Optical: systems and elements – Mirror – Plural mirrors or reflecting surfaces
Reexamination Certificate
1998-12-31
2001-02-13
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Mirror
Plural mirrors or reflecting surfaces
C359S852000, C355S067000
Reexamination Certificate
active
06186632
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to lithography systems for semiconductor fabrication, and more specifically to condenser systems for ring-field deep ultraviolet and extreme ultraviolet projection optics.
BACKGROUND OF THE INVENTION
Photolithography has become a critical enabling technology in the fabrication of modern integrated circuit (IC) devices. The photolithography process typically involves exposing a patterned mask to radiation to produce patterned radiation. The patterned radiation is then passed through an optical reduction system, and the reduced patterned radiation or mask image is projected onto a substrate, typically a silicon wafer, that is coated with photoresist. The radiation exposure changes the properties of the photoresist and allows subsequent processing of the substrate.
Photolithography machines or steppers, use two common methods of projecting a mask image onto a substrate, “step and repeat” and “step and scan”. The step and repeat method sequentially exposes portions of a substrate to a mask image and uses a projection field which is large enough to project the entire mask image onto the substrate. After each image exposure, the substrate is repositioned and the process is repeated.
Step and scan optical systems typically utilize ring fields to project the mask image onto the substrate, and are more commonly used in IC fabrication processes. The step and scan method scans a mask image onto a substrate through a narrow arc-shaped slit of light. Referring to
FIG. 1
, a ring-field lithography system
100
for use in the step and scan method is shown. A moving mask
101
is illuminated by a radiation beam
103
, which reflects from the mask
101
and is directed through a reduction ring-field optical system
107
. Within the optical system
107
the image is reversed and an arc shaped reduced image beam
109
is projected onto a moving substrate
111
. The arc-shaped reduced image beam
109
can only project a portion of the mask
101
at one time, and is thus moved over time to scan the complete mask
101
onto the substrate
111
. The mask
101
and substrate
111
are moved synchronously in relation to one another to perform the scanning process.
FIG. 2
is a more detailed illustration of the arcuate ring field slit
201
produced by the photolithography system illustrated in FIG.
1
. Ring field arc
201
corresponds to the projection of the arc-shaped reduced image beam
109
in
FIG. 1
, as it is seen on the surface of substrate
111
. Ring field arc
201
is projected over angle
209
and is geometrically described by a ring field radius
203
, a ring field width
205
, and a ring field length
207
. In general, ring field coverage is unlimited in azimuth.
As the degree of circuit integration has increased, the feature sizes of IC's have dramatically decreased. To support future semiconductor fabrication requirements, lithography systems using extremely short wavelength radiation have been developed to overcome the inherent limitations of traditional optical systems. Extreme Ultraviolet (EUV) lithography using 10 to 14 nm soft x-ray photons has emerged as a promising technology for fabrication of integrated circuits with design rules requiring sizes of 100 nm or less. Deep Ultraviolet (DUV) lithography using 100-300 nm radiation has also proven to be a viable technology for IC fabrication lithography systems.
In lithography, where it is crucial that the image characteristics are invariant across the imaging field, the condenser optical system is a critical component. Because refractive optics absorb all EUV radiation, only reflective optical elements are suitable for EUV optical systems. In EUV lithography, therefore, the condenser is likely to be all-reflective and must gather as much light as possible from the source, in order to reduce exposure time to an economical level.
Most projection optics image over ring fields, whereas most EUV sources are small and circular. In order to make the most efficient use of the EUV source, a ring-field system requires a condenser that illuminates only the required arc of the ring field. The angular distribution of the illumination at any field point must have a particular profile (for example, a uniform circular disc) and must be directed into the center of the entrance pupil of the projection optical system.
The illumination system for a ring-field projection camera must satisfy a number of criteria. These criteria are driven by the lithographic requirements of the camera, namely, critical dimension (CD) uniformity and minimal pattern placement error across the field, the ability to image at high contrast, and the ability to print at a high rate. To satisfy these requirements, the condenser used in such an optical system must satisfy minimum performance characteristics, such as, shape and uniformity of the pupil fill; lack of any variation of illumination characteristics across the field (stationarity); telecentricity; EUV throughput; and compensation of camera errors. Of these, the pupil fill and stationarity are most important, since they encompass the main purpose of an illuminator which is to provide illumination of the mask in a way that optimizes its aerial image by scanning and minimizes any variation of CD across the field. Furthermore, the EUV throughput must be maximized due to the economic need to minimize exposure times. In addition to these optical metrics, there are also those of the scalability to higher numerical aperture (NA) imaging systems, and the ease of manufacturability of the condenser.
Present, known condenser designs have proven unable to meet all of the requirements demanded by EUV and DUV lithography in which the condenser must collect as much light as possible from a source such as a laser-produced plasma, discharge, or synchrotron source, and transform it into an arc at the mask plane with the desired pupil fill at the pupil plane of the projection optics. Present EUV lithography condensers exhibit certain critical drawbacks, including inefficiency due to the inability to use a large proportion of the EUV light, and non-uniformity of the pupil fill. These drawbacks have led to condensers that are unable to provide both the necessary arc illumination and angular distribution, thus leading to changes in imaging quality across the field and for different object orientation.
SUMMARY AND OBJECTS OF THE INVENTION
It is an object of embodiments of the present invention to provide a condenser for a ring-field optical system that exhibits uniform critical dimension distribution and minimal pattern placement error across the projection field.
It is a further object of embodiments of the invention to provide a condenser that enables the projection optics to generate a high contrast image of the mask pattern.
It is yet a further object of embodiments of the invention to provide a condenser that is readily scalable to imaging systems with different numerical apertures.
A condenser for use with a ring-field deep ultraviolet and extreme ultraviolet lithography system is described. In one embodiment of the present invention, the condenser includes a ripple-plate mirror which is illuminated by a collimated beam at grazing incidence. The ripple plate comprises a plate mirror that includes a series of channels along an axis of the mirror to form a series of concave and/or convex surfaces in an undulating pattern. The light is incident along the channels of the mirror and is reflected onto a series of cones. The distribution of slopes on the ripple plate leads to a distribution angles of reflection of the incident beam. This distribution has the form of an arc, with the extremes of the arc given by the greatest slope in the ripple plate. An imaging mirror focuses this distribution to a ring-field arc at the mask plane.
Other objects, features, and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.
REFERENCES:
patent: 4521087 (1985-06-01), Hayes et al.
patent: 5339346 (1994-0
Chapman Henry N.
Nugent Keith A.
Grzybicki Daryl S.
Robinson Mark A.
Spyrou Cassandra
The Regents of the University of California
Thompson Alan H.
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