A UV Direct-Write Approach for Formation of Embedded Structures in Photostructurable Glass-Ceramics
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P.D. FUQUA*, D.P. TAYLOR*, H. HELVAJIAN**, W.W. HANSEN", M.H. ABRAHAM** *Materials Processing and Evaluation Department, Space Materials Laboratory **Center for Microtechnology; The Aerospace Corporation, Los Angeles, CA 90009-2957 ABSTRACT Photostructurable glass-ceramics are a promising class of materials for MEMS devices. Previous work micromachining these materials used conventional photolithography equipment and masking techniques; however, we use direct-write CAM tools and a pulsed UV laser micromachining station for rapid prototyping and enhanced depth control. We have already used this class of materials to build components for MEMS thrusters, including fuel tanks and nozzles: structures that would prove difficult to build by standard microfabrication techniques. A series of experiments was performed to characterize process parameters and establish the processing trade-offs in the laser exposure step. The hypothesis that there exists a critical dose of UV light for the growth of an etchable crystalline phase was tested by exposing the material to a fluence gradient for a variety of pulse train lengths, and then processing as usual. By measuring the dimensions of the etched region, we were able to determine the dose. We found that the dose is proportional to the square of the per-pulse fluence. This has allowed us to create not only embedded structures, but also stacked embedded structures. This also implies that we can embed tubes and tunnels with a single exposure inside a monolithic glass sample. We feel that this technique has promise for a number of applications, including microfluidics. INTRODUCTION One of the many factors that drives progress in Microelectromechanical Systems (MEMS) is the improvement in materials and materials processing. Much of the work is based
on silicon; however, semiconductors are not optimal for some MEMS applications. One particularly interesting alternative class of materials is photostructurable glass ceramics. Stookey and others did the earliest work at Corning in the 1950s. [1,2] That work nucleated a number of wildly successful commercial product lines including Corningware. These materials are typically exposed by ultraviolet lamps and patterns are created using shadowmasks. Recent work in this laboratory has involved the use of ultraviolet lasers to create MEMS components for space applications. [3,41 The use of laser light in the exposure process has several advantages to conventional techniques. Our laser micromachining station moves the work piece relative to the laser beam according to a predefined computer program. The exposure process is maskless and thus amenable to rapid prototyping. Depth control is achieved by the proper choice of exposure wavelength and dosing. As one tunes the laser further into the material's UV absorption band, the absorption of light increases and the penetration depth decreases resulting in shallower features. Furthermore, the beam can be shaped and the resulting structures will maintain the shape. For example, a collimat
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