U.S. patent application number 17/296739 was filed with the patent office on 2021-12-23 for reducing impact of cross-talk between modulators that drive a multi-channel aom.
This patent application is currently assigned to Mycronic AB. The applicant listed for this patent is Mycronic AB. Invention is credited to Anders SVENSSON.
Application Number | 20210397066 17/296739 |
Document ID | / |
Family ID | 1000005813298 |
Filed Date | 2021-12-23 |
United States Patent
Application |
20210397066 |
Kind Code |
A1 |
SVENSSON; Anders |
December 23, 2021 |
REDUCING IMPACT OF CROSS-TALK BETWEEN MODULATORS THAT DRIVE A
MULTI-CHANNEL AOM
Abstract
The disclosed technology teaches a method of reducing the impact
of cross-talk between transducers that drive an acousto-optic
modulator. The method includes operating the transducers, which are
mechanically coupled to an acousto-optic modulator medium, with
different frequencies applied to adjoining transducers and
producing a time-varying phase relationship between carriers on
spatially adjoining modulation channels emanating from the
adjoining transducers, with a frequency separation between carriers
on the adjoining channels of 400 KHz to 20 MHz. The disclosed
technology also includes operating 5 to 32 modulators, which are
mechanically coupled to the acousto-optic modulator crystal, and
varying the different frequencies applied to the modulators in a
sawtooth pattern, varying the different frequencies over a range
and then repeating variation over the range. Also included is
varying the frequencies applied to the modulators in a rising or
falling pattern applied progressively to the spatially adjoining
transducers.
Inventors: |
SVENSSON; Anders;
(Sollentuna, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mycronic AB |
Taby |
|
SE |
|
|
Assignee: |
Mycronic AB
Taby
SE
|
Family ID: |
1000005813298 |
Appl. No.: |
17/296739 |
Filed: |
December 13, 2019 |
PCT Filed: |
December 13, 2019 |
PCT NO: |
PCT/EP2019/085022 |
371 Date: |
May 25, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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16221296 |
Dec 14, 2018 |
|
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17296739 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03F 7/704 20130101;
G02F 1/113 20130101; G02F 1/332 20130101 |
International
Class: |
G02F 1/33 20060101
G02F001/33; G02F 1/11 20060101 G02F001/11; G03F 7/20 20060101
G03F007/20 |
Claims
1. A method of reducing impact of cross-talk between transducers
that drive a multibeam acousto-optic modulator, abbreviated AOM,
including: operating the transducers, which are coupled to an
acousto-optic medium for driving separate modulation channels
within the acousto-optic medium, with different single-frequency
signals applied to adjoining transducers and thereby producing a
time varying phase relationship between carriers on spatially
adjoining modulation channels emanating from the adjoining
transducers.
2. The method of claim 1, further including operating the
transducers with the single-frequency signals having differences
between pairs of adjoining transducers of at least 100 KHz and a
maximum difference of 20 MHz.
3. The method of claim 2, wherein frequency differences between
pairs of adjoining transducers is in a range of 400 KHz to 10
MHz.
4. The method of claim 1, further including operating between 5 and
32 of the transducers to produce 5 to 32 modulation channels in the
acousto-optic medium.
5. The method of claim 1, wherein the different frequencies between
the spatially adjoining modulation channels are arranged in a
sawtooth pattern.
6. The method of claim 1, wherein the different frequencies between
the spatially adjoining modulation channels are arranged in a
rising or falling pattern applied progressively to the adjoining
transducers.
7. The method of claim 1, wherein the different frequencies vary
between pairs of adjoining transducers by an amount in a range of
plus or minus three percent from an average frequency applied to
the transducers.
8. A multibeam acousto-optic modulator, abbreviated AOM, with
reduced impact of cross-talk between transducers that are part of
the AOM, including: an acousto-optic medium; a plurality of
transducers physically coupled to the acousto-optic medium, spaced
apart to drive separate modulation channels within the
acousto-optic medium; and a signal synthesizer coupled to the
transducers that drives the transducers at different
single-frequency signals to produce a time varying phase
relationship between spatially adjoining modulation channels.
9. The multibeam AOM of claim 8, further including operating the
transducers with the single-frequency signals having differences
between pairs of adjoining transducers of at least 100 KHz and a
maximum difference of 20 MHz.
10. The multibeam AOM of claim 9, wherein frequency differences
between pairs of adjoining transducers are in a range of 400 KHz to
10 MHz.
11. The multibeam AOM of claim 8, further including operating
between 5 and 32 of the transducers to produce 5 to 32 modulation
channels in the acousto-optic medium.
