Jim C. Cheng

Low temperature, low pressure nanoimprinting of chitosan as a biomaterial for bionanotechnology applications

Micro- and nanoscale structures of chitosan were fabricated by nanoimprinting lithography and biochemically functionalized for bionanodevice applications. Chitosan solutions were prepared and a nanoimprinting process was developed for it, where chitosan solution is used as a functional resist for nanoimprinting lithography. A low temperature (90°C) and low pressure (5–25psi) nanoimprinting with polydimethylsiloxane mold could achieve not only microscale structures but also nanoscale features such as nanowire and nanodots down to 150nm dimensions. The nanoimprinted structures were chemically modified and used for the immobilization of protein molecules.

High-Resolution Direct Patterning of Gold Nanoparticles by the Microfluidic Molding Process

A novel microfluidic molding process was used to form microscale features of gold nanoparticles on polyimide, glass, and silicon substrates. This technique uses permeation pumping to pattern and concentrate a nanoparticle ink inside microfluidic channels created in a porous polymer template in contact with a substrate. The nanoparticle ink is self-concentrated in the microchannels, resulting in dense, close-packed nanoparticle features. The method allows for better control over the structure of printed features at a resolution that is comparable to inkjet printing, and is purely additive with no residual layers or etching required. The process uses low temperatures and pressures and takes place in an ambient environment. After patterning, the gold nanoparticles were sintered into continuous and conductive gold traces.

Photolithographic Process for Integration of the Biopolymer Chitosan Into Micro/Nanostructures

This paper describes a novel photolithographic method to fabricate MEMS structures from chitosan, which is a beta-1,4-linked polysaccharide. The biopolymer chitosan is the partially deacetylated product of chitin. Chitin is used in nature as robust bioscaffolds and infrared absorbers. Chitosan has been shown to have similar properties and may additionally form cationic hydrogels. Patterning methods for chitosan in the literature include imprinting, electrodeposition, and mold-casting. These methods have numerous substrate and surface topography limitations and occur in aqueous solutions, which for most hydrogels, are when they are most susceptible to chemical changes. In order to surmount these obstacles, the authors have adopted low-temperature photolithography for the chitosan that uses the following: 1) a polymethyl-methacrylate chemical barrier layer and 2) a dry anisotropic oxygen plasma etch for direct transfer of features-as small as 1 mum.

Rigid, vapor-permeable poly(4-methyl-2-pentyne) templates for high resolution patterning of nanoparticles and polymers.

Soft lithography methods are emerging as useful tools for high-resolution, three-dimensional patterning of polymers and nanoparticles. However, the low Youngs modulus of the standard template material, poly(dimethylsiloxane) (PDMS), limits attainable resolution, fidelity, and alignment capability. While much research has been performed to find other more rigid polymer template materials, the high solvent and vapor permeability that is characteristic of PDMS is often sacrificed, preventing their use in those processes reliant on this property. In this work, a highly rigid, chemically robust, optically transparent and vapor-permeable poly(4-methyl-2-pentyne) template is developed. The combination of high rigidity and high vapor permeability enables high resolution patterning with simplified ink handling. This material was nanopatterned to create a template for patterning polymers and nanoparticles, achieving a resolution of better than 350 nm.

Multi-temperature zone, droplet-based microreactor for increased temperature control in nanoparticle synthesis.

Microreactors are an emerging technology for the controlled synthesis of nanoparticles. The Multi-Temperature zone Microreactor (MTM) described in this work utilizes thermally isolated heated and cooled regions for the purpose of separating nucleation and growth processes as well as to provide a platform for a systematic study on the effect of reaction conditions on nanoparticle synthesis.

Nanowire-integrated microporous silicon membrane for continuous fluid transport in micro cooling device

We report an efficient passive micro pump system combining the physical properties of nanowires and micropores. This nanowire-integrated microporous silicon membrane was created to feed coolant continuously onto the surface of the wick in a micro cooling device to ensure it remains hydrated and in case of dryout, allow for regeneration of the system. The membrane was fabricated by photoelectrochemical etching to form micropores followed by hydrothermal growth of nanowires. This study shows a promising approach to address thermal management challenges for next generation electronic devices with absence of external power.

