Method for producing a sheet from a melt by imposing a periodic change in the rate of pull

Abstract

A method of forming crystalline sheets using a Horizontal Ribbon Growth process, where the sheet of material formed in the process is withdrawn from a crucible in a specified manner to reduce instabilities in the process and to regulate crystal growth dynamics.

Claims

1. A method for growing a sheet of material comprising: melting the material in a crucible to form a melt; cooling a top surface of the melt using a cooling device to form a solid sheet on the top surface of the melt; and withdrawing the sheet from the crucible in a first direction, wherein a rate of the withdrawal is changed periodically in a sinusoidal manner relative to a constant velocity.

2. The method of claim 1, wherein withdrawing the sheet comprises introducing a successive back and forth motion by periodically reversing a direction of the withdrawal relative to the first direction.

3. The method of claim 1, further comprising translating the sheet side-to-side relative to the first direction.

4. The method of claim 1, wherein the crucible is translated back and forth.

5. The method of claim 1 further comprising: rotating the crucible, wherein the crucible is circular.

6. The method of claim 1, wherein the material is selected from the group consisting of silicon and germanium.

7. The method of claim 1, wherein the material is selected from the group consisting of gallium-arsenide, silicon-carbides, oxides of aluminum, and gallium oxides.

8. The method of claim 1, wherein the sheet defines an angle of 2 degrees or more with respect to the top surface of the melt.

9. The method of claim 8, wherein a mechanism for oscillation of the sheet depends on the angle of the sheet with the melt.

10. The method of claim 1, wherein the cooling device comprises jets of an inert gas.

11. The method of claim 10, wherein the rate of the withdrawal of the sheet depends on the flow of inert gas from the jets.

12. The method of claim 1 where the atmosphere above the melt contains nitrogen.

13. The method of claim 10, wherein the inert gas comprises He or Ar.

14. A method for growing a sheet of material comprising: melting the material in a crucible to form a melt; cooling a top surface of the melt using a cooling device to form a solid sheet on the top surface of the melt, wherein a solidification front forms at a boundary between the solid sheet and the melt; and withdrawing the sheet from the crucible in a first direction, wherein oscillations are introduced to create a relative movement of the solidification front.

15. The method of claim 14, wherein the oscillations are sinusoidal.

16. The method of claim 14, wherein the oscillations are side-to-side relative to the first direction.

17. The method of claim 1, wherein a change in the rate of withdrawal comprises a composition of sines.

Description

BRIEF SUMMARY OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 shows a device used to perform the method of the present invention, according to one embodiment.

(2) FIG. 2 shows the interface between a monocrystalline sheet and the melt.

DETAILED DESCRIPTION

(3) According to embodiments of the present invention is a float process for producing crystalline materials. The materials can include electronic materials such as silicon and germanium, gallium-arsenide, silicon-carbide, oxides such as sapphire (i.e. aluminum oxide), gallium oxides, and other materials that can be fabricated as a single crystal for opto-electronic applications. As shown in FIG. 1, the float process occurs in a crucible 100. In a first zone 101 of the crucible 100, solid pellets 201 of the material are loaded into the crucible 100. The temperature in the first zone 101 is above the melting temperature of the pellets 201, causing them to melt. The molten material, or melt 202, then flows to a stabilization zone 102. In a third zone 103, the material cools and a sheet 203 of solid material forms on top of the melt 202. A cooling device 110 is positioned above the third zone 103 to control the heat flux. In the embodiment shown in FIG. 1, the cooling device 110 comprises a cooling plate 111 through which a liquid coolant flows. In an alternative embodiment, the cooling device 110 may comprise a jet of inert gas, such as helium (He).

