Method and device for electroplating in cylindrical geometry

Abstract

A method and device for electrodeposition in cylindrical geometry. A method for electrochemically depositing a thin layer on a flexible substrate, comprising: providing, in an electrolysis bath, a first closed cylinder in a second hollow cylinder, applying the flexible substrate to one of the surfaces chosen from the outer surface of the first cylinder and the inner surface of the second, the flexible substrate forming a first electrode, providing, in the electrolysis bath, a second electrode, and applying a potential difference between the first electrode and the second electrode in order to electrodeposit the thin layer on the flexible substrate.

Claims

1. A method for plating a thin layer on a flexible substrate, by electrochemistry, comprising: providing, in an electrolytic bath, a first closed cylinder inside a second hollow cylinder, the electrolytic bath being filled with an electrolytic solution, the electrolytic solution being comprised in a volume delimited between the first cylinder and the second cylinder, applying the flexible substrate on one surface among an outer surface of the first cylinder and an inner surface of the second cylinder, said flexible substrate forming a first electrode, providing, in said electrolytic bath, at least one second electrode, and applying a potential difference between the first electrode and the second electrode to electroplate the thin layer on the flexible substrate.

2. The method of claim 1, further comprising: rotating the first cylinder around an axis thereof during electroplating.

3. The method of claim 1, further comprising: rotating the second electrode.

4. The method of claim 1, further comprising: providing said first, closed cylinder coaxial, being with said second, hollow cylinder.

5. The method of claim 1, wherein another surface among the outer surface of the first cylinder and the inner surface of the second cylinder is the second electrode.

6. The method of claim 1, further comprising: providing a second soluble electrode.

7. The method of claim 1, further comprising: applying the flexible substrate on the outer surface of the first cylinder, and providing a mobile carrier arm connected to the first cylinder.

8. The method of claim 7, further comprising: displacing the first cylinder from the electrolytic bath in the second cylinder to at least one tank in a third cylinder.

9. The method of claim 7, further comprising: moving the first cylinder towards an annealing enclosure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The method which is the subject of the invention will be better understood by reading the description and observing the drawings below in which:

(2) FIG. 1 illustrates a sample electroplating device with cylindrical geometry that can come from the method that is the subject of the invention;

(3) FIG. 2 shows the four principal steps of the electroplating method that is the subject of the invention;

(4) FIG. 3a is a schematic perspective representation of a cylindrical geometry electroplating facility according to an embodiment;

(5) FIG. 3b is a schematic representation of a cylindrical geometry electroplating facility according to the embodiment of FIG. 3a seen from above;

(6) FIG. 4 is a graphic showing the volume of electrolytic solution in liters used in parallelepiped and cylindrical shaped electrolytic baths for three different substrate sizes;

(7) FIG. 5 illustrates the 16 steps of a photovoltaic panel on flexible substrate production method according to an all wet embodiment.

DETAILED DESCRIPTION

(8) As shown in FIG. 1, the invention makes use of a cylindrical geometry electroplating device including a substantially cylindrical tank 2 in which a closed cylinder 1 is inserted. As shown in FIG. 1, the closed cylinder 1 includes a flexible substrate 3 on a portion of the outer surface thereof. This substrate is connected to a supply 9 in order to form a first electrode 8, which advantageously forms a cathode. As shown in FIG. 1, the tank 2 is also connected to the supply 9 in order to form a second electrode 7, which is advantageously a counter-electrode or anode 7. A reference electrode 4, which serves as an independent potential probe, can also be provided in the electrolytic bath between the closed cylinder 1 and the tank 2.

(9) In a first step, the electrolytic bath delimited by tank 2 is filled with an electrolytic solution whose concentration C is chosen on the basis of specific plating parameters. The electroplating starts by applying an electric current I or a voltage between the substrate and the reference electrode or even a voltage applied between the substrate and the anode generated by a supply 9 between the anode 7 and the cathode 8. The closed cylinder 1 is then rotated at an angular velocity , using the motor 5 actuating an arm 6. The angular velocity will subsequently be designated as being the rotation speed of the closed cylinder 1 around the axis thereof in tank 2.

