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The entire dental implant microenvironment is successfully replicated in the microfluidic model via unprecedented high-aspect-ratio microchannels separated by closely-spaced micropillars.

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.

Endovascular aneurysm repair (EVAR) is an established and reliable surgical treatment of abdominal aortic aneurysm (AAA), where a covered stent is implanted to prevent blood flow in the aneurysm sac. Although current EVAR is an effective technique, it requires long-term monitoring for the post-operative complications (e.g., endoleak, stent occlusion), which usually requires expensive radiologic imaging techniques such as ultrasonography, computed tomography (CT), or magnetic resonance imaging (MRI). In this paper, we present an accessible and continuous post-EVAR surveillance scheme by developing a smart stent that features ultrasonic powering, blood flow sensing, and integrated wireless electronics. As the body of the smart stent, a piezoelectric membrane with a custom pore architecture is utilized. The benefits of the embedded pore architectures include the expansion and contraction to comply with a minimally invasive procedure and enhanced ultrasonic powering and sensing capability. The smart stent is able to expand up to 112% in diameter when stretched due to the negative Poisson’s ratio. The ultrasound generates 0.23 mW of electrical power, which can sufficiently operate a low power wireless electronics. A comprehensive operation of the smart stent is demonstrated in vitro . The integrated flow sensor can measure blood flow ranging between 30 mL/min to 980 mL/min with an average sensitivity of 0.11 mV/mL/min. The wireless electronics transmit and receive such blood flow rate data by modulating the carrier frequency from 81 to 88 MHz from 1 m away. The developed smart stent prototype is the first step towards minimizing post-EVAR complications by providing early and frequent access to diagnostic information to patients and physicians.

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 simple in-house fabrication of a solenoid inductor using 3D printing, selective electroless plating, and electroplating for the inductor, and pressing for a highly laminated metal core. The frame of the solenoid inductor was 3D printed, where the design of the frame has a solenoid trench and ridge for wiring and insulator, respectively. The ridge of the frame was selectively coated with a novolac resin by dipping and rolling. The unique selective electroless plating showed the direct patterning to form a solenoid conductive path that came from the different surface activation. Electroplating to thicken the solenoid wire completes the inductor fabrication. A pressing and stacking of a nickel film form the highly laminated magnetic core for the inductor. A 10-turn solenoid inductor with a wire thickness of was successfully fabricated. An average inductance of 133 nH was measured in air-core condition. A 120 nickel layer cores were integrated into the inductor and showed the boosted inductance of 252 nH. The proposed simple fabrication has a great potential for various RF passive 3D devices including antennas, filters, and waveguides.

This paper presents frequency-selective 3D vertical pillars arrays that have been tuned to 15 GHz as a center frequency of the Ku-Band (12-18 GHz) for satellite applications. The vertical pillars were designed with a quarter wavelength height and a half lambda period at 15 GHz. Simple in-house fabrication methods including electroless and electro platings were introduced to fabricate the frequency-selective 3D vertical pillars arrays. An array of pillars was 3D printed together on the substrate. The electroless plating was introduced to deposit a seed copper layer over the device, where the pillar and the substrate were metalized simultaneously. An additional copper layer was electroplated over the sample to thicken the copper layer considering the skin depth at 15 GHz. The completed sample was electrically characterized using a network analyzer with two horn antennas operating at 15 GHz. Various 3D structures were successfully fabricated and showed great potentials for frequency selective surface applications.

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.

Microneedle arrays have been proposed in a wide range of biomedical applications, such as transdermal drug delivery and sensing. However, a scalable manufacturing process of precise microneedle fabrication of the microneedle has been challenged. This paper demonstrates UV-lithography-based one-step fabrication of fine-tuned bell-tip microneedles using a combination of light diffraction and the self-aligned lens effect. Microscale photopatterns can derive the predictive diffraction patterns where the higher light intensity at the center of the photopattern solidifies the liquid photoresist and forms a microlens shape in a self-aligned manner. The light through the microlens focuses down to a sharp point to form a conical shape for the body of the microneedle. Then light propagation through the vertex of the cone causes light emission, creating a fine bell-tip. The described light propagation behavior was characterized and explained in terms of the light intensity distribution from the diffraction based on the extended Fresnel–Kirchhoff diffraction model. The optics finite element analysis software was used to verify the light propagation and the intensity distribution. The step-by-step fabrication process was demonstrated using biocompatible photosensitive resins and validated the light attenuation and the cross-linking energy. The 20 × 20 bell-tip microneedles' array was able to be fabricated from the predicted model. Finally, a microneedle array with various shapes and heights on the same substrate was fabricated by single light exposure, demonstrating numerous achievable shapes using the proposed microneedle fabrication method.

