Method for fabricating a phase shifting mask

A method for fabricating a phase shifting mask suitable for positive photoresist process. The method includes the steps of: (a) forming a plurality of opaque layer patterns (44) in an array at a fixed interval from each other in their width direction on a substrate (41); (b) coating an interlayer (45) on and covering the opaque layer patterns; (c) forming interlayer patterns (45) on the substrate at both longitudinal sides of each opaque layer pattern by etching the interlayer; (d) forming a plurality of insulation films (46) on the substrate between adjacent pairs of the opaque layer patterns on which the interlayer patterns are formed; (e) removing the remaining interlayer under each of the insulation films; and (f) forming a phase shifter (47) having a ninety degree area (47-2) in a region where the interlayer has been removed and a one hundred and eighty degree area (47-1) in the remainder of the region by heating the insulation film.

FIELD OF THE INVENTION 
This invention relates to a method for fabricating a phase shifting mask, 
more particularly to a method for fabricating a phase shifting mask 
suitable for positive photo resist process. 
BACKGROUND OF THE INVENTION 
Shown in FIG. 1 is a common mask 10 for patterning, and shown in FIG. 2(a) 
is amplitude of light on the mask 10, FIG. 2(b) is amplitude of light on a 
wafer and FIG. 2(c) is intensity of light on the wafer. 
Referring to FIG. 1, a common patterning mask 10 includes structure having 
opaque layers 12 of chrome spaced from each other in a certain interval 
formed on a transparent substrate 11 of quartz. 
As shown in FIG. 2(a), the amplitude of light on the mask 10 overlaps 
offsetting each other on both edges of the opaque layer 12, and the 
amplitude, and the intensity of light on the wafer behaves as shown in 
FIGS. 2(b) and 2(c), respectively. 
Therefore, in case the mask 10 is used, it is impossible to obtain a clear 
patterning due to smaller difference of intensity of light, exhibiting 
unclear shade, at both edges of the opaque layer 12. 
Moreover, because the rate of offset of light on both edges of the opaque 
layer is greater in a microscopic patterning, a microscopic patterning 
process can not be carried out with the foregoing mask 10. 
Recently, as all elements are highly integrated, requirements on masks for 
carrying out super microscopic patterning in the order of submicron has 
risen. 
To meet such a requirements, the phase shifting masks have been developed. 
Shown in FIG. 3 is a section of a conventional phase shifting mask, and 
shown in FIG. 4 is wave of light of the phase shifting mask of FIG. 3. 
Referring to FIG. 3, a common phase shifting mask 20 includes a structure 
having opaque layers 22 of chrome spaced from each other in a certain 
interval formed on a transparent substrate 21 of quartz, between which 
opaque layers 22 phase shifters 23 are formed. 
These phase shifters 23 serve to shift the phase of amplitude of light 
incident to the substrate. 
Shown in FIG. 4(a) is amplitude of light on the phase shifting mask 20 of 
FIG. 3. 
In FIG. 4(a), curve A indicates amplitude of light incident to the 
substrate 21 when there is no phase shifting mask 23, and curve B 
indicates amplitude of light incident to the substrate 21 when there is 
the phase shifter 23. 
According to FIG. 4(a), it can be shown that the phase of the amplitude of 
light incident to the substrate 21 has been shifted 180 degrees by the 
phase shifter 23. 
In FIG. 4(a), the phase difference .delta. between the graph A and the 
graph B can be represented in following formula (1). 
##EQU1## 
where, n is the refraction index of the phase shifter 23, d is the 
thickness of the phase shifter 3, and nO is the refraction index of air. 
In formula(1), it can be known that the phase difference should be 180 
degrees ie., .pi. for shifting the phase of an amplitude of light 
completely. 
When the phase difference .delta. is substituted by .pi. in formula(1), the 
thickness of the phase shifter 23 for shifting the phase completely can be 
represented in following formula. 
##EQU2## 
A phase shifting mask 20 having conventional phase shifter 23 includes the 
phase shifter 23 formed between two adjacent opaque layers 22, which phase 
shifter 23 shifts the phase of incident light onto a substrate 21 180 
degrees as shown in FIG. 4(a). 
Accordingly, though the phase shifting mask 20 is used, overlap of light at 
both edges of the opaque layers 23 does not occur, which makes light 
having an amplitude as shown in FIG. 4(b) incident onto a wafer. 
