WO2015156661A1

WO2015156661A1 - Method of producing a patterned nanowires network

Info

Publication number: WO2015156661A1
Application number: PCT/NL2014/050218
Authority: WO
Inventors: Teppei Araki; Rajesh Mandamparambil; Iryna YAKIMETS; Katsuaki Suganuma; Jeroen Van Den Brand; Jinting Jiu
Original assignee: Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno; Osaka University
Priority date: 2014-04-07
Filing date: 2014-04-07
Publication date: 2015-10-15

Abstract

The prior art has failed to provide an adequate method of producing a patterned nanowires network, for which the invention seeks a solution. To this end, it is proposed to provide a method of producing a patterned nanowires network, the method comprising the steps of: providing a nanowire donor film comprising a donor layer of nanowire material mixed with an adhesive epoxy; aligning a laser beam of a laser system and guiding the nanowire donor film distanced from a substrate surface; impinging the laser beam on the nanowire donor film; in such a way that the nanowire donor film is activated to cover a selected part of substrate surface with nanowire matter transferred from the donor layer; and wherein the laser beam is restricted in timing and energy, in such a way that the nanowire matter is sintered into a patterned nanowires network. Accordingly, a patterning method is provided, wherein the nanowire network can be transferred to an acceptor substrate with a desired pattern, and at the same time providing for a sintering process during transferring by suitable tuning of the patterned radiation.

Description

Title: Method of producing a patterned nanowires network

FIELD OF INVENTION

The invention relates to a method of producing a patterned nanowires network. In particular, the invention concerns a method for manufacturing conductive patterns with sub-micron thicknesses, for example, on flexible substrates for example for producing ultrathin electrode layers.

BACKGROUND

In the art, it is known to provide a liquid dispersion in which metal nanowires are dispersed is deposited by application onto a substrate and drying. Subsequently, metal nanowires are sintered e.g. by irradiation with pulsed light to manufacture a transparent conductive pattern through joining of the metal nanowires at intersections thereof. While nanowires as such are known as such in the art, they may be characterized as elongated structures with a diameter in the order of 0,01-1 micrometer and lengths in the order of 1 to several hundreds of micrometers. Some of these structures are grown in ionic liquid compositions, and extracted therefrom for further processing steps.

Conductive and semiconductive nanowires (including nanotubes, nanofibers, etc.) have got much attention as promising materials for flexible or stretchable electronics, in view of their robust conductivity properties. In recent years research and development on nanowires have been focused on achieving relatively higher electrical and optical performances with solution process compatibility. Deposition process for nanowires are based on spin coating, drop casting, and wire bar coating. However, using these techniques, the pattered deposition on any shaped surface has limitations. For example, due to the vulnerability of the nanowires in unsintered form, screen and stencil printing will mostly result in damaging the nanowires before further processing resulting in faulty conductive behavior of the processed conductive nanowire layer.

In this respect, it is found especially difficult to provide for methods of patterned nanowire layers, partly due to the hazardous risks of nanop articles in processing then.

WO2013133420 discloses a method for manufacturing transparent conductive patterns of improved conductivity, through irradiation by pulsed light. SUMMARY OF THE INVENTION

The prior art has failed to provide an adequate method of producing a patterned nanowires network, for which the invention seeks a solution. To this end, it is proposed to provide a method of producing a patterned nanowires network, the method comprising the steps of:

- providing a nanowire donor film comprising a donor layer of nanowire material mixed with an adhesive material;

- aligning a laser beam of a laser system and guiding the nanowire donor film distanced from the substrate surface;

- impinging the laser beam on the nanowire donor film; in such a way that the nanowire donor film is activated to cover a selected part of substrate surface with nanowire matter transferred from the donor layer; and

- wherein the laser beam is restricted in timing and energy, in such a way that the nanowire matter is sintered into a patterned nanowires network and wherein the nanowire matter is sintered simultaneously during transfer.

