Fundamental limits to force detection using quartz tuning forks

This paper explores the fundamental limits of the use of quartz tuning fork s as force detectors in scanned probe microscopy. It is demonstrated that a t room temperature, pressure, and atmosphere these force sensors have a noi se floor of 0.62 pN/root Hz and exhibit a root mean square Brownian motion of only 0.32 pm. When operated as a shear force sensor both dissipative and reactive forces are detected on approach to the sample. These forces are s ufficient to reduce the amplitude of motion of the probe nearly to zero wit hout physically contacting the surface. It is also demonstrated that conven tional proportional-integral feedback control yields closed loop responses at least 40 times faster than their open loop response. (C) 2000 American I nstitute of Physics. [S0034-6748(00)02907-5].