The air-gap structure between integrated LiNbO3 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 LiNbO3 resonant-type optical modulator chips on the Si substrate. To prevent this substrate effect, an air-gap was formed between the LiNbO3 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.
© 2011 OSA
Silicon (Si) offers a promising platform because of its excellent electrical, mechanical and thermal properties, and it can be widely used for microelectronics, microelectromechanical systems (MEMS), Si photonics and optoelectronic packaging. Recently, Si-on-insulator waveguide modulators have been investigated for high-speed and compact photonic systems [1–5]. However, Si does not provide the essential electro-optical (EO) effect for reliable high-speed optical modulation. A solution that overcomes this problem is the hybrid integration of well-developed lithium niobate (LiNbO3) high-speed EO modulator chips on Si platforms.
Eutectic AuSn (80 wt% Au, 20 wt% Sn, eutectic temperature: 280 °C) has been widely used for the chip bonding of optical components, such as laser diode chips because of its excellent mechanical and thermal properties [6–12]. However, AuSn bonding processes require a high temperature > 300 °C, and this high temperature processing produces cracks during bonding because of the large coefficients of thermal expansion (CTE) between LiNbO3 (CTE: 14.4 (Z cut) −7.5 (X, Y cut) × 10−6/K)  and Si (CTE: 2.6 × 10−6/K). As an alternative to conventional AuSn bonding, a low-temperature solid-state bonding technique using a surface-activated bonding (SAB) method and gold (Au) as an intermediate layer has been reported [14–17]. Recently, we demonstrated the surface mounting of LiNbO3 chips on Si substrates using a combination of visual index alignment and the low-temperature bonding of Au micro-bumps .
Using this technology, LiNbO3 optical modulator chips can be integrated on Si platforms. Additionally, direct metal-metal bonding has the potential to provide metal interconnections between the optical modulators and Si complementary metal-oxide semiconductor (CMOS) microelectronic circuits on the Si substrates. However, the performance of the optical modulator on the Si substrate is influenced by the high permittivity of the Si substrate.
In this paper, we investigate in detail the influences of the Si substrate on the modulation characteristics of LiNbO3 resonant-type optical modulators for the integration of LiNbO3 on Si. Our solution to this problem is to incorporate of an air-gap that exhibited a low permittivity of close to 1.0 between the LiNbO3 structure and Si substrate. As LiNbO3 is difficult to fabricate using conventional microfabrication processes, the concept of air-gap integration was generated using a combination of low-temperature flip-chip bonding and Si micromachining. It is possible to integrate LiNbO3 modulators on Si substrates without the substrate effect.
2. Device Structure and Sample Information
2.1. Hybrid Structure
To generate a high-speed electro-optic phase-modulation function on a Si substrate, air-gap structures between the LiNbO3 optical modulators integrated and micromachined Si substrates are fabricated. Figure 1 shows a schematic structure of the device. LiNbO3 optical phase modulator chips are passively aligned and mounted on Si substrates. Passive alignment techniques offer many advantages when compared with active alignment techniques, such as lower equipment expense, faster processing time and smaller device size . However, the highly precise positioning of optical components is required to achieve low-loss optical coupling between each single-mode waveguide component. Thus, a high precision flip-chip bonder has been used to bond the LiNbO3 chips with the waveguide side down onto the Si substrates for passive alignment. This bonder consists of a pretreatment chamber for plasma activation and a bonding chamber for alignment and bonding . After the organic contaminants on the Au are removed using argon radio frequency (RF) plasma, a surface activation process, direct Au-Au bonding is performed.
The Si substrate has V-grooves for aligning the optical fibers and a groove for the formation of the air-gap between the LiNbO3 modulator chip and Si substrate. Two single-mode optical fibers (diameter: 125 µm) on the V-grooves are attached to both end faces of the LiNbO3 chip with a single-mode waveguide. The center of the optical fiber core is located 6.3 µm from the surface of the SiO2 layer on the Si substrate. In the following section, the LiNbO3 optical modulator chip and Si substrate fabrication will be described in detail.
