WO2010128107A1

WO2010128107A1 - Process for manufacturing conductive tracks

Info

Publication number: WO2010128107A1
Application number: PCT/EP2010/056189
Authority: WO
Inventors: Ko Hermans; Josué Jean Philippe VALETON
Original assignee: Neodec B.V.
Priority date: 2009-05-07
Filing date: 2010-05-06
Publication date: 2010-11-11

Abstract

The invention pertains to a process for manufacturing conductive tracks comprising a coating step in which an organometallic compound, defined by MX where M is metal cation and X is an organic anion, is applied from a solution onto a substrate, an exposure step, during which the organometallic compound is exposed to electromagnetic radiation, and a heating step.

Description

Process for manufacturing conductive tracks

Description:

The invention pertains to a process for the manufacturing of conductive tracks..

Conductive tracks are currently used as one of the main components in a wide variety of applications ranging from electronic circuitry (e.g. in computers and displays), and antennas to anti-static films. These tracks are made from conductive metals (e.g. copper, silver or gold), ceramics (e.g. ITO) or polymers (e.g. poly(3,4-ethylenedioxythiophene)) and can be manufactured via different methods in a wide range of sizes and shapes.

A conventional method to create conductive tracks comprises the steps of depositing a conductive layer on a substrate by chemical or physical vapor deposition, for example. Next, a photosensitive coating is deposited on top of the conductive layer and the coating is locally exposed to light. During the local exposure a difference in solubility is created in exposed and non-exposed areas of the photosensitive layer and the soluble areas are subsequently removed using an appropriate solvent. In a following step, the conductive layer is exposed to an aggressive etching fluid and the parts, which are not shielded by the remaining photosensitive coating, are removed. In a final step, the remaining photosensitive coating is removed by using another solvent. Although this method can be used to create well-defined conductive tracks with small features on a large variety of substrates, the process is comprehensive and very expensive. Furthermore, most of the processing steps can only be applied batch-wise and the technique is not suitable for roll-to-roll processing. To reduce the number of steps described in the previous process an alternative method is known from US 2003/207568, wherein a special organometallic compound as a photosensitive coating and a conductive metal precursor is disclosed. This compound is defined by the following formula MLL'X, where M is a metal, L is a neutral ligand, L' is negatively charged ligand and X is an anion. Said compound is applied from solution to a substrate and locally exposed to light through a photolithographic mask. The exposure is applied to locally decompose the compound into insoluble species and thus create a difference in solubility between the exposed and non-exposed areas. In a final step, the non-exposed, soluble, areas are removed by exposing the material to a suitable solvent. It is possible to improve the conductivity of the obtained pattern by applying a temperature annealing step or reducing step.

Another method is based on the concept of electroless plating. In this method, a substrate is exposed to several subsequent baths. First the substrate is etched and neutralized to ensure good adhesion and activated to ensure the absorption of a catalyst. Next it is exposed to a catalyst to promote the deposition of the conductive layer and exposed to an accelerator, which improves the deposition of the conductive layer. Finally the substrate is exposed to a solution of conductive material to create the conductive layer. This method can be used to create highly conductive tracks. However, the process is very complex and the solutions are often toxic and very aggressive. Furthermore, the method is very sensitive to the processing parameters and a slight deviation can result in spontaneous precipitation of the conductive material in non-desired areas.

A more convenient method to manufacture conductive tracks is based on the printing (e.g. ink-jet printing, offset printing, screen printing, or flexo printing) of conductive ink. These inks are usually made up from polymer solutions or nano- dispersions of colloidal metal particles. These methods have the advantage that the conductive tracks are created in the desired size and shape without any additional patterning steps. However, after applying the colloidal metal particles or polymers to the substrate a thermal treatment is required to make the applied materials conductive. In the case of the colloidal metal particles an organic coating, which is applied to the metal particles to form a stable dispersion, is removed and the particles are sintered together into densely packed conductive metal tracks. In the case of a polymer, the heat treatment is applied to remove any remaining solvent. These thermal treatments require relatively high temperatures (typically above 150 ⁰C) for a long period of time (typically longer than 30 minutes). These techniques are therefore not suitable when applying conductive tracks on most common polymer substrates. As a result, only inorganic (e.g. glass or silicium) or expensive specialized polymer substrates (e.g. polyimide) can be used. Furthermore, the conductivity of conductive polymers is relatively poor compared to their inorganic counterparts.

It is therefore an objective of the invention to provide a fast process to create conductive tracks, which is compatible with the above-mentioned printing techniques. The conductive tracks, created by this process, exhibit very high conductivity and excellent mechanical properties.

