Screen printing apparatus including support bars, and methods of using same

A method is disclosed for electroforming metal screen. The method deposits photoresist over a mandrel, and then exposes the photoresist with light through a plurality of openings in a photo tool to form hardened resist pillars. Unexposed photoresist is removed without affecting the resist pillars. The method then electroforms the metal screen in areas free of the hardened resist pillars such that the metal screen forms apertures defined by each of the resist pillars, a space between at least two of the resist pillars defining a support bar that forms at a reduced thickness as compared to portions of the metal screen that are not between the resist pillars. Finally, the method detaches the metal screen from the mandrel.

BACKGROUND

Photovoltaic (PV) power generation is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of cells (called “solar cells” herein) containing a photovoltaic material. Cell materials include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide (CIGS). Due to the growing demand for renewable energy sources, manufacturing of solar cells and photovoltaic arrays has advanced considerably in recent years. Solar cells include thin film solar cells and crystalline solar cells. Thin film solar cells are normally much larger than crystalline cells. Both crystalline solar cells and thin film solar cells include an electrode structure that collects electrons freed by photon impingement on the solar cell. The electrode structure includes finger electrodes and buss electrodes. Generally, the buss electrodes are much larger than the finger electrodes.

Thick film screens made with woven stainless steel wires are conventionally used to form solar cell electrode structures, which are often referred to as prints. A printing process produces the prints by using conductive paste or ink printed through the thick film screens (the conductive material will be called “paste” herein; it is understood that the teachings herein are equally applicable to use of thinner conductive material that may be called “ink”). A typical screen for solar printing is 325 mesh with a 0.9 mils wire diameter. The thick film screen mesh wire may stretch during printing life cycles such that printed images may distort. Thick film screens typically have an open print area of about 42%, with the best screens having an open print area of about 50%. Typical thick film screens have a base wet print thickness of about 1 mil. A common emulsion (used to block the transfer of paste in areas that are not to be printed) is about 0.5 mils thick and requires the wet print thickness to be about 1.5 mils.

As solar cells become larger and finger electrodes become smaller, thick film screens run into some limitations. When the widths of the finger electrodes approach 50 μm to 75 μm (about 2 to 3 mils), the wire diameter of the thick film screens becomes an issue in print line definition. The mesh structure becomes a greater portion of the open line width, leading to sawtooth edges where images formed by the mesh wires intersect with the intended edge. A practical limit for line resolution in the thick film screens is about 3 mils wide. As the area of the printed image gets larger, trampoline screens are required to provide precise mounting and uniform tension. However, trampoline screens are very expensive and prone to distortion during their printing life cycles. Screens can also deform under tension throughout their useful lives, leading to dimensional instability.

There remains a need for developing new screens and processes to resolve issues associated with thick film screens and trampoline screens. A simple wish list for an advanced screening technique is that sawtooth edges should be eliminated, the percentage of open area in printed areas should increase, and the image should be dimensionally stable throughout its useful life.

SUMMARY

In an embodiment, a method is disclosed for electroforming metal screen. The method deposits photoresist over a mandrel, and then exposes the photoresist with light through a plurality of openings in a photo tool to form hardened resist pillars. Unexposed photoresist is removed without affecting the resist pillars. The method then electroforms the metal screen in areas free of the hardened resist pillars such that the metal screen forms apertures defined by each of the resist pillars, a space between at least two of the resist pillars defining a support bar that forms at a reduced thickness as compared to portions of the metal screen that are not between the resist pillars. Finally, the method detaches the metal screen from the mandrel.

In an embodiment, a method is provided for fabricating electrodes of solar cells. The method includes electroforming a metal screen that forms a plurality of finger apertures therethrough, the metal screen including support bars that cross the finger apertures; placing the metal screen over a substrate of the solar cells; printing conductive paste through the finger apertures of the metal screen; and solidifying the conductive paste to form the electrodes.

In an embodiment, a method is provided for electroforming a metal screen. The method includes forming photoresist features on a conductive mandrel; electroforming a first metal layer on the mandrel, the first metal layer forming a plurality of finger apertures defined by the photoresist features, the first metal layer including support bars across the finger apertures at intervals; and detaching the metal screen from the mandrel.

In an embodiment, a metal screen is provided for printing solar cells. The metal screen includes a metal layer forming a plurality of finger apertures and a plurality of support bars spaced apart in a first direction, wherein the support bars are formed across the finger apertures in a second direction substantially perpendicular to the first direction.

In an embodiment, a metal screen is provided for printing solar cells. The metal screen includes a metal layer having a plurality of support bars spaced apart in a first direction and a first plurality of finger apertures, and an emulsion layer disposed over the metal layer. The emulsion layer has a second plurality of finger apertures aligned with the first plurality of finger apertures of the metal layer in the first direction, wherein the support bars cross the first plurality of finger apertures in a second direction that is substantially perpendicular to the first direction.

In an embodiment, a method is provided for forming screens for printing finger electrodes onto a solar cell. The method includes electroforming a first metal screen having finger electrode features defined therein, each finger electrode feature forming an open printing area across the width of the screen except for one or more first support bars that cross the finger electrode features in a region corresponding to one or more buss bars of the solar cell. The method also includes electroforming a second metal screen having features defined therein corresponding to the one or more buss bars.

In an embodiment, a method of screen printing electrodes onto a solar cell is provided, including printing finger electrodes onto the solar cell utilizing a first metal screen having finger electrode features defined therein, each finger electrode feature forming an open printing area across the width of the screen except for one or more large support bars, each support bar crossing the open printing area in a region corresponding to a buss bar of the solar cell, and one small support bar crossing the open printing area between and equidistantly from each pair of the buss bars in the solar cell. The method also includes printing buss bars onto the solar cell utilizing a second metal screen having buss bar features defined therein.

