U.S. patent application number 11/562472 was filed with the patent office on 2007-05-24 for method of forming optical images, apparatus for carrying out said method and process for manufacturing a device using said method.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS, N.V.. Invention is credited to Antonius Johannes Maria NELLISSEN.
Application Number | 20070115555 11/562472 |
Document ID | / |
Family ID | 29225669 |
Filed Date | 2007-05-24 |
United States Patent
Application |
20070115555 |
Kind Code |
A1 |
NELLISSEN; Antonius Johannes
Maria |
May 24, 2007 |
METHOD OF FORMING OPTICAL IMAGES, APPARATUS FOR CARRYING OUT SAID
METHOD AND PROCESS FOR MANUFACTURING A DEVICE USING SAID METHOD
Abstract
An optical image is formed in a radiation-sensitive layer (5),
by a number of sub-illuminations, in each of which an array of
light valves (21-25) and a corresponding array of converging
elements (91-95) are used to form a pattern of spots (111-115) in
the resist layer in accordance with a sub-image pattern. Between
the sub-illuminations, the resist layer is displaced relative to
the arrays. Bright and well-defined spots are obtained by using
refractive lenses (43) as converging elements. The
radiation-sensitive layer may be a resist layer on top of a
substrate wherein a device is to be configured by means of
lithography or an electrostatic charged layer used in a
printer.
Inventors: |
NELLISSEN; Antonius Johannes
Maria; (Eindhoven, NL) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS,
N.V.
GROENEWOUDSEWEG 1
EINDHOVEN
NL
5621 BA
|
Family ID: |
29225669 |
Appl. No.: |
11/562472 |
Filed: |
November 22, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10510785 |
Oct 12, 2004 |
7154674 |
|
|
11562472 |
Nov 22, 2006 |
|
|
|
Current U.S.
Class: |
359/619 |
Current CPC
Class: |
B41J 2/465 20130101;
G03F 7/70291 20130101; G03F 7/70358 20130101; G03F 7/70275
20130101 |
Class at
Publication: |
359/619 |
International
Class: |
G02B 27/10 20060101
G02B027/10 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 14, 2003 |
IB |
PCT/IB03/01372 |
Apr 15, 2002 |
EP |
EP02076459.3 |
Claims
1-18. (canceled)
19. A method of forming an optical image in a radiation-sensitive
layer, the method comprising the steps of: providing a radiation
source; providing a radiation-sensitive layer; positioning a
two-dimensional array of individually controlled light valves
between the radiation source and the radiation-sensitive layer;
positioning a two-dimensional array of radiation-converging
elements between the array of light valves and the
radiation-sensitive layer, wherein each of plurality of the
radiation-converging elements corresponds to a different one of the
light valves and serves to converge radiation from the
corresponding light valve in a spot area in the radiation-sensitive
layer; simultaneously writing image portions in areas of the
radiation-sensitive layer by scanning said layer areas, and the
associated light valves/converging element pairs, relative to each
other and switching each light valve between an "on" and an "off"
state in dependence upon the image portion to be written by the
light valve; wherein at least a portion of the converging elements
are refractive lenses, and the array of radiation-converging
elements and the array of light valves are used for forming a
matrix array of spots, said matrix having a pitch which is
substantially larger than the spot size; and wherein the intensity
of a spot at the border of an image feature is adapted to the
distance between the feature border and a neighboring feature.
20. The method of claim 19 comprising part of a lithographic
process for producing a device in a substrate, wherein the
radiation-sensitive layer is a resist layer provided on the
substrate, and the image pattern corresponds to a pattern of
features of the device to be produced; and the optical image is
divided into sub-images each belonging to a different level of the
device to be produced, and during formation of the different
sub-images, the resist layer surface is set at different distances
from the array of refractive lenses.
21. An apparatus comprising: a radiation source; positioning means
for positioning a radiation-sensitive layer relative to a radiation
beam from the source; a two-dimensional array of individually
controllable light valves arranged between the source and the
location for the radiation-sensitive layer; and an imaging element
comprising an array of radiation-converging elements arranged
between the array of light valves and the location of the
radiation-sensitive layer, each converging element corresponding to
a different one of the light valves and serving to converge
radiation from the corresponding light valve in a spot area in the
resist layer, wherein the converging elements are refractive
lenses, and wherein a projection lens is arranged between the array
of light valves and the array of refractive lenses.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is a divisional of application Ser. No. 10/510,785,
filed Oct. 12, 2004, which issued Dec. 26, 2006, as U.S. Pat. No.
7,154,674.
[0002] The invention relates to a method of forming an optical
image in a radiation-sensitive layer, the method comprising the
steps of:
[0003] providing a radiation source;
[0004] providing a radiation-sensitive layer;
[0005] positioning a two-dimensional array of individually
controlled light valves between the radiation source and the
radiation-sensitive layer;
[0006] positioning a two-dimensional array of radiation-converging
elements between the array of light valves and the
radiation-sensitive layer, such that each converging element
corresponds to a different one of the light valves and serves to
converge radiation from the corresponding light valve in a spot
area in the radiation-sensitive layer;
[0007] simultaneously writing image portions in radiation-sensitive
layer areas by scanning said layer, on the one hand, and the
associated light valves/converging element pairs, on the other
hand, relative to each other and switching each light valve between
an on and an off state in dependence upon the image portion to be
written by the light valve.
[0008] The invention also relates to an apparatus for carrying out
this method and to a method of manufacturing a device using this
method.
[0009] An array of light valves, or optical shutters, is understood
to mean an array of controllable elements, which can be switched
between two states. In one of the states radiation incident on such
an element is blocked and in the other state the incident radiation
is transmitted or reflected to follow a path that is prescribed in
the apparatus of which the array forms part.
[0010] Such an array may be a transmissive or reflective liquid
crystal display (LCD)-or a digital mirror device (DMD). The
radiation-sensitive layer is, for example, a resist layer used in
optical lithography, or an electrostatic charged layer used in a
printing apparatus.
[0011] This method and apparatus may be used, inter alia, in the
manufacture of devices such as liquid crystal display (LCD) panels,
customized-ICs (integrated circuits) and PCBs (printed circuit
board). Currently, proximity printing is used in the manufacture of
such devices. Proximity printing is a fast and cheap method of
forming an image in a radiation-sensitive layer on a substrate of
the device, which image comprises features corresponding to device
features to be configured in a layer of the substrate. Use is made
of a large photomask that is arranged at a short distance, called
the proximity gap, from the substrate, and the substrate is
illuminated via the photomask by, for example, ultraviolet (UV)
radiation. An important advantage of the method is the large image
field, so that large device patterns can be imaged in one image
step. The pattern of a conventional photomask for proximity
printing is a true, one-to-one copy, of the image required on the
substrate, i.e. each picture element (pixel) of this image is
identical to the corresponding pixel in the mask pattern.
[0012] Proximity printing has a limited resolution, i.e. the
ability to reproduce the points, lines etc., in general the
features, of the mask pattern as separate entities in the sensitive
layer on the substrate. This is due to the diffractive effects,
which occur when the dimensions of the features decrease with
respect to the wavelength of the radiation used for imaging. For
example, for a wavelength in the near UV range and a proximity gap
width of 100 .mu.m, the resolution is 10 .mu.m, which means that
pattern features at a mutual distance of 10 .mu.m can be imaged as
separate elements.
