U.S. patent application number 12/275952 was filed with the patent office on 2009-06-18 for device and method for microstructuring a storage medium and storage medium comprising a microstructured region.
This patent application is currently assigned to tesa scribos GmbH. Invention is credited to Steffen Noehte, Robert Thomann.
Application Number | 20090154318 12/275952 |
Document ID | / |
Family ID | 38124141 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090154318 |
Kind Code |
A1 |
Noehte; Steffen ; et
al. |
June 18, 2009 |
Device and method for microstructuring a storage medium and storage
medium comprising a microstructured region
Abstract
A device providing for microstructuring a storage medium
includes a radiation source for producing an at least partially
coherent beam from electromagnetic radiation, a modulator-provided
with a plurality of individually switchable modulator elements, a
beam-forming optical element for illuminating the modulator, a
reducing optical element for reducing a beam radiated by the
modulator, and a transport table for displacing the storage medium
in relation to the reducing optical element. The device solves
technical problems caused by writing of microstructuring and
individual diffractive optical elements, in particular
computer-generated holograms having a high speed and high writing
energy. The device is achieved in that the reducing optical element
is configured with limited diffraction and produces a surface
reduction of at least 25 from a surface of the individually
switchable modulator elements.
Inventors: |
Noehte; Steffen; (Weinheim,
DE) ; Thomann; Robert; (Heidelberg, DE) |
Correspondence
Address: |
PROSKAUER ROSE LLP
ONE INTERNATIONAL PLACE
BOSTON
MA
02110
US
|
Assignee: |
tesa scribos GmbH
Heidelberg
DE
|
Family ID: |
38124141 |
Appl. No.: |
12/275952 |
Filed: |
November 21, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12295695 |
Nov 14, 2008 |
|
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PCT/EP2007/053328 |
Apr 4, 2007 |
|
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12275952 |
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Current U.S.
Class: |
369/53.35 ;
369/103; G9B/20.046; G9B/7 |
Current CPC
Class: |
G02B 21/245 20130101;
G02B 19/0052 20130101; G03H 2001/048 20130101; G02B 27/0081
20130101; G03H 2001/0224 20130101; G02B 21/08 20130101; G02B
27/0955 20130101; G02B 19/0014 20130101; G03F 7/70291 20130101;
G03H 1/0891 20130101 |
Class at
Publication: |
369/53.35 ;
369/103; G9B/7; G9B/20.046 |
International
Class: |
G11B 20/18 20060101
G11B020/18; G11B 7/00 20060101 G11B007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 4, 2006 |
DE |
10 2006 015 609.9 |
Claims
1. A device for autofocusing an imaging optical unit of a writing
device for microstructuring a storage medium, wherein a distance
and/or a tilting of the imaging optical unit relative to the
storage medium is regulated, comprising: at least two light
sources, at least one beam splitter for directing light beams
generated by the at least two light sources onto the imaging
optical unit, at least two detectors for analyzing light beams
reflected by the storage medium, at least one beam splitter for
directing a light that is reflected by the storage medium and runs
through the imaging optical unit onto the at least two detectors,
and a lens for focusing the light reflected by the storage medium
onto the at least two detectors.
2. The device as claimed in claim 1, wherein one of the at least
two detectors is assigned to each of the at least two light
sources.
3. The device as claimed in claim 1, wherein the at least two
detectors are formed as a 4-quadrant detector, and wherein at least
one optical element that generates an astigmatism is assigned to
each of the at least two detectors.
4. The device as claimed in claim 1, wherein the at least two light
sources and the at least two detectors are provided with actuators
for alternating a spatial position.
5. A method for autofocusing an imaging optical unit of a writing
device for microstructuring a storage medium, wherein a distance of
the imaging optical unit relative to the storage medium is
regulated, wherein light beams are generated by at least two light
sources and directed onto the imaging optical unit, the light beams
are focused onto a surface of the storage medium at a scanning
point and reflected by the storage medium, focus error signals are
generated by at least two detectors for analyzing light beams
reflected by the storage medium, a light that is reflected by the
storage medium and runs through the imaging optical unit is
directed onto the at least two detectors by at least one beam
splitter, the light reflected by the storage medium is focused onto
the at least two detectors, and regulation of the distance of the
imaging optical unit relative to the storage medium is carried out
in a manner dependent on the focus error signal of at least one of
the at least two detectors.
6. The method as claimed in claim 5, wherein each reflected light
beam is analyzed by an assigned detector.
7. The method as claimed in claim 5, wherein a regulation is
carried out with at least two light beams spatially and/or
temporally independently of one another.
8. The method as claimed in claim 5, wherein with the aid of the
writing device, a writing beam is focused onto the surface of the
storage medium for producing a micro structuring, an areal
microstructuring is produced by a relative movement between the
imaging optical unit and the storage medium, and depending on a
direction of movement, at least one scanning point which is
positioned before the writing beam in the direction of movement is
produced on the surface of the storage medium.
9. The method as claimed in claim 5, wherein an exposure line is
produced as focused writing beam.
10. A method for autofocusing an imaging optical unit of a writing
device for microstructuring a storage medium, wherein a distance
and a tilting of the imaging optical unit relative to the storage
medium are regulated, wherein a light beam is generated by at least
one light source and directed onto the imaging optical unit, the
light beam is directed onto a surface of the storage medium and
reflected by the storage medium, the distance between the imaging
optical unit and the surface of the storage medium is determined at
least two points of the surface, and measured distances are used to
determine and regulate the distance and the tilting.
11. The method as claimed in claim 10, wherein with the aid of the
writing device, a writing beam is focused onto the surface of the
storage medium for producing a micro structuring, an areal
microstructuring is produced by a relative minimum between the
imaging optical unit and the storage medium, and a region
illuminated by the light beam is positioned before the writing beam
in a direction of movement.
12. The method as claimed in claim 10, wherein at least two light
beams are focused onto the surface of the storage medium at
scanning points, a focus error signal is generated by at least two
detectors for analyzing the at least two light beams that are
reflected by the storage medium, for scanning points, the distance
between the imaging optical unit and the surface of the storage
medium is determined from the focus error signals, and a correction
of the tilting of the storage medium relative to the imaging
optical unit is carried out depending on the focus error signals
for the scanning points.
13. The method as claimed in claim 10, wherein the light beam is
focused onto the surface of the storage medium at a scanning point
and reflected by the storage medium, a focus error signal is
generated by a detector for analyzing the light beams reflected by
the storage medium, the scanning point is scanned over a section of
the surface of the storage medium, and a distance profile is
produced from measured focus error signals.
14. The method as claimed in claim 10, wherein a scanning direction
is oriented transversely with respect to a direction of a relative
movement between the imaging optical unit and the storage
medium.
15. The method as claimed in claim 10, wherein the light beam is
directed onto the surface of the storage medium and reflected by
the storage medium, and the distance between the imaging optical
unit and the storage medium is determined at a plurality of points
by a wavefront detector for analyzing the light beams reflected by
the storage medium.
16. A method for determining an operating point of a device for
autofocusing an imaging optical unit of a writing device for
microstructuring a storage medium, wherein a distance and/or a
tilting of the imaging optical unit relative to the storage medium
is regulated, wherein a predetermined region of a surface of the
storage medium is structured with a predetermined pattern, along
one axis an intensity and along the other axis an exposure height
are altered continuously, and afterward an exposed region is
analyzed microscopically and the operating point is defined.
17. The method as claimed in claim 16, wherein a value range of 16
different gray shades is used for the intensity, and the exposure
height is varied over a region of a predetermined S curve.
18. The method as claimed in claim 16, wherein an optimum exposure
height where a structuring is sufficiently achieved with extremely
low intensity of a writing beam is established for a setting.
19. The method as claimed in claim 16, wherein a focus error signal
is used as a reference for the exposure height.
20. The method as claimed in claim 16, wherein a focus error signal
is used as a reference for the operating point.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. application Ser.
No. 12/295,695, filed on Oct. 1, 2008, which claims the benefit of
and priority to International Application No. PCT/EP2007/053328,
filed on Apr. 4, 2007, and German Patent Application No. DE 10 2006
015 609.9, filed on Apr. 4, 2006. The disclosure of the above
applications are incorporated herein by reference in their
entirety.
FIELD OF THE INVENTION
[0002] The invention relates to a device and a method for
microstructuring a storage medium and a storage medium comprising a
microstructured region. The invention likewise relates to a device
and a method for autofocusing an imaging optical unit of a writing
device for microstructuring a storage medium. Furthermore, the
invention relates to a novel writing strategy for producing a
microstructuring on a storage medium.
BACKGROUND OF THE INVENTION
[0003] Microstructurings comprise a plurality of dots and/or lines
that are used to alter an optical property of a surface or of one
or more layers of a storage medium. A change in reflectivity,
transmission, absorption, scattering behavior, phase of the
reflected light or a combination of all the effects can be achieved
through microstructurings. In this case, the spatial resolution may
be less than 10 .mu.m down to dot or line dimensions of less than 1
.mu.m. Microstructurings of this type are used for storing
information; and in particular, computer-generated holograms,
microimages or microscripts can be produced thereby.
[0004] Computer-generated holograms comprise one or more layers of
dot matrices or dot distributions which, in the event of
illumination with a preferably coherent light beam, lead to a
reconstruction of items of information coded in the hologram. In
this case, the dot distribution may be calculated as an amplitude
hologram, phase hologram or as a kinoform Fresnell or Fourier
hologram. In order to produce computer-generated holograms, the
holograms are first calculated and subsequently written to a
storage medium by a suitable writing device by means of dotwise
introduction of energy. The resolution of the dot matrix that
arises may lie within the range down to less than 1 .mu.m, as
already discussed. Consequently, holograms having a high resolution
can be written in a confined space, the information of which
holograms can only be read out by illumination with a light beam
and reconstruction of a diffraction pattern. In this case, the size
of the holograms may be between a few square millimeters and
multiple square centimeters.
