Abstract:
A process for etching the surfaces of semiconductor substrates utilizes a texturing tank which introduces a process fluid through a circulating system. The process fluid is heated to a desired temperature and maintained at a desired concentration prior to entering a processing area where laminar flow is produced to more quickly and uniformly roughen the surface of semiconductor substrates. The texturing tank permits removal of bubbles and eliminates temperature stratification in the processing area.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an etching bath for etching the surfaces of semiconductor substrates, primarily silicon. In particular, the present invention relates to a texturing tank and a process used therein in the fabrication of a non-reflective surface on silicon solar cells. 
     BACKGROUND OF THE INVENTION 
     Solar cells are devices that convert light energy incident to its surface into electrical energy. Past efforts and ongoing research have led to improved efficiency by finding methods to convert more of the incident light energy into electrical energy. Several such improvements involve processes for roughening or texturing the surface of the solar cell substrate in order to reduce its reflective quality and therefore absorb more incident light for conversion. The texturing procedures improve the silicon solar efficiency by forming minute projections and recesses on the surface so that incident light is absorbed rather than reflected. 
     Various methods have been proposed in the prior art to achieve the desired textured silicon surface on a solar cell. The following patents describe some of the methods developed to texture a solar cell substrate. 
     U.S. Pat. No. 7,128,975 to Inomata entitled Multicrystalline Silicon Substrate and Process for Roughening Surface Thereof describes a process involving a wet etching step to form large irregularities followed by a dry etching step to form a multiplicity of fine textures over the large irregularities. The use of both wet etching and dry etching is a drawback 
     U.S. Pat. No. 6,752,897 to Jang et al. entitled Wet Etch System With Overflow Particle Removing Feature describes a wet etch system for removing particulate impurities from an etching liquid. The impurities are removed by causing a displacement of a significant amount of etchant into an overflow section of the tank by inserting a wafer carrier with a fixed volume into a full process tank. Fresh etchant is poured into the etch bath chamber prior to a subsequent etch cycle. The Jang patent does not recirculate used etchant. 
     U.S. Pat. No. 6,663,944 to Park et al. entitled Textured Semiconductor Wafer for Solar Cell describes a method for forming a plurality of random grooves on a surface of a semiconductor wafer. The grooves are formed by randomly depositing a protector on the surface and then dipping the wafer into an isotropic etching solution to etch the surface where the protector is not deposited. The need to deposit and later remove a protector layer adds extra steps. 
     U.S. Pat. Nos. 6,391,145 and 6,156,968 to Nishimoto et al. both entitled Solar Cell, a Method of Producing the Same and a Semiconductor Producing Apparatus describe a method whereby projections and recesses are uniformly formed on the surface of a silicon substrate by dipping in a wet etching solution. Spherical projections and recesses are formed on the surface of the substrate. The apparatus describes a multi tank configuration wherein one tank has recirculation of the etching solution and a means to maintain concentration of processed chemicals. The use of multiple tanks creates unneeded complexity. 
     U.S. Pat. No. 6,228,211 to Jeong entitled Apparatus for Etching a Glass Substrate describes an apparatus wherein a container includes a bubbling plate at the bottom thereof. The glass substrate to be etched is placed in the container where a liquid etchant is passed through holes in a porous plate to react with the surface of the substrate. Bubbles produced by the bubbling plate also pass through the holes in the porous plate to mix the liquid etchant and remove impurities from the surface of the glass substrate. The Jeong patent creates turbulent flow in the processing area and generates bubbles that can hinder the etching process. 
     U.S. Pat. No. 6,207,890 to Nakai et al. entitled Photovoltaic Element and Method for Manufacture Thereof describes an element that directly converts optical energy into electrical energy. The process involves forming many uneven sections on the surface of a silicon substrate then isotropically etching the surface. The bottoms of recessed sections are rounded and a p-type amorphous silicon layer is formed on the surface. The rounded bottom recesses allow an amorphous silicon layer to be deposited in a uniform thickness. Hydrogen bubbles created during the process attach to the substrate and prevent the alkali from reaching the surface at the point of attachment. 
