Hf Silicate Research Articles

Characteristics of the HfO2 Thin Films Grown by Remote Plasma Atomic Layer Deposition Method on the Plasma Oxidized Si Substrate

We investigated the electrical and physical characteristics of HfO2/SiO2/Si structure grown by remote plasma atomic layer deposition (RPALD) method. The surface preparation of Si (100) substrate was performed on HF-cleaned Si substrate by remote plasma oxidation (RPO) process and yielded about 0.5-nm-thick SiO2 layer. HfO2 films were deposited on both HF-cleaned and RPO-treated Si substrates by the RPALD method, respectively. Results show that the RPO-treated substrate effectively suppressed the diffusion of oxygen or/and silicon into the film during film deposition. Therefore, the RPO-treated samples have thinner initial Hf silicate layers than those without RPO treated substrate. And we have also observed a decreasing of the hysteresis and effective charge density (Qeff) as well as an improvement of the equivalent oxide thickness (EOT) in RPO treated samples.

Annealing effects on the properties of HfO 2 films grown by metalorganic molecular beam epitaxy

We present the annealing effects on the properties of HfO 2 dielectrics grown by metalorganic molecular beam epitaxy. From the comparison of the line shapes of core-level spectra for the N 2-annealed samples with different annealing temperatures, the concentrations of SiO and Hf–silicate with high dielectric constant are found to be highest for HfO 2 film annealed at 700 °C. Therefore, this result supports that the accumulation capacitance of the sample annealed at 700 °C in N 2 ambient is not deteriorated in spite of the steep increase of interfacial layer thickness. For the O 2-annealed samples, SiO 2 with low dielectric constant is widely spread throughout the samples as the annealing temperature increases. Therefore, the accumulation capacitance density is degraded due to the increase in the thickness of the interfacial layer which mainly consists of SiO 2.

Mechanism of Hf-silicide formation at interface between poly-Si electrode and HfO2∕Si gate stacks studied by photoemission and x-ray absorption spectroscopy

We have investigated the mechanism for silicidation by chemical reactions at polycrystalline-Si (poly-Si)∕HfO2∕Si gate stacks by annealing in ultrahigh vacuum using photoemission spectroscopy and x-ray absorption spectroscopy. Si 2p, Hf 4f, and O 1s high-resolution photoemission spectra have revealed that a Hf-silicide formation starts at as low temperature as 700°C and that a Hf silicate is also formed at the interface between poly-Si electrodes and HfO2. The metallic Hf silicide is formed at the interface between HfO2 and Si substrates, which changes the band offsets on Si substrates. We have found that poly-Si electrodes promote the interfacial reaction between HfO2 and Si substrates, while the crystallization in a HfO2 layer is independent of the silicide formation. The silicidation mechanism based on photoemission spectra is also confirmed from the thermodynamical analysis considering the Gibbs’ free energy.

Role of oxygen vacancy in HfO2∕SiO2∕Si(100) interfaces

We have investigated the interface states in HfO2∕SiO2∕Si(100) systems that were prepared by using the in situ pulsed laser deposition technique. X-ray photoelectron spectroscopy data revealed that when the HfO2 film thickness exceeds 11Å, the film composition undergoes a systematic change from Hf silicate to oxygen-deficient HfOx<2. Furthermore, we determined that the evolution of the interface states clearly depends on the oxygen condition applied during the film growth and that the oxygen vacancy is an important parameter for Hf silicate formation.

Characterization of mixed-signal properties of MOSFETs with high-k (SiON/HfSiON/TaN) gate stacks

