U.S. patent application number 13/608454 was filed with the patent office on 2013-08-08 for link processing with high speed beam deflection.
This patent application is currently assigned to GSI GROUP CORPORATION. The applicant listed for this patent is James J. Cordingley, Jonathan S. Ehrmann, Joseph J. Griffiths, Shepard D. Johnson, Michael Plotkin, Donald J. Svetkoff. Invention is credited to James J. Cordingley, Jonathan S. Ehrmann, Joseph J. Griffiths, Shepard D. Johnson, Michael Plotkin, Donald J. Svetkoff.
Application Number | 20130200050 13/608454 |
Document ID | / |
Family ID | 40468746 |
Filed Date | 2013-08-08 |
United States Patent
Application |
20130200050 |
Kind Code |
A1 |
Ehrmann; Jonathan S. ; et
al. |
August 8, 2013 |
LINK PROCESSING WITH HIGH SPEED BEAM DEFLECTION
Abstract
The present invention relates to the field of laser processing
methods and systems, and specifically, to laser processing methods
and systems for laser processing multi-material devices. Systems
and methods may utilize high speed deflectors to improve processing
energy window and/or improve processing speed. In some embodiments,
a deflector is used for non-orthogonal scanning of beam spots. In
some embodiment, a deflector is used to implement non-synchronous
processing of target structures.
Inventors: |
Ehrmann; Jonathan S.;
(Sudbury, MA) ; Griffiths; Joseph J.; (Winthrop,
MA) ; Cordingley; James J.; (Littleton, MA) ;
Svetkoff; Donald J.; (Ann Arbor, MI) ; Johnson;
Shepard D.; (Andover, MA) ; Plotkin; Michael;
(Newton, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ehrmann; Jonathan S.
Griffiths; Joseph J.
Cordingley; James J.
Svetkoff; Donald J.
Johnson; Shepard D.
Plotkin; Michael |
Sudbury
Winthrop
Littleton
Ann Arbor
Andover
Newton |
MA
MA
MA
MI
MA
MA |
US
US
US
US
US
US |
|
|
Assignee: |
GSI GROUP CORPORATION
Bedford
MA
|
Family ID: |
40468746 |
Appl. No.: |
13/608454 |
Filed: |
September 10, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12233476 |
Sep 18, 2008 |
8269137 |
|
|
13608454 |
|
|
|
|
60994404 |
Sep 19, 2007 |
|
|
|
Current U.S.
Class: |
219/121.69 |
Current CPC
Class: |
B23K 26/0344 20151001;
B23K 26/0648 20130101; Y10T 29/49156 20150115; B23K 26/0853
20130101; H01L 21/76894 20130101; B23K 26/082 20151001; H01L
23/5258 20130101; B23K 26/361 20151001; H01L 27/10844 20130101;
B23K 2101/40 20180801; B23K 26/40 20130101; B23K 2103/16 20180801;
B23K 2103/50 20180801; B23K 26/0869 20130101; B23K 26/064 20151001;
B23K 26/0624 20151001; B23K 26/06 20130101; B23K 26/0665 20130101;
H01L 2924/0002 20130101; H01L 2924/0002 20130101; H01L 2924/00
20130101 |
Class at
Publication: |
219/121.69 |
International
Class: |
B23K 26/08 20060101
B23K026/08; B23K 26/36 20060101 B23K026/36 |
Claims
1-64. (canceled)
65. A method for laser processing a multi-material device including
a substrate and at least one target structure, the method
comprising: grouping microstructures into sets of N structures;
producing P laser pulses, combined pulses, pulse groups or pulse
bursts for each set of N microstructures in a pass of a laser beam
waist position over said sets of target structures, wherein P is
not equal to N; processing at least one target structure during
said pass.
66. The method of claim 65, wherein P is less than N.
67. The method of claim 65, wherein N=MP, wherein M is an
integer.
68. The method of claim 65, wherein at least one microstructure is
a member of more than one group.
69. The method of claim 65, wherein N=2, and P=1.
70. The method of claim 65, wherein N=3, and P=1.
71. The method of claim 70, wherein some of said microstructures
are members of only one group, and some of said microstructures are
members of two groups.
72-85. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 12/233,476, filed on Sep. 18, 2008, and entitled Link
Processing with High Speed Beam deflection, which application
claims priority under 35 U.S.C. Section 119(e) to Provisional
Application No. 60/994,404, filed on Sep. 19, 2007. Both of these
applications are incorporated by reference in their entireties.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to the field of laser
processing methods and systems, and specifically, to laser
processing methods and systems for laser processing multi-material
devices.
[0004] 2. Description of the Related Art
[0005] Lasers can be used in the processing of microstructures in
memory and integrated circuit devices. For example, laser pulses
can be used to ablate conductive links or link portions in a memory
device, such as DRAMs in order to substitute working redundant
memory cells for defective memory cells during memory
manufacture.
[0006] Recently, the use of new materials, such as aluminum, gold,
and copper, coupled with the small geometry of these devices, have
made the problem of link removal more difficult. Economics and
device performance goals have driven the size for the DRAMs and
logic devices to very small physical dimensions. Thus, it can be
increasingly difficult to irradiate a target structure without
damaging surrounding components such as the substrate and adjacent
circuitry and links. Furthermore, as more links need to be
processed for a given area of semiconductor circuitry, the time
required to process a given die increases.
[0007] When a single laser pulse or burst of pulses is used to
irradiate and sever each link designated for removal, the beam path
of laser pulses may move relative to the substrate during the
process of irradiation in an "on-the-fly" link blowing process.
This relative movement may include moving the substrate and/or
moving the beam, although substrate motion on an X-Y stage in
conjunction with a vertically oriented and stationary beam is a
currently common approach. In conventional laser processing
systems, groups of arrayed microstructures are processed. The array
may be links in a row, links in closely spaced rows, links in
staggered rows and similar regularly spaced arrangements. The
conventional processing is generally carried out with either an
energy on demand system (e.g. pulse equalization) or an energy
picking system (e.g. pulse picking). In the energy on demand
system, an irradiation period is timed to coincide with a moving
target and the processing rate is limited by a minimum period
between energy on demand irradiation periods. In the energy picking
system, the laser is pulsed in a continuously repeating sequence at
a predetermined repetition rate (e.g. at a q-rate, pulse rate, or
burst rate) and the arrayed microstructures in a group are moved
synchronously with the repetition rate so that energy is available
to process any microstructure in a particular group. The processing
rate is limited by a period associated with the maximum repetition
rate, and an acousto-optic device or other optical switching device
blocks energy from reaching the substrate except when processing a
selected synchronized target.
[0008] The conventional energy picking process is illustrated in
FIGS. 1 and 2. A repeating sequence of laser pulses, for example
pulses from a q-switched laser, pulses from a sequence of pulse
bursts, or a sequence of temporally shaped pulses is generated at a
predetermined repetition rate. A group of links 10 having a
characteristic spacing d is put in motion relative to a processing
head at a predetermined velocity V by moving a stage 12 under
control of a control computer 14. As adjacent links move relative
to the processing head, there is an associated transit time T1 such
that after a period equal to T1, the substrate has moved by an
amount equal to the characteristic spacing of the links. Put
another way, the link to link period at velocity V relative to the
processing head is T1.
[0009] In a conventional processing system links and pulses are
synchronized. T1 and the period of the laser pulse repetition rate
(e.g. the pulse to pulse period of a q-switched laser controlled by
trigger signals from the control computer 14) are made equal. With
this method, a pulse is available to process every link. Pulses
that are synchronized with links to be processed, such as links
10a, 10d, and 10f of FIG. 2, are allowed to reach the targets and
process the respective links. Pulses that are synchronized with
links that are to remain intact are blocked from reaching the
targets by an energy control and pulse picking system 16 of FIG. 1,
as indicated by dashed circles in FIG. 2 where the beam would
strike if it was not blocked.
[0010] It will be appreciated that the time required to process a
given set of links within a group of a row or a column of links is
approximately the number of links times the time period T1, which
in these systems equals the laser pulse repetition rate. If the
laser used has a maximum pulse rate of 50 kHz, for example,
completing the pass of the beam across the 11 links of FIG. 1 will
require at least 200 microseconds.
[0011] Although the above embodiment was described in terms of
single pulse link processing, link blowing systems have been
described that apply multiple pulses to each link to sever the
link. FIG. 3 shows a system which applies a burst or sequence of
two pulses to each link. In this embodiment, the pulse selector 16
selects groups of pulses rather than individual pulses for link
processing. In some embodiments, the laser itself produces
separated bursts of pulses where the pulse to pulse separation
within the burst is much less than the separation between bursts.
In these embodiments, the pulse picker 16 selectively passes or
blocks pulse bursts. Other known embodiments use multiple lasers or
split and re-combined pulses to produce a variety of intensity
profiles of the laser energy applied to a link for processing. It
will therefore be appreciated that all of the discussion throughout
this document related to applying a pulse to a target structure for
processing includes applying a sequence of pulses, pulse groups,
combined pulses, or pulse bursts, or any other irradiance intensity
profile for performing a complete or partial target processing
function.
[0012] In many advantageous embodiments, the pulse picker 16 is an
acousto-optic modulation device, but may be an electro-optic
switch, a fast steering mirror or any other type of optical switch
with sufficient speed and accuracy.
