Embodiments of the invention generally relate to an apparatus and method for forming thin substrates from an ingot. The present invention also relates to a system including a plurality of wire sawing devices that can saw multiple ingots at once to form the thin substrates. The invention is particularly useful for fabrication of thin crystalline silicon solar cell substrates from a formed crystalline ingot.
Conventional wire sawing devices generally contain a plurality of the wires that are moved in a single direction relative to an ingot or piece that is to be sawed. Wire sawing devices are generally used in the electronics industry to saw ingots including ferrites, quartz and silica, to obtain thin slices of material, such as polysilicon or monocrystalline silicon, or even new materials such as GaAs, InP, GGG or else quartz, synthetic sapphire, ceramic materials. The high price of the materials renders wire sawing more attractive compared to other techniques such as diamond disc sawing.
In the known devices, the sawing region is constituted by an assembly of multiple cylinders in parallel. These cylinders, called wire guides, are engraved with grooves defining the interval between the wires of the layer, namely the thickness of the slices. The piece to be sawed is fixed on a support which moves perpendicularly to the layer of wires. The speed of movement of the piece defines the cutting speed. Renewal of the wire, as well as control of its tension, takes place in a so-called management region for the wire located beyond the sawing region where the ingots or pieces are cut. The abrasive agent which effects the cutting is either an abrasive fixed on the wire, for example for a diamond wire, or a free abrasive provided in the form of a slip or slurry. The wire acts only as a carrier for the abrasive material. During cutting of the piece to be sawed into thin slices, the tensioned wire is both guided and tensioned by the wire guide cylinders.
For numerous applications, the sawed slices, or also referred herein as wafers, are of a very small thickness relative to the cross-section, or diameter, of the piece to be sawed. The sawed slices thus have a substantial flexibility and can flex and curve to come into contact with adjacent slices. This flexing is undesirable for precision and flatness of cutting and can give rise to undulations, striations and undesirable irregularities on the surface of the sawed slices. These irregularities, even of several micrometers, are enough to render the slices unusable for certain applications, such as silicon for the solar industry and for semiconductors. The deformations of the slices can even lead to micro ruptures and ruptures, especially near the coupling point where the slice are connected to their support.
Wire sawing techniques have gained favor in the process of forming photovoltaic type substrates. Photovoltaics (PV), or solar cells, are devices which convert sunlight into direct current (DC) electrical power. A typical PV cell includes a ptype silicon wafer, substrate, or sheet particularly less than about 0.185 mm thick with a thin layer of an n-type silicon material disposed on top of the p-type substrate. In general, silicon substrate based solar energy technology follows two main strategies to reduce the costs of solar electricity by use of PV solar cells. One approach is increasing the conversion efficiency of single junction devices (i.e., power output per unit area) and the other is lowering costs associated with manufacturing the solar cells. Since the effective cost reduction due to conversion efficiency is limited by fundamental thermodynamic and physical limits, the amount of possible gain depends on basic technological advances, such as aspects of the invention disclosed herein. The other strategy to make commercially viable solar cells lies in reducing the manufacturing costs required to form the solar cells.
In order to meet these challenges, the following solar cell processing requirements generally need to be met: 1) the cost of ownership (CoO) for substrate fabrication equipment needs to be improved (e.g., high system throughput, high machine up-time, inexpensive machines, inexpensive consumable costs), 2) the area processed per process cycle needs to be increased (e.g., reduce processing per Wp) and 3) the quality of the formed layers and film stack formation processes needs to be well controlled and be sufficient to produce highly efficient solar cells. Therefore, there is a need to cost effectively form and manufacture thin silicon substrates for solar cell applications.
Further, as the demand for solar cell devices continues to grow, there is a trend to reduce cost by increasing the substrate throughput, increase the size of the solar cell substrate to increase the amount of power that can be collected during operation of the solar cell, increase the wafer saw system throughput (MWatts/yr), reduce the cost of consumables, and improve the quality of the deposition processes performed on the substrate. To cut down the substrate formation cost, it is desirable to design a novel wafer sawing system and wafer sawing processing sequence that have a high substrate throughput and improved wafer sawing process yield.
In light of the above, the wafer sawing system according to independent claim 1, and the method of sawing an ingot in wafer sawing system according to independent claim 8 are provided. Further advantages, features, aspects and details are evident from the dependent claims, the description and the drawings.