Therefore, using the foregoing phase shifting mask 20 is advantageous for 
super microscopic patterning due to the great difference of intensity of 
light at both edges of the opaque layers 23 providing clear shade of light 
incident onto a wafer. 
There are spatial frequency modulation type, edge emphasis type and cut-off 
effect emphasis type in kinds of phase shifting masks. 
Of the foregoing phase shifting masks, a spatial frequency modulation type 
phase shifting mask has, as known well, disadvantage of carrying out 
removing unnecessary bridge pattern film formed at the edges of the phase 
shifter at the time of fabrication. 
Recently, as a solution for this problem, a process for fabricating a phase 
shifter which can shift the phase at the edges of a phase shifter by 90 
degrees instead of shifting 180 degrees, is under development. 
In a phase shifting mask having a 180 degrees area for shifting the phase 
at a main area of the phase shifter by 180 degrees, and a 90 degrees area 
for shifting the phase at edges of the phase shifter by 180 degrees, the 
phase shifter has to have different thicknesses in the 180 degrees area 
and the 90 degrees area from each other in order for a phase shifter, 
shifting phase 180 degrees, to shift the phase 90 degrees only at the 
edges thereof. 
As for the method for fabricating a phase shifting mask having 180 degrees 
area and 90 degrees area, there is a method for fabricating a phase 
shifter having different thicknesses in 90 degrees area and 180 degrees 
area from each other through two times of deposition processes and two 
times of patterning processes. 
The foregoing method is described in detail in SPIE Vol. 1604, 11th Annual 
BACUS symposium on Photomask Technology, 1991, pp 265 to 273. 
However, such a method is cumbersome due to two times of deposition 
processes in forming the phase shifter and two times of photo etching 
processes in forming the 90 degrees area and the 180 degrees area. 
As an another method, there is a method for fabricating a phase shifting 
mask having 90 degrees area at the edges of the 180 degrees area by 
forming 90 degrees area through etching of the edges of the 180 degrees 
area after a phase shifter of 180 degrees area having been formed. 
Referring to FIG. 5, the foregoing Levenson type phase shifting mask is to 
be explained in detail. 
FIGS. 5(a) to 5(c) show a structure of a conventional Levenson type phase 
shifting mask, of which FIG. 5(a) shows a plan view of the phase shifting 
mask, FIG. 5(b) shows a section across line A--A' of FIG. 5(a), and FIG. 
5(c) shows a section across line B--B' of FIG. 5(c). 
Referring FIGS. 5(a) to 5(c), a conventional Levenson type phase shifting 
mask 30 is provided with a structure having a plurality of chrome patterns 
34 formed spaced from each other on a quartz substrate 31 as opaque layers 
and a plurality of phase shifters 37 formed between and partially 
overlapped with adjacent two chrome patterns 34 on the substrate 31. 
Each phase shifter 37 has 180 degrees area 37-1 and 90 degrees area 37-2, 
wherein the part corresponding to the opaque layer pattern 34 is 180 
degrees area 37-1, and the portion corresponding to the edge parts of the 
opaque layer pattern 34 is 90 degrees area 37-2. 
Referring to FIGS. 6 and 7, a method for fabricating a conventional 
Levenson type phase shifting mask having a structure as shown in FIG. 5 is 
to be explained hereinafter. 
FIGS. 5(a) to 6(m) and 7(a) to 7(m) show a method for fabricating a 
conventional Levenson type phase shifting mask, of which, FIGS. 6(a) to 
6(m) are sections across line A--A' of FIG. 5(a) showing the fabrication 
method, and FIGS. 7(a) to 7(m) are sections across line B--B' of FIG. 5(a) 
showing the fabrication method. 
First, as shown in FIGS. 6(a) and 7(a), a chrome layer 32 is formed on a 
quartz substrate 31, and a negative photoresist film 33 is coated thereon. 
Through exposure (FIGS. 6(b) and 7(b)) and development (FIGS. 6(c) and 
7(c)), the negative photoresist film 33 is patterned with a predetermined 
pattern. 
Then, the chrome layers are etched using the patterned negative photoresist 
film 33 (FIGS. 6(d) and 7(d)), and the remained photoresist film 33 is 
removed to form a plurality of opaque layer patterns 34 (FIGS. 6(e) and 
7(e)). 
The plurality of opaque layer patterns 34 are positioned in array with a 
certain interval from each other in the direction of the width thereof 
(direction of line A--A'). 
