Abstract:
An improved, lower cost method of processing substrates, such as to create solar cells is disclosed. In addition, a modified substrate carrier is disclosed. The carriers typically used to carry the substrates are modified so as to serve as shadow masks for a patterned implant. In some embodiments, various patterns can be created using the carriers such that different process steps can be performed on the substrate by changing the carrier or the position with the carrier. In addition, since the alignment of the substrate to the carrier is critical, the carrier may contain alignment features to insure that the substrate is positioned properly on the carrier. In some embodiments, gravity is used to hold the substrate on the carrier, and therefore, the ions are directed so that the ion beam travels upward toward the bottom side of the carrier.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation application of U.S. application Ser. No. 12/895,927, filed Oct. 1, 2010 and entitled “Integrated Shadow Mask/Carrier for Patterned Ion Implantation,” the disclosure of which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    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 is a device having 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. 
         [0003]      FIG. 1  shows a cross section of a representative solar cell  100 . Photons  101  enter the solar cell  100  through the top surface  105 , as signified by the arrows. These photons pass through an anti-reflective coating  110 , designed to maximize the number of photons that penetrate the solar cell  100  and minimize those that are reflected away from the solar cell  100 . 
         [0004]    Internally, the solar cell  100  is formed so as to have a p-n junction  120 . This junction is shown as being substantially parallel to the top surface  105  of the solar cell  100  although there are other implementations where the junction may not be parallel to the surface. The solar cell is fabricated such that the photons enter the substrate through the n-doped region, also known as the emitter  130 . While this disclosure describes p-type bases and n-type emitters, n-type bases and p-type emitters can also be used to produce solar cells and are within the scope of the disclosure. The photons with sufficient energy (above the bandgap of the semiconductor) are able to promote an electron within the semiconductor material&#39;s valence band to the conduction band. Associated with this free electron is a corresponding positively charged hole in the valence band. In order to generate a photocurrent that can drive an external load, these electron hole (e-h) pairs need to be separated. This is done through the built-in electric field at the p-n junction. Thus any e-h pairs that are generated in the depletion region of the p-n junction get separated, as are any other minority carriers that diffuse to the depletion region of the device. Since a majority of the incident photons are absorbed in near surface regions of the device, the minority carriers generated in the emitter need to diffuse across the depth of the emitter  130  to reach the depletion region and get swept across to the other side. Thus to maximize the collection of photo-generated current and minimize the chances of carrier recombination in the emitter  130 , it is preferable to have the emitter  130  be very shallow. 
         [0005]    Some photons  101  pass through the emitter region  130  and enter the base  140 . These photons  101  can then excite electrons within the base  140 , which are free to move into the emitter  130 , while the associated holes remain in the base  140 . As a result of the charge separation caused by the presence of this p-n junction  120 , the extra carriers (electrons and holes) generated by the photons  101  can then be used to drive an external load to complete the circuit. 
         [0006]    By externally connecting the emitter  130  to the base  140  through an external load, it is possible to conduct current and therefore provide power. To achieve this, contacts  150   a  and  150   b,  typically metallic, are placed on the outer surface of the emitter  130  and the base  140 . Since the base  140  does not receive the photons  101  directly, typically its contact  150   b  is placed along the entire outer surface of the base  140 . In contrast, the outer surface of the emitter  130  receives photons  101  and therefore cannot be completely covered with contacts  150   a.  However, if the electrons have to travel great distances to the contact, the series resistance of the cell increases, which lowers the power output. In an attempt to balance these two considerations; the distance that the free electrons must travel to the contact  150   a  or  150   b,  and the amount of exposed emitter surface  160  illustrated in  FIG. 2 ; most applications use contacts  150   a  that are in the form of fingers.  FIG. 2  shows a top view of the solar cell of  FIG. 1 . The contacts  150   a  are typically formed so as to be relatively thin, while extending the width of the solar cell  100 . In this way, free electrons need not travel great distances, but much of the outer surface of the emitter is exposed to the photons. Typical contacts  150   a  on the front side of the substrate are 0.1 mm wide, with an accuracy of approximately +/−0.1mm. These contacts  150   a  are typically spaced between 1-5 mm apart from one another. While these dimensions are typical, other dimensions are possible and contemplated herein. 
