Patent Application
20120125466
This invention relates generally to the field of treating the surfaces of substrates in a furnace to enhance the subsequent deposition of thin films or to enhance the reaction of the surfaces with gas phase components.
Early integrated circuits used diffusion processes to incorporate dopant atoms into the silicon surface to change the electrical characteristics and the conductivity of the silicon. Common dopant atoms included phosphorous, boron, arsenic, and antimony, but others were used as well. This step in the integrated circuit manufacturing process is commonly known as the “diffusion” step. The diffusion behavior of common dopant atoms has been well characterized in both multi-crystalline and single crystal silicon substrates. Factors that influence the diffusion behavior of dopant atoms comprise dopant source concentration, silicon crystal structure (i.e. multi-crystalline or single crystal), crystal surface orientation for single crystal applications, the presence and concentration of pre-existing doping in the substrate surface, dopant exposure time, peak temperature and temperature history, time held at peak temperature, surface cleanliness, and the presence of a thin film at the substrate surface (i.e. native oxide or thin deposited layers). The resulting concentration of the dopant atom as a function of depth into the substrate surface after the diffusion process is well known in the art.
The advancement of integrated circuit technology and the subsequent requirements for high concentrations of dopant atoms constrained to shallow depths from the surface led to the diffusion step being replaced by an ion implantation process followed by a rapid thermal anneal (RTA) process used to activate the implanted dopant species. Therefore, diffusion processes have not been used in leading edge integrated circuits for many years.
Photovoltaic (PV) solar cells based on silicon technology have been known for many decades and have recently begun to be manufactured in high volume. Wafer based PV solar cells based on other material systems are also well known, but have not progressed to high volume manufacture. Recent trends such as high costs of conventional energy sources, lower materials costs, lower equipment costs, improved solar cell efficiency, and an increased adoption of renewable energy technologies has allowed the use of wafer based PV solar cells to become cost competitive with conventional energy sources in the generation of electricity. The concentration of the dopant atom as a function of depth into the substrate surface for wafer based PV solar cells is not as stringent as that of silicon based integrated circuits. Therefore, diffusion processes are often used to introduce dopant atoms into the wafer surface to change the electrical characteristics and the conductivity of the substrate.
The efficiency of the wafer based PV solar cells is sensitive to many factors. Some of these factors comprise the presence of contaminants in the bulk silicon, silicon crystal structure (i.e. multi-crystalline or single crystal), the presence and concentration of pre-existing doping in the substrate surface, dopant exposure time, peak temperature and temperature history, time held at peak temperature, surface cleanliness, the presence of a thin film at the substrate surface (i.e. native oxide or thin deposited layers), the concentration of carrier recombination centers on the surface, the concentration of carrier recombination centers within the bulk of substrate, and the PV solar cell architecture.
The profitability of the wafer based PV solar cell manufacturing process and the ultimate cost per watt of electricity produced (i.e. $/watt) is sensitive to the number and complexity of the various steps used to produce the wafer based PV solar cell. A balance and compromise must be made between optimized efficiency and the manufacturing cost to produce the solar cell. Trade-offs must be made between complicated and costly cleaning and pre-treatment steps before the diffusion step and the final manufacturing cost of the solar cell. For example, the success of the diffusion step may be enhanced by ensuring that the surfaces of the substrates are freshly cleaned and free of contaminants and native oxide layers. This can be accomplished by cleaning the substrates in a hydrofluoric acid (HF) solution (for silicon based PV solar cells) and then completing the diffusion process within a specified amount of time. This may be impractical or too costly in a large PV solar cell factory where tens of millions of solar cells are manufactured each year. A great benefit would be realized if fast, cost effective processes could be developed to properly treat the surface of the substrates before or during the solar cell manufacturing process.
Therefore, a need exists in the art for the development of apparatus and methods that facilitate the pre-treatment, cleaning, conditioning, etc. of the surface of the substrates prior to many of the steps used in the manufacture of wafer based PV solar cells.
Accordingly and advantageously the present invention provides apparatus and methods that allow the surface of the substrate to be pre-treated, cleaned, conditioned, etc. prior to a diffusion, annealing, or deposition step in the manufacturing process of a wafer based solar cell. The apparatus and methods also facilitate the inclusion of a pre-treatment, cleaning, conditioning, etc. step as an integral part of a multi-step process that might be accomplished within a single diffusion, annealing, or deposition apparatus.