2.2. LiNbO3 Optical Modulator Chips
In this experiment, we integrated compact LiNbO3 resonant-type optical phase modulator chips [21,22] with micromachined Si substrates for high-speed optoelectronic systems. Figure 2 shows a schematic view of the LiNbO3 chip. The design value of the peak resonant frequency and operation wavelength are 10 GHz and 1.55 µm, respectively. A resonant-type optical modulator chip has a single-mode optical waveguide, symmetric modulating electrodes (L1 in Fig. 2(a): 1.6 mm) and stub electrodes (L2 in Fig. 2(a): 0.6 mm) . The voltage of the modulating electrode can be enhanced by the modulating electrode and stub resonance, which increases the short electrode modulation efficiency at the designed resonant frequency. The modulator size is smaller than that of conventional traveling-wave-type optical modulators, which is suitable for integrated modulators. Additionally, compared with conventional traveling-wave-type optical modulators, the modulation characteristics of a resonant-type optical modulator tend to depend largely on the device structure.
The LiNbO3 chips were prepared from commercially available, double-sided, polished, 3-inch Z-cut LiNbO3 wafers (thickness: 500 µm). The single-mode waveguides were fabricated via Ti (thickness: 80 nm, width: 8 µm) diffusion at a temperature of approximately 1,030 °C at the surface of the LiNbO3 wafer. A layer of SiO2 (thickness: 0.55 µm) was deposited on the LiNbO3 wafer using plasma chemical vapor deposition (CVD). The Au electrodes and Au alignment marks for visual index alignment were fabricated using electron-beam (EB) evaporation (thickness: 0.55 µm) and a lift-off process. The end faces of the waveguides are prepared using a dicing saw. The wafers were cut into 6 mm × 6 mm chips using a dicing saw.
The surface roughness is an important factor in achieving good direct bonds. The root-mean-square (rms) surface roughness of the Au electrode on the LiNbO3 chips was approximately 4.3 nm (scanning area: 2.5 µm × 2.5 µm), as measured by atomic force microscopy (AFM).
2.3. Si Micromachined Substrates
The Si substrates were prepared from double-sided, polished, 4-inch (100) wafers (thickness: 500 µm). Currently, wet/dry etching and mechanical machining are widely applied to the fabrication of Si substrates. In this experiment, to couple the LiNbO3 waveguides with the optical fibers using a passive alignment technique, V-grooves for aligning the optical fibers were precisely fabricated via wet anisotropic etching. The V-groove was defined with (111) planes (angled 54.7° to the substrate surface). However, the air gap grooves under the LiNbO3 chip were fabricated using a dicing process because of the facile generation of deep grooves. The Si substrate was then covered with a 1.0-µm SiO2 layer. Then, the bonding area was deposited with 40 nm of Ti and 2.0 µm of Au using EB evaporation. Next, Au micro-bumps and alignment marks for visual index alignment were made using photolithography and a dry etching process. The diameter and pitch of the Au micro-bumps on the Si substrates were approximately 5 µm and 25 µm, respectively, as shown in Fig. 3 . The Si wafers were then cut into 12 mm × 12 mm chips. The rms surface roughness of the Au micro-bumps was approximately 6.6 nm (scanning area: 2.5 µm × 2.5 µm), as measured by AFM.
3. Results and Discussion
3.1. Hybrid Integration
Direct bonding between the Au electrodes on the LiNbO3 chip and the Au micro-bumps on the Si substrate was conducted in ambient air at 100 °C with an applied bonding pressure of 300 MPa (contact load: 441 N), plasma irradiation of 150 s and a bonding time of 30 s. Applied bonding pressure is normalized using the Au microbump areas. These bonding conditions were optimized for strong bonding strength. The tensile strength was approximately 100 N. This tensile strength exceeded the failure criteria of MIL-STD-883E .
Figure 4 shows a scanning electron microscope (SEM) image of the area around the end face of a LiNbO3 optical modulator chip on a micromachined Si substrate. Alignment tolerance between a single-mode Ti-diffused waveguide and a single-mode optical fiber is approximately ± 1.0 µm to achieve low excess-loss (within approximately 1.0 dB) . Excess-loss was defined as the optical coupling loss due to misalignment between the Ti-diffused single-mode LiNbO3 waveguide and single-mode optical fiber with flat ends. In this experiment, the bonding accuracies (3σ) in the horizontal and vertical directions were within ± 0.8 µm and ± 0.2 µm, respectively (number of measurements: 10). Low excess losses, ranging from 0.2 dB to 0.6 dB per interface, were achieved (wavelength: 1.55 µm, number of measurements: 10).
