Synchrotron measurement of the effect of linewidth scaling on stress in advanced Cu/Low-k interconnects

The stress of Cu/low-k interconnects with linewidths scaled to 50 nm was determined using precision lattice parameter measurement at an advanced light facility. Grazing incidence and theta-2 theta diffraction geometries were used to gain a direct measurement of the strain tensor, showing an increase in stress as the linewidth is reduced an order of magnitude from 500 to 50 nm. This increase in stress contrasts existing predictions of finite element simulations, which predict a decrease in stress as the line aspect ratio increases above one. Our simulations, considering the low-k stack, have shown this decrease should occur at lower aspect ratios; however, neither trend is reflected in the measured data. All of the lines showed a strong [111] texture suggesting their stiffness was not affected at the scaled dimensions. The narrower lines show a more bamboolike structure compared to a more polycrystalline structure in the wider lines. In the narrow lines, the grains become pinned in the linewidth reducing stress relaxation through grain growth or reorder. This leads to the observed increase in stress with linewidth scaling and the increase in stress for bamboo lines during fabrication. This work demonstrates the grain structure of interconnects has a significant effect on stress and stress evolution.