FIELD OF THE INVENTION

[0001] The present invention relates to semiconductor fabrication. More particularly, the present invention relates to fabrication of contacts for semiconductor substrates.

BACKGROUND OF THE INVENTION

[0002] A photovoltaic solar cell typically includes a substrate in the form of a wafer of a semiconductor material (e.g. silicon) with a pn junction formed near its front side. When the front side of the photovoltaic cell is exposed to light, such as solar radiation, an electric current may be produced in the junction. This current may be collected by electrical contacts that are deposited on the front (light-facing) and back sides of the photovoltaic cell. Thus, the energy of the sunlight may be converted to electricity, which may be utilized.

[0003] In addition to the pn junction, a solar cell may include additional layers for improving the efficiency of the photovoltaic cell. For example, each surface of the photovoltaic cell that is designed to form an electrical connection to the deposited electrical contacts may include a conducting layer of highly-doped (e.g. n+ or p+) semiconductor. The highly-doped semiconducting layer may have increased conductivity. The increased conductivity may decrease resistive losses at the electrical contacts (ohmic contact).

[0004] Reflection of light off the front surface of the photovoltaic cell may prevent incident light from impinging on the pn junction and being converted to electricity. Therefore, the front surface of a solar cell may be covered by a thin antireflection (AR) coating. The AR coating may typically be electrically insulating.

[0005] During manufacture of such a solar cell, the electrically insulating AR coating is typically applied prior to deposition of the electrical contacts on the front surface of the photovoltaic cell. Thus, deposition of the electrical contact material must be performed in such a manner as to ensure that the deposited electrical contact material is in electrical (ohmic) contact with the layer of highly-doped semiconductor (or similar electrically conducting layer). One possibility is to chemically or otherwise remove the AR coating from those regions (typically lines) on the front surface where electrical contact material is to be deposited. Another possibility, sometimes referred to as a "fire through" technique, is to deposit the electrical contact material on top of the AR coating in the form of a metal paste or ink that is impregnated with glass frits. When heated, the glass frits in the material may break through the AR coating so as to bring the material into contact with electrically conducting layer below.

[0006] The concentration of the glass frits per unit area of the photovoltaic cell surface (hereinafter referred to as an "area density" of glass frits) may determine how deeply the material penetrates into the AR coating, and possibly into the layers below. Typically, the frit concentration may be controlled such that the material penetrates to the conducting layer, but no deeper.

In particular, the electrical contact material may be deposited in the form of lines that cross one another. For example, the electrical contacts may be deposited in the form of conducting fingers for collecting electrical current produced in the photovoltaic cell. The width of the fingers may be small so as to minimize shading of the pn junction. ). Wider bus bars or strips may be deposited across (typically at right angles to) the conducting fingers. The bus bars connect the fingers to one another and to an external connector or tab. Thus, the bus bars may conduct the generated electricity to the edge of the photovoltaic cell for connection to another cell or to an external device. The bus bars cross the conducting fingers, and electrically connect with them, at intersections.

SUMMARY OF THE INVENTION

[0007] There is thus provided, in accordance with some embodiments of the present invention, a method for fabricating electric contacts for a semiconductor substrate. The method includes: depositing a first line of a first conducting material, on the semiconductor substrate; and depositing a second line of a second conducting material on the semiconductor substrate, so as to cross the first line; wherein the first conducting material and the second conducting material differ from one another in that the first conducting material and the second conducting material penetrate into the semiconductor substrate to different depths when subjected to a heat treatment, or in that an electrical connection formed between the semiconductor substrate and the first conducting material when subjected to the heat treatment differs in its contact resistivity from an electrical connection formed between the semiconductor substrate and the second conducting material when subjected to the heat treatment.

[0008] Furthermore, in accordance with some embodiments of the present invention, a concentration of glass frits in the first conducting material differs from a concentration of glass frits in the second conducting material. [0009] Furthermore, in accordance with some embodiments of the present invention, a composition of glass frits included in the first conducting material differs from a composition of glass frits included the second conducting material.

[0010] Furthermore, in accordance with some embodiments of the present invention, the compositions of glass frits differ in their concentrations of a component selected from a group of components consisting of oxides of lead, bismuth, titanium, and chromium.

