U.S. patent application number 12/607612 was filed with the patent office on 2010-05-06 for optical arrangement for three-dimensionally patterning a material layer.
This patent application is currently assigned to Carl Zeiss SMT AG. Invention is credited to Heiko Feldmann.
Application Number | 20100112465 12/607612 |
Document ID | / |
Family ID | 42062810 |
Filed Date | 2010-05-06 |
United States Patent
Application |
20100112465 |
Kind Code |
A1 |
Feldmann; Heiko |
May 6, 2010 |
OPTICAL ARRANGEMENT FOR THREE-DIMENSIONALLY PATTERNING A MATERIAL
LAYER
Abstract
The disclosure relates to an optical arrangement for
three-dimensionally patterning a radiation-sensitive material
layer, such as a projection exposure apparatus for
microlithography. The optical arrangement includes a mask for
forming a three-dimensional radiation pattern, a substrate with the
radiation-sensitive material layer, and a projection optical unit
for imaging the three-dimensional radiation pattern from the mask
into the radiation-sensitive material layer. The optical
arrangement is designed to compensate for spherical aberrations
along the thickness direction of the radiation-sensitive material
layer in order to generate a stigmatic image of the
three-dimensional radiation pattern.
Inventors: |
Feldmann; Heiko; (Aalen,
DE) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
Carl Zeiss SMT AG
Oberkochen
DE
|
Family ID: |
42062810 |
Appl. No.: |
12/607612 |
Filed: |
October 28, 2009 |
Current U.S.
Class: |
430/5 ; 355/53;
430/322 |
Current CPC
Class: |
G02B 5/32 20130101; G03F
7/70258 20130101; G02B 5/0236 20130101; G03F 7/201 20130101; G02B
5/0284 20130101; G03F 7/0037 20130101; G03F 7/70341 20130101; G03F
7/70416 20130101 |
Class at
Publication: |
430/5 ; 355/53;
430/322 |
International
Class: |
G03F 7/20 20060101
G03F007/20; G03B 27/42 20060101 G03B027/42; G03F 1/00 20060101
G03F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 30, 2008 |
DE |
102008043324.1 |
Claims
1. An optical arrangement, comprising: a mask configured to form a
three-dimensional radiation pattern; a substrate; a
radiation-sensitive material layer supported by the substrate, the
radiation-sensitive material layer having first and second planes
in a thickness direction of the radiation sensitive material layer,
the first plane being different from the second plane; and a
projection optical unit configured to image the three-dimensional
radiation pattern from the mask into the radiation-sensitive
material layer, wherein the optical arrangement is configured to
compensate for spherical aberrations at the same time at least
within the first and second planes to generate a stigmatic image of
the three-dimensional radiation pattern in the radiation-sensitive
material layer, and wherein the optical arrangement is configured
to be used in a projection exposure apparatus for
microlithography.
2. The optical arrangement according to claim 1, wherein the first
and second planes are oriented perpendicular to the thickness
direction of the material layer.
3. The optical arrangement according to claim 1, wherein the first
and second planes are separated from each other by a distance that
corresponds to more than twice a depth of focus of the projection
optical unit.
4. The optical arrangement according to claim 1, wherein the mask
has a three-dimensionally patterned material layer.
5. The optical arrangement according to claim 4, wherein, for an
imaging scale .beta. of the projection optical unit, a refractive
index n.sub.r of the radiation-sensitive material layer and a
refractive index n.sub.m of the material layer of the mask, the
following holds true: .beta.=n.sub.m/n.sub.r.
6. The optical arrangement according to claim 4, wherein an imaging
scale .beta. of the projection optical unit can be set in a manner
dependent on a refractive index of the radiation-sensitive material
layer and a refractive index of the material layer of the mask.
7. The optical arrangement according to claim 4, wherein an imaging
scale .beta. of the projection optical unit can be set in a manner
dependent on a refractive index of the radiation-sensitive material
layer and a refractive index of the material layer of the mask
where 0.8<.beta.<1.2
8. The optical arrangement according to claim 4, further comprising
an illumination system configured to introduce radiation into the
material layer of the mask, wherein the optical arrangement is
configured to allow only a portion of the radiation that is
scattered at structures in the material layer of the mask to enter
into the projection optical unit.
9. The optical arrangement according to claim 8, wherein the
structures in the material layer of the mask are reflective.
10. The optical arrangement according to claim 8, wherein the
illumination system is configured to generate dark field
illumination.
11. The optical arrangement according to claim 1, wherein the mask
has a holographic structure configured to at least partly
compensate for spherical aberrations along the thickness direction
of the radiation-sensitive material layer.
12. The optical arrangement according to claim 11, wherein the
holographic mask has a diffraction grating configured to diffract
radiation from an illumination system into the projection optical
unit.
13. The optical arrangement according to claim 1, wherein the mask
is displaceable in a direction perpendicular to an object plane of
the projection optical unit by a magnitude of more than 3
.lamda./NA.sub.O.sup.2, and/or the light-sensitive material layer
is displaceable perpendicular to an image plane of the projection
optical unit by a magnitude of more than 3 .lamda./NA.sub.B.sup.2,
where .lamda. is a wavelength of the radiation used during
operation of the optical arrangement, NA.sub.B is an image-side
numerical aperture of the projection optical unit, and NA.sub.0
denotes an object-side numerical aperture of the projection optical
unit.
