METHOD OF PRODUCING BRICKS FROM A SILICON INGOT BACKGROUND OF THE INVENTION

Cross-Reference To Related Applications

The present application claims the benefit of U.S. Patent Application No. 61/579,856, filed December 23, 2011.

1. Field of the Invention.

[0001] The present invention relates to a method of dividing a silicon ingot into a collection of silicon bricks comprising monocrystalline and multicrystalline bricks, as well as to the collection of silicon bricks produced.

2. Description of the Related Art.

[0002] Crystal growth apparatuses or furnaces, such as directional solidification systems (DSS) and heat exchanger method (HEM) furnaces, involve the melting and controlled resolidification of a feedstock material, such as silicon, in a crucible to produce a crystalline material, often referred to as an ingot. Production of a solidified ingot from molten feedstock occurs in several identifiable steps over many hours. For example, to produce a silicon ingot by the DSS method, solid silicon feedstock is provided in a crucible, often contained in a graphite crucible box, and placed into the hot zone of a DSS furnace. The feedstock is then heated to form a liquid feedstock melt, and the furnace temperature, which is well above the silicon melting temperature of 1412°C, is maintained for several hours to ensure complete melting. Once fully melted, heat is removed from the melted feedstock, often by applying a temperature gradient in the hot zone, in order to directionally solidify the melt and form a silicon ingot. By controlling how the melt solidifies, an ingot having greater purity than the starting feedstock material charged to the crucible can be achieved. This material can then be used in a variety of high end applications, such as in the semiconductor and photovoltaic industries.

[0003] In a typical solidification of silicon feedstock, the resulting solidified silicon ingot is generally multicrystalline, having random small crystal grain sizes and orientations. It has also been shown that a silicon ingot comprising monocrystalline (i.e. single crystal) silicon can also be formed. For example, to produce a monocrystalline silicon ingot using either a DSS or HEM process, one or more solid seeds of monocrystalline silicon can be placed along the bottom of a crucible, along with the silicon feedstock, and then heated to melt. If at least a part of the seeds is maintained after the feedstock has fully melted, directional crystallization of the melt occurs corresponding to the crystal orientation of the monocrystalline seed.

[0004] Typically, as the directional solidification of a monocrystalline silicon ingot occurs, regions of multicrystalline silicon also form, most often along the outside edges of the ingot (sometimes referred to as edge growth) particularly when a seed is placed in the center of the crucible bottom. This results, for example, when crystals nucleate from surfaces other than the seed. In order to form as large a region of monocrystalline material as possible, the entire bottom of the crucible can be covered with a single seed or a plurality of smaller seeds placed against each other (also called tiling), but this entails higher costs. Furthermore, due to conditions used to grow the crystalline material, it has been observed that edge growth may still occur as crystals nucleate from the cooling edges.

[0005] Since producing larger regions of monocrystalline material has come with higher costs, it has therefore become important to maximize the availability of the monocrystalline region of a silicon ingot that is produced. However, this has been a challenge. For example, the silicon ingots are generally placed into a cutting device, such as a wire saw or band saw, and sliced vertically into bricks of nearly identical size. The resulting bricks are then sliced horizontally into thin wafers that can be further processed and used in solar cells. Thus, the size of the wafers determines the size of the bricks, which, in turn, dictates how the silicon ingot is cut. To obtain typical wafers that are 15.6 cm square, an ingot that is 84 cm square would be cut into 25 bricks in a 5x5 pattern. However, cut in this way, only the center 9 bricks would be considered monocrystalline and useful for producing monocrystalline wafers. The edge and corner bricks would also contain multicrystalline material, which, when sliced, would result in wafers having both monocrystalline and multicrystalline characteristics, sometimes referred to as hybrid wafers. These are not as beneficial as monocrystalline wafers.

[0006] Therefore, there is a need in the industry for a method in which the amount of monocrystalline region of a silicon ingot is better utilized, forming a maximum number of bricks from which substantially fully monocrystalline wafers can be cut. SUMMARY OF THE INVENTION

[0007] The present invention relates to a method of sawing a silicon ingot, which has a substantially square cross-section and comprises a monocrystalline silicon center region and a multicrystalline silicon perimeter region, into a collection of bricks. The method comprises the steps of squaring the silicon ingot on a saw capable of cutting through the silicon ingot; and vertically cutting the silicon ingot into the collection of silicon bricks comprising a) a plurality of center bricks of substantially similar size and having a substantially square cross-section, each comprising at least 90% monocrystalline silicon, b) a plurality of edge bricks having a substantially rectangular cross-section, each comprising monocrystalline silicon and multicrystalline silicon, and c) four corner bricks having a substantially square or substantially rectangular cross-section, each comprising multicrystalline silicon. The edge bricks and corner brick have a size smaller than the size of the center bricks. Preferably the silicon ingot is cut into the collection of silicon bricks simultaneously. The present invention further relates to this collection of silicon bricks as well as to a method of forming substantially monocrystalline wafers from the resulting monocrystalline silicon bricks of this collection.

