Title: Radiation correction method for electron beam lithography
Abstract: A method for forming a patterned microelectronics layer employing electron beam lithography in a sensitive material upon a substrate with optimal correction for proximity effects resulting from electron back scattering into the resist material. There is provided a substrate having formed thereon a layer of resist material sensitive to electron beam exposure. There is then exposed the sensitive layer to a vector scan shaped electron beam to write a primary pattern with dose correction of the beam dose for proximity effects due to electron scattering at each point in the primary pattern. There is then written a secondary pattern which is a negative reversed image of the primary pattern in a secondary exposure employing a vector scan shaped focused electron beam at an exposure dose substantially below the primary beam dose, there being provided a gap between the primary pattern and the secondary pattern. There is then developed the primary pattern in the sensitive resist layer to form the final corrected pattern on the substrate. The patterned layer of resist material may be employed directly on the substrate on which it is formed, or alternatively the patterned resist layer may be employed formed over an opaque layer upon the transparent substrate and subsequently the pattern etched into the opaque layer to form a photomask.
Patent Number: 6,872,507 Issued on 03/29/2005 to Tzu, et al.
| Inventors:
|
Tzu; San-De (Taipei, TW);
Chiu; Ching Shiun (Hsinchu, TW);
Chou; Wei-Zen (Jungle, TW);
Wu; Chia Fang (Tainan, TW)
|
| Assignee:
|
Taiwan Semiconductor Manufacturing Company (Hsin-Chu, TW)
|
| Appl. No.:
|
286231 |
| Filed:
|
November 1, 2002 |
| Current U.S. Class: |
430/296; 430/394; 430/942 |
| Intern'l Class: |
G03C 005//00 |
| Field of Search: |
430/296,394,942
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Young; Christopher G.
Attorney, Agent or Firm: Thomas, Kayden, Horstemeyer & Risley
Claims
What is claimed is:
1. A method for electron beam lithography comprising: providing a substrate
having formed thereupon a layer of material sensitive to electron beam
exposure;
writing a primary pattern employing an electron beam exposure of the
sensitive layer;
writing a secondary or "ghost" pattern employing electron beam exposure of
the sensitive layer, which "ghost" pattern is a negative of the primary
pattern, while providing a gap at the border of separation between primary
and secondary patterns.
2. The method of claim 1 wherein the primary pattern is written with a
vector scan electron beam where the dose for each pattern element is
corrected for proximity effects.
3. The method of claim 1 wherein the secondary or "ghost" pattern is
written with a vector scan shaped focused electron beam at a dose which is
substantially equivalent to the maximum dose absorbed by the resist in the
primary pattern.
4. The method of claim 1 wherein the gap at the border separating the
periphery of the primary pattern from the secondary or "ghost" pattern is
from about 2 to about 6 microns.
5. The method of claim 1 wherein the substrate is formed from material
selected from the group consisting of: microelectronics conductor
materials;
microelectronics semiconductor materials; and microelectronics dielectric
materials.
6. A method for forming a patterned photomask employing electron beam
lithography of an electron beam sensitive resist layer on a photomask
substrate comprising:
providing a transparent substrate having formed thereupon an opaque layer
over which is formed an electron beam sensitive resist layer;
exposing the substrate to an vector scan electron beam in a primary pattern
where there has been corrected the electron beam dose for mutual proximity
and self proximity effects;
exposing the substrate to an electron beam employing a vector scan shaped
focused beam in a secondary pattern which is the inverted negative of the
primary pattern, where there is provided a gap between the borders of the
primary pattern and the secondary pattern;
developing the exposed resist pattern and etching the opaque layer to form
the photomask with corrected pattern.
7. The method of claim 6 wherein the transparent substrate is a fused
quartz substrate.
8. The method of claim 6 wherein the opaque layer is formed of chromium.
9. The method of claim 6 wherein the gap at the border between the primary
and secondary pattern is from about 2 to about 6 microns.
10. The method of claim 6 wherein the photomask is fabricated so as to
provide a binary photomask.
