Abstract:
A flame stabilizing apparatus is disclosed for application to a burner for a flame hydrolysis deposition (FHD) process. In the flame stabilizing apparatus, a co-flow diffusion flame burner emits a flame onto a substrate and a flame stabilizer, installed around the burner coaxially, isolates the emitted flame from an instable ambient flow to stabilize the flame and a particle flow.

Description:
CLAIM OF PRIORITY 
     This application claims priority to an application entitled “Flame Stabilizer for Burner for Flame Hydrolysis Deposition” filed in the Korean Industrial Property Office on Jul. 21, 2001 and assigned Ser. No. 2001-44004, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a fine particle fabricating burner using flame hydrolysis deposition (FHD), and in particular, to a flame stabilizer for a burner to stably form a thin film with fine particles to be used as a light waveguide on a wafer by flame. 
     2. Description of the Related Art 
     In general, FHD is a technology developed from fiber preform techniques and modified to produce planar waveguide geometry. A vapor mixture of halides (SiCl4, GeCl4, etc) is reacted in an oxy-hydrogen flame to form fine glass particles, which are deposited directly onto a suitable substrate, e.g., silicon. The high-silica technology offers the potential of integrating a number of passive functions on a silicon chip, as well as the possibility of hybrid integration of both active and passive devices onto a silicon motherboard. 
     Flame-using fine particle formation is performed by oxygen-hydrogen flames generated from a co-flow diffusion flame burner having a plurality of concentric nozzles. Being fed with flames, a source material forms fine particles by means of flame hydrolysis or oxidation. The fine particles migrate with the flames, grown by collision-caused coagulation, and adhere to a substrate in various forms by thermophoresis. The fine particles then are sintered and dried according to their use, for example, by OVD (Outside Vapor Deposition) or VAD (Vapor-phase Axial Deposition) in fabricating an optical fiber, and FHD in fabricating a planar optical waveguide thin film. 
     Planar optical waveguide technology has added the growth of the Internet and multimedia communications. These areas are also driving the demand for more and more capacity on networks, and the medium of choice for high-bandwidth is optical fiber. There are three technologies to increase the overall data rate in an optical link: space division multiplexing (SDM) using multiple fibers in parallel, time division multiplexing (TDM), i.e. higher bit rate per channel and fiber, and wavelength division multiplexing (WDM), i.e., more channels per fiber by making use of different wavelengths for the transport of data. 
     Long-haul backbone networks already use all three strategies to exploit the enormous bandwidth of the optical fiber. Transmission rates in the Terabit per second (Tb/s) range over more than 100 WDM channels have been demonstrated in laboratories around the world. 
     Planar optical waveguide technology, for example, enables functions such as adding and filtering out specific wavelengths to be performed efficiently and flexibly at low cost. 
     FIGS. 1 and 2 are schematic views of a co-flow diffusion flame burner  100  having a plurality of concentric nozzles for use in the conventional FHD process. The term ‘diffusion flame’ can imply that the fuel and oxidizer (air) gas streams mix together in a chemical reaction zone by mass diffusion. 
     Referring to FIGS. 1 and 2, a flame F is generated by spraying source materials and a carrier gas from the center of the co-flow diffusion flame burner  100  in a direction A, with combustion of hydrogen (H 2 ) and oxygen (O 2 ) outside the center, respectively in directions B and C. The source materials for forming particles usually include SiCl 4 , GeCl 4 , and POCl 3 . Since these materials are liquid at room temperature, they are used after bubbling by the carrier gas. Hydrolysis occurs where H 2 O or OH resulting from the combustion of H 2  and O 2  meets the source materials diffused from the center of the burner  100 . The resulting particles migrate with the flame F and stick to a silicon wafer W by thermophoresis. 
     The particles generated and grown in the flame F are as small as several nanometers to tens of nanometers. For particles this small, the influence of inertia can be neglected. The velocity of the particles in the flame F is the sum of the velocity of the gas and a thermophoretic velocity over a temperature gradient. 