We report a novel fabrication method of creating thin flexible Parylene-based microelectrodes. The fabrication technique described here is based on inkjet printing and thus does not require conventional photolithography and thin-film deposition processes. First, silver microelectrodes are printed on poly-dimethylsiloxane (PDMS) spin-coated on a silicon wafer. Then a thin Parylene layer is deposited on top of the PDMS layer followed by peeling-off. The silver microelectrode patterns are flawlessly transferred to the Parylene film during this process. Finally, sintering of the silver pattern is achieved in vacuum oven. Narrow (40–100 μm) silver microelectrodes were perfectly transferred from PDMS onto Parylene film without structural damages or significant drop in electrical resistance (Note that direct printing of silver microelectrode patterns on Parylene is very challenging due to the surface property of Parylene.). In fact, the silver microelectrodes transferred onto Parylene showed lower electrical resistance and better adhesion than the original silver microelectrodes printed on PDMS. In order to demonstrate the utility of thin flexible Parylene-based silver microelectrodes, we fabricated diverse microelectrodes and conducted dielectrophoretic (DEP) manipulation of microbeads with them. We believe that this rapid and low-cost fabrication method of creating thin flexible Parylene-based microelectrodes can be used in a variety of applications in flexible electronics, neural engineering, medical implants as well as MEMS and lab on a chip.

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.

Recent progress in 3D printing has brought many interests in developing functional devices in various applications by enabling the exploration of complex 3D structures. This paper presents the first fully metal printed standalone 3D solenoid microinductors using a high-power laser with a biocompatible Ti-alloy. Fine powder-based 6AL-4V Eli Titanium was directly printed using an automated gantry laser system with 60 μm resolution to form the inductors without any substrate. A substrate-less 10-turn solenoid as a testbed was printed out and characterized, and additional designs of 20 and 40 turn inductors were further explored. The wire of the inductor was designed to have a 700-micron square cross-section and a winding gap of 300 microns. The successfully fabricated 10-turn titanium-alloy solenoid inductor showed an average inductance of 130 nH, a low-frequency resistance of 0.8 ohms at 0.3 MHz, and a quality factor of 10.5 at 30 MHz respectively. Additional electrodeposition of copper to the 10-turn inductor demonstrated process compatibility with the conventional micromachining process. The standalone inductor printing method saves a significant volume from where the conventional substrate dimensions often take more than the volume of the device. Also, the standalone inductor can directly be used as a sensor device. Examples of force-displacement sensing applications were presented using the 10-turn and 19-turn round edge solenoid inductors.

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 fabrication of extremely thick SU-8 photoresist process in a height range Of $\mathbf{1000}\ \mu \mathbf{m}$ to $\mathbf{2000}\ \mu \mathbf{m}$ high-aspect-ratio microstructures. To differentiate conventional high-aspect-ratio tall structures less than $\mathbf{1000}\ \mu \mathbf{m}$, a new term, microtower, was adopted for the microstructure with a height of $\mathbf{1000}\ \mu \mathbf{m}$ or taller. In fabrication process, to avoid a long softbaking process from the thick photoresist and provide uniform film thickness, an SU-8 dry film was adopted. Ultra-violet (UV) light exposure with an acrylic filter was investigated in terms of transparency of the applicable wavelength spectrum. An array of the $\mathbf{1000}-\mu \mathbf{m}$ tall microtower array was fabricated without the acrylic filter resulting in wavy sidewalls. Whereas an array of $\mathbf{2000}-\mu \mathbf{m}$ tall microtower was fabricated with the acrylic filter resulting straight sidewalls. An array of $\mathbf{1500}-\mu \mathbf{m}$ -long inclined V-pillars was also successfully fabricated by adopting the UV exposure with the acrylic filter, and the fabricated microstructures have potential use as a microfilter in microfluidics. A state-of-art 100-turn toroid inductor was successfully fabricated and electrically characterized resulting an average inductance of 950 nH in the frequency range of 0.1 to 10 MHz. A low-frequency resistance of $\mathbf{5.4}\ \Omega$ at 0.1 MHz and a quality factor of 22 at 60 MHz were measured.

