Abstract:
An improved method of doping a substrate is disclosed. The method is particularly beneficial to the creation of interdigitated back contact (IBC) solar cells. A patterned implant is performed to introduce a first dopant to a portion of the solar cell. After this implant is done, an oxidation layer is grown on the surface. The oxide layer grows more quickly over the implanted region than over the non-implanted region. An etching process is then performed to remove a thickness of oxide, which is equal to the thickness over the non-implanted regions. A second blanket implant is then performed. Due to the presence of oxide on portions of the solar cell, this blanket implant only implants ions in those regions which were not implanted previously.

Description:
FIELD 
     This invention relates to solar cells and, more particularly, to solar cells formed using ion implantation. 
     BACKGROUND 
     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. 
     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. 
     Solar cells typically consist of a p-n semiconducting junction.  FIG. 1  is a cross-sectional view of an interdigitated back contact (IBC) solar cell. In the IBC solar cell, the p-n junction is on the back or non-illuminated surface of the solar cell. Photons  10  enter the solar cell  100  through the top (or illuminated) surface, as signified by the arrows. These photons  10  pass through an anti-reflective coating  104 , designed to maximize the number of photons  10  that penetrate the substrate  100  and minimize those that are reflected away from the substrate. The ARC may be comprised of an SiN x  layer. Beneath the ARC  104  may be a SiO 2  layer  103 , also known as a passivation layer. Of course, other dielectrics may be used. On the back side of the solar cell  100  is an emitter region  204 . 
     Internally, the solar cell  100  is formed so as to have a p-n junction. This junction is shown as being substantially parallel to the top surface of the solar cell  100 , although there are other implementations where the junction may not be parallel to the surface. In some embodiments, the solar cell  100  is fabricated using an n-type substrate  101 . The photons  10  enter the solar cell  100  through the n+ doped region, also known as the front surface field (FSF)  102 . 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 to the depletion region and get swept across to the other side. 
     As a result of the charge separation caused by the presence of this p-n junction, the extra carriers (electrons and holes) generated by the photons can then be used to drive an external load to complete the circuit. 
     The doping pattern is alternating p-type and n-type dopant regions in this particular embodiment. The n+ back surface field  204  may be between approximately 0.1-0.7 mm in width and doped with phosphorus or other n-type dopants. The p+ emitter  203  may be between approximately 0.5-3 mm in width and doped with boron or other p-type dopants. This doping may enable the p-n junction in the IBC solar cell to function or have increased efficiency.  FIG. 8  shows a commonly used pattern for the back side of the IBC solar cell. The metallic contacts or fingers  220  are all located on the bottom surface of the substrate. Certain portions of the bottom surface may be implanted with p-type dopants to create emitters  203 . Other portions are implanted with n-type dopants to create more negatively biased back surface field  204 . The back surface is coated with a passivating layer  210  to enhance the reflectivity of the back surface. Metal fingers  220   b  are attached to the emitter  203  and fingers  220   a  attaches to the BSF  204 . 
     Thus, to form the IBC solar cell, two patterned doping steps may be required. These patterned doping steps need to be aligned to prevent the p+ emitter  203  and the n+ back surface field  204  from overlapping. Poor alignment or overlapping may be prevented by leaving a gap between the p+ emitter  203  and the n+ back surface field  204 , but this may degrade performance of the IBC 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 require extra process steps. 
       FIG. 2  is a block diagram of a first method to form an IBC solar cell according to the prior art. This process requires two patterned diffusion steps (shown as “Screen Print Patterned Resist”) which must be well aligned to produce the pattern of  FIG. 8 .  FIG. 8  shows one example of an IBC pattern; others include a grid, “dots”, or a hexagonal pattern.  FIG. 3  is a block diagram of a second method to form an IBC solar cell. This embodiment performs a first blanket diffusion. The emitter is then etched to expose underlying silicon. The etch mask and the diffusion mask can be the same, although different chemistries are used to etch the oxide mask and to dope the underlying silicon. 
     The embodiments of  FIGS. 2-3  both require a large number of expensive process steps to form an IBC solar cell. 
     Therefore, there is a need in the art for an improved method of doping for solar cells and, more particularly, an improved method of doping for IBC solar cells using ion implantation. 
