Process Variations in Microsystems Manufacturing by Michael Huff

Author:Michael Huff
Language: eng
Format: epub
ISBN: 9783030405601
Publisher: Springer International Publishing

6.2 Residual Stress and Young’s Modulus

The mechanical material properties that often are of the greatest interest in deposited thin-film layers are the residual stress and Young’s modulus [1]. The interest in these two thin-film material properties is based on several reasons, the principal ones being many MEMS microsystems devices have a mechanical functionality wherein these two material properties have a large impact on the resultant device behavior; and, both of these material properties can vary over large range of values depending on the exact processing conditions used during fabrication. In comparative terms however, the variations seen in the residual stresses are considerably larger than seen in Young’s modulus. That is, it is not uncommon to see variations in residual stresses range over 100s of percent as well as change sign from compressive to tensile or vice versa depending on the processing conditions used in processing the thin-film layers.

This is not to imply that other material properties do not matter in MEMS microsystems. Indeed, depending on the device application and design, other mechanical properties, as well as the electrical and optical properties, may be more important. For example, the behavior of a MEMS thermal device will depend heavily on the thermal conductivity and thermal capacitance values of the material layers used in the device. However, the amount of variation typically seen in the thermal properties of most thin-film material layers will usually be limited to a relatively small amount (e.g., often less than ±10% of the nominal bulk value) depending on the processing conditions. It is useful to review the definition of Young’s modulus and residual stress in deposited material layers.

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