减少聚合物阵列波导光栅损耗的蒸气回溶技术

阵列波导光栅(AWG)器件是波分复用(WDM)系统的一种关键器件,其中,聚合物阵列波导光栅由于其制备工艺、器件集成等方面的优势而受到人们的日益关注。侧壁散射损耗是聚合物阵列波导光栅损耗的一个主要因素,减少阵列波导光栅波导的侧壁损耗对制备低损耗阵列波导光栅具有重要意义。一种蒸气回溶技术被用来有效地减少硅基聚合物阵列波导光栅的散射损耗,该技术的机理是饱和溶剂分子融入并软化波导侧壁,增加其流动性,从而降低波导侧壁粗糙度。用扫描电镜方法验证了用该技术能获得更光滑的波导侧壁。对直波导和阵列波导光栅样品进行回溶处理,测试后得到直波导的侧壁散射损耗减少2.1 dB/cm,阵列波导光栅中心信道和周边信道的插入损耗分别减少5.5 dB和6.7 dB,串扰减少2.5 dB。     

Particle‐in‐cell simulation of the effect of curved magnetic field on wall bombardment and erosion in a hall thruster

A curved, convex towards the channel bottom magnetic field is an important feature of an advanced Hall thruster that allows confining the plasma flow in the channel center, reducing the divergence angle of the ejected ion beam, and improving the discharge performance. In this article, the discharge behaviour of a Hall thruster in magnetic fields with different degrees of curvature is simulated with a particle‐in‐cell numerical method, and the effect of curved magnetic field on the ion bombardment and wall erosion and the associated mechanisms are studied and analysed. The results show that, as the curvature of the magnetic field increases, the propellant ionization becomes more confined at the channel center, the potential drop inside the channel decreases, and the acceleration region shifts outside the channel, which lead to the attenuation of the ion energy bombarding the wall and the deviation of the bombardment angle from the optimal sputtering angle. Conversely, the ion flux bombarding the wall near the channel exit increases. Nevertheless, the bombardment energy and angle are the dominant factors for the wall erosion, and the wall erosion rate clearly decreases with the increasing curvature of the magnetic field. These findings are closely related to the behaviour of electron conduction under a curved magnetic field; the relevant mechanisms are clarified in this article.     

提高离子束刻蚀亚微米光栅侧壁陡直度的方法

现代亚微米光栅的应用通常要求栅脊侧壁陡直。通过比较两种配备不同离子源的刻蚀机的反应离子束刻蚀结果,认为影响亚微米光栅侧壁陡直度的一个重要因素是离子束发散角(束散角),且小束散角有利于获得陡直的光栅侧壁。国内应用最广泛的双栅考夫曼刻蚀机束散角较大(大于13°),致使用常规方法获得的熔石英光栅的侧壁倾角仅为77°。针对此刻蚀机,尝试了三种提高侧壁陡直度的方法:旋转倾斜刻蚀法、交替倾斜刻蚀法和二次金属掩模法,分别把侧壁倾角提高到86°、86°和82°。最后从掩模侧壁收缩速率和槽底部与顶部离子通量的差异对束散角对侧壁陡直度的影响给予解释,并说明了上述三种方法的工作机理。     

In situ endpoint detection of reactive ion-beam etching of dielectric gratings with an etch-stop layer using downstream mass spectrometry

Downstream mass spectrometry is successfully used in the reactive ion-beam etching of dielectric diffraction gratings of deep grooves with vertical sidewalls to achieve in situ endpoint detection. Silica (SiO₂) gratings with a Sc₂O₃ etch-stop layer are fabricated by reactive ion-beam etching with CHF₃ as etchant, and the mass spectrometric signal of SiF₃⁺ produced by the reactive etching of the SiO₂ grating material is monitored. When the etch-stop layer is reached, a drop of this signal occurs. By comparing the monitoring curves and resulting gratings of different etching methods, we find that the decrease of the monitored signal is strongly influenced by the sidewall steepness of the etched grating grooves. All conditions being equal, the greater sidewall steepness renders the faster decrease of the signal. Consequently, the proposed approach of endpoint detection applies well to the gratings with steep sidewalls. With the help of two previously developed methods, the sidewall steepness of grating grooves is increased, and the optimal endpoint is detected. Employing the proposed technique, we have reproducibly fabricated dielectric gratings with proper groove depth and even groove bottom.     

