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.     

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.     

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.     

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.     

Effective formation of interface controlled Y2O3 thin film on Si(1 0 0) in a metal-(ferroelectric)-insulator-semiconductor structure

Yttrium was deposited on the chemical oxide of Si and annealed under vacuum to control the interface for the formation of Y₂O₃ as an insulating barrier to construct a metal-ferroelectric-insulator-semiconductor structure. Two different pre-annealing temperatures of 600 and 700 °C were chosen to investigate the effect of the interface state formed after the pre-annealing step on the successive formation of Y₂O₃ insulator and Nd₂Ti₂O₇ (NTO) ferroelectric layer through annealing under an oxygen atmosphere at 800 °C. Pre-anneal treatments of Y-metal/chemical-SiO₂/Si at 600 and 700 °C induced a formation of Y₂O₃ and Y-silicate, respectively. The difference in the pre-anneal temperature induced almost no change in the electrical properties of the Y₂O₃/interface/Si system, but degraded properties were observed in the NTO/Y₂O₃/interface/Si system pre-annealed at 600 °C when compared with the sample pre-annealed at 700 °C. C-V characteristics of the NTO/Y₂O₃/Si structured system showed a clockwise direction of hysteresis, and this gap could be used as a memory window for a ferroelectric-gate. A smaller hysteric gap and electrical breakdown values were observed in the NTO/Y₂O₃/Si system pre-annealed at 600 °C, and this was due to an unintentional distribution of the applied field from the presence of an interfacial layer containing Y-silicate and SiO₂ phases.     

Atomic layer deposition of tantalum oxide thin films for their use as diffusion barriers in microelectronic devices

Atomic Layer Deposition (ALD) was used for the deposition of tantalum oxide thin films in order to be integrated in microelectronic devices as barrier to copper diffusion. The influence of deposition temperature, number of cycles and precursor pulse time on the film growth was discussed. The conformity of thinnest deposited films was shown. Copper diffusion through ALD Ta₂O₅ thin films, 20 nm in thickness, was investigated, for three temperatures from 600 to 800 °C, using X-ray Photoelectron Spectroscopy. The failure of such films was detected after a thermal treatment at 700 °C.     

Effects of annealing schedule on orientations of Bi3.2Nd0.8Ti3O12 ferroelectric films prepared by metalorganic solution deposition

Fatigue-free Bi_3.2Nd_0.8Ti₃O₁₂ ferroelectric thin films were successfully prepared on p-Si(1 1 1) substrate using metalorganic solution deposition process. The orientation and formation of thin film under different annealing schedules were studied using XRD and AFM. XRD analysis indicated that (2 0 0)-oriented films with degree of orientation of I₍₂₀₀₎/I₍₁₁₇₎ = 2.097 and 0.466 were obtained by preannealing the film at 400 °C for 10 min followed by rapid thermal annealing at 700 °C for 3 min, 10 min and 20 min, respectively, (0 0 8)-oriented film with degree of orientation of I₍₀₀₈₎/I₍₁₁₇₎ = 1.706 were obtained by rapid thermal annealing the film at 700 °C for 3 min without preannealing, and (0 0 8)-oriented film with degree of orientation of I₍₀₀₈₎/I₍₁₁₇₎ = 0.719 were obtained by preheating the film from room temperature to 700 °C at 20 °C/min followed by annealing for 10 min. The a-axis and c-axis orientation decreased as increase in annealing time due to effects of (1 1 1)-oriented substrate. AFM analysis further indicated that preannealing at 400 °C for 10 min followed by rapid thermal annealing at 700 °C for 3 min resulted in formation of platelike crystallite parallel to substrate surface, however rapid thermal annealing at 700 °C for 3 min without preannealing resulted in columnar crystallite perpendicular to substrate surface.     

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.     

Infrared spectroscopy and X-ray diffraction studies on the crystallographic evolution of La2O3 films upon annealing

Rare earth oxides (REOs) have lately received extensive attention in relation to the continuous scaling down of non-volatile memories (NVMs). In particular, La₂O₃ films are promising for integration into future NVMs because they are expected to crystallize above 400 °C in the hexagonal phase (h-La₂O₃) which has a higher κ value than the cubic phase (c-La₂O₃) in which most of REOs crystallize. In this work, La₂O₃ films are grown on Si by atomic layer deposition using La(C₅H₅)₃ and H₂O. Within the framework of the h-La₂O₃ formation, we systematically study the crystallographic evolution of La₂O₃ films versus annealing temperature (200-600 °C) by Fourier transform infrared spectroscopy (FTIR) and grazing incidence X-ray diffraction (GIXRD). As-grown films are chemically unstable in air since a rapid transformation into monoclinic LaO(OH) and hexagonal La(OH)₃ occurs. Vacuum annealing of sufficiently thick (>100 nm) La(OH)₃ layers induces clear changes in FTIR and GIXRD spectra: c-La₂O₃ gradually forms in the 300-500 °C range while annealing at 600 °C generates h-La₂O₃ which exhibits, as inferred from our electrical data, a desirable κ ∼ 27. A quick transformation from h-La₂O₃ into La(OH)₃ occurs due to H₂O absorption, indicating that the annealed films are chemically unstable. This study extends our recent work on the h-La₂O₃ formation.     