11. The method of claim 6 wherein the photomask is further fabricated to
provide a phase shift mask (PSM).
12. A method for electron beam lithography comprising:
providing a substrate having formed thereupon a layer of material sensitive
to electron beam exposure;
writing a primary pattern employing an electron beam exposure of the
sensitive layer;
writing a secondary or "ghost" pattern employing electron beam exposure of
the sensitive layer, which "ghost" pattern is a negative of the primary
pattern, while providing a gap at the border of separation between primary
and secondary patterns; and
developing the layer of material sensitive to electron beam exposure,
leaving the primary pattern corresponding to an electron beam written
image pattern corrected for proximity effects.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of manufacture of microelectronics
devices employing patterned etch mask resist layers to form patterns. More
particularly the invention relates to the employment of electron beam
microlithography fabrication methods to form patterned etch resist mask
layers and patterned photomasks.
2. Description of the Related Art
Microelectronics fabrications consist of multiple layers of
microelectronics materials formed on a substrate. Many of the
microelectronics layers are patterned and must be not only accurate and
precise in themselves, but must also be registered with great precision to
other patterned layers. These objectives are met by employing
photolithographic methods in which the desired patterns are first formed
as patterned stencil photomasks with opaque and transparent regions. This
pattern is then transferred to a photosensitive layer or photoresist by
illumination through the patterned photomask, causing a chemical
difference in the illuminated and non-illuminated regions. This difference
may be exploited by subsequent chemical development of the pattern image
in the photoresist layer which is then employed as a patterned photomask
layer for fabrication purposes such as, for example, subtractive etching
of an underlying material layer to transfer the pattern.
In order to fabricate photomasks for pattern formation in microelectronics
fabrication, it is necessary to start with a master image of the desired
pattern. In the early stages of development of microelectronics
fabrication technology, such master image patterns were generally enlarged
versions of the pattern which were then reduced by photographic methods to
the final dimensions on the working photomask This process was tedious and
costly, and has been largely replaced by the use of direct exposure of the
photomask opaque blank substrate, coated with a layer of sensitive resist
material, to the desired pattern. The exposure is normally done with a
directed electron beam to obtain the required precision and fine
dimensions required. The energy absorbed from the directed electron beam
is integrated by the resist material layer into a chemical change which
can be exploited to develop the exposed pattern in the resist layer such
that the pattern can then be transferred into the underlying opaque
substrate layer.
Because the resist layer integrates all energy to which it is exposed, not
only is the directly incident electron beam energy stored in the resist,
but also any stray electrons from scattering processes elsewhere are
capable of registering their effect on the resist. Thus the total energy
absorbed by a given exposed region is not only a function of the electron
energy dose intentionally delivered by design to that region, but also a
function of the electrons absorbed from those delivered nearby and
scattered back into the intended exposed region. This so-called "proximity
effect" on the actual energy dose absorbed by the resist layer in a given
region from the design nominal electron dose delivered to a region and
that absorbed from electrons from nearby regions due to scattering is a
significant effect on the accuracy with which the developed resist image
follows the designed pattern of electron beam energy delivery. The
proximity effect may be divided into "mutual proximity " effects from
nearby electrons scattered sideways from adjacent pattern elements, and
"self-proximity" effects from electrons delivered directly to the desired
region which after passage through the resist layer and into the substrate
are fortuitously scattered backwards at lower energy.
The correction for proximity effects to improve on the accuracy of electron
beam exposed resist patterns is generally accomplished by adjustment of
the actual electron energy dose delivered for exposure after taking into
account the pattern of nearby exposures and estimating the degree of extra
electron energy from scattering, and reducing the delivered dose
accordingly. Although effective for many purposes, this dose correction
method is not without problems, particularly with respect to being costly
and time-consuming.
