The present invention relates generally to wire sawing and more particularly relates to method and apparatus of cutting and cleaning wafers in a wire saw.
Wire saws are extensively used to slice silicon for solar and micro-electronics applications. The wire saws are also used for slicing a variety of other materials including sapphire, gallium arsenide (GaAs), indium phosphide (InP), silicon carbide (SiC), glass, lithium tantalate (LiTaO3) Z-cut crystals, lithium niobate (LiNbO3), lithium triborate (LiB3O5), quartz crystals, ceramics like aluminum nitride (ALN) and lead zirconate titanate (PZT), magnetic materials/parts, optical parts and the like material. The wire saws typically use a 120-180 micron diameter steel wire, which is several hundred kilometers long (
The work piece or the ingot 150, which needs to be sliced, is first glued to a plate 160 and then mounted on the wire saw. Then the ingot 150 is pressed with a vertical motion (top to bottom or bottom to top) against the horizontally moving wire web 140. The wire travels at a speed of about 15 meters/sec (or even higher) during slicing of wafers. Abrasive slurry, mainly made up of silicon carbide grains and a lubricant (e.g., polyethylene glycol or mineral oil), is introduced over the wire web 140. The abrasive slurry 210 coats the wire and travels to the cutting zone as shown in
Also, it can be seen in
Typically, slicing is achieved by slowly pushing the ingot 150 against the wire web 140. Furthermore, as cutting progresses, very fine silicon particles are loaded into the slurry. These particles in the slurry can increasingly adhere to the wafer surface as a function of time during the process. This is particularly true for very thin wafers, which require a much longer time to cut. Therefore, prompt cleaning is essential in all wire saw operations.
Slicing is completed when the ingot 150 completely passes through the wire web 140. At this point, the wafer stack which is held to the plate 160 is slowly pulled out of the wire web 140. After completing slicing and removing the stack of wafers from the wire saw the wafers are then cleaned immediately with water and other solvents to remove the abrasive slurry 210, otherwise the abrasive slurry 210 may stain the wafers thereby making them unusable in downstream processes. Further, the slurry remaining between the wafers needs to be removed quickly otherwise the slurry between the wafer can harden and hold the wafers together tightly and can make it difficult to remove and in some instances can break the wafers.
The current wire saws generate heat during slicing. Further, as the wafers become thinner, the cutting surface area increases significantly and as a result this can significantly increase the amount of heat generated during slicing. Also, the current wire saws cannot dissipate such heat generated during slicing. Further, lesser area is generally available for heat dissipation by radiation during slicing due to the slurry getting loaded between the wafers. This can lead to significant thermal stress in the wafers. Furthermore, the heat generated during slicing can soften the glue holding the stack of wafers to the plate 160. This can result in wafers dislodging from the plate 160 and breaking during slicing.
Further, as the silicon wafers are manufactured to thinner specifications, the sensitivity of these thinner wafers to any stress is significantly increased and these wafers can readily break. Currently, the standard for the solar industry is wafers sliced to a thickness of about 200 micrometers (microns; μm).
A method and apparatus of cutting and cleaning wafers in a wire saw is disclosed. According to one aspect of the present invention, a wire sawing apparatus includes a horizontal ingot feeding wire slicing apparatus for slicing wafers, a frame for holding the horizontal ingot feeding wire slicing apparatus, and a control panel for operating the wire sawing apparatus.
Further, the horizontal ingot feeding wire slicing apparatus includes a vertical wire web such that sawing wires of the vertical wire web are located substantially in a vertical plane and move in a substantially vertical direction and at least one first top outlet and second top outlet for applying fluids during sawing, wherein the at least one first top outlet and second top outlet being located in a top position with respect to at least one work piece, such that the fluids flow in a substantially downward vertical direction under a gravitational force, and where the first top outlet supplies an abrasive slurry and the second top outlet supplies at least one cleaning fluid.
Also, the horizontal ingot feeding wire slicing apparatus includes at least one chute for removing the fluids, such that the at least one chute is located substantially below the at least one work piece for receiving the fluids, where the at least one work piece is impelled against the vertical wire web by movement in a horizontal direction, and where the fluids are applied to the top of the at least one work piece and the fluids flow in a vertical direction against and into the at least one work piece for slicing and cleaning wafers.
