The superlayer test configuration

Bagchi et al. (1994) have used the superlayer test to determine the separation energies of Cu thin films on silica substrates. In their approach, a thin strip of carbon is first deposited onto the substrate so as to create a debond precrack over an area along the interface. The desired thickness hf of Cu film is then deposited onto the substrate over a region that includes the prepared debond zone. Subsequently, a Cr superlayer of thickness hs is deposited on top of the first film by electron beam evaporation. When the Cr layer thickness exceeds a certain critical value, spontaneous delamination ensues. Upon delamination, the bilayer film develops a curvature k which, for the case when Mf = Mgi and Cm = 0, depends on the system parameters according to 

Knowledge of this curvature, along with the values of other system parameters, can be used to extract the interface separation energy F using the 

A rough estimate of the thickness ratio that results in = 0 can be obtained on the basis of the following reasoning. For the case considered by Suo and Hutchinson (1989), the elastic mismatch strain across the interface prior to delamination is zero, and the substrate is unstrained. Therefore, the conditions that the extensional strain of the film surface z = 0 after delamination must still be zero may correspond to a relatively small value of shear stress on the interface at the delamination front a nonsingular value of shear stress axz = 0 corresponds to the condition that ip = 0, provided that the normal stress azz is still singular and positive. The methods of Section 2.2 can be used to show that this condition on strain implies that / f// sl must have the value 2. While the estimate is crude, it may provide a useful indicator of behavior for arbitrary combinations of material parameters and film thicknesses in cases for which other indicators are not available. 

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