Two‐Polymer Microtransfer Molding for Highly Layered Microstructures

Abstract

wafers using a hydrothermal method. Electron microscopy shows the tubes to have outer diameters of 20-40 nm and wall thicknesses in the range 5-15 nm. HRTEM images and SAED analysis shows the tubes to be single-crystalline. Annealing the nanotube arrays in vacuum, at low (300°C) temperature, causes substantial enhancement of the UV PL following 325 nm excitation, and much reduced green-yellow emission. ExperimentalThin-ZnO-film-coated Si wafers were used as substrates for subsequent growth of nanorod and nanotube arrays. The ZnO thin films were formed via PLD (with the T sub in the range 25-450°C, and deposition times of 1-45 min). The PLD system has been described elsewhere [22]. 50 mL of 0.1 M aqueous solutions of zinc nitrate (Alfa Aesar, 99 %) and methenamine (Alfa Aesar, 99+ %) were each maintained at constant temperature (90°C) using a thermostatically controlled oil bath, and then mixed in a sealed glass bottle. For nanorod growth, and the growth of nanotubes on nanorods, the substrates were immersed in the reactive solution immediately after mixing. Typical growth times were t < 3 h for nanorod arrays, and t > 6 h for the growth of nanotubes on nanorods. The highest-density nanorod/nanotube arrays were grown on very thin ZnO template layers (1-5 min deposition time) on Si. These substrates were pre-heated (to ∼ 90°C) and immersed in the reactive solution that had been previously maintained at 90°C for a user-selected time s = 10-60 h after mixing. The as-grown samples were rinsed in deionized water, and then dried in air at room temperature. Deposited products were characterized and analyzed using SEM (JEOL 6300 LV), TEM (JEOL 1200EX), and HRTEM (JEOL 2010). PL spectra were measured at room temperature following excitation with a continuous-wave He-Cd laser (k = 325 nm, power ∼ 3 mW). Two-Polymer Microtransfer Molding for Highly Layered Microstructures**By Jae-Hwang Lee,* Chang-Hwan Kim, Kai-Ming Ho, and Kristen Constant* Microfabrication techniques for highly layered microstructures have attracted much attention, since highly layered microstructures permit fabrication of three-dimensional (3D) devices with functionality not possible in planar devices. With the growing demands of highly layered microstructures for a number of applications, including artificial bones, [1,2] integrated microfluidic systems, [3][4][5] polymer-based integrated optical circuits, [6,7] photonic crystals, [8,9] and catalytic reactors, [10,11] reliable low-cost microfabrication methods are desirable. Traditional fabrication methods using photolithography are usually slow and costly. Over the last few years, a number of new approaches for fabricating 3D microstructures have been reported as alternatives to conventional photolithography such as microtransfer molding (lTM), [12] two-photon polymerization, [13,14] holographic lithography, [15,16] and nanoimprinting.[17] Among these techniques, lTM showed a number of advantages, including low cost, capability for non-periodic 3D structures, a wide range of materials compatibil...

Photonic crystal: energy-related applications

“…29 μTM has a number of advantages, including low cost, potential for nonperiodic 3-D structures, compatibility with a wide range of materials, and flexibility in design. Our group has developed an advanced technique called two-polymer μTM [30][31][32][33] to create high-quality layer-by-layer PC structures on an optical scale. In this technique two different UV-curable prepolymers are used, one as a filler (prepolymer A) and another as an adhesive (prepolymer B), to build a 3-D structure layer by layer (the schematic procedure is shown in Fig.…”

“…This layer-by-layer woodpile PC was first proposed and fabricated at microwave scale in the 1990s, 6 then in the IR 34 and most recently on an optical scale. 30 Such structures have attracted much interest because it has a full band gap and can also be built by layer-by-layer semiconductor processing. 34,35 Using the multilayer polymer structure as shown in Fig.…”

Photonic crystal: energy-related applications

“…2. 30 The area of the fabricated structure is 4 by 4 mm and the orientation of each layer is perpendicular to the one below. This layer-by-layer woodpile PC was first proposed and fabricated at microwave scale in the 1990s, 6 then in the IR 34 and most recently on an optical scale.…”

Semicrystalline woodpile photonic crystals without complicated alignment via soft lithography

“…In the schematic illustration in Fig. 1, the initial polymer template is fabricated by a soft-lithographic technique called twopolymer microtransfer molding 21 on a sacrificial-layer ͑AZ 6612KE, Clariant͒-coated glass plate using alignment by diffracted moiré fringes. 22 As we stack the rods of each layer perpendicular to those of the layer below without considering lateral position, the fabricated template has semicrystalline structure including face-centered-tetragonal ͑FCT͒, tetragonal, and amorphouslike phases.…”