METHOD FOR MODIFYING AN ELECTRICALLY CONDUCTIVE OXIDE SURFACE, USE FOR ELECTRODEPOSITION OF COPPER ON SAID SURFACE

Abstract

The present invention relates to a method for modifying the surface of a substrate made of electrically conductive metal oxide and notably made of ITO, comprising the following steps consisting in: (i) bringing into contact said surface with a solution containing copper ions (Cu.sup.2+) and ammonia then washing and optionally drying the surface thus obtained; and (ii) bringing into contact the surface obtained following step (i) with a solution containing sodium tetraborohydride then washing and optionally drying the surface of said conductive metal oxide substrate. The present invention relates to the use of such a method within the scope of the metallisation by copper of a conductive metal oxide substrate as well as the surfaces of a modified and metallised conductive metal oxide substrate thus obtained.

Claims

1. Method for modifying the surface of an electrically conductive metal oxide substrate, said method comprising the following steps consisting in: i) bringing into contact the surface of said electrically conductive metal oxide substrate with a solution containing at least one copper ion (Cu.sup.2+) and at least ammonia then washing and optionally drying the surface of said electrically conductive metal oxide substrate thus obtained; ii) bringing into contact the surface of said electrically conductive metal oxide substrate obtained following step (i) with a solution containing sodium tetraborohydride then washing and optionally drying the surface of said electrically conductive metal oxide substrate.

2. Method according to claim 1, characterised in that, during said step (i), the Cu.sup.2+ ion is, in said solution, in the form of a copper salt, advantageously selected from the group consisting of a nitrate, a sulphate, an acetate, a halide, a tetrafluoroborate and any of the hydrated forms thereof.

3. Method according to claim 2, characterised in that said copper salt is present, in said solution, in a quantity comprised between 0.05 and 0.25 mol/L and advantageously between 0.06 and 0.2 mol/L.

4. Method according to claim 1, characterised in that, during said step (i), ammonia is present, in said solution, in a quantity comprised between 1 and 6 mol/L and advantageously between 1.5 and 5 mol/L.

5. Method according to claim 1, characterised in that said step (i) is carried out at a temperature comprised between 10° C. and 30° C., advantageously between 15° C. and 25° C. and, more particularly, at room temperature and generally for 1 min to 1 h, notably for 5 min to 30 min and, in particular, of the order of 15 min.

6. Method according to claim 1, characterised in that, during said step (ii), sodium tetraborohydride is present in a quantity comprised between 0.03 and 0.15 mol/L, advantageously between 0.06 and 0.1 mol/L and, in particular, of the order of 0.08 mol/L.

7. Method according to claim 1, characterised in that said step (ii) is carried out at a temperature comprised between 30° C. and 60° C., advantageously between 35° C. and 50° C. and, more particularly, at a temperature of the order of 40° C. and generally for 1 to 15 min, notably for 2 and 10 min and, in particular, of the order of 5 min.

8. Method for forming a film of copper metal on the surface of an electrically conductive metal oxide substrate, said method comprising the following steps consisting in: a) preparing a surface of an electrically conductive metal oxide substrate modified in accordance with the modification method such as defined in claim 1; b) electrodepositing copper metal on the surface of the modified substrate prepared during said step (a).

9. Method according to claim 8, characterised in that said step (b) implements an electrodeposition bath comprising Cu.sup.2+ ions being in the form of a copper salt.

10. Method according to claim 9, characterised in that said electrodeposition bath is an aqueous acid solution containing Cu.sup.2+ ions.

11. Method according to claim 8, characterised in that the film of copper metal is formed on the surface of the electrically conductive metal oxide substrate according to a predetermined pattern and in that said method comprises: prior to said step (a), steps consisting in depositing on the surface of the electrically conductive metal oxide substrate a layer of photosensitive resin then eliminating, by photolithography, the resin layer at given sites thus creating said pattern and once said step (b) has been carried out, a step consisting in eliminating the remaining photosensitive resin whereby the electrically conductive metal oxide substrate no longer has resin on the surface thereof.

