Thickness & Depth Measurements
Characterization of thin films is a common practice for many optical techniques. However, accurately measuring large thickness poses a challenge with a number of unique aspects.
Reflectometry is especially suitable for this kind of measurement. Light is reflected from the two surfaces (either the two wafer sides or from the wafer top and Via bottom), and interferences are incurred upon collection. The light going through the longer path acquires an additional phase. This phase difference leads to an oscillatory behavior of the reflected intensity, with a frequency directly related to thickness.
This method is very robust
relatively insensitive to details of the measured application, and allows precisions of a few tens of nm.
No prior knowledge of the measured sample is required
so that samples of a few µm thickness or depth are measured through the exact same scheme and algorithmic analysis as those with depths exceeding 100µm.
Short acquisition times
unlike interferometric methods, no scan is involved in the measurement. This dramatically reduces the acquisition time.
Since the interaction of light with the sample is well-known, the interpretation of the spectral reflectometry signal is highly accurate.