Anisotropic Profiles Research Articles

Field-controlled optical profile of a waveguide having a liquid-crystalline core

The optical properties of a nematic liquid-crystal waveguide in a hybrid configuration have been dynamically reshaped in a straightforward and controlled way by applying an external electric field. The anisotropic refractive index profile of the waveguide has been determined for different electric-field strengths by means of m-lines spectroscopy. The experimental results are in very good agreement with the values predicted by a simple model that takes into account the dielectric, optical, and elastic properties of the liquid-crystal guiding layer.

Effect of H2 on the etch profile of InP/InGaAsP alloys in Cl2/Ar/H2 inductively coupled plasma reactive ion etching chemistries for photonic device fabrication

This study demonstrates etch profile engineering of InP, In1−xGaxAs1−yPy, and In0.53Ga0.47As heterostructures results from adding H2 to standard Cl2/Ar inductively coupled plasma-reactive ion etching chemistries. Etch rate curves of bulk InP, In1−xGaxAs1−yPy, and In0.53Ga0.47As show a general parabolic trend as a function of the H2 component of the Cl2/Ar/H2 ratio. Three distinct etching profiles of InP/InGaAsP layers were realized by varying the Cl2/Ar/H2 ratio. Highly anisotropic profiles result for Cl2/Ar/H2 ratios between 2/3/1 and 2/3/2. Waveguiding structures fabricated using this technology are presented with a loss as low as 2 dB/cm. An InP racetrack resonator with a quality factor (Q)>8000 is also presented.

Shape‐dependent molecular polarizabilities

AbstractIn a series of papers we have considered the overall effects of geometrical shape on molecular electronic properties. Molecules are mapped into ellipsoids, based on tabulated bond lengths and van der Waals radii. The main advantage of the approach is to make predictions of the influence of shape itself on important physical properties and their trends, particularly for systems which are computationally difficult or time‐consuming. The results include insight into the effect of shape on molecular stopping anisotropy, stopping in ultra‐thin films, and anisotropic Compton profiles. In this paper we extend the treatment to polarizability anisotropies and to the scaling of the polarizability of long objects in terms of the number of basic building blocks. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Anisotropic self-focusing of intense lasers in undercritical plasma

A variational principle approach to study the anisotropic self-focusing of ultrarelativistic laser beams in the undercritical plasma is performed in this paper. The presence of the anisotropic self-focusing laser intensity profile is viewed as a result of unstabilization of the isotropic profile. The isotropic laser profile is solved and then its stability relative to the anisotropic one is discussed. Numerical results show that parameters of the isotropic profile are important for its possibility of anisotropic unstabilization.

The formation of volume elements for microelectromechanical systems in polyimide by method of reactive ion beam etching

The main problem related to the creation of microelectromechanical systems—the formation of an anisotropic profile with vertical side walls and clean flat bottom in a thick resist film—is experimentally solved by optimizing the parameters of the reactive ion beam etching process for polyimide (PI) layers with a thicknesses up to 50 μm. The optimized technique was used to form the volume parts of a microelectromotor structure by etching a PI film with an oxygen ion beam (beam current density j=0.5 mA/cm2; ion extraction energy U=800 V).

Cryogenic etching of deep narrow trenches in silicon

Deep and narrow anisotropic etching of silicon structures has been investigated in a low-pressure high density plasma reactor working with a cryogenic chuck. We have previously demonstrated the feasibility of this technique on such structures. Improvement of etch rate and profiles has been studied and new results show 2 μm wide trenches etched to a depth of 50 μm at an average etch rate of 5 μm/min with highly anisotropic profiles and very high selectivity (>500:1) toward the SiO2 mask. An evaluation of a commercially available reactor from Alcatel has been carried out and similar results are obtained. A phosphosilicate glass mask has been used to study the effect on profiles. It is shown that undercut is reduced while bowing is independent of the mask material. Since surface temperature strongly affects the profiles, wafer deformations in our cryogenic chuck have been measured and temperature evolution across the wafer has been estimated. A significant temperature difference of 10 °C between the chuck and the wafer is expected for thin wafers (210 μm).

