NL8420337A - METHOD FOR MANUFACTURING SOLAR CELLS - Google Patents

Description

ö 'L u o o / VO 7315

Method for manufacturing solar cells.

The invention relates to the manufacture of photo-voltage cells and more particularly in an improved inexpensive manner for the production of polycrystalline silicon solar cells in which the damaged surface layer formed during the hydrogen passivation is used as a plating mask for the metalization of the front surface electrodes is used.

Heretofore, a conventional method of manufacturing silicon solar cells has included the following steps: forming a PN junction by diffusing a suitable dopant into the front of a silicon wafer or strip, a grid electrode pattern in a protective dielectric masking layer formed on the front face, depositing a nickel plating on all the silicon released by the etching, dipping in solder or overplating the nickel with copper and tin, removing the remainder of the dielectric masking layer of the front face, and applying an anti-reflective coating to the newly released portions of the front face.

Although such a procedure can be used with both monocrystalline and polycrystalline silicon, cost considerations make it desirable to manufacture the solar cells from the latter material. However, as is known, because of the minority carrier losses at grain boundaries, dislocations and the like, the efficiencies obtained in polycrystalline silicon solar cells are generally lower than those of monocrystalline cells. This condition has been improved by introducing a monovalent element such as hydrogen into the system in combination with the free bonds associated with the structural defects, thereby minimizing the carrier-recombining loss.

As is well known, an important consideration in designing a cell processing series is that the combination of time and temperature in a step after the hydrogen passivation step should not cause the hydrogen supplied to the silicon to diffuse out of the passivated substrate again. For example, it has been found that a 8420337 -2 hydrogen passivated cell which is subjected to a temperature of 600 ° C for half an hour in vacuo essentially loses all bound hydrogen and returns after its pre-passivation level, as shown from the observed electron beam-induced current activity thereof. In this connection, it should be noted that the junction diffusion step in solar cell manufacturing involves, in particular, temperatures of the order of 900 ° C.

It has further been found that in the hydrogen passivation the cell is typically heated to such a high temperature that base metals, such as copper, migrate through the junction, resulting in a "soft" diode or short circuit. As indicated, for example, by C.H. Seager, Q, J. Sharp, JKG Panitz, and R.V. D'Aiello in Journal of Vacuum Science and Technology, Vol. 20, no.3, pp. 430-435 (March 1982), the passivation of the polycrystalline silicon can take place with an ion source of the Kaufman type, which is used for generating a hydrogen ion beam in the kilo-electron volt energy range.

Relatively short exposure times (e.g. between 0.5 and 4 minutes) at 2 high ion energy and high flux (e.g. 1-3 mA / cm) appear to be optimal. Such exposures generally result in an increase in the temperature of the substrate to at least approximately 275 ° C if the substrate is carefully contacted with a suitable heat sink. If this is not the case, temperatures of more than 400 ° C easily occur. It is important, however, that the temperatures be limited to less than about 300 ° C in order to avoid rapid migration of base metals into the silicon matrix. A manipulation of the substrate and the heat sink to effect thermal control however, during passivation, the rate limiting factor readily becomes a factor in higher flow processing at such ion sources. Therefore, it is desirable to avoid heat dissipation to obtain an inexpensive, high flow rate process. In addition, EPG type silicon strips, which can be economically manufactured, make surface irregularities and heat dissipation difficult.

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Furthermore, the hydrogen passivation is most effective when the base silicon surface is released. Therefore, any plating mask, such as the silicon nitride layer used in known processes, must be removed before the passivation takes place. Therefore, the front surface electrodes must be deposited before the passivation.

As described in U.S. Patent Application Serial No. 563061, the modified surface layer formed upon hydrogen ion beam passivation can be used as a plating mask for subsequent rationalization steps. In more detail, a preferred embodiment of the process described in detail therein as applied to the manufacture of silicon solar cells includes the following steps: (1) forming a plating mask of a dielectric material on the front face of a silicon strip with shallow junction to to release those areas of the silicon to be covered later by the front surface electrodes, (2) applying a thin layer of nickel (or similar material) to the exposed silicon, (3) removing the plating mask, ( 4) passivating the junction side of the strip with hydrogen, (5) sintering. nickel for partially forming a nickel silicide, (6) plating extra metal or extra metals on the metal-coated parts of the cell, and (7) applying an anti-reflective coating on the exposed surface of the silicon . The silicon can then be further processed, for example to prepare it for connection to electrical chains.

The passivation changes the exposed surface of the junction side of the substrate to serve as a mask for the secondary plating step (6). In an alternative process, heating the sample during passivation supplies at least a portion of the energy for the nickel sintering step.

This procedure is further simplified in U.S. Patent Application Serial No. 563292, which shows that a negative plating mask (ie, a mask covering only those areas of the front face, which are to be covered later by the front face electrode), which is applied to the front face of the substrate is applied, can be used to control the size of the modified surface layer obtained by the ion beam, allowing passivation 8420337-4- for any front surface metallization, the mask after passivation and before the metallization is removed.

Both procedures just described allow for a passivation with hydrogen with consequent improvement in the cellular performance of polycrystalline substrates, with a preferred temperature protocol. However, even the simplest of the two procedures just described requires several steps of the order of a dozen. Both procedures further require that a surface layer on the substrate be formed in a temporary removable mask with the associated cost of both time and non-recoverable material.

Therefore, the invention aims at eliminating steps in the manufacture of solar cells from which a strip of the EFG type or similar substrates is eliminated.

A further object of the invention is to provide a treatment series for the manufacture of solar cells, which comprises a hydrogen passivation step, which is carried out after the treatment steps with high temperature, but before any base metals are incorporated into the structure, the use of which of non-recoverable material is reduced to a minimum.

This object and other objects are achieved according to the invention in that a removable and reusable mechanical mask is used to utilize the ion beam used for hydrogen passivation with shadowing on the front face of the substrate. The modified surface layer, which is obtained by the ion beam passing through the openings in the mask, forms a plating mask, which delimits the areas of the subsequent front surface metallization by dip plating.

It will be appreciated that using a removable mechanical mask to determine the surfaces of the substrate to be exposed to ion beam, the chemical steps used to form and remove the plating masks in the known treatment series .will be avoided. Furthermore, the mask according to the invention can be reused. As a result, a highly simplified and more cost efficient process is enabled.

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The invention will be explained in more detail below with reference to the drawing, which shows a number of steps which occur in the manufacture of the solar cells according to a preferred embodiment of the invention.

5 In the drawing, like parts are provided with like references. Furthermore, in the drawing, the thicknesses and depths of the different coatings and areas are not accurately represented for their illustration according to their relative proportions.

The preferred embodiment according to the invention shown in the drawing relates to the production of solar cells from EFG-grown silicon strips of the P-type. As a first requirement of the process, one side (hereinafter referred to as "front side") of a pre-cleaned EFG silicon strip 2 with a P-type guide is subjected to a phosphor diffusion process calculated to be relatively shallow junction 4 is obtained (ie a junction with a depth between about 3000 and about 7000 8), and an N-type guide region 6 is obtained. For example, a silicon strip and a 20 P-type guide, manufactured by the EFG process and cleaned with a specific resistance of about 5 ohm-cm, is etched in a solution of HNO ^ (70 %): HF (49%) in a ratio of between about 4: 1 and 9: 1 for about one to three minutes at a temperature of about 25 ° C. The strip is then subjected to a phosphorus diffusion process, as is known per se.

a layer of phosphorsilicate glass 8, formed as detailed in U.S. Patent 4,152,824, is used as a source of the phosphorus dopant.

Then, the phosphorsilicate glass layer 8 is etched away by immersing the substrate in a buffered HF solution.

Therefore, (Ρ205 ^ χ ^ · * "02 ^ γ 'a phosphorsilicate glass may be removed from the substrate by the latter at a temperature between about 25 ° C and about 40 ° C for a period of time between about 15 seconds and 2 minutes, immerse in 1HF (40%): 1HF.

Next, the back of the substrate is coated with an 8420337-6 layer 10 of an aluminum paste. The aluminum paste used to form a layer 10 preferably comprises aluminum powder in a volatile organic carrier, such as terpineol, which can be removed by evaporation.

This step is then followed by an alloying step, wherein the substrate is heated at a temperature higher than about 575 ° C for about 0.25 to 2.0 minutes to remove any volatile or pyrolysable organic components from the paste and alloy the aluminum in the paste with the silicon substrate. In the alloying step, the aluminum cladding 10 alloys with the back of the substrate to form a P + region 12 with a depth of about 1 to about 5 microns.

The cell is then passivated in hydrogen. A preferred method is to expose the front surface of the substrate 2 to the hydrogen ion beam of a (wide) beam ion source of the Kaufman type, which is located at a distance of about 15 cm from the substrate. A mask 14 is placed between the ion source and the substrate.

This ion source is operated at a pressure ranging between about 20 and 50 millitorr (hydrogen) at a hydrogen-20 flow rate of the order of about 25-40 sccper minute, at a potential of about 1700 V DC between the source and the substrate, and with a beam of current comprised between about 1 and 3 mA / cm 2 at the substrate. It has been found that an exposure time of between about 1 and about 4 minutes is adequate to cover both the minority carrier recombining losses, which is more particularly the case with EFG type silicon cells (providing a passivation region with a depth of about 20 to 80 microns or about 100 times as deep as the junction 4) and at the same time provide a modified surface layer 18 with a depth of approximately 200 & at the exposed portions of the substrate 2.

