Pure HfO2 Research Articles

Plasma etching of Hf-based high-k thin films. Part I. Effect of complex ions and radicals on the surface reactions

The effect of ion and radical compositions in BCl3/Cl2 plasmas was assessed in this work with a focus on the formation of etch products in patterning hafnium aluminate, a potential high-k gate oxide material. The plasma composition became increasingly more complex as the percentage of boron trichloride was increased, which led to the formation of a significant amount of boron-containing species including B+, BCl+, BCl2+, BCl3+, B2Cl3+, and B2OCl3+ in the plasma. The BCl2+ ions were found to be the dominant species in BCl3 containing plasmas at most conditions; however, increasing the pressure or decreasing the power led to an increase in the formation of higher mass ions. Several compositions of Hf1−xAlxOy thin films ranging from pure HfO2 to pure Al2O3 were etched in BCl3/Cl2 plasmas as functions of ion energy and plasma composition. The etch product distributions were measured and the dominant metal-containing etch products were HfClx and AlClx in a Cl2 plasma and HfClx, HfBOCl4, and AlxCly in a BCl3 plasma, and their concentrations increased with increasing ion energy. Oxygen was detected removed in the form of ClO in Cl2 and as trichloroboroxin ((BOCl)3) in BCl3. Both the etch rate and the etch product formation are enhanced in BCl3/Cl2 plasmas, as compared to those in Cl2 plasmas, due to the change in the composition and reactivity of the dominant ions and radicals.

HfO2-based Thin Films Deposited by RF Magnetron Sputtering

AbstractHfO2-based layers prepared by RF magnetron sputtering were studied by X-ray diffraction, infrared absorption spectroscopy and transmission electron microscopy techniques. The effect of the deposition parameters and post-deposition annealing treatment on the properties of the layers was investigated. The amorphous-crystalline transformation of pure HfO2 layers is observed to be stimulated by annealing treatment at 800 ° C. It was found that the incorporation of silicon in HfO2 matrix allows to prevent crystallization of the layers and to shift the crystallization temperature to values up to 900 °C.

Effect of Al and Y incorporation on the structure of HfO2

We have investigated the structural changes in HfO2 thin films upon Y and Al incorporation. The crystallinity of the films is examined by transmission electron diffraction. The local coordination symmetries of the Hf atoms in the films are revealed by the profile of the oxygen K-edge measured in the electron energy loss spectroscopy (EELS). The EEL spectra are then simulated using real-space multiple-scattering method. We found a good agreement between the simulated and the experimental EELS result of the pure HfO2. The Y incorporation indeed stabilized HfO2 to a cubic structure. Nevertheless, it also contributed to possible lattice distortion and/or creation of complex defect states other than O vacancies, causing discrepancies between the experimental and the simulated results. As a comparison, the local symmetry of Hf is largely degraded upon the incorporation of Al, which not only amorphorized HfO2, but also introduced significantly amount of O vacancies in the film.

Universal Correlation Between Mobility and NBTI on Advanced High-k/Metal Gate Stacks

Negative Bias Temperature Instabilities (NBTI) and mobility were extensively studied on a wide range of technological options for advanced CMOS gate stack technologies including pure HfO2, silicates, nitrided silicate, HfZr compounds; metal gates included TiN, TaN, TaC, WN with various deposition modes and thicknesses; bottom oxide was pure SiO2 or SiON. The effect of Nitrogen on the two main components of NBTI, fast reversible and slow, is first investigated. Then we show that NBTI and mobility show extensive process variations leading in worst case to bad drivability and reliability. The cause was clearly identified as the Nitrogen incorporated in the stack with cumulative effects with the various process steps. The importance of metal gate is particularly emphasized. By controlling the amount of N in the layers, it is possible to obtain a mobility close to 100% of the universal SiO2 mobility as well as a good reliability

Effects of Y doping on the structural stability and defect properties of cubic HfO2

