Abstract:
An improved method of doping a workpiece is disclosed. The method is particularly beneficial to the creation of interdigitated back contact (IBC) solar cells. A patterned implant is performed on one surface of the workpiece. A self-aligned masking process is then performed, which is achieved by exploiting the changes in surface properties caused by the patterned implant. The masking process includes applying a coating that preferentially adheres to the previously implanted regions. A blanket implant is then performed, which serves to implant the portions of the workpiece that are not covered by the coating. Thus, the blanket implant is actually a complementary implant, doping the regions that were not implanted by the first patterned implant. The coating is then optionally removed from the workpiece.

Description:
FIELD 
       [0001]    This disclosure relates to a method of processing a workpiece and, more particularly, to the creation of a self-aligned mask for creating complementary doped regions using ion implantation. 
       BACKGROUND 
       [0002]    Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. 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 workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. 
         [0003]    Solar cells are one example of a device that uses silicon workpieces. Any reduced cost to the manufacture or production of high-performance solar cells or any 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. 
         [0004]    The doping pattern for certain types of solar cell, such as interdigitated back contact (IBC) solar cells, may include both p-type and n-type dopant regions disposed on the same surface of the solar cell. Certain portions of the surface may be implanted with p-type dopants to create emitters. Other portions are implanted with n-type dopants to create more negatively biased back surface fields. The surface may then be coated with a passivating layer to enhance the reflectivity. Metal fingers are attached to the emitter and to the BSF. 
         [0005]    Thus, to form the IBC solar cell, two patterned doping processes may be performed. These patterned doping processes are aligned to prevent the p-type emitters and the n-type back surface fields from overlapping. Poor alignment or overlapping may be prevented by leaving a gap between the p-type emitter and the n-type back surface field, but this may degrade performance of the solar cell. Even when properly aligned, such patterned doping may have large manufacturing costs. For example, photolithography or hard masks (such as an oxide) may be used, but both are expensive and use extra fabrication processes. 
         [0006]    Therefore, it would be beneficial if there were an improved method of high precision doping for solar cells and, more particularly, an improved method of high precision doping for IBC solar cells using ion implantation. 
       SUMMARY 
       [0007]    An improved method of doping a workpiece is disclosed. The method is particularly beneficial to the creation of interdigitated back contact (IBC) solar cells. A patterned implant is performed on one surface of the workpiece. A self-aligned masking process is then performed, which is achieved by exploiting the changes in surface properties caused by the patterned implant. The masking process includes applying a coating that preferentially adheres to the previously implanted regions. A blanket implant is then performed, which serves to implant the portions of the workpiece that are not covered by the coating. Thus, the blanket implant is actually a complementary implant, doping the regions that were not implanted by the first patterned implant. The coating is then optionally removed from the workpiece. 
         [0008]    In one embodiment, a method of creating complementary implanted regions on a workpiece is disclosed. The method comprises performing a patterned implant, where a mask, disposed separate from the workpiece, is used to block first ions from reaching portions of the workpiece, creating first implanted regions and unimplanted regions; applying a coating directly on the workpiece to only the first implanted regions of the workpiece; and performing a second implant while the coating is disposed on the first implanted regions, so that second ions of the second implant are implanted into the unimplanted regions, forming second implanted regions. In certain embodiments, the coating is applied using molecular layer deposition. In certain further embodiments, applying the coating comprises: applying a first material to a surface of the workpiece, wherein the first material adheres to the first implanted regions; and alternately depositing a first precursor and a second precursor on the first material so as to form a polymeric compound on the first implanted regions. 
         [0009]    In one embodiment, a method of creating complementary implanted regions on a workpiece is disclosed. The method comprises performing a patterned implant, where a mask is used to block first ions from reaching portions of the workpiece, creating first implanted regions and unimplanted regions; applying a coating to the workpiece, wherein the coating adheres to the first implanted regions and does not adhere to the unimplanted regions; and performing a second implant while the coating is disposed on the first implanted regions, so that second ions of the second implant are implanted into the unimplanted regions, forming second implanted regions, and wherein the coating blocks the second ions from penetrating the first implanted regions. In certain embodiments, the first implanted regions are made hydrophilic by the patterned implant, and the coating adheres to hydrophilic surfaces. In certain embodiments, the first ions comprise a dopant species and a surface modifying species. In certain embodiments, a second patterned implant is performed, wherein the patterned implant implants a dopant species and the second patterned implant implants a surface modifying species. 