[0008] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed.

BRIEF DESCIPTION OF THE DRAWINGS [0009] FIG 1 is a cross-sectional view of a silicon ingot comprising both monocrystalline and multicrystalline regions sliced into bricks using the currently available method in the art. FIG 2 and FIG 3 are cross-sectional views of the same silicon ingot sliced into bricks using embodiments of the method of the present invention. FIG 4 is a cross-sectional view of a larger silicon ingot sliced into bricks using the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention relates to bricks and wafers produced from a silicon ingot having a monocrystalline silicon region, and a method of cutting the silicon ingot.

[0011] In the method of the present invention, a silicon ingot is provided comprising a monocrystalline silicon center region and a multicrystalline silicon perimeter region. The ingot may have any cross-sectional shape known in the art, such as round, rectangular, or square, but preferably the method involves providing a silicon ingot having a substantially square cross-section. It is also contemplated that such an ingot can be provided by removing portions of an ingot produced having a different shape, such as a round ingot that has been cut to have a square cross-section or a larger square ingot that has been cut to a desired size by removing edge, top, and bottom sections of less pure material.

[0012] The silicon ingot can be produced using any method known in the art. In particular, the silicon ingot can be provided by solidification in a crystal growth apparatus, which is a furnace, in particular a high-temperature furnace, capable of heating and melting a solid feedstock, such as silicon, at temperatures generally greater than about 1000°C and subsequently promoting resolidification of the resulting melted feedstock material to form the crystalline materials. For example, the crystal growth apparatus can be a directional solidification system (DSS) crystal growth furnace or a heat exchanger method (HEM) crystal growth furnace, but is preferably a DSS furnace. [0013] In particular, the crystal growth apparatus can be a DSS furnace comprising an outer furnace chamber or shell and an interior hot zone within the furnace shell. The furnace shell can be any known in the art used for high temperature crystallization furnaces, including a stainless steel shell comprising an outer wall and an inner wall defining a cooling channel for circulation of a cooling fluid, such as water. The hot zone of the crystal growth apparatus is an interior region within the furnace in which heat can be provided and controlled to melt and resolidify a silicon feedstock material. The hot zone is surrounded by and defined by insulation, which can be any material known in the art that possesses low thermal conductivity and is capable of withstanding the temperatures and conditions in a high temperature crystal growth furnace, and also comprises at least one heating system, such as multiple heating elements to provide heat to melt feedstock placed in a crucible. The temperature in the hot zone may be controlled by regulating the power provided to the various heating element.

[0014] The crystal growth apparatus further comprises at least one means for removing heat from the hot zone. When the apparatus is a DSS furnace, the means for removing the heat can comprise movable sections of the insulation that surrounds the hot zone. For example, top and side insulation panels of the hot zone can be configured to move vertically while the bottom insulation panel is configured to remain stationary. Alternatively, as another example, the top and side insulation panels may be configured to remain stationary while the bottom insulation panel is configured to move vertically. Other combinations are also possible. When the apparatus is a HEM furnace, the means for removing heat from the hot zone can be a heat exchanger, such as a helium-cooled heat exchanger, provided to be in thermal communication with the bottom of the crucible placed within the hot zone.

[0015] The silicon ingot can be prepared in a crucible having specified dimensions that is placed into the hot zone of the crystal growth apparatus. The crucible can optionally be contained within a crucible box, which provides support and rigidity for the sides and bottom of the crucible and is particularly preferred for crucibles made of materials that are either prone to damage, cracking, or softening, especially when heated. The shape of the crucible determines the shape of the silicon ingot. Preferably, to produce a silicon ingot, the crucible is made of silica and has a cube or cuboid shape. For example, the crucible can have a square cross-sectional shape having dimensions capable of forming a silicon ingot that is 60-90 cm square, such as between about 65-70 cm square or 80-85 cm square.