[0013] To increase the resolution in optical lithography, a real
projection apparatus is used, i.e. an apparatus having a real
projection system like a lens projection system or a mirror
projection system. Examples of such apparatus are wafer steppers or
wafer step- and scanners. In a wafer stepper, a whole mask pattern,
for example, an IC pattern is imaged in one go by a projection lens
system on a first IC area of the substrate. Then the mask and
substrate are moved (stepped) relative to each other until a second
IC area is positioned below the projection lens. The mask pattern
is then imaged on the second IC area. These steps are repeated
until all IC areas of the substrate are provided with an image of
the mask pattern. This is a time-consuming process, due to the
sub-steps of moving, aligning and illumination. In a
step-and-scanner, only a small portion of the mask pattern is
illuminated at once. During illumination, the mask and the
substrate are synchronously moved with respect to the illumination
beam until the whole mask pattern has been illuminated and a
complete image of this pattern has been formed on an IC area of the
substrate. Then the mask and substrate are moved relative to each
other until the next IC area is positioned under the projection
lens and the mask pattern is again scan-illuminated, so that a
complete image of the mask pattern is formed on the next IC area
These steps are repeated until all IC areas of the substrate are
provided with a complete image of the mask pattern. The
step-and-scanning process is even more time-consuming than the
stepping process.
[0014] If a 1:1 stepper, i.e. a stepper with a magnification of
one, is used to print a LCD pattern, a resolution of 3 .mu.m can be
obtained, however, at the expense of much time for imaging.
Moreover, if the pattern is large and has to be divided into
sub-patterns, which are imaged separately, stitching problems may
occur, which means that neighboring sub-fields do not fit exactly
together.
[0015] The manufacture of a photomask is a time-consuming and
cumbersome process, which renders such a mask expensive. If much
re-design of a photomask is necessary or in case customer-specific
devices, i.e. a relatively small number of the same device, have to
be manufactured, the lithographic manufacturing method using a
photomask is an expensive method.
[0016] The paper: "Lithographic patterning and confocal imaging
with zone plates" of D. Gil et al in: J. Vac. Sci. Technology B
18(6), November/December 2000, pages 2881-2885, describes a
lithographic method wherein, instead of a photomask, a combination
of a DMD array and an array of zone plates is used. If the array of
zone plates, also called Fresnel lenses, is illuminated, it
produces an array of radiation spots, in the experiment described
in the paper: an array of 3.times.3 X-ray spots, on a substrate.
The spot size is approximately equal to the minimum feature size,
i.e. the outer zone width, of the zone plate. The radiation to each
zone plate is separately turned on and off by the micromechanic
means of the DMD device, and arbitrary patterns can be written by
raster scanning the substrate through a zone plate unit cell. In
this way, the advantages of maskless lithography are combined with
a high throughput due to parallel writing with an array of
spots.
[0017] Zone plates, or other similar elements which may be used
instead, are diffraction elements, i.e. they split an incident
radiation beam into sub-beams of different diffraction orders. The
geometry of the diffraction elements is designed in such a way that
radiation portions of the different diffraction orders
constructively interfere in a small spot area and destructively
interfere outside this spot area, so that theoretically a small
radiation spot is formed. In practice, however, constructive
interference also occurs outside said spot area so that the spot is
smeared out. In other words, the diffraction element does not
provide a sharp focus. Moreover, diffraction elements are designed
for a specific wavelength, and if the illumination beam comprises a
wavelength component, which is different from said specific
wavelength image aberrations, chromatic aberrations will occur.
This means that a broad wavelength source, such as a mercury arc
lamp, which is conventionally used in this kind of lithographic
apparatus, can no longer be used. Also the wavelength of radiation
emitted by a laser source may show small variations, which may
affect the performance of a lithographic imaging apparatus using
diffraction elements, because of the small size of the spots to be
formed.
[0018] It is an object of the present invention to solve the
above-mentioned problems and to provide an accurate and
radiation-efficient lithographic imaging method, which may employ
different kinds of radiation sources. This method is characterized
in that use is made of converging elements in the form of
refractive lenses, and in that the two arrays are used for forming
a matrix array of spots having a pitch, which is substantially
larger than the spot size.
[0019] The performance of refractive lenses is considerably less
sensitive to wavelength variations, so that chromatic aberrations
can be avoided. These lenses have a sharper focus than diffraction
elements, because they show no order splitting.
[0020] A matrix array of spots is understood to mean a
two-dimensional array having a comparable, albeit not necessarily
the same, number of spots in two, mutually perpendicular
directions. The matrix pitch may be of the order of a hundred times
the spot size.
[0021] It is remarked that U.S. Pat. No. 6,288,830 discloses an
optical image-forming method and device wherein a digital mirror
device and an array of microlenses are used. According to the known
method, the image is written line-by-line, and in order to obtain a
high pixel density, each image line is written by means of a number
n, for example six, mirror rows. The mirrors of each row are
shifted over a distance p/n with respect to the light valves of the
other lines, wherein p is the pitch of the mirrors in one row. In
the known method, a single image pixel is written by means of
corresponding pixels of all of the n rows, which row pixels are
shifted relative to each other in the row direction. In the method
of the present invention, each light valve is used to successively
write a large number of pixels, i.e. all pixels of, for example a
radiation-sensitive layer area having dimensions corresponding to
the matrix pitch.
[0022] A first embodiment of the method is characterized in that
said scanning is such that each spot scans its own associated layer
area, which area has dimensions corresponding to the matrix
pitch.
[0023] According to this method, each light valve is used to write
only one layer area, hereinafter referred to as light valve area,
by two-dimensionally scanning the spot from this light valve across
this associated light valve area. After a spot has scanned a line
within the light valve area, this spot and the area are moved
relative to each other in a direction perpendicular to the scanning
direction, where after a subsequent line within this area is
scanned.
[0024] A second embodiment of the method is characterized in that
the matrix of spots and the radiation-sensitive layer are scanned
relative to each other in a direction at a small angle to the
direction of the lines of spots in the matrix, and in that the
scanning is carried out over a length, which is substantially
larger than the matrix pitch.
[0025] According to this embodiment, all spots of all lines are
used to scan different lines, and a layer area having a width
corresponding to the total number of spots times the size of a spot
and an arbitrary length can be scanned by means of one scanning
action, without movement in a direction perpendicular to the
scanning direction.
[0026] The method of the present invention may be further
characterized in that, between successive sub-illuminations, the
radiation-sensitive layer and the arrays are displaced relative to
each other over a distance which is at most equal to the size of
the spots formed in the radiation-sensitive layer.
[0027] In this way, image, i.e. pattern, features can be written
with a constant intensity across the whole feature. The spots may
have a circular, square, diamond or rectangular shape, dependent on
the design of a beam-shaping aperture present in the apparatus. The
size of the spot is understood to mean the size of the largest
dimension within this spot.
