A vibrating reed apparatus to measure the natural frequency of multilayered thin films

摘要

An apparatus for measuring the natural frequency of sub-micrometric layered films in cantilever beam configuration is presented. The instrument comprises a specially designed test rig with a sample holder, an electronic excitation source, a vibration sensor and an automated software for the excitation and data recollection. The beam is excited by means of an air pulse and the oscillation amplitude of its free end is measured through a laser diode-photosensor arrangement. The instrument provides a very low uncertainty (similar to 1 mHz, for frequencies of the order of tens Hz) for repeated sequential tests and the major source of uncertainty (similar to 0.2 Hz, corresponding to a coefficient of variation of 0.18%) arises from the difficulty of placing the sample in an exactly identical location upon clamping. This high sensitivity renders the capability of measuring very small frequency shifts upon deposition of sub-micrometric films over thicker substrates. In order to assess the reliability of the apparatus, cantilever beams of 125 mu m thick neat Kapton (substrate) and thin layered films of Au/Kapton and Al/Au/Kapton of 200-250 nm film thickness were fabricated and their natural frequency and damping factor were measured. Calculations of the natural frequency of such beams by finite element analysis further support the accuracy of the experimental measurements.