Optical properties of ceramic-like layers obtained by low energy ion beam irradiation of polysiloxane films

In the present paper we report on the modifications induced by low energy particle beams (1-5 keV He+, Ar+, N-2(+), He-0, N-2(0) and H-2(0)) on the chemical structure, optical properties and surface morphology of silicon-based polymer, polyhydroxy-methyl-siloxane (PHMSO), The in situ XPS analysis shows that ion irradiation induces depletion of C atoms and progressive enrichment of Si and O atoms within the irradiated layers, yielding a ceramic-like SiOxCyHz phase of variable composition. For a given projectile, a steady state composition is reached in any case above the fluence of 1 x 10(16) particles/cm(2). The most efficient conversion to a ceramic-like layer, with a final composition SiO1.85-1.89C0.3-0.4 H-z, is obtained by using 5 keV He+ beams, while N-2(+) and Ar+ seem less effective, At variance of this, a dramatic carbon enrichment is observed when the PHMSO films are irradiated with fast neutral particles (FAB treatments). The optical measurements show that in general the beam-converted layers remain practically absorption-less, while the relevant features of the reflectivity spectra (positions of maxima and minima) critically depend upon the type of projectile. Thus, 5 keV He+ ion irradiation induces the shift of the reflectivity maxima to shorter wavelengths (blue shift), while 5 keV N-2(+) ions induce no shift and irradiation with N-2 or H-2 neutral beams induce a red shift. The AFM measurements show that also the surface morphology critically depends on the nature of the irradiating particles, Thus, 5 keV He+ irradiation produces films as flat as the original polymer surface, while irradiation with Ar+ (inducing a lower degree of conversion) increases the roughness, N-2(+) irradiation induces characteristic undulations of the surfaces and FAB treatments induces a much higher surface roughening. The experiments show clearly that both the compositional modifications and the irradiation-induced nanometer scale morphological features critically determine the optical properties of the irradiated materials. (C) 1998 Elsevier Science B,V. All rights reserved.