METAL WIRING AND CONDUCTIVE SHEET BOTH EXCELLENT IN BENDING RESISTANCE, AND METAL PASTE FOR FORMING THE METAL WIRING

Abstract

The present invention relates to a metal wiring, to be formed on a flexible substrate, including a sintered body of silver particles. The sintered body constituting the metal wiring has a volume resistivity of 20 μΩ.Math.cm or less, hardness of 0.38 GPa or less, and a Young's modulus of 7.0 GPa or less. A conductive sheet provided with the metal wiring can be produced by applying/calcinating, on a substrate, a metal paste containing, as a solid content, silver particles having prescribed particle size and particle size distribution, and further containing, as a conditioner, an ethyl cellulose having a number average molecular weight of 10,000 or more and 90,000 or less. The metal wiring of the present invention is excellent in bending resistance with change in electrical characteristics suppressed even through repetitive bending deformation.

Claims

1. A metal wiring, to be formed on a flexible substrate, comprising a sintered body of silver particles, wherein the sintered body has a volume resistivity of 20 μΩ.Math.cm or less, hardness of 0.38 GPa or less, and a Young's modulus of 7.0 GPa or less.

2. The metal wiring according to claim 1, wherein an average particle size of the silver particles is 100 nm or more and 200 nm or less.

3. The metal wiring according to claim 1, wherein a standard deviation of a particle size of the silver particles is 40 nm or more and 120 nm or less.

4. The metal wiring according to claim 1, wherein the sintered body has a thickness of 1 μm or more and 20 μm or less.

5. A conductive sheet, comprising a flexible substrate, and a metal wiring formed on at least one surface of the substrate, wherein the metal wiring defined in claim 1 is formed as the metal wiring.

6. A metal paste to be used for forming the metal wiring defined in claim 1, comprising a solid content comprising silver particles, a solvent, a conditioner, and an optional organic additive, comprising the solid content including silver particles, the solvent, and the optional organic additive, wherein the solid content contains silver particles having an average particle size of 100 nm or more and 200 nm or less, and a standard deviation of a particle size of 40 nm or more and 120 nm or less, in the silver particles constituting the solid content, at least one amine compound having 4 or more and 8 or less carbon atoms is bound as a protective agent, and the conditioner is an ethyl cellulose having a number average molecular weight of 10,000 or more and 90,000 or less.

7. The metal paste according to claim 6, wherein the conditioner is an ethyl cellulose having a number average molecular weight of 10,000 or more and 60,000 or less.

8. The metal paste according to claim 7, further comprising, as the conditioner, an ethyl cellulose having a number average molecular weight over 60,000 and 90,000 or less.

9. The metal wiring according to claim 2, wherein a standard deviation of a particle size of the silver particles is 40 nm or more and 120 nm or less.

10. The metal wiring according to claim 2, wherein the sintered body has a thickness of 1 μm or more and 20 μm or less.

11. The metal wiring according to claim 3, wherein the sintered body has a thickness of 1 μm or more and 20 μm or less.

12. A conductive sheet, comprising a flexible substrate, and a metal wiring formed on at least one surface of the substrate, wherein the metal wiring defined in claim 2 is formed as the metal wiring.

13. A conductive sheet, comprising a flexible substrate, and a metal wiring formed on at least one surface of the substrate, wherein the metal wiring defined in claim 3 is formed as the metal wiring.

14. A conductive sheet, comprising a flexible substrate, and a metal wiring formed on at least one surface of the substrate, wherein the metal wiring defined in claim 4 is formed as the metal wiring.

15. A metal paste to be used for forming the metal wiring defined in claim 2, comprising a solid content comprising silver particles, a solvent, a conditioner, and an optional organic additive, comprising the solid content including silver particles, the solvent, and the optional organic additive, wherein the solid content contains silver particles having an average particle size of 100 nm or more and 200 nm or less, and a standard deviation of a particle size of 40 nm or more and 120 nm or less, in the silver particles constituting the solid content, at least one amine compound having 4 or more and 8 or less carbon atoms is bound as a protective agent, and the conditioner is an ethyl cellulose having a number average molecular weight of 10,000 or more and 90,000 or less.