The present invention generally provides a wafer sawing system, including an ingot input module, an ingot output module, two or more wire sawing chambers that each include two wire guide cylinders, at least one wire disposed across both of the wire guide cylinders, a support table that is configured to receive a single ingot, and an ingot positioning system that is configured to urge the single ingot disposed on the support table against the at least one wire, and a robot that is configured to transfer the single ingot between the ingot input module, at least one of the two or more wire sawing chambers and the ingot output chamber.
Embodiments of the invention may further provide a method of sawing an ingot in a wafer sawing system, including transferring a single ingot from an input module to one of a plurality of wire sawing chambers that are positioned relative to a transferring region of a transfer chamber, sawing the single ingot in the wire sawing chamber, wherein sawing the single ingot includes receiving the single ingot from a robot disposed in the transfer chamber, urging the single ingot against a layer of wires disposed across two wire guides, and moving the layer of wires relative to the single ingot, and transferring the sawed ingot from the wire sawing chamber to an output module.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
For clarity, identical reference numerals have been used, where applicable, to designate identical elements that are common between figures. It is contemplated that features of one embodiment may be incorporated in other embodiments without further recitation.
The present invention generally provides a wafer sawing system including a plurality of wire sawing chambers that each can independently saw an ingot to form thin substrates. The wafer sawing system can be used to form different types of substrates, or wafers, such as solar cell substrates, semiconductor substrates, or other useful substrates from a larger piece, such as an ingot, boule or block. In one configuration, the wafer sawing system is configured to accept a crystalline silicon (c-Si) ingot and perform all of the processing steps needed to form clean and dry substrates. For ease of discussion and to avoid confusion, the phrase ingot will be used herein to broadly signify a larger piece, or un-cut element, that is to be sawed in the wafer sawing system.
The central robot 120 generally includes a conventional robotic device, such as a SCARA robot or a six-axis robot as is shown in
The input module 102 generally includes one or more storage shelves (not shown) that are configured to receive an un-cut type of ingot 317 (
The output module 110 generally includes one or more storage shelves (not shown) that are configured to receive an ingot 317 and mounting plate 376 after the wafer sawing process has been performed on the ingot 317. The output module 110 may also include a rinsing device (e.g., DI water delivery system) or rinsing station that is able to keep the sawed ingots wet so that the slurry used in the sawing process will not dry on the processed ingots. In one embodiment, the output module 110 may include a rinsing device/system that cleans the sawed ingots to remove any contamination found on the surface of the formed substrates.
In general, the system controller 128 is used to control one or more components and processes performed in a wafer sawing system. The system controller 128 is generally designed to facilitate the control and automation of the wafer sawing system and particularly includes a central processing unit (CPU) (not shown), memory (not shown), and support circuits (or I/O) (not shown). The CPU may be one of any form of computer processors that are used in industrial settings for controlling various system functions, substrate movement, chamber processes, process timing and support hardware (e.g., sensors, robots, motors, timing devices, etc.), and monitor the processes (e.g., chemical concentrations, processing variables, chamber process time, I/O signals, etc.). The memory is connected to the CPU, and may be one or more of a readily available memory, such as random access memory (RAM), read only memory (ROM), floppy disk, hard disk, or any other form of digital storage, local or remote. Software instructions and data can be coded and stored within the memory for instructing the CPU. The support circuits are also connected to the CPU for supporting the processor in a conventional manner. The support circuits may include cache, power supplies, clock circuits, input/output circuitry, subsystems, and the like. A program, or computer instructions, readable by the system controller 128 determines which tasks are performable on a substrate. Preferably, the program is software readable by the system controller 128 that includes code to perform tasks relating to monitoring, execution and control of the movement, support, and/or positioning of an ingot within the wafer sawing system, along with the various wafer sawing recipe tasks and various wafer sawing chamber process recipe steps being performed in each of the wire sawing chambers 300 in the wafer sawing system.
The linear robot 220 is used to transfer un-cut ingots from the input module 202 to one of the wire sawing chambers 300, and then after performing the sawing process, the sawed ingots are transferred to the washing station 209 or the output module 210. In one embodiment, the washing station 209 is used to clean and dry the slurry material from the sawed ingot before it is delivered to the output module 210 by the linear robot 220. The robot 220 generally includes a conventional robotic device, such as a SCARA robot or a six-axis robot, that is configured to move along a rail 221 (i.e., direction “M” in
In one embodiment, only a single ingot 317 is positioned and sawed at a time within each of the wire sawing chambers 300 disposed in a wafer sawing system 100, 200. It is believed that processing a single ingot using a single layer 319 of wires has significant advantage over conventional wire sawing systems that try to maximize ingot throughput by sawing multiple ingots at once using a relatively slow wire cutting speed (e.g., ingot movement speed 200-400 μm/min and wire speed of 15 m/s). While the “load”, or length 374 (
Accordingly, according to some embodiments, which can be combined with other embodiments described herein, the ingot length can be 310 mm to 370 mm, e.g. 330 mm to 350 mm. Thereby, a beneficial compromise between sawing speed, number of wafers, yield and complexity of the sawing device can be realized to improve the CoO.
