Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-12-26
2004-06-01
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S311000
Reexamination Certificate
active
06743553
ABSTRACT:
BACKGROUND OF THE INVENTION
i) Field of the Invention
The present invention relates to a phase shit mask which can enhance a resolution of a transfer pattern by utilizing an interference action caused to occur among light waves in a phase shifter. In particular, the present invention relates to a material of the phase shift mask, and a method of manufacturing the phase shift blank and mask, and more particularly to a halftone phase shift mask, blank, and a method of manufacturing the halftone phase shift mask and blank.
ii) Description of Related Art
For DRAM, a mass production system of 256 Mbits has been established at present, and a higher integration to a gigabit grade from a megabit grade is going to be attempted nowadays. Accordingly, a design rule of an integrated circuit has increasingly been minute. It is a matter of time that a fine pattern with a line width (half pitch) of 0.10 &mgr;m or less is demanded.
As one means for handling such a fine pattern, a resolution of the pattern has been improved by shortening a wavelength of an exposure light source. As a result, a KrF excimer laser (248 nm) or an ArF excimer laser (193 nm) is mainly used in the exposure light source in an existing photolithography method.
Although the shortened exposure wavelength improves the resolution, a depth of focus becomes shallow. Therefore, a heavy burden tends to be imposed onto a design of an optical system including a lens, or adverse influences such as deterioration of stability of a process takes place in such a design.
To solve the problems, a phase shift method has been used, in the phase shift method, a phase shift mask is used as a mask for transferring the fine pattern.
The phase shift mask is constituted, for example, of a phase shifter portion that serves as a pattern portion on the mask, and a non-pattern portion in which the phase shifter portion does not exist. A phase of light transmitted through both the portions is shifted by 180° relative to each other, then mutual interference of light, namely, light waves is caused to occur at pattern boundary portions. This is effective to enhance contrast of a transferred image. It is known that a phase shift amount &phgr; (rad) of a light wave passed through the phase shifter portion depends on a real part n of a complex refractive index and a film thickness d of the phase shifter portion and that a relationship of the following equation (1) is established.
&phgr;=2
&pgr;d
(
n
−1)/&lgr; (1)
Here, &lgr; denotes a wavelength of an exposure light. Therefore, to shift the phase by 180°, the film thickness d may be set as follows.
d
=&lgr;/{2(
n
−1)} (2)
The phase shift mask achieves an increase of the depth of focus for obtaining a necessary resolution, and it is possible to establish both improvement of the resolution and enhancement of applicability of the process without changing the exposure wavelength.
The phase shift mask is practically roughly classified into a perfect transmission (Levenson type) phase shift mask and a halftone phase shift mask in accordance with a light transmission property of the phase shifter portion forming a mask pattern. The former has a transmittance of the phase shifter portion which is equal to that of the non-pattern portion (light transmission portion), and is a mask substantially transparent to the exposure wavelength. It is generally said that the mask is effective for transferring a line and space. On the other hand, in the latter halftone type, the phase shifter portion (light semi-transmission potion or light translucent portion) has a transmittance of about several percentages to several tens of percentages, as compared with the transmittance of the non-pattern portion (light transmission portion). It is said that the halftone type is effective for preparing a contact hole or an isolated pattern.
An example of the halftone phase shift masks includes a single-layer type halftone phase shift mask which is simple in structure and which is easy to manufacture. Such single-layer halftone phase shift masks can be exemplified by SiO
x
-based and SiO
x
N
y
-based masks described in Japanese Patent Application Laid-Open No. 199447/1995, SiN
x
-based mask described in U.S. Pat. No. 5,477,058, and the like.
In order to achieve higher integration by the photolithography method in future, even by using the phase shift method, It is still necessary to further shorten the exposure wavelength. As a next-generation exposure light source following the KrF excimer laser and ArF excimer laser, F
2
excimer laser (157 nm) has already been tried and investigated. However, shortening the wavelength of the exposure light source imposes the burden onto the optical system as described above, and also brings about difficulty in development/preparation of a photo mask. As a result, development of the halftone phase shift mask for the F
2
excimer laser has hardly been started in the present situation. The following will be made about causes of the problems hereinafter.
First in many solid materials, a degree of light absorption increases as the wavelength becomes short. Taking this into account, it is assumed that a transmission film material and a translucent film material which are used in the KrF excimer laser and ArF excimer laser may be applied to the halftone phase shift mask for the F
2
excimer laser. In this event, a film should be thick in order to obtain a predetermined phase angle. As a result the transmittance substantially indicates a value close to zero. Moreover, when the degree of absorption of the exposure light is high, the film forming the phase shifter portion is liable to suffer from damages by the exposure light. The damages herein mean changes of optical properties (transmittance, refractive index, and the like) of the film, film thickness change, film properties deterioration, and the like because of a defect generated in the film forming the phase shifter portion by absorption of the exposure light, and a cleavage in a bond.
Additionally, consideration should also be made as a film material of the phase shifter portion about problems, such as etching selectivity of the phase shifter film influencing a processing precision, a chemical durability to acid or alkali used in a cleaning step of a manufacturing process, and the like.
SUMMARY OF THE INVENTION
The present invention has been developed under the aforementioned background, and an object thereof is to provide a halftone phase shift mask which can be used in an exposure wavelength range of 140 nm to 200 nm including a wavelength of an F
2
excimer laser of 157 nm, and a halftone phase shift mask blank for forming the halftone phase shift mask.
To achieve the aforementioned object, the present inventor et al have performed an intensive research/development. As a result, it has been found out that SiNx makes a matrix of a film dense by the virtue of an Si—N bond. This brings about the facts that SiN
x
has a high irradiation durability against an exposure light and a high chemical durability against detergent or the like. On the other hand, SiO
x
has a relatively high transmittance even on a short wavelength side. Under the circumstances, attention has been directed to SiO
x
N
y
that has advantages of both materials mentioned above. Furthermore, it has been found that using SiO
x
N
y
makes it possible to obtain a phase shifter film suitable for use in an exposure light having a short wavelength by controlling a composition of SiO
x
N
y
. The present invention has been made in consideration of the above-mentioned facts.
Now, the phase shifter film is basically different in a composition range from a conventional SiO
x
N
y
-based film, and is further different in film properties (e.g., physical properties such as k) from the conventional SiO
x
N
y
-based film. A combination of the composition range and film properties can realize a phase shifter film which has a transmittance of 3 to 40% and a refractive index (related to a film thickness for shifting the phase by a predetermined angle
Nozawa Osamu
Shiota Yuki
Hoya Corporation
Young Christopher G.
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