Method for depositing a metallic material on a major surface of a substrate

Abstract

An embodiment relates to a method for depositing a metallic material on at least one major surface of a planar substrate (120) according to a pre-determined pattern. The method includes providing at least one planar substrate (120) having a major surface with one or more conductive regions arranged according to said pre-determined pattern. The method also includes dispensing a fluid stream (320) containing metal ions from a plurality of jetting orifices (135) to form a dispensed fluid stream (320) that first intercepts an electrode surface (310) and is redirected by the electrode surface to contact the major surface of the planar substrate (120); and forming an electrochemical circuit between the electrode surface (310) and the one or more conductive regions of the planar substrate (120) such that the metal ions applied to the major surface of the planar substrate by the dispensed fluid stream are electrochemically reduced to form a metallic material on the major surface according to the pre-determined pattern.

Description

[0001] Method for depositing a metallic material on a major surface of a substrate

[0002] Technical Field

[0003] The present invention relates generally to electrodepositing a metallic material according to a predetermined pattern on a major surface of a substrate.

[0004] Backoround

[0005] Printed circuit boards, integrated circuits and devices such as solar cells require metal structures to form electrodes which can transfer electrical current to and from a device or between components of a device. Copper electroplating has been successfully used to form metallic patterns and interconnects for printed circuit boards, integrated circuits and packing of electronic devices. However, for some devices, such solar cells, use of copper electroplating has been limited by high cost and low processing throughput. There is hence a need to both reduce cost and increase processing throughput compared to current known electroplating methods, whilst not sacrificing deposition quality.

[0006] Most electroplating methods require large volumes of electrolyte and employ agitation to ensure that metal ions do not become depleted at the surface(s) to be electroplated. In order to increase throughput and reduce handling, substrates are often conveyed through a tank, commonly referred to as an electroplating bath, containing an electroplating electrolyte which contains the source of metal ions. The volume of electrolyte required by these electroplating baths can be as high as 800 to 1000 L for production systems.

[0007] Substrates to be electroplated can be oriented horizontally and conveyed across the surface of the electroplating electrolyte using sets of rollers or similar conveying mechanisms. Electrical contact is made to the upper surfaces of each substrate, whilst metal is deposited on regions of the under surfaces of the substrates which are exposed to the electroplating electrolyte through a pattern of openings in an insulating masking layer. The upper surfaces of the substrates are typically conductive or partially conductive and are maintained dry during the electroplating process.

[0008] Alternatively, substrates can be conveyed through the bath in a vertical orientation. In the latter arrangement, both substrate major surfaces can be in contact with the electroplating electrolyte and electrical contact is typically made via a conductive (e.g., metal) seed layer which extends across at least one of the major surfaces of each substrate and is exposed to the electroplating electrolyte through openings in an insulating masking layer.

[0009] In both arrangements, the negative terminal of a power source is in electrical contact with the substrate and the positive terminal of the power source is connected to an anode, which is immersed in the electroplating electrolyte. Provision of an electrical potential between the positive and negative terminals of the power source initiates electrochemical deposition at the cathodic regions of the substrates which are exposed to the metal ions of the electroplating electrolyte.

[0010] A key advantage of the vertical orientation is that both major surfaces of the substrate can be electroplated at the same time. Whereas for the horizontal arrangement, typically each substrate needs to pass through the bath twice to form metal electrodes on both its major surfaces. However, countering the advantage of the vertical orientation is the requirement for a conductive seed layer on the major surfaces of the substrate being electroplated, the seed layer conducting the electrical current across the surface of the substrate. Consequently, both horizontal and vertical electroplating arrangements exist in the market today.

[0011] However, both electroplating orientations typically require large volumes of electroplating electrolyte. These large volumes introduce complexities. Electroplating baths can be expensive to make-up with fresh electrolyte for new processes. In the event that the electroplating electrolyte becomes contaminated with waste products, which can arise from the use of soluble anodes and / or the chemical decomposition of electroplating additives, the electrolyte may need to be filtered, chemically treated or, in worse cases, disposed of and replaced. This can result in a significant cost due to both the high make-up cost as well as the cost of disposing of the contaminated electroplating electrolyte.

[0012] Further complexities are introduced by the need to pump electrolyte through the electroplating system. The flow of the electrolyte past the moving surface to be electroplated serves to replenish the source of metal ions in the surface layers of the device thereby ensuring that there are always sufficient metal ions to sustain the required electroplating rate. Fast electroplating rates can determine the economic viability of processes such as through silicon vias (TSVs) for wafer level packaging of semiconductor devices.

[0013] Electrolyte agitation with eductors, or other flow conditioners, can be used in addition to or in place of the relative electrolyte movement across the surface to be plated generated by the device moving, to ensure sufficient metal ion supply at the substrate surface. If high rates of electrochemical deposition are required to sustain a high manufacturing throughput, active electrolyte agitation is typically required in addition to the flow generated by conveying.

[0014] The power consumed by fluid pumping in electroplating systems can be as large as 300 - 500 kW for an annual manufacturing throughput of -100 MW of solar cells. In addition, if an electroplating process requires that more than one metal is deposited to form a metal electrode on the substrate, the substrate must be either transferred to a second bath or transitioned to a new zone in the electroplating system via a weir or similar draining construct which can increase substantially the pumping power and factory footprint required for the electroplating system.

[0015] What is required to reduce the cost and increase the processing throughput of existing electroplating systems for planar substrates, such as wafers for wafer level packaging and solar cells, are new electroplating processes and equipment which can significantly reduce the volume of operating electrolyte. This can directly reduce the operational costs and waste management demands of current electroplating systems, thereby lowering the cost of using copper metallization.

[0016] Summary

[0017] An embodiment provides a method for depositing a metallic material on at least one major surface of a planar substrate according to a pre-determined pattern, the method comprising: providing at least one planar substrate having a major surface with one or more conductive regions arranged according to said pre-determined pattern; dispensing a fluid stream containing metal ions from a plurality of jetting orifices to form a dispensed fluid stream that first intercepts an electrode surface and is redirected by the electrode surface to contact the major surface of the planar substrate; and forming an electrochemical circuit between the electrode surface and the one or more conductive regions of the planar substrate such that the metal ions applied to the major surface of the planar substrate by the dispensed fluid stream are electrochemically reduced to form a metallic material on the major surface according to the pre-determined pattern.

