Abstract:
An apparatus for cutting a substantially cylindrical work piece in a direction generally perpendicular to a longitudinal axis of the work piece includes a wire having a plurality of cutting elements affixed thereto and a wire drive mechanism for driving the wire across and through the work piece. The wire drive mechanism includes a capstan to move the wire orthogonally across a longitudinal axis of the work piece, a rotational drive to oscillate the wire around the longitudinal axis and an advancing drive to advance the wire perpendicularly through the longitudinal axis of the work piece. In a particular embodiment disclosed herein, the apparatus comprises imparts a substantially rocking motion to the wire drive mechanism about the longitudinal axis of the work piece and the cutting elements of the wire are impregnated diamonds.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. Pat. application Ser. No. 09/108,864, filed Jul. 1, 1998, now U.S. Pat. No. 6,024,080, which is a continuation-in-part application of U.S. Pat. application Ser. No. 08/993,007, filed Dec. 18, 1997, now U.S. Pat. No. 5,964,210, which is a continuation-in-part application of U.S. Pat. application Ser. No. 08/888,952, filed Jul. 7, 1997, now U.S. Pat. No. 5,878,737, and claims the benefit of United States Provisional Patent Application Ser. No. 60/129,331 filed Apr. 14, 1999. 
    
    
     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&#39;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 position above or proximate an outer surface of the work piece to a second position proximate the outer surface on the opposite side of the work piece. The angle of the arc varies and is typically up to about an included angle of 60 degrees. The angle is varied depending on the depth of the cut through the ingot. For example, at the beginning of the cut through the ingot, the wire drive mechanism is not rotated at all, but is held stationary at a position in which the saw wire passes horizontally across the surface of the ingot. As the cut deepens, rotation begins with the arc starting off very small, only a few degrees and then is progressively increased as the cut progresses. This rocking, reciprocal, movement of the wire drive mechanism about the ingot permits the kerf to provide lateral guidance to the wire during the cut while maintaining the wire substantially tangential to the bottom of the cut and advantageously minimizes the effects created by surface irregularities on the ingot on the precision of the cut. 
     Another feature of the invention is a unique capstan arrangement that eliminates the potential for broken wire from becoming entangled in the wire drive mechanism which has previously been experienced. This capstan arrangement provides a complete enclosure of the capstan drive members thus precluding entanglement of wire with grease laden drive members. 
     Another feature of the invention is automatic coordination of the wire drive mechanism, the wire drive mechanism rotation mechanism, and wire drive mechanism advancing mechanism preferably based on maintaining a constant predetermined wire force on the ingot at the bottom of the cut in the ingot. This feature is accomplished through the use of a continuous deflection detector which measures the deflection distance of the wire saw either entering or leaving the work piece cut, with respect to an index position of the wire prior to engagement with the work piece. The advancing rate is adjusted to maintain a predetermined amount of deflection, and thus downward force exerted by the wire saw wire against the ingot material at the bottom of the cut. This downward force may be fixed or may be varied in accordance with a programmed schedule depending on the position of the wire saw in the cut. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a front perspective view of an apparatus for slicing a work piece in accordance with the present invention; 
     FIG. 2 is a rear perspective view of the apparatus in accordance with the present invention shown in FIG. 1; 
     FIG. 3 is a front view of the apparatus shown in FIG. 1 with the wire drive mechanism raised above the work piece; 
     FIG. 4 is a front view of the apparatus shown in FIG. 1 with the wire drive mechanism positioned to begin slicing a work piece; 
     FIG. 4A is a perspective view of a backside of a wire drive mechanism in accordance with the present invention; 
     FIG. 4B is a perspective view of a front side of the wire drive mechanism of FIG. 4A; 
     FIG. 4C is a perspective view of a frame guide in accordance with the present invention; 
