1. Background
Grating couplers are key optical interfaces in modern photonic integrated circuits (PICs). They enable efficient coupling of light between on-chip waveguides and external optical fibers, which is critical for scalable photonic systems in data communication, sensing, and optical computing.
Structurally, a grating coupler consists of a periodic array of etched lines or grooves that act as a diffraction grating, converting light between an in-plane guided mode and an out-of-plane free-space or fiber mode. The performance of these devices—specifically their coupling efficiency, angular response, and spectral bandwidth—is determined by geometric parameters such as the grating period (pitch), duty cycle (fill factor), and etch depth.
In many PIC platforms, grating couplers are buried under silicon or encapsulated within multilayer semiconductor stacks to route light between chip layers, as illustrated in Figure 1. This buried configuration complicates direct optical inspection and metrology, since conventional visible-light microscopy cannot penetrate opaque semiconductor materials, while conventional infrared microscopy, though capable of imaging through silicon, lacks the spatial resolution needed to clearly resolve submicron grating features.
As photonic integration advances, chips are becoming increasingly complex and multilayered, enabling vertical optical interconnects and denser circuit architectures. Both major industry players such as Oracle [1,2] and leading research groups [3–7] are investigating grating-coupler-based solutions to directly link optical signals between layers. These efforts highlight the growing importance of imaging technologies capable of resolving buried photonic features with high fidelity.
Figure 1. a) Light is transferred between the two silicon chips by aligning their grating couplers face-to-face for optical coupling [1]. b) Photograph of two bonded silicon chips featuring a grating-to-grating optical interconnect, tested using optical fibers [5].
2. Objective
Accurate characterization of grating couplers is critical because even minor fabrication deviations can strongly influence optical performance—shifting the peak coupling wavelength, altering efficiency, or increasing back reflections. While such nanometer-scale variations cannot be directly resolved with optical or infrared microscopy, higher-resolution imaging remains essential for identifying larger-scale indicators of process drift, non-uniformity, or damage.
Through-silicon imaging enables engineers to examine the physical integrity of these devices once they are embedded or bonded between layers—without cross-sectioning or delayering. This approach is particularly valuable for inspecting grating regions and waveguides after flip-chip or wafer-bonding processes, where alignment errors, bonding defects, or thermal stress can distort or misalign buried structures.
By capturing high-contrast images of the buried features, it becomes possible to assess:
- Pitch uniformity and periodicity : detection of pattern distortions or process drift across the device
- Etch completeness and trench definition : identification of collapsed or contaminated regions
- Overlay alignment : verification of registration between lithography or bonding layers
- Local and global non-uniformities : visualization of across-wafer process variations or bonding stress zones
This capability provides valuable feedback during fabrication and packaging development, supporting correlation between optical test results and structural integrity -helping to pinpoint root causes of coupling loss or yield degradation.
3. Methodology
- Sample : integrated silicon chip sandwiched between two 500um thick silicon wafers
- System under test : Jay Photonics’ infrared imaging system
- Reference system : high-end industrial IR microscope
- Acquisition conditions : identical samples, silicon chip with an unpolished backplane and silicon wafers double-side polished
4. Observations and analysis
Infrared images obtained with the Jay Photonics Si-Through-HR system reveal the fine periodic lines (400 nm wide) of grating couplers and adjacent waveguides with exceptional clarity and contrast. Compared to conventional IR microscopes, the Jay Photonics system provides enhanced spatial resolution and depth discrimination across magnifications of 20X, 40X, and 100X.
This clarity enables detailed inspection of:
- Grating pitch and etch uniformity
- Missing or collapsed grooves
- Particulate contamination on buried surfaces
- Overlay alignment between successive process layers
- Across-wafer pattern uniformity and process variation
Figure 2. Infrared comparison images illustrating the Si-Through-HR microscope’s performance versus conventional IR imaging at 20X, 40X, and 100X magnifications.
Figure 3. Image of an integrated silicon chip encapsulated between two 500 µm silicon wafers, demonstrating high-definition imaging of buried layers with exceptional precision.
5. Conclusion
The Jay Photonics Si-Through-HR microscope provides a powerful, non-destructive method for visualizing photonic grating couplers and other buried structures through silicon. By combining deep-silicon transparency, sub-micron resolution, and real-time acquisition, it delivers clear, high-fidelity images of features that directly influence optical performance.
For grating couplers, this visibility is especially valuable. It allows engineers to correlate coupling efficiency with actual physical parameters—such as pitch uniformity and overlay accuracy—without resorting to destructive cross-sectioning. Early detection of fabrication drifts or structural defects helps optimize coupling strength, which are critical to both device yield and system performance.
By providing this immediate, structural feedback, the Si-Through-HR microscope accelerates process refinement, reduces development cycles, and ensures that photonic components perform as designed once integrated into full systems.
6. Curious to See the Difference?
If you’re developing 3D photonic integrated circuits, flip-chip assemblies, or bonded and stacked PIC architectures, seeing the Si-Through-HR system in action can reveal details that conventional inspection tools simply can’t capture. Its ability to image through silicon enables direct visualization of buried grating couplers, alignment interfaces, and bonding layers—helping engineers verify structural integrity without destructive preparation.
Jay Photonics offers live virtual demonstrations, where our engineers walk you through the imaging process in real time. We can show buried grating patterns, verify uniformity across a die, or visualize overlay alignment directly through silicon. You can choose to use our standard samples or, under NDA, your own photonic devices.
Interested? Reach out to schedule a session.
References
[1] Yao, Jin, et al. "Grating-coupler based low-loss optical interlayer coupling." 8th IEEE International Conference on Group IV Photonics. IEEE, 2011.
[2] Yao, Jin, et al. "Grating-coupler-based optical proximity coupling for scalable computing systems." Optoelectronic Interconnects and Component Integration XI. Vol. 7944. SPIE, 2011.
[3] Pashkova, Tatiana, and Peter O'Brien. "Integrated photonic chip to chip interconnection utilising grating coupler technology." Optical Interconnects XX. Vol. 11286. SPIE, 2020.
[4] Wang, Jinghao, et al. "Efficient and tolerant chip-to-chip optical coupling via silicon nitride grating couplers." AOPC 2023: AI in Optics and Photonics. Vol. 12966. SPIE, 2023.
[5] Bernabé, Stéphane, et al. "Chip-to-chip optical interconnections between stacked self-aligned SOI photonic chips." Optics express 20.7 (2012): 7886-7894.
[6] Yang, Zhonghua, Wenbo Luo, and Yu Sun. "Computational Study of the Coupling Performances for a Long-Distance Vertical Grating Coupler." Photonics. Vol. 11. No. 1. MDPI, 2023.
[7] Zhang, Yang, et al. "On-chip intra-and inter-layer grating couplers for three-dimensional integration of silicon photonics." Applied Physics Letters 102.21 (2013).