硅薄膜导热系数微尺度效应的临界尺寸

基于灰介质假设及拟合的色散关系,采用蒙特卡洛方法对微纳米硅薄膜内的声子热输运特性进行了模拟.将3μm厚的硅薄膜导热系数的计算结果与文献结果进行了对比,二者吻合较好.分析了厚度对薄膜内温度场及导热系数的影响.结果表明,当厚度小于某一尺度时,薄膜导热系数会随着厚度的减小而降低,呈现尺度效应;薄膜内温度场也不再是线性分布,而是在边界处呈现阶跃特性,且随着厚度减小,温度阶跃增大.基于该方法求得了100~400 K温度区间内,硅薄膜法向导热系数出现明显尺度效应的临界尺寸.计算发现,随着温度的升高,临界尺寸变化范围很大,平均温度为100 K时,临界尺寸约为50μm,平均温度为400 K时,临界尺寸约为2.5μm,前后相差了一个数量级.     

High speed scanning electron beam annealing of ion-implanted silicon layers

Sawtooth scanning with a 40 keV electron probe at frequencies of 10⁵–10⁶ Hz can be used for generating in a silicon surface a quasi-line-shaped temperature field. With a power of 70–80 W and a spot diameter of 50 μm this yields a surface temperature of about 1500 K. The procedure was tentatively used for solid state annealing of ⁷⁵As-ion-implanted silicon layers to a depth of 1000 Å. These beam parameters permit an areal throughput of 10 cm² s^-1 without back side cooling since the substrate temperatures at the back side can be kept below 800 K.     

Silicon coating treatment to improve high temperature corrosion resistance of 9%Cr steels

Abstract

A silicon coating process is described which confers good protection on 9% Cr steels and alloys in CO₂ based atmospheres at high temperatures and pressures. The coatings are formed by decomposition of silane at temperatures above 720 K. Protective layers are typically up to 1 μm thick. The optimum coating conditions are discussed. The chemical state of the coatings has been investigated by X-ray photoelectron spectroscopy and has demonstrated the importance of avoiding silicon oxide formation during processing. Corrosion testing has been carried out for extended periods, up to 20 000 h, at temperatures between 753 and 853 K, in a simulated advanced gas cooled reactor gas at 4 MPa pressure. Benefit factors of up to 60 times have been measured for 9% Cr steels. Even higher values have been measured for 9Cr–Fe binary alloy on which a 1 μm coating was sufficient to eliminate significant oxidation over 19 000 h except at the specimen edges. The mechanism of protection is discussed. It is suggested that a silicon surface coating for protecting steels from high temperature corrosion has some advantages over adding silicon to the bulk metal.

MST/1090     

Resistivity and photoconductivity of plasma-sprayed polycrystalline silicon

Polycrystalline silicon layers of area 8 cm² and thickness t ranging from 100 μm to 2 mm were prepared on alumina substrates by plasma spraying silicon powder. For t > 500 μm, these layers could be detached from the substrate. The conductivity could be made n or p type by in situ doping. The microstructure, impurity content and resistivity as a function of both phosphorus and boron doping concentrations were studied. The effect of heat treatment on the resistivity, Hall mobility and the photoconductivity is reported. The data are explained qualitatively on the basis of existing models of the transport behaviour of polycrystalline silicon.     

Fabrication of (TiB/Ti)-TiAl composites with a controlled laminated architecture and enhanced mechanical properties

