FORMULATIONS AND PROCESSES FOR PRODUCING HIGHLY CONDUCTIVE COPPER PATTERNS

Abstract

Provided are formulations and processes for obtaining conductive patterns of copper onto a substrate.

Claims

1.-56. (canceled)

57. An ink formulation comprising copper nanoparticles, at least one copper-oxidizing agent, and CuH.

58. The ink formulation of claim 57, comprising at least 0.0001 wt % of CuH.

59. The ink formulation of claim 57, wherein said copper-oxidizing agent is selected from organic acids, inorganic acids and anhydrides, alcohols, aldehydes, and hydroxyamines.

60. The ink formulation of claim 59, wherein the inorganic acid or anhydride is a phosphorous-containing compound.

61. The ink formulation of claim 60, wherein the phosphorous-containing compound is selected from hypophosphorous acid, phosphorous acid, phosphoric acid, pyrophosphoric acid (H.sub.4P.sub.2O.sub.7), tripolyphosphoric acid (H.sub.5P.sub.3O.sub.10), tetrapolyphosphoric acid (H.sub.6P.sub.4O.sub.13), trimetaphosphoric acid (H.sub.3P.sub.3O.sub.9), phosphoric anhydride (P.sub.4O.sub.10), polyphosphoric acid, hypophosphoric acid (H.sub.4P.sub.2O.sub.6), pyrophosphorous acid (H.sub.4P.sub.2O.sub.5), and metaphosphorous acid (HPO.sub.2), and mixtures thereof.

62. The ink formulation of claim 61, wherein the phosphorous-containing compound is hypophosphorous acid (HPA).

63. The ink formulation of claim 57, wherein the formulation comprises between about 10 and 90 wt % of copper nanoparticles.

64. The ink formulation of claim 57, wherein the formulation comprises between about 0.001 and 20 wt % of said copper-oxidizing agent.

65. The ink formulation of claim 57, wherein the ratio between the copper-oxidizing agent and the copper nanoparticles is between about 0.001 and about 0.2 (wt/wt).

66. The ink formulation of claim 57 being in the form of a dispersion or a paste.

67. A kit for preparing an ink formulation of claim 57, comprising: a first container comprising a dispersion or a paste of copper nanoparticles in a liquid carrier; and a second container comprising a solution of at least one copper-oxidizing agent.

68. A printed pattern comprising the ink formulation of claim 57.

69. A sintered printed pattern comprising copper and up to 0.1 mol % phosphorous.

70. A process for obtaining a pattern onto a substrate, the process comprising: (a) applying a dispersion or a paste that comprises copper nanoparticles in a liquid carrier onto at least a surface region of the substrate; (b) applying at least one copper-oxidizing agent to said substrate; (c) permitting at least a portion of the copper nanoparticles to react with said copper-oxidizing agent for transforming Cu.sup.0 into CuH, thereby obtaining a pattern of an ink.

71. The process of claim 70, wherein step (b) is carried out before step (a).

72. A process for obtaining a conductive copper pattern onto a substrate, the process comprising: printing an ink formulation of claim 57 onto at least a surface region of said substrate to obtain a pattern-bearing substrate; exposing said pattern-bearing substrate to conditions permitting decomposition of CuH and sintering of copper, said exposing being for a period of time of between about 0.01 and 600 seconds, to thereby obtain a conductive copper pattern.

73. A process for obtaining a conductive copper pattern onto a substrate, the process comprising: (i) printing a dispersion or a paste that comprises copper nanoparticles in a liquid carrier onto at least a surface region of the substrate; (ii) applying at least one copper-oxidizing agent to said substrate; (iii) permitting at least a portion of the copper nanoparticles to react with said copper-oxidizing agent for transforming Cu.sup.0 into CuH, to thereby obtain a pattern-bearing substrate, the pattern comprising the ink formulation of claim 57; (iv) exposing said pattern-bearing substrate to conditions permitting decomposition of CuH and sintering of copper, said exposing being for a period of time of between about 0.001 and 600 seconds, to thereby obtain a conductive copper pattern.

74. The process of claim 72, wherein said conditions permitting decomposition of CuH and sintering of copper comprise exposing said pattern-bearing substrate to a temperature of at least 125 C.

