Stone working – Sawing – Endless
Reexamination Certificate
2000-02-04
2001-08-28
Banks, Derris H. (Department: 3723)
Stone working
Sawing
Endless
C125S016020, C125S016010, C140S112000
Reexamination Certificate
active
06279564
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates, in general, to the field of an apparatus and method for accurately sawing a work piece into two or more sections. More particularly, the present invention relates to an apparatus and method for cropping and/or slicing crystalline ingots, such as relatively large diameter polysilicon and single crystal silicon ingots, with great accuracy, speed and efficiency.
The vast majority of current semiconductor and integrated circuit devices are fabricated on a silicon substrate. The substrate itself is initially created utilizing raw polycrystalline silicon having randomly oriented crystallites. However, in this state, the silicon does not exhibit the requisite electrical characteristics necessary for semiconductor device fabrication. By heating high purity polycrystalline silicon at temperatures of about 1400 degrees, a single crystal silicon seed may then be added to the melt and a single crystalline ingot pulled having the same orientation of the seed. Initially, such silicon ingots had relatively small diameters of on the order of from one to four inches, although current technology can produce ingots of 150 mm (six inches) or 200 mm (eight inches) in diameter. Recent improvements to crystal growing technology now allow ingots of 300 mm (twelve inches) or 400 mm (sixteen inches) in diameter to be produced.
Once the ingot has been produced, it must be cropped (i.e. the “head” and “tail” portions of the ingot must be removed) and then sliced into individual wafers for subsequent processing into a number of die for discrete or integrated circuit semiconductor devices. The primary method for cropping the ingot is through the use of a band saw having a relatively thin flexible blade. However, the large amount of flutter inherent in the band saw blade results in a very large “kerf” loss and cutting blade serration marks which must then be lapped off.
At present, there are two primary techniques for slicing an ingot into wafers: the ID (inner diameter) hole saw and the slurry saw. The former is used predominantly in the United States in order to slice single crystal silicon and is so named due to the fact that the cutting edge of the blade adjoins a centrally located hole at its inner diameter in an attempt to reduce the flutter of the blade and resultant damage to the crystalline structure. Among the disadvantages inherent in this technique is that as silicon ingots increase in diameter, the ID hole saw must increase to three times the ingot diameter to allow it to cut all the way through the ingot to a point at which it becomes unwieldy if not unworkable.
As previously mentioned, an alternative technique also utilized in the United States but used primarily in the Pacific Rim countries is the slurry saw. The slurry saw comprises a series of mandrels about which a very long wire is looped and then driven through the ingot as a silicon carbide or boron carbide slurry is dripped onto the wire. Wire breakage is a significant problem and the saw down time can be significant when the wire must be replaced. Further, as ingot diameters increase to 300 mm to 400 mm the drag of the wire through the ingot reaches the point where breakage is increasingly more likely unless the wire gauge is increased resulting in greater “kerf” loss. Importantly, a slurry saw can take many hours to cut through a large diameter ingot.
As is the case with the ID hole saw technique as well, excessive “kerf” loss results in less wafers being able to be sliced from a given ingot with a concomitant greater cost per wafer. Moreover, the score marks of the ID hole saw and less than even cutting of the slurry saw wires result in an increased need for lengthy and expensive lapping operations to make the surfaces of the wafer smooth and parallel as well as to remove other surface markings and defects. This excessive lapping also requires even greater amounts of silicon carbide and oil or aluminum oxide slurries, the ultimate disposal of which gives rise to well known environmental concerns.
Laser Technology West, Limited, Colorado Springs, Colo., a manufacturer and distributor of diamond impregnated cutting wires and wire saws, has previously developed and manufactured a proprietary diamond impregnated wire marketed under the trademarks Superwire™ and Superlok™. These wires comprise a very high tensile strength steel core with an electrolytically deposited surrounding copper sheath into which very small diamonds (on the order of between 20 to 120 microns) are uniformly embedded. A nickel overstrike in the Superlok wire serves to further retain the cutting diamonds in the copper sheath. The technique of cutting fixed work pieces with a direction reversing diamond wire is one that has been utilized, to date, primarily in a laboratory environment and not in a production process due to the inherently very slow cutting speed involved.
SUMMARY OF THE INVENTION
Disclosed herein is an apparatus and method for slicing a work piece, in particular, a polysilicon or single crystal silicon ingot utilizing a diamond impregnated wire in which the work piece is held stationary and the wire saw drive mechanism is reciprocally rotated or rocked back and forth through an arc about the work piece longitudinal axis relative to the diamond wire as the diamond wire is driven orthogonally to the longitudinal axis of the work piece. This motion produces a vertical cut in the work piece that has an arcuate bottom with the wire continually being maintained in a substantially tangent relation to the bottom of the cut. The wire drive mechanism is advanced from a position adjoining the outer diameter (“OD”) of the ingot through the ingot as the kerf or cut deepens. In this manner, the diamond wire cuts through the work piece at a point substantially tangential to the circumference of the cut, i.e., tangential to the bottom of the kerf along the length of the cut. The speed of advancement of the wire drive mechanism is controlled preferably automatically to maintain a constant force of the wire saw wire against the polysilicon at the bottom of the kerf. This is accomplished by maintaining a constant amount or angle of deflection of the saw wire as it travels through the cut. Through use of this technique, polysilicon or single crystal silicon ingots of 300 mm to 400 mm or more may be sliced into wafers relatively quickly, with minimal “kerf” loss and less extensive follow-on lapping operations than with conventional machines.
The presently preferred embodiment of the apparatus comprises a frame, a work piece support mechanism attached to the frame for positioning, leveling and holding a work piece beneath a wire having a plurality of cutting elements affixed thereto, a wire drive mechanism for moving the wire orthogonally with respect to a longitudinal axis of the work piece, a wire drive mechanism rotation mechanism coupled to the wire drive mechanism for rotating the wire drive mechanism about the work piece's longitudinal axis, and a wire advancing mechanism mounted on the frame which positions the wire drive mechanism and thus the cutting wire from a first tangential position proximate an outer surface of the work piece, sequentially through the work piece, to a second tangential position proximate the opposite side of the outer surface of the work piece.
The work piece, in particular, a silicon ingot, is preferably held stationary and leveled in a support mechanism which includes a pair of computer controlled “V” blocks on a computer controlled indexing bed connected to the frame and which is positioned beneath the wire drive mechanism. The frame includes a pair of spaced apart upright members. An inverted, U shaped yoke is movably fastened to and between the upright support members. Rotatably fastened to this yoke is the wire drive mechanism. The wire drive mechanism is reciprocally rotated or rocked through a predetermined arc about the work piece by the rotation mechanism while the wire drive mechanism advancing mechanism advances the wire drive mechanism vertically from a first posit
Hodsden John B.
Luedders Steven M.
Banks Derris H.
Burton Carol W.
Hogan & Hartson LLP
Kubida William J.
Shanley Daniel
LandOfFree
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