Research Papers

ArF Excimer Laser Micromachining of MEMS Materials: Characterization and Applications

[+] Author and Article Information
Kewei Liu

Lightspeed ADL™ Application Development Lab Resonetics LLC.,
Nashua, NH 03060
e-mail: liu_kewei@hotmail.com

Yoontae Kim

Mechanical Engineering and Mechanics,
Drexel University,
Philadelphia, PA 19104
e-mail: yk373@drexel.edu

Hongseok (Moses) Noh

Mechanical Engineering and Mechanics,
Drexel University,
Philadelphia, PA 19104
e-mail: mosesnoh@coe.drexel.edu

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received September 9, 2013; final manuscript received February 26, 2014; published online April 11, 2014. Assoc. Editor: Stefan Dimov.

J. Micro Nano-Manuf 2(2), 021006 (Apr 11, 2014) (13 pages) Paper No: JMNM-13-1069; doi: 10.1115/1.4027121 History: Received September 09, 2013; Revised February 26, 2014

Excimer laser ablation is a versatile technique that can be used for a variety of different materials. Excimer laser ablation overcomes limitations of conventional two-dimensional (2D) microfabrication techniques and facilitates three-dimensional (3D) micromanufacturing. Previously, we have reported a characterization study on 248 nm KrF excimer laser micromachining. This paper extends the study to 193 nm ArF excimer laser micromachining on five representative micro-electro-mechanical systems (MEMS) materials (Si, soda-lime glass, SU-8, polydimethylsiloxane (PDMS), and polyimide). Relations between laser parameters (fluence, frequency and number of laser pulses) and etch performances (etch rates, aspect ratio, and surface quality) were investigated. Etch rate per shot was proportional to laser fluence but inversely proportional to number of laser pulses. Laser frequency did not show a notable impact on etch rates. Aspect ratio was also proportional to laser fluence and number of laser pulses but was not affected by laser frequency. Materials absorbance spectrum was found to have important influence on etch rates. Thermal modeling was conducted as well using numerical simulation to investigate how the photothermal ablation mechanism affects the etching results. Thermal properties of material, primarily thermal conductivity, were proved to have significant influence on etching results. Physical deformation in laser machined sites was also investigated using scanning electron microscopy (SEM) imaging. Element composition of redeposited materials around ablation site was analyzed using energy dispersive x-ray spectroscopy (EDXS) analysis. Combined with our previous report on KrF excimer laser micromachining, this comprehensive characterization study provides guidelines to identify optimized laser ablation parameters for desired microscale structures on MEMS materials. In order to demonstrate the 3D microfabrication capability of ArF excimer laser, cutting and local removal of insulation for a novel floating braided neural probe made of polyimide and nichrome was conducted successfully using the optimized laser ablation parameters obtained in the current study.

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Grahic Jump Location
Fig. 1

Schematic drawing of excimer laser micromachining station (RapidX 250)

Grahic Jump Location
Fig. 2

Side view images of excimer laser cuts on (a) Si, (b) glass, (c) SU8, (d) PDMS, and (e) polyimide

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Fig. 3

Absorbance spectrum of Si, glass, SU-8, PDMS, and Polyimide in UV range (200–300 nm)

Grahic Jump Location
Fig. 4

Etch rates in the direction perpendicular (a)–(c) and parallel (d)–(f) to material surface as a function of (a) and (d) fluence (F varied from 6.24 to 29.59 J/cm2), (b) and (e) frequency (f varied from 10 to 100 Hz), and (c) and (f) number of shots (N varied from 10 to 100). Baseline conditions were F = 16.76 J/cm2, f = 50 Hz, and N = 100).

Grahic Jump Location
Fig. 5

Aspect ratio as a function of (a) fluence (F varied from 6.24 to 29.59 J/cm2), (b) frequency (f varied from 10 to 100 Hz), and (c) number of shots (N varied from 10 to 100). Baseline conditions were F = 16.76 J/cm2, f = 50 Hz, and N = 100).

Grahic Jump Location
Fig. 6

3D simulation results of temperature profile for excimer laser cuts at t = 0.2 s on (a) Si, (b) glass, (c) SU8, (d) PDMS, and (e) polyimide. Baseline conditions were: F = 16.76 J/cm2, f = 50 Hz, and N = 10.

Grahic Jump Location
Fig. 7

SEM images of nine laser machined channels on polyimide surface

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Fig. 8

SEM images of laser machined channels in (a) Si, (b) glass, (c) SU-8, (d) PDMS, and (e) polyimide with different fluences (8.15, 16.76, and 29.59 J/cm2)

Grahic Jump Location
Fig. 9

EDXS analysis of atomic composition of the ablation debris for different substrates: (a) silicon, (b) glass, (c) SU-8, (d) PDMS, and (e) polyimide

Grahic Jump Location
Fig. 10

SEM images of KrF and ArF excimer laser machined holes with different fluences (KrF: 7.740, 36.622, and 66.183 J/cm2; ArF: 8.15, 16.76, and 29.59 J/cm2) and frequency of 10 Hz on (a) Si, (b) glass, (c) PDMS, and (d) SU-8

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Fig. 11

ArF micromachining of flexible braided neural probe: (a) SEM image of a bundle of braided wires cut by excimer laser, (b) schematic drawing of a single nichrome wire with polyimide insulation layer, (c) microscopic image of a braided nichrome wires cut by scissors, (d) SEM image of local removal of insulation layer on single nichrome wire using ArF excimer laser, (e) SEM image of local removal of insulation layer on single nichrome wire using KrF excimer laser, (f) SEM image of single nichrome wire cut by ArF excimer laser, and (g) SEM image of single nichrome wire cut by KrF excimer laser



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