Bipolar Voltage Stresses Research Articles

Endurance of magnetic tunnel junctions under dynamic voltage stress

MgAl2O4 (MAO)-based magnetic tunnel junctions (MTJs) with an MAO thickness of ∼1.25 nm are fabricated and their cycling characteristics under dynamic voltage stress are evaluated. The speed of breakdown strongly depended on the pulse polarities used, bipolar, positive (+) unipolar, and negative (−) unipolar. The bipolar condition yielded more rapid breakdown under cycling. Between the two unipolar conditions, positive bias yielded more rapid breakdown than negative bias; the difference between these is understood to arise from the conditions of the interface between the MAO and ferromagnetic layers. Among apparently normal MTJ cells showing little resistance drift, 20% were degraded during a long cycling test in the bipolar stress condition. Thus, the use of bipolar voltage stress is essential to screen for potentially defective MTJs, and the asymmetric condition at the interface is minimised by process control for application of the simple unipolar bias condition.

Characterization of Al2O3–HfO2–Al2O3 sandwiched MIM capacitor under DC and AC stresses

In this paper, electrical properties and reliability of high capacitance density Metal–Insulator–Metal (MIM) capacitor with sandwiched hafnium-based dielectric is analyzed using three kinds of voltage stress; constant voltage stress (CVS), unipolar voltage and bipolar voltage stresses. The fabricated MIM capacitor shows not only high capacitance density but also low leakage current density of about ∼10nA/cm2 at room temperature and 1V. The relative variation of capacitance (ΔC/C0) increases and the variation of voltage linearity (α/α0) gradually decreases with stress-time due to the charge trapping effect in the high-k dielectric. The relative variations of capacitance and voltage linearity show the greater change by the bipolar voltage stress than CVS and unipolar voltage stresses.

Experimental isolation of degradation mechanisms in capacitive microelectromechanical switches

DC and bipolar voltage stresses are used to isolate mechanical degradation of the movable electrode from charging mechanism in microelectromechanical capacitive switches. Switches with different metals as the movable electrode were investigated. In titanium switches, a shift in the pull-in voltages is observed after dc stressing whereas no shift occurs after the bipolar stressing, which is to be expected from charging theory. On switches with similar dielectric but made of aluminium, the narrowing effect occurs regardless if dc or bipolar stressing is used, which indicates the mechanical degradation as the mechanism responsible.

Constant-current study of dielectric breakdown of Pb(Zr,Ti)O3 ferroelectric film capacitors

This paper addresses the problem of time-dependent dielectric breakdown of Pb(Zr,Ti)O3 (PZT) thin films. It is shown by using constant-current breakdown measurements, that for PZT film capacitors with Pt and SrRuO3 (SRO) electrodes thebreakdown onset is controlled by charge to breakdown (QBR) rather than the voltage applied to the capacitor. The QBR value for the asymmetrical Pt/SrRuO3/PZT/Pt capacitors is found to be much higher that for the Pt/PZT/Pt system. Polarization fatigue caused by bipolar voltage stress provokes a substantial decreasein charge to breakdown. These results can be interpreted in terms of percolation modelused for breakdown in SiO2, where the QBR is associated with injected-carrier-assisted creation of the critical concentration of defects required for formation of the breakdown paths. The observed variation of the QBR values depending on the electrode material (i) and polarization fatigue (ii) are inteipreted in terms of current redistribution across the area of the capacitor, specifically current redistribution between grains and grain boundaries (case (i)) or current redistribution between fatigued and nonfatigued areas (case(ii)).

Degradation and breakdown of thin silicon dioxide films under dynamic electrical stress

Thin oxide MOS capacitors have been subjected to dynamic voltage stresses of different characteristics (shape, amplitude and frequency) in order to analyze the transient response and the degradation of the oxide as a function of the stress parameters. The current transients observed in dynamic voltage stresses have been interpreted in terms of the charging/discharging of interface and bulk traps. As for the oxide degradation, the experimental data has been interpreted in terms of a phenomenological model previously developed for dc stresses. According to this model, the current evolution in voltage stresses is assumed to be related to the oxide wearout. The evolution of the current during bipolar voltage stresses shows the existence of two different regimes, the degradation being much faster at low frequencies than at high frequencies. In both regimes, the frequency dependence is not significant, and the change from one regime to the other takes place at a threshold frequency which depends on the oxide field. These trends are also observed in time-to-breakdown versus frequency data, thus suggesting a strong correlation between degradation and breakdown in dynamic stresses. The experimental results are discussed in terms of microscopic degradation models.

Frequency dependence of degradation and breakdown of thin SiO2 films

AbstractThin oxide MOS capacitors are subjected to bipolar voltage stresses of different amplitudes and frequencies. According to a previously proposed breakdown model, the evolution of the current with the stress time has been considered to be due to the degradation of the oxide, i.e. to the generation and partial occupation of electron traps. When log(J) is represented versus the stress time, the slope of the plot and the magnitude of the current (which tends to decrease during constant‐voltage tests) are taken as indicators of the oxide degradation rate and degradation level, respectively. Our results suggest lower degradation rates, and consequently lower degradation levels for the same stress times, at high frequencies. This is consistent with the increase of time‐to‐breakdown with stress frequency observed by other authors, and confirms that, also for dynamic stresses, the relation between degradation and breakdown is fundamental to understand the physics of these phenomena. The slower degradation rates confirm the improvement of oxide reliability under dynamic AC stress conditions.