The present invention relates to furnaces for crystal growth and directional solidification, and more particularly to a crystal growth apparatus having a load-centered aperture, and a device and method for controlling heat extraction from a crucible contained in the crystal growth apparatus.
Directional solidification systems (DSS) are used for the production of silicon ingots, for example, for use in the photovoltaic industry. A DSS furnace can be used for crystal growth and directional solidification of a starting material such as silicon. In DSS processes, silicon feedstock can be melted and directionally solidified in the same furnace. Conventionally, a crucible containing a charge of silicon is placed in a furnace, and at least one heating element is arranged near the crucible.
In directional solidification processes, a volume of silicon feedstock material is melted in a crucible at about its melting point temperature of 1412° C., thus forming a silicon melt. As heat is extracted from the bottom of the crucible, a bottom layer of the silicon melt begins to solidify, and forms a first layer of solid silicon at the bottom of the crucible. As heat is further removed from the bottom of the crucible, the solidified silicon continues to grow. The process continues until substantially the entire volume of the silicon melt is solidified, i.e., an ingot is produced. In this process, the direction of heat extraction is opposite to the direction of silicon growth; that is, as heat is extracted from the bottom of the crucible, solidification of the silicon melt advances toward the top of the crucible. Thus, impurities are “pushed” to the top and edges of the crucible, where solidification is last to occur. Directional solidification can be used as a purification process, i.e., since most impurities are more soluble in liquid than in the solid phase during solidification, impurities will be “pushed” by the solidification front, resulting in a lower concentration of impurities in the ingot that is formed as compared to the feedstock material.
Two typical solid-liquid interfaces may occur in a directional solidification process: a convex profile in which impurities are moved to corners of the silicon ingot, and a concave profile in which impurities are formed at the center and corners of the silicon ingot. A convex profile of the silicon ingot is more desirable, as it can provide maximum usable material of a substantially uniform shape.
German Patent DE 100 21 585 discloses an arrangement for producing a silicon melt, and directionally solidifying the silicon melt, in which a plurality of heating rods are arranged beneath a mold containing the silicon melt, and a cooling facility is arranged below the heating rods and separated from the heating rods by an insulating slide, such that during a solidification phase, the insulating slide is moved in a horizontal direction away from the mold, and radiant heat from the mold is transferred to the cooling facility. According to this German patent, the insulating slide is either open or closed, i.e., closed such that the silicon melt is heated by the heating rods during a heating phase, or open and removed away from the mold during the solidification phase.
U.S. Patent Application Publication US 2009/0280050 to Ravi et al. discloses an apparatus and method for forming a multi-crystalline silicon ingot by directional solidification including the use of horizontally movable heat shields arranged below a crucible, which purportedly results in controlled ingot growth and a convex profile.
According to the various embodiments of the published patent application to Ravi et al., either four independently movable heat shields, or two pivoting and/or overlapping heat shields, are used. However, in each embodiment, a complex control mechanism and/or multiple drive mechanisms are required. In particular, each of the heat shields is independently and separately movable in order to produce an opening of the desired size and/or shape.
It would be desirable to provide a crystal growth apparatus, and a device and method for controlling heat extraction from a crucible contained in the crystal growth apparatus, in which radiant heat from the crucible is allowed to pass through an aperture that can open in varying amounts, and where a simplified drive mechanism is provided for controlling a size of the aperture, such that a silicon melt can be cooled from its bottom center to produce a silicon ingot having a convex profile.
A crystal growth apparatus, and a device and method for controlling heat extraction from a crucible contained in the crystal growth apparatus are provided, where the crystal growth apparatus preferably includes at least two plates that move in a coordinated manner to form a symmetrical aperture centered with respect to an ingot being formed in a crucible, and a drive mechanism is provided to drive the plates with one degree of freedom. The plates are arranged to form an aperture that is load centered with respect to the ingot being formed in the crucible, in order to promote directional solidification of the ingot being formed, and thus achieve a desired convex profile of the ingot. The crystal growth apparatus can be a directional solidification furnace in which a charge of silicon is placed in the crucible, and at least one heating element is arranged near the crucible. In particular, the charge can be silicon feedstock, or silicon feedstock with a monocrystalline silicon seed.
A crystal growth apparatus according to the subject invention preferably includes a crucible for receiving a charge; a support mechanism configured to support the crucible; at least one heating element for heating and at least partially melting the charge; and a device for controlling heat extraction from the crucible including at least two plates being movable in a coordinated manner to form a symmetrical aperture substantially centered with respect to an ingot being formed in the crucible, and a drive mechanism configured to drive the at least two plates with a single degree of freedom.