[0016] The silicon ingot provided in the method of the present invention comprises a monocrystalline region (having a single crystal orientation throughout) and a multicrystalline region (having various crystal orientations and sizes throughout). The monocrystalline region is in the center of the ingot, with the multicrystalline region being around the perimeter. While centrally located within the ingot, the monocrystalline region may not necessarily be symmetric and, further, may not be aligned with the geometric center of the silicon ingot. For example, due to asymmetries in the heating elements of the hot zone of a crystal growth apparatus, it is possible that the monocrystalline region formed may not be centrally symmetric and also that the center of the monocrystalline region may not align with the center of the ingot. However, for the method of the present invention, the silicon ingot preferably does not comprise monocrystalline silicon along the outer perimeter of the ingot. Also, preferably the amount of monocrystalline silicon present in the silicon ingot is greater than the amount of multicrystalline silicon. For example, the silicon ingot may comprise at least about 60% monocrystalline silicon, such as at least about 70% and at least about 80% monocrystalline silicon.

[0017] To form this type of silicon ingot, silicon feedstock can be provided as a charge to the crucible along with at least one monocrystalline silicon seed. The silicon feedstock material can be in any form known in the art, including powder, pellets, or larger chunks or pieces. A plurality of monocrystalline seeds can be used and arranged along the bottom of the crucible. Any type of seed crystal known in the art can be used. For example, the monocrystalline seeds may be circular or polygonal, such as square or rectangular, in cross-sectional shape. The number of monocrystalline seeds can vary depending, for example, on the inner dimensions of the crucible used and on the size of the seeds. For example, from 2 to 36 square monocrystalline seeds can be arranged around the interior crucible bottom. As a particular example, 25 square seeds can be arranged in a 5 by 5 pattern on the bottom of the crucible. The monocrystalline seeds can range in size from about 10 cm to about 85 cm along any edge. The seeds can be arranged in a pattern to substantially fully cover the interior surface of the crucible bottom, being placed as close to the inside edges and corners of the crucible as is practically possible. Such a placement is sometimes referred to as tiling. Thus, when a plurality of monocrystalline seeds are used, these can be arranged or tiled along the inside bottom surface of the crucible so that each seed is in contact with a neighboring or adjacent seed, forming a close- packed arrangement. The thickness of the seeds can also vary, depending on availability and cost. For example, the seeds may have a thickness of about 0.5 cm to about 5 cm, including from about 1 cm to about 4 cm and from about 2 cm to about 3 cm. Preferably, all of the seeds are substantially similar in size, shape, and thickness.

[0018] In the method of the present invention, the silicon ingot is placed onto a cutting device such as a saw that is capable of cutting through the silicon ingot, and then vertically cut or sliced into a collection of silicon bricks, described in more detail below. The cutting device can be any saw known in the art that can be configured to cut the silicon ingot. For example, the saw may be a band saw, which sequentially cuts an ingot into bricks, or it may be a wire mesh saw, which simultaneously cuts an ingot into bricks. The cutting device may also be used to vertically cut and remove outer sections of the ingot that are not used or included as part of the desired collection of bricks. This is sometimes referred to as ingot cropping and can be done prior to or simultaneously with the cutting of the ingot into the collection of bricks.

[0019] The vertical cutting of the silicon ingot using the method of the present invention results in a collection of silicon bricks of three distinct types - a plurality of center bricks, a plurality of edge bricks, and four corner bricks. In particular, the center bricks are substantially similar in size and have a substantially square cross- sectional shape. Furthermore, these bricks result from the center monocrystalline region of the silicon ingot, and, as such, therefore each comprise at least about 90% monocrystalline silicon, preferably at least about 95% monocrystalline silicon, and more preferably at least about 99% monocrystalline silicon. In essence, these bricks would be considered to be full monocrystalline bricks.

[0020] The plurality of edge bricks have a substantially rectangular cross- sectional shape rather than the square cross-sectional shape of the center bricks. Each edge brick comprises multicrystalline silicon and can further comprise monocrystalline silicon as well. However, it is preferred that the amount of monocrystalline silicon in the edge bricks is minimized, thereby maximizing the amount present in the desired center bricks. Thus, preferably, each edge brick comprises most about 50% monocrystalline silicon, more preferably at most about 30% monocrystalline silicon, and most preferably at most about 10% monocrystalline silicon. Also, each of the edge bricks has a size smaller than the size of the center bricks. In particular, since the edge bricks have a rectangular cross-sectional shape, this brick has either a length or a width smaller than the size of the center bricks (which, being square, have the same length and width). Preferably, the edge bricks are substantially similar in size to each other and have one dimension (length or width) that is half the size of the center bricks, with the other dimension substantially similar to the size of the center bricks.

