Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-12-14
2002-10-01
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06458496
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
This application claims the priority of Japan patent application Serial No. P11-355522 filed on Dec. 15, 1999 and Japan patent application Serial No. P2000-154687 filed on May 25, 2000.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photomask that is used for producing high density integrated circuits like LSI and VLSI or for forming other microscopic patterns, and to a blank for producing the such a photomask, especially relates to a halftone phase shift photomask by which projection image in fine dimension can be obtained and to a blank for producing the halftone phase shift photomask.
2. Description of the Related Art
Semiconductor integrated circuits, including IC, LSI and VLSI, are produced by repeating the lithography process of using a photomask, especially in case of forming fine circuit patterns, it is studied to use phase shift photomasks disclosed in, for example, Japanese Patent Application Laid-Open No. 58-173744, Japanese Patent Application Publication No. 62-59296 and others.
Although phase shift photomasks with various types of constitution are pro posed, among them, for example, halftone phase shift photomasks disclosed in Japanese Patent Application Laid-Open No. 4-136854, U.S. Pat. No. 4,890,309 and others attract attentions from the viewpoint of possibility of being put to practical use in early time.
As for halftone phase shift photomasks, in literatures, including, for example, Japanese Patent Application Laid-Open Nos. 5-2259 and 5-127361, the improvement of yield rates by reducing the number of production processes, the constitution with possibility of reducing the cost, preferable materials and others have been proposed.
In the following, a common halftone phase shift method and a common halftone phase shift photomask will be briefly explained with reference to the accompanying drawings.
FIG.
14
(
a
) to FIG.
14
(
d
) are views showing the principle of a halftone phase shift method (a halftone phase shift photo lithography), and FIG.
15
(
a
) to FIG.
15
(
d
) are views showing the principle of a photomasking method using a photomask except a phase shift photomask. FIG.
14
(
a
) and FIG.
15
(
a
) are sectional views of a photomask, FIG.
14
(
b
) and FIG.
15
(
b
) show the amplitude of light on a photomask, FIG.
14
(
c
) and FIG.
15
(
c
) show the amplitude of light on a wafer, and FIG.
14
(
d
) and FIG.
15
(
d
) show the strength of light on a wafer. Reference numerals
911
and
921
denote substrates,
912
denotes a halftone phase shift film in which the phase of incident light is substantially shifted by 180 degree and the transmittance of transmitted light is within the range of 1 to 50%,
922
denotes a 100% light shielding film, and
913
and
923
denote incident light.
In a conventional photomasking method, as shown in FIG.
15
(
a
), the 100% light shielding film
922
made of chromium or the like is formed on the substrate
921
consisting of quartz glass to form a light transmission part (an aperture) in a desired pattern. In this case, the distribution of light strength on a wafer is broadened toward the end as shown in FIG.
15
(
d
), resulting in inferior resolution.
On the other hand, in a halftone phase shift method, because the phase of light transmitted through the halftone phase shift film
912
is substantially inverted to that of light transmitted through the aperture, light strength on boundary parts of patterns on a wafer becomes zero as shown in FIG.
14
(
d
), which can prevent light from broadening toward the end. In this case, therefore, resolution can be improved.
Here, it should be noted that, in phase shift photo lithography that belongs in types except a halftone phase shift method, because a light shielding film and a phase shifter film are formed in different patterns with different materials, the plate making process is needed to be repeated at least 2 times, while because it is enough to form only one pattern in the halftone phase shift photo lithography, it is essentially needed to carry out the plate making process only once and this is a big advantage in halftone phase shift lithography.
In the halftone phase shift film
912
of a halftone phase shift photomask, two functions, that is, phase inversion and permeability control are needed. Out of them, as for the phase inversion function, it is sufficient that phase will be substantially inverted between exposure light transmitting through the halftone phase shift film
912
and exposure light transmitting through the aperture. Here, if the halftone phase shift film
912
is considered as an absorption film that is shown, for example, in pages 628 to 632 of “Principles of Optics” written by M. Born and E. Wolf, since multiplex interference can be ignored, phase change öof vertically transmitted light will be calculated using the following expression. And when the value of &phgr; is within the range of n&pgr;±&pgr;/3 (n is an odd number), the above-mentioned phase shift effect will be obtained.
φ
=
∑
k
=
1
m
-
1
x
k
,
k
+
1
+
∑
k
=
2
m
-
1
2
π
(
u
k
-
1
)
d
k
/
λ
Expression (1)
Further, in expression (1), &phgr; is a phase change caused to light vertically transmitting through a photomask in which a multilayer film of (m−2) layers is formed on the substrate, &khgr;
k,k+1
is a phase change occurring in the interface between a k
th
layer and a (k+1)
th
layer, u
k
and d
k
are the refractive index and film thickness of the k
th
layer, respectively and &lgr; is the wavelength of exposure light, providing that the layer of k=1 is the above mentioned transparent substrate and the layer of k=m is air.
On the other hand, the transmittance of exposure light transmitted through the halftone phase shift film
912
for obtaining a halftone phase shift effect is determined by the dimension, area, arrangement, shape and the like of a transcription pattern, and differs depending on patterns.
In order to substantially obtain the above-mentioned effect, the transmittance of exposure light transmitted through the halftone phase shift film
912
should be within the range of the optimum transmittance±some percents, where the center value is the optimum transmittance determined by the pattern.
Generally, this optimum transmittance greatly varies within the wide range of 1 to 50% depending on transcription patterns when the transmittance in the aperture of the halftone phase shift film is set to 100%. That is, in order to adapt to all patterns, halftone phase shift photomasks having various transmittances are needed.
In a practical situation, the phase inversion function and the transmittance control function are determined by a complex refractive index (a refractive index and an extinction coefficient) and film thickness of a material forming the halftone phase shift film. In case of a multilayer structure, the phase inversion function and the transmittance control function are determined depending on a complex refractive index and a film thickness of each layer. In other words, it is possible to use a material adjustable its film thickness so as to control phase difference calculated by the above mentioned expression (1) within the range of n&pgr;±&pgr;/3 (n is an odd number) as a halftone phase shift film of a halftone phase shift photomask.
As thin film materials for photomask patterns, tantalum based materials are commonly known as shown in, for example, Japanese Patent Application Laid-Open No. 57-64739, Japanese Patent Application Publication Nos. 62-51460 and 62-51461. Because tantalum based materials are extremely excellent in processing properties, chemical stability after being processed, and others, they have been vigorously studied and tried to apply them in halftone phase shift films by oxidizing ornitriding tantalum as disclosed in, for example, Japanese Patent Application Laid-Open Nos. 5-257264, 7-134396 and 7-281414. Furthermore, with the advancement o
Fujikawa Junji
Kinase Yoshinori
Mohri Hiroshi
Motonaga Toshiaki
Nakagawa Hiro-o
Dai Nippon Printing Co. Ltd.
Keefer Timothy J.
Rosasco S.
Wildman Harrold Allen & Dixon
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