Method for manufacturing electrical conductors, and electrical conductors manufactured according to same

Abstract

A method for manufacturing an electrical conductor includes: depositing a solid metal conductive layer or film on a substrate 30; depositing a liquid metal on the solid layer; and allowing the liquid metal and the solid layer 40 to alloy by diffusion of the liquid metal into the solid layer or film so as to form a solid conductive layer or film of the alloy; as well as allowing the liquid metal to further infiltrate the alloy so as to form percolating paths and/or droplets of the liquid metal in the the solid conductive layer or film, thus forming a biphasic conductive layer.

Claims

1. A method for manufacturing an electrical conductor, said method comprising: depositing a solid metal layer on a substrate; depositing a liquid metal on said solid metal layer; allowing said liquid metal and said solid metal layer to form an alloy by diffusion of said liquid metal into said solid metal layer so as to form a solid conductive layer of said alloy; allowing said liquid metal to further wet and infiltrate said alloy so as to form percolating paths of said liquid metal in said solid conductive layer, thus forming a biphasic conductive layer comprising said alloy as a solid phase and said percolating paths of said liquid metal as a liquid phase dispersed in the solid phase; allowing said liquid metal to further accumulate into bulges, locally yet randomly, on or within said biphasic conductive layer; and controlling a ratio n between a total number of atoms of said liquid metal and a total number of atoms of said solid metal in said biphasic layer to be between 2 and 50.

2. The method according to claim 1, wherein said liquid metal is deposited on said solid metal layer by thermal vapour deposition of said liquid metal.

3. The method as claimed in claim 1, wherein said liquid metal comprises one of gallium or a gallium-based alloy.

4. The method as claimed in claim 1, wherein said solid metal layer is sputtered on said substrate.

5. The method as claimed claim 1, wherein said solid metal layer is made of one of Au, Pd, Pt, Ir, or an alloy thereof.

6. The method according to claim 1, wherein said solid metal layer has a thickness between 10 and 1000 nm.

7. The method as claimed in claim 1, wherein said substrate is an elastomeric substrate.

8. The method as claimed in claim 1, wherein said method further comprises patterning said biphasic conductive layer or film so as to form at least one biphasic strip.

9. The method as claimed in claim 8, wherein said biphasic conductive layer is patterned using one of stencil and photolithography.

10. The method as claimed in claim 1, wherein said method further comprises encapsulating said biphasic conductive layer by forming an encapsulation layer on said biphasic conductive layer.

11. The method as claimed in claim 10, said method further comprising forming at least one through via in said encapsulation layer so as to expose at least a portion of said biphasic conductive layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, description will be given of the embodiments of the present invention depicted in the drawings. It has however to be noted that the present invention is not limited to the embodiments depicted in the drawings and described below; to the contrary, the present invention comprises all those embodiments which fall within the scope of the appended claims.

(2) In the drawings:

(3) FIGS. 1, 2, 3 and 4 depict method steps of a method according to a first embodiment of the present invention;

(4) FIGS. 5, 5a and 6 show schematic representations or views of a conductive film according to one embodiment of the present invention;

(5) FIGS. 7, 7a and 8, 8a show scanning electron microscope (SEM) images of a conductive film according to an embodiment of the present invention;

(6) FIGS. 9, 9a and 10, 10a show SEM images of the conductive film depicted in FIGS. 7, 7a and 8, 8a, respectively, when subjected to a strain or stretch.

(7) FIG. 11 shows scanning electron microscope (SEM) images of gallium deposited by thermal evaporation on a PDMS substrate and gallium deposited by thermal evaporation on a PDMS substrate previously coated with a layer of 60 nm of gold; scale bar is 5 m.

(8) FIG. 12 shows sheet resistance, gauge factor and relative increase in resistance at rest after stretching to 50% as a function of the ratio n between the number of atoms of liquid metal and the number of atoms of solid metal in the resulting biphasic (solid-liquid) layer or film.

DETAILED DESCRIPTION

(9) As depicted in FIG. 1, during a first method step, a substrate 30 is provided. The substrate 30 may be an elastomeric substrate, 20 for instance a PDMS substrate. The thickness of the substrate 30 may range from 1 m to 1 mm according to the needs and/or circumstances. Still by way of non-limiting example, providing the substrate 30 may comprise spin coating a PDMS layer on a carrier 100 (not depicted), for instance a silicon carrier (wafer or the like), curing the layer 30 at 80 C. and removing the carrier.

(10) During a second step, as depicted in FIG. 2, the substrate 30 is coated with a thin film 40 of a solid conductive metal; by way of example, the film 40 may be provided on the substrate 30 by sputtering gold (Au) on the substrate 30. Still by way of example, the final thickness of the gold film 40 may amount to 60 nm.

(11) During a third step as depicted in FIG. 3, one or more liquid metals are deposited on the film 40 by thermal evaporation.

(12) During the third step, the liquid metal (for instance gallium Ga) diffuses into the solid film 40 and alloys with the solid film to 5 form an alloy (for instance AuGa.sub.2 in case Au is used for forming the solid conductive layer 40 and Ga is used as a liquid metal). Moreover, once the solid film is totally alloyed, the liquid metal starts to accumulate so that percolating paths and bulges of liquid metal are formed.

(13) The situation at the end of the third step is therefore that depicted in FIGS. 3 and 4, namely with a conductive film or layer 43 comprising both a solid phase (the alloy) 41 and a liquid phase, namely the percolating paths and/or bulges 42 of liquid metal dispersed in the alloy 41.

