Characterisation and passivation of interface defects in (1 0 0)-Si/SiO2/HfO2/TiN gate stacks

The density and energy distribution of electrically active interface defects in the (1 0 0)Si/SiO₂/HfO₂ system are presented. Experimental results are analysed for HfO₂ thin films deposited by atomic layer deposition and metal-organic chemical vapour deposition on (1 0 0)Si substrates. The paper discusses the origin of the interface states, and their passivation in hydrogen over the temperature range 350–550 °C.     

Influence of the manufacturing process on the electrical properties of thin (<4 nm) Hafnium based high-k stacks observed with CAFM

In this work, the dependence of the electrical characteristics of some thin (<4 nm) HfO₂, HfSiO and HfO₂/SiO₂ stacks on their manufacturing process is studied at the nanoscale. Topography, current maps and current–voltage (I–V) characteristics have been collected by conductive atomic force microscope (CAFM), which show that their conductivity depends on some manufacturing parameters. Increasing the annealing temperature, physical thickness or Hafnium content makes the structure less conductive.     

Degradation pattern of thin HfO2 films on Si(100) under ultrahigh-vacuum annealing: An investigation by x-ray photoelectron spectroscopy and low-energy ion scattering

The evolution of microstructure and phase structure of ultrathin HfO₂ films on Si(100) under ultrahigh-vacuum annealing is investigated in situ by x-ray photoelectron spectroscopy (XPS) and low-energy ion scattering (LEIS). The onset temperature of degradation is found to depend on film thickness. It is established that, for HfO₂ (4 nm)/SiO₂ (1 nm)/Si(100) specimens, 5-min annealing at about 900°C causes silicon (LEIS evidence) to appear on the surface, the silicon being uncombined with oxygen or the metal (XPS evidence). A longer annealing at the same temperature produces HfSi_x; annealing at 950°C converts the entire HfO₂ film into polycrystalline silicide whose grains are partly oriented as the Si substrate. With respect to annealing in a low-oxygen environment, the experimental results support a model whereby the degradation of an ultrathin HfO₂ film starts with the formation of nanopores by clustering of oxygen vacancies, whose density increases sharply due to partial desorption of oxygen; HfO_x with x < 2 then forms in the vicinity of vacancy clusters. It is concluded that the formation of hafnium silicide, the end product of HfO₂ degradation, starts in Si surface areas at the bottom of nanopores.     

Characterization of interfacial reaction and chemical bonding features of LaOx/HfO2 stack structure formed on thermally-grown SiO2/Si(1 0 0)

A stack structure consisting of ～1.5 nm-thick LaO_x and ～4.0 nm-thick HfO₂ was formed on thermally grown SiO₂ on Si(1 0 0) by MOCVD using dipivaloymethanato precursors, and the influence of N₂ annealing on interfacial reaction for this stack structure was examined by using X-ray photoelectron spectroscopy and Fourier transform infrared attenuated total reflection. We found that compositional mixing between LaO_x and HfO₂ becomes significant from 600 °C upwards and that interfacial reaction between HfLa_yO_z and SiO₂ proceeds consistently at 1000 °C in N₂ ambience.     

Electrical characterization of metal-oxide-high-k dielectric-oxide-semiconductor (MOHOS) structures for memory applications

Conventional SONOS (polysilicon-oxide-nitride-oxide-silicon) non-volatile memory devices use silicon nitride as the charge storage layer. In this work, metal-oxide-high-k dielectric-oxide-silicon (MOHOS) structures are fabricated using HfO₂ and Dy₂O₃ high-k dielectrics as the charge storage layer. The Al/SiO₂/Dy₂O₃/SiO₂/Si capacitors have a C–V memory window of 1.88 V and a leakage current density of 10⁻⁸ A/cm². This leakage current is lower than those of Al/SiO₂/HfO₂/SiO₂/Si capacitors and other similar capacitors reported in the literature. A minimum detection window of 0.5 V for MOHOS capacitors can be maintained up to 2 × 10⁸ s using as-deposited Dy₂O₃. The better performance of the Al/SiO₂/Dy₂O₃/SiO₂/Si structure over Al/SiO₂/HfO₂/SiO₂/Si is attributed to the larger conduction band offset at the Dy₂O₃/SiO₂ interface (2.3 eV) versus 1.6 eV at the HfO₂/SiO₂ interface.     

