WO2014197672A1

WO2014197672A1 - Insitu sneeze valve for clearing exhaust of a czochralski growth chamber

Info

Publication number: WO2014197672A1
Application number: PCT/US2014/041045
Authority: WO
Inventors: Deedra May MORGAN
Original assignee: Sunedison, Inc.
Priority date: 2013-06-05
Filing date: 2014-06-05
Publication date: 2014-12-11

Abstract

A system for growing a crystal ingot from a silicon melt includes a furnace for supplying heat to liquefy the silicon melt. The furnace has an exhaust outlet connected with the furnace to remove at least one of inert gas, contaminants, and evaporated species from within the furnace. A sneeze valve is connected with the exhaust outlet for inhibiting flow through the exhaust outlet. A programmable device is connected with the sneeze valve for regulating the operation of the valve.

Description

I SITU SNEEZE VALVE FOR CLEARING EXHAUST OF

A CZOCHRALSKI GROWTH CHAMBER

CROSS REFERENCE

[0001] This application claims priority to U.S. Provisional Application No. 61/831,452 filed June 5, 2013, the disclosure of which is hereby incorporated by reference in its entirety.

FIELD

[0002] This disclosure generally relates to systems and methods for the production of ingots of semiconductor or solar material, and more particularly to systems and methods for reducing contaminants in the exhaust outlets by providing increased pressure into the exhaust outlets.

BACKGROUND

[0003] Single crystal silicon, which is the starting material in most processes for the fabrication of semiconductor electronic components, is commonly prepared by the so-called Czochralski ("Cz") method. In this method, polycrystalline silicon ("polysilicon") is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon, and then a single crystal is grown by slow extraction.

[0004] Solar cells may be fabricated from monocrystalline silicon substrates produced by the Czochralski method. Czochralski grown monocrystalline silicon substrates may be grown by either standard, i.e., batch, or continuous.

[0005] During the crystal growth process, silicon dioxide powder is produced within the growth chamber and is expelled through exhaust outlets. Silicon dioxide powder is a contaminant that may clog the exhaust outlets. Prior systems for growing crystals required exhaust outlet geometry designed to increase gas flow through the exhaust outlets. Clogged systems were required to be shut down while the exhaust outlets were manually cleaned out. Thus, there exists a need for a more efficient and effective system and method to clear silicon dioxide powder from the exhaust outlets during normal operation of the system.

[0006] This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

BRIEF DESCRIPTION

[0007] A first aspect is a system for growing a crystal ingot from a silicon melt. The system includes a furnace for supplying heat to liquefy the silicon melt. The furnace has an exhaust outlet connected with the furnace to remove at least one of inert gas, contaminants, and evaporated species from within the furnace. A sneeze valve is connected with the exhaust outlet for inhibiting flow through the exhaust outlet. A programmable device is connected with the sneeze valve for regulating the operation of the valve.

[0008] Another aspect is a method for growing a crystal ingot in a growth chamber having an exhaust outlet. The method includes supplying an inert gas to the growth chamber. The method also includes removing contaminates from the growth chamber by gas flow through the exhaust outlet. The method further includes obstructing the exhaust outlet to inhibit gas flow therethrough to increase pressure within the growth chamber and exhaust outlet, and allowing gas flow through the exhaust outlet.

[0009] Various refinements exist of the features noted in relation to the above-mentioned aspects. Further, features may also be incorporated in the above-mentioned aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments may be incorporated into any of the above-described aspects, alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 is a side cross sectional view of a growth chamber having a crystal growth chamber in accordance with one embodiment;

[0011 ] FIG. 2 is a front perspective view of exhaust outlets connected with the crystal growth chamber of FIG. 1 ;

[0012] FIG. 3 is a side perspective view of the exhaust outlets of FIG. 2; and [0013] FIG. 4 is a perspective view of a vacuum system connected to the exhaust outlets shown in FIGS. 2 and 3.

[0014] Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

[0015] According to a process of one embodiment, a monocrystalline silicon ingot is grown by the Czochralski method. In some embodiments, a monocrystalline silicon ingot is grown by the batch Czochralski method. In other embodiments, a monocrystalline silicon ingot is grown by the continuous Czochralski method.

