Controlled formation and mixing of two-dimensional fluids

We introduce a novel technique for the controlled spreading and mixing of lipid monolayers from multilamellar precursors on surfaces covered by the hydrophobic epoxy resin SU-8. The lipid spreads as a monolayer as a result of the high surface tension between SU-8 and the aqueous environment. A micropatterned device with SU-8 lanes, injection pads, and mixing regions, surrounded by hydrophilic Au, was constructed to allow handling of lipid films and to achieve their mixing at controlled stoichiometry. Our findings offer a new approach to dynamic surface functionalization and decoration as well as surface-based catalysis and self-assembly.     

金属基底上三维可动微机构的制作

在无背板生长工艺的基础上,以SU-8胶为牺牲层,直接在金属Ni基底上制作出了一种三维可动微机构．该器件的制作主要采用了SU-8厚胶光刻工艺、高深宽比结构的微电铸工艺以及牺牲层工艺．就SU-8胶与金属基底结合差、易产生气泡、去除困难,铸层与金属基底之间以及铸层与铸层之间结合不牢等问题进行了讨论,采用了过渡层工艺、梯度升温工艺以及酸洗等方法予以解决．对于在金属基底上通过微电铸制作微结构的工艺而言,由于基底可以直接作为微结构的一部分,因此该工艺的优势在于节省了微结构的电铸时间,降低了铸层的生长应力,提高了结构的整体强度,使微器件的可靠性大大增加．电铸时间由原来的几天甚至几周缩短为二十几个小时,避免了出现由铸层内应力引起的铸件结构变形的问题．     

Study of a chemically amplified resist for X-ray lithography by Fourier transform infrared spectroscopy

Future applications of microelectromechanical systems (MEMS) require lithographic performance of very high aspect ratio. Chemically amplified resists (CARs) such as the negative tone commercial SU-8 provide critical advantages in sensitivity, resolution, and process efficiency in deep ultraviolet, electron-beam, and X-ray lithographies (XRLs), which result in a very high aspect ratio. In this investigation, an SU-8 resist was characterized and optimized for X-ray lithographic applications by studying the cross-linking process of the resist under different conditions of resist thickness and X-ray exposure dose. The exposure dose of soft X-ray (SXR) irradiation at the average weighted wavelength of 1.20 nm from a plasma focus device ranges from 100 to 1600 mJ/cm(2) on the resist surface. Resist thickness varies from 3.5 to 15 mum. The cross-linking process of the resist during post-exposure bake (PEB) was accurately monitored using Fourier transform infrared (FT-IR) spectroscopy. The infrared absorption peaks at 862, 914, 972, and 1128 cm(-1) in the spectrum of the SU-8 resist were found to be useful indicators for the completion of cross-linking in the resist. Results of the experiments showed that the cross-linking of SU-8 was optimized at the exposure dose of 800 mJ/cm(2) for resist thicknesses of 3.5, 9.5, and 15 microm. PEB temperature was set at 95 degrees C and time at 3 min. The resist thickness was measured using interference patterns in the FT-IR spectra of the resist. Test structures with an aspect ratio 3:1 on 10 microm thick SU-8 resist film were obtained using scanning electron microscopy (SEM).     

Simple surface modification techniques for immobilization of biomolecules on SU-8

SU-8, an epoxy based negative photoresist polymer has found wide range of applications in the field of microfabrication based biosensors. SU-8 surfaces need to be modified in order to immobilize bioreceptors. We studied the possibility of grafting desired functional groups by means of simple chemical treatments under normal laboratory conditions. These chemical treatments involve the use of crosslinkers that are expected to react with epoxy groups or hydroxyl groups generated by acid/alkali treatment. Here, a comparison of the results obtained on surface modification using glycine and 11-mercapto undecanoic acid as crosslinkers is presented. Human Immunoglobin G (HIgG) was covalently immobilized to carboxylic acid on SU-8 surface using carbodiimide/succinimide chemistry. The activity of immobilized HIgG was verified by using fluorescence imaging of FITC tagged goat anti HIgG bound to the surface. Fluorescence imaging was used to determine the chemistry best suited to functionalize SU-8 surface for biosensor applications.     

