The use of angle resolved XPS to measure the fractional coverage of high-k dielectric materials on silicon and silicon dioxide surfaces

Angle resolved XPS (ARXPS) is a powerful tool for the determination of the thickness of ultra-thin films. In the case of high-k dielectric layers, the technique is capable of measuring the thickness of both the high-k layer and intermediate layers of silicon dioxide or metal silicate. The values for layer thickness are in close agreement with those generated by a variety of other techniques. As well as knowing the thickness of these layers, it is important to determine whether the layers are continuous or whether the coverage of the high-k layer is only partial. Using ARXPS, a method has been developed to determine whether the coverage of the high-k material is continuous and, if not, to calculate the fraction of the surface that is covered. The method is described with reference to the layers of Al₂O₃ grown on SiO₂ using atomic layer deposition (ALD). The method is then applied to HfO₂ layers produced using ALD on silicon wafers whose surfaces had received three different types of surface treatment. The way in which the layers grow and the nature of the resulting layer were found to depend upon the pre-treatment method. For example, growth on a thermal silicon dioxide surface resulted in complete coverage of HfO₂ after fewer ALD cycles than layers grown on an H-terminated surface. The results from ARXPS are compared with those obtained from ToF SIMS that have been shown earlier to be a valuable alternative to the LEIS analysis [1].     

Microstructure and interfacial properties of HfO2-Al2O3 nanolaminate films

High-k HfO₂-Al₂O₃ composite gate dielectric thin films on Si(1 0 0) have been deposited by means of magnetron sputtering. The microstructure and interfacial characteristics of the HfO₂-Al₂O₃ films have been investigated by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and spectroscopic ellipsometry (SE). Analysis by XRD has confirmed that an amorphous structure of the HfO₂-Al₂O₃ composite films is maintained up to an annealing temperature of 800 °C, which is much higher than that of pure HfO₂ thin films. FTIR characterization indicates that the growth of the interfacial SiO₂ layer is effectively suppressed when the annealing temperature is as low as 800 °C, which is also confirmed by spectroscopy ellipsometry measurement. These results clearly show that the crystallization temperature of the nanolaminate HfO₂-Al₂O₃ composite films has been increased compared to pure HfO₂ films. Al₂O₃ as a passivation barrier for HfO₂ high-k dielectrics prevents oxygen diffusion and the interfacial layer growth effectively.     

X-ray photoemission and X-ray absorption studies of Hf-silicate dielectric layers

Photoelectron spectroscopy and X-ray absorption spectroscopy (XAS) measurements have been performed on HfSi_xO_y and HfSi_xO_yN_z dielectric layers, which are potential candidates as high-k transistor gate dielectrics. The hafnium silicate layers, 3-4 nm thick, were formed by codepositing HfO₂ and SiO₂ (50%:50%) by MOCVD at 485 °C on a silicon substrate following an IMEC clean. Annealing the HfSi_xO_y layer in a nitrogen atmosphere at 1000 °C resulted in an increase in the Si⁴⁺ chemical shift from 3.5 to 3.9 eV with respect to the Si⁰ peak. Annealing the hafnium silicate layer in a NH₃ atmosphere at 800 °C resulted in the incorporation of 10% nitrogen and the decrease in the chemical shift between the Si⁴⁺ and the Si⁰ to 3.3 eV. The results suggest that the inclusion of nitrogen in the silicate layer restricts the tendency of the HfO₂ and the SiO₂ to segregate into separate phases during the annealing step. Synchrotron radiation valence band photoemission studies determined that the valence band offsets were of the order of 3 eV. X-ray absorption measurements show that the band gap of these layers is 4.6 eV and that the magnitude of the conduction band offset is as little as 0.5 eV.     

Fabrication and electrical characterization of a MOS memory device containing self-assembled metallic nanoparticles

Floating gate devices with nanoparticles embedded in dielectrics have recently attracted much attention due to the fact that these devices operate as non-volatile memories with high speed, high density and low power consumption. In this paper, memory devices containing gold (Au) nanoparticles have been fabricated using e-gun evaporation. The Au nanoparticles are deposited on a very thin SiO₂ layer and are then fully covered by a HfO₂ layer. The HfO₂ is a high-k dielectric and gives good scalability to the fabricated devices. We studied the effect of the deposition parameters to the size and the shape of the Au nanoparticles using capacitance–voltage and conductance–voltage measurements, we demonstrated that the fabricated device can indeed operate as a low-voltage memory device.     