[0016] A Czochralski growth chamber or system suitable for growing the

monocrystalline silicon ingots described herein is indicated generally at 100 in FIG. 1. The growth chamber 100 includes a crucible 102 for holding a melt 104 of semiconductor or solar- grade material, such as silicon, surrounded by a susceptor 106 contained within a furnace 108. The semiconductor or solar-grade material is melted by heat provided from a heating element 110 surrounded by insulation 1 12. Heat shields or reflectors 114 may be disposed above the melt surface 1 16 to provide better thermal distribution within the growth chamber 100.

[0017] A pulling mechanism 118 is provided within the growth chamber for growing and pulling ingots out of the melt 104. The pulling mechanism includes a pulling cable 120, a seed holder or chuck 122 disposed at the end of the pulling cable, and a seed crystal 124 coupled to the seed holder or chuck 122 for initiating crystal growth.

[0018] The growth chamber 100 may also include one or more gas inlets 126 for introducing or supplying an inert gas into the growth chamber. The gas inlets 126 may be disposed anywhere along the growth chamber 100. The gas inlets 126 in FIG. 1 are disposed above the melt surface 116. Gas introduced through the gas inlets 126 flows over the surface of the silicon melt and the surface of the growing ingot. The inert gas flow over the surface of the silicon melt tends to increase evaporation of species from the silicon melt, particularly volatile dopants, such as indium. This effect is amplified when the inert gas flows through narrow channels above the silicon melt, such as those created by heat shields or reflectors 114.

Accordingly, the gas inlets 126 may be attached to one or more flow controllers 128 for controlling the flow rate of the incoming inert gas, described in more detail below. [0019] The growth chamber 100 includes one or more exhaust outlets 130 connected with the furnace for removing fluids, such as inert gases and evaporated species from the melt, and contaminant particles from the growth chamber. The exhaust outlets 130 may be attached to one or more pressure regulating devices 132, described in more detail below, for regulating the internal chamber pressure of the growth chamber 100. The exhaust outlets may also be attached to a pump 134, described in more detail below, which may be operated independent of or in conjunction with the pressure regulating devices 132 to regulate the internal chamber pressure of the growth chamber 100.

[0020] During the growth process, an inert gas may be introduced into the growth chamber from one or more gas inlets 126 disposed above the silicon melt 104 to remove contaminants and evaporated species produced by the melt, and reduce the risk of contaminants being incorporated into the grown ingot. The inert gas flows over the melt surface 116 and the surface of the growing ingot 136, thereby preventing contaminant particles from being able to reach the phase boundary 138 at which the single crystal is growing. Suitable inert gasses include argon, helium, nitrogen, neon, mixtures thereof, and any other suitably inert gas.

[0021 ] In particular embodiments, the inert gas flow rate and the internal chamber pressure may be adjusted such that the ratio of the inert gas flow rate and the internal chamber pressure is between about 0.05 and about 1.40 normal-liters per minute per millibar, between about 0.10 and about 1.00 normal-liters per minute per millibar, or between about 0.10 and about 0.80 normal-liters per minute per millibar. In yet further embodiments, the flow rate of the inert gas may be varied between about 20 normal-liters per minute and about 200 normal-liters per minute, between about 30 normal-liters per minute and about 140 normal-liters per minute, or between about 30 normal-liters per minute and about 80 normal-liters per minute. In yet further embodiments, the internal pressure of the growth chamber may be varied between about 20 millibar and about 400 millibar, between about 30 millibar and about 200 millibar, or between about 30 millibar and about 100 millibar. In yet further embodiments, the internal pressure of the growth chamber may be about 20 millibar, 33 millibar, 39 millibar, or 35 millibar. As used herein, the term "normal-liter" refers to one liter of the referenced gas at 273.15 Kelvin and 101.325 kPa.

[0022] The flow rate of the inert gas flowing through the gas inlets 126 is controlled using one or more flow controllers 128 attached to the gas inlets 126. The flow controllers 128 may be any device suitable for regulating the flow rate of inert gas passing through the gas inlet 126 and into the growth chamber 100, such as mass flow controllers, volumetric flow controllers, throttle valves, or butterfly valves. The flow controllers 128 may be automated or manually controlled. Automated flow controllers may be controlled by one or more programmable devices or processors 140 capable of adjusting the flow rate of the inert gas based on user defined conditions (e.g., after a certain amount of time has elapsed). The flow controllers 128 shown in FIG. 1 are automated mass flow controllers controlled by a programmable device 140 capable of adjusting the flow rate of the inert gas based on user defined conditions.

