Transistor characteristics with Ta2O5 gatedielectric

As the gate oxide thickness decreases below 2 nm, the gate leakage current increases dramatically due to direct tunneling current. This large gate leakage current will be an obstacle to reducing gate oxide thickness for the high speed operation of future devices. A MOS transistor with Ta₂O₅ gate dielectric is fabricated and characterized as a possible replacement for MOS transistors with ultra-thin gate silicon dioxide. Mobility, I_d-V_d, I_d-V_g, gate leakage current, and capacitance-voltage (C-V) characteristics of Ta₂O₅ transistors are evaluated and compared with SiO₂ transistors. The gate leakage current is three to five orders smaller for Ta₂O₅ transistors than SiO₂ transistors     

Ta2O5/silicon barrier height measured fromMOSFETs fabricated with Ta2O5 gate dielectric

N-channel metal oxide semiconductor field effect transistors with Ta₂O₅ gate dielectric were fabricated. The Ta₂O₅/silicon barrier height was calculated using both the lucky electron model and the thermionic emission model. Based on the lucky electron model, a barrier height of 0.77 eV was extracted from the slope of the ln(I_g/I_d) versus ln(I_sub/I_d) plot using an impact ionization energy of 1.3 eV. Due to the low barrier height, the application of Ta₂ O₅ gate dielectric transistors is limited to low supply voltage preferably less than 2.0 V     

Fabrication and characterization of metal-oxide-semiconductorfield-effect transistors and gated diodes using Ta2O5 gate oxide

N-channel metal oxide semiconductor field effect transistors (MOSFETs) using Ta₂O₅, gate oxide were fabricated. The Ta₂O₅ films were deposited by plasma enhanced chemical vapor deposition. The I_DS-V_DS and I_DS-V_GS characteristics mere measured. The electron mobility was 333 cm²/V·s. The subthreshold swing was 73 mV/dec. The interface trapped charge density, the surface recombination velocity, and the minority carrier lifetime in the field-induced depletion region measured from gated diodes were 9.5×10¹² cm^-2 eV^-1, 780 cm/s and 3×10^-6 sec, respectively. A comparison with conventional MOSFETs using SiO₂ gate oxide was made     

Effects of the metal gate on the stress-induced traps in Ta2O5/SiO2 stacks

The degradation of Ta₂O₅-based (10 nm) stacked capacitors with different top electrodes, (Al, W, Au) under constant current stress has been investigated. The variation of electrical characteristics after the stress is addressed to gate-induced defects rather than to poor-oxidation related defects. The main wearout parameter in Ta₂O₅ stacks is bulk-related and a generation only of bulk traps giving rise to oxide charge is observed. The post-stress current–voltage curves reveal that stress-induced leakage current (SILC) mode occurs in all capacitors and the characteristics of pre-existing traps define the stress response. The results are discussed in terms of simultaneous action of two competing processes: negative charge trapping in pre-existing electron traps and stress-induced positive charge generation, and the domination of one of them in dependence on both the stress level and the gate used. The charge build-up and the trapping/detrapping processes modify the dominant conduction mechanism and the gate-induced defects are precursors for device degradation. It is concluded that the impact of the metal gate on the ultimate reliability of high-k stacked capacitors should be strongly considered.     

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.     

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.     

Polyimide/Ta2O5/polyimide antiresonantreflecting optical waveguides

A novel antiresonant reflecting optical waveguide (ARROW) fabricated using both the organic and dielectric thin film technologies is presented for the first time. The ARROW consisted of the fluorinated polyimide and tantalum pentoxide (Ta₂O₅) hybrid layers deposited on a Si substrate. For TE polarized light, the propagation loss of the waveguide as low as 0.8 dB/cm is obtained at 632.8 nm. The propagation loss for TM polarized light is 2.7 dB/cm. Some design considerations of the hybrid ARROW are also discussed in this work     

