lineWLI’s measurement capabilities go beyond just height information. The device can also measure the thickness of transparent thin films. This posts revisits a sample from a previous post. In the previous post we used lineWLIs surface profilometry mode to measure the depth of a pad.
The pad itself is an opening in the top SiOx. Measurements with lineWLI in thin film mode show that the bottom of the pad is made of the same material as the top. The SiOx layer is just thinner at the bottom of the pad.
Microscopy image of the test structure
SiOx film thickness of the test structure
The thickness profile of the surface (left) matches the surface profile from the previous post (right) with one notable exception. The alignment structure is visible only in the surface profile. This means that that the alignment structure is not etched into the top SiOx but lies below it. It shows up in the surface profile because the thin film covers the top and sidewalls. The roundness of the surface profile around the edge of the alignment marks corroborates this hypothesis.
Thickness profile of the top SiOx
Surface profile of the pad
The difference in the thickness of the oxide within the pad and outside of it given the depth of the etch process. The result of the sidewall height matches well with the result from the surface profile measurement.
Thin Film: Side wall profile
Thin Film: Bottom floor
Profile: Bottom floor
Both measurements deviate by about 10 nm. Thin film measurements require knowledge of the refractive index of the material. While the refractive index of thermal SiO2 is well known, deposited oxide, e.g. TEOS, may show small variations depending on process parameters. 10 nm for a 1500 nm thick film translates into a refractive index change of 0.01. This is a reasonable deviation compared to the dispersion curve on file.