Reduction projection aligner system

In a reduction projection aligner system wherein a pattern on a reticle is formed directly on a wafer by reducing, projecting and printing it, a positioning pattern on the wafer is optically magnified and projected and then focused onto a focal plane where a slit scans the projected image. The distance from a mechanical origin provided on the supporting body of the system to the positioning pattern on the wafer is then measured on the basis of the movement of the slit, and the reticle is then relatively moved and positioned so as to coincide with the position of the wafer relative to the body of the system, thereby bringing the wafer and the reticle into coincidence.

BACKGROUND OF THE INVENTION 
This invention relates to improvements in a reduction projection aligner 
system for use in semiconductor production. 
The reduction projection aligner system is typically used for semiconductor 
production and forms the patterns of semiconductor integrated circuits, 
such as ICs and LSIs, directly on wafers. In many conventional reduction 
projection aligner systems, the wafer and a reticle (a glass plate on 
which an original pattern is depicted) are positioned independently of 
each other. More specifically, the reticle is positioned by visually 
detecting two typical portions on the reticle with a reticle positioning 
optical microscope and is fixed onto the body of the apparatus, while the 
wafer is positioned by visually detecting two typical portions on the 
wafer with a wafer positioning optical microscope and is fixed onto the 
same body. With the method in which the whole wafer is positioned on the 
basis of the two typical portions on the wafer in this manner, the 
precision of the alignment between the reticle and the wafer depends 
principally on the positioning precision of the wafer, which forms a 
serious hindrance to the enhancement of the alignment precision. In 
addition, it is impossible to align the reticle and the wafer if an error 
in the arrangement of the pattern on the wafer attributed to the 
anisotropic expansion or contraction of the wafer occurs during the 
manufacturing of the wafer. 
Therefore, in order to eliminate such disadvantages and to make high 
precision alignment of both the reticle and the wafer possible, there has 
been devised a method as shown in FIG. 1 wherein the position of a wafer 
is relatively detected through a reticle as well as a reduction lens 
(refer to Japanese patent application Laid Open No. 54-93974). 
In the reduction projection aligner system shown in FIG. 1, a pattern to be 
used for positioning is formed on the wafer 4 by a preceding step and this 
pattern is illuminated by a light guide 7 through a reference pattern 5 
formed on an edge of the reticle 2 and further through a reduction lens 3. 
Using the reflected light from the wafer 4, the positioning pattern on the 
wafer 4 is focused on the reticle 2. On the other hand, in order to detect 
the position of the reticle 2, the reference pattern 5 on the reticle 2 is 
illuminated by a light guide 6. Thus, the reflected lights from the wafer 
4 and the reticle 2 are projected onto the position occupied by a slit 10 
by means of a magnifying optical system 9, the distance of movement of the 
slit 10 is measured by a uniaxial movable table 12 and a measuring machine 
13, and the output from a photo-detector 11 corresponding to the position 
of the slit 10 is detected, whereby the relative positions of the reticle 
2 and the wafer 4 are detected. 
With this system, however, the predetermined reference pattern 5 needs to 
be formed in a specified position on the reticle 2 separately from a 
circuit pattern carried by the reticle and a reticle positioning pattern 
which is used for positioning the reticle 2 with respect to a holder 14. 
This provision of the reference pattern 5 forms a serious hindrance in 
preparing the reticle. Moreover, any arrangement error of the reference 
pattern 5 with respect to the circuit pattern on the reticle results in an 
alignment error between the reticle 2 and the wafer 4. Further, since the 
relative positions of the reticle 2 and the wafer 4 are detected, an 
optical system for illuminating the reticle surface which consists of the 
light guide 6 and a mirror 8 must be installed; however, such installation 
is very difficult in practice. In the figure, numeral 1 designates a 
condensing lens in the primary optical path of the system. 
