Warlink Kona ----- Germanium to silicon nitride mid-infrared integrated photonics waveguides
Introduction
A germanium platform with large core-cladding index contrast, silicon nitride germanium waveguide, was demonstrated at mid-infrared wavelength. The feasibility of this structure is verified by simulation. This structure is achieved by first bonding germanium-on-silicon donor wafers deposited with silicon nitride to silicon substrate wafers, and then obtaining the germanium-on-silicon nitride structure by layer transfer method, which is scalable to all wafer sizes.
Introduce
Silicon-based photonics has received a lot of attention in recent years due to its compatibility with CMOS processes and its potential for integration with microelectronics. Researchers have been trying to extend the operating wavelength of photonics to the mid-infrared (MIR), defined here as 2-15 μm, because there are promising applications in MIR, such as next-generation communications, biochemical sensing, environmental monitoring, and more. Silicon on standard insulators (SOI) are not suitable for MIR because the material loss for burying oxide layers becomes very high at 3.7lm and above. Many efforts have been made to find an alternative material system that could work on Mir. Silicon on Sapphire (SOS) waveguide technology has been pursued to extend the operating wavelength range to 4.4lm. Silicon nitride (SON) waveguides, which provide a wide transparency range of 1.2-6.7 μm, have also been proposed. Germanium (ge) has wide transparency and many optical properties, making it a good alternative to SOI.
Germanium on Insulator (GOI) has been proposed, and passive waveguides and active germanium modulators have been manufactured on the platform, but as mentioned above, burying oxide layers actually limits the transparency of the platform. Germanium on SOI has also been reported to have electrical advantages. The germanium on Silicon (GOS) platform is currently widely used in photonics research and has already achieved a number of impressive achievements. The lowest propagation loss germanium waveguide on this platform is only reported to have a loss of 0.6dB/cm. However, germanium (n. 4. The refractive index is 3.8 μm. Therefore, the bending radius of the GOS must be correspondingly greater than the bending radius of the SOI, resulting in the coverage area of the devices on the GOS chip usually greater than the SOI. What is needed is a better alternative germanium waveguide platform that will provide greater core cladding refractive index contrast than GOS, as well as useful transparency and a smaller channel bending radius.
In order to achieve these goals, the structure proposed and implemented in this work is germanium nitride on silicon, here called GON. The refractive index of our PECVD silicon nitride (SiNx) was measured by ellipsometry at 3.8lm. The transparency of SiNx is usually up to about 7.5 mm. So the exponential contrast in GON is. Once this Ge platform operating in the MIR range is implemented, there will be many passive photonic devices that can be manufactured with a compact footprint, such as MachZehnder interferometers, microring resonators, and so on. In order to make a compact ring, a small bending radius is required, which is only possible in high-contrast waveguides with strong optical limitations. Moving forward, compact sensing devices can also be realized based on microring resonators with such germanium platforms. Most importantly, we have developed a viable and scalable wafer bonding and layer transfer technology to implement GON.
Experiment
Germanium/silicon platforms can be manufactured through several technologies. These techniques include germanium condensation, liquid phase epitaxy, 20, and layer transfer techniques.21 However, when germanium is grown directly on silicon nitride, the quality of germanium crystals is expected to be poor and a high density of defects is formed
Graph. 2. Compared with GOS, the simulated bending loss of Nepal government is lower, indicating that the waveguide bending loss of Nepal government is lower.
Because SiNx is amorphous. As a result, these defects increase scattering losses. In this work, we utilize wafer bonding and layer transfer techniques to fabricate GON as shown in Figure 2. Silicon donor wafers use reduced pressure chemical vapor deposition (RPCVD) and a three-step germanium growth process.22 The germanium epitaxial layer is then coated with silicon nitride and transferred to another silicon substrate to obtain GON wafers. For comparison, some germanium silicon (GOS) chips (which grow in a similar way but do not transfer) were included in subsequent experiments. The final germanium layer usually has a penetration dislocation density (TDD) of < 5106cm2, surface roughness < 1nm, and tensile strain of 0.2%.23 In addition, the donor wafer is cleaned to obtain a surface free of oxides and contaminants, then rinsed with deionized water (DI water) and N2 drying. After the cleaning process, the donor wafers are loaded into the Cello PECVD system for the deposition of tension strain SiNx. Annealing for a few hours after deposition ensures that gases trapped in the wafer are released during deposition.
All heat treatments are performed at temperatures below 40 ° C. In addition, another 1 mm SiNx is deposited on the back of the wafer to compensate for the bending effect. By low temperature plasma chemical vapor deposition, the binding layer of 300 nm is finally deposited. The bonding layer is silica, making it easy to bond with another silicon-treated wafer. Due to the use of hydrophilic bonding in this work, water molecules are formed in the bonding reaction. Therefore, silica was chosen as the bonding layer because it can absorb these water molecules, thus providing a high bonding quality.24 The bonding layer is chemically mechanically polished (chemo-mechanical polished) to 100 nm to reduce the surface roughness and make it suitable for wafer bonding. The donor wafer can then be bonded to a silicon substrate wafer. Prior to bonding, both wafer surfaces are exposed to O2 plasma for about 15s to improve surface hydrophilicity.
After that, the Adi washing step is added to increase the density of the surface hydroxyl group, thereby triggering the binding. The bonded wafer pairs are then annealed for about 4 hours after bonding at temperatures below 30 ° C to improve bonding strength. Bonding wafers are examined using infrared imaging to check for interfacial void formation. To complete the layer transfer process, the top silicon donor wafer is ground in order to transfer the germanium/silicon nitride layer stack on the substrate wafer. This is followed by wet etching using tetramethylammonium hydroxide (TMAH) to completely remove the silicon donor wafer. Considering the high selectivity of silicon to germanium, the etching stop occurs at the original germanium/silicon interface.
The germanium/silicon interface layer is then removed by chemical and mechanical polishing. Our process uses two silicon wafers, silicon donor wafers and silicon substrate wafers, so it is scalable to all chip sizes. X-ray diffraction (XRD) analysis was used to characterize the quality of germanium thin films, referring to GOS after the manufacture of Gunn chips, and the results are shown in Figure 4. XRD analysis shows that the crystal quality of Germanium epitaxial layer has no obvious change, and its peak strength and curve shape are similar to that of Germanium on silicon wafer.
Graph. 4. XRD pattern of Geng and GOS germanium epitaxial layer.
Sum up
In summary, defective layers containing mismatched dislocations can be exposed by layer transfer and removed by chemical-mechanical polishing, thus providing a high-quality germanium layer on SiNx under the coating. Simulations were performed to investigate the feasibility of the GON platform providing a smaller channel bend radius. Waveguides are manufactured on GON wafers and characterized at 3.8lm wavelengths. The bending loss at a GON with a radius of 5 mm is 0.1460.01 dB/bend and the propagation loss is 3.3560.5 dB/cm. These losses are expected to be further reduced by using advanced processes (such as electron beam lithography and deep reactive ion etching) or by not structuring to improve sidewall quality.
