Effect of top metallization on the fatigue and retention properties of Sol-gel PZT thin films

Abstract

Fatigue and retention problems pose significant hurdles towards successful implementation of ferroelectric (FE) memory. The PZT-substrate or PZT-electrode interface is well known to affect the FE properties and specifically the fatigue behaviour. Bottom electrode (or the substrate) is typically a noble metal such as Pt or Au. Pt-PZT-Pt capacitors exhibited fatigue resistance up to 10⁸ - 10⁹ cycles while RuO₂, La_0.5Sr_0.5CoO₃ and YBa₂Cu₃O₇ electrodes yielded fatigue-free devices up to 10¹¹ - 10¹² cycles. In an early study which compared the fatigue behaviour of bulk PLZT ceramics with various metal electrodes, it was found that In offered the best fatigue performance (up to 10⁹ cycles). These films were fired to 700C to achieve single-phase FE perovskite films. Monolithic capacitors were obtained by depositing top electrodes of Ag, Al, Bi, Cr, Cu, Mg, Ni, Pb, Pd, Pt, Sn, V and Zn. Work function difference alone did not fully explain the dependence of the observed electrical properties and fatigue behaviours on the choice of metal electrodes. Rather, the reactivity of the metal and interfacial states at the PZT-metal junctions impacted more on the device characteristics. Zn as the top electrode seemed to offer the optimal fatigue resistant devices.