Anodic oxides etching

M. Hirita, S. Suwazono, and H. Tanigawa, Diaphragm thickness control in silicon pressure sensors using an anode oxidation etch-stop, J. Electrochem. Soc. 134, 2037, 1987. 

C. Gourgon, L.S. Dang, H. Mariette, C. Vieu, F. Muller, Optical-properties of CdTe/ CdZnTe wires and dots fabricated by a final anodic-oxidation etching. Appl. Phys. Lett. 66(13), 1635-1637 (1995)... 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

Barrier anodic oxides covering the surface of aluminum etched foil are usually formed in borate or phosphate solutions. To improve capacitor characteristics, high-purity aluminum is desirable with as low a concentration of impurities as is acceptable in terms of cost. 

Anodic oxide films formed under different kinetic conditions vary in structure, composition and property (e.g., etch rate) and they change with time during the anodization. 

For the case of Si02 etching, HF, (HF)2 and HF2- are assumed to be the active species [Vel, Jul]. If HC1 is added to the solution the concentration of the HF2-ion becomes negligible, which leaves HF and its polymers to be the active species [Ve3]. Because for high current densities the electrochemical dissolution of silicon occurs via a thin anodic oxide layer it can be concluded that, at least for this regime, the same species are active. This is supported by the observation that F- is... 

A passivating oxide is formed under sufficiently anodic potentials in HF, too. However, there are decisive differences to the case of alkaline and fluoride-free acidic electrolytes. For the latter electrolyte the steady-state current density prior to passivation is zero and it is below 1 mA cnT2 for alkaline ones, while it ranges from mA cm-2 to A cm-2 in HF. Furthermore, in HF silicon oxide formation does not lead to passivation, because the anodic oxide is readily etched in HF. This gives rise to an anodic I-V curve specific to HF, it shows two current maxima and two minima and an oscillatory regime, as for example shown in Fig. 4.7. 

In contrast to acidic electrolytes, chemical dissolution of a silicon electrode proceeds already at OCP in alkaline electrolytes. For cathodic potentials chemical dissolution competes with cathodic reactions, this commonly leads to a reduced dissolution rate and the formation of a slush layer under certain conditions [Pa2]. For potentials slightly anodic of OCP, electrochemical dissolution accompanies the chemical one and the dissolution rate is thereby enhanced [Pa6]. For anodic potentials above the passivation potential (PP), the formation of an anodic oxide, as in the case of acidic electrolytes, is observed. Such oxides show a much lower dissolution rate in alkaline solutions than the silicon substrate. As a result the electrode surface becomes passivated and the current density decreases to small values that correspond to the oxide etch rate. That the current density peaks at PP in Fig. 3.4 are in fact connected with the growth of a passivating oxide is proved using in situ ellipsometry [Pa2]. Passivation is independent of the type of cation. Organic compounds like hydrazin [Sul], for example, show a behavior similar to inorganic ones, like KOH [Pa8]. Because of the presence of a passivating oxide the current peak at PP is not observed for a reverse potential scan. 

The easiest way to have different parts of the electrode surface under different bias is to disconnect them by an insulator. This method is elucidated by an experiment in which an electrochemical etch-stop technique has been used to localize defects in an array of trench capacitors. In a perfect capacitor the polysilicon in the trench is insulated from the substrate whereas it is connected in a defect capacitor, as shown in Fig. 4.15 a. If an anodic bias is applied the bulk silicon and the polysilicon in the defect trench will be etched, while the other trenches are not etched if an aqueous HF electrolyte is used, as shown in Fig. 4.15b. The reverse is true for a KOH electrolyte, because the only polysilicon electrode in the defect trench is passivated by an anodic oxide, as shown in Fig. 4.15 c. 

The morphology of the anodic oxide is sensitive to oxidation parameters and electrolyte composition. At water concentrations below about 5% in the electrolyte the oxide is usually more dense and shows a lower etch rate in HF than for concern... 

Fig. 5.10 Voltage-time curve (solid line) for an n -type silicon electrode (3 mficm) anodized with a constant current density of 6.25 mA crrf2 for t> 0 (sample at OCP for t<0) in 0.3 mol kg- NH4F (pH = 3.5). The thickness of the anodic oxide was measured by ellipsometry (open circles, broken line fitted as a guide to the eye). The etch rate of the anodic oxide in the electrolyte was measured (values above arrows) at different...

ELECTROLYTIC ETCHING OF metals produces various results intergranular attack, attack of crystalline surfaces which is orientation dependent formation of etch pits, and anodic oxide films. The behavior of a metal or alloy depends on composition, temperature of the electrolyte, and above all on the electrode potential which varies with the metal. Applications to Al, Fe, stainless steel, Ti, 2r, U, and their alloys will be discussed. 

In contrast, after electroetching the growth of the anodic oxide is epitaxial and its colors vary with orientation, la die case of the electrolytes of Ence and Margolin or Hiltz (except for (0001) grains), the weak dependence of color on orientation is probably due to the lack of electroetching prior to the anodic oxidation. The authors recommend chemical etching prior to oxidation in order to improve the color contrasts but they do not specify the nature of this etching. 

Two well known processes have been used to prepare inorganic membranes with nearly straight pores anodic oxidation and track etching. The former method has been practiced commercially while the latter remains as a laboratory investigation. 

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