The exact nature of the modified surface layer 18 is not known. It is believed that the layer forms a damaged zone in which the crystal structure is slightly broken, the silicon partially forming 35 SiH or SiH 2 with the hydrogen from the ion beam, but the 8420337 material being optionally amorphous. It appears that a small amount of carbon or one or more hydrocarbons is required to form the desired modified surface layer. At the first installation, the Kaufman type ion source used was provided with a graphite support with a diameter of about 13 cm on which the substrates, more particularly from 5 - 10 cm, were centrally arranged. In some instances where a silicon support replaced the graphite support, the modified layer did not behave like a plating mask the way it does when the graphite support is used. On this basis, it is believed that carbon or hydrocarbon vapor generated by the incidence of the hydrogen ion beam on the graphite support may form a dielectric layer on the surface of the substrate. Whatever the nature of this layer, it has been found that a modified surface layer 18 formed in accordance with this procedure at accelerating voltages between about 1400 and about 1700 V and exposure times of e.g. 1 minute is sufficient to prevent the subsequent dip plating metallization of the substrate over the modified layer 18.

The mask 14 consists of a metallic grid with a pattern of the desired multi-fingered grid electrode, e.g. an electrode of the shape disclosed in U.S. Pat. No. 3,686,036. The mask 14 is preferably made of molybdenum, although other metals, such as invar, stainless steel, titanium, nickel or the like, or graphite or similar high temperature non-metallic materials may also be used.

The mask 14 is positioned relative to the front surface substrate 2 and the ion beam source such that the desired electrode grid pattern is cast on the front surface 20 as a shadow when the ion beam source is activated. That is, the mask 14 interferes with the ion beam 16 according to the desired electrode grid pattern, while irradiating the substrate in the regions located between the electrodes.

The cell is then metallized. Both 35 sides of the substrate are plated with nickel plating 8420337 -δέη an adhesive nickel deposit forms a nickel layer 22 on the back over the entire surface of the aluminum cladding 10, while the adhesive nickel deposit on the front a nickel layer 20 directly on the surface of the substrate 2, only 5 is formed over those areas which are free from the modified surface layer 18. In this plating step, the modified surface layer 18 of the silicon forms a plating mask to which the nickel does not adhere close. Immersion plating of the nickel layers can be done in various ways. Preferably, this takes place in accordance with a dip-nickel plating process as or corresponding to the process described in U.S. Pat. 4,321,283.

The term "dip plating" as used herein refers to a process in which an object is plated without using an external electric field by immersing the object in a plating bath containing no reducing agent, and the plating includes a displacement reaction . In contrast, the term "electrodeless plating" refers to a plating without using an externally applied electric field by immersing the object to be plated in a plating bath containing a reducing agent.

As a preliminary step, the cleaned silicon substrate surface is pre-activated with an appropriate agent. This pre-activation procedure is desirable since the silicon surface will often not support the electrodeless plating process itself, and nickel plated on an untreated surface will generally only adhere to it in a very moderate manner. Preferably, the activating agent is gold chloride, although platinum chloride, tin chloride palladium chloride, or other known activators may be used, such as e.g. is described in U.S. Pat. No. 3,489,603. Thereafter, both sides of the silicon strip are coated with a layer of nickel, preferably by dipping the strip in an aqueous bath as described in the aforementioned U.S. Patent 4,321,283, or in an aqueous bath of nickel sulfamate and ammonium fluoride at a pH of about 2.9 and about at room temperature for a period of about 2 to 6 minutes.

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After the nickel is applied, the substrate is heated in an inert atmosphere or in a mixture of, for example, nitrogen and hydrogen, to a temperature and for a time sufficient to sinter the nickel layers and cause the nickel layer 20 to form on the front side of the nickel. the substrate reacts with the adjacent silicon to form a nickel silicon ohmic contact. For this purpose, the substrate is preferably heated to a temperature of about 300 ° C for a period of about 15 to about 40 minutes.

This provides a nickel silicide layer with a depth of about 10 300 aan at the interface between the nickel layer 20 and the substrate 2. The nickel layer 22 at the back forms an alloy with the aluminum layer 10. The temperature of this sintering step should be 300 ° Do not exceed C strongly since higher temperatures lead to a nickel layer 20 of poor quality and, as already noted, may cause part of the passivating hydrogen to diffuse from the substrate material. Preferably, the deposition and sintering of the nickel is controlled such that the nickel layer 20 at the front of the substrate has a thickness of no more than about 750.

