Yong Son

Nanoscale Electronics: Digital Fabrication by Direct Femtosecond Laser Processing of Metal Nanoparticles

For various applications in the electronics industry, the fabrication of electrically conductive nanoand micropatterns has become important. Conventional vacuum metal deposition and photolithography processes are widely used for high-resolution metal patterning of microelectronics. However, those conventional approaches require expensive vacuum conditions, high processing temperatures, many steps, and toxic chemicals to fabricate one layer of a metal pattern. Furthermore, it is almost impossible to change the design of the expensive photomask once it is fabricated. For these reasons, the development of alternative maskless, direct, high-resolution patterning techniques to fabricate conductive microand nanopatterns at atmospheric pressure and low temperature without using vacuum deposition or photolithography has attracted wide attention in recent years. One of the most promising alternatives is the direct patterning of solution-deposited metal nanoparticles (NPs). The development of metal NP solution ink enabled 1) an inexpensive solution-based metal deposition approach without using expensive vacuum deposition and 2) a low-temperature metal deposition process, which allows using heat-sensitive and inexpensive polymer as the substrate. Examples of NP-inkbased direct metal patterning include screen printing, [ 1 ] direct nanoimprinting, [ 2 , 3 ] microcontact printing, [ 4 , 5 ] inkjet printing, [ 6 , 7 ]

Photopatternable quantum dots forming quasi-ordered arrays.

We have functionalized core-shell CdSe/ZnS quantum dots (QDs) with a photosensitive monolayer, rendering them solution processable and photopatternable. Upon exposure to ultraviolet radiation, films composed of this material were found to polymerize, forming interconnected arrays of QDs. The photoluminescence properties of the nanocrystal films increased with photocuring. The material was found to be suitable for spin casting and was used as the active layer in a green electroluminescent device. The electroluminescence efficiency of devices containing a photocured active layer was found to be largely enhanced when compared to devices containing nonphotocured active layers. The material also showed excellent adhesion to both organic and inorganic substrates because of the unique combination of a siloxane and a photopatternable layer as ligands. The pristine functionalized nanocrystals could easily be used for two-dimensional patterning on organic and inorganic substrates. The photopatternable quantum dots were uniformly dispersed into a photopolymerizable resin to fabricate QD embedded three-dimensional microstructures.

Autofocusing method using fluorescence detection for precise two-photon nanofabrication

We propose a method capable of focusing a laser beam on a substrate automatically via fluorescence detection from the resin of a two-photon nanofabrication system. When two-photon absorption (TPA) occurs by focusing the laser beam in the resin, fluorescence is emitted from the focusing region in the visible range. The total pixel number above the threshold value of the fluorescence images obtained by a CCD camera is plotted on a graph in accordance with the focus position. By searching for the position when the total pixel number undergoes an abrupt change in the pre-TPA region, the correct configuration of the focused laser beam can be found. Through focusing tests conducted at four vertices of a 500 μm x 500 μm square placed arbitrarily inside SCR500 resin, the errors of the autofocusing method were found to range from -100 nm to + 200 nm. Moreover, this method does not leave any polymerized marks. To verify the usefulness of the autofocusing method, the fabrication of a pyramid structure consisting of 20 layers was attempted on a coverglass. It was completely fabricated without losing a layer.

Proportional enlargement of movement by using an optically driven multi-link system with an elastic joint

Diverse movements using optical manipulation have been introduced. These are generally performed in the focal region of the laser beam. To achieve a wider range of movements based on precise motion transformation, an effective method for optical manipulation that overcomes the important obstacles such as small optical trapping forces, friction, and the viscosity of fluids is required. A multi-link system with an elastic joint is introduced that provides precise motion transformation and amplification. By considering the physical properties of the structure and the optical trapping force, an elastic micron-scale joint with the simple shape of a thin plate was designed. As a further example of a multi-link system with an elastic joint, a double 4-link system for motion enlargement was designed and fabricated. By performing experimental evaluations of the fabricated structures, it was confirmed that multi-link systems with an elastic joint were effective tools for precise motion transformation through optical manipulation.

