AN IMPROVED METHOD OF FABRICATING CONTACTS FOR SOLAR CELLS

Abstract: A solar cell fabrication procedure is described in which a silicon substrate (2) having a layer of silicon nitride (10) on one side is selectively coated with a metal paste (16). The paste contains a selected metal and a lead silicate glass frit. Upon heating, the glass penetrates the silicon nitride and the substrate surface is metallized by the selected metal.

Description: AN IMPROVED METHOD OF FABRICATING CONTACTS FOR SOLAR CELLS This invention relates to the manufacture of solid state semiconductor devices. More particularly, the invention relates to an improved process for forming a conductive metal contact on the front side of a silicon wafer coated with silicon nitride. BACKGROUND OF THE INVENTION Heretofore a common method of fabricating silicon solar cells has included the steps of: (1) diffusing phosphine into P-type silicon ribbon so as to form a shallow junction, (2) forming on the junction side of the ribbon a thin coating of silicon nitride, (3) forming a grid electrode pattern in the silicon nitride coating by photolithography (using a suitable photoresist composition and etching) , (4) coating the other side of the silicon ribbon with an aluminum paste, (5) heating the silicon so as to alloy the aluminum and silicon, (6) plating exposed silicon on both sides of the ribbon with nickel, (7) sintering the nickel to form a nickel suicide, and (8) plating additional metal (s) onto the metal covered portions of the silicon. A more detailed description of such a process can be found in U. S. Patent 4451969. The steps of forming a grid electrode by photolithography, which necessarily involves removal of residual resist, is extremely effective. However, SUBSTITUTESHEET photolithography involves several time consuming steps and adds a disproportionally high share to the overall cost of forming solar cells. Illustrating the state of the prior art are U.S. Patents Nos. 4375007, 4388346 and 4640001, and the references cited therein. U.S. Patent 4375007 discloses a silicon solar cell with aluminum-magnesium alloy low resistance contacts. The Al-Mg alloy is mixed with Ni-Sb alloy or Al, in powder form, to form a thick-film metallizing paste useful for making low resistance electrically conductive contacts to a silicon solar cell coated with Si N . 3 4 U.S. Patent 4388346 discloses electrodes for solid state devices. The electrodes are formed by screen-printing through a mask a pattern of ink containing a dispersion of lower melting, sinterable metal coated base metal particles dispersed in a liquid vehicle including a vapor zable binder polymer and a fluorocarbon polymer. On firing the screen-printed device, the coated metal particles sinter without being oxidized to form a contact electrode. An anti-reflection coating is then applied to the cell. More recently, U.S. Patent 4640001 (Koiwai et al) discloses a solar cell manufacturing method in which a silicon wafer is coated with an anti-reflection coating of silicon nitride by means of plasma CVD deposition at a temperature between 250 degrees C and 600 degrees C. The silicon nitride coating makes the surface of the silicon cell unacceptable for firing screen-printed electrodes. Therefore, the coating is partially removed by etching in the regions where the electrodes are to be formed to insure that the electrodes will bond to the silicon wafer. At the time this invention was conceived, it was widely recognized that widespread use of photovoltaic solar cells was dependent upon the development of fabrication techniques capable of producing reliable solar cells with a conversion efficiency of 12% or higher at a relatively low cost. The cost of solar cells, like other semiconductor devices, depends on the cost of the starting materials and the cost of converting the starting materials into the finished product. The conversion of silicon into at least 12% efficient silicon solar cells at low cost cannot be accomplished without a suitable low-cost metallization technique for forming the grid electrode. OBJECTS OF THE INVENTION Accordingly, the primary object is to provide a low cost processing sequence for the fabrication of solar cells in which a metal paste is fired through a silicon nitride layer to form a conductive contact. Another object of this invention is to provide an improved low cost method of making photovoltaic semiconductor junction devices in which a metal paste is fired through a silicon nitride coating to form a conductive contact with the substrate, while leaving the surrounding silicon nitride to serve as an anti-reflection layer and also as an environmental protector for the substrate. Still another important object is to provide an improved method of making semiconductor solar cells in which a layer of silicon nitride serves as an anti-reflection coating on each cell, and formation of a metal contact is accomplished through the silicon nitride layer. Yet another object of the present invention is to provide a method of making solar cells in which a paste employing nickel or silver is fired through a silicon nitride layer to form a conductive contact. An additional object of this invention is to provide an improved method of fabricating a silicon solar cell with a silicon nitride coating which achieves the previous objects without (a) etching a grid pattern in the silicon nitride layer in preparation for application