         [0007]    A further enhancement to solar cells is the addition of heavily doped substrate contact regions.  FIG. 3  shows a cross section of this enhanced solar cell. The solar cell  100  is as described above in connection with  FIG. 1 , but includes heavily n-doped contact regions  170 . These heavily doped contact regions  170  correspond to the areas where the contacts  150   a  will be affixed to the solar cell  100 . The introduction of these heavily doped contact regions  170  allows much better contact between the solar cell  100  and the contacts  150   a  and significantly lowers the series resistance of the solar cell  100 . This pattern of including heavily doped regions on the surface of the substrate is commonly referred to as selective emitter (SE) design. These heavily doped regions may be created by implanting ions in these regions. Thus, the terms “implanted region” and “doped region” may be used interchangeably throughout this disclosure. 
         [0008]    A selective emitter design for a solar cell also has the advantage of higher efficiency cells due to reduced minority carrier losses through recombination due to lower dopant/impurity dose in the exposed regions of the emitter layer. The higher doping under the contact regions provides a field that collects the majority carriers generated in the emitter and repels the excess minority carriers back toward the p-n junction. 
         [0009]    Such structures are typically made using traditional lithography (or hard masks) and thermal diffusion. An alternative is to use implantation in conjunction with a traditional lithographic mask, which can then be removed easily before dopant activation. Yet another alternative is to use a shadow mask or stencil mask in the implanter to define the highly doped areas for the contacts. All of these techniques utilize a fixed masking layer (either directly on the substrate or upstream in the beamline). 
         [0010]    All of these alternatives have drawbacks. For example, the processes enumerated above all contain multiple process steps. This causes the cost of the manufacturing process to be prohibitive and may increase substrate breakage rates. These options also suffer from the limitations associated with the special handling of solar cells, such as aligning the mask with the substrate and the cross contamination with materials that are dispersed from the mask during ion implantation. 
         [0011]    Consequently, efforts have been made to reduce the cost and effort required to dope a pattern onto a substrate. While some of these efforts may be successful in reducing cost and processing time, often these modifications come at the price of reduced accuracy. Typically, in semiconductor processes, masks are very accurately aligned. Subsequent process steps rely on this accuracy. For example, referring to  FIG. 4 , after the heavily doped regions  170   a - c  have been implanted, contacts  150   a  are pasted to the substrate. Each of these processes is usually performed relative to some reference mark or fiducial. This mark may be an edge or corner of the substrate, or a specific mark or feature on the substrate. Since each of these process steps is typically referenced to a specific point, it is imperative that a high degree of accuracy be maintained. These efforts to reduce cost and processing steps degrade this accuracy, thereby potentially impacting the performance and yields of the devices made using these methods. 
         [0012]    Therefore, there exists a need to produce solar cells where the number and complexity of the process steps is reduced, while maintaining adequate accuracy so that subsequent process steps are correctly positioned. While applicable to solar cells, the techniques described herein are applicable to other doping applications. 
       SUMMARY OF THE INVENTION 
       [0013]    An improved, lower cost method of processing substrates, such as to create solar cells is disclosed. In addition, a modified substrate carrier is disclosed. The carriers typically used to carry the substrates are modified so as to serve as shadow masks for a patterned implant. In some embodiments, various patterns can be created using the carriers such that different process steps can be performed on the substrate by changing the carrier or the position with the carrier. In addition, since the alignment of the substrate to the carrier is critical, the carrier may contain alignment features to insure that the substrate is positioned properly on the carrier. In some embodiments, gravity is used to hold the substrate on the carrier, and therefore, the ions are directed so that the ion beam travels upward toward the bottom side of the carrier. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0014]      FIG. 1  shows a cross section of a solar cell of the prior art; 
           [0015]      FIG. 2  shows a top view of the solar cell of  FIG. 1 ; 
           [0016]      FIG. 3  shows a cross section of a solar cell using selective emitter design; 
           [0017]      FIG. 4  shows a top view of the solar cell of  FIG. 3 ; 
           [0018]      FIG. 5  shows a representative coordinate system; 
           [0019]      FIG. 6  is a representative illustration of an ion implanter used in accordance with an embodiment; 
           [0020]      FIG. 7  shows a shadow mask being used to form the doped regions shown in  FIG. 4 ; 
           [0021]      FIGS. 8A-C  show several embodiments in accordance with the present disclosure; 
           [0022]      FIG. 9  shows the implant process according to one embodiment; 
           [0023]      FIGS. 10A-B  are embodiments of a carrier having two different patterns; 
           [0024]      FIG. 11  shows another embodiment of a carrier having two different patterns; 
           [0025]      FIG. 12  is a table showing the results of shifting substrates in a carrier having a plurality of different patterns; 
           [0026]      FIG. 13  shows another embodiment of a carrier having two different patterns; and 
           [0027]      FIG. 14  shows another embodiment of a carrier having two different patterns suitable for an IBC solar cell. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]      FIG. 4  shows a top view of the solar cell manufactured using the methods of the present disclosure. The solar cell is formed on a semiconductor substrate  100 . The substrate can be any convenient size, including but not limited to circular, rectangular, or square. Although not a requirement, it is preferable that the width of the solar cell  100  be less than the width of the ion beam used to implant ions in the solar cell  100 . However, no such limitation exists with respect to the orthogonal direction of the solar cell. In other words, a solar cell  100  can be arbitrarily long, and can be in the shape of a roll of solar cell material. Typically, the substrates for solar cells  100  are very thin, often on the order of 300 microns thick or less. 
