Etch Rate and Surface Morphology of Plasma Etched Glass Substrates
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Etch Rate and Surface Morphology of Plasma Etched Glass Substrates Junting Liua, Nikolay I. Nemchuka, Dieter G.Asta, J. Gregory Couillardb Cornell University, 214 Bard Hall, Ithaca, NY 14853, USA b Corning Incorporated, Corning, NY 14831,USA
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ABSTRACT Micro-machined transparent components are of interest for optical MEMS and miniaturized biological systems. The glass ceramic GC6 developed by Corning is optically transparent, has a softening point in excess of 900 oC, and a thermal expansion coefficient matched to silicon. These properties make it useful for the construction of devices that combine thin film silicon electronics with MEMS systems. Both the ceramic precursor (green glass) and the glass ceramic etch at a similar rate, about 1/3 to 1/4 of that of SiO2 etched under the same conditions, indicating that chemistry rather than microstructure control the etch rate. The cleaning steps used to clean the glass precursor profoundly influence the degree of surface roughness that develops during subsequent plasma etching. In glass ceramics, the morphology of plasma etched surface is always very smooth and independent of the cleaning steps used. Assuming that the removal of spinel crystals is the rate limiting step in plasma etching glass ceramics can explain this observation. INTRODUCTION Transparent substrates permit fabrication of novel MEMS devices with promising applications in optical communication and microbiological systems. To be useful, the transparent substrate must be compatible with conventional lithographic and micromachining techniques. Compatibility with thin film silicon electronics is desired as this permits one to build intelligent systems. Corning Inc. recently developed a novel glass-ceramic GC6 [1] meeting all of the above requirements. In particular, this substrate matches the thermal expansion coefficient of Si and has been used successfully to fabricate high quality polycrystalline silicon thin film transistors at processing temperatures as high as 900oC [2]. The degree to which this versatile material can be micromachined is therefore of interest. To be useful in optical applications, the topography of the etched surfaces must be smooth (1/10 to 1/20 of the wavelength of light) to minimize scattering loss and cross talk. Microfluidic biological systems also require smooth surfaces. Thus, both the etch rate and surface morphology of dry-etched glass and glass ceramics are of interest. In this publication, we report on the etch rate and surface roughness of GC6 as a function of etching conditions and cleaning steps used. Since the glass ceramic is a complex, multiphase system consisting of a mixture of very small, ~ 10 nm sized spinel crystals and an amorphous SiO2 matrix, two reference substrates were studied as well. The first one was very pure fused silica. The second one was “green glass”, the amorphous precursor of the glass ceramic. When “green” glass is heated, spinel crystals form, transforming it into a glass ceramic containing ~30 vol% spinel crystals and ~70 vol% of almost pure a
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