     FIG. 5 is a front view of the apparatus shown in FIG. 1 with the wire drive mechanism positioned after an initial cut of a work piece; 
     FIG. 6 is a front view of the apparatus shown in FIG. 1 with the wire drive mechanism rotated counterclockwise during the cutting of the work piece; 
     FIG. 7 is a front view of the apparatus shown in FIG. 1 with the wire drive mechanism rotated in a clockwise direction during the cutting of the work piece; 
     FIG. 8 is a front view of the apparatus shown in FIG. 1 with the wire drive mechanism positioned at the end cut of the work piece; 
     FIG. 9 is an enlarged perspective view of a capstan in accordance with the present invention; 
     FIG. 10 is a sectional view of the capstan taken along the line  10 — 10  in FIG. 9 with the capstan drum  30  at a mid position; 
     FIG. 11 is a sectional view of the capstan taken along the line  10 — 10  in. FIG. 9 with the capstan drum  30  at one end of the wire position; 
     FIG. 12 is a sectional view of the capstan taken along the line  10 — 10  in FIG. 9 with the capstan drum  30  at the other end of the wire position; 
     FIG. 13 is a sectional view of the capstan taken along the line  13 — 13  in FIG. 10; 
     FIG. 14 is a sectional view of the capstan taken along the line  14 — 14  in FIG. 10; 
     FIG. 15 is a sectional view of the capstan taken along the line  15 — 15  in FIG. 10; 
     FIG. 16 is a sectional view of the capstan taken along the line  16 — 16  in FIG. 10; 
     FIG. 17 is a sectional view of the apparatus taken along the line  17 — 17  in FIG. 3; 
     FIG. 18 is a sectional view of the apparatus taken along the line  18 — 18  in FIG. 17; 
     FIG. 19 is a sectional view of the apparatus taken along the line  19 — 19  in FIG. 3; 
     FIG. 19A is a sectional view of the apparatus taken along the line  19 A— 19 A in FIG. 19; 
     FIG. 20 is a front view of a control console in accordance with the present invention. 
     FIG. 21 is an enlarged view of the touch screen of the control console of FIG. 20; 
     FIG. 22 is an enlarged view of an inductive proximity sensor and cutting wire of the apparatus shown in FIG. 1 illustrating the position of the sensor to detect the bow of the cutting wire as it moves from an unbowed position substantially tangential to the cross-sectional face of the sensor to where a greater chord across the face of the sensor is detected; and 
     FIG. 23 is an enlarged view of an alternative embodiment of an inductive proximity sensor and cutting wire as shown in FIG. 1 illustrating the position of the sensor to detect the bow of the cutting wire as it moves from an unbowed position substantially bisecting the cross-sectional face of the sensor at the diameter thereof to where a lesser chord across the face of the sensor is detected. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawing, an apparatus  10  in accordance with the present invention is shown in a front view in FIGS. 1 through 4. Not shown in FIGS. 1-4 is a computer control console  2000 , which can readily be seen with reference to FIGS. 20 and 21. 
     Referring primarily to FIGS. 1-4, the apparatus  10  (best shown in FIG. 1) comprises, in pertinent part, a cutting wire  12  (FIG.  1 ), which could be, for example, a diamond impregnated wire such as the Superwire™ or Superlok™ series of cutting wires available from Laser Technology West Limited, Colorado Springs, Colo. The wire  12  accurately and rapidly crops and saws a silicon ingot  14  (FIG. 3) into multiple wafers for subsequent processing into discrete or integrated circuit devices. Apparatus  10  enhances the sawing ability of wire  12  by rocking, i.e., rotating, the saw back and forth through an arc relative to the stationary ingot  14  and advancing the saw vertically through the ingot  14  as the cut progresses while driving wire  12  across ingot  14 . 
     The apparatus  10  includes a stationary, generally rectangular frame  15  (FIG.  1 ), an indexing bed  17  (FIG. 1) for supporting and positioning the ingot  14  and a wire drive mechanism  16  (FIG. 3) for both moving either a continuous wire  12  in a single direction or moving a length of wire  12  in a reciprocating fashion with respect to the ingot  14  and for advancing wire  12  through ingot  14 . Indexing bed  17  is equipped with at least two clamps  17   a  (FIG. 2) preferably in a “V” shape, to hold the ingot  14  substantially still during the cutting of ingot  14 . Indexing bed  17  could be equipped (but not shown here) with a motor to incrementally advance the ingot  14  along axis subsequent cuts. 
