Process and reliability assessment of plasma-based copper etch process

摘要

The plasma-based etching processes of copper (Cu) and titanium tungsten (TiW)thin films, and the electromigration of the copper lines patterned by above etchingprocesses were studied. Instead of vaporizing the plasma/copper reaction product, adilute hydrogen chloride solution was used to dissolve the nonvolatile reaction product.The plasma/copper reaction process was affected by many factors including themicrostructure of the copper film and the plasma conditions. Under the same chlorineplasma exposure condition, the copper conversation rate and the copper chloride (CuClx)formation rate increased monotonically with the Cu grain size. The characteristics of theCu etching process were explained by diffusion mechanisms of Cl and Cu in the plasmacopperreaction process as well as microstructures of Cu and CuClx. The Cu chlorinationprocess was also affected by the additive gas in the Cl2 plasma. The additive gas, such asAr, N2, and CF4, dramatically changed the plasma phase chemistry, i.e., the Clconcentration, and the ion bombardment energy, which resulted in changes of the Cuchlorination rate and the sidewall roughness. TiW thin films, used as the diffusion barrier layer for the Cu film, were reactiveion etched with CF4/O2, CF4/Cl2, and CF4/HCl plasma. Process parameter such as feedgas composition, RF power, and plasma pressure showed tremendous effects on the etchrate and the etch selectivity. The TiW etch rate was a function of the sum of Cl and Fconcentrations and the ion bombardment energy. Cu/diffusion barrier metal stack wassuccessfully patterned by above plasma etch processes. The electromigration (EM)performance of the Cu lines was evaluated by the accelerated isothermal test. Theactivation energy of 0.5~0.6 eV and the current density exponent of 2.7 were obtained.Failure analysis showed that both copper-silicon nitride cap layer interface and thecopper grain boundary were active diffusion paths. The EM induced stress caused thecap layer crack and affected the reliability of Cu lines.The processes studied in this dissertation can be applied in advancedmicroelectronic fabrication including large area flexible microelectronics.