Investigation of the laser material interaction for glass micromachining

Cover glass made of chemically surface strengthened Gorilla glass is currently used for many electronic applications such as mobile phones, laptops etc. The glass is required to be machined to varied shape and size for a specific performance. For many years, machining glass is a big challenge due to its brittle nature even with advanced, non-contact technologies such as laser machining. This study investigated the influence of laser parameters on cutting of 700 μm thick Gorilla glass using picosecond (ps) near-infrared laser pulses with wavelength of 1064 nm for crack free laser cutting. The laser beam was focused using an objective lens with numerical aperture (NA) of 0.1. The laser parameters investigated were the position of the laser focus inside the glass, laser pulse frequency and laser scanning speed. The study revealed that at cutting speeds from 0.5 mm/s to 6 mm/s, when the laser focus was placed at 500 μm below the top surface for a 700 μm thick glass, namely more than half of the glass thickness, the glass could be well-separated into two pieces by non-linear absorption induced ablation, melting, plasma explosion inside the glass. When the laser focus was placed near the top surface, V-shaped ablation grooves were generated without glass separation. When the laser focus was placed inside the glass and near to the glass bottom surface, the glass could also be separated by scribing-induced cracking throughout the glass entire thickness. The optimal pulse frequency for well-separation of the glass was at a frequency of around 200 KHz, namely a proper laser fluence of 1.19 J/cm2 at the glass top surface. Low pulse frequencies of 50 KHz and 100 KHz produced higher laser fluences and 3.77 J/cm2 and 2.18 J/cm2 respectively. Glass top surface was ablated without glass separation at the optimal focuses inside the glass. At high pulse frequencies of 300 KHz and 400 KHz, cracks were produced and the glass was separated into multiple pieces. At pulse frequency of 500 KHz, the glass cracked into multiple pieces with top surface ablation, melting and bottom surface ablation observed. When pulse frequency was increased to 600 KHz, optical breakdown induced voids were observed. The voids were elongated, extending from laser focus to slightly below the top surface. When the pulse frequency was further increased to 700 KHz to 1000 KHz, modified region, resembling striations, was observed within the cutting kerf and the glass was cracked. Scanning speeds from 0.5 mm/s to 6 mm/s could achieve a well-separation of the glass without cracking into multiple pieces. However, the optimal laser scanning speed for minimal chipping size of 54-55 μm was found to be either low speed of 0.5 mm/s or high speed of 6 mm/s. At moderate speed of 3-4 mm/s, the maximum chipping size was observed to be larger. The scanning speed, together with pulse frequency, determines the amount of total laser energy deposited into the glass. The time dependent energy amount could be described using laser energy deposition rate. The optimal laser deposition rate at 200 KHz for cutting through the glass was found to be 1.79x105 μJ/μm3s at 0.5 mm/s and 1.49x104 μJ/μm3s at 6 mm/s. When the cutting speed was increased above 6 mm/s, the laser energy deposition rate decreased below the required value for glass separation which results in glass cracking into multiple pieces. The cutting speeds of 0.5 mm/s to 6 mm/s may not sufficient to meet the industry requirement of high cutting throughput. Hence, future work may include the use of high numerical aperture lens as well as harmonics of the ps laser such as second harmonic of 532 nm and third harmonic of 355 nm to achieve smaller laser focus spot and higher laser fluence. The smaller spot size may be able to reduce the chipping size along the cutting edge and higher laser fluence could allow for higher speed cutting.Bachelor of Engineering (Materials Engineering