The influence of N containing plasmas on low-k films

The use of low-k materials is essential for improving the quality of integrated circuits. Subsequent process steps may however modify this film to the extent that the final result is unacceptable. Organosilicate-based low-k films, with a nominal k-value of 2.3, were exposed to different post-CMP cleaning plasmas used for copper reduction. The resulting plasma damage was investigated and is reported in this paper. All the studied plasmas increased the density of the low-k film. TOFSIMS and FTIR analyses showed that they all removed CH3 groups from the bulk, leading to water incorporation. The carbon depletion was more pronounced and deeper (100 nm) from a NH3 plasma than from any other investigated plasma. N2 + H2 plasma removed somewhat less carbon from the low-k film (83 nm deep). The N2 plasma removed carbon down to a depth of 60 nm into the film, while a pure H2 plasma removed the least carbon of all the investigated plasmas, to a depth of only 35 nm. The combination of TOFSIMS and XPS indicated the incorporation of a significant amount of N in the films treated with the pure N2 plasma. C–V measurements showed an increase of the dielectric constant, again mostly for the NH3 plasmas. There was an intermediate and approximately equal increase of the dielectric constant for all N2 containing plasmas, and the least increase was for the H2 plasma. This increase of the dielectric constant was caused by the increase of density of the film, incorporation of water, and in the case of the N2 plasma also the incorporation of N. This shows that the presence of N2 in plasma may significantly damage low-k materials, and it should not therefore be treated as a mere carrier gas.