Optics as a tool for metrology of surfaces

Modeling of in-situ optical measurements, ellipsometry, confocal Raman microscopy, and laser induced heat propagation

Multi-scale characterization of surface morphology with setups like AFM, contact profilometer and optical profilometer is rather precise but extremely long, since a good resolution can generally be obtained at small surface area and thus sufficient measurement statistics must be done.

Optics-based methods such as basic spectroscopic reflectivity and more elaborate spectroscopic (generalized) ellipsometry allow rapid and at least mm-scale analysis of the surface. Moreover, in some cases like physical vapor deposition or dewetting, only in-situ tools can provide the insight on the different stages and dynamics of sample evolution during the process. Optics offers an elegant non-perturbative solution for in-situ study of such processes.

Modeling of in-situ optical measurements (stress and differential spectroscopy of thin films)

One of the optics-based approaches is used for the in situ study of silver growth on dielectric substrates. For most substrates of interest, silver nano-particles are formed during the growth, which support so-called localized surface plasmon resonances (LSPR). These resonances alter significantly the interaction with light. 

In surface differential reflection spectroscopy (SDRS), the relative change in the reflectivity of the substrate is recorded before and during the growth. Thanks to the LSPRs of the particles support, the SDRS-spectra have a non-trivial structure. Within the theoretical framework of GranFilm, for example, the features observed in the measured SDRS-spectra can be related to morphological parameters of the particles, like, radius, aspect ratio, density, and contact angle, but also particle-particle distance, etc.

Modeling of rapid (ellipsometry) or non-destructive (confocal Raman microscopy) optical measurements

Recently, we developed an advanced optical metrological method capable of achieving large-area inspection capabilities of periodic samples. This approach is based on the measurement of the spectrosopic and azimuthal dependencies of the so-called Mueller matrix elements of the sample. When combined with a fast and reliable state-of-the-art simulation technique (based on the reduced Rayleigh equation), the measured data can be inverted successfully with respect to the surface morphology.

Measured (lower halves) and simulated (upper halves) Mueller Matrix elements for the sample (reproduced from)

We are also actively developing an optical model for the prediction of confocal Raman microscopy depth profiles for several thin films on the substrate, and its integration to SVI-developed Raman cartography processing toolbox.

Laser-induced heat propagation in thin films and substrates

Modeling of temperature-dependence of heat distribution due to laser treatment of a thin film on a substrate