(10) The electroplating method of the invention includes four main steps shown in FIG. 2. A first step, S1, consists in placing the flexible substrate 3 on a cylinder. Two notable cases can arise.

(11) In a first embodiment, it is advantageous to place the substrate 3 on the outer surface of the closed cylinder 1. This substrate can be kept in place by means of toothed discs 10, or any other attachment means for a flexible substrate on a cylindrical surface, such as for example application of an adhesive, holding by depressurization under the substrate 3 or even holding by a mechanical jaw of material that is inert in chemical solution. The curved surface of the closed cylinder 1 can furthermore be the substrate 3 itself, on the condition that the substrate provides a tight seal and does not allow the electrolytic solution to go inside the closed cylinder 1. When the substrate 3 is not an intrinsic part of the closed cylinder 1, the closed cylinder 1 can have an electrically insulating outer surface, in order to avoid electroplating on areas outside the substrate 3. In the opposite case, an electrically insulating material can be applied in the areas outside the substrate 3 exposing an electrically conducting surface of the closed cylinder 1.

(12) An alternative embodiment consists in placing the flexible substrate 3 on the inner surface of the tank 2. In this embodiment, the closed cylinder 1 can be electrically conducting and form a second electrode, which can be a counter-electrode or anode 7. The closed cylinder 1 can also be at least partially covered with a conducting material to form a second electrode, counter-electrode or anode 7. The tank 2 can advantageously be electrically insulating, or, in the opposite case, the exposed electrically conducting areas can be covered with an electrically insulating material.

(13) A third alternative can consist in placing the substrate 3 on the outer surface of the closed cylinder 1 and placing a second, substantially cylindrical, electrode 7 in the substantially cylindrical tank 2. This second electrode 7 can be a closed cylinder at least partially covered with a conducting element connected to the supply 9. These two cylinders can be rotated around their respective axes and move in the tank 2 so as to mix the electrolytic solution during the electroplating method.

(14) After this first step S1 of installation of the flexible substrate 3 on a cylinder, it is appropriate in step S2 to put the closed cylinder 1 into position in the hollow cylinder 2. This placement can advantageously be done such that the closed cylinder 1 and the tank 2 are substantially co-axial. By placing the closed cylinder 1 in the tank 2 such that the two cylinders are co-axial, it is possible to benefit from specific hydrodynamics for homogenizing the electrolytic solution.

(15) The electrolytic bath is advantageously prepared in the following step S3. This preparation includes pouring a liquid electrolyte solution in the volume located between the closed cylinder 1 and the tank 2. It also involves applying electrical contacts connecting the substrate 3 to an electrical supply 9 and also the counter-electrode 7, which could be the tank 2, to this same electric supply 9. It is also advantageous to arrange in the space located between the anode 7 and the cathode 8 a reference electrode 4, which serves as an independent potential probe. The flexible substrate 3, covered with a metal layer, for example molybdenum, can be electrically connected with a copper ribbon. In order to avoid depositing elements on this ribbon, the exposed surface thereof can be covered with electrically insulating material.

(16) It is also possible to invert the steps S2 and S3.

(17) According to an advantageous embodiment, the tank 2 is not the second electrode 7, and this second electrode 7 is an electrode soluble in the electrolytic solution and made up of the material that is intended to be plated on the substrate 3.

(18) The electroplating, strictly speaking, starts once an electric current I is applied by the supply 9 between the two electrodes 7 and 8, for example between the anode 7 and the cathode 8. This current is delivered in step S4. Because of this current, the cations, for example at least one element from columns 11, 12, 13, 14 or 16, present in the electrolytic solution, migrate from the second electrode, for example the anode 7, to the substrate 3, forming cathode 8. When the counter-electrode 7 is soluble, the application of a current progressively dissolves the anode 7 in the electrolytic bath. For example, the anode 7 can be copper immersed in an electrolytic solution of copper sulfate or nitrate. During the electroplating, the plating of copper on the substrate 3 by reduction of ions in the solution is accompanied by the dissolution of the same quantity of copper from the anode 7.

