Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1998-10-13
2001-07-03
Arroyo, Teresa M. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C359S365000, C378S034000
Reexamination Certificate
active
06255661
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a mirror projection system for use in a step-and-scan lithographic projection apparatus for imaging a mask pattern, present in a mask, on a substrate by means of a beam of EUV radiation, which beam has a circular segment-shaped cross-section, said projection system being constituted by six imaging mirrors having ordinal numbers 1-6 from the object side to the image side, the first, the second, the fourth and the sixth mirror being concave and the fifth mirror being convex.
The invention also relates to a lithographic apparatus for step-and-scan imaging of a mask pattern on a number of areas of a substrate, which apparatus comprises such a mirror projection system.
EP-A 0 779 528 describes a mirror projection system for use in a step-and-scan lithographic apparatus with which an IC mask pattern is imaged on a number of areas of a semiconductor substrate, using EUV radiation. EUV, extreme ultraviolet, radiation is understood to mean radiation having a wavelength in the range between several nm and several tens of nm. This radiation is also referred to as soft X-ray radiation. The use of EUV radiation provides the great advantage that extremely small details, of the order of 0.1 &mgr;m or less, can be imaged satisfactorily. In other words, an imaging system in which EUV radiation is used has a very high resolving power without the NA of the system having to be extremely large, so that also the depth of focus of the system still has a reasonably large value. Since no suitable material of which lenses can be made is available for EUV radiation, a mirror projection system must be used for imaging the mask pattern on the substrate, instead of a hitherto conventional lens projection system.
The lithographic apparatuses currently used in the production of ICs are stepping apparatuses. In these apparatuses, a full field illumination is used, i.e. all areas of the mask pattern are illuminated simultaneously and these areas are simultaneously imaged on one IC area of the substrate. After a first IC area has been illuminated, a step is made to a subsequent IC area, i.e. the substrate holder is moved in such a way that the next IC area will be positioned under the mask pattern, whereafter this area is illuminated, and so forth until all IC areas of the substrate of the mask pattern are illuminated. As is known, it remains desirable to have ICs with an increasing number of components.
It is attempted to meet this desire not only by reducing the dimensions of these components but also by enlarging the surface areas of the ICs. This means that the, already relatively high, NA of the projection lens system must be further increased and, for a stepping apparatus, the image field of this system must also be further increased. This is practically impossible.
It has therefore been proposed to change from a stepping apparatus to a step-and-scan apparatus. In such an apparatus, a rectangular or circular segment-shaped sub-area of the mask pattern and hence also such a sub-area of an IC area of the substrate is illuminated, and the mask pattern and the substrate are moved synchronously through the illumination beam, taking the magnification of the projection system into account. A subsequent circular segment-shaped sub-area of the mask pattern is then imaged each time on a corresponding sub-area of the relevant IC area on the substrate. After the entire mask pattern has been imaged on an IC area in this way, the substrate holder performs a stepping movement, i.e. the beginning of a subsequent IC area is introduced into the projection beam and the mask is set to its initial position, whereafter said subsequent IC area is scan-illuminated via the mask pattern. This scan-imaging method may be used to great advantage in a lithographic apparatus in which EUV radiation is used as the projection radiation.
The embodiment of the projection system described in EP 0 779 528, intended for use with EUV radiation having a wavelength of 13 nm, has an NA of 0.20 at the image side. The annular image field has an inner radius of 29 mm and an outer radius of 31 mm and a length of 30 mm. The resolution of the system is 50 nm and the aberrations and distortions are sufficiently small to form a good image of a transmission mask pattern on an IC area of a substrate by way of a scanning process. The third mirror of this projection system is concave. A first pair of mirrors, consisting of the first and the second mirror, constitutes a magnified image of the object or the mask pattern. This image is transported by a second pair of mirrors, constituted by the third and the fourth mirror, and presented to a third pair of mirrors, constituted by the fifth and the sixth mirror, which provides the desired telecentric image with the required aperture NA=0.20. In this projection system, an intermediate image is formed between the third and the fourth mirror, and a diaphragm is situated on the second mirror.
In the known projection system, the mirror sections which constitute the third and the fourth mirror must be located at a relatively large distance from the optical axis of the system. This may cause alignment and stability problems. Moreover, the known system has a small free working distance of the order of 17 mm. In practice, a larger working distance is often required, for example, in connection with building in measuring systems.
It is an object of the present invention to provide a projection system of the type described in the opening paragraph, having a relatively large free working distance and being stable. To this end, the projection system according to the invention is characterized in that the third mirror is convex.
SUMMARY OF THE INVENTION
In the projection system in accordance with the novel concept, only one mirror element, the fourth mirror, is situated at a relatively large distance from the optical axis. The free working distance is, for example, a factor of six larger than that of the system described in EP-A 0 779 528. Now, an intermediate image is formed by the first four mirrors, which intermediate image is present at a position between the fourth and the fifth mirror. This intermediate image is directly imaged in the image plane by the fifth and the sixth mirror. In the novel projection system, the first mirror is placed close to the third mirror, whereas in the system in accordance with EP-A 0 779 528 the second mirror is placed close to the fourth mirror.
It is to be noted that U.S. Pat. No. 5,686,728 describes a six-mirror projection system for a step-and-scan apparatus. However, this projection system is designed for wavelengths in the range between 100 nm and 300 nm, i.e. not for EUV radiation. In U.S. Pat. No. 5,686,728 it is noted that such mirror projection systems are not suitable for EUV radiation. In the embodiment using six mirrors of the projection system described in U.S. Pat. No. 5,686,728, the first mirror is convex.
Within the above-mentioned novel design of the projection system, there is still some freedom of choice of the parameters of numerical aperture, magnification and size of the image field.
An embodiment of the projection system is characterized in that the system has a numerical aperture of the order of 0.20 nm at the image side, a magnification M=+0.25 and a circular segment-shaped image field having a width of 1.5 mm.
This projection system is suitable for imaging details having a size of the order of 50 nm.
The projection system is further preferably characterized in that all mirrors have aspherical surfaces.
An aspherical surface is understood to mean a surface whose fundamental shape is spherical but whose actual surface locally deviates from this fundamental shape so as to correct aberrations of the optical system of which this surface forms part.
By making all mirrors aspherical, a satisfactorily corrected system can be obtained with said image field and said numerical aperture.
The projection system is preferably further characterized in that it is telecentric at the image side.
Consequently, magn
Arroyo Teresa M.
Biren Steven R.
Quash Anthony
U.S. Philips Corporation
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