The correlation between the stoichiometry of HfSiON gate dielectrics and mixed-signal properties of low-power MOSFETs is investigated. MOSFETs with gate length L down to 100 nm were fabricated in a conventional fabrication flow with a thermal budget of 1000/spl deg/C. The equivalent oxide thickness values of the gate stacks ranged from 12.0 to 14.4 /spl Aring/. The inversion thickness t/sub ox//sup inv/ ranged between 16.2 and 18.9 /spl Aring/. Accumulation leakage current density at V/sub g/=V/sub fb/-1 V ranged from 3.1/spl times/10/sup -3/ to 3.9/spl times/10/sup -2/ A/spl middot/cm/sup -2/, corresponding to a leakage reduction of around 1000 times compared with that of SiON/TaN. The inversion leakage current density at V/sub g/=V/sub t/+0.7 V was found to be between 0.18 and 0.66 A/spl middot/cm/sup -2/, i.e., 20 times lower than that of SiON/TaN. The threshold voltage instability (/spl Delta/V/sub t/) of the high-k gate stacks was found to be below 10 mV, corresponding to a (bulk) oxide trap density N/sub ot/<3/spl times/10/sup 10/ cm/sup -2/. Normalized input-referred gate noise spectra S/sub vg/ showed minor dependence on the Hf content of the gate dielectric. This is attributed to the fact that the main sources for the low-frequency (LF) 1/f noise are defects located at the interfaces rather than bulk defects in the gate dielectric. Moderate high- and low-field mobilities of 50%-70% of a SiON/TaN reference device were found in MOSFETs with Hf-based gate dielectric. Of the investigated layers, stoichiometric (55% Hf) and Hf-rich (70% Hf) silicates on SiO/sub 2/ interface show the best compromise in terms of gate leakage reduction, threshold voltage instability, LF noise, drive current, and analog voltage gain. For an HfSiON silicate dielectric layer with 70% Hf and SiO/sub 2/ interface, a low-field peak electron mobility /spl mu//sub eff/=148 cm/sup 2//V/spl middot/s, an oxide trap density N/sub ot/<1.4/spl times/10/sup 10/ cm/sup -2/, and a drive current I/sub ON/=459 /spl mu/A//spl mu/m at I/sub OFF/=9 pA//spl mu/m were found. The analog voltage gain A/sub v/ of a MOSFET with this gate dielectric and W/L=1/0.45 was found to be A/sub v/=121. To meet specifications for mixed-signal properties, optimization of dopant engineering along with tuning of the gate stack properties is required to improve the performance of high-k-based MOSFETs.

Investigation of Wet Etching Properties and Annealing Effects of Hf-Based High-k Materials

Wet etching properties of Hf based high- dielectrics of , , Hf oxynitride and Hf silicate were investigated, using various chemicals employed for the current CMOS device fabrication. Experimental results show that fluorides species such as , and are very effective for dissolving Hf and in acids. is highly soluble in with a solubility of , which is much higher than the calculated result, and its solubility increases with reduced pH value. Etching properties of those high- dielectrics in and annealing effects on their microstructure were explored. It is observed that the etch rates of Hf and increase with increasing concentration and after etch, Hf and surfaces become F terminated. Although the mechanisms responsible for the change in etch rates after the Hf based high- films have undergone annealing are dependent on their chemical property, it is universally observed that the formation of the crystalline phase of is the primary attributor to the reduced etch rates. The etching of Hf based high- films occurs via weak points in the films, e.g., , and bonds, as well as amorphous areas of .

Effects of nitrogen atom doping on dielectric constants of Hf-based gate oxides

We have theoretically shown that doping of N atoms increases the dielectric constant of the Hf silicates, enhancing both the electronic and lattice polarization contributions. The enhancement of the lattice contribution is dominant and is attributed to low-frequency vibration modes induced by the N doping. It is found that Hf and Si ions bonded to N atoms located at O vacancies largely vibrate in the modes and play a crucial role in the enhancement. Doped N atoms are shown to also improve the electric characteristic of the silicates, elevating O vacancy levels appearing in the band gap of the silicates.

Comparision of MOSFET Characteristics Between ALD and MOCVD TiN Metal Gate on Hf Silicate

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Correlation of Phase Segregation and Electrical Properties of Low-Power MOSFETs with Hf-based Silicate Gate Dielectric Layers and TaN Metal Gates

