Patent Publication Number: US-6035845-A

Title: Saw strip for fixing a crystal and process for cutting off wafers

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a saw strip for fixing a crystal of semiconductor material when cutting wafers from this crystal using a wire saw. The invention also relates to a process for cutting off wafers using the saw strip is used. 
     2. The Prior Art 
     When cutting wafers from a crystal of semiconductor material using a wire saw, undesirable flutes or saw marks may be formed on the sides of the wafers following transverse deviations of the saw wire. The crystal is fixed on a saw strip into which the saw wire penetrates to a depth of a few millimeters after the wafers have been cut off. The saw strip usually consists of a solid graphite block, and the crystal is cemented onto this block using, for example, an epoxy resin adhesive. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a saw strip that reliably avoids the formation of flutes when cutting wafers from a crystal of semiconductor material using a wire saw. 
     This object is achieved by a saw strip for fixing a crystal of semiconductor material when cutting wafers from this crystal using a wire saw. The strip has a section that adjoins the crystal and has a hardness that essentially corresponds to the hardness of the crystal. 
     Tests have established that the formation of flutes is closely related to the use of solid saw strips made of graphite. In comparison with semiconductor material, such as for example silicon, graphite is a relatively soft material. When cutting off the wafers, the saw wire has to overcome a certain resistance in the direction of advance. When the saw wire enters a saw strip made of graphite, this resistance suddenly becomes weaker. As a result, the saw wire may be deflected transversely, leaving behind flutes in the edge region of the wafers. 
     In order to prevent this, the present invention proposes a saw strip made from a material having a hardness that corresponds to the hardness of the semiconductor material, or has an approximately equal hardness. Saw strips made of a material having a hardness in the range of 4 to 7 on the Mohs scale, such as saw strips made of glass or silicon, have proven particularly suitable. It is also advantageous if the material used is an electrical insulator. In this case, the saw wire may be subjected to a weak electric current during the cutting operation and a crack in the wire or a ground contact can be detected by the resulting change in current intensity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained below with reference to figures which diagrammatically show preferred embodiments of the saw strip, in cross-section. The drawings are for illustration only and do not define the limits of the invention. 
     In the drawings, wherein similar reference numbers denote similar elements throughout the several views: 
     FIG. 1 shows a solid saw strip according to the invention; 
     FIG. 2 shows a saw strip according to the invention designed as a layered body; and 
     FIG. 3 shows a saw strip according to the invention having a hollow cross-section. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now in detail to the drawings and in particular, FIG. 1, there is shown a saw strip 1 underneath a crystal 2. Saw strip 1 is made from a material whose hardness essentially coincides with the hardness of the semiconductor material from which crystal 2 is made. Due to this material selection, the saw wire remains in the planned cutting plane even when it enters the saw strip, so that the edges of the wafers are not damaged. 
     In the embodiment shown in FIG. 2, saw strip 1 consists of a layered, structured composite body. Section 3, which adjoins the crystal, consists of a material whose hardness is equal or similar to the hardness of crystal 2. Section 3 preferably consists of a glass or silicon shell that is matched to the shape of the crystal, if the crystal consists of silicon. This shell is adjoined by at least one further section 4 which is made of a material that is significantly softer than the material of section 3. It is particularly preferable for section 4 to consist of graphite or to have a similar hardness to that of graphite. The sections of the layered, structured composite body are preferably adhesively bonded to one another. Using a saw strip of this nature avoids the formation of flutes. It is also particularly simple to break the cut wafers from the saw strip, because after the wafers have been cut off, the join to the saw strip which remains and is formed by a section of the saw strip is comparatively soft and is suitable as a desired breaking position. 
     In accordance with the embodiment illustrated in FIG. 3, saw strip 1 is designed in cross section as a hollow section. The hollow section may be of any desired shape, for example rectangular, circular or polygonal in some other shape. After the wafers have been cut off, the hollow section provides that a comparatively narrow web 5, by which the wafers remain joined to the saw strip, remains behind. By breaking this join, the wafers can be freed. As a result, it is possible to avoid the need to return the saw wire along the cut produced when cutting off the wafers, thus avoiding damage to the wafers. 
     If a solid saw strip is used, the join to the saw strip which remains after the wafers have been cut off is comparatively wide, so that it is not possible to prevent a wafer from being damaged when the join is broken. By contrast, a remaining web which is narrow due to the hollow section is particularly suitable as a desired breaking position. 
     The invention also relates to a saw strip which is designed, in accordance with FIG. 2, as a composite body and which also has a hollow section in accordance with FIG. 3. 
     Thus, while only a few embodiments have been shown and described, it is obvious that many modifications may be made thereunto without departing from the spirit and scope of the invention.