Method and apparatus for manufacturing semiconductor wafers and cutting wire apparatus for use therein

Wafers are manufactured from an ingot of semiconductor material by first machining a front face of the ingot to provide a substantially planar reference surface and then slicing the ingot using looped cutting wire apparatus. The cutting wire apparatus includes a looped cutting wire and at least two drive rollers over sectors of which the cutting wire is wrapped. Each drive roller is provided with its own drive motor which has shunt motor characteristics. The speed and torque of each motor is adjustable relative to the load applied to its drive roller by the cutting wire such that all of the drive rollers participate in driving the cutting wire with substantially the same reliability against slippage so that wear is distributed uniformly over all of the drive rollers.

BACKGROUND OF THE INVENTION 
This invention relates generally to methods and apparatus for manufacturing 
semiconductor wafers and cutting wire apparatus for use in such methods 
and apparatus. 
The particular properties of hard, brittle non-metallic semiconductor 
material having a Vickers hardness of up to HV 15,000 N/mm.sup.2 result in 
rigorous demands being placed on the cutting or slicing process by which 
wafers are manufactured from the material. 
In the manufacture of semiconductor wafers, elongated bars or ingots are 
cast from or pulled out from a melt of the semiconductor material. Further 
processing of the material requires a slice-by-slice separation of these 
ingots into wafers. The separation process is characterized by at least 
the following two requirements: 
(a) cutting losses must be minimized since the material is very expensive 
due to the complex manner in which it is obtained in highly pure form, 
e.g., the width of the cut should be significantly less than the thickness 
of the wafer, i.e., about 1 mm; and 
(b) the sliced wafers should have surfaces which are as planar and as 
parallel to each other as possible. 
Regarding the manner of cutting or slicing, arrangements in which looped 
cables are driven to cut through material have been known in principle for 
many years. For example, such arrangements have been used to cut rocks 
into square blocks. Generally, the moving cable is pulled against the 
workpiece while an abrasive agent and coolant-lubricant is applied whereby 
the cable slices through the workpiece. Although the cutting capacity of 
such arrangements is relatively modest, they are still in use today in 
quarries and in other applications in which precise cutting tolerances are 
not required. 
Significant improvements in productivity of looped cable cutting processes 
has been achieved by providing abrasive agent directly onto the cable 
either in the form of discrete elements, such as by clamping a sleeve 
having an abrasive outer surface firmly onto the cable, or by fixing the 
abrasive agent onto the cable itself. 
Since the precision of the cut obtained by a cutting cable as well as the 
cutting capacity of the cable both increase with increasing cable tension, 
cables having higher tensile strength are now used in many cable cutting 
applications. Indeed, traditional hemp rope no longer plays a role in 
modern industrial practice. 
The present state of technology is such that it is quite possible to cut or 
slice very hard materials by means of cutting wire arrangements in which 
diamond coated wires or wires which function to carry loosely added 
cutting medium are used. A serious problem, however, arises from the 
demand for a minimum cutting width, such as in the case of manufacturing 
semiconductor wafers. In particular, whereas wires having a diameter as 
small as 1 mm enable the requisite tensile and cutting forces to be 
transferred to the wire using conventional drive mechanisms, the cutting 
width required in slicing wafers from ingots of semiconductor material 
necessitates reducing the wire diameter to only a few tenths of a 
millimeter. Such extremely thin cutting wires can only be used, however, 
if the tensile strength of the wire is approached in receiving the tensile 
and cutting forces. In this respect, the design of the arrangement by 
which the driving forces are applied to the cutting wire assumes special 
significance, i.e., the drive arrangement must be designed so that the 
tension in the wire, which is limited by the strength and thickness of the 
wire, will generate a maximum cutting force. 
Countervailing considerations exist in connection with the tension and 
no-load forces acting on a cutting wire during a slicing operation. On the 
one hand, the tension and no-load forces must be sufficient to effect 
sliding of the wire within the cut being formed in the workpiece in order 
to complete a separation process. On the other hand, however, the same 
tension and no-load forces must not effect sliding of the wire over any of 
the drive components since this would cause the wire to cut the drive 
component itself, especially in the case where extremely thin cutting 
wires are used. This problem is magnified due to the widely fluctuating 
magnitude of the friction between the cutting wire and the workpiece 
and/or between the cutting wire and the drive mechanism. 