[0013] Other uses in addition to pulse picking for acousto-optic
modulators, fast steering mirrors, or other forms of high speed
deflectors have also been described. One such use is for beam
position correction along the direction of beam motion if a long
pulse or pulse burst is being applied to process a link. Without
correction, if the pulse or pulse burst is short relative to the
transit time of the beam spot across the link during the pulse or
pulse burst duration, the beam spot will not move appreciably
during the above described relative beam and link motion during the
on-the-fly processing. However, as shown in FIG. 4, if the burst
has many pulses or includes relatively long inter-pulse spacing,
the beam spot can walk off the center of the link 18 over the
course of the pulse sequence. In U.S. Patent Publication
2002/0167581 to Cordingley et al., a deflector (e.g., a high-speed
deflector) is described to deflect laser pulses to improve the
coincidence of the pulses with the target structures in this
situation. As described in this Publication, the deflector can act
to oppose the relative movement of the beam spot across the link.
In one embodiment generally illustrated in FIG. 5, the deflector 20
would be operatively coupled to the relative positioning system.
The deflector 20 is preferably solid state and may be a single axis
acousto-optic device which has a very fast "retrace"/access time.
Alternatively, a higher speed electro-optic deflector (e.g., a
gradient index reflector or possibly a digital light deflector) may
be used. The time-bandwidth product (number of spots) can be traded
for response time on an application basis. Alternatively, an
electro-optic modulator may be used with a separate acousto-optic
deflector operated in a "chirp mode" (e.g., linear sweep as opposed
to random access mode) and synchronized (triggered) based on the
positioning system coordinates. A modulator 22 may be used for
intensity control and pulse gating/selection, to select pulses 24
at times t.sub.1, t.sub.2, t.sub.3 for target structure processing.
As the beam path moves further across and beyond an edge of a
target structure during a sweep of a multiple pulse processing
function, the deflector 20 would more strongly deflect the beam
path in a direction opposing the relative movement. Thus, a
plurality of pulses would irradiate approximately the same portion
of the target structure. U.S. Publication Number 2002/0167581 is
hereby incorporated by reference in its entirety.
[0014] Another application of an acousto-optic modulator in a beam
path of a laser processing machine is also described in U.S.
Publication Number 2002/0167581. In an embodiment described therein
with reference to FIG. 20, an acousto-optic modulator is used as a
beam splitter to allow the processing of more than one target
structure at a time. The beam paths of the resulting beams may be
controlled by controlling the frequencies applied to the
acousto-optic modulator to simultaneously position each resulting
beam accurately on the targets to be processed.
[0015] Although high speed beam scanning within a dominant
fundamental beam trajectory has been utilized in a variety of
contexts such as described above, additional uses of high speed
deflectors in link blowing systems would be useful in the
field.
[0016] For further reference, the following co-pending U.S.
applications and issued patents are assigned to the assignee of the
present invention, describe many additional aspects of laser link
blowing, and are hereby incorporated by reference in their
entirety: [0017] 1. U.S. Pat. No. 5,300,756, entitled "Method and
System for Severing [0018] Integrated-Circuit Connection Paths by a
Phase Plate Adjusted Laser beam"; [0019] 2. U.S. Pat. No.
6,144,118, entitled "High Speed Precision Positioning Apparatus";
[0020] 3. U.S. Pat. No. 6,181,728, entitled "Controlling Laser
Polarization"; [0021] 4. U.S. Pat. No. 5,998,759, entitled "Laser
Processing"; [0022] 5. U.S. Pat. No. 6,281,471, entitled "Energy
Efficient, Laser-Based Method and System for Processing Target
Material"; [0023] 6. U.S. Pat. No. 6,340,806, entitled
"Energy-Efficient Method and System for Processing Target Material
Using an Amplified, Wavelength-Shifted Pulse Train"; [0024] 7. U.S.
Pat. No. 6,483,071, entitled "Method and System For Precisely
Positioning A Waist of A Material-Processing Laser Beam To Process
Microstructures Within A Laser-Processing Site", filed 16 May 2000,
and published as WO 0187534 A2, December, 2001; [0025] 8. U.S. Pat.
No. 6,300,590, entitled "Laser Processing"; and [0026] 9. U.S. Pat.
No. 6,339,604, entitled "Pulse Control in Laser Systems." [0027]
10. U.S. Pat. No. 6,639,177, entitled "Method and System For
Processing One or More Microstructures of A Multi-Material
Device"
[0028] The subject matter of the above referenced applications and
patents is related to the present invention. References to the
above patents and applications are cited by reference number in the
following sections.
SUMMARY OF THE INVENTION
[0029] In one embodiment, the invention comprises a method for
laser processing a multi-material device including a substrate and
at least one target structure. The method comprises producing
relative motion in a first direction between a beam path and a
substrate having at least one target structure thereon, generating
a first pulse and irradiating a first portion of the target
structure with the first pulse wherein a first beam waist
associated with the first pulse and the at least one microstructure
substantially coincide. A second pulse is generated, the second
pulse being delayed a predetermined time relative to the first
pulse. The second pulse is deflected in a second direction, wherein
the second direction is non-parallel to the first direction, and a
second portion of the at least one microstructure is irradiated
with the deflected second pulse wherein a second beam waist
associated with the deflected second pulse and the second portion
of the at least one microstructure substantially coincide.
[0030] In another embodiment, such a method comprises producing
relative motion in a first direction between a beam path and a
substrate having at least one target structure thereon, generating
a first pulse and irradiating a first portion of the target
structure with the first pulse wherein a first beam waist
associated with the first pulse and the at least one microstructure
substantially coincide. A second pulse is generated, the second
pulse being delayed a predetermined time relative to the first
pulse. In this embodiment, the second pulse is deflected so as to
irradiate the at least one target structure with the deflected
second pulse, wherein a second beam waist associated with the
deflected second pulse and a longitudinally distinct second portion
of the at least one target structure substantially coincide.
[0031] In another embodiment, a method for laser processing a
multi-material device including a substrate and a target structure
comprises scanning a plurality of applied laser pulses in a
direction substantially parallel to the length of the target
structure as the substrate moves in a direction that is
non-parallel to the length of the target structure.
[0032] Another method of processing a multi-material device
including a substrate and at least one microstructure comprises
producing relative motion between a beam path and a substrate
having at least one target structure thereon, generating a first
pulse, deflecting the first pulse in a first direction, and
irradiating the at least one microstructure with the first
deflected pulse wherein a first beam waist associated with the
first deflected pulse and the at least one microstructure
substantially coincide. A second pulse is generated, the second
pulse being delayed a predetermined time relative to the first
pulse. The second pulse is deflected in a second direction, wherein
the second direction is non-parallel to the first direction. The at
least one microstructure is irradiated with the deflected second
pulse wherein a second beam waist associated with the deflected
second pulse and the at least one microstructure substantially
coincide.
[0033] In another embodiment, a method for laser processing a
multi-material device including a substrate and at least one target
structure comprises producing relative motion at a predetermined
average velocity V between a beam delivery subsystem and a
substrate having a plurality of adjacent microstructures thereon,
the plurality including the at least one target structure, and
generating a pulsed laser output comprising a sequence of pulses,
pulse groups, combined pulses, or pulse bursts, wherein the
sequence is characterized by a minimum synchronization period T.
The plurality of microstructures are separated by a spatial
distance S in the direction of relative motion and t.sub.1 is the
transit time S/V between at least one pair of adjacent
microstructures. In this embodiment, t.sub.1 is not equal to the
predetermined period T.
[0034] Another embodiment of a method for laser processing a
multi-material device including a substrate and at least one target
structure comprises grouping microstructures into sets of N
structures and producing P laser pulses, combined pulses, pulse
groups or pulse bursts for each set of N microstructures in a pass
of a laser beam waist position over the sets of target structures.
In this embodiment, P is not equal to N. The method further
comprises processing at least one target structure during the
pass.
[0035] In yet another embodiment, a method for laser processing a
multi-material device including a substrate and at least one target
structure comprises receiving data corresponding to coordinates of
target microstructures to be processed, determining a processing
trajectory segment including a plurality of target microstructures
along a row or column of microstructures. A pulsed laser output is
generated comprising a sequence of pulses, pulse groups, combined
pulses, or pulse bursts, wherein the sequence is at least partially
unsynchronized with respect to the trajectory. The method further
comprises determining a relative displacement between each target
microstructure and a corresponding laser pulse in the sequence,
generating a deflection command sequence corresponding to the
relative displacements, deflecting the pulsed laser output
according to the deflection command sequence during relative motion
between the substrate and a beam positioning subsystem to
synchronize the target microstructures and the corresponding laser
pulses, and irradiating each target microstructure with at least a
portion of the pulsed laser output.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a block diagram illustrating several conventional
components of a laser processing system.
[0037] FIG. 2 is a plan view of a row of links illustrating the
application of laser pulses to selected links.
[0038] FIG. 3 is a block diagram of a multiple pulse laser
processing system.
[0039] FIG. 4 is a plan view of offset laser spots applied to a
link.
[0040] FIG. 5 is a block diagram of a laser processing system
configured to correct for relative motion during link
processing.
[0041] FIG. 6 is a block diagram of a laser processing system with
non-orthogonal beam deflection capability.
[0042] FIG. 7 is a plan view of link rows an columns illustrating
axes of relative beam notion and deflection directions.
[0043] FIGS. 8A and 8B is a plan view of a laser spot on a link
showing vectors of motion and deflection.
[0044] FIGS. 9A and 9B show spot motion on links.
[0045] FIGS. 10A and 10B show spot motion on links in another
embodiment.
[0046] FIGS. 11A and 11B show spot motion on links in another
embodiment
[0047] FIG. 12 shows non-orthogonal deflection used to blow links
in adjacent rows or columns in a single pass.
[0048] FIG. 13 shows multiple beam processing of three parallel
rows or columns of target structures;
[0049] FIG. 14 shows multiple beam processing of two staggered
parallel rows or columns of target structures;
[0050] FIG. 15 shows multiple beam processing of two fanned out
parallel rows or columns of target structures;
[0051] FIG. 16 shows another embodiment of multiple beam processing
of two fanned out parallel rows or columns of target
structures;
[0052] FIG. 17 shows multiple beam processing of four fanned out
parallel rows or columns of target structures;
[0053] FIG. 18 illustrates spot motion over an alignment target in
one embodiment.
[0054] FIG. 19 is a block diagram of a laser processing system
configured to deflect non-synchronous pulses to targets for
processing.
[0055] FIG. 20 shows non-synchronous beam spot positions prior to
correction.
[0056] FIG. 21 shows non-synchronous beam spot positions following
correction.
[0057] FIG. 22A shows non-synchronous pulses applied to a mixed
pitch link row.
[0058] FIG. 22B shows non-synchronous pulses applied to a mixed
phase link row.
[0059] FIG. 23 shows link severing during a non-constant velocity
segment of a trajectory.
[0060] FIG. 24 is a block diagram of a laser processing system
implementing multiple link blowing channels.
[0061] FIG. 25 is a plan view of a link row illustrating link
groups.
[0062] FIGS. 26A-26E illustrate additional link grouping options in
a link row.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0063] Methods and systems described herein relate to laser
processing target structures of multi-material devices utilize
multiple laser pulses. While this invention is well suited for
improvements in processing closely spaced metal and non-metal links
on semiconductor substrates, other types of multi-material devices
may be laser processed with various benefits including, but not
limited to increased process throughput, increased accuracy,
reduced substrate damage, and reduced damage to adjacent
devices.
[0064] General Aspects of Laser Processing of Target
Microstructures
[0065] Many aspects of processing links on a multi-material device
that may be used advantageously with this invention are described
in sections [0115] to [0159], [0169] to [0175], and [0213] to
[0225] of U.S. Patent Publication 2002/0167581, which is mentioned
above.
[0066] A multi-material device may include a plurality of target
structures positioned over a substrate. A pulsed laser beam, the
beam having pre-determined characteristics for processing of
microscopic structures, is used to cleanly remove at least a
portion of a target structure. An application of the method and
system of the present invention is severing of conductive links
which are part of a high speed semiconductor memory device. The
targets may have a sub-micron dimension, including a dimension
below the wavelength of the laser beam. The target may be separated
from a semiconductor substrate by a multi-layer stack, which may
have several dielectric layers. Furthermore, both the temporal and
spatial characteristics of the pulse may be selected or controlled
based on the thermal and optical properties of the microscopic
target, underlying layer materials, and the three-dimensional
layout of the device structure, including the spacing of target
structures and functional inner conductor layers.
[0067] In many embodiments of the present invention, one or more
laser pulses irradiate a generally rectangular target structure or
microstructure. In one embodiment, an output from a laser system is
generated to produce a sequence of pulses, pulse groups, combined
pulses, or pulse bursts. In some embodiments, each pulse has a rise
time fast enough to efficiently couple energy into a highly
reflective target structure. The total duration of irradiation is
sufficient to process at least a portion of the target structure
wherein at least a portion of the structure is cleanly removed
without leaving operationally significant amounts of residue, slag,
or other debris. The fall time is preferably fast enough to avoid
creating undesirable damage to the layers or substrate.
[0068] The temporal pulse shape may be selected, in part, based on
physical properties of the target microstructure, for instance,
thickness, optical absorption, thermal conductivity, or a
combination thereof. In some embodiments of the invention, the
processing will occur with a single pulse having a fast leading
edge relative to a selected pulse duration of several nanoseconds.
In some embodiments, the laser output used to process a target
structure may be a series of narrow q-switched Gaussian shape or
rectangular pulses, with very fast rise time, for example 800 ps
pulses representative of the output of commercially available
q-switch micro-lasers. The pulses may be delayed with respect to
each other so as to provide a burst of pulses to irradiate the
target structure. The laser output may be generated with a
combination of a high bandwidth seed laser diode and fiber optic
amplifier with Raman shifting, or with a waveguide amplifier
system. Alternatively, a desirable pulse characteristic may be
provided with various modified q-switched systems or with the use
of high speed electro-optic modulators. Other pulse shapes may be
selected for the material processing requirements. For instance, a
sequence of closely spaced pulses having duration from a few
picoseconds to several nanoseconds is taught in Reference 5.
[0069] In one embodiment, a high bandwidth MOPA configuration is
used to amplify the laser output of a high speed semiconductor
diode. Generation of various pulse shapes and duration with direct
modulation of the diode is considered advantageous, provided any
affect associated with variable amplitude drive waveforms does not
affect overall performance. Further details of various aspects of
pulse generation and amplification can be found in references 5 and
6 (e.g., in '471--Reference 5--FIG. 5 and columns 14-16).
[0070] As indicated above, embodiments of the laser system may
include fiber optic amplifiers which amplify the preferred square
pulse shape generated by a seed laser. The seed laser may be a high
speed semiconductor diode or the shaped output of a modified
q-switched system. The amplified output may be matched in
wavelength to the input or Raman-shifted as taught in References 4
and 6 (e.g., in Reference 6, FIGS. 12-13 and column 14, line
57--column 19, line 3). Wavelength shifting of a short pulse
q-switched laser output is generally taught in '759 Reference
4.
[0071] In an alternative arrangement the seed laser is a
semiconductor diode and the optical amplifier is a waveguide
amplifier. Advantages of an embodiment with a waveguide amplifier
when compared to a fiber system include avoidance of Raman
shifting, lower pulse distortion at the speed of operation, and,
with proper design, minimal thermal lensing. A precision anamorphic
optic system is used to optimize coupling between the seed and
amplifier. Basic description of waveguide amplitude and lasers can
be found in product literature provided by Maxios, Inc. and in the
article "CW and passively Q-switched Cladding Pumped Planar
Waveguide Lasers," Beach et. al. Yet another amplifier system
including a 28 DB planar waveguide amplifier for use at 1.064
micron wavelengths was developed by University of Southhampton and
described in "A Diode Pumped, High Gain, Planar Waveguide,
Nd:Y3A15O12 Amplifier."
[0072] In an alternative arrangement, for generation of a fast
rising pulse or other desirable shape, a plurality of q-switched
micro-lasers can be used. The modules produce a q-switched waveform
with pulse durations of about 1 nanosecond or less, for example 800
ps to 2 ns for commercially available units. An example of a
commercially available laser is the AOT-YVO-1Q available from
Advanced Optical Technology (AOTLasers.com). These recently
developed short pulse, active q-switch lasers can be triggered with
a TTL pulse at a variable repetition rate while maintaining
specified sub-nanosecond timing jitter. In general, the pulse shape
incident on the target microstructure will vary significantly at
repetition rates approaching the maximum rate. Reference 9 teaches
methods of maintaining a constant pulse shape despite variations in
the temporal spacing of pulses incident on a target (e.g., see the
figures and associated specification). AOT offers a pulsewidth of 2
nanoseconds available at a repetition rate of 20 KHz. Frequency
doubled versions are also available (532 nm). IMRA America reports
800 ps pulses with the PicoLite system, and high peak power was
obtained with fiber amplification at repetition rates up to 10 KHz.
Shorter pulsewidths, for instance about 1 ns or less, are available
at slower repetition rates. Laser processing with pulses in the
1-100 picosecond range has been described and performed with more
recently developed laser systems.
[0073] As known in the art and illustrated in Reference 5 (e.g.,
FIGS. 1C, 2), the q-switched waveforms may approximate (at least to
1st order) a symmetric Gaussian shape, or a fast rising pulse with
an exponential tail, depending on the stored energy. With reference
to Publication 2002/0167581, a series of devices, with appropriate
delays introduced by a plurality of triggering signals, or delays
of a trigger signal with a delay line, may be used to generate a
series of spaced apart pulses. The optical outputs are preferably
combined with appropriate bulk optics (polarization sensitive),
fiber optics, or waveguides to form a single output beam. The
resultant addition of the q-switched waveforms produces a fast rise
time characteristic and relatively short duration. An optical
amplifier may be used to increase the output power as needed.
Shaped pulses may be produced, for example, by using a beam
combiner to deliver the output of two lasers (or the output of a
single laser split by a beam splitter) to an amplifier or using a
modulator approach to chop the leading edge or tail of the pulse
but with a two-stage or shaped modulation voltage pulse.
[0074] During system operation for memory repair, position
information, obtained with a precision measurement system, is used
to relatively position the focused beam waist of the pulsed laser
at a location in space to substantially coincide with the
three-dimensional coordinates (Xlink,Ylink,Zlink) of the target. A
trigger pulse, generated to produce a laser pulse at a time where
the laser beam waist and target width position substantially
coincide, operates in conjunction with the laser and associated
control circuitry in the laser subsystem to produce an output
pulse.
[0075] References 2 and 7 describe details of a method and system
for precision positioning, including three-dimensional beam waist
positioning. Reference 7 describes an embodiment for producing an
approximate diffraction limited spot size with a range of spot size
adjustment (e.g., FIGS. 7-9 of WO0187534 ('534) and the associated
specification), and a method and system for three-dimensional
positioning of the beam waist. Three-dimensional (height)
information is obtained, for instance with focus detection, and
used to estimate a surface and generate a trajectory (e.g., FIGS.
2-5 of '534 and the associated specification). The laser is pulsed
at a location substantially corresponding to the three-dimensional
position of the link (Xlink, Ylink, Zlink) (e.g., FIGS. 10a-b of
'534 and the associated specification).
