Patent Application
20120255488
This application is related to co-pending application “Methods and apparatuses for roll-on coating”, of the same inventors.
In photovoltaic devices, electrical energy is converted from light via the photoelectric effect and resulting charges are collected via pn junctions in semiconductor substrates for current generation. PN junctions can be formed by means of diffusing dopants into the bulk semiconductor material. The diffusion process can occur from a dopant vapor ambient, for example, phosphine or POCl3, or from solid source releasing dopant vapor. Another process can employ a dopant layer coated on the substrate, which, upon heating or firing, causes the dopant to diffuse from the dopant layer into the substrate. Another important process in solar cell fabrication is thin film deposition, such as the coating of passivation or absorber layer.
In-line fabrication processes, such as in-line doping diffusion or in-line thin film deposition, are preferred processes for minimizing cost and toxicity. For example, in a typical in-line diffusion process, the substrate is coated with a dopant containing layer and subsequently, the dopants from the dopant layer are diffused into the substrate in a furnace. The coating of the dopant source can be accomplished by spraying, dipping, spin-on, or condensation of a dopant-containing chemical, in liquid or gaseous form, with or without solvent, and with or without carrier gas. Such systems can be difficult to control with respect to uniformity and doping levels, particularly on textured surfaces. Also these processes often require significant excess chemicals, driving up the cost of production.
The present invention relates to methods and apparatuses for coating substrates, such as coating thin films or dopant layers on single crystalline or multicrystalline silicon substrates. In an embodiment, the present invention discloses a roll-on coating process for coating a layer on the substrate surfaces, for example, a passivation or absorber layer, or a dopant layer in which the dopants are then diffuse into the substrate after a high temperature anneal, forming a pn junction for solar cells.
In an embodiment, the present invention discloses a system for roll-on coating a substrate, comprising at least a roller accepting a fluid flow and then migrating the fluid to the roller surface. Upon contacting a substrate surface, the fluid on the roller's surface is transferred to the substrate surface, forming a coating layer. The fluid can comprise active chemicals in a solution mixture, which is subsequently dried to form a solid thin film layer on the substrate. The fluid can comprise a dopant chemical, which is continuously supplied to the roller for coating a plurality of substrate surfaces by rolling contact. The dopant chemical can contain boron, arsenic or phosphorus chemical in a solution or mixture form.
In an embodiment, the present invention discloses a fluid supplying roller, comprising a cylindrical structure spanning across a large substrate or a number of smaller substrates. The substrates can be positioned above the roller, under the roller, or sandwiched between two rollers, and moved relative to a rotation of the roller. The fluid supplying roller can be configured to accept a fluid flow at one end of the roller and to supply fluid at the roller's surface. The roller can be hollow or can have inner channels for guiding the fluid along the length of the roller. The surface of the roller can have a plurality of holes distributed along the length of the roller's core and connected with the inner channels for transferring the fluid to the outer surface of the roller. In one embodiment, the roller can further comprise a soft porous layer covering the outer surface of the roller, which is wetted with the fluid at the roller's surface, for example, the fluid coming from the holes distributed along the length of the roller's core. Upon contacting a substrate, the wetted porous layer can transfer the fluid to the substrate surface, effectively coating the substrate surface with layer of the fluid. In an embodiment, the fluid supplied roller is configured for delivering a uniform coating across the substrate surface, for example, by a uniform distribution of the holes along the length of the roller's core, or by a uniform pore density of the porous layer along the length of the roller. In another embodiment, the roller is configured to tailor the delivery of the fluid, effectively providing different coating thickness at different portions of the substrates, for example, by a non-uniform distribution of holes or a non-uniform pore density of the porous layer. This thickness non-uniformity can be used to compensate for subsequent process non-uniformity, for example, the non-uniformity of the anneal temperature in a diffusion furnace during the dopant drive-in process.
In an embodiment, temperature control devices such as heaters or coolers can be provided for heating or cooling the fluid, either at the fluid reservoir, at the fluid delivery lines, and/or at the fluid within the roller. In addition, heaters/coolers can be provided to heat/cool the roller and/or to heat/cool the substrates, for example, in the coating process.
