Patent Description:
A thermoelectric device is a semiconductor-based, solid-state device that can convert a temperature difference into electrical energy, in which voltage is generated through Seebeck effect. When multiple units of thermoelectric devices are electrically connected in series, a high voltage output can be obtained. Alternatively, a thermoelectric device can be used for modifying temperature using electrical energy.

Current thermoelectric device industries focus mainly on the rigid thermoelectric devices based on ceramic substrates. However, such thermoelectric devices are not suitable for applications involving non-flat surfaces, such as curved surfaces, tubes etc. In particular, while there is growing interest in using thermoelectric devices to convert body heat into electrical energy or use electrical energy for cooling, the application of rigid thermoelectric devices in wearable electronics is limited by their rigidity. Besides the disadvantages caused by the rigidity of the ceramic substrate, high thermal resistance from the ceramic substrate to the circuit where the thermoelectric elements are attached will reduce the thermoelectric performance.

Flexible thermoelectric devices have been designed, where the N-type and P-type thermoelectric elements are placed in the same axis with the bending direction of the flexible thermoelectric devices. However, in conventional designs, such as one shown in <FIG>, compression and tension forces created during bending acts on different directions of the thermoelectric elements, solder joints and the flexible circuit connecting the thermoelectric elements. This causes issues of cracks on the solder joints, broken thermoelectric elements and peeling of the flexible circuit. Furthermore, conventional flexible circuits are typically limited to <NUM> in length only due to standard UV exposure size for photoresist patterning. The international patent application <CIT> shows another example of a flexible thermoelectric device.

A need therefore exists to provide solutions that seek to address at least one of the problems above or to provide a useful alternative.

According to a first aspect of the present disclosure, there is provided a method of fabricating a thermoelectric device. The method includes:.

The step of forming the patterned circuits may include forming a first patterned photo-resist layer on the metal layer, disposing a metal on the metal layer based on the first patterned photo-resist layer, and removing the photo-resist layer and non-functional metal from the sub-assembly.

The step of forming the first patterned photo-resist layer on the metal layer may include providing a first photomask, and aligning registration spots of the first photomask with corresponding registration holes of the metal layer.

The step of forming the blind vias may include forming a second patterned photo-resist layer on the dielectric layer, and removing the dielectric layer.

The step of forming the second patterned photo-resist layer on the dielectric layer may include providing a second photomask, and aligning registration spots of the second photomask with corresponding registration holes of the dielectric layer.

The method in accordance with the first aspect may further include joining multiple sub-assemblies to form an assembly extending in the first direction before forming the patterned circuits and blind vias. The step of joining the multiple sub-assemblies includes aligning markers on an edge of one sub-assembly with corresponding markers on an adjoining edge of another sub-assembly.

Additionally, the method may further include forming through vias in the dielectric layer at the step of forming blind vias to form a gap between the two adjacent sub-assemblies.

The step of fabricating first and second thermoelectric elements may include screen printing of the first and second thermoelectric elements.

The method may further include providing metal contacts to connect the thermoelectric units within the sub-assembly in series. In one implementation, the ends of adjacent sub-assemblies may be connected in series in a serpentine arrangement.

The dielectric layer may be made of a material selected from a group consisting of polyimide, polyesters, thermoplastic polyimide, polyamide, polyolefin film, liquid crystalline polymer film, and flexible FR4 based film. The first thermoelectric element may be made of a material selected from a group consisting of bismuth telluride, silicon germanium, calcium cobalt oxides based materials, sodium cobalt oxides based materials, and strontium titanium oxides based materials. The second thermoelectric element may be made of a material selected from a group consisting of antimony telluride, lead tellurides, and Mg<NUM>X (X = Si, Ge, and Sn) based materials.

Embodiments of the invention are provided by way of example only, and will be better understood and readily apparent to one of ordinary skill in the art from the following written description and the drawings, in which:.

Embodiments of the present invention will be described, by way of example only, with reference to the drawings. Like reference numerals and characters in the drawings refer to like elements or equivalents.

