U.S. patent application number 12/758554 was filed with the patent office on 2010-08-05 for illumination system for a microlithography projection exposure installation.
This patent application is currently assigned to Carl Zeiss SMT AG. Invention is credited to Markus Brotsack, Damian Fiolka, Jess KOEHLER, Manfred Maul, Wolfgang Singer, Johannes Wangler.
Application Number | 20100195077 12/758554 |
Document ID | / |
Family ID | 34314995 |
Filed Date | 2010-08-05 |
United States Patent
Application |
20100195077 |
Kind Code |
A1 |
KOEHLER; Jess ; et
al. |
August 5, 2010 |
ILLUMINATION SYSTEM FOR A MICROLITHOGRAPHY PROJECTION EXPOSURE
INSTALLATION
Abstract
An illumination system for a microlithography projection
exposure installation is used to illuminate an illumination field
with the light from a primary light source (11). The illumination
system has a light distribution device (25) which receives light
from the primary light source and, from this light, produces a
two-dimensional intensity distribution which can be set variably in
a pupil-shaping surface (31) of the illumination system. The light
distribution device has at least one optical modulation device (20)
having a two-dimensional array of individual elements (21) that can
be controlled individually in order to change the angular
distribution of the light incident on the optical modulation
device. The device permits the variable setting of extremely
different illuminating modes without replacing optical
components.
Inventors: |
KOEHLER; Jess; (Immenstaad,
DE) ; Wangler; Johannes; (Koenigsbronn, DE) ;
Brotsack; Markus; (Notzing, DE) ; Singer;
Wolfgang; (Aalen, DE) ; Fiolka; Damian;
(Oberkochen, DE) ; Maul; Manfred; (Aalen,
DE) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W., SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
Carl Zeiss SMT AG
Oberkochen
DE
|
Family ID: |
34314995 |
Appl. No.: |
12/758554 |
Filed: |
April 12, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10571475 |
Feb 12, 2007 |
7714983 |
|
|
PCT/EP04/10188 |
Sep 13, 2004 |
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12758554 |
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Current U.S.
Class: |
355/67 |
Current CPC
Class: |
G03F 7/70116 20130101;
G03F 7/702 20130101 |
Class at
Publication: |
355/67 |
International
Class: |
G03B 27/54 20060101
G03B027/54 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 12, 2003 |
DE |
103 43 333.3 |
Feb 26, 2004 |
DE |
10 2004 010 571.5 |
Claims
1. An illumination system for a microlithography projection
exposure installation for illuminating an illumination field with
light from a primary light source, comprising: a light distribution
device which receives light from the primary light source and which
produces a two-dimensional intensity distribution in a
pupil-shaping surface of the illumination system, wherein the light
distribution device variably sets the two-dimensional intensity
distribution, wherein the light distribution device comprises at
least one optical modulation device which controllably changes the
angular distribution of the light incident on the optical
modulation device, and wherein a space between the optical
modulation device and the pupil-shaping surface is free of optical
components.
2. The illumination system according to claim 1, wherein a distance
between the optical modulation device and the pupil-shaping surface
is so great that the pupil-shaping surface lies in the far-field
region of the optical modulation device.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional Application of U.S.
application Ser. No. 10/571,475 filed on Feb. 12, 2007, which is a
National Stage Entry of International Application No.
PCT/EP2004/010188, filed on Sep. 13, 2004 in the European Patent
Office, which claims priority from DE 103 43 333.3, filed on Sep.
12, 2003 in the German Patent Office, and DE 10 2004 010 571.5,
filed on Feb. 26, 2004 in the German Patent Office. The entire
disclosures of the prior applications are hereby incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to an illumination system for a
microlithography projection exposure system for illuminating an
illumination field with the light from a primary light source.
[0004] 2. Description of the Related Art
[0005] The efficiency of projection exposure systems for the
microlithographic production of semiconductor components and other
finely structured components is determined substantially by the
imaging properties of the projection objectives. Furthermore, the
image quality and the wafer throughput that can be achieved with
the system are also determined substantially by properties of the
illumination system arranged upstream of the projection objective.
This must be capable of preparing the light from a primary light
source, for example a laser, with the highest possible efficiency
and, in the process, of producing the most uniform intensity
distribution possible in an illumination field of the illumination
system. Moreover, it should be possible to set various illuminating
modes (settings) of the illumination system, in order for example
to optimize the illumination in accordance with the structures of
the individual patterns (e.g. on masks, reticles) to be imaged.
Possible settings between different conventional settings with
various degrees of coherence and also annular field illumination
and dipole or quadrupole illumination are usual. The unconventional
illumination settings for producing off-axis, oblique illumination
can be used inter alia to increase the depth of focus by means of
two-beam interference and also to increase the resolving power.
[0006] EP 0 747 771 describes an illumination system having a
zoom-axicon objective, in whose object plane there is arranged a
first diffractive raster element with a two-dimensional raster
structure. This raster element is used to increase the geometric
light guidance valve (or extendue) of the incident laser radiation
slightly and to change the form of the light distribution in such a
way that, for example, the result is an approximated circular
distribution, annular distribution or quadrupole distribution. In
order to change between these illuminating modes, first raster
elements are interchanged. A second raster element, which is
located in the exit pupil of the objective, is illuminated by the
corresponding light distribution and forms a rectangular light
distribution, whose form corresponds to the entry surface of a
following rod-like light mixing element. By means of adjusting the
zoom-axicon, the annularity of the illumination and the size of the
area that is lit up (illuminated) can be adjusted.
[0007] EP 1 109 067 (corresponding to US 2001001247) describes an
illumination system in which a changing device is provided for the
optional changing of different diffractive optical elements in the
light path of the illumination system. By interchanging the
diffractive optical elements, various illuminating modes can be
set. The system manages without a zoom-axicon module.
[0008] Other known possibilities for achieving off-axis
illumination are shown, for example, in the U.S. Pat. No.
5,638,211, EP 500 393 B1 (corresponding to U.S. Pat. No.
5,305,054), U.S. Pat. No. 6,252,647 or U.S. Pat. No. 6,211,944.
[0009] In the case of illumination systems which operate with
interchangeable optical elements (for example diffractive optical
elements or spatial filters) in order to set different illuminating
modes, the number of different illumination settings is limited by
the number of different changeable elements. Appropriate changing
devices can be constructionally complex.
[0010] DE 199 44 760 A1 discloses an illumination device for
printing plates which permits modulation of the illumination
intensity in the integrated digital screen imaging process (IDSI).
