Using a novel RF biasing technique, the energy of positive ions at a depositing substrate is controlled, independently of other parameters. Under bias conditions which gave the maximum and minimum ion energies, plasmas of propionic and acrylic acid were investigated using mass spectrometry, an ion flux probe, quartz crystal microbalance, and X-ray photoelectron spectroscopy (XPS). For both compounds investigated, the ion energy affects the deposition rate but leaves the neutral gas-phase chemistry and positive ion fluxes unchanged. The chemistry of the polymer deposit for acrylic acid is unaffected by the change in ion energy, but the chemistry of the propionic acid plasma polymer changes markedly. We argue that the results presented are consistent with the hypothesis that, under the plasma conditions explored, the carbon-carbon double bond present in acrylic acid plays a significant role in the formation of the polymer. Conversely, the absence of this bond in propionic acid leads us to conclude that positive ions contribute significantly to film formation for this compound.

Near edge X-ray absorption fine structure (NEXAFS) has been employed to provide insight into the chemical nature of nitrogen in deposits formed from plasmas of allylamine and propylamine. The nitrogen K-edge spectra of these materials unambiguously demonstrate the presence of significant quantities of sp or sp(2) hybridized nitrogen. This finding, in conjunction with carbon K-edge spectra, strongly indicates that there is a substantial level of dehydrogenation during the plasma polymerization process resulting in the formation of imine groups and, at high power, nitrile groups in addition to sp(3) hybridized amines. Comparison with standard polymers indicates that amide formation (following a few days exposure to atmosphere) is negligible. These findings suggest that the hydrolysis of aminated plasma polymers may be important in their long-term aging.

Organic surfaces of specific chemistry can be fabricated using plasma polymerisation. A recently developed deposition approach allows gradients in surface carboxyl group concentration to be fabricated over a distance of 13 mm from a mixed acrylic acid/octadiene plasma. The utility of these surfaces in the investigation of the chemisorption of volatile specie using small area XPS analysis has been demonstrated using halogen-substituted epoxide molecules. The combination of automated small area XPS analysis and chemical gradients has been used to reduce the number of samples required by a factor of 20 compared with the traditionally employed one data point-one sample approach. We believe that chemical gradients are a flexible and widely applicable route to reducing the sample preparation load in such a study of adsorbate - surface reactivity. The reactivity of epoxy functionalities with surfaces is of great importance in the field of adhesion science; most high performance paints and adhesives are based on epoxy resins. Here, the inter-relationship between the amount of epoxide chemisorption and the carboxylic acid concentration is determined. It is apparent that the level of chemisorption is strongly dependent on the carboxyl concentration, exhibiting a threshold surface composition below which reaction was not observed. The influence of the halogen tag atom is made clear; it is shown that a fluorine tagged epoxide has enhanced reactivity compared to chlorine or bromine substituted epoxides. It is also observed that degradation of the epifluorohydrin molecule occurs to form an inorganic fluorine species at the plasma polymer surface.

We describe the development and diagnosis of a two-stage plasma discharge in which a source plasma is separated from a secondary process plasma by a fine mesh. Through the application of a DC bias potential to the mesh the electron temperature Te and mean ion bombarding energy Ei to an electrically isolating substrate can be well controlled in the secondary discharge. Electrical probe and retarding field analyser measurements have shown that Te and Ei can be varied in the range 0.2–4 and 1–26 eV, respectively, and are found to be spatially uniform. Using this system, the bombarding ion flux can also be varied in the range from 1017 to 1019 m−2s−1, independently of the ion energy through the combined variation of the primary discharge power, and pressure and substrate location. These types of plasmas may find applications in the controlled plasma modification of materials through tailoring of the ion energy and flux.

A new methodology for creating surface chemical gradients is reported. By way of a demonstration of the technique, gradients of amine/carboxyl and of hydrocarbon/ carboxyl functionality are deposited over a distance of 11 mm. The gradient chemistries are examined by X-ray photoelectron spectroscopy (XPS) and chemical derivatisation of acid functional groups by TFE labelling.

A novel technique, which allows the importance of ion energy in plasma polymer film growth to be investigated, without perturbation of any other plasma parameter (particle densities or temperatures) or, in principle, perturbation of particle (neutral or ion) fluxes is applied in the plasma polymerisation of acrylic acid and new insight into polymer formation is gleaned.