[0011] Furthermore, in accordance with some embodiments of the present invention, the method includes subjecting the semiconductor substrate with the deposited lines to the heat treatment

[0012] Furthermore, in accordance with some embodiments of the present invention, a mechanical bond that is formed between the semiconductor substrate and the first conducting material when subjected to the heat treatment differs iii its adhesive strength from a mechanical bond formed between the semiconductor substrate and the second conducting material when subjected to the heat treatment.

[0013] Furthermore, in accordance with some embodiments of the present invention, a line of the first line and the second line represents a conducting finger and the other line represents a bus strip.

[0014] Furthermore, in accordance with some embodiments of the present invention, the line representing the conducting finger is deposited before the line representing the bus strip.

[0015] Furthermore, in accordance with some embodiments of the present invention, the line representing the bus strip is deposited before the line representing the conducting finger.

[0016] Furthermore, in accordance with some embodiments of the present invention, the first line and the second line are deposited concurrently.

[0017] Furthermore, in accordance with some embodiments of the present invention, depositing the first line and the second line includes operating an inkjet apparatus concurrent with relative motion between the semiconductor substrate and the inkjet apparatus.

[0018] Furthermore, in accordance with some embodiments of the present invention, the first conducting material and the second conducting material are dispensed by different heads of the inkjet apparatus.

[0019] There is further provided, in accordance with some embodiments of the present invention, a method for fabricating electric contacts for a semiconductor substrate. The method includes: depositing a first line of a conducting material on the semiconductor substrate, the first line including a zone of reduced thickness; and depositing a second line of a conducting material on the semiconductor substrate, so as to cross the first line at the zone of the reduced thickness.

[0020] Furthermore, in accordance with some embodiments of the present invention, the conducting material, when subjected to a heat treatment, penetrates into the semiconductor substrate, forms an electrical connection to the semiconductor substrate, or forms a mechanical bond with the semiconductor substrate,.

[0021] Furthermore, in accordance with some embodiments of the present invention, the method includes subjecting the semiconductor substrate with the deposited lines to the heat treatment

[0022] Furthermore, in accordance with some embodiments of the present invention, the conducting material of the first line differs from the conducting material such that when subjected to the heat treatment, the first line differs of the second line in its depth of penetration into the semiconductor substrate, in a contact resistivity of its electrical connection to the semiconductor substrate, or in an adhesive strength of its mechanical bond to the semiconductor substrate.

[0023] Furthermore, in accordance with some embodiments of the present invention, the method includes plating the deposited lines with a metal.

[0024] Furthermore, in accordance with some embodiments of the present invention, in the depositing of the first line, no material is deposited in the zone of reduced thickness.

[0025] Furthermore, in accordance with some embodiments of the present invention, the first line represents a conducting finger and the second line represents a bus strip.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to better understand the present invention, and appreciate its practical applications, the following Figures are provided and referenced hereafter. It should be noted that the Figures are given as examples only and in no way limit the scope of the invention. Like components are denoted by like reference numerals;

[0027] Fig. 1 is a schematic illustration of a contact material deposition system, in accordance with embodiments of the present invention.

[0028] Fig. 2A shows a portion of a cross section along a conducting finger and through an intersection with a bus strip, in accordance with some embodiments of the present invention. [0029] Fig. 2B shows the cross section shown in Fig. 2 A after subjection to a heat treatment.

[0030] Fig. 2C shows a portion of a cross section along a bus strip and through an intersection with a conducting finger, where material for formation of the conducting finger is deposited on top of material for formation of the bus strip, in accordance with some embodiments of the present invention.

[0031] Fig. 3 A illustrates control of material thickness in a portion of a cross section through an intersection of crossing conducting lines, in accordance with some embodiments of the present invention.

[0032] Fig. 3B illustrates deposition of a material across the intersection in the cross section shown in Fig. 3 A.

[0033] Fig. 3C illustrates the cross section shown in Fig. 3B after subjection to a heat treatment.

[0034] Fig. 3D illustrates metallic plating of the cross section shown in Fig. 3C.

[0035] Fig. 4A illustrates a gap in deposited material in a portion of a cross section through an intersection of crossing conducting lines, in accordance with some embodiments of the present invention.

[0036] Fig. 4B illustrates deposition of a material across the intersection in the cross section shown in Fig. 4A.

[0037] Fig. 4C illustrates the cross section shown in Fig. 4B after heating and metallic plating.