Fabrication of Nano-scale Conductors by Selective Femtosecond Laser Sintering of Metal Nanoparticles

For various applications in electronic devices, metal nanopatterns fabrication on various substrates, such as glass and flexible polymers has become important. Recently, selective laser sintering of nanomaterials has emerged as a promising approach to the fabrication of precise metal nanopatterns without using any conventional vacuum deposition or photolithography. In this study, we present the experimental results on the fabrication of subwavelength scale silver nanopatterns using a femtosecond laser for the first time. By applying a femtosecond laser for the selective sintering of metal nanoparticles on a heat sensitive substrate, the heat-affected zone during the laser sintering process can be minimized. Through this work, metal conductors with nanometer features and a high conductivity were successfully fabricated.

Improvement of two-photon induced photoreduction by using a metal ion solution with a high concentration of silver ions

Fabrication of three-dimensional metallic microstructures is an important issue for various applications in electronics and nanophotonics. Precise metallic micropatterns were fabricated by the two-photon induced photoreduction (TPPR) process. The process employs the photoreduction of silver ions in a metal ion solution composed of silver nitrate (AgNO3) and water-soluble polymer (poly(4-styrenesulfonique acid)). To improve the resolution and the uniformity of metallic micropatterns, a metal ion solution with a high concentration of silver ions was used, and the continuous forming window (CFW) of fabrication conditions was obtained by considering the mechanism of TPPR and heat effects during the process. Through this work, continuous silver lines and micropatterns with a minimum width of 560 nm were fabricated and their quality was verified.

Fabrication of submicron-sized metal patterns on a flexible polymer substrate by femtosecond laser sintering of metal nanoparticles

The femtosecond laser sintering of metal nanoparticles was studied in order to fabricate submicron-sized metal patterns on flexible polymer substrates for various applications in the electronic and photonic industries. In this process, a mode-locked Ti:sapphire laser beam was tightly focused on silver nanoparticles. To achieve a homogeneous dispersion of the silver nanoparticles, the nanoparticles were prepared using a two-phase reduction method wherein the silver nanoparticles were encapsulated by functional surfactants. The key advantage of the femtosecond laser sintering process is that it reduces the heat-affected zone during sintering, as the femtosecond (10-15s) laser pulse is shorter than the heat diffusion time (picosecond: 10-12s). Therefore, sintering of metal nanoparticles is limited to the laser focal spot and the thermal diffusion effect is suppressed, enabling the realization of submicron-sized metal patterns on flexible polymer substrates. Through this process, metal conductors with submicron-sized features and high conductivity were successfully fabricated. As demonstrated by the obtained results, the femtosecond laser sintering of metal nanoparticles is a novel process that offers direct, low-temperature, ultra-high-resolution results, and which will have numerous further applications in electronics and photonics.

Highly flexible micro-joint with large rotational movement based on elastic design concept

As nano/micro devices become smaller, it is important to manipulate them with highly accurate control. The design of micro-joints is important for highly accurate movement. An elastic joint, one of type of micro-joint that is commonly used for rotational movement, however, has a limitation in terms of its small rotational range. This paper discusses a new micro-joint based on an elastic design concept for large rotational movement. The proposed micro-joint was designed with a simple shape of two thin plates. For large rotational movement with small force, a modified micro-joint with a helical structure was proposed. Through numerical simulation, the proposed micro-joint is estimated to have a large rotation that can be obtained with only a small force.

Fabrication of Microstructures Containing High Refractive Index Materials by Two-Photon Lithography

Here we review our work on incorporating high refractive index inorganic materials into microstructures by two-photon lithography. We describe the direct writing of silver microstructures from dispersions of silver salt solutions, as well as the fabrication of silver nanoparticle embedded microstructures by a combination of UV exposure and thermal curing. The last section summarizes the functionalization of semiconductor quantum dots for their incorporation into microstructures. These materials pave the way for new investigations into photonic properties of hybrid inorganic nanomaterial incorporated microstructures.

Synthesis and Characterization of Two-Photon Absorbing Dithienothiophene Derivative with Silyl End Group

Efficient two-photon absorbing chromophore based on dithienothiophene has been synthesized and its two-photon absorption cross section (σ2) was evaluated by two-photon induced fluorescence excitation spectroscopy. The chromophore obtained shows a sizable two-photon absorption cross section value of 618 GM at 700 nm. By using one- and two-photon spectroscopic experiment, two-photon absorption properties of the chromophore with the electronic transition dipole moment and one- and two-photon transition energies were evaluated. Besides, we have demonstrated the successful application of this compound in fabrication of 2D and 3D microobjects by using two-photon microfabrication technique.