Abstract:
An apparatus and method for producing a crystalline ribbon continuously from a melt pool of liquid feed material, e.g. silicon. The silicon is melted and flowed into a growth tray to provide a melt pool of liquid silicon. Heat is passively extracted by allowing heat to flow from the melt pool up through a chimney. Heat is simultaneously applied to the growth tray to keep the silicon in its liquid phase while heat loss is occurring through the chimney. A template is placed in contact with the melt pool as heat is lost through the chimney so that the silicon starts to “freeze” (i.e. solidify) and adheres to the template. The template is then pulled from the melt pool thereby producing a continuous ribbon of crystalline silicon.

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 60/827,246, filed on Sep. 28, 2006. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for producing thin silicon substrates suitable for manufacturing photovoltaic cells, and in one of its aspects relates to a method and apparatus for laterally pulling a long crystalline ribbon from its melt which is established from unseeded growth and which is accomplished under strongly variable radiation control of heat loss. 
     BACKGROUND OF THE INVENTION 
     Development of energy sources that are alternatives to fossil fuels has become more and more important over the years. One such alternate source is solar energy wherein photovoltaic cells or the like directly convert solar energy into useful electrical energy. Typically, a plurality of these photovoltaic cells are encased between a transparent sheet (e.g. glass, plastic, etc.) and a sheet of backing material to form a flat, rectangular-shaped module (sometimes also called “laminate” or “panel”) which, in turn, is installed onto the roof of an existing structure (e.g. a house, building, or the like) to provide all or at least a portion of the electrical energy used by that structure. 
     A majority of photovoltaic cells are comprised of silicon-based, crystalline substrates. These substrates can be wafers cut from ingots of silicon or from ribbons of silicon where the ingots or ribbons are “grown” from batches of molten silicon. In the early development of ribbon crystal growth and solar energy, several apparatuses were developed to produce silicon bodies or substrates with good crystalline quality. Unfortunately, however, most of these ribbon growth apparatuses focused too intently (a) on producing perfect crystalline quality of the ribbon (e.g. the net shape of the substrates which were typically between 200 and 400 microns in thickness) or (b) on perfecting the combination of melt and growth necessary for a continuous growth operation. This resulted in only small crystalline areas being grown or, alternately, the entire operation was unstable due to dual melt isotherms in contact with the growth region. 
     As part of these efforts, significant focus was also placed on the application of cooling gas, primarily helium, to the growth region to effect the proper heat extraction required for the desired growth of the crystals. However, difficulties arose in maintaining the proper volume of cooling gas during a growth operation. That is, insufficient gas flow did not allow enough heat to be extracted during the growth of the crystalline substrate to enable sufficiently high growth rates to be readily economically competitive. On the other hand, merely increasing the flow of cooling gas to a rate sufficient to achieve the required extraction of heat resulted in the increased gas flow disrupting the melt surface of the substrate thereby preventing the formation of a desired flat sheet of crystal. Still another attempt to correct this problem involved decreasing the heater input power but, unfortunately, this resulted in poor thermal gradient control; i.e. the ability to maintain a stable crystal growth front without a proclivity to form dendrites. 
     More recently, other approaches have been proposed to overcome some of the above-described problems related to the growth of a silicon-based, crystalline substrate. For example, U.S. Pat. No. 4,329,195 describes a technique for growing a thin and wide substrate at relatively high rates by using a cooling gas (i.e. mixture of argon and hydrogen or helium) and a seed crystal to start the growth of the substrate. However, the supply of molten silicon is replenished by a silicon feed rod which, in turn, is positioned into same melt zone that produces the nearby crystalline sheet, thereby creating an undesirable dual melt isotherm dilemma which presents control problems. Another technique involves directly contacting the substrate with a heat sink to control the extraction of heat from the substrate during growth; see U.S. Pat. No. 3,681,033 and “Float Zone Silicon Sheet Growth”, C. E. Bleil, Final Report-DOE Grant No. DE-FG45-93R551901, Sep. 23, 1993 to Dec. 31, 1996 which may cause unevenness in the surface of the substrate. 
