Photoresist Microstructures Research Articles

Fabricating SU-8 Photoresist Microstructures with Controlled Convexity–Concavity and Curvature through Thermally Manipulating Capillary Action in Poly(dimethylsiloxane) Microholes

We present a simple, robust, and cheap microfabrication method, based on thermally manipulating capillary action in poly(dimethylsiloxane) (PDMS) microholes, for preparing SU-8 curved microstructures. The microstructure morphology including convexity-concavity and curvature can be controlled via tuning the formation temperature. The convex SU-8 microspherical crowns with a height of 40 μm were formed at 10 °C, whereas the concave SU-8 microspherical crowns with a height of 90 μm were formed at 100 °C. The morphology of the microstructures is dictated by the thermally controlled combination of the pressure difference across the interface, contact angle, and surface tension. The fabricated microstructures with a spherical surface can be used as a microlens array or a mold for producing a microlens array. The clear and uniform images were observed using the generated microlens arrays. The equilibrium morphology of the microstructures can be predicted by numerical simulation, which can lessen the number of experiments and thus the design cost. The proposed method has the potential to find applications in industrial fields.

Precision-Microfabricated Fiber-Optic Probe for Intravascular Pressure and Temperature Sensing.

Small form-factor sensors are widely used in minimally invasive intravascular diagnostic procedures. Manufacturing complexities associated with miniaturizing current fiber-optic probes, particularly for multi-parameter sensing, severely constrain their adoption outside of niche fields. It is especially challenging to rapidly prototype and iterate upon sensor designs to optimize performance for medical devices. In this work, a novel technique to construct a microscale extrinsic fiber-optic sensor with a confined air cavity and sub-micron geometric resolution is presented. The confined air cavity is enclosed between a 3 μm thick pressure-sensitive distal diaphragm and a proximal temperature-sensitive plano-convex microlens segment unresponsive to changes in external pressure. Simultaneous pressure and temperature measurements are possible through optical interrogation via phase-resolved low-coherence interferometry (LCI). Upon characterization in a simulated intravascular environment, we find these sensors capable of detecting pressure changes down to 0.11 mmHg (in the range of 760 to 1060 mmHg) and temperature changes of 0.036 °C (in the range 34 to 50 °C). By virtue of these sensitivity values suited to intravascular physiological monitoring, and the scope of design flexibility enabled by the precision-fabricated photoresist microstructure, it is envisaged that this technique will enable construction of a wide range of fiber-optic sensors for guiding minimally invasive medical procedures.

Microfabrication of Ni-Fe Mold Insert via Hard X-ray Lithography and Electroforming Process

In this research, a Ni-Fe mold insert for the efficient replication of high aspect-ratio microstructure arrays was fabricated via hard X-ray lithography and an electroforming process. For the X-ray exposure on a photoresist, a gold-based X-ray mask was prepared with conventional UV photolithography. The gold thickness was designed to be over 15 μm to prevent development underneath the absorber and to enhance the adhesion strength between the photoresist and substrate. By using the X-ray mask, a positive-type photoresist was selectively exposed to X-ray under an exposure energy of 4 kJ/cm3. Thereafter, the exposed region was developed in a downward direction to effectively remove the residual photoresist from the substrate. During the evaporation process, deionized water mixed with a surface additive prevented the bending and clustering of the photoresist microstructure arrays by lowering the capillary force, resulting in a defect-free mother structure for electroforming. Lastly, the mother structure was uniformly Ni-Fe electroformed on a conductive substrate without the formation of any pores or detachment from the substrate. Based on the proposed microfabrication process, a Ni-Fe mold insert with a 183 μm pattern size, 68 μm gap size, 550 μm height, 2116 microcavities and a hardness of 585 Hv was precisely manufactured. It can be utilized for the mass production of high aspect ratio metal and ceramic microstructure arrays in micro molding technologies.