AbstractThe need for nitridation of Hf silicate is controversial. On one hand, it has not been proven that the nitridation is mandatory to have working devices and on the other hand, it is known to increase the charge density. In this paper, we present a detailed comprehensive study of the role and the need for nitridation of Hf-based silicates deposited by Atomic Layer Deposition (ALD). The results are based on a correlation of Fourier-Transformed Infrared Spectroscopy (FT-IR), X-ray Photoelectron Spectroscopy (XPS), High-resolution Transmission Electron Microscopy (HR-TEM) and electrical measurements (gate leakage and mobility).It was observed that the phase segregation in gate dielectrics is not detrimental for the gate leakage density at room temperature. However, the leakage current is significantly increased at higher temperature. The incorporation of nitrogen was either done by NH3 anneal (at 800C) or by Decoupled Plasma Nitridation (DPN – 25.9kJ). While the DPN or NH3 anneal prevent phase segregation for 50% Hf silicate, only the NH3 anneal helps prevent the phase segregation of Hf-rich silicate. Furthermore, the NH3 anneal increases the interfacial thickness, which produces a very low gate leakage with only 10% loss in mobility at high field. Interestingly, DPN followed by O2 anneal leads to an advantageous phase segregation of the Hf-rich silicate by transforming the silicate in a HfO2/SiO2-like stack.As a conclusion, not only the phase segregation of the silicate does not always lead to shorted devices, but it can be beneficial in terms of mobility. However, the phase segregation seems to be responsible for an enlarged trap-assisted conduction mechanism at high temperature. But even if the 50% Hf silicates non-nitrided leads to working devices, the incorporation of nitrogen in the stack improves the Jg/CET trends and is therefore beneficial.

Effects of Remote Plasma Pre-oxidation of Si Substrates on the Characteristics of ALD-Deposited HfO[sub 2] Gate Dielectrics

We investigated the physical and electrical characteristics of a HfO 2 /ultrathin SiO 2 (∼0.5 nm)/Si structure grown by a remote plasma atomic layer deposition (RPALD). The HF-cleaned Si substrate was oxidized by a remote plasma oxidation (RPO) process and yielded a ∼0.5 nm thick SiO 2 layer. HfO 2 films were deposited on both H-terminated and RPO-treated Si substrates by the RPALD. During HfO 2 film deposition, the RPO-treated sample showed more effectively retarded formation of initial Hf silicate layers than the sample on H-terminated Si. RPO treatment also improves electrical properties including hysteresis, effective fixed oxide charge density (Q f,eff ) and equivalent oxide thickness (EOT).

An asymmetry of conduction mechanisms and charge trapping in thin high-k HfxTiySizO films

The electrical behavior of high-permittivity (high-k) hafnium titanium silicate (HfxTiySizO) layers with different Hf:Ti ratios in the films is investigated. The films were deposited by metal-organic chemical-vapor deposition using a mixture of two single-source precursors for the deposition of Hf silicate and Ti silicate. The attention is focused on the gate bias asymmetry of leakage currents, conduction mechanisms, and trapping behavior dependent on film composition. The pure Hf silicate, the Ti silicate, and the sample with the lowest Hf content reveal a gate bias symmetry of the conduction process, implying the formation of a single-layer structure, whereas samples containing both Hf and Ti in significant amounts exhibit an asymmetrical behavior to the gate polarity. A continuous change from Poole-Frenkel conduction in pure Hf silicates to phonon-assisted tunneling in pure Ti silicates is established. It is found that a strong negative charge trapping occurs in the samples containing significant amounts of both Hf and Ti. With an increase of Ti content in the films, the density of the trapped charge increases and its centroid moves farther away from the dielectric/Si interface. Two possible processes are considered to explain the observed asymmetry of the conduction process—negative charge trapping and formation of a double-layer structure. It is suggested that both phenomena occur and are consequences of one and the same structural process, phase separation, i.e. the formation of TiO2, HfO2, and SiO2 islands in the film. The degree of phase separation depends strongly on the film composition and defines the process, which has the dominating role in the gate bias asymmetry of the conduction mechanism.

Annealing Effects on the Chemical Configuration of Uncapped and (Poly-Si)-Capped HfOxNy Films Deposited on Si(001)

We use spatially resolved spectroscopy in a scanning transmission electron microscope to study the thermal stability of the system with and without an in situ capping layer of silicon. The films were deposited by metallorganic chemical vapor deposition using the amide precursor tetrakis(diethylamido)hafnium with NO as the oxidant. A interfacial layer is observed at the interface for films directly exposed to air. In addition, the N loss in the film for the uncapped sample is significant. In contrast, in situ capping is found to reduce the thickness of the interfacial layer and to keep the N content in the final dielectric film high. The capped films are quasi-amorphous, whereas uncapped films are polycrystalline following exposure to air. Oxinitridation at both interfaces is observed following a rapid thermal annealing process ( in for ) of the capped film. However, the interfacial layers remain thin and a significant amount of N is present in the film. The rapid thermal annealing leads to the partial crystallization of the film and the Si capping layer. No Hf silicate is detected on a scale of in the electron energy loss spectroscopy analysis.

