Classifications

• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B15/00—Single-crystal growth by pulling from a melt, e.g. Czochralski method
  • C30B15/007—Pulling on a substrate
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/02—Elements
  • C30B29/06—Silicon

Definitions

• the invention generally relates to ribbon crystals and, more particularly, the invention relates to grain boundaries of wafers formed from ribbon crystals.
• String ribbon crystals such as those discussed in U.S. Patent No. 4,689,109 (issued in 1987 and naming Emanuel M. Sachs as the sole inventor), can form the basis of a variety of electronic devices.
• Evergreen Solar, Inc. of Marlborough, Massachusetts forms solar cells from conventional string ribbon crystals.
• the wafers When used to form solar cells, the wafers often have backside electrodes to transmit electrons. Due to the fluctuating and relatively unknown shape of the edges, however, those in the art typically do not form the backside electrodes on much of the area of the wafer. Instead, those in the art typically form the backside electrode in a smaller area of the wafer; namely, spaced a relatively large distance from the edges of the wafer. Accordingly, this practice further reduces the full electrical efficiency of the wafer.
• a method of processing a ribbon crystal provides a string ribbon crystal, and removes at least one edge of the string ribbon crystal.
• the method also can remove the string with the edge, or remove the portion between the string and the edge.
• removal of the edge can form a substantially planar edge or non-planar edge on the crystal.
• the method also can remove two or more edges of the string ribbon crystal.
• the method can separate the ribbon crystal into a plurality of individual wafers after removing at least one edge. After forming the wafers, the method can form a back-surface contact on at least one of the wafers.
• the method can first form a back-surface contact on the string ribbon crystal before removing at least one edge of the string ribbon crystal, and then separate the ribbon crystal into a plurality of individual wafers.
• removal of the original edge forms a new edge
• the back-surface contact may substantially extend to the new edge.
• the back-surface contact is spaced from the new edge.
• the ribbon crystal may be provided by growing the ribbon crystal from molten silicon (e.g., polysilicon).
• molten silicon e.g., polysilicon
• the method preferably removes the edge of the ribbon crystal at a point that improves ultimate device performance. For example, if the ribbon crystal has a grain boundary, then the method may remove at least a portion of the grain boundary.
• a string ribbon wafer having a body with larger grains.
• the body also may be free of string on at least one side and have an edge that is substantially planar or, in some embodiments, has an irregular pattern and no string.
• a method of processing a ribbon crystal provides a string ribbon crystal, and then separates the crystal into a plurality of wafers. After separating the crystal, the method removes at least one edge of at least one of the plurality of wafers.
• a string ribbon wafer has a body with a plurality grains, which includes a plurality of large grains and a plurality of small grains. The plurality of large grains have smallest outer dimensions that are greater than about two times the diffusion length of the carriers within the wafer. The majority of the plurality of grains are large grains and the body is substantially free of string.
• Figure 1 schematically shows a partially cut-away view of a silicon ribbon crystal growth furnace that may participate in implementation of illustrative embodiments of the invention.
• Figure 2 schematically shows an example of a string ribbon crystal without its edges removed.
• Figure 3 schematically shows an example of the string ribbon crystal of Figure 2 with its edges removed.
• Figure 4 shows a method of forming a wafer in accordance with illustrative embodiments of the invention.
• a wafer fabrication method removes an edge of a string ribbon crystal, or an edge of a wafer cut from the string ribbon crystal, to substantially mitigate the above noted problems.
• this method may both generally planarize the crystal/ wafer edge and remove at least a portion of the smaller grains that act as electron traps. Accordingly, the resultant wafers 1) have improved electrical properties, 2) may be positioned in closer proximity to neighboring wafers, and 3) maximize the area of a back-surface contact.
• removal of the smaller grains should improve the aesthetic appearance to some observers. Details of illustrative embodiments are discussed below.
