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This paper presents a computer-controlled multidirectional UV-LED lithography system for 3-D microfabrication. The presented UV-LED system has adopted adjustable or programmable high-intensity UV-LEDs as a light source enabling for photopatterning both thin and thick SU-8 photoresist process. The prototype of the proposed system comprises 5-by-5 surface-mounted type LEDs with customized collimation lens, a tilt-rotational sample holder for introducing multidirectional light exposure, and a computer-control asset for synchronized controls of the light source and the sample holder. An adjustable light intensity provides an ease of lithography both for a few micron feature size patterning and a millimeter thick photopatterning by providing optimal light exposure condition by considering diffraction, and absorption of the UV light. Together with the tilt-rotational sample holder, the multidirectional UV-LED lithography can fabricate various 3-D microstructures in a wide range of the photoresist film thickness. The 3-D fabrications include an array of micro triangle slabs, 1-mm tall pillar array, the different scale same 3-D geometry structures on the same substrate.

Hollow microneedles are extremely attractive to drug delivery domains with high demands from clinics and industry. However, its complicated fabrication processes have impeded its wide adoption. This paper presents a simple one-step fabrication method for hollow microneedles based on diffraction UV lithography and solid-liquid light propagation. The fabrication process utilizes bottom-up exposure of a liquid photosensitive resin through photomask patterns comprising a plurality of apertures. Hollow microneedles with various heights were fabricated in a range of 400 µm to 600 µm from a few minutes of UV exposure. The fabricated hollow microneedles were characterized with force-displacement tests showing a good tip strength of 0.35 N per single unit. A hollow fluidic test on a pig cadaver skin showed great potential for drug delivery. Also, batch fabrication with multiple height microneedles on a single substrate has demonstrated compatibility with the manufacturing process.

This paper presents a tiltable UV-LED lithography system for 3D microfabrication. Collimated light with adjustable intensity from 7-by-7 405-nm UV-LEDs was used as a light source. An orbital rotation of the UV-LED array provided the uniform light intensity. The key advantages of the proposed system create 3D light traces without the sample's tilt-rotational movement, which is used to cause non-uniform light exposure and sample instability. Also, the tiltable UV-LED lithography can directly pattern on a liquid photosensitive state resin that provides a rapid process without soft baking and post-exposure baking. Several 3D microstructures in hundreds-microns to millimeter heights were successfully fabricated including vertical pillars, inclined pillars, triangular slabs, single and dual microneedle, and multi-angled petals. The tiltable UV-LED lithography has a great potential for a 3D micromanufacturing area with various 3D MEMS applications.

This paper presents a microlens fabrication process using the timed-development-and-thermal-reflow process, which can fabricate various types of aperture geometry with a parabolic profile on a single substrate in the same batch of the process. By controlling the development time of the uncrosslinked negative photoresist, a state of partial development of the photoresist is achieved, called the timed development process. The thermal reflow process is followed after the timed development, which allows the photoresist to regain its liquid state to form a smooth meniscus trench surrounded by a crosslinked photoresist sidewall. Microlens with larger aperture size forms deeper trench with constant development time. With constant aperture size, longer developing time shows deeper meniscus trench. The depth of the meniscus trench is modeled in the relationship of the development time and aperture size. Other characteristics for the microlens including the radius of curvature, focal length, and the parabolic surface profile are modeled in the relationship of the microlens thickness and diameter. Microlens with circular, square, and hexagonal bases have been successfully fabricated and demonstrated where each geometry of the lens-bases shows different fill factors of the lens arrays. To test the fabricated lenses, a miniaturized projection lithography scheme was proposed. A centimeter-scale photomask pattern was photo-reduced using the fabricated microlens array with a ratio of 133, where the smallest linewidth was measured as 2.6 &micro, m.

