Negative ions and particle formation in low-pressure halocarbon discharges

The growth of particles in a radiofrequency (RF) (13.56 MHz) plasma at pressures from 25 to 200 mTorr in mixtures of CF4, CF 2Cl2 and argon has been studied experimentally. A planar configuration was used, with a silicon wafer on the powered electrode. The electron density has been measured with microwave resonance spectroscopy using a cylindrical cavity surrounding the plasma. The same geometry has been used to measure the density of various species of negative ions by detecting the extra electrons created by laser-induced photo detachment. It appears that the negative ion density is much larger in the case of CF2Cl2 than in the case of CF4. There seems to be hardly any dependence of the negative ion concentration on the CF2Cl2/Ar partial pressure ratio in the range where powder growth occurs. However, the attachment rate to chlorine is found to be much higher than to fluorine. Furthermore the gas phase discharge chemistry has been studied using infrared absorption spectroscopy. Both a tunable diode laser system, and a Fourier transform spectrometer have been applied. The CF2 concentration appears to decrease strongly when powder growth occurs. The SiF4 concentration then has a maximum. The results indicate that the presence of chlorine in the plasma feed gas is essential. In CF4 no particle formation is detected. The wafer surface is blackened during powder formation. SEM inspection indicates that this is caused by micromasking. Considering all the information, we arrive at the conclusion that particle growth is initiated by micromasking at the Si surface combined with a highly directional etching process. Due to residual isotropic etching the particles are released from the surface and enter the plasma, where they start coalescing and growing under the influence of CF2 polymerization.