Scaling effects and brittle fracture mechanisms in laser punching of PECVD SiO₂films

We experimentally investigate the removal of several microns thick plasma-enhanced chemical vapour deposition SiO2 films by a localized dynamic fracture due to confined laser-matter interaction with the silicon substrate (punching) using 10 ps laser pulses at 355 nm. A near order of magnitude increase in the punching threshold fluence (from ∼0.1 to ∼1 J cm-2) is observed as the ratio between the spot size and the film thickness is scaled down, in order to produce high aspect ratio openings in the film. An opening radius of about twice the film thickness appears to be an approximate practical limit. A high aspect ratio opening is created by a cone fracture of the film and the ejection of a conoid film segment (flyer). We discuss mechanisms of brittle fracture that may lead to the observed patterns.