The requirements described above are simultaneously achieved by providing 
that the extent to which the cutting wire wraps around sectors of the 
drive rollers of the drive mechanism on the driving side of the wire is 
sufficiently large. A large wrap-around curvature is generally obtained by 
providing several drive rollers over sectors of which the cutting wire is 
wrapped. However, the provision of several drive rollers in order to 
distribute forces over their respective sectors is useful on a practical 
basis only if the driving effect of the individual rollers are precisely 
adjusted with respect to each other. A proportionate division of torque 
resulting from an arrangement in which a common motor drives all of the 
drive rollers is not satisfactory since expansion slip results in 
unavoidable wear, particularly in the partial load region and primarily on 
the first drive roller. Such wear increases the maintenance intervals and, 
therefore, the down time of the machine. Moreover, a mechanical coupling 
of the drive rollers with each other is not practical using presently 
available technology. For example, a mechanical coupling of the drive 
rollers, such as by a toothed gear arrangement, limits the speed of 
rotation of the drive rollers, due to lubrication and other 
considerations, to speeds well below those required for wire cutting 
processes. The division of torque between the drive rollers obtained by 
friction couplings is not satisfactory since slippage between the drive 
rollers is unavoidable. Such slippage will of course be transmitted to the 
drive rollers wrapped by the cutting wire thereby resulting in increased 
wear of one or more of the drive rollers. 
To the present, no solution has been found to the problems discussed above, 
and for these reasons looped cutting wire arrangements for use in 
applications where large cutting forces, minimum cutting widths and high 
precision are critical have not progressed beyond the experimental stage 
and have not been practical in industrial processes. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide new and 
improved methods and apparatus for manufacturing semiconductor wafers 
using looped cutting wire arrangements. 
Another object of the present invention is to provide new and improved 
drive apparatus for looped cutting wire arrangements which facilitate 
their use in cutting or slicing operations in which large cutting forces, 
minimum cutting widths and/or high precision are required. 
A further object of the present invention is to provide new and improved 
methods and apparatus for manufacturing semiconductor wafers including a 
substantially planar reference surface. 
Briefly, in accordance with the present invention, these and other objects 
are obtained by providing in looped cutting wire apparatus including a 
looped cutting wire and at least two drive rollers, a number of motors 
corresponding to the number of drive rollers, with each motor driving a 
respective one of the drive rollers, each motor including means for 
adjusting its rate of rotation and angular torque characteristics, either 
electrically or mechanically, in a manner so that the total force applied 
by the cutting wire to the individual drive rollers is distributed in a 
manner such that, 
(a) the individual drive rollers participate in the driving of the cutting 
wire with substantially the same reliability against slippage with respect 
to the cutting wire so that the driving force transmitted by each of the 
rollers is maximized; and 
(b) wear caused by expansion slip is distributed substantially uniformly 
with respect to all of the drive rollers. 
With regard to the second of the two requirements for semiconductor wafer 
manufacture mentioned above, i.e., for planar parallel wafer surfaces, the 
use of a looped cutting wire to slice wafers from an ingot cannot achieve 
a precisely planar surface since fluctuating process forces and the 
non-uniform cutting capacity of the cutting wire due to continuing wear 
and tear cause the wire to deviate in direction during the cutting 
operation. In order to provide the wafer with a precisely planar reference 
surface to enable the other wafer surface to be precisely planed in 
parallel relationship with the reference surface in a subsequent 
processing step, the front face of the ingot is machined prior to slicing 
the wafer to provide a substantially planar surface. A wafer including the 
previously planed surface is then sliced from the ingot by the looped 
cutting wire.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings wherein like reference characters designate 
identical or corresponding parts throughout the several views, and more 
particularly to FIG. 1, looped cutting wire apparatus in accordance with 
the invention, generally designated 10, is illustrated. Generally, a 
looped cutting or sawing wire 1 is driven by drive rollers 3 in the 
direction of arrow A and engages a cylindrical ingot 2 of semiconductor 
material as the ingot advances in a radial or transverse direction, 
designated by arrow B, to form a cut C in the ingot. 
