Abstract:
An improved method of producing solar cells utilizes a mask which is fixed relative to an ion beam in an ion implanter. The ion beam is directed through a plurality of apertures in the mask toward a substrate. The substrate is moved at different speeds such that the substrate is exposed to an ion dose rate when the substrate is moved at a first scan rate and to a second ion dose rate when the substrate is moved at a second scan rate. By modifying the scan rate, various dose rates may be implanted on the substrate at corresponding substrate locations. This allows ion implantation to be used to provide precise doping profiles advantageous for manufacturing solar cells.

Description:
RELATED APPLICATIONS 
       [0001]    This patent application claims priority to U.S. Provisional Patent Application Ser. No. 61/232,821 filed Aug. 11, 2009 which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the invention relate to the field of device fabrication. More particularly, the present disclosure relates to a scanning method for ion implantation utilizing a shadow mask. 
         [0004]    2. Discussion of Related Art 
         [0005]    Ion implantation is a standard technique for introducing conductivity-altering impurities into substrates. A precise doping profile in a substrate and associated thin film structure is critical for proper device performance. Generally, a desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the substrate. The energetic ions in the beam penetrate into the bulk of the substrate material and are embedded into the crystalline lattice of the substrate material to form a region of desired conductivity. 
         [0006]    Such an ion implanter may be used to form solar cells. Solar cells are typically manufactured using the same processes used for other semiconductor devices, often using silicon as the substrate material. A semiconductor solar cell has an in-built electric field that separates the charge carriers generated through the absorption of photons in the semiconductor material. This electric-field is typically created through the formation of a p-n junction (diode) which is created by differential doping of the semiconductor material. Doping a part of the semiconductor substrate (e.g. surface region) with impurities of opposite polarity forms a p-n junction that may be used as a photovoltaic device converting light into electricity. These solar cells provide pollution-free, equal-access energy using a recurring natural resource. Due to environmental concerns and rising energy costs, solar cells are becoming more globally important. Reducing cost to manufacture or increasing production capability of these high-performance solar cells or other efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology. 
         [0007]    Solar cells may require doping to improve efficiency. This may be seen in  FIG. 1  which is a cross-sectional view of a selective emitter solar cell. It may increase efficiency of a solar cell to dope the emitter  200  and provide additional dopant to the regions  201  under the contacts  202 . More heavily doping the regions  201  improves conductivity and having less doping between the contacts  202  improves charge collection. The contacts  202  may only be spaced approximately 2-3 mm apart. The regions  201  may only be approximately 50-300 μm across. 
         [0008]      FIG. 2  is a cross-sectional view of an interdigitated back contact (IBC) solar cell. In the IBC solar cell, the junction is on the back of the solar cell. The doping pattern is alternating p-type and n-type dopant regions in this particular embodiment. The p+ emitter  203  and the n+ back surface field  204  may be doped. This doping may enable the junction in the IBC solar cell to function or have increased efficiency. 
         [0009]    In the past, solar cells have been doped using a dopant-containing glass or a paste that is heated to diffuse dopants into the solar cell. This does not allow precise doping of the various regions of the solar cell and, if voids, air bubbles, or contaminants are present, non-uniform doping may occur. Solar cells could benefit from ion implantation because ion implantation allows precise doping of the solar cell. Ion implantation of solar cells, however, may require a certain pattern of dopants or that only certain regions of the solar cell substrate are implanted with ions. Previously, implantation of only certain regions of a substrate has been accomplished using photoresist and ion implantation. However, use of photoresist, adds an extra cost to solar cell production because extra process steps are involved. Other hard masks on the solar cell surface are expensive and likewise require extra process steps. There are advantages in implanting small regions of solar cells and having a lower sheet resistance between implanted regions to improve series resistance. Both may be accomplished through the use of ion implantation. Accordingly, there is a need in the art for an improved method of implanting through a shadow mask and, more particularly, a scanning method for ion implantation that uses a shadow mask with solar cell fabrication. 