14. The optical arrangement according to claim 1, further
comprising an illumination system configured to produce a relative
numerical aperture .sigma. of less than 0.1.
15. The optical arrangement according to claim 1, wherein the
optical arrangement is configured to be used with radiation having
a wavelength of 400 nm or less.
16. The optical arrangement according to claim 1, wherein the
projection optical unit has an image-side aperture of 1.2 or
more.
17. The optical arrangement according claim 1, further comprising
an immersion liquid is between a last optical element of the
projection optical unit and the radiation-sensitive material
layer.
18. A mask configured to be used in the optical arrangement of
claim 1, the mask having a holographic structure configured to form
a three-dimensional radiation pattern, the holographic structure at
least partly compensating for spherical aberrations along the
thickness direction of the radiation-sensitive material layer.
19. A method, comprising: operating a projection exposure apparatus
for microlithography by: forming a three-dimensional radiation
pattern; and imaging the three-dimensional radiation pattern into a
radiation-sensitive material layer, wherein, while forming the
three-dimensional radiation pattern and/or imaging the
three-dimensional radiation pattern, spherical aberrations at least
within a first plane and a second plane, which are located at
different positions along a thickness direction of the
radiation-sensitive layer, are compensated for to generate a
stigmatic image of the three-dimensional radiation pattern in the
radiation-sensitive material layer.
20. The method according to claim 19, further comprising, prior to
imaging, displacing the mask perpendicular to an object plane
and/or displacing the radiation-sensitive material layer
perpendicular to an image plane of a projection optical unit used
during imaging, by an amount defined in a manner dependent on the
mask respectively chosen.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C. .sctn.119
to German Patent Application 10 2008 043 324.1, filed Oct. 30,
2008, the contents of which are hereby incorporated by reference in
their entirety.
FIELD
[0002] The disclosure relates to an optical arrangement, such as a
projection exposure apparatus for microlithography, for
three-dimensionally patterning a radiation-sensitive material
layer, to a mask for use in such an optical arrangement, and to a
method for three-dimensionally patterning a radiation-sensitive
material layer.
BACKGROUND
[0003] Photonic crystals, interconnect layers of semiconductor
components and micromechanical elements drive the demand for
three-dimensional patterning of elements in the field of
microelectronics. Conventionally lithographic methods are used for
producing three-dimensional structures, which methods involve
producing the semiconductor elements layer by layer using different
masks. In this case, firstly a photoresist (resist) as
radiation-sensitive material layer is applied to a carrier
(substrate) and exposed by a first lithographic mask. This is
followed by a chemical development step, in which the illumination
pattern produced in the course of the preceding exposure in the
photoresist is transferred into a physical structure in the
photoresist. In order to achieve a three-dimensional patterning,
the abovementioned steps, namely the application of a photoresist
layer, the exposure of the layer and the subsequent chemical
development, are typically to be repeated a number of times using
different lithography masks. This method can be very complicated
and hence time- and cost-intensive.
SUMMARY
[0004] In some embodiments, the disclosure allows for
three-dimensional exposure of a comparatively thick
radiation-sensitive material layer with a high resolution in three
dimensions.
[0005] In certain embodiments, the disclosure provides an optical
arrangement that includes a mask for forming a three-dimensional
radiation pattern, and a substrate with the radiation-sensitive
material layer. The optical arrangement also includes a projection
optical unit for imaging the three-dimensional radiation pattern
from the mask into the radiation-sensitive material layer. The
optical arrangement is designed to compensate for spherical
aberrations at the same time at least within a first plane and a
second plane, which are located at different positions along the
thickness direction of the radiation-sensitive material layer
(e.g., along a direction perpendicular to the radiation-sensitive
material layer), to generate a stigmatic image of the
three-dimensional radiation pattern in the radiation-sensitive
material layer.
[0006] The disclosure proposes exposing all at once a volume in the
radiation-sensitive material whose extent in the thickness
direction corresponds to more than twice (e.g., five times, or even
more than ten times) the depth of focus (given by
.lamda./NA.sub.B.sup.2, .lamda.: wavelength of the radiation used,
NA.sub.B: image-side numerical aperture of the projection optical
unit) during the imaging of a two-dimensional structure. Typically,
in this case in a predetermined position of the substrate, the
entire radiation-sensitive material layer over a thickness of, for
example, approximately 10 .mu.m is exposed simultaneously.
[0007] If a conventional optical imaging system for imaging a
three-dimensional object into a radiation-sensitive material layer
having a high thickness is used, it may not be possible to image
the entire three-dimensional object with good quality, for example
due to the fact that the spherical aberrations in the volume to be
imaged in the object space may not correspond to the spherical
aberrations in the imaged volume in the image space. The disclosure
proposes compensating for the variations of the spherical
aberration along the thickness direction of the radiation-sensitive
material layer, which occur in a manner dependent on the vertical
focus position on the part of the mask. This can be done by
suitably designing the mask and/or the projection optical unit, as
is explained in greater detail below.
