SE456624B - PROCEDURE FOR THE PRODUCTION OF SOLAR CELLS - Google Patents

Description

15 20 25 30 35 456 624 2 ensure that the combination of time and temperature in any of the steps following the hydrogen passivation step does not cause the hydrogen introduced into the silicon to diffuse back out of the passivated substrate. For example, it has been found that a hydrogen passivated cell, which is exposed to a temperature of 600 ° C for half an hour in vacuum, loses substantially all of the bound hydrogen and returns to its pre-passivated level, as evidenced by observation of the current activity induced by an electron beam. In this context, it should be noted that the transition diffusion step in the manufacture of solar cells typically involves temperatures of the order of 900 ° C.

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

Seager, D.J. Sharp, J.K.G. Panitz and R.V. D'Aiel1o 20, No. 3, pp. 430-435 (March 1982), is carried out with a jon- in Journal of Vacuum Science and Technology, Vol. of the Kaufman type, which is used to produce hydrogen jets in the kiloelectron voltage range. Relatively short exposure times (ie between 0.5 and 4 min) in an interval with high ion energy and large ion flux (ie 1-3 mA / cm 2) appear 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 connected to a suitable cooler. Otherwise temperatures above 400 ° C are easily reached. However, it is important that the temperature be limited to less than about 3Û0 ° C to avoid the rapid migration of base metals into the silicon matrix. However, the manipulation of the substrate and the cooling device to perform heat control during passivation easily becomes the factor limiting the rate of highly productive treatment with such ion sources. Accordingly, it is desirable to avoid cooling to provide a cheap and highly productive process. In addition, cooling of EFG-type silicon strips ("edge defined film-fed growth"), which are economically advantageous to produce, is difficult due to surface roughness.

Hydrogen passivation is also most effective when the silicon base surface is exposed. Thus, any plating mask used to define the front grid electrode pattern should be removed before or applied after passivation.

Accordingly, an object of the present invention is to provide a treatment sequence for the manufacture of solar cells, wherein the hydrogen passivation step is inserted after the steps of treatment at high temperature and still before any base metals are built into the structure.

A further object of the present invention is to insert the hydrogen passivation step so that the passivation of the silicon and the nickel silicide, which lies beneath the front electrode structure in a partially manufactured solar cell, is improved.

A further object of the present invention is to insert a hydrogen passivation step in the processing of EFG-type silicon strips into solar cells in such a way that the need for cooling of the substrate during the passivation is eliminated.

These and other objects are achieved by the method according to claim 1, which method in a preferred embodiment, when applied in the manufacture of silicon sol- (1) to form a thin grid electrode pattern of nickel cells, comprises, inter alia, the following measures: (or similar materials ) on the front of a shallow transition silicon belt, (2) hydrogen passivating the transition side of the cell, (3) sintering the nickel so that it partially forms a nickel silicide, (4) plating one or more additional metals on the metal-covered portions of the cell, and (5) to cover the exposed surface of the silicon with an anti-reflective agent. The silicon can then be further processed, for example to prepare it for connection to electrical circuits. 456 624 10 15 20 25 30 35 4 In an alternative method, heating the sample during the passivation adds energy to the nickel sintering step.

These manufacturing sequences have several features.

First, the applicant has discovered that the hydrogen ions incident during hydrogen passivation change the surface of the silicon wafer in such a way that plating of metal by immersion plating on the altered surface is prevented. Any plating mask initially used to define the front electrode grid pattern can, as a result, be removed after a basic metal layer has been plated on the substrate. The passivation can now be performed on an exposed silicon base layer, whereby the passivation not only improves the cell's electrical performance but also, by changing the surface layer, serves as a second plating mask for the subsequent immersion plating steps. It follows that the passivation of the exposed silicon base can be completed prior to the application of base metals without the need for an additional masking step prior to immersion plating metallization. The applicant has further found that passivation can take place through thin layers of metal, such as nickel. The silicon and nickel silicide during the first thin nickel plating of an electrode on the front can thus be passivated.

Other objects of the invention will in part be obvious and in part will become apparent from the following.

Accordingly, the invention includes the multiple steps and the relationship between one or more such steps and each of the other steps exemplified in the following detailed description and the scope of the application will be set forth in the claims.

For a more complete understanding of the principle and objects of the present invention, reference is made to the following detailed description which is to be considered in conjunction with the accompanying drawing, which illustrates a number of the steps which, according to a preferred embodiment, are preferred. The embodiment of the invention is included in the manufacture of solar cells.

In the drawing, like reference numerals refer to like structures.

In the drawing, for the sake of simplicity, the thickness and depth of the several layers and areas are not shown to scale or with their mutual correct proportions.