• FIG. 1 schematically shows a partially cut-away view of a silicon ribbon crystal growth furnace 10 that may implement illustrative embodiments of the invention.
• the furnace 10 has, among other things, a housing 12 forming a sealed interior that is substantially free of oxygen (to prevent combustion). Instead of oxygen, the interior has some concentration of another gas, such as argon, or a combination of gasses.
• the housing interior also contains, among other things, a crucible 14 and other components for substantially simultaneously growing four silicon ribbon crystals 16.
• the ribbon crystals 16 may be any of a wide variety of crystal types, such as multi-crystalline, single crystalline, polycrystalline, microcrystalline or semi-crystalline.
• a feed inlet 18 in the housing 12 provides a means for directing silicon feedstock to the interior crucible 14, while an optional window 16 permits inspection of the interior components.
• silicon ribbon crystals 16 is illustrative and not intended to limit all embodiments of the invention.
• the crystals 16 may be formed from a material other than silicon, or a combination of silicon and some other material.
• An interior platform 20 within the housing 20 supports the crucible 14.
• This embodiment of the crucible 14 has an elongated shape with a region for growing silicon ribbon crystals 16 in a side-by-side arrangement along its length.
• the crucible 14 is formed from graphite and resistively heated to a temperature capable of maintaining silicon above its melting point.
• the crucible 14 has a length that is much greater than its width.
• the length of the crucible 14 may be three or more times greater than its width.
• the crucible 14 is not elongated in this manner.
• the crucible 14 may have a somewhat square shape, or a nonrectangular shape. String holes (not shown) through the crucible 14 enable strings to pass through molten silicon and thus, form the crystals 16.
• FIG 2 schematically shows an example of a string ribbon crystal 16 produced by the furnace 10 shown in Figure 1.
• This ribbon crystal 16 still has its original edges 24, which were formed as the crystal 16 was slowly drawn from the molten silicon in the crucible 14.
• the edges 24 of the ribbon crystal 16, which are not drawn to scale, are irregularly shaped. In some embodiments, however, the original edges 24 are not irregularly shaped. Instead, in such embodiments, the edges 24 are generally planar and generally parallel with the strings 26 (discussed immediately below) of the ribbon crystal Figure 2 also shows a pair of strings 26, which normally are encapsulated by the silicon.
• FIG. 2 shows what appears to be a significant area between the strings 26 and their respective edges 24, it is anticipated that the strings 26 will be very close to their respective edges 24 and thus, effectively form the edges 24.
• Figure 2 also shows dashed lines identifying the boundary of wafers 28 ultimately to be produced. Conventional methods cut along the dashed lines to form each wafer 28.
• Each wafer 28 also has a back-surface contact 30. As its name suggests, the back-surface contacts 30 are formed on a side of the ribbon crystal 16 that ultimately will be the back surface of the wafers 28 (i.e., if used as a solar cell).
• edges 24 of prior art ribbon crystals reduced the mobility for carriers within wafers 28 ultimately formed from the ribbon crystal 16.
• a prior art ribbon crystal would be less electrically efficient than it would be if it did not have such edges 24.
• the inventor took an approach that is contrary to what they understood to be the conventional wisdom — they removed at least a portion of the edge 24.
• the inventor removed many of the smaller grains, which produce a high concentration of grain boundaries.
• removal of the edges 24 improved the electrical efficiency in solar cells (e.g., carrier mobility), which is critical in the viability of photovoltaics.
• edge 24 removes a significant amount of the polysilicon, which currently is in low supply and has a corresponding high cost. The inventor nevertheless was surprised to discover that resultant efficiency improvements more than offset the costs associated with material loss caused by edge removal.
• edge 24 requires an additional process step or a plurality of additional steps, further increasing production costs.
• additional steps/ cuts required to perform this process increase the likelihood of crystal breakage, thus reducing yield.
