Plasma Technology for Advanced Devices

Ion-Ion Plasmas

Formation:

Ion–ion plasmas are plasmas in which the negative charges are negative ions instead of electrons.

Ion–ion plasmas may form in the (temporal) afterglow of pulsed discharges in electronegative gases.

Ion–ion plasmas may also form in a flowing plasma, sustained by a continuous wave power source. Far enough downstream of the plasma generation zone, the electron concentration can decay substantially to form an ion–ion plasma.

The electron decay is mainly by electron attachment to electronegative gases (e.g., chlorine, sulfur hexafluoride and oxygen) which leads t the formation of negative ions and diffusion to the walls of the reactor.

Reference: “Fundamentals and applications of ion-ion plasmas”; Demetre J. Economou; Appl. Surf. Sci. 253 (2007) 6672

Characteristics:

The plasma potential does not have to be the most positive potential in the system.

Ion–ion plasmas are characterized by significantly weaker electrostatic fields with a plasma potential determined by the ion temperature (in conventional plasmas it is determined by the electron temperature).

If positive and negative ions have equal mass and temperature, ion–ion plasmas are characterized by the absence of electrostatic fields, zero plasma potential, and no sheaths. Both species are able to diffuse freely to the walls.

The Debye length in an ion–ion plasma is determined by the ion Both positive ions and negative ions can be readily extracted from the plasma.temperature. It is proportional to the square root of the negative ion temperature.

Both positive ions and negative ions can be readily extracted from the plasma.

In the absence of electrons, heavy particle reactions dictate the plasma chemistry.

A Langmuir probe will give an almost symmetric I–V characteristic in an ion–ion plasma.

Reference: “Fundamentals and applications of ion-ion plasmas”; Demetre J. Economou; Appl. Surf. Sci. 253 (2007) 6672

Slide 1 shows the calculated time variation of species fluxes striking an unbiased electrode in a high density pulsed plasma through chlorine (V. Midha, D.J. Economou, Plasma Sources Sci. Technol. 9 (2000) 256).

The electron density shows a large variation of several orders of magnitude between the activeglow and the afterglow, while the negative ions are much less modulated. During the afterglow, electrons are lost rapidly in the early afterglow by ambipolar diffusion to the walls and dissociative attachment. Once the electron density becomes low enough (see below), negative ions become the dominant negative charge carrier in the plasma and the transition from an electron-dominated plasma to an ion–ion plasma occurs.

Slide 2 shows the calculated time evolution of the negative ion and positive ion flux bombarding an electrode in contact with an ion–ion plasma under the influence of a 100 kHz bias (V. Midha, D.J. Economou, J. Appl. Phys. 90 (2001) 1102).

At 100 kHz operating frequency, the powered electrode is bombarded alternately by positive ions (dashed line) and negative ions (solid line) during each half of the RF cycle. The peak ion flux during a half cycle is comparable to twice the diffusion flux of ions without a bias (dotted line). The total ion flux integrated over a full cycle is equivalent to the diffusion flux of ions without a bias. The flux of ions ions bombarding the electrode is limited by the ion diffusion flux.

Slide 3 shows the ion fluxes for a bias frequency of 10 MHz. The bias frequency of 10 MHz is close to the ion plasma frequency evaluated at the sheath edge. Effects of the transit time of ions through the sheath start to play a role. At this bias frequency, there is insufficient time for the applied bias to be fully shielded in the sheath regions. Due to the presence of relatively high electric fields in the bulk plasma, the ion flux to the walls exceeds the diffusion flux. A bias frequency approaching the ion plasma frequency is required to extract ions at a faster rate than the diffusion flux from the plasma.

A bias frequency of 60 MHz is investigated in slide 4. Here, the ions are no longer able to respond to the applied frequency, the perturbation in the ion velocities due to the applied bias is small. The electrode is now continuously bombarded by both positive and negative ions throughout the cycle with an average flux equal to the diffusion flux of the ions.

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