Etch Tunnels Research Articles

Atomic Force Microscopy Study of Anodic Etching of Aluminum: Etching Morphology Development and Caustic Pretreatment

Atomic force microscopy (AFM) was used to investigate the topographic characteristics of pitting sites during anodic galvanostatic etching of high-purity aluminum foils in hot baths containing HCl and Pit sites near pre-existing ridges formed by rolling are preferred, and the distribution of sites depends on the foil fabrication process. Etch tunnels are found only within shallow pits formed by surface dissolution, which may be a prerequisite for tunnel formation. Pit sites are more numerous and evenly distributed after pretreatment of foils by immersion in 1 N NaOH. AFM imaging during acid dissolution of NaOH treated foils reveal submicrometer cavities formed as a result of NaOH treatment. Some cavities appear to form near oxide particles or inclusions. It is argued that these cavities are subsurface voids exposed by uniform dissolution in the acid. If the void surface is oxide-free, they may dissolve as pits when exposed by the shallow surface dissolution shown to take place in the etchant bath. © 2001 The Electrochemical Society. All rights reserved.

Structural Features of Crystalline Anodic Alumina Films

Aluminum capacitor foil with a tunnel etch structure was reacted with boiling water to deposit a layer of psuedoboehmite and was then anodized to voltages between 55 and 540 V in a neutral hot borate solution. The resulting composite oxide film consists of two layers of crystalline barrier γ′-oxide containing a high density of nanovoids, covered with a layer of residual hydrous oxide. A uniformly thin 50 nm section was prepared by focused ion beam. The specimen was cut parallel to the foil surface so the oxide shell within an etch tunnel appears in cross section. These were examined by transmission electron microscopy to reveal oxide structural features with a resolution of a few nanometers. The void structure is different in inner and outer crystalline layers, reflecting different growth mechanisms, and the void distribution changes as the film thickens. The crystalline region develops initially in the center of the anodic oxide, with a layer of amorphous anodic alumina on either side. Crystalline fronts spread toward both interfaces until all anodic alumina is transformed to γ′-oxide and reaction with psuedoboehmite transforms that nonbarrier layer to barrier γ′-oxide. Details of film growth and structure are different for oxides grown in etch tunnels and on a smooth substrate. The field coefficient, for 500 nm thick crystalline oxides is not significantly different than that for amorphous barrier oxides. © 2000 The Electrochemical Society. All rights reserved.

Behaviour of bismuth during simulated processing of model aluminium capacitor foils

The production of high specification, aluminium-based, electrolytic capacitors requires optimization of material composition and heat and surface treatments in order to maximize the area available for formation of the dielectric, anodic film. Commercial foils contain low additions of copper and lead in order to achieve this goal. The present study examines the effects of heat and surface treatments on aluminium foil containing either 50 or 1000 ppm bismuth, as a replacement for lead, by a combination of Rutherford backscattering spectroscopy and scanning electron microscopy. Heat treatment at 823 K results in segregation of bismuth to the surface regions of the foils, with enrichments in the range 4–8 × 1014 Bi atoms cm−2, localized mainly just beneath the thermal oxide, for the selected treatment conditions. The enrichment reduces following alkaline etching, to the range 1–4 × 1014 Bi atoms cm−2. This level of enrichment is maintained during subsequent anodizing, with the enrichment partitioned between the metal and the outermost layers of the anodic films. The enrichment of the metal is about 4 × 1013 Bi atoms cm−2. Electropolishing in perchloric acid solution eliminates the enrichment developed during the heat treatment, probably due to activation during the polishing process. The enrichment remains very low or negligible during subsequent anodizing. The general behaviours of bismuth and lead are similar in aluminium foils subject to the selected heat and surface treatments. However, additional studies are needed of tunnel etching to determine the feasibility of substitution of lead by bismuth in commercial foils.

