Observation of nanoscale dynamics in cantilever sensor arrays

Silicon micro cantilevers are used as transducers for a wide range of physical, chemical and biochemical stimuli, where they exhibit exquisite sensitivity (10(-18) g, 10(-15) J, 10(-9) M, etc) over a wide range of temperatures (100 mK-1300 K). This is accomplished by inducing static bending in the cantilever structure or by changing the cantilever's resonant behaviour, both easily measurable responses. There is increasing interest in using higher-order resonant modes to achieve extra sensitivity; however, this raises the question of exactly which modes are excited in the cantilever. Using strobed interferometric microscopy we are able to probe the dynamic behaviour of individual (100 x 500 x 1 mu m(3)) cantilevers in an eight cantilever array over frequencies from 0 to 1 MHz. We present data that enable spatial visualization of 16 cantilever modes with nanometre-scale amplitudes; combined with finite element analysis calculations we directly assign mode indices. We discuss the reversal of specific modes between experiment and theory, the uniformity of individual cantilevers in the array and the clear relationship between boundary conditions and resonant behaviour. Our conclusion is that the assignment of a resonant frequency spectrum is fairly complex and does not necessarily follow simple intuition.