Si Multilayer Structure Research Articles

Multiphysics Modeling and Analysis of Sc-Doped AlN Thin Film Based Piezoelectric Micromachined Ultrasonic Transducer by Finite Element Method.

This paper presents a Piezoelectric micromechanical ultrasonic transducer (PMUT) based on a Pt/ScAlN/Mo/SiO2/Si/SiO2/Si multilayer structure with a circular suspension film of scandium doped aluminum nitride (ScAlN). Multiphysics modeling using the finite element method and analysis of the effect of different Sc doping concentrations on the resonant frequency, the effective electromechanical coupling coefficient (keff2) and the station sensitivity of the PMUT cell are performed. The calculation results show that the resonant frequency of the ScAlN-based PMUT can be above 20 MHz and its keff2 monotonically rise with the increasing doping concentrations in ScAlN. In comparison to the pure AlN thin film-based PMUT, the static receiving sensitivity of the PMUT based on ScAlN thin film with 35% Sc doping concentration is up to 1.61 mV/kPa. Meanwhile, the static transmitting sensitivity of the PMUT is improved by 152.95 pm/V. Furthermore, the relative pulse-echo sensitivity level of the 2 × 2 PMUT array based on the Sc doping concentration of 35% AlN film is improved by 16 dB compared with that of the cell with the same Sc concentration. The investigation results demonstrate that the performance of PMUT on the proposed structure can be tunable and enhanced by a reasonable choice of the Sc doping concentration in ScAlN films and structure optimization, which provides important guidelines for the design of PMUT for practical applications.

Study of Selective Dry Etching Effects of 15-Cycle Si0.7Ge0.3/Si Multilayer Structure in Gate-All-Around Transistor Process.

Gate-all-around (GAA) structures are important for future logic devices and 3D-DRAM. Inner-spacer cavity etching and channel release both require selective etching of Si0.7Ge0.3. Increasing the number of channel-stacking layers is an effective way to improve device current-driving capability and storage density. Previous work investigated ICP selective etching of a three-cycle Si0.7Ge0.3/Si multilayer structure and the related etching effects. This study focuses on the dry etching of a 15-cycle Si0.7Ge0.3/Si multilayer structure and the associated etching effects, using simulation and experimentation. The simulation predicts the random effect of lateral etching depth and the asymmetric effect of silicon nanosheet damage on the edge, both of which are verified by experiments. Furthermore, the study experimentally investigates the influence and mechanism of pressure, power, and other parameters on the etching results. Research on these etching effects and mechanisms will provide important points of reference for the dry selective etching of Si0.7Ge0.3 in GAA structures.

Space charge capacitance study of GaP/Si multilayer structures grown by plasma deposition

Microcrystalline gallium phosphide (GaP)/Si multilayer structures grown on GaP substrates using combination of plasma enhanced atomic layer deposition (PE-ALD) for GaP and plasma-enhanced chemical vapor deposition for Si layers deposition are studied by three main space charge capacitance techniques: capacitance versus voltage (C-V) profiling, admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS), which have been used on Schottky barriers formed on the GaP/Si multilayer structures. C-V profiling qualitatively demonstrates an electron accumulation in the Si/GaP wells. However, quantitative determination of the concentration and spatial position of its maximum is limited by the strong frequency dependence of the capacitance caused by electron capture/emission processes in/from the Si/GaP wells. These processes lead to signatures in AS and DLTS with activation energies equal to 0.39 ± 0.05 and 0.28 ± 0.05 eV, respectively, that are linked to the energy barrier at the GaP/Si interface. It is shown that the value obtained by AS (0.39 ± 0.05 eV) is related to the response from Si/GaP wells located in the quasi-neutral region of the Schottky barrier, and it corresponds to the conduction band offset at the GaP/Si interface, while DLTS rather probes wells located in the space charge region closer to the Schottky interface where the internal electric field yields to a lowering of the effective barrier in the Si/GaP wells. Two additional signatures were detected by DLTS, which are identified as defect levels in GaP. The first one is associated to the SiGa + VP complex, while the second was already detected in single microcrystalline GaP layers grown by PE-ALD.

