurization tests demonstrate that SU-8 interconnects hold water pressure to 200 kPa without leaking and PDMS interconnects hold air pressure to 21 kPa and water pressure to roughly 6.9 kPa.

Fabrication of vertical digital silicon optical micromirrors on suspended electrode for guided-wave optical switching applications

This paper presents a novel technological and design study of optical guided-wave MicroElectroMechanical Systems (MEMS) switch. We work on a concept of hermetic assembly in which a MEMS wafer is covered (top–down) by a crossing optical waveguide wafer. The mirrors are designed to be pulled up and inserted at waveguide intersection for N × N optical switching. Vertical micromirrors have been fabricated in SU8 photopolymer or in single crystalline silicon (on SOI substrate). The SU8 mirrors, coated with gold layer, exhibit vertical and smooth sidewalls that will be sufficient for optical use. For integration of silicon mirrors, we have developed a new simple two-step deep reactive ion etching (DRIE) pattern transfer that allow simultaneous fabrication of good quality silicon mirrors (20 μm height and 2 μm thickness) and of recessed mechanical platform (5 μm thick). The platform has been analysed through its resonance behaviour. The strong effect of mechanical non-linearity which is observed in clamped suspension can be exploited to increase the control range of the electrostatic actuator.

Investigation of the bond strength between the photo-sensitive polymer SU-8 and gold

We present the results from a thorough investigation of the bond strength between the photo-polymer SU-8 and Au. The data were obtained by pull-test experiments, below the glass transition temperature of the polymer. The different aspects that we investigated were: (i) the effect of using different adhesion promoters between the SU-8 and Au; (ii) the influence of the processing sequence, keeping either Au or SU-8 as the bottom layer; (iii) the importance of the UV exposure dosage of the SU-8. For comparison, also the bond strength between SU-8 and other materials was measured. For SU-8 and Au without any adhesion promoter we find a bond strength of 4.8 ± 1.2 MPa for a 7.5-μm-layer of SU-8. The value of the bond strength can be increased additionally by up to 75% using an adhesion promoter and fully optimising the processing conditions of the SU-8.

Fluidic encapsulation in SU-8 μ-reservoirs with μ-fluidic through-chip channels

A process has been developed that allows encapsulation of an aqueous solution within a micrototal analysis system (μ-TAS) device. This device, a microtransdermal glucose sensor called B-FIT is cast in the photopolymer SU-8. A 50 nl μ-reservoir is filled and sealed by means of bonding two cured SU-8 surfaces using a thin layer of uncured SU-8. The μ-reservoir is bound by two membranes of 5 and 20–40 μm thick. Upon electrolysis of the encapsulated fluid by applying 2.6 V (or more, depending on the desired rate of fluid release) to two electrodes within the μ-reservoir, expansion of the created gas instantly ruptures the 5 μm membrane in a controlled fashion at a scoring mark that was fabricated by thinning the 5 μm membrane to 2–3 μm by reactive ion etching. The fluid exits the μ-reservoir and moves up the adjacent μ-capillaries through the entire chip. The technique of encapsulating fluid within MEMS devices offers possibilities for drug delivery systems.

Photo-Polymer Wafer Bonding for Double Layer Transfer

AbstractTemporary bonding is necessary for the two-stage transfer of thin films from one substrate to another while preserving the original orientation of the film. The process developed in this study uses an SU-8 photo-polymer both as a bonding material and as a delamination material for layer transfer. SU-8 was spun onto an optically transparent handle wafer (e.g., quartz or glass) and then bonded to the layer to be transferred. The processing parameters such as pre-baking temperature and UV exposure dose affect the bonding strength. UV curing of the SU-8 through the backside of the handle wafer could be performed to obtain the ultimate bond strength at low temperatures. A laser pulse from a KrF excimer laser (λ = 248 nm and λ = 38 ns) at a fluence of 50-150 mJ/cm2 was used to delaminate the SU-8 bonding layer to complete the layer transfer.

Large Area Mems: Materials Issues and Applications

AbstractConventional MEMS devices are based on silicon micro-machining and their maximum size is limited by the wafer. In contrast, we are exploring micro-machining for large area applications on substrates such as glass using polymeric materials. Our research is focused on the photopolymer SU-8, and we apply the MEMS fabrication technology to large area image sensors and displays. There are many challenges concerning the materials and processes, since large area compatibility is essential and integration with large area electronics may be required. The adhesion of SU-8 to the underlying layers as well as stress in the SU-8 are important issues and surface treatments have been investigated. Two applications of SU-8 MEMS are discussed to illustrate large area applications:. First, in the fabrication of an X-ray imager, high aspect ratio SU-8 walls form a micro-patterned phosphor screen to increase the image resolution. Second, a similar approach of patterning SU-8 into arrays of micro-cells is applied to an electrophoretic display.