Background of the Invention A conventional grinding operation includes rotating a grinding wheel against a workpiece until the workpiece has a desired dimension or flatness. For example, a grinding operation may be used in a semiconductor wafer processing method to grind the wafer to a desired flatness. In such a method, the wafer is cut from an ingot by a wiresaw, one or both surfaces of the wafer are ground to a desired flatness, and the wafer is thereafter subjected to a polishing operation, among other operations. However, a thinner workpiece, such as a semiconductor wafer, often deforms or bows due to its own weight during grinding so that the wafer does not attain the desired flatness. To prevent the weight of the workpiece from causing deformation or bowing, a horizontal axis grinding apparatus may be used. In a horizontal axis grinding apparatus, a grinding wheel is mounted for rotation about a horizontal axis and the workpiece to be ground is positioned vertically. Further, the grinding processing time may be reduced by use of a double surface grinding apparatus in which the workpiece is held between two opposing grinding wheels.

During grinding, a grinding fluid is applied to the grinding wheel to cool the workpiece and the wheel, and to flush away ground or waste particles formed during the operation. An important factor in a grinding step is proper application of the grinding fluid. It is difficult, especially in horizontal axis wafer grinding, to ensure that a sufficient amount of grinding fluid is applied to the grinding wheel to cool the wafer and the teeth, and to flush away waste particles. If the grinding fluid is not appropriately selected or the supply of fluid is not appropriate, serious problems are likely to occur. The grinding apparatus may become overloaded

due to an increase in the electrical current required to rotate the grinding wheel, possibly causing the grinding operation to be stopped. Defects in the wafer, such as scratches caused by waste particles, may result in the wafer being scrapped, and more seriously, the wafer may be broken during grinding. In addition, an increase in amount of material abraded from the grinding wheel decreases the life of the grinding wheel and thereby increases the frequency of replacement of the grinding wheel. As will be understood, these problems increase the cost of the grinding operation.

Typically, grinding fluid flow to the wheel is limited by the pump capacity of the apparatus. Accordingly, one possible method of ensuring a sufficient amount of grinding fluid is to increase the fluid pump capacity. However, this solution is not preferred because the apparatus becomes significantly larger, more complicated and more expensive.

Summary of the Invention Among the several objects of the present invention may be noted the provision of a grinding wheel that inhibits damage to the workpiece; the provision of such a grinding wheel that is stable during grinding; the provision of such a grinding wheel that helps to ensure sufficient cooling of the workpiece and the discharge of waste particles ; the provision of such a grinding wheel which reduces power consumed during grinding; the provision of such a grinding wheel that has a long useful life, and the provision of such a grinding wheel which reduces the cost of the grinding operation.

Briefly, one aspect of this invention is directed to a grinding wheel adapted to be mounted on a grinding apparatus for grinding a workpiece. The wheel comprises a plate adapted for rotation about a rotation axis, a supply port extending through the plate for supplying a grinding fluid to the plate, and a plurality of spaced-apart grinding teeth on a surface of the plate for grinding the workpiece. The teeth are disposed radially outward from the supply port so that fluid impinges upon the teeth as the plate is rotated. Each tooth has a width, and gaps between adjacent grinding teeth have a total length between 2% and 10% of a sum of the total length of the gaps and a total width of the teeth so as to reduce

flow of the grinding fluid radially outwardly of the wheel between the teeth for use in maintaining adequate fluid between the grinding wheel and the workpiece in operation.

Brief Description of the Drawings Figure 1 is a partially schematic perspective view of an embodiment of a grinding wheel of the present invention; Figure 2 is an enlarged fragmentary plan view of the grinding wheel of Fig.

1 showing grinding teeth of the wheel ; Figure 3 is an enlarged plan view similar to Fig. 2 but showing grinding teeth of another configuration; Figure 4 is an enlarged plan view showing grinding teeth of yet another configuration; Figure 5 is an enlarged plan view showing grinding teeth of a further configuration; Figure 6 is an enlarged plan view showing grinding teeth of a still further configuration; Figure 7 is an enlarged plan view showing grinding teeth of another configuration; Figure 8 is a schematic Mercator projection view of exemplary grinding teeth; Figure 9 is a graph of electrical current powering a grinding apparatus using grinding wheels of this embodiment as compared to current powering the apparatus using prior art grinding wheels; and Figure 10 is a graph comparing an abrasion amount from grinding teeth of a wheel of this invention to an abrasion amount from grinding teeth of a prior art grinding wheel.

