INFLUENCE OF ELECTROLYTES IN THE ELECTRICAL CHARACTERISTICS OF ANODIC FILMS ON TANTALUM

The electrical characteristics of anodic oxide films formed on tantalum are investigated in anodes oxidized in the standard electrolytes for low-voltage (0.01% H PO in water) and high-voltage (same plus ethylene glycol) applications. It is found that small additions (about 0.1%) of certain organic acids such as citric acid to the above electrolytes greatly improves the leakage current, the scintillation voltage and the dielectric losses of tantalum capacitors. Furthermore, the use of these organic acids makes it possible to extend to higher voltages the use of the low-voltage electrolyte, ahd allows, in the case of the high-voltage electrolyte, a substantial diminution in the ethylene glycol concentration without impairing the characteristics of the resulting capacitors. Finally, the effect of the citric acid and the ethylene glycol in the anodizing electrolytes is discussed.


INTRODUCTION
The growth and properties of Ta20s thin films formed by the anodic oxidation of tantalum have been extensively studied. Phosphoric acid, H3PO4, is probably the most widely used electrolyte in the formation of anodic dielectric films for use in tantalum electrolytic capacitors, z The phosphorus ions which are incorporated in the Ta20s film during the oxidation in H3PO4 strongly influence several of the oxide's properties such as chemical stability and dielectric constant. It is usual practice in the tantalum capacitor industry, for voltages lower than 200 V, to form the oxide in a diluted aqueous solution of H3PO4 (from 0.01 to 0.1%). However, for higher voltages, other solvents such as ethylene glycol are added in high concentrations (about 50%) in order to increase the electrolyte's scintillation voltage. 4 This is also the case in the anodic oxidation of other metals (e.g. A1, Bi, Mo, Ti) and semiconductors (e.g. Si, AsGa). 6 In addition to the above electrolytes, other electrolytes based on organic acids seem to form very good quality Taz Os films. Among the organic acids, citric acid is extensively used in the anodization of sputtered tantalum films 7 and has also been proposed as an additive for the working electrolyte in aluminum electrolytic capacitors.
In spite of the extensive use of the electrolytes which we have mentioned, there does not exist a 205 thorough study of some of the basic properties of the oxide (scintillation voltage, leakage current, dielectric losses) which influence the resulting capacitor. In particular, it might be interesting to find mixtures of electrolytes which would optimize the quality of the resulting oxide, as well as reduce the amount of the expensive ethylene glycol used in the high voltage electrolyte.

EXPERIMENTAL
Two types of sintered tantalum anodes, provided by two different suppliers, were used in this work: (i) Ta anodes with a capacity-voltage product CV of 164/2F.V and a weight of 0.04 g.; and (ii) Ta anodes with CV 188/aF. V and weighing 0.09 g. The difference in the relation between CV product and weight is due to the different pressed density of the anode (green density) as well as to the size of the  This effect is more important at low ethylene glycol concentrations, as shown in Table I for tantalum anodes with CV 164/sF-V anodized at 300 V. As in the case of the L.V. electrolyte, a concentration of 0.15% citric acid added to the H.V. electrolyte was also the most appropriate in order to get a minimum in the value of the leakage current.
For an electrolyte formed with a given concentration of ethylene glycol in water (varying from 0 to 55%), 0.15% citric acid, and enough HaPO4 to give a resistivity of 0 440 2cm, the leakage current was next investigated as a function of ethylene glycol concentration. This was done with the object of finding electrolytes with much less ethylene glycol concentration than the H.V.  It might also be significant to study the behaviour of the leakage current as a function of the voltage of polarization and therefore better show the effect of adding citric acid, in appropriate concentrations, to the high voltage electrolyte. Figure 2 shows such characteristic curves for Ta anodes oxidized in the high voltage electrolyte at various ethylene glycol concentrations. It can be again observed that for a given ethylene glycol concentration the use of the electrolytes with citric acid results in a substantially smaller leakage current. In addition, since the characteristic curves of log Ii, vs V show a smaller slope for the electrolytes with citric acid, their relative behaviour improves as the polarizing voltage increases. Figure 3 shows the influence of the citric acid, added to the L.V. electrolyte (0 120 2cm), on the scintillation voltage. It can be observed that the addition of small amounts of citric acid produces an increase in Vb. The relation between Vo and the log of the electrolyte's resistivity is linear, but with negative slope. This last fact contradicts the result found with a large number of electrolytes, which show a small positive slope. 9 The addition of citric acid to the H.V. electrolyte was examined for the anodes with CV 188/F. V (Table II). When citric acid is added to the H.V. electrolyte there appear two beneficial effects: a) The scintillation voltage can be increased by about 100 V, and b) The increase is most effective for ethylene glycol concentrations of the order of 25%, thus allowing a reduction in the recommended amount of 5 5% when no use of citric acid is made.

Dielectric Losses
The addition of citric acid to the L.V. electrolyte has little effect on tan 6, but it is more significant in the H.V. electrolyte as it can be observed in Table III for two different voltages. This table also shows the effect of the citric acid in the high voltage electrolytes with different ethylene glycol concentrations. For an ethylene glycol concentration around 25% the value of tan 6 shows a minimum.  (Table I) Tables II and III). As checked by the authors, the addition of 0.1% to 0.2% of citric acid to the L.V. electrolyte allows its use to be extended by about 50 V. Therefore, this electrolyte without ethylene glycol can, in principle, be substituted for the more expensive H.V. electrolytes up to formations of around 240 V to 250 V, producing anodes of essentially the same characteristics. In the case of oxidations above 250 V,  Due to the effect of the ethylene glycol, the H.V. electrolyte, when used at high voltages, show better anodizing characteristics than the L.V. electrolyte, i.e. the resulting anodes have smaller It and higher Vb. This might be due to the fact that the ethylene glycol traps free water molecules from the electrolyte, resulting in better characteristics and efficiency of the anodization process. 11 Moreover, it has been shown by Jackson that the use of ethylene glycol as a solvent for anodizing electrolytes produces a strong inhibition of the oxide's electric-field induced crystallization. 2 This fact can explain the observed decrease in the leakage current for electrolyte A (Table I)and the generally accepted fact that the scintillation voltage is higher for electrolytes with ethylene glycol. 4