Surface and Coatings Technology 184 (2004) 116–122 0257-8972/04/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.10.005 Using heated probes in plasma polymerising discharges Marshal Dhayal, James W. Bradley* Department of Physics, UMIST, Sackville Street, P.O. Box 88, Manchester M60 1QD, UK Received 11 June 2003; accepted in revised form 10 October 2003 Available Online 5 March 2004 Abstract Using a heated probe, a technique has been developed to measure the spatial variation of the electron temperature (T ) plasma e density (N ), floating (V ), and plasma potentials (V ), and the electron energy distribution function (eedf) in low-pressure e f p 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 y1 depending on the discharge conditions. To obtain a Langmuir probe characteristic (and derive the quantities N , T , V and eedf), e e f 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 . The technique has been applied to a new two-stage p 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 fall from 1.7=10 to 2=10 m and T falls from 3.7 to 1.3 eV. However, at a 14 13 y3 e e fixed distance of 70 mm, with increasing discharge power, T remains constant, while N increases over an order of magnitude. e e 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.  2003 Elsevier B.V. All rights reserved. Keywords: Plasma polymerisation; Langmuir probe; Emissive probe; Electron energy distribution 1. Introduction The synthesis of thin films in plasma polymerisation of organic compounds has been a subject of intensive research efforts over many years w1x. Plasma techniques are versatile since through variation of the external discharge parameters such as the power (for continuous and pulsed modes), pressure and flow rate, the plasma conditions can be controlled to yield functional thin films with tailored surface properties w2–7x. In both low-pressure continuous wave (CW) RF w8x and pulsed RF plasmas w3,9,10x, films retaining a high degree of chemical functionality and structure of the starting compound (often described as the monomer) can be deposited, particularly at low effective power w8x. *Corresponding author. Tel.: q44-161-200-8702; fax: q44-161- 200-3941. E-mail address: j.w.bradley@umist.ac.uk (J.W. Bradley). There have been some detailed studies of the plasma species present at the substrate in such devices, for instance recently, Haddow et al. w11x carried out mass spectrometric and ion energy measurements on a CW plasma developed for polymerisation of acrylic acid. Neutral particle analysis revealed substantial fragmen- tation of the acrylic acid, even at low plasma powers (-1W). While positive ion mass spectrometry, showed that oligomers of the series (nMqH) as high as ns4 q are present. It has been proposed that under certain operating conditions that cationic oligomeric and other positively charged species may contribute up to 50% of the deposited mass, assuming a sticking co-efficient approaching unity. The remaining coating is derived from grafting of intact acrylic acid. These two process (gas-phase oligomerisation and grafting) give rise to a highly functionalised plasma polymer, containing pre- dominately carboxylic groups. A further study using mass spectrometry was carried out on the same system, also using acrylic acid, however in pulsed mode w12x.