Nondestructive investigation of interface states in high‐k oxide films on Ge substrate using X‐ray absorption spectroscopy

The unoccupied electronic structures of 5 nm thick high permittivity (k) oxides (HfO₂, ZrO₂, and Al₂O₃) and SiO₂ films on Ge substrates were examined using O K‐edge X‐ray absorption spectroscopy. Comparative studies with those on Si substrates showed contrasts in the conduction bands, which should be due to the formation of interface states. In the Al₂O₃ and SiO₂ films, GeO₂ layers are formed at the interface and they suppress in part the formation of detrimental germanate phases. In contrast, in the HfO₂ and ZrO₂ films, no signature of the Ge‐oxide phase is observed but some germanate phases are expected to prevail, suggesting a degradation of the gate oxide characteristics. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical properties of multilayer structures

The possibility of using the ellipsometry method for investigation of the optical properties of multilayer films and structures is shown. The optical properties of structures HfO₂/SiO₂/Si, HfO₂/Si, ZrO₂/Si, Ta₂O₅/Si, and Al₂O₃/Si are studied. It is found that a layer of hafnium silicate is formed at the interface between the HfO₂ film and Si. Annealing of the structures in oxygen shows that oxides studied are oxygen-permeable and that the thickness of SiO₂ at the film-substrate interface increases. The growth rate of SiO₂ layers depends on the chemical nature of an oxide. Al₂O₃ films are impermeable for oxygen diffusion. The production of layers of alloys (Al₂O₃) _x (HfO₂)_{1 ? x} is optimized, which allows one to obtain layers with a homogeneous distribution of elements over the thickness.     

LaTiON/LaON as band-engineered charge-trapping layer for nonvolatile memory applications

Charge-trapping characteristics of stacked LaTiON/LaON film were investigated based on Al/Al₂O₃/LaTiON-LaON/SiO₂/Si (band-engineered MONOS) capacitors. The physical properties of the high-k films were analyzed by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. The band profile of this band-engineered MONOS device was characterized by investigating the current-conduction mechanism. By adopting stacked LaTiON/LaON film instead of LaON film as charge-trapping layer, improved electrical properties can be achieved in terms of larger memory window (5.4 V at ±10-V sweeping voltage), higher program speed with lower operating gate voltage (2.1 V at 100-μs +6 V), and smaller charge loss rate at 125 °C, mainly due to the variable tunneling path of charge carriers under program/erase and retention modes (realized by the band-engineered charge-trapping layer), high trap density of LaTiON, and large barrier height at LaTiON/SiO₂ (2.3 eV).     

Study of interfacial reactions and phase stabilization of mixed Sc, Dy, Hf high-k oxides by attenuated total reflectance infrared spectroscopy

Grazing angle attenuated total reflectance Fourier transform infrared spectroscopy is applied to study ultrathin film Hf⁴⁺, Sc³⁺ and Dy³⁺oxides, due to its high surface sensitivity. The (multi)metal oxides studied, are of interest as high-k dielectrics. Important properties affecting the permittivity, such as the amorphous or crystalline phase and interfacial reactions, are characterized.Dy₂O₃ is prone to silicate formation on SiO₂/Si substrates, which is expressed in DyScO₃ as well, but suppressed in HfDyO_x. Sc₂O₃, HfScO_x and HfO₂ were found to be stable in contact with SiO₂/Si. Deposition of HfO₂ in between Dy₂O₃ or DyScO₃ and SiO₂, prevents silicate formation, showing a buffer-like behavior for the HfO₂.Doping of HfO₂ with Dy or Sc prevents monoclinic phase crystallization. Instead, a cubic phase is obtained, which allows a higher permittivity of the films. The phase remains stable after anneal at high temperature.     