The nickel layers 20 and 22 are then preferably subjected to an etching treatment, e.g. with nitric acid, and a further metallization, such as with a second layer of nickel and one or more layers of copper. The additional nickel is applied by dip plating, preferably in the manner previously described for the formation of layers 20 and 22, since the added nickel will flatten only layers 20 and 22 but not the exposed areas of the modified layer 18. Preferably, kopa: is applied by dip plating and / or electroplating, ie in a known manner. No masking of the modified layer 18 is required, since copper formed by dip plating or electroplating will not adhere to the modified surface 18.

After the metallization, the cell edges (not shown) are trimmed and an anti-reflective coating 24 is applied to the front face of the cell. This can be done in one of a number of known ways, for example by a chemical vapor deposition or evaporation of, for example, TiCl 2. For example, an anti-reflective coating 24 may also be formed by the plasma nitride plasma deposition at a temperature of about 150 ° C, as known per se.

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By way of example, the preferred method of the invention includes the individual steps described above in the preferred mode described in detail for each step and in the order specified.

It has been found that the solar cells made from EGG grown strips according to the above process show an increase in average efficiency between 10 and 20%. In addition, for this material, the hydrogen passivation step narrows the distribution of the cell yields in a pronounced manner.

It is clear that the method according to the invention greatly simplifies the production of solar cells. The method not only reduces the number of steps required in the process, but also eliminates the use of a large amount of non-recoverable material. Therefore, by eliminating the use of a photoresist to form a plating mask for the front surface electrodes, the method of the invention eliminates the need to coat, expose and develop such a varnish, along with the required step of removing the residual paint. Similar considerations apply to equivalent chemical milling processes.

It is clear that the above-described process 20 can be modified within the scope of the invention. Therefore, although in the preferred embodiment of the process of the invention, the modified layer formed by the hydrogen passivation is used to mask subsequent plating, except for the previously plated nickel, the method may be used with metals other than nickel. For example, it will be appreciated by those skilled in the art that the initial layer of the front surface electrodes may be applied to a shallow junction silicon device by dip-plating any of a number of low reactivity materials capable of (preferably at low temperature) to form an ohmic contact and act as a barrier to the diffusion of copper or other base material to be applied 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, no silicide layer is essential. However, it is important that the initial metal layer adheres properly, as an ohmic contact, and acts as a barrier to the migration of 8420337 * * -η- Λ any metal applied later, while the material is - * Either does not migrate significantly to the junction itself.

Within the scope of the invention, other modifications may also be made by, for example, (a) forming the P + back-5 side region of the cell using flame-sprayed aluminum instead of aluminum paste, or (b) using different methods for applying the second and subsequent coatings of nickel or other low reactivity material such as palladium, platinum, cobalt and rhodium, or (c) forming the junction by ion implantation. If no masking layer is applied to the modified layer, the additional nickel (or other low reactivity metal of the type described) should be applied by dip plating.

The method according to the invention is of course not limited to the production of solar cells from EFG substrates. For example, cast polycrystalline substrates, epitaxial silicon on metallurgical grade silicon or high quality polysilicon layers formed by chemical or physical vapor deposition can be used to form relatively high efficiency solar cells according to the invention. Furthermore, the method applies to monocrystalline silicon. The method can furthermore also be applied to both N-type and P-type substrates.

It is clear that in each of these cases the junction can be formed by different processes and not just by a phosphorus diffusion.

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Claims (9)

1. A method of manufacturing a solid state semiconductor device characterized by providing a silicon substrate with opposed first and second surfaces, positioning a mechanical mask with selected apertures next to the first surface, and exposing the first surface on a hydrogen ion beam through the selected apertures at an intensity and for a period of time sufficient to form a surface layer on said first surface corresponding to said apertures and to which metals will only adhere poorly. 2. Process according to claim 1, characterized in that after exposing those parts of the front face which were masked by the mask to the hydrogen ion beam, they are metallized. Method according to claim 2, characterized in that the entire front surface is exposed to a metal during the metallization. Method according to claim 1, characterized in that a junction is formed in the substrate at the first surface before said surface is exposed to the hydrogen beam. A method according to claim 1, characterized in that the first surface is exposed to the hydrogen ion beam for a time at an intensity sufficient to reduce the minor carrier losses in the substrate. A method according to claim 2, characterized in that the metallization is carried out using a metal selected from the group of metals comprising nickel, palladium, cobalt, platinum and rhodium. Method according to claim 1, characterized in that an immersion plating takes place to metalize those parts of the front surface which were masked by the mask. 8. A method according to claim 7, characterized in that the metallizing step takes place such that the second surface of the substrate 30 is also metallized. A method according to claim 2, characterized in that the metallization comprises applying nickel from a bath containing a nickel salt and fluoride ions. 8420337