1. Background
In wafer bonding and chip stacking applications, sub-micron alignment accuracy is critical. To validate positioning, engineers rely on alignment Verniers—microscopic reference marks embedded into silicon layers. These Verniers allow operators to detect lateral offsets between bonded wafers or components.
However, when embedded under multiple layers of silicon, Vernier structures become extremely challenging to visualize using conventional infrared imaging setups. Contrast is often poor, and key features may become indistinct, making alignment validation unreliable.
2. Objective
This study demonstrates the performance of the infrared imaging technology developed by Jay Photonics in visualizing alignment Verniers beneath multiple layers of silicon. A comparison is made with a high-end commercial system, anonymized for confidentiality.
3. Methodology
- Sample: Silicon chips featuring alignment Verniers
- Material: Full-thickness, doped silicon (~700 µm)
- System under test: Jay Photonics’ infrared imaging system
- Reference system: high-end industrial IR imaging setup
- Acquisition conditions: Identical samples, no sample preparation or polishing
4. Observations & Analysis
The imaging comparison presented in this section was conducted in collaboration with C2MI, whose support and facilities played a key role in capturing high-quality comparative data.
Figure 1 shows the cross-section diagram of the silicon chip.
Figure 2A shows the Vernier region (between levels 20 and 40, which are buried under a 70 µm-thick layer of heavily doped silicon (10 mΩ·cm)), captured using a conventional IR setup. The Vernier structures are visible, though subtle, and require careful interpretation.
Figure 2B presents the same region imaged with the Jay Photonics system. The Vernier lines are sharp and distinct, with significantly improved contrast, allowing for easy differentiation of aligned and misaligned states.
Figure 3A and 3B show vertical and horizontal box-in-box alignment (between levels 40 and 80) using the conventional IR microscope and the Jay Photonics' system, respectively.
Manual Verification Limitations
In the conventional setup, assessing Vernier alignment typically involves manual interpretation of low-quality images. Operators must rely on experience and subjective judgment to decide if the marks are correctly aligned. This process can lead to delays, variability, and even alignment errors.
With the Jay Photonics' system, the clarity of the image makes lateral offset immediately visible, reducing ambiguity and improving inspection confidence. The result is faster, more consistent decision-making—especially in bonding, packaging, and photonic integration workflows.
5. Pro tip for more accurate alignment measurements
To minimize parallax effects, which can introduce errors in alignment measurements, a well‑calibrated, parallel illumination system is ideal. However, depending on the level of precision required, perfect alignment is never truly achievable!
That said, there’s a simple trick widely used in industry that improves measurement accuracy regardless of the illumination setup or system calibration: rotate the sample by 180° and average the two alignment measurements.
As long as the illumination and imaging conditions remain unchanged between the two measurements (which is the case if you simply rotate a flat sample), this method effectively cancels out parallax errors.
Bonus note: Even if you have a perfectly calibrated, parallel‑illumination system, this trick can also help detect placement errors. For example, if a dust particle trapped under the sample is causing a slight tilt, the two measurements will differ significantly after the 180° rotation—indicating something is off. In that case, you know you need to inspect and re‑mount the sample, rather than trusting a single measurement.
6. Conclusion
Jay Photonics’ infrared imaging technology enables clear, high-contrast visualization of embedded alignment Verniers through >2 mm of silicon. Its higher resolution and transmission-based imaging approach minimize guesswork. This makes it a valuable tool for any workflow where alignment accuracy is critical.
7. Curious to See the Difference?
If you're working with alignment structures, stacked chips, or MEMS integration, seeing the results firsthand 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.
Acknowledgment
Special thanks to C2MI for providing the sample and conducting the comparative tests.