Composition Segregation of Ge-Rich GST and Its Effect on Reliability

PCRAM SET/RESET cycling caused GST component segregation and void formation, thus leading to cell failure. We compared electrical cell characteristics and failure analysis of GST with various Ge compositions to understand the GST material segregation and how they may affect the PCM cell reliability. Ge-rich GST424 and GST612 films have an inferior GST segregation uniformity compared to Ge
2
Sb
2
Te
5
(GST225) as revealed from the TEM in-situ anneal analysis. This chemical inhomogeneity could be a problem for PCM array when designing the programming operations as the Ge-rich cells, while improve retention, may not gain endurance due to the broader resistance distributions induced by the GST segregation. Using STEM/ EELS analysis, we also showed that PCM endurance failure is due to the amorphous-like “porous-GST” creation, while the Ge-rich GST doesn't really help much on slow down the GST segregation, as the cycling-dependent I-V subthreshold slope (STS) and voltage-dependent activation energy (Ea) of the Poole-Frenkel transport characteristics remain similar for GST with different Ge concentrations.