Method of exposure employing phase shift mask of attenuation type

By the method of exposure using attenuation type phase shift mask, based on the phase angle of exposure light passing through second and third light transmitting portions and on hole diameters of the second and third light transmitting portions, an optimal value between first and second distances h.sub.1 and h.sub.2 can be calculated. Therefore, a resist film can be exposed at optimal focal position. As a result, even when there is a step in the material to be exposed, a desired pattern can be exposed with high precision through the same steps in every region of the material to be exposed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of exposure employing phase shift 
mask of attenuation type and, more particularly, to a method of exposure 
for exposing a material having a stepped portion. 
2. Description of the Background Art 
Recently, semiconductor integrated circuits have been remarkably improved 
in the degree of integration and miniaturization. Accordingly, circuit 
patterns formed on the semiconductor substrate has been rapidly reduced in 
size. Photolithography is widely recognized as basic technique in 
patterning. Though various developments and improvements have been made to 
date, there is stronger demand in improved resolution of patterns, as 
patterns have come to be smaller and smaller. 
The resolution limit R (nm) in photolithography employing the magnification 
exposure method is represented as 
EQU R=K.sub.1 .cndot..lambda.(NA) (1) 
where .lambda. represents wavelength (nm) of the light beam used, NA 
represents numerical aperture of the lens, and K.sub.1 is a constant 
dependent on the resist process. 
As can be seen from equation (1), in order to improve resolution limit, the 
values of K.sub.1 and .lambda. should be reduced while the value of NA 
should be increased. In other words, the constant dependent on the resist 
process should be made smaller while the wavelength of the light beam 
should be reduced and the numerical aperture should be enlarged. However, 
improvement in the light source and the lens is technically difficult and, 
in addition, if the wavelength is made shorter and NA is increased, depth 
of focus .delta. (.delta.=K.sub.2 .cndot..lambda./(NA).sup.2) becomes 
shallower, resulting in undesirable lowering of the resolution. 
Referring to FIGS. 11(a), (b) and (c), cross section of a photomask, 
electric field of the exposure light on the photomask and intensity of the 
exposure light on the semiconductor wafer will be described. 
First, referring to FIG. 11(a), cross sectional structure of a photomask 
100 will be described. On a quartz glass substrate 110, a metal mask 
pattern 120 formed of chromium or the like and a light transmitting 
pattern 130 exposing the quartz glass substrate 110 are formed. Referring 
to FIG. 11(b), the electric field of the exposure light immediately after 
the passage through photomask 100 will be described. The electric field of 
the exposure light on photomask 100 is conforming to the photomask 
pattern. Referring to FIG. 11(c), light intensity on the semiconductor 
wafer will be described. As to the intensity of exposure light on the 
semiconductor wafer, especially when fine pattern is to be transferred, 
the beams of exposure light which have passed through the photomask are 
overlapped at adjacent pattern images because of diffraction and 
interference of light as shown in the figure, and thus the intensity is 
increased. As a result, the difference in the intensity of light on the 
semiconductor wafer becomes smaller, resulting in poor resolution. 
In order to solve this problem, Japanese Patent Laying-Open No. 57-62052 
and 58-173744 proposed a phase shift exposure method employing a phase 
shift mask. Referring to (a), (b) and (c) of FIG. 12, the phase shift 
exposure method using a phase shift mask disclosed in Japanese Patent 
Laying-Open No. 58-173744 will be described. Referring to FIG. 12(a), the 
cross sectional structure of phase shift mask 200 will be described. The 
phase shift mask 200 includes a chromium mask pattern 220 formed on a 
glass substrate 210, and a light transmitting portion 230 at which glass 
substrate 210 is exposed. Further, at every other light transmitting 
portions 230, a phase shift 240 formed of a transparent insulating film 
such as a silicon oxide film, is provided. 
Referring to FIG. 12(b), the electric field of the exposure light on the 
phase shift mask will be described. The electric field of the exposure 
light on the phase shift mask provided by the exposure light which has 
passed through the phase shift mask 200 has its phase inverted by 
180.degree. alternately. Therefore, the light beam offset with each other 
at adjacent pattern images where exposure light beams overlap with each 
other, because of light interference. The intensity of exposure light on 
the semiconductor wafer will be described with reference to FIG. 12(c). As 
shown in the figure, the difference in intensity of exposure light on the 
semiconductor wafer is distinctive, and thus resolution of the pattern 
image can be improved. 
The phase shift exposure method employing the phase shift mask is very 
effective for periodic patterns having lines and spaces, for example. 
