Method of determining dimensions of features of a subsurface topography, scanning probe microscopy system and computer program

A method of determining one or more dimensions of subsurface features from a measurement of subsurface topography of a sample, wherein the measurement of the subsurface topography is obtained using a scanning probe microscopy system, and wherein the method comprises:

• applying, using a transducer, an acoustic input signal to the sample;
• sensing in a plurality of locations on a surface of the sample an acoustic output signal using a probe, the probe including a cantilever and a probe tip, wherein the probe tip is in contact with the surface, wherein the acoustic output signal is representative of acoustic waves responsive to the acoustic input signal that are measurable at the surface;
• and
• obtaining, for each location of the plurality of locations, a measurement value of the acoustic output signal, and providing a location dependent subsurface topography signal based on the measurement values of at least a part of the plurality locations, the at least part of the plurality locations including N locations;
• wherein the subsurface features include one or more features having a spatial periodicity, and wherein the method further comprises:
• providing an autocorrelation matrix by performing, for each location of the at least part of the plurality of locations, a cross-correlation of the subsurface topography signal in respect of each further location of the at least part of the plurality of locations using a location-shifted version of the subsurface topography signal which is shifted over distance equal to the distance between the respective location and the further location, such as to yield the autocorrelation matrix having a size N*N;
• performing an Eigenvalue decomposition of the autocorrelation matrix for obtaining a plurality of Eigenvalues, and selecting from the Eigenvalues a subset of Eigenvalues having the largest values;
• constructing a frequency estimation function from the subset of Eigenvalues;
• and
• obtaining, from the frequency estimation function, at least one output value being indicative of the spatial periodicity.