Temperature coefficient of resistance of beta-tantalum films and mixtures with b.c.c.-tantalum

Beta-tantalum is the preferred thin film material for the anode of thin film tantalum oxide capacitors,but further information is required about its basic electronic characteristics. The original report on its properties characterized it as having: a resistivity at room temperature of 200 ± 20 μΩ cm; a temperature coefficient of resistance (α), measured between room and liquid nitrogen temperatures, of ± 100 p.p.m./°C; and a superconducting transition of 0.57 -0.47 K. In the present study, beta-tantalum films obtained from various bell jar systems in Bell Telephone Laboratories were studied for their resistivity characteristics down to 4.2 K. Films which had no detectable b.c.c.-tantalum had linear α values of -150±30 p.p.m./°C over the temperature range of 4.2 to 380 K. Films with increasing amounts of b.c.c.-tantalum tended to have an increasing positive α value in the temperature range of 4.2–100 K, and with sufficient b.c.c.-tantalum, a maximum in resistivity can occur. The pure beta-tantalum films with the negative constant α values had superconducting transitions of 0.96-0.67 K. Mixed beta- and b.c.c.-tantalum films have been prepared by nitrogen doping, and these films confirm that the presence of b.c.c.-material causes an increasing positive α value at low temperatures. The conversion temperature of beta- to b.c.c.-tantalum, determined on stripped film material, has been confirmed to have a high value of 800°–850°C. Despite this high value, the shape of the low temperature resistivity curve indicates that mixtures of b.c.c.- and beta-tantalum can be deposited by sputtering at substrate surface temperatures of 400°–465°C. The α-values between 4.2 and 77 K or between 77 and 196 K are particularly sensitive to the amount of b.c.c.-material in the film. The α-value method of analysis to detect b.c.c.-material is probably more sensitive than the X-ray method.