Acrylic acid plasma polymers have been fabricated from pulsed 13.56 MHz RF plasmas in a "capacitively" coupled plasma deposition chamber. Plasma "on" and "off" (t(on/off)) times in the millisecond (ms) pulse time regime have been investigated using a peak plasma power of 50 W. Employing a fixed t(on) (5 ms), the effect of t(off) (2-1000 ms) on the solid-phase plasma polymer has been investigated using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF SIMS). For the first time, mass spectrometry has been employed to monitor the neutral and charged species in pulsed plasma (gas-phase) as a function of tiff. These measurements are supported by quartz crystal mass balance (QCMB) deposition rate measurements. Throughout the study, comparison with continuous (CW) wave plasmas of equivalent "average" power has been made XPS analyses revealed that with increased t(off) (i.e., lower "average" power) the extent of carboxyl retention increased from <33% at a t(off) of 5 ms to ca. 66% at a t(off) of 1000 ms. The SIMS data indicate at low-average power (0.2 W) a linear polymer, closely resembling conventional acrylic acid, is deposited. The mass spectral data indicate that the principal affect of t(off) is on the amount of intact (uncharged) acrylic acid in the system. This is shown by monitoring the signal from the molecular ion of acrylic acid (m/z 72, M.+) in the electron impact mass spectra of the neutral plasma species. Positively charged species were readily detected with ions of m/z >73 [M + H](+) present at all t(off). These ions correspond to oligomers of the form [nM + H](+), where n = 2 or 3, or fragments resulting from the loss of, for example, H2O. The extent of oligomerization is inversely proportional to the average plasma power. In pulsed plasma, the deposition rate (ng/s) increased with average plasma power. A plot of deposition mass per cycle (ng/cycle) shows the increasing importance of radical chemistry in plasma polymer growth at longer t(off). Deposition continues for up to 500 ms after the plasma is switched off. The gas-phase mass spectrometric and QCMB data substantiate the importance of radicals in plasma polymer growth at longer t(off) and we estimate that under the plasma operating conditions employed the radicals' (average) lifetime t(1/2) is about 250 ms.

The plasma polymerization of acrylic acid has been probed by means of mass spectrometry (neutrals and positively charged species) and ion energy spectroscopy. The interpretation of mass spectra is facilitated by the comparison of mass spectra obtained from plasmas of a C-13-labeled acrylic acid (with the label at the carboxyl C) and plasmas of an unlabeled acrylic acid. Mass spectrometry of the neutrals shows that under the action of plasma, even at low plasma power inputs, P, there is substantial fragmentation of the acrylic acid. Fragmentation is homolytic at the carboxyl functional group, and this is attributed to energy transfer from electrons. In the positive ion mass spectrometry, oligomers of the series (nM + H)(+) as high as n = 4 were detected. Low ion energies were measured at low P and are consistent with minimal fragmentation on arrival at a self-biased surface. On the basis of these measurements, we speculate that at low P, gas-phase cationic oligomers may be responsible for up to 50% of the deposit's total mass. Grafting of "intact" acrylic acid accounts for the remaining mass, and the two processes (gas-phase oligomerization and grafting) give rise to a highly functionalized. plasma polymer, containing predominately carboxyls. Irrespective of the exact weighting given to these alternative mechanisms, it is the abundance of intact acrylic acid which determines the extent of functional group retention. It follows that as P increases and the amount of intact acrylic acid declines steeply, high levels of functional group retention cannot be achieved.

Using an RF-driven collecting surface, mounted on the front end of a high-resolution energy-resolved mass spectrometer, the ion energy distribution functions (IEDFs) within a 13.56 MHz argon discharge have been measured and controlled. A technique of RF signal feedback has been developed, in which the RF amplitude and phase (fundamental and first two harmonics) in the sheath are varied, so manipulating the mean bombarding ion energies and widths of the IEDFs. For high RF sheath potentials, the IEDFs are broad and bi-modal. However as the sheath potential drop is reduced (with imposed matched RF signal) the IEDFs narrow, eventually becoming single peaked when no RF potential drop is present in the sheath. The observed widths in the IEDFs broadly agree with simple modelling predictions for the `high frequency' RF sheath regime; however we calculate, from the ion spectral data, marginally thinner sheaths than given by the Child-Langmuir law (derived using the mean DC sheath potential).

Absolutely calibrated vacuum ultraviolet (VUV) spectroscopy has been used to determine the energy fluxes of VUV photons at an electrically floating substrate in a low-pressure 13.56-MHz radiofrequency plasma reactor used for polymer surface treatments. These fluxes have been compared with the positive ion flux that was reported in an earlier study. At the typical operating parameters of 10-mTorr pressure and 10-W power, the total VUV energy flux is 2.2 mW cm(-2), compared with a value of 3.3 mW cm(-2) from the ions. With increasing power (from 0.5 to 12 W), both the ion and VUV energy fluxes increase monotonically. However, as the pressure increases. (1-100 mTorr), the ion energy flux declines, while the VUV component increases. At discharge powers of 10 W, and pressures greater than 25 mTorr, the greater part of the energy flux to the surface is from the VUV photons. These measurements are used to determine which of the plasma components, VUV or ions, will be most effective in the treatment of polystyrene surfaces in a pure Ar plasma. Because of the low VUV absorption coefficient of polystyrene most of the photon flux penetrates through the outermost 2 nm Of the polymer Surface without attenuation. Consequently, we calculate that under typical plasma operating parameters, the ions play the major role in the modification of polystyrene surfaces because of their shorter stopping distances.

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.