U.S. patent application number 11/643746 was filed with the patent office on 2008-04-24 for methods and systems for precisely relatively positioning a waist of a pulsed laser beam and method for controlling energy delivered to a target structure.
Invention is credited to James J. Cordingley, Joseph J. Griffiths, Shepard D. Johnson, Donald V. Smart.
Application Number | 20080094640 11/643746 |
Document ID | / |
Family ID | 23069846 |
Filed Date | 2008-04-24 |
United States Patent
Application |
20080094640 |
Kind Code |
A1 |
Cordingley; James J. ; et
al. |
April 24, 2008 |
Methods and systems for precisely relatively positioning a waist of
a pulsed laser beam and method for controlling energy delivered to
a target structure
Abstract
A method and system for locally processing a predetermined
microstructure formed on a substrate without causing undesirable
changes in electrical or physical characteristics of the substrate
or other structures formed on the substrate are provided. The
method includes providing information based on a model of laser
pulse interactions with the predetermined microstructure, the
substrate and the other structures. At least one characteristic of
at least one pulse is determined based on the information. A pulsed
laser beam is generated including the at least one pulse. The
method further includes irradiating the at least one pulse having
the at least one determined characteristic into a spot on the
predetermined microstructure. The at least one determined
characteristic and other characteristics of the at least one pulse
are sufficient to locally process the predetermined microstructure
without causing the undesirable changes.
Inventors: |
Cordingley; James J.;
(Littleton, MA) ; Griffiths; Joseph J.; (Winthrop,
MA) ; Johnson; Shepard D.; (Andover, MA) ;
Smart; Donald V.; (Boston, MA) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET
FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
23069846 |
Appl. No.: |
11/643746 |
Filed: |
December 21, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11247541 |
Oct 11, 2005 |
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11643746 |
Dec 21, 2006 |
|
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10107702 |
Mar 27, 2002 |
7027155 |
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11247541 |
Oct 11, 2005 |
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60279644 |
Mar 29, 2001 |
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Current U.S.
Class: |
356/614 ;
219/121.72; 219/121.73; 219/121.76; 219/121.77; 219/121.85 |
Current CPC
Class: |
B23K 26/389 20151001;
B23K 26/082 20151001; B23K 26/16 20130101; B23K 26/361 20151001;
H01L 2924/0002 20130101; B23K 26/04 20130101; B23K 2101/40
20180801; B23K 26/064 20151001; B23K 26/0648 20130101; B23K 26/0665
20130101; B23K 26/0736 20130101; B23K 2103/12 20180801; B23K
2103/50 20180801; B23K 26/0613 20130101; H01L 23/5258 20130101;
B23K 26/0624 20151001; B23K 26/032 20130101; H01L 22/12 20130101;
B23K 26/0622 20151001; B23K 26/0643 20130101; H01L 2924/00
20130101; H05K 3/0026 20130101; B23K 2103/10 20180801; H01L
2924/0002 20130101; H01L 21/485 20130101; B23K 26/40 20130101; B23K
26/067 20130101; B23K 2101/38 20180801; B23K 26/10 20130101; H01L
21/76888 20130101; B23K 2103/172 20180801; B23K 26/0652 20130101;
B23K 26/043 20130101; H01L 21/76894 20130101 |
Class at
Publication: |
356/614 ;
219/121.72; 219/121.77; 219/121.85; 219/121.73; 219/121.76 |
International
Class: |
G01B 11/14 20060101
G01B011/14; B23K 26/38 20060101 B23K026/38; B23K 26/06 20060101
B23K026/06 |
Claims
1. A method for precisely relatively positioning a waist of at
least one pulsed laser beam to compensate for microscopic
positional variations of one or more predetermined target
structures to be laser processed, the method comprising: measuring
a position of at least one alignment target formed at a
predetermined measurement location to obtain at least one
measurement; predicting a relative location of one or more
predetermined target structures and the at least one laser beam
based at least in part on the at least one measurement to obtain
one or more predicted relative locations; generating at least one
laser beam including at least one pulse; inducing relative motion
between the one or more predetermined target structures and the at
least one laser beam based on the predicted relative location;
updating the predicted relative location during relative motion
based at least in part on updated position information, the updated
position information being obtained during the relative motion; and
irradiating the at least one pulse onto a spot on the one or more
predetermined target structures to process the one or more
predetermined target structures based at least in part on the
updated position information.
2. The method of claim 1, comprising processing multiple target
structures with multiple laser beams.
3. The method of claim 2, comprising: updating the predicted
relative location of multiple laser beams and multiple target
structures during relative motion based at least in part on updated
position information, the updated position information being
obtained during the relative motion; and irradiating the at least
one pulse onto multiple spots on the multiple target structures to
process multiple target structures based at least in part on the
updated position information.
4. The method of claim 1, comprising obtaining three-dimensional
information at a plurality of predetermined locations about the
target material, the locations being a combination of at least one
predetermined location at which an alignment target is formed and
at least one other predetermined location at which an alignment
target is not formed and which is suitable for an optical
measurement; predicting the location of the target material
relative to a laser beam waist position based on the
three-dimensional information including three-dimensional
information obtained at the at least one other predetermined
location to obtain a three-dimensional location prediction;
inducing motion of the target material relative to a laser beam
waist position based on the prediction; generating a laser beam
including at least one pulse; and irradiating the at least one
pulse into a spot on the target material to process the target
material.
5. A system for precisely relatively positioning a waist of at
least one pulsed laser beam to compensate for microscopic
positional variations of one or more predetermined targets to be
laser processed, the method comprising: means for measuring a
position of at least one alignment target formed at a predetermined
measurement location to obtain at least one measurement; means for
predicting a relative location of one or more predetermined targets
and the at least one laser beam based at least in part on the at
least one measurement to obtain one or more predicted relative
locations; means for generating at least one laser beam including
at least one pulse; means for inducing relative motion between the
one or more predetermined targets and the at least one laser beam
based on the predicted relative location; means for updating the
predicted relative location during relative motion based at least
in part on updated position information, the updated position
information being obtained during the relative motion; and means
for irradiating the at least one pulse onto a spot on the one or
more predetermined targets to process the one or more predetermined
targets based at least in part on the updated position
information.
6. The system of claim 5, wherein the means for updating comprises
a position encoder.
7. The system of claim 5, wherein the means for updating comprises
an optical sensor.
8. A method for laser processing a multi-material device including
a substrate and at least one microstructure designated for removal
within a 1D or 2D array of microstructures, the microstructures of
the array being disposed at a pre-determined spacing, the
positioning subsystem inducing relative motion between the device
and laser beam waists, the processing to remove at least one
microstructure designated for removal within the array without
causing undesirable damage to the substrate, adjacent
microstructures, or any layer disposed between a microstructure and
the substrate, the method comprising: obtaining information to
identify microstructures designated for removal within a row or
column of the array of microstructures, and to identify alignment
targets about the microstructures, each target having a
predetermined location relative to the microstructures; scanning at
least one measurement beam relative to an alignment target to
obtain at least one signal representative of the alignment target
location, and processing the at least one signal to determine a
target location estimate; predicting a location of at least one
designated microstructure based on the estimate; planning and
generating a trajectory to position one or more pulsed laser beams
relative to one or more designated microstructures so that the
microstructures and pulsed laser beam waists substantially coincide
at a location along a laser beam centerline; generating a single
laser pulse; forming at least two spatially separated pulses from
the first pulse wherein the step of forming includes spatially
splitting the single pulse to selectively direct the separated
pulses along multiple directions, and wherein the step of forming
includes utilizing pre-determined spacing information, and at least
one of an alignment target estimate and a predicted location to
adjust the separation between the spatially split pulses so as to
cause a waist of each spatially split pulse to coincide with a
designated microstructure; irradiating at least one microstructure
with the spatially separated pulses wherein a beam waist of each
spatially separated pulse and a microstructure substantially
coincide; wherein the processing of the at least one microstructure
with the at least two spatially separated pulses during relative
motion of the at least one microstructure and the beam waists
precisely positioned relative to the at least one microstructure,
whereby throughput of the processing system is substantially
improved.
9. The method of claim 8, wherein the processing occurs in a single
pass operation controlled with a positioning subsystem of a laser
processing system.
10. The method of claim 8, wherein said trajectory defines
trajectories of a plurality of pulsed laser beams.
11. The method of claim 8, wherein the trajectory is based on three
dimensional information.
12. The method of claim 8 wherein the step of planning and
generating comprises: producing a first motion profile
corresponding to a first trajectory segment, the profile
representative of approximately constant velocity of relative
motion of the at least one microstructure and corresponding waist
locations generally along a row or column of the 1D or 2D array,
the motion occurring during a first specified time interval, or
over a first specified distance, at which one or more
microstructures are to be processed with the at least one laser
pulse, and producing a second motion profile corresponding to a
second trajectory segment, the second motion profile representative
of an acceleration or deceleration during a second specified time
interval, or over a second specified distance, at which the
microstructures and corresponding waists are accelerated to achieve
a predetermined velocity for processing with at least one laser
pulse; and moving the microstructures relative to a laser beam axis
in accordance with the trajectory plan.
13. The method of claim 8 wherein the step of splitting
simultaneously produces the separated pulses.
14. The method of claim 8 wherein exactly two spatially separated
pulses are formed, and wherein the two pulses irradiate one or two
microstructures, or none, with the two spatially separated
pulses.
15. A method for use in processing structures on or within a
semiconductor substrate using N series of laser pulses to obtain a
throughput benefit, wherein N.gtoreq.2, the structures being
arranged in a plurality of substantially parallel rows extending in
a generally lengthwise direction, the N series of laser pulses
propagating along N respective beam axes, the method comprising
planning at least one trajectory for simultaneously moving in the
lengthwise direction the N laser beam axes substantially in unison
relative to the semiconductor substrate so as to process structures
on or within the semiconductor substrate with the respective N
series of laser pulses, whereby the trajectory is such that the
throughput benefit is achieved while ensuring that the trajectory
represents feasible velocities for each of the N series of laser
pulses and for each of the respective structures processed with the
N series of laser pulses.
16. The method of claim 12, wherein said trajectory comprises one
or more acceleration and velocity profiles.
17. A method for use in processing structures on or within a
semiconductor substrate using N series of laser pulses to obtain a
throughput benefit, wherein N.gtoreq.2, the structures being
arranged in a plurality of substantially parallel rows extending in
a generally lengthwise direction, the N series of laser pulses
propagating along N respective beam axes until incident upon
selected structures in N respective distinct rows, the method
comprising: determining a joint velocity profile for simultaneously
moving in the lengthwise direction the N laser beam axes
substantially in unison relative to the semiconductor substrate so
as to process structures in the N rows with the respective N series
of laser pulses, whereby the joint velocity profile is such that
the throughput benefit is achieved while ensuring that the joint
velocity profile represents feasible velocities for each of the N
series of laser pulses and for each of the respective N rows of
structures processed with the N series of laser pulses.
18. The method of claim 17, wherein the determining step comprises:
determining for each of the N rows a velocity profile for moving in
the lengthwise direction the respective laser beam axis relative to
the semiconductor substrate so as to process structures with the
respective series of laser pulses, thereby resulting in N
individual velocity profiles.
19. The method of claim 18, wherein the joint velocity profile does
not exceed the minimum value of the N individual velocity profiles
while a structure is processed with a laser pulse.
20. The method of claim 17, wherein the joint velocity profile
includes one or more sections of constant velocity.