Improved charge trapping flash device with Al2O3/HfSiO stack as blocking layer

In this paper, we investigate an Al₂O₃/HfSiO stack as the blocking layer of a metal-oxide-nitride-oxide-silicon-type (MONOS) memory capacitor. Compared with a memory capacitor with a single HfSiO layer as the blocking layer or an Al₂O₃/HfO₂ stack as the blocking layer, the sample with the Al₂O₃/HfSiO stack as the blocking layer shows high program/erase (P/E) speed and good data retention characteristics. These improved performances can be explained by energy band engineering. The experimental results demonstrate that the memory device with an Al₂O₃/HfSiO stack as the blocking layer has great potential for further high-performance nonvolatile memory applications.     

Growth and interfacial properties of atomic layer deposited Al0.7Ti0.3O y high-k dielectric on Ge substrate

Growth and interfacial properties of atomic layer deposited Al_0.7Ti_0.3O _y on Ge have been investigated as a potential high-k gate dielectric for future Ge-based metal oxide semiconductor devices. A sandwich structure of Al₂O₃/TiO₂ stack is proposed for Al₂O₃/TiO₂ intermixing and high-k/Ge interfacial passivation. The film thicknesses and interface microstructure are characterized by spectroscopy ellipsometry and high-resolution transmission electron microscopy. X-ray photoelectron spectrometry is used to analyze the chemical composition and bonding states, and to reveal the band alignment of high-k/Ge heterojunctions. Metal-oxide-capacitors are formed by depositing aluminum electrodes to perform capacitance–voltage measurements for electrical characteristics. All evidences show a positive prospect of employing atomic layer deposited Al_0.7Ti_0.3O _y as high-k gate dielectric for future Ge-based devices.     

AlGaN/GaN-based HEMT on SiC substrate for microwave characteristics using different passivation layers

In this paper, a new gate-recessed AlGaN/GaN-based high electron mobility transistor (HEMT) on SiC substrate is proposed and its DC as well as microwave characteristics are discussed for Si₃N₄ and SiO₂ passivation layers using technology computer aided design (TCAD). The two-dimensional electron gas (2DEG) transport properties are discussed by solving Schr?dinger and Poisson equations self-consistently resulting in various subbands having electron eigenvalues. From DC characteristics, the saturation drain currents are measured to be 600?mA/mm and 550?mA/mm for Si₃N₄ and SiO₂ passivation layers respectively. Apart from DC, small-signal AC analysis has been done using two-port network for various microwave parameters. The extrinsic transconductance parameters are measured to be 131.7?mS/mm at a gate voltage of V _gs?= ?0.35?V and 114.6?mS/mm at a gate voltage of V _gs?= ?0.4?V for Si₃N₄ and SiO₂ passivation layers respectively. The current gain cut-off frequencies (f _t) are measured to be 27.1?GHz and 23.97?GHz in unit-gain-point method at a gate voltage of ?0.4?V for Si₃N₄ and SiO₂ passivation layers respectively. Similarly, the power gain cut-off frequencies (f _max) are measured to be 41?GHz and 38.5?GHz in unit-gain-point method at a gate voltage of ?0.1?V for Si₃N₄ and SiO₂ passivation layers respectively. Furthermore, the maximum frequency of oscillation or unit power gain (MUG = 1) cut-off frequencies for Si₃N₄ and SiO₂ passivation layers are measured to be 32?GHz and 28?GHz respectively from MUG curves and the unit current gain, ?O?h ₂₁??O?=?1 cut-off frequencies are measured to be 140?GHz and 75?GHz for Si₃N₄ and SiO₂ passivation layers respectively from the abs ?O?h ₂₁??O curves. HEMT with Si₃N₄ passivation layer gives better results than HEMT with SiO₂ passivation layer.     