An Analytical Model for Thermal Effect of Microcantilever-DNA Biosensors

The thermal effect of microcantilever-DNA biosensors is investigated by the energy method. Based on a liquid crystal theory for DNA solutions and a two-variable method for laminated cantilevers, an analytical model for nanomechanical cantilever motion under the combination of bio-interactions and thermal loadings is provided and then it is extended to T-shaped cantilevers. Then, the effects of chemo-physical properties of DNA biofilm (i.e., grafting density, nucleotide number, and ionic strength) and temperature change on deflections are discussed. In order to reduce noise signals, the controlling temperature and size optimization of cantilevers with different substrate materials and ionic strengths are also studied. Results show that SU-8 polymer cantilevers can preserve the sensitivity of molecule adsorption and thermal stability, which agrees well with the related experiments; the layer-to-layer thickness ratio of SU-8 polymer cantilevers should be as small as possible, while for silicon nitride cantilevers, there exists an optimal value. These results help to understand the sensitivity and reproducibility of biosensors.     

Durable diamond-like carbon templates for UV nanoimprint lithography

The interaction between resist and template during the separation process after nanoimprint lithography (NIL) can cause the formation of defects and damage to the templates and resist patterns. To alleviate these problems, fluorinated self-assembled monolayers (F-SAMs, i.e.?tridecafluoro-1,1,2,2,tetrahydrooctyl trichlorosilane or FDTS) have been employed as template release coatings. However, we find that the FDTS coating undergoes irreversible degradation after only 10 cycles of UV nanoimprint processes with SU-8 resist. The degradation includes a 28% reduction in surface F atoms and significant increases in the surface roughness. In this paper, diamond-like carbon (DLC) films were investigated as an alternative material not only for coating but also for direct fabrication of nanoimprint templates. DLC films deposited on quartz templates in a plasma enhanced chemical vapor deposition system are shown to have better chemical and physical stability than FDTS. After the same 10 cycles of UV nanoimprints, the surface composition as well as the roughness of DLC films were found to be unchanged. The adhesion energy between the DLC surface and SU-8 is found to be smaller than that of FDTS despite the slightly higher total surface energy of DLC. DLC templates with 40?nm features were fabricated using e-beam lithography followed by Cr lift-off and reactive ion etching. UV nanoimprinting using the directly patterned DLC templates in SU-8 resist demonstrates good pattern transfer fidelity and easy template-resist separation. These results indicate that DLC is a promising material for fabricating durable templates for UV nanoimprint lithography.     

Photoresist with low fluorescence for bioanalytical applications

The negative photoresist SU-8 has found widespread use as a material in the fabrication of microelectrical-mechanical systems (MEMS). Although SU-8 has been utilized as a structural material for biological MEMS, a number of SU-8 properties limit its application in these bioanalytical devices. These attributes include its brittleness, nonspecific adsorption of biomolecules, and high fluorescence in the visible wavelengths. In addition, native SU-8 is a poor substrate for cellular adhesion. Photoresists composed of resins with epoxide side groups and photoacids were screened for their ability to serve as a low-fluorescence photoresist with sufficient resolution to generate microstructures with dimensions of 5-10 microm. The fluorescence of structures formed from 1002F photoresist (1002F resin combined with triarylsulfonium hexafluoroantimonate salts) was as much as 10 times less fluorescent than similar SU-8 microstructures. The absorbance of 1002F in the visible wavelengths was also substantially lower than that of SU-8. Microstructures or pallets with an aspect ratio as high as 4:1 could be formed permitting 1002F to be used as a structural material in the fabrication of arrays of pallets for sorting adherent cells. Several different cell types were able to adhere to native 1002F surfaces, and the viability of these cells was excellent. As with SU-8, 1002F has a weak adhesion to glass, a favorable attribute when the pallet arrays are used to sort adherent cells. A threshold, laser pulse energy of 3.5 microJ was required to release individual 50 microm, 1002F pallets from an array. Relative to SU-8, 1002F photoresist offers substantial improvements as a substrate in bioanalytical devices and is likely to find widespread use in BioMEMS.     

In situ curing of sliding SU-8 droplet over a microcontact printed pattern for tunable fabrication of a polydimethylsiloxane nanoslit

A tunable process for polydimethylsiloxane (PDMS) nanoslit fabrication is developed for nanofluidic applications. A microcontact printing (μCP) of a laterally spreading self-assembled hexadecanethiol (HDT) layer, combined with in situ curing of a sliding SU-8 droplet, enables precise and independent tuning of a nanoslit-mold width and height using a single μCP master mold. The SU-8 nanoslit-mold is replicated using a hard-soft composite PDMS to prevent channel collapse at low (<0.2) aspect ratio (height over width). The fluidic characteristics as well as dimensions of nanoslits fabricated with various conditions are analyzed using a fluorescein sample and AFM images. Finally, concentration polarization-based sample preconcentration is successfully demonstrated at the nanoslit boundary where an electric double-layer is overlapped.     