Detailed Description of the Preferred Embodiment Referring now to the drawings and in particular to Fig. 1, a grinding wheel of an embodiment of the present invention is generally designated in its entirety by the reference numeral 11. The grinding wheel comprises a ring-shaped plate

13 having a generally flat upper surface 15 and a peripheral edge 17. In this embodiment, a supply port 19 for supplying grinding fluid to the plate is a hole through the plate 13 and is disposed generally in a center of the plate.

Spaced-apart grinding teeth 21 a are disposed radially outwardly from the supply port 19 so as to receive the grinding fluid as the plate is rotated. The teeth form a generally ring-shaped pattern and are disposed adjacent the peripheral edge 17 of the plate 13, though other positions for the teeth are contemplated within the scope of this invention.

In preferred embodiments shown in Figs. 1-3, grinding teeth 21a, 21b are shaped for reducing flow of the grinding fluid radially outward of the plate from between the teeth as the plate 13 is rotated during grinding. More particularly, each grinding tooth 21 a, 21 b has a cross-sectional shape which opens up toward the center of the plate 13 so that some of the grinding fluid is retained or inhibited from flowing to the edge 17 due to the shape of the tooth. For example, the grinding teeth 21 a of Figs. 1-2 have an inverted V-shape which opens up toward the center of the plate 13. As shown in Fig. 3, teeth 21 b are channel-shaped and open up toward the center of the plate.

In contrast, teeth 21 c shown in Figs. 4-5 and 7 open up toward the peripheral edge 17 of the plate 13, rather than toward the center. The teeth need not necessarily open up toward the edge 17 or toward the center. For example, teeth 21 d (Fig. 6) are simply rectangular with an elongate flat surface generally facing the center of the plate 13. Additionally, the plate 13 may include a combination of different shapes of grinding teeth as shown in Fig. 5 wherein an inner ring of grinding teeth 21 c open up toward the peripheral edge 17 of the plate 13, and an outer ring of grinding teeth 21 a open up toward the center of the plate.

In the embodiment of Fig. 5, the teeth are preferably disposed so that the inner ring of teeth 21 c tend to channel the grinding fluid into the outer ring of teeth 21 a so that the fluid is inhibited from flowing directly through the gap between the outer ring teeth 21 a.

The teeth are suitably made of diamond abrasive grains and binding materials, and are attached to the surface of the plate 13 with an adhesive.

Alternatively, the teeth may be integrally formed as one piece with the plate. It is

to be understood that the teeth of the invention may have other shapes than those depicted herein.

Figure 4 shows an embodiment in which a continuous wall 25 is disposed radially outwardly from the teeth 21 c. In this embodiment, the wall 25 is spaced about 1mm from the grinding teeth 21 c. During grinding, grinding fluid is temporarily retained inside the wall 25 even when the fluid is drained out through gaps (A) between the grinding teeth. The wall 25 preferably has a height less than a height of the teeth 21 c so that the fluid will eventually flow over the wall.

As will be understood by those of ordinary skill in the art, the height of the teeth will be reduced or ground down over the useful life of the wheel to a minimum height. Preferably, the wall height is no greater than the minimum height of the teeth. For example, if virgin, unground teeth have a height of 5 mm and will be ground to a minimum height of 1 mm, the wall height is no greater than 1 mm.

Due to the wall 25, some amount of the grinding fluid which passes through the gap between teeth is retained adjacent the contact area between the grinding teeth contact the workpiece so as to cool the workpiece and the teeth. The wall 25 is especially suited for teeth that do not open toward the center of the plate 13, such as teeth 21 c, 21 d, but may be used in combination with any type of grinding teeth within the scope of this invention.

Referring to Fig. 8, a width of an exemplary grinding tooth 21 of this invention is referred to as (A) and a gap length between the adjacent grinding teeth is referred to as (B). An individual gap ratio of the gap length to a sum of the gap length and the width of an adjacent tooth is expressed by the equation: B / (A + B) X 100% and is preferably within a range between 2% and 10%.

Moreover, the overall ratio of the total of the gaps (sum of all gap lengths or"total gap length") is between 2% and 10% of a sum of the total gap length and a total width of all of the teeth. Preferably, each individual gap ratio and the overall ratio is within a range between 4% and 8%, and more preferably between 5% and 7%.