     Therefore, in the case where the flow of the flame F is a laminar flow that can define a streamline, the particles in the flame F show a similar velocity distribution. On the other hand, if the flame flow is a turbulent flow with velocity fluctuation that cannot define a streamline, the particles in the flame F move at an unstable velocity along with the flow. As a result, the deposition efficiency and deposition uniformity of the particles is reduced. 
     The co-flow diffusion flame F generated from the burner  100  is generally unstable for two reasons. First, a shear flow is formed due to the velocity difference between the gases sprayed from the concentric nozzles and determined by process conditions, that is, formation of the flame F, source materials, and the flow rate of the carrier gas. The second reason is that entrainment of an ambient gas and a shear flow caused by the velocity and pressure differences between the flame F and the ambient gas. The entrainment of the ambient gas and the shear flow depend on the flow outside the flame F. 
     In the general FHD process, the flame F attracted by gravitation is positioned by rotation or linear movement over the silicon wafer W having a relative velocity in a direction D with respect to the burner  100 . An exhaust flow is absorbed in a direction E into a discharge tube  12  for discharging non-deposited particles. The flame F exists in the vicinity of the flame F. A natural convection phenomenon is observed due to heating of the silicon wafer W and the flame F itself is influenced by buoyancy in a direction G. Hence, the flame F and the flow around the flame F are vulnerable to instability. 
     A flame shield tube  10  is usually provided to the periphery of the co-flow diffusion flame burner  100  to prevent introduction of ambient air and thus stabilize the flame F. 
     However, this method is not effective in fundamentally overcoming the instability of the flame F formed after passing through the flame shield tube  10 . Therefore, high particle deposition efficiency and uniformity cannot be expected. Moreover, a vortex arising from an unstable flow causes contamination by allowing glass soot particles that are not deposited on the silicon wafer W to stick to the outside of the burner  100  or inside a deposition chamber. For at least these reasons, an improved burner design is needed 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an improved flame burner. 
     It is another object of the present invention to provide a flame stabilizer for a burner, which stabilizes a flow around a co-flow diffusion flame to prevent the disturbance of the flame and a particle flow in the flame caused by the ambient flow with increased particle deposition efficiency and stable and uniform particle deposition. 
     It is another object of the present invention to provide a flame stabilizer for a burner, which prevents the burner and a deposition chamber from being contaminated with glass soot due to a vortex and a flow disturbance around a flame. 
     The foregoing and other objects of the present invention are achieved by providing a flame stabilizing apparatus for application to a burner for an FHD process. In the flame stabilizing apparatus, a co-flow diffusion flame burner emits a flame onto a substrate and a flame stabilizer, installed around the burner coaxially, isolates the emitted flame from an instable ambient flow to stabilize the flame and a particle flow. 
     Another aspect of the present invention relates to a method for stabilizing a flame from an instable ambient flow and particle flow. The method includes the steps of inserting source materials into a center of a co-flow diffusion flame burner and inserting an inert gas that is not involved in flame hydrolysis through a plurality of channels juxtaposed to the center of the burner. The method also includes the step of allowing the insert gas to develop into a flow in the same direction as an emitted flame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic view of a conventional burner for FHD, illustrating formation of a thin film on a silicon wafer by spraying a flame from the burner; 
     FIG. 2 is a frontal view of the burner shown in FIG. 1; 
     FIG. 3 is a sectional view of a burner with a flame stabilizer according to a preferred embodiment of the present invention; and 
     FIG. 4 is a schematic view of the burner with the flame stabilizer according to the preferred embodiment of the present invention, illustrating formation of a thin film on a silicon wafer by feeding a flame from the burner. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the present invention. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. 
     FIG. 3 is a sectional view of a burner with a flame stabilizer according to a preferred embodiment of the present invention. 
     FIG. 4 is a schematic view of the burner with the flame stabilizer, illustrating formation of a thin film on a silicon wafer by spraying a flame from the burner. 