The looped cutting wire 1 is guided by guide rollers 4 which do not affect 
the tensile force acting on the wire 1. In accordance with the invention, 
the cutting wire 1 is driven by three drive rollers 3, each of which is 
driven by its own drive motor, schematically shown at 3a. It is desireable 
to maximize the angle or length of the sector of each drive roller 3 
around which the cutting wire 1 wraps, i.e., the total wrap-around angle. 
To this end, the three drive rollers 3 are arranged close to each other 
with their parallel axes of rotation located at the corners of an 
equilateral triangle. 
Still referring to FIG. 1, the tension force acting on the wire 1 in a 
segment extending between adjacent rollers is designated S. Due to the 
process forces created during operation, a frictional force R acts on the 
cutting wire 1 within cut C of ingot 2. The frictional force R represents 
the difference in the magnitude between the larger tension force S.sub.1 
on the drive side of the cutting wire loop and the smaller tension force 
S.sub.4 on the no-load side of the cutting wire loop. The no-load tension 
force S.sub.4 is maintained at a constant magnitude, such as through a 
weight-and-spring mechanism 5. 
The magnitude of the tension forces S acting on the segments of the cutting 
wire 1 between successive drive rollers 3 are intermediate of the tension 
forces S.sub.4 and S.sub.1 and are of diminishing value in the direction 
of run of wire 1. In particular, the magnitude of the tension force 
S.sub.2 in the section of wire 1 between drive rollers 3.sub.1-2 and 
3.sub.2-3 is less than the magnitude of S.sub.1 while the magnitude of 
tension force S.sub.3 in the section of wire 1 between drive rollers 
3.sub.2-3 and 3.sub.3-4 is less than the magnitude of tension force 
S.sub.2, but greater than the no-load tension force S.sub.4. The actual 
values of the tension forces S.sub.2 and S.sub.3 depend upon the extent to 
which each of the drive rollers participates in driving the cutting wire 
1. In order to function in accordance with the invention, the torque and 
speed of each of the drive motors 3a are adjusted to provide a reliable 
friction force transmission from its associated drive roller to the 
cutting wire to eliminate slippage between them to thereby maximize the 
participation of each of the drive rollers in driving the cutting wire. 
Referring to FIG. 2, the graphical illustration shows a manner of 
calculating the desired intermediate tensions S.sub.2 and S.sub.3 in order 
to provide for a substantially uniform distribution of the difference 
between the driving tension force S.sub.1 and the no-load tension force 
S.sub.4 (i.e., the friction force R) over the three drive rollers 
3.sub.1-2, 3.sub.2-3, and 3.sub.3-4. 
Referring to FIG. 2, quadrant I may be considered to represent the drive 
arrangement as a whole, i.e., the driving tension force S.sub.1 in wire 1 
as it approaches the first drive roller 3.sub.1-2, the no-load tension 
force S.sub.4 acting on the wire as it leaves the last drive roller 
3.sub.3-4, and the intermediate tension forces S.sub.2 and S.sub.3. As 
noted above, the no-load tension force S.sub.4 is maintained constant 
under all operating conditions. It will be understood that if the cutting 
wire does not engage the workpiece, all of the tension forces, including 
S.sub.1, will be of magnitudes equal to that of tension force S.sub.4 and 
the operating condition of the cutting assembly would be designated by the 
dot-dash line 6. However, when the cutting wire engages the workpiece, a 
frictional force R is introduced in the cutting wire and a force of equal 
magnitude must be generated by the drive apparatus in the form of a total 
peripheral force, designated U.sub.ges in FIG. 2, which increases the 
driving tension force S.sub.1 relative to the no-load tension force 
S.sub.4 This total peripheral force U.sub.ges, which is equal to the 
difference between the drive tension force S.sub.1 and the no-load tension 
force S.sub.4, can on the one hand be as large as the frictional forces 
produced by the drive rollers as determined by the length of the wrapped 
sectors of all of the driving rollers. The extreme case wherein S.sub.1 is 
at the slide or slip limit critical for the force transmission is 
represented by the line designated 7 in quadrant I. 