In an embodiment, the present invention discloses an in-line coating system, comprising a plurality of rotating fluid supplied rollers for coating a substrate. The fluid supplied roller can be positioned above the substrate for coating the top surface of the substrate. The wetted roller can be positioned under the substrate for coating the bottom surface of the substrate. Two rollers can sandwich the substrate to coat the top and bottom surfaces of the substrate simultaneously. The rotating rollers can double as a transporting mechanism, continuously moving a plurality of substrates from an input to an output stage of the in-line coating system. The in-line coating system can further comprise means of controllably reducing the amount of dopant or carrier fluid on the substrates, such as additional liquid absorbing intrinsically dry rollers.
In an embodiment, the in-line coating system further comprises additional coating mechanisms, such as spraying nozzles for delivering additional fluid onto the substrate surfaces. Further the in-line coating system can comprise additional roller wetting mechanisms, such as providing an outer belt to deliver fluid to the outer surface of the roller, or providing a fluid-filled pan for mounting the roller.
In an embodiment, the present invention discloses an integrated in-line processing system, comprising an in-line coating system for coating a dopant layer on the substrates feeding an in-line furnace anneal system for driving the dopant into the substrates from the surface coating layer. The coating system and the furnace anneal system can be disposed next to each other, or can be separated, for example, by a transport line. The system may also consist of a pre-conditioning step prior to processing such as ozone treatment or chemical oxidation.
The present invention relates to methods and systems for uniform deposition of materials on a flat substrate, such as photovoltaic substrates, for creation of thin layers by a roll-on technology. The thin layers can be any layer deposited by a liquid material. For example, the thin layers can comprise Phosphorus or Boron, deposited on a semiconductor layer for emitter formation after subsequent treatments. The thin layers can also be absorber layers, for example CdS or ZnS, in thin film photovoltaic processing.
The present invention further pertains to the manufacture of photovoltaic cells, such as in crystalline solar cell manufacturing, including improved dopant coating processes and systems for producing photovoltaic emitter junctions by dopant diffusion. The substrate is preferably single crystal or multicrystalline (or polycrystalline) substrate, but other semiconductor substrate can also be utilized. The present invention can provide high performance junctions for cost reduction and efficiency improvement in photovoltaic cells and related devices. In an embodiment, the substrate is first exposed to a fluid-containing surface comprising dopant material, such as phosphorus, arsenic, or boron compounds, such as phosphoric acid (H3PO4). The exposure will form a dopant coating on the substrate surface. Afterward, the dopant coating maybe subjected to a high temperature ambient to diffuse the dopant into the substrate or annealed in a furnace at high temperature, for example, between 600 and 1000 C.
In another embodiment, the present invention discloses a deposition process by liquid roll-on technology, such as absorber layers in thin film photovoltaic, including controlled heating of the substrates for deposition of the layer constituents from the coating fluid, for example by crystallization.
The substrate 10 is then exposed to a dopant coating process 15, which forms a dopant layer 11 on the substrate 10. The dopant layer 11 is preferably a solid dopant source, comprising the appropriate dopant for forming a pn junction with the semiconductor substrate 10. The dopant layer 11 can be formed by contact with a fluid surface to generate a liquid layer coated on the substrate, and then dried in an elevated temperature ambient. The coating process comprises a dopant precursor, such as phosphorus-containing chemicals (phosphorus acid, phosphine), boron-containing chemicals, or arsenic-containing chemicals. The dopant precursors can be delivered in liquid or semi-liquid form, and with or without solvent or carrier gas. Other dopant sources in liquid form can be used, including solutions and mixtures.
In an embodiment, the dopant material is phosphorus, for example, from phosphorus acid solution. For example, phosphorus acid solution is applied to the core of the roller, which then rolls on the substrate to form a phosphoric acid coating. An exhaust or a hood, and a capture pan can be included to capture any phosphorus acid not adhered to the substrate.
The substrate may then be dried 11* and annealed 16 in a diffusion furnace, or the substrate may be directly annealed without a dry step. The anneal temperature may be about 800 to 900 C to drive phosphorus into the substrate to form a doping layer 12.
In an embodiment, the present invention discloses a system for coating a substrate, comprising a rotated roller with outer surface wetted from a fluid supplied through an inner core. In an embodiment, the present system comprises a plurality of rotated soft porous rollers for liquid media deposition, wherein the rollers is wetted with the liquid media supplied from one end of the rollers. The rotated soft porous rollers can be used to simultaneously transport the substrates, for example, in an in-line conveyor mechanism.
