Abstract:
A system and method of exposing photoresist on the surface of the solar cell to light so as to create an appropriate mask is disclosed. A microcavity array is used to expose the photoresist to UV light in a pattern that matches the desired pattern on the solar cell. Microcavity arrays consist of an array of cavities, which may include tens of thousands of cavities. When an appropriate potential is applied to an electrode, a plasma is formed in the activated cavity. If the cavity contains a suitable gaseous environment, these activated cavities will emit light in the near ultraviolet spectrum. By properly configuring the locations of the activated cavities, a UV source may be created that exposes the photoresist in a desired pattern. The desired pattern can be created by selectively activating cavities, disabling certain cavities, or filling certain cavities so that they cannot create a plasma.

Description:
FIELD 
       [0001]    This invention relates to implantation of ions in silicon substrates and, more particularly, to a system and method for creating photoresist masks for solar cells. 
       BACKGROUND 
       [0002]    Ion implantation is a standard technique for introducing conductivity-altering impurities into substrates. 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. 
         [0003]    Solar cells provide pollution-free, equal-access energy using a free natural resource. Due to environmental concerns and rising energy costs, solar cells, which may be composed of silicon substrates, are becoming more globally important. 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]    Doping may improve efficiency of solar cells.  FIG. 1  is a cross-sectional view of a selective emitter solar cell  210 . It may increase efficiency (e.g. the percentage of power converted and collected when a solar cell is connected to an electrical circuit) of a solar cell  210  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 100-300 μm across.  FIG. 2  is a cross-sectional view of an interdigitated back contact (IBC) solar cell  220 . In the IBC solar cell, the junction is on the back of the solar cell  220 . 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. 
         [0005]    As shown in  FIG. 3 , the doping pattern includes alternating p-type and n-type dopant regions in this particular embodiment. The p+ emitter  203  and the n+ back surface field  204  are appropriately doped. This doping may enable the junction in the IBC solar cell to function or have increased efficiency. 
         [0006]    Some solar cells, such as IBC solar cells, require that different regions of the solar cell be p-type and others n-type. It may be difficult to align these various regions without overlap or error. For example, the p+ emitter  203  and n+ back surface field  204  in  FIG. 3  must be doped. If overlap between the p-type regions  203  and the n-type regions  204  exists, counterdoping may occur. Any overlap or misalignment also may affect the function of the solar cell. For solar cells that require multiple implants, particularly those with small structure or implant region dimensions, the alignment requirements can limit the use of a shadow mask or proximity mask. In particular, as shown in  FIG. 3 , an IBC solar cell requires alternating lines doped with, for example, B and P. Therefore, any shadow mask or proximity mask for the B implant has narrow, long apertures that are carefully aligned to the small features that were implanted with P using a different proximity mask or shadow mask. 
         [0007]    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. Currently, the use of photoresist, however, would add an extra cost to solar cell production because extra process steps are involved. For example, a shadow or proximity mask must be created and used to illuminate a portion of the photoresist, such that a hardened mask is created on the surface of the solar cell. 
         [0008]    Accordingly, there is a need in the art for an improved method of implanting a solar cell and, more particularly, a system and method of exposing the photoresist on the surface of the solar cell to light so as to create the appropriate mask. 
       SUMMARY 
       [0009]    A system and method of exposing photoresist on the surface of the solar cell to light so as to create an appropriate mask is disclosed. A microcavity array is used to expose the photoresist to UV light in a pattern that matches the desired pattern on the solar cell. Microcavity arrays consist of an array of cavities, which may include tens of thousands of cavities. When an appropriate potential is applied to an electrode, a plasma is formed in the activated cavity. If the cavity contains a suitable gaseous environment, these activated cavities will emit light in the near ultraviolet spectrum. By properly configuring the locations of the activated cavities, a UV source may be created that exposes the photoresist in a desired pattern. The desired pattern can be created by selectively activating cavities, disabling certain cavities, or filling certain cavities so that they cannot create a plasma. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
           [0011]      FIG. 1  is a cross-sectional view of a selective emitter solar cell; 
           [0012]      FIG. 2  is a cross-sectional view of an interdigitated back contact solar cell; 
           [0013]      FIG. 3  is a view of an interdigitated back contact solar cell; 
           [0014]      FIG. 3  is a cross-sectional view of implantation through a mask; 
           [0015]      FIG. 4  a cross-sectional view of one embodiment of a microcavity; 
           [0016]      FIG. 5  shows a top view of a microcavity array; 
           [0017]      FIG. 6  shows a top view of an addressable microcavity array; 
           [0018]      FIG. 7  illustrates the use of a microcavity to expose photoresist to ultraviolet light; and 
           [0019]      FIG. 8  illustrates the use of a glass surface as a lens to focus the ultraviolet light. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Embodiments of this system are described herein in connection with solar cells. However, the embodiments of this system can be used with, for example, semiconductor substrates or flat panels. Thus, the invention is not limited to the specific embodiments described below. 