[0023] The internal pressure of the growth chamber 100 and the flow rate through the exhaust outlets 130 may be controlled using one or more pressure regulating devices 132 in fluid communication with the exhaust outlets of the growth chamber. The pressure regulating devices 132 may be any type of device suitable for regulating the pressure within the growth chamber 100, including electronic pressure controllers, throttle valves, butterfly valves, and ball valves. The pressure regulating devices 132 may be automated or manually controlled to adjust pressure within the exhaust outlet 130 and the furnace 108 during growth of the crystal ingot. Automated pressure regulating devices may be controlled by one or more programmable devices 140 capable of adjusting the internal pressure of the growth chamber 100 based on user defined conditions (e.g., after a certain amount of time has elapsed). The programmable device 140 used to control the pressure regulating device 132 may be separate from or the same as the programmable device 140 used to control the flow controller 128. The pressure regulating device shown in FIG. 1 is automated and controlled by a programmable device 140 capable of adjusting the internal pressure of the growth chamber based on user defined conditions.

[0024] With additional reference to FIGS. 3 and 4, the programmable device 140 is also connected to a temperature sensor 142 and a sneeze valve 144. Both the temperature sensor 142 and the sneeze valve 144 are connected with the exhaust outlets 130. The temperature sensor 142 measures a temperature of the gas flow through the exhaust outlet 130 and provides a signal to the programmable device 140 that corresponds to the measured temperature. As contaminates accumulate within the exhaust outlets 130, the flow through the exhaust outlets is restricted, causing the temperature of the exhaust outlets to rise.

[0025] The programmable device 140 determines whether the exhaust outlet 130 is operating within predetermined limits or whether the exhaust outlet maybe completely or partially clogged by contaminants based on the measured temperature signal it receives from the temperature sensor 142. This determination is based on comparing the received measured temperature with the predetermined limits. As contaminates collect in the exhaust outlet 130, the flow through the exhaust outlet 130 is restricted and causes an increase in temperature. The programmable device 140 is programmed with the predetermined limits and determines that the exhaust outlet 130 is partially or completely clogged based on receiving a measured temperature signal from the temperature sensor 142 that meets or exceeds a predetermined limit.

[0026] The sneeze valve 144 has an open position and a closed position. The sneeze valve 144 allows flow through the exhaust outlets 130 in the open position, and inhibits or obstructs flow through the exhaust outlet 130 in the closed position. The sneeze valve 144 is connected with and may be controlled by the programmable device 140 to allow the

programmable device to regulate the position and operation of the sneeze valve. Based on a determination that the exhaust outlet 130 is either completely or partially clogged by

contaminants, the programmable device closes the sneeze valve 144. In some embodiments, the sneeze valve may be a pressure relief valve.

[0027] As shown in FIG. 4, the exhaust outlets 130 of the growth chamber 100 are in fluid communication with a first filter 146, a second filter 148, and the pump 134. The pump 134 may be used to pump or remove inert gas, contaminants, and evaporated species from the silicon melt (e.g., SiO and dopant related species) out of the growth chamber 100 through the Active Filtration System and then into the filters 146 and 148. The pump 134 may be operated independent of or in conjunction with the pressure regulating device 132 to regulate or adjust operational pressure within the exhaust outlet 130 and the growth chamber 100 during growth of the crystal ingot. The pump 134 may be controlled by a programmable device 140 capable of adjusting the settings of the pump based on user defined conditions (e.g., after a certain amount of time has elapsed). The programmable device 140 may be the same as or separate from the programmable device used to control the flow controller 128 and/or the pressure regulating device 132.

[0028] To clear clogged or partially clogged exhaust outlets during a crystal growing process, including in a continuous process (CCz), the pressure in the growth chamber 100 is increased and then the sneeze valve 144 is quickly opened to release the pressure into the exhaust outlets 130. The opening of the sneeze valve 144 rapidly releases the pressure in the growth chamber 100 through the exhaust outlets 130, causing a burst or a sneeze of high pressure gas through the exhaust outlets. The release of pressure increases the flow rate through the exhaust outlets 130 to a level that is substantially greater than that of the operational flow rate through the exhaust outlets during the growth process, at which the contaminates are deposited and accumulate within the exhaust outlets. Thus, any clogs or accumulations of contaminates in the exhaust outlets are cleared by sending the high velocity gas through the exhaust outlets.

