Lithium niobate-on-insulator (LNOI) waveguides fabricated on a silicon wafer using a room-temperature bonding method have potential application as Si-based high-density photonic integrated circuits. A surface-activated bonding method using a Si nanoadhesive layer was found to produce a strong bond between LN and SiO2/Si at room temperature, which is sufficient to withstand both the wafer-thinning (LN thickness <5 μm) and surface micromachining processes used to form the strongly confined waveguides. In addition, the bond quality and optical propagation characteristics of the resulting LNOI waveguides were investigated, and the applicability of this bonding method to low-loss LNOI waveguide fabrication is discussed. The propagation loss for the ridged waveguide was approximately 2 dB/cm at a wavelength of 1550 nm, which was sufficiently low for the device application. The results of the present study will be of significant use in the development of fabrication techniques for waveguides with any bonded materials using this room-temperature bonding method, and not only LN core/SiO2 cladding waveguides.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Lithium niobate (LiNbO3: LN) is a unique ferroelectric material with excellent electro-optical (EO), non-linear optical (NLO), and acousto-optical characteristics, in addition to a wide transmission range from the ultraviolet to the mid-infrared. LN is thus widely utilized for numerous photonics applications, including ultra-fast optical modulators, wavelength converters, and optical microsensors. Thin-film LN-on-insulator (LNOI) has recently received significant attention as a platform capable of realizing high-density photonic circuits that can fully exploit the unique properties of LN [1-6]. Compared to conventional Ti-diffused LN waveguides, LNOI waveguides exhibit a high refractive index difference between LN and SiO2, which results in much stronger optical confinement. The much stronger confinement leads to improved performance of conventional LN devices and allows for device miniaturization. The combination of LNOI waveguides with Si, which is a promising platform for optical benches, microelectromechanical systems (MEMS), large-scale integration (LSI), and Si photonics, is an interesting approach to producing highly functional and multi-functional opto-electro-mechanical platforms. Therefore, it is desirable to develop techniques for the direct bonding of LN and SiO2/Si.
Direct bonding of LN and Si wafers has been a challenge due to the large coefficient of thermal expansion (CTE) mismatch between these two materials: 14.4 (a, b-axes)–7.5 (c-axis) × 10−6 K−1 and 2.6 × 10−6 K−1 for LN and Si, respectively, at room temperature . Conventional plasma-activated bonding has been employed for the direct bonding of LN and Si ; however, annealing (typically performed above 200 °C for a few hours ) is required for the bonded wafers to withstand post-bond processing, particularly wafer thinning, which is required for waveguide fabrication. To avoid this large CTE mismatch, typical LNOI wafers have been fabricated by plasma-activated bonding of a LN wafer to a SiO2 layer deposited on another LN wafer. Our previous research suggested that the post-bonding annealing temperature for a directly bonded LN/Si wafer should be limited to approximately 100 °C, due to the large CTE mismatch [10,11]. Local laser annealing has also been developed recently to minimize the thermal stress in the bonded LN/Si wafer [11,12]. On the other hand, organic adhesives such as benzocyclobutene (BCB) have been used as a low-temperature intermediate bonding layer for LN and Si [13-15]. Although this organic adhesive layer also acts as a cladding layer for the LNOI waveguide, the refractive index difference between LN and BCB is lower than that for LN and SiO2, which results in weaker optical confinement. In addition, organic adhesives tend to be unsuitable for hermetically sealed packaging.
As an alternative, surface-activated bonding (SAB) [7,16-21] and atomic diffusion bonding  are promising methods for the low-temperature bonding of dissimilar materials. To date, SAB has been utilized to achieve low-temperature bonding of LN and Si by direct bonding [7,16,17] and Au microbump bonding [19-21]. However, it is difficult to apply the conventional SAB method using a Ar fast atom beam (FAB) for direct bonding of ionic materials such as SiO2, sapphire, and SiN . To overcome this limitation, an SAB method that employs a nanoadhesive layer has been proposed [24-27]. We have recently performed SAB using an Fe nanoadhesive layer to directly bond LN wafer to a SiO2 layer on a Si wafer at room-temperature . In LNOI waveguide applications, an Fe nanoadhesive layer between the LN core and SiO2 cladding can lead to a large propagation loss of the guided light due to the high optical absorption by Fe. Therefore, Si would be a good choice as an adhesive material because it exhibits a wide transmission range in the infrared wavelength region. This SAB method could thus be applied using a Si nanoadhesion layer to bond LN and SiO2 for the fabrication of low-loss LNOI waveguides.
Here, we demonstrate LNOI waveguides fabricated on a Si wafer by room-temperature SAB using a Si nanoadhesive layer. The Si nanoadhesion layer achieves a strong bond between LN and SiO2 at room temperature, which is sufficient to withstand the wafer-thinning (LN thickness <5 μm) process for thin-film LNOI/Si hybrid wafers. Low-loss LNOI waveguides are produced on a Si wafer, which would be suitable for photonic integrated circuits. The bond quality and the propagation characteristics of the resulting LNOI waveguides are investigated, and the applicability of this bonding method to low-loss waveguide fabrication is discussed.