Accordingly, a patterning method is provided, wherein the nanowire network can be transferred to an acceptor substrate with a desired pattern, and at the same time providing for a sintering process during transferring by suitable tuning of the patterned radiation. The method has as advantage that no subtractive methods such as etching methods on the acceptor substrate need be carried out for forming the patterned nanowires network. The nanowire matter that is to be transferred can be formed by a matrix of a epoxy with low viscous or adhesive properties. During transfer of the nanowire matter from the donor layer to the substrate surface, the laser beam may be restricted in timing and energy, in such a way that that the nanowire matter is sintered into a network structure. It is understood that the nanowire matter that the nanowire matter is sintered into a network structure when the resulting patterned nanowires network has sintered connections between adjacent nanowires, substantially without disintegration of the nanowire. In this way an efficient patterning method is provided wherein nanowire matter is sintered into a network structure of any selected patterning by suitable irradiation of the donor layer.

The method is especially suitable for flexible or stretchable substrates as used in this text referring in particular to substrates that are bendable enough to be used in a reel to reel process. In other words a flexible substrate as used in this text is a substrate that is flexible enough to allow bending over a certain curvature, e.g. with a radius of 1 - 100 centimeters (depending on the reel diameter), without the substrate losing essential functionality.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1. shows various SEM images of the structures provided by the invention.

Figure 2 shows results of stretching experiments of the produced structures

Figure 3 shows a schematic set up for the transferring process. Figure 4. shows the set up in a roll to roll feed system

DETAILED DESCRIPTION

In an aspect, a patterning method is provided for providing patterned nanowire networks, e.g. for electrode layers that are robust and, depending on the applications, transparent for visible light. With the attainable resolution of the disclosed methods and systems, that a resolution spot size of transferred nanowire material may be attained with a spot diameter of the transferred nanowire material that is smaller than 50 micron.

The nanowires networks can be patterned additively to form conductive or semiconductive, and passive components and structures (circuits, transparent electrode, resisters, capacitors etc.) from a flat donor substrate to a shaped substrates (3D).

Stretchable nanowires networks can be formed by sintering the nanowire matter to produce waved nanowire structures. This is carried out by converting straight nanowires networks to wavy nanowires networks e.g. by heating at certain wavelength or through thermo-mechanical shocks.

The advantage of wavy nanowires networks is that it can be stretched to a greater extend than the straight nanowires networks. No other known techniques are available to achieve this during the deposition process. This enables precise control of stretchability of the substrates.

Modification of the donor layer specifically tuned for the pattered deposition of the nanowires networks. The use of only already existing sacrificial layer will not ensure a proper transfer.

Nanowires can be sintered in the transfer process with keeping nanowires network.

In an embodiment, the nanowire donor film is provided on an organic substrate, without the need for an additional release layer such as a triazene layer, in the art also known as a dynamic release layer. This is because the nanowire donor film can be transferred with minimal impact, due to the low viscous properties of the mixture of nanowire material mixed and adhesive epoxy ( molecular weight range 50 to 250 g/mol) of viscosity range from 1 to 10 mPa.s preferably a viscosity of water. The adhesive can be diluted from higher viscosity values from up to 1000 mPa.s. Preferably, the boiling point of the adhesive is chosen below the Tg of the substrate, so that the adhesive can be evaporated during transfer. A typical boiling point at room temperature conditions is 150 0C or lower. An example of the conductive adhesive epoxy is loctite or Hysol or PEDOT:PSS conductive polymer PEDOT:PSS or poly(3,4-ethylenedioxythiophene), which is advantageous because of its conductive properties, together with a matrix function for the nanowires. Poly(styrenesulfonate) is known as a polymer mixture of two ionomers. One component in this mixture is made up of sodium polystyrene sulfonate which is a sulfonated polystyrene. Part of the sulfonyl groups are deprotonated and carry a negative charge. The other component poly(3,4-ethylenedioxythiophene) or PEDOT is a conjugated polymer and carries positive charges and is based on polythiophene. Together the charged macromolecules form a macromolecular salt.

In an embodiment the donor film substrate is provided a quartz substrate on which silver networks that are fabricated by drop casting nanowires solution air-dried and followed by deposition of a transparent resin (suspended in diethylene glycol monoethyl ether acetate) in the wet state. In order to transfer the nanowires onto the substrate by illumination from quartz side, the resin in which the nanowires networks is present is transparent to the light but the DRL layer will ensure that it absorbs light to transfer the matrix. The matrix can be polymers, conductive polymers, and solutions (regardless of wet and dry state). The matrix thickness can be tuned to get better networks connections between transferred dots. On the quartz substrate a dynamic release layer (DRL) is provided, which may be a separate layer or may be part of the donor layer. For example, nanowires networks may be provided without or in a matrix (wet resin, dry triazene polymer or dry polyvinyl alcohol) on a quartz substrate with DRL spin coated over it. The donor film is illuminated by light in order to get deformation of nanowires itself by Shockwave or to get it of matrix during transferring process.

