Anodized Tungsten Thin-Film Capacitors and Resistors

Capacitors with tungsten oxide as the dielectric were prepared by electrochemical oxidation in dilute sulfuric acid of thin tungsten films deposited on glass substrates. Evaporated gold counter-electrodes were used. Tungsten capacitors did not exhibit a definite capacitance-forming voltage relationship, and the capacitance often changed markedly with time. When the inverse of the capacitance C is plotted against layer thickness d for capacitors anodized to the same voltage, the points are found to drift toward a line given by C
^-1
= B(d -- d
o
). The dielectric constant calculated from the slope B is 40. This suggests that a limited portion do of the total oxide thickness is gradually converted into relatively conductive material. Thin-film resistors have been prepared by the anodization of vacuum-deposited tungsten metal. The resistors are provided with almninum contact tabs to maintain a durable, low resistance contact. Anodization of the tungsten film forms a protective oxide layer and adjusts the metal thickness to provide precisely a desired resistance value. Resistance values with 1 per cent precision are obtained by automatically monitoring the anodization process. Heat treating the films in vacuum for short periods results in stabilization without any appreciable change in resistance. Sheet resistivities as high as 2000 ohms/sq have been obtained with a temperature coefficient of resistance no greater than --200 ppm/°C, and sheet resistivities as high as 575 ohms/sq have been obtained with zero temperature coefficient of resistance. Stability is maintained within 1 per cent per 1000 hours while dissipating 2 w/in
²
at 25°C for the 2000 ohms/sq films, and 8 w/in
²
for 250 ohms/sq films. These characteristics made tungsten an attractive choice over other refractory metals for use as thin-film resistors in microelectronic circuits.