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International audienceUsing Langmuir probe-assisted laser photodetachment, the temporal evolution of the O- density has been determined in the bulk plasma of a unipolar pulsed-dc magnetron. The source was operated in reactive mode, at a fixed nominal on-time power of 100 W, sputtering Ti in argon-oxygen atmospheres at 1.3 Pa pressure, but over a variation of duty cycles from 5 to 50 % and oxygen partial pressures of 10 and 50 % of the total pressure. In the plasma on-time, for all duty cycles the negative ion density (n-) rises marginally reaching values typically less than 2 x 10 15 m -3 with negative ion-to-electron density ratios, < 1. However, immediately after the transition from pulse on-to-off, n- falls by about 20- 30% as fast O- species created at the cathode exit the system, however this is followed a rapid rise in n- to values at least 2 or 3 times that in the on-time. The rate of rise of n- and its maximum value both increase with decreasing duty cycle. In the off-time, the electron density falls rapidly (initial decay rates of several 10's of µs), and therefore the afterglow plasma becomes highly electronegative, with reaching 4.6 and 14.4 for 10 and 50 % oxygen partial pressure respectively. The rapid rise in n- in the afterglow (in which the electron temperature falls from about 5 to 0.5 eV) is attributed to the dissociative attachment of highly-excited oxygen matastables, which themselves are created in the pulse on-time. At the lowest duty of 5 %, the long-term O- decay times are several 100's of µs. Langmuir probe characteristics show the clear signature that negative ions dominate over the electrons in the off-time. From the ion and electron saturation current ratios, has been estimated in some chosen cases and found to agree within a factor between 2 and 10 with those obtained more directly from the photodetachment method

A study of the plasma electronegativity in an argon–oxygen pulsed-dc sputter magnetron