DETAILED DESCRIPTION OF EMBODIMENTS

[0038] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, modules, units and/or circuits have not been described in detail so as not to obscure the invention.

[0039] Embodiments of the invention may include an article such as a computer or processor readable medium, or a computer or processor storage medium, such as for example a memory, a disk drive, or a USB flash memory, encoding, including or storing instructions, e.g., computer-executable instructions, which when executed by a processor or controller, carry out methods disclosed herein. [0040] In accordance with embodiments of the present invention, a pattern of electrically conducting material (e.g. a conducting ink that includes a volatile organic solvent in which silver nano-particles are suspended) may be deposited on as surface of a substrate, such as a photovoltaic solar cell. In the case of a photovoltaic cell (or similarly constructed device based on electrically connected semiconductor wafers), the electrically conducting material may be deposited on top of an insulating layer (e.g. an anti-reflection coating) on a surface of a semiconducting substrate (such as a silicon wafer with variously doped layers).

[0041] The pattern may include lines of conducting material that cross at intersections. For example, an electrical contact material may be printed on a substrate surface using a material deposition system such as an inkjet or aerosol jet deposition system. A printing head or heads of the material deposition system may be configured to first deposit material (e.g. a conducting ink having a volatile liquid component with suspended silver nano-particles) in parallel lines having a first orientation, and then deposit parallel lines of material having a second orientation (e.g. perpendicular to the first orientation.)

[0042] The deposited electrical contact material may include a penetrating conducting material that includes a concentration of a penetration-assisting component. The electrical contact material is designed to penetrate, when heated, into the substrate, e.g. through the electrically insulating anti-reflection coating (such as silicon nitride) to a conducting layer. Typically, the penetration-assisting component includes glass frits (to be understood to be representative of any penetration-assisting component of a penetrating conducting material).

[0043] The deposited electrical contact material may be designed to promote mechanical adhesion of the electrical contact material to the substrate. For example, an adhesion- promoting component (e.g. a combination of a chromium oxide and a titanium oxide) may be incorporated into glass frits that are suspended in the electrical contact material. The adhesion- promoting component may promote adhesion by promoting a mechanical bond with the anti- reflection coating when heated. The mechanical bond between the electrical contact material and the substrate may be characterized by its adhesive strength (e.g. a force required to separate a unit area of the material from the substrate). For example, the adhesive strength of a mechanical bond between a bus strip and the substrate may be designed to be stronger than a bond between a conducting finger and the substrate.

[0044] Typically, the depth to which the electrical contact material penetrates may increase as the area density (defined here as a quantity, e.g. mass or number, per unit area of substrate surface) of the glass frits increases. (The area density of glass frits may be determined by the thicknesses of the applied materials, as well as by concentrations of glass frits in each of the applied materials.) An excessive area density of the glass frits on a region of the substrate surface may result in excessive penetration of the electric contact material at that region. Patterns of electrical contact material may be deposited on the substrate surface such that the patterns partially cross or overlap. In accordance with embodiments of the present invention, the deposition is to be controlled such that the area density of glass frits at the regions of overlap does not exceed a predetermined value. For example, the predetermined value may determine that when heated, a penetrating conducting material penetrates the substrate to the depth of a conducting layer of the substrate (e.g. of n+ doped silicon), but not deeper. In particular, the penetrating conducting material may not penetrate the substrate to a pn junction layer between a layer of w-type silicon (sometimes referred to as the emitter) and a layer of p- type silicon.

[0045] According to embodiments of the present invention, a pattern for creation of conducting fingers may be printed using a finger electrical contact material containing a relatively large concentration of glass frits. Thus, when heated, the finger electrical contact material may penetrate an insulating anti-reflection coating to the depth of a conducting layer below the anti-reflection coating. The electrical contact material may thus form a metal seed layer on the substrate, and in contact with the conducting layer. On the other hand, a bus strip may be printed using a bus electrical contact material having a relatively low concentration of glass frits. Such a material may penetrate the anti-reflection coating to only a relatively shallow depth when heated (e.g. only to a depth sufficient to prevent the bus strip from separating from then substrate surface). Thus, at an intersection where the bus contacts cross the conducting fingers, the area density of glass frits, and thus a penetration depth of the deposited electrical contact material, may be primarily determined by the glass frit density of the finger electrical contact material.

