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Research Papers

# Examination of Selective Pulsed Laser Micropolishing on Microfabricated Nickel Samples Using Spatial Frequency Analysis

[+] Author and Article Information
Tyler L. Perry, Dirk Werschmoeller, Neil A. Duffie, Xiaochun Li, Frank E. Pfefferkorn

Department of Mechanical Engineering, University of Wisconsin-Madison, 1513 University Avenue, Madison, WI

J. Manuf. Sci. Eng 131(2), 021002 (Feb 24, 2009) (9 pages) doi:10.1115/1.3075874 History: Received November 09, 2007; Revised September 06, 2008; Published February 24, 2009

## Abstract

The precision of parts created by microfabrication processes is limited by surface roughness. Therefore, as a means of improving surface roughness, pulsed laser micropolishing on nickel was examined numerically and experimentally. A one-dimensional finite element method model was used to estimate the melt depth and duration for single 50–300 ns laser pulses. The critical frequency was introduced to predict the effectiveness of polishing in the spatial frequency domain. A 1064 nm Nd:YAG laser with 300 ns pulses was used to experimentally investigate pulsed laser polishing on microfabricated nickel samples with microscale line features. A microfabricated sample with $2.5 μm$ wide and $0.2 μm$ high lines spaced $5 μm$ apart and one with $5 μm$ wide and $0.38 μm$ high lines spaced $10 μm$ apart were polished with 300 ns long pulses of $47.2 J/cm2$ and $44.1 J/cm2$ fluences, respectively. The critical frequency for these experimental conditions was predicted and compared with the reduction in the average surface roughness measured for samples with two different spatial frequency contents. The average surface roughness of $5 μm$ and $10 μm$ wavelength line features were reduced from $0.112 μm$ to $0.015 μm$ and from $0.112 μm$ to $0.059 μm$, respectively. Four regimes of pulsed laser micropolishing are identified as a function of laser fluence for a given pulse width: (1) at low fluences no polishing occurs due to insufficient melting, (2) moderate fluences allow sufficient melt time for surface wave damping and significant smoothing occurs, (3) increasing fluence reduces smoothing, and (4) high fluences cause roughening due to large recoil pressure and ablation. Significant improvements in average surface roughness can be achieved by pulsed laser micropolishing if the dominant frequency content of the original surface features is above the critical spatial frequency for polishing.

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Copyright © 2009 by American Society of Mechanical Engineers
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## Figures

Figure 1

Schematic representation of molten surface asperity attenuation

Figure 2

Schematic of a transient 1D FEM model for PLμP

Figure 5

Critical frequency fcr values as a function of laser pulse width

Figure 6

Surface wave oscillation times for different laser pulse widths

Figure 12

Intensity map of microfabricated sample after PLμP with 300 ns pulse of 47.2 J/cm2

Figure 13

Surface profiles of sample before (dotted: Ra=0.112 μm) and after (solid: Ra=0.015 μm) PLμP

Figure 14

Spatial frequency plot of sample before (dotted) and after (solid) PLμP

Figure 9

Microfabricated sample with 2.5 μm wide and 0.2 μm high lines spaced 5 μm apart (before polishing)

Figure 10

Normalized average surface roughness as a function of laser fluence (300 ns pulse)

Figure 11

Microfabricated sample with 2.5 μm wide and 0.2 μm high lines spaced 5 μm apart (after polishing with 300 ns pulse of 47.2 J/cm2)

Figure 15

Histogram of profile data from sample before (white) and after (black) polishing

Figure 16

Microfabricated sample with 5 μm wide and 0.38 μm high lines spaced 10 μm apart (after polishing with 300 ns pulse of 44.1 J/cm2)

Figure 17

Intensity map of microfabricated sample after PLμP with 300 ns pulse of 44.1 J/cm2

Figure 18

Effect of varying laser fluences on surface roughness

Figure 3

Predicted energy window: the dotted and dashed curves indicate the onset of melting and boiling, respectively

Figure 4

(a) Power profile of absorbed laser pulse, (b) temperature profile of the sample at various depths, and (c) melt front evolution

Figure 7

Schematic of experimental setup for PLμP (not to scale)

Figure 8

Microfabrication process (not to scale)

Figure 19

Surface profiles of sample before (dotted: Ra=0.180 μm) and after (solid: Ra=0.059 μm) PLμP

Figure 20

Spatial frequency plot of sample before (dotted) and after (solid) PLμP

Figure 21

Histogram of profile data from sample before (white) and after (black) polishing

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