Stone working – Sawing – Reciprocating
Reexamination Certificate
2000-06-20
2002-03-05
Rose, Robert A. (Department: 3723)
Stone working
Sawing
Reciprocating
C451S164000
Reexamination Certificate
active
06352071
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a wire saw slicing apparatus and a slicing method for slicing a workpiece, such as a silicon semiconductor single crystal ingot, into wafers with low warp.
BACKGROUND OF THE INVENTION
Wire saws are used to slice cylindrical workpieces, such as silicon crystal ingots, into a plurality of wafers. The wire saw provides many parallel lines of wire that are evenly spaced apart, and an abrasive-containing fluid is continuously supplied to the point of contact with the workpiece. The wire is then continuously rotated, or more commonly reciprocated, across the workpiece, thereby slicing the workpiece into wafers using a grinding technique. The workpiece is advanced through the wires as the slicing progresses at a controlled rate to help provide a uniform slice without kerf variations on the surface of the wafers.
The wires are tensioned between several wire guides that are parallel to each other. The wire guides are made of metal, and typically have an outer sleeve that is made of synthetic material such as polyurethane. The outer surface of the synthetic sleeve is manufactured with grooves that precisely position where the wire seats, thereby controlling the axial distance between the wires, and thus the thickness of the wafers being sliced. As slicing is performed, the synthetic sleeve surface is degraded and worn away by the wire and abrasive, and must be resurfaced or replaced on a periodic basis. Various methods exist, and are well known in the art, of attaching the outer sleeve of the wire guide to the metal substructure, including overlaying techniques, gluing, mechanical attachments such as bolts or screws, and molding techniques. Similarly, well known methods exist for resurfacing the outer sleeve including placing resurfacing tools inside the slicing machine, removing and retooling the outer sleeve only while leaving the metal substructure in place, and removing the entire wire guide to resurface the outer sleeve.
Each wire guide has one or more sets of bearings that support the wire guide, and allow it to rotate around its longitudinal axis as the wire advances or reciprocates on its surface. The bearings supporting the wire guide experience friction from the rotation of the wire guide, and the friction is increased by wire wound around the wire guides and placed in tension. As the slicing process begins, the bearings are at one temperature, and as the slicing progresses, the temperature of the bearings will increase by significant amounts. This added temperature is passed from the bearings to the metal substructure, and on to the polyurethane outer sleeve.
The production of semiconductor wafers, such as silicon wafers, requires very tight process control for a wire saw. The desired outcome of the slicing process is to produce a plurality of wafers, all of which are the same thickness to within several microns, and have front and back surfaces that are as flat, smooth, and parallel to one another as possible. Unfortunately, however, wafers deviate from the desired surface outcome. Often wafer surfaces exhibit bow, the deviation of the center point of a wafer from a selected plane, or warp, the sum of the deviations of the highest and lowest points on the wafer surface from the same selected plane. The surface deformations of bow and warp cannot be removed in later processes such as lapping or surface grinding, because the wafer undergoes elastic deformation from the forces exerted on the wafer in those processes. Therefore, it is critical to control and minimize bow and warp in the slicing process.
It is well known in the art that warp and bow are generated due to variations of pitch of the wire wound around the wire guides. These variations of pitch are caused by thermal expansion of the main wire guides caused by frictional heat generated during the slicing process, both from the bearings supporting the metal substructure of the wire guide and the interaction between the wire and the synthetic outer sleeve. For example, the thermal coefficient of expansion for steel is approximately 6.0*10
−6
in/in° F. Likewise, the thermal coefficient of expansion for polyurethane is approximately 9*10
−5
in/in° F. Current manufacturing methods slice crystal ingots of lengths up to 1000 mm in length. If, then the temperature of the outer synthetic sleeve raises a mere 100° F. due to friction, the expansion would be approximately 9 mm. Using a wire thickness of 200 microns, and a wafer thickness of 800 microns, each wire guide groove would experience expansion of approximately 9 microns. In view of the progressive miniaturization of electronic circuitry, this 9 microns of expansion per wire groove is significant to the acceptable bow and warp measurements of a wafer.
To address the problem of thermal expansion, various means for cooling the wire guides during slicing have been proposed. For example, U.S. Pat. No. 5,269,285 discloses a method of slicing wafers wherein the abrasive-carrying fluid applied to the wires is also flushed through the bearings of the wire guides and acts as a coolant for the bearings. The system provides temperature sensors for the working fluid coming from the slicing interface and from the bearings independently, and heat exchangers for controlling the temperatures of each. This technology can reduce thermal variations, but cannot eliminate them completely. Further, the apparatus is somewhat cumbersome and both equipment and space intensive.
U.S. Pat. No. 5,377,568 discloses an improvement that includes a thread guide detection system with cooling fluid. The cooling fluid flows through the center of the wire guide. The detection system measures the position of thread guide at one end, and as movement is detected at the end of the thread guide due to thermal expansion or contraction, the temperature of the cooling fluid is changed to offset the change in temperature. The obvious disadvantage of this system, however, lies in the fact that the detection system can only monitor the movement at one precise position, and cannot compensate for expansion or contraction along the length of the wire guide.
U.S. Pat. No. 5,616,065 provides a method of monitoring the location of the workpiece to be sliced, and a means of compensating the workpiece to move into the proper location. The apparatus, however, is very complex, and requires the use of many interrelated parts to monitor and adjust crystal placement. Further, the material forming the wire guide outer sleeve is described to have a coefficient of thermal expansion of ≦1.0*10
−6
in/in° F., and preferably ≦0.1*10
−6
in/in° F., such as glass-ceramic materials. Although these materials have a very small coefficient of thermal expansion, these materials are expensive to manufacture and re-work compared to synthetic materials, and cause increased stresses on the wire, leading to earlier wire failure.
U.S. Pat. No. 5,810,643 provides a method of controlling wafer surface by coordinating a cyclical workpiece advancement to the wires with the cyclical reciprocation speed of the wire such that the workpiece advancement is maximized at the maximum wire reciprocation speeds, and is minimized at the inflection points between reciprocation of the wire in the two directions. This patent does not address thermal expansion of the wire guides, however.
Finally, U.S. Pat. No. 5,910,203 discloses a removable outer sleeve of a wire guide, wherein a synthetic coating is mounted over a metal subplate, and the outer sleeve is broken into a plurality of sections that mount in unison to completely cover the wire guide. As retooling of the wire guide is needed, the sections of the outer sleeve are removed and replaced with new or newly resurfaced sections. Although the removable outer sleeve of the wire guide has a metal subplate having a low coefficient of thermal expansion, there is nothing to constrain the thermal expansion of the outer synthetic coating. Since the synthetic coating has a large coefficient of thermal expansion, the workpiece will exper
Kononchuk Oleg V.
Preece George
Anderson Douglas G.
Rose Robert A.
SEH America Inc.
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