     U.S. Pat. No. 4,980,017 to Kaji et al. entitled Method for Recirculating High-Temperature Etching Solution describes a method for recirculating a high temperature etching solution wherein the portion of an etching solution contained in an etching bath is continuously removed and a predetermined amount of pure water for adjusting the concentration of the etching solution is injected into the removed solution. The removed solution is then reheated to a predetermined temperature and recirculated into the etching bath. The described method does not produce a uniform flow rate or remove bubbles. 
     U.S. Pat. No. 4,252,865 to Gilbert et al. entitled Highly Solar-Energy Absorbing Device and Method of Making the Same uses sputtering to create a device wherein the surface of an amorphous semiconductor material has a particular characterization in that an array of outwardly projecting structural elements of relatively high aspect ratio and at effective lateral spacings on the order of magnitude of wave lengths within the solar energy spectrum. 
     U.S. Pat. No. 4,229,233 to Hansen et al. entitled Method for Fabricating Non-Reflective Semiconductor Surfaces by Anisotropic Reactive Ion Etching describes a process that takes place in a reactive ion etching tool, typically a diode configured system employing ambient gases which react with the silicon. 
     There are a number of drawbacks involved with the above-described methods and devices. Several of the processes utilize a dry etch or employ a wet etch system that does not involve a pump or recirculation of the etching solution. Other systems require the deposition of a protector on the surface to be etched. Still other processes describe the use of turbulent flow and a bottom tank inlet. In such prior art devices it is difficult to obtain a uniform flow rate across the semiconductor surface. Still other prior art patents describe an etching process that includes sputtering or a reactive ion etching chamber. 
     Accordingly, there is a need for an etching process and apparatus that enhances and speeds up the etching by providing more control over critical variables such as temperature uniformity, bubble removal and flow rate across the surface to be etched. In addition, a process is needed that produces laminar flow across the surface of the substrate. Further, a means of preventing attachment of bubbles or particles to the surface of the substrate is needed. The following invention provides these and other related advantages. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a process for etching the surface of a semiconductor substrate and an apparatus embodying said process. The process comprises the steps of introducing a process fluid having an initial concentration of an isotropic etching agent to a texturing tank. The process fluid is degassed and a uniform, laminar flow is produced in the process fluid. The semiconductor substrate to be etched is placed into a laminar flow segment of the process fluid and the surface of the semiconductor substrate is etched in the laminar flow segment of the process fluid. 
     The process further comprises the step of dispersing the process fluid through an inlet baffle to produce turbulent flow through a heater area of the texturing tank prior to the degassing step. The process fluid is heated in this heater area prior to the degassing step. The temperature of the process fluid is monitored in the texturing tank. 
     The degassing step comprises the step of introducing the process fluid to a preprocessing area. The process fluid flows through a vertical chamber in the preprocessing area. Gasses and bubbles in the process fluid are allowed to escape from an air interface surface in this vertical chamber. Uniform, laminar flow of the process fluid is produced by passing the process fluid through an entry diffuser after the degassing step. 
     The process fluid is premixed with an interface active agent prior to being introduced into the texturing tank. After the etching step the process fluid passes through an exit baffle. An isotropic etching agent is added to the process fluid after the etching step in order to restore the process fluid to the initial concentration. The process fluid is recirculated from the exit baffle back to the introducing step. 
     The apparatus for employing the above method comprises a texturing tank having an inlet and an outlet and defining a flow path therebetween for a process fluid. The flow path includes a preprocessing area and a downstream processing area. A degasser in the preprocessing area removes gasses and bubbles in the process flow. An entry diffuser in the flow path disposed between the preprocessing area and the processing area creates a uniform, laminar flow through the processing area. The semiconductor substrate to be etched is introduced into the preprocessing area. The substrate is introduced to the processing area by a conveyor system, a wafer basket or similar means. 
     A mixing area is further included in the flow path subsequent to the processing area. The mixing area being for mixing chemicals into the process fluid. The apparatus also includes a pump in the flow path subsequent to the processing area. The pump being for recirculating the process fluid back to the tank inlet. 
     An inlet baffle in the flow path subsequent to the tank inlet disperses the process fluid across a width of the tank and creates turbulent flow in the process fluid. A heater area in the flow path subsequent to the inlet baffle heats the process fluid while in turbulent flow. The heater area includes heater elements, all of which are coated with perfluoroalkoxy. A thermocouple in the processing area measures the temperature of the process fluid. 
