Abstract
This research focused on (1) the effect of substrate (oxide) on the preferred orientation of Ti, and the relationship of orientation among Ti, TiN and Al films, (2) the effect of processing parameter on the Al film properties and electromigration performance, (3) the comparison of electromigration performance between hot Al and cold Al processing,and the analysis of failure mechanism . The Ti, TiN and Al films are deposited using multi chamber dc magnetron sputtering system . Ti film was deposited on SiO2 substrate at room temperature, 100℃and 200 ℃. The results of x-ray diffraction (XRD) showed that the crystallization of Ti improved with increasing substrate temperature. The preferred orientation of TiN is also correlated to the crystallization of Ti. Since the Al film deposited on the TiN was much thicker than TiN film, the preferred orientation of Al film was not significantly affected by the underlayer TiN film . When “reflectance,” “sheet resistance” and “stress” were identified as the indeices of film properties parameters, the change of Ar pressure (from 15 sccm to 50 sccm) did not distinctly influence the film properties . The electromigration performance can be correlated to the grain size of Al film . High power results in smaller grain size with higher reflectance ; low power results in larger grain size and lower reflectance . Al film with large grain size had fewer diffusion paths and therefore the electromigration performance increased . The reflectance was found to be decreased with increasing base pressure, implying the increase of impurity in film and thus results in lower performance of EM . The temperature of hot Al process during Al deposition is higher (450℃) than that of cold Al (300 ℃) process . Experiment results showed that Al wired deposited by hot Al process tended to fail earlier in the EM test . It was found that there are three steps for the occurrence of open circuit: (1) microvoid nucleation: the grooving occurred at the interface between Al and TiN providing a microvoid nucleation center. The surface tension of the Al grain boundary is calculated to be 1.438 J/m2. (2) microvoid growth: the direction of microvoid growth can be vertical or horizontal. Horizontal growth tended to form free surface, which can accelerate the migration of Al atoms. (3) open circuit: the formation of voids rapidly increased the resistance and induced joule heating and thereby accelerating diffusion rate . The final failure of the metal line was caused by a auto catalytic process consisting of increasing resistivity due to reducing cross section of the wire, joule heating, diffusion rate increasing due to increasing locally temperature, and final melting of metal line .