Patent Publication Number: US-2013234271-A1

Title: Conductive compositions and processes for use in the manufacture of semiconductor devices

Description:
FIELD OF THE INVENTION 
     This invention is directed primarily to a silicon semiconductor device. In particular, it is directed to a conductive silver paste for use in the front side of a solar cell device. 
     BACKGROUND OF THE INVENTION 
     The present invention can be applied to a broad range of semiconductor devices, although it is especially effective in light-receiving elements such as photodiodes and solar cells. The background of the invention is described below with reference to solar cells as a specific example of the prior art. 
     A conventional solar cell structure with a p-type base has a negative electrode that is typically on the front-side or sun side of the cell and a positive electrode on the backside. It is well-known that radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate hole-electron pairs in that body. Because of the potential difference which exists at a p-n junction, holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metallized, i.e., provided with metal contacts that are electrically conductive. 
     Most terrestrial electric power-generating solar cells currently are silicon solar cells. Process flow in mass production is generally aimed at achieving maximum simplification and minimizing manufacturing costs. Electrodes in particular are made by using a method such as screen printing to form a metal paste. An example of this method of production is described below in conjunction with  FIG. 1 .  FIG. 1  shows a p-type silicon substrate,  10 . 
     In  FIG. 1(   b ), an n-type diffusion layer,  20 , of the reverse conductivity type is formed by the thermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride (POCl 3 ) is commonly used as the phosphorus diffusion source. In the absence of any particular modification, the diffusion layer,  20 , is formed over the entire surface of the silicon substrate,  10 . This diffusion layer has a sheet resistivity on the order of several tens of ohms per square (Ω/□), and a thickness of about 0.3 to 0.5 μm. 
     After protecting one surface of this diffusion layer with a resist or the like, as shown in  FIG. 1(   c ), the diffusion layer,  20 , is removed from most surfaces by etching so that it remains only on one main surface. The resist is then removed using an organic solvent or the like. 
     Next, a silicon nitride film,  30 , is formed as an anti-reflection coating on the n-type diffusion layer,  20 , to a thickness of about 700 to 900 Å in the manner shown in  FIG. 1(   d ) by a process such as plasma chemical vapor deposition (CVD). 
     As shown in  FIG. 1(   e ), a silver paste,  500 , for the front electrode is screen printed then dried over the silicon nitride film,  30 . In addition, a backside silver or silver/aluminum paste,  70 , and an aluminum paste,  60 , are then screen printed and successively dried on the backside of the substrate. Firing is then carried out in an infrared furnace at a temperature range of approximately 700° C. to 975° C. for a period of from several minutes to several tens of minutes. 
     Consequently, as shown in  FIG. 1(   f ), aluminum diffuses from the aluminum paste into the silicon substrate,  10 , as a dopant during firing, forming a p+ layer,  40 , containing a high concentration of aluminum dopant. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell. 
     The aluminum paste is transformed by firing from a dried state,  60 , to an aluminum back electrode, 61. The backside silver or silver/aluminum paste,  70 , is fired at the same time, becoming a silver or silver/aluminum back electrode, 71. During firing, the boundary between the back side aluminum and the back side silver or silver/aluminum assumes an alloy state, and is connected electrically as well. The aluminum electrode accounts for most areas of the back electrode, owing in part to the need to form a p+ layer,  40 . Because soldering to an aluminum electrode is impossible, a silver back electrode is formed over portions of the back side as an electrode for interconnecting solar cells by means of copper ribbon or the like. In addition, the front electrode forming silver paste,  500 , sinters and penetrates through the silicon nitride film,  30 , during firing, and is thereby able to electrically contact the n-type layer,  20 . This type of process is generally called “fire through.” This fired through state is apparent in layer  501  of  FIG. 1(   f ). 
     JP-A 2001-313400 to Shuichi et al. teaches a solar cell which is obtained by forming, on one main surface of a semiconductor substrate, regions that exhibit the other type of conductivity and forming an antireflection coating on this main surface of the semiconductor substrate. The resulting solar cell has an electrode material coated over the antireflection coating and fired. The electrode material includes, for example; lead, boron and silicon; and additionally contains, in a glass frit having a softening point of about 300 to 500° C., and one or more powders from among titanium, bismuth, cobalt, zinc, zirconium, iron, and chromium. 
     U.S. Pat. No. 4,737,197 to Nagahara et al. discloses a solar cell including a semiconductor substrate, a diffused layer provided in the semiconductor substrate by diffusion of dopant impurities, and a contact made of metal paste formed on the diffusion layer. The metal paste includes metal powder, which functions as the main contact material, glass frits, an organic binder, a solvent, and an element belonging to the fifth group of the periodic table. 
     Although, as noted, various methods and compositions for forming solar cells exist, there is an on-going effort to provide compositions which are Pb-free while at the same time maintaining electrical performance. The present inventors create a novel composition and method of manufacturing a semiconductor device which provides such a Pb-free system and maintains electrical performance and solder adhesion. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a thick film conductive composition comprising: (a) electrically conductive silver powder; (b) zinc-containing additive; (c) glass frit wherein said glass fit is lead-free; dispersed in (d) organic medium. 
     The present invention is further directed to an electrode formed from the composition above wherein said composition has been fired to remove the organic medium and to sinter said glass particles. Still further, the invention is directed to a method of manufacturing a semiconductor device from a structural element composed of a semiconductor having a p-n junction and an insulating film formed on a main surface of the semiconductor comprising the steps of (a) applying onto said insulating film the thick film composition detailed above; and (b) firing said semiconductor, insulating film and thick film composition to form an electrode. Additionally, the present invention is directed to a semiconductor device formed by the method detailed above and a semiconductor device formed from the thick film conductive composition detailed above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a process flow diagram illustrating the fabrication of a semiconductor device. 
     
