Most processes for fabricating semiconductor electronic components start with monocrystalline, or single crystal, silicon in the form of wafers. In general, semiconductor wafers are produced by thinly slicing or cutting a generally cylindrical single crystal silicon ingot. After slicing, each wafer undergoes a number of processing operations to shape the wafer, reduce its thickness, remove damage caused by the slicing operation, and to create a highly reflective surface.

Known wafering processes include slicing an ingot into individual wafers with a cutting apparatus, such as a wire saw. In general, the wire saw uses a wire mounted on rollers for cutting the ingot. The wire saw's drive mechanism moves the wire back and forth in a lengthwise direction around the rollers at an average speed of, for example, 600-900 meters per minute. The saw slices the ingot in a direction normal to the ingot's longitudinal axis to produce as many as several hundred thin, disk-shaped wafers. In a typical slicing operation, the reciprocating wire contacts the ingot while a liquid slurry containing abrasive particles (e. g., grains of silicon carbide) is supplied to the contact area between the ingot and the wire.

As the wire rubs the abrasive particles in the slurry against the ingot, the saw removes silicon crystal and gradually slices the ingot. The wire saw provides a gentle mechanical method useful for cutting silicon crystal, which is brittle and more likely damaged by other types of saws (e. g., conventional internal diameter saws).

Wire saws generally have three or four rollers that are rotatably mounted on a frame, each roller having a plurality of circumferential guide grooves for receiving segments of wire.

Multiple parallel lengths of the wire extend between two of the rollers to form a wire web for slicing. the ingot into multiple wafers. The space between adjacent wires in the web generally corresponds to the thickness of one wafer before processing. The frame is adjustable to change the spacing between the rollers for adjusting the tension in the wire. The saw's drive mechanism includes spools for supplying and taking up the wire as it travels back and forth around the rollers.

The wire saw apparatus also includes an ingot support fixture for holding the silicon ingot in position for cutting. Preferably, the fixture is adjustable to accurately align an orientation of the crystalline structure of the ingot relative to the saw's cutting plane and has a mount to which the ingot is bonded during slicing. A rack typically extends upward from the support fixture and a motor-driven pinion engages the rack for advancing and retracting the ingot. In other words, the fixture is moveable in translation to bring the ingot into contact with the wire web.

A fluid dispensing system having a pump, tubing, and at least one nozzle, manifold, or other dispenser transports slurry from a nearby slurry container and dispenses it onto the wire web. A portion of the slurry then moves with the wire into a contact area between the wire and the ingot where the silicon crystal is cut. For example, two nozzles may be positioned on opposite sides of the ingot holder so that slurry is dispensed onto the web on both sides of the ingot, thus facilitating delivery of slurry to the cutting region for either direction of travel of the reciprocating wire. Each nozzle is positioned above the wire web at close spacing and configured to dispense slurry in a generally thin, linear distribution pattern, forming a curtain or sheet of slurry. The slurry curtain extends across a full width of the wire web so that slurry is delivered to every reach of wire and every slice in the ingot.

Commonly assigned U. S. Patent Nos. 5,735,258,5,827,113, and 6,006,736, the entire disclosures of which are incorporated herein by reference, discloses wire saw apparatus for slicing silicon wafers.

Unfortunately, when cutting a relatively large cylindrical workpiece (e. g., having a diameter of 200 mm or more) and/or when the slurry contains grains of a relatively high count (e. g., No. 1500 or higher), the wire saw is susceptible to having its wire break or jump out of one or more of the guide grooves on the saw's rollers. This is particularly problematic when the saw is advancing the ingot toward the wire at conventional cutting speeds.

Although the cutting speed can be set at the beginning of the cutting operation to avoid these problems, it necessarily increases the cutting time and, thus, decreases throughput.

For these reasons, a method and an apparatus are desired for slicing wafers from relatively large ingots using high count grains while reducing incidents of wire breakage and groove skipping but without decreasing throughput.