• the inventor believes that reducing the width of the ribbon crystal 16, and/ or removing the string 26, can lead to additional breakage/ yield problems. Despite these and other obstacles teaching away from their solution, the inventor removed the edges 24 to discover the improved benefits.
• a grain is considered to be "large” when it has a smallest outer dimension that is greater than about two times the diffusion length of carriers (e.g., holes and electrons) within the crystal 16.
• carriers e.g., holes and electrons
• grains having a smallest outer dimension of between about 2-5 times the diffusion length of the carriers should suffice.
• Grains having smallest outer dimensions of greater than three times should provide even better results. In fact, it is anticipated that larger grain sizes, even five or more times the carrier diffusion length, should provide even better results.
• the substantial majority of all grains remaining in the crystal 16 are large grains — leaving only trace amounts of small grains.
• the removal step preferably removes a majority of the small grains, which generally concentrate around the string 26.
• Figure 3 schematically shows the ribbon crystal 16 of Figure 2 with both of its edges 24 removed.
• the (new) edges (identified by reference number 32) of the ribbon crystal 16 are substantially planar. In alternative embodiments, however, the new edge 32 may be a non-planar shape, or irregularly shaped. In either case, the ribbon crystal 16 of Figure 3 has substantially no small grains or very few small grains when compared to the ribbon crystal 16 before the edge 24 is removed.
• the back-surface contacts 30 each extend to the new edge 32 of the ribbon crystal 16.
• the ribbon crystals 16 in Figures 2 and 3 are illustrative of but one of a number of different embodiments.
• the back-surface contact 30 may be added after the ribbons are separated/ cut into individual wafers 28, and/ or not extend to the new edge 32.
• only one edge 24 may be removed, and/ or the edge 24 may be removed after the ribbon crystal 16 is separated/cut into individual wafers 28.
• Figure 4 shows a method of forming a wafer 28 in accordance with illustrative embodiments of the invention.
• this method is a simplified summary of the overall process of forming a wafer 28 and thus, does not include a number of other steps that may be included, such as wafer testing and preparation of certain equipment and the silicon. Moreover, some steps may be performed in a different order or, in some instances, omitted.
• the method begins while a ribbon crystal growth furnace 10 draws a ribbon crystal 16 from a molten material. Specifically, at step 400, the method determines if the back-surface contact 30 is to be added to the ribbon crystal 16 before or after removing one or both of the edges 24 (for simplicity, this method refers to one or both edges 24 in the singular; as an "edge 24"). In some instances, if it is formed after removing the edge 24, the back-surface contact 30 undesirably may extend around the new edge 32, which could cause a short circuit. Consideration of this possibility therefore should be used in making this determination. If step 400 determines that the back-surface contact 30 is to be formed first, then the method continues to step 402, which adds the back-surface contact 30 to the ribbon crystal 16.
• conventional processes may screen print the back-surface contact 30 on one side of the ribbon crystal 16.
• the back-surface contact 30 may be screen printed onto the ribbon crystal 16 as a plurality of separate blocks, as shown in Figures 2 and 3, or as a solid block spanning more than one wafer 28.
• the method determines at step 404 if the edge(s) 24 should be removed while in the form of a ribbon crystal. In other words, the method may remove the edge(s) 24 either before or after the ribbon crystal 16 is separated into individual wafers 28.
• the method separates the ribbon crystal 16 along the dashed lines of Figure 2 to form individual wafers 28 (step 406).
• the conventional sawing or dicing processes may cut the ribbon along the dashed lines shown in Figures 2 and 3.
• a laser may cut along the dashed lines as discussed in the above incorporated patent application.
• step 408 removes one or both edges 24 of the ribbon crystal 16 (if continuing from step 404) or the wafers 28 (if continuing from step 406).
• conventional sawing/ dicing processes may remove the entire string 26 and many other smaller grains inward of the string (if any).
• Experimental processes may determine how far to remove the edge 24 inward of the string 26.
• the removal device e.g., a laser or saw
• the string 26 may not be positioned perfectly straight from top to bottom of the crystal 16.
• the string may be more straight than the cut.