2. Room-temperature bonding using Si nanoadhesive layer
Commercially available 3-inch LN (Z-cut) wafers and 3-inch Si wafers with a thermally grown 1 μm thick SiO2 layer were procured. In the LNOI waveguide, the 1-μm-thick SiO2 layer acts as a cladding layer for the optical confinement of the LN core. The thicknesses of the LN and Si wafers were 500 and 360 μm, respectively. SAB using a Si nanoadhesion layer was performed to bond the LN and SiO2/Si wafers using Ar FAB irradiation, as shown in the schematic illustration in Fig. 1. A LN wafer, Si wafer with a thermally grown 1-μm-thick SiO2 layer, and a Si blanket wafer were set in the bonding chamber. The Si wafer with the thermally grown SiO2 layer was set on the upper side, while the Si blanket wafer was set on the lower side as a sputtering target. The surface activation process with Ar FAB bombardment was started when the background pressure reached less than 1 × 10−5 Pa. The FAB source generates a neutralized Ar atom beam with a voltage of 1.8 kV and a current of 100 mA. FAB irradiation of the Si blanket wafer was conducted for 10 min, and the sputtered Si thin film was then deposited on the Si/SiO2 wafer. The Si blanket wafer on the lower wafer holder was then replaced with the LN wafer, without opening the bonding chamber to the atmosphere. FAB irradiation of the Si thin film deposited on the surface of the SiO2/Si wafer was performed for 2 min, and an ultrathin Si film was deposited on the surface of the LN wafer. The Si-deposited surfaces of LN and SiO2 were then brought into contact at room temperature with an applied load of approximately 22 MPa. Dicing and blade tests were performed to evaluate the bond strength of the bonded wafer. The bond state between LN and SiO2 was investigated by analysis of the nanostructure using high-resolution transmission electron microscopy (TEM) and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX).
3. Thin film lithium niobate on insulator
3.1. Bond strength between LN and SiO2
Figure 2 shows a photograph of the resulting LN and SiO2/Si hybrid wafer bonded at room temperature, from which it is clear that almost void-free bonding without crack generation was achieved. The bonded wafer was cut into 10 mm × 10 mm dies and then cut into 0.5 mm × 0.5 mm dies, as shown in Fig. 3(a). This perfect dicing indicates that a strong bond can be achieved at the LN/SiO2 interface, which is sufficient to withstand the dicing process. The bond strength was evaluated using the crack-opening method . After bonding, a razor blade was inserted at the bonding interface and the crack length on the edge of the bonded wafer was measured through the LN wafer. Figure 3(b) shows a photograph of the LN and SiO2/Si bonded wafer after crack propagation. The heterogeneous surface energy, which represents the bond strength, is estimated from the crack length. Immediately after blade insertion, the measured crack length was approximately 8.0 mm, and the estimated surface energy of the bonded wafer at room temperature was approximately 2.2 J/m2, which was close to 2.5 J/m2, the theoretical energy for a Si(100) surface .
The thickness of the LN in the resulting bonded wafer was reduced from 500 μm to less than 5 μm by conventional mechanical polishing. Figure 4 shows a cross-sectional scanning electron microscopy (SEM) micrograph of the resulting thin-film LNOI/Si wafer. No peeling was observed over the entire bond interface during polishing. These results indicate that a strong bond was achieved between the LN and SiO2, which may be sufficient to withstand post-bonding processes such as wafer thinning by mechanical polishing.
3.2. Bond interface state
Herein, we focus on the bonding interface state between LN and SiO2 by investigating the nanostructure across the bonding interface. Figure 5 show a cross-sectional high-resolution TEM image of the LN/SiO2 bonding interface; no cracks or voids were present at a nanoscale level. In addition, an approximately 7-nm-thick uniform amorphous layer was observed, which is deposited during the surface activation process. A sputter-deposited Si film is well known to have an amorphous structure. No interface corresponding to the original amorphous Si surface was observed, although the original bonding surface exists within this layer. These observations suggest that close atomic-level contact between the LN and SiO2 films can be achieved at room temperature. The bonding interface state provides evidence that the bond is sufficiently strong to withstand wafer thinning by mechanical polishing.
