Surface contamination-induced resonance frequency shift of cantilevers

Nanoelectromechanical cantilevers have achieved unprecedented sensitivity in the detection of displacement, mass, force and charge. Although resonating cantilevers are demonstrated to be excellent mass sensors, environmental effects like humidity, ambient gases and contamination result in uncertainty in the calculation of either adsorbed mass or mechanical properties due to shifts in resonance frequencies. In this work the resonance frequency shift due to surface contamination has been studied. Single crystalline silicon (SCS) cantilevers of various dimensions (1025, 340, and 93 nm-thick) were fabricated and their resonance frequencies due to thermal noise were measured in air and in low vacuum. A resonance frequency shift was seen in air after keeping the samples in vacuum. Our measurement shows that this shift comes from surface contamination. The thinner cantilever showed more sensitive behavior to the air conditions. These results can be used to decrease the errors in the calculation of adsorbed mass and mechanical properties of nanostructures. The calculated equivalent mass-induced resonance frequency shifts of our experiments were measured to be 183times10^-15 gram (183 femtogram) and a mass sensitivity of about 6.5times10^-18 g/Hz (6.5 ag/Hz) was obtained. Our results and analysis indicate that mass sensing of individual molecules will be realizable when taking into account the surface contamination.