[0008] In some embodiments, the first plane and the second plane
are oriented perpendicular to the thickness direction of the
material layer.
[0009] In some embodiments, the first plane and the second plane
are separated from each other by a distance that corresponds to
more than twice (e.g., five times, or even more than ten times) the
depth of focus.
[0010] In certain embodiments, the mask for forming the
three-dimensional radiation pattern has a three-dimensionally
patterned material layer. This three-dimensionally patterned
material layer can have mask structures that are distributed along
the thickness direction of the mask over a region of, for example,
approximately 10 .mu.m (over a region that is considerably larger
than in conventional masks used for two-dimensional
lithography).
[0011] In some embodiments, for the imaging scale .beta. of the
projection optical unit, the refractive index n.sub.r of the
radiation-sensitive material layer and the refractive index n.sub.m
of the material layer of the mask, the following holds true:
.beta.=n.sub.m/n.sub.r. From fundamental considerations, it is
possible to formulate the rule that the beam angles should be
identical in the object space and in the image space. This is
possible if the imaging scale .beta. (magnification) of the
projection optical unit, which is defined in the customary way as
.beta.=image size/object size, meets the above condition. It goes
without saying that a departure can be made from the above
condition, if appropriate, for reasons of structural engineering or
other reasons, where the condition is still considered to be met
provided that deviations between the two sides of the equation are
less than approximately 50%, such as less than 10%. Through
suitable choice of the imaging scale .beta. for given refractive
indices of the material of the mask or the resist, the
contributions thereof to the spherical aberration can precisely be
compensated for. A typical material used for the mask is quartz
glass, which has a refractive index n.sub.m of approximately 1.56
at a wave length of 193 nm. A typical material for a lithographic
photoresist has a refractive index n.sub.r of approximately 1.7,
thus resulting in an imaging scale .beta. of approximately 0.918 in
order to comply with the above condition. A lithographic projection
objective as a projection optical unit having an imaging scale of
approximately 1:1 is known for example from US 2006/0268253, which,
with regard to this aspect, is herein incorporated by
reference.
[0012] In certain embodiments, the imaging scale .beta. of the
projection optical unit can be set in a manner dependent on the
refractive index of the radiation-sensitive material layer and the
refractive index of the material layer of the mask, such as in an
interval between 0.8<.beta.<1.2. Besides the materials
mentioned above, it is also possible, of course, to use materials
having a different refractive index, such that it is advantageous
to make the imaging scale of the projection optical unit adjustable
for adaptation to these different materials.
[0013] In some embodiments, the optical arrangement includes an
illumination system for introducing radiation into the material
layer of the mask, where the optical arrangement is designed to
allow only a portion of the radiation that is scattered at the
structures in the material layer of the mask to enter into the
projection optical unit.
[0014] Some components, for example photonic crystals, have a
substantially periodic three-dimensional structure. In this case,
the specific functionality of the structure is coded in
deliberately chosen deviations from the periodic structure. By way
of example, waveguides or resonator modes can be produced in this
way. Compared with the underlying periodic structure, these
intentionally introduced defects are comparatively rare. An
economic approach for constructing such structures is to use a
simple method for producing the basic structure, such as, for
example, full-beam interference or the use of densely packed
colloids for producing an inverse opal structure. The defects then
have to be introduced in a second step using three-dimensional
lithography.
[0015] In this case, the desired accuracy of the lithographic
exposure are generally still high, though the number of structures
to be imaged is itself typically comparatively low, such that a
large volume region of the material layer of the mask is
transparent and contains no structures to be imaged. In order
nevertheless to ensure a high contrast during imaging, the
situation in which radiation from these transparent regions
impinges on the resist should be avoided. Therefore, illumination
configurations are desired in which no unscattered light reaches
the radiation-sensitive material layer. Two such configurations are
described below.
[0016] In certain embodiments, the structures in the material layer
of the mask are embodied in reflective fashion and the illumination
system can be designed to generate coherent illumination. The use
of a mask having reflective structures makes it possible to prevent
that portion of the illumination radiation which is transmitted at
the transparent regions of the mask from entering into the
projection objective. In this case, a coherent illumination can be
used in order to radiate the radiation of the illumination system
at every point almost perpendicularly onto the mask, such that the
illuminated solid angle range is very small. This illuminated solid
angle range in this case forms a central obscuration (shading) in
the entrance pupil of the projection optical unit.
[0017] In some embodiments, the illumination system is designed to
generate dark field illumination. In this case, the mask is
typically operated in transmission and the illumination radiation
impinges on the mask at solid angles which are chosen to be greater
than the acceptance angle of the projection optical unit, such that
likewise only the radiation scattered at the structures in the mask
can enter into the projection optical unit.
[0018] In certain embodiments, the mask for forming the
three-dimensional radiation pattern has a holographic structure,
which at least partly compensates for spherical aberrations along
the thickness direction of the radiation-sensitive material layer.
It goes without saying that the holographic structure of the mask
can be embodied as a phase hologram, amplitude hologram or a
combination of the two.
[0019] It is known to carry out three-dimensional lithography by a
holographic mask instead of a three-dimensionally patterned mask.
