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

Abstract 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 0 2 and H 0 2 ) on the chemical structure, optical properties and surface morphology of silicon-based polymer, poly-hydroxy-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 SiO x C y H z phase of variable composition. For a given projectile, a steady state composition is reached in any case above the fluence of 1 × 10 16 particles/cm 2 . The most efficient conversion to a ceramic-like layer, with a final composition SiO 1.85–1.89 C 0.3–0.4 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.