Method for producing metal structures

Abstract

A method for producing metal structures includes an initiator composition comprising a photocatalytic substance being applied to a substrate. A precursor composition that can be reduced to a metal by the photocatalytic activity of the nanodusts is applied to the layer. A structure template is then applied, partially displacing the precursor composition, and then the substrate is illuminated. Structured metal structures are thus generated.

Claims

1. A process for producing metallic structures, comprising the following steps: (a) applying an initiator composition to a substrate, the composition comprising a photocatalytically active inorganic substance comprising nanorods having a ratio of length to diameter of between 1000:1 to 2:1 as an initiator, to form an initiator layer; (b) applying a precursor composition comprising a precursor compound for a metal layer to the substrate; (c) applying a structure template to the initiator layer with partial displacement of the precursor composition; and (d) reducing the precursor compound to the metal by electromagnetic radiation through the structure template on the initiator, thereby forming a single metal layer comprising the metallic structures on the substrate.

2. The process as claimed in claim 1, wherein the photocatalytically active inorganic substance comprises TiO.sub.2 or ZnO.

3. The process as claimed in claim 2, wherein the photocatalytically active substance comprises TiO.sub.2.

4. The process as claimed in claim 1, wherein the precursor compound comprises a silver, gold or copper complex.

5. The process as claimed in claim 1, wherein the application of the initiator composition is preceded by pretreatment of the surface of the substrate, said pretreatment comprising a plasma treatment, corona treatment, flame treatment and/or the application and curing of an organic-inorganic coating.

6. The process as claimed in claim 1, wherein the precursor composition is applied in step (b) to the structure template which is applied to the initiator layer in step (c) together with the structure template.

7. The process as claimed in claim 1, wherein the nanorods have a length between 30 and 100 nm, with a ratio of length to diameter between 10:1 and 3:1.

8. The process as claimed in claim 1, wherein the metallic structures have an at least partly transparent appearance.

9. The process as claimed in claim 3, wherein the TiO.sub.2 is doped with a metal compound.

10. A process for producing metallic structures, comprising: applying an initiator composition comprising TiO.sub.2 nanorods to a substrate to form an initiator layer, said substrate comprising a polyethylene terephthalate film, polyimide film, or glass; applying a precursor composition comprising a metal complex solution to the initiator layer; applying a stamp comprising polydimethylsiloxane to a surface of the initiator layer, thereby displacing the precursor composition into depressions of the stamp; and reducing the precursor compound by electromagnetic radiation through the stamp on the initiator composition, thereby forming a single layer of metallic structures on the substrate.

11. The process as claimed in claim 1, wherein the nanorods have a diameter of less than 100 nm and a length of less than 500 nm.

12. The process as claimed in claim 1, wherein the nanorods have a diameter of 30-100 nm and a length of 200-500 nm.

13. The process as claimed in claim 10, wherein the metallic structures have a width of 10 to 100 m.

14. The process as claimed in claim 10, further comprising placing a UV-transparent quartz glass sheet onto the PDMS stamp and said electromagnetic radiation is through the UV-transparent quartz glass sheet and the PDMS stamp.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 TEM image of nanorods of TiO.sub.2;

(2) FIG. 2 electron diffractogram of the nanorods;

(3) FIG. 3A schematic diagram of one embodiment of the process according to the invention (30: structure template; 32: precursor composition; 34: initiator layer; 36: substrate; 38 UV light);

(4) FIG. 3B image of the PDMS stamp template used, produced by means of an impression operation from a commercially available nickel master;

(5) FIG. 3C light micrograph of a surface structure of the PDMS stamp shown in FIG. 3B;

(6) FIG. 4 light micrograph of a metallic structure produced (reflected light image). The light-colored regions are the silver depositions;

(7) FIG. 5 light micrograph of a metallic structure produced (transmitted light image). The dark-colored regions are the silver depositions;

(8) FIG. 6 photograph of an experimental setup for demonstration of an interference pattern involving passing laser radiation through a metallic structure produced;

(9) FIG. 7 light micrograph (transmitted light image) of linear metallic deposition produced using TiO.sub.2 nanorods. The dark-colored areas show silver depositions. The scale corresponds to 10 m;

(10) FIG. 8 light micrograph (transmitted light image) of structured metallic depositions produced using TiO.sub.2 nanorods. Dark-colored areas show silver depositions. The scale corresponds to 10 m.

