Optimization of edge strength in femtosecond laser cutting of ultra-thin glass

Abstract

Glass has outstanding properties including low surface roughness, optical transparency, high hardness, and resistance to chemicals. Ultra-thin glass of thickness ≤ 100 μm is developed for flexible devices because thin glass sheets have good bending characteristics. Accordingly, mechanical robustness of ultra-thin glass sheets has become a key factor in manufacturing of flexible devices. More specifically, the edge strength is an important criterion that depends mainly on the edge quality after a cutting process. In this regard, the process to cut ultra-thin glass should to be optimized to minimize the defect generation. This thesis reports the optimal condition to cut ultra-thin glass using femtosecond laser irradiation, maximizing the edge strength of diced samples.

First, in direct ablation cutting of ultra-thin glass, the edge strength was optimized, with emphasis on the correlation between defect generation and edge strength. No damage was observed on the front surface, whereas severe chipping was generated on the back surface unless the overlap ratio was 99%. Back-surface chipping substantially decreased the back-side edge strength. Relatively smooth cut surface was achieved at low fluences and high overlap ratios. The inter-pulse distance had to be shorter than the wavelength of the laser beam to produce smooth cut surface. At the optimal condition, the front-side and back-side edge strengths were as high as 285 MPa and 230 MPa, repectively. At the optimal condition, no chipping was observed on the back surface; the back-surface ablation was minimized; smooth cut surface was obtained.

Secondly, a bottom-up cutting method was proposed as a novel technique to eliminate the subsurface and back-surface damages, and thus to maximize the edge strength. Subsurface damages are generated by refraction of the laser beam at the V-shaped ablation groove into glass in typical top-down cutting processes. Furthermore, our analysis indicates that interference of the refracted beam with its reflected portion at the back surface induces back-surface damages. On the other hand, refraction of the beam does not occur in the proposed bottom-up cutting method as the ablation groves are inverted V-shaped. The concept of bottom-up cutting was verified as a means to maximize the edge strength, showing that no subsurface/back-surface damages were generated in the bottom-up cutting process. The maximum front-side and back-side edge strengths of the glass sample cut by the bottom-up cutting method were ~330 and ~380 MPa, respectively, which were substantially higher than those achieved in conventional top-down cutting.

Thirdly, the femtosecond laser internal-scribing process using a Bessel beam was optimized to maximize the edge strength. The defects induced by laser internal modification and those by mechanical breaking, i.e., separation, were analyzed. Excessively high pulse energy ablated the front surface and hindered uniform modification. The size of internal modification increased with the pulse energy, being largest at the focal position where the intensity was the highest. Proper separation required internal modification without ablation and its size had to be slightly larger than the modification interval. Too sparse internal modification or too small size of internal modification lead to cutting-path deviations due to irregular crack propagation. Too dense internal modification and/or defects such as ablation, also generated cutting-path deviations, decreasing the edge strength. Once the samples were properly separated, the edge strength inversely proportional to the size of internal modification, i.e., to the defect level. Consequently, the optimal condition corresponded to relatively low pulse energy, high focal position, and moderate modification interval. The maximum front-side and back-side edge strengths were 370 MPa and 400 MPa, respectively.

In this work, the edge strength of thin glass was optimized for three femtosecond laser cutting processes: direct ablation cutting, bottom-up ablation cutting, and internal scribing and breaking. Direct ablation cutting is the simplest technique, but the inevitably induced sub-surface damages lead the lowest edge strength of 230 MPa (42 % of the ideal strength) among the three processes. Bottom-up cutting eliminated the subsurface and back-surface damages, increasing the edge strength. The edge strength of the bottom-up cutting was as high as 330 MPa (60% of the ideal strength). However, the cutting speed was the lowest because a large number of scans were required. In addition, bottom-up cutting could cut the glass in any desired shaped. Internal scribing using a Bessel beam showed the highest edge strength of 370 MPa (67 % of the ideal strength). The cutting speed was also the highest, because it cut a glass sheet in a single scan. However, cutting along a the sharp-edged cut path was difficult unlike the other two methods.