WO2012072263A1

WO2012072263A1 - Method for making electric circuit pattern

Info

Publication number: WO2012072263A1
Application number: PCT/EP2011/006051
Authority: WO
Inventors: Albert R. Liberski; Joseph T. Delaney; Jolke Perelaer; Ulrich Schubert; Aleksandra M Liberska
Original assignee: Stichting Dutch Polymer Institute
Priority date: 2010-12-03
Filing date: 2011-12-02
Publication date: 2012-06-07

Abstract

The present invention relates to a method for providing a circuit pattern onto a substrate comprising the steps of: a) providing a removable electrically conductive layer onto the substrate, b) depositing a toner on the conductive layer of the substrate by a laser-printing technology to define a first, non-removable region having the shape of the circuit pattern covered by the toner and a second region not covered by the toner and c) removing the second region of the electrically conductive layer optionally using a solvent.

Description

METHOD FOR MAKING ELECTRIC CIRCUIT PATTERN

FIELD OF THE INVENTION

The present invention relates to a method for providing a circuit pattern onto a substrate.

BACKGROUND

Thin, printable electronic interconnects with high conductivity have a wide range of applications in the manufacture of electronics. In particular, for investigating new circuit layouts, especially during early design stages of a new device, the ability to cheaply and rapidly prepare high resolution circuit patterns without the need for specialized equipment is particularly desirable: ideally, using an approach that embraces the additive, maskless rapid protyping philosophy.

Presently, there are a number of technical barriers associated with currently employed printed circuit board fabrication that make 'do-it-yourself (D.I.Y.) circuitry on flexible substrates impractical or even impossible.

An example of methods for manufacturing and/or prototyping of electric circuit boards or printed circuit boards (PCB's) is a multi-step photo-lithographic process comprising the steps of: 1) providing a substrate; 2) applying a metal to a surface of the substrate (usually by sputtering); 3) applying a photo resist layer on top of the metal layer; 4) providing a mask of the desired circuit pattern; 5) positioning of the mask on top of the photo resist layer and illuminate the photo resist; 6) one or more (wet) etch steps to remove unreacted photoresist and the underlying metal layer; 7) remove the photo resist layer which is present on top of the circuit pattern; 8) optionally several intermediate rinsing steps.

A disadvantage of such a process is that the process is rather complex and specialized equipment is required to perform most of the process steps. It often involves wet etching steps that are not practical in all labs. It is another disadvantage of photolithography that for each change in the circuit board design a new mask has to be made.

Another method known in the art comprises printing of conductive inks which contain micro- and/or nanospheres of conductive metals (e.g. silver, copper, nickel, platinum, palladium, or gold). A desired circuit pattern may be printed with e.g.

seriography (i.e. screen-printing) or inkjet printing (e.g. with a drop on demand method).

Upon drying, the circuit lines of the circuit pattern are typically non- conductive, and must be sintered at elevated temperatures before they are useful for most electronics applications. Alternatively, lines may be prepared using semiconductive organic materials such as derivatives of poly(3,4-ethylenedioxythiophene) (PEDOT), though their conductivity is limited. Also, since these polymers are frequently solvent- cast, the processing conditions used to deposit PEDOT may not be compatible with polymeric systems that are soluble or swellable under similar conditions. The electronic properties of semiconductive materials are highly process-dependent, and may be prone to chances by subsequent steps.

C. J. Easley, R. K. Benninger, J. H. Shaver, W. Steven Head and D. W. Piston, Lab. Chip, 2009, 9, 1119-1127 discloses the use of standard laser-printed patterns on transparencies for rapid prototyping of micro devices by photomasking. The patterned transparency may be used in a photo-lithographic process described above (step 4 ). The toner being applied as a photo-negative (i.e. the desired circuit pattern remains transparent) onto the transparency causes blocking of the electromagnetic radiation. Upon radiation, only the unpolymerized liquid prepolymer solutions (photo resist) in areas unprotected by the mask start to react. Afterwards several (wet) etching process steps may be performed as described above (step 6).

