SE456625B - SET TO MANUFACTURE SOLAR CELLS - Google Patents

Description

456 625 10 15 20 25 30 35 2 Considering when designing a cell fabrication sequence that the combination of time and temperature at any step following the hydrogen passivation should not cause the hydrogen introduced into the silicon to diffuse back out of the passivated substrate. Thus, for example, it has been found that a hydrogen passivated cell, which has been exposed to a temperature of 600 ° C for half an hour in vacuum, loses substantially all of the bound hydrogen and returns to its previous level of passivation, as evidenced by the induction of an electron beam. - reduced power activity. In this regard, it should be noted that the transition diffusion step in solar cell production typically involves temperatures of the order of 900 ° C.

It has also been found that hydrogen passivation normally heats the cell to a sufficiently high temperature to cause base metals, such as copper, to migrate through the junction, thereby causing a "soft" diode or short circuit. As shown, for example, by C. H.

Seager, DJ Sharp, JKG Panitz, and RV D'Aiello in Journal of Vacuum Science and Technology, vol 20, no. 3, pages 430 - 435 (March 1982), polycrystalline silicon passivation can be performed with a Kaufman-type ion source, which is used to generate a hydrogen ion beam in the kiloelectron voltage range. Relatively short exposure times (eg between 0.5 and 4 min) in an interval with high ion energy and large flux (eg 1 - 3 mA / cm 2) prove to be optimal. Such exposures generally result in the temperature of the substrate rising to at least about 275 ° C, if the substrate is carefully contacted with a suitable cooler. Otherwise, temperatures in excess of 400 ° C are easily reached. However, it is important that temperatures are limited to less than about 300 ° C to avoid rapid migration of base metals in the silicon matrix. However, manipulation of the substrate and coolant to provide heat control during passivation easily becomes the rate limiting factor in highly productive treatment with such ion sources. Accordingly, it is desirable to avoid cooling in order to obtain a cheap, highly productive process. In addition, the surface irregularities of EFG-type silicon strips, which can be manufactured profitably, make cooling difficult.

Hydrogen passivation is also more effective when the silicon base surface is exposed. Thus, no "positive" plating mask, which is used to define the electrode grid pattern of the front surface by covering the intermediate electrode surfaces of the front surface, should be in place during passivation.

The altered surface layer produced by hydrogen ion beam passivation can be used as a plating mask for subsequent metallization steps, which includes immersion plating of a selected metal.

A preferred embodiment of the method, which is applied in the manufacture of silicon solar cells, comprises, inter alia, the following steps: (1) forming a plating mask of a dielectric material on the front surface of a silicon strip with a shallow transition, to leave these silicon surfaces exposed to later covered by the front surface electrode, (2) depositing a thin layer of nickel (or similar material) on the exposed silicon, (3) removing the plating mask, (4) hydrogen passivating the transition side of the cell, (5) sintering the nickel to partially forming a nickel silicide, (6) immersion plating additional nickel on the metal-coated parts of the cell, (7) electroplating a copper layer on the nickel, and (8) applying an anti-reflective coating over the exposed silicon surface. Thereafter, the silicon can be further processed, for example by preparing it for connection to electrical circuits.

In an alternative process, heating the sample during passivation provides at least some of the energy for the nickel sintering step.

Although it is recognized that such a method (1) allows the removal of the initial plating mask I prior to passivation (allows better passivation) and (2) allows passivation prior to the application of base metals without the requirement of a additional masking steps before metallization (both elimination of the danger of destroyed cells, caused by migration of the base metal during passivation, and simplification of the production process by eliminating the need for either careful thermal regulation of the substrate during passivation or of a photolithographic step after passivation) , the procedure can be further improved. Thus, although the heat regulation required to prevent the migration of base metals (temperatures preferably below about 300 ° C) is eliminated, there is still a risk of destroyed cells due to migration, albeit at a slower diffusion rate, of nickel or nickel silicide. in the matrix of the substrate.

Furthermore, the method just specified requires the formation of a plating mask, such as an additional layer on the substrate, prior to the initial metallization, and to this extent requires additional treatment and materials.

Object of the Ugg Invention Accordingly, an object of the present invention is to provide a treatment sequence for the manufacture of solar cells, which treatment sequence comprises a hydrogen passivation step after the treatment step at high temperature before any front surface metallization.

Another object of the present invention is such a treatment sequence which does not require the formation of a plating mask to prevent metallization of the exposed passivated surface after hydrogen passivation.