     SUMMARY 
     An improved method of doping a substrate is disclosed. The method is particularly beneficial to the creation of solar cells that require patterning like the interdigitated back contact (IBC) solar cells. A patterned implant is performed to introduce a first dopant to a portion of the solar cell. After this implant is done, an oxide or nitride layer is grown on the surface. The oxide or nitride layer grows more quickly over the implanted region than over the non-implanted region. An etching process is then performed to remove a thickness of oxide or nitride, which is equal to the thickness over the non-implanted regions. A second blanket implant is then performed. Due to the presence of oxide or nitride on portions of the solar cell, this blanket implant only implants ions in those regions which were not implanted previously. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
         FIG. 1  is a cross-sectional view of an IBC solar cell; 
         FIG. 2  is a block diagram of a first method to form an IBC solar cell according to the prior art; 
         FIG. 3  is a block diagram of a second method to form an IBC solar cell according to the prior art; 
         FIG. 4  is a chart showing the relationship between silicon dopant concentration and oxide thickness; 
         FIGS. 5A-I  are cross-sectional views of a first method to form an IBC solar cell; 
         FIGS. 6A-C  are cross-sectional views of a second method to form an IBC solar cell; 
         FIGS. 7A-G  are cross-sectional views of a third method to form an IBC solar cell; and 
         FIG. 8  is a bottom view of the IBC solar cell of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments of the solar cell are described herein in connection with an ion implanter. Beamline ion implanters, plasma doping ion implanters, focused beam ion implanters, or flood ion implanters may be used. However, the 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 (interdigitated back contact) 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 invention is not limited to the specific embodiments described below. 
     It is known that doped silicon oxidizes at a higher rate than undoped or lightly doped silicon.  FIG. 4  shows the relationship between grown oxide thickness and the dopant concentration in the underlying silicon. This data is based on the following test conditions. The temperature of the environment was about 840° C., and the duration was roughly 40 minutes. As seen in  FIG. 4 , in this test environment an oxide thickness of 5.3 nm is grown on undoped silicon. However, as the dopant in the silicon increases, the grown oxide thickness increases as well. As dopant concentrations of 1E+15/cm 2 , the thickness is 10 nm, while at dopant concentrations approaching 1E+16/cm 2  the thickness grows to over 25 nm. Since this grown oxide is effective at blocking ions from penetrating into the underlying silicon, this grown oxide layer can be used as a mask for subsequent process steps. Similar results can be achieved with the use of nitrogen or ammonia to produce an uneven nitride layer on the substrate. 
       FIGS. 5A-I  are cross-sectional views of a first method to form an IBC solar cell. In  FIG. 5A , a substrate  300  is shown. This substrate may be an n-type substrate, such as n-silicon. 
     In  FIG. 5B , a mask  301  is positioned between the ion beam and the substrate  300 . This mask  301  is used to block a region of the substrate  300  from receiving implantation during the subsequent implant step. This mask may be any suitable mask, including a stencil, proximity or shadow mask. 
     In  FIG. 5C , ions of a dopant of a first species  310  is introduced to the substrate  300 . In some embodiments, the ions are implanted using a beam-line ion implanter. In other embodiments, other methods are used, such as plasma doping, are used. The first species  310  may be boron and may form the p+ emitter  203 . In other embodiments, other Group III elements may be used as the p-type dopant. The mask  301  may serve to substantially prevent the species  310  from being implanted into the regions of the solar cell  300  beneath the mask  301 . 
     After ions of the first species  310  have been implanted in the substrate  300 , the mask is removed as shown in  FIG. 5D . The substrate is subjected to a thermal process, such as an anneal cycle. The anneal cycle may operates at approximately 800-900° C. in one embodiment. The thermal process may be done in the presence of an oxidizing environment. This oxidizing environment is defined as an environment in which sufficient oxygen is present to allow the growth of an oxide layer on the substrate  300 . During the thermal process, the oxide  303  grows on the surface of the substrate  300 , as shown in  FIG. 5E . The oxide  303   a  which is grown over the previously unimplanted region has a first thickness (T 1 ). The oxide  303   b  grown over the previously implanted emitter regions  203 , has a second thickness (T 2 ). As explained above, the thickness of the oxide  303   b  will be greater than that of oxide  303   a , due to the difference in the underlying dopant concentration in the substrate  300 . 
     In another embodiment, the thermal process is preformed in an environment conducive to the growth of a nitride layer, such as an environment containing nitrogen or ammonia. It is understood that while the disclosure describes the creation and subsequent etching of an oxide layer  303 , this technique is equally applicable to a nitride layer. Thus, the thermal process is performed in an environment conducive to the growth of a film, where the film is either an oxide or nitride layer. 
