Fabrication of periodic microstructures on glass and polymer using low power CO2 laser

Abstract

Micromachining on glass and polymers has been a widely attractive approach during the past few decades. In laser micromachining of materials, carbon dioxide (CO2) laser is one of the most significant lasers used. This thesis describes direct laser writing (DLW) scheme for the fabrication of periodic structures on glass and polymers. The periodic structures are important components in diffractive optics and microfluidic devices. The DLW technology is a modern day machining tool which helps to experimentally investigate the behavior of high power lasers on glass and polymers without lithographic and mask-based techniques. The DLW scheme gives great advantages, making it an efficient and cost effective approach for inducing periodic structures. The experimental observations in this research have urged the use of low power (2.5 W) CO2 laser irradiation to obtain narrow and fine patterns. The laser power and scanning speed play a vital role in the fabrication process. The current investigation focuses on glass and acrylic for the generation of regular and tidy periodic structures. The whole DLW process is controlled by a computer software program. The structure to be written by the laser is first coded and input into the CAD software, before being written on an actual workpiece. The Gaussian CO2 laser beam with a maximum power of 2.5 W has been targeted to the workpiece which is placed on the moveable xy translational stage. The laser power used in this process ranged from 1 to 2.5 W and the scanning speed, from 0 to 5 mm/s. A scanning electron microscope (SEM), an optical microscope and a surface profiler were used for observing the surface morphology and the channel cross section. A 632.8 nm HeNe laser was used for observing diffraction patterns of the fabricated periodic structures. The formation of periodic structures depends on laser power and scanning speed. The depth and width of the formed channels for glass ranged from 35 to 45 µm and from 15 to 25 µm, respectively. This research has shown the potential to fabricate periodic structures with a period of 1.5 µm which is less than the laser wavelength of 10.6 µm. These results were analyzed using a high precision, non-contact surface profiler technique developed by Taicaan, United Kingdom. In the case of polymethyl methacrylate (PMMA), the depth of the channels increases with increasing laser power, reaching a maximum value of 2349 µm at a laser power of 2.5 W. The formed structure exhibits the properties of diffraction gratings and hence can be used for diffraction experiments. The direct laser writing technique for the formation of microstructures, proves to be an efficient and effective method. A model for heat transfer inside the material is developed using the COMSOL Multiphysics software. Results from the simulated model give the temperature distribution inside the workpiece and are in good agreement with the experimental data obtained.