Non-Contact Atomic Force Microscopy (NC-AFM)
Non-contact atomic force microscopy (NC-AFM), also known as dynamic force microscopy (DFM), is the AFM mode that historically has achieved the highest microscopy resolution, down to sub-atomics levels, in real space. From an instrumentation point of view, the following 3 aspects need to be considered while aiming at the highest performance:
- The steepest phase slope at resonance is directly related to the highest sensitivity.
NC-AFM operation works best with high-Q resonators, such as quartz or MEMS-based sensors or in vacuum environments which limits intrinsic loss.
- Excellent servo loop electronics speed up relaxation.
High-Q means small resonator natural bandwidth, proportional to f/2Q, and in order to achieve reasonable pixel dwell time, an optimized phase-locked loop (PLL) will provide the best trade-off between speed and resolution. This comes in contrast to tapping mode or AM-AFM technique where the phase can freely vary, with longer time for the amplitude to settle.
- Linearizing the system gives access to quantitative, accurate measurements.
In addition to phase, amplitude can also be tracked simultaneously, providing additional information on dissipation processes. Careful tracking of the resonance ensures that the amplitude measurements are always performed at the peak, thus providing maximum response and constant gain amplification for quantitative analysis.
Let's look at the AFM probe as a mechanical amplifier. The oscillating motion of the tip convolutes the small tip-sample interaction together with a robust and stable resonator, thus being sensitive to the Force gradient and canceling out static contributions, providing enhanced sensitivity. Amplitude and Phase of this oscillatory motion are measured by a lock-in technique and fed into 2 different PID loops. NC-AFM operation means that both phase-locked loop and automatic gain controller (AGC) work together by acting on the drive frequency and excitation voltage that results in a drive signal for the mechanical resonator. The same principle would also work with optomechanical resonators or micro/nano electromechanical systems (MEMS/NEMS).
The aim of the PLL and AGC is to lock this drive signal in phase while keeping the amplitude at its peak resonance value. The Zurich Instruments PLL/PID option provides simple steps to optimize one or multiple feedback loops, making use of simulation tools to determine P, I, D values for a given target loop bandwidth. The operator is then left with a single adjustable parameter, to be set in accordance with the experiment, typically faster than the Z-Controller feedback for imaging. This PID Advisor, provided in the LabOne user interface, relies on quantitative DUT transfer function models (freely available in the user manual) for better transparency.
While scanning, all signals available internally such as phase, amplitude, frequency, and excitation voltage can be output to analog BNC or simply recorded digitally using the LabOne GUI or API. This data can be aligned to make an image provided some end of line trigger (EOL) or fast scan axis is fed as a trigger signal to the instrument. In this way multiple images can be acquired at once, even over multiple eigenmodes or harmonics.
Such NC-AFM implementation is quantitative because the phase is locked at 90° so that the output frequency shift from the phase measurements and the drive excitation from the amplitude measurements are orthogonal. This means that conservative (in-phase) and dissipative (in-quadrature) interaction can be unambiguously separated. In addition, this gives rise to dissipation Kelvin probe modes where the electrostatic contribution from the interaction can be projected onto the dissipation axis, hence de-coupled from mechanical modes and therefore topography.
The Benefits of Choosing Zurich Instruments
- Find the best trade-off between resolution and speed quickly with the PID/PLL Advisor: be quantitative.
- Save all internal signals digitally by synchronising the acquistion with the scan engine thanks to the LabOne data acquisition (DAQ) module.
- Open up to a wealth of new modes where multi-frequency and direct sideband detection can be added to your setup's capabilities.
- Zurich Instruments' products work with third-party microscopes: increase the performance of your favorite microscope with a simple add-on.
- Zurich Instruments' PLL/PID can be integrated with third-party software through our data server APIs.