       SUMMARY OF THE INVENTION 
       [0010]    Exemplary embodiments of the present invention are directed to an apparatus and method for implanting ions into a substrate in an ion implanter. In an exemplary method, an ion beam is directed through an aperture of a shadow mask toward a substrate support configured to support a target substrate. A first portion of the substrate aligned with the aperture of the mask is exposed to the ion beam. The substrate support is moved with respect to the ion beam at a first scan rate when the first portion of the substrate is exposed to the ion beam. A second portion of the substrate aligned with the aperture of the mask is exposed to the ion beam. The substrate support is moved with respect to the ion beam at a second scan rate when the second portion of the substrate is exposed to the ion beam, wherein the first scan rate and the second scan rate are different. 
         [0011]    In an exemplary embodiment, an ion implanter includes an ion source, a beam line assembly, a shadow mask and a scanning assembly. The beam line assembly is configured to extract ions from the ion source to form an ion beam and direct the ion beam toward a substrate disposed on a substrate support. The mask is disposed in front of the substrate and has a plurality of apertures to allow respective portions of the ion beam through the mask toward the substrate. The scanning assembly is configured to move the substrate with respect to the ion beam at a first scan rate when first portions of the substrate are aligned with the plurality of apertures and at a second scan rate when second portions of the substrate are aligned with the plurality of apertures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a cross-sectional view of a selective emitter solar cell; 
           [0013]      FIG. 2  is a cross-sectional view of an interdigitated back contact solar cell; 
           [0014]      FIG. 3A  is a block diagram of a representative ion implanter in accordance with an embodiment of the present disclosure; 
           [0015]      FIG. 3B  is a cross-sectional view of ion implantation through a shadow mask; 
           [0016]      FIG. 4  is a front perspective view of a shadow mask; 
           [0017]      FIG. 5  is example of a velocity profile for a substrate in accordance with an embodiment of the present disclosure; 
           [0018]      FIGS. 6A-6E  represent the implant method corresponding to the velocity profile of  FIG. 5  in accordance with an embodiment of the present disclosure; and 
           [0019]      FIG. 7  is a front perspective view of a substrate that results from the implant method illustrated in  FIGS. 6A-6E  in accordance with an embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout. 
         [0021]      FIG. 3A  is a block diagram of an ion implanter  115  including an ion source chamber  120 . A power supply  121  supplies the required energy to source chamber  120  which is configured to generate ions of a particular species. The generated ions are extracted from the source through a series of electrodes  114  and formed into a beam  101  which passes through a mass analyzer magnet  116 . The mass analyzer is configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer for maximum transmission through the mass resolving slit  117 . Ions of the desired species pass from mass slit  117  through deceleration stage  118  to corrector magnet  119 . Corrector magnet  119  is energized to deflect ion beamlets in accordance with the strength and direction of the applied magnetic field to provide a ribbon beam targeted toward a work piece or substrate ( 100  in  FIG. 3B ) positioned on support (e.g. platen)  102 . In some embodiments, a second deceleration stage  122  may be disposed between corrector magnet  119  and support  102 . The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy. A mask  104  (shown in  FIG. 3   b ) is disposed proximate the substrate (shown in  FIG. 3   b  at  100 ) in a process chamber which houses platen  102 . The mask  104  may be referred to herein as a shadow or proximity mask. The mask has a plurality of apertures ( 105  in  FIG. 3B ) which allows portions of the ion beam aligned with the apertures to travel towards the substrate and blocks portions of the ion beam not aligned with the apertures  105 . 
         [0022]    The ion beam  101  has a height (Y direction) that is smaller than its width (X direction) as it travels toward the substrate (in the Z direction). Since the height of the ion beam is smaller than the width, only a portion of the substrate is exposed to the ion beam. Thus, in order to scan the entire substrate, the ion beam  101  (and consequently the mask  104 ) must move relative to the substrate or the substrate must be moved relative to the ion beam  101 . However, if the ion beam and mask move in order to scan the surface of the substrate, complicated cooling and grounding connections must accommodate this scanning movement in the process chamber where space is already at a premium. Thus, it is less complicated to move the substrate with respect to the ion beam  101 . A scanning assembly  102   a  is coupled to platen  102  and is configured to move the substrate with respect to the ion beam. In particular, the scanning assembly moves the substrate at a first scan rate when first portions of said substrate are aligned with the plurality of apertures and at a second scan rate when second portions of said substrate are aligned with said plurality of apertures. 
