Advanced multidirectional UV lithography for three dimensional (3-D) micro-/nano structures

Computer controlled dynamic mode multidirectional ultraviolet (UV) lithography (DMUL) has been introduced as micro and nano scale three-dimensional (3-D) fabrication process. The system for DMUL consists of a conventional collimated UV light source, a movable substrate holder, two stepper motors, a computer, and an interface box. A movable stage of substrate holder is placed under the collimated UV light source and is activated by two stepper motors where the one control the tilting angle of the substrate and the other does the rotational angle. The routine of the movable stage is commanded by the computer programming. The interface box makes a role of converting the language of the computer program command to controlling signals to stepper motors. During the UV exposure, collimated UV light creates various traces on a photoresist layer placed on the movable stage which is followed the routine from the program. Summation of traces is finally turned into the 3-D structures after subsequent processes such as baking and developing in case of negative photoresist. Various microstructures are demonstrated such as the four leaf clover horn, the cardiac horn, a vertical triangular slab, a screwed wind vane, and arbitrary shape horns. Since the inclined angle of the microstructure in the conventional air environment was found the limit as approximately 35° due to the difference of refractive indices of air and SU-8, a liquid-state refractive index matching medium is introduced to extend the limit of the inclined angle of the three-dimensional (3D) microstructures by dynamic mode multidirectional ultraviolet (UV) lithography. The refractive index matching medium is filled with an isolated container which is placed between the UV light source and the SU-8 layer. By filling the glycerol as an index matching medium, difference of refractive indices between the SU-8 and the glycerol could be minimized as 0.13 which is 81.16% reduction from the conventional air environment. Direct exposure to a liquid state photoresist method is introduced by using the computer controlled dynamic mode UV lithography. A customized container filled with a liquid state negative photoresist called as LF55GN is enclosed with photomask substrate where it is attached to a movable stage of dynamic mode multidirectional UV lithography for the complex 3-D microstructure fabrication. Since the fabrication procedure is not required softbaking and post exposure baking, 3-D structures by dynamic mode multidirectional UV lithography can be realized within an hour. The feasible fabrication size of the dynamic mode multidirectional UV lithography has been widely ranged from hundreds nanometers to a several millimeters scale. Lithographically defined very tall 3-D structures by dynamic mode multidirectional UV lithography have been introduced such as 6mm inverted triangular slab and 9mm uniformly tapered pillar by utilizing LF55GN. Also, dynamic mode multidirectional UV lithography was performed on nano scale patterned photomask by e-beam lithography. The fabricated nano scale 3-D structures include the triangular slab, the screwed wind vane, the quadruple triangular slab, and the horn where those widths of introduced 3-D nano structures are varied in a few hundred nanometers and heights are ranged from 1µm to 4.5µm. As an application, the air-lifted bow-tie antenna has been demonstrated. The antenna backbone is made of LF55GN with 4.5mm tall and metalized by copper. The air-lifted bow-tie antenna has been tested with return loss measurement between 8 and 15 GHz. The resonant radiation frequency was shown at 12.34GHz and a maximum return loss was measured as 36 dB with 7% bandwidth

Photolithography of SU-8 microtowers for a 100-turn, 3-D toroidal microinductor

Abstract 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

Ku-Band Frequency Selective 3D Vertical Pillar Array

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

Effects of Supplemental UV-A LEDs on the Nutritional Quality of Lettuce: Accumulation of Protein and Other Essential Nutrients

Light plays an important role in influencing the nutritional quality of food crops, especially with regard to the health-promoting phytochemicals. However, its role in affecting the nutritional quality with regard to the essential nutrients is not well understood. In this study, the effects of preharvest UV-A treatment on the nutritional quality of lettuce (Lactuca sativa, cv. red-leaf ‘New Red Fire’ and green-leaf ‘Two Star’) in relation to the essential nutrients and health-promoting phytochemicals were explored. Lettuce plants were grown in a growth chamber and were subjected to supplemental UV-A LEDs (peak wavelength 375 nm) for a brief period (3–6 days) prior to harvest. UV-A LEDs were equipped with lenses to control the light dispersion. Many growth indices such as shoot fresh mass, leaf area, and leaf number were unaffected by supplemental UV-A in both varieties while shoot dry mass decreased in response to a 6-day UV-A treatment compared to the control. Leaf chlorophyll and carotenoid concentrations increased significantly in green-leaf lettuce after 3 or 6 days of UV-A treatment, but only after 3 days of UV-A treatment in red-leaf lettuce compared to the control. Leaf protein concentration increased significantly in both lettuce varieties along with a number of essential nutrients such as phosphorus, potassium, calcium, manganese, and sulfur in response to supplemental UV-A. Supplemental UV-A increased the accumulation of protein by approximately 48% in green-leaf lettuce and 31% in red-leaf lettuce compared to the control plants. Moreover, in addition to the above essential nutrients, green-leaf lettuce accumulated higher amounts of magnesium, copper, and zinc compared to the control plants, indicating that green-leaf lettuce was more responsive to preharvest supplemental UV-A treatment than red-leaf lettuce. However, the accumulation of total phenolic compounds and flavonoids in both varieties was lower under supplemental UV-A. Furthermore, the use of LED lenses did not have a consistent impact on most of the plant responses studied. Overall, the results indicate that a brief preharvest exposure of both red- and green-leaf lettuce varieties to UV-A increased their nutritional quality by enhancing the accumulation of protein and other major essential nutrients

Experimental Validation of Diffraction Lithography for Fabrication of Solid Microneedles

Microneedles are highly sought after for medicinal and cosmetic applications. However, the current manufacturing process for microneedles remains complicated, hindering its applicability to a broader variety of applications. As diffraction lithography has been recently reported as a simple method for fabricating solid microneedles, this paper presents the experimental validation of the use of ultraviolet light diffraction to control the liquid-to-solid transition of photosensitive resin to define the microneedle shape. The shapes of the resultant microneedles were investigated utilizing the primary experimental parameters including the photopattern size, ultraviolet light intensity, and the exposure time. Our fabrication results indicated that the fabricated microneedles became taller and larger in general when the experimental parameters were increased. Additionally, our investigation revealed four unique crosslinked resin morphologies during the first growth of the microneedle: microlens, first harmonic, first bell-tip, and second harmonic shapes. Additionally, by tilting the light exposure direction, a novel inclined microneedle array was fabricated for the first time. The fabricated microneedles were characterized with skin insertion and force-displacement tests. This experimental study enables the shapes and mechanical properties of the microneedles to be predicted in advance for mass production and wide practical use for biomedical or cosmetic applications

Size- and Composition-Dependent Radio Frequency Magnetic Permeability of Iron Oxide Nanocrystals

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 Mössbauer 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

A Technology Overview of the PowerChip Development Program

Abstract—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 200 V) and low-to-moderate power levels (e.g., up to 50 W) 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 highfrequency magnetics and magnetic materials, and high-frequency converter architectures are summarized. Index Terms—Gallium nitride, high frequency (HF), integrated magnetics, integrated power converter, light-emitting diode (LED) driver, PwrSoC. I