Electrical characterization of high-k gate dielectrics fabricated using plasma oxidation and post-deposition annealing of a Hf/SiO2/Si structure

High permittivity (high-k) gate dielectrics were fabricated using the plasma oxidation of Hf metal/SiO₂/Si followed by the post-deposition annealing (PDA), which induced a solid-phase reaction between HfO_x and SiO₂. The oxidation time and PDA temperature affected the equivalent oxide thickness (EOT) and the leakage current density of the high-k dielectric films. The interfacial structure of the high-k dielectric film/Si was transformed from HfO_x/SiO₂/Si to HfSi_xO_y/Si after the PDA, which led to a reduction in EOT to 1.15 nm due to a decrease in the thickness of SiO₂. These high-k dielectric film structures were investigated by X-ray photoelectron spectroscopy. The leakage current density of high-k dielectric film was approximately four orders of magnitude lower than that of SiO₂.     

Full-band tunneling in high-κ dielectric MOS structures

High-κ oxides such as ZrO₂ and HfO₂ have attracted great interest, due to their physical properties, suitable to replacement of SiO₂ as gate dielectric materials. In this work, we investigate the tunneling properties of ZrO₂ and HfO₂ high-κ oxides, by applying quantum mechanical methods that include the full-band structure of Si and oxide materials. Semiempirical sp³s*d tight-binding parameters have been determined to reproduce ab initio band dispersions. Transmission coefficients and tunneling current have been calculated for Si/ZrO₂/Si and Si/HfO₂/Si MOS structures, showing a very low gate leakage current in comparison to SiO₂-based structures with equivalent oxide thickness.     

Structural and interfacial properties of high-k HfOxNy gate dielectric films

The thermal stability and interfacial characteristics for hafnium oxynitride (HfO_xN_y) gate dielectrics formed on Si (1 0 0) by plasma oxidation of sputtered HfN films have been investigated. X-ray diffraction results show that the crystallization temperature of nitrogen-incorporated HfO₂ films increases compared to HfO₂ films. Analyses by X-ray photoelectron spectroscopy confirm the nitrogen incorporation in the as-deposited sample and nitrogen substitution by oxygen in the annealed species. Results of FTIR characterization indicate that the growth of the interfacial SiO₂ layer is suppressed in HfO_xN_y films compared to HfO₂ films annealed in N₂ ambient. The growth mechanism of the interfacial layer is discussed in detail.     

Evidence that N2O is a stronger oxidizing agent than O2 for both Ta2O5 and bare Si below 1000 °C and temperature for minimum low-K interfacial oxide for high-K dielectric on Si

N₂O is known to be the stronger oxidizing agent than O₂ for the post-deposition annealing of Ta₂O₅·N₂O should also be stronger than O₂ for Si oxidation. However, NO released from N₂O is also a nitridation agent which can produce silicon oxynitride at a temperature above 1000 °C and silicon oxynitride can be a diffusion barrier for oxygen. Below 1000 °C, SiO sublimation can make the comparison of N₂O oxidation and O₂ oxidation of Si difficult. Below 750 °C, N₂O is obviously the faster oxidizing agent than O₂ for bare Si. Furthermore, our results show that minimum interfacial SiO_x, which has a low dielectric constant, occurs at about 800 °C or 950 °C for high-K metallic oxide gate insulator for future generations of CMOS because rapid thermal oxidation at these two temperatures can help to reduce leakage current or charge trapping by suppressing oxygen vacancies without too much low-K interfacial SiO_x formation.     

Admittance spectroscopy of traps at the interfaces of (1 0 0)Si with Al2O3, ZrO2, and HfO2

Admittance (ac) measurements were carried out to determine the interface trap density (D_it) as a function of energy E in the Si bandgap at interfaces of Si with different insulating oxides (Al₂O₃, ZrO₂, HfO₂). The results are compared to those of the conventional thermal SiO₂/Si interface. The results show that a significant portion of the interface trap density in the as-deposited and de-hydrogenated samples is related to the amphoteric Si dangling bond defects (P_b0 -centers). The D_it is much enhanced for the Al-containing insulators as compared to Si/SiO₂ but can be reduced by annealing in O₂. As to annealing in H₂, efficient passivation of P_b0 centers by hydrogen is achieved for Si/ZrO₂ and Si/HfO₂ interfaces, yet it fails for Si/Al-containing oxide entities. Among the insulators studied, the results suggest HfO₂ to be the best choice of an alternative insulator.     

