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Research Papers

Effect of Proximity of Features on the Damage Threshold During Submicron Additive Manufacturing Via Two-Photon Polymerization

[+] Author and Article Information
Sourabh K. Saha

Materials Engineering Division,
Lawrence Livermore National Laboratory,
7000 East Avenue,
P. O. Box 808,
Livermore, CA 94550
e-mail: saha5@llnl.gov

Chuck Divin, Jefferson A. Cuadra, Robert M. Panas

Materials Engineering Division,
Lawrence Livermore National Laboratory,
7000 East Avenue,
P. O. Box 808,
Livermore, CA 94550

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received August 5, 2016; final manuscript received March 24, 2017; published online May 12, 2017. Assoc. Editor: Nicholas Fang.The United States Government retains, and by accepting the article for publication, the publisher acknowledges that the United States Government retains, a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States Government purposes.

J. Micro Nano-Manuf 5(3), 031002 (May 12, 2017) (10 pages) Paper No: JMNM-16-1029; doi: 10.1115/1.4036445 History: Received August 05, 2016; Revised March 24, 2017

Two-photon polymerization (TPP) is a laser writing process that enables fabrication of millimeter scale three-dimensional (3D) structures with submicron features. In TPP, writing is achieved via nonlinear two-photon absorption that occurs at high laser intensities. Thus, it is essential to carefully select the incident power to prevent laser damage during polymerization. Currently, the feasible range of laser power is identified by writing small test patterns at varying power levels. Herein, we demonstrate that the results of these tests cannot be generalized, because the damage threshold power depends on the proximity of features and reduces by as much as 47% for overlapping features. We have identified that this reduction occurs primarily due to an increase in the single-photon absorptivity of the resin after curing. We have captured the damage from proximity effects via X-ray 3D computed tomography (CT) images of a nonhomogenous part that has varying feature density. Part damage manifests as internal spherical voids that arise due to boiling of the resist. We have empirically quantified this proximity effect by identifying the damage threshold power at different writing speeds and feature overlap spacings. In addition, we present a first-order analytical model that captures the scaling of this proximity effect. Based on this model and the experiments, we have identified that the proximity effect is more significant at high writing speeds; therefore, it adversely affects the scalability of manufacturing. The scaling laws and the empirical data generated here can be used to select the appropriate TPP writing parameters.

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Figures

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Fig. 1

Schematic of the two-photon polymerization process demonstrating that a spot smaller than the width of the laser beam and lying in the interior of the resist can be polymerized

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Fig. 2

Designed versus fabricated part with internal cavities. (a) Lateral cross section of the designed part. (b) Discretized model of the part for 3D printing illustrating the part build up to the level of the designed cavities. (c) Phase contrast X-ray CT image of the fabricated part. The dark-bright edge pairs in the image represent the physical edges between the void space and the cured resin.

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Fig. 3

Scanning electron microscope (SEM) images of a set of individual lines written directly on top of a glass slide surface (coated with a thin film of indium tin oxide) at different writing speeds and laser powers. (a) Lines written at a constant average power of 35 mW and at writing speeds varying from 100 μm/s to 63 mm/s. (b) Lines written at a constant writing speed of 100 μm/s and at average powers varying from 8 mW to 50 mW. The small line at the left edge is a marker to identify the first pair of lines.

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Fig. 4

Damage threshold power during writing of widely spaced individual lines with IP-DIP photoresist. (a) Effect of writing speed on the damage threshold; (b) optical image of the writing process illustrating formation of bubbles at high laser power; and (c) optical image of the same region 3 s after writing. Damaged lines were identified by the presence of bubbles or discontinuity in the lines. Lines from left to right were written at the same speed but at progressively higher power. Scale bars are 20 μm long.

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Fig. 5

Effect of proximity of features on the damage threshold power during writing of square pillars with IP-DIP photoresist. (a) Damage threshold power versus in-plane feature overlap spacing; (b) model of the test pillar showing the tool paths and the rendered voxels; and (c) schematic of the overlapping line features.

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Fig. 6

Damage threshold power during writing of widely spaced individual lines with TMPTA prepolymer resin on top of cured parts fabricated out of IP-DIP photopolymer

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Fig. 7

Schematic demonstration of the laser–material interaction zone during printing of overlapping voxel features

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Fig. 8

Normalized variable component of the single-photon absorptivity at different overlap spacings. The y-axis represents the Gauss error function part of Eq. (8), i.e., the part within the outer brackets on the right-hand side of Eq. (8).

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Fig. 9

Verification of the effect of feature proximity on damage threshold for non-Cartesian voxel layout. (a) Scanning electron micrograph of the printed structures showing the top view of the truncated right-circular cones; (b) truncated cone geometry of the designed parts; and (c) discretization of the parts for 3D printing.

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