[0046] Alternatively, a single electrical contact material may be used to print both the conducting fingers and the bus strips. A smaller amount of material for one of the crossing contact lines (typically the conducting finger) may be deposited in a zone near the intersection. Thus, an approximately uniform area density of glass frits, and an approximately uniform penetration depth, may be ensured.

[0047] The composition of glass frits incorporated into an electrical contact material may be adapted to an intended purpose of the electrical contact material. For example, glass frits may be incorporated into an electrical contact material intended for forming a conducting finger. These glass frits may be designed to assist the material in penetrating into the anti-reflection coating and in forming electrical contact with the conducting layer below the anti-reflection coating. On the other hand, glass frits may be incorporated into electrical contact material intended for forming a bus strip. In this case the glass frits may be designed to promote adhesion to the substrate, while not promoting electrical contact with the substrate.

[0048] Fig. 1 is a schematic illustration of a contact material deposition system, in accordance with embodiments of the present invention. Material deposition system 10 included a material deposition device 18. For example, material deposition device 18 may include a printer or one or more separately controllable printing heads of an inkjet or aerosol jet deposition system. A printing head of material deposition device 18 may be configured to deposit a line or strip of material in one direction, while another may be configured to deposit material in another (e.g. perpendicular) direction.

[0049] Material deposition device 18 may include a material dispensing element 19 (which may also be understood as representing a row or array of individual, an jointly or separately controllable, material dispensing nozzles or printing heads), for dispensing a material. Material deposition device 18 may be configured to deposit a material on a substrate 20. For example, substrate 20 may include a multilayered semiconducting wafer for fabrication as a photovoltaic cell. The material deposited by material deposition device 18 may include a conducting material, e.g. a penetrating conducting ink, paste, or material, for formation of a pattern of conducting lines for collecting electrical current produced by a photovoltaic cell when exposed to light.

[0050] Material deposition device 18 may be controlled by a processor 22. For example, processor 22 may include a processor that is incorporated into material deposition device 18. Alternatively or in addition, processor 22 may include one or more computers that communicate with material deposition device 18. Processor 22 may control operation of material deposition device 18. For example, processor 22 may control relative motion or rotation between material deposition device 18 and substrate 20, a rate of material deposition, a material deposited (e.g. if material deposition device 18 includes multiple heads, each configured to dispense a different material, or multiple reservoirs of material connected to a single head), or a configuration (e.g. line width or thickness) of deposited material.

[0051] Typically, material may be deposited on substrate 20 in the form of a pattern that includes lines or strips of material that may cross at intersections. (A line or strip of material should be understood as referring any straight or curved elongated pattern of deposited material.) A typical pattern of conductors for a photovoltaic solar cell may include a parallel arrangement of parallel conducting fingers 12, and a perpendicular arrangement of parallel bus strips 14. In a typical inkjet printing method, material deposition device 18 may be controlled to first deposit material for formation of conducting fingers 12.

[0052] For example, an array of parallel nozzles of material dispensing element 19 may concurrently deposit material in the form of parallel lines. Material deposition device 18 may then be controlled to deposit a material to form a parallel arrangement of bus strips 14. For example, substrate 20 may be transported to an array of nozzles of material dispensing element 19 that are configured to deposit material in the direction of bus strips 14. As another example, substrate 20 may be rotated by 90 degrees, or a direction of motion of material dispensing element 19 may be changed, to enable deposition of material for forming bus strips 14 that are perpendicular to conducting fingers 12.

