Photocopying – Projection printing and copying cameras – With temperature or foreign particle control
Reexamination Certificate
1999-10-25
2003-02-18
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
With temperature or foreign particle control
C355S053000, C355S067000
Reexamination Certificate
active
06522384
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates generally to an exposure apparatus for projecting and printing a circuit pattern formed on a mask, onto a substrate being coated with a photosensitive material, in a reduced scale. More particularly, the invention is concerned with an exposure apparatus which uses deep ultraviolet light or an excimer laser as an exposure light source.
Reduction type projection exposure apparatuses are used in a process of manufacturing a semiconductor device which is formed with a very fine pattern such as LSI or VLSI. Miniaturization of a pattern has been required strongly due to increases in the integration density of a semiconductor device, and exposure apparatuses have been modified to meet such miniaturization, as well as improvements in a resist process.
The resolving power of an exposure apparatus can be improved by two methods, that is, a method in which the exposure wavelength is shortened, and a method in which the numerical aperture (NA) of a projection optical system is enlarged. Generally, the resolution is proportional to the exposure wavelength and it is inversely proportional to the NA. Besides the improvement of resolution, many attempts have been made to keep the depth of focus of a projection optical system. Generally, the depth of focus is proportional to the exposure wavelength, and it is inversely proportional to the square of the NA. Thus, improving the resolution and keeping the depth of focus are contradictory matters. As an attempt to solve such a problem, a phase shift method and a FLEX (Focus Latitude Enhancement Exposure) method, for example, have been proposed.
As regards the exposure wavelength, recently, KrF excimer lasers having an emission wavelength of about 248 nm are prevalently used in place of i-line of 365 nm. Also, ArF excimer lasers having an emission wavelength of about 193 nm are currently being developed, as a next generation exposure light source.
From the viewpoint of the production cost of a semiconductor device, further improvements in the throughput of an exposure apparatus have been attempted. For example, the power of an exposure light source is enlarged to thereby shorten the exposure time per one shot. Another example is enlarging the exposure area to thereby increase the number of chips per one shot.
In recent years, in order to meet the requirement of enlargement in chip size of a semiconductor device, the stream is shifting from step-and-repeat type exposure apparatuses (steppers) in which a mask pattern is printed sequentially in association with stepwise motion, to step-and-scan type exposure apparatuses in which a mask and a wafer are scanningly exposed in synchronism with each other, followed by stepwise motion to place a subsequent shot. In such step-and-scan type exposure apparatuses, the exposure field has a slit-like shape and, therefore, the exposure area can be enlarged without enlargement in size of the projection optical system.
Where ultraviolet light is used as an exposure light source, as described above, there may occur a phenomenon that, due to long-period use, ammonium sulfate (NH
4
) or silicon dioxide (SiO
2
) is deposited on the surface of an optical element disposed on the light path, to cause considerable degradation of the optical characteristic. The deposition is produced because of chemical reaction of ammonia (NH
3
), sulfurous acid (SO
2
) or silicon compound contained in the surrounding ambience caused in response to irradiation with ultraviolet light. In order to prevent such deterioration of optical elements, conventionally, the whole of the light path is purged by use of a clean dry air or an inert gas such as nitrogen.
As regards deep ultraviolet light, particularly, ArF excimer lasers having a wavelength of about 193 nm, it is known that there are plural absorbing bands for oxygen (O
2
) in the bandwidth about that wavelength. Also, ozone (O
3
) will be produced when oxygen absorbs light, and this ozone acts to increase light absorption, causing considerable decrease of transmission factor. Additionally, various products, as described above, attributable to the ozone will be deposited on the surface of an optical element, thus causing a decrease of the efficiency of the optical system.
In consideration of it, in an exposure optical system for projection exposure apparatuses having a deep ultraviolet light source such as an ArF excimer laser, for example, purge means using an inert gas such as nitrogen, for example, may be provided to keep the oxygen density along the light path at a low level.
An example of such inert gas purge means for an illumination optical system in a projection exposure apparatus, will be described with reference to FIG.
8
.
As illustrated in the drawing, there are an excimer laser
201
, and a container or housing
202
for the illumination optical system. Further, there are a reticle
203
and mirrors
204
,
205
and
206
. Denoted at
207
is a beam shaping optical system, and denoted at
208
is an optical integrator. Also, there are condenser lenses
209
,
210
and
211
.
A laser beam emitted by the excimer laser
201
is shaped by the beam shaping optical system
207
into a predetermined beam shape. Thereafter, the light enters the optical integrator
208
and, in response, secondary light sources (not shown) are produced near the light exit surface of the optical integrator
208
. The light rays from the secondary light sources are directed through the condenser lenses
209
,
210
and
211
to uniformly illuminate the reticle
203
. Thus, the arrangement provides a Koehler illumination optical system.
In order to provide an inert gas ambience around the optical elements described above and along the light path of them, inert gas supply means (not shown) supplies a nitrogen gas, for example, into the housing
202
through a gas inlet port
202
a
. The thus applied inert gas flows through the interior of the illumination optical system. After substitution to remove any residual gas such as atmospheric gas, for example, the inert gas is discharged outwardly through a gas outlet port
202
b
, by gas discharging means (not shown).
The gas supply quantity may be controlled so as to minimize the substitution time by the inert gas, to thereby increase the system throughput, or minimize the consumption quantity of the inert gas after substitution, to thereby decrease the system running cost (Japanese Laid-Open Patent Application, Laid-Open No. 216000/1994).
On the other hand, a currently prevailing illumination method is a variation illumination method (e.g., Japanese Laid-Open Patent Application, Laid Open No. 204114/1994) wherein the distribution of the secondary light source as described above is changed in various ways. This is to accomplish both a high resolution and a large depth of focus. In order that the illumination condition is made variable, many optical elements of an illumination optical system should be made interchangeable. With the above-described inert gas substitution method, on that occasion, it is very difficult to forcibly substitute the inside space of a mechanism (barrel) for holding optical elements to be interchanged. Particularly, in a case where an ArF excimer laser having an emission wavelength about 193 nm is used, there is a problem, as described, that the light absorption occurs due to any oxygen remaining along the light path which causes a serious decrease of optical efficiency. Therefore, forcible substitution of the interior of the movable barrel, if desired, needs a complicated structure for the gas flow passageway, and it causes an increase of the system cost as well as prolongation of the time for completion of the substitution which results in a decrease of the system throughput.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to accomplish reduction of a substitution time to an inert gas ambience along an exposure light path, still with a minimum cost, and thereby to increase the system throughput.
In accordance with an aspect of the present inven
Adams Russell
Fuller Rodney
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