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Micro & Nanoscale Thermosciences

Exploration of Thermolithography  
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    We are exploring a broad class of lithography techniques that utilize controlled or localized heating to create micro- and nanoscale patterns. Heat conduction in highly disordered polymer films is a very slow process compared with propagation of electromagnetic waves or energetic material beams. Heat diffusion in highly disordered materials also does not exhibit diffraction, interference, or near field effects that can complicate pattern transfer. These attributes potentially offer intriguing leverage in developing novel micro- and nano-fabrication processes.

    Our recent exploration of a thermolithography employs localized heating to induce thermo-chemical crosslinking of photoresist layers. The thermal transport properties of photoresist layers are measured and kinetics of cross-linking reactions is studied using microfabricated heaters as localized heat sources. It is demonstrated that polymer films can be patterned in a controlled manner using heater temperature, heating duration, as well as UV exposure dose as control variables. The data and experimental approaches will help systematic evaluation and development of thermolithography schemes, such as nanoparticle-based thermolithography and laser thermolithography.


Characterization of Thermal Properties of Polymer Thin-Films  
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    Polymer thin films have attracted tremendous attention in recent years. Their flexibility, manufacturability, light weight, biocompatibility, low cost, and other superior properties have made them a promising candidate in many novel applications including microelectronics, optoelectronics, Bio-/MEMS/NEMS, and micro/nanofabrication. Thermal transport properties of polymer thin films are very important in many applications, for example, the resists used in thermolithography, low-k dielectric films in microelectronics, surface coating or membrane in bio-chip device for temperature sensitive applications. Unfortunately, most of the data is not available in part due to the difficulty in measurement.

    We have developed several measuring techniques for in-plan and out-of-plan thermal transport properties. A membrane based microfabricated device is especially useful for chemically or mechanically "weak" polymers. This measurement does not require post-processes after sample deposition, which prevents possible damages to the polymer films and the effect due to thermal interface resistance. Process dependent thermal conductivity of photoresist layers is studied using this technique.


Controlled Heating of Nanoparticles  
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    Nanoparticles are one of the most widely used building blocks of nanotechnology. We are interested in thermal phenomena involving nanoparticles for a wide variety of nanomanfuacturing, biological, and scientific applications. Pulsed laser heating of nanoparticles suspended in liquids allows control of their size and shape. Nanoparticles dispersed in macromolecules can also serve as highly localized heat sources for novel thermolithography and related nano-manufacturing schemes. Targeted heating of biological molecules or cells bound to nanoparticles opens intriguing biomimetic as well as therapeutic applications.

    In this project, we are studying heat conduction in and around metallic nanoparticles embedded or suspended in a soft medium, such as polymer or liquid. The cooling time constant of gold nanoparticles subjected to ultra-short pulsed laser heating has been observed to be a strong function of particle diameter. Our study has shown that such strong dependence is due in part to spatial non-equilibrium between electrons and phonons that strongly affects heat conduction across nanoparticle-medium interfaces. Such spatial nonequilibrium gives rise to the particle size dependence of the apparent thermal interface resistance. The impact of electron-phonon spatial nonequilibrium is believed to be most pronounced for particles made of Au, which are bio-compatible and very promising for biomedical applications. We employ the two-fluid heat diffusion model to represent interactions between electrons and phonons in metallic nanoparticles. Our prediction agrees well with the previously reported experimental data.





Thermal Transport in Graphite Nanoplatelet(GNP)-reinforced Nanocomposites
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    Composites reinforced with nanoscale fillers are called nanocomposites , which have the potential to achieve superior thermal, mechanical and electrical characteristics for lightweight, energy-efficient, and multifunctional structures. One of the promising candidates is graphite nanoplatelet (GNP), which has comparable characteristics as carbon nanotube but much less expensive.
    Heat transport in GNP nanocomposites was strongly affected by the thermal interface resistance between GNPs and the polymer matrix. The interfacial bonding can be tailored by chemically or physically modifying the surfaces of GNPs. This allows us to investigate how interfaces affect heat conduction in nanocomposites.





 
UCLA Department of Mechanical and Aerospace Engineering, Last update: January 31st, 2008