However, if a hologram is produced for this purpose and imaged by a
projection optical unit whose imaging scale deviates significantly
from one, for example if the hologram is used in a wafer stepper
with an imaging scale 1:4, the same problem as described above can
occur. If a correction is performed for a focus position, the image
of the three-dimensional radiation pattern at locations deviating
from this position in the thickness direction of the
radiation-sensitive material has spherical aberrations and is not
stigmatic.
[0020] The disclosure proposes implementing this effect at least
partly (e.g., completely) by suitable patterning of the holographic
structure coded in the hologram. This is possible particularly when
a computer generated hologram (CGH) is used which is patterned by a
laser beam or an electron beam. The program steps that can be used
to produce such a computer generated hologram are in this case
known in principle to the person skilled in the art. The hologram
produces in this case a three-dimensional radiation pattern whose
direct (virtual or real) image is blurred. It is as a result of the
imaging by a projection optical unit, typically having a high
demagnification, a high numerical aperture in the image region and
a refractive index in the image region of greater than one, that a
sharp image is produced in the radiation-sensitive material
layer.
[0021] The holographic mask can have a diffraction grating to
diffract radiation from an illumination system, which can be
designed to generate oblique illumination, into the projection
optical unit. As already explained above in connection with the
three-dimensionally patterned mask, it is favorable for light
transmitted by the mask not to impinge on the radiation-sensitive
material layer. This can be achieved by setting at the illumination
system an illumination setting in the case of which radiation is
radiated in from a fixed "oblique" direction outside the
entrance-side aperture of the projection optical unit. In order to
diffract the radiation into the projection optical unit in this
case, a diffraction grating is provided at the holographic mask.
This procedure can be referred to as carrier frequency method since
the hologram approximately corresponds to a holographic recording
with an obliquely incident reference wave. In the mask plane, a
spatial frequency dependent on angle of incidence and wavelength
corresponds to an obliquely incident wave.
[0022] In certain embodiments, the mask can be displaced
perpendicular to an object plane of the projection optical unit by
a magnitude of more than 3 .lamda./NA.sub.O.sup.2 (e.g., more than
10 .lamda./NA.sub.O.sup.2, more than 20 .lamda./NA.sub.O.sup.2),
and/or the light-sensitive material layer can be displaced
perpendicular to an image plane of the projection optical unit by a
magnitude of more than 3 .lamda./NA.sub.B.sup.2 (e.g., more than 10
.lamda./NA.sub.B.sup.2, more than 20 .lamda./NA.sub.B.sup.2), where
.lamda. denotes the wavelength of the radiation used, NA.sub.0
denotes the object-side and NA.sub.B the image-side numerical
aperture of the projection optical unit. The magnitude of the
displacement and hence the defocusing of the reticle or of the
wafer from the respective focal plane of the projection optical
unit exceeds the range of the depth of focus of a two-dimensional
object or image, given by .lamda./NA.sub.O.sup.2 or respectively by
.lamda./NA.sub.B.sup.2, at least twice. In this case, the degree of
defocusing increases normally with the thickness to be exposed of
the radiation-sensitive material layer. Via the defocusing, it is
possible to influence the region ("effective radius"), in which the
given mask structure has effects on the image in the
radiation-sensitive material layer. A small defocus leads to a
local effect, whereas a large defocus enables the energy of a
larger area in the object to be concentrated onto a smaller area in
the image. When an excessively large defocus is used, however, the
complexity for calculating the hologram, which increases as the
effective radius becomes larger since an "inverse problem" has to
be solved in the calculation, increases. Furthermore, in the case
of an excessively large defocus, the sensitivity of the holographic
structure toward coherence effects in the illumination
increases.
[0023] The optical arrangement can include an illumination system
designed to produce a relative numerical aperture .sigma. of less
than 0.1 (e.g., less than 0.05, less than 0.03). In this case, the
relative numerical aperture .sigma. (degree of coherence) is
defined as the ratio of the object-side numerical aperture of the
projection optical unit to the exit-side numerical aperture of the
illumination system. As already explained above, it is favorable
for the illumination to be coherent to a significantly greater
extent than is currently customary, in order that the hologram can
represent the desired intensity profile well over a larger depth
range, that is to say that the illuminated solid angle range has to
be very small. In the case of conventional illumination (incidence
of the illumination light perpendicular to the object plane), this
can be ensured for the above-specified ranges of the relative
numerical aperture.
[0024] In some embodiments, the optical arrangement is designed for
operation with radiation at a wavelength of 400 nm or less (e.g.,
200 nm or less, such as at 193 nm). With the use of typical
wavelengths for microlithography of, for example, 193 nm, it is
possible to use conventional projection exposure apparatuses for
microlithography as optical arrangements for three-dimensional
lithography, provided that they are suitably modified for this
application. In order to reduce the extent of the focus in the
thickness direction of the radiation-sensitive material layer, it
may be appropriate also to use comparatively large wavelengths of,
such as 365 nm or higher, and to achieve the high lateral
resolution by a very large numerical aperture of NA 0.6 or
greater.
