Pulse biasing during the growth of thin films by magnetron sputtering

The control of the energy of an ionic flux from a plasma is of critical importance to both the growth and modification of thin films undergoing plasma based processing. An asymmetrical pulse biasing technique is discussed for the control of charged particles extracted from a plasma to a surface coupled capacitively to the pulse. The asymmetrically pulsed bias offers a unique advantage over typical radio frequency (rf) techniques through the ability to modify the asymmetry of the pulse. Through the proper application of the pulse bias, such as properly setting the frequency, the duty and the amplitude, a desirable ion energy distribution can be obtained.; A quasi-static treatment of the sheath relates the potential evolution of a pulsed surface exposed to the plasma to the shape of the applied pulse, the capacitive coupling, and the flux of charge from the plasma in response to the pulse. Both measured and predicted timescales associated with the charging of an electrode exposed to an ionic flux extracted from a 600 mA discharge were on the order of 2 ms when a 50 Volt pulse is coupled to the surface through a 10 nF coupling capacitance. Measured and predicted energy spectra of the ions to a pulsed surface show good correlation as the magnetron discharge was varied from 100 mA to 600 mA, the coupling capacitance was varied from 1 nF to 100 nF, the frequency of the pulse was varied from 500 Hz to 100 kHz, and the duty of the pulse was varied from 50% to 90%. Based on predicted behavior of the sheath in response to a pulsed bias, frequencies below 1 MHz can be treated with the quasi-static treatment.; To examine the transient behavior of the sheath, the temporal-spatial evolution of the sheath is obtained through the simultaneous solution of the Poisson equations, the ionic fluid equations, and a Boltzmann treatment of the electrons. Calculations are performed to demonstrate the displacement currents associated with the restructuring of the sheath and the lag in the response of the ions extracted from the sheath occurs over timescales below 1 μs. This finite response of the plasma to the applied pulse influences the ion energy distribution. Comparison between the predictions made by the quasi-static treatment of the sheath and the predictions made considering the transient nature of the sheath illustrate how the transient phenomena distort the ion energy spectra for pulse frequencies above 100 kHz.; As an example of the effectiveness of the asymmetrical biasing technique, thin copper films are sputter deposited onto silicon dioxide surfaces under the influence of ion bombardment. The preferential crystallographic orientation, the deposition rate, the surface roughness, and the electrical resistivity are shown to be functions of the ion energy. Copper films grown under the influence of energetic bombardment driven by a 70-volt bias pulsed at 50 kHz with a duty of 70% have lower resistivity (3.7 μΩ-cm at 40 nm) and possess larger grains (>1000 nm2 at 5 nm) compared to the resistivity (4.8 μΩ-m at 40 nm) and grain sizes (<500 nm 2 at 5 nm) of copper films grown with out the pulsed bias.