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Research Papers

Characterization of “Bulk Lithography” Process for Fabrication of Three-Dimensional Microstructures

[+] Author and Article Information
Prasanna Gandhi

Associate Professor
Department of Mechanical Engineering,
Indian Institute of Technology Bombay,
Mumbai, Maharashtra 400076, India
e-mail: gandhi@me.iitb.ac.in

Kiran Bhole

Research Scholar
Suman Mashruwala Advance
Micro-engineering Laboratory,
Department of Mechanical Engineering,
Indian Institute of Technology Bombay,
Mumbai, Maharashtra 400076, India
e-mail: kiranbhole@iitb.ac.in

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF Micro- AND Nano-Manufacturing. Manuscript received January 29, 2013; final manuscript received September 16, 2013; published online October 10, 2013. Assoc. Editor: Nicholas Fang.

J. Micro Nano-Manuf 1(4), 041002 (Oct 10, 2013) (8 pages) Paper No: JMNM-13-1012; doi: 10.1115/1.4025461 History: Received January 29, 2013; Revised September 16, 2013

Various ways of fabricating a three-dimensional (3D) component in a single-layer exposure using spatial variation of exposure dose have been presented in the literature. While some of them are based on dynamic mask process, more recently, a process based on varying intensity of a scanning Gaussian laser beam termed as “bulk lithography” has been proposed. In bulk lithography, the entire varying depth 3D microstructure gets fabricated because of spatial variation of intensity of laser imposed at every point in single layer scan. For the bulk lithography process, this paper first presents experimental characterization of unconstrained depth photopolymerization of resin upon exposure to Gaussian laser beam. Experimental characterization carried out for two resins systems: namely 1,6 hexane diol-diacrylate (HDDA) and trimethylolpropane triacrylate (TMPTA), over relatively wider range of Ar+ laser exposure dose and time, show behavior well beyond Beer–Lambert law. A unified empirical model is proposed to represent the nondimensional depth variation with respect to the time and energy of exposure for both resins. Finally, using these models, successful fabrication of several microstructures including micro-Fresnel lens, textured curved surface, otherwise difficult or impossible to fabricate, is demonstrated. Several advantages of the bulk lithography as compared to other similar processes in the literature are highlighted.

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Figures

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Fig. 2

Experimental results of dimensionless cured depth against dimensionless energy at constant duration (s) of exposure for TMPTA-based resin (a) under lower energy exposure and comparison with published results and (b) under wide range of energy exposure [19]

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Fig. 3

Experimental results of dimensionless cured depth against dimensionless energy at constant duration (s) of exposure along with proposed cure depth model: (a) resin, TMPTA and (b) resin, HDDA

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Fig. 4

Variation of parameter “a” and “b” against exposure time for TMPTA-based resin

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Fig. 5

SEM image of lateral surface of the 3D microstructure fabricated using conventional scanning MSL

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Fig. 1

Schematic representations of (a) CAD model slicing strategy in bulk lithography and (b) cured depth experimental setup and 3D microfabrication using bulk lithography [19]

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Fig. 7

Fabrication of microstructure having variable cured depth along radial direction

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Fig. 6

SEM image of the variable depth microstructure (sine wave) using bulk lithography corresponding to (a) regime I and (b) regime II

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