Miyuki Uomoto

Invited) Room Temperature Bonding of Wafers with Thin Nanocrystalline Metal Films

The bonding of two nanocrystalline metal films that are sputter-deposited on micro-polished wafers [1,2] is a promising candidate process to achieve wafer bonding at room temperature, in addition to the surface activated bonding [3,4]. In this study, bonding between two flat wafers with thin metal films was studied for 16 thin metal films with various crystal structures and self-diffusion coefficients, and we discuss the potential of this bonding method for applications.

High-output-power deep ultraviolet light-emitting diode assembly using direct bonding

We fabricated high-output-power 255 and 280 nm light-emitting diodes (LEDs) using direct bonding. The LED chips were bonded to sapphire lenses at room temperature using either atomic diffusion bonding or surface-activated bonding. The LEDs with lenses had a higher light extraction efficiency than conventionally structured LEDs. As a result, at a forward current of 350 mA, the output power of the 255 nm LED increased by a factor of 2.8, reaching 73.6 mW, while that of the 280 nm LED increased by a factor of 2.3, reaching 153 mW.

Room temperature bonding of sapphire with sapphire or metal substrates in Air using Au films

Room temperature bonding of sapphire-sapphire and sapphire-metal substrates was achieved in Air using Au films. This bonding technique can support expansion of the potential applications of sapphire in diverse industries.

Lasing and thermal characteristics of Yb:YAG/YAG composite with atomic diffusion bonding

We demonstrated the laser performance of an Yb:YAG/YAG composite ceramic laser medium mounted on an aluminium heatsink via atomic diffusion bonding (ADB) technique using nanocrystalline metal films at room temperature in air. The surface temperature rise of the ADB bonded laser medium was linear with 57 °C lower than that of the commercially available soldered Yb:YAG thin disk at the pump power of 280 W. Moreover, the ADB disk was pumped 1.5 times higher (7.3 kW cm−2) than the typical damage threshold of the soldered disk without any sign of damage. The undoped capping may be effective for the suppression of ASE heating; however, according to the in situ OPD measurement it induces strong thermal lensing. The CW laser output power of 177 W was obtained at the pump power of 450 W with the optical-to-optical efficiency of 40% using V-shape cavity.

Novel diffraction gratings for next generation spectrographs with high spectral dispersion

As a transmission grating, a surface-relief (SR) grating with sawtooth shaped ridges and volume phase holographic (VPH) grating are widely used for instruments of astronomical observations. However the SR grating is difficult to achieve high diffraction efficiency at high angular dispersion, and the VPH grating has low diffraction efficiency in high diffraction orders. We propose novel gratings that solve these problems. We introduce the hybrid grism which combines a high refractive index prism with a replicated transmission grating, which has sawtooth shaped ridges of an acute apex angle. The birefringence VPH (B-VPH) grating which contains an anisotropic medium, such as a liquid crystal, achieves diffraction efficiency up to 100% at the first diffraction order for natural polarization and for circular polarization. The quasi-Bragg (QB) grating which consists of long rectangular mirrors aligned in parallel precisely, like a window blind, achieves diffraction efficiency of 60% or more in higher than the 4th diffraction order. The volume binary (VB) grating with narrow grooves also achieves diffraction efficiency of 60% or more in higher than the 6th diffraction order. The reflector facet transmission (RFT) grating which is a SR grating with sawtooth shaped ridges of an acute apex angle achieves diffraction efficiency up to 80% in higher than the 4th diffraction order.

Room temperature bonding of wafers in air using Au-Ag alloy films

Room temperature bonding of wafers in air using Au-Ag alloy films was studied. Experimental results show that the bonding using Au-Ag alloy films containing Ag content up to 70-80 at% shows almost equal bonding performance in air to that obtained using Au films.

Interface structure of InGaAs wafers bonded using thin amorphous Ge films in vacuum

This study examines room-temperature bonding of InGaAs wafers in vacuum using thin amorphous Ge (a-Ge) films. InGaAs wafers bonded using 0.5–1-nm-thick a-Ge film on respective sides show great bonding strength comparable to the fracture strength of InGaAs after annealing at 340 °C. High-resolution images obtained using transmission electron microscopy show no vacancy at the bonded a-Ge/a-Ge interface and no clear damage of the deposited a-Ge films on the crystal lattices of the InGaAs surfaces. Analysis using electron energy-loss spectroscopy reveals a slight inter-diffusion of Ge and In at the a-Ge/InGaAs interface.

Novel gratings for next-generation instruments of astronomical observations

We will introduce current status of development of a birefringence volume phase holographic (B-VPH) grating, volume binary (VB) grating and reflector facet transmission (RFT) grating developing as the novel dispersive optical element for astronomical instruments for the 8.2m Subaru Telescope, for next generation 30 m class huge ground-based telescopes and for next generation large space-bone telescopes. We will also introduce a hybrid grism developed for MOIRCS (Multi-Object InfraRed Camera and Spectrograph) of the Subaru Telescope and a quasi-Bragg (QB) immersion grating. Test fabrication of B-VPH gratings with a liquid crystal (LC) of UV curable and normal LCs or a resin of visible light curable are performed. We successfully fabricated VB gratings of silicon as a mold with ridges of a high aspect ratio by means of the cycle etching process, oxidation and removal of silicon oxide. The RFT grating which is a surface-relief (SR) transmission grating with sawtooth shaped ridges of an acute vertex angle. The hybrid grism, as a prototype of the RFT grating, combines a high-index prism and SR transmission grating with sawtooth shape ridges of an acute vertex angle. The mold of the SR grating for the hybrid grism on to a work of Ni-P alloy of non-electrolysic plating successfully fabricated by using our ultra-precision machine and a single-crystal diamond bite. The QB immersion grating was fabricated by a combination of an inclined QB grating, Littrow prism and surface reflection mirror.

Demonstration of laser oscillation of an Yb-doped Y 2 O 3 composite disk by use of atomic diffusion bonding in room temperature

We demonstrate the laser oscillation of an Yb-doped Y2O3 composite disk by use of atomic diffusion bonding (ADB) in room temperature. The continuous wave laser power was observed to be 6 W. We also measured the behavior of the wavefront produced by ADB.

Room temperature bonding of InGaAs wafers using thin Ge films

Room temperature bonding of InGaAs wafers using thin Ge films was studied. Wafers were bonded even with 0.5 nm thick Ge film on each side. Bonded wafers showed strong bonding force after annealing at 340 oC, with no vacancy at the bonded interface in TEM images.