[0005] A major advantage of the computer-generated holograms is
that each hologram can be calculated individually without a high
outlay. Consequently, holograms comprising consecutive numbers or
production parameters, for example, can be generated in series.
Holograms of this type can therefore be used in particular as
security features or in logistics for product tracking on
packaging, credit cards, entrance tickets or the like. By a
suitable read-out device, the security features of the hologram can
be read out and the authenticity and individuality of the security
feature can be checked in a simple manner.
[0006] The computer-generated holograms described above can be
combined with a directly visible item of information (i.e.
microscripts and microimages). In addition, with a microstructuring
mentioned in the introduction, the above-mentioned microimages and
microscripts themselves can also be written independently of
computer-generated holograms. The dot distributions can also be
produced as dot matrix holograms, wherein in each case individual
small area portions are produced as different diffraction
structures of the dot matrix hologram. In addition, it is also
possible to produce a diffractive optical element (DOE) per se with
the microstructuring.
[0007] When writing or read-out by a light beam as described below,
a laser beam in the visible wavelength range is generally
preferred. Nevertheless, the present invention is not restricted to
the application of visible light. In principle, the invention can
be applied with electromagnetic radiation in a wide wavelength
range.
[0008] The prior art furthermore discloses a plurality of writing
devices for writing computer-generated holograms which write the
optical structures of the holograms in planar storage media. By way
of example, reference is made in this respect to documents WO
02/079881, WO 02/079883, WO 02/084404, WO 02/084405 and WO
03/012549. These writing devices use a laser beam that successively
scans each individual dot of the dot matrix and each may or may not
introduce light energy into the storage medium.
[0009] A plurality of reading devices are likewise known which are
suitable, by illuminating the hologram area by a light beam and a
suitable optical unit, for making the reconstruction visible or
electronically representable by recording means and evaluable. By
way of example, in this context, reference is made to documents DE
101 37 832, WO 02/084588 and WO 2005/111913.
[0010] By contrast, EP 1 094 352 A2 discloses an optical writing
device for image generation with a light source comprising a
plurality of laser diodes arranged in a series. The light emitted
by said light source is directed, by an optical unit, onto a
grating light valve (GLV) from Silicon Licht Machines, by which a
diffraction takes place for each point of the GLV. The GLV can also
be referred to as a line light modulator. An image arises line by
line as a result of progressive exposure. The resolution in the
written image that is generated by the writing device is specified
as 2400 dpi corresponding to a dot size of about 10 .mu.m.
[0011] DE 198 02 712 A1 discloses a device for exposure of a
computer-generated hologram on a storage medium. A conditioned
laser beam impinges on a digital light processor (DLP), by which a
2D light field is deflected onto the storage medium. Structures are
produced in a storage medium by small-area individual mirrors of
the DLP. The hologram size is therefore determined by the DLP used
and a scaling of the imaging.
[0012] EP 1 202 550 A1 shows a writing device comprising a line
light modulator (GLV) and comprising an imaging optical unit. A
polarizer is arranged in-a beam path of a laser beam, said
polarizer producing a linear polarization of the beam. The
preferred direction of the line light modulator is utilized as a
result, such that high intensities are achieved on a writing
medium. A writing of photosensitive and heat-sensitive materials is
thus accelerated.
[0013] EP 1 480 441 A1 shows a construction of a writing device
comprising a beam multiplier that generates a multiplicity of
individual beams from a single-mode laser beam. Said individual
beams then impinge on a multichannel spatial light modulator and
are individually modulated there. The individual beams reflected by
the SLM are each substantially single-mode beams and a downstream
optical unit images this plurality of individual beams onto a
surface of a light-sensitive workpiece. The method described serves
for example to produce circuits.
[0014] WO 01/79935 discloses a device which can be used to produce
photomasks for semiconductor elements and display devices.
Semiconductor elements, displays, integrated optical elements and
electronic connecting structures can likewise be written directly.
In order to produce individual dots, a spatial light modulator
(SLM) is in a beam path of a laser beam. A downstream imaging
optical unit then images a light modulator on a storage medium in
order to write a structure. In order to control an energy
fluctuation between individual writing pulses and thus to enable
the structure to be written uniformly, it is proposed to use a very
fast control of a switch that enables the light energy to be
switched off actually during a laser pulse. In this case, WO
01/79935 takes an average writing power of 10-100 mW as a
basis.
[0015] The systems described above each has at least one of the
following problems, such as an excessively low spatial resolution
and low accuracy of the written structuring, an excessively low
throughput or writing speed, no individually producible structures
or an excessively low writing energy to allow the structuring of
materials that are not very photosensitive or heat-sensitive.
[0016] Systems of the type described above require for operation in
progress an active autofocusing in order to correct relative
deviations between the imaging optical unit and the storage medium.
In this case, one problem for a very accurate and quickly operating
autofocus system is that the structuring of a storage material with
a resolution in the range of the wavelength or with a corresponding
numerical aperture (NA) necessitates an active height regulation.
This results from the limited depth of field, which is related to
the NA used as follows:
.DELTA. z = 0.8 .lamda. NA 2 . ##EQU00001##
[0017] It can be seen here that as NA increases, which means
smaller structures, the depth of field .DELTA.z decreases
reciprocally to the square of NA. For structures in the visible
wavelength range and given structure sizes of about 1 .mu.m, a
depth of field in the single-digit micrometer range is thus
calculated. On account of manufacturing tolerances of the storage
medium or on account of inaccuracies as a result of the mounting of
the storage medium, a height variation of the storage material in
the two-digital micrometer range should be expected. This
necessitates an active height regulation.
[0018] The requirement made of the height regulation consists in
keeping the writing beam within the depth of field over its entire
region. In the case of systems known from the prior art it was
assumed that the height changes are little within the exposure
width or region and a one-point height regulation is thus
sufficient. However, experiments have repeatedly shown that
unstructured locations, particularly at the edge of the exposure,
occur on account of inaccuracies in manufacturing and as a result
of misalignment of the sample or storage material mounting. This
results in the requirement for a height regulation over the entire
exposure region which becomes all the more urgent, the larger the
exposure region becomes.
SUMMARY OF THE INVENTION
[0019] In one aspect, the present invention is related to writing
microstructurings and individual diffractive optical elements
(DOE), in particular computer-generated holograms, at high speed,
with high precision and with a high writing energy.
[0020] Another aspect of the present invention relates to
specifying a system which can be used to write dot or line
structures having a structure size in the micrometer range (50
.mu.m to 1 .mu.m) or in the submicron range (1.0 .mu.m to 0.01
.mu.m).
[0021] A further aspect of the present invention relates to
specifying a system which can be used to write dot or line
structures at high speed (typically 100 Mdots/s or higher).
[0022] A further aspect of the present invention relates to
specifying a system which can be used to write large-area
microstructurings in the range of 1 mm.sup.2 to 1 m.sup.2,
typically 1 cm.sup.2.
[0023] A further aspect of the present invention relates to
specifying a system which can be used to write microstructurings to
a material that is optically not very sensitive to light or heat
(e.g. thin aluminum layers, metals, polymers, and rare earths).
[0024] In an embodiment, a device for writing microstructurings
includes: [0025] a radiation source for generating an at least
partly coherent beam of electromagnetic radiation, [0026] a
modulator having a plurality of individually switchable modulator
elements, [0027] a beam shaping optical unit for illuminating the
modulator, [0028] a reducing optical unit for reducing a beam
emitted by the modulator, [0029] an arrangement for converting a
phase modulation into an intensity modulation, for example by a
spatial frequency filter, and [0030] a transport device, in
particular a transport table or a transport belt, for moving a
storage medium relative to the reducing optical unit.
[0031] The device is exemplified below on the basis of a transport
table, but this embodiment is not intended to restrict the
invention to this specific embodiment. A transport belt or any
other suitable transport device is for example also possible in
application.
[0032] In one embodiment in accordance with the invention, the
reducing optical unit is formed in diffraction-limited fashion, and
the reducing optical unit produces an area reduction of at least 25
proceeding from an area of the individual modulator elements.
[0033] By the beam shaping optical unit, the individually
switchable modulator elements are in each case irradiated with a
plane wave or a substantially plane wave, such that an optimum
result, i.e. very small diameters of individual reduced dots are
achieved by the diffraction-limited reducing optical unit. For a
delimitation of the lack of planarity of a wave it is possible to
employ the Rayleigh criterion, which states that across each
modulator element the deviation from the plane wave is not greater
than .lamda./4 where .lamda. is the wavelength of the light.
[0034] Owing to the use of a modulator operating in parallel, for
example a line light modulator, a multiplicity of individual dots
or lines of an individual structure can be written to the storage
medium in one exposure step. In this case, the diffraction-limited
reducing optical unit and the great reduction associated therewith
afford the possibility of being able to write very small structures
with a high writing energy. This enables fast and massively
parallel writing of microstructurings to materials which previously
cannot be written to. Owing to the reduction of typically 250 or at
least 25 and diffraction-limited beam guiding, high energy
densities that would otherwise destroy the modulator itself can be
controlled with the modulator.
[0035] In another embodiment, without a downstream development
step, DOE, in particular computer-generated holograms, can thus be
produced rapidly and cost-effectively, and they can additionally be
produced individually and in the micrometer or submicron range. The
writing speed can reach as much as 100 Mdots/sec, for example. A
further advantage is that a high positioning accuracy of the
diffractive exposure points can be achieved. Finally, the
microstructuring can also be written with a different intensity,
i.e. for example with different gray shades, by a suitable driving
of the modulator.
[0036] The term diffraction monitoring is described according to
DIN ISO 10110-5 as follows. A measure of the imaging quality of a
system is the "Strehl definition brightness" or the "Strehl
intensity ratio", i.e. the ratio of the intensity in the center of
the point image to what is defined for the aberration-free optical
system. This ratio states how much light in comparison with the
theoretically possible light is combined in the diffraction disk,
also called Airy disk, and how much light is situated outside. A
Strehl value of 1 corresponding to 100% means that the light
present within the diffraction disk reaches the theoretically
maximum value. Such a system would be free of imaging aberrations.