This paper presents a diffraction ultra-violet (UV) lithography method for fabricating various micro-cone shape microneedle structures. A direct UV exposure to a liquid state photosensitive resin through a photomask generates a unique diffraction pattern of the light, in which the exposed area of the photosensitive resin becomes the needle structures. The change of liquid to solid-state of the resin by crosslinking also works as a light waveguide to create a novel sharp tip on top of the cone. Secondary and tertiary harmonic cone shapes are formed with further UV energy exposure. The proposed method is unique and versatile as it enables the formation of various cone-shaped microstructures with a straight or curved sidewall, such as tip-integrated cone and multiple harmonic cones having different heights and base shapes. The relationship between the microneedle height formed at different exposure energy and the corresponding geometry was also discussed. Insertion test and force-displacement test were conducted to demonstrate the functionality of the fabricated microneedle, and the results showed that the tip of each tested microneedle can withstand up to 0.15 N before breakage occurs. The fabricated cone-shaped microneedle with a sharp tip has a great potential for transdermal drug delivery microneedle application.

This paper reports transparency characteristics of the h-line in the microlithography for the fabrication of ultra-tall SU-8 microstructures. Since the h-line (405 nm) in the lithography system has higher transparency to the SU-8 photoresist compared to that of the i-line (365 nm), the h-line is more suitable for fabrication of ultra-tall SU-8 microstructures for the expanded design capability of the microdevices. The transparency of the h-line through the SU-8 was experimentally verified as a function of film thickness and exposure time. Approximately 62% to the initial intensity of the h-line was observed at the 2.4 mm thick SU-8 film. The transparency of the SU-8 was also decreased by the exposure time. The h-line exposure to the 2.4 mm thick SU-8 film for 3500 seconds decreased the SU-8 transparency to the h-line by 17.7% due to the crosslinking of the SU-8. With the proposed exposure scheme, an array of 2-mm tall micropillar array and 1.5-mm long inclined micropillar array were successfully fabricated. Also, 7-mm tall micropillar was fabricated as a state-of-art microstructure. As an application, an array of $1000-\mu \mathrm{m}$ -tall micropillars were successfully fabricated to form a 100-turn 3-D toroid inductor. The fabricated inductor showed average inductance of 950 nH in the frequency range of 0.1 to 10 MHz.

We present a photolithography scheme for ultra-tall, high-aspect-ratio microstructures. While increased height of microstructures can expand the design capability of various microdevices, it has been challenging to achieve the ultra-tall microstructure, 1 mm or higher, using a well-known negative photoresist, SU-8. One of the reasons is the high absorption rate of 365-nm ultra-violet light during the exposure process, although it used to be recommended for the SU-8 process. We report on optical characteristics of microlithography, in particular the 365- and 405-nm wavelengths, and present the lithography method for ultra-tall micropillars with a height of 1 mm or higher, called microtowers. While the 365-nm wavelength is experimentally validated with its high attenuation inside the SU-8, higher transparency of the 405-nm wavelength with a thicker SU-8 is reported to be suitable for ultra-tall micropillar structures. Assuming exposure time causes the color change of the SU-8, transparency of the SU-8 as a function of exposure time is measured with a thick SU-8. SU-8 microtowers with various heights are reported, including an array of 2000-µm-tall microtowers and a state-of-the-art 7000-µm microtower. To demonstrate usefulness of the proposed fabrication method, an array of 1000-µm-tall microtowers are successfully fabricated to form a 100-turn, 3-D toroid inductor. The fabricated inductor shows average inductance of 950 nH in the frequency range of 0.1 to 10 MHz, a low-frequency resistance of 5.4 Ω at 0.1 MHz, and a quality factor of 22 at 60 MHz.

This paper presents a double-side-exposure scheme based on computer-numerical-controlled (CNC) UVLED lithography for fine 3-D microfabrication. The CNC UV-LED lithography manipulates the exposure angle of the UV light where the system comprises a switchable, movable UV-LED array as a light source, a motorized tilt-rotational sample holder, and a computer-control unit. The double-sided exposure scheme utilizes an additional photomask on top of the photoresist-bearing photomask/substrate. The CNC UV-LED apparatus programmably introduces tilt-rotational UV light through the two photomasks via sequential exposure of each side by use of a synchronized switchable UV light source. This creates user-definable 3-D light traces, which are transferred into the photosensitive resist. This approach enables the fabrication of new 3-D structures that have previously been impossible to implement using single-side exposure lithography. Sample fabricated 3D microstructures include a micro spear, a micro golf club, a micro folder, and a micro bird. Arrays of micro ‘y’ and micro mushroom structures are demonstrated as batch fabrications.

