Patent Application
20100080902
The present invention relates generally to chemical vapor deposition (CVD) reactor, and more particularly relates to low cost production of polycrystalline silicon.
One of the widely practiced conventional methods of polysilicon production is by depositing polysilicon in a CVD reactor, and is generally referred as Siemens method. In this method, polysilicon is deposited in the CVD reactor on high-purity, electrically heated thin silicon rods called “slim rods”. The reactor used for this purpose is referred to as a “cold walled reactor”.
The reactor walls are maintained by circulating water around the periphery of the reactor to take away the heat generated in the reactor by the hot silicon rods. The silicon rods are kept at temperature well above 1000° C. Since no other surface in the reactor can be kept hot as silicon can deposit on any hot surface approximately above 450° C., cooling the reactor walls is generally required to prevent silicon from depositing on the reactor walls. Further, insulating media cannot be used in the reactor for the same reason as the insulating media can get heated, resulting in possibility of contaminating the product.
While circulating cold water solves the above problems and has been the generally practiced state of the art for the past few decades, the water may also take away significant amount of energy needed to heat the silicon rods and hence the reactor may require more electrical energy to heat the silicon rods and keep them in the operating temperature. Generally, it takes several tens of kilowatt hours of energy to produce a kilogram of silicon thus making the cost of production of silicon also significantly expensive. For large polysilicon plants, it becomes necessary to set up captive power plants to operate the reactors to produce polysilicon. This can cause a significant additional capital expense and operating cost for the polysilicon plant.
In addition, it can be seen that the above process may require large amount of water to operate the reactors during polysilicon production. Even though most of the water is re-circulated, when cooled through a cooling tower to remove the heat that is extracted, a considerable amount of water evaporates and the polysilicon plant can require replenishing the water for continuous use. Furthermore, the water has to be treated for correct mineral content and pH values, which can also significantly increase the cost of polysilicon production
A method and apparatus for low cost production of polysilicon using Siemen's reactors is disclosed. According to an aspect of the present invention, a chemical vapor deposition (CVD) reactor assembly includes a CVD reactor, a steam generator, and a steam turbine/generator. Further, the CVD reactor includes a base plate including a process gas inlet port and a process gas outlet port coupled to a process gas inlet valve and a process gas outlet valve, respectively, a reactor forming a stainless steel envelope attached to the base plate so as to form a closed stainless steel enclosure, one or more power electrodes attached to the base plate, one or more silicon rods disposed substantially in the stainless steel envelope and electrically coupled to the one or more power electrodes, and at least one heating element disposed substantially in the middle of the one or more silicon rods and coupled to the base plate.
Further, the stainless steel envelope is designed to receive a thermal fluid at room temperature and maintain a reactor wall temperature up to 450° C. For example, the thermal fluid is capable of maintaining reactor wall temperature of up to 450° C. Also, the reactor includes a thermal fluid inlet port and a thermal fluid outlet port. The at least one heating element emits radiant heat having a color temperature of at least 1800° C.
The enclosed CVD reactor assembly also includes the steam generator configured to receive the thermal fluid having a temperature of up to 450° C. from the reactor and to generate a low pressure steam around 350° C. to 450° C. upon the reactor wall reaching sufficient temperature during operation of the CVD reactor assembly. In one embodiment, the low pressure steam is used to generate electricity using low RPM (revolutions per minute) steam turbines/generators. In some embodiments, the low pressure steam is converted to super-heated steam by using an external heat source. In another embodiment, the super-heated steam is used to generate power using conventional steam turbines/generators. In one example embodiment, the enclosed CVD reactor assembly includes the steam turbine/generator configured to receive the low pressure steam/super-heated steam and to generate electricity.
Furthermore, the temperature drop in the low pressure steam/super-heated steam, which is used to operate the steam turbine/generator, manifests itself as water (i.e., condensed steam) and this condensed steam can be re-circulated back to the steam generator to exchange the heat from the thermal fluids. In addition, the thermal fluid taken out from the steam generator can be re-circulated back to the CVD reactor.
According to another aspect of the present invention, a method for production of bulk polysilicon in the CVD reactor assembly includes circulating a thermal fluid substantially around a reactor wall of the stainless steel envelope and through a steam generator to maintain the reactor wall temperature up to 450° C., evacuating the stainless steel envelope to have substantially low oxygen content, applying sufficient current using a high-voltage power supply to raise the one or more silicon rods to a firing temperature (e.g., in the range of 1000° C. to 1400° C.), applying sufficient current using a low-voltage power supply to the at least one heating element until the one or more silicon rods reach a deposition temperature (e.g., 1100° C.) of the process gas and upon a silicon reactant material reaching the firing temperature, and turning off the high-voltage power supply upon the one or more silicon rods reaching the firing temperature.