12. The multibeam AOM of claim 8, wherein the different frequencies
between the spatially adjoining modulation channels are arranged in
a sawtooth pattern.
13. The multibeam AOM of claim 8, wherein the different frequencies
between the spatially adjoining modulation channels are arranged in
a rising or falling pattern applied progressively to the adjoining
transducers.
14. The multibeam AOM of claim 8, wherein the different frequencies
vary between pairs of adjoining transducers in an amount in a range
of plus or minus five percent from an average frequency applied to
the transducers.
15. A microlithographic laser writer comprising an AOM of claim
8.
16. A microlithographic laser writer configured to perform the
method of claim 1.
Description
FIELD OF THE TECHNOLOGY DISCLOSED
[0001] The disclosed invention relates to pattern generation,
direct-write lithography and to optical writing of patterns on a
photosensitive surface in general. In particular it relates to the
patterning of photomasks, wafers, printed circuit boards (PCBs),
fine-pitch interconnection substrates, flexible substrates with or
without active components (transistors) and/or of panels for
displays, photovoltaics and illumination. Other patterns with line
widths from 0.03 to 10 microns may also use the technology
disclosed. In particular the technology relates to high-precision
pattern generators and direct writers using acousto-optic
modulation.
BACKGROUND
[0002] The subject matter discussed in this section should not be
assumed to be prior art merely as a result of its mention in this
section. Similarly, a problem mentioned in this section or
associated with the subject matter provided as background should
not be assumed to have been previously recognized in the prior art.
The subject matter in this section merely represents different
approaches, which in and of themselves may correspond to
implementations of the claimed technology also.
[0003] Streaming video from smartphones and tablets require
high-resolution displays, which are only possible with the use of
advanced manufacturing tools, including laser mask writers for
photomask production. Display mask writers are the de facto used in
the industry for production of all high-resolution thin film
transistor (TFT), liquid crystal display (LCD) and active-matrix
organic light-emitting diode (AMOLED) displays worldwide.
[0004] Pattern generators are used to write microscopic images onto
photomasks which then function as templates for mass production of
displays, integrated circuits and electronic packaging. The
manufacturing process, called microlithography, is similar to the
way in which photographs are reproduced with the help of a
negative. A microlithographic laser writer uses a laser beam to
pattern a latent image in a photosensitive surface, such as resist
on a mask, which is used, in turn, to pattern wafers or large area
displays. In the photomask manufacturing industry, stringent
requirements are placed on critical dimensions (CD).
[0005] Acousto-optic modulation is commonly used in laser scanners,
providing a reasonable compromise between cost, speed and
efficiency. The laser scanner using an acousto-optic modulator
(AOM) may have a single beam or multiple beams and after the
modulation of the beam it may be scanned by electro-optic or
mechanical means. Prior art exists in the form of polygon scanners,
and acousto-optic, all of them employing acousto-optic multibeam
modulation.
[0006] The pattern line width measurement is a critical dimension
(CD) also referred to as edge roughness, which varies as a result
of variations in the signal for the laser dose, which is controlled
using acousto-optic modulation. An opportunity arises to improve
the stability of line widths, and thereby the critical dimensions
for pattern generation, direct-write lithography and for optical
writing of patterns on a photosensitive surface.
SUMMARY OF THE INVENTION
[0007] As a first aspect of the invention, there is provided a
method of reducing impact of cross-talk between transducers that
drive an acousto-optic modulator, abbreviated AOM, including:
operating the transducers, which are coupled to an acousto-optic
medium, with different frequencies applied to adjoining transducers
and producing a time varying phase relationship between carriers on
spatially adjoining modulation channels emanating from the
adjoining transducers.
[0008] The method may be for reducing impact of cross-talk between
transducers that drive an AOM in a microlithographic laser
writer.
[0009] The AOM may comprise a plurality of transducers. The
transducers may be configured for creating an acoustic wave in the
acousto-optic medium. The transducers may for example be
piezoelectric transducers.
[0010] In embodiments, the method is further comprising operating
the transducers with the frequencies having differences between
pairs of adjoining transducers of at least 100 KHz and a maximum
difference of 20 MHz. As an example, the frequency differences
between pairs of adjoining transducers may be in a range of 400 KHz
to 10 MHz. As a further example, the frequency differences between
pairs of adjoining transducers may be less than 5 MHz, such as less
than 2 MHz, such as less than 1 MHz, such as less than 500 kHz. As
an example, the frequency differences between pairs of adjoining
transducers may be between 400 kHz and 1 MHz.