    
    
     Reference numerals shown in  FIG. 1  are explained below.
           10 : p-type silicon substrate     20 : n-type diffusion layer     30 : silicon nitride film, titanium oxide film, or silicon oxide film     40 : p+ layer (back surface field, BSF)     50 : silver paste formed on front side     51 : silver front electrode (obtained by firing front side silver paste)     60 : aluminum paste formed on backside     61 : aluminum back electrode (obtained by firing back side aluminum paste)     70 : silver or silver/aluminum paste formed on backside     71 : silver or silver/aluminum back electrode (obtained by firing back side silver paste)     80 : solder layer     500  silver paste formed on front side according to the invention     501 : silver front electrode according to the invention (formed by firing front side silver paste)       

     An example of this method of production is described below in conjunction with  FIG. 1 .  FIG. 1(   a ) shows a p-type silicon substrate,  10 . 
     In  FIG. 1(   b ), an n-type diffusion layer,  20 , of the reverse conductivity type is formed by the thermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride (POCl 3 ) is commonly used as the phosphorus diffusion source. In the absence of any particular modification, the diffusion layer,  20 , is formed over the entire surface of the silicon substrate,  10 . This diffusion layer has a sheet resistivity on the order of several tens of ohms per square (Ω/□), and a thickness of about 0.3 to 0.5 μm. 
     After protecting one surface of this diffusion layer with a resist or the like, as shown in  FIG. 1(   c ), the diffusion layer,  20 , is removed from most surfaces by etching so that it remains only on one main surface. The resist is then removed using an organic solvent or the like. 
     Next, a silicon nitride film,  30 , is formed as an anti-reflection coating on the n-type diffusion layer,  20 , to a thickness of about 700 to 900 Å in the manner shown in  FIG. 1(   d ) by a process such as plasma chemical vapor deposition (CVD). 
     As shown in  FIG. 1(   e ), a silver paste,  500 , for the front electrode is screen printed then dried over the silicon nitride film,  30 . In addition, a backside silver or silver/aluminum paste,  70 , and an aluminum paste,  60 , are then screen printed and successively dried on the backside of the substrate. Firing is then carried out in an infrared furnace at a temperature range of approximately 700° C. to 975° C. for a period of from several minutes to several tens of minutes. 
     Consequently, as shown in  FIG. 1(   f ), aluminum diffuses from the aluminum paste into the silicon substrate,  10 , as a dopant during firing, forming a p+ layer,  40 , containing a high concentration of aluminum dopant. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The main components of the thick film conductor composition(s) are electrically functional silver powders, zinc-containing additive(s), and Pb-free glass frit dispersed in an organic medium. Additional additives may include metals, metal oxides or any compounds that can generate these metal oxides during firing. The components are discussed herein below. 
     I. Inorganic Components 
     The inorganic components of the present invention comprise (1) electrically functional silver powders; (2) Zn-containing additive(s); (3) Pb-free glass frit; and optionally (4) additional metal/metal oxide additive selected from (a) a metal wherein said metal is selected from Zn, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu and Cr; (b) a metal oxide of one or more of the metals selected from Zn, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu and Cr; (c) any compounds that can generate the metal oxides of (b) upon firing; and (d) mixtures thereof. 
     A. Electrically Functional Silver Powders 
     Generally, a thick film composition comprises a functional phase that imparts appropriate electrically functional properties to the composition. The functional phase comprises electrically functional powders dispersed in an organic medium that acts as a carrier for the functional phase that forms the composition. The composition is fired to burn out the organic phase, activate the inorganic binder phase and to impart the electrically functional properties. 
     The functional phase of the composition may be coated or uncoated silver particles which are electrically conductive. When the silver particles are coated, they are at least partially coated with a surfactant. The surfactant may be selected from, but is not limited to, stearic acid, palmitic acid, a salt of stearate, a salt of palmitate and mixtures thereof. Other surfactants may be utilized including lauric acid, palmitic acid, oleic acid, stearic acid, capric acid, myristic acid and linolic acid. The counter-ion can be, but is not limited to, hydrogen, ammonium, sodium, potassium and mixtures thereof. 
     The particle size of the silver is not subject to any particular limitation, although an average particle size of no more than 10 microns, and preferably no more than 5 microns, is desirable. The silver powder accounts for 70 to 85 wt % of the paste composition, and generally 90 to 99 wt % of the solids in the composition (i.e., excluding the organic vehicle). 
     B. Zn-Containing Additive(s) 
     The Zn-containing additive of the present invention may be selected from (a) Zn, (b) metal oxides of Zn, (c) any compounds that can generate metal oxides of Zn upon firing, and (d) mixtures thereof. 
     In one embodiment, the Zn-containing additive is ZnO, wherein the ZnO has an average particle size in the range of 10 nanometers to 10 microns. In a further embodiment, the ZnO has an average particle size of 40 nanometers to 5 microns. In still a further embodiment, the ZnO has an average particle size of 60 nanometers to 3 microns, 
     Typically, ZnO is present in the composition in the range of 2 to 10 weight percent total composition. In one embodiment, the ZnO is present in the range of 4 to 8 weight percent total composition. In still a further embodiment, the ZnO is present in the range of 5 to 7 weight percent total composition. 
     In a further embodiment the Zn-containing additive (for example Zn, Zn resinate, etc.) is present in the total thick film composition in the range of 2 to 16 weight percent. In a further embodiment the Zn-containing additive is present in the range of 4 to 12 weight percent total composition. 
     In a further embodiment, the Zn-containing additive has an average particle size of less than 0.1 μm. In particular the Zn-containing additive has an average particle size in the range of 7 nanometers to less than 100 nanometers. 
     C. Glass Frit 
     Typical glass frit compositions (glass compositions) of the present invention are listed in Table 1 below. It is important to note that the compositions listed in Table 1 are not limiting, as it is expected that one skilled in glass chemistry could make minor substitutions of additional ingredients and not substantially change the desired properties of the glass composition of this invention. In this way, substitutions of glass formers such as P 2 O 5  0-3, GeO 2  0-3, V 2 O 5  0-3 in weight % maybe used either individually or in combination to achieve similar performance. It is also possible to substitute one or more intermediate oxides, such as TiO 2 , Ta 2 O 5 , Nb 2 O 5 , ZrO 2 , CeO 2 , SnO 2  for other intermediate oxides (i.e., Al 2 O 3 , CeO 2 , SnO 2 ) present in a glass composition of this invention. It is observed from the data that generally higher SiO 2  content of the glass degrades performance. The SiO 2  is thought to increase glass viscosity and reduce glass wetting. Although not represented in the Table 1 compositions, glasses with zero SiO 2  are expected to perform well, as other glass formers such as P 2 O 5 , GeO 2  etc. may be used to replace the function of low levels of SiO 2 . The CaO, alkaline earth content, can also be partially or fully replaced by other alkaline earth constituents such as SrO, BaO and MgO although CaO maybe preferred. 
     The glass compositions in weight percent total glass composition are shown in Table 1. Preferred glass compositions found in the examples comprise the following oxide constituents in the compositional range of: SiO 2  0.1-8, Al 2 O 3  0-4, B 2 O 3  8-25, CaO 0-1, ZnO 0-42, Na 2 O 0-4, Li 2 O 0-3.5, Bi 2 O 3  28-85, Ag 2 O 0-3 CeO 2  0-4.5, SnO 2  0-3.5, BiF 3  0-15 in weight percent total glass composition. The more preferred composition of glass being: SiO 2  4-4.5, Al 2 O 3  0.5-0.7, B 2 O 3  9-11, CaO 0.4-0.6, ZnO 11-14, Na 2 O 0.7-1.1, Bi 2 O 3  56-67, BiF 4-13 in weight percent total glass composition. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Glass Composition(s) in Weight Percent Total Glass Composition 
               
