image: The optical setup uses a high-numerical-aperture objective and camera to capture film thickness information in a single snapshot, eliminating the need for slow mechanical rotation.
Credit: ©Science China Press
The challenge: measuring invisible layers at high speed
Modern computer chips are complex 3D structures containing dozens of thin layers, each precisely controlled at the atomic scale. A slight thickness variation—less than the width of a single atom—can render a chip useless.
The semiconductor industry relies on a technique called ellipsometry to measure these layers. It works by shining light on a surface and analyzing how the polarization changes upon reflection. But conventional ellipsometry is slow. It typically takes a few seconds per measurement point, making it impractical to scan entire wafers with high density and speed.
"With 3D chip architectures like NAND flash now stacking 200+ layers, we need metrology tools that can keep up with manufacturing speed," said Professor Shiyuan Liu, who led the research. "Waiting seconds per point is no longer acceptable."
A snapshot solution
The team developed a new approach called snapshot Fourier ellipsometry. Instead of mechanically rotating components to modulate light—the time-consuming part of conventional methods—they use a high-numerical-aperture objective lens that captures angle-resolved information in a single camera image. This enables dynamic measurement during continuous scanning, rather than stop-and-measure operation at each point.
“Think of it as taking a photograph versus scanning a document line by line. Both capture the same information, but the photograph is instant.” said Doctor Lihua Peng, co-corresponding author.
The key innovation is a sophisticated mathematical model and in-situ calibration procedure that corrects for optical imperfections in the system—polarization errors, aberrations, and depolarization effects that have historically limited the accuracy of this approach.
Dramatic performance gains
The results are striking:
Accuracy improved by 95%: Measurement errors dropped from >3 nm to ~0.2 nm when compared to a commercial ellipsometer
Picometer precision: The system can detect thickness variations as small as 3-10 picometers (1 picometer = one-thousandth of a nanometer) in a single snapshot
100x faster throughput: A 60,000-point scan of a 4-inch wafer completes in 10 minutes—over 100 times faster than commercial tools requiring ~8 seconds per point
The system achieved these results on standard silicon dioxide films, as well as more challenging materials like silicon nitride and indium tin oxide (ITO) used in displays and solar cells. This marks the first time a dense wafer-level film thickness map completes in ten minutes while maintaining picometer precision.
Why it matters
As semiconductor manufacturing moves toward 3D architectures with atomic-scale control, high-throughput metrology becomes a bottleneck. "You can't control what you can't measure," said Professor Jian Wang, who designed the system. " Our method gives manufacturers a tool that can keep pace with their production lines while providing atomic-level accuracy for next-generation devices."
The technology could also find applications in other fields requiring thin-film measurements, such as display manufacturing, optical coatings, and solar cell production.
What's next
The team is working on further improvements, including better autofocus systems for more robust dynamic measurements and algorithms to handle more complex film structures. They also see potential for extending the technique to other wavelengths and integrating it directly into semiconductor manufacturing equipment.
Method of Research
Experimental study