Abstract:
A method and apparatus for heat treating a photovoltaic device. The apparatus includes a heating module, a processing module, and a cooling module in which the operating temperatures of the modules may be controlled separately. The heating module is configured to pre-heat a substrate and stabilize the substrate at the desired target temperature, the processing module is configured to thermally process the substrate, and the cooling module is configured for post-treatment cooling of the substrate.

Description:
[0001]    This application claims the benefit of priority of U.S. Provisional Patent Application No. 61/590,616, filed Jan. 25, 2012, entitled: “Method and Apparatus Providing Separate Modules For Processing a Substrate,” the entirety of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Embodiments described herein relate generally to a method and apparatus for preheating, processing, and cooling down a photovoltaic module during fabrication. 
       BACKGROUND OF THE INVENTION 
       [0003]    A photovoltaic device converts the energy of sunlight directly into electricity by the photovoltaic effect.  FIG. 1  is a cross-sectional view of a portion of one example of a thin-film photovoltaic module  10  that can be built in layer sequence on a glass substrate  110 , e.g. soda-lime glass. A multi-layered transparent conductive oxide (TCO) stack  150  can be used as an n-type front contact. The TCO stack  150  has several functional layers including a barrier layer  120 , a TCO layer  130  and a buffer layer  140 . The front contact can affect various device characteristics such as visual quality, conversion efficiency, stability and reliability. Window layer  160 , which is a semiconductor layer, is formed over front contact  150 . Absorber layer  170 , which is also a semiconductor layer, is formed over window layer  160 . Window layer  160  and absorber layer  170  can include, for example, a binary semiconductor such as group II-VI or III-V semiconductors, such as, for example, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InS, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb or mixtures thereof. An example of a window layer and absorbing layer can be a layer of CdS and a layer of CdTe, respectively. Back contact  180  is formed over absorber layer  170 . Back contact  180  may also be a multi-layered stack similar to front contact  150 . Back support  190 , which may also be a glass, is formed over back contact  180 . 
         [0004]    The various layers of the photovoltaic devices may undergo a variety of processes, including surface modification, doping activation, and heat treatment. Further, a variety of deposition processes may be used, each of which may require heating the device to a processing temperature, treating the device at the processing temperature, and then cooling the device to an ambient temperature before proceeding to the final processing steps, which may include packaging, shipping, etc. 
         [0005]    Currently, most thermal treatments are performed in a single oven. However, such ovens are not specifically designed for handling the successive steps of heating, processing, and cooling the device thereafter and therefore lack flexibility to perform each function efficiently and effectively. What is needed is a system to perform the specific functions of heating, processing, and cooling a device under fabrication efficiently and effectively. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a cross-sectional view of a portion of an example of a photovoltaic device. 
           [0007]      FIG. 2  shows a system for heat treating a semiconductor on a glass sheet substrate according to an embodiment described herein. 
           [0008]      FIG. 3  shows a temperature feedback control loop for a heating module according to an embodiment described herein. 
           [0009]      FIG. 4  shows a heating module according to an embodiment described herein. 
           [0010]      FIG. 5  shows a processing module according to an embodiment described herein. 
           [0011]      FIG. 6  shows a temperature feedback control loop for a processing module according to an embodiment described herein. 
           [0012]      FIG. 7  shows a cooling module according to an embodiment described herein. 
           [0013]      FIG. 8  shows a temperature feedback control loop for a cooling module according to an embodiment described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. It should be understood that like reference numbers represent like elements throughout the drawings. Embodiments are described in sufficient detail to enable those skilled in the art to make and use them, and it is to be understood that structural, material, electrical, and procedural changes may be made to the specific embodiments disclosed, only some of which are discussed in detail below. 
         [0015]      FIG. 2  shows an embodiment of a modularized oven  200  that includes three discrete modules optimized for specific purposes. The modules include a heat-up and stabilization module, referred to herein as heating module  220 , an activation, treatment and deposition zone, referred to herein as processing module  210 , and a post-treatment and cooling zone, referred to herein as cooling module  230 . The heating module  220 , processing module  210 , and cooling module  230  are modular so that they may be coupled together and taken apart as needed for particular fabrication applications. For example, a particular module oven  200  could include or lack a heating module  220  and/or a cooling module  230 , and could include one or more processing modules  210 . 