     The wire drive mechanism  16 , in the embodiment shown, has a capstan  18  that uses a servomotor  26  (FIG. 2) to drive the length of wire  12  continuously in one direction for the length of wire  12  or back and forth in a reciprocating fashion, simultaneously winding and unwinding the length of wire on a capstan drum  30  (FIG.  1 ). Alternatively, if one or more individual continuous loops of wire  12  are utilized instead of a single linear length of wire, capstan  18  may drive the wire  12  continuously in a single direction without reversal. 
     The wire  12  is guided in the proximity of the ingot  14  by a pair of idler pulleys  20  (FIG.  3 ), with proper tensioning of the wire  12  being maintained by a constant tension pulleys  22  (FIG.  3 ). Attached to constant tension pulleys  22  are constant force torque motors  22   a  (FIG.  3 ). Torque motors  22   a , the capstan  18 , idler pulleys  20 , and tension pulleys  22  are all mounted to a generally upside down “U” shaped wire drive mechanism frame  24  (FIG.  2 ). Tension pulleys  22  are actually slideably mounted so that they can move in response to torque motors  22   a  to maintain wire  12  at a constant tension, which is explained in more detail below. The capstan  18  is driven by a computer controlled servomotor  26  (FIG. 2) mounted to a frame  28  (FIG. 2) of the capstan  18  which is in turn fastened to the frame  24  of the wire drive mechanism  16 . The wire  12  is wrapped onto the capstan  18  such that as a capstan drum  30  (FIG. 1) rotates in one direction, wire is conveyed around one of the constant tension pulleys  22 , one of the idler pulleys  20 , across the ingot  14  and around the other idler pulley  20 , around the other constant tension pulley  22 , and back to the capstan drum  30 . Approximately 600 feet of wire can be wound onto the capstan drum  30  in the presently preferred embodiment. However, more or less wire may be provided depending on the length and size of the capstan drum  30  utilized. Preferably, approximately one full layer of wire is wrapped on the capstan drum  30 . 
     The use of the servomotor  26  as the drive motor for the capstan  18  permits the capstan drum  30  to be accurately positioned and reversed at the end of a directional rotation to within half a wrap of wire  12  remaining on the capstan drum  30 . In particular, servomotor  26  sends a signal to computer console  2000 . Computer console  2000  uses this signal to determine how much wire has been unwound/wound on capstan drum  30  based on the number of revolutions of capstan drum  30 . When the computer console  2000  determines that capstan drum  30  has turned a preset number of revolutions (which corresponds to a length of wire that has wrapped and unwrapped) the console  2000  sends a signal that reverses the direction of servomotor  26 . This maximizes the use of saw wire. Conventionally, wire saw capstans require a number of wraps, spanning about a half inch on the capstan drum  30 , to remain on the capstan drum  30  to account for imprecision in the number of capstan rotations needed to reverse direction. As will be explained in more detail below, by utilizing the servomotor  26  as the capstan drive motor, accurate tracking of the angular position of the capstan drum  30  is always maintained and thus the capstan drum  30  can be precisely stopped and reversed. 
     Referring to FIGS. 9 and 10, the capstan  18  is mounted to frame  24 . Capstan  18  includes the servomotor  26 , the capstan frame  28 , the capstan drum  30 , and capstan shaft  30   a . Servomotor  26  is mounted to an end  1002  of frame  28 . End  1002  has an opening  1006  that a servomotor shaft  1008  extends into. 
