1. Background
Integrated photonic chips rely on networks of waveguides whose structural integrity is critical to device performance. Discontinuities, breaks, and other irregularities can affect optical propagation and reduce overall device reliability. [1]
Many photonic structures are embedded within silicon substrates and cannot be observed using conventional visible-light microscopy because silicon is opaque in the visible spectrum. At suitable infrared wavelengths, however, silicon becomes partially transparent, enabling non-destructive visualization of buried structures. [2]
Infrared transmission microscopy is therefore a valuable inspection approach for semiconductor and photonic devices, particularly when internal features must be examined without cleaving, thinning, or cross-sectioning the sample. [3]
2. Objective
This application note demonstrates through-silicon imaging of buried waveguide structures in stacked photonic chips using the Jay Photonics infrared transmission microscope.
The objective was to visualize the overall photonic circuit and identify localized waveguide defects through multiple silicon layers without destructive sample preparation.
3. Methodology
Sample: The sample consisted of two vertically stacked silicon photonic chips, each 700 µm thick. Two photonic circuits were positioned between the silicon substrates.
The upper surface of the stack was polished, while the lower surface remained unpolished. Under visible illumination, the silicon substrates obscure the embedded structures. Infrared transmission imaging was used to visualize the photonic circuits through the stacked silicon layers.
Figure 1. Schematic of the stacked photonic sample. Two silicon substrates, each 700 µm thick, contain two buried photonic circuits between them. The upper surface is polished, while the lower surface is unpolished.
System under test: The experiment was performed using the Jay Photonics Si-Through-HR infrared transmission microscope. Images were acquired using 10X and 40X objectives to evaluate both large-area circuit visibility and localized waveguide detail.
4. Observations and Analysis
4.1 Large-Area Visualization of the Photonic Circuit
The image acquired using the 20X objective provides an overview of the buried photonic circuit through the stacked silicon substrates.
The main circuit layout and large-scale waveguide structures remain visible, enabling inspection of the overall device architecture and identification of regions requiring more detailed analysis.
4.2 High-Magnification Inspection of Waveguide Structures
At 50X magnification, finer details of the buried waveguides become visible. The increased magnification enables closer inspection of localized structures and reveals discontinuities, breaks, and irregularities along the waveguide paths.
These features would be difficult or impossible to observe using conventional visible-light microscopy without destructive access to the internal layers.
Figure 2. Through-silicon infrared images of buried photonic structures acquired using (a) a 20X objective, showing the overall photonic circuit layout, and (b) a 50X objective, revealing finer waveguide details and localized structural irregularities.
5. Conclusion
The Jay Photonics infrared transmission microscope successfully visualized buried photonic structures through two stacked silicon substrates, each approximately 700 µm thick. The 10X image provided a large-area view of the photonic circuit, while the 40X image revealed localized waveguide discontinuities and structural irregularities. The structures remained visible despite the total silicon thickness and the presence of an unpolished surface.
These results demonstrate the potential of through-silicon infrared microscopy for non-destructive waveguide inspection, failure analysis, process monitoring, and quality control in integrated photonic devices.
6. Curious to See the Difference?
Working with buried photonic structures or stacked silicon devices?
Contact Jay Photonics to discuss your inspection challenge or submit your samples for a non-destructive imaging evaluation.
References
[1] G. R. Bhatt, U. D. Dave, J. Rocha-Rodrigues, M. Zadka, I. Datta, A. Asenjo-Garcia, and M. Lipson, “Influence of discontinuities on photonic waveguides,” Optics Letters, vol. 49, no. 14, pp. 3918–3921, 2024.
[2] K. Agarwal, R. Chen, L. S. Koh, C. J. Sheppard, and X. Chen, “Crossing the resolution limit in near-infrared imaging of silicon chips: Targeting 10-nm node technology,” Physical Review X, vol. 5, no. 2, 021014, 2015.
[3] P. Aryan, S. Sampath, and H. Sohn, “An overview of non-destructive testing methods for integrated circuit packaging inspection,” Sensors, vol. 18, no. 7, 1981, 2018.