12. Modified surface of an electrically conductive metal oxide substrate capable of being obtained following the modification method such as defined in claim 1.

13. Surface of an electrically conductive metal oxide substrate coated with a film of copper metal optionally according to a predetermined pattern capable of being obtained following the metallisation method such as defined in claim 8.

14. Use of a modified surface of an electrically conductive metal oxide substrate according to claim 12 in the field of photovoltaic cells; liquid crystal screens, plasma screens, touch screens; OLEDs; antistatic deposits as well as optical, reflective coverings, and anti-reflection coatings.

15. Method according to claim 1, characterised in that said electrically conductive metal oxide is ITO.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0099] FIGS. 1A and 1B propose a schematic representation of the methods of metallisation of an ITO surface of the prior art, namely the Ni—Cu ECD method (FIG. 1A) and the Cu ECD method (FIG. 1B).

[0100] FIG. 2 is a schematic representation of the method of metallisation of an ITO surface according to the present invention.

[0101] FIGS. 3A and 3B present scanning electron microscopy (SEM) micrographs of a heterojunction PV cell with surface view (FIG. 3A) and profile view highlighting the ITO layer of a thickness of 80 nm (FIG. 3B).

[0102] FIGS. 4A and 4B present the profilometry of a line of the mask photolithographied on the heterojunction PV cell before electrodeposition (FIG. 4A) and after electrodeposition (FIG. 4B).

[0103] FIG. 5 presents the UV spectra of a non-treated sample of heterojunction PV cell (ITO ref), a sample treated uniquely with the ammoniacal solution only containing copper sulphate (ITO-CuSO.sub.4/NH.sub.4OH), a sample of cell treated uniquely with the solution of NaBH.sub.4 (ITO-NaBH.sub.4) and a sample treated in accordance with the method according to the invention (ITO-CuSO.sub.4/NH.sub.4OH—NaBH.sub.4).

[0104] FIG. 6 presents the OCP (Open Circuit Potential) measurements on Si of the ITO samples alone (ITO), ITO treated by copper sulphate alone (ITO-CuSO.sub.4), ITO treated by ammonia alone (ITO-NH.sub.4OH) and ITO treated by a solution containing copper sulphate and ammonia (ITO-CuSO.sub.4/NH.sub.4OH).

[0105] FIGS. 7A, 78, and 7C present SEM micrographs of a sample of cell modified according to the method of the invention, i.e. immersed in the ammoniacal solution of Cu(II) then in the solution of NaBH.sub.4, with the surface of a line after treatment (FIG. 7A), highlighting the surface structuring (FIG. 7B) and the profile of the ITO layer thus modified (FIG. 7C).

[0106] FIGS. 8A, 8B, and 8C present SEM micrographs of a sample of cell modified according to the method of the invention, i.e. immersed in the ammoniacal solution of Cu(II) then in the solution of NaBH.sub.4, with surface views at different enlargements (FIGS. 8A and 8B) as well as the energy dispersive (EDX) analysis spectrum associated with the SEM micrograph of FIG. 8B (FIG. 8C).

[0107] FIG. 9 presents an X-ray diffractometry (XRD) analysis of a deposition of ITO on Si having undergone the surface modification treatment according to the invention.

[0108] FIG. 10 presents the profilometry of a copper line on heterojunction PV cell formed by electrodeposition following the implementation of the method according to the invention.

[0109] FIGS. 11A and 11B present the characterisation by SEM microscopy of an electrodeposition of copper on ITO without modification of the ITO surface (FIG. 11A) and of an electrodeposition of copper on ITO following the modification of the ITO surface in accordance with the method of the invention (FIG. 11B).

[0110] FIGS. 12A and 12B present the SEM (FIG. 12A) and EDX (FIG. 12B) analyses of the interface between a line of copper formed by the metallisation method according to the invention and the ITO film at the surface of the heterojunction PV cell.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0111] I. Metallisation of the ITO Surface According to the Method of the Invention.

[0112] I.1. Photolithography Step.

