Dissertation/Thesis Abstract

Measurement of thermo-fluid parameters, mass removal, and physical opening mechanisms during nanosecond laser interaction with metal films
by Hendijanifard, Mohammad, Ph.D., Southern Methodist University, 2011, 284; 3453984
Abstract (Summary)

Laser-matter interactions are frequently studied by measuring the propagation of shock waves caused by the rapid laser-induced material removal. An improved method for calculating the thermo-fluid parameters behind shock waves is introduced in this work. Shock waves in ambient air, induced by pulsed Nd:YAG laser ablation of aluminum films, are measured using a shadowgraph apparatus. Normal shock solutions are applied to experimental data for shock wave positions and are used to calculate pressure, temperature, and velocity behind the shock wave. The results of the normal shock solution and the Taylor-Sedov similarity solution are compared to show that the Taylor-Sedov solution under-predicts pressure when the Mach number of the shock wave is small.

Scientists and engineers working with shock waves have frequently employed the Taylor-Sedov blast solutions to predict the thermo-fluid parameters behind the shock wave and the shock wave energy. Taylor-Sedov solution implicitly assumes the shock wave is of a blast type, in other words, the background pressure is zero. This research reveals the conditions and criterion for which the more general form of the Rankine-Hugoniot relations for thermo-fluid parameters simplifies to the thermo-fluid parameters employed in the similarity solutions proposed by Sedov and Taylor. This work emphasizes the importance of the dimensionless product, γM2 term, in which γ is the specific heat ratio and M is the Mach number.

Several laser application processes such as Copper Gallium de Selenide (CIGS) photovoltaic cell cutting and micro drilling of ink jet nozzles require the knowledge of the amount of mass removed during laser process. A combined theoretical and experimental treatment of mass removal during short pulse laser ablation of thin films is presented in this work. Theoretical scaling is used to show the importance of the dimensionless group, m out/&PSgr;1, the ratio of the removed mass to the mass irradiated by the laser. Scaling analysis shows that for laser fluences above a certain threshold, mout/&PSgr;1 is a linear function of the incident laser fluence. Experiments are performed with Nd:YAG laser ablation of films of aluminum, nickel, and tungsten and confirm the existence of the linear scaling above a threshold fluence. Plotting measured values of mout/&PSgr;1 as a function of the incident laser fluence reveals the threshold for hole opening and the threshold for the linear mass removal region. The thresholds for hole opening and the linear ablation region are measured for 200 nm and 1,000 nm aluminum films, 1,000 nm nickel films, and 200 nm tungsten films.

The process of hole opening during the interaction of nanosecond lasers with films of metals is not completely understood. The current work reports time-resolved measurements of the rapid hole-opening process during laser micromachining of 200 nm aluminum films deposited on glass substrates. The films are ablated by a Nd:YAG laser with a pulse duration of 7 ns and wavelength of 1064 nm. The hole opening process is observed by time-resolved imaging of a transmitted dye laser beam for a wide range of laser fluences. The images are analyzed by MATLAB image analysis toolbox. Two stage empirical functions are fitted to the transient dimensionless hole opening. The first stage of the function is linear in time with a time delay equal to the laser hole opening starting time. Magnitude analysis of the vapor recoil force is performed to verify that the rapid hole opening as the first stage of hole formation is due to recoil vapor force. The second stage follows an inverse function in time and is a slower hole opening mechanism due to a combination of driving forces possibly the reduced vapor recoil, surface tension, and Marangoni forces. It is observed that for each laser fluence, the first stage transitions to the second stage when the hole radius reaches 70% of the final hole radius. The transient hole opening is also compared to results with the ablation model explained earlier to verify the observed linear ablation mechanism. (Abstract shortened by UMI.)

Indexing (document details)
Advisor: Willis, David A.
Commitee: Ajaev, Vladimir, Antohe, Bogdan V., Krueger, Paul S., Otugen, Volkan
School: Southern Methodist University
Department: Mechanical Engineering
School Location: United States -- Texas
Source: DAI-B 72/07, Dissertation Abstracts International
Subjects: Applied Mathematics, Mechanical engineering
Keywords: Backside imaging, Blast waves, Laser ablation, Laser induced shock waves, Recoil pressure, Thin film micromachining
Publication Number: 3453984
ISBN: 978-1-124-62645-1
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