[0028] If features of the image to be written are very close to
each other, these features may broaden and blend with each other,
which phenomenon is known as the proximity effect. An embodiment of
the method, which prevents proximity effects from occurring, is
characterized in that the intensity of a spot at the border of an
image feature is adapted to the distance between this feature
border and a neighboring feature.
[0029] The method can be used in several applications. A first
application is in the field of optical lithography. An embodiment
of the method, which is suitable to form part of a lithographic
process for producing a device in a substrate, is characterized in
that the radiation-sensitive layer is a resist layer provided on
the substrate, and in that image pattern corresponds to the pattern
of features of the device to be produced.
[0030] This embodiment of the method may be further characterized
in that the image is divided into sub-images each belonging to a
different level of the device to be produced, and in that, during
formation of the different sub-images, the resist layer surface is
set at different distances from the array of refractive lenses.
[0031] This embodiment of the method allows imaging on different
planes of the substrate and thus production of multiple level
devices.
[0032] A second application is in the field of printing. An
embodiment of the method, which is suitable to form part of a
process for printing a sheet of paper, is characterized in that the
radiation-sensitive layer is a layer of electrostatically charged
material.
[0033] The method may be further characterized in that the array of
light valves is positioned to directly face the array of refractive
lenses.
[0034] The two arrays are positioned close to each other, without
imaging means being arranged between them, so that the method can
be performed by compact means. If the array of light valves is an
array of LCD cells, which modulate the polarization of incident
radiation, a polarization analyzer is arranged between the LCD and
the array of diffraction cells.
[0035] Alternatively, the method may be characterized in that the
array of light valves is imaged on the array of diffraction
cells.
[0036] Imaging one array on the other by a projection lens provides
advantages with respect to stability, thermal effects, and
crosstalk.
[0037] The invention also relates to an apparatus for carrying out
the method described above. This apparatus comprises:
[0038] a radiation source;
[0039] positioning means for positioning a radiation-sensitive
layer relative to the radiation beam;
[0040] a two-dimensional array of individually controllable light
valves arranged between the source and the location for the
radiation-sensitive layer; and
[0041] an imaging element comprising an array of
radiation-converging elements arranged between the array of light
valves and the location of the radiation-sensitive layer, such that
each converging element corresponds to a different one of the light
valves and serves to converge radiation from the corresponding
light valve in a spot area in the resist layer.
[0042] This apparatus is characterized in that the converging
elements are refractive lenses.
[0043] With this apparatus, arbitrary image patterns can be written
by scanning the radiation-sensitive layer with a number of sharp
spots simultaneously, wherein efficient use is made of the
available radiation.
[0044] A first embodiment of the apparatus, suitable for forming an
image in a resist layer on a substrate, which image comprises
features corresponding to device features to be configured in said
substrate, is characterized in that the radiation-sensitive layer
is a resist layer, and in that the positioning means is a substrate
holder carried by a substrate stage.
[0045] This embodiment may be adapted to allow sub-images to be
formed in different planes of the substrate and is then
characterized in that it comprises means for adapting the distance
between the resist layer surface and the array of refractive lenses
when forming different sub-images.
[0046] A second embodiment of the apparatus is suitable for
printing data on a sheet of paper, is characterized in that the
radiation-sensitive layer is a layer of electrostatically charged
radiation-sensitive material, and in that the positioning means are
means for moving said layer relative to the array of light valves
and the array of refractive lenses and for sustaining said layer at
the location of the image field of these arrays.
[0047] The term data is understood to encompass all visual
information that can be printed on paper, such as text, graphics,
photos, etc.
[0048] The apparatus may be further characterized in that the
imaging element is arranged behind the array of light valves
without intervening imaging means.
[0049] The gap, for example an air gap, may be very small so that
this embodiment has a sandwich shape. If the array of light valves
is a LCD, a polarization analyzer is arranged between the array of
light valves and the imaging element.
[0050] An embodiment of the apparatus, which is alternative to the
sandwich embodiment, is characterized in that a projection lens is
arranged between the array of light valves and the array of
refractive lenses.
[0051] The projection lens images each light valve on its
associated refractive lens in the imaging element so that
crosstalk, optical aberrations and temperature effects are
eliminated. Moreover, the substrate of the imaging element may be
relatively thick so that the apparatus is more stable.
[0052] The invention also relates to a method of manufacturing a
device in at least one process layer of a substrate, the method
comprising the steps of:
[0053] forming an image, comprising features corresponding to
device features to be configured in the process layer, in a resist
layer provided on the process layer; and
[0054] removing material from, or adding material to, areas of the
process layer which areas are delineated by the image formed in the
resist layer.
[0055] This method is characterized in that the image is formed by
means of the method as described above.
[0056] Devices, which can be manufactured by means of this method
and apparatus, are liquid crystal display devices,
customer-specific ICs, electronic modules, printed circuit boards
and MOEMS (integrated Micro-Optical-Electrical-Mechanical System),
etc. An example of such a device is an integrated optical
telecommunication device comprising a diode laser and/or detector,
a light guide, an optical switch and possibly a lens between the
light guide and the diode laser, or the detector.
[0057] These and other aspects of the invention are apparent from
and will be elucidated, by way of non-limitative example, with
reference to the embodiments described hereinafter.
[0058] In the drawings:
[0059] FIG. 1 shows schematically a conventional proximity printing
apparatus;
[0060] FIG. 2 shows an embodiment of an imaging apparatus according
to the invention;
[0061] FIG. 3a is a top view of a portion of a refractive lens
array used in this embodiment;
[0062] FIG. 3b is a top view of a portion of a light valve array
used in this embodiment;
[0063] FIG. 3c is a top view of a portion of the array of spots
formed in the resist layer by means of this embodiment;
[0064] FIG. 4 shows a first method of producing the lens array;
[0065] FIG. 5 shows an embodiment of an apparatus for producing a
mould, which is used for manufacturing the lens array by
replication;
[0066] FIGS. 6a-6c show, in a cross-sectional view, different
moments of the printing process;
[0067] FIGS. 7a-7c show, in a top view, different moments of the
printing process;
[0068] FIG. 8 shows the principle of skew scanning of an array of
spots and a resist layer relative to each other;
[0069] FIGS. 9a-9c show an array of spots formed with different
widths of the gap between the lens array and the resist layer;
[0070] FIG. 10 shows an embodiment of the imaging apparatus
including a projection lens between the array of light valves and
the lens array, and
[0071] FIG. 11 shows an embodiment of a printing apparatus wherein
the invention can be used.