In some embodiments of the present invention, an exhaust system is provided as part of a furnace system that allows the pressure within the tube to be reduced to facilitate the introduction of a gas used to pre-treat, clean, condition, etc. the surface of the substrates. The exhaust system comprises components that allow the pre-treatment, cleaning, conditioning, etc. exhaust flow to be diverted to a specific gas treatment system while the exhaust flow during the diffusion, annealing, or deposition process is diverted to a separate gas treatment system. The selection of the pathway of the exhaust flow is typically controlled by the control system of the furnace and may be selected for each step of a typical recipe.
In some embodiments of the present invention, a door closure system is provided as part of a furnace system that provides an active inert gas purge of the door region during a diffusion, annealing, deposition, pre-treatment, cleaning, conditioning, etc. step of a furnace process. The inert gas purge acts to prevent reactive gases from escaping the door seal of the furnace during the above mentioned processes. Additionally, the inert gas purge acts to prevent the possible deposition of material in the door seal region of the furnace or the corrosion of the door seal region of the furnace. In some embodiments of the present invention, several examples of inert gas purge systems are provided.
In some embodiments of the present invention, a door closure system is provided as part of a furnace system that allows the pressure within the tube to be reduced to facilitate the introduction of a gas used to pre-treat, clean, condition, etc. the surface of the substrates. The door closure system typically provides a seal with a leak rate that falls within a range selected during the design of the furnace. The leak rate range is selected so that the leak rate range is less than between one-quarter and one-half of the exhaust flow imparted by the exhaust system. This provides a seal that prevents the pre-treatment, cleaning, conditioning, etc. gas from escaping the tube in the door region. Furthermore, this provides a seal that facilitates a flow of air from outside the tube, through the seal, and into the tube to act as a purge of the door seal area during the pre-treatment, cleaning, conditioning, etc. step. In some embodiments of the present invention, several examples of door closure systems are provided.
These and other advantages are achieved in accordance with the present invention as described in detail below.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The drawings are not to scale and the relative dimensions of various elements in the drawings are depicted schematically and not to scale.
The techniques of the present invention can readily be understood by considering the following detailed description in conjunction with the accompanying drawings, in which:
After considering the following description, those skilled in the art will clearly realize that the teachings of the invention can be readily utilized in the design and operation of apparatus used in diffusion, annealing, and deposition steps of the manufacture of wafer based solar cells.
During the diffusion, anneal, or deposition steps, the exhaust gases are conveyed along the exhaust tubing and exit through the process exhaust valve, 104. During the diffusion, anneal, or deposition steps, valve 107 is open and valve 106 is closed. The exhaust line leading from process exhaust valve, 104, leads to exhaust gas treatment facilities configured to safely treat the exhaust gases present during the diffusion, anneal, or deposition steps. Examples of suitable gas treatment facilities include high volume dilution, wet scrubbers, burn boxes, gas reaction columns, gas absorbers, and the like. Drain trap, 105, is used to collect condensable by-products from the diffusion, anneal, or deposition process step. Pressure monitor, 103, is used to monitor the pressure of the process tube during this step. For example, during the doping of silicon using phosphorous oxy-chloride (POCl3) as a source of phosphorous, condensable by-products are typically trapped in a system such as drain trap, 105.
During the pre-treatment, cleaning, conditioning, etc. step, process exhaust valve, 104, is closed and valve 107 is closed. Valve, 106, is opened and the exhaust gases are conveyed along the exhaust tubing and exit through the valve, 106. The exhaust line leading from process exhaust valve, 106, leads to a source of vacuum and then to exhaust gas treatment facilities configured to safely treat the exhaust gases present during the pre-treatment, cleaning, conditioning, etc. step. Examples of suitable vacuum include high flow process exhaust fans, venture pumps, mechanical vacuum pumps, and the like. Examples of suitable gas treatment facilities include high volume dilution, wet scrubbers, burn boxes, gas reaction columns, gas absorbers, and the like. Pressure monitor, 108, is used to monitor the pressure in the exhaust line and the furnace tube. Typically, it is advantageous to reduce the pressure within the tube below one atmosphere during the pre-treatment, cleaning, conditioning, etc. step.