However, if the pattern is complicated, positioning of the phase shift is 
very troublesome, and therefore this method cannot be applied to every 
desired pattern. 
As a phase shift mask solving this problem, an attenuation type phase shift 
mask is disclosed, for example, in JJAP Series 5 Proc. of 1991 Intern. 
Micro Process Conference pp. 3-9 and in Japanese Patent Laying-Open No. 
4-136854. The phase shift mask of attenuation type disclosed in Japanese 
Patent Laying-Open No. 136854 will be described in the following. 
Referring to FIG. 13(a), the structure of the attenuation type phase shift 
mask 300 will be described. The attenuation type phase shift mask 300 
includes a phase shift pattern, which is a prescribed exposure pattern, 
that includes a quartz substrate 310 transmitting exposure light, light 
transmitting portions 330 formed on the main surface of the quartz 
substrate 310 and exposing the main surface of the quartz substrate 310, 
and a phase shift film 320 for shifting the phase of the exposure light 
passing therethrough by 180.degree. with respect to the phase of the 
exposure light passing through said transmitting portions 330. The phase 
shift film 320 has a two-layered structure consisting of a chromium layer 
320a having the light transmittance of 5 to 20%, and a shift layer 320b 
providing phase difference of 180.degree. between the light passing 
therethrough and the light passing through the light transmitting portions 
330. Recently, chromium oxide, chromium nitride oxide, chromium nitride 
carbide oxide, molybdenum silicide oxide, and molybdenum silicide nitride 
oxide have come to be used as a one layered structure phase shift portion, 
instead of the aforementioned phase shifter portion 320. 
Provision of the attenuation type phase shift mask allows exposure of even 
a complicated pattern, and therefore this has become dominant in the 
recent exposure methods. 
However, semiconductor devices have very complicated cross sectional 
structures these days. Therefore, when a resist film is to be exposed by 
using the aforementioned attenuation type phase shift mask, there is often 
a step in the material underlying a resist film. Therefore, optimal focal 
position may differ from one position to the other, even in one and the 
same step of exposure. 
For example, let us consider an example in which there is a step in an 
underlying substrate 12, a resist film 14 is formed on the substrate 12 
having the step and resist film 14 is to be exposed, with reference to 
FIGS. 14 to 16. 
First, referring to FIG. 14, assume that the focal position is at the left 
side region of resist film 14. In this case, at the left side region of 
resist film 14, a desired pattern 14a can be formed. However, in the right 
side region of resist film 14, a pattern 14b is formed with the diameter 
being smaller than desired. 
Referring to FIG. 15, let us assume that the focal position is in the right 
side region of resist film 14. In this case, a desired pattern 14b can be 
formed in the right side region of resist film 14. However, in the left 
side of resist film 14, a pattern 14a is formed with its diameter made 
larger than desired. 
Referring to FIG. 16, assume that the focal position is at the intermediate 
portion of the step. In this case, in the left side region of resist film 
14, pattern 14a is formed with its diameter being larger than desired, 
while in the right side region of resist film 14, pattern 14b is formed 
with its diameter made smaller than desired. 
As described above, when there is a step in the resist film, optimal focal 
position cannot be obtained for every region in one and same step of 
exposure, and hence desired pattern cannot be formed in the resist film. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method of exposure 
employing a phase mask of attenuation type which allows highly precise 
exposure of desired patterns in every region even when there is a step in 
the material to be exposed. 
According to one aspect, the present invention is directed to a method of 
exposure using an attenuation type phase shift mask in which, by using an 
exposure apparatus having an attenuation type phase shift mask including 
first and second light transmitting portions exposing a transparent 
substrate and a phase shift mask formed on the aforementioned transparent 
substrate to surround the first and second light transmitting portions, a 
material having a first exposure region which is positioned at a first 
distance from a projection lens of the exposure apparatus corresponding to 
the region of the first light transmitting portion and a second exposure 
region positioned at a second distance from the projection lens of the 
exposure apparatus corresponding to the region of the second light 
transmitting portion, characterized in that the method includes a step of 
providing a prescribed phase angle to the exposure light passing through 
the second light transmitting portion, and the step of calculating optimum 
value of difference between the first and second distances, based on the 
aforementioned phase angle and the hole size of the second light 
transmitting portion. 
