Thermal stress cleavage of a single-crystal round sapphire bar by carbon dioxide laser

Highlights

• The first attempt of applying thermal stress cleavage to a single-crystal round sapphire bar was demonstrated.

• Thermal stress cleavage utilising the CO₂ laser beam enabled the complete separation of a single-crystal round sapphire bar by preparing a V-shaped groove on the surface.

• The preparation of a sharp V-shaped groove helped in enhancing the quality of the cleaved surface, and the high aspect ratio of the V-shaped groove enabled high-quality cleavage.

• The crack propagation direction was controlled by considering the tip morphology of the V-shaped groove.

Abstract

This study is the first attempt of applying thermal stress cleavage to a single-crystal round sapphire bar having a morphologically continuous circumference surface. Previously, thermal stress cleavage was only applied to brittle material substrates or wafers owing to the crack propagation behaviour induced by thermal stress distribution. To overcome this issue, a V-shaped groove was prepared on the circumference as an initial point for crack propagation, and a continuous-wave carbon dioxide (CO₂) laser was used as the heat source. The thermal stress behaviour was simulated via finite element analysis, and the effect of the initial crack morphology on the thermal stress behaviour was evaluated. In addition, the crack propagation was monitored using an acoustic emission sensor, and the separation mechanism of the round sapphire bar was investigated. Results indicated that the thermal stress cleavage using the CO₂ laser beam allowed the complete separation of the round sapphire bar with the V-shaped groove without any final edge remaining. The crack propagation behaviour was determined from the relationship between the thermal stress distribution and morphology change during crack propagation. The fabricated sharp V-shaped groove improved the quality of the cleaved surface, and the high aspect ratio of the groove enabled a high-quality cleavage. The initial crack propagation immediately after the start of laser beam irradiation was affected by the tip of the V-shaped groove, and the crack propagation direction could be controlled through the tip morphology of the V-shaped groove.

Introduction

Single-crystal sapphire is a hard and brittle material with high optical transparency to visible light, high thermal conductivity, and a Mohs hardness of 9 (Pishchik et al., 2009). It can be applied as a substrate for light-emitting diodes (Chen et al., 2000), windows for optical devices and wristwatches (Allen et al., 2012), optical fibres (Ghazanfari et al., 2016), and laser devices (Chichkov et al., 1996). However, processing single-crystal sapphire by traditional subtractive machining methods using a diamond blade or wire saw is difficult owing to the hardness of the material and microcracks on the processed surface, which result in a decrease in the defect rate. The tool wear of machining tools causes an increase in machining cost; therefore, the development of an efficient machining process and improvement in productivity are required. To improve the machining quality of single-crystal sapphire, Mizumoto et al. (2017) investigated the brittle–ductile transition behaviour during subtractive machining and reported that the quality of the machined surface depended on the anisotropic ductility of the cleavage plane. Cheng and Wu (2017) investigated the effect of crystal orientation in sapphire wafers on slot grinding using a mounted wheel and concluded that the fracture size and subsurface crack size differed according to the crystal orientation and threshold on which the surface fracture size increased rapidly. Wang et al. (2020) proposed the application of ultrasonic-vibration-assisted cutting for the improvement of the surface quality of m-plane sapphire and concluded that the vibrations inhibited the formation of median cracks on the machined surface and improved the surface quality. Mohammadi and Patten (2017) proposed laser-assisted machining (LAM) of single-crystal sapphire and concluded that excessive laser heating causes more fracture owing to the thermal stress and forms a rough surface. Langan et al. (2019) also investigated the application of LAM to single-crystal sapphire. They showed that the use of LAM decreased the compressive residual stress owing to the formation of chipping and cracks, which resulted in reduced machining time and cost. Kim et al. (2013) investigated the influence of reciprocating motion in multiwire sawing on the machinability of single-crystal sapphire. They showed that the initial wear of the wire saw affected the shape quality of the machined single-crystal sapphire ingot, and the surface quality could be controlled by minimising the break-in zone and wear. Wang et al. (2016) proposed a step diamond core drill for rotary ultrasonic machining to reduce the edge chipping during hole manufacturing. They concluded that the use of a step drill was effective in reducing the edge chipping owing to the reprocessing effect of the step face.

