Electrical characteristics of a ZrO2/SiO2 modified tunnel barrier for low-temperature non-volatile memory

The electrical characteristics of nonvolatile memory, which consists of an asymmetrical ZrO₂/SiO₂ (ZO) modified tunnel barrier, a high-k HfO₂ trapping layer and an Al₂O₃ blocking layer, were investigated for the application of a tunnel barrier engineered nonvolatile memory at low process temperatures. The efficiency of the ZO modified tunnel barrier on the charge trap flash (CTF) memory cell was compared to a conventional single SiO₂ tunnel barrier. The ZO tunnel barrier revealed field sensitivity larger than the single SiO₂ tunnel barrier. The programming and erasing speeds as well as the retention and endurance characteristics of CTF memory were largely enhanced. Moreover, the forming gas annealing process in 2% diluted H₂/N₂ ambient improved the charging trapping property and tunneling sensitivity of the ZO modified tunnel barrier.     

Plasmons in double interfaces system of Si3N4/SiO2/Si irradiated by Co

The first level plasmons of Si in the pure Si state, in the SiO₂ state and in the Si₃N₄ state (corresponding to bonding energy 116.95, 122.0 and 127.0 eV) were investigated directly with X-ray photoelectron spectroscopy before and after ⁶⁰Co radiation. The experimental results demonstrate that there existed two interfaces, one consisted of plasmons of Si in the Si₃N₄ and SiO₂ states, while another was made of plasmons of Si in the pure Si state and in the SiO₂ state. When the Si₃N₄-SiO₂-Si samples were irradiated by ⁶⁰Co, the interface at Si₃N₄/SiO₂ was extended and at the same time the center of this interface moved towards the surface of Si₃N₄. The concentration of plasmon for silicon in the SiO₂ state is decreased at the SiO₂-Si interface, and the effects of radiation bias field on plasmons in the SiO₂-Si interface are observable. Finally, the mechanism of experimental results is analyzed by the quantum effect of plasmon excited by the photoelectron.     

Properties of polyimide/Al2O3 and Si3N4 deposited thin films

Polyimide (PI) nanocomposites with different proportions of Al₂O₃ were prepared via two-step reaction. Silicon nitride (Si₃N₄) was deposited on PI composite films by a RF magnetron sputtering system and used as a gas barrier to investigate the water vapor transmission rate (WVTR). The thermal stability and mechanical properties of a pure PI film can be improved obviously by adding adequate content of Al₂O₃. At lower sputtering pressure (4 mTorr), the PI/Al₂O₃ hybrid film deposited with Si₃N₄ barrier film exhibits denser structure and lower root mean square (RMS) surface roughness (0.494 nm) as well as performs better in preventing the transmission of water vapor. The lowest WVTR value was obtained from the sample, 4 wt.%Al₂O₃-PI hybrid film deposited with Si₃N₄ barrier film with the thickness of 100 nm, before and after bending test. The interface bonding, Al-N and Al-O-Si, was confirmed with the XPS composition-depth profile.     

Structure of double interfaces system of Si3N4/SiO2/Si irradiated by γ-rays

The interfacial structures of double interfaces system of Si₃N₄/SiO₂/Si were examined using X-ray photoelectron spectroscopy (XPS) before and after ⁶⁰Co radiation. The experimental results demonstrate that there existed two interfaces, one consisted of Si₃N₄ and SiO₂, while another was made of Si and SiO₂, the interface between SiO₂ and Si was extended towards the interface of the Si₃N₄/SiO₂ meanwhile the center of the former interface was removed in the direction of the latter interface by ⁶⁰Co. The concentration of silicon in the Si₃N₄ state (BE 101.8 eV) was decreased with the variation of radiation dosage as well as bias field within the SiO₂-Si interface, remarkably. The mechanism for the experimental results is analyzed.     