         [0021]      FIG. 4  is a cross-sectional view of one embodiment of a microcavity. In this embodiment, cavities  400  are created in a substrate  401 . A first electrode  410  and a second electrode  420 , having different potentials are formed around the cavity. These electrodes  410 ,  420  are separated, such as by dielectric layer  430 . A protective layer  440 , such as a second dielectric layer, is located above the second electrode  420 . In operation, a gas is injected into the cavity  400 . When the electrodes  410 ,  420  are activated, a potential difference appears across the height of the cavity  400 . This potential difference causes the injected gas to become plasma  403 . If a suitable gas is selected, this plasma will emit light in the ultraviolet spectrum. Such gasses include, for example, argon, xenon, xenon-neon, argon-deuterium and nitrogen. 
         [0022]      FIG. 5  shows a top view of a microcavity array  407 . In this embodiment, the cavities  400  are arranged in rows and columns to form an array. In some embodiments, the microcavities  400  are formed through the use of photolithography. For example, photoresist is not deposited in those areas that will form the cavities  400 . An etching process is then performed which removes material in the exposed regions of the substrate, thereby creating the cavities  400 . Linear cavities have been built as small as 5 um in width, and point cavities with spacings of 100 um have been made. Therefore, a resolution of 100 um is readily achievable with current technology. 
         [0023]    The first electrode  410  and second electrode  420  may be configured in a number of ways. In one embodiment, the first electrodes  410  for all cavities are connected together. Similarly, the second electrodes  420  for all cavities  400  are connected together. In this embodiment, either all of the microcavities  400  are activated or none of the microcavities  400  are activated. In another embodiment, shown in  FIG. 6 , all first electrodes  410  in each column  411  are connected together. Similarly, all second electrodes  420  in each row  421  are connected together. In this way, the selection of a particular row  421  and column  411  activates exactly one microcavity. Of course, other configurations can be created whereby groups of microcavities may be addressable. For example, multiple rows  421  or columns  411  may be electrically connected such that clusters of cavities are activated simultaneously. 
         [0024]    The use of microcavity arrays allows new methods of exposing photoresist to ultraviolet light, for purposes of creating a mask on the substrate. 
         [0025]    In one embodiment, a microcavity array having individually addressable microcavities (or addressable groups of microcavities) is used. A photoresist is applied to the surface of the substrate. The microcavity array is then brought in close proximity to the surface of the substrate. In some embodiments, this distance is approximately 1 mm. 
         [0026]    In some embodiments, the environment in which the microcavity array is placed is filled with a suitable gas, such as nitrogen. In other embodiments, shown in  FIG. 7 , a surface  460 , such as a glass surface, is placed over the microcavity array  407 , so as to form a tight seal. The individual cavities  400  are filled with the desired gas, which is contained in the volume defined by the cavities  400  and by the surface  460 . Such a configuration may be advantageous if the gas used is rare or expensive. In this embodiment, the microcavity array may be constructed such that the surface  460  is sealed to the array  407  and gas is injected prior to the sealing of the surface  460 . 
         [0027]    The desired microcavities  400  are then activated, which causes a plasma  403  to form in these desired cavities. This plasma emits ultraviolet light, which exposes the photoresist  480  located directly beneath the plasma. If a positive photoresist is used, the photoresist located beneath the activated cavities  471  becomes hardened. If a negative photoresist is used, the photoresist located beneath the unactivated cavities  472  becomes hardened. 
         [0028]    In another embodiment, the pattern of light is predetermined. In this embodiment, the microcavity array is created having cavities only in those regions where light is desirable. Microcavity arrays are produced using semiconductor processes, such as photolithography. In one case, a grid of thin photoresist lines is deposited on a silicon wafer, and an anisotropic etch is applied. The etch then creates inverted pyramids between each line in the photoresist. These pyramids become the microcavities. By proper application of photoresist, arrays having microcavities only in particular locations can be fabricated. The inactive parts of the array may be covered with photoresist, such that no inverted pyramids are created in the appropriate regions. This creates a specific pattern of cavities and can be particularly effective for patterns that are commonly used. For example,  FIG. 3  shows the patterns used for IBC solar cells. One or more specially designed microcavity arrays can be designed to create the masks needed to implant regions  203 ,  204 . 