[0018] The dispensed fluid stream may be directed at an angle ranging from 20° to 80°, and more preferably from 55° and 70°, to a perpendicular line extending from the planar substrate. The dispensed fluid stream may be directed in a direction substantially perpendicular to the planar substrate after being redirected by the electrode surface. The electrode surface may be substantially planar. The term “substantially perpendicular” as used herein means a direction perpendicular or close to perpendicular such as a transverse angle ±10° from perpendicular. The planar substrate may be in a substantially vertical orientation when forming the dispensed fluid stream and forming the metallic material on the major surface. The fluid may be dispensed by one of hydrodynamic flow, ultrasonic or thermal dispensing means.

[0019] The jetting orifices may be arranged in at least one vertically oriented linear array and a spacing between individual jetting orifices in the linear array is less than 5 mm, such as less than 3 mm. the jetting orifices may comprise piezoelectric actuators. The actuators may be actuated at a frequency ranging from 100 to 300 kHz, preferably ranging from 150 to 200 kHz.

[0020] The method may further comprise conveying the planar substrates past the electrode surface at a speed ranging from 0.2 m / min to 1 .5 m / min. The metallic material may be one of nickel, silver, copper, tin and alloys thereof. An insulating material may cover a portion of the major surface of the planar substrate and exposes the one or more regions of a conductive layer which extends across the planar substrate below the insulating material. The at least one conductive region on the planar substrate may be connected to a negative terminal of a power supply. The at least one conductive region may be connected to the negative terminal using a lead having a clip that clips onto the planar substrate to contact the at least one conductive region.

[0021] The planar substrate may be a printed circuit board, a substrate used for the interconnecting of electronic circuits, or other material such as glass or a ceramic material designed to support electronic circuits or components of electronic circuits. The planar substrate may be a semiconductor device. The planar substrate may be a semiconducting wafer. The planar substrate may be a solar cell.

[0022] The planar substrate may have a first major surface and a second major surface and the metallic pattern comprises an electrode which is formed at least on the first major surface or on both major surfaces of the substrate. The second major surface may have a conductive region that is in electrical connection with a negative terminal of a power source. The method may further comprise forming the metallic pattern on the first major surface of the solar cell due to current flowing through the planar substrate in a direction which is substantially perpendicular to the major surfaces of the substrate. Where the planar substrate comprises a p-n junction, and the first major surface may have an n-type polarity, and the second major surface may have a p-type polarity, the method may further comprise illuminating the substrate. The generated photocurrent induces a voltage which forward biases the semiconductor junction of the substrate so that a metallic pattern is formed on one or more conductive regions exposed on the n-type surface of the substrate.

[0023] The method may further comprise affixing a plurality of planar substrates to a plate with electrode regions. The step of applying metal ions to the major surface of the plurality of planar substrates may form a metallic pattern on the first major surfaces of each of the plurality of planar substrates. The conductive region may include a metal, a transparent conductive oxide, and a silicon material including a polysilicon material.

[0024] An embodiment provides an electroplating system for depositing a metallic material on at least one major surface of a planar substrate according to a pre-determined pattern, the electroplating system comprising: a body having a chamber that can receive a planar substrate having a major surface with one or more conductive regions arranged according to the predetermined pattern; a dosing system having a plurality of jetting orifices for providing a fluid stream containing metal ions; an electrode surface spaced from the plurality of jetting orifices and arranged such that the fluid stream of metal ions emitted from the plurality of jetting orifices is reflected off the electrode surface to be redirected to the planar substrate; and an electrical system configured to electrically connect the electrode surface and the one or more conductive regions on the planar substrate and configured such that metal ions applied to the one or more conductive regions are electrochemically reduced to form a metallic material on the major surface of the planar substrate according to the pre-determined pattern.

[0025] The chamber may have an open first end and an open second end such that the planar substrate can be conveyed through the chamber in a direction extending from the first open end to the second open end. The chamber may be substantially vertical such that the planar substrate is orientated vertically when the plurality of jetting orifices provide the fluid stream containing metal ions. The plurality of jetting orifices and the electrode surface may be positioned on a sidewall of the chamber. The jetting orifices may be integrated in or attached to the sidewall of the chamber. A longitudinal direction of each jetting orifice may be arranged perpendicular to the sidewall of the chamber. A plurality of jetting orifices may be configured such that the fluid stream of metal ions is directed at an angle ranging from 20° to 80°, and preferably from 55° to 70°, relative the longitudinal direction. The electrode surface may be configured such that the reflected fluid stream travels along a direction that is substantially parallel to the longitudinal direction. The plurality of jetting orifices may be arranged in a vertically oriented linear array. A spacing between individual jetting orifices in the linear array may be less than 5 mm, and preferably less than 3 mm.

[0026] The electroplating system may further comprise a plurality of vertically oriented linear arrays each having a plurality of jetting orifices. Adjacent linear arrays may be spaced apart by a distance ranging from 5 mm to 50 mm. The electrode surface may be substantially planar.

[0027] The dosing system may be configured such that the fluid stream of metal ions is dispensed by one of hydrodynamic flow, ultrasonic or thermal dispensing means. The jetting orifices may comprise piezoelectric actuators. The electrical system may comprise a clip for electrically connecting the one or more conductive regions on the planar substrate to the negative terminal of a power source. The electrode surface may be electrically connected to the positive terminal of the power source.

[0028] The electroplating system may further comprise a light source for illuminating the planar substrate. The light source may be configured so that, when the planar substrate is received in the chamber, illumination of the planar substrate generates a photocurrent which induces a voltage that forward biases the semiconductor junction of the planar substrate such that a metallic pattern is formed on an n-type surface of the planar substrate. The light source may include a plurality of light sources and the electrode surface includes a plurality of electrode surface. Each light source of the plurality of light sources may be positioned an adjacent electrode surface of the plurality of light sources. The planar substrate may be a solar cell.