         [0010]    In one embodiment, a method of creating complementary implanted regions on a workpiece is disclosed. The method comprises performing a patterned implant using first ions, wherein the first ions comprise a dopant species and a surface modifying species, so as to create first implanted regions and unimplanted regions, wherein the first implanted regions become hydrophilic; applying a polymeric coating to the workpiece, using molecular layer deposition, wherein the polymeric coating adheres to the first implanted regions and does not adhere to the unimplanted regions; and performing a second implant using second ions while the polymeric coating is disposed on the first implanted regions, wherein the polymeric coating blocks the second ions from the first implanted regions and the second ions are implanted into the unimplanted regions, forming second implanted regions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
           [0012]      FIG. 1  shows an apparatus that may be used to process the workpiece according to one embodiment; 
           [0013]      FIG. 2  is a flowchart illustrating the fabrication sequence used to create the complementary doped regions; 
           [0014]      FIGS. 3A-3D  show a first workpiece being processed using the sequence of  FIG. 2 ; and 
           [0015]      FIGS. 4A-4D  show a second workpiece being processed using the sequence of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The embodiments are described herein in connection with an ion implanter. Beamline ion implanters, plasma doping ion implanters, focused plasma systems, systems that modulate a plasma sheath, or flood ion implanters may be used. However, gaseous diffusion, furnace diffusion, laser doping, other plasma processing tools, or other methods known to those skilled in the art also may be used. While specific n-type and p-type dopants are listed, other n-type or p-type dopants may be used instead and the embodiments herein are not limited solely to the dopants listed. Furthermore, while one particular embodiment of a solar cell is specifically listed, embodiments of this process may be applied to other solar cell designs or other workpieces, such as semiconductor wafers or flat panels. Thus, the disclosure is not limited to the specific embodiments described below. 
         [0017]      FIG. 1  shows a representative ion implantation system  100  that may be used to process the workpiece  10  so as to form a solar cell. The ion implantation system  100  may include an ion source  110 , a workpiece support  120  and a mask  130 . An ion beam  190  is extracted from the ion source  110  and directed toward the workpiece  10 , which is disposed on the workpiece support  120 . A mask  130  may be disposed between the ion source  110  and the workpiece  10  and may serve to block a portion of the ion beam  190  from reaching the workpiece  10 . 
         [0018]    In one particular embodiment, the ion source  110  may include an ion source chamber  111  having an extraction aperture  112 . On the side opposite the extraction aperture  112  may be a dielectric window  113 . An RF antenna  114  may be disposed proximate the dielectric window  113 . An RF power supply  115  may be in communication with the RF antenna  114 . Energy supplied by the RF antenna  114  may cause gasses within the ion source chamber  111  to form a plasma. In some embodiments, the ions may be extracted from the ion source chamber  111  using an electrode  116  disposed proximate the extraction aperture  112 , on the outside of the ion source chamber  111 . The electrode  116  may have an aperture, aligned with the extraction aperture  112 . The electrode  116  may be negatively biased so that positively charged ions are attracted from the ion source chamber  111 . The ions pass through the extraction aperture  112  and the aperture in the electrode  116  and may form an ion beam  190 . 
         [0019]    In certain embodiments, a mass analyzer (not shown) may be used to separate the various ions extracted from the ion source chamber  111  so that only ions of a particular charge and mass are directed toward the workpiece  10 . In other embodiments, a mass analyzer is not used such that all extracted ions reach the workpiece  10 . 
         [0020]    In certain embodiments, an electrode  116  is not used. Rather, the workpiece support  120  may be negatively biased to attract positively charged ions from the ion source chamber  111 . 
         [0021]    The ion beam  190  extracted from the ion source  110  may be a spot ion beam, a ribbon ion beam, or an ion beam of another shape. In certain embodiments, the ion beam  190  is scanned or otherwise manipulated prior to reaching the workpiece  10 . Of course, other types of ion sources may be used, and the disclosure is not limited to this, or any particular, implementation. 