Performance of SU-8 microchips as separation devices and comparison with glass microchips

Effective analytical performance of native, all-SU-8 separation microdevices is addressed by comparing their performance to commercial glass microdevices in microchip zone electrophoresis accompanied by fluorescence detection. Surface chemistry and optical properties of SU-8 microdevices are also examined. SU-8 was shown to exhibit repeatable electroosmotic properties in a wide variety of buffers, and SU-8 microchannels were successfully utilized in peptide and protein analyses without any modification of the native polymer surface. Selected, fluorescent labeled, biologically active peptides were baseline resolved with migration time repeatability of 2.3-3.6% and plate numbers of 112,900-179,800 m(-1). Addition of SDS (0.1%) or SU-8 developer (1.0%) to the separation buffer also enabled protein analysis by capillary zone electrophoresis. Plate heights of 2.4-5.9 microm were obtained for fluorescent labeled bovine serum albumin. In addition, detection sensitivity through SU-8 microchannels was similar to that through BoroFloat glass, when fluorescence illumination was provided at visible wavelengths higher than 500 nm. On the whole, the analytical performance of SU-8 microchips was very good and fairly comparable to that of commercial glass chips as well as that of traditional capillary electrophoresis and chromatographic methods. Moreover, lithography-based patterning of SU-8 enables straightforward integration of multiple functions on a single chip and favors fully microfabricated lab-on-a-chip systems.     

Micropallet arrays for the separation of single, adherent cells

The selection and collection of single cells from within a heterogeneous population is required to produce genetically engineered cell lines, to develop new stem cell lines, and for single-cell studies. We describe a new platform for the positive selection of single live mammalian cells while the cells remain adherent to their growth surface. Cells were grown on arrays of microfabricated, releasable elements composed of SU-8 polymer termed "cell pallets". The presence of air between the elements restricted the cells to the top surfaces of the pallets. Single pallets situated within large arrays of pallets were released on demand using a single, focused, laser pulse. The laser pulses were low in energy (2-5 muJ) and did not detach nearby, nontargeted pallets. Since the SU-8 pallets and the underlying glass substrate were optically transparent, the cells on the pallets could be visualized by microscopy before and after release. Over 90% of cells remained attached to the pallet during laser-based release. The feasibility of growing the cells from the released pallets into clonal colonies was demonstrated. The pallet array system permits adherent cells to be inspected using conventional microscopy and selected cells released for further analysis. The ability to assess cells while they remain adherent to a surface will broaden the number of attributes that can be utilized for cell separation, for example, cell shape, cytoskeletal properties, and other attributes.     

Fabrication of 3D copper oxide structure by holographic lithography for photoelectrochemical electrodes

We fabricated three-dimensional copper oxide structure by holographic lithography and electroless deposition. A five-beam interference pattern defined a woodpile structure of SU-8. The surface modification of SU-8 structure was achieved by multilayer coating of polyelectrolyte, which is critical for activating the surface for the reduction of copper. Copper was deposited onto the surface of the structure by electroless deposition, and subsequent calcinations removed the SU-8 structure and simultaneously oxidized the copper into copper oxide. The porous copper oxide structure was used as a photoelectrochemical electrode. Because of the highly porous structure, our structure showed higher photocurrent efficiency.     

Epoxy based photoresist/carbon nanoparticle composites

We have fabricated composites of SU-8 polymer and three different types of carbon nanoparticles (NPs) using ultrasonic mixing. Structures of composite thin films have been patterned on a characterization chip with standard UV photolithography. Using a four-point bending probe, a well defined stress is applied to the composite thin film and we have demonstrated that the composites are piezoresistive. Stable gauge factors of 5–9 have been measured, but we have also observed piezoresistive responses with gauge factors as high as 50. As SU-8 is much softer than silicon and the gauge factor of the composite material is relatively high, carbon nanoparticle doped SU-8 is a valid candidate for the piezoresistive readout in polymer based cantilever sensors, with potentially higher sensitivity than silicon based cantilevers.     

Fabrication of alignment structures for a fiber resonator by use of deep-ultraviolet lithography

We present a novel method to mount and align an optical-fiber-based resonator on the flat surface of an atom chip with ultrahigh precision. The structures for mounting a pair of fibers, which constitute the fiber resonator, are produced by a spin-coated SU-8 photoresist technique by use of deep-UV lithography. The design and production of the SU-8 structures are discussed. From the measured finesses we calculate the coupling loss of the SU-8 structures acting as a kind of fiber splice to be smaller than 0.013 dB.     

The anatase phase of nanotopography titania plays an important role on osteoblast cell morphology and proliferation

The surface properties of biomaterials play a vital role in cell morphology and behaviors such as cell adhesion, migration, proliferation and differentiation. Three different crystal phases of titania film (rutile, anatase and amorphous titania) with similar roughness were successfully synthesized by DC reactive magnetron sputtering. The surface roughness of each film was about 8-10 nm. Primary rat osteoblasts were used to observe changes in morphology and to evaluate cell behavior at the film surface. The number of the osteoblasts on anatase film was significantly higher than rutile and amorphous films after 36 and 72 h incubation. More importantly, synthesis of alkaline phosphatase was significantly greater by osteoblasts cultured on anatase film than on rutile and amorphous films after 7 and 14 days. In addition, the cells grown on the anatase phase film had the largest spreading area; the actin filaments in cells with regular directions were well defined and fully spreaded. The results indicate that the anatase phase of titania with nanoscale topography yield the best biological effects for cell adhesion, spreading, proliferation and differentiation. There are strong therapeutic prospects for this biomaterial film for osteoblast proliferation, with possible applications for orthopedic and dental implant.     

Microfabricated solid-state dye lasers based on a photodefinable polymer

We present a solid polymer dye laser based on a single-mode planar waveguide. The all-polymer device is self-contained in the photodefinable polymer SU-8 and may therefore easily be placed on any substrate and be integrated with polymer-based systems. We use as the active medium for the laser the commercially available laser dye Rhodamine 6G, which is incorporated into the SU-8 polymer matrix. The single-mode slab waveguide is formed by three-step spin-coating deposition: a buffer layer of undoped SU-8, a core layer of SU-8 doped with Rhodamine, and a cladding layer of undoped SU-8.     

SiC nanowire-based SU-8 with enhanced mechanical properties for MEMS structural layer design

In addition to being used for pattern transfer, the negative photoresist SU-8 is widely used as a structural material in microelectromechanical systems(MEMS). Due to its good photopatternability, SU-8 has lower manufacturing costs than many other materials, but its mechanical properties are relatively weak to some extent, which limits its performance. The mechanical properties of epoxy-like SU-8 can be enhanced by adding micro-or nano-fillers such as carbon nanotube, clay, and SiC nanowire, which have superior elastic modulus. In this study, SiC nanowires were used to improve the mechanical properties of SU-8 while the SU-8 retains its photopatternability.The SiC nanowires were uniformly dispersed in SU-8 by stirring and ultrasonication. SU-8 materials with different SiC nanowire contents were fabricated into dog bone samples by lithography. The elastic modulus, storage modulus, and damping factor of the samples were measured by the Dynamic mechanical analysis(DMA)Q800. The experiment result shows that the rigidity and toughness increased, and the damping reduced. The2 wt% SiC nanowires-reinforced SU-8 had a 73.88% increase in elastic modulus and a 103.4% increase in elongation at break. Furthermore, a spring component made by SiC-doped SU-8 could withstand greater acceleration.The SiC nanowires-reinforced SU-8 has the potential to meet higher requirements in the design and manufacture of MEMS and greatly reduce the manufacturing costs of MEMS devices.     

Single-mode rib optical waveguides on SOG/SU-8 polymer and integrated Mach-Zehnder for designing thermal sensors

This paper presents a successful design, realization,and characterization of single-mode rib optical waveguides on SOG/SU-8 polymers in order to highlight a new approach to designing heat sensors. The basic principle of this new thermal-sensing method relies on the differential thermal behavior regarding both acting arms of a micro Mach-Zehnder Interferometer(MZI). First, two families of single-mode straight rib waveguides composed of SOG/SU-8 polymers are analyzed. Hence, optical losses for TE/sub 00/ and TM/sub 00/ optical modes for structures on Si/SiO/sub 2//SU-8 have been estimated respectively as 1,36 /spl plusmn/ 0,02 and 2,01/spl plusmn/0,02 dB/spl middot/cm/sup -1/, while the second one composed of Si/SiO/sub 2//SOG/SU-8 presented losses of 2,33 /spl plusmn/ 0,02 and 2,95/spl plusmn/0,02 dB/spl middot/cm/sup -1/. Then, owing to modeling results, an experimental sensor is realized as an integrated device made up of SU-8 polymer mounted on a standard silicon wafer. When subjected to a radiant source, as a laser light (980 nm) is injected across the cleaved input face of the MZI, the significant change of output signal allows us to consider a new approach to measuring radiant heat flowrate. Experimental results are given regarding the obtained phase shift against the subjected thermal power. According to the modeling results, one can expect new highly sensitive devices to be developed in the next coming years, with advantageous prospective industrial applications.     

Biocompatible SU-8-Based Microprobes for Recording Neural Spike Signals From Regenerated Peripheral Nerve Fibers

A biocompatible neural microprobe constructed using well-established SU-8 microfabrication techniques is described that was designed to record fiber spike signals from regenerated axons within peripheral nerves. These microprobes features bipolar longitudinal gold electrodes recessed below the surface within “grooves” designed to guide the growth of regenerating axons along the length of the grooves and limit the number of fibers that come in contact with the longitudinal electrodes. In addition, screening microprobe toxicity using cultures of human skin fibroblasts, the biocompatibility of these SU-8 microprobes for neural interface applications, in particular, was specifically verified using primary cultures of two sensitive cell types found in peripheral nerves: purified Schwann cells and explanted dorsal root ganglion (DRG) neurons and their fibers. The SU-8 microprobes were surgically implanted into transected rat Sciatic nerves within a unique peripheral nerve regeneration tube. Long-term fiber spike signals were recorded with these SU-8 microprobes in 13 chronically implanted rats for periods from 4 to 51 weeks without any signs of tissue damage or inflammatory reaction.      

Electrophoretic transport in surfactant nanotube networks wired on microfabricated substrates

Nanofluidic devices are rapidly emerging as tools uniquely suited to transport and interrogate single molecules. We present a simple method to rapidly obtain compact surfactant nanotube networks of controlled geometry and length. The nanotubes, 100-300 nm in diameter, are pulled from lipid vesicles using a micropipet technique, with multilamellar vesicles serving as reservoirs of surfactant material. In a second step, the nanotubes are wired around microfabricated SU-8 pillars. In contrast to unrestrained surfactant networks that minimize their surface free energy by minimizing nanotube path length, the technique presented here can produce nanotube networks of arbitrary geometries. For example, nanotubes can be mounted directly on support pillars, and long stretches of nanotubes can be arranged in zigzag patterns with turn angles of 180 degrees. The system is demonstrated to support electrophoretic transport of colloidal particles contained in the nanotubes down to the limit of single particles. We show that electrophoretic migration velocity is linearly dependent on the applied field strength and that a local narrowing of the nanotube diameter results from adhesion and bending around SU-8 pillars. The method presented here can aid in the fabrication of fully integrated and multiplexed nanofluidic devices that can operate with single molecules.     

Imprinted laminate wafer-level packaging for SAW ID-tags and SAW delay line sensors

We have developed a wafer-level packaging solution for surface acoustic wave devices using imprinted dry film resist (DFR). The packaging process involves the preparation of an imprinted dry film resist that is aligned and laminated to the device wafer and requires one additional lithography step to define the package outline. Two commercial dry film solutions, SU-8 and TMMF, have been evaluated. Compared with traditional ceramic packages, no detectable RF parasitics are introduced by this packaging process. At the same time, the miniature package dimensions allow for wafer-level probing. The packaging process has the great advantage that the cavity formation does not require any sacrificial layer and no liquids, and therefore prevents contamination or stiction of the packaged device. This non-hermetic packaging process is ideal for passive antenna modules using polymer technology for low-cost SAW identification (ID)-tags or lidding in low-temperature cofired ceramic (LTCC) antenna substrates for high-performance wireless sensors. This technique is also applicable to SAW filters and duplexers for module integration in cellular phones using flip-chip mounting and hermetic overcoating.