Patent Publication Number: US-2010129955-A1

Title: Protection layer for fabricating a solar cell

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/106,561 entitled “PROTECTION LAYER FOR FABRICATING A SOLAR CELL” filed Apr. 21, 2008, which claims the benefit of U.S. Provisional Application No. 60/930,800, filed May 17, 2007, the entire contents of which are hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present invention are in the field of Semiconductor Fabrication and, in particular, Solar Cell Fabrication. 
     BACKGROUND 
     Photovoltaic cells, commonly known as solar cells, are well known devices for direct conversion of solar radiation into electrical energy. Generally, solar cells are fabricated on a semiconductor wafer or substrate using semiconductor processing techniques to form a p-n junction near a surface of the substrate. Solar radiation impinging on the surface of the substrate creates electron and hole pairs in the bulk of the substrate, which migrate to p-doped and n-doped regions in the substrate, thereby generating a voltage differential between the doped regions. The doped regions are coupled to metal contacts on the solar cell to direct an electrical current from the cell to an external circuit coupled thereto. 
     Typically, the surface of the solar cell to receive radiation is textured and/or coated with a layer or coating of an anti-reflective material to decrease the reflection of light, thereby increasing the efficiency of the solar cell. The fabrication of such solar cells, in particular the formation of the p-n junction and contacts thereto, involves a number of complicated process steps including the deposition, doping and etching of many different layers of material. These process steps are performed or carried out with low variation tolerances using many different processing tools under controlled environmental conditions. 
     Accordingly, there is a need for a simplified process for fabricating solar cells that reduces the number of separate steps needed, thereby reducing the time and cost of fabricating solar cells. It is further desirable that the method eliminates entirely the need for one or more processing tools, thereby further reducing the cost of fabricating solar cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a Flowchart representing a series of operations in a method for fabricating a solar cell, in accordance with an embodiment of the present invention. 
         FIG. 2A  illustrates a cross-sectional view of a substrate, corresponding to operation  102  from the Flowchart of  FIG. 1 , in accordance with an embodiment of the present invention. 
         FIG. 2B  illustrates a cross-sectional view of a substrate having an anti-reflective coating (ARC) layer formed thereon, corresponding to operation  104  from the Flowchart of  FIG. 1 , in accordance with an embodiment of the present invention. 
         FIG. 2C  illustrates a cross-sectional view of a substrate having a protection layer formed thereon, corresponding to operation  106  from the Flowchart of  FIG. 1 , in accordance with an embodiment of the present invention. 
         FIG. 2D  illustrates a cross-sectional view of a substrate having a masking layer formed thereon, in accordance with an embodiment of the present invention. 
         FIG. 2E  illustrates a cross-sectional view of a substrate having a plurality of contact openings formed thereon, in accordance with an embodiment of the present invention. 
         FIG. 2F  illustrates a cross-sectional view of a substrate having the protection layer and the masking layer removed, in accordance with an embodiment of the present invention. 
         FIG. 2G  illustrates a cross-sectional view of a substrate having a plurality of contacts formed in the plurality of contact openings, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Methods to fabricate a solar cell are described herein. In the following description, numerous specific details are set forth, such as specific dimensions, in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known processing steps, such as patterning steps, are not described in detail in order to not unnecessarily obscure the present invention. Furthermore, it is to be understood that the various embodiments shown in the Figures are illustrative representations and are not necessarily drawn to scale. 
     Disclosed herein is a method to fabricate a solar cell. A substrate having a light-receiving surface may be provided in a process chamber. In an embodiment, an anti-reflective coating (ARC) layer is then formed, in the process chamber, above the light-receiving surface of the substrate. Finally, without removing the substrate from the process chamber, a protection layer (also known as an etch mask) may then be formed above the ARC layer. In one embodiment, the protection layer comprises amorphous carbon. In another embodiment, the protection layer comprises amorphous silicon. 
     Formation of a protection layer on an ARC layer may enable preservation of the ARC layer during various process operations in the fabrication of a solar cell. For example, in accordance with an embodiment of the present invention, a protection layer is used to maintain the integrity of an ARC layer disposed on a solar cell substrate during exposure of the solar cell substrate to a buffered oxide etch (BOE). In order to reduce the number of processing steps required to fabricate a completed solar cell, the protection layer may be fabricated in the same process tool as the ARC layer. For example, in accordance with an embodiment of the present invention, an ARC layer is first formed on a solar cell substrate in a process chamber. Then, without removing the substrate from the process chamber, the protection layer is formed on the ARC layer. 
     A protection layer may be utilized in the fabrication of a solar cell.  FIG. 1  depicts a Flowchart  100  representing a series of operations in a method for fabricating a solar cell, in accordance with an embodiment of the present invention.  FIGS. 2A-2G  illustrate cross-sectional views representing operations in the fabrication of a solar cell, in accordance with an embodiment of the present invention. 