[0072] FIG. 1 shows, very schematically, a conventional proximity
printing apparatus for the manufacture of, for example a LCD
device. This apparatus comprises a substrate holder 1 for carrying
a substrate 3 on which the device is to be manufactured. The
substrate is coated with a radiation-sensitive, or resist, layer 5
in which an image, having features corresponding to the device
features, is to be formed. The image information is contained in a
mask 8 arranged in a mask holder 7. The mask comprises a
transparent substrate 9, the lower surface of which is provided
with a pattern 10 of transparent and non-transparent strips and
areas, which represent the image information. A small air gap 11
having a gap width w of the order of 100 .mu.m separates the
pattern 10 from the resist layer 5. The apparatus further comprises
a radiation source 12. This source may comprise a lamp 13, for
example, a mercury arc lamp, and a reflector 15. This reflector
reflects lamp radiation, which is emitted in backward and sideways
directions towards the mask. The reflector may be a parabolic
reflector and the lamp may be positioned in a focal point of the
reflector, so that the radiation beam 17 from the radiation source
is substantially a collimated beam. Other or additional optical
elements, like one or more lenses, may be arranged in the radiation
source to ensure that the beam 17 is substantially collimated. This
beam is rather broad and illuminates the whole mask pattern 10
which may have dimensions from 7.5.times.7.5 cm.sup.2 to
40.times.40 cm.sup.2. For example, illumination step has a duration
of the order of 10 seconds. After the mask pattern has been imaged
in the resist layer, this is processed in the well-known way, i.e.
the layer is developed and etched, so that the optical image is
transferred in a surface structure of the substrate layer being
processed.
[0073] The apparatus of FIG. 1 has a relatively simple construction
and is very suitable for imaging in one go a large area mask
pattern in the resist layer. However, the photomask is an expensive
component and the price of a device manufactured by means of such a
mask can be kept reasonably low only if a large number of the same
device is manufactured. Mask making is a specialized technology,
which is in the hands of a relatively small number of mask
manufacturing firms. The time a device manufacturer needs for
developing and manufacturing a new device or for modifying an
existing device is strongly dependent on delivery times set by the
mask manufacturer. Especially in the development phase of a device,
when redesigns of the mask are often needed, the mask is a
capability-limiting element. This is also the case for low-volume,
customer-specific devices.
[0074] Direct writing of a pattern in the resist layer, for example
by an electron beam writer or a laser beam writer, could provide
the required flexibility, but is not a real alternative because
this process takes too much time.
[0075] FIG. 2 shows the principle of a maskless method and
apparatus by means of which an arbitrary and easily changeable
image pattern can be formed in a resist layer within a reasonable
time. FIG. 2 shows very schematically and in a vertical
cross-section a small portion of the means which are used for
performing the method and form part of the apparatus. The apparatus
comprises a substrate holder 1 for accommodating a substrate, which
is coated with a resist layer 5. Reference numeral 20 denotes a
light valve device, for example a liquid crystal display (LCD),
which is currently used in a display apparatus for displaying
information, either in direct-view or in projection. Device 20
comprises a large number of light valves, also called pixels
(picture elements) of which only a few are shown in FIG. 2 and
denoted by reference numerals 21 to 25. The light valve device is
controlled by a computer configuration 30 (not on scale) wherein
the pattern, which is to be configured in a substrate layer, is
introduced in software. The computer thus determines, at any moment
of the writing process and for every light valve, whether it is
closed, i.e. blocks the portion of the illuminating beam 17
incident on this light valve, or is open, i.e. transmits this
portion to the resist layer. An imaging element 40 is arranged
between the array of light valves 20 and the resist layer 5. This
element comprises a transparent substrate 41 and an array 42 of
radiation-converging elements 43. The number of these elements
corresponds to the number of light valves, and the array 42 is
aligned with the array of light valves so that each converging
element belongs to a different one of the light valves.
[0076] As the radiation source, the substrate holder and the mask
holder are less relevant for understanding the new method, these
elements will not be described in detail.
[0077] According to the invention, the converging elements 43 are
refractive lenses. Such lenses allow focusing of radiation from
corresponding light valves in spots, which are smaller than those
obtained with diffraction lenses. Moreover the optical performance
of these lenses is substantially less dependent on the wavelength
of the radiation than that of a diffraction lens element.
[0078] FIGS. 3a and 3b are top views of a portion of the array 42
of refractive microlenses 43 and the corresponding portion of the
array 20 of light valves 21 to 25 and further light valves 27. The
array 42 includes a number of cells 45 each comprising a central
transmission portion 43, shaped as a microlens, and a surrounding
border portion 47. The border portion of a cell blends with the
border portion of the neighboring cells, thereby constituting a
black matrix. Such a black matrix reduces crosstalk between the
beam portions passing through the individual lenses. The border
portions of all cells may be constituted by a radiation-absorbing
or reflecting layer. The size of the spots formed in the resist
layer and the depth of focus of the beam portions forming these
spots is determined by the power of the lenses 43. By means of a
spot-shaping aperture (not shown) arranged in the radiation source
unit or illumination system supplying the illumination beam 17, the
shape of the produced spots can be adapted to a required
application. These spots may be, for example, round, rectangular,
square or diamond-shaped. The geometric structure of the lens array
42 of the imaging element 40 is adapted to the geometric structure
of the light valve array. The imaging element 40 is arranged at a
distance from the array, such that as much as possible of the
radiation from a light valve passes through an associated lens 43
and is concentrated in the spot produced by this lens and a minimum
amount of background radiation occurs.
[0079] FIG. 3c shows an array 50 of spots 52 obtained by means of
the lens array of FIG. 3a if the corresponding portion of the light
valve array is illuminated with radiation having a wavelength of,
for example 365 nm and all light valves of the portion are open.
The distance 44 between the diffraction structure 42 and the resist
layer 5 is, for example, 250 .mu.m. Spots 52 have a size of the
order of, for example, 2 mu.m.sup.2.
[0080] The microlenses 43 are usually spherical lenses, i.e. their
curved surface is a portion of a perfect sphere. If necessary,
aspherical lenses may be used. An aspherical lens is understood to
mean a lens, whose basic surface is spherical, but whose actual
surface deviates from spherical in order to correct for spherical
aberrations which a spherical lens may produce.
[0081] The spots 52 shown in FIG. 3c are rectangular spots. As
mentioned hereinbefore, these spots may also be round or square or
may have any other shape, which is deemed appropriate.
[0082] The array of refractive microlenses, i.e. the imaging
element 40, may be manufactured by means of techniques known per
se. A first, lithographic, technique is shown in FIGS. 4a-4e. This
technique starts from a transparent, for example a quartz,
substrate 60, which is coated with a, for example polymer, resist
layer 61, as shown in FIG. 4a By means of radiation 63, an exposure
pattern corresponding to the pattern of microlenses is formed in
the resist layer 61, as illustrated in FIG. 4b. The exposure
pattern consists of a two-dimensional array of cylindrical exposed
resist portions 65. The exposure may be performed, for example, by
optical radiation of different kinds or by a charged particle
(electron or ion) beam. After the resist has been developed and the
non-exposed portions removed, a configuration of cylindrical resist
portions 65 on top of the substrate 60 remains, as shown in FIG.
4c. As a next step, this configuration is heated, for example to
200.degree. C. dependent on the kind of resist used, which causes
flowing of the resist material, such that the cylinders 65 are
reshaped to spherical segments 67, as shown in FIG. 4d. By reactive
ion etching, the pattern of resist segments 67 is then transferred
to a similar pattern of spherical segments 69 at the upper side of
the substrate, as shown in FIG. 4e. In this way, a lens plate 70 is
obtained, wherein the geometry of a microlens 69 and the difference
between the index of refraction of this lens and that of the
ambient medium determine its optical power. Finally, as shown in
FIG. 4f, the substrate surface areas between the microlenses are
coated with an absorbing or reflective layer 73, such as a chromium
layer.
