The present invention relates to a system and method of growing silicon ingots from seeds in a crucible, and the method of manufacturing the seeds used therein.
Crystal growth apparatuses or furnaces, such as directional solidification systems (DSS), involve the melting and controlled resolidification of a feedstock material, such as silicon, in a crucible to produce 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 loaded into a crucible and placed into the hot zone of a DSS furnace. The feedstock charge is then heated using various heating elements within the hot zone 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 can be achieved, which can then be used in a variety of high end applications, such as in the semiconductor and photovoltaic industries.
For the preparation of a monocrystalline silicon ingot using a DSS process, single crystal seed tiles (seeds) are typically placed in a layer at the bottom of the crucible that is being used for the production of silicon ingots. The silicon feedstock is then loaded on top of the seeds and melted from top down. Once the melt reaches the top of the seeds, the process transitions to a directional solidification stage. The seeds remain at least partially solid throughout the process and serve as a template for the crystallization of the melted feedstock. That is, although the surface of the seeds may partially melt, since the growth (solidification of the melt) starts at the surface of the seeds, the crystalline structure of the seeds is replicated throughout the resulting grown ingot.
These seeds typically range in thickness from about 5 mm to 35 mm and cover a significant area of the crucible bottom, usually the entire bottom of the crucible. The dimensions of a standard crucible can be quite large and thus, a significant number of seeds are required in order to grow the above described silicon ingots.
Since the cost of the seeds required are typically quite high (e.g., about $2,000 to $10,000 per ingot), the manufacturing costs of each individual ingot are extremely high as well as a result. Therefore, there is a need for a system and method of producing silicon ingots that reduces the manufacturing costs of each ingot and simplifies the orientation process to produce a higher quality ingot at a lower cost.
Systems and methods to reduce the overall cost of producing a silicon ingot are provided herein. More specifically, one or more surface pieces may be sliced from a silicon boule in relation to a plurality of nodes at a particular orientation. These one or more surface pieces may then be formed into one or more seeds having a specific length, width and thickness usable in a silicon ingot growth process. By utilizing these pieces to form one or more seeds, pieces of a boule which would have been previously discarded may now be used to form high quality seeds for use in a silicon ingot growth process.
Preferably, the surface pieces may be one or more residual pieces that are a direct result of a process for squaring a silicon boule into a brick. These surface pieces may then be arranged in a bottom of a crucible in various orientations.
In some exemplary embodiments of the present invention, the surface pieces may also be cropped by cutting each surface piece a plurality of times. The cropped surface pieces may then be sliced into a plurality of seeds and placed in the bottom of the crucible. The seed or seeds may then be placed in the crucible in a particular orientation which is configured to prevent or reduce the formation and spread of subgrain boundaries within the grown ingot.
Other aspects and embodiments of the invention are discussed below.
For a better 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 instant 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 “crystal growth apparatus” as described herein refers to any device or apparatus 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 a crystalline material, such as a monocrystalline silicon ingot, for photovoltaic (PV) and/or semiconductor applications.
A “cropping device” as described herein is any device that is capable of precisely cutting silicon into one or more pieces. A cropping device may be for example a knife, an automated saw, or any other specifically configured device known in the art for effecting precise cutting techniques.
A “squaring device” as described herein is any device that is capable of precisely squaring silicon into one or more pieces having a square cross sectional shape, such as a brick. A squaring device may be for example an automated computer-aided manufacturing machine or a saw that is specifically configured to square silicon boules.
A “surface piece” as described herein is a semi-cylindrical piece that is sliced from a cylindrical boule, sometimes referred to as a cylindrical segment. These surface pieces include at least one surface that originated as a portion of the surface of the original cylindrical boule before the boule was cut, sliced, or in some instances even sectioned. These surface pieces preferably are residual pieces that are a result of squaring a boule during a typical squaring process. However, residual pieces may also be removed from the boule on an individual basis before or after the boule has been sectioned. Accordingly, the illustrative embodiments of the present invention are not limited as such.
Preferred embodiments of the subject invention are described below with reference to the accompanying drawings, in which like reference numerals represent the same or similar elements.
The subject invention relates to systems and methods of growing silicon ingots in a crucible of a directional solidification process. As stated above, single crystal seed tiles (seeds) are placed in a layer at the bottom of a crucible (e.g., a casting crucible) that is being used for the production of silicon ingots. The silicon feedstock is then loaded on top of the seed(s) and melted from top down. Once the melt reaches the top of the seeds, the process transitions to a directional solidification stage. Most of the bulk of the seeds remain solid (or mostly solid) throughout the casting process and serve as a template for the crystallization of the melted feedstock. Since the growth (solidification of the melt) starts at the surface of the seed, the crystalline structure of the seed is replicated throughout the resulting ingot.