       FIG. 2A  illustrates a cross-sectional view of a substrate, corresponding to operation  102  from Flowchart  100 , in accordance with an embodiment of the present invention. Referring to operation  102  of Flowchart  100  and corresponding  FIG. 2A , a substrate having a light-receiving surface is provided in a process chamber. 
     Referring to  FIG. 2A , a substrate  200  has a light-receiving surface  202  and a back surface  204 . In an embodiment, light-receiving surface  202  is textured, as depicted in  FIG. 2A , to mitigate undesirable reflection during solar radiation collection efficiency. A plurality of active region  206  is formed at back surface  204  of substrate  200 . In accordance with an embodiment of the present invention, the plurality of active regions  206  includes alternating N+ and P+ regions, as depicted in  FIG. 2A . In one embodiment, substrate  200  is composed of silicon, the N+ regions include phosphorous dopant impurity atoms and the P+ regions include boron dopant impurity atoms. A dielectric layer  208  is disposed on back surface  204  of substrate  200 . In one embodiment, dielectric layer  208  is composed of a material such as, but not limited to, silicon dioxide. 
       FIG. 2B  illustrates a cross-sectional view of a substrate having an anti-reflective coating (ARC) layer formed thereon, corresponding to operation  104  from Flowchart  100 , in accordance with an embodiment of the present invention. Referring to operation  104  of Flowchart  100  and corresponding  FIG. 2B , an ARC layer is formed above light-receiving surface  202  of substrate  200  in the process chamber. 
     Referring to  FIG. 2B , an ARC layer  220  is formed above and conformal with light-receiving surface  202  of substrate  200 . In one embodiment, ARC layer  220  is composed of a material such as, but not limited to, silicon nitride, silicon dioxide or titanium oxide. In a specific embodiment, ARC layer  220  is a multi-layer stack including a silicon dioxide portion directly adjacent to light-receiving surface  202  and a silicon nitride portion directly adjacent to the silicon dioxide portion. ARC layer  220  may be formed by any technique suitable to dispose a conformal layer above light-receiving surface  202 , as depicted in  FIG. 2B . In accordance with an embodiment of the present invention, at least a portion of ARC layer  220  is formed by a technique such as, but not limited to, chemical vapor deposition, plasma-enhanced chemical vapor deposition, atmospheric-pressure chemical vapor deposition or physical vapor deposition. In a specific embodiment, ARC layer  220  is composed of silicon nitride deposited by a plasma-enhanced chemical vapor deposition process and formed to a thickness approximately in the range of 10-100 nanometers. 
       FIG. 2C  illustrates a cross-sectional view of a substrate having a protection layer formed thereon, corresponding to operation  106  from Flowchart  100 , in accordance with an embodiment of the present invention. Referring to operation  106  of Flowchart  100  and corresponding  FIG. 2C , without removing substrate  200  from the process chamber, a protection layer is formed above ARC layer  220 . 
     Referring to  FIG. 2C , a protection layer  230  is formed above and conformal with ARC layer  220 . Protection layer  230  may be composed of a material and formed by a technique suitable to provide conformal coverage of ARC layer  220 . In accordance with an embodiment of the present invention, protection layer  230  is composed of amorphous carbon. In one embodiment, protection layer  230  is formed by vapor deposition using a gas such as, but not limited to, methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ), ethylene (C 2 H 4 ) or propylene (C 3 H 6 ). In one embodiment, protection layer  230  is formed by using a liquid hydrocarbon precursor such as, but not limited to, toluene (C 7 H 8 ) transported by a carrier gas such as, but not limited to, argon (Ar), nitrogen (N 2 ), helium (He) or hydrogen (H 2 ). In a specific embodiment, protection layer  230  is composed of amorphous carbon and is formed at a temperature of less than approximately 500 degrees Celsius and, more preferably, at a temperature of less than approximately 400 degrees Celsius. In accordance with another embodiment of the present invention, protection layer  230  is composed of amorphous silicon. In one embodiment, protection layer  230  is formed by vapor deposition using a gas such as, but not limited to, silane (SiH 4 ) gas. Protection layer  230  may be formed to a thickness suitable to provide a pin-hole-free coverage of ARC layer  220  while being sufficiently easy to remove at a subsequent processing step. In one embodiment, protection layer  230  is formed to a thickness approximately in the range of 1-30 nanometers. In a specific embodiment, protection layer  230  is resistant to a BOE. 
     In accordance with an embodiment of the present invention, protection layer  230  is formed directly after the formation of, and in the same process chamber as, ARC layer  220 . For example, in an embodiment, ARC layer  220  is first formed in the process chamber and then, without removing substrate  200  from the process chamber, protection layer  230  is formed on ARC layer  220 . Thus, in an embodiment of the present invention, at least one complete process step is eliminated from an integration scheme for fabricating a solar cell. In one embodiment, ARC layer  220  and protection layer  230  are formed by the same technique such as, but not limited to, chemical vapor deposition, plasma-enhanced chemical vapor deposition, atmospheric-pressure chemical vapor deposition or physical vapor deposition. In a specific embodiment, ARC layer  220  and protection layer  230  are formed by first flowing, in a process chamber, at least a first process gas and a second process gas to form ARC layer  220  above light-receiving surface  202  of substrate  200 . Then, without removing substrate  200  from the process chamber, at least the first process gas, but not the second process gas, is flowed to form protection layer  230  above ARC layer  220 . In a particular embodiment, ARC layer  220  is composed of a material such as, but not limited to, silicon nitride, silicon oxy-nitride or carbon-doped silicon oxide, protection layer  230  is composed of amorphous silicon, the first process gas is silane (SiH 4 ) and the second process gas is ammonia (NH 3 ). 