We present a pressing and punching method for a highly nanolaminate metal-insulator structure as a simple and cost-effective fabrication technique. The proposed nanolaminate metal-insulator structure is suitable for a magnetic core of a solenoid inductor. Cobalt-Nickel-Iron as a soft magnet metal was deposited on a polyethylene film which serves as an insulator. The Cobalt-Nickel-Iron bearing polymer film was folded multiple times and pressed with heat. The pressed sample was then comprised of multiple submicron magnetic metal layers and insulating polymer layers. The sample was cut into pieces by a punching with a predetermined shape and wire-wound for inductor application. Each nanolaminate magnetic layer suppresses the space of the eddy-current while the stacked magnetic layers will provide enough magnetic volume for the performance as an inductor. As a test vehicle, a multilayer structure of submicron Cobalt-Nickel-Iron with polyethylene film as an insulation layer was fabricated and tested. The dimension of the multilayer structure was 4 mm width, 10 mm length, and 3 mm thickness. 3000 layers in a combination of CoNiFe and polyethylene film were formed. A 10-turn with a standard magnet wire was wound to form a solenoid inductor. The test result through an impedance analyzer measurement exhibited an average inductance of 290 nH stably shown up to 100 MHz with peak quality factor of 130 at 95 MHz. The proposed method is a substrate-less process and therefore it is versatile, cost-effective, introducing easing fabrication and reducing processing time while it can achieve thousands of magnetic core layer.

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.

We present a microfluidic network-based combinatorial dilution device to generate on-demand combinatorial dilutions of all input samples in the range of a 3D simplex-centroid. The device consists of an initial concentration control module and a combinatorial dilution module. In the initial concentration control module, the concept of using a single common channel has been incorporated to generate desirable concentrations of each sample, diluted independently in response to variable input flow. Then, the diluted samples flow into the combinatorial dilution module to generate a full set of seven combinations from the three samples. First, we investigated the performance of the initial concentration controller by computational simulation (CFD-ACE+). The simulated output concentrations are extremely close to the expected theoretical values. Further, a PDMS-based initial concentration controller was fabricated, and its linearity and independency were tested with fluorescent dye. Then, we designed, simulated, and tested a combinatorial dilution device integrated with the initial concentration controller. Finally, as proof-of-concept, we performed a simple combinatorial cytotoxicity test with three drugs (Mitomycin C, Doxorubicin, and 5-FU) for MCF-7 cancer cells.

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 fabrication technique to develop highly laminated structures comprising stacked thin films, in which the structures are based on surface tension-driven assembly at the liquid-air interface. When multiple metallic films are removed from a liquid solution, there is a surface tension-driven coalescence and self-alignment of the wetted films, resulting in thick metallic microstuctures comprised of many layers of metallic nanosheets after evaporation of the liquid. If the liquid contains a dissolved material, each sheet can further be coated with the material prior to assembly. Based on this technique, we developed laminated structures comprising hundreds of nanoscale layers of alternating metallic film and non-conducting polymer. Electroplated Co 44 Ni 37 Fe 19 (Cobalt-Nickel-Iron alloy) and a commercial Novec 1700 solution (3M, Minnesota) were utilized for the metallic film and the liquid solution, respectively. However, the assembly process inherently allows a variety of materials to be exploited. Theoretical analysis and experimental results were compared, demonstrating a critical gap between the metallic films, below which capillary force is sufficient for driving self-assembly of the films. As an exemplary application of this technique, highly laminated magnetic cores comprising 600 layers of 500 nm thick CoNiFe that are insulated by 100 nm thick polymer were prepared. A 15-turn toroidal inductor with the fabricated magnetic core exhibited a constant inductance of 2.5 µH up to 30 MHz with a quality factor over 70 at 1 MHz.

