U.S. patent application number 16/911147 was filed with the patent office on 2020-10-15 for method for laser processing a transparent material.
This patent application is currently assigned to Rofin-Sinar Technologies Inc.. The applicant listed for this patent is Rofin-Sinar Technologies Inc.. Invention is credited to S. Abbas HOSSEINI.
Application Number | 20200324368 16/911147 |
Document ID | / |
Family ID | 1000004916868 |
Filed Date | 2020-10-15 |
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United States Patent
Application |
20200324368 |
Kind Code |
A1 |
HOSSEINI; S. Abbas |
October 15, 2020 |
METHOD FOR LASER PROCESSING A TRANSPARENT MATERIAL
Abstract
Systems and methods are described for forming continuous laser
filaments in transparent materials. A burst of ultrafast laser
pulses is focused such that a beam waist is formed external to the
material being processed without forming an external plasma
channel, while a sufficient energy density is formed within an
extended region within the material to support the formation of a
continuous filament, without causing optical breakdown within the
material. Filaments formed according to this method may exhibit
lengths exceeding up to 10 mm. In some embodiments, an aberrated
optical focusing element is employed to produce an external beam
waist while producing distributed focusing of the incident beam
within the material. Various systems are described that facilitate
the formation of filament arrays within transparent substrates for
cleaving/singulation and/or marking. Optical monitoring of the
filaments may be employed to provide feedback to facilitate active
control of the process.
Inventors: |
HOSSEINI; S. Abbas;
(Orlando, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rofin-Sinar Technologies Inc. |
Plymouth |
MI |
US |
|
|
Assignee: |
Rofin-Sinar Technologies
Inc.
Plymouth
MI
|
Family ID: |
1000004916868 |
Appl. No.: |
16/911147 |
Filed: |
June 24, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14336912 |
Jul 21, 2014 |
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16911147 |
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61861880 |
Aug 2, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C03C 14/002 20130101;
B23K 26/0006 20130101; H01L 21/78 20130101; B23K 2103/00 20180801;
C03C 2214/02 20130101; B23K 26/082 20151001; B23K 26/0626 20130101;
C03B 33/04 20130101; B23K 26/0648 20130101; B23K 26/53 20151001;
B23K 26/0624 20151001; B23K 2103/56 20180801; B23K 2103/50
20180801; C03B 33/0222 20130101; C03B 33/033 20130101; B23K 2103/54
20180801; C03B 33/09 20130101; B23K 2103/42 20180801; B23K 26/38
20130101; B23K 2103/52 20180801; C03B 33/07 20130101 |
International
Class: |
B23K 26/06 20060101
B23K026/06; B23K 26/53 20060101 B23K026/53; B23K 26/38 20060101
B23K026/38; B23K 26/00 20060101 B23K026/00; B23K 26/082 20060101
B23K026/082; B23K 26/0622 20060101 B23K026/0622; C03B 33/02
20060101 C03B033/02; C03B 33/07 20060101 C03B033/07; C03C 14/00
20060101 C03C014/00; H01L 21/78 20060101 H01L021/78; C03B 33/09
20060101 C03B033/09 |
Claims
1. A method of laser processing a material, comprising the steps
of: providing a laser beam having bursts of laser pulses, the laser
pulses having a pulse width of less than 100 picoseconds, the
material being transparent to the laser beam; focusing the laser
beam using one or more optical components having spherical
aberration, the optical components inducing aberration and focusing
the laser beam in a distributed manner along a longitudinal axis of
the laser beam, the focused laser beam having sufficient energy
density in the material to self-focus and form a laser filament
therein, laser energy deposited along the laser filament creating a
modification in the material, the distributed focus maintaining
sufficient intensity to accomplish material modification over a
desired length therein, the material modification having a shape
defined by the laser filament; and translating the focused laser
beam laterally to form an array of closely positioned
filament-induced modifications in the material.
2. The method of claim 1, wherein the laser filament extends over a
portion of the thickness of the material.
3. The method of claim 1, wherein the laser filament extends over
the full thickness of the material.
4. The method of claim 1, wherein the modified material is radially
compressed over the entire length of the laser filament.
5. The method of claim 1, wherein the material modification is a
void.
6. The method of claim 5, wherein the void extends through the
entire thickness of the material.
7. The method of claim 1, wherein the material modification is one
of defects, color centers, stress, micro-channels, micro-voids, and
micro-cracks.
8. The method of claim 1, wherein the material modification is
formed without laser ablation damage at the top and bottom surfaces
of the material.
9. The method of claim 1, wherein the ultrafast laser pulses have a
pulse width of less than 25 picoseconds.
10. The method of claim 1, wherein the laser pulses within each
burst have a relative delay that is less than a timescale for
relaxation of all material-modification dynamics.
11. The method of claim 1, wherein the focused laser beam has a
waist located below the material.
12. The method of claim 1, wherein the focused laser beam has a
waist located above the material.
13. The method of claim 1, wherein after the translating step, the
material is cleaved along the array of filament-induced
modifications.
14. The method of claim 13, wherein the cleaving step separates the
material by one of additional laser processing, heating, cooling,
and mechanical pressure.
15. The method of claim 13, wherein the edge roughness of the
cleaved material is controlled by selecting the degree-of-overlap
or discrete-spacing of the filament-induced modifications.
16. The method of claim 13, wherein the root-mean-square roughness
of the cleaved surfaces is less than 10 micrometers.
17. The method of claim 1, wherein the laser filament has a length
of greater than 1 millimeter.
18. The method of claim 1, wherein the focused laser beam has a
uniform energy distribution along the longitudinal axis of the
laser beam.
19. The method of claim 1, wherein the laser processing is
accomplished in a single pass during the translating step.
20. The method of claim 1, wherein the material is scribed and
separated during the translating step.
Description
[0001] This application is a continuation of U.S. Ser. No.
14/336,912, filed Jul. 21, 2014, which in turn claims priority to
and the benefit of U.S. provisional patent application Ser. No.
61/861,880 filed Aug. 2, 2013 the disclosures of which are
incorporated herein in their entirety by reference hereto.
FIELD OF THE INVENTION
[0002] The invention is in the field of laser filamentation within
transparent materials.
BACKGROUND OF THE INVENTION
[0003] The present disclosure is related to systems and methods for
the laser processing of materials. More particularly, the present
disclosure is related to systems and methods for the singulation
and/or cleaving of wafers, substrates, and plates containing
passive or active electronic or electrical devices created upon
said materials.
[0004] In current manufacturing, the singulation, dicing, scribing,
cleaving, cutting, and facet treatment of wafers or glass panels is
a critical processing step that typically relies on diamond or
conventional, ablative or breakdown (stealth) laser scribing and
cutting, with speeds of up to 30 cm/sec for LEDs, LED devices (such
as lighting assemblies) and illuminated devices (such as LED
displays) as some examples.
[0005] In the diamond cutting process, after diamond cutting is
performed, a mechanical roller applies stress to propagate cracks
that cleave the sample. This process creates poor quality edges,
microcracks, wide kerf width, and substantial debris that are major
disadvantages in the lifetime, efficiency, quality, and reliability
of the product, while also incurring additional cleaning and
polishing steps. The cost of de-ionized water to run the diamond
scribers are more than the cost of ownership of the scriber and the
technique is not environmentally friendly since water gets
contaminated and needs refining, which further adds to the
production cost.
[0006] Laser ablative machining has been developed for singulation,
dicing, scribing, cleaving, cutting, and facet treatment, to
overcome some of the limitations associated with diamond cutting.
Unfortunately, known laser processing methods have disadvantages,
particularly in transparent materials, such as slow processing
speed, generation of cracks, contamination by ablation debris, and
moderated sized kerf width. Furthermore, thermal transport during
the laser interaction can lead to large regions of collateral
thermal damage (i.e. heat affected zone).