[0025] In certain embodiments, the projection optical unit has an
image-side aperture of 1.2 or more, such as 1.4 or more. Even if
the lateral dimensions of the three-dimensional radiation pattern
to be imaged do not require it, it is favorable to use a largest
possible image-side numerical aperture in order to reduce the
extent of the focus in the thickness direction.
[0026] An immersion liquid can be introduced between a last optical
element of the projection optical unit and the radiation-sensitive
material layer. The immersion liquid can contribute to increasing
the numerical aperture. Water, for example, can be used as the
immersion liquid. Since the refractive index of the immersion
liquid should ideally correspond to the refractive index of the
photoresist, which is approximately 1.7, for example, as described
above, it is also possible to use immersion liquids having a high
refractive index in order to further increase the image-side
numerical aperture.
[0027] In some embodiments, the disclosure provides a mask for use
in an optical arrangement as described above. The mask has, for
forming a three-dimensional radiation pattern, a holographic
structure, which at least partly compensates for spherical
aberrations along the thickness direction of the
radiation-sensitive material layer. As explained further above, the
holographic mask produces an only partly stigmatic
three-dimensional radiation pattern, which is converted into a
stigmatic image in the radiation-sensitive material layer by the
imaging by the projection optical unit.
[0028] In certain embodiments, the disclosure provides a method for
three-dimensionally patterning a radiation-sensitive material
layer. The method includes forming a three-dimensional radiation
pattern, and imaging the three-dimensional radiation pattern into
the radiation-sensitive material layer. In the course of forming
the three-dimensional radiation pattern and/or in the course of
imaging the three-dimensional radiation pattern, spherical
aberrations along the thickness direction of the
radiation-sensitive layer are compensated for in order to obtain a
stigmatic image of the three-dimensional radiation pattern in the
radiation-sensitive material layer. As explained above, both by
suitable shaping of the three-dimensional radiation pattern and
during the imaging of this pattern, it is possible to compensate
for the spherical aberrations in the thickness direction of the
radiation-sensitive material layer, such that the resolution of the
image generated in the radiation-sensitive material layer is of the
order of magnitude of approximately 100 nm in all three spatial
directions.
[0029] In some embodiments, prior to imaging, the mask is displaced
perpendicular to an object plane and/or the light-sensitive
material layer is displaced perpendicular to an image plane of a
projection optical unit used during imaging, by an amount defined
in a manner dependent on the type of mask. Each (holographic) mask
used can be assigned defocus values for the mask and/or for the
substrate. These "design defocus values" define the magnitude of
which the mask and/or the substrate are to be defocused in relation
to the respective focal planes. The defocus values are then set by
suitable actuating devices on the optical arrangement before the
three-dimensional structure is imaged.
[0030] Further features and advantages of the disclosure emerge
from the following description of exemplary embodiments of the
disclosure, with reference to the figures of the drawing that show
details essential to the disclosure, and from the claims. The
individual features can each be realized individually by themselves
or as a plurality in any desired combination in a variant of the
disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] Exemplary embodiments are illustrated in the schematic
drawings, in which:
[0032] FIGS. 1a-c show schematic illustrations of an optical
arrangement (a) and also the latter's object region (b) and image
region (c) with the use of a conventional projection optical
unit;
[0033] FIGS. 2a-c show schematic illustrations of an optical
arrangement (a) and also the latter's object region (b) and image
region (c) with the use of a projection optical unit for generating
a stigmatic image;
[0034] FIGS. 3a-b show a three-dimensionally patterned mask
operated in reflection (a) in the case of coherent illumination and
also the entrance pupil of the projection optical unit (b) in the
case of this type of illumination;
[0035] FIGS. 4a-b show a three-dimensionally patterned mask
operated in transmission in the case of dark field illumination (a)
and also the entrance pupil of the projection optical unit (b) in
the case of this type of illumination;
[0036] FIGS. 5a-c show a mask having a holographic structure (a),
the mask in the case of coherent illumination (b), and also the
image of the three-dimensional radiation pattern that is generated
in the resist during the imaging of the mask;
[0037] FIGS. 6a-b show a holographic mask with a diffraction
grating in the case of oblique illumination (a) and also the
entrance pupil of a projection optical unit (b) in the case of this
type of illumination; and
[0038] FIG. 7 shows a schematic illustration of a projection
exposure apparatus according to the disclosure for microlithography
for three-dimensionally patterning a resist.
DETAILED DESCRIPTION
[0039] FIG. 1a schematically shows an optical arrangement 1
including a mask 2 and a resist 3 as a radiation-sensitive material
layer. Arranged between the mask 2 and the resist 3 is a projection
optical unit 4, which has an imaging scale .beta. of 1:4 that is
typical of lithography optical units, and which is designed for
imaging two-dimensional structures formed on the mask 2 onto the
resist 3. In this case, the thickness of the mask 2 and the
thickness of the resist 3 are increased by comparison with the
thicknesses conventionally used in lithography for imaging
two-dimensional structures, in order to be able to image a
three-dimensional object onto a three-dimensional image. In the
case of the three-dimensional patterning, too, the intention is to
ensure a good quality during imaging independently of the position
along a thickness direction 5 of the resist 3.