DETAILED DESCRIPTION OF INVENTION

(11) FIG. 1 shows a TEM image (transmission electron microscope) of inventive nanorods of TiO.sub.2. The elongation thereof is clearly evident.

(12) FIG. 2 shows a diffractogram of nanorods of TiO.sub.2. The reflections demonstrate the crystalline structure of the nanorods.

(13) FIG. 3A shows a schematic diagram of one embodiment of the process according to the invention. A substrate (36) is coated with a photocatalytically active initiator layer (34). To this layer is applied a layer of the precursor composition (32). To a substrate coated in this way is applied a structure template (30), the structure template (30) being brought into at least partial contact with the initiator layer (34). This results in displacement of the precursor composition at these sites. The precursor compound (32) is displaced into the depressions of the structure template (30) which are not in contact with the initiator layer (34). Then exposure is effected with UV light (38) through the structure template. This reduces the precursor compound and deposits a metal layer at the sites where the precursor composition has not been displaced.

(14) FIG. 3B shows a PDMS stamp template used in the examples, produced via an impression operation from a nickel master. The production of such stamps forms part of the specialist knowledge of the person skilled in the art.

(15) FIG. 3C shows a light micrograph of a surface structure of the stamp shown in FIG. 3B. The stamp has cylindrical elevations in a regular hexagonal pattern. The metallic structures produced with this structure template thus show a regular arrangement of circular uncoated regions.

(16) FIG. 4 shows a light micrograph of a metallic structure produced by the process according to the invention. The sharp delimited regions of the deposition of the metallic structures are clearly evident. The sharpness of the metallic structure also shows that much finer structures are possible by the process than the structures shown having structuring of less than 10 m.

(17) FIG. 5 shows a light micrograph of a metallic structure produced by the process according to the invention. In contrast to FIG. 4, a structure template having circular pimples, which appear as nonmetallized circles on the image, was used. Even with structure templates of this kind, a high resolution can be achieved.

(18) FIG. 6 shows an experimental setup in which the interference of a transparent inventive metallic structure is shown. This involves exposure with a laser through a substrate structured in accordance with the invention. The homogeneous structure on the surface results in interference phenomena which are visible on the surface set up at a particular distance. The concentric rings are clearly evident. This involved attenuating the laser light spot on the surface by means of a filter placed there, in order to make the rings visible for the photograph. This experimental setup firstly shows the transparency of the coatings obtained, and also the high imaging accuracy in the case of use of regular structure templates. Only in the case of very high imaging accuracy is there interference.

(19) FIG. 7 shows a light micrograph of a linear metallic structure which has been produced using a corresponding stamp.

(20) FIG. 8 shows a metallic structure with a circular pattern analogous to FIG. 5.

(21) Numerous modifications and developments of the working examples described can be implemented.

EXAMPLES

(22) (1) Substrates Used

(23) The substrates used were various films and glasses. For instance, the films used were polyethylene terephthalate films or polyimide films, polycarbonate substrates and PMMA substrates, and the glasses used were soda-lime glass or borosilicate glass. The size of the substrates varied between 5 cm5 cm and 10 cm10 cm. The thickness of the substrates varied between 0.075 mm and 5 mm.

(24) (2) Production of the Nanorods

(25) Method taken from: Jia, Huimin et al., Materials Research Bulletin, 2009, 44, 1312-1316, Nonaqueous sol-gel synthesis and growth mechanism of single crystalline TiO.sub.2 nanorods with high photocatalytic activity.

(26) 240 ml of benzyl alcohol were initially charged in a 500 ml Schott bottle with a stirrer flea. Subsequently, everything (benzyl alcohol, syringe, titanium tetrachloride) was introduced into a glovebag under argon, the benzyl alcohol bottle was opened and the bag was flushed twice with argon (=filled with Ar and partly emptied and filled again) while stirring vigorously. By means of a 20 ml syringe and a long cannula, 12 ml of TiCl.sub.4 were withdrawn, the cannula was removed from the syringe and the TiCl.sub.4 was added dropwise to the benzyl alcohol while stirring vigorously.