Optical units having a Strehl value of better than 0.8 are deemed
to be diffraction-limited according to this standard.
[0037] The measure according to Strehl is directed toward measuring
the intensity distribution at the focus relative to the diffraction
disk. There is a further possibility for determining the
diffraction limitation, which relates, however, to the form or to
the dimension of the diffraction disk. In this respect there exists
the formula
d=.lamda./NA
[0038] wherein [0039] d is the diameter of the diffraction disk
measured as the distance between the diffraction minima on both
sides of the diffraction disk, [0040] .lamda. is the wavelength of
the focused light, and [0041] NA is the numerical aperture of the
reducing. optical unit.
[0042] If the dimension of the diffraction disk deviates by less
than 20% from this value d according to the abovementioned formula
toward larger values, then the optical unit used is
diffraction-limited.
[0043] In another embodiment, it is furthermore preferred for the
area reduction of the reducing optical unit to be between 25 and
1000, preferably between 50 and 1000, in particular 250. In this
value range it is possible to produce particularly small and
intensive light structures or light spots line by line or areally
which can enable even less light-sensitive materials to be altered
sufficiently to produce a microstructuring. Examples of such
materials include thin aluminum layers, metals, polymers, and rare
earths, in which restructurings, displacement processes,
condensation processes, ablation processes or evaporation processes
are initiated by point wise introduction of energy.
[0044] In a further preferred embodiment, the reducing optical unit
reduces the radiation intensity diffracted by each modulator
element to a size of less than 10 .mu.m diameter, in particular
less than 1 .mu.m diameter. As a result it is possible to produce
microstructures with a resolution which have hitherto been
unattainable particularly in the case of less photosensitive
materials.
[0045] In another embodiment of the invention, the radiation source
is advantageously formed as a laser. However, this configuration of
the light source does not represent a restriction of the invention.
It is already apparent from the prior art, however, that the laser
is the preferred means for generating an at least partly coherent
beam of electromagnetic radiation. A pulsed laser is preferably
used, the pulses of which have a higher energy density than the
laser beam of a continuous wave laser.
[0046] Typically a pulsed laser has such characteristics that the
pulse durations are in the nanosecond range and that the energy of
a pulse is in the region of 100 microjoules. This means, given 1024
points of a line light modulator, an energy per pulse and modulator
element of about 0.1 microjoule. This results in a sufficient
energy density for material structuring which depends only to a
small extent on the thermal conductivity of the material.
[0047] An embodiment of producing microstructurings is therefore
realized optimally if high-energy laser pulses in the visible or
adjacent wavelength ranges are used to microstructure the storage
medium. This results in a broader variety of usable materials. The
use of a cw laser is possible in principle, but is not as
advantageous owing to the lower energy density than other
lasers.
[0048] Furthermore, it is preferred for the laser to generate a
monomode laser beam, such that, on the one hand, the coherence
properties of the light beam are optimal and, on the other hand, a
regular intensity distribution of the laser beam transversely with
respect to the propagation direction is present.
[0049] By contrast, it may also be sufficient for the laser to
generate a multimode laser beam with partial coherence. Even if the
intensity distribution of the laser beam generated is not optimal
and possibly has to be compensated for, it is sufficient if the
beam directed onto the modulator has a spatial coherence extending
at least across some of the individually switchable modulator
elements. It is therefore not of importance for all the
individually switchable modulator elements to be irradiated with
mutually coherent light.
[0050] A diode-pumped solid-state laser in the visible spectral
range is an example of a pulsed laser system.
[0051] An essential element for generating an intensity-modulated
beam is the modulator. The individually switchable modulator
elements can be controlled electronically so as to adopt at least
in each case one of two possible settings. Generally, the modulator
is switchable in a continuously variable manner and acts as a
spatial phase modulator. Single-line or else two-dimensional
modulators can be used in this case. The two-dimensional modulators
have the disadvantage, however, that a two-dimensional illumination
of the area of the modulator is necessary, which is more difficult
than a one-dimensional illumination. A further disadvantage of
two-dimensional modulators is that interference effects are
manifested to a greater extent in two-dimensional exposure in the
case of specific constructions of the storage media. Consequently,
it is a simplification of the system if a single-line construction
is chosen and a homogenization of the beam intensity is necessary
only in one dimension.
[0052] In one embodiment, a single-line line light modulator
comprising separately (i.e. individually) switchable modulator
elements is preferred. If said single-line line light modulator is
successively driven in stepwise fashion such that a different
intensity distribution is produced after each new setting, then
different microstructuring lines can be successively written to the
storage medium. This gives rise to an area or a region with a
microstructuring which can be configured individually. This is
achieved because the light modulator has to be driven anew for each
writing operation, which allows it to be driven differently for
each writing operation.
[0053] In another embodiment, the line structure of the area light
modulator is preferably imaged onto the storage medium
perpendicular to the scanning movement, in order to achieve a
maximum width of the written strip. A modified embodiment provides
a non-orthogonal orientation, which makes it possible to obtain a
finer resolution although with a reduced writing width.
[0054] One example of a single-line light modulator is a grating
light valve (GLV) from Silicon Light Machines, CA, USA. This line
light modulator has separately switchable diffraction gratings.
Each modulator element has a plurality of ribbons which run
parallel to one another and are arranged in one plane, and of which
for example every second ribbon can be adjusted electrostatically
out of the plane. Consequently, the modulator element acts either
as a mirror or as a small diffraction grating that diffracts the
light intensity in a direction that deviates from the reflection
direction. As a result, each modulator element, depending on the
driving, can deflect the impinging light to different extents in
the reflection or diffraction direction.
[0055] The beam shaping optical unit serves for illuminating the
single-line or two-dimensional light modulator. It is desired for
the illumination of the modulator to be as uniform as possible in
order that a homogeneous intensity distribution is achieved which,
given identical driving of the individually switchable modulator
elements, leads to identical writing intensities on the storage
medium. To put it in general terms, the beam shaping optical unit
is intended to generate a beam profile adapted to the area of the
modulator from the beam profile generated by the radiation
source.
[0056] In one embodiment, when a line light modulator is used, it
is preferred for the beam shaping optical unit to have a Powell
lens. A Powell lens is known from U.S. Pat. No. 4,826,299 and
generates, from a Gaussian intensity distribution, for example, an
intensity distribution substantially approximated to a rectangular
form.
[0057] Consequently, a beam shaping optical unit has the function
of generating from the beam profile of the light source or laser a
beam profile adapted to the geometry of the line light
modulator.
[0058] A further preferred embodiment consists in the reducing
optical unit being a microscope objective. A conventional
technology can thus be used for focusing the intensity distribution
generated by the modulator onto the storage medium and for reducing
it.
[0059] Preferably, in another embodiment the reducing optical unit
has at least two objectives having a different focal length and/or
numerical aperture. For this purpose, an interchangeable microscope
objective can be used, for example, such that the objectives in
light microscopes can be used without high cost expenditure of a
special development. By altering the focal length or numerical
aperture, the size of the individual structures or dots can be
altered in a simple manner. An easy changeover between different
structure sizes and pitch is then possible when an interchangeable
optical unit is employed. Reference may also be made to a fast
changing of the pitch. In this case, by changing the focal length
of the optical unit, the pitch, i.e. the distance between the
individual dots, is altered, while changing the numerical aperture
alters the dot size. Therefore, the pitch and the dot size do not
change in the same way when the objective is changed.
[0060] In another embodiment it is furthermore preferred for the
reducing optical unit to have a spatial frequency filter. The
function of the spatial frequency filter is to convert a phase
modulation into an intensity modulation. In this case, the spatial
frequency filter is preferably combined with the reducing optical
unit, wherein the apertured diaphragm required is arranged within
the reducing optical unit.
[0061] The reducing optical unit with combined spatial frequency
filter therefore constitutes nothing more than arranging a filter
in the form of an apertured diaphragm in the Fourier plane in order
to convert the phase modulation of the modulator, in particular of
the line light modulator, into an intensity modulation in the
writing beam.
[0062] For transporting the storage medium relative to the reducing
optical unit, the transport device is provided. It is preferred for
the transport device to be formed as a transport table and to be
adjustable preferably in at least one axis, in particular in two
axes. The storage medium can thus be moved in any direction
transversely with respect to the propagation direction of the
focused light beam. The transport device can also be formed as a
transport belt.
[0063] A movement of the storage medium relative to the writing
head in two directions makes it possible to write a plurality of
tracks of a microstructuring alongside one another and thus to be
able to process large areas.
[0064] In an embodiment, the transport table preferably has a high
accuracy in the position determination, but not in the movement
sequence itself. Through massively parallel exposure, the speed of
the movement can be correspondingly reduced since high speeds are
not of importance when the storage medium is moved. A transport
table with a roll drive, in particular with a position feedback
system, is preferably used.
[0065] As will be explained in more detail further below, the exact
position detection is used for triggering the writing process
itself; i.e. that the transport table, by virtue of its movement
and its exact position determination, is the means controlling the
synchronization of the writing operation. In this system,
therefore, the material movement triggers the laser system and
causes the controller to supply the modulator with the next
exposure information. In particular the accuracy primarily in
large-area exposures is increased by the synchronization.
[0066] Alternatively, it is also possible for a free-running
driving of the transport table to be used and for the device to be
activated at predetermined times and to perform the
microstructuring line by line or portion by portion.
[0067] A further preferred embodiment consists in the fact that a
movable optical element, in particular a beam splitter or mirror,
is provided in the beam path upstream of the reducing optical unit
for the purpose of shifting the reduced intensity pattern on the
storage medium. By introducing an actuator, e.g. for moving the
beam splitter or the mirror, the exposure line generated by the
modulator or the exposure portion can be moved relative to the
storage medium.
[0068] If a movement along the direction of the material transport
is realized, then a fluctuation brought about, inter alia, by a
speed fluctuation of the transport table can advantageously be
compensated for as a result.