[0011] In embodiments, the method comprises operating between 5 and
32 of the transducers to produce 5 to 32 modulation channels in the
acousto-optic medium.
[0012] In embodiments, the different frequencies between the
spatially adjoining modulation channels are arranged in a sawtooth
pattern.
[0013] In embodiments, the different frequencies between the
spatially adjoining modulation channels are arranged in a rising or
falling pattern applied progressively to the adjoining
transducers.
[0014] In embodiments, the different frequencies vary between pairs
of adjoining transducers by an amount in a range of plus or minus
three percent from an average frequency applied to the
transducers.
[0015] As a second aspect of the invention, there is provided an
acousto-optic modulator, abbreviated AOM, with reduced impact of
cross-talk between transducers that are part of the AOM, including:
[0016] an acousto-optic medium; [0017] a plurality of transducers
physically coupled to the acousto-optic medium, spaced apart to
drive separate modulation channels within the acousto-optic medium;
and [0018] a signal synthesizer coupled to the transducers that
drives the transducers at different frequencies to produce a time
varying phase relationship between spatially adjoining modulation
channels.
[0019] The AOM may further comprise acoustic absorbers for
preventing reflection of waves generated by the transducers back to
through the acousto-optic medium.
[0020] The acousto-optic medium may for example be silica, quartz
or glass.
[0021] The AOM may have a beam entrance surface and a beam exit
surface. Each transducer may be adapted to modulate a laser beam
within a specific modulation zone that traverses the laser beam
path between the beam entrance surface and the beam exit
surface.
[0022] The signal synthesizer may be adapted to generate a
radiofrequency signal (RF signal), and the plurality of transducers
may be adapted to convert the RF signals from the signal
synthesizers into acoustic waves that transverse through the
AOM.
[0023] In embodiments of the second aspect, the AOM is further
including operating the transducers with the frequencies having
differences between pairs of adjoining transducers of at least 100
KHz and a maximum difference of 20 MHz. Thus, the signal
synthesizer may be configured to operate the transducers with the
frequencies having differences between pairs of adjoining
transducers of at least 100 KHz and a maximum difference of 20
MHz.
[0024] In embodiments of the second aspect, frequency differences
between pairs of adjoining transducers are in a range of 400 KHz to
10 MHz. The signal synthesizer may be configured to operate the
transducers with frequency differences between pairs of adjoining
transducers that are in a range of 400 KHz to 10 MHz.
[0025] The phase relationship between spatially adjoining
modulation channels may be between 2/3.pi. and 4/3.pi..
[0026] In embodiments of the second aspect, the AOM includes
operating between 5 and 32 of the transducers to produce 5 to 32
modulation channels in the acousto-optic medium. The signal
synthesizer may be configured to operate between 5 and 32 of the
transducers to produce 5 to 32 modulation channels in the
acousto-optic medium.
[0027] In embodiments of the second aspect, different frequencies
between the spatially adjoining modulation channels are arranged in
a sawtooth pattern.
[0028] In embodiments of the second aspect, the different
frequencies between the spatially adjoining modulation channels are
arranged in a rising or falling pattern applied progressively to
the adjoining transducers.
[0029] In embodiments of the second aspect, different frequencies
vary between pairs of adjoining transducers in an amount in a range
of plus or minus five percent from an average frequency applied to
the transducers.
[0030] As third aspect of the invention, there is provided a
microlithographic laser writer comprising an AOM according to the
second aspect above.
[0031] As third aspect of the invention, there is provided a
microlithographic laser writer configured to perform the method
according to the first aspect above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] In the drawings, like reference characters generally refer
to like parts throughout the different views. Also, the drawings
are not necessarily to scale, with an emphasis instead generally
being placed upon illustrating the principles of the technology
disclosed. In the following description, various implementations of
the technology disclosed are described with reference to the
following drawings.
[0033] FIG. 1A shows a multibeam laser scanner, as known in prior
art.
[0034] FIG. 1B shows the radio frequency (RF) driver of FIG. 1A in
detail, as known in prior art.
[0035] FIG. 2A shows an example spatial-dimension graph for a
multi-channel AOM for an array of fifteen channels of transducers
integrated with a single acousto-optic crystal, operated at a
carrier frequency of 220 MHz.
[0036] FIG. 2B shows a 3-dimensional pattern for sound waves
leaking from adjacent AOM channels.
[0037] FIG. 3A shows a graph of measured line width critical
dimension (CD) per unit time for three different patterning runs
represented using three different colors.