         [0029]    In another embodiment, the microcavity array is manufactured so as to create a complete array, as shown in  FIG. 5 . Certain cavities are then disabled, such as by filling them with a suitable material to prevent a plasma from forming in specific cavities. In one embodiment, inkjet or screen print technologies are used to dispense a material, such as an organic material, to effectively “clog” the inactive cavities. Alternately, photolithography could be used to set the resist in the appropriate regions of the array. This may be more flexible than the first inkjet approach because the coating could be removed and re-printed to change the pattern. The resulting pattern, much like that described in the previous embodiment, is useful for commonly needed patterns, such as the back surface of an IBC solar cell. 
         [0030]    In another embodiment, the electrical connections to the cavities that need to be deactivated can be broken mechanically to render a set of cavities inactive. This technique may work best when the active cavities are contiguous, but by choosing positive or negative photoresist, there is some flexibility in this choice. In one embodiment, a laser can be used to ablate the dielectric layer  430  and the electrode  420  on select parts of the array. This would be between cavities  400  where the laser can be easily focused and the electrode  420  readily accessed. Etching through a mask may accomplish the same result. In this case, a mask would be, for instance, inkjet printed over the array and the dielectric and electrode removed. 
         [0031]    In summary, several methods are disclosed to modify the operation of a traditional microcavity array for the purpose of creating ultraviolet light for exposure to photoresist. First, the power to one or more cavities can be controlled. This can be done using addressable cavities, or by separating one or more electrodes from the power source. Secondly, gas can be prevented from entering one or more cavities, such as by applying a material to fill certain cavities. Thirdly, the cavities can be eliminated, such as by manufacturing the microcavity array without one or more of the cavities. 
         [0032]    In order to achieve smaller features than the cavity size, the glass surface of  FIG. 7  can be used as a lens.  FIG. 8  shows an embodiment, where the glass surface is used as a lens. The diverging light  490  emitted by the cavity  400  becomes focused as it passes through the lens  475 . Such a technique is possible, as the scale (size) of the microcavity array is roughly the same as the size of a CMOS sensor. This lens structure allows better collection of the emitted light to improve the fidelity of the transfer of the pattern of microcavities to the substrate. 
         [0033]    While this form of lithography may find many applications for structures in the scale of tens of microns, a primary application would be for the manufacture of silicon solar cells. In solar cell manufacture, this lithography method can be used for various processes and solar cell architectures. 
         [0034]    In the case of implanting ions into the substrate, the photoresist can be used as a soft mask for ion implantation to allow patterned doping of the substrate. 
         [0035]    In the case of etching, the photoresist can be used as an etch resist to allow etching. Patterned etching can be used to make holes in passivating dielectrics (for example the front side anti-reflective coating on a standard solar cell design) or to etch back the silicon substrate (for example to remove the heavily doped surface between the metal lines on the emitter of a standard solar cell design). 
         [0036]    In the case of metallization, the photoresist can be used to liftoff a metallization that covers the entire face of the solar cell, such as evaporation or sputtering. 
         [0037]    In one application, when doping an interdigitated back cell, the same pattern on the microcavity array can be used with negative and positive photoresists to create complementary regions of p-type and n-type dopants. The fact that the same array of UV sources is used to create each pattern removes most of the problems of relative alignment. 
         [0038]    Relative to conventional proximity masking the microplasma exposure offers several advantages. First, the UV source is very close to the wafer, and the UV light is created with some level of parallelism. By contrast, when using a proximity mask the UV source must either be very far from the substrate to ensure that the light is parallel, or expensive optics must be used to make the light parallel. Secondly, because almost all the emitted UV light will be absorbed in the photoresist the power required for the microcavity array is much smaller than that required for a proximity mask where most of the UV light will be absorbed in the mask, and the light source may need to be far away from the wafer. The lower power reduces costs, but also reduces heating and thermal expansion. Finally, depending on the technology used, the proximity mask can be expensive. The microcavity array can be manufactured very inexpensively and is very reliably. 
         [0039]    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.