Chemical interaction of Fe, Ni and Au with poly(vinyl chloride) and poly(tetrafluoroethylene) during thermal evaporation and the effect of post-metallization X-ray irradiation studied by in situ X-ray photoelectron spectroscopy - Polyimides and Other High Temperature Polymers - page 354

The production of metal halides by X-rays is postulated to be a consequence of
the secondary electrons produced from X-ray-substrate interaction [28, 29]. These
secondary electrons initiate C-X (X = Cl, F) bond breaking in the polymer, pro-
ducing “active” halogen species, including F - and Cl - which can migrate into the
metal/polymer interface. Subsequent reactions between halogen-containing free
radicals e.g. -ĊF-, -CH 2 -ĊH- as well as ionic species with the metal lead to the
production of MX n (X = Cl, F; n = 1 or 2) (Scheme 2).

R-X + e - R • + X/X - (R = -ĊF-, -CH 2 -ĊH-, X = F, Cl)

→

n X/X - + M → MX n (n = 1 or 2)

Scheme 2. X-ray induced metal halide production.

The results shown in Figures 2-4 also illustrate that the conversion of nickel
and iron to MX n (X = Cl or F: n = 1 or 2) during post-metallization X-ray treat-
ment of the PVC and PTFE system does not proceed to completion. One possible
explanation for this behavior is that the metal halide salts formed (Scheme 2) in-
hibit the diffusion of active halogen species to the metallic overlayer. As the
thickness of the MX n interlayer increases, the rate of metal halide formation will
decrease and the maximum concentration of metal halide that can be produced
will be limited. The importance of a metallic overlayer in post-metallization X-
ray irradiation is also illustrated in Figure 4. This clearly shows that in the ab-
sence of a metallic overlayer little change is observed in the chemical composi-
tion of the metal/polymer interface. In contrast, significant changes are evidenced
in the presence of a metallic overlayer.
Based on a comparison of the results obtained in the present investigation and
those of previous studies [16, 17, 19], it appears that the effects of both X-ray and
Ar + ion post-metallization treatment strategies are sensitive to the chemical nature
of both the metal and the polymer. For example, the Ni/PTFE system is sensitive
to post-metallization X-ray irradiation, while the Ni/PVC system is relatively un-
affected. Results of other related studies in our laboratory on the Ti/PTFE system
indicate that metallic titanium overlayers can be almost completely converted
from Ti to TiF 3 as a result of post-metallization X-ray treatment [19]. This illus-
trates the fact that, in general, reactive metals such as Fe and Ti are most suscep-
tible to modification under the influence of post-metallization X-ray treatment.
Kinetic control may also be important in certain systems where the production
of metal fluoride is an activated process requiring post-metallization modification
by X-rays or energetic ions. In this context, it should be noted that CuF 2 formation
has been observed during sputter deposition onto another fluoropolymer, Teflon
AF1600 [30]. The kinetic energy of the copper atoms, however, (several thousand
kJ mol -1 ) is much larger than when Cu is thermally evaporated, and is of suffi-
cient magnitude to activate C-F bond cleavage within the fluoropolymer.