[0007] Laser ablation processes can be improved by selecting lasers
with wavelengths that are strongly absorbed by the medium (for
example, deep UV excimer lasers or far-infrared CO.sub.2 laser).
However, the aforementioned disadvantages cannot be eliminated due
to the aggressive interactions inherent in this physical ablation
process. This is amply demonstrated by the failings of UV
processing in certain LED applications where damage has driven the
industry to focus on traditional scribe and break followed by etch
to remove the damaged zones left over from the ablative scribe or
the diamond scribe tool, depending upon the particular work-around
technology employed.
[0008] Alternatively, laser ablation can also be improved at the
surface of transparent media by reducing the duration of the laser
pulse. This is especially advantageous for lasers that are
transparent inside the processing medium. When focused onto or
inside transparent materials, the high laser intensity induces
nonlinear absorption effects to provide a dynamic opacity that can
be controlled to accurately deposit appropriate laser energy into a
small volume of the material as defined by the focal volume. The
short duration of the pulse offers several further advantages over
longer duration laser pulses such as eliminating plasma creation
and therefor plasma reflections thereby reducing collateral damage
through the small component of thermal diffusion and other heat
transport effects during the much shorter time scale of such laser
pulses.
[0009] Femtosecond and picosecond laser ablation therefore offer
significant benefits in machining of both opaque and transparent
materials. However, in general, the machining of transparent
materials with pulses even as short as tens to hundreds of
femtosecond is also associated with the formation of rough
surfaces, slow throughput and micro-cracks in the vicinity of
laser-formed kerf, hole or trench that is especially problematic
for brittle materials like alumina (Al.sub.2O.sub.3), glasses,
doped dielectrics and optical crystals. Further, ablation debris
will contaminate the nearby sample and surrounding devices and
surfaces. Recently, multi-pass femtosecond cutting has been
discussed in Japan, utilizing a fiber laser approach. This approach
suffers from the need to make multiple passes and therefore results
in low processing throughput.
[0010] Although laser processing has been successful in overcoming
many of the limitations associated with diamond cutting, as
mentioned above, new material compositions have rendered the wafers
and panels incapable of being laser scribed. Furthermore, the size
of the devices and dice on the wafers are getting smaller and
closer to each other that limit the utility of both diamond and
conventional laser-based scribing. For example, 30 .mu.m is a
feasible scribing width, while 15 .mu.m is challenging for these
conventional methods. Moreover, as diamond scribing uses mechanical
force to scribe the substrate, thin samples are very difficult to
scribe. Due to the use of increasingly exotic and complex material
stacks in the fabrication of wafer-based devices, the laser
scribing techniques previously applied will simply no longer work
due to the opacity of the stack.
SUMMARY OF THE INVENTION
[0011] Systems and methods are described for forming continuous
laser filaments in transparent materials. A burst of ultrafast
laser pulses is focused such that a beam waist is formed external
to the material being processed, such that a primary focus does not
form within the material, while a sufficient energy density is
formed within an extended region within the material to support the
formation of a continuous filament, without causing optical
breakdown within the material. Filaments formed according to this
method may exhibit lengths exceeding up to 10 mm with a 1:1
correspondence in the length of the modified zone (in that the
filament is the agent of modification, so the modified zone tracks
1:1 with the extent of the filament) and a taper-free profile when
viewed with the long axis in cross-section. In some embodiments, an
uncorrected or abberrated optical focusing element is employed to
produce an external beam waist while producing distributed focusing
of the incident beam within the material. Various systems are
described that facilitate the formation of filament arrays within
transparent substrates for cleaving/singulation and/or marking.
Optical monitoring of the filaments may be employed to provide
feedback to facilitate active control of the process.
[0012] Accordingly, in a first aspect, there is provided a method
of laser processing a transparent material, the method
comprising:
[0013] providing a laser beam comprising a burst of laser
pulses;
[0014] externally focusing the laser beam relative to the
transparent material to form a beam waist at a location that is
external to the transparent material while avoiding the formation
of an external plasma channel;
[0015] wherein the laser pulses are focused such that a sufficient
energy density is maintained within the transparent material to
form of a continuous laser filament therein without causing optical
breakdown.
[0016] In another aspect, there is provided a method of processing
a transparent material, comprising the steps of:
[0017] providing a laser beam, said laser beam having a plurality
of bursts and each of said bursts include a plurality of
pulses;
[0018] generating an initial waist of said laser beam outside said
transparent material;
[0019] generating a weakly focused laser beam distributed within
said transparent material; and,
[0020] producing a spatially extended and spatially homogenous
filament in said transparent material.
[0021] In another aspect, there is provided a method of processing
a transparent material, said transparent material has a metal layer
formed within or on a surface thereof, said laser beam is a
filament-forming laser beam, comprising the steps of:
[0022] prior to forming said continuous laser filament within said
transparent material:
[0023] providing a low-power laser beam comprising a burst of laser
pulses by reducing the power of said filament-forming laser beam
below the threshold for the formation of a filament within said
transparent material, while maintaining sufficient power to ablate
said metal layer; and irradiating said metal layer with the
low-power laser beam at one or more locations such that said metal
layer is locally ablated by said laser beam thereby producing one
or more ablative markings within the metal layer.
[0024] In another aspect, there is provided a transparent material
having a continuous laser filament formed therein, the continuous
laser filament having a length exceeding approximately 1 mm.
[0025] In another aspect, there is provided a transparent substrate
exhibiting a post-cleave or post-singulation break strength that
exceeds approximately 50 MPa.
[0026] In another aspect, there is provided a system for laser
processing a transparent material, the system comprising:
[0027] a laser source configured to provide a laser beam comprising
bursts of laser pulses;
[0028] one or more focusing elements configured to externally focus
the laser beam relative to the transparent material to form a beam
waist at a location that is external to the transparent material
while avoiding the formation of an external plasma channel and
internal plasma centers, wherein the laser beam and the one or more
focusing elements are configured to produce a sufficient energy
density within the transparent material to form of a continuous
laser filament therein without causing optical breakdown;
[0029] means for varying a relative position between the laser beam
and the transparent material; and
[0030] a control and processing unit operatively coupled to the
means for varying the relative position between the laser beam and
the transparent material, wherein the control and processing unit
is configured to control the relative position between the laser
beam and the transparent material for the formation of an array of
continuous laser filaments within the transparent material.
[0031] A further understanding of the functional and advantageous
aspects of the disclosure can be realized by reference to the
following detailed description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] Embodiments of the disclosure will now be described, by way
of example only, with reference to the drawings, in which:
[0033] FIGS. 1(a)-(e) illustrate various optical configurations for
the formation of filaments. FIGS. 1(a) and (b) illustrate the
formation of filaments using previously known methods involving the
formation of a beam waist within the material. FIGS. 1(c)-1(e)
illustrate various embodiments in which long homogeneous filaments
are formed by focusing the beam energy such that it is "dumped"
into a focus above and/or below the target material (forming an
"optical reservoir") in order to modulate the amount of energy
passed into the desired filament zone.
[0034] FIG. 1(f) is an example image of filaments formed according
to the methods disclosed herein, demonstrating the formation of a
spatially extended filament with a homogeneous and continuous
profile, where the depth and location of the filament within the
material is determined by the relative positioning of the beam
focus (each filament corresponds to a vertical offset of 25
microns).
[0035] FIG. 1(g) is a microscope image of a glass sample
illustrating the difference between the stealth dicing and
filamentation processing methods.
[0036] FIGS. 2(a) and (b) depict (a) a long filament zone on the
scales of millimeters to tens of millimeters; while (b) illustrates
the ability for the beam to pass through intermediate layers
without damaging them.
[0037] FIG. 3 illustrates an example lens arrangement employing a
scanner. A conventional scan lens (e.g. telecentric or otherwise)
is employed with the inclusion of an aspheric plate placed either
before or after the scanner. This embodiment enables coordinated
motion and constant velocity processing over curved pathways.
