Chemical Mechanical Polishing of Ruthenium in Spin-Transfer-Torque Magnetic Random Access Memory

Abstract

CMP is an essential step for planarizing multi-level interconnect structures in nano-scale integrated circuit fabrication in recent years. As the increasing demand for next generation memories and development of new structures and novel materials, the CMP process, as a robust planarization method, has become the fastest growing technologies in next generation memories and conventional memories such as DRAM and NAND in 10x nm design rule. In these emerging applications, however, increasingly stringent requirements for the CMP performance, which is mainly determined by the slurry, are proposed due to the susceptibility and fragility of device function to defects originated during the CMP process. One of the state-of-the-art CMP technologies is proposed in STT-MRAM, where the CMP process is adopted to form a Ru bottom electrode plug before constructing MTJ on it. This Ru CMP requires a high polishing rate, a high polishing selectivity between Ru and SiO2, low dishing and erosion, and especially an extremely low surface roughness. This work aims to research and develop the CMP slurry for the Ru CMP in STT-MRAM in two parts, namely in the colloidal silica slurry and in the titania slurry. At the former part of this dissertation, we study effects of oxidizer, pH, and corrosion inhibitor on performances of the Ru CMP in the colloidal silica slurries. We conducted CMP and corrosion experiments, and found that the Ru film has the highest polishing rate and corrosion rate with NaIO4 at the neutral pH range. The key factor that determines the polishing rate and corrosion rate is the formation of RuO3/RuO2 oxide layer on the surface of the Ru film, which is demonstrated by XPS measurement. Also, reaction byproducts of RuO4/RuO4- in the slurry is proved to have a direct influence on the formation of the RuO3/RuO2 oxide layer. A band diagram based on the work function of Ru electrode and hydrated redox couple in the electrolyte is proposed to explain the electron transfer process between the Ru film and oxidizer in the slurry. In addition, both equivalent circuit models obtained from EIS measurement and surface morphology by SEM indicate that the RuO3/RuO2 oxide layer has an extent of porosity, resulted from pitting corrosion. This porosity results in a rough surface roughness after CMP. In order to improve the surface roughness, we verify effects of corrosion inhibitors on the Ru CMP performance in the colloidal silica slurry with NaIO4. We find EDTA with both amine and carboxylic functional groups effectively suppress pitting corrosion while maintain a relatively high polishing rate. EDTA is supposed to enable the Ru film on a subtle equilibrium state that has both chelated EDTA layers and the porous RuO3/RuO2 oxide layer. We also investigate and evaluate the titania slurry with H2O2 to replace the conventional silica slurry with NaIO4. We find that the polishing rate of the Ru film has a strong dependency on the H2O2 concentration in the titania slurry. Surprisingly, the CMP mechanism is rather close to the direct polishing mechanism by analyzing corrosion behaviors and surface morphologies, which is mainly observed in dielectric CMP without oxidizer in the slurry. Adsorption of H2O2 on the titania particle is considered to promote the formation of a ‘chemical tooth’ like the case of glass polishing with the ceria slurry. The results show that the titania slurry with H2O2 has the same CMP performance as the silica slurry with NaIO4 including the high polishing rate, the low corrosion current density, and the low surface roughness, while eliminates serious problems such as the high ionic strength, byproduct generation of a toxic gaseous RuO4 during CMP, and remaining polished debris on the pad after CMP.