The(TiB/Ti)-TiAl composites with a laminated structure composing of alternating TiB/Ti composite layers,α₂-Ti₃Al interfacial reaction layers of andγ-TiAl layers were successfully pre pared by spark plasma sintering of alternately stacked Tib₂/Ti powder layers and TiAl powder layers.And the influence of thickness ratio of Tib₂/Ti powder layers to TiAl powder layers on microstructure evolution and mechanical properties of the re sulting(TiB/Ti)-TiAl laminated composites were investigated systemically.The results showed that the thickening ofα₂-Ti₃Al layers which originated from the reaction of Ti and TiAl was significantly hindered by introducing Tib₂particles into starting Ti powders.As the thickness ratio of Tib₂/Ti powder layers to TiAl powder layers increased,the bending fracture strength and fracture toughness at room temperature of the final(TiB/Ti)-TiAl laminated composites were remarkably improved,especially for the(TiB/Ti)-TiAl composites prepared by Tib₂/Ti powder layers with thickness of 800μm and TiAl powder layers with thickness of 400μm,whose fracture toughness and bending strength were up to 51.2 MPa·m^1/2and 1456 MPa,respectively,293%and 108%higher than that of the monolithic TiAl alloys in the present work.This was attributed to the addition of high-performance network TiB/Ti composite layers.Moreover,it was noteworthy that the ultimate tensile strength at 700℃of(TiB/Ti)-TiAl composites fabricated by 400μm thick Tib₂/Ti powder layers and 400μm thick TiAl powder layers was as high as that at 550℃of network TiB/Ti composites.This means the service temperature of(TiB/Ti)-TiAl laminated composites was likely raised by 150℃,meanwhile a good combination of high strength and high toughness at ambient tempe rature could be maintained.Finally,the fracture mechanism of(TiB/Ti)-TiAl laminated composites was proposed.     

阳极氧化电流密度和时间对多孔硅形貌和电子发射特性的影响

为了应用于场发射显示器,采用电化学阳极氧化,快速热氧化和磁控溅射等方法制备出了金属/多孔硅/硅基底/金属结构的多孔硅电子发射体,并运用扫描电子显微镜观察了多孔硅的微观形貌,结果发现多孔硅的孔径随着电化学阳极氧化电流密度的增加而增加,多孔硅层的厚度随着阳极氧化电流密度和时间乘积的增加而增加。在真空系统中测量了多孔硅的电子发射特性,电子发射的阈值电压Vth随着多孔硅层厚度的增加而增加;最大的发射效率η为7.5‰,此效率出现在孔径6~16 nm,多孔硅层厚度为11.06μm的样品中,对应的器件电压Vps为30V。     

Study of a solid state microdosemeter based on a monolithic silicon telescope: irradiations with low-energy neutrons and direct comparison with a cylindrical TEPC

A silicon device based on the monolithic silicon telescope technology coupled to a tissue-equivalent converter was proposed and investigated for solid state microdosimetry. The detector is constituted by a ΔE stage about 2 μm in thickness geometrically segmented in a matrix of micrometric diodes and a residual-energy measurement stage E about 500 μm in thickness. Each thin diode has a cylindrical sensitive volume 9 μm in nominal diameter, similar to that of a cylindrical tissue-equivalent proportional counter (TEPC). The silicon device and a cylindrical TEPC were irradiated in the same experimental conditions with quasi-monoenergetic neutrons of energy between 0.64 and 2.3 MeV at the INFN-Legnaro National Laboratories (LNL-INFN, Legnaro, Italy). The aim was to study the capability of the silicon-based system of reproducing microdosimetric spectra similar to those measured by a reference microdosemeter. The TEPC was set in order to simulate a tissue site about 2 μm in diameter. The spectra of the energy imparted to the ?E stage of the silicon telescope were corrected for tissue-equivalence through an optimized procedure that exploits the information from the residual energy measurement stage E. A geometrical correction based on parametric criteria for shape-equivalence was also applied. The agreement between the dose distributions of lineal energy and the corresponding mean values is satisfactory at each neutron energy considered.     

R.f.-sputtered SiC coatings on carbon fibres

R.f. sputtering as a new method for the deposition of SiC layers onto carbon fibre substrates was applied at temperatures below 400°C and deposition rates higher than 1 μm h^-1. Optimum conditions for high quality SiC films were selected by variation in the r.f. power, r.f. peak voltage, substrate temperature and gas pressure. The SiC layers were characterized using electron probe microanalysis, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, transmission high energy electron diffraction, IR and UV transmission spectroscopy as well as Vickers' hardness measurements. Amorphous SiC layers were obtained. In the case of discontinuous SiC deposition onto carbon monofilaments the film thickness precalculated from the sputtering parameters was achieved, whereas the continuous process results in film thickness of about one-third of the precalculated value. When fibre bundles with different numbers of monofilaments were used no influence on the resulting SiC layer thickness could be observed. Increasing the SiC film thickness, however, led to a strong decrease in the fibre tensile strength for layers more than 50 nm thick.     

