High Spatial Resolution Grain Orientation and Strain Mapping in Thin Films using Polychromatic Submicron X-ray Diffraction

The availability of high brilliance synchrotron sources, coupled with recent progress in achromatic focusing optics and large area 2D detector technology, have allowed us to develop a X-ray synchrotron technique capable of mapping orientation and strain/stress in polycrystalline thin films with submicron spatial resolution. To demonstrate the capabilities of this instrument, we have employed it to study the microstructure of aluminum thin film structures at the granular and subgranular level. Owing to the relatively low absorption of X-rays in materials, this technique can be used to study passivated samples, an important advantage over most -electron probes given the very different mechanical behavior of buried and unpassivated materials. Deposited metal thin films patterned into micron-scale structures are ubiquitous in integrated circuits and other modern technologies’. Working with laboratory X-ray sources many authors2,3Y4 have provided valuable information on the average behavior of thin films obtained over a length scale of millimeters. Individual grains in such films are usually in the micron size range, a thousand times smaller than the spatial resolution available with laboratory x-ray instruments. In this letter, we describe the application of a technique using submicron synchrotron-based X-ray diffraction, developed at the Advanced Light Source, to characterize both the orientation and strain/stress state of blanket thin films and patterned, passivated interconnect lines, including the change in stress state as they undergo thermal cycling. Similar or related techniques have also been developed at other synchrotron facilities to measure strain in Al interconnect lines5T697. There are many ways’ to produce x-ray microbeams. Our technique requires white instead of monochromatic radiation to rapidly determine the orientation of each illuminated grain by taking a Laue pattern in reflection mode. To form the required high quality white light focus, we use a pair of orthogonal elliptical Kirkpatrick-Baez (KB) mirrors9*10y’ ’ in grazing incidence for point to point imaging. The elliptical shape of our KB mirrors are produced by controlled bending of a flat substrate, which has a specific width variation allowing us to generate a 0.8 x 0.7 w white X-ray beam by demagnifying a bending magnet synchrotron radiation source”. Each Laue pattern is recorded with a large area X-ray charge coupled device (CCD) camera. Custom software based on previous algorithms’* permits us to rapidly index the white-beam