A. Paskaleva

Challenges of Ta2O5 as high-k dielectric for nanoscale DRAMs.

The present status, successes, challenges and future of Ta₂O₅, and mixed Ta₂O₅-based high-k layers as active component in storage capacitors of nanoscale DRAMs are discussed. The engineering of new Ta₂O₅-based dielectrics (doped Ta₂O₅ and multicomponent high-k dielectrics) as well as of metal/high-k interface in MIM capacitor configuration are identified as critical factors for further reduction of EOT value below 1 nm

Tunneling atomic-force microscopy as a highly sensitive mapping tool for the characterization of film morphology in thin high-k dielectrics

High-k dielectric layers (HfSixOy and ZrO2) with different film morphologies were investigated by tunneling atomic-force microscopy (TUNA). Different current distributions were observed for amorphous and nanocrystalline films by analyzing TUNA current maps. This even holds for crystalline layers where highly resolved atomic-force microscopy cannot detect any crystalline structures. However, TUNA enables the determination of morphology in terms of differences in current densities between nanocrystalline grains and their boundaries. The film morphologies were proven by high-resolution transmission electron microscopy. The investigations show TUNA as powerful current mapping tool for the characterization of morphology in thin high-k films on a nanoscale.

Oxygen annealing modification of conduction mechanism in thin rf sputtered Ta2O5 on Si

Abstract The effect of oxygen annealing at high temperature (873, 1123 K; 30 min) on the insulating properties and conduction mechanism of rf sputtered Ta2O5 (25–80 nm) on Si has been investigated. It is found that the oxygen heating significantly reduces the oxide charge (Qf The conduction mechanism of the as-deposited films is found to be of Poole–Frenkel (PF) type for a wide range of applied fields. A change of the conduction mechanism for the annealed films at medium fields (0.8–1.3 MV/cm) is established. This transition from PF process to the Schottky emission limited current is explained with an annealing of bulk traps (oxygen vacancies and nonperfect bonds). It is concluded that the dominant conduction mechanism in the intermediate fields can be effectively controlled by appropriate technological steps.

Breakdown fields and conduction mechanisms in thin Ta2O5 layers on Si for high density DRAMs

Abstract The conduction mechanisms and the microstructure of rf sputtered Ta2O5 on Si, before and after oxygen annealing at high temperatures (873, 1123 K; 30 min) have been investigated. The as-deposited and annealed at 873 K layers are amorphous whereas crystalline Ta2O5 (orthorhombic β-Ta2O5 phase) was obtained after O2 treatment at 1123 K. The results (electrical, X-ray diffraction, transmission electron microscopy) reveal the formation of an interfacial ultrathin SiO2 layer under all technological regimes used. The higher (493 K) substrate temperature during deposition stimulates the formation of amorphous rather than crystalline SiO2. It is found that the oxygen heating significantly reduces the oxide charge ( Q f 10 cm −2 ) and improves the breakdown characteristics (the effect is more pronounced for the higher annealing temperature). It is accompanied by an increase of the effective dielectric constant (up to 37 after 1123 K treatment). It is established that the influence of the oxygen treatment on the leakage current is different depending on the film thickness, namely: a beneficial effect for the thinner and a deterioration of leakage characteristics for thicker (80 nm) films. A leakage current density as low as 10−7 A/cm2 at 1 MV/cm applied field for 26 nm annealed layers has been obtained. The current reduction is considered to be due to a removal by annealing of certain structural nonperfections present in the initial layers. Generally, the results are discussed in terms of simultaneous action of two opposite and competing processes taking place at high temperatures––a real annealing of defects and an appearance of a crystal phase and/or a neutral traps generation. The contribution of the neutral traps also is involved to explain the observed weaker charge trapping in the as-fabricated films compared to the annealed ones. The conduction mechanism of the as-deposited films is found to be of Poole–Frenkel (PF) type for a wide range of applied fields. A change of the conduction mechanism for the annealed films at medium fields (0.8–1.3 MV/cm) is established. This transition from PF process to the Schottky emission limited current is explained with an annealing of bulk traps (oxygen vacancies and nonperfect bonds). It is concluded that the dominant conduction mechanism in the intermediate fields can be effectively controlled by appropriate technological steps.

Influence of oxidation temperature on the microstructure and electrical properties of Ta2O5 on Si

The effect of the oxidation temperature (673 - 873 K) on the microstructural and electrical properties of thermal Ta 2 O 5 thin films on Si has been studied. Auger electron spectroscopy and X-ray photoelectron spectroscopy results revealed that the films are non-stoichiometric in the depth; an interfacial transition layer between tantalum oxide and Si substrate, containing presumably SiO 2 was detected. It has been found by X-ray diffraction that the amorphous state of Ta 2 O 5 depends on both the oxidation temperature and the thickness of the films-the combination of high oxidation temperature (> 823 K) and thickness smaller than 50 nm is critical for the appearance of a crystal phase. The Ta 2 O 5 layers crystallize to the monoclinic phase and the temperature of the phase transition is between 773 and 823 K for the thinner layers (< 50 nm) and very close to 873 K for the thicker ones. The electrical characterization (current/voltage; capacitance/voltage) reveals that the optimal oxidation temperature for achieving the highest dielectric constant (∼32) and the lowest leakage current (10 -8 A/cm 2 at 1 MV/cm applied field) is 873 K. The results imply that the poor oxidation related defects are rather the dominant factor in the leakage current than the crystallization effects.

