New developments in the adhesion promotion of electroless Ni or Cu films to polyimide substrates - Polyimides and Other High Temperature Polymers - page 292

formed colloidal particles and inhibits the action of the electroless bath. As a re-
sult, this operation bares the palladium sites which then become available for the
initiation of the metal deposition in the electroless bath. On the other hand, the al-
ternative approaches described in Figure 1 (B), only involve, before activation,
gas phase treatments through plasma or VUV-assisted processes, the activation
being carried out successively in acidic SnCl 2 and PdCl 2 solutions or in a simple
acidic PdCl 2 solution when the polymer surfaces are functionalized in oxygenated
or nitrogenated environments, respectively. In these experiments, various polymer
surfaces were subjected either to plasma treatments in O 2 , N 2 or NH 3 atmosphere
[22-24, 27-33] or to UV/VUV irradiation in O 2 , air or NH 3 atmosphere using
Nd:YAG (5th harmonic, = 193 nm) lasers [25,

26] or an Xe 2 * excimer lamp [32, 33]. Note that, in the case of the UV/VUV irra-
diation, the molecular nitrogen N 2 cannot be used as a gas source allowing the
grafting of nitrogenated species. Indeed, the coherent and incoherent photon
sources emitting at 213, 193 and 172 nm are not energetic enough (5.8, 6.4 and
7.2 eV, respectively) to break in the gas-phase the N

λ = 213 nm) and ArF* excimer ( λ

N triple bond of the nitrogen
molecule (9.8 eV). On the contrary, they are quite able to break the N-H single
bond (4.0 eV) and O=O double bond (5.1 eV) of ammonia and oxygen molecules,
respectively.
For illustrating the new approaches to the EP process described in Figure 1 (B)
some XPS survey spectra relative to Kapton ® HN substrates plasma-treated in NH 3
and O 2 environments are shown in Figures 2 and 3, respectively. Figure 2 repre-
sents XPS survey spectra of Kapton ® samples, after cleaning in ethanol (a) then af-
ter NH 3 plasma for 1 min (b) and immersion either in a simple acidic PdCl 2 solu-
tion (c) or successively in acidic SnCl 2 and PdCl 2 solutions (d). In a similar way,
Figure 3 represents XPS survey spectra of substrates subjected to the same treat-
ments as those described for Figure 2 except that the plasma treatment was carried
out for 1 min in O 2 instead of NH 3 . As can be seen from (b) spectra, O 2 plasma
causes grafting of oxygenated functionalities (Fig. 3) while NH 3 plasma is respon-
sible for the uptake of nitrogenated functionalities (Fig. 2) which add to those natu-
rally present at the surface of PI substrates. In other respects, spectra (c) relative to
plasma-treated samples dipped in a simple acidic PdCl 2 solution show that only
traces of palladium are attached on the O 2 plasma-treated surface (Fig. 3) while a
significant Pd amount (1.4 at.% in this experiment) is anchored onto the NH 3
plasma-treated surface (Fig. 2). Finally, when the plasma-treated samples are
dipped successively in the acidic SnCl 2 and PdCl 2 solutions (spectra (d)), a signifi-
cant amount of tin is attached onto the O 2 -plasma treated surface (Fig. 3) and not at
all on the NH 3 -plasma treated surface (Fig. 2). Note also that Sn is not found on
NH 3 -plasma treated surfaces when the latter are treated either in a simple acidic
SnCl 2 solution or simultaneously in an equimolecular SnCl 2 and PdCl 2 acidic solu-
tion. On the other hand, palladium is attached at a relatively low surface concentra-
tion (0.8 at.% in this experiment) on samples which are O 2 plasma-treated, and
then sensitized by immersion in the acidic SnCl 2 solution (Fig. 3, spectrum (d)).
This palladium attachment on tin species is in agreement with the mechanism of

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