Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-06-25
2004-04-27
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S011000, C430S022000, C430S030000, C430S311000
Reexamination Certificate
active
06727025
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a photomask used in a lithography process for producing fine patterns of electronic devices such as semiconductor integrated circuits, image pickup devices (CCDs etc.), liquid crystal displays and thin film magnetic heads, and the present invention also relates to an exposure method using this photomask. The present invention is preferably used when light having a wavelength of about 200 nm or less is used as illumination light for exposure.
BACKGROUND ART
Conventionally, in a lithography process for forming a fine pattern of an electronic device such as a semiconductor integrated circuit and a liquid crystal display, there is used a method in which a reticle as a photomask on which an original pattern obtained by enlarging a pattern to be formed four to five times is disposed on a projection exposure apparatus, and under predetermined illumination light for exposure (exposure light), the original pattern is reduced in size and projected and transferred onto a wafer (or glass plate or the like) as a substrate to be exposed on which a photoresist is applied.
When such a pattern of the reticle is transferred, a line width of the resist pattern formed on the wafer after development is increased or reduced in accordance with integrated exposure amount of the exposure light with respect to the wafer. Thereupon, in order to obtain a designed line width over the entire surface of the resist pattern, the illumination distribution of the exposure light to the pattern of the reticle is maintained uniform extremely precisely so that an error of the distribution is in a range of, for example, +1% within the illumination region.
In the projection exposure apparatus, in order to enhance resolution to meet finer semiconductor integrated circuits, wavelength (exposure wavelength) of the exposure light tends to be shorter. At the present, 248 nm of a KrF excimer laser becomes mainstream as the exposure light wavelength, but 193 nm of an ArF excimer laser having shorter wavelength will soon be in practical use. Further, research has been conducted to develop a projection exposure apparatus using an F
2
laser having shorter wavelength (wavelength is 157 nm).
If the exposure light wavelength is in vacuum ultraviolet region (VUV region) of wavelength of about 200 nm or less, e.g., 157 nm by the F
2
laser, kinds of preferable materials as substrate material of the reticle which allow exposure light of such short wavelength to pass therethrough are limited. For example, fluorite (calcium fluoride) has excellent transmissivity at that wavelength, but since the linear expansion coefficient is as great as about 20×10
−6
/K, the fluorite is not always preferable as the substrate material of reticle. With such a great linear expansion coefficient, the substrate of the reticle is expanded by the illumination heat of the exposure light generated when the exposure and transfer, and the positional precision of a pattern to be transferred is deteriorated. Therefore, in order to use the fluorite, it is necessary to enhance the cooling function of the reticle for example.
As explained above, in the projection exposure apparatus, the exposure light wavelength tends to be shorter, but if the exposure light wavelength becomes about 157 nm, there is conventionally almost no material for the substrate of the reticle having high transmissivity and relatively small linear expansion coefficient.
In this regard, an attempt to use quartz and a synthetic quartz (quartz glass) doped with fluorine as the substrate material which is substantially transparent with respect to light of wavelength of about 157 nm has been made.
However, also with respect to the quartz and the quartz glass doped with fluorine, the wavelength of about 157 nm is close to the absorbing end (wavelength from which material-inherent abrupt absorption starts) of light for each material, and there is an adverse possibility that the transmissivity distribution of the material is largely varied at the wavelength near 157 nm due to slight non-uniformity in composition of the material, stress deformation generated in the material or the like. Due to these factors, if the transmissivity distribution inside the reticle substrate becomes uneven (non-uniformity), the uniformity in line width of the resist pattern to be transferred onto the wafer is deteriorated as in the case in which the illumination distribution of the exposure light for illuminating the reticle becomes non-uniform. The deterioration in uniformity in line width of the resist pattern brings about non-uniformity in circuit line width in an electronic device to be produced, which largely deteriorates operating speed of the electronic device for example.
In view of the above circumstances, it is a first object of the present invention to provide a substrate for a photomask having highly uniform transmissivity distribution and to provide the photomask.
It is a second object of the invention to provide a substrate for a photomask having a. relatively high transmissivity with respect to light of short wavelength of about 200 nm or less, for example, and having a uniform transmissivity distribution, and to provide the photomask.
It is a third object of the invention to provide an exposure method and a producing method of a device capable of producing an advanced device using the photomask.
Disclosure of the Invention
A substrate for a photomask according to the present invention is a substrate for a photomask on which an original pattern is formed, and the substrate is provided with a transmisslvity compensating member which compensates for non-uniformity of a transmissivity distribution inside the substrate.
According to the present invention, if the photomask is irradiated with exposure illumination light (exposure light) of a predetermined wavelength, a reduction amount of the transmissivity by the transmissivity compensating member is reduced in a portion of the substrate where the transmissivity of the substrate itself is lower than the average value under that wavelength, and the reduction amount of the transmissivity by the transmissivity compensating member is increased in a portion of the substrate where the transmissivity of the substrate itself is higher than the average value. With this arrangement, the non-uniformity of the transmissivity distribution of the substrate is compensated, and it is possible to secure sufficiently high uniformity of the transmissivity distribution to be used as a substrate for the photomask.
In this case, it is preferable to. form the substrate of quartz, quartz glass (e.g., synthetic quartz glass having hydroxyl (OH group) of concentration of 1000 ppm or more), quartz glass doped with a predetermined impurity (e.g., fluorine (F
2
) or the like), sapphire (Al
2
O
3
) or magnesium fluoride (MgF
2
). These materials have relatively high transmissivity even in vacuum ultraviolet region of wavelength of about 200 nm or less, and since the quartz to the sapphire have linear expansion coefficient smaller than that of fluorite, the quartz to the sapphire are suitable as the substrate of the photomask which is irradiated with exposure light of such short wavelength. The magnesium fluoride can allow light of shorter wavelength to pass through as compared with the fluorite.
One example of the transmissivity compensating member is a thin film provided on one surface of the substrate, and a film thickness distribution of this thin film is set in accordance with the transmissivity distribution of the substrate. In this case, the transmissivity distribution of the substrate can be compensated only by controlling the film thickness distribution.
The transmissivity compensating member may be formed by reforming, in at least one surface of the substrate, a vicinity of the one surface of the substrate, or the transmissivity distribution may be formed by providing another substrate other than the substrate with a transmissivity distribution which substantially compensates for the non-uniformity of the transmissi
Huff Mark F.
Nikon Corporation
Sagar Kripa
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