Self-bias positive

Non-thennal plasmas in contact with insulating walls (substrate) have an important property. The plasma with the hot electrons is positively charged relative to the wall (self-bias). A sheath with a positive space charge and an electric field is fonned between the wall and the plasma. The hot electrons travel faster to the wall than the heavy... 

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... 

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. 

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. 

FlQUre 31 Raman spectra at different self-bias DC voltages showing the shift in the D-line and G-line peak position for films deposited at 7 mtorr [72]. (Reproduced from Diamond and Related Materials, 7, Yoon, S. R, et al., The effects of self-generated DC bias on the characteristics of diamond-like carbon films prepared using ECR-CVD, pp. 1213-1218. Copyright 1998, with permission from Elsevier Science.)... 

It is possible for a surface in contact with a plasma to generate a positive self-bias. This occurs when electrons are kept from the surface by a magnetic field but positive ions reach the surface by diffusion. An example is in the post cathode magnetron sputtering configuration with a floating substrate fixture, which can attain a positive self-bias. 

Typically, an ECR discharge is established at 1 kW, 2.45 GHz, 800-1000 gauss, and O.l-lOmTorr gas pressure with an electron density of lO -lO electrons/cm and a self-bias (plasma potential) of 10-20 volts in the remote substrate position. Auxiliary magnetic fields may be used in the vicinity of the substrate to increase plasma uniformity over the substrate surface. The ECR sources suffer from the difficulty in scaling them up to large-area sources. 

Bias, self (plasma technology) An electrical potential on a surface generated by the accumulation of excess electrons (negative self-bias) or positive ions (positive self-bias). See also Sheath potential. 

Okawa and Aono [50] demonstrated under ambient conditions a diacetylen chain polymerization induced in a self assembled monolayer of 10,12-nonacosadiyonic acid on graphite. First an artificial defect was created with the STM tip by applying a positively pulsed sample bias, the polymerization of a single diacetylene monolayer chain was initiated at another surface location with a negative voltage pulse. After progression of the chain reaction, the polymer chain was terminated at the artificial defect site. 

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