[0083] By changing, during the etching process, the ratio of the
etching rate of the resist and of the substrate, i.e, changing the
selectivity of the etching process, this method allows production
of aspherical microlenses.
[0084] A second method of manufacturing a microlens plate for use
with the new method uses a replication-from-a-mould technique.
[0085] FIG. 5 schematically shows an apparatus that is preferably
used for manufacturing a mould suitable for this replication
technique. The apparatus comprises a workpiece holder 76 on which a
mould 75 to be manufactured can be placed and fastened by clamping
means (not shown). The holder with the mould can be accurately
moved and positioned by means of X and Y-slides (not shown). The
apparatus further comprises a tool holder 77, which is fastened to
a Z-slide 78. The holder 77 is provided with a circular-cylindrical
stepped chamber 79 comprising a first chamber portion 80 and a
second chamber portion 81, which has a smaller diameter than the
first chamber portion 80. The second chamber portion 81 constitutes
a straight guide for a round shaft 82 of a die holder 83, which is
guided with sliding possibility parallel to the Z-direction
relative to the second chamber portion 81 and is provided with an
end disc 84. The end disc bears on a stop 85, formed by a shoulder
of the stepped chamber 79, under the influence of a previously
defined pre-tensioning force of a mechanical helical spring 85
arranged in the first chamber portion. As FIG. 5 shows, a die 86 is
fastened to the holder 83, which die has a smooth surface 87 and is
manufactured from, for example, a hard steel or sapphire. The tool
holder 77 and the die holder 83 are jointly displaceable relative
to the workpiece holder 76 in directions parallel to the
X-direction, Y-direction and Z-direction.
[0086] The mould 75 is manufactured from a ductile metal such as,
for example, copper, aluminum, zinc or an alloy comprising these
metals. The surface 88 of the mould 75 has been given a surface
smoothness of optical quality desired for the lens plate or imaging
element 40 to be manufactured. The shape of the surface 87 of the
die corresponds to that of one lens 43 of the lens plate 40.
[0087] FIG. 5 represents the situation after the die has been
pressed several times into the mould 75 and several imprints 89
have been made. To form the next imprint, the die 86 is displaced
to the next position through a displacement of the X-slide and/or
Y-slide, during which the holder 83 rests against the stop 85 of
the chamber 79. Then, the die is pressed into the mould 75 through
a displacement of the Z-slide 78. The end disc 84 of the die holder
83 then clears the stop 85, so that the die is imprinted in the
mould 75 with a force which corresponds to the predetermined
pre-tensioning force of the mechanical helical spring 85. It is
achieved through a favorable choice of the pre-tensioning force and
of the elasticity modulus of the helical spring 85 that the
increase of the pre-tensioning force of the spring upon clearance
of the end disc 84 is negligibly small. The force with which the
die 86 is imprinted in the mould 75 is thus substantially
independent of the position occupied by the Z-slide 78 and the tool
holder 77 parallel to the Z-direction during imprinting. No
stringent requirements need accordingly be imposed on the
positioning accuracy of the Z-slide 78.
[0088] While the die 86 is being imprinted in the mould 75, the
ductile metal of the mould present below the die surface 87 is
plastically deformed. The metal at this position assumes a shape
corresponding to that of the die surface 87, i.e. to that of a lens
43 of the lens plate 40. The complete mould shape is formed by
successively imprinting the die surface shape at all required
positions in the mould.
[0089] For more details about the method of and apparatus for
manufacturing the mould, reference is made to WO 96/07523, which
discloses said method and apparatus for other applications.
[0090] After the mould has been finished, it can be positioned in a
replication apparatus. The mould is covered with a transparent
material in a sufficiently viscous state, for example a liquid
polymer, such that it fills up the imprints 89. After the
transparent material has been hardened, for example by UV
radiation, whereby the surface profile of the mould is transferred
to transparent material, the plate of transparent material can be
removed from the mould and the lens plate is obtained.
[0091] By giving the die surface 87 an aspherical shape, a lens
plate with aspherical microlenses can be produced.
[0092] Instead of chromium, other non-transmission materials can be
used for the selective coating of the lens plate.
[0093] As shown in FIG. 3c, each spot 52 occupies only a small,
point-like portion of the resist layer area belonging to the light
valve, which determines whether this spot is present or not.
Hereinafter, the point-like resist areas will be called spot areas
and the resist area belonging to a light valve will be called valve
area. To obtain full features, i.e. lines and areas, of the image
pattern corresponding to the device features to be produced, the
substrate with the resist layer, on the one hand, and the two
arrays, on the other hand, should be displaced relative to each
other. In other words, each spot should be moved in its
corresponding valve area such that this area is fully scanned and
illuminated at prescribed, i.e. feature-determined, positions. Most
practically, this is realized by displacing the substrate stepwise
in a gridlike pattern. The displacement steps are of the order of
the size of the spots, for example of the order of 1 .mu.m or
smaller. A portion of the valve area belonging to a given spot,
which portion is destined for an image feature or part thereof, is
illuminated in flashes. For displacing the substrate holder in
steps of 1 .mu.m or smaller with the required accuracy, use can be
made of servocontrolled substrate stages, which are used in
lithographic projection apparatus and operate with an accuracy of
well below 1 .mu.m, for example of the order of 10 nm.
[0094] The illumination process of flashing and stepping is
illustrated in FIGS. 6a-6c, which show a small portion of the array
of light valves, the array of refractive lenses arid the resist
layer. In these Figures, the reference numeral 17 denotes the
illuminating beam incident on the light valves 21-25. Reference
numerals 101 to 105 denote the sub-beams passed by open light
valves and converged by the corresponding refractive lenses 91 to
95. FIG. 6a presents the situation after a first sub-illumination
has been made with all light valves open. A first set of spot areas
111 to 115, one spot area in each light valve area has been
illuminated. FIG. 6b presents the situation after the substrate has
made one step to the right and a second sub-illumination has been
made also with all light valves open so that a second set of spot
areas 121 to 125 has been illuminated. FIG. 6c presents the
situation after the substrate has made five steps and six
sub-illuminations have been made. During the fourth
sub-illumination light valves 23 and 25 were closed so that spot
areas 133 and 135 have not been illuminated. During the fifth
sub-illumination, the light valves 24 and 25 were closed so that
spot areas 144 and 145 have not been illuminated. All other spot
areas have been illuminated.