Microelectroforming of freestanding metallic microcomponents using silver-coated poly(dimethylsiloxane) molds

Microelectroforming is not only a key step of LIGA (Lithographie, Galvanoformung, and Abformung) and LIGA-like techniques, but also a widely used method for fabricating freestanding metallic microcomponents. Currently, the commonly used molds, consisting of photoresist microstructures which are prepared by lithographic techniques and conductive substrates, limit the development of microelectroforming to a certain degree due to their high cost and complicated processing steps. In this paper, a novel low-cost and net-shaped microelectroforming method using silver (Ag)-coated poly(dimethylsiloxane) (PDMS) molds is demonstrated to fabricate freestanding metallic microcomponents. Pristine PDMS molds are first replicated from a microstructured master using soft lithography. Then polydopamine is adhered to the surfaces of the pristine PDMS molds and followed by electroless deposition of Ag nanoparticle coatings. Subsequently, metal atoms are electrodeposited on the Ag-coated PDMS molds to form metallic microcomponents. After microelectroforming, the Ag coatings are peeled off from the molds and adhered to the surfaces of the metallic microcomponents during the demolding process. However, the molds can be repaired repeatedly using electroless deposition of Ag coatings on the polydopamine films, and the Ag coatings adhered to the surfaces of metallic microcomponents can be completely removed by immersion in ammonia water. Therefore, the Ag-coated PDMS molds can be reused several times. The application of this method for fabricating freestanding metallic microcomponents is demonstrated by producing nickel microcomponents. The results show that freestanding nickel microcomponents with precise dimensions and low surface roughnesses can be reliably achieved. We believe that this method has great potential for fabricating metallic microcomponents, microparts, and micropatterns.

Design, simulation, and fabrication of three-dimensional microsystem components using grayscale photolithography

Grayscale lithography is a widely known but underutilized microfabrication technique for creating three-dimensional (3-D) microstructures in photoresist. One of the hurdles for its widespread use is that developing the grayscale photolithography masks can be time-consuming and costly since it often requires an iterative process, especially for complex geometries. We discuss the use of PROLITH, a lithography simulation tool, to predict 3-D photoresist profiles from grayscale mask designs. Several examples of optical microsystems and microelectromechanical systems where PROLITH was used to validate the mask design prior to implementation in the microfabrication process are presented. In all examples, PROLITH was able to accurately and quantitatively predict resist profiles, which reduced both design time and the number of trial photomasks, effectively reducing the cost of component fabrication.

A Microchip Electrophoresis Device Integrated with the Top-bottom Antiparallel Electrodes of Indium Tin Oxide to Detect Inorganic Ions by Contact Conductivity.

A microchip electrophoresis (ME) device with off-channel contact conductivity detection (C2D) was constructed using top-bottom antiparallel indium tin oxide (ITO) electrodes and a cross-type microchannel. The 500-m wide top-bottom antiparallel decouplers were found to effectively decrease the interference of the electrophoretic current. The cross-type microchannel was formed by bonding the patterned negative photoresist microstructures and the two top-bottom opposed ITO-deposited glass substrates. Five seconds of 150 V/cm injection field and the 100 V/cm separation field equipped with the C2D of AC 200 mV excitation voltage provided adequate ME operational parameters to obtain the K+ and Na+ peaks separation. The ME devices obtained good coefficients in a range of 11000 μM for the K+ and Na+ detection. The calculated limit of detection was 1 μM. This design for off-channel and top-bottom antiparallel electrodes shows that C2D ME devices have great potential for the measurement of inorganic ions.

Fabrication of three-dimensional microstructures in positive photoresist through two-photon direct laser writing

Three-dimensional (3D) microstructures with micron scale are fabricated in photoresist using two-photon direct laser writing with an infrared femtosecond laser at 800 nm. The positive photoresist of Novolak/diazonaphthoquinone (DNQ) is used for the fabrication of line structures and 3D microstructures. Linewidths of line structures are fabricated with laser power ranging from 1 to 15 mW and scanning speeds ranging from 5 to 50 μm s−1. The obtained linewidth is analyzed using an exposure kinetics model of DNQ for two-photon absorption. Both 3D inversed woodpile structures and helical structures are fabricated. Two-photon direct laser writing is used to fabricate microstructures in positive photoresist. Microstructures with helical and inversed woodpile shapes were fabricated with positive photoresist Novolak/DNQ using two-photon direct laser writing.