Improved insight in charge trapping of high-k ZrO2/SiO2 stacks by use of tunneling atomic force microscopy

In this work, tunneling atomic force microscopy (TUNA) is used to describe the charge trapping in high-k ZrO2 dielectric stacks at nanoscale dimensions by analyzing the alteration of the I-V curves and the I-V hysteresis phenomena with repeated measurements (up to 100 curves) at a single spot of only several nm2 in area. TUNA is also suggested as a powerful technique to correlate the electrical characteristics to the physical properties of the stacks. In particular, the influence of the thin SiO2 interfacial layer thickness and its modification with annealing conditions on the electrical properties is demonstrated. Furthermore, the appearance of an I-V hysteresis and its relation to degradation mechanisms in high-k dielectric stacks are explained. Trapping at pre-existing traps is evidenced.

Conduction mechanisms and reliability of thermal Ta2O5–Si structures and the effect of the gate electrode

The effect of metal gate electrodes (Al, W, Au, TiN, and TiN∕W) on dielectric properties, leakage currents, conduction mechanisms, and reliability characteristics of metal-oxide-semiconductor capacitors with thermal Ta2O5 is investigated. The results are discussed in terms of the relative influence of the gate deposition techniques and the intrinsic properties of the electrode material and the former appears to be more pronounced. It is found that some parameters such as interface state density, breakdown fields, and charge trapping are defined mainly by the properties of Ta2O5 itself. The global dielectric constant of the stack dielectric, oxide charge, leakage current level at high applied fields, charge-to-breakdown, and stress-induced leakage currents are remarkably affected by the upper electrode. It is concluded that the nature and spatial distribution of the gate deposition-induced defects are sensitive to the technological process (evaporation or sputtering); this effect is so strong that it tends...

Constant current stress of Ti-doped Ta2O5 on nitrided Si

The response of Ti-doped Ta2O5 stacked films (4‐6nm) to constant current stress (CCS) under gate injection has been investigated. Two doping methods (‘surface’ and ‘bulk’ doping of Ta2O5) as well as two Si-surface nitridation processes (rapid thermal annealing in N2O and NH3) have been used to prepare the stacks. The effect of doping approach, Si-surface nitridation and the metal electrode (Al and W) on the dielectric degradation has been discussed in terms of stress-induced leakage current (SILC), stress-induced flat band voltage shifts, charge trapping and defect generation. CCS generates charges in the bulk dielectric and in slow states, whose sign and amount depend on both the nitridation process and the doping approach. The method of doping has the strongest impact on the SILC whose behaviour is found to be substantially different from that in SiO2. The method of doping plays a crucial role in the CCS degradation and can be used as an enabling tool for control of this degradation. The gate-deposition-induced defects are another key factor which controls the stress degradation. These defects are very susceptible to electrical stress and could lead to serious reliability concerns. (Some figures in this article are in colour only in the electronic version)

Dielectric properties of rf sputtered Ta2O5 on rapid thermally nitrided Si

Dielectric properties, including leakage currents of stacked structures composed of rf sputtered Ta2O5 and ultrathin SiOxNy grown by rapid thermal nitridation (RTN) in N2O or NH3 ambient, have been investigated. It was previously established that the leakage current in the structures is controlled by both hopping conduction and tunnelling in the SiOxNy layer and Poole-Frenkel emission in Ta2O5. The analysis shows that the RTN in N2O gives higher barriers, lower SiOxNy layer thickness and lower permittivity than in NH3. It is concluded that RTN of the Si surface before deposition of Ta2O5 is a beneficial method for remarkable improvement of the parameters of the Ta2O5/Si system in terms of the permittivity and the leakage currents.

Detailed leakage current analysis of metal–insulator–metal capacitors with ZrO2, ZrO2/SiO2/ZrO2, and ZrO2/Al2O3/ZrO2 as dielectric and TiN electrodes

ZrO2-based metal–insulator–metal capacitors are used in various volatile and nonvolatile memory devices as well as for buffer capacitors or radio frequency applications. Thus, process optimization and material tuning by doping is necessary to selectively optimize the electrical performance. The most common process for dielectric fabrication is atomic layer deposition which guarantees high conformity in three dimensional structures and excellent composition control. In this paper, the C–V and J–V characteristics of ZrO2 metal–insulator–metal capacitors with TiN electrodes are analyzed in dependence on the O3 pulse time revealing the optimum atomic layer deposition process conditions. Moreover, a detailed study of the leakage current mechanisms in undoped ZrO2 compared to SiO2- or Al2O3-doped ZrO2 is enclosed. Thereby, the discovered dependencies on interfaces, doping, layer thickness, and crystalline phases enable the detailed understanding and evaluation of the most suitable material stack for dynamic ra...