16. A metal paste to be used for forming the metal wiring defined in claim 3, comprising a solid content comprising silver particles, a solvent, a conditioner, and an optional organic additive, comprising the solid content including silver particles, the solvent, and the optional organic additive, wherein the solid content contains silver particles having an average particle size of 100 nm or more and 200 nm or less, and a standard deviation of a particle size of 40 nm or more and 120 nm or less, in the silver particles constituting the solid content, at least one amine compound having 4 or more and 8 or less carbon atoms is bound as a protective agent, and the conditioner is an ethyl cellulose having a number average molecular weight of 10,000 or more and 90,000 or less.

17. A metal paste to be used for forming the metal wiring defined in claim 4, comprising a solid content comprising silver particles, a solvent, a conditioner, and an optional organic additive, comprising the solid content including silver particles, the solvent, and the optional organic additive, wherein the solid content contains silver particles having an average particle size of 100 nm or more and 200 nm or less, and a standard deviation of a particle size of 40 nm or more and 120 nm or less, in the silver particles constituting the solid content, at least one amine compound having 4 or more and 8 or less carbon atoms is bound as a protective agent, and the conditioner is an ethyl cellulose having a number average molecular weight of 10,000 or more and 90,000 or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0091] FIG. 1 is a diagram for explaining a curvature radius (R) in bending deformation of a conductive sheet.

[0092] FIG. 2 is a diagram illustrating a result (relation between the number of times of bending and a resistance value) obtained in a repetitive bending test performed on a metal wiring produced in First Embodiment.

[0093] FIG. 3 illustrates SEM photographs of cross-sectional structures of metal wirings No. 1 to No. 6 of Second Embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0094] First Embodiment: A preferred embodiment of the present invention will now be described. In the present embodiment, silver particles having appropriate average particle size and particle size distribution were produced, and a metal paste containing the silver particles and a low molecular weight ethyl cellulose dispersed in a solvent was produced. This metal paste was used for forming a metal wiring on a resin substrate, and bending resistance was evaluated.

[0095] [Production of Silver Particles]

[0096] As a silver compound used as a raw material, 102.2 g of silver carbonate (silver content: 80.0 g) was used. The silver compound was prepared in a wet state with 37.3 g of water (36.4% by weight with respect to 100 parts by mass of silver carbonate) added thereto. To the silver compound, 3-methoxypropylamine (in a 6-fold amount in a molar ratio of the silver mass in the silver compound) was added as an amine compound of a protective agent to produce a silver-amine complex. The silver compound and the amine were mixed at room temperature, and an area of an uncomplexed portion of the silver compound was properly reduced.

[0097] In the silver-amine complex, water was added if necessary in consideration of a water content. A water content in a reaction system was checked before heating. In the reaction system in which the water content had been checked, the silver-amine complex was decomposed by heating from room temperature to precipitate silver particles. As for a heating temperature at this point, a decomposition temperature of the complex was assumed as 110 to 130° C., which was set as an achieving temperature. Besides, a heating rate was set to 10° C./min. In the heating process, generation of carbon dioxide was found from the vicinity of the decomposition temperature. The heating was continued until the generation of carbon dioxide was stopped.

[0098] After the heating process, in cooling a reaction solution to room temperature after stopping the heating, the temperature was restored to room temperature with the temperature controlled to obtain a cooling rate of about 0.4° C./min. After the precipitation of silver particles, methanol was added to the reaction solution for washing, and the resultant was centrifuged. The washing and centrifugation were performed twice.

[0099] [Production of Metal Paste]

[0100] Texanol used as a solvent was kneaded with the silver particles produced as described above, and a low molecular weight ethyl cellulose was further added thereto to produce a metal paste (silver paste). As the low molecular weight ethyl cellulose, a commercially available ethyl cellulose (ETHOCEL(registered trademark) STD7, manufactured by The Dow Chemical Company (number average molecular weight: 17,347)) was used. The content of the silver particles was set to 70% by mass with respect to the entire paste, and the content of the low molecular weight ethyl cellulose was set to 1.95% by mass with respect to the entire paste.