(27) Every drop of TiCl.sub.4 added caused a noise like a crack or bang, and significant evolution of smoke was observed. At the same time, the solution turned an intense red and heated up. On completion of addition, the solution was an intense orange-yellow color and red agglomerates had formed. The mixture was left to stir with the lid open under an Ar atmosphere for another 1 h and then taken out of the glovebag. The solution was then intense yellow in color with several small and somewhat thicker white/yellow agglomerates.

(28) Under a fume hood, the mixture was then left open to stir for another 1 h, before being divided into two Teflon vessels without the thicker lumps (130 g each) and autoclaved (pressure digestion: in block A; time: 223 h 59 min; temp.: 80 C.)

(29) The supernatant in both Teflon vessels was removed by means of a pipette, and the gel-like white precipitate was slurried, introduced into centrifuge tubes and centrifuged (15 min; at 2000 RCF; at RT; braking power: 0). The centrifugate was decanted and chloroform was added to the residue. The mixtures were left to stand overnight.

(30) The centrifuge tubes were balanced out in pairs with chloroform, shaken properly until no larger agglomerates were observable any longer, and centrifuged (15 min; 3000 RCF; RT; braking power: 0). The centrifugate was decanted again and chloroform was again added to the residue. Subsequently, the further procedure was as described above (without leaving to stand overnight). Overall, the particles were washed three times with chloroform.

(31) After the last decanting operation, the centrifuge tubes were left open to stand under a fume hood overnight and, the next morning, the dried nanorods were transferred into a snap-lid bottle.

(32) (3) Preparation of the Silver Complex Solution

(33) 0.1284 g (1.06 mmol) of TRIS (tris(hydroxymethyl)aminomethane) was dissolved in 0.5 g (27.75 mmol) of deionized H.sub.2O and 0.5 g (10.85 mmol) of EtOH. In addition, 0.0845 g (0.5 mmol) of AgNO.sub.3 was dissolved in 0.5 g (27.75 mmol) of deionized H.sub.2O and 0.5 g 10.85 mmol) of EtOH. The AgNO.sub.3 solution was added to the first solution while stirring. The solution of the metal complex formed was colorless and clear. The solution can also be prepared in pure deionized water.

(34) (4) Lyothermal Synthesis of TiO.sub.2 Particles (lyo-TiO.sub.2)

(35) 48.53 g of Ti (O-i-Pr).sub.4 were added to 52.73 g of 1-PrOH (n-propanol). To this solution was slowly added dropwise a solution of hydrochloric acid (37%, 3.34 g) and 10.00 g of 1-PrOH. To this solution was then added dropwise a mixture of 4.02 g of H.sub.2O and 20.00 g of 1-PrOH. The solution obtained may be pale yellow in color and was transferred to a pressure digestion vessel (approx. 130 g). In this vessel, the solution was treated at 210 C. for 2.5 h.

(36) The mixture was decanted and the particles obtained were transferred to a flask and the solvent was removed at 40 C. in a rotary evaporator under reduced pressure.

(37) For further use, the particles obtained were suspended in water.

(38) (5) General Use

(39) The steps which follow were conducted for each sample. The substrates were pre-cleaned with ethanol, propanol and lint-free tissues. The various suspensions were applied either by flow coating or by knife coating. The TiO.sub.2 layers obtained were dried in an oven at temperatures between 100 C. and 150 C., especially at 120 C. or 140 C., for 5 to 30 minutes. Thereafter, the substrates were rinsed with deionized water to remove residues, and dried with compressed air.

(40) Thereafter, the solution of the silver complex was applied. The structure template, usually a PDMS stamp, was applied to the surface and irradiated with UV radiation. Thereafter, the excess silver complex was removed by rinsing with deionized water and the coated substrates were dried with compressed air. The light source used was a mercury-xenon lamp (LOT-Oriel solar simulator, 1000 W, focused onto an area of 10 cm10 cm). The intensity of the lamp was measured with the UV-Integrator digital measuring instrument (BELTRON) and was 55 mmW/cm.sup.2 within the spectral range from 250 to 410 nm.

