Porous low-k thin films
Diffusion Barrier Integrity
A simple method to directly probe the continuity of barrier layers on open-pored films is illustrated in Figure 1. Positrons are injected into a dielectric film and form Ps, which diffuses rapidly throughout the interconnected pores. Despite many collisions with the barrier, a continuous overlayer should prevent Ps from escaping into vacuum. Hence, detection of Ps in vacuum through its telltale 140 ns vacuum lifetime clearly indicates leakage through pin-holes or discontinuities in the barrier.
Figure 1. Illustration of Ps diffusion barrier testing in blanket (upper) and patterned (lower) samples. The patterned samples have trenches, but they are not filled with Cu.
We have studied a series of Ta- and TaN-capped A10C blanket films (with no trenches or vias) deposited on Si substrates. The thickness of the capping layer is 25 nm, 35 nm or 45 nm for each of the two barrier materials. The intensities of Ps in vacuum are plotted in Figure 2.
Figure 2.Intensities of Ps vacuum component in Ta and TaN-capped A10C silica films.
Barrier thermal Stability
An effective barrier is certainly required to be continuous at room temperature. Moreover, it must be able to maintain its integrity at high temperatures (at least 350 °C) encountered in the subsequent device processing. TaN barriers with thickness of 35 nm and 45 nm on A10C, continuous at room temperature, were selected for demonstrating PALS capability of thermal stability test. These samples, as well as the oxide-capped control sample, were progressively annealed in vacuum up to 500 °C for 1 hour at each temperature.
Figure 3. The lifetimes of Ps annihilating gin the pores of annealed TaN-capped A10C films.
The study demonstrates that PALS is capable of detecting two failure modes of diffusion barriers on low-k films with open porosity.