AN IMPEDANCE BASED NONCONTACT FEEDBACK-CONTROL SYSTEM FOR SCANNING PROBE MICROSCOPES

We describe a non-contact, non-optical distance feedback control syste m for scanning probe microscopes that detects the surface damping of a vibrating probe. The feedback signal is derived from an electrical im pedance change in a dithering piezoelectric element with attached scan ning tip. The system incorporates an arbitrary-impedance bridge that m aximizes detection sensitivity of the surface damping-induced impedanc e change as the tip approaches and interacts with the sample. In addit ion, an auxiliary circuit greatly improves reliability by making the f eedback signal insensitive to the phase of the impedance change. The c omplete detection network can sense changes of -80 to -100 dB down to the level of 1 mu V in a bandwidth of >1 kHz. The feedback system has demonstrated topographic height sensitivity of similar to 0.5 Angstrom and dynamic range of >60 dB. (C) 1996 American Institute of Physics.