[0021] The collection of bricks further comprises four corner bricks, which would result from a substantially square silicon ingot. The corner bricks can have a substantially square or a substantially rectangular cross-sectional shape and are smaller in size than either the center bricks or the edge bricks. For example, when the corner bricks have a substantially rectangular cross-sectional shape, the length or width of these bricks is smaller than the length or width of the edge bricks and therefore smaller than the dimensions of the center bricks. Preferably, the corner bricks are substantially similar in size to each other and have a substantially square cross-sectional shape, with a size that is half of the longest side of the rectangular edge bricks and, therefore, ¼ of the size of the center bricks. Furthermore, each of the corner bricks comprises multicrystalline silicon, although some monocrystalline material may also be present. Preferably, the corner bricks comprise less than about 25% monocrystalline, material, more preferably less than about 10% monocrystalline material, and, most preferably, less than about 5% monocrystalline material.

[0022] The quantity of each type of brick in the resulting collection can vary depending on the size of the silicon ingot and the desired size of the bricks. Brick size is typically dictated by the size of the wafers to be cut from the bricks. As discussed above, silicon wafers used for solar cells are typically 14-16 cm square. As a result, the center bricks, which contain primarily monocrystalline silicon, are preferably also of this size and shape, which determines the number of bricks that are to be cut. For example, the method of the present invention can result in a collection of 25 silicon bricks of which 9 are center bricks as described above, 12 are edge bricks as described above, and 4 are corner bricks, as described above. In addition, the collection can be 36 bricks (16 center brick, 16 edge bricks, and 4 corner bricks), 49 bricks (25 center bricks, 20 edge bricks, and 4 corner bricks), or 64 bricks (36 center bricks, 24 edge bricks, and 4 corner bricks).

[0023] This is most clearly shown in FIG 1-4. FIG 1 is a cross-sectional view of a silicon ingot comprising both monocrystalline and multicrystalline regions sliced into bricks using the currently available method in the art. FIG 2 and FIG 3 are cross- sectional views of the same silicon ingot sliced into bricks using embodiments of the method of the present invention. FIG 4 is a cross-sectional view of a larger silicon ingot sliced into bricks using the method of the present invention. It should be apparent to those skilled in the art that these are merely illustrative in nature and not limiting, being presented by way of example only. Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the present invention. In addition, those skilled in the art should appreciate that the specific configurations are exemplary and that actual configurations will depend on the specific system. Those skilled in the art will also be able to recognize and identify equivalents to the specific elements shown, using no more than routine experimentation.

[0024] As shown in FIG 1-3, silicon ingot 10 has a substantially square cross- section and comprises two distinct regions separated by line 11 - a monocrystalline region in the center surrounded by a multicrystalline region around the perimeter. For these examples, the ingot is 84 cm square. In order to cut bricks having the size needed for standard silicon wafers, this ingot would typically be placed on a saw and then cut vertically in a 5x5 array as shown in FIG 1, with each brick being 15.6 cm square. This allows for a standard kerf of approximately 0.3 cm and removal of approximately 2.4 cm around each perimeter to ensure product purity. However, as can be seen, only 9 substantially square center bricks 1A of the desired size are fully within the monocrystalline region. The 12 edge bricks IB and 4 corner bricks 1C, which are all have a substantially square cross-section, have both monocrystalline and multicrystalline regions. Thus, using the standard method of sawing an ingot, the resulting collection of bricks could only provide 9 bricks useful for producing monocrystalline silicon wafers.

[0025] By comparison, using the method of the present invention, a significantly larger number of substantially square monocrystalline bricks can be produced. This is shown in FIG 2. Thus, the same silicon ingot shown in FIG 1 is placed in a saw and squared in such a way that 16 substantially square center bricks 2A can be cut from the monocrystalline region. In this way, 16 edge bricks of similar size 2B, having a substantially rectangular cross-section, are produced, each having both monocrystalline and multicrystalline silicon regions. Likewise, the 4 corner bricks of similar size 2C, also contain both types of silicon, but are substantially square in cross-section. Thus, using the method of the present invention, a significantly larger number of bricks useful for producing monocrystalline silicon wafers can be cut from this silicon ingot.

