Patent Application
20080011134
The invention is described in more detail below on the basis of figures:
The invention is thus directed to by a sawing strip 1 for fixing a substantially cylindrical workpiece 2 when cutting off slices from this workpiece 2 with a wire saw, the sawing strip 1 having a first face 4, which is concavely curved perpendicular to its longitudinal direction and is intended for connecting to the workpiece 2, a second face 5, which lies opposite the first face 4 and is intended for connecting to a mounting plate, and two side faces 6, 7, which connect the first face 4 and the second face 5, the two edges 8, 9 of the sawing strip 1 at which the side faces 6, 7 meet the first face 4 being at a distance a from each other, an imaginary line 10 on the first face 4 marking its minimum distance d from the second face 5, and the side faces 6, 7 being at a distance b, measured at the height of the line 10 and perpendicular to the distance d, wherein the distance b is less than the distance a.
The object is also achieved by a method for simultaneously cutting off a multiplicity of slices from a substantially cylindrical workpiece, the workpiece, connected to a sawing strip, and a wire frame of a wire saw performing with the aid of a feeding device a relative movement directed perpendicular to the longitudinal axis of the workpiece, by which the workpiece is guided through the wire frame, wherein a sawing strip according to the invention is used.
A sawing strip is an elongate strip which is produced from a suitable material, for example from graphite, glass, plastic or the like, and is intended for fixing a workpiece during the wire sawing process. A sawing strip according to the prior art is distinguished by a substantially rectangular cross section, but the face which is intended for fixing the cylindrical workpiece has a concave curvature corresponding to the workpiece, so that the form of the sawing strip is adapted to the form of the workpiece. According to the invention, in a way similar to according to the prior art, the fixing of the workpiece on the sawing strip is preferably performed by cementing or gluing it on. The adaptation to the form of the workpiece achieves an adherend surface area that is as large as possible, and therefore a connecting force between the workpiece and the sawing strip that is as great as possible. The form of a sawing strip can be generally described as follows:
First it is defined that the longitudinal direction of the sawing strip 1 is understood as meaning the direction parallel to the longitudinal axis 3 of the workpiece 2 connected to it. As described, the sawing strip 1 has a first face 4, which is concavely curved perpendicular to its longitudinal direction and is intended for connecting to the workpiece 2. Lying opposite the first face 4 is a second face 5, which is intended for connecting to a mounting plate (not represented). The faces 4 and 5 are connected by two side faces 6, 7. The two edges 8, 9, at which the side faces 6, 7 meet the first face 4, are at a distance a from each other. In the central region of the first face 4, a line 10 can be defined, running in the longitudinal direction through all the points on this face that are at a minimum distance d from the second face 5. Expressed another way, this line 10 runs in the longitudinal direction (i.e. parallel to the longitudinal axis 3 of the workpiece connected to the sawing strip) where the sawing strip 1 has the smallest thickness, this minimum thickness being synonymous with the distance d. The line 10 lies at the location at which the wire frame leaves the workpiece 2 at the end of the sawing process. A further dimension that is characteristic of the sawing strip is the length b of a line which intersects the line 10, is perpendicular to the distance d and the end points of which lie on the side faces 6, 7. Expressed another way, b is the distance between the side faces 6, 7, measured at the height of the line 10.
A sawing strip 1 according to the invention (
It is preferred for the relationship 0.5·a<b<0.96·a to apply. It is particularly preferred for the relationship 0.6·a<b<0.75·a to apply.
By contrast, in the case of a sawing strip according to the prior art (
The use of the sawing strip according to the invention surprisingly leads to a significantly reduced waviness in the sawing-out region. It is not clear on what this effect is based. However, the following observation was made in the course of the investigations carried out in connection with the present invention:
During the sawing operation, sawing suspension is applied to the wire frame. The portions of wire transport the sawing suspension at high speed in the direction of the workpiece and into the sawing gaps, where it displays its abrasive action. As soon as the wire frame penetrates into a sawing strip according to the prior art, which has a substantially rectangular cross section, it can be observed that some of the sawing suspension is flung back far in the direction opposed to the movement of the wire by the impact with the straight side faces of the sawing strip, some of the sawing suspension that is flung back again hitting the portions of wire of the wire frame that are running in the direction of the workpiece. By contrast, when sawing into a sawing strip according to the invention, it is observed that some of the sawing suspension is flung back substantially vertically upward, but not in the opposed direction, by the impact with the sloping side faces of the sawing strip. The sawing suspension flung back onto the wire frame possibly causes uneven application of sawing suspension to the portions of wire or uncontrolled lateral deflection of the portions of wire in the longitudinal direction of the workpiece. It is conceivable that the reduction in the waviness in the sawing-out region is attributable to the extensive elimination of this effect. However, other explanations are also conceivable.
The sawing strip according to the invention is preferably symmetrical to a plane running through the longitudinal axis 3 of the workpiece 2 and the line 10. It is likewise preferred for the side faces 6, 7 to be planar faces. It is also preferred for the second face (5) to be a planar face.