In this case, the light from a light source falls onto a digital
light modulator having a two-dimensional array of cells, which can
be activated and deactivated via a computer-controlled system in
order to deflect a specific pattern onto a light-sensitive
substrate, which is moved relative to the light modulator. In one
embodiment, the light modulator comprises a micro-mirror
arrangement (digital mirror device, DMD) having a large number of
individual mirrors that can be driven individually. During
printing, those mirrors which are not used for the exposure of the
light-sensitive material are tilted in such a way that they deflect
the light beam falling on them away from light-sensitive material.
By means of the control system, the number of individual mirrors
used in the exposure is thus changed. A similar system is disclosed
by WO 00/36470.
SUMMARY OF THE INVENTION
[0011] The object of the invention is thus to provide an
illumination system for a microlithography projection exposure
system which, with a simple construction, permits great variability
in setting different illuminating modes.
[0012] In order to achieve this object, the invention provides an
illumination system having the features of Claim 1. Advantageous
developments are specified in the dependent claims. The wording of
all the claims is incorporated by reference in the content of the
description.
[0013] An illumination system according to the invention has an
optical axis and a light distribution device for receiving light
from a primary light source and for producing a two-dimensional
spatial intensity distribution which can be set variably in a
pupil-shaping surface of the illumination system. The light
distribution device has at least one optical modulation device for
the controllable changing of the angular distribution of the
radiation incident on the optical modulation device. The optical
modulation device can comprise an array of individual elements that
can be driven individually, which are able to effect a specific
change in the angle or angular spectrum of the radiation in each
case at the location of their installed position. The optical
modulation device is also designated a locally variant optical
modulation device, since the extent of the angular change can be
set as a function of location. The array is preferably
two-dimensional, for example having a plurality of rows and columns
of individual elements. In this case, the individual elements are
preferably driven in such a way that, in all the set illuminating
modes, all of the light intensity incident on the individual
elements of the optical modulation device is deflected into the
usable region of the pupil-shaping surface and can thus contribute
to the illumination of the illumination field. By means of the
optical modulation device, a location-dependent redistribution of
the light intensity can thus be effected, without incident light
being "discarded". Therefore, an in principle largely loss-free,
variable setting of different illuminating modes is possible.
[0014] As a result of the possibility according to the invention of
deflecting small sections of the light incident on the optical
modulation device specifically into predeterminable regions of the
pupil-shaping surface, virtually any desired illumination intensity
distributions can be set in the pupil-shaping surface. These
include, for example, round, e.g. essentially circular,
illumination spots of different diameters centred around the
optical axis in the case of the conventional illumination settings
and, in the case of the unconventional, off-axis types of
illumination, annular illumination and polar intensity
distributions, for example dipole illumination or quadrupole
illumination. However, with illumination systems according to the
invention, intensity distributions differing from this are also
possible, for example multipole illumination with more than four
poles, for example hexapole illumination. The illumination
distributions do not necessarily have to have any symmetry in
relation to the optical axis.
[0015] The pupil-shaping surface of the illumination system, in
which the desired intensity distribution is to be present, in an
illumination system incorporated in a microlithography projection
exposure system can be positioned at or close to a position which
is optically conjugate with a pupil plane of a following projection
objective. In general, the pupil-shaping surface can correspond to
a pupil surface of the illumination system or be located in its
vicinity. If the interposed optical components operate so as to
maintain angle (angle conserving elements), the spatial light
distribution in the pupil of the projection objective is thus
determined by the spatial light distribution (location
distribution) in the pupil-shaping surface of the illumination
system. If the illumination system comprises, for example, a fly's
eye condenser as light-mixing element (light integrator), then the
pupil-shaping surface can be located in the vicinity of its entry
side or can coincide with the latter. In the case of systems which
comprise one or more rod-like light integrators (rod integrators)
operating with internal reflection, the pupil-shaping surface can
be a plane that is Fourier-transformed in relation to the entry
surface of the light integrator or can be located in its vicinity.
Systems are also possible in which none of the aforementioned
classical light mixing elements is present. Here, homogenization of
the intensity distribution can, if appropriate, be carried out by
means of suitable superimposition of partial beams by means of
prisms or the like.
[0016] The terms "radiation" and "light" in the sense of this
application should be interpreted widely and in particular are
intended to cover electromagnetic radiation from the ultraviolet
range, for example at wavelengths of about 365 nm, 248 nm, 193 nm,
157 nm or 126 nm. Also covered is electromagnetic radiation from
the extreme ultraviolet range (EUV), for example soft X-rays with
wavelengths of less than 20 nm.
[0017] In one development, the optical modulation device is formed
as a mirror arrangement having an array of individual mirrors that
can be controlled individually for changing the angular
distribution of the radiation incident on the mirror arrangement.
The individual mirrors which form the individual elements of the
modulation device can be arranged in the manner of a raster in a
one-dimensional or two-dimensional array. According to another
development, the optical modulation device is formed as an
electro-optical element, which preferably comprises a
one-dimensional or two-dimensional field arrangement (array) of
controllable diffraction gratings or a corresponding array of
acousto-optical elements. Each of these individual elements, which
are arranged in that manner of a raster and accordingly can also be
designated raster elements, introduces at the location of the
raster element a specific angle or angular spectrum of the
radiation output, as a rule a beam deflection of the incident
radiation, that is to say a change in the propagation direction,
being introduced. By means of driving the individual elements
electrically, for example, the angular distribution of the
radiation output can be set variably.
[0018] The space between the light modulation device and the
pupil-shaping surface can be free of optical components, such as
lenses or other imaging elements. In this case, it is beneficial to
choose the distance between light modulation device and
pupil-shaping surface to be so great that the pupil-shaping surface
lies in the far-field region of the light modulation device. Under
these conditions, the desired spatial intensity distribution is
established automatically in the pupil-shaping surface.
[0019] In one development, an optical system for converting the
incident angular distribution into a spatial distribution
(distribution in the location space) in the pupil-shaping surface
is provided between the optical modulation device and the
pupil-shaping surface. This optical system is thus intended to
carry out a Fourier transformation of the angular distribution into
the pupil-shaping surface. In this case, it can be an individual
optical element, for example a lens with a fixed focal length and
therefore a defined magnification. The optical system used for the
Fourier transformation preferably has a focal length that can be
set variably. It can be configured as a zoom objective. As a
result, with a given illumination distribution, the size of the
region in the pupil-shaping surface that can be lit up with this
illumination distribution can be set, preferably continuously. If
an axicon system with conical surfaces is provided between the
optical modulation device and the pupil-shaping surface, a desired
level of the annular field character (annularity) of the
illumination can be set, continuously if appropriate, by adjusting
the axicon system. In one embodiment, a zoom-axicon objective,
whose structure can correspond for example to the construction of
the zoom-axicon objective described in EP 0 747 772, can be
arranged between the optical modulation device and the
pupil-shaping surface. In this case, the optical modulation device
can be used in the place of the first diffractive raster element
shown there. The disclosure content of EP 0 747 772 is incorporated
by reference in the content of this description.