Fabrication of high-aspect-ratio microstructures in polymer microfluid chips for in vitro single-cell analysis

Technologies and methods of prototyping microfluidic devices are widely used in solving many biological problems and testing of operability of new microanalytic systems. This study is devoted to analyzing the features of the formation of microstructures in SU-8 photoresist and the preparation of replicas in polydimethyl siloxane by the soft lithography method. It has been shown that the aspect ratio of the resultant microstructures is determined by their shape, size, and the force of resist adhesion to the silicon substrate and the efficiency of the circulation of the developer around microstructures. In the replication of complex microstructures, an aspect ratio of ~25 is attained. The technology considered here is used to prepare microfluidic chips with mechanical traps for fixation and the in vitro analysis of living cells.

Development of low-fluorescence thick photoresist for high-aspect-ratio microstructure in bio-application.

In this study, we propose and evaluate a novel low-auto-fluorescence photoresist (SJI photoresist) for bio-application, e.g., in gene analysis and cell assay. The spin-coated SJI photoresist has a wide thickness range of ten to several hundred micrometers, and photoresist microstructures with an aspect ratio of over 7 and micropatterns of less than 2 μm are successfully fabricated. The emission spectrum intensity of the SJI photoresist is found to be over 80% less than that of the widely used SU-8 photoresist. To evaluate the validity of using the proposed photoresist in bio-application for fluorescence observation, we demonstrate a chromosome extension device composed of the SJI photoresist. The normalized contrast ratio of the SJI photoresist exhibits a 50% improvement over that of the SU-8 photoresist; thus, the SJI photoresist is a versatile tool for bio-application.

Fabrication of a micro-tip array mold using a micro-lens mask with proximity printing

In this study, a mold for a micro-tip array is fabricated using a microlens array mask with proximity exposure. The micro-tip array uses a microlens array mask with geometrical optics. Light passing through a microlens is focused at the focal points. There is microlens on the mask and the pattern that results from the light passing through the mask is directly projected onto the photoresist surface. A concave profile is developed using a positive photoresist and the remaining photoresist microstructures are formed after the development process. By changing the distance between the mask and the photoresist and the radius of curvature of the microlens, various tip shapes can be fabricated. The exposure gap is calculated using the microlens array mask and the geometry of the mold of micro-tip array is established using the irradiance absorption maps for the different levels. These methods respectively use the model of the positive photoresist and optical software. When electroforming a metallic micro-tip copy of the patterned photoresist, masters are created. The metal micro-tip array is used membrane probe card.

Optimized SU-8 Processing for Low-Cost Microstructures Fabrication without Cleanroom Facilities

The study and optimization of epoxy-based negative photoresist (SU-8) microstructures through a low-cost process and without the need for cleanroom facility is presented in this paper. It is demonstrated that the Ultraviolet Rays (UV) exposure equipment, commonly used in the Printed Circuit Board (PCB) industry, can replace the more expensive and less available equipment, as the Mask Aligner that has been used in the last 15 years for SU-8 patterning. Moreover, high transparency masks, printed in a photomask, are used, instead of expensive chromium masks. The fabrication of well-defined SU-8 microstructures with aspect ratios more than 20 is successfully demonstrated with those facilities. The viability of using the gray-scale technology in the photomasks for the fabrication of 3D microstructures is also reported. Moreover, SU-8 microstructures for different applications are shown throughout the paper.

Study on Fabrication Process of SU8 Photoresist Microstructures and Evaluation of Stress Gradient

A fabrication process to manufacture SU8 photoresist microstructures is presented in which BP212 positive photoresist was used as sacrifice layer and SU8 was used as structure layer. No crack has been observed in the obtained microstructures. The relation between PEB temperature and stress gradient in SU8 film has been studied by measuring radii of released SU8 cantilevers made at different PEB temperatures.