Another method for improving the accuracy of electron beam exposure of
resist layers is known as the "ghost" correction method. In this method,
the desired pattern of exposed resist is written in two steps: a first
pattern which is the desired pattern written at a fixed dose, and a second
pattern which is a negative reversal of the first pattern and written at a
lower dose, generally with a defocused electron beam. The method relies on
the total dose at the edge of a first pattern feature to have its slightly
lower actual dose increased by the background exposure dose of the second
pattern exposure to provide the desired pattern exposure dose for proper
pattern image development.
Although the method of dose correction of the written pattern or the
"ghost" correction method are in general satisfactory for general use in
electron beam lithography, neither method is entirely without problems.
Densely populated patterns require inordinately long and costly
calculation of incremental dose correction adjustments for each pattern
element in the dose correction method. For small features and/or sparsely
populated designs, the time required for the second exposure of the
"ghost" correction method is time consuming and the defocused beam may
cause resolution problems.
It is thus towards the goal of forming patterned resist mask layers and/or
photomasks by irradiation of sensitive material layers employing electron
beam lithography with correction of proximity effects to improve pattern
accuracy that the present invention is generally directed.
Various methods have been disclosed for the formation of mask layers and
masks by electron beam pattern generation with correction for proximity
effects.
For example, Abe et al., in U.S. Pat. No. 5,451,487, disclose a method for
correction of electron beam exposure of patterns based on the "ghost"
method which greatly decreases the time for correction. The method
calculates a dose required for a representative figure combining a number
of smaller pattern features, and then supplies the required dose employing
a defocused beam to write the inverted pattern.
Further, Pan et al., in U.S. Pat. No. 5,510,214, disclose a method for
forming a double destruction phase shift mask (PSM) which eliminates the
spurious "ghost" line in the mask image which may occur in conventional
phase shift masks. The method combines transparent phase shifting regions
with attenuating phase shifting regions to form interference patterns
which reduce the light intensity transmitted to nearly zero in the pattern
elements of the mask.
Still further, Ham, in U.S. Pat. No. 5,582,938, discloses a method for
forming a phase shift mask which prevents the formation of a "ghost" image
due to interference and diffraction of light with a phase angle of 0 and
180 degrees. The method employs a photoresist layer.
Yet still further, Veneklasen et al., in U.S. Pat. No. 5,847,959, disclose
a method for correcting an electron beam pattern for proximity effects due
to electron scattering, heating and thermal expansion effects. The method
employs a raster scanning electron beam in which calculated corrections
for the various proximity effects are applied to the delivered dose as
correction factors.
Finally, Ohnuma, in U.S. Pat. No. 5,885,748, discloses a method for
correcting photomask patterns for proximity effects due to electron beam
scattering or light exposure employing the photomask. The method utilizes
correction of the pattern by forming a mesh and determining if another
portion of the pattern is close enough to cause a proximity effect. If so,
the dose is corrected to result in a final exposure pattern which is close
to the design pattern.
Desirable in the art of microelectronics fabrication are further methods
for forming patterned resist mask layers, bipolar photomasks and phase
shift photomasks with electron beam exposure of patterns with correction
for proximity effects.
It is towards these goals that the present invention is generally and more
specifically directed.
SUMMARY OF THE INVENTION
It is a first object of the present invention to provide a method for
fabrication of patterned microelectronics layers employing electron beam
exposure of sensitive material layers with optimal correction for
proximity effects due to scattered electrons in said sensitive material
layer.
It is a second objective of the present invention to provide a method in
accord with the first object of the present invention where there is
formed a patterned photomask employing electron beam lithography of
sensitive resist materials with optimal correction of the pattern for
proximity effects due to electron scattering back into the sensitive
resist material.
It is a third object of the present invention to provide a method in accord
with the first object of the present invention and the second object of
the present invention, where an optimal pattern correction for proximity
effects is achieved with reduced computational effort compared to
conventional electron beam lithographic correction methods.
It is a fourth object of the present invention to provide a method in
accord with the first object of the present invention, the second object
of the present invention and the third object of the present invention,
where the method is readily commercially implemented.
