An anomaly in a thin film will disturb the phase of a light wave passing through it. Normal cameras only register the amplitude of the reflected light but are insensitive to the phase. LineWLI, however, is highly sensitive to phase differences in the reflected light. Hence, it can detect anomalies that are invisible to a normal camera.
To simulate voids and other anomalies in the thin film, a glass slide pushed down on water droplet on a wafer. The water droplet spread into a thin film. Small air bubbles in the film created voids. On the opposite side, there are small water droplets on the silicon wafer. These figures show the expected features in a schematic drawing and in an RGB picture.
Analyzing the local variations in phase delay, lineWLI shows variations that a regular camera cannot. A scratch in he surface will produce a thicker apparent film because there’s more material within the optical path. Likewise, a particle on the bottom surface or a void will reduce the apparent optical path length.
An anomaly detector looks at deviation from the thickness. Blue indicates where regions are thicker than expected and red regions that are thinner as expected. Normal regions are green.
Looking closer at a specific area, we see two crossing lines. The blue color indicates that they are thicker than expected. The assumption these are water filled scratches on the surface. Also there’s a chain of oval features that appear to be thinner than expected. These could be air bubbles. As the air has a lower index of refraction, the optical path is short in air than for the surrounding water. What’s most important is that both features do not show up in the contrast enhanced RGB image. A regular camera does not see them whereas lineWLI can easily identify them.
Looking at the cross section demonstrates the high sensitivity of lineWLI. For the identified features, the thickness deviation is in the single digits nanometers yet they’re clearly visible
