Sputtered Tantalum Films Research Articles

Tantalum Films Deposited by Asymmetric A‐C Sputtering

The new technique of asymmetric a‐c sputtering has been evaluated for the deposition of tantalum films in argon, oxygen‐argon, and nitrogen‐argon gas mixtures. This method utilizes a lower sputtering potential on the tantalum substrate holder every half cycle compared to the potential on the tantalum cathode. At an optimum current ratio, tantalum films of three to five times that of bulk resistivity are obtained compared to twenty to forty times bulk resistivity for normal d‐c glow discharge sputtering. The reactive sputtering experiments confirm that the normal high resistivity of sputtered tantalum films are caused by impurities gettered during the deposition and that the greater tolerance to reactive gas pressure with asymmetric a‐c sputtering may be due to ion and electron bombardment during deposition. Further work is required to confirm this latter supposition.

The preparation of microcircuit stencils and patterns by photomechanics and electron beam machining

Photomechanics is at present the preferred method of defining the geometry of finely detailed two-dimensional microcircuit structures. A number of alternative techniques based on photon and electron irradiation have been described. In the authors' laboratories an existing photomechanical service is operated alongside a prototype 60 kV, 2 μ spot size electron beam machining apparatus. In Part 1 of the paper, representative photomechanical results are described. Part 2 deals with the application of the electron beam apparatus to the thermal etching of tracks in thin supported films and foils using power densities of the order of 10 6 W/cm 2. Typical microcircuit materials have been machined and the resolution and edge definition attainable determined. Using fast scanning of the beam parallel sided isolating tracks 5–10 μ wide have been obtained at spacings of 20 μ or less when machining evaporated nickel chromium, gold and sputtered tantalum films on borosilicate glass substrates. These results are sufficiently promising to justify further investigation of the feasibility of making precision film resistors and other devices in this way. A “flash evaporation” machining mechanism has been derived from the experimental results and classical electron penetration theory. It is concluded that photomechanics will continue to be the preferred shaping method where fixed images containing detail above the diffraction limit must be reproduced cheaply. Electron beam thermal machining offers slightly better resolution and can be used with refractory materials which are difficult to etch chemically. Its main advantage is, however, the possibility of using feed-back from monitored samples to control the beam's action. Other suggested applications of the electron beam apparatus include welding, localized alloying, sintering and recrystallization.

233. The oxidation of sputtered tantalum films: H. Basseches, J. Electrochem. Soc., 109, June 1962, 475–479

233. The oxidation of sputtered tantalum films: H. Basseches, J. Electrochem. Soc., 109, June 1962, 475–479

The Oxidation of Sputtered Tantalum Films

The oxidation of sputtered tantalum films has been studied over a temperature range of 100°–600 °C in oxygen at a gas pressure of 7.6 cm of Hg. The results obtained by means of a vacuum microbalance are compared with data on bulk tantalum, and it is found the rate of oxidation and amount of oxidation is considerably smaller than the bulk. A logarithmic relation was found to represent the data best in the range 400°–600°C. The factors which may give rise to the differences found between the film and the bulk are discussed.

The Oxidation of Sputtered Tantalum Films and Its Relationship to the Stability of the Electrical Resistance of These Films

A study of the oxidation of sputtered tantalum films was carried out to help explain the changes in electrical resistance of such films when heated in air at 100°C. A microbalance was used to obtain the oxidation data over a range of temperatures from 100-600°C in pure oxygen at a pressure of 7.6 cm of Hg. The electrical resistance of films was measured over a temperature range up to 200°C. Some results were also obtained when heating was carried out in a vacuum of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">--6</sup> mm of Hg. From a consideration of the oxidation data and the electrical resistance data, it appears that oxidative effects based on a simple model of oxide film formation do not play a major role in affecting the stability of the electrical resistance of the films under the test conditions. Oxidative effects at grain boundaries may have more of an influence. However, structural changes, not yet characterized, may be equally important in accounting for the resistance changes.

Tantalum Printed Capacitors

It has previously been considered either impossible or impractical to make tantalum oxide capacitors by applying a metal counter electrode directly to a tantalum oxide film. Attempts in this direction have led to either direct shorts or low breakdown strength. For this reason, it has been presumed that it was essential to use either an electrolyte or a semiconductor in the cathode structure. Using sputtered tantalum films as the base for the anodized oxide film, however, excellent results have been achieved employing evaporated metal counter electrodes. Many of the properties of units made in this way are superior to those of other types of tantalum capacitors. Capacitances obtained are comparable to the capacitance-area relationships for tantalum electrolytic capacitors formed to the same voltages. DC leakages, however, have been found to be much lower than values reported for those of tantalum electrolytic units. Another advantage is that these units are capable of withstanding higher voltages than will tantalum solid electrolytic capacitors formed to the same voltage. Indeed, voltages equal to the anodizing voltage may be maintained on the capacitor without impairment. Capacitors have also been produced with thin films of other anodizable metals, evaporated aluminum in particular. This type of capacitor should find many applications in the lower capacitance areas, and seems ideally suited for printed circuit applications.