The horizontal ingot feeding wire slicing apparatus further includes at least two wire guide cylinders, such that the sawing wires are stretched between the at least two wire guide cylinders and held substantially in the vertical plane by a defining interval between the sawing wires, a tension control unit for controlling tension of the sawing wires, a support table for carrying the at least one work piece to be sliced, and a power driver for driving the at least two wire guide cylinders.
According to another aspect of the present invention, a method for producing wafers includes cutting a work piece including at least one ingot by impelling the work piece into a substantially vertical wire web, in which sawing wires of the substantially vertical wire web are located in a substantially vertical plane and move in a substantially vertical direction and in which the work piece is moved in a substantially horizontal direction into the substantially vertical wire web, and contacting the moving work piece for slicing wafers separately with at least two fluids including an abrasive slurry and a cleaning fluid, such that the at least two fluids flow in a substantially downward vertical direction under a gravitational force, in which the cleaning fluid cleans wafers of the resulting wafer stack during the impelling process, wherein moving the work piece and contacting with the at least two fluids slice the thin wafers secured at one end to a plate.
The methods and apparatuses disclosed herein may be implemented in any means for achieving various aspects. Other features will be apparent from the accompanying drawings and from the detailed description that follows.
Embodiments of the present invention are illustrated by way of an example and not limited to the figures of the accompanying drawings, in which like references indicate similar elements and in which:
Other features of the present embodiments will be apparent from the accompanying drawings and from the detailed description that follows.
A method and apparatus for cutting and cleaning wafers in a wire saw is disclosed. In the following detailed description of the embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.
The terms “slicing”, “sawing”, “watering”, and “cutting” are used interchangeably throughout the document.
In one exemplary implementation, the work piece 150 is attached to the plate 160 by glue 310. It is appreciated that gluing of the work piece 150 to the plate 160 ensures secured holding of sliced wafers to the plate 160. One skilled in the art can envision that the work piece 150 can be attached to the plate 160 using other techniques that are well known in the art.
It can be seen in
Further, as shown in
In operation, the work piece 150 is impelled against the vertical wire web 140 by movement in a horizontal direction (e.g., as shown by reference numeral 440) for slicing wafers. In one embodiment, the work piece 150 including a plurality of ingots is impelled substantially simultaneously to the vertical wire web 140. In an alternate embodiment, the work piece 150 including the plurality of ingots is impelled substantially serially to the vertical wire web 140. It is appreciated that the sawing wires of the vertical wire web 140 are adapted to move in a substantially vertical alternating or continuous direction while impelled against the work piece 150.
Further, in accordance with the above-described embodiments, the first top outlet 410 and the second top outlet 450 are located in a top position with respect to the work piece 150 for applying fluid 420 and a cleaning fluid 460 respectively, during the sawing operation. In one exemplary implementation, the first top outlet 410 is located and oriented to substantially flow the fluid 420 over the top of the work piece 150 as the work piece 150 is impelled against the vertical wire web 140 and during the slicing of the wafers. For example, the fluid 420 is abrasive slurry.
Further, as shown in
Further, the second top outlet 450 is located and oriented to substantially spray the cleaning fluid 460 over wafers as cut wafers emerge from the vertical wire web 140. In some embodiments, the cleaning fluid 460 includes a surfactant such that the wafers in the vertical wire web 140 are maintained in a separate condition by electrostatic repulsion. In these embodiments, the cleaning fluid 460 also includes water or high heat dissipating fluids. As shown in
It can be seen in
According to the one or more embodiments described above, the method for producing wafers using the above-described horizontal ingot feeding wire slicing and cleaning apparatus 400 includes cutting the work piece 150 that includes one or more ingots by impelling the work piece 150 substantially into the vertical wire web 140 and contacting the moving work piece 150 for slicing thin wafers separately with the fluid 420 (e.g., an abrasive slurry) and the cleaning fluid 460. The cleaning fluid 460 cleans wafers of the resulting wafer stack during the impelling process. Further, moving and contacting the work piece 150 with the fluid 420 and the cleaning fluid 460 slice the thin wafers secured at one end to the plate 160. In addition, the method includes dissipating heat during cleaning by adjusting slicing rate and thermal properties of the abrasive slurry 420 and the cleaning fluid 460.