[0076] In practice, the three-dimensional measurement and
positioning are used to compensate for topographical variations
over a wafer surface, or other position variations introduced in a
system (mis-alignment). These variations are generally system or
application dependent and may exceed several microns, which in turn
may exceed the depth of focus of the focused laser beam. In some
micro-machining applications the system positioning requirements
may be relaxed if certain tolerances are maintained, or if external
hardware manipulates the device position, as might be done with a
micro-positioning sub-system. The device may comprise a miniature
part (e.g., single die) which is positioned by an external
micro-positioning subsystem to a predetermined reference location.
Similarly, if a miniature part has a pre-determined tolerance the
positioning may be based on single measurement at a reference
location or perhaps a single depth measurement combined with a
lateral (X,Y) measurement. For processing of multilevel devices on
wafers, (e.g.: 300 mm) at high speed it is expected that densely
sampled three-dimensional information will improve performance,
particularly as link dimensions shrink.
[0077] In applications requiring very high speed operation over a
large surface (e.g., 300 mm wafer), an alternative method is to
combine information which may be predetermined (e.g., the plane of
a wafer chuck relative to a beam positioner plane of motion
measured during a calibration process) with dimensional information
obtained from each part to be processed. For example, in '534,
FIGS. 1-2, a fraction of the tilt of region 28 may be associated
with fixturing). For example, the steps may include (a) obtaining
information identifying microstructures designated for removal, (b)
measuring a first set of reference locations to obtain
three-dimensional reference data, (c) generating a trajectory based
on at least the three-dimensional reference data to obtain a
prediction of beam waist and microstructure surface locations, (d)
updating the prediction during relative motion based on updated
position information, the updated position information obtained
from a position sensor (e.g., encoder) and/or from data acquired
during the relative motion. The additional data may be measurement
data acquired at additional alignment target or at other locations
suitable for an optical measurement (e.g., dynamic focus).
Reference 2 describes a system wherein a precision wafer stage is
used to position a wafer at high speed. A method of obtaining
feedback information with resolution of a fraction of one nanometer
is disclosed wherein interferometric encoders are used, and such a
high precision method is preferred. In Reference 2 it was noted
that other conventional laser interferometers may also be used.
FIGS. 9-11 and columns 5-6 of Reference 2 describe aspects of the
precision measurement subsystem associated with the precision
positioning apparatus. Additionally, designated reference locations
on the workpiece (e.g., wafer) which may be an x,y alignment target
or a region suited for a three-dimensional measurement may be used
for various applications. It should also be noted that height
accuracy of about 0.1 .mu.m was reported in "In-situ height
correction for laser scanning of semiconductor wafers," Nikoonhad
et al., Optical Engineering, Vol. 34, No. 10, October 1995, wherein
an optical position sensor obtained area averaged height data at
high speeds. Similarly, a dynamic focus sensor (e.g., astigmatic
systems used for optical disk tracking and control) may be used to
obtain height information provided the data rate is fast enough to
support "on the fly" measurement.
[0078] Various combinations of the above technologies can be used
depending upon the application requirements. A combination may be
based on the number and typical distribution over a device of
microstructures designated for removal. When a large number of
repair sites are distributed across a device, the throughput may be
maximized by providing updates "on the fly."
[0079] In an application of the invention, the target structure is
provided as a part of a multi-material, multi-layer structure
(e.g., redundant memory device). The multi-layer stack having
dielectric layers provides spacing between the link and an
underlying substrate. In one type of multi-layer memory device,
alternating layers of Silicon Dioxide and Silicon Nitride may be
disposed between a link target structure and a Silicon substrate.
The target structure is generally located in proximity to other
similar structures to form a 1-D or 2-D array of fuses which are
designated for removal. In addition to the link structure,
underlying conductors disposed as part of the functional device
circuitry, may be in proximity to the link structure, and arranged
in a series of patterns covered by relatively thin (<0.1 .mu.m
typical) Silicon Nitride and thicker (1 .mu.m typical) Silicon
Dioxide materials.
[0080] The irradiance distribution at the link may substantially
conform to a diffraction limited, circular Gaussian profile. In
another useful embodiment, the beam has an approximate elliptical
Gaussian irradiance profile, as might be produced with an
anamorphic optical system, or with a non-circular laser output
beam. In one embodiment, the incident beam has a non-uniform aspect
ratio. Alternatively, rectangular or another chosen spatial
profiles may be implemented in a lateral dimension. For example,
Reference 10 discloses various advantageous methods and optical
systems for spatially shaping of laser beams for application to
memory repair and Reference 1 discloses various advantageous
methods and optical systems for "non-Gaussian" irradiance
distribution of laser beams for application to memory repair.
[0081] With the nearly diffraction limited elliptical Gaussian
case, the preferable minimum beam waist dimension at location
approximates the narrow target width dimension of FIG. 1B, which,
in turn, produces high pulse energy density at the link. Further,
with this approach, a high fraction of the laser energy is coupled
to the link and background irradiance is reduced.
[0082] A typical link used in a present memory has width and
thickness of about 1 .mu.m or less, for example, 0.6 .mu.m, and
length of about five microns. Future memory requirements are
expected to further reduce the scale of target dimensions. The
minimum beam waist dimension Wyo at will typically overfill the
sub-micron link to some degree, whereas aspect ratio Wxo/Wyo with
Wxo a few microns along the link, can facilitate clean link
removal. Additionally, rapidly decreasing energy density on the
layers and substrate may be achieved through defocus of the high
numerical aperture beam portion.
[0083] Pulse energies in the range of about 0.1 to 3 .mu.j have
been found effective, with a preferred typical range of about 0.1-5
.mu.j considered sufficient margin for spot shape and process
variations. The preferred pulse duration may be selected based upon
the nominal link thickness specifications, or based on a model of
the dissimilar thermal and optical properties of adjacent
materials. Lower energies per pulse may be utilized when the target
is processed with bursts of multiple pulses.
[0084] Hence, a combination of the spatial characteristics (e.g.,
beam waist shape and position) and the temporal (e.g., rise time,
flatness, and duration) pulse characteristics avoids undesirable
cracking of lower layers, avoids significant pulse interaction with
inner layer conductor, and limits substrate heating.
[0085] Regarding laser wavelength, near IR (Infrared) wavelengths
conveniently correspond to wavelengths where high bandwidth laser
diodes are available, and to the spectral range where optical
amplification of the pulsed laser beam can be efficiently produced
with fiber and waveguide amplifiers. Those skilled in the art will
recognize that amplified laser diode outputs, having a desired
temporal pulse shape, may also be frequency multiplied to produce
visible laser outputs when advantageous. The fast rise time of
semiconductor diodes is particularly advantageous for producing a
fast rise time, square pulse characteristic. Future developments in
visible diode and optical amplifier technology may support direct
pulse amplification in the visible range.
[0086] In some systems for link blowing, the link width is a
fraction of one micron and the link spacing (pitch) is a few
microns with present process technology. The link width may
correspond to a wavelength of visible light. Further, at the
microscopic scale of operation, where the lateral and/or thickness
dimensions of the materials are on the order of the laser
wavelength, the thickness and indices of refraction of the stack
materials can significantly affect the overall optical
characteristics of the stack.
[0087] In one embodiment of the invention, a preferred reduced
wavelength is selected in the visible or near infrared range
wherein a non-absorptive optical property of the layers (e.g.,
interference or reflection loss) is exploited.
[0088] U.S. Pat. No. 6,300,690 (Reference 8) describes a system and
method for vaporizing a target structure on a substrate. The method
includes providing a laser system configured to produce a laser
output at the wavelength below an absorption edge of the substrate.
Furthermore, Reference 4 discloses benefits of a wavelength less
than 1.2 .mu.m for processing links on memory devices wherein the
substrate is Silicon, namely smaller spot size and shorter laser
pulse widths. In accordance with the present invention, improved
performance can be realized by exploiting the non-absorbing stack
properties with wavelength selection. Furthermore, at least one of
precision positioning of a high numerical aperture beam, spatial
shaping of the spot, or temporal pulse shaping also will provide
for reduced energy at the substrate. The result corresponds to a
relatively low value of energy expected to be deposited in the
substrate, despite an incident beam energy necessary to deposit
unit energy in the target structure sufficient to vaporize the
target structure.
[0089] The factors affecting the energy deposited in the substrate
are, in effect, multiplicative. Likewise, at short visible
wavelengths, copper is absorbing (e.g., about 50% at 500 nm, 70% in
the near UV, compared to 2% at 1.064 .mu.m) so less energy is
required for clean removal, at least an order of magnitude. The
preferred identified wavelength corresponding to a relatively low
value of the energy expected to be deposited in the substrate is
within a visible of near IR region of the spectrum. A model-based
approach may be used to estimate the shortest wavelength with
sufficient margin for a specified dielectric stack, spot position,
tolerance, temporal and three-dimensional spatial pulse
characteristics.
[0090] For processing on links on multilevel devices with Silicon
substrates, the limiting wavelength corresponding to a relatively
low value of the energy expected to be deposited in the substrate
(e.g., below the image threshold) may be within the green or near
UV region of spectrum, but the use may require tightly controlled
system parameters, including possible control of the stack layer
thickness or index of refraction.
With wavelength selection in accordance with the present invention,
where the internal transmission and preferably reflection of the
stack is at or near a maximum, stack layer damage is avoided.
Furthermore, decreasing substrate irradiance, while simultaneously
providing a reduced spot size for link removal (at or near
diffraction limit), is preferred provided irradiation of functional
internal layers is within acceptable limits. Spectral transmission
curves for typical large bandgap dielectric materials generally
show that the transmission decreases somewhat at UV wavelengths.
For example, in HANDBOOK OF LASER SCIENCE AND TECHNOLOGY, the
transmission range of Silicon Dioxide is specified as wavelengths
greater than 0.15 .mu.m. The absorption coefficient of both Silicon
Nitride and Silicon Dioxide remains relatively low in the visible
range (>400 nm) and gradually increases in the UV range.
