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Abstract
An electronic instrument to control anodization of tantalnm thin film used in the fabrication of resistors and capacitors for hybrid microcircuits has been developed.
In the case of resistors, tantalum pentoxide layer which is grown on the surface during anodization reduces the resistors track thickness thereby increasing its resistance. During anodization the resistance is sensed continuously using a wheatstone bridge excited from an a.c. voltage source. The four arms of the bridge consist of two equal resistors, the tantalum resistor being anodized and a reference resistor become. When the tantalum resistor becomes equal to the reference resistor the output of the bridge at this moment is used to get a step output which in turn is used to stop the anodization process.
For capacitors tantalum pentoxide itself provides the dielectric (Σr=25). Its thickness is proportional to the voltage applied across the anodization cell. The anodization is carried out first at constant current and then it is automatically switched over to constant voltage which is preset to give desired thickness of tantalum pentoxide.
Tentalum resistors with tolerance better than ± 1 per cent and capacitors with tolerance better than ± 10 per cent have been obtained using this instrument.