[0040] FIG. 1b shows three beams 6a, 6b, 6c, which enter into an
air-filled space below the mask from the upper end, the center and
the lower end of the mask 2 and form a three-dimensional radiation
pattern in this case. This radiation pattern is imaged into the
resist 3 by the projection optical unit 4, in the manner as shown
in FIG. 1c, which illustrates the image region of the optical
arrangement 1 with the resist 3. Water as the immersion liquid is
arranged above the resist 3 in order to produce an image-side
numerical aperture NA.sub.B of approximately 1.2. As can likewise
be discerned in FIG. 1c, the two imaged beams 6b', 6c' do not
generate a stigmatic image on account of spherical aberrations in
the radiation-sensitive material layer, that is to say that, unlike
the first imaged beam 6a', the beams 6b', 6c' do not combine at one
point.
[0041] In order to produce a stigmatic imaging of all the beams 6a
to 6c, it is desirable for the beam angles in the object region and
in the image region to substantially correspond. This can be
achieved if the imaging scale .beta., defined as the ratio of image
width to object width, with respect to the refractive index n.sub.m
of the material layer 2a of the mask 2 and the refractive index
n.sub.r of the resist 3, has the following relationship:
.beta.=n.sub.m/n.sub.r.
[0042] As is shown in FIGS. 2a-c for an optical arrangement 1'
which meets the above condition, such a coordination of the imaging
scale .beta. with the refractive indices of the mask 2 and of the
resist 3 leads to a stigmatic imaging in the course of which the
images 6a' to 6c' of the beams 6a to 6c each combine at the same
time at one point in the resist 3. The images 6a' of the beams 6a
combine at one point within a plane 34, which for example coincides
with one surface of the resist 3. The images 6b' of the beams 6b
combine at one point within a plane 35, which for example is
situated inside of the resist 3. The images 6c' of the beams 6c
combine at one point within a plane 36, which for example coincides
with another surface of the resist 3 that is situated in beam
direction downstream the planes 34 and 35. All planes 34, 35 and 36
may be oriented perpendicular to the thickness direction 5 of the
resist 3 and may be located at different positions along the
thickness direction 5 of the resist 3. For the imaging it is
possible in this case to use radiation at a wavelength of 193 nm,
at which a material layer 2a composed of quartz glass has a
refractive index n.sub.m of 1.56. With the use of a resist 3 having
a refractive index n.sub.r, this results in an imaging scale .beta.
of 0.918 for meeting the above condition.
[0043] It goes without saying that, depending on the materials used
for the resist 3 and the material layer 2a of the mask 2, the
imaging scale .beta. has to be adapted, and so it is expedient to
design a projection optical unit 4' such that its imaging scale is
variable. This can be done, for example, by virtue of the fact that
the distance between the two lenses (not shown) of a lens telescope
in the projection optical unit 4' can be set by suitable
devices.
[0044] In particular in the patterning of preprocessed photonic
crystals, that is to say in the case of crystals which already have
a basic structure into which only a small number of defect sites
are intended to be introduced in the targeted manner by the
three-dimensional lithography, the three-dimensionally patterned
mask 2 of the optical arrangement 1' shown in FIG. 2a can be
transparent to the greatest possible extent and have just a small
number of structures 7a-c distributed over the thickness of the
material layer 2a, as is shown in FIG. 3a. In this case, in order
to obtain a high contrast during the imaging, it is expedient if
radiation which passes through these transparent regions does not
impinge on the resist 3.
[0045] As illustrated in FIG. 3a, this can be achieved by operating
the mask 2 in reflection, that is to say by the structures 7a-c
with the material layer 2a being embodied in reflective fashion.
For the radiation 8 impinging on the mask 2 from an illumination
system (not shown), a coherent illumination setting was chosen in
this case, that is to say that the radiation 8 impinges on the mask
2 substantially perpendicularly at each point, such that the
illumination radiation is distributed over a small solid angle
range 9a, which is shown in the entrance pupil 10 of the projection
optical unit 4' in FIG. 3b.
[0046] In the case of the illustration of the pupil 10 in FIG. 3b,
as is generally customary, illumination radiation having a small
angle with respect to the optical axis is illustrated as lying
radially further inward, while radiation that forms a larger angle
with the optical axis is shown as lying radially further outward.
In the case shown, the illumination radiation 8 in the solid angle
range 9a does not enter into the projection optical unit 4' because
the entrance pupil 10 has a so-called central obscuration 11. The
latter arises by virtue of the fact that that portion of the beams
6a-c which is backscattered at a small solid angle at the
reflective structures 7a-c is scattered into the illumination
device provided for producing the coherent illumination and,
consequently, only radiation backscattered at the structures 7a to
7c in a solid angle range between the central obscuration 11 and
the edge 10a of the entrance pupil 10 can enter into the projection
optical unit 4'.
[0047] An alternative possibility for preventing light transmitted
by the mask 2 from entering into the projection optical unit 4' is
shown in FIGS. 4a,b. In this case, the mask 2 is operated in
transmission, that is to say that the illumination radiation 8 is
scattered forward at the structures 7a-c. In this case, however,
the solid angles of the radiation 8 emitted by the illumination
system are chosen to be so steep that the latter cannot enter into
the projection optical unit 4': as is shown in FIG. 4b, the solid
angle range 9b at which the radiation 8 impinges on the projection
optical unit 4' lies outside the acceptance angle range thereof,
given by the edge 10a of the entrance pupil 10. This type of
illumination is also referred to as dark field illumination and
ensures that, in the case of the arrangement shown in FIGS. 4a,b,
only radiation scattered at the structures 7a-c can enter into the
projection optical unit 4'.
