Leakage Current Conduction Mechanisms Research Articles

Direct tunneling stress-induced leakage current in ultrathin HfO2∕SiO2 gate dielectric stacks

The conduction mechanism(s) and behavior of direct tunneling stress-induced leakage current (SILC) through ultrathin hafnium oxide (HfO2)/silicon dioxide (SiO2) dual layer gate stack in metal-oxide-semiconductor (MOS) devices have been experimentally investigated in-depth. Both transient and steady-state SILCs have been studied after constant voltage stress (CVS) and constant current stress (CCS) in n-MOS capacitors with negative bias on the tantalum nitride (TaN) gate. The present report clearly indicates that the observed steady-state SILC is due to assisted tunneling via both monoenergetic trapped positive charges and neutral electron traps generated in the HfO2 layer during either CVS or CCS. SILC measured immediately after stress decays slowly due to tunnel detrapping of stress-induced trapped holes in the HfO2 layer. Furthermore, the mechanisms for stress-induced charge carrier generation/trapping and trap creation in the dielectric have been discussed. Our analysis also shows that CVS degrades the dielectric integrity more severely than CCS in the 4.2nm physically thick HfO2∕SiO2 stack.

EFFECT OF MANGANESE DOPING ON THE ELECTRICAL CHARACTERISTICS OF SOL-GEL DERIVED LEAD ZIRCONATE TITANATE THIN FILMS

ABSTRACT Manganese (Mn) doped Pb(Zr,Ti)O3 thin films were prepared by sol-gel technique and their dielectric, ferroelectric and dc leakage current characteristics were studied in detail. There was a decrease in the dielectric constant, phase transition temperature and increase in the leakage current with the increase in the concentrations of the Mn ions. The decrease in the transition temperature is due to the breaking down of translation symmetry of the polarization, thereby establishing the local dipole moments on suppressing the long-range ferroelectric order. The dielectric dispersion was large for the higher concentration of Mn ions in PZT thin films at lower frequencies and is attributed to the space charge accumulation. The loss tangent initially increases and then decreases with the increase in the Mn ions concentrations indicating the disappearance of the conducting boundary layers. The undoped and Mn doped PZT thin films exhibit the hysteresis loops which confirms the ferroelectric behavior. The dc leakage current conduction mechanism obeyed to space charge limited mechanism and the activation energy calculated from the dc leakage current is attributed to shallow traps.

Effects of annealing on the electrical properties of TiO2 films deposited on Ge-rich SiGe substrates

Titanium oxide (TiO2) high-k dielectric layers were deposited on the Si0.3Ge0.7 substrates by plasma enhanced chemical vapor deposition (PECVD) at low temperature (150°C). To study the effects of annealing on the electrical properties, the TiO2 films were annealed in pure nitrogen ambient in the temperature range of 400–600°C. Good electrical performance for the gate dielectrics was observed for the dielectric films annealed up to 500°C, in terms of interface state density, leakage current, and charge trapping properties. Annealing at 600°C is found to degrade the electrical properties due to Ge segregation and subsequent diffusion into the TiO2 layer. Schottky emission was found to be the dominant leakage current conduction mechanism in PECVD TiO2 films.

High-Reliability Ta2O5 Metal–Insulator–Metal Capacitors with Cu-Based Electrodes

The properties of tantalum oxide metal–insulator–metal (MIM) capacitors with bottom electrodes were investigated. An ultrathin Al film successfully suppresses oxygen diffusion in the MIM capacitor with the Cu-based electrode. The electrical characteristics and reliability of MIM capacitors are improved by addition of ultrathin Al films. MIM capacitors have low leakage current density ( at ) and high breakdown field ( at ). The decrease in leakage current is attributed to the formation of a dense and uniform layer, which has self-protection property and stops further oxygen diffusion into the tantalum contact. The dominant conduction mechanism of leakage current is the Poole–Frenkel effect at electric fields above .