Optimal phase shift mask and multilayer stack with the evaluation of imaging performance and process latitude in extreme ultraviolet high numerical aperture

In high numerical aperture (NA) extreme ultraviolet lithography, which is used to implement a finer linewidth of 10 nm or lower, serious problems arise in patterning as the NA increases. To alleviate such problems, a thin absorber and a multilayer with good reflective efficiency and improved pattern quality are required. To develop an effective EUV photomask for the commercialization of high-NA systems, we determined the optimal ruthenium (Ru)/silicon (Si) multilayer structure using a phase-shift mask (PSM) absorber. A Ru/Si multilayer using PSM as an absorber has a smaller best-focus range and placement error compared to the molybdenum (Mo)/silicon (Si) multilayer. At the same time, it provides improved image contrast, enabling more stable patterning. Even when the number of layers of the Ru/Si multilayer was reduced, it was confirmed that the reflectance efficiency and image quality were maintained.

Preferential sputtering in quantitative sputter depth profiling of multi-element thin films

Preferential sputtering plays an important role in the quantification of measured depth profiles. The distortion of the measured depth profile as compared to the original one has to be evaluated upon preferential sputtering in depth profiling. In the framework of the mixing-roughness-information depth (MRI) model, the influences of preferential sputtering on the depth profile of multi-element thin films are quantitatively evaluated as demonstrated for a layered structure of three elements with different sputtering rates and subsequent interfaces as A/BC/A. Moreover, the mass conservation upon Auger electron spectroscopy/X-ray photoelectron spectroscopy depth profiling of multi-element structure is discussed with respect to the preferential effect. Finally, as an example, the measured glow discharge optical emission spectroscope depth profiling data of 60x(3.0 nm Mo/0.3 nm B4C/3.7 nm Si)/Si(111) multilayer structure are quantitatively evaluated using the extended MRI model.

Enhanced ferroelectric properties of P(VDF-TrFE) thin films from Nb nanopin electrodes

We demonstrate the influence of Nb nanopin electrodes on ferroelectric properties of poly(vinylidene fluoride-ran-trifluoroethylene) (P(VDF-TrFE)) thin films deposited using a spin-coating technique. The Nb nanopin electrodes consisting of an array of regular hexagon nanopins were formed by virtue of a multiple anodizing process and an etching process of an Al/Nb/SiO2/Si multilayer structure. An electric field enhancement of the Nb nanopin electrodes contributed to the improved ferroelectric polarizations, low coercive electric fields, and fast switching characteristics of the P(VDF-TrFE) thin-film capacitors. Futhermore, the Nb nanopin electrodes made a significant contribution to low activation energy of domain wall motion. In particular, the study of domain wall motion by piezoresponse force microscopy support that the fast switching properties of the P(VDF-TrFE) capacitors is attributed to the low activation energy for the domain wall motion. This work provides a solution to solve problems of the high coercive and switching electric fields that have been considered obstacles in electronic devices and applications of the P(VDF-TrFE) thin films.

Investigation of tip-depletion-induced fail in scanning capacitance microscopy for the determination of carrier type

Scanning capacitance microscopy (SCM) was performed on an n-type Si multilayer structure doped by phosphorus whose concentration ranges from 2×1017 to 2×1019cm−3. Three types of tips were used, i.e. fresh Pt/Ir coated tip, worn Pt/Ir coated tip and non-coated commercial Si tip. The use of fresh Pt/Ir coated tips produces SCM result in good agreement with the doping profile including the correct identification of the carrier type. In contrast, a worn Pt/Ir coated tip which has lost its metal coating and a non-coated tip will fail to recognize successfully the carrier type for phosphorus dopant concentration above 8×1018cm−3 (identifying as p instead of n) due to the tip depletion effect. These results alert us to carefully interpret the SCM results, especially in the case for identification of carrier type inside the sample of interest which is unknown.

