Experimental setup for measuring the dispersion forces by the adhered cantilever method

Dispersion forces start to play role in modern micro/nanoelectromechanical devices, but the methods to measure these forces at distances close to contact (<50 nm) suffer from pull-in instability. The method of adhered cantilever proposed recently has no instability and is able to make measurements at short separations. To measure the force at the average distance between surfaces in contact, one has to know the shape of an elastic beam with one end fixed at a height of 1-10 μm and the other end adhered to the substrate. The maximum contribution to this shape from the dispersion forces is in a range of 30-100 nm, which is well measurable by the interferometric methods. This paper describes the instrument, measurements, and data processing that make possible the reconstruction of the beam shape with an accuracy of 1 nm in a height range of at least 5000 nm. Critical steps of the fabrication procedure of cantilevers that are 12 mm long, 200 μm wide, and 10 μm thick are described. The interferometer measures the shape based on the differential interference-contrast method; the scanning is realized by a stage with a step of 0.1 μm. The signal recorded from the adhered cantilever has a noise level of 0.33 nm at a maximum sensitivity in a frequency band of 20 MHz. It is concluded that the instrument and data processing algorithm can be used to measure the dispersion forces and adhesion energies between rough surfaces in unloaded contact.