Abstract:
A low cost method is described for forming a textured Si surface such as for a solar cell which includes forming a dielectric layer containing pinholes, anisotropically etching through the pinholes to form inverted pyramids in the Si surface and removing the dielectric layer thereby producing a high light trapping efficiency for incident radiation.

Description:
BACKGROUND 
       [0001]    The present invention relates to surface texturing to improve light trapping in solar cells, and more specifically, to low cost surface texturing of a Si containing substrate to form inverted pyramids using chemical etching and a low quality dielectric layer. 
         [0002]    Texturing and anti-reflection coatings are commonly used to increase the efficiency of light absorption in solar cells. Upright pyramid formation on the surface of mono-crystalline silicon wafers is a standard technique for texturing the surface to maximize light absorption into a solar cell. It is well known that, by texturing a solar cell surface, photon utilization, light trapping or quantum efficiency can be improved by as much as 20% compared to a solar cell with a flat polished surface. Hemispherical reflectance of 10 to 13% has been reported from the upright pyramids formed by anisotropic chemical etching. The density of upright pyramids and their geometry both affect the light trapping efficiency. To achieve uniform and dense upright pyramids, isopropanol (IPA) can be added into alkaline etching solutions. 
         [0003]    In order to achieve inverted pyramid patterns, photolithography followed by anisotropic etching is a standard sequence. Even if a more effective textured surface with less reflectance can be achieved by this method, the photolithography step adds to the cost of the process. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    In accordance with the present invention, a method for forming a textured surface on a Si containing substrate is described comprising forming a dielectric layer on the surface having a plurality of pinholes through the dielectric layer, anisotropic etching the surface through the pinholes to form inverted pyramid patterns and removing the dielectric layer. One embodiment of the invention provides a random distribution of inverted pyramid patterns having a hemispherical reflectance of less than 13%. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0005]    These and other features, objects, and advantages of the present invention will become apparent upon consideration of the following detailed description of the invention when read in conjunction with the drawing in which: 
           [0006]      FIG. 1  is a cross-section view of a structure having a Si layer or Si containing layer and a porous oxide and/or nitride dielectric layer with a high density of pinholes thereon. 
           [0007]      FIG. 2  is a cross-section view of a structure after etching the upper surface of a Si layer or of a Si containing layer through a porous oxide and/or nitride dielectric layer. 
           [0008]      FIG. 3  is a cross-section view of the structure of  FIG. 2  after removing the porous oxide and/or nitride layer. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    A method is described to form inverted pyramid patterns on a Si or Si containing surface with minimum process cost. The method offered does not require extensive etching of Si materials and a cost-inefficient photolithography step. The method for generating an inverted pyramid pattern on a Si surface is achieved through depositing one or more low quality porous dielectric layers on a Si surface followed by anisotropic etching of the Si surface with an alkaline solution where it penetrates the low quality porous dielectric layer or layers. A high density of pinholes having been formed in a low quality dielectric layer or layers is used as a mask for anisotropic etching. A random distribution of inverted pyramids are then formed in the Si or Si containing surface having a hemispherical reflectance of less than 13%. 
         [0010]    Referring now to the drawing,  FIGS. 1 through 3  illustrate the process flow to form inverted pyramid patterns on a Si or Si containing surface.  FIG. 1  is a cross-section view of structure  10  having a substrate  12  which may, for example, comprise Si, a Si containing material, a glass, a metal or a polymer. A layer  14  of Si or a Si containing crystalline material is formed over substrate  12 . Layer  14  has an upper surface  15  which may be initially smooth. A smooth Si or Si-containing surface is a surface having a surface roughness of less than 1 nm root mean square (RMS). Surface  15 , if layer  14  is Si, may have a (100) crystal orientation. 