Accordingly, the first region to be exposed comes to be positioned at the 
optimal focal position of the exposure light which passed through first 
light transmitting portion, and the second region to be exposed comes to 
be positioned at the optimal focal position of the exposure light which 
has passed through the second light transmitting portion. Therefore, both 
the first and second regions to be exposed which are at different 
distances from the projection lens of the exposure apparatus can be 
exposed at the optimal focal positions in respective regions, and thus a 
desired pattern can be exposed. 
According to another aspect, the present invention relates to a method of 
exposure using a phase shift mask of attenuation type in which, by using 
an exposure apparatus including an attenuation type phase shift mask 
having first and second light transmitting portions exposing a transparent 
substrate and a phase shift mask formed on the transparent substrate and 
surrounding the first and second light transmitting portions, a material 
to be exposed having a first region to be exposed positioned at a first 
distance from a projection lens of the exposure apparatus corresponding to 
the region of the first light transmitting portion and a second region to 
be exposed positioned at a second distance from the projection lens of the 
exposure apparatus corresponding to the region of the second light 
transmitting portion is exposed, characterized in that the method includes 
the steps of providing a prescribed phase angle to the exposure light 
passing through the second light transmitting portion, and calculating 
optimal value of difference between the first and second distances based 
on the phase angle, hole size of the second light transmitting portion and 
transmittance of the phase shift film near the periphery of the second 
light transmitting portion, with the transmittance of the phase shift film 
near the periphery of the second light transmitting portion made different 
from the transmittance of the phase shift mask near the periphery of the 
first light transmitting portion. 
Accordingly, the first region to be exposed comes to be positioned at the 
optimal focal position of the exposure light which has passed through the 
first light transmitting portion, and the second region to be exposed 
comes to be positioned at the optimal focal position of the exposure light 
which has passed through the second light transmitting portion. As a 
result, both the first and second regions to be exposed which are 
positioned at different distances from the projection lens of the exposure 
apparatus can be exposed at optimal focal positions respectively, and thus 
desired patterns can be exposed in respective regions. 
According to a still another aspect, the present invention relates to a 
method of exposure employing a phase shift mask of attenuation type in 
which by using an exposure apparatus including an attenuation type phase 
shift mask having a phase shift mask formed on a transparent substrate, a 
first pattern in which a plurality of first light transmitting portions 
exposing the transparent substrate are arranged at prescribed positions on 
the phase shift mask with prescribed pitch between each other and a second 
pattern in which a plurality of second light transmitting portions 
exposing the transparent substrate are arranged with a prescribed pitch 
between each other, a material having a first region to be exposed 
positioned at first distance from a projection lens of the exposure 
apparatus corresponding to the region of the first pattern and a second 
region to be exposed positioned at a second distance from the projection 
lens of the exposure apparatus corresponding to the region of the second 
pattern is exposed, characterized in that the method includes the step of 
providing a prescribed phase angle to the exposure light transmitting 
through the second light transmitting portion, and calculating optimal 
value of a difference between the first and second distances based on the 
phase angle, hole size of the second light transmitting portion and the 
space between the second light transmitting portions. 
Accordingly, the first region to be exposed comes to be positioned at the 
optimal focal position of the exposure light which has passed through the 
first pattern, and the second region to be exposed comes to be positioned 
at the optimal focal position of the exposure light which has passed 
through the second pattern. As a result, both the first and second regions 
to be exposed which are positioned at different distances from the 
projection lens of the exposure apparatus can be exposed at optimal focal 
positions respectively, and thus desired patterns can be exposed at 
respective regions. 
The foregoing 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 
A first embodiment of the method of exposure employing an attenuation type 
phase shift mask in accordance with the present invention will be 
described with reference to the figures. 
Referring to FIG. 1(a), cross sectional structure of a phase shift mask 100 
of attenuation type used in the present embodiment will be described. At 
prescribed positions of a quartz substrate 2, first and second light 
transmitting portions 6 and 8 each having hole diameter of 0.40 .mu.m 
through which quartz substrate 2 is exposed, and a third light 
transmitting portion 10 having hole diameter of 0.35 .mu.m through which 
the substrate is exposed are provided, and remaining region is covered 
with a phase shift film 4. In the present embodiment, a single layer film 
formed of chromium oxide, chromium nitride oxide, chromium nitride carbide 
oxide, molybdenum silicide oxide, molybdenum silicide nitride oxide or the 
like is used as phase shift mask 4, which provides a phase angle of 
180.degree. to the exposure light transmitting therethrough, and its has 
transmittance of 10%. 
