Potential self-bias

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] ... 

FIG. 44. Plasma parameters as deduced from the lEDs and material properties as a function of power delivered to the SiHa-Ar discharge at an excitation frequency of 50 MHz and a pressure of 0.4 mbar (a) the plasma potential Vp (circles) and dc self bias (triangles), (b) the sheath thickness d, (c) the maximum ion flux r ax. (d) the growth rate r,/. (e) the microstructure parameter R. and (f) the refractive index ni ev- (Compiled from E. A. G. Hamers. Ph.D. Thesis, Universiteit Utrecht, Utrecht, the Netherlands. 1998.)... 

The plasma potential is about 25 V (Figure 63a). This value of the plasma potential is typical for the silane plasmas in the asymmetric capacitively coupled RF reactors as used in the ASTER deposition system, and is also commonly found in argon or hydrogen plasmas [170, 280, 327]. From the considerable decrease of the dc self-bias with increasing frequency (Figure 63a) it is inferred that the... 

The value of the self-bias potential was reported to strongly depend on the RF power and deposition pressure, following a (VF/p) /- dependence [30]. The... 

This picture was found to be consistent with the comparison of Raman spectra and optical gap of a-C H films deposited by RFPECVD, with increasing self-bias [41], It was found that both, the band intensity ratio /d//g and the peak position (DQ increased upon increasing self-bias potential. At the same time, a decrease on the optical gap was observed. Within the cluster model for the electronic structure of amorphous carbon films, a decrease in the optical gap is expected for the increase of the sp -carbon clusters size. From this, one can admit that in a-C H films, the modifications mentioned earlier in the Raman spectra really correspond to an increase in the graphitic clusters size. 

Figure 5 Amplitude Reduction Factor and Self-Bias Potential Versus Power Density at 35mTorr...

Fig. 11.1. Parallel component of the real part of the refractive index np as a function of the resulting hydrogen content H/(H+C) of the deposited films. Seven different hydrocarbon feed gases and three different dc self-bias voltages (Vb) were used. Solid circles correspond to deposition at floating potential, solid squares to Vb = —30 V, and solid triangles to Vb = —200 V. In addition, one data point for a 1 2 mixture of acetylene with hydrogen deposition at floating potential is included (blank circle). The thick gray line is only a guide to the eye...

The sheath potential (voltage) is the difference between the plasma potential and the electrode (or wall) potential. The potential of the sheath over the grounded electrode (right electrode in Fig. 8) is equal to the plasma potential. The potential of the sheath over the powered electrode (left electrode in Fig. 8) is the difference between the plasma potential and the electrode potential. The time-average potential of the powered electrode (with respect to ground) is called the DC self-bias or DC bias. Actually, the DC-bias is the difference of the time-average potential of the sheath... 

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... 

In an RF discharge at 13.56 MHz, the ions respond not to the applied RF field but rather to the mean field, that is, the self-bias potential at the cathode which is less than the applied RF voltage. Furthermore, collisions will occur as ions fall through this self-bias potential. Actual... 

---