An advantage of the invention includes the ability to combine the transport of the substrates in an inline process system with the deposition process. The substrates are fed through the system in single or multiple parallel lanes by a sequence of rotated rollers, which can be positioned on top and/or bottom of the substrates. Rotating the rollers will move the substrates linearly from the input of the in-line process system to the output end. Liquid media is supplied to the rollers at one end, and traveled through inner channels of the rollers to wet the outer surface. Liquid layers are then dispensed with ultrathin thickness onto the substrates while moving through the equipment.
In an embodiment, the porous rollers comprise a rigid hollow core surrounded by a layer of porous material such as a sponge material. The rigid core preferably has adequate stiffness and horizontal flatness across the deposition area of the substrates to ensure uniform deposition of the coating layer. The core material can be metal, alloys, carbon, glass, ceramic, or plastic such as PVC, PP or fluorocarbons.
The porous material can be polymer or polymer foams or sponge, such as PVA, Poly urethane and poly olefin, or can be any materials with pores to allow the liquid media to pass through. In addition, the porous material can be a soft material, which can help in relaxing the requirement of the horizontal flatness of the rigid core.
The roller 31B can comprise a soft porous layer 35B at the outer surface of the roller for ease of fluid transfer between the roller and the substrate (
Uniformity of the coating layers can be controlled by adjusting the liquid media available at the surfaces of the rollers. In general, an even distribution of the liquid media along the length of the roller can create a uniform coating on the substrates along the direction perpendicular to the path of travel. Using multiple rollers, the uniformity along the direction of travel can be further improved.
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In an embodiment, the non-uniform profile of the thickness across the doper is designed to compensate for the non-uniform profile of a subsequent process. An example is the dopant diffusion process. After coating the substrates with a dopant layer, the substrates are annealed in a furnace to drive the dopant into the substrates. For in-line processing, after dopant coating, the substrates will move into an inline furnace with the same arrangements (rows) as they have been in the doper. It is costly to make the temperature in the furnace perfectly uniform in the direction perpendicular to the path of the substrates. For example, the temperature can be higher at the center than at the edge due to the boundary heat loss. This temperature difference can result in different substrate qualities according to their locations inside the furnace, for example, more dopant can be diffused to the substrates at the center than at the edges of the furnace.
The present rollers having core design with multiple types of perforations to apply a non-uniform coating layer on the substrates can compensate for this temperature difference to generate a homogeneous quality of the substrates after the furnace. For example, more chemical can be applied to the substrates at the edges of the doper, resulting in a thicker layer of coating, as compared to the substrates at the center of the doper. Thicker coating layers contain more dopant, which can be used to compensate for the lower temperature in the anneal furnace. Thus substrates positioned at the edges and at the center of the in-line conveyor (which is used to move the substrates from one location to another location) can have similar dopant concentration regardless of the temperature profile in the anneal furnace. Further, the coating layers can be removed after the anneal process, and thus the substrates remain uniform in surface topology.
In an embodiment, the present system comprises one or more temperature control devices, such as heaters or coolers, to provide thermal energy to the substrates or to the chemicals. Some chemicals may require high temperature (e.g., higher than room temperature) before a successful coating, thus heaters can be used to heat the chemicals to a desired temperature. Some chemicals may require low temperature (e.g., temperature lower than room temperature) for coating or for preserving the chemicals, thus coolers can be used to chill the chemicals to a desired temperature. For example, the heaters/coolers can be disposed at the chemical reservoir, at the liquid delivery line, or at the rollers to heat or cool the chemicals directly. In addition, or alternatively, the heaters/coolers can also be disposed at or near the rollers to heat/cool the rollers' surfaces, thus heating or cooling the chemicals when they reach the rollers' surfaces.
Heating or cooling can also be directed to the substrates. For example for deposition of emulgated solid salts to form absorber layers like CdS and ZnS in thin film solar cell manufacturing, the chemicals is heated after deposition, to accelerate the reactions of the chemicals, to dry the liquid chemicals, or to anneal the coating layers. The heaters can be disposed at or near the substrates to heat the substrates' surface, including IR or UV lamps disposed on top and/or bottom of the substrates, or between the rollers. In addition, both heaters and coolers can be used. For example, in deposition of absorber layers, the chemicals are preferably cooled, e.g., by coolers disposed at or near the chemical reservoir or delivery line, to around or below room temperature to preserve the life time of the chemicals. The chemicals are then heated, e.g., by IR heating the substrates, after deposited on the substrates to form the absorber layers.
