Design of CMP Slurry of Polycrystalline Ge2Sb2Te5 Film for Phase Change Memory Device

Abstract

Phase-change random-access memory (PRAM) is increasingly becoming one of the more promising candidates for next-generation nonvolatile memory owing to its large sensing signal, fast writing speed, outstanding endurance, and long data-retention time compared to commercial NAND flash memory. As the design rule shrank to below 30 nm, the memory-state distributions and reduction of reset current have become tighter and the thermal-cross-talk between adjacent cells has increased. The PRAM cell structure was thus evolved from a conventional T-shape, to a partially confined structure, and finally to a fully confined structure. In the mass production of high-density PRAM, chemical mechanical polishing (CMP) is essentially adopted because it can perform local and global planarization within a wafer as well as remove the overburden Ge2Sb2Te5 (GST) on dielectric. However, there are some important issues arose from material properties of GST film. Since the GST film is composed of ternary alloy, each element in the film has a different etching rate according to its electronegativity (Ge: 2.01: Sb: 2.05: and Te: 2.1 on the Pauling scale). Thus, the selective corrosion rate on pc-GST film is increased, and many corrosion pits are formed at grain boundary region in aqueous solution resulting in deterioration of surface roughness and variation of composition of pc-GST film after the CMP process. Optimum CMP slurry of a pc-GST film is essentially needed for improving operation characteristics of memory and the efficiency of production. This thesis focused on chemical additives as corrosion-inhibition agents such as oxidizer and nonionic inhibitor to inhibit the selective corrosion of pc-GST film in colloidal silica abrasive-based slurry to decrease CMP-induced defect such as pitting corrosion, scratch as well as high polishing rate of pc-GST film and selectivity of polishing rate between pc-GST to SiO2 film. Then, the chelating agent for pc-GST film was investigated to enhance the polishing rate of pc-GST and obtain high surface quality. The colloidal silica-based slurries for PRAM were synthesized, and then polishing performance was evaluated using the slurries. In particular, the polishing mechanism depending on the slurry component (such as oxidizer, nonionic polymer, and chelating agent) was revealed as follows. Fist, we develop new alkaline slurry added with KMnO4 used as an oxidizer. Second, the corrosion inhibition effect of different nonionic polymers, which are polyacrylamide (PAM), polyvinyl pyrrolidone (PVP), and polyvinyl alcohol (PVA), on (pc-GST) was demonstrated in an alkaline media for chemical mechanical planarization. In addition, the corrosion inhibition efficiency and polishing rate of PVP with molecular weights of 3.5k, 25k, and 1300k g/mol were evaluated. Finally, the role of chelating agent focused on β-cyclodextrin comparing to citric acid in H2O2 containing slurry for pc-GST film after polishing was investigated to enhance polishing rate with high surface quality of pc-GST film.