Process for manufacturing semiconductor wafers having deformation ground in a defined way

A process for manufacturing semiconductor wafers having deformation ground in a defined way.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to semiconductor wafers having deformation 
ground in a defined way, and a method of producing such semiconductor 
wafers. 
2. The Prior Art 
In the production of semiconductor wafers, appreciable effort is expended 
on obtaining wafers which are as flat as possible. Only an extremely flat 
wafer surface ensures that the photolithographic application of electronic 
components to the wafer surface is achieved in a fault-free manner in the 
integration density which is now standard. Starting with the sawing of a 
wafer out of a rod-like single crystal and proceeding through to the 
surface finishing of the semiconductor wafer by lapping and polishing 
methods, the emphasis is therefore on obtaining semiconductor wafers 
having at least one plane surface. 
In the further course of the processing of a semiconductor wafer, however, 
method steps are generally necessary which impair the flatness of the 
semiconductor wafer achieved up to that point. Because of the circular 
wafer shape, a rotationally symmetrical deformation of the semiconductor 
wafer is frequently to be observed after a single-sided treatment of a 
wafer surface. Thus, for example, the application or creation of one or 
more material layers (for example, by epitaxy or oxidation) on a preferred 
wafer surface results in stresses in the semiconductor material which 
cause a rotationally symmetrical deformation of the semiconductor wafer. A 
similar deformation occurs if a semiconductor wafer is etched, doped or 
subjected to a so-called damage treatment (the creation of defects in the 
crystal lattice) on one side. A precise inspection shows a semiconductor 
wafer treated in this way to be bowed. 
A recognized measure of the deformation of a semiconductor wafer is the 
"warp." The warp specifies the difference between the maximum and the 
minimum distance of the center plane of the semiconductor wafer from a 
reference plane. It can be determined, for example, in a method of 
measurement in accordance with the U.S. Standard ASTM F 657-80. The 
one-sided treatment of a semiconductor wafer described above increased the 
warp. For ideally flat and plane-parallel wafer surfaces, the warp is 
equal to zero, unless the semiconductor wafer was already deformed in a 
defined way prior to the one-sided treatment so as to virtually or 
completely compensate for the additional rotationally symmetrical 
deformation. In the latter case, the warp would become smaller and the 
flat wafer geometry necessary for a fault-free photolithographic 
application of electronic components would be achieved. 
The total thickness variation (TTV value) of a semiconductor wafer is 
particularly suitable for assessing the parallelism of the wafer surfaces. 
Referred to as the TTV value is the absolute amount of the thickness 
values of a semiconductor wafer determined from a multiplicity of point 
measurements. The TTV value does not therefore change if a semiconductor 
wafer deforms as a consequence of a one-sided treatment of its surface. A 
prerequisite for this is, however, that the treatment does not alter the 
wafer thickness non-uniformly. 
German Published Application No. 3,906,091 A1 and corresponding U.S. Pat. 
No. 4,991,475, disclose that deformed semiconductor wafers can be created 
by influencing certain parameters during the cutting of semiconductor 
wafers from crystal rods by means of annular saws. However, the precision 
of this method is not sufficient to consistently reproduce semiconductor 
wafers having the same identical defined deformation or warp. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide semiconductor wafers 
having a defined deformation (i.e., having a defined warp), and to provide 
a method by which such semiconductor wafers can be produced. 
The semiconductor wafers according to the invention have a rotationally 
symmetrical curved front surface and a back surface which extends parallel 
to the front surface. The curvature of the front surface is ground into 
the surface of the semiconductor wafers. Preferably, the rotationally 
symmetrical curvature is concavely, convexly or conically formed. The 
degree of deformation of the semiconductor wafers as expressed by their 
warp is adjusted in a defined way. The preadjustment of the warp is made 
possible by a method which is described below. 
First, semiconductor wafers are cut in a manner known per se from the end 
face of a rod-like, or rod-shaped semiconductor crystal, for example by 
means of an annular saw. It should be ensured that the chosen sawing 
method generates at least one wafer surface which is as flat as possible. 
Preferably, a surface-ground surface is created on one side of the 
semiconductor wafer produced by a prior end-face grinding of the crystal 
face. Such a sawing method is disclosed in German Patent specification 
3,613,132 C2 and in corresponding U.S. Pat. No. 4,896,459. 