Memory characteristics of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/LaAlO<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> multilayer structures with tunnel oxide thickness variation

Charge trap flash (CTF) memory devices are candidates to replace NAND flash devices. In this study, Pt/Al₂O₃/LaAlO₃/SiO₂/Si multilayer structures with lanthanum aluminate charge traps were fabricated for nonvolatile memory device applications. An aluminum oxide film was used as a blocking oxide for low power consumption in the program/erase operation and to minimize charge transport through the blocking oxide layer. The thickness of SiO₂ as tunnel oxide layer was varied from 30 to 50 Å. Thicknesses of oxide layers were confirmed by high resolution transmission electron microscopy (HRTEM) and all the samples showed amorphous structure. From the C–V measurement, a maximum memory window of 3.4 V was observed when tunnel oxide thickness was 40 Å. In the cycling test for reliability, the 30 Å tunnel oxide sample showed a relatively large memory window reduction by repeated program/erase operations due to the high electric field of ~10 MV/cm through tunnel oxide. The other samples showed less than 10% loss of memory window during 10⁴ cycles.     

Quantitative analysis of chemical compositions in ultra-thin oxide-nitride-oxide stacked films having wet oxidized blocking layer

2nd-derivative Auger electron spectra analyses and time-of-flight secondary ion mass spectroscopy depth profiles are employed for analyzing quantitatively chemical compositions in ultra-thin oxide-nitride-oxide (ONO) stacked films. Tunnel oxide of 2.3 nm is grown on silicon substrate in nitrogen-diluted oxygen ambient. Nitride of 5.7 nm is immediately deposited on the tunnel oxide by a low-pressure chemical vapor deposition. Blocking oxide is subsequently grown by oxidizing the oxide-nitride structure in a wet O₂ ambient. Some chemical mixing occurs during the ONO formation. That is, the tunnel oxide formed on the silicon substrate changes into SiO_1.11N_0.67 by nitrogen substitution. Oxygen diffuses into the nitride layer, and converts some of the nitride layer into SiO_1.11N_0.67 during wet oxidation. We think the SiON and Si₂NO species exist near the tunnel oxide-nitride and the nitride-blocking oxide interfaces are segments of the Si₂N₂O. These are evidence of dangling bonds as unstable chemical species which may act as charge traps in the oxide-nitride interfaces.     

Combined spectroscopic ellipsometry and attenuated total reflection analyses of Al2O3/HfO2 nanolaminates

The microstructure of thin HfO₂-Al₂O₃ nanolaminate high κ dielectric stacks grown by atomic vapor deposition has been studied by attenuated total reflection spectroscopy (ATR) and 8 eV spectroscopic ellipsometry (SE). The presence of Al₂O₃ below HfO₂ prevents the crystallisation of HfO₂ if an appropriate thickness is used, which depends on the HfO₂ thickness. A thicker Al₂O₃ is required for thicker HfO₂ layers. If crystallisation does occur, we show that the HfO₂ signature in both ATR and 8 eV SE spectra allows the detection of monoclinic crystallites embedded in an amorphous phase.     

The etching characteristics of Al2O3 thin films in an inductively coupled plasma

In this study, the etching characteristics of ALD deposited Al₂O₃ thin film in a BCl₃/N₂ plasma were investigated. The experiments were performed by comparing the etch rates and the selectivity of Al₂O₃ over SiO₂ as functions of the input plasma parameters, such as the gas mixing ratio, the DC-bias voltage, the RF power, and the process pressure. The maximum etch rate was obtained at 155.8 nm/min under a 15 mTorr process pressure, 700 W of RF power, and a BCl₃ (6 sccm)/N₂ (14 sccm) plasma. The highest etch selectivity was 1.9. We used X-ray photoelectron spectroscopy (XPS) to investigate the chemical reactions on the etched surface. Auger electron spectroscopy (AES) was used for the elemental analysis of the etched surfaces.     