This paper presents a computer-controlled multidirectional UV-LED (ultraviolet light-emitting diode) lithography system for three-dimensional (3-D) microfabrication, introducing the concept of CNC-lithography. The system comprises a switchable, movable UV-LED array as a light source and a motorized tilt-rotational sample holder, in which each system element is computer-controlled. This approach enables a relatively small size and overall portability of the system. The proposed system has two unique features; (1) the movable LED array improves the uniform UV light distribution over the substrate area; and (2) the switchable function of the LEDs is synchronized with the movement of the tilt-rotational sample holder, enabling the creation of 3-D patterns of substantial complexity. Unlike layer-by-layer additive approaches such as stereolithography, the CNC-lithography system can form fine shapes with no layering artifacts in a batch-compatible manner, at the expense of true arbitrariness of formable shape. When compared to conventional inclined lithography, CNC-lithography greatly increases ease of fabrication by eliminating multiple manual exposure steps and also enables the fabrication of new 3-D structures that would have been challenging to implement previously. Demonstration lithographic shapes, including a micro-‘pipe,’ a micro-‘hi,’ a micro-‘Calla lily,’ and a micro-‘cyborg’, are fabricated to illustrate the range of the process.

This paper presents a CMOS-compatible microfabrication process for on-silicon racetrack power inductors using prism-assisted lithography with an ultraviolet light-emitting diode (UV-LED) array as a light source. This combination of lithographic techniques was applied in making 60-μm-tall SU-8 structures with 45° sidewalls to achieve high performance from a sputtered nanogranular magnetic core material, Co-Zr-O. Microfabricated magnetic components with Co-Zr-O cores were designed, built, and tested for use in a 25-W dc-dc power converter operating at a switching frequency between 5-30 MHz. Fabricated inductors achieved a peak quality factor of 18 and an inductance of 660 nH at 15 MHz.

This paper presents a computer-numerical-controlled ultraviolet light-emitting diode (CNC UV-LED) lithography scheme for three-dimensional (3D) microfabrication. The CNC lithography scheme utilizes sequential multi-angled UV light exposures along with a synchronized switchable UV light source to create arbitrary 3D light traces, which are transferred into the photosensitive resist. The system comprises a switchable, movable UV-LED array as a light source, a motorized tilt-rotational sample holder, and a computer-control unit. System operation is such that the tilt-rotational sample holder moves in a pre-programmed routine, and the UV-LED is illuminated only at desired positions of the sample holder during the desired time period, enabling the formation of complex 3D microstructures. This facilitates easy fabrication of complex 3D structures, which otherwise would have required multiple manual exposure steps as in the previous multidirectional 3D UV lithography approach. Since it is batch processed, processing time is far less than that of the 3D printing approach at the expense of some reduction in the degree of achievable 3D structure complexity. In order to produce uniform light intensity from the arrayed LED light source, the UV-LED array stage has been kept rotating during exposure. UV-LED 3D fabrication capability was demonstrated through a plurality of complex structures such as V-shaped micropillars, micropanels, a micro-'hi' structure, a micro-'cat's claw,' a micro-'horn,' a micro-'calla lily,' a micro-'cowboy's hat,' and a micro-'table napkin' array.