H. Cho, M. Parameswaran and H.-Z. Yu, Sens. Actuators, 8, 2007, B123, 749-756 discloses printing alkylthiol protective layers onto gold films, followed by chemical etching using nitric acid.

US5196286 discloses a method for patterning a substrate. In US5196286. a laminate of an electrically non-conductive substrate, an electrically conductive material, and an additional electrically non-conductive material is xerographically processed so as to leave a toner image formed upon the surface of one of the non-conductives. The non- conducive and conductive are successively removed chemically in those areas not coated by the toner, leaving a conductive pattern replica of the original toner image. The removal is performed by immersion into a selective solvent chosen for the selective dissolution of the non-conductive material with little dissolution of the toner.

US2003/016 959 discloses a method for fabricating an inorganic resistor on an organic substrate. The method comprises the steps of: a) applying a flowable precursor composition to an organic substrate wherein said precursor composition comprises a molecular precursor to a conductive phase and a powder of an insulating material; b) heating said substrate to a temperature of not greater than about 350 °C to convert said molecular precursor to said conductive phase and form a resistor wherein said resistor has a resistivity of at least about 100 μΩ-cm.

The known processes require specialized equipments and tend to require complex processes. It is an object of the present invention to obviate or at least mitigate the above stated and/or other disadvantages.

SUMMARY OF THE INVENTION

According to the present invention, a method is provided for providing a circuit pattern onto a substrate comprising the steps of:

a) providing a removable, electrically conductive layer onto the substrate, b) depositing a toner on the conductive layer of the substrate by a laser- printing technology to define a first, non-removable region having the shape of the circuit pattern covered by the toner and a second region not covered by the toner and

c) removing the second region of the electrically conductive layer optionally using a solvent.

It is herein understood that the term 'removable' layer means that the layer can be removed by wiping it off from the substrate, possibly using a suitable solvent, if no further measure is taken to secure its adhesion to the substrate.

It is herein understood that the term 'to wipe' is used in a normal sense, i.e. to clean a surface by rubbing something on it.

The toner deposited by the laser-printing technology in the desired circuit pattern acts as a resist, masking the electrically conductive layer underneath it from being removed during the subsequent removal step. Removing the second region not covered by the toner leaves behind the desired circuit pattern.

It is an advantage of the method according to the present invention that the toner to be printed does not need to contain highly conductive particles, since the conductivity of the circuit is primarily given by the electrically conductive layer underneath the toner. This allows that readily available toners which do not contain highly conductive particles may be used for defining the circuit pattern, which allows use of a wide variety of laser printers. Typical toners are a mixture of carbon powder and a polymer such as, for example, a styrene acrylate copolymer, a polyester resin, a styrene butadiene copolymer.

Surprisingly, the circuit pattern obtained by this method shows a very high conductivity. Although the inventors do not wish to be bound by any particular theory, it is believed that the heat applied during the laser printing serves to anneal the electrically conductive layer. In this way, the laser printing step not only provides a protective layer for the conductive layer, but also increases the conductivity of the circuit pattern.

In the conventional technology where metal nanoparticles such as silver nanoparticles are inkjet printed to define a circuit pattern, a problem was known that such formulations may clog inkjet printers. Consequently, there are practical limits to the nozzle size that must be taken into consideration, which may consequently constrain achievable limits to feature resolution. Beyond these technical challenges, the silver nanoparticle formulations are uniformly proprietary, costly, have special storage considerations, and have a limited lifetime. According to the method of the present invention, no such problems are encountered since an ordinary laser printer using an ordinary toner may be used.

The invention also takes advantage of the robust, well-developed ink and toner technology of standard office printers. The inks and printheads used for printing toner-based documents have been rigorously developed in relation to each other for decades, resulting in systems that deliver high performance, as well as a high degree of ruggedness and robustness. The printing resolution achieved by office printers using toner are commonly very advanced. For example, the HP 4700 Color LaserJet printer has a feature resolution of 1200 DPI, or 21 pm dot size, for unmodified surfaces. As a comparison, the dot size commonly reported by drop-on-demand printing of silver inks on non-modified surfaces is typically on the order of 50 pm dot size, which corresponds to a resolution of approximately 500 DPI.