            
           
           
               
               
            
               
                 Glass ID 
                 Glass Component (wt % total glass composition) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 SiO 2   
                 Al 2 O 3   
                 B 2 O 3   
                 CaO 
                 ZnO 
                 Na 2 O 
                 Li 2 O 
                 Bi 2 O 3   
                 Ag 2 O 
                 CeO 2   
                 SnO 2   
                 BiF 3   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Glass I 
                 4.00 
                 2.50 
                 21.00 
                   
                 40.00 
                   
                   
                 30.00 
                   
                   
                 2.50 
                   
               
               
                 Glass II 
                 4.00 
                 3.00 
                 24.00 
                   
                 31.00 
                   
                   
                 35.00 
                   
                   
                 3.00 
                   
               
               
                 Glass III 
                 4.30 
                 0.67 
                 10.21 
                 0.55 
                 13.35 
                 0.94 
                   
                 57.85 
                   
                   
                   
                 12.12 
               
               
                 Glass IV 
                 4.16 
                 0.65 
                 9.87 
                 0.53 
                 12.90 
                 0.91 
                   
                 66.29 
                   
                   
                   
                 4.69 
               
               
                 Glass V 
                 7.11 
                 2.13 
                 8.38 
                 0.53 
                 12.03 
                   
                   
                 69.82 
                   
                   
                   
                   
               
               
                 Glass VI 
                 5.00 
                 2.00 
                 15.00 
                 0.50 
                   
                 2.00 
                 3.00 
                 70.00 
                   
                 2.50 
                   
                   
               
               
                 Glass VII 
                 4.00 
                   
                 13.00 
                   
                   
                 3.00 
                 1.00 
                 75.00 
                   
                 4.00 
                   
                   
               
               
                 Glass VIII 
                 2.00 
                   
                 18.00 
                   
                   
                 0.50 
                   
                 75.00 
                 2.50 
                 2.00 
                   
                   
               
               
                 Glass IX 
                 1.50 
                   
                 14.90 
                   
                   
                 1.00 
                 1.00 
                 81.50 
                   
                 0.10 
                   
                   
               
               
                 Glass X 
                 1.30 
                 0.11 
                 13.76 
                   
                   
                 0.54 
                 1.03 
                 82.52 
                   
                 0.74 
               
               
                   
               
            
           
         
       
     
