Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
1998-08-28
2001-03-13
Berman, Jack (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
Reexamination Certificate
active
06201251
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to charged particle beam projection systems and more particularly to the imaging sections of an electron or ion beam column.
2. Description of Related Art
In a particle beam projection lithography system with sufficient beam current to provide useful tool throughput there is a problem that the Coulomb interaction causes a shift in the position of the image plane and a change in nominal magnification which vary depending on the transmitted beam current, i.e., exposure pattern density.
For a hypothetical electron beam column under consideration, Monte Carlo calculations of the effect suggest image shifts of approximately 50 microns and magnification changes of approximately 0.1%, for a 20 microAmpere change in transmitted beam current. Tolerable image shifts and magnification changes are about 10-100 times smaller than these values.
Dynamic refocusing to correct shifts of the image plane of an electron beam system in response to beam current changes is known from shaped beam lithography systems, and character projection systems, but to our knowledge, compensation of magnification effects has never before been addressed.
SUMMARY OF THE INVENTION
Definitions
Imaging section
-
Those lenses located between the reticle
plane and the target (work piece) plane
of a particle beam projection system.
Doublet
-
A pair of lenses in an optical system,
e.g. a collimating lens and a final lens.
Lens position
-
the axial position of the image side
principal plane of a lens.
While refocussing of the image shift has been suggested in the past, the present invention is new and has the advantage that it provides a correction for the change in magnification as well as compensation of the image shift.
In accordance with this invention, the optical effects of variations in beam current transmitted through an imaging system are analyzed and compensated as follows:
Space charge effects are represented by a thin diverging lens (space charge lens) at a fixed position in the optical system, the thin lens having a focal length which varies inversely with the beam current.
The imaging system is analyzed to obtain expressions for the image shift and magnification in terms of the positions and focal length of the lenses comprising the imaging system including the space charge lens.
Numerical data on the image shift and magnification as a function of beam current are used in equations to solve for the position and focal length of the space charge lens.
A compensating lens is inserted into the optical system at the position of the space charge lens, the compensating lens having a focal length which varies inversely with the beam current, the focal length being equal in magnitude and opposite in sign to the focal length of the effective space charge lens for all beam currents.
To provide a solution to the problem caused by the effective space charge lens a compensating lens is aligned with the location of the effective space charge lens adjusted to reverse the image shift and magnification change created by the effective space charge lens.
As an example of the analysis and space charge compensation of the imaging section of a particle beam column, we have derived the formulae for the lens configuration known as a doublet, which is typically preferred for the imaging section of a particle beam projection system, either in the form of a magnetic doublet for an electron beam system, or for an electrostatic doublet for an ion beam system. The doublet is comprised of a collimating lens with focal length f
1
positioned at a distance f
1
from the object plane (reticle plane), and a final lens of focal length f
2
positioned at a distance f
2
from the image plane (work piece plane), the two lenses being separated by a distance f
1
+f
2
.
For one configuration of an imaging section, with the compensating lens located between the reticle plane and the collimating lens of the imaging system, the image shift, and magnification in the presence of space charge effects is given by the following equations:
d
=
f
2
2
(
L
-
f
1
)
2
f
1
2
(
f
1
-
L
-
f
s
)
m
=
-
f
2
*
f
s
f
1
(
L
+
f
s
-
f
1
)
The equations can be solved for f
s
and L, as follows:
f
s
=
d
*
f
1
3
*
m
f
2
(
f
2
+
f
1
*
m
)
2
L
=
f
1
(
f
2
2
+
(
f
2
*
f
1
*
m
)
-
(
d
*
f
1
)
)
f
2
(
f
2
+
f
1
*
m
)
where:
d is the distance by which the image is shifted from the nominal image plane
m is the magnification of the imaging doublet
L is the distance from the final input lens to the effective space charge lens.
f
1
is the focal length of the collimating lens IL
f
2
is the focal length of the final lens FL
f
s
is the focal length of the effective space charge lens SCL, SCL′ (a negative quantity)
Preferably, the compensating lens is a rotation-free dynamic lens at the position of the effective space charge lens, the compensating lens having a focal length equal and opposite to that of effective space charge lens thereby providing approximate compensation for the defocussing and magnification effects caused by the effective space charge lens.
Preferably, in accordance with another aspect of this invention, the imaging section comprises a reticle, a collimating lens, a final lens and a work piece plane, and the compensating lens is located between the collimating lens position and the final lens position. Preferably, the imaging section comprises a reticle; a magnetic doublet comprising a collimating lens and a final lens; and a work piece plane, and the compensating lens is located between the collimating lens position and the final lens position.
In another aspect of this invention the compensating lens is located between the final lens position and the work piece plane, with the space charge compensating lens placed within the imaging system of the particle beam column, and adjusting the focal length of the compensating lens to eliminate image shifts and/or magnification changes due to variations in transmitted beam current.
Preferably the compensating lens is a rotation-free dynamic focus lens at the position of an effective space charge lens, the compensating lens having a focal length equal and opposite to that of effective space charge lens thereby providing approximate compensation for the defocussing caused by the effective space charge lens. Preferably the imaging system comprises a doublet.
In accordance with another aspect of this invention a method is provided for compensating the imaging section of a particle beam column for space charge effects as described by the following steps. Determine the image position and magnification of the imaging section in the absence of space charge effects. Determine the image position and magnification of the imaging section for a known high beam current sufficient to produce space charge effects. Use the image shift and magnification data for a transmitted beam current to determine the position of the effective space charge lens, and the relationship between the beam current and the focal length of the effective space charge lens. Incorporate into the imaging system a compensating lens aligned with the location of the effective space charge lens, and adjust the focal length of the space charge compensating lens to be equal in magnitude and sign to the focal length of the space charge lens for all beam currents.
Provide an imaging system which is a doublet and position and energize the compensating lens accord to formulae and measurements of calculated image shift and magnification data.
Position the compensating lens between a reticle plane and a collimating lens and energize the compensating lens accord to formulae and measurements of calculated image shift and magnification data.
Alternatively, position the compensating lens between a collimating lens and a final lens, and energize the compensating lens accord to formulae and measurements of calculated image shift and magnification data.
In still another aspect of the invention, position the compensating lens between a final lens and a work piece plane
Enichen William Albert
Golladay Steven Douglas
Berman Jack
Jones II Graham S.
Nikon Corporation
Smith Johnnie
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