         [0022]    The mask  130  is disposed between the ion source  110  and the workpiece  10 , and serves to block a portion of the ions in the ion beam  190  from reaching the workpiece  10 . The mask  130  may be disposed a distance from the workpiece  10  and be separate from the workpiece  10 . This mask  130  may be referred to as a shadow mask or a proximity mask. For example, the mask  130  may be held in place by a separate apparatus (not shown), or may be connected to the workpiece support  120 . The mask  130  may be a material suitable for implantation, such as, for example, graphite. Further, the mask  130  may have one or more apertures which define the implant pattern that the workpiece is exposed to. Additionally, the mask  130  may be movable, such that some implants, referred to as patterned implants, may be performed with the mask  130  in place, while other implants, referred to as blanket implants, are performed without the mask  130 . 
         [0023]    To create certain semiconductor devices, such as, for example, IBC solar cells, two dopant regions are created on a single surface. These dopant regions may be, for example, an n-type doped region and a p-type doped region. The n-type dopant region and the p-type dopant region may be complementary, implying that each location on the surface of the workpiece is either an n-type doped region or a p-type doped region. In other words, there are no regions on the surface of the workpiece that are not doped. Further, there are no regions on the surface of the workpiece that are doped by both n-type and p-type dopants. 
         [0024]    To achieve complementary dopant regions, the implanted regions of the n-type dopant and p-type dopant may be precisely controlled. Masks may be used to perform two patterned implants. The process of performing two patterned implants using one or more masks may be performed. However, aligning the patterns optically and/or mechanically can be challenging at times depending on the dimensions of the dopant regions. 
         [0025]    The present disclosure describes a novel method of creating two complementary dopant regions using self-aligned masks. The implanting of certain species into a workpiece may affect surface properties of that workpiece, such as its morphology, crystallinity, chemical characteristics, hydrophobicity and hydrophilicity. For example, many different species, such as CF 4 , CHF 3 , C 3 F 6 , BF 3 , SF 6 , SiF 4 , NF 3 , He, and O 2 , can be implanted for the purpose of modulating surface energies to make the surface of the workpiece either more hydrophobic or more hydrophilic. For example, surfaces can be made more hydrophobic with the implant of certain species, such as CF 4 . However, other species, such as fluorinated carbons (including CHF 3  and C x F y ), BF 3 , SF 6  and SiF 4  may also be used to create a more hydrophobic surface. Surfaces can be made more hydrophilic with an implant of NF 3 . However, other species, such as O 2  and He, may also be used. This list of species is not intended to be inclusive, but rather illustrates some of the species that can be used to affect surface energy of a workpiece. By exploiting these changes in surface properties, it is possible to coat the surface of the workpiece using an organic or inorganic coating such that the coating adheres to only one of the hydrophobic and hydrophilic regions of the workpiece. Once in place, this coating serves as a self-aligned mask for a second implant, and blocks ions from the second implant from penetrating the regions that were previously implanted. 
         [0026]      FIG. 2  shows a representative process that may be used to create a workpiece having complementary doped regions according to one embodiment. First, as shown in Process  200 , a patterned implant is performed. This patterned implant may be performed using a mask  130 , as shown in  FIG. 1 . The pattern that is created is not limited by the present disclosure. The patterned implant is used to implant first ions into the workpiece. 
         [0027]      FIGS. 3A-3D  show a first workpiece being processed using the sequence of  FIG. 2 . In this embodiment, the workpiece is being implanted using a mask having a plurality of striped apertures.  FIGS. 4A-4D  show a second workpiece being processed using the sequence of  FIG. 2 . In this embodiment, the workpiece is being implanted using a mask having a plurality of circular apertures. After the patterned implant, each workpiece has first implanted regions  310  and unimplanted regions  320 . The first implanted regions  310  of the workpiece are created using a patterned implant, where the mask  130  (see  FIG. 1 ) has apertures that correspond to the first implanted regions  310 . The shape of the first implanted regions  310  is not limited by this disclosure.  FIGS. 3A and 4A  illustrate two different shapes, however other shapes are also possible. For example, in certain embodiments, oval or elliptical shapes may also be used. 