21. The method of claim 17, wherein N=2.
22. The method of claim 17, wherein the first and second series of
laser pulses have respective first and second sets of optical
properties, and wherein the first and second sets are different
from one another.
23. The method of claim 17, wherein the first laser beam axis is
offset from the second laser beam axis by some amount in a
direction parallel to the lengthwise direction of the rows.
24. The method of claim 17, further comprising: generating the N
series of laser pulses; and moving in the lengthwise direction the
N laser beam axes in unison relative to the semiconductor
substrate, in accordance with the joint velocity profile, so as to
selectively irradiate structures in the N rows with the respective
N series of laser pulses.
25. The method of claim 24, wherein the generating step comprises:
generating the N laser beams from N respective lasers.
26. The method of claim 24, wherein the generating step comprises:
generating a single laser beam from a single laser; and splitting
the single laser beam to form the N laser beams.
27. The method of claim 24, wherein the generating step is
commenced based upon a trigger signal.
28. The method of claim 27, wherein the trigger signal is generated
based upon a timing signal.
29. The method of claim 27, wherein the trigger signal is generated
based upon a comparison of one or more desired target locations and
one or more locations at which the laser beam axes intersect the
semiconductor substrate.
30. The method of claim 24, further comprising: during the moving
step, dynamically adjusting the relative spacing among two or more
of the N laser beam axes.
31. The method of claim 30, wherein: the adjustment of the relative
spacing is in a direction substantially perpendicular to the
lengthwise direction of the rows.
32. The method of claim 24, wherein the moving step comprises:
moving the N laser beam axes.
33. The method of claim 24, wherein the moving step comprises:
moving the semiconductor substrate.
34. The method of claim 24, further comprising: selectively
blocking one or more of the N laser beams from reaching the
semiconductor substrate.
35. The method of claim 24, wherein the N laser beams reach the
workpiece at a substantially simultaneous time.
36. The method of claim 17, wherein the determining step comprises:
generating a set of master coordinates; determining for each
structure in the N rows to be laser irradiated a relative offset
coordinate from a master coordinate; and determining a joint
velocity profile for the N rows based on the set of master
coordinates.
37. The method of claim 17, wherein the structures comprise
electrically conductive links and the irradiation of a link results
in severing that link.
38. The method of claim 21, wherein the first laser beam axis is
offset from the second laser beam axis by some amount in a
direction parallel to the lengthwise direction of the rows.
39. A link processing system comprising: one or more lasers
producing a first laser beam; a splitter receiving said first laser
beam as an input and generating a separated plurality of laser
beams; an orientation controller receiving said separated plurality
of laser beams and controlling a relative orientation of said
separated plurality of laser beams; optics configured to focus said
separated and oriented plurality of laser outputs onto a
corresponding plurality of links of an array of links; and a
trajectory planner configured to produce one or more acceleration
and/or velocity profiles for positioning beam waists of said
separated and oriented plurality of laser outputs at link locations
coincident with pulses of said separated plurality of laser
beams.
40. A method of laser processing one or more links on an electronic
device, said method comprising: forming N spatially separated laser
beams along N different beam paths, wherein N.gtoreq.2; planning
one or more trajectories of said N spatially separated laser beams
along a scan beam path; forming one or more beam spots on said
electronic device with said N spatially separated laser beams;
moving said one or more beam spots relative to said electronic
device in accordance with said one or more trajectories; wherein
multiple spatially separated laser beam spots process locations on
said electronic device during a pass of said beam spots over said
electronic device.
41. A system for laser processing a multi-material device including
a substrate and at least one microstructure designated for removal
within a 1D or 2D array of microstructures, the microstructures of
the array being disposed at a pre-determined spacing, the
positioning subsystem inducing relative motion between the device
and laser beam waists, the processing to remove at least one
microstructure designated for removal within the array without
causing undesirable damage to the substrate, adjacent
microstructures, or any layer disposed between a microstructure and
the substrate, the system comprising: means for obtaining
information to identify microstructures designated for removal
within a row or column of the array of microstructures, and to
identify alignment targets about the microstructures, each target
having a predetermined location relative to the microstructures;
means for scanning at least one measurement beam relative to an
alignment target to obtain at least one signal representative of
the alignment target location, and processing the at least one
signal to determine a target location estimate; means for
predicting a location of at least one designated microstructure
based on the estimate; means for planning and generating a
trajectory to position one or more pulsed laser beams relative to
one or more designated microstructures so that the microstructures
and pulsed laser beam waists substantially coincide at a location
along a laser beam centerline; means for generating a single laser
pulse; means for forming at least two spatially separated pulses
from the first pulse wherein the step of forming includes spatially
splitting the single pulse to selectively direct the separated
pulses along multiple directions, and wherein the step of forming
includes utilizing pre-determined spacing information, and at least
one of an alignment target estimate and a predicted location to
adjust the separation between the spatially split pulses so as to
cause a waist of each spatially split pulse to coincide with a
designated microstructure; means for irradiating at least one
microstructure with the spatially separated pulses wherein a beam
waist of each spatially separated pulse and a microstructure
substantially coincide; wherein the processing of the at least one
microstructure with the at least two spatially separated pulses
during relative motion of the at least one microstructure and the
beam waists precisely positioned relative to the at least one
microstructure, whereby throughput of the processing system is
substantially improved.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/247,541, filed on Oct. 11, 2005, which is a
divisional application of 10/107,702, filed on Mar. 27, 2002, both
entitled Methods and Systems for Precisely Relatively Positioning a
Waist of a Pulsed Laser Beam and Method for Controlling Energy
Delivered to a Target Structure, and also claims priority to U.S.
provisional application Ser. No. 60/279,644, filed Mar. 29, 2001.
The entire disclosures of these applications are hereby
incorporated by reference.
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 thermal-based laser processing
multi-material devices.
[0004] 2. Background Art
[0005] In the repair of memory integrated circuits such as DRAMs
and laser programming of high-density logic devices, the use of new
materials, such as aluminum, gold, and copper, coupled with the
small geometry of these devices, make the problem of link removal
difficult. The new materials are typically metals or highly
conductive composites having reflectivity that is well over 90% in
the visible and near infrared wavelength regions. Aluminum, for
example, reflects greater than 90% of the laser energy over the
range from the UV through to the near infrared. Gold and copper
reflects even more strongly in the near infrared, the wavelengths
of choice used by most of the lasers repairing memories in
production.
[0006] Further, economics and device performance have driven the
size for the DRAMs and logic devices to very small physical
dimensions. Not only are the devices small, but the interconnects
and links thickness have also decreased dramatically in recent
years.
[0007] Thermal laser processing of links relies on the differential
thermal expansion between the oxide above the link and the link
itself. This differential expansion results in a high pressure
build-up of the molten link contained by the oxide. The oxide over
the link is necessary to contain the link in a molten state long
enough to build-up sufficient pressure to crack the oxide and
explosively expel the link material. If the pressure is too low,
the link will not be removed cleanly. Alternative laser wavelengths
and laser control strive to increase the laser "energy window"
without damaging the substrate and material contiguous to the
link.
[0008] Descriptions of an all-copper, dual-Damascene process
technology can be found in "Benefits of Copper--Copper Technology
is Here Today in Working Devices," NOVELLUS DAMASEUS, Dec. 20,
2001; and "Preventing Cross-Contamination Caused By Copper
Diffusion and Other Sources," P. Cacouvis, MICRO, July 1999.
[0009] FIGS. 2a and 2b illustrate prior art laser processing of
multi-layer structure wherein a target structure is located in
proximity to a substrate, with a q-switched pulse 20 from a
conventional solid state laser 21 irradiating and overfilling a
target structure 23. A laser spot size is typically significantly
larger than the (target) link size which relaxes precision
positioning requirements. A laser wavelength is typically selected
based on substrate 27 (commonly Silicon) transmission so as to
allow for higher peak laser power or other system and process
variations. In certain cases, a layer 28, 25 absorption coefficient
is controlled (e.g., as a transition or protective layer) and/or a
wavelength selected wherein substrate damage is avoided.
[0010] Further information is available regarding link blowing
methods and systems, including material processing, system design,
and device design considerations, in the following representative
U.S. patents and published U.S. applications: U.S. Pat. Nos.
4,399,345; 4,532,402; 4,826,785; 4,935,801; 5,059,764; 5,208,437;
5,265,114; 5,473,624; 6,057,180; 6,172,325; 6,191,486; 6,239,406;
2002-0003130; and 2002-0005396.
[0011] Other representative publications providing background on
link processing of memory circuits or similar laser processing
applications include: "Laser Adjustment of Linear Monolithic
Circuits," Litwin and Smart, ICAELO, (1983); "Computer Simulation
of Target Link Explosion In Laser Programmable Memory," Scarfone,
Chlipala (1986); "Precision Laser Micromachining," Boogard, SPIE
Vol. 611 (1986); "Laser Processing for Application Specific
Integrated Circuits (asics)," SPIE Vol. 774, Smart (1987); "Xenon
Laser Repairs Liquid Crystal Displays," Waters, Laser and
Optronics, (1988); "Laser Beam Processing and Wafer Scale
Integration," Cohen (1988); "Optimization of Memory Redundancy Link
Processing," Sun, Harris, Swenson, Hutchens, Vol. SPIE 2636,
(1995); "Analysis of Laser Metal Cut Energy Process Window,"
Bernstein, Lee, Yang, Dahmas, IEEE Trans. On Semicond. Manufact.,
Vol 13, No. 2. (2000).
[0012] Also, the following co-pending U.S. applications and issued
patents are assigned to the assignee of the present invention and
are hereby incorporated by reference in their entirety: [0013] 1.
U.S. Pat. No. 5,300,756, entitled "Method and System for Severing
Integrated-Circuit Connection Paths by a Phase Plate Adjusted Laser
beam"; [0014] 2. U.S. Pat. No. 6,144,118, entitled "High Speed
Precision Positioning Apparatus"; [0015] 3. U.S. Pat. No.
6,181,728, entitled "Controlling Laser Polarization"; [0016] 4.
U.S. Pat. No. 5,998,759, entitled "Laser Processing"; [0017] 5.
U.S. Pat. No. 6,281,471, entitled "Energy Efficient, Laser-Based
Method and System for Processing Target Material"; [0018] 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"; [0019] 7. U.S. Ser. No.
09/572,925, 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; [0020] 8. U.S. Pat.
No. 6,300,590, entitled "Laser Processing"; and [0021] 9. U.S. Pat.
No. 6,339,604, entitled "Pulse Control in Laser Systems."
[0022] However, it is to be understood that this listing is not an
admission that any of the above references are prior art under the
Patent Statute.
[0023] 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
[0024] An object of the present invention is to provide improved
methods and systems for thermal-based laser processing
multi-material devices.
[0025] In carrying out the above object and other objects of the
present invention, a method for thermal-based laser processing a
multi-material device including a substrate and at least one
microstructure is provided. The processing occurs with multiple
pulses in a single pass operation controlled with a positioning
subsystem of a thermal processing system. The positioning subsystem
induces relative motion between the device and laser beam waists.
The processing removes the at least one microstructure without
damaging the substrate. The method includes generating a first
pulse having a first predetermined characteristic, and irradiating
the at least one microstructure with the first pulse wherein a
first beam waist associated with the first pulse and the at least
one microstructure substantially coincide. The step of irradiating
at least initiating processing of the at least one microstructure.
The method also includes generating a second pulse having a second
predetermined characteristic. The second pulse is delayed a
predetermined time relative to the first pulse. The method further
includes irradiating the at least one microstructure with the
second pulse wherein a second beam waist associated with the second
pulse and the at least one microstructure substantially coincide.