(19) Advantageously, the method includes an additional step of rotating the closed cylinder 1 relative to the hollow cylinder 2. With this rotation it is possible to generate a specific hydrodynamics in the electrolytic solution so as to homogenize the solution and thereby guarantee a more uniform plating of the chemical elements on the substrate 3.

(20) Furthermore, rotating tank 2 around its axis instead of rotating the closed cylinder 1 around its axis is conceivable. The resulting homogenization effect is equivalent.

(21) The electroplating on flexible substrate 3 is thus controlled by three parameters: the cation concentration C in the electrolytic solution, the intensity I of the electric current delivered by the supply 9 or the plating potential V between the substrate and the reference electrode 4, and the angular velocity of the closed cylinder 1 around its axis in the tank 2.

(22) Through a close determination of these three parameters, it is possible to guarantee an electrochemical plating controlled in composition and in thickness.

(23) Advantageously, the electrochemical plating is done in more than one step in order to build up a complex device, for example a photosensitive panel on flexible substrate 3. The devices involved in the production of such a panel according to the method that is the subject of the invention are shown in FIGS. 3a and 3b. The production of such a panel advantageously includes several successive liquid phase electroplatings. In a first part, copper, indium and gallium can be plated. The resulting layer can next advantageously undergo reducing annealing in gaseous phase in an annealing enclosure 201. To do that, the closed cylinder 1 comprising the flexible substrate 3 can be moved using a carrier arm 60, which is intended to undergo a translation along the axis of the cylinder and a rotation around an axis substantially parallel to that of the closed cylinder 1 and located outside of the tank 2. In this way, it is possible to install several electrolytic baths in cylindrical tanks 2, 220, 230 advantageously arranged in a circle around the carrier arm 60. The closed cylinder 1 carrying the flexible substrate 3 can then be moved from one bath to another by translation and rotation of the carrier arm 60. Furthermore, the carrier arm 60 can advantageously also move by radial translation, together with the two modes of movement mentioned above, thereby allowing movement in the three spatial directions.

(24) The step of reducing annealing, for example under hydrogen atmosphere, can be done in an annealing enclosure 201 in which the flexible substrate 3 undergoes thermal treatment by hot gas propulsion, as described in patents FR 2,975,223 and FR 2,975,107. Advantageously, the carrier arm 60 then includes a collar forming a cover 11 installed above the closed cylinder 1 and suited to close the tanks for the electrolytic baths 2, 220, 230, as well as the reducing enclosure 201. Closing the reducing enclosure 201 is particularly advantageous considering that the presence of hydrogen could react on contact with oxygen present in the air. By closing the tanks 2, 220, 230, it is possible to make a primary vacuum or else inject a neutral gas into the tanks in order to avoid oxidation of the walls of the electrodes and cylinders not immersed in the electrolytic solution in addition to those which are immersed in the electrolytic solution.

(25) The reducing annealing step can advantageously be followed by a selenization or sulfurization step done in the same enclosure 201 in vapor phase and at temperatures over 400 C.

(26) Subsequently, the resulting device, including for example a Cu(In, Ga)Se.sub.2 type absorber layer, undergoes two other platings in liquid phase. These platings can be: a first plate, by chemical route, of cadmium sulfide (CdS), forming a buffer layer, and a second zinc oxide (ZnO) electroplating, forming a transparent conducting layer corresponding to the upper electric contact of the photosensitive panel, where the initial metal layer of the flexible substrate 3, for example of molybdenum, forms the rear contact.

(27) Beyond the electroplating method, the invention also relates to an electroplating device with cylindrical geometry for flexible substrate 3.