The semiconductor wafer cut from the crystal rod is then placed by means of 
its first surface on the pickup of a surface-grinding machine. The side of 
the wafer which has the greater flatness is regarded as the first surface 
of the semiconductor wafer. If the end face of the crystal rod was 
surface-ground prior to cutting the semiconductor wafer, the semiconductor 
wafer is placed by means of the surface-ground surface on the wafer 
pickup. The opposite, second surface of the semiconductor wafer is then 
ground in a first grinding step. Suitable surface-grinding machines are, 
in particular, single-wafer grinding machines which employ the operational 
principle of surface rotation grinding disclosed in European Patent 
Application No. 272,531 A1. In this case, not only the wafer pickup with 
the semiconductor wafer fixed thereon rotates, but the axially fed 
grinding tool (for example, a disk wheel or peripheral grinding wheel) 
also rotates. During the grinding of the second surface of the 
semiconductor wafer in the first grinding step, it must be ensured that 
the axis of rotation of the grinding tool is inclined with respect to the 
axis of rotation of the wafer pickup. As a result of this procedure, a 
semiconductor wafer having a rotationally symmetrically curved surface is 
produced. The curvature of the wafer surface depends on the angle of 
inclination which the axes of rotation of wafer pickup and grinding tool 
assume with respect to one another during the surface rotation grinding 
and can therefore be adjusted in a defined way. 
After the first grinding step of the method of the invention, the 
semiconductor wafer is turned over and fixed on a wafer pickup by means of 
the second surface, which has just been ground. The entire second surface 
of the semiconductor wafer must rest tangentially on the wafer pickup. 
Preferably, the semiconductor wafer is held by suction onto a vacuum 
pickup so that it completely conforms to the pickup face. In this 
position, the first wafer surface is now ground parallel to the pickup 
face of the wafer pickup in a second grinding step of the method of the 
invention. In addition to the surface rotation grinding already mentioned, 
plunge-cut grinding and surface side grinding methods can be used. In the 
latter case, only the grinding tool rotates. 
When released from the wafer pickup after the second grinding step, the 
semiconductor wafer springs back into a shape which is essentially defined 
by the first grinding step. The wafer surfaces are curved in a 
rotationally symmetrical manner and extend parallel to one another. The 
semiconductor wafer has a deformation (i.e., a warp), ground in a defined 
way which is determined in the first grinding step by the choice of angle 
of inclination which the axes of rotation of the grinding tool and the 
wafer pickup assume with respect to one another. 
In another embodiment of the method according to the invention, a surface 
side grinding machine is used in the first grinding step and the tool is 
moved in such a way that the ground surface appears saddle-shaped. After 
the second grinding step, the semiconductor wafer is also deformed in a 
saddle shape and has parallel surfaces. The curvature imparted to the 
wafer surface in the first semiconductor wafer grinding step does not 
therefore necessarily have to be rotationally symmetrical. 
The method of the invention can be applied to all semiconductor wafers 
which are sufficiently elastic and can be placed without crushing on a 
wafer pickup in the second grinding step in such a way that the wafer 
surface facing the pickup face rests completely tangentially on the 
latter. Using the method it is possible to set a warp which is greater, 
the greater the diameter and the smaller the thickness of the 
semiconductor wafer used. For silicon wafers, which are particularly well 
suited for the method, however, deformations can be achieved which are a 
multiple of the warp to be expected as a result of a one-sided treatment 
of a wafer surface. For silicon wafers which have been treated on one 
side, the warp is typically up to approximately 50 .mu.m for wafers having 
a diameter of 200 mm and up to approximately 150 .mu.m for wafers having a 
diameter of 100 mm. 
The precision with which the warp of a semiconductor wafer can be adjusted 
by the method according to the invention depends on the flatness of the 
semiconductor wafer prior to the first grinding step and on the precision 
with which the axis of rotation of the grinding tool can be aligned during 
the first grinding step. With machines which are commercially available at 
the present time, a warp adjustment having a precision of less than 2 
.mu.m is possible for silicon wafers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Turning now in detail to the drawings, FIG. 1 shows the cutting of a 
semiconductor wafer from the end face of a rod-like semiconductor crystal 
1. It is irrelevant whether the end face 2 of the rod is ground before or 
while the saw blade of the annular saw 3 is cutting the semiconductor 
wafer. The resultant semiconductor 4 has a surface-ground first surface 5 
and a normally less flat second surface 6 (FIG. 2). The semiconductor 
wafer is placed by means of the first surface onto the pickup 7 of a 
rotary surface grinding machine and fixed thereto, for example, by 
applying a vacuum. The pickup rotates along with the semiconductor wafer 
placed onto it during the subsequent surface grinding. 
It can be seen from FIG. 3 that the axis of rotation 8 of the grinding tool 
is inclined with respect to the axis of rotation 9 of the pickup 7 during 
this first grinding step of the method. In principle, the axis of rotation 
of the grinding tool can be tilted in one spatial direction, for example 
to the right or to the rear, or in two spatial directions, for example to 
the right and to the rear, with respect to the axis of rotation of the 
wafer pickup. The ground wafer surface acquires a curved appearance as a 
result of the first grinding step. The curvature of the surface is 
rotationally symmetrical and, depending on the inclined arrangement of the 
axes of rotation set, it is concave, convex (shape of a spherical segment 
with inclination of the axis of rotation of the grinding tool in two 
spatial directions) or conical (with inclination of the axis of rotation 
of the grinding tool in one spatial direction). 
According to FIG. 4, the semiconductor wafer is turned over and placed by 
means of the second surface 6, which is now curved, onto a wafer pickup 7. 
