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Room-Temperature Direct Bonding for Integrated Optical Devices
Abstract
We report a room-temperature direct bonding technique for wafer combinations of InP-Ce:YIG, Si-InP and Si-LiNbO<sub>3</sub>. This technique is versatile for integrating optical devices that are composed of different crystals.
... Thus, lowering the annealing temperature is of great importance. The surface activated direct bonding technique was developed to integrate Ce:YIG into III-V compound semiconductors, silicon, and LiNbO 3 [12,34,35]. ...
This paper reviews the direct bonding technique focusing on the waveguide optical isolator application. A surface activated direct bonding technique is a powerful tool to realize a tight contact between dissimilar materials. This technique has the potential advantage that dissimilar materials are bonded at low temperature, which enables one to avoid the issue associated with the difference in thermal expansion. Using this technique, a magneto-optic garnet is successfully bonded on silicon, III-V compound semiconductors and LiNbO 3 . As an application of this technique, waveguide optical isolators are investigated including an interferometric waveguide optical isolator and a semileaky waveguide optical isolator. The interferometric waveguide optical isolator that uses nonreciprocal phase shift is applicable to a variety of waveguide platforms. The low refractive index of buried oxide layer in a silicon-on-insulator (SOI) waveguide enhances the magneto-optic phase shift, which contributes to the size reduction of the isolator. A semileaky waveguide optical isolator has the advantage of large fabrication-tolerance as well as a wide operation wavelength range.
... Because of this, a magnetooptic garnet with good crystallinity, sufficiently large magnetooptic effect, and practically low optical absorption has yet to be grown on the semiconductors [9]- [11]. To overcome this, we developed the surface-activated direct bonding technique for integrating a magnetooptic garnet on III-V compound semiconductors and silicon [12], [13]. ...
A wafer direct bonding technique enables one to integrate dissimilar crystals like a magnetooptic garnet on Si and III-V semiconductors, which facilitates fabrication of optical nonreciprocal devices on commonly used waveguide platforms. The surface-activated direct bonding technique is described, focusing on the change of surface roughness due to the surface activation process with oxygen or argon plasma irradiation. The interferometric waveguide optical isolator that uses magnetooptic nonreciprocal phase shift is fabricated by directly bonding a magnetooptic garnet onto a silicon rib waveguide. An isolation of 21 dB is obtained at a wavelength of 1.56 mu m. The interferometric optical isolator can be modified to a waveguide optical circulator. The calculated performance of the waveguide optical circulator is also shown in this paper.
... The bonding at wafer level is an industrial proven fabrication process very common in packaging of MEMS devices. The available bonding techniques used for industrial or research applications can be classified as: silicon fusion bonding [1], low temperature direct bonding [2], anodic bonding [3], eutectic bonding [4], glass-frit bonding [5] and adhesive bonding6789. Silicon fusion bonding requires processing at high temperature (usually 800-1100 O C) also the roughness of the surface is a very important parameter. ...
The present work proposes an adhesive bonding technique, at wafer level, using SU-8 negative photoresist as intermediate layer. The adhesive was selective imprint on one of the bonding surface. The main applications are in microfluidic area where a low temperature bonding is required. The method consists of three major steps. First the adhesive layer is deposited on one of the bonding surface by contact imprinting from a dummy wafer where the SU-8 photoresist was initially spun, or from a Teflon cylinder. Second, the wafers to be bonded are placed in contact and aligned. In the last step, the bonding process is performed at temperatures between 100(O)C and 200(O)C, a pressure of 1000 N in vacuum on a classical wafer bonding system. The results indicate a low stress value induced by the bonding technique. In the same time the process presents a high yield: 95-100%. The technique was successfully tested in the fabrication process of a dielectrophoretic device.
Waveguide optical isolators are investigated for an application to integrated optics. A nonreciprocal loss shift isolator has the advantage of high compatibility with optical active devices. The interferometric waveguide optical isolator that uses nonreciprocal phase shift brought about by the first-order magneto-optic effect is applicable to a variety of waveguide platforms. In a silicon-on-insulator (SOI) waveguide, the low refractive index of buried oxide layer enhances the magneto-optic phase shift, which contributes to reduce the device size of isolator. We developed the surface activated direct bonding technique to integrate a magneto-optic garnet crystal onto dissimilar crystals. The performance of SOI waveguide optical isolator was demonstrated with an optical isolation of 21 dB at a wavelength of 1559 nm.
A semi-leaky isolator was fabricated by bonding LiNbO3 onto a magneto-optic garnet waveguide. A 1.6mm-long device provides an isolation of 20.2dB at a wavelength of 1.55mum.