[0026] The shape and size of the edge and corner bricks resulting from the method of the present invention are determined by the symmetry and location of the monocrystalline region of the silicon ingot to be cut. As discussed above, due to asymmetries in the heating elements of the hot zone of a crystal growth apparatus, it is possible that the monocrystalline region formed may not be centrally symmetric and, further, that the center of the monocrystalline region will not align with the center of the ingot. This is shown in FIG 3 showing ingot 30 which is the same size ingot as in FIG 1 and FIG 2 and has the same shape monocrystalline region (shown by line 11), but wherein the monocrystalline region in not centered in the ingot. In this example, using the method of the present invention, 16 substantially square center bricks 3A can be cut from this off-centered monocrystalline region, along with 16 edge bricks 3B, having a substantially rectangular cross-section, each having both monocrystalline and multicrystalline silicon regions, and 4 corner bricks 3C, also containing both types of silicon. As can be seen, the edge bricks from one side of the ingot different in size and shape from those of the other sides. Also, the corner bricks are not each substantially square in cross-section. Nevertheless, even with a silicon ingot having a non-centrosymmetric monocrystalline region, 16 bricks useful for producing monocrystalline silicon wafers can be cut using the method of the present invention. Only 9 would be expected using the standard method. By locating the center of the monocrystalline region of the silicon ingot, centering the desired brick pattern over the center of the monocrystalline region, and sawing the ingot into that pattern, a significantly larger number of bricks useful for producing monocrystalline silicon wafers can be cut.

[0027] Ingots of various sizes (smaller or larger) can also be cut using the method of the present invention. For example, a silicon ingot that is 1100 cm square would typically be cut into 15.6cm square bricks in a 6x6 array using the standard method in the art, thereby producing 16 substantially square center bricks from the center monocrystalline silicon region, 16 substantially square edge bricks of the same size from the perimeter, and 4 substantially square corner bricks. Only the 16 center bricks would be useful for producing monocrystalline silicon wafers, since the edge and corner bricks would comprise both moncrystalline and multicrystalline silicon. However, as shown in FIG 4, this same larger ingot 40, having a center monocrystalline region and a perimeter multicrystalline region (shown by line 41) can be cut using the method of the present invention to produce 25 substantially square monocrystalline center bricks 4A, along with 20 substantially rectangular edge bricks 4B and 4 substantially square corner bricks 4C. The larger number of center bricks represents a significant improvement over the method known in the art.

[0028] Thus, as can be seen, using the method of the present invention, more bricks having nearly full monocrystalline characteristics can be cut from a silicon ingot, which overcomes an ongoing and yet unsolved problem in the industry of how to take advantage of the desirable monocrystalline region produced in a silicon ingot. The present method dramatically maximizes utilization of the desirable monocrystalline region of the silicon ingot, which would not be possible using the standard and currently available methods of sawing an ingot. This is particularly important for silicon ingots having large center monocrystalline regions, such as greater than 80% monocrystalline silicon.

[0029] The method of the present invention produces a collection of bricks from a single silicon ingot that may be further cut into silicon wafers. Thus, the present invention further relates to a collection of bricks produced by this method and comprises the types and numbers of bricks described above. Such a collection is not possible using standard or currently known methods and differs significantly from current collections of bricks produced from a single silicon ingot in size, shape, composition, and number. In particular, for the collection of bricks of the present invention, resulting from the method described above, more than 50% of the cross- sectional area of the bricks can be used to cut monocrystalline wafers. For example, as shown in FIG 1, the collection of bricks resulting from the currently known method produces a total of 25 square bricks of substantially similar size, only 9 of which can be used for producing monocrystalline silicon wafers (approximately 36% of the total area). By comparison, since the collection of bricks of the present invention, as shown in FIG 2, includes 16 bricks from the monocrystalline region of the desired size, this represents 64% of the area, which is a dramatic improvement over the collection of bricks than can currently be produce.

[0030] In addition, as discussed above, since the collection of bricks can be used to produce silicon wafers, the present invention also relates to a method of forming substantially monocrystalline silicon wafers. This method comprises the steps of vertically cutting a silicon ingot using the method of the present invention, and then horizontally cutting the center bricks, comprising monocrystalline silicon, into monocrystalline silicon wafers. Any method known in the art can be used to cut the center bricks. Furthermore, the method may further comprise the step of cropping to remove top and bottom portions prior to or simultaneous with cutting into wafer. The resulting wafers may be any thickness desired in the industry, including, for example, from about 0.05 to about 0.25 mm in thickness, such as from about 0.1 to about 0.22 mm and from about 0.15 to 0.20 mm in thickness.

[0031] The foregoing description of preferred embodiments of the present invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings, or may be acquired from practice of the invention. For example, while the present method is described in relation to a silicon ingot, this method may be used to cut an ingot of a variety of different materials produced in a crystal growth furnace and having a center monocrystalline region and perimeter multicrystalline region. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.

[0032] What is claimed is: