Femtosecond Laser Micromachining of BioMEMS

BioMEMS Lab Mechanical and Aerospace Engineering University of Texas Arlington

Comparison of Micromachining Processes Process Mechanical Resolution

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m 100 Surface Roughness

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m 6.3-1.6

Side Effects Burring, requires polishing EDM Chemical Etch 100 250 4.75-1.6

6.3-1.6

Electrode wear, rough finish, slow and unclean process Undercutting LIGA Nd: YAG Laser 5 50 1-2 1 Synchrotron source: very expensive Redeposition Excimer Laser 5 > 1

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m (nm range) Recast Layer, aspect ratios Ti:sapphire Ultrafast Laser < 1 nm range Higher power ranges may require vacuum environment

Laser Micromachining Process

Advantages of Laser Micromachining • Non-contact machining • Very high resolution, repeatability and aspect ratios • Localized heating, minimal redeposition • No pre/post processing of material • Wide range of materials: fragile, ultra-thin and highly reflective surfaces • Process can be fully automated

Effect of Laser Micromachining Process Parameters

Process Parameters

Wavelength, Focal length of lens Beam shape (Gaussian/square wave) Beam energy, Pulse width Depth of focus Vacuum or inert gas environment

Effect

Feature size Feature shape Size of heat affected zone Aspect ratio Amount of redeposition, size of recast layer

Characteristics of Femtosecond Laser Micromachining  Very high peak powers in the range 10 13 W/cm 2 provide for minimal thermal damage to surroundings  Very clean cuts with high aspect ratios  Sub-micron feature resolution  Minimal redeposition  Possible to machine transparent materials like glass, sapphire etc

Ultrashort Pulses vs. Long Pulse Micromachining Courtesy: Sandia National Labs

Ti: sapphire,120fs a) air b) vacuum c) Nd:YAG, 100ns

Extremely short pulses provide for minimal thermal damage to surroundings

Femtosecond Laser System at BioMEMS Lab  Spectra Physics Laser

Hurricane

Femtosecond Ti: sapphire  Pulse width: 106fs  Wavelength range: 750nm-850nm  Average energy: 1mJ/pulse  Beam profile: Gaussian  Polarization: linear, horizontal

Femtosecond Laser Micromachining

(Preliminary Experimental Testbed)

Additional Equipment for Femtosecond Laser Micromachining     Ultra-high precision 3-axis linear stage assembly by Aerotech Inc .

Ultrafast High Energy Beam Attenuator by Newport Corporation.

Power Meter by Scientech Inc.

2GHz Oscilloscope by Hewlett Packard Under development  10 -3 Torr, 1m 3 Vacuum Chamber with inert gas and electrical and power ports   Fully automated multiple lens changer LabView based control environment

Preliminary Experimental Results Micromachining in 18μm Thick Aluminum Foil (a) Array of shots (b) Thru-hole drilled after 33 shots at a pulse energy of 14μJ

Preliminary Experimental Results Single Shots in 18μm Thick Aluminum Foil Focal position Off-focal position

Preliminary Experimental Results Thru-holes Drilled in 25μm Thick Brass Foil 56μJ/pulse 27μJ/pulse

Ablation Rate vs. Energy Density in 18  m Thick Aluminum Foil

1.6

1.4

1.2

1 0.8

0.6

0.4

0.2

0 0 10 20 30 Energy Density (J/cm 2 ) 40 50

Optimization of Pulse Energy Required to Drill Thru-Holes

1000 900 800 700 600 500 400 300 200 100 0 0 10 20 30 Energy Density (J/cm 2 ) 40 50

Femtosecond Laser Bonding of Optically Transparent Materials •Explore femtosecond laser bonding of optically transparent PMMA or glass to a substrate • Automatic lens changer will be used to study the effect of variable focal length on the bond strength

Laser Intensity Distribution in PMMA Focal length of 9mm Focal length of 40mm

Through the Thickness Intensity Distribution of Transmitted Laser Beam in PMMA Focal length of 9mm Focal length of 40mm

Automation of Laser Micromachining Process

Conceptual Solid Model of Laser Micromachining Setup