Self-bias negative

FIG. 4. Schematic representation (a) of a parallel-plate, capacitively coupled RF-discharge reactor, with unequal-size electrodes. The potential distribution (b) shows the positive plasma potential Vp and the negative dc self-bias voltage... 

In most systems the substrate electrodes are larger than the powered electrodes. This asymmetric configuration results in a negative dc self-bias voltage Vdc on the powered electrode. Without that, the difference in electrode areas would result in a net electron current per RF period [134, 169]. It has been shown that the ratio of the time-averaged potential drops for the sheaths at the grounded (V g) and the powered electrode (Vsp) are inversely proportional to a power of the ratio of the areas of the two electrodes (Ag, Ap) [134, 170-172] ... 

Finally, when an AC discharge is operated with a blocking capacitor between the power supply and one of the electrodes, that electrode will assume negative self bias. Such an average negative voltage on this electrode can serve to accelerate ions toward it with considerable energies. 

Figure 5 Creation of negative self bias in AC discharge.14...

Yokoyama H., Okamoto M. et al.. Effects of a negative self-bias on the growth of cubic boron nitride prepared by plasma chemical vapor deposition, Jpn. J. Appl. P/2yj., 30 (1991) pp. 344-348. 

In the case of a tubular reactor made of silica with rf discharge excitation by means of an external coil, the average potential distribution over the chamber diameter is symmetricaL A dc negative self-bias occurs on the inside wail of the reactor (under the coil turn). 

Rather than use either microwave (MW, 2.45 GHz) or radio frequency (RF, 13.56 MHz) power to sustain our plasma, we often combine the two power sources to generate a so-called "mixed" (or dual-) frequency plasma, as shown in a schematic view of the plasma tqrparatus. Fig. 3 [15]. While MW excitation generates a high concoitration of active i redes in the gas phase, as pointed out above, the role of the RF power is to create a negative DC self-bias voltage Vg on the powered, electrically isolated substrate holder electrode. This causes ions to be accelerated 1 the potential drop (Vp-Vg) across the RF-induced plasma sheath, to their maximum kinetic energy... 

By definition, plasma is an ionized gas that is spacially neutral. That is, there are an approximately equal munber of positive and negative charges in a given volume. It should also be stressed that these are radio-frequency plasmas, not direct current plasmas. The terms anode and cathode have no meaning, on the time scale of diffusion, because the polarity of the electrodes is reversing every 37 nsec (at 13.56 MHz). Because there is only a few V/cm gradient in the plasma, and because of the enormous mass difference between electrons and ions, the motion of the ions is almost unaffected by the RF field. However, ions may be affected by the build-up of a DC self-bias on the electrodes if the electrodes are of different areas. 

Ion attack or sputtering does become an important effect if the electrical ener is capacitively coupled to the electrodes and if the areas of the two electrodes are not the same. In that case the smaller electrode will develop a negative self-bias that can be... 

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