     Glass frits useful in the present invention include ASF1100 and ASF1100B which are commercially available from Asahi Glass Company. 
     An average particle size of the glass frit (glass composition) of the present invention is in the range of 0.5-1.5 μm in practical applications, while an average particle size in the range of 0.8-1.2 μm is preferred. The softening point of the glass frit (Ts: second transition point of DTA) should be in the range of 300-600° C. The amount of glass frit in the total composition is in the range of 0.5 to 4 wt. % of the total composition. In one embodiment, the glass composition is present in the amount of 1 to 3 weight percent total composition. In a further embodiment, the glass composition is present in the range of 1.5 to 2.5 weight percent total composition. 
     The glasses described herein are produced by conventional glass making techniques. The glasses were prepared in 500-1000 gram quantities. Typically, the ingredients are weighed then mixed in the desired proportions and heated in a bottom-loading furnace to form a melt in platinum ahoy crucibles. As is well-known in the art, heating is conducted to a peak temperature (1000° C.-1200° C.) and for a time such that the melt becomes entirely liquid and homogeneous. The molten glass is quenched between counter rotating stainless steel rollers to form a 10-20 mil thick platelet of glass. The resulting glass platelet is then milled to form a powder with its 50% volume distribution set between 1-3 microns. 
     D. Additional Metal/Metal Oxide Additives 
     The additional metal/metal oxide additives of the present invention may be selected from (a) a metal wherein said metal is selected from Zn, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, and Cr; (b) a metal oxide of one or more of the metals selected from Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu and Cr; (c) any compounds that can generate the metal oxides of (b) upon firing; and (d) mixtures thereof. 
     The particle size of the additional metal/metal oxide additives is not subject to any particular limitation, although an average particle size of no more than 10 microns, and preferably no more than 5 microns, is desirable. 
     In one embodiment, the particle size of the metal/metal oxide additive is in the range of 7 nanometers (nm) to 125 nm. In particular, MnO 2  and TiO 2  may be utilized in the present invention with an average particle size range (d 50 ) of 7 nanometers (nm) to 125 nm. 
     The range of the metal/metal oxide additives and ZnO in the total composition is in the range of 0 to 8 weight percent. In one embodiment, the metal/metal oxide additives are present in an amount of 4 to 8 weight percent total composition. When metal/metal oxide additives and ZnO are present in the composition, the metal/metal oxide additives are present in the range of 1 to 5 weight percent total composition and the ZnO is present in the range of 2 to 5 weight percent total composition. 
     E. Organic Medium 
     The inorganic components are typically mixed with an organic medium by mechanical mixing to form viscous compositions called “pastes”, having suitable consistency and rheology for printing. A wide variety of inert viscous materials can be used as organic medium. The organic medium must be one in which the inorganic components are dispersible with an adequate degree of stability. The rheological properties of the medium must be such that they lend good application properties to the composition, including: stable dispersion of solids, appropriate viscosity and thixotropy for screen printing, appropriate wettability of the substrate and the paste solids, a good drying rate, and good firing properties. The organic vehicle used in the thick film composition of the present invention is preferably a nonaqueous inert liquid. Use can be made of any of various organic vehicles, which may or may not contain thickeners, stabilizers and/or other common additives. The organic medium is typically a solution of polymer(s) in solvent(s). Additionally, a small amount of additives, such as surfactants, may be a part of the organic medium. The most frequently used polymer for this purpose is ethyl cellulose. Other examples of polymers include ethylhydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose and phenolic resins, polymethacrylates of lower alcohols, and monobutyl ether of ethylene glycol monoacetate can also be used. The most widely used solvents found in thick film compositions are ester alcohols and terpenes such as alpha- or beta-terpineol or mixtures thereof with other solvents such as kerosene, dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexylene glycol and high boiling alcohols and alcohol esters. In addition, volatile liquids for promoting rapid hardening after application on the substrate can be included in the vehicle. Various combinations of these and other solvents are formulated to obtain the viscosity and volatility requirements desired. 
     The polymer present in the organic medium is the range of 8 wt. % to 11 wt. % of the total composition. The thick film silver composition of the present invention may be adjusted to a predetermined, screen-printable viscosity with the organic medium. 
     The ratio of organic medium in the thick film composition to the inorganic components in the dispersion is dependent on the method of applying the paste and the kind of organic medium used, and it can vary. Usually, the dispersion will contain 70-95 wt % of inorganic components and 5-30 wt % of organic medium (vehicle) in order to obtain good wetting. 
     Description of Method of Manufacturing a Semiconductor Device 
     Accordingly, the invention provides a novel composition(s) that may be utilized in the manufacture of a semiconductor device. The semiconductor device may be manufactured by the following method from a structural element composed of a junction-bearing semiconductor substrate and a silicon nitride insulating film formed on a main surface thereof. The method of manufacture of a semiconductor device includes the steps of applying (typically, coating and printing) onto the insulating film, in a predetermined shape and at a predetermined position, the conductive thick film composition of the present invention having the ability to penetrate the insulating film, then firing so that the conductive thick film composition melts and passes through the insulating film, effecting electrical contact with the silicon substrate. The electrically conductive thick film composition is a thick-film paste composition, as described herein, which is made of a silver powder, Zn-containing additive, a glass or glass powder mixture having a softening point of 300 to 600° C., dispersed in an organic vehicle and optionally, additional metal/metal oxide additive(s). 