SUMMARY OF THE INVENTION The invention meets the above needs and overcomes the deficiencies of the prior art by providing an improved method for cutting a cylindrical workpiece such as a silicon single crystal. Among the several objects and features of the present invention may be noted the provision of a method and apparatus that reduce the incidence of wire malfunctions when beginning slicing of relatively large work pieces; the provision of such method and apparatus that reduce the incidence of wire malfunctions when beginning slicing using slurry having high count grains; the provision of such apparatus and method that improve throughput; and the provision of such method and apparatus that are economically feasible and commercially practical.

Briefly described, an embodiment of the present invention is directed a method of slicing an ingot of semiconductor material into wafers using a wire saw. The wire saw includes a holder for supporting the ingot and a wire, movable in a forward direction and in a reverse direction, for slicing the ingot. The method includes the step of feeding the holder and the wire relatively toward each other along a substantially linear feed path to force the

ingot and the wire into contact during slicing. The holder and wire are fed relatively toward each other at a first cutting speed for an initial interval and at a second cutting speed thereafter. According to the method, the first cutting speed is substantially constant and the second cutting speed is variable. The method further includes providing a slurry containing abrasive particles to a contact area between the wire and the ingot during slicing.

A wire saw apparatus embodying aspects of the invention includes a wire movable in lengthwise forward and reverse directions for slicing an ingot of semiconductor material into wafers. The saw apparatus also includes a plurality of grooved guide rollers for supporting and guiding the wire during slicing. The wire is supported by the rollers in a reach between adjacent rollers. The reach defines a cutting web that includes multiple generally parallel lengths of the wire for cutting multiple wafers from the ingot. In addition, the apparatus includes a holder for supporting the ingot in registration with the cutting web and with a longitudinal axis of the ingot generally perpendicular to the lengths of the wire in the cutting web. The holder supports the ingot for relative motion along a substantially linear feed path such that the ingot passes through the cutting web as the wire is driven in the lengthwise directions for substantially simultaneous slicing of wafers from the ingot by the wire. The holder and wire are fed relatively toward each other at a substantially constant first cutting speed for an initial interval and at a variable second cutting speed thereafter. The apparatus also includes a slurry delivery system for providing a slurry containing abrasive particles to a contact area between the wire and the ingot during slicing.

Yet another embodiment of the invention is directed to a method of slicing an ingot of semiconductor material into wafers using a wire saw. The wire saw includes a holder for supporting the ingot and a wire, movable in a forward direction and in a reverse direction, for slicing the ingot. The method includes the step of feeding the holder and the wire relatively toward each other along a substantially linear feed path to force the ingot and the wire into contact during slicing. The holder and wire are fed relatively toward each other at a first cutting speed less than or equal to approximately 250 Fm/min. for an initial interval and at a

second cutting speed thereafter. According to the method, the first cutting speed is substantially constant and the second cutting speed is variable. The method further includes providing a slurry containing abrasive particles to a contact area between the wire and the ingot during slicing. The abrasive particles contained in the slurry have an average diameter of less or equal to approximately 8 um.

Alternatively, the invention may comprise various other methods and apparatuses.

Other objects and features will be in part apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. I (a) to I (c) illustrate cutting a cylindrical workpiece according to a preferred embodiment of the present invention. In particular, FIG. l (a) is a schematic diagram showing a site of a wire in cutting the cylindrical workpiece; FIG. l (b) is a graph showing a 1 correlation between cutting speed of the cylindrical workpiece and cutting time; and FIG. l (c) is a graph showing a correlation between secondary differentiation of cutting speed of the cylindrical workpiece and cutting time.

FIGS. 2 (a) and 2 (b) illustrate cutting a cylindrical workpiece according to a conventional method. In particular, FIG. 2 (a) is a schematic diagram showing a site of wire in cutting the cylindrical workpiece; and FIG. 2 (b) is a graph showing a correlation between cutting speed of the cylindrical workpiece and cutting time.

