Stress In SiO2 Research Articles

Understanding and Control of Stress at Si-SiO<sub>2</sub> Interface

This paper presents the results of the investigation of stress relaxation at the Si-SiO2 interface using electron paramagnetic resonance (EPR) spectra, scanning electron microscopy (SEM) image technique, deflection analysis, X-ray photoelectron spectroscopy (XPS) and C-V characterisation of metal oxide semiconductor (MOS) structures. It has been shown, on the basis of EPR, XPS, C-V, and deflection data, that the mechanism of stress relaxation depends on the oxidation conditions: temperature, cooling rate and oxide thickness. In the Si-SiO2-Si3N4 system the stress relaxation occurs due to the difference in the thermal expansion coefficient of SiO2 and Si3N4 material. With an appropriate choice of oxidation conditions compressive stresses in SiO2 and tensile stresses in Si are almost equal and stress can be reduced considerably at the interface.

Oxidation induced stress in SiO2/SiC structures

Physical stress in SiO2/SiC stacks formed by the thermal oxidation of SiC is studied experimentally through both room temperature ex-situ and variable temperature (25–1150 °C) in-situ investigations. Mechanisms giving rise to the stress are a thermal component, associated with differences in thermal expansion coefficients of the oxide and the substrate, and an intrinsic component associated with the different atomic densities and structure of the film and substrate. Ex-situ results show a ∼108 Pa compressive stress in the SiO2 film in a SiO2/SiC stack with a strong crystal face dependence (C face(000ī) and Si face (0001)) and processing (temperature, growth rate) dependence. Real-time stress determination demonstrates that at temperatures above ∼900 °C, the total intrinsic stress and a portion of the thermal stress may be relieved. On the basis of these findings, a viscous model is proposed to discuss the stress relaxation.

Crack suppression of silica glass formed by zoned F2 laser-induced photochemical surface modification of hard silicone thin film coating on polycarbonate

The surface layer of a hard silicone thin film coating on polycarbonate was modified to silica glass (SiO2) through F2-laser-induced photochemical reactions. To obtain samples with higher abrasion resistances, SiO2 films of 1 µm thickness and over were successfully formed without cracking, by zoning the laser-irradiated area of micrometer order. With the conversion of silicone to SiO2, the volumetric shrinkage of the sample was induced, which simply depended on the number of photons, by varying the single-pulse fluence and irradiation time of a F2 laser. The ratio of volumetric shrinkage to the original silicone was estimated to be approximately 0.85, generating tensile stress in SiO2. The stress could be suppressed to be lower than 48 MPa for typical SiO2 by reducing the laser-irradiated area to be of micrometer order. Also, when the length of one side of the irradiated area is 1 mm, the thickness of the SiO2 film is expected to increase to approximately 5 µm.

Reduction of residual stress in SiO2‐matrix silicon nano‐crystal thin films by a combination of rapid thermal annealing and tube‐furnace annealing

AbstractIn this work, SRO thin films were prepared by magnetron sputtering and converted to Si nano‐crystals (Si‐NCs) by rapid thermal annealing (RTA). Further tube‐furnace annealing was performed on the Si‐NC films in order to reduce residual stress. The residual stress in the Si‐NC thin films after RTA at 1100 and 1200 °C were determined as 2.41 and 1.87 GPa, respectively, and the following tube‐furnace annealing can obviously decrease the residual stress. The residual stress decreased with increasing annealing temperature and time. The following tube‐furnace annealing at 1100 °C for 60 min and 1000 °C for 180 min reduced the residual stress to 1.46 and 1 GPa, respectively, for the Si‐NC thin films with prior RTA at 1200 °C. The reduction of residual stress was ascribed to the larger density of extended defects and the lower surface‐to‐volume ratio of the thin films.

