Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
2000-05-24
2003-09-09
Nguyen, Kiet T. (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S3960ML
Reexamination Certificate
active
06617585
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to charged particle beam projection systems and more particularly to a method of design and manufacture of Charged Particle Beam Projection Systems (CPBPS).
2. Description of Related Art
For the purpose of lithography (among other things) in semiconductor electronics fabrication, a Charged Particle Beam Projection System as described in U.S. Pat. No. 5,466,904 of Pfeiffer et al. for “Electron Beam Lithography System” and U.S. Pat. No. 5,545,902 of Pfeiffer et al. for “Electron Beam Lithography System” employs Large Area Reduction Projection Optics with beam Scanning (LARPOS). LARPOS optical systems are based on providing a doublet of lenses for imaging a large object (integrated circuit pattern in a reticle), in combination with deflectors for positioning the image on the target (wafer) within a given range (scan field). There are numerous conceivable different combinations for providing such imaging/deflection, each characterized by the operating (input) requirements and performance in terms of image fidelity and positioning or exposure speed.
In turn, image fidelity is defined by the edge acuity, with which the pattern features are delineated in the exposure sensitive material (e.g. electron beam sensitive resist) on the wafer, as well as the trueness, with which the feature shape is reproduced. The former is often referred to in terms of the negative aspect of image “blur”. The result of lack of trueness of shape is often referred to as image “distortion”. Both of these performance criteria are determined by the (charged particle) optical aberrations of the system, as well as Coulomb interactions between the charged particles (see below).
Among all CPBPS systems, LARPOS systems are subject to the largest number of aberrations or deviations of the individual particle trajectories from ‘ideal’ or Gaussian optics. A class of aberrations is related to the lens system. Another class of aberrations is related to the deflection system. A third class of aberrations, so-called ‘hybrid’ terms, is related to both.
The main goal for the designer of such a LARPOS system is to minimize the overall impact of the aberrations, under the specific condition of maximizing the beam current, as beam current (among other factors) is the primary factor determining the exposure speed. The consequence is that beam current by affecting the exposure speed ultimately affects both the throughput and the practical viability of a Charged Particle Beam Projection Systems (CPBPS) for industrial applications.
However, a problem with such a LARPOS system is that as the beam current increases the image blur and/or distortion, which become more pronounced. This is due to the effects of forces of electrostatic repulsion between charged beam particles, commonly referred to as Coulomb interactions. These Coulomb interaction effects are strongly dependent on several configuration and operating parameters, in particular, on the path length of the particles, and become worse as the path length increases. Thus, a major incentive for the design of such a system is to minimize the object-to-image distance of the system, i.e to design a tool in which the object-to-image distance is as short as possible.
This predicates overlapping of lens and deflection fields almost completely, giving rise to the need to optimize what is now a Curvi-Linear Variable Axis (CVA). The problem is to determine the optimum CVA in terms of blur and distortion.
SUMMARY OF THE INVENTION
In accordance with this invention, a method is provided for manufacturing a structure with an optimal CVA design by determination of the criteria of and for selecting the optimum CVA in terms of blur and distortion from a virtually infinite number of possible methods.
Further in accordance with this invention, a method is provided for making an optimized charged particle beam projection system by the following steps. Specify lens configuration and first order optics and then calculate lens excitations. Then configure the lens system thereby providing lens field distributions,and the beam landing angle and axis cross-over of the principal off-axis imaging ray. Then provide an input of a deflector configuration including an axial location of the deflectors, then solve linear equation set, and thereby provide a curvilinear variable axis and associated deflection field distributions, then calculate the third order aberration coefficients yielding a list of a plurality of (up to 54) aberration coefficients. Then provide an input of dynamic correctors. Next, calculate excitations to eliminate quadratic aberrations in deflection, then calculate third and fifth order aberrations, providing image blur and distortion vs. deflection, best focal plane, and depth of focus and calculate the total current consumption of all deflectors. Then test to determine whether the current result is better than the previous result, if YES then change the input for the axial location of the deflectors to solve the linear equation set again, if NO, then proceed to test whether the current result is acceptable, if NO, then provide a new deflector configuration input to again solve the linear equation set and continue through the steps thereafter, if YES, check as to whether the current consumption by all of the deflectors is higher than that of the preceding configuration. If YES, then change the input again. If NO then END the process.
Preferably when providing an input of a deflector configuration including the step of providing a beam trajectory, the radial component of which decreases monotonically from a reticle to an aperture placed at the axis cross-over location and increases from the aperture to a target.
Preferably, the lens system comprises an antisymmetric doublet which is preferably telecentric.
Definitions
“Doublet”
The term “doublet” as used herein denotes a pair of lenses operated under a specific symmetry condition, established in the following way:
A source (of particles) illuminates an object in front of a lens pair. The object is located precisely in the back focal plane of the first lens. The first lens generates an image of the source between the pair, and an image of the (closer) object at infinity. This effectively collimates the rays of particles emerging from the object. Accordingly the first lens is labeled “collimator”. The second lens is positioned exactly such that its back focal plane coincides with the front focal plane of the first lens. The second lens focuses the collimated, therefore parallel rays at its front focal plane, which then becomes the image plane for the object. Since the object is now projected into the image plane, the second lens is referred to as “projector”. Under this condition the optical magnification of the lens pair is given by the ratio of the focal lengths of projector to collimator, M=f
2
/f
1
. Simultaneously, the object—image distance, is given by L=2(f
1
+f
2
). If lenses of the same shape are used, then their sizes scale with their respective focal lengths. For example, if f
1
=4f
2
, the collimator must be four times as large as the projector to maintain congruency of the lenses. Consequently, the plane of coincidence of focal planes between the lenses, located at a distance along the system (Z) axis from the object of z
1
=2f
1
, and by z
2
=2f
2
from the image, constitutes a plane of symmetry. The doublet is “symmetric” about the coincidence plane. In the special case of f
1
=f
2
or unity magnification, the doublet is “mirror-symmetric”.
If the source is placed infinitely far upstream of the doublet, its image will appear at the coincidence or symmetry plane. As a consequence, all rays originating from any one point on the source or its intermediate image at the symmetry plane will be parallelized by the projector. In that case, the doublet is characterized as a “telecentric symmetric doublet”. If the lenses are of the magnetic type, their field polarities are generally chosen to be opposite to each oth
Jones II Graham S.
Nguyen Kiet T.
Nikon Corporation
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