Hf-doped Ta2O5 stacks under constant voltage stress

The stress-induced leakage current in Hf-doped Ta₂O₅ layers (7; 10 nm) under constant voltage stress at gate injection was investigated in order to assess the mechanisms of conduction, the traps involved and the effect of Hf doping. The amount of Hf is found to affect the conduction mechanisms, the temperature dependence of the leakage current and the current response to the stress. A significant leakage current increase is observed only when the stress voltage and/or stress time exceed the corresponding threshold values, where the charge trapping at the pre-existing traps dominates below and defect generation above these threshold values. The energy levels of the traps responsible for the current transport are estimated. The stress effect on dominant conduction mechanisms appears quite weak, and the nature of the traps controlling the current transport before and after the stress seems to be nearly identical. The results indicate that the constant voltage stress affects the pre-existing traps in Hf-doped Ta₂O₅ and modifies their parameters, but there is no evidence for stress-induced generation of traps with completely new nature different from oxygen-vacancy related defects.     

Investigation of Ta2O5/SiO2/4H-SiC MIS capacitors

Tantalum pentoxide (Ta₂O₅) deposited by pulsed DC magnetron sputtering technique as the gate dielectric for 4H-SiC based metal-insulator-semiconductor (MIS) structure has been investigated. A rectifying current-voltage characteristic was observed, with the injection of current occurred when a positive DC bias was applied to the gate electrode with respect to the n type 4H-SiC substrate. This undesirable behavior is attributed to the relatively small band gap of Ta₂O₅ of around 4.3 eV, resulting in a small band offset between the 4H-SiC and Ta₂O₅. To overcome this problem, a thin thermal silicon oxide layer was introduced between Ta₂O₅ and 4H-SiC. This has substantially reduced the leakage current through the MIS structure. Further improvement was obtained by annealing the Ta₂O₅ at 900 °C in oxygen. The annealing has also reduced the effective charge in the dielectric film, as deduced from high frequency C-V measurements of the Ta₂O₅/SiO₂/4H-SiC capacitors.     

脉冲激光对石英基底Ta2O5/SiO2滤光膜的损伤效应研究

随着高能激光系统的发展,对光学薄膜抵抗激光损伤能力的要求越来越高,而激光脉宽是脉冲激光对薄膜损伤行为的重要影响因素。针对Ta₂O₅/SiO₂多层膜,基于1-on-1测试方法,分析其在飞秒、皮秒、纳秒激光作用下的损伤特性。测得800 nm飞秒激光作用下的损伤阈值为1.67 J/cm₂;532 nm和1 064 nm皮秒激光作用下的损伤阈值分别为1.08 J/cm²和1.98 J/cm²;532 nm和1 064 nm纳秒激光作用下的损伤阈值分别为9.39 J/cm₂和21.57 J/cm₂,并使用金相显微镜观察了滤光膜的损伤形貌。实验结果表明：飞秒激光对滤光膜的损伤机理主要是多光子电离效应,而皮秒和纳秒激光对滤光膜的损伤机制主要是热效应。滤光膜在飞秒激光作用下的损伤阈值与皮秒激光作用下的损伤阈值相当,纳秒激光作用下的损伤阈值要高一个数量级,透射通带外损伤阈值约为通带内损伤阈值的2倍。     

Fabrication and characterization of Si-MOSFET's with PECVDamorphous Ta2O5 gate insulator

Silicon MOS transistors having amorphous Ta₂O₅ insulator gates have been fabricated. The Ta₂O₅ films were deposited using a low pressure (a few mtorr) plasma-enhanced CVD process in a microwave (2.45 GHz) excited electron cyclotron resonance reactor. The source gas was TaF₅. Electrical characteristics of p-channel Al gate transistors are presented     

Hybrid polyimide/Ta2O5/polyimide antiresonantreflecting optical waveguides

A hybrid polyimide/Ta₂O₅/polyimide antiresonant reflecting optical waveguide (ARROW) is presented. The ARROW consists of fluorinated polyimide and tantalum pentoxide hybrid layers deposited on Si substrates. The propagation losses of the device are 0.6 and 2.6 dB/cm at 1.3 μm for TE and TM polarised lights, respectively     

Metal gates and gate-deposition-induced defects in Ta2O5 stack capacitors

The effect of various electrodes (Al, W, TiN) deposited by evaporation (Al) and sputtering (W, TiN) on the electrical characteristics of Ta₂O₅ stack capacitors has been investigated. The leakage currents, breakdown fields, mechanism of conductivity and dielectric constant 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. 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. The sputtered W top electrode is a good candidate as a gate of storage capacitors in DRAMs, but sputtering technique is less suitable for deposition of TiN due to the introduction of radiation defects causing deterioration of leakage current. Although some reaction between Al and Ta₂O₅ occurs, the resulting electrical properties of the capacitors are still acceptable.     

