Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-04-10
2004-03-23
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06709791
ABSTRACT:
The present invention relates generally to a photomask used for the fabrication of high-density integrated circuits such as LSIs, a photomask blank for the fabrication of such a photomask and a pattern-formation process using the same, and more particularly to a halftone phase shift photomask that enables projected images of minute size to be obtained, a halftone phase shift photomask blank for the fabrication of this halftone phase shift photomask and a pattern-formation process using the same.
BACKGROUND ART
Semiconductor integrated circuits such as LSIs are fabricated by the repetition of a so-called lithography process using a photomask. Possible applications of such phase shift masks as set forth in JP-A 58-173744, JP-B 62-59296, etc. to the formation of microcircuits in particular have already been under investigation. Among others, the so-called halftone phase shift photomask such as one set forth in U.S. Pat. No. 4,890,309, etc. has attracted attention in view of expedited practical applications. For instance, as disclosed in JP-A's 05-2259 and 05-127361, some proposals have been made in respect of arrangements and materials improved in yields and reduced in costs for the purpose of practical applications.
Here a typical halftone phase shift photomask is briefly explained with reference to
FIGS. 14 and 15
. FIGS.
14
(
a
) through
14
(
d
) are illustrative of the principles of halftone phase shift lithography, and FIGS.
15
(
a
) through
15
(
d
) are illustrative of a conventional process. FIGS.
14
(
a
) and
15
(
a
) are each a sectional view of a photomask, FIGS.
14
(
b
) and
15
(
b
) are each illustrative of the amplitude of light on the photomask, FIGS.
14
(
c
) and
15
(
c
) are each illustrative of the amplitude of light on a wafer, and FIGS.
14
(
d
) and
15
(
d
) are each illustrative of the intensity of light on the wafer. Reference numerals
911
and
921
are each a transparent substrate,
922
a 100% light-blocking film,
912
a halftone phase shift film, and
913
and
923
are each incident light. By the term “halftone phase shift film” used herein is intended a film, in a single layer or multilayer form, having functions of substantially reversing the phase of transmitting exposure light with respect to the phase of exposure light passing through the air having the same optical length and attenuating the intensity of the light. According to the conventional process, the 100% light-blocking film
922
formed of Cr or the like is provided on the substrate
921
formed of quartz glass or the like, as shown in FIG.
15
(
a
), thereby achieving a simple arrangement where a light transmitting portion of any desired pattern is formed. The light on the wafer has such a fan-shaped intensity distribution as shown in FIG.
15
(
d
), resulting in poor resolution. With the halftone phase shift lithography, on the other hand, it is possible to achieve improvements in resolution, because the phase of light transmitting through the halftone phase shift mask
912
is substantially reversed with respect to the phase of light transmitting through its opening, so that the intensity of light at pattern boundaries on the wafer is reduced to zero and, hence, such a fan-shaped intensity distribution is prevented.
It is here worthy of note that various factors needed for a conventional photomask such as the dimensional accuracy, alignment accuracy, etc. of the pattern formed on the mask as well as its phase angle and transmittance are of vital importance so as to make use of the effect of the halftone phase shift photomask, and these factors are determined by the refractive index, extinction coefficient and thickness of the single layer or multilayer forming the halftone phase shift film.
In general, the optimum phase angle is 180°, and the optimum transmittance is in the range of 1 to 20% (100% for the opening) as determined depending on the pattern to be transferred, transfer conditions, etc. A halftone phase shift mask must be fabricated in such a way as to meet its optimum phase angle and transmittance. Any deviation from the optimum values leads to changes in the correct exposure, etc., ending up with dimensional accuracy drops, a limited tolerance to focus, etc. Thus, the refractive index, extinction coefficient, thickness accuracy and thickness stability of the single layer or multilayer forming the halftone phase shift mask are of vital importance.
FIGS. 17 and 18
are illustrative of the results of simulation of what influences are produced by transmittance and phase difference changes on the depth of focus, transfer size and best focus change in lithography using a halftone phase shift mask.
In this regard, it is noted that as the pattern to be formed becomes fine, it is required to make short the wavelength of exposure light used for lithography, and the KrF excimer lasers of 248 nm wavelength are now put to practical use for patterns finer than the so-called 0.25 &mgr;m design rule. With expected further size reductions in view, ArF excimer lasers of 193 nm wavelength are under investigation. For halftone phase shift films used for halftone phase shift masks, too, the development of materials capable of achieving the optimum phase angle and transmittance for these wavelengths and having stable refractive indices and extinction coefficients are in great demand.
To meet such demands, for instance, it has been proposed to use a film composed mainly of chromium and containing fluorine for a halftone phase shift mask as typically disclosed in JP-A 07-110572. The merits of this film are that the ranges of phase angle and transmittance needed at those wavelengths cannot only be ensured but blank fabrication, mask-making processing, etc. can also be achieved as is the case with a conventional photomask. For these reasons, the film has already been passed from an early investigation stage into a practical stage.
A problem with the halftone phase shift mask composed mainly of chromium and containing fluorine is, however, that its refractive index and/or its extinction coefficient change upon long-term irradiation with exposure light such as an ArF excimer laser. In other words, with a resist pattern formed using a halftone phase shift photomask having the halftone phase shift film composed mainly of chromium and containing fluorine, its transmittance and/or its phase angle change whenever it is used.
From the results of simulation shown in
FIGS. 17 and 18
, however, it is found that even slight fluctuations in the phase difference and transmittance of a halftone phase shift photomask result in large changes in focus position and tolerance to focus, to say nothing of transfer size.
That is, whenever this mask is used, the proper exposure changes with limited dimensional accuracy and limited tolerance to focus. Alternatively, even when the mask is used once alone, there is a possibility of changes in these factors, resulting in a limited tolerance to pattern formation and deterioration of pattern shape.
SUMMARY OF THE INVENTION
In view of such states of the prior art as mentioned above, an object of the present invention is to provide a halftone phase shift mask which does not vary in its refractive index and phase angle even upon irradiation with excimer laser used for exposure over an extended period of time, a blank for the fabrication of the same, and an image-formation process using the same.
The present invention has been accomplished as a result of studies made so as to develop a halftone phase shift film which does not vary in its refractive index and phase angle even upon irradiation with excimer laser used for exposure over an extended period of time.
According to the first aspect of the present invention, there is provided a halftone phase shift mask blank comprising a halftone phase shift film containing at least chromium and fluorine on a transparent substrate, characterized in that:
optical characteristic changes upon irradiation with an exposure excimer laser have been reduced by application of light of a wavelength substantially absorbed by said h
Hatsuta Chiaki
Hayashi Naoya
Ito Norihito
Matsuo Takahiro
Mohri Hiroshi
Dai Nippon Printing Co. Ltd.
Dellett & Walters
Rosasco S.
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