Potential plasma

Figure 12. Variation of the plasma parameters of a CH4/H2 plasma with pressure, (a) Plasma potential, (b) Electron temperature, (c) Electron density. Reprinted with permission from [88], K. Okada et al., /. Vac. Sci. TechnoL, A 17, 721 (1999). 1999, American Institute of Physics.

Figure 14. Variation of the plasma parameters of a CH4/CO/H2 plasma with [CO] content, (a) Plasma potential, (b) Electron temperature, (c) Electron density.

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... 

The electrons follow the oscillations in the electric field, and experience the time-dependent plasma potential. Due to the capacitor through which the RF power is coupled to the electrodes, no dc current flows through the plasma. The ion and electron currents towards each of the electrodes balance each other over one RF period. 

The plasma potential is the maximum value with which ions can be accelerated from the edge of the sheath towards the substrate, located at the grounded electrode. This may cause ion bombardment, which may induce ion-surface interactions such as enhancement of adatom diffusion, displacement of surface atoms, trapping or sticking of incident ions, sputtering, and implantation see Section 1.6.2.1. 

When a probe is inserted into a plasma, it will experience electrons and ions colliding with its tip. Due to the high mean speed of electrons, the flow of electrons is higher than the flow of ions. Consequently, the tip will charge up negatively until the electrons are repelled, and the net current then is zero. The probe potential then is the floating potential, Vfl. The electron current density Je then balances the ion current density 7,. At potentials lower than Vfl the ion current cannot increase further—in fact, only ions are collected from the plasma—and the ion saturation current /,s is measured. The plasma potential Vpi is defined as the potential at which all electrons arriving near the probe are collected and the probe current equals the electron current. Note that the plasma assumes the plasma potential in the absence of a probe hence probe perturbation at Vpi is... 

The plasma potential determined from I-V data for argon increases linearly from about 24 to 27 V with increasing power at a pressure of 0.05 mbar. At the highest pressure of 0.35 mbar these values have shifted downwards by about 2 V only. For the hydrogen discharge similar behavior is observed, with an increase... 

The energy position Cp of peak p in the lED of an ion with mass m is seen to be dependent on the plasma potential Vpi, the RE period T, and the ion plasma frequency cd, = yje n j m(o). Equation (48) can be used to determine the (net) charge carrier density in the sheath and the time-averaged potential Vpi from measured lEDs. The mean position Xp follows from combining Eq. (47) and Eq. (48) ... 

In silane discharges several ions are observed to be involved in a charge exchange process, and therefore maxima in their ion energy distribution at distinct energies are observed. The charge carrier density and the plasma potential that result from the fit of the lED allow for the quantification of the related parameters sheath thickness and ion flux. This method has been be used to relate the material quality of a-Si H to the ion bombardment [301. 332] see also Section 1.6.2.3. 

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.)... 

In the presented range of pressure variation. Hamers [163] also has studied the influence of the substrate temperature on the plasma and the material. It was found that in the temperature range of 200 to 300°C the trends of the bias voltage, the plasma potential, and the growth rate as functions of pressure all are the same, while the absolute magnitude depends on the temperature. The trends in material properties are similar to the ones reported above at a temperature of 200°C the material quality is worse than at higher temperatures. The a-y transition occurs at a lower pressure than at a temperature of 250°C. This has been observed before [248]. 

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... 

In contrast, Heintze and Zedlitz [236] also presented data on the plasma potential as function of frequency in silane plasmas the plasma potential varies from about 27 V at 35 MHz to about 20 V at 180 MHz. Moreover, Dutta et al. [284] used a symmetric capacitively coupled RF reactor and estimated the plasma potential in their system from the applied voltage at the powered electrode. A decrease of the plasma potential from 45 V at 13.56 MHz to only 15 V at 70 MHz is observed. This difference in behavior is thought to be solely due to the different reactor geometries. 

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