Currently the total cost of just the seeds for producing one ingot of monocrystalline silicon through the above process is in the order of $2000-$10000/ingot. This cost is substantially due to the value of the material from which the seeds are prepared. In particular, seeds are cut from bricks from which wafers would have been sliced. Thus, a process which reduces the overall cost of each individual ingot by reducing the cost of the seeds while at the same time taking into consideration sub-grain boundary formations would be extremely beneficial to the industry as a whole.
The exemplary embodiment of the present invention includes systems and methods for reducing the overall cost of producing a silicon ingot by reducing the cost of the seeds used in growing the ingot. More specifically, costs associated with silicon ingot growth manufacturing are reduced by utilizing one or more surface pieces that are sliced from a boule. These surface pieces may be residual pieces that are a result of squaring a boule during a silicon brick manufacturing process or alternatively may be individually sliced from any one of four sides of cylindrical silicon boule in relation to a plurality of nodes at a particular orientation on the boule. As noted above, since the surface pieces include the a segment of the surface of a cylinder, each piece should have a one curved surface and thee flat surfaces when they are originally sliced from the boule. For example,
For example, a boule formed using the Czochralski (Cz) process contains an upper “neck” section and bottom “tail” section, which is generally sectioned or removed to form a relatively cylindrical boule having a desired or target length. The resulting cylindrical boule is then typically further processed by slicing off the rounded sides to form a brick having a square or pseudo-square cross sectional shape (often referred to as squaring), which can then be further sliced into wafers for use in a solar cell. This is illustrated in
Conventionally, the surface pieces (especially residual pieces resulting from squaring a boule) are broken into smaller pieces by the brick manufactures and recycled for use as feedstock either in CZ or directional solidification system (DSS) growth processes. Advantageously, however, the exemplary embodiment of the present invention utilizes these surface/residual pieces, which are considered waste and are therefore inexpensive, in order to manufacture at least one seed for use in a silicon ingot growth process, thereby reducing the overall costs associated with ingot manufacture.
Alternatively, as mentioned above, these surface pieces may be individually sliced from the cylindrical silicon boule as part of an intentional process to obtain a surface piece 204 that is not a result of the squaring process discussed above. Accordingly, the illustrative embodiment of the present invention is not limited to surface pieces that are obtained only from a squaring process, as the squaring process is merely a preferable method of obtaining the surface pieces that would normally be discarded.
Regardless of whether the surface pieces are residual pieces resulting from squaring or individual surface pieces that are intentionally sliced from the boule, the slice should be made in relation to at least two nodes at a particular orientation on the boule. That is, the slicing planes should be made with consideration of the orientation of the crystal (i.e., due to the relationship with the plurality of nodes) within the boule. For example, with a silicon boule, doing so insures that the surface pieces themselves preferably have a <100> orientation in the vertical direction when placed with the flat surface facing downward. The surface pieces also preferably have a <100> orientation along the length of any one surface piece as this is parallel to the growth orientation of the boule. This also implies that a direction normal to the first two directions (i.e., the vertical direction and the direction along the length of the residual piece) also has a <100> family plane due to the cubic symmetry of a silicon crystal lattice.
That is, in the exemplary embodiment of the present invention, when a silicon boule is squared in relation to a plurality of specifically identified nodes located on the outer surface of the boule, a vector drawn normal from the axis of growth through any node is a <110> direction. Thus, by squaring the boule in relation to these nodes, a proper surface orientation of the resulting brick can be assumed. Therefore, a <100> orientation may be obtained along the side surface of a pseudo square brick as well as along the flat surfaces of the residual pieces.
In the above exemplary embodiment of the present invention, the boule should preferably have a diameter of about 170 mm-220 mm and more preferably has a diameter of about 205 mm. The resulting silicon brick preferably has a square or pseudo square cross sectional shape having sides of about 150 mm-170 mm and preferably 150-155 mm. Preferably the length of the brick is between about 350 mm-450 mm and more preferably between about 375-425 mm, such as about 400 mm. Each surface piece may have a width W that is similar to that of the resulting brick (e.g., preferably 90 mm-105 mm, and more preferably 95-100 mm) and a thickness (from the flat surface to the curved surface) from about 0.1 mm-24 mm). Alternatively, each of the surface pieces may be sectioned into a plurality of surface pieces that are each smaller than the cylindrical boule from which they were sliced.