of a metal contact, and (b) degradation of cell performance in the course of contact formation. Another object of this invention is to provide an improved method of making silicon solar cells which is less expensive but no less reliable than prior methods of forming nickel or silver contacts. The foregoing objects are achieved by a solar cell fabrication procedure of the character described which involves the following steps: (1) providing a silicon substrate which has a P-N junction and a layer of silicon nitride on its front surface adjacent the junction, (2) selectively coating the silicon nitride layer with a selected metal paste so that the layer forms a selected contact pattern on the silicon nitride, and (3) heating the substrate to rapidly and efficiently cause the metal coating to penetrate the silicon nitride layer and form a metal ohmic contact on the front surface of the substrate. The novel method may be practiced with a silicon solar cell which already has a back ohmic contact or the latter contact may be formed after the front surface contact has been formed. A preferred embodiment involves, inter alia, the following steps: (1) providing a silicon substrate which has a P-N junction and a layer of silicon nitride on the front surface, (2) coating the back side of the silicon substrate with an aluminum paste, (3) selectively coating the front side of the silicon substrate with a selected silver or nickel paste and (4) heating the silicon substrate to rapidly and efficiently cause (a) the aluminum to form an adherent conductive back side contact and (b) the metal and frit components in the paste to penetrate the silicon nitride layer and form an adherent conductive front side contact. Thereafter, the substrate may be further processed, e.g., to prepare it for connection to electrical circuits. The metal paste that is fired into the substrate must have the following characteristic. First, it must be capable of rapidly penetrating the silicon nitride layer at relatively low temperatures at which the solar cell integrity is not destroyed, i.e., temperatures below about 900 degrees C. Second it must not diffuse into the silicon substrate so as to alter its P-N junction, and thus degrade cell performance. Third, it must not cause the silicon nitride layer to crack during thermal treatment. Fourth, it must not significantly alter the ability of the silicon nitride layer to act as an anti-reflection element. Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims. BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description which is to be considered together with the accompanying drawing which illustrate a number of the steps involved in making solar cells according to a preferred form of the invention. Throughout the drawing, like reference numbers refer to similar structure. In the drawing, the thicknesses and depths of the several coatings and regions are neither shown to scale nor shown exactly in accordance with their relative proportions, for convenience of illustration. DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawing, the preferred embodiment of the invention relates to the production of solar cells from EFG grown P-type silicon ribbon. For this embodiment, there is provided as a starting piece a partially finished cell 1. Partially finished cell 1 has a substrate 2 , preferably comprising a length of P-type conductivity silicon ribbon, one side (hereafter the "front side") of which has been provided with a relatively shallow junction 4 (e.g., a junction of between about 3,000 and about 7,000 Angstrom units deep) , an N-type (typically N-f) conductivity region 6, and a silicon nitride layer 10. The other side (hereinafter the "rear side") of the substrate is preferably provided with a layer of aluminum 12 alloyed to the substrate and a P+ region 14. The P+ region 14 preferably has a depth of from about 1 to about 5 microns. Partially finished cell 1 may be fabricated by any of a number of means well known in the art. For instance, junction 4 and N+ conductivity region 6 may be formed in a P-type silicon substrate 2 by the diffusion of phosphorus. Layer 12 and P+ region 14 may be formed by coating the rear side of the substrate with a layer of an aluminum paste comprising aluminum powder in a volatile organic vehicle, such as terpineol, diethylene glycol monobutyl ether acetate or ethylcellulose, that can be removed by evaporation or combustion, and then heating the substrate to remove any volatile or pyrolyzable organic components of the paste and to alloy the aluminum to the substrate and form the P+ region. However, other forms of substrate, junction, and rear electrode, and other methods of fabrication, may equally well be employed to provide partially finished cell 1. Of course, the process provided by this invention is not limited to the production of solar cells from EFG substrates. Thus, for example, cast polycrystalline substrates, epitaxial silicon on metallurgical grade silicon or fine grade polysilicon layers formed by chemical or physical vapor deposition can be used to form solar cells using the present invention. Further, the process is applicable to single crystal silicon. Then, too, the process may be practiced with N-type as well as P-type material. Also, the invention may be applied to subεtrates other than ribbons or similar flat stock, e.g., circular pieces of material or substrates having an