Note that the ratio is calculated prior to use of the grinding wheel (in other words the tooth is in virgin, unused condition). The gap (B) does not exceed 10% because an abrasion amount from the grinding wheel significantly increases, as described in the Example below. The gap (B) is not less than 2% because such a

small gap does not allow sufficient flow therethrough. Note that where two or more rings of teeth are on the same plate (as shown, for example, in Fig. 5) the overall ratio for at least one of the rings of grinding teeth 21 a, 21 b is within the range from 2% to 10%, but the teeth in all rings need not necessarily be in the range.

During a typical grinding operation using the grinding wheels of this invention to grind a semiconductor wafer, the grinding wheels are rotated about a horizontal axis at a high speed, typically about 5000 rpm or more, and the wafer is also rotated at about 5-50 rpm. The grinding fluid is introduced through the supply port 19 at the center of the plate 13 at a flow rate of about five liters/minute and due to a centrifugal force caused by the rotation of the grinding wheel, the fluid flows generally to the area where the grinding teeth contact the workpiece.

(See lines FF of Fig. 1). The wheels are moved toward the wafer so that the grinding teeth contact the wafer and grind material therefrom. The grinding operation is stopped after a predetermined period of time sufficient for the wafer to attain a desired flatness.

Any kind of grinding fluid can be used as long as the fluid is suitable for the workpiece and for the grinding conditions. Among the commonly used grinding fluids are water, oil, or a slurry prepared by dispersing metallic powders, ceramic powders, or polymeric powders into water or oil. The grinding wheel of the invention may be adapted to grind workpieces of metal or non-metals such as ceramics, polymers or a composite of these materials.

Example In an example of a method of the invention, grinding wheels 11 of the embodiment of Figs. 1-2 are mounted on a simultaneous double side grinding apparatus, Model DXSG300A, manufactured by Koyo Machine Industries Co. ,<BR> Ltd. , Osaka, Japan. The apparatus was operated to simultaneously grind both sides of semiconductor (silicon) wafers having a diameter of 300 mm. The grinding wheels were rotated at a speed of 6000 rpm and ultra pure water (grinding fluid) was supplied at a rate of about 5 liters/minute. The gap (B)

between the grinding teeth 21 in this example was 5.3%. About 200 wafers were processed for this example.

A comparison test was performed by mounting conventional grinding wheels (Model No. SD3000VPN, manufactured by A. L. M. T. Corp. , Tokyo, Japan) on the double side grinding apparatus and grinding both sides of semiconductor (silicon) wafers under the same conditions described above. A schematic diagram similar to Figure 8 was created also for these conventional wheels, and it was determined that the gap length ratio was 11.4%. About 200 comparison wafers were processed using the conventional wheels.

Referring to Fig. 9, electrical current used during grinding to rotate the grinding wheels of this invention and the conventional wheels was measured and plotted. The bar graph shows an average measurement value, while a lower end of a line segment shows a minimum value and an upper end of the line segment shows a maximum value. As shown by Fig. 9, the electrical current required to rotate the wheels of this invention is reduced compared to those of the prior art.

Similarly, with reference to Fig. 10, an abrasion amount from a prior art wheel and an abrasion amount from the grinding wheel of this invention after grinding the wafers was measured and plotted. This result shows that less material is abraded from the grinding wheel of the present invention and thereby lengthens the useful life of the wheel, as compared to abrasion and resulting useful life for the conventional grinding wheel under substantially similar grinding conditions.

The grinding wheel of this invention reduces flow of the grinding fluid radially outwardly of the wheel between the teeth for use in maintaining adequate slurry between the grinding wheel and the workpiece in operation. The flow of the grinding fluid is reduced or inhibited from flowing away from the teeth 21 so that more of the fluid is retained adjacent the teeth at the contact area with the workpiece so as to cool the workpiece and the teeth. However, the gaps are also sized so that a sufficient amount of the fluid flows therethrough so that the waste particles caused by grinding are carried away from the teeth and do not scratch or damage the workpiece. Further, an embodiment of the new grinding wheel was shown to be stable during grinding, to reduce the power consumption of the grinding operation and to reduce the abrasion amount from the grinding wheel.

Consequently, the workpiece attains the desired flatness, scratching of the workpiece by waste particles is inhibited, the life of the grinding of the grinding wheel is increased and the cost of the processed workpiece is reduced.

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

When introducing elements of the present invention or the preferred embodiment (s) thereof, the articles"a","an","the"and"said"are intended to mean that there are one or more of the elements. The terms"comprising", "including"and"having"are intended to be inclusive and mean that there may be additional elements other than the listed elements.

As various changes could be made in the above constructions 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.