Laser machining of special designed photopolymers-photochemical ablation mechanism Page: 3 of 11
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Laser Machining of Special Designed Photopolymers
-Photochemical Ablation Mechanism-
Thomas Lippert'*, J. T. Dickinson2, S. C. Langford2, H. Furutani3, H. Fukumura3, H.
Masuhara3 , T. Kunz4 , A. Wokaun4
1Los Alamos National Laboratory, MS J 585, Los Alamos, NM 87545, USA,
2Department of Physics, Washington State University, Pullman, WA 99164-2814, USA
30saka University, Department of Applied Physics, Osaka 565, Japan
4Paul Scherrer Institute, 5232 Villigen, Switzerland.
ABSTRACT
Photopolymers based on the triazeno chromophore group (-N=N-N<) have been developed. The absorption properties
can be tailored for a specific irradiation wavelength (e.g. 308 nm XeCl laser). The photochemical exothermic
decomposition yields high energetic gaseous products which are not contaminating the surface. The polymer can be
structured with high resolution. No debris has been found around the etched corners. Maximum ablation rates of
about 3 m / pulse were achieved due to the dynamic absorption behavior (bleaching during the pulse). No physical
or chemical modifications of the polymer surface could be detected after irradiation at the tailored absorption
wavelength, whereas irradiation at different wavelengths resulted in modified (physical and chemical) surfaces. The
etching of the polymer starts and ends with the laser pulse, shown by ns-interferometry, confirming that the acting
mechanism is mainly photochemical. TOF-MS revealed fragments which are also totally compatible with a
photochemical decomposition mechanism.
Keywords: Photopolymer, laser ablation, mechanism, excimer laser, triazenopolymer
1. INTRODUCTION
During the last decade, processing of polymers has become an important field of applied and fundamental research '.
One of the most important fields is laser ablation involving various techniques and applications. Laser ablation is
used as an analytical tool for MALDI (matrix assisted laser desorption ionization) 2, and LIBS (laser induced
breakdown spectroscopy) " s or as a preparative tool for PLD (pulsed laser deposition) 6. 7 of synthetic polymer
films. Another application is surface modification of polymers 8; if low fluences are applied, the polymer surface
can be either chemically modified to improve adhesion 9, or it can be changed physically. This can be either a
random increase of the surface area or it can result in LIPMS (laser induced periodic microstructures in the nm
range) 10' " which have been suggested to be used for the alignment of liquid crystals. The application of high
energy UV lasers lead to the discovery of a process termed ablative photodecomposition (APD) in 1982 12, ". One
of the most promising proposals for APD was for its application as a dry etching technique in photo-lithography.
APD promised a higher resolution, due to the use of the shorter laser wavelength as compared to the traditional UV
lamp techniques, and a lower number of processing steps. For a 'real' dry etching technique there would be no need
for a wet development. This would also eliminate a part of the liquid waste. Until now APD is mainly used for
producing via-holes in polyimide (PI) on MCM (multi chip modules) '" and not as a dry etching technique in
photolithography. This is partly due to several disadvantages of APD compared to conventional photolithography.
One reason is the comparison of APD using standard polymers such as PI, PET (polyethylene-terephthalate) or
PMMA (poly-methyl-methacrylate) '0','1'6 which are designed for totally different applications, but are compared to
the highly developed and specialized photoresists.' correspondence author: lippert@lanl.gov, phone ++1-505-665-4379, fax: ++1-505-665-4817
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Lippert, Thomas; Dickinson, J. T.; Langford, S. C.; Furutani, H.; Fukumura, H.; Masuhara, H. et al. Laser machining of special designed photopolymers-photochemical ablation mechanism, article, August 1, 1997; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc690305/m1/3/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.