Sheet production apparatus for removing a crystalline sheet from the surface of a melt using gas jets located above and below the crystalline sheet
09677193 ยท 2017-06-13
Assignee
Inventors
Cpc classification
C30B15/00
CHEMISTRY; METALLURGY
C30B35/00
CHEMISTRY; METALLURGY
C30B15/22
CHEMISTRY; METALLURGY
Y10T117/1048
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
C30B15/34
CHEMISTRY; METALLURGY
C30B11/00
CHEMISTRY; METALLURGY
Y10T117/1092
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
International classification
C30B15/22
CHEMISTRY; METALLURGY
C30B35/00
CHEMISTRY; METALLURGY
C30B11/00
CHEMISTRY; METALLURGY
C30B9/00
CHEMISTRY; METALLURGY
Abstract
In one embodiment, a sheet production apparatus comprises a vessel configured to hold a melt of a material. A cooling plate is disposed proximate the melt and is configured to form a sheet of the material on the melt. A first gas jet is configured to direct a gas toward an edge of the vessel. A sheet of a material is translated horizontally on a surface of the melt and the sheet is removed from the melt. The first gas jet may be directed at the meniscus and may stabilize this meniscus or increase local pressure within the meniscus.
Claims
1. A sheet production apparatus comprising: a vessel configured to hold a melt of a material; a cooling plate disposed proximate said melt, said cooling plate configured to form a sheet of said material that is horizontal on said melt proximate said cooling plate; a first gas jet configured to direct a gas toward a meniscus at a juncture of a bottom surface of the sheet and an edge of said vessel; and a pressure cell that is separate from and downstream of the cooling plate in a direction in which the melt flows, the pressure cell defined by a top section including a ceiling and adjoining sidewalls disposed above the sheet over the first gas jet and defining a gap of less than 0.5 millimeters between a lower extent of the sidewalls and the sheet, the pressure cell further defined by a bottom section including a floor disposed below the sheet and integral with the first gas jet, wherein the pressure cell encloses a gas at a higher pressure than a surrounding atmosphere and wherein the gas is allowed to escape the pressure cell through the gap.
2. The apparatus of claim 1, further comprising a pump associated with the vessel and oriented to circulate said melt within said vessel.
3. The apparatus of claim 1, further comprising a second gas jet configured to direct said gas toward said edge of said vessel, said second gas jet positioned opposite said first gas jet and integral with the top section.
4. The apparatus of claim 1, further comprising a support table disposed adjacent said first gas jet, said support table configured to support said sheet.
5. The apparatus of claim 1, wherein said material is silicon or silicon and germanium.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:
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DETAILED DESCRIPTION
(13) The embodiments of the apparatus and methods herein are described in connection with solar cells. However, these also may be used to produce, for example, integrated circuits, flat panels, LEDs, or other substrates known to those skilled in the art. Furthermore, while the melt is described herein as being silicon, the melt may contain germanium, silicon and germanium, gallium, gallium nitride, other semiconductor materials, or other materials known to those skilled in the art. Thus, the invention is not limited to the specific embodiments described below.
(14)
(15) This vessel 16 defines at least one channel 17. This channel 17 is configured to hold the melt 10 and the melt 10 flows from a first point 18 to a second point 19 of the channel 17. The melt 10 may flow due to, for example, a pressure difference, gravity, a pump, or other methods of transport. The melt 10 then flows over the spillway 12. This spillway 12 may be a ramp, a weir, a ledge, a small dam, or a corner and is not limited to the embodiment illustrated in
(16) In one particular embodiment, the vessel 16 may be maintained at a temperature slightly above approximately 1685 K. For silicon, 1685 K represents the freezing temperature or interface temperature. By maintaining the temperature of the vessel 16 to slightly above the freezing temperature of the melt 10, the cooling plate 14 may function using radiation cooling to obtain the desired freezing rate of the sheet 13 on or in the melt 10. The cooling plate 14 in this particular embodiment is composed of a single segment or section but also may include multiple segments or sections. The bottom of the channel 17 may be heated above the melting temperature of the melt 10 to create a small vertical temperature gradient in the melt 10 at the interface to prevent constitutional supercooling or the formation of dendrites, or branching projections, on the sheet 13. However, the vessel 16 may be any temperature above the melting temperature of the melt 10. This prevents the melt 10 from solidifying on the vessel 16.
(17) The sheet-forming apparatus 21 may be maintained at a temperature slightly above the freezing temperature of the melt 10 by at least partially or totally enclosing the sheet-forming apparatus 21 within an enclosure. If the enclosure maintains the sheet-forming apparatus 21 at a temperature above the freezing temperature of the melt 10, the need to heat the sheet-forming apparatus 21 may be avoided or reduced and heaters in or around the enclosure may compensate for any heat loss. This enclosure may be isothermal with anisotropic conductivity. In another particular embodiment, the heaters are not disposed on or in the enclosure and are rather in the sheet-forming apparatus 21. In one instance, different regions of the vessel 16 may be heated to different temperatures by embedding heaters within the vessel 16 and using multi-zone temperature control.
(18) The enclosure may control the environment where the sheet-forming apparatus 21 is disposed. In a specific embodiment, the enclosure contains an inert gas. This inert gas may be maintained at above the freezing temperature of the melt 10. The inert gas may reduce the addition of solutes into the melt 10 that may cause constitutional instabilities during the formation of the sheet 13.