In accord with the objects of the present invention, there is provided a
method for forming a patterned microelectronics layer employing electron
beam exposure of a sensitive layer on a substrate with optimal correction
of the pattern for proximity effects in the sensitive material layer due
to scattered electrons. To practice the invention, there is provided a
substrate having formed thereupon a layer of material sensitive to
electron beam exposure. There is then exposed the sensitive layer to a
vector scan shaped electron beam to write a primary pattern with dose
correction of the beam for proximity effects of the primary pattern at
each point in the pattern. There is then written a secondary pattern which
is a negative reverse image of the primary pattern in a second exposure
employing a vector scan shaped focused electron beam at an exposure dose
substantially equivalent to the maximum exposure dose employed in the
primary pattern exposure, there being provided a gap between the
boundaries of the first pattern and the second pattern. There is then
developed the primary pattern written in the sensitive layer to form the
final corrected pattern. The patterned layer of resist material may be
formed directly on a substrate within which the microelectronics device is
fabricated, or alternately the patterned resist layer may be formed over
an opaque layer on a blank photomask substrate for further processing by
subtractive etching into a patterned photomask.
The present invention provides a method for forming a patterned resist
layer employing electron beam writing of a pattern in the resist layer
with correction for proximity effects due to electron scattering employing
dose correction and ghost pattern correction methods. The method is
suitable for formation of a patterned resist layer upon a substrate for
employment in microelectronics fabrication processes such as subtractive
etching, deposition, etc. The substrate may be formed of material chosen
from the group consisting of microelectronics conductor materials,
microelectronics semiconductor materials and microelectronics dielectric
materials. The substrate may be employed within a microelectronics
fabrication chosen from the group including but not limited to
microelectronics integrated circuit fabrications, charge coupled device
microelectronics fabrications, solar cell microelectronics fabrications,
optoelectronics display microelectronics fabrications, radiation emitting
microelectronics fabrications, ceramic substrate microelectronics
fabrications and flat panel display microelectronics fabrications.
Alternatively, the method may be applied equally well to the formation of
a resist layer upon a blank photomask substrate for subsequent etching of
a patterned photomask. Both bipolar photomasks and phase shift photomasks
may be fabricated by the present invention.
The present invention employs methods and materials for pattern generation
and fabrication as are known in the art of microelectronics fabrication,
but in a novel order and arrangement. The method of the invention is
therefore readily commercially implemented.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects, features and advantages of the present invention are
understood within the context of the Description of the Preferred
Embodiments as set forth below. The Description of the Preferred
Embodiments is understood within the context of the accompanying drawings,
which form a material part of this disclosure, wherein:
FIG. 1, FIG. 2, FIG. 3, and FIG. 4 show a series of schematic diagrams
which illustrate the formation of a patterned resist layer upon a
substrate employing electron beam lithography with optimal proximity
effect pattern correction in accord with a general embodiment of the
present invention which is a first preferred embodiment of the present
invention.
FIG. 5, FIG. 6, FIG. 7 and FIG. 8 show a series of schematic
cross-sectional diagrams illustrating the formation of a patterned
photomask employing electron beam lithography of a sensitive resist
material with optimal proximity effect correction for electron back
scattering into the resist material in accord with a more specific
embodiment of the present invention which is a second preferred embodiment
of the present invention.
FIG. 9 and FIG. 10 are schematic illustrations of patterns having sparsely
populated and densely populated features such as contact holes (FIG. 9)
and line-space patterns (FIG. 10). FIG. 9 and FIG. 10 illustrate both
primary first patterns and secondary negative "ghost" patterns for each
type of feature.
FIG. 11 and FIG. 12 are graphs of the linearity of critical dimensions in
patterns formed in electron beam sensitive resist materials.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides a method for forming a patterned resist
layer for microelectronics fabrication wherein the pattern is formed
employing vector scan electron beam exposure with optimal proximity effect
correction by dose correction and "ghost" image inversion correction with
a focused beam.
