This difference in the incident-light angle leads to differences in the cost, complexity, and capabilities of the two measurement techniques. Since an ellipsometer’s light is incident at an angle, it must analyze the polarization of the reflected light, as well as its intensity, which gives it better capabilities for measuring very thin and complicated film stacks. Analyzing the polarization, however, also means that expensive, precision-moving optical components are required.
By using light that is perpendicular to the film, SR can ignore polarization effects (since most films are rotationally symmetric). Thus, SR can be realized without any moving components, which results in much simpler and lower-cost instruments. SR systems can also easily include transmittance analysis for even greater power.
Referring to the table below, spectral reflectance is usually the preferred thickness measurement technique for films greater than 10um, while spectroscopic ellipsometry is generally preferred for films thinner than 10nm thick. Between these two thicknesses, there are many applications where both techniques can be used. When this is the case, spectral reflectance is often chosen due to its speed, simplicity, and low cost.
|Spectral Reflectance||Spectroscopic Ellipsometry|
|Thickness Measurement Range:||1nm - 3mm (non-metals)
0.5nm - 50nm (metals)*
|1nm - 1mm (non-metals)
0.5nm - 50nm (metals)
|Thickness Required for Index Measurement:||>20nm (non-metals)
5nm - 50nm (metals)
|Measurement Speed:||~0.1 - 5 seconds per location||~1 - 300 seconds per location|
|Special Training:||No||Required for most applications|
|Moving Parts:||No||Moving precision optics|
|Price of Basic System:||~15K||~55k|
|*film stack dependent|