Radiant energy – With charged particle beam deflection or focussing – With target means
Reexamination Certificate
1999-03-16
2002-04-23
Berman, Jack (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
With target means
C250S492210, C250S3960ML, C250S492200
Reexamination Certificate
active
06376842
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to microlithography systems that employ a charged particle beam to transfer a pattern (such as a circuit pattern for an integrated circuit) onto a sensitive substrate (such as a semiconductor wafer). More specifically, the invention pertains to methods and apparatus for reducing aberrations in such systems.
BACKGROUND OF THE INVENTION
There has been much recent progress in semiconductor processing technology, including so-called “microlithography” technology. Current optical microlithography apparatus utilize light (typically ultraviolet light) for exposure. To further improve the resolution of microlithography apparatus, light having increasingly shorter wavelengths is preferably used. However, there are practical limits to the extent to which the wavelength of light can be reduced for microlithography.
To solve such problems, X-ray microlithography has been under development. However, X-ray technology for microlithography is plagued with many serious problems, including a current inability to fabricate an acceptable reticle for X-ray lithography. Consequently, practical X-ray microlithography has not yet been realized.
Another approach to improving the resolution of microlithography has been the use of charged-particle-beam (CPB) apparatus, such as electron-beam apparatus. Fortunately, many aspects of CPB optics are well understood in view of many years' experience in using CPB optics in instruments such as electron microscopes and the like. With respect to CPB microlithography, various optical configurations have been proposed, such as MOL (Moving Objective Lens; Goto et al.,
Optik
48:255, 1977), VAL (Variable Axis Lens; Pfeiffer and Langner,
J. Vac. Sci. Technol
. 19:1058, 1981), and VAIL (Variable Axis Immersion Lens; Sturans et al.,
J. Vac. Sci. Technol
. B
8:1682, 1982).
However, whenever a circuit pattern is exposed using a CPB optical system such as any of the listed systems, image blur generated by geometric aberrations of the CPB optical system is very high, which is problematic for practical applications. In addition, in CPB exposure apparatus based on a multiple-deflection theory (directed to reducing third-order aberrations, Hosokawa,
Optik
56:21, 1980), a number of deflectors equal to the number of aberrations to be eliminated is used. The principal aberrations discussed in the Hosokawa paper are longitudinal coma aberration, radial coma aberration, astigmatism, and chromatic aberrations; hence, at least four deflectors are required to reduce these aberrations. An additional deflector is normally used for controlling the angle of incidence of the beam onto the reticle and substrate. Hence, five deflectors are typically required. In the Hosokawa paper, a respective linear equation is used to define the current applied to each deflector, and the linear equations are combined to form a set of simultaneous linear equations. The set of simultaneous linear equations is then solved to provide a respective current for each deflector. Unfortunately, however, in such a scheme the trajectory of the beam is unnatural. (“Unnatural” in this context means that the trajectory W
m
of the beam changes greatly. The CPB optical system has three principal rays: W
a
, W
b
, and W
m
. Chu et al.,
Optik
61:121, 1982. W
a
and W
b
are determined by lens conditions, and W
m
is determined by both the lenses and the deflectors.) As a result, higher-order aberrations (i.e., higher order than third order) increase sharply even when third-order aberrations are largely eliminated. Moreover, the solutions to the set of simultaneous linear equations change continuously and thus require constant recalculation.
Furthermore, in conventional systems as summarized above, aberrations are calculated only up to the third-order aberrations; no calculations are performed of fifth-order or higher-order aberrations. However, in order to achieve the requisite imaging accuracy using a CPB microlithography apparatus to expose a pattern having a critical feature dimension of less than 0.15 &mgr;m, fifth-order and higher-order aberrations must be reduced as much as possible.
By way of example, in conventional SMD optical systems, whenever the magnification (M) exhibited by such a system is equal to 4 (i.e., whenever M=4) and the inside radius of a deflector on the image side is 25 mm, the inside radius of a deflector on the object side is 100 mm. In conventional SMD systems, deflectors are usually situated along the optical axis inside one or both lenses. The example situation just mentioned would require that the inside radius of the lens on the object side be very large. As a result, not only must the size and the weight of the lens itself be very large, but also the radius of a “lens column” (i.e., lens housing) containing the lens must be even larger. This results in a corresponding increase in the size and complexity of the microlithography apparatus utilizing such a lens. Other detriments with such size increases are greater deviations from design specification due to assembly errors and the like (e.g., positional errors of the optical axis and displacements of the respective rotation angle and the like of the deflectors).
Also, with conventional SMD optical systems, the number of deflectors for correcting aberrations is limited, which correspondingly limits the degrees of freedom for aberration-reducing adjustments. Consequently, blur due to aberrations is very sensitive to any displacement of the axial position of the deflectors and any displacement of the angular orientation of each deflector radially about the optical axis. This situation greatly increases the difficulty of performing aberration-reducing micro-adjustments of the SMD optical system. The end result is that achieving a level of performance from a conventional CPB microlithography apparatus has tended to fall substantially below design specifications.
SUMMARY OF THE INVENTION
In view of the foregoing shortcomings of the prior art, an object of the invention is to provide projection-optical systems for CPB microlithography apparatus that exhibit satisfactorily reduced aberrations and blur, even with a large main field.
According to one aspect of the invention, apparatus are provided for projecting an image of a pattern, defined by a reticle, onto a substrate using a charged particle beam. An embodiment of such an apparatus comprises a projection-optical system situated and configured to receive a charged particle beam passing through an illuminated region (i.e., a region irradiated by the charged particle beam) of the reticle and form an image of the illuminated region on a corresponding region of the substrate. The apparatus also comprises six deflectors associated with the projection-optical system. The deflectors are configured and situated to correct an on-axis aberration of the beam and a corresponding off-axis aberration of the beam, wherein the off-axis aberration is corrected substantially equally with the correction of the on-axis aberration. The projection-optical system in such an embodiment preferably satisfies a Symmetric Magnetic Doublet (SMD) condition. Also, the deflectors preferably have axial positions, relative to an axial position of the substrate, of Z1-Z6, respectively, that satisfy the expressions:
Z1=(−L)−M(Z6)
Z2=(−L)−M(Z5)
Z3=(−L)−M(Z4)
wherein an axial direction leading from the substrate to the reticle is regarded as a negative axial direction, “L” is the “column length” of the projection-optical system, and 1/M is the demagnification ratio of the projection-optical system. (An axial direction leading from the substrate to the reticle is regarded as a negative axial direction.)
According to another embodiment, an apparatus is provided that comprises a projection-lens system configured and situated so as to perform the following: (1) receive a charged particle beam passing through an illuminated region of the reticle and form an image of the illuminated region on a corresponding r
Berman Jack
Fernandez Kalimah
Klarquist & Sparkman, LLP
Nikon Corporation
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