Ion-beam induced atomic mixing in isotopically controlled silicon multilayers

Implantation of germanium (Ge), gallium (Ga), and arsenic (As) into crystalline and preamorphized isotopically controlled silicon (Si) multilayer structures at temperatures between 153 K and 973 K was performed to study the mechanisms mediating ion-beam induced atomic mixing. Secondary-ion-mass-spectrometry was applied to determine concentration-depth profiles of the stable isotopes before and after ion implantation. The intermixing is analytically described by a depth-dependent displacement function. The maximum displacement is found to depend not only on temperature and microstructure but also on the doping type of the implanted ion. Molecular dynamics calculations evaluate the contribution of cascade mixing, i.e., thermal-spike mixing, to the overall observed atomic mixing. Calculated and experimental results on the temperature dependence of ion-beam mixing in the amorphous and crystalline structures provide strong evidence for ion-beam induced enhanced crystallization and enhanced self-diffusion, respectively. On the other hand, the former process is confirmed by channeling Rutherford backscattering analyses of the amorphous layer thickness remaining after implantation, the latter process is consistently attributed to the formation of highly mobile Si di-interstitials formed under irradiation and in the course of damage annealing. The observed ion-beam mixing in Si is compared to recent results on ion-beam mixing of Ge isotope multilayers that, in contrast to Si, are fully described by thermal-spike mixing only.

Shear-horizontal surface acoustic wave characteristics of a (110) ZnO/SiO2/Si multilayer structure

Shear-horizontal (SH) surface acoustic wave (SAW) propagation characteristics in IDT/(110)ZnO/SiO2/Si multilayered structure were theoretically investigated using 3-dimensions (3D) finite element analysis (FEA). X-ray diffraction result shows that the prepared ZnO films have preferred (110) orientation with its c-axis parallel to the substrate. 3D FEA results and the out of plane vibration experiment confirmed that SH-SAW was successfully excited in the multilayered structure. The phase velocities Vp, electromechanical coupling coefficients K2, and temperature coefficient of frequency (TCF) dispersion properties of mode 0 were studied considering various thicknesses of ZnO and SiO2 thin films, which were verified by experiments. SiO2 film with a positive TCF not only compensate the negative TCF of the (110) ZnO but also enhance the K2. A large K2 of 3.37 and nearly zero TCF of −0.1 ppm/°C was achieved in SH-SAW device based on IDT/(110)ZnO(2 μm)/SiO2(1 μm)/Si multilayer structure, which is promising for high sensitive and temperature stable bio-sensing application.

Periodic oscillation in the reflection and photoluminescence spectra of suspended two-dimensional crystal flakes

Suspended two-dimensional (2D) materials have been widely used to improve the device performances in comparison with the case of supported 2D materials. To realize such a purpose, 2D materials are mainly suspended on the holes of substrates, which are usually used to support 2D materials. The holes beneath the 2D materials are usually full of air. The air layer with the thickness identical to the hole depth will affect the spectral features of the reflection and photoluminescence spectra of suspended 2D materials because there exist multiple optical interferences in the air/2D-flakes/air/Si multilayer structures. However, it is not clear that how the spectral features depend on the hole depth. In this paper, the reflection spectra of suspended multilayer graphene and MoS2flakes as well as the photoluminescence spectra of suspended multilayer MoS2flakes are systematically studied. The reflection spectra of suspended multilayer graphene flakes exhibit obvious oscillations, showing the optical characteristic with periodic oscillations in wavenumber. The oscillation period decreases with increasing the hole depth (or the thickness of the air layer), but is independent of the thickness of suspended graphene flakes. This can be well explained by the model based on multiple optical interferences in the air/graphenes/air/Si multilayer structures, which have been successfully utilized to understand the Raman intensity of ultrathin 2D flakes and substrate beneath the ultrathin 2D flakes dependent on the thickness of 2D flakes, the thickness of SiO2 layer, the laser wavelength and the numerical aperture of objective. The theoretical simulation shows that the oscillation is obviously observable only when the hole depth reaches up to the value on the order of microns. For suspended multilayer MoS2flakes, the reflection and photoluminescence spectra show similar periodic oscillations in wavenumber and the oscillation period is also dependent on the hole depth. The hole depth is measured by the surface profiler. It is found that the calculated oscillation period based on the measured hole depth and multiple optical interference model is usually larger than the experimental one, which is attributed to the existence of the dielectric layer in the holes. The dielectric layer may be the residues after the hole etching process, which have much smaller refractive indexes than substrates and 2D flakes. This results in an increase of the effective hole depth, which becomes larger than the one measured by the surface profiler. The observation of oscillation period in the reflection and photoluminescence spectra of different flakes of 2D materials demonstrates that the periodic oscillation is a general optical characteristic for optical spectra of suspended 2D materials. In principle, the electroluminescence spectra of suspended 2D materials may also exhibit similar periodic oscillations in wavenumber. These findings may be helpful for understanding the optical spectra of various suspended 2D materials and monitoring the existence of the residues in the holes of substrate after the etching process.