[0048] In addition or as an alternative to the possibility of
obtaining the stigmatic image of a three-dimensional radiation
pattern in the resist by correspondingly adapting the projection
optical unit 4', it is possible to alter the three-dimensional
radiation pattern formed at the mask in such a way that the
spherical aberrations occurring in the radiation-sensitive layer
are precisely compensated for. Such a modification of the radiation
pattern can be effected, for example, by the mask 2' being embodied
as a phase and/or amplitude hologram and having holographic
structures 12a, 12b (in its virtual image) from which emerge beams
13a, 13b which do not combine at one point, as is illustrated in
the enlarged detail illustration for the second holographic
structure 12b in FIG. 5a. It goes without saying that holographic
structures which generate a stigmatic virtual image can also be
provided at the mask 2. Thus, for example, the first beam 13a
combines at one point at the holographic structure 12a.
[0049] As is illustrated in FIG. 5b, the mask 2' having the
holographic structures 12a, 12b is illuminated coherently with
illumination radiation 8 and the beams 13a, 13b emerging therefrom
are imaged onto the resist 3 (cf. FIG. 5c) with the aid of the
projection optical unit 1 from FIG. 1a on a scale of 1:4. In this
case, for those beams 13b which do not generate a stigmatic virtual
image at the mask 2, a correction of the spherical aberrations is
performed by the demagnifying projection and also the properties of
the resist 3, the correction having the effect that a stigmatic
image 13a', 13b' of the three-dimensional radiation pattern formed
by the beams 13a, 13b arises at each point along the thickness
direction of the resist 3, for example at points located within
plane 34 and also at points located within plan 35.
[0050] As already described with regards to FIGS. 3a,b and also
FIGS. 4a,b in connection with a three-dimensionally patterned mask
2, it is expedient in the case of the holographic mask 2', too, if
only a small portion of the radiation transmitted by the mask
impinges on the resist 3. This can be achieved by setting an
oblique illumination, as shown in FIG. 6a, in the case of which
illumination the angles of the illumination radiation 8 are chosen
in such a way that the solid angle range 9c of the illumination
radiation 8 lies outside the edge 10a of the entrance pupil 10 (cf.
FIG. 6b), the radiation 8 also being radiated in substantially from
a single, constant direction with regard to the azimuthal angle.
Such an illumination setting makes it possible to provide a
diffraction grating 14 at the holographic mask 2', at which
diffraction grating the radiation 8 is diffracted into the entrance
pupil 10.
[0051] Independently of the type of illumination chosen, the
compensation of the spherical aberrations that is desired to
generate a stigmatic image is taken into account with the aid of a
computer program in the creation of the holographic mask 2'. If the
spherical aberrations are completely compensated for by the mask
2', the imaging can indeed be carried out by a conventional
projection optical unit, for example, by a projection objective for
microlithography, but it is more expedient for the projection
optical unit and/or the illumination system used to be suitably
modified.
[0052] FIG. 7 shows such a modified projection exposure apparatus
20 for microlithography in the form of a wafer stepper for
producing large scale integrated semiconductor components. The
projection exposure apparatus 20 includes an excimer laser 21
having an operating wavelength of 193 mm as a light source, other
operating wavelengths, for example 248 nm, or 365 nm, also being
possible. An illumination system 22 displaced downstream produces
in its exit plane a large, sharply delimited, very homogeneously
illuminated image field adapted to the desired telecentricity
properties of a downstream projection objective 23 serving as a
projection optical unit.
[0053] A device 24 for holding and manipulating a
three-dimensionally patterned mask 2 or a holographic mask 2' is
arranged downstream of the illumination system 22 in such a way
that the mask lies in the vicinity of the object plane 25 of the
projection objective 23 and can be moved in this plane for step
and/scan operation in a departure direction indicated by an arrow
26. Either pure stepper operation or combined step and scan
operation can be provided in this case.
[0054] Downstream of the plane 25, also referred to as mask plane,
there follows the projection objective 23, which images an image of
a three-dimensional radiation pattern, generated during the
illumination of the mask 2, 2', with a reduced scale, for example
on a scale of 1:4 or 1:5 or 1:10, onto a wafer 27 coated with a
photoresist layer 3. The wafer 27 is arranged in such a way that
the planar surface 28 of the photoresist layer 3 substantially
coincides with the image plane 29 of the projection objective 23.
In this case, the wafer 27 is held by a device 30 including a
scanner drive in order to move the wafer 27 synchronously with the
mask 2, 2' and (anti-)parallel to the latter. The device 30 also
includes manipulators in order to move the wafer both in the z
direction parallel to an optical axis 31 of the projection
objective 23 and in the x and y directions perpendicular to the
axis.