FIGS. 3 (a) and 3 (b) illustrate cutting a cylindrical workpiece according to another conventional method. In particular, FIG. 3 (a) is a schematic diagram showing a site in cutting the cylindrical workpiece ; and FIG. 2 (b) is a graph showing a correlation between cutting speed of the cylindrical workpiece and cutting time.

FIG. 4 is a schematic view of an apparatus for cutting a cylindrical workpiece.

FIG. 5 is a perspective view of portions of the apparatus of FIG. 4.

Corresponding reference characters indicate corresponding parts throughout the drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIGS. I (a)-l (c), a single crystal ingot 1 (i. e., the generally cylindrical workpiece) is positioned for cutting. A slurry supply tube 2 (see FIG. 4) sprays slurry 3 on a wire 4, which is running at a relatively high speed to cut the ingot 1 into wafers.

As is known in the art, the wire saw includes a plurality of rollers 5 (see FIGS. 4 and 5) having a plurality of guide grooves 5a for arranging the wire 4 in a plurality of substantially parallel, regularly spaced lines defining a cutting web. The rollers 5 preferably support wire 4 in a reach between adjacent rollers. An ingot support fixture, or work bench, 6 preferably holds ingot 1 at a predetermined site using a holder, or beam, 7. The wire saw begins slicing ingot 1 into wafers by reciprocating its wire 4 in a back and forth motion as wire 4 and ingot 1 feed relatively toward each other along a substantially linear feed path (i. e., the cutting direction). Preferably, the workbench 6 is movable for feeding ingot 1 toward wire 4, forcing the ingot and wire together during slicing.

Those skilled in the art recognize that a conventional wire saw operates at a cutting speed that decreases with the depth of cut in the workpiece until the depth of cut reaches half of the overall depth of cut (i. e., the middle of ingot 1). For example, FIGS. 2 (a)-2 (b) show cutting at a speed that is inversely proportional to the width or chord length of the cylindrical workpiece, ingot 1, at site. In other words, the cutting speed is greater when the width of the portion of ingot 1 being sliced is smaller (i. e., at the beginning and ending of the cutting operation) than when the width of the portion of ingot 1 being sliced is greater (i. e., at the middle of the cutting operation). By varying cutting speed in this manner, the wire saw aims

to provide a relatively uniform cutting force during cutting at all sites. Another conventional cutting, method, such as shown in FIGS. 3 (a) and 3 (b), suppresses the cutting speed in a vicinity c near the end of cutting of the cylindrical workpiece in an attempt to reduce vibrations and, thus, avoid breaking wafer (s). As used herein, the term"cutting speed,"also known as"feed rate,"refers to the relative speed at which ingot 1 moves toward wire 4 during cutting while the term"wire speed"refers to the speed at which wire 4 travels back and forth.

FIGS. 1 (a) to 1 (c) illustrate cutting ingot 1 according to a preferred embodiment of the present invention. In particular, FIG. I (a) is a schematic diagram showing a site of wire 4 in cutting ingot 1 ; FIG. 1 (b) is a graph showing a correlation between cutting speed of ingot 1 and cutting time ; and FIG. l (c) is a graph showing a correlation between secondary differentiation of cutting speed of ingot 1 and cutting time.

In contrast to conventional cutting methods, the present invention provides for cutting at most about 40% of a cutting plane of ingot 1 while the wire saw maintains a substantially constant cutting speed. In this embodiment, the problem of breaking wire 4 is not caused when beginning to cut ingot 1 at the initial cutting speed. This reduces the likelihood that wire 4 will break or jump over a groove on the saw's main roller 5 when slurry 3 contains grains of high count (e. g., No. 1500 or higher) or when ingot 1 has a relatively large diameter (e. g., 200 mm or more). In addition, the invention of FIGS. l (a)-1 (c) is found to be economically excellent.

A main characteristic of the method of the present invention is that ingot 1 is cut at a substantially constant speed from the beginning of cutting to a predetermined time of cutting as shown in FIG. l (b). In other words, if a function of cutting speed y at the beginning of cutting of ingot 1 and cutting time t is represented by y = f (t), then the secondary differential equation F' (t) preferably has a negative region as shown in FIG. l (c). In contrast to the present invention, conventional cutting methods call for the cutting speed to be inversely proportional to the chord length of a cylindrical workpiece at site.

