Influence of two reactive gases on the instabilities of the reactive sputtering process

Since the process using two reactive gases is much more complex than one reactive gas, the transition zone of the reactive mode must be defined taking into account oxygen and nitrogen mass flow rates. A two-dimensional representation delimiting boundaries of the instability region and depending on both mass flows is proposed for RF and DC polarisation of the titanium target. This diagram allows to determine operating conditions favourable to deposit oxynitride coatings with a modulated oxygen or nitrogen composition. It is also shown that the supply of one reactive gas affects consumption and behaviours of the other gas as well as its own characteristics. A critical region is then defined in which the process is trapped in reactive mode and cannot go back to elemental conditions by changing mass flow rate of only one reactive gas. Kinetics and pollution of the titanium surface is discussed from measurements of DC potential for various operating points. It is shown that comportments of oxygen and nitrogen towards metallic or poisoned target are interdependent and a surprising evolution of the time of pollution against oxygen and nitrogen introduction is observed.