An STEM-EDX analysis was performed across the bonding interface, and peaks corresponding to C, Cu, Nb, Si, O, and Ar were observed. The C peak is due to contamination during TEM observations, and the Cu peak is due to the copper mesh sample holder. The atomic ratio for each atom was determined, and the compositional distribution of Nb, Si, O, and Ar atoms across the bonding interface is shown in Fig. 6. Our previous research showed that Fe was effective as an adhesion layer between LN and SiO2 . The STEM-EDX analysis detected no metal atoms present in this amorphous Si nanoadhesion layer. The small amount of Ar atoms is due to Ar FAB bombardment during the surface activation process. The surface energy estimated during this study was approximately 2.2 J/cm2, which is close to the value of 1.8 J/cm2 for the bond using an Fe nanoadhesive layer . These results suggest that amorphous Si also functions well as a nanoadhesive layer to form a strong bond between LN and SiO2. In the case of the room-temperature bonding of amorphous Si films, the interface corresponding to the original surface is present because the self-diffusion of amorphous Si is not considerable . Figure 6 shows the diffusion of Nb and O atoms into the Si nanoadhesive layer. This diffusion and the crystal structure appear to be responsible for the lack of an interface corresponding to the original surface, as shown in Fig. 5, so that a strong bond is achieved at room temperature.
4. Waveguide fabrication and optical propagation characteristics
In Section 3, the fabrication of a LNOI/Si hybrid wafer using the SAB method and the resultant bond quality were discussed. As a next step, ridged LNOI waveguides were fabricated on the wafer by micromachining the LN thin film. To date, surface micromachining of LN has been difficult due to its high etching resistance. As an alternative to dry/wet etching, several micromachining techniques have been investigated for the fabrication of ridged LN waveguides with smooth sidewalls, including dicing [4,31], ductile mode cutting [32,33], and femtosecond laser micromachining . In the present study, a dicing process was adopted, which is able to produce smooth sidewalls through polishing by rotation of the dicing blade. Although dicing is unable to produce bent structures, it is considered to be a relatively simple micromachining technique. Figure 7 shows SEM images of the resulting LNOI ridged waveguides. After the bonded wafer was cut into chips, the ridged waveguides were fabricated on the chips without interfacial debonding due to the applied stress during dicing. The width and height of the LN ridge structures were approximately 5-7 μm and 5 μm, respectively.
Next, the propagation loss for the fabricated LNOI waveguides was assessed using the cutback method . Waveguides with lengths of 10 and 20 mm were prepared, and both end faces of each waveguide were polished by dicing. Single-mode fibers held by micro-positioners were optimally located using the active alignment method at both ends of the LNOI waveguide. The power transmitted through the fiber-LNOI waveguide-fiber system was measured using a power meter connected to the fiber at the output end. The fiber at the input end was a polarization-maintaining fiber connected to a 1550 nm light source coupled to a polarization controller. The propagation loss was estimated by subtracting the transmitted power for the 20-mm-long waveguide from that for the 10-mm-long waveguide and dividing the result by the difference in length (10 mm). The estimated propagation losses were approximately 2.0 dB/cm for transverse-electric (TE) polarized light and 2.1 dB/cm for transverse-magnetic (TM) polarized light. The measured propagation losses are close to that typical of a ridged LN waveguide fabricated using a conventional dicing process [4,30]. Figure 8 shows near-field patterns for the 20 mm long LNOI waveguide (width: approximately 5 μm) in the TE and TM modes. The near-field pattern for the fundamental mode of the guided light in the LNOI waveguide could be observed in both modes. The full width at half maximum (FWHM) in the horizontal and vertical direction was approximately 5 μm. These results demonstrate that this room-temperature bonding method with a Si nanoadhesion layer is effective for the fabrication of low-loss LNOI waveguides on Si. In future work, we will measure the propagation losses more precisely by the Fabry–Pérot method  to avoid the reflection at the both end faces of the LNOI waveguide.
LNOI waveguides were fabricated on a 3-inch Si wafer using a room-temperature bonding method with a Si nanoadhesive layer. This bonding method provided a strong bond between the LN core and SiO2 cladding at room temperature, which was sufficient to withstand the required wafer thinning process (LN thickness <5 μm) by mechanical polishing. TEM observation revealed an approximately 7-nm-thick amorphous Si layer at the bonding interface between LN and SiO2, which appears to function well as a nanoadhesive to facilitate the formation of a strong bond. The measured propagation loss for the LNOI waveguide formed on the Si wafers by surface micromachining (dicing) was approximately 2.0 dB/cm and 2.1 dB/cm for TE and TM polarization, respectively, at a wavelength of 1550 nm. These results provide evidence that a Si nanoadhesive layer is a good choice for the fabrication of low-loss LNOI waveguides on Si substrates. The LNOI waveguides produced in this study serve as a demonstration of the potential for Si-based high-density photonics integrated circuits with large electro-optical, non-linear optical, and acousto-optical effects. The results obtained in the present study will be of significant use for the application of this bonding method to waveguides constructed from any bonded materials, and not limited only to LN core/SiO2 cladding waveguides.
Japan Society for the Promotion of Science (JSPS) KAKENHI (JP17H04925).
The authors would like to acknowledge Mitsubishi Heavy Industries Machine Tool Co., Ltd. for assistance with the surface-activated bonding experiments, and Prof. Tetsuya Kawanishi and Dr. Yuya Yamaguchi at Waseda University and Dr. Akihiro Ikeda at Kyushu University for helpful discussions.
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