Multiple exposure method

Details Multiple exposure method Multiple exposure method

1999-06-28

2002-06-11

Huff, Mark. F. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Plural exposure steps

C430S005000, C430S311000, C430S396000

Reexamination Certificate

active

06403291

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure method and an exposure apparatus, and more particularly to an exposure method and an exposure apparatus for exposing a photosensitive substrate to light with a minute circuit pattern. The exposure method and the exposure apparatus according to the invention are adapted for the manufacture of various devices such as semiconductor chips like an IC and an LSI, a display element like a liquid crystal panel, a detecting-element like a magnetic head, an image pickup element like a CCD, etc.
2. Description of Related Art
In the manufacture of an IC, and LSI, a liquid crystal element and the like devices by photolithography, a projection exposure method and a projection exposure apparatus are used to carry out exposures by projecting, through a projection optical system, a circuit pattern of a photomask, a reticle or the like (hereinafter referred to as a mask) onto a photosensitive substrate, such as a silicon wafer or glass plate, coated with a photoresist or the like (hereinafter referred to as a wafer).
It is a general trend to increase the rate of integration of the above-stated devices. To meet this trend, the pattern to be transferred to the wafer is required to be more minutely and finely prepared for a higher degree of resolution and the area of each chip on the wafer is required to be increased. Therefore, the projection exposure method or the projection exposure apparatus which is most important in the art of finely and minutely processing the wafer is now under the efforts of making it possible to form, over a wider area, an image measuring not greater than 0.5 &mgr;m in line width for the purpose of an improvement in resolution and in exposure area.
FIG. 18
schematically shows the arrangement of a projection exposure apparatus conventionally employed. In
FIG. 18
, there are illustrated an excimer laser
191
used as a light source for a far-ultraviolet ray exposure, an illumination optical system
192
, illumination light
193
, a mask
194
, object-side exposure light
195
which comes from the mask
194
to be incident on an optical system
196
, the optical system
196
which is a demagnifying projection optical system, image-side exposure light
197
which comes from the optical system
196
to be incident on a photosensitive substrate
198
, the photosensitive substrate
198
which is a wafer, and a substrate stage
199
which is arranged to hold and carry the photosensitive substrate
198
.
A laser beam emitted from the excimer laser
191
is led to the illumination optical system
192
by a delivery optical system. The laser beam is adjusted and converted by the illumination optical system
192
into the illumination light
193
having a light intensity distribution, a luminous distribution, an opening angle (a numerical aperture NA), etc., which are predetermined. The mask
194
is thus illuminated by the illumination light
193
.
A minute and fine pattern to be eventually formed on the wafer
198
is beforehand formed on the mask
194
with chromium or the like applied to a quartz substrate. The minute pattern is formed in a size measuring a reciprocal number of times (two, four or five times) as much as the projection magnification of the projection optical system
196
. The incident illumination light
193
passes and is diffracted through the minute pattern of the mask
194
to become the object-side exposure light
195
. The projection optical system
196
converts the object-side exposure light
195
into the image-side exposure light
197
to image the minute circuit pattern on the wafer
198
at the predetermined projection magnification and with sufficiently small aberrations. As shown in an enlarged view at the lower part of
FIG. 18
, the image-side exposure light
197
then converges on the wafer
198
at the predetermined numerical aperture NA (=sin &thgr;) to form an image of the minute pattern on the wafer
198
. In a case where the minute pattern is to be formed in a plurality of shot areas (to be used for one or a plurality of chips) of the wafer
198
one after another, the substrate stage
199
is moved step by step along the image plane of the projection optical system
196
to vary the relative positions of the wafer
198
and the projection optical system
196
.
However, with the projection exposure apparatus which is generally arranged to use an excimer laser as a light source as described above, it is difficult to form a pattern of a line width not greater than 0.15 &mgr;m.
The attainable resolution of the projection optical system
196
is limited by a trade-off between the optical resolution and the depth of focus due to the wavelength of exposure light used for an exposure. The resolution R of a pattern resolvable by the projection exposure apparatus and the depth of focus DOF are expressed by the following Rayleigh's formulas (1) and (2):
R
=k
1
(&lgr;/NA)  (1)
DOF
=k
2
(&lgr;/NA
2
)  (2)
where “&lgr;” is the wavelength of the exposure light, “NA” is a numerical aperture on the image side indicating the brightness of the projection optical system
196
, “k
1
” and “k
2
” are constants which are determined by the characteristic of a developing process for the wafer
198
, etc., and are normally within a range from 0.5 to 0.7.
According to the formulas (1) and (2), the value of resolution R may be made smaller, for a higher degree of resolution, by making the numerical aperture NA larger. In actually carrying out an exposure, however, the depth of focus DOF of the projection optical system
196
must be at least at a certain value. This requirement imposes some limitation on the possible increase of the numerical aperture NA. It is thus apparent that the numerical aperture NA cannot be increased beyond the limit. It is also apparent that, in order to improve the resolution, the wavelength &lgr; of the exposure light must be shortened.
The efforts to shorten the wavelength, however, encounter a serious problem. This problem lies in the difficulty of finding an optical material suitable for the lenses of the projection optical system
196
. Most of known optical materials have their transmission factors close to zero in the far ultraviolet region. Although there is some fused quartz material manufactured as an optical material to have an exposure wavelength of about 248 nm for an exposure apparatus, the transmission factor of the fused quartz material abruptly drops for the exposure wavelengths not greater than 193 nm. It is thus extremely difficult to obtain an optical material actually usable for the region of exposure wavelengths not greater than 150 nm corresponding to the minute pattern of line width not greater than 0.15 &mgr;m. Besides, any glass material that is to be used for the region of far ultraviolet rays must meet various requirements other than the transmission factor. The requirements include durability, the uniformity of refractive index, optical strain, workability and so on. Such being the requirements, the availability of any optical material that is practically usable is in doubt.
As mentioned above, in order to form on the wafer
198
a pattern not greater than 0.15 &mgr;m in line width, the conventional method and apparatus for a projection exposure necessitate a reduction in exposure-light wavelength at least down to 150 nm or thereabout. However, there is no optical material usable for such a wavelength region. Therefore, it has been impossible to form on the wafer
198
any pattern that is not greater than 0.15 &mgr;m in line width.
A method for forming a minutely fine pattern by making a two-light-flux interference exposure has been disclosed in U.S. Pat. No. 5,415,835. According to the two-light-flux interference exposure, a pattern of line width not greater than 0.15 &mgr;m can be formed on the wafer.
FIG. 14
shows the principle of the method for the two-light-flux interference exposure. Referring to
FIG. 14
, a laser
151
emits a laser beam composed of coherent

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