[0038] FIG. 3B shows a graph of measured line width critical
dimension (CD) per unit time results for a second test pattern in
which the set of transducer beams were turned on at the same
time.
[0039] FIG. 4 shows spread carrier frequency approach results, for
two tests in which 15 transducer channels, each fed by independent
electronics, were assigned unique carrier frequencies. In the first
test, only one of the exposure beams was turned on at any given
moment. For the second test, all of the exposure beams were turned
on at the same time.
[0040] FIG. 5 shows spread carrier frequency approach results for
the measured critical dimension for a highlighted segment with all
of the transducers turned on at the same time.
[0041] FIG. 6 shows a simulation of interference from neighboring
transducers that affects a center transducer.
[0042] FIG. 7 displays a simulation of average energy disturbance
that will be imprinted in the energy distribution over the laser
beam, from two neighboring channel signals.
[0043] FIG. 8 displays a simulation of the average energy
disturbance that will be imprinted in the energy distribution over
the laser beam for three different phase relationships.
[0044] FIG. 9 shows an example sawtooth distribution of channel
frequencies in an AOM with 15 transducer channels.
[0045] FIG. 10 shows an example step distribution of channel
frequencies in an AOM with 15 transducer channels.
[0046] FIG. 11 depicts a block diagram of an exemplary system for
generating modulated RF signals for driving AOM channels, according
to one implementation of the technology disclosed.
DETAILED DESCRIPTION
[0047] The following detailed description is made with reference to
the figures. Sample implementations are described to illustrate the
technology disclosed, not to limit its scope, which is defined by
the claims. Those of ordinary skill in the art will recognize a
variety of equivalent variations on the description that
follows.
[0048] A multibeam laser scanner, as known in prior art, is shown
in FIG. 1A. The scanner of FIG. 1A could for example be a
microlithographic laser writer for writing a pattern on a
photomask, or for direct writing of a pattern on a substrate such
as a printed circuit board. A laser 100 emits a beam 113 which is
divided into multiple beams 116 by a beam-splitter 114. Each beam
is modulated by a multibeam acousto-optic modulator (AOM) 112 and
deflected by a deflector 110, (could also be a polygon, mirror
galvanometer, etc.) so that it scans 108 over the surface of the
workpiece. The optics is symbolically shown as a single lens 116.
The multibeam acousto-optic modulator 112 accepts a modulated RF
signal 130 for each beam which is generated in the RF driver 128
which modulates the video 126 on an RF carrier. The RF is typically
amplitude modulated with the carrier. The input pattern is stored
in memory 120 and converted by a rasterizer 124 to the video used
by the RF driver.
[0049] FIG. 1B shows the RF driver in more detail. The input is a
digital signal 126 which contains gray values for each pixel. The
gray values are converted to an analog voltage 156 by the DAC 150.
The analog voltage, typically ranging from 0 to 1 volt, modulates
the carrier 160 from the local oscillator to produce a low-level
modulated RF 158 which is then amplified in an RF amplifier to a
power level suitable for the acousto-optic modulator, often 1-5
watts per channel.
[0050] An acousto-optic modulator (AOM) uses sound waves within a
crystal to create a diffraction grating. As the power of the
applied RF signal is varied, the amount of diffracted light varies
proportionally. Acousto-optic multi-channel modulators allow
multiple beams to be modulated independently by integrating an
array of transducers with a single acousto-optic crystal. FIG. 2A
shows an example spatial dimension graph for a multi-channel AOM
with an array of fifteen channels of transducers integrated with a
single acousto-optic crystal, operated at a carrier frequency of
220 MHz 242. A multi-channel AOM operates on the same principles as
a typical AOM, and is fabricated using an array of electrodes on
the transducer substrate so that a parallel array of beams can be
simultaneously controlled. The AOM is typically operated with the
same carrier frequency on all channels. A different acoustic wave
diffracts each input beam independently to modulate its intensity.
As the number of channels incorporated into a single device
increases, so does the crosstalk between the various modulation
channels.
[0051] As used herein, "crosstalk" may refer to acoustic crosstalk
and/or electric crosstalk. The electric crosstalk may be capacitive
(electrostatic) and/or inductive (electromagnetic).
[0052] Under conditions of acoustic crosstalk, sound waves leaking
from adjacent AOM channels will interact and create a 3-dimensional
pattern such as the one shown in FIG. 2B, within the AOM-crystal,
consisting of volumes with constructive and destructive
interference. This pattern is static in time and will imprint an
amplitude distortion on the laser beams being modulated by the AOM.
