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