         [0028]    The first implanted regions  310  may be formed using only one ion species, such as when mass analysis is performed. In other embodiments, the first implanted regions  310  are created using a plurality of different ion species. In one embodiment, multiple ion species may be implanted by using an ion source  110  without mass analysis. In other embodiments, multiple ion species may be implanted by performing two or more patterned implants, where each patterned implant introduces one ion species. 
         [0029]    In certain embodiments where more than one ion species is implanted, the first implanted regions  310  may contain a dopant species, such as a p-type or n-type dopant, and a surface modifying species. In certain embodiments, the dopant species and the surface modifying species may be the same species. In certain embodiments, the patterned implant of  FIG. 2  may comprise one or more patterned implants. The patterned implant serves to implant first ions into the first implanted regions  310  of the workpiece, where the first ions may comprise a dopant species and a surface modifying species. 
         [0030]    After the patterned implant, a coating is applied to the surface of the workpiece, as shown in Process  210  of  FIG. 2 . This coating is selected to adhere only to the first implanted regions  310 . For example, the coating may only adhere to hydrophilic surfaces.  FIGS. 3B and 4B  show a coating  330  being applied only over the first implanted regions  310 . 
         [0031]    Unlike the mask  130 , the coating  330  is deposited directly on the workpiece  10 . The coating  330  may be of such a thickness so as to block subsequently implanted ions from penetrating the coating  330  and penetrating the underlying first implanted regions  310 . 
         [0032]    In some embodiments, the coating  330  is selected based on the surface properties of the first implanted regions  310 . In certain embodiments, the coating may be water soluble or soluble in commonly available organic solvents. Examples of water-soluble coatings may include, but are not limited to, water-soluble color dyes/compounds. Other examples include water-soluble polymeric compounds such as polyethylene glycol (PEG), which can form a polymeric coating selectively on hydrophilic surfaces. The coating maybe applied by spin coating, dip coating or other polymer deposition techniques, such as molecular layer deposition (MLD).Of course, other methods of applying the coating may be used, such as, but not limited to, using brushes, roller or another applicator. 
         [0033]    MLD is a process that may be suited to produce a coating that meets the selectivity and thickness criteria. The first implanted regions  310  may react with 3-aminoproyltriethoxysilane (APTES) or 3-aminopropyltrimethoxysilane (APTMS) to provide an amine-terminated surface, which can react with precursor A and precursor B sequentially and repetitively to form a thicker polymeric layer. Precursor A may include bifunctional organic molecules with acidic functionalities, such as adipoyl dichloride (111-50-2), pyromellitic acid anhydride (PMDA) (89-32-7), 1,4-diisocyanatobenzene (104-49-4), 1,4-diisothiocyantobenzene (4044-65-9), 1,4-benzenedicaboxaldehyde (623-27-8) and 1,4-benzenedicarbonyldichloride (100-20-9). Precursor B may include bifunctional organic molecules with amino or hydroxyl functionalities, such as 1,6-hexanediamine (124-09-4), 1,4-butanediamine (110-60-1), 1,4-butynediol (110-65-6), 1,4-benzenediamine (106-50-3), ethylene glycol (107-21-1) (CAS numbers are given in parentheses). The resultant polymeric compounds include polyamide, polyimide, polyurea, polythiourea, polyurethane, polyazomethine and polyester. This in-situ process may be carried out in liquid phase by dip coating. 
         [0034]    3-cyanopropyltrichlorosilane (1071-27-8) can also be used to selectively bind to the first implanted region  310  with a cyano group protruding from the surface. The cyano group can be hydrolyzed in aqueous acid to carboxylic acid or reduced to an amino group using various reducing agents, such as, for example, NaBH 4  in ethanol. The carboxylic or amino function can be used as template for further polymerization through MLD, as described above. 
         [0035]    In another embodiment, 1,2-Bis(trimethoxysilyl)ethane (91166-50-6) can also be used to selectively bind to first implanted regions  310 . However, unlike the compounds described above, this particular compound has the potential to undergo self polymerization to meet the criteria of self-aligned masking. The coating may be formed by attaching a plurality of molecules of the same compound on top of one another. 