Thermal-stress stability of yttrium oxide as a buffer layer of metal-ferroelectric-insulator-semiconductor field effect transistor

The effect of thermal stress on the electrical properties of ferroelectric/semiconductor structure was investigated when introducing Y₂O₃ film as a barrier layer in the structure. Two different thermal stress states could be obtained by fast (400 °C/min) or slow (30 °C/min) cooling of sputter-deposited Y₂O₃ films on silicon wafer from 800 °C. The formation of interfacial layer containing Y-Si-O and SiO₂ layers while annealing could be characterized by using a spectroscopic ellipsometry. The introduction of strain-induced defects from thermal stress of the fast cooled sample showed a soft breakdown at low applying voltage. In the capacitance-voltage relation, a flat band voltage shift, hysteresis, and stretch-out phenomena were also observed. Nd₂Ti₂O₇ was spin deposited using sol-gel procedure on the Y₂O₃/Si to form a metal-ferroelectric-insulator field effect transistor structured sample. These Nd₂Ti₂O₇/Y₂O₃/Si samples were also furnace-annealed at 800 °C and cooled down to room temperature fast or slowly. One order lower value of leakage current, 1E−8 A/cm² was observed with these samples when comparing with the Y₂O₃/Si samples. A soft breakdown of the fast cooled sample seemed to have the same origin as the fast cooled Y₂O₃/Si sample, i.e., the strain-induced defects in the interfacial layer containing Y-Si-O and SiO₂ phases. Hysteretic gaps of the Nd₂Ti₂O₇/Y₂O₃/Si samples showed a possibility to be used as a memory window for ferroelectric gate.     

Applying RF current harmonics for end-point detection during etching multi-layered substrates and cleaning discharge chambers with NF3 discharge

The present paper reports the results of studying the characteristics of the etching process of multi-layered materials (Si₃N₄/SiO₂/Si and SiO₂/Si) and of cleaning technological chambers covered with silicon nitride films (Si₃N₄) in a NF₃ RF capacitive discharge. The process of chamber cleaning was monitored with a mass spectrometer. The gas pressure, RF voltage amplitude, current-voltage phase shift, ohmic current as well as the second harmonic of the RF current were also recorded. The opportunity of using these parameters for end-point detection of etching and plasma cleaning is discussed. It is found that the second harmonic of the RF current may be successfully used for end-point detection of multi-layered materials etching and to monitor the cleaning process of technological chambers. The cleaning of chambers of complicated design may possess a double-stage pattern.     

Effects of physical growth conditions on the structural and optical properties of sputtered grown thin HfO2 films

HfO₂ thin films were prepared by reactive DC magnetron sputtering technique on (100) p-Si substrate. The effects of O₂/Ar ratio, substrate temperature, sputtering power on the structural properties of HfO₂ grown films were studied by Spectroscopic Ellipsometer (SE), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrum, and X-ray photoelectron spectroscopy (XPS) depth profiling techniques. The results show that the formation of a SiO_x suboxide layer at the HfO₂/Si interface is unavoidable. The HfO₂ thickness and suboxide formation are highly affected by the growth parameters such as sputtering power, O₂/Ar gas ratio during sputtering, and substrate temperature. XRD spectra show that the deposited films have (111) monoclinic phase of HfO₂, which is also supported by FTIR spectra. XPS depth profiling spectra shows that highly reactive sputtered Hf atoms consume some of the oxygen atoms from the underlying SiO₂ to form HfO₂, leaving Si-Si bonds behind.     

Correlation between surface damage and micro-defects in Si covered with insulating layer by implantation of He and H ions

Cz n-type Si (100) wafers covered with a 220 nm SiO₂ layer or a 170 nm Si₃N₄ layer were singly implanted with 160 keV He ions at a dose of 5 × 10¹⁶/cm² or successively implanted with 160 keV He ions at a dose of 5 × 10¹⁶/cm² and 110 keV H ions at a dose of 1 × 10¹⁶/cm². Surface morphologies together with defect microstructures have been studied by means of several techniques, including optical microscopy, atomic force microscopy, and cross-sectional transmission electron microscopy (XTEM). Only surface blistering has been observed for He and H sequentially implanted SiO₂/Si samples after annealing in temperature range up to 1000 °C. However, as for the He and H implanted Si₃N₄/Si samples, surface features including blistering and the localized exfoliation of both the top Si₃N₄ layer and the implanted Si layer have been well demonstrated during subsequent annealing. XTEM observations reveal quite different defect morphologies in two kinds of materials under the same implantation and annealing conditions. The possible mechanisms of surface damage in two kinds of materials have been discussed and presented based on the XTEM results.     