[0029] During the crystal growth process, a contaminant such as silicon dioxide powder forms within the growth chamber 100 and accumulates within the exhaust outlets 130. Therefore, the exhaust outlets 130 must be cleaned to remove the contaminant. However, it is undesirable to shut down the growth chamber 100 and changes in operating conditions during the crystal growth process may cause the growing ingot to have an undesirable atomic structure. As a result, the exhaust outlets 130 are cleared between the growths of crystals.

[0030] Between crystal growths, growing of a first crystal ingot and growing of a second crystal ingot, and while the growth chamber 100 is operational, the operator monitors the temperature of the exhaust outlet 130. Increased temperature of the exhaust outlet 130 corresponds to an increase of accumulated contaminates within the exhaust outlets. When the temperature of the exhaust outlet 130 exceeds a preset or predetermined threshold or value, the operator takes steps to clear the accumulated contaminates from within the exhaust outlets between crystal growths.

[0031 ] In a method of one embodiment, the operator first turns off the oxidation to the growth chamber 100 and closes the sneeze valve 144, obstructing the exhaust outlet 130 to inhibit gas flow therethrough to increase pressure within the growth chamber 100 and exhaust outlet. The sneeze valve 144 is closed as the active filtration approaches the midpoint of the "feathering phase," both filters are opening and closing at the same time and at midpoint the filter that was closing will then be reopened. Both filters 146 and 148 are equally opened, the inlets 126 are opened, and the exhaust outlets 130 are half opened, causing the filters to drop in pressure. As the pressure in the filters 146 and 148 drop, the pressure in the growth chamber 100 increases and the pressure regulating devices 132 will open. The pressure within the growth chamber 100 increases to a predetermined set-point, determined by the process engineer. [0032] In some embodiments, the predetermined set-point is about 80 millibar, 133, millibar, about 266 millibar, about 400 millibar, about 533 millibar. In yet other embodiments, the predetermined set-point is between about 20 millibar and about 533 millibar, between about 20 millibar and about 400 millibar, between about 30 millibar and about 200 millibar, or between about 30 millibar and about 100 millibar.

[0033] When the pressure within the growth chamber 100 reaches or increases to the predetermined pressure or set-point, the sneeze valve 144 is opened, allowing gas flow through the exhaust outlet, and the pressure is released into the exhaust outlets 130. The predetermined set-point is greater than operational pressure of the growth chamber 100. The increased pressure within the exhaust outlets 130 forces the silicon dioxide powder that had accumulated within the exhaust outlets downstream and out of the exhaust outlets. The pressure within the growth chamber 100 is then reduced from the predetermined set-point to a target value. The target value may be less than approximately 35% of the predetermined set-point. In some embodiments, the target value is approximately 25% of the predetermined set-point. In other embodiments, the target value is between approximately 25% and approximately 35%.

[0034] The predetermined set-point and the target value may be dependent upon the crystal growth process that was running prior to the cleaning operation.

[0035] The "feathering phase" is continued without interruption. Upon the pressure within the growth chamber 100 reaching the target value, the operator checks the temperature of the exhaust outlets 130 and if the exhaust outlets are clear, the oxidation is turned on and the crystal growth process is restarted. If the temperature of the exhaust outlets 130 is above a predetermined value, which means the pipes are clogged, the growth chamber 100 is shut down and the exhaust outlets are cleaned manually.

[0036] In a method of another embodiment, the cleaning operation of the exhaust outlets 130 is automated. In operation, the operator activates an "Auto Insitu Sneeze" (AIS) button between crystal growth processes. The AIS button is connected with the programmable device 140 that is programed to turn off the oxidation, close the gas inlets 126, shut off a feeder (not shown), and close the sneeze valve 144. The pressure within the growth chamber 100 then increases to the predetermined set-point. [0037] When the pressure within the growth chamber 100 reaches the predetermined set- point, the sneeze valve 144 is opened and the pressure is released into the exhaust outlets 130, as discussed above.

[0038] In some embodiments, the programmable device is programmed to automatically perform the insitu sneeze cleaning operation after each crystal growth process. In other embodiments, the programmable device is programmed to automatically perform the insitu sneeze cleaning operation after a predetermined set of crystal growths. In still other

embodiments, the programmable device is connected with temperature sensors connected with the exhaust outlets and is configured to automatically perform the insitu sneeze cleaning operation after a crystal growth process when the temperature sensor measures a temperature that indicates the exhaust outlets are clogged or contain an accumulation of contaminates.