         [0016]    The heating module  220  is configured to heat up a substrate  20  in a rapid and uniform manner and stabilize the substrate  20  at a desired target temperature. The heating module  220  may include a plurality of rollers  222  to transport the substrate  20  there-through. The spacing between the plurality of rollers  222  and their low thermal mass allows heat to reach the substrate  20 , providing a rapid and even heating process. In other embodiments, the rollers  222  could be replaced with a different transport mechanism, so long as the transport mechanism allows heat to rapidly and evenly reach the substrate  20 . For example, the transport mechanism could be a wire mesh belt transport. On-board metrology of the heating module  220  may measure the position, dimensions, and temperature of the substrate  20  as it is transported through the heating module  220 . 
         [0017]    The heating module  220  may include heaters  224  arranged inside the module  220  on both the top and bottom portions of the module  220 . The distance between the heaters  224  above the substrate  20  and below the substrate  20  may be equal to provide equal amounts of heat to the substrate  20 . The distance may be, for example, approximately 2 to 6 inches, which facilitates rapid and even heating of the substrate  20 . In various embodiments, a plurality of heating elements of the heaters  224  may be oriented in a direction that is parallel or perpendicular to the path of travel A of the substrate  20  through section  220  to achieve greater temperature uniformity. 
         [0018]    In addition to, or in lieu of the heaters  224 , the temperature of the heating module  220  may be controlled using heated gas (e.g., an inert gas) introduced through a gas injector  320  ( FIG. 3 ). By this method, heated inert gas may be injected into the heating module  220  to displace oxygen and to heat the substrate  20 . 
         [0019]    The temperature of the heating module  220  is controlled independently of the processing module  210  and cooling module  230  to allow independent optimization of the heating conditions.  FIG. 3  shows a temperature feedback control loop  300  based on an in-situ temperature control to obtain the desired temperature within the heating module  220 . The in-situ metrology serves to monitor and adjust for deviations in substrate temperature from the target temperature to achieve greater consistency in temperature prior to the substrate  20  entering the processing module  210 . The feedback control loop  300  includes a controller  330  to control the temperature of the heaters  224  or the temperature and output of the gas from the heated gas injector  320 , depending on which is used for heating. Alternatively, both heaters  223  and gas injector  320  can be used simultaneously. The controller  330  may receive input from the heaters  224  and the gas injector  320  that indicates the temperature of the heaters  224  and the temperature and output volume of the gas from the gas injector  320 . The controller  330  may also receive input from a substrate temperature sensor  340  that monitors the temperature of the substrate  20 . The substrate temperature sensor  340  may, for example, be a thermal imager in a spot configuration or line scanner configuration. In another embodiment, the substrate temperature sensor  340  may be a spectrometer and could monitor black body radiation using a black body curve. The controller  330  may also receive input from an ambient temperature sensor  350  that measures the internal temperature of the atmosphere inside the heating module  220 . In one embodiment, the ambient temperature sensor  350  may monitor air temperature inside the heating module  220  at various locations to measure heat loss from the various parts of the module  220  and to monitor changes that result therefrom. Using the various sensor inputs and controlling the output of the heaters  224  and/or the gas injector  320 , the temperature feedback control loop may be optimized to maintain a +/−1° C. control of the substrate  20  temperature prior to the substrate  20  entering the processing module  210 . 
         [0020]    Referring back to  FIG. 2 , the heating module  220  may also include one or more catch trays  226  arranged underneath the rollers  222  for removing substrates  20  that may have been broken due to defects in the substrate or because of the high temperatures within the heating module  220 . In one embodiment, each catch tray  226  may be made of wire mesh to allow heat to easily pass through to the substrate  20 . In another embodiment, each catch tray  226  may be arranged below the lower heater  224  so as to not block heat from reaching the substrate  20 .  FIG. 4  shows a heating module  220  that includes a hydraulic lift  228  to lift up the top  229  of the module  220  from the bottom  231  of the module  220 . The heating module  220  may also include side latches and/or hinges  233  to release the top  229 . 
         [0021]    After the substrate  20  is heated in the heating module  220 , the substrate may be transported along the rollers  222  into the processing module  210  ( FIG. 2 ). The processing module  210  is configured to process substrate  20  and/or a film stack arranged on substrate  20 . This processing may include a thermal processing of the substrate  20 . The processes carried out in the processing module  210 , which inherently require thermal processing may include, for example, exposing the substrate  20  to vapor deposition, surface etching, dopant introduction and/or activation, film deposition, and surface passivation, among others. 