[0053] Alternatively, a pattern of variously oriented lines may be deposited by a single set of nozzles while the relative orientation between substrate 20 and material deposition device 18 remains constant. For example, material dispensing element 19 of deposition device 18 may be controlled to deposit material in a pattern of closely spaced lines (similar to control of an inkjet printing head when printing text or graphics using standard ink on a paper substrate). While printing each line, material dispensing element 19 may be controlled so as to dispense material only when positioned over an area of substrate 20 onto which a line of the pattern is to be deposited. In this manner, material for formation of both conducting fingers 12 and bus strips 14 may be deposited concurrently, e.g. during a single controlled scan of material deposition element 19 over the surface of substrate 20. In the case that material for formation of conducting fingers 12 differs from material for formation of bus strips 14, different nozzles or printing heads of material deposition element 19 may be configured to deposit the different materials (e.g. each nozzle connected to, or each printing head including, a different reservoir of conducting material). The different nozzles may be controlled to deposit only the material required at each location on the surface of substrate 20. Thus, conducting fingers 12 and bus strips 14 that include different materials may be deposited concurrently. For example, separate heads of a head assembly of an inkjet printing apparatus may dispense different materials at different locations on the surface of substrate 20 as the head assembly is scanned over the surface of substrate 20 (or as substrate 20 is moved past the head assembly, or during a combination of relative motions). [0054] A material pattern may include a pattern for formation of conducting fingers 12. Material for formation of conducting fingers 12 typically includes a penetrating electrically conducting material, e.g. a metal such as silver, deposited with a typical thickness of up to about 10 μηι. Achieving a desired thickness may require several passes of material deposition device 18 (or dispensing printing material from several aligned nozzles during a single pass of material deposition device 18).

[0055] A penetrating conducting material for formation of conducting fingers 12 may also include a penetration-assisting component that enables the material to penetrate an insulating layer of the substrate when heated. For example, in the case of manufacture of photovoltaic solar cells, the penetration-assisting component may include glass frits. Glass frits may typically include an oxide of one or more of the elements lead, silicon, boron, bismuth, zinc, zirconium, titanium, phosphorus, or antimony. When subjected to a heat treatment (e.g. heated to a temperature of about 900°C for a period of few seconds, e.g. less than about 6 seconds), the penetrating conducting material may penetrate an insulating layer of substrate 20. The penetrating conducting material may penetrate the insulating layer to form an ohmic contact with a conducting layer below the insulating layer, forming a metal seed layer. For example, an oxide of lead or bismuth that is a component of the glass frits, when catalyzed by titanium oxide component, may assist in reducing the contact resistivity (resistivity of a unit area of contact surface) of an electrical connection between the penetrating conducting material and (a conducting layer of) the substrate.

[0056] The seed layer may be metallized, e.g. by electrochemical plating, so as to form conducting fingers 12 having suitable electrical conducting properties. For example, a seed layer that is about 3 μηι thick may be plated with an additional layer of conducting material that is about 15 μηι thick. The resulting conducting fingers 12 may conduct electricity away from the conducting layer. A typical spacing between conducting fingers 12 may be about 2 mm, with a width per conducting finger 12 of less than 100 μηι. The configuration of conducting fingers 12 may be determined by balancing such considerations as increasing conduction of electricity produced in substrate 20, on the one hand, and reducing shading of substrate 20 from incident light, on the other.

[0057] A material pattern may also include material for formation of bus strips 14. A bus strip 14 may be configured to conduct electricity from conducting fingers 12 to an edge of substrate 20. For example, an end of a bus strip 14 at an edge of substrate 20 may be connected (e.g. soldered) to a tab, ribbon, or other connector. The connector may enable an electrical connection between bus strip 14 and an external device, such as another solar cell or a device to be powered. The width of a typical bus strip 14 may be in the range of 0.5 mm to 3 mm.

[0058] A bus strip 14 crosses a conducting finger 12 at an intersection 16. Electrical contact may be formed at intersection 16 between bus strip 14 and conducting finger 12. Typically, the deposited patterns of conducting fingers 12 and bus strips 14 are such that bus strips 14 cross conducting fingers 12 at intersections 16 at approximately right angles. In accordance with embodiments of the present invention, deposition of material for conducting fingers 12 and bus strips 14 may be controlled by processor 22 such that the area density of glass frits at intersections 16 may be approximately equal to the area density of glass frits at other points of the pattern.

[0059] After deposition of patterns of electrical contact material, and after heating substrate 20 with the deposited penetrating conducting material so as to enable penetration of the material into an insulating layer of the substrate, additional conducting material may be deposited over some or all of the conducting pattern (over the metal seed layer). For example, conducting fingers 12, bus strips 14, or intersections 16 may plated with an additional layer of metal. An additional layer of metal may improve electrical contact between conducting fingers 12 and bus strips 14, as well as improve the electrical conducting capabilities of the conducting pattern (e.g. increase conductivity of each conducting finger 12 or bus strip 14).