[0091] In this context of laser based target processing, various
embodiments of methods and apparatus for processing target
structures will now be described.
[0092] Tilted Scan Processing and Alignment
[0093] FIG. 6 is a block diagram of one embodiment of a link
blowing apparatus configured according to one embodiment of the
invention. This system is similar in structure to FIG. 20 of U.S.
2002/0167581 described above. In the system of FIG. 20 of U.S.
2002/0167581, an acousto-optic modulator is used as a beam
splitter. In some embodiments of the present invention, an
acousto-optic modulator 36 is used to sweep the beam axis in a
direction that is neither parallel to nor orthogonal to the
fundamental direction of beam motion during an on-the-fly beam pass
over the device being processed. As described in more detail below,
the AOM may be configured to deflect pulses at an angle
substantially equal to 45 degrees to a processing axis. In some
embodiments, more than one deflector is present or a
two-dimensional AOM may be used to sweep the beam axis at angles
that are substantially perpendicular to each other and tilted with
respect to the processing trajectory.
[0094] Preferably, one or more acousto-optic modulators achieve
fast, accurate and stable power control with a wide dynamic range.
However, other types of modulators, electro-optic modulators and
other suitable modulators can be employed, where preferably
deflection is fast enough to position a beam on a target by target
basis. Modulation may be combined with other functions to reduce
system complexity, for example, beam steering, beam switching, beam
blocking, chirp mode focusing or pulse picking.
[0095] Thermally stable acousto-optic devices may be used in at
least one embodiment of the invention to reduce beam pointing
instability or to reduce optical aberrations. A thermally
stabilized acousto-optic device may be used for pulse picking from
a continuous pulse sequence or for beam positioning. The stabilized
acousto-optic device is driven by one or more transducers at one or
more frequencies with RF power. The first-order diffracted beam is
deflected to a laser processing path as is well-known in the art.
The frequency of the RF may be varied dynamically to position a
deflected laser spot to a precise work piece location. The
amplitude of the RF may be varied to change the diffraction
efficiency of the acousto-optic device and modulate the beam
energy. During pulsed processing and between processing pulses the
acousto-optic cell receives RF power and a near constant thermal
load. When a pulse is not needed for processing, the RF power is
interrupted in coincidence with the non-processing pulse to allow a
single non-processing pulse to pass into the zero order beam to a
beam dump and then the near constant RF is resumed. Alternately,
the RF frequency is interrupted in coincidence with the
non-processing pulse to allow a single non-processing pulse to pass
into a different order beam to a beam dump or into a deflected beam
to a beam dump and then the near constant RF is resumed. Therefore,
the duty cycle of the RF is typically high and thermal instability
due to an intermittent RF load is reduced. RF power may be
modulated between laser pulses to control thermal loading to the
acousto-optic cell, for example to maintain a constant average
thermal load.
[0096] Various configurations of acousto-optic beam deflectors
(AOBD) as know in the art may be used. The AOBD may use single or
multiple transducers attached to a single or multiple acoustic
cells. Acoustic interaction zones may be superimposed or
distributed serially along the optical path and may be in the same
plane or in transverse planes. Any of the well-know geometries and
configurations for acousto-optic scanning, such as those taught
over the past 30 years or more, may be employed. For example, a
first transducer and a second transducer can be mounted to be
co-planar, parallel, orthogonal, or tilted on adjacent orthogonal
or opposing surfaces. Transducers may be tilted or stepped and used
as phased array configuration to enhance performance of the
deflector.
[0097] Each transducer used is mounted to a surface of an acoustic
cell and is driven with an RF signal as is well know in the field.
The transducer is driven with an RF driver, and the RF driver is
controlled with a frequency controller. Routine optimization of the
AOBD includes maximizing Bragg efficiency, increasing modulation
depth, increasing time-bandwidth product (to scan a large number of
spots), and aligning entrance angle and exit angles. For example,
the RF signal can be modulated in amplitude and or frequency to
compensate for Bragg efficiency variation and pointing drift.
[0098] The AOBD may be constructed with acoustic materials
generally offered in commercially available products such as fused
silica, glass, TeO2 and the like from Crystal Technologies, NEOS
and others. On axis device or an off-axis device for example using
a TeO2 crystal may be used to achieve a high time-bandwidth
product. Material choice and orientation is generally a matter of
routine selection to match performance characteristics with the
deflection application considering the beam size, scanning speed,
scanning range, wavelength, efficiency and other characteristics as
generally published in product literature and reviewed in technical
articles, and text books readily available to workers of all skill
levels in the field of optical scanning.
[0099] In at least one embodiment, the AOM is configured to sweep
the processing beam in a direction that is non-orthogonal and
non-parallel to a processing axis. In some embodiments, the
deflector is oriented substantially at 45 degrees to a processing
axis. As shown in FIG. 7, the AOM may be configured such that the
deflection angle is oriented substantially 45 degrees to both the
horizontal and a vertical processing axes.
[0100] In one embodiment, a parallel component of the sweep
velocity vector component produces motion in the direction of a
substrate positioning axis, and may be used to maintain spot
alignment across the link. As shown in FIG. 8A, the spot may be
deflected at a velocity S that is 1.414 times the link velocity V
produced by the underlying on-the-fly pass, so that the component
spot velocity across the link L precisely matches the link velocity
(V=L=S/1.414). However, in the orthogonal direction, a beam spot
velocity A at the link processing velocity results along the link
(A=V=S/1.414). Therefore, during link processing, the beam spot
moves along the length of the link, rather than across the
link.
[0101] The link blowing process may be extremely sensitive to
misalignment across the link, for example about 50 to 250
nanometers misalignment may adversely affect the process window
with decreased pulse energy thresholds for substrate and/or
neighbor link damage. But, it is recognized that this error along
the link may be considerably larger without adversely affecting the
process window, for example from 0.5 to 5 microns. The link blowing
process window is more tolerant to this type of error and pulse
energy thresholds for substrate and/or neighbor link damage may not
be adversely affected.
[0102] Furthermore, an elongated spot along the link may improve
the process window, so lateral motion with a 45 degree oriented
deflector may improve the process window by elongating the spot as
integrated over the burst to form a temporally shaped spatial spot
irradiance profile. Spot elongation may be increased with an
increased link velocity, an increased irradiance period or a
combination.
[0103] In some embodiments, the beam spot may be swept in a
plurality of directions. For example, as illustrated in FIG. 8B, a
two-dimensional deflector may be configured to deflect pulses in
two orthogonal directions, S.sub.1 and S.sub.2, both of which are
oriented at about 45 degrees with respect to the X and Y processing
axes. In this instance, deflection of the pulse at 1.414*V along
the first direction S.sub.1 results in movement up the link along
the direction A.sub.1. Deflection of the pulse at 1.414*V along the
second direction S.sub.2 results in movement down the link along
the direction A.sub.2.
[0104] Applications of the tilted beam scan for link processing
with multiple pulses per link are illustrated in FIGS. 9, 10, and
11. For example, FIG. 9A illustrates the effect of relative
movement in a first direction 25 between a plurality of initial
beam paths 26a, 28a, 30a and 32a and a target structure 34: the
later pulses (e.g., 32a) are less coincident with the target
structure than the earlier pulses (e.g., 26a). In FIG. 9B, three of
the four pulses 26a, 28a, 30a and 32a are deflected to produce the
resultant pulses 26b, 28b, 30b and 32b both in a second direction
36 to oppose the relative movement of the beam paths and in a third
direction 38 perpendicular to the relative movement.
[0105] The non-parallel direction may comprise a component that is
perpendicular to the direction of relative movement of the beam
paths. The pulses may also be deflected in a direction parallel to
the direction of relative movement of the beam paths. For example,
a first deflector may act to oppose the relative movement of the
beam paths, while a second deflector acts to deflect the pulses in
a direction perpendicular to the relative movement of the beam
paths.
[0106] In some embodiments, two or more pulses are deflected. The
magnitude and/or the direction of the deflection may differ across
pulses. For example, the relative movement between the beam paths
and the target substrate may cause a first initial pulse 32a to be
further from the target structure than a second initial pulse 28a.
The first initial pulse 32a may therefore undergo a stronger
deflection than the second initial pulse 28a.
[0107] The number and/or frequency of pulses and/or the dimensions
of the target structure may at least partially determine the
direction of deflection. For example, the magnitude and/or
direction of the deflection may be determined in accordance with
overlap between portions of the target structure irradiated by a
plurality of pulses and/or in an effort to control the total area
irradiated by the plurality of pulses.
[0108] In some instances, a system or method herein may produce
resultant pulses from initial pulses. The initial pulses may
comprise deflected or undeflected pulses. Resultant pulses may
comprise one, two or more of the initial pulses that have been
deflected in a direction non-parallel to the direction of relative
movement of the beam paths. Resultant pulses may also comprise one,
two, or more of the initial pulses that have not been deflected in
a direction non-parallel to the direction of relative movement of
the beam paths. The resultant pulses may include pulses deflected
in a direction non-parallel to the direction of relative movement
as well as pulses that are not deflected in the non-parallel
direction. For example, in FIG. 9B, resultant pulse 26b corresponds
to the same spatial location as initial pulse 26a, while resultant
pulses 28b, 30b and 32b correspond to different spatial locations
than initial pulses 28a, 30a and 32a.
