Vacuum Ultraviolet Laser Ablation of Teflon (PTFE)
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VACUUM ULTRAVIOLET LASER ABLATION OF TEFLON (PTFE) PETER R. HERMAN, BOYI CHEN, AND DAVID J. MOORE Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada M5S-1A4.
ABSTRACT Vacuum-ultraviolet laser ablation of Teflon is reviewed. The 157 nm irradiation of Teflon produces clean ablation sites well suited to micromachining applications in the electronics and medical fields. At 193 nm, etching profiles are poorly defined, showing swelling characteristics commonly produced by longer wavelength lasers. Comprehensive new 193 nm ablation data are presented showing the first evidence of incubation effects for Teflon. A computer model was developed to include ablation, swelling and incubation processes. The computer2 results satisfactorily model the experimental data over a large fluence range of 0.6 to 13 J/cm with three adjustable parameters.
INTRODUCTION The unique physical, dielectric and chemical properties of Teflon (polytetrafluoroethylene; PTFE) are ideally suited for numerous semiconductor processing and medical applications, but present difficulties in micromachining fine feature sizes. Teflon exhibits a remarkable resilience to ultraviolet ablation, undergoing swelling and thermal degradation at high fluences of -1 J/cm 2 . Clean ultraviolet etching of Teflon has only been demonstrated by inducing two-photon absorption with 300 fs pulses from a novel KrF laser [1], or by artificially enhancing the linear absorption coefficient with dopants of perfluoro acromatic compounds [2] or polyimide [3]. To avoid the complexities of sub-ps lasers or material doping, vacuum-ultraviolet (VUV: 100 - 200 nm) laser radiation that is strongly absorbed by Teflon must be employed [4-6]. Teflon remains transparent to ArF (193nm) laser radiation and subsequently produces [4,5] poor ablation characteristics. Cleanly formed ablation sites at low laser fluences have been demonstrated by this group [4], Obara and coworkers [5], and Basting et al. [6] by using 157 nm fluorine excimer laser radiation. Corresponding to a Beer's law energy deposition, the ablation rates follow the usual logarithmic dependence on fluence that is characteristic of most opaque insulators over a moderate fluence range. The diverse sensitivity displayed by Teflon to the ablation wavelength suggests several commercial opportunities. For the electronics industry, Teflon masks, readily micro-patterned with 157 nm radiation, offer a natural high-damage resistance to subsequent photo-processing steps even under deep ultraviolet irradiation at moderate fluence levels. Furthermore, the contrasting behaviour displayed for 193 and 157 nm wavelength sources is motivation for further study of underlying ablation mechanisms, particularly since photochemical bond breaking processes are expected [7] to play a role due to the correspondingly high-photon energies of 6.4 eV and 7.8 eV, respectively. In the present paper, the ablation of Teflon with 157 nm and 193 nm wavelength lasers is reviewed, including presentation of comprehensive
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