Mechanical Engineering Journal (May 2023)
Novel measurement method of internal stress in thin films using micro spring structure
Abstract
As the application areas of MEMS (microelectromechanical systems) expand, more advanced functions and performance are being demanded from MEMS. To meet these demands, materials with various functionalities such as mechanical strength and shape memory, which are difficult to achieve with Si-based materials alone, are being used in MEMS. When applying new materials to MEMS, it is essential to establish micromachining techniques and control internal stresses in the same way as for Si-based thin films. In this research, a method for measuring the internal stress of thin film structures formed by microfabrication technology was developed. The thin film structure sample to be measured is in the form of a beam with fixed ends. The uniaxial strain due to internal stress is measured using a micro spring, and the internal stress is calculated. A process for fabricating a device that realizes the novel measurement method was devised and the device was fabricated. The microfabrication technique used was a reverse lift-off process, which can form thin film structures with rectangular cross-sections and uniform film thickness. Thin film metallic glass, an amorphous alloy with higher strength and lower Young's modulus than Si-based materials, was used as the new material to be measured. Using the developed measurement method, the internal stress change of the thin film was measured as a function of annealing temperature. As a result, it was confirmed that the internal stress of the thin film changed from the compressive direction to the tensile direction with increasing annealing temperature. The internal stress values measured by the novel method were compared with those measured by Stoney's equation, and the two measurement methods were close, demonstrating the usefulness of the new measurement method.
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