[0069] If a movement transverse to the direction of the material
transport is realized, this degree of freedom permits compensation
of faults or inaccuracies in the material guidance. Inter alia, in
the case of an areal exposure composed of the plurality of exposure
lines or exposure strips, it is necessary for the exposure lines to
be placed against one another in a controlled manner. By
identifying the position of the preceding exposure of the storage
medium, for example by a camera system described below, it is
possible for the scanning device to place the tracks or strips
against one another accurately. In general, any desired detection
device can be used.
[0070] The detection of an edge or marking can be detected by a
camera. However, owing to the restricted repetition rate of the
camera, a detection of an edge during the transport movement, for
example by a photodetector arrangement, may be advantageous and
thus preferred.
[0071] Likewise, by a movement of the exposure line or the exposure
portion transverse to the direction of movement of the storage
medium, the exposure can be effected at a position predefined in
the material of the storage medium. This in turn presupposes a
detection of the predefined position, e.g. an embossed marking or
track, for which purpose the camera system or the photodetector
arrangement can again advantageously be used.
[0072] In an embodiment, the device therefore preferably has a
camera system for monitoring the microstructuring, said camera
system being integrated into the beam path such that it can detect
the surface of the storage medium by means of a separate
illumination and observe and evaluate the structuring or marking of
said storage medium. For this purpose, a beam splitter is
preferably provided which directs, on the one hand, the beam
deflected by the modulator and, on the other hand, the light from
or into the optical branch of the camera system. For further
explanation, reference is made to the detailed description of the
invention.
[0073] In another embodiment, the camera system can likewise be
provided for monitoring a predetermined orientation or alignment
relative to a predetermined or already written microstructuring or
marking. It can be utilized in particular for improved stringing
together of the exposure strips of a plurality of writing
operations.
[0074] In various embodiments, it is preferred for a controller to
be provided, which synchronizes the laser, the modulator and the
transport table. The manner of synchronization by the movement of
the transport table has already been discussed above, wherein the
transport table, with its exact position determination, generates a
trigger signal that is taken up and processed further by the
controller. A free movement of the transport table is likewise
possible, wherein the controller then generates, at suitable time
intervals corresponding to the speed of the transport table,
control signals for the driving of the laser and of the modulator
and transmits them thereto. Likewise, the control system can also
be used for fast data processing and transmission to the modulator
or the light modulator.
[0075] In one embodiment, the controller is preferably based on two
coupled computers (a control computer and a data computer). The
control computer provides the user interface and supervises all
non-time-critical sequences. These include all initiation tasks,
calibration routines, diagnostic tools, etc. The data computer is
responsible for providing the data to be processed. The two
computers communicate with one another via a connection.
Time-critical functions such as the triggering of the laser and the
driving of the modulator are performed by a suitable processor
card. Preferably the position signal of the transport table, in
particular of the X-Y linear table, is used as central clock
generator of the entire lithographic system. The sequences during
the exposure are controlled according to the position signal.
[0076] Another aspect in accordance with the invention relates to a
method for controlling an above-described device for
microstructuring a storage medium, wherein [0077] during the
movement of the transport table, a master trigger T.sub.0 is
generated for a predetermined time duration .DELTA.T before the
position of a new structuring to be written in the storage medium
is reached, [0078] the radiation source is driven in order to
generate a radiation pulse at the instant T.sub.0+.DELTA.T, [0079]
the modulator is set in accordance with the information to be
written, before the instant T.sub.0+.DELTA.T is reached, and [0080]
the radiation source generates, at the instant T.sub.0+.DELTA.T, a
radiation pulse whose intensity distribution is varied in
accordance with the setting of the modulator.
[0081] In an embodiment, the respective microstructurings can be
produced successively by the modulator or light modulator.
Particularly in the case of a linear single-line light modulator,
the dots of the structuring are written to the storage medium line
by line at predetermined distances. In this case, uniform distances
between the individual lines are primarily of importance in order
to achieve a uniform arrangement of the dots or lines in a
predetermined grid.
[0082] Preferably, the transport table is moved continuously, and
the master triggers T.sub.0 are generated by the movement of the
transport table by a predetermined distance in each case. In this
way, the transport table is moved continuously and not driven for
individual movement steps.
[0083] Another aspect in accordance with the invention relates to a
method for controlling an above-described device for
microstructuring a storage medium, wherein [0084] a first
structuring track is written to the storage medium in a writing
direction, [0085] at least a second structuring track is written to
the storage medium in a manner parallel to the preceding
structuring track, and [0086] the at least second structuring track
is written counter to the writing direction of the preceding
structuring track.
[0087] A major advantage afforded in embodiments of this method is
that a large area of the storage medium has individual structuring
tracks written successively to it, wherein the writing is possible
with alternating writing directions without the device having to be
returned to a starting position.
[0088] In one embodiment, the camera system mentioned above can
preferably be used to measure the position of the first structuring
track, i.e. to detect the edge of the first structuring track.
Consequently, a continuous tracking of the device can be effected
during the writing of the second structuring track. Likewise, as
mentioned above, a photodetector arrangement can also be used.
[0089] Preferably, the transport table can be moved transversely
with respect to the writing direction such that the second
structuring track is written in a manner directly adjoining the
edge of the first structuring track. In this case, the camera
system can also be utilized in order to enable an alignment, i.e.
an assignment to prestructured materials, in particular to prior
structures produced by printing, embossing, partial metallizations
and already existing holograms.
[0090] For tracking the position of the exposure line or the
exposure portion, it is also possible for example to use the
abovementioned adjustable optical element, for example in the form
of the beam splitter or mirror. With the movement of this optical
element, it is possible in particular to achieve a movement
transversely with respect to the direction of movement of the
transport table relative to the reducing optical unit. Additionally
or alternatively, the modulator can also be shifted or adjusted by
a piezo-drive.
[0091] Another aspect in accordance with the invention relates to a
storage medium that has been microstructured in particular by
embodiments described above, comprising an optically variable
layer, wherein the layer is microstructured in a predetermined
region, wherein the dimension of the microstructured region is at
least 10 mm at least in one direction. Preferably, the dimension is
at least 50 mm. In this case, the resolution of the
microstructuring can lie in the micrometer range and possibly in
the submicron range.
[0092] Through embodiments described above, it is possible for the
first time to write to such large areas having a resolution such
that applications not possible hitherto arise.
[0093] By way of example, the microstructured region can be written
in the form of a strip having a length corresponding to a multiple
of the width of the strip. Consequently, the length ratio of the
side edges of the microstructured region is altered, such that a
strip with a plurality of items of microstructured information that
can be read out and monitored for the purpose of checking
authenticity can be stored for example on a security document. In
this case, the length can preferably correspond to at least three
times the width, preferably five times the width, in particular ten
times the width.
[0094] Likewise, for the specified size of the microstructuring it
is possible for the microstructured region to have at least two
different information contents, wherein preferably at least one
information content is a computer-generated hologram. The
information content can likewise be a dot matrix hologram, a
microimage or a microscript or a microcoding.
[0095] Another aspect in accordance with the invention relates to a
device having: [0096] at least two light sources, [0097] at least
one beam splitter for directing a light beam generated by the light
sources onto an imaging optical unit, [0098] at least two detectors
for analyzing light beams reflected by a storage medium, [0099] at
least one beam splitter for directing a light that is reflected by
the storage medium and runs through the imaging optical unit onto
the at least two detectors, and [0100] a lens for focusing a
reflected light onto the at least two detectors.
[0101] By providing at least two scanning points, it is possible,
during the writing of a microstructuring, in particular by means of
an exposure line, for both directions of movement of the writing
operation, to detect that portion of the surface of the storage
medium which has not yet been written to, with at least one
scanning point. In addition or as an alternative thereto, it is
possible to use at least two scanning points in order to establish
not only a distance but also a tilting of the storage medium
relative to the imaging optical unit.
[0102] Further configurations and advantages of the device
according to the invention are presented in the detailed
description of exemplary embodiments.
[0103] Another aspect in accordance with the invention relates to a
method for autofocusing an imaging optical unit of a writing device
for microstructuring a storage medium, wherein a distance of the
imaging optical unit relative to the storage medium is regulated,
wherein [0104] light beams are generated by at least two light
sources and directed onto the imaging optical unit, [0105] the
light beams are focused onto a surface of the storage medium at a
scanning point and reflected there, [0106] focus error signals are
generated by at least two detectors for analyzing light beams
reflected by the storage medium, [0107] a light that is reflected
by the storage medium and runs through the imaging optical unit is
directed onto the at least two detectors by at least one beam
splitter, in which the reflected light is focused onto the at least
two detectors, and [0108] regulation of the distance of the imaging
optical unit relative to the storage medium is carried out in a
manner dependent on the focus error signal of at least one of the
at least two detectors.
[0109] The method described above utilizes the at least two
scanning points as previously explained. Further configurations and
advantages of the method according to the invention are presented
in the detailed description of exemplary embodiments.
[0110] Another aspect in accordance with the invention relates to a
method for autofocusing an imaging optical unit of a writing device
for microstructuring a storage medium, wherein a distance and a
tilting of the imaging optical unit relative to the storage medium
are regulated, wherein [0111] a light beam is generated by at least
one light source and directed onto the imaging optical unit, [0112]
the light beam is directed onto a surface of the storage medium and
reflected there, [0113] the distance between the imaging optical
unit and the surface of the storage medium is determined at least
two points of the surface, and [0114] measured distances are used
to determine and regulate the distance and the tilting.
[0115] The method mentioned above makes it possible to determine,
during the writing operation, not only the distance but also a
possible tilting between the surface of the storage medium and the
imaging optical unit, in order to carry out an active regulation.
Further configurations and advantages of the method according to
the invention are presented in the detailed description of
exemplary embodiments.
[0116] Another aspect in accordance with the invention relates to a
method for determining an operating point of a device for
autofocusing an imaging optical unit of a writing device for
microstructuring a storage medium, wherein the distance and/or the
tilting of the imaging optical unit relative to the storage medium
is regulated, wherein [0117] a predetermined region of the surface
of the storage medium is structured with a predetermined pattern,
[0118] along one axis the intensity and along the other axis the
exposure height are altered continuously, and [0119] afterward the
exposed region is analyzed microscopically and the operating point
is defined.