     As can be seen, shaft  30   a  actually comprises several components. The external portion of shaft  30   a  comprises a portion  1010  connected to end  1002  of frame  28  on one side and an end  1004  of frame  28  on the other side. A portion  1012  of shaft  30   a  is connected on both sides of drum  30 . Shaft portions  1010  and  1012  are slideably coupled over area  1014  such that portion  1012  can move relative to portion  1010 . A drum rotator  1016  is mounted internal to portions  1010  and  1012  and coupled to servomotor shaft  1008  at end  1002  and rotatably mounted at end  1004 . Drum rotator  1016  is coupled to drum  30  also. When servomotor shaft  1008  rotates, drum rotator  1016  transfers the rotation to drum  30  causing drum  30  and shaft portion  1012  to rotate. Note that shaft portion  1010  remains fixed and does not rotate. The servomotor shaft  1008  has an arm  1020  that is connected, by a rotation transfer bearing  1018 , to drum rotator  1016 . Also mounted to rotation transfer bearing  1018  is a worm gear mount  1022 . Worm gear mount  1022  does not rotate. Worm gear mount  1022  has teeth  1024  mounted in the drum  30  rotational area. Mounted on drum  30  are corresponding teeth  1026 . When drum  30  rotates teeth  1024  and  1026  engage and act as a worm gear to move drum  30  relative to shaft portion  1010 . This allows wire  12  to play out in a substantially constant position aligned with the cut in ingot  14 . FIGS. 11 and 12 show the capstan drum  30  at extreme ends of movement in either direction. Notice that wire  12  plays out at substantially the same position regardless of the position of drum  30 . Because wire  12  plays out at substantially the same position, it would be possible, and preferred, to enclose the capstan with a casing (not shown) that has an ingress and egress for wire  12 . The casing would inhibit the wire from becoming tangled in the unlikely event of wire breakage. While having wire  12  play out at the same position is preferable, it is not necessary because the idler pulleys  20  ensure wire  12  properly aligns with the cut on ingot  14 . 
     The wire drive mechanism  16  is mounted on a wire drive mechanism frame  24 . Frame  24  comprises a generally U shaped metal plate that is rotatably mounted on an arcuate advancing frame plate  32  (FIG. 4) via a bull gear  200  (FIG. 4A) and pinion drive gear  202  (FIG. 4C) arrangement (FIGS.  4 A- 4 C). Referring to FIGS. 17 and 18, the bull gear  200  is shown engaged with pinion drive gear  202 . Bull gear  200  is mounted on a shaft  206  (FIG. 17) of stepper motor  34 , which is mounted on wire drive mechanism frame  24 . Stepper motor  34  receives signals from console  2000 , as is explained in more detail below, that cause the stepper motor  34  to drive bull gear  200  in clockwise, counterclockwise, or some combination thereof, rotation. The rotation of bull gear  200  causes bull gear  200  to move relative to pinion gear  202 , which is stationary and mounted on advancing frame plate  32 . Holding pinion gear  202  stationary allows the rotation of bull gear  200  to cause wire drive mechanism frame  24  to rotate in counterclockwise and clockwise rotation about ingot  14 . Thus, the wire drive mechanism  16  rocks the wire  12  about ingot  14  during cutting. 
     The advancing frame plate  32  is mounted to two stationary upright guide rods  36  (FIG. 4) that are mounted to the stationary frame  15 . A drivemotor, not shown in the drawing figures, raises and lowers the plate  32  to raise and lower the wire drive mechanism  16 . Referring to FIGS. 4A-4C and  19  and  19 A. The wire drive mechanism  16  is mounted on frame  24 . Frame  24  has a plurality of spring loaded ball bearings  204  (FIG. 4B) and fixed ball bearings  206  (FIG.  4 B). A generally U shaped frame guide  208  (FIG. 4C) that has a wire drive mechanism track  210  (FIG. 4C) mounted on frame  24  that is connected such that the ball bearings  204  and  206  allow frame  24  to ride in track  210 . Frame  208  is mounted to advancing frame plate  32 . Stepper motor  34  (FIG.  4 ), mounted on frame  24 , receives drive signals from console  2000  to drive bull gear  200 . Driving bull gear  200  causes the bull gear  200  to engage pinion gear  202  and cause wire drive mechanism  16  to rotate about ingot  14 . Console  2000  drives bull gear  200  in alternating directions so that wire drive mechanism  16  can rotate in a clockwise then counterclockwise direction about ingot  14 . 
     The wire drive mechanism  16  is rocked or rotated about the longitudinal axis of the ingot  14  via stepper motor  34  which turns the bull gear  200  that engages the pinion gear  202  fastened to the advancing frame plate  32 . The wire drive mechanism frame  24  thus rocks counterclockwise, as shown in FIG. 6, and then clockwise, as shown in FIG. 7, as the cut through the ingot  14  progresses. 