A further disadvantage of printing of conductive inks, e.g. silver nanoparticle inks with seriography or inkjet printing, is the low adhesive strength of the deposited nanoparticles when printed onto an organic surface. This causes that the printed patterns can be inadvertently damaged. Furthermore, the required high- temperature sintering step limits the variety of substrate materials that can be used, since many desirable substrates, such as common sheet plastics like cellophane,

polypropylene, polystyrene, and terephthalate polyesters, are not able to withstand sintering temperatures commonly reported (>200 °C). According to the present invention, since a sintering step is not required, a wide variety of substrates may be used.

Accordingly, in preferred embodiments of the present invention, the substrate has an organic layer and the electrically conductive layer is provided on the organic layer. Any organic material may be used for the organic layer as long as its electrical conductivity is low enough to allow its use as the substrate of a circuit. Such organic material may be chosen from the group consisting of polyolefin, polyester, polyamide, polycarbonate, polyimide, polyether ether ketone and a copolymer thereof. The organic material may also be a biopo!ymer, such as cellulose derivatives. Especially preferred examples of the organic material include polyethylene, polypropylene, polyethylene terephtalate, polybutylene terephthalate, cellophane,

polymethylmethacrylate, polystyrene, polyether ether ketone and polydimethylsiloxane. It is advantageous that the process of these embodiments uses readily available materials. The substrate may consist of the organic layer, or the organic layer may be a coating on a further layer. In the latter example, the further layer may either be inorganic or organic. A particularly preferred example is a paper coated with the organic layer as described above, such as a glossy paper.

Preferably, the substrate is a flexible sheet having a thickness of between

0.05 and 0.3 mm. This has an advantage that the laser printing may be performed by an ordinary laser printer. Preferably, the flexible sheet has a standard paper size, e.g. A5, A4, A3, Letter (US). It will be appreciated that the flexible sheet may be either transparent or non-transparent and colored or non-colored.

Removing the second region of the electrically conductive layer may be performed using a solvent. Suitable solvents may be chosen by the skilled person according to the type of the electrically conductive layer, toner and the substrate so that the second region is removed while the first region is not removed. Examples of suitable solvents include alcohol such as methanol, ethanol, isopropanol and butanol and ether such as dimethoxyethane, acetone and a mixture thereof. The solvent may also be diluted with water.

Before removing the second region, the substrate may optionally be heated at a moderate temperature for a short period of time. For example, the temperature may be e.g. 80-120 °C, preferably 100-115 °C. The duration may be e.g. 10-60 seconds, preferably about 30 seconds. This optional heating step improves the attachment between the toner and the electrically conductive layer underneath and may also increase the conductivity of the conductive layer. It will be appreciated that the conditions of this optional heating step should be chosen so that the substrate is not damaged.

Preferably, the removable electrically conductive layer comprises electrically conductive particles. The electrically conductive particles form a structure having a sufficient continuity to give electrically conductivity, but the particles are fragmented enough that the structure can still be removed by wiping. Preferably, the electrically conductive particles comprise metal particles. Preferably, the metal particles are chosen from the group consisting of silver, copper, nickel, platinum, palladium and gold. Silver particles are most preferred.

The particles may be microparticles or nanoparticles. The particles may have an average a size of between 1 nm to 10 pm. Preferably, the average particle size is at least 5 nm, 10 nm, 50 nm, 100 nm, 500 nm, 1 pm, 2 pm or 5 pm. Preferably, the average particle size is at most 5 pm, 2 pm, 1 pm, 500 nm, 100 nm, 50 nm or 10 nm.