[0113] The heterojunction PV cell implemented has on the surface thereof a pyramidal structure (FIG. 3A) stemming from the chemical etching of crystalline silicon. On the surface, the layer of 80 nm of ITO is observed by SEM and illustrated in FIG. 3B.

[0114] The heterojunction PV cell has firstly been protected on the rear face by means of a photosensitive resin (positive resin AZ 4562). A mask stemming from the same photosensitive resin was also applied on the front face by photolithography.

[0115] I.2. Step of Modification of the ITO Surface.

[0116] The heterojunction PV cell photolithographied beforehand is immersed in a bath of Pegactiv A2 (Pegastech), at room temperature, for 15 minutes. This commercially available bath is an alkaline copper solution containing mainly ammonia (<10%), copper sulphate (<10%) and nickel sulphate (<2.5%). This bath was then replaced by other ammoniacal solutions containing only copper sulphate. The concentrations of ammonia used vary from 1.5 mol/L to 5 mol/L and those of CuSO.sub.2.5H.sub.2O vary from 0.06 to 0.2 mol/L. All the solutions used enabled the modification of the ITO with the same efficiency and are stable over time.

[0117] Once the 1.sup.st bath carried out, the heterojunction PV cell is rinsed with deionised water.

[0118] The heterojunction PV cell is next immersed in an aqueous solution containing 0.08 M of sodium tetra borohydride (NaBH.sub.4, Sigma Aldrich ≧96%), at 40° C., for 5 minutes. The surface of the cell goes rapidly from the original blue to light grey.

[0119] Once the 2.sup.nd bath carried out, the heterojunction PV cell is also rinsed with deionised water.

[0120] I.3. Step of Electrodeposition of Copper.

[0121] Once the ITO modification method has been carried out, the cell is next immersed in the copper electrodeposition bath.

[0122] In order to apply the current necessary for the reduction of Cu.sup.2+ ions into particles of copper metal on the ITO surface, the cell is connected by means of two contact pick-ups situated at the edge thereof.

[0123] The current applied is characteristic of the bath used and is 2.5 A/dm.sup.2 for a given rate of growth of the film of 0.5 μm/min. The anode used is a platinized titanium grid anode. The electrodeposition bath is constituted of 75 g/L of copper sulphate pentahydrate (CuSO.sub.4.5H.sub.2O, Sigma Aldrich), 180 g/L of sulphuric acid (H.sub.2SO.sub.4, (Sigma Aldrich), 70 ppm of hydrochloric acid (HCl, Sigma Aldrich) and 2.5 ml/L of an organic brightening agent (Copper Gleam PC Additive, Pegastech).

[0124] The cell is then metallised by electrodeposition of copper at 2.5 A/dm.sup.2 for 1 hour. It is then removed from the bath and rinsed with deionised water. An annealing of 30 minutes at 50° C. is finally applied.

[0125] The thickness of the copper film produced is controlled by profilometry. The lines measure around 15 urn deep before metallisation (FIG. 4A) whereas, after metallisation, the photolithographied mask is entirely filled with dense copper metal (FIG. 4B).

[0126] I.4. Resin Revelation Step.

[0127] The final step is the revelation of the photolithography mask. The cell is immersed successively and for several minutes in two baths of dimethyl sulphoxide (Sigma Aldrich), then in a bath of ethanol (Sigma Aldrich) and finally in a bath of isopropanol (Sigma Aldrich). The cell is finally dried under a slight current of nitrogen, then in an oven at 50° C. for 10 minutes.

[0128] II. Characterisation of the Surfaces Obtained.

[0129] II.1. Characterisation of the ITO Surface Obtained Following Step I.2.

[0130] Once the heterojunction PV cell has been immersed successively in the two baths (step I.2 as defined previously), its surface rapidly goes from the original blue to a light grey.

[0131] By way of comparison, a non-treated cell sample (ITO ref), a cell sample treated uniquely with the ammoniacal solution only containing copper sulphate (ITO-CuSO.sub.4/NH.sub.4OH) and a cell sample treated uniquely with the solution of NaBH.sub.4 (ITO-NaBH.sub.4) are used. This latter sample was used notably with the goal of determining whether the 2.sup.nd bath damaged the optical and electrical properties of the ITO.