Conduction band-edge d-states in high-k dielectrics due to Jahn–Teller term splittings

X-ray absorption spectroscopy (XAS) is used to study conduction band edge electronic structure of high-k transition metal (TM) and trivalent lanthanide series rare earth (RE) oxide dielectrics. Empty TM/RE d-states are studied by intra-atomic transitions originating in core level spin-orbit split p-states, and conduction band states are studied in inter-atomic transitions which originate in the oxygen atom 1s core level state. In non-crystalline Zr and Hf silicate alloys, the local bonding symmetry, or crystal field splits these d-states into doubly and triply degenerate features. In nano-crystalline oxides, there are additional d-state splittings due to contributions of more distant neighbors that completely remove d-state degeneracies via the Jahn–Teller effect mechanism. This gives rise to highly localized band edge states that are electronically active in photoconductivity, internal photoemission, and act as bulk traps in metal oxide semiconductor (MOS) devices.

Depth profiling the electronic structures at HfO2∕Si interface grown by molecular beam epitaxy

The depth profile of the HfO2∕Si interface grown by molecular beam epitaxy (MBE) has been investigated by high-resolution x-ray photoelectron spectroscopy using synchrotron radiation beam and low energy Ar+ sputtering. At the interfacial layer, the Hf 4f spectra show a Hf silicide state without a Hf silicate signal. The O 1s and Si 2p spectra show weakly two silicon oxidation states of SiO and SiO2. The data suggest an inward reaction of the Hf and O species into the Si substrate during the MBE growth. The valence band offset (ΔEV) of ∼3.5eV is obtained for the HfO2∕Si interface by measuring the valence-band edges of HfO2 and Si.

Surface treatment for the atomic layer deposition of HfO2 on silicon

The atomic layer deposition (ALD) of HfO2 on silicon with a Cl2 surface treatment is investigated by physicochemical and electrical techniques. The specificity of this treatment is to create, on a HF-dipped silicon surface, the nucleation sites necessary for the ALD growth. The growth rates obtained by spectroscopic ellipsometry and total x-ray fluorescence spectroscopy indicate that the nucleation sites (i.e., the –OH groups), which are necessary to perform some bidimensional ALD growth, are generated during this surface treatment. After deposition of thin HfO2 layers (from a few monolayers up to 8.7nm), a very thin parasitic SiOx layer, underneath 1 monolayer of Hf silicate, is observed by x-ray photoelectron spectroscopy. Nevertheless, an equivalent oxide thickness of 1.1nm is obtained with an as-deposited 3.7nm thick HfO2 layer.

Dopant penetration studies through Hf silicate

We present a study of the penetration of B, P, and As through Hf silicate (HfSixOy) and the effect of N incorporation in Hf silicate (HfSixOyNz) on dopant penetration from doped polycrystalline silicon capping layers. The extent of penetration through Hf silicate was found to be dependent upon the thermal annealing budget for each dopant investigated as follows: B(T⩾950°C∕60s), P(T⩾1000°C∕20s), and As (T⩾1050°C∕60s). We propose that the enhanced diffusion observed for these dopants in HfSixOy, compared with that of SiO2 films, is related to grain boundary formation resulting from HfSixOy film crystallization. We also find that, as in the case of SiO2, N incorporation inhibits dopant (B, P, and As) diffusion through the Hf silicate and thus penetration into the underlying Si substrate. Only B penetration is clearly observed through HfSiON films for anneals at 1050 °C for durations of 10 s or longer. The calculated B diffusivity through the HfSixOyNz layer is D0=5.2×10−15cm2∕s.

Atomic Layer Chemical Vapor Deposition (ALCVD) of Hf and Zr Silicate and Aluminate High-k Gate Dielectric for Next Generation Nano Devices

Among many deposition techniques, atomic layer deposition (ALD) is well suited for highly conformal and uniform ultrathin films with accurate thickness control over a large area. A number of ALD deposition processes have been used to grow mixed oxides like Hf or Zr silicate and aluminate thin films for high-k gate dielectrics needed in the next generation silicon transistors. Nano laminate mixing of each oxide by switching between two metal precursors with an oxidant like water can be used and film composition can be controlled by adjusting the number of each single oxide cycles. An alternating injection of metal alkoxides and chlorides without the injection of extra oxidants is another method. Single metal source, which contains both metal and Si (or Al), also can be adopted to form silicate or aluminate films. These processes will be overviewed in this article with our recent work on atomic layer deposition (ALD) of high-k gate dielectrics. It is important to select a proper set of precursors and each process should be evaluated and compared in terms of the reliability and film properties. In-situ diagnostic technique such as FT-IR is also helpful for the process development.