Operating in accordance with FIGS. l (a)-l (c) reduces the load on wire 4 caused at the moment wire 4 contacts ingot 1 in the case of using grains (e. g., fine grains having average diameter of 8 pm or less) of high count (e. g., No. 1500 or higher) or in the case of cutting a cylindrical workpiece having a relatively large diameter (e. g., 200 mm or more). As a result, the present invention reduces the risk of breaking wire 4 upon cutting and the risk of having wire 4 jump over a groove on main roller 5.

FIG. l (a) illustrates an initial interval a in which the cutting speed is substantially constant. In a preferred embodiment, the saw cuts at most approximately 40% of a cutting plane of ingot 1 while maintaining the substantially constant cutting speed, as shown in the cutting speed profile of FIG. l (b). In other words, the initial interval is less than or equal to approximately 40% of the total depth of cut or the cutting plane of ingot 1. The minimum depth of cut at the first, substantially constant, cutting speed is preferably about 10%.

Operating in this manner reduces the risk of wire-related malfunctions caused at the beginning of cutting of ingot 1, such as breaking wire 4 or causing it to skip a guide groove.

As an example, when slurry 3 contains fine grains having an average diameter of 8 um or less, a preferred cutting speed at the beginning of cutting is between approximately 230 gm/min and approximately 250 pm/min.

Although maintaining a constant cutting speed throughout most, if not all, of the cutting operation can shorten the overall cutting time, such a process fails to yield satisfactory wafers. Maintaining a constant cutting speed for too long makes it difficult to obtain satisfactory mechanical quality (flatness, parallelism, warp, and the like) of each wafer cut from ingot 1. Nonetheless, the present invention performs the cutting operation in an economical manner because cutting speed in a portion where cutting speed becomes lowest may be increased to a certain degree so that cutting time is not extended unnecessarily. In other words, beginning the cutting operation with an interval of substantially constant speed permits continued operation over the varying speed portion of the cutting time to be at a

higher speed than the corresponding portion when cutting performed according to a conventional method.

The present invention is described in greater detail with respect to the following Examples 1 and 2 and Comparative Examples 1 and 2. However, the present invention is by no means limited to these Examples.

EXAMPLES The silicon ingot 1 (i. e., the cylindrical workpiece), having a diameter of approximately 300 mm, was subjected to cutting according to the schedule shown in FIG. l (b) using the wire saw of FIG. 4. Table I, below, shows the cutting conditions.

The slurry 3 used in the Examples had an average particle diameter of approximately 6.7 llm. Regarding parameters such as missing ratio, flow rate, temperature, and the like, known ones were used.

TABLE I Beginning cutting speed (llm/min) Lowest cutting speed* (pm/min) Example 1 225 170 Example 2 225 213 Comp. Ex. 1 294 170 Comp. Ex. 2 250 170 *cutting speed at a site approximately 201 mm from the starting point of the ingot (300 mm in diameter).

In Examples 1 and 2, proper wafers were obtained from ingot 1 without breaking wire 4 or having it jump over a groove on main roller 5. On the other hand, in Comparative Example 1, wire 4 broke at the moment it contacted ingot 1 and, as a result, cutting of ingot 1 could no longer be continued. In Comparative Example 2, wire 4 did not break but proper wafers could not be obtained from ingot 1 since wire 4 jumped over a groove on roller 5 (e. g., one groove no longer held a portion of wire 4 while another groove held two portions).

As described above, a wire saw operating in accordance with a preferred embodiment of the present invention suppresses breakage of wire 4 as well as reduces the likelihood that wire 4 will jump or skip out of one or more of the grooves on the saw's roller 5 when slurry 3 contains grains of high count, such as No. 1500 or higher, and/or when ingot 1 has a relatively large diameter, such as 200 mm or more, but is also economically excellent.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.