Radiation imagery chemistry: process – composition – or product th – Plural exposure steps
Reexamination Certificate
2001-03-02
2004-08-24
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Plural exposure steps
C430S312000, C430S396000
Reexamination Certificate
active
06780574
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to an exposure method, an exposure apparatus and a device manufacturing method and, more particularly, to a multiple exposure technology for lithographically printing a very fine circuit pattern on a photosensitive substrate. The present invention is suitably usable, for example, in the production of various patterns to be used for various devices and micro-mechanics 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), for example.
Generally, the manufacture of microdevices such as ICs, LSIs or liquid crystal panels through photolithographic processes uses a projection exposure method and a projection exposure apparatus wherein a circuit pattern formed on a photomask or a reticle (hereinafter, “mask”), for example, is projected to and thus transferred (photoprinted) to a photosensitive substrate such as a silicon wafer or a glass plate (hereinafter, “wafer”), for example, through a projection optical system.
With increasing density (integration) of these devices, further miniaturization of a pattern to be transferred to a wafer is required. Namely, improvements in the resolution as well as enlargement of the area of a single chip on the wafer are required. In this respect, projection exposure apparatuses and projection exposure methods which are the core of the wafer micro-processing technology are required to assure formation of an image of a size (linewidth) of 0.1 micron or less in a wider region, particularly, a circuit pattern of 80 nm or less.
FIG. 33
is a schematic view of a general structure of a projection exposure apparatus. Denoted in the drawing at
191
is an excimer laser which is an 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 impinging on a reduction projection optical system
196
. Denoted at
197
is image-side exposure light emitted from the optical system
196
and impinging on a photosensitive substrate
198
which is a wafer. Denoted at
199
is a substrate stage for holding the photosensitive substrate.
The laser light emitted from the excimer laser
191
is directed by a guiding optical system to the illumination optical system
192
, by which the light is adjusted and transformed into the illumination light
193
having a predetermined light intensity distribution, orientation distribution, opening angle (numerical aperture NA) and the like. The illumination light
193
illuminates the mask
194
. The mask
194
has a fine pattern corresponding to a fine pattern to be produced on the wafer
198
. The mask pattern is formed on a quartz substrate by use of chromium, for example, and the pattern has a size corresponding to the inverse of the projection magnification of the projection optical system, that is, 2×, 4× or 5×, for example. The illumination light
193
is transmissively diffracted by the fine pattern of the mask
194
, whereby object-side exposure light
195
is provided. The projection optical system
196
functions to transform the object-side exposure light
195
into image-side exposure light
197
for imaging the fine pattern of the mask
194
upon the wafer
198
in accordance with the above-described projection magnification and with a sufficiently small aberration. As best seen in an enlarged view at the bottom of
FIG. 33
, the image-side exposure light
197
is converged upon the wafer
198
with a predetermined numerical aperture NA (=sin&thgr;), whereby an image of the fine pattern is produced on the wafer
198
. When fine patterns are to be produced sequentially on different regions of the wafer, each corresponding to single or plural chips, the substrate stage
199
moves stepwise along the image plane of the projection optical system, to change the position of the wafer
198
with respect to the projection optical system
196
. Denoted at
200
is a pupil position of the projection optical system.
With currently prevailing projection exposure apparatuses having an excimer laser as a light source, such as described above, however, it is not easy to produce a pattern of a linewidth of 0.15 micron or narrower.
In the projection optical system
196
, there is a limit of resolution due to the tradeoff between the depth of focus and the optical resolution attributable to the exposure wavelength (wavelength used for the exposure process). The resolution R of a resolving pattern and the depth of focus DOF in a projection exposure apparatus can be expressed by Rayleigh's equations 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 of the projection optical system which represents the brightness thereof, and k
1
and k
2
are constants which are determined by the developing process characteristic of the wafer
198
, for example, and which are generally about 0.5 to 0.7. From equation (1) above, it is seen that improvements in resolution (making the value of resolution R smaller) can be attained by enlarging the numerical aperture NA (NA enlargement). Also, from equation (2), it is seen that improvements in resolution anyway necessitate shortening the exposure wavelength (wavelength shortening) because the projection optical system
196
should have a certain value of the depth of focus DOF in the practical exposure such that the NA enlargement cannot be done unlimitedly.
However, the wavelength shortening encounters a critical problem. Namely, there is no glass material usable for the projection optical system
196
. Most glass materials have a substantially zero transmission factor with respect to the deep ultraviolet region. Fused silica is a glass material which can be produced by a special process, for use in an exposure apparatus having an exposure wavelength of about 248 nm. However, even the transmission factor of fused silica largely decreases with respect to the exposure wavelength of 193 nm or shorter. In the exposure wavelength region of 150 nm or shorter, corresponding to a fine pattern of 0.15 micron or less, development of a practical glass material is very difficult. Further, glass materials to be used in the deep ultraviolet region should satisfy not only the transmission factor but also various conditions such as durability, refractive index uniformness, optical distortion, and machining easiness, for example. In consideration of these factors, development of practical glass materials is difficult to accomplish.
As described above, in conventional projection exposure method and projection exposure apparatuses, the wavelength shortening to an exposure wavelength of about 150 nm or less is necessary to enable formation of a pattern of 0.15 micron or narrower on a wafer, whereas there is no practical glass material to be satisfactorily used in such a wavelength region.
A dual exposure method has been proposed by the same assignee of the subject application, which method comprises a combination of a dual-beam interference exposure and a multiple-value exposure, to enable formation of a pattern of 0.15 micron or less with use of a currently available exposure apparatus.
The dual exposure method is a method in which a multiple value exposure process based on an ordinary or standard exposure as well as a periodic pattern exposure process based on a dual beam interference exposure are performed without intervention of a development process. More specifically, a periodic pattern may be photoprinted with a level less than the exposure threshold level of a resist and, thereafter, a standard exposure process having a multiple level exposure amount distribution is performed. As regards the exposure amount by the standard exposure, different exposure amount distributions are produced in diff
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