Angle detector device for silicon wafers

An angle detector for determining the orientation of the crystal axes of silicon wafers is disclosed. A portion of an X-ray beam generated by a synchrotron for exposing a wafer is directed onto the back surface of the wafer via a pair of monocrystalline silicon plate diffraction gratings and a slit having a pin-hole for collimating the X-ray. The X-ray beams diffracted by the wafer form a diffraction pattern on a two-dimensional photosensor array, from which pattern the angular position of the wafer is determined. The angle is determined by an image processor, a memory for storing a diffraction pattern corresponding to a predetermined angular position of the wafer, and a comparison of the image processor output and the memory data.

BACKGROUND OF THE INVENTION 
This invention relates to an angle detector device for detecting the 
angular position or orientation of a semiconductor wafer, and especially 
to an angle detector device for a precise determination of the angular 
position of a silicon wafer with respect to the crystal axes thereof in a 
photolithography step in the LSI production process. 
In the production of LSI's, etc., fine patterns are formed on silicon 
wafers by means of photolithography. Before the photoresist on the wafer 
is exposed in the photolithography process, the angular position of the 
wafer must be determined and adjusted so that the photolithography 
exposure pattern is aligned with the crystal axes of the wafer. 
Referring first to FIG. 1, let us describe a conventional method for 
determining the angular position of a silicon wafer 30 around its central 
axis. A disk-shaped silicon wafer 30 is supported on a holder (not shown) 
rotatable in the direction 31. The determination of the angular position 
of the wafer 30 is effected by means of a flat surface 32 (referred to 
hereinafter as orientational flat) formed on the edge of the wafer. A 
laser oscillator 33 generates a laser beam 34 directed toward a peripheral 
portion of wafer 30. The wafer 30 is rotated while the laser beam 34 
irradiates on the edge of the wafer 33. The laser beam 34 is transmitted 
through the edge of the wafer 30 and hence is received by a photosensor 35 
opposing the laser oscillator 33 only when the wafer 30 is at such a 
rotational position that the laser beam 34 falls on the orientational flat 
32. Thus, the angular position of the wafer 30 can be determined and 
adjusted on the basis of the profile of the output signal of the 
photosensor 35. 
The above method of detecting the angular position of silicon wafers, 
however, has the following disadvantage. Namely, according to the above 
conventional method, the determination of the directions of the crystal 
axes of the wafer is effected only indirectly by means of the 
orientational flat 32. Thus precise alignment of the crystal axes of the 
wafer to the photolithography pattern is hard to effect. In addition, the 
precision of the angular position of the wafer in the photolithography 
step is further reduced due to the fact that the wafer may be moved a 
number of times after the angle thereof is set. 
SUMMARY OF THE INVENTION 
It is therefore a primary object of this invention to provide an angle 
detector device for detecting the angular position of crystalline 
semiconductor wafers by directly determining the directions of the crystal 
axes of the wafer such that the reticle of the photolithography exposure 
device can be aligned precisely with the crystal axes of the wafer. In 
addition, this invention provides an angle detector device for enhancing 
the precision of the angular setting of the wafer in the photolithography 
process. 
The above objects are accomplished according to the principle of this 
invention by an angle detector device which comprises: X-ray irradiation 
means for generating an X-ray beam which is directed onto the wafer and 
diffracted thereby sensor means for detecting a Laue diffraction pattern 
formed by the X-rays diffracted by the wafer; and angle determination 
means, coupled to the output of the sensor means, for determining the 
angular position of the wafer on the basis of the Laue diffraction pattern 
detected by the sensor means. 
Preferably, the sensor means comprises a two-dimensional photosensor array 
for detecting the Laue diffraction pattern, and the angle determination 
means comprises image processing means, coupled to an output of the 
photosensor array, for generating an output corresponding to the Laue 
diffraction pattern detected by the photosensor array; memory means for 
storing a Laue diffraction pattern corresponding to a predetermined 
angular position of the wafer; and calculation means, coupled to the image 
processing means and the memory means, for determining the deviation of 
the detected Laue diffraction pattern from the stored Laue diffraction 
pattern, the calculation means thereby determining the angular position of 
the wafer with respect to crystal axes thereof. 
The precise alignement of the crystal axes of the wafer with the 
photolithography exposure pattern is particularly important in the case of 
the newly developed semiconductor devices in which quatum effects come 
into play. The angle detector device according to this invention is 
especially suited for the precise determination of the angular position of 
a wafer that is required in the production of such semiconductor devices.

