A study on the effect of Cu reflector in glass drilling using a pulsed NIR laser

This paper reports on the investigation of the effect of Cu reflector on glass drilling using a pulsed ytterbium laser with a near-infrared (NIR) wavelength (1068 nm). In this study, a Cu reflector is held below the glass at a certain distance so as to reflect a laser beam back on to the glass in order to maximize the absorption of laser energy by the glass. The effect of the Cu reflector is evaluated at different values of laser pulse widths, glass thicknesses, and distance between the Cu reflector and the glass. From the results of the experiments, it is found that glass drilling is significantly improved when the Cu reflector is used owing to more laser energy absorbed by the glass. This behavior is confirmed at various laser pulse widths and glass thicknesses. More specifically, the size of the bottom hole increases by 78.2% for a glass of 1.1 mm thickness when the Cu reflector is held below the glass at 1 mm distance, and 4 ms pulse is used. It is also observed that the Cu reflector assists drilling considerably when it is held below the glass at a distance of 1–2 mm. However, the effect of the Cu reflector is insignificant when it is held at distances greater than 2 mm because of the reduced intensity of the re-absorbed beam. The increase in the beam diameter with the increased distance reduces the intensity of the re-absorbed beam. Energy dispersive spectroscopy (EDS) analysis confirms that the plasma plume produced at the rear side of the glass substrate contaminates (by deposition of evaporated particles and molten debris in the plasma plume) the surface of the Cu reflector during the drilling process. This result suggests that the Cu reflector absorbs more laser beam (transmitted via the glass) energy than the value obtained by theoretical calculation because of the optical property modification of the Cu surface. The damage observed in the Cu surface indicates that the laser beam transmitted via the glass ablates the Cu surface, and therefore a plasma plume is also produced from the Cu surface. However, there is no clear evidence that the glass is contaminated by this plasma plume. Thus, it is inferred that its influence on the re-absorption of the reflected beam is very limited.