Analysis of sputtering discharge by optical and mass spectrometry. II. Platinum and tantalum sputtered in argon/nitrogen mixtures

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Abstract
The gas composition during sputtering of tantalum and platinum in argon‐nitrogen mixtures in a dc diode system has been investigated using optical and mass spectrometry. Both methods show that, for sputteringtantalum, the water‐vapor pressure is significantly reduced by the addition of nitrogen, although no nitrogen is observed until a flow rate of 0.3 cm3 min−1 is reached. For platinum, however, nitrogen is observed at much lower flow rates and no reduction in the water vapor takes place. After sputteringtantalum in the presence of nitrogen, ammonia is the main residual gas and it is suggested that the reactions required to form it take place at a tantalum surface. For nitrogen flow rates less than 0.3 cm3 min−1, the β‐Ta phase is replaced by the bcc phase with a consequent reduction in film resistivity. The nitrogen apparently prevents the incorporation of oxygen in the film, but some nitrogen is probably dissolved in the bcc phase. At flow rates just above 0.3 cm3 min−1, the Ta2N phase is formed and is replaced by the TaN phase at higher flow rates. The increase in resistivity with increasing flow is probably due to the presence of an insulating amorphous nitride phase. The tantalumdeposition rate decreases with increasing nitrogen flow because of the reduction in the hydrogen concentration, the replacement of argon by nitrogen as the sputtering species, and, possibly, the formation of nitrides on the cathode. The ratio of the intensity emitted by excited tantalum atoms to the tantalumdeposition rate increases rapidly with nitrogen flow and this may be due to the formation of nitrides on the cathode. The film density also decreases from 14.0 to 6.5 g cm−3 as the nitrogen flow rate is increased.