We investigate the size- and composition-dependent ac magnetic permeability of superparamagnetic iron oxide nanocrystals for radio frequency (RF) applications. The nanocrystals are obtained through high-temperature decomposition synthesis, and their stoichiometry is determined by Mossbauer spectroscopy. Two sets of oxides are studied: (a) as-synthesized magnetite-rich and (b) aged maghemite nanocrystals. All nanocrystalline samples are confirmed to be in the superparamagnetic state at room temperature by SQUID magnetometry. Through the one-turn inductor method, the ac magnetic properties of the nanocrystalline oxides are characterized. In magnetite-rich iron oxide nanocrystals, size-dependent magnetic permeability is not observed, while maghemite iron oxide nanocrystals show clear size dependence. The inductance, resistance, and quality factor of hand-wound inductors with a superparamagnetic composite core are measured. The superparamagnetic nanocrystals are successfully embedded into hand-wound inductors to function as inductor cores.

An approach to the ultimate integration and miniaturization of MEMS-based 3-D magnetic components involves embedding the volume of the magnetic structures within the volume of the silicon wafer itself, exploiting microfabricated windings to create current paths, and utilizing embedded magnetic cores within the limited footprint of these components to boost the magnetic performance. However, this embedding approach imposes volumetric and microfabrication constraints that require an unusual magnetic component optimization methodology compared to wire-wound inductors and PCB inductors. These constraints dictate embedded toroidal inductors with non-overlapping windings and thin magnetic cores, and impose additional limitations on inductor design parameters such as pattern resolution, the number of winding turns and winding thickness; these constraints complicate the trade-offs to be made in designing core-integrated inductors. A design methodology encompassing these constraints is therefore needed. For a targeted inductance value within a given footprint, our design methodology addresses an inductor with a maximized quality factor based on the trade-offs between copper loss and core loss. To illustrate this methodology, silicon-embedded inductors with iron powder cores are designed and fabricated; a quality factor of 24 is achieved at 30 MHz.

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.

We investigate the ac magnetic behavior of solution processable, non-stoichiometric zinc ferrite nanocrystals with a series of sizes and zinc concentrations. Nearly monodisperse ZnxFe3−xO4 nanocrystals (x = 0–0.25) with an average size ranging from 7.4 nm to 13.8 nm are synthesized by using a solvothermal method. All the nanocrystals are in a superparamagnetic state at 300 K, which is confirmed by Superconductive Quantum Interference Device magnetometry. Due to the doping of non-magnetic Zn2+ into A site of ferrite, the saturation magnetization of nanocrystals increases as the size and Zn concentration increases. The ac magnetic permeability measurements at radio frequencies reveal that the real part of the magnetic permeability of similarly sized ferrite nanocrystals can be enhanced by almost twofold as the Zn2+ doping level increases from 0 to 0.25. The integration of 12.3 nm Zn0.25Fe2.75O4 nanocrystals into a toroidal inductor and a solenoid inductor prepared via a simple solution cast process yields a...

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.

The PowerChip research program is developing technologies to radically improve the size, integration, and performance of power electronics operating at up to grid-scale voltages (e.g., up to 200V) and low-to-moderate power levels (e.g., up to 50W) and demonstrating the technologies in a high-efficiency light-emitting diode driver, as an example application. This paper presents an overview of the program and of the progress toward meeting the program goals. Key program aspects and progress in advanced nitride power devices and device reliability, integrated high-frequency magnetics and magnetic materials, and high-frequency converter architectures are summarized.

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.

This paper presents a CMOS-compatible process for fabrication of 3D structures embedded in the volume of a silicon wafer, and capable of interconnection to circuitry on the wafer surface. The key challenge of embedding structures in the silicon substrate is processing inside deep silicon trenches. This difficulty is overcome by means of several key techniques: multilevel wafer etching; cavity shaping; fine proximity lithography at the bottom of trenches; and laminated dry-film lithography on complex 3D structures. As a technology demonstration, a topologically complex 3D toroidal inductor is fabricated in a deep silicon trench, and is coupled to the wafer surface with high-power, electroplated through-wafer interconnect. Inductors fabricated in these trenches achieved an overall inductance of 60 nH, dc resistance of 399 MΩ, and quality factor of 17.5 at 70 MHz.