Wafer level reliability and leakage current modeling of PZT capacitors

Over the last few years, thin films of PbZr x Ti 1− x O 3 (PZT) have been the focus of extensive researches for high-k capacitor applications. However, some electrical properties such as leakage current conduction and degradation mechanisms need to be better understood. From Constant Voltage Stress experiments, we identified two distinct failure mechanisms depending on the applied voltage levels. The existence of these two failure mechanisms makes it impossible to extrapolate lifetime results from high voltage to low voltage. Since the typical operating voltage for decoupling capacitors is around 3 V the reliability study has to be focused on the low voltage breakdown. A good opportunity to learn about the low voltage failure mechanisms is to investigate the characteristics of leakage current. The time evolution of leakage current is mainly controlled by the resistance degradation phenomenon. A quantitative analytical model has already been developed to account for this effect. We propose a more complete model that also includes dielectric relaxation and trapping effects. The resulting model is combined to a charge-influenced thermoionic emission model that fits fairly well the voltage and temperature dependence of leakage current. The static and dynamic parts of our model are found to be consistent, especially in terms of barrier lowering effect induced by the resistance degradation process. We believe that the low voltage breakdown is related to a trapping-induced creation of defects in the film.

The Leakage Current Conduction Mechanisms of Cu/Porous Silica Damascene Schemes with Nano-cluster TaSix Barrier

Leakage current conduction mechanisms of Cu/porous silica damascene structures with TaSix (20 nm) barrier were investigated. Polycrystalline TaSi0.54 and nano-cluster TaSi0.42 barriers were prepared by dc sputtering with various power and pressure. The newly developed silica based slurry was employed for the metal-CMP of TaSix films. The incorporation of traditional NH4OH solution and megasonic were applied to perform post-CMP cleaning. Auger mapping revealed clearly straight Cu and Ta lines in these TaSix samples. The line-to-line leakage current density (JL) was plotted versus the electric field (E) at various temperatures. The Ln(JL)–(E)1/2 plots showed that Ln(JL) is linearly dependent on (E)1/2, corresponding to Schottky emission mechanism. The slope of straight lines were determined from the data points of the Ln(JL/T2)–(1000/T) plots. It revealed the magnitude of barrier height. The maximum height of the potential barrier of nano-cluster TaSix/porous silica is about 0.12–0.30 eV. The bias-temperature-stress induced leakage current was measured for reliability study.

CVD of Tantalum Oxide Dielectric Thin Films for Nanoscale Device Applications

A metallorganic chemical vapor deposition (MOCVD) process has been developed for the growth of films from the reaction of tantalum penta-ethoxide and oxygen in a 200 mm wafer tool platform. An optimized process was identified by performing a systematic study of the reaction-rate-limiting steps as a function of substrate temperature, partial pressure, and partial pressure. Subsequent chemical and microstructural analyses showed that as-deposited films were stoichiometric, amorphous, pure, and exhibited a smooth surface morphology. Carbon contamination was below the detection limits of the analytical techniques employed (<1 atom %). Typical as-deposited film dielectric constant and leakage current density were measured to be 27 and (at 1 V), respectively. Annealing at 650°C for 30 min in decreased both the dielectric constant and leakage current, while a similar anneal in Ar resulted in increased dielectric constant and associated decrease in leakage current. It was determined that Schottky emission was the dominant leakage current conduction mechanism in the MOCVD films. © 2004 The Electrochemical Society. All rights reserved.

Electrical conduction mechanism and phase transition studies using dielectric properties and Raman spectroscopy in ferroelectric Pb0.76Ca0.24TiO3 thin films

We have studied the phase transition behavior of Pb0.76Ca0.24TiO3 thin films using Raman scattering and dielectric measurement techniques. We also have studied the leakage current conduction mechanism as a function of temperature for these thin films on platinized silicon substrates. A Pb0.76Ca0.24TiO3 thin film was prepared using a soft chemical process, called the polymeric precursor method. The results showed that the dependence of the dielectric constant upon the frequency does not reveal any relaxor behavior. However, a diffuse character-type phase transition was observed upon transformation from a cubic paraelectric phase to a tetragonal ferroelectric phase. The temperature dependency of Raman scattering spectra was investigated through the ferroelectric phase transition. The soft mode showed a marked dependence on temperature and its disappearance at about 598 K. On the other hand, Raman modes persist above the tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive above the phase transition temperature. The origin of these modes must be interpreted in terms of a local breakdown of cubic symmetry by some kind of disorder. The lack of a well-defined transition temperature suggested a diffuse-type phase transition. This result corroborate the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in the thin film. The leakage current density of the PCT24 thin film was studied at elevated temperatures, and the data were well fitted by the Schottky emission model. The Schottky barrier height of the PCT24 thin film was estimated to be 1.49 eV.