The advantage of sintered nanowires in a transferring process is simplifying a process of deposition and sintering. There have been two stages for fabrication materials in the existing process such as printing. Sintering/curing process for materials always comes after deposition and patterning process. To provide a simple technique, sintered nanowires were obtained in a process of patterned transfer without a necessity for subsequent process stages. Deposition, pattering, and sintering in a transfer process thus may lead to the simplified process. The details for transferring process are disclosed below with the results.

Figure 1 shows an embodiment wherein silver nanowires networks were additively patterned by deposition on a polyurethane (PU) substrate by using the transfer process as presently described.

In Figure 1A an example image is provided of a after transfer in sintered form.

In Figure IB a patterned wavy nanowire layer was provided. The electrical resistance is measured after the transfer process and may be found to be 10-4 Qcm. The typical dimension of the tracks is in the order of several millimeter long and are fabricated by stacking transferred nanowires networks. By stitching the transferred spots conductive tracks are fabricated. In Figure 1C the network is imaged having a tuned irradiance, wherein the networks are produced with a Polyvinyl alcohol , wherein the nanowires are curled and produced as wavy networks.

Figure 2 shows results of stretching experiments carried out to quantify the electrical resistance of the transferred tracks and found to have stable resistance during cyclic measurement. Buckled Ag nanowires lines were fabricated by pre-streching show stable electrical resistance up to 100% strain because of mechanically flexible nanowires. According to existing reports, buckled lines with bulk metal can be stretched up to several tens strain but could not realize such high stretch ability because of no flexibility in bulk metals.

Figure 3 shows an exemplary transfer setups for transferring die nanowire matter 1511 from a donor film 15 to an acceptor substrate 20 for the method as presently disclosed. In the embodiment, a method is disclosed of producing a patterned nanowires network, the method comprising the steps of:

- providing a nanowire donor film 15 comprising a donor layer 151 of nanowire material mixed with an adhesive epoxy;

- aligning a laser beam 75 of a laser system and guiding the nanowire donor film distanced from the substrate surface;

- impinging the laser beam on the nanowire donor film 15; in such a way that the nanowire donor film 15 is activated to cover a selected part of substrate surface 20 with nanowire matter transferred from the donor layer; and

-restricting the laser beam 75 in timing and energy, in such a way that the nanowire matter is sintered into a patterned nanowires network 40. In the setup, a laser spot may be formed with a spot size D of about 20-200 micron, in particular, an beam shaped Nd:YAG or excimer laser with fluencies of 20-300 mJ/cm², more particular, 40 - 150 mJ/cm².

The spot is aimed on a transparent carrier substrate 70 in particular, in the example, a quartz glass for a 248 nm KrF excimer and PET or Soda Lime Glass for a 355nm Nd:YAG laser. On the substrate 70 a donor film 15 is provided comprising a nanowire material layer 151 and a dynamic release layer 152 adjacent to the nanowire material layer 151. Dynamic release layers are well known in the art and typically comprise a composition formed in a layer, that abruptly locally transforms in gaseous substance, when locally irradiated.so that dynamic release is provided by propulsion of the gaseous substance. In the example, the dynamic release layer 152 is formed by a Triazene layer of about 100 nm thickness which functions as a sacrificial dynamic release layer (DRL), and comprises a polymer that, when photoactivated decomposes into nitrogen and other organic volatile gases 1521. Other compositions to the effect that a dynamic release is provided of the substance provided on the dynamic release layer 152, i.e. dynamic release of nanowire material of a curable conductive adhesive or flux based solder paste from the dynamic release layer 152 to a selected part of the connection pad structure 40 may be equally suitable. A typical peak absorption is found at 290-330 nm and the ablation threshold: 22-32 mJ/cm² at 308 -248 nm is quite low so that the donor film layer is not thermally loaded and the nanowire matrix remains intact after transfer. For example, the laser beam may be restricted in timing and energy, in such a way that the nanowire matter is sintered into a patterned nanowires network 40 during transfer. In another example, sintering may take place by continued irradiation after transfer. Accordingly a desired material property of the nanowire donor film can be retained during transfer from the dynamic release layer 152 to acceptor substrate 20 by impinging the laser beam on the dynamic release layer 152 adjacent to the nanowire material layer 15; in such a way that the dynamic release layer 152 is activated to direct the nanowire matter 1511 to cover a selected part of the substrate 20 with transferred conductive die nanowire material 40.