Next, an insulation film 35 is coated on all over the opaque layers 34 
(FIGS. 6(f) and 7(f)), on which insulation film 35 a negative photoresist 
film 36 is coated (FIGS. 6(g) and 7(g)), which is subjected to patterning 
through exposure (FIGS. 6(h) and 7(h)) and development (FIGS. (7i) and 
7(i)) using predetermined pattern. 
The insulation film is etched using the patterned negative photoresist film 
36 (FIGS. 6(j) and 7(j)), and the remained photoresist film 36 is removed 
forming a plurality of phase shifters 37 between and partially overlapped 
with two adjacent opaque layer patterns 34 (FIGS. 6(k) and 7(k)). 
This phase shifter 37 is a 180 degrees phase shifter. 
Then an etching process is carried out for etching the edges of the phase 
shifters 37 to a predetermined thickness so as to form the 90 degrees area 
shifting the phase 90 degrees at the edges of each phase shifters. 
Again, a negative photoresist film 38 is coated on all over the surface, 
which is undertaken a patterning with predetermined pattern exposing both 
edges of the phase shifters 37 in the longitudinal direction(direction of 
line B--B' of FIG. 5(a) thereof (FIGS. 6(l) and 7(l)). 
The exposed edge part of the phase shifter 37 is etched to a certain 
thickness using the patterned negative photoresist film 38. 
Thus, fabrication of a conventional phase shifting mask is completed by 
forming phase shifters 37 which can shift the phase 90 degrees at both 
edges not overlapping with the opaque layer patterns 37 and shift the 
phase 180 degrees in rest of the part. 
As such, the foregoing method for fabricating a conventional Levenson type 
phase shifting mask has a cumbersome process of carrying out photo etching 
process once more to form the 90 degrees area 37-2 which shifts the phase 
90 degrees at edges of the phase shifters, and a problem of difficulty in 
controlling the process for etching the phase shifters of the edge part to 
shift the phase 90 degrees exactly. 
SUMMARY OF THE INVENTION 
The object of this invention designed to solve foregoing problems, is 
providing a method for fabricating a phase shifting mask suitable for 
photoresist process, with which quality phase shifters can be obtained. 
These and other objects and features of this invention can be achieved by 
providing a method for fabricating a phase shifting mask including steps 
for forming a plurality of opaque layer patterns in array at a fixed 
interval from each other in the direction of width thereof on a substrate, 
coating an interlayer on all over the substrate to cover the plurality of 
the opaque layer patterns, forming interlayer patterns on the substrate at 
both sides in the longitudinal direction of each opaque layer by etching 
the interlayers, forming a plurality of insulation films on the substrate 
between adjacent one pair of the opaque layer patterns the interlayer 
patterns formed thereon, removing the remained interlayers under each of 
the insulation film, and forming a phase shifter having 90 degrees area in 
a part where the interlayer has been removed and 180 degrees area in rest 
of the part by making the insulation film cause to flow with heat 
treatment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 8(a) to 8(c) show a structure of a phase shifting mask in accordance 
with a first embodiment of this invention, wherein FIG. 8(a) is a plan 
view, FIG. 8(b) is a section across line C--C' of FIG. 8(a), and FIG. 8(c) 
is a section across line D--D' of FIG. 8(a). 
FIGS. 9(a) to 9(i) show a process for fabricating a phase shifting mask 
across line C--C' of FIG. 8(a), and FIGS. 10(a) to 10(i) show a process 
for fabricating a phase shifting mask across line D--D' of FIG. 8(a). 
First, referring to FIGS. 9(a) to 9(i) and 10(a) to 10(i), a glass 
substrate 41 is coated with a chrome layer 42, on which a resist film 43 
for electron beam lithography is coated (FIGS. 9(a) and 10(a)). 
The resist film 43 is undertaken patterning to a desired pattern using an 
electron beam lithography system (FIGS. 9(b) and 10(b)), which resist film 
43 pattern is used for etching the chrome layer, and the remained resist 
film 43 is removed to form a plurality of opaque layer patterns 44 (FIGS. 
9(c) and 19(c)). 
The plurality of the opaque layer patterns 44 is positioned in array with a 
fixed interval from each other. 
Then, an electron beam resist film 45 is coated again on all over the 
substrate to cover the opaque layers 44 completely (FIGS. 9(d) and 10(d)), 
which is subjected to a patterning to from a desired pattern using an 
electron beam lithography system (FIGS. 9(e) and 10(e)). 