Brief Description of the Invention These and other objects are realized by the method according to claim 1, which method in a preferred embodiment used in the manufacture of silicon solar cells includes, inter alia, the following steps: (1) diffusion of phosphorus in p-type silicon strip to form a shallow transition, the diffusion process simultaneously forming a layer of phosphosilicate glass on the surface of the strip adjacent to the transition, (2) forming an electrode grid pattern in the phosphosilicate layer by photolithography (using a suitable photoresist composition and etching) in the form of a "negative" plating mask (ie leaving phosphosilicate glass only in those areas of the substrate where it is desirable to subsequently attach the front surface electrodes), (3) coating the other side of the silicon strip with an aluminum paste, (4) heating the silicon to alloy the aluminum, (5) hydrogen passivation of the transition side of the cell while simultaneously forming an altered layer s on the unexposed silicon substrate between the phosphosilicate glass electrode pattern, (6) etching the remaining phosphosilicate glass, and (7) metallizing both the unchanged exposed silicon and the aluminum with a selected metal, such as nickel, by immersion plating. The silicon can then be further treated, for example by preparing it for connection to electrical circuits, and provided with an anti-reflective coating.

As used herein, the term "immersion plating" means a process in which an object without the use of an externally applied electric field is plated with a metal by immersing that object in a plating bath containing no reducing agent, and the plating involves an displacement reaction. Immersion plating differs from electroplating in that the latter involves a plating bath containing a reducing agent.

This manufacturing sequence has several significant advantages. By delaying the deposition of the front surface electrodes until after passivation, all danger of the cells being destroyed by migration of the electrode material to the transition during subsequent treatment steps is greatly reduced. Since heat regulation during passivation is less critical, cooling can be avoided. Therefore, the present process allows highly productive ion beam passivation. In addition, by using the glass layer formed as a result of the method of forming the transition as the body of a negative plating mask, the preferred method eliminates both an etching step (to initially remove the glass) and a coating step (to provide material for a plating mask). ), in which case the glass is instead removed in two steps.

Other objects of the invention will be understood in part and in part in the following. Accordingly, the invention includes the particular steps and the relationship between one or more such steps with reference to each of the others, which steps are exemplified in the following detailed description and in the scope of the claims set forth in the application.

Brief Description of the Drawing For a more complete understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, which show a number of steps included in the drawing. manufacture of solar cells according to a preferred form of the invention.

Throughout the drawing, the same reference numerals refer to similar structure.

The drawing does not show the thicknesses and depths of the particular layers and areas exactly in accordance with their relative proportions, in order to provide a more convenient illustration.

Detailed Description of the Invention With reference to the drawing, the preferred embodiment of the invention relates to the production of solar cells from p-type EFG-cultured silicon strips.

As a first process claim, a side (hereinafter referred to as the "front side") of a pre-cleaned p-type EFG silicon strip 2 is subjected to a phosphodiffusion process which is calculated to give a relatively shallow transition 4 (i.e. a transition with a depth of between about 3000 and about 7000 Å), an area 6 of the n-non-conductive type, and a rather thick (for example in the order of at least 1500 Å) layer of phosphosilicate glass 8.

As an example, a p-conductive type silicon strip, produced by the edge-defined film-fed growth process and with a resistivity of about 5 ohm-cm, is cleaned by etching in a solution of HNO 3 (70 %): HF (49%) in a ratio of between about 4: 1 and 9: 1, for about 1-3 minutes at about 25 ° C. Thereafter, the strip is exposed to phosphorus diffusion in an oxygen-rich atmosphere.

In this example, silica and oxygen react to form silica, phosphorus and oxygen react to form phosphorus pentoxide, phosphorus pentoxide and silica react to form a phosphosilicate glass, and phosphorus pentoxide and silica react to form phosphorus and silica.

Alternatively, a phosphosilicate glass formed as detailed in U.S. Patent No. 4,152,824 may be used as the source of phosphorus.

The next step involves coating the front of the tape with a negative photoresist 10, which is well known in the art. The resist can be applied in a suitable manner, for example by spraying, and then dried to drive away solid is typically achieved by heating the photoresist to between about 80 and 110 ° C for between about 35 and about 60 minutes.

The photoresist layer is then covered with a negative mask with the pattern of a multi-fingered electrode grid, such as the organic solvents, and cause the resist to adhere to the phosphosilicate glass. This drying is a mask with transparent areas which correspond to the fingers of the desired electrode and which is opaque in other places. As an example of a suitable electrode pattern, reference is made to U.S. Patent No. 3,686,036. The grid mask is then irradiated with ultraviolet light of suitable intensity and for a sufficient time to cause the illuminated portion of the photoresist to polymerize. Thereafter, the photoresist is developed by treatment with one or more suitable developing agents, such as, for example, by contact with toluene and propanol or other suitable solvents. This developing process removes those parts of the resist that have not been irradiated and consequently have not been polymerized. Thus, a portion 10A of the photoresist remains in the pattern of the desired electrode grid configuration.

