Abstract:
A carbon deposition chamber is provided with several advantages. The gas mixture used within the deposition process is expressed from tubing through three zones, which are each individually determined with needle valves. The substrate is permitted to rotate back-and-forth to permit more even deposition of carbon films onto the substrate. The heating filaments are permitted to expand and contract without breakage by permitting the electrode attached to one end of the filaments to move freely as the filaments change in length. The substrate and the heating filaments are cooled to a temperature to prevent carbonization by permitting a cooling fluid to be passed through tubing connected to these elements in a heat sink like manner.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This non-provisional application for a patent is a continuation of the provisional application for a patent entitled “HOT FILAMENT CHEMICAL VAPOR DEPOSITION CHAMBER AND METHOD OF USE” filed on Aug. 30, 1999 by the inventor, Zhidan L. Tolt and granted U.S. Serial No. 60/151,420. This non-provisional application relates to two non-provisional applications filed Feb. 23, 2000 by inventor, Zhidan L. Tolt, entitled A Substrate Support for Use in a Hot Filament Chemical Vapor Deposition Chamber, Ser. No. 09/511,572, and A Heating Element for Use in a Hot Filament Chemical Vapor Deposition Chamber, Ser. No. 09/511,021. 
    
    
     TECHNICAL FIELD 
     This invention relates to the chemical vapor deposition of diamond or carbon films, and more particularly, to an apparatus and a method of use in such deposition. 
     BACKGROUND INFORMATION 
     One class of methods developed in recent years for carbon deposition consists of the chemical vapor deposition (“CVD”) methods. For a general summary of various deposition methods including CVD methods, reference is made to Chemical &amp; Engineering News, 67(20), 24-39 (May 15, 1989), incorporated herein by reference. 
     In the CVD methods, a mixture of hydrogen and a hydrocarbon gas such as methane is introduced into a chamber and is then heated or thermally activated. Some of the hydrogen gas is disassociated into atomic hydrogen which reacts with the hydrocarbon to form growth species. These species deposit on the substrate in the form of a carbon film when they come into contact with a cooler substrate. This process is schematically illustrated in FIG.  1 . 
     One of the many of the CVD coating methods, hereinafter referred to as “filament” methods, employ one or more resistance heating units including heated wires or filaments, typically at temperatures of at least 2000° C. to provide the high activation temperatures at which these disassociations take place. This method is known in the art as hot filament assisted chemical vapor deposition (“HFCVD”). 
     Many reaction and mass transport processes occur on the substrate surface. The substrate temperature is therefore crucial to optimize growth of the film. 
     The integrity of the resistance heating filaments is critical in achieving and maintaining uniform temperature during the deposition. If a heating filament sags, the temperature in the vicinity of the heating element differs, thereby creating thermal and species nonuniformity on the substrate. If filaments break frequently, maintenance is needed more often. The condition and position of heating elements is therefore a critical factor for reactor operation. Resistance heating filaments are delicate and are easily broken during operation of the reactor. They expand when heated from room temperature to an operating temperature of 2000° C. and are thus subject to thermally-induced stresses that can break the filaments. Also, during carbon deposition, resistance heating filaments can react with carbon-bearing gases contacting the filaments and form a carbide, further lengthening and embrittling the filaments. 
     Horizontal filaments which are typically used in HFCVDs present a number of problems. The filaments severely deform and sag after they are first heated up so that uniform and repeatable film deposition become impracticable. As a result, intense maintenance work becomes necessary and makes it impractical to use most conventional HFCVD reactors for manufacturing purposes. 
     Because of tremendous thermal gradients present in a hot filament reactor, a large ceramic substrate plate breaks easily. For large area deposition, a uniform gas distribution is also necessary to achieve uniform deposition. It is thus important to be able to control the temperature of the substrate. Furthermore, graphitic carbon on non-substrate (i.e., the elements of the reactor) accumulates in HFCVD reactors. This accumulation requires frequent removal resulting in higher maintenance costs. 
     What is needed, therefore, is a device that can maintain a substantially constant force on the filaments to eliminate sagging of the filaments. Preferably, such a device would minimize maintenance and equipment set-up between cycles of operation. What is also needed is a device to control the gas flow. Preferably, such a device would also control the deposition of the carbon on the substrate and control the temperature of the substrate to prevent breakage of the substrate. Furthermore, a preferred device would also result in reduced maintenance costs by minimizing carbon accumulation. 
     SUMMARY OF THE INVENTION 
     The previously mentioned needs are fulfilled with the present invention. Accordingly, there is provided, in a first form, a disclosed CVD reactor which substantially reduces or eliminates the disadvantages and shortcomings associated with the prior art techniques. 
     To prevent filament sagging, the filaments in the present invention are arranged in the reactor vertically instead of the typical horizontal configuration. The filament assembly is configured such that the lower end of each filament can freely contract or extend vertically while constrained in all other directions. 