Referring to FIG. 16(a), the resist film 45 patterns are formed spaced with 
the opaque layer patterns 44 in a fixed interval. 
Then, a spin on glass film 46 is formed on all over the substrate to a 
thickness of .lambda./2(n-1), a thickness capable to shift the phase 180 
degrees (FIGS. 9(f) and 10(f)). 
The SOG (Spin On Glass) film 46 is subjected to an etching to form a 
desired pattern (FIGS. 9(g) and 10(g)). 
Next, the patterns of the electron beam resist film 45 formed beyond each 
of the opposing sides of each one pair of the opaque layer patterns 44 
(FIGS. 9(h) and 10(h)) are removed, and, which is undertaken a heat 
treatment at a temperature about 200 to 400 degrees C. 
When the SOG film 46 is subjected to a heat treatment to cause flow, the 
SOG film 46 at the edge parts the electron-beam resist film 45 having been 
removed therefrom (FIG. 8(a) and E parts of FIG. 10(i)) become linear on 
the substrate 21, finally obtaining a phase shifter 47. 
This phase shifter 47 has vertical sides in the longitudinal direction of 
each opaque layer pattern 44 (D--D' direction) as shown in FIGS. 8(b) and 
9(i), and linear sides in the direction of width of each opaque layer 
pattern 44 (C---C' direction) as shown in FIGS. 8(a) and 10(i). 
That is, in the first embodiment, the phase shifter has the 90 degrees area 
47-1 formed only on both edges of the longitudinal direction (D--D') 
thereof. 
Thus, a phase shifter 47 having 180 degrees area 47-1 and 90 degrees area 
47-2 can be obtained by appropriately controlling the thickness of the 
electron beam resist film 45. 
That is, where the refraction index is n, the thickness is d, and .lambda. 
is the wave length of transmission light of a phase shifter 47, the 
thickness of a phase shifter d=.lambda.k/2(n-1). 
In this time, the thickness d of the phase shifter 47 is deposited so that 
the phase can be shifted 180 degrees in the main pattern areas 
corresponding to upper surfaces of adjacent one pair of opaque layer 
patterns 44. 
On the other hand, since the phase has to be shifted 90 degrees at the 
edges of the phase shifter 47, the electron beam resist film 45 has to be 
coated to have a thickness of .lambda./4(n-1), one half of the thickness 
of the phase shifter 47 because the 90 degrees areas 47-2 have to be 
formed on both sides of adjacent opaque layer patterns 44 by flow process 
after removing the resist film formed thereon as shown in FIGS. 10(f) to 
10(h). 
FIGS. 11(a) to 11(c) show a structure of a phase shifting masks in 
accordance with a second embodiment of this invention, and FIGS. 12(a) to 
12(i) show a process for fabricating a phase shifting mask across line 
D--D' of FIG. 11(a). 
In FIGS. 11(a) to 11(c) and 12(a) to 12(i), identical reference numbers are 
given to the identical or corresponding parts with FIGS. 8 to 10 for the 
first embodiment. 
The method for fabricating a phase shifting mask in accordance with the 
second embodiment is identical to the method for fabricating a phase 
shifting mask in accordance with the first embodiment. 
However, as shown in FIG. 16(a), though the pattern of the electron beam 
resist films 45 are formed at lower part of the edges of the phase shifter 
47 spaced with the opaque layer pattern 44 in a fixed distance in first 
embodiment, as shown in FIG. 16(b), the pattern of the electron beam 
resist films 45 are formed on the lower part of the edges of the phase 
shifter 47 excluding the edges of the phase shifter 47 overlapped with the 
adjacent one pair of opaque layer patterns 44 in the second embodiment. 
That is, the process for fabricating phase shifting mask across line C--C' 
of FIG. 11(a) is identical to the process in accordance with the first 
embodiment (FIGS. 9(a) to 9(i)), and the process for fabricating a phase 
shifting mask across line D--D' of FIG. 11(a) is as shown in FIGS. 12(a) 
to 12(i). 
Referring to FIGS. 12(a) to 12(i), the process for fabricating a phase 
shifting mask in accordance with the second embodiment is the same with 
the process in accordance with the first embodiment, in which a plurality 
of opaque layer patterns 44 are formed on a substrate 41, on which an 
electron resist film 44 is coated (FIGS. 12(a) to 12(d)). 