First-principles calculations have been performed to study the structural and electronic properties of pure and Y-doped cubic HfO2. It is found that Y doping in HfO2 would increase the stability of the cubic phase relative to the monoclinic phase by reducing the energy difference and the phase transition pressure. This result is consistent with the observed stabilization of the cubic phase of HfO2 by the addition of Y. The calculated formation energy of the VO–YHf complex defect in different charged states indicates that the single positively charged state (VO–YHf)+ is more stable than the neutral state (VO–YHf)0 and the double positively charged state (VO–YHf)++ in Y-doped cubic HfO2. Because the number of d-electrons of Y is less than that of Hf by one and substitutional Y for Hf introduces holes in the oxygen p-band, Y doping would make the highest occupied defect level induced by (VO–YHf)+ fall into the valence band rather than the energy gap, which explains the experimental observation that gap states related to oxygen vacancy defects become nondetectable in Y-doped HfO2 films.

Rational Hydrothermal Route to Monodisperse Hf1-xEuxO2-x/2 Solid Solution Nanocrystals

Highly dispersed and nearly uniformly sized Hf1-xEuxO2-x/2 (x = 0.05, 0.1, and 0.15) solid solution nanoparticles with particle sizes between 2.2 and 5.2 nm were produced via the hydrothermal treatment of a metal−oleate complex precursor at 200 °C for 6 h. The obtained nanocrystals were characterized by IR, EDX, XRD, Raman scattering, TEM, and PL spectroscopy. It was demonstrated that the proposed synthetic technique led to the formation of a Hf1-xEuxO2-x/2 solid solution with a thermodynamically metastable phase (tetragonal HfO2 phase). Pure HfO2 nanocrystals were rod-like in shape with a monoclinic phase, while the doping of HfO2 by Eu3+ resulted in the nearly spherically shaped nanocrystals. The PL spectroscopy showed significant quenching of Eu3+ 5Do → 7Fn radiative transitions for the as-prepared samples, whereas the annealed sample at 400 °C for 5 h showed clear red luminescence.

Low Voltage Operation of High-κ HfO2/TiO2/Al2O3 Single Quantum Well for Nanoscale Flash Memory Device Applications

Memory characteristics of the high-κ HfO2/TiO2/Al2O3 single quantum well in a p-Si/Al2O3/HfO2/TiO2/Al2O3/platinum capacitor structure have been investigated. All high-κ films have been deposited by atomic layer deposition. A significant memory window of ∼1.6 V at a low gate voltage operation of 2 V, a high charge storage density of ∼3.6×1012/cm2, a moderate leakage current density of 1×10-6/cm2 at a gate voltage of -2 V, and a negligible charge loss of <5% up to 104 s of retention for the high-κ HfO2/TiO2/Al2O3 single quantum well memory capacitor have been observed as compared with those of pure HfO2, pure TiO2, and pure Al2O3 charge trapping layers. Excellent memory characteristics of the high-κ HfO2/TiO2/Al2O3 single quantum well device are obtained due to charge storage in the quantum well. The high-κ HfO2/TiO2/Al2O3 quantum well device with a low gate voltage operation of <2 V can be used in future nanoscale flash memory device applications.

Atomic layer deposition of HfO2, Al2O3, and HfAlOx using O3 and metal(diethylamino) precursors

