FR2956242A1 - Substrate i.e. P-type silicon substrate, realizing method for forming photovoltaic cell, involves realizing diffusion heat treatment to form first and second volumes doped respectively from sources of dopants - Google Patents

Abstract

A first dopant layer (2) is deposited on one side of a substrate (1). The first dopant source (2) is structured to form, on the face of the substrate (1), a first zone (3) covered by the first dopant source (2) and a second free zone (4). A second dopant source (5) is deposited at the second free zone (4). A diffusion heat treatment is applied to form the first (6) and second (7) doped volumes respectively from the first source of dopants (2) and the second dopant source (5).

Description

Method for producing first and second doped volumes in a substrate Technical field of the invention

The invention relates to a method for producing a substrate comprising first and second doped volumes. State of the art

In the field of photovoltaic cells, the research focuses mainly on improving the conversion efficiency of the cell and on simplifying its production method.

Conventionally, a photovoltaic cell is formed by a diode, for example a p / n type junction made of a photovoltaic material such as silicon. The diode then comprises a zone doped with a p-type impurity, for example boron, which is in contact with a zone doped with an n-type impurity, for example phosphorus. The photodiode can therefore be made simply by forming an n-type doped zone within a p-type substrate.

One way of improving photovoltaic cells comes from the use of the so-called "selective emitter" structure instead of a homogeneous doped structure. The "selective emitter" structure is characterized by a variation of the dopant concentration on the surface of the substrate according to the desired functionalities. There are then, in the photovoltaic cell, 30 different zones which have between them different dopant concentrations and specific electrical and photovoltaic properties. Thus, high doping is used in close proximity to the electrodes, the "emitters", which cause the current to flow out of the cell and low doping is used on the rest of the cell. The low doping makes it possible to obtain a high short-circuit current whereas the high contact doping makes it possible to reduce the contact resistance for a good collection.

This promising structure is also characterized by a production process which is longer and more complex than a conventional structure with homogeneous doping, which inevitably results in a decrease in the overall efficiency of this technological sector.

OBJECT OF THE INVENTION The subject of the invention is a method which is simple to implement and which comprises a reduced number of technological steps to limit the losses of mechanical efficiency.

The method according to the invention is characterized in that it comprises on one side of the substrate: the deposition of a first source of dopants, the structuring of the first source of dopants to form, on the face of the substrate, a first zone covered by the first dopant source and a second free zone, the deposition of a second dopant source at the second free zone, the application of a diffusion heat treatment.

BRIEF DESCRIPTION OF THE DRAWINGS Other advantages and features will emerge more clearly from the following description of particular embodiments of the invention given by way of nonlimiting example and represented in the accompanying drawings, in which: FIGS. 1 to 4 are schematically in section, the main steps of making a first embodiment of a substrate according to the invention, Figures 5 and 6 show, schematically, in section, a 1 o step of realization two embodiments of a substrate according to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

As illustrated in FIG. 1, a first source of dopants 2 is deposited on the surface of a substrate 1. The substrate 1 is, for example, of semiconductor material or of any other suitable material. The substrate 1 may be a substrate with a flat surface or an at least partially textured surface.

The first source of dopants 2 comprises a first type of doping impurities in a first concentration. The first dopant source 2 is, for example, a doped dielectric material such as a doped oxide or a solution of SOD type P506 from Filmtronics (comprising 6% by weight of SiO 2, 3% by weight of phosphorus and a solvent). The first source of dopants 2 is formed by any suitable technique, for example by spin coating or plasma enhanced chemical vapor deposition. The first source of dopants 2 is formed on a temperature basis in order to avoid any parasitic diffusion of the first type of doping impurities in the substrate 1. The first source of dopant is deposited on a textured zone or a flat zone of the substrate.

As illustrated in FIG. 2, the first dopant source 2 is then structured by any suitable technique to form at least one void area on the surface of the substrate which releases the surface of the substrate. In this way, during the structuring, there is at least one formation of a first zone 3 and a second zone 4 on the surface of the substrate 1. The first zone 3 is a part of the surface of the substrate 1 which is covered by the first dopant source 2 while the second zone 4 is an empty zone, that is to say a portion of the surface of the substrate 4 which is left free.

