Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-02-14
2002-03-05
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S067000, C359S859000
Reexamination Certificate
active
06353470
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
i
·&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 multiple layers 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. With the reflectivity of the multilayer coatings approximating 70%, it is desirable to use as few optical components as possible in applications such as EUV projection objective microlithography 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 numerical aperture (NA)=0.20 have been used.
The 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 aperture to NA=0.2, an asphericity of 25 &mgr;m from peak to peak remains, with little reduction in the angle of incidence. Such asphericities and angles of incidence are not practicable in the EUV region according to the present state of the art because of the higher requirements on surface quality and reflectivity of the mirrors in these latter systems.
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 lying closest to the wafer. This short distance allows only very thin mirrors to be used in the U.S. Pat. No. 5,686,728 apparatus. Because of the extreme stresses on the coatings of the multilayer systems suitable for the 11 nm and 13 nm wavelengths in question, such thin mirrors are very unstable.
A projection objective with six mirrors for use in EW lithography, even at 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. Unfortunately, in the EP 779,528 arrangement, the optical free working distance between the mirror next to the wafer and the wafer is so small that either instabilities occur or a negative mechanical free working distance is obtained.
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 an aspect of the invention, the shortcomings of the prior art are overcome by using a projection objective with six mirrors where the mirror nearest to a wafer to be illuminated is arranged in such a way that an image-side numerical aperture NA≧0.15. Furthermore, the mirror nearest to the wafer is arranged in such a way that the image-side optical free working distance corresponds at least to the used diameter of the mirror next to the wafer; the image-side optical free working distance is at least the sum of one-third of the used diameter of this nearest mirror and a length between 20 and 30 mm; or the image-side optical free working distance is at least 50 mm. In a preferred embodiment, the image-side optical free working distance is 60 mm.
According to another aspect of the invention, a projection objective that includes six mirrors is characterized by an image-side numerical aperture, NA, is greater than 0.15 and the arc-shaped field width, W, at the wafer lies in the range 1.0 mm≦W, and the peak-to-valley deviation, A, of the aspheres is limited with respect to the best fitting sphere in the used range on 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 on 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 angle of incidence AOI relative to the surface normal is limited for all beams on all mirrors by:
AOI≦
23°−35°(0.25−
NA
)−0.2°/mm (2 mm−
W
).
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-valley (PV) deviation, A, of the aspherical surfaces with respect to the best fitting sphere in the used range. The aspherical surfaces are approximated in the examples by using a sphere with center on the figure axis vertex of the mirror and which intersects the asphere in the upper and lower endpoint of the useful range in the meridian section. The data regarding the angles of incidence always refer to the angle between the incident beam and the normal to the surface at the point of incidence. The largest angle of any incident light occurring on any of the mirrors is always given, i.e., the angle of a bundle-limiting beam. The used diameter will be defined here and below as the envelope circle diameter of the used region, which is generally not circular.
Preferably, the wafer-side optical free working distance is 60 mm.
The objective can be used not only in the EUV, but also at other wavelengths, without deviating from the scope of the invention. In any respect, however, to avoid degradation of image quality, especially degradation due to central shading, the mirrors of the projection objectives should be arranged so that the light path is obscuration-free. Furthermore, to provide easy mounting and adjusting of the system, the mirror surfaces should be designed on a surface which shows rotational symmetry to a principal axis (PA). Moreover, to have a compact design with an accessible aperture and to establish an obscuration-free path, the projection objective devices are designed to produce an intermediate image, preferably formed after the fourth mirror. In such systems, it is possible for the aperture stop to lie in the front, low-aperture objective pa
Adams Russell
Marshall Gerstein & Borun
Nguyen Hung Henry
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