In one embodiment, the single ingot 317 is mounted on a support table 312 by means of a temporary support, such as the mounting plate 376 (
The periphery of the wire guide cylinders 321, 322 is engraved with grooves 333 (
The wire 323 is stretched and both guided and tensioned by the wire guide cylinders 321, 322 so as to move with reciprocating or continuous single direction movement. This wire 323 may include a spring steel with a diameter included between 0.1 and 0.2 mm so as to saw ingots of hard material, or of more particular composition, such as silicon, ceramic, compounds of the elements of groups III-V and II-VI, GGG (gadolinium gallium garnet), sapphire, etc., in slices of desirably about 300 μms or less, or preferably for next generation substrates 180 μms or less in thickness. An abrasive agent is generally a commercial product, such as diamond, silicon carbide, alumina, or other useful material that is used to improve the ingot sawing process. The abrasive agent may be fixed to the wires 323, or be in a free form that is in suspension in a liquid (e.g., PEG), such as a slurry, which serves as a transport for the abrasive particles. To reduce the downtime of the wire sawing chamber 300, in some embodiments it is desirable to weld, or join, a new wire to the end of a nearly completely used wire 323 that is disposed on the supply bobbin 326, thus allowing the wire sawing chamber to continue to process ingots 317 without being taken down to replace the used wire 323.
The wire sawing chamber 300 is typically provided with a holding device 377 arranged so as to hold, in the course of sawing, the partially or entirely sawed slices 317B (e.g., unformed substrates) substantially parallel to each other, such that the width of the sawing gaps 317A are maintained substantially constant during sawing of the slices.
In one embodiment of the wire sawing chamber 300, as briefly discussed above, a fluid delivery system 375 is configured to deliver a fluid, such as DI water, a coolant, or a gas to channels (not shown) formed in the mounting plate 376 to cool the ingot 317 during processing or rinse the slices (or substrates) after the wafer sawing process has been completed.
Accordingly, according to aspects of the present disclosure, the wafer sawing system as disclosed herein may include a clamping assembly for connecting to a cylindrical wire guide of a wire saw for cutting wafers. The clamping assembly may include a shaft-side connector adapted to connect to a shaft of a wire saw with the shaft having an axis of rotation. The shaft-side connector includes an outer surface which is normal to the axis and adapted to abut a complementary outer surface of a complementary connector of the wire guide, and a conical surface between the outer surface and the axis, the conical surface being disposed symmetrically about the axis, and adapted to abut a complementary surface of the wire guide.
It is also possible that the clamping assembly for connecting a cylindrical wire guide to a shaft of a wire saw includes an outer surface which is normal to the axis and adapted to abut a complementary outer surface of a complementary connector of the shaft, and a conical surface between the outer surface and the axis, the conical surface being disposed symmetrically about the axis, and adapted to abut a complementary surface of the complementary connector. The wire saw is adapted to cut wafers. The shaft has an axis of rotation.
According to embodiments, the outer surface of the clamping assembly is planar. In addition or alternatively, according to embodiments, the outer surface of the clamping assembly is adjacent to the conical surface. It may be annularly shaped.
It is possible that a portion of the outer surface is located at a distance of at least about 65%, 70%, 75% or 80% of the radial distance from the axis to a radially outer edge of the shaft-side connector. The outer surface may be annularly shaped; it is possible that the outer surface optionally includes 1, 2, 3, or 4 sections.
The conical surface of the shaft-side connector may include a deformable material. Generally and not limited to any embodiment, the conical surface may include 1, 2, 3, or 4 conical sections.
According to embodiments, the shaft-side connector of the clamping assembly is hollow. The shaft side connector may be female. The clamping assembly may include the shaft which is connected to the shaft side connector.
The clamping assembly may include a holding mechanism that is typically selected from a hydraulic system, a pneumatic system, a screw, and combinations thereof.
According to embodiments, the conical surface of the clamping assembly may be attached to the wire guide. The outer surface may be adjacent to the conical surface.
The cylindrical wire guide may include a carbon fiber reinforced polymer section.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
Number | Date | Country | Kind |
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12185456 | Sep 2012 | EP | regional |
12185459 | Sep 2012 | EP | regional |