[0029] The electroplating system may further comprise a conductive plate configured to be received in the chamber and to electrically bond to a plurality of planar substrates such that the plurality of substates bonded to the conductive plate can be received in the chamber. The chamber may comprise a reservoir in which redirected fluid that has contacted with the planar substrate can flow down off the planar substrate to fall into the reservoir. The reservoir may be in fluid communication with the jetting orifices such that fluid collected in the reservoir can be recirculated back to the jetting orifices. The electroplating system may further comprise a filter for removing particulate matter from fluid being recirculated from the reservoir to the jetting orifices. The conductive region may include a metal, a transparent conductive oxide, and a silicon material including a polysilicon material.

[0030] A major advantage of an embodiment of the present disclosure is that the total volume of electrolyte required for electroplating is significantly reduced and can be < 10% that of prior art methods which use large recirculating electrolyte baths. The power required to pump electrolyte through the electroplating system may also be significantly reduced due to the reduced volume of electrolyte. Furthermore, the increased control over the delivery of the electrolyte to the surface may enable uniform metal electrodeposition across an entire major surface of a semiconductor device at high electrodeposition rates.

[0031] Brief Description of the Drawings

[0032] Embodiments will now be described by way of example only with reference to the accompanying non-limiting Figures, in which:

[0033] Figure 1 is an embodiment of a schematic cross-sectional representation in which an array of dispensing jets is applied to supply an electrolyte containing metal ions which are reduced to form a metallic pattern on both major surfaces of a planar substrate such as a solar cell;

[0034] Figure 2A is an embodiment of a schematic cross-sectional in which an array of dispensing jets is applied to supply an electrolyte containing metal ions which are reduced to form a metallic pattern on a major surface of two planar substrates, the substrates being supported on a central cathode plate which is electrically connected to the negative terminal of a power source;

[0035] Figure 2B is a schematic representation of a solar cell surface supported on a cathode plate such as shown in Figure 2A;

[0036] Figure 2C is a schematic representation of a major surface of two separate solar cells which are supported on a cathode plate such as shown in Figure 2A;

[0037] Figure 3 is a schematic top view representation of an individual dispensing jet needle showing the redirection of the jet stream from the jet needle orifice onto first an electrode portion and then the substrate on which the metallic pattern is to be formed;

[0038] Figure 4A is a schematic side view showing three dispensing jet arrays integrated into the outer walls of an electroplating chamber and redirecting their individual jet streams into the surface of aligned electrode portions;

[0039] Figure 4B is a variation of the arrangement depicted in Figure 4A, in which arrays of light sources are additionally located on the side wall of the electroplating chamber between the jet arrays.

[0040] Detailed Description of Embodiments

[0041] Embodiments relate to a method for depositing a metallic material on at least one major surface of a planar substrate such as a solar cell by electrodeposition from a solution of metal ions. The metal is typically deposited such that an electrode pattern is formed on a surface of the planar substrate allowing current to be extracted to / from an electronic device or between components of an electronic device. Embodiments also relate to an electroplating system used to perform an electroplating method.

[0042] The embodiments are be described with reference to a planar substrate comprising a solar cell; however, it should be clear to a person skilled in the art that embodiments could also be directed to forming patterns of metal structures on wafers used for other semiconductor devices such as integrated circuits and substrates which integrate individual electronic devices, such as printed circuit boards and substrates used for 2.5D and 3D integration of electronic circuits (e.g., silicon, glass and ceramics). For solar cells, the formed metal pattern enables solar cells to be electrically connected to other solar cells to form a solar PV module.

[0043] The term “electroplating” is used throughout this specification to refer to the general process of electrodeposition of a material onto a surface. Electroplating requires an electrochemical circuit between negative (cathode) and positive (anode) terminals of a power source. The surface on which a metallic pattern is to be formed is in electrical contact with the cathode and metal ions supplied to that surface are reduced to form a metallic pattern. An electrochemical oxidation reaction occurs at the anode to complete the circuit.

[0044] In conventional electroplating baths, metal ions are typically supplied by immersing the cathode (with the substrate or surface to be electroplated) and the anode in a bath of electrolyte and applying a potential across the terminals of the power supply. The cathode and anode support the reduction and oxidation reactions of the electrochemical reaction, respectively, with ion flow in the electrolyte completing the electrochemical circuit.

[0045] Anodes can be classified as soluble (which dissolve to provide a source of metal ions) or insoluble (support an oxidation reaction that typically releases a gas). Insoluble anodes typically comprise meshes or wire patterns of a metal such as titanium which are coated with a surface layer capable of catalysing a reaction that, most commonly, generates a gaseous product such as 02(g) or CI2 (g). Typically, insoluble anodes are used when rapid electroplating rates are required.

[0046] Although embodiments are described below with reference to the formation of copper electrode patterns on silicon solar cells, it should be clear to a person skilled in the art that the method could also be applied to other solar cells, including thin film solar cells, such as solar cells comprising cadmium telluride (CdTe), copper indium gallium selenide (CIGS), perovskite structures and various tandem and multijunction devices.

[0047] For silicon solar cells, the electrode pattern typically comprises a plurality of thin metal fingers, which are intercepted substantially at right angles by one or more busbars. Depending on the design of a solar cell and, in particular, the lateral conductivity of the electrical carrier collection layers, the thin metal fingers can be spaced between 1 .5 mm and 0.5 mm apart. The number of busbars per solar cell can be varied, though for larger wafer sizes the number of busbars is typically greater than or equal to 12. An important advantage of having a larger number of busbars is that the amount of metal, which is required to form the metallic electrode pattern, can be reduced as the area from which electrical current is collected in each finger is reduced. However, use of too many busbars results in increased shading of the solar cell. Hence an electrical optimization should be performed as is described, for example, in “Solar Cells: Operating Principles, Technology and System Applications (The Red Book)” by M. Green (ISBN: 0858235803).

[0048] In many industrially produced silicon solar cells, the metal busbars are often replaced by a series of solder pads connected by narrower metal lines. The solder pads provide solid metal regions to which interconnection wire can be reliably soldered to interconnect a series of solar cells in an electrically connected string. The strings of cells can then be laminated in a solar PV module.

[0049] The electroplated metallic electrode pattern can be formed on just one major surface of the solar cell, or on both major surfaces (bifacial) of a solar cell. The bifacial arrangement is advantageous because it allows the metallised solar cells to be interconnected into bifacial modules, which can receive light from both major surfaces. Bifaciality is especially advantageous in ground-mounted PV installations where highly reflective backgrounds can result in larger electricity generation over a period of time.

