In a photomask, a first and a second rectangular openings are formed in a shield layer which provided on a transparent plate. The first rectangular opening is disposed so as to deviate off normal lines of respective sides of the second rectangular opening. Thus, no diffraction images of the first and the second rectangular openings overlap zero-order diffraction images of the second and the first rectangular openings, respectively.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a photomask used in a photolithography 
process which is one of the manufacturing processes of a semiconductor 
device. 
2. Description of the Background Art 
A lithography technique is usually used for forming a circuit pattern to be 
provided on a semiconductor device. This technique fundamentally consists 
of a coating step for coating a substrate such as wafer with photoresist 
to form a photoresist layer, an exposure step for impinging light (for 
example, light in an ultraviolet region) from a suitable light source upon 
a photomask having a predetermined pattern to transfer the pattern to the 
photoresist layer and a development step for developing the photoresist 
layer to obtain a photoresist layer having the predetermined pattern. 
FIG. 1 is a perspective view of a conventional photomask 70 used in the 
exposure step. In FIG. 1, numeral 71 represents a glass substrate, on 
which a shield layer 72 of chromium is formed. The shield layer 72 is 
provided with two adjacent rectagular light-transmission patterns 73 and 
74 formed by rectangular openings in parallel. In addition, the term 
"rectangular" means a shape including a square and a oblong. 
FIG. 2 is a schematic structural view of an exposure device. As shown in 
FIG. 2, the exposure device is provided with a light source 81 for 
emitting light in an ultraviolet region downwardly. The light from the 
light source 81 is directed through a lens 82 to the photomask 70. Some of 
the incident light passes through the rectangular light-transmission 
patterns 73 and 74, further to be introduced through a lens 83 onto a 
photoresist surface 84. On the other hand, the light impinging upon the 
shield layer 72 is shielded by the shield layer 72. Thus, exposure 
patterns corresponding to the rectangular light-transmission patterns 73 
and 74 are transferred to the photoresist surface 84. 
As in FIG. 1, the rectangular light-transmission patterns 73 and 74 are 
adjacent to each other; More particularly, one rectangular 
light-transmission pattern 74 (or 73) is disposed on the normal lines 
N.sub.3 (or N.sub.4) of the other rectangular light-transmission pattern 
73 (or 74). When the exposure process is carried out with the photomask 
70, diffraction light from the rectangular light-transmission pattern 73 
(or 74) exerts adverse effect on the form of the transferred pattern which 
is formed by focusing diffraction light from the rectangular 
light-transmission pattern 74 (or 73) on the photoresist surface 84; in 
brief, the patterns transferred on the photoresist surface 84 are deformed 
by the diffraction light. The closer the rectangular light-transmission 
patterns 73 and 74 are in proximity to each other, the larger the 
deformation grows. The deformation causes the deterioration of transfer 
accuracy of the pattern to the photoresist surface 84. In particular, in 
these days of advancing high integration, the rectangular 
light-transmission patterns 73 and 74 are disposed in further closer 
proximity to each other. Accordingly, the influence of the diffraction 
light is a serious problem. 
SUMMARY OF THE INVENTION 
The present invention is directed to a photomask. The photomask comprises: 
a transparent plate; and a shield layer formed on the transparent plate, 
the shield layer having a first and a second rectangular openings each of 
which serves as a rectangular light-transmission pattern, wherein the 
first rectangular opening is disposed so as to deviate off normal lines of 
respective sides of said second rectangular opening. 
Accordingly, an object of the present invention is to provide a photomask 
in which two exposure pattern adjacent to each other can be accurately 
transferred to an exposure surface. 
These and other objects, features, aspects and advantages of the present 
invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Prior to explaining a preferred embodiment of a photomask according to the 
present invention, the cause why the above-mentioned problem is generated 
will be examined. 
First, considered is the case where a single rectangular light-transmission 
pattern is formed on a photomask. 
