How to Gauge Sensor Noise in Closed-Loop AFMs

Atomic Force Microscopy (AFM) is a key tool in nanotechnology, providing unprecedented high resolution images and allowing site specific measurements at the nanoscale. The “motors” that almost all AFMs use to create the nanometer-scale motions required are piezo actuators. While they can move smoothly down to atomic scales, these actuators are plagued with other problems like non-linearity, hysteresis, cross-coupling and creep that prevent precise positioning. The most common fix involves adding a sensor and a feedback loop on each axis to create a closed-loop AFM. Unfortunately, this introduces sensor noise which is now transmitted directly to the actuator, diminishing image resolution and positioning abilities.

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To provide a meaningful noise specification, you should ask manufacturers for three numbers: the noise value (often expressed in nanometers RMS or Adev), the bandwidth of the measurement, and the length of time of the measurement. Some manufacturers claim “sub-nanometer” sensor noise, but this number itself is meaningless. One way to get a lower number is by averaging (lowering the bandwidth), but this might mean you get sub-nanometer noise only at very slow scan speeds.
The Asylum Research MFP-3D AFM uses a Nanopositioning System (NPS) with the lowest noise levels in the industry. The NPS uses a proprietary, low noise, inductive sensor that eliminates the Barkhausen noise that limits traditional
inductive LVDT sensors. NPS noise figures are 0.3 nm Adev in Z, 0.6 nm Adev in X and Y, in a 10-sec measurement, and in a 1-kHz bandwidth.
The MFP-3D’s NPS, coupled with the exclusive low noise controller, provides unprecedented precision for force measurements, nanolithography; can zoom and offset accurately; and ensures image clarity.