[0050] In order to electroplate a metallic electrode pattern on a substrate such as a solar cell surface using any of the abovementioned arrangements, a pattern of openings is typically formed through a masking layer to allow the electroplating electrolyte to contact the electrically conductive regions of the solar cell only where the metallic contact pattern is required.

[0051] In an embodiment, the conductive region includes a conductive material, a conductive substrate, a semiconductor, a metal, a transparent conductive oxide, and a silicon material including a polysilicon material. However, the disclosure is not limited to these conductive regions and may include additional types of conducting regions.

[0052] The masking material may include an inorganic material such as SiC>2 or SiNxor an organic material, such as a resin polymer or hotmelt ink. Patterning of this masking material can be formed using photolithography, a laser or a printer, such as an inkjet printer. Alternatively, the mask can be formed by screen printing the masking material according to a predetermined pattern of openings.

[0053] Forming a pattern of openings in a resin polymer layer is described for example in: Z. Li et al., Patterned masking using polymers: insights and developments from silicon photovoltaics, International Material Reviews, 61 :6, 416-435, 2016). Another polymer masking method comprises the direct printing of a hot melt wax mask. For this method, the wax is melted in the printhead and, when encountering the substrate, the wax solidifies in a mask pattern (for further details we refer to: A. Descoeudres et aL, Low-temperature processes for passivation and metallization of high-efficiency crystalline silicon solar cells, Solar Energy, 175, 54, 2018).

[0054] Alternatively, a thin inorganic mask can be used with patterning being achieved using laser ablation or inkjet removal of the inorganic material in the pattern of the desired metallic grid (for further details we refer to T. Hatt et aL, Advances with resist-free copper plating approaches for the metallization of silicon heterojunction solar cells, AIP Conference Proceedings 2156, 020010, 2019). In practice any of these masking / patterning methods can be used for embodiments of the present disclosure.

[0055] Embodiments will now be described in detail with reference to the figures.

[0056] Figure 1 shows a schematic cross-sectional representation of an arrangement in accordance with an embodiment where a metallic pattern is formed on one or more major surfaces of a single solar cell 120 (the substrate). The solar cell has a conductive seed layer 125 exposed through the patterned openings in a masking layer 130 on both major surfaces of the solar cell 120. The conductive seed layer 125 is electrically connected to the negative terminal of a power source 105 via a clipping structure 110. In alternative arrangements, where a metal electrode pattern is only required on one of the major surfaces, then the conductive seed layer 125 can be formed on a single major surface of the solar cell 120 and the clipping structure 110 need only contact regions of the conductive seed layer on that major surface.

[0057] Typically for larger solar cells, the clipping structure 110 comprises multiple electrical connections which may be arranged at different exposed conductive seed layer locations on the solar cell surface(s). The structure can be composed of metal, conductive brushes and films and numerous examples exist in currently available electroplating tools. Preferably, any outer surfaces of the clipping structure 110, which may be exposed to the solution of metal ions, are coated with an inert material such as polypropylene or Teflon. Alternatively, the surfaces can be coated with a racking masking material such as “Enplate Stop Off”, so as to avoid metal also being deposited on those surfaces.

[0058] In the preferred arrangement, the clipping structure 110 is connected to a conveying gantry which conveys individual solar cells 120 in a vertical orientation through an electroplating chamber.

[0059] The side walls of the electroplating chamber 155 house linear arrays of dispensing orifices, also referred to as jetting needles. The longitudinal axis of each jetting needle extends into the electroplating chamber substantially perpendicular to the walls of the chamber 155 and the tips of the needles 135 are exposed through the surface of a jet array housing 140, the latter containing the electrical and fluid controls for the individual jets in the array. If a metal pattern is to be formed on both surfaces of the solar cell 120, then jet array housings 140 with associated needle arrays are provided on both the vertical side walls of the electroplating chamber, as shown in Figure 1.

[0060] If multiple lanes of solar cells are to be electroplated simultaneously, then the jet array housings 140 in Figure 1 can be configured to contain arrays of jets which dispense in opposite directions (i.e., left and right in Figure 1) so as to enable the compact electroplating of solar cells 120 in adjacent lanes of an electroplating system.

[0061] The jet array housings 140 are integrated with or placed close to a series of anodes 115 as shown in Figure 4. A fluid stream is emitted by the jetting needles at an angle to a direction which is perpendicular to the solar cell’s major surface such that the fluid stream intersects an anode surface 115 from where it is redirected to the solar cell 120 surface to form the metallic pattern. Figure 3 shows in more detail the redirection of the fluid stream by an anode plate 310 (also indicated as 115 in Figure 1). Fluid stream 320 is jetted from a scallop 308 at the tip of the needle 305 at an angle A to a direction which is perpendicular to the solar cell’s major surface (indicated by 306 in Figure 3). The stream intersects the anode plate 310 from where it is redirected as fluid stream 325 to intersect the solar cell’s surface.

[0062] The dispensing orifices (also referred to as jets) dispense a fluid stream containing metal ions which are electrochemically reduced at the exposed conductive regions of the solar cell 120 to form a metallic pattern (e.g., 350 in Figure 3). The oxidation reaction which completes the electrochemical circuit occurs at the anode plate 310 (which is described in more detail below. Under gravity, the fluid at the surface of the solar cell then drains to an electrolyte reservoir 150 in stream 330.

[0063] In alternative arrangements, other needle end shapes can be used and / or the needles can be positioned at an angle to the longitudinal axis 306, such that the fluid stream 320 can be reflected by an anode surface 310 to intersect with the surface of the solar cell 120.

[0064] The angle A of the generated fluid stream 320 is between 20 and 80° and more preferably between 55 and 70°, allowing the anode plate 310 to be angled such that the reflected fluid stream 325 is perpendicular to the substrate (e.g., solar cell) surface.

[0065] In an alternative arrangement, the jetted fluid streams can be redirected from the needle tips 305 using a compressed air (or gas) flow generated within the jetting array housing 140. The air flow can also be used to accelerate the fluid stream 320, however if the air / gas flow rate is too high it can ‘dry’ the particles of the fluid stream which is undesirable, as described above.