[0020] The optical modulation device can operate reflectively and
can be aligned obliquely with respect to the optical axis in the
manner of a deflection mirror in order, for example, to achieve on
average an approximately 90.degree. deflection (or a deflection by
a smaller or larger angle).
[0021] For the function of the optical components of the
illumination system which follow the pupil-shaping surface, it is
generally beneficial if the angles at which the rays are incident
in the pupil-shaping surface are as small as possible. For this
purpose, in preferred embodiments, provision is made to select the
optical distance between optical modulation device and the
pupil-shaping surface to be so great that the angles between the
optical axis and light beams of the angular distribution in the
region of the pupil-shaping surface are less than about 5.degree.,
in particular less than about 3.degree.. The smaller the angles are
selected, the steeper is it possible, for example, for the flanks
at the light/dark transition between the illuminated region and the
adjacent non-illuminated region to be.
[0022] A finely divided, specific setting of various forms of an
area of the pupil-shaping surface to be illuminated can be of great
benefit particularly in systems which use one or more fly's eye
condensers as light mixing elements. In such systems, as is known,
the desired evening out of the intensity distribution downstream of
the fly's eye condenser (honeycomb condenser) can be achieved only
if the individual radiation channels formed by the "honeycombs" are
either used completely or not used at all. On the other hand, the
radiation from an only partly used radiation channel impairs the
uniformity. For this reason, conventional systems operate with
masks, in order for example to block partially illuminated channels
at the edge of an illumination region. This can lead to light
losses.
[0023] In an embodiment of the invention having at least one fly's
eye condenser, in which the pupil-shaping surface normally lies in
the region of the entry surface of the fly's eye condenser or in a
surface which is optically conjugate therewith, the spatial
intensity distribution in the pupil-shaping surface can on the
other hand be controlled or set in such a way that specifically
only completely illuminated and completely non-illuminated channels
(or honeycombs) exist and partially illuminated "honeycombs" are
avoided. It is then possible to dispense with the use of aperture
stops for blocking individual channels. Thus, with a simplified
construction, largely loss-free illumination control becomes
possible.
[0024] To this end, in one embodiment the light distribution device
has at least one diffractive optical element arranged optically
between the optical modulation device and the pupil-shaping surface
for receiving light emerging from the optical modulation device and
for modifying the light by introducing an angular distribution
according to an effect function defined by the configuration of the
diffractive optical element. Due to this construction the angular
distribution of light created by the optical modulation device is
optically folded with the angular distribution created by the
diffractive optical element in the optical far field.
[0025] The effect function may be such that a beam emerging from an
individual element of the optical modulation device is shaped by
the diffractive optical element to conform to the shape and size of
one single optical channel or a group of adjacent optical channels
of the fly's eye condenser. Particularly, the diffractive structure
of the diffractive optical element can be such that a rectangular
illumination field can be created from a beam emerging from a
single optical element of the optical modulation device. The
rectangular shape can be dimensioned to conform to one single
optical channel of the fly's eye condenser. In other embodiments
the illuminated area can be dimensioned to cover a group of
neighbouring optical channels.
[0026] According to one embodiment, the diffractive optical element
is a computer generated hologram (CGH) having diffractive
structures periodically repeating across the cross section of the
diffractive optical element.
[0027] If a mirror arrangement of the optical modulation device is
used, then the minimum size of the illuminated surfaces which are
produced by the individual mirrors of the mirror arrangement is
normally determined substantially by the size of the individual
mirrors which, for example, can be flat mirrors. It is possible to
reduce the minimum extent of the light spots produced by the
individual mirrors not being formed as flat mirrors but as curved
mirrors with a finite mirror focal length. The focal length can be
dimensioned such that the radiation incident on the individual
mirrors is incident on the pupil-shaping surface in substantially
focused form. As a result, very differentiated setting of various
spatial intensity distributions in the pupil-shaping surface is
possible.
[0028] The individual mirrors of the mirror arrangement can all
have the same shape and size, which may be beneficial in terms of
fabrication. It is also possible for the individual mirrors to
comprise a first mirror group and at least a second mirror group
each having one or more individual mirrors, and for the individual
mirrors of the mirror groups to have a different size and/or
different shape and/or different curvature. If, for example, the
size of the individual mirrors is varied, then this can be used to
divide up the tasks among the individual mirrors of the mirror
arrangement. For instance, the individual mirrors with a larger
area can produce the greater proportions of the light spots
produced, while smaller individual mirrors permit the production of
a fine structure of the light distribution.
[0029] In general, the individual mirrors can in each case be
viewed as producers of specific basic light distributions, which
are then assembled to form the desired distribution in the
pupil-shaping surface of the illumination system, by the light
distributions produced being displaced relative to one another. The
variation in the angular distribution, and therefore the
displacement of spots of light in the pupil-shaping surface, can be
achieved, for example, by suitable tilting of individual mirrors
about at least one tilt axis.
[0030] A further degree of freedom in the production of spatial
light distributions can be created by at least some of the
individual mirrors having a diffractive optical structure or a
structure with a comparable effect for forming the distribution of
the radiation reflected from the individual mirror. As a result,
the "basic distribution" which is produced by this individual
mirror can still intrinsically be shaped. For example, an
individual mirror can be designed in such a way that it produces a
basic distribution which can consist of a plurality of spots of
light, which do not have to be contiguous.
[0031] The individual mirrors of the mirror arrangement preferably
adjoin one another directly, so that they form a faceted,
substantially closed, contiguous reflecting surface. In order to
facilitate relative mobility of adjacent individual mirrors, it may
be beneficial if, between the individual mirrors, there are small
distances or gaps, which result in narrow, non-reflective regions.
In particular in the case of such embodiments, it is beneficial if
in front of the mirror arrangement there is arranged an optical
device for concentrating radiation incident on the optical device
onto individual mirrors of the mirror arrangement. The optical
device may be a two-dimensional raster arrangement of optical
elements or a diffractive optical array generator. In this way,
incident light, for example from a laser, can be led onto the
individual mirrors in focused form, by which means reflection
losses at the mirror arrangement can be reduced to a minimum.
[0032] A two-dimensional raster arrangement can comprise, for
example, a two-dimensional array having telescope lens systems,
which is preferably arranged in the largely collimated beam path
upstream of the mirror arrangement. There may be a 1:1 association
between the optical elements of the raster arrangement and the
following individual mirrors.
[0033] In particular in conjunction with individual mirrors of
different shape and/or size, it may be beneficial likewise to
design the individual optical elements of the raster arrangement
differently. If, for example, regions of different size of the
widened beam coming from the light source are focused to form light
beams which are then aimed at the individual mirrors, a variation
in the light energy on the individual mirrors of the mirror
arrangement can be achieved. In this way, the radiation energy
content of the individual basic light distributions can be changed.
