Optimized Cantilevers for Highly Sensitive and Spatially Resolved Measurements of Magnetic, Electric, and Dielectric Properties in Lift-Mode operated AFM Techniques

P. De Wolf, R. Poddar, K. Kaja, B. Pittenger, S. Hu
United States

Keywords: MFM, EFM, KPFM, dielectric constant, multimodal


Probes play a paramount role in shaping the quality of atomic force microscopy (AFM) measurements. In this work, we present the development of novel AFM probes, specially designed for enhanced sensitivity and spatial resolution in electric (EFM) and magnetic (MFM) force microscopy experiments. The measurement in EFM and MFM relies on detecting the phase (∆φ) or frequency (∆f) shifts of an oscillating cantilever, scanning the sample surface at a fixed tip-sample (lift) height. Its sensitivity to phase and frequency variations scales directly with the cantilever’s quality factor (Q) and detection frequency (f), and inversely with the spring constant (k), according to ∆φ~Q/k∙∂F/∂z and ∆f~f/2k∙∂F/∂z, respectively. It follows that lowering the cantilever’s spring constant, and/or increasing its quality factor and resonance frequency are key to enhancing the measurements’ sensitivity. In the design strategy of our novel probes, we deliberately focused on the cantilever’s spring constant to promote their use at generally low force regimes, unlocking their applicability in force mapping-based imaging modes. For this, we drastically reduce the cantilever’s thickness and width, using a spoon-shaped profile to support conventional laser spot used in AFM. These design features led to clear improvements in the sensitivity of EFM and MFM measurements, even for short cantilevers, showing a three- and twelve-folds increase in phase and frequency sensitivities, respectively. Further improvements were made to the low-moment magnetic coating of probes for MFM measurements, featuring an optimized process for minimizing stress-induced cantilever bending, which could be an issue on soft, thin cantilevers. Our preliminary test results on a 20Tb hard disk drive and Pt/Co/Pt multilayer samples demonstrate a significant enhancement of the measurements’ sensitivity, showing superior performances compared to conventionally used low magnetic moment probes in MFM. Additionally, our findings show that increasing the sensitivity enables operations at lower oscillation amplitudes and smaller lift heights, leading to higher spatial resolution measurements. Further improvements in spatial resolution could be achieved through additional optimizations of the magnetic coating process, such as adopting a side-coating approach (in contrast to full-tip coating). Similar probes were used for demonstrating simultaneous measurements of the mechanical (i.e., adhesion, deformation, and modulus), electrical (i.e., surface potential), and dielectric (i.e., permittivity) properties in the Peakforce KPFM mode. This method has been recently shown to seemingly measure the second-order derivative of the tip-sample capacitance, enabling the mapping of local variations in dielectric constant with a direct discrimination of subsurface properties [1]. Interestingly, this method could be coupled to fast force volume operation, providing direct access to mapping the tip-sample capacitance derivatives at different lift heights, which paves the path toward promising dielectric nano-tomography methods for characterizing buried and embedded systems and dielectric interfaces. Therefore, the optimized AFM probe design presented in this work is expected to unlock new opportunities to unveil local magnetic, electric, and dielectric properties at unprecedented levels of sensitivity and high resolution. [1] K. Kaja, et al, Adv. Mat. Int. 2023; 11(2): 2300503. https://doi.org/10.1002/admi.202300503