     The composition has a glass powder content of less than 5% by weight of the total composition and a Zn-containing additive combined with optional additional metal/metal oxide additive content of no more than 6% by weight of the total composition. The invention also provides a semiconductor device manufactured from the same method. 
     The invention may also be characterized by the use of a silicon nitride film or silicon oxide film as the insulating film. The silicon nitride film is typically formed by a plasma chemical vapor deposition (CVD) or thermal CVD process. The silicon oxide film is typically formed by thermal oxidation, thermal CFD or plasma CFD. 
     The method of manufacture of the semiconductor device may also be characterized by manufacturing a semiconductor device from a structural element composed of a junction-bearing semiconductor substrate and an insulating film formed on one main surface thereof, wherein the insulating layer is selected from a titanium oxide silicon nitride, SiN x :H, silicon oxide, and silicon oxide/titanium oxide film, which method includes the steps of forming on the insulating film, in a predetermined shape and at a predetermined position, a metal paste material having the ability to react and penetrate the insulating film, forming electrical contact with the silicon substrate. The titanium oxide film is typically formed by coating a titanium-containing organic liquid material onto the semiconductor substrate and firing, or by a thermal CVD. The silicon nitride film is typically formed by PECVD (plasma enhanced chemical vapor deposition). The invention also provides a semiconductor device manufactured from this same method. 
     The electrode formed from the conductive thick film composition(s) of the present invention is typically fired in an atmosphere that is preferably composed of a mixed gas of oxygen and nitrogen. This firing process removes the organic medium and sinters the glass frit with the Ag powder in the conductive thick film composition. The semiconductor substrate is typically single-crystal or multicrystalline silicon. 
       FIG. 1(   a ) shows a step in which a substrate of single-crystal silicon or of multicrystalline silicon is provided typically, with a textured surface which reduces light reflection. In the case of solar cells, substrates are often used as sliced from ingots which have been formed from pulling or casting processes. Substrate surface damage caused by tools such as a wire saw used for slicing and contamination from the wafer slicing step are typically removed by etching away about 10 to 20 μm of the substrate surface using an aqueous alkali solution such as aqueous potassium hydroxide or aqueous sodium hydroxide, or using a mixture of hydrofluoric acid and nitric acid. In addition, a step in which the substrate is washed with a mixture of hydrochloric acid and hydrogen peroxide may be added to remove heavy metals such as iron adhering to the substrate surface. An antireflective textured surface is sometimes formed thereafter using, for example, an aqueous alkali solution such as aqueous potassium hydroxide or aqueous sodium hydroxide. This gives the substrate,  10 . 
     Next, referring to  FIG. 1(   b ), when the substrate used is a p-type substrate, an n-type layer is formed to create a p-n junction. The method used to form such an n-type layer may be phosphorus (P) diffusion using phosphorus oxychloride (POCl 3 ). The depth of the diffusion layer in this case can be varied by controlling the diffusion temperature and time, and is generally formed within a thickness range of about 0.3 to 0.5 μm. The n-type layer formed in this way is represented in the diagram by reference numeral  20 . Next, p-n separation on the front and backsides may be carried out by the method described in the background of the invention. These steps are not always necessary when a phosphorus-containing liquid coating material such as phosphosilicate glass (PSG) is applied onto only one surface of the substrate by a process, such as spin coating, and diffusion is effected by annealing under suitable conditions. Of course, where there is a risk of an n-type layer forming on the backside of the substrate as well, the degree of completeness can be increased by employing the steps detailed in the background of the invention. 
     Next, in  FIG. 1(   d ), a silicon nitride film or other insulating films including SiN x :H (i.e., the insulating film comprises hydrogen for passivation during subsequent firing processing) film, titanium oxide film, and silicon oxide film,  30 , which functions as an antireflection coating is formed on the above-described n-type diffusion layer,  20 . This silicon nitride film,  30 , lowers the surface reflectance of the solar cell to incident light, making it possible to greatly increase the electrical current generated. The thickness of the silicon nitride film,  30 , depends on its refractive index, although a thickness of about 700 to 900 Å is suitable for a refractive index of about 1.9 to 2.0. This silicon nitride film may be formed by a process such as low-pressure CVD, plasma CVD, or thermal CVD. When thermal CVD is used, the starting materials are often dichlorosilane. (SiCl 2 H 2 ) and ammonia (NH 3 ) gas, and film formation is carried out at a temperature of at least 700° C. When thermal CVD is used, pyrolysis of the starting gases at the high temperature results in the presence of substantially no hydrogen in the silicon nitride film, giving a compositional ratio between the silicon and the nitrogen of Si 3 N 4  which is substantially stoichiometric. The refractive index falls within a range of substantially 1.96 to 1.98. Hence, this type of silicon nitride film is a very dense film whose characteristics, such as thickness and refractive index, remain unchanged even when subjected to heat treatment in a later step. The starting gas used when film formation is carried out by plasma CVD is generally a gas mixture of SiH 4  and NH 3 . The starting gas is decomposed by the plasma, and film formation is carried out at a temperature of 300 to 550° C. Because film formation by such a plasma CVD process is carried out at a lower temperature than thermal CVD, the hydrogen in the starting gas is