With reference to the drawing, the preferred embodiment of the invention relates to the manufacture of solar cells based on EFG-cultured p-type silicon strips. For this embodiment, a partially completed cell 1 is used as a starting piece. The partially completed cell comprises a substrate 2, which preferably consists of a β-conducting silicon strip, one side (hereinafter referred to as "the front side") has been provided with a relatively shallow transition 4 (ie a transition that is between 3000 and 7000 Ångström depth), an n-conducting area 6 and a mask 8.

The mask consists of a material (e.g. a dielectric) in which metals, such as nickel, adhere poorly and is configured to expose portions of the front of the substrate 2 with a pattern for a multi-finger grid patterned electrode (e.g. a electrode with the shape shown in US 3,686,036). The other side of the substrate (hereinafter referred to as the "back side") is preferably provided with a layer 10 of aluminum, which is alloyed at the substrate, and a p + region 12. The depth of the P + region 12 is preferably 1-5 μm.

The partially completed cell 1 can be manufactured in any of a number of ways well known in the art. The transition 4 and the area 6 can be provided, for example, in a p-type silicon substrate 2 by diffusion of phosphorus and a mask 8 can be provided on its front side by photolithography or printing. The layer 10 and the p + region 12 can be formed by covering the back of the substrate with a layer of aluminum paste comprising aluminum powder in a volatile organic carrier medium, such as terpineol, which can be removed by evaporation, and by the substrate is then heated to remove any volatile or pyrolysable organic component in the paste and by alloying the aluminum to the substrate and forming the p + region. However, other types of substrates, transitions and back electrodes and other manufacturing methods may equally well be used to provide the partially completed cell 1.

Starting from such prefabricated sides with nickel, a piece of nickel-aluminum layer is first plated on both substrates, with an adhesive deposit of nickel layer 14 on the back of the piece over the whole 10, while the adhesive deposit of nickel on the front side forms a layer 16 directly. on the surface of the substrate 2 only in the areas exposed by the mask 8.

The plating of the nickel layers 14 and 16 can be done in different ways. Preferably, it is carried out in accordance with a known electrofree or immersion plating process, i.e. an immersion plating process, which is similar or similar to that described in US 4,321,283, Kirit Patel et al. The term "electroplating" as used herein refers to plating from a bath containing a reducing agent without the use of an externally applied electric field, and the term "immersion plating" means a process in which an object is plated with a metal without the use of an externally applied electric field by immersion in a plating bath containing no reducing agent, and the plating involves a displacement reaction.

As a preparatory step, the cleaned silicon substrate surface is pre-activated by a suitable agent. This pre-activation process is desirable because the silicon surface itself often does not support the electrofree plating process and all nickel plated on an untreated surface generally adheres to it very poorly. Preferably, gold chloride is used as activating agent, while platinum chloride, stannous chloride-palladium chloride or other well-known activators can be used, for example in US 3,489,603. Thereafter both sides of the silicon band are covered with a layer of nickel. preferably by immersion plating the strip in an aqueous bath as described in US 4,321,283 or an aqueous bath of nickel sulphate and ammonium fluoride at a pH of as described about 2.9 and at approximately room temperature for a period of about 2 to 6 minutes.

At this stage, the mask 8 is peeled off from the substrate 2. Depending on the type of mask, this can be done in any of a number of well known ways, such as for example by the use of a buffered etchant. As a result of the removal of the mask, the front side of the substrate 2 is exposed through a grid pattern consisting of the nickel layer 16.

The cell is then hydrogen passivated. A preferred method is to expose the front of the substrate 2 (and the nickel layer 16) to a hydrogen ion beam from a Kaufman-type ion source (wide beam), which is arranged about 15 cm from the substrate. This ion source is preferably operated at a pressure of about 20-50 millitorr (of hydrogen), with a hydrogen flow rate of 25-40 cm 3 / min, with a potential of about 1700 volts dc between the source and the substrate and with a current in the jet of between about 1 and 3 mA / cm 2 at the substrate. It has been found that an exposure time between about 1 and about 4 minutes is adequate both to minimize losses due to minority carrier recombination, which losses typically occur with EFG-type silicon cells (thereby providing a passivation zone which is approximately 20-80 μm deep or about 100 times as deep as the transition 4 while an altered surface layer 18, which is about 200 Angstroms deep is produced simultaneously on the exposed parts of the substrate 2. It has also been found that the use of a mechanical mixer for pulsing the ion beam on and off with a pulse ratio of 50% results in a minimal temperature increase for the substrate during the passivation.10 The exact nature of the altered layer 18 is not known. however, be a damaged area in which the crystal structure has been partially destroyed, the silicon partially forming SiH or SiH2 with hydrogen from the ion beam and in which the material is still possibly amorphous.

A small amount of carbon or one or more hydrocarbons may be necessary in the vacuum system to form the desired altered surface layer. The Kaufman ion source originally used was equipped with a mounting device which consisted of graphite and was about 13 cm (5 ") in diameter and on which the substrates, which were typically 5 cm x 10 cm (2" x4 ") on a side was placed in the middle. When the graphite mounting device was in some cases replaced with a silicon mounting device, the formed, altered layer did not work as well as the plating mask as when the graphite device was used. On the basis of this, it has been assumed that carbon or hydrocarbon vapor formed by the action of the hydrogen ion jet on the graphite device can improve the formation of a dielectric layer on the surface of the substrate. Regardless of the nature of the altered surface layer 18, it has been found that an altered surface layer generated in accordance with this method has acceleration voltages between about 1400 and about 1700 volts and exposure times as short as one minute are sufficient to prevent subsequent immersions. plating the silicon substrate between the nickel layers 16.

Thereafter, the substrate is heated in an inert atmosphere or nitrogen atmosphere to a temperature and for a period of time sufficient to sinter the nickel layers and cause the nickel layer 16 on the front of the substrate to react with the adjacent silicon to form an ohmic contact consisting of nickel silicide. . For this purpose, the substrate is preferably heated to a temperature of about 300 ° C for between about 15 and about 40 minutes.

This produces a nickel silicide layer 20, the depth of which is - about 300 Angstroms, in the interface between the nickel layer 16 and the substrate 2. The nickel layer 14 on the back forms an alloy with the aluminum layer 10. The temperature at this sintering step should not exceed 300 ° C too much as higher temperatures cause the nickel layer 16 to penetrate too far into the silicon. If this heat treatment is performed in forming gas (95% nitrogen and 5% hydrogen), it also appears to expel hydrogen loosely bonded to the nickel layer 16, thereby improving the adhesion of the subsequent plating.

After this, the nickel in layers 14 and 16 is subjected to etching with hot dilute nitric acid, followed by ultrasonic cleaning, to remove excess nickel from both sides of the substrate. In the nickel etching, not only excess nickel but also part of the aluminum alloy formed on the back of the substrate during the sintering step is removed. After the nickel etching step, the layer 14 is characterized by a layer of nickel-aluminum alloy, which lies on top of the aluminum electrode layer 10, while the layer 16 has been peeled off to expose the nickel silicide layer 20 corresponding to the preselected electrode grid pattern.

Thereafter, the nickel silicide layer 20 and the nickel-aluminum alloy layer 14 are metallized with one or more layers 22 and 24, respectively, to provide suitable contacts.

In these metallization steps, the altered surface layer 18 of the substrate 2 acts as a plating mask to prevent metal from adhering to the surface of the substrate between the pattern formed by the already attached nickel silicide layer 20.

Preferably but not necessarily, this further metallization involves the application of a second layer of nickel to layers 14 and 20. The additional nickel layers are applied by immersion plating in the manner described above in connection with the formation of nickel layers 14 and 16, since in immersion plating nickel will be plated. layers 14 and 16 but not the altered surface 18. Immediately thereafter, one or more copper layers (by immersion plating and / or electroplating, by techniques well known in the art) are applied to the expo 10 15 20 25 30 35 456 624 10 nickel on both sides of the substrate to bond the nickel layers and thereby protect them from oxidation, and to ensure high conductivity. No masking of the altered layer 18 is required for the copper plating, as the copper will not adhere to the altered layer. Thereafter, the device can be subjected to other treatments for known purposes, for example layers of tin and solder can be applied successively over the previously applied metal layers.

After metallization, the cell edges are polished (not shown) and an anti-reflective coating 26 is applied to the front of the cell. This last step can be carried out by any of a number of known methods, such as by chemical vapor deposition of, for example, TiO2. Alternatively, the anti-reflective coating 26 may be formed by plasma deposition of silicon nitrite.

As an example, the preferred method of practicing the present invention consists in performing the individual steps described above in the preferred manner described in detail above for each step and in the sequence described.

It will be appreciated that the preferred method of the present invention is to perform the individual preferred steps described in detail above, these steps being performed in the order just indicated.

It has been established that solar cells, according to the previous process from EFG-grown tapes, which are manufactured show an increase in the average efficiency of between 10 and 20%. it has also been found that the hydrogen passivation step noticeably additionally for this material has limits the distribution of cell efficiencies.