Optimization of silicon nanocrystals growth process by low pressure chemical vapor deposition for non-volatile memory application

The processes of silicon nanocrystals (Si-NCs) growth on both SiO₂ and Si₃N₄ substrates by low pressure chemical vapor deposition have been systematically investigated. A two-step process was adopted for Si-NCs growth: nucleation at a high temperature (580-600 °C) and growth at a low temperature (550 °C). By adjusting the pre-deposition waiting time and deposition time, the density, size and uniformity can be effectively controlled. Compared to the growth of Si-NCs on SiO₂, the coalescence speed of Si-NCs on Si₃N₄ is faster. Uniform Si-NCs with a high density of 1.02 × 10¹² cm^− 2 and 1.14 × 10¹² cm^− 2 have been obtained on SiO₂ and Si₃N₄, respectively. Finally, a Si-NCs-based memory structure with a 2.1 V memory window was demonstrated.     

Plasma etching of high-k and metal gate materials

Etching characteristics of high-k dielectric materials (HfO₂) and metal electrode materials (Pt, TaN) have been studied in high-density chlorine-containing plasmas at pressures around 10 mTorr. The etching of HfO₂ was performed in BCl₃ without rf biasing, giving an etch rate of about 5 nm/min with a high selectivity of >10 over Si and SiO₂. The etching of Pt and TaN was performed in Ar/O₂ with high rf biasing and in Ar/Cl₂ with low rf biasing, respectively, giving a Pt etch rate of about several tens nm/min and a TaN etch rate of about 200 nm/min with a high selectivity of >8 over HfO₂ and SiO₂. The etched profiles were outwardly tapered for Pt, owing to the redeposition of etch or sputter products on feature sidewalls, while the TaN profiles were almost anisotropic, probably owing to the ion-enhanced etching that occurred.     

Lightweight geopolymer made of highly porous siliceous materials with various Na2O/Al2O3 and SiO2/Al2O3 ratios

The syntheses of lightweight geopolymeric materials from highly porous siliceous materials viz. diatomaceous earth (DE) and rice husk ash (RHA) with high starting SiO₂/Al₂O₃ ratios of 13.0-33.5 and Na₂O/Al₂O₃ ratios of 0.66-3.0 were studied. The effects of fineness and calcination temperature of DE, concentrations of NaOH and KOH, DE to RHA ratio; curing temperature and time on the mechanical properties and microstructures of the geopolymer pastes were investigated. The results indicated that the optimum calcination temperature of DE was 800 °C. Increasing fineness of DE and starting Na₂O/Al₂O₃ ratio resulted in an increase in compressive strength of geopolymer paste. Geopolymer pastes activated with NaOH gave higher compressive strengths than those with KOH. The optimum curing temperature and time were 75 °C and 5 days. The lightweight geopolymer material with mean bulk density of 0.88 g/cm³ and compressive strength of 15 kg/cm² was obtained. Incorporation of 40% RHA to increase starting SiO₂/Al₂O₃ and Na₂O/Al₂O₃ ratios to 22.5 and 1.7 and enhanced the compressive strength of geopolymer paste to 24 kg/cm² with only a marginal increase of bulk density to 1.01 g/cm³. However, the geopolymer materials with high Na₂O/Al₂O₃ (>1.5) were not stable in water submersion.     

High-quality YBa2Cu3O7 films on 4-in. Wafers of sapphire, gallium arsenide, and silicon

Our technique of reactive thermal co-evaporation has been extended to fabricate large films (up to 4 in.) of YBa₂Cu₃O₇ with high quality. A rotating substrate holder is used to separate the deposition and oxidation processes. This allows free access of the metal vapors. As large substrate wafers we use Al₂O₃, Si, and GaAs with buffer layers of CeO₂, YSZ, and MgO, respectively. On all substrates, the uniformity of thickness and composition was better than 2%. Inductively measuredT _c andj _c (77 K) were 87.5±0.2 K and >1×10⁶ A/cm², respectively, across the full wafer area. This holds also for GaAs substrates due to a new procedure of capping by Si₃N₄.This work was supported by the German Bundesminister für Forschung und Technologie.     