Preferably, the at least two plates are moved at the same rate so as to vary a size, but more preferably not substantially change the shape, of the aperture, where the at least two plates are movable between a fully closed position and a fully open position, and more preferably the at least two plates are movable in a plurality of discrete partially open positions between the fully closed position and the fully open position. Also, the at least two plates can be interlocking, such that the at least two plates are engaged and interlocked in the fully closed position. Further, the at least two plates can be configured to slide toward or away in approximately equal amounts from a bottom center of the crucible. In other words, the aperture formed by the at least two plates preferably is load centered, i.e., the at least two plates are arranged such that their installation center corresponds to a bottom center of the crucible where the ingot is being formed. The at least two plates can form the aperture having a shape selected from at least the following shapes: square, rectangular, circular, parabolic, rhombic, and elliptical, or the shape can be defined by the relationship y=f(x), where x and y refer to distances along an X-axis and Y-axis, respectively. In certain embodiments, the at least two plates include triangular sections that form an aperture having the shape of a square, rectangle, or rhombus. The at least two plates preferably are movable so as to allow passage of radiant heat through the aperture in a controlled manner, and thereby achieve thermal gradient profiles resembling the contour of an ingot being formed.
According to the subject invention, the crucible preferably is contained in a crucible box, which can directly contact the support mechanism. The support mechanism is a block made of graphite or a similar material, and may be formed as a solid block. Alternatively, the block can include a plurality of holes that extend through the block. As a further alternative, the support mechanism can be formed as a plurality of supports, beams, and/or columns.
The crystal growth apparatus of the subject invention optionally can include a heat exchanger arranged in the crystal growth apparatus, where the heat exchanger preferably receives heat radiated from a bottom of the support mechanism. A diffusion plate optionally can be arranged between the support mechanism and the heat exchanger to provide a substantially even temperature distribution.
A device according to the subject invention for controlling heat extraction from a crucible contained in a crystal growth apparatus can include: at least two plates being movable in a coordinated manner to form a symmetrical aperture substantially centered with respect to an ingot being formed in the crucible; and a drive mechanism configured to drive the at least two plates with one degree of freedom.
A method according to the subject invention for controlling heat extraction from a crucible contained in a crystal growth apparatus can include steps of: providing a crucible for receiving a charge; heating and at least partially melting the charge contained in the crucible; providing at least two plates that are movable in a coordinated manner to form a symmetrical aperture substantially centered with respect to an ingot being formed in the crucible; and driving the at least two plates with a single degree of freedom.
Other aspects and embodiments of the invention are discussed below.
For a fuller understanding of the nature and desired objects of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawing figures wherein like reference character denote corresponding parts throughout the several views and wherein:
The subject invention is most clearly understood with reference to the following definitions:
As used in the specification and claims, the singular form “a”, “an” and “the” include plural references unless the context clearly dictates otherwise.
A “furnace” or “crystal growth apparatus” as described herein refer to any device or apparatus used to promote crystal growth and/or directional solidification, including but not limited to crystal growth furnaces and directional solidification (DSS) furnaces, where such furnaces may be particularly useful for growing silicon ingots for photovoltaic (PV) and/or semiconductor applications. The term “furnace” also refers to any device used for heating, including those suitable for high temperature applications in which operating temperatures exceed about 1000° C.
A crystal growth apparatus, and a device and method for controlling heat extraction from a crucible contained in the crystal growth apparatus are provided, in which at least two plates are arranged so as to be movable in a coordinated manner to form a symmetrical aperture substantially centered with respect to an ingot being formed in a crucible, and a drive mechanism is configured to drive the at least two plates with one degree of freedom. Preferably, the at least two plates are arranged under the crucible such that an installation center of the at least two plates corresponds to a bottom center of the crucible, and an aperture formed by the at least two plates will be load centered with respect to the ingot being formed in the crucible. According to the subject invention, the aperture is approximately symmetrical, where the opening shape of the aperture can be any of a variety of shapes, depending on the selection of the shape of the at least two plates, where a suitable shape of the aperture can be, for example: square, rectangular, circular, parabolic, rhombic, and elliptical, among other shapes. Also, the aperture optionally can have a nonlinear shape, and can be defined by the relationship y=f(x), where x and y refer to distances along an X-axis and Y-axis, respectively. Further, the aperture can be formed by at least two plates having triangular sections that form an aperture shaped as a square, rectangle, or rhombus. Preferably the at least two plates are moved at the same rate so as to vary a size of the aperture, and more preferably not substantially change the shape of the aperture. The at least two plates can be moved between a fully closed position and a fully open position.