     An etching process, such as a controlled isotropic oxide wet-etch, is then performed. The purpose of this etch is to remove a fixed thickness of oxide from the surface of the substrate  300 . The thickness of oxide to be removed is substantially equal to the thickness of oxide  303   a . In one embodiment, a dilute buffered hydrofluoric (HF) acid is used, which etches slower and allows better thickness control than other acids. Of course, other acids may be used. In other embodiments, this etch may be a plasma etch or any other etch chemistry. The result, shown in  FIG. 5F , is that no oxide exists over the previously unimplanted regions, while a layer of oxide  303   b  still remains on previously implanted emitter regions  203 . The thickness of the remaining oxide may be approximately equal to T 2 −T 1 . 
     Ions of a dopant of a second species  320  are then blanket implanted into the substrate  300 , as shown in  FIG. 5G . As described above, the ions of the second species  320  may be introduced using a beam-line implanter, a plasma doping system, or any other suitable means. The second species  320  may be an n-type dopant, such as phosphorus, or any other Group V element. This blanket implant of second species  320  may be used to create n+ back surface fields  204 , as shown in  FIG. 5H . 
     The remaining oxide  303   b  is then removed with a second etching step, as shown in  FIG. 5I . Less precision may be required in this etching process, so a less expensive and time-consuming process may be employed. In other embodiments, the same etching process as was used above is employed. 
     In some embodiments, this blanket implant of second species  320 , shown in  FIG. 5G , can be used to implant both surfaces of the substrate  300 . These blanket implants may be sequential or at least partially simultaneous. This second method is shown in  FIG. 6A . In this embodiment, a species  320  of n-type dopant, such as phosphorus, is implanted on both surfaces of the substrate  300 . This blanket implant results in the creation of an n+ front surface field  102  and n+ back surface fields  204 , as shown in  FIG. 6B . 
     As described above, the oxide  303   b  is then removed from the back side of the substrate  300 , using an etching process, as seen in  FIG. 6C . 
     While this process has been described with the p-type dopant being used as the first species  310 , other embodiments are possible. For example, the first species (i.e.  FIG. 5C ) may be an n-type dopant, such as phosphorus. In this embodiment, the simultaneous implant of the front surface (as shown in  FIG. 6A ) would be performed at this point of the process. In addition, in this embodiment, the second species  320  may be a p-type dopant. Other steps of the process would remain similar. 
     In other words, this second embodiment would include the following steps, illustrated in  FIG. 7 . First, as shown in  FIG. 7A , a mask  401  is placed over region of a substrate  400  which will subsequently become the p+ emitter  203 . Second, as illustrated in  FIG. 7B , an n-type dopant  410  is implanted through the mask  401  on bottom surface. It should be noted that the blanket implant of the front surface may be done at this time as well. These implants create the n+ front surface fields  102  and back surface field regions  204 . Third, as shown in  FIG. 7C , the mask  401  is removed and the substrate is subjected to a thermal process in an oxygen rich environment to grow oxide on the back surface. As described above, and shown in  FIG. 7D , the thickness of the oxide  403   b  on the previously implanted regions  204  (T 2 ) is greater than the thickness of the oxide  403   a  on the previously unimplanted regions (T 2 ). Next, a thickness of oxide is etched from the back surface where this thickness is sufficient to expose the previously un-implanted regions, as illustrated in  FIG. 7E . Next, a blanket implant of p-type dopant  420  to create the p+ emitters  203  is performed on the back surface, as shown in  FIG. 7F . Finally, as illustrated in  FIG. 7G , a second etch process is performed to remove remaining oxide from back surface. As noted above, the thermal process of  FIG. 7C  may be performed in a nitrogen rich environment or in the presence of ammonia to create a nitride layer, rather than an oxide layer. 
     In both cases, another oxidation process may be performed to passivate both sides of the inter-digitated back contact solar cell. 
     While this process describes the creation of IBC solar cells, it can be used for other processes as well. For example, while this disclosure describes species  310  and species  320  as being opposite conductivity, this is not a requirement of the present disclosure. For example, dopants of the same conductivity may be used for the two implants. This may be done when the species of the two implants differs, such as phosphorus and arsenic for n-type implants or boron and aluminum or gallium for p-type implants. Different dopants of the same conductivity may be used to take advantage of the different implant or diffusion characteristics of the various species. 
     In another embodiment, the same species may be used for both implants, where there is a difference in the implant parameters between the two implants. For example, the desired concentration of the two implants may be different, or the implant energy of the two implants may differ. In such a case, a patterned implant is done at the first set of operating parameters. Afterwards, the oxide is grown and etched, as described above. The blanket implant, using the same species, but different implant parameters, is then performed. 
     In addition, while IBC solar cells were used as an exemplary implementation of the process, other devices may also be processed in this manner. For example, conventional solar cells, or selective emitter solar cells can also be made using this process. In addition, other semiconductor devices which require alignment of one doped region to a subsequently doped region may also use this process. 
     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.