Chemical structure and electrical properties of sputtered HfO2 films on Si substrates annealed by rapid thermal annealing

The chemical structure and electrical properties of HfO₂ thin film grown by rf reactive magnetron sputtering after rapid thermal annealing (RTA) were investigated. The chemical composition of HfO₂ films and interfacial chemical structure of HfO₂/Si in relation to the RTA process were examined by X-ray photoelectron spectroscopy. Hf 4f and O 1s core level spectra suggest that the as-deposited HfO₂ film is nonstoichiometric and the peaks shift towards lower binding energy after RTA. The Hf-Si bonds at the HfO₂/Si interface can be broken after RTA to form Hf-Si-O bonds. The electrical characteristics of HfO₂ films were determined by capacitance-voltage (C-V) and current density-voltage (J-V) measurements. The results showed that the density of fixed charge and leakage current density of HfO₂ film were decreased after the RTA process in N₂ atmosphere.     

Synthesis of monodisperse spherical core-shell SiO2-SrAl2Si2O8: Eu2+ phosphors by hydrothermal homogeneous precipitation method

Abstract

Nanocrystalline SrAl₂Si₂O₈ :Eu²⁺ phosphor layers were coated on nonaggregated, monodisperse and spherical SiO₂ particles using a hydrothermal homogeneous precipitation. After annealing at 1100 °C, core-shell SiO₂@SrAl₂Si₂O₈ :Eu²⁺ particles were obtained. They were characterized with x-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and photoluminescence techniques. XRD analysis confirmed the formation of SiO₂ @SrAl₂Si₂O₈ :Eu²⁺ particles; it indicated that the SrAl₂Si₂O₈ :Eu²⁺ shells on SiO₂ particles consisted of hexagonal crystallites. The core-shell phosphors obtained are well-dispersed submicron spherical particles with a narrow size distribution. The thickness of the coated layer is approximately 20–40 nm. Under ultraviolet excitation (361 nm), the particles emit blue light at about 440 nm due to the Eu²⁺ ions in their shells.     

Synthesis of monodisperse spherical core-shell SiO2-SrAl2Si2O8: Eu2+ phosphors by hydrothermal homogeneous precipitation method

Nanocrystalline SrAl₂Si₂O₈ :Eu²⁺ phosphor layers were coated on nonaggregated, monodisperse and spherical SiO₂ particles using a hydrothermal homogeneous precipitation. After annealing at 1100 °C, core-shell SiO₂@SrAl₂Si₂O₈ :Eu²⁺ particles were obtained. They were characterized with x-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and photoluminescence techniques. XRD analysis confirmed the formation of SiO₂ @SrAl₂Si₂O₈ :Eu²⁺ particles; it indicated that the SrAl₂Si₂O₈ :Eu²⁺ shells on SiO₂ particles consisted of hexagonal crystallites. The core-shell phosphors obtained are well-dispersed submicron spherical particles with a narrow size distribution. The thickness of the coated layer is approximately 20–40 nm. Under ultraviolet excitation (361 nm), the particles emit blue light at about 440 nm due to the Eu²⁺ ions in their shells.     

The photoluminescence of Ce-doped Lu4Si2O7N2 green phosphors

Cerium-doped Lu₄Si₂O₇N₂ green phosphors were synthesized by a high-temperature solid-state reaction method under nitrogen atmosphere. Compared with Ce-doped Y₄Si₂O₇N₂ phosphors, Ce-doped Lu₄Si₂O₇N₂ phosphors showed longer wavelengths of both excitation and emission, lower Stokes shift, and much stronger emission intensity. Based on the first-principles calculations of the two phosphors, the strong emission intensity originates from the large density of states. At last, the effects of Ce³⁺ concentration on photoluminescence were also examined.