Laser Chemical Vapor Deposition
Laser chemical vapor deposition (LCVD) is a process by which an organometallic compound in the form of a precursor gas is induced, by light or heat from a laser source, to deposit in the form of a solid on a substrate. Deposition can occur through a photo
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3.1 Introduction Laser chemical vapor deposition (LCVD) is a process by which an organometallic compound in the form of a precursor gas is induced, by light or heat from a laser source, to deposit in the form of a solid on a substrate. Deposition can occur through a photolytic or pyrolytic mechanism, or a combination of both mechanisms. In photolytic LCVD, deposition occurs when the precursor gas is dissociated directly by light. The dissociation can occur over the surface (when the laser beam is parallel to the surface) or at the surface (when the beam is incident to the surface as in direct write photolysis). The deposition rate in photolytic LCVD is very low (usually in Angstroms/unit time) and is feasible for depositing thin films without heating the substrate. It has been employed for the manufacture and repair of integrated circuit interconnects (Osgood 1983; Baum 1992). In pyrolytic LCVD, the precursor gas is induced to deposit on the surface through laser-induced surface heating of the substrate. The deposition rate is much higher than that of photolytic LCVD, which makes it better suited as an important technique in freeform fabrication of high aspect ratio microstructures. While pyrolytic LCVD has been widely explored for depositing thin metallic films, the deposition of three-dimensional microstructures has only recently become of widespread interest. The LCVD process is computer controlled and is capable of producing microstructures of arbitrary shapes. Furthermore, the process is rapid, flexible, and relatively inexpensive to operate. The pyrolytic LCVD set-up consists of a laser, a vacuum chamber, and a moveable target as shown schematically in Fig. (3.1.1). The laser beam is focused through a chamber window onto a substrate target. Scanning of the substrate surface by the laser beam is accomplished through a computercontrolled movement of the stage in the x y plane. A precursor gas is introduced into the chamber where it reacts at or near the focal spot of the laser on the substrate, leaving behind a solid deposit. Precursors are chosen so that the by-products of the reaction are volatile and return to the surrounding gas
R. Nassar et al., Modelling of Microfabrication Systems © Springer-Verlag Berlin Heidelberg 2003
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3 Laser Chemical Vapor Deposition
mixture (Maxwell 1996). Using this technique, a solid can be deposited on a spot in the form of a fiber or rod. By scanning the beam in one or in two dimensions in the xy plane, solids can be deposited to form two-dimensional or three-dimensional microstructures of various shapes.
Fig. 3.1.1. LCVD schematic diagram (reprinted from Dai et al 1999b by permission of Taylor & Francis, Inc., http://www.routledge-ny.com)
Determining the rate of deposition on the substrate surface is crucial for modeling the process and for assessing its feasibility for the fabrication of microstructures. The deposition rate in pyrolytic LCVD is influenced by the surface temperature, by the reaction activation energy, and by the rate of transport of the precursor gas an
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