[0120] The method described above makes it possible to realize a
very accurate and effective calibration of writing intensity and
distance regulation. Further configurations and advantages of the
method according to the invention are presented in the detailed
description of exemplary embodiments.
[0121] The above-described autofocusing systems and methods for
adapting the distance of the storage medium from the imaging
optical unit enable a sufficient tracking of the distance between
the storage medium and the imaging optical unit, which is necessary
on account of the intense focusing and the associated short extent
of the focus along the propagation direction.
[0122] Another aspect in accordance with the invention relates to a
method comprising-the following steps: [0123] a structuring track
is written to a storage medium in a writing direction line by line,
[0124] an individual pixel is formed with a predetermined maximum
intensity, and [0125] at least two pixels that are to be written in
adjacent fashion are formed with an intensity that is less than the
predetermined maximum intensity.
[0126] Further configurations are presented in the detailed
description.
[0127] However, the exposure strategy is not dependent on the
precise configuration of the device, but rather concerns all
exposure operations in which microstructurings are introduced into
a storage medium line by line or areally.
[0128] Various embodiments in accordance with the invention
therefore serve for correcting so-called proximity effects that
occur during the writing operation.
[0129] Preferably, for this purpose a control system is used for
correcting the proximity effects during the running time together
with an algorithm comprising a splitting of the line profile into a
portioned component and into a differential component. The
correction depth can be set depending on parameters which can be
chosen in a variable manner and which can be material-specific,
inter alia.
[0130] In such embodiments, a specific pulse strategy is used as a
degree of freedom in the material structuring, which can preferably
be set electronically, in particular in a manner dependent on
whether the exposure line is oriented orthogonally or obliquely
with respect to the direction of movement.
[0131] In such embodiments, the degree of structuring of individual
exposure points in a scanned pattern, i.e. for example the
intensity of the conversion--effected by the exposure--of the
optical property of the material of the storage medium, is intended
to be independent of the structuring or exposure of the
surroundings.
[0132] In order to meet this requirement, so-called proximity
effects can be taken into consideration in the driving of the
pixels and thus in the choice of exposure intensity. Two different
effects have to be assumed here in the lithographic device
described above. Firstly, within an exposure line, intensity
increases occur in the case of points placed alongside one another
in comparison with an individual placed point; secondly, on account
of heat propagation, different material properties occur depending
on whether a point was placed in the vicinity shortly beforehand
(microsecond). In other words, proximity effects are to be taken
into account not only in the direction of the exposure line, but
also in the writing direction. This second effect of temporal
proximity is less dominant at low writing speeds and can generally
be disregarded.
[0133] The effect of the different placement of the storage points
can be compensated for by preprocessing the data to be written.
[0134] A final aspect in accordance with the invention relates to a
method comprising the following steps: [0135] a structuring track
is written to a storage medium in a writing direction line by line,
and [0136] in each line individual pixels arranged at a distance
from one another are formed, with a predetermined maximum
intensity, in each exposure track.
[0137] Further configurations are presented in the detailed
description.
[0138] The exposure strategy likewise consists in writing separate
pixels that have no direct neighbors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0139] The invention is explained in detail below on the basis of
exemplary embodiments in connection with the following drawings, in
which:
[0140] FIG. 1 shows an exemplary device in accordance with the
invention for microstructuring a storage element with optical
elements for generating a writing beam,
[0141] FIG. 2 shows an exemplary device in accordance with the
invention for microstructuring a storage medium with the optical
elements for monitoring and controlling the writing operation,
[0142] FIG. 3 shows a schematic illustration of the writing of two
structuring tracks lying alongside one another,
[0143] FIG. 4 shows a schematic illustration of an exposure wedge
for determining the focus error and determining the operating
point,
[0144] FIG. 5 shows a schematic illustration for elucidating a
one-point height regulation,
[0145] FIG. 6 shows a schematic illustration for elucidating a
two-point height regulation,
[0146] FIG. 7 shows a schematic illustration for elucidating a
scanning height regulation,
[0147] FIG. 8 shows a schematic illustration for elucidating a
height regulation on the basis of a wavefront sensor,
[0148] FIG. 9 shows a schematic illustration for elucidating a
height regulation on the basis of a simple wavefront sensor,
[0149] FIG. 10 shows a schematic illustration for elucidating a
height regulation on the basis of a position-dependent phase
shift,
[0150] FIG. 11 shows a schematic illustration for elucidating a
first writing strategy, and
[0151] FIG. 12 shows a schematic illustration for elucidating a
second writing strategy.
DETAILED DESCRIPTION OF THE INVENTION
[0152] FIG. 1 shows the construction of the optical components of a
device for microstructuring a storage medium 2.
[0153] A laser 4 serves as a radiation source for generating an at
least partly coherent light beam 14.
[0154] A light modulator 6 is arranged in the form of a line light
modulator in the beam path, said modulator having a plurality of
individually switchable modulator elements.
[0155] A beam shaping optical unit, which is identified overall by
the reference symbol 8, serves for illuminating the light modulator
6.
[0156] A reducing optical unit, which is identified overall by the
reference symbol 10, serves for reducing the intensity distribution
emitted by the light modulator 6.
[0157] A transport table 12 is provided in order to move the
storage medium 2 relative to the reducing optical unit 10.
[0158] According to the invention, the reducing optical unit 10 is
formed in diffraction-limited fashion, and the reducing optical
unit 10 produces an area reduction of at least, preferably 50
proceeding from the area of the individually switchable modulator
elements. As already described above, the range of favorable
reducing factors is between 50 and 1000, in particular 250.
[0159] The reducing optical unit 10 thus reduces the radiation
intensity diffracted by each modulator element to a size of less
than 10 .mu.m diameter, in particular less than 1 .mu.m diameter.
The structure sizes that can thereby be obtained on the surface of
the storage medium can therefore be classified in the micrometer
range down to the submicron range. Such small structurings in area
sizes of up to 1 m.sup.2 which can be written to by the device
according to the invention lead to a multiplicity of new
applications.
[0160] In the present exemplary embodiment, the laser 4 is formed
as a pulsed diode-pumped solid-state laser in the visible spectral
range and generates a monomode laser beam 14.
[0161] The light modulator 6 is a single-line light modulator
comprising separately switchable modulator elements, said modulator
also known as a grating light valve (GLV). The individually
switchable modulator elements each comprises a plurality of
individual strips which can be adjusted alternately from their
planar arrangement by electronic driving. By corresponding
electronic driving, the adjustable strips can be arranged in the
plane of the non-adjustable strips in such a way that a mirroring
surface arises. If the strips are adjusted, by contrast, then a
modulator having an emission characteristic that deviates from the
mirroring surface arises. As a result, the emission characteristic
of the individually switchable modulator elements can be altered in
a targeted manner and a characteristic line pattern can thus be set
across the multiplicity of modulator elements. A grating light
valve typically has 1000 or more modulator elements. The dimension
of such a line light modulator can typically be 30 mm.times.25
.mu.m, wherein the individual modulator elements have dimensions of
about 25 .mu.m in length.
[0162] The beam shaping optical unit 8 generates, from the round
beam profile of the laser beam 14 with a Gaussian radial intensity
distribution, a beam profile adapted to the area of the single-line
light modulator.
[0163] For this purpose, the beam shaping optical unit 8 firstly
has two lenses 16 and 18, which expand the laser beam 14 and which
are arranged at the distance of the sum of both focal lengths from
one another. In this exemplary embodiment, the focal length of the
lens 16 is smaller than the focal length of the lens 18. The laser
beam therefore has a larger diameter in the portion 14a of the beam
path than at the output of the laser 4.
[0164] The beam path then has two cylindrical lenses 20 and 22,
which generate, from the expanded rotationally symmetrical
intensity distribution of the laser beam 14, an elliptical
intensity distribution in the portion 14b of the beam path.
[0165] The beam shaping optical unit 8 then has a Powell lens 24,
which generates a substantially rectangular intensity distribution,
also referred to as flat-top, from the elliptical intensity
distribution. The Powell lens 24 is known per se from the prior
art, as has already been indicated in the general description. In
the portion 14c of the beam path, therefore, the laser beam has a
substantially uniform intensity distribution along the
perpendicular direction illustrated in FIG. 1.
[0166] A collimation lens 26 generates a parallel beam bundle from
the divergent beam that leaves the Powell lens 24, said beam bundle
being focused by the downstream cylindrical converging lens 28. A
mirror 30 directs the focused beam onto the line light modulator 6.
In this exemplary embodiment, the modulator elements of the line
light modulator 6 are essentially arranged in the focal plane of
the converging lens 28.
[0167] As has already been explained above, one requirement made of
the optical system is for each modulator element to be illuminated
with a plane wave, the deviation of which from the plane wave does
not exceed a maximum value of .lamda./4. In this exemplary
embodiment, it is preferred for adjacent pixels to be irradiated
with light whose plane wave properties do not deviate from one
another more than .lamda./4. It is particularly favorable, however,
if the whole light projected onto the modulator elements can be
characterized as a plane wave.
[0168] Therefore, the beam shaping optical unit 8 is preferably
formed in diffraction-limited fashion overall.
[0169] The reducing optical unit 10 is arranged in the beam path
downstream of the line light modulator 6. For this purpose, firstly
a collimation optical unit comprising two lenses 32 and 34 is
provided, which generates parallel beams in the portion 14e from
the individual single beams emitted by the virtually point-type
modulator elements in the portion 14d. These parallel beams are
then focused onto the surface of the storage medium 2 by an
objective 36. The focused beam can then also be referred to as a
writing beam because it introduces the writing energy required for
microstructuring into the storage medium 2.
[0170] Although only one objective 36 is illustrated in FIG. 1, the
reducing optical unit 10 can have at least two different objectives
having different focal lengths and/or numerical apertures in order
to obtain a fast changeover between dot sizes of different
magnitudes during the production of the microstructuring. The
objectives are preferably formed as microscope objectives.
