The yield strength of thin copper films on Kapton

Thin films of copper, with thickness between 0.1 and 3 mum, are vapor-deposited on 12.7 or 7.6-mum-thick polyimide (Kapton) substrates, are tested in a microtensile tester in which the strain is measured by optical diffraction from a microlithographically applied grid. It is found that the yield stress depends strongly on the film thickness and is fit by sigma(sub y) = 116 + 355(t)(super -0.473, where t is the thickness in mum and sigma(sub y) is in MPa.