[0038] FIG. 4 illustrates an example embodiment similar to that
shown in FIG. 3, employing a specialized scan lens (telecentric or
non-telecentric) without an aspheric plate.
[0039] FIGS. 5(a)-(e) illustrate the characteristics of an example
burst pulse train. In some embodiments, the spacing between the
sub-pulses and the burst packets can be controlled, and the number
of pulses in the burst pulse train can be controlled.
[0040] FIG. 6 illustrates the ability of the methods disclosed
herein to create parts with curved edges and shapes, formed by
tracing out paths on the target material to generate the desired
shape. Arbitrary control of the location and orientation of the
filaments can be achieved by suitable choice of system
components.
[0041] FIG. 7(a) is a schematic layout of an example apparatus for
performing the methods disclosed herein, and where the system
includes, a stage, scanner, lens array, and servo controlled XYZ
positioner.
[0042] FIG. 7(b) is a block diagram of an example control and
processing system.
[0043] FIGS. 8(a) and 8(b) illustrate the relationships of axes
relative to the processed part in an example embodiment,
illustrating how the apparatus may be controlled to process a wide
array of substrate shapes and orientations, using (a)
non-telecentric and (b) telecentric lenses.
[0044] FIG. 8(c) illustrates an example embodiment in which the
stage supporting the material being processed is rotated to produce
angled filaments.
[0045] FIG. 9 depicts an example system for producing parts using
the methods disclosed herein. Such an embodiment may be employed
for the singulation of substantially transparent media with high
edge quality and speed.
[0046] FIGS. 10(a)-(c) illustrate example embodiments using a theta
stage for the positioning of the apparatus described herein, with a
non-orthogonal (i.e. <90.degree. or >90.degree. with respect
to the target surface), for the creation of angled filament cleave
planes. Such an embodiment enables the production of parts with
edges that are not perpendicular to the surface (e.g. a chamfered
part). For example, using the theta stage, a circular path can be
traced producing a part with a cut-out having an angled edge
characteristic.
[0047] FIG. 10(d) and (e) illustrates the formation of a chamfered
edge via processing with multiple filament forming beams at
different angles.
[0048] FIGS. 10(f)-(i) illustrate the processing of sodalime glass
using multiple cuts to obtain chamfered edges.
[0049] FIGS. 10(j) to (l) show edge-on views of a chamfered facet
at different zoom levels.
[0050] FIGS. 10(m) to (o) show the processing of a part with three
cuts to obtain an edge having an intermediate vertical edge and two
chamfered edges.
[0051] FIG. 11(a) illustrates a schematic of an example rotary
processing tool for processing wafers according to the methods
disclosed herein.
[0052] FIG. 11(b) illustrates an example implementation of the
processing stage shown in FIG. 11(a), providing multi-substrate,
multi-beam, and multi-laser head capability.
[0053] FIGS. 11(c)-11(f) illustrate an example implementation of a
system for performing laser filament processing on four wafers
using a single laser system.
[0054] FIG. 12 illustrates an example implementation of a processed
material with complex edges and shapes within cutouts from mother
sheets with rounded corners having an optionally variable
radius.
[0055] FIGS. 13(a) and (b) illustrates an example embodiment
showing variable cut edge roughness by selection and control of the
filament spacing.
[0056] FIGS. 14(a)-(c) show the break strength testing protocol as
described in ASTMC158 for determining the as processed break
strength of the materials thus singulated. FIGS. 14(a) and (b) show
two example break strength measurement configurations, while FIG.
14(c) shows an example Weibull plot for determining the
characteristic strength.
[0057] FIGS. 15(a)-(d) illustrate an example embodiment showing the
formation of complex spline parts from curved targets by servoing
the z and "steering the beam" via adaptive optics, which would also
be servo'd. The beam (FIG. 15(a)) and/or part (FIG. 15(b, c)) can
be rotated, tilted or otherwise manipulated to create a very wide
process window and capability for producing parts with complex
surface curvature. FIG. 15(d) provides an example implementation of
such an embodiment, showing a glass part processed via filament
formation to exhibit a rounded edge.
[0058] FIG. 16(a) illustrates an example embodiment in which a
multilayer substrate can be cut or processed in a single pass,
cutting at normal and/or non-normal angles.
[0059] FIG. 16(b) illustrates the processing of a triple layer
laminated glass substrate having a thickness of 2.1 mm.
[0060] FIG. 16(c) shows an electron microscope image,
post-cleavage, of a filament-processed multi-layer device including
two air gaps an intermediate adhesive layer.
[0061] FIG. 16(d) shows microscope images of a laminated liquid
crystal display substrate, in which the top surface is processed
via a V-groove, and the bottom surface is processed via filament
formation.
[0062] FIGS. 17(a)-(d) illustrates several example embodiments
showing the use of an imaging device for process control, where
output from the imaging device is processed to provide feedback.
Output from the imaging device(s) is provided to a process control
computing device.
[0063] FIGS. 18(a)-(c) illustrate a method of processing a
semiconductor substrate having an array of devices formed therein
by processing the back portion of the substrate to produce ablating
markings, and subsequently using the ablative markings as fiducial
reference points when performing filament processing from above the
substrate.
[0064] FIG. 18(d) is an overhead image of a LED wafer processed
according to this method, in which burst laser pulses were employed
to process all layers, including the metal layer (low power
marking), the DBR layer, the PSS layer, and the sapphire and GaN
layers.
[0065] FIGS. 18(e) and (f) show the post-processed substrates with
intact dicing tape, and FIGS. 18(g) and (h) show the processed
substrates after the removal of the dicing tape.
[0066] FIG. 19 shows a micrograph of a facet edge of a glass
substrate after filament processing and singulation.
[0067] FIG. 20 shows post-singulation surface roughness
measurements of an example substrate in orthogonal directions.
[0068] FIG. 21 shows measurements of surface roughness obtained of
a post-singulated sapphire sample, with measured values as low as
approximately 200 nm RMS.
DETAILED DESCRIPTION OF THE INVENTION
[0069] Various embodiments and aspects of the disclosure will be
described with reference to details discussed below. The following
description and drawings are illustrative of the disclosure and are
not to be construed as limiting the disclosure. Numerous specific
details are described to provide a thorough understanding of
various embodiments of the present disclosure. However, in certain
instances, well-known or conventional details are not described in
order to provide a concise discussion of embodiments of the present
disclosure.
[0070] As used herein, the terms, "comprises" and "comprising" are
to be construed as being inclusive and open ended, and not
exclusive. Specifically, when used in the specification and claims,
the terms, "comprises" and "comprising" and variations thereof mean
the specified features, steps or components are included. These
terms are not to be interpreted to exclude the presence of other
features, steps or components.
[0071] As used herein, the term "exemplary" means "serving as an
example, instance, or illustration," and should not be construed as
preferred or advantageous over other configurations disclosed
herein.
[0072] As used herein, the terms "about" and "approximately" are
meant to cover variations that may exist in the upper and lower
limits of the ranges of values, such as variations in properties,
parameters, and dimensions. In one non-limiting example, the terms
"about" and "approximately" mean plus or minus 10 percent or
less.
[0073] Unless defined otherwise, all technical and scientific terms
used herein are intended to have the same meaning as commonly
understood to one of ordinary skill in the art. Unless otherwise
indicated, such as through context, as used herein, the following
terms are intended to have the following meanings.
[0074] As used herein, the term "filament modified zone" refers to
a filament region within a substrate characterized by a region of
compression defined by the optical beam path.
[0075] As used herein, the phrases "burst", "burst mode", or "burst
pulses" refer to a collection of laser pulses having a relative
temporal spacing that is substantially smaller than the repetition
period of the laser. It is to be understood that the temporal
spacing between pulses within a burst may be constant or variable
and that the amplitude of pulses within a burst may be variable,
for example, for the purpose of creating optimized or
pre-determined filament modified zones within the target material.