present as well in the resulting silicon nitride film. Also, because gas decomposition is effected by a plasma, another distinctive feature of this process is the ability to greatly vary the compositional ratio between the silicon and nitrogen. Specifically, by varying such conditions as the flow rate ratio of the starting gases and the pressure and temperature during film formation, silicon nitride films can be formed at varying compositional ratios between silicon, nitrogen and hydrogen, and within a refractive index range of 1.8 to 2.5. When a film having such properties is heat-treated in a subsequent step, the refractive index may change before and after film formation due to such effects as hydrogen elimination in the electrode firing step. In such cases, the silicon nitride film required in a solar cell can be obtained by selecting the film-forming conditions after first taking into account the changes in film qualities that will occur as a result of heat treatment in the subsequent step. 
     In  FIG. 1(   d ), a titanium oxide film may be formed on the n-type diffusion layer,  20 , instead of the silicon nitride film,  30 , functioning as an antireflection coating. The titanium oxide film is formed by coating a titanium-containing organic liquid material onto the n-type diffusion layer,  20 , and firing, or by thermal CVD. It is also possible, in  FIG. 1(   d ), to form a silicon oxide film on the n-type diffusion layer,  20 , instead of the silicon nitride film  30  functioning as an antireflection layer. The silicon oxide film is formed by thermal oxidation, thermal CVD or plasma CVD. 
     Next, electrodes are formed by steps similar to those shown in  FIGS. 10(   e ) and ( f ). That is, as shown in  FIG. 1(   e ), aluminum paste,  60 , and back side silver paste,  70 , are screen printed onto the back side of the substrate,  10 , as shown in  FIG. 1(   e ) and successively dried. In addition, a front electrode-forming silver paste is screen printed onto the silicon nitride film,  30 , in the same way as on the back side of the substrate,  10 , following which drying and firing are carried out in an infrared furnace at typically at a set point temperature range of 700 to 975° C. for a period of from one minute to more than ten minutes while passing through the furnace a mixed gas stream of oxygen and nitrogen. 
     As shown in  FIG. 1(   f ), during firing, aluminum diffuses as an impurity from the aluminum paste into the silicon substrate,  10 , on the back side, thereby forming a p+ layer,  40 , containing a high aluminum dopant concentration. Firing converts the dried aluminum paste,  60 , to an aluminum back electrode, 61. The backside silver paste,  70 , is fired at the same time, becoming a silver back electrode, 71. During firing, the boundary between the backside aluminum and the backside silver assumes the state of an alloy, thereby achieving electrical connection. Most areas of the back electrode are occupied by the aluminum electrode, partly on account of the need to form a p+ layer,  40 . At the same time, because soldering to an aluminum electrode is impossible, the silver or silver/aluminum back electrode is formed on limited areas of the backside as an electrode for interconnecting solar cells by means of copper ribbon or the like. 
     On the front side, the front electrode silver paste,  500 , of the invention is composed of silver, Zn-containing additive, glass frit, organic medium and optional metal oxides, and is capable of reacting and penetrating through the silicon nitride film,  30 , during firing to achieve electrical contact with the n-type layer,  20  (fire through). This fired-through state, i.e., the extent to which the front electrode silver paste melts and passes through the silicon nitride film,  30 , depends on the quality and thickness of the silicon nitride film,  30 , the composition of the front electrode silver paste, and on the firing conditions. The conversion efficiency and moisture resistance reliability of the solar cell clearly depend, to a large degree, on this fired-through state. 
     EXAMPLES 
     The thick film composition(s) of the present invention are described herein below in Table 2-6 
     Paste Preparation 
     Paste preparations were, in general, accomplished with the following procedure: The appropriate amount of solvent, medium and surfactant was weighed then mixed in a mixing can for 15 minutes, then glass frits and metal additives were added and mixed for another 15 minutes. Since Ag is the major part of the solids of the present invention, it was added incrementally to ensure better wetting. When well mixed, the paste was repeatedly passed through a 3-roll mill for at progressively increasing pressures from 0 to 400 psi. The gap of the rolls was adjusted to 1 mil. The degree of dispersion was measured by fineness of grind (FOG). A typical FOG value is generally equal to or less than 20/10 for conductors. 
     The ASF1100 glass frit (available from Asahi Glass Company) used in the following examples was not used as supplied. It was milled to a D 50  in the range of 0.5-0.7 microns prior to use. 
     Test Procedure-Efficiency 
     The solar cells built according to the method described above were placed in a commercial IV tester for measuring efficiencies (ST-1000). The Xe Arc lamp in the IV tester simulated the sunlight with a known intensity and radiated the front surface of the cell. The tester used a four contact method to measure current (I) and voltage (V) at approximately 400 load resistance settings to determine the cell&#39;s I-V curve. Both fill factor (FF) and efficiency (Eff) were calculated from the I-V curve. 
     Paste efficiency and fill factor values were normalized to corresponding values obtained with cells contacted with industry standard PV145 (E.I. du Pont de Nemours and Company). 
     Test Procedure-Adhesion 
     After firing, a solder ribbon (copper coated with 96.5 Sn/3.5 Ag) was soldered to the bus bars printed on the front of the cell. Solder reflow was typically achieved at 365° C. for 5 seconds. Flux used was non-activated Alpha-100. The soldered area was approximately 2 mm×2 mm. The adhesion strength was obtained by pulling the ribbon at an angle of 90° to the surface of the cell. Normalized adhesion strength was calculated to compare vs. a minimum adhesion value of 300 g. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Effect of Glass Composition on Thick Film Silver Paste 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 % 
                   