(41) (6) Suspensions of TiO.sub.2 nanorods in H.sub.2O/EtOH

(42) First of all, the TiO.sub.2 nanorods were suspended in deionized water. Thereafter, an appropriate amount of ethanol was added. In all suspensions, the ratio of H.sub.2O and EtOH in the suspensions was H.sub.2:EtOH.fwdarw.20:80 in % by weight or 10:90 in % by weight. For the production of TiO.sub.2 layers, the following suspensions were prepared: 1.5% by weight of TiO.sub.2 nanorods in H.sub.2O/EtOH
(7) Application of the TiO.sub.2 Layer to a Porous SiO.sub.2 Layer

(43) The suspension of TiO.sub.2 nanorods was applied by flow coating to a porous SiO.sub.2 layer on glass. For this purpose, a standard SiO.sub.2 sol was used.

(44) (8) Production of a Structure Template

(45) Production of an embossing stamp as a cast of a nickel master There follows a description of the production of an embossing stamp made from PDMS (silicone rubber) as a cast of a nickel master.

(46) 1. Description of the Nickel Master and of the Casting Mold

(47) The nickel master is a nickel foil produced by electroplating and of dimensions 100 mm100 mm, to which a microstructure (cylindrical depressions in regular arrangement having a diameter of 5 m, a height of 10 m and a separation of 5 m) has been applied. This nickel foil is bonded to the base of a casting mold produced from aluminum or a similar material, or mounted by means of ferromagnetic adhesive film. It should be noted here that the nickel master should be applied with absolute planarity, since any unevenness will be reflected in the later stamp.

(48) In addition, the casting mold has to be placed in a very substantially horizontal position in order that the embossing stamp later has a homogeneous thickness.

(49) 2. Mixing of the Silicone Rubber and Mold Casting

(50) The two components of the PDMS (e.g. Sylgard 184 from Dow Corning) are combined in the appropriate ratio (e.g. 10:1) and the two components are mixed by stirring. The amount to be made up is guided by the desired thickness of the embossing stamp (typical stamp thickness: 2 to 4 mm). The mixing vessel should have 3 times the volume of the mixture in order to prevent overflow in the subsequent degassing operation.

(51) To remove the air bubbles mixed in during stirring, the mixture is introduced into a vacuum drying cabinet (at room temperature) and evacuated until all air bubbles have been removed.

(52) The degassed PDMS mixture is then introduced into the casting mold and the mixture is allowed to cure. In most cases, it is advisable to accelerate the curing by temperature control of the casting mold. Typically, heating of the casting mold to 70 C. over one hour leads to full curing of the PDMS.

(53) 3. Demolding Operation

(54) The demolding of the PDMS stamp is accomplished by using a scalpel or another sharp blade to cut the PDMS away from the vertical wall of the casting mold over the entire circumference and then lifting it away from the edge with a flat and blunt tool (for example a flat spatula) and then cautiously detaching it from the nickel master.

(55) Irregular sites at the edge can then be cut away with a sharp blade (for example carpet knife).

(56) The reason for the use of PDMS as the stamp material is the transparency of this material for UV light, which is used for the photochemically implemented metallization. In this context, a suitable stamp material is also any other material which meets the demands of UV transparency and demoldability according to point 3. of the abovementioned example of the production of the embossing stamp as a cast of a nickel master.

(57) (9) Production of the Silver Microstructures

(58) The substrates (e.g. glass, PMMA, PET, PVC, PS, . . . ) were coated with TiO.sub.2 nanorods. A transparent coating was obtained.

(59) Thereafter, the coated surface was wetted with the solution with the silver complex. A PDMS stamp (polymethyldisiloxane) was pressed onto the substrate. In the course of this, the elevations of the stamp and the points which come into contact with the photocatalytic layer displace the solution with the silver complex at these points. The substrate was then irradiated through the transparent stamp with UV light (e.g. LOT-Oriel solar simulator, 1000 W Hg(Xe) light source, focused onto an area of 1010 cm.sup.2) for 20 s to 2 minutes. The stamp was removed and the excess silver complex was removed by washing. Thereafter, the substrates were dried. It is also possible to effect thermal aftertreatment, for example at temperatures between 50 C. and 200 C. Optionally, a further protective layer was applied or laminated on. This operation gives structures having a resolution of up to 1 m.