A comparative effect could also be achieved by means of a suitable
transmission filter upstream and/or downstream of the
two-dimensional raster arrangement, but light losses would have to
be tolerated.
[0034] If a diffractive optical array generator is used, the
transformation of an incoming beam into a plurality of light beams
concentrated on individual optical elements of an array-type
optical modulation device can be achieved with an optical element
with simple construction. Optical array generators suitable for
creating a number of separate light fluxes from one incoming light
flux are described e.g. in the article "Beam Shaping with Optical
Array Generators" by N. Streibl, J. Mod. Optics 36 (1989) pages
1559-1573. In one embodiment an optical array generator designed as
so called "Dammann grid" is used. High efficiency multiple phase
holograms of this type are described in the article "High
Efficiency in-line Multiple Imaging by means of Multiple Phase
Holograms" by H. Dammann, K. Gortler in: Optics Commun. 3 (1971)
pages 312-315. Using diffractive optical array generators for
concentrating light on individual elements of an array-type optical
modulation device provides high efficiency (low light loss) at low
cost.
[0035] For the construction and/or the driving of the individual
mirrors of the mirror arrangement, recourse can be had to known
concepts from the prior art, adaptations to the respective
illumination system possibly having to be made with regard to
dimensioning. Mirror arrangements having individual mirrors that
can be driven individually, which are frequently also designated a
digital mirror array (DMD), are known for example from systems for
maskless lithography (cf., for example, U.S. Pat. No. 5,523,193;
U.S. Pat. No. 5,691,541; U.S. Pat. No. 5,870,176 or U.S. Pat. No.
6,060,224).
[0036] Some measures, explained using the example of the individual
mirrors, for configuring the output radiation produced by the
optical modulation device can also be provided with the same effect
in an electro-optical element having controllable diffraction
gratings or acousto-optical elements. These include the possibility
of tilting the individual elements relative to one another, the
possibility of influencing a basic distribution of the light output
from an individual element by means of suitable configuration of
the individual elements, or the measure of providing optical
elements for concentrating the incident radiation onto the
angle-changing individual elements upstream of the corresponding
optical modulation device, for the efficient use of controllable
diffraction gratings or acousto-optical elements. The individual
elements of the electro-optical element can be configured
identically or differently from one another.
[0037] In particular in embodiments of illumination systems for
microlithography, it is beneficial to use a light mixing device in
the illumination system, in order to achieve a high level of
uniformity or homogeneity of the illumination falling onto the
illumination field. In illumination systems according to the
invention, both light mixing devices having fly's eye condensers
and light mixing devices having one or more integrator rods or
light mixing rods or combinations thereof can be used. Such light
mixing devices are available both respectively in a refractive
design (fly's eye condenser having lens elements, integrator rod
made of transparent material) and in a reflective design (fly's eye
condenser having concave mirrors, hollow rod with internal
reflection).
[0038] The invention also relates to a method of illuminating an
illumination field with the light from a primary light source, the
illumination field being in particular the object plane of a
microlithography projection objective or a plane conjugate
therewith. The illumination method comprises changing the angular
distribution of the light incident in the illumination field in the
light path between light source and illumination field. The change
is brought about by the light from a primary light source being
guided onto an optical modulation device having at least two
individual elements that can be varied independently of one another
and these individual elements being set suitably relative to one
another. This setting can comprise, for example, tilting at least
one of the individual elements with respect to the other individual
element about one or more tilt axes or changing the diffractive
properties of diffraction elements. The result, downstream of the
light modulation device, is an angular distribution of the light
which depends on the relative setting of the individual elements
and which, by means of following optical components, is transformed
into an angular distribution of the light incident in the
illumination field. The light output by the light modulation device
preferably has considerably more than two beams of light that can
be set independently of one another, for example at least 10 or at
least 50 or at least 100 beams of light that can be set
individually.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] The above and further features emerge from the description
and the drawings as well as from the claims, its being possible for
the individual features to be implemented in each case on their own
or in a plurality in the form of sub-combinations in embodiments of
the invention and in other fields and to represent embodiments
which are advantageous and intrinsically capable of protection.
[0040] FIG. 1 shows a schematic overview of an embodiment of an
illumination system for a microlithography projection exposure
system, having an embodiment of an optical modulation device which
comprises a mirror arrangement with many individual mirrors;
[0041] FIG. 2 shows a schematic illustration to explain the
function of the mirror arrangement; and
[0042] FIG. 3 shows a simple embodiment of an illumination system
in which the desired light distribution is produced without any
optical projection elements in the far field of the optical
modulation device.
[0043] FIG. 4 shows a schematic overview of another embodiment of
an illumination system for a microlithography projection exposure
system, in which the optical modulation device comprises a raster
arrangement of controllable diffraction gratings and a raster
element arranged in the pupil-shaping surface serves as the light
mixing device;
[0044] FIG. 5 shows a schematic overview of another embodiment of
an illumination system for a microlithography projection exposure
system having a light distribution device with diffractive optical
elements for concentrating light on an array type optical
modulation device and for forming light spots adapted to optical
channels of a fly's eye condenser;
[0045] FIG. 6-9 show schematic diagrams of light distributions on
an entrance side of a fly's eye condenser in the embodiment
according to FIG. 5; and
[0046] FIG. 10 shows a section of another embodiment of an
illumination system including a polarizing beam splitter.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION
[0047] FIG. 1 shows an example of an illumination system 10 of a
projection exposure system for microlithography, which can be used
in the production of semiconductor components and other finely
structured components and, in order to achieve resolutions down to
fractions of micrometres, operates with light from the deep
ultraviolet range. The light source 11 used is an F.sub.2 excimer
laser having an operating wavelength of about 157 nm, whose light
beam is aligned coaxially with respect to the optical axis 12 of
the illumination system. Other UV light sources, for example ArF
excimer lasers with 193 nm operating wavelength, KrF excimer lasers
with 248 nm operating wavelength or mercury vapour lamps with 365
nm or 436 nm operating wavelength or light sources with wavelengths
below 157 nm are likewise possible.
[0048] The light from the light source 11 is firstly incident into
a beam expander 13, which widens the laser beam and, from the
original beam profile with a cross section of 20 mm.times.15 mm,
produces a widened profile 14 with a cross section of 80
mm.times.80 mm. In the process, the divergence angles decrease from
about 4 mrad.times.2 mrad to about 1 mrad.times.0.4 mrad.
[0049] Downstream of the beam expander there follows a
two-dimensional raster arrangement 15 of telescope lens systems 16,
which, from the widened beam 14, produces a set of regularly
arranged, mutually parallel beams 17, which each have a lateral
spacing from one another.