     The wire saw  12  cuts a curved cut with the wire saw  12  substantially tangent to the bottom of the cut throughout the cut through the diameter of the ingot  14 . The wire saw  12  maintains the tangential cut as it advances almost entirely through the ingot  14 , as shown in FIG.  8 . Further, the arc angle or arc length of the reciprocal rotation of the wire drive mechanism  16  may be varied in a predetermined manner throughout the duration of the cutting operation, and may vary depending on the depth of cut. For example, the arc angle in each direction may be small at the beginning and end of the cut through the diameter of the ingot  14  and larger, e.g. about 30 degrees toward the middle of the cut through the ingot  14 . The purpose of the rotation, however, remains the same. That is, to maintain the wire saw substantially tangent to the cut. This minimizes the side forces on the wire saw caused by imperfections or undulations in and on the outer surface of the ingot  14 . 
     Referring now to FIGS. 4,  6 ,  6 ,  7  and  8 , the operation of cutting and the motion of the wire drive mechanism  16  is described in more detail. In FIG. 4, wire drive mechanism  16  is lowered by the drive mechanism advancing mechanism so as to just touch the surface of the ingot  14 . A piece of tape, or its equivalent (not shown), placed on the surface of ingot  14  is used as a sacrificial kerf starter. The kerf starter prevents the wire  12  from wandering over the surface of ingot  14  as the cut is commenced. At this point, there is little or no deflection of the wire  12  as wire  12  is moved across ingot  14  to start the cut, as described in more detail below. 
     Referring to FIG. 5, apparatus  10  has begun cutting ingot  14 . As can be seen, after the initial cut (shown in FIG.  4 ), wire  12  begins deflecting, but maintains a cutting surface  50  substantially tanget to ingot  14 . As explained in more detail below, proximity sensor  40  registers the deflection of wire  12  and generates a corresponding analog voltage signal that is sent to console  2000 . Console  2000  uses the voltage to determine the tension on wire  12 . Based on the tension in wire  12 , torque motors  22   a  move tension pulleys  22  to increase or decrease the tension. Wire drive mechanism  16  does not rock during the initial cut. When advancing frame plate  32  lowers wire drive mechanism  16  deeper into the cut of ingot  14 , as shown in FIG. 6, console  2000  send a rocking drive signal to stepper motor  34  that cause stepper motor  34  to drive bull gear  200 . Bull gear  200  walks along pinion gear  202  in one direction and then the other. For example, wire drive mechanism  16  moves counterclockwise, in FIG. 6, and the clockwise in FIG.  7 . Proximity sensor  40  operates during the rocking to ensure wire  12  maintains the proper tension. Wire drive mechanism track  210  is set so that cutting surface  50  remains substantially tanget to ingot  14  as the wire drive mechanism rocks about ingot  14 . 
     FIG. 7 shows wire drive mechanism  16  advanced even further through ingot  14 . As before, the tension of wire  12  is maintained by proximity sensor  40  sending a signal to console  2000  that causes console  2000  to move tension pulleys  22 , via torque motor  22   a . FIG. 7 show wire drive mechanism rocked in the clockwise direction. Again, cutting surface  50  remains substantially tanget to ingot  14 . 
     FIG. 8 shows wire drive mechanism  16  advanced substantially through ingot  14 . Advancing frame plate  32  has been lowered almost to its bottom most position. Also, console  2000 , has stopped driving stepper motor  34 , which in turn stops the rocking motion of the saw. Again, proximity sensor  40  maintains a constant tension on wire  12 . 
     As can be seen from the sequence of FIGS. 4,  5 ,  6 ,  7  and  8 , the rocking of the saw starts at a minimum rocking motion, which is preferably zero (FIG.  5 ). Preferably, the rocking gradually increases after the initial cut and increases towards a maximum towards the middle of the cut (FIG.  7 ). Preferably, at the maximum rocking, the wire drive mechanism  16  rocks 30° in either direction off about the centerline (i.e., about 90° of rotation), but more or less motion is possible. From the maximum rocking towards the center position, the rocking motion gradually decreases back to a minimum, which is preferably zero rocking, at the bottom of the cut (FIG.  8 ). 