Large diameter radial semiconductor drift chambers for low noise radiation detection

A semiconductor drift chamber with cylindrical symmetry has been investigated theoretically for its suitability as a low noise radiation detector. The signal/noise of a silicon drift chamber at room temperature is likely to be determined by the detector dark current and the drift chamber should be superior in performance to a conventional pn junction detector of equivalent dimensions. A device of 48 mm diameter and 300 μm thickness is predicted to be capable of an energy resolution of less than 5 keV fwhm at 300 K. This is about seven times better than that predicted for its conventional counterpart.     

Silicon micromechanical structures fabricated by electrochemical process

Silicon micromechanical structures were fabricated by means of sacrificial layers defined with porous silicon and masked by hydrogen ion implantation with adequate thermal annealing. The fabrication process to remove the porous silicon layers with diluted potassium hydroxide at room temperature does not cause damage to the remaining silicon microstructures, which are less than 1 /spl mu/m in thickness controlled by process parameters.     

Growth conditions and properties of evaporated semicrystalline silicon layers

Semicrystalline silicon layers 50–100 μm thick were vacuum deposited onto stainless steel and Pyrex glass substrates using an electron beam evaporation process. Because of the ultrahigh deposition rate (20–50 μm min^-1) the silicon layers had a columnar structure on substrates that had been previously coated with aluminium.The layers on stainless steel contained both grain boundaries and twin planes whereas the layers on Pyrex glass showed only twin planes. This difference in structure can be explained by considering the crystallinity of the substrates and the difference in surface temperatures during growth due to the different thermal properties of stainless steel and Pyrex glass.     

Dielectric breakdown of reactively sputtered silicon nitride

Silicon nitride layers on silicon substrates were prepared by reactive sputtering of silicon in nitrogen under conditions which led to layers of maximum dielectric strenght. Dielectric breakdown and its dependence on temperature, pulse width and layer thickness were investigated. It is shown that conversion from thermal to electronic breakdown can be achieved at low temperatures and short pulse widths. This enables the thickness dependence of breakdown to be observed. From this dependence the mean free time of carriers was found to be 1.1×10^-15 sec.     

Formation of carbide layers on AISI H13 and D2 steels by treatment in molten borax containing dissolved both Fe–Nb and Fe–Ti powders

In this work, AISI H13 and D2 tool steels were treated in molten borax, containing dissolved ferro-niobium, ferro-titanium and aluminum, at 1020 °C for 4 h. Samples were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD) and Vickers microhardness. Well defined layers were obtained with excellent thickness regularity. For the AISI H13 steel the layer measured 9 μm and for the AISI D2 steel the layer thickness was 18 μm. Their microhardness values were at about 2600 HV_0.050. The layers consisted of niobium carbide according to XRD analysis. EDS results showed the predominance of niobium and absence of iron in the layers on both steels. The presence of titanium was detected, just in small amounts, in the region of the layer next to substrate.     

钽表面的甲烷等离子渗碳改性技术研究

为了增强钽表面的高温抗氧化、抗腐蚀性能, 使用基于空心阴极效应的离子渗碳法, 以氩气和甲烷作为渗碳气体对钽片表面进行渗碳实验, 利用X射线衍射、扫描电镜、俄歇电子能谱分别对改性层进行成分、形貌及元素化学状态等分析。实验结果表明, 在渗碳温度1300℃, 渗碳时间20 min的条件下可以得到由Ta C与Ta2C两相组成、厚度约为6μm的渗碳层, 其中表层Ta C厚度约为3.5μm, 过渡层中碳含量呈梯度分布。渗碳后样品改性层表面变得致密并消除了基体的孔洞缺陷。碳与钽的俄歇峰位变化说明渗碳过程中碳元素进入了样品表面并与钽结合, 渗碳过程中表层组分逐渐由金属钽向Ta C转变。     

Electrical conduction in solution-grown polycarbonate films

The electrical conduction in solution-grown polycarbonate films was studied at different voltages between 1 and 10⁴V as a function of thickness from 25 to 65 μm at a constant temperature of about 385 K and as a function of temperature in the range 328–430 K at a constant thickness of about 50 μm. The results show two distinct regions corresponding to different types of conduction: region I at low or moderate fields where the slope of the current-voltage characteristics is about 2 and region II at high fields where the slope is about 4. The Richardson-Schottky effect was suggested as the dominant mode of conduction at moderate fields whereas at high fields the currents were found to be space charge limited.     