An alternate door closure system incorporating a known leak rate range capability is also illustrated in
An alternate door closure system incorporating a known leak rate range capability is also illustrated in
In some embodiments of the present invention, methods of implementing the exhaust system disclosed above are used to facilitate the use different process steps with different pressure and gas treatment requirements in a furnace. Typically, diffusion and annealing steps in furnaces for the manufacture of wafer based solar cells are executed at about atmospheric pressure. Simple exhaust systems are used to convey the exhaust gases to appropriate gas treatment systems. Typically, deposition steps in furnaces for the manufacture of wafer based solar cells are executed at reduced pressures. Simple vacuum systems are used to convey the exhaust gases to appropriate gas treatment systems. If a different step in the diffusion, annealing or deposition process such as a pre-treatment, cleaning, conditioning, etc. step requires a different pressure or an alternate gas treatment technology, some embodiments of the present invention allow the exhaust gases to be re-directed to an alternate exhaust path with additional equipment for controlling the process pressure and appropriate gas treatment technologies. Using
Method-1—Surface Pre-Treatment, Cleaning, Conditioning, etc. Before the Diffusion, Annealing, or Deposition Step
Method-2—Surface Pre-Treatment, Cleaning, Conditioning, etc. During Multiple Diffusion, Annealing, or Deposition Steps
In this example, “Method-1” as described above and the apparatus illustrated in
Silicon based wafers are commonly used as substrates for wafer based solar cells. The silicon wafers may be multi-crystalline or single crystal. Typically, there is a diffusion step that is used to dope the wafer with phosphorous atoms during the solar cell manufacturing process. POCl3 is commonly used as the source of the phosphorous atoms. This process is typically executed at about atmospheric pressure. Before the POCl3 diffusion process, the wafers are typically cleaned in wet chemical equipment that is well known in the industry. However, it is not uncommon for the wafers to arrive at the POCl3 diffusion step with a thin native oxide (SiOx) layer on the surface that grows due to the exposure to air. The POCl3 diffusion process and ultimate solar cell efficiency may be enhanced if the diffusion step can be executed in a silicon surface that is free of contaminants and native oxide layers. One method to realize this goal is to implement an apparatus and method that allows the surface of the silicon wafer to be pre-treated, cleaned, conditioned, etc. before the POCl3 diffusion step.
Typically, the silicon wafers are removed from cassettes and placed into quartz boats. The quartz boats are then loaded into the furnace at about 600 degrees Celsius. The furnace is purged with an inert gas such as N2 to remove any air that may have been introduced during the loading process. The temperature is then ramped to the desired temperature for the pre-treatment, cleaning, conditioning, etc. step. In this example, a cleaning step will be described. Valve 107 is closed and valve 106 is opened to direct the exhaust flow to the proper pressure control equipment and gas treatment equipment. The cleaning step is typically executed at reduced pressure and is facilitated by directing the exhaust flow to a vacuum pump as is well known in the art. The vacuum pump serves to reduce the pressure in the process tube to the desired level. The effluent of the vacuum pump is directed to an appropriate gas treatment technology for the cleaning gas employed. Once the flow, pressure, and temperature of the furnace are established for the cleaning step, the inert gas flow is stopped and the reactive gas is introduced to begin the cleaning process. NF3 is a good candidate as a cleaning gas because it will thermally decompose at 600 degrees Celsius and provide reactive fluorine atoms. The fluorine atoms are effective at removing the native oxide layer. The NF3 is typically diluted in a suitable inert. Other examples of cleaning gases comprise HF, CF4, F2, Cl2, ClF3, etc. Door closure systems as illustrated in
At the end of the cleaning step, the reactive gas flow is stopped and an inert gas purge is initiated to remove the reactive gas from the process tube. During this step, the temperature is ramped to the diffusion, annealing, or deposition step temperature. In this example, a diffusion step will be described. Once the target temperature is reached and stabilized, valve 106 is closed and valves 107 and 104 are opened to direct the exhaust flow to the proper pressure control equipment and gas treatment equipment for the diffusion step. Typically, the pressure is increased to about atmospheric pressure for a diffusion step. The inert gas flow is stopped and the reactive gas is introduced to begin the diffusion process. POCl3 is a good candidate as a diffusion gas because it will thermally decompose and provide reactive phosphorous atoms. Other reactive gases such as O2 are also typically used with the POCl3 to facilitate the diffusion process. Other examples of diffusion gases comprise PH2, BBr3, B2H6, AsH3, etc. Door closure systems as illustrated in
At the end of the diffusion step, the reactive gas flow is stopped and an inert gas purge is initiated to remove the reactive gas from the process tube. During this step, the temperature is ramped to the unload temperature. When the unload temperature is reached, the quartz boats are removed from the furnace and allowed to cool. After cooling, the wafers are removed from the quartz boats and loaded into cassettes for transport to the next manufacturing process step.