(19) The cooling plate 14 allows heat extraction that enables the sheet 13 to form on the melt 10. The cooling plate 14 may cause the sheet 13 to freeze on or in the melt 10 when the temperature of the cooling plate 14 is lowered below the freezing temperature of the melt 10. This cooling plate 14 may use radiation cooling and may be fabricated of, for example, graphite, quartz, or silicon carbide. Disturbances to the melt 10 may be reduced while the sheet 13 forms to prevent imperfections in the sheet 13. Cooling a sheet 13 on the surface of the melt 10 or a sheet 13 that floats on the melt 10 allows the latent heat of fusion to be removed slowly and over a large area while having a relatively large sheet 13 extraction rate.
(20) After the sheet 13 is formed on the melt 10, the sheet 13 is separated from the melt 10 using the spillway 12. The melt 10 flows from the first point 18 to the second point 19 of the channel 17. The sheet 13 will flow with the melt 10. This transport of the sheet 13 may be a continuous motion. In one instance, the sheet 13 may flow at approximately the same speed that the melt 10 flows. Thus, the sheet 13 may form and be transported while at rest with respect to the melt 10. The shape of the spillway 12 or orientation of the spillway 12 may be altered to change the velocity profile of the melt 10 or sheet 13.
(21) The melt 10 is separated from the sheet 13 at the spillway 12. In one embodiment, the flow of the melt 10 transports the melt 10 over the spillway 12 and may, at least in part, transport the sheet 13 over the spillway 12. This may minimize or prevent breaking a sheet 13 because no external stress is applied to the sheet 13. Of course, the sheet 13 also may be pulled or have some external force applied. The melt 10 will flow over the spillway 12 away from the sheet 13 in this particular embodiment. Cooling may not be applied at the spillway 12 to prevent thermal shock to the sheet 13. In one embodiment, the separation at the spillway 12 occurs in near-isothermal conditions. The sheet 13 may tend to go straight beyond the spillway 12 in one embodiment. This sheet 13 may be supported after going over the spillway 12 in some instances to prevent breakage.
(22) Of course, different cooling temperatures across the length of the cooling plate 14, different flow rates of the melt 10 or pull speeds of the sheet 13, the length of the various sections of the sheet-forming apparatus 21, or the timing within the sheet-forming apparatus 21 may be used for process control. If the melt 10 is silicon, the sheet 13 may be polycrystalline or single crystal sheet using the sheet-forming apparatus 21.
(23) A meniscus interface forms when a liquid is in contact with a gas. This interface follows the Young-Laplace Equation. In two dimensions, it takes the form:
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where P is the difference in pressure across the interface, is the surface tension of the liquid, and R is the radius of curvature of the surface.
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(27) The concave shape of the meniscus 27 illustrated in
(28) Gas jets may be used to stabilize the meniscus by increasing local pressure in the melt. For example, local pressure on the gas side of the meniscus may be increased. Meniscus stabilization using the embodiments described herein is independent of any melt flow so that crystal initialization may occur before the melt begins to flow, which simplifies seeding in systems that use melt flow. The sheet may be grown horizontally using the embodiments described herein, which eliminates the complicated balance of growth rate (i.e., heat removal) against pull speed at an angle. Sheet growth may be upstream of the edge of the vessel where separation from the melt occurs.
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The Young-Laplace Equation in this case is a second order differential equation that requires two boundary conditions. In the embodiments of
(31) The magnitude of the pressure at the exit of the gas jet 22 depends on the flow of gas and the width of the opening in the gas jet 22 that allows the flow of gas. The opening may be, for example, a slit jet. This may be at least partly estimated using conservation of momentum. So at the stagnation point where the gas bounces off the meniscus 27, the pressure would be:
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where .sub.g, u.sub.g, and Q.sub.g are the gas density, velocity, and volume flow rate, respectively. The following example calculates the flow of argon needed to obtain a pressure of 40 Pa at the meniscus 27 through an opening in the gas jet 22 that is 0.5 mm in width. The density of the argon at the temperature of the melt 10, which is 1412 C. for silicon, is 0.32 kg/m.sup.3.
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(34) The pressure estimated here may only exist at the exit of the gas jet 22.
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The pressure may fall off axially and transversely. The pressure distribution may be approximated as an elliptical Gaussian.
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(37) Thus, by using gas impingement, a stable meniscus 27 can be pinned to the wall of the vessel 16 at least 2.5 mm below the sheet 13 with a contact angle of approximately 11. Even if the contact angle were as low as 0, a stable meniscus 27 could still be maintained 1 mm below the wall of the vessel 16. The gas jet impingement also may compensate for any drag caused by viscous forces. The pressure of the gas jet impingement can be configured to stabilize the meniscus 27 or assist in maintaining the pinning of the meniscus 27 to the vessel 16.
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(44) The seed wafer 28 is translated in
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(46) Meniscus stabilization using gas jets has multiple advantages. It may be applied to a horizontal sheet formation or horizontal ribbon growth (HRG) system and may be used to avoid LASS in one instance. The sheet 13 may be pulled horizontally in one embodiment, thereby allowing the crystal forming region to be upstream of the meniscus 27. This minimizes perturbations caused by a pulling mechanism from affecting the sheet 13 while it forms. The melt 10 flow speed may be controlled independently of the speed of the sheet 13. This may enable a simpler seeding process. Furthermore, spills of the melt 10 may be reduced or prevented.
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(48) In the embodiments disclosed herein, the gas jet 22 may direct gas at a particular temperature. The gas may be heated to prevent the meniscus from freezing. The gas also may be cooled to prevent the sheet from being melted or to otherwise cool the sheet.
(49) The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.