1. Background
In the semiconductor industry, it is generally assumed that materials like Germanium (Ge) are opaque to cameras based on silicon or standard InGaAs sensors, as these detectors only operate at wavelengths below Ge’s ~1.8 µm transmission cutoff. This has historically made non-destructive inspection of internal structures in devices built on Germanium substrates extremely challenging, if not impossible, without resorting to prohibitively expensive, low-resolution technologies. Consequently, validating critical processes such as the fabrication of Through-Substrate Vias (TSVs) or inspecting internal junctions has often relied on complex and invasive methods.
Recent research is focused on the miniaturization of power devices, such as solar cells, which are frequently fabricated on Germanium substrates. The article by de Lafontaine et al., published in Cell Reports Physical Science, illustrates this need by describing the fabrication of III-V/Ge triple-junction micro solar cells with 3D interconnects (TSVs). The inspection of these complex, deeply buried structures is vital for R&D and quality control.
2. Objective
This application note demonstrates the capability of Jay Photonics' infrared imaging technology to clearly visualize internal structures through a Germanium substrate, challenging the prevailing assumption of opacity. It shows how this capability enables effective inspection of advanced devices like III-V/Ge micro solar cells.
3. Methodology
• Sample: A III-V/Ge triple-junction micro solar cell with through substrate via (TSV) contacts, fabricated using the method described in the de Lafontaine et al. article, was used for this experiment.
• Composition: The device includes a Germanium substrate approximately 20 µm thick and other semiconductor layers (III-V) around 5-8 µm thick, all bonded on a 300 µm-thick quartz substrate.
• Imaging System: Jay Photonics' infrared imaging system.
• Conditions: No sample preparation was required.
4. Observations & Analysis
The Jay Photonics system successfully produced clear, high-contrast images of the micro solar cell. The internal structure, including the complex 3D interconnects and Through-Substrate Vias (TSVs) that were previously invisible, was imaged with high resolution and clarity.
Figure 1A : An optical microscope image of the III-V/Ge micro solar cell (source: de Lafontaine et al. article).
Figure 1B: An image of the same device, captured with the Jay Photonics system through the Germanium substrate. The contours of the TSVs are sharp and distinct, revealing the quality of their fabrication.
Figure 1C: Schematic of the solar cell.
This non-destructive visualization was a first for the researcher, revealing the internal structure through a material previously considered opaque to traditional IR imaging. Jay Photonic system's ability to image with high resolution through both Germanium (20 µm) and other semiconductor layers (5-8 µm) is a major breakthrough.
5. Conclusion
Jay Photonics’ infrared imaging technology overcomes the limitation of Germanium’s opacity, providing an unexpected solution for non-destructive characterization. This technology delivers crucial insights into the fabrication and quality of miniaturized devices like 3D interconnected micro solar cells, where internal layer inspection is vital.
This discovery significantly broadens the use cases for IR microscopy on Germanium-based devices, encouraging new applications in the R&D and manufacturing of advanced optoelectronic devices.
6. Curious to See the Difference?
If you're working with power devices, 3D interconnects, or any other application involving complex materials like Germanium, a live demo can be eye-opening.
We offer virtual live demos where we walk you through the system in real-time, either using our samples or, even better, your own (under NDA). It’s the easiest way to see how our system performs on your actual structures.
Interested? Reach out to schedule a session.
Acknowledgement
Special thanks to Professor Mathieu de Lafontaine from the University of Ottawa, his collaborators and the SUNLAB for providing the sample and context for this application note. Their work was published in the article:
de Lafontaine, M., Bidaud, T., Gay, G., et al. (2023). 3D interconnects for III-V semiconductor heterostructures for miniaturized power devices. Cell Reports Physical Science, 4(12), 101701.
The full list of collaborators on the publication includes: Thomas Bidaud, Guillaume Gay, Erwine Pargon, Camille Petit-Etienne, Artur Turala, Romain Stricher, Serge Ecoffey, Maïté Volatier, Abdelatif Jaouad, Christopher E. Valdivia, Karin Hinzer, Simon Fafard, Vincent Aimez, and Maxime Darnon.