At the second light transmitting portion 8, the transparent quartz 
substrate 2 is dug to the depth of d.sub.2 so as to provide phase angle of 
20.degree. to the exposure light transmitting therethrough, and at the 
third transmitting portion 10, quartz substrate 2 is dug to the depth of 
d.sub.1 so as to provide phase angle of 10.degree. to the exposure light 
transmitting therethrough. 
Referring to FIG. 1(b), the resist film which is exposed/developed by using 
the phase shift mask 100 of attenuation type mentioned above will be 
described. The resist film 14 is formed on a stepped substrate 12, and the 
step is reflected to the surface of resist film 14. Therefore, the 
distance between the region A.sub.1 on the left side of the resist film 14 
to a projection lens 101 is different from the distance between the region 
A.sub.2 on the right side to the projection lens 101 (represented by 
h.sub.1 and h.sub.2 in the figure), with the step being the boundary. 
At a position corresponding to the first light transmitting portion 6 of 
resist film 14, a hole 16 having the diameter of 0.35 .mu.m is formed, at 
a position corresponding to the second light transmitting portion 8 of 
resist film 14, a hole 18 having the diameter of 0.35 .mu.m is formed, and 
at a position corresponding to the third light transmitting portion 10 of 
resist film 14, a hole 20 having the diameter of 0.30 .mu.m is formed. 
Generally, when a resist film is to be exposed by using an attenuation 
type phase shift mask and a hole having the diameter of 0.30 .mu.m is to 
be provided in the resist film, it is known that the hole diameter of the 
light transmitting portion on the side of the attenuation type phase shift 
mask should be set to 0.35 .mu.m, that is, larger by 0.05 .mu.m. 
Referring to FIG. 2, relation between focus shift and the mask hole 
diameter formed in attenuation type phase shift mask 100 when a prescribed 
phase angle is provided to the exposure light transmitting through the 
second and third light transmitting portions 6 and 8 will be described. 
FIG. 2 shows the relation between the focus shift (.mu.m) and the mask 
hole diameter (.mu.m) when wavelength (.lambda.) of the exposure light is 
365 nm, numerically aperture (NA) of the exposure apparatus is (NA)=0.57, 
coherency (.sigma.) is (.sigma.)=0.4 and phase angle is 20.degree.. 
According to the present invention, it can be understood from the graph of 
FIG. 2 that focus shift (.mu.m) is 0.3 .mu.m when the mask hole diameter 
is 0.40 .mu.m. Therefore, referring to FIG. 1, the optimal focal position 
of region A.sub.2 is shifted by 0.3 .mu.m with respect to the optimal 
focal position of the region A.sub.1. As a result, when the step between 
the regions A.sub.1 and A.sub.2 is 0.3 .mu.m, it is possible to expose 
both the regions A.sub.1 and A.sub.2 simultaneously through the same step 
of exposure at optimal focal positions, with the mask hole diameter of 
0.40 .mu.m. 
When mask hole diameter is 0.35 .mu.m, the focus shift (.mu.m) is 0.6 
.mu.m, as can be seen from FIG. 2. The relation between the phase angle 
provided to the exposure light and the focus shift is approximately in 
direct proportion, so long as the phase angle is within the range of 
0.degree. to 20.degree., as shown in FIG. 3. Therefore, when the phase 
angle provided to the exposure light transmitting through the third light 
transmitting portion having the hole diameter of 0.35 .mu.m is set to 
10.degree., the focus shift would be 0.3 .mu.m, and therefore the same 
result as provided to the exposure light passing through the second light 
transmitting portion can be obtained. 
According to the first embodiment, it becomes possible to determine optimal 
focal positions corresponding to the step in the substrate, that is, the 
difference between the distances from region A.sub.1 to the projection 
lens and from region A.sub.2 to the projection lens of the exposure 
apparatus, based on the phase angle of the exposure light passing through 
the second and third light transmitting portions and on the hole diameters 
of the second and third light transmitting portions. As a result, even 
when there is a step in the resist film, exposure can be done at optimal 
focal positions simultaneously by using the same attenuation type phase 
shift mask. 
The second embodiment of the method of exposure using the attenuation type 
phase shift mask based on the present invention will be described with 
reference to the figures. First, referring to FIG. 4(a), cross sectional 
structure of the attenuation type phase shift mask 110 used in the present 
embodiment will be described. At prescribed positions of a quartz 
substrate 2, first and second light transmitting portions 22 and 24 having 
hole diameter of 0.45 .mu.m through which quartz substrate 2 is exposed, 
and a third light transmitting portion 26 having hole diameter of 0.4 
.mu.m are provided, and remaining region is covered with a phase shift 
mask 4. Phase shift mask 4 is formed of the same material as in the first 
embodiment described above, and this film provides a phase difference of 
180.degree. to the exposure light transmitting therethrough, and has 
transmittance of 10%. 
