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 Ta₂O₅ 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 SiO₂ was detected. It has been found by X-ray diffraction that the amorphous state of Ta₂O₅ 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 Ta₂O₅ 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/cm² 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.     

Thermal Ta2O5––alternative to SiO2 for storage capacitor application

Tantalum pentoxide thin layers (10–100 nm) obtained by thermal oxidation of rf sputtered Ta films on Si have been investigated with respect of their dielectric, structural and electric properties. It is established that stoichiometric Ta₂O₅ detected at the surface of the layers is reduced to tantalum suboxides in their depth. The oxide parameters are discussed in terms of a presence of an unavoidable ultrathin SiO₂ between Si and Ta₂O₅ and bond defects in both the oxide and the interface transition region. Conditions which guarantee obtaining high quality tantalum oxide with a dielectric constant of 32–35 and a leakage current less than 10⁻⁷–10⁻⁸ A/cm² at 1.5 V (SiO₂ equivalent thickness of 2.5–3 nm) are established. These specifications make the layers obtained suitable alternative to SiO₂ for high density DRAMs application.     

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

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<10¹⁰ 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.     

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.     

Structural and electrical properties of radio frequency magnetron sputtered tantalum oxide films: Influence of post-deposition annealing

The as-deposited and annealed radio frequency reactive magnetron sputtered tantalum oxide (Ta₂O₅) films were characterized by studying the chemical binding configuration, structural and electrical properties. X-ray photoelectron spectroscopy and X-ray diffraction analysis of the films elucidate that the film annealed at 673 K was stoichiometric with orthorhombic β-phase Ta₂O₅. The dielectric constant values of the tantalum oxide capacitors with the sandwich structure of Al/Ta₂O₅/Si were in the range from 14 to 26 depending on the post-deposition annealing temperature. The leakage current density was <20 nA cm^?2 at the gate bias voltage of 0.04 MV/cm for the annealed films. The electrical conduction mechanism observed in the films was Poole–Frenkel.     

Influence of the metal electrode on the characteristics of thermal Ta2O5 capacitors

The effect of various electrodes (Al, W, TiN) deposited by evaporation (Al) and sputtering (W, TiN) on the electrical characteristics of thermal thin film (15-35 nm) Ta₂O₅ capacitors has been investigated. The absolute level of leakage currents, breakdown fields, mechanism of conductivity, dielectric constant values are discussed in the terms of possible reactions between Ta₂O₅ and electrode material as well as electrode deposition process-induced defects acting as electrically active centers. The dielectric constant values are in the range 12-26 in dependence on both Ta₂O₅ thickness and gate material. The results show that during deposition of TiN and Al a reaction that worsens the properties of Ta₂O₅ occurs while there is not an indication for detectable reduction of Ta₂O₅ when top electrode is W, and the leakage current is 5-7 orders of magnitude lower as compared to Al and TiN-electroded capacitors. The high level of leakage current for TiN and Al gate capacitors are related to the radiation defects generated in Ta₂O₅ during sputtering of TiN, and damaged interface at the electrode due to a reaction between Al and Ta₂O₅, respectively. It is demonstrated that the quality of the top electrode affects the electrical characteristics of the capacitors and the sputtered W is found to be the best. The sputtered W gate provides Ta₂O₅ capacitors with a good quality: the current density <7 × 10⁻¹⁰ A/cm² at 1 V (0.7 MV/cm, 15 nm thick Ta₂O₅). W deposition is not accompanied by an introduction of a detectable damage leading to a change of the properties of the initial as-grown Ta₂O₅ as in the case of TiN electrode. Damage introduced during TiN sputtering is responsible for current deterioration (high leakage current) and poor breakdown characteristics. It is concluded that the sputtered W top electrode is a good candidate as a top electrode of storage capacitors in dynamic random access memories giving a stable contact with Ta₂O₅, but sputtering technique is less suitable (favorable) for deposition of TiN as a metal electrode due to the introduction of radiation defects causing both deterioration of leakage current and poor breakdown characteristics.     

Effect of a silicon nitride buffer layer on the electrical properties of tantalum pentoxide films

Ta₂O₅ films with a buffer layer of silicon nitride of various thicknesses were deposited on Si substrate by reactive sputtering and submitted to annealing at 700 °C in nitrogen atmosphere. The microstructure and the electrical properties of thin films were studied. It was found that with a buffer layer of silicon nitride the electrical properties of Si_xN_y/Ta₂O₅ film can be improved than Ta₂O₅ film. When the thickness of the buffer layer was 3 nm, the Si_xN_y/Ta₂O₅ film has the highest dielectric constant of 27.4 and the lowest leakage current density of 4.61 × 10⁻⁵ A/cm² (at −1 V). For the Si_xN_y (3 nm)/Ta₂O₅ film, the conduction mechanism of leakage current was also analyzed and showed four types of conduction mechanisms at different applied voltages.     