[0101] The metal paste produced in the present embodiment was measured for the average particle size and the particle size distribution of the silver particles. In this measurement, the metal paste was appropriately sampled for SEM observation, and in a SEM image thus obtained, respective particle sizes of 500 silver particles were measured by a biaxial average method to calculate an average value (median diameter) and a standard deviation. The silver particles of the metal paste produced in the present embodiment had an average particle size of 120 nm, and a standard deviation of 71.3 nm.

[0102] [Production of Conductive Sheet]

[0103] The thus produced metal paste was used to produce a conductive sheet. As a substrate, a transparent resin substrate (dimension: 150 mm×150 mm, 38 μm in thickness) constituted of polyethylene terephthalate (PET) was used. The metal paste produced as described above was screen printed on the substrate through a SUS screen mask. Thereafter, the resultant was levelled for 10 minutes, and calcinated at 120° C. for 1 hour, and thus, a conductive sheet provided with a metal wiring of a sintered body of the silver particles was produced. In the present embodiment, a plurality of screen masks were used to form metal wirings (60 mm in length) each having a line width of 0.1 mm, 0.2 mm or 0.5 mm in parallel on the substrate. It is noted that the metal wirings all had a thickness of about 4.1 μm.

[0104] [Repetitive Bending Test]

[0105] Each conductive sheet (line width: 0.1 mm, 0.2 mm, or 0.5 mm) of the present embodiment produced as described above was subjected to a repetitive bending test to examine bending resistance of the metal wiring. In the repetitive bending test, in an arbitrary one wiring on the produced conductive sheet, terminals were connected to positions 10 mm away from one end and 10 mm away from the other end. In the repetitive bending test, a resistance value of the metal wiring before the test was first measured with a digital tester.

[0106] Then, with a curvature radius of bending deformation set to 1.0 mm, the bending deformation was applied along a center line of the substrate of the conductive sheet. In the present embodiment, the bending deformation was applied with the metal wiring disposed inside as illustrated in FIG. 1. When a bent state in which the film was stretched into a V-shape was obtained, the number of times of bending was counted as once, and the bending deformation was applied for 100,000 times. In this repetitive bending test, a resistance value of the metal wiring was measured with a digital tester every 20,000 times of bending. A graph plotting the number of times of bending and a change of the resistance value obtained in this repetitive bending test is illustrated in FIG. 2.

[0107] As is understood from FIG. 2, the metal wiring produced in the present embodiment was little changed in the resistance value even after the repetitive bending deformation (curvature radius: 1.0 mm) was applied 100,000 times, and exhibited good bending resistance. Specifically, the maximum resistance value (R.sub.max) observed through the deformation process of 100,000 times was about 1.4 times as large as a resistance value (R.sub.1) obtained at an initial stage of the test. It was confirmed that this excellent bending resistance was exhibited even if the metal wiring had a fine line width of 0.1 mm.

[0108] Second Embodiment: In the present embodiment, 6 types of silver particles respectively having average particle sizes of 80 nm to 180 nm (respectively referred to as lots a to f; the silver particles of First Embodiment corresponding to the lot c), and two types of silver particles having a sharp particle size distribution with a small standard deviation (respectively referred to as lots g and h) were produced to produce metal pastes. In the production of these silver particles, for adjusting the average particle sizes (in the lots a to f), the amount of a water content to be added was changed in the production process of First Embodiment to adjust the particle sizes. Besides, for adjusting the standard derivation of the particle size (in the lots g and h), a reaction tank was rapidly cooled with cold water in decreasing the temperature after the heating process in the production process of First Embodiment. The average particle sizes and the standard deviations of the particle sizes of the silver particles produced in the present embodiment are shown in Table 1 below.