Leakage current characteristics of laser-ablated SrBi2Nb2O9 thin films

We have studied the leakage current conduction mechanism of pulsed-laser-deposited SrBi2Nb2O9 (SBN) thin films on platinized silicon substrates. The time-dependent dc leakage current densities of SBN thin films do not follow Curie–von Schweidler power law. Instead the contribution of conduction current is predominantly electronic. At lower fields, the leakage current follows the ohmic behavior, and it increases exponentially for higher fields. The leakage current density of the SBN thin films was studied at elevated temperatures, and the data were fitted with the Schottky emission model. The effective Richardson’s constant was calculated to be about 8.7×10−6 A/cm2 K2. The Schottky barrier height of the SBN thin films was estimated to be 1.37 eV.

Improved model for the stress-induced leakage current in thin silicon dioxide based on conduction-band electron and valence-band electron tunneling

This article presents a detailed investigation on the stress-induced leakage current (SILC) conduction mechanism via conduction-band electron (CBE) and valence-band electron (VBE) tunneling in thin oxides. An improved SILC model that is able to reproduce the experimental SILC over a wide range of oxide fields, and yet give a realistic level of extracted neutral trap concentration, is proposed. Calculations performed with the improved SILC model suggest that SILC conduction via neutral traps is accompanied by energy relaxation (i.e., an inelastic mechanism), irrespective of the origin (i.e., whether CBE or VBE) of the tunneling species. For both CBE and VBE tunneling, inelastic tunneling with energy relaxation (Erelax) of 1.5 and 0.8 eV, was found to fit the experimental measurements well. These values of Erelax agree with those reported in the literature.

Nanoscale radiative striations of sol-gel-derived spin-coating films

Bi3.25La0.75Ti3O12 (BLT) is one of the prospective candidate materials for data storage devices application. In order to understand the leakage current conduction mechanism in BLT we have studied Mn doped Bi3.25La0.75Ti3O12 (BLT) thin films with varying concentration of Mn. These films were prepared by chemical solution deposition technique. The structure and phase analysis of these films was performed at room temperature by X-ray diffraction, and scanning electron microscope was used to investigate the surface morphology. Different leakage current conduction mechanisms, e.g., Schottky emission, Modified Schottky, Poole–Frenkel emission and space charge limited (Lampert’s theory) conduction were examined to explain the true nature of charge transport phenomena in BLT and Mn doped BLT thin films. It was found that BLT and Mn doped BLT thin films leakage current conduction mechanism follows the Lampert’s theory of space charge limited conduction in an insulator with traps.

Conduction mechanism and origin of stress-induced leakage current in thin silicon dioxide films

The conduction mechanism and origin of the electrical stress-induced leakage current (SILC) in thin silicon dioxide (SiO2) films thermally grown on silicon substrate were clarified from various electrical properties. The properties examined consisted of the I-V characteristics, the oxide trap charge buildup, the generation of the Si/SiO2 interface states, and the generation of the neutral oxide traps. The electrical properties were obtained from films of different oxide thicknesses fabricated by different oxidation processes. The conduction mechanism of SILC was investigated from the viewpoint of oxide thickness dependence, using 92- and 56-Å-thick oxide films. From the oxide-thickness-dependent studies it was found that the SILC phenomenon was not correlated with the oxide trap charge buildup and interface state generation, but rather closely correlated with neutral electron trap generation. The conduction mechanism for nonequilibrium SILC was theoretically deduced from one-dimensional ballistic triangular barrier tunneling that occurred only during the filling process. The tunneling was directed from a leakage spot at the electron-injecting cathode to neutral electron trap sites uniformly generated within the oxide at a trap level (≊1.17 eV from the cathode conduction band and ≊2.0 eV from the SiO2 conduction band) lower than the SiO2 barrier height during only the filling process. The origin of the SILC was also investigated from the viewpoint of oxidation process dependence, using both wet and dry oxides of 86 and 50 Å thicknesses. The oxidation-process-dependent studies revealed that the SILC associated with a wet oxide after the stress application was less than that of a stressed dry oxide. The oxide trap charge buildup and the interface state generation associated with a wet oxide after the stress application was, however, greater than that of a stressed dry oxide. This result suggested that the SILC originated not from water-related chemical reactions, but from the distortion of the thermally grown SiO2 bond structure during electrical stressing. The SILC of both wet and dry oxides after the application of stress were well fitted by Fowler-Nordheim lines, confirming that the leakage conduction mechanism is independent of the oxidation process.