FIG. 3 is a perspective view of a photomask 30 so formed. FIG. 4 
schematically shows images transferred on an exposure surface 41 when 
parallel light L having uniform light intensity distribution is vertically 
impinged upon the surface of the photomask 30. In FIG. 4, the exposure 
surface 41 is disposed a predetermined distance apart from the photomask 
30. 
FIG. 5 is a graph showing a light intensity in the x-direction of the 
exposure surface 41 of FIG. 4. In FIG. 5, the abscissa shows a distance 
from the point 0 (shown in FIG. 4) in the x-direction, and the ordinate 
shows a ratio of the light intensity as the light intensity at the point 0 
is taken as "1". As will be understood from FIG. 5, peaks due to 
Fraunhofer diffraction are seen around the primary peak. Accordingly, not 
only a rectangular pattern 42 corresponding to a rectangular 
light-transmission pattern 31 but also superfluous diffraction images 43 
to 45 are formed on the exposure surface 41. 
In FIG. 4, the relatively brighter image 42 formed substantially at the 
center of the exposure surface 41 is a zero-order diffraction image. 
First-order diffraction images 43, second-order diffraction images 44 and 
third-order diffraction images 45 are formed in series from the image 42 
in the directions normal to the respective sides of the rectangular 
light-transmission pattern 31, that is, in the x-direction and the 
y-direction of FIG. 4. 
Next, considered is the case where the parallel light L is vertically 
impinged upon the surface of the photomask 70 shown in FIG. 1. FIG. 6 
schematically shows images formed on an exposure surface 61. In FIG. 6, 
the exposure surface 61 is disposed a predetermined distance apart from 
the photomask 70. For easy understanding, the diffraction images of the 
rectangular light-transmission pattern 73 are indicated by solid lines 
while those of the rectangular light-transmission pattern 74 are indicated 
by alternate dashed-and-dotted lines. As shown in FIG. 6, the diffraction 
images (indicated by solid lines) 62a, 63a, 64a and 65a of the rectangular 
light-transmission pattern 73 are the same as the diffraction images (in 
FIG. 4) in the case of impinging the parallel light to the single 
rectangular light-transmission pattern. Centered on the zero-order 
diffraction image 62a corresponding to the rectangular light-transmission 
pattern 73, the first-order diffraction images 63a, the second-order 
diffraction images 64a and the third-order diffraction images 65a are 
formed in series in the directions of the normal lines N.sub.3 of the 
respective sides of the rectangular light-transmission pattern 73, that 
is, in the x-direction and the y-direction of FIG. 6. Also the diffraction 
images (indicated by alternate dashed-and-dotted lines) 62b, 63b, 64b and 
65b of the rectangular light-transmission pattern 74 are formed on the 
exposure surface 61 in the same manner as the diffraction images 
(indicated by solid lines) 62a, 63a, 64a and 65a of the rectangular 
light-transmission pattern 73. 
In the photomask 70, the rectangular light-transmission pattern 74 is 
formed on the normal line N.sub.3 of the rectangular light-transmission 
pattern 73. Accordingly, the diffraction images 62a, 63a, 64a and 65a of 
the rectangular light-transmission pattern 73 and the diffraction images 
62b, 63b, 64b and 65b of the rectangular light-transmission pattern 74, 
which are formed in the y-direction, overlap partially. As shown in FIG. 
6, for example, the third-order diffraction image 65b of the rectangular 
light-transmission pattern 74 overlaps the zero-order diffraction image 
62a of the rectangular light-transmission pattern 73, so that the image 
62a is deformed. The zero-order diffraction image 62b of the rectangular 
light-transmission pattern 74 is also deformed due to the diffraction 
image of the other rectangular light-transmission pattern 73. 