In some embodiments, a burst of pulses may be formed with
variations in the intensities or energies of the pulses making up
the burst.
[0076] As used herein, the term "transparent" refers to a material
having an absorption spectrum and thickness such that at least a
portion of the incident beam is transmitted in the linear
absorption regime. For example, a transparent material may have a
linear absorption spectrum, within a bandwidth of the incident
beam, and a thickness, such that the percentage of light
transmitted through the material is greater than 10%, greater than
25%, greater than 50%, greater than 75%, greater than 80%, greater
than 85%, greater than 90%, greater than 95%, greater than 98%, or
greater than 99%.
[0077] As used herein, the phrase "geometric focus" refers to the
calculated or estimated focus produced by an optical focusing lens
or assembly, where the calculation is made without incorporating or
considering nonlinear effects within the material being processed
(e.g. with a beam waist position determined according to the simple
lens equation). This phrase is used to distinguish between the
expected location of the optical focus based on the position of the
lenses, and the optical constriction events created within the
material being processed that are caused by non-linear beam
refocusing, which provides, in effect, a quasi-Rayleigh length on
the order of up to approximately 10 mm.
[0078] Embodiments of the present disclosure provide devices,
systems and methods for the processing of materials by laser
filamentation. Unlike previously known methods of laser
filamentation, some embodiments of the present disclosure utilize
an optical configuration that focuses the incident beam in a
distributed manner along the longitudinal beam axis. This
distributed focusing method enables the formation of filaments over
distances well beyond those achieved to date using previously known
methods, while maintaining a sufficient laser intensity to
accomplish actual, uniform modification and compression of material
over the entire length of the filament zone. Such filaments (and
filamentation processes) involve the self-propagating beam of light
within the material being processed such that a balance between
thermal processes is responsible for compression, while avoiding
the optical breakdown that is employed in other known ablative and
other known processing methods. For example, as further described
below, the distributed focusing methods disclosed herein support
the formation of filaments with lengths well beyond one millimeter
(even as long as 10 mm) and yet maintain an energy density beneath
the optical breakdown threshold of the material.
Laser Filamentation
[0079] It is known that laser filaments can be formed in
transparent materials using ultrafast laser pulses that are focused
within the material. For example, as taught in Patent Cooperation
Treaty Application No. PCT/CA2011/050427, titled "Method of
Material Processing by Laser Filamentation", filaments can be
formed by focusing, with an objective lens, a burst-train of short
duration laser pulses inside a transparent substrate. The burst of
laser pulses produces internal microstructural modification with a
shape defined by the laser filament volume. By moving the sample
relative to the laser beam during pulsed laser exposure, a
continuous trace of filament tracks is permanently inscribed into
the glass volume as defined by the curvilinear or straight path
followed by the laser in the sample.
[0080] As noted in PCT Application No. PCT/CA2011/050427, it is
believed that filaments (also referred to as "plasma channels") are
produced by the weak focusing of laser pulses having a high
intensity and short duration, such that the pulses can self-focus
by the nonlinear Kerr effect due to the formation of a plasma. This
high spatio-temporal localization of the light field can deposit
laser energy in a long and narrow channel, while also being
associated with other complex nonlinear propagation effects such as
white light generation and formation of dynamic ring radiation
structures surrounding this localized radiation. PCT Application
No. PCT/CA2011/050427 teaches that laser filaments may be formed
over length scales on the order of hundreds of microns by focusing
the laser beam such that the focal point (e.g. initial beam waist)
lies within the material.
[0081] In contrast to known filament forming methods, the present
disclosure provides methods for forming spatially extended and
spatially homogeneous filaments in transparent materials. According
to one embodiment, a burst of ultrafast laser pulses is focused
such that an external beam waist is formed outside the target
material, without forming a plasma channel outside of the material,
and weak distributed focusing of the incident beam occurs within
the target material, thereby forming a high density electric field
within the material and creating a zone of compression along the
incident path of the laser. This zone of compression results in a
phase change (confirmed by etch rate experiments) in a narrow
curtain of material extending uniformly and radially from the
center of the propagation axis.
[0082] As described further below, the length and position of the
filament is readily controlled, for example, by the positioning of
the focusing apparatus, the numerical aperture of one or more
focusing elements, the laser pulse energy, wavelength, duration and
repetition rate, the number of laser pulses and bursts applied to
form each filament track, and the optical and thermo-physical
properties of the transparent medium. Collectively, these exposure
conditions (power, repetition rate, translation speed and the
degree to which the wavefronts has been distributed/aberrated to
extend the interaction zone) can be manipulated to create
sufficiently long and intense filaments to extend or nearly extend
over the full thickness of the processed material.
[0083] Accordingly, embodiments disclosed herein harness short
duration bursts of laser pulses (preferably with a pulse duration
less than about 100 ps) to generate a filament inside a transparent
medium. The method avoids plasma generation such as through optical
breakdown that can be easily produced in tight optical focusing
conditions as typically applied and used in femtosecond laser
machining (for example, as disclosed by Yoshino et al.,
"Micromachining with a High repetition Rate Femtosecond Fiber
Laser" (2008), Journal of Laser Micro/Nanoengineering Vol. 3, No. 3
pgs. 157-162).
[0084] In the weak and distributed focusing embodiments disclosed
herein, the nonlinear Kerr effect is believed to create an extended
laser interaction focal volume that exceeds the conventional depth
of field, overcoming the optical diffraction that normally diverges
the beam from the small self-focused beam waist. It is in this
so-called filament zone, formed via distributed or extended
focusing, that the material undergoes a phase transition induced by
photo-acoustic compression, in a substantially symmetrical,
substantially cylindrical region centered about the axis of beam
propagation in the material.
[0085] This modification requires energy densities above a certain
characteristic threshold for each material, ideally chosen by the
highest threshold of the materials present in a non-homogeneous
complex stack. This modification can occur at normal and non-normal
angles of incidence relative to the top of the substrate and
persist for distances only limited by the power available in the
incident beam.
[0086] Furthermore, it is believed that optical breakdown does not
occur in the material during processing, as this would create
discrete damage centers instead of the long continuous and
homogenous filament modified zones of the present disclosure. While
laser energy deposited along the filaments leads to internal
material modification that can be in the form of defects, color
centers, stress, microchannels, microvoids, and/or micro
cracks--experimental results have shown that the modification is
substantially uniform and symmetric in its appearance, with an
interior surface that is substantially homogenous in its physical
characteristics. This is believed to be achieved by presenting a
very high intensity electric field that possesses a very uniform
energy distribution along the length of the filament.
Formation of Extended Filaments via Distributed Focusing of Laser
Beam
[0087] In contrast to the methods and apparatus disclosed in PCT
Application No. PCT/CA2011/050427, the present disclosure provides
methods, apparatus and systems for the controlled formation of
filaments in transparent materials using an optical focusing
configuration in which the incident beam is directed onto the
material such that an external waist is formed without forming a
plasma channel outside of the material, and such that the beam
energy is focused in a distributed manner throughout a region
within the material. It is believed that the distributed focusing
configuration without the formation of an internal beam waist
provides conditions that sustain the formation of the laser
filament over longer distances, with more controllable geometries
and mechanical properties, as further described below.
[0088] Referring now to FIG. 1(a), a focusing arrangement disclosed
in PCT/CA2011/050427 is illustrated, in which focusing lens 100 is
employed to focus a burst of ultrafast laser pulses within a
material 110 for the formation of a filament 120. Focusing lens 100
is positioned relative to material 110 such that the focus of lens
100 is located within material 110. Incident beam 130 is focused by
focusing lens 100 to form converging beam 140, which is focused
within material 110 and maintains a focused configuration, forming
filament 120 prior to expanding and de-focusing. As described
above, the confinement of the optical power within material 110,
while forming filament 120, is achieved through self-focusing via
self-phase modulation. Beam 140 expands beyond the filament forming
region due to loss of optical beam power, such that self-phase
modulation is no longer sufficient to support self-focusing and to
counter the defocusing caused by the presence of the heating and
subsequent index change in the target material. As shown in FIG.