In an embodiment, a method is provided for electroforming a metal screen. The method includes forming a photo tool with a pattern having a number of openings. The method also includes depositing a photoresist layer over a mandrel and placing the photo tool over the photoresist layer. The method further includes exposing the photoresist layer to a collimated light through the plurality of openings to form hardened resist pillars. The method also includes removing a portion of the photoresist layer being unexposed to the light to leave the resist pillars over the mandrel. The method further includes electroforming a metal layer in areas of the removed portion of photoresist layer and peeling off the metal layer from the mandrel. The metal layer has a plurality of finger apertures interleaved with a plurality of support bars, each support bar being formed between two respective resist pillars such that the support bars have a lower height than the finger apertures.

In an embodiment, a method for fabricating electrodes of solar cells is provided. The method includes electroforming a metal screen with a plurality of finger apertures and support bars. The method also includes placing the metal screen over a substrate with photo cells. The method further includes printing conductive paste through the finger apertures of the metal screen, and solidifying the conductive paste to form electrodes over the substrate.

In an embodiment, a method is provided for electroforming a metal sheet. The method includes forming a photo tool with a pattern having a plurality of openings. The method also includes electroforming a first metal layer over a mandrel, the first metal layer having a pattern for a plurality of support bars and finger apertures. The method also includes depositing a photoresist layer over the first metal layer. The method further includes aligning the plurality of the openings of the photo tool over the photoresist layer with the pattern for the plurality of finger apertures of the first metal layer, and exposing the photoresist layer to a collimated light through the plurality of openings to form hardened resist pillars. The method still includes removing a portion of the photoresist layer being unexposed to the light to leave the resist pillars over the first metal layer, and forming a second metal layer in areas of the removed portion of photoresist layer. The method further includes peeling off the metal sheet including the first and second metal layers from the mandrel.

In an embodiment, a metal screen is provided for printing solar cells. The metal screen includes a metal layer having a plurality of finger apertures and a plurality of support bars spaced apart in a first direction. The support bars are interleaved with the finger apertures in a second direction substantially perpendicular to the first direction.

In an embodiment, a metal screen is provided for printing solar cells. The metal screen includes a metal layer having a plurality of support bars spaced apart in a first direction and a first plurality of finger apertures. The metal screen also includes an emulsion layer disposed over the metal layer. The emulsion layer has a second plurality of finger apertures aligned with the first plurality of finger apertures of the first metal layer in the first direction. The support bars are interleaved with the first plurality of finger apertures in a second direction substantially perpendicular to the first direction.

In an embodiment, a method of forming screens for printing electrodes onto a solar cell includes electroforming a first metal screen having finger electrode features defined therein, each finger electrode feature forming an open printing area across the width of the screen except for one or more first support bars that cross the finger electrode features in a region corresponding to one or more buss bars of the solar cell, and electroforming a second metal screen having features defined therein corresponding to the one or more buss bars.

In an embodiment, a method of screen printing electrodes onto a solar cell includes printing finger electrodes onto the solar cell utilizing a first metal screen having finger electrode features defined therein, each finger electrode feature forming an open printing area across the width of the screen except for one or more large support bars, each support bar crossing the open printing area in a region corresponding to a buss bar of the solar cell, and one small support bar crossing the open printing area between and equidistantly from each pair of the buss bars in the solar cell. The method also includes printing buss bars onto the solar cell utilizing a second metal screen having buss bar features defined therein.

In an embodiment, a screen for printing electrodes onto a solar cell includes a metal foil having finger electrode apertures defined therein, each finger electrode aperture forming an open printing area across the width of the screen except for one or more first support bars, each support bar crossing the open printing area in a region corresponding to a buss bar of the solar cell.

Additional embodiments and features are set forth in the description that follows, and still other embodiments will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention.

DETAILED DESCRIPTION

The present disclosure may be understood by reference to the following detailed description, taken in conjunction with the drawings as described below. It is noted that, for purposes of illustrative clarity, certain elements in the drawings may not be drawn to scale. Reference numbers for items that appear multiple times may be omitted for clarity. Where possible, the same reference numbers are used throughout the drawings and the following description to refer to the same or similar parts.

This disclosure advances the art by providing a cost effective method for fabricating electroformed screens for use in screen printing. Certain electroformed screens are made and sold under the trade name “AccuScreen.”

In one embodiment, an electroformed product includes support bars that are formed across narrow apertures to allow up to about 90% open print area. The support bars are narrow, such that conductive paste printed on both sides of the support bar will merge together to form a continuous printed shape. When used for solar cell fabrication, this embodiment may have an electroformed mesh in areas that print wider buss bar electrodes, or may have support bars across the buss bar areas, like the narrow apertures. The narrow support bars facilitate large open print areas (about 90%) and thus provide better paste transfer and bleed under the support bar than prior art screens. In this embodiment, large open areas feature a flat solid metal foil webbing structure such that minimal dimensional distortion is expected during their print life cycle. Line widths of 2 mils are achievable with this embodiment.

Another embodiment is similar to the embodiment described above, except that an additional emulsion layer is added. (“Emulsion” is a substance that can be patterned so as to be present in certain areas and absent in other areas; emulsion can be for example photoresist. Typically, but not by way of limitation, “emulsion” is used to describe a substance that is present as a patterned layer in a finished screen suitable for screen printing, while “photoresist” is used to describe a photosensitive substance used as a temporary mask during fabrication of the screen.) The finger widths may are as small as 50 μm in some embodiments. Screens disclosed herein may be formed using a two-layer process, with at least one electroformed metal layer and a second layer that may also be metal or may be emulsion. The second layer, when present, provides clearance for support bars present in the first layer, to make it easier for conductive paste that is forced past the support bars and through the narrow apertures during printing to merge together to form a continuous printed shape.