         [0029]    As described above, the solar cell has an n-doped emitter region and a p-doped base. The substrate is typically p-doped and forms the base, while ion implantation is used to create the emitter region. A block diagram of a representative ion implanter  600  is shown in  FIG. 6 . Of course, one skilled in the art will recognize that numerous other ion implanter designs exist and may be used. An ion source  610  generates ions of a desired species, such as phosphorus or boron. A set of electrodes (not shown) is typically used to attract the ions from the ion source  610 . By using an electrical potential of opposite polarity to the ions of interest, the electrodes draw the ions from the ion source  610 , and the ions accelerate through the electrode. These attracted ions are then formed into an ion beam  650 , which then passes through a source filter  620 . The source filter  620  is preferably located near the ion source  610 . The ions within the ion beam  650  are accelerated/decelerated in column  630  to the desired energy level. A mass analyzer magnet  640 , having an aperture  645 , is used to remove unwanted components from the ion beam  650 , resulting in an ion beam  650  having the desired energy and mass characteristics passing through resolving aperture  645 . 
         [0030]    In certain embodiments, the ion beam  650  is a spot beam. In this scenario, the ion beam  650  passes through a scanner  660 , preferably an electrostatic scanner, which deflects the ion beam  650  to produce a scanned beam  655  wherein the individual beamlets  657  have trajectories which diverge from scanner  660 . In certain embodiments, the scanner  660  comprises separated scan plates in communication with a scan generator. The scan generator creates a scan voltage waveform, such, as a sine, sawtooth or triangle waveform having amplitude and frequency components, which is applied to the scan plates. In a preferred embodiment, the scanning waveform is typically very close to being a triangle wave (constant slope), so as to uniformly expose the scanned beam  655  at every position of the substrate for nearly the same amount of time. Deviations from the triangle are used to make the beam uniform. The resultant electric field causes the ion beam to diverge as shown in  FIG. 6 . 
         [0031]    An angle corrector  670  is adapted to deflect the divergent ion beamlets  657  into a set of ion beamlets  657  having substantially parallel trajectories. Preferably, the angle corrector  670  comprises a magnet coil and magnetic pole pieces that are spaced apart to form a gap, through which the ion beamlets  657  pass. The coil is energized so as to create a magnetic field within the gap, which deflects the ion beamlets  657  in accordance with the strength and direction of the applied magnetic field. The magnetic field is adjusted by varying the current through the magnet coil. Alternatively, other structures, such as parallelizing lenses, can also be utilized to perform this function. 
         [0032]    Following the angle corrector  670 , the scanned beam  655  is targeted toward the substrate, such as the solar cell to be processed. The scanned beam typically has a height (Y dimension) that is much smaller than its width (X dimension). This height is much smaller than the substrate, thus at any particular time, only a portion of the substrate is exposed to the ion beam. To expose the entire substrate to the scanned beam  655 , the substrate may be moved relative to the beam location. 
         [0033]    The substrate, such as a solar cell, is attached to a substrate holder  675 . The substrate holder  675  may provide a plurality of degrees of movement. For example, the substrate holder  675  can be moved in the direction orthogonal to the scanned beam  655 . A sample coordinate system in shown in  FIG. 5 . Assume the beam is in the XZ plane. This beam can be a ribbon beam, or a scanned spot beam. The substrate holder can move in the Y direction. By doing so, the entire surface of the solar cell  100  can be exposed to the ion beam, assuming that the solar cell  100  has a smaller width than the ion beam (in the X dimension). 