The step of irradiating the at least one microstructure with the
second pulse further processing the at least one microstructure
wherein the processing of the at least one microstructure with the
first and second pulses occurs during relative motion of the at
least one microstructure and the beam waists in a single pass
whereby throughput of the thermal processing system is
substantially improved.
[0026] The device may be a semiconductor memory including a silicon
substrate and the at least one microstructure may be a metal link
of the semiconductor memory separated from the silicon substrate by
at least one oxide layer.
[0027] At least one of the pulses may have a duration of greater
than a few picoseconds to several nanoseconds.
[0028] The pulses may be generated by a mode-locked laser system
and amplified with an optical amplifier.
[0029] At least one of the pulses may be generated by a q-switched
microlaser having a pulsewidth less than 5 nanoseconds.
[0030] The first and second pulses may be propagated along
different optical paths so that the second pulse is delayed for the
predetermined time relative to the first pulse based on a
difference in optical path length.
[0031] The pulses may have a temporal spacing less than or
approximately equal to the predetermined time. The method further
includes selecting the second pulse to irradiate the at least one
microstructure.
[0032] The predetermined time may be determined by a thermal
property of the substrate wherein substrate temperature is
substantially reduced after the predetermined time compared to the
temperature of the substrate during the step of irradiating the at
least one microstructure with the second pulse.
[0033] The substrate temperature may be substantially reduced to
approximately room temperature.
[0034] The first and second predetermined characteristics may
include a substantially square temporal pulse shape having a rise
time of less than about 2 nanoseconds and a pulse duration of about
10 nanoseconds.
[0035] The predetermined time may be in the range of about 20-50
nanoseconds, or may be in the range of about 30 nanoseconds.
[0036] Two pulses may be used to completely process the at least
one microstructure, and laser energy of each of the pulses is about
60-70% of laser energy required for laser processing the at least
one microstructure with a single pulse.
[0037] Relative position change between the pulses at the at least
one microstructure may be less than about 10% of a dimension of the
at least one microstructure to be processed.
[0038] At least one of the first and second predetermined
characteristics may include a substantially square pulse.
[0039] At least one of the predetermined characteristics may
include a non-circular spatial profile based on a selected
numerical aperture and shape of a spot and the spot and the at
least one microstructure are substantially correlated in at least
one dimension whereby percent of laser energy delivered to the at
least one microstructure is increased and irradiance of the
substrate is decreased.
[0040] A spatial beam shape of the second pulse may be in the form
of a cleaning beam having an energy density lower than energy
density of the first pulse.
[0041] The cleaning beam may have an attenuated central region and
a higher energy outer region so as to remove debris surrounding a
target site on the at least one microstructure.
[0042] The steps of generating may include directing a portion of a
laser pulse through an optical subsystem having opposing,
spaced-apart, corner cube reflectors and polarization rotators so
as to align a pulsed laser beam, and to control delay and amplitude
of the second pulse relative to the first pulse.
[0043] The steps of generating may further include providing an
optical subsystem having multiple lasers wherein delay between
trigger pulses to the optical subsystem determines the
predetermined time.
[0044] A fiber optic delay line may delay the second pulse for the
predetermined time and the predetermined time may be about several
nanoseconds.
[0045] Relative position change between the pulses at the at least
one microstructure may be either greater than about 10% of a
dimension of the at least one microstructure to be processed or
greater than about 1/2 of either of the beam waists and may further
include a high speed beam deflector operatively coupled to the
positioning subsystem to compensate for relative motion between the
pulses. The second pulse may be deflected by the deflector to also
substantially irradiate the at least one microstructure with the
second pulse.
[0046] The predetermined time may be in the range of about 10 ns to
10 .mu.s.
[0047] The beam deflector may be a single axis acousto-optic
device.
[0048] The first and second predetermined characteristics may be
based on physical properties of the multi-material device.
[0049] The first pulse may irradiate a first portion of the at
least one microstructure and the second pulse may irradiate a
second portion of the at least one microstructure, and relative
position change between the first and second portions of the at
least one microstructure may be less than 1/4 of either of the beam
waists.
[0050] The step of providing may also provide at least one optical
amplifier optically coupled to at least one of the lasers.
[0051] The at least one microstructure and the beam waists may be
relatively positioned during relative motion based upon
three-dimensional information.
[0052] The steps of generating may include generating a single
pulse and forming the first and second pulses from the single
pulse.
[0053] The step of forming may delay the second pulse for the
predetermined time relative to the first pulse.
[0054] The step of forming may include splitting the single pulse
with a multi-frequency deflector to form the first and second
pulses.
[0055] First and second microstructures may be irradiated by the
first and second pulses, respectively.
[0056] Further in carrying out the above object and other objects
of the present invention, a system for thermal-based laser
processing a multi-material device including a substrate and at
least one microstructure is provided. The processing occurs with
multiple pulses in a single pass operation controlled with a
positioning subsystem which induces relative motion between the
device and laser beam waists. The processing removes the at least
one microstructure without damaging the substrate. The system
includes means for generating a first pulse having a first
predetermined characteristic, and means for irradiating the at
least one microstructure with the first pulse wherein a first beam
waist associated with the first pulse and the at least one
microstructure substantially coincide. The first pulse at least
initiating processing of the at least one microstructure. The
system also includes means for generating a second pulse having a
second predetermined characteristic. The second pulse is delayed a
predetermined time relative to the first pulse. The system further
includes means for irradiating the at least one microstructure with
the second pulse wherein a second beam waist associated with the
second pulse and the at least one microstructure substantially
coincide. The second pulse further processing the at least one
microstructure wherein the processing of the at least one
microstructure with the first and second pulses occurs during
relative motion of the at least one microstructure and the beam
waists in a single pass whereby throughput of the system is
substantially improved.
[0057] The means for generating may include a mode-locked laser
system and may further include an optical amplifier for amplifying
the pulses.
[0058] At least one of the means for generating may include a
q-switched microlaser having a pulsewidth less than 5
nanoseconds.
[0059] The pulses may have a temporal spacing less than or
approximately equal to the predetermined time. The system may
further include means for selecting the second pulse to irradiate
the at least one microstructure.
[0060] The predetermined time may be determined by a thermal
property of the substrate wherein substrate temperature may be
substantially reduced after the predetermined time compared to the
temperature of the substrate during irradiation of the at least one
microstructure with the second pulse.
[0061] The means for generating the first and second pulses may
include an optical subsystem having opposing, spaced-apart, corner
cube reflectors and polarization rotators so as to align a pulsed
laser beam, and to control delay and amplitude of the second pulse
relative to the first pulse.
[0062] The means for generating the first and second pulses may
also include an optical subsystem having multiple lasers wherein
delay between trigger pulses to the optical subsystem determines
the predetermined time.
[0063] The means for generating the first and second pulses may
further include means for generating a single pulse and means for
forming the first and second pulses from the single pulse.
[0064] The means for forming may include a multi-frequency
deflector for splitting the single pulse to form the first and
second pulses.
[0065] Still further in carrying out the above object and other
objects of the present invention, a method for thermal-based laser
processing a multi-material device including a substrate and a
microstructure is provided. The method includes generating the at
least one laser pulse having at least one predetermined
characteristic based on a differential thermal property of
materials of the device. The method also includes irradiating the
microstructure with the at least one laser pulse wherein a first
portion of the at least one pulse increases a difference in
temperature between the substrate and the microstructure, and a
second portion of the at least one pulse further increases the
difference in temperature between the substrate and the
microstructure to process the multi-material device without
damaging the substrate.
[0066] The first and second portions may be portions of a single
pulse, or may be portions of different pulses.
[0067] The first portion of the at least one pulse may increase
temperature of the microstructure.
[0068] The first portion may be a high density leading edge portion
of the at least one pulse.
[0069] The leading edge portion may have a rise time of less than
two nanoseconds.
[0070] The rise time maybe less than one nanosecond.
[0071] The first and second portions of the at least one pulse may
be sufficient to remove the microstructure.
[0072] The microstructure may be a metal link having reflectivity,
and the leading edge portion of the at least one pulse may reduce
the reflectivity of the metal link.
[0073] The substrate may be silicon and the device may be a
semiconductor memory.
[0074] The second portion of the at least one pulse may further
increase the temperature of the microstructure.
[0075] The step of irradiating may be completed in a period between
5 and 75 nanoseconds.
[0076] The period may be between 10 and 50 nanoseconds.
[0077] Yet still further in carrying out the above object and other
objects of the present invention, a system for thermal-based laser
processing a multi-material device including a substrate and a
microstructure is provided. The system includes means for
generating the at least one laser pulse having at least one
predetermined characteristic based on a differential thermal
property of materials of the device. The system also includes means
for irradiating the microstructure with the at least one laser
pulse wherein a first portion of the at least one pulse increases a
difference in temperature between the substrate and the
microstructure, and a second portion of the at least one pulse
further increases the difference in temperature between the
substrate and the microstructure to process the multi-material
device without damaging the substrate.