1b, this method can result in the formation of a filament 120
within material 110 having a length on the scale of hundreds of
microns.
[0089] FIG. 1(c) illustrates an example embodiment of spatially
extended filament generation in a transparent material through the
distributed focusing of a burst of ultrafast laser pulses. Unlike
the configuration shown in FIG. 1(a), in which incident beam 130 is
focused by focusing lens 100 to form a well-defined initial waist
within material 110, the configuration shown in FIG. 1(c) employs
distributed focusing element 150 to focus incident beam 160 such
that the resulting converging beam 165 is focused to an initial
external waist 175, and is also weakly focused in a distributed
manner within material 115. The external formation of the initial
waist prevents excessive focusing and optical breakdown within the
material, avoiding deleterious effects such as optical breakdown.
The distributed focusing configuration causes the focused beam to
be directed onto material 115 such that the optical power is
extended over a range of locations, unlike known methods in which
external focusing produces a narrow external plasma channel,
thereby distributing the incident laser within material 115, as
opposed to forming a waist within the material with a tight and
well-defined location. Such a distributed focusing configuration is
capable of producing a filament 170 having controlled geometrical
properties and a length on the millimeter scale. Distributed
focusing element 150 may include one or more lenses that are formed
(e.g. ground or molded) to produce what appears to be a distributed
focus (not necessarily an evenly distributed focus), with a waist
residing above or in front of the surface of the material, adjacent
to a surface of the material, providing a very weakly focused spot
at the material surface in the absence of an external plasma
channel. In one embodiment, the waist is located at an offset of at
least approximately 10 .mu.m from an external surface of the
material. In another embodiment, the waist is located at an offset
of at least approximately 20 .mu.m from an external surface of the
material. In another embodiment, the waist is located at an offset
of at least 50 .mu.m from an external surface of the material.
Accordingly, the present embodiment avoids the need to form a
primary beam waist within the material by altering the focal
properties of the lenses, offering a wide range of processing
options, such that an external waist is formed above, below or, for
example, in between layers of target materials in an interstitial
space.
[0090] Without intending to be limited by theory, it is believed
the distributed focusing configuration of the present disclosure
produces longer filaments due to the spatially replenishment of the
optical beam power from the additional focal regions. As optical
power within the narrow filament forming region is initially
depleted during beam propagation, through interaction with the
complex (non-linear) index changes formed via the nonlinear
processes, additional optical power is provided by the distributed
focusing of the beam along its length, such that the beam can
propagate further in a self-focused manner while forming the
filament, prior to defocusing. As noted above, in some embodiments
this approach produces the desired self-focusing and compression
without actually forming a plasma.
[0091] Referring again to FIG. 1(c), the illustrated example
embodiment is shown with focusing element 150 positioned such that
a least a portion of the converging beam 165 is focused in front of
material 115, for example at location 175 in the Figure. In
particular, in the example implementation shown, the high numerical
aperture rays encountering distributed focusing element 150 are
focused in front of material 115. By focusing a portion of the
incident optical power in front of material 115, the intensity
profile formed immediately within the material is not too high or
too low, which therefore allows the formation of a filament having
a substantially uniform cross section over its length.
[0092] As noted above, the advantage of creating a beam waist above
or in front of the material, instead of below, lies in the desire
to avoid exceeding the optical breakdown threshold of the material.
It also enables a larger process window by giving the user more
options relative to process set-up and sample configuration.
[0093] FIG. 1(d) illustrates an alternative example embodiment in
which distributed focusing element 150 is positioned such that a
portion of converging beam 180 is focused behind material 115 at
location 185 for forming filament 190.
[0094] Referring now to FIG. 1(e), another example embodiment is
illustrated in which distributed focusing element 150 is positioned
such that a portion of converging beam 200 is focused in front of
material 115 (e.g. at location 205), and another portion of
converging beam 200 is focused behind material 115 (e.g. at
location 210), for forming filament 220. In this configuration, the
beam energy can be distributed along a greater length and still
maintain enough energy along the path through the target to create
acoustic compression from the electric field induced heating
effects formed by the laser. This embodiment may be employed to
enable a higher degree of control and depth in forming filaments.
As shown in the Figure, such an embodiment may be useful in forming
filaments throughout the material, through transparent substrates
with thicknesses of millimeters to tens of millimeters.
[0095] The benefit of the distributed focusing configuration
employed in methods disclosed herein can be understood as follows.
If the incident beam is focused to a waist within the material, the
filament progression stops after a short distance, as in the
previously known methods. However, if the incident power is focused
outside of the material, forming an optical reservoir, and the
material is allowed to act as the final lens as it undergoes
thermally induced changes in the index (complex index in
particular), then the filament can be formed with a substantially
homogeneous cross sectional profile, and spatially extended over
millimeters in length, as illustrated in FIG. 1(f), which
demonstrates the formation of homogeneous filaments with lengths
exceeding 1 mm in soda lime glass. FIG. 1(f) further illustrates
the control of relative vertical positioning of the filaments by
varying the axial position of the beam focus (each filament
corresponds to an offset of 25 microns). Such processing can
produce high quality edges substantially absent of chipping >10
.mu.m.
[0096] FIG. 1(g) is a microscope image of a glass substrate
processed and cleaved according to the stealth dicing method. A
filament array had also been formed in a direction perpendicular to
the stealth dicing line (the sample shown had not yet been cleaved
according to the filament array line). As shown in the Figure,
stealth dicing line 20 shows the telltale signs of optical
breakdown, which produces a very smooth top edge, but a very rough
face overall. The roughness of the faced edge obtained by cleaving
along the stealth dicing line was found to be 53 microns in the
vertical direction and 85 microns in the horizontal direction. In
contrast, filament 10 illustrates a continuous material
modification extending through the substrate, facilitating cleavage
that results in a smoother facet. As described below, the cleavage
of the sample along the filament array line can produce surface
roughness values on the order of 1 to 10 microns for glass
materials.
[0097] As described in further detail below, the filaments formed
according to the methods disclosed herein can be formed with a
length that is much longer than previously reported filaments.
Furthermore, the present methods can be employed to produce
filaments that are continuous, radially symmetric and uniform in
dimension. A key consideration for the formation of extended
filaments within the transparent material is the supply of the
requisite fluence, treated below, while at the same time avoiding
the optical breakdown threshold of the material. It has been found
that the filament length is tied into the total energy supplied to
the material and the material's linear absorption.
[0098] For example, experimental studies have shown that filaments
that are 6 mm long can be formed in borosilicate glass using the
following conditions: an average power of approximately 50 W; a
wavelength of 1064 nm; a pulse width of less than approximately 50
picoseconds; a burst profile of approximately 15 pulses, a pulse
profile decreasing in amplitude, increasing in amplitude, and/or
level in amplitude; and a spot size of approximately 10 .mu.m.
[0099] The position of the stop and start of the filament can be
controlled by selecting the position of the geometric focus, or
beam waist, as predicted by the geometrics of the lens group or
focusing assembly. The balance between power and size, also
explained below, provides the ability to avoid the formation of a
beam waist within the material.
[0100] FIGS. 2(a) and 2(b) show the flexibility can be achieved by
controlling the location of the beam waist locus. FIG. 2(a) depicts
a long filament zone 250 on the scales of millimeters to tens of
millimeters (not to scale; filaments of up to 10 mm have been
produced by the inventor). The ability for the beam to pass through
layers without damaging them is illustrated. The critical diameter
range 260 (which may vary, for example, accordingly for each
material, focusing conditions, and laser power) is also indicated,
which is defined as the range of laser spot diameters above which
filaments do not form and below which optical damage occurs. In one
example implementation, the critical diameter range for soda lime
glass has been found to lie near 8 .mu.m. The critical ratio is
also noted, which equals the ratio of the diameter of the incident
laser spot on the material to the diameter of the filament. For
example, in one example range, the critical ratio may vary between
approximately 0.01 and 1000; in another example range, the critical
ratio may vary between approximately 0.01 and 10, in another
example range, the critical ratio may vary between approximately 10
and 50, in another example range, the critical ratio may vary
between approximately 50 and 500, and in another example range, the
critical ratio may vary between approximately 1 and 1000.