     Further, many prior techniques for forming crystalline substrates rely on the formation of a meniscus to act as the sole support of the growth process; see U.S. Pat. No. 2,927,008. However, many of such free-standing meniscus shaping and related techniques use graphite or silicon carbide dies for ribbon definition and accordingly, have the difficulty of providing a stable and uniform ribbon thickness under varying conditions. Also, continuous and direct contact with one of these carbon sources contributes to melt contamination and limits the performance of an eventual solar cell. These types of techniques also rely on vertical pulling from the melt and since heat will be lost along the length of the growing ribbon perpendicular to the growth front, the total amount of heat which can be extracted is severely limited. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus and method for producing or forming a sheet or ribbon of silicon, preferably a sheet or ribbon of crystalline silicon, continuously from a melt pool of liquid silicon. The silicon is melted in a melt chamber and flowed into a growth tray where it forms a level melt pool of liquid silicon. Heat is passively extracted by allowing heat to flow from the melt pool up through a chimney which, in turn, is positioned downstream of the melt chamber. Heat is simultaneously applied to the growth tray to keep the silicon in its liquid phase while heat loss is occurring through the chimney. One end of a template is placed in contact with the melt pool at a point where heat loss has occurred so that the silicon starts to “freeze” (i.e. solidify) and adheres to the template. The template is pulled from the melt pool thereby producing a continuous ribbon of silicon, preferably crystalline silicon. 
     More specifically, the present invention provides an apparatus and method wherein silicon is melted within a crucible which, in turn, is positioned within a pressurized, water-cooled furnace. The crucible is supported on a growth plate which also supports a heat-extracting means (e.g. chimney) downstream of the crucible. The melted silicon flows through an outlet in the crucible and into a growth tray. Preferably, a dimple in the growth tray below the outlet and a mid-tray ridge aids in smoothing out the melted silicon as it fills the tray and forms a substantially uniform, level melt pool within the tray. 
     Adjustable means, e.g. throttle plates, are provided in the chimney by which the heat flow from the melt pool through the chimney can be regulated. Radiated heat (i.e. heat loss) flows up through the chimney and is dissipated at the water-cooled walls of the furnace. A template is positioned into contact with the melt pool at a point where the controlled heat loss causes the silicon to start to solidify whereupon it adheres to the template. The template is extracted from the growth tray thereby pulling a sheet or ribbon of the now-solidifying silicon along with it. Although this invention is described with respect to using silicon as a feed material to form a ribbon of silicon, preferably crystalline silicon, which is suitable for manufacturing solar cells, it is to be understood that the apparatus and method of this invention can be used to convert other feed materials to ribbons of such feed materials. Such other feed materials can comprise, for example, other semiconductor materials. Preferably, the other feed materials have a melting temperature of at least about 1200° C., a high surface tension, and a liquid phase that is denser that the solid phase. When a feed material other than silicon is used, the material used to construct the growth tray and crucible will be selected so they are compatible with that particular liquid feed material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The actual construction operation, and apparent advantages of the present invention will be better understood by referring to the drawings, not necessarily to scale, in which like numerals identify like parts and in which: 
         FIG. 1  is a simplified illustration of a furnace, partly broken away to show an embodiment of the apparatus of the present invention positioned therein for producing a crystalline substrate; 
         FIG. 2  is an enlarged view of the apparatus of  FIG. 1 ; and 
         FIG. 3  is an exploded view of the apparatus of  FIG. 2 . 
     
    
    
     While the invention will be described in connection with its preferred embodiments, it will be understood that this invention is not limited thereto. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalents that may be included within the spirit and scope of the invention, as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings,  FIG. 1  illustrates a furnace  50  in accordance with the present invention. Furnace  50  is comprised of a vacuum/pressure vessel  51  having water-cooled walls  52  which allows control of the ambient conditions in the furnace. Positioned and supported within vessel  51  is apparatus  10  which is used to produce a thin, continuous, preferably crystalline substrate or ribbon (used interchangeably throughout) in accordance with the method of the present invention. 