As shown in
As shown in
Further, as shown in
The second top outlet 450 is located and oriented to substantially spray the cleaning fluid 460 over the wafers as the cut wafers emerge from the vertical wire web 140. Also, as shown in
According to the above-described embodiments, the tension control unit 610 controls tension of the sawing wires, the support table 220 carries the work piece 150 to be sliced and cleaned and the power driver 620 drives the wire guide cylinders 130. It is appreciated that the support table 220 along with other elements form a horizontal ingot feeding device in the horizontal ingot feeding wire slicing and cleaning apparatus 400. In one exemplary implementation, the horizontal ingot feeding device is arranged to maintain, during slicing, partially or completely sliced wafers substantially parallel to each other and such that the width of the sawing gaps is held substantially constant during slicing of the wafers.
Further, the horizontal ingot feeding wire slicing and cleaning apparatus 400 and a method thereof is described in greater detail with respect to
With reference to the above-described wafer cutting technique, a plurality of zones is envisioned with respect to the work piece 150 and the vertical wire web 140, in terms of placement of sources of the fluid 420, and collection of the fluid 420. Thus, there is a “pre-saw” zone in which the fluid 420 is contacted to the work piece 150.
An embodiment of the present invention, designed to solve the hydrodynamic stress problem without breakage of wafers, provides a design of the wire sawing apparatus in which the work piece is impelled against the vertical wire web by movement in a horizontal direction. Unlike conventional wire saws, in which the fluid is fed over a horizontal wire web, in the present method and apparatus, the fluids are fed downward across the vertical wire web (as illustrated in
An advantage of gravity assisted vertical fluid flow in the design of the wire sawing apparatus herein is that, particles do not settle out of the abrasive slurry. Rather the entire abrasive slurry is forced to pass through the cutting zone of the work piece. Use of the vertical wire sawing apparatus further enables use of low viscosity slurries, which imparts lower stress on the wafers. Further, by use of the above-described wire sawing apparatus, a larger number of abrasive particles are introduced into the cutting zone, and efficiency of the cutting process is thereby significantly increased. Further, the consumable cost of the slicing process is decreased. However, viscosity of the abrasive slurry still increases once the slicing process is initiated, as silicon fines become loaded into the fluid. Also, in spite of the high viscosity of the abrasive slurry, the abrasive slurry still do not entirely prevent particles from settling, a result which can prevent cutting. Therefore, for processes of manufacture of very thin wafers as described herein address the problem of the necessity to remove trapped abrasive slurry, a step which would allow the wafers to be pulled away from the vertical wire web without breaking them.
A major advantage of the design of the system herein, is accomplishing cleaning of the wafers as the cut wafers emerge out of the vertical wire saw. By cleaning the wafers this way, according to the design of the apparatus and method herein, separating the cut and finished wafers from each other when the cutting is complete is facilitated, as there is no slurry remaining between the wafers to obstruct the wire movement. In addition, surfactants such as sodium silicate are added to the cleaning fluid, which electro-statically keeps the wafers separate from each other. Cleaning of the wafers in the wire saw substantially reduces breakage of wafers. One skilled in the art will appreciate that prompt cleaning of the wafers removes foreign particles adhering to the wafers.
A skilled person will recognize that many suitable designs of the systems and processes may be substituted for or used in addition to the configurations described above. It should be understood that the implementation of other variations and modifications of the embodiments of the invention and its various aspects will be apparent to one ordinarily skilled in the art, and that the invention is not limited by the exemplary embodiments described herein and in the claims. Therefore, it is contemplated to cover the present embodiments of the invention and any and all modifications, variations, or equivalents that fall within the true spirit and scope of the basic underlying principles disclosed and claimed herein. The contents of all references cited are incorporated herein by reference in their entireties.
This application claims priority under 35 U.S.C. 119 to U.S. Provisional Application No. 61/117605, entitled “METHOD AND APPARATUS FOR CUTTING AND CLEANING WAFERS IN A WIRE SAW” by Cambridge Energy Resources, Inc., filed on Nov. 25, 2008, which is incorporated herein its entirety by reference.
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