Constant Amplitude AFM

Applications: contact and non-contact AFM, Kelvin probe force microscopy (KPFM), high-speed imaging, dynamic force spectroscopy

Measurements in atomic force microscopy are moving towards higher cantilever frequencies, multi-frequency, and multi-mode operation. Applications that previously performed on lock-in amplification below 1 MHz are moving into the regions of tens of MHz. Scientists demand scanning at ever increasing speeds and thus require instrumentation with small time constants, in order to capture very fast events.

Objectives: performing lock-in amplification, cantilever frequency tracking, at multiple modes and on various harmonics simultaneously
Benefits: integrated solution for high-speed imaging, including 2 phase-locked loops, 2 lock-in amplifiers, 2 frequency generators
Relevant products: HF2PLL, HF2LI-UHS

HF2PLL Constant Amplitude AFM Mode

In this AFM application, the cantilever is driven at resonance by a variable amplitude and variable frequency signal. Due to interactions between the cantilever and the sample, the signal at the detector varies both in amplitude and phase. In order to keep the cantilever in resonance, the phase shift is tracked with a high-speed phase-locked loop (PLL) and the drive signal is applied at 90 degrees shift compared to the sense signal, whereas the output amplitude is regulated (by means of a fast PID controller) to have the cantilever readout at constant amplitude. The frequency deviation of the PLL is a direct measure of the topography, whereas the dissipation is a direct measure of the energy added to the cantilever to keep its oscillation constant, therefore a relation of electrostatic / magnetic / chemical / molecular forces applied to the cantilever.

Key Features of the HF2PLL Phase-locked Loop for this Application

  • 2 phase-locked loops
  • 2 lock-in amplifiers
  • 2 signal generators
  • 4 independent programmable PID controllers
  • 1 µHz to 50 MHz frequency range
  • Up to 50 kHz PLL regulation bandwidth
  • 6 dual-phase demodulators
  • Integrated frequency response analyzer, FFT spectrum analyzer, and oscilloscope
  • Ultra-high stability oscillator option

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