                   
                   
                   
                   
               
               
                 Glass ID No. 
                 Frit 
                 % ZnO 
                 FF (%) 
                 Norm FF 
                 Eff (%) 
                 Norm Eff 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Glass I 
                 1.8 
                 6 
                 54.7 
                 74.7 
                 9.8 
                 74.8 
               
               
                 Glass II 
                 1.8 
                 6 
                 59 
                 80.6 
                 10.3 
                 78.6 
               
               
                 Glass III 
                 1.8 
                 6 
                 73.6 
                 100.5 
                 13.3 
                 101.5 
               
               
                 Glass IV 
                 1.8 
                 6 
                 71.8 
                 98.1 
                 13.1 
                 100.0 
               
               
                 Glass V 
                 1.8 
                 6 
                 63.1 
                 86.2 
                 11.2 
                 85.5 
               
               
                 Glass VI 
                 1.8 
                 6 
                 50.7 
                 69.3 
                 8.0 
                 61.1 
               
               
                 Glass VII 
                 1.8 
                 6 
                 56.7 
                 77.5 
                 9.3 
                 71.0 
               
               
                 Glass 
                 1.8 
                 6 
                 67.2 
                 91.8 
                 12.0 
                 91.6 
               
               
                 VIII 
                   
                   
                   
                   
                   
                   
               
               
                 Glass IX 
                 1.8 
                 6 
                 70.0 
                 100.0 
                 12.8 
                 97.7 
               
               
                 Glass X 
                 1.8 
                 6 
                 65.7 
                 93.9 
                 11.8 
                 90.1 
               
               
                 Control I 
                   
                   
                 73.2 
                 100.0 
                 13.1 
                 100.0 
               
               
                 (PV145)* 
                   
                   
                   
                   
                   
                   
               
               
                 Control II 
                   
                   
                 70.0 
                 100.0 
                 13.1 
                 100.0 
               
               
                 (PV145)* 
               
               
                   
               
               
                 *Control I and Control II represent PV145 a high performance thick film composition comprising a Pb bearing glass frit commercially available from E. I. du Pont de Nemours and Company. 
               
            
           
         
       
     
     Percents of glass frit and ZnO given in Table 2 are given in percent total thick film composition. 
     Thick films containing Glasses III, IV, VIII, and IX achieved especially good contact to the solar cell as demonstrated by good cell performance similar to that of the Control I and Control II thick film paste compositions. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Effect of ZnO Additions on Thick Film Silver Paste 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Add 
                 ASF 1100* 
                   
                 FF Norm 
                   
                 Eff Norm 
               
               
                 Add 
                 % 
                 % Frit 
                 FF(%) 
                 to PV145 
                 Eff (%) 
                 to PV145 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 None 
                 0 
                 1.8 
                 29.6 
                 38.8 
                 3.3 
                 23.9 
               
               
                 ZnO 
                 4 
                 1.2 
                 72.6 
                 95.3 
                 13.0 
                 94.2 
               
               
                 ZnO 
                 4 
                 2.4 
                 71.2 
                 93.4 
                 13.3 
                 96.4 
               
               
                 ZnO 
                 6 
                 1.8 
                 76.3 
                 100.1 
                 14.1 
                 102.2 
               
               
                 ZnO 
                 8 
                 1.2 
                 76.4 
                 100.3 
                 13.7 
                 99.3 
               
               
                 ZnO 
                 8 
                 2.4 
                 75.8 
                 99.5 
                 13.9 
                 100.7 
               
               
                 PV145 
                   
                   
                 76.2 
                 100.0 
                 13.8 
                 100.0 
               
               
                 Control 
               
               
                   
               
               
                 *ASF1100 glass frit is commercially available from Asahi Glass Company 
               
            
           
         
       
     
     Percents of glass frit and additive given in Table 3 are given in percent total thick film composition. 
     Thick film silver paste compositions containing ZnO have superior electrical performance as compared to silver paste with no ZnO. With addition of ZnO, silver pastes attain electrical performance similar to or better than high performance control paste PV145 commercially available from E.I. du Pont de Nemours and Company. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Effect of Various Zn Additions on Thick Film Silver Paste 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 ASF 
                   
                 FF Norm 
                   
                   
               
               
                   
                   
                 1100 
                 FF 
                 to 
                 Eff 
                 Eff Norm 
               
               
                 Add 
                 Add % 
                 % Frit 
                 (%) 
                 PV145 
                 (%) 
                 to PV145 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 None 
                 0 
                 1.8 
                 29.6 
                 40.4 
                 3.3 
                 25.6 
               
               
                 Zn 
                 6 
                 1.8 
                 74 
                 101.0 
                 13.2 
                 102.3 
               
               
                 ZnO Fine 
                 5.4 
                 1.8 
                 74.3 
                 101.4 
                 12.5 
                 96.9 
               
               
                 ZnO Fine 
                 6 
                 1.8 
                 72.4 
                 98.8 
                 12.7 
                 98.4 
               
               
                 Zn 
                 12 
                 1.2 
                 67.9 
                 92.6 
                 12.1 
                 93.8 
               
               
                 Resinate 
                   
                   
                   
                   
                   
                   
               
               
                 Zn 
                 16 
                 1 
                 69.3 
                 94.5 
                 11.8 
                 91.5 
               
               
                 Resinate 
                   
                   
                   
                   
                   
                   
               
               
                 PV145 Control 
                   
                   
                 73.3 
                 100.0 
                 12.9 
                 100.0 
               
               
                   
               
            
           
         
       
     
     Percents of glass frit and additive given in Table 4 are given in percent total thick film composition. 
     Experiments conducted and detailed in Table 4 Illustrate the use of various types of Zn-containing additives and their effect on the thick film composition. Thick film silver paste compositions containing other forms and particle sizes of Zn and ZnO also achieve excellent electrical contact to Si solar cells. The Zn resinate used was 22% Zinc Hex-Cem obtained from OMG, Cleveland; OH. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Effect of Mixed Oxide Additions on Thick Flim Silver Paste 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 ASF 
                   
                   
                   
                 Eff Norm 
               
               
                   
                   
                 1100 % 
                 FF 
                 FF Norm 
                 Eff 
                 to 
               
               
                 Add 
                 Add % 
                 Frit 
                 (%) 
                 to PV145 
                 (%) 
                 PV145 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 None 
                 0 
                 1.8 
                 29.6 
                 42.3 
                 3.3 
                 25.2 
               
