An investigation was made of tantalum films sputtered in argon containing individual small amounts of nitrogen, methane, and oxygen. With argon alone, 1000-Å bcc tantalum films deposited on glass substrates heated to 400°C had a specific resistivity four times the bulk value, with the increase attributable to a reactive gas residual pressure of 10−6 to 10−5 Torr. The mixed argon-nitrogen experiments produced initially hcp Ta2N, and at higher pressures an fcc phase attributed to a new TaN structure. With methane only fcc TaC was obtained, while with oxygen an amorphous phase began to grow initially, and at higher pressures was dominant with a structure identical to anodically formed Ta2O5. The nitrides and carbide have specific resistivities from 200–300×10−6 Ω cm, and temperature coefficients between +3×10−4 and −2×10−4 deg−1. With increasing oxygen content, the specific resistivity increases approximately exponentially; the positive temperature coefficient decreases, becomes zero before the precipitation of Ta2O5, and then increases negatively at a rapid rate at the appearance of Ta2O5.

About five years ago when one wanted to become acquainted with thin film deposition techniques and the properties of thin films there were two standard books which covered the literature published on these subjects quite adequately. These books were written by Holland and Mayer. However, in the past few years the number of papers published annually on all aspects of films has increased tremendously and it is next to impossible for one author to keep abreast of all the new developments in this rapidly expanding field. Since it is likely that the research in this area which includes optical, electrical, magnetic, and superconducting properties as well as various surface reactions and surface phenomena, just to name a few, will grow at an even faster rate in the future, there was a definite need for up-to-date survey articles on important advances in this field.The first volume of the series "Physics of Thin Films" serves this purpose very well, indeed. Quite appropriately the first article by H. L. Caswell is on ultrahigh vacuum evaporators and residual gas analysis with emphasis on the importance of the vacuum in which films are deposited in determining their characteristics. The second article by

Results are presented for the thermal oxidation behavior of four types of sputtered tantalum films between 100° and 525°C. These films are bcc tantalum, β-tantalum, a porous β-tantalum, and tantalum nitride (Ta2N). All four types of films show parabolic oxidation kinetics and the formation of adherent Ta2O5 (tantalum oxy-nitride for Ta2N films) with bright interference colors. The first three types of films dissolve about 12 at.% oxygen, which is much higher than the oxygen solubility found in bulk tantalum, and the films show a different degree of suboxide formation than bulk tantalum. The activation energies for oxygen diffusion in the metal and for oxide growth are 1.2 and 1.4 eV, respectively. The dielectric properties of the thermal oxide are discussed in terms of the defects in the oxide.

The technique of reactive sputtering has been used for producing tantalum films with various concentrations of interstitial additives like oxygen, carbon, nitrogen, and hydrogen. A number of compounds between tantalum and these nonmetallic elements also have been deposited. The dielectric properties of anodic films on the tantalum formed in an aqueous solution of citric acid have been related to the composition of the tantalum. It was found that the capacitance density of 130v anodic films remains unaffected by the presence of oxygen and hydrogen, while it decreases as the atomic concentration of nitrogen and carbon in the tantalum increases. The dielectric constant, ε, ofanodic films on Ta2N is approximately 50% of the value of anodic Ta2O5 formed on pure tantalum.

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