[0039] In still other embodiments, the operational pressure within the growth chamber and the exhaust outlet is increased during growth of a crystal ingot. In these embodiments, the increased operational pressure is greater than normal operational pressure of the growth chamber but is less than the set-point of the sneeze valve. This increased operational pressure causes an increase in the flow of gas through the exhaust outlet for maintaining the exhaust outlet within normal operational limits.

[0040] Use of the above embodiments reduces the amount of contaminates in the exhaust outlets and thereby reduces the need to shut down the growth chamber for cleaning the exhaust outlets. As described above, the operation may be automated to inhibit the accumulation of contaminates within the exhaust outlets. Additionally, use of the above embodiments significantly reduces the time the crystal growth system is inoperative. This reduction in risk and improved efficiency not only increases the overall production of the crystal growing system, but also lowers overall operational costs.

[0041] When introducing elements of the present disclosure or the embodiments thereof, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of the elements. The terms "comprising", "including" and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of terms indicating a particular orientation (e.g., "top", "bottom", "side", etc.) is for convenience of description and does not require any particular orientation of the item described. [0042] As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

What is claimed is:

1. A system for growing a crystal ingot from a silicon melt, the system comprising:

a furnace for supplying heat to the silicon melt;

an exhaust outlet connected with the furnace to remove at least one of inert gas, contaminants, and evaporated species from within the furnace;

a sneeze valve connected with the exhaust outlet for inhibiting flow through the exhaust outlet;

a programmable device connected with the sneeze valve for regulating the operation of the valve.

2. The system of claim 1, further comprising a temperature sensor connected with the exhaust outlet, the programmable device being connected with the temperature sensor to receive a measured temperature signal therefrom for determining the exhaust outlet is one of clogged or partially clogged by contaminants.

3. The system of claim 1 or 2, wherein the programmable device is connected with the sneeze valve to close the sneeze valve based on a determination that the exhaust outlet is one of clogged or partially clogged.

4. The system of claim 2 or 3, wherein the programmable device is programmed to close the sneeze valve upon receiving a measured temperature signal from the temperature sensor that meets or exceeds a predetermined threshold.

5. The system according to any of the claims 1-4, further comprising a pressure regulating device connected with the exhaust outlet to adjust operational pressure within the exhaust outlet and the furnace.

6. The system of claim 5, wherein the programmable device is connected with the pressure regulating device to adjust operation pressure within the exhaust outlet and the furnace during growth of the crystal ingot.

7. The system according to any of the claims 1-6, further comprising a pump to remove at least one of inert gas, contaminants, and evaporated species from within the furnace.

8. The system of claim 7, wherein the programmable device is connected with the pump to adjust operational pressure within the exhaust outlet and the furnace during growth of the crystal ingot.

9. A method for growing a crystal ingot in a growth chamber having an exhaust outlet, the method comprising:

supplying an inert gas to the growth chamber;

removing contaminates from the growth chamber by gas flow through the exhaust outlet;

obstructing the exhaust outlet to inhibit gas flow therethrough and to increase pressure within the growth chamber and exhaust outlet; and

allowing gas flow through the exhaust outlet.

10. The method of claim 9, wherein the exhaust outlet includes a sneeze valve for obstructing the exhaust outlet.

11. The method of claim 10, wherein the step of allowing gas flow includes opening the sneeze valve at a predetermined pressure.

12. The method of claim 1 1, wherein opening the sneeze valve is controlled by a programmable device.

13. The method according to any of the claims 9-12, further comprising measuring a temperature of the gas flow through the exhaust outlet to determine that the exhaust outlet is clogged or partially clogged by contaminants.

14. The method of claim 13, wherein the step of obstructing is performed upon measuring a temperature that exceeds a predetermined threshold.

15. The method of claim 14, wherein the exhaust outlet includes a sneeze valve for obstructing the exhaust outlet, and the step of obsructing is performed by closing the sneeze valve.

16. The method according to any of the claims 1-15, further comprising growing a first crystal ingot and growing a second crystal ingot, the steps of obstructing and allowing gas flow is performed between growing the first crystal ingot and growing the second crystal ingot.

17. The method according to any of the claims 1-16, further comprising increasing the pressure within the growth chamber to a predetermined set-point, the

predetermined set-point being greater than operational pressure of the growth chamber.

18. The method according to any of the claims 1-17, further comprising increasing operational pressure within the growth chamber and the exhaust outlet during growth of a crystal ingot.

19. The method of claim 18, wherein the step of increasing operational pressure is performed by one of a pressure regulating device and a pump connected with the exhaust outlet.