Time-dependent-dielectric-breakdown characteristics of Hf-doped Ta2O5/SiO2 stack

The Time-Dependent-Dielectric Breakdown (TDDB) characteristics of MOS capacitors with Hf-doped Ta₂O₅ films (8 nm) have been analyzed. The devices were investigated by applying a constant voltage stress at gate injection, at room and elevated temperatures. Stress voltage and temperature dependences of hard breakdown of undoped and Hf-doped Ta₂O₅ were compared. The doped Ta₂O₅ exhibits improved TDDB characteristics in regard to the pure one. The maximum voltage projected for a 10 years lifetime at room temperature is −2.4 V. The presence of Hf into the matrix of Ta₂O₅ modifies the dielectric breakdown mechanism making it more adequate to the percolation model. The peculiarities of Weibull distribution of dielectric breakdown are discussed in terms of effect of three factors: nature of pre-existing traps and trapping phenomena; stress-induced new traps generation; interface layer degradation.     

Characteristics of metal-ferroelectric-insulator-semiconductor structure using Sr0.8Bi2.2Ta2O9 and Sr0.8Bi2.2Ta2O9-BaZrO3 for ferroelectric gates

Characteristics of BaZrO₃ (BZO) modified Sr_0.8Bi_2.2Ta₂O₉ (SBT) thin films fabricated by sol-gel method on HfO₂ coated Si substrates have been investigated in a metal-ferroelectric-insulator-semiconductor (MFIS) structure for potential use in a ferroelectric field effect transistor (FeFET) type memory. MFIS structures consisting of pure SBT and doped with 5 and 7 mol% BZO exhibited memory windows of 0.81, 0.82 and 0.95 V with gate voltage sweeps between −5 and +5 V, respectively. Leakage current density levels of 10⁻⁸ A/cm² for BZO doped SBT gate materials were observed and attributed to the metallic Bi on the surface as well as intrinsic defects and a porous film microstructure. The higher than expected leakage current is attributed to electron trapping/de-trapping, which reduces the data retention time and memory window. Further process improvements are expected to enhance the electronic properties of doped SBT for FeFET.     

Lightly Al-doped Ta2O5: Electrical properties and mechanisms of conductivity

Lightly Al-doped Ta₂O₅ films (10;15 nm) obtained by rf sputtering have been studied with respect to their dielectric and electrical properties. The formed metal-high-k dielectric-semiconductor capacitors have been characterized by capacitance-voltage and temperature-dependent current-voltage characteristics. It was established that the introduction of small amount (5 at.%) Al into the matrix of Ta₂O₅ improves dielectric constant, introduces negative oxide charge, suppresses deep oxygen-vacancy centers in Ta₂O₅ but creates shallow traps and changes the dominant conduction mechanism in the stacks. The doping produces more leaky films at room temperature and lower current at high temperature as compared to the case of pure Ta₂O₅. It is concluded that the strong contribution of tunneling processes through shallow traps in the conductivity of doped films could explain the observed current degradation at room temperature and its improved temperature stability at high temperatures. The energy levels of the traps responsible for the current transport are estimated.     

Effect of microwave radiation on the properties of Ta2O5–Si microstructures

The paper presents results of the effect of microwave irradiation at room temperature on the properties of thin layers of tantalum pentoxide deposited on Si by rf sputtering. Electrical characterization is performed in conjunction with Auger electron spectroscopy and atomic force microscopy. Among exposure times used (1; 5; 10 s), treatment of about 5 s shows the best promise as an annealing step––an improvement of number of parameters of the system Ta₂O₅–Si is established (dielectric constant and surface morphology; stoichiometry and microstructure of both the bulk oxide and the interfacial transition region; electrical characteristics in terms of oxide charge density, leakage current and breakdown fields). At the same time the microwave irradiation is not accompanied by crystalization effects in Ta₂O₅ and/or additional oxidation of Si substrate. It is concluded that the short-time microwave irradiation can be used as an annealing process for Ta₂O₅–Si microstructures and it has a potential to replace the high-temperature annealing processes for high-k insulators.     