According to the subject invention, the at least two plates are provided in the fully closed position during heating and melting of a charge contained in the crucible of the crystal growth apparatus. The crystal growth apparatus can be a directional solidification furnace in which a charge of silicon is placed in the crucible, and at least one heating element is arranged near the crucible. In particular, the charge can be silicon feedstock, or silicon feedstock with a monocrystalline silicon seed. After heating and melting of the charge, the charge is gradually solidified in the crucible during a solidification phase. During solidification, the at least two plates are moved or stopped at desired positions with selected velocities between and including the fully closed position and the fully open position. Between the fully closed position and the fully open position, the at least two plates can be opened in discrete amounts, such that a plurality of intermediate partially open positions are attainable. By opening the at least two plates gradually, directional solidification is promoted, and the ingot being formed can achieve a desired convex profile. The at least two plates preferably are movable to allow passage of radiant heat through the aperture in a controlled manner, and thereby achieve thermal gradient profiles resembling a contour of the ingot being formed.
Preferably at least two plates are moved to form a desired aperture shape, which can approximate a shape of the ingot being formed in the crucible, and more preferably, only two plates are used. However, it is possible to use more than two plates, for example, by reconfiguring and/or replacing one or more of the plates with multiple plates. Preferably the plates are interlocking and/or overlapping, such that they are configured to be interlocked and engaged in the fully closed position. The interlocking of the at least two plates can occur in a variety of configurations, such as a “sandwich” construction or a “staggered” construction, depending on how the plates are positioned. For example, according to a sandwich construction, a first plate is at least partially received between top and bottom portions of a second plate. According to a staggered construction, first and second plates are arranged in a staggered and overlapping manner with respect to each other.
Referring to
The crucible 12 can be formed with four side plates and one bottom plate, although other arrangements may be suitable. Preferably the crucible 12 is made of fused silica or a suitable substitute material. The crucible 12 preferably is contained in the crucible box 13, which can be made of graphite or a suitable substitute material. The crucible box 13 is supported by the support mechanism 14, such that the support mechanism 14 preferably directly contacts the crucible box 13, which conducts heat from the crucible 12. Preferably a surface area of the support mechanism 14 is greater than or about equal to a surface area of the crucible box 13 and the bottom of the crucible 12, in order to adequately conduct heat from substantially the entire bottom surface of the crucible 12.
As shown in
Referring to
Referring to
Referring to
According to the subject invention, the at least two plates 18 depicted in
According to the subject invention, the at least two plates 18 can be moved between a fully closed position and a fully open position, with a plurality of discrete “partially open” positions formed between the fully closed and fully open positions. Referring to
Alternatively, in other embodiments, by changing the shapes of the plates, the resulting aperture formed in the fully open and partially open positions can be rectangular or rhombic (see, e.g.,
Although the plate 30 is depicted as a single plate with separate top and bottom “sandwiching” portions, the plate 30 could be formed as a plurality of plates, and the total number of plates 30, 32 depicted in
Referring to
Therefore, the plates 30, 32 depicted in
The at least two plates of varying shapes can be used to form apertures of different shapes.
Referring to
According to the subject invention, a drive mechanism is configured to drive the at least two plates with one degree of freedom. Referring to
Referring to
The subject invention further relates to a device for controlling heat extraction from a crucible contained in a crystal growth apparatus that can include: at least two plates movable in a coordinated manner to form a symmetrical aperture substantially centered with respect to an ingot being formed in the crucible; and a drive mechanism configured to drive the at least two plates with one degree of freedom.
The subject invention further relates to a method for controlling heat extraction from a crucible contained in a crystal growth apparatus that can include steps of: providing a crucible for receiving a charge; heating and at least partially melting the charge contained in the crucible; providing at least two plates that are movable in a coordinated manner to form a symmetrical aperture substantially centered with respect to an ingot being formed in the crucible; and driving the at least two plates with one degree of freedom.
Although preferred embodiments of the invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.
The entire contents of all patents, published patent applications and other references cited herein are hereby expressly incorporated herein in their entireties by reference.
This application claims the benefit of copending application U.S. Provisional Application Ser. No. 61/313,347 filed on Mar. 12, 2010, the disclosure of which is expressly incorporated herein by reference in its entirety.
Number | Date | Country | |
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61313347 | Mar 2010 | US |