3.2. Numerical Simulation
The electrical reflection (S11) parameter was calculated using the electromagnetic simulator based on a 3-D finite element method (HP HFSS ver.11.2). Figure 5 shows the cross-sectional view of the 3-D simulation model. The LiNbO3 chip size and electrode shape in the simulation model was identical to that of the chip used in the experiment, as shown in Fig. 2(a). The air-gap height between the LiNbO3 chip and Si substrate (size: 6 mm × 6 mm, thickness: 500 µm) with a SiO2 layer (1.0 µm) was changed to evaluate the Si substrate effect. The following parameters were used for numerical calculations: εAir, 1.0; εSi, 11.9; εSiO2, 4.0; εLiNbO3[x], 43.0; εLiNbO3[y], 28.0; εLiNbO3[z], 28.0 and σAu, 4.3 × 107(Ωm)−1, where εAir, εSi and εSiO2 are the permittivity of air, SiO2 and Si, respectively, εLiNbO3 [x], εLiNbO3 [y] and εLiNbO3 [z] denote the permittivity of LiNbO3 in the x, y and z directions, respectively, and σAu is the Au electrode conductivity.
Figure 6 shows the simulated resonant frequency as a function of the air-gap height. The peak resonant frequency shifts to the lower frequency side when the Si substrate approaches the LiNbO3 chip. When the LiNbO3 chip is contacted perfectly with the Si substrate (air-gap height: 0 µm), the peak resonant frequency changes from 10.0 GHz to 8.4 GHz. When the air-gap height is 1.0 µm, peak resonant frequency is 9.0 GHz. However, when the air-gap height is > 50 µm, the peak resonant frequency does not change. Therefore, incorporation of a thick air-gap between the LiNbO3 chip and Si substrate reduces the substrate effect.
3.3. Device Performance
To characterize the modulation performance of the device, the S11 parameter of the LiNbO3 chips was measured using a network analyzer (Agilent technologies, E8361C). Figure 7 shows the measured S11 parameter before and after mounting the LiNbO3 chips onto the micromachined Si substrate. The resonant frequency of each LiNbO3 chips before mounting ranged from 10.1 GHz to 10.4 GHz because of fabrication errors. When the air-gap height is 1 µm, peak resonant frequency changed from 10.4 GHz to 9.8 GHz, as shown in Fig. 7(a). As the air-gap height increased (air-gap height: ~50 µm), the shift of resonant frequency decreased from 10.1 GHz to 10.0 GHz, as shown in Fig. 7(b). For an extended air-gap (air-gap height: ~100 µm), the peak resonant frequency did not changed, as shown in Fig. 7(c). These experimental results closely resemble the simulation results (Fig. 6) and show that the air-gap significantly helps reduce the substrate-induced peak resonant frequency shift.
The optical modulation characteristic measurements were recorded. Figure 8 shows the experimental setup for measuring the optical modulation characteristics before and after mounting the LiNbO3 chips onto the micromachined Si substrate. The Au electrodes on the samples were connected to a RF connector (K type) via Au wire bonding. The RF signal from the signal generator was then transmitted to the samples through a RF connector and Au wire. The spectral data of the output were performed using an optical spectrum analyzer (Ando, AQ6317B). Figure 9 shows the measured optical spectra of the LiNbO3 optical modulators before and after mounting onto the micromachined Si substrate (RF signal: 10 GHz, transverse-magnetic polarization). Small changes in the optical modulation characteristics occur because the resonant-type optical modulator is very sensitive to the location of the Au bonding wire between the electrode and RF connector. The measured results show that the substrate-induced shift is negligible in the case of the air-gap structure between the LiNbO3 optical modulators and micromachined Si substrates.
In conclusion, the hybrid integration of 10 GHz LiNbO3 optical modulator chips and micromachined Si substrates by low-temperature flip-chip bonding is demonstrated. Low-loss coupling between the optical fiber and LiNbO3 optical modulator chip using passive alignment was achieved. Additionally, the optical modulation characteristics of the integrated LiNbO3 optical modulator chips on the high permittivity Si substrates were discussed in detail. The calculated and experimental results show that the air-gap structure between the LiNbO3 optical modulators on the Si substrates is useful for high-speed optical modulation.
The authors acknowledge T. Hare (Toray Engineering) for technical support and for operating the flip-chip bonder and the Photonics Device Laboratory staff (National Institute of Information and Communications Technology) for their technical support in the LiNbO3 fabrication process. This work was supported in part by the Industrial Technology Research Grant Program in 2006 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, Global COE Program (Global Center of Excellence for Mechanical System Innovation) and Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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