X-ray or gamma ray systems or devices – Source
Reexamination Certificate
2001-12-07
2003-05-13
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Source
C250S50400H, C315S111310
Reexamination Certificate
active
06563907
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the production of radiation, particularly extreme ultraviolet and soft x-rays, with a shaped, extended capillary electric discharge source for projection lithography.
BACKGROUND OF THE INVENTION
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. “Long” or “soft” x-rays (a.k.a. Extreme UV) (wavelength range of &lgr;=100 to 200 Å (“Angstrom”)) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. Soft x-ray radiation, however, has its own problems. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that there are no transparent, non-absorbing lens materials for soft x-rays and most x-ray reflectors have efficiencies of only about 70%, which in itself dictates very simple beam guiding optics with very few surfaces.
Projection lithography has natural advantages over proximity printing. One advantage is that the likelihood of mask damage is reduced because the mask does not have to be positioned within microns of the wafer as is the case for proximity printing. The cost of mask fabrication is considerably less because the features are larger. Imaging or camera optics in-between the mask and the wafer compensate for edge scattering and, so, permit use of longer wavelength radiation. Use of EUV radiation in bands at which multilayer coatings have been developed (i.e., &lgr;=13.4 nm, &lgr;=11.4 nm) allows the use of near-normal reflective optics. This in turn has lead to the development of lithography camera designs that are nearly diffraction limited over useable image fields. The resulting system is known as extreme UV (“EUVL”) lithography (a.k.a., soft x-ray projection lithography (“SXPL”)).
A favored form of EUVL projection optics is the ringfield camera. All ringfield optical forms are based on radial dependence of aberration and use the technique of balancing low order aberrations, i.e., third order aberrations, with higher order aberrations to create long, narrow arcuate fields of aberration correction located at a fixed radius as measured from the optical axis of the system (regions of constant radius, rotationally symmetric with respect to the axis). Consequently, the shape of the corrected region is an arcuate or curved strip rather than a straight strip. The arcuate strip is a segment of the circular ring with its center of revolution at the optic axis of the camera. See
FIG. 4
of Jewell et al., U.S. Pat. No. 5,315,629 for an exemplary schematic representation of an arcuate slit defined by width, W, and length, L, and depicted as a portion of a ringfield defined by radial dimension, R, spanning the distance from an optic axis and the center of the arcuate slit. The strip width defines a region in which features to be printed are sharply imaged. Outside this region, increasing residual astigmatism, distortion, and Petzval curvature at radii greater or smaller than the design radius reduce the image quality to an unacceptable level. Use of such an arcuate field allows minimization of radially-dependent image aberrations in the image and use of object:image size reduction of, for example, 4:1 reduction, results in significant cost reduction of the, now, enlarged-feature mask.
Sweatt al. al. U.S. Pat. No. 6,118,577 discloses a condenser system that couples radiation from a small diameter source to a ringfield camera. The condenser system typically includes six substantially equal radial segments of a parent aspheric mirror, each having one focus at the radiation source and line focus filling the object field of the camera at the radius of the ringfield and each producing a beam of radiation. The condenser system also includes a corresponding number of sets of correcting mirror means which are capable of translation or rotation, or both, such that all of the beams of radiation pass through the real entrance pupil of the camera and form a coincident arc image at the ringfield radius.
The overall layout of an EUV lithography system used with the Sweatt condenser is shown in FIG.
4
. The radiation is collected from the source
22
by mirror segments
30
(referred to collectively as the “C
1
” mirrors) which create arc images that are in turn are rotated by roof mirror pairs illustrated collectively as mirrors
40
and
50
(referred to as the “C
2
” and “C
3
” mirrors, respectively). Beams of radiation reflected from mirrors
50
are reflected by a toric mirror
60
(or C
4
mirror) to deliver six overlapped ringfield segments onto reflective mask
70
. Mirror
31
creates an arc image and roof mirror pair
41
and
51
rotates the arc image to fit the slit image and translate it to the proper position. Similar arc images are created and processed by mirror combinations
32
,
42
, and
52
, and so on. Mirrors
41
,
42
, and
43
are parts of different and unique channels; and the group of mirrors
44
,
45
, and
46
is a mirror image of the group of mirrors
41
,
42
, and
43
, respectively. An illustrative arc
71
is shown on mask
70
. The EUV lithography system further includes a ringfield camera
77
having a set of mirrors which images the mask using the radiation onto wafer
78
.
Despite the advantages of the Sweatt condenser system, the art is still searching for improved efficiency. Achieving sufficient EUV flux at the wafer to support a high wafer throughput commercial EUV lithography “step-and-scan” exposure tool is a significant challenge. Of the many elements that impact tool throughput, EUV source power and condenser efficiency both have tremendous leverage. For example, eliminating a single mirror in a condenser can increase flux at the wafer by a factor of (R
mirror
)
−1
or approximately 1.5x.
SUMMARY OF THE INVENTION
The present invention is based in part on the recognition that employing a source of radiation, such as an electric discharge source, which is equipped with a capillary region that is configured into some predetermined shape, such as an arc or slit, can significantly improve EUV flux. One reason is that the condenser which delivers critical illumination to the reticle can be simplified from five or more reflective elements to a total of three or four reflective elements thereby increasing condenser efficiency. In this regard, preferably the dimensions of the non-circular shaped capillary bore correspond to that of the desired image that is focused by the camera. In the case where the inventive capillary discharge source is used in an EUV lithography system where the camera focuses arc or slit shaped images, the capillary discharge source has a bore having a length to width ratio that substantially matches that of the arc or slit shaped image that is focused by the camera. This enables the employment of a simpler condenser with fewer mirrors since the magnification parallel and perpendicular to the arc or slit can be approximately equal.
Accordingly, in one embodiment the invention is directed to a capillary discharge source that includes:
a body constructed from a dielectric material that defines a capillary with a bore having a non-circular shaped cross section; and
a gaseous species inserted into the capillary, wherein the capillary is used to generate radiation discharges.
In a preferred embodiment, the bore has a proximal end and a distal end and the source further includes:
(i) a source of gas that is in communication with the capillary bore;
(ii) a first electrode positioned at the distal end of the bore;
(iii) a second electrode at a reference potential positioned at the proximal end of the bore; and
(iv) a source of electric potential that is selectively connectable to the first electrode.
In another embodiment, the invention is directed to a source of radiation that includes:
means for generating radiation; and
Kubiak Glenn D.
Sweatt William C.
Dunn Drew A.
EUV LLC
Fliesler Dubb Meyer & Lovejoy LLP
Kiknadze Irakli
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