Exposure method based on multiple exposure process

Details Exposure method based on multiple exposure process Exposure method based on multiple exposure process

2003-05-14

2004-03-23

Baxter, Janet (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Radiation modifying product or process of making

Radiation mask

C430S022000, C430S394000

Reexamination Certificate

active

06709794

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ART
This invention relates to an exposure method, an exposure apparatus and a device manufacturing method. More particularly, the invention concerns an exposure method and apparatus for transferring a very fine circuit pattern onto a photosensitive substrate through multiple exposures. The exposure method and apparatus of the present invention are suitably usable for the manufacture of various devices such as semiconductor chips (e.g., ICs or LSIs), display devices (e.g., liquid crystal panels), detecting devices (e.g., magnetic heads), or image pickup devices (e.g., CCDs), or for the production of patterns to be used in micro-mechanics.
The manufacture of microdevices such as ICs, LSIs or liquid crystal panels, for example, use a projection exposure method and a projection exposure apparatus wherein a circuit pattern formed on a photomask or reticle (hereinafter, “mask”) is projected through a projection optical system onto a photosensitive substrate such as a silicon wafer or a glass plate (hereinafter, “wafer”) which is coated with a photoresist, for example, by which the circuit pattern is transferred (photoprinted) to the wafer.
In order to meet enlargement of integration of a device (chip), miniaturization of a pattern to be transferred to a wafer, that is, improvements in resolution, as well as enlargement in area of each chip have been desired. Thus, in a projection exposure method and projection exposure apparatus which play a main role in the wafer microprocessing procedure, many attempts have been made to improve the resolution and to enlarge the exposure area in order that an image of a size (linewidth) of 0.5 micron or less can be formed in a wider range.
FIG. 22
is a schematic view of a conventional projection exposure apparatus, wherein denoted at
191
is an excimer laser which is a deep ultraviolet light exposure light source. Denoted at
192
is an illumination optical system, and denoted at
193
is illumination light. Denoted at
194
is a mask, and denoted at
195
is object side exposure light emitted from the mask
194
and entering an optical system
196
which is a reduction projection optical system. Denoted at
197
is image side exposure light emitted from the optical system
196
and impinging on a substrate
198
which is a photosensitive substrate (wafer). Denoted at
199
is a substrate stage for holding the photosensitive substrate.
Laser light emitted from the excimer laser
191
is directed by a guiding optical system to the illumination optical system
192
, by which the laser light is adjusted to provide the illumination light
193
having a predetermined light intensity distribution, a predetermined orientation distribution, and a predetermined opening angle (numerical aperture NA), for example. The illumination light
193
then illuminates the mask
194
.
The mask
194
has formed thereon a pattern of a size corresponding to the size of a fine pattern to be formed on the wafer
198
but as being multiplied by an inverse of the projection magnification of the projection optical system
196
(namely, 2×, 4× or 5×, for example). The pattern is made of chromium, for example, and it is formed on a quartz substrate. The illumination light
193
is transmissively diffracted by the fine pattern of the mask
194
, whereby the object side exposure light
195
is provided. The projection optical system
196
serves to convert the object side exposure light
195
to the image side exposure light
197
with which the fine pattern of the mask
194
can be imaged upon the wafer
198
at the projection magnification and with a sufficiently small aberration. As shown in a bottom enlarged view portion of
FIG. 22
, the image side exposure light
197
is converged on the wafer
198
with a predetermined numerical aperture NA (=sin &thgr;), whereby an image of the fine pattern is formed on the wafer
198
. The substrate stage
199
is movable stepwise along the image plane of the projection optical system to change the wafer
198
position relative to the projection optical system
196
, such that fine patterns are formed sequentially on different regions on the wafer
198
(e.g., shot regions each covering one or more chips).
However, with projection exposure apparatuses currently used prevalently and having an excimer laser as a light source, it is still difficult to produce a pattern image of 0.15 micron or less.
As regards the resolution of the projection optical system
196
, there is a limitation due to a “trade off” between the depth of focus and the optical resolution attributable to the exposure wavelength (used for the exposure process). The resolution R of a pattern to be resolved and the depth of focus DOF of a projection exposure apparatus can be expressed by Rayleigh's equation, such as equations (1) and (2) below.
R=k
1
(&lgr;/
NA
)  (1)
DOF=k
2
(&lgr;/
NA
2
)  (2)
where &lgr; is the exposure wavelength, NA is the image side numerical aperture which represents the brightness of the projection optical system
196
, and k
1
and k
2
are constants which are determined by the development process characteristics, for example, and which are normally about 5-0.7. From equations (1) and (2), it is seen that, while enhancement of resolution, that is, making the resolution R smaller, may be accomplished by enlarging the numerical aperture NA (NA enlarging), since in a practical exposure process the depth of focus DOF of the projection optical system
196
cannot be shortened beyond a certain value, increasing the numerical aperture NA over a large extent is not attainable, and also that, for enhancement of resolution, narrowing the exposure wavelength &lgr; (band-narrowing) is anyway necessary.
However, such band-narrowing encounters a critical problem. That is, there will be no glass material available for lenses of the projection optical system
196
. In most glass materials, the transmission factor is close to zero, with respect to the deep ultraviolet region. Although there is fused silica, which is a glass material produced for use in an exposure apparatus (exposure wavelength of about 248 nm) in accordance with a special method, even the transmission factor of fused silica largely decreases with respect to the exposure wavelength not longer than 193 nm. It is very difficult to develop a practical glass material for a region of an exposure wavelength of 150 nm or shorter, corresponding to a very fine pattern of 0.15 micron or less. Further, glass materials to be used in the deep ultraviolet region should satisfy various conditions, other than the transmission factor, such as durability, uniformness of refractive index, optical distortion, easiness in processing, etc. In these situations, the availability of practical glass materials is not large.
As described, in conventional projection exposure methods and projection exposure apparatuses, the band-narrowing of the exposure wavelength to about 150 nm or shorter is required for formation of a pattern of 0.15 micron or less upon a wafer
198
whereas there is no practical glass material for such a wavelength region. It is, therefore, very difficult to produce a pattern of 0.15 micron or less on a wafer.
Recently, an exposure method and apparatus for performing a dual exposure process, comprising a periodic pattern exposure and a standard (ordinary) exposure, to a substrate (photosensitive substrate) to be exposed, has been proposed in an attempt to produce a circuit pattern including a portion of 0.15 micron or less.
Here, the term “standard exposure” or “ordinary exposure” refers to an exposure process by which an arbitrary pattern can be photoprinted although the resolution is lower than that of the periodic pattern exposure. A representative example of it is the exposure process to be performed by projection of a mask pattern with a projection optical system.
A pattern to be printed by the standard exposure (hereinafter, “standard exposure pattern”) may include a very fine pattern less than the re

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