Non-coincidence effect of aromatic ring vibrations

The non-coincidence of the maxima of the isotropic and the anisotropic profiles in Raman spectra has always been considered an efficacious probe of the structure and dynamics of polar liquids. An anomalous, negative non-coincidence effect has been observed in ring breathing modes of aromatic molecules, without dipole moment (p-xylene, mesitylene) or with a weak permanent dipole (toluene, o-xylene, m-xylene). Temperature effects and isotopic dilution have also been performed in toluene and o-xylene, confirming the presence of a resonant coupling mechanism. Previous analogous results of other aromatic non-polar liquids (benzene, hexafluorobenzene) have been confirmed. It has been hypothesized that quadrupolar intermolecular forces and steric repulsive interactions balance each other giving final negative values of non-coincidence effect.

Correlated photofragment product distributions in the photodissociation of NO 2 at 212.8 nm

The 212.8 nm photolysis of NO 2 has been studied using REMPI–TOF and LIF techniques. Photodissociation occurs via a parallel transition on a time scale which is rapid compared to molecular rotation, producing anisotropic TOF profiles. Analysis of state selected NO flight profiles gives the correlated photofragment product distributions and shows that the predominant photodissociation channel produces vibrationally excited NO in v=3 with O 1 D as the co-fragment. If NO is formed in a rovibrational level for which O 1 D production is energetically allowed then it is the exclusively produced co-fragment.

Improvement of sidewall roughness in deep silicon etching

The recently developed High Aspect Ratio Si Etch (HARSE) process is widely used for applications requiring silicon structures with high aspect ratios. This process relies on the alternation of sidewall passivation and silicon etching phases and enables the obtainment of high silicon etch rates and highly anisotropic profiles. This paper reports an innovative approach to improve the sidewall roughness through a multiple-step HARSE process using an ICP system. Unlike the standard HARSE process, the etching conditions for this new process are gradually altered in order to reinforce the silicon etch efficiency as a function of the silicon depth previously etched. Trenches with aspect ratios as high as 40 can be achieved. The sidewall roughness along the entire etching depth is less than 8 nm rms. In comparison with the standard HARSE process in which ripples appear on the trenches sidewall, the sidewall roughness is improved by a factor of 4.

High Etch Rate Gallium Nitride Processing Using an Inductively Coupled Plasma Source

The ongoing development of III-nitride laser diode and light emitting diode fabrication has been made possible by dry etching techniques. Conventional wet etching of these materials has proved difficult and while Reactive Ion Etch (RIE) plasma systems can be used as an alternative, the Inductively Coupled Plasma (ICP) offers an enhanced dry etch route. In this paper, high etch rate (>1.25 μm/min) gallium nitride (GaN) processing is reported using the Surface Technology Systems (STS) ICP system. This is the fastest etch rate published for an ICP system. The etched material exhibits smooth anisotropic profiles and is free from residues. The effect of plasma parameters such as power, pressure and flow of reactive and inert process gases are presented. The use of both chlorine (Cl2) and boron trichloride (BCl3) as reactive etch gases is also studied.

Reactive ion etching for mesa structuring in HgCdTe

Both wet chemical and dry plasma etching techniques have been investigated for mesa structuring in n- and p-type HgCdTe. Scanning electron microscopy (SEM) confirms the isotropic nature of a bromine-based wet chemical etching solution, and the anisotropic profile that results from reactive ion etching. Laser-beam-induced-current (LBIC) measurements reveal no significant modifications to the electrical properties for chemically etched HgCdTe material, but clearly demonstrate a p- to n-type conversion in p-type samples and n+ doping in n-type samples for reactive ion etching (RIE) (processing conditions: 400 mTorr, CH4/5H2, 0.4 W/cm2). LBIC measurements following low-temperature (200 °C) mercury annealing of RIE-processed samples indicate the full restoration of electrical properties to that of the initial as-grown wafers, thus preserving the beneficial aspects of RIE for anisotropic mesa structuring in HgCdTe.

Influence of Coil Power on the Etching Characteristics in a High Density Plasma Etcher

We present the effects of applied coil power on silicon etching in a high density inductively coupled plasma etcher in both the isotropic and anisotropic regimes. The experimentally obtained response surfaces were generated while changing four etching variables for the isotropic case and eight etching variables for the anisotropic case. The measured performance contained in this report serves as a guide to tailor etching conditions to produce isotropic profiles while etching with pure , and also to optimize anisotropic profiles when using time multiplexed deep etching. The response surfaces contained herein are useful for micromachining high aspect ratio structures. © 1999 The Electrochemical Society. All rights reserved.

