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Research Papers

Nanosecond Time-Resolved Measurements of Transient Hole Opening During Laser Micromachining of an Aluminum Film

[+] Author and Article Information
David A. Willis

e-mail: dwillis@lyle.smu.edu
Department of Mechanical Engineering,
Southern Methodist University,
Dallas, TX 75275

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received July 12, 2012; final manuscript received January 15, 2013; published online July 26, 2013. Assoc. Editor: Pamela M. Norris.

J. Heat Transfer 135(9), 091202 (Jul 26, 2013) (7 pages) Paper No: HT-12-1364; doi: 10.1115/1.4024389 History: Received July 12, 2012; Revised January 15, 2013

Laser micromachining of an aluminum film on a glass substrate is investigated using a time-resolved transmission imaging technique with nanosecond resolution. Micromachining is performed using a 7 ns pulse-width Nd:YAG laser operating at the 1064 nm wavelength for fluences ranging from 2.2 to 14.5 J/cm2. A nitrogen laser-pumped dye laser with a 3 ns pulse-width and 500 nm wavelength is used as a light source for visualizing the transient hole area. The dye laser is incident on the free surface and a CCD camera behind the sample captures the transmitted light. Images are taken from the back of the sample at various time delays with respect to the beginning of the ablation process, allowing the transient hole area to be measured. For low fluences, the hole opening process is delayed long after the laser pulse and there is significant scatter in the data due to weak driving forces for hole opening. However, for fluences at and above 3.5 J/cm2, the starting time of the process converges to a limiting minimum value of 12 ns, independent of laser fluence. At these fluences, the rate of hole opening is rapid, with the major portion of the holes opened within 25 ns. The second stage of the process is slower and lasts between 100 and 200 ns. The rapid hole opening process at high fluences can be attributed to recoil pressure from explosive phase change. Measurements of the transient shock wave position using the imaging apparatus in shadowgraph mode are used to estimate the pressure behind the shock wave. Recoil pressure estimates indicate pressure values over 90 atm at the highest fluence, which decays rapidly with time due to expansion of the ablation plume. The recoil pressure for all fluences above 3.1 J/cm2 is higher than that required for recoil pressure driven flow due to the transition to explosive phase change above this fluence.

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Figures

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Fig. 1

Illustration of physical mechanisms of hole opening during laser interaction with a metal film on an insulating substrate

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Fig. 2

Experimental apparatus for transmission imaging

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Fig. 3

A sequence of snapshots at F = 12.2 J/cm2 for 200 nm aluminum films. Transient photos are shown on the top row with the corresponding final hole image is shown below each transient photo. The horizontal and vertical scale bars on the pictures are equivalent to 52 μm and 54 μm, respectively.

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Fig. 4

Final radii of opened holes measured by imaging apparatus

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Fig. 5

Optical micrographs of holes formed at (a) 2.6 and (b) 6.6 J/cm2

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Fig. 6

Square of the ratio of the equivalent radii of the hole during opening and the hole long after being opened

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Fig. 7

Two-curve empirical fit function to data points for F = 12.2 J/cm2 (two parameter iteration)

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Fig. 8

Shock wave pressure as a function of time and fluence. Pressure values are nondimensionalized with respect to atmospheric pressure.

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Fig. 9

Laser-induced recoil pressure for F = 14.5 J/cm2 (from Ref. [27] for 200 nm Al) and dimensionless hole opening for F = 14.5 J/cm2 versus time on the same graph

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