In this example, “Method-2” as described above and the apparatus illustrated in
Silicon based wafers are commonly used as substrates for wafer based solar cells. The silicon wafers may be multi-crystalline or single crystal. Typically, there is a diffusion step that is used to dope the wafer with phosphorous atoms during the solar cell manufacturing process. POCl3 is commonly used as the source of the phosphorous atoms. This process is typically executed at about atmospheric pressure. The POCl3 diffusion process may be used multiple times to dope the surface of the wafers with varying concentrations of phosphorous atoms. The exposure of the wafer surface to POCl3 and O2 results in the formation of an SiO2 layer that is doped with phosphorous atoms. This layer must be removed before the next step in the manufacturing process. If a second diffusion step is desired, the wafer must be removed from the furnace before the second diffusion step is executed. This increases the time and cost for the manufacture of the solar cell. One method to realize this goal is to implement an apparatus and method that allows the surface of the silicon wafer to be pre-treated, cleaned, conditioned, etc. between the two POCl3 diffusion steps.
Typically, the silicon wafers are removed from cassettes and placed into quartz boats. The quartz boats are then loaded into the furnace at about 600 degrees Celsius. The furnace is purged with an inert gas such as N2 to remove any air that may have been introduced during the loading process. The temperature is then ramped to the desired temperature for the first diffusion, annealing, or deposition step. In this example, a diffusion step will be described. During this step, the temperature is ramped to the first diffusion step temperature. During this step, valves 107 and 104 are open and valve 106 is closed to direct the exhaust flow to the proper pressure control equipment and gas treatment equipment for the first diffusion step. Typically, the pressure is about atmospheric pressure for a diffusion step. The inert gas flow is stopped and the reactive gas is introduced to begin the diffusion process. POCl3 is a good candidate as a diffusion gas because it will thermally decompose and provide reactive phosphorous atoms. Other reactive gases such as O2 are also typically used with the POCl3 to facilitate the diffusion process. Other examples of diffusion gases comprise PH2, BBr3, B2H6, AsH3, etc. Door closure systems as illustrated in
At the end of the first diffusion step, the reactive gas flow is stopped and an inert gas purge is initiated to remove the reactive gas from the process tube. During this step, the temperature is ramped to the pre-treatment, cleaning, conditioning, etc. step. The temperature is then ramped to the desired temperature for the pre-treatment, cleaning, conditioning, etc. step. In this example, a cleaning step will be described. Valve 107 is closed and valve 106 is opened to direct the exhaust flow to the proper pressure control equipment and gas treatment equipment. The cleaning step is typically executed at reduced pressure and is facilitated by directing the exhaust flow to a vacuum pump as is well known in the art. The vacuum pump serves to reduce the pressure in the process tube to the desired level. The effluent of the vacuum pump is directed to an appropriate gas treatment technology for the cleaning gas employed. Once the flow, pressure, and temperature of the furnace are established for the cleaning step, the inert gas flow is stopped and the reactive gas is introduced to begin the cleaning process. NF3 is a good candidate as a cleaning gas because it will thermally decompose and provide reactive fluorine atoms. The fluorine atoms are effective at removing the native oxide layer. The NF3 is typically diluted in a suitable inert. Other examples of cleaning gases comprise HF, CF4, F2, Cl2, ClF3, etc. Door closure systems as illustrated in
At the end of the cleaning step, the reactive gas flow is stopped and an inert gas purge is initiated to remove the reactive gas from the process tube. During this step, the temperature is ramped to the second diffusion, annealing, or deposition step temperature. In this example, a second diffusion step will be described. Once the target temperature is reached and stabilized, valve 106 is closed and valves 107 and 104 are opened to direct the exhaust flow to the proper pressure control equipment and gas treatment equipment for the second diffusion step. Typically, the pressure is increased to about atmospheric pressure for the second diffusion step. The inert gas flow is stopped and the reactive gas is introduced to begin the second diffusion process. POCl3 is a good candidate as a diffusion gas because it will thermally decompose and provide reactive phosphorous atoms. Other reactive gases such as O2 are also typically used with the POCl3 to facilitate the diffusion process. Other examples of diffusion gases comprise PH2, BBr3, B2H6, AsH3, etc. Door closure systems as illustrated in
At the end of the second diffusion step, the reactive gas flow is stopped and an inert gas purge is initiated to remove the reactive gas from the process tube. During this step, the temperature is ramped to the unload temperature. When the unload temperature is reached, the quartz boats are removed from the furnace and allowed to cool. After cooling, the wafers are removed from the quartz boats and loaded into cassettes for transport to the next manufacturing process step.
In the examples discussed above, cleaning and diffusion steps were described. Those skilled in the art will understand that the cleaning step may be substituted with other known pre-treatment or conditioning steps and still fall within the scope of the present invention. Likewise, those skilled in the art will understand that the diffusion step may be substituted with other known annealing or deposition steps and still fall within the scope of the present invention.
Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.
This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/405,418, filed Oct. 21, 2010, entitled “Apparatus and Method for Surface Treatment in a Furnace”, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61405418 | Oct 2010 | US |