Foams in contact with solid boundaries: Equilibrium conditions and conformal invariance

A liquid foam in contact with a solid surface forms a two-dimensional foam on the surface. We derive the equilibrium equations for this 2D foam when the solid surface is curved and smooth, generalising the standard case of flat Hele-Shaw cells. The equilibrium conditions at the vertices in 2D, at the edges in 3D, are invariant by conformal transformations. Regarding the films, conformal invariance only holds with restrictions, which we explicit for 3D and flat 2D foams. Considering foams confined in thin interstices between two non-parallel plates, normal incidence and Laplace’s law lead to an approximate equation relating the plate profile to the conformal map. Solutions are given for the logarithm and power laws in the case of constant pressure. The paper concludes on a comparison with available experimental data.     

Structure and stability of annular sheared channel flows: effects of confinement,curvature and inertial forces – waves

The structure and stability of the flows in an annular channel sheared by a rotating lid are investigated experimentally, theoretically and numerically. The channel has a square section, and a small curvature parameter: the ratio Γ of the inter-radii to the mean radius is 9.5%. The sidewalls and the bottom of the channel are integral and can rotate independently of the lid, permitting pure shear, co-rotation and counter-rotation cases. The basic flows obtained at small shear are characterized. In the absence of co-rotation, the centrifugal force linked with the curvature of the system plays an important role, whereas, when co-rotation is fast, the Coriolis force dominates. These basic flows undergo some instabilities when the shear is increased. These instabilities lead to supercritical traveling waves in the pure shear and co-rotation cases, but to weak turbulence in the counter-rotation case. The Reynolds number for the onset of instabilities, constructed with the velocity difference between the lid and bottom at mid-radius, and the height of the channel, increases from 1000 in the counter-rotation case to 1260 in the pure shear case and higher and higher values when co-rotation increases, i.e., when the Coriolis effect increases. The relevance of uni-dimensional Ginzburg-Landau models to describe the dynamics of the waves is studied. The domain of validity of these models turns out to be quite narrow.     

Hydrodynamic instability of premixed flame propagating in narrow planar channel in the presence of gas flow

The paper analyses the hydrodynamic instability of a flame propagating in the space between two parallel plates in the presence of gas flow. The linear analysis was performed in the framework of a two-dimensional model that describes the averaged gas flow in the space between the plates and the perturbations development of two-dimensional combustion wave. The model includes the parametric dependences of the flame front propagation velocity on its local curvature and on the combustible gas velocity averaged along the height of the channel. It is assumed that the viscous gas flow changes the surface area of the flame front and thereby affects the propagation velocity of the two-dimensional combustion wave. In the absence of the influence of the channel walls on the gas flow, the model transforms into the Darrieus–Landau model of flame hydrodynamic instability. The dependences of the instability growth rate on the wave vector of disturbances, the velocity of the unperturbed gas flow, the viscous friction coefficients and other parameters of the problem are obtained. It is shown that the viscous gas flow in the channel can lead, in some cases, to a significant increase in instability compared with a flame propagating in free space. In particular, the instability increment depends on the direction of the gas flow with respect direction of the flame propagation. In the case when the gas flow moves in the opposite direction to the direction of the flame propagation, the pulsating instability can appear.     