         [0036]    Other compounds or molecules may be used, depending on the implanted species. For example, polymeric siloxane molecules may be spin coated or dip coated. This process may not involve in-situ polymerization and selectivity depends on whether the process is self-assembled or not. Examples include poly(dimetylsiloxane), hydroxyl terminated (Av Mn=550), poly(dimethylsiloxane), vinyl terminated (Av Mn=25000), poly(dimethylsiloxane-co-diphenylsiloxane), dihydroxy terminated, poly(dimethylsiloxane), chlorine terminated (Av Mn=3000). 
         [0037]    To further improve the selectivity of the coating between the first implanted regions  310  and the unimplanted regions  320 , the unimplanted regions  320  may optionally be further functionalized. In one embodiment, the unimplanted regions  320  may be treated using known and commonly found chemical agents, such as hexamethyldisliazane (HMDS), that render the surface of the workpiece completely hydrophobic. For example, HDMS molecules may be applied to the surface of the workpiece. The first implanted regions  310  may reject the HDMS molecules, while these molecules interact with the unimplanted regions  320 . 
         [0038]    As described above, the coating  330  is applied to the surface of the workpiece, and adheres only to first implanted regions  310 . After the coating has been applied, a second implant is performed, as shown in Process  220  of  FIG. 2 . This second implant is performed without the mask  130  disposed between the ion source  110  and the workpiece  10 . Consequently, this implant may be referred to as a blanket implant. However, the coating  330  serves as a hard mask, which is defined as a mask that is disposed directly on the workpiece being implanted. 
         [0039]    This second implant may be performed using second ions having the opposite conductivity of the dopant ions used for the patterned implant. For example, if the dopant species used for the patterned implant (Process  200 ) was an n-type dopant, such as phosphorus, the second ions used for the second implant may be a p-type dopant, such as boron. Thus, the previously unimplanted regions  320  are now implanted by the second implant, forming second implanted regions  340 , while the coating  330  blocks the second ions from the second implant from penetrating the first implanted regions  310 . The degree of blocking afforded by the coating  330  may be based on various factors, including the thickness of the coating, the temperature of the workpiece, and the plasma stability. The coating  330  may block most of the second ions from penetrating the first implanted regions  310 . In certain embodiments, the blocking efficiency, which is defined as the percentage of second ions that are blocked by the coating  330 , may be at least 90%. Thus, after Process  220 , the workpiece has second implanted regions  340  and first implanted regions, which remain covered by the coating  330 . 
         [0040]    Sometime after the second implant, the coating  330  is removed, as shown in Process  230  of  FIG. 2 . In the case of a water-soluble coating, the workpiece  10  may be rinsed with water to remove the coating  330 . In other embodiments, a suitable solvent may be used to rinse the coating  330  from the workpiece  10 . In certain embodiments, the coating  330  may be removed by the application of heat to the workpiece. In certain embodiments, the coating  330  is removed immediately after Process  220 . In other embodiments, one or more subsequent processes may be performed while the coating  330  is still in place. 
         [0041]    After the coating  330  is removed, the workpiece comprises a first implanted region  310  and a second implanted region  340 , where the sum of the first implanted region  310  and the second implanted region  340  constitutes the entirety of the surface of the workpiece. Further, due to the self-aligning nature of the coating process, there is no overlap between the first implanted region  310  and the second implanted region  340 . Additionally, the first implanted region  310  comprises first ions and the second implanted region  340  comprises second ions, where the conductivity of the second ions may be opposite the conductivity of the dopant species contained within the first ions. 
         [0042]    By exploiting the surface properties of the first implanted regions  310 , it is possible to create a hard mask, in the form of a polymer coating, which covers only the first implanted regions  310 . Because of this, the second implant process implants only the regions that were not implanted by the first implant. This creates a workpiece with complementary dopant regions with relatively high precision. 
         [0043]    The embodiments of this disclosure have many advantages. First, the use of a coating that adheres only to the first implanted regions provides a self-aligned mask. This self-aligned mask has a high degree of precision. Second, this approach yields a simplified process flow and should considerably reduce the extra costs associated with the in-situ masking and alignment hardware as well as result in a higher throughput implant process. Finally, the self-aligned mask allows maximizing the dopant coverage for a given cell architecture, regardless of the complexity of the different structures/shapes. 
         [0044]    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.