Cu is considered as the most promising alternative to Al-based alloy for the interconnection material in Si-based integrated circuits due to its low resistivity and superior resistance to electromigration. However, Cu is quite mobile and creates a deep trap level in Si, which causes degradation of device reliability. Therefore, it is inevitable to use diffusion barrier between Si and Cu which can suppress the diffusion of Cu into Si.
Although TiN which is currently used in Al metallization technology is chemically stable with Cu and Si, it has been reported that Cu diffuses through the grain boundary of TiN, resulting in barrier failure. Therefore, current research in the field of diffusion barrier for Cu metallization has been directed to the development of the amorphous barrier material. In the present work, the barrier performance and failure mechanism of Ta-Si and Ta-Si-N amorphous thin films were investigated.
All barrier films were deposited using rf magnetron cosputtering. Various sputtering power of Si target and/or $N_2$ flow ratio were adopted to change the composition of Ta-Si and Ta-Si-N barrier films. All barrier films are x-ray amorphous and show acceptable range of resistivity.
After annealing of Ta/Cu/Ta-Si/Si, Cu/Ta-Si-N/Si, and $Cu/Ta-Si-N/SiO_2/Si$ structures in $10%H_2/Ar$ ambient for 1 hour, various analytical techniques were employed to investigate the failure mechanism and evaluate the barrier performance of Ta-Si and Ta-Si-N thin films. In all cases, 300Å thick barrier films are used.
In the case of Ta-Si amorphous barrier, the barrier properties of four compositions ranging from $Ta_{74}Si_{26}$ to $Ta_{56}Si_{44}$ were investigated using X-ray diffraction(XRD), Auger electron spectroscopy (AES), and cross-sectional transmission electron microscope(XTEM). The Ta-Si barriers contained Si less than 37 at.% are failed with the formation of tantalum silicides and $Cu_3Si$, but there is no evidence of crystallization below the failure temperature. In the other hand, small $Cu_3Si$ precipitates are found at Cu/Ta-Si and Ta-Si/Si interfaces even below the crystallization temperature when using Ta-Si barriers contained Si more than 40 at.%. These results are consistent with the expectation from Ta-Si-Cu ternary phase diagram. According to phase diagram, Ta-Si alloy contained Si more than 37.5 at.% has a reactivity with Cu. Therefore, relatively Si-rich Ta-Si films are sacrificial barriers between Cu and Si. Among all compositions investigated, $Ta_{63}Si_{37}$ film is the most stable barrier to suppress the formation of $Cu_3Si$ up to 575℃ between Si and Cu.
Ta-Si-N amorphous thin films of ten compositions were studied as a diffusion barrier between Cu and Si or $SiO_2$. The failure mechanism of Ta-Si-N barrier was investigated using sheet resistance measurement, XRD, XTEM, high resolution TEM, scanning electron microscope(SEM), AES, and energy dispersive spectroscopy(EDS). Ta-Si-N barriers contained N less than 47 at.%(Group A) are failed with the formation of tantalum silicides and $Cu_3Si$, but there is no evidence of crystallization below the crystallization temperature. However, Ta-Si-N barriers contained N more than 51 at.%(Group B) shows the formation of $Cu_3Si$ crystalline phase at Cu/Ta-Si-N and Ta-Si-N/Si interface even below the crystallization temperature. This results indicate that the films in Group B are sacrificial barriers between Cu and Si, which agree well with the previous report. In the case of $Ta_{43}Si_4N_{53}$ barrier(Group C), the formation temperature of tantalum silicide and $Cu_3Si$ are above 800℃, but TaN-like crystalline phase is found after annealing at relatively low temperature above 500℃. Therefore, Cu diffusion through grain boundary is possible. Leakage current measurement of $Cu/Ta-Si-N/n^+p Si$ diode was used to evaluate the barrier performance of 300Å thick Ta-Si-N barriers. When using Ta-Si-N barriers in Group B, leakage current never increase up to 500℃.
C-V test was used to evaluate the barrier performance of 300Å thick Ta-Si-N barriers between Cu and $SiO_2$. When using Ta-Si-N barriers in Group B, C-V curve does not change up to 525℃. Therefore, Ta-Si-N amorphous thin films in Group B are the most stable barriers among all compositions investigated in this work, and thought promising barrier material in future Cu metallization.