High-k HfO2-Ta2O5 mixed layers: Electrical characteristics and mechanisms of conductivity

Electrical properties of mixed HfO₂-Ta₂O₅ films (10;15 nm) deposited by rf sputtering on Si have been studied from the view point of their applications as high-k layers, by standard capacitance-voltage and temperature dependent current-voltage characteristics. The effect of HfO₂ addition to the Ta₂O₅ is thickness dependent and the thicker layers exhibit advantages over the pure Ta₂O₅ (higher dielectric constant, enhanced charge storage density and improved interface quality). The process of HfO₂ and Ta₂O₅ mixing introduces negative oxide charge, tends to creates shallow bulk traps and modifies the dominant conduction mechanisms in the stack capacitors as compared to the Ta₂O₅-based one (a contribution of tunneling processes through traps located below the conduction band of mixed layers to the leakage current in the HfO₂-Ta₂O₅ stacks is observed). The traps involved in both Poole-Frenkel and tunneling processes are identified.     

Examination and evaluation of La2O3 as gate dielectric for sub-100 nm CMOS and DRAM technology

High-k gate dielectric La₂O₃ thin films have been deposited on Si(1 0 0) substrates by molecular beam epitaxy (MBE). Al/La₂O₃/Si metal-oxide–semiconductor capacitor structures were fabricated and measured. A leakage current of 3 × 10⁻⁹ A/cm² and dielectric constant between 20 and 25 has been measured for samples having an equivalent oxide thickness (EOT) 2.2 nm. The estimated interface state density D_it is around 1 × 10¹¹ eV⁻¹ cm⁻². EOT and flat-band voltage were calculated using the NCSU CVC program. The chemical composition of the La₂O₃ films was measured using X-ray photoelectron spectrometry and Rutherford backscattering. Current density vs. voltage curves show that the La₂O₃ films have a leakage current several orders of magnitude lower than SiO₂ at the same EOT. Thin La₂O₃ layers survive anneals of up to 900 °C for 30 s with no degradation in electrical properties.     

Structural and electrical characteristics of RF-sputtered HfO2 high-k based MOS capacitors

The HfO₂ high-k thin films have been deposited on p-type (1 0 0) silicon wafer using RF magnetron sputtering technique. The XRD, AFM and Ellipsometric characterizations have been performed for crystal structure, surface morphology and thickness measurements respectively. The monoclinic structured, smooth surface HfO₂ thin films with 9.45 nm thickness have been used for Al/HfO₂/p-Si metal-oxide-semiconductor (MOS) structures fabrication. The fabricated Al/HfO2/Si structure have been used for extracting electrical properties viz dielectric constant, EOT, barrier height, doping concentration and interface trap density through capacitance voltage and current-voltage measurements. The dielectric constant, EOT, barrier height, effective charge carriers, interface trap density and leakage current density are determined are 22.47, 1.64 nm, 1.28 eV, 0.93 × 10¹⁰, 9.25 × 10¹¹ cm⁻² eV⁻¹ and 9.12 × 10⁻⁶ A/cm² respectively for annealed HfO₂ thin films.     

Microstructure and electrical characteristics of Ba0.65Sr0.35TiO3 thin films etched in CF4/Ar/O2 plasma

Radio frequency magnetron sputtered Ba_0.65Sr_0.35TiO₃ (BST) thin films were etched in CF₄/Ar/O₂ plasma by magnetically enhanced reactive ion etching technique. The etching characteristics of BST films were characterized in terms of microstructure and electrical properties. Atomic force microscopy and X-ray diffraction results indicate that the microstructure of the etched BST film is degraded because of the rugged surface and lowered intensities of BST (1 0 0), (1 1 0), (1 1 1) and (2 0 0) peaks compared to the unetched counterparts. Dielectric constant and dielectric dissipation of the unetched, etched and postannealed-after-etched BST film capacitors are 419, 346, 371, 0.018, 0.039 and 0.031 at 100 kHz, respectively. The corresponding dielectric tunability, figure of merit and remnant polarization are 19.57%, 11.56%, 17.25%, 10.87, 2.96, 5.56, 3.62 μC/cm², 2.32 and 2.81 μC/cm² at 25 V, respectively. The leakage current density of 1.75 × 10⁻⁴ A/cm² at 15 V for the etched BST capacitor is over two orders of magnitude higher than 1.28 × 10⁻⁶ A/cm² for the unetched capacitor, while leakage current density of the postannealed-after-etched capacitor decreases slightly. It means that the electrical properties of the etched BST film are deteriorated due to the CF₄/Ar/O₂ plasma-induced damage. Furthermore, the damage is alleviated, and the degraded microstructure and electrical properties are partially recovered after the etched BST film is postannealed at 923 K for 20 min under a flowing O₂ ambience.     