75. A conductive copper pattern having % bulk conductivity of at least 5%, the pattern obtained by the process of claim 72.

76. A conductive copper pattern substantially free of copper oxide, the pattern obtained by the process of claim 72.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0114] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0115] FIG. 1 shows the % bulk conductivity of sintered copper patterns obtained at different sintering temperatures after a 2-steps printing process in which HPA was applied by dipping.

[0116] FIG. 2 shows sintering for various periods of time, at a constant temperature of 150 C. in air after a 2-steps printing process in which HPA was applied by dipping.

[0117] FIGS. 3A-3D show SEM images of non-sintered printed pattern and patterns sintered at 150, 200 and 225 C., respectively.

[0118] FIG. 4 shows the results of an XRD analysis of a pattern sintered at 200 C.

[0119] FIG. 5 shows sheet resistance as a function of HPA/Cu weight ratio for sintered patterns prepared from 60 wt % Cu paste.

[0120] FIG. 6 shows an E52 antenna printed on paper using formulations of this disclosure.

[0121] FIG. 7 shows the results of an EDX analysis of a pattern sintered at 150 C., with Cu and P peaks.

DETAILED DESCRIPTION OF EMBODIMENTS

[0122] In the following examples, Cu nanoparticles (NPs) were used. Methods for preparing such nanoparticles are known, for example from [12].

EXAMPLE 1

Single-Step Printing

[0123] Single-Step Printing Ink Formulations

[0124] A copper NPs dispersion was washed in an ultrafiltration membrane (CO=100 KDa, PES). In some formulations, the water was exchanged with various liquid carriers to obtain formulations having 30-60 wt % copper in the liquid carrier. The obtained red color ink was easily filtered through a 1 m syringe filter. Hypophosphorous acid (HPA) was added to the mixtures for obtaining formulations suitable for single-step printing, as detailed in Table 1.

[0125] In all of the formulations below the addition of HPA induced formation of CuH.

TABLE-US-00001 TABLE 1 formulations for single-step printing Formula Cu Acid # (wt %) (wt %) Carrier Binder Form 1 NPs HPA water Liquid (30%) (0.1%) dispersion 2 NPs HPA dipropylene 1 wt % Liquid (30%) (0.1%) glycol methyl polyvinyl dispersion ether pyrrolidone (MW = 40,000) 3 NPs HPA dipropylene 1 wt % Liquid (30%) (0.6%) glycol methyl polyvinyl dispersion ether butyral 4 NPs HPA dipropylene Paste (60%) (2%) glycol methyl ether 5 NPs HPA dipropylene 1 wt % Paste (60%) (2%) glycol methyl polyvinyl ether butyral 6 NPs HPA dipropylene Paste (30%) (2%) glycol methyl 1-5 m ether MPs (30%) 7 NPs HPA dipropylene 1 wt % Paste (30%) (2%) glycol methyl polyvinyl 1-5 m ether butyral MPs (30%) 8 NPs HPA Terpineol 2 wt % Paste (60%) (15%) ethyl cellulose 9 NPs HPA Water/glycerol 2 wt % Paste (70%) (15%) mixture polyvinyl pyrrolidone (MW = 36,000) 10 NPs HPA dipropylene Paste (52%) (1.4%) glycol methyl ether 11 NPs HPA dipropylene Paste (45%) (10%) glycol methyl ether 12 NPs HPA dipropylene Paste (32%) (8.7%) glycol methyl MPs ether (26%) 13 NPs HPA dipropylene Paste (64%) (1.6%) glycol methyl ether 14 NPs HPA Diethylene Paste (64%) (1.6%) Glycol n-Butyl Ether 15 NPs HPA Terpineol Paste (64%) (1.6%) 16 NPs HPA dipropylene Paste (64%) (0.7%) glycol methyl ether 17 NPs HPA Diethylene Paste (64%) (0.7%) Glycol n-Butyl Ether 18 NPs HPA Terpineol Paste (64%) (0.7%) 19 NPs HPA Terpineol Paste (64%) (2.5%) 20 NPs HPA water Paste (64%) (1.6%) 21 NPs HPA water Paste (64%) (2.5%) 22 NPs HPA water Inkjet ink (30%) (0.01%) 23 NPs HPA dipropylene 1 wt % Inkjet ink (30%) (0.01%) glycol methyl polyvinyl ether pyrrolidone (MW = 40,000) 24 NPs HPA dipropylene 1 wt % Inkjet ink (30%) (0.06%) glycol methyl polyvinyl ether butyral

