Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-04-03
2004-08-24
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06780547
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a photomask used in the production of a density integrated circuit such as an LSI, blanks for the photomask for the production of the photomask and a method for forming a pattern by using the halftone phase shifting photomask, and in particular the invention relates to a halftone phase shifting photomask by which a projected image having fine dimensions can be obtained, blanks for the halftone phase shifting photomask for the production of the halftone phase shifting photomask and a method for forming the pattern by using the halftone phase shifting photomask.
(2) Description of the Prior Art
Semiconductor integrated circuits such as an LST are produced by repeating the so-called lithographic process in which-a photomask is used. Particularly, for the formation of products having fine dimensions, the use of a phase shifting photomask such as that described for example in Japanese Patent Laid-Open No. 173744 of 1983 (Showa 58), Japanese Patent Publication No. 59296 of 1987 (Showa 62) and others are studied. Especially, the so-called halftone phase shifting photomask such as that described for example in U.S. Pat. No. 4,890,309 attracted the attention of people from a viewpoint of early practical application. Some proposals are made as to the improvement in yield and the structure and material that result in a decrease in cost which proposals are described in Japanese Patent Laid-Open No. 2259 of 1993 (Heisei 5), Japanese Patent Laid-Open No. 127361 of 1993 (Heisei 5) and others, and making such proposals practicable is proceeding.
A halftone phase shifting photomask is explained briefly referring to figures.
FIG. 1
comprises explanatory views showing the principle of halftone phase shifting lithography.
FIG. 2
comprises explanatory views showing a conventional method. FIG.
1
(
a
) and FIG.
2
(
a
) are each a sectional view of a photomask, respectively. FIG.
1
(
b
) and FIG.
2
(
b
) show the amplitude of light on the photomask, respectively. FIG.
1
(
c
) and FIG.
2
(
c
) show the amplitude of light on a wafer, respectively. FIG.
1
(
d
) and FIG.
2
(
d
) show the strength of light on the wafer, respectively. Numerals
11
and
21
(FIG.
1
(
a
)) designate a transparent substrate. Numeral
22
FIG.
2
(
a
)) designates 100% lightproof film. Numeral
12
designates a halftone phase shifting film. Numerals
13
and
23
designate incident rays. Halftone phase shifting film is a film having the function that the phase of applied rays which have passed through the film is inverted in fact against the phase of applied rays passing through air with the same length of light paths and the optical strength of the former is attenuated against the optical strength of the latter, which is formed of a layer or multiple layers. In a conventional method, as shown in FIG.
2
(
a
), 100% lightproof film
22
made of chromium and others is formed on substrate
21
made of quartz glass and others, by which a light-transmitting area having a desired pattern is only formed, wherein as shown in FIG.
2
(
d
), the distribution of optical strength shows the spread of edges so that the resolution is decreased. On the contrary, in the halftone phase shifting lithography, since the phase of rays which have transmitted through halftone phase shifting film
12
of semitransparent film is inverted in fact against phase of rays which have passed through openings of the halftone phase shifting film
12
, as shown in FIG.
1
(
d
), the optical strength in the border of the pattern formed on the halftone phase shifting photomask becomes 0 on the wafer so that the spreading of both edges is suppressed, therefore the solution being improved.
Special attention should be paid to that in order to obtain the effect of halftone phase shifting photomask, the phase angle and the transmittance of halftone phase shifting film become very important, adding to various characters such as the dimensional tolerances, the positional tolerances and others of pattern formed on the photomask which are requested to a conventional photomask, and further that the phase angle and transmittance are determined according to refractive index, coefficient of extinction in the wavelength of the applied rays and the thickness of film of a layer or multiple layers forming the halftone phase shifting film.
Generally, the most suitable value for the phase angle is 180° . However, the most suitable value for the transmittance is in the region of 1 through 20% supposing the transmittance in the opening area of halftone phase shifting film is 100%, which is determined according to the pattern to be transferred, transferring conditions and others. As to the halftone phase shifting photomask, it is requested to set the phase angle and the transmittance to the most suitable values thereof. When values of them deviate from the most suitable value, a proper quantity of the change of applied rays appears, which results in the lowering of the dimensional tolerance, and the lowering of the focal latitude or allowable extent and others. Accordingly, refractive index, coefficient of extinction, the accuracy of the thickness of film and stability are important needless to say.
FIGS. 3 and
,
4
show the results of simulation made about the effect of transmittance change and phase difference change in the lithography using halftone phase shifting photomask on a change of focal depth, transferring length and best focus.
Together with a miniaturizing pattern to be formed, it is needed to shorten the wavelength of rays used in lithography. As the miniaturization over the so-called 2.5 micron design rule proceeds, the practical application of a KrF excimer laser (wavelength: 248 nm) starts, and further the length of pattern is miniaturized. In consideration of the above-mentioned conditions, the use of ArF excimer laser is studied. As to a halftone phase shifting film used in a halftone shifting photomask, the development of material is requested which enables the most suitable phase angle and transmittance to also be realized for these wavelengths and has the stable refractive index and coefficient of extinction.
Accordingly, it was proposed to use film containing mainly chromium and including fluorine as disclosed in Japanese Patent Laid-Open No. 110572 of 1995 (Heisei 7) as halftone phase shifting film. This film enables the range of values of suitable phase angle and transmittance against the above-mentioned wavelength to be realized. Further, this film has the advantage of the production of blanks, the manufacturing process of the mask and others being possible in the same manner as these of conventional photomasks. Therefore, the investigation of this film was begun from the early stage, and practical application was made.
However, the halftone phase shifting film containing mainly chromium and including fluorine has a problem that by applying rays used in the application of rays such as ArF excimer laser, over a long time, the refractive index, and coefficient of extinction, or any of the two of the halftone phase shifting film change(s). Therefore, in the case of forming a resist pattern using the halftone phase shifting photomask having the halftone phase-pattern film that contains mainly chromium and includes fluorine, the refractive index and phase angle, or any of the two, changes over time as the halftone phase shifting photomask is used.
In this case, as is evident from the result of a simulation shown in
FIGS. 3 and 4
, even if the phase difference and transmittance slightly change, focusing position, and latitude as well as transferring length change greatly. Namely, these changes result in the possibilities that a proper quantity of the change of applied rays, the lowering of dimensional tolerance and the lowering of focal latitude appear every time a photomask is used, and that even in one usage of photomask, these change, which results in a decrease in latitude of pattern forming and the worsening of the shape of pattern.
BRIEF SUMMARY OF THE IN
Fujikawa Junji
Hatsuta Chiaki
Hayashi Naoya
Ito Norihito
Matsuo Takahiro
Dainippon Printing Co., Ltd.
Flynn ,Thiel, Boutell & Tanis, P.C.
Rosasco S.
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