X-ray microdiffraction examines stress distributions

08/01/1997

X-ray microdiffraction examines stress distributions

Researchers at Columbia University and IBM Research report that they have used white beam x-ray radiation to study thermal and electromigration strain distributions in real time. According to authors P.C. Wang, et al., in a paper for the TMS Outstanding Student Paper contest, the microdiffraction technique uses broad spectrum synchrotron radiation, concentrated through borosilicate glass capillaries. The sample can be heated, and current applied, while it is scanned under the x-ray beam. Thus, the researchers obtained spatially-resolved lattice spacing measurements over time as electromigration progressed.

Thermal and electromigration stresses directly affect interconnect performance, especially at submicron dimensions. Thermal stresses result from thermal expansion mismatch between metal lines, the silicon substrate, and the oxide dielectric when cooled from processing temperatures, and are responsible for void formation in interconnects. Electromigration, which causes mass transport from the cathode to the anode end of a conductor line, creates both hillocks and voids and can eventually limit the lifetime of a device. This study examined thermal and electromigration stress distributions in 10-?m-wide, 200-?m-long sputtered aluminum lines with sputtered tungsten contact pads.

First, lattice spacings were measured at room temperature, 139?C, and 267?C in order to determine thermal strain distributions. The W pads were under highly compressive in-plane stress, consistent with previous results for sputtered tungsten. The observed thermal stress in the aluminum lines was less than calculated values, perhaps due to yielding during cooling, or relaxation during storage after deposition. The variation of lattice spacing with temperature suggested an equi-biaxial stress state for the aluminum lines.

Next, to study electromigration, the researchers passed 0.6?106 ?/cm2 through the aluminum line for about 69 hrs at 267?C. Sample resistance started to increase after about 10 hrs, then continued to increase linearly for the rest of the test. After the first 15 hrs, lattice spacings decreased at the cathode end of the line. At the anode end, lattice spacings increased during the first 15 hrs, then dropped during the rest of the electromigration test.

Wang interpreted these measurements as electromigration within grain boundaries: the line was under biaxial compressive stress at 267?C before electromigration. As electromigration occurred, the grain boundaries served as paths for fast diffusion. Depletion of atoms from grain boundaries at the cathode end of the line, in turn, reduced the in-plane compressive stress. Once these stresses were completely relaxed, further depletion caused void growth. Meanwhile, accumulation of atoms at the anode end of the line increased the compressive stresses there. Beyond a certain point, the Al line deformed, reducing stress. - K.D.