[0066] The dispensed and re-directed jet stream continually replaces the fluid adherent to the solar cell 120 surfaces which are being electroplated, thereby ensuring that a constant fresh supply of metal ions is supplied at the surface as the solar cell 120 is conveyed through the narrow electroplating chamber. This arrangement (depicted in Figure 1) differs from conventional electroplating baths in that the vertically oriented jet arrays 140 are the only source of electroplating electrolyte supplied to the solar cell 120 surface.

[0067] In the preferred arrangement where the solar cells 120 are conveyed through an electroplating chamber, linear jetting arrays 140 are vertically aligned and spaced along the sidewalls of the electroplating chamber 155. Slower conveying speeds < 0.5 m / min) require that the arrays are more closely spaced (e.g., 5 to 25 mm apart) and typically lead to shorter electroplating chambers. Conveying speeds in excess 1 m / min enable higher processing throughput and allow the arrays to be spaced further apart (e.g., 30 to 50 mm), though typically this arrangement results in larger tool footprints.

[0068] If the solar cells 120 are fabricated on rectangular wafers, then preferably they are conveyed through the electroplating chamber in portrait orientation (i.e., leading long edge) to increased processing throughput.

[0069] The length (or height) of the jet arrays is approximately equal to the height of the solar cells being conveyed through the electroplating chamber. So, for a solar cell 120 fabricated on a half-cut M12 wafer which is being transported in portrait orientation through the electroplating chamber 155, the height of the linear jetting array 140 would be ~ 210 mm. To ensure that the upper region of the solar cell 120 is uniformly plated, the linear jetting array 140 is positioned slightly higher (~ 5 mm) than the top of the solar cell due to some loss of jet stream height before it reaches the solar cell 120 surface.

[0070] The jetting needles 135 have a linear spacing of between 5 and 50 dots per inch (dpi) and more commonly between 10 and 30 dpi. Two dimensional arrays can also be used to increase the density of the volume of electroplating electrolyte delivered to the solar cell surface, although the large number of individual jetting needles can increase the cost of the jetting array.

[0071] If a sufficiently high resolution of closely spaced jets in a two-dimensional array, where the spacing between individual jetting needles is similar, is used, then conveying of substrates is not required. Such a 2D array can allow jetting control on an individual nozzle basis to ensure that electrolyte is delivered to specific structures on a substrate (e.g., TSVs in interposers or vias in solar cells).

[0072] The individual jetting needles 135 are preferably piezoelectric actuated, although thermal inkjet technology can also be used without departing from the scope of the present disclosure. The jet firing frequency is in the range of 100 to 300 kHz, and preferably between 150 and 200 kHz, with each jetting needle delivering a volume of 1 to 2 mL / min.

[0073] Alternative jetting arrays comprising ultrasonic nebulisers, hydraulic nozzles (e.g., veejets) or two-phase nozzles can also be used. However, these jetting / spaying methods can result in significant ‘drying’ of the electrolyte requiring tuning of the electrolyte to ensure that the concentration of electrolyte components is not significantly increased at the surface of the solar cell 120. Variations in electrolyte concentration can affect the uniformity of the electroplated structures formed across the surface of the solar cell 120. This can impact the electrical performance of the solar cell 120 as well as potentially reducing the yield and reliability of the cell interconnection process. Unreliable cell interconnection can impact a solar PV module’s durability in the field.

[0074] More angular jetting / spraying methods (e.g., wide angle veejets) can also result in a different droplet size distribution at the surface subtended by the different angles. Particles generated at the wider-angle extremities travel a longer path to reach the surface and therefore will experience more solvent loss, which is also referred to as particle drying.

[0075] To sustain fast rates of copper deposition, it may be desirable to ensure a constant and high copper ion concentration at the reaction surface. If too much solvent (water) evaporates during solution delivery, copper salts (e.g., copper sulphate) can precipitate in the particles, be deposited on the solar cell surface and become trapped in the electroplated metal. These impurities increase the electrical resistivity of the electroplated structures and can potentially induce additional stress in the electroplated fingers and busbars.

[0076] Excess evaporation of solvent (water) can also change the concentrations of electroplating additives which are used to control the electroplated structure morphology and microstructure. Additives need to be maintained within well controlled ranges in electroplating electrolytes and any misbalances between different classes of additive alter the size of metal grains, the smoothness of the metal surface and, in some cases, the width of the metal fingers in the electrode pattern.

[0077] The anode (depicted as an anode plate 310 in Figure 3) may be composed of titanium and have a smooth reflective surface which is coated with a mixed metal oxide (MMO) comprising one or more of titanium oxide, tantalum oxide, ruthenium oxide and iridium oxide. Titanium anodes with MMO coatings can be obtained from a number of sources such as Hebei Boni Tech Co Ltd or Hele Titanium. Other insoluble anodes, such as platinized titanium, titanium oxide or tantalum oxide may also be used. Preferably, the thickness of the MMO coating is between 0.5 and 20 mm, and more preferably between 3 and 20 mm with thicker coatings having the benefit of longer operating life in the electroplating chamber.

[0078] Unlike the mesh insoluble anodes, which are frequently used in conventional electroplating baths, the anodes used for the arrangement in Figure 1 are composed of connected thin rectangular plates which are aligned at a fixed angle to the main axis of the jetting needles such that the majority of the dispensed fluid stream 320 is reflected to perpendicularly intercept the solar cell 120 surface as shown in Figure 3.

[0079] Figure 4A shows an arrangement of connected anode plates 310 aligned with the jetting arrays 140 on a side wall of the electroplating chamber 155. In an embodiment, the anode plate structures 310 are held in alignment by periodically placed horizontal struts or plates 400 and 410 as shown in Figure 4A. The struts or plates 400 and 410 may form part of a support structure or body for housing the anode plate structure 310. In an embodiment, the entire anode structure comprising anode plates 310 and struts or plates 400 and 410 are composed of titanium and coated with a MMO layer as described earlier. An advantage of having the anode plate 310 be separate from the array of needles 135 is that it can reduce production costs of the needles 135 and make replacement of the anode plates 310 easier.

[0080] The anode structures, such as depicted in Figure 4A, line the side walls of the electroplating chamber 155. If only one major surface of a device such as a solar cell is to be electroplated, then the anode structures 310 are only required on one side wall of the chamber 155. Preferably, the anode structures 310 are connected to the side wall of the electroplating chamber 155 in segments so that individual sections of the anode structures can be replaced separately.