The structuring of the first dopant source 2 may be carried out, for example, by means of photolithography followed by wet-type or plasma-type etching or by means of etching by laser radiation or by any other suitable technique. It is also possible to deposit a masking material on the surface of the substrate prior to the deposition of the first dopant source 2. The masking material comprises free zones which are then covered by the first dopant source 2. A polishing step mechanical-chemical then allows to structure the first source of dopants 2 and locate it in the old empty areas of the masking material. The masking material is removed to leave it on the surface as the first source areas of dopants.

As illustrated in FIG. 3, once the surface of the substrate 1 has first 3 and second 4 zones, a second dopant source 5 is deposited at least on the second zone 4, that is to say the free zone. . The second source of dopants 5 is, for example, a doped dielectric material such as a doped oxide or a SOD solution P509 from Filmtronics. The deposition of the second dopant source 5 may be carried out by any suitable technique, for example by spin coating, by depositing

plasma enhanced chemical vapor phase. The second dopant source 5 comprises a second type of dopant impurities in a second concentration. The localization of the second dopant source 5 is then carried out conventionally, for example by means of a photolithography step followed by an etching step, or directly by an etching step if the second doping source has a greater thickness. At the second zone 4, again, the location of the second dopant source can be achieved by chemical mechanical polishing.

The surface of the substrate 1 is then covered by the first dopant source 2 in the first zone 3 and by the second dopant source 5 in the second zone 4.

In a particular embodiment (FIG. 3), the second dopant source 5 can also be deposited on the assembly, that is to say on the first free zone 3 of the substrate 1 and on the second zone 4 above the first source of dopants 2. The deposition of the second source of dopants 5 on the entire substrate is advantageous because, it allows the use of conventional and cheap deposition techniques such as those described above. Moreover, the lack of localization makes it possible to reduce the technological steps of the production process, which also contributes to obtaining reduced cycle times and to a robust and economical process.

Depending on the desired applications, the first and second types of doping impurities have the same or different conductivity types (N-type or P-type).

As illustrated in FIG. 4, the assembly then undergoes a diffusion heat treatment that will migrate a part of the doping impurities of the first 2 and second 5 dopant sources to the substrate 1. The doping impurities of the first dopant source 2 will form a first doped volume 6, that is to say a portion of the substrate 1 doped with the first type of doping impurities and in a third concentration. At the same time, the doping impurities of the second dopant source 5 will diffuse and form a second doped volume 7, that is to say a portion of the substrate 1 doped with the second type of doping impurities and according to a fourth concentration. By way of example, the fourth dopant concentration is greater than the third dopant concentration, the opposite is also possible. The diffusion heat treatment is carried out by any suitable technique, for example by laser radiation.

The values of the third and fourth dopant concentrations are a function of the values of the first and second dopant concentrations, the dopant diffusion modes, the materials to be passed through during the diffusion and the thermal budget incurred.

The first doped volume 6, that is to say the portion of the substrate 1 doped with the first type of doping impurities, is located at the level of the first zones 3, therefore below the surface of the substrate 1 covered by the first source of dopants. 2. The second doped volume 7, that is to say the portion of the substrate 1 doped by the second type of doping impurities is located at the second zones 4 and therefore under the surface of the substrate 1 covered by the second source of dopants 5.

Once the heat treatment has been performed and the first 6 and second 7 doped volumes formed, the first 2 and / or second 5 dopant sources are eliminated or at least partially eliminated if access to the doped volumes 6 and 7 is required. The use of a single heat treatment to form the first and second doped volumes 6 and 7 makes it possible to reduce the duration of the process and thus to obtain a cheaper product. This treatment 7

thermal single also allows to achieve better performance because there is reduction in the number of steps.

In the variant embodiment in which the second doping source is also deposited on the first doping source, during the heat treatment, the first dopant impurities are diffused from the first zone 3 to the substrate 1 and the second dopant impurities are diffused. second zone 4 towards the substrate 1. There is also diffusion of the second doping impurities from the first zone 3 towards the substrate 1 through the first dopant source 2.

As at the level of the first zone 3, the substrate 1 is covered by the first doping source 2 and then by the second doping source 5, the second doping impurities must pass through the first doping source 2 before reaching the substrate 1. This results that this additional distance to be traveled delays the moment when the second doping impurities will diffuse inside the second doped volume 7.