[0101] As shown in FIG. 2(b), the incident beam 270, focused by
final lens 275, can be made to pass through one or more
substantially transparent layers 272 above the desired target
location of the filament zone, without forming either a focus or
any filaments, with an external waist 290 that is formed beyond the
final layer 276. Filament(s) 280 then forms within the desired
layer or layers (274, 276) as a result of the critical fluence not
being achieved until that position along the incident path through
the target stack.
[0102] In order to control the fluence in the incident beam, the
power contained within the beam diameter at the surface of the
material can be altered, programmed in fact, by varying the size of
the beam diameter. In particular, the inventor has discovered a
correlation between the material, the critical fluence and filament
formation efficacy.
[0103] In one example implementation, the properties of the
filaments can be controlled by varying the first incident fluence
(the fluence on the first incident surface) and the energy
contained within each sub pulse. It will be understood that in
embodiments involving multiple surfaces upon which the laser is
incident (described further below), each surface will have its own
fluence, and each fluence will depend upon the first incident
fluence due to absorption and scattering.
[0104] It has been found that an incident spot size of
approximately 100 .mu.m leads to filaments with a diameter of 2.5
.mu.m and a length of several mm. Some characteristics of the
filament can be controlled by changing the pulse energy and spot
size ratio. Different materials have different propensities toward
extended filament formation.
[0105] In order for the filament to create a continuous and
substantially uniform channel, the energy intensity must also be
such that once deposited, the burst pulse induced intensity,
refreshed at the burst pulse frequency rate, is capable of forming
a shock wave of the requisite intensity to radially compress the
material. Once this phase change occurs (or in certain materials,
simply a density change), the filament zone functions as a cleavage
plane, either immediately, after some programmable delay or via the
application of a subsequent cleavage step. A suitable energy can be
empirically determined for a given material by producing filaments
at various beam energies, observing or measuring the filament
depth, and selecting a beam energy that produces filaments of a
suitable length. In one non-limiting example implementation, the
energy in the incident beam (the energy of all pulses in a burst)
may be between approximately 10 .mu.J and approximately 2000
.mu.J.
[0106] It will be understood that a wide range of laser beam
parameters may be varied to obtain filaments with different
characteristics and properties. Furthermore, the beam parameters
suitable for forming a filament with a given set of properties will
vary among different materials. The present disclosure provides
some example ranges of beam parameters that may be employed to form
filaments in some selected materials. Such example ranges are not
intended to be limiting, and it will be understood that other
ranges may be suitable for forming filaments with other properties
in the example materials, or in other materials.
[0107] It is to be understood that the burst repetition rate,
together with the translation speed of the material relative to the
incident beam, defines the spacing between neighboring filaments.
In one non-limiting example, this range may be from approximately 1
kHz to approximately 2 MHz.
[0108] As shown in FIGS. 1(c) to 1(e), the incident beam is focused
such that the focal volume is distributed over an extended region
within the sample. The distributed focal volume may be sufficiently
longer than the Rayleigh range obtained from a non-distributed
focusing element having a similar or equivalent numerical aperture.
For example, the focal region under distributed focusing may be
20%, 30%, 50%, 100%, 200%, 500%, 1000% or greater than the
corresponding Rayleigh range obtained without distributed
focusing.
[0109] The distributed focal volume may be sufficiently long that
the filament formed by the beam under distributed focusing
conditions is significantly longer than the filament that would be
formed from a non-distributed focusing element having a similar or
equivalent numerical aperture. For example, the filament formed
under distributed focusing may be 20%, 30%, 50%, 100%, 200%, 500%,
1000% or greater than the corresponding filament formed without
distributed focusing.
[0110] It is to be understood that the distributed focal assembly
may include one or more optical components/elements, such as an
optical train including two or more optical components. In one
embodiment, the distributed focal assembly is configured to focus
the optical beam in a non-distributed manner in one lateral
dimension, and to focus the beam in a distributed manner in the
other lateral dimension.
[0111] With appropriate beam focusing, manipulating the focal
length or beam expansion ratio, for instance, laser filaments can
terminate and cause the laser beam to exit the glass bottom surface
at high divergence angle such that laser machining or damage is
avoided at the bottom surface of the transparent plate. It is also
possible to create filaments in the middle of a multi-sheet stack,
without inducing damage in the sheets located above and below the
target sheet, but while damaging the top and bottom surface of the
target sheet, as described further below.
[0112] In some embodiments, the long filament length may be created
by a combination of negative and positive lenses that optimize the
filling of the apertures on each optical element, maintaining high
power efficiency within the optical train and maintains laser
intensity to radially compress the target material as has been
previously described.
Example Distributed Focusing Element: Aberrated Element
[0113] In some embodiments, the distributed focal assembly may
include one or more optical components configured to induce
aberrations in the focused optical beam, such that the focused
optical beam is focused in a distributed manner over a longitudinal
focal volume without forming a waist within the material.
[0114] One or more optical components may include spherical
aberration. In some embodiments, the distributed focal assembly may
include one or more aberrated optical components, and one or more
substantially non-aberrated optical components. In some
embodiments, aberrations are induced by the distributed focal
assembly in one dimension. In other embodiments, aberrations are
induced by the distributed focal assembly in two dimensions.
[0115] Long filaments can be created by the use of aberrated
optical assembly (one or more aberrated optical elements) such that
a long series of quasi-focal points can be achieved, even though no
beam-waist forms within the material due to the distortion wrought
by the target itself and the electric field heating created along
the incident axis. Creating a large spot with >1 .mu.m diameter
and creating at least one external beam waist (a "reservoir
region"), in front of the target material and/or behind it (as
illustrated in FIGS. 1(c)-(e)), enables "dumping" energy into focal
spots outside the target material or layer, where a beam waist is
formed in the air without forming a plasma channel in the air, and
no ablative work is accomplished.
[0116] The present embodiment provides a beam path with non-uniform
distribution of energy outside of the material, while also forming
a uniform beam path contained within the target material that
produces a filament containing no beam waist along its length.
[0117] Using one or more aberrated elements, one can choose to
distribute the energy in such a fashion as to avoid the beam waist
event within the material and yet create a uniform filament and
"dump" the extra energy into one, two or more external waist
regions, without forming an external plasma channel, in order to
maintaining the required fluence to promote filament formation,
long uniform modification and avoid optical breakdown. In other
words, the strong focus of the one or more aberrated elements may
be employed to act as an external beam dump, and the remaining rays
may be employed to create a strong burst pulse filament within the
material.
[0118] The aberration of an optical focusing component, assembly,
or system may be measured in waves (or fractions thereof, related
to the wavelength of light being used). For example, the aberration
may be specified according to the ratio of waves not arriving at
the same spatial point (or volume) as defined by the ideal lens to
the ratio of waves arriving at the same point. In some non-limiting
example implementations, the aberration in the optical focusing
assembly may be greater than approximately 0.1% aberration and less
than approximately 80% aberration.
[0119] The nature of the aberration can be variable provided the
energy density at the first incident surface stays above that
required to form filaments and below the optical breakdown
threshold for the target material. In one particular example, the
optical elements can be formed such that two primary foci form,
separated by at least a distance corresponding to the thickness of
the target material(s) or layer(s) (see, for example, FIG.
1(e)).