This in turn has a negative impact on the line-width control of the
exposed pattern. The overall effect on the total transmitted
optical energy is small, typically less than 1%, confirmed in
previous tests. The crosstalk modulates the shape of the Gaussian
laser beam, giving relatively large impacts on the measured line
width of exposed pattern structures. The disclosed technology
reduces the impact of cross-talk between transducers that drive a
multi-channel AOM.
[0053] Experimental results that led to the disclosed technology
are described next. Unexpected CD variation was encountered that
needed to be diagnosed. The test patterns written to investigate
this problem showed a periodic variation in CD accuracy across
beams that varied between patterning runs. The periodicity led to
investigation of AOM performance and discovery of a problem
resulting from cross-talk between neighboring transducers driving
modulation channels of the AOM. Careful study and simulation led to
an understanding of coherence effects within the AOM crystal that
were impacting CD accuracy and to the disclosed approach to
addressing the coherence effects that were discovered.
[0054] Signal amplitudes for individual beams in the multibeam scan
are calculated to give the same exposure dose, and should result in
the same line width, written via the different beams. With control
over the exposure dose, it is expected that the scanner will
measure the same line width, written via different beams.
[0055] Researchers used two types of test patterns to evaluate the
relative dose in the different exposure beams. For the test
patterns, a measure of the exposure dose is the width of the
structures exposed by the different beams. Researchers used skew
pattern A and measured line width, with only one of the exposure
beams on at any given moment. FIG. 3A shows a graph of the results
for pattern A, of measured line width critical dimension (CD) per
unit time 324 for exposures across three different patterning runs
312 represented using three different colors. The results for the
three different exposure patterns are very similar, with the same
range of CD of between +10 and -10 nm. The measurement of static
crosstalk resulted in power modulations of less than 1%.
[0056] As a second test with pattern B, researchers used a stable
and repeatable transducer input signal and measured line width,
with all beams exposed (turned on) at the same time. From job to
job the exposure dose did not change. For a multi-channel
acousto-optic modulator, some kind of cross-talk is expected when
sending acoustic energy into the crystal. In this experiment, 15
transducers were mounted, spaced separated by 0.9 nm, on a single
monolithic quartz crystal the size of a sugar cube and fed by
independent electronics. FIG. 3B shows the results for pattern B.
The results changed a lot from job to job, for three different
patterning runs, represented in the graph using three different
colors. For the inputs described, the critical dimensions (CD) per
unit time 384 for the width of structures exposed by the different
beams covered a range between approximately -20 nm 274 and +30 nm
354. When all 15 transducer beams were exposed at the same time, a
fully developed crosstalk situation ensued. The typical
relationship between CD for line width and dose for this exposure
mode is 4 nm per percent, which translates to a spread in exposure
dose of up to 12% between consecutive exposures of the pattern. 12%
corresponds to 48 nm in line-width variability, which is
unacceptable. If interpreted as a true dose, pattern B indicates a
dose variability of 10-15%.
[0057] The results for the two test pattern types show very
different beam dose signatures. Cross-talk-driven exposure dose
variations are not viewable in the static situation in which a
single signal is active at any given time. Cross-talk is dependent
on the phases of multi-channel input signals. If the phase is
random from job to job, then the CD is random from job to job.
[0058] Phase-dependent cross talk between acousto-optic modulator
(AOM) channels affects the CD: when the phase relationship between
neighbor transducer channels changes, the impact on the apparent
beam dose changes. In the research example, the 220 MHz carrier
signal introduces a random phase relationship between the different
transducer beams for each job, which is a cause of the variability
between jobs. The phase relationship remained constant during a job
but changed between jobs. That is, inter-channel cross-talk between
modulators in a multi-channel AOM can have an adverse impact on
critical dimensions (CD). When this problem was identified and
fixed, the beam dose remained constant over jobs, reducing the
impact of acoustic cross-talk between the modulators in the
multi-channel AOM and thereby improving the critical dimensions for
pattern generation, direct-write lithography and for optical
writing of patterns on a photosensitive surface.
[0059] Careful study and simulation led to an understanding of
coherence effects within the AOM crystal that were impacting CD
accuracy. The spread frequency approach transforms static CD
differences impacting CD-uniformity, into periodically changing CD
along the sweep direction of the pattern writer. Experiments have
shown that the nature of the crosstalk in the AOM is strongly
dependent on the relative phase of the 220 MHz carrier signal in
the different channels. By applying different frequencies in
different AOM channels the phase relationship is constantly
changing. This in turn modulates the crosstalk signature over time
and smears the impact on exposed structures.