Quantitative reconstruction of Ta/Si multilayer depth profiles obtained by Time-of-Flight-Secondary-Ion-Mass-Spectrometry (ToF-SIMS) using Cs+ ion sputtering

Measured sputter depth profiles of Ta/Si multilayers consisting of 10 alternating layers of Si (10.5nm thickness) and Ta (7.5nm thickness) obtained by sputtering with Cs+ ions of 1keV energy, impinging under an angle of 45°, were re-evaluated by application of the Mixing-Roughness-Information depth (MRI) model for profile reconstruction. To be able to perform the latter, a decrease of the sputtering rate with sputtered depth, a strong and sputtering time dependent matrix effect of the detected Si+ ion intensity as well as preferential sputtering of Si have to be taken into account. The results disclose a similar depth resolution for the Si and Ta layers composed of atomic mixing and roughness contributions. The depth resolution increases with the sputtered depth from 4.2 to 6.8nm at 50 and 300s sputtering time, respectively. Except for the first and the last two layers (Nos. 19 and 20), by application of the MRI model a full reconstruction of the measured profiles of the Si and Ta multilayer structure was obtained with a mean error of about ±5%.

High specific capacity retention of graphene/silicon nanosized sandwich structure fabricated by continuous electron beam evaporation as anode for lithium-ion batteries

A graphene/silicon (Si) multilayer sandwich structures are fabricated using electron beam (EB) deposition without air exposure. The graphene and Si thin films are formed on Cu current correctors through a continuous process in high-vacuum EB chamber. Synthesized graphene should be suggested to the stacked multiple layer from Raman analysis. The fabricated multilayer films are used as anodes. In the beginning, the half-cell, which used a seven-layer of each thickness 50-nm graphene and Si film, exhibits good specific capacity retention over 1000mAhg−1 after 30 charge/discharge cycles. The capacity value changed with the number of graphene and Si layers. In this study, the number of layers that exhibited optimal properties is seven. Morphological investigation showed a fine layer-by-layer structure. The relationship between different thicknesses of graphene and Si is investigated at 7L. A 100-nm thickness exhibited optimal properties. Finally, the optimal 7L and 100-nm thick graphene/Si exhibited high discharge capacitance >1600mAhg−1 at a current density of 100mAg−1 after 30 cycles. Initial coulombic and reversible efficiencies exceed 84%. The capacity retention (30th/1st discharge value) at 100nm and 7L exceeds 90%. Finally, the soft package battery is assembled by combining the fabricated graphene and Si electrode as anode, LiCoO2 as cathode, separator and liquid electrolyte. It can be used for commercial light-emitting diode (LED) lighting even under bending status.