DETAIELD DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIGS. 2 through 4 of the drawings, an angle detector 
device according to an embodiment of this invention for detecting the 
angular position or orientation of a silicon wafer is described. The angle 
detector device determines the angular position of the wafer for the 
adjustment of the orientation thereof in a photolithography exposure step 
by means of X-rays. 
As shown in FIG. 2, the silicon wafer 30 having an orientational flat 32 as 
described above by reference to FIG. 1 is supported on a goniometer (not 
shown) for adjusting the angular position or orientation of the wafer 30 
both with respect to the rotational position of the wafer 30 around its 
central axis and with respect to the direction of the main surface of the 
wafer 30. The wafer 30 has a photoresist formed on the main front surface 
thereof (at the left side in FIG. 2) which is to be exposed by an X-ray 
beam 11 generated by a synchrotron 10. The exposure of the wafer 30 is 
effected in the subsequent photolithography step. The synchrotron 10 also 
serves as an X-ray source for generating an X-ray beam 12 for the 
determination of the angular position of the silicon wafer 30. Thus, the 
X-ray generated by the synchrotron 10 is divided into two portions: an 
X-ray beam 11 for exposing the photoresist formed on a main surface of the 
wafer 30, and an X-ray beam 12 for determining the angular position of the 
wafer 30. The angle detector device according to this invention comprises, 
in addition to the synchrotron 10 as its X-ray source, a pair of 
monocrystalline silicon plates 13a and 13b serving as diffraction 
gratings, a slit 14 having a pin-hole for collimating the X-ray beam 12, a 
two-dimensional photosensor array 15, and an angle determination means 
coupled to the output of the photosensor array 15, as described in detail 
below. 
The determination of the angular position of the wafer 30 is effected by 
the angle detector device as follows. First, an approximate angular 
position of the wafer 30 is determined utilizing the orientational flat 32 
as described above in reference to FIG. 1. Then, as shown in FIG. 2, the 
X-ray beam 12 for angle determination generated by the synchrotron 10 is 
reflected by a pair of monocrystalline silicon plates 13a and 13b and 
directed, via a slit 14 and a central hole 15a in the two-dimensional 
photosensor array 15, onto a central portion of the back surface of the 
wafer 30 opposite to the main surface thereof on which the photoresist 
which is to be exposed in the subsequent exposure step is formed. Thus, 
the photoresist on the wafer 30 remains unexposed during this angle 
determination. The X-ray beams 16 diffracted at the wafer 17 fall on the 
two-dimensional photosensor array 15 to form a back Laue diffraction 
pattern thereon. 
The Laue diffraction pattern formed on the photosensor array 15 has a 
configuration which is characteristic of the orientation of the wafer 30 
as well as the crystal structure thereof. FIG. 3 shows in a diagramatic 
perspective view the Laue diffraction pattern (as viewed from the back 
side) which is formed on the photosensor array 15 in the case where the 
main surface of the wafer 30 corresponds to a (111) plane of a crystalline 
silicon wafer. Thus, the diffraction pattern comprises spots 20 through 26 
having a six-fold symmetry around the center, i.e., the pattern 20 through 
26 is invarient under a rotation of 2.pi./6 around the center. (In the 
case where the main surface of the silicon wafer 30 is a (100) crystal 
plane, the Laue diffraction pattern has a four-fold symmetry around the 
center and hence is invariant under a rotation of 2.pi./4.) 
When the angular position or orientation of the silicon wafer 30 deviates 
from the predetermined position, this deviation of the orientation of the 
wafer 30 appears in the Laue diffraction pattern formed on the photosensor 
array 15. Namely, when the rotational position of the disk-shaped wafer 30 
around its central axis deviates by an angle .theta. from the 
predetermined position, then the diffraction pattern 20 through 25 is also 
rotated around its center by the same angle .theta. with respect to the 
normal positions thereof as represented in FIG. 3. Thus, in the case shown 
in FIG. 3, the rotational position of the wafer 30 around its central axis 
can be determined, for example, from the positions of the spots 20 and 21 
on the photosensor array 15. On the other hand, when the direction of the 
main surface of the wafer 30 is deviated from the predetermined direction, 
such deviation results in an asymmetry of the Laue pattern 20 through 25 
with respect to its center. Thus, in the case shown in FIG. 3, the 
deviation of the orientation of the main surface of the wafer 30 in the 
horizontal rotational direction can be determined from the observation of 
the positions of the spots 22 through 25. 
FIG. 4 shows an example of the implementation of the angle determination 
means 40 for effecting the determination of the angular position or 
orientation of the wafer 30 according to the above principle. The angle 
determination means 40 comprises image processing means 40a, memory means 
40b, and calculation means 40c. The output signal S of the photosensor 
array 15 is processed by the image processor 40a to obtain an output A 
corresponding to the Laue pattern 20 through 25 formed on and detected by 
the sensor array 15. On the other hand, the memory means 40 stores the 
information B corresponding to the Laue pattern which is to be formed on 
the photosensor array 15 when the wafer 30 is at the predetermined angular 
position where the crystal axes of the wafer 30 are precisely aligned with 
the exposure device. On the basis of the information A and B received from 
the image processor 40a and the memory 40b, the calculation means 40c 
determines the deviation of the detected Laue pattern from that stored in 
the memory 40b, and generates an output signal C corresponding to the 
angular position of the wafer 30. 
The advantages of the angle detector device according to this invention are 
as follows. First, since the directions of the crystal axes of the wafer 
can be determined directly, precise orientation of the wafer becomes 
possible. Further, since the adjustment of the angular position of the 
wafer 30 can be effected while the wafer 30 is mounted on the wafer stage 
on which the wafer 30 is exposed in the subsequent photolithography step, 
the setting precision of the wafer is not reduced by intervening movements 
of the wafer 30 which otherwise take place after the angular position 
thereof is set. The angle detector device according to this invention is 
thus especially suited for the determination and adjustment of the angular 
position of silicon wafers before the exposure thereof in the production 
of quantum-effect semiconductor devices. 
While description has been made of the particular embodiment of this 
invention, it will be understood that many modifications may be made 
without departing from the spirit thereof. For example, although a 
synchrotron was utilized as the X-ray source in the above embodiment, 
other X-ray sources may be used as well. Further, the silicon plates 13a 
and 13b may be replaced by other types of mirrors. Furthermore, although 
the angle determination is effected by means of an X-ray beam incident on 
the back surface of the wafer, an X-ray beam incident on a portion of the 
front surface on which semiconductor device chips are not formed may be 
utilized as well for the angle determination. The appended claims are 
contemplated to cover any such modifications as fall within the true 
spirit and scope of this invention.