Polarity dependence of dielectric breakdown in scaled SiO/sub 2/

Abstract

We have investigated polarity dependence of dielectric breakdown under constant current stress in scaled S io2 dielectrics. Results show that high-field-induced interface state generation is reduced as oxide thickness scales down and charge-to-breakdown (QBD) for positive gate bias (+V,) increases with decreasing oxide thickness. However, QBD for negative gate bias (-Vg) shows an opposite vend to QBD(+Vg), i.e. QBD(-V~) decreases dramatically with decreasing oxide thickness. Therefore, there is an increased polarity dependence of dielectric breakdown when oxide thickness scales down. A physical model based on the degradation of structural transition layer (STL) and interface is proposed to explain the observed trends in QBD.techniques, i.e. 1050°C rapid-thermal processing (RTP) oxidation and 850°C furnace oxidation, with thicknesses ranging from 42 to 120 A. At the same time, N20-oxides with a similar thickness range were also rapid-thermally grown in pure N 2 0 ambient at 1050°C. Polysilicon was deposited and doped with POCl3, followed by lithography, reactive ion etching and forming gas anneal. QBD was measured by constant current stress. Interface trap density (Dit) was determined from high-frequency C-V (HFCV) and quasi-static C-V (QSCV) measurements. The oxide thicknesses were determined from QSCV measurements. Capacitors with the smallest gate area ( 5~1 0 -~ cm2) were used throughout this study in order to identify only intrinsic breakdown. RESULTS AND DISCUSSION INTRODUCTIONThe reliability of ultrathin gate or tunnel oxides under both injection polarities is an important issue for ULSI technology. Another important issue is the dependence of catastrophic dielectric breakdown on oxide thickness. Much work has been directed toward investigating and modeling of oxide wearout and the impact of thickness scaling on the breakdown mechanism [ 1-61. However, there appears to be several different theories regarding the actual mechanism for dielectric breakdown. Most researchers have developed their model based on a single polarity (usually +V,) measurement results, not much attention has been paid to the investigation of breakdown mechanism under both polarities simultaneously. In this paper, QBD of scaled Si02 under both polarities is studied with thicknesses ranging from 42 to 120 A. The oxides are prepared by using two different techniques, i.e., rapid thermal processing (RTP) oxidation and furnace oxidation. The results clearly show that QBD(+V%) increases with decreasing oxide thickness while QBD(-V~) decreases with decreasing oxide thickness, i.e. greater polarity dependence of dielectric breakdown when oxide thickness scales down. Based on the endurance of the interface and the structural transition layer (STL), we present a model which fully explains the polarity dependence of scaled Si02 breakdown. EXPERIMENTALMOS capacitors with n+-polysilicon gate were fabricated on 5-10 R-cm, p-type Si(100) substrates. Thermal oxide were prepared in dry 0 2 ambient by using two different Figs. 1 and 2 show QBD as a...

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