Scanning white-light interferometry (sWLI) and phase-shifting WLI (pWLI) reconstruct surface topography from the coherence envelope or phase of a broadband interference signal acquired point-by-point or frame-by-frame as the reference arm is mechanically scanned through focus. On bare surfaces this works well, but the presence of a thin film introduces a second partially coherent reflection from the film–substrate interface that distorts both the coherence envelope peak and the phase, causing systematic height errors that are difficult to separate from genuine topography without additional a priori knowledge of film thickness. A further complication arises on surfaces with adjacent metal and dielectric regions — common in advanced packaging and semiconductor substrates — where the large reflectivity contrast between materials causes sWLI and pWLI to produce phase errors that vary discontinuously across the material boundary, corrupting step-height measurements at precisely the transitions that matter most.
lineWLI avoids both classes of ambiguity by operating in the spectral domain: each camera pixel records the full reflectance spectrum across the illumination bandwidth, and a proprietary signal processing pipeline simultaneously retrieves surface height and film thickness from a single spectral acquisition — no mechanical scan, no envelope fitting, no phase unwrapping. The pipeline handles mixed-reflectivity pixels natively, recovering accurate topography across metal–dielectric boundaries without material-specific recalibration. Because surface height and film parameters are solved jointly rather than sequentially, the crosstalk that corrupts sWLI and pWLI measurements on filmed surfaces is eliminated by construction, not corrected in post-processing. Critically, this joint retrieval remains effective down to film thicknesses of approximately 400 nm — well below the ~1.5–3 µm coherence envelope separation floor that limits commercial sWLI instruments — making lineWLI applicable to a range of advanced thin-film applications that are inaccessible to scanning or phase-shifting approaches.
To demonstrate the capabilities of lineWLI, we used a PTB thickness standard. The PTB is the German national metrology institute and they provide various film thickness standards. Through a reciprocity agreement with the US NIST, all their standards are NIST traceable as well. The standards consist of thermally grown SiO2 and they have various features etched into the top film. This application note looks at these etched structures. Because of its spectral working principle, lineWLI measures the spectral contribution from both the interferometric signal from topography and the spectral signal from thin film interference in the SiO2 film. The etched structures provide an excellent way to distiguish both signals. At the film edge the difference in elevation from the interferometric signal must be equal to the difference in film thickness.
The first measurement looks at a 1000 nm standard. With 1015 nm, the film is slightly thicker than the nominal thickness. Within the etched regions, the film was full removed and it is assumed that there’s only a 2 nm thick layer of native oxide. Such a thin layer is beyond the capabilities of a spectral reflectometry instrument in the visible range. The cross section shows the surface profile variation along the bar of the “T” in “PTB”
The topography information from the interferometric signal will ideally follow the thickness profile. Subtracting the thickness information from the topography of the film surface reveals the topography of the underlying substrate. For an ideal setup, there is no offset at the film edge since both signal capture the step identically. The scale of the topography map now only spans ±10 nm. Minute edge artifacts on the order of 2 nm or less reveal the position of the etched sections. This makes it easier to see how the surface texture extends from the etched into the covered regions. The cross section now shows the calculated surface elevation at the same location as before. The profile clearly shows that there’s virtually no offset between the measured surface elevation within the etched region and the calculated surface profile outside of it. This demonstrates lineWLI excellent accuracy of its combined surface topography and film thickness measurement.
The spectral interferometry technology behind lineWLI allows for the measurement of thinner films as well. Here’s an example with another PTB standard with a nominal thickness of 380 nm SiO2. The film thickness is only 40% of the other measurement. Hence, the surface texture leads shows as color variations in the surface map.