         [0023]    The scanning assembly is used to move the substrate at a variable speed in the Y direction as the ion beam implants a desired ion dose amount on the substrate. Alternatively, the scanning assembly can move the substrate at a variable speed in the X direction as the ion beam implants a desired ion dose amount on the substrate. In this manner, the scanning speed may slow down to have longer dwell times at particular portions of the substrate to heavily dope contact regions of a solar cell and move faster to have shorter dwell times to lightly dope exposed emitter regions of the cell. 
         [0024]      FIG. 3B  is a cross-sectional view of implantation through a mask. When a specific pattern of ion implantation in a substrate  100  is desired, a mask  104  may be placed in front of a substrate  100  in the path of an ion beam  101 . This mask  104  may be a shadow or proximity mask. The substrate  100  may be, for example, a solar cell placed on platen  102  Typical substrates used for solar cells are very thin, often on the order of  300  microns thick or less. The substrate  100  is retained in position on the platen  102  using electrostatic or physical forces. Although not required, it is preferable that the width of the substrate in the X direction is less than the width of the ion beam  101 . However, no such limitation is preferred with respect to the orthogonal direction of the substrate. 
         [0025]    The mask  104  has one or more apertures  105  that correspond to the desired pattern of ion implantation. Use of the shadow mask  104  eliminates process steps, such as screen printing or lithography, required for other ion implantation techniques. As previously stated, there is an advantage to having high dopant levels under the contact portions of the solar cell, such as seen in the regions  201  in  FIG. 1 . While a selective emitter solar cell is discussed, embodiments of this method may be applied to other solar cell designs. Point contacts will minimize the metal-to-silicon contact area. For these point contacts, dopant should be localized under the contacts  202  to provide an electrical field to shield the contacts from minority carriers. The dopant must be localized because highly-doped regions are detrimental under the passivated surfaces that may exist between the contacts  202 . 
         [0026]    Point contacts may be doped by implanting through a mask that has points on it. However, the series resistance of a solar cell with such a doping structure may still be limited by the amount of dopant between the contacts  202 . In an implant through the mask  104 , the mask  104  must travel with the substrate  100  when the substrate  100  is scanned. As mentioned above, this movement increases the difficulty in cooling and grounding the mask  104 . Additionally, a mask  104  with small apertures  105  will block much of the ion beam  103  which decreases productivity and device throughput. 
         [0027]    A fixed mask  104  having a plurality of apertures  105  is configured to cover the height (Y direction) of the ion beam  101 . This may be, for example, the height of a ribbon ion beam  101  or the maximum and minimum vertical extent of a scanned ion beam  101 . Such a design provides positional definition in one dimension, such as the dimension in which the ion beam  101  is scanned (e.g. Y direction) or the long dimension of a ribbon ion beam  101  (e.g. X direction). Positional definition in a second dimension, such as the direction in which the substrate  100  is scanned, is provided by changing the scan speed of the substrate  100 . 
         [0028]      FIG. 4  is a front perspective view of a shadow mask  404  having seven apertures  405  and is fixed (i.e., does not translate or scan) with respect to the ion beam  101 . While seven apertures  405  are illustrated, the mask  404  is not solely limited to seven apertures  405  and other numbers are possible. An ion beam, such as ion beam  101 , is incident on the mask  404  throughout the implant process and the mask  404  is stationary or fixed relative to this ion beam. The substrate  100  is scanned behind the mask  404 . Portions of the mask  405   a  defined between the apertures  404  and at the ends of the mask  405   b  and  405   c  block ion beam from implanting on the substrate  100 . 
         [0029]      FIG. 5  is example of a corresponding scanning velocity profile for an exemplary substrate. This velocity profile has the substrate  100  moving for a large percentage of the velocity profile at a first speed, but the substrate  100  slows down to a second speed when the ion beam  101  is on each of the five evenly-spaced locations on the substrate  100 . In particular, the substrate  100  moves at a first speed S 1  for an exemplary time interval defined between t N  and t N-1  represented by reference  501 . The movement speed of substrate  100  changes to speed S 2  for an exemplary time interval defined between t N-1  and t N-2  represented by reference  500 . Since the scan speed at S 1  is slower than the scan speed S 2 , the dose rate of ions from ion beam  101  on the surface of substrate  100  at speed S 2  will be lower since the substrate is moving faster through the ion beam. Thus, the dose implanted between these five locations on substrate  100  (represented by  501 ) will be lower than the dose at the five locations on the substrate (represented by  500 ) because of the higher scan speed S 1 . This may be used to align the higher dose rate locations of the substrate to the point metal contacts on an exemplary solar cell. 