Metal Dissolution Kinetics in Aluminum Etch Tunnels

The potential dependence of the metal dissolution current density of aluminum in etch tunnels was measured, using pulsed increases of current during galvanostatic etching experiments. Etching was carried out in 1 N HCl solution at 65°C, and was followed by analysis of the topographic development of the dissolving surfaces using scanning electron microscopy. By manipulation of the current waveform, the dissolving area and the applied current were varied independently. Parallel experiments with current interruptions were used to help identify the area of the dissolving surfaces during the anodic pulse, which were shown to be submicron size patches on the tunnel tip surfaces. After correction for solution‐phase potential drops, the current was found to obey an exponential Tafel dependence on potential. Measurements of the dissolved depth vs. time show that the rate of cathodic hydrogen evolution is significantly increased during the anodic pulse, by comparison with that at the more negative potentials during constant current etching. Good agreement of the present current‐potential relation was obtained with that derived from the measurements of pit currents in thin films by Frankel et al. [J. Electrochem. Soc., 143, 1834 (1996)]. © 2000 The Electrochemical Society. All rights reserved.

Kinetic Model for Oxide Film Passivation in Aluminum Etch Tunnels

Aluminum etch tunnels are micrometer‐wide corrosion pits with large length‐width aspect ratios, in which dissolution proceeds from the tip or end surfaces, while the sidewalls are covered by oxide films. The dynamics of oxide film passivation in etch tunnels has been investigated using decreasing current ramps superimposed on the otherwise constant applied current during anodic etching in 1 N HCl at 70°C. The ramps cause the dissolving area on the tip to be continuously reduced by passivation around its perimeter. Analysis of potential transients along with tunnel width profiles shows that two additive processes contribute to the passivation rate, expressed as the rate of decrease of actively dissolving area: a potential‐dependent Tafel‐type kinetic expression and a term proportional to the time derivative of the potential. The potential driving force is the “repassivation overpotential,” the difference between the potential at the dissolving surface and the repassivation potential there. The kinetic model for passivation is consistent with both potential transients and tunnel width profiles, over a range of current ramp rates. The rate‐controlling step of passivation is considered to be potential‐dependent removal of chloride ions from the dissolving surface. © 2000 The Electrochemical Society. All rights reserved.

Initial stages of the anodic etching of aluminum foils studied by atomic force microscopy

The modification of the surface morphology of two types of aluminum foils during galvanostatic etching in hot HCl⧹H 2SO 4 solution was studied as a function of etch time by Atomic Force Microscopy (AFM) and complementary electrochemical measurements. Particular emphasis was placed on the nucleation and growth of etch pits and tunnels. Three stages of the etch process with characteristic differences in the surface morphology can be identified: for etch times up to 5–10 s predominantly large etch pits with surface areas 0.5 mm 2 are formed (stage I), followed by the nucleation of small rectangular pits and channels (0.01–0.5 mm 2 area) for etch times up to 20 s (stage II), and finally by the formation and lateral growth of surface areas with a rough, strongly etched surface morphology for etch times >20 s (stage III). The distribution of smaller etch pits and tunnels and the rough morphology in stage III differ strongly for the two different materials, which can be correlated with differences in the sub-micrometer scale morphology of the foils prior to etching. The etch process is accompanied by the deposition and redissolution of characteristic hemispherical hills.

電解コンデンサ用アルミニウム箔の直流エッチング挙動に及ぼす結晶性酸化物の影響

The electrode potential of high-purity aluminum foils for electrolytic capacitors moves from a high potential period to a stability period during early DC etching in hot hydrochloric acid. We observed changes in pit structures in transmission electron micrographs of films stripped from etched foils, and measured a potential change using a digital oscilloscope. Pitting attacks occurred around MgAl2O4 spinel or γ-Al2O3. Pits varied from hemispherical to half-cubic, and the etch tunnel growth was promoted by crystallographic dissolutions of the (100) faces. Pit wall surfaces of a half-cubic pit and part of the tips of etch tunnels were passivated. Both MgAl2O4 spinel and γ-Al2O3 crystals were observed at the metal ridges on the aluminum substrate by transmission electron micrographs of ultramicrotomed sections. Pits are considered to be initiated at cracks associated with oxide film crystallizations. MgAl2O4 spinel crystals were observed in a foil containing 123 wt-ppm magnesium after annealing at 445°C for 5h in an Ar gas atmosphere. At 575°C, γ-Al2O3 crystals were only observed in a foil containing 1 wt-ppm magnesium. The growth of MgAl2O4 crystals was accelerated above 1 wt-ppm magnesium.

