Excimer laser machining of microvias in glass substrates for the manufacture of high density interconnects

Machining of microvias in 100-50 μm thick CMZ glass using an excimer laser (248 nm) was investigated. The effect of various laser process parameters: pulse energy, repetition rate, irradiation time were studied to optimise the microvia drilling process and a process window was identified. Through-hole drilling of 100 μm diameter (entry hole) microvias was achieved at a fluence (energy density) as low as 2.3 J/cm with an irradiation time of 30-40 s at a repetition rate of 20 Hz, giving a taper angle between 22-24° relative to the vertical. However, by increasing the fluence to 4.5 J/cm , this reduced the machining time to 5-10 s and taper angle to 14°, giving an exit hole diameter of around 45-50 μm. With 50 μm thick glass, it was possible to machine through-hole microvias with smaller entry hole diameters down to 40 μm. Machined microvias were characterised to investigate debris, recast layer and microcrack formation. Debris and recast layer around the machined features was minimised by using a protective photoresist layer coating on the glass and through appropriate operating parameter selection. Microcracks along the sidewalls of the microvias could not be avoided, but their severity depended on the laser machining parameters used. © 2012 Springer-Verlag.