The Hardness and Wear of Electrochemically Grown Tantalum Oxide
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controlled growth of the oxide to a pre-determined thickness. EXPERIMENTAL METHODS
The electrochemical growth of tantalum oxide on a Ta substrate has been studied and shown to provide a linear relationship between the voltage applied to the sample in the electrochemical cell and the thickness of the oxide layer formed [8-10]. The rate of the oxide formation is dependent on the composition of the electrolytic solution and the current density applied to the electrochemical cell [10]. Therefore, it is necessary to calibrate any system before use. Anodic Polarization Tantalum sheet (0.25 mm thick) was cut into 10 mm X 20 mm samples. The samples were degreased with minimal amounts of acetone, washed with 90% ethanol, washed with distilled water and then placed in a glass cell. The anodic polarization of the samples was carried out using 0. IM sulphuric acid as the electrolytic solution and a platinum wire gauze as the counterelectrode. The voltage applied to the sample was controlled using a D.C. power supply and monitored using a digital multimeter. All samples were started at 0 V and then ramped at 1V/sec until the desired voltage was reached. Once the desired voltage was attained the anodic polarization was permitted to continue until the current density of the circuit was 0 A/cm 2 . Following the completion of the reaction, all samples were washed with distilled water and allowed to dry. 211 Mat. Res. Soc. Symp. Proc. Vol. 505 01998 Materials Research Society
Ta2Q5 Layer Thickness Measurement The thickness of the oxide layers was calibrated using the coulometric measurement method [10]. The charge density through the electrochemical cell was measured for the entire reaction using a chart recorder. The equation: d=QM/nFp
(1)
3
were Q is the charge density C/m , M is the molecular weight of tantalum oxide (408.9 g/mol), n is the number of electrons involved in the reaction (10), F is Faraday's constant (96,485 C/mol) and p is the density of tantalum oxide (7930 kg/m3) [10] was used to determine the relationship between thickness and applied voltage. The thickness of the oxide layers was also calibrated using the potentiostatic method of film thickness measurement. With the potentiostat we made capacitance measurements from 1 Hz to 10 kHz using an electrolytic capacitor bridge.
The measurements were made using the same
electrolytic solution and counterelectrode that was used to grow the films. capacitance was then used in the equation: d=(E Eox A)/C
The measured
(2)
where E is the permittivity of free space (8.85 X 10-12 F/m), Eox the dielectric constant of the Ta20 5 film (27.6) [10], A is the surface area of the reacted sample, C is the measured capacitance of the film and d is the thickness of the oxide film. Indentation Testing of Ta2Q 5 Films Microindentation measurements were made using a LECO M400 Hardness Tester. Samples were mounted on a glass slide to provide stability and to prevent the sample sliding during the indentation tests. Indentations were made using both Knoop and Vickers tips under load
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