This paper presents a UV lithography method that utilizes a UV-LED (ultraviolet light-emitting diode) array as a micropatterning light source for high aspect ratio polymer microstructure fabrication. The sidewalls of commercial 5-mm-diamater UV-LEDs were coated with an opaque polymer to suppress the side propagation of UV light and to enhance UV collimation, resulting in a maximum flare angle of 15° . The UV-LEDs were then assembled into a 10x10 array to form the light source. Although the flare angle of most conventional UV lithography tools is 2~3° , resulting in a slightly better light collimation quality, the polymer structures microfabricated with the UV-LED approach showed similar reproduction fidelity and similar exposure times to patterns fabricated using conventional lithography. The UV-LED approach has the potential for cost reduction as well as simplicity of initial system setup and management. This latter feature is of particular importance in advanced, inclined three-dimensional (3-D) lithography schemes to create complex 3-D structures. The UV-LED system has been demonstrated to perform satisfactorily in these advanced 3-D lithography schemes.

Three dimensional (3-D) nanofabrication using the dynamic mode multidirectional ultraviolet (UV) lithography has been explored, where the size of the photomask pattern is compatible to or smaller than the wavelength of the UV source and therefore the diffraction effect is prominent in photopatterning. Ray trace taking into account the effect of refraction, diffraction, and absorption has been simulated using an optical numerical analysis tool (COMSOL, Inc.). Subwavelength patterning with a pattern diameter of 300nm has been performed on a chromium coated glass substrate using E-beam lithography to form a photomask. A thin layer of SU-8, a negative tone photoresist, with a thickness of 1∼10 microns has been coated on the photomask and multidirectional UV lithography is performed through the mask, where the photomask servs as a substrate as well. Nanoscale pillars with a pattern diameter of 300nm have been fabricated with different optical doses and simulation results show good correlation with simulation results in terms of the shape and the height. Various 3-D structures including an inclined pillar array, a vertical triangular slab, a tripod embedded horn, and a triangular slab embedded horn have been successfully fabricated. A vertical triangular slab array has been demonstrated for a terahertz (THz) selective surface application.

Multiple height pillar arrays as well as various circular or parabolic microstructures are fabricated by the combination of the timed-development-and-thermal-reflow process and an additional ultraviolet (UV) exposure step. First, uncrosslinked photoresist (SU-8) in a confined region such as a via or trench from the initial lithography step is partially developed in a timed manner to obtain a different development depth while a subsequent thermal treatment above the glass transition temperature of the remaining uncrosslinked polymer allows the polymer to reflow to form an uneven but smooth round shape surface profile, which can be well described using the Young-Laplace equation. Second, an additional UV exposure step to the prepared uneven surface by the back side exposure scheme through a transparent substrate with prepatterned Cr patterns is performed to have pillar arrays with various heights. A 2 by 5 micropillar array with gradually changing heights is successfully demonstrated. Also, a microfluidic channel with a triangular cross section is fabricated for particle sorting.

A vertically placed monopole type bow-tie antenna has been fabricated using dynamic mode multidirectional ultraviolet (UV) lithography and polymer metallization on a non-transparent printing wiring board. A photopatternable liquid state polyurethane is utilized to define a bow-tie antenna backbone with a height of 4.5mm, a top portion width of 3mm, a bottom portion width of 800µm, and a flare angle of 30°, which allows a resonant radiation frequency of 12 GHz. The printing wiring board with a thickness of 508 µm has a drilled hole with a diameter of 800 µm through which backside tilting exposure has been performed to define the vertical bow-tie slab. Conductor backed coplanar waveguide (CB-CPW) has been used for feeding the air-lifted bow-tie antenna. The measurement results show a resonant radiation frequency of 12.34GHz and a 10dB-bandwidth of 7%. It shows an omnidirectional far-field radiation pattern based on simulation. Different shapes of air-lifted backbone for 12GHz resonant radiation has been fabricated and the performance has been simulated using a high frequency structure simulator tool.

A method to use a liquid-state refractive index matching medium is introduced to overcome the limit of the inclined angle for the three-dimensional (3D) microstructures by dynamic mode multidirectional ultraviolet (UV) lithography. The proposed approach uses an isolated container for an index matching medium without direct contact between the index matching medium and the sample, reducing the chance of contamination, simplifying the fabrication process, and broadening the selection of an index matching medium. In addition, the liquid container is designed to allow in-situ adjustable refractive index matching performance during a dynamic mode operation. A refracted angle of 58.5° for an incident angle of 67.5° has been obtained for an SU-8 structure using glycerol as an index matching medium. UV lithography using water, solvent, acid, oil, and starch syrup as an index matching medium has been demonstrated. Various microstructures with large inclined or flare angles such as a chevron shape, an ellipsoidal horn, and a chained wind vane are successfully demonstrated with dynamic mode multidirectional UV lithography.

