Investigating radio frequency plasmas used for the modification of polymer surfaces

By use of Langmuir probes and energy-resolved mass spectrometry, the properties of a cold plasma suitable for the surface treatment of polymers are investigated. The 13.56 MHz radio frequency (rf) excitation is provided by an external coil, and we demonstrate that a plasma of Ar gas is capacitively coupled to the source coil. The spatial distributions of plasma and floating (self-bias) potentials, electron temperature T-e, and plasma density n(e) have been investigated for a range of input powers (1-50 W) and gas pressures (10(-3)-10(-1) Torr) using compensated Langmuir probes. Estimates of the rf potential amplitudes are also given, The energy distribution of plasma ions at plasma boundaries has also been measured, and the effect of the perturbation to the plasma parameters due to the presence of the polymer sample and the spectrometer has been quantified. A feature of these plasmas is the presence of large rf potentials (up to about 25T(e)) and high self-bias potentials (up to 80 V). We show that the presence of a mass spectrometer changes the plasma potential and alters the level of rf fluctuation in the plasma and thereby affects the ion energy distribution function at the sample surface. Estimates of ion and photon energy fluxes are made, and the relative importance of these two fluxes in terms of polymer modification at a pressure of 10(-2) Torr is discussed.