Paper NS-MoM8
Fabrication and Characterization of Ferroelectric BiFeO₃ Nanocapacitors for Next Generation FeRAMS

Monday, October 18, 2010, 10:40 am, Room La Cienega

Low density (≤ 4 Mb) non-volatile ferroelectric random access memories (FeRAMs) are now in the market in “smart cards” competing successfully against established non-volatile memories such as FLASH and EEPROM. The next frontier in fundamental and applied science relevant to FeRAMs is to develop film synthesis processes, materials integration strategies, device architecture, and fabrication processes needed to produce ferroelectric nanocapacitors and study ferroelectric performance at the nanoscale in real device conditions. Critical parameters that need to be measured are polarization switching, fatigue, retention, and imprint and leakage currents, since all these parameters play a critical role in the performance of a reliable FeRAM. Until now, most studies of polarization domain configuration and dynamics in ferroelectric nanocapacitors have been done using the Atomic Force Microscope piezoresponse force microscopy (PFM) technique that images the polarization via the piezoelectric deformation of the polarized region. However, nanocapacitors performance relevant to high density (≥ 1 Gb) FeRAMS need to be evaluated via measurements of all the parameters mentioned above. This paper describes our effort to fabricate BiFeO₃ nanocapacitors (≤ 200 nm in diameter) in a matrix address configuration to measure the ferroelectric properties described above at the nanoscale. BiFeO₃ is explored in this work as the ferroelectric capacitor layer, because it exhibits high polarization (90-120 µC/cm²), it does not contain Pb as PbZr_xTi_1-xO₃ (PZT), and has much higher polarization than SrBi₂Ta₂O₉ (SBT) (~ 30 µC/cm²), which is a leading ferroelectric material in “smart cards” based on FeRAMS. The BFO nanocapacitors involve a SrRuO₃ (SRO)/BFO/SRO layered film structure grown on Strontium Titanate (STO) single crystal substrate. All films are grown by off-axis magnetron sputter-deposition. Nanocapacitors are fabricated with top and bottom electrodes in a matrix line configuration, using electron beam lithography and reactive ion etch processes developed by our group. The BFO nanostructure is at the crossing of the top and bottom electrode lines. The characterization of the nanocapacitor’s performance involves measurement of polarization using electrical excitation via voltage applied between the bottom and top electrodes. Correlations between the film thickness and diameter with the nanocapacitor’s polarization characteristics will be discussed.

Use of the Center for Nanoscale Materials is supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.