X-ray or gamma ray systems or devices – Specific application – Lithography
Reexamination Certificate
2001-10-25
2003-12-02
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Lithography
C378S145000
Reexamination Certificate
active
06658084
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns an illumination system for wavelengths ≦193 nm, particularly for EUV lithography, a method for adjusting the illumination in an exit pupil of an illumination system, as well as a projection exposure comprising such an illumination system.
2. Description of the Prior Art
In order to allow even further reduction in the line widths for electronic components, particularly in the submicron range, it is necessary to reduce the wavelength of the light used for the microlithography. For example, with wavelengths less than 193 nm, lithography with soft X-rays, so-called EUV lithography is possible.
An illumination system suitable for EUV lithography should illuminate homogeneously, i.e., uniformly, the field used in EUV lithography, particularly the ring field of an objective, with as few reflections as possible, and furthermore, the pupil of the objective should be illuminated up to a particular filling ratio a, independently of the field, and the exit pupil of the illumination system should lie in the entrance pupil of the objective.
From U.S. Pat. No. 5,339,346 an illumination system for a lithography device that uses EUV radiation has been made known. For uniform illumination in the plane of the reticle and filling of the pupils, U.S. Pat. No. 5,339,346 proposes a condenser, which is constructed as a collector lens and comprises at least four mirror facets arranged in pairs and symmetrically. The light source used is a plasma light source.
In U.S. Pat. No. 5,737,137 there is shown an illumination system with a plasma light source comprising a condenser mirror. In U.S. Pat. No. 5,737,137 an illumination of a mask or reticle is achieved by means of spherical mirrors.
U.S. Pat. No. 5,361,292 shows an illumination system in which a plasma light source is used. The point-like plasma light source is imaged into a ring-shaped illuminated surface by means of a condenser having five aspheric mirrors arranged eccentrically. The ring-shaped illuminated surface is then imaged in the entrance pupil by means of a special sequence of grazing-incidence mirrors.
From U.S. Pat. No. 5,581,605 an illumination system in which a photon emitter is divided into a plurality of secondary light sources by means of a honeycomb condenser is known. In this way, a uniform illumination is achieved in the plane of the reticle. The imaging of the reticle on the wafer to be exposed is done with conventional reduction optics. Exactly one rastered mirror with elements of equal curvature is arranged in the illumination beam path.
EP 0 939, 341 shows a Köhler illumination system for wavelengths <200 nm, especially for the EUV range, with a first optical integrator comprising a plurality of first raster elements and a second optical integrator, comprising a plurality of second raster elements. The light distribution in the exit pupil is controlled by a diaphragm wheel, which involves a considerable loss of light. As an alternative to this, for a quadrupole distribution of light, it is proposed to split the light beam after the source and before the first optical integrator into four light beams. Various other illumination settings can also be achieved according to EP 0 939,341 without the use of diaphragms, for example, by changing optics. This type of variation of the illumination settings is on the one hand very costly, and on the other hand limited to certain types of illumination settings, namely, annular and quadrupolar illumination.
From DE 199 03,807 A1 there is another known EUV illumination system. The system according to DE 199 03 807 comprises two mirrors or lenses with raster elements. Such systems are also known as double-faceted EUV illumination systems. The disclosure content of DE 199 03,807 A1 is fully incorporated into the present application.
In DE 199 03,807 A1, the basic construction principle of a double-faceted EUV illumination system is illustrated. The illumination in the exit pupil of the illumination system is determined, according to DE 199 03,807, by the arrangement of the raster elements on the second mirror. A variable controlling of the illumination in the exit pupil or the adjustment of a predetermined distribution of the illumination in the exit pupil is not described by simple means according to DE 199 03,807.
SUMMARY OF THE INVENTION
The object of the invention is to indicate the most simple possible construction of a double-faceted illumination system, which allows a variable adjustment of any illumination distribution in the exit pupil with substantially no losses of light, as well as a method for adjusting an illumination distribution in such an illumination system.
According to the invention, the object is solved in that, in an illumination system for wavelengths ≦193 nm, particularly for EUV lithography, a predetermined illumination in an exit pupil is adjusted by altering points of incidence of light channels traveling from a light source to the exit pupil. Thanks to such an adjustment of the light distribution in the exit pupil, any given distributions can be realized and losses of light, such as occur for example in the solutions using diaphragms, can be avoided.
While the system is designed to be purely reflective at wavelengths in the EUV range, i.e., designed exclusively with mirror components, refractive components such as lenses or lens arrays are used as so-called optical integrators in 193-nm or 157-nm systems.
Thus, the invention also provides an illumination system in the 193-nm and 157-nm range, with which the illumination of the exit pupil can be adjusted and changed in a simple manner.
With the illumination system of the invention, the field in the plane of the reticle is illuminated homogeneously and with a partially filled aperture. Furthermore, the exit pupil of the illumination system is illuminated variably.
As described below, several light distributions in the exit pupil can be obtained with the help of the invention.
For circular illumination, the distribution of light or the illumination setting in the exit pupil, which in the present case coincides with the objective pupil, is defined by the filling factor &sgr;:
Filling factor: &sgr;=
r
illumination
/R
objective aperture
Here, r
illumination
means the radius of the illumination and R
objective aperture
is the radius of the objective aperture.
By definition, for &sgr;=1.0, the objective pupil is completely filled; and, for example, &sgr;=0.6 corresponds to less than complete filling.
For an annular distribution of light, the objective pupil is illuminated in annular fashion. To describe this, one can use the following definition of &sgr;
out
/&sgr;
in
:
σ
out
=
r
(
90
)
R
(
NA
max
)
whereby
∫
0
r
(
90
)
l
(
r
)
r
ⅆ
r
=
0.9
·
∫
0
R
(
NA
max
)
l
(
r
)
r
ⅆ
r
σ
in
=
r
(
10
)
R
(
NA
max
)
whereby
∫
0
r
(
10
)
l
(
r
)
r
ⅆ
r
=
0.1
·
∫
0
R
(
NA
max
)
l
(
r
)
r
ⅆ
r
Another light distribution is the so-called quadrupole illumination for imaging of “Manhattan structures”, for example.
According to the invention, all of the above-described settings can be realized at the same time in double-faceted illumination systems. In a first embodiment of the invention, on the second optical element with raster elements, hereinafter also called pupil raster elements or pupil honeycombs, the distribution of the second raster elements on the second optical element for all possible illuminations in the exit pupil are available.
By replacing the first optical element or lens with raster elements, hereinafter also called field raster elements or field honeycombs, or by changing the tilt of the raster element on the plate of the first optical element, then only the pupil raster elements of a particular setting, such as the quadrupole setting, can be illuminated o
Bruce David V.
Carl Zeiss SMT AG
Ohlandt Greeley Ruggiero & Perle LLP
Thomas Courtney
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