[0109] In some instances, the direction or magnitude of the
deflection at least partially depends on one or more of the
frequency or number of pulses, the speed of the relative movement
between the beam paths and the substrate, and the separation
between target locations. For example, FIG. 10A shows three initial
pulses 40a, 42a and 44a and their coincidence to a target structure
46. The initial pulses 42a and 44a can be deflected in a first
deflection direction 48 to produce the resultant pulses 42b and
44b. In FIG. 10B, there are five initial pulses 50a, 52a, 54a, 56a
and 58a. Deflecting the initial pulses in the same direction would
cause the later pulses (e.g., 56a and 58a) to extend beyond the
length of the target structure 46. Therefore, these initial pulses
50a, 52a, 54a, 56a and 58a are deflected in a second deflection
direction 60 that is different from that of the first deflection
direction 48.
[0110] The resultant pulses may coincide with portions of the
target structures that are spatially distinct and partially
overlapping. In some embodiments, the portions are non-overlapping.
The portions may extend across a length of a target structure,
wherein the length of a target structure may be a direction
perpendicular to the direction of relative movement between the
beam paths and the substrate.
[0111] FIG. 11A illustrates an example in which initial pulses 72a,
74a, 76a, 78a, 80a, and 82a are moved relative to a target
structure 70 in a first direction. The initial pulses 72a, 74a,
76a, 78a, 80a, and 82a are aligned in a second direction
perpendicular to the first direction to be aligned with an end of
the target structure 70. The initial pulses 74a, 76a, 78a, 80a, and
82a can then be deflected in deflecting directions 84, 86, 88, 90,
and 92 non-parallel to the direction of motion to produce the
resultant pulses 72b, 74b, 76b, 78b, 80b, and 82b. In some
instances, the deflecting directions 84, 86, 88, 90, and 92 are the
same across pulses, while the magnitude of the deflection differs.
In other instances, the deflecting directions 84, 86, 88, 90, and
92 differ. The latter option may be advantageous, for example, if
the middle of the target structure 70 is more sensitive to
irradiation or is less sensitive to misalignment.
[0112] In FIG. 11B, initial pulses 94a, 96a, 98a, 100a, 102a, and
104a are similarly moved relative to a target structure 70 in a
first direction. However, the initial pulses 94a, 96a, 98a, 100a,
102a, and 104a are aligned in a second direction perpendicular to
the first direction to be aligned with a middle portion of the
target structure 70. The initial pulses 94a, 96a, 98a, 102a, and
104a can then be deflected in deflecting directions 106, 108, 110,
112 and 114 non-parallel to the direction of motion to produce the
resultant pulses 94b, 96b, 98b, 100b, 102b, and 104b. The
deflecting directions 106, 108, and 110 may include components in
the direction of motion in addition to components perpendicular to
the direction of motion. Meanwhile, the deflecting directions 112,
and 114 may include components opposite to the direction of motion
in addition to components perpendicular to the direction of motion.
Thus, one or more of the pulses may undergo a deflection in
different direction that one or more other pulses.
[0113] In some embodiments, it may be desirable to increase or
decrease spot elongation in a temporally shaped spot irradiance
profile during extended irradiation periods while using a single
deflector and image rotation. Referring back to FIG. 6, by using an
image rotator 37 such as a dove prism to orient the deflection
angle, the deflection orientation angle may be arbitrarily set
relative to the positioning system. In this case the spot alignment
across the link can be compensated by considering the vector
component of motion across the link (sin .theta.*S=V=L) and the
rotation angle can be adjusted to increase or decrease the lateral
spot motion along the link (cos .theta.*S=A). For example, if the
deflector scans at about 26 degrees to the link axis, the lateral
motion along the length of the link will be twice the corrected
link motion.
[0114] The scan rate along the link can be nonlinear as shown, for
example, in FIG. 11B, or can be a discontinuous pattern within
bandwidth limitations of the beam deflector. For example, multiple
scans along a link within one burst may be possible. Of course many
other patterns are possible for integrated spot shaping with long
bursts. The energy painted on the link with this type of patterning
over a dense burst of many pulses provides spot shaping with the
energy integrated over the length of the burst.
[0115] The AOBD scanner may be used with closely spaced multiple
input frequencies to cause multiple simultaneous output spots to
merge into a single shaped spot. The shaped spot may be somewhat
linear with a flattop cross section in the long axis. With two axis
AOBD deflection and multi-frequency inputs, rapid spot shaping can
be accomplished in either the x or y scan axis or in both axes
combined.
[0116] As shown in FIG. 12, two or more rows of links oriented in
either the x or y axis can be addressed with a single AOBD oriented
at an intermediate angle such as 45 degrees. In this case, a link
to link diagonal between rows may not be precisely 45 degrees. This
alignment error can be accommodated with auxiliary beam rotation,
pulse timing correction, trajectory path planning or a preferably
with secondary scan axis. The seqondary scan axis may have a small
range sufficient to correct for spot placement errors of a larger
range scan axis Using a secondary AOBD scan axis can eliminate the
need for any image rotation when simultaneous processing is not
employed. Since the secondary scan axis range can be small, on the
order of one or two link pitches, errors generated from separation
of the two AOBDs without intermediate relay optics are reduced. The
main 45 degree scan axis pupil is imaged to the objective lens, the
secondary AOBD is located close to the main AOBD and small pupil
errors may generate acceptably small position errors over the
secondary scan range, for example position errors resulting from
small errors in focus height.
[0117] Use of a tilted primary scanner with a small range secondary
scanner can provide improved performance compared to conventional
non-tilted 2-axes scanning. For example, the smaller range of the
secondary scanner can reduce undesirable scanning consequences
including efficiency variation with scan angle and beam motion
across the scanner input pupil especially when the secondary
scanner is upstream of the primary scanner in the optical path. The
scanners may be optimized differently according to scan range, or
they may be similar devices. The scanning range may be set
according to command signals such that the smaller scan range is a
sub-range of a larger scan range. In this case the roles of primary
and secondary scanner can be reversed by the system controller to
switch the large and small scan ranges for example to change the
orientation of the primary scan axis without using beam path
switching or image rotation. The tilted primary scan axis may
require a larger scan range than an orthogonal scanner, and
additional trajectory overhead at the beginning and ending of each
processing pass may be required.
[0118] Tilted 2-axis scanning can also be used to minimize
undesirable consequences of compound-angle scanning where a first
scanner deflects the beam through in a first scan plane and a
second scanner deflects deflected beam through a second
non-parallel scan plane. The resulting deflection is in a compound
angle which can affect beam characteristics. For example, in a
conventional orthogonal and square scan field, the worst case
compound angle is in the corners of the field. When 45 degree
tilted scanners are used to scan the same field size, the worst
case compound angle scanning is at the mid-point of each edge of
the scan field, a lesser compound angle, whereas at the corners of
the field the scan angle have minimal compound angles.
[0119] Furthermore, the image rotator can be used to provide a
flexible system that can selectively scan along either the x or y
axes in addition to intermediate angles. Since the deflecting
orientation is rotated at twice the rate of the beam rotator, a 45
degree rotation of the image rotator permits 90 rotation of the
deflection orientation, for example from the x axis to the y axis
through 45 degrees. A 67.5 degree rotation of the image rotator
produces a 135 degree rotation of the deflection orientation, for
example from the x axis to 45 degrees to the y axis to -45 degree.
With bidirectional deflection, a 90 degree rotation of the
deflector permits deflection along any azimuth.
[0120] In some embodiments, it is desirable to have at least a
small amount of image rotation to align an axis of deflection with
a device to be processed. When wafers are mounted onto a wafer
chuck, typically there is a residual rotation of the wafer and a
small rotation of the scan axis can be used to correct for
resulting alignment errors. For example, the axis of an AOBD is
aligned to be orthogonal to a row or column of links. In this case,
spots may be laterally offset to coincide with one or more
collinear links without significant errors due to relative rotation
of the scan axis and the device geometry.
[0121] Image rotation is useful when multiple spots are used
simultaneously to irradiate one or more links. In these
embodiments, multiple laser beam paths are simultaneously produced
and may be directed to multiple spots on the device being
processed. Typically, each separate beam can be blocked so that
none, one, two, or more separate locations can be simultaneously
processed. The multiple spots may be on a single target structure,
or may be distributed among several target structures. The scan
axis can be aligned to accommodate to the geometry so that two or
more links can be processed with a single pulse or a single group
of pulses. This is illustrated in FIG. 13, where a beam path in the
X-direction over three parallel rows of links is split to
selectively process any one or more of the three links along the
orthogonal Y direction. Control of the precise orientation of the
scan axis perpendicular to the rows is useful to minimize errors
along the width of the link for links at the ends of the scan
path.
[0122] A tilted scan angle with controlled orientation can be
useful for multiple beam processing of adjacent staggered link rows
as shown in FIG. 14. In this case, the scan axis along which a pair
of beam paths diverge is neither parallel nor perpendicular to the
rows of links. Image rotation in this embodiment can align the scan
axis with any variation in link pitch and row separation.
[0123] In some fuse bank geometries, the average link pitch is
reduced by using fanned out groups of staggered links. For example,
groups of three farmed out links alternate in two staggered rows.
One such arrangement is shown in FIG. 15. These types of link
arrays are also illustrated, for example, in U.S. Pat. No.
5,636,172 to Prall et al. FIG. 15 shows an embodiment with two
beams separated along a scan path to process none, one, or both
links of staggered pairs of links in the array. Image rotation may
align a scan axis with different combinations of links in fanned
out geometries. For example, FIG. 16 illustrates a three beam path
embodiment for processing two rows of fanned out links in a single
pass. FIG. 17 illustrates a three beam path embodiment for
processing four rows of fanned out links in a single pass.