<FIG> shows a flowchart <NUM> illustrating a method for fabricating a flexible thermoelectric device according to an example embodiment. At step <NUM>, a metal layer is deposited on a dielectric layer to form a flexible sub-assembly. In an example embodiment, the metal layer is made of copper (Cu) to form a flexible Cu clad substrate. At step <NUM>, patterned circuits are formed on the metal layer using photolithographic process, which function as electrodes for connecting the thermoelectric elements. At step <NUM>, blind vias are formed in the dielectric layer, where the thermoelectric elements are fabricated subsequently. In some embodiments, at step <NUM>, both blind vias and through vias are formed in the dielectric layer. The through vias are arranged in a way that gaps can be created between two thermoelectric elements for improved flexibility.

At step <NUM>, one or more first thermoelectric elements are fabricated in a first series of the blind vias formed at step <NUM>. At step <NUM>, one or more second thermoelectric elements are fabricated in a second series of the blind vias formed at step <NUM>. The one or more second thermoelectric elements form one or more thermoelectric units with the one or more first thermoelectric elements fabricated at step <NUM>, connected by the patterned circuits formed at step <NUM>. The sub-assembly is configured to be joined to an adjacent sub-assembly along a first direction to form a strip of multiple sub-assemblies. The first and second thermoelectric elements are arranged in a manner such that the first and second thermoelectric elements of each thermoelectric unit are aligned in a second direction substantially perpendicular to the first direction.

When the strip of sub-assemblies is flexed or bent in the first direction, the tension and compression forces acting on the solder joints, the thermoelectric elements and the flexible patterned circuits can be reduced due to the alignment of the thermoelectric elements in the second direction. Additionally, the gaps formed by the through vias can further increase the flexibility and durability of the thermoelectric device.

An example implementation of the above method is now described with reference to <FIG>. <FIG> shows a polyimide (PI) layer <NUM> provided as the dielectric layer. Thickness of the PI layer <NUM> can be in the range of <NUM>-<NUM>. One can appreciate that the dielectric layer can be made of other materials, including but not limited to thermoplastic polyimide, liquid crystalline polymer film, polyolefin film, polyester, polycarbonate, polyamide, or the like.

<FIG> shows a thin copper (Cu) layer <NUM> deposited on the PI layer <NUM> forming a flexible sub-assembly, corresponding to step <NUM> of the <FIG>. The Cu layer <NUM> can be deposited by Cu sputtering followed by electroplating method. Thickness of the Cu layer <NUM> can be in the range of <NUM>-<NUM>. One can appreciate that the Cu layer <NUM> can be deposited by other methods, such us lamination of Cu foil film, casting of polymeric film in a molten state on the Cu foil, or the like. Besides Cu, the metal layer can be made of other materials, including but not limited to gold, silver, platinum, aluminum, and steel.

In the example embodiment, patterned circuits and blind vias are formed using a photolithography method. Photoresist is first laminated on both the top surface of the Cu layer <NUM> and the bottom surface of the PI layer <NUM>, followed by UV patterning and developing to create a first patterned photoresist layer 314a (on the Cu layer <NUM>) and a second patterned photoresist layer 314b (on the PI layer <NUM>), as shown in <FIG>.

<FIG> shows Cu <NUM> deposited on the Cu layer <NUM> based on the first patterned photoresist layer 314a using electroplating method. In the example embodiment, Cu was deposited by Cu electroplating process. One can appreciate that the Cu can be deposited by both electroplating and electroless plating method. The function of the plated Cu <NUM> is to provide electrical connection between the first and second thermoelectric elements in a thermoelectric unit. Besides Cu, the deposited metal can include, but is not limited to gold, silver, platinum, aluminum and steel and is typically of the same material as the layer <NUM>.

<FIG> shows the both blind vias <NUM> and through vias <NUM> created on PI layer <NUM> based on the second patterned photoresist layer 314b. In the example embodiment, the blind vias <NUM> and through vias <NUM> are created by chemical etching of PI. Besides chemical etching, there are other methods of removing the PI or other dielectric materials to create blind vias, such as mechanical punching, and laser etching. The blind via angle varies from <NUM>-<NUM> degree depending on the etching method. In the example embodiment, the blind via angle is typically between <NUM>-<NUM> degrees.