         [0034]    Substrates are moved into and from the process chamber through the use of carriers. In some embodiments, the carriers are rectangular, such as box shaped, and are capable of holding a plurality of substrates. In other embodiments, a separate, typically flat, carrier is used for each substrate. In one embodiment, the substrate is removed from the carrier and placed on the substrate holder  675  in preparation of processing, such as by a robotic arm. One reason to remove the substrate from the carrier may be to minimize cross-contamination for multi-species  processes. After processing has been completed, the robotic arm returns the substrate to the carrier. The substrate, contained within the carrier, can now be transported outside the chamber. In another embodiment, the substrate remains in the carrier during the implant process. This allows the carrier to serve as an alignment reference for the substrate. This also allows the carrier to have a pattern on it which will serve as a mask in the presence of an ion beam. 
         [0035]    There may be additional reasons to utilize a carrier. For example, a carrier supports the substrate in multiple axis during transport. If the substrate, for example, resides in a pocket of the carrier, the tool can move the substrate/carrier combination faster than if it relied exclusively on friction to the substrate (e.g., backside pads). Thus, with a fragile substrate constrained in the carrier, the handling system may be passively (e.g., pins) or actively (e.g., grabbers) held for more secure transport. 
         [0036]    In addition, in some embodiments, a carrier can be made of conductive materials to form an electrical ground path to the substrate. In some embodiments, a carrier can be used to apply an electrical voltage to the substrate, such as a pulsed voltage for a plasma tool. 
         [0037]    A carrier can easily be either adapted or replaced in an implanter to enable the handling of alternate substrate sizes or shapes. 
         [0038]    Finally, once a substrate is rigidly constrained within a carrier, reliable positional references can be made to the carrier alone. In other words, locating to a kinematic pin feature in the carrier can be done repeatedly very accurately, and without risking substrate breakage. 
         [0039]    In addition to beam line ion implanters, plasma doping systems can also be used. A plasma doping system forms a plasma containing the dopant using an electron cyclotron resonance plasma source, a helicon plasma source, a capacitively coupled plasma source, an inductively coupled plasma source, a DC glow discharge, a microwave source, or an RF source, as examples. The substrate, which is located within a chamber containing this plasma, is then biased using either a pulse or DC voltage and ions are accelerated into the surface of the substrate. Other ion implanters, including those with or without mass analysis, may be used. 
         [0040]    There are a number of methods that can be used to create the doping pattern shown in  FIG. 4 . In some embodiment, the pattern is created by traditional implantation techniques. For example, an ion beam can be used to implant the surface of the solar cell  100  which is exposed to the beam. In some embodiments, the emitter  160  is doped using an ion implantation across the entire surface, also known as a blanket implant. The more heavily doped region  170  is then created using a mask.  FIG. 7  shows a mask  12  disposed between the source of ions and the solar cell  100 . The mask  12  includes one or more apertures  14  that allow the passage of ions  13 . The mask  12  will block ions  13  that do not pass through the apertures  14 . Those areas which are exposed to the ion beam become implanted or doped regions  170 . 
         [0041]    However, the use of a traditional shadow mask requires precise alignment processes. In some embodiments, the shadow mask is placed between the ion beam and the substrate holder  675 , while the substrate is clamped to the substrate holder. In this embodiment, there is an alignment process that must be completed to properly orient the shadow mask to the clamped substrate. In some embodiments, the substrate is moved while the shadow mask is held stationary. In other embodiments, the substrate is held stationary while the shadow mask is moved to perform the alignment. 