Preferably, a vacuum pickup is used which applies a suction to hold the 
semiconductor wafer on, so that it rests by means of the entire second 
surface tangentially on the pickup face. As indicated in FIG. 5, due to 
the elasticity of the semiconductor material, the curvature of the vacuum 
held wafer surface is transmitted to the opposite, first surface 5 of the 
semiconductor wafer. This is only the case for as long as the 
semiconductor wafer is fixed on the wafer pickup. 
FIG. 6 shows how the free, first surface of the semiconductor wafer vacuum 
held onto the vacuum pickup is ground over in a second grinding step 
parallel to the pickup face. Although the pickup face 10 is shown as flat 
and horizontal in the figure, this does not necessarily have to be the 
case. The pickup face could also have another shape, for example be 
conically formed. In such cases, it is only more complex to grind the 
first wafer surface parallel to the pickup face because the axes of 
rotation of the pickup and of the grinding tool have to be inclined in a 
suitable manner. 
If the semiconductor wafer is taken from the vacuum pickup after being 
ground over, as shown in FIG. 7, it springs back into its unstressed 
shape. In that case, the wafer surfaces 5, 6 remain parallel to one 
another. The semiconductor wafer is deformed in a defined way in 
accordance with the curvature of the second wafer surface as a consequence 
of the first grinding step. The warp of the semiconductor wafer resulting 
after the second grinding step can be adjusted in a defined way in the 
first grinding step by means of the inclined position of the axes of 
rotation of grinding tool and pickup. 
The deformation of the semiconductor wafer is impressed relative to the 
crystal structure, so that the semiconductor wafers can be treated further 
using standard manufacturing methods, such as lapping, etching or 
polishing, without the set deformation being impaired in the process. 
The method according to the invention advantageously produces 
surface-ground semiconductor wafers whose warp is chosen so as to 
compensate for the wafer deformation to be expected in a one-sided wafer 
treatment, so that, after such a treatment, semiconductor wafers are 
provided which have plane-parallel surfaces. 
The invention will now be further explained by reference to the following 
example which is non-limitative of the invention. 
EXAMPLE 
A total of 18 semiconductor wafers were cut from a rod-like silicon single 
crystal by means of an annular saw. The end face of the rod was 
surface-ground prior to every sawing operation. The semiconductor wafers 
produced had a diameter of 200 mm and a thickness of 760 .mu.m. Groups of 
6 of these semiconductor wafers were each to be provided with a defined 
warp of 25 .mu.m, 45 .mu.m and 75 .mu.m by the method according to the 
invention. During the rotary surface grinding in the first grinding step, 
the axis of rotation of the grinding tool was inclined in two spatial 
directions (to the right and to the rear) with respect to the axis of 
rotation of the wafer pickup, so that the ground wafer surface assumed the 
shape of a spherical segment. During this grinding step, the semiconductor 
wafers rested on a vacuum pickup by means of their surface which had been 
surface-ground during the end-face grinding of the crystal. The angle of 
inclination of the axes of rotation was chosen so that the ground 
semiconductor wafer had a TTV value corresponding to the required warp. 
The dome shape and the dome height were checked with the aid of an 
inductive differential measurement probe. 
A wafer ground in this manner was then turned over and vacuum held onto the 
vacuum pickup so that the wafer surface facing the pickup was completely 
in contact with the pickup face. In a second grinding step, the axis of 
rotation of the rotating grinding tool was aligned so that the 
semiconductor wafer was ground parallel to the pickup face. The full 
ground semiconductor wafers were subjected to a standard etching treatment 
which removes material uniformly from the wafer surface in order to remove 
subsurface damage as a consequence of the grinding treatment. Finally, the 
TTV value of these wafers was determined as a measure of the parallelism 
of a wafer and the warp as a measure of the deformation of the wafer. The 
warp was measured in an area-covering manner using a commercial warp 
measuring instrument. From the measurement results listed in the Table it 
can be inferred that the TTV values set in the first grinding step 
correlate well with the warp of the full ground semiconductor wafers and 
vary only to a minimum extent from the required warp. Furthermore, the 
surfaces of the semiconductor wafers created are virtually parallel. 
TABLE 
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Wafer No. TTV 1 TTV 2 Warp 2 
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1 74 2.4 72.5 
2 76 2.7 70.4 
3 74 2.8 71.7 
5 74 3.1 72.7 
6 79 3.3 73.2 
7 47 2.7 46.8 
8 48 2.8 45.6 
9 47 2.3 44.4 
10 47 2.9 44.1 
11 47 2.4 44.7 
12 46 3.2 43.0 
13 26 2.3 24.4 
14 27 2.4 25.6 
15 25 1.9 24.8 
16 27 2.1 25.7 
17 26 2.6 24.0 
18 27 2.8 24.9 
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TTV 1: TTV value after the first grinding step, in .mu.m 
TTV 2: TTV value after the second grinding step and etching, in .mu.m 
Warp 2: warp after the second grinding and the etching treatment, in .mu. 
 
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.