Method for fracturing crystalline materials

Abstract

This disclosure relates to a method for fracturing crystalline materials whereby thin wafers may be produced with minimum waste of the crystal.

Description

FIELD OF THE INVENTION

This invention relates to a method for fracturing solid materials, and more particularly to a method for fracturing a single crystal whereby thin wafers or slices having a desired thickness and fracture surface configuration are produced.

BACKGROUND OF THE INVENTION

In the manufacture of transistors and other solid state devices crystals or rods of semiconductor material are generally cut into thin slices or wafers by a saw blade for further processing. These slices are typically in the order of 0.010 to 0.015 inch thick which is about the same thickness as the cutting blade. In many cases the thickness of the slice is nearly the same as the cutoff wheel kerf loss which results in an immediate loss of up to 50 percent of the semiconductor material. Also the slicing operation is in itself time consuming and results in considerable scrap due to breakage of the slices.

SUMMARY OF THE INVENTION

The present invention is a method of fracturing a crystal (such as silicon or germanium) in a transverse manner in order to produce thin wafers.

The basic method for creating the desired fracture is by imparting a desired stress distribution to the solid which predetermines the direction of crack growth and then by initiating the fracture at the desired location.

The major advantages of this process are the realization of material savings in creating slices of semiconductor crystals and reducing manufacturing costs of making semiconductor devices.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a silicon crystal prepared for fracture.

FIG. 2 is a perspective view of the silicon crystal.

DETAILED DESCRIPTION OF THE INVENTION

One method that has been employed to create flat fracture surfaces of silicon rods is by application of a tensile load, shown as L in FIG. 1, to the material 1 and then the fracture is initiated at the predetermined fracture plane 2 by forcing a wedge 3 into a previously formed notch 4 on one or both sides of the material 1. Several items critical to the method include the method of applying the tensile load, shown as L in FIG. 1, the size and shape of the notch 4 and the wedge 3, the magnitude of the tensile load L and the force F on the wedge and, if only one wedge is used, the type of support 5 opposite the wedge. The method illustrated in FIG. 1 has been employed to create thin slices 6 from a semiconductor grade single crystal 1 of silicon. A flat 7 is cut on one side of the crystal and a notch 4 is cut across this flat perpendicular to the axis 8 of the rod as illustrated in FIG. 2. The blocks 9 for imparting the tensile load are adhered to the crystal 1 with mounting wax 10. The tensile load L is applied and then the wedge 3 is forced into the notch 4 to initiate the fracture. Various types of supports have been used including a wedge with an additional notch in the material (not shown). A spherical support 5 has given the best experimental results but other support configurations will also give satisfactory results.

Several methods for adhering the blocks to the crystal can be used. Similarly, several methods for making the notch can be used. One or more notches can be used with or without more than one wedge. Scribe lines on the surface of the crystal and in the notch can be used to aid in initiating and guiding the fracture. The geometry of the notch as well as the wedge can be varied. A flat on none, one, or more sides can be used. Impact loading to the wedge may also be used. In addition, it is possible to initiate the fracture without a precut notch by using a sharp wedge.

There are a large number of variations that may be applied to this method, for example the preloading force L described above may also be compressive, a bending moment, a torsional moment, shear or any combination of these loadings which may be applied to the crystal before and/or during fracture so long as the forces applied to the crystal surface produce an internal force in the crystal that acts substantially perpendicular to the desired fracture plane. Any loading configuration may be used to create a controlled stress pattern of the desired form in the crystal.

The wedge used in initiating the fracture may be substituted for in several fashions. In the static or quasi-static case, expanding materials can be placed in the notch and made to expand sufficiently in the notch to initiate fracture. Other mechanical and thermal methods may also be used. In dynamic cases, exploding wires, exploding materials, stress waves or impact loads can be introduced into the notch or crystal to initiate the fracture. This novel method is divided into three essential parts, the step of introducing a preselected stress concentration into the material, the step of applying an internal tensile stress acting normally upon the desired fracture plane, and the step of fracturing the crystal by application of predetermined forces acting substantially perpendicular to the predetermined fracture plane. Examples of specific modes of accomplishing the various steps of the method are as follows:

A. the step of introducing a preselected stress concentration into the crystal may be accomplished by:

1. A notch.

2. A scribe line.

3. Any physical deformation of the crystal that produces the desired stress concentration therein.

B. the step of applying an internal tensile stress acting normally upon the desired fracture plane may be accomplished by:

1. Tensile force.

2. Compressive force.

3. Shear force.

4. Bending moment.

5. Torsional moment.

6. Any combination of the foregoing forces that produces internal tensile force normal to the fracture plane.

C. the step of fracturing the crystal may be accomplished by:

1. A wedge, or wedges.

2. Expanding material in a notch.

3. Thermal expansions and shock.

4. An exploding wire.

5. A stress wave.

6. An impact load.

7. Other similar mechanical means or combinations of the above.

Other combinations of mechanical expedients can be employed to obtain the desired slices or wafers as will be readily appreciated by those skilled in the art once the force and stress relationships necessary to accomplish the method are realized.

Claims

What is claimed is:

1. A method for producing thin wafers from crystalline material characterized by:

the step of introducing a preselected stress concentration into a crystal along a line to establish a predetermined fracture plane that will produce a thin wafer whereby the location of the fracture initiation is predetermined;

the step of applying a continuing tensile stress acting normally upon the predetermined fracture plane; and

the step of initiating fracturing of the crystal while maintaining the application of tensile stress by application of a sudden acting fracturing force acting substantially perpendicular to the predetermined fracture plane whereby said applied fracturing force and tensile stress enables said thin wafers to be produced at said location predetermined by said introduced stress concentration.

2. The method according to claim 1 in which the stress concentration is accomplished by a mechanical expedient causing physical deformation of the crystal to produce a predetermined force concentration.

3. The method according to claim 1 in which the step of introducing the tensile stress internally in the material is accomplished by application of a loading mode to the crystal exterior surface.

4. The method according to claim 3 in which said loading mode is applied by application of at least one external force acting upon said crystal.

5. The method according to claim 1 in which the step of fracturing the crystal is accomplished by a force applied to the material by a mechanical expedient.

6. The method according to claim 5 in which said force applied by said mechanical expedient is by means of a wedge applied in the direction of the predetermined fracture plane.

7. A method for producing thin wafers from a crystalline rod characterized by the steps of:

providing a flat side on the rod, which flat is disposed normally to a line extending from the mid-point of said flat to the center of said rod;

notching said flat surface along a line where it is desired to initiate fracture of the rod;

applying tensile stress to the rod stock in an axial direction; and

applying a fracturing force into said notch acting substantially perpendicular to the longitudinal rod axis whereby the rod is fractured substantially transversely with respect to the longitudinal axis of the rod.

8. The method of claim 7 in which the fracture is initiated by driving a wedge into the notch and a support force is applied to prevent movement of the rod in a transverse direction.