[0050] The light divided up into beams 17 or partial beams 17
strikes a mirror arrangement 20 serving as a location-variant (or
space variant) optical modulation device, which is aligned
macroscopically at an angle of about 45.degree. with respect to the
optical axis 12 and, in the manner of a deflection mirror, on
average effects 90.degree. folding of the optical axis. Other
angular positions and deflection angles are also possible. One
advantage of small angles is the fact that the object plane of the
following zoom system is located more beneficially, and thus the
expenditure for the zoom system can be reduced. The mirror
arrangement 20 comprises a large number of individual, small,
individual mirrors 21 which are flat in the example, which adjoin
one another directly with very small interspaces and impart a
faceted mirror surface overall to the mirror arrangement 20. Each
of the individual mirrors 21 can be tilted about two tilt axes
aligned perpendicular to each other, independently of the other
individual mirrors. The tilting movements of the individual mirrors
can be controlled by a control device 22 via electrical signals to
corresponding individual drives. The mirror arrangement 20 is a
substantial constituent part of a light distribution device 25 and
is used for the purpose of changing the angular distribution of the
radiation incident on the mirror arrangement specifically in a
locally resolved manner.
[0051] The mirror arrangement 20 is arranged in the region of the
object plane of a zoom-axicon objective 30, which is arranged
downstream thereof in the beam path and in whose exit pupil 31 a
diffractive optical raster element 32 is arranged. The exit pupil
31 is here also designated the pupil-shaping surface of the
illumination system. The components arranged upstream thereof in
the light path are used to set a two-dimensional spatial intensity
distribution, which can be set variably, in this pupil-shaping
surface.
[0052] In detail, this basic structure can be implemented as
follows, for example. The widened laser beam 14 striking the
telescope lens array 15 is divided up by the segments of the
telescope array into a large number of individual beams. A
subregion of 4 mm.times.4 mm of the widened laser beam is in this
case reduced by a telescope segment of the telescope array to a
beam 17 having the dimensions of 2 mm.times.2 mm. In this way,
20.times.20=400 partial beams or beams 17 are produced. These
strike the associated individual mirrors 21 of the mirror
arrangement, which are in each case flat and have a size of 3
mm.times.3 mm. Each of the individual mirrors is located in a
square region of 4 mm.times.4 mm. These regions are located beside
one another on a square grid, so that there is a total of
20.times.20=400 individual mirrors.
[0053] In the embodiment, the axial spacing between the telescope
lens array 15 and the mirror arrangement 20 is about 100 mm. The
axial spacing between the mirror arrangement and the pupil-shaping
surface 31, in which the refractive optical grid element 31 is
situated, is more than 1000 mm. The maximum diameter of the region
which is lit up in the pupil-shaping surface 31 is designed to be
about 100 mm. Given this geometry, only relatively small beam
angles with values of less than about 2.9.degree. enter the
pupil-shaping surface 31. This can be achieved under the assumption
that the individual mirrors located above the optical axis (in FIG.
2) influence only the light distribution in the upper half of the
pupil-shaping surface 31, and the individual mirrors located below
the optical axis influence only the lower half of this illumination
region. A part beam or an individual beam is normally widened by at
most about 1.1 mm on the light path of about 1100 mm. This value
limits the minimum size of the spot of light which is produced in
the pupil-shaping surface 31 by an individual beam reflected from
an individual mirror.
[0054] Injection optics 40 (coupling-in optics) arranged downstream
of the pupil-shaping surface 31 transmit the light of the intensity
distribution to the rectangular entry surface 44 of a rod-like
light integrator 45 which is fabricated from synthetic quartz glass
(or calcium fluoride) and which mixes and homogenizes the light
passing through by means of multiple internal reflection. The
pupil-shaping surface 31 is a Fourier-transformed plane in relation
to the entry surface 44, so that a spatial intensity distribution
in the plane 31 is transformed into an angular distribution at the
rod entry 44. Immediately at the outlet surface 46 of the rod 45
there is an intermediate field plane 47, in which a reticle masking
system (REMA) 50 is arranged, which is used as an adjustable field
stop. The following objective 55 projects the intermediate field
plane 47 having the masking system 50 onto a plane 65, which is
also designated the reticle plane here. In the reticle plane 65
there is arranged a reticle 66. The plane 47 of the reticle masking
system and the reticle plane 65 are planes in which an illumination
field of the illumination system lies. The reticle plane 65
coincides with the object plane of a projection objective 67, which
projects the reticle pattern into its image plane 68, in which a
wafer 69 coated with a photoresist layer is arranged. The objective
55 contains a first lens group 56, an intermediate pupil plane 57,
into which filters or aperture stops can be introduced, a second
and a third lens group 58, 59 and a deflection mirror 60 which is
located between them and which makes it possible to incorporate the
large illumination device horizontally and to mount the reticle
horizontally.
[0055] Together with the projection objective 67, the illumination
system 10 forms an adjustable reticle holder, which holds the
reticle 66 in the object plane 65 of the projection objective, and
an adjustable wafer holder of a projection exposure system for the
microlithographic production of electronic components, but also of
optically diffractive elements and other microstructured parts. The
illumination system can be used both in a wafer stepper and in a
wafer scanner.
[0056] The illumination system is constructed in such a way that it
introduces the complete etendue (geometric light guidance valve) in
a number of stages. Because of the extensive parallelism of the
radiation and of the small beam cross section, the light beam
emitted by the laser has a very low etendue, which is possibly
increased by the beam widening and by the division of the beam with
the aid of the telescope array 15. Depending on the position of the
individual mirrors 21 and the angular distribution which can be
achieved as a result, the etendue is increased further by the
mirror arrangement 20, the shape of the distribution of the
radiation also being changed. The zoom-axicon system 30 is designed
for a projection at infinity. The arrangement arranged in the
region of the front focal plane of the zoom-axicon system 30,
together with the zoom-axicon optics, prepares a two-dimensional
intensity distribution of variable size in the exit pupil 31 of the
zoom system, which serves as the pupil-shaping surface. The
refractive raster element 32 arranged here has a rectangular
emission characteristic, produces the main proportion of the
etendue and adapts the etendue via the injection optics 40 to the
field size, that is to say to the shape of the rectangular entry
surface 44 of the rod integrator 45.
[0057] The tilting positions of the individual mirrors 21 are set
by the control device 22 via suitable electrical signals, any
desired orientations of the individual mirrors being possible on
account of the possibility of tilting about two axes. However, the
tilting actions are limited mechanically or electronically to small
tilting angles such that, during each possible setting of the
individual mirrors, all of the radiation reflected from the mirror
arrangement can enter the objective 30. By means of the tilting of
the individual mirrors 21, the beams originating from the latter
are reflected to various locations of the pupil-shaping surface 31
(a pupil plane of the illumination system). The characteristics of
the two-dimensional light distributions 35 which can be produced in
this way are in principle limited only by the size of the
individual spots of light. The desired size of the spots of light
that can be produced can be achieved, for example, by means of
suitable curvature of individual mirrors. It would also be
conceivable to design the individual mirrors as adaptive mirrors,
in which the shape of the mirror surface can be varied to a limited
extent via suitable actuators, for example piezoelectrically.
