FUNDAMENTAL CONCEPTS OF PLASMA
2.7 Basic Plasma Properties
2.7.5 Self bias and plasma potential
To see how this comes about, let us consider a discharge system with one small electrode connected to an RF power source through a coupling capacitor shown in Fig.
2.8. The characteristic response of the plasma to a voltage V is given by the curve in Fig. 2.9. Owing to the much greater mobility of the electrons compared to the ions, a given positive voltage will result in a much larger electron current than the ion current which flows for the same negative voltage. In effect, the plasma behaves like a leaky diode, showing a much larger effective resistance for ion current than for electron current [35].
l-l
r v,
1
Figure 2.8 Schematic of electrode configuration for an RF glow discharge.
Ion saturation
I Electron saturation
v
Figure 2.9 Electron and Ion current as functions of the applied potential V[40J.
Let us now apply a square wave with peak amplitude Vo (Fig. 2.10). Initially, when the applied voltage goes to +Vo, the potential across the plasma is Vo. The capacitor will be charged through the effective resistance of the plasma for electron current flow, and the plasma potential V2 will drop as shown in Fig. 2.11. When the power supply changes sign, the voltage across the plasma drops instantaneously by - 2 Vo, after which the voltage decays with the longer time constant associated with the higher effective resistance ion current flow. As shown in Fig. 2.11, this continues until the time average electron and ion currents are equal, a condition which results in time- average negative bias on the electrode [35]. Although the derivation was presented with a square wave power source, a similar effect holds for a sine wave, as in Fig. 2.12.
Implicit in this derivation is the fact that the area of the large electrode was sufficient to permit all of the necessary currents to flow during each cycle. In other words, the limiting impedance for current flow on both half cycles occurs at the small area electrode. It is important to note here that the features of the RF discharge which resulted in the self bias were the presence of the coupling capacitor, and the fact that one of the electrodes was much smaller than the other [35].
The driven electrode is not necessarily the one where the bias occurs. The location of the capacitor is similarly irrelevant in determining the bias. In fact, if the apparatus were symmetric and totally decoupled from ground, there would be no self bias. The grounded electrode can have a bias if there is a coupling capacitor somewhere in the circuit and the grounded electrode is smaller than the driven electrode [35].
26
r
+vo
o
-vo -
t
Figure 2.10 The square wave voltage used inthe circuit shown in Fig. 2.8 [35].
v
2I
-vo
Slow decay (ion current)
Equal electron and ion currents
t
Figure 2.11 The waveshape of voltage V,appeared on small electrode.
/ Appliedvoltage,VI
LOltage on small electrode,V,
t
Figure 2.12 Self bias for a sine wave driven system [35].
The plasma prefers to be more positive that the most positive surface. Then, for the case of a large bias, we would expect the plasma potential to behave as shown in Fig. 2.13.
Even with the self bias, the small electrode is positive for some fraction of a cycle, so the time average plasma potential is usually higher than for the DC case. Ions, which cannot respond to the fast RF cycle, will bombard the small electrode with an energy given by the difference between the time-average plasma potential and the time-average self bias. The magnitude of the bias depends on the neutral pressure [35]. As the pressure is increased with constant power into the discharge, the bias will decrease. This is due in part to decreases in the RF voltage, because the plasma impedance decreases as the neutral density increases. Another way of looking at this phenomenon is that at high densities, the discharge does not require as high a sheath field to sustain itself, because it is able to put energy directly into the glow electrons [35].
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Plasma
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Figure 2.13 The plasma potential and voltage on the small electrode [35].
The asymmetric RF discharge configuration, which results in high bias, is the chosen configuration for reactive ion etching [40], where ion bombardment produces anisotropic etching. The highest bombardment energies are obtained as the pressure is lowered. Since excessive ion energy can result in damage to a wafer, some reactive ion etch processes are operated at higher pressures (200-300 mTorr). At higher pressure the ion energy is reduce both through a lower bias and collisions in the sheath. Higher collisions, however, tends to destroy the directionality of the bombarding ions, which reduces their utility for anisotropic etching.
For the case of a symmetric discharge i.e., equal area electrodes, there is no self bias and the plasma potential appears as in Fig. 2.14. Here the time average plasma potential is much greater than for the DC case, even though there is no bias [35]. There will be energetic ion bombardment which will occur at both electrodes. In this apparatus, energetic ion bombardment occurs primarily due to the large plasma potential [35].
Vo
-Vo
Instantaneous plasma potential
t
Figure 2.14 Plasma potential and self bias voltage for a symmetric discharge configuration [35].