[0060] When different carrier frequencies are used on the different
AOM channels the interference pattern will no longer be static. The
interference pattern will travel through the crystal with a speed
given by the frequency difference between adjacent AOM channels.
The amplitude imprint on the laser beam will change over time and
the impact on the exposure result will change as well. If these
changes occur quickly enough, the overall impact on the exposure
result will be an improvement compared to the default configuration
with the same frequency on all channels.
[0061] FIG. 4 illustrates a spread carrier frequency approach, with
each of 15 transducer channels 455 assigned a unique carrier
frequency ranging from 216.5 to 223.5 MHz, centered around a
frequency of 220 MHz 442 and fed by independent electronics. FIG. 4
pattern A 422 shows results for a test in which a single exposure
beam for one of the 15 transducer channels 455 is turned on at any
given moment. The graph shows measured line width, for the scenario
in which only one of the exposure beams is turned on at any given
moment. The graph shows measured line width critical dimensions
(CD) per unit time for repeated exposures across three different
exposure patterns represented using three different colors in the
graph. The results for the three different exposures are very
similar. FIG. 4 also shows results for test pattern B 438 in which
all of the exposure beams are turned on at the same time, again
using very slightly different carrier frequencies for each of the
15 transducers, so the pattern types were nearly in agreement on
the beam doses. For the inputs described, the critical dimensions
(CD) per unit time for the width of structures exposed by the
different beams covered a range of less than 10 nm at any given
moment in time. That is, the modulation of the line width
corresponds to the frequency difference between two adjacent AOM
channels, in the spread frequency approach.
[0062] Inter channel crosstalk in a multi-channel AOM causes the
phase relationship between neighboring channels to have an impact
on the beam dose. The crosstalk may be both electric and/or
acoustic crosstalk. During acoustic crosstalk, each beam
experiences the effects of an interference wave pattern generated
by acoustic energy spread from at least the closest transducer
channel in the AOM crystal. With a change in phase of the carrier
frequency, the standing wave pattern will move and the impact on
the beams will change. With different carrier frequencies on the
channels, the standing wave pattern will move and smear the effect
over time.
[0063] FIG. 5 shows the sweep position in micrometers, as a
function of the exposed line width in nanometers 522. In FIG. 5,
the uppermost line in the graph corresponds to beam 1, the second
line from the top corresponds to beam 2, and so forth. This
illustrates spread carrier frequency approach results for the
measured critical dimension, when the fifteen transducers are
turned on at the same time, for the test in which 15 transducer
beams were each given unique carrier frequencies, ranging from
216.5 to 223.5 MHz. The modulation of the line width corresponds to
the frequency difference between two adjacent AOM channels.
[0064] AOM bandwidth affects exposure results, which will benefit
from faster moving interference patterns in the AOM crystal
achieved by increasing the difference in carrier frequency between
adjacent AOM channels. There is a limit to how much the carrier
frequency can deviate from the designed 220 MHz utilized in the
described example. The limitation can be understood by considering
the bandwidth of the AOM and the effects of impedance matching of
the electronics in the AOM. Offsetting the carrier frequency very
far from the most efficient frequency attenuates the optical
transmission, resulting in lower available writing power. This
result may be compensated by increasing the laser power. There is
however a limit to the feasibility of this approach. Traditionally
AOMs have been designed to a narrow bandwidth, according to the
classical requirements. Note that the design of the AOM may be
changed to increase the bandwidth, enabling a larger spread in
carrier frequency.
[0065] Simulations led to further understanding of coherence
effects, within the AOM crystal, that impact CD accuracy. When many
transducers are mounted on a single monolithic quartz crystal, the
side-lobes of neighboring transducer signals interact to create a
complex interference pattern of sound waves, even when the
transducers are fed by independent electronics. The neighboring
channel crosstalk impacts the beam dose. The simulation shows the
effect of side lobes of nearby transducer signals, with coherence
effects caused by the phase relationship of crosstalk between
neighboring channels in the multi-channel AOM.
[0066] FIG. 6 shows a simulation of interference from neighboring
transducers 622, 662 that affects a center transducer 642. The
effects of the side lobes of two neighboring transducer signals
become visible in the enhanced contrast image, with side lobe
interference 628 caused by transducer one 622 and side lobe
interference 668 caused by transducer three 662.
[0067] FIG. 7 displays a simulation of average energy disturbance
that will be imprinted in the energy distribution over the laser
beam, from two neighboring channel signals. The simulation
calculates the effect of subtracting the energy of an undisturbed
single transducer channel signal 724 from the average acoustic
energy in the crystal 722, which is calculated by integrating over
the period of the 220 MHz signal. The subtraction yields the
average energy disturbance from the two neighbor channels 726. This
pattern 746 will be imprinted in the energy distribution over the
laser beam. The color scales 762, 765, 768 are displayed to the
right of each of the color energy maps.