     An exit baffle in the flow path subsequent to the processing area impedes the process flow as it exits the processing area. A mixer in the flow path prior to the tank inlet mixes the process fluid with an interface active agent. 
     All wetted surfaces in the texturing tank are coated with polyvinylidene fluoride or perfluoroalkoxy. The preprocessing area includes a generally vertical chamber whereby bubbles and gasses in the process fluid leave the process fluid and pass through a degassing area. 
     Other features and advantages of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate the invention. In such drawings: 
         FIG. 1  is a cutaway perspective view of a texturing tank embodying the present invention; 
         FIG. 2  is a schematic representation of a texturing tank embodying the present invention; 
         FIG. 3  is a schematic representation of a texturing tank, including an IPA mixing apparatus; 
         FIG. 4  is a schematic representation of a texturing tank embodying the present invention, including an in-line heater; 
         FIG. 5  is a perspective view of a texturing tank embodying the present invention, including silicon substrates in a wafer basket placed in the processing area; 
         FIG. 6  is a perspective view of a texturing tank embodying the present invention, including silicon substrates on a conveyor system through the processing area; and 
         FIG. 7  is a top view of a texturing tank embodying the present invention, illustrating process fluid flowing through the processing area. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to a process and apparatus for etching the surfaces of semiconductor substrates, primarily silicon. In particular, the present invention relates to a recirculating texturing tank and the process employed therein. The inventive texturing tank shortens the processing time required to etch the surfaces of semiconductor substrates by creating a more uniform flow across the surface of the substrates to be etched. In current methods, manufacturers use either a static tank in which the substrates see no flow, or an overflow tank in which some substrates see a greater flow than others. By introducing flow in a controlled, uniform manner the substrates are etched quicker and more consistently. Such a system facilitates mass production utilization because greater production can be achieved in a shorter amount of time with more uniform quality. These flow characteristics also act to remove and eliminate bubbles and other particulates from the surfaces of the substrates to be etched. 
     As illustrated in  FIG. 1 , the texturing tank  10  consists of a housing  12  defining a processing area  14 . The tank  10  includes an inlet  16  and an outlet  18 , both of which pass through the housing  12 . Preferably the inlet  16  and outlet  18  are positioned proximate to each other on the tank  10  to facilitate the circulatory process described below. 
     A pump  20  forces process fluid through a series of pipes  22  into the inlet  16 . As described below, one or more of various components may be included in line with the pipes  22 . 
     The fluid flow as depicted by arrows  24  passes through the inlet  16  entering an area of the texturing tank  10  beneath an intermediate floor  26 . This intermediate floor  26  separates a heater area  28  of the tank  10  from the processing area  14 . As the process fluid enters the heater area  28  it passes through an inlet baffle  30  that spans the width of the heater area  28 . The inlet baffle  30  includes a series of slots or openings  32 . These slots  32  force the process fluid to spread across the entire width of the heating area  28  as it is forced to flow through the inlet baffle  30 . 
     The heater area  28  includes a series of heating elements  34  in a coil configuration to maximize surface area, occupy less space, and provide lower wattage density. The heating elements  34  are coated with perfluoroalkoxy (Teflon™). The inlet baffle  30  creates features in the flow of the process fluid through the heater area  28  such that the flow is even and the temperature is consistent throughout the width of the tank  10 . This configuration for the heater area  28  provides more efficient and uniform process fluid temperature, thus avoiding temperature gradients in the process area  14 . Having a uniform temperature improves the etch rate and uniformity across substrates. 
     After being heated, the process fluid moves to a preprocessing area  36  which consists generally of a vertical space  38  along a wall of the tank  10  opposite the inlet  16 . As the process fluid enters the preprocessing area  36  and enters the vertical space  38 , any bubbles or gasses trapped in the process fluid quickly rise to the top of this vertical space  38 . The tank  10  is configured such that process fluid does not completely fill the tank to the top and there is a liquid/air interface at which the entrapped gasses and bubbles escape the process fluid. This liquid/air interface is part of a degassing area  40  designed to remove bubbles from the process fluid before it enters the processing area of the tank. The intermediate floor  26  may be angled slightly upward in the direction of process flow  24  such that any bubbles or gasses in the heater area  28  move toward the preprocessing area  36  and are thus permitted to escape through the degassing area  40 . 