Effect of ion-beam assisted deposition on the film stresses of TiO2 and SiO2 and stress control

Based on Hartmann-Shack sensor technique, an online thin film stress measuring system was introduced to measure the film stresses of TiO2 and SiO2, and comparison was made between the film stresses prepared respectively by the conventional process and the ion-beam assisted deposition. The effect of ion-beam assisted deposition on the film stresses of TiO2 and SiO2 was investigated in details, and the stress control methodologies using on-line adjustment and film doping were put forward. The results show that the film stress value of TiO2 prepared by ion-beam assisted deposition is 40 MPa lower than that prepared by conventional process, and the stress of TiO2 film changes gradually from tensile stress into compressive stress with increasing ion energy; while the film stress of SiO2 is a tensile stress under ion-beam assisted deposition because of the ion-beam sputtering effect, and the film refractive index decreases with increasing ion energy. A dynamic film stress control can be achieved through in-situ adjustment of the processing parameters based on the online film stress measuring technique, and the intrinsic stress of film can be effectively changed through film doping.

Growth stress in SiO2 during oxidation of SiC fibers

A method to calculate growth stress in SiO2 scales formed during SiC fiber oxidation was developed. Calculations were done for Hi-NicalonTM-S SiC fibers using previously measured Deal-Grove oxidation kinetics parameters. Initial compressive stresses in SiO2 of ∼25 GPa from the 2.2× oxidation volume expansion are rapidly relaxed to lower levels by flow of silica with a shear-stress dependent viscosity. At >1200°, viscous flow of silica relaxes stress to negligible levels. At 700° – 900 °C, compressive axial and hoop stress at the GPa level persist, but radial stresses are much smaller. The decrease in growth stress with increase in temperature is a consequence of larger activation energy for silica viscosity than for oxidation kinetics. Radial expansion of the outer scale eventually causes hoop stress to become tensile in the outer scale, and axial stress becomes tensile from the Poisson effect. Tensile hoop stresses can be >1 GPa for thick scales formed at <1000 °C. Approximate analytical expressions for growth stress for some limiting cases are discussed. Growth stresses were also calculated for crystallized SiO2 scales; these were qualitatively consistent with microstructural evidence of stress. Assumptions and limitations of the calculation method are discussed, along with the possible effects of growth stress on SiC oxidation kinetics and on SiC fiber strength.

Growth Stress in SiO2 Formed by Oxidation of SiC

ABSTRACTGrowth stresses in amorphous SiO2 scales formed during SiC fiber oxidation were calculated. A numerical method using Deal-Grove oxidation kinetics and shear-stress dependent SiO2 viscosity was used. Initial compressive stresses in SiO2 of ∼25 GPa from the 2.2× oxidation volume expansion rapidly relaxes. At >1200°C, viscous flow of amorphous SiO2 further relaxes stress to negligible levels. At 700° - 900°C, axial and hoop stress at the GPa level persist in SiO2 near the SiC-SiO2 interface. Radial expansion of the scale causes hoop stress to become tensile, and axial stresses are driven to tensile values by the Poisson effect. These tensile stresses can be >1 GPa for thick scales formed at lower temperatures on surfaces with high curvature. Approximate analytical expressions for growth stress are discussed. Effects of viscosity variation as well as other assumptions and limitations of the calculation method are discussed.

Interface States in Hf-Based Stacks and their Impact on the Reliability of these Alternative Oxides

This paper investigates interface states in Hf-based devices with metal gate. By electrical measurements we first show that the interface defects of HfO2 stacks are very similar to those of conventional SiO2 oxides. To passivate these defects forming gas anneals FGA under N2/H2 can be performed at high temperature T>500{degree sign}C in Hf-based oxide. We also propose for the first time the use of an atomic H0 plasma performed at low temperature T~300{degree sign}C to passivate these interface defects. Finally we investigate the impact of the interface states on the reliability of High-K dielectrics. The interface state generation in HfO2 oxides annealed by FGA is similar to the one induced by NBTI stress in SiO2. On the other hand, in H0 annealed samples, degradation is enhanced due to plasma charging in the oxide. But some of the interface degradations induced by PID can be cured by standard thermal treatments.