         [0011]    A dielectric layer  18  is formed on crystalline silicon surface  15  of layer  14 . Dielectric layer  18  contains pores or pinholes  20  and functions as a mask layer for etching. Dielectric layer  18  may be low density, low quality, and/or porous and may be SiO 2 , SiN x  or combinations of SiO 2  and SiN x . Typically, dielectric oxides or nitrides deposited using plasma enhanced chemical vapor deposition (PECVD) are less dense than those formed by other methods such as by thermal oxidation, atomic layer deposition (ALD), low pressure chemical vapor deposition (LPCVD) and sputtering. In other words, PECVD oxides or nitrides may include more pores or pinholes  20  in dielectric layer  18 . The density of pores or pinholes  20  in dielectric layer  18  may be greater than 10 6  pinholes/cm 2  and can be controlled by manipulating the deposition parameters of PECVD. For example, PECVD deposition parameters of low temperature in the range from 25° C. to 250° C., low power density in the range from 1 mW/cm 2  to 100 mW/cm 2 , and low pressure in the range from 10 mtorr. to 1000 mtorr. leads to the deposition of porous or low quality oxides/nitrides which include a high density of pores or pinholes  20 . The other methods mentioned above can also be used as long as the density of pinholes can be greater than 10 6  pinholes/cm 2 . 
         [0012]      FIG. 2  is a cross-section view of structure  10 ′ which is formed by chemically etching structure  10 . Structure  10  with porous oxides/nitrides as dielectric layer  18  as shown in  FIG. 1  may be dipped into an alkaline solution such as KOH, tetra methyl ammonium hydroxide (TMAH) or NH 3 OH for a time in the range from 10 sec to 20 min to anisotropically etch surface  15  of layer  14  which may be originally smooth. The temperature of the anisotropic etching process, which includes the temperature of the etching solution, may be in the range from 23° C. for a slow etch rate to an aqueous alkaline solution boiling temperature of about 100° C. for a fast etch rate. The etching time of the anisotropic etching should be less than the time required for growing inverted pyramid patterns that impinge or overlap an adjacent inverted pyramid on the surface. 
         [0013]    The etching solution passes or penetrates through pores or pinholes  20  in layer  18 , widens up original pinholes  20  shown as  20 ′ in  FIG. 2 , and reaches surface  15  of layer  14  to anisotropically etch Si or Si containing surface  15  of layer  14  to form inverted pyramid patterns having a depth in the range from 100 nm to 10 μm and a width in the range from 200 nm to 20 μm. Therefore, increasing the density of pores or pinholes  20  in layer  18  in the range from 10 6 /cm 2  to 10 8 /cm 2  achieves a higher density of inverted pyramids in the same range from 10 6  inverted pyramids/cm 2  to 10 8  inverted pyramids/cm 2  since surface  15 ′ of layer  14 ′ is exposed to the etching solution through pores or pinholes  20 ′ in dielectric layer  18 ′ forming inverted pyramids. Dielectric layer  18  may have a thickness in the range from 10 nm to 100 nm. The thickness of dielectric layer  18  such as an oxide must be thick enough in the range from 10 nm to 100 nm so as not to be removed during the anisotropic etching process, and yet not too thick in the range from 100 nm to 1 μm prohibit the etching solution from penetrating through pinholes  20  in dielectric layer  18 . After finishing anisotropic etching of Si surface  15 ′ comprised of inverted pyramid patterns, Si surface  15 ′ of layer  14 ′ is ready to serve as a textured surface for solar cell applications. 
         [0014]    It should be noted that pinholes  20  in dielectric layer  18  are not necessarily physical openings in dielectric layer  18  at the time dielectric layer  18  is deposited. For example, some or all of pinholes  20  may comprise pinhole-sized regions in dielectric layer  18  that are less resistant than the rest of the dielectric layer to the anisotropic etch used to etch layer  14 , with the result that some or all of physical openings  20 ′ would be formed at early stages of the anisotropic etch, rather than being present originally. 
         [0015]      FIG. 3  is a cross-section view of structure  10 ″ after removing porous oxide and/or nitride layer  18 ′ shown in  FIG. 2 . Layer  18 ′ can be removed by etching with hydrofluoric acid followed by rinsing Si or Si containing surface  15 ′ with deionized (DI) water to provide a clean textured Si or Si containing surface  15 ′ comprised of inverted pyramid patterns. 
         [0016]    While there has been described and illustrated a method for forming a textured Si surface comprised of inverted pyramid patterns via anisotropic etching through a porous dielectric layer containing pores or pinholes, it will be apparent to those skilled in the art that modifications and variations are possible without deviating from the broad scope of the invention which shall be limited solely by the scope of the claims appended hereto.