TABLE-US-00001 TABLE 1 Ag Particle Average Particle Standard Lot Size (nm) Deviation (nm) a 80 44.5 b 100 41.9 c 120 71.3 d 140 78.3 e 160 77.4 f 180 107.1 g 80 34.5 h 120 32.1

[0109] Then, each of the 8 types of silver particles described above was dispersed together with a conditioner in a solvent to produce a metal paste. In the present embodiment, in the production of the metal pastes, metal pastes different in the amount of the silver particles to be mixed, and in the type of conditioner (one of, or a combination of both of a low molecular weight ethyl cellulose having a number average molecular weight of 10,000 or more and 60,000 or less, and a high molecular weight ethyl cellulose having a number average molecular weight over 60,000 and 90,000 or less) were produced.

[0110] Then, each of the plurality of metal pastes thus produced was used to form a metal wiring to produce a conductive sheet, and bending resistance was evaluated. The metal wiring was produced in the same manner as in First Embodiment. As for the line width, however, the metal wiring was produced in a line width of 0.1 mm, which was the most severe condition in the evaluation of bending resistance. The evaluation test for bending resistance was performed in the same manner as in First Embodiment. In some examples, a conductive sheet was produced with a calcinating temperature employed after applying the metal paste set to 150° C.

[0111] For evaluation of a test result, a ratio (R.sub.max/R.sub.i) of a maximum resistance (R.sub.max) obtained during the test to a resistance value (R.sub.i) obtained at the initial stage of the test was calculated, and a metal wiring having R.sub.max/R.sub.i of 1.5 or less was determined as “excellent”, a metal wiring having R.sub.max/R.sub.i of 1.5 or more and 2.0 or less was determined as “good”, a metal wiring having R.sub.max/R.sub.i of 2.0 or more and 5.0 or less was determined as “fair”, and a metal wiring having R.sub.max/R.sub.i of 5.0 or more was determined as “poor”. It is noted that a metal wiring disconnected during the 100,000 times of bending was also determined as “poor”. The evaluation results of the bending resistance of the various metal wirings produced in the present embodiment are shown in Table 2.

TABLE-US-00002 TABLE 2 Metal Paste Ag Particles Physical Properties of Metal Average Wiring (line width: 0.1 mm) Ag Particle Standard Ethyl Cellulose Calcinating Young's Evaluation of Test content Size Deviation Addition Temperature Thickness Hardness Modulus Bending Test No. lot (wt %) (nm) (nm) Type *.sup.1 Amount *.sup.2 (° C.) (μm) (GPa) (GPa) (R = 1.0 mm) 1 a 70 80 44.5 Standard 1 120 5.0 0.48 6.70 poor 2 b 100 41.9 4.8 0.38 6.90 fair 3 c 120 71.3 4.9 0.36 7.00 good 4 d 140 78.3 4.6 0.25 5.50 excellent 5 e 160 77.4 4.4 0.27 5.50 excellent 6 f 180 107.1 4.3 0.29 5.90 excellent 7 c 120 71.3 STD200 5.0 0.31 6.70 fair 8 STD100 5.9 0.32 6.10 good 9 MIX A 6.1 0.32 6.50 good 10 STD7 4.1 0.26 5.80 excellent*.sup.3 11 STD50 5.4 0.36 6.90 good 12 MIX B 6.1 0.38 6.90 good 13 g 80 34.5 Standard 4.0 0.52 7.70 poor 14 h 120 32.1 6.5 0.36 7.80 poor 15 c 120 71.3 — 0 5.5 0.12 5.20 poor*.sup.4 16 Standard 0.3 3.9 0.18 5.20 fair 17 0.5 4.2 0.27 5.40 excellent 18 0.7 3.9 0.26 5.40 excellent 19 1.1 5.2 0.32 5.70 good 20 b 75 100 41.9 1 11.0 0.36 6.40 good 21 c 120 71.3 10.8 0.35 7.00 good 22 f 180 107.1 15.8 0.38 6.30 good 23 c 70 120 71.3 1 150 4.7 0.34 6.10 good *.sup.1 Number average molecular weights of ethyl celluloses are as follows: STD200: number average molecular weight: 80,733 STD100: number average molecular weight: 63,420 STD50: number average molecular weight: 56,489 STD7: number average molecular weight: 17,347 Standard: mixture of STD100 and STD7 (C.sub.H/C.sub.L = 6.8) MIX A: mixture of STD100 and STD50 (C.sub.H/C.sub.L = 1.0) MIX B: mixture of STD100 and STD20 (number average molecular weight: 38,984) (C.sub.H/C.sub.L = 9.0) *.sup.2 The addition amount of ethyl cellulose is shown as a relative amount assuming the addition amount of No. 1 is 1. The addition amount of ethyl cellulose of No. 1 (Standard) is STD100: 1.70 wt % + STD7: 0.25 wt % *.sup.3Test No. 10 corresponds to the result of First Embodiment (using the Ag particles of the lot c). *.sup.4The resistance value could not be measured because the metal wiring was broken to be disconnected through bending applied once.