Alteration of internal stresses in SiO 2/Cu/TiN thin films by X-ray and synchrotron radiation due to heat treatment

A break of wiring by stress-migration becomes a problem with an integrated circuit such as LSI. The present study investigates residual stress in SiO 2/Cu/TiN film deposited on glass substrates. A TiN layer, as an undercoat, was first deposited on the substrate by arc ion plating and then Cu and SiO 2 layers were deposited by plasma coating. The crystal structure and the residual stress in the deposited multi-layer film were investigated using in-lab. X-ray equipment and a synchrotron radiation device that emits ultra-high-intensity X-rays. It was found that the SiO 2 film was amorphous and both the Cu and TiN films had a strong {1 1 1} orientation. The Cu and TiN layers in the multi thick (Cu and TiN:1.0 μm)-layer film and multi thin (0.1 μm)-layer film exhibited tensile residual stresses. Both tensile residual stresses in the multi thin-layer film are larger than the multi thick-layer film. After annealing at 400 °C, these tensile residual stresses in both the films increased with increasing the annealing temperature. Surface swelling formations, such as bubbles were observed in the multi thick-layer film. However, in the case of the multi thin-layer films, there was no change in the surface morphology following heat-treatment.

Stress map for ion irradiation: Depth-resolved dynamic competition between radiation-induced viscoelastic phenomena in SiO2

The dynamic competition between structural transformation, Newtonian viscous flow, and anisotropic strain generation during ion irradiation of SiO2, leads to strongly depth-dependent evolution of the mechanical stress, ranging between compressive and tensile. From independent in situ stress measurements during irradiation, generic expressions are derived of the nuclear stopping dependence of both the structural transformation rate and the radiation-induced viscosity. Using these data we introduce and demonstrate the concept of a “stress map” that predicts the depth-resolved saturation stress in SiO2 for any irradiation up to several MeV.

Improved interatomic potential for stressed Si, O mixed systems

We propose an improved formula of a previous interatomic potential for Si, O mixed systems. The new potential is designed so as to more accurately reproduce the structural property of compressively strained SiO 2 structures, by reducing unnatural steric hindrance caused by a long-range part of a three-body term. As the results of the improvement, (1) compressive stress in SiO 2 film, which was highly overestimated to be 13 GPa by the earlier potential, is reduced to 2.7 GPa, and (2) a spurious peak in Si–O pair correlation function of SiO 2 film disappeared. A limitation of the conventional interatomic potentials and its solution are also discussed.

Optical properties and mechanical stress in SiO 2/Nb 2O 5 multilayers

The aim of this work is the formation of stress-controlled reflection layers which are suitable for application on micromechanical elements without bending them out of shape. Thin films of Nb 2O 5 and SiO 2 have been deposited by magnetron sputtering of an oxide target (SiO 2) and reactive sputtering of a niobium target in an oxygen containing atmosphere, respectively. Silicon and niobium oxide had been selected on the basis of preliminary experiments out of nine different thin film oxides since they yielded sufficiently low absorption coefficients of magnetron sputtered films. Both optical parameters and film stress have been investigated in dependence on deposition parameters. In particular, by varying the substrate bias voltage (for Nb 2O 5) and the sputtering pressure (for SiO 2) the film stress could be influenced to a large extent without deterioration of the optical properties. In Nb 2O 5, the stress could be varied between compressive and zero stress, whereas in SiO 2 compressive stress was always obtained. Hence, a complete stress-compensation in the multilayers necessitates the application of an additional underlying metal layer having tensile stress. We found magnetron-sputtered chromium films most suitable for that purpose since they provided the highest line stress in comparison to their additional mass per area. To demonstrate our approach we present as an example a layer stack of six SiO 2 and six Nb 2O 5 films which were designed for maximum reflection at the wavelengths 446, 532 and 629 nm. The film stack was formed after a chromium film (nominal thickness 143 nm) was deposited onto a thin (thickness 30 μm) single-crystal silicon mirror plate. Reflectivity at the wavelengths given above was between 96 and 98%. The multilayer was highly stress-compensated with a typical residual bow of 0.4 μm for a 3.5×3.5 mm 2 mirror.