         [0022]    To transport the substrate  20 , the processing module  210  may include a belt transport  212  having a solid belt upon which the substrate  20  rests. The belt transport  212  may serve a dual purpose of protecting the bottom of the substrate  20  from chemical vapors introduced into the processing module  210  and to increase the thermal mass of the processing module  210  to maintain a steady temperature. In other embodiments, other transport mechanisms could be used. 
         [0023]    The processing module  210  may include heaters  214  arranged outside muffle  218  of the module  210 . The muffle  218 , which is the enclosed treatment box portion of the processing module  210 , may be made of metal such as Inconel, molybdenum, stainless steel, tungsten, and alloys thereof. The metal of the muffle  218  may transmit the heat from the heaters  214  into the interior of the processing module  210 . The belt transport  212  may be situated so that the top of the muffle  218  is about 1 to 3 inches from the substrate  20 . 
         [0024]      FIG. 5  shows a processing module  210  according to another embodiment. As shown in  FIG. 5 , the muffle  218  may include local exhaust ports  217 , local separating gas introduction ports  219 , and local process gas ports  215  that provide the capability for gas segregation within the muffle  218 . While the muffle  218  does not include interior walls to physically separate the various processing gases, the processing gases may nonetheless be separated by the use of gas separation curtains, which are fast moving streams of gas. For example, processing gas may be introduced into the muffle  218  through local processing gas ports  215  into processing zones C and E and excess gas may be removed from zones C and E by exhaust ports  217  within the respective zones. The processing gasses may be the same or different within the different zones. Separating gas may be introduced into gas separation curtain zones B, D, F through local separating gas introduction ports  219  and removed by exhaust ports  217 , creating a fast moving stream of gas that acts as a gas curtain separating the different processing zones C and E from each other. The gas separation curtains allow the muffle  118  to include multiple processing zones C, E, having incompatible gases without causing detrimental or dangerous reactions to occur between them. Hence, various process gases and vapors, for example, inert, toxic, oxidizing, reducing, and reactive gasses may simultaneously be used in the muffle  118 . For example, in one embodiment, the muffle  218  may include multiple processing gas injectors  215  to allow for one or more of pre-treatment, deposition, activation, doping, and post-treatment sections within the same muffle  218 . In addition to local introduction ports  219  and exhausts  217 , the muffle  218  may also include outer introduction ports  216  and exhausts  213 , which may be located on the outer edges of the muffle  218  to create outer gas curtains that block outside gas contamination from entering the muffle  218 . Note that in the present embodiment, the separating gas used is an inert gas such as nitrogen gas. 
         [0025]    The processing module  210  may be of a modular design to allow for a plurality of the modules  210  to be interlocked together in cascading fashion so that the output of one processing module  210  may become the input of the next processing module  210 . 
         [0026]    The temperature of the processing module  210  is controlled independently from that of the heating module  220  and the cooling module  230  to allow independent optimization of the processing conditions therein. In addition to the use of the gas separation curtain zones B, D, F described above to provide different processing zones C, E within the processing module  210 , different portions of the heaters  214  may be heated to different temperatures to provide different amounts of heat to the substrate  20  within the different processing zones C, E. In addition to or in lieu of heaters  214 , heated gas can also be injected into the module  210  to set a desired temperature within each processing zone in the muffle  218 . 
         [0027]      FIG. 6  shows a temperature feedback control loop  600  based on an in-situ temperature control to obtain the desired temperature within the processing module  210 . The in-situ metrology serves to monitor and adjust for deviations in substrate temperature from the target temperature to achieve greater temperature consistency during the various thermal processes. The feedback control loop  600  includes a controller  630  to control the temperature of the heaters  214 , the temperature of the gas output from the gas injectors  620 , and the flow of the gas output from the gas injectors  620 . Gas injectors  620  may include the local gas introduction ports  219 , and local process gas ports  215 . The controller  630  may be the same or different controller from controller  330 . The controller  630  may receive input from the heaters  214  and the gas injectors  620  that indicates the temperature of the heaters  214  and the temperature and output volume of the gas from the gas injectors  620 . The controller  630  may also receive input from a substrate temperature sensor  640  that monitors the temperature of the substrate  20 . The substrate temperature sensor  640  may, for example, be a thermal imager in a spot configuration or line scanner configuration or a spectrometer. The controller  630  may also receive input from an ambient temperature sensor  650  that measures the internal temperature of the atmosphere inside the heating module  220 . In one embodiment, the ambient temperature sensor  650  may monitor air temperature inside the various processing zones C, E. Various detectors  660 , including but not limited to gas-phase fourier transform infrared spectroscopy (FTIR), optical emission spectroscopy (OES) and in-situ mass-spec etc., may be used to measure the quantity of chemical vapor in a processing zone C, E and send the information to the controller  630 , which will maintain specific chamber ambient conditions by adjusting the quality of gas introduced through gas injectors  620  and/or the amount of gas removed through exhaust ports  217 . 