         [0042]    To eliminate these alignment processes, the present disclosure uses the substrate carrier as the shadow mask. In one embodiment, the substrates are placed flat on the substrate carrier. In some embodiments, alignment features are used to insure that the substrate is properly positioned on the carrier. In addition, one surface of the carrier, typically the bottom surface, has apertures or openings in the shape of the desired doping pattern.  FIG. 8A  shows a substrate carrier  800 , which supports a single substrate. The substrate carrier  800  includes a plurality of slots or apertures  805 , through which ions may pass, thereby allowing the exposed regions of the substrate resting on the carrier to be implanted. In some embodiments, the slots or apertures are the result of the removal of the material used to construct the carrier. In other embodiments, the slots are the result of the use of a material which allows the transmission of ions through it. In another embodiment, the bottom surface of the carrier  800  may be constructed by combining stacks of thin material and spacers, which form the desired pattern. In another embodiment, the solid parts of the bottom surface of the carrier may be wires tensioned across the substrate. In yet another embodiment, the carrier  800  may have a bottom surface that is substantially open. The carrier  800  could support or be independently aligned to a separate mask, which is positioned between the bottom surface of the carrier  800  and the substrate. In this embodiment, the carrier  800  supports the substrate, and the carrier  800  and/or mask are registered to a mask within the implanter. This “dual-registration” approach may allow the option of repeatably registering multiple masks to one substrate. 
         [0043]    The carrier  800  may be constructed of any material capable of withstanding the ion implantation process, such as graphite, Silicon carbide or silicon. In some embodiments, the slots  805  are between  50  μm and  800  μm in width, and are used to create the highly doped selective emitter regions  170 , as described in connection with  FIG. 3 . While  FIG. 8A  shows a substrate carrier  800  suitable for holding a single substrate, other embodiments are possible.  FIG. 8B  shows a substrate carrier  810  which is able to accommodate four substrates, located in positions  811 - 814 .  FIG. 8C  shows a substrate  820  carrier configured to hold as many as  20  substrates. The size of the carrier and the number of substrates that can be supported is not limited by the present disclosure. Similarly, while  FIGS. 8A-C  show the pattern as a series of slots, other patterns may also be used and are within the scope of the disclosure. 
         [0044]    As described above, one or more alignment features may be included in the carrier to properly position the substrate relative to the pattern. These alignment features may be on the side opposite that impacted by the ion beam. In one embodiment, two points are used to align to the edge of the substrate to reference the location of the pattern. The substrate could be referenced in two dimensions as well for two dimensional patterning. The referencing of the substrate to the carrier may be done by tipping the carrier and allowing the substrate to slide against the alignment feature due to the force of gravity. In another embodiment, alignment features are not used and can be replaced by an optical recognition system to align the doped lines to the metal lines at the metallization step. The alignment features may be in the carrier, in the mask or within the implanter. While gravity may be used to slide the substrates gently against an alignment feature, an active device, such as a robot, could be used as well. 
         [0045]    In operation, the substrate is placed on a substrate carrier. The carrier may hold any number of substrates, as shown in  FIGS. 8A-8C , although it is preferable that each substrate is positioned over a corresponding pattern. As described above, the pattern may be a series of apertures or slots that allow ions to be implanted into the substrate. The pattern of apertures or slots matches that which is to be implanted in the substrate. For example, the pattern of  FIG. 8A  can be used to implant the heavily n-doped contact regions  170  of  FIG. 3 . The substrate is placed on the carrier and may be aligned using alignment features located on the carrier, such as on the top surface of the carrier. The substrates may be held in position by, for example, gravity. In other words, the patterns of  FIG. 8A-C  are created on the bottom surface of the carrier. The populated carrier is then placed in the process chamber. 
         [0046]    In some embodiments, as shown in  FIG. 9 , the carrier  850  remains horizontally oriented, such that its bottom surface  851  is parallel to ground. The ion beam  870  is then incident on the bottom surface  851  of the carrier, such that the dopant ions are implanted into the substrates  860  through the pattern of apertures on the bottom surface  851 . In some embodiments, the carrier  850  is scanned through the ion beam  870 , such that it moves while the beam is held stationary. In one particular instance, this carrier  850  is part of a conveyor belt. In other embodiments, the carrier  850  is held stationary and the ion beam  870  scans across the carrier. In the case of a pulsed plasma implant, the carrier  850  and substrate are stationary relative to the beam. 
         [0047]    The ion beam  870  can be at any angle relative to the bottom surface  851  of the carrier  850 , although in some embodiments, an ion beam  870  normal to the bottom surface  851  may be desirable. The angle of incidence can be modified by either changing the direction of the ion beam  870 , tilting the carrier  850 , or a combination of the two actions. In embodiments where gravity is used to hold the substrate  860  in place, the maximum angle of tilt may be limited. 