First Preferred Embodiment
FIG. 1 to FIG. 5 are a series of schematic diagrams illustrating the
results of forming upon a substrate a patterned resist layer by vector
scan electron beam exposure with optimal proximity effect exposure dose
correction and "ghost" pattern correction with a shaped focused beam FIGS.
1-2 and FIGS. 4-5 are schematic cross-sectional diagrams illustrating
progressive stages in the fabrication of the patterned resist layer; FIG.
3 is a schematic plan view of the patterned resist layer corresponding to
FIG. 2. FIG. 1 is a schematic cross-sectional diagram of the substrate at
an early a stage in its fabrication in accord with a first preferred
embodiment of the present invention.
Shown in FIG. 1 is a substrate 10 having formed thereupon a sensitive
resist layer 12.
With respect to the substrate 10 shown in FIG. 1, the substrate 10 is
formed employing material selected from the group consisting of
microelectronics conductor materials, microelectronics semiconductor
materials and microelectronics dielectric materials. The substrate 10 may
be the substrate itself employed in a microelectronics fabrication, or
alternatively the substrate 10 may be any of several microelectronics
material layers formed upon a substrate employed within a microelectronics
fabrication.
With respect to the sensitive layer 12 shown in FIG. 1, the sensitive layer
12 may be chosen from the group of electron beam sensitive materials
including but not limited to polybutene sulfone (PBS), chain scission
positive tone solvent developable resist formulations (e.g., ZEP 7000,
manufactured by Nippon Zeon Co., Ltd.), and medium-resolution positive
resists (e.g., EBR 9HS31. Preferably, the sensitive resist layer is formed
employing PBS resist obtained by Nippon Co., under the trade name ZEON,
2-6-1, Marunouchi, Chiyoda-ku, Tokyo 100-00t, Japan.
Referring now more particularly to FIG. 2, there is shown a schematic
cross-sectional diagram illustrating the results of further processing of
the substrate whose schematic cross-sectional diagram is shown in FIG. 1
in accord with the first preferred embodiment of the present invention.
Shown in FIG. 2 is a substrate otherwise equivalent to the substrate shown
in FIG. 1, but where there has been written in the resist layer 12' a
pattern 14 by electron beam lithography employing focused beam 16a.
Pattern 14 has been written with a beam dose corrected for proximity
effects. A second "ghost" pattern 15 has been written by electron beam
lithography employing a focused beam 16b such that pattern 15 is an
inverted negative of pattern 14. A gap 17 has been left between patterns
14 and 15 in order to minimize sub-field boundary overlap and field
stitching effects between the two patterns. Preferably the gap is about
the width of several beam widths, and is about 2 to about 6 microns.
Referring now more particularly to FIG. 3, there is shown a schematic
diagram showing a plan view of the first pattern 14 and second pattern 15
written in the resist layer 12' employing electron beam lithography
patterning processes 16a and 16b respectively.
Referring now more particularly to FIG. 4, there is shown a schematic
cross-sectional diagram illustrating the results of further processing of
the substrate whose schematic cross-sectional diagram is shown in FIG. 2
in accord with the first preferred embodiment of the present invention.
Shown in FIG. 4 is a substrate otherwise equivalent to the substrate shown
in FIG. 2, but where there has been developed the pattern 14 in the
exposed sensitive resist layer 12' to form the patterned sensitive resist
layer 12", where the pattern 14 corresponds to an electron beam written
image pattern corrected for proximity effects.
With respect to the developed patterned layer 12" shown in FIG. 4, the
developed patterned layer 12" has been formed employing methods and
materials as are known in the art of photolithography employed in
microelectronics fabrication.
The first preferred embodiment of the present invention provides a method
for forming upon a substrate a patterned resist layer employing electron
beam lithography to write a first pattern wherein the electron energy dose
is corrected for proximity effects, followed by writing a second inverted
negative "ghost" pattern of the first pattern to complete the correction
of the first pattern. A gap is left unexposed between the first pattern
and the "ghost" pattern to minimize sub-field boundary and stitching
effects.