In contrast, another embodiment may be electroformed by a single layer process and does not require aligning two layers like the above embodiment. The single layer process is generally easier and cheaper than a multi-layer process. In this embodiment, the support bars are of partial thickness, such that conductive paste that is forced past the support bars and through the narrow apertures during printing merges together to form a continuous printed shape.

The present disclosure provides methods and structures of electroformed screens and methods for fabricating electrodes over solar cells. Electroforming is a metal forming process that forms thin metal parts by an electroplating process. A part is produced by plating a thin metal layer, such as nickel or another electroplatable metal, around resist pillars on a conductive base form, commonly called a mandrel (photoresist is sometimes referred to a simply “resist” herein and has the same meaning). The mandrel is removed after plating. The mandrel is conductive and is made either of a metal or a non-conductive material covered with a conductive coating.

FIG. 1illustrates hardening photoresist pillars. Collimated light112is directed toward a multilayer structure100, which includes a photo tool102, a photoresist layer106under the photo tool102, and a mandrel layer108under the resist layer106. Photo tool102has a number of openings to allow collimated light112to pass through and harden a portion of resist layer106, thereby forming hardened resist pillars110as part of a resist image on the mandrel. The unexposed photoresist is removed, such that only hardened resist pillars110are left on mandrel108.

FIG. 2illustrates generation of a screen by electroplating. An electroforming device200includes an electroforming tank202containing electrolytic solution204and a voltage supply206. A cathode of the voltage supply206is connected to a plating part208and an anode of the voltage supply206is connected to an anode bag210that provides a plating material that may be for example nickel pellets. The plating part208including mandrel108is oriented toward anode bag210, such that the hardened resist pillars110face toward anode bag210. Screen212is plated onto mandrel108in areas not covered by hardened resist pillars110, as further discussed below.

FIGS. 3A-3Cillustrate a process for forming a metal screen212.FIG. 3Aschematically shows a cross section of mandrel108(FIGS. 1, 2) with resist pillars110thereon. Metal screen212electroplates onto mandrel108that are not covered by resist pillars110.FIG. 3Bis an enlarged detail of a portion ofFIG. 3Aduring the electroplating process, and schematically shows how thickness of metal screen212varies in proximity of a single resist pillar110. At a distance from mandrel108and resist pillar110, a flux of metal ions220approaching mandrel108is roughly constant in cross section. However, as metal ions220approach mandrel108closely, ions220do not plate onto resist pillar110, so that the flux of ions220increases in areas where ions220can plate. This causes thickened plating in locations215near resist pillar110. After plating, metal screen212is peeled off of mandrel108and resist pillars110, as shown inFIG. 3C.

FIG. 4illustrates a mechanism for forming support bars. When two resist pillars402A and402B are close enough to each other, metal layer404between the resist pillars402A and402B does not plate to the full thickness achieved by metal layers406A and406B in more open areas. This is due to diffusion effects within the plating electrolyte. Specifically, the electrolyte within volume408between resist pillars402A and402B becomes depleted of metal ions after some of them plate out into layer404, and the metal ion concentration within volume408does not come back to equilibrium with the metal concentration in the rest of the electrolyte as quickly as in larger volumes, limiting the metal ions available for deposition in layer404.

Metal layer404may be utilized as a support bar in electroformed stencils. It should be noted that the side of the plating shown inFIG. 4against mandrel410becomes the squeegee side of the eventual screen during printing, while the top of the plated layers (the uneven side, due to the partial thickness support bars) becomes the contact side of the screen. Therefore, the use of partial thickness support bars converts what is usually thought of as a nuisance feature (incomplete plating in narrow areas) to a useful feature. A support bar provides some mechanical stability, but purposefully does not extend down to the bottom of the contact side of the screen, so that paste can flow around the support bar and fill underneath what would otherwise be a small gap due to the support bar blocking the paste. However, other embodiments herein can advantageously utilize the mandrel side of the screen as the contact side of the screen during printing, as discussed further below.

FIG. 5Ais a schematic plan view of a portion500of an electroformed screen that is formed of metal (e.g., nickel). Broken lines5B-5B′ and5C-5C′ indicate the locations of cross-sectional views shown inFIGS. 5B and 5Crespectively. Most areas of portion500are covered by a second layer506that may be formed of emulsion or of metal (e.g., nickel). Portion500also includes a metal layer504that underlies second layer506in the view ofFIG. 5A, except for support bars502, see alsoFIGS. 5B, 5C(not all instances of support bars502are labeled inFIGS. 5A through 5C, for clarity of illustration). Second layer506and metal layer504form apertures516having a width512, with support bars502spaced at a pitch514crossing apertures516, and each support bar502having a width518, as shown. Thus, first metal layer504has a plurality of support bars502spaced apart in a first direction (an X-axis, inFIG. 5A). Second layer506is disposed over first metal layer504. Second layer506has a plurality of apertures516aligned with the plurality of support bars502in the first direction. Support bars502cross apertures516in a second direction along a Y-axis substantially perpendicular to the first direction. The electroformed screen represented by portion500may be utilized as a stencil for printing fine structures such as finger electrodes, by forcing conductive paste through apertures516.

FIG. 5Bis a schematic cross-sectional view of portion500of an electroformed screen, taken along lines5B-5B′ inFIG. 5A. As illustrated, support bars502are formed as part of a first layer504that is formed of metal (e.g., nickel). A second layer506is disposed over first layer504and forms an opening corresponding to aperture516and extending above each support bar502. Support bar502has a thickness508which is the same as a thickness of first metal layer504, and second layer506has a thickness510. Aperture516has a width512. As shown, each aperture516extends from first metal layer504to second layer506and has a height equal to thickness510of second layer506plus thickness508of metal layer504.