According to the invention this object is achieved by a process for the manufacturing of conductive tracks, comprising a coating step, in which an organometallic compound, defined by MX where M is metal cation and X is an organic anion, is applied from a solution onto a substrate, an exposure step, during which the organometallic compound is exposed to electromagnetic radiation, and a heating step.

Preferably a part of the organometallic compound is activated via locally exposing said compound to electromagnetic radiation.

Preferably the manufacturing is carried out at a temperature range between 0 ⁰C and 350 ⁰C, more preferably between 20 ⁰C and 200 ⁰C. More preferably the heating step is carried out at a temperature range between 50 ⁰C and 350 ⁰C, more preferably between 70 ⁰C and 200 ⁰C.

A conductive track is to be understood as a pattern of a conductive material, which can be in any shape or size, with the restriction that the conductivity of the track should be more than 1000 Siemens per meter (SIm). Said track can cover the complete substrate on which it is deposited, but also only a part of it. Furthermore said track can be considered as a single feature or plurality of features.

In the course of this invention an organometallic compound is to be understood as any compound consisting of a metal cation (M) and an organic anion (X). Although a variety of metals are known to form an organometallic compound, for the invention the following metals (M) either alone or in combination with each other are preferred: copper, aluminum, platinum, palladium, silver or gold. The organic part (X) of said compound should be large enough to allow sufficient dissolution in the preferred solvent. It should also be small enough not to have a limiting effect on the final properties of the conductive tracks. In a preferred embodiment of the invention the organometallic compound belongs to the class of metal carboxylates and/or metal thiolates, in which preferably the number of carbon atoms lies between 4 and 20. In a more preferred embodiment of the invention the organometallic compound belongs to the class of metal carboxylates, in which preferably the number of carbon atoms lies between 8 and 12, and in which the carbon backbone is branched. In an even more preferred embodiment of the invention the organometallic compound is a metal neodecanoate, most preferably silver neodecanoate.

In an alternative embodiment of the invention the film formation and/or the conductivity of the conductive tracks is improved by adding a solid organic compound to the organometallic solution. Said compound can be an oligomeric or polymeric or monomeric component which can preferably be dissolved in the same solvent as the organometallic compound. The molecular weight of said compound should be at least 500 gram/mol. Oligomers or polymers which can be used are known to those skilled in the art, and can for example be, but are not limited to: polycarbonate (PC,) polystyrene (PS), polymethylmethacrylate (PMMA), polyethylene (PE), polypropylene (PE) or poly(ethylene terephthalate) (PET).

In a preferred embodiment the solid organic compound is a mixture of a monomeric compound and a polymerization initiator.

In another embodiment of the invention the solid organic compound is a polymeric precursor. The polymeric precursor should have at least one active group, which can react to other active groups with or without the addition of an initiator. Examples of these polymeric precursors are known to one skilled in the art and are understood to be any species, which can form a polymer or polymer network upon polymerization, such as, but not limited to, methylmethacrylate, ethylene, propylene or butadiene.

The polymerization initiator is preferably a UV initiator, which is capable of reacting with the polymeric precursors upon absorption of UV light. In principle any known UV initiator can be used like for example any Irgacure® initiator of Ciba Specialty Chemicals in a typical concentration range of about 1 to 10 wt-%.

To apply the organometallic compound to a substrate it is dissolved in a suitable solvent. The organometallic compound should be sufficiently soluble in said solvent to allow the required adjustment of processing parameters, such as viscosity and surface tension, for a desired processing method. In a preferred embodiment of the invention the solvent is an organic, aromatic solvent like for example toluene or xylene. In a other preferred embodiment the organometallic compound is dissolved into an apolar solvent. Although the solvent is evaporated after applying the organometallic compound to a substrate, it is also possible that residual solvent remains present in the organometallic layer.

The organometallic compound can be applied from solution to the substrate via any known method, which can be used to apply thin layers from solution to a substrate. For the method of deposition such methods are preferred that allow the pattern of the intended conductive track to be deposited directly without the need for any additional patterning steps. Methods which can for example be used to apply the track in a desired pattern include ink-jet printing, solution casting, offset printing, screen printing, flexoprinting, spin coating, doctor blading, dip coating, capillary filling or spray coating.

