Electron shower in glass2013-05-04
A simulation project at Fraunhofer ILT is advancing the use of picosecond laser ablation as a cutting technique.
The perfect glass for a display would be a two-dimensional, infinitely hard surface with no thickness whatsoever. Display manufacturers are constantly striving to move ever closer to this ideal; the glass sheets used in the modern displays are extremely hard and, in many cases, little more than 0.3 millimeters thick. This pushes conventional cutting processes to their limits. They produce mechanical stress in the thin layers of glass and form potentially critical microfissures which can only be removed by grinding the edges in a post-processing step.
Cutting – the right way
Laser ablation with ultrashort pulsed lasers — a process in which laser pulses ablate material and cut the glass without subjecting the workpiece to thermal or mechanical stress — is increasingly being recognized as a promising solution. In practice, however, this method can sometimes yield surprising results such as unpredictable ablation rates and unexpected damage. That is why Fraunhofer ILT and TRUMPF decided to develop a simulation to enhance our understanding of the interactions between the glass and the incident laser pulses.
Many wide-band-gap dielectrics such as glass and water are transparent to near-infrared and visible laser light. However, the extremely high intensity of ultrashort laser pulses results in multi-photon ionization (MPI) followed by a cascade mechanism which produces a large number of free electrons. Once a critical electron density is reached, the dielectric exhibits a metal-like behavior and laser absorption is initiated. After the laser pulse, ablation occurs as a result of the electrons releasing their energy. The process is therefore dependent on the distribution of freeelectron density — and it was this aspect that the project team took as a starting point for their numerical model.
The model – step by step
The model focuses on a two-dimensional cross-section through the workpiece, perpendicular to the cutting direction. In the first step, the maximum density of the ionized electrons is calculated as a function of laser intensity. In the second step, the laser beam is propagated into the workpiece to establish the intensity distribution and the distribution of free-electron density inside the material. The third step determines where the laser will produce ablation or permanent modification of the material by applying the predefined ablation and modification criteria, using the results of the second step. The result is a simulation of the ablation crater and the heat-affected zone. The simulation repeats the second and third steps for each further pulse, gradually displacing the surface of the glass with each pulse.
In a series of tests, the project group compared the results of the simulation program with the results of a real-life application using a frequencydoubled, mode-locked picosecond laser. The laser delivers pulses with a peak pulse energy of up to 60 microjoules and a pulse duration of 10 picoseconds in the green spectral region. During the experiments, the laser was operated at a pulse energy of 40 microjoules at a focal length of 63 millimeters and a beam waist diameter of 6.5 micrometers. The substrate comprised 0.3-millimeter-thick boroaluminosilicate glass (Corning Eagle XG). The average ablation rate in the experiments was 2.9 microns per pulse — almost three times greater than the ablation rate of a femtosecond laser. The pulses steadily increased the size of the crater in the glass, with the angle of the crater wall exceeding 80 degrees after the tenth pulse.
Cutting glass with ultrashort pulsed lasers: Scientist are beginning to shed some light on the process chains inside the material.
What happens inside the glass – pulse by pulse
A cross-section of the crater walls reveals a ray-like modification region. The image produced in the experiment corresponds closely to the simulation results, as do the crater dimensions and ablation rates. The simulation also provides information on what is actually happening inside the material. The first pulse hits the flat glass surface and releases electrons within the resulting optical penetration depth. In accordance with the Gaussian profile of the laser pulse, the initial pulse leaves a parabolic crater in the planar surface. In the subsequent pulses, the crater walls diffract and deflect the incident electric field, scattering the field into the crater walls and ionizing the material at an angle to the beam direction.
This produces a ray-like interference pattern in the distributions of both intensity and electron density. Modified regions are formed by these interferences, particularly in the crater wall but also deeper within the material. Although these regions are not ablated, the free-electron density is sufficient to permanently change the material’s refractive index and to generate defects such as F-Centers. These interferences are also a possible source of microfissures. The numerical model used in the simulation has shown itself to be a promising tool for studying processes within the material, diagnosing sources of defects in the ablation process, and investigating the effects of parameter changes.
The damage mechanism and the future
The first series of simulations and experiments has already suggested a key damage mechanism: Peaks in intensity distribution create peaks in the distribution of free electrons, thereby leading to thermo-mechanical loads or thermal damage in the material. Even if the model does not succeed in revealing the exact damage mechanism, the results have shown that this is not absolutely necessary to predict the distribution and the magnitude of the resulting damage. Based on these findings, the model will be used to improve the micro-machining process in terms of both quality and speed. This will lead to changes in strategies for cutting glass with picosecond lasers and will help support and advance the development of the process and of laser manufacturing systems suitable for industrial use.