Synthesis and characterization of gold nanoparticle/SU-8 polymer based nanocomposite

A novel nanocomposite material was designed and synthesized for potential use in energy storage devices. It comprises of gold nanoparticles of diameter 5nm and SU-8 polymer. Scanning Electron Microscopy, Transmission Electron Microscopy and Energy Dispersive X-ray Spectroscopy techniques were used for the characterization of the nanocomposite material. The presented work represents a major first step toward creation of a superior metal-polymer nanocomposite solid-state dielectric for the development of a high energy and power density capacitor. A uniform dispersion of nanoparticles with low particle agglomerations and number of voids has been achieved. It is critical to reduce the number of voids through the nanocomposite film so the dielectric leakage can be minimized. Capacitor devices with nanocomposite material as dielectric exhibited higher dielectric constant values than the polymer only samples.

Lithographic patterning of immobilized enzymes in chitosan thin films for multi-layer, chemical/biological sensors

Patterning of immobilized enzyme in spin-cast chitosan thin films will enable the creation of multi-layer biologically-active devices. Bulk immobilized enzymes used in multi-layer devices, in contrast to surface functionalized devices; enable the implementation and characterization of multi-component assays. This paper demonstrates a procedure to immobilize enzymes into an aqueous chitosan solution, spin-cast that solution into a thin-film, and, finally, pattern that thin film using photolithography and oxygen plasma. Enzymes immobilized, deposited, and patterned using this process retain functionality, as shown with the example assay of szlig-D-galactosidase (szlig-Gal) and fluorescein di-szlig-D-galactopyranoside (FDG), a fluorogenic substrate for szlig-Gal. Hydrogel thicknesses of 200 nm to 1.5 mum (dry) were achieved and line widths down to 2 mum were observed. Finally, a multi-layer stack of chitosan hydrogel is demonstrated using a stopping layer of sputtered silicon dioxide (SiO2).

4H-Silicon Carbide p-n Diode for High Temperature (600 °C) Environment Applications

In this work, we demonstrate the stable operation of 4H-silicon carbide (SiC) p-n diodes at temperature up to 600 °C. In-depth study methods of simulation, fabrication and characterization of the 4H-SiC p-n diode are developed. The simulation results indicate that the turn-on voltage of the 4H-SiC p-n diode changes from 2.7 V to 1.45 V as the temperature increases from 17 °C to 600 °C. The turn-on voltages of the fabricated 4H-SiC p-n diode decreases from 2.6 V to 1.3 V when temperature changes from 17 °C to 600 °C. The experimental I-V curves of the 4H-SiC p-n diode from 17 °C to 600 °C agree with the simulation ones. The demonstration of the stable operation of the 4H-SiC p-n diodes at high temperature up to 600 °C brings great potentials for 4H-SiC devices and circuits working in harsh environment electronic and sensing applications.

Device Packaging Techniques for Implementing a Novel Thermal Flux Method for Fluid Degassing and Charging of a Planar Microscale Loop Heat Pipe

A novel two-port thermal flux method is implemented for degassing a microscale loop heat pipe (mLHP) and charging it with a working fluid. The mLHP is fabricated on a silicon wafer using standard MEMS micro-fabrication techniques, and capped by a Pyrex wafer, using anodic bonding. For these devices, small volumes and large capillary forces render conventional vacuum pump-based methods quite impractical. Instead, we employ thermally generated pressure gradients to purge non-condensible gases from the device, by vapor convection. Three different, high-temperature-compatible, MEMS device packaging techniques have been studied and implemented, in order to evaluate their effectiveness and reliability. The first approach uses O-rings in a mechanically sealed plastic package. The second approach uses an aluminum double compression fitting assembly for alignment, and soldering for establishing the chip-to-tube interconnects. The third approach uses a high temperature epoxy to hermetically embed the device in a machined plastic base package. Using water as the working fluid, degassing and filling experiments are conducted to verify the effectiveness of the thermal flux method.Copyright