The use of the sawing strip according to the invention is particularly advantageous when working with a sawing suspension containing particles of hard material, which is sprayed onto the wire frame with the aid of at least one nozzle unit during the cutting-off of slices from the workpiece. The sawing strip according to the invention may, however, also be used when using a sawing wire with bound particles of hard material, to which a liquid cooling lubricant is applied with the aid of at least one nozzle unit.
A nozzle unit refers to all the nozzles which apply sawing suspension or cooling lubricant to the wire frame on one side of the workpiece. A nozzle unit may for example be an elongate slot-shaped nozzle running parallel to the axes of the wire guiding rollers and to the axis of the workpiece, which is preferred. If a number of such nozzles are provided on one side of the workpiece above the wire frame, these nozzles together form a nozzle unit. A nozzle unit may also comprise a preferably linearly arranged row of individual nozzles, this row running parallel to the axes of the wire guiding rollers and to the axis of the workpiece and each nozzle having for example a round cross section and applying sawing suspension or cooling lubricant to a portion of wire of the wire frame.
If a sawing suspension is used, it is preferred to reduce the flow of the sawing suspension at the end of the cut, as disclosed in DE 102005007312 A1.
It is likewise preferred to increase the temperature of the sawing suspension over the last 10% of the cutting distance, in order to reduce the viscosity of the sawing suspension and consequently the back pressure gradient. The temperature of the sawing suspension is preferably increased by up to 20 K over the last 10% of the cutting distance.
The cutting distance is the distance covered altogether in the workpiece by the wire frame during the entire cutting operation, that is to say the entire feeding displacement in the workpiece. In the case of workpieces which have the form of a circular cylinder, the cutting distance corresponds to the diameter of the workpiece.
The best effect is achieved by a combination of the use of a sawing strip according to the invention with an increase in the temperature of the sawing suspension and simultaneous reduction of the flow of the sawing suspension at the end of the cut.
In order to investigate the effect of the use of the sawing strips according to the invention, a considerable number of cylindrical pieces of monocrystalline silicon ingot with a diameter of 300 mm and a length of 80 mm to 355 mm were cut by means of a commercially available four-roller wire saw into slices with a thickness of approximately 930 μm. A sawing suspension which contained particles of hard material, comprising silicon carbide, suspended in dipropylene glycol was applied to the sawing wire. At the end of the cut, the amount of sawing suspension was reduced, as described in DE 102005007312 A1. In half of the sawing operations, a sawing strip according to the prior art (comparative example) was used and in the other half a sawing strip according to the invention (example) was used.
On each of the silicon slices or wafers produced in this way, the waviness was determined in the sawing-out region. Waviness refers to dimensional deviations (peak to valley) in the spatial wavelength range of 2 mm to 10 mm, without the thickness variation component. The sawing-out region is defined as the last 50 mm of the cutting distance.
The waviness in the sawing-out region is determined as follows:
The measuring head of the measuring device, fitted with a pair of capacitive distance measuring sensors (one for the front side and one for the rear side of the silicon wafer), is guided over the front side and the rear side of the silicon wafer along the line running in the cutting direction through the center of the wafer. The cutting direction refers to the direction of the relative movement between the workpiece and the wire frame during the wire sawing operation. In this process, the distance between the sensors and the front or rear side of the silicon wafer is measured and recorded every 0.2 mm. The surface roughness in the spatial wavelength range of <2 mm is eliminated by a lowpass filter (Gaussian filter). After these steps, the evaluation curves for the front side and the rear side of the silicon wafer are available.
To determine the waviness in the sawing-out region, a window of 10 mm in length is then allowed to run over the last 50 mm, seen in the cutting direction, of each of the two evaluation curves for the front side and the rear side (rolling boxcar filtering). The maximum deviation (peak to valley) within the window is referred to as the waviness at the location of the center of the window. The greatest of all waviness on the front side and the rear side, determined over the last 50 mm of the evaluation curves, is referred to in the following comparative example and in the example as the waviness of the sawing-out region.
Symmetrical sawing strips according to the prior art were used, the distances a and b (see
Symmetrical sawing strips according to the invention with a=170 mm, b=114 mm and d=14.5 mm were used. Altogether, likewise approximately 1000 silicon wafers were produced in this way and the waviness of the sawing-out region was determined as prescribed above.
The results of these measurements were statistically evaluated. The statistical evaluation is represented in
The application range of the invention extends to all sawing methods in which cylindrical workpieces are divided up into a multiplicity of slices by means of a wire saw and with a sawing suspension being supplied, and for which a high degree of planarity and a low degree of waviness of the products are important. The invention is preferably used for the production of semiconductor wafers, in particular silicon wafers. The term “cylindrical” is to be understood as meaning that the workpieces have a substantially circular cross section, certain deviations, for example orientation notches or flats applied to the lateral surface, being immaterial.
While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 032 432.3 | Jul 2006 | DE | national |
