Manabu Yasui

Micro Press Molding of Borosilicate Glass Using Plated Ni–W Molds

The hot embossing of glass with a size on the submicron or micron level is a target of interest for the industrial production of products such as microdevices. For fluidic micro chip applications, polymer materials have been used with the advantage of a relatively low cost of fabrication. However, glass is suitable for high-temperature applications such as in microreactors. Although glass is also a good candidate material for optical devices because of its enhanced optical properties, the development of mold materials has not been established for the hot embossing of glass. In this study, we used Ni–W as a mold material for the hot embossing of glass. A plated Ni–W film has a high heat resistance and a linear expansion coefficient, properties that are similar to those of glass materials. Focused ion beam (FIB) machining was employed for the micron and submicron structurings of a Ni–W mold material. Borosilicate glass, D263, was used as glass material. Glass patterns of 0.4 µm width were obtained by hot embossing with the Ni–W mold.

Large constriction of lattice constant in epitaxial magnesium oxide thin film: Effect of point defects on lattice constant

Epitaxial thin films prepared using an MgO target on silicon substrate often show constriction of lattice constant (a∼4.1 A). Detailed investigation of the crystal structure excluded the possibility that the epitaxial films are either cubic spinel MgO or magnesium silicate (a/2∼4.1 A). With such a constriction in rock salt MgO structure point defects must be induced into structure. An ab initio method with semicore pseudopotentials predicted such constrictions on configurations of Schottky type defects. The Schottky defects with random distribution throughout lattice cites satisfied constricted lattice constant, mass density, and crystallography experimentally observed on the epitaxial MgO films.

Development of Ni–P-Plated Inconel Alloy Mold for Imprinting on Pyrex Glass

We fabricated microlenses and the logo of the National Institute of Advanced Industrial Science and Technology (AIST) on Pyrex glass by employing thermal nanoimprint technology. The mold material used for imprinting on Pyrex glass was an amorphous Ni–P alloy that was deposited on Inconel-600 by electroless plating technology. The complete fabrication technique consisted of highly accurate processing by focused-ion-beam (FIB) on material that involved a high-temperature thermal treatment that has the advantage of improving the hardness of the mold. An amorphous Ni–P alloy layer on an Inconel-600 substrate was characterized by measuring its X-ray diffraction spectrum. Using this technique we successfully developed a low-cost mold for imprinting on Pyrex glass instead of using a more expensive glass-like carbon mold that is commonly used for this purpose. Microlenses with concave curvatures having radii of 12 and 20 µm were created on the mold by a FIB system equipped with three-dimensional computer-aided-design (CAD) software. This mold was used for thermal imprinting on Pyrex glass substrates to fabricate microlenses and the AIST logo. When polished Inconel-600 was used as a substrate for molds, the accuracy of the Ni–P mold proved to be of higher quality than a mold made of unpolished Inconel-600. The microlenses made using Ni–P/polished-Inconel-600 molds showed lubricious surfaces that were not possible to achieve using Ni–P/unpolished-Inconel-600 molds. Moreover, some of the parameters in Ni–P electroless plating were changed in order to make three kinds of molds with P content ratios of 4, 8, and 16 wt %. The micro-vickers hardness caused by thermal treatment and the differences among the transcript values on Pyrex glass were also evaluated experimentally.

Nano-strip grating lines self-organized by a high speed scanning CW laser.

After a laser annealing experiment on Si wafer, we found an asymmetric sheet resistance on the surface of the wafer. Periodic nano-strip grating lines (nano-SGLs) were self-organized along the trace of one-time scanning of the continuous wave (CW) laser. Depending on laser power, the nano-trench formed with a period ranging from 500 to 800 nm with a flat trough between trench structures. This simple method of combining the scanning laser with high scanning speed of 300 m min(-1) promises a large area of nanostructure fabrication with a high output. As a demonstration of the versatile method, concentric circles were drawn on silicon substrate rotated by a personal computer (PC) cooling fan. Even with such a simple system, the nano-SGL showed iridescence from the concentric circles.