(28) With the cylindrical geometry electroplating device it is possible to realize substantial savings in volume of electrolytic solution compared to parallelepiped geometry electroplating devices. Indeed, the electrolytic solution is included in the volume delimited by the closed cylinder 1 on the one hand and the tank 2 on the other. It hence appears that the cylindrical geometry makes it possible to reduce the quantities of electrolytic liquid used by increasing the size of the closed cylinder 1. The larger the size of the substrate 3and therefore the larger the outer surface of the closed cylinder 1the larger is the savings in the volume of solution. FIG. 4 is a chart showing various electrolytic baths, some with parallelepiped geometry and others with cylindrical geometry, used with three different sizes of substrate 3: 1010 cm.sup.2, 1515 cm.sup.2 and 3060 cm.sup.2. This graph demonstrates the advantage of making use of a cylindrical geometry electrochemical device for large substrate 3 surfaces. Indeed, to make an electroplating on a substrate 3 whose surface has an area of 3060 cm.sup.2, the cylindrical device geometry requires about 55 L compared to about 200 L for the parallelepiped geometry bath. With the cylindrical geometry in this example it is possible to achieve a savings of about a factor of four in the volume of electrolytic solution used.

(29) The electroplating device which is the subject of the present invention, for example as shown in FIG. 1, advantageously includes a hollow cylindrical substrate carrier, closed at both ends by two toothed discs 10. The electrical contacts connecting the supply 9 to the substrate 3 forming cathode 8 are routed by a hollow shaft 6 advantageously arranged above the closed cylinder 1. The electrical contact for the cathode can thus follow the rotation of the substrate 3 without being twisted. Preferably it involves a turning electrical contact. Advantageously, the hollow shaft 6 can contain, on the upper portion of the closed cylinder 1, a collar forming a cover 11 intended to close the upper end of the tank 2. This makes it possible to avoid evaporation of the electrolytic solution during electroplating phases or else to avoid possible splashes that could otherwise occur. Furthermore, as described above, the presence of a collar 11 that forms a cover serves to seal the tank 2 and inject a neutral gas into it in order to limit the insertion of oxygen present in the outside atmosphere included between the cover and the solution height into the electrolytic solution. This way oxidation of both the submerged parts and non-submerged parts can be avoided at the same time.

(30) The tanks 2, 220, 230, can include openings through which to continuously, or at chosen intervals, inject electrolytic solution. In particular it is possible to provide one opening as an inlet for adding electrolytic solution or rinsing liquid and a second opening as an outlet for evacuating the electrolytic solution or rinsing liquid. With these openings a tank can be reused for plating different chemical elements, which can require electrolytic solutions of different compositions.

(31) On the outer surface thereof, the flexible substrate 3 includes a metal conductor which can be for example molybdenum, titanium, aluminum, copper or any other material commonly used to serve as a conducting metal in an electrolytic bath. Electroplating can advantageously include several steps of plating different chemical elements. Typically, in the production of photosensitive panels, producing a stack of thin layers of different materials is intended, for example a stack of layers including: copper, indium, gallium, selenium, cadmium sulfide and zinc oxide.

(32) Production of a stack of layers calls for more than one electroplating step. Furthermore, the plating of different materials can involve several tanks holding electrolytic baths and annealing enclosures suited to each material to be plated. Consequently, the invention also relates to a facility for electroplating on flexible substrate 3, such as shown for example on FIGS. 3a and 3b.

(33) As shown in FIG. 3a, the closed cylinder 1 is rigidly connected to a carrier arm 60 having an axis of rotation located outside of the tank 2 and substantially parallel to the axis of the first 1 and second 2 cylinders. The attachment of the carrier arm 60 to the closed cylinder 1 can be done with different connection means, like for example, screwing, a weld or clipping. As indicated above, the carrier arm 60 advantageously has, above cylinder 1, a collar 11 forming a cover intended to close the upper ends of the electrolytic tank 2, 201, 220, 230. The arrangement of the tanks 2, 220, 230 and annealing enclosure 201 is advantageously circular so as to make the movement of the carrier arm 60 easier and to reduce the space occupied by the facility.

(34) The carrier arm 60 can turn around an axis outside the tanks 2, 220, 230, translate along the axis of rotation thereof and also move radially relative to the axis of rotation thereof. With such a displacement system for the carrier arm 60, it is consequently possible to route the substrate 3 to any point in the facility.