Covering the selective part with conductive die nanowire material clearly may function for providing a suitable electrically conductive structure. In the example, a transparent epoxy commercially obtained from Henkel, suspended in diethylene glycol monoethyl ether acetate 151 is transferred with a viscosity of 1 to 10 mPa.s . The mixture of nanowire material mixed and adhesive epoxy of low viscosity 151 is provided as homogenous layer of 20-30 micron, in particular, 25 micron thick provided on the dynamic release layer 152. The thickness is controlled to be around 25 um or 50 um but could be theoretically be of any thickness. The donor nanowire materials are held at a distance of about 13-350 micron away from the substrate by spacer shims 80.

The patterned nanowires network 40 may exhibit electrical conductivity of typically 1-10 E-4 Ohm. cm

In the disclosed reel to reel method of Figure 4, a substrate 20 or carrier web is unwound from a first reel 265 and guided via a set of (contactless) guide rollers 240 to a second reel 270 to be wound up. In the unwound condition, various sub processes can be carried. In particular, these sub processes may involve transferring a patterned nanowire network 40 as disclosed here above

A further embodiment involves a repeated steps of transferring material to provide for stacked nanowire structures.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. In particular, unless clear from context, aspects of various embodiments that are treated in various embodiments separately discussed are deemed disclosed in any combination variation of relevance and physically possible and the scope of the invention extends to such combinations.

Other variations to the disclosed embodiments can be understood and by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. The structures as presently disclose may have following advantages, in whole or in part: no physical contact provides production flexibility (stacking, repairing, etc.), in particular for roll-to-roll applications. Due to the additive nature of the patterning methods, nanowires layers are only produced when needed, reducing hazard risks of waste particles. The transfer process is virtually independent on the transfer substrate since the nanowire network may be provided on any material substrate, in particular, any organic material substrate. In addition, selected shapes (square, circle, triangle, area from tens of microns to hundreds of microns, etc. ) of nanowire networks may be provided in sub micron resolutions.

Claims

1. A method of producing a patterned nanowires network on a substrate, the method comprising the steps of:

- providing a donor film comprising a donor layer of nanowire material mixed with an adhesive material;

- aligning a laser beam of a laser system and guiding the donor film distanced from the substrate surface;

- impinging the laser beam on the donor film; in such a way that the donor film is activated to cover a selected part of substrate surface with nanowire matter transferred from the donor layer; and

- wherein the laser beam is restricted in timing and energy, in such a way that the nanowire matter is sintered into a network structure and wherein the nanowire matter is sintered simultaneously during transfer.

2. A method according to claim 1, wherein the nanowire matter is sintered to produce waved nanowire structure.

3. A method according to claim 1, wherein the adhesive is a transparent epoxy material.

4. A method according to claim 1, wherein the donor film is a wet material.

5. A method according to claim 1, wherien the conductive polymer material is formed by a PEDOT; PSS material or adhesive conductive epoxy

6. A method according to claim 5, wherein the donor film is formed on a PET release layer.

7. A method according to any of the preceding claims wherein the substrate is a flexible substrate (20) having a radius of curvature of at least 1 cm.

8. A method according to any of the preceding claims, wherein the substrate is prestretched.

9. A method according to any of the preceding claims, wherein the distance to the die surface is kept in a range of 1-200 micron.

10. A method according to any of the preceding claims, wherein the die nanowire material layer has a thickness in a range between 10-50 micron.

11. A method according to any of the preceding claims, wherein the donor film is provided with a premachined patterning.

12. A method according to claim 7, wherein premachined patterning forms a grid with a grid size that coincides or is smaller than a laser spot size.