A self-packaged antenna obtained by folding a normal patch antenna into rectangular waveguide shape has been recently reported by the authors. In this work, by applying electric vias at the edge of the folded patch as well as inductive dents on the edge, the folded antenna dimension is greatly reduced. At the same radiation frequency of 2.4GHz, the vertical length of the newly proposed antenna is reduced from 40mm to 19mm which is 52.5% size reduction. The cross section area is 20mm × 20mm. Since this architecture allows PCB layout or other electronics inside the cavity of the folded antenna, it is considered a self-packaged structure. If designed carefully, this antenna may take zero footprint and offers electromagnetic interference shielding to the PCB enclosed in its cavity. Meanwhile, this antenna does not require additional impedance matching circuits, which makes it very useful for compact devices. A quasi-omni-directional radiation pattern is obtained. Theoretical analysis, numerical simulation, fabrication and measurement are performed. Good agreement is observed between simulation and measurement data.

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.

A photoresist dispense and self-planarization (DSP) approach as a simple and cost-effective photoresist coating method has been exercised on a surface property modified substrate, where the oxygen plasma treatment of a substrate through a shadow mask selectively modifies the open area to a hydrophilic property. The non-plasma-treated area repels the dispensed photoresist while one on the plasma treated surface spontaneously spreads out to the boundary between the plasma treated and not-treated surface, allowing to form pre-defined polymer islands. Photoresist islands with various thicknesses on a single substrate have been successfully demonstrated by dispensing an appropriate amount of photoresist on each area with its dimension known. Also photoresist has been tilted to some degree to obtain a non-planar surface and a pillar array with multiple heights has been demonstrated.

A corrugated substrate integrated waveguide (SIW) with dual non-Bragg resonance frequencies at 4.8 GHz and 8 GHz is presented, where the two sidewalls of the SIW are not straight but corrugated in sinusoidal shape. Theory has been described and backed up by numerical simulation. In addition, test devices are fabricated and characterized. The measurement results show good agreement with those of theoretical calculation and simulation. Design parameters for dual bands include the amplitude and period of the corrugations, and the phase difference between two corrugation boundaries. The presented architecture has flexibility for broadband or multi-band applications. Design guidelines are discussed.

Recently, an omni-directional patch antenna has been demonstrated by folding the patch in a rectangular wave guide shape, and therefore the inside of the folded patch secures a good electromagnetic shielding zone. However, because of the folding architecture, the antenna gain (for transmitting) and sensitivity (for receiving) has been greatly reduced. In this work, the patch antenna is integrated with an active low noise amplifier in the cavity formed by the folded patch, demonstrating an improved gain and sensitivity, which is attributed to the power gain of the integrated amplifier. Since the amplifier has been self-packaged in the cavity shielded by the ground plane of the patch antenna, it does not interfere with the antenna radiation and any other external circuitry. It preserves its omni-directional radiation characteristic, which finds great usage for many biomedical applications and sensor systems such as a biomedical wireless endoscope. Also, since it is folded, its footprint has been reduced compared to its planar counterpart. In addition, the gap between the edges of the folded patch is utilized for impedance matching, not requiring an additional impedance matching circuit between the antenna and the feeding circuit. Integrated with a low noise amplifier circuit consisting of NBB-400 (RFMD, Inc.) and some passives in the cavity area, a self-packaged omnidirectional active antenna operating at 2.4 GHz with a dimension of 19mm × 18.4mm × 18.4mm (length × width × height) is successfully demonstrated. The footprint has been reduced by more than 50% compared with its counterpart. The structure has been simulated using a high frequency structure simulator (HFSS, Ansys Inc.) and the results show good agreement with measurement results.

Electromagnetic (EM) wave propagation in periodic structure is an interesting research topic in electronic, optical and laser systems. Conventionally, wave propagation in periodic media can be characterized by the Bragg's law where the resonance frequency can be determined by the periodicity of the media and a wave incident angle. However, there is also a kind of non-Bragg resonance which cannot be characterized by the traditional Bragg's law. A non-Bragg resonance has been observed in a rectangular waveguide with corrugated walls by the Cheng et al. [1]. The resonance frequency depends on the waveguide height (or thickness), the periods and amplitudes of the corrugations, and a phase shift between the corrugated walls. Since non-Bragg resonance only occurs under specific geometric conditions, it is also known as geometric resonance [1].