Numerical Simulation of Glass Imprinting for Molding Temperature Prediction

For optimizing the conditions of glass imprinting, many researchers commonly use the trial-and-error method. Therefore, it is advantageous to carry out a preliminary analysis of glass imprinting. The viscoelastic property of glass is necessary for MEMS-ONE in which a viscoelastic model is used. Assuming glass materials to be viscoelastic boies, the relaxation share modulus was measured by the creep test based on traditional thermo-viscoelastic theory. The Williams–Landel–Ferry (WLF) equation is applied using the temperature dependence of liquid viscosity. We compared experimental results with the analytic results of MEMS-ONE simulation under the conditions of fixed pressure (3.56 MPa) and time (10 min). The object of evaluation is the height of the central position prong. The molding temperature can be predicted within 10 °C error by the simulation.

Effects of post exposure bake temperature and exposure time on SU-8 nanopattern obtained by electron beam lithography

SU-8 is a photoresist imaged using UV rays. However, we investigated the characteristics of an SU-8 nanopattern obtained by electron beam lithography (EBL). In particular, we studied the relationship between post-exposure bake (PEB) temperature and exposure time on an SU-8 nanopattern with a focus on phase transition temperature. SU-8 residue was formed by increasing both PEB temperature and exposure time. To prevent the formation of this, Monte Carlo simulation was performed; the results of such simulation showed that decreasing the thickness of SU-8 can reduce the amount of residue from the SU-8 nanopattern. We confirmed that decreasing the thickness of SU-8 can also prevent the formation of residue from the SU-8 nanopattern with EBL.

Property Variation of Ni--W Electroformed Mold for Micro-Press Molding

We proposed a simple method to fabricate a Ni–W electroformed mold for glass micro-press molding. For example, borosilicate glass (D263) was molded using the Ni–W electroformed mold. A Ni–W electroformed mold with a fine line was fabricated by photolithography and electroforming technology. Additionally, the Ni–W electroformed mold did not require a release layer. As the result of molding D263 at 883 K, the minimum pitch of the glass pattern was the same as that of the Ni–W electroformed mold. We argue that the crystallization of amorphous Ni–W occurred with the activation energy derived from the heating of micro-press molding. The heating temperature was 833 K. Additionally, the release characteristics of a Ni–W film were improved by increasing the percentage of W. In terms of the thermochemical stability and high content rate of W, we indicated that Ni–W electroformed molds can be used repeatedly for glass micro-press molding.

Growth of Nanocubic MgO on Silicon Substrate by Pulsed Laser Deposition

Magnesium oxide (MgO) prepared by both pulsed laser deposition and sputtering methods showed constriction of lattice constants. To emphasize the effect of the constriction of lattice constants, MgO prepared at high oxygen atmosphere and high substrate temperature, resulted in the growth of cubic-shaped magnesium oxide (MgO) nanoparticles on a Si substrate. In oxygen atmosphere, the nanocubic MgO was scattered on the substrate without the Si surface being covered by a MgO thin film. Interestingly, the growth of nanocubic MgO was restrained on the samples prepared in nitrogen atmosphere. The formation of nanocubic MgO is related to the deposition pressure as well as the etching effect provided by oxygen atmosphere.

Large Lattice Misfit on Epitaxial Thin Film: Coincidence Site Lattice Expanded on Polar Coordinate System

The growth of an epitaxial film with a large lattice misfit to substrates is interpreted as domain growth with domain matching by the coincidence site lattice (CSL), in which it is assumed that (m×n) film lattice units are superimposed on (k×l) substrate lattice units. The domain matching with all combinations of two lattice units was expanded on a polar coordinate system (polar CSL figure), where the radius and angle are indicative of the domain size and the lateral angle between two superimposed lattices, respectively. The polar CSL figure enable us to visualize the domain matching of all combinations between the two lattice units (k×l) and (m×n). The cubic-on-cubic growth of magnesium oxide on a silicon substrate and the lateral rotation of bismuth cuprate oxide were taken as examples of visualization using the polar CSL figure.

Periodic nano trench structure fabricated by high-speed scanning CW laser

We report periodic nanostructure on solid material irradiated by scanning continuous wave (CW) laser. Long periodic nano strip grating lines (nano-SGL) formed, not in a spot, but along the trace of the beam scan, literally parallel to each other with a at trough between the strip lines. The period of nanostructure was varied with the laser power between 500 nm and 800 nm, which equals to wavelengths used for laser scanning of green and infrared lasers. Thermal simulation and Raman spectra indicated the temperature of target exceeded the melting temperature to form the periodic nanostructure on target materials.