TECHNICAL FIELD OF THE INVENTION

The present invention concerns a method for manufacturing a thermoelectric device, which thermoelectric device comprises a plurality of thermoelectric junctions where each thermoelectric junction, on the one hand, is arranged between a first thermal side and a second thermal side of the thermoelectric device and, on the other hand, comprises a first element doped according to a first doping type extending between said first and second thermal sides and a second element doped according to a second doping type and extending between said first and second thermal sides, the first element and the second element being electrically connected together at one of said first and second thermal sides and allowing generating a thermoelectric effect.

The invention allows manufacturing a thermoelectric device where each thermoelectric junction of the thermoelectric device is configured to generate a thermoelectric effect, in particular during the operation of the thermoelectric device. In the Seebeck mode, the thermoelectric effect is such that the at least one thermoelectric junction enables the generation of an electrical energy when it is subjected to a temperature difference applied to the thermoelectric device between its first thermal side and its second thermal side. In the Peltier mode, the thermoelectric effect is such that the at least one thermoelectric junction enables the generation of a thermal energy (temperature difference between the first and second thermal sides) when it is subjected to an electrical energy electrically powering said at least one thermoelectric junction.

STATE OF THE ART

In a known manner, and as illustrated inFIG. 1, a thermoelectric device100may include a set of first pads101made of a thermoelectric material and a set of second pads102made of a thermoelectric material different from the thermoelectric material of the first pads101. The first and second pads101,102are electrically connected so as to form thermoelectric junctions103between a first side104, for example called hot side, of the thermoelectric device100and a second side105, for example called cold side, of the thermoelectric device100. Each thermoelectric junction103includes one of the first pads101and one of the second pads102. For each thermoelectric junction103, an electrical linking element106allows electrically connecting the first pad101and the second pad102of the corresponding thermoelectric junction103, also called NP junction in the field. Connecting elements107allow electrically connecting the thermoelectric junctions103together in series and/or in parallel. The thermoelectric device100ofFIG. 1is composed by several first and second pads101,102electrically connected in series and thermally connected in parallel so as to form four thermoelectric junctions103. In particular, in the field, the first and second pads101,102are also called N pads and P pads, respectively, a N pad being a pad with N-type conductivity (that is to say the material composing it has a strictly negative Seebeck coefficient) and a P pad being a pad with P-type conductivity (that is to say the material composing it has a strictly positive Seebeck coefficient).

In general, considering that the dimensions (in particular the height and the section) of the first and second pads are identical, the electrical performances of the thermoelectric device100used as an energy generator, in particular electrical energy generator, could be determined in a simplified manner by:an internal electrical resistance, denoted Rint, determined for example in the following manner (denoted equation (1) later on):
Rint=N×pnp×H/A

with N the number of thermoelectric junctions of the thermoelectric device, H the height of one of the first pads101of one of the thermoelectric junctions of the thermoelectric device100, this height H being measured according to an axis orthogonal to the first side104and to the second side105, A the surface area of the section of said first pad101considered orthogonally to the direction in which H is measured, and pnp corresponding to the electrical resistivity of said first pad101(this electrical resistivity being considered to be identical for each first pad101and each second pad102);an output voltage, denoted V, which could be determined in the following manner (denoted equation (2) later on):
V=N×Snp×ΔT

with Snp the Seebeck coefficient associated to one of the thermoelectric junctions of the thermoelectric device, Snp being the difference between Sp and Sn, ΔT the effective temperature difference between the first side104and the second side105to which the thermoelectric junctions are subjected, N the number of thermoelectric junctions of the thermoelectric device, Sp the Seebeck coefficient of the P-type material used for example to form each second pad102, and Sn the Seebeck of the N-type material used to form each first pad101;a useful electric power Puwhich could be determined in the following manner (denoted equation (3) later on):

Pu=Vo⁢c24×Ri⁢n⁢t

The equation (1) neglects the electrical contact resistances and the electrical resistances of the connecting elements107and of the linking elements106which generally have negligible contributions in comparison with the electrical resistances of the N and P pads.

Hence, to obtain a high useful power, there should be a high output voltage and a low electrical resistance simultaneously.