An electrically pumped light source on silicon is a key element needed for photonic integrated circuits on silicon. Here we report an electrically pumped AlGaInAs-silicon evanescent laser architecture where the laser cavity is defined solely by the silicon waveguide and needs no critical alignment to the III-V active material during fabrication via wafer bonding. This laser runs continuous-wave (c.w.) with a threshold of 65 mA, a maximum output power of 1.8 mW with a differential quantum efficiency of 12.7 % and a maximum operating temperature of 40 degrees C. This approach allows for 100's of lasers to be fabricated in one bonding step, making it suitable for high volume, low-cost, integration. By varying the silicon waveguide dimensions and the composition of the III-V layer, this architecture can be extended to fabricate other active devices on silicon such as optical amplifiers, modulators and photo-detectors.
A sequential plasma activation process consisting of oxygen reactive ion etching (RIE) plasma and nitrogen radical activation is proposed for wafer direct bonding at room temperature. The Si wafer surface is activated by oxygen RIE plasma and subsequently exposed to nitrogen radicals. The activated wafers by the two-step process were brought into contact in air followed by keeping them in air for 24 h. The wafers were bonded throughout the whole area and the bonding strength of the interface is as strong as bulk Si without any post-annealing process and wet chemical cleaning steps. XPS study indicates that the silicon surface has thermodynamically unstable characteristics. IR transmission images reveal a considerable amount of water is absorbed in the wafer surfaces during exposure to air after the plasma activation process. The high bonding strength is thought to be due to a diffusion of absorbed water into the wafer surface and a reaction between silicon oxynitride layers on the opposing wafer. TEM images show that an intermediate amorphous layer with thickness of 15 nm is formed across the interface. The bonding is so intimate that no micro-voids are found at the bonding interface. Furthermore, strong bonding of crystalline quartz and fused quartz at room temperature was also obtained by the sequential activation process.
Chemically cleaned InP(100) surfaces in aqueous HF solutions at 20°C have been studied using X-ray photoelectron spectroscopy (XPS) spectroscopic ellipsometry (SE), atomic force microscopy (AFM) and contact-angle measurements. The XPS data clearly indicated that the solutions caused the removal of the native oxide and left behind InP surface terminated by atomic fluorine. The SE data also indicated the immediate removal of the native oxide upon immersing the sample in the solutions. The AFM rms roughnesses for the HF-cleaned surfaces were ∼0.1–0.2 nm which were considerably smaller than those obtained from the SE data (∼0.6 nm), the difference may be due to the SE technique being sensitive to both the surface microroughness and adsorbed chemical species (and/or a newly grown oxide film), while AFM was sensitive only to the surface microroughness. The as-degreased InP surface was, if anything, hydrophilic, while the HF-cleaned surfaces were hydrophobic.
We present the experimental study of an optical isolator with a semiconductor guiding layer that was obtained by use of a nonreciprocal phase shift. The isolator is equipped with an optical interferometer composed of tapered couplers, nonreciprocal phase shifters, and a reciprocal phase shifter. The nonreciprocal phase shifter was constructed by wafer direct bonding between the semiconductor guiding layer and the magneto-optic cladding layer. The isolator, designed for the 1.55-mum wavelength, was fabricated to investigate the characteristics of each component. By applying an external magnetic field to the nonreciprocal phase shifter, we achieved an isolation ratio of approximately 4.9 dB in the interferometric isolator.
The applicability of wafer bonding as a tool to integrate the
dissimilar material system InP-to-Si is presented and discussed with
recent examples of InP-based optoelectronic devices on Si. From there,
the lowering of annealing temperature in wafer bonding by
plasma-assisted bonding is the essence of this review paper. Lower
annealing temperatures would further launch wafer bonding as a
competitive technology and enable a wider use of it. Oxygen plasma
treatment has been proven to be very feasible in achieving a strong
bonding already at low temperatures. It was also seen that in our
experimental setups the results depended on what plasma parameters that
were used, since different plasma parameters create different surface
conditions
1.3-μm InP-InGaAsP lasers have been successfully fabricated on
Si substrates by wafer bonding. InP-InGaAsP thin epitaxial films are
prepared by selective etching of InP substrates and then bonded to Si
wafers, after which the laser structures are fabricated on the bonded
thin films. The bonding temperature has been optimized to be 400°C
by considering bonding strength, quality of the bonded crystal, and
compatibility with device processes. Room-temperature continuous-wave
(RT CW) operation has been achieved for 6-μm-wide mesa lasers with a
threshold current of 39 mA, which is identical to that of conventional
lasers on InP substrates. Additionally, the lasers fabricated on Si have
exhibited higher output powers than the lasers on InP, which is due to
higher thermal conductivity of Si substrates. From these results, the
wafer bonding is thought to be a promising technique to integrate
optical devices on Si and implement optical interconnections between Si
LSI chips