[0078] The above object and other objects, features, and advantages
of the present invention are readily apparent from the following
detailed description of the best mode for carrying out the
invention when taken in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0079] FIG. 1a is a block diagram of a laser system which generates
a laser pulse in response to a trigger signal obtained from a
control system, the pulse having a temporal shape including a fast
rise and fall time, and a duration selected for the material
processing application of the present invention;
[0080] FIGS. 1b and 1c are views partially broken away illustrating
a multi-layer, multi-material device wherein a laser pulse with
pre-determined temporal and spatial characteristics irradiates the
device; FIG. 1b is a first side sectional view of a portion of the
device, showing a target structure having a rectangular
cross-section, wherein a high numerical aperture laser beam, having
a non-unity aspect ratio, is incident on the target structure
having a plurality of layers forming a stack; FIG. 1c is a second
side sectional view of a portion of the device, orthogonal to the
first, showing a rectangular target structure, wherein a high
numerical aperture laser beam, having a non-unity aspect ratio, is
incident on the target structure;
[0081] FIG. 2a is a block diagram of a prior art laser system which
shows a conventional q-switched or Gaussian pulse;
[0082] FIG. 2b is a view of a conventional multi-layer structure
having a single oxide layer between the link and substrate,
therefore being located in proximity to a substrate, with a
conventional q-switched laser pulse irradiating and substantially
overfilling the narrow dimension of the target structure;
[0083] FIG. 3 is a graph of reflection as a function of wavelength
of a multi-layer stack having 28 layers in 14 pairs, the stack
representative of a device processed with a method and system of
the present invention;
[0084] FIGS. 4a and 4b are top views and associated graphs which
illustrate the effect of irradiating the target structure with
laser beam profiles of varying dimension with respect to the target
structure; FIGS. 4a and 4b show the result of truncating a
representative non-uniform Gaussian shaped laser spatial profile,
wherein the energy enclosed by the target structure is strongly
affected, the energy at the target edge varies, and potential stray
radiation effects result from energy not absorbed by the target
structure;
[0085] FIG. 4c is a side schematic view of a plurality of
microstructures formed on a layer and which illustrate that for
decreasing spacing (pitch) inter-reflections and stray energy
result in irradiation of neighboring target structures;
[0086] FIGS. 5a and 5b are graphs which show the reduction in
irradiance on the device as a function of depth resulting from
precise position control of a high numerical aperture beam (at the
top surface), wherein the position and depth of focus of the beam
provides for processing of the target structure without creating
undesirable changes to other materials; In particular, FIG. 5a
illustrates the increase in spot area with for various spherical
and elliptical Gaussian irradiance distributions, for a
representative multi-layer stack used in a copper memory
process;
[0087] FIG. 5b normalizes the defocus function relative to the
energy density (fluence) at the target location;
[0088] FIGS. 6a and 6b are schematic views of a stack of layers
formed on a wafer substrate and which illustrate exemplary results
obtained with a ray trace simulation used to estimate the level of
radiation impinging on the internal layers and adjacent links with
a specified beam numerical aperture;
[0089] FIGS. 7a, 7b, 8 and 9 are views of images taken from
detectors and which illustrate, on a continuous scale spanning 5
decades, simulated patterns of radiation at the surface, substrate,
and with the stack removed respectively;
[0090] FIG. 10 is a schematic diagram of a system for measuring
fiducials or other alignment targets;
[0091] FIG. 11 is a graph of reflectivity versus outer layer
thickness;
[0092] FIG. 12 shows a pair of graphs of reflectivity versus
thickness of the outer oxide layer for two different laser beam
wavelengths;
[0093] FIG. 13 is a schematic diagram of a system for automatically
controlling pulse energy based on a thickness measurement;
[0094] FIG. 14a shows schematic and graphical representations of an
effect of debris on signal fidelity during alignment
measurements;
[0095] FIG. 14b shows similar representations with improved signal
fidelity after cleaning with a pulsed laser beam;
[0096] FIGS. 15a-15c show various arrangements for combining laser
pulses or generating a sequence of closely spaced pulses using
optical or electronic delay methods; FIG. 15a illustrates use of
multiple lasers with delayed triggering; FIG. 15b illustrates a
basic arrangement with a single laser and an optical delay path;
and FIG. 15c illustrates yet another modular optical delay line
providing for pointing stability and simplified alignment;
[0097] FIG. 16 is a graph of temperature versus time which
illustrates simulation results for metal link (top) and substrate
(bottom) irradiance with a pair of delayed pulses wherein the
substrate temperature decays rapidly exhibiting a differential
thermal property of the materials; the two laser pulses each had a
square temporal shape;
[0098] FIG. 17 is a series of schematic views of a metal link which
illustrate a multiple pulse sequence wherein: (1) a first pulse
irradiates the metal link; (2) debris is left after removing the
link; (3) a second pulse with a spatial pulse shape is used wherein
the central zone is attenuated, the second pulse having a lower
peak energy density than the first pulse; and (4) 25 ns after the
start of the first pulse the debris is removed;
[0099] FIG. 18 is a block diagram of a system which generates and
controllably selects pulses;
[0100] FIG. 19 is a block diagram of a system of the present
invention wherein a portion of a high repetition rate pulse train
(e.g., 1 MHz) is selected and a high speed beam deflector (e.g.,
electro-optic or acousto-optic device) synchronized with
microstructure positions is used to process a single microstructure
with multiple pulses during relative motion; and
[0101] FIG. 20 is a block diagram of another system of the present
invention wherein a beam deflector is used to spatially split a
single pulse so as to irradiate either one or two microstructures
(or none) with a pair of pulses during relative motion.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0102] One aspect of the invention is removal of a microscopic
target structure which is part of a multilayer, multimaterial
device, wherein laser energy is incident on several materials
having dissimilar optical and thermal properties. One application
is memory repair. A new fabrication process (Damascene) includes a
copper target structure, multiple dielectric layers in the form of
a "stack," and functional circuitry disposed at the dielectric
layers. The target structure and layers are typically formed on a
silicon substrate. This is illustrated in FIGS. 1b and 1c and
corresponds to a device processed with an embodiment of the present
invention. This will be referred to as a "multilevel" process.
[0103] With the use of more complex structures at finer scale
(e.g., at or below a wavelength of visible light), considerations
for reliable operation of laser processing system increase to meet
the standards for high yield in the semiconductor industry.
[0104] Aspects of the invention include methods and subsystems for
operation of the laser processing system. At the microscopic scale,
the laser beam waist diverges rapidly due to the small spot size
and depth of focus. The materials within the 3D beam location may
include functional circuitry. In an automatic system, robust
measurement of target locations is used in conjunction with
database information to position a laser beam in three dimensions
at high speed. The interaction of a laser beam within the
multilevel device influences yield. Modeling of thermal interaction
is useful of understanding and predicting performance in the
thermal processing regime. However, at the microscopic scale, a
more detailed understanding of interaction based on physical optics
is also beneficial.
[0105] In the following sections, detailed aspects of spatial and
temporal pulse shaping, three-dimensional measurement and
prediction, device modeling and process design are disclosed with
emphasis on solving the problem of cleanly removing links on a
multilevel device, wherein damage is avoided to inner layers and
functional circuitry between a link and the substrate. However,
various methods, subsystems, and experimental results may also be
applied for link processing of conventional single inner layer
devices, and generally for processing microstructures surrounded by
materials having dissimilar thermal or optical properties.
Processing Links on a Multilevel Device
[0106] 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 highly reflective copper links which are part of a high
speed semiconductor memory device. The method and system of the
present invention is particularly advantageous for processing of
targets having a sub-micron dimension, including targets with a
dimension below the wavelength of the laser beam. The target is
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.
[0107] FIGS. 1a-1c generally show an embodiment of the present
invention. A laser pulse 3 irradiates a rectangular target
structure or microstructure 10, side views of which are shown in
FIGS. 1b and 1c, with a focused beam. In a preferred embodiment, an
output from short pulse amplified laser system 1 is generated to
produce the pulse 3 which has a rise time 4 fast enough to
efficiently couple energy into a highly reflective target
structure. The duration 5 is sufficient to process the target
structure wherein at least a portion of the structure is cleanly
removed without leaving residue, slag, or other debris. The fall
time 6 is preferably fast enough to avoid creating undesirable
damage to the layers or substrate.
[0108] The temporal pulse shape is selected, in part, based on
physical properties of the target microstructure 10, for instance,
thickness, optical absorption, thermal conductivity, or a
combination thereof. In an advantageous embodiment of the
invention, the processing will occur with a single pulse having a
fast edge leading relative to a selected pulse duration of several
nanoseconds. In an alternative embodiment, the laser output may be
a series of narrow q-switched 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.
[0109] 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).
[0110] 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.
[0111] 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 .mu.m
wavelengths was developed by University of Southhampton and
described in "A Diode Pumped, High Gain, PlanarWaveguide,
Nd:Y3A15O12 Amplifier."
[0112] 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.
[0113] 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 FIGS. 15a-15c, a series of devices, with appropriate delays
introduced by a plurality of triggering signals, or delays of a
trigger signal with a delay line, is 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
122 may be used to increase the output power as needed.
[0114] FIG. 15a shows a schematic of one basic embodiment with bulk
optics, where a beam combiner 123 is used to deliver the output of
two lasers 120,121 to an amplifier 122. A delay circuit 126, which
may be programmable, controls triggering. Polarization optics 127,
128 are used to provide the proper input to the beam combiner. In
one arrangement the pulses are spaced apart and appear as a high
frequency burst 124. In a second arrangement triggering of the
second pulse occurs at a slightly delayed (but controlled) position
which produces a characteristic approximating a square pulse shape
125. In the latter arrangement the controlled delay is about 50% of
the FWHM. Those skilled in the art will recognize that alternative
arrangements may be used with multiple amplifiers, combiners, with
bulk, fiber, or integrated optic arrangements.
[0115] Generation of multiple pulse waveforms may also include some
form of active q-switching of two separate microlasers or detecting
a first pulse from a passively q-switched laser and subsequently
triggering an actively q-switched laser or MOPA relative to the
first pulse.
[0116] FIG. 15-b is a basic schematic showing the use of a single
laser 140 wherein the laser output is divided by beam splitter 142,
whereby a portion of the beam propagates along a path 141, followed
by combining with combiner 143, after polarization adjustment with
rotator 146 which may be a half-wave plate. An optional optical
amplifier 145 may then be used to produce higher output power.
[0117] In an arrangement using a single laser and an optical delay
line, the optical system will preferably be stable and easy to
align. FIG. 15c shows an exemplary embodiment wherein the use of
opposing corner cube retroreflectors 130 makes the setup
insensitive to tilt of the folding elements. The angular alignment
of the delayed beam paths 131,132 is very stable even in a high
vibration environment. One of the corner cubes in each pair of
retroreflectors 130 is initially adjusted in the X/Y translation
and Z rotation to get the transverse position of the delayed beam
path centered. Each of the .lamda./2 retarders 133 in the main beam
path is adjusted so that vertical or horizontally polarized light
will have its polarization rotated by 45 degrees. The .lamda./2
retarder 133 in the second delay loop is adjusted so that vertical
or horizontal polarized light will have the polarization rotated by
90 degrees causing the delayed pulse in the second loop to
circulate twice before exiting. The peak-to-peak spacing of the
output waveform 135 (e.g., 4 combined pulses) is controlled by the
length of the delay loops. If non-equal amplitudes for the delayed
pulses are desired, the .lamda./2 retarders 133 in the main beam
can be set for a polarization of other than 45 degrees. Likewise,
the pulse shape can be varied at the time a system is setup or
possibly in operation by manually or automatically controlling the
spacing. Those skilled in the art of laser pulse generation and
shaping will appreciate the advantages of the compact and modular
arrangement for short pulse for typical delays ranging from a few
nanoseconds to tens of nanoseconds. For instance, U.S. Pat. No.
5,293,389 to Hitachi describes a polarization-based fiber delay
line for generating laser pulses of decreasing amplitude for
generating longer pulses, for instance 100 ns or longer.
[0118] Another means of producing a shaped pulse is to use the
modulator approach to chop the leading edge or tail of the pulse
but with a two-stage or shaped modulation voltage pulse. For
example: with a 10 ns q-switched pulse, the modulator could have
100% transmission for the first 1-5 ns followed by 25% transmission
for the remainder of the pulse. Early pioneering work by Koechner
(U.S. Pat. No. 3,747,019) and Smart (U.S. Pat. No. 4,483,005)
demonstrate exemplary amplitude and pulse shape control methods
using electro-optic modulators.
[0119] The multiple pulses shown in FIGS. 15a-15c may or may not
have the same wavelength, and the temporal shape of a pulse may be
varied depending upon specific requirements. For example, in
certain embodiments an output may be a q-switched pulse of short
duration and high peak power combined with a lower power square
pulse shape.
[0120] Referring to FIGS. 1a and 1b, 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 7,8,9 to
substantially coincide with the target 10 three-dimensional
coordinates (Xlink,Ylink,Zlink). A trigger pulse 2, generated at a
time where the laser beam waist and target position substantially
coincide, operates in conjunction with the laser and associated
control circuitry in laser subsystem 1 to produce an output
pulse.
[0121] References 2 and 7 describe details of a method and system
for precision positioning, including three-dimensional beam waist
positioning. Reference 7 describes a preferred 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 preferred 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).
[0122] 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
exceeds 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.
[0123] 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.
[0124] 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."
[0125] In an application of the invention, the target structure 10
is provided as a part of a multi-material, multi-layer structure
(e.g., redundant memory device). The multi-layer stack having
dielectric layers 14,15 provides spacing between the link and an
underlying substrate 17. In one type of multi-layer memory device,
alternating layers of Silicon Dioxide 15 and Silicon Nitride 14 may
be disposed between a copper link target structure 10 and a Silicon
substrate 17. The copper 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 copper
link structure, underlying conductors 16 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 14 and
thicker (.about.1 .mu.m typical) Silicon Dioxide 15 materials.
[0126] 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 12,11 as also illustrated in FIG. 4b (e.g., 3:1).