More preferably, the average particle size is 5 nm to 1 pm. The size of the particles may be determined by measurement methods suitable for the relevant size range, for example according to ASTM B822-10, or by scanning electron microscopy.

In preferred embodiments, the substrate has an organic layer and the removable electrically conductive layer provided on the organic layer comprises metal particles having an average size of 10 nm to 1 pm. This combination allows an especially suitable adhesion strength for the provision of the removable electrically conductive layer on the substrate. The metal particles adhere weakly to the organic surface, forming a continuous film of electrically conductive material.

The removable layer comprising the silver particles may e.g. be provided by depositing a Tollens' reagent on the substrate. In some preferred embodiments of the present invention, step a) comprises the substeps of

at) treating the surface of the substrate with one or more solutions containing a silver salt precursor and

a2) reducing the silver precursor to obtain silver particles deposited on the substrate.

This treatment of the surface with a sequence of one or more solutions containing a metal (e.g. silver) precursor, (silver salt or complex) is based on the reduction of metal (e.g. silver) salts by appropriately selected reducing agents. Such (silver) metal precursor salts or complexes may be chemically transformed (i.e. reduced) with such known compounds as aldehydes based on the well-known Tollens reaction, or the reduction with the aid of sodium or potassium borohydride, or reduction using hydrazine or its reductively active derivatives, or reduction using dimethylformamide (D F), or reduction using polyols, reduction using ascorbic acid, monosaccharides, or through biosynthetic reduction by an organism or purified enzyme.

Alternatively, step a) comprises the substeps of

a1) treating the surface of the substrate with one or more solutions containing a silver precursor and

a3) radiating the treated surface with actinic or ultrasonic radiation to obtain silver particles deposited on the substrate.

Additional components, such as polycations or polyanions, may be used to enhance formation of metallic nanoparticles into different morphologies, often including continuous, conductive metallic nanostructures. Polycation that have been used to modify the formation of silver nanoparticles include such materials as DNA, PVP, PVA, polyacrylamides, proteins and polylallylamine. Many other polycations, such as

polyoxazolines, and poly(ethylene amine), may also be employed in a similar fashion. Alternatively, the diamminesilver cation itself may be replaced with a polyamine, where the silver coordinates to the backbone of the amine, and then is reduced to yield metallic silver following the chain length of the polymer. Polyamine complexes of silver are well- known, and so this may also be an alternative. Polyanions, such as poly(sodium styrene sulfonate) (PSS), polyacrylic acid, and chitosan. may be used in an analgous fashion.

The circuit pattern may be designed using any software. Examples of standard office drawing software packages are known in the art, such as MS Paint or AutoCad.

In some embodiments of the present invention, the method further comprises the step of d) removing the toner on the circuit pattern after step c) using a toner removing agent different from the solvent in step c). The toner removing agent comprises a solvent which may be different from the solvent in step c) in the type of the solvent or the concentration of the solvent. Suitable toner removing agent may be chosen by the skilled person according to the type of the electrically conductive layer, the toner and the substrate so that only the toner is removed and the underneath electrically conductive layer is not removed. For example, a mixture of acetone and water having a weight ratio of 25:75-75:25 may be used as the toner removing agent for an example where the substrate is PET, the metal in the conductive layer is silver and the toner is mainly polyester. A further example of the toner removing agent includes chloroform.

The exposed circuit pattern obtained this way may be advantageous for some applications.

In some embodiments of the present invention, the substrate is made of an inorganic substrate and the method further comprises the step of e) annealing the substrate after step c). Preferably, the annealing is performed at a temperature of at least 200 °C. The inorganic substrate may be made of silica, including quartz and amorphous silica, borosilicate glass, aluminum oxide, or ceramic materials. Step e) may be performed in addition to or alternatively to the optional step d). If steps d) and e) are both employed, step e) may be performed before, during and/or after step d). Annealing the pattern of nanoparticles increases the conductivity of the pattern.

