Polishing Sapphire Substrates by 355？nm Ultraviolet Laser

This paper tries to investigate a novel polishing technology with high efficiency and nice surface quality for sapphire crystal that has high hardness, wear resistance, and chemical stability. A Q-switched 355？nm ultraviolet laser with nanosecond pulses was set up and used to polish sapphire substrate in different conditions in this paper. Surface roughness of polished sapphire was measured with surface profiler, and the surface topography was observed with scanning electronic microscope. The effects of processing parameters as laser energy, pulse repetition rate, scanning speed, incident angle, scanning patterns, and initial surface conditions on surface roughness were analyzed. 1. Introduction Sapphire is a kind of important photoelectronic material with excellent properties such as high hardness, wear resistance, and chemical stability. So it is applied not only as important substrate for GaN and then LED, but also as important components for solid lasers, IR window and precision bearing. All these applications require sapphire surface with high machining quality and thin damaged layer [1]. Conventional polishing techniques as abrasive polishing, chemical mechanical polishing (CMP), and chemical polishing [2] are very difficult to get high quality for sapphire crystal polishing due to machining defects like mechanical scratch, low efficiency, and yield. Laser polishing technology shows potential for sapphire crystal because of high polishing quality and efficiency. Recently, series of studies on laser polishing of diamond film, polymer, ceramics, fiber, and metal have been conducted [3–7]. However, the research on laser polishing of sapphire crystal is still lack. Polishing sapphire substrate [8] by ultraviolet laser with picosecond pulses had been conducted in our previous research. The results indicated that better surface quality can be achieved via shorter wavelength and pulse duration. However, the high cost of picosecond ultraviolet lasers affects its further industrialization. Currently, the Q-switched ultraviolet laser with wavelength of 355？nm and nanosecond pulses is widely introduced to industry. Such type of ultraviolet lasers can be used to machine and remove sapphire material in micrometer scale. Thus, a Q-switched 355？nm ultraviolet laser with nanosecond pulses was used to polish sapphire substrate in this paper. The effects of processing parameters as laser energy, pulse repetition rate, laser scanning speed, incident angle, scanning patterns, and initial sapphire surface conditions on surface quality were analysed according to the