         [0048]    In some embodiments, as described above, gravity is used to hold the substrate  860  in place in the carrier  850 . In other embodiments, the substrate  860  is held in place, such as by an electrostatic or mechanical clamp, so that the carrier  850  can be tilted to a greater extent, such as completely vertically. For example, the pattern of the mask may serve as the active clamping surface. 
         [0049]      FIGS. 8B-C  show a plurality of locations in which the substrates may be placed. In these figures, each location has an identical pattern. However, other embodiments are also possible. For example,  FIG. 10A  shows a carrier  830 , which is configured to hold two substrates. The first location  831  has a pattern similar to that shown in  FIGS. 8A-C , which may be used to implant the heavily n-doped contact regions  170  of  FIG. 3 . The second location  832  has almost all of the material removed, such the almost the entire surface of the substrate located in second location  832  is implanted by the ion beam. Small tabs  833  may be used to support the substrate when in this position. A border or other edge also may be used. This second location  832  may be used to perform a blanket implant on the substrate. 
         [0050]    In some embodiments, the carrier  830  may be loaded with two substrates, such that the first is positioned in first location  831  so that it is pattern implanted while the second is positioned in second location  832  and is blanket implanted. After the implant is completed, the positions of the substrates in the carrier  830  may be switched, such that the first substrate is now in second location  832  and is blanket implanted, while the second is pattern implanted in first location  831 . Such an arrangement allows two separate process steps (blanket and pattern implantation) to be performed on two substrates using a single tool. The substrates may be changed using, for example, a substrate handling robot. 
         [0051]    In another embodiment, a single substrate is loaded into carrier  830 , such as in first location  831 . The carrier  830  is then placed so as to be impacted by the ion beam. After the blanket implant is completed, the carrier  830  is shifted from first location  831  to second location  832 . In some embodiments, the carrier is tilted such that the substrate slides from first location  831  to second location  832 . In other embodiment, a substrate handling robot is used. At this point, the carrier  830  is again moved so as to be impacted by the ion beam. The substrate is now pattern implanted. In this way, a single substrate can have two implants performed on, it using a single tool. This allows a substrate to receive both a blanket implant to create an emitter region  130  and a pattern implant to create heavily n-doped contact regions  170 . 
         [0052]    To improve, alignment, the carrier  830  may be tilted toward first location  831  so that the substrate slides to the end of the carrier. After the first implant, the carrier  830  may be tilted toward second position  832  so that the substrate slides to the opposite end of the carrier  830 . This method insures that the substrate is aligned, with the patterns on the bottom surface of the carrier  830 . 
         [0053]    To improve yield, each location  831 ,  832  may be extended to form rows so as to hold a plurality of substrates, each substrate adjacent to the other, as shown in  FIG. 10B . In this embodiment, the carrier  835  has two rows  836 ,  837 , each capable of holding five substrates. The tilting process described above may be used to align all substrates located in row  836  first. After the implant is completed, the carrier  835  may be tilted slide the substrates into row  837 . Note also that other methods may be user to shift the substrates from one row to the second row. 
         [0054]      FIG. 11  shows a larger carrier  840 , having four rows  841 - 844  having two different patterns that can be used to simultaneously implant  20  substrates. The ten substrates in rows  841 ,  843  may be pattern implanted. The ten substrates in rows  842 ,  844  may be blanket implanted. After this implant is completed, the substrates may be moved to another row to allow each substrate to receive both types or patterns of implants. 
         [0055]    In another embodiment, the lowest row  844  is left vacant, such that substrates are only loaded into the top three rows  841 - 843  of the carrier  840 . The carrier  840  is then moved so as to be impacted by the ion beam. After the implant is completed, the substrates are caused to shift downward by one row. In other words, the substrates in top row  841  are shifted to second row  842 . Similarly, the substrates in rows  842 ,  832  and shifted to rows  843 ,  844  respectively. The carrier  840  is then moved so as to be impacted by the ion beam. In this way, the substrates each now receive a second implant, of a different type than the first implant. In other words, those which were blanket implanted in row  842  during the first implant are now pattern implanted in row  843 . Those that were pattern implanted in row  841 ,  843  are now blanket implanted in rows  842 ,  844 . 
