Molecular adsorption and the chemistry of plasma-deposited thin organic films: Deposition of oligomers of ethylene glycol |
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Authors: | Gabriel P López Buddy D Ratner |
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Institution: | (1) Department of Chemical and Nuclear Engineering, University of New Mexico, 87131 Albuquerque, New Mexico;(2) Department of Chemical Engineering and Center for Bioengineering, University of Washington, Box 351750, 98195 Seattle, Washington |
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Abstract: | Weakly ionized, radio-frequency, glow-discharge plasmas formed from methyl ether or the vapors of a series of dimethyl oligo(ethylene
glycol) precursors (general formula: H-(CH2OCH2)n-H;n=1 to 4) were used to deposit organic thin films on polytetrafluoroethylene. X-ray photoelecton spectroscopy (XPS) and static
secondary ion mass spectrometry (SIMS) of the thin films were used to infer the importance of adsorption of molecular species
from the plasma onto the surface of the growing, organic film during deposition. Films were prepared by plasma deposition
of each precursor at similar deposition conditions (i.e., equal plasma power (W), precursor flow rate (F), and deposition duration), and at conditions such that the specific energy (energy/mass) of the discharge (assumed to be
constrained byW/FM, whereM=molecular weight of the precursor) was constant. At constantW/FM conditions, two levels of plasma power (and, hence, twoFM levels) and three substrate temperatures were examined. By controlling the energy of the discharge (W/FM) and the substrate temperature, these experiments enabled the study of effects of the size and the vapor pressure of the
precursor on the film chemistry. The atomic % of oxygen in the film surface, estimated by XPS, and the intensity of theC-O peak in the XPS Cls spectra of the films, were used as indicators of the degree of incorporation of precursor moieties into the plasma-deposited
films. Analysis of films by SIMS suggested that these two measures obtained from XPS were good indicators of the degree of
retention in the deposited films of functional groups from the precursors. The XPS and SIMS data suggest that adsorption of
intact precursor molecules or fragments of precursor molecules during deposition can have a significant effect on film chemistry.
Plasma deposition of low vapor pressure precursors provides a convenient way of producing thin films with predictable chemistry
and a high level of retention of functional groups from the precursor. |
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Keywords: | Plasma-polymerized films structure retention X-ray photoelectron spectroscopy temperature effects poly(ethylene oxide) |
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