[0124] In one example for processing N rows of links, a first pulse
is split along a first row to irradiate one or more links in a
selected group of links in the first row. A second pulse is
likewise split and deflected to a second group of links in a second
row (as in FIG. 22). Referring to FIG. 24, third and fourth pulses
are split and deflected to third and fourth rows. A fifth pulse is
split and deflected back to the first row to continue a repeating
sequence. The pulses can be split into M spots where M=N to
irradiate all structure in N rows of an array, M<N when there
are gaps between groups, or >N to redundantly address links.
[0125] Thus, when pulse splitting is employed with an AOBD to
simultaneously process 2 or more links, the spots are located along
the scan axis of the AOBD, and image rotation may be useful to
align the spots to multiple links especially when multiple links
are not located along a single row and hence rotation error of the
scan axis generates spot position errors relative to the width of
the link. Image rotation may not be required when multiple links
being processed are in a single row and rotation generates small
tolerable errors along the length of the link.
[0126] It will be appreciated that in mutli-beam processing
embodiments, single beams can be split or multiple beam paths may
be combined and critically aligned to process links. In both of
these cases, delivered pulse energy is preferably controlled to
provide consistent processing. Beam splitting and beam combining
generally require power split calibrations. Splitting can reduce
available energy by a factor or 2 or more and splitting or
switching may affect beam polarization control. Reference 10
discloses various advantageous methods and optical systems using
beam switching for application to memory repair, for example in
addition to or in place of beam rotation.
[0127] Embodiments of this invention may use a 2 cascaded single
axis deflectors or a 2 axis deflector to deflect the beam in more
than one axis. In this 2 axis deflector case, linear scanning at 45
degrees or at an arbitrary orientation can be performed as
described for the image rotation case.
[0128] Pupil errors of cascaded deflector can be reduced with 45
degree scanning and a small secondary axis (as shown in FIG. 13 and
described below). In other cases, relay optics may be used to image
each deflector window. A first deflector window may be imaged to a
second deflector, or anamorphic cylinder optics may be used. For
example an image relay may include two sets of crossed cylinder
lenses with pairs of aligned cylinders relaying each deflector
window independently. The relay may also include spherical optical
elements, for example to allow a weak set of crossed cylinders
elements to be used along with a spherical element.
[0129] The system of FIG. 20 of U.S. Patent Publication
2002/0167581 described above may be used in one or more embodiments
of this invention. In at least one other embodiment, the deflector
system is used to generate at least 2 slightly diverging beams and
may generate more than two slightly diverging beams. The beams may
be generated simultaneously or generated in a sequence. The
deflected angle of each beam corresponds to a driven frequency in a
range that is a small percentage of the center RF signal frequency.
The position of each spot is proportional to the driven
frequency.
[0130] Other deflector configurations maybe used. For example,
cascaded acousto-optic modulators or a single crystal two axis
modulator can be used to position each beam without using a beam
rotator. Furthermore, any type of beam positioning system with
sufficient bandwidth to accurately locate a sequence of pulses may
be employed.
[0131] Zoom optics may form an image of the acousto-optic window at
the entrance pupil of the objective lens to minimize or eliminate
beam input offset errors at the objective lens. Optionally, fixed
relay optics may be used between the acoustic window and the zoom
optics to form an intermediate image of the acoustic window near
the input of the zoom optics. For two axis AOBD scanning a pair of
AOBDs may be stacked in a crossed configuration and the mid point
between the acoustic windows of each device may be imaged to the
objective. Positioning errors due to residual angle errors can be
determined using ray tracing or system measurement. Alternatively a
pair of AOBDs may be spaced apart and the window of the first AOBD
can be imaged onto the window of the second AOBD with the second
window imaged or relayed onto the input of the zoom telescope using
cylindrical and/or spherical elements as mentioned.
[0132] When two single-axis spaced-apart AOBDs are used, the
position of the relay beam expander may be adjusted to image one of
two AOBD windows onto the objective lens. For example, if two AOBDs
are used for x and y beam positioning, the beam expander is
adjusted axially to image either the x or the y AOBD window onto
the objective. The relay beam expander may be adjusted during
processing of a processing site to change the orientation of AOBD
scanning. Alignment scans may be performed after the adjustment to
accommodate residual positioning errors. In this way the system can
be precisely adjusted for x or y beam deflecting without a relay
optical system between the x and t beam deflectors
[0133] The zoom optics may be a variable Keplerian telescope with
three groups of elements. The first and second groups may be single
optical elements and the third group may be a doublet. Each group
may be moved independently according to predetermined positional
data to provide a selected beam expansion and collimation. Element
groups of the zoom optic may be pre aligned to minimize or
eliminate beam steering effects as the expansion ration is varied.
Alignment of element groups may also ensure that beam centering is
maintained at the entrance pupil of the objective lens.
[0134] Residual alignment variations through the zoom range of the
zoom optics may be corrected with auxiliary beam steering devices
such as remotely controllable precision adjustable turning mirrors
or scanning mirrors. Correction values may be stored for
predetermined zoom settings and corrections may be made
automatically for different zoom setting to maintain overall system
calibration and alignment accuracy.
[0135] One or more spatial filters may be used at beam waist planes
of the optical path for picking off unused diffraction orders and
to block scattered energy and noise.
[0136] By using a tilted deflection oriented at or rotated to 45
degrees, other advantages are possible in a laser processing
system. The tilted deflection can be used in a system calibration
mode to perform edge scans for alignment as is known in the field
of precision laser processing equipment. In the same way that
scanning for processing in 2 orthogonal axes can be performed with
a tilted deflection, so to can alignment in 2 orthogonal axes be
performed with a single deflector oriented at 45 degrees. When used
with typical, orthogonal alignment targets 125 such as illustrated
in FIG. 18, the 45 degrees scan is able to traverse either an x or
a y edge, where the scan line is at 45 degrees to the alignment
edge. Furthermore, if an alignment scan is performed near an inside
or outside corner of an alignment target, then orthogonal edges can
be scanned in a single sweep. The latter is illustrated in FIG. 18,
where the beam spot moves along a scan axis at velocity V, and
after scanning one leg of the alignment target, the tilted scan
produced motion at speed V in the direction A to scan across the
orthogonal leg of the alignment target.
[0137] Two axis scanning can also be used for alignment scans. The
laser spot can be dithered in a direction parallel to the alignment
target edge to sample multiple locations along the edge and average
out the effects of edge defects.
[0138] Alignment aspects of the invention for example as described
in sections [130], [169] and FIG. 10 of U.S. Publication
2002/0167581 may also be applied to laser systems with high pulse
rates such as mode locked lasers. While q-switched laser operate in
the range of about one kilohertz to a few hundred kilohertz, mode
locked laser typically operate at many tens of megahertz. Various
scanning and target sampling strategies can be employed. Edges can
be scanned at higher rates with dense sampling depend on the
bandwidth limit of the detector response. Using fast pulse picking,
a pulse sequence can be multiplexed to multiple targets for
alignment.
[0139] Non-Synchronous Processing
[0140] In one embodiment of the invention, processing speed is
increased by eliminating the requirement that the time between
pulses be equal to the link to link transit time T1 described above
with reference to FIG. 2. This embodiment is described with
reference to FIGS. 19, 20, and 21. A pass of the beam across the
same eleven links of FIG. 2 is illustrated in FIG. 20. However, in
FIG. 20, only nine laser pulses (or pulse bursts) are used instead
of eleven. In the embodiment of FIG. 2, the laser pulses may be
produced at the same rate as in FIG. 2, (e.g. 50 kHz pulse rate),
but the relative velocity between the beam axis and the links is
increased so the nine pulses are still spread approximately evenly
over all eleven links. To successfully process links 10a, 10d, and
10f, a high speed deflector is used to deflect the processing beam
and offset the laser spot in the direction of the arrows on FIG. 20
to so that the spot and a link substantially coincide when a given
link is to be processed. The result is illustrated in FIG. 21.
Blocked pulses or pulse bursts, as shown by dotted lines on FIGS.
20 and 21, need not coincide with any potential target structure. A
precise offset for processing pulses can be predetermined based on
the link position and the timing of the pulse sequence. If the same
50 kHz laser pulse rate is available in the process of FIGS. 20 and
21 as the conventional example of FIG. 2, the pass can be completed
in 160 microseconds, for about a 20% processing speed improvement.
An apparatus suitable for performing this technique is illustrated
in FIG. 19. During processing, a deflected spot offset is
determined by the controller for each link to be processed relative
to a selected pulse. The high speed deflector applies the offset
for processing.
[0141] It is apparent that when T1 is less than the laser pulse to
pulse period, there will be insufficient available pulses in the
sequence to have a pulse available for every link. However, in an
exemplary memory repair application, a relative few links, for
example 1 in 10 links are processed and a majority of links remain
intact. Therefore, without the constraint of 1 to 1 link to pulse
synchronization, generally a sufficient number pulses are available
to process a device at an increased velocity. If, for example, the
time between laser pulses is 1.25 times T1, a series of up to about
eight adjacent links could be processed if the deflector was
capable of deflecting the pulse forward or backward up to the
distance d between each link. In practice, it is relatively rare
that even four or five consecutive adjacent links are processed.
Thus, using high speed deflection and determined spot offsets,
relative velocity between links and a laser processing head can be
increased and can exceed conventional limits. In other cases,
rather than increasing processing speed, other benefits can be
obtained by using lower pulse repetition rates while maintaining
conventional processing speeds. For example, it may be desirable to
operate a laser at a reduced repetition rate for increased pulse to
pulse stability, to use alternate pulse widths, or to accommodate
different positioning velocities without altering the repetition
rate such as processing near the edge of a stage travel range.