Electrical properties and interfacial chemical environments of in situ atomic layer deposited Al2O3 on freshly molecular beam epitaxy grown GaAs

Interfacial chemical analyses and electrical characterization of in situ atomic layer deposited (ALD) Al₂O₃ on freshly molecular beam epitaxy (MBE) grown n- and p- GaAs (001) with a (4 × 6) surface reconstruction are performed. The capacitance-voltage (C-V) characteristics of as-deposited and 550 °C N₂ annealed samples are correlated with their corresponding X-ray photoelectron spectroscopy (XPS) interfacial analyses. The chemical bonding for the as-deposited ALD-Al₂O₃/n- and p-GaAs interface is similar, consisting of Ga₂O (Ga¹⁺) and As-As bonding (As⁰) without any detectable arsenic oxides or Ga₂O₃; the interfacial chemical environments remained unchanged after 550 °C N₂ annealing for 1hr. Both as-deposited and annealed p-GaAs metal-oxide-semiconductor capacitors (MOSCAPs) exhibit C-V characteristics with small frequency dispersion (<5%). In comparison, n-GaAs MOSCAPs shows much pronounced frequency dispersion than their p-counterparts.     

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.     

Composition and electrical characteristics of Al2O3-HfO2-HfTiO nanolaminates on Si

A dielectric constant of 27 was demonstrated in the as deposited state of a 5 nm thick, seven layer nanolaminate stack comprising Al₂O₃, HfO₂ and HfTiO. It reduces to an effective dielectric constant (k_eff) of ∼14 due to a ∼0.8 nm interfacial layer. This results in a quantum mechanical effective oxide thickness (EOT) of ∼1.15 nm. After annealing at 950 °C in an oxygen atmosphere k_eff reduces to ∼10 and EOT increases to 1.91 nm. A small leakage current density of about 8 × 10⁻⁷ and 1 × 10⁻⁴ A/cm², respectively at electric field 2 and 5 MV/cm and a breakdown electric field of about 11.5 MV/cm was achieved after annealing at 950 °C.     

Effects of annealing conditions on the properties of tantalum oxide films on silicon substrates

The formation of a SiO₂ layer at the Ta₂O₅/Si interface is observed by annealing in dry O₂ or N₂ and the thickness of this layer increases with an increase in annealing temperature. Leakage current of thin (less than 40 nm thick) Ta₂O₅ films decreases as the annealing temperature increases when annealed in dry O₂ or N₂. The dielectric constant vs annealing temperature curve shows a maximum peak at 750 or 800° C resulting from the crystallization of Ta₂O₅. The effect is larger in thicker Ta₂O₅ films. But the dielectric constant decreases when annealed at higher temperature due to the formation and growth of a SiO₂ layer at the interface. The flat band voltage and gate voltage instability as a function of annealing temperature can be explained in terms of the growth of interfacial SiO₂. The electrical properties of Ta₂O₅ as a function of annealing conditions do not depend on the fabrication method of Ta₂O₅ but strongly depend on the thickness of Ta₂O₅ layer.     

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.     

Post deposition UV-induced O2 annealing of HfO2 thin films

UV-assisted annealing processes for thin oxide films is an alternative to conventional thermal annealing and has shown many advantages such as low annealing temperature, reducing annealing time and easy to control. We report in this work the deposition of ultra-thin HfO₂ films on silicon substrate by two CVD techniques, namely thermal CVD and photo-induced CVD using 222 nm excimer lamps at 400 °C. As-deposited films of around 10 nm in thickness with refractive indices from 1.72 to 1.80 were grown. The deposition rate measured by ellipsometry was found to be about 2 nm/min by UV-CVD, while the deposition rate by thermal CVD is 20% less than that by UV-CVD. XRD showed that the as-deposited HfO₂ films were amorphous. This work focuses on the effect of post deposition UV annealing in oxygen on the structural, optical and electrical properties of the HfO₂ films at low temperature (400 °C). Investigation of the interfacial layer by FTIR revealed that thickness of the interfacial SiO₂ layer slightly increases with the UV-annealing time and UV annealing can convert sub-oxides at the interface into stoichiometric SiO₂, leading to improved interfacial qualities. The permittivity ranges in 8–16, are lower than theoretical values. However, the post deposition UV O₂ annealing results in an improvement in effective breakdown field and calculated permittivity, and a reduction in leakage current density for the HfO₂ films.     