(35) The facility as shown in FIGS. 3a and 3b has the advantage of considerably reducing the footprint of a facility for production of photosensitive devices. For example, to make a panel on a 3060, 30120 or even 60120 cm.sup.2 substrate, the tank 2 can typically have a 34 cm radius. By supposing that two electrolysis tanks are installed on the same diameter of travel of the carrier arm 60 the footprint of the two reactors would be nearly 70 cm. To leave room for the arm 60 and operators of the facility, it could be advantageous to take four times this dimension, or about 3 m. With such a dimension for the facility, the presence of rinsing reactors between the Cu, In and Ga electroplating and reducing annealing can even be considered, and also between the CdS plating and ZnO electroplating.

(36) It is also conceivable to configure an electroplating device or even a facility in horizontal position instead of vertical. Advantageously, it is then possible to stack the tanks one over the other and to move the carrier arm 60 along the vertical axis to move the substrate 3 from one tank to another. Such a configuration has the advantage of optimizing the floor space by an upward layout.

Example Implementation

(37) FIG. 5 illustrates a specific implementation example of the invention in 16 steps.

(38) During a first step S500, the flexible substrate comprising a 50 m thick molybdenum coating is placed in a 10 cm radius and 150 cm high closed cylinder 1.

(39) In step S501, a soluble copper anode is placed in a 34 cm radius and 150 cm high electrically insulating tank 2. A reference electrode 4 is also called for in tank 2.

(40) In step S502, the closed cylinder 1 is placed into cylindrical tank 2, such that the two cylinders are substantially coaxial. Electrical contacts are made to connect a supply 9 both to the flexible substrate 3, to form a cathode, and also to the counter-electrode 7 to form an anode.

(41) In step S503, a 0.25 mol/L concentration sulfuric acid H.sub.2SO.sub.4 electrolytic solution containing 1 mol/L of CuSO.sub.4 is poured in tank 2.

(42) At step S504, a potential of 1 V relative to the reference potential or a current I of 450 mA is applied between the anode 7 and the cathode 8.

(43) In the following step S505, the closed cylinder 1 is rotated around its axis at a speed of 10 RPM for 15 minutes.

(44) At the end of this step, the copper present in the solution covers the flexible substrate 3 and the copper layer is thus formed.

(45) Because of the progression of the copper ion concentration in the solution, the copper anode 7 is made to dissolve and thus result in a bath with a closely regulated concentration.

(46) It is then followed with a rinsing step S506 of tank 2.

(47) After this rinsing step, a new indium anode 7 is placed in the electrolytic bath filled with sulfuric acid and indium sulfate in step S507.

(48) During step S508, an indium electroplating is then done as previously described.

(49) Analogously to that described above, rinsing is done in tank 2 in step S509, followed by introduction of a soluble gallium anode 7 in step S510 and gallium electroplating in step S511.

(50) Subsequently, the closed cylinder 1 is moved with the carrier arm 60 to the reducing annealing enclosure 201. In step S512 a high temperature reducing annealing under hydrogen atmosphere is done.

(51) This step is followed in step S513 by high temperature selenization in the same enclosure 201 as the previous step.

(52) Next, the closed cylinder 1 is moved to a tank 220 where chemical plating with CdS is done in step S514.

(53) Finally, the closed cylinder 1 is moved to an electrolytic tank 230 in which the photosensitive panel is made through electroplating of a ZnO layer.

(54) The invention is not limited to the embodiments described above, and can include equivalent embodiments.

(55) For example, it is possible to use substantially cylindrical tanks of noncircular section. It is also possible to vary the electroplating parameters during the process, by dynamically modifying the current I, the potential V, the angular velocity and the cation concentration C.

(56) The layout of the various elements of the device and the facility can differ from that presented above, in particular in order to increase the ergonomics of the facility. It is also possible to move the substrate 3 using a carrier arm 60 mobile by translation along the three spatial directions.

(57) Providing a simultaneous rotation of tank 2, 220, 230 and the closed cylinder 1 in opposite directions or in the same direction is also conceivable. When the counter-electrode is not the closed cylinder 1 or the tank 2, 220, 230, it is possible to rotate this counter-electrode 7 in the electrolysis bath, around substrate 3 and around the axis thereof.

(58) The filling rate of the tanks can vary from one plating to another. It is thus possible to only partially fill the tanks with electrolytic solution, or to completely fill them.