[0058] In the application of the invention illustrated here, it is
of critical importance that the distribution of the light in the
pupil-shaping surface 31 (a pupil plane of the illumination system)
can be set as a function of the structure of the mask 66 in the
reticle plane 65. By means of suitable, computer-controlled
alignment of the individual mirrors, all the familiar
two-dimensional illuminating light distributions can be set in the
first pupil-shaping surface 31, for example conventional
illuminations with different diameters, annular settings,
quadrupole or dipole settings. Differing from other systems, it is
moreover also possible to set any other desired light distributions
variably in the pupil-shaping surface 31. No replacement of optical
components is necessary for the change between the settings. Above
all, the light distribution in the pupil plane 31 can be set
without the aid of filters, aperture stops or other elements that
cause light losses. This also applies in particular to other
embodiments in which the light mixing element used is a fly's eye
condenser, whose entry side should preferably be arranged in the
region of the pupil-shaping surface 31. The ability specifically to
set virtually any desired light distributions in the pupil-shaping
surface 31 can also be used for the purpose of influencing some
pupil properties such as pupil ellipticity or polar balance. This
can be very advantageous, since the intensity distribution of
conventional laser beams in no way has the desired form with a
sharp light-dark transition (the form of a top-head function). In
the embodiment, the angles at which the light beams open into the
pupil plane 31 are at most about 3.degree.. This has a positive
effect on the filling of the rod integrator 45.
[0059] A simplified embodiment of an illumination system will be
described by using FIG. 3. In the illumination system 100, the
light from a laser light source 111 is incident at an angle of
incidence of about 25.degree. on a mirror arrangement 120 which is
aligned obliquely with respect to the optical axis 112 and has a
large number of individual mirrors 121 that can be driven
individually and respectively tilted about two tilt axes. Here, the
smaller the angle of attack of the mirror arrangement is in
relation to the irradiation direction, the lower are the light
losses in this embodiment, since there are no means of focusing the
radiation onto the individual mirrors. The mirror arrangement 120
is used as a location-variant optical modulation device and forms
the light distribution device 125 of this system, is driven by the
control device 122 and is at such a great distance from the
pupil-shaping surface 131 of the illumination system, in which the
desired two-dimensional intensity distribution is to be present,
that the pupil-shaping surface 131 lies in the region of the far
field of the mirror arrangement 120. In this case, the desired
intensity distribution is automatically established in the region
of the pupil-shaping surface 131 without the angular distribution
output from the mirror arrangement 120 having to be converted into
a spatial distribution by Fourier transformation by means of a lens
or optics of comparable effect. A field lens 140 arranged
downstream of the pupil-shaping surface 131 transforms the
intensity distribution into a following field plane 165 in which,
for example, a mask to be illuminated is located and is illuminated
from the desired direction. Following projection optics 170 project
the pattern of the reticle onto a wafer covered with a
light-sensitive coating in the image plane 180 of the projection
objective 170.
[0060] The structure of the illumination system 210 in FIG. 4 is
derived from the structure of the illumination system shown in FIG.
1, for which reason mutually corresponding features and components
have corresponding reference symbols, increased by 200. Differences
from the system according to FIG. 1 exist, firstly, in the
structure of the location-variant optical modulation device 220
and, secondly, in the concept of the light mixing. In this regard,
it is worth noting that the illumination system 210 is constructed
without a special light mixing element, that is to say without an
integrator rod or fly's eye condenser. As in the embodiment
according to FIG. 1, the light from the laser light source 211,
after passing through a beam expander 213 and a two-dimensional
raster arrangement 215 of telescope lens systems, is present as a
set of regularly arranged, mutually parallel beams 217, which in
each case have a lateral spacing from one another. The beams or
part beams 217 are in each case aimed at individual elements 221 of
the optical modulation device 220. The latter is constructed as an
electro-optical element and has a large number of controllable,
reflective diffraction gratings 221, which form the individual
elements of the optical modulation device, are arranged physically
in a two-dimensional raster or grid and can be set and changed
independently of one another with respect to their diffraction
properties by the control device 222. With the aid of electrical
signals, it is thus possible to set the angular distribution of the
radiation reflected from the optical modulation device 220 in the
direction of the zoom-axicon objective 230 in a variable manner. In
another embodiment, the individual elements of the optical
modulation device are formed by acousto-optical elements.
[0061] The optical modulation device 220 is fitted in the region of
the object plane of the zoom-axicon objective 230, whose exit pupil
231 is the pupil-shaping surface of the illumination system. In the
pupil-shaping surface 231 or in its vicinity there is arranged a
raster element 232 having a two-dimensional arrangement of
diffractive or refractive optical elements which, in this
embodiment, has a plurality of functions. Firstly, by means of the
raster element 232, the incoming radiation is shaped in such a way
that, after passage through the subsequent injection optics 240 in
the region of the field plane 250 of the illumination system, it
lights up a rectangular illumination field. The raster element 232
with rectangular emission characteristics in this case produces the
main part of the etendue and adapts this to the desired field size
and field shape in the field plane 250, which is optically
conjugate with the reticle plane 265 and in which the reticle-mask
system is arranged. The raster element 232 can be implemented as a
prism array, in which the individual prisms arranged in a
two-dimensional array introduce locally determined angles in order
to light up the field plane 250 as desired. The Fourier
transformation carried out by the injection optics 240 has the
effect that each specific angle at the exit from the raster element
232 corresponds to a location in the field plane 250, while the
location of the raster element, that is to say its position in
relation to the optical axis 212, determines the illumination angle
in the field plane 250. The beams leaving the individual raster
elements are in this case superimposed in the field plane 250. By
means of suitable design of the raster element and its individual
elements, it is possible for the rectangular field in the field
plane 250 to be lit up substantially homogeneously. The raster
element 232 thus serves also to homogenize the field illumination,
so that it is possible to dispense with a separate light mixing
element, such as the integrator rod 45 of the embodiment according
to FIG. 1. Since no separate light mixing element is required
between the pupil-shaping surface 231 and the exit plane 265 of the
illumination system (reticle plane), illumination systems of this
type can be designed particularly compactly in this area.
[0062] A field-shaping and homogenizing element of the type of the
raster element 232 which, in combination with Fourier optics
connected downstream, is used firstly to set a field size and shape
and, secondly, to homogenize the illumination in this field, can of
course also be used in the embodiment according to FIG. 1 in
combination with a mirror arrangement as optical modulation device.