[0112] It was confirmed, based on Table 2, that a sintered body (metal wiring) using a metal paste obtained by adding, as a conditioner, an ethyl cellulose having a number average molecular weight of 10,000 or more and 90,000 or less with optimizing the average particle size and the standard deviation of the particle size of silver particles could exhibit good bending resistance. In addition, it is deemed that the resistance value can be retained through repetitive bending deformation when both the hardness and the Young's modulus of the metal wiring are equal to or less than prescribed values.

[0113] Referring to the respective constitutions of the metal paste in detail, the metal wiring of the metal paste No. 1 using the silver particles having an average particle size less than 100 nm (80 nm) had too high hardness, and hence was inferior in bending resistance. Besides, the metal pastes containing the silver particles having a small standard deviation of the particle size and having even particle sizes provide metal wirings having high hardness and Young's modulus, and low bending resistance (Nos. 13 and 14). These trends cannot be improved even when the average particle size of the silver particles is optimized.

[0114] As for the ethyl cellulose used as the conditioner, the metal wiring using the metal paste containing a low molecular weight ethyl cellulose (number average molecular weight: 60,000 or less) had particularly good bending resistance (No. 10; First Embodiment). Even in using the metal paste containing only a high molecular weight ethyl cellulose (number average molecular weight: over 60,000), the effect of improving the bending resistance of the metal wiring could be probably obtained by optimizing the average particle size and the standard deviation of the silver particles (Nos. 7 and 8). The metal wirings formed using the metal pastes containing both the low molecular weight ethyl cellulose and the high molecular weight ethyl cellulose exhibited good bending resistance (Nos. 2 to 6, and Nos. 9 and 12).

[0115] The metal wiring using the metal paste not containing an ethyl cellulose as the conditioner (No. 15) had low hardness, and showed a result of particularly poor bending resistance. In the conductive sheet of this No. 15, the wiring was found broken to be disconnected through bending applied once. On the contrary, in the other conductive sheets evaluated as “poor” in the bending test (Nos. 1, 13 and 14), the resistance values of the wirings were increased after bending, but the wirings were not disconnected. Considering these results, it is deemed that the ethyl cellulose contained in the metal paste was an essential constitution from the viewpoint of preventing wiring disconnection through bending. In order to suppress variation of the resistance value through bending, it is necessary to adjust the particle size and the particle size distribution of the silver particles with the ethyl cellulose added to the metal paste.

[0116] Besides, as for the content of the ethyl cellulose, assuming that the addition amounts of No. 1 etc. (1.70% by mass+0.25% by mass) are regarded as a standard addition amount, it is deemed, based on the result obtained by a 0.3-fold addition amount (No. 16), that the content is preferably 0.50% by mass or more, and that a content larger than this is preferable for the addition (Nos. 17 to 19).

[0117] It is noted that the bending resistance could be secured also when the content of the silver particles in the metal paste was increased to form the metal wiring of 10 μm or more (Nos. 20 to 22). Besides, when an appropriate metal paste was used, the bending resistance of the resultant metal wiring was good even when the calcinating temperature was 150° C. (No. 23).