On the other hand, avoidance of sliding of the cutting wire with respect to 
any one of the drive rollers, and the damage to the roller which would 
result therefrom, is ensured only if the frictional limit of each 
individual roller is not exceeded. To illustrate this point, the operating 
state of the three drive rollers 3.sub.1-2, 3.sub.2-3 and 3.sub.3-4 are 
represented in quadrants II, III and IV of FIG. 2. The slide limit 
represented by the solid lines in each of the quadrants II-IV is of 
significantly lower slope than the slope of the solid line 7 in quadrant I 
since by using a critical frictional value of identical magnitude for all 
of the drive rollers in the first quadrant, the total wrap-around angle of 
all three rollers is taken into consideration. On the other hand, in each 
of the three other quadrants, only that fraction of the total wrap-around 
angle represented by the angle of the sector of the particular drive 
roller of that quadrant is taken into account. 
It is evident from the foregoing that the peripheral forces U acting at 
each of the individual drive rollers may become only as large as the 
no-load tension force S.sub.4. The sum of the peripheral forces U.sub.1-2, 
U.sub.2-3 and U.sub.3-4 produced at the individual drive rollers 
3.sub.1-2, 3.sub.2-3 and 3.sub.3-4 respectively, represents the total 
peripheral force U.sub.ges produced by the drive mechanism. For optimum 
operation in accordance with the invention, the distribution of the total 
peripheral force U.sub.ges between the individual components U.sub.1-2, 
U.sub.2-3 and U.sub.3-4, independent of the no-load tension force S.sub.4, 
should have a specific relationship. It also follows that in order to 
utilize a greater total peripheral force U.sub.ges, the no-load tension 
force S.sub.4 must be increased. The extent to which the no-load tension 
force S.sub.4 can be increased is of course limited to the point at which 
the driving tension force S.sub.1 reaches the tensile strength of the 
cutting wire. 
In order to obtain a condition in which all three of the drive rollers wear 
uniformly and at the same time enable the three drive motors to have 
substantially identical capacity, the total peripheral force U.sub.ges 
should be divided into three individual components of equal magnitude. 
However, this condition can only be approximated under actual operating 
conditions. 
For these reasons, the three drive rollers are arranged in a 
non-symmetrical manner as illustrated in FIG. 1 to thereby increase the 
wrap-around angle over which the cutting wire extends around the third 
drive roller 3.sub.3-4. This in turn increases the otherwise reduced 
magnitude of the peripheral force U.sub.3-4 provided by the third drive 
roller 3.sub.3-4. By increasing the wrap-around angle of the third drive 
roller 3.sub.3-4, the wrap-around angle of the first drive roller 
3.sub.1-2 is reduced which similarly results in reducing the otherwise 
larger peripheral force provided by the first drive roller. 
Thus, an arrangement in accordance with the invention includes means for 
individually and independently adjusting the peripheral forces provided at 
each of the three drive rollers by varying the characteristics of the 
particular motor which drives the same. This step is preferably 
accomplished in a simple manner without the requirement for measurement 
and/or control apparatus. 
In accordance with the invention, the division of the total peripheral 
force in a substantially uniform manner over the various drive rollers is 
accomplished through adjusting the electromechanical properties of the 
individual motors relative to the particular force transmission 
requirements. 
Referring to FIG. 3, the rotational speed (n) to torque (M) characteristics 
of each of the three drive motors 3a are plotted in quadrant I. It is 
important that each of the motors have shunt motor characteristics since 
the motor would otherwise operate at excessively high speeds at no-load 
conditions. The rotational speed of each motor multiplied by the 
associated roller radius provides the peripheral speed of the drive roller 
which is to be equated with the wire speed (quadrant IV). 
The moment of each motor divided by the associated roller radius yields the 
peripheral force provided by that drive roller (quadrant II). Both of 
these inter-relationships are linear and with appropriate scaling, the 
same straight line can extend within both the second and fourth quadrants. 