In this case, it is possible to dispense with the integrator rod
45. Secondly, the mirror arrangement according to FIG. 1 can also
be replaced by an electro-optical optical modulation device having
controllable diffraction gratings or opto-acoustic elements. As an
alternative to the reflective diffraction gratings according to
FIG. 4, the use of transmission diffraction gratings in an optical
modulation device is also possible.
[0063] In FIG. 5 another embodiment of an illumination system 310
using a fly's eye condenser 380 as a light mixing element is shown
schematically. Linearly polarized radiation emanating from a laser
light source 311 is shaped by a light distribution device 325 into
variably selectable two-dimensional intensity distributions in a
pupil-shaping surface 331 arranged in the vicinity of the entry
side of the fly's eye condenser 380. The light distribution device
325 includes, a beam expander 313, a first diffractive optical
element (DOE1) 315, a mirror arrangement 320 serving as location
variant (or space variant) optical modulation device (which is
aligned macroscopically at an angle of about 45.degree. with
respect to the optical axis 312 and has a large number of
individual mirrors 321 tiltable about two mutually perpendicular
tilt axes), a second diffractive optical element (DOE2) 390
arranged downstream of the mirror arrangement 320, and optionally
an optical system 330 for transforming an angular distribution of
radiation entering the optical system 330 into a spatial
distribution of light in the pupil shaping surface 331.
[0064] The fly's eye condenser 380 consists of a first raster
arrangement 381 of first cylindrical lens 383 and a second raster
arrangement 382 having second cylindrical lens 384. The lenses 383
of the first raster arrangement 381 have identical refractive power
and rectangular cross section where the rectangular shape of the
cylindrical lens 383 corresponds to the rectangular shape of the
illumination field 351 to be illuminated. Therefore the first
lenses 383 are also named "field honeycombs". The cylindrical
lenses 383 are arranged immediately adjacent to one another in a
rectangular raster substantially filling the area in or in the
vicinity of the pupil-shaping surface 331 (which is a pupil plane
of the illumination system).
[0065] The first cylindrical lenses 383 have the effect that light
incident on the plane 331 is divided up into a number of beams of
light corresponding to the number of cylindrical lenses 383 that
are illuminated, where those light beams are focused on to a field
plane 332 of the illumination system that lies in the focal plane
of these cylindrical lenses 383. In this field plane, or in the
vicinity thereof, the second raster arrangement 382 is positioned
having second cylindrical lenses 384 of rectangular cross section
and positive, identical refractive power. Each cylindrical lens 383
of the first raster arrangement 381 project the light source 311 on
to a respectively associated second cylindrical lens 384 of the
second raster arrangement 382 so that a large number of secondary
light sources is produced. The cylindrical lenses 384 of the second
raster arrangement are frequently also designated "pupil
honeycombs". A pair of mutually associated first and second
cylindrical lenses 383, 384 of the first and second raster
arrangements 381, 382 form an optical channel. The first and second
raster arrangement 381, 382 form the fly's eye condenser 380, which
has a large number of optical channels arranged in an
two-dimensional array.
[0066] The cylindrical lenses 384 of the second raster arrangement
382 are arranged in the vicinity of the respective secondary light
sources and project the rectangular first cylindrical lenses 383
onto an intermediate field plane 347 of the illumination system via
a field lens 385 arranged downstream of the fly's eye condenser.
The field lens 385 is a zoom lens designed to vary the focal length
continuously. The rectangular images of the first cylindrical
lenses (field honeycombs) 383 are superimposed in this intermediate
field plane. This superimposition has the effect of homogenizing or
evening out the light intensity in the region of this intermediate
field plane, whereby a homogeneous illumination of the rectangular
illumination field 351 is obtained.
[0067] As in the embodiments mentioned above, a reticle-masking
system (REMA) 350 is arranged in the intermediate field plane 347
serving as an adjustable field stop. The following objective 355
projects the intermediate plane 347 onto the illumination surface
365 where a reticule (the mask or the lithography original) can be
positioned.
[0068] One characterizing feature of this embodiment is the first
diffractive optical element 315 arranged at a distance upstream of
the mirror arrangement 320 between the light source 311 and the
mirror arrangement 320. The diffractive optical element 315 is
designed as an optical array generator for concentrating radiation
incident on the diffractive optical element onto the individual
mirrors 321 of the mirror arrangement, whereby potential light loss
caused by illuminating gaps between the usable mirror surfaces of
the individual mirrors 321 can be avoided. The first diffractive
optical element 315 is designed as a diffractive fan-out element
creating, from a single parallel light beam impinging on the
diffractive optical element, a number of focused light beams 317
corresponding to the number of individual mirrors 321 and directed
such that the light beams are concentrated in the optically active
area of the individual mirrors 321 (see inset FIG. 5(a)). A fan-out
element in the form of "Damann grating" creating a multitude of
diffraction orders positioned at the individual mirrors 321 is used
for this purpose. Although a telescope lens array such as described
in connection with FIG. 1 may be used instead of the diffractive
optical element 315, a diffractive optical element may be
preferable due to the simpler construction as compared to a
telescope lens array. Also installation space maybe conserved.
[0069] Using one or more diffractive optical element for
concentrating light on individual elements of an optical modulation
device having an array structure of individual elements that can be
driven individually to change the angle or angular spectrum of the
radiation incident on the individual elements can also be used in
connection with other optical modulation devices, such as an
electro-optical element having a large number of controllable,
reflective diffraction gratings, such as described in connection
with FIG. 4 or with an array of acoustic-optical elements.
[0070] The light distribution device 325 is specifically adapted
for use in connection with a fly's eye integrator 380 and allows
illumination of the pupil shaping surface 331 at the entrance of
the fly's eye integrator 380 such that individual optical channels
of the fly's eye condenser are either substantially completely
irradiated or are substantially non-irradiated, whereby a high
uniformity of light mixing can be obtained with a minimum of light
loss. This effect is obtained by the combined action of the
controllable mirror arrangement 320 and the second diffractive
optical element 390 arranged downstream thereof between the mirror
arrangement and the pupil-shaping surface 331 as follows. The
individual light beams 317 reflected by the individual mirrors 321
of the mirror arrangement 320 have specific angles with respect to
the optical axis depending on the orientation of the individual
mirrors 321. These angles are designated as "offset-angles" in the
following. Each offset-angle corresponds to a defined position in
the optical far field of the mirror arrangement, i.e. in the
pupil-shaping surface 331. The second diffractive optical element
390 is designed as a computer generated hologram (CGH) such that it
creates, at each location of the diffractive optical element
illuminated by a light beam 317, a defined angular distribution 391
around the light beam 317, wherein this defined angular
distribution corresponds, in the pupil shaping surface 331, to the
size and shape of a single lens element 383 of the entry side
raster arrangement 383 of the fly's eye condenser. In other words:
in this embodiment the angular distribution 391 created by the
second diffractive optical element 390 corresponds to a rectangular
illumination area in the pupil shaping surface 331. In the combined
action of the mirror arrangement 320 and the second diffractive
optical element, the position of an illuminated area corresponding
to a single light beam 317 in the pupil-shaping surface 331 can be
controlled by tilting the individual mirrors 321 to the desired
offset-angles, and the shape and size of the illuminated areas is
essentially formed by the diffractive optical element 390.
