Femtosecond Laser Ablation of Sapphire

INSTRUCTION Sapphire is widely used in the field of optical components and micromechanical devices owing to its useful mechanical, optical, and electrical properties. However, sapphire is mechanically and chemically difficult to machine because of its high hardness and chemical stability. The difficulty of machining sapphire has restricted the development of advanced device structures. As sapphire is inert to most types of wet chemical and dry etching, laser ablation has been proposed as a potential machining method. The interaction of laser pulses with sapphire materials has been investigated for many years [1-3]. However, a thermal effect is unavoidable, even in ultraviolet laser processing. Recently, the femtosecond laser ablation of sapphire has been attracting much attention because femtosecond laser ablation can produce precise, well-defined micrometer-sized structures in materials that cannot be processed with nanosecond pulsed lasers. A number of studies on the femtosecond laser ablation of sapphire have been carried out [4-7], and a mechanism for the femtosecond laser ablation of sapphire has been proposed [8-10]. The advanced development of III–V nitride devices such as those of GaN and InGaN [11,12] has prompted research into a suitable method of patterning sapphire. However, further experiments need to be carried out to obtain fine structures on sapphire. In this study, experiments on the femtosecond laser ablation of sapphire were carried out focusing on several aspects including the investigation of laserinduced periodic surface structures (LIPSS) on sapphire, and the relationship between ablation depth and the number of pulses, and a method of improving the surface quality of sapphire. As a result of our investigations, a sample of sapphire with parallel microgrooves on the surface was fabricated. EXPERIMENTS In the experiments, we used a commercially available amplified Ti:sapphire laser system that generated 164 fs laser pulses with a maximum pulse energy (Ep) of 1 mJ at a 1 kHz repetition rate and with a central wavelength of λ = 780 nm. The laser beam had a Gaussian profile with a diameter of 6 mm. The incident laser beam was irradiated onto a sapphire sample through a 4 mm aperture positioned normal to the sample and a 10× or 50× microscope objective lens. The sample was placed on a scanning stage and translated at various scanning speeds (ν) under computer control during irradiation. The scanning direction was perpendicular or parallel to the laser polarization direction. All experiments were carried out in air at atmospheric pressure and room temperature. After irradiation, the sample was rinsed for 30 minutes with acetone in an ultrasonic cleaner to remove any debris from the ablation process. The morphology of the structures was examined by scanning electron microscopy (SEM). The sample was coated with a gold layer of approximately 22 nm thickness to ensure conduction in the SEM observation. The surface profile was measured by atomic force microscopy (AFM).