具有低电压、低“迟滞效应”的C60薄膜晶体管

以C60为激活层,同时以聚合物/高K氧化物双绝缘层结构研制了N型有机场效应晶体管。结果表明,采用这种双层结构的绝缘层能够很好的将Ta2O5和PMMA的优点结合在一起,即既利用了Ta2O5的高介电常数又利用了PMMA与半导体层良好的界面接触特性。与采用单一Ta2O5或这PMMA绝缘层的器件相比,这种具有双层结构的器件性能大幅提升。最终研制了能够在10V低电压下正常工作的C60晶体管,其场效应迁移率、阈值电压和开关电流比分别为0.26 cm2/Vs, 3.2V和8.31×104。同时,利用修饰绝缘层PMMA的疏水性大大降低了这种具有双层结构的N型有机晶体管的“迟滞效应”,从而让器件工作时有较好的稳定性。     

Ferroelectric and leakage current characteristics of (Pb0.72La0.28)Ti0.93O3 thin films prepared by a sol-gel process

(Pb_1 − xLa_x)Ti_1 − x/4O₃(x = 28 mol%, denoted as PLT) thin films were grown on Pt/Ti/SiO₂/Si substrates by using a sol-gel process. The Pt/PLT/Pt film capacitor showed well-saturated hysteresis loops at an applied electric field of 500 kV/cm with spontaneous polarization (P_s), remanent polarization (P_r) and coercive electric field (E_c) values of 9.23 μC/cm², 0.53 μC/cm² and 19.7 kV/cm, respectively. At 100 kHz, the dielectric constant and dissipation factor of the film were 748 and 0.026, respectively. The leakage current density is lower than 1.0 × 10⁻⁷ A/cm²over the electric field range of 0 to 200 kV/cm. And the Pt/PLT interface exist a Schottky emission characteristics.     

Effects of fluorine-implanted treatment on Ba0.7Sr0.3TiO3 films

The effects of F-ion implantation on the leakage and dielectric properties of the Ba_0.7Sr_0.3TiO₃ (BST) films were investigated. The BST film implanted with 1×10¹⁵ cm^–2 shows the optimum leakage performance. The leakage current density can be decreased by one order of magnitude as compared to that of the non-implanted sample at an applied voltage of 2 V. On increasing the implanted dose from 5×10¹⁴ to 5×10¹⁵ cm^–2, the dielectric constant first increases and then decreases due to the deteriorated crystallinity. It is found that the suitable F-ion dose can reduce the –OH contaminants and improve the dielectric and leakage properties.     

Temperature dependence of refractive index of Ta2O5 Dielectric Films

The temperature dependence of refractive index of tantalum pentoxide (Ta₂O₅) dielectric films was investigated experimentally. The films were formed by a magnetron radio frequency sputtering technique on the Si substrates. After deposition, the film was fabricated into an antiresonant reflecting optical waveguide using a novel wet etching technique. The thermal variation of refractive index of Ta₂O₅ was characterized by measuring the index-vs-temperature coefficient of the antiresonant reflecting optical waveguide with a Mach-Zehnder interferometry system. The measured result was 2.3 × 10^-61/K at 632.8 nm from 298 to 328K. This result indicates that index-vs-temperature coefficient of the Ta₂O₅ dielectric films is about ten times less than those of conventional III-V semiconductors.     

Carrier mobility in inversion layers of Si–thin Ta2O5 structures

The behaviour of carrier mobility in the inversion channel of gateless p-MOSFETs with thin (7-50 nm) Ta₂O₅ layers, having a dielectric constant of (23-27) and prepared by rf sputtering of Ta in an Ar-O₂ mixture, has been investigated. It is shown that independently of the high dielectric constant of the layers, the transport properties in the channel are strongly affected by defects in Ta₂O₅/Si system in the form of oxide charge and interface states. These defects act as scattering centers and are responsible for the observed minority carrier mobility degradation. Both, the oxide and the interface state charges are virtually independent on the oxygen content (in the range 10-30%) during the sputtering process. A reduction of the oxide charge and the density of interface states with increasing Ta₂O₅ film thickness was found, which results in the observed increase of the inversion channel mobility with thickness. It is assumed that the bond defects (broken or strained Ta-bonds as well as weak Si-O bonds in the transition region between Ta₂O₅ and Si) are much more probable sources of defect centers rather than Ta and O vacancies or impurities.