In recent decades, thermal machining of the single-crystal sapphire substrates using laser beam irradiation is also proposed. Chen and Darling (2005) investigated the laser ablation efficiency of single-crystal sapphire using a pulsed Nd:YAG laser with a wavelength of 355 nm. They concluded that the laser ablation was caused by the combination of thermal and chemical processes, and the ablation rate was affected by plasma shielding during the laser beam irradiation. Gu et al. (2004) proposed the application of a pulsed copper vapour laser with a wavelength of 255 nm for separating a single-crystal sapphire wafer through laser dicing. They indicated that a single-crystal sapphire wafer with a thickness of 430 μm could be separated with a high aspect ratio. Tam et al. (1989) proposed micromachining of single-crystal sapphire in an air atmosphere using a picosecond pulsed excimer laser, and showed that single-crystal sapphire was machined without surface cracks or debris deposition. Shamir and Ishaaya (2013) reported that the use of a femtosecond laser allowed large-volume ablation without any cracks on the fabricated surface. The use of pulsed lasers is effective for machining the single-crystal sapphire substrates without any mechanical or thermal damage, and several unique processes, such as hybrid machining using a combination of different laser fluences (Xie et al., 2011), laser scribing of single-crystal sapphire wafers by laser-induced plasma-assisted ablation (Lee et al., 2000), and stealth dicing using a femtosecond near-infrared pulsed laser (Yadav et al., 2017), have been proposed.

Thermal stress cleavage, proposed by Lumley (1969) for separating brittle materials using a controlled fracture technique, also has a great potential for separating single-crystal sapphire substrates. When a crack is initiated at the surface edge or surface, it propagates due to the tensile stress generated by the heat input to the substrate surface, and the substrate is separated by controlling the heat input position. Therefore, thermal stress cleavage does not result in microcracks or heat-affected zones, although this technique is limited to thin substrates such as wafers (Yang et al., 2012). Xu et al. (2018) investigated the thermal stress cleavage of a 120-μm thick single-crystal sapphire wafer covered with a 5-μm thick gallium nitride film. They used a carbon dioxide (CO₂) laser as a heat source for the occurrence of thermal stress distribution inside the wafer and showed that the cleavage surface was smooth with no defects. Rapp et al. (2017) proposed the irradiation of a Bessel beam from an ultra-short-pulsed laser for the separation of a 150-μm thick single-crystal sapphire wafer, and concluded that the crack morphology depended on the polarisation, crystalline axes, and laser scan direction. Furthermore, it has been reported that a V-shaped groove acts as a guide for crack propagation, thus enabling the cleavage of a 5-mm thick single-crystal sapphire substrate (Kawabe et al., 2019). However, the application of thermal stress cleavage to brittle materials is limited to substrates or wafers because the initial cracks on the substrate edges or surfaces must be prepared, and cracks propagate from the substrate edge. Therefore, applying thermal stress cleavage to round bars is difficult owing to the morphological continuity of the round bar surface.

This study describes the first attempt to apply thermal stress cleavage to a single-crystal round sapphire bar. A round bar with a diameter of 20 mm, on which a V-shaped groove was prepared on the entire circumference, was used. Moreover, a continuous CO₂ laser was applied as a heat source. The thermal stress distribution during laser beam irradiation on the circumference was simulated using finite element (FE) analysis, and the influence of the initial crack morphology on the thermal stress distribution was evaluated. In addition, the crack propagation during the laser beam irradiation was monitored by an acoustic emission sensor attached to the edge of the round bar, and its corresponding separation mechanism was clarified.