Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-03-24
2003-07-01
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S067000
Reexamination Certificate
active
06587182
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to illumination apparatuses that are suitable for photo optical systems, particularly for projecting and exposing semiconductor or liquid-crystal patterns, which are formed onto masks, onto photosensitive substrates. This invention further pertains to projection and exposure apparatuses and exposing methods that use said illumination apparatuses.
BACKGROUND OF THE INVENTION
In the recent years, projection and exposure apparatuses that use a KrF excimer laser as a light source, particularly semiconductor manufacturing projection and exposure apparatuses (KrF excimer stepper) have been produced.
FIG. 6
is a schematic diagram of a projection and exposure apparatus with prior art illumination apparatus equipped. After a beam generated from a KrF excimer laser
100
has been expanded by using a beam expander
102
, said expanded light beam is reflected with a vibration mirror
104
, and said reflected light enters a fly eye lens
106
. The beam whose wave front is divided by using the fly eye lens
106
illuminates a reticle
110
via a condenser lens
108
. A circuit pattern displayed onto reticle
110
is transferred onto a wafer
114
by using a projecting lens
112
. In this case, a single exposure is performed by several tens of pulse radiations. The fly eye lens
106
is a necessary element for correction of an nonuniform intensity of the Gaussian distribution which the laser beam has. However, the beam whose wave front is divided by the fly eye lens
106
, is overlapped onto the reticle again, an interference noise is generated onto the reticle. As for a first prior art projection and exposure apparatus shown in
FIG. 6
, the angle of vibration mirror
104
is adjusted per pulse radiation by using a mechanism as not shown in the drawing, the interference noise is averaged and from this, the distribution of the illumination intensity on the reticle is made to be uniform.
FIG. 7
is a schematic diagram of an illumination apparatus disclosed in Japanese unexamined patent application No. S63-216338, which is a second projection and exposure apparatus. A light beam radiated from an excimer laser
200
is converted into a group of beams parallel to the line direction, which are as the same number as that of element lenses of a fly eye lens
204
, by using a multi-reflection mirror system (a multi-beam optical system)
202
that comprises a total reflection mirror
202
C and a partial reflection mirror
202
R. Each beam enters each element lens of fly eye lens
204
. A beam radiated from a secondary light source
206
that is formed corresponding to each element lens is radiated onto a mask (a reticle)
210
by a condenser lens
208
. As for said prior art projection and exposure apparatus, by adjusting the distance between total reflection mirror
202
C and partial reflection mirror
202
R, the difference in length of the beam passage of each beam is determined so as to be the distance that can be interfered by excimer laser
200
or longer. When the difference is determined as described above, beam waves generated from the fly eye lens
204
do not interfere with each other; as a result, an interference noise is not generated onto a mask (a reticle).
FIG. 8
is a diagram illustrating a third illumination apparatus and a projection and exposure apparatus that equips said illumination apparatus as disclosed in Japanese unexamined patent application No. H10-125585. A two-dimensional multi-beam forming optical system
308
that comprises a first one-dimensional multi-beam forming optical system
310
and a second one-dimensional multi-beam forming optical system
312
forms a group of N×M numbers of two-dimensional beams from a beam generated from a laser light source
300
. The group of said N×M numbers of said two-dimensional beams enter a fly eye lens
320
a
, a condenser lens
322
a
, a fly eye lens
320
b
, a condenser lens
322
b
, and a reticle
328
in that order. The first multi-beam optical system
310
and the second multi-beam optical system
312
are orthogonally arranged; the difference in length of the light passages of the first and second multi-beam optical systems is optimized such that all the beams do not interfere with each other. The reflection ratio of each section of the partial reflection mirror that is a component of the first and the second multi-beam optical systems is also optimized such that each intensity of two-dimensional beam arrays that are subsequently generated. Each beam generated from the multi-beam optical system is made to enter the fly eye lens
320
a
while it is expanded to an effective diameter of the element lens of the fly eye lens or larger by using diffusion plates
314
a
and
314
b.
As for excimer lasers which are light sources for projection and exposure apparatuses, the width of the wave length has been reduced. For said reason, in addition to the time coherence, the spatial coherence of recent excimer lasers has also increased in comparison with that of conventional excimer lasers. As the spatial coherence increases, the contrast of interference noises by using the fly eye lenses increases. Said interference noises cause pattern transferring errors when they are superimposed onto circuit patterns. More specifically, ununiform exposure components that have a fine structure increase. When an exposure apparatus is structured with the first prior art illumination apparatus as described above, using a KrF excimer laser having a narrower width of wave length and when an exposure is made with several tens of pulses, the Gaussian intensity distribution of the laser beam can be averaged; however, fine interference noises cannot be sufficiently averaged, which is a disadvantage of the prior art. When the number of exposure pulses (an average number) is increased while reducing the exposure intensity, the interference noises are reduced; however, the throughput also decreases.
The second prior art is a method to reduce the effect of a spatial coherence without increasing the number of exposure pulses. The second prior art method aims to obtain an effect equivalent to the increase of exposure pulses, by converting a beam from a light source into multiple beams that do not interfere with each other.
However, when a projection and exposure apparatus is provided by using the illumination apparatus, it is necessary to increase the number of multiple beams to 50 or more in order to reduce the effect of an interference noise when a single reflection layer is used. For said reason, the size of a multi-beam optical system increases in the reflecting direction of the beams. Additionally, because each beam is projected to an element lens of a fly eye lens without expanding it, the nonuniformity of the intensity with the Gaussian distribution is presented, which is specific to laser beams. It is also difficult to project a beam having a uniform shape to each element lens of a fly eye lens. Therefore, when the illumination apparatus is used, an illumination with a practical uniformity cannot be obtained as similarly to the other case as described above; the uniformity of intensity on a reticle cannot be sufficiently improved; as a result, a pattern transfer error occurs. More specifically, as for the embodiments shown in
FIGS. 6 and 7
, either of the disadvantages, such as the nonuniformity of the Gaussian intensity distribution or the interference noise generated by a fly eye lens, can be solved; however, but not both at the same time.
The third prior art illumination apparatus is compact and does not reduce the throughput; said illumination apparatus also has a structure such that both ununiformity of the Gaussian intensity distribution and interference noise of a fly eye lens can be reduced at the same time. As for the third prior art illumination apparatus, in order to reduce ununiformity of the Gaussian intensity distribution, the effective diameter of each beam generated is expanded to the effective diameter of an element lens of the fly eye lens or larger. In order to generate multipl
Birch & Stewart Kolasch & Birch, LLP
Nguyen Henry Hung
Nikon Corporation
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