[0095] FIGS. 7a-7c are top views of the resist layer during
subsequent sub-illumination steps. In these Figures, the grey spot
areas have already been illuminated in preceding sub-illumination
steps and the blank spot areas, i.e. areas 151 to 154 and 156 to
160 in FIG. 7a, are being illuminated in the present illumination
step. The portion of the resist layer being illuminated comprises
two rows of five light valve areas. In the situation depicted in
FIG. 7a, a relatively large number of spot areas of the upper row
and a lower number of spot areas in the lower row have already been
illuminated. During a first sub illumination step four of the five
light valves belonging to the upper row of light valve areas are
open and the fifth, the most right one, is closed so that spot
areas 151 to 154 are momentarily illuminated and spot area 155 is
not. All of the five light valves belonging to the lower row of
light valve areas are open so that spot areas 156 to 160 are
momentarily illuminated. FIG. 7b shows the situation after the
substrate has made one step and a second sub-illumination is being
carried out. Again, four of the five light valves of the upper row
are open and the fifth light valve of this row is closed so that
spot areas 161 to 164 are momentarily illuminated and spot area 165
is not. All of the five light valves of the lower row are open so
that spot areas 166 to 170 are momentarily illuminated. FIG. 7c
shows the situation during the sixth sub-illumination, i.e. after
the substrate has made five steps. During the six sub-illumination,
the fifth light valve of the upper row was (is) closed. During the
fourth sub-illumination the third light valve of the upper row and
the third and fifth light valves of the lower row were closed so
that spot areas 181, 182 and 183 have not been illuminated. During
the fifth sub-illumination, the fourth and fifth light valves of
the lower row were closed so that spot areas 184 and 185 have not
been illuminated. During the sixth sub-illumination, all light
valves, except the fifth one of the upper row are open so that all
spot areas 191 to 200 are momentarily illuminated, except spot area
195.
[0096] FIGS. 6a-6c and 7a-7c show how the required image patterns
are produced in ten light valve areas simultaneously by the
successive steps of displacing the resist layer and opening and
closing the ten corresponding light valves. Opening and closing of
each light valve is controlled individually. As shown in the upper
right portion of FIG. 7a, scanning of a valve area 150 with a spot
52 can be performed serpentine-wise. A first line of the area is
scanned from left to right, a second line from right to left, a
third line from left to right again, etc.
[0097] Instead of the stepping mode, illustrated in FIGS. 6a-6c and
FIGS. 7a-7c, also a scanning mode can be used to produce the
required image patterns. In the scanning mode the resist layer, on
the one hand, and the arrays of light valves and refractive lenses,
on the other hand, are continuously moved with respect to each
other, and the light valves are flashed when they face a prescribed
position on the resist layer. The flash time, i.e. the open-time of
the light valve, should be smaller than the time during which the
relevant light valve faces said position.
[0098] In a practical embodiment of the proximity printing
apparatus shown in FIG. 2, the several parameters have the
following values:
[0099] 1 Illuminated field: 10 .times. 10 mm.sup.2; Radiation
source: Mercury-arc lamp; Intensity of the illumination beam: 20
mW/cm.sup.2; Beam collimating angle: 0.2 degrees; Transmission of
the light valves: 25%; Shutter speed of the light valves: 1 ms.;
Spot area in the resist layer: 1 .times. 1 .mu.m.sup.2;
Spot-to-spot distance: 100 .mu.m; Number of light valves;
1,000,000; Intensity of the spots: 50 W/cm.sup.2; Exposure dose;
100 mJ/cm.sup.2; Total exposure time: 20 sec.; Gap width: 250
.mu.m; Scan speed: 0.5 mm/sec.
[0100] The exposure dose is the amount of illumination radiation
energy deposited in a spot area of the resist. The intensity of the
illumination beam and the opening time of the light valve determine
this dose. For a mercury discharge lamp, it holds that 40% of the
radiation emitted by this lamp has a wavelength of 365 nm, 20% of
this radiation has a wavelength of 405 nm and 40% of this radiation
has a wavelength of 436 nm. The effective contribution to the image
formation of this lamp radiation is 60% by the 365 nm component,
15% by the 405 nm component and 25% by the 436 nm component, due to
the absorption in the resist layer.
[0101] The invention can also be implemented with other radiation
sources, preferably lasers, especially lasers used currently or to
be used in the near future in wafer steppers and wafer-step-and
scanners, emitting radiation at wavelengths of 248, 193 and 157 nm,
respectively. Lasers provide the advantage that they emit a beam,
which has a single wavelength and is collimated to the required
degree. Essential for the present imaging method is that the
illumination beam is substantially a collimated beam. The best
results are obtained with a fully collimated beam, i.e. a beam
having an aperture angle of 0.sup.0. However satisfactory results
can also be obtained with a beam having an aperture angle which is
smaller than 1.sup.0.
[0102] The required movement, with respect to each other, of the
resist layer, on the one hand, and the array of light valves and
the array of microlenses, on the other hand, is most practically
performed by movement of the substrate stage. Substrate stages
currently used in wafer steppers are very suitable for this
purpose, because they are more than accurate enough. It will be
clear that movement of the substrate stage, for either the stepping
mode or the scanning mode, should be synchronized with switching of
the light valve. To that end, the computer 30 of FIG. 2, which
controls the light valve array, may also control the movement of
the stage.
[0103] An image pattern larger than the illumination field of one
array of light valves and one array of refractive lenses can be
produced by dividing, in the software, such a pattern into
sub-patterns and successively transferring the sub-patterns to
neighboring resist areas having the size of the image field. By
using an accurate substrate stage, the sub-image patterns can be
put together precisely so that one non-interrupted large image is
obtained.
[0104] A large image pattern can also be produced by using a
composed light valve array and a composed refractive lens array.
The composed light valve array comprises, for example, five LCDs,
each having 1000.times.1000 light valves. The LCDs are arranged in
a series to cover, for example, the width of the image pattern to
be produced. The composed refractive lens array is constructed in a
corresponding way to fit to the composed light valve array. The
image pattern is produced by first scanning and illuminating a
resist area having a length covered by a single array of light
valves and a width covered by the series of light valve arrays.
Subsequently, the substrate with the resist layer and the series of
arrays are displaced relative to each other in the longitudinal
direction over a distance covered by a single array. Then a second
resist area, which now faces the composed arrays is scanned and
illuminated, etc. until the whole image pattern has been
produced.
[0105] In the embodiment of the method described above, a spot
formed by a light valve is stepped or scanned in two dimensions
across the valve area belonging to this spot to write this area.
This is no longer the case in an alternative embodiment of the
method. According to this embodiment, each spot is used to write a
resist area, which in one direction has a dimension which is
considerably larger than that of said valve area, whilst a number
of spots are used to write said resist area in the other direction.
The principle of said alternative method is shown in FIG. 8. The
left-hand part of FIG. 8 shows a small portion 240 of a matrix of
spots. This portion comprises four rows of five spots each 241 to
245, 246 to 250, 251 to 255 and 256 to 260, respectively. The
right-hand part of FIG. 9 shows the portion of the resist layer 5,
which can be written, by the spots 241 to 260. The direction 282 of
the lines of spots is now at a small angle .gamma. with respect to
the direction 285 along which the spots and the resist layer are
moved relative to each other. This angle is chosen to be such that
the spots of one line of spots when projected upon the Y-axis fit
within the Y-interspace between this line of spots and the next
line of spots and fill up this interspace. When the substrate is
scanned in the X direction, each spot scans its own line across the
resist layer over a length which is equal to the length of the
image field of the light valve array and refractive lens array
assembly. The lines 261 to 265 in the right-hand part of FIG. 8 are
the center lines of the small, for example 1 .mu.m wide, strips
scanned by the spots 241 to 245, respectively. Spots 246 to 250
scan lines 266 to 270, respectively, and so on.