Mechanical instabilities of bubble clusters between parallel walls

We have carried out a systematic study of buckling-like mechanical instabilities in simple two- (2D) and three-dimensional (3D) symmetric foam clusters sandwiched between parallel planar walls. These instabilities occur when the wall separation w is reduced below a critical value, w^*, for which the foam surface energy E reaches its minimum, E^*. The clusters under investigation consist of either a single bubble, or of twin bubbles of fixed equal sizes (areas A in 2D or volumes V in 3D), which are either free to slide or pinned at the confining walls. We have numerically obtained w^* for both free and pinned 2D and 3D clusters. Furthermore, we have calculated the buckled configurations of 2D twin bubbles, either free or pinned, and of 3D free twin bubbles, whose energy is independent of w and equal to the minimum energy E^* of the unbuckled state. Finally, we have also predicted the critical w_t^* at which the terminal configurations under extension of 2D and 3D single and twin bubbles are realised. Experimental illustrations of these transitions under compression and extension are presented. Our results, together with others from the literature, suggest that a bubble cluster bounded by two parallel walls is stable only if the normal force it exerts on the walls is attractive, i.e., if dE/dw > 0; clusters that cause repulsion between the walls are unstable. We correlate this with the distribution of film orientations: films in a stable cluster cannot be too parallel to the confining walls; rather, their average tilt must be larger than for a random distribution of film orientations.     

Quasi-one-dimensional foam drainage

Foam drainage is considered in a froth flotation cell. Air flow through the foam is described by a simple two-dimensional deceleration flow, modelling the foam spilling over a weir. Foam microstructure is given in terms of the number of channels (Plateau borders) per unit area, which scales as the inverse square of bubble size. The Plateau border number density decreases with height in the foam, and also decreases horizontally as the weir is approached. Foam drainage equations, applicable in the dry foam limit, are described. These can be used to determine the average cross-sectional area of a Plateau border, denoted A, as a function of position in the foam. Quasi-one-dimensional solutions are available in which A only varies vertically, in spite of the two-dimensional nature of the air flow and Plateau border number density fields. For such situations the liquid drainage relative to the air flow is purely vertical. The parametric behaviour of the system is investigated with respect to a number of dimensionless parameters: K (the strength of capillary suction relative to gravity), α (the deceleration of the air flow), and n and h (respectively, the horizontal and vertical variations of the Plateau border number density). The parameter K is small, implying the existence of boundary layer solutions: capillary suction is negligible except in thin layers near the bottom boundary. The boundary layer thickness (when converted back to dimensional variables) is independent of the height of the foam. The deceleration parameter α affects the Plateau border area on the top boundary: weaker decelerations give larger Plateau border areas at the surface. For weak decelerations, there is rapid convergence of the boundary layer solutions at the bottom onto ones with negligible capillary suction higher up. For strong decelerations, two branches of solutions for A are possible in the K = 0 limit: one is smooth, and the other has a distinct kink. The full system, with small but non-zero capillary suction, lies relatively close to the kinked solution branch, but convergence from the lower boundary layer onto this branch is distinctly slow. Variations in the Plateau border number density (non-zero n and h) increase individual Plateau border areas relative to the case of uniformly sized bubbles. For strong decelerations and negligible capillarity, solutions closely follow the kinked solution branch if bubble sizes are only slightly non-uniform. As the extent of non-uniformity increases, the Plateau border area reaches a maximum corresponding to no net upward velocity of foam liquid. In the case of vertical variation of number density, liquid content profiles and Plateau border area profiles cease to be simply proportional to one another. Plateau border areas match at the top of the foam independent of h, implying a considerable difference in liquid content for foams which exhibit different number density profiles. Received 3 July 2001     

Horseshoe vortices in inhomogeneous flows

The formation of a horseshoe-shaped vortex from an initial straight cylindrical vortex occurring in the viscous layer of a retarding fluid flow in a channel with a nonuniform cross section is studied experimentally. The spatial bends of the vortex filament and the effect of the ambient fluid pressure first form an arch with supports at the bottom of the channel, then a horseshoe-shaped vortex, and finally a circular vortex with the destruction of the supports.     

