IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Electronics Monday Sessions
       Session EL-MoM

Invited Paper EL-MoM6
Recent Progress of Ferroelectric Memory Materials and FET-type FeRAMs

Monday, October 29, 2001, 11:20 am, Room 130

Session: Ferroelectric
Presenter: H. Ishiwara, Tokyo Institute of Technology, Japan
Correspondent: Click to Email
A new class of ferroelectric materials was synthesized by adding sol-gel solution of a dielectric material to that of conventional ferroelectric materials such as PbZr@sub 1-X@Ti@sub X@O@sub 3@, SrBi@sub 2@Ta@sub 2@O@sub 9@, and Bi@sub 4@Ti@sub 3@O@sub 12@. It was found that the crystallization temperature of the new materials was decreased by 150°C to 200°C compared to the original materials and that the surface of the crystallized film was extremely flat. It was also found that the ferroelectric properties were almost the same as or even better than the original ones. Thus, a 13-nm-thick Bi@sub 4@Ti@sub 3@O@sub 12@-based film with a saturation polarization voltage of 0.5 V was obtained. Concerning the FET-type FeRAMs (ferroelectric random access memories), improvement of the data retention characteristics is most important. To improve the retention time, the buffer layer material, which was necessary to insert between the ferroelectric gate film and a Si substrate for preventing interdiffusion of the constituent elements, was optimized, as well as the structure of the ferroelectric-gate FET was carefully designed. As a result, MFIS (metal-ferroelectric-insulator-semiconductor) diodes with excellent characteristics were fabricated by combination of a Si@sub 3@N@sub 4@ buffer layer and a c-axis-oriented Bi@sub 4@Ti@sub 3@O@sub 12@ film. In order to further improve the retention characteristics, a prototype of the 1T2C cell was also fabricated, in which two ferroelectric capacitors with the same area were connected to the gate terminal of an MOSFET so that the depolarization field was not generated. In this cell, nondestructive readout operation up to 10@super 4@ times and the excellent data retention up to 17 hours were realized.