E. Atanassova

AES and XPS study of thin RF-sputtered Ta2O5 layers

Abstract The elemental composition and the chemical bonding in RF-sputtered tantalum oxide (4–75 nm) on Si, obtained by reactive sputtering of Ta in an ArO2 mixture, have been studied by AES and XPS. The influence of the gas composition (O2 content in the gas mixture 0.1%–50%) and of the substrate temperature (300 or 493 K) during the film deposition have been studied. The effect of post-deposition annealing in dry O2 has also been considered. The results indicate that a content of 10% O2 favours the formation of stoichiometric Ta2O5 and an abrupt interface transition region between Si and Ta2O5. The annealing effect depends strongly on the quality of the as-deposited layer, it leads basically to homogenization of the layers and improves additionally the stoichiometry of non-perfect oxides.

X-ray photoelectron spectroscopy of thermal thin Ta2O5 films on Si

Abstract X-ray photoelectron spectra (XPS) have been recorded for thermally grown thin tantalum pentoxide films on Si. The peak decomposition technique was employed to identify the composition and the chemical states throughout the film. It is established that stoichiometric Ta2O5 detected at the surface of the layer is reduced to tantalum suboxides in its depth which amount increases in the depth of Ta2O5 film. The results show that even the films obtained at low oxidation temperature (673 K) have a two-layer structure (i.e. silicon dioxide at the interface and tantalum oxide above it) suggesting oxidation of silicon substrate in addition to oxidation of the tantalum film on Si. The interface between Si and this ultra thin SiO2 is not abrupt and the coexistence of Si–O and Ta–O bonding states in close proximity to the interface is found. The thicknesses of both, the tantalum oxide and the SiO2, as well as the width of the Si–SiO2 interface transition region are evaluated.

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

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.

Thin RF sputtered and thermal Ta2O5 on Si for high density DRAM application

Tantalum pentoxide thin films on Si prepared by two conventional for modern microelectronics methods (RF sputtering of Ta in Ar + O2 mixture and thermal oxidation of tantalum layer on Si) have been investigated with respect to their dielectric, structural and electric properties. It has been found that the formation of ultra thin SiO2 film at the interface with Si, during fabrication implementing the methods used, is unavoidable as both, X-ray photoelectron spectroscopy and electrical measurements, have indicated. The initial films (as-deposited and as-grown) are not perfect and contain suboxides of tantalum and silicon which act as electrical active centers in the form of oxide charges and interface states. Conditions which guarantee obtaining high quality tantalum oxide with dielectric constant of 32–37 and leakage current density less than 10−7 A/cm2 at 1.5 V applied voltage (Ta2O5 thickness equivalent to about 3.5 nm of SiO2) have been established. These specifications make the layers obtained suitable alternative to SiO2 for high density DRAM application.

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.

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)

Electrical properties of thin Ta2O5 films obtained by thermal oxidation of Ta on Si

Abstract Tantalum pentoxide films (13–260 nm) on p-type Si have been prepared by thermal oxidation at 673–873 K of rf sputtered Ta films and have been studied using Al–Ta 2 O 5 –Si capacitors. Both dielectric constant and refractive index were found to depend on the thickness of the Ta 2 O 5 layers. Layers with a dielectric constant of 25–32 were obtained. A decreasing trend in the leakage current was found upon increasing oxidation temperature from 673 to 873 K. Leakage current density of (10 −8 to 3×10 −7 ) A cm −2 at 1 MV cm −1 effective field was achieved.