(4) Controlled oscillations are introduced at the boundary 300 between the melt 202 and solid sheet 203 by translating the sheet 203 and/or the crucible 100 in conjunction. The oscillations promote redistribution of solid 203 and melt 202 around the interface 300, which reduces in-homogeneity around the growing crystal. The oscillations introduced into the system can vary depending on the type of material being produced and the cooling device 110 used. For example, in one embodiment, the direction of the pull is periodically reversed, creating a push/pull motion. In an alternative embodiment, the sheet is translated side-to-side relative to the direction of the pull. In yet another alternative embodiment, movement is created in the crucible. Other movements may be used that create a relative movement between the sheet 203 and the melt 202. Periodic perturbation of one or more process parameters such as the withdrawal rate of the sheet or the cooling or heating rates can produce a more robust wafer that exhibits a limited degree of mechanical flexibility. For example, the sheet 203 produced by the method, according to one embodiment, has a bending radius of about 3 inches or more.

(5) The formation of a thin and uniform wedge, as shown in FIG. 2, produces a high-quality sheet 203. The wedge allows for appropriate thickness control by balancing heating, cooling, and pull-rate. Moreover, crystallization is nearly orthogonal to the process flow when the wedge is sharp. High production rate can then be achieved while crystallization is nearly epitaxial, as required to achieve a high-quality, crystalline sheet 203.

(6) During a float process with constant pulling speed, the leading edge of the sheet forms a smooth facet. Facets grow via nucleation which have slow growth kinetics and require significant sub-cooling to maintain growth. Sub-cooling requires an intense amount of heat removal at the leading edge to sustain the pulling. In contrast, the method of the present invention utilizes a control method, in one embodiment, where the sheet 203 is pushed into the melt 202 and then pulled in the opposite direction at its regular speed. The solid at the leading edge of the sheet 203 can be redistributed to form a roughened surface, as shown in FIG. 2. This redistribution of solid causes the smooth surface to become roughened, which stays roughened for the remainder of the process, thereby improving crystal growth dynamics. The control method also eliminates the sub-cooling zone, creating better quality crystals with low defects.

(7) When the mono-crystalline sheet 203 is pulled at a constant velocity, there is a definite upper limit in the production rate of the sheet. In one embodiment, the control method increases the upper limit on the production rate of the monocrystalline sheet 203.

(8) Another objective of controlling the movement of the sheet 203 relative to the melt 202 is to homogenize the melt 202 so that the impurity boundary layer near the solidification front 301 diffuse faster into the melt 202. This combined with the loss of sub-cooling reduces the possibility of dendritic growth thereby making the process more stable.

(9) Industry has stringent commercial standard for thickness of silicon wafers to be used for semi-conductor or electronic applications. The method, according to an embodiment of the present invention, makes it easier for thickness control by breaking the faceted wedge at the leading edge of the sheet 203, allowing the wedge to take an acute angle depending on the velocity, the period, and amplitude of the sheet pulling rate. This makes it possible for the crystal to grow uniformly along the length of the sheet 203, decoupling the direction of pulling from the direction of the solidification, which happens in the vertical direction in prior art HRG processes. Also since the mechanism for heat removal at the leading edge 301 is from the cooling device 110 instead of the pool of sub-cooled liquid, the process is easier to model and control.

(10) It can also be advantageous to allow the sheet 203 to have an angle relative to the surface of the liquid melt 202. Such an angle stabilizes the meniscus at the point where the sheet 203 is removed from the melt 202 and helps prevent the instability and down-growth reported in previous embodiments of the HRG process. For example, in one embodiment, the sheet 203 has an angle of about 2 degrees relative to the top surface of the melt 202.

(11) In one embodiment, the atmosphere above the melt typically consists of an inert gas such as Argon and cooling is achieved by radiation to a cold surface used as the cooling device 110. Additional cooling may be achieved by inserting jets of cold inert gases such as helium.

(12) It may also be advantageous to introduce a small percentage of other gases into the process. For example, introducing nitrogen into the process atmosphere leads to the formation of a very thin layer of silicon-nitride on top of the melt 202. This layer stabilizes the formation of microscopic surface waves and isolates the melt 202 chemically from impurities that may be present in the atmosphere.

(13) While the disclosure has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modification can be made therein without departing from the spirit and scope of the embodiments. Thus, it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.