Tetrakis-diethylamino hafnium (TDEAH), tris-diethylamino aluminum (TDEAA), and ozone were used for the atomic layer deposition (ALD) of HfO2, Al2O3, and HfAlOx films. The ALD rates were measured to be 1.1 Å/cycle for HfO2 and 1.3 Å/cycle for Al2O3. The ALD temperature windows were found to be between 200 and 325 °C for TDEAA, and between 200 and 275 °C for TDEAH. The overlap of these ALD windows between 200 and 275 °C is critical for ALD of the composite film, HfAlOx. In addition to the overlapping ALD temperature windows, the two metal precursors have similar thermal characteristics, as shown by TGA and differential scanning calorimetry. As-deposited films and films postannealed at 600 and 800 °C films were analyzed using Fourier transformed infrared (FTIR) spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffraction (XRD) techniques. FTIR spectra revealed interfacial oxide growth during deposition of both HfO2 and Al2O3 whose thickness increased with annealing temperature. The FTIR data also indicated hydroxyl and nitrate groups in the films; these species were removed after annealing in Ar at a temperature of ⩾600 °C. Both FTIR and XRD results indicated the crystallization of pure HfO2 after annealing at temperatures as low as 600 °C. On the other hand, pure Al2O3 remained amorphous after annealing at temperatures up to 800 °C. XRD data of the composite HfAlOx film show that films deposited by alternating five cycles of HfO2 and one cycle of Al2O3 remained amorphous after annealing at 600 °C. Rutherford backscattering analysis of HfAlOx deposited with a varied number of alternating HfO2 and Al2O3 cycles demonstrated a strong correlation between the cyclic dosage of TDEAA and TDEAH and the film composition.

Suppression of oxygen vacancy formation in Hf-based high-k dielectrics by lanthanum incorporation

The effects of lanthanum incorporation into HfO2 dielectrics were studied using first-principles total energy calculations. The author’s computational result clearly showed that the formation energy of a neutral oxygen vacancy (VO0) in the vicinity of substitutional La atoms at Hf sites is 0.7eV larger than that in pure HfO2, indicating that the concentration of VO0’s is drastically reduced by La incorporation. This effect is understood to be caused by the decrease in the local dielectric constant κL around La atoms due to the strong ionic character of the La–O bond compared to the Hf–O bond.

Higher permittivity rare earth doped HfO2 for sub-45-nm metal-insulator-semiconductor devices

Rare earth (RE) doping (Gd, Er, Dy) of HfO2 reduces leakage current by three orders of magnitude compared with pure HfO2. The key to reducing HfO2 leakage current and equivalent oxide thickness (EOT) is stabilization of the higher permittivity tetragonal phase. RE doping of 10–20at.% stabilizes tetragonal HfO2 and increases permittivity. The maximum permittivity achieved for HfREOx is 28. The maximum permittivity for ZrREO is 32. HfGdO metal-insulator-semiconductor capacitors with EOT=1.93nm and leakage current &amp;lt;1×10−8A∕cm2 after 1070°C have been demonstrated.

Charge storage characteristics of atomic layer deposited RuOx nanocrystals

The charge storage characteristics of atomic layer deposited RuOx nanocrystals embedded in high-k HfO2∕Al2O3 films in a metal/Al2O3∕RuOx∕HfO2∕SiO2∕n-Si structure have been investigated. The size and density of RuOx nanocrystals have been measured using transmission electron microscopy. The RuOx nanocrystals show a density of ∼1×1012∕cm2 and a diameter of 5–8nm. A large hysteresis memory window of ∼13.3V at a gate voltage of 9V has been observed for RuOx nanocrystal memory capacitors. A hysteresis memory window of 0.7V has also been observed under a small sweeping gate voltage of 1V. A promising memory window of RuOx nanocrystals has been observed as compared with those of pure HfO2 and Al2O3 charge trapping layers, due to charge storage in the RuOx metal nanocrystals. The RuOx nanocrystal memory capacitor has similar leakage current with the pure HfO2 and Al2O3 charge trapping layers. The RuOx memory capacitor has a large breakdown voltage of ∼13.8V.