The method described above has a number of other advantages. By utilizing the substrate during hydrogen passi-? The surface layer produced as a mask for the subsequent plating as described above, the method first of all allows passivation of an exposed substrate before such later metallization. This enables the passivation of a clean substrate (rather than passivation through a plating mask layer), eliminates "soft" or short-circuited cells (derived from base metals that migrate during passivation) and saves in the steps between the passivation and the subsequent metallization by immersion plating as no further masking step is required. It should be noted that the hydrogen passivated surface also serves as a mask that prevents the deposition of copper during immersion plating or electroplating. In addition, by passivating through a thin layer of material on the front electrodes, the substrate under the front electrodes can also be passivated, provided that the first nickel layer is not more than about 750 Ångström thick. It will also be appreciated that the process of the present invention involves passivation at a stage of solar cell production where the subsequent treatment of the cell does not adversely affect the effects of the passivation. It will be appreciated that the method may be modified in a number of ways without departing from the scope of the invention described herein. Thus, although the nickel sintering step in the preferred embodiment is performed after the passivation, it can also be performed just before the passivation. It will be appreciated that in such a case it is desirable for cells with shallow transitions to have short ion beam exposures with or without thermal control of the substrate with a suitable coolant for protection against the nickel silicide migrating to the transition. Such a control also generates Ni2Si rather than other silicides (NiSi or NiSi2) whereby less silicon per silicide molecule is built in and it is ensured that the silicide does not penetrate into the n + region. It will also be appreciated that if the nickel sintering is performed prior to passivation, a drying step may be required after passivation to expel the loosely bound hydrogen from the nickel prior to further treatment.

It is also obvious that the heating of the cell during the passivation can be used to perform at least part of the nickel sintering step. Furthermore, while the preferred embodiment of the process of the present invention utilizes the modified layer formed during hydrogen passivation to mask subsequent immersion nickel plating except for previously plated nickel, the method may further be used in conjunction with other metals other than nickel. The first layer of the front electrodes on a silicon device with shallow transitions can, for example, as will be appreciated by those skilled in the art, be deposited by plating in various ways well known to those skilled in the art of any number of low reactive materials which (preferably at low temperature) may form an ohmic contact and serve as a barrier to the diffusion of copper or other counterfeit metal, which is deposited at a later stage. Metals that are suitable for use with copper are, for example, 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 first metal layer adheres properly, serves as an ohmic contact and acts as a barrier for the migration of each later deposited metal as well as that it does not to any great extent itself migrate to the transition.

The process accomplished by this invention is, of course, not limited to the production of solar cells from EFG substrates. Thus, cast polycrystalline line substrates, epitaxial silicon upon metallurgical grade silicon or fine grade polysilicon layers which are formed by chemical or physical vapor deposition can be used to form relatively high efficiency solar cells according to the present invention. Furthermore, the process is applicable to single crystalline silicon. In this case, the method can be used with materials of both n- and p-type.

Since these and other changes can be made in the procedure without departing from the scope of the invention, it is intended that everything in the description above and everything shown in the accompanying drawing should be construed as exemplary and not as limiting. n;

Claims (10)

10 15 20 25 30 456 624 13 PATENT REQUIREMENTS A method of manufacturing semiconductor devices, characterized in that it sequentially comprises the steps of: (a) providing a silicon substrate with opposite first and second surfaces, (b) metallizing selected areas on the first surface with a layer of nickel or similar material, (c) exposing the first surface excluding the selected areas to a hydrogen ion beam of sufficient intensity and for a sufficiently long time to (1) form a surface layer on the first surface to which they are exposed; the selected metals adhere poorly, and (2) reduce the minority carrier losses of the substrate, and (d) metallize the selected areas on the first surface with at least one of these selected metals. A method according to claim 1, characterized in that it further comprises the step of forming a pn junction in the substrate adjacent the first surface. 3. A method according to claim 1 or 2, characterized in that the devices are photoelectromotive. Process according to any one of the preceding claims, characterized in that the metallization is carried out by means of a metal selected from the group of metals which includes nickel, palladium, cobalt, platinum and rhodium. A method according to any one of the preceding claims, characterized in that the metallization step comprises the steps of: applying a nickel coating to the selected areas of the first surface, and coating the nickel coating with at least one layer of selected conductive metals. 10 15 20 456 62LÉ 14 6. A method according to claim 5, characterized in that said layer is applied by (a) contacting the nickel coating with an etchant for removing unbound nickel, and (b) coating the nickel coating with copper. 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 the first surface. 8. A method according to claim 5, characterized in that said layer of conductive metals comprises a nickel layer formed by means of an immersion plating method. 9. A process according to claim 8, characterized in that a bath comprising a nickel salt and fluoride ions is used in the plating process. 10. A method according to claim 5 or 6, characterized in that said further conductive metal layer comprises a copper layer formed by immersion plating or electroplating. 'll ß