Constrained Violent Relaxation to a Spherical Halo

Violent relaxation during the collapse of a galaxy halo is known to be incomplete in realistic cases such as cosmological infall or mergers. We adopt a physical picture of strong but short-lived interactions between potential fluctuations and particle orbits, using the broad framework outlined earlier for incorporating incompleteness of the relaxation. We are guided by results from plasma physics, viz., the quasilinear theory of Landau damping, but allow for significant differences in our case. Crucially, wave particle scattering does not drive the system to an equilibrium distribution function of the exponential type, even in regions of phase space allowed by the constraints. The physical process is mixing without friction in action space, for which the simplest possible model is a constant phase space density modulated by the constraints. Our distribution function does not use the exponential functions of the energy prevalent in other work, which we regard as inappropriate to collisionless systems. The dynamical constraint of a finite short period of the relaxation process is argued to lead to a 1/Tr factor in the distribution function, where Tr is the radial period. The notion of strong potential fluctuations in a core is built in as a cutoff in pericenter (which we find preferable to one angular momentum, the other alternative we explored). The halo of the self-consistent, parameter-free solutions show an r-4 behavior in density at large r, an r1/4 surface brightness profile in the region 0.1re-8re, and a radially anisotropic velocity dispersion profile outside an isotropic core. The energy distribution seen in simulations N(E) singles out the pericenter cutoff model as the most realistic among the variants we have explored. The results are robust to modifications of the period dependence keeping the same asymptotic behavior or to the use of binding energy raised to the power of 3/2 in place of 1/Tr.

Inductively coupled plasma etching of III–V antimonides in BCl3/Ar and Cl2/Ar

Inductively coupled plasma (ICP) etching characteristics of GaSb and AlGaAsSb have been investigated in BCl3/Ar and Cl2/Ar plasmas. The etch rates and selectivity between GaSb and AlGaAsSb are reported as functions of plasma chemistry, ICP power, rf self-bias, and chamber pressure. It is found that physical sputtering desorption of the etch products plays a dominant role in BCl3/Ar ICP etching, while in Cl2/Ar plasma, the chemical reaction dominates the etching. GaSb etch rates exceeding 2 μm/min are achieved in Cl2/Ar plasmas with smooth surfaces and anisotropic profiles. In BCl3/Ar plasmas, etch rates of 5100 and 4200 Å/min are obtained for GaSb and AlGaAsSb, respectively. The surfaces of both GaSb and AlGaAsSb etched in BCl3/Ar plasmas remain smooth and stoichiometric over the entire range of plasma conditions investigated. This result is attributed to effective removal of etch products by physical sputtering. For a wide range of plasma conditions, the selectivity between GaSb and AlGaAsSb is close to unity, which is desirable for fabricating etched mirrors and gratings for Sb-based midinfrared laser diodes.

Group-III Nitride ETCH Selectivity IN BCl /Cl ICP Plasmas

Patterning the group-III nitrides has been challenging due to their strong bond energies and relatively inert chemical nature as compared to other compound semiconductors. Plasma etch processes have been used almost exclusively to pattern these films. The use of high-density plasma etch systems, including inductively coupled plasmas (ICP), has resulted in relatively high etch rates (often greater than 1.0 µm/min) with anisotropic profiles and smooth etch morphologies. However, the etch mechanism is often dominated by high ion bombardment energies which can minimize etch selectivity. The use of an ICP-generated BCl3 /Cl2 plasma has yielded a highly versatile GaN etch process with rates ranging from 100 to 8000 Å/min making this plasma chemistry a prime candidate for optimization of etch selectivity. In this study, we will report ICP etch rates and selectivities for GaN, AlN, and InN as a function of BCl3/Cl2 flow ratios, cathode rf-power, and ICP-source power. GaN:InN and GaN:AlN etch selectivities were typically less than 7:1 and showed the strongest dependence on flow ratio. This trend may be attributed to faster GaN etch rates observed at higher concentrations of atomic Cl which was monitored using optical emission spectroscopy (OES).