[0071] Since the tilting orientation of the individual mirrors can
be individually controlled by the control device 322, individual
optical channels of the fly's eye condenser 380 can be addressed
selectively such that an optical channel is either completely
illuminated or completely left out from the illumination. In order
to demonstrate this capability FIG. 6 shows schematically a view
along the optical axis 312 on the entry side of the fly's eye
integrator 380, where the rectangular lenses 383 of the first
raster arrangement 381 can be seen. In FIG. 6, one particular
optical channel 383i (i.e. one selected field honeycomb 383i) is
illuminated (dark area) whereas all other optical channels 383ni
are not illuminated. FIG. 7 shows an example where an annular
setting is obtained by controlling the tilting angles of the
individual mirrors 321 such that an essentially ring-shaped
illuminated area (dark areas) is created on the entry surface of
the fly's eye condenser. Again, all rectangular cylindrical lenses
383 of the first raster arrangement 381 are either completely
illuminated (dark areas 383i) or completely non-illuminated of
(bright rectangles 383ni). FIG. 8 shows a corresponding example for
a conventional setting where the illuminated area on the entry side
of the fly's eye condenser 380 (in the pupil-shaping surface 331)
is essentially circular. Again, the macroscopically circular shape
of the illuminated area is divided into small rectangles
corresponding to single optical channels of the fly's eye
condenser, where a number of optical channels 383i centred around
the optical axis are completely illuminated whereas optical
channels 383ni lying outside the desired area are completely
non-illuminated.
[0072] The angular distribution created by the second diffractive
optical element 390 is adapted to the shape of the microlenses
constituting the field honeycombs 383. This shape, in turn, is a
miniature shape of the rectangular shape of the illumination field
351. In illumination systems used for scanner-systems this shape
has a high width-to-height aspect ratio to obtain a slit shaped
illumination field. A typical aspect ratio between width and height
of a rectangular illumination field may be in the range between 2:1
and 8:1, for example. On the other hand, it may be advantageous in
some illumination settings (for example conventional settings or
annular settings) to obtain a two-dimensional intensity
distribution in the pupil-shaping surface 381 which is essentially
symmetric around the optical axis. In these cases it may be
preferable to use second diffractive optical elements 390 having an
effect function that allows the shaping of the exiting light beam
391 such that the illuminated area on the entry side of the fly's
eye condenser 380 includes more than one "honeycombs" for each
individual light beam 317. FIG. 9 shows an example where a second
diffractive optical element 390 is designed such that a light beam
originating from one individual mirror 321 is formed such that the
light covers a block or group of six adjacent rectangular lenses
383 of the first raster 381i to obtain an almost square shaped
illumination area with an aspect ratio close to unity.
[0073] FIG. 10 shows schematically a section of another embodiment
of an illumination system including a mirror arrangement 420
serving as an optical modulation device and a second diffractive
optical element 490 downstream of the mirror arrangement. The
construction upstream and downstream of the section depicted in
FIG. 10 may be constructed similarly or identically to the
illumination system shown in FIG. 5. This embodiment is adapted for
linearly polarized light provided for e.g. by a laser. In
contra-distinction to the embodiments shown above, the reflective
light modulator 420 comprising the individual mirror elements 421
is arranged generally perpendicular to the optical axis 412. The
optical axis is folded at a polarization-selective splitter surface
450 inclined at 45.degree. to the optical axis and arranged
geometrically between the mirror arrangement 420 and the second
diffractive optical element 490, which is oriented parallel to the
mirror arrangement. An optical retardation device formed as a
.lamda./4-plate 460 is arranged immediately ahead of the mirror
arrangement 420 between the mirror arrangement and the polarizer
450. The polarizer 450 may be a thin-film polarizer. The
polarization beam splitter surface may be arranged on a thin
transparent plate or within a block of transparent material.
[0074] The incoming laser beam is polarized such that the field
vector of the electrical field oscillates perpendicular to the
incidence plane on the planar beam splitter surface 450 (s
polarization). The polarizing layer structure is designed such that
it essentially reflects light with s-polarization and substantially
transmits light with p-polarization (where the electrical field
vector oscillates parallel to the incident plane). S-polarized
beams 417 impinging on the beam splitter 450 are reflected towards
the mirror arrangement 420. The linear polarization is transformed
to circular polarization upon transiting the quarterwave-plate 460
such that circularly polarized light is reflected by the mirrors
421. The reflected beams having a desired off-set angle with
respect to the optical axis transit the quarterwave-plate 460 which
transforms circular polarization into p-polarization. The
p-polarized beams 417 are then transmitted through the polarizer
450 and are then incident on the second diffractive optical element
490 which introduces an angular spectrum adapted to the size and
shape of the optical channels of the fly's eye integrator. In this
arrangement all individual laser beams 417 have essentially the
same optical path length between the light source and the second
diffractive element 490, which is arranged in the vicinity or at a
field plane of the illumination system. Therefore, this plane is
the same for all beams with respect to the optical imaging
occurring in the illumination system. Similar arrangements with
polarizing beam splitter may also be used in some or all other
embodiments described above.
[0075] In the embodiments of FIGS. 5 and 10 the mirror arrangement
320, 420 can be regarded as a first diffuser element of the
illumination system, since a particular angular distribution of
light is created depending on the construction and setting of the
individual mirrors of the mirror arrangement. The second
diffractive optical element 390, 490 can be considered as a second
diffuser element since a particular angular distribution is created
by this element. By arranging the second diffractive optical
element downstream of the mirror arrangement, the angular
distribution created by the first diffuser element 320, 420 is
optically folded with the angular distribution created by the
second diffuser element 390, 490 in the optical far field (in the
pupil-shaping plane). The first diffuser element 320, 420 is
illuminated with an essentially collimated laser beam and is
positioned in the vicinity of a field plane of the illumination
system which may be the first field plane of the illumination
system. The second diffuser element 390, 490 is positioned between
this field plane and a pupil plane of the illumination system being
Fourier-conjugated to the field plane where the first diffuser is
positioned. It is a particular feature that the first diffuser
element is a dynamical element where the effect function can be
controlled dynamically by the action of a control device.
* * * * *