Fences – Flood
Reexamination Certificate
1998-11-13
2003-01-28
Lee, John R. (Department: 2881)
Fences
Flood
C250S492230
Reexamination Certificate
active
06511048
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an electron beam lithography apparatus and a semiconductor device pattern forming method for use therewith, the apparatus and method being arranged to write precisely patterns near the periphery of a cell mask so that large scale integrated circuits and fine structure devices may be fabricated at high yield rates.
Patterns of a semiconductor device are typically formed by an electron beam lithography apparatus as follows: digital data are first converted to a voltage or current signal by a DA converter. The converted voltage or current signal is amplified and fed as a deflection signal to an electrostatic deflector or a magnetic deflector whereby an electron beam is deflected. The deflected electron beam is controlled for exposure position on a target such as a semiconductor device. The target is then exposed to the beam.
For writing on the target with an electron beam, the so-called cell projection method has been used extensively to boost the throughput of fine electron beam lithography. The method has some disadvantages as discussed in “Journal of Vacuum Science and Technology; Vol. B11, No. 6, 1993” (pp. 2357-2361). That is, a significantly high degree of deflection on the mask causes substantial aberration in a crossover image. The aberration can provoke problems such as current density fluctuations in the electron beam on the target. The publication cited above describes ways to bypass the bottleneck through correction of the aberration.
FIG. 3
is a schematic view of electron trajectories in a projection lens unit of a conventional electron beam lithography apparatus, illustrating how the aberration of a crossover image occurs.
FIGS. 4A and 4B
are schematic views of electron beams entering targets, showing effects of the aberration of a crossover image.
FIG. 5
is a schematic view depicting a typical constitution of a conventional electron beam lithography apparatus operating on the cell projection method.
In
FIG. 5
, an electron beam from an electron gun
1
is projected directly onto a first mask
2
. An image of the first mask
2
is formed on a second mask
5
by two-stage projection lenses
3
-
1
and
3
-
2
. Located between the two masks, a cell selection deflector
4
selects a desired aperture (i.e., pattern) from among a plurality of apertures on the second mask
5
.
Referring to
FIG. 3
, trajectories of the electron beam EB inside the projection lens unit will now be described in detail. The electron beam EB from the electron gun
1
first passes through a rectangular aperture of the first mask
2
before being projected onto the second mask
5
by the first and second projection lenses
3
-
1
and
3
-
2
. This forms an image of the rectangular aperture of the first mask
2
on the second mask
5
. At this point, a first crossover image (i.e., rectangular aperture image of the first mask) CR
1
formed by the first projection lens
3
-
1
is moved in deflective fashion by the cell selection deflector
4
onto the second mask
5
. The image thus moved is arranged to coincide with an appropriate cell aperture for cell projection, whereby the pattern (aperture) to be written is selected. The electron beam EB thus passes off the axis of the second projection lens
3
-
2
. This gives rise to a significant degree of aberration in a second crossover image CR
2
.
As shown in
FIG. 5
, the electron beam past the second mask
5
is contracted by a two-stage demagnification lens arrangement
8
. The contracted electron beam passes through an objective lens
14
and is focused eventually on a sample
17
. The aberration developed in the first and the second crossover image CR
1
and CR
2
causes the electron beam passing through both extremes of the pattern (aperture) to vary its transiting position within the lens. Because the electron beam position differs at a halfway objective aperture
9
within the pattern, irregularities in current density take place inside the pattern. In addition, not all electrons within the pattern can pass through the lens center, resulting in resolution non-conformity.
Other effects of the aberration will now be described with reference to
FIGS. 4A and 4B
.
FIG. 4A
shows beam paths in effect when there is no aberration in the crossover image, while
FIG. 4B
depicts beam paths in effect when the crossover image involves aberration. In the case of
FIG. 4A
, the electron beams EB-
1
and EB-
2
passing at both ends of the pattern enter the sample
17
at about the same angle. Because the electron beams EB-
1
and EB-
2
each enter the sample
17
at a beam center BC, there is no change in the size of a projected pattern P
1
. In the case of
FIG. 4B
, on the other hand, electron beams EB-
1
′ and EB-
2
′ passing at both extremes of the pattern enter the sample
17
at different incidence angles. (In
FIG. 4B
, the electron beam EB-
2
′ enters the sample at a tilted incidence angle.) As a result, attempts at focus correction to reduce the Coulomb effect vary the size of a pattern P
2
on a focus correction plane FC. (In
FIG. 4B
, the pattern P
2
is seen enlarged.) These adverse effects can be reduced by correcting the aberration of the crossover image, but coma aberration and chromatic aberration are difficult to correct. Where focus correction is not carried out, focusing errors still occur in practice and can degrade pattern size accuracy.
Since aberration increases in proportion to the distance of the aperture (pattern) from the lens center (i.e. from the optical axis), the above-described effects become more pronounced the closer the aperture (pattern) in question is to the periphery of a group of apertures.
Field curvature and astigmatism may be corrected but not to a satisfactory degree. Coma aberration and chromatic aberration are difficult to correct, as described above. In sum, thorough correction cannot be expected from the conventional setup.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to overcome the above and other deficiencies and disadvantages of the prior art and to provide an electron beam lithography apparatus arranged to write precisely patterns close to the periphery of a cell mask through reduction of aberration-induced adverse effects.
It is another object of the present invention to provide a pattern forming method for fabricating large scale integrated circuits and fine structure devices at high yield rates.
In carrying out the invention and according to one aspect thereof, there is provided a pattern forming method whereby apertures within a single aperture group on a cell mask are located closer to the periphery of the group the lower the aperture rate is for each aperture (pattern). Where patterns are to be written for the fabrication of large scale integrated circuits, the same aperture group may include both line pattern apertures and hole pattern apertures. In such a case, the hole pattern apertures should be placed outside the line pattern apertures for effective fabrication.
The same improvement is expected where, within the same aperture group on the cell mask, apertures involving shorter pattern spacing (aperture spacing) are located closer to the periphery of the group than apertures having longer pattern spacing.
Like benefits are expected when, within the same aperture group on the cell mask, apertures involving shorter pattern lengths (aperture lengths) are arranged to be located closer to the periphery of the group than apertures having longer pattern lengths.
It is also effective, within the same aperture group on the cell mask, to establish outside a cell figure a second cell figure comprising part or all of the patterns constituting the cell figure inside.
It is preferred that apertures each having a single pattern (i.e., apertures having no periodicity) for use in writing peripheral circuits be arranged to be located closer to the periphery of the aperture group.
Further benefits are gained when the peripheral regions of an aperture group having large aberration are arranged to com
Matsuzaka Takashi
Nakayama Yoshinori
Ohta Hiroya
Saitou Norio
Sohda Yasunari
Lee John R.
Mattingly Stanger & Malur, P.C.
Quash Anthony
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