Formation of submicron buckling patterns in thin metal film on micro-scale substrate by pyrolysis of photoresist

A method for fabricating micro-scale structure with submicron buckling pattern on the surface is proposed. In the fabrication, photoresist substrate is prepared using photolithography; a metal film with specified thickness is sputtered on the substrate. Then the photoresist micro-structure is pyrolyzed. During the pyrolysis, the micro-structure shrinks, and compressive stress is created in the metal film. When the compressive stress exceeds a critical value, the metal film buckles and various patterns appear. The influence of process parameters on the morphology, wavelength, and amplitude of the buckling pattern is investigated, and the mechanism of buckling is discussed based on the experiment results.

Control of inclination angle of glass-like carbon mold by defocus UV exposure on Si-containing photoresist

The authors fabricated micropatterns with inclined sidewalls in a glass-like carbon (GC) mold to thermal imprint on a glass substrate, and succeeded in controlling its inclination angle. The technology comprised three features: (1) A Si-containing photoresist was used with its etching resistance higher than that of the conventional photoresist; (2) the inclination angle of pattern's sidewalls was controlled by a defocus UV exposure technique; and (3) a GC substrate was etched to form a mold by using a Si-containing photoresist structure with inclined sidewalls as a masking layer; the technique was also used to control the inclination angle of the trench's sidewalls in the GC mold. The authors input different focus offset values to defocus the image in order to control the intensity distribution of UV lights within a film of Si-containing photoresist that was spin-coated on a polished GC wafer. In the next step, GC was etched by a 19:1 mixture of O2 and CHF3 which then created a trench structure with inclined sidewalls according to the variations in local masking thicknesses. By changing the focus offset in the UV exposure system from 0 to −12 μm, the inclination angle of the Si-containing photoresist microstructures was varied from 17° to 40°. With these changes, the range of the angles of GC molds became 6° to 17°. Then by thermal imprinting on Pyrex glass and quartz, these numbers expanded from 10° to 32°.

Deformation Mode Construction Using Photoresist Microstructure Devices Produced with Nanoindention Technology

In this study, we try to produce SU-8 photoresist microstructure devices using nano-imprint technology, and try to conduct nano-indention tests on SU-8 photoresist with nano-indention detector, in order to describe the behaviors and characteristics of nano-indentions on SU-8 microstructure devices and establish the deformation mode for the indention under nano-meter level. The tests tell us that, after nano-indention tests, the result indention hardness increases with the loading rate, indention repeats, and reduction of load or depth. Similarly, the indention hardness decreases because of reduction of loading rate, extension of loading time, and increase of load, and depth. Finally, we propose a deformation mode for nano-indention. This mode can also be used to explain the deformation behavior of SU-8 under nano-indention.

Replication and comparison of concave and convex microlens arrays of light guide plate for liquid crystal display in injection molding

AbstractA back light unit (BLU) is a key module of a thin‐film transistor liquid crystal display (TFT‐LCD), frequently utilized in various mobile displays. In this study, we experimentally characterize transcription and optical properties of concave and convex microlens arrays (MLAs) of light guide plate (LGP) fabricated by injection molding with polycarbonate as a LGP substrate material. Nickel mold inserts were manufactured by electroforming on the MLA, which was fabricated by the thermal reflow of photoresist microstructures patterned by UV‐photolithography. For the case of convex microlens, the height of replicated microlens was less than that of the mold insert while maintaining almost the same microlens diameter of the mold insert as the location of the microlens is far from the gate. In contrast, for the concave microlens, the diameter of replicated microlens was larger than that of mold insert, while showing almost the same microlens height as the mold insert. From the optical examination of replicated convex and concave MLAs, it was found that a higher luminance of the LGP was achieved by the concave MLAs compared to the convex MLAs with the same number density of microlenses (about 30% enhancement in this case) due to the utilization of a larger amount of light provided by the light sources. POLYM. ENG. SCI., 50:1696–1704, 2010. © 2010 Society of Plastics Engineers

