Process optimisation and characterization of excimer laser drilling of microvias in glass

Thin glass sheets may be a suitable material from which substrates for high density electrical interconnect can be produced. Since glass is extremely brittle, micromachining of thin glass substrates to create microvias and tracks is a significant challenge for which laser machining technologies offer a potential solution. This work aims to optimize and characterize the process of microvia drilling in 50-100 mu m thick CMZ glass using a KrF excimer laser (wavelength = 248 nm: pulse length = 34 ns). The effect of various laser process parameters: pulse energy, pulse repetition rate and irradiation time on the geometry, debris and remelt formation of the microvias were studied. Through-hole drilling of 100 square m diameter (entry hole) microvias was achieved successfully at a fluence (energy density) of 3 J/cm(2) with repetition rates as low as 20 Hz; within 20 s, giving a taper profile of 19-24 degrees. Microvia drilling was improved by adjusting the optical set-up to increase the fluence to 4.5 J/cm(2) which reduced the taper angle to 14 degrees giving an exit hole diameter of around 50 mu m. It was possible to achieve through hole microvias with smaller diameters down to 40 mu m in 50 mu m. thick glass. Debris and remelt formation around microvias was a key issue which was minimized by using a protective layer coating on the glass. In addition to laser operating parameters, laser beam intensity distribution also strongly influenced the microvia drilling process.