FIG. 5Cis a schematic cross-sectional view of portion500of an electroformed screen, taken along lines5C-5C′ inFIG. 5A, that is, down the middle of one of apertures516. InFIG. 5C, metal layer504forms a number of support bars502that cross apertures516; the cross-sectional plane ofFIG. 5Ccuts through support bars502, while the solid part of layer504is visible behind the cross-sectional plane. Second layer506is also visible behind the cross-sectional plane.

FIG. 6Ais a schematic plan view of a portion600of an electroformed screen during fabrication of the screen. Broken lines6B-6B′ and6C-6C′ indicate the locations of cross-sectional views shown inFIGS. 6B and 6Crespectively. A metal (e.g., nickel) layer604forms most of the area of portion600except where masked by resist pillars620. Layer604forms apertures616having a width612. Support bars602having a width618cross apertures616at a pitch614, see alsoFIGS. 6B, 6C(not all instances of support bars602or resist pillars620are labeled inFIGS. 6A through 6C, for clarity of illustration). Each support bar602forms between a pair of resist pillars620that limit a height of support bar602relative to a thickness of the rest of layer604, as previously discussed in connection withFIG. 4. Thus, metal layer604has a plurality of support bars602spaced apart in a first direction (an X-axis, inFIG. 6A). Metal layer604is formed of electroplated metal that is masked by resist pillars620to form a plurality of apertures616. Support bars602cross apertures616in a second direction along a Y-axis substantially perpendicular to the first direction. Resist pillars620are removed after formation of layer604and support bars602. The electroformed screen represented by portion600may be utilized as a stencil for printing fine structures such as finger electrodes, by forcing conductive paste through apertures616.

FIG. 6Bis a schematic cross-sectional view of portion600of an electroformed screen, taken along lines6B-6B′ inFIG. 6A. As illustrated, support bars602are formed with a support bar height606that is less than a thickness608of layer604. Support bars602are located within apertures616.

FIG. 6Cis a schematic cross-sectional view of portion600of an electroformed screen, taken along lines6C-6C′ inFIG. 6A, that is, down the middle of one of apertures616. As shown inFIG. 6C, each support bar602forms between two resist pillars620. Resist pillars620are only used in the manufacturing process and are removed in the final device, but are shown inFIG. 6Cfor illustration. Support bars602have a width618. Apertures616have a width612, as shown inFIG. 6B.

By using the electroformed screen exemplified by portion600, fine structures (e.g., electrode fingers) may be printed. The difference between the support bar height606and the metal layer thickness608is the gap that allows conductive paste to flow under the support bar602to form a continuous structure.

FIG. 7shows a photo700of a cross-sectional view of an electroformed screen exemplified by portion600. Photo700shows a support bar602with a lower height than the bulk thickness of metal layer604. Support bar602is formed as described earlier with respect toFIG. 4.

FIG. 8shows a photo800of a top view of an electroformed screen exemplified by portion600. Note that the screen has about support bar802(dark area between light areas) that is 0.5 mils wide, across a finger aperture804(light areas) that is about 2 mils wide, formed in metal foil806(dark area) that is about 2 mils thick.

As disclosed herein, support bars may have a range of widths and heights that are related to finger aperture width and bulk metal thickness for electroformed screens, and resist thickness utilized to form the screens, as exemplified by portions500and portion600. These dimensions scale together, because the photoresist that defines both the apertures and the support bars must be patterned with reasonable integrity, which urges a decreased photoresist thickness, but mechanical integrity of the electroformed metal improves with increased thickness, which requires a minimum photoresist thickness so that the metal does not begin to electroplate over top of the photoresist.

For example, to form an electroformed screen in one embodiment, that has an electroformed bulk metal thickness of 1.5 to 2 mils (e.g., in large areas where proximity to photoresist pillars does not apply) the photoresist must typically be at least about 2.4 mils thick to keep the metal from plating over the photoresist. Small finger aperture widths in the screen (defined by areas of exposed, hardened photoresist) of about 1 to 3 mils can be imaged in the photoresist with good pattern integrity, with support bars (spaces in the photoresist) of about 0.5 to 1 mil crossing the finger aperture shapes. During electroplating, support bars with heights of about 0.7 to 1.5 mils will form in the narrow support bar spaces, due to metal ion depletion in the narrow spaces, as opposed to the bulk metal thickness of 1.5 to 2 mils.

In another embodiment, that has an electroformed bulk metal thickness of 2 to 3 mils, the photoresist must typically be at least about 3.5 mils thick, the support bar width ranges from 0.7 mils to 1.2 mils, and the support bar height ranges from 1.1 mils to 2.4 mils. The finger width ranges from 1.5 mils to 3.5 mils. In a further embodiment, that has an electroformed bulk metal thickness of 2.5 to 3.5 mils, the photoresist must typically be at least about 4.2 mils thick, the support bar width ranges from 0.9 mils to 1.7 mils and the support bar height ranges from 2 mils to 3 mils. The finger width ranges from 2 mils to 4 mils.

Table 1 summarizes key dimensional ranges in the electroformed screens for the above embodiments having small, medium and large electrode fingers respectively. The bulk metal thickness of each embodiment sets a lower bound for the required photoresist thickness, while the finger widths require photoresist to be thin for good pattern integrity. The photoresist thickness sets a lower bound for both the finger width and support bar width. Below the finger width ranges indicated, the required photoresist thickness will have imprecise width control and/or will disappear completely. Above the finger width ranges indicated, the electrodes will be unnecessarily wide and will block more light than desirable for high solar cell efficiency. Below the support bar width ranges indicated, the required photoresist thickness will have imprecise width control and/or will not develop out to the bottom of the resist layer at the mandrel, such that support bars will not electroform at all. Above the support bar width ranges indicated, the support bars will become too thick to properly print under (the support bar areas will not be narrow enough to suppress electroplating due to metal ion depletion).

