1. Background
Thin metallic layers such as chromium (Cr) are widely used in micro‑ and nano‑fabrication as adhesion layers, hard masks, electrodes, and alignment markers. Chromium is commonly deposited with thicknesses on the order of a few tens of nanometers and is often combined with glass or dielectric substrates in photonics, MEMS, and semiconductor processing. [1]
From an inspection standpoint, metallic layers present a significant challenge. Even when they are extremely thin, metals are generally opaque in the visible range and strongly reflective (mirrors are commonly fabricated using metal layers), making it difficult to inspect underlying structures using conventional optical microscopy. While infrared (IR) wavelengths can penetrate certain materials more effectively, the presence of metal layers is typically assumed to block transmission‑based imaging altogether.
Demonstrating the ability to image through thin metallic films is therefore important for applications such as process monitoring, layer‑stack verification, alignment inspection, and defect analysis in multilayer assemblies. Since the Jay Photonics microscope is widely used for inspection through semiconductors with metal layers, in this application note, we investigate the capability and limitations of the Jay Photonics Si‑Through‑HR infrared microscope to image through chromium layers while maintaining sufficient spatial resolution to observe fine calibration features.
2. Objective
The objective of this application note is to reveal the performance of the Jay Photonics infrared microscopy system when imaging through thin chromium films deposited on glass substrates. Specifically, this work aims to demonstrate:
- The ability to observe a standard calibration slide through a single 50 nm chromium layer deposited on glass
- The feasibility of imaging through multiple stacked chromium‑coated glass layers
3. Methodology
The experiment was conducted in two parts using the Jay Photonics Si‑Through‑HR infrared microscope. For the first part, we have used a standard calibration slide and overlayed with one glass slide coated with a 50 nm thick chromium layer to demonstrate the ability of Jay Photonics microscope in transmission mode for observing through Chrome layer. In the second part, the same system was used and by progressively increasing the number of chromium layers in the optical path, we assess how image quality changes and identify practical limits for transmission‑based inspection through chrome films.
Sample: Standard calibration slide (10 µm per division) as the bottom layer and a stack of glass slides coated with a 50 nm thick chromium layer
System under test: Jay Photonics’ infrared imaging system
4. Observations and analysis
Imaging through a single chromium layer
Infrared images acquired through a single 50 nm chromium layer clearly reveal the underlying calibration features. Across all tested magnifications, the calibration patterns remain visible with high contrast, demonstrating that the thin chromium film does not fully obstruct infrared transmission when combined with the Jay Photonics imaging approach.
As the objective magnification increases, finer calibration details become resolvable, indicating that the system maintains effective spatial resolution even in the presence of a metallic layer.
Figure 1: 10 µm-per-division calibration slide imaged through 50 nm chrome layer with Jay Photonics Si-Through-HR Infrared Microscope using 4X and 10X magnification.
Imaging through multiple chromium layers
When stacking several chromium‑coated glass slides, the calibration slide remains observable. Although overall transmission is reduced and image contrast decreases compared to the single‑layer case, the calibration features are still discernible with a practical limit of five layers (250 nm total chromium thickness).
This result demonstrates that even with multiple metallic layers in the optical path, the Jay Photonics system can recover meaningful structural information. Such capability is particularly relevant for inspecting multilayer stacks, bonded assemblies, or process monitors where several thin metal films are present.
Figure 2: Images through a stack of five chrome‑coated glass slides with 4X magnification using Jay Photonics Si-Through-HR infrared microscope. (a) no chrome-coated layer. (b) one chrome-coated layer. (c) two chrome-coated layers. (d) three chrome-coated layers. (e) four chrome-coated layers. (f) five chrome-coated layers.
5. Conclusion
This study demonstrates that the Jay Photonics Si‑Through‑HR infrared microscope can successfully image through thin chromium films. With a single 50 nm chromium layer, calibration features are clearly visible across multiple magnifications.
The imaging results demonstrate that the Jay Photonics microscope maintains the ability to resolve calibration features through progressively thicker chrome layers. Even with five 50 nm chrome coatings stacked on top of the calibration slide, the underlying line patterns remain visible, confirming that imaging through chrome is feasible using the Jay Photonics illumination and detection configuration. As expected, increasing the total chrome thickness leads to a gradual reduction in image contrast and effective resolution. This degradation is attributed to increased absorption and scattering within the chrome layers, which reduces the signal-to-noise ratio. Nonetheless, the persistence of recognizable calibration features indicates that sufficient optical information is still transmitted to enable meaningful imaging.
These results highlight the robustness of the Jay Photonics imaging approach for transmission‑based inspection in the presence of thin metal films. This capability opens opportunities for non‑destructive inspection of multilayer structures, process control samples, and bonded stacks where conventional optical microscopy would fail due to metallic opacity.
By enabling visualization through chromium without sample preparation or delayering, the Jay Photonics system provides valuable structural insight while preserving the integrity of delicate samples.
6. Curious to See the Difference?
If your process involves thin metal films, multilayer stacks, or buried structures that are difficult to inspect with conventional microscopy, the Jay Photonics Si‑Through‑HR system can provide a unique perspective.
Jay Photonics offers live virtual demonstrations, where we image real samples in real time and discuss achievable resolution, contrast, and limitations for your specific application. Standard samples or your own chromium‑ or metal‑coated samples, under NDA, can be used for the virtual demonstration.
Contact us to schedule a demonstration and explore what advanced infrared imaging through metal can reveal.
References
[1] Mulloni, Viviana. "Chromium in MEMS technology." Materials Science Research Journal 5.2/3 (2011): 211.