Intermodulation AFM a novel multifrequency technique for material insight

D. Forchheimer
Intermodulation Products AB,

Keywords: atomic force microscopy, multifrequency


Conventional single-frequency Atomic Force Microscopy imaging modes such as "Tapping mode" or frequency modulated AFM (FM-AFM) operate using the resonance frequency of the AFM cantilever and therefor offer a very sensitive interaction with the surface. The resonance however also works as a frequency filter, making the AFM blind to response at other frequencies. This is fine for measuring surface topography, but not suitable when it comes to detecting other material properties that can vary across the surface such as elasticity, visco-elasticity ,adhesion etc. These material properties will be reflected in the higher harmonics of the AFM signal which are filtered out. Intermodulation AFM (ImAFM) combines the high sensitivity of the cantilever-resonance with the material characterization properties of the higher harmonics by using a multi-frequency drive and detection scheme. The cantilever is operated in such a way that the information that exist in the higher harmonics is "folded down" and appears as a frequency comb near resonance, where each frequency in the comb can be measured with good signal-to-noise ratio. With ImAFM one can routinely measure amplitude and phase of over 20 frequencies above the noise floor, each of these amplitude and phase signals being delicate probes of various aspects of the the surface mechanical response. These new big datasets allow and even necessitate new methods for data analysis. We will present methods inspired by big data and machine learning as well as methods rooted more in first principle understanding of the physics and material science at hand, each showing how ImAFM can lead to improved insight into the surfaces studied and of atomic force microscopy in general.