An impedance based non-contact feedback control system for scanning probe microscopes

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Abstract
We describe a non‐contact, non‐optical distance feedback control system for scanning probe microscopes that detects the surface damping of a vibrating probe. The feedback signal is derived from an electrical impedance change in a dithering piezoelectric element with attached scanning tip. The system incorporates an arbitrary‐impedance bridge that maximizes detection sensitivity of the surface damping‐induced impedance change as the tip approaches and interacts with the sample. In addition, an auxiliary circuit greatly improves reliability by making the feedback signal insensitive to the phase of the impedance change. The complete detection network can sense changes of −80 to −100 dB down to the level of 1 μV in a bandwidth of ≳1 kHz. The feedback system has demonstrated topographic height sensitivity of ∼0.5 Å and dynamic range of ≳60 dB.