[0171] An apertured diaphragm 37 is arranged as a further beam
shaping element in the beam path upstream of the objective 36. The
apertured diaphragm 37 realizes a spatial frequency filter. By
means of the collimation optical unit composed of the lenses 32 and
34, the image of the modulator elements of the modulator 6 is
Fourier-transformed into the Fourier plane in the region of the
apertured diaphragm 37. Higher diffraction orders that arise in the
Fourier plane are blocked by the apertured diaphragm 34, such that
substantially only the light of the zeroth diffraction order is
focused by the objective 36.
[0172] An alternative construction can be realized by virtue of the
zeroth order being blocked instead of the first order, and by
virtue of one of the first orders being transmitted. As a result, a
higher contrast can be achieved, although at the expense of the
intensity of the writing beam.
[0173] The apertured diaphragm 37 preferably has a rectangular
aperture since the beam generated by the line light modulator 6
assumes a substantially rectangular form, in particular in the
Fourier plane. By contrast, it is also possible to utilize an
elliptical or round aperture form.
[0174] The transport table 12 illustrated below the storage medium
2 is adjustable in two axes and can therefore be moved in a plane
transversely with respect to the beam propagation of the focused
beam below the microscope objective 36.
[0175] In addition, the transport table 12 can move the storage
medium 2 in the direction of the reducing optical unit 10 in order
to enable a height adaptation. This degree of freedom is
advantageous for carrying out an autofocusing, mentioned further
below, in order to keep the storage medium as precisely as possible
at the focus of the reducing optical unit 10.
[0176] A dichroic beam splitter 38 is arranged in the beam path
between the lenses 32 and 34 of the collimation optical unit and
the objective 36, and can be moved along the propagation direction
of the beam 14e by means of an actuator (not illustrated). By a
movement of the beam splitter 38, the focused line of exposure
points can be adjusted and aligned transversely with respect to the
movement of the storage medium 2, which is represented by the arrow
A.
[0177] FIG. 2 shows the optical construction for monitoring and
controlling the writing operation, in particular also for the
alignment of the structures to be exposed with respect to already
existing structures, on the storage medium 2. Firstly a camera
system 40, inter alia for monitoring the microstructuring, and
secondly an autofocusing system 42 are provided for this
purpose.
[0178] Both systems 40 and 42 begin at the dichroic beam splitter
38 described above. Above the latter system, a further, preferably
likewise dichroic beam splitter 44 is provided, which splits the
two beam paths for the camera system 40, on the one hand, and the
autofocusing system 42, on the other hand.
[0179] The camera system firstly has an illumination device in the
form of a light source 46, which is formed as a diode lamp (LED) or
as a halogen lamp. An adjustable diaphragm 48 regulates the
intensity of the quantity of light radiated onto the surface of the
storage medium 2. Via the beam splitter 44, the light beam is
directed downward in the direction of the storage medium 2, wherein
the light beam is focused by the objective 36. In this respect,
reference is made to the illustration in FIG. 1, which at 39
rudimentarily shows the beam running in the direction of the camera
system 40 and the autofocus system 42.
[0180] The light reflected from the surface of the storage medium 2
passes again, but in the opposite direction, through, the objective
36, the beam splitters 38 and 44 and impinges on a, preferably
50:50 acting, beam splitter 50. Proceeding from the beam splitter
50, the light beam (shown offset in FIG. 2) passes through an
objective 52 and a polarizer 54 and impinges on a camera 56.
[0181] The arrows B indicate that the camera 56 can be moved
transversely with respect to the propagation direction of the beam.
This freedom of movement serves to be able to observe a
sufficiently large region of the surface of the storage medium 2
with a sufficiently high resolution. If, by way of example, the
line light modulator 6 has about 1000 modulator elements and if the
imaged points on the surface of the storage medium or the written
microstructure is intended to be observed with a resolution of 4
pixels in each case, then chips having a quantity of 4000 pixels in
one direction are required, which are only available as special
productions. By contrast, if conventional chips are used, then
their quantity of pixels is insufficient, such that the resolution
deficiency can be compensated for by the described movement
transversely with respect to the light propagation. The region of
the storage medium 2 that is to be observed is therefore scanned by
the movement of the camera 56.
[0182] The camera branch 40 described above can fulfill three
tasks. The camera branch 40 can be used for preliminary examination
of the storage medium 2 to be exposed, can be used for a
microscopic examination of the structured 30 region of the storage
medium 2, and can be used for the calibration of the modulator 6,
for example of the line light modulator.
[0183] The calibration of the modulator 6 is discussed in detail
further below. At any rate the polarizer 54 arranged upstream of
the camera 56 can serve for a continuously variable intensity
attenuation during the calibration of the line light modulator.
[0184] The camera branch 40 can furthermore advantageously be used
for the fast but also accurate alignment of the overall system. It
is primarily possible to use the positioning of the below-described
laser diodes or laser diodes for height regulation relative to the
writing laser or the exposure line.
[0185] The branch of the optical construction that is arranged
above the beam splitter 44 in accordance with FIG. 2 represents an
autofocusing system 42 for adapting the distance of the storage
medium 2 from the reducing optical unit 10.
[0186] For this purpose, a first laser diode 60 is provided, the
light from which is collimated by a lens 62 and is directed through
two preferably 50:50 beam splitters 64 and 66, next through the
dichroic beam splitter 38 and lastly through the objective 36 onto
the storage medium 2. The reflected light then passes in the
opposite direction and part of the reflected light is directed
through the beam splitter 66 in the direction of a 4-quadrant
detector 68, wherein a focusing lens 70 and a further preferably
50:50 beam splitter 72 are provided in the further beam path. An
astigmatism is produced by the coplanar plate 74, such that the
signal generated by the 4-quadrant detector 68 has an item of
direction information with respect to the defocusing of the light
beam on the surface of the storage medium 2.
[0187] By the defocusing information, a distance deviation of the
storage medium 2 from the reducing optical unit 10, i.e. from the
objective 36, can be determined and the corresponding signal can be
used for regulating the distance, i.e. for an autofocusing.
[0188] A second laser diode 76 with a further lens 78 and also a
further 4-quadrant detector 80 are provided in parallel with the
first laser diode 60 and the first 4-quadrant detector 68. The
light from the second laser diode 76 is directed onto the surface
of the storage medium 2 via the beam splitter 64 in the same way as
the light from the first laser diode 60 and is reflected from said
surface. The reflected light is then directed through the beam
splitter 72, which then acts as a coplanar plate, and, provided
with an astigmatism, onto the 4-quadrant detector 80, where a
distance deviation of the storage medium 2 from the objective 36
can then likewise be established on the basis of the direction
information of the defocusing.
[0189] The application of two laser diodes and two 4-quadrant
detectors means that an autofocusing can be carried out both
temporally and spatially independently of one another. In this
exemplary embodiment, the construction described has the effect
that a maximum number of optical components is used jointly for
both beam paths and a small space requirement thus arises.
[0190] FIG. 2 also illustrates at the bottom that a mirror 82 can
also be used in place of the dichroic beam splitter 38, which
mirror then directs the light modulated by the line light modulator
6 onto the camera system 40 and thus enables a direct observation
of the functioning of the line light modulator 6. The beam splitter
38 and mirror 82 are adjustable jointly along the direction
represented by the arrow C, such that a changeover between the two
operating modes is easily possible.
[0191] FIG. 3 shows a schematic illustration of writing to the
storage medium 2 two adjacent strips 90 and 92 with
microstructurings which are intended to form for example a
continuous computer-generated hologram.
[0192] As can be discerned in FIG. 3, the edge of the strip 90
deviates from the straight line illustrated in dashed fashion, the
size of the deviation being exaggerated in the illustration. The
strip 92 to be newly written is intended to be written as close as
possible to the first strip. For this purpose, in the present
exemplary embodiment, with the camera system 40 that has been
described in connection with FIG. 2, the right-hand edge of the
strip 90 is observed and for example the beam splitter 38 is
adjusted such that the line 94 that is currently to be written is
adjusted toward the left in accordance with the arrow D by a
distance determined by the evaluation of the camera 15 system 40.
It is thus possible for the two strips 90 and 92 to be written in a
manner virtually adjoining one another.
[0193] Ultimately, it is thereby possible for a plurality of strips
to be written on a relatively large area.
[0194] The function and application of the autofocus system
42--which has already been explained above with reference to FIG.
2--are described in more detail below.
[0195] The height regulation system preferably operates with a
plurality of scanning points, in particular with two scanning
points.
[0196] In the application of various embodiments for
microstructuring, the goal of which is exposures of two or more
square centimeters, an areal height regulation is particularly
advantageous. The autofocusing described below is nevertheless not
restricted to application explained herein and can also be applied
in comparable devices.
[0197] As has already been mentioned in FIG. 2, a dichroic beam
splitter 38 separates the exposure branch from the camera system 40
and from the autofocus system 42 of the optical construction.
[0198] A height regulation with a direction-dependent measurement
point is preferred. For this purpose, the autofocus for finding and
keeping the focus of the writing beam in the region of the storage
layer of the storage medium 2 comprises an autonomous system based
on the astigmatism method. In this case, an optical unit is used
which has different focal lengths in two directions--which are
usually at right angles to one another--transversely with respect
to the light propagation direction. Thus, for the two directions at
different distances with respect to the optical unit, focal planes
are obtained which bring about a distortion of the beam diameter in
the respective other direction. The beam cross section is round
only in the region of half the path between the two focal planes. A
4-quadrant detector can then establish the extent to which and the
direction in which the beam cross section picked up deviates from
the round form. A reset signal can then be generated from the
deviation in order to set a uniform illumination of the 4-quadrant
detector.
[0199] The two laser diodes 60 and 76 illustrated in FIG. 2 serve
as light sources, said laser diodes operating independently of one
another and their reflected light being captured by two 4-quadrant
detectors 68 and 80.