arcuate or polygonal cross-section. Starting with such a prefabricated piece as shown in Fig. 1, the front side is coated with a paste 16 containing a selected conductive metal, a frit of lead silicate glass or a lead borosilicate glass, and a low boiling liquid organic vehicle such as ethylene glycol monoethyl ether or terpineol in amounts up to 2-10 wt. % of the paste. Preferably, for cost and other considerations, the metal is nickel or silver. The paste is applied according to a predetermined electrode pattern, e.g., in the form of a grid electrode comprising a plurality of narrow fingers attached at one end to a bus bar or runner. One form of suitable grid electrode is illuminated in U.S. Patent No. 3686036. Paste 16 may be applied using any of the known techniques which are commonly used in the electronics industry. Screen printing and direct write-on coating techniques are satisfactory. Once paste 16 has been applied and dried it is ready to be fired. The firing is conducted in a furnace with an oxygen atmosphere. The firing temperature may vary, but in any case it should be adequate to volatize or pyrolyze the organics and melt the glass frit until the frit fires through the silicon nitride layer 10. The time in the furnace will vary with the firing temperature, generally the overall time in the furnace may be between 0.25 and 2.0 minutes, including the time required to bring the substrate up to firing temperature. The concentration of metal, glass frit and organics in paste 16 may vary. TE SHEc CETτT Preferably, however, the paste consists of 20 to 70 wt. % metal, 5 to 30 wt. % glass frit, and 1 to 12 wt. % organics. Most preferably, the paste has only about 5 wt. % glass frit and between 1 wt. % to 2.0 wt. % ethylene glycol monoethyl ether or between 1.0 wt. % to 10 wt. % terpineol. The glass frit comprises 5-80 wt. % lead and up to 40 wt. % silicon oxide in the case of a lead silicate glass, and 5-80% lead, 1 to 40 % silicon oxide and 1-30% boron oxide in the case of a lead boroεilicate glass. Temperatures of 650-850 degrees C are suitable for firing the paste, but a temperature of about 800 degrees C is preferred. Thus, for example, a furnace containing ambient air heated to 800 degrees C has been found sufficient to produce the desired results in 15-60 seconds at the firing temperature for pastes which contained silver or nickel as the selected conductive metal. If nickel is the selected metal contained in paste 16, the ambient air is preferably substantially replaced with nitrogen, e.g., 90-100% nitrogen and 0-10% oxygen. During firing the glass frit in paste 16 becomes fluid and fires through the silicon nitride layer 10 to form depressions 17 and ohmic contact 18. The organics in paste 16 volatize or burn off and the conductive metal and glass frit bond to the front surface of the substrate, forming ohmic contact 18. In an alternate preferred embodiment, the rear side of the substrate has been coated with an aluminum paste. In this embodiment, the firing of the metal paste through silicon nitride layer 10 on the front side is accomplished simultaneously with firing of the aluminum paste to alloy " aluminum to the rear side of the silicon substrate. In the preferred embodiment of the invention, the conductive metal is silver or nickel. Paste 16 may contain other metals in addition to or in place of silver and nickel. These other metals are palladium, platinum and rhodium. Like nickel and silver, they are capable of forming an ohmic contact, preferably at a relatively low temperature. The following example illustrates the preferred mode of practicing the invention. EXAMPLE A partially finished cell of P-type EFG-grown silicon ribbon is provided having a conductivity of about 1-5 ohm-cm. The cell has a shallow P-N junction about 6,000 angstroms deep, and a silicon nitride layer of about 800 angstroms on its front surface. The rear side of the cell has a layer of aluminum alloyed to the silicon. A layer of silver-containing paste is selectively screen printed onto the silicon nitride layer. The layer of paste is patterned as a multi-fingered grid electrode having the form illustrated in U.S. Patent No. 3686036. The paste contains silver metal, a lead borosilicate glass frit and terpineol as the liquid vehicle. The silver metal is in powder form and the glass frit contains 5-80% lead, 5-30% boron oxide, and 5 to 40% silicon oxide. The silver metal, the glass frit and the vehicle constitute about 70-80 wt. %, 5 wt. % and 15-25 wt. % respectively of the paste. Once the paste has dried (which may but need not involve heating the paste to evaporate any volatile " vehicle) , the silicon substrate is passed into an air furnace and heated in air to about 800 degrees C. It is held at that firing temperature for about 15 seconds. Then it is removed from the furnace and allowed to cool to room temperature. The overall time in the furnace is about 2 minutes. The firing step has been found to be sufficient to cause any organics in the paste to volatize and burn and the lead borosilicate glass frit to become sufficiently fluid to fire through the silicon nitride coating and bond to the front surface of the silicon substrate. The glass frit carries the silver with it and permits the silver to make an ohmic contact with the substrate. EFG substrate solar cells having contacts formed according to the present invention, and exemplified by the foregoing