A subsequent development which dries at about 140 ° C has been found to improve the etching resistance of the remaining photoresist in the following treatment step.

After exposure and development of the photoresist (and drying after development), the assembly is subjected to etching with a buffered oxide, which consists of, for example, a solution of HF and NH4F, whereby the exposed layers of phosphosilicate glass 8 are removed. Thus, (P 2 O 5) x (SiO 2) y, a phosphosilicate glass, can be removed from the substrate by immersing the latter in 10ONH 4 F (40%): 1HF at a temperature of about 25 ° C for a period of between about 15 seconds and 2 minutes. This leaves an intact layer 12 of phosphosilicate glass in the pattern of the electrode grid configuration below the polymerized, non-removing portion 10A of the photoresist.

Thereafter, the back of the substrate is coated with a layer 14 of an aluminum paste. The aluminum paste used to form the layer 14 preferably comprises aluminum powder in a volatile organic carrier medium, such as terpineol, which can be removed by evaporation.

This step is then followed by an alloying step, where the substrate is heated for about 0.25 - 2.0 minutes to a temperature higher than 575 ° C to remove any volatile or pyrolizable organic components in 456 625 9 components in the paste, and to alloy the aluminum into the paste with the silicon substrate.

In the alloying step, the aluminum coating 14 alloys with the back of the substrate to provide a p + region 16 with a depth of from about 1 to about 5 microns. The alloy step also serves to remove the remaining resist portion 10A by pyrolysis.

The cell is then hydrogen passivated. A preferred method is to expose the front surface of the substrate 2 to the hydrogen ion beam from the Kaufman-type ion source (wide beam), which is located about 15 cm from the substrate.

This ion source is preferably operated at a pressure of between about 20 and 50 millitorr (hydrogen), at a hydrogen flow rate in the order of about 25 - 40 cm 3 per minute, at a potential of about 1700 volts DC between the source and the substrate, and at a beam current of between about 1 and 3 mA / cmz on the substrate. An exposure time of between about 1 and about 4 minutes has been found to be suitable both to minimize recombination losses of minority carriers, which typically occur in EFG-type silicon cells (a passivation zone that is about 20 ~ 80 μm deep or about 100 times as deep as the transition 4 is provided), while at the same time an altered surface layer 18 with a depth of about 200 A is provided on the exposed parts of the substrate 2.

The exact nature of the altered surface layer 18 is not known. However, it is assumed that there is a damaged zone where the crystal structure has been somewhat interrupted, the silicon partially forming SiH or SiH2 with the hydrogen from the ion beam, and where the material is still possibly amorphous. A small amount of carbon or one or more hydrocarbons proves to be necessary for the formation of the desired altered surface layer. As initially installed, the Kaufman ion source was equipped with a graphite mounting device about 13 cm (about 50 ") in diameter on which the substrates, typically 5 cm x 10 cm (2" x 4 ") on one side, were centrally located. 10 15 20 25 30 35 -l 456 625 10 In some cases, such as when a silicon mounting device replaced the graphite device, the altered layer did not appear as a plating mask, as when the graphite device was used. It has been assumed that carbon or hydrocarbon vapor formed by the action of the hydrogen ion beam on the graphite device can increase the formation of a dielectric layer on the surface of the substrate.Independent in nature, it has been found that an altered surface layer 18 produced by this process with acceleration stresses between about 1400 and about 1700 volts and exposure times as short as 1 minute, are sufficient to prevent subsequent metallization of the exposed altered surface layer 18, where the metallization includes im mercury plating of a material, such as nickel.

After passivation, the remaining phosphosilicate glass layer 12 is removed by immersing the substrate in a buffered solution of 10NH 4 F (40%): 1HF at a temperature of between about 25 and about 40 ° C. As a result, the front surface of the substrate 2 is now completely exposed.