By judiciously choosing the thermal budget and the materials to be scanned, it is possible to prevent the second doping impurities from passing through the first dopant source 2 and thus reaching the second doped zones 7 of the substrate 1.

In this way, it is possible to easily obtain first and second doped volumes which have opposite conductivities. It is also possible to obtain first and second doped volumes which are of the same conductivity, for example by means of the same dopant impurity present in the two dopant sources, and which have different concentration levels to form, for example, volumes doped and overdoped. 8

A structure which has doped and overdoped volumes is particularly advantageous for forming a "selective emitter" type solar cell. The first doped volume 6 is weakly doped in order to favor the photovoltaic effect of the p / n junction formed with the substrate 1, whereas the second doped volume corresponds to an overdoped volume which favors a low access resistance for the extraction current out of the solar cell. For example, the doped volume 6 has a resistivity of the order of 100 to 180 O / D while the overdoped volume 7 has a resistivity of the order of 40%.

Advantageously, the first concentration of first doping impurities is less than the second concentration of doping seconds or even much lower impurity to facilitate obtaining a first doped volume 6 having a dopant concentration lower than that of the second doped volume 7, which the doping impurities are identical or different.

Obtaining a third concentration which is lower than the fourth dopant concentration is not necessarily linked to the fact that the first concentration of first doping impurities is lower than the second concentration of second doping impurities. This result essentially depends on the types of doping impurities incorporated in the first 2 and second 5 dopant sources, the mode of diffusion of these doping impurities, the materials constituting the substrate 1 and the dopant sources 2, 5 and also the temperature. heat treatment.

Thus, a judicious choice of the thickness of the first dopant source 2 and / or the adapted thermal budget may be sufficient to obtain the desired concentrations. It may also be advantageous to use, for the first dopant source 2, a material in which the second doping impurities have a low diffusion coefficient. Those skilled in the art can then adapt the thickness of the first doping source 2, its constituent materials and the thermal budget (time / temperature pair) so that the second doping impurities do not pass through the first dopant source 2.

In an alternative embodiment, an additional thermal diffusion treatment is performed after the deposition of the first dopant source 2 and before the deposition of the second dopant source 5. The additional heat treatment can be performed before or after the step of structuring of the first source of dopants 2.

In a first variant embodiment illustrated in FIG. 5, the additional heat treatment is performed before the step of structuring the first dopant source 2. As previously, the first doped volume 6 is produced on the whole of the substrate 1. covered by the first dopant source 2. The first dopant source 2 is then structured and the second dopant source 5 is deposited. The diffusion heat treatment then forms the second doped volumes 7 under the parts of the substrate 1 covered by the second dopant source 5 and ends up forming the first doped volume 6.

The second doped volumes 7 are formed by the diffusion of the second doping impurities located in the second zones 4. In this way, the second doped volume 7 comprises the first and second doping impurities. Depending on the doping impurities used and their respective concentrations, it is then possible to obtain first 6 and second 7 doped volumes of the same type of conductivity or of opposite types of conductivities.

In the case where the conductivity types are identical, it is particularly advantageous to form a second doped volume which has a higher dopant concentration than the first doped volume 6. Thus, the first doped volume 6 is covered by the second doped volume 7 10

during the additional heat treatment of diffusion. It is also possible to form a second doped volume 7 which has a dopant concentration lower or of the same order of magnitude as the first doped volume. In this way, the concentration profile of first and second doping impurities can be controlled.

If the first and second doping impurities are of opposite types, and depending on the values of the concentrations between these two types of impurities, there can be a compensation between the impurities of type N and type P. As the first doped volume is formed at least partially by means of the additional heat treatment, it is possible to have a greater margin of maneuver on the concentration profiles of the two doping impurities. In a second variant embodiment illustrated in FIG. 6, the additional heat treatment is performed after the step of structuring the first dopant source 2. The first doped volume 6 is then produced under the first zones 3 of the surface of the substrate 1 because they are covered by the first source of dopants 2. As in the previous embodiment, there is great flexibility on the additional heat treatment operating conditions because the second source of dopants 5 is not yet deposited. .

