Introduction
The insatiable demand for higher data transmission rates in applications like autonomous driving, smart farming, and telemedicine is driving the need for multi-Tb/s optical communication systems by 2030. Conventional single-material photonic technologies like InP monolithic integration and silicon (Si) photonics have limitations in meeting the requirements for high-speed operation and low power consumption simultaneously. The III-V/Si hybrid integration technology emerges as a promising solution to overcome these limitations.
This approach enables not only the large transmission capacity facilitated by the multiple-channel integration of high-speed and efficient III-V active devices, but also superior characteristics over single-material devices by leveraging the advantages of both III-V and Si materials. While InP-based gain region/Si waveguide hybrid lasers fabricated via wafer-to-wafer direct bonding have been demonstrated previously, the chip-on-wafer (CoW) bonding process is indispensable for enabling the monolithic integration of various types of III-V epitaxial structures and diverse functionalities.
CoW Hydrophilic Bonding Process
The CoW bonding process, as illustrated in Figure 1, facilitates the integration of different III-V active regions on a Si waveguide circuit. Initially, InP chips with epitaxial layers undergo a mega-sonic cleaning process, and their surfaces are made hydrophilic through ultra-violet (UV)-ozone treatment, ensuring sufficient hydrophilization without increasing surface roughness.
The InP chips, with epitaxial layers facing down, are then bonded onto a silicon-on-insulator (SOI) wafer using a pick-and-place technique. To enhance the bonding strength, the CoW assembly is annealed at 150°C under mechanical pressure in a vacuum chamber.
Integration of Various Epitaxial Structures
Leveraging this process, the monolithic integration of III-V active regions with various epitaxial structures and different thicknesses is demonstrated, as shown in the photomicrograph in Figure 2.
A Si waveguide circuit designed for an optical transceiver chip for digital coherent transmission, including a laser (LD), semiconductor optical amplifiers (SOAs), modulators (MODs), and photodetectors (PDs), is fabricated on a Si wafer. InP chips with dimensions of 1.8 mm × 0.5 mm are bonded onto each active region.
After the CoW bonding, the InP substrate is removed through a combination of chemical mechanical polishing (CMP) and wet chemical etching, leaving only the InP-based epitaxial layers on the Si wafer, as shown in the scanning electron microscope (SEM) image in Figure 3(a).
The photoluminescence (PL) measurement results for these epitaxial layers, shown in Figure 3(b), reveal PL spectra with different peak wavelengths around 1.20, 1.53, and 1.61 μm, corresponding to the GaInAsP active layer in the LD/SOA region, the GaInAsP core layer in the MOD region, and the GaInAs absorption layer in the PD region, respectively.
These results demonstrate the capability of the CoW bonding process to enable the monolithic integration of III-V active regions with different epitaxial structures on a Si photonics platform.
Aging Test of Hybrid Lasers
To investigate the reliability of active devices fabricated via the CoW bonding process, an aging test was performed on hybrid Fabry-Perot (FP) lasers, as shown in the schematic in Figure 4.
In the gain region, InP chips including a GaInAsP active layer are directly bonded onto a straight Si waveguide, and an InP shallow ridge waveguide structure is formed. The widths of the InP ridge stripe (WInP) and Si waveguide (WSi) are 2.2 μm and 1.0 μm, respectively. The Si waveguide region is optically connected to both sides of the gain region through a two-step taper structure consisting of n-type and p-type tapers. The cavity is configured by the facets on the Si waveguide, with a total cavity length of 2.5 mm, including a gain-region length of 1.0 mm.
The aging conditions from the beginning to 2,000 hours involved an injection current of 200 mA and a temperature of 85°C. After that, an aging test with a temperature condition of 110°C was carried out for 550 hours, resulting in a total aging time of 2,550 hours.
Figure 5 shows the light output and current (L-I) characteristics of a hybrid FP laser measured at 25°C, before and after the aging test.
No significant degradation is observed in the L-I curves, and the change in the threshold current is within 10%. Consequently, the stable continuous wave (CW) operation of the hybrid laser is verified in the aging test, demonstrating the high quality of the III-V active region on the Si waveguide via the CoW bonding process.
Conclusion
The monolithic integration of III-V active regions with different epitaxial structures on a Si wafer has been demonstrated using the CoW hydrophilic bonding process. Furthermore, the stable CW operation of hybrid FP lasers has been validated in an aging test spanning 2,550 hours. These results highlight the promising potential of the CoW bonding technique for realizing photonic devices with various III-V active regions monolithically integrated on the Si photonics platform, paving the way for high-performance, power-efficient optical communication systems.
Reference
[1] T. Kikuchi, M. Kurokawa, N. Fujiwara, N. Inoue, T. Mitarai, H. Fujikata, T. Hiratani, T. Nitta, Y. Itoh, T. Watanabe, C.-Y. Lee, A. Furuya, T. Horikawa, N. Nishiyama, and H. Yagi, "Monolithic integration of various-type III-V epitaxial structures on silicon-photonics platform using chip-on-wafer hydrophilic bonding process," presented at the IEEE SiPhotonics, Photonics Electronics Technology Research Association (PETRA), Sumitomo Electric Industries, Ltd., and Tokyo Institute of Technology, Tokyo, Japan, 2024.
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