Alternatively, rectangular or another chosen spatial profiles may
be implemented in a lateral dimension. For example, Reference 1
discloses various advantageous methods and optical systems for
"non-Gaussian" spatially shaping of laser beams for application to
memory repair.
[0127] With the nearly diffraction limited elliptical Gaussian
case, the preferable minimum beam waist dimension at location 11
approximates the narrow target 10 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.
[0128] A typical copper 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 11 will typically overfill the
sub-micron link to some degree, whereas aspect ratio Wxo/Wyo 12,11
with Wxo a few microns along the link, can facilitate clean link
removal. Additionally, rapidly decreasing energy density on the
layers 14,15 and substrate 17 is achieved through defocus of the
high numerical aperture beam portion 11.
[0129] The graphs of FIGS. 5a and 5b illustrate the estimated
defocus for various aspect ratios, relative to a circular Gaussian
and an elliptical beam at best focus. FIG. 5a shows the very rapid
falloff of a 1.6 .mu.m circular Gaussian (0.002 mm numerical
divisions=2 .mu.m). FIG. 5b shows a normalized result to scale the
energy density at best focus for the different spot shapes. These
results indicate that with precision beam positioning in depth,
wherein the power density is maximized at the target site, at
relative reduction in energy density of more than one decade occurs
at the substrate level for an exemplary multi-layer stack used in a
copper based process for memory fabrication. Further, the rapid
defocus relative to the waist Wy0 is beneficial for avoiding inner
layer damage, provided the "tails" of the incident beam irradiate
functional inner layer 16 (e.g., copper) at a low level.
[0130] In one embodiment for processing a multilevel device, copper
link removal is initiated with application of the fast rise time
pulse, having a nominal 10-90% rise time 4 in a preferred range of
less than 1 nanosecond to about 2 nanoseconds. A pulse duration 5
in the range of about 2 nanoseconds to 10 nanoseconds is preferable
to sever the link while limiting thermal diffusion. Pulse energies
in the range of about 0.1 microjoules (.mu.j) to 3 .mu.j were
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.
During the pulse duration, thermal shock of top layer 13 and
thermal expansion of the target 10 result in explosion of the link
through ruptured top oxide layers 13, which in turn reduces the
stress at the lower corner of the link structure adjacent to the
layer 14. The laser pulse is rapidly terminated, preferably within
a few nanosecond fall time 6 after the explosion, at a time just
after the thin link is cleanly severed, and prior to a time the
lower corner of the link results in cracking of at least layer 14.
Further details and results related to the interaction of a laser
pulse with a metal link and overlying layers is disclosed in
references 4 and 5. The '471 patent and the associated
specification describe the interaction process (e.g., FIGS. 1a, 1b,
11a, 11b, and in column 18).
[0131] Hence, a combination of the spatial characteristics (e.g.,
beam waist shape and position) and the temporal (e.g., rise time 4,
flatness, and duration 5) pulse characteristics avoids undesirable
cracking of lower layers 14,15, avoids significant pulse
interaction with inner layer conductor 16, and limits substrate 17
heating. Hence, despite the high reflectivity of the copper link at
visible and near infrared wavelengths, and the expectation in the
prior art of incomplete removal and damage to surrounding
structures and substrate, the target structure is processed without
undesirable damage to other structures. It is also known that
copper, in addition to having nearly maximum reflectance in the
near IR, is also more reflective than other link materials (e.g.,
aluminum, platinum). Nevertheless, due to the optical interaction
of the near IR beam with the target and the optical and thermal
properties of adjacent (overlying) layers, the preferred copper
material can be processed.
[0132] Furthermore, near IR (Infrared) wavelengths also
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.
[0133] In a preferred system for copper 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
typically correspond to a wavelength of visible light. Further, at
the microscopic scale of operation, where the lateral and/or
thickness dimensions of the materials of FIGS. 1b and 1c 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.
[0134] 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. The device structure
of FIGS. 1a and 1b can be damaged with substantial absorption
within the lower layers, such damage is prohibitive because of the
presence of adjacent circuitry. This is in contrast to link
processing with the prior art system of FIG. 2b where inner layer
damage is not generally detrimental to overall device
performance.
[0135] 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 um for processing links on memory devices wherein the
substrate is Silicon, namely smaller spot size and shorter laser
pulsewidths. 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.
[0136] 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 um) 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 or 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.
[0137] 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.
[0138] 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.
[0139] FIG. 3 is a graph which illustrates the estimated back
reflection produced by a representative multi-layer stack of 14
Silicon Dioxide 15 and Silicon Nitride 14 pairs over a range of
near IR wavelengths, where the thickness of the layers is about 1
.mu.m and 0.07 microns, respectively. In accordance with the
present invention, a large number of layers can be accommodated,
and may range from about 4-28 dependent upon the process (e.g.,
sometimes multiple layers may separate a functional conductor
layer).
[0140] By way of example, it is shown that significant reflection
occurs over relatively broad wavelength range. A single layer
disposed as an internal layer 14 will typically reflect roughly 2%
at each surface over the visible and near IR spectrum. It is well
known in the art of link and semiconductor processing that Silicon
absorption varies by orders of magnitude in the near IR spectral
range. Further, studies of Silicon material processing have shown
that the absorption is unstable and non-linear with increased laser
power and substrate heating at wavelengths near the absorption
edge, as taught in reference 4. However, as stated above, the
shorter wavelengths are preferred to produce smaller spots
(references 4-6, and 8) and higher energy concentration at the link
position.
[0141] In accordance with the present invention, exploiting the
layer reflection with wavelength can further enhance the system
performance and supplement the benefits associated with temporal
and spatial control of the pulse in a preferred short wavelength
range. Such wavelength selection is regarded as particularly
advantageous at wavelengths where the substrate absorption would
otherwise greatly increase, and significant margin can be obtained
when the number of layers 14,15 disposed between the link and
substrate substantially exceeds the number of overlying layers 13.
A preferred structure for processing will comprise a substantial
number of layers, with large reflectance at a predetermined short
wavelength, the wavelength being well matched for generation of the
preferred fast square temporal pulse shape.
[0142] Standard laser wavelengths in the range of FIG. 3 include
1.047 .mu.m and 1.064 .mu.m, the latter being a standard wavelength
of semiconductor diodes. Further, custom wavelengths include 1.08
.mu.m, and other wavelengths generated with Raman shifting. Those
skilled in the art will recognize that frequency multiplication of
the near IR wavelengths can be used to generate short wavelengths,
and with appropriate design multiple wavelengths may be provided in
a single system. For instance a preferred temporal pulse shape,
with a fast rise time, may be generated in the green portion of the
visible spectrum by frequency doubling a near IR laser.
[0143] In an alternative embodiment, wavelength tuning is used to
match the wavelength to the approximate peak reflectance of the
stack. Such an arrangement may be particularly advantageous for
adjustment of a laser wavelength at the edge of the reflectance
range (i.e., "cutoff" range) over a limited wavelength range,
whereby sensitivity to tolerances in the material thickness and
index of refraction are avoided. As noted above, further discussion
of laser amplifier systems and application to other link structures
can be found in references 4-6.
[0144] Generation of the pulsed laser beam may include the step of
shifting the wavelength of the laser beam from a first wavelength
to a predetermined wavelength. The predetermined wavelength may be
based on material characteristics comprising at least one of: (1)
coupling characteristics of the microstructure, (2) multi-layer
interference, and (3) substrate reflectivity.
[0145] Experimental results have shown that at a wavelength of
1.047 .mu.m, where the absorption of Silicon is orders of magnitude
higher than at 1.2 .mu.m, substrate damage is avoided with a short
q-switched (standard) pulse and the stack characteristic of FIG. 3.
However, the results with a standard laser having a q-switched
temporal pulse shape showed cracking of an oxide layer 14 below the
link. The relatively slow rising q-switched pulse shape, which for
a Gaussian approximation is a substantial fraction of the duration,
was considered a limiting factor for link removal without cracking
of the inner layer based on experimental results. However, based on
the teachings of the prior art, severe damage to the Silicon
substrate would be expected at the 1.047 .mu.m wavelength because
the absorption is orders of magnitude higher than at a wavelength
corresponding to maximum transmission. In accordance with the
teachings of the present invention, the spatial pulse
characteristics and the stack reflection are important factors to
consider so as to avoid inner layer and substrate damage and short
wavelengths of operation (which also provide for a smaller spot
size and higher energy concentration at the link). Further, in
accordance with the present invention, a predetermined square pulse
shape generated at a laser wavelength of 1.047 .mu.m would be
expected to produce clean removal without undesirable changes to
the stack and substrate.
Laser Processing and Process Design at the Sub-Micron Scale
[0146] Furthermore, in an exemplary advantageous embodiment for
short wavelength processing of reflective microscopic structures, a
specification for a multi-layer stack may be considered in process
design. For example, a quarter-wave stack of alternating
dielectrics or other suitable arrangement having a large difference
in the index of refraction, and high transmission within each
layer, is specified at a selected wavelength. It can be shown that
very high reflectance is achievable, the quarter-wave stack being
easily computed in closed form and modeled. Hence, the method and
system of the present invention can be used effectively with other
aspects of process design, and may be advantageous where the
absorption of deeply buried layers and the substrate is relatively
high, or where the width of a target structure is well below the
laser wavelength.
[0147] The design of the device structure may have certain
constraints related to the layout of the circuitry. As such,
certain thickness and material for a certain layer may be defined,
for instance an insulator in a plane of a conductor having the
approximate thickness of the conductor, or related to the thickness
of the conductor. It may be possible to select a material having a
different index of refraction than the specified layer. A specified
thickness may be based on the estimated reflection at an
advantageous laser wavelength which may reduce or eliminate a
requirement for special laser equipment operating at "exotic"
wavelengths where the lasers are difficult to manufacture with high
yield. The reflection may be estimated using a model wherein the
thickness is a variable, and an estimate made to maximize the
reflection, subject to other device constraints.
[0148] Thickness of the layers can be tuned to a wavelength in as
much as the wavelength (or angle) can be tuned to the layers. Index
of refraction could be used to fine-tune over a limited range, but
the range may not be significant for small changes in index. Even
with all thicknesses fixed by the process, the addition of a
variable thickness tuning layer or layers with predetermined
thickness could be used to significantly affect reflectivity of the
whole stack. For example, a layer not constrained by metallization
requirements could be used as a precision spacer between an upper
and a lower stack portion. This could be a very powerful tool for
tuning the process with adjustment of perhaps only one layer.
Physical Optics and Laser Processing of Multi-Level Devices
[0149] Other controllable laser characteristics may be exploited,
alternatively or in conjunction with wavelength selection, to
provide further improvements in the processing energy window.
Reference 3 describes an advantageous method and system for
polarization control, including dynamic polarization selection and
computer control so as to align the polarization with a link
orientation (e.g., details shown in FIG. 4 and the associated
description in the reference). The polarization can be selected on
the basis of the target coupling characteristics, the film
reflectance, or a combination thereof.
[0150] With a link dimension below the spot size, effects like
diffraction, scattering, and edge reflection should be considered
as physical phenomena which can have either advantageous or
detrimental results depending upon the device geometry and beam
characteristics. Likewise, at high energy density, non-linear
absorption may affect results, with particular concern of
semiconductor material damage.
[0151] An additional important consideration with fine pitch
(spacing) of adjacent links and circuitry is collateral damage.