At second and third light transmitting portions 24 and 26, transparent 
quartz substrate 2 is dug to the depth of d.sub.1, so as to provide a 
phase angle of 10.degree. to the exposure light passing therethrough. 
A transmittance adjusting film 28 formed of chromium or the like having 
transmittance of 50% is formed on phase shift film 4 near the periphery of 
third light transmitting portion 26. Accordingly, substantial 
transmittance of the region on which transmittance adjustment film 28 is 
formed is 5%. 
The resist film exposed/developed by using the attenuation type phase shift 
mask 110 will be described with reference to FIG. 4(b). The resist film is 
formed on a stepped substrate 12, and the step is reflected to the surface 
of resist film 14. Therefore, the distance from the left region A.sub.1 of 
the resist film 14 to the projection lens 101 differs from the distance 
from the right region A.sub.2 to the projection lens 101 (represented by 
h.sub.1 and h.sub.2 in the figure), with the step being the boundary. 
At a position corresponding to first light transmitting portion 6 of resist 
film 14, a hole 29 having hole diameter of 0.4 .mu.m is formed, at a 
position corresponding to second light transmitting portion 24 of resist 
film 14, a hole 30 having hole diameter of 0.4 .mu.m is formed, and at a 
position corresponding to third light transmitting portion 26 of resist 
film 14, a hole 32 having hole diameter of 0.35 .mu.m is formed. 
Referring to FIGS. 5 and 6, relation between transmittance of phase shift 
film 4 near the periphery of third light transmitting portion 26 and focus 
shift with certain mask hole diameter formed in the attenuation type phase 
shift mask 110 and prescribed phase angle (in the present embodiment, 
10.degree.) applied to the exposure light transmitting through the second 
and third light transmitting portions 24 and 26 will be described. 
FIG. 5 shows relation between transmittance (%) of the phase shift mask in 
the periphery of the third light transmitting portion and focus shift 
(.mu.m) when the mask hold diameter is 0.45 .mu.m, wavelength .lambda. of 
the exposure light is .lambda.=365 nm, numerical aperture (NA) of the 
exposure apparatus is (NA)=0.57 and the coherency (.sigma.) of the 
exposure apparatus is (.sigma.)=0.4. FIG. 6 shows the relation between 
transmittance (%) of the phase shift mask in the periphery of the third 
light transmitting portion and focus shift (.mu.m) under the same 
conditions as in the example of FIG. 5 except that the mask hole diameter 
is 0.40 .mu.m. 
Referring to FIG. 5, when the mask hold diameter is 0.4 .mu.m and the mask 
transmittance (%) is 10%, the focus shift is 0.23 .mu.m. Therefore, if the 
step between the regions A.sub.1 and A.sub.2 is 0.23 .mu.m, both the 
regions A.sub.1 and A.sub.2 can be exposed through the same step of 
exposure simultaneously at optimal focal positions. When the mask hole 
diameter is 0.4 .mu.m and the same amount of focus shift is to be 
obtained, the mask transmittance (%) should be about 5%. Therefore, by 
setting the transmittance near the third light transmitting portion of the 
attenuation type phase shift mask 110 to 5% as in the present embodiment, 
every region can be exposed at optimal focal position. 
As described above, according to the second embodiment, it becomes possible 
to expose a stepped resist film at optimal focal positions, based on the 
phase angle of the exposure light passing through the second and third 
light transmitting portions, the hole diameters of the second and third 
light transmitting portions, the hole diameters of the second and third 
light transmitting portions and the transmittance of the phase shift 
portion near the periphery of the second and third light transmitting 
portions. Further, as compared with the first embodiment, by adjusting 
transmittance of the attenuation type phase shift mask near the periphery 
of the third light transmitting portion, the depth of trenches dug in the 
substrate for providing phase angle to the second and third light 
transmitting portions can be made the same, and therefore manufacturing of 
the attenuation type shift mask is facilitated. 
A third embodiment of the method of exposure employing phase shift mask of 
attenuation type of the present invention will be described with reference 
to the figures. 
Referring to FIG. 7(a), cross sectional structure of the phase shift mask 
130 of attenuation type used in the present embodiment will be described. 