Co gamma irradiation effects on electrical characteristics of Al/Y2O3/n-Si/Al capacitors

Al/Y₂O₃/n-Si/Al capacitors were irradiated by using a ⁶⁰Co gamma ray source and a maximum dose up to 8.4 kGy. The effect of an annealing treatment performed at 600 or 900 °C on the yttrium oxide (Y₂O₃) films was investigated by XRD and Raman spectroscopy. High-frequency capacitance-voltage (C-V) and conductance-voltage (G-V) measurements as well as quasi-static measurements of the MOS structures were analysed. The annealing improves the crystalline state of the Y₂O₃ thin film material and decreases the values of the flat-band voltage and of the interface trap level density indicating an improvement of the electrical properties of the interface thin film-substrate. But at this interface, the formation of an yttrium-silicate layer was also evidenced. After gamma irradiation, the values of the flat-band voltage and of the interface trap level density related to the Al/Y₂O₃/n-Si/Al structure increase and especially for the structure made with the materials annealed at 900 °C for 1 h. In that case, the structure is very sensitive to a gamma irradiation dose up to 8.4 kGy.     

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.     

Improvement on low-temperature deposited HfO2 film and interfacial layer by high-pressure oxygen treatment

In this study, high-pressure oxygen (O₂ and O₂ + UV light) technologies were employed to effectively improve the properties of low-temperature-deposited metal oxide dielectric films and interfacial layer. In this work, 13 nm HfO₂ thin films were deposited by sputtering method at room temperature. Then, the oxygen treatments with a high-pressure of 1500 psi at 150 °C were performed to replace the conventional high temperature annealing. According to the XPS analyses, integration area of the absorption peaks of O-Hf and O-Hf-Si bonding energies apparently raise and the quantity of oxygen in deposited thin films also increases from XPS measurement. In addition, the leakage current density of standard HfO₂ film after O₂ and O₂ + UV light treatments can be improved from 3.12 × 10⁻⁶ A/cm² to 6.27 × 10⁻⁷ and 1.3 × 10⁻⁸ A/cm² at |Vg| = 3 V. The proposed low-temperature and high pressure O₂ or O₂ + UV light treatment for improving high-k dielectric films is applicable for the future flexible electronics.     

Comparison between the properties of amorphous and crystalline Ta2O5 thin films deposited on Si

In this study, the structural and electrical properties of amorphous and crystalline Ta₂O₅ thin films deposited on p-type Si by low pressure metalorganic chemical vapour deposition from a Ta(OC₂H₅)₅ source have been investigated. The as-deposited layers are amorphous, whereas crystalline Ta₂O₅ (hexagonal phase) was obtained after post-deposition O₂-annealing at 800°C. Physico-chemical analysis of our layers shows that the O₂-treatment leads to the growth of a thin (1 nm) interfacial SiO₂ layer between Ta₂O₅ and Si but, contrary to other studies, was not sufficient to reduce the level of carbon and hydrogen contaminants. Crystalline Ta₂O₅ shows better leakage current properties than amorphous Ta₂O₅. The conduction mechanism in amorphous Ta₂O₅ is clearly attributed to the Poole–Frenkel effect with a barrier height separating the traps from the conduction band of 0.8 eV. For crystalline Ta₂O₅, the situation remains unclear since no simple law can be invoked due to the presence of the SiO₂ interlayer: a double conduction process based on a tunnelling effect in SiO₂ followed by a trap-modulated mechanism in Ta₂O₅ may be invoked. From capacitance–voltage measurements, the permittivity was found to be 25 for amorphous samples, but values ranging from 56 to 59 were found for crystalline layers, suggesting a high anisotropic character.     

Fabrication and characterization of Zr-rich Zr-aluminate films for high-κ gate dielectric applications

Two kinds of Zr-rich Zr-aluminate films for high-κ gate dielectric applications with the nominal composition of (ZrO₂)_0.8(Al₂O₃)_0.2 and (ZrO₂)_0.9(Al₂O₃)_0.1, were deposited on n-type silicon wafer by pulsed laser deposition (PLD) technique at different deposition conditions. X-ray diffraction (XRD) reveals that the (ZrO₂)_0.8(Al₂O₃)_0.2 film could remain amorphous after being rapid thermal annealed (RTA) at the temperature above 800 °C, while the other one displays some crystalline peaks at 700 °C. The energy gap calculated from optical transmittance spectrum of (ZrO₂)_0.8(Al₂O₃)_0.2 film on quartz is about 6.0 eV. Sputtering depth profile of X-ray photoelectron spectroscopy and Auger electron spectroscopy indicate that a Zr-Si-O interfacial layer was formed at the near surface of the silicon substrate. The dielectric constant of the (ZrO₂)_0.8(Al₂ O₃)_0.2 film has been determined to be 22.1 by measuring a Pt/(ZrO₂)_0.8(Al₂ O₃)_0.2/Pt MIM structure. An EOT of 1.76 nm with a leakage current density of 51.5 mA/cm² at 1 V gate voltage for the film deposited in N₂ were obtained. Two different pre-treatments of Si substrates prior to depositions were also carried out and compared. The results indicate that a surface-nitrided Si substrate can lead to a lower leakage current density. The amorphous Zr-rich Zr-aluminate films fabricated by PLD have promising structure and dielectric properties required for a candidate material for high-κ gate dielectric applications.