Characterization of Strain In Thin-Film Structures With Microdiffraction

We have constructed a microdiffraction endstation for use at the monochromatic synchrotron beamline X20A of the National Synchrotron Light Source at Brookhaven National Laboratory. It cosists of a tapered capillary for microfocusing and a limited four-circle diffractometer with high-precision sample positioning. It was designed for the characterization of strain, mosaic broadening, and orientation in samples that are interesting to the microelectroincs industry, including thin metal film structures and semiconductor films. The unique combination of micron-scale diffraction imaging, fluorescence imaging, and high-precision lattice parameter determination make this a valuable tool for both basic materials studies and applied research.
We illustrate the capabilities of the instrument with a recent study that characterized thin-film/substrate interfaces. The quality of adhesion between 1mm diameter thin metal film "dots" and a silicon wafer substrate was assessed on a microscopic scale. The Shear-Lag model predicts maximum shear-stress at the film edge, smoothly decreasing toward the center of the dot. Using a micro focussed x-ray beam and recording the Si(004) reflection intensity, topographic images of the Si around and under an Al dot were constructed. Results show a striking difference between the stress transfer observed experimentally and that predicted by the model. The Al/Si interface is not fully coupled, and despite a uniform thickness profile and absence of surface cracks, shows piecewise disruptions in the stress transfer occurring at intervals from 60 to 250 um.