HfO2/HfAlO/HfO2 Nanolaminate Charge Trapping Layers for High-Performance Nonvolatile Memory Device Applications

A high-κ HfO2/HfAlO/HfO2 nanolaminate charge trapping layer in a p-Si/SiO2/[HfO2/HfAlO/HfO2 nanolaminate]/Al2O3/platinum memory capacitor has been investigated. High-κ HfO2, Al2O3, and HfO2/HfAlO/HfO2 nanolaminate charge trapping layers are deposited by atomic layer deposition. Well-behaved counter clockwise capacitance–voltage hysteresis characteristics are observed for all memory capacitors. A large memory window of ∼10 V, a very low leakage current density of ∼5×10-9 A/cm2 at a gate voltage of -5 V, a high charge trapping density of ∼1.6×1013/cm2, and a low charge loss of ∼20% after 10 years of retention for the HfO2/HfAlO/HfO2 nanolaminate charge trapping layer are observed as compared with those of pure HfO2 and pure Al2O3 charge trapping layers. Excellent memory characteristics of HfO2/HfAlO/HfO2 nanolaminate layers are obtained owing to the layer-by-layer charge storage. High-κ HfO2/HfAlO/HfO2 nanolaminate charge trapping layers can be used in future nanoscaled high-performance nonvolatile memory device applications.

Effect of Al addition on the microstructure and electronic structure of HfO2 film

We have investigated the microstructures and electronic structures of a series of hafnium aluminate (HfAlO) films with Al concentration ranging from 0% to 100%. When the films evolve from pure HfO2 to pure Al2O3 by increasing the aluminum content, we find changes in their radial distribution functions, which disclose the short-range order of the materials, despite the amorphous nature of all films. The HfAlO films (with Al∕Hf ratio ranging from 0.25 to 5.8) appear to be a single glassy phase of Hf, Al, and O, instead of simple mixtures of HfO2 and Al2O3. The Hf (Al)–O, Hf (Al)–Al, and Hf–Hf bonds are observed to be insensitive to the amount of Al in the film, except when the Al concentration is large (Al∕Hf∼5.8), in which case the bonding is similar to that in pure Al2O3. Although the local symmetry of Hf in amorphous HfO2 is suggested by the electron energy-loss spectrum taken at an oxygen K edge, it is largely disrupted when Al is introduced. The valence electron energy-loss spectroscopy reveals three distinct evolving features as the Al content increases, which we discuss in terms of the electronic structure of HfO2.

Doped HfO2 for Higher-k Dielectrics

In this work, we discuss doping effects of other metal oxides into HfO2 as a function of annealing temperature or doping concentration, and demonstrate a dramatic change of dielectric properties of doped HfO2 films. In addition to the relative dielectric constant (k-value), crystallization/amorphous characteristics have been also studied. One case induces easier crystallization than the pure HfO2, while the other case keeps amorphous up to 900 ºC without nitrogen introduction. From those observations, we discuss a general guideline for obtaining higher-k dielectric films.

Effect of nitrogen incorporation on the electronic structure and thermal stability of HfO2 gate dielectric

The effect of nitrogen incorporation on the electronic structure and thermal stability of HfO2 gate dielectric was investigated by using photoemission study and first-principles calculation. Hafnium oxynitride (HfON) dielectric shows higher thermal stability in comparison to pure HfO2 on Si. Atomic N can passivate O vacancies in the dielectrics during nitridation process, but the N atoms incorporated into interstitial sites cause band gap reduction. Postnitridation annealing is required to activate interstitial N atoms to form stable N–Hf bonds, which will increase the band gap and band offset of as-nitrided dielectric film.

Effects of Al addition on the native defects in hafnia

Two occupied native defect bands are experimentally detected in pure HfO2. The density of states of band one in the middle of the band gap reduces drastically with the Al addition, while that of band two slightly above the valence-band maximum remains rather unaffected. We attribute the two bands to the charged oxygen vacancy, and the oxygen-interstitial-related defect states of the HfO2, respectively. We demonstrate that the added Al passivates the VO+ induced midgap states but has little effect on other aspects of the electronic structure of the material.