The method further includes flowing process gas (H2) ladened with the silicon reactant material via the process gas inlet port, generating low pressure steam using the steam generator upon the reactor wall reaching sufficient temperature during operation of the CVD reactor assembly, and inputting the generated low pressure steam into a steam turbine/generator to generate electricity, depositing silicon on the one or more silicon rods to form a bulk polysilicon product, flowing gaseous byproducts of the CVD process out through the process gas outlet port, and removing the bulk polysilicon product from the closed stainless steel enclosure.
The systems and apparatuses disclosed herein may be implemented in any means for achieving various aspects. Other features will be apparent from the accompanying drawings and from the detailed description that follows.
Example embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:
Other features of the present embodiments will be apparent from the accompanying drawings and from the detailed description that follows.
A method and apparatus for low cost production of polysilicon using Siemen's reactors is disclosed. In the following detailed description of the embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.
As shown in
Moreover as shown in
In one embodiment, the steam generator 170 is configured to receive the thermal fluid having the temperature of up to 450° C. from the reactor 120 and to generate a low pressure steam around 350° C. to 450° C. In one embodiment, the low pressure steam is used to generate electricity using low RPM (revolutions per minute) steam turbines or low pressure steam turbines/generators, such as the steam turbine/generator 175 shown in
As shown in
Also, the CVD reactor 102 includes the heating element 110 disposed substantially in the middle of the silicon rods 105. As shown in
In one example embodiment, the heating element 110 is a thin filament made from high purity tungsten, tantalum, molybdenum, or silicon carbide. Further, the thin filament is coated with a substantially thin layer of silicon to prevent any exposure of element to process gases. In these embodiments, the process gas is hydrogen (H2). Further, the thin filament is coupled to the power electrodes 115 that supply power. For example, the thin filament is disposed in spiral, elliptical, rectangular, square shapes and the like.
Further as shown in
In operation, the heating element 110 is used for heating the silicon rods 105 during startup, in the CVD reactor 102. In these embodiments, the heating element 110 is configured to be disposed substantially in the middle of the silicon rods 105. For example, the heating element 110 emits radiant heat having a color temperature of approximately 1800° C. Further, the thermal fluid is circulated substantially around a reactor wall of the stainless steel envelope and through the steam generator 170 to maintain the reactor wall temperature up to 450° C.
Further in operation, current sufficient for raising the silicon rods 105 to a firing temperature is applied to the heating element 110 using the high voltage power supply (e.g., the high/low voltage power supply 155). In one example embodiment, the firing temperature is in the range of about 1000° C. to 1400° C. Further, the low-voltage power supply (e.g., the high/low voltage power supply 155) applies sufficient current to the heating element 110 until the silicon rods 105 reach a deposition temperature of the process gas and upon a silicon reactant material reaching the firing temperature. In one example embodiment, the deposition temperate is about 1100° C. In one embodiment, the high voltage power supply is turned off upon the one or more silicon rods 105 reaching the firing temperature.
As shown in
Furthermore, the temperature drop in the low pressure steam/super-heated steam, which is used to operate the steam turbine/generator 175, manifests itself as water (i.e., condensed steam) and this condensed steam can be re-circulated back to the steam generator 170 to exchange the heat from the thermal fluids. In addition, the thermal fluid taken out from the steam generator 170 can be re-circulated back to the CVD reactor 102.
Further in operation, the process gas (i.e., H2) ladened with the silicon reactant material is flown through the process gas inlet port 130 coupled to the process gas inlet valve 140. In these embodiments, the gaseous byproducts obtained during the CVD process are flown out through the process gas outlet port 135. Finally, the bulk polysilicon product obtained during the CVD process in the CVD reactor 102 is removed from the closed stainless steel enclosure.
In the example embodiment illustrated in
In another embodiment, the hot thermal fluid (e.g., up to 450° C.) that is removed from the reactor 120 is sent to the steam generator 170 where heat from the thermal fluid is exchanged with the water to raise the water temperature from 30° C. to a low pressure steam temperature of 350° C. to 450° C. Further, the low pressure steam is converted to super-heated steam by using heat from the external source 180 and various hot gasses generated during production of bulk polysilicon. The generated low pressure steam/super-heated steam is then sent to the steam turbine/generator 175 which converts the low pressure steam/super-heated steam to electric power. As shown in
For example, a typical 250 MT capacity reactor can require about 3500 KWh/hr of energy. Assuming that about 60% of the heat from the reactor is removed using the thermal fluid, one skilled in the art can understand that, about 2000 kWh/hr of energy is removed from the reactor. This is during normal operation of the reactor. In one example embodiment, by maintaining heat at 400° C., lesser amount of heat is being removed from the reactor walls since the radiation loss will be considerably less. This results in using significantly lesser power for running the CVD reactor 102. The above mentioned process can be used to reactor of any size and energy extracted depending on the design of the reactor.