An electromechanically steerable directional antenna with a floating pillar array has been demonstrated for 2.4 GHz in-plane communication. A linear array of three floating pillars with a space of 0.06λ each other, a height of 0.38λ, a gap of 0.01λ from ground, and a distance of 0.06λ from the monopole is held in a piece of Styrofoam with a permanent magnet attached. The Styrofoam is rotatable in 360° in-plane scanning range around the monopole with the monopole as a rotating axis, where a permanent magnet integrated motor underneath the substrate is used to move pillars. The antenna shows a 10 dB-bandwidth of 10% and an average gain of 7 dBi with VSWR

Among short-range wireless communication systems, Ultra-wideband (UWB) technology, with a frequency allocation of 3.1–10.6 GHz, has gained a lot of popularity from researchers and the wireless industry alike due to the high data transfer rate (110–200 Mb/s) and low power consumption. However, since UWB covers the frequency spectrum of Wireless Local Area Network (WLAN) operating at 5.2 GHz (5150–5350 MHz) and 5.8 GHz (5725–5825MHz), a frequency notched function in the UWB antenna design is greatly desirable to avoid interferences from WLAN. Since 2002, various antenna structures [1, 2] and methods to notch a specific frequency band [1, 3, 4] have been introduced. Moreover, a planar integrated antenna working on both Bluetooth and UWB has been recently introduced for systems operating in those two communication systems [5, 6].

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 parabolic reflector antenna consisting of an air-lifted monopole, a reflector pillar array, and a director on a ground plane is demonstrated for 2.4 GHz applications. The monopole is fed by conductor backed coplanar waveguide (CB-CPW) for easy connectivity with other on-board components. The apex of the parabolic reflector is located 0.2 lambda from the monopole and the reflector pillar array has a space of approximately 0.1 lambda each other. The antenna with 7 pillar reflectors shows a directivity of 8.0 dBi, has a less reflector weight compared to the solid wall counter part by approximately 75%, and has less cross talk with the CPW feeding line. With an additional pillar in the opposite side of the apex 0.2 lambda from the monopole, an enhanced directivity has been obtained when the lateral reflector span is small e.g. for 3, 5 pillars, while little or degraded directivity has been observed when the reflector span is large e.g. for 7 pillars. Transmission test using two identical antennas show a transmission gain of -23.9 dB with a distance of 3lambda.

Automated multidirectional UV exposure has been demonstrated using a collimated UV source and a movable stage equipped with two computer controlled motors and a microcontroller for complex three dimensional (3-D) microstructures. The stage is tilted and rotated using independently controlled motors during UV exposure as programmed by the user. Various dynamic operations with the synchronized angular velocities of tilting and rotation have been performed using an SU-8 substrate and a reverse-side exposure scheme. Fabricated structures using the dynamic mode include a vertical reverse triangular slab, a quadruple reverse triangular slab, a cardiac horn, screwed wind vane shapes with double blades and quadruple blades. Ray tracing simulation using mathematical equations has been performed to analytically confirm the shapes of resultant structures.

This paper presents a rectangular, anisotropic nanolaminated CoNiFe core that possesses a magnetically hard axis in the long geometric axis direction. Previously, we have developed nanolaminated cores comprising tens to hundreds of layers of 300–1000 nm thick metallic alloys (i.e. Ni80Fe20 or Co44Ni37Fe19) based on sequential electrodeposition, demonstrating suppressed eddy-current losses at MHz frequencies. In this work, magnetic anisotropy was induced to the nanolaminated CoNiFe cores by applying an external magnetic field (50–100 mT) during CoNiFe film electrodeposition. The fabricated cores comprised tens to hundreds of layers of 500–1000 nm thick CoNiFe laminations that have the hard-axis magnetic property. Packaged in a 22-turn solenoid test inductor, the anisotropic core showed 10% increased effective permeability and 25% reduced core power losses at MHz operation frequency, compared to an isotropic core of the identical geometry. Operating the anisotropic nanolaminated CoNiFe core in a step-down dc–dc converter (15 V input to 5 V output) demonstrated 81% converter efficiency at a switching frequency of 1.1 MHz and output power of 6.5 W. A solenoid microinductor with microfabricated windings integrated with the anisotropic nanolaminated CoNiFe core was fabricated, demonstrating a constant inductance of 600 nH up to 10 MHz and peak quality factor exceeding 20 at 4 MHz. The performance of the microinductor with the anisotropic nanolaminated CoNiFe core is compared with other previously reported microinductors.