FIG. 7 is a perspective view of a preferred embodiment of a photomask 
according to the present invention. As shown in FIG. 7, a photomask 10 
comprises a shield layer 12 formed on a glass substrate 11. The shield 
layer 12 has two rectangular openings each of which serves as a 
light-transmission patterns. The terms "rectangular light-transmissions 13 
and 14" refer to the openings. The rectangular light-transmission pattern 
14 is provided in a position deviating off the normal lines N.sub.1 of the 
respective sides of the rectangular light-transmission pattern 13. 
Likewise, the rectangular light-transmission pattern 13 is provided in a 
position deviating off the normal lines N.sub.2 of the respective sides of 
the rectangular light-transmission pattern 14. Assuming that the 
rectangular light-transmission patterns 13 and 14 of FIG. 1 are rotated by 
45.degree. around the vertical axis, the rotated rectangular 
light-transmission patterns coincide with the rectangular 
light-transmission patterns 13 and 14, respectively. Accordingly, the 
normal lines N.sub.1 of the respective sides of the rectangular 
light-transmission pattern 13 and the normal lines N.sub.2 of the 
respective sides of the rectangular light-transmission pattern 14 
intersect at right angles. 
FIG. 8 shows typically images formed on an exposure surface 21 when 
parallel light L is vertically impinged upon the surface of the photomask 
10. In FIG. 8, the exposure surface 21 is disposed a predetermined 
distance apart from the photomask 10. Similarly to the aforesaid case, for 
easy understanding in FIG. 8, diffraction images of the rectangular 
light-transmission pattern 13 are indicated by solid lines while 
diffraction images of the rectangular light-transmission pattern 14 are 
indicated by alternate dashed-and-dotted lines. As shown in FIG. 8, 
centered on a zero-order diffraction image 22a of the rectangular 
light-transmission pattern 13, first-order diffraction images 23a, 
second-order diffraction images 24a and third-order diffraction images 25a 
thereof are formed in series in the direction of the normal lines N.sub.1 
of the respective sides of the rectangular light-transmission pattern 13, 
that is, in the x-direction and the y-direction of FIG. 8. Also 
diffraction images 22b, 23b, 24b and 25b of the rectangular 
light-transmission pattern 14 are formed in the x-direction and the 
y-direction of FIG. 8 in the same manner as the diffraction images 
(indicated by solid lines) 22a, 23a, 24a and 25a of the rectangular 
light-transmission pattern 13. 
In the photomask 10, however, since the rectangular light-transmission 
pattern 14 is provided in a position deviating off the normal lines 
N.sub.1 of the respective sides of the rectangular light-transmission 
pattern 13, the first-through third-order diffraction images 23a, 24a and 
25a formed in the direction of the normal lines N.sub.1 do not overlap the 
zero-order diffraction image 22b of the rectangular light-transmission 
pattern 14. Therefore, the image 22b of the rectangular light-transmission 
pattern 14 can be formed on the exposure surface 21 accurately without 
being affected by the diffraction light of the rectangular 
light-transmission pattern 13. The image 22a of the rectangular 
light-transmission pattern 13 can be also formed on the exposure surface 
21 accurately without being affected by the diffraction light of the 
rectangular light-transmission pattern 14, similarly to the case of the 
rectangular light-transmission pattern 14. 
In the above preferred embodiment, the rectangular light-transmission 
patterns 13 and 14 are disposed so that the normal lines N.sub.1 and 
N.sub.2 intersect at right angles. The intersection angle, however, is not 
limited to 90.degree.. The similar effect can be obtained if the 
rectangular light-transmission pattern 14 is provided in a position 
deviating off the normal lines N.sub.1 and the rectangular 
light-transmission pattern 13 is provided in a position deviating off the 
normal lines N.sub.2. 
Furthermore, the similar effect can be obtained in a photomask in which one 
or both of the rectangular light-transmission patterns 13 and 14 are 
filled with phase shift members for shifting the phase of transmission 
light. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation. The spirit 
and scope of the present invention should be limited only by the terms of 
the appended claims.