[0142] As described above, recent developments have included
multiple beam systems that can process two or more link sites
simultaneously. Although this increases throughput, it is likely
that in most applications, a similar or better throughput increase
can be obtained with the non-synchronous processing described
herein. Thus, spot offsets can be used in a sequential process and
can reduce requirements for simultaneous processing which can
require more complicated beam control. Combining the two
techniques, e.g. using multiple non-synchronous pulsed beams could
provide still further throughput enhancements.
[0143] Mixed Pitch Processing
[0144] The deflector 130 of FIG. 19 can also be used to process
rows of links with gaps or sections of different pitch. FIG. 22A
shows a series of links with different link separations. In some
cases such as in FIG. 22A, the spacing between links is different
in different areas, but the pitch phase is constant, which will
occur if the distance between any two links is an integer multiple
of the smallest pitch. In FIG. 22A, for example, s.sub.1 is 3d long
and s.sub.2 is 2d long. The same nine pulses of FIG. 20 can still
be used to process this structure.
[0145] Mixed Phase Processing
[0146] FIG. 22B shows another link arrangement where two sets of
links in a common row both have the same pitch, but are out of
phase with each other. This will occur when the distance s is not
an integer multiple of pitch distance d. Conventionally, such link
arrangements were time consuming to process. If the entire row
including both sets of links was processed in a single pass, twice
as many pulses were produced so that the pulses were synchronized
with both pitches for the entire pass. Alternatively, two separate
passes at full speed could be used, which takes time to set up
between passes.
[0147] With the deflector 130, the pulses associated with one of
the link sets can be offset during the pass to match the phase of
that group, as shown by the arrows of FIG. 22B. The deflector 130
can thus perform on-the-fly phase correction and process such link
arrangement much faster than conventional systems. Phase correction
can be used to stabilize laser pulse energy by minimizing changes
to pulse to pulse periods and reducing or eliminating associated
settling times.
[0148] Link Severing During Acceleration
[0149] These techniques can also be used to sever links during
non-constant velocity segments of a beam trajectory. As shown in
FIG. 23, during an acceleration segment of a trajectory, a pulse
period T will produce a different beam spot spacing between pulses
at different points in the trajectory. Beam deflection can be used
to correct beam spot position to match the link positions for links
to be processed. The ability to process links during acceleration,
deceleration, and other non-constant velocity portions of a beam
trajectory can reduce processing time.
[0150] Multi-Channel Processing for Link Buffering
[0151] FIGS. 24 and 25 illustrate another system and method for
processing links at a faster rate than conventional processing. In
this embodiment, the deflector 130 selects a channel 141, 142, or
143 that causes the pulse to hit the substrate being processed at
three different locations. The distance between beam spots for
adjacent channels can be set equal to the link pitch d. The links
are assigned to link groups 150a-e, each containing N links, where
in this embodiment N is equal to 3. Depending on the channel
selected for beam spot 152, link 154, 156, or 158 can be processed
with that pulse. If the groups overlap, some links can be processed
with either of two pulses. This embodiment provides a potential
doubling of processing speed, as only one pulse is produced for
every two links. Thus, in a pass over M links, only P pulses are
produced, where P is half of M. However, although any pair of
adjacent links can be processed, there are not enough pulses
available to process a series of three or more adjacent links. As
described above, though, in some cases this may not be present in a
link group being processed in a pass. However, groups with very
large numbers of links may have occurrences of adjacent links even
when a small percentage of links are processed.
[0152] It will be appreciated that information regarding the number
and location of links to be processed in a pass can be used to
determine the optimum size and positioning of the link groups. FIG.
26A shows a plurality of structures indicated by thick lines. The
black lines correspond to non-target structures 105a, and the grey
lines correspond to target structures 105b.
[0153] As illustrated in FIG. 26B, structures may be grouped into
sets 305 of two structures 105. Within each set 305, the number of
structures 105 that are target structures 105b may be determined.
For example, sets 305d and 305h each include one target structure
105, while set 305a does not include any target structures 105b. In
total for this example, 4 of the 12 sets contain no target
structures 105b, 8 of the 12 sets contain only one target structure
105b, and none of the 12 sets contain more than one target
structure 105b.
[0154] In some embodiments, groups of different sizes are examined
prior to performing a pass. For example, in FIG. 26C, the same
structures are grouped into sets of three structures. As before,
the number of target structures in each group can be determined. In
this instance, none of the 8 sets contain 0 target structures, 8 of
the 8 sets contain only 1 target structure, and none of the sets
contain 2 or 3 target structures. Therefore, grouping structures
into sets of three provides fewer sets containing no target
structures (0 of 8 sets) than would grouping structures into sets
of two (5 of 12 sets). If one pulse is produced for each set, the
efficiency and/or the speed of processing may be further increased
when using sets of three rather than sets of two.
[0155] Notably, the effect of grouping may depend upon the phase of
the groups. For example, FIG. 26D shows another grouping into sets
of 3 structures, wherein each grouping is shifted compared to that
in FIG. 26C. Therefore, while the grouping of FIG. 26C included no
sets with no target structures, the grouping of FIG. 26D includes
two sets with no target structures. Additionally, while the
grouping of FIG. 26C did not include any sets with two target
structures, that of FIG. 26D includes two sets with two target
structures. This arrangement could not be processed with a single
pulse for each set. In some embodiments, for each set size, the
groupings may be shifted in order to determine whether a particular
grouping offset provides preferred target structure
distributions.
[0156] FIG. 26E shows structures 105 grouped into sets 515 of four
structures, wherein each set 515 overlaps with the previous set 515
by two structures. As before, one or more pulses 520 may be
generated for each set. However, in this embodiment, any given
structure may be irradiated by more than one pulse 520 depending on
the deflection. In this example, each pulse may be deflected to
irradiate one of four structures. The size of the sets 515 and the
overlap can determine the number of pulses 520 which may irradiate
any given structure 105.
[0157] Although the previous examples are described for a single
axis deflection, the buffering/channeling technique can be used in
multiple axes, for example one or more channels may correspond to
different link types in a set of links in a fanned-out staggered
array of links. In some cases the links are unevenly spaced and are
located in multiple rows. Various embodiments of this invention may
be practiced at the same time, for example one or more of, spot
shaping, and link buffering. In some embodiment, each pass is
analyzed beforehand to determine whether non-synchronous, mixed
pitch, mixed phase, or channeled processing would produce the
optimum processing speed enhancement for the given pass. Groups of
links may be analyzed to predetermine the best operational mode and
slow or speed the scan rate as required in conjunction with the
system trajectory planner, while maintaining a constant laser pulse
rate, severing a single link per pulse. Adjacent channels may
correspond to adjacent links, however greater spatial channel
separations may be used to hit the same group on pitch and multiple
channels may be used to access a group at multiple locations.
[0158] To deflect the beam into different channels, preferably the
RF applied to the deflector when it is an AOM is generated using a
multi-channel Direct Digital Synthesizer (DDS) such as Model AODS
20160 from Crystal Technologies. Alternately a group of fixed
frequency drivers or a single or multiple variable frequency
drivers may be used. Buffering capability of several preset
frequency channel values and rapid switching between channels
provides enhanced capability for laser processing of memory
devices. For example, frequencies may be switched among channels of
a multi-channel DDS. Frequencies may be set corresponding to
spatial properties of a work piece for example, link pitch or row
spacing in an array of memory repair links. Frequencies may
correspond to adjacent links, a sequence of links, adjacent row,
non-adjacent links, and non-adjacent rows. Frequencies may
correspond to a lateral offset along or across one or more links or
to a measured or predicted position error. Frequencies may
correspond to temporal properties of a memory repair process, for
example, a frequency may correspond to a delay associated with an
inter-pulse period, a pulse burst parameter, a position timing
error correction value or a measured or predicted position phase
lag. Generally, channel switching occurs at rates on the order of
the link velocity divided by the link pitch or a higher rates
especially with multi-pulse processing. Switching time maybe less
than 20 ns in a DDS driver. It is to be understood that actual
switching time of beam steering angles is limited by well-know
acoustic propagation parameters of the AO cell.
[0159] In all embodiments described herein, corrective optics may
be used to compensate for the well-known cylindrical lensing effect
as well as other aberration artifacts of the AOBD. When the AOBD is
operated in chirp mode, the cylindrical lensing effect increases
with the scan rate, and correction may include using fixed or
variable optical elements to cancel astigmatism and tilt artifacts
of off-axis devices generated at the AOBD. Correction of AOBD
aberrations is generally taught in published U.S. Patent
Application 2006/0256181, which is hereby incorporated by reference
in its entirety.
[0160] Other aberrations of AOBD devices can include chromatic
aberration introduced by dispersion of the AOBD. Especially with
broad band width fiber laser sources which may have bandwidths of
16 nm or more, pre-dispersion correct techniques can be employed to
minimize lateral chromatic aberration in the image plane. For
example, dispersion prisms and or diffraction grating can be used
to compensate for the center frequency dispersion of the AO cell.
Axial chromatic aberration may also be significant and focusing
optics may be corrected over the full laser bandwidth to maintain
small spot size and consistent spot shape.
[0161] Auxiliary detectors responsive to beam position and or
energy may be used to sense position and or energy and to generate
signals used for feedback in conjunction with AOBD scanning.
[0162] While the best modes for carrying out the invention have
been described in detail, those familiar with the art to which this
invention relates will recognize various alternative designs and
embodiments for practicing the invention as defined by the
following claims. All described methods and systems herein may used
in any combination. Multiple pulses or single pulses may be applied
to single target structures, multiple pulses or single pulses may
be simultaneously applied to multiple target structures. In a
processing pass, the laser pulse rate may correspond to the rate of
a single row single pass processing or may be asynchronous with
spatial offsets and/or buffering.
* * * * *