Process and reliability assessment of plasma-based copper etch process
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The plasma-based etching processes of copper (Cu) and titanium tungsten (TiW) thin films, and the electromigration of the copper lines patterned by above etching processes were studied. Instead of vaporizing the plasma/copper reaction product, a dilute hydrogen chloride solution was used to dissolve the nonvolatile reaction product. The plasma/copper reaction process was affected by many factors including the microstructure of the copper film and the plasma conditions. Under the same chlorine plasma exposure condition, the copper conversation rate and the copper chloride (CuClx) formation rate increased monotonically with the Cu grain size. The characteristics of the Cu etching process were explained by diffusion mechanisms of Cl and Cu in the plasmacopper reaction process as well as microstructures of Cu and CuClx. The Cu chlorination process was also affected by the additive gas in the Cl2 plasma. The additive gas, such as Ar, N2, and CF4, dramatically changed the plasma phase chemistry, i.e., the Cl concentration, and the ion bombardment energy, which resulted in changes of the Cu chlorination rate and the sidewall roughness. TiW thin films, used as the diffusion barrier layer for the Cu film, were reactive ion etched with CF4/O2, CF4/Cl2, and CF4/HCl plasma. Process parameter such as feed gas composition, RF power, and plasma pressure showed tremendous effects on the etch rate and the etch selectivity. The TiW etch rate was a function of the sum of Cl and F concentrations and the ion bombardment energy. Cu/diffusion barrier metal stack was successfully patterned by above plasma etch processes. The electromigration (EM) performance of the Cu lines was evaluated by the accelerated isothermal test. The activation 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 the copper grain boundary were active diffusion paths. The EM induced stress caused the cap layer crack and affected the reliability of Cu lines. The processes studied in this dissertation can be applied in advanced microelectronic fabrication including large area flexible microelectronics.