It is appreciated that although the methods and electroformed screens disclosed herein are discussed in terms of applicability to printing electrodes for solar cells, these methods and screens can be advantageously utilized for printing narrow, high aspect ratio features (e.g., narrow features that are at least 10 times as long as wide) for any purpose.

FIG. 9is a flow chart illustrating the steps for fabricating a metal foil or sheet using a single layer process. Method900includes forming a photo tool with a pattern having a number of openings at step902. The method also includes depositing a photoresist layer over a mandrel at step904, placing the photo tool over the photoresist layer, and exposing the photoresist layer to light through the openings to form hardened resist pillars at step908. The mandrel may be made of stainless steel, copper alloy or a non-conductive material with a conductive coating. The method further includes removing an unhardened portion of the photoresist layer at step910and electroforming a metal layer over the mandrel at step912, followed by peeling off the metal layer from the mandrel at step914. The method also includes adding a nano-coating over the metal foil or sheet before fabricating a product, such as solar cell electrodes.

Electroformed screens can be used for solar printing applications. For example, electroformed screens may be used for producing solar cell electrodes. In an embodiment, by placing an electroformed screen over a solar cell substrate, and flowing conductive paste through the electroformed screen, finger electrodes would be formed. This process is called printing. The electroformed screens and a metal layer define finger traces that can be as thick as 75 to 100 μm. A mesh pattern is electroformed to define the mesh pattern over the open finger traces.

Printing requirements typically differ between crystalline solar cells and thin film solar cells. Normally, crystalline solar cells require a conductive glass frit type paste fired at high temperatures. On the other hand, thin film photo panels normally require silver filled printable epoxy. The conductive paste may include silver pastes. For example, polymer based silver paste, such as epoxy based silver paste, is typically used in thin film solar cells. Polymer based silver paste may be provided by, for example, Indium Corporation. Glass frit based silver paste is commonly used for crystalline solar cells. Glass frit based silver paste may be provided by, for example, Heraeus or Dupont.

FIG. 10Ais a simplified, schematic cross-sectional system diagram illustrating fabricating a product using an electroformed screen. System1000positions substrate1010and electroformed screen1008in an X-Y plane (each of the substrate and screen is reduced to a line in the X-Z plane shown inFIG. 10A). Printing direction, squeegee direction, and paste spreading direction are along the Y-axis. System1000includes a printer1002, which is normally automated with controls for squeegee speed, squeegee pressure, screen off contact spacing, and flood print or print flood cycles. System1000also includes frame1004for mounting electroformed screen1008to printer1002. Frame1004holds electroformed screen1008taut. Conductive paste1016(e.g. silver paste) is placed on a top surface1009of electroformed screen1008. System1000also includes a squeegee1006that spreads conductive paste1016on a surface of electroformed screen1008by moving front to back in a squeegee direction along the Y-axis. This process is referred to as a flood process. Next, the squeegee1006comes in contact with the surface of electroformed screen1008and moves paste1016from back to front in an opposite direction from the paste spreading direction along the Y-axis. There may be a spacing between a bottom surface of electroformed screen1008and substrate1010called off-contact distance or snap-off distance, which allows electroformed screen1008to break away from substrate1010during a printing operation.

FIG. 10Bis an enlarged perspective view of respective portions of electroformed screen1008and substrate1010ofFIG. 10A. In the portions shown inFIG. 10B, electroformed screen1008has an aperture1012that is crossed by support bars1026spaced along aperture1012. Support bars1026do not extend all the way to the bottom of screen1008(e.g., the surface of screen1008nearest substrate1010), thus leaving a gap1024for paste1016to spread under support bars1026and thus form continuous printing lines on substrate1010.

The amount of paste spread may be determined by viscosity of the conductive paste. The conductive paste may have various viscosities. Generally, low viscosity paste spreads more than high viscosity paste. To achieve small finger line width less than 100 μm, higher viscosity paste is used to promote low paste spreading. However, very high viscosity past may result in broken conductor lines. Very low viscosity paste, on the other hand, may spread too much, resulting in excessively wide print lines that block photons from striking a solar cell surface, thereby reducing solar cell efficiency.

FIG. 11Ais a photo illustrating an untreated electroformed screen surface andFIG. 11Bis a photo illustrating a treated electroformed screen surface with a nano-coating in an embodiment. The nano-coating is a finishing process for an electroformed screen. The advantages of the finishing process include reduced production cost, faster production cycle, lower product defects and higher yield, for example, in solar cell electrodes. The photos were taken after measured drops of flux1100,1100′ were placed on uncoated and coated surfaces1101and1103respectively.FIGS. 11A and 11Bdemonstrate that an uncoated surface1101produces a smaller contact angle1102than a contact angle1104produced by coated surface1103. Clearly, a higher flux contact angle is obtained with coated surface1103.

FIG. 11Cillustrates cleaning effects on a nano-coating treated surface and an untreated surface. The photograph labeled1106shows a print made from a screen that has been used to generate 14 prints without cleaning. The photograph labeled1108shows a print made from a screen that has a nano-coating, that has also been used to generate 14 prints without cleaning. Screens1110and1112show prints made from the same screens as screens1106and1108respectively, after the screens have been cleaned. As shown inFIG. 11C, the print made from uncoated screen1110looks similar to the print made from coated screen1112after cleaning. However, uncoated screen1106requires a longer cleaning cycle than coated screen1108.

Referring toFIG. 10Bagain, a nano-coating is applied on a bottom surface1022of electroformed screen1008that makes contact with substrate1010. The nano-coating also flows up into the slits or apertures1012. However, the squeegee side or top surface1020of electroformed screen1008does not have a nano-coating. The finger slits or apertures1012should be aligned generally parallel to the squeegee stroke so that the print direction is down the long length of the finger apertures or slits in the Y-axis.

FIG. 12is a flow chart illustrating the steps for fabricating products using an electroformed screen, in an embodiment. Method1200includes electroforming an electroformed screen with finger apertures and support bars at step1202. Method1200may optionally include applying a nano-coating over the electroformed screen at step1204, as shown in dash-line. Method1200also includes placing the electroformed screen over a solar cell substrate such that the nano-coating is disposed between the electroformed screen and the substrate at step1206. Method1200also includes printing conductive paste through the finger apertures of the electroformed screen at step1208. Method1200further includes solidifying the conductive paste at step1210and forming electrodes over the substrate at step1212.

FIGS. 13A-Care photos that illustrate print results of electroformed screen according to portion600.FIG. 13Ashows a print when the support bar is 1.1 mils wide.FIG. 13Bshows a print with support bars of 0.7 mils wide.FIG. 13Cshows a print with support bar of 0.5 mils wide. As shown inFIGS. 13A-13C, the finger electrode paste print1302obtained with wider support bars is narrower than the finger electrode paste prints1304and1306obtained with narrower support bars such that the finger electrode paste prints1304and1306inFIGS. 13B and 13Care not broken but the finger electrode paste prints1302inFIG. 13Aare broken. These results illustrate that if the support bar is too wide and thus tall, such as the print obtained with a 1.1 mil support bar as shown inFIG. 13A, the finger electrode paste print1302will be broken. When the support bar is narrow and has a lower height, such as 0.5 mils or 0.7 mils, the support bar allows paste to flow under the support bar. If the resist pillars shown inFIG. 4are too far apart, resulting in a wide and tall support bar, a second layer as shown inFIGS. 5A-5Bhelps the paste bridge the gap beneath the support bar.

In an embodiment, an electroformed screen has an electroformed mesh with an electroformed build-up defining a print area. The mesh may be a hexagonal mesh. An example of this configuration is shown inFIG. 14, which is a photo of an electroformed screen with 1 mil mesh1404and 2 mils electroformed build-up1402. Since the mesh is electroformed, it is flat like a stencil, without the weave knuckles that characterize stainless steel mesh. This flatness promotes dimensional image stability during use and prolonged usage life. However, the open area is very similar to thick film screens, i.e. only about 50% of the “open” area is open.

Certain electroformed screens the electrode fingers are formed differently. Instead of the hexagonal mesh pattern, narrow support bars stretch across the open fingers at a repetitive interval. In this manner, the open area can be as much as approximately 90%. As before, the first electroformed layer is of the order of 1 mil and the electroformed build-up is of the order of 1 to 2 mils. The buss electrodes may be configured the same way as the finger electrodes with support bars. In an alternative embodiment, if the buss electrodes are wide, they may have a hexagonal mesh.FIG. 15is a photo of an electroformed screen with 2 mil wide fingers1502, 2 mil wide support bars1504, and 1.5 mil electroformed build-up1506A-B which are first and second metal layers. In this case, the buss electrode is wide so that a mesh pattern is employed.FIG. 16is a photo of an electroformed screen with support bars1608for finger electrodes1602and support bars1610for buss electrodes1604. Buss electrodes1604are larger than finger electrodes1602, as shown inFIG. 16.

In certain electroformed screen embodiments, a layer of emulsion is added on top of the electroformed image build-up area. This configuration is shown inFIG. 17, which is a photo of an electroformed screen with 1 mil thick for finger support bars1710and a buss mesh1702, 1.5 mil electroformed build-ups1704and1706(first and second metal layers) for finger and buss electrodes, 0.5 mil emulsion build-up1706for the finger and buss electrodes.

An example of an electroformed screen for printing electrodes on a crystalline wafer is shown inFIG. 18, which is a photo of an electroformed screen for a five inch by five inch (about 125 mm) crystalline solar cell with 3.15 mil (80 μm) wide fingers. The emulsion used in the electroformed screen aids in forming a tight gasket between the electroformed screen and the substrate. This is useful for conductive epoxies with low viscosities, to keep the printed epoxy paste from spreading to unwanted areas.

The printing areas for crystalline solar cells and thin film solar cells are quite different, typically 25 square inches for crystalline solar cells and about 300 square inches for thin film solar cells.FIG. 19is a photo of finger and buss prints for 4 mil wide fingers and 80 mil wide buss electrodes for a crystalline cell. Note that there is a slight narrowing of the finger line width at the support bar location (see circled area1902).

FIG. 20is a photo of finger and buss prints for a thin film solar cell. A semi-transparent dummy panel shows 5 mil wide finger prints2002and 15 mil wide buss prints2004. The finger prints and buss prints include conductive epoxies. The total print area is about two hundred-and-eighty square inches.