In an embodiment, the present invention discloses an in-line deposition system comprising rollers accepting a liquid flow to the inside of the rollers and distributing the liquid to the outer surfaces. The liquid media is protected by the delivery lines, and the only liquid exposed to the environment is the liquid adhered to the roller surfaces. This minimum exposure can improve safety for the deposition system, especially for hazardous liquids such as phosphorous acid or boron trifluoride.
The deposition coating with the present rollers can be performed on both sides of the substrate, or can be applied to only one side. In addition, other types of coating can be included. The substrates can be transported with an in-line transport, such as any type of conveyor or ceramic rollers, to the liquid rollers to be coated, and then to an outlet in-line transport to subsequent processes such as an anneal furnace. Other components can be implemented, for example, exhaust and isolation to prevent hazardous gas from escaping, temperature isolation to provide wall safety, carrier gas and curtain gas for atmospheric isolation and purging, substrate conditioning before removal from the process chamber, such as a drying environment, and spray cleaning system for system cleaning
The liquid media flowing to the rollers 91A/91B can be controlled to perform a desired coating on the substrate 96 with minimum excess waste. In addition, the separation of the rollers can be controlled to exert appropriate contact pressure to the substrate 96 to minimize the wasted liquid. For example, a spring can be positioned on top roller 91A to provide a desired pressure to the substrate 96. The multiple independent control mechanisms can eliminate or minimize the excess fluid on the substrate surfaces, such as providing a uniform coating without any liquid droplets. The bottom roller 91B can be fixed around the axis of rotation, and rotated to coat the bottom surface of the substrate, together with forward substrate movement. Optionally, pan 99 can be used to capture any residue liquid.
In an embodiment, all rollers can accept the same liquid media from a single reservoir and pumping system. Alternatively, different rollers can accept different liquid media from multiple reservoirs. Different chemicals can be deposited on top and bottom surfaces. Different chemicals can be deposited on a same surface for a coating mixture. Filling rollers, e.g., rollers without accepting a liquid flow, can be included for other purposes, such as improved media distribution. The liquid media can be all distributed from a single reservoir, or can be individually provided from multiple reservoirs. Flow distribution mechanisms can be included, such as pumping mechanism, pressure controller, flow controller and distribution manifold. Recirculating mechanism and automatic refilling can also be included.
The liquid media can be provided to the rollers from one or more reservoirs. For example, a reservoir 104A containing chemical liquid can provide liquid media to a pump 104B, which can push the liquid media to the rollers 106. A pressure controller 104C and a flow controller 104D can be added to regulate the pressure and flow to the rollers. Manifold 104E serves to distribute the liquid media to the rollers. Temperature controller 104F can provide heat or cooling to the liquid media and/or the substrates. For simplicity, the manifold is shown distributing the same liquid media with same pressure and flow to the bottom rollers 106, but the invention is not so restricted. Different configurations can be used, such as a manifold distributing liquid media to all rollers, or different delivery systems can be added to deliver different chemicals with different pressures and different flows to different rollers.
The flow and pressure controllers can regulate the liquid media that is to be provided to the substrate surfaces, which can serve to optimize the coating process, such as minimizing the excess liquid to the substrates, or regulating a desired coating to the substrates with liquid media having different properties such as higher or lower viscosity or higher or lower reaction rates with the substrates. The present liquid delivery system can further prevent reactions in the delivery lines, such as at the rollers or at the liquid source. For example, the present liquid roller can be used to deposit an absorber layer for solar cell device, with crystallization occurring at the substrate surfaces instead of at the roller surfaces. The multiple independent controllers of flow rate, flow pressure, roller pressure, chemical temperature and substrate temperature can provide an optimized coating on the substrate surfaces, for example, a uniform coating layer with zero or minimum liquid droplet or excess liquid waste.
The top and bottom rollers can accept different liquid media for different coating material.
Additional rollers can be added. For example, one or more dry rollers can be provided for better liquid media distribution. The dry rollers can be disposed alternate to the liquid rollers, or can be disposed at selected locations. The dry rollers can be rollers without any wetting liquid, roller without accepting liquid from the inside, or rollers with vacuum suction (instead of liquid flowing) to dry the porous layer, or brush rollers.