Since all three drive rollers are coupled by the common cutting wire, the 
peripheral speed of all three drive rollers for a given operating 
condition must be identical. Since in the illustrated embodiment, all of 
the drive rollers have the same diameter, all of the three motors will 
therefore run at the same rotational speed. As illustrated in quadrant III 
of FIG. 3, the peripheral forces U.sub.1-2, U.sub.2-3 and U.sub.3-4, each 
of which can be individually located on the abscissa, can be plotted as 
the sum U.sub.ges =U.sub.1-2 +U.sub.2-3 +U.sub.3-4 on the ordinate. The 
optimum peripheral force distribution determined according to FIG. 2 can 
then be represented as a set of straight lines in the third quadrant of 
FIG. 3. 
The relationship between U.sub.1-2, U.sub.2-3 and U.sub.3-4 should maintain 
a specific value independent of the total peripheral force U.sub.ges. The 
torques provided at the drive rollers by the respective peripheral forces 
acting over the radius of the drive rollers should become effective at 
substantially the same rotational speed. From this fact, the adjustment of 
the torque and peripheral speed characteristics of each of the motors can 
be obtained in the first quadrant of FIG. 3 according to the relationship 
represented in quadrant III. 
As an example, a driving mechanism for a cutting wire apparatus comprising 
three drive rollers with specified boundary conditions can transmit a 
total peripheral force of nearly 8N without adjustment of the individual 
drive motors in accordance with the invention. However, by adjusting the 
speed and torque characteristics of the individual drives, the total 
peripheral force can be increased to nearly 10N. 
Whereas the wear of the drive rollers of a three-roller arrangement, 
without adjustment in accordance with the invention, is distributed in a 
manner such that the first roller exhibits 37.7% of the wear, the second 
roller exhibits 43.2% of the wear, and the third roller exhibits 19.1% of 
the wear. Upon adjustment of the speed and torque characteristics of the 
individual drive motors in accordance with the invention, the distribution 
of wear independent of the actually transmitted force is 34.0% for the 
first drive roller, 38.9% for the second drive roller, and 27.2% for the 
third roller. The substantially uniform distribution of wear obtained in 
accordance with the invention becomes even more important where the load 
decreases to about 70 percent of its maximum value in which case, without 
adjustment in accordance with the invention, 46.6% of the wear is 
exhibited by the first roller and 53.4% of the wear is exhibited by the 
second roller, while the third drive roller is not subjected to any wear. 
If the load decreases to less than one-quarter of its maximum value, the 
wear is exhibited substantially exclusively on the first drive roller. 
Thus, the substantially uniform distribution of wear becomes particularly 
important since cutting wire arrangements of the type with which the 
invention is concerned are generally operated such that the drive tension 
force is somewhat less than the maximum and in the occasional case where 
greater loads act on the wire, the slippage problem is generally handled 
by appropriate dimensioning of the drive arrangement components. 
As noted above, another requirement in the manufacture of wafers of 
semiconductor material is that the wafer should have surfaces which are as 
planar and as parallel to each other as possible. This requirement becomes 
more difficult to achieve where the wafers are obtained by slicing from an 
ingot using looped cutting wire apparatus. The cutting wire tends to 
migrate from its intended path during the slicing operation under the 
effect of process forces as well as the non-uniform cutting capability 
exhibited by the tool as it is subjected to wear and tear. The resulting 
surfaces of the wafer cut from the ingot are therefore neither planar nor 
parallel to each other, but are rather bowed or warped. 
Referring to FIG. 4, it is seen that the bowing or warping of a wafer 
cannot be corrected even through additional conventional processing steps. 
The separated wafer 20 (FIG. 4a) has two uneven surfaces 21 and 22 which 
give rise to a warp which can be up to a few hundredths of a millimeter. 
If the surface 22 of wafer 20 is clamped, such as by suction, to a planar 
table (FIG. 4b), the free surface 21 can be machined to a substantially 
planar state (FIG. 4c) so that two substantially planar and parallel 
surfaces 21' and 22 exist. However, once the wafer 20 is unclamped, the 
surface 22 of the wafer which was clamped to the flat table will assume 
its original warped shape (FIG. 4d) due to its elasticity. Additional 
processing steps cannot rectify this problem. 
On the other hand, the problem can be solved through an integration of 
slicing and planing steps. In this connection, reference is made to DE-OS 
No. 36 13 132 of the applicant. 