     The reactant gas is introduced into the reactor chamber through a gas dispersion system. The gas dispersion system, mounted within the reactor chamber, has a configuration to introduce the gas into three separate feeding zones. Each feeding zone has an independent control of the gas feeding rate. The extent of radial gas flow in the reactor, therefore, is controlled. Thus, the typical nonuniform distribution of gas is reduced by utilizing a three-zone feed gas distribution so that a uniform deposit of material can be achieved over the entire surface of the substrate. 
     A substrate support is mounted within the reactor chamber. During the operation of the reactor, the substrate support continually rotates 180 degrees back and forth. This constant rotation also helps ensure a uniform distribution of the carbon onto the surface of the substrate. 
     To prevent substrate breakage, there is also a feature to preheat the substrate before the actual deposition process is started. This preheating reduces the thermal gradient created by the filaments. There is also provided a means to cool the substrate holder when the reactor is fully operational. This cooling mechanism allows the user to independently control the temperature of the substrate. 
     Furthermore, the filament support structure and other relatively hot surfaces inside the reactor are water-cooled so that little, if any, graphitic carbon can accumulate on these surfaces. Thus, maintenance costs due to this accumulation are greatly reduced. 
     These and other features, and advantages, will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. It is important to note the drawings are not intended to represent the only form of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic view showing the chemical reactions and processes of a typical hot filament vapor deposition process; 
     FIG. 2 is a side-sectional view of the chamber illustrating one embodiment of the present invention; 
     FIG. 3 is a cross-sectional view of the chamber illustrating the filament configuration of one embodiment of the present invention; 
     FIG. 4 is another cross-sectional view of the chamber illustrating the gas dispersion configuration of one embodiment of the present invention; 
     FIG. 5 is a detail plan view illustrating the configuration of a substrate heating system of one embodiment of the present invention; 
     FIG. 6 is a schematic view of the operational aspects of the gas dispersion system illustrated in FIG. 4; and 
     FIG. 7 is a detail plan view illustrating the configuration of a substrate support cooling system of one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in FIGS. 2-8 of the drawings, in which like numbers designate like parts. In the following description, well-known elements are presented without detailed description in order not to obscure the present invention in unnecessary detail. For the most part, details unnecessary to obtain a complete understanding of the present invention have been omitted inasmuch as such details are within the skills of persons of ordinary skill in the relevant art. Details regarding control circuitry or mechanisms used to control the rotation of the various elements described herein are omitted, as such control circuits are within the skills of persons of ordinary skill in the relevant art. 
     Referring now to FIG. 2, there is depicted a side view of reaction chamber  200  showing the interior features of reaction chamber  200  of the HFCVD reactor according to the present invention. Chamber  200  is the housing where the chemical reactions and depositions occur. All of the features disclosed in the present invention are enclosed in reaction chamber  200  which is air-tight and thus capable of being maintained at reduced pressure and is fitted with a suitable gas inlet and an exhaust port (not shown). All portions of the apparatus which are present in the reaction chamber  200  are constructed of suitable heat-resistant materials, as necessary to withstand high temperatures. Stainless steel is an illustrative heat-resistant material suitable for this purpose. 
     FIG. 2 is a simplified sectional view of the HFCVD reactor  200  according to the present invention. In the following description of the invention, the reactor  200  is oriented horizontally and gas flow is from right to left. However, this does not imply that the invention is limited to this arrangement of flow. Vertical upward flow, and in some cases vertical downward flow, are included as embodiments of the present invention. 
     The key elements of the improved HFCVD reactor  200  comprise a multi-zone gas dispersion system  202  (see FIG.  4 ), a filament array  206  (see FIG.  3 ), a rotating substrate holder  208 , a substrate cooling system  212 , and a substrate heater  210 . Gas dispersion system  202  and substrate holder  208  are oriented within the reactor so that their surfaces are perpendicular to the axis of the gas flow through the reaction zone. The substrate  216  to be coated is supported by substrate holder  208  which rests on substrate heater  210 . Under substrate heater  210  is substrate cooler  212  which is mounted to substrate holder base  214 . Substrate heater  210  is provided with a lead (not shown) to which an electrical heating current is conducted from a suitable external power source (not shown). Substrate holder  208  is also provided with a thermocouple (not shown) to measure substrate temperature and a connecting electrical lead (not shown) through which the thermocouple output may be transmitted to an external indicator or read-out (not shown). 
     Substrate holder base  214  is connected to turning axle  215 , which is hollow to permit the passage of fluid conduits  251  and  252 , which will pass the cooling fluid for substrate cooler  212 , along with the electrical lead (not shown) for the thermal couple (not shown) and the substrate heater  210 . Motor  253  turns axle  215 , resulting in the turning of the substrate  208 . This is performed in a back-and-forth motion where the substrate is rotated 180° clockwise and then returned 180° counterclockwise. Note that a lateral displacement of the substrate holder is also permitted. 
