Multiple exposure method

Details Multiple exposure method Multiple exposure method

1999-06-14

2003-04-08

Huff, Mark F. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Making electrical device

C430S005000, C430S022000, C430S394000, C430S396000

Reexamination Certificate

active

06544721

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 a method and an apparatus adapted for exposing a photosensitive substrate to light with a minute circuit pattern so as to manufacture devices of varied kinds, including semiconductor chips such as an IC and an LSI, display elements such as a liquid crystal panel, detection elements such as a magnetic head, and image pickup elements such as a CCD.
2. Description of Related Art
In manufacturing devices such as an IC, an LSI, a liquid crystal panel, etc., by using photolithography techniques, a projection exposure method or a projection exposure apparatus is used to project, through a projection optical system, a circuit pattern formed on a photomask, a reticle or the like (hereinafter referred to as a mask) onto a photosensitive substrate, such as a silicon wafer or a glass plate, coated with a photoresist (hereinafter referred to as a wafer) and to transfer the circuit pattern to the wafer (i.e., expose the wafer to light with the circuit pattern).
To meet a recent trend of increasing the degree of integration of the above-stated devices, a circuit pattern to be transferred to the wafer is required to be more finely and minutely prepared. In other words, it is desired to have a higher resolution and to permit an increase of the area of one chip on the wafer. Hence, the projection exposure method or the projection exposure apparatus which plays a main role in the art of accomplishing minute work on the wafer is being developed these days to increase the resolution and the exposure area in such a way as to form the image of the circuit pattern at a line width not greater than 0.5 &mgr;m over a larger range.
FIG. 39
schematically illustrates the arrangement of a projection exposure apparatus conventionally employed. The illustration of
FIG. 39
includes an excimer laser
191
which is a light source used for a far ultraviolet ray exposure, an illumination optical system
192
, illumination light
193
radiated from the illumination optical system
192
, a mask
194
, object-side exposure light coming through the mask
194
to be incident on a projection optical system
196
, the projection optical system
196
which is a demagnification exposure type optical system, image-side exposure light
197
coming from the projection optical system
196
to be incident on a substrate
198
, the substrate
198
which is a wafer as a photosensitive substrate, and a substrate stage
199
arranged to hold the wafer (substrate)
198
.
A laser beam emitted from the excimer laser
191
is led to the illumination optical system
192
through delivery optics (
190
a
and
190
b
). The laser beam is adjusted by the illumination optical system
192
to be converted into the illumination light
193
which has a light intensity distribution, a luminance distribution, an aperture angle (numerical aperture NA), etc., which are predetermined. Then, the mask
194
is illuminated with the illumination light
193
. On a quartz substrate of the mask
194
, there is formed, with chromium or the like, a pattern which corresponds to a minute pattern to be formed on the wafer
198
. The pattern on the mask
194
is in such a size that is obtained by multiplying the size of the minute pattern on the wafer
198
by a reciprocal number of the projection magnification of the projection optical system
196
(for example, two, four or five times). The illumination light
193
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
, which forms an image of the minute pattern of the mask
194
on the surface of the wafer
198
at the above-stated projection magnification and with sufficiently small aberration. As shown in an enlarged view at the lower left part of
FIG. 39
, the image-side exposure light
197
converges on the wafer
198
at a predetermined numerical aperture NA (=sin &thgr;) to form the image of the minute pattern on the wafer
198
. When the minute pattern is to be formed on a plurality of different areas (shot areas which become one or a plurality of chips) of the wafer
198
one after another, the substrate stage
199
is moved stepwise along the image plane of the projection optical system
196
in such a way as to vary the position of the wafer
198
relative to the projection optical system
196
.
The projection exposure apparatus which uses the excimer later as a light source and is most popularly in use these days has a high projection resolving power. However, it is technically difficult to form a pattern image of a line width not greater than, say, 0.15 um with the projection exposure apparatus.
The resolution of the projection optical system
196
is limited by a trade-off between the optical resolution and the depth of focus which depend on the exposure wavelength (wavelength of light used for exposure). The resolution R and the depth of focus DOF of the projection exposure apparatus are expressed by the following Rayleigh's formulas (1) and (2):
R=k
1
·(&lgr;/
NA
)  (1)
DOF=k
2
·(&lgr;/
NA
2
)  (2)
In these formulas, “&lgr;” represents the exposure wavelength, “NA” represents a numerical aperture on the image side indicating the brightness of the projection optical system
196
, and k
1
and k
2
represent constants which are normally between 0.5 and 0.7 and are determined by the characteristic of a developing process on the wafer
198
, etc. According to the formulas (1) and (2), in order to make the value of the resolution R smaller for a higher resolution, it is conceivable to increase the numerical aperture NA. However, in actually making an exposure, it is difficult to increase the numerical aperture NA more than a certain extent, because the depth of focus DOF of the projection optical system
196
must be kept above a certain value. In order to increase the resolution, therefore, it is necessary to make the exposure wavelength &lgr; smaller for shorter wavelength after all.
The attempt to shorten the exposure wavelength, however, encounters a serious problem. The problem lies in that it becomes hardly possible to find any optical material for lenses which form the projection optical system
196
. Almost all optical materials have their transmission factors near to “0” in the far ultraviolet region. Although there is a fused quartz material which is manufactured by a special method for an exposure apparatus to have an exposure wavelength of about 248 nm, the transmission factor of the fused quartz also abruptly drops for the exposure wavelength not greater than 193 nm. It is thus extremely difficult to develop any optical material practically usable for an exposure wavelength not greater than 150 nm required for a minute pattern of line width not greater than 0.15 &mgr;m. Besides, any optical material to be used within the far ultraviolet region is required to satisfy a plurality of conditions relative to durability, uniform refractive index, optical strain, workability, etc., in addition to the transmission factor. Accordingly, it is doubted whether there exits any practically-usable optical material.
Use of some of optical materials, such as CaF
2
, MgF
2
, etc., that have a fairly good transmission factor even for the wavelength of 150 nm or thereabout has recently come to be considered. However, the exposure wavelength is of course preferred to be longer.
As described above, in order to form on the wafer a pattern of a line width not greater than 0.15 &mgr;m, the conventional projection exposure method and apparatus necessitate the exposure wavelength to be shortened down to a wavelength not greater than 150 nm. For this wavelength region, however, no practically-usable optical material is obtainable at present. Therefore, it has been hardly possible to form on the wafer any pattern of a line width not great

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