[0106] For a matrix of 100.times.100 spots each having a dimension
of 1.times.1 .mu.m.sup.2, which matrix covers an image field of
10.times.10 mm.sup.2, the spot period is 100 .mu.m in the X and
Y-directions. In order to achieve that one hundred spots of one row
scan one hundred successive lines in the resist layer, the angle
.gamma. between the scan direction and the direction of lines of
the spots should be: .gamma.=arctan( 1/100)=0.57.sup.0. By scanning
each spot in the X-direction over 10 mm, the whole field of
10.times.10 mm.sup.2 can be written, without moving the spots and
the resist layer relative to each other in the Y-direction. Due to
run-in and run-out of the spots, the total scanning distance is
larger, for example 20 mm, than the effective scanning distance of
10 mm, for example 20 mm. The scanning distance needed for run-in
and run-out is dependent on said angle .gamma.. For a larger matrix
of spots, for example 1000.times.1000 spots, the ratio of effective
scanning distance and total scanning distance is considerably
increased.
[0107] By decreasing the distance between the spots, the centers of
the strips written by the spots can be decreased and the density of
the written pattern can be increased. This allows imparting
redundancy to the system and avoiding that a spot failure results
in a hard error.
[0108] Skew scanning may also be used in a system for imaging a
large pattern and comprising a composed light valve array and a
corresponding composed refractive lens array. For example, with a
system comprising five LCD arrays arranged in series in the
Y-direction and each producing, within an image field of
100.times.100 mm.sup.2, 1000.times.1000 spots with lines of spots
at the above-mentioned angle of 0.570, a resist area of
500.times.100 mm.sup.2 can be written by scanning the resist layer
10 mm in the X-direction. After the resist layer has been moved 90
nm in the X-direction, the same scan can be repeated. In this way,
a resist area of 500.times.1000 mm.sup.2 can be written by scanning
and moving ten times in the X-direction only.
[0109] The number of scans and intermediate movements needed for
writing a given area depends on the number of light valves, and
thus the number of spots, in the X and Y-directions. For example,
with an array of 5000.times.100 spots, a resist area of 500 mm in
the Y-direction can be written by continuously scanning in the
X-direction without intermediate movement. The scan length
determines the length in the X-direction of the written area.
[0110] An essential parameter for the imaging process is the gap
width 44 (FIG. 2). Gap width is one of the input parameters for
computing the required power of the refractive lenses and is
determined by the required image resolution. If a refractive lens
array is computed and manufactured for a given gap width and
resolution, this resolution can only be obtained for the given gap
width. If, in real circumstances, the gap width deviates from said
given gap width, the required resolution cannot be achieved. This
can be demonstrated, on an analogy base, with reference to FIGS.
9a, 9b and 9c, which have been copied from a co-pending patent
application. The latter application relates to a maskless
lithography apparatus wherein an array of specific diffraction
lenses, instead of refractive lenses, is used to focus sub-beams
from the light valves to spots in the resist layer. FIGS. 9a, 9b
and 9c show the spots formed in the resist layer by means of the
same array of microlenses, designed for a gap width of 50 .mu.m,
and under the same illumination conditions, but with different gap
widths. FIGS. 9a shows a pattern 210 of spots 52', which are
obtained when the gap width is 40 .mu.m. FIG. 9b shows a pattern
220 of spots 52 obtained with a gap width of 50 .mu.m and FIG. 8c
shows a pattern 230 of spots 52'' obtained with a gap width of 60
.mu.M. Only the spots of FIG. 9b obtained with a gap width which is
equal to the design gap width have the required sharpness and
intensity. The performance of a maskless lithographic apparatus,
which uses refractive lenses, shows a similar dependence on the gap
width.
[0111] For an apparatus with a larger design gap width of, for
example 250 .mu.m, the requirements for the real gap width can be
lessened. With an increasing design gap width the NA of the
sub-beams (101 to 105 in FIG. 6a) from the refractive lenses
decreases. As the depth of focus is proportional to the inverse of
the squared NA, the depth of focus increases with an increasing
design gap width. This means that, for larger design gap widths,
larger gap width variations are tolerable than for smaller design
gap widths. From a tolerances point of view, a larger gap width of,
for example 250 .mu.m, is preferred to a smaller gap width of, for
example 50 .mu.m.
[0112] The minimum size of the spots is also related to the gap
width. If the gap width is reduced, this size can be decreased, for
example below 1 nm. A smaller gap width, and thus a smaller spot
size, requires a better control of this width.
[0113] The present method is suitable for the manufacture of a
device composed of sub-devices, which are positioned at different
levels. Such a device may be a purely electronic device or a device
that comprises two or more different kinds of features from a
diversity of electrical, mechanical or optical systems. An example
of such a system is a micro-optical-electrical-mechanical system,
known as MOEMS. A more specific example is a device comprising a
diode laser or a detector and a light guide and possibly lens means
to couple light from the laser into the light guide or from the
light guide into the detector. The lens means may be planar
diffraction means. For the manufacture of a multilevel device, a
substrate is used that has a resist layer deposited on different
levels.
[0114] In principle, a multiple level device could be manufactured
by means of an apparatus having a microlens array, which comprises
collections of refractive lenses, which collections differ from
each other in that the focal plane of the refractive lenses of each
collection is different from that of the other collections. Such an
apparatus allows simultaneous printing in different planes of the
substrate.
[0115] A more practical, and thus preferred method of producing
multiple-level devices is to divide software-wise the total image
pattern into a number of sub-images each belonging to a different
level of the device to be produced. In a first sub-imaging process,
a first sub-image is produced, wherein the resist layer is
positioned at a first level. The first sub-imaging process is
performed according to the, scanning or stepping, method and by the
means described hereinbefore. Then the resist layer is positioned
at a second level, and in a second sub-imaging process, the
sub-image belonging to the second level is produced. The shifting
of the resist layer in the Z-direction and the sub-imaging
processes are repeated until all sub-images of the multiple-level
device are transferred to the resist layer.
[0116] The method of the invention can be carried out with a robust
apparatus that is, moreover, quite simple as compared with a
stepper or step-and-scan lithographic projection apparatus.
[0117] In the apparatus, schematically shown in FIG. 2, the array
of light valves 21 to 25, i.e. a LCD, is arranged as close as
possible to the imaging element, or lens plate, 40 comprising the
refractive lenses 43. The size of the light valves, or pixels, of
this LCD may be relatively large, for example 100.times.100
mu.m.sup.2. In a LCD device, a polarization analyzer, also called
analyzer, is needed to convert polarization states, introduced by
the light valves, into intensity levels. If a commercially
available LCD panel, currently applied in video projectors working
with visible light, is used, the visible light analyzer should be
removed from the panel and a separate UV or DUV analyzer should be
arranged between the light valves and the imaging element 40.