A Mathematical Model for the Initiation of Aluminum Etch Tunnels

A mathematical simulation is presented which predicts the spontaneous shape evolution of cubic etch pits on aluminum as they develop into etch tunnels during anodic etching in chloride solutions. The simulation is based on a model for oxide passivation, according to which the rate of oxide film coverage increases as the potential at the dissolving surface is made more negative than the critical repassivation potential, which depends on the local chloride ion concentration. Mass transport calculations are used to predict the electrolyte concentration and potential in the pit, which in turn determine the rate of oxide advance and hence the shape change of the pit. The model predicts the pit‐tunnel transformation, as well as width expansion of tunnels near their mouths. The occurrence of these features is independent of the choice of the passivation rate constant, an adjustable parameter in the simulation. Tunnel width oscillations were found at relatively low values of the rate constant. In the model, the pit‐tunnel transformation is produced by rapid pit sidewall passivation, which is due to the relatively slower increase of the pit electrolyte concentration relative to the ohmic drop, during the pit's initial growth. A fully quantitative comparison of the model and experiment is possible with independent experimental information on passivation kinetics.

The effect of indium impurity on the DC-etching behaviour of aluminum foil for electrolytic capacitor usage

The effects of indium impurity on the etched morphology of high purity aluminium foils for electrolytic capacitor usage were investigated. The indium impurity was present either in as-received aluminium foils or deposited purposely on the foil surface through immersion-reduction reaction. The rolling line effect can be improved through introducing deposited indium on foil surfaces by immersion of the foil into In(NO 3) 3 solution prior to DC-etching. Under a high immersion temperature and more concentrated In(NO 3) 3 condition, surface etching competes with tunnel etching, and the foil thickness clearly decreases. Under low temperature and dilute In(NO 3) 3 condition, surface etching is inhibited and tunnel etching prevails. The different DC-etching behaviour under the two In-deposition conditions is explained through the energy state difference between the two deposited-In and the activation energy difference between surface etching and tunnel etching. The modification of etchability by indium on aluminium surface is stronger than that of lead. This facilitates employing lower temperature and dilute In(NO 3) 3 condition in the immersion-reduction process to optimize the etchability.

Tunnel Morphology in Anodic Etching of Aluminum

High purity, high cubicity aluminum foils were electrochemically etched in HCl‐based etchants at constant anodic current density to produce etch tunnels. Tunnel widths were measured as a function of the etching depth, for temperatures in the range 60 to 95°C and HCl concentrations in the range 0.1 to 9 N. The effect of sulfate and glycerol additions was examined. For HCl concentrations above 0.5 N, over most of the tunnel length, the widths w taper exponentially with depth x, i.e., w = w0 exp(−x/2L), where w0 is the tunnel width at the surface, and L is a characteristic length, here called attenuation length. For a defined set of experimental conditions, each tunnel has a unique value of w0 but the same value of L. The tunnel taper increases with increasing temperature. Tunnel velocity and attenuation length vary with temperature in an Arrhenius relationship, in such a way that their product is almost independent of temperature. The relationship between current density and tunnel taper is similar to a criterion reported for stable pit growth, suggesting a common mechanism. A quantitative model of tunnel growth, in which a salt film is assumed to be present at the tunnel tip, is developed. It predicts a relationship between the tunnel velocity, taper and saturation concentration similar to that observed.

A model for the ?100? crystallographic tunnel etching of aluminium

When high purity aluminium foil is subjected to DC electrochemical etching in hot aqueous chloride solution at temperatures above 60 °C, the 〈100〉 crystallographic tunnel growth of an etch pit is achieved. This crystallographic tunnel growth behaviour arises due to the following reason. The tunnel tip region where the active corrosion reaction takes place tends to be made of {111} planes which minimize the surface energy, whilst the subsequent tunnel wall is made of {100} planes, because the passive oxide film formed on the {100} planes, along which the biaxial elastic modulus is the smallest, would be least attacked by corrosion.