The process according to the present invention provides a simple way of producing conductive patterns on a temperature/solvent-sensitive substrate using common lab reagents, without the need for expensive masks or complicated processing steps. Both the raw material used in the method and the process equipment (standard office printer) are cheap and portable. This enables rapid prototyping of circuit patterns onto cheap substrates with excellent resolution.

According to a further aspect of the invention, the substrate provided with a circuit obtainable by the method according to the present invention is provided. According to a further aspect of the invention, the substrate provided with a removable electrically conductive layer used in the method of the present invention is provided. In particular, a flexible sheet is provided having a thickness of between 0.05 and 0.3 mm, wherein the flexible sheet is made of an organic material chosen from polyolefin, polyester, polyamide, polycarbonate, polyimide, polyether ether ketone and a copolymer thereof , preferably polyethylene, polypropylene, polyethylene terephtalate, polybutylene terephthalate, cellophaneand wherein the sheet is provided with a removable electrically conductive layer comprising electrically conductive particles of a material selected from the group consisting of silver, copper, nickel, platinum, palladium and gold. In a particularly preferred embodiment, the flexible sheet is a polyethylene terephtalate provided with a layer of silver particles. A further aspect of the invention provides use of the flexible sheet according to the present invention for making a circuit pattern.

It is noted that the invention relates to all possible combinations of features recited in the claims.

The invention will now be explained with reference to the following figures in which :

FIG.1 is a schematic representation of the method according to the present invention and

FIG. 2 is an example of a circuit pattern provided according to the method of the present invention.

The method of the present invention will be illustrated referring to an exemplary embodiment referring to Figure 1. A substrate 1 is provided, which may be standardized polyethylene terephtalate transparency sheets, polyester films, polyethylene films, polypropylene films. The substrate 1 is thin, flexible and printable. Prior to use, the sheet material is prepared such that it can be used in a standard office printer, which means that it is cut into a standard paper size, e.g. A4, A3, Letter (US).

Step a): A thin layer of conductive particles 2 is deposited onto the substrate 1 , such as by deposition of silver particles onto the surface by means of the Tollens reaction. The substrate 1 is treated with solutions that deposit silver nanoparticles or microparticles that adhere weakly to the surface, forming a continuous film of electrically conductive material. The substrate is subsequently treated by an appropriately selected reducing agents to form a removable conductive layer. Step b): A toner is printed onto the metal coated substrate using a standard office laser printer using an ordinary toner, defining a first region 3 covered by the toner and a second region 4 not covered by the toner.

Step c): Once the pattern has been printed, selective removal of superfluous nanoparticles on the non-printed areas of the substrate may be achieved by gently rubbing or wiping the printed feature, leaving only the first region 3 on the substrate 1. A (paper) towel involving a solvent such as ethanol or isopropanol may assist the removal of the superfluous unprotected metal layer, leaving behind the desired pattern. EXAMPLE

A standard office transparency sheet {A4, PET) was placed onto a sheet of aluminum foil. The edges of the PET sheet were fixed to the aluminum foil using an adhesive tape in order to prevent any liquid flowing between the aluminum foil and the PET sheet. Next, the edges of aluminum foil were bent up to form a tray.

Then, 100 mL of a 20 wt% water solution of fructose was dispensed onto the PET sheet fixed at the bottom of the Al tray. A mixture of 0.5 g silver inflate and 0.6 mL of 25 wt% ammonium hydroxide was filtrated and added dropwise uniformly onto the PET sheet using a pipette. The system was allowed to stand while Tollens reaction took place, resulting in silver deposition. After 3h, the fructose solution was removed and the PET sheet was placed in 5 L of water. After 10 min washing, the PET foil was dried using N₂ stream.

The as-prepared PET sheet with the deposited silver layer of 100 nm was placed in the paper reservoir of a laser printer (HP Color LaserJet 4700dn, Hewlett- Packard Company (CA, USA)). A designed circuit layout was then printed in black and white mode using only a black toner (Q5950A). The obtained circuitry was heated for 30 s at 110 °C, after which the excess of silver was removed using ethanol and a paper tissue. Finally, the circumference of the circuitry design was cut out to its final shape.