U型槽刻蚀工艺对GaN垂直沟槽型金属-氧化物-半导体场效应晶体管电学特性的影响

U型槽的干法刻蚀工艺是GaN垂直沟槽型金属-氧化物-半导体场效应晶体管(MOSFET)器件关键的工艺步骤,干法刻蚀后GaN的侧壁状况直接影响GaN MOS结构中的界面态特性和器件的沟道电子输运.本文通过改变感应耦合等离子体干法刻蚀工艺中的射频功率和刻蚀掩模,研究了GaN垂直沟槽型MOSFET电学特性的工艺依赖性.研究结果表明,适当降低射频功率,在保证侧壁陡直的前提下可以改善沟道电子迁移率,从35.7 cm^2/(V·s)提高到48.1 cm^2/(V·s),并提高器件的工作电流.沟道处的界面态密度可以通过亚阈值摆幅提取,射频功率在50 W时界面态密度降低到1.90×10^12 cm^-2·eV^-1,比135 W条件下降低了一半.采用SiO2硬刻蚀掩模代替光刻胶掩模可以提高沟槽底部的刻蚀均匀性.较薄的SiO2掩模具有更小的侧壁面积,高能离子的反射作用更弱,过刻蚀现象明显改善,制备出的GaN垂直沟槽型MOSFET沟道场效应迁移率更高,界面态密度更低.     

Statics and dynamics of adhesion between two soap bubbles

An original set-up is used to study the adhesive properties of two hemispherical soap bubbles put into contact. The contact angle at the line connecting the three films is extracted by image analysis of the bubbles profiles. After the initial contact, the angle rapidly reaches a static value slightly larger than the standard 120^° angle expected from Plateau rule. This deviation is consistent with previous experimental and theoretical studies: it can be quantitatively predicted by taking into account the finite size of the Plateau border (the liquid volume trapped at the vertex) in the free energy minimization. The visco-elastic adhesion properties of the bubbles are further explored by measuring the deviation Δθ_d(t) of the contact angle from the static value as the distance between the two bubbles supports is sinusoidally modulated. It is found to linearly increase with Δr _c/r _c , where r_c is the radius of the central film and Δr _c the amplitude of modulation of this length induced by the displacement of the supports. The in-phase and out-of-phase components of Δθ_d(t) with the imposed modulation frequency are systematically probed, which reveals a transition from a viscous to an elastic response of the system with a crossover pulsation of the order 1rad · s^-1. Independent interfacial rheological measurements, obtained from an oscillating bubble experiment, allow us to develop a model of dynamic adhesion which is confronted to our experimental results. The relevance of such adhesive dynamic properties to the rheology of foams is briefly discussed using a perturbative approach to the Princen 2D model of foams.     

Investigations on the radially free full circle arc

An argon arc, burning between two horizontal, plane-parallel, insulating plates, is bent circularily by an external and its own magnetic field. Except for the small electrode zone, one gets a stationary radially free full circle arc for experimental investigations of magnetohydrodynamic and thermodynamic effects under well defined conditions. The local temperature distributions in the arc cross-section are detected spectroscopically as functions of the arc current and the arc radius or curvature, respectively. By means of the basic equations of conservation of energy, mass, and charge and the known transport properties of argon at atmospheric pressure, the mass flow field in the arc is evaluated. In the arc core it follows the direction of the Lorentz force radially outward counter-balancing the tendency of the curved arc to move inwards thermodynamically. Due to the symmetry to the centre-plane of the arc chamber and the vanishing net flow for the whole system, the gas flow has to stream back in the cold outer arc zones, thereby forming a double whirl. By reasons of the experimental arrangement even a quadruple whirl occurs. Additionally the evaluation yields the specific radiationu(T) of argon being compared with results in the recent literature.     

Multi-relaxation time lattice Boltzmann simulation of inertial secondary flow in a curved microchannel

The inertial secondary flow is particularly important for hydrodynamic focusing and particle manipulation in biomedical research.In this paper,the development of the inertial secondary flow structure in a curved microchannel was investigated by the multi relaxation time lattice Boltzmann equation model with a force term.The numerical results indicate that the viscous and inertial competition dominates the development of secondary flow structure development.The Reynolds number,Dean number,and the cross section aspect ratio influence significantly on the development of the secondary vortexes.Both the intensity of secondary flow and the distance between the normalized vortex centers are functions of Dean numbers but independent of channel curvature radius.In addition,the competition mechanism between the viscous and inertial effects were discussed by performing the particle focusing experiments.The present investigation provides an improved understanding of the development of inertial secondary flows in curved microchannels.     