[0081] The titanium core of the anode structure is connected to the positive terminal of the power supply (105 or 205) and, when electroplating is initiated, current supplied to the MMO surface catalyses the oxidation of water carried in the dispensed fluid stream 320 to generate 02(g). The majority of the generated 02(g) is released upward to the ambient environment, however some generated gas can be carried in the reflected fluid stream 325.

[0082] A polypropylene filter can be optionally inserted between the anode plates 310 and the solar cell 120 surface to prevent the flow of larger gas bubbles to the surface of the solar cell. Meshes or nets comprising polypropylene (such as provided by Industrial Netting; see between 30 and 65% can be used.

[0083] To minimize dispersion of the fluid stream 325 delivered to the surface of the solar cell 120, the anode plates (shown collectively as 115 in Figure 1 and 310 in Figure 4A) are located within 20 mm, and preferably less than 10 mm, of the needle tips 305. The distance between the anode plate 310 and the solar cell 120 surface is less than 20 mm and more preferably < 5 mm.

[0084] The process of forming a metallic pattern on a solar cell 120 will now be described with reference to Figure 1 , Figure 3 and Figure 4A. The jetting arrays 140 are initiated to dispense or fire when a solar cell 120 surface is conveyed into the fluid stream’s target delivery area. The linear array of needles effectively coats a vertical slice of the solar cell’s major surface, displacing any previously supplied electrolyte and providing a fresh supply of metal ions for electrochemical reduction. On surface impact, metal ions in the electrolyte are reduced to form metal regions 350 on the conductive seed layer exposed through the masking layer 130 on the solar cell 120 surface. The electrolyte begins to drain downwards 330 to be collected in a reservoir 150 at the base of the electroplating chamber.

[0085] Monitoring and dosing systems operate on the electroplating electrolyte in the reservoir 150 to ensure that metal ion, electroplating additive concentration and pH are maintained at desired levels. The electroplating electrolyte is returned to the jetting arrays 140 via external tubes or pipes 145. Preferably, the electrolyte is filtered to remove carbon and particulate contaminants before or during this transfer to the jetting arrays 140.

[0086] The electroplating chamber 155 can comprise a plurality of zones, with a separate metal being electroplated in each zone (e.g., nickel, copper and silver). Additional zones can be used for rinsing steps if more than one metal is to be deposited. The rinsing zones can use similar linear jet arrays as used for the electroplating of the metal patterns. Alternatively, wide area spray jets can also be used.

[0087] The length of the separate zones, and the speed of the conveyor are tuned to achieve a desired mass of electroplated metal. The conveying speed can vary between rates of 0.2 m per min to faster rates of 1 .5 or even 2 m per min. Slower rates can result in a smaller tool footprint but at the expense of reduced processing throughput.

[0088] The electrolytes used for electroplating are essentially the same as used in conventional electroplating baths providing that the distance between the jetting needle orifices 305 and the solar cell 120 surface is < 20 mm. If larger standoff distances are being used, then the electroplating electrolyte may require some dilution to allow for electrolyte drying. Evaporative losses from electroplating electrolyte in the electroplating chamber can be minimized by ensuring that the chamber is fully enclosed.

[0089] Proprietary electroplating electrolytes can be used for all electroplating steps. For example, nickel can be electroplated in a Watts Nickel or Barret SN1 electrolyte. The most- commonly used copper chemistry used for solar cell metallization is a CUSO4 / H2SO4 plating electrolyte with proprietary suppressor, accelerator and leveler additives. Sulphuric acidbased copper plating electrolytes can be commercially sourced from many suppliers, including Technic, Inc. and MKS / Atotech. Alternatively, copper plating electrolytes comprising nitric acid can also be used (see, for example, TWI490376).

[0090] Where very high plating rates are required (e.g., to fill vias in substrates such as solar cells or interposers for 2.5 / 3D integration), then low acid CuSC electrolytes where a Cu loading of between 100-300 g / L can be used.

[0091] Figure 2A shows a cross-sectional representation of an arrangement in accordance with an embodiment where a metallic pattern is formed on a major surface of one or more substrates, such as solar cells 220. In this arrangement, solar cells 220a and 220b are affixed to one or both of the major surfaces of a central cathode plate 210, as shown in Figure 2B.

[0092] Each solar cell, indicated by 220a and 220b in Figure 2A, is in electrical contact with a planar electrode surface 235 on the cathode plate 210 and cathode plates 210 are conveyed through an electroplating chamber, substantially as described for the arrangement depicted in Figure 1. Electrolyte fluid streams are produced from the jet arrays 140 located in the side walls of the electroplating chamber 155 and electrochemical current flows from the anode plates 310 via the reflected fluid stream 325 to the solar cell(s) 220 surface and then through the solar cell(s) to the electrode surface(s) 235 of the cathode plate 210.

[0093] This embodiment does not require the solar cells 220a and 220b to have a conductive seed layer 125 as described for the arrangement in Figure 1. However, if more than one solar cell surface is to be electroplated, then all solar cells must be affixed to the cathode plate 210 in the same semiconductor (i.e., n-p) orientation.

[0094] The surface regions of solar cells 220a and 220b, exposed in the openings of the masking layer 130, are typically conductive and can comprise transparent conducting oxide (TCO) if the solar cell is a silicon heterojunction cell or a doped silicon layer if the solar cell is a passivated emitter and rear cell (PERC) or a tunnel-oxide passivated contact solar cell (TOPCon). The arrangement shown in Figure 2A reduces cost as it reduces the need to first deposit the seed layer (before forming the masking layer 130) and then remove the seed layer in the non metallised regions once the metal pattern has been formed on the solar cell 220.

[0095] To form a metallic pattern on the p-type major surfaces of the solar cells 220a and 220b, the n-type surfaces of the solar cells are placed in intimate electrical connection with the conductive surfaces(s) 235 on the cathode plate 210. When an electrical current flows between the anode(s) 115 and cathode plate 210 in the presence of electrolyte supplied by the reflected fluid stream 325, a metal pattern is simultaneously electroplated on the p-type surfaces of each of the solar cells as all solar cells are forward-biased by the applied potential.

