# Patent

## Title
Conductive layers and fabrication methods thereof

## Abstract

Methods for forming conductive layers. A layer of metal composite is applied on a substrate, comprising a plurality of metal flakes, a plurality of nanometer metal spheres, and a plurality of mixed metal precursors. The plurality of mixed metal precursors comprises a mixture of inorganic salts and organic acidic salts. The layer of metal composite is cured to induce an exothermic reaction, thereby forming a conductive layer on the substrate at a relatively low temperature (<200° C.).

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a division of U.S. patent application Ser. No. 11/002,509, filed Dec. 2, 2004, now U.S. Pat. No. 7,135,394.


BACKGROUND
The invention relates to conductive layers and fabrication methods thereof and, more particularly, to methods for fabricating high conductive layers at a relatively low curing temperature.
Typically, the thermal resist temperature or operative temperature for fabricating a conductive layer on substrates of conventional electronic products is less than 350° C. for polyimide substrates, 290° C. for printed circuit boards (PCBs), and lower than 200° C. for other plastic substrates. The curing temperature of conductive pastes for conventional low temperature products are constrained to the thermal resist temperature or operative temperature of the substrates. The resistivity of the conductive layer such as silver on the substrate is 10-50 times that of pure silver, resulting in more power consumption, signal loss, and reduction of transmission distance during signal transmission.
U.S. Pat. No. 6,036,889 (Kydd et. al.), the entirety of which is hereby incorporated by reference, discloses a method of fabricating a metal layer. A conductive paste comprising organic acidic salt and metal flakes is formed on a substrate. The organic acidic salts such as silver 2-ethylhexanoate (C8H15O2Ag) are decomposed at 250-350° C. to create interlinks between the metal flakes, thereby improving conductivity of the metal layer. Moreover, U.S. Pat. No. 6,036,889 (Kydd et. al.), the entirety of which is hereby incorporated by reference, further discloses a method of fabricating a metal layer. Organic acidic salts such as silver 2-ethylhexanoate (C8H15O2Ag) are decomposed at 250-350° C. to create interlinks between nanometer colloidal metal flakes, thereby improving conductivity of the metal layer.
The decomposition temperature of conventional organic acidic salts, however, is still relatively high, and cannot be held under 200° C. for use in fabrication of conductive layers on plastic substrates.
SUMMARY
Conductive layers and fabrication methods thereof employing relatively low curing temperature are provided. An exemplary embodiment of a conductive layer comprises metal flakes and nanometric metal spheres forming densified compact structure. The conductive layer further comprises inorganic salts and organic acidic salts. Decomposition of the inorganic salts can be exothermic at low temperature to induce another high exothermic reaction of the organic acidic salts, thereby forming continuous conductive layer skeletal network structure, at a temperature of less than 200° C. The resistivity of the conductive layer can achieve less than 7 μΩcm.
Some embodiments of the invention provide a method for forming a conductive layer on a substrate comprising applying a layer of metal composite on the substrate. The metal composite comprises a plurality of metal flakes, a plurality of nanometer metal spheres, and a plurality of mixed metal precursors. The layer of metal composite is cured to induce an exothermic reaction, thereby forming a conductive layer on the substrate at a relatively low temperature.
An exemplary embodiment of a conductive layer structure comprises a main layer formed on a substrate. The main layer comprises a mixture of a plurality of different aspect ratios of metal flakes and a plurality of different diameters of metal spheres. A continuous network skeleton is interposed within the main layer. The continuous network skeleton comprises a plurality of inorganic salts and a plurality of metal precursors.



DESCRIPTION OF THE DRAWINGS
Conductive layers and fabrication methods thereof will be better understood reference to the descriptions to be read in conjunction with the accompanying drawings, in which:
FIG. 1 is a schematic cross-section of an embodiment of a conductive layer on a substrate; and
FIG. 2 is graph illustrating curves of curing temperature dependent of energy release of exothermic reaction.



DETAILED DESCRIPTION
As the metal paste becomes thinner and thinner, the continuity of the metal powders must be good enough to ensure lower conductivity of the metal layer. Since the sintering temperature of the metal paste layer, such as silver, is higher than 450° C., low temperature sintered metal layer cannot reach high densification whiling maintaining high conductivity.
Referring to FIG. 1, a layer of metal paste 15 is provided on a substrate 10. The metal paste layer 15 comprises a main layer comprising a mixture of a plurality of different aspect ratios of metal flakes 12 such as in a range of approximately larger than 1 and less than 20 and a plurality of different diameters of metal spheres 14. A continuous network skeleton is interposed within the main layer. The network skeleton matrix comprises a plurality of inorganic salts and a plurality of metal precursors 16.
The plurality of metal precursors 16 comprises a mixture of organic acidic salts, such as silver 2-ethylhexanoate (C8H15O2Ag) and inorganic salts. The ratio of the mixture is shown in Table 1.The materials of the metal flakes 12 are selected from the group consisting of Cu, Pt, Ni, Ag, and Au. The ratio of the metal flakes in the mixture is in a range of 60-80%. The ratio of the metal spheres relative to the metal flakes is in a range of approximately 30-60%.