(14) For the sake of clarity, reference is made to FIGS. 5, 5a and 6 (with FIG. 5a showing an enlarged view of a portion of FIG. 5) showing schematic views of the film 43.

(15) As apparent in particular from FIGS. 5 and 5a, the biphasic conductive film or layer 43, as anticipated above, comprises the 20 solid metallic alloy 41 (for instance AuGa.sub.2) and the percolating paths and bulges 42 of liquid metal (for instance Ga).

(16) The advantages of a biphasic conductive film of the kind disclosed above are schematically depicted in FIG. 6; it appears in fact clearly from FIG. 6, that when the film 43 is subjected to strain (for instance by subjecting to strain or stretch or the like the substrate 30), microcracks eventually formed in the solid phase 41 (the alloy) are filled by the liquid phase, so that the electrical conductivity of the layer or film 43 is maintained (electrical current flows across both the solid and liquid phases).

(17) As to the liquid metals, gallium and a gallium-based alloy may be used according to the present invention; however, gallium has revealed to provide the best results since the stoichiometry of the evaporated film is the same as the stoichiometry of the original material. Moreover, as to the deposition of the liquid metal, thermal evaporation of one or both of the above mentioned gallium and gallium-based alloy has revealed to be the most preferred solution.

(18) As to the solid metal(s) to be used for the formation of the 10 alloying layer 40, one of Au, Pd, Pt and Ir and an alloy thereof may be used according to the present invention.

(19) Moreover, within the frame of the present invention several parameters of the thermal evaporation step (FIG. 3) have been investigated. For instance, the atomic ratio n.sub.Ga/n.sub.Au (i.e. the ratio between the number of gallium atoms and the number of gold atoms in the film 43) offering the best results in terms of elevated conductivity and stretchability of the biphasic film 43 has been investigated. In this respect, a ratio n.sub.Ga/n.sub.Au2, more particularly corresponding to 13, has revealed to offer the best results.

(20) The most convenient thickness for the alloying solid layer or film 40 has been investigated too; in this respect, for each of Au, Pt, Pd and Ir a thickness of about 60 nm (10 nm and 1000 nm) offered the best electromechanical properties and/or results.

(21) According to the present invention, during an optional further step (not depicted in the drawings), the biphasic film 43 may be patterned so as to obtain the desired conductive arrays and/or paths, films, lines or the like wherein for the purpose of patterning the film 43, for instance one or both of stencil and photolithography may be used. Still according to the present invention, and depending on the needs and/or circumstances, the biphasic film or array 43 can be encapsulated during a further step (not depicted in the drawings), for instance by spin-coating a further PDMS layer.

(22) Still according to the present invention and depending on the needs and/or circumstances, during a further optional step not depicted in the drawings, one or more through vias can be formed in the upper encapsulation layer so as to expose one or more portions of the conductive biphasic film or array 43, wherein the exposed portions can be used for instance as contacting pads for electrical connection of the film 43, for instance wiring connection or stacking and connecting multiple metallized layers.

(23) The above mentioned patterning and/or encapsulation and/or 15 wiring steps are not disclosed in detail for the sake of conciseness.

(24) In the following, reference is made to FIGS. 7, 7a, 8, 8a, 9, 9a, 10 and 10a, wherein FIGS. 7a, 8a, 9a and 10a show enlarged view of portions of the biphasic film depicted in FIGS. 7, 8, 9 and 10, respectively. Moreover, in FIGS. 7, 7a, 8 and 8a the biphasic film showed therein is not subjected to any strain, whilst to the contrary, in FIGS. 9, 9a, 10 and 10a the biphasic film is subjected to a strain amounting to 50%.

(25) Moreover, it has to be noted that, within the meaning of the following disclosure, a 50% applied strain has to be understood as meaning that a 50% strain was applied to the elastomeric substrate underlying the biphasic film, for instance the substrate depicted in FIG. 1.

(26) The real SEM images of the drawings relate to a AuGa.sub.2 biphasic 30 conductive film with a ratio n.sub.Ga/n.sub.Au corresponding to 13 formed according to the present invention. Comparing the images 9, 9a, 10 and 10a with the images 7, 7a, 8 and 8a, respectively, confirms that, in the case of a biphasic conductive film according to the present invention, when the biphasic film is subjected to a strain, for instance when the film is stretched, the liquid metal (for instance gallium) is able to flow and fill in the cracks eventually induced by the stretch. In particular, no degradations of its electrical performance (essentially its conductivity) arise up to 80% mechanical strain.

(27) It has therefore been demonstrated with the above description that biphasic films and manufacturing methods thereof according to the present invention allow to obtain the wished results, thus overcoming the drawbacks affecting the prior art conductive films (either solid or liquid) and manufacturing methods thereof.

(28) It has in particular been demonstrated by means of the above disclosure that according to the present invention, both stretchable and non-stretchable conductive interconnections (in particular conductive micro interconnections) may be formed. By way of example, as described, stretchable interconnections may be formed by forming the biphasic film according to the present invention on a stretchable substrate, for instance a rubber substrate, in particular a PDMS substrate. However, non-stretchable interconnections may be formed as well, for instance by forming the biphasic film on a non-stretchable substrate.

(29) Whilst the present invention has been clarified by means of the above description of its embodiments as depicted in the drawings, the present invention is not limited to the embodiments depicted in the drawings and/or described above.