When attempting to achieve that, there are two main methods using thin film technologies allowing making microelectronic thermoelectric devices with thin films compatible with microelectronics processes.

The first method is of the «2D» («two-dimensional») technology type, in other words a planar technology type. In this method, the first pads101and the second pads102mentioned hereinbefore in connection withFIG. 1are in the form of rows, respectively N and P doped, connected electrically in series and thermally in parallel. In general, this technology is carried out only by microelectronics thin film forming techniques, that is to say for thickness values comprised between 1 nm and 100 μm.

This method type is well suited for the manufacture of thermoelectric devices such as microsensors, but it is not suited for the manufacture of thermoelectric devices intended to generate electricity according to Seebeck effect. Indeed, the two thermal sides, positioned in the plane of the thermoelectric device obtained in 2D, are away from each other by a distance corresponding to the length of the rows forming the first and second pads. Thus, to obtain a high ΔT of the equation 3 (the effective temperature difference between the first thermal side and the second thermal side to which the thermoelectric junctions are subjected), the thermal sides should be as away as possible from each other and thermally-insulated from each other. This involves the formation of rows forming the longest possible first and second pads. But in this case, the electrical resistance according to the equation 1 increases, which burdens the useful power according to the equation 3, which is not satisfactory.

In addition, from a manufacturing perspective, this requires the sequential completion of several deposition (thermoelectric materials, insulating materials, electrically-conductive materials . . . ), lithography, etching, annealing steps. All these operations are long, tedious and delicate to implement, besides being expensive.

The second method is of the «3D» («three-dimensional») technology type, in other words orthogonal technology type. This method could be obtained using one single bulk substrate, or by stacking of substrates where each one is obtained by the previously-described 2D technology. The architecture of the obtained thermoelectric device allows exploiting, in the context of electrical energy generation, a hot source (placed on the hot side) and a cold source (placed on the cold side) arranged opposite, in particular orthogonally, to the plane of the substrate(s). A cold source has a temperature strictly lower than the temperature that the hot source has. In other words, the hot and cold sources could extend on either side of a wall constituted by a plate formed by the substrate(s).

Thus, even though such an architecture has the advantage of offering a low electrical resistance thanks to the height H of each of the first and second pads generally amounts to a few micrometers, that is to say comprised between 1 and 100 microns, in the microelectronics field, it has the drawback of being difficult to obtain a satisfactory temperature difference to generate a high output voltage since the latter depends on the temperature difference, which temperature difference depending on the temperature of the hot source and on the temperature of the cold source. However, the closer are the hot source and the cold source in the case of a very thin wall, the more obtaining a high temperature difference to be exploited in the context of Seebeck effect will be difficult to obtain.

In addition, from a manufacturing perspective, in the case where the wall consists of a stack of substrates each obtained by a 2D technology, this requires the sequential completion of several deposition (thermoelectric materials, insulating materials, electrically-conductive materials . . . ), lithography, etching, annealing steps. All these operations are long, tedious and delicate to implement, besides being expensive.

Still from a manufacturing perspective but in the case where the wall consists of a bulk substrate, herein again, it is unfortunately necessary to sequentially complete a lot of sintering, polishing, cutting, metallization, annealing transfer, also called «pick and place» according to the dedicated terminology, steps, etc . . . . This consists of a complex manufacture which, furthermore, limits a lot the geometry and the modularity of the manufactured thermoelectric device.

In this respect, it should be understood that there is a need for providing a manufacturing method allowing obtaining a thermoelectric device, suited in particular to constitute an energy generator for electrical energy production by exploiting a temperature difference to which the thermoelectric device is subjected, which has a very good efficiency, a low electrical resistance and which could be used with a large temperature difference between the hot and cold sources, where in addition, this method advantageously comprises less steps than is the case in the mentioned state of the art, is more simple and more economical, allows for a great shape and design modularity of the manufactured thermoelectric device, reduces the material losses, facilitates integration and interfacing, allows implementing complex assemblies, is quick to implement.