(60) For the metallic structures shown in the figures, a TiO.sub.2 layer of nanorods was applied as a suspension without a binder system in a thickness of approx. 60 nm, and a solution of the silver complex was applied. Application of a PDMS stamp of appropriate shape was followed by exposure for 20 s to 60 s. The stamp had a structure depth of <10 m, which then corresponds roughly to the thickness of the layer of the solution of the silver complex during the exposure.

(61) The process enables production of metallic structures in a simple manner with only two coating compositions.

(62) (10) Production of Metallic Structures with PDMS Stamps

(63) 1.

(64) An initiator-coated substrate according to (5) is placed under the exposure unit and wetted (overcoated) with the suspension of the precursor composition as described in Example (5).

(65) 2.

(66) A PDMS embossing stamp produced as described in (8) is then moved toward the wetted substrate at an angle of 45 on one side and placed on gradually until it lies completely planar on the substrate. This serves to prevent possible inclusions of air which could have an adverse effect on the quality of the structured metallization.

(67) 3.

(68) The embossing stamp is then weighted down over the full area in such a way that the beam path for a later exposure of the initiator layer remains unimpeded. This is achieved by placing a UV-transparent quartz glass sheet, the dimensions of which are greater than the embossing stamp, onto the latter over the full area. Onto this is placed a frame made of any material which leaves the beam path through the PDMS stamp unimpeded and serves merely to weigh down the quartz glass sheet and hence also the stamp. The aim of this process is to completely displace the suspension of the precursor composition under the raised regions of the structured stamp between the latter and the substrate coated with initiator layer. The minimum pressure needed for that purpose depends on the structure type and size of the stamp and is determined empirically for every stamp used. The maximum pressure which may be exerted without adversely affecting the quality of the metallization result is at first likewise determined empirically. This depends not only on the structure type and size but also on the aspect ratio of the structures as a result of the deformability of the stamp material. An adverse effect of the metallization result in the context of the application is reduced resolution or altered shape of the metallic structures resulting from plastic deformation of the stamp or else a lower resulting thickness of the metallic layer resulting from displacement of the suspension of the precursor composition from the non-raised regions (equivalent to interstices) of the embossing stamp.

(69) 4.

(70) The initiator layer is then irradiated through the UV-transparent quartz glass layer, the UV-transparent embossing stamp and the UV-transparent suspension of the precursor composition, as described in (5).

(71) 5.

(72) After sufficient irradiation time, the beam path at the light source is blocked by means of a shutter and, simultaneously or successively, the weighting frame, the quartz glass sheet and the embossing stamp are removed from the substrate and the latter is rinsed as described in (9) and dried and optionally thermally aftertreated as described in (9).

(73) 6.

(74) Alternatively to point 1. of this example, the stamp can also first be applied as described in point 2. of this example to the substrate, except that it is unwetted, and weighted as described in point 3. Subsequently, the suspension of the precursor composition is applied to the substrate at any number of sites on the edge of the stamp until the interstices between stamp and substrate have been completely filled with the same suspension owing to the capillary forces and the air has been removed therefrom. For this purpose, it may be necessary or advantageous to tilt the substrate along with the stamp, quartz glass and frame.

(75) Subsequently, the further procedure is according to points 4. and 5. of this example.

REFERENCE NUMERALS

(76) 30 Structure template 32 Precursor composition 34 Initiator layer 36 Substrate 38 UV light

LITERATURE CITED

(77) U.S. Pat. No. 5,534,312 US 2004/0026258 A1 US 2005/0023957 A1 US 2006/0144713 A1 Noh, C.-, et al., Advances in Resist Technology and Processing XXII, Proceedings of SPIE, 2005, 5753, 879-886, A novel patterning method of low-resistivity metals. Noh, C.-, et al., Chemistry Letters, 2005, 34(1), 82-83, A novel patterning method of low-resistivity metals. US 2009/0269510 A1 Jia, Huimin et al., Materials Research Bulletin, 2009, 44, 1312-1316, Nonaqueous sol-gel synthesis and growth mechanism of single crystalline TiO.sub.2 nanorods with high photocatalytic activity.