example, have been tested and have yielded pleasant results. Cells made according to this invention have exhibited the following characteristics: an open circuit voltage of 589 mV; a short circuit current density of 30.2 mA per square centimeter; a fill factor of 0.78; and an efficiency of 13.8 percent. The foregoing values are comparable with results obtained with current processing methods which use etching and plating techniques to form ohmic contacts. The best results were obtained from contacts which received a brief (10-20 seconds) bath in a solution containing fluorine ions after firing. The average values from 8 cells subjected to the bath were calculated as follows: The average open circuit voltage was 588 mV, short circuit current was 29.5 mA per square centimeter; the fill factor was 0.75; and the efficiency was 12.9 SUBSTITUTE SHEET percent. Silicon nitride remaining on the front side of the finished devices serves as an effective anti-reflective coating. In this connection it is to be noted, as previously mentioned, that the silicon nitride coating does not crack during the thermal treatment involved in the heating step. It should be noted that the back contact may be made of silver which may contain a small amount of aluminum instead of aluminum when the front contact is made of silver. The back contact may be applied immediately after application of the paste defining the front contact has been applied and dried. Since these and other changes may be made in the above processes without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative. The present invention is indicated by the broad general meaning of the terms in which the appended claims are expressed. SUBSTITUTESHΞΞi

Claims: WHAT IS CLAIMED IS: 1. A method of fabricating solid state semiconductor devices comprising in sequence the steps of: (a) providing a silicon substrate having (1) oppositely-facing first and second surfaces, and (2) a layer of silicon nitride on said first surface; (b) selectively covering said layer of silicon nitride with a paste containing (1) a selected metal from the class consisting of nickel, silver, palladium, platinum and rhodium, (2) glass frit comprising a lead silicate glass or a lead borosilicate glass; and (3) an organic vehicle; and (c) heating said substrate so as to cause said paste to penetrate said silicon nitride and form an ohmic contact on said first surface. 2. A method according to claim 1 wherein said silicon substrate has a preformed PN junction adjacent said first surface. 3. A method according to claim 1 wherein said devices are photovoltaic. 4. A method of claim 1 wherein said metal is silver. 5. A method according to claim 1 wherein said metal is nickel. 6. A method according to claim 1 wherein said glass frit is a lead silicate glass. " 7. A method according to claim 1 wherein said glass contains approximately 5 to 80 wt. % lead. 8. A method according to claim 7 wherein said glass contains approximately up to about 30 wt. % boron oxide. 9. A method according to claim 1 wherein said paste is applied by screen printing. 10. A method according to claim 1 wherein heating step (c) is conducted in an oxygen-containing atmosphere. 11. A method according to claim 1 in which said metal is silver. 12. A method according to claim 1 in which said heating step is accomplished in a furnace under ambient air. 13. A method according to claim 12 in which said heating step is accomplished at a temperature between 650-850 degrees C. 14. A method according to claim 13 in which said heating step involves firing the substrate at a temperature of 650-850 degrees C for a period of 15-60 seconds. 15. The method of claim 14 in which said metal is nickel. 16. The method of claim 1 in which said metal is palladium. 17. The method of claim 1 in which said metal is platinum. 18. A method according to claim 1 in which said metal is rhodium. 19. A method according to claim 1 in which said substrate has a second ohmic contact on said second surface. 20. A method according to claim 19 in which said second ohmic contact comprises aluminum or silver metal. 21. A method according to claim 1 in which a second aluminum-containing coating is applied to said substrate. 22. A method according to claim 1 wherein said paste comprises terpineol or ethylene glycol monoethyl ether in an amount not exceeding 10 wt. % of the paste in the case of terpineol and 2.0 wt. % in the case of the ether. 23. A solid state semiconductor device formed by the process of: (a) providing a silicon substrate having (1) opposing first and second surfaces; and (2) a layer of silicon nitride on said first surface; (b) selectively covering said first surface with a paste containing (1) at least one selected electrically conductive metal; (2) a lead silicate glass frit and (3) an organic vehicle; and (c) heating said substrate to cause said paste to penetrate said silicon nitride so that said selected metal will form an ohmic contact on said first surface. 24. The device of claim 23 in which said silicon substrate has a preformed PN junction adjacent said first surface. 25. The device of claim 24 in which said paste contains a metal chosen from the group consisting of nickel, silver, palladium, platinum and rhodium.

Inventors: HANOKA JACK I, HANOKA, JACK, I.
Applicants: MOBIL SOLAR ENERGY CORP, MOBIL SOLAR ENERGY CORPORATION
Application year: 1989
Main classification: H01L27
IPC codes: H01L31