Second Preferred Embodiment
Referring now more particularly to FIG. 5 to FIG. 8, FIG. 5 to FIG. 8 show
a series of schematic cross-sectional diagrams illustrating the results of
forming a patterned photomask employing electron beam lithography of an
electron beam resist material with optimal correction for proximity
effects due to electron back scattering into the resist layer. FIG. 5 is a
schematic cross-sectional diagram of a photomask substrate at an early
stage in its fabrication in accord with a more specific embodiment of the
present invention which constituted a second preferred embodiment of the
present invention. Shown in FIG. 5 is a photomask substrate 30 over which
is formed an opaque layer 31 and a sensitive resist layer 32.
With respect to the photomask substrate 30 shown in FIG. 5, the photomask
substrate 30 is analogous to the substrate 10 shown in FIG. 1 of the first
embodiment of the present invention, and is formed of transparent
material. Preferably the substrate 30 is formed of optically perfect fused
quartz (silica)
With respect to the opaque layer 31 shown in FIG. 5, the opaque layer 31 is
formed employing methods and materials as are known in the art of
photomask fabrication. Preferably the opaque layer 31 is formed of
chromium metal employing vacuum evaporation.
With respect to the sensitive layer 32 shown in FIG. 5, the sensitive layer
32 is analogous or equivalent to the sensitive layer 12 shown in FIG. 1 of
the first preferred embodiment of the present invention.
Referring now more particularly to FIG. 6, there is shown a schematic
cross-sectional diagram illustrating the results of further processing of
the photomask substrate whose schematic cross-sectional diagram is shown
in FIG. 5 in accord with the second preferred embodiment of the present
invention. Shown in FIG. 6 is a photomask substrate otherwise equivalent
to the photomask substrate shown in FIG. 5, but where there has been
written a first pattern 34 in the sensitive layer 32' employing beam
lithography with an electron energy dose corrected for proximity effects
36a, followed by a second pattern 35 written in the sensitive layer 32' by
electron beam lithography which is a "ghost" negative of the first pattern
34 and is written with a uniform electron dose equivalent to the maximum
dose correction determined for first pattern 34.
With respect to the first pattern 34 and the second "ghost" pattern 35
written by electron beam lithography processes 36a and 36b respectively
shown in FIG. 6, the patterns 34 and 35 and the electron beam processes
36a and 36b are analogous or equivalent to the patterns 14 and 15 and the
electron beam lithography processes 16a and 16b shown in FIG. 1 of the
first preferred embodiment of the present invention.
Referring now more particularly to FIG. 7, there is shown a schematic
cross-sectional diagram illustrating the results of further processing of
the photomask substrate whose schematic cross-sectional diagram shown in
FIG. 6 in accord with the second preferred embodiment of the present
invention. Shown in FIG. 7 is a photomask substrate otherwise equivalent
to the photomask substrate shown in FIG. 6, but where there has been
developed the first pattern 34 in the sensitive resist layer.
With respect to the development of the patterned image 32" of the corrected
first pattern in the sensitive layer shown in FIG. 7, the patterned
corrected image 32" is analogous to the patterned image 12" shown in FIG.
4 of the first preferred embodiment of the present invention.
Referring now more particularly to FIG. 8, there is shown a schematic
cross-sectional diagram illustrating the final results of the processing
of the photomask substrate whose schematic cross-sectional diagram is
shown in FIG. 7 in accord with the second preferred embodiment of the
present invention. Shown in FIG. 8 is a photomask substrate otherwise
equivalent to the photomask substrate shown in FIG. 7, but where there has
been subtractively etched the pattern of the patterned resist layer 35'
into the opaque layer 31 to form the patterned opaque layer 31' on the
transparent substrate 30, followed by stripping of the resist layer 32" to
complete the photomask.
With respect to the etching of the opaque layer 31 and stripping of the
patterned resist layer 35 to form the completed opaque layer pattern 31'
on the transparent substrate shown in FIG. 8, the etching and stripping
are performed employing methods and materials as are known in the art of
photolithography as employed in microelectronics fabrication.