Effect of sulphate anions on tunnel etching of aluminium

Electrochemical etching of hard aluminium foil was studied at 100°C in NaCl solutions without and with Na2SO4 in concentrations up to 1.0m. Addition of Na2SO4 resulted in an increase in electric capacitance, in refinement of etch configuration and in an increase in tunnel density per unit volume. A decrease in the number of pits from which the tunnels grew also occurred. The capacitance increased with increasing concentration of Na2SO4 up to about 0.351 m, and then decreased. Sulphate ions depressed the formation of pits on the outer oxide-covered surface, but enhanced the growth of the pits and the formation of tunnels from the pits. It is suggested that the retardation of pit nucleation and the acceleration of tunnel growth in the presence of SO 4 2− ions can be explained by a partial replacement of Cl− ions from the oxide and metal surface, respectively. Smaller diameter tunnels may be due to the formation of Al2(SO4)3 which can, in part, replace more aggressive AlCl3, and to an easier formation of a passivating film on the tunnel walls owing to their slower dissolution in the presence of Al2(SO4)3.

Evolution of Microscopic Surface Topography during Passivation of Aluminum

The time evolution of microscopic topography on corroding aluminum surfaces during oxide film passivation was characterized. Passivation was studied after galvanostatic etching in 1N at 65°C, in both aluminum etch tunnels (by scanning electron microscopy) and micron‐size cubic etch pits (by atomic force microscopy). Step reductions of applied current initiated passivation. At times of 1 to 300 ms after current steps, the corroding surface was microscopically heterogeneous, consisting of a number of small corroding patches 0.1 to 1 μm in width, which were surrounded by passive surface. As some patches grew by dissolution, others were passivated, until eventually only one patch remained on the pit or tunnel surface. The topography of the corroding surface was controlled by the potential: the surface dissolved uniformly at the repassivation potential , while partial passivation to produce patches occurred at potentials more cathodic than. Patches were unstable at potentials below and would ultimately passivate. Topographic evolution during passivation was very similar for pits as for tunnels, except that the time scale of the process is much longer for tunnels than for pits (200–300 ms vs. 11–20 ms). The difference of time scales was due to the different corroding surface areas of pits and tunnels. Patches are probably defined by a surface layer which inhibits passivation.

Initial Events during the Passivation of Rapidly Dissolving Aluminum Surfaces

The early stages of oxide film passivation of corroding surfaces in aluminum etch tunnels and pits were investigated. Passivation was initiated by step reductions in applied current. A mathematical model for passivation was used to predict experimentally measured potential transients during the first millisecond after the current step. The experimental transients had a characteristic potential undershoot at about 70 μs after the current step; according to the model, the undershoot was directly related to the partial coverage of the corroding surface with an oxide layer. The time and extent of the undershoot were in quantitative agreement with the theoretical predictions, suggesting that the fractional actively dissolving area in the pit at these times is a linear function of potential and adjusts rapidly to changes of the potential. This reconfiguration of the active area occurs at times when the extent of passive film formation is still small. A chemical mechanism for passivation which is consistent with the model is one in which the fractional active area is controlled by the coverage of specifically adsorbed chloride ions. Agreement between experimental and predicted potential transients was also observed at room temperature, where no tunnels are found, but only corrosion pits with irregular shapes.

<I>In situ</I> Observations of Growing Pits during Tunnel Etching of Aluminum

A new technique for studying the mechanisms of tunnel etching is described. During the electrochemical etching of an aluminum foil in a chloride solution, the propagation of initiated pits to 〈100〉 crystallographic directions on the specimen surface was observed with an optical microscope and dynamic TV images of the pit growth were recorded on a video tape. In situ observations revealed that tunnel pits grow continuously at a constant rate without hydrogen gas evolution and then stop growing abruptly at a limited length. Frequent changes in the growth directions through right angles were also detected clearly. At the moment of growth cessation, it was observed frequently that the electrolyte inside the pits convected out rapidly from the tunnel tip toward the base and a gas phase was formed.

