Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-03-10
2002-12-17
Berman, Jack (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C378S034000, C359S359000, C359S366000, C359S856000, C359S857000, C359S858000, C359S861000
Reexamination Certificate
active
06495839
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 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 numerical aperture to NA=0.2, an asphericity of 25 &mgr;m from peak to valley remains, with little reduction in the 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 EUV lithography, even at wavelengths of 13 nm and 11 nm, has become known from EP 779,528. This projection objective also has the disadvantage, however, that at least two of the six mirrors have very high asphericities of 26 and 18.5 &mgr;m. Furthermore, 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.
Four-mirror projection objectives have become known from the following publications:
U.S. Pat. No. 5,315,629
EP 480,617
U.S. Pat. No. 5,063,586
EP 422,853
In U.S. Pat. No. 5,315,629, a four-mirror projection objective with NA=0.1, 4×, 31.25×0.5 mm
2
is claimed. From EP 480,617, two NA=0.1, 5×, 25×2 mm
2
systems have become known. The system according to U.S. Pat. No. 5,063,586 and EP 422,853 have a rectangular image field of at least 5×5 mm
2
. These generally decentered systems exhibit very high distortion values. Therefore, the objectives can only be used in steppers with distortion correction on the reticle. However, the high level of distortion makes such objectives impractical at wavelengths below 100 nm.
From U.S. Pat. No. 5,153,898, overall arbitrary three to five-multilayer mirror systems have become known. The disclosed embodiments, however, all describe three-mirror systems with a rectangular field and small numerical aperture (NA<0.04). Therefore, the systems described therein can only image structures above 0.25 &mgr;m in length. The distortion of most examples lies in the &mgr;m range.
Furthermore, reference is made to T. Jewell: “Optical system design issues in development of projection camera for EUV lithography”, Proc. SPIE 2437 (1995) and the citations given there, the entire disclosure of which is incorporated by reference.
Thus, it is desirable to provide a projection objective suitable for lithography with short wavelengths, preferably smaller than 100 nm, which does not have the disadvantages of the state of the art mentioned above, and which has as few optical elements as possible and a sufficiently large numerical aperture.
SUMMARY OF THE INVENTION
According to the invention, the short comings of the prior art are overcome by using a projection objective device which has five mirrors. By omitting a mirror from the known six-mirror systems—according to the five-mirror system of the invention—one can achieve a transmission which is at least 30% higher at wavelengths in the EUV region, if a reflectivity of the multilayer coating system of 70% is assumed for this radiation. In addition, numerical apertures of NA>0.10 can be realized. The five-mirror objective according to the invention is thus characterized by high resolution, low manufacturing costs and high throughput.
In a first embodiment of the invention, the mirror closest to the wafer is arranged in such a way that the image-side numerical aperture NA is greater than or equal to 0.10. Furthermore, the mirror closest to the wafer is arranged such that (1) the image-side free optical working distance corresponds at least to the used diameter of the mirror closest to the wafer, (2) the image-side optical free working distance is at least the sum of one-third of the used diameter of the mirror closest the wafer and a length which lies between 20 and 30 mm, and/or (3) the image-side optical free working distance is at least 50 mm. Preferably, the optical free working distance is 60 mm.
In a second embodiment of the invention, the image-side numerical aperture NA is greater than or equal to 0.10, the annular field width W at the wafer lies in the region 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 useful region on all mirrors, by:
A≦
24 &mgr;m−129 &mgr;m(0.20−
NA
)−2.1 &mgr;m/mm(2 mm−
W
).
In a preferred embodiment, the peak-to-valley deviation A of the aspheres on all mirrors is limited by:
A≦
9 &mgr;m−50 &mgr;m(0.20−
NA
)−0.4 &mgr;m/mm(2 mm−
W
).
In a third embodiment of the invention, with a numerical aperture in the range NA≧0.10 and an image-side width of the annular field in the range W≧1 mm, the angles of incidence, AOI, on all mirrors, relative to the normal of the surface of a given mirror, is limited by:
AOI≦
22°−2°(0.20
−NA
)−0.3°/mm(2 mm−
W
).
Combinations of the above may also be used according to the invention. F
Berman Jack
Carl-Zeiss Stiftung
Marshall Gerstein & Borun
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