     The index or undeflected position of the wire saw  12  is detected by an inductive proximity sensor  40  (FIGS. 3 and 22) positioned adjacent the wire  12  on the wire drive mechanism frame  24 . This sensor  40  generates an analog voltage proportional to the angle of wire deflection from the index position. The analog voltage is sent to console  2000  as a tension signal. As downward force on the ingot  14  increases, the deflection increases (FIG.  23 ). The change in deflection corresponds to a proportional change in the analog voltage signal from sensor  40 . Because the constant force torque motors  22   a  supporting the tension pulleys  22  maintain a constant tension on the wire, the deflection distance becomes an accurate measurement of the force being exerted by the wire saw  12  on the semiconductor crystal in the cut. 
     The force on wire  12  must be monitored carefully to minimize wire breakage and optimize the cutting operation. The computer console  2000  monitors the analog voltage and maintains a constant deflection by sending positioning signals to torque motors  22   a  during rocking of the wire drive mechanism  16  and advancement of the advancing mechanism to optimize the cutting operation. FIGS. 22 and 23 show idler pulley  20  with proximity sensor  40  connected in the vicinity of wire  12 . 
     FIG. 22 shows wire  12  in a non-cutting or an initial cutting position. In this position, wire  12  passes just off of a tangential position with respect to the circumference of the circular cross-sectional face of the inductive proximity sensor  40  as maintained in position by bracket  2200 . This position causes the proximity sensor  40  to generate a reference or idle voltage that corresponds to zero downward cutting force. Proximity sensor  40  sends this analog voltage to console  2000 . The wire  12  will be slightly deflected upwards (indicated by the other dashed positions of the wire  12  in the direction of the arrow), in response to beginning a downward cutting force. Because of this deflection, wire  12  then begins to be sensed by the proximity sensor  40  and a lesser analog voltage is then generated by the sensor  40  that corresponds to the greater downward force. As the downward force increases, the chord the wire  12  forms over the face of the proximity sensor  40  increases resulting in proximity sensor  40  generating an even lower voltage. Console  2000  receives this lower voltage and sends a signal that causes torque motors  22   a  to reposition tension pulleys  22  to decrease the tension on wire  12 . Conversely, if the downward force decreases, the chord formed by the wire  12  with respect to the face of the proximity sensor  40  decreases resulting in proximity sensor  40  generating a higher voltage. Console  2000  receives the higher voltage and sends a signal that causes torque motors  22   a  to reposition tension pulleys  22  to increase the tension on wire  12 . Other proximity sensor configurations would work also. While it is preferred to precisely control the position of tension pulleys  22  using a motor  22   a , other embodiments to maintain tension would also work. For example, tension pulleys  22  could be positioned by a spring that compresses and expands to maintain a constant tension on the wire  12 . 
     With reference to FIG. 23, an alternative configuration of the sensor  40  and wire  12  is shown wherein the initial position of the wire with respect to the face of the sensor  40  is substantially along the diameter thereof. In this instance, as the wire is deflected from this position by a downwards cutting force, a lesser chord due to the wire&#39;s presence is sensed and a correspondingly greater voltage level is then produced by the sensor  40 . The functionality previously described with respect to the wire tensioning of the embodiment of FIG. 22 could then be applied. The embodiment of this figure has the advantage of being able to sense the absence of the wire  12  due, for example, to potential wire breakage. In the event no wire  12  is sensed by the sensor  40 , the voltage output of the sensor  40  would be greater than that of the initial value when it is positioned substantially along the diameter thereof. 
     As shown in FIG. 3, tubes  500  and nozzles  550  are mounted on wire drive mechanism  16 . Tubes  600  and nozzles  550  blow a water and air mixture onto the wire  12  as it enters and/or leaves the kerf of ingot  14  to aid in keeping the kerf free of debris. Tubes  500  are preferably mounted on frame  24  to blow air into the kerf at a constant angle to the cutting surface  50 . The water and air mixture is kept in a pressurized tank, not shown, and connected to tubes  500 . 
     While there have been described above the principles of the present invention in conjunction with specific apparatus and wire sawing techniques, it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation to the scope of the invention. Particularly, it is recognized that the teachings of the foregoing disclosure will suggest other modifications to those persons skilled in the relevant art. Such modifications may involve other features which are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure herein also includes any novel feature or any novel combination of features disclosed either explicitly or implicitly or any generalization or modification thereof which would be apparent to persons skilled in the relevant art, whether or not such relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as confronted by the present invention. The applicants hereby reserve the right to formulate new claims to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.