• the removal step may leave a portion of the string 26 behind in the crystal 16.
• one skilled in the art can select an appropriate distance to cut the ribbon crystal 16 (or wafer 28, as the case may be) inward from the string.
• one skilled in the art can set the width of the crystal 16 and measure outwardly from a generally longitudinal point of the crystal 16. For example, to yield a crystal 16 with about a 100 millimeter width, one skilled in the art can cut along generally parallel lines about 50 millimeters from a general longitudinal portion of the crystal 16.
• the ribbon crystal 16 is grown to have a significant amount of area outward of the string 26, then some embodiments may remove a portion of the crystal 16 outward of the string 26, thus keeping the string 26 in the crystal 16. It nevertheless is anticipated that removal of the string 26 in such a crystal 16 will yield more efficient wafers. It should be noted that a wafer produced by the discussed techniques and in the described manners is considered to be a string ribbon wafer even if the string 26 is partially or completely removed.
• the method may perform step 408 in a number of different manners.
• the method may automate the process as the ribbon crystal 16 grows.
• the furnace 10 may be retrofitted to include a saw or laser (not shown) to remove the edge(s) 24 from the growing ribbon crystal 16 in real time.
• the ribbon crystal 16 first may be manually scribed to remove it from the furnace 10, and then manually or automatically moved to another machine that cuts the edge(s) 24 in the prescribed manner.
• some embodiments remove the edge(s) 24 by means of an operator manually scribing the edge(s) 24 of the ribbon crystal 16.
• the method may use either automatic or manual means to remove the edge(s) 24.
• removal of one or both edges 24 removes the smaller grains (i.e., the area with high grain density). This should leave relatively larger grains in the resulting wafers 28, which improves electrical efficiency.
• the method concludes by adding the back-surface contact 30 to the ribbon crystal 16 or wafers 28, depending on their form, if such feature was not already added (step 410), and separating the ribbon crystal 16 into wafers 28 if not already in that form (step 412).
• the back-surface contact 30 may be formed at a number of different points in the overall fabrication of a solar cell.
• the method could add the back-surface contact 30 before any fabrication steps are executed, or add the back-surface contact 30 after performing a number of solar cell fabrication steps that were not discussed.
• planar edges 32 with few or no grain boundary regions.
• These planar edges 32 may form approximately ninety degree angles with their adjacent sides (i.e., the intersection of the top edge and the new side edge 32 of the ultimate wafers 28).
• these planar edges 32 may form acute and/ or obtuse angles with their adjacent sides.
• such embodiments may form new edges 32 having a variety of shapes (e.g., irregularly shaped).
• many such wafers 28 should 1) have improved electrical properties due to removal of many of the high grain concentrations near the crystal edge, 2) may be positioned in closer proximity to neighboring wafers, and 3) maximize the area of a back-surface contact 30.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Crystallography & Structural Chemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Crystals, And After-Treatments Of Crystals (AREA)
• Photovoltaic Devices (AREA)
• Processing Of Stones Or Stones Resemblance Materials (AREA)
• Silicon Compounds (AREA)
• Mechanical Treatment Of Semiconductor (AREA)

Priority Applications (5)

Applications Claiming Priority (2)

Publications (1)

Family

ID=39791565

Family Applications (1)

Country Status (9)

Families Citing this family (4)

Citations (6)

Family Cites Families (11)

• 2008
  • 2008-07-25 EP EP08782387A patent/EP2195475B1/en not_active Not-in-force
  • 2008-07-25 ES ES08782387T patent/ES2399465T3/es active Active
  • 2008-07-25 JP JP2010518410A patent/JP2010534610A/ja active Pending
  • 2008-07-25 US US12/179,972 patent/US20090025787A1/en not_active Abandoned
  • 2008-07-25 CN CN200880023563.5A patent/CN101688322B/zh not_active Expired - Fee Related
  • 2008-07-25 MY MYPI20100162 patent/MY150483A/en unknown
  • 2008-07-25 KR KR1020107002383A patent/KR20100039386A/ko not_active Ceased
  • 2008-07-25 WO PCT/US2008/071179 patent/WO2009018145A1/en not_active Ceased
  • 2008-07-25 CA CA002689519A patent/CA2689519A1/en not_active Abandoned

Patent Citations (6)

Non-Patent Citations (6)

Also Published As

Similar Documents

Legal Events