         [0056]    In some embodiments, the substrates are shifted from one row to an adjacent row by tilting the carrier. In this embodiment, the substrates slide until touching against substrates in an adjacent row or an alignment feature. In other embodiments, the substrates may be mechanically pushed from one row to another. Alternatively, the substrates may be held stationery, such as by electrostatic clamping, while the carrier is advanced. 
         [0057]    The above description shows two patterns, where one is a series of slots and the second is for a blanket implant. However, the disclosure is not limited to these patterns. For example, two different patterns, each having a series of slots (perhaps oriented in different directions) may be used. Similarly, other types of patterns may be used for the various rows. 
         [0058]    For example, it is possible to create two separate pattern features by referencing two different alignment features within the carrier. This technique would duplicate the same implant pattern on two different positions of the substrate. An advantage of this technique is that the irregularities contained within the pattern will always be matched by the duplicate. This technique may help with tolerancing between patterns. 
         [0059]    In addition, the disclosure is not limited to only two patterns. Three or more different patterns can be used with a single carrier. Furthermore, if desired, each substrate may receive patterned implants using each of the patterns on the carrier. For example, a carrier may utilize three different patterns, A, B and C. These patterns may be arranged in adjacent rows, such as A, B, C, A, B. If substrates are placed in the first three rows, after two shifts and three implants, all substrates would have been implanted with patterns A, B and C. If desired, the substrates may be shifted fewer times, thereby creating substrates with different doping patterns on them.  FIG. 12  shows the various doping patterns that can be achieved using adjacent rows having three different patterns, A, B and C. As can be seen, the doping patterns implanted in the substrates after 0 shifts or 1 shift are unique to the row in which the substrate was originally placed. Thus, using fewer than 2 shifts, it is possible to create substrates of different patterns. However, if the substrates are shifted 2 times, then all substrates are ultimately implanted with all three patterns. This concept can be expanded to an arbitrary number of patterns if desired, for example, if N patterns are used, N-1 shifts can be used to produce identical doping patterns on all substrate. A fewer number of shifts will create unique substrates. To allow N-1 shifts, it is necessary that at least this number of rows were not populated with substrates, thus allowing the placed substrates to be able to shift to unpopulated or vacant rows. 
         [0060]    In addition, although  FIGS. 10 and 11  show that all patterns in a particular row are identical, this is not a limitations of the present disclosure. For example, in  FIG. 13 , the carrier  900  has two rows  901 ,  902 . Each row has five positions of columns  910 - 914 . The patterns of row  901  are arranged from left to right as A, B, A, B, A. To achieve uniform doping of all substrates, it may be desirable to have the opposite alternating set of patterns in row  902 . For example, row  902  may have patterns B,A,B,A,B, such that each column  910 - 914  has both patterns in it. The patterns do not have to be arranged in alternating fashion as shown in  FIG. 13 . To achieve uniform doping of all substrates, it is only important that each column  910 - 914  has each type of pattern in it. 
         [0061]    In addition, any of the above embodiments can be used for applications where there is a need to have successive implants of different species. For example, one pattern may be implanted with a first species, such as phosphorus, and another pattern may be implanted with a second species, such as boron. The carrier  840 , such as those shown in  FIGS. 10A-B  and  11 , may be used for each implant with the position of the substrates shifting in the carrier between implant steps. The successive implants may be performed in different implanters, or they may be performed in two implanters that have been clustered into a single vacuum system, or they may be performed in the same process chamber from separate ion sources, or from a single ion source that can switch quickly between species. 
         [0062]      FIG. 14  shows a carrier  990 , with a first row  991  and a second row  992 . The pattern in the first row  991  differs from that in the second row  992 . In this embodiment, the patterns are created so as to implant an interdigitated back contact (IBC) for a solar cell. In one embodiment, the substrates are populated in first row  991 . The substrates are then exposed to an ion beam, containing one species of dopant, such as for example n-type dopants. The substrates are then moved to the second row  992 , using any of the methods described above and aligned. The substrates are then exposed to an ion beam, containing a second species of dopant, such as for example p-type dopants. The two patterns are created so as to create non-overlapping interdigitated regions of highly doped material. Such a doping pattern may be used on the back side of an IBC solar cell. 
         [0063]    The terms and expressions which have been employed herein are used as terms of description and riot of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described (or portions thereof). It is also recognized that various modifications are possible within the scope of the claims. Other modifications, variations, and alternatives are also possible. Accordingly, the foregoing description is by way of example only and is not intended as limiting.