SUMMARY OF THE INVENTION 
This invention has been made in view of the above drawbacks, and has for 
its object to provide a reduction projection aligner system in which a 
positioning pattern on a wafer obtained through a reduction lens is 
detected by the use of a detecting optical system which is easy to 
install, thereby to permit a high precision detection of the wafer 
position relative to the body of the system. 
In order to accomplish the stated object, according to this invention, in a 
reduction projection aligner system which forms the patterns of 
semiconductor integrated circuits, such as ICs and LSIs, directly on 
wafers, when a pattern on a wafer already formed by a preceding step and a 
pattern to be formed anew are to be aligned, the positioning pattern on 
the wafer is optically magnified and projected and then focused, the focal 
plane is scanned by a slit having its mechanical origin on the body of the 
system, the brightness of light passing through the slit is 
photoelectrically converted in correspondence with movement of the slit, 
and the distance from the mechanical origin of the slit to the positioning 
pattern of the wafer is measured, thereby to calculate the position of the 
wafer relative to the body of the reduction projection aligner system, and 
a reticle having the pattern to be formed anew is relatively moved so as 
to coincide with the wafer, to execute the so-called alignment of the 
reticle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereunder, this invention will be described in detail with reference to 
various exemplary embodiments. 
A conceptual diagram of a reduction projection aligner system according to 
this invention is shown in FIG. 2. The system of this invention greatly 
differs from the system shown in FIG. 1 in the following points. A part of 
the holder 14 which serves to carry a reticle 2' thereon is provided with 
a hole 15 so as to illuminate a pattern on the wafer 4 by light projected 
from the light guide 7 through the reduction projection lens 3 similarly 
to the system shown in FIG. 1. Therefore, a reticle 2' to be placed on the 
holder 14 does not need to be provided with the special reference mark 5 
as shown in FIG. 1, and it may merely have a structure adapted to transmit 
light through the edge thereof. Further, elimination of the special 
reference mark 5 means that the light guide 6 for illuminating the 
reference mark 5 is unnecessary, and one mirror 8' suffices in this 
system. In addition, the pattern (for example, a rectilinear mark usually 
having a width of 6 .mu.m or so) on the wafer as focused on the reticle is 
projected onto the position of the slit 10 through the optical system 9. 
At this time, by means of an origin sensor 16 disposed in the movable 
range of the slit 10, the distance from the origin of the slit 10 to the 
positioning pattern of the wafer is measured by the measuring machine 13. 
The origin sensor 16 is fixed on the body of the system, so that the 
distance of movement of the slit to the positioning pattern on the wafer 
after the slit has passed through the origin sensor 16 is gauged as the 
position of the wafer relative to the body of the system. 
The details of the pattern detecting portion are shown in FIG. 3. On the 
uniaxial movable table 12, there are placed the slit 10, a member 
to-be-detected 21 for the original sensor 16, and a glass scale 13'. The 
glass scale 13' is movable only in the direction of arrow 22 with the 
movement of the uniaxial movable table 12. An index scale 19 is fixed at a 
predetermined spacing with respect to the glass scale 13'. The glass scale 
13' and the index scale 19 are overlaid with gratings at equal spacings, 
and they are sandwiched inbetween a light source 17 and lens 18, on the 
one hand, and a photodetector 20, on the other hand. Light projected from 
the light source 17 towards the photodetector 20 repeats in brilliance and 
darkness each time the gratings of the glass scale 13' move one pitch with 
respect to the index scale 19, and this modulated light is taken out as a 
sinusoidal signal. Since the output signal from the photodetector 20 is 
feeble as it is, it is amplified by a preamplifier 23. Further, the 
amplified signal is passed through a waveform shaping circuit 24, as well 
as a direction discriminator 28, and is indicated as a digital quantity by 
a digital counter 29. On the other hand, as the origin sensor 16, there is 
employed by way of example a magnetic tranducer which applies the 
principle of magnetic recording. When the magnetic conductor 21 which is 
the member to-be-detected placed on the movable table 12 moves in the 
sense of the arrow 22, an analog output of, for example, about 1 mV/.mu.m 
is produced via the magnetic transducer 16 as well as a detecting circuit 
25, and it is turned into a pulse output by a waveform changing circuit 
26. Herein, the system is operated under the condition that the spacing 
between the magnetic transducer 16 and the magnetic conductor 21 is set 
at, for example, below 0.5 mm. Thus, when the magnetic conductor 21 placed 
on the movable table 12 passes in front of the magnetic transducer 16, a 
pulse output is obtained at a reproduction precison of approximately 1 
.mu.m. 