               
                 ZnO + 
                   4/1.5 
                 1.8 
                 63.4 
                 90.6 
                 11.4 
                 87.0 
               
               
                 FeO 
                   
                   
                   
                   
                   
                   
               
               
                 ZnO + 
                 4.5/2.3 
                 1.8 
                 70.8 
                 101.1 
                 13.2 
                 100.8 
               
               
                 SnO2 
                   
                   
                   
                   
                   
                   
               
               
                 ZnO + 
                 4.5/1.5 
                 1.8 
                 69.6 
                 99.4 
                 12.7 
                 96.9 
               
               
                 GdO 
                   
                   
                   
                   
                   
                   
               
               
                 PV145 Control 
                   
                   
                 70.0 
                 100.0 
                 13.1 
                 100.0 
               
               
                   
               
            
           
         
       
     
     Percents of glass frit and additive given in Table 5 are given in percent total thick film composition. 
     Thick film silver paste compositions comprising a mixture of oxide frits also demonstrate much improved performance. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Effect of Other Oxide Additions on Thick Film Silver Paste 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 ASF 
                   
                 FF Norm 
                   
                 Eff Norm 
               
               
                   
                   
                 1100* 
                 FF 
                 to 
                 Eff 
                 to 
               
               
                 Add 
                 Add % 
                 % Frit 
                 (%) 
                 PV145 
                 (%) 
                 PV145 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 None 
                 0 
                 1.8 
                 29.6 
                 41.6 
                 3.3 
                 26.0 
               
               
                 TiO2 
                 6 
                 1.8 
                 53.4 
                 75.1 
                 9.2 
                 72.4 
               
               
                 Cr2O3 
                 6 
                 1.8 
                 55.5 
                 78.1 
                 10.1 
                 79.5 
               
               
                 MnO 
                 6 
                 1.8 
                 26.8 
                 37.7 
                 1.6 
                 12.6 
               
               
                 MnO 
                 3 
                 1.8 
                 33.3 
                 46.8 
                 5.1 
                 40.2 
               
               
                 MnO2 
                 6 
                 1.8 
                 28.7 
                 40.4 
                 2.3 
                 18.1 
               
               
                 FeO 
                 6 
                 1.8 
                 59.4 
                 83.5 
                 10.5 
                 82.7 
               
               
                 CoO 
                 6 
                 1.8 
                 50.6 
                 71.2 
                 8.9 
                 70.1 
               
               
                 Cu2O 
                 6 
                 1.8 
                 44.4 
                 62.4 
                 7.6 
                 59.8 
               
               
                 ZnO 
                 6 
                 1.8 
                 72 
                 101.3 
                 12.8 
                 100.8 
               
               
                 ZrO2 
                 6 
                 1.8 
                 30.5 
                 42.9 
                 4.4 
                 34.6 
               
               
                 MoO3 
                 4 
                 1.8 
                 25.8 
                 36.3 
                 1.4 
                 11.0 
               
               
                 RuO2 
                 6 
                 1.8 
                 34 
                 47.8 
                 5.8 
                 45.7 
               
               
                 SnO2 
                 6 
                 1.8 
                 58.4 
                 82.1 
                 9.7 
                 76.4 
               
               
                 SnO2 
                 9 
                 1.8 
                 58.9 
                 82.8 
                 10.1 
                 79.5 
               
               
                 WO3 
                 4 
                 1.8 
                 52.3 
                 73.6 
                 9.0 
                 70.9 
               
               
                 CeO2 
                 6 
                 1.8 
                 54 
                 75.9 
                 9.4 
                 74.0 
               
               
                 GdO 
                 6 
                 1.8 
                 62 
                 87.2 
                 11.2 
                 88.2 
               
               
                 FeCoCrOx 
                 6 
                 1.8 
                 61.2 
                 86.1 
                 10.7 
                 84.3 
               
               
                 CoCrOx 
                 6 
                 1.8 
                 38.2 
                 53.7 
                 5.7 
                 44.9 
               
               
                 CuCrOx 
                 6 
                 1.8 
                 59 
                 83.0 
                 10.6 
                 83.5 
               
               
                 CuRuO3 
                 6 
                 1.8 
                 54 
                 75.9 
                 9.5 
                 74.8 
               
               
                 PV145 Control 
                   
                   
                 71.1 
                 100.0 
                 12.7 
                 100.0 
               
               
                   
               
               
                 *ASF1100 glass frit is commercially available from Asahi Glass Company 
               
            
           
         
       
     
     Percents of glass frit and additive given ire Table 6 are given in percent total thick film composition. 
     All oxide additions, detailed in Table 6 above, to thick film silver paste result in solar cell performance improvement. 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Effect of ZnO Additive Level on Thick 
               
               
                 Film Silver Paste Adhesion to Si 
               
            
           
           
               
               
               
               
            
               
                 % ASF1100 Frit 
                 % ZnO 
                 Adh (g) 
                 Normalized Adh (%) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 1.2 
                 4 
                 558 
                 186 
               
               
                 2.4 
                 4 
                 486 
                 155 
               
               
                 1.8 
                 6 
                 441 
                 147 
               
               
                 1.2 
                 8 
                 332 
                 111 
               
               
                 2.4 
                 8 
                 282 
                 94 
               
               
                   
               
               
                 *ASF1100 glass frit is commercialiy available from Asahi Glass Company 
               
            
           
         
       
     
     Percents of glass frit and additive given in Table 7 are given in percent total thick film composition.