Etching of SiO 2 with CO 2 and CO 2 + Ar + Lasers
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ETCHING OF SiO2 WITH CO2 AND CO2 + Ar+ LASERS D. PAN , B.T. DAI, B.S. AGRAWALLA, K. IMEN AND S.D. ALLEN Center for Laser Studies, DRB 17, University of Southern California, Los Angeles, California 90089-1112
ABSTRACT Dry etching of fused Si0 2 in the presence of several etchants using CO2 0 and CO2 + Ar+ lasers yields controlled, rapid removal rates of 10 A - 500 ,m/sec and smooth, high quality surfaces. Etching occurs mainly by thermal ablation of Si0 2 due to strong CO2 laser absorption. The addition of the Ar+ laser, which is absorbed by Br 2 etchant to yield Br atoms, increases the etch rate as a result of a combination of photochemical and gas-phase heating effects.
INTRODUCTION A localized, direct write, dry etching process for Si0 2 would find applications not only in microelectronics but in micromachining of fiber optics and other surfaces. The ideal surface micromachining technique should have fast, controllable etch rates, yield the required spatial resolution, and result in high quality, smooth surfaces. The laser is an ideal localized energy source for micromachining and can be used to drive local removal reactions in several ways: a) Thermal - Deposition of the laser energy into the substrate surface produces a localized high temperature region which, on reaching the evaporation temperature, results in ablative removal of the substrate. This mechanism is the basis of laser cutting of metals and scribing of ceramic substrates. b) Thermochemical - Localized heating in a reactive gas may drive a thermochemical reaction of the gas with the substrate, producing volatile products for the appropriate choice of reactants. c) Photochemical - Reactive removal may also be accomplished via localized photochemical generation of reactive species either by gas-phase or substrate absorption which lead to volatile products on reaction with the substrate with minimal or no temperature rise. d)
Combinations of the above.
We have investigated the laser etching of Si0 2 using ablation and subsequent gettering of the removed material in a reactive ambient. The LACE (laser ablative chemical etching) process combines the rapid removal rates obtainable with ablation with the smooth, debris-free surfaces 3 produced in thermochemical processes [1]. At the rapid (105 - 107 Am /s) rates of LACE removal, control of etch depth becomes critical, particularly if optical surfaces are desired. A possible control technique is the combination of photochemical and LACE mechanisms, using Ar+ laser decomposition of Br 2 combined with CO2 laser heating of the Si0 2 substrate. *
Permanent address: Sichuan,
Department of Physics, Sichuan University,
China.
Mat. Res. Soc. Symp. Proc. Vol. 75. 1987 Materials Research Society
Chengdu,
396
NaCl
C02 LASER
SiO2
Br2 V ZnSe
E w
Art
LASER
Fig.
1.
Irradiation geometry for two-wavelength etching of SiO2 .
EXPERIMENTAL The experimental technique for CO2 laser etching of SiO2 has already Modifications to the apparatus for twobeen described in detail [1]. wavelength experiments are show
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