The leakage current mechanism of PZT thin films deposited by in-situ sputtering

Abstract

PZT thin film has been expected to replace conventional dielectric as a charge storage element in DRAM without use of a sophisticated structure such as a trench, because PZT has a dielectric constant much higher than silicon dioxide [1]. However, PZT thin film has relatively high leakage current [2]. To improve the high leakage current, the mechanism must be studied. Some studies have reported that the leakage current mechanism of PZT thin films on platinum showed Schottky emission [3, 4]. Their interpretation depends on the linearity of l n J versus E 1/2 (J: current density, E: electric field). This method is ,not able to distinguish Schottky emission from Poole-Frenkel emission, which may be one of the main mechanisms in PZT thin films. The equation for Poole-Frenkel emission predicts a straight line graph In J against E 1/2. That is, under most circumstances, experimental results can be plotted as a good straight line on either type of graph. Therefore, the mechanism cannot be separated by this means. To tell them apart, it must be determined that the leakage current results from a bulk-limited process or a barrier limited process, Poole-Frenkel emission being bulk limited and Schottky emission being barrier limited [5]. In this study, we measure J V curves for two different polarities using different electrodes, for example, upper electrode aluminium and bottom electrode platinum. In this case, if the leakage current depends on the barrier limited process, the J V curve will be asymmetrical when electrode polarity is reversed because the work functions of aluminium and platinum are different [6]. If leakage current depends on the bulk limited process, the J V curve shows symmetrical characteristics. Platinum film (250nm) with an intervening titanium (300 nm) layer as an adhesion layer were deposited as the bottom electrode by DC sputtering on thermally surface oxidized Si substrate, and then PZT films were deposited by RF magnetron sputtering, the films having a compositional Zr/Ti ratio of 50/50 and a thickness of 200 nm. The deposition conditions are shown in Table I. The aluminium top electrode of radius 100 ~m was deposited by thermal evaporation. X-ray diffractometry revealed that the film was of (1 1 1)-oriented perovskite phase. Leakage current at various voltages was measured using a HP 4145B semiconductor parameter measurement equipment.