[0132] Sheet resistance measurements using a 4-point measurement system were carried out on the samples and the values are grouped together in Table 1. Only the sample having undergone the two steps of the modification method according to the invention (ITO-CuSO.sub.4/NH.sub.4OH—NaBH.sub.4) show a resistance 4 times higher (328.4Ω/□) compared to the other samples (79-80Ω/□). The electrical properties of the ITO are thus not modified by the single bath of NaBH.sub.4 but by the association of the two successive baths.

TABLE-US-00001 TABLE 1 Sheet resistance measurements of the samples. Samples R (Ω/□) ITO ref 79.38 ITO-CuSO.sub.4/NH.sub.4OH 79.88 ITO-NaBH.sub.4 79.95 ITO-CuSO.sub.4/NH.sub.4OH—NaBH.sub.4 328.4

[0133] The solution of NaBH.sub.4 does not modify either the optical properties of the ITO. The UV-visible spectrum remains unchanged for ITO-CuSO.sub.4/NH.sub.4OH and ITO-NaBH.sub.4 (FIG. 5). Only the association of the two steps of the modification method according to the invention modifies the absorbance of the ITO in the visible domain.

[0134] The surface modification produced on the ITO is thus made possible exclusively thanks to two successive baths: the CuSO.sub.4—NH.sub.4OH solution (ammoniacal copper) and the solution containing NaBH.sub.4.

[0135] In order to show the influence of copper sulphate independently of ammonia, samples of non-textured monocrystalline silicon and covered with a deposition of ITO of 100 nm of thickness were immersed in an aqueous solution containing copper sulphate and/or containing ammonia.

[0136] These samples designated ITO (reference), ITO-CuSO.sub.4 (copper sulphate alone), ITO-NH.sub.4OH (ammonia alone) and ITO-CuSO.sub.4/NH.sub.4OH (solution containing copper sulphate and ammonia) were subjected to an electrochemical analysis by OCP (Open Circuit Potential). The open circuit voltage (without application of current), was measured directly in the solution containing NaBH.sub.4 at 40° C.

[0137] The voltage measurements in open circuit presented in FIG. 6 reveal an identical potential for ITO, ITO-CuSO.sub.4 and ITO-NH.sub.4OH of around E=0.1 V and a potential difference of around ΔE=0.5 V between ITO and ITO-CuSO.sub.4/NH.sub.4OH. This potential difference shows that in the presence of NaBH.sub.4, a surface reaction only takes place in the presence of both CuSO.sub.4 and NH.sub.4OH.

[0138] This surface modification reaction of the ITO thus involves copper-ammonia [Cu(NH.sub.3).sub.4].sup.2+ complexes. To date, the functioning of this step remains unknown.

[0139] The ITO surface that has undergone the two steps of the modification method according to the invention is thus the only one to show differences at the electrical and optical level. FIG. 7A shows an open line of the photolithography mask after treatment, a clear boundary is observed between the zone protected by the resin during treatment and the zone having reacted to the two successive baths. The zoom made on this zone (FIG. 7B, FIG. 8A and FIG. 8B) shows a uniform layer constituted of grains of 80 nm to 200 nm diameter. The profile SEM micrograph, presented in FIG. 7C, shows a layer of ITO which is granular compared to the initial deposition of ITO.

[0140] The EDX analysis presented in FIG. 8C shows that the structured layer is constituted exclusively of ITO. No energy peak associated with copper was observed. The method according to the invention has thus modified the physical-chemical properties of the ITO film.

[0141] The morphological modification of the deposition of ITO has been highlighted by SEM and EDX. To study the change of structure of the ITO, X-ray diffraction analyses were carried out at grazing angle on a sample of non-textured silicon, covered with ITO and having been immersed successively in the two baths. FIG. 9 represents the XRD diagram of the modified ITO sample.