     Apparatus  10  is comprised of a melt chamber  11  which, in turn, is comprised of a pour crucible susceptor  12  which has pour crucible  13  positioned therein and has an insulation layer  14  therearound. The term “susceptor” as used throughout, refers to a structure that provides a stable body which (a) acts as a mechanical support at high temperature and (b) is electrically conductive to receive power from the induction heating coils. Positioned around insulation layer  14  is a heating means (e.g. electrical induction heating coil  15 ) for supplying the power necessary to melt silicon within pour crucible  13 . A cover  16  closes the top of melt chamber  11  and has an opening  17  therethrough for feeding solid silicon (not shown) into pour crucible  13  as will be discussed more fully below. 
     Pour crucible  13  is supported on growth plate  18  which, in turn, is supported on growth tray susceptor  19 . As shown in  FIG. 3 , growth plate  18  is comprised of a graphite material that has been machined to provide support areas which are integral with areas of thermal isolation for selective heat extraction as will be explained below. An upper insulation layer  20 , a lower insulation layer  21 , and a bottom insulation layer  22  are positioned around growth tray susceptor  19  and growth tray heating means (e.g. electrical induction heating coil  23 ) encircles lower insulation layer  21  for a purpose described below.  FIG. 3  does not show growth tray susceptor  19  separate from insulation  20 ,  21 , and  22 . Further, a forked-shaped layer  25  of insulation (see  FIG. 3 ) is positioned above and along the length of growth plate  18  (see  FIG. 2 ). Pour crucible  13  has an outlet  26  at its lower end which passes through openings in both layer  25  and in growth plate  18  and opens into growth tray  27  which, in turn, is supported on growth tray susceptor  19 . A baffle  26   a  is positioned at the outlet  26  to effectively act as a filter to allow only molten silicon (i.e. melt pool  33 — FIG. 2 ) to flow through the outlet while blocking any unmelted silicon or other solids. 
     Growth tray  27  has a dimple  28  directly below outlet  26  and a ridge  29  downstream from the outlet for a purpose described later. Growth plate  18  also supports heat-extracting means, e.g. chimney  30 , which is spaced from but is in thermal communication with growth tray  27  as will be described below. The chimney  30  is positioned downstream of the outlet  26  and has an adjustable means (e.g. at least one throttle plate  31  (two shown) pivotably mounted therein) to control the heat flow through the chimney as will be further described below. 
     While the precise details of the present furnace are not critical and may vary substantially in different operations, the following discussion merely sets forth examples of typical dimensions and materials used and are not to be considered limiting in any way. As illustrated, pour crucible  13  may have a cylindrical shape (e.g. typical diameter of greater than 10 cm and a height of more than 15 cm to accommodate a quantity of feed material) and may be comprised of a material that is compatible with the material to be melted; i.e. to melt silicon, material is preferably comprised of silica. Growth plate  18  is a rectangular plate made of heat conductive material (e.g. carbon-graphite) and if as shown in the FIGS., might have typical dimensions of 30 cm long and 15 cm wide. The specific dimensions are free to be scaled to any width required for the desired ribbon sheet width and the length for the desired ribbon sheet pulling speed. Chimney  30  is preferably rectangular shaped to match the growth area of the growth plate  18  and is preferably primarily comprised of graphite, graphite insulation, and ceramics. Growth tray  27  is also preferably rectangular and, for example, can be 60 cm by 25 cm and approximately 30 mm deep, and is preferably comprised of at least 98% silica. With the construction of apparatus  10  having been described, the method of the present invention will now be set forth. 
