Physical properties of contamination particle traps in a process plasma

Abstract

This article describes the results of measurements of the plasma parameters, plasma potential Φ, positive ion density n+ , electron density ne, electron temperature Te, and electron energy distribution function (EEDF) f(ε) within and near the electrostatic particle traps in an argon process plasma. The measurements have been made with a tuned Langmuir probe at a constant set of system parameters: 250 W, 13.56 MHz power, 15 mTorr, and 10 sccm flow of Ar. The electrostatic traps have precisely defined boundaries characterized by a sharp increase in Φ of about 6 V as we have described elsewhere. We show here that during an initial mapping period of 0–10 min after the rf power is turned abruptly on (10 min are required to map 48 points), n+ is continuous across a trap boundary as are ne and Te. kTe/e≊6.5 eV uniformly throughout the mapped region. If the mapping is repeated later at t=30–40 min while n+ is still continuous and does not show any change in its values in time, ne has decreased by 42% within the trap compared to the region outside the trap which we call the ambient plasma region. Also, kTe/e has increased within the trap to 8 eV while the ambient plasma remains at 6.5 eV. If the mapping is again repeated at t=60–70 min, ne has decreased by 56% of its value in the ambient plasma and kTe/e has increased within the trap to 10 eV. This picture is consistent with the postulation that we have made previously that the trap initially contains few particles and is created by the system configuration and not particles; the trap then slowly fills with negatively charged particles. In order to maintain charge neutrality within the trap ne must decrease. Also, since the particles allow recombination to occur on their surfaces, the electron temperature must increase within the trap in order that generation can keep up with increased recombination. Finally, the EEDF appears to be Maxwellian in the ambient plasma but non-Maxwellian within the trap. In both cases, the EEDF has an attenuated high energy tail.