     An entry diffuser  42  is disposed between the preprocessing area  36  and the processing area  14 . The entry diffuser  42  impedes and scatters the process fluid to create a uniform and laminar flow across the processing area  14 . The entry diffuser  42  is a vertical wall and creates this laminar flow through a series of slots  44  therein. The slots  44  control how quickly the process fluid passes through the entry diffuser  42  and are conducive to creating the uniform laminar flow. 
     A thermocouple  46  is positioned in the processing area  14  for measuring the temperature of the process fluid. The thermocouple  46  has a feedback loop to the heating elements  34  so that the temperature of the process fluid can be adjusted in real time. 
     There are various embodiments for introducing semiconductor substrates to the processing area  14 .  FIG. 5  illustrates an embodiment wherein a plurality of semiconductor substrates  48  are positioned in the processing area  14  by immersion of a basket  50  or similar device therein. The basket  50  may be placed in and/or removed from the processing area  14  simply by lifting the basket  50  through an open top of the texturing tank  10 .  FIG. 6  illustrates an alternate embodiment wherein the plurality of substrates  48  are moved through the processing area  14  on a conveyor system  52  or similar device. The conveyor system  52  passes the plurality of substrates  48  through opposite sidewalls of the texturing tank  10  such that the substrates  48  pass between the entry diffuser  42  and an exit baffle  54  described below. The conveyor system  52  orients the substrates  48  to achieve laminar flow across the surface thereof. 
     The exit baffle  54  is a vertical wall positioned adjacent to the processing area  14  and includes a series of slots  56  through which the process fluid passes as it leaves the processing area  14 . The purpose of the exit baffle  54  is to impede the flow  24  of the process fluid so that it does not leave the processing area  14  before sufficient etching has occurred. Once through the exit baffle  54  the process fluid exits the tank  10  through the outlet  18 . The processing fluid then circulates back to the pump  20  for reintroduction to the system. 
     At some point between the exit baffle  54  and the entry diffuser  42  a mixing area  58  should be included to introduce additional etching chemicals to assure that the process fluids are at an ideal concentration for etching the substrates  48 . The mixing area  58  must be positioned in the flow of process fluid such that the process fluids will be sufficiently mixed prior to reaching the processing area  14  so as not to chemically shock the substrates  48 . Preferably, this mixing area  58  immediately follows the exit baffle  54  to allow a maximum amount of time for mixing. 
       FIGS. 2 ,  3 ,  4  and  7  illustrate the process flow  17  through the circulatory system and tank  10 . As illustrated in  FIG. 2 , the system may consist of the texturing tank  10  and the pump  20  to complete the circulatory system. As illustrated in  FIG. 3 , the system may also include a mixer  60  and a source  62  of interface active agent, preferably isopropyl alcohol (IPA). The interface active agent is to prevent particulates and bubbles from attaching to the surface of the substrates  48  as described in the prior art above. An agent like IPA should be introduced in a measured way to achieve a uniform and proper mix. The mixer  60  and mixing area  58  are preferably at different locations in the flow path but may be one and the same. As illustrated in  FIG. 4 , the circulatory system may also include an in-line heater  64  for heating the process fluid prior to entering the tank  10 . The process flow  17 , as clearly shown in  FIG. 7  is parallel to the substrates to be etched. This orientation is conducive to the laminar flow being generated. 
     The process fluid preferably consists of an isotropic etching agent such as sodium hydroxide (NaOH) or potassium hydroxide (KOH). The isotropic etching agent is preferably delivered in an aqueous solution. All of the wetted surfaces in the tank  10  are preferably coated with a material to protect from damage caused by the process fluids. Protective materials include polyvinylidene fluoride (PVDF) or perfluoroalkoxy (PFA), or any such materials compatible with the etching solutions. The heaters are preferably jacketed with PFA and all other wetted surfaces with PVDF. 
     Although several embodiments have been described in detail for purposes of illustration, various modifications may be made without departing from the scope and spirit of the invention. Accordingly, the invention is not to be limited, except as by the appended claims.