The circuit pattern obtained by this example is shown in Figure 2.

A part of the conductive silver layer was peeled off form the circuit pattern chip using a standard office adhesive tape (Scotch tape; Scotch, Crystal Cat. No. 34- 8509-2927-3). The thickness as well as the cross-sectional area of the silver layer was measured using Wyko NT9100 optical profilometer (Veeco, Tucson, USA,

2.5x magnification, coupled with a 2.0x FOV). Scanning electron microscopy images were taken using Field Emission Scanning Electron Microscope by JEOL JSM-6700F.

The electrical resistance p of the pattern was calculated from the resisatance R, the length /, and the cross sectional area A of the line, using the formula p = R A/I. The resistance was measured to be 6.0 ± 0.2 Ω over 1.0 cm using a standard multimeter (Voltcraft Plus VC 960, Conrad, Germany).

The resistivity of the obtained circuit pattern was measured to be 4.4 x 10^"8 Ohm.m. Thus, a circuit pattern was obtained which has almost 50% of the conductivity of bulk Ag.

Claims

1. A method for providing a circuit pattern onto a substrate comprising the steps of:

a) providing a removable electrically conductive layer onto the substrate, b) depositing a toner on the conductive layer of the substrate by a laser- printing technology to define a first, non-removable region having the shape of the circuit pattern covered by the toner and a second region not covered by the toner and

c) removing the second region of the electrically conductive layer optionally using a solvent,

2. The method according to claim 1 , wherein the substrate has an organic layer and the electrically conductive layer is provided on the organic layer.

3. The method according to claim 2, wherein the organic layer is made of a material is chosen from the group consisting of polyolefin, polyester, polyamide, polycarbonate, polyimide, polyether ether ketone and a copolymer thereof.

4. The method according to any one of claims 1-3, wherein the substrate is a flexible sheet having a thickness of between 0.05 and 0.3 mm.

5. The method according to any one of claims 1-4, wherein the removable electrically conductive layer comprises electrically conductive particles.

6. The method according to any one of claims 5, wherein the conductive particles are metal particles, preferably selected from the group consisting of silver, copper, nickel, platinum, palladium and gold.

7. The method according to claim 5 or 6, wherein the conductive particles have an average size of 5 nm to 5 μητ

8. The method according to any one of claims 1-7, wherein step a) comprises the sub-steps of:

a1 ) treating the surface of the substrate with one or more solutions containing a silver precursor and a2) reducing the silver precursor to obtain silver particles deposited on the substrate.

9. The method according to claim 8, wherein step a2) involves reduction with aldehydes, sodium borohydride, potassium borohydride, dimethylformamide (D F), glycerol and other polyols, hydrazine, ascorbic acid, sodium sulfite, hydroquinone, or monosaccharides or biosynthetic reduction.

10. The method according to any one of claims 1-7, wherein step a) comprises the sub-steps of:

11. The method according to any one of claims 1-10, further comprising the step of d) removing the toner on the circuit pattern after step c) using a toner removing agent different from the solvent used in step c).

12. The method according to any one of claims 1-10, wherein the substrate is made of an inorganic substrate and the method further comprises the step of e) annealing the substrate after step c).

13. The substrate provided with a circuit obtainable by the method according to any one of claims 1-12.

14. A flexible sheet having a thickness of between 0.05 and 0.3 mm, wherein the flexible sheet is made of an organic material chosen from the group consisting of from polyethylene, polypropylene, polyethylene terephtalate, polybutylene terephtha!ate, cellophane, polymethylmethacrylate, polystyrene, polyether ether ketone and

polydimethylsiloxane and wherein the sheet is provided with a removable electrically conductive layer comprising electrically conductive particles of a material selected from the group consisting of silver, copper, nickel, platinum, palladium and gold.

15. Use of the flexible sheet according to claim 14 for making a circuit pattern.