[0126] Single-Step Printing

[0127] A dipropylene glycol methyl ether based dispersion of Example 1.1 with 30 wt % copper was mixed with HPA at a weight ratio 0.08 (copper/HPA). That ink formulation was inkjet-printed using a DMC dimatix inkjet head. The printing was found to be stable. The obtained patterns were sintered at 300 C. at air atmosphere and the resistance was measured. The calculation of the resistivity (according to the obtained line profile) led to 7 cm, which is equal to 24% of the copper bulk conductivity. XRD analysis showed 100% fcc copper in the obtained patterns with no oxides.

EXAMPLE 2

Two-Steps Printing

[0128] In the two-steps printing, the substrate was first printed with a dispersion of copper nanoparticles obtained by the synthesis described in example 1.1 in the carrier liquid to obtain a pattern. The pattern was dried at a temperature of 20-150 C. for a period of a few seconds up to a few minutes.

[0129] Then a solution of HPA is applied onto the pattern, allowed to react with the copper NPs for forming CuH, thereby forming a pattern of the ink formulation onto the substrate. Subsequent to formation of CuH, the pattern is heated, resulting in a sintered conductive copper pattern.

[0130] Formulations similar to those detailed in Table 1, however without the inorganic acid, may be used as a dispersion for printing the copper NPs' pattern onto the substrate.

[0131] 10-50 wt % solutions of the HPA were prepared by dissolving HPA in the liquid carrier, for example in dipropylene glycol methyl ether, and then applied onto the printed pattern to allow the oxidation of copper to CuH, and subsequently sintered.

[0132] Application of HPA by Printing

[0133] A solution of 10 wt % HPA in dipropylene glycol methyl ether was inkjet-printed onto a pre-printed copper NPs pattern. Then the pattern was heated to 140 C. for 10 seconds in air. Resistances of 0.05 to 1 (Ohms) were measured along 1 cm line.

[0134] Application of HPA by Dipping

[0135] Copper NPs printed patterns were dipped for 1-10 sec in a 50 wt % HPA/water solution. Then the pattern was washed for up to 1 min in water and dried at 60-150 C. for 5-60 sec. Then, the pattern was sintered for 1 sec to 20 minutes in air at a temperature of 125-500 C. FIG. 1 shows the change in electrical resistance as a function of sintering temperature for a liquid dispersion ink formulation printed on paper and then dipped in a 50 wt % HPA for 5 sec, washed for 30 sec in water and then sintered for 60 seconds in air. Without wishing to be bound by theory, at sintering temperatures higher than 120 C., CuH decomposes to form a local H.sub.2 atmosphere that prevents the formation of copper oxides and permits metal-metal interactions for sintering.

[0136] Application of HPA by Fumigation

[0137] Copper NPs printed patterns were placed in a container together with a 50 wt % HPA/water solution and heated to 20-130 C. As the temperature increased, more HPA vapor was formed, resulting in a shorter required exposure time in order to obtain CuH. The exposure time was in the range of 10 sec-3 hrs.

[0138] In an exemplary process, 30 wt % copper ink (as described in example 1.1) was used to print copper patterns on paper and Kapton (polyethylene imide). The patterns were exposed to HPA vapor at (i) 130 C. for 30 sec, or (ii) 50 C. for 3 hrs. After the exposure, the patterns were sintered at 250 C. for 2 sec. The obtained sheet resistance was less than 0.1 /sq.

[0139] Effect of Sintering Process Parameters on Conductivity

[0140] Effect of Sintering Temperature

[0141] As noted above, FIG. 1 shows the % bulk conductivity of sintered copper patterns obtained at different sintering temperatures; all samples were sintered for 60 seconds in air. As can be seen, values of at least 20% bulk-conductivity were obtained, and increased with increase in sintering temperature, seemingly due to the increased rate and extent of thermal decomposition of CuH.

