Instrumentation-level improvements in shear-force near-field acoustic microscopy

The recently introduced Shear-force Near-field Acoustic Microscopy (SANM) brings a new sensing mechanism to the scanning probe microscopy family. SANM's ability to simultaneously monitor, in real time, several physical sample's responses presents some challenges for ensuring optimal operation; namely, avoid "cross-talk" among the multiple signals, address the compensation of thermal drift to ensure reproducibility of the measurements, and measuring the typical low-level signals obtained from nanometer-sized tested regions. Here, several improvements relevant to SANM, but valid for SPM in general, are addressed. i) The probe's coarse approach is performed via stepper motors, which are controlled either by a computer software interface, or simply by a user-friendly RF remote control. ii) The inherent mechanical drift of the stage is evaluated first (by monitoring the feedback voltage that acts on the sample to maintain the probe-sample distance constant), and then automatically compensated (via linear interpolation) in the immediate subsequent probe's approach/retraction measurements. iii) To determine the absolute position of the substrate relative to the probe, the probe-substrate contact current is measured with circuitry properly shielded from eventual electrical ground loops. Buffer amplifiers are used to drive the quartz tuning fork (that holds the probe) and to measure the AC and DC tunneling currents between the tip and sample. iv) To improve the signal to noise ratio, a Kalman filter is implemented into the SANM's field-programmable gate array board, which processes the signals in real time. Finally, v) to simplify the operation of the microscope, an intuitive LabVIEW host program is developed to control the whole system and offer the user a visualization of the data in real time.