[0118] [Cross-Sectional Structure of Metal Wiring]

[0119] FIG. 3 illustrates photographs, obtained through SEM observation, of cross-sectional structures of the metal wirings No. 1 to No. 6 (respectively using the silver particles of the lots a to f). As is understood from FIG. 3, the metal wiring of the silver particles having an average particle size of 80 nm was formed by a dense sintered body, and included a very small number of voids. As the average particle size of the silver particles was increased, the number of voids was increased, but when the average particle size was larger than 140 nm (No. 4), it seems that there was not a large difference in the average particle size and the number (area) of voids. When the cross-sectional structure was similarly observed in other metal wirings in addition to these metal wirings No. 1 to No. 6, it was observed that the cross-sectional structure of a metal wiring having a good result of the repetitive bending test (evaluated as good or excellent) had similar voids.

[0120] Third Embodiment: In the present embodiment, the conductive sheets of Nos. 4 to 6, 8, 10, 11 and 21, which showed good results in the repetitive bending test (R=1.0 mm) of Second Embodiment, were subjected to the repetitive bending test under a more severe condition of a curvature radius R of 0.5 mm. The constitutions of the respective conductive sheets (metal wirings) and the other test conditions were the same as those employed in Second Embodiment. Results are shown in Table 3.

TABLE-US-00003 TABLE 3 Metal Paste Ag Particles Physical Properties of Metal Average Wiring (line width: 0.1 mm) Ag Particle Standard Ethyl Cellulose Young's Evaluation of Test Content Size Deviation Addition Thickness Hardness Modulus Bending Test No. lot (wt %) (nm) (nm) Type *.sup.1 Amount *.sup.2 (μm) (GPa) (GPa) (R = 0.5 mm) 24 d 70 140 78.3 Standard 1 4.6 0.25 5.50 excellent 25 e 160 77.4 4.4 0.27 5.50 excellent 26 f 180 107.1 4.3 0.29 5.90 good 27 c 120 71.3 STD100 5.9 0.32 6.10 fair 28 STD7 4.1 0.26 5.80 excellent 29 STD50 5.4 0.36 6.90 good 30 75 Standard 10.8 0.35 7.00 fair *.sup.1 Number average molecular weights of ethyl celluloses are as follows: STD100: number average molecular weight: 63,420 STD50: number average molecular weight: 56,489 STD7: number average molecular weight: 17,347 Standard: mixture of STD100 and STD7 (C.sub.H/C.sub.L = 6.8) *.sup.2 The addition amount of ethyl cellulose is shown as a relative amount assuming the addition amount of No. 1 is 1. The addition amount of ethyl cellulose of No. 1 (Standard) is STD100: 1.70 wt % + STD7: 0.25 wt %

[0121] It is understood, from Table 3, that the metal wirings of Nos. 24, 25 and 28 (Nos. 4, 5 and 10) exhibited good bending resistance with the resistance values little varied through the bending deformation under the very severe condition of the curvature radius of 0.5 mm (R=0.5 mm). Referring to No. 27 (No. 8), in the metal wiring using the metal paste in which the high molecular weight ethyl cellulose alone was added as the conditioner, change in the resistance value increased by making severe the condition of the bending test. Even when the high molecular weight ethyl cellulose was contained, however, the severe bending deformation could be coped with by containing also the low molecular weight ethyl cellulose (Nos. 24 and 25 (Nos. 4 and 5)). While in the case where the content of the silver particles in the metal paste was rather high (75% by mass), the bending resistance tended to decrease through bending deformation with a very small curvature radius (No. 30 (No. 21)). It is deemed that a metal paste having a large silver particle content is suitable for forming a thick wiring, but consideration should be paid to the degree of bending deformation to be applied to a conductive sheet.

INDUSTRIAL APPLICABILITY

[0122] As described so far, a metal wiring including a sintered body of silver particles of the present invention is good in bending resistance, and is little changed in electrical characteristics even through repetitive bending deformation. As for this bending deformation, very severe deformation with a curvature radius of 0.5 mm can be endured. A conductive sheet of the present invention is provided with a very fine metal wiring, has good bending resistance, and is applicable to a constituent member of a foldable display or a wearable device.