[0142] Analysis by X-ray diffraction shows that the crystallographic structure of the ITO deposition is modified. In addition to ITO, centred quadratic indium metal (reference ICDD 04-004-7737) is highlighted. Taking into account the intensity of the main rays of each of the two compounds (ray (222) for ITO and ray (101) for indium), the composition by weight of each of the two phases in the deposition was estimated at 16% for indium metal and 84% for ITO. It may thus be affirmed that the action alone of each of the two baths does not modify the structure of the ITO deposition, but that their successive action makes it possible to reduce around 16% of indium in the metal form thereof.

[0143] The combination of the different analyses carried out has made it possible to note a real modification of the ITO deposition. The modified ITO sample shows changes not only of the optical and electrical properties thereof, but also the morphological properties thereof: the surface becomes granular, which increases the specific surface; and of the chemical properties thereof: presence of indium metal within the ITO deposition.

[0144] The inventors have also been able to show that copper sulphate and ammonia, used independently, had no effect on the transformation of the ITO. Only the action of copper-ammonia complexes such as [Cu(NH.sub.3).sub.4].sup.2+ is efficient. The most probable hypothesis could be that these complexes, under the action of NaBH.sub.4, play a role of catalyst, thus enabling a transfer of energy capable of modifying the ITO deposition.

[0145] II.2. Characterisation of the Electrodeposition of Copper on an ITO Surface Obtained Following Step I.4.

[0146] The line thicknesses of copper metal obtained following the implementation of the method defined in paragraphs I.1 to I.4 above, controlled by profilometry, are comprised between 18 and 24 μm (FIG. 10).

[0147] By way of comparison, FIG. 11A shows a profile micrograph of an electrodeposition of copper carried out on a heterojunction PV cell without modification of the latter, the empty zones present at the interface show perfectly the absence of a veritable contact between the copper and the ITO inducing a loss of adherence and a poor electrical contact.

[0148] On the contrary, FIG. 11B shows a profile micrograph of the electrodeposition of copper on a heterojunction PV cell using the method of the invention i.e. with modification of the ITO surface. A layer corresponding to the modification of the ITO surface and serving as adhesion layer is present. It thus enables good adherence of the metal on the ITO, which generates an electrical contact of very good quality. The presence of this adhesion layer is revealed by SEM (FIG. 12A) and by EDX analyses, (FIG. 12B).

[0149] Once the method of metallisation of the cell was finished, contact resistance measurements were carried out. The measurements presented in Table 2 hereafter show that even if the Cu-ITO contact resistances of the cells metallised in accordance with the method according to the invention are of the order of 10 times greater than the reference values obtained for cells metallised according to the Cu ECD method (reference method), they remain all the same low and testify to a good electrical contact at the Cu-ITO interface.

TABLE-US-00002 TABLE 2 TLM measurements determining the Cu-ITO contact resistance Sheet R Contact R Specific contact R Width of a plot Rsheet Rc Rhoc Lt (Ohm) (Ohm) (Ohm .Math. cm.sup.2) (μm) Invention 76 1.26 8.43E−03 99 method Cell 1 Invention 78 2.02 1.38E−03 99 method Cell 2 Cu ECD 57 0.0989 1.71E−04 17 method

[0150] Tests of the electrical performances of the cells were finally carried out after laser cutting of the cells. The laser cutting defines around the grid an active surface of 107 cm.sup.2, in order to insulate the active part or the metal grid from the front contact. The electrical efficiencies are very satisfactory (above 20%), and comparable to those obtained by the Cu ECD method. The electrical characteristics of the cells tested are grouped together in Table 3 below.

TABLE-US-00003 TABLE 3 Electrical measurements I(V) on heterojunction PV cells with front contact made of copper by the method according to the invention and the Cu ECD method (reference protocol). 1 sun IV curve Voc Jsc FF η Plate [mV] [mA .Math. cm.sup.−2] [%] [%] Invention method Cell 1 723.9 37.4 77.9 21.1 Invention method Cell 2 714.1 37.4 77.0 20.6 Invention method Mean 719.0 37.4 77.5 20.9 Cu ECD method Mean 714.1 37.4 78.1 20.8

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