[0200] Two height regulation systems are necessary since the
detection point, depending on the direction of movement 35 of the
storage medium 2 relative to the writing beam, has to be situated
either on the left-hand side or on the right-hand side of the
exposure region. This is because a meaningful evaluation of the
focus error signal is possible only in a non-structured, i.e.
unexposed region of the storage medium 2. Moreover, the autofocus
point is intended to be determined in a manner leading the writing
operation in order to be able subsequently to correct a change in
the height position of the surface of the storage medium 2.
[0201] For setting the illumination points of the two laser diodes
60 and 76, the two laser diodes are arranged such that each of the
two illumination points is arranged on either side of the focus
line of the focused writing beam on the surface of the storage
medium 2. For this purpose, the position of the two laser diodes 60
and 76 can be set mechanically and/or electronically.
[0202] Furthermore, the two independent autofocus systems are
arranged in a manner interleaved in one another in space-saving
fashion and utilize a plurality of optical components in common.
Primarily the branches with the 4-quadrant detectors 68 and 80
share the focus lens 70.
[0203] Instead of the two laser diodes 60 and 76 described above,
it is also possible to provide further light sources in order to
enable further illumination points for further autofocusing
systems. For this purpose, for each light source there is
correspondingly a 4-quadrant detector for measuring the deviation
of the illumination point from the predetermined position. By way
of example, two or three illumination points can be arranged on
either side of the focused writing beam on the surface of the
storage medium 2. The principle described here is therefore not
restricted to two light sources.
[0204] If more than one illumination point is provided on one side
of the focused writing beam, then it is thereby possible to
establish not only a deviation of the height position but also a
detection of the material position, e.g. height and tilting.
[0205] The above-described system with a plurality of scanning
points or illumination points can also be referred to as a
multipoint astigmatism height regulation system. Depending on the
direction of movement of the transport table 12, different height
regulation signals are then evaluated or the system changes the
position of the scanning beam by an adjustment of the position of
the light sources or of an optical means, such as beam splitter or
mirror, arranged in the beam path.
[0206] The operating point is determined for the accurate
positioning of the distance between the objective 36 and the
storage medium 2. The operating point determination can be
automated, particularly in the case of continuous exposures, by
virtue of the diffraction efficiency of a written test structure
being analyzed in temporal and spatial proximity by means of,
preferably, a photodetector or a camera with varying setting of the
operating point. The operating point setting for which the best,
but not necessarily highest, diffraction efficiency was achieved
can then be deduced by way of the location or time. Additional
information can also be obtained here by evaluating higher
diffraction orders. Furthermore, the operating point can be
effected, preferably automatically, by the evaluation of a contrast
function of the exposed structure.
[0207] If a tilting is detected by the autofocus system with at
least two illumination points, i.e. for example with a two-point
autofocus regulation, then a tilting correction of the storage
medium can be effected during the running time by means of an
actuator which tilts the diffraction grating 6 in the corresponding
direction.
[0208] The spatial offset as a result of the process-dictated delay
during the writing process has an effect primarily during
bidirectional writing. In order to avoid an impairment of the
exposed structure, an electronic offset correction can be provided.
In principle, the offset correction consists in an adjustable
additional delay time and/or a spatial pretrigger of the laser.
This compensates for the time period that elapses between the
measurement of the height position or the tilting and the writing
instant, in which the storage medium is adjusted between the
measurement position and the writing position. Furthermore, the
tilting is likewise realized by an additional adjustable
delay--here rising linearly, however, whereas it is fixed in the
case of the offset compensation.
[0209] The height regulation is based on the astigmatism principle
also used in CD or DVD drives. The extent and the orientation of
the defocusing can be detected on the basis of the orientation of
the reflected-back beam that is deformed by the astigmatism and
imaged onto a 4-quadrant detector. This information serves for
driving an actuator that tracks the sample or the optical system in
accordance with the height profile of the storage material.
[0210] The method for determining the operating point is based on
producing an exposure wedge. In this respect reference is made to
FIG. 4. In this exemplary embodiment, a region of the material is
structured in which along one axis the intensity and along the
other axis the exposure height are altered continuously. A value
range of 16 different gray shades is preferably used for the
intensity. The height is varied over that region of the S curve
(about 20 .mu.m) which was defined beforehand in the adjustment of
the S curve. A microscopic analysis of the exposed region leads to
a wedge, see FIG. 4. The optimum exposure height is assumed where
structuring still occurs in conjunction with very low intensity.
Since the exposure height does not have a fixed relationship with
the drive voltage of the actuator on account of the hysteresis, the
focus error signal (FES) is preferably used as a reference for an
optimum exposure. The FES voltage is therefore the operating point
of the height regulation.
[0211] In order to be able to relate the exposure wedge to the
focus error signal, it is necessary to rapidly generate and store
the S curve synchronously with the exposure timing. The exposure of
the wedge should be effected at normal exposure speed in order to
increase the meaningfulness for the later large-area exposure. The
exposure of the gray-scale value wedge and the determination of the
operating point result as follows:
[0212] Firstly, a region for the gray-scale value wedge is selected
by means of the microscopy function in the camera branch of the
device. This is done by zeroing a position counter in the drive
electronics.
[0213] The exposure pattern of the gray-scale value wedge (16
gray-scale values) is loaded.
[0214] The transport table is set back by a defined position in
order to have a run-up for the exposure operation.
[0215] The height regulation module is put into the gray-scale
value wedge mode. This has the effect that the values of the S
curve are stored in a memory in a manner triggered by the laser
pulse. In this case, the memory can be situated either preferably
in the height regulation module or alternatively in the drive
electronics.
[0216] The exposure of the gray-scale value wedge is preferably
effected by means of 16 gray-scale value or intensity and 128
height steps, where the numerical values mentioned should be
understood by way of example and are only preferred. In this
exemplary embodiment, the drive voltage of the piezo actuator of
the height adjustment of the microscope objective is altered in the
ranges defined beforehand. The range restriction is effected during
the adjustment of the height of the S curve by user intervention.
At the same time as the exposure and the changing of the
piezo-voltage, the value of the S curve is determined and
stored.
[0217] The microscopy mode of the device is activated after the
exposure. In this mode, the tip of the exposure wedge is brought to
the center of the camera image or target mark by the movement of
the linear table. Here the current position of the transport table
is read out from the drive electronics. The stored S curve data are
likewise read out. That S curve value at which an optimum exposure
occurs can be read out on the basis of the current position in the
read-out S curve data. This value is the operating point of the
height regulation.
[0218] A one-point height regulation or a bidirectional writing
requires an illumination or detection point on either side of the
exposure lines, i.e. of the focused writing beam, since the
detection functions effectively and in a manner free of
disturbances only in the unstructured storage medium. This
situation is illustrated in FIG. 5, wherein the exposure line is
illustrated as a box pattern and the two illumination points are
illustrated as circles.
[0219] In accordance with the direction of movement, only one of
the two illumination points and the respectively assigned
4-quadrant detector are used for the evaluation. Through the
distance between the exposure line and the detection point, the
system looks into the future or the system can compensate for the
reaction time of the electronics and of the actuator. Preferably,
the detection points are situated in the center of the exposure
line, but it is also possible to choose any other position if this
is advantageous.
[0220] However, by the arrangement shown in FIG. 5 with one
detection point on either side of the illumination line, in a
direction of movement, no tilting of the storage material along the
exposure line can be detected and thus compensated for. This system
relies on the storage material and the focus region (depth of
field) of the exposure beam being parallel. A height regulation can
be effected only transversely with respect to the orientation of
the exposure line.
[0221] The construction of the two-point height regulation is
illustrated in FIG. 6 and is similar to the one-point height
regulation described above. In order to be able to detect a tilting
between the exposure line and the storage medium along the exposure
line, instead of one detection point or scanning point, two
detection points or scanning points are used for each direction of
movement. If the detection points are arranged as in FIG. 6, then a
tilting can be detected. The advantage of this system, moreover, is
that independent height regulations can be used for each detection
point. One detection point, e.g. the respective upper detection
point in FIG. 6, can be incorporated in a closed height regulation
loop with the piezo actuator at the microscope objective, wherein
the other detection point, e.g. the respective lower detection
point in FIG. 6, can act in a regulating loop with a tilting
mechanism, e.g. at the fixing of the line light modulator.
[0222] It is thereby possible to use customary linear standard
regulators, for example a PID regulator.
[0223] The tilting component can be separated from the z component
electronically by a coordinate transformation and can then be
passed to the respective actuator control loops.
[0224] A further preferred alternative to the previous systems and
methods for detecting the position of the storage layer with one or
two fixed detection points is a system which operates with a
scanning detection point and thus realizes a scanning height
regulation. The principle of the scanning height regulation is
illustrated in FIG. 7.
[0225] At a specific distance from the exposure line, a detection
point is scanned along the exposure line or perpendicular to the
direction of movement, whereby a substantially sinusoidal waveform
is produced, for example. In combination with the movement of the
exposure line and of the detection point relative to the storage
material, the material is thereby areally exposed and
simultaneously scanned.
[0226] By the astigmatism method described above, the detection
point obtains information about the height profile of the storage
material before the exposure line. This information is composed of
the error signal including the voltage of the piezo actuator of the
microscope objective. If the two items of information are. combined
with the driving or current position of the detection point, then a
height profile is obtained. The goal of a regulation is then to
minimize the integral of the focus error by the regulation firstly
adapting the piezo actuator of the microscope objective and
secondly adjusting the actuator for the tilting of the exposure
line relative to the storage material.
[0227] The advantage of this solution is that a height profile can
be recorded over the entire writing region or at least part of the
writing region. An ideal setting of the height and tilting is
possible as a result. Errors, on account of dirt particles at the
detection point, can be detected as such and delimited.
Furthermore, a curvature of the storage material that is not
detected in the case of a two-point detection can be taken into
consideration. Moreover, it is possible to create a height profile
of the storage medium for quality assurance tasks.
[0228] However, the evaluation of the height profile and the
driving of the actuators are more complex and consequently require
a higher computational complexity.