Subsequently, the first and second photoresist layers 314a, 314b are removed or stripped from the Cu layer <NUM> and the PI layer <NUM>, respectively, as shown in <FIG>.

From <FIG>, the non-functional copper is removed from the sub-assembly. Flash etching process is used in the example embodiment to remove the non-functional copper. One can appreciate that other methods can be used such as subtractive etching using chemical process.

The patterned Cu circuits <NUM> and blind vias <NUM> in the PI layer <NUM> correspond to steps <NUM> and <NUM> of the method of <FIG>. While a photoresist patterning method is used, one can appreciate that the patterned circuits <NUM> can be formed by other methods, such as screen printing, lithoprinting, or the like.

<FIG> shows the P-type thermoelectric elements <NUM> (positive) fabricated in alternating blind vias, corresponding to step <NUM> of the method of <FIG>. The P-type thermoelectric elements <NUM> are fabricated using screen printing method, which prints P-type semiconductor ink in the alternating blind vias. Annealing process is performed after the screen print. The P-type thermoelectric elements <NUM> are made of a material including but not limited to bismuth telluride, homogeneous oxides such as SrTiOx, NaCo<NUM>O<NUM> Ca<NUM>CoaO<NUM>, and silicon-germanium materials.

<FIG> shows the N-type thermoelectric elements <NUM> (negative) fabricated in the remaining alternating blind vias to form thermoelectric units with the P-type thermoelectric elements <NUM> and the patterned Cu circuits <NUM>. The N-type thermoelectric elements <NUM> are fabricated using screen printing method, which prints N-type semiconductor ink in the alternating blind vias. Annealing process is performed after the screen print. The N-type thermoelectric elements <NUM> are made of a material including but not limited to antimony telluride, lead tellurides, and Mg<NUM>X (X = Si, Ge, and Sn) based materials.

Besides screen printing method, other methods can be used to fabricate the P-type and N-type thermoelectric elements <NUM>, <NUM> such as attaching the P- and N-type semiconductor chips in the range of thickness <NUM> - <NUM> using solder, metallic paste, ACP (anisotropic conductive paste) bonding process, thermal adhesive, etc.. One can also use eutectic bonding process for the gold plated P- and N-type chips with gold or silver plated circuit. When the thermoelectric elements are fabricated by the above methods, the P- and N-type semiconductor chips tend to be less flexible than the thermoelectric elements fabricated by screen printing. When joined into a flexible assembly of multiple thermoelectric units (i.e., sub-assemblies), preferably, dielectric layer between two adjacent sub-assemblies can be partially or completely etched using chemical etching process during the blind via formation. Advantageously, a gap can be created between the adjacent sub-assemblies, which increases the flexibility of the assembly so that the assembly can be bent more easily.

<FIG> shows a thermoelectric unit <NUM> formed by a P-type thermoelectric element <NUM>, a N-type thermoelectric element <NUM>, and the Cu circuit <NUM> (in dash circle). When the thermoelectric unit <NUM> is used as a generator, the Cu circuit <NUM> side of the thermoelectric unit <NUM> may be in thermal contact with the heat source. Once being heated, the P- and N-type carriers in these thermoelectric elements <NUM>, <NUM> are diffused from their hot side towards their cool side due to the temperature difference. Consequentially, P-type carriers will be concentrated on the cooling side of the P-type thermoelectric element <NUM> while N-type carriers will be concentrated on the cooling side of the N-type thermoelectric element <NUM>. Therefore, the electric potential at the cooling side of the thermoelectric unit <NUM> will be built up during heating, thus creating a thermal-electric battery.

