Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2001-12-03
2003-07-29
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S071000, C359S859000
Reexamination Certificate
active
06600552
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a microlithography objective, a projection exposure apparatus containing the objective, and a method of manufacturing an integrated circuit using the same.
BACKGROUND OF THE INVENTION
Using a lithography system operating with wavelengths below 193 nm for imaging structures of below 130 nm resolution has been proposed. In fact, such lithography systems have been suggested for the extreme ultraviolet (EUV) range with wavelengths of &lgr;=11 nm or &lgr;=13 nm producing structures of below 100 nm. The resolution of a lithographic system is described by the following equation:
RES=k
1
·&lgr;/NA
where k
1
is a specific parameter of the lithographic process, &lgr; is the wavelength of the incident light, and NA is the image-side numerical aperture of the system. For example, if one assumes a numerical aperture of 0.2, then the imaging of 50 nm structures with 13 nm radiation requires a process with k
1
=0.77. With k
1
=0.64, the imaging of 35 nm structures is possible with 11 nm radiation.
For imaging systems in the EUV region, substantially reflective systems with multilayer coatings are available as optical components. Preferably multilayers of Mo/Be are used as multilayer coating systems for systems operating at &lgr;=11 nm, whereas Mo/Si systems are used for &lgr;=13 mm. Since the reflectivity of the multilayer coatings is approximating 70%, it is desirable to use as few optical components as possible in e.g. an EUV projection microlithography objective to achieve sufficient light intensity. Specifically, to achieve high light intensity and to allow for the correction of imaging errors, systems with six mirrors and a image side numerical aperture (NA)=0.20 have been used.
Six-mirror systems for microlithography have become known from the publications U.S. Pat. No. 5,686,728, EP 779,528 and U.S. Pat. No. 5,815,310. The projection lithography system according to U.S. Pat. No. 5,686,728 has a projection objective with six mirrors, where each of the reflective mirror surfaces has an aspherical form. The mirrors are arranged along a common optical axis in such a way that an obscuration-free light path is achieved. Since the projection objective known from U.S. Pat. No. 5,686,728 is used only for UV light with a wavelength of 100-300 nm, the mirrors of this projection objective have a very high asphericity of approximately ±50 &mgr;m as well as very large angles of incidence of approximately 38°. Even after reducing the image side aperture to NA=0.2, an asphericity of 25 &mgr;m from peak to peak remains, with a barely reduced angle of incidence. Such asphericities and angles of incidence are not practicable in the EUV region due to the high requirements for surface quality and reflectivity of the mirrors.
Another disadvantage of the objectives disclosed in U.S. Pat. No. 5,686,728, which precludes their use with wavelengths below 100 nm such as the 11 nm and 13 nm wavelengths desirable for EUV microlithography, is the short distance between the wafer and the mirror arranged next to the wafer. In the case of U.S. Pat. No. 5,686,728, due to this short distance between the wafer and the mirror next to the wafer, the mirrors could be made only very thin. Due to the extreme layer stress in the multilayer systems discussed for 11 nm or 13 nm wavelengths, such thin mirrors are very unstable.
A projection objective with six mirrors for use in EUV lithography, particularly also for wavelengths of 13 nm and 11 nm, has become known from EP 779,528. This projection objective also has the disadvantage that at least two of the six mirrors have very high asphericities of 26 and 18.5 &mgr;m. However, even in the EP 779,528 arrangement, the optical free working distance between the mirror next to the wafer and the wafer itself is so small that either instabilities occur or the mechanical free working distance is negative.
Thus, it is desirable to provide a projection objective for lithography with short wavelengths, preferably smaller than 100 nm, which does not have the disadvantages of the state of the art described above.
SUMMARY OF THE INVENTION
According to one aspect of the invention, the shortcomings of the prior art are overcome by a projection objective having an object plane and an image plane and a light path for a bundle of light rays from the object plane to the image plane. The six mirrors of the objective are arranged in the light path from the object plane to the image plane. According to the invention the mirror closest to the image plane where e.g. an object to be illuminated such as a wafer is situated is arranged in such a way that an image-side numerical aperture is NA≧0.15. In this application the image-side numerical aperture is understood to be the numerical aperture of the bundle of light rays impinging onto the image plane. Furthermore, the mirror arranged closest to the image plane of the objective is arranged in such a way that the image-side free working distance corresponds at least to the used diameter of the mirror next to the wafer. In a preferred embodiment the image-side free working distance is at least the sum of one-third of the used diameter of the mirror next to the image plane and a length between 20 and 30 mm. In an alternative embodiment the image-side free working distance is at least 50 mm. In a particularly preferred embodiment, the image-side free working distance is 60 mm. In this application the free working distance is defined as the distance of the vertex of the surface of the mirror next to the image plane and the image plane. All surfaces of the six mirrors in this application are rotational-symmetric about a principal axis (PA). The vertex of a surface of a mirror is the intersection point of the surface of a mirror with the principal axis (PA). Each mirror has a mirror surface. The mirror surface is the physical mirror surface upon which the bundle of light rays traveling through the objective from the object plane to the image plane impinge. The physical mirror surface or the used area of a mirror can be an off-axis or an on-axis mirror segment relative to the principal axis (PA).
According to another aspect of the invention, a projection objective that comprises six mirrors is characterized by an image-side numerical aperture, NA, greater than 0.15 and an arc-shaped field width, W, at the wafer in the range 1.0 mm≦W. The peak-to-valley deviation, A, of the aspheres are limited with respect to the best fitting sphere of the physical mirror surface of all mirrors by:
A≦
19
&mgr;m
−102
&mgr;m
(0.25−
NA
)−0.7
&mgr;m/mm (
2
mm−W
).
In a preferred embodiment, the peak-to-valley distance A of the aspheres is limited with respect to the best fitting sphere of the off-axis segments of all mirrors by:
A≦
12
&mgr;m
−64
&mgr;m
(0.25−
NA
)−0.3
&mgr;m/mm
(2
mm−W
).
According to yet another aspect of the invention, a projection objective that includes six mirrors is characterized by an image-side numerical aperture NA≧0.15 and an image-side width of the arc-shaped field W≧1 mm, and the angles of incidence AOI are limited for all rays of the light bundle impinging a physical mirror surface on all six mirrors S
1
, S
2
, S
3
, S
4
, S
5
, S
6
by:
AOI≦
23°−35°(0.25
−NA
)−0.2°/
mm
(2
mm−W
)
wherein the angles of incidence AOI refer to the angle between the incident ray and the normal to the physical mirror surface at the point of incidence. The largest angle of any incident bundle of light rays occurring on any of the mirrors is always given by the angle of a bundle-limiting ray.
Preferably, an embodiment of the invention would encompass all three of the above aspects, e.g., an embodiment in which the free optical working distance would be more than 50 mm at NA=0.20 and the peak-to-valley deviation of the aspheres, as well as the angles of incidence, would lie in the regions defined above.
The asphericities herein refer to the peak-to-va
Carl-Zeiss SMT AG
Marshall Gerstein & Borun
Nguyen Henry Hung
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