Characterization of a Time Multiplexed Inductively Coupled Plasma Etcher

We report the experimentally obtained response surfaces of silicon etching rate, aspect ratio dependent etching (ARDE), photoresist etching rate, and anisotropy parameter in a time multiplexed inductively coupled plasma etcher. The data were collected while varying eight etching variables. The relevance of electrode power, pressure, and gas flow rates is presented and has been found to agree with observations reported in the literature. The observed behavior presented in this report serves as a tool to locate and optimize operating conditions to etch high aspect ratio structures. The performance of this deep reactive ion etcher allows the tailoring of silicon etching rates in excess of 4 μm/min with anisotropic profiles, nonuniformities of less than 4% across the wafer, and ARDE control with a depth variation of less than 1 μm for trenches of dissimilar width. Furthermore it is possible to prescribe the slope of etched trenches from positive to reentrant. © 1999 The Electrochemical Society. All rights reserved.

Dry and Wet Etching for Group III – Nitrides

The group-III nitrides have become versatile semiconductors for short wavelength emitters, high temperature microwave transistors, photodetectors, and field emission tips. The processing of these materials is significant due to the unusually high bond energies that they possess. The dry and wet etching methods developed for these materials over the last few years are reviewed. High etch rates and highly anisotropic profiles obtained by inductively-coupled-plasma reactive ion etching are presented. Photoenhanced wet etching provides an alternative path to obtaining high etch rates without ion-induced damage. This method is shown to be suitable for device fabrication as well as for the estimation of dislocation densities in n-GaN. This has the potential of developing into a method for rapid evaluation of materials.

Comparison of plasma etch techniques for III–V nitrides

Fabrication of group-III nitride devices relies on the ability to pattern features to depths ranging from ∼1000 Å to >5 μm with anisotropic profiles, smooth morphologies, selective etching of one material over another and a low degree of plasma-induced damage. In this study, GaN etch rates and etch profiles are compared using reactive ion etch (RIE), reactive ion beam etching (RIBE), electron cyclotron resonance (ECR) and inductively coupled plasma (ICP) etch systems. RIE yielded the slowest etch rates and sloped etch profiles despite dc-biases >−900 V. ECR and ICP etching yielded the highest rates with anisotropic profiles due to their high plasma flux and the ability to control ion energies independently of plasma density. RIBE etch results also showed anisotropic profiles but with slower etch rates than either ECR or ICP possibly due to lower ion flux. InN and AlN etch characteristics are also compared using ICP and RIBE.

Solutions of Unsaturated Seepage Problem by Power-Series Method

An analytical approximation method is suggested for solving the unsaturated steady-state seepage equations in a two-dimensional anisotropic profile where the lateral scale, s, is large compared with the normal scale, n. The method is based on the assumption that the fluxes (rain or drainage) passing through the upper and lower lateral boundaries of the profile can be estimated as a power series of s. The analytical result found by using this power-series method is a polynomial of the large-scale variable s with coefficients being functions of the variable n. This solution always satisfies the equations precisely; hence the mass is preserved completely. A group of solutions is presented for profiles with horizontal and sloped boundaries for anisotropic, layered soils where boundary conditions are constant flux or pressure or piesometric head. It is found that stream functions in these cases are linear functions of the large-scale coordinate s. The power-series method is explained in general cases when the ...

The electrodynamic responses of the atmosphere and ionosphere to the lightning discharge

The purpose of this study was to use the complete set of Maxwell’s equations to simulate the electromagnetic response of the atmosphere and the ionosphere during and after the lightning discharge. A three-dimensional numerical model is used to calculate the quasistatic electric field over the 10 ms–2 s range. The model contains an isotropic conductivity profile below 70 km and an anisotropic conductivity profile from 70 km–150 km (different latitudes, day/night conditions have been considered), a time-varying charge distribution (representing the thunderstorm source function), and a perfectly conducting ground surface. Both vertical and horizontal transient electric fields and upward Maxwell currents in the ionosphere have been calculated for a vertical discharge. The results show that the relaxation time of the electric field due to the lightning discharge, which is no longer than ε 0/ σ, will decrease with an increase of the altitude of observation and have little change with the horizontal distance from the lightning discharge. It also shows that the Maxwell current from the thundercloud spreads out to the ionosphere during a time interval of several milliseconds following the lightning discharge. This current flows along directions both parallel and perpendicular to the geomagnetic field lines and mainly propagates horizontally above 70 km. A good agreement between the simulation results and the measurement data has been achieved. These results show the importance of the use of the complete set of Maxwell’s equations and the inclusion of an anisotropic ionospheric conductivity in the lightning response simulation. The conclusion of this paper also may help to explain some of the long-standing controversies between measurements and theoretical predictions.