[0229] A further preferred configuration of the height regulation
consists in a wavefront height regulation, for which purpose, for
the height regulation or detection of the storage surface, it is
possible to use a system that operates similarly to a wavefront
sensor, as is illustrated in FIG. 8.
[0230] A collimated laser beam 100 impinges on a storage medium 102
after deflection by a beam splitter 101. The reflected-back wave is
directed onto a one-dimensional lens array 108, which preferably
has cylindrical lenses 110, via an intermediate focus produced by
the lens pair 104, 106. On account of the Fourier transformation
property of the lenses 110, the position of the focus is dependent
on the angle of the incident beam 112 and thus on the tilting of
the surface of the storage medium 102.
[0231] The shift of each focus of the individual lenses 110 can be
used for the detection of the position and orientation of the
surface of the storage medium 102. Moreover, this method affords a
possibility for obtaining areal information about the surface of
the storage medium 102.
[0232] If, instead of a one-dimensional lens array 108, a
two-dimensional lens array with a two-dimensional detector,
preferably a camera chip, is employed, then it is possible to drive
the actuators in anticipatory fashion for height and tilting of the
storage medium relative to the objective, for example the objective
36 in FIG. 1.
[0233] However, this type of height measure requires an additional
absolute height measurement at least one point since the system
described above obtains only gradients of the height rather than
absolute values.
[0234] In a further preferred alternative it is possible to realize
a simple wavefront sensor with the construction shown in FIG. 9. In
this exemplary embodiment, a plurality of sample beams 120 are
directed at different angles onto the schematically illustrated
microscope objective 122, which may correspond for example to the
objective 36 in FIG. 1, and the storage medium 102.
[0235] As a result of a tilting or deformation of the storage
medium 102, each sample beam 120 experiences its own offset, which
is illustrated by small arrows at the top in FIG. 9. An areal
sensor (camera chip) can determine the offset with respect to a
reference position by centroid calculation of the measured
intensity distribution, from which offset in turn the gradient of
the surface of the storage medium 102 can be deduced. An absolute
height determination at one point is once again necessary here.
[0236] A further preferred exemplary embodiment obtains the height
information by means of a position-dependent phase shift. This
system is illustrated schematically in FIG. 10 and operates with a
plurality of sample beams 120 in the same way as previously
described. A deformation of the reflection plane, i.e. of the
surface of the storage medium 102, produces a translation in the
Fourier plane. In this exemplary embodiment, the Fourier
transformation is produced by two focusing lenses 130 and 132. If
an optical element 134 that alters the phase depending on the
position, for example a spherical or alternatively an aspherical
lens or a cylindrical lens, is introduced in the Fourier plane
between the two lens, then this results in a shift in the foci in
the focal plane of the second lens 132. By way of the shift in the
foci, the deformation of the surfaces of the storage medium 102 can
then in turn be deduced. As in the systems described above, an
absolute measurement of the heights at one point is necessary here,
too.
[0237] A multiplicity of possible and unavoidable inadequacies in
the optical, mechanical and electrical construction can be
compensated for by a calibration of the light modulator 6. The
preferred line light modulator 6 is generally mentioned without
wishing to exclude a two-dimensional modulator.
[0238] In an exemplary embodiment, each individual pixel is
measured. The measurement result is stored for each pixel
separately in a table in a computer. This table is used to
calculate a look-up table, which is stored in the drive electronics
of the line light modulator 6. The central element is a CMOS camera
or some other suitable camera system which is used for the local
intensity measurement. By an upstream image processing, a region
around the focus of an imaged pixel of the line light modulator is
identified which is subsequently used for the intensity
determination. This ensures that no artifacts such as light
reflections are taken into account in the measurement. Moreover, a
fine adjustment of a photodiode that is usually used for such
measurements is obviated.
[0239] Furthermore, the use of a CMOS camera permits the position
detection of the current focus. If the latter migrates from a
central region, then a directed repositioning can be effected by
means of the transport table.
[0240] A further method for determining the intensity of an
individual pixel of the line light modulator is based on the use of
a small aperture, preferably having a diameter of about 10 .mu.m,
under which a photodetector is positioned, and an X-Y positioning
device. By means of the X-Y positioning device in the writing plane
(plane of the storage material), the pin hole, i.e. the small
aperture, is positioned centrally with respect to a writing point
to be examined. The intensity of the impinging writing beam is
measured by means of the photodiode and an evaluation electronic
unit, which preferably operates logarithmetically.
[0241] The calibration specified above is effected in a relatively
complicated manner and compensates for inaccuracies and temporal
fluctuations in the illumination of the line light modulator 6.
[0242] The following functions are provided in the calibration:
[0243] submicron measurement of the writing focus [0244]
correlation to a pixel of the line light modulator [0245]
measurement of the transfer function [0246] deviation of uniform
intensity steps over all pixels [0247] storage in a look-up table
[0248] correction of the driving of the line light modulator during
the running time [0249] on-line monitoring and calibration of the
correction of the line light modulator or illumination optical unit
[0250] feedback of the exposure quality (holographic and pattern
based quality) and on-line calibration of the exposure.
[0251] During scanning operation, in connection with the
construction described in FIG. 1 and the writing path achieved
therewith, it is assumed that the exposure line is fixed relative
to the objective or to the device, in particular to the objective
36. By introducing an actuator e.g. for moving the beam splitter 38
"dichroic beam splitter", it is possible to move the exposure line
relative to the objective 36.
[0252] If a movement of the exposure line in the direction of the
material transport is realized, it is thereby possible to
compensate for a jitter brought about, inter alia, by a speed
fluctuation of the transport table 12.
[0253] If a movement transversely with respect to the material
transport is realized, this degree of freedom permits compensation
of faults or inaccuracies in the material guidance. Inter alia, in
the case of an areal exposure composed of a plurality of exposure
lines, it is necessary for the exposure lines to be placed against
one another in a controlled manner; in this respect, see the
description of FIG. 3 presented above. By a detection of the
preceding exposure, it is possible for the tracks to be placed
exactly against one another by means of the scanning device.
[0254] Likewise by a movement of the exposure line transversely
with respect to the direction of movement of the storage medium 2,
the exposure can be effected at a position predefined in the
material of the storage medium 2. This in turn presupposes a
detection of the predefined position, for example by an embossed
track.
[0255] A detailed explanation is given below of preferred writing
strategies or exposure strategies in the application of a device
for microstructuring which concern a control of the intensity
during the writing operation. However, the exposure strategy is not
dependent on the precise configuration of the device as described
above, but rather relates to all exposure operations in which
microstructures are introduced into a storage medium line by line
or areally.
[0256] Preferably, for this purpose use is made of a control system
for correcting the proximity effects during the running time
together with an algorithm comprising a splitting of the line
profile into a portioned component and into a differential
component. The correction depth can be set depending on parameters
which can be chosen in a variable manner and which can be
material-specific, inter alia.
[0257] In an exemplary embodiment, a specific pulse strategy is
used as a degree of freedom in the material structuring, which can
preferably be set electronically, in particular in a manner
dependent on whether the exposure line is oriented orthogonally or
obliquely with respect to the direction of movement.
[0258] In an exemplary embodiment, the degree of structuring of
individual exposure points in a scanned pattern, i.e. for example
the intensity of the conversion--effected by the exposure--of the
optical property of the material of the storage medium, is intended
to be independent of the structuring or exposure of the
surroundings.
[0259] In order to meet this requirement, so-called proximity
effects can be taken into consideration in the driving of the
pixels and thus in the choice of exposure intensity. Two different
effects have to be assumed here in the lithographic device
described above. Firstly, within an exposure line, intensity
increases occur in the case of points placed alongside one another
in comparison with an individual placed point; secondly, on account
of heat propagation, different material properties occur depending
on whether a point was placed in the vicinity shortly beforehand
(microsecond). In other words, proximity effects are to be taken
into account not only in the direction of the exposure line, but
also in the writing direction. This second effect of temporal
proximity is less dominant at low writing speeds and can generally
be disregarded.
[0260] The effect of the different placement of the storage points
can be compensated for by a preprocessing of the data to be
written.
[0261] FIG. 11 illustrates an example of an exposure strategy which
takes a preprocessing as a basis. The individual pixel is ascribed
the maximum energy for formation. By means of the gray-scale value
option of driving, in the case of a double pixel the intensity of
the two adjacent pixels is reduced. In the case of three adjacent
pixels, the intensity is reduced further in the case of the central
pixel. In the case of further adjacent exposure points, the central
pixels are exposed with identical intensity.
[0262] As a result, exposure points are fashioned largely
identically. Under certain circumstances, the third intensity
reduction can be dispensed with. The writing strategy described can
be employed both within an exposure line and from exposure line to
exposure line. In this exemplary embodiment, it can furthermore be
taken into account that the individual pixels in an exposure line
are written simultaneously and that adjacent pixels from different
exposure lines are written in a temporally offset manner. An
exposure strategy from line to line can be obviated, however,
depending on the material properties.
[0263] A further preferred exposure strategy consists in writing
separate pixels which have no direct neighbors. For this purpose,
an exposure pattern with isolated exposure points is preferably
written in, wherein one pixel of a pair of possible exposure points
is in each case activated during a desired exposure, while the
other pixel remains blanked. This ensures that each pixel is
surrounded by a non-exposed region.
[0264] In order to compensate for the pitch spacing that increases
as a result, it is possible to achieve a double reduction by
replacing the objective. Computer-generated holograms that are
exposed according to this method exhibit a smaller edge roll-off in
the reconstruction. This is because the smaller the written dots
are in relation to the entire structure, the larger is the region
having sufficiently high intensity in the reconstruction of the
hologram. The region that decreases in intensity toward the edge
lies further outward and thus leads to improvement of the quality
of hologram.
[0265] A pitch spacing that is larger only by the factor 2 is
obtained if the orientation of the line light modulator is tilted
with respect to the direction of movement. This is achieved by
changing the active pixel of a pairing from line to line; in this
respect, see FIG. 12.
* * * * *