Organic thin film transistors with HfO2 high-k gate dielectric grown by anodic oxidation or deposited by sol–gel

We report here on pentacene based organic field effect transistors (OFETs) with a high-k HfO₂ gate oxide. HfO₂ layers were prepared by two different methods: anodic oxidation and sol–gel. A comparison of the two processes on the electrical properties of OFETs is given. Ultra thin nanoporous (20 nm) sol–gel deposited oxide films were obtained following an annealing at 450 °C. They lead to high mobility and stable devices (μ = 0.12 cm²/V s). On the other hand, devices with anodic HfO₂ revealed a little bit more leaky and show some hysteresis. Anodization, however, presents the advantage of being a fully room temperature process, compatible with plastic substrates. Stability and response to a bias stress are also reported.     

Nanoscale electrical characterization of ultrathin high-k dielectric MOS stacks: A conducting AFM study

Extreme scaling in both silicon and alternative channel CMOS has highlighted the importance of localized characterization on the nanometer scale. We have used a conductive-contact atomic force microscopy (C-AFM) technique in ultra high vacuum (UHV) conditions to analyze and compare intrinsic stack degradation mechanisms leading to breakdown (BD) for ultrathin high-k dielectric films of (4 nm) Hf_xSiO_y/SiO₂ on Si and (2 nm) ZrO₂/GeO₂ on Ge. Simultaneous nanoscale current–voltage I–V characteristics, topography, tunneling current and relative tip–surface contact interactions as normal and lateral force maps revealed localized injected charge dependence on electrical stress. It is shown that the charge can propagate laterally. Successive voltage scanning is related to the overall post-BD conductivity for pre- to post-BD degradation propagation. In contrast with SiO₂ interface, an increased GeO₂ interlayer reactivity yielding more active interface defects is suggested.     

Intrinsic bonding defects in transition metal elemental oxides

Gate dielectrics comprised of nanocrystalline HfO₂ in gate stacks with thin SiO₂/SiON interfacial transition regions display significant asymmetries with respect to trapping of Si substrate injected holes and electrons. Based on spectroscopic studies, and guided by ab initio theory, electron and hole traps in HfO₂ and other transition metal elemental oxides are assigned to O-atom divacancies, clustered at internal grain boundaries. Three engineering solutions for defect reduction are identified: i) deposition of ultra-thin, <2 nm, HfO₂ dielectric layers, in which grain boundary formation is suppressed by effectively eliminating inter-primitive unit cell π-bonding interactions, ii) chemically phase separated high HfO₂ silicates in which inter-primitive unit cell p-bonding interactions are suppressed by the two nanocrystalline grain size limitations resulting from SiO₂ inclusions, and iii) non-crystalline Zr/Hf Si oxynitrides without grain boundary defects.     

Thermal stability of high-k Er-silicate gate dielectric formed by interfacial reaction between Er and SiO2 films

We fabricated a high-k Er-silicate gate dielectric using interfacial reaction between Er and SiO₂ films and investigated its thermal stability. The reduced capacitance with increasing annealing temperature is associated with the chemical bonding change of Er-silicate from Er-rich to Si-rich, induced by a reaction between Er-silicate and Si during thermal treatment. Further an increase in the annealing temperature (>500 °C) causes the formation of Si dangling bonds, which is responsible for an increased interface trap density.     

Electrical and structural properties of hafnium silicate thin films

Thin (4 nm) hafnium silicate (HfO₂)_x(SiO₂)_1−x/SiO₂ gate stacks (0 < x < 1) grown by metal organic chemical vapour deposition (MOCVD) are investigated in this study. The focus is on extracting the optical constants, and hence bandgaps as well as dielectric constants. The VUV (vacuum ultraviolet) spectroscopic ellipsometry (VUV-SE) technique in the spectral range 140–1700 nm, together with current–voltage and capacitance–voltage techniques were used for studying the optical and electrical properties of the layers, respectively. The bandgap was found to increase from 5.24 eV for HfO₂ to 6 eV for Hf-silicate with 30% Hf. The permittivity was reduced from 21 for HfO₂ layers to 8 for Hf-silicate with x = 0.3. The results suggest that the optimal Hf content is above 0.6, for which the permittivity higher than 10 can be achieved.     