Structural and optical properties on thulium-doped LHPG-grown Ta2O5 fibres

Structural, spectroscopic and dielectric properties of thulium-doped laser-heated pedestal Ta₂O₅ as-grown fibres were studied. Undoped samples grow preferentially with a single crystalline monoclinic structure. The fibre with the lowest thulium content (0.1 at%) also shows predominantly a monoclinic phase and no intra-4f¹² Tm³⁺ recombination was observed. For sample with the highest thulium amount (1.0 at%), the appearance of a dominant triclinic phase as well as intraionic optical activation was observed. The dependence of photoluminescence on excitation energy allows identification of different site locations of Tm³⁺ ions in the lattice. The absence of recombination between the first and the ground-state multiplets as well as the temperature dependence of the observed transitions was justified by an efficient energy transfer between the Tm³⁺ ions. Microwave dielectric properties were investigated using the small perturbation theory. At a frequency of 5 GHz, the undoped material exhibits a dielectric permittivity of 21 and for thulium-doped Ta₂O₅ samples it decreases to 18 for the highest doping concentration. Nevertheless, the dielectric losses maintain a very low value.     

Thermally robust Ta2O5 capacitor for the256-Mbit DRAM

The thermal degradation of the Ta₂ O₅ capacitor during BPSG reflow has been studied. The cause of deterioration of Ta₂O₅ with the TiN top electrode was found to be the oxidation of TiN. By placing a poly-Si layer between TiN and BPSG to suppress oxidation, the low leakage current level was maintained after BPSG reflow at 850°C. The Ta₂O₅ capacitor with the TiN/poly-Si top electrode was integrated into 256-Mbit DRAM cells and excellent leakage current characteristics were obtained     

Dependence of electrical properties on interfacial layer of Ta2O5 films

The change in the thickness and chemical states of the interfacial layer and the related electrical properties in Ta₂O₅ films with different annealing temperatures were investigated. The high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy analyses revealed that the 700 °C-annealed Ta₂O₅ film remained to be amorphous and had the thinnest interfacial layer which was caused by Ta-silicate decomposition to Ta₂O₅ and SiO₂. In addition, the electrical properties were improved after annealing treatments. Our results suggest that an annealing treatment at 700 °C results in the highest capacitance and the lowest leakage current in Ta₂O₅ films due to the thinnest interfacial layer and non-crystallization.     

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.     

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.     

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 Ta₂O₅-based 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.     

Conduction mechanisms in Ta2O5 stack in response to rapid thermal annealing

The influence of the rapid thermal annealing (RTA) in vacuum at 1000 °C on the leakage current characteristics and conduction mechanisms in thermal Ta₂O₅ (7-40 nm) on Si has been studied. It was established that the effect of RTA depends on both the initial parameters of the films (defined by the oxidation temperature and film thickness) and annealing time (15-60 s). The RTA tends to change the distribution and the density of the traps in stack, and this reflects on the dielectric and leakage properties. The thinner the film and the poorer the oxidation, the more susceptible the layer to heating. The short (15 s) annealing is effective in improving the leakage characteristics of poorly oxidized samples. The RTA effect, however, is rather deleterious than beneficial, for the thinner layers with good oxygen stoichiometry. RTA modifies the conduction mechanism of Ta₂O₅ films only in the high-field region. The annealing time has strong impact on the appearance of a certain type of reactions upon annealing resulting to variation of the ratio between donors and traps into Ta₂O₅, causing different degree of compensation, and consequently to domination of one of the two mechanisms at high fields (Schottky emission or Poole-Frenkel effect). Trends associated with simultaneous action of annealing and generation of traps during RTA processing, and respectively the domination of one of them, are discussed.