Photoablation Studies of Polymers, Quartz, and Semiconductors with Vacuum Ultraviolet Laser Radiation
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PHOTOABLATION STUDIES OF POLYMERS, QUARTZ, AND SEMICONDUCTORS WITH VACUUM ULTRAVIOLET LASER RADIATION PETER R. HERMAN, BOYI CHEN, DAVID J. MOORE, AND MARK CANAGA-RETNAM Department of Electrical Engineering, University of Toronto, Toronto, Ont., Canada M5S-1A4.
ABSTRACT Excimer lasers sources of 193nm and 157 nm wavelength were used to obtain new photoablation etching rates for several materials of interest to the microelectronics industry. The harder 157nm radiation provided lower ablation rates and smaller threshold fluences for Polyimide and Polymethyl Methacrylate (PMMA) than with 193nm. For normally robust materials like quartz and Teflon (PTFE), the 157nm laser produced clean and smooth ablation 2 sites with low threshold fluences of 620mJ/cm 2 and 68mJ/cm , respectively, features impossible to obtain with conventional excimer lasers at longer wavelengths. The data should help define new micromachining applications of these two materials for the electronic, optical or medical industry. Results are also reported for GaAs and InP based materials which are found 2 to undergo moderate etch rates of 30-80nm/pulse at fluences of -3J/cm , but suffer thermal damage and material segregation due to surface melting. INTRODUCTION Current excimer laser processing techniques can be extended to a broader and more diverse range of materials by moving to the vacuum ultraviolet (VUV; 100-200nm) spectral region. Most materials are highly opaque in the VUV and the hard photon energies readily break chemical bonds, minimizing thermal loading at target surfaces. With appropriate optics, the short wavelength can offer 0.2gm feature sizes that are currently in demand by the electronics industry. Further incentive is the recent 10 fold improvement in the output energy of the F 2 laser (157nm) to -100mJ per pulse [1], a level that can permit large area processing of semiconductor wafers. Another motivation for the VUV study is the paucity of literature available on F 2 laser based material processing; only a handful of publications exist centering on self developing resists [2], contact [3] and projection lithography [4], and photoablation of organic resists
[5].
Driven by the hope of reducing the large number of steps in resist based photolithography, researchers have published much data on excimer laser ablation of polymers, insulators and semiconductors in the ultraviolet spectral region. The purpose of this paper is an extension of this investigation into the VUV spectral region based on 193nm and 157nm laser sources. Results are presented for PMMA and Polyimide materials, selected because of their common use in industry and to contrast results with the comprehensive ultraviolet data already available in the literature. Robust materials like Teflon and quartz, which have poor etch characteristics at 193nm or longer wavelength, were studied to test their resilience to the much harder 157nm radiation (7.9eV). GaAs and InP were included in this survey because of the need for improved processing techniques to produce optoelectronic
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