Preferably the substrate - on which the organometallic compound is applied - is a polymeric substrate. Further material for the substrates are ceramic, glassy or metallic, whereby for all substrates is the restriction essential that the substrate should not dissolve in the solution containing the organometallic compound, during the time said substrate is exposed to this solution. Due to the high speed of the manufacturing process of the conductive track it is possible to use flexible, commodity polymeric substrates such as poly(ethylene terephthalate) (PET) or comprising at least 80% of poly(ethylene terephthalate) and triacetyl cellulose (TAC) or comprising at least 80% of triacetyl cellulose, which tend to degrade and/or deform at elevated temperature.

The substrate on which the organometallic compound is applied can be any shape or size. The substrate can for example be a sheet, slide, foil, plate, fiber or a porous membrane. The required exposure step after the deposition of the organometallic compound consists of an exposure to electromagnetic radiation. Said exposure can be performed under ambient conditions or in any inert atmosphere without any significant change in the final properties of the conductive tracks. The exposure occurs preferably with electromagnetic irradiation in a wavelength range between 200 - 1000 nm, more preferably between 250-450 nm. In a most preferred embodiment of the invention the light applied for the exposure is UV light.

In an alternative embodiment only a part of organometallic compound is activated by locally exposing said compound to electromagnetic radiation. This can for example be done by using a photolithographic UV mask exposure or a holographic UV exposure. As a result of this patterned exposure a latent image is created in the organometallic film. This latent image is characterized by a difference in speed of development during the heating step. During the heating step, the exposed areas are therefore more easily reduced into a conductive track than the non- exposed areas. It is preferred to remove the material remaining in the non- exposed areas by using a proper solvent such as for example toluene, xylene or isopropanol.

During the heating step the layer comprising the organometallic and (where required the) solid organic compound is heated to preferable between 50 to 350 ⁰C and more preferably between 70 to 200 ⁰C. The heating step can be carried out by any means by which the temperature of organometallic compound or the forming conductive track is increased. This does not necessarily mean that the substrate on which the organometallic compound is applied should be at the same temperature. The heating can for example be done by using infra red (IR) radiation, hot gas, oxygen or argon plasma, microwave radiation or direct contact with a hot object. The invention is further elucidated by one example, which is given below.

Example 1

40 wt. -% of silverneodecanoate was dissolved into toluene. This solution was spincoated at 2000 rpm onto a cleaned glass substrates (4 x 20mm). Next, the coated substrates were exposed for 100 second to UV light with a wavelength of 320 - 390 nm. One sample was exposed 20 mW/cm², one sample was exposed to 2 mW/cm² and one sample was not exposed. After this step the three samples were heated to 175 ^°C and the resistance over the coated surface of the samples was measured during the heating procedure. The results are shown in figure 1. From these results it can be concluded that the UV exposure reduced the heating time required for obtaining a certain conductivity significantly. In figure 1 shows the X-axis the heating time in minutes and the Y-axis shows the resistance in Ω.

Claims

Process for manufacturing conductive tracksClaims:

1. A process for manufacturing conductive tracks comprising a coating step, in which an organometallic compound, defined by MX where M is metal cation and X is an organic anion, is applied from a solution onto a substrate; an exposure step, during which the organometallic compound is exposed to electromagnetic radiation and a heating step.

2. A process according to claim 1 in which only a part of the organometallic compound is activated via locally exposing said compound to electromagnetic radiation.

3. The process according to claim 1 to 2, wherein the manufacturing is carried out at a temperature range between 0 ⁰C and 350 ⁰C, preferably between 20 ⁰C and 200 ⁰C.

4. The process according to claims 1 to 3, wherein the organometallic compound is a metal carboxylate and/or a metal thiolate and/or a silver neodecanoate.

5. The process according to any of the preceding claims, wherein the organometallic compound is dissolved into an apolar solvent.

6. The process according any of the preceding claims, wherein the substrate is polymeric.

7. The process according to claim 6, wherein the substrate comprises at least 80% of poly(ethylene terephthalate).

8. The process according to claim 6, wherein the substrate comprises at least 80% of thacetyl cellulose.

9. The process according to any of the preceding claims, wherein the substrate has any shape and size, such as a sheet or a fiber.

10. The process according to any of the preceding claims, wherein the exposure occurs with electromagnetic radiation in a wavelength range between 200-1000 nm.

11.The process according to claim 10, wherein the wavelength range is between 250-450 nm.

12. The process according to any of the preceding claims, wherein a solid organic compound is added to the solution containing the organometallic compound.

13. The process according to claim 12, wherein said solid organic compound is polymeric or oligomeric or monomeric.

14. The process according to claims 12 or 13, wherein said solid organic compound is a mixture of the monomeric compound and a polymerization initiator.

15. The process according to claim 14 wherein said polymerization initiator is a UV initiator.