TABLE 1




















The preferable ratios of the mixture are shown in the gray area of the Table 1. When a mixture of 20% C8H15O2Ag and the ratios of Ag2C2O4/C8H15O2Ag are in a range of 5% to 35% at 190° C. 15 sec, the conductivity of the metal layer has relatively low resistivity.

An embodiment of a method for forming a conductive layer on a substrate at relative low temperature is provided. A layer of metal paste is applied on the substrate. The metal paste comprises a plurality of metal flakes, a plurality of nanometer metal spheres, and a plurality of mixed metal precursors. The layer of metal composite is cured to induce an exothermic reaction, thereby forming a conductive layer on the substrate at a relatively low temperature. The inorganic salt comprises a silver oxalate (Ag2C2O4). The organic acidic salt comprises a silver 2-ethylhexanoate (C8H15O2Ag).
FIG. 2 is graph illustrating curves of curing temperature dependent on the energy release of an exothermic reaction. In FIG. 2, curve 24 exerts exothermic effect 20 at 180° C., compared with conventional method curve 26 at 250° C. The reaction equations are:

Ag2C2O4→2Ag+2CO2+ΔH1  Eq. 1

AgC8H15O2+(43/4)O2+ΔH2→Ag+8CO2+(15/2)H2O  Eq. 2

In Eq. 1, Ag2C2O4 is reduced into silver and CO2, releasing heat ΔH1. In Eq. 2, C8H15O2Ag absorbs heat ΔH1 released from Eq. 1 and is reduced into silver, CO2, and H2O at a temperature less than 200° C. The overall reaction is an exothermic reaction.
The decomposition temperature of the mixture of metal precursors is reduced from 250° C. to 180° C. by adding Ag2C2O4 into C8H15O2Ag. The heat released from reduction of Ag2C2O4 can trigger decomposition of C8H15O2Ag at a temperature less than 200° C., thereby forming continuous silver layer with resistivity less than 7 μΩcm.
Conductive layers and fabrication methods thereof provide the following potential advantages. First, by mixing a plurality of different sizes of metal particles with a plurality of different sizes of metal flakes, the continuity of pure metal in the main layer can be improved. Second, using the mixture of the metal precursors, such as silver oxalate and long chain metal carboxyl compounds, can reduce decomposition at a temperature less than 200° C. Nanometric silver can be potentially reduced from the metal precursors, comprising excellent wieldability at relatively low temperature resulting in increasing the effective conductive traces of pure metal and the conductivity of metal film.
While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the inventions is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Thus, the scope of the appended claims should be accorded the broadest interpretations so as to encompass all such modifications and similar arrangements.

## Claims

1. A conductive structure, comprising:
a substrate;
a main layer formed on the substrate, comprising a mixture of a plurality of different aspect ratios of metal flakes and a plurality of different diameters of metal spheres; and
a continuous network skeleton interposed within the main layer, the network skeleton comprising a plurality of inorganic salts and a plurality of metal precursors,
wherein the plurality of metal precursors comprise a mixture of metal precursors, and ratio of the mixture of the metal precursors comprises:
3-35 wt % of inorganic metal salt relatively to long chain metal carboxyl groups, having decomposition temperature less than 200° C.; and
20-50 wt % of long chain metal carboxyl groups relatively to metal flakes, having decomposition temperature in a range of approximately 200-300° C.

2. The conductive layer structure as claimed in claim 1, wherein the aspect ratios are in a range of approximately larger than 1 and less than 20.

3. The conductive layer structure as claimed in claim 1, wherein the ratio of the metal flakes in the mixture is in a range of 60-80%.

4. The conductive layer structure as claimed in claim 1, wherein the diameters of the metal spheres are less than approximately 0.2 μm.

5. The conductive layer structure as claimed in claim 1, wherein the ratio of the metal spheres relatively to the metal flakes is in range of approximately 30-60%.

6. The conductive layer structure as claimed in claim 1, wherein the materials of the metal flakes are selected from the group consisting of Cu, Pt, Ni, Ag, and Au.

7. The conductive layer structure as claimed in claim 1, wherein the decomposition temperature of the metal precursors is in a range of approximately 150-300° C.

8. The method as claimed in claim 1, wherein the inorganic metal salt comprises a silver oxalate (Ag2C2O4).

9. The method as claimed in claim 1, wherein the long chain metal carboxyl groups comprise a silver 2-ethylhexanoate (C8H15O2Ag).