Forced diffusion via electrically induced crystallization for fabricating ZnO–Ti–Si structures

Electrically induced crystallization (EIC) is a recently developed process for material modification. This study is applied to EIC to fabricate ZnO–Ti–Si multi-layer structures of various thicknesses to dope Ti into ZnO thin film and to form TiSix intermetallic compound (IMC) in a single step. The IMC layer was confirmed using transmission electron microscopy images. The Ti layer thickness was more than 40nm, which enhanced electron transmission and decreased the total electrical resistance in the structure. Finally, the diffusion mechanisms of EIC and the annealing process were investigated. This study shows that the EIC process has potential for industrial applications.

Structural, dielectric, ferroelectric and optical properties of PBCT, PBST and PCST complex thin films on LaNiO3 metallic conductive oxide layer coated Si substrates by the CSD technique

Ferroelectric thin films and LaNiO3 (LNO) metallic conductive oxide thin films were prepared by a chemical solution deposition (CSD) method. PBCT60, PBST60 and PCST60 ferroelectric thin films were grown on different structures such as LNO/Si and single-crystalline quartz SiO2 (X-cut) substrates. The LNO layer acts as the bottom electrode for the electrical measurements. X-ray diffraction (XRD) analysis shows that LNO thin films on Si substrates and PBCT60, PBST60 and PCST60 thin films on LNO/Si structures are polycrystalline with a moderate (110)-texture and a complete perovskite phase. LNO, PBCT60, PBST60 and PCST60 thin films have a continuous, dense and homogenous microstructure with a grain size on the order of 50–80nm. Electrical resistivity-dependence temperature data confirm that LNO thin films display a good metallic character over a wide large range of temperatures. Optical characteristics of PBCT60, PBST60 and PCST60 thin films have also been investigated using ultraviolet–visible (UV–vis) spectroscopy in the wavelength range of 200–1100nm. Ferroelectric thin films show a direct allowed optical transition with optical band gap values on the of order of 3.54, 3.66 and 3.89eV for PBCT60, PCST60 and PBST60 thin films deposited on a SiO2 substrate, respectively. Good dielectric and ferroelectric properties are reported for ferroelectric thin films deposited on the LNO layer as bottom electrodes. Au/PBCT60/LNO/Si, Au/PBST60/LNO/Si and Au/PCST60/LNO/Si multilayer structures show a hysteresis loop with remnant polarization, Pr, of 9.6, 6.6 and 4.2μC/cm2 at an applied voltage of 6V for PBCT60, PBST60 and PCST60 thin films, respectively.

Contribution of elastic photoelectron scattering to the shape of the measured XPS intensity in‐depth profile

The in‐depth profiles obtained from Auger electron spectroscopy or X‐ray photoelectron spectroscopy (XPS) experiments are known to differ from an actual concentration profiles due to limitations of surface‐sensitive techniques, e.g. finite sampling depth, or atomic mixing during sputtering. However, the monitored signal intensities are also affected by elastic scattering of signal electrons. Calculations performed for idealized multilayer structures indicate that elastic scattering of photoelectrons may significantly influence the in‐depth signal intensity. In the present work, an attempt is made to evaluate the contribution of elastic photoelectron scattering to measured signal intensity profile for samples with diffused interfaces. The in‐depth profile measured for the 5x(Au/Ni)/Si multilayer structure has been submitted to this analysis. A relatively simple procedure to evaluate the contribution of elastic scattering to the profile shape has been proposed. The effect of elastic scattering is found to be a reduction of the XPS intensity by 5–10% in the investigated cases. Copyright © 2014 John Wiley & Sons, Ltd.

Synchrotron study of the formation of nanoclusters in Al2O3/SiO x /Al2O3/SiO x /…/Si(100) multilayer nanostructures

Al2O3/SiO x /Al2O3/SiO x /…/Si(100) multilayer nanoperiodic structures (MNS) are studied by X-ray absorption near-edge structure spectroscopy (XANES). Experimental XANES spectroscopy spectra are obtained using synchrotron radiation. The formation of Si nanoclusters in the surface layers of the structures during their high-temperature annealing is observed. The structures featured intense size-dependent photoluminescence in the wavelength region near 800 nm. At the same time, it is shown that the formation of aluminum silicates is possible. The inversion effect of the intensity of the XANES spectra during the interaction of synchrotron radiation with MNSs is revealed.