On this exposed surface, altered surface layers 18 define a pattern of in this case unchanged n + conductive silicon, with the configuration of the desired front electrode structure. 'Then the cell is metallized. The substrate is immersion plated with nickel, an adhesive deposit of nickel forming a nickel layer 22 on the back of the entire surface of the aluminum coating 14, while the adhesive deposit of nickel on the front side forms a layer 20 directly on the surface of the substrate 2 over only the surfaces from which the phosphosilicate glass layer 12 was removed after passivation. In this plating step, the altered silicon surface layer 18 forms a plating mask to which the nickel does not adhere. Plating of the nickel layers can be done according to different immersion plating methods. Preferably, it is carried out according to an immersion plating process which is the same as or similar to the process described in U.S. Patent No. 4,321,283. As a preliminary step, the surface of the cleaned silicon substrate is preactivated by a suitable agent. This pre-activation process is desirable because the silicon surface often does not in itself favor the electroless plating process, and any nickel plated on an untreated surface generally adheres only weakly thereto. Preferably, gold chloride is used as the activating agent, although platinum chloride, tin chloride - palladium chloride, or other well known activators may be used, as described, for example, in U.S. Patent No. 3,489,603. Both sides of the silicon belt are then coated with a layer of nickel, preferably by immersion in a water bath, as described in the aforementioned U.S. Patent No. 4,321,283, or in a water bath of nickel sulfamate and ammonium fluoride at a pH of about 2.9 and at about room temperature for a period of about 2 to 6 minutes. Once applied, the substrate is heated in an inert atmosphere or a nitrogen atmosphere to a temperature and for a time sufficient to sinter the nickel layers and cause the nickel layer 20 on the front side of the substrate to react with the adjacent silicon to form an ohmic contact of nickel silicide. For this purpose, the substrate is preferably heated to a temperature of 300 ° C for between about 15 and about 40 minutes. provides a nickel-silicide layer with a depth of about 300 Å at the interface between the nickel layer 20 and the substrate 2. The nickel layer 18 on the back forms an alloy with the aluminum layer 12. The temperature at this sintering step should not greatly exceed 300 ° C, since higher temperatures turns lead to excessive penetration into the silicon by the nickel layer 20. Preferably, the deposition and sintering of the nickel is controlled so that the nickel layer 20 on the front side of the substrate has a thickness of about 1000 Å.

Thereafter, the nickel in the layers 456 625 is preferably subjected to etching, such as with hydrochloric acid, and to further metallization, such as with a second nickel layer by immersion plating and with one or more copper layers by immersion plating and / or or electroplating, by well known techniques.

For the copper plating, no masking of the altered layer is required, as copper does not adhere to the destroyed layer.

After metallization, the cell edges (not shown) are trimmed, and an anti-reflective coating 24 is applied to the front surface of the cell. This can be done by any of a number of known methods, such as by chemical vapor deposition or evaporation of eg TiO2. Alternatively, the anti-reflection coating 24 may be formed by plasma deposition of silicon nitride at a temperature of about 150 ° C, as is well known in the art.

As will be shown by way of example, the preferred method of applying the present invention involves performing the individual steps described above in the preferred manner described in detail for each step and in the order described.

It has been determined that solar cells manufactured according to the previous procedure of EFG-cultured tapes show between 10 and 20% increase in average efficiency.

In addition, for this material, it has also been found that the hydrogen passivation step significantly limits the distribution of cell efficiencies.

The present method has a number of advantages.

First, it eliminates the risk of transition 4 being destroyed by the migration of nickel or nickel silicide during passivation, which could occur in cells with shallow transitions if an original nickel coating was deposited on the front surface prior to passivation. Apart from reducing the risk of damaged cells, the requirements for careful heat regulation (such as by cooling) during passivation are reduced. This therefore enables a highly productive ion beam passivation step in the manufacturing process. It will be appreciated that the present invention may be modified within the scope of the description. Although the front electrode grid pattern in the preferred embodiment is formed in the plating mask by photolithography, it will be appreciated that it could also be formed by other methods commonly used in chemical etching (eg screen printing).

Although the preferred method utilizes the phosphosilicate glass formed during the n + diffusion to form the ion beam mask for the purpose of defining the damaged layer pattern, other materials or methods may be used for the front surface layer. Thus, an n-type substrate could be used as a starting material, the transition being formed, for example, by diffusion with boron, thereby providing a borosilicate glass layer.

If the starting material were to be provided with a transition and without a glass layer, a suitable layer would have had to be provided. It will be appreciated that the resulting layer may either be etched to form a suitable plating mask or be deposited itself in the appropriate configuration.

