Nathan P. Sandler

Properties of amorphous and crystalline Ta2O5 thin films deposited on Si from a Ta(OC2H5)5 precursor

In this work, the structural and electrical properties of amorphous and crystalline Ta2O5 thin films deposited on p-type Si substrates by low-pressure chemical vapor deposition from a Ta(OC2H5)5 precursor have been investigated. The as-deposited layers are amorphous, whereas crystalline Ta2O5 was obtained after postdeposition O2 treatment at 800 °C. As evidenced by x-ray diffraction, a hexagonal structure was obtained in the latter case. Physicochemical analysis of our layers shows that the O2-annealing step leads to the growth of a thin (∼1 nm) interfacial SiO2 layer but was not sufficient to reduce the level of hydrocarbon contamination. The dominant conduction mechanism in amorphous Ta2O5 is clearly due to the Poole–Frenkel effect, whereas the situation remains unclear for crystalline Ta2O5 for which no simple law can be invoked to correctly describe its conduction properties. From capacitance–voltage measurements, the dielectric constant was found to be ∼25 for amorphous samples, but values ranging fr...

Detection of oxygen vacancy defect states in capacitors with ultrathin Ta2O5 films by zero-bias thermally stimulated current spectroscopy

Defect state D (0.8 eV) was experimentally detected in Ta2O5 capacitors with ultrathin (physical thickness <10 nm) Ta2O5 films using zero-bias thermally stimulated current spectroscopy and correlated with leakage current. Defect state D can be more efficiently suppressed by using N2O rapid thermal annealing (RTA) instead of using O2 RTA for postdeposition annealing and by using TiN instead of Al for top electrode. We believe that defect D is probably the first ionization level of the oxygen vacancy deep double donor. Other important defects are Si/O-vacancy complex single donors and C/O-vacancy complex single donors.

Evidence that N2O is a Stronger Oxidizing Agent than O2 for the Post-Deposition Annealing of Ta2O5 on Si Capacitors

Cross-sectional transmission electron microscopy (XTEM), secondary ion mass spectrometry (SIMS) and capacitance measurements were used to study the effect of post-deposition annealing on Ta2O5/Si structures. A significantly thicker SiOx interfacial layer was formed at the Ta2O5/Si interface, if N2O was used instead of O2 for post-deposition annealing. This indicates that N2O is a stronger oxidizing agent than O2. It is known that the leakage current of Ta2O5 capacitors is greatly reduced if N2O is used instead of O2 for post-deposition annealing. This may also be partially explained by postulating that N2O annealing is more effective in the suppression of oxygen vacancies. Furthermore, the suppression of Si diffusion from the Si substrate into Ta2O5 due to the thicker SiOx interfacial layer can be another factor. The basic reason for the superiority of N2O is that the energy required to produce free O atoms is lower than that for O2. From this point of view, we can also predict that the use of NO will be worse than that of O2 because the energy required to produce free O atoms is higher than that of O2.

Detection of defect states responsible for leakage current in ultrathin tantalum pentoxide (Ta2O5) films by zero-bias thermally stimulated current spectroscopy

Defect states responsible for leakage current in ultrathin (physical thickness <10 nm) tantalum pentoxide (Ta2O5) films were measured with a novel zero-bias thermally stimulated current technique. It was found that defect states A, whose activation energy was estimated to be about 0.2 eV, can be more efficiently suppressed by using N2O rapid thermal annealing (RTA) instead of using O2 RTA for postdeposition annealing. The leakage current was also smaller for samples with N2O RTA than those with O2 RTA for postdeposition annealing. Hence, defect states A are quite likely to be important in causing leakage current.

Mechanism of leakage current reduction of tantalum oxide capacitors by titanium doping

In this letter, the authors will point out that defect states related to oxygen vacancies in tantalum oxide capacitors can be suppressed by titanium doping, resulting in significant leakage current reduction. The theory is that titanium forms an acceptor which can move at high temperature and neutralize other donors. However, defect states which cannot be suppressed by titanium doping were detected. These are explained by H2O-related contamination occurring at low temperature (<400°C) during the cooling down period.

Characterization of Defect States Responsible for Leakage Current in Tantalum Pentoxide Films for Very-High-Density Dynamic Random Access Memory (DRAM) Applications

Defect states in tantalum pentoxide films grown by low-pressure metal-organic chemical vapor deposition on silicon wafers have been studied with Al/Ta 2 O 5 /p + -Si and Al/Ta 2 O 5 /n + -Si capacitor structures by the zero-bias thermally stimulated current technique. It was demonstrated that a shallow band of defect states is responsible for leakage current. The shallow band of defect states can be suppressed by low-temperature post metallization annealing, resulting in a reduction of leakage current for both positive gate bias and negative gate bias. However, the reduction in leakage current for positive gate bias is much stronger than that for negative gate bias

Defect states responsible for leakage current in Ta2O5 films on Si due to Si contamination from the substrate

A defect state, defect B, was found in Ta2O5 after postdeposition annealing in O2 by a novel zero‐bias thermally stimulated current spectroscopy technique. The activation energy ET was estimated to be about 0.3 eV. Evidence is given that defect B is a hole trap. We believe that defect B is an acceptor level in Ta2O5 due to Si substituting for Ta. The presence of Si contamination in Ta2O5 due to diffusion of Si from the Si substrate into Ta2O5 was confirmed by secondary‐ion‐mass spectrometry.

A Comparison of Defect States in Tantalum Pentoxide (Ta2O5) Films after Rapid Thermal Annealing in O2 or N2O by Zero-Bias Thermally Stimulated Current Spectroscopy

The concentration of shallow defect states in Ta2O5 films was found to be greatly reduced, resulting in much less leakage current in Al/Ta2O5/Si capacitors, if N2O was used instead of O2 for post-deposition annealing. The superiority of N2O is explained by the formation of a slightly thicker SiOx diffusion barrier, which can reduce Si contamination coming from the Si substrate into the Ta2O5 film.

The Superiority of N 2O Plasma Annealing over O 2 Plasma Annealing for Amorphous Tantalum Pentoxide (Ta 2O 5) Films

As-deposited tantalum pentoxide (Ta2O5) films are amorphous. The films will remain amorphous after low-temperature O2 or N2O plasma annealing. High-temperature annealing will produce polycrystalline films where grain boundaries can generate leakage current. It was found that N2O plasma annealing is superior to O2 plasma annealing in terms of leakage current reduction. This can be easily explained by the lower energy required to break the nitrogen-oxygen bond in a N2O molecule compared to the energy required to break the O=O bond in an O2 molecule. We also observed that there is less Si contamination, which may lead to leakage current, in the sample with N2O plasma annealing compared to the sample with O2 plasma annealing.

Mechanism of leakage current reduction of tantalum oxide capacitors by postmetallization annealing

In this letter, the authors will point out that defect states related to impurities or structural defects in tantalum oxide capacitors can be passivated by hydrogen during postmetallization anneal (PMA) while oxygen vacancies are enhanced by PMA such that some will observe a decrease while other may observe an increase in the leakage current after PMA. The PMA process can be tuned such that the hydrogen passivation of defect states dominates over the enhancement of oxygen vacancies, resulting in significant leakage current reduction.