On the other hand, the brilliance and darkness of the light which has 
passed through the slit is photoelectrically converted by the 
photomultiplier 11, the output of which is applied to an analog-to-digital 
converter 27. The A-D converter 27 digitizes the photomultiplier output in 
synchronism with the counter output from the digital counter 29, and 
applies the digital signal to a central processing unit 30. After having 
received the pulse output from the magnetic transducer 16, the CPU 30 
receives the A-D converted value of the photomultiplier output in 
proportion to the count quantity of the digital counter 29 and stores it 
therein. 
FIG. 4 shows an example of the positioning pattern 31 on the wafer 
projected on the position of the slit 10. FIG. 5 shows examples of the 
intensity of signals stored in the CPU 30 at this time. The abscissa 
represents the position X of the slit, while the ordinate represents the 
A-D converted value Y of the photomultiplier output indicative of the 
intensity or the brilliance and darkness of the light passing through the 
slit. The digital value of the photomultiplier output begins to be 
received at a position X.sub.o at which the pulse from the origin sensor 
is received. When data is received in proportion to the count quantity of 
the digital counter 29, the input data at the i-th count value X.sub.i 
becomes Y.sub.i. By processing the signals shown in FIG. 5 and evaluating 
the position X.sub.c of the slit indicative of the center of the 
positioning pattern 31 on the wafer, the distance of movement of the slit 
10 from the origin thereof is obtained. 
The central coordinates of the positioning pattern on the wafer as included 
in this output signal can be evaluated with, for example, the following 
method (refer to U.S. Pat. No. 4,115,762). An arbitrary position X.sub.i 
of the slit is supposed as a tentative center, and data on both the sides 
thereof amounting to 2.m is superposed to calculate Z.sub.i. Here, letting 
Y.sub.i denote an output signal at the position X.sub.i, Z.sub.i is 
calculated as follows: 
##EQU1## 
Among the changes of Z thus obtained, a point which gives the minimum 
value of Z becomes the central position of the pattern on the wafer. 
In this manner, the positioning pattern on the wafer is magnified and 
focused through the optical lens, and the focal plane is scanned by the 
slit having its mechanical origin on the body of the system so as to 
photoelectrically detect the intensity of the light passing through the 
slit in accordance with the position of the movement of the slit, thereby 
to measure the position of the wafer relative to the body with a high 
precision, whereupon the wafer and the reticle positioned to the body with 
the separate detecting optical system are aligned by movement of the 
reticle, for example, in the manner described in U.S. Pat. No. 4,153,371. 
As set forth above, according to this invention, the position of the 
positioning pattern on the wafer is detected in the position in which the 
pattern is magnified and projected by the optical system of simple 
construction including the reduction lens, whereby a high-precision 
positional detection has become possible. Experiments have revealed that 
the detection and reproduction precision of the positioning pattern on the 
wafer in accordance with this invention is enhanced to double the same 
precision in the system shown in FIG. 1. 
While we have shown and described an embodiment in accordance with the 
present invention, it is understood that the same is not limited thereto 
but is susceptible of numerous changes and modifications as are known to 
those skilled in the art, and we therefore do not wish to be limited to 
the details shown and described herein but intend to cover all such 
changes and modifications known to one of ordinary skill in the art.