Atomic layer deposition of nanolaminate oxide films on Si

Among the methods for depositing thin films, atomic layer deposition is unique for its capability of growing conformal thin films of compounds with a control of composition and thickness at the atomic level. The conformal growth of thin films can be of particular interest for covering nanostructures since it assures the homogeneous growth of the ALD film in all directions, independent of the position of the sample with respect to the incoming precursor flow. Here we describe the technique for growing the HfO₂/Al₂O₃ bilayer on Si substrate and our in situ approach for its investigation by means of synchrotron radiation photoemission. In particular, we study the interface interactions between the two oxides for various thickness compositions ranging from 0.4 to 2.7 nm. We find that the ALD of HfO₂ on Si induces the increase of the interfacial SiO₂ layer, and a change in the band bending of Si. On the contrary, the ALD of Al₂O₃ on HfO₂ shows negligible interaction between layers as the binding energies of Hf4f, Si2p, and O1s core level peaks and the valence band maximum of HfO₂ do not change and the interfacial SiO₂ does not increase.     

Improved characteristics for MOHOS memory with oxygen-rich GdO as charge storage layer annealed by NH3

Characteristics of metal–oxide–high-k–oxide–silicon (MOHOS) memories with oxygen-rich or oxygen-deficient GdO as charge storage layer annealed by NH₃ or N₂ are investigated. Transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction are used to analyze the cross-sectional quality, composition and crystallinity, respectively, of the stacked gate dielectric with a structure of Al/Al₂O₃/GdO/SiO₂/Si. The MOHOS capacitor with oxygen-rich GdO annealed in NH₃ exhibits a good trade-off among its memory properties: large memory window (4.8 V at ±12 V, 1 s), high programming speed (2.6 V at ±12 V/100 μs), good endurance and retention properties (window degradation of 5 % after 10⁵ program/erase cycles and charge loss of 18.6 % at 85 °C after 10 years, respectively) due to passivation of oxygen vacancies, generation of deep-level traps in the grain boundaries of the GdO layer and suppression of the interlayer between GdO and SiO₂ by the NH₃ annealing.     

A two-dimensional threshold voltage analytical model for metal-gate/high-k/SiO2/Si stacked MOSFETs

In this paper the influences of the metal-gate and high-k/SiO 2 /Si stacked structure on the metal-oxide-semiconductor field-effect transistor（MOSFET） are investigated.The flat-band voltage is revised by considering the influences of stacked structure and metal-semiconductor work function fluctuation.The two-dimensional Poisson＇s equation of potential distribution is presented.A threshold voltage analytical model for metal-gate/high-k/SiO 2 /Si stacked MOSFETs is developed by solving these Poisson＇s equations using the boundary conditions.The model is verified by a two-dimensional device simulator,which provides the basic design guidance for metal-gate/high-k/SiO 2 /Si stacked MOSFETs.     

Effect of thermal annealing sequence on the crystal phase of HfO2 and charge trap property of Al2O3/HfO2/SiO2 stacks

In this study, we investigated the effect of a post annealing sequence on the HfO₂ crystal phase and the memory window of charge trap devices with TiN-Al₂O₃-HfO₂-SiO₂-Si stacks. The charge trap dielectrics of HfO₂ were deposited by atomic layer deposition and were annealed in an oxygen environment with or without Al₂O₃ blocking oxides. X-ray diffraction analysis showed that, after thermal annealing, the predominant crystal phase of HfO₂ is divided into tetragonal and monoclinic phase depending on the presence or absence of Al₂O₃ blocking oxide. In addition, deconvolution of X-ray diffraction spectra showed that, with increasing annealing temperature, the fraction of the tetragonal phase in the HfO₂ film was enhanced with the Al₂O₃ blocking oxide, while it was reduced without the Al₂O₃ blocking oxide. Finally, measurements of program/erase and increase-step-pulse programming showed that the charge trap efficiency and the memory window of the charge trap devices increased with decreasing fraction of tetragonal HfO₂.     

