A Study on Chemical Mechanical Planarization Characteristics of Polycrystalline Chalcogenide Material

Abstract

Phase-change Random Access Memory (PCRAM) is increasingly becoming one of the most candidates for nonvolatile memories in the future memory market. In the fabrication process for the confined cell structure below 30nm design rule, chemical mechanical polishing (CMP) should be adopted after the deposition of Ge2Sb2Te5 GST on the bottom electrode contact followed by patterning the confined cell. The effect of slurry pH and hydrogen peroxide (H2O2) on polycrystalline Ge2Sb2Te5 (pc-GST) CMP performance was investigated. The polishing rate of pc-GST film increased when the slurry pH decreased or increased from neutral pH. In addition, pc-GST polishing rate was highly affected by adding H2O2, resulting in a high pc-GST polishing rate at pH 2-11. However, a noticeable difference in surface roughness of pc-GST film polished was observed between acidic (pH 2) and alkaline (pH 11) regions. Low RMS of roughness as well as a high polishing rate of pc-GST film was obtained in the acidic pH region with 1wt% H2O2. In contrast, in the alkaline region a high RMS of roughness of pc-GST film was observed owing to enhanced selective corrosion between Ge, Sb, and Te elements. And the role of H2O2 in alkaline slurry on the polishing rate of pc-GST film was investigated. The polishing rate of pc-GST films increases sharply with the H2O2 concentration up to 0.2 wt % and then increases slightly with a further increase in H2O2 concentration up to 3.0 wt%. However, the polishing rate of SiO2 films very slightly increases with H2O2 concentration up to 3 wt%. Polishing selectivity of pc-GST films to SiO2 films of over 100:1 can therefore be achieved by adding H2O2 to the slurry. And the chemical mechanical planarization (CMP) mechanism of pc-GST using nitric acid slurry without or with 1.0 wt% H2O2 was investigated. Without H2O2, the pc-GST film surface undergoes selective corrosion of Ge and Sb, resulting in pitting corrosion. Otherwise, with H2O2, the pc-GST film surface produces a Ge, Sb, Te oxide layer, thereby introducing a cyclic polishing process such as chemical oxidation, chemical and mechanical polishing, and sequential chemical oxidation. As a result, the polishing rate increased up to ~1000 Å/min and the surface roughness was decreased 0.86 nm.