     In operation, a desired amount of solid silicon (in chunks, small pieces, rods, or the like) is loaded through opening  17  into pour crucible  13  onto the top of baffle  26   a  and furnace  50  is closed and evacuated to remove oxygen that may contaminate or degrade the ribbon-growing process. Once a sufficient vacuum is established, the process chamber is heated to 300° C., then to 500° C., to assist in removing any moisture that may be adsorbed on the components of the furnace. The furnace is then backfilled with an inert gas (e.g. argon) to ambient pressure and the ribbon exit slot  53  is opened to enable a ribbon template  40  to be inserted into the furnace. Ribbon template  40  is preferably a thin sheet of a carbon-based material, such as a graphite coated carbon or graphite fabric, having approximately the same width as that of the desired ribbon which approximately matches the width of the lower end of chimney  30 . A constant flow of argon is supplied through gas injector  34  to maintain positive pressure and outward flow from the head space between the melt pool  33  of molten silicon and the growth plate  18  and to prevent back-streaming of air into the furnace chamber through exit slot  53 . 
     The temperatures of both the pour crucible  13  and the growth tray susceptor  19  are raised to and stabilized at approximately 1400° C. One end of ribbon template  40  is connected to any type of known extraction mechanism  54  and the other end is moved to a position near the melt pool  33  in growth tray  27 , after which the temperature of growth tray susceptor  19  is raised to approximately 1420° C. 
     The power input to electrical induction heating coil  15  around the pour crucible susceptor  12  is increased by approximately 2000 watts, in this example, to begin melting the solid silicon in the pour crucible  13 . At melt temperature (e.g. 1412° C.), approximately 1.1 grams of silicon should melt per second. The amount of power added is changed proportionally to the amount of silicon required for makeup. As melting occurs, the more dense molten or liquid silicon moves to the bottom of crucible  13  and out through outlet  26  into growth tray  27 . Baffle  26   a  allows only the liquid silicon to pass through outlet  26  while blocking flow of any solids. 
     The liquid silicon passes through outlet  26  and directly on dimple  28  in growth tray  27 . Dimple  28  reduces the splashdown depth of silicon and thereby limits ripple formation. Mid tray ridge  29  limits wave propagation into the growth area thereby providing a smoother surface to the liquid silicon melt pool  33  in tray  27 . The molten silicon will spread out into a shallow pool of approximately 10 mm in depth. A shallow melt pool helps to suppress convection currents that cause localized hot spots and uneven thickness growth rate of the ribbon. By nature, molten silicon has a very high surface tension that may cause it to pull together and not flow freely into the desired thin layer of liquid. If this becomes a problem, it can be alleviated by flowing a gas, e.g. nitrogen, onto the melt pool  33  through gas port  34  to modify the surface properties of the melt pool. 
     It should be noted that, in a preferred embodiment of the invention, the gas supplied through gas port  34  is not to aid in cooling the melt pool but is to sweep or purge the surface of the melt pool and remove gaseous and small particle impurities that may exist in the space between the melt pool  33  and the growth plate  18 . As will be further explained below, growth plate  18  provides for the removal of heat, preferably all of the heat, from the molten silicon required to reach the desired growth parameters. Since the ribbon to be formed is very thin, there will, preferably, only be a very small thermal gradient (e.g. approximately 0.3° C.) between its top and bottom surfaces at the point of separation with the melt pool  33 . 
     When the melt pool  33  reaches a pre-determined depth (e.g. 10 mm), the power to coil  15  is reduced by 2000 watts and template  40  is moved into contact with melt pool  33  at contact point  40   a  to wet the edge of the template. Throttle plates  31  are opened to modulate the amount of radiation heat that can be extracted from the growth plate  18 . The growth plate acts as a gas barrier and moderator between the melt and cold wall of the furnace with its modulation controlled by the open/closed angle of the throttle plates. This heat will radiate from the surface of the template  40  and/or growing ribbon  40   b  to the growth plate  18 , through chimney  30 , and on to the water-cooled walls  52  of the furnace  50  where it is dissipated. Preferably, the throttle plates are opened in sequence with the throttle plate nearest the contact point  40   a  being opened first to thereby allow the thin, silicon crystalline ribbon  40   b  to begin growth at the region  40   a  where the template  40  contacts the liquid silicon. 