[0142] Effect of Sintering Duration

[0143] FIG. 2 shows sintering for various periods of time, at a constant temperature of 150 C. in air. As evident from FIG. 2, sintering occurs within a few seconds (as fast as 2 seconds of exposure to 150 C.), as no significant change in %-bulk conductivity was observed over time. This also attests to a mechanism of sintering that does not involve the formation and decomposition of copper oxides, as higher temperatures (above 150 C., typically about 250-300 C.) and significantly longer sintering duration (at least 1 hour) are required for decomposition copper oxide.

[0144] FIGS. 3A-3D show, respectively, SEM images of non-sintered printed pattern and patterns sintered at 150, 200 and 225 C. These images show the formation of a continuous grid of copper particles, that results in a conductive sintered pattern. As evident from the XRD analysis shown in FIG. 4, the sintered pattern contains 100% metallic copper, with no detected copper oxides.

[0145] Effect of HPA/Cu Ratio

[0146] The effect of the HPA/Cu weight ratio on the obtained resistivity of a 60 wt % Cu paste prepared according to example 1 was evaluated.

[0147] That paste was screen printed on Kapton film and sintered at 300 C. for 2 sec.

[0148] As seen in FIG. 5, HPA/Cu weight ratio of 3-4% (i.e. HPA/Cu wt/wt=0.03-0.04) led to the lowest sheet resistance.

[0149] Stability of Sintered Patterns

[0150] Durability test results at 85% humidity and 85 C. revealed that the sintered patterns are stable. Without any coating, the resistivity increased by 44% after 408 hrs, and 26% with a sealant layer. These results indicate that the sintered patterns obtained from the formulations and processes of this disclosure are very stable, and very high conductivities were maintained even after exposure to extreme conditions.

[0151] NIR Sintering

[0152] Near infra-red (NIR) sintering of formulations of this disclosure was also evaluated. Lamps of 800 W were used to sinter samples prepared according to examples 3 and 4 in Table 1. The samples were exposed to the NIR lamp for 0.5 to 5 sec and yielded sheet resistance down to 0.07 /sq.

[0153] Such a sintering approach enables also selective sintering by exposing the sample to NIR lamp through a protection mask. Selective sintering was carried out by NIR using an aluminum foil mask placed on the printed pattern; it was found that only the unmasked area was sintered.

[0154] Similar selective sintering may be obtained by using laser scanning of a specific area of a printed pattern.

EXAMPLE 3

Printing Conductive Patterns on Various Substrates

[0155] The paste as describes in formulation #16 in example 1 was printed by screen printing on various substrates to form conductive lines:

[0156] (a) Paper

[0157] (b) Silicon wafer

[0158] (c) Silicon wafer coated by ITO

[0159] (d) Glass coated by ITO

[0160] (e) Kapton (polyimide film)

[0161] After printing, the patterns were dried for 30-120 sec on a hot plate (60-120 C.). Then, the printed patterns were sintered by inserting the dried patter between two hot plates (300 C.) for 5 sec. High conductivities of up to 30% bulk Cu were obtained.

EXAMPLE 4

Forming Various Working Devices

[0162] 4.1 RFID antenna: An E52 RFID antenna was printed on paper as described in example 3. Then, a chip was attached to the antenna and the performance of the antenna was characterized. It was found that the antenna performance is comparable to those of etched antennas with about 6.5 dBm. FIG. 6 shows a photo of such an E52 antenna printed on paper.

[0163] 4.2 NFC antenna: A standard NFC antenna was printed on paper as described in example 3. Then, an NFC chip was attached to the antenna. The NFC performance was evaluated by placing the device near a smartphone. The smartphone responded to the NFC and enabled the storage of data for example a link to a website.

[0164] 4.3 Solar cells front electrode: Conductive lines were printed on top of a heterojunction solar cell (Si wafer coated by ITO) as described in example 3.

[0165] 4.4 HDTV antennas: HDTV antennas were printed according to a specific design on paper as described in example 3.

EXAMPLE 5

Phosphor Detection

[0166] In order to detect phosphor in the sintered pattern, EDX analysis was carried out. The analysis was carried out for a pattern printed by formulation #21 (Table 1) and sintered at 150 C. for 30 sec.

[0167] The EDX results presented in FIG. 7 clearly indicate the presence of Copper (with a very large peak at 0.93 keV) and Phosphor (with a peak at 2.013 keV).