[0060] Fig. 2A shows a portion of a cross section along a conducting finger and through an intersection with a bus strip, in accordance with some embodiments of the present invention. As shown in Fig. 2A, substrate 20 is configured to function as a photovoltaic cell. Substrate 20 includes a junction layer formed between two layers of different semiconducting materials. Typically, pn junction 28 is formed between «-type silicon layer 26 facing the front side (from which electricity-generating light impinges), and ?-type silicon layer 24 facing the back side. Light impinging on pn junction 28 may generate electron-hole pairs, forming an electric potential.

[0061] A rearward-facing surface of />-type silicon layer 24 may be in contact with rear conducting layer 34 containing p+ doped silicon. Enhanced conductivity of rear conducting layer 34 may improve electrical conductivity (form an ohmic contact) when rear conducting layer 34 is in electrical contact with an electrical conductor bonded to, or in contact with, the rear surface of substrate 20.

[0062] Similarly, a frontward-facing surface of n-type silicon layer 26 may be in contact with a conducting layer 30 of n+ doped silicon. Conducting layer 30 may enable minimizing resistivity when connected to an electrical contact on the front surface of substrate 20 (to form an ohmic contact).

[0063] An outward-facing surface of conducting layer 30 may be covered with an anti- reflection coating 32. Typically, anti-reflection coating 32 electrically insulates conducting layer 30 from the outer surface of anti-reflection coating 32. For example, anti-reflection coating 32 may include a layer of silicon nitride (Si ₄ N ₄ ) having a thickness in the range of 80 rum to 100 nm.

[0064] Conducting finger material 36 (shown in a section along the long axis of the conducting finger) may be deposited on the outward-facing surface of anti-reflection coating 32. Conducting finger material 36 may include a sufficient concentration of glass frits to enable penetration of anti-reflection coating 32 to conducting layer 30 when heated (so as to form an ohmic contact). At the point of the fabrication method illustrated in Fig. 2A, conducting finger material 36 may be electrically isolated from conducting layer 30.

[0065] Bus strip material 38 (shown as a cross section perpendicular to the long axis of the bus strip) may be deposited on the outward-facing surface of anti-reflection coating 32 after deposition of, and perpendicular to, conducting finger material 36. At intersection 16, bus strip material 38 covers conducting finger material 36. The composition of bus strip material 38 may be different from that of conducting finger material 36. Bus strip material 38 is typically not required to form a direct electrical connection with conducting layer 30, but only with conducting finger material 36. However, bus strip material 38 may be required to physically adhere to substrate 20 (e.g. to anti-reflection coating 32 of substrate 20). A concentration of glass frits in bus strip material 38 may be lower than in conducting finger material 36. Alternatively or in addition, a composition of glass frits in bus strip material 38 may differ from the composition of glass frits included in conducting finger material 36.

[0066] For example, the concentration of glass frits in bus strip material 38 may be sufficient, and of such composition, to enable mechanical bonding between bus strip material 38 and anti-reflection coating 32. The combined area density of glass frits at intersection 16 is determined by the separate area densities of glass frits of both bus strip material 38 and conducting finger material 36. The concentration of glass frits in bus strip material 38 may be sufficiently small such that the combined area density of glass frits at intersection 16 is less than a predetermined maximum area density (e.g. sufficiently small that such that the material does not penetrate deeper into substrate 20 than conducting layer 30). (This is as opposed to the prior art, in which the concentration of glass frits in the bus strip material may have been approximately equal to that in conducting finger material. In that case, a combined area concentration at an intersection may have caused the material to penetrate deeper than a conducting layer. This may have had a detrimental effect on conversion efficiency of a photovoltaic solar cell by introducing a shunt parasitic resistance, for example, by the conducting material contacting a /in junction layer.) Bus strip material 38 may include a metal, a metal-organic compound, or a metal oxide of an element such as titanium, zirconium, palladium, or nickel.

[0067] Fig. 2B shows the cross section shown in Fig. 2 A after subjection to a heat treatment. Conducting finger material 36 (Fig. 2 A) has penetrated anti-reflection coating 32 (Fig. 2 A) to form seed conducting finger 36'. Seed conducting finger 36' is in contact with conducting layer 30. Similarly, bus strip material 38 (Fig. 2A) has formed seed bus strip 38'. No material at intersection 16 has penetrated deeper than conducting layer 30.