[0096] To form a metal contact pattern on the n-type major surfaces of the solar cells 220a and 220b, the p-type surfaces are placed in intimate electrical connection with the conductive surface(s) 235 on the cathode plate 210 and a set of light sources 430 are provided in the side walls of the electroplating chamber 155, as shown in Figure 4B, to illuminate the solar cells as they are conveyed through the chamber. The light sources 430 generate a photocurrent in the solar cell(s) which induces a voltage that forward biases the semiconductor junction of the solar cells 220a and 220b, so that the conductive solar cell n- type regions, which are exposed through the mask 130 to the electroplating electrolyte, are cathodic, thereby allowing a metal electrode pattern to be electroplated to those surfaces.

[0097] The light sources 430 are in this embodiment light emitting diodes (LEDs), however alternative light sources such as incandescent lights, mercury or xenon arc-lamps or tungsten-halogen lamps, can also be used. The LED light sources are sealed in quartz or epoxy resin tubes which are arranged on the outside walls of the electroplating chamber. For copper plating, a wavelength in the range from 400 to 700 nm can be used. However, white LEDs can also be used given their lower cost. The radiant flux of the light source 430 should be sufficient to ensure that the light intensity incident on a solar cell 220 surface can induce a photocurrent that is at least equal to the electroplating current.

[0098] Figure 2B shows a solar cell 220a affixed to a cathode plate 210. The solar cell contains openings in a masking layer 130 for the formation of finger 270 and busbar 275 regions of a metal electrode pattern. The solar cell is held to the cathode plate 210 by a sealing rim 265 which prevents electrolyte from penetrating between the solar cell 220a and the conductive plate 235 of the cathode plate 210.

[0099] Figure 2C shows a variation of the cathode plate 210 in which two solar cells 220a1 and 220a2 are affixed to separate conductive plate regions 235 on the cathode plate 210. Processing throughput can be increased by carrying solar cells on both major surfaces of the cathode plate 210 and carrying more than one solar cell on each major surface of the cathode plate 210.

[0100] Methods for forming the sealing rim 265 are disclosed in AU2023900831 , the contents of which are incorporated in its entirety herein by reference. The sealing rim 265 may be formed by depositing a heated thermoplastic material such as a phenolic resin around the perimeter of the solar cell(s) 220. The deposited material of the sealing rim 265 solidifies substantially on contact and holds the solar cell(s) 220 to the cathode plate 210. It also prevents any electroplating electrolyte from penetrating behind the solar cell(s) 220 and contacting the conductive plate 235 and / or the solar cell(s)’ second major surface during the electroplating steps.

[0101] Alternatively, gasket seals, located within grooves or recesses behind the solar cells (e.g., 220a and 220b), can be used to hold solar cell(s) to the cathode plate 210 such that electroplating electrolyte does not penetrate behind the solar cell(s).

[0102] The process of forming a metallic pattern, such as the electrode pattern depicted in Figures 2B and 2C, is substantially as described for the arrangement shown in Figure 1. However, in this case, the metal pattern is formed directly on the surface of the solar cell and not on a seed layer. Once all the plating and rinsing steps have been completed the solar cell(s) are removed from the cathode plate 210.

[0103] It should be clear to a person skilled in the art that the processes described above with reference to Figure 1 and Figure 2A to form a metallic pattern on one or more solar cells can also be used to form metal patterns and structures (e.g., blind vias or through substrate vias) on other substrates such as semiconductors wafers, or carrier materials for electronic circuit such as printed circuit boards, glass or ceramic materials.

[0104] Throughout this specification the term "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[0105] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.

Claims

Claims:1 . A method for depositing a metallic material on at least one major surface of a planar substrate according to a pre-determined pattern, the method comprising: providing at least one planar substrate having a major surface with one or more conductive regions arranged according to said pre-determined pattern; dispensing a fluid stream containing metal ions from a plurality of jetting orifices to form a dispensed fluid stream that first intercepts an electrode surface and is redirected by the electrode surface to contact the major surface of the planar substrate; and forming an electrochemical circuit between the electrode surface and the one or more conductive regions of the planar substrate such that the metal ions applied to the major surface of the planar substrate by the dispensed fluid stream are electrochemically reduced to form a metallic material on the major surface according to the pre-determined pattern.

2. The method according to claim 1 , wherein the dispensed fluid stream is directed at an angle ranging from 20° to 80°, and more preferably from 55° and 70°, to a perpendicular line extending from the planar substrate.

3. The method according to claim 1 or 2, wherein the dispensed fluid stream is directed in a direction substantially perpendicular to the planar substrate after being redirected by the electrode surface.

4. The method according to any one of claims 1 to 3, wherein the electrode surface is substantially planar.

5. The method according to any one of claims 1 to 4, wherein the planar substrate is in a substantially vertical orientation when forming the dispensed fluid stream and forming the metallic material on the major surface.

6. The method according to any one of claims 1 to 5, wherein the fluid is dispensed by one of hydrodynamic flow, ultrasonic or thermal dispensing means.

7. The method according to any one of claims 1 to 6, wherein the jetting orifices are arranged in at least one vertically oriented linear array and a spacing between individual jetting orifices in the linear array is less than 5 mm, and more preferablyless than 3 mm.

8. The method according to any one of claims 1 to 7, wherein the jetting orifices comprise piezoelectric actuators.

9. The method according to claim 8, wherein the actuators are actuated at a frequency ranging from 100 to 300 kHz, preferably ranging from 150 to 200 kHz.

10. The method according to any one of claims 1 to 9, further comprising conveying the planar substrates past the electrode surface at a speed ranging from 0.2 m / min to1 .5 m / min.11 . The method according to any one of claims 1 to 10, wherein the metallic material is one of nickel, silver, copper, tin and alloys thereof.

12. The method according to any one of claims 1 to 11 , wherein an insulating material covers a portion of the major surface of the planar substrate and exposes the one or more regions of a conductive layer which extends across the planar substrate below the insulating material.

13. The method according to any one of claims 1 to 12, wherein the at least one conductive region on the planar substrate is connected to a negative terminal of a power supply.

14. The method according to claim 13, wherein the at least one conductive region is connected to the negative terminal using a lead having a clip that clips onto the planar substrate to contact the at least one conductive region.