<FIG> shows silver (Ag) paste <NUM> coating, which functions to connect one P-type thermoelectric element <NUM> with a N-type thermoelectric element <NUM> of an adjacent thermoelectric unit <NUM> such that all thermoelectric units <NUM> can be connected in series. When used as a generator, a higher voltage output can be generated in a series connection. In some embodiments, ends of adjacent sub-assemblies are connected in series in a serpentine arrangement. Nevertheless, the thermoelectric units <NUM> can also be connected in parallel, depends on the needs and the applications. The Ag paste <NUM> can be applied by screen printing, direct lithoprinting, or selective plating of Ag. Besides Ag paste <NUM>, other contacts can be used to connect the thermoelectric units <NUM>, such as Cu paste or other similar metallic pastes.

In an example embodiment, each of the sub-assemblies as described above is configured to be joined to an adjacent sub-assembly along a first direction. When the P-type and N-type thermoelectric elements <NUM>, <NUM> are fabricated, the P-type and N-type thermoelectric elements <NUM>, <NUM> of each thermoelectric unit <NUM> are aligned in a second direction substantially perpendicular to the first direction to provide better flexibility and durability.

In order to make a long flexible thermoelectric device (e.g., a strip) with multiple sub-assemblies, the P-type and N-type thermoelectric elements <NUM>, <NUM> need to be attached to a long flexible circuit. <FIG> shows diagrams of the top view of three sub-assemblies for making a <NUM> strip using a registration/alignment technique according to an example embodiment. Three photomasks <NUM>, <NUM>, <NUM> are provided with registration spots <NUM> and alignment markers <NUM> on each photomask. In the example embodiment, four registration spots are provided on each photomask, but it would be appreciated that a minimum of three spots are necessary. Photomask <NUM> on the left is further provided with two alignment markers close to the right edge <NUM>, and photomask <NUM> on the right is further provided with two alignment markers close to the left edge. Photomask <NUM> in the middle is further provided with four alignment markers, two close to the left edge and two close to the right edge.

The four registration spots <NUM> on each photomask <NUM>, <NUM>, <NUM> are used to match the top Cu pattern with the bottom PI pattern. The alignment markers are used to align the left, middle and right Cu or PI pattern for making <NUM> strip.

<FIG> shows a diagram of the top view of a <NUM> flexible thermoelectric strip formed by the three sub-assemblies in <FIG> according to the example embodiment. Firstly, four registration holes <NUM> are created on each sub-assembly using a punch machine so that the related four registration spots <NUM> on the photomasks can be matched with the registration holes <NUM>. The middle pattern on the middle sub-assembly <NUM> can be created by UV exposure, then the left and right patterns on sub-assemblies <NUM> and <NUM> can also be created with their photomasks, respectively. The alignment markers <NUM> are used for alignment between left, middle and right patterns of Cu or PI side. In detail, for the Cu side, the after creating middle pattern, the alignment markers <NUM> on this pattern can be used as reference for alignment of the left and the right pattern, which means the location of the left and the right pattern can be adjusted based on their registration spots/holes and alignment markers. Similarly, the patterns on PI side can also be created by using the same method. In addition, the matching between Cu and PI pattern can also be proceeded by these registration spots/holes.

<FIG> shows a diagram of the cross-sectional view of the <NUM> flexible thermoelectric strip in <FIG> according to the example embodiment. The sub-assembly has a substrate structure of a Cu layer <NUM> and a PI layer <NUM>. Photoresist layers <NUM> are laminated on both sides of the substrate. The registration holes <NUM> are created by punch machine. The left, middle and right expose patterns are aligned/ matched together on both Cu layer <NUM> and PI layer <NUM> for creating the <NUM> strip. The three aligned sub-assemblies with an overall length of <NUM> as a whole is provided for depositing the Cu (<FIG>) and onward processing. In a preferred embodiment, the Cu deposited by Cu plating provides sufficient stability to hold the sub-assemblies together. Advantageously, no extra step of stitching or connecting is required to join the sub-assemblies.

With this method, a <NUM> flexible thermoelectric strip can be created by short length (<NUM>) exposer. Longer strip, such as <NUM> meters or more, can be fabricated by repeating the above procedure. Besides the design of the registration spots, registration holes and alignment markers described above, designs with other shapes, number of spots/holes/markers and different distances can also be used to achieve the same effect. Besides using a punch machine, the registration holes can be created by other methods, such as laser drilling, 3D printing or screen printing to create registration marks.

