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PROPOSED ELECTRODE MATERIAL FOR FERROELECTRIC NONVOLATILE MEMORY CAPACITORS

IP.com Disclosure Number: IPCOM000008905D
Original Publication Date: 1999-Jan-01
Included in the Prior Art Database: 2002-Jul-23
Document File: 2 page(s) / 108K

Publishing Venue

Motorola

Related People

Beth Baumert: AUTHOR [+3]

Abstract

In order to store charge in a ferroelectric non- volatile memory, capacitors comprised of an elec- trode/ ferroelectric/ electrode structure will be used. One of the materials currently being researched for this use is strontium bismuth tantalate (SBT) because of its high write speed and low-voltage write, which would make it advantageous for use in contactless smartcards.

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Technical Developments

PROPOSED ELECTRODE MATERIAL FOR FERROELECTRIC NONVOLATILE MEMORY CAPACITORS

by Beth Baumert, Tse-Lun Tsai and Li-Hsin Chang

BACKGROUND

  In order to store charge in a ferroelectric non- volatile memory, capacitors comprised of an elec- trode/ ferroelectric/ electrode structure will be used. One of the materials currently being researched for this use is strontium bismuth tantalate (SBT) because of its high write speed and low-voltage write, which would make it advantageous for use in contactless smartcards.

PROBLEM

  An electrode is needed which is compatible with SBT and which will increase endurance and stabili- ty. Most researchers deposit SBT on PuTiiSiO$Si, with which endurances of up to 1011 cycles have been demonstrated [l]. However, upon annealing in hydrogen, Bi diffusion from the ferroelectric layer through the bottom Pt occurs, which results in delamination of the bottom electrode and conse- quent catastrophic failure of the capacitor. Films of SBT have also been deposited on CeO$Si [2], but it was found that Si diffused through the CeO2 layer. In addition, Ce02 has a resistivity of 667 @*cm and is therefore not sufficiently conductive to be an

electrode material.

SOLUTION

  The use of Bi-Sr-Ca-Cu-0 (BSCCO) electrodes is proposed. This material is a superconductor but exhibits metallic behavior at room temperature. Both BSCCO and SBT are perovskites and have similar structures, thus rendering the BSCCO a use- ful template for the ferroelectric film. In addition, the lattice parameters of the two materials are very similar (for SBT, a=5.504 A, b=5.488 A, and c=25.06 A, [3], whereas for Bi2Sr$aCu20,,

a=5.420 A, b=5.412 A, and c=30.68 A [4]), which will allow growth of highly-oriented SBT films and will thus enhance electrical properties. Films orient- ed with the a and b axes in the growth direction of the film, for example, will have higher polarizations
[5], whereas c-axis oriented films will have lower switching fields. The chemical compatibility of SBT and BSCCO is also an advantage (both contain Bi and Sr). Furthermore, the use of BSCCO as the electrode material can potentially improve the endurance of the ferroelectric thin film capacitor. It has been demonstrated that PZT thin film capacitors with oxide electrodes, such as (La,Sr)Co03 (LSCO)
[6], RuOz [7], and IrO, [S], essentially can exhibit fatigue-free behavior. It has been suggested in the literature that the occurrence of fatigue is due to the accumulation of oxygen vacancies near the elec- trodes...