     Chamber  200  also includes view ports  220  and  221  for looking at the internal contents of the chamber  200 . Additionally, chamber  200  includes the hinged lid  218 , which can be opened to access the internal components of chamber  200 , 
     Label  204  indicates that the gas dispersion system  202  can be moved back-and-forth to be closer or farther away from the substrate  208 . The remainder of the components noted in FIG. 2 are discussed in further detail with respect to FIGS. 3-7. 
     FIG. 3 illustrates the vertical array  206  of heating filaments  302 . Filaments  302  are made of a material that heats upon passing an electrical current through it. Means for applying an electrical current are connected to electrodes  304  and  306 . Illustrative materials are metallic tungsten, tantalum, molybdenum, and rhenium, with tungsten being preferred. Although the embodiment in FIG. 3 shows preferred vertical filaments  302 , several features of the embodiments of this invention are also applicable to a reactor which utilizes horizontal filaments instead of or in addition to the vertical filaments. 
     Resistance heating filaments  302  are of approximately the same length as the other filaments and are attached to two electrodes (filament bars)  304  and  306  to form the array of filaments  206 . The top electrode  304  has its position fixed for reactor operation. 
     Electrode  306  is also connected to filaments  302 . However, as more clearly illustrated in FIG. 2, electrode  306  is permitted to slide between positioning posts  260  and  261 , which are affixed to the chamber  200  wall by base  361 . Base  360  affixes electrode  304  to the chamber wall also. Positioning posts  260  and  261  are made of a material, such as a ceramic, which is not only insulative, but also has a surface that permits the slidable movement of electrode  306  in a vertical direction as the filaments  302  lengthen from heat and carbonization, or shorten as they are cooled. This alleviates the longitudinal stress upon the filaments  302  as the temperature in the chamber  200  changes. 
     Cooling of the electrodes  304  and  306  is provided by the cooling fluid passing through conduits  352 - 354 , which pass the cooling fluid from flexible tubing tubings  350  and  351 , which receive and send the cooling fluid to and from a source external to the chamber  200 . 
     During operation, filaments grow from carburization and from thermal expansion. The gravity force on the movable electrode maintains the filaments taut. The movable electrode of the array is also preferably so designed that it rigidly clamps its filaments&#39; ends, thereby preventing substantial vibration of the filaments at the spring end. For vertical filaments, it is preferable to provide guide tracks on each end of the movable electrode to prevent unwanted spiraling and bouncing of the movable electrode and filaments that could be caused by the flow of hot gases around the filaments and movable electrode. 
     A conventional HFCVD reactor design utilizes a flat rectangular or circular substrate holder over which a small substrate is placed for CVD coating. The gas flow adjacent to the substrate surface is therefore generally radial outward from a stagnation point, resulting in non-uniform deposition. 
     FIG. 4 illustrates the gas dispersion system  202  whereby three separate feedlines  611 - 613  supply the reaction mixture of gases in three separate zones further depicted in FIG.  6 . These three zones are labeled as  615 , 616  and  617 , which zones are concentric. Each of the pipes associated with each zone are perforated for expulsion of the gas towards the substrate in their respective zones. 
     Gas is fed to a mixing chamber  604  through a valve  603  from a gas source or reservoir  601 - 602 . The mixed gas is then passed through needle valves  605 - 607  to the respective gas feedlines  611 - 613  to zones  615 - 617 , respectively, which lie within the chamber. The barometric pressure can be measured at gauge  615  through open/close valves  608 - 610  individually for each gas feedline  611 - 613 . The needle valves fine control the flow rate of each line to the different zones of the reactor to optimize the gas distribution. The shape of the gas dispersion elements or perforated tubes can be different for different substrate geometries. As described above, the distance between the perforated gas line assembly and the filaments is also adjustable. 
     FIGS. 5 and 7 illustrate front and side views of cooling tubing configurations for the substrate cooler  212 , whereby the cooling fluid is passed to either one of these types of configurations for cooling the substrate. 
     In operation, the reaction chamber  200  is maintained at a pressure up to about  760  torr, typically on the order of  10  torr. A mixture of hydrogen and a hydrocarbon, most often methane is passed into the chamber  200  and a current is passed through the electrodes  304 ,  306  and filaments  302  to heat the filaments  302  to a temperature of at least about 2000° C. 
     The heat sink(s) is maintained at a distance from the substrate and water passage through the tubing associated therewith is maintained at a rate to provide a substrate temperature in the range of about 800-1000° C., depending on the desired film structure and growth rate. 
     During the CVD operation, filaments  302  undergo thermal expansion and expansion due to carburization. By reason of their prestressed condition, however, such expansion merely causes them to lengthen. Thus, they are not prone to distort in other directions or break. Using this combination of elements, the filaments will have a longer life span. 
     Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention.