Method of producing semiconductor wafers

A method for producing semiconductor wafers is by a repeated sequence of grinding the end face of a monocrystal using a grinding tool and cutting a semiconductor wafer having a thickness from the monocrystal using a cutting tool, a grinding abrasion of a specified depth being produced during grinding and the semiconductor wafer being cut in a cutting plane which is as parallel as possible to the ground end face. The method includes (a) simultaneously grinding a part of the surface of an auxiliary body, to produce a ground surface of the auxiliary body and the end face of the monocrystal lying substantially in one plane and the thickness of the material abraded from the auxiliary body by grinding being substantially equal to the grinding abrasion; (b) cutting into the auxiliary body in the cutting plane using the cutting tool and producing a cut section which has a ground part and an unground part, and (c) determining the grinding abrasion, either (1) as the distance between the ground surface of the auxiliary body and the surface of the auxiliary body before grinding, or (2) as the difference in the thickness of the cut section in the unground part and the thickness of the semiconductor wafer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for producing semiconductor 
wafers by a repeated sequence of grinding the end face of a monocrystal 
using a grinding tool and cutting a semiconductor wafer from the 
monocrystal using a cutting tool, a grinding abrasion of a specified depth 
being produced during grinding, and the semiconductor wafer being cut in a 
cutting plane which is as parallel as possible to the ground end face. 
2. The Prior Art 
The grinding of the end face of the monocrystal produces a first flat side 
face on the semiconductor wafer. After the semiconductor wafer is 
separated from the monocrystal, this first flat side face is used as a 
reference face and the opposite side face is ground parallel to this 
reference face. The desired result is a semiconductor wafer having flat 
and parallel side faces. U.S. Pat. No. 4,967,461 describes how the 
grinding of the end face of the monocrystal eliminates unevenness which is 
attributable to deflections of the saw blade during the cutting of a 
semiconductor wafer and which would result in warped semiconductor wafers. 
Conventionally, the saw blade of an annular saw is used as the cutting 
tool. While the saw blade is working through the monocrystal, sawing 
forces occur which deflect the saw blade from the intended cutting plane 
situated parallel to the ground end face of the monocrystal. The actual 
cutting plane is therefore not completely flat, but slightly curved and 
this unevenness is reflected in the quality of the end face of the 
monocrystal. Before the next semiconductor wafer can be cut, the end face 
has to be ground flat again. 
The repeated sequence of grinding the end face of the monocrystal and 
cutting a semiconductor wafer from the monocrystal applies stresses both 
to the grinding tool and to the cutting tool. The condition of the 
grinding tool can be determined with a perthometer. However, this requires 
a fairly long shutdown time for the grinding machine since the 
investigation is carried out on the grinding tool and much time has to be 
expended on dismantling and reassembly and also on the readjustment of the 
grinding tool. Wear phenomena or changes in operating parameters may 
result in the deflection behavior of the saw blade altering over the 
course of time. These changes may alter the cutting pattern when a 
semiconductor wafer is being cut, and lead to a different cutting pattern 
from the cutting pattern that results when a subsequent semiconductor 
wafer is later being cut. Sensors were therefore developed to enable 
alterations in the cutting pattern to be observed and followed even during 
the cutting operation. Suitable sensors are described, for example, in 
U.S. Pat. No. 4,991,475. 
When the end face of the monocrystal is being ground, an attempt is made to 
keep the grinding abrasion to a minimum. The grinding abrasion is 
specified in a length unit. It corresponds to the distance between the 
ground end face and the highest material elevation on the unground end 
face. If the resulting grinding abrasion is too high, material is wasted 
and the yield of semiconductor wafers per monocrystal is reduced. If the 
grinding abrasion is too low, the unevenness on the end face of the 
monocrystal is not adequately removed and the semiconductor wafers 
produced are defective. 
A specified grinding abrasion can be set very accurately with the aid of 
the feed unit which moves the grinding tool and/or moves the monocrystal 
for the purpose of grinding. A predetermined required grinding abrasion 
may, however, soon prove to be too low, for example, because the magnitude 
of the deflection of the saw blade has increased during the course of the 
wafer production. On the other hand, the saw gap in the monocrystal due to 
the saw blade may become thinner, for example, as a consequence of wear of 
the saw blade. This wear will cause the result that, if the original 
grinding abrasion is maintained, more material would be ground from the 
end face of the monocrystal than necessary. 
Since the reference grinding abrasion and the necessary actual grinding 
abrasion may be different, there is an urgent requirement to be able to 
determine the grinding abrasion at least from time to time. However, this 
is difficult to do. One known method is to cut a semiconductor wafer from 
the monocrystal at regular intervals without the end face of the 
monocrystal being previously ground beforehand. The grinding abrasion is 
then measured directly from the thickness of the semiconductor wafer, 
provided the cutting conditions have not changed since the penultimate 
cutting of a semiconductor wafer. Constant cutting conditions may be 
assumed if sensors do not reveal any substantial alteration in the cutting 
pattern during the cutting of two consecutive semiconductor wafers. A 
disadvantage of this known method described above, in particular, is that 
the semiconductor wafer investigated becomes a reject and reduces the 
yield, because it lacks the flat reference face and cannot be used as 
intended. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide improvement 
in the production method for semiconductor wafers and, in particular, to 
provide a more advantageous method for determining the grinding abrasion. 
The present invention achieves the above object by providing a method for 
producing semiconductor wafers by a repeated sequence of grinding an end 
face of a monocrystal using a grinding tool and cutting a semiconductor 
wafer having a thickness from the monocrystal using a cutting tool, a 
grinding abrasion of a specified depth being produced during grinding, and 
the semiconductor wafer being cut in a cutting plane which is as parallel 
as possible to the ground end face of the monocrystal, which method 
comprises the steps of 
(a) grinding a part of a surface of an auxiliary body to produce a ground 
surface of the auxiliary body and the surface of the auxiliary body and 
the end face of the monocrystal lying substantially in one plane, and 
thickness of material abraded from the auxiliary body by grinding being 
substantially equal to the grinding abrasion; 
(b) cutting into the auxiliary body in the cutting plane using the cutting 
tool and producing a cut section which has a ground part and an unground 
part; and 
(c) determining the grinding abrasion, either (1) as the distance between 
the ground surface of the auxiliary body and a surface of the auxiliary 
body before grinding or (2) as the difference in thickness of the cut 
section in the unground part and the thickness of the semiconductor wafer. 
Thus, there are two method embodiments regarding the process steps (c)(1) 
and (c)(2) above for determining the grinding abrasion in the present 
invention. 
The grinding abrasion can be determined at intervals, for example, whenever 
a specific number of semiconductor wafers has been produced. However, it 
is also possible to determine the grinding abrasion for every 
semiconductor wafer produced. The grinding abrasion is preferably 
determined when the monitoring of the saw blade has revealed that the 
cutting conditions have not changed substantially during the cutting of 
two consecutive semiconductor wafers. Neither is the wafer production 
procedure delayed nor is the yield of semiconductor wafers impaired.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Turning now in detail to the drawings, FIG. 1 shows that the grinding can 
be carried out either before, or simultaneously with, the cutting of a 
semiconductor wafer. FIG. 1 shows not only the grinding of the end face of 
the monocrystal 1, but also shows the grinding of the surface of an 
auxiliary body 2. This surface is substantially in the same plane as the 
unground end face. The choice of type of auxiliary body is substantially 
unlimited. A possibility exists, however, to use a bar composed, for 
example, of carbon for this purpose, which is in any case conventionally 
used as an auxiliary in cutting semiconductor wafers. This bar is joined 
to the monocrystal, for example by means of an adhesive, and prevents a 
semiconductor wafer from being damaged during the cutting from the 
monocrystal. 
In addition to the end face of the monocrystal, a part of the surface of 
the auxiliary body is also ground with the grinding tool 11, the grinding 
abrasion in the case of the monocrystal being substantially equal to the 
grinding abrasion in the case of the auxiliary body. Consequently, the 
ground surface 3 of the auxiliary body and the ground end face 4 of the 
monocrystal are located in the same plane. 
As shown in FIG. 2, the distance C between the unground surface 5 of the 
auxiliary body 2 and the ground surface 3 of the auxiliary body 2 is equal 
to the grinding abrasion actually produced. In principle, this distance C 
can be measured by inspecting the depth of the step 6 produced during 
grinding. It is, however, preferable to use the semiconductor wafer 8 cut 
by the cutting tool 7 for this determination. The cutting plane 12 in 
which the cutting tool works through the monocrystal should be as parallel 
as possible to the ground end face 4. In this cutting plane, the auxiliary 
body 2 is also cut into by using the cutting tool so that a cut section 9 
is produced on the auxiliary body 2 on whose front side there is the step 
6. The grinding abrasion C actually produced is given by the difference in 
the thickness B of the cut section 9 and the thickness A of the auxiliary 
body. The thickness B is equal to the distance between unground surface 5 
of the auxiliary body and the back 10 of the cut section cut out by the 
cutting tool. 
In practice, after the cutting tool has been used, the join between the cut 
section and the remainder of the auxiliary body is parted using the 
grinding tool and the semiconductor wafer and the cut section are picked 
up by a wafer pickup means. Optionally, the thickness of the two objects 
is measured after cutting. Also multiple measurements are possible, such 
that a mean value for the thickness of the semiconductor wafer and for the 
cut section are determined in each case. An automated thickness 
measurement is preferred. 
The cut section 9 is also useful in other respects. Its ground surface 3 
can be investigated with a perthometers. On the basis of this 
investigation, it is possible to draw conclusions about the profile of the 
grinding tool and any wear of the grinding tool can be revealed as 
promptly as possible. 
According to a further embodiment of the method of the invention, it is 
possible to observe and to keep track of the deflections of the saw blade 
from the intended cutting plane situated parallel to the ground end face 
of the monocrystal and to determine on the basis thereof, a required 
grinding abrasion by calculation. The required grinding abrasion is equal 
as much as possible to that grinding abrasion which is just sufficient to 
achieve a flat end face by grinding. If the required grinding abrasion 
determined differs from that reference grinding abrasion to which the feed 
unit was originally set, then the feed unit is reprogrammed automatically 
or manually to a new reference required grinding abrasion. The grinding 
abrasion which then actually takes place is determined in the manner 
described above and compared with the new reference required grinding 
abrasion. 
While several embodiments of the present invention have been shown and 
described, it is to be understood that many changes and modifications may 
be made thereunto without departing from the spirit and scope of the 
invention as defined in the appended claims.