Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-02-25
2004-08-03
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492100, C250S492220, C250S492230, C250S493100, C250S306000, C250S365000, C378S034000
Reexamination Certificate
active
06770894
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an illumination system, and more particularly to an illumination system for EUV lithography with wavelengths less than 193 nm.
2. Description of the Prior Art
To reduce the structural widths for electronic components, especially in the submicron range, it is necessary to reduce the wavelength of the light used for microlithography. For example, lithography with soft x-rays is conceivable with wavelengths smaller than 193 nm. U.S. Pat. No. 5,339,346 disclosed an arrangement for exposing a wafer with such radiation. An illumination system for soft x-rays, so-called EUV radiation, is shown in U.S. Pat. No. 5,737,137, in which illumination of a mask or a reticle to be exposed is produced using three spherical mirrors.
Field mirrors that show good uniformity of output of an exposure beam at a wafer in a lithographic system have been disclosed in U.S. Pat. No. 5,142,561. The exposure systems described therein concern the contact exposure of a wafer through a mask with high-energy x-rays of 800 to 1800 eV.
EUV illumination systems for EUV sources have been disclosed in EP 99 106 348.8 (U.S. application Ser. No. 09/305017) and PCT/EP99/02999. These illumination systems are adapted to synchrotron, wiggler, undulator, Pinch-Plasma or Laser-Produced-Plasma sources.
Scanning uniformity is a problem of the aforementioned scanning exposure systems in illuminating a slit, particularly a curved slit. For example, the scanning energy obtained as a line integral over the intensity distribution along the scan path in a reticle or wafer plane may increase toward the field edge despite homogeneous illumination intensity because of the longer scan path at the field edge for a curved slit. However, scanning energy and with it scanning uniformity may also be affected by other influences, for example coating or vignetting effects are possible. The curved slit is typically represented by a segment of a ring field, which is also called an arc shaped field. The arc shaped field can be described by the width delta r, a mean Radius R
0
and the angular range 2·&agr;
0
. For example, the rise of the scanning energy for a typical arc shaped field with a mean radius of R=100 mm and an angular range of 2·&agr;
0
=60° is 15%.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an illumination system for a projection exposure system in which the scanning energy is uniform, or can be controlled to fit a predetermined curve.
This and other objectives of the present invention are achieved by shaping a field lens group in an illumination system of a generic type so that the illuminated field is distorted in an image plane of the illumination system perpendicular to the scanning direction. In this plane the mask or reticle of a projection exposure system is located.
The term “field lens group” is taken to describe both field mirror(s) and field lens(es). For wavelengths &lgr;>100 nm the field lens group typically comprises refractive field lens(es), but mirrors are also possible. For wavelengths in the EUV region (10 nm<&lgr;<20 nm) the field lens group comprises reflective field mirror(s). EUV lithography uses wavelengths between 10 nm and 20 nm, typically 13 nm.
According to the present invention it is possible to determine the necessary distortion to obtain a predetermined intensity distribution. It is advantageous for a scanning system to have the capability of modifying the intensity distribution perpendicular to the scanning direction to get a uniform distribution of scanning energy in the wafer plane. The scanning energy can be influenced by varying the length of the scanning path or by modifying the distribution of the illumination intensity. The present invention relates to the correction of the distribution of the illumination intensity. In comparison to stepper systems where a two-dimensional intensity distribution has to be corrected, a scanner system only requires a correction of the distribution of the scanning energy.
In one embodiment of the present invention, the illumination intensity decreases from the center of the field to the field edges by means of increasing distortion. The intensity is maximum at the field center (&agr;=0°) and minimum at the field edges (&agr;=±&agr;
0
). A decrease of the illumination intensity towards the field edge permits a compensation for an increase of the scan path so that the scanning energy remains homogeneous.
The present invention also provides for the illumination intensity to increase from the center of the field to the field edges by means of decreasing distortion. This correction can be necessary if other influences like layer or vignetting effects lead to a decreasing scanning energy towards the field edges.
Preferably, the field lens group is designed so that uniformity of scanning energy in the range of ±7%, preferably ±5%, and very preferably ±3%, is achieved in the image plane of the illumination system.
The field lens group is shaped so, that the aperture stop plane of the illumination system is imaged into a given exit pupil of the illumination system. In addition to the intensity correction, the field lens group achieves the correct pupil imaging. The exit pupil of the illumination system is typically given by the entrance pupil of the projection objective. For projection objectives, which do not have a homocentric entrance pupil, the location of the entrance pupil is field dependent. In such a case, the location of the exit pupil of the illumination system is also field dependent.
The shape of the illuminated field according to this invention is rectangular or a segment of a ring field. The field lens group is preferably shaped such that a predetermined shaping of the illuminated field is achieved. If the illuminated field is bounded by a segment of a ring field, the design of the field lens group determines the mean radius R
0
of the ring field.
It is advantageous to use a field lens group having an anamorphotic power. This can be realized with toroidal mirrors or lenses so that the imaging of the x- and y-direction can be influenced separately.
In EUV systems the reflection losses for normal incidence mirrors are much higher than for grazing incidence mirrors. Accordingly, the field mirror(s) is (are) preferably grazing incidence mirror(s).
In another embodiment of the present invention the illumination system includes optical components to transform the light source into secondary light sources. One such optical component can be a mirror that is divided into several single mirror elements. Each mirror element produces one secondary light source. The mirror element can be provided with a plane, spherical, cylindrical, toroidal or an aspheric surface. Theses single mirror elements are called field facets. They are imaged in an image plane of the illumination system where the images of the field facets are at least partly superimposed.
For extended light sources or other purposes it can be advantageous to add a second mirror that is divided in several single mirror elements. Each mirror element is located at a secondary light source. These mirror elements are called pupil facets. The pupil facets typically have a positive optical power and image the corresponding field facets into the image plane.
The imaging of the field facets into the image plane can be divided into a radial image formation and an azimuthal, image formation. The y-direction of a field facet is imaged in the radial direction, and the x-direction is imaged in the azimuthal direction of an arc shaped field. To influence the illumination intensity perpendicular to the scanning direction the azimuthal image formation will be distorted.
The imaging of the field facets is influenced by the field lens group. It is therefore advantageous to vary the azimuthal distortion by changing the surface parameters of the components of the field lens group.
The field lens group is shaped such that the secondary light sources produc
Carl Zeiss SMT AG
El-Shammaa Mary
Lee John R.
Ohlandt Greeley Ruggiero & Perle LLP
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