These needs also arise for the manufacture of a thermoelectric device adapted to operate in Peltier mode, that is to say one that could be supplied with electrical energy so as to cool down one of its sides amongst the first thermal side or the second thermal side.

OBJECT OF THE INVENTION

The present invention aims at providing a method for manufacturing a thermoelectric device addressing the above-mentioned problems.

This object could be achieved thanks to the implementation of a method for manufacturing a thermoelectric device, which thermoelectric device comprises a plurality of thermoelectric junctions where each thermoelectric junction, on the one hand, is arranged between a first thermal side and a second thermal side of the thermoelectric device and, on the other hand, comprises a first element doped according to a first doping type extending between said first and second thermal sides and a second element doped according to a second doping type and extending between said first and second thermal sides, the first element and the second element being electrically connected together at one of said first and second thermal sides and allowing generating a thermoelectric effect, the manufacturing method comprising:a step a) of manufacturing at least one first part formed in a first material doped according to the first doping type and shaped like a comb delimiting a first base and a plurality of first branches substantially parallel to one another and extending substantially orthogonally from the first base,a step b) of manufacturing at least one second part formed in a second material doped according to the second doping type shaped like a comb delimiting a second base and a plurality of second branches substantially parallel to one another and extending substantially orthogonally from the second base,an assembly step c) during which the first part and the second part are mechanically assembled together to form a self-supporting set,an electrical connection step d) in which the distal end of all or part of the first branches of the first part are electrically connected to the second base of the second part and in which the distal end of all or part of the second branches of the second part are electrically connected to the first base of the first part,a cutting step e) in which the first base is sectioned into at least one first area located between the root of one of the first branches and the contact with one of the adjacent second branches and in which the second base is sectioned into at least one second area located between the root of one of the second branches and the contact with one of the first branches, on completion of which step e) each first branch and each second branch separated by a first area respectively constitute the first element and the second element of a thermoelectric junction electrically connected via the portion of the second base linking said first branch and said second branch separated in this manner by this first area, whereas each first branch and each second branch separated by a second area respectively constitute the first element and the second element of a thermoelectric junction electrically connected via the portion of the first base linking said first branch and said second branch separated in this manner by this second area.

Some preferred, yet non-limiting, aspects of this manufacturing method are the following ones, these could be implemented separately or in combination.

After step e), the manufacturing method comprises an electrical connection step f) in which the thermoelectric junctions created at step e) are electrically connected together so as to connect them in series and/or in parallel.

During step e), the arrangement of a first area and of a second area on either side of one of the first branches, or of one of the second branches, creates a thermoelectric junction in series with any other thermoelectric junction also created at step e).

Steps c) and d) are obtained in one single operation, in which the distal end of at least one of the first branches is electrically and mechanically connected with the second base and in which the distal end of at least one of the second branches is electrically and mechanically connected with the first base.

Step a) consists of an additive manufacturing method in which a material is deposited through successive passes on a tray, the result of the successive depositions during the different passes comprising a first comb whose shape corresponds to the shape of each first part.

A plurality of first combs secured together so as to form a first block in one-piece are obtained during step a) and step a) comprises a fractionation step a1) in which said first block is fractionated in order to deliver said plurality of first combs shaped in unitary manner and separated from one another.

The material used in the additive manufacturing method in step a) is the first material in which each first part is made.

Step b) consists of an additive manufacturing method in which a material is deposited through successive passes on a tray, the result of the successive depositions during the different passes comprising a second comb whose shape corresponds to the shape of each second part.

A plurality of second combs secured together so as to form a second block in one-piece are obtained during step b) and step b) comprises a fractionation step b1) in which said second block is fractionated in order to deliver said plurality of second combs shaped in unitary manner and separated from one another.

The material used in the additive manufacturing method in step b) is the second material in which each second part is made.

The material used in the additive manufacturing method of step a) and the material used in the additive manufacturing method of step b) are identical, and step a) comprises a step of doping by implantation or by heat treatment in order to transform the material used in the additive manufacturing method of step a) into the first material in which the at least one first part is made, whereas step b) comprises a step of doping by implantation or by heat treatment in order to transform the material used in the additive manufacturing method of step b) into the second material in which the at least one second part is made.

