Gravimetric sensing of metallic deposits using an end-loaded microfabricated beam structure

In this communication, the gravimetric sensitivities of two geometrically different microfabricated beam structures are reported. Each was loaded serially with gold followed by chromium metal by vapor depositing through a shadow mask exposing only the end of the structures. The change in fundamental resonance frequency was found to be solely a function of the density of the deposited for deposits less than 50 nm in thickness. Therefore, the change in the resonance frequency of the mechanical beam is solely a function of the end-loaded mass. The gravimetric sensitivities of the fundamental mode of excitation for these structures, measured under ambient excitation conditions, is of the order of 10(3) cm(2) g(-1) while the minimum detectable mass density with this type of structure can be as small as approximately 0.5 ng cm(-2). A theoretical relationship is provided that enables further enhancement of the gravimetric sensitivity of these structures through decreasing the thickness and density of the microsensor. Through establishing a quantitative comparison between these structures and other, more traditional acoustic wave sensors, it will be shown that these micromechanical devices offer potential advantages in environmental monitoring applications. (C) 1998 Elsevier Science S.A. All rights reserved.