Then, the electron beam resist film 45 is etched using an electron beam 
lithography system so that the pattern of the electron beam resist film 45 
can be formed on the substrate 41 contacting with both edges in the 
longitudinal direction of the opaque layer pattern 44 (FIG. 12(e)). 
After carrying out patterning of the electron beam resist film 45, a SOG 
film 46 is coated on all over the substrate, which SOG film 46 is 
undertaken a patterning over and between one pair of adjacent opaque layer 
patterns 44 (FIGS. 12(f) and 12(g)). 
Referring to FIG. 16(b), in the second embodiment, the electron beam resist 
film 45 is formed at lower part of the edges of the SOG film 46. 
In this time, the electron beam resist film 45 does not exist at the lower 
part of edges of the SOG film formed on one pair of adjacent opaque layer 
patterns 44. 
Then, the pattern of the electron beam resist film 45 is removed (FIG. 
12(h)), the SOG film 46 is made to cause flow to form a phase shifter 47 
(FIG. 12(i)). 
The phase shifter 47 in accordance with the second embodiment has 90 
degrees area 47-2 formed on all edges excluding the part overlapped with 
one pair of the adjacent opaque layer patterns 44. 
FIG. 13(a) is a plan view of a phase shifting mask in accordance with a 
third embodiment of this invention, FIG. 13(b) is a section across line 
C--C' of FIG. 13(a), and FIG. 13(c) is a section across line D--D' of FIG. 
13(a). 
Referring to FIGS. 13(a) to 13(c) for the phase shifting mask in accordance 
with the third embodiment, though the 90 degrees area 47-2 is formed only 
at the edge part in the longitudinal direction of the phase shifter 47 in 
accordance with the first embodiment, the 90 degrees area 47-2 is formed 
at all edges of the phase shifters in accordance with the third 
embodiment. 
FIGS. 14(a) to 14(i) show a process for fabricating a phase shifting mask 
across line C--C' of FIG. 13(a), and FIGS. 15(a) to 15(i) show a process 
for fabricating a phase shifting mask across line D--D' of FIG. 13(a). 
Referring to FIGS. 14(a) to 14(i) and 15(a) to 15(i), the process for 
fabricating a phase shifting mask in accordance with the third embodiment 
has, same as the first embodiment, a plurality of opaque layer patterns 44 
formed on a substrate 41, and an election beam resist film 45 coated on 
all over the substrate the opaque layer patterns 44 have been formed 
thereon (FIGS. 14(a) to 14(d) and 15(a) to 15(d)). 
Then, as shown in FIGS. 14(e) and 15(e), the electron beam resist film 44 
is undertaken patterning using an electron beam lithography system. 
As shown in FIGS. 15(f) to 15(g) and 16(f) to 16(g), a SOG film 46 is 
coated on all over the substrate the resist film 45 and the opaque layer 
patterns 44 have been formed thereon, and is undertaken patterning. 
Referring to FIG. 16(c), in the third embodiment, the pattern of the 
electron beam resist film 45 is formed at lower part of all the edges of 
the SOG film 46. 
Next, the electron beam resist film 45 formed at lower part of the SOG film 
46 is removed totally (FIGS. 14(h) and 15(h)), and a phase shifter 47 is 
formed over one pair of the adjacent opaque layer patterns 44 (FIGS. 14(i) 
and 15(i)). 
Referring to FIGS. 14(i) and 15(i), center of the phase shifter 47 is a 180 
degrees area 47-1 and all the edges thereof are a 90 degrees area 47-2. 
As the foregoing description, this invention has following advantages over 
the conventional method for fabricating a phase shifting mask. 
First, there is easiness in process because forming of the 90 degrees area 
is carried out with heat treatment in this invention, while there is 
difficulty in obtaining exact 90 degrees area because a phase shifter 
having 90 degrees area is formed by etching both edges of a phase shifter 
in conventional method. 
And, as the heat treatment is carried out at a temperature range of 200 to 
400 degrees C., a low temperature heat treatment is viable. 
Second, because this invention can control the thickness of a photoresist 
film to form an exact 90 degrees phase shifter easily, it is possible to 
prevent a phase shifter from forming bridge pattern films generated at the 
side walls thereon. 
Third, in case this phase shifting mask is to be used in a positive 
photoresist process, it is possible to obtain a good quality pattern 
profile.