Further, each reactor can be running for about 100 to 180 hours per batch, depending upon the efficiency of the process, the types of gases used, and so on. It can be seen that nearly 300 MWh of energy can be produced for each cycle, assuming an average of 150 hour process time for each reactor.
Further, the power produced at the steam turbine/generator 175 depends on the generated steam temperature. Generally, for low power generation, approximately 22 tons of low pressure steam is required to produce about 5 MW of power. Obtaining 22 tons of low pressure steam is an attractive proposition for large polysilicon plants, operating with a number of reactors. Further, the heat output from several reactor banks can be tied together to the steam generator 170 and to the steam turbine/generator 175 to produce additional power. Further, the additional power produced by the steam turbine/generator 175 can be fed back to the grid to significantly lower the cost of production of polysilicon. The above process can result in savings of at least 60% in the energy cost, which can reduce the cost of producing polysilicon by at least 20%.
As shown in
Further as shown in
In one example embodiment, the stainless steel envelope is designed to receive the thermal fluid at room temperature (e.g., through the thermal fluid inlet port 160) and maintain a reactor wall temperature up to 450° C. The thermal fluid having a temperature of up to 450° C. is extracted from the reactor 120 upon the reactor wall reaching sufficient temperature during operation of the CVD reactor assembly 200 and sent to the steam generator 170 to generate low pressure steam. In one embodiment, the steam generator 170 is configured to receive the thermal fluid having the temperature of up to 450° C. from the reactor 120 and to generate the low pressure steam around 350° C. to 450° C. In one embodiment, the low pressure steam turbine/generator 175 is used to convert the low pressure steam into electric power. In some embodiments, the low pressure steam is converted to super-heated steam by using heat from the external source 180 and various hot gasses generated during the production of bulk polysilicon. In one embodiment, the super-heated steam is used to generate power using conventional steam turbine/generators. Further, the steam turbine/generator 175 is configured to receive the generated low pressure steam/super-heated steam and to convert the low pressure steam/super-heated steam to electric power.
The CVD reactor 202 further includes the one or more power electrodes 115 attached to the base plate 125. Also, the CVD reactor 202 includes one or more silicon rods 105 disposed substantially in the stainless steel envelope and electrically coupled to the one or more power electrodes 115. In addition, the CVD reactor 202 includes the heat radiation system 205 that is annularly disposed in the reactor 120 having at least one heating element which emits thermal radiation having a color temperature of at least 2000° C.
In operation, the heat radiation system 205 irradiates the silicon rods 105 with thermal radiation having a color temperature of at least 2000° C. The radiant heat is applied using the at least one heating element to the closed stainless steel enclosure sufficient for raising the one or more silicon rods 105 to a firing temperature. The irradiation is terminated when a particular electrical voltage applied to the silicon rods 105 causes a specified current to flow. The method of production of bulk polysilicon is similar to the method illustrated in
In the example embodiment illustrated in
The thermal fluid at the thermal fluid outlet port 165 of the reactor wall, which typically is around 400° C., is inputted to the steam generator 170. The steam generator 170 exchanges the heat from the thermal fluid (e.g., up to 450° C.) to raise the water temperature from 30° C. to the low pressure steam temperature of 350° C. to 450° C.
Further, the low pressure steam is converted to super-heated steam by using the external heat source 180 and various hot gasses generated during the production of bulk polysilicon. Further, the low pressure steam/super-heated steam is then sent to the steam turbine/generator 175 where energy supplied by the low pressure steam/super-heated steam operates the steam turbine/generator 175 to produce electricity. As shown in
Particularly,
In operation, radiant heat is applied by the electric heater assembly 310 until the silicon tube 305 reaches the deposition temperature. Further, radiant heat penetrates through quartz envelope 315 to the silicon tube 305. When the silicon tube 305 reaches the deposition temperature, a process gas is fed into the CVD reactor 302 through the process gas inlet 345.
Further in operation, a thermal fluid around 30° C. is circulated around the base plate 340 and also around any other metal part of the CVD reactor that is exposed to the heat through the thermal fluid inlet port 330 and the thermal fluid around 450° C. is extracted at the thermal fluid outlet port 335. In one example embodiment, the thermal fluid is capable of maintaining reactor wall temperature of up to 450° C. Further, the thermal fluid extracted from the quartz envelope 315 is sent to the steam generator 170 to generate low pressure steam which is fed to the low pressure steam turbine/generator 175.
