Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1999-03-30
2002-04-23
Mathews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C359S701000
Reexamination Certificate
active
06377336
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection exposure apparatus for use to form a pattern of a semiconductor integrated circuit, or a liquid crystal device, or the like.
2. Related Background Art
When a circuit pattern of a semiconductor device or the like is formed, so-called photolithography technology is required. In this process, a method, in which a reticle (a mask) pattern is formed on a substrate such as semiconductor wafer, is usually employed. The surface of the substrate is applied with photosensitive photoresist so that a circuit pattern is transferred to the photoresist in accordance with an image irradiated with light, that is, in accordance with the shape of the pattern corresponding to a transparent portion of the reticle pattern. In a projection exposure apparatus (for example, a stepper), the image of a circuit pattern drawn on the reticle so as to be transferred is projected on the surface of the substrate (wafer) via a projection optical system so as to be imaged.
In an irradiation optical system for irradiating the reticle with light, an optical integrator such as a fly-eye type optical integrator (a fly-eye lens) and a fiber is used so as to uniform the distribution of the intensities of irradiation light with which the surface of the reticle is irradiated. In order to make the aforesaid intensity distribution uniform optimally, a structure which employs the fly-eye lens is arranged in such a manner that the reticle-side focal surface (the emission side) and the surface of the reticle (the surface on which the pattern is formed) hold a substantially Fourier transformed relationship. Also the focal surface adjacent to the reticle and the focal surface adjacent to the light source (the incidental side) hold the Fourier transformed relationship. Therefore, the surface of the reticle, on which the pattern is formed, and the focal surface of the fly-eye lens adjacent to the light source (correctly, the focal surface of each lens of the fly-eye lens adjacent to the light source) hold an image formative relationship (conjugated relationship). As a result of this, irradiation light beams from respective optical elements (a secondary light source image) of the fly-eye lens are added (superposed) because they pass through a condenser lens or the like so that they are averaged on the reticle. Hence, the illuminance uniformity on the reticle can be improved. Incidentally, there has been disclosed an arrangement capable of improving the illuminance uniformity in U.S. Pat. No. 4,497,015 in which two pairs of optical integrators are disposed in series.
In a conventional projection exposure apparatus, the light quantity distribution of irradiation beams to be incident on the optical integrator, such as the aforesaid fly-eye lens, has been made to be substantially uniform in a substantially circle area (or in a rectangular area), the center of which is the optical system of the irradiation optical system.
FIG. 14
illustrates a schematic structure of a conventional projection exposure apparatus (stepper) of the above described type. Referring to
FIG. 14
, irradiation beams L
140
pass through a fly-eye lens
41
c
, a spatial filter (an aperture diaphragm)
5
a
and a condenser lens
8
so that a pattern
10
of a reticle
9
is irradiated with the irradiation beams L
140
. The spatial filter
5
a
is disposed on, or adjacent to a Fourier transformed surface
17
(hereinafter abbreviated to a “pupil surface”) with respect to the reticle side focal surface
414
c
of the fly-eye lens
41
c
, that is, with respect to the reticle pattern
10
. Furthermore, the spatial filter
5
a
has a substantially circular opening centered at a point on optical axis AX of a projection optical system
11
so as to limit a secondary light source (plane light source) image to a circular shape. The irradiation light beams, which have passed through the pattern
10
of the reticle
9
, are imaged on a resist layer of a wafer
13
via the projection optical system
11
. In the aforesaid structure, the number of apertures of the irradiation optical system (
41
c
,
5
a
and
8
) and the number of reticle-side apertures formed in the projection optical system
11
, that is a value is determined by the aperture diaphragm (for example, by the diameter of an aperture formed in the spatial filter
5
a
), the value being 0.3 to 0.6 in general.
The irradiation light beams L
140
are diffracted by the pattern
10
patterned by the reticle
9
so that 0-order diffracted light beam Do, +1-order diffracted light beam Dp and −1-order diffracted light beam Dm are generated from the pattern
10
. The diffracted light beams Do, Dp and Dm, thus generated, are condensed by the projection optical system
11
so that interference fringes are generated. The interference fringes, thus generated, correspond to the image of the pattern
10
. At this time, angle &thgr; (reticle side) made by the 0-order diffracted light beam Do and ±1-order diffracted light beams Dp and Dm is determined by an equation expressed by sin &thgr;=&lgr;/P (&lgr;: exposure wavelength and P: pattern pitch).
It should be noted that sin &thgr; is enlarged in inverse proportion to the length of the pattern pitch, and therefore if sin &thgr; has become larger than the number of apertures (NA
R
) formed in the projection optical system
11
adjacent to the reticle
9
, the ±1-order diffracted light beams Dp and Dm are limited by the effective diameter of a pupil (a Fourier transformed surface)
12
in the projection optical system
11
. As a result, the ±1-order diffracted light beams Dp and Dm cannot pass through the projection optical system
11
. At this time, only the 0-order diffracted light beam Do reaches the surface of the wafer
13
and therefore no interference fringe is generated. That is, the image of the pattern
10
cannot be obtained in a case where sin &thgr;>NA
R
. Hence, the pattern
10
cannot be transferred to the surface of the wafer
13
.
It leads to a fact that pitch P, which holds the relationship sin &thgr;=&lgr;/P≅NA
R
, has been given by the following equation.
P≅&lgr;/NA
R
(1)
Therefore, the minimum pattern size becomes about 0.5·&lgr;/NA
R
because the minimum pattern size is the half of the pitch P. However, in the actual photolithography process, some considerable amount of focal depth is required due to an influence of warp of the wafer, an influence of stepped portions of the wafer generated during the process and the thickness of the photoresist. Hence, a practical minimum resolution pattern size is expressed by k·&lgr;/NA
R
, where k is a process factor which is about 0.6 to 0.8. Since the ratio of the reticle side number of articles NA
W
and the wafer side number of articles NA
R
is the same as the imaging magnification of the projection optical system, the minimum resolution size on the reticle is k·&lgr;/NA
R
and the minimum pattern size on the wafer is k·&lgr;/NA
W
=k·&lgr;/B·NA
R
(where B is an imaging magnification (contraction ratio)).
Therefore, a selection must be made whether an exposure light source having a shorter wavelength is used or a projection optical system having a larger number of apertures is used in order to transfer a more precise pattern. It might, of course, be considered feasible to study to optimize both the exposure wavelength and the number of apertures.
However, it is so far difficult for the projection exposure apparatus of the above described type to shorten the wavelength of the irradiation light source (for example, 200 nm or shorter) because a proper optical material to make a transmissive optical member is not present and so forth. Furthermore, the number of apertures formed in the projection optical system has approached its theoretical limit at present and therefore it is difficult to further enlarge the apertures. Even if the aperture can be further enlarged, the focal depth expressed by ±&lgr;/2NA
2
rapidly decreases with an increa
Kamiya Saburo
Kudo Yuji
Shiraishi Naomasa
Mathews Alan A.
Miles & Stockbridge P.C.
Nikon Corporation
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