Referring to FIG. 5, the non-planar front face 25 of the ingot remaining 
from a previous slicing operation (stage 1) is planed (stage 2) by a 
suitable machining process. Although grinding is the preferred technique, 
other processes can be used to obtain planar surface 25', such as milling, 
turning, and electrolytic and erosive cutting. A wafer 26 is then formed 
by slicing the ingot (stage 3) by means of cutting wire apparatus in 
accordance with the invention. This leaves a new non-planar face 25 in the 
ingot as well as a non-planar surface 27 on the wafer 26. However, since 
the surface 25' of the separated wafer 26 is substantially planar, it can 
function as a planar reference surface and be clamped to a flat table 
without any warp whereupon the opposite surface 27 can be machined to a 
surface 27' which is substantially planar and parallel to the planar 
reference surface 25' (stage 4). When the wafer 26 is then removed from 
its clamping site, it will no longer warp. The non-planar front face 25 of 
the ingot is then machined to a planar condition preparatory to slicing 
the next wafer. 
It is not important in the method described above whether the front face of 
the ingot is perpendicular to the ingot axis or in a slanted position. 
Thus, an arrangement in which the looped cutting wire is driven by a 
mechanism in accordance with the foregoing description, and wherein the 
wafer separation and surface machining steps are in accordance with the 
method described above in connection with FIG. 5, meets both of the 
initially stated requirements for manufacturing semiconductor wafers. It 
is most efficient for the apparatus to comprise a single unit including a 
combination of looped cutting wire apparatus and surface machining 
apparatus. It will also be understood that for purposes of savings in 
time, it is preferred that the machining of the front face of the ingot 
and the slicing of the wafer not occur sequentially, but, rather, overlap 
in time with each other. 
Reference will now be made to FIGS. 6-10 in which a combined 
slicing-grinding unit is illustrated for performing the above-described 
method. 
Referring to FIG. 6 in which the apparatus and ingot 2 are illustrated in 
their initial relative positions, the ingot 2 from which a wafer is to be 
sliced generally has a non-planar front face 25. The ingot is fixed in a 
clamp of the grinding device so that its front region projects into an 
annular grinding body 11 of a rotating abrasive-cup wheel 12 so that the 
surface 25 is positioned entirely within the annular grinding body 11. The 
ingot 2 begins its advance movement (FIG. 7) in a substantially radial or 
transverse direction whereupon the grinding body 11 removes material at 
the end of the ingot so that the front face of the ingot begins to obtain 
a planar configuration despite its original geometry. 
As the advance movement of ingot 2 continues, the ingot engages the cutting 
wire 12 (FIG. 8) whereupon the cutting wire 12 begins to cut or slice 
through the ingot 2. The cut is performed to form a wafer having a 
thickness which is somewhat greater than that designed for the finished 
wafer. 
With continued advancement of the ingot (FIG. 9), the grinding body 11 
becomes disengaged from the front surface of the ingot while the slicing 
operation is still in progress. The bottom face 25' of the ingot is now 
substantially planar. Upon completion of the advancement of the ingot 
(FIG. 10), the wafer 13 is completely sliced from the ingot and includes a 
substantially planar reference surface 25'. On the other hand, since the 
cutting wire 12 has drifted under the effects of the processing forces as 
described above, the opposite surface 27 of the wafer 13 is neither planar 
nor parallel to surface 25'. Similarly, the newly formed front surface 25 
of the ingot has a non-planar geometry. At the end of the slicing process, 
the wafer 13 is transported from the processing zone. This transport is 
particularly simple in the case where the apparatus utilizes a looped 
cutting wire since there is nothing to obstruct the movement of the wafer 
from the processing zone. The reference surface 25' of wafer 13 is then 
clamped to a planar clamping table whereupon the non-planar surface 27 is 
machined to a planar geometry substantially parallel to surface 25'. This 
last machining step is preferably accomplished by grinding. 
Obviously, numerous modifications and variations of the present invention 
are possible in the light of the above teachings. Therefore, it is to be 
understood that within the scope of the claims appended hereto, the 
invention may be practiced otherwise than as specifically disclosed 
herein.