アルミニウム箔のトンネルエッチングに及ぼす硫酸の影響

Aluminum foil for high voltage aluminum electrolytic capacitor is etched by D. C. current in the hot chloric solution. The several studies of tunnel growth have ever studied using the hot hydrochloric acid. Effects of sulfuric acid in the hot hydrocloric acid on the tunnel etching of aluminum were studied. The concentration of sulfuric acid has no effect on the initial tunnel growth rate, but has definite effect on the limit of tunnel length. As increase in the concentration of sulfuric acid causes a decrease in the tunnel length. Initial etching pit sites scatter partially on the surface by adding sulfuric acid. The multiplication of etch tunnels occurs in sequence after previous tunnels have reached their limits of length.

Mesa-isolated GaAs Schottky-barrier photodiodes

The design, fabrication and characterisation of GaAs Schottky-barrier phtodiodes with evaporated, free-standing metal airbridges is reported. The photodiodes were farbicatied using all dry-etching techniques. Anisotropic chemically assisted ion beam etching was used to etch vertical sidewall measas, and istropic reactive ion etching was used to etch a lateral tunnel. A free-standing-metal airbridge created by the lateral tunnel etch results in isolation of the active area at the same time, providing free-standing-metal interconnection to the contact pad.

アルミニウムのトンネルエッチングのその場観察

アルミニウムのトンネルエッチングのその場観察

Passivation of Surfaces within Aluminum Etch Tunnels

Transient events accompanying passivation of active surfaces in aluminum etch tunnels are investigated. Passivation is induced by pulsed reductions of the anodic etching current, of several milliseconds duration. Scanning electron microscopy is used to measure the area passivated. Potential transients are analyzed to identify a possible potential driving force for passivation. For the experimental conditions of this work, at 0.13 ms after the current step, there is a temporary reduction in the metal dissolution current in tunnels. The change in dissolution current is proportional to the cathodic surface overpotential, relative to the critical repassivation potential. The transient reduction in dissolution current is associated with passivation, since no variation in dissolution current with potential is observed during steady tunnel growth. Surface changes resulting in permanent passivation occur later, at times between 3 and 12 ms after the current step.

Dissolution of quartz in aqueous basic solution, 106–236°C: Surface kinetics of “perfect” crystallographic faces

Past work on dissolution kinetics of minerals has focused on particulates and the total transfer of material to solution. We look instead at local topography of single crystals. Etching of quartz in alkali hydroxide solutions produces four types of features: flat-bottomed negative crystals with facets for walls, believed to nucleate on microfractures; jumbo pits, giant etch pits cored by dislocations and often with hollow cores; small pits, also believed to be dislocation-cored; and etch tunnels extending from some jumbo pits. A transition from selective etching (revelation of intrinsic defects) to nonselective etching (most intrinsic defects dissolving at the same rate as bulk crystal) occurs as a function of ionic strength, with small pits forming only at ionic strengths below ~ 0.005 m. This transition is attributed to the decrease in Debye-Huckel length with increasing solution strength. We introduce the negative crystal method, whereby dissolution rates of nominally perfect crystal faces are obtained by measuring the size of individual negative crystals during a sequence of dissolution steps. Rates are obtained to ~4% accuracy, with temperature control of ±1.25°C. Using this method, we find apparent activation energies for dissolution of the prism and for an average of the rhombs to be 86.39 and 90.19 kJ/mol, respectively, with corresponding pre-exponential factors of 2.99 × 10 9 and 1.43 × 10 10 ° μm/h. The closeness of these two activation energies argues for a single atomic site controlling the rate, while site density varies with the crystal orientation. At a constant ionic strength of 0.01 m, the rates vary in a complex manner with hydroxide activity, increasing as activity to the ~0.5 power up to ~0.005 m, above which the rate is nearly independent of the alkalinity. Very similar etching behavior is observed in all KOH-KCl mixtures, as well as in LiOH, NaOH, RbOH, and CsOH, although the detailed morphology of small pits is controlled by the chemistry of the etchant. The lower atomic weight hydroxides have nearly identical etch rates; RbOH and CsOH dissolve quartz roughly twice as quickly. We also note a factor of ~ 1.6 enhancement in dissolution rate in certain synthetic over natural samples.