Study of oxidation process of Cr/Cu/Cr thin film electrodes

Understanding the oxidation process of Cr/Cu/Cr laminated thin film electrode is important for developing new oxidation-resistant electrodes. By studying the evolution of crystal structure, morphology and electric resistivity of Cr/Cu/Cr thin films and electrodes that were post-annealed at different temperatures, the oxidation mechanism and process of Cr/Cu/Cr electrode were proposed. Copper is first oxidized into Cu₂O at low temperatures (<310 °C), and converts to CuO phase at higher temperatures. Two pathways for oxygen diffusion were identified: diffusion from the protective Cr layer, and diffusion from the sidewall of electrode, of which the latter one leads to total oxidation of copper interlayer at high temperatures (>310 °C). As a result, the passivation of Cu metal at electrode sidewalls is crucial in reducing oxidation of Cr/Cu/Cr electrodes or designing new copper-containing oxidation-resistant electrodes.     

Kink properties on curved manifold

In the framework of the two-dimensional field model the influence of the curvature on kink width is discussed. Breading of the kink width in a curved region of the manifold is observed. Examples of kinks on curved manifolds are studied analytically and numerically as well. The deformation of the kink front in the form of the travelling waves propagating along the curved surfaces are found. Enlarging of the travelling wave speed in a curved regions of the manifold is predicted.     

Meniscus instability in a thin elastic film

A new kind of meniscus instability leading to the formation of stationary fingers with a well-defined spacing has been observed in experiments with elastomeric films confined between a plane rigid glass and a thin curved glass plate. The wavelength of the instability increases linearly with the thickness of the confined film, but it is remarkably insensitive to the compliance and the energetics of the system. However, lateral amplitude (length) of the fingers depends on the compliance of the system and on the radius of curvature of the glass plate. A simple linear stability analysis is used to explain the underlying physics and the key observed features of the instability.     

Semi-Analytical Simulation of Titanium-Indiffused Lithium Niobate–Integrated Optic Directional Couplers Consisting of Curved Waveguides

Integrated optic directional couplers consisting of curved waveguides are simulated analytically by solving the Riccati equation. The coupling coefficient between the curved waveguides with a parabolically varying gap and the condition of total power transfer between the waveguides are derived. In order to compute the overall coupling coefficient and hence the power distribution along the waveguides for Ti:LiNbO ₃ curved waveguide directional couplers, the coupling coefficient for straight waveguide couplers is computed for different gaps using the effective-index-based matrix method (EIMM). Finally, the power distribution in the curved waveguides along the length is computed. The method is mostly analytical except the effective-index method and is computationally simple.     

Optical management in high‐efficiency thin‐film silicon micromorph solar cells with a silicon oxide based intermediate reflector

In the effort to increase the stable efficiency of thin film silicon micromorph solar cells, a silicon oxide based intermediate reflector (SOIR) layer is deposited in situ between the component cells of the tandem device. The effectiveness of the SOIR layer in increasing the photo‐carrier generation in the a‐Si:H top absorber is compared for p–i–n devices deposited on different rough, highly transparent, front ZnO layers. High haze and low doping level for the front ZnO strongly enhance the current density (J_sc) in the μc‐Si:H bottom cell whereas J_sc in the top cell is influenced by the angular distribution of the transmitted light and by the reflectivity of the SOIR related to different surface roughness. A total J_sc of 26.8 mA/cm² and an initial conversion efficiency of 12.6% are achieved for 1.2 cm² cells with top and bottom cell thicknesses of 300 nm and 3 μm, and without any anti‐reflective coating on the glass. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)