Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2002-06-14
2004-03-16
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S078000
Reexamination Certificate
active
06707538
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a near-field exposure system which exposes a fine pattern on a photosensitive material such as a photoresist.
2. Description of the Related Art
The development in the photolithographic technology has been supported by the development in the reduction-projection exposure technology and the resist technology. In the reduction-projection exposure technology, performance is mainly determined by two fundamental quantities, the resolution RP and the focal depth DOP, which are respectively expressed as RP=k
1
&lgr;/NA and DOP=k
2
&lgr;/NA
2
, where &lgr; is an exposure wavelength of a projection optical system, NA is a numerical aperture of a projection lens, and k
1
and k
2
are coefficients. In order to increase the resolution in lithography, it is important to reduce the wavelength &lgr; and increase the numerical aperture NA of the projection lens. Although the resolution is improved with increase in the numerical aperture NA, the focal depth DOP is reduced in inverse proportion to the square of the numerical aperture NA. Therefore, currently, the reduction in the wavelength &lgr; is required for realizing fine lithography. Thus, the exposure light used in lithography has been changed from the g line having the wavelength of 436 nm to the i line having the wavelength of 365 nm, and currently use of the excimer laser light having the wavelength of 248 or 193 nm is becoming mainstream.
However, in the lithography using light, the resolution limit is the diffraction limit of the exposure light. Therefore, it is said that the finest line width obtained by use of the excimer laser light having the wavelength of 248 or 193 nm and a lens-series optical system is 100 nm. Further, in order to achieve a resolution of the nanometer order, it is necessary to use an electron-beam or X-ray lithography technique. In particular, the synchrotron orbital radiations (SORs) are used in the X-ray lithography.
The electron-beam lithography enables highly precise control for forming a pattern including nanometer-scale structures. In addition, the electron-beam lithography can achieve a considerably greater focal depth than the lithography using an optical system. Further, the electron-beam lithography enables direct drawing on a wafer without use of a mask. However, because of low throughput and high cost, the current electron-beam lithographic techniques are far from the level which enables mass production.
On the other hand, the X-ray lithography can achieve about one order of magnitude higher resolution and precision than the excimer laser exposure either when 1:1 projection exposure through a 1:1 mask is used, or when a reflective imaging X-ray optical system is used. However, it is not easy to realize the X-ray lithography since production of masks is not easy. In addition, equipment cost is high.
In order to solve the above problems, a method for exposing a photosensitive material such as a photoresist to near-field light has been proposed. In this method, exposure light is applied to an exposure mask in which a pattern of openings having dimensions smaller than the wavelength of the exposure light is formed, so that near-field light emerges through the openings of the exposure mask, and the photosensitive material is exposed to the near-field light. According to the above method, a fine pattern of the same order of magnitude as the openings in the exposure mask can be formed in the photosensitive material regardless of the wavelength of the exposure light. An example of an exposure system executing the above method is disclosed in Japanese Unexamined Patent Publication No. 2000-112116.
It is known that when a fine pattern in an exposure mask is transferred to a photosensitive material such as a photoresist by exposure to near-field light as above, and the fine pattern is constituted by straight lines extending in an identical direction as in the case of a diffraction grating, and the exposure light is linearly polarized in the same direction as the direction of the straight lines, thickening of straight lines formed on the photosensitive material can be prevented so that a finer pattern can be formed on the photosensitive material.
Based on the above knowledge, JUPP No. 2000-112116 discloses a near-field exposure mask in which an opening pattern and a grid polarizer are stacked on a mask substrate, where the opening pattern has dimensions smaller than the wavelength of exposure light, and the grid polarizer polarizes light in a direction parallel to the direction of the openings constituting the opening pattern.
However, when a polarizer is produced for each exposure mask as above, the exposure mask becomes expensive, and thus the cost of fine pattern exposure increases.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a near-field exposure system which can expose a photosensitive material such as a photoresist to near-field light so as to form a fine pattern including thin linear portions, and suppress the cost of the exposure.
(1) According to the first aspect of the present invention, there is provided a near-field exposure system comprising: a light source which emits exposure light having a predetermined wavelength and being unpolarized; a polarizer plate which linearly polarizes the exposure light; an exposure mask which has a pattern of openings each having a dimension smaller than the wavelength of the exposure light, and is placed at such a position that the exposure light is applied to the exposure mask; an exposure table which holds a photosensitive material sensitive to the exposure light, at a position which near-field light emerging from the openings reaches; and a polarizer-plate holding means which holds the polarizer plate in such a manner that the polarizer plate can be moved between first and second positions, where the first position is in an optical path of the exposure light from the light source to the exposure mask, and the second position is outside the optical path.
The near-field exposure system according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iv).
(i) The polarizer-plate holding means may hold the polarizer plate in such a manner that the polarizer plate can be rotated and directed to a direction in which polarizer plate linearly polarizes the exposure light.
(ii) The near-field exposure system the first aspect of the present invention may further comprise an indication means for indicating a direction in which the polarizer plate linearly polarizes the exposure light. For example, the indication means is an arrow indication provided on the polarizer plate.
(iii) The exposure light which is applied to the exposure mask may have a first component which is linearly polarized in a desired direction and a second component which is linearly polarized in a direction perpendicular to the desired direction, and a ratio of the second component to the first component may be 25% or smaller.
(iv) In the near-field exposure system having the above feature (iii), the ratio may be 15% or smaller.
(2) The advantages of the near-field exposure system according to the first aspect of the present invention are explained below.
The near-field exposure system according to the first aspect of the present invention comprises the polarizer-plate holding means, so that the polarizer plate which linearly polarizes the exposure light can be moved between the first position in the optical path of the exposure light from the light source to the exposure mask and the second position outside the optical path. That is, the polarizer plate can be selectively inserted in the optical path to the exposure mask.
In the case where the exposure mask has an opening pattern constituted by only lines extending in an identical direction, thickening of lines formed on the photosensitive material can be prevented when the exposure mask is placed in an optical path of the linearly polarized e
Fuji Photo Film Co. , Ltd.
Nguyen Henry Hung
Sughrue & Mion, PLLC
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