         [0028]    Referring again to  FIG. 2 , after the substrate  20  is processed in one or more processing modules  210 , the substrate  20  may be transported along the belt  212  into the cooling module  230 .  FIG. 7  illustrates the cooling module  230  in greater detail. The cooling module  230  is configured for post-treatment cooling of the substrate  20 . The temperature of the cooling module  230  is controlled independently of the processing module  210  and heating module  220  to allow for independent optimization of the cooling and/or quench rate to maintain an optimal stress/strain state within the substrate  20 . In various embodiments, the cooling module  230  may be air and/or water cooled and may provide a rapid quench and/or slow cooling by injecting air and/or water through a plurality of inputs  239 . 
         [0029]    The cooling module  230  may include a plurality of rollers  232  to transport the substrate  20  through the module  230 . The spacing between the plurality of rollers  232  allows heat to dissipate from the substrate  20 , which provides a rapid and even cooling process. The rollers  232  have a further advantage over bulkier transport mechanisms in that they have a lower thermal mass. In other embodiments, the rollers  232  could be replaced with a different transport mechanism, so long as the transport mechanism allows heat to rapidly and evenly dissipate from the substrate  20 . For example, the transport mechanism could be a wire mesh belt transport. The rollers  232  may be arranged within the cooling module  230  to position the substrate  20  so that there is symmetrical access from the top and bottom of the substrate  20  to allow cooling at an even rate, which may reduce thermal stress and breakage. 
         [0030]    The temperature of the cooling module  230  is controlled independently of the processing module  210  and heating module  220  to allow independent optimization of the cooling conditions.  FIG. 8  shows a temperature feedback control loop  800  based on an in-situ temperature control to obtain the desired temperature within the cooling module  230 . The feedback control loop  800  includes a controller  830 , which may be the same or different than controllers  330  and  630 , to control the input of the coolant gas from the gas injector  820 . It should be understood that the gas injector  820  could also be used to inject a liquid coolant, for example, water. The controller  830  may receive input from the coolant gas injector  820  that indicates the temperature and output volume of the gas from the gas injector  820 . The controller  830  may also receive input from a substrate temperature sensor  840  that monitors the temperature of the substrate  20 . The substrate temperature sensor  840  may, for example, be a thermal imager in a spot configuration or line scanner configuration or a spectrometer. The controller  830  may also receive input from an ambient temperature sensor  850  that measures the internal temperature of the atmosphere inside the cooling module  230 . In one embodiment, the ambient temperature sensor  850  may monitor air temperature inside the cooling module  230  at various locations. Using the various sensor inputs and controlling the output of the coolant gas injector  820 , the temperature feedback control loop may provide for optimized cooling of the substrate  20 . 
         [0031]      FIG. 7  also shows how cooling module  230  may be arranged into different zones. As shown in  FIG. 7 , the cooling module  230  may include two discrete cooling zones G, H. The first zone H may be an initial cooling zone that cools the substrate  20  down below a critical temperature in an inert atmosphere, for example, using argon or nitrogen injected through a coolant input  239  and exhausted through exhaust port  237 . The second zone G may be a subsequent cooling zone that cools the substrate  20  down to a post processing temperature, for example, using clean dry air injected through a coolant input  239  and exhausted through exhaust port  237 . In other embodiments, the same gas could be used in both the first H and second G zones. The first H and second G zones may use the same or different cooling rates. The cooling module  230  may also have a dual containment body  231 , i.e., a second body  231  arranged around the cooling module  230 , to prevent the escape of process byproducts and/or reactants from the processing module  210 . 
         [0032]    In the embodiment shown in  FIG. 2 , a heating module  220 , a processing module  210 , and a cooling module  230  are coupled sequentially to each other. In other embodiments, the modules  210 ,  220 ,  230  may be arranged in different orders and/or may include additional modules depending on the particular process needs. 
         [0033]    While disclosed embodiments have been described in detail, it should be readily understood that the invention is not limited to the disclosed embodiments. Rather, the disclosed embodiments can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described.