A rapid three-dimensional (3-D) ultraviolet (UV) lithography process for the fabrication of millimeter-tall high aspect ratio complex structures is presented. The liquid-state negative-tone photosensitive polyurethane, LF55GN, has been directly photopatterned using multidirectionally projected UV light for 3-D micropattern formation. The proposed lithographic scheme enabled us to overcome the maximum height obtained with a photopatternable epoxy, SU8, which has been conventionally most commonly used for the fabrication of tall and high aspect ratio microstructures. Also, the fabrication process time has been significantly reduced by eliminating photoresist-baking steps. Computer-controlled multidirectional UV lithography has been employed to fabricate 3-D structures, where the UV-exposure substrate is dynamically tilt-rotating during UV exposure to create various 3-D ray traces in the polyurethane layer. LF55GN has been characterized to provide feasible fabrication conditions for the multidirectional UV lithography. Very tall structures including a 6-mm tall triangular slab and a 5-mm tall hexablaze have been successfully fabricated. A 4.5-mm tall air-lifted polymer-core bowtie monopole antenna, which is the tallest monopole structure fabricated by photolithography and subsequent metallization, has been successfully demonstrated. The antenna shows a resonant radiation frequency of 12.34 GHz, a return loss of 36 dB, and a 10 dB bandwidth of 7%.

This paper presents the fabrication and modeling of three-dimensional (3D) nanostructures by automated multidirectional ultraviolet (UV) lithography, which is a fast, cost-effective, manufacturable fabrication method. Multidirectional UV exposure is performed using a static UV light source equipped with a tilt-rotational substrate holder. A glass substrate with a nanopatterned chrome layer is utilized as both a photomask and a substrate, for which a backside UV exposure scheme is used. For the analytical modeling of the shape of fabricated nanostructures, UV exposure dosage, diffraction and refraction effects, and absorption rate are taken into account. For more accurate process predictive models, a commercially available multiphysics simulation tool is used. The structural shapes predicted from analytical calculation and simulation are compared with the fabricated ones for which various 3D nanoscale test structures are fabricated such as an inclined nanopillar array and a vertical triangular slab. Also, nanostructures with multiple heights are successfully implemented from single layer photoresist by controlling the UV exposure dosage and tilt angles. A tripod embedded horn and a triangular-slab embedded horn are demonstrated.

Computer-controlled dynamic mode multidirectional ultraviolet (UV) lithography has been demonstrated using a collimated UV light source, a substrate-holding stage equipped with two stepper motors (one for tilting and the other for rotation), a controller with programming software and a laptop computer. The tilting and rotational angles of the stage in motion are accurately controlled during UV exposure as programmed by the user to produce complex three-dimensional (3D) microstructures. Process parameters include the initial and final tilting and rotational angles of the stage, and the relative angular velocities of the two motors in addition to the normal fabrication process parameters of UV lithography such as optical dose, baking time, and developing time and condition. Symmetric patterns can be generated by a simple synchronous mode dynamic operation, where both the angular velocities of the tilting motion and the rotating motion are set equal or harmonically related. More complex and non-symmetric patterns can be obtained using a piecewise synchronous mode, where the relationship between the angular velocities of the two motors is described not with a single coefficient but with a set of coefficients. 3D structures fabricated from the synchronous mode operation include the four-leaf clover horn and the cardiac horn while the ones from the piecewise synchronous mode are a vertical triangular slab, a screwed wind vane and arbitrary shape horns. Ray trace simulation has been performed using a mathematical tool in a spherical coordinate system and the simulated 3D patterns show good agreement with the fabricated ones.