Investigating the penetration of a plasma polymer film into porous silicon with helium ion microscopy

Plasma polymerisation is a thin film deposition technique that is frequently used to modify surfaces for a variety of biomedical and industrial applications. Many materials have some degree of porosity and generally it is not well understood how effectively plasma polymer films penetrate pores with varying pore diameters. Plasma polymerised acrylic acid (ppAAc) films with thicknesses of approximately 50, 100 and 150 nm were deposited onto four porous silicon (pSi) substrates with pore sizes of 13–25 nm, 32–58 nm, 120–180 nm and 0.9–1.2 μm, to investigate the relationship between pore size and film penetration into the pores. Film chemistry was characterised by X-ray photoelectron spectroscopy while film thicknesses on non-porous substrates deposited at the same time as the pSi substrates were measured using spectroscopic ellipsometry. Helium ion microscopy (HIM) was used to image the ppAAc films on the pSi substrates and also to determine film thickness. For the three pSi substrates, with pores diameters <180 nm, negligible film penetration into the pores was observed in the HIM images. Continuous films were observed, completely occluding the pores when the ppAAc films were thicker than the diameter of the largest pores. For pSi with pore diameters of 0.9–1.2 μm, ppAAc film was observed up to 2 μm inside the pore. These results indicate the pore size scales required for the penetration of plasma polymer films into a porous structure, which is an important consideration when applying plasma polymer coatings to porous substrates for any application.