Failure Analysis of Nitrogen-Doped Ge2Sb2Te5 Phase Change Memory

Phase change memory (PCM) devices, consisting of nitrogen-doped Ge
₂
Sb
₂
Te
₅
(NGST) chalcogenide, are fully integrated in the 110-nm complementary metal-oxide-semiconductor technology. By combining high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS), we thoroughly analyze the original fail cells. The direct results of structural observation and compositional analysis of the phase change cells are displayed. The original fail cells, which are initially at the extra low-resistance state, cannot be operated to high-resistance state when applying the normal reset condition as most of the successfully reset cells. Hexagonal Ge
₂
Sb
₂
Te
₅
(GST) and TiTe
₂
phases are observed in the microstructure of the original fail cells. The element analysis indicates that the atomic composition of the fail cells changes to nonstoichiometric phase. Titanium atoms incorporate into the GST-based film, whereas tellurium and antimony atoms diffuse into the titanium nitride adhesive layer. The germanium atoms accumulate at the bottom electrode. It is believed that phase change materials, which undergo very high thermal budget during back-end processing at 350°C or even high at 400°C during the fabrication, can inevitably cause intrinsic problems in stability of the PCM devices. The irreversible modification of the NGST composition and these hexagonal GST and TiTe
₂
phases will directly result in the early failure of the PCM.