The preferred second embodiment of the present invention provides a method
for forming a patterned opaque layer on a transparent photomask substrate,
employing electron beam lithography to write a pattern in a sensitive
resist layer on the photomask substrate corrected for proximity effects.
Subsequent development and etching of the corrected pattern is employed to
produce a photomask for photolithography.
The photomask produced employing the second preferred embodiment of the
present invention may be a binary photomask, or alternately there may be
employed other fabrication methods and materials in conjunction with those
of the present invention to produce more complex photomasks such as, for
example, a phase shift mask (PSM).
Experimental
The benefits of the present invention are exemplified by the experimental
results obtained employing the methods of the present invention. A series
of patterns were written and developed on three different sensitive resist
layer materials employing MEBES 4500 electron beam lithography system
obtained from ETEC System, Inc. 26460 Corporate Avenue, Mail Drop B/44F2,
Hayward, Calif., USA, with both dose adjustment and "ghost" pattern
methods for proximity effect correction, and subsequent measurements of
pattern dimensions were performed employing scanning electron microscopy
(SEM). The patterns employed were a series of lines and spaces (FIG. 9)
and contact holes (FIG. 10) as well as isolated line and contact features,
and the conditions for the three resist materials examined are given in
Table I:
TABLE I
Comparison of "Ghost" Correction Effects for Three
Resist Materials MEBES 4500 System (10 Kev);
Cr on quartz
ZEP 7000 PBS EBR9HS31
Thickness, A 3000 2500 4000
Dose uC/cm.sup.2 8 2 4
Address size, nm 50 50 50
Beam size, nm 120 80 80
Defocused beam 700 700 700
size, Ghost, nm
Correction/primary 0.30 0.40 0.37
dose ratio, Qc/Qp
Etching condition dry wet wet
The results for patterns formed employing the conditions in Table I for
"ghost" correction alone are summarized in FIG. 11 and FIG. 12. FIG. 11
shows the results obtained for uniformity in terms of three standard
deviations for line widths in patterns formed in PBS resist which are
typical of all three resist materials. It is seen that "ghost" correction
alone is not as accurate as writing the pattern with a primary dose for
this experiment. In FIG. 12, the results for uniformity of line widths in
terms of three standard variations is shown for ZEP 7000 resist as a
function of the ratio Qc/Qp of the correction dose Qc employed for the
"ghost" pattern to the primary dose Qp. As this ratio increases, the
deviation or non-uniformity becomes larger.
When the primary pattern is written with a system which allows dose
correction such as the Hitachi system (obtained from Hitachi Co., 1-24-14
Nishi-Shimbashi Minato-ku, Tokyo 1058717, Japan), combining a "ghost"
pattern with dose correction provides an improvement as shown in Table II:
TABLE II
Comparison of "Ghost" Correction Alone, Dose
Correction Alone and Combined Dose
plus "Ghost" Correction for Pattern
Formation by Electron Beam Lithography
Dose + ghost
Ghost correction Dose correction correction
Proximity correction "ghost" dose correction dose + "ghost"
Electron Beam MEBES raster Hitachi vector Hitachi vector
Beam Shape Gaussian shaped beam shaped beam
Beam voltage, KeV 10 50 50
Ghost beam focus defocused focused focused
Reverse (negative) yes no yes (gap)
Uniformity poor good good
Isolated/dense poor for fair good
proximity effect 1/s < 0.5 u
Dimension linearity fair (>0.5 u) poor < 0.72 u good
Throughput poor good poor
It is readily seen that the third column, which represents the present
invention, provides improved results in terms of improved pattern
formation and accuracy.
As is understood by a person skilled in the art, the preferred embodiment
of the present invention is illustrative of rather than limiting of the
present invention. Revisions and modifications may be made to materials,
structures and dimensions through which is provided the preferred
embodiment of the present invention while still providing embodiments
which are within the spirit and scope of the present invention, as defined
by the appended claims.
*
