Photostructurable glass-ceramics (PSGCs) present an attractive alternative to silicon as substrates for microfabrication.
Moving a laser beam with a focal volume a few microns across and a few tens of microns high through a transparent PSGC induces a cascade of reactions that results in selective crystallization in the laser-exposed regions. The process offers excellent 3-D shaping control. Crystal formation alters many material properties, including opacity, index of refraction, etch rates, density, stiffness, and strength. Presented here are the results of bulk mechanical measurements of the mass density and the velocity of sound in several phases of Foturan, a commercially available PSGC. The measurements are nondestructive and easily repeatable at many stages of processing. From the velocity and the mass density, we calculated the elastic modulus for each Foturan phase. The measured samples included native, amorphous Foturan; exposed Foturan that was not thermally treated; and exposed, thermally treated Foturan. Results show that Foturan becomes somewhat stiffer with higher crystal content; the elastic modulus of Foturan rises from about 78 GPa in the original amorphous glass state to about 88 GPa in a crystal-rich, exposed, baked state. The speed of sound in Foturan rises from about 5.8 km/s to 6.1 km/s.
Photostructurable glass-ceramics (PSGCs), although not yet widely used, are well suited to many micro-optical and
micromechanical applications. Their appeal stems from the combination of the physical properties of glass-ceramics
with the excellent three-dimensional shaping control that can be achieved by laser-patterning a transparent
photostructurable material. The PSGCs are both mechanically and thermally robust. Exposure with a focused 355-nm
pulsed laser beam initiates a cascade of reactions that ends in crystallization of a different phase of the glass-ceramic.
The crystal-rich phase etches chemically much faster than the original crystal-free phase. In this experiment, we
examined the dependence of the chemical etch rate on the aspect ratios and sizes of structures made from Foturan, a
commercially available PSGC. We fabricated several types of test structures in 1-mm-thick Foturan samples. We tested
the initial and long-term etch behavior of Foturan etched in 5% HF as a function of the size of the etched structure. An
aspect ratio of 100 for a 10-μm-wide trench etched through a 1000-μm-thick sample was achieved.
For the past few years we have been investigating the photophysical and photostructurable properties of
Foturan, a photostructurable glass ceramic (PSGC) manufactured by Schott Glass Corp. In this paper, we discuss
results on using Foturan as a MEMS and MOEMS substrate. Microfabrication in Foturan is possible through
patterning by a pulsed UV laser, a subsequent heat treatment step, and chemical etching. In Foturan, the exposed areas
undergo a selective phase change in which the native amorphous glass phase converts to a crystalline lithium silicate
phase. The degree and type of crystallization are both sensitive functions of the irradiation and thermal processing
procedures. Under high exposure dose, the crystallized areas etch up to 30 times faster than the unexposed material in
HF, with the etch rate varying with irradiation dose. Because Foturan is transparent at visible through IR wavelengths,
direct-write XYZ exposure with a pulsed laser can pattern complex 3-D structures within a sample. Devices made from
Foturan may be glass, a glass-ceramic composite, or ceramic, with the final material composition depending on the
irradiation and thermal processing procedures. Excellent aspect ratios (>30:1) have already been demonstrated in
Foturan. Our interest is in making simple 3-D MEMS structures by implementing cost-effective manufacturing
solutions that produce consistent results with a resolution on the order of ten microns.
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