Literature review on laser machining of transparent materials
2.4 Laser-Induced Plasma Assisted Ablation (LIPAA)
2.4.1 Experimental investigation on LIPAA
Literature reports several investigations carried out on the experimental studies on LIPAA of various materials. Zhang et al. (1998) reported the novel method of LIPAA for micromachining fused quartz using the fourth harmonic of Nd:YAG laser. A series of experiments were performed to find the dependence of ablation rate and threshold laser
fluence on parameters such as laser fluence, the number of pulses and the distance between the fused quartz and the metal target. Zhang et al. (1999) also fabricated high quality micro-grating structures and holes in fused quartz and pyrex glass and reported the dependence of ablation depth on laser fluence and distance between the target and the substrate material. Increasing the distance between the target and the substrate and also the laser fluence, results in decrease and increase in ablation depth respectively. They indicated that either energetic species in the plasma or thin metal film deposited is responsible for glass ablation. Hong et al. (2001) focused at the various applications of LIPAA such as different colour and tones of laser printing by varying the target metal and the gap distance, picture printing, non-crack surface printing and metallization on glass substrate. Hanada et al. (2004) reported an improvement in the selective metallization with the use of an encapsulated film of oil based black ink. The same research group further utilised the LIPAA process for scribing of sapphire substrate (Hanada et al., 2005).
Pan et al. (2017) reported a combined process of LIPAA with chemical corrosion to fabricate micro-channels with micro-texture surface on glass. They investigated the effect of chemical corrosion parameters on the micromorphology of micro-texture. Also the effect of scanning cycles, scanning speed, pulse power density and the gap distance between the glass and the target metal on the channel geometry and chemical corrosive rate was investigated. Recently, Xu et al. (2017) also proposed a combined process of LIPAA with successive electroplating to fabricate an electrofluidic device. They fabricated microchannel on the glass by the laser plasma plume using graphite as the target metal. A graphite film thus sputter coated on the rear side of the glass results in strong adhesion of Ni pattern after being electro plated. The electroplating was found to be helpful in enhancing the conductivity of the metallized pattern. Lorenz et al. (2014) reported a laser-induced front side etching process for fabrication of sub-µm structures on glass fibre surfaces using a thin absorbing metal layer of high absorbing co-efficient.
The analysis of the surface structures presented that the laser-induced front side etching method allowed successful fabrication of well-defined periodic sub-µm structures.
Furthermore, the structuring process was simulated by a thermodynamic equation including an approach of the laser–plasma interaction.
Investigations were also carried out for backside wet etching where the metal targets are replaced by strong absorbing aqueous solution. Kopitkovas et al. (2007) fabricated complex structures on UV transparent materials like quartz using conventional
XeCl excimer laser and an organic solution. They reported that roughness of the etched surface varied between 10 to 200 nm and depends on the laser fluence and pulse duration. Also the influence on the etching mechanism due to the formation of carbon deposits was discussed. Niino et al. (2006) fabricated well-defined deep trenches without crack on fused silica plates. Kim and Park (2017) fabricated circuit patterned glass by backside wet etching followed by electroless plating. Further, Vass et al. (2015) carried out comparative study on grating fabrication by backside wet etching and direct ablation by ultrashort laser pulses. Tsvetkov et al. (2017) investigated the mechanism involved in single-pulse laser-induced backside wet etching of silicate glass. The study showed significant differences in the mechanism of crater formation in the “soft” mode (laser fluence < 150-170 J/cm2) and in the “hard” mode (at higher laser fluencies). Accurate craters with good pulse to pulse reproducibility of their shape and smooth walls were obtained in “soft” mode which was not obtained in “hard” mode. Hopp et al. (2009) carried out a comparative study for laser-induced backside dry and wet etching of transparent material using solid and molten tin as absorbers. It showed that etch rate was same for both the method for the first pulse. However for successive laser pulses, etch rate was seen decreasing for dry etching while for wet etching it was maintained constant. It was found that decreasing etch rate was due to residual tin incorporation near the fused silica surface. Ehrhardt et al. (2017) studied the confinement effect of thin liquid and solid hydrocarbon absorber films with different thickness at the laser induced backside etching. It was noted that the phase of the absorber (solid or liquid) does not influence the principle backside etching characteristics, whereas the thickness of the absorber at the interface influences the etching rate.
Observations
From the available literature, it can be noted that most of the research related to laser processing of transparent materials employ ultrashort pulsed lasers or lasers of short wavelength. Though, the ultrashort lasers produce clean and quality micro-channels, they are expensive and provide low material removal rate. Moreover, their ablation capability is also restricted to a few hundreds of nano meter. Very scant work hasbeen reported on the use of LIPAA process in the fabrication of microchannels on PC. Also, it has been obsevered that limited work on the study of the effect of the laser process parameters on the channel dimensions and surface quality during LIPAA process has been reported.
Moreover, the application of millisecond pulsed lasers during LIPAA for the fabrication
of microchannels on PC was also found to be very limited. It can also be observed that there has been limited research on the investigation of the effect of the metal target properties on the geometrical characteristics of the microchannels produced during the LIPAA process.