The magnetron discharge pulse excitation

Abstract

Pulse mode is widely used for deposition technology of thin films. These regimes have the advantages over traditional magnetron deposition. During pulsed sputtering nonlinear effects may occur also. But there may be difficulties in excited of a pulsed magnetron discharge. It is due to the generation of free electric charge carriers at the initial stage of the discharge.

The purpose of the work is to determine the conditions when the electron energy corresponds to the maximum cross-section of argon atoms ionization for hundreds of nanoseconds from begin. During this period a Townsend discharge develops. The research is conducted for voltages from 300 V to 1800 V and a pressure of 5 10-3Torr, which are typical for magnetron discharge.

The magnetron model has the following characteristics:

• The magnetic field has a distribution as an experimental sample;
• The magnetic field of the magnetron is balanced;
• The voltage at the cathode-target varies from -300 V to -1800 V with 250 V step.
• The anode is located at a distance of 60 mm from the cathode-target;
• The anode has the potential of the ground;
• The pressure of the working gas (argon) is 5∙10-3Torr;
• Trajectories of 100 electrons were modeled. They were located on the cathode-target surface in a random manner and had zero kinetic energy at the beginning.

The Townsend discharge is beginning when the electrons has an energy corresponding to the maximum cross-section of argon atoms ionization by electrons. It corresponds to the interval of electron energy from 30 eV to 200 eV.

The calculations are made for the time interval from 0.01 μs to 0.1 μs after voltage was applied to the discharge gap. They show that the energy of the main part of electrons (over 70%) has the following meanings:

• For Uk = -300 V, the electron energy is up to 10 eV;
• For Uk = -550 V, the electron energy is from 5 eV to 15 E;
• For Uk = -800 V, the electron energy is from 10 eV to 25 E;
• For Uk = -1050 V, the electron energy is from 10 eV to 30 E;
• For Uk = -1300 V, the electron energy is from 15 eV to 40 E;
• For Uk = -1800 V, the electron energy is from 20 eV to 55 E,
• whereUk is voltage on the cathode-target.

The trajectories of electrons for the cathode-target voltage from 300V to 1800V were calculated. The samples of the calculation results are shown in Figures 3 and 4. It shows that with increase of voltage at the cathode-target, the elec-tronic trap is move down to the surface of it. When the Townsend discharge passes into a magnetron discharge, this leads to the phenomenon of rarefaction of the working gas over of the cathode- target surface.

As a result of calculating the distribution of electron energies, the following were determined: 1) The voltage of a self-maintained pulsed magnetron discharge stable excitation. This voltage should be more than 1300 V. 2) The voltage range at which the excitation of a self-maintained pulsed magnetron discharge is unstable and can lead to a delay in the excitation of the discharge during the pulse, or to their passes. This is the voltage range is from 800 V to 1300 V. 3) The voltage at which a pulsed magnetron discharge can’t be excited without external sources of free electric charge carri-ers.

Ref. 12, Fig. 5.