     As known, the phase change between solid and liquid silicon is an isothermal process (both at 1412° C.), and much like water, significant amounts of energy (heat) can be removed or added without a change in temperature. Only part of the heat released on solidification is radiated to the growth plate  18  (useful work) while the balance is conducted away through the liquid in contact with the solid that has just formed. When the throttle plates  31  are opened, a fraction of the radiated power is added to coil  23  around growth susceptor  19  to assure a proper melt temperature gradient and to prevent formation of dendrites on the growing crystal. As used herein, “radiated power” is meant to be that power lost through radiation as opposed to conduction or convection. 
     It should be noted that in the preferred mode of operation, throttle plates  31  are the sole source of cooling control of the melt as the ribbon is formed. The throttle plates are actively controlled to begin growth and to control the thickness of the ribbon being formed. The opening of throttle plates allow modulation of heat flow from the melt pool  33  at its freezing point without significantly lowering the temperature of the liquid silicon present below the solid ribbon. As noted above, coil  23  provides an inductive source of heating to growth plate susceptor  19  which, in turn, heats and maintains the body of the melt pool  33  in a molten state as the ribbon is pulled from the melt pool. 
     Once the ribbon begins to form, the template  40  is extracted from the melt pool  33  and is advanced at a rate that is proportional to heat loss through the growth plate  18 . The template  40  with the ribbon  40   b  attached thereto is extracted at a takeoff angle α that provides for the formation of the ribbon such as, for example and angle of from about 1° to about 15° (e.g. approximately 4°) (see  FIG. 1 ) from horizontal to maintain a balance of forces against the buoyancy/weight of the liquid silicon, its surface tension, and triple-point meniscus between liquid melt pool  33 , the solid ribbon, and the gas ambient. Since the present method of forming the silicon ribbon does not require using a single crystal seed as a template, it can avoid the many problems normally associated therewith. Also, induced stress in the ribbon can be reduced by allowing the ribbon to take a natural catenary shape coming off the melt pool  33 . The cooling profile of the formed ribbon can be adjusted independently of the growth rate through various insulation means around extraction guides  54 , thus allowing custom tailoring of silicon electrical properties. 
     Since the ribbon exits the furnace  50  shortly after formation (e.g. about 3 minutes or less), a direct thickness measurement can be used to make minor adjustments (a) to the throttle plates  31  to modulate the amount of heat loss through chimney  30 , (b) to the pull speed to alter residence time, and/or (c) to the amount of power to the heating coils to affect net heat loss. As the ribbon forms, the extraction guides  54  can be used to maintain a smooth and even take-off angle while the arm or other extraction mechanism  55  (e.g. opposed rollers, “hand over hand” pinch grips, or other continuous feed devices) external to the furnace can be adjusted to maintain rate and continuous ribbon growth. Once a section of continuous ribbon has progressed past the extraction mechanism, it can be scribed and removed from the ribbon being formed, sectioned to finished substrate size, or left as part of the continuous ribbon for further processing in downstream operations. 
     Further, as should be understood in the art, a solid silicon feed mechanism (not shown) can cooperate with opening  17  in cover  18  on pour crucible  13  to add pieces of silicon into the pour crucible as needed to keep the operation continuous. The silicon pieces are normally added during the downtime between melt additions so that the solid silicon is brought to pre-melt temperature before the next cycle begins. Adjustment of the resistivity of the formed ribbon can be altered by varying the properties (dopants) of the solid silicon addition. Because the residence time of the system is low, this adjustment can be accomplished relatively quickly to maintain the formed ribbon within desired resistivity range. Also, if needed for growth stability and ribbon template formation, a heterogeneous seed surface can be formed in situ by the adding a small amount of nitrogen in the argon ambient through gas port  34 . This forms a very thin (&lt;10 mm) skin of silicon nitride/silicon oxynitride on the surface of the melt pool  33  to template the crystalline silicon growth underneath. 
     The continuous process will be interrupted when the growth tray or pour crucible have reached the end of their useful life, or the silicon melt pool has concentrated impurities or dopants to the point that adequate quality ribbon can no longer be produced. At this point, the furnace can be cooled, the worn components and silicon replaced, and the method restarted. 
     U.S. Provisional Patent Application No. 60/827,246, filed on Sep. 28, 2006, is incorporated herein by reference in its entirety.