Thermal strain in lead thin films II: strain relaxation mechanisms

A deformation mechanism map was constructed to study the mechanisms of strain relaxation in lead thin films which were deposited on oxidized silicon wafers at room temperature and which were then thermally cycled between room temperature and liquid helium temperature. The stress level, which was calculated from the strain measured by an X-ray diffraction technique, was plotted on the map. By comparing the calculated and experimental stress levels the following observations were obtained. In the cooling process the strain was relaxed rapidly in a field of dislocation glide mechanism for films of greater than 0.2 μm thickness. In the heating process most of the strain was again believed to be relaxed by the glide mechanism. For a film 0.5 μm thick the stress (after the primary relaxation was completed) was found to be (1–1.5) × 10⁹ dyn cm^-2 for the cooling process and (0.17–0.24) × 10⁹ dyn cm^-2 for the heating process at temperatures around 200–280 K. Slow secondary relaxations were observed after the primary relaxations were completed. The measured compressive strain relaxation rate at room temperature was very close to the rate calculated on the assumption of grain boundary diffusion creep. This suggested that the secondary relaxation mechanism of compressive strain was grain boundary diffusion creep at temperatures near room temperature. These suggestions were supported by scanning electron microscopy observations in which dislocation slip lines were observed inside grains and hillocks were observed on grain boundaries.     

Improvement of electro-physical properties of ultra-thin PECVD silicon oxynitride layers by high-temperature annealing

Ultra-thin (5 and 6 nm) silicon oxynitride layers have been fabricated by the plasma-enhanced chemical vapour deposition (PECVD) process. Split experiments with annealing of the deposited dielectric layers were performed using the RTP reactor and a standard furnace, both at 900 °C. Possible changes in properties, structure and chemical composition of the obtained layers were investigated by means of spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and electrical characterisation of manufactured test structures (metal-insulator-semiconductor (MIS) capacitors and MISFETs). The results achieved have shown that annealing at high temperature causes improvement of the properties of ultra-thin silicon oxynitride layers (e.g. lower interface traps density, lower leakage currents within the dielectric layer and lower charge-pumping currents of the MISFETs). The observed improvement in electro-physical properties can be attributed to the increase of the SiON phase. Moreover, comparison between the physical thickness and the equivalent oxide thickness (EOT) of the layers shows a decrease in physical thickness obtained by using the silicon oxynitride layer instead of the classical silicon dioxide. These findings are important for the consideration of chances of PECVD oxynitride layer application for CMOS technology.     

Laser generation in microdisc resonators with InAs/GaAs quantum dots transferred on a silicon substrate

Microdisc resonators based on InAs/GaAs quantum dots separated from a GaAs substrate by selective etching and fixed to a silicon substrate by epoxy glue are studied using luminescence spectroscopy. A disc resonator 6 μm in diameter exhibits quasi-single-mode laser generation at a temperature of 78 K with a threshold power of 320 μW and λ/Δλ ～ 27000.     

Experimental study of superconducting transition in a molybdenum–copper thin film structure showing the proximity phenomenon and the estimation of the sensitivity of TES bolometers on the basis of such a structure

Mo/Cu bi-layers with different layer thickness values showing superconducting transitions in the temperature range 0.08–1.0 K have been experimentally studied. The temperature of the superconducting transition can be adjusted to the stable temperatures of refrigerators used at the space radio telescopes using the proximity phenomenon by varying the thickness of Mo and Cu layers. The estimation of the noise equivalent power of TES bolometers on the basis of such structures using measured dependences of Mo/Cu structure resistances on temperature and electron energy balance equation is fulfilled.