         [0030]      FIGS. 6A-6E  illustrate results of the implant method corresponding to the velocity profile of  FIG. 5 . In  FIG. 6A  the ion beam  101  implants substrate  100  through the apertures  405  in the mask  404  in a first position. Lower dose regions  601  correspond to that portion of the velocity profile of  FIG. 5  where the scan speed is faster (S 1 ). In  FIG. 6B , the ion beam  101  implants through the apertures  405  in the mask  404  in a second position where the higher-dose implanted regions  600  correspond to that portion of the velocity profile having a slower scan speed (S 2 ). Thus,  FIG. 6B  illustrates the higher dose implanted regions  600 , lower-dose implanted regions  601  from the first position and the undoped regions  602  corresponding to the areas of the mask  405   a  between the apertures  405 . 
         [0031]    The substrate  100  may be scanned behind the mask  404  continuously. However, as seen in  FIG. 5 , the substrate  100  scans more slowly at the second speed when the ion beam  101  is implanting the higher-dose implanted regions  600  as compared to the lower-dose regions  601  of the substrate  100 . Thus, the substrate  100  moves faster at the first speed between the first position and second position (not illustrated) than while implanting in the first position and second position illustrated in  FIGS. 6A-6B . Since the mask is fixed with respect to the ion beam  101 , there is no need to move the mask with the substrate during scanning of the substrate which provides increased manufacturing throughput especially at lower dose levels. 
         [0032]      FIGS. 6C-6E  represent the results of ion beam  101  implanted through apertures  405  in the mask  404  in a third position, fourth, position, and fifth position respectively corresponding to portions of the substrate  100  as it moves in the exemplary Y direction.  FIG. 7  is a front perspective view of a substrate that results from the implant method illustrated in  FIGS. 6A-6E . As illustrated in  FIG. 7 , a series of higher-dose implanted regions  600  have been formed in the substrate  100  with lower-dose implanted regions  601  in between. Undoped regions  602  correspond to portions of the substrate that are aligned behind the mask areas  405   a,    405   b  and  405   c  between the apertures  405  and at the periphery of the mask  404 . The higher-dose implanted regions  600  may correspond to future point contacts in a solar cell that are added during a metallization step. Since the mask  404  is stationary or fixed in  FIGS. 6A-6E , this results in less-complicated cooling and grounding connections because the mask  404  does not move like the substrate  100 . This also may improve the effectiveness and reliability of the cooling and grounding connections for the mask  404 . 
         [0033]    The substrate  100  of  FIG. 7  could be produced by a shadow mask, such as mask  104 , that scans with the substrate  100 . However, the entire substrate  100  would have to remain in the ion beam  101  for the same amount of time as each of the implant steps shown in  FIGS. 6A-6E . This means that the ion beam  101  dose required to perform the implants would be increased by a factor of S h /((B h )(n)) where S h  is the height of the substrate  100 , B h  is the height of the ion beam  101 , and n is the number of rows of the higher-dose implanted regions  600 . For an ion beam  101  with a height of 4 mm, a substrate  100  height of 156 mm, and four rows of higher-dose implanted regions  600 , the total dose could be ten times lower using the method illustrated in  FIGS. 6A-6E  than an implant method with a scanning mask  104 . This reduced dose results in increased throughput and lower production costs. The reduced dose also may lead to less thermal load on the mask  404 . 
         [0034]    The lower-dose implanted regions  601  between the higher-dose implanted regions  600  may lower the series resistance of any resulting solar cell. Majority carriers must be transported from generation sites to the contacts and the resistance these encounter during transportation reduces the output of a solar cell. For majority carriers that originate between the contact points, a slightly higher dose between the contacts will lower the resistance. This may result in better internal series resistance in the solar cell. 
         [0035]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.