Pulsed rf plasma source for materials processing

A pulsed rf plasma source was evaluated for materials processing. A pulsed rf discharge of carbon tetrafluoride (CF\sb4), sulfur hexafluoride (SF\sb6), oxygen (O\sb2), or acetylene (C\sb2H\sb2) created the plasmas. The frequency and duration of the rf discharge were about 290 kHz and 30 μsec, respectively. The repetition rate was 1 discharge per minute. Plasma diagnostics included Langmuir probes, a photodiode dectector, an optical multichannel analyzer (OMA), and a microwave interferometer. Langmuir probe measurements showed that at a position 67 cm away from the rf coil, CF\sb4 plasma arrived in separate packets. Plasma densities and electron temperatures at this position were in the range 4 × 10¹¹ cm⁻³ to 1.8 × 10¹³ cm⁻³ and 2 eV to 8.3 eV, respectively. The OMA measurements identified neutral atomic fluorine in the CF\sb4 plasma and neutral atomic oxygen in the O\sb2 plasma. A plasma slab model of the microwave interferometer was applied to predict the interferometer response. The measured response was found to be almost identical to the predicted response. The influence of different reactor parameters on plasma parameters was studied. Metal barriers of different geometry were used to control the ratio of charged particles to atomic neutrals in the plasma chamber. Four plasma structures were identified: precursor plasma, shock induced plasma, driver plasma, and delayed glow plasma. Pulsed CF\sb4 and SF\sb6 plasmas were used to etch silicon dioxide (SiO\sb2) grown on silicon wafers. The SF\sb6 plasma etched SiO\sb2 at a rate of about 0.71 A per discharge and the CF\sb4 plasma deposited a non-uniform film (possibly polymer) instead of etching. The C\sb2H\sb2 plasma deposited plasma polymerized acetylene on a KBr pellet with a deposition rate of 127 A per discharge. An FT-IR spectrum of the deposited film showed that carbon-to-carbon double bonds as well as carbon-to-hydrogen bonds were present. This device can be used in plasma assisted deposition and/or synthesis of materials.