On prescribed positions of quartz substrate 2, there are a first pattern 
P.sub.1 in which first light transmitting portions 34 having hole diameter 
of 0.45 .mu.m through which transparent quartz substrate 2 is exposed are 
arranged with a pitch of 2.0 .mu.m; a second pattern P.sub.2 in which 
second light transmitting portions 36 having hole diameter of 0.45 .mu.m 
through which quartz substrate 2 is exposed are arranged at a pitch of 2.0 
.mu.m; and a third pattern P.sub.3 in which third light transmitting 
portions 38 having hole diameter of 0.45 .mu.m through which quartz 
substrate 2 is exposed are arranged at a pitch of 0.7 .mu.m. Phase shift 
film 4 is formed of the same material as in the first and second 
embodiments, it provides a phase angle of 180.degree. to the exposure 
light passing therethrough, and it has transmittance of 10%. At the second 
light transmitting portion 36, quartz substrate 2 is dug to the depth of 
d.sub.1 so as to provide a phase angle of 10.degree. to the exposure light 
transmitting therethrough, and at the third light transmitting portion 38, 
quartz substrate 2 is dug to the depth of d.sub.2 so as to provide phase 
angle of 20.degree. to the exposure light passing therethrough. 
The resist film exposured/developed by using the attenuation type phase 
shift mask 130 will be described with reference to FIG. 7(b). 
The resist film 14 is formed on a stepped substrate 12, and the step is 
reflected to the surface of resist film 14. Therefore, the distance from 
the left region A.sub.1 of the resist film 14 to projection lens 101 is 
different from the distance from the right region A.sub.2 to the 
projection lens 101 (h.sub.1 and h.sub.2 in the figure), with the step 
being the boundary. 
At a position corresponding to the first pattern P.sub.1 of resist film 14, 
holes 40 having the diameter of 0.4 .mu.m are formed at a pitch of 2.0 
.mu.m, at the position corresponding to the second pattern P.sub.2 of 
resist film 14, holes 42 having the diameter of 0.4 .mu.m are formed at a 
pitch of 2.0 .mu.m, and at the position corresponding to the third pattern 
P.sub.3 of resist film 14, holes 44 having the diameter of 0.4 .mu.m are 
formed at a pitch of 0.7 .mu.m. 
Relation between the distance between the second and third light 
transmitting portions of the second and third patterns and focus shift 
with prescribed phase angle of the exposure light passing through the 
second and third patterns and prescribed hole diameters of the second and 
third patterns will be described with reference to FIG. 8. FIG. 8 shows 
the relation between the pitches of the mask holes and the focus shift 
when the wavelength of the exposure light is 248 nm, numerical aperture 
(NA) of the exposure apparatus is (NA)=0.5, coherency (.sigma.) is 
(.sigma.)=0.4, phase angle is 10.degree. and the mask hole diameter is 
0.45 .mu.m. It can be seen from this figure that when the pitch of the 
mask holes is 2.0 .mu.m, the focus shift is 0.2 .mu.m. Therefore, if the 
step between the regions A.sub.1 and A.sub.2 is 0.2, the regions A.sub.1 
and A.sub.2 can be exposed simultaneously through the same step of 
exposure with optimal focal positions. 
If the pitch of the mask hole is 0.7, for example, it can be seen that the 
focus shift is 0.1 .mu.m from FIG. 8. Therefore, as shown by the graph of 
FIG. 3 showing the relation between the phase angle and the focus shift, 
as the phase angle and the focus shift is in direct proportion as long as 
the phase angle is 0.degree. to 20.degree., the focus shift can be set to 
0.2 .mu.m by setting the phase angle 20.degree., with the pitch of the 
mask hole being 0.7. Therefore, by using the attenuation type phase shift 
mask 130 shown in FIG. 7(a), every region of the resist film 14 having the 
step of 0.2 .mu.m shown in FIG. 7(b) can be exposed at the optimal focal 
position. 
In the first, second and third embodiments above, the numerical aperture 
(NA) of the exposure apparatus and the coherency (.sigma.) are fixed. 
However, there is such a relation as shown in FIG. 9 between the numerical 
aperture (NA) and the focus shift. Therefore, it should be noted that when 
the numerical aperture changes, the focus shift changes accordingly. 
Further, there is such a relation as shown in FIG. 10 between the value of 
coherency (.sigma.) and the focus shift. Therefore, it should be noted 
that when coherency (.sigma.) changes, the focus shift changes 
accordingly. 
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 being limited only by the terms of the 
appended claims.