Enhanced rear‐side reflection and firing‐stable surface passivation of silicon solar cells with capping polymer films

Low refractive index polymer materials have been investigated with a view to form the back surface mirror of advanced silicon solar cells. SiO_x:H or AlO_y SiO_x:H polymer films were spun on top of an ultra‐thin (<10 nm) atomic‐layer‐deposited (ALD) Al₂O₃ layer, itself deposited on low‐resistivity (1 Ω cm) p‐type crystalline silicon wafers. These double‐layer stacks were compared to both ALD Al₂O₃ single layers and ALD Al₂O₃/plasma‐enhanced chemical vapour deposited (PECVD) SiN_x stacks, in terms of surface passivation, firing stability and rear‐side reflection. Very low surface recombination velocity (SRV) values approaching 3 cm/s were achieved with ALD Al₂O₃ layers in the 4–8 nm range. Whilst the surface passivation of the single ALD Al₂O₃ layer is maintained after a standard firing step typical of screen printing metallisation, a harsher firing regime revealed an enhanced thermal stability of the ALD Al₂O₃/SiO_x:H and ALD Al₂O₃/AlO_y SiO_x:H stacks. Using simple two‐dimensional optical modelling of rear‐side reflection it is shown that the low refractive index exhibited by SiO_x:H and AlO_y SiO_x:H results in superior optical performance as compared to PECVD SiN_x, with gains in photogenerated current of ～0.125 mA/cm² at a capping thickness of 100 nm. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of corona pre-treatment on the performance of gas barrier layers applied by atomic layer deposition onto polymer-coated paperboard

The effect of corona pre-treatment on the performance of Al₂O₃ and SiO₂ gas barrier layers applied by atomic layer deposition onto polymer-coated paperboards was studied. Both polyethylene and polylactide coated paperboards were corona treated prior to ALD. Corona treatment increased surface energies of the paperboard substrates, and this effect was still observed after several days. Al₂O₃ and SiO₂ films were grown on top of the polymer coatings at temperature of 100 °C using the atomic layer deposition (ALD) technique. For SiO₂ depositions a new precursor, bis(diethylamido) silane, was used. The positive effect of the corona pre-treatment on the barrier properties of the polymer-coated paperboards with the ALD-grown layers was more significant with polyethylene coated paperboard and with thin deposited layers (shorter ALD process). SiO₂ performed similarly to Al₂O₃ with the PE coated board when it comes to the oxygen barrier, while the performance of SiO₂ with the biopolymer-coated board was more moderate. The effect of corona pre-treatment was negligible or even negative with the biopolymer-coated board. The ALD film growth and the effect of corona treatment on different substrates require further investigation.     

Band edge electronic structure of transition metal/rare earth oxide dielectrics

This article addresses band edge electronic structure of transition metal/rare earth (TM/RE) non-crystalline and nano-crystalline elemental and complex oxide high-k dielectrics for advanced semiconductor devices. Experimental approaches include X-ray absorption spectroscopy (XAS) from TM, RE and oxygen core states, photoconductivity (PC), and visible/vacuum ultra-violet (UV) spectroscopic ellipsometry (SE) combined with ab initio theory is applied to small clusters. These measurements are complemented by Fourier transform infra-red absorption (FTIR), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). Two issues are highlighted: Jahn-Teller term splittings that remove d-state degeneracies of states at the bottom of the conduction band, and chemical phase separation and crystallinity in Zr and Hf silicates and ternary (Zr(Hf)O₂)_x(Si₃N₄)_y(SiO₂)_1−x−y alloys. Engineering solutions for optimization of both classes of high-k dielectric films, including limits imposed on the continued and ultimate scaling of the equivalent oxide thickness (EOT) are addressed.     

Characterization of the interface between the Hf-based high-k thin film and the Si using spatially resolved electron energy-loss spectroscopy

The interfacial structures of HfO₂ and HfAlO thin films on Si have been investigated using spatially resolved electron energy-loss spectroscopy. We have found that interfaces are not atomically sharp, and variation in the symmetry of the local atomic coordination lasts for a couple of monolayers for both the as-deposited HfO₂ and the HfAlO samples. Annealing of the HfO₂ film in the oxygen environment leads to the formation of a thick SiO₂/SiO_x stack layer in-between the original HfO₂ and the Si substrate. As a comparison, the interfacial stability is significantly improved by Al incorporation into the HfO₂ film (forming HfAlO), which effectively reduced/eliminated the interfacial silicon oxide formation during the oxygen annealing process. The mechanism of the high-k film/substrate stabilization by Al incorporation is discussed based on the experimental results.