A Comparative Study of Amorphous and Crystal Ge2Sb2Te5 on Performance of Chemical Mechanical Planarization in Phase Change Memory

Abstract

Phase change memory (PRAM) has been considered as one candidate of the nonvolatile memories due to its fast switching speed, good endurance, and compatibility with the conventional CMOS logic manufacturing process. Nevertheless, the high reset current is bottleneck problems of developing high density PRAM. For the phase change material, Ge2Sb2Te5 (GST) has been generally considered as the best phase change material used as data the storage media in PRAM due to its high crystallization rate and small reset current for the phase change. Doping with oxygen or nitrogen in the standard GST material has shown that it is an effective means to reduce the reset current by increase the resistance of phase change material. For the cell structure, it is essential to decrease contact dimension of the phase change material with the bottom electrode (BEC). To decrease this phase change volume, the PRAM cell structure has evolved from the conventional T-shape structure to the confined cell structure. To produce a well-planarized surface and remove surplus GST without introducing etching damage, the chemical mechanical planarization has been adopted to produce the surface planarity of deposited GST and facilitate mass production. The phase of as-deposited GST film is mainly determined by the deposition conditions: including deposition method, chamber temperature etc., and can be switched from each other by simply applying an appropriate current pulse, and maybe accompanied by a quenching process. To determine which phases of the GST film are more suitable for applying CMP, it is necessary to systematically investigate CMP behaviors and surface characteristics of the amorphous and polycrystalline GST after CMP performance. Due to the small phase change volume as the cell structure changed to the confined structure, it is critical to maintain a relatively high polishing rate of GST, while suppress the polishing rate of the isolation material of SiO2, allowing auto stopping of CMP process as SiO2 film is exposed. Among the commonly used alkaline agents, including sodium hydroxide, potassium hydroxide, ammonium hydroxide, and tetramethyl ammonium hydroxide (TMAH), TMAH has been proved to have the best surface roughness of GST after etching, while maintain a very low etching rate of both GST and SiO2 film. Thus, TMAH was chosen as the alkaline agent in our investigation. . In the presence of 0.12wt% TMAH, the amorphous GST shows an overwhelmingly larger polishing rate than the crystalline GST. By measuring hardness of both GST films, it was confirmed that the mechanical hardness is not the main factor determines this great polishing rate difference during GST CMP. On the contrary, a chemically more reactive nature of the amorphous GST in the slurry was found by measuring corrosion potential. By the analysis of X-ray photoelectron Spectroscopy and Energy-Dispersive X-ray Spectroscopy, a selective etching behavior of Te atoms in the amorphous phase was observed. A model based on Valence-Alternation Pairs is proposed to explain this selective etching behavior of Te atoms in the amorphous phase.