[0120] One example method for providing an aberrated optical
configuration is to employ an optical arrangement of at least two
elements, where one optical component is a normal field-corrected
scan lens, and where a second optical component is a corrective
window designed to deliver light focused as described above. FIG. 3
illustrates such an example lens arrangement employing first and
second lenses L1 and L2 for beam relaying and collimation, a
scanning mechanism 300 and a final focusing lens 305. Final
focusing lens 305 may be a telecentric lens. An aspheric plate 310
is provided below final lens 305 (although it may alternatively be
located before scanning mechanism 300) in order to generate an
aberrated focused beam, such that a portion of the beam forms an
initial waist in front of material 315. The separation between the
components may be determined based on the thickness of the target
substrate and the desired length of the filament zone. In some
example embodiments, the filament properties can be controlled or
prescribed by controlling the ratio of the lens focal lengths of
focusing lenses employed. For example, in some example
implementations, ratio of the focal lengths of L1/L2 can be -300 to
+300.
[0121] FIG. 4 illustrates an alternative example embodiment,
employing a specialized scan lens 320 (telecentric or
non-telecentric) without an aspheric plate. Specialized scan lens
320 is configured to induce an aberrated focused beam as described
above.
Burst Pulse Characteristics
[0122] It has been found by the present inventor that the use of
burst pulses in a distributed focusing configuration supports the
formation of long filaments (such as, but not limited to, filaments
having a length >15% of the total target material thickness, for
example, in glass applications, a length >100 .mu.m and up to
>10 mm), with homogenous properties (for example, filaments
having substantially the same diameter over a substantial length
thereof, and substantially the same diameter at the entrance and
exit faces of the material for filaments that traverse the material
thickness). The distributed focusing of a burst of pulses also
supports the formation of smooth surfaces after cleaving along a
filament array. For example, the beam and focusing conditions
disclosed herein have been employed to provide segmented samples
with cut face surface roughness (RMS) that is less than
approximately 10 .mu.m, and sometimes as low as 200 nm, or less.
Such filaments can be formed in brittle materials.
[0123] FIGS. 5(a)-(e) illustrates multiple embodiments showing the
temporal nature of the burst pulses 350 and the degree of control
the laser source can provide on the timing and sequencing of the
pulse. FIGS. 5(a) and 5(b) illustrates the optional control over
burst repetition rate 360 and inter-pulse temporal spacing 375. For
example, the timing between the pulses can be controlled by
manipulating the EO switch timing, to create various multiples of
the main oscillator signal, generating the variable pulse timing.
FIG. 5(b) is an example illustration showing the degree of
variability over which the pulses can be delivered and a schematic
of the pulses being generated within the laser head 370. It is to
be understood that in some embodiments, the pulses could be
modulated along the optical train, for example, by inclusion of an
optical switch or electro-optical switch to develop user selectable
pulse (and or pulse envelopes) profiles (rising or falling or
equal), changing the amplitude of the energy in the pulse (and or
pulse envelopes) and deciding to what degree it is divided among
smaller burst pulses where the total number of pulses is user
selectable.
[0124] As shown in FIG. 5(c), the user/operator may manipulate the
pulse profile 380 to control the process based on the desired
material properties of the parts generated in a system equipped
with such a laser and associated optics.
[0125] FIG. 5(d) illustrates the ability to control the net energy
delivered to the material, based on the integrated power in the
burst. FIG. 5(e) illustrates the ability to control the number of
pulses in a given burst 390. In one example implementation, the
burst of laser pulse is delivered to the material surface with a
pulse train containing between 2 and 20 sub pulses into which the
laser pulse is divided. This division may be created within the
laser head according to one of several known approaches.
[0126] It is to be understood that any one or more of the pulse
parameters shown in FIGS. 5(a)-5(e) may be employed to control the
formation of filaments within the processed material.
Materials
[0127] The filamentation methods disclosed herein may be employed
for the processing of a wide range of materials that are
transparent to the incident laser beam, including glasses,
crystals, selected ceramics, polymers, liquid-encapsulated devices,
multi-layer materials or devices, and assemblies of composite
materials. Substrates processed according to the methods disclosed
herein may include glasses, semiconductors, transparent ceramics,
polymers, transparent conductors, wide bandgap glasses, crystals,
crystalline quartz, diamond, and sapphire, rare earth formulations,
metal oxides for displays and amorphous oxides in polished or
unpolished condition with or without coatings.
[0128] It is further to be understood that the spectral range of
the incident laser beam is not limited to the visible spectrum, but
may also lie in the vacuum ultraviolet, ultraviolet, visible,
near-infrared, or infrared spectra. For example, silicon is
transparent to 1500 nm light but opaque to visible light. Thus,
laser filaments may be formed, for example, in silicon with short
pulse laser light generated at this 1500 nm wavelength either
directly (e.g. via erbium-doped glass lasers) or by nonlinear
mixing (e.g. via optical parametric amplification) in crystals or
other nonlinear medium. Suitable performance can be expected with
light ranging from 1200-3000 nm for a wide array of brittle
materials, such as Si, SiC GaAs, GaN, and other compound and
complex compound semiconductors (for example, II-VI and similar
band gap engineered materials) as well as display-related
compounds, such as ITO, IPS, IGZO, etc.
Pulse Energies and Power
[0129] In order to form filaments and to sustain self-focusing, the
pulse energy is selected to be lie within the nonlinear regime,
such that burst generated filament formation is possible. According
to one non-limiting example, it has been found that for the
processing of soda lime glasses, pulse energies between
approximately 10 .mu.J and 2 mJ are suitable for achieving the
electric field intensity needed to reach a state where it can be
sustained for self-focusing to occur. In some example
implementations, the average power of the laser may lie within
approximately 3 W and 150 W (or more), although it will be
understood that the average power required for filament formation
will depend on the pulse energies, number of pulses per burst, and
repetition rate, for example.
[0130] In one example embodiment, the pulse repetition rate may
range between 10 kHz and 8 MHz in terms of the pulse repetition
frequency as defined by the pulse picked oscillator. These may be
subsequently down-selected into bursts of less energy and delivered
to the material with sub-pulse spacing equal to 1 fs or greater, up
to 1 millisecond. In some example embodiments, the laser possess a
beam quality, M.sup.2, of less than approximately 5. An M.sup.2 of
approximately 1 may be employed, for example, in embodiments where
the optical components are configured to create more than one focal
point along the axis, and while less strict M.sup.2 embodiments can
be tolerated to the extent that the downstream optics are
compensating for the beam's native shape. In some example
embodiments, in which a filament is to be formed throughout the
thickness of the material, the laser beam should be transmitted
through the material (including any intervening gaps of
inhomogeneous and dissimilar materials) with a transmitter power
exceeding a pre-selected threshold (for example, at least
approximately 50%) in order to provide sufficient luminous
intensity along the beam path.
Collimation, Focal Length, Clear Aperture
[0131] In some embodiments, the optical train of the system
includes one or more optical components for collimating the beam
prior to focusing, in order to accommodate a variable path length
between the distributed focusing element and the laser source. In
some example embodiments, the numerical aperture of the collimating
components is between approximately 0.1 and 0.88 NA, with an
effective focal length between approximately 4.5 mm and 2.0 m. In
some example embodiments, the diameter of the clear aperture may be
between approximately 2 and 10 mm.
Use of Regenerative Amplifier
[0132] In one embodiment, a regenerative amplifier is employed to
provide a flexible apparatus, as the regenerative amplifier may be
easily reconfigured to change the burst train characteristics for
filament formation. For example, the regenerative amplifier may be
reconfigured for a subsequent exposure step for singulation (based
on another source for heat or cold to provide a thermal gradient)
in the case of incompletely separated materials following the first
laser exposure. Such a laser system can produce full or partial
length filaments tailored to the particular application with
variable or constant pulse timing, and auto focus coordinated with
beam timing and speed such that the velocity of the part is
invariant throughout processing--in effect keeping the spacing
between adjacent filament zones constant. This in turn enables
singulation with the lowest cut-face roughness presently available
in a laser process, namely approximately 1-3 .mu.m immediately
following cutting, compared with 10-100 .mu.m as obtained using
conventional laser cutting processes. Auto focus can be achieved by
pre-scanning the part, sensing the head height in situ (for
example, optically) or determining the position using a machine
vision system.
