The basic structure of the 1T-1C cell requires the bottom electrode of the ferroelectric capacitor to be in direct electrical contact with the source/drain of the transistor for highly integrated memory devices. To establish the structure of stack-type capacitor, diffusion barrier is needed between ferroelectric film and poly-Si plug, because both high temperature and oxygen ambient processes result in various problems. Especially oxygen diffuses though electrode grain boundaries and then oxidizes poly-Si, underlying layer at high temperature. It results in the degradation in contact resistance of the capacitor and if there are reactions between layers, these will cause the film‘s peeling off by a large volume expansion and high leakage current of the ferroelectric film due to rough surface morphology of electrodes.
In this study, we analyzed the characteristics of Ir and Ru as diffusion barrier directly measuring electrical property of layers from poly-Si to conducting oxide electrode and investigating reactions between layers.
Ir,Ru, LSCO and PZT all the films are deposited by DC magnetron sputtering method. We used highly doped poly-Si(∼$10^{21}$) as the substrate, LSCO which can help to progibit oxygen diffusion as a bottom electrode and TiN as a adhesion layer between poly-Si and Ir.
We found Ir were ohmically connected with poly-Si by less than 700℃ and Ru were ohmically connected with poly-Si by 650℃ when they were used with LSCO. TiN which is used as a adhesion layer between Ir and poly-Si improved crystallinity of Ir and LSCO/Ir/TiN/poly-Si showed low reaction at their interfaces by 700℃.
LSCO thin film(100㎚) was deposited at 450℃, low temperature and then PZT thin film(200㎚) was deposited on LSCO/Ir/TiN/poly-Si/SiO2/Si and LSCO/Ru/poly-Si/SiO2/Si at 550℃, PZT thin film on LSCO/Ir/TiN/poly-Si/SiO2/Si showed highly (001) preferred orientation, densely packed uniform grains and remanent polarization(2Pr) of about 6 5∼70μC/㎠. But PZT thin film on LSCO/Ru/poly-Si/SiO2/Si showed random orientation and lower remanent polarization(2Pr=47∼52μC/㎠).
Fatigue characteristics of all the structures were good because we used LSCO as the bottom electrode. Leakage current density of the former structure remained on the order of $10^－7$A/㎠ at 5V, the applied voltage. On the other hand in the case of the latter suructure, break down occurred at 3.5V becaues of a rougher surface and small work funtion, but we can use this structure enough because generally the operating voltage of memory devices is less than 3V.
