Tailored Femtosecond Laser Beam Micro/Nano Processing of Brittle Materials

  • Zhaoqing Li

Student thesis: Phd

Abstract

This Thesis explores how the optical light fields of a femtosecond laser beam can be tailored and used to improve the control of laser-materials interactions during micro- and nano-scale manufacturing. Special attention is given to the three fundamental optical parameters of a laser beam: wavefront, polarisation, and amplitude. The chosen target workpieces are brittle materials as these are typically the most challenging in high precision processing applications. Laser processing of BOROFLOAT 33 glass, quartz, sapphire, silicon, and alumina ceramic have been carried out for stealth dicing, full depth cutting, blind hole drilling, and surface micro processing. In addition, an extremely unusual type of light field has been produced - the longitudinal field, characterized by its polarisation orientation parallel to the optical propagation axis. The longitudinal field has been induced by combining wavefront, polarisation, and amplitude tailoring technologies and focusing these shaped optical fields using a high Numerical Aperture (NA) objective lens. The initial part of this work focused on wavefront tailoring, which was used to improve the quality of the sidewalls produced when cutting transparent materials, using a highly uniform short length Bessel-Gaussian beam. The beam fields were shaped by passing the diameter reduced femtosecond laser beam through an axicon lens with a physical angle of 20 degrees. Under the control of laser beam parameters, very low surface roughness sidewalls in both straight line and curve cutting, free from debris and crack with a zero taper, have beam obtained on BOROFLOAT 33 glass (0.5 mm, 1 mm, and 1.5 mm in thickness), quartz (1 mm and 0.5 mm in thickness), and sapphire (0.38 mm, 1 mm, and 1.5 mm in thickness), without losing the uniformity. The second part of the work focused on characterizing the impact of polarisation has on laser- material interactions. Linear polarisation has been applied to cut silicon wafer sheets with a 200 μm in thickness, using a full depth cutting process. By focusing the laser beam with a low NA objective lens and introducing parallel scan lines, much better sidewall quality than that reported previously has been obtained. The polarisation tailoring aspect is also concluded in some of the other light field tailoring aspects, i.e., circular polarisation combined with Bessel beam, cylindrical vector beam combined with flattop beam. As the next topic of study, amplitude shaping has been realized in several ways. The first method was putting an iris diaphragm in the beam path to block the out area of the Gaussian beam to make it flatter. The shaped beam has been used for alumina ceramic blind hole drilling to reduce the taper of the holes. Besides, with the combination of cylindrical vector beam with vortex wavefront and focusing under high NA lens, the footprint with the desired shape was obtained on polished silicon. The most significant findings in this research have been obtained by combining the light fields tailoring technologies introduced above. This was achieved by using a novel experimental method based on double spatial light modulators (SLMs) to realize annular beam amplitude tailoring for apodization function and tailored Fresnel wavefront for spherical aberration correction. By using an S-waveplate to convert linear polarisation into radial polarisation and focusing the laser beam with a 0.95 NA objective lens, a high-quality longitudinal field with a purity of 94.7% has been generated and imprinted on the surface of polished silicon, copper, and sapphire, a feature size of 10 nm is produced on sapphire. Both experiments and theoretical modelling have been carried out to verify and understand the findings. The longitudinal field has the potential application of nanolithography.
Date of Award31 Dec 2021
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorLin Li (Supervisor) & Olivier Allegre (Supervisor)

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