In step 410, the stainless steel envelope is evacuated to have substantially low oxygen content. In step 415, sufficient current is applied using a high-voltage power supply (e.g., the high/low voltage power supply 155 of
In step 420, the sufficient current is applied using a low-voltage power supply (e.g., the high/low voltage power supply 155 of
In step 430, a process gas (e.g., Hydrogen (H2)) ladened with the silicon reactant material is flown via a process gas inlet port 130. For example, the silicon reactant material includes silane, trichlorosilane, dichlorosilane or silicon tetrachloride. In one example embodiment, the steam generator 170 generates low pressure steam using the thermal fluid extracted from the reactor wall upon the reactor wall reaching sufficient temperature during operation of the CVD reactor assembly. In another example embodiment, various hot gasses generated during production of bulk polysilicon are inputted into the steam generator 170 to generate the low pressure steam. In step 435, the generated low pressure steam is inputted into a low pressure steam turbine/generator 175 to generate electricity. In step 440, power is supplied to an electrical grid using the generated electricity.
In step 445, gaseous byproducts of the CVD process are flown out through the process gas outlet port 135. In step 450, silicon is deposited on the one or more silicon rods 105 to form a bulk polysilicon product. In step 455, the bulk polysilicon product is removed from the closed stainless steel enclosure.
If the heating element 110 is not coated with silicon, then the steps 520 to 535 are performed for coating the heating element 110 with silicon. In step 520, sufficient current is applied (e.g., using a power supply) to the heating element 110 of the closed stainless steel enclosure, sufficient for raising the heating element 110 to a deposition temperature. In one example embodiment, the deposition temperate is about 1100° C. In step 525, a process gas ladened with a silicon reactant material is flown via a process gas inlet port 130. In some embodiments, the process gas is H2 and the silicon reactant material is silane, trichlorosilane, dichlorosilane, silicon tetrachloride, etc.
In step 530, a substantially thin coating of silicon, sufficient to prevent metal exposure on the heating element 110 is formed. In step 535, flow of the silicon reactant material is stopped upon forming the substantially thin coating of silicon, sufficient to prevent the metal exposure on the heating element 110.
In step 515, if the heating element 110 is coated with silicon, then step 540 is performed directly without performing the steps 520 to 535. The process 500 goes to the step 540 either from step 515 or from step 535, based on the determination made in step 515.
In step 540, radiant heat using the at least one heating element (e.g., through the heat radiation system 205 of
In step 565, process gas ladened with silicon reactant material is flown via the process gas inlet port 130. In step 570, gaseous byproducts of the CVD process are flown out through a process gas outlet port 135. In step 575, the bulk polysilicon product is removed from the closed stainless steel enclosure. In one example embodiment, silicon is deposited on the one or more silicon rods 105 to form a bulk polysilicon product.
In step 615, sufficient current is applied using a high-voltage power supply (e.g., low/high voltage power supply 155 of
In step 625, the high-voltage power supply is turned off upon the one or more silicon rods 105 reaching the firing temperature. In step 630, the process gas ladened with the silicon reactant material is flown via the process gas inlet port 130. In step 635, various hot gasses generated during production of bulk polysilicon are inputted along with an external heat source 180 into the steam generator 170 to generate super heated steam. In step 640, the generated super heated steam is inputted into a steam turbine/generator 175 to generate electricity.
In step 645, power is supplied to an electrical grid using the generated electricity. In step 650, gaseous byproducts of the CVD process are flown out through the process gas outlet port 135. In step 655, silicon is deposited on the one or more silicon rods 105 to form a bulk polysilicon product. In step 660, the bulk polysilicon product is removed from the closed stainless steel enclosure.
The above described method generates power from polysilicon reactors during production of polysilicon using the Siemen's process. The above described method also saves water that is lost by evaporation to the atmosphere at the cooling tower. Currently, power is similarly generated in nuclear reactors. However, the operating temperatures in nuclear reactors are significantly higher and the fluids used for heat exchange are different. Further, the operating pressure is also high. In the above described process, the operating temperature is not very high and hence the operating pressures are low and much more stream is required to generate the steam for power generation.
Generally, polysilicon production is a batch process and therefore a number of reactors can be coupled together and their outputs are sent to a single steam generator and a steam turbine/generator. In one example embodiment, a plant operating with about 50 reactors can have 10 reactors coupled together to each steam generator and a steam turbine/generator. In this case, 5 steam turbine/generators can be coupled to a common grid.
Further, the polysilicon plant will have a number of other types of reactors for producing various gases which are used in the polysilicon reactors. It can be seen that all the heat generated in the plant can be diverted to these steam generators to produce more power, thereby, significantly reducing the auxiliary power load for the entire plant.
Although the present embodiments have been described with reference to specific example embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the various embodiments. For example, the various devices, modules, analyzers, generators, etc. described herein may be enabled and operated using hardware circuitry (e.g., CMOS based logic circuitry), firmware, software and/or any combination of hardware, firmware, and/or software (e.g., embodied in a machine readable medium).
