the photoresist and the niobium film. Buckling occurs when the detached segment is bounded by an attached portion of the film, and at least part of the film thickness is in compression. Curling occurs when the detachment extends to the end of the line and there is a stress gradient in the film. In this way, buckling and curling on the same sample and even in the same line is not inconsistent. The photoresist/niobium bilayer buckled once an interface crack was formed, due to the compressive stress in the niobium film. When the buckle formed, the previously tensile photoresist was driven even more tensile. This is the case for the buckles observed next to the curls in these lines. However, once one end of the buckle was detached from the substrate and the constraint from the substrate was removed, the photoresist/niobium bilayer, subjected to the tensile stress in the photoresist and the compressive stress in the niobium, curled in the opposite direction. No clear trend of the effect of silver on interface adhesion can be established based on the results from delamination of patterned lines. All the PVD samples that had interface delamination in the form of either buckles or curls have silver at the interface, ranging from 2.1 monolayers to 6.5 monolayers. The sample without silver at the interface did not show any interface failure, indicating a stronger interface. However, two other samples that have a relatively high interfacial silver level (4.2 and 7.6 monolayers) did not have any interface failure either. Effect of IBAD on Interface Fracture Toughness Ion beam assisted deposition (IBAD) was used to establish the interfacial orientation relationships of either (110) Nb || (0001) sapphire and [001] Nb || [11 00] sapphire or (110) Nb || (0001) sapphire and [001] Nb || [11 ] sapphire. Silver was introduced at the interface of these samples so that a comparative study could be conducted between the samples with only silver at the interface and samples with both silver and strong inplane orientation relationship at the interface. The silver level ranged from 0.7 to 6.5 monolayer for the set with the nominal thickness of 100 nm, and 0 to 6 monolayers for the set that had nominal film thickness of 500 nm. This silver level range is similar to that in the study of the effect of silver on interface fracture toughness in PVD samples. 2 0 No interface fracture was observed in any of the IBAD samples using the microscratch test. The stress state in the IBAD films was more compressive than that in the PVD films [14], and compressive stress is expected to promote interface delamination in the microscratch test. Therefore, the absence of delamination in the IBAD films indicates a strong interface. All the scratch tracks were similar to that shown in Fig. 1b in which the indenter tip simply ploughed into the niobium layer and eventually broke through the interface to make contact with the sapphire substrate. Even samples with the highest silver level (6.5 monolayers) exhibited strong interfacial adhesion such that there was no observable delamination. Results of the microwedge scratch test revealed a stronger interface for IBAD samples as compared to the PVD sample as well. Only the sample with the highest amount of silver at the interface (11.4 monolayers) showed delamination. The interface fracture toughness for this sample was 107.91 J/m2, much higher than that of the PVD samples measured with the same technique (4.54 J/m2 and 12.44 J/m2). The IBAD sample without silver at the niobium/sapphire interface showed no interface delamination. The wedge tip ploughed into the niobium film, indicating a strong interface. Delamination of patterned lines on sapphire substrate also indicated a stronger interface for the IBAD samples. Six out of nine PVD samples showed line delamination in the forms of either buckles or curls, while only the one sample with the highest silver level at the interface (6 monolayers) of the four IBAD samples had line detachment from the substrate after the dry etch step. It was found that the delamination occurred at the niobium/sapphire interface. Failure occurred by curling at the end of the lines, similar to that shown in Fig. 4b. The average radius of curvature of the curls was 28 µm. Using the stress of -184.9 MPa obtained from substrate curvature measurement, the interface fracture toughness was found to be 3.22 J/m2, almost 3.5 times higher than that for the PVD sample with less silver (2.1 monolayer) measured by the same technique (from curls). Another cause of the strong interface for IBAD samples is ion mixing at the interface. Interface mixing is a common observation in IBAD samples due to ion bombardment during film deposition. Interface mixing is determined via Rutherford backscattering (RBS) analysis. RBS analysis of both PVD and IBAD samples showed that the interface of the IBAD sample was mixed, compared to that for the PVD sample. Mixing at
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