FIG. 21is a flow chart illustrating the steps for fabricating metal foil or a metal sheet using a two-layer process, in an embodiment. Method2100includes forming a photo tool with a pattern having a plurality of openings at step2102. Method2100also includes electroforming a first metal layer over a mandrel at step2104. The first metal layer has a pattern for a plurality of support bars interleaved with a plurality of finger apertures. Method2100also includes depositing a photoresist layer over the first metal layer at step2106and aligning the plurality of the openings of the photo tool over the photoresist layer with the pattern for the plurality of electrode finger apertures of the first metal layer at step2108. Method2100further includes exposing the photoresist layer to a collimated light through the plurality of openings to form hardened resist pillars and removing a portion of the photoresist layer being unexposed to the light to leave the photoresist layer pillars over the first metal layer at step2110, and forming a second metal layer in areas of the removed portion of photoresist layer at step2112. Method2100still further includes peeling off the metal sheet including the first and second metal layers from the mandrel at step2114.

In still other embodiments, finger structures can be applied to solar cells in a first pass screening, followed by a second pass screening to apply coarser features orthogonal to the finger structures. Separation of the first and second pass screening allows each screening to be optimized for simplicity, best solar cell characteristics, and cost savings, as compared to prior art apparatus and methods. These and other embodiments can also provide screens with openings in the metal about the periphery of the screen, with emulsion over the openings, to provide strain relief to features closer to the middle of the screen without printing through the openings. These embodiments are now described.

FIG. 22Aschematically shows a screen2200for printing finger structures onto solar cells in a first screen printing step. Screen2200is an electroformed foil that is formed, e.g., using method2300described below. Screen2200includes openings2210in locations that correspond to the finger structures of the solar cells. Openings2210are not shown to scale; in particular, openings2210are shown in greater width than they would typically have in proportion to an overall size of screen2200, for clarity of illustration.FIG. 22Aalso shows locations2220in dashed outline, where buss bars will be formed on the solar cells in a second screen printing step. Openings2210cross screen2200except at support bars2230and2240, which occur in lines through screen2200, noted by arrows inFIG. 22A(that is, each arrow points not just to a single occurrence of support bars2230or2240respectively, but to a line of such support bars occurring across screen2200).

FIG. 22Bis an enlarged schematic view of a region denoted as A inFIG. 22A. Openings2210are interrupted at support bars2230within locations2220that correspond to buss bars in the finished solar cells, as shown. Support bars2230allow metal of screen2200to traverse openings2210, for improved structural integrity of screen2200. Support bars2230may be of a width that is sufficient to increase structural integrity of screen2200but is also less than a width of locations2220, so that once buss bars are printed over the finger structures, the buss bar material meets the finger electrode material printed on both sides of each support bar2230. For example, support bars2230may be 100 to 300 mils wide, and especially about 170 mils wide.

Openings2210are also interrupted at support bars2240that are located halfway between locations2220. Support bars2240also allow metal of screen2200to traverse openings2210, for further improved structural integrity of screen2200. Support bars2240may result in corresponding breaks in finger electrodes printed through openings2210. However, when support bars2240are small and are halfway between locations2220, the resulting solar cell performance does not degrade significantly, because charge carriers in this region can easily travel the small distance through the solar cell material to either nearby finger electrode section to a corresponding, adjacent buss bar. That is, the overall resistance penalty for the solar cell when gaps are present in the finger electrode material is low when such gaps are small (in comparison to the spacing between adjacent finger structures) and are equidistant from adjacent buss bars. Support bars2240may be approximately as narrow as a resolution limit of the photolithography process utilized to generate screen2200, and may be approximately as wide as spacing between adjacent finger electrodes before the resistance penalty of the resulting solar cell starts to be significant. Therefore support bars2240may be approximately 5 to 200 mils wide, and most likely about 10 to 20 mils wide.

When screen2200is utilized for printing finger electrodes, it may be advantageous to utilize the mandrel side of the screen during electroforming, as the contact side of the screen during printing. This is because the electroformed openings are typically narrowest at the mandrel during electroforming, as seen inFIGS. 3A-3CandFIG. 4. The sharp mandrel side edge in screens thus formed helps keep printed finger traces tall and narrow, resulting in high conductivity with minimal light blockage of the solar cell material. The same advantages do not necessarily apply to a screen that prints buss bars, which are generally much wider than the finger electrodes and thus well above the minimum printing resolution supported by screen2200. An electroformed screen that prints the buss bars may therefore utilize a mandrel side of the screen as a squeegee side of the screen during printing. Also, screen2200is one embodiment that includes both support bars2230and2240, across the buss bar locations and equidistant from buss bar locations respectively; it is contemplated that other embodiments may include support bars only across the buss bar locations, or only equidistant from the buss bar locations.

FIG. 23Aschematically shows a screen2200′ for printing finger structures onto solar cells in a first screen printing step, andFIG. 23Bis an enlarged schematic view of a region denoted as B inFIG. 22A. Screen2200′ is an electroformed foil that is formed, e.g., using method2300described below. The features of screen2200′ are identical to those of screen2200(FIG. 22), howeverFIG. 23Aadditionally shows locations2250where the buss bar screen is configured to print conductive paste to connect finger electrodes printed through openings2210. Only representative locations2250are labeled withinFIGS. 23A and 23B, for clarity of illustration. Locations2250may be designed into the buss bar screen and subsequently printed when a resistance penalty for breaks due to support bars2240is determined to be too high. Locations2250are shown slightly oversized inFIGS. 23A and 23Brelative to the breaks due to support bars2240, to allow for looser screen printing feature sizes and alignment tolerance that are expected to apply in the case of the buss bar screen. Printing locations2250using the buss bar screen imposes the requirement for aligning locations2250with the breaks due to support bars2240, but opens up the possibility that support bars could be provided at more locations than being equidistant between buss bars, for increased mechanical integrity of screen2200′.

FIG. 24is a flowchart of a method2300for generating screens and for screen printing electrodes connected by buss bars onto solar cells. Method2300prints finger electrodes separately from buss bars so that features of the screens, and the printing processes utilizing the screens, can be separately optimized, as discussed below. In a first step2302, a first metal screen is electroformed as described above, with finger electrode features that are open (e.g., are not crossed by the electroformed screen) except for support bars at two types of locations. The first type of location, denoted as (a) is where buss bars will eventually be printed. Since buss bars will be printed over these areas, breaks in the finger electrodes can be tolerated because the buss bar material will contact the finger electrode material on both sides of the breaks. Large support bars can therefore be formed across the open finger electrode areas of the first metal screen in these areas; such large support bars may be for example 100 to 300 mils wide, and especially about 170 mils wide. The second type of location, denoted as (b) is where the finger electrodes are equidistant from the buss bars. For lowest possible resistance, it is most desirable to print the finger electrodes across these areas, so small support bars (e.g., 5 to 200 mils wide, especially about 10 to 20 mils wide) can be placed across the finger electrodes. Since electrons generated by the solar cell can travel to either adjacent buss bar, there is a negligible resistance penalty for breaking a finger electrode at these points. Therefore, in embodiments, even relatively large support bars can be placed across the finger electrodes at these points.

Step2302may be optimized to produce the first metal screen that is optimized for the finest possible printing, to achieve small finger electrode size. Small finger electrode size is advantageous in that it preserves the maximum area in a solar cell that can actually interact with light to produce electrical power. For example, a photoresist utilized to mask deposition of metal may be optimized to produce small features, and an overall thickness of metal that is electroplated to form the screen may be tailored for screen printing of the small features. Step2302of method2300may also include optional steps2304,2306,2308described further below.

Step2310of method2300electroforms a second metal screen with buss bar features. The second screen may vary significantly from the first screen formed in step2302. For example, the buss bars are much coarser features than the fine finger electrodes, so the printing step that utilizes the second screen may benefit from the second screen being thicker than the first screen.

Step2312prints finger electrode features onto a solar cell utilizing the first screen. The finger electrodes may or may not break at the locations of the support bars corresponding to buss bar locations and equidistant from the buss bars. Step2312is optimized for printing fine features to reduce the area of the solar cell in which the finger electrodes block light from reaching the solar cell material. For example, step2312may be performed by utilizing the mandrel side of the screen during electroforming, as the contact side of the screen during printing.

Step2314prints buss bars onto the solar cell. Step2312is optimized for printing coarser features than step2312. Since the buss bars print at the locations where support bars were placed across the printing area of the finger electrodes, the buss bars connect across any breaks that were formed at that time. As noted above, breaks that form across the finger electrode printing areas equidistant from the buss bars may have inconsequential effect on resistance of the finished solar cell. An optional step2316prints features to bridge breaks in the electrode features caused by support bars. Step2316occurs at the same time as step2314and simply results when the buss bar screen includes open spaces at the locations of the support bar breaks in the electrode features. An example of step2316is utilizing a buss bar screen that includes open spaces at locations2250,FIGS. 23A and 23B.

In embodiments, steps2302and2310form a screen generation subprocess2320of method2300, and steps2312and2314form a solar cell printing subprocess2340of method2300, as shown inFIG. 24. That is, subprocess2320of generating screens forms one embodiment herein, while subprocess2340of utilizing such screens to print solar cells forms a second embodiment.

It has been discovered that metal screens can deform when stretched for printing, with deformation usually most notable in areas of the screen that are closest to the edges of the screen in the direction of tension.FIG. 25schematically illustrates a screen2400that is under tension in a direction denoted as Z for printing. Screen2400includes open shapes2410intended, for example, to print finger electrodes. Areas indicated as2420,2420′ schematically show where screen deformation might be expected; however, actual areas of deformation in a given screen will depend on various factors such as the screen tension in direction Z, where open shapes are located on the screen, thickness of the screen and other factors. In particular, deformation is often found in the nearest open shapes to the edge of the screen, likely because solid metal areas resist deformation better than the open areas.

FIG. 26shows a screen2500that is similar to screen2400, but includes features designed to reduce deformation in printing areas of the screen. Like screen2400, screen2500includes open shapes2410. Screen2500also includes dummy shapes2510that are similar in size and orientation to shapes2410, but are located closer to edges of screen2500in direction Z. Because shapes2510are closer to edges of screen2500in direction Z than shapes2410, deformation of screen2500under tension will be concentrated in the area of shapes2510so that shapes2410are not deformed. Areas2520,2520′ are patterned with emulsion such that shapes2510do not print on a solar cell that is printed using screen2500. It is appreciated that emulsion may already be patterned on screen2500for other reasons (e.g., to increase standoff distance of screen2500to a solar cell during printing, to promote transfer of a thick paste layer to a solar cell during printing). Thus, simply adding dummy shapes2510to the pattern utilized to form screen2500, and ensuring that areas2520,2520′ are patterned with emulsion on screen2500, mitigates deformation of shapes24W under tension.

Referring back toFIG. 24, optional steps2304through2306create screen2500shown inFIG. 26. Step2304generates dummy open areas (for example, shapes2510) in a nonprinting area of screen2500, simultaneously with generation of features that are intended to print (for example, shapes2410). Step2306patterns emulsion on the screen; as discussed previously, emulsion may also be patterned for other reasons as well, such as to increase standoff distance of the screen during printing). When step2306is performed, step2308can simultaneously cover the dummy open areas with the emulsion.

Having described several embodiments, it will be recognized by those skilled in the art that various modifications, alternative constructions and equivalents may be used without departing from the spirit of the disclosure, for example, variations in sequence of steps and configuration, etc. Additionally, a number of well known mathematical derivations and expressions, processes and elements have not been described in order to avoid unnecessarily obscuring the present disclosure. Accordingly, the above description should not be taken as limiting the scope of the disclosure.