Different rollers can be used on a same surface.
In an embodiment, the present invention discloses deposition processes using continuous liquid media supply.
In an embodiment, the present invention discloses methods for depositing liquids on substrates, comprising using a roller accepting liquid media at at least one end and migrating the liquid to the outer surfaces for contact coating. The end of the roller accepting the liquid media can be an extreme end, such as the end surface of a cylindrical roller, or can be near the extreme end, such as on the outer surface of the roller close to the end surface. The roller has channels to bring the liquid media to the outer surface of the roller, such as the circumference surface of a cylindrical roller.
In operation 131, the substrates enter a coating zone comprising a plurality of liquid rollers, some of which also acting as transport rollers to move the substrates forward. Liquid media is flowing to at least one end of the liquid rollers, where the liquid media is transferred to the outer surface of the rollers. Optional foam material covering the rollers can be used for improving the coating process, such as reducing substrate damage and improving distribution of liquid media on the substrates. The rollers can have channels, such as hollow cylindrical tubes, or grooves along the length of the rollers, to guide the liquid media.
In operation 132, the rollers are rotating so that the liquid media from the roller surfaces coats the substrate surfaces by contact. In addition, the rotating transport rollers move the substrates to the end of the coating zone. Means for improving the coating layers can be included, such as dry rollers disposed after the liquid rollers for better media distribution.
In optional operation 133, the coating layers are conditioned, for example, drying by thermal energy such as IR lamps or by other forms of energy excitation to the liquid coatings, or by one or more dry rollers. The drying zone can be disposed after the coating zone, for example, by disposing IR heaters after the liquid rollers. Alternatively, the drying zone can be integrated with the coating zone, for example, by disposing IR heaters alternatedly with the liquid rollers.
Additional elements can be used in the deposition process. For example, for deposition processes requiring hot chemicals, heaters can be used to heat the liquid supplier before reaching the substrates. For deposition processes requiring high temperature substrates for activating chemical reactions, heaters can be used to heat the substrates. In addition, temperature control devices can be used to keep the chemicals in appropriate temperature, such as cooling or heating.
In operation 138, the substrates enter a diffusion furnace to drive the dopant to the substrates. The furnace can comprise a plurality of heaters to heat the coating layers to a high temperature, such as between 600 and 1000 C. The furnace can comprise a pre-heating zone, acting to form a transition temperature zone between the hot furnace zone and the room temperature ambient. The pre-heating zone can also act as a drying zone for drying the liquid coating layers.
In an embodiment, the present invention discloses improvements to the roll-on coating process using liquid supplied rollers. For example, pressing mechanisms can be included to apply a desired pressure to the liquid rollers, enabling wet coating on the substrates without or with minimum excess liquid. The liquid media can be regulated to compensate for different liquid properties, such as viscosity, evaporation property, density, or reactivity, using active mechanism such as pumps and controllers.
In operation 142, at least one of the liquid temperature, the liquid flow rate, the fluid concentration, the fluid temperature,and the liquid pressure are regulated to achieve a desired coating layer on the substrate surfaces. Further, in the case of coating using top and bottom rollers, the contact pressure between the top and bottom rollers can be controlled to obtain optimum coating conditions, such as having a desired coating, minimizing excess fluid waste, or compensating for the variations of liquid media properties. The multiple control mechanisms can offer significant advantages, for example, enabling a deposition process having zero or minimum droplets or excess fluid media, allowing a uniform coating layer with reduced consumable chemicals. The consistency of the fluid, and other fluid properties, such as concentration or temperature, can be controlled to ensure consistency and desired properties of the deposited layer.
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In an embodiment, the present invention discloses different process conditions for achieving a desired coating using liquid rollers. For example, top and bottom rollers can accept different liquid media to enable coating different layers on the top and bottom of the substrates. Different liquid media can be applied to alternating rollers to enable mixing layers or laminate layers.
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While the present invention has been described with respect to a preferred mode thereof, it will be apparent that numerous alterations and modifications will be apparent to those skilled in the art without departing from the spirit of the invention. As in all such obvious alterations and modifications, it is desired that they be included within the purview of my invention, which is to be limited only by the scope, including equivalents, of the following appended claims.