<FIG> shows a diagram of the side view of a flexible thermoelectric strip <NUM> fabricated in accordance with the method <NUM> according to an example embodiment. The sub-assemblies are joined along a first direction D. In the example embodiment, P- and N-type semiconductor chips are used as the P- and N- type thermoelectric elements. As shown in the figure, portions of PI between adjacent sub-assemblies are etched out to form gaps <NUM>, which can further improve the flexibility of the strip <NUM>.

<FIG> shows a schematic diagram of the side view of a flexible thermoelectric strip <NUM> in a bent form. As shown in the figure, the gaps <NUM> advantageously improve the flexibility of the strip <NUM> when being bent.

<FIG> shows a schematic diagram of the perspective view of a flexible thermoelectric strip <NUM>. One may note that, for each thermoelectric unit, the P- and N-type semiconductor chips are aligned in a second direction substantially perpendicular to the first direction D. With the layout of the thermoelectric units as described above, the compression and tension forces acting on the P- and N-type semiconductor chips, solder joints and the flexible circuit <NUM> can be reduced when the strip <NUM> is bent, which allows better flexibility and durability of the device. The flexibility may be further improved by etching the dielectric layer between two adjacent sub-assemblies joined along the first direction D to create a gap <NUM>. In other words, the gaps <NUM> are in the second direction substantially perpendicular to the first direction. The dielectric layer can be partially or completely etched. As a result, an array of thermoelectric units in the second direction (i.e. width direction) does not connect to the neighboring arrays of thermoelectric units except at the bottom interconnect layer. In other words, the arrays of thermoelectric units are minimally connected in the first direction, thereby allowing a high flexibility and durability when the strip <NUM> is bent in a manner shown in <FIG> and <FIG>.

Referring to <FIG>, an example assembly of a bottom interconnected layer <NUM> (<FIG>) and a top interconnect layer <NUM> (<FIG>) is depicted in accordance with the embodiment. As shown in <FIG>, arrays of thermoelectric units are aligned in the second direction on the bottom interconnected layer <NUM>. To connect these arrays of thermoelectric units, two rows of thermoelectric units are provided along the edges of the strip <NUM> in the first direction. As shown in <FIG>, Cu electrodes <NUM> are arranged in the top interconnect layer <NUM> to connect the thermoelectric units on the bottom interconnected layer <NUM>. Advantageously, upon assembly, the arrays of thermoelectric units aligned in the second direction are minimally connected in the first direction only along the edges on the bottom interconnected layer (e.g. in a serpentine arrangement as shown in <FIG>), such that a very high flexibility of the strip can be achieved. One may appreciate that other ways of assembling the bottom and top interconnected layers can be used, depending on the design of the thermoelectric units on the strip.

Embodiments of the present invention provide a long flexible thermoelectric device with thermoelectric units connected in series. Attributed to its length and flexibility, it can be wrapped around any non-flat surface or tubes with higher diameter. It also allows easier thermal management than the conventional rigid thermoelectric devices which needs to be connected in series.

Claim 1:
A method of fabricating a thermoelectric device, the method comprising:
disposing a metal layer (<NUM>; <NUM>) on a dielectric layer (<NUM>; <NUM>) to form a sub-assembly;
forming patterned circuits (<NUM>) on the metal layer (<NUM>; <NUM>);
forming blind vias (<NUM>) in the dielectric layer (<NUM>; <NUM>);
fabricating first thermoelectric elements (<NUM>) in a first series of blind vias (<NUM>); and
fabricating second thermoelectric elements (<NUM>) in a second series of blind vias (<NUM>) to form thermoelectric units (<NUM>) with the first thermoelectric elements (<NUM>) and the patterned circuits (<NUM>),
wherein the sub-assembly is configured to be joined to an adjacent sub-assembly along a first direction, characterised in that fabricating the first and second thermoelectric elements (<NUM>, <NUM>) comprises aligning the first and second thermoelectric elements (<NUM>, <NUM>) of each thermoelectric unit in a second direction substantially perpendicular to the first direction.