[0068] As a result of subjection to the heat treatment, an electrical connection between seed conducting finger 36' and conducting layer 30 may differ from (typically have a lower contact resistance than) an electrical connection between seed bus strip 38' and conducting layer 30. Similarly, a mechanical bond between seed bus strip 38' and substrate 20 may differ (typically have a greater adhesive strength than) a mechanical bond between seed conducting finger 36' and substrate 20.

[0069] Seed conducting finger 36' and seed bus strip 38' may be covered with a conduction metal layer (similar to conduction metal layer 44 in Fig. 3D). For example, the conduction metal layer may be deposited using electrochemical metal plating. The conduction metal layer may further improve electrical conduction properties of seed conducting finger 36' and seed bus strip 38'.

[0070] Alternatively, a process of depositing a pattern of conducting material may include depositing further layers of additional conducting material on top of bus strip material 38 and conducting finger material 36 (Fig. 2 A). For example, the additional conducting material may not include glass frits. When heated, a resulting conducting finger or bus strip may have a desired thickness (e.g. about 20 μηι). In this case, the resulting conducting finger or bus strip may have suitable conducting properties without a need for an additional conduction metal layer.

[0071] Thus, composition of bus strip material 38 may differ from that of conducting finger material 36 in the concentration of, in the composition of, or both, of glass frits. After subjection to a heat treatment, seed bus strip 38' may differ from seed conducting finger 38' (Fig. 2B) in one or more of the following: depth of penetration into substrate 20, quality of an electrical connection (e.g. characterized by a contact resistivity) to substrate 20, or strength of a mechanical bond to substrate 20.

[0072] In accordance with some embodiments of the present invention, material for formation of a conducting finger may be deposited on top of material for formation of a bus strip. In this case, the material for formation of the conducting finger, and designed to penetrate into the substrate when heated, is not initially in contact with the substrate at intersections. This configuration may further reduce penetration into the substrate at the intersections between conducting fingers and bus strips.

[0073] Fig. 2C shows a portion of a cross section along a bus strip and through an intersection with a conducting finger, where material for formation of the conducting finger is deposited on top of material for formation of the bus strip, in accordance with some embodiments of the present invention. Bus strip material 39 (shown along a longitudinal cross section) has been deposited on top of anti-reflection coating 32. Conducting finger material 37 (shown in cross section perpendicular to the long axis of bus strip material 39) may be deposited on the outward-facing surface of anti-reflection coating 32 after deposition of, and perpendicular to, bus strip material 37. Conducting finger material 37 crosses bus strip material 39 at intersection 17. At intersection 17, conducting finger material 37 is separated from anti- reflection coating 32 by the thickness of bus strip material 39.

[0074] Alternatively to using different materials to form conducting fingers and bus strips, a single material containing a uniform concentration of glass frits may be used. The thickness of the deposited material may be controlled so as to ensure that the area density of glass frits is approximately uniform, including at intersections.

[0075] Fig. 3A illustrates control of material thickness in a portion of a cross section through an intersection of crossing conducting lines, in accordance with some embodiments of the present invention. Conducting material layer 46 may be deposited on an outward facing surface of ant-reflection coating 32. For example, conducting material layer 46 may represent deposited material for formation of a conducting finger.

[0076] Conducting material layer 46 includes zone of reduced thickness 40, in which the thickness of deposited material is less than in the remainder of conducting material layer 46. For example, conducting material layer 46 may be formed in several stages. During the first stages, material may be deposited on substrate 20 to a uniform thickness equal to the thickness of zone of reduced thickness 40, e.g. 1 μηι to 2 μπι. During subsequent stages (e.g. subsequent passes of a material deposition device), material may continue to be deposited along conducting material layer 46, except in zone of reduced thickness 40. For example, the thickness of conducting material layer 46 may be increased to 5 μηι to 10 μπι.

[0077] For example, in order to form zone of reduced thickness 40, a nozzle that is depositing material on conducting material layer 46 may be controlled to stop dispensing material when located above zone of reduced thickness 40. Alternatively, some nozzles of an array of nozzles may be deactivated when crossing zone of reduced thickness 40.

[0078] Fig. 3B illustrates deposition of a material across the intersection in the cross section shown in Fig. 3A. Cross conducting material layer 42 may be deposited on top of zone of reduced thickness 40 of conducting material layer 46, thus forming intersection 16. For example, cross conducting material layer 42 may represent deposited material for a strip of material that is oriented perpendicular to conducting material layer 46 (e.g. for a bus strip).