Structure and magnetic properties of pure and Gd-doped HfO2 thin films

Pure HfO2 and Gd-doped HfO2 thin films have been grown on different single crystal substrates (silicon, R-Al2O3, and LaAlO3) by pulsed laser deposition. X-ray diffraction (XRD) patterns show that the pure HfO2 thin films are of single monoclinic phase. Gd-doped HfO2 films have the same XRD pattern except that their diffraction peaks have a shift toward lower angles, which indicates that Gd dissolves in HfO2. Transmission electron microscopy images show a columnar growth of the films. Very weak ferromagnetism is observed in pure and Gd-doped HfO2 films on different substrates at 300 and 5K, which is attributed to either impure target materials or signals from the substrates. The magnetic properties do not change significantly with postdeposition annealing of the HfO2 films. In addition to the films, HfO2 powders were annealed in pure hydrogen flow, and a ferromagnetic signal was not observed.

Growth of highly oriented HfO2 thin films of monoclinic phase on yttrium-stabilized ZrO2 and Si substrates by pulsed-laser deposition

We report on the growth of highly oriented HfO2 thin films of monoclinic phase stabilized by 5% Co doping on (001) yttrium-stabilized zirconia (YSZ) using pulsed-laser deposition at 700°C at an oxygen partial pressure of 10−4Torr. On the other hand, pure HfO2 of such quality did not grow on YSZ in wide range of growth parameters. Rutherford backscattering-ion channeling in this film showed a 24% minimum yield (χmin) indicating highly oriented film growth, while hardly any ion channeling was observed in the undoped sample. High-resolution transmission electron microscopy revealed a sharp interface, and no signature of Co clusters. Electron energy loss spectroscopy showed that Co is in the 2+ state. Attempts were also made to grow films on a (001) Si substrate, and the results showed a very low ion channeling yield (∼8%).

Electrical and material characterizations of high-permittivity HfxTi1−xO2 gate insulators

High-permittivity hafnium titanate (HfxTi1−xO2) films of various compositions have been investigated. Chemical-vapor deposited thin films have well-controlled composition and a smooth surface. Thicker films show large, highly anisotropic grains with their long axis in the plane of the film. Electrical measurements showed that the interfacial layer equivalent oxide thickness (EOT) decreases linearly with increasing Hf content. Electron energy-loss spectroscopy in scanning transmission electron microscopy indicates that the interfacial layer is primarily SiO2. In the composition range of 0.3&amp;lt;x&amp;lt;0.5, the permittivity of the high-κ films increases linearly with increasing Ti content. For values of x&amp;lt;0.3, the film’s permittivity is close to that of pure HfO2(∼20), while for Ti-rich films the permittivity is close to that of pure TiO2(∼50). For films with a physical thickness less than 10 nm, the leakage current decreases as Ti increases for a fixed EOT.

Formation of dual-phase HfO2–HfxSi1−xO2 dielectric and its application in memory devices

In this paper, we studied the phase-separation phenomenon of Hf0.5Si0.5O2 film deposited on SiO2 or sandwiched by SiO2, by x-ray photoelectron spectroscopy and transmission electron microscopy. The Hf0.5Si0.5O2 film underwent phase separation to form a doublet-phase HfO2–HfxSi1−xO2 (x&amp;lt;0.5) film, and was used as a trapping layer in a metal-blocking oxide-silicon nitride-tunnel oxide-silicon-type memory structure, where the dual-phase HfO2–HfxSi1−xO2 (DPHSO) film replaces the conventional silicon nitride (Si3N4) trapping layer. The charge storage properties of the DPHSO film were investigated and compared with HfO2 and Si3N4. It was found that for a given electric field applied to the tunnel oxide, the programming speed of memory devices using a DPHSO or HfO2 film as a trapping layer is faster than that using Si3N4. This indicates the higher electron-capture efficiency of the DPHSO and HfO2 films. In addition, the double-phase microstructure of the DPHSO film also provided better retention property than pure HfO2.