[0068] Another simulation of phase relationships on coherence
effects within the AOM crystal also led to the disclosed approach
for addressing the coherence effects that were discovered.
Continuing with phase-based simulations, when the phase of one of
the neighbor channels is changed, the geometry of the interference
pattern changes. FIG. 8 displays a simulation of the average energy
disturbance that will be imprinted in the energy distribution over
the laser beam for three different phase relationships. FIG. 8
shows the average energy disturbance when there is no phase change
822 which results in energy disturbance 872. FIG. 8 also shows the
effect when there is a phase relationship of 2/3 pi 825, resulting
in energy disturbance 875, and when there is a phase relationship
of 4/3 pi 828 resulting in energy disturbance 878. This simulation
sheds light on the effect on the CD of carrier frequency phase
changes of transducer signals. This simulation explains the effect
on the line width of exposed structure changes with carrier
frequency phase.
[0069] FIG. 9 illustrates a saw-tooth configuration for channel
distribution that utilizes the concept that crosstalk is limited in
space and fades as distance increases from the transducer in the
AOM. This addresses the limit of how much the carrier frequency may
spread through the AOM before resulting in a lower optical
transmission, as described supra. A large carrier frequency offset
for the nearest neighbors in the AOM crystal mitigates the effects
of the crosstalk. A distant neighbor may, due to the large
distance, be close in frequency. The impact from this distant
neighbor is weak enough not to be a problem due to its large
spatial separation.
[0070] FIG. 10 illustrates an example stair configuration of
channel frequency distribution in an AOM. The x axis shows the
fifteen different AOM channels, in the spatial dimension and the y
axis shows the AOM carrier frequency for each of the channels,
centered on a 220 Mhz carrier frequency. In this arrangement with a
staircase distribution of frequencies, repeated pairs of channels,
separated only by one channel in the spatial dimension, utilize the
same carrier frequency, so these pairs will be the weakest link in
the design.
[0071] Next, we describe a computer system usable for generating
modulated RF signal 130 for driving the AOM channels.
Computer System
[0072] FIG. 11 is a simplified block diagram of a computer system
1100 that can be used for generating modulated RF signals for
driving the AOM channels, according to one implementation of the
technology disclosed.
[0073] Computer system 1100 includes at least one central
processing unit (CPU) 1172 that communicates with a number of
peripheral devices via bus subsystem 1155. These peripheral devices
can include a storage subsystem 1110 including, for example, memory
devices and a file storage subsystem 1136, user interface input
devices 1138, user interface output devices 1176, and a network
interface subsystem 1174. The input and output devices allow user
interaction with computer system 1100. Network interface subsystem
1174 provides an interface to outside networks, including an
interface to corresponding interface devices in other computer
systems.
[0074] User interface output devices 1176 can include a display
subsystem or non-visual displays such as audio output devices. The
display subsystem can include an LED display, a flat-panel device
such as a liquid crystal display (LCD), a projection device, a
cathode ray tube (CRT), or some other mechanism for creating a
visible image. The display subsystem can also provide a non-visual
display such as audio output devices. In general, use of the term
"output device" is intended to include all possible types of
devices and ways to output information from computer system 1100 to
the user or to another machine or computer system.
[0075] Memory subsystem 1122 used in the storage subsystem 1110 can
include a number of memories including a main random-access memory
(RAM) 1132 for storage of instructions and data during program
execution and a read only memory (ROM) 1134 in which fixed
instructions are stored. A file storage subsystem 1136 can provide
persistent storage for program and data files, and can include a
hard disk drive, a floppy disk drive along with associated
removable media, a CD-ROM drive, an optical drive, or removable
media cartridges. The modules implementing the functionality of
certain implementations can be stored by file storage subsystem
1136 in the storage subsystem 1110, or in other machines accessible
by the processor.
[0076] Bus subsystem 1155 provides a mechanism for letting the
various components and subsystems of computer system 1100
communicate with each other as intended. Although bus subsystem
1155 is shown schematically as a single bus, alternative
implementations of the bus subsystem can use multiple busses.