15. The method according to any one of claims 1 to 14, wherein the planar substrate is a printed circuit board or a substrate used for the interconnecting of electronic circuits.

16. The method according to any one of claims 1 to 14, wherein the planar substrate is a semiconducting wafer.

17. The method according to any one of claims 1 to 14, wherein the planar substrate isa solar cell having a first major surface and a second major surface and the metallic pattern comprises an electrode which is formed at least on the first major surface of the solar cell.

18. The method according to claim 17, wherein a metallic pattern is formed on both major surfaces of the solar cell.

19. The method according to claim 17 or 18, wherein the second major surface has a conductive region that is in electrical connection with a negative terminal, wherein the method further comprises forming the metallic pattern on the first major surface of the solar cell due to current flowing through the solar cell in a direction which is substantially perpendicular to the major surfaces of the solar cell.

20. The method according to any one of claims 17 to 19, wherein the first major surface has an n-type polarity, and the second major surface has a p-type polarity, wherein the method further comprises illuminating the solar cell to generate a photocurrent which induces a voltage that forward biases a semiconductor junction of the solar cell so that a metallic pattern is formed on one or more conductive regions exposed on the n-type surface of the solar cell.21 . The method according to any one of claims 1 to 20, further comprising affixing a plurality of planar substrates to a plate with electrode regions, wherein the step of applying metal ions to the major surface of the plurality of planar substrates forms a metallic pattern on the first major surfaces of each of the plurality of planar substrates.

22. The method according to any one of claims 1 to 21 , wherein the conductive region includes a metal, a transparent conductive oxide, and a silicon material including a polysilicon material.

23. An electroplating system for depositing a metallic material on at least one major surface of a planar substrate according to a pre-determined pattern, the electroplating system comprising: a body having a chamber that can receive a planar substrate having a major surface with one or more conductive regions arranged according to the predetermined pattern;a dosing system having a plurality of jetting orifices for providing a fluid stream containing metal ions; an electrode surface spaced from the plurality of jetting orifices and arranged such that the fluid stream of metal ions emitted from the plurality of jetting orifices is reflected off the electrode surface to be redirected to the planar substrate; and an electrical system configured to electrically connect the electrode surface and the one or more conductive regions on the planar substrate and configured such that metal ions applied to the one or more conductive regions are electrochemically reduced to form a metallic material on the major surface of the planar substrate according to the pre-determined pattern.

24. The electroplating system according to claim 23, wherein the chamber has an open first end and an open second end such that the planar substrate can be conveyed through the chamber in a direction extending from the first open end to the second open end.

25. The electroplating system according to claim 23 or 24, wherein the chamber is substantially vertical such that the planar substrate is orientated vertically when the plurality of jetting orifices provide the fluid stream containing metal ions.

26. The electroplating system according to any one of claims 23 to 25, wherein the plurality of jetting orifices and the electrode surface are positioned on a sidewall of the chamber.

27. The electroplating system according to claim 26, wherein the jetting orifices are integrated in or attached to the sidewall of the chamber.

28. The electroplating system according to claim 26 or 27, wherein a longitudinal direction of each jetting orifice is arranged perpendicular to the sidewall of the chamber.

29. The electroplating system according to claim 28, wherein plurality of jetting orifices are configured such that the fluid stream of metal ions is directed at an angle ranging from 20° to 80°, and preferably from 55° to 70°, relative the longitudinal direction.

30. The electroplating system according to claim 28 or 29, wherein the electrode surface is configured such that the reflected fluid stream travels along a direction that is substantially parallel to the longitudinal direction.31 . The electroplating system according to any one of claims 23 to 30, wherein the plurality of jetting orifices is arranged in a vertically oriented linear array.

32. The electroplating system according to claim 31 , wherein a spacing between individual jetting orifices in the linear array is less than 5 mm, and preferably less than 3 mm.

33. The electroplating system according to claim 31 or 32, comprising a plurality of vertically oriented linear arrays each having a plurality of jetting orifices, wherein adjacent linear arrays are spaced apart by a distance ranging from 5 mm to 50 mm.

34. The electroplating system according to any one of claims 23 to 33, wherein the electrode surface is substantially planar.

35. The electroplating system according to any one of claims 23 to 34, wherein the dosing system is configured such that the fluid stream of metal ions is dispensed by one of hydrodynamic flow, ultrasonic or thermal dispensing means.

36. The electroplating system according to any one of claims 23 to 35, wherein the jetting orifices comprise piezoelectric actuators.

37. The electroplating system according to any one of claims 23 to 36, wherein the electrical system comprises a clip for electrically connecting the one or more conductive regions on the planar substrate to the negative terminal of a power source, wherein the electrode surface is electrically connected to the positive terminal of the power source.

38. The electroplating system according to any one of claims 23 to 37 further comprising a light source for illuminating the planar substrate, the light source being configured so that, when the planar substrate is received in the chamber, illumination of the planar substrate generates a photocurrent which induces a voltage that forward biases a semiconductor junction of the planar substrate suchthat a metallic pattern is formed on an n-type surface of the planar substrate.

39. The electroplating system according to claim 38, wherein the planar substrate is a solar cell.

40. The electroplating system according to claim 38 or 39, wherein the light source includes a plurality of light sources, and the electrode surface includes a plurality of electrode surface, wherein each light source of the plurality of light sources is positioned adjacent to one electrode surface of the plurality of electrode surfaces.41 . The electroplating system according to any one of claims 23 to 40, further comprising a conductive plate configured to be received in the chamber and to electrically bond to a plurality of planar substrates such that the plurality of substates bonded to the conductive plate can be received in the chamber.

42. The electroplating system according to any one of claims 23 to 41 , further comprising a reservoir in which redirected fluid that has contacted with the planar substrate can flow down off the planar substrate to fall into the reservoir.

43. The electroplating system according to claim 42, wherein the reservoir is in fluid communication with the jetting orifices such that fluid collected in the reservoir can be recirculated back to the jetting orifices.

44. The electroplating system according to claim 43, further comprising a filter for removing particulate matter from fluid being recirculated from the reservoir to the jetting orifices.

45. The electroplating system according to any one of claims 23 to 44, wherein the conductive region includes a metal, a transparent conductive oxide, and a silicon material including a polysilicon material.

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