[0055] The projection objective 23 has, as terminating element
adjacent to the image plane 29, a transparent planoconvex lens 32,
which is arranged at a working distance above the substrate surface
28. Arranged between the planoconvex lens 32 and substrate surface
28 is an immersion liquid 33, in the present case water, which
increases the output-side numerical aperture of the projection
objective 23 to approximately 1.2. As an alternative, it is also
possible to use liquids having a high refractive index such as, for
example, IF131, IF132, IF169, IF175, n-decane, n-hexane,
cyclodecane or cyclohexane, by which a higher image-side numerical
aperture can be produced. Via the immersion liquid 33, the imaging
of three-dimensional radiation patterns can be effected with a
higher resolution than is possible if the interspace between the
optical element 32 and the wafer 27 is filled with a medium having
a lower refractive index, for example, air.
[0056] The projection exposure apparatus shown in FIG. 7 has the
following special characteristics for generating a stigmatic image
of the radiation pattern produced at the mask 2 or 2' into the
resist layer 3 of the wafer 27:
[0057] Firstly, the illumination system 22, for producing the
desired intensity profile over a large depth range of the resist 3,
is designed for generating illumination radiation that is coherent
to a significantly greater extent than is usual in conventional
illumination systems. For this purpose, the relative numerical
aperture .sigma. chosen in the case of perpendicular incidence (cf.
FIGS. 4a and 5b) has to be as small as possible and should
typically be less than 0.1 (e.g., less than 0.05, less than 0.03),
such that the illuminated solid angle range turns out to be very
small. An illumination system for producing illumination settings
with such a small relative numerical aperture can contain for
example a pinhole diaphragm in a diaphragm plane, the form of which
predetermines the illumination setting. As an alternative, the
illumination setting can also be provided as early as at the
designing of the illumination system. It goes without saying that
the illumination system 22 can additionally or alternatively also
be configured for producing oblique illumination, such as dark
field illumination and/or an illumination setting in the case of
which the illumination radiation runs in a defined solid angle
range outside the entrance pupil of the projection objective 23
(FIGS. 6a, 6b). The way in which the illumination system 22 has to
be designed in order to produce the illumination settings described
above is known in principle to the person skilled in the art, and
so it will not be discussed in any greater detail at this
point.
[0058] Particularly when a holographic mask 2' is used, the device
24 for retaining the mask 2' should furthermore be displaceable in
a range .DELTA.Z.sub.m in the Z direction around the object plane
25 of the projection objective 23 which is more than 3 .lamda./
NA.sub.O.sup.2 (e.g., more than 10 .lamda./NA.sub.O.sup.2, more
than 20 .lamda./NA.sub.O.sup.2) and which can extend downward
and/or upward from the object plane 25, the range .DELTA.Z.sub.m
specifying the respective deviation from the nominal object plane
25 in the corresponding direction (bottom/top). The same applies to
the device 30 for retaining the wafer 27, which is likewise
displaceable perpendicular to the image plane 29 of the projection
objective 23 by a magnitude .DELTA.Z.sub.r of more than 3
.lamda./NA.sub.B.sup.2 (e.g., more than 10 .lamda./NA.sub.B.sup.2,
more than 20 .lamda./NA.sub.B.sup.2) in at least one direction.
This displaceability enables a defocusing of the mask 2,2' and of
the resist 3, respectively, such that the effective radius, that is
to say the region in which a given mask structure influences the
image during the imaging, can be chosen in variable fashion.
[0059] By way of example, a highly defocused holographic mask can
focus the light of a large mask region onto an individual point,
while a weakly defocused mask is limited to a greater extent during
the focusing of the light. On the other hand, in the design of
highly defocused masks it is desirable to take account of the
larger transverse range of the diffractive structures, such that
the production of numerous fine structures becomes more difficult.
In this case, the desired defocus range .DELTA.Z.sub.m and
.DELTA.Z.sub.r respectively, is dependent on the thickness of the
resist 3 and is all the larger, the thicker the resist 3. In this
case, each mask 2 can be assigned a defocus value for the
displacement of the mask 2 and/or of the resist 3, the defocus
value being set automatically or manually on the projection
exposure apparatus 20 by suitable devices (not shown).
[0060] It goes without saying that the projection objective 23 can
also be designed for setting a variable imaging scale .beta., which
can vary in a range of between 0.8 and 1.2, for example, in order
to use a three-dimensionally patterned mask 2, as described above
in connection with FIGS. 2a-c. In this case, the projection
objective 23 has a pair of lenses (not shown) serving as a beam
telescope. It goes without saying that in this case the projection
objective 23 can be designed for producing an imaging scale of
approximately 1:1, for example, by choosing a design such as is
described in US 2006/0268253.
[0061] Overall, in the manner described above, it is possible to
generate an image of a three-dimensional radiation pattern in a
radiation-sensitive material layer, the extent of which image in
the thickness direction is significantly above that of a
conventional two-dimensional imaging, without an impairment of the
imaging quality on account of spherical aberration arising in this
case. A high image-side numerical aperture, such as can be
achieved, for example, by the immersion liquid and/or the use of
comparatively large wavelengths, which can also lie above 365 nm,
if appropriate, are expedient for the above applications.
* * * * *