Tungsten nitride ($WN_x$) is a promising material in the future Cu metallization schemes of Si VLSI technology as Cu diffusion barrier because it is relatively stable even at thickness less than 30 nm and applicable of not only PVD but also CVD process. Also, $WN_x$ films are being studied as a gate material of GaAs MESFET and a top electrode material in the capacitor structure.
In this study, thermal stabilities and microstructures of reactively sputtered $WN_x$ films were studied as a diffusion barrier between Cu and Si. Barrier stabilities of $WN_x$ films against Cu during high temperature annealing were investigated as a function of film thickness and composition, and their relationship with microstructures was discussed.
$WN_x$ films were prepared by reactive sputtering from W target in Ar+$N_2$ ambient. By changing the ratio of $N_2$ to Ar+$N_2$ gas flow rate, $WN_x$ films with seven different compositions were deposited. Depending on the nitrogen concentration in films, amorphous $WN_x$ and poly-$W_2N$ films were obtained. Amorphous $WN_x$ films were obtained for the films below 32% nitrogen content and polycrystalline $W_2N$ films above that.
To test the barrier properties of $WN_x$ films, the Cu overlayer with thickness of 130 nm was sputter-deposited on the $WN_x$ barrier without breaking vacuum and the samples were annealed in 10% $H_2$-Ar forming gas ambient. Amorphous $WN_x$ films with thickness of 100 nm maintained their barrier properties up to 800℃ and failed at 850℃. However, poly-$W_2N$ with the same thickness failed at the temperature lower than 800℃. Even though the thickness of $WN_x$ barrier was reduced to 5 nm, amorphous $WN_x$ films prevented Cu diffusion up to 600℃ for 1 hr in electrical C-V test. Among the amorphous $WN_x$ films, $W_68N_32$ was the most stable composition in various thickness.
Amorphous films started to crystallize at temperature ranging from 450℃ to 600℃ and transformed to a two phase mixture of W and $W_2N$ between 600℃ and 700℃. Polycrystalline $W_2N$ films maintained the initial structure up to 800℃ but they have the columnar structure having a possible diffusion path of Cu. Over 800℃, all films changed to W over 800℃ due to the nitrogen release.
The microstructural changes of amorphous $WN_x$ films during crystallization were investigated closely. The amorphous $W_79N_21$ film changed directly to a two-phase mixture of W and $W_2N$ through the eutectic decomposition mechanism. The crystallization-starting temperature was highest at this composition and had a tendency to decrease as the film composition became close to the composition of α-W or $β-W_2N$. With this composition as a boundary, amorphous $W_74N_26$ and $W_68N_32$ films with the higher nitrogen content than $W_79N_21$ have undergone the primary crystallization of $W_2N$, followed by the eutectic decomposition, resulting in a cellular structure with a mixture of W and $W_2N$ microcrystallites in the intergranular boundary region when fully-crystallized. Whereas, the amorphous $W_84N_16$ film with the lower nitrogen content have undergone the primary crystallization of W, followed by the eutectic decomposition. Therefore, the resulting microstructure was composed of the primary W grains with the intergranular boundary region of a mixture of W and $W_2N$ microcrystallites.
From these results, it can be concluded that amorphous $WN_x$ films prevent the grain boundary diffusion of Cu effectively even after crystallization because it does not have the direct diffusion path of Cu to Si. On the other hand, the polycrystalline $W_2N$ films have a columnar structure and the change of the film composition occurs by the partial nitrogen release below 800℃, which are expected to cause the early failure of the films.
