Plasma is typically a partially ionized gas and is considered to be a distinct state of matter, in contrast to gases because of its unique properties.
The reactions to form plasma in a vacuum system include dissociation,
in which electrons emitted by two parallel electrode plates under DC or AC bias collide with the gas X2 to form plasma.
Plasma generation requires electrons with high kinetic energy K:
K = F ∙ d = q ∙ Ε ∙ λ = q ∙ V
de ∙ λ ….…….…………
where E is electric field, λ is mean free path, V is electrode voltage and de is electrode distance. Thus, plasma is generated in high voltage bias to increase V and in low pressure to increaseλ . Take argon (Ar) for example; the process of plasma generation can be conceptually diagramed as in Fig. 2.18.
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Figure 2.18 The process of Ar plasma generation.
Bombarded by accelerating ions with high energy in the plasma, the surface particles of target are then sputtered and deposit on the substrate.
(Reference [47] detailed the mechanism of nucleation and growth of thin film.)
The two electrode plates of DC sputter have to be conductors or the charges will accumulate on the non-conductive plates and then the plasma reaction will stop. AC sputter can prevent charges from accumulating on the electrode plates by changing the electrode polarity periodically.
Therefore, DC sputter is suitable for sputtering conductive material such as metal while AC sputter is suitable for both conductive and insulating material such as SiO2. The lowest frequency for AC sputter must be higher than 100 Hz to maintain plasma reaction. The AC sputter system of which frequency is 13.56 MHz is called an RF (radio frequency) sputter.
In RF case, the electrons obtain kinetic energy through oscillation in the field.
An RF magnetron sputter system applies a magnetic field so that electrons can accelerate electrons and increase collision of electrons with gases in a helix path between electrode plates, as illustrated in Fig. 2.19.
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Figure 2.19 Magnetron sputtering.
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Chapter 2 Bibliography
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