Relation between Stress-Induced Leakage Current and Dielectric Breakdown in SiN-Based Antifuses

We report on the degradation process of metal-to-metal antifuses that use thin silicon nitride film as the dielectric layer under high electric field stress. Stress-induced leakage current was observed in all samples, and it flows through local spots. Two-level fluctuations were found on the leakage current well below the stress voltage, and large and complex fluctuations were observed near the stress voltage. The conduction mechanism of the stress-induced leakage current was the Poole-Frenkel type. It was found that the dielectric constant of the path became large and that the breakdown and the anomalous current depended on the barrier metal thickness. Considering these results, the stress-induced leakage current and the breakdown are thought to be caused by electromigration of electrode material to the SiN film.

Low pressure chemical vapor deposition of silicon dioxide below 500 °C by the pyrolysis of diethylsilane in oxygen

Low pressure chemical vapor deposition (LPCVD) of SiO2 in a horizontal LPCVD furnace using liquid diethylsilane and oxygen has been studied. A temperature deposition window ranging from 425–500 °C was observed resulting in a maximum deposition rate of ∼275 Å/min. The pressure dependence of the deposition rate revealed a threshold of ≳950 mTorr for gas phase reactions at a deposition temperature of 450 °C. Analysis of the films by Rutherford backscattering spectroscopy has indicated that as-deposited films are stoichiometric SiO2 for deposition temperatures ≤450 °C. Best case across wafer uniformity was ±5% for a caged boat. Wet chemical and reactive ion etch rates were found to be comparable to those of thermal oxides after annealing. Cross-sectional scanning electron microscopy images of the SiO2 films deposited on 2 μm deep 1 μm wide silicon trenches revealed a conformality of ∼80%. The electrical properties of films deposited at 450 °C were studied. The electrical properties of the films were studied as-deposited and after annealing the films in a cold-wall rapid thermal annealing (RTA) system. RTAs were performed at temperatures ranging from 950 to 1100 °C in Ar, N2, or O2 ambients. Current–voltage, current–temperature, and capacitance–voltage measurements were performed for the electrical characterization. Catastrophic breakdown field measurements have shown electric field strengths of 9.5 MV/cm for as-deposited 500 Å films. A study of the leakage current conduction mechanisms has indicated that as-deposited films exhibit trap conduction mechanisms at high electric fields and temperatures. However, if deposition is followed by a RTA in Ar or O2, the leakage current follows closely the Fowler–Nordheim mechanism and yields a leakage-current electric field dependence comparable to a thermal oxide. Results have shown that values as low as 6 × 1010/cm2 for fixed charge density can be obtained if oxide deposition is followed by a RTA in Ar or N2.

The heating in vacuum of silicon substrates to temperatures greater than 800°C : J. Wales and A. J. Jeal, Journal of Scientific Instruments41, 521 (1964)

HfAlO/Ge gate stack with Al2O3 passivation layer and different Al2O3 concentration in HfAlO layer has been deposited on Ge substrate by atomic layer deposition. Based on measurements from capacitance-voltage and current density-voltage curves, the evolution of the electrical properties of Ge-based MOS capacitors based on HfAlO/Al2O3/Ge gate stack has been determined as functions of precursor ratio and annealing temperature. It has been found that high annealing temperature and high Al2O3 incorporation in HfAlO films can obtain improved electrical properties such as low hysteresis, low flat-band, low flat-band shift and low interface-state density. However, the oxide-charge density obviously increases with rising the annealing temperature and decreasing Al2O3 incorporation in HfAlO film. By analysis and calculation, it can be noted that the samples annealed at 500 °C holds the lowest leakage current density and the change of the precursor ratio has no apparent effect in 500 °C-annealed Ge MOS capacitor. To investigate current conduction mechanism of Ge MOS capacitors employing HfAlO/Al2O3 gate stacks, current density-voltage measurements have been carried out as functions of annealing temperature and precursor ratio. The involved leakage current conduction mechanisms of the as-deposited and 500 °C-annealed HfAlO/Al2O3/Ge MOS capacitors for substrate injection also have been discussed in detail.