     Following formation of protection layer  230 , dielectric layer  208  may be patterned to form a plurality of contact openings to the plurality of active regions  206  at back surface  204  of substrate  200 .  FIG. 2D  illustrates a cross-sectional view of a substrate having a mask layer formed thereon, in accordance with an embodiment of the present invention. Referring to  FIG. 2D , a mask layer  240  is disposed on dielectric layer  208 . In an embodiment, the pattern of mask layer  240  determines the location where a plurality of contact openings will subsequently be formed. In one embodiment, mask layer  240  is composed of a material such as, but not limited to, an organic ink or an organic photo-resist. 
       FIG. 2E  illustrates a cross-sectional view of a substrate having a plurality of contact openings formed thereon, in accordance with an embodiment of the present invention. Referring to  FIG. 2E , a plurality of contact openings  250  is formed in dielectric layer  208  in regions determined by mask layer  240 . In accordance with an embodiment of the present invention, the plurality of contact openings  250  is formed by etching dielectric layer  208  using a BOE. In one embodiment, protection layer  230  protects ARC layer  220  during the forming of the plurality of contact openings  250  with the BOE. In a specific embodiment, the BOE is composed of an aqueous solution that includes hydrofluoric acid (HF) and ammonium fluoride (NH 4 F). In a particular embodiment, the HF:NH 4 F ratio is approximately in the range of 1:4-1:10 and the BOE is applied to dielectric layer  208  for a duration approximately in the range of 3-10 minutes at a temperature approximately in the range of 30-40 degrees Celsius. 
       FIG. 2F  illustrates a cross-sectional view of a substrate having the protection layer and the masking layer removed, in accordance with an embodiment of the present invention. Referring to  FIG. 2F , protection layer  230  is removed to re-expose the top surface of ARC layer  220  and mask layer  240  is removed to re-expose the top surface of dielectric layer  208 . Thus, in accordance with an embodiment of the present invention, protection layer  230  need only be retained throughout the patterning of dielectric layer  208  to form the plurality of contact openings  250 . In one embodiment, protection layer  230  and mask layer  240  are removed in the same process step. For example, in a specific embodiment, protection layer  230  is composed of amorphous carbon and is removed by using a wet etchant that includes sulfuric acid (H 2 SO 4 ) and hydrogen peroxide (H 2 O 2 ) and is applied for a duration in the range of 10-30 seconds. In another specific embodiment, protection layer  230  is composed of amorphous silicon and is removed by using a wet etchant that includes potassium hydroxide (KOH) and water and is applied for a duration sufficiently long to completely remove the amorphous silicon protection layer, but sufficiently short as to mitigate any detrimental loss of silicon from the exposed portions of back surface  204  of substrate  200 . In a particular embodiment, the loss of silicon from the exposed portions of back surface  204  of substrate  200  is targeted to be less than approximately 10 nanometers. 
       FIG. 2G  illustrates a cross-sectional view of a substrate having a plurality of contacts formed in the plurality of contact openings, in accordance with an embodiment of the present invention. Referring to  FIG. 2G , a plurality of contacts  260  is formed by depositing a metal-containing material into the plurality of contact openings  250 . In one embodiment, the metal-containing material is composed of a metal such as, but not limited to, aluminum, silver, palladium or alloys thereof. In accordance with an embodiment of the present invention, a back side contact solar cell  290  is thus formed. Back side contact solar cells are also disclosed in U.S. Pat. Nos. 5,053,083 and 4,927,770, the entire contents of which are hereby incorporated by reference herein. 
     Thus, a method for fabricating a solar cell has been disclosed. In accordance with an embodiment of the present invention, a substrate having a light-receiving surface is provided in a process chamber. An ARC layer is then formed, in the process chamber, above the light-receiving surface of the substrate. Finally, without removing the substrate from the process chamber, a protection layer is formed above the ARC layer. In one embodiment, the protection layer comprises amorphous carbon. In another embodiment, the protection layer comprises amorphous silicon. 
     The advantages of the method for fabricating solar cells of the present invention over previous or conventional cells and methods may include: (i) substantial savings in the cost of fabricating solar cells through the elimination of the need for a dedicated tool to form a protection layer for an ARC layer, (ii) significant reduction in the time needed to fabricate solar cells through the combining of the ARC layer and protection layer deposition steps, and (iii) improved yield through the reduced handling of the substrate achieved through the deposition of the protection layer in the same process chamber used to form an ARC layer.