Impact of open de-ionized water thin film laminar immersion on the liquid immersed ablation threshold and ablation rate of features machined by KrF excimer laser ablation of bisphenol A polycarbonate

Dowding, Colin and Lawrence, Jonathan (2009) Impact of open de-ionized water thin film laminar immersion on the liquid immersed ablation threshold and ablation rate of features machined by KrF excimer laser ablation of bisphenol A polycarbonate. Optics and Lasers in engineering, 47 (11). pp. 1169-1176. ISSN 0143-8166

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Impact of open de-ionized water thin film laminar immersion on the liquid immersed ablation threshold and ablation rate of features machined by KrF excimer laser ablation of bisphenol A polycarbonate.
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Official URL: http://dx.doi.org/10.1016/j.optlaseng.2009.06.009

Abstract

Debris control and surface quality are potential major benefits of sample liquid immersion when laser micromachining; however, the use of an immersion technique potentially modifies the ablation mechanism when compared to an ambient air interaction. To investigate the machining characteristics, bisphenol A polycarbonate has been laser machined in air and under a controllable open liquid film. To provide quantitative analysis, ablation threshold, ablation rate and the attenuation coefficient of the immersing DI water fluid were measured. In ambient air the threshold fluence was measured to be 37 mJ.cm-2. Thin film immersion displayed two trends: threshold fluences of 58.6 mJcm-2 and 83.9 mJcm-2. The attenuation of DI water was found to be negligible; thus, the change in ablation rate resulted from increased confinement of the vapour plume by the liquid medium, generating higher Bremsstrahlung attenuation of the beam, lowering the laser etch rate. Simultaneously, splashing motivated by the confined ablation plume allowed release of plume pressure before plume etching commenced. This contributed to the loss of total etching efficiency. Two interaction scenarios were obsereved as a result of splashing: (i) intermediate threshold fluence, where splashing occured after every pulse in a mode that interrupted the flow entirely, leaving an ambient air interaction for the following pulse; (ii) high threshold fluence, where splashing occured for every pulse in a mode that allowed the flow to recommence over the image before the next pulse causing every pulse to experience Bremsstrahlung attenuation. Since attenuation of the immersion liquid was negligible, it is the action of the constrained ablation plume within a thin flowing immersion liquid, the resultant Bremsstrahlung attenuation and splashing events that are the critical mechanisms that modify the primary ablation characteristics.

Item Type:Article
Additional Information:Debris control and surface quality are potential major benefits of sample liquid immersion when laser micromachining; however, the use of an immersion technique potentially modifies the ablation mechanism when compared to an ambient air interaction. To investigate the machining characteristics, bisphenol A polycarbonate has been laser machined in air and under a controllable open liquid film. To provide quantitative analysis, ablation threshold, ablation rate and the attenuation coefficient of the immersing DI water fluid were measured. In ambient air the threshold fluence was measured to be 37 mJ.cm-2. Thin film immersion displayed two trends: threshold fluences of 58.6 mJcm-2 and 83.9 mJcm-2. The attenuation of DI water was found to be negligible; thus, the change in ablation rate resulted from increased confinement of the vapour plume by the liquid medium, generating higher Bremsstrahlung attenuation of the beam, lowering the laser etch rate. Simultaneously, splashing motivated by the confined ablation plume allowed release of plume pressure before plume etching commenced. This contributed to the loss of total etching efficiency. Two interaction scenarios were obsereved as a result of splashing: (i) intermediate threshold fluence, where splashing occured after every pulse in a mode that interrupted the flow entirely, leaving an ambient air interaction for the following pulse; (ii) high threshold fluence, where splashing occured for every pulse in a mode that allowed the flow to recommence over the image before the next pulse causing every pulse to experience Bremsstrahlung attenuation. Since attenuation of the immersion liquid was negligible, it is the action of the constrained ablation plume within a thin flowing immersion liquid, the resultant Bremsstrahlung attenuation and splashing events that are the critical mechanisms that modify the primary ablation characteristics.
Keywords:KrF excimer laser, Open Thin Film Liquid Immersion, Etch Rate, Ablation Threshold
Subjects:H Engineering > H680 Optoelectronic Engineering
F Physical Sciences > F361 Laser Physics
H Engineering > H700 Production and Manufacturing Engineering
H Engineering > H300 Mechanical Engineering
Divisions:College of Science > School of Engineering
ID Code:4029
Deposited By: Colin Dowding
Deposited On:17 Feb 2011 20:56
Last Modified:13 Mar 2013 08:56

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