[0077] Computer system 1100 itself can be of varying types
including a personal computer, a portable computer, a workstation,
a computer terminal, a network computer, a television, a mainframe,
a server farm, a widely-distributed set of loosely networked
computers, or any other data processing system or user device. Due
to the ever-changing nature of computers and networks, the
description of computer system 1100 depicted in FIG. 11 is intended
only as a specific example for purposes of illustrating the
preferred embodiments of the present invention. Many other
configurations of computer system 1100 are possible having more or
less components than the computer system depicted in FIG. 11. The
computer system can be used to control a microlithography laser
writer, such as a laser writer for large area masks or smaller,
semi-conductor masks. The microlithography writer can be a
multi-beam writer.
[0078] The preceding description is presented to enable the making
and use of the technology disclosed. Various modifications to the
disclosed implementations will be apparent, and the general
principles defined herein may be applied to other implementations
and applications without departing from the spirit and scope of the
technology disclosed. Thus, the technology disclosed is not
intended to be limited to the implementations shown, but is to be
accorded the widest scope consistent with the principles and
features disclosed herein. The scope of the technology disclosed is
defined by the appended claims.
Some Particular Implementations
[0079] Some particular implementations and features are described
in the following discussion.
[0080] In one implementation, a disclosed method of reducing impact
of cross-talk between transducers that drive an acousto-optic
modulator (AOM) includes operating the transducers, which are
coupled to an acousto-optic medium, with different frequencies
applied to adjoining transducers and producing a time varying phase
relationship between carriers on spatially adjoining modulation
channels emanating from the adjoining transducers.
[0081] The method described in this section and other sections of
the technology disclosed can include one or more of the following
features and/or features described in connection with additional
methods disclosed. In the interest of conciseness, the combinations
of features disclosed in this application are not individually
enumerated and are not repeated with each base set of features. The
reader will understand how features identified in this method can
readily be combined with sets of base features identified as
implementations.
[0082] The disclosed method also includes operating the transducers
with the frequencies having differences between pairs of adjoining
transducers of at least 100 KHz and a maximum difference of 20 MHz.
For some implementations of the disclosed method frequency
differences between pairs of adjoining transducers is in a range of
400 KHz to 10 MHz.
[0083] Some implementations of the disclosed technology include
operating between 5 and 32 of the transducers to produce 5 to 32
modulation channels in the acousto-optic medium.
[0084] In one implementation of the disclosed method, the different
frequencies between the spatially adjoining modulation channels are
arranged in a sawtooth pattern.
[0085] In another implementation of the disclosed method, the
different frequencies between the spatially adjoining modulation
channels are arranged in a rising or falling pattern applied
progressively to the adjoining transducers.
[0086] For some implementations of the disclosed method, the
different frequencies vary between pairs of adjoining transducers
by an amount in a range of plus or minus three percent from an
average frequency applied to the transducers.
[0087] For one implementation of the disclosed technology, an
acousto-optic modulator (AOM) with reduced impact of cross-talk
between transducers that are part of the AOM, includes an
acousto-optic medium, a plurality of transducers physically coupled
to the acousto-optic medium, spaced apart to drive separate
modulation channels within the acousto-optic medium, and a signal
synthesizer coupled to the transducers that drives the transducers
at different frequencies to produce a time varying phase
relationship between spatially adjoining modulation channels.
[0088] The disclosed AOM can include operating the transducers with
the frequencies having differences between pairs of adjoining
transducers of at least 100 KHz and a maximum difference of 20 MHz
in one case. In another implementation, the disclosed AOM includes
frequency differences between pairs of adjoining transducers in a
range of 400 KHz to 10 MHz.
[0089] One implementation of the disclosed AOM includes operating
between 5 and 32 of the transducers to produce 5 to 32 modulation
channels in the acousto-optic medium. For some implementations, the
different frequencies between the spatially adjoining modulation
channels are arranged in a sawtooth pattern. In other
implementations of the disclosed AOM, the different frequencies
between the spatially adjoining modulation channels are arranged in
a rising or falling pattern applied progressively to the adjoining
transducers. In some implementations of the disclosed AOM, the
different frequencies vary between pairs of adjoining transducers
in an amount in a range of plus or minus five percent from an
average frequency applied to the transducers.
[0090] The technology disclosed can be practiced as a system,
method, or article of manufacture. One or more features of an
implementation can be combined with the base implementation.
Implementations that are not mutually exclusive are taught to be
combinable. One or more features of an implementation can be
combined with other implementations.
[0091] While the technology disclosed is disclosed by reference to
the preferred embodiments and examples detailed above, it is to be
understood that these examples are intended in an illustrative
rather than in a limiting sense. It is contemplated that
modifications and combinations will readily occur to those skilled
in the art, which modifications and combinations will be within the
spirit of the innovation and the scope of the following claims.
* * * * *