Electret-Condensor-Microphone used as a very sensitive Force Sensor

The pressure sensitivity of commercially available electret microphones is typically about 1 mV/µbar. A pressure of 1 µbar corresponds to a force of 10-6 N acting on the diaphragma. We show that, with suitable electronics, it is possible to detect — at the center of the diaphragm — even forces in the range of 10-9 N.This sensitivity is in an interesting range, because the interaction forces occuring in standard tunneling microscopy between the tip and sample are of the same order of magnitude.We present data of STM experiments under ambient air conditions on samples of graphite (H0P6) and of gold mounted at the center of the diaphragm. During the tunnel process the height of the tip was modulated (z-direction) with an amplitude of about 1 nm at a frequency of 2 kHz. We were able to measure the vibrations of the diaphragm caused by the periodic interaction forces between tip and sample.In summary, we present a new method of simultaneous tunneling and force-microscopy.