The effect of oxygen in Ru gate electrode on effective work function of Ru/HfO2 stack structure

Effective work function (φ_m,eff) values of Ru gate electrode on SiO₂ and HfO₂ MOS capacitors were carefully examined and discussed from the viewpoint of an effect of oxygen incorporation in Ru gate electrode on φ_m,eff. Annealing at 400 °C in the reduction (3%H₂) and the oxidation (1%O₂) ambient resulted in similar changes in the φ_m,eff of Ru/HfO₂/SiO₂ and Ru/SiO₂ MOS capacitors. Furthermore, the Ru gate MOS capacitor after annealing in the oxidation condition have shown almost the same φ_m,eff value to that of RuO₂ gate MOS capacitors. The oxygen concentration in the Ru/HfO₂ interface after annealing in oxidizing atmosphere is approximately one order of magnitude higher than that after annealing in reducing atmosphere as confirmed by secondary ion mass spectroscopy analysis. Furthermore, the higher oxygen concentration at the Ru/dielectric interface leads to the higher φ_m,eff value, regardless of SiO₂ or HfO₂ dielectrics. This indicates that φ_m,eff of Ru gate MOS capacitor is dominantly determined by the oxygen concentration at the Ru/dielectric layer interface rather than the dipoles originated from the oxygen vacancy in HfO₂.     

Fabrication and electrical characteristics of ultrathin (HfO2)x(SiO2)1−x films by surface sol-gel method and reaction-anneal treatment

Hafnium oxide (HfO₂) films were deposited on Si substrates with a pre-grown oxide layer using hafnium chloride (HfCl₄) source by surface sol-gel process, then ultrathin (HfO₂)_x(SiO₂)_1−x films were fabricated due to the reaction of SiO₂ layer with HfO₂ under the appropriate reaction-anneal treatment. The observation of high-resolution transmission electron microscopy indicates that the ultrathin films show amorphous nature. X-ray photoelectron spectroscopy analyses reveal that surface sol-gel derived ultrathin films are Hf-Si-O alloy instead of HfO₂ and pre-grown SiO₂ layer, and the composition was Hf_0.52Si_0.48O₂ under 500 °C reaction-anneal. The lowest equivalent oxide thickness (EOT) value of 0.9 nm of film annealed at 500 °C has been obtained with small flatband voltage of −0.31 V. The experimental results indicate that a simple and feasible solution route to fabricate (HfO₂)_x(SiO₂)_1−x composite films has been developed by means of combination of surface sol-gel and reaction-anneal treatment.     

Suppression of Ge–O and Ge–N bonding at Ge–HfO2 and Ge–TiO2 interfaces by deposition onto plasma-nitrided passivated Ge substrates: Integration issues Ge gate stacks into advanced devices

A study of changes in nano-scale morphology of thin films of nano-crystalline transition metal (TM) elemental oxides, HfO₂ and TiO₂, on plasma-nitrided Ge(1 0 0) substrates, and Si(1 0 0) substrates with ultra-thin (0.8 nm) plasma-nitrided Si suboxide, SiO_x, x < 2, or SiON interfacial layers is presented. Near edge X-ray absorption spectroscopy (NEXAS) has been used to determine nano-scale morphology of these films by Jahn-Teller distortion removal of band edge d-state degeneracies. These results identify a new and novel application for NEXAS based on the resonant character of the respective O K₁ and N K₁ edge absorptions. This paper also includes a brief discussion of the integration issues for the introduction of this Ge breakthrough into advanced semiconductor circuits and systems. This includes a comparison of nano-crystalline and non-crystalline dielectrics, as well as issues relative to metal gates.     

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.     

Electrical reliability aspects of HfO2 high-k gate dielectrics with TaN metal gate electrodes under constant voltage stress

The dielectric breakdown property of ultrathin 2.5 and 5.0 nm hafnium oxide (HfO₂) gate dielectric layers with metal nitride (TaN) gate electrodes for metal oxide semiconductor (MOS) structure has been investigated. Reliability studies were performed with constant voltage stressing to verify the processing condition effects (film thicknesses and post metal annealing temperatures) on times to breakdown. The leakage current characteristics are improved with post metal annealing temperatures (PMA) for both 2.5 and 5.0 nm HfO₂ physical thicknesses. However, it is more prominent (2 orders of magnitudes) for 2.5 nm HfO₂ film thickness. The values of oxide-trapped charge density and interface-state density are also improved for 2.5 nm HfO₂ film. The different stages of charge-trapping behaviors, i.e., stress-induced leakage current, soft and hard breakdown mechanisms have been detected. During constant voltage stress of the MOS capacitors, an increase in the time-dependent gate current is observed, followed by the occurrence of several fluctuations. The amplitude of the fluctuations is much larger in the 5.0 nm HfO₂ gate dielectric layer compared to the 2.5 nm HfO₂ layer. After the occurrence of such fluctuations, the current–voltage characteristics exhibited an increased in gate current compared to the fresh (unstressed) devices.