Influence of thickness of Hf buffer layer on the interfacial structures of sputtered HfO 2 on SiO 2/Si

The influence of thickness of Hf–metal buffer layer on the interfacial diffusion and reaction was investigated using in situ X-ray photoelectron spectroscopy, scanning Auger microscope and grazing incident X-ray reflectivity. Hf–metal firstly reacted with native Si oxide forming Hf silicates, and all Si–O was further reduced to be Si 0 after 1 nm Hf–metal deposition. The Hf–metal and Hf-suboxide in Hf(1 nm)/SiO 2/Si structure were further oxidized to be Hf 4+ during HfO 2 sputtering deposition and post-deposition annealing. Si diffused out and reacted with HfO 2 during annealing. The Hf(1 nm) buffer layer exhibited a better performance than the Hf(0.3 nm) buffer layer in suppressing the diffusion of Si and the reaction between diffused Si and HfO 2.

Thermal stability of the HfO2∕SiO2 interface for sub-0.1μm complementary metal-oxide-semiconductor gate oxide stacks: A valence band and quantitative core-level study by soft x-ray photoelectron spectroscopy

Synchrotron-radiation photoelectron spectroscopy is used to study the valence-band structure and the core-level photoemission spectra of HfO2 ultrathin films grown onto SiO2∕Si substrates by atomic layer deposition (ALD). We determine the band offsets (valence and conduction) of HfO2 to Si as a function of postdeposition annealing treatments (under an inert N2 atmosphere or in situ in ultrahigh vacuum) and find a significant evolution, the conduction-band offset remaining larger than 1.5eV. The Si2p and the Hf4f core-level spectra give detailed information on the composition and the spatial extent of the interfacial Hf silicate layer formed between the SiO2 bottom oxide and the HfO2 ALD thin film. By a quantitative treatment of the Si2p core-level intensities, we examine the thermal stability of the interface silicate after postdeposition annealing under N2 and in situ annealing in ultrahigh vacuum (UHV), both at 800°C. The as-deposited layer gives rise to a HfO2∕Hf0.35Si0.65O2∕SiO2 stack with corresponding thicknesses of 0.74∕0.51∕0.73nm. After postdeposition annealing at 800°C in a N2 atmosphere, this becomes a HfO2∕Hf0.31Si0.69O2∕SiO2 stack with corresponding thicknesses of 0.71:0.58:0.91nm. In situ annealing in UHV, on the other hand, gives a HfO2∕Hf0.35Si0.65O2∕SiO2 stack with corresponding thicknesses of 0.65:0.70:0.76nm. The former favors an extension of both the silicate and the SiO2 interface layers, whereas the latter develops only the silicate layer.

Atomic scale characterization of HfO2∕Al2O3 thin films grown on nitrided and oxidized Si substrates

One and three bilayers of HfO2(9Å)∕Al2O3(3Å) thin films were grown by atomic layer chemical-vapor deposition on Si(001) substrates whose surfaces were nitrided or oxidized. The films as-grown and postannealed in an ultrahigh vacuum were analyzed by atomic force microscopy, photoelectron spectroscopy, and medium energy ion scattering. For the one- and three-bilayer films grown on the nitrided Si substrates, the HfO2 and Al2O3 layers are mixed to form Hf aluminates at temperatures above 600°C. The mixed Hf aluminate layer is partly decomposed into HfO2 and Al2O3 grains and Al2O3 segregates to the surface by postannealing at 900°C. Complete decomposition takes place at 1000°C and the surface is covered with Al2O3. The surfaces are uniform and almost flat up to 900°C but are considerably roughened at 1000°C due to the complete decomposition of the Hf aluminate layer. In contrast, for one- bilayer films stacked on the oxidized Si substrates, Hf silicate layers, including Hf aluminate, are formed by annealing at 600–800°C. At temperatures above 900°C, HfSi2 grows and Al oxide escapes from the surface.