Although the preferred embodiment of the method of the present invention utilizes the altered layer formed by hydrogen passivation to mask subsequent plating, in addition to previously plated nickel, the method may be used with metals other than nickel. As will be appreciated by those skilled in the art, for example, the original layer of the front surface electrodes on a shallow transition silicon device can be deposited by plating any of a number of materials with low reactivity and capable of forming (preferably at low temperature) an ohmic contact, and operating as a barrier to the diffusion of copper or other base metals which are deposited at a later stage. Suitable metals for use with copper include palladium, platinum, cobalt and rhodium, as well as nickel.

Although all of these materials form silicides, a silicide layer is not necessary. However, it is important that the original metal layer adheres properly, acts as an ohmic contact and acts as a barrier to the migration of any later deposited metals, as well as to insignificant migration to the transition itself. These materials can be applied by immersion plating techniques in the same way as nickel.

Still other changes may be made without departing from the main principles of the invention, such as (a) forming the p + back region of the cell by using flame-sprayed aluminum instead of an aluminum paste, or (b) using different means of applying the other and the subsequent coatings of nickel or other low reactivity materials, such as palladium, platinum, cobalt and rhodium, or (f) formation of the transition by ion implantation. If no additional mesh layers are deposited on the passivated surfaces, the nickel layer (or other low reactivity metals of the type described) must be applied by immersion plating and the additional layers of copper must be applied by electroplating or immersion plating.

The process accomplished by this invention is, of course, 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 layer formed by chemical or physical vapor deposition can be used to form relatively high efficiency solar cells according to the present invention.

In addition, the process is applicable to single crystalline silicon, and can be used with both n-type and p-type silicon.

In addition, the method could be used in the manufacture of Schottky barrier devices, whereby the altered surface layer acts as a mask for the metallization. For such applications, it will be appreciated that the metal-substrate interface constitutes the transition, and consequently that the substrate would not be provided with a transition by / diffusion of phosphorus or the like. Since these and other changes must be made in the above procedures without departing from the scope of the invention included herein, it is intended that all material contained in the above description or shown in the accompanying drawings be construed as illustrative and not restrictive.

Claims (13)

456 625 10 15 20 25 30 16 PATENT REQUIREMENTS A method of manufacturing semiconductor devices, characterized in that it comprises in sequence the steps of: (a) providing a silicon substrate having opposite first and second surfaces and having a mask on the first surface in the form of a layer with a predetermined , two-dimensional pattern, thereby exposing selected portions of the first surface, (b) exposing the first surface to a hydrogen ion beam with an intensity and for a sufficiently long time to reduce the minority carrier losses of the substrate and forming a surface layer on the selected exposed portions of the first surface, to which surface layer metals only weakly adheres, 7 (c) removing the mask, and (d) subjecting the first surface to metal plating, so that a metal layer is formed on all parts of the first surface except the selected, exposed parts. 2. A method according to claim 1, characterized in that it further includes the step of forming a pn junction adjacent to the first surface before the exposure to the hydrogen jet. 3. A method according to claim 1 or 2, characterized in that the devices are photoelectromotive elements. 4. A method according to any one of the preceding claims, characterized in that it further comprises the step of applying an anti-reflective coating to cover the first surface. 5. A method according to any one of the preceding claims, characterized in that the surface layer is a glass layer formed as a result of the diffusion of a transition in the silicon substrate adjacent to the first surface. 10 15 20 25 30 35 456 625 17 6. A method according to any one of the preceding claims, characterized in that the transition is formed by diffusion of phosphorus into the silicon substrate, whereby a phosphosilicate glass surface layer is formed. 7. A method according to any one of the preceding claims, characterized in that the mask is formed by photolithography using a photoresist covering the first surface. 8. A method according to any one of the preceding claims, characterized in that it further comprises the steps of applying a coating of aluminum to the second surface before removing the photoresist, and subsequently heating the silicon substrate to a temperature and for a time which is sufficient to (1) cause the aluminum of the coating to alloy with the silicon substrate and (2) effect removal of the photoresist by pyrolysis. IN 9. A method according to any one of the preceding claims, characterized in that both the first and the second surface are metallized. 10. A method according to any one of the preceding claims, characterized in that the first surface layer is a dielectric. 11. ll. A method according to any one of the preceding claims, characterized in that it further includes the step of applying a coating of aluminum for alloying with the second surface before metallizing the first surface, and heating the aluminum to a temperature and for a time sufficient to cause the aluminum to alloy with the silicon substrate. 12. A method according to any one of the preceding claims, characterized in that the metal plating is carried out using a metal selected from the group of metals which includes nickel, palladium, cobalt, platinum and rhodium. 13. A method according to any one of the preceding claims, characterized in that the metal plating comprises per plating of nickel from a bath comprising a nickel salt and fluoride ions. J?