Once the second dopant source 5 has been deposited, the diffusion heat treatment then forms the second doped volumes 7 under the parts of the substrate 1 covered by the second dopant source 5 and ends the first doped volumes 6. The second doped volumes 7 are then formed by the second doping impurities. Advantageously, the additional heat treatment is carried out at a temperature greater than that of the diffusion heat treatment. Also advantageously, it may be envisaged to modulate the thickness of the first dopant source 2 as a function of the thermal budget envisaged for the heat treatment linked to the second dopant source 5. However, the thickness of the first source of Dopants 2 has an influence on the thermal budget which must be imposed during the additional heat treatment to obtain the concentration of desired doping impurities and their distribution profile. Advantageously, the depth of the second doped volume 7 in the substrate 1 is greater than the depth of the first doped volume 6.

By way of example, to form a structure compatible with a photovoltaic cell with a selective collector, the first and second doping impurities are of the same type of conductivity, for example of the N type if the substrate is of P-type silicon. To obtain a particularly simple and robust embodiment, the first 2 and second 5 sources of dopants are spin-on dopant solutions, respectively the SOD solutions P506 and P509 mentioned above. The SOD P506 solution contains 3% by weight of phosphorus and the P509 solution contains 10.5% by weight of phosphorus.

The first doping source of SOD type P506 can be deposited by means of a type RC8 spinning device from SÜSS MicroTec. Once the first dopant source 2 has been deposited, it is annealed at 175 ° C for about 10 minutes to remove the solvents. Substrate 1 is then annealed for 30 seconds at 1025 ° C. in a Centrotherm IR oven, which makes it possible to form the first doped volume 6. The structuring of the first source 3o of dopants 2 is carried out by etching with the aid of laser radiation with a current of 25A, a coverage of 98% and a running speed 10 12

equal to 3m / s. The second SOD doping source P509 is then deposited and annealed under conditions similar to those previously noted for the first dopant source. The final structure has first doped volumes having a resistance of 100% and second doped volumes having a resistance of 40%.

In a second example, the additional heat treatment of the first dopant source 2 has been suppressed, the other operating conditions have been preserved. The final structure has first doped volumes 6 having a resistance of 100% and second doped volumes having a resistance of 40%.

In another variant embodiment, it may be envisaged to use a third source of dopants. Thus, during the diffusion heat treatment, the surface of the substrate 1 has in addition a third zone covered by the third dopant source.

The first and second doped volumes being formed by diffusion of the doping impurities from the dopant sources 2, 5 to the substrate 1, it should be noted that the first 6 and second 7 doped volumes extend respectively beyond the first and second zones. Therefore, if the first 3 and second 4 areas cover the entire surface of the substrate 1, the first and second volumes deviate very slightly from this distribution. As a result, this technique is poorly adapted to the formation of doped volumes which have a depth of the same order of magnitude as the smallest longitudinal and transverse dimension of the first and second zones. Depending on the dopant concentrations involved, one of the doped volumes may be masked by its neighbors.

The first and second doped volumes are formed from the surface of the substrate. They are offset laterally with respect to each other

say that in top view, one of the doped volumes can not hide the other. The first and second doped volumes may have different thicknesses, even if they have undergone the same thermal budget.

Claims (8)

Revendications1. Process for producing a substrate (1) comprising first (6) and second (7) doped volumes, characterized in that it comprises, on one side of the substrate (1): the deposition of a first dopant source ( 2), structuring the first dopant source (2) to form, on the face of the substrate (1), a first zone (3) covered by the first dopant source (2) and a second free zone (4) depositing a second dopant source (5) at the second free zone (4), applying a diffusion heat treatment. 2. Method according to claim 1, characterized in that an additional thermal diffusion treatment is performed before the structuring of the first dopant source (2). 3. Method according to claim 2, characterized in that the thermal budget of the additional heat treatment is greater than that of the diffusion heat treatment. 4. Method according to any one of claims 1 to 3, characterized in that the second dopant source (5) is deposited on the first dopant source (2). 5. Method according to any one of claims 1 to 4, characterized in that the first source of dopants (2) is deposited on a textured area of the substrate (1). 1415 6. Method according to any one of claims 1 to 5, characterized in that the first dopant source (2) and the second dopant sources (5) comprise the same dopant impurity. 7. Process according to claim 6, characterized in that the concentration of the dopant impurity in the first dopant source (2) is less than the concentration of the doping impurity in the second dopant source (5). 10 8. Method according to any one of claims 1 to 7, characterized in that the diffusion heat treatment is performed by laser radiation. 15