Ryo Takigawa

Room-Temperature Bonding of Vertical-Cavity Surface-Emitting Laser Chips on Si Substrates Using Au Microbumps in Ambient Air

The feasibility of room-temperature (RT) bonding of vertical-cavity surface-emitting laser (VCSEL) chips on silicon (Si) substrates with Au microbumps was demonstrated by Au–Au surface-activated bonding. The diameter at the top, the height, and the pitch of Au microbumps measured approximately 5, 2, and 10 µm, respectively. Following activation of the Au surfaces with argon radio-frequency plasma, Au–Au bonding was carried out using contact at RT in ambient air. The measured results of light–current–voltage (L–I–V) characteristics indicated no significant degradation of the VCSEL chips after bonding.

Passive Alignment and Mounting of LiNbO

In this study, passive alignment and mounting of lithium niobate (LiNbO3) chips, with a large mismatch in the coefficient of thermal expansion with most semiconductors, are demonstrated for hybrid-integrated optical devices. LiNbO3 chips were aligned passively using the visual index alignment method and were subsequently bonded on the Si substrates by low-temperature solid-state bonding with Au microbumps, which allow for electrical connections and heat dissipation. Au-Au bonding was carried out at 100°C in ambient air after surface activation by argon RF plasma. The vertical bonding accuracy was determined by assessing the height variations of the Au microbumps due to the plastic deformation in the bonding process. The bonding accuracies in the horizontal and vertical directions were estimated to be within ±1 μm. Average excess loss due to misalignment between titanium-diffused single-mode LiNbO3 waveguides and V-groove-guided single-mode fibers was about 0.5-dBm per interface (wavelength: 1.55 μm).

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The air-gap structure between integrated LiNbO(3) optical modulators and micromachined Si substrates is reported for high-speed optoelectronic systems. The calculated and experimental results show that the high permittivity of the Si substrate decreases the resonant modulation frequency to 10 GHz LiNbO(3) resonant-type optical modulator chips on the Si substrate. To prevent this substrate effect, an air-gap was formed between the LiNbO(3) modulator and the Si substrate. The ability to fabricate the air-gap structure was demonstrated using low-temperature flip-chip bonding (100 °C) and a Si micromachining process, and its performance was experimentally verified.

Waveguide Chips on Si Substrates by Low-Temperature Solid-State Bonding of Au

This paper demonstrates the application of ultra-precision cutting to the fabrication of ridged LiNbO₃ waveguides for use in low-loss photonic integrated circuits. Ridged waveguides with sidewall verticality of 88° and ultra-smooth sidewalls were obtained in LiNbO₃ crystals using this technique. In addition, the possibility of fabricating bent ridged waveguides via this mechanical micromachining method was examined. The root mean square surface roughness of the machined sidewall was 4.5 nm over an area of 2.5 × 10 µm, which is sufficiently low so as to minimize scattering losses of guided light. The propagation loss of the ridged waveguide produced during this work was well below 1 dB/cm at a wavelength of 1550 nm. The present technique should have significant applicability to the micromachining of ferroelectric materials and the fabrication of highly confined optical waveguides such as ridged waveguides and photonic wires.

Air-gap structure between integrated LiNbO3 optical modulators and micromachined Si substrates.

A lithium niobate (LiNbO3)/silicon (Si) hybrid structure has been developed by the surface-activated bonding of LiNbO3 chips with gold (Au) thin film to Si substrates with patterned Au film. After organic contaminants on the Au surfaces were removed using argon radiofrequency plasma, Au-to-Au bonding was carried out in ambient air. Strong bonding at significantly low temperatures below 100◦C without generating cracks has been demonstrated. key words: low-temperature bonding, Au-to-Au bonding, surface-activated bonding, lithium niobate/silicon structure, hybrid integration

Lithium niobate ridged waveguides with smooth vertical sidewalls fabricated by an ultra-precision cutting method

We newly introduce a compliant rim to realize hermetic sealing of electronic components at low temperature. The compliant rim easily deforms under pressing load owing to its cone-shaped cross section and, therefore, intermetallic bonding can be performed at low temperature. We demonstrate the room-temperature vacuum sealing using the compliant rim made of Au with the aid of ultrasonic vibration of submicron amplitude. A test vehicle fabricated using silicon and glass showed that the air leak rate of the room-temperature sealing was well below 1 ? 10?12 Pa?m3/s, which is sufficiently low for use in vacuum packaging.

Low-temperature Au-to-Au bonding for LiNbO3/Si structure achieved in ambient air

We have proposed and developed a fluorescence detector in which a thick SiO2/Ta2O5 multilayer optical interference filter is monolithically integrated on a hydrogenated amorphous silicon (a-Si:H) PIN photodiode. Combined with the optical transparency of the detectors glass substrate, the annular shape of the detector will enable us to construct a module containing a detector and an excitation source. Based on a detection platform that comprises a fluorescence-collecting microlens and an annular fluorescence detector while using an external laser source, we have demonstrated single molecule DNA detection capability when combined with a polymerase chain reaction (PCR). We also describe our work towards the integration of an excitation source to fabricate a heterogeneously-integrated laser-induced fluorescence detection (LIF) device comprising a laser diode, micro-lenses and the integrated a-Si:H fluorescence detector.

Room-temperature hermetic sealing by ultrasonic bonding with Au compliant rim

Surface activated bonding (SAB) method with atmospheric-pressure plasma treatment is an effective approach to develop low cost, low damage, and low temperature bonding technology. In this research, not only conventional low-pressure plasma treatment (Ar RF plasma) but also atmospheric-pressure plasma treatment (Ar+O<inf>2</inf>, Ar+H<inf>2</inf>) was investigated for low-temperature Au-Au surface-activated bonding (150°C). In the case of Au thin film to Au thin film bonding, enough bonding strength was not obtained with Ar+O<inf>2</inf> atmospheric-pressure plasma treatment due to Au<inf>2</inf>O<inf>3</inf> formed on Au surface. However, by using Au microbump (diameter at the top: 5 μm, height: 2 μm, and pitch: 10 μm), strong bonding strength was obtained with all these plasmas. Semiconductor laser diodes chips were successfully bonded to Si substrates wiht Au microbumps at low temperature (150°C) in ambient air using Ar+H<inf>2</inf> atmospheric-pressure plasma treatment.

Heterogeneously integrated laser-induced fluorescence detection devices: Integration of an excitation source

The direct current pumping from highly doped silicon microwire to InP-based III-V active layer for spontaneous light emission was realized by air ambient plasma-assisted direct bonding. The semi-conductive properties of the hetero-integration and the effects of plasma-assisted bonding process on InGaAsP multiple quantum well (MQW) were measured and discussed. The electrical pumping from silicon microwire to InGaAsP MQW material for spontaneous light emission was successfully demonstrated afterwards.

Low-temperature bonding of laser diode chips on Si substrates with oxygen and hydrogen atmospheric-pressure plasma activation

In this study, we demonstrate room-temperature vacuum sealing by combining Cu compliant rim with ultrasonic assist.