Stone working – Sawing – Reciprocating
Reexamination Certificate
1999-11-04
2001-10-02
Hail, III, Joseph J. (Department: 3723)
Stone working
Sawing
Reciprocating
C125S012000, C125S016010, C125S035000
Reexamination Certificate
active
06295977
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for simultaneously cutting off a multiplicity of wafers from a hard, brittle workpiece. The invention also relates to a device which is suitable for carrying out the method.
2. The Prior Art
The invention is useful in particular for the production of semiconductor wafers. For such applications, wire saws which are able to cut off a multiplicity of wafers from a workpiece in a single operation are increasingly being used. U.S. Pat. No. 5,771,876 describes the operating principle of a wire saw which is suitable for producing semiconductor wafers. The workpiece is guided through the wire web of the wire saw by a feed movement and, when it penetrates through the wire web, is divided into wafers. Wire saws which operate with a slurry are known, as well as other wire saws in which abrasive grains are fixedly bonded to the sawing wire. When cutting off semiconductor wafers from a crystal, it is usual for the crystal to be fixed at a mounting beam. The sawing wire then cuts the crystal at the end of the method. The mounting beam is a supporting body which is adhesively bonded or cemented to the peripheral surface of the crystal. After they have been cut off, the semiconductor wafers formed remain fixed to the mounting beam in the manner of the teeth of a comb and can thus be removed from the wire saw. Later, the residual mounting beam is removed from the semiconductor wafers.
When working with this kind of wire saws, the sawing wire may leave marks and waves on the sides of the wafers, which is undesirable. This is because they require more material to be removed during subsequent processes and because they make it difficult to measure the thickness of the wafers, and also constitute deviations from the desired wafer shape. For this reason, they have to be removed, for example by grinding, resulting in the loss of additional material.
JP-08-85053 A describes a method in which the workpiece is cemented to a mounting beam and, when the wafers are being cut off, carries out a pivoting movement, so that the sawing wire leaves a curved base in the sawing gap when penetrating into the workpiece.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved sawing method.
The invention relates to a method for simultaneously cutting off a multiplicity of wafers from a hard, brittle workpiece which has a longitudinal axis and a peripheral surface, the workpiece being guided, by means of a translational relative movement, directed perpendicular to the longitudinal axis, between the workpiece and a wire web of a wire saw with the aid of a feed device, through the wire web which is formed by a sawing wire, wherein the workpiece is rotated about the longitudinal axis while the wafers are being cut off.
The invention furthermore relates to a wire saw for carrying out the method, which saw has a device for holding and for rotating the workpiece about the longitudinal axis.
In addition to the advantage that there are fewer marks made when cutting off wafers and the fact that the wafer yield increases, since less material has to be sacrificed when treating the sides using abrasive tools, there are further advantages associated with the invention. Examples include improvements in characteristic variables which, as with TTV (total thickness variation) and thickness distribution, describe the shape of the wafers produced. Therefore, wafers produced according to the invention can be directly subjected to double side polishing, without an intermediate material-removing treatment. Furthermore, the full slicing capacity increases when the method is employed, since a feed path which corresponds to half the diameter of the workpiece is sufficient to allow the wafers to be cut off completely. Shorter sawing times can be achieved by suitably selecting the length over which the sawing wire acts and the feed rate. When cutting off semiconductor wafers, it is possible to dispense with the attachment of a mounting beam before cutting and the removal of the residual mounting beam after cutting. Finally, the invention can also be used to obtain semiconductor wafers with a defined bow. To achieve this, the rotational movement can be combined with an axial feed movement of the sawing wire or of the workpiece, resulting, with a radially changed material-removal volume, in a corresponding abrasion of material. The axial feed movement of the sawing wire or of the workpiece may, for example, be achieved by measures which are described in the German Patent Application Serial Number 197 39 966.5.
The invention provides further advantages compared to known methods in which a slurry is employed. During rotation of the workpiece, the slurry is distributed within the sawing gap more successfully, thus ensuring that the sawing gap is provided with sufficient slurry. The rotation of the workpiece about the longitudinal axis may also lead to direct substantial contact between the sawing wire and the workpiece in the cutting gap and thus to a high sawing pressure. In this way, and by possibly increasing the relative speed of the sawing wire when workpiece and sawing wire are rotating in opposite directions, it is possible to accelerate the speed at which the wafers are cut off. Any reduction in the peripheral speed of the sawing wire that may be required can be compensated for by a corresponding increase in the rotational speed of the workpiece. The above advantages overall make the method according to the invention more economical.
Although the invention offers a particularly large number of advantages when cutting off semiconductor wafers from a crystal, it is not limited to this area. It may also be employed with particular preference for cutting off wafers which are to be processed further to form hard disks. In contrast to crystals, which are present in the form of solid bodies, wafers which are to be processed to form hard disks are cut off workpieces which, since they have an axial hole, are rotationally symmetrical hollow bodies.
The workpieces are preferably made from hard brittle material, such as silicon or gallium arsenide, if semiconductor material is involved, and from silicon carbide if material for producing hard disks is involved.
In the context of the invention, a longitudinal axis of the workpiece is to be understood as meaning the geometric center of the workpiece. The workpiece extends in a rotationally symmetrical manner about this axis. A crystal made from semiconductor material is generally ground in such a way that this axis coincides with a preferred crystal axis or forms a defined angle with respect to the preferred crystal axis.
The invention provides for the workpiece to be rotated about the longitudinal axis when wafers are being cut off. Thus there are various options available to the user in this context. For example, the direction of rotation may be maintained or may be changed periodically or according to a defined program. In the event of the direction of rotation being changed, the workpiece may be rotated for longer in one direction than in the opposite direction, or may be rotated for equal lengths of time in each direction of rotation.
If the workpiece is a solid crystal made from semiconductor material, it is preferable to divide this crystal into wafers without the crystal being joined to a mounting beam. It is possible that the mounting beam may be cemented to the outer peripheral surface of the solid crystal. However, in this case, the angle of rotation is to be limited.
In the case of a workpiece which is designed as a hollow body, a support body, for example a bar made from glass, graphite, metal or plastic, is to be joined to the inner peripheral surface of the workpiece in the cavity in the workpiece. This support body is required in order to rotate the workpiece.
When using a mounting beam or some other form of support body, the wafers are cut off until the sawing wire of the wire web starts to cut into the support body. The wafers which have been cut off thus rema
Andrae Christian
Egglhuber Karl
Greim Jochen
Kolker Helmut
Lundt Holger
Collard & Roe P.C.
Hail III Joseph J.
McDonald Shantese
Wacker - Chemie GmbH
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