In the exemplary embodiment of the present invention a crystal growth apparatus configured to promote monocrystalline growth by directional solidification may be provided and at least one seed and a feedstock material are preferably placed in a crucible in the crystal growth apparatus. The crystal growth apparatus is then heated to melt the feedstock material, preferably without substantially melting the seed(s), contained in the crucible. At least one heating element in the crystal growth apparatus controls the melting in order to achieve the desired ingot growth as described above. The seeds that are placed in the bottom of the crucible may contain one or more seeds formed from at least one surface piece which may be obtained from any one of the above process.
In the exemplary embodiment of the present invention, these surface pieces may additionally be cropped or left un-cropped depending upon how one or more seeds are to be disposed in the bottom of a crucible. More specifically, in the exemplary embodiment of the present invention, as can be seen from
For example, when the residual pieces are not cropped, as is shown in
The cross section of a surface piece may also be cropped to provide a trapezoidal cross sectional shape, which can also be placed in a crucible using this alternating pattern, as is illustrated in
Alternatively, as illustrated in
Furthermore, as shown in
As stated above, the planks 600′ may be sliced into a plurality of seeds. However, how these seeds are sliced/cut may also affect properties of the boundary formed between adjacent seeds, thereby affecting the properties of the crystal ingot grown.
In particular, as shown in
In addition, as shown in
The above process may be iterated a number of times until the desired number of diamond shaped seeds are produced. The resulting diamond shaped seeds in this exemplary embodiment of the present invention make up a first group of diamond shaped seeds (e.g., 700a) having at least one side surface with a <110> orientation and at least one side surface with a <100> orientation, and a second group of diamond shaped seeds (e.g., 700b) having orientations that mirror the first group.
As shown in
Sub-grains may also initiate from inclusions of silicon carbide and nitride. As the ingot grows, these inclusion induced sub-grains can invade an entire volume of the grown ingot. These defects have a significant impact on the efficiency of each solar cell produced from the resulting ingot. However, as can be seen from the minority carrier lifetime scan of a brick face of
It should be noted, however, that the above orientations are merely exemplary and other orientations using the above described diamond shaped seeds and rectangular seeds formed from the surface pieces of squaring a boule of silicon may be used without departing from the present invention.
Using the above techniques,
The seeds are arranged in the bottom of the crucible (Step 908), and feedstock material is loaded thereon (Step 910). If the seeds are not to be cropped, the at least one seed formed from a surface piece proceeds to Steps 908 and 910. Finally, the feedstock material is melted, without substantial melting of the seeds, and the melt is then solidified to form the silicon ingot (Step 912).
Furthermore, in a system for performing the above method, a squaring device, such as first ban saw or wire saw mounted with specific blade for squaring, may be used for squaring the boule. The squaring process in the exemplary embodiments of the present invention may be automated or manually driven depending up on the process being used. Likewise, any known cropping mechanism, such as a second band saw or wire saw mounted with a specific blade for cropping, may be used in the exemplary embodiment of the present invention to crop and slice the surface pieces into one or more seeds respectively. Furthermore, the first and second cutting devices may each include a plurality of saws that are each able to be configured to cut or slice the material at a preconfigured angle. Accordingly, the above described embodiments of the present invention are not limited thereto.
Advantageously, by utilizing these surface pieces the overall cost of producing a single silicon ingot of silicon can be reduced from, for example, $4000/ingot to $400/ingot. Furthermore, by specifically cropping, arranging and orientating the seeds formed from the surface pieces in the crucible as described above, sub-grain boundaries between two neighboring seeds can be greatly reduced if not prevented entirely.
The seeds produced by the above method may be utilized in a crystal growth apparatus, such as a directional solidification furnace, to form an ingot having a targeted crystal orientation. For example, as shown in
A silicon ingot can be prepared using this crystal growth apparatus by heating and melting the feedstock material, using top heater 20a and side heaters 20b, monitored using thermocouple 21 preferably without substantially melting the seeds (although some partially seed melt back may be possible, especially for embodiments in which the seeds have a curved upper surface), and removing heat from the crucible to form the silicon ingot. If the seeds are placed in the crucible all having the same orientation, the resulting ingot would be a monocrystalline ingot (having the same crystal orientation throughout). If the seed are arranged to have alternating patterns of crystal orientations, then, while the portion of the ingot above the seed would be monocrystalline, the ingot as a whole would be a geometrically-ordered multicrystalline ingot (having multiple regions of different monocrystalline materials in an defined or ordered pattern).
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.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/684,331, filed Aug. 17, 2013. The entire contents of this application are hereby incorporated by reference herein.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/US2013/055323 | 8/16/2013 | WO | 00 |
Number | Date | Country | |
---|---|---|---|
61684331 | Aug 2012 | US |