Furthermore, functional circuitry in a plane of the layers must not
be damaged. With an increasing trend toward fine pitch and high
density memory, the three-dimensional structure of the device
should be considered and may affect a choice of beam spatial and
temporal characteristics. By way of example, FIGS. 4a-4c illustrate
effects of reflection and diffraction associated with sub-micron
width link 10 resulting in truncated 43,44 Gaussian beams 11, where
the spot size (as measured at the 13.5% point) is wider than the
link by varying degrees. The sketches are representative of a
diffraction limited beam waist at a near IR wavelengths. The
central lobe is clipped by the link, which appears as a near field
obscuration, resulting in transmitted beam portions which are
truncated 43,44. Energy which is not incident on the link may
propagate at wide angles into the layers 49 which may be
advantageous from the standpoint of avoiding damage to the
substrate 17 as shown in FIG. 1. In any case, there will be some
correlation of neighbor irradiance with spot size. Large spots with
a relatively large depth of focus have reduced divergence and
neighbor irradiance can be small, provided that the link spacing is
large enough that the non-absorbed energy 43 of the incident beam
impinging on an adjacent structure is weak, for instance
corresponding to level 44. With a higher N.A. and smaller spot
size, the reflected beam diameter at the link location 46 is
increased. There will be a maximum value for some spot size 41,42.
Then irradiance at a neighboring link 48 decreases as the reflected
energy grows larger in area.
[0152] Simultaneously there is an angular variation in internal
reflection. Hence, the stack layer thickness can also effect the
irradiance of adjacent structures, including the internal
structures 16 of FIG. 1. Furthermore, polarization variations with
angle are expected to produce variations. FIGS. 6a and 6b
illustrate by way of example geometric ray tracing effects of
internal reflections propagating over an extended area.
[0153] Similarly, as shown in FIG. 4c, if a portion 45 of the laser
beam incident on the edge of the link 46 is considered, the energy
which is not coupled into the link structure may also be scattered
and/or specularly reflected to the adjacent links 48. The
inter-reflection 47 occurs as a consequence of at least the link 46
physical edge profiles which may be slightly curved or sloped.
[0154] An additional consideration is the three-dimensional spacing
between an inner conductor layer 16 of FIG. 1, the beam waist 11,
and the adjacent links 48 of FIG. 4c. A large numerical aperture
beam waist 11, producing the smallest spot size at the link, while
diverging and reflecting in a manner so as to avoid significant
interaction with the inner layer 16 is preferred. Examination of
FIGS. 4a-4c suggest a reduced spot size with controlled precision
3D waist positioning is expected to reduce collateral damage by
maximizing energy coupled into the link. With high enclosed energy
within the link and a low intensity transmitted profile 44, edge
reflection is minimized. The spatial profile should also be
selected subject to the constraint of only low level, negligible
interaction between the beam angular distribution at 16.
[0155] It is preferred that the interaction mechanisms associated
with a portion of the three-dimensional device structure be modeled
for selection of at least a spatial pulse characteristic, such a
characteristic may be the N.A. and position of the beam waist.
Preferably, the model will include an estimate of the irradiance
seen by each adjacent link structures 48, internal layer 16, and
substrate 17. Whereas damage to adjacent link structures may be
relatively apparent with conventional microscopy, assessment of
inner layer 16 and substrate 17 damage may be considerably more
difficult with the 3-D device structure.
[0156] With link widths below 1 .mu.m, and pitch of a few microns,
precise, sub-micron alignment is required to compensate for
variations between wafers, and local variations within a wafer, and
system tolerances (e.g., 300 mm wafer with 25 .mu.m of
topographical variation, and 5 .mu.m of manufacturing tolerances,
for instance). In accordance with the present invention, a
precision positioning method and system is used to relatively
position the beam waist so as to provide high laser energy
concentration at the link. Also, one important consideration for
precision positioning is predicting accurate (Xlink, Ylink)
location information. The prediction is subsequently used by a
motion control and positioning system to generate a laser output
via trigger 2 at the target coordinates, during relative motion of
the target 10 and laser beam. A preferred embodiment includes a
polarization insensitive scanning and detection system as described
hereinbelow, wherein a region containing an alignment target
location is imaged to obtain reference data. The target location is
often covered by a dielectric layer of Silicon Dioxide, Silicon
Nitride, or other insulating material. Experiments have indicated
that polarization insensitive detection is advantageous to avoid
spurious measurements. The results led to a hypothesis that
birefringence is introduced in the insulating layers by polishing
or other process operations, which is manifested by polarization
variations in the reflected beam. These variations reduce the
signal-to-noise ratio and appear to induce position distortion. The
digital output data from each target location is used by an
8-parameter least squares alignment algorithm to estimate and
correct position information affected by offset, angle, scale,
orthogonality, and trapezoidal variations over the wafer containing
the links to be processed.
[0157] Given the variations in the received beam at the target
location, concerns arise that process variations may affect layer
optical properties near the target structure. Furthermore, in
practice, variations occur in the thickness and reflectivity of the
target and layers, either over a water to be processed or from
batch-to-batch. Measurement of the thickness and reflectivity is
useful for process monitoring, and can also be used to determine
adjustments for the laser power and wavelength to increase the
energy window. For instance, any variation in the reflectivity of
the link can affect the energy required for processing. A preferred
method and system for adaptive energy control is also described
hereinbelow.
[0158] As dimensions of links and other microscopic structures
continue to rapidly shrink, those skilled in the art will
appreciate the benefits of multi-parameter modeling. A model-based
approach leads to selection and precision control of the spatial
and temporal characteristics of the laser output, resulting in
controlled three-dimensional interaction of the laser with complex
multi-layer, multi-material structures.
Polarization Insensitive Detection and X,Y Reference
Measurements
[0159] Commercial laser systems of the assignee of the present
invention use a beamsplitter to pick off a portion of the reflected
light from the work surface (e.g., a multi-layer memory device) as
the laser is relatively positioned 152 over the alignment targets
(e.g., fiducials). A block diagram of the subsystem is shown in
FIG. 10. The reflectance/transmission (R/T) split of the
beamsplitter 150 depends on the laser that is being used. In cases
where the laser has low total energy and as much transmission as
needed is necessary, the split of 90% transmission and 10%
reflectance is made. This gives 90% going to the work surface on
the way in and the 10% reflected is dumped. But this only picks off
10% of the reflected light, 90% of the reflected light is
transmitted back down the laser path. When possible, the split
70/30 is made. This gives less total energy to the work surface but
gives higher reflected signal.
[0160] Regardless of the R/T split, the specification is the R/T
for S polarization=R/T for P polarization (within 5%). This is
accomplished with a special dichroic coating, which produced good
results. Because any polarization state can be thought of as a
vector sum of S and P, the beamsplitter works at the correct R/T
ratio for any polarization.
[0161] This is important because switching polarization to any
desired state is done in the preferred link processing system to
improve link cutting efficiency. For example, co-pending U.S.
application Ser. No. 10/013,956, filed 13 Dec. 2001, a continuing
application of U.S. Pat. No. 6,181,728 (Reference 3) and assigned
to the assignee of the present invention reports results wherein a
process window improvement occurs with polarization perpendicular
to the link, particularly as the spot size is reduced. The
preferred polarization controller disclosed in the '728 patent is
used to switch states.
[0162] The method and system of the present invention are
advantageous when there are oxide layers over the targets to be
scanned and measured. The oxide layer may affect the polarization
of the beam. This may happen because the oxide layer is stressed
and creates birefringence. With the polarization insensitive
arrangement this is not a problem, no matter how the polarization
is changed one gets the same reflectance from the beamsplitter and
the same signal level. If a more typical polarizing beamsplitter or
simpler coating is used for the beamsplitter, the changed
polarization will result in a change in the reflected signal. If
the stress in the oxide layer varies, especially where it is over
the target microstructure (it may be stressed because it is going
over a target edge) then the polarization may vary as the beam
scans the target. Again, this is not a problem because of the
coating. In the polarizing beamsplitter case the reflected signal
151 measured at the detector would vary because the polarization is
varying at the same time that one is trying to gather edge data,
skewing the results uncontrollably and unpredictably.
[0163] This polarization insensitive technique is regarded as the
most robust method and is preferred for measuring targets covered
by at least one oxide layer. However, other imaging and edge
location methods may be used, but may require more complex
measurement algorithms to accurately measure the targets in the
presence of multiplicative image noise.
Measurement with Anomalous Reflectivity Variations--
Cleaning with a Pulsed Laser Beam
[0164] A typical alignment target 100 is depicted in the schematic
drawings of FIGS. 14a and 14b. The target 100 is typically covered
with one or more passivation layers, these may correspond to the
layer 13 in FIGS. 1b and 1c, but are not so restricted. During
experiments with link removal on a multi-level, the preferred
polarization insensitive measurement method was used to obtain X,Y
target locations. However, it was discovered that debris 1001
within the target area 100, possibly from residual solder flux from
nearby solder deposits (solder balls), significantly affected the
reflected signals obtained with a detector resulting in noisy
profiles 101. The impact on the measurement was manifested as a
large residual in the least squares fit algorithm used to estimate
location. In this illustration, the target area is shown as a
positive contrast (e.g., higher measured intensity) region, but
those skilled in the art will recognize that contrast reversal is
acceptable provided that the contrast between the target 100 and
the background is adequate for measurement.
[0165] A pulsed beam with lower peak power was used to remove the
debris. An enhanced exemplary signal profile 102 (e.g., associated
with relatively uniform intensity and a mostly debris-free region)
was obtained as a result of the cleaning operation, as shown in
FIG. 14b. Representative energies for cleaning were on the order of
0.01 .mu.j, for instance, 0.005 .mu.j. This is well below the
damage threshold of the materials, and well below the typical
energies used for removal of links 12.
[0166] In one embodiment, a single linear scan or a plurality of
linear scans 104 across the target 100 are used to obtain reflected
intensity data which is analyzed statistically to measure fidelity,
for instance by determining the % intensity variation or standard
deviation. In an exemplary embodiment, data is taken along the
line(s) 104 at about every 0.001''. However, the sample space may
be finer or coarser depending upon the signal fidelity obtained. If
the spacing is too fine, additional "texture noise" may be
introduced. If too coarse, an edge contrast 107 will be reduced or
errors introduced by undersampling. If the variation is excessive,
a cleaning operation is initiated with a pulsed beam. Preferably,
the laser power is controlled with an acousto-optic modulator
(i.e., "energy control" of FIG. 13) which is a standard part of the
laser processing equipment. The operation of the modulator for
intensity control and pulse selection within a link blowing system
is described in more detail in U.S. Pat. No. 5,998,759 (e.g.,
Reference 4, col. 7, and the associated drawings). Those skilled in
the art will recognize that such modulators provide for intensity
control over a wide dynamic range, e.g., 100:1. A relatively simple
user interface can provide for operator interaction to initiate
operation, based on "pass/fail" or other criteria.
[0167] In another embodiment, the linear scan(s) may be done
automatically and the cleaning operation performed at each
measurement location.
[0168] In a preferred arrangement, only an adjustment of the energy
will be needed, and other system parameters unaltered during the
cleaning operation or as a result of cleaning. Those skilled in the
art of measurement will be able to make various adjustments in the
system parameters based on correlation of the results with other
process parameters.
[0169] In a preferred arrangement, the cleaning operation will be
applied only to scanned regions as needed. In one arrangement, the
process is iterative with a measurement goal of obtaining suitable
residuals in the least squares fit algorithm. If the residuals are
above a designated value, scans of at least one region are obtained
and cleaning occurs. In some cases, it may be desirable to adjust
the positions of the scan lines (e.g., if cleaning is difficult). A
fidelity measurement (e.g., contrast, standard deviation) may be
used to guide the cleaning operation. Preferably, no more than one
pass will be required.
[0170] It is to be understood that numerous arrangements could be
used to practice the cleaning invention. For instance, an array
camera could be used with different wavelength illumination to
identify regions of non-uniform intensity. These regions could then
be designated for cleaning. Those skilled in the art of optical
measurement will be able to implement such arrangements, and such
arrangements are within the scope of the present invention.
Reflectivity Measurement and Power
Adjustment--Case 1: Single Wavelength
[0171] The above discussion related to a preferred measurement
method and system for locating and measuring X,Y reference
locations. An additional option to further improve the process
energy window is measurement and control concept to adjust the
laser energy and power as required by the material to be processed.
If the reflectivity is high, then the energy is to be increased to
compensate for these reflection losses. If the reflection is low,
then the energy and power is to be decreased since more energy is
being coupled into the workpiece or target microstructure. There
are a number of ways that one can adjust this power and energy. The
simplest is to measure the reflectance from the surface and adjust
the energy and power control for optimum energy coupling.
[0172] Light interference between metal and oxide layers can
greatly affect the reflection and hence the absorption in the metal
links (see FIGS. 11 and 12). Even though the process engineer tries
to optimize the absorption in the link by designing the best oxide
thickness, the necessary thickness tolerance is difficult to
control. Typically, the thickness of a layer may vary by 10% and
there may be several layers of oxide between the top layer and the
metal layer to be processed.
[0173] If the thickness and index refraction over the link could be
determined, then the energy required to process the link could be
calculated and adjusted accordingly. There are two methods of
determining the optical constants of a film. These are ellipsometry
and spectral analysis. Ellipsometry uses the change in polarization
as a light beam either transmits or reflects from a surface. The
amount of change in polarization determines the index of refraction
of the material and thickness of the material that the light beam
traverses. The spectrometric method measures the reflection from a
surface at different wavelengths to determine the same optical
constants. In commercial versions of the spectrometer, the
reflected light is sensed at 256 different wavelengths and
calculations made on thickness, index of refraction and extinction
coefficient (absorptivity) of the layers to very high accuracy.
[0174] Another method is to measure the reflectance at two
different wavelengths and calculate the thickness of the oxide. If
the index of refraction of the oxide used for the device could be
measured, then the reflectance and hence the fraction of the laser
radiation absorbed over the link can be calculated. Knowing this
absorption, the optimum laser energy to remove the link can be
programmed into the laser system. This second method is more
accurate for thin film trimming where the material to be trimmed is
thin and some of the energy is transmitted through the film.
[0175] The implementation of the thickness measurement and energy
control is as shown in FIG. 13. The laser 160 used to remove the
link provides one of the laser wavelengths for the thickness
measurement. The energy delivered to the part is controlled by an
acousto-optical modulator (i.e., "energy control") 161 as shown in
FIG. 13 and is reduced to a level to measure the reflectance
without damaging the part. The other wavelength to measure the
reflectivity can be provided by a red laser diode (i.e., 670 nm
diode) 162 added into the optical path as shown. Beamsplitters
166,167 (e.g., dichroic mirrors) are generally used to transmit the
two wavelengths to the device surface and to direct the reflected
beams to the photodiode detectors 164,165. The reflectance can be
monitored by the two photodiodes 164,165 as shown in FIG. 13. From
the reflectance intensity of the two photodiodes (i.e., the 670 nm
diode and 1047 nm detectors) and knowing the index of refraction of
the oxide layers, the thickness of the oxide is uniquely
determined. Once the thickness and the index of refraction is
known, then the absorption in the link material can be calculated
and the optimum energy programmed into the acousto-optic energy
control device by the computer.
[0176] For the highest accuracy, the size of the spot and the link
dimensions can be used in the calculation. Referring to FIGS. 4a
and 4b, one sees that there is some energy that will fall off the
link and therefore the difference in the reflected light that does
not fall on the link has to be calculated. Hence, two measurements
have to be made to accommodate for the reflected energy that is not
covered by the link. These measurements can be made on each die if
required and the energy per pulse can be varied as the thickness of
the oxide varies across the wafer. Alternatively, the method could
be selectively applied on a wafer-by-wafer basis for process
monitoring, for instance. This technique will reduce the
requirement to use a laser processing energy that is on the high
side to account for the variations in absorption in the link due to
interference effects.
Reflectivity Measurement and Power Adjustment--
Case 2: Tunable or Adjustable Wavelength
[0177] The process energy window may be improved in certain cases
by adjusting the wavelength over a range wherein the coupling of
energy to the target is improved, the stack reflectance is
increased by way of the interference effect, or where the substrate
reflectivity increases. Special solid state tunable lasers--Optical
Parametric Oscillators (OPO), Raman, or other tunable lasers may be
used provided that power and repetition rate requirements are met
for a given application. For example, parametric oscillators may be
used which are of fixed wavelengths, that use 2 or 3 crystals at
the same time. Under certain circumstances, tunable lasers are
operable. Published U.S. patent application 2001-0036206 describes
a tunable laser diode having a 40 nm range developed for the
telecommunications industry (i.e. 1.55 .mu.m wavelength). Standard
OPO lasers provide for high power and narrow pulses but generally a
very slow rep rate, but may be suitable for certain applications.
However, 10 KHz versions have been demonstrated and proposed for 20
KHz repetition rates. U.S. Pat. Nos. 6,334,011 and 5,998,759
(Reference 4), and U.S. Pat. No. 6,340,806 (Reference 6) disclose
various combinations of shifters. As disclosed in the '759 patent,
Fosterite lasers have a tunable region that essentially straddles
the absorption edge region of silicon, and can permit operation
both beyond and below the absorption edge of silicon. At the
present state of the art, they do not appear to be as efficient as
they may become in time. As materials and improvements are being
continually developed in the laser field, it is within the
invention to use such devices and obtain corresponding benefits for
processing. For instance, the multilayer thickness and reflectivity
measurements may be extended to select a wavelength range which
will provide for an improved energy window.
Application to a Cu Link with a Single Layer Between the Substrate
and the Link
[0178] It should be noted that the above teachings can also be
selectively applied to conventional link structures (see FIG. 2-B),
for instance processing of high reflectivity copper links separated
from the substrate by a single layer dielectric layer. Production
trends are pushing away from polysilicon structures and toward
metal structures of Al and Cu, which poses on-going challenges for
link processing systems to avoid reliability problems and to
increase yield. As discussed above, many Cu-based devices have a
multi-layer stack wherein substrate and stack damage can be avoided
with wavelength selection, spatial beam shaping, or temporal
shaping in accordance with the above teachings. However, some
manufacturers etch all the dielectrics under the copper link and
build the fuse on a single layer dielectric, with no SiN layers
between the link and substrate. With conventional laser processing,
the likelihood of substrate damages increases due to the high power
required for Cu processing.
[0179] In certain cases processing with multiple pulses ("double
blast") has been used to process metal fuses. However, there is
generally a throughput problem for the double blast approach
because two passes are required in present on-line memory
processing systems. Simulated results and experiments indicate a
second blast may open the link completely even if 1st blast failed,
despite extended time between first and second blasts. In specific
cases improved yield was reported. According to the simulation
results, double blast with 50% energy of a single blast energy was
very interesting; it was observed that the Si substrate acts as a
heat sink and cools down very fast. As shown in FIG. 16, the
results indicated only 10 to 20 ns are needed for the Si substrate
201 to stabilize to room temperature. The copper target 202
recovery was much slower indicating a significant differential
thermal property. The second pulse will also clear debris at cut
site resulting in an "open circuit". It is estimated that about
60-70% of the energy used in a "single blast" is needed for each
pulse of "double blast." The pulse energy may be varied with each
pulse. In this example, the pulse delay was 50 ns, but it is clear
that a much shorter delay may be possible.
[0180] In one embodiment, a delay line arrangement of FIG. 15a may
be used to avoid any delay in throughput. For example, with a
preferred positioning system of the '118 patent (i.e., Reference 2)
assume about 150 mm/sec for fine stage speed movement. With 30 ns
between two pulses, the change in beam position at the link
location would be only 0.0045 um which is negligible. In an optical
delay line (FIGS. 15b and 15c, for instance), 9 meters of extended
path in air for the beam will delay 30 ns for the second pulse.
Alternatively, as shown in FIG. 15a, a second laser could be used
with a 30 ns or other controllable delay between the trigger
pulses, and the trigger delay may be generated with a programmable
digital delay line. The temporal pulse shape may be a fast rising,
square pulse (as was used in the simulation) generated with a seed
laser diode, for instance.
[0181] Numerous options for generating the pulse combinations may
be implemented based on the teachings herein. For example, at least
one pulse may have a duration of greater than a few picoseconds to
several nanoseconds. The pulses may be amplified mode locked
pulses. At least one pulse may generated with a q-switched
microlaser having a pulsewidth less than 5 nanoseconds. At least
one pulse may propagate along a second optical path whereby the
pulse delay is determined by a difference in optical path length as
shown in FIGS. 15b,c. Multiple laser and/or amplifiers may be used
as shown in FIG. 15a.
[0182] As shown in FIG. 18, the generated pulses 275 may have a
repetition rate and a corresponding temporal spacing approximately
equal to or shorter than a pre-determined delay (e.g., 60 MHz mode
locked system) and a modulator is used to select the at least
second pulse irradiating the microstructure or groups of pulses
276. U.S. Pat. No. 5,998,759 (e.g., Reference 4, col. 7, and the
associated drawings) teaches the use of a modulator to allow pulses
to irradiate a link on demand. At very high speed repetition rates
an electro-optic modulator is preferred.
[0183] Additional optics may be used to spatially shape at least
one of the delayed pulses, prior to combining for instance. For
instance, as shown in FIG. 17, a first pulse 210 may be an
elliptical or circular Gaussian spatial shape, or a top hat along
the length of the link. The second pulse 212 may have a different
aspect ratio, or may be a special form of a "cleaning pulse"
wherein the central zone of the spot is attenuated with an
apodizing filter or effectively removed with a central obscuration.
In such a case, the energy will be concentrated at the link
periphery to remove debris 211 around the link location resulting
from processing with the first pulse, thereby completing the
processing 213. (For clarity, this "on-the-fly" link site cleaning
step is to be distinguished from the "cleaning for measurement"
method described above). Reference 1 provides at least one example
of beam shaping for link blowing applications, wherein a uniform
distribution rather than Gaussian spot profile is disclosed.
[0184] In certain cases, the relative motion between the
microstructure and the laser beam may be significant between the
pulses, e.g., greater than 25% of the spot size. This may be the
result of a slower repetition rate (with increased pulse energy),
faster motion speed, a longer pre-determined delay, or decreased
target area. For example, an ultrashort or other short pulse laser
system with amplified pulses with output energy in the range of
several microjoules-millijoules may have a 100 KHz-10 MHz
repetition rate whereas a system with 10-40 nanojoule output may
have 50 MHz repetition rate. In the former case, a high speed,
small angle beam deflector may be used to compensate for the motion
and deflect a delayed pulse to substantially irradiate the first
microstructure at the slower repetition rate during relative motion
258.
[0185] In one embodiment generally illustrated in FIG. 19, the
deflector would be operatively coupled to the relative positioning
system controller 251 in a closed loop arrangement. The deflector
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. The deflector would preferably be
used for intensity control and pulse gating/selection, as taught in
Reference 4 (col. 7, and associated drawings). Alternatively, an
electro-optic modulator may be used with a separate acousto-optic
deflector operated in a "chirp mode" 252 (e.g., linear sweep as
opposed to random access mode) and synchronized (triggered) 253
based on the positioning system coordinates 254. The positioning
system coordinates are, in turn, related to the time at which the
laser pulses are gated by the modulator to irradiate the same
single microstructure 256 at times t.sub.1, t.sub.2, t.sub.3
corresponding to the selected pulses 259 during relative motion
258.
[0186] In yet another embodiment, a single laser pulse is used to
blast up to two links at one time (e.g., no, one or two links).
Referring to FIG. 20, two focused spots 306,307 are formed on two
links by spatially splitting the single collimated laser beam 310
into two diverging collimated beams 309. The use of acousto-optic
devices for spatially splitting beams in material processing
applications is known in the art. For example, patent abstract JP
53152662 shows one arrangement for drilling microscopic holes using
a multi-frequency deflector having selectable frequencies f.sub.1 .
. . f.sub.n.
[0187] A laser 300 is pulsed at a predetermined repetition rate.
The laser beam goes through relay optics 302 that forms an
intermediate image of the laser beam waist into the acoustic optic
modulator (AOM) aperture. The AOM 303, which operates in the Bragg
regime, preferably is used to controllably generate the two
slightly diverging collimated first order diffraction laser beams
and control the energy in each beam. The AOM is driven by two
frequencies, f.sub.1 and f.sub.2 where f.sub.1=f.sub.0+.DELTA.f and
f.sub.2=f.sub.0-.DELTA.f where .DELTA.f is a small percentage of
the original RF signal frequency f.sub.0. The angle between the two
beams is approximately equal to the Bragg angle for f.sub.0
multiplied by 2(.DELTA.f/f.sub.0). The AOM controls the energy in
each of the laser beams by modulating the signal amplitudes of two
frequency components, f.sub.1 and f.sub.2, in the RF signal and
making adjustments for beam cross-coupling.
[0188] After exiting the AOM, the beams go through the beam
rotation control module 313 to rotate the beam 90 degrees on axis
with links orientated in either the X or Y. In one embodiment, a
prism is used for this rotation, though many rotation techniques
are well known as described in the regular U.S. application noted
in the Cross-Reference to Related Applications section.
[0189] Next, the beam goes through a set of optics to position the
beam waist and set the beam size to be appropriate for the zoom
optics and the objective lens 305. Note, the zoom optics also
modify the angle between the two beams, therefore the angle between
the two beams exiting the AOM has to be adjusted depending on the
zoom setting to result in the desired spot separation at the focal
plane. Next, the laser beams enter the objective lens 305 which
provides a pair of focused spots 306,307 on two links. The two
spots are separated by a distance that is approximately equal to
the focal length of the lens times the angle between the two beams.
In one exemplary embodiment, a 80 MHz AOM center frequency with a
sweep range of about 2.3 MHz (77.7-82.3 MHz) may be used to produce
a spot size of about 1.8 .mu.m on a pair of adjacent links spaced
apart by about 3 .mu.m. As mentioned earlier, these links may
have--a dimension on the order of a laser wavelength (e.g., 1
micron) which, at very high speed operation, require the precision
positions of the laser beam and microstructure.
[0190] Summary of Some General Aspects of Invention
[0191] In summary, one aspect of the invention is a method of
selective material processing of a microscopic target structure
with a pulsed laser beam. The target structure is separated from a
substrate by a plurality of layers which form a multi-layer stack.
The target structure, layers, and substrate have dissimilar thermal
and optical properties. The method includes generating a pulsed
laser beam with an energy density; irradiating the target structure
with at least one pulse. Undesirable changes to the stack structure
and substrate are avoided by selection of at least one pulse
characteristic.
[0192] A portion of the stack may be irradiated with the laser beam
during the processing of the target structure, yet undesirable
damage to the layers, substrate, and functional circuitry in a
plane of the inner layers is avoided.
[0193] Undesirable damage of the stack structure includes cracking,
induced by thermal stress, of inner dielectrics. Undesirable damage
to inner layer conductors of the stack includes thermal damage
caused by irradiation. Undesirable damage to the substrate may
arise from laser irradiation and resulting thermal diffusion.
[0194] The dielectric layers may include Silicon Nitride or Silicon
Dioxide. The substrate may be Silicon.
[0195] The target structure is preferably copper, and may have
thickness or width below one micron, with a dimension at or below
wavelengths of visible light. Alternatively, the target structure
may be a metal link, for instance aluminum, titanium, platinum, or
gold.
[0196] An aspect of the invention is selection or control of the
spatial and temporal beam characteristics of the pulse, which
allows the target structure to be cleanly processed while avoiding
undesirable damage to the layers, substrate, and functional
circuitry in a plane of the inner layers.
[0197] A temporal characteristic of the pulse is the pulse shape.
The pulse shape includes a rise time fast enough to efficiently
couple laser energy into the target, a duration sufficient to
cleanly remove a portion of the target structure, and a fall time
fast enough to avoid undesirable damage caused by subsequent
optical transmission. A preferred pulse rise time for link
processing is less than 1 nanosecond (ns) to about 2 ns. A
preferred duration is less than 10 ns. A fall time of less than 3
ns is preferred. The pulse shape may be substantially square, with
ringing or variation between the rising and falling edges of about
+-10%. A single pulse or multiple pulses in the form of a rapid
burst may be used. Alternatively, a series of q-switched pulses
spaced apart in time, with varying output power if desired, may be
combined to form a pulse shape having a fast leading edge with high
peak power, followed by a second pulse with lower power. In yet
another embodiment of the present invention the q-switched pulses
may have approximately the same output power and combined to
produce a substantially square pulse shape.
[0198] Another temporal pulse characteristic is the pulse power at
the leading edge. If the irradiance on the target structure is
greater than about 10.sup.9 W/cm.sup.2, the reflectivity of the
target structure is reduced and coupling of the laser energy is
improved.
[0199] A fast rising pulse characteristic avoids undesirable damage
of a dielectric stack of a memory device having a metal target
structure. Cracking of the upper corner occurs during the pulse
duration which lowers the stress on the lower corner adjacent to
underlying layers of the stack.
[0200] A spatial characteristic of the beam is the irradiance
profile at a controlled beam waist position. The irradiance profile
may approximate a circular Gaussian beam, an elliptical Gaussian
beam, a rectangular profile in one direction and Gaussian in the
orthogonal direction. The beam may be nearly diffraction limited. A
spatial shape and beam numerical aperture may be selected to
control the interaction of the pulsed laser beam with the target
and underlying structures of the 3D device structure to avoid
undesirable damage. The material interaction may further be
controlled by precision positioning of the beam waist of the pulsed
laser beam. The numerical aperture and beam shape may be selected
so the spot size and link size are substantially matched in at
least one dimension.
[0201] One aspect of the invention is a method of selection of a
pulse characteristic based on a model of pulse interaction within a
portion of the three-dimensional device structure. The
three-dimensional device includes a target structure, stack, and
substrate with a dissimilar optical property. A series of
structures are disposed at a predetermined spacing to form an
array, with at least one structure not designated as a target
structure. A specification may further include information
regarding the material and spacing of functional circuit elements
in a plane of the stack. The method includes determining the
optical propagation characteristics of a portion of an incident
pulsed laser beam which is not absorbed by the target structure.
The method further includes specifying a laser pulse characteristic
to avoid undesirable damage to any non-target structures, stack,
and substrate.
[0202] The interactions mechanisms which result in selection of a
pulse characteristic include reflection from the target surface,
layer surface and internal reflections, polarization, interference
effects, near field diffraction, scattering and absorption or a
combination thereof. A thermal model may be used in conjunction
with an optical model.
[0203] The energy in a pulse used for processing a copper link
target structure of a semiconductor memory device may be in the
range of about 0.1-5 microjoules. The energy density corresponds to
an area of the irradiance profile of the beam waist. The area may
be in the range of less than 20 square microns, and preferably less
than 10 square microns.
[0204] Another controllable laser pulse characteristic is
polarization. The polarization may be controlled or selected based
on the relative reflectance of the layers and optical coupling of
laser energy into the target structure at a wavelength.
[0205] A wavelength of the laser pulse may be selected based on the
reflectance of the multi-layer stack (interference effect). The
preferred wavelength corresponds to a spectral region where the
stack reflection is substantial, for example 60%, and where the
internal transmission of within a layer of the stack is high,
approaching a maximum. Short wavelengths are preferred for maximum
control of the spatial characteristics of the beam (for example,
the smallest achievable beam waist with an option for controllably
selecting a larger beam waist and depth of focus). The laser
wavelength may be fixed, or may be varied with wavelength shifting
or harmonic generation. A measurement of the thickness or
reflectance may be used to select or adjust the wavelength.
[0206] In at least one embodiment, the target structure may be
substantially reflective at the laser wavelength. The laser
wavelength may be below the absorption edge of the substrate and
correspond to an absorbing or reflecting region. The laser
wavelength is above the absorption edge of the dielectrics layers
of the stack, and corresponds to a substantially maximum
transmitting region.
[0207] A selected wavelength corresponds to the near UV, visible
and near IR spectrum, from below 0.4 .mu.m to about 1.55 .mu.m. The
lower limit may be determined by the absorption of a layer. With
silicon substrates, both absorption and reflection increase at
shorter wavelengths. For Silicon Dioxide and Silicon Nitride, the
internal transmission and single surface reflectance are
substantially constant throughout the visible and near IR ranges.
The upper limit corresponds to a range of preferred laser
wavelengths of laser diodes, optical amplifiers. An amplifier
output may be either wavelength preserved or Raman shifted.
[0208] Another aspect of the invention is a method of selective
material processing of a microscopic target structure of a
multi-material, multi-layer device with a pulsed laser beam. The
target structure, layers, and substrate have a dissimilar thermal
and optical property. The beam has a focused beam waist with a
centerline. An alignment pattern is included at one of a plurality
of predetermined measurement locations associated with the device.
The alignment pattern is covered by at least one layer. The target
structure is separated from a substrate by a plurality of layers
which form a multi-layer stack. The method includes measuring the
position of the alignment target in at least one dimension;
predicting the relative location of the target structure and
centerline based on the measurement; inducing relative motion
between the target structure and the centerline based on the
measurement; generating a pulsed laser beam with an energy density;
irradiating the target structure with at least one pulse.
Undesirable changes to the stack structure and substrate are
avoided with by selection of a pulse characteristic.
[0209] The measurement of a position may include a method and
system for polarization insensitive detection to avoid spurious
measurements resulting from reflected signal variations. The signal
variations may result from process induced optical characteristics,
including birefringence.
[0210] The relative location of the target structure, beam waist,
and centerline may be predicted based on multi-parameter least
squares fit.
[0211] A cleaning process may be used to enhance data used for
measurement by removing contaminants which produce multiplicative
variations (reflection noise).
[0212] Three-dimensional (depth) measurements may be done using the
alignment target, wafer, or other suitable material. The
measurement may be used to predict the relative location of the
target structure relative to the beam waist, the beam waist being
located along the centerline of the pulsed laser beam. A surface
may be estimated from the three-dimensional measurements. A
numerical offset may be introduced to compensate for a depth
difference between a measurement location and the target structure,
based on the thickness of the stack.
[0213] An aspect of the invention includes measurement of the layer
thickness or reflectivity at a location, and use of the measurement
to control a pulse characteristic. The pulse characteristic may be
the pulse energy, pulse width, or wavelength. The location may be a
single location on the device or a plurality or locations.
[0214] 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.
* * * * *