Epitaxial Si and Gd2O3 Heterostructures - Distributed Bragg Reflectors with Stress Management Function for GaN on Si Light Emitting Devices

Tensile stress in GaN layers grown directly on Si is a serious obstacle for the implementation of this technology for electronic and photonic devices. The problem can be solved by stress engineering using epitaxial buffer layers grown on a Si-substrate. Heteroepitaxial Si and Gd2O3 multilayer structures that can be used both as a tensile strain compensating buffer for GaN epitaxial layers and an efficient reflector for light emitting devices are demonstrated in this work. A three-periods distributed Bragg reflector has been fabricated. It exhibits 82% reflectivity at the design wavelength of 450 nm. In situ curvature measurements of the 200 mm diameter wafers with the grown structure reveal compressive stress in the Gd2O3 -Si multilayer structure. The compressive stress compensates the tensile stress which arises during subsequent growth and cooling of the GaN layer.

Structural properties of subnanometer thick Y layers in extreme ultraviolet multilayer mirrors

We studied the structure and optical properties of B(4)C/Mo/Y/Si multilayer systems. Using extended x-ray absorption fine structure measurements at the Y and Mo K-edge, the structure of the subnanometer thick Y layer and the underlying Mo layer were analyzed. It was found that even a 0.2 nm thick Y layer significantly reduced silicon diffusion toward Mo, thus reducing Mo silicide formation. Hard x-ray reflectometry showed that the difference in average interface roughness of the B(4)C/Mo/Y/Si multilayer structure compared to Mo/Si and B(4)C/Mo/B(4)C/Si multilayer structures was negligible. Soft x-ray reflectometry showed optical improvement of B(4)C/Mo/Y/Si with respect to Mo/Si and B(4)C/Mo/B(4)C/Si multilayer structures.

Investigation of surface acoustic waves propagating in ZnO–SiO2–Si multilayer structure

The investigations of the propagation of the SAW in the multilayered structures have been of great interest since the combination of the surface acoustic wave (SAW) technology with the microelectromechanical system (MEMS) technology comes true. In this paper, the recursive asymptotic method (RAM) is applied to analysis the propagation of the SAW in the ZnO–SiO2–Si multilayered structure. The influences of the ZnO layer thickness and the SiO2 layer thickness to the phase velocity and the electro-mechanical coupling coefficient for the Rayleigh wave and Love wave are discussed. The Love mode wave is found to be predominantly generated since the c-axis of the ZnO film is generally perpendicular to the substrate. In order to prove the theoretical results, a series of Love mode SAW devices based on the ZnO–SiO2–Si multilayered structure is fabricated by micromachining, and their frequency responses are detected. The experimental results are found to be mainly consistent with the theoretical ones except the little larger velocities induced by the residual stresses produced in the fabrication process of the films. The deviation of the experimental results from the theoretical ones is reduced by thermal annealing.

Epitaxial Si and Gd2O3 Heterostructures: Distributed Bragg Reflectors with Stress Management Function for GaN on Si Light Emitting Devices

Tensile stress in GaN layers grown directly on Si is a serious obstacle for the implementation of this technology for electronic and photonic devices. The problem can be solved by stress engineering using epitaxial buffer layers grown on a Si-substrate. Heteroepitaxial Si and Gd2O3 multilayer structures that can be used both as a tensile strain compensating buffer for GaN epitaxial layers and an efficient reflector for light emitting devices are demonstrated in this work. A three-period distributed Bragg reflector has been fabricated. It exhibits 82% reflectivity at the design wavelength of 450 nm. In situ curvature measurements of the 200 mm diameter wafers with the grown structure reveal compressive stress in the Gd2O3 – Si multilayer structure. The compressive stress compensates the tensile stress which arises during subsequent growth and cooling of the GaN layer.