Using heated probes in plasma polymerising discharges

Abstract

Using a heated probe, a technique has been developed to measure the spatial variation of the electron temperature ( T e ) plasma density ( N e ), floating ( V f ), and plasma potentials ( V p ), and the electron energy distribution function (eedf) in low-pressure polymerising plasma. During the non-data acquisition times, the probe was heated using an external current, so minimising deposition of an insulating layer on its surface. Typically the insulating film deposition rate was between 2 and 4 nm s −1 depending on the discharge conditions. To obtain a Langmuir probe characteristic (and derive the quantities N e , T e , V f and eedf), the heating current was switched off and the data collected over a short time (10 to 30 s). By heating the probe to strong electron emission, it was also possible to obtain an accurate measurement of V p . The technique has been applied to a new two-stage reactor (source and diffusion chambers separated by a mesh) in which acrylic acid is plasma polymerised to form thin films with functional surface chemistry at low pressure (5.2 Pa). The probe results show that in the polymerising chamber, with increasing distance from the mesh, (15 to 70 mm), N e fall from 1.7×10 14 to 2×10 13 m −3 and T e falls from 3.7 to 1.3 eV. However, at a fixed distance of 70 mm, with increasing discharge power, T e remains constant, while N e increases over an order of magnitude. The low plasma densities and low electron temperatures obtainable, give rise to conditions, similar to those obtained in pulsed RF discharges, and therefore favourable conditions for the control of surface functionality in the deposited film.