Richard Koops and Vincent Fokkema, “An approach towards 3D sensitive AFM cantilevers”, Meas. Sci. Technol. 25 (2014) 044001 (8pp).
The atomic force microscope (AFM) tapping mode is a highly sensitive local probing technique that is very useful to study and measure surface properties down to the atomic scale. The tapping mode is mostly implemented using the resonance of the ﬁrst bending mode of the cantilever and therefore provides sensitivity mainly along the direction of this oscillation. Driven by the semiconductor industry, there is an increasing need for accurate measurements of nanoscale structures for side wall characterization by AFM that requires additional sensitivity in the lateral direction.
We present an approach towards true 3D sensitivity for AFM cantilevers based on simultaneous excitation and optical detection of multiple cantilever resonance modes along three axes. Tuning the excitation of the cantilever to speciﬁc frequencies provides a mechanism to select only those cantilever modes that have the desired characteristics. Additionally, cantilever engineering has been used to design and create a substructure within the cantilever that has been optimized for speciﬁc resonance behavior around 4 MHz. In contrast to the conventional approach of using a piezo to actuate the cantilever modulation, we present results on photo-thermal excitation using an intensity modulated low-power laser source. By tightly focusing the excitation spot on the cantilever we were able to attain a deﬂection efﬁciency of 0.7 nm/μW for the ﬁrst bending mode. The presented approach results in an efﬁcient all optical excitation and deﬂection detection scheme where both the position of the spot to excite the cantilever and the spot position of the read-out beam provide additional parameters to fully control and optimize the multi-mode structure required for 3D AFM measurements.
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