Moreover the substrate 41 of this element has some thickness. As a
consequence, there is some distance between the light valves and
the lenses of the imaging element. This distance should be taken
into account when designing the apparatus in order to prevent that
a non sharp image of the light valves is formed in the resist layer
and that crosstalk occurs between radiation from different light
valves due to said distance and diffraction effects.
[0118] To reduce the distance between the light valves and the
refractive lenses and to prevent annoying crosstalk, the array of
refractive lenses may be arranged on the lower surface of the
polarizer and/or the polarizer may be arranged on the light valve
structure.
[0119] FIG. 10 shows an alternative embodiment of the apparatus,
which is attractive in view of the above remarks. This apparatus
comprises a projection lens, which images the array of light valves
on the array of refractive lenses, whereby each light valve is
conjugated with a corresponding refractive lens. The use of a
projection lens allows more freedom of design than allowed in the
sandwich design of the FIG. 2 apparatus.
[0120] The left part of FIG. 10 shows an illumination system 300,
which may also be used in the apparatus of FIG. 2. This
illumination system comprises a radiation source, for example a
mercury lamp 13 and a reflector 15, which may have the shape of a
half sphere. The reflector may be arranged with respect to the lamp
such that no central obstruction of the illumination beam occurs. A
laser may replace lamp 13 and reflector 15. The beam from the
radiation source 13,15 is incident on a wavelength selective
reflector, or dichroic mirror, 302, which reflects only the beam
component with the required wavelength, for example UV or DUV
radiation, and removes radiation of other wavelengths, such as IR
or visible radiation. If the radiation source is a laser, no
selective reflector is needed and either a neutral reflector can be
arranged at the position of the reflector 302 or the laser can be
arranged in line with the rest of the optical path. A first
condenser lens system, for example comprising a first condenser
lens 304 and a second condenser lens 306 arranged before and after
the reflector 302, respectively, converges the illumination beam 17
on a radiation shutter 310. This shutter is provided with a
diaphragm 312. The shape of this diaphragm determines the shape of
the spots formed in the resist layer 5 and this diaphragm thus
constitutes the spot-shaping aperture mentioned hereinbefore. A
second condenser system, for example comprising condenser lenses
314,316 concentrates the radiation passed by diaphragm 312 in the
pupil 322, or diaphragm, of a projection lens 320, i.e. it images
diaphragm 312 in the plane of the pupil of the projection lens 320.
The beam passing condenser lens 316 illuminates LCD 20, which is
arranged between condenser lens 316 and projection lens 320. This
lens images the LCD on imaging element 40, described herein before,
such that each light valve (pixel) of the LCD is conjugated with a
corresponding refractive lens of the imaging element 40. If a light
valve is open, the radiation from this valve is incident on the
conjugated refractive lens only. The imaging element may be
arranged at a distance of 600 mm from the LCD. The distance between
the imaging element and the resist layer 5 may be of the order of
100 to 300 .mu.m.
[0121] The LCD 20 may have a pixel size of 20 .mu.m and the
projection lens may image the LCD pixel structure on the imaging
element with a magnification of 5.times.. For such imaging no large
numerical aperture (NA) for the projection lens is required. To
improve the collimation of the illumination beam incident on the
imaging element, a collimator lens 324 may be arranged in front of
the imaging element. The projection lens and a refractive lens
together image the diaphragm opening into a spot. For example, a
diaphragm opening of 1 mm is imaged in a spot with a dimension of 1
.mu.m. As the operation of the LCD is based on changing the
polarization state of incident radiation, a polarizer, which gives
the radiation the required initial polarization state is needed.
Also needed is a polarization analyzer, which converts the
polarization state into an intensity. This polarizer and analyzer
are denoted by reference numerals 308 and 318, respectively. The
polarizer and the analyzer are adapted to the wavelength of the
illumination beam. Although not shown in FIG. 2, a polarizer and an
analyzer are also present in the apparatus according to this
Figure.
[0122] As the image of the LCD pixel structure is focused on the
imaging element 40 in an apparatus with a projection lens,
practically no crosstalk will occur in such an apparatus. Moreover,
the imaging element may comprise a thick substrate so that it is
more stable. When an apparatus having a LCD light valve array is in
use, the polarizer and the analyzer absorb radiation and produce
heat. If the polarizer and analyzer are arranged close to the LCD,
which is usually the case, this may cause thermal effects. An
apparatus wherein a projection lens is arranged between the LCD and
the imaging element allows arranging the analyzer 308 remote from
the LCD. In this way it is prevented to a high degree that thermal
effects will occur. As shown in FIG. 10, also the analyzer 318 may
be arranged at some distance from the LCD 20. Moreover, the design
of FIG. 10 allows separate cooling of the LCD. A LCD light valve
array may comprise spacers in the form of small, for example 4
.mu.m, spheres of a polymer material. Such spheres may cause
optical disturbances. In an apparatus with a projection lens the
effects of the spacers are reduced because the projection lens,
which has a relatively small NA functions as a spatial filter for
the high frequency disturbances.
[0123] When using a projection lens, it becomes easy to replace a
transmission light valve array by a reflective array, such as a
reflective LCD or a digital mirror device (DMD). An apparatus
wherein a DMD is used should be provided with spatial filtering
means. These means should ensure that only radiation having a
predetermined direction, i.e. radiation which is reflected by
mirrors having a predetermined orientation, reaches the imaging
element 40 and the resist layer. A projection lens provides such a
filtering function.
[0124] The apparatus of FIG. 10 is only one example of an apparatus
with a projection lens. Many modifications of the FIG. 10 apparatus
are possible.
[0125] In practice, the method of the invention will be applied as
one step in a process for manufacturing a device having device
features in at least one process layer of a substrate. After the
image has been printed in the resist layer on top of the process
layer, material is removed from, or added to, areas of the process
layer, which areas are delineated by the printed image. These
process steps of imaging and removing or adding material are
repeated for all process layers until the whole device is finished.
In those cases where sub-devices are to be formed at different
levels and use can be made of multiple level substrates, sub-image
patterns associated with the sub-devices can be imaged with
different distances between the imaging element and the resist
layer.
[0126] The invention can be used for printing patterns of, and thus
for the manufacture of display devices like LCD, Plasma Display
Panels and PolyLed Displays, printed circuit boards (PCB) and micro
multiple function systems (MOEMS).
[0127] The invention cannot only be used in a lithographic
proximity printing apparatus but also in other kinds of
image-forming apparatus, such as a printing apparatus or a copier
apparatus.
[0128] FIG. 11 shows an embodiment of a printer, which comprises an
array of light valves and a corresponding array of refractive
lenses according to the invention. The printer comprises a layer
330 of radiation-sensitive material, which serves as an image
carrier. The layer 330 is transported by means of two drums, 332
and 333, which are rotated in the direction of arrow 334. Before
arriving at the exposure unit 350, the radiation-sensitive material
is uniformly charged by a charger 336. The exposure station 350
forms an electrostatic latent image in the material 330. The latent
image is converted into a toner image in a developer 338 wherein
supplied toner particles attach selectively to the material 330. In
a transfer unit 340 the toner image in the material 330 is
transferred to a transfer sheet 342, which is transported by a drum
344.
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