Silicon nitride layers of 140 nm thickness were deposited on silicon wafers by low pressure chemical vapour deposition (LPCVD) and irradiated at GANIL with Pb ions of 110 MeV up to a maximum fluence of 4 x 10^13 cm-2. As shown in a previous work these irradiation conditions, characterized by a predominant electronic slowing-down (Se = 19.3 keV.nm-1), lead to damage creation and formation of etchable tracks in Si3N4. In the present study we investigated other radiation-induced effects like out of plane swelling and refractive index decrease. From profilometry, step heights as large as 50 nm were measured for samples irradiated at the highest fluences (> 10^13 cm-2). From optical spectroscopy, the minimum reflectivity of the target is shifted towards the high wavelengths at increasing fluences. These results evidence a concomitant decrease of density and refractive index in irradiated Si3N4. Additional measurements, performed by ellipsometry, are in full agreement with this interpretation.