[0079] Resolution of a material deposition device may be limited. In order to prevent overlap between cross conducting material layer 42 and conducting material layer 46, zone of reduced thickness 40 may be designed to be somewhat wider than the width of cross conducting material layer 42. Thus, a gap 41 may be formed on either side of cross conducting material layer 42. For example, gap 41 may be approximately 10 μηι across.

[0080] The material deposited to form cross conducting material layer 42 and to form cross conducting material layer 46 may contain similar concentrations of glass frits. Alternatively, material deposited to form a bus strip (e.g. cross conducting material layer 42) may include a lower concentration of glass frits. For example, the concentration of glass frits in material for a bus strip may be sufficient to enable stable bonding to anti-reflection coating 32, without penetration to conducting layer 30.

[0081] Fig. 3C illustrates the cross section shown in Fig. 3B after subjection to a heat treatment. The conducting material, in the form of seed conductor ribbon 46' and seed conductor strip 42', is in contact with conducting layer 30 of substrate 20. However, at gaps 4Γ, the electrical connection between seed conductor ribbon 46' and seed conductor strip 42' may not be reliable (minimal contact area). Therefore, the seed conductors, seed conductor ribbon 46' and seed conductor strip 42', may be plated or otherwise covered with a metal or similar conductor.

[0082] Fig. 3D illustrates the metallic plating of the cross section shown in Fig. 3C. Seed conductor ribbon 46' and seed conductor strip 42', including gaps 4Γ, have been covered by conduction metal layer 44. For example, an electrochemical plating process for creating conduction metal layer 44 may selectively coat only those areas of the surface of substrate 20 that had been previously coated with a metallic seed layer.

[0083] As a special case of the situation shown in Fig. 3 A, the thickness of zone of reduced thickness 40 may be zero. In other words, no material may be deposited in a gap in a conducting material layer (e.g. a conducting finger) near an intersection where a crossing strip of conducting material is to be deposited.

[0084] Fig. 4A illustrates a gap in deposited material in a portion of a cross section through an intersection of crossing conducting lines, in accordance with some embodiments of the present invention. Conducting material layer 48 may be deposited on an outward facing surface of ant- reflection coating 32. For example, conducting material layer 48 may represent deposited material for formation of a conducting finger. Conducting material layer 48 includes gap 50, in which no conducting material was deposited. For example, a nozzle that is depositing material on conducting material layer 48 may be controlled to stop dispensing material when located above gap 50.

[0085] Fig. 4B illustrates deposition of a material across the intersection in the cross section shown in Fig. 4A. Cross conducting material layer 52 may be deposited in gap 50 to form intersection 16. For example, cross conducting material layer 52 may represent deposited material for a strip of material oriented perpendicular to conducting material layer 48 (e.g. for a bus strip). Gap 50 may be designed to be somewhat wider than the designed width of cross conducting material layer 52, forming a gap 51 on either side of cross conducting material layer 52.

[0086] The material deposited to form cross conducting material layer 52 and to form conducting material layer 48 may contain similar concentrations of glass frits. Alternatively, material deposited to form a bus strip (e.g. cross conducting material layer 52) may include a lower concentration of glass frits. For example, the concentration of glass frits in material for a bus strip may be sufficient to enable stable bonding to anti-reflection coating 32, without penetration to conducting layer 30.

[0087] Fig. 4C illustrates the cross section shown in Fig. 4B after heating and metallic plating. The conducting material, in the form of seed conductor ribbon 48' and seed conductor strip 52', is in contact with conducting layer 30 of substrate 20. However, at gaps 51, there is no direct contact between seed conductor ribbon 48' and seed conductor strip 52'. [0088] Conduction metal layer 54 covers seed conductor ribbon 48' and seed conductor strip 52'. Conduction metal layer 54 also connects seed conductor ribbon 48' and seed conductor strip 52' across gaps 51.

[0089] It should be clear that the description of the embodiments and attached Figures set forth in this specification serves only for a better understanding of the invention, without limiting its scope.

[0090] It should also be clear that a person skilled in the art, after reading the present specification could make adjustments or amendments to the attached Figures and above described embodiments that would still be covered by the present invention.