Laser microplasma as a spot tool for glass processing: Focusing conditions

Abstract

Indirect laser-based surface nano- and microprocessing of optical materials demonstrate promising applications in microoptics, microfluidics and electrofluidic. The methods principle is based on the conversion of laser energy to a plasma plume, which effectively and precisely etchуы the glass backside. However, there are a lot of variable occurs during fabrication, which we discuss in this research to improve the processing quality and maintain the repeatability of the LIMP method. Experimental methodologies supported with geometrical calculations of a plasma plume expansion and calculation of a laser beam distortion by the microlens formed on the glass backside are presented. The study contributes to understanding the importance of experimental conditions - an air gap and focus plane position. The results make LIMP method to be robust, low-cost, and easily implementable in the industry.

Introduction

Glass microstructuring with nanoscale changes of the surface relief is extremely important for optical, micromechanical, and microfluidic devices and systems. Zappe (2012) reviewed the relevance of surface multilevel relief structuring to produce refractive and diffractive optical elements, for example Fresnel lenses, microlens array, and laser beam splitter. Special attention is now paid to laser-based methods for glass surface structuring. The utmost advantage involves the processing resolution achieved by radiation focusing in a micron-sized spot. Sugioka and Cheng (2014) demonstrated that direct ultrafast laser glass processing is quite technically studied, and the resolution depends on the suppression of thermal diffusion to the surroundings and accuracy of sample positioning. On the contrary, the cluster of indirect laser technologies - laser-induced plasma-assisted ablation (LIPAA), laser-induced backside dry etching (LIBDE), laser-induced microplasma (LIMP), aims to process glass with utilizing industrial lasers with less energy consumption.

These technologies have much in common, for example, the realization is achieved when a glass plate is placed in contact with a target, which strongly absorbs laser irradiation causing plasma plume arising. The plasma plume is a concentrated bunch of excited particles (ions, electrons, and molecules), which effectively and precisely etch the glass backside. Zhang et al. (1998) was among the first to high-speed fused quartz machining using LIPAA. Then, Hong et al. (2002) investigated LIPAA to real-time monitor the glass processing and obtained different color labels on the surface. Hopp et al. (2006) proposed LIBDE method for gratings production with submicrometer periods. Veiko et al. (2017) studied the properties of the plasma plume in the LIMP scheme and influence of the confinement mode when glass is tightly pressed to the target.

In addition to studying the physical foundations of methods, a wide class of micro-sized elements and devices was proposed. Kostyuk et al. (2016) fabricated microlens array with high speed and quality on a fused silica surface using LIMP. Sarma and Joshi (2020) reported experimental investigations of micro-channels formation on polycarbonate by using LIPAA process. Xu et al. (2017) suggested a combination of LIPAA with successive electroplating to fabricate the electrofluidic devices including a micro-heater chip and droplet-based microfluidic device. However, whenever a novel structure is formed, the researcher has to provide a routine procedure to obtain the required depth, geometry, and roughness of the final structure. That procedure implies a semi-empirical approach due to the presence of some variables resulting from a technical implementation, which have been paid no attention up to this day. Investigating variables may unlock the method potential and spread it across the industry.

We emphasize that the plasma plume possesses characteristics such as geometry, divergence, and temperature. The focusing lens in a laser setup defines the plasma divergence, however, one more variable occurs during fabrication - a microrelief on the glass surface, which brings unexpected changes in focusing conditions of the transmitted laser radiation. Fig. 1 schematically demonstrates these variables during LIMP glass processing: (a) the air gap size between the glass and target; (b) the need to shift the focal plane position due to the formation of curvature in the glass treated area; the formed microrelief distorts the laser radiation while fabricating a subsequent track with a high overlap ratio. Being aware of these parameters we can predict the plasma plume behavior and glass processing quality and efficiency. Up today, it is still unclear which of these parameters is primary and which is secondary, thus we have decided to conduct the research presented in this article.

Here, we provide a detailed experimental study of glass processing by LIMP. The results are supported with geometrical calculations of a plasma plume expansion and calculation of a laser beam distortion by the microlens formed on the glass backside. The study contributes to understanding the importance of experimental conditions - an air gap and focus plane position, to improve the quality of the LIMP method. First, applying a few lenses, we experimentally demonstrate the focus plane shift and discuss possible reasons. Then, we determine and discuss the air gap distance influence on the glass processing resolution. Moreover, we conduct a model experiment to show the influence of a microlens geometry on the processing results.