A versatile diffractive maskless lithography for single-shot and serial microfabrication

We demonstrate a diffractive maskless lithographic system that is capable of rapidly performing both serial and single-shot micropatterning. Utilizing the diffractive properties of phase holograms displayed on a spatial light modulator, arbitrary intensity distributions were produced to form two and three dimensional micropatterns/structures in a variety of substrates. A straightforward graphical user interface was implemented to allow users to load templates and change patterning modes within the span of a few minutes. A minimum resolution of ~700 nm is demonstrated for both patterning modes, which compares favorably to the 232 nm resolution limit predicted by the Rayleigh criterion. The presented method is rapid and adaptable, allowing for the parallel fabrication of microstructures in photoresist as well as the fabrication of protein microstructures that retain functional activity.

Micromachining of electroformed nickel mold using thick photoresist microstructure for imprint technology

In order to fabricate polymer-based microstructures with feature sizes on the order of micrometers, we have been developing a microimprint technology with a fine nickel (Ni) mold instead of a conventional photolithography technique. The Ni mold was successfully fabricated by electroforming using a positive thick photoresist microstructure patterned on a silicon substrate as a replication master. The photoresist microstructure with excellent edge quality can be obtained under irradiation with single wavelength (g line) selected from a high-pressure mercury lamp. In addition, its sidewall angle in the range of 65° to 84° can be controlled precisely by varying the distance between a photomask and a photoresist surface. On the structured photoresist master, Ni was electroplated up to a thickness of about 110 μm, and then removed from the master. In this process, two-step electroplating at different current densities was carried out in order to prevent deformation of the photoresist master due to stress generated in a Ni electrodeposit. With the Ni mold, fine patterns with a width of 10 or 30 μm and a depth of 24 μm were almost completely transferred to polymetric materials (PMMA). The geometrical dimensions of the fabricated PMMA microstructures were found to be only about 10% reduction against the Ni mold.

Enhancement on forming complex three dimensional microstructures by a double-side multiple partial exposure method

This study presents a novel batch process based on standard lithography technology, called the double-side multiple partial exposure (DoMPE) method, which enhances the fabrication capability of three dimensional (3D) photoresist microstructures. By incorporating gray-tone lithography and double-side exposure techniques, the proposed DoMPE scheme extends the multilevel morphology on both the front side and back side of the suspended photoresist microstructures. Back-side gray-tone lithography is achieved by depositing various appropriate thicknesses of Ti film on glass substrate that acts as the gray-tone mask. The process parameters, including metal film thickness, developed depth, exposure dosage, development time, and soft-bake time, are experimentally characterized. Different 3D photoresist microstructures with multiple levels on the front and back sides are successfully fabricated and presented here to show the enhancement effect using the proposed technique, including a microinductor structure and a vertical comb drive structure that demonstrate potential applications even on electrically conductive devices.

Generation of submicrometer structures by photolithography using arrays of spherical microlenses

This paper describes the fabrication of arrays of spherical microlenses by self-assembly of microspheres and the use of these arrays for nearfield photolithography to generate repetitive microstructures in photoresist. We used these arrays of microspheres to fabricate two types of elastomeric membranes: (i) membranes that have microspheres embedded in their surface and (ii) membranes that have hemispherical wells in their surface. Both types of membranes act as amplitude masks that pattern the intensity of illumination in the near field incident on the photoresist. Microspheres in the first type of membrane act as convergent lenses that generate recessed microstructures in positive photoresist. Hemispherical wells in the second type of membrane act as divergent lenses that produce protrusive microstructures in positive photoresist. This method can generate dense, regular arrays of microstructures with a variety of profiles—circular or hexagonal holes, circular posts, hollow posts, and cones—depending on the sizes and refractive indices of the spherical lenses and the distance between the lenses and the photoresist. This technique provides a simple route to large areas (>4 cm 2) of repetitive, simple microstructures.