Formation Voltage

A significant influence of the formation voltage on the film properties was found. The aluminium oxide films were formed at a constant current density of 0.5 mA/cm. The formation voltage was raised in steps to 5, 10, 15, 30, 60 and 100 V. When the formation voltage was reached, the potentiostat was switched 

For low formation voltages, the current density approached a value of 0.01 mA/cm after an anodisation time of 2400 s. However, above 30 V the current density reached values 0.01 mA/cm and up to 0.02 mA/cm at a voltage of 100 V. 

The transition voltage depends on the formation current density. At formation current densities above 2.5 mA/cm, the transition voltage was greater than 60 V. Chang et al. [11] reported that the transition voltage was 150 V when the films were formed at 25 mA/cm in 0.83 M ammonium adipate solution. 

The transformation from an amorphous film to a crystalline film at 30 V was also observed for Ti02 films [20]. The Raman spectrum of anatase was observed when the formation voltage was raised to 30 V. The Raman intensities increased with increasing formation voltages. A similar phenomenon can explain the change of the structural properties for the aluminium oxide films formed at voltages 30 V. 

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Depending on substrate orientation and formation condition, individual pores may have different shapes. The shape of the pores formed on (100) substrate is a square bounded by 011 planes with comers pointing to the <100> directions.14,77 The shape of individual pores formed on n-Si tends to change from circular to square to star-like and to dendrite-like with increasing potential.20 Low formation voltage tends to favour circular shape while high voltage favours star-like shape. Near perfect square shape of pores can be obtained for the PS formed on n-Si under certain conditions. 

The barrier-layer thickness (AB in Figure 2.15), cell diameter and pore diameter are directly proportional to the formation voltage. 

Curve showing the variation of capacity with formation-voltage of a chemical condenser, using aluminum plates. 

To obtain the large capacities required for B battery eliminator circuits, we must use very large plates or a very thin dielectric. The electrolytic condenser, on account of its extremely thin gas-film dielectric, has an enormous capacity when only small plates are used. C. I. Zimmermon found that the thickness of the film is between 1/50,000 and 1/500,000 of an inch, depending upon the formation-voltage. The dielectric constant of the film is about 10, so that a capacity of to j4-mfd. per square inch of electrode surface is easily obtained. 

The total effective area of the ten plates is 503 square inches, and the length of the boundary lines 3 inches. The residual current, or leakage current, at 110 volts was. 0005 amperes, which leaked through the insulation at the boundary line. The distance between the plates was about 3/16 of an inch. In this condenser, five plates were used for each side of the circuit and as each set of plates is of aluminum, it makes no difference which way the condenser is connected in the circuit. The following table shows the measured capacities with different formation-voltages ... 

Russian workers have looked at acoustic waves produced during the electrochemical oxidation of antimony [ 147], almost a reverse application of sonoelectrochemistry. Antimony was anodized in aqueous H3B03 solutions galvanostatically (2.2 x 1(T3 A/cm2) and isothermally (292 K). The formation voltage increased to >200 V with time, which is characteristic of the valve metals. Acoustic waves were observed in this electrochemical oxidation with amplitudes that did not differ essentially from the very beginning of the oxidation. The energy of the acoustic wave had only one sharply distinct peak which coincided in time with the appearance of the electrochemical breakdown products. 

SEM was used for morphological studies of anodic aluminium oxide films, formed at various formation current densities up to a formation voltage of 60 V. The total thickness of the films was determined by cross-section SEM micrographs as shown in Figure 23.1, but it should be emphasised that one can not clearly identify barrier and porous layers of the oxide film by using this technique. 

Figure 23.1 High-resolution cross-sectional SEM micrograph of an anodic aluminium oxide film of 66 nm formed at 0.3 mA/cm and a formation voltage of 60 V (Al thickness remaining 633 nm).

Figure 23.3 Bode plots of natural and anodic aluminium oxide films. The aluminium oxide film was formed at 2.5 mA/cm and a formation voltage of 60 V, experimental (points) and fitting (lines).

Figure 23.6 Bode plots of aluminium oxide films formed at 0.5 inA/cm, different formation voltages 5, 10 and 15 V, experimental points and fitting lines.

The significant reduction of the resistance i 2 of the aluminium oxide film fi om 166 MQ/cm at 30 V to 8.2 MQ/cm at 100 V could be explained by increasing crystallinity inside the amorphous phase. The resistance of the film decreases from 2.8 MQ/cm at 30 V to 1.6M 2/cm at 100 V. Figure 23.9 shows the effect of the formation voltage on the capacitance and the barrier layer thickness. The capacitance decreases with increasing formation voltage, while the barrier layer thickness approaches a constant value around 40% of total thickness. 

Figure 23.8 Model of anodic alumimum oxide film formed at formation voltage of 30-100 V (A) and equivalent circuit (B).

Figure 23.9 Effect of the formation voltage on capacitance and barrier layer thickness.

The above results showed that an aluminium oxide film with the best dielectric properties was prepared in neutral electrolyte of 0.01 M tartaric acid at low current densities and formation voltages < 30 V. 

The film is formed on E-200-Al/glass at a current density of 0.5 mA/cm for the anodisation time of 1800 s. The film thickness was controlled by the formation voltage of 10 V. The electrical measurements were carried out on different electrode areas ranging from 0.0025 to 0.04 cm (see Figure 23.12). Below breakdown, low leakage currents of less than 13 nA/cm at 1.67 MV/cm could be observed. The breakdown of the dielectric film occurs when the electrical field strength approaches 8 MV/cm. 

The average capacitance and specific resistivity of the barrier aluminium oxide films are determined to be 430. .. 470nF/cm and 1.3. .. 2.4 10 " Qcm, respectively. By using the anodisation factor of 1.2 nm/V for the films formed at low formation voltage, dielectric constants of 5.8. .. 6.4 are calculated from the measured capacitance values. The comparatively low dielectric constant is in agreement with the formation of an amorphous anodic aluminium oxide film as discussed above rather than a crystalline structure for which a higher dielec-... 

Figure 23.12 Current density (/) vs. field strength (E) of the harrier aluminium oxide film/Al(200 nm)/glass formed at a current density of 0.5 mA/cm, a formation voltage of 10 V and an anodisation time of 1800 s.

The thickness and properties of the barrier aluminium oxide layer were investigated by eleetrochemieal impedanee speetroscopy. The total thickness of the films was determined by seanning electron microscopy of cross-sections. Then, the thiekness of eaeh layer within the aluminium oxide films was calculated. Formation eurrent density, formation voltage, anodization time, and sur-faee roughness of the substrate influenced the electrical and structural properties of the barrier aluminium oxide layer. 

The anodic treatment process was carried out in a combined regime first, galvanostatically at a current density of 1 mA/cm up to the formation voltages (Uform) of 10-50 V, and then, when the given Uform was reached, potentiostatically, up to the current of 0.01-0.02 mA. To measure characteristics of the anodically oxidized silicon (AOS) (thickness, dielectric properties, and their surface distribution) the anodic treatment of the silicon wafers was done in a cell with the anodization area of 5 cm. Single-crystal boron-doped (100) silicon wafers of resistivity 0.3 Ohm cm, phosphorus-doped (100) and (111) silicon substrates of resistivities 0.1 and 4.5 Ohm-cm, respectively, and boron-doped (100) and( 111) silicon wafers of resistivity 4.5 Ohm-cm were used as silicon anodes. 

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