Gate Materials and Fabrication-Processes of Metal-Ferroelectric-Insulator-Semiconductor Memory FETs with Long Data Retention

Abstract

A recently-developed Pt/SrBi2Ta2O9/(HfO2)x(Al2O3)1-x /Si FET (x=0.75) shows outstanding data retention characteristics. The drain current ratio between the on- and off-states is more than 2 x 106 after 12 days, and the decreasing rate of this ratio is so small that the extrapolated ratio after 10 years is larger than 1 ×105. Among various materials researched for this decade, the combination of SrBi2Ta2O9 and (HfO2)x(Al2O3)1-x or Hf-Al-O is regarded as the best choice presently. Device performance by changing the Hf and Al composition ratio is investigated, and an Hf-rich-side region around x = 0.75 is found most suitable. Pure HfO2 (x=1) is also a good candidate, but the gate leakage current increase due to HfO2 crystallization is observed. Very recently, we fabricated a self-aligned gate Pt/SrBi2Ta2O9/Hf-Al-O/Si FET, and measured the retention characteristics for 33.5 day that is the longest period measured so far. This also shows a sufficiently large ratio of 2.4 × 105 after 33.5 day which will be 6.5 × 104 after 10 years by extrapolation. The development of a self-aligned gate FeFET is an inevitable step of FeFET miniaturization according to the LSI scaling rule.