Strain gradient measurements of thin films using an Optical Interferometer ASTM E2246

Strain gradient measurements of thin films using an Optical Interferometer ASTM E2246

The ASTM E2246 test method determines the strain gradient of thin, reflecting films in microelectromechanical systems by a non-contact optical interferometric microscope that can form topographical 3-D data sets. This optical interferometric microscope does not record readings from the cantilevers touching the underlying layer.

    Scope:

    ASTM E2246 test method defines a standard for calculating the strain gradient of thin, reflecting films that help design and fabricate the microelectromechanical systems.  Moreover, the strain gradient value can contribute to the application of Young’s Modulus. It obtains 3-D data sets by using a non-contact optical interferometric microscope which can help determine the properties of the cantilever. The strain gradient value impacts the structural and conductivity properties of thin films.

    Procedure:

    A surface-micromachined cantilever is fabricated and made to bend out of the plane in the z-direction. An optical interferometer forms a topographical 3-D data set, resulting in 2-D data traces for calculating strain gradient. The transitional edges are obtained and aligned, along with determining the attachment position of cantilevers. The endpoints and uncertainties are determined, and the misalignment is adjusted. A circular arc is used to specify the misalignment of cantilevers. The strain gradient is obtained from the circle’s radius by taking the average of the strain gradient values from the traces.

    Specimen size

    The specimen size should be as required, that is 5 µm wide and 350 µm long.

    Data:

    The strain gradient sg is obtained by taking the average of the residual strain values of the traces b, c, and d, as follows:

    sg=sgb+sgc+sgd‌/3

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