     Referring to FIGS. 3 and 4, a flame stabilizing apparatus includes the co-flow diffusion flame burner  100  and a flame stabilizer  30  for the burner  100 . Source materials and a carrier gas for bubbling the source materials are sprayed from the center of the burner  100  in the direction A. The flame F is generated by combustion of a mixture of hydrogen and a diluting gas for temperature control in the direction B. As a oxidizer, oxygen flows in the direction C outside the center. Additionally, a flame shield tube  10  (see FIG. 4) is provided to prevent the introduction of air around the burner  100  and thus to stabilize the flame F. 
     The source materials for forming particles usually include SiCl 4 , GeCl 4 , POCl 3 , and BCl 3 . 
     The flame stabilizing apparatus according to one embodiment of the present invention includes the co-flow diffusion flame burner  100  and the flame stabilizer  30  around the burner  100 . The flame stabilizer  30  sprays a gas to stabilize the flame F against an ambient flow. The flame stabilizer  30  is hollow and substantially surrounds the burner  100  coaxially. However, it should be noted that other configurations may also used as long as they are juxtaposed to the center of the co-flow diffusion flame burner  100 . For example, the flame stabilizer  30  need not completely surround the burner  100 . Also, the outer shape of the flame stabilizer  30  may vary. 
     More specifically, the flame stabilizer  30  is composed of a plurality of preferably ceramic honeycomb tubes  32 , which are stacked in the outer circumferential direction. Other configurations for the flame stabilizer  30  may also be used, e.g. a plurality of channels or multi-branching passages. In addition, other flame retardant and/or resistant materials may be used other than ceramic. The diameter of the plurality of tubes  32  need not be the same at both ends. It is also noted that a plurality of cross-sectional shapes can be used for the honeycomb tubes, e.g., circular, square, oval, hexagon, etc. The tubes  32  may be arranged in a random pattern or stacked in a grid-like array pattern. 
     In operation, the flame stabilizer  30  sprays inert gases (see FIG. 4) that are not involved in flame hydrolysis, such as O 2 , N 2 , Ar, He, etc. through the ceramic honeycomb tubes  32  (Shown in FIG.  5 ). Passing through the small diameter (relative to the diameter of the flame shield tube  10 ) honeycomb tubes  32 , the inert gases become fully developed flow. Then, the fully developed inert gases have only a velocity component in the same direction as that in which the flame F is emitted at the outlet of the flame stabilizer. 
     This arrangement causes a velocity component perpendicular to the flame emitting direction to be removed from the fully developed inert gases. If the inert gases are directed in multiple layers in a direction H around the flame F, the flows in the direction perpendicular to the flame emitting direction, caused by the introduction of stationary air, can be blocked or minimized. Also, the instability of the flame F due to a shear flow and a suction flow caused by the relative velocity difference in the direction D between the silicon wafer W and the burner  100  can be minimized. 
     In particular, spraying, forcing or inserting the inert gases around the flame F from the co-flow diffusion flame burner  100  through the multi-layered honeycomb tubes  32  prevents the flame F from the instability caused by the natural convection and the suction flow. It should be understood by one of ordinary skill in the art that the burner  100  with the flame stabilizer  30  shown in FIG. 4 is a sectional view in flame F emitting direction, and the stabilizer  30  surrounds the burner  100  coaxially. 
     As described above, in application to a co-flow diffusion flame burner for FHD, the inventive flame stabilizing apparatus uses a flame stabilizer for spraying inert gases with a velocity in the flame emitting direction in multiple layers around a flame. This has the effect of removing instability caused by factors other than the flame and thus maintaining the flame and a particle flow in the flame at the state of a laminar flow. Consequently, the thermophoresis effect with respect to a temperature gradient is maximized and the particle mass deposited on a silicon wafer and particle deposition uniformity are increased when source materials of the same mass are used. Furthermore, suppression of flow disturbance around the flame prevents contamination of the FHD burner and a deposition chamber with glass soot. The resulting obviation of periodical cleansing of the burner prevents damages to the burner and reproducibility deterioration. 
     While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.