Mechanical Stresses in SiO2 of Polycrystalline Si

Mechanical Stresses in SiO2 of Polycrystalline Si

Measurements of residual stresses in thin films deposited on silicon wafers by indentation fracture

A semi-empirical formula is proposed to assess residual stresses in SiO 2 and Cr thin films deposited on Si wafers using Vickers indentation fracture tests. The formula indicates that the ratio of the indentation load to the cubic of square root of the crack length is linearly proportional to the reciprocal of the square root of the crack length, the magnitude of the residual stress, and the film thickness. Wafer curvature measurements are conducted to calibrate the dimensionless parameters in the proposed formula. The experimental results agree well with the theoretical prediction and the residual stresses in the SiO 2 and Cr thin films are evaluated to be −358 and 1095 MPa, respectively.

Effect of Growth Conditions on the Reliability of Ultrathin MOS Gate Oxides

AbstractIn this work we demonstrate that in MOS devices the reliability of ultrathin (< 100Å) gate oxide is a strong function of growth conditions, such as, temperature and the growth rate. In addition, for constant current gate injection the degradation of SiO2 is enhanced as the thickness is reduced. We attribute this to physical stress in SiO2 resulting from the growth process. The degradation is always more for those growth conditions which result in higher physical stress in SiO2. Higher temperatures and slower oxidation rates allow stress relaxation through viscous flow and hence result in SiO2 of better reliability. We also found that for constant current stressing, the interface damage is more at the collecting electrode than at the injecting electrode. ΔDit (stress induced interface state generation) can be reduced after a high temperature Ar post anneal after the gate oxide growth.

Determination of thickness, refractive indices, optical anisotropy of, and stresses in SiO 2 films on silicon wafers

In a previous paper we demonstrated a simple yet accurate method for determining the thickness and refractive index of thin SiO 2 films on silicon substrates, the only equipment required being an Abbe refractometer. The films were assumed to be isotropic. In the present communication we show that the method described, used with s- and p-polarized light, also permits to determine the optical anisotropy of the films which are optically uniaxial with their optic axis oriented normally to the film. The measured ordinary and extraordinary refractive indices of 2–10 μm thick SiO 2 films are reported. The anisotropy is induced photoelastically by compressive stresses in the SiO 2 films. From the measured anisotropy the stresses are determined to be σ ‖∑−290 MPa.

Stress Compensation Techniques in Thin Layers Applied to Silicon Micromachining

ABSTRACTStress in thin films is one of the mayor technological problems of silicon micromachining. Therefore stress reduction by a compensation method increases the technological use of a material. Three methods to obtain stress free films are investigated:1. Stress in SiO2 and Si3N4 can be compensated by ion implantation.2. Sandwich layers of SiO2 and Si3N4 can be made internally stress compensated by variation of the thickness ratio.3. Polysilicon is stress reduced by appropriate doping and annealing.The film stress is evaluated by the measurement of the bending of the substrate. Examples of processed membranes, bridges and cantilevers made of stress compensated films are given.

Study of mechanical strains in implanted silicon slices

In this paper the influence of Ar + and He + ion bombardment on mechanical stress formation in silicon and SiSiO 2 structures is studied. Ion bombardment was carried out at energies of 50–175 keV, ion flux densities of 0.1–10 μA cm −2 and fluences up to 10 17 cm −2. Double-beam laser interferometry was used for contactless registration with high accuracy of deformation of the samples during the process of ion implantation. We obtained that the stresses induced by ion implantation in silicon are a complex function of ion kind, fluence and flux density. The sign of strains for the oxidized slices is opposite to that for the unoxidized ones. Large deformations are obtained during He + implantation in the oxidized samples. The experimental results are directly dependent on ion damage level. The formation of mechanical stresses in SiO 2 is related to the amount of SiO and SiSi bonds which changes under ion bombardment.

Strain-induced anisotropy measurement in oxide films grown on silicon

A multiangle single-wavelength ellipsometric method has been applied to the evaluation of anisotropy and stress in SiO(2) films grown on silicon substrate. The method is nondestructive, and it does not require a priori knowledge of the dielectric functions of the oxide layer. A novel theoretical anisotropic multiple-layer program is used to calculate the Fresnel reflection coefficients at various angles of incidence; and an iterative least-squares algorithm compares the theoretical calculations to the experimental data. The data-fitting process yields the index, the thickness, and the anisotropy of the oxide layer and also the index and thickness of the SiO(x) transition layer.