[0133] The regenerative amplifier design enables precise timing
control in terms of how many round trips are taken prior to
ejection of the pulse. Pulse-to-pulse or burst-to-burst timing can
be manipulated with stage speed to provide very finely tuned facet
edges (roughness, for instance) according to the specific
application. In particular, the laser system is especially well
suited for glass parts with complex shapes or mother glass sheets
where a complex spline is present. In one example implementation,
the Rofin MPS platform may be readily modified to include the above
embodiments.
Filament Formation Mechanisms
[0134] The present methods for forming filaments therefore support
new material processing applications for transparent materials that
were hitherto not possible. Although there have been previous
investigations into filament formation in solid materials, the
present disclosure represents the first reduction to practice
wherein extremely long filaments are generated, by virtue of burst
mode timing and distributed focusing.
[0135] Known laser processing methods, such as those employed in
the Stealth Dicing and the Accuscribe systems, are driven by
modifications such as those by the Yoshino et al. ["Micromachining
with a High repetition Rate Femtosecond Fiber Laser" (2008),
Journal of Laser Micro/Nanoengineering Vol. 3, No. 3 pgs. 157-162],
are processes governed primarily by optical breakdown, where the
primary mode of material removal is ablation via small explosions
creating voids of variable lateral dimensions and of limited
longitudinal length. Optical breakdown is the result of a tightly
focused laser beam inside a transparent medium that forms a
localized and dense plasma around the geometrical focus created by
the material to be singulated. The plasma generation mechanism is
based on initial multi-photon excitation of electrons; followed by
inverse Bremsstrahlung, impact ionization, and electron avalanche
processes. The Columbic explosion is responsible for creation of
the localized voids and other modifications described in the
literature. Such processes and systems, underscore the refractive
index and void formation processes described above [U.S. Pat. No.
6,154,593; SPIE Proceedings 6881-46,], and form the basis of most
short-pulse laser applications for material processing. In this
optical breakdown domain, the singulation, dicing, scribing,
cleaving, cutting, and facet treatment of transparent materials has
disadvantages such as slow processing speed, generation of cracks,
low-strength parts, contamination by ablation debris, and large
kerf width--all of which require further processing to complete the
part's journey to assembly into handheld electronic devices such as
computers, tablets and or phones.
[0136] In contrast, laser filamentation processing and the systems
disclosed herein overcome the disadvantages of the previously known
methods for internal laser processing of transparent materials, and
can avoid ablation or surface damage (if desired), dramatically
reduce kerf width, avoid crack generation, and speed processing
times for such scribing applications. Further, high repetition rate
lasers equipped with regenerative amplifiers and fast electro-optic
switching allow for the enhancement of the formation of laser beam
filaments with minimal heat accumulation and other transient
responses of the material on time scales much faster than thermal
diffusion out of the focal volume (typically <10 microseconds).
The focal volume produced according to the methods of the present
disclosure can be manipulated by optical components in the beam
path to extend many times the calculated depth of focus (DOF).
[0137] As shown in the examples below, using picosecond pulse
bursts, the present inventor has demonstrated that when the pulse
focuses in a distributed manner, it remains confined for an axial
distance ranging from approximately 20 .mu.process conditions
chosen. This enables dense, localized sonic pressure formation
useful for via drilling with substantially zero taper, in materials
where substantially non-ablative processes are responsible for
removing or compressing most of the material.
[0138] Without intending to be limited by theory, it is believed
that the filaments are produced by weak focusing, high intensity
short duration laser light, which can self-focus by the nonlinear
Kerr effect, thus forming a so-called filament. This high
spatio-temporal localization of the light field can deposit laser
energy in a long narrow channel, while also being associated with
other complex nonlinear propagation effects such as white light
generation and formation of dynamic ring radiation structures
surrounding this localized radiation.
[0139] Heating by the rapid laser pulses temporarily lowers the
refractive index in the center of the beam path causing the beam to
defocus and break up the filament. The dynamic balance between Kerr
effect self-focusing and index shifting modulated defocusing can
lead to multiple re-focused laser interaction filaments through to
formation of a stable filament
[0140] Unlike known methods of filament modification, embodiments
disclosed herein support the formation of continuous filaments that
are extendable over a wide range of depths within a substrate. For
example, in some embodiments, filaments are produced such that they
are substantially continuous in nature along their longitudinal
axis. This is to be contrasted with known filament processing
methods that produce disconnected, discreet damage centers with
insufficient radiation intensity (laser fluence or power) to affect
any changes in the material. Accordingly, embodiments described
below include methods for forming a continuous zone of photo
acoustic compression along the path of a processing beam, such that
the material properties of the substrate differ compared to regions
not exposed to this phenomenon. In some embodiments, a continuous
via is formed within the substrate by a radially uniform
compression of material within the substrate.
[0141] On the simplest level, the filamentation process is believed
to depend mainly on two competing processes. First, the spatial
intensity profile of the laser pulse acts like a focusing lens due
to the nonlinear optical Kerr effect. This causes the beam to
self-focus, resulting in an increase of the peak intensity. This
effect is limited and balanced by increasing diffraction as the
diameter decreases until a stable beam diameter is reached that can
propagate distances many times longer than that expected from a
simple calculation of the confocal beam parameter (or depth of
focus) from this spot size. The other key distinguishing feature is
the extremely small filament size achieved by this technique.
[0142] This regime of filament formation is new, as experiments
described herein have shown filaments with a length well in excess
those obtained using prior filament forming methods. For example,
according to selected embodiments of the present disclosure,
radially compressive filaments--wherein the material is compressed
revealing a cylindrical void extending through the entire thickness
of the material--with a length of 10 mm or longer may be formed in
suitably transparent media. Without intending to be limited by
theory, the mechanism appears to involve a shockwave compression
created by rapid heating via tightly spaced successive pulses of
laser light (the burst pulse phenomenon) centered along the beam
propagation axis in the material. Provided that the filament
forming beam has sufficient intensity, it is capable of crossing
air gaps and gaps of material with substantially lower indices of
refraction (real and complex) and forming filaments when entering
other transparent layers. White light generation and x-ray emission
confirm the highly non-linear processes at work. Gurovich and Fel
[ArXiv 1106.5980v1], writing about related phenomena, observed
shock wave formation in the presence of ionic and electron
collisions in a medium.
[0143] A further clue to the photoacoustic nature of the filament
forming process resides in deep ablative drilling studies carried
out by Paul, et al. [Proceedings SPIE vol. 6107, 610709-1 (2006)]
wherein their method of measurement involved a photoacoustic signal
generated by void formation using multiple laser pulses. The
present approach, involving the distributed focusing of bursts,
appears to generate an even more intense photoacoustic signal while
avoiding the plasma formation and material ablation common to other
techniques. Furthermore, although a mildly thermal entrance and
exit are formed at the initial and final surfaces of the target
materials, the internal surfaces of the filament have been shown to
be substantially free from any disturbances associated with
ablative micromachining.
[0144] It is further noted that the extreme pressures associated
with solid state machining using plasma assisted laser ablation are
reported upon by Kudryashov, et al. [Appl. Phys. Lett. 98, 254102
(2011)]. In their work they report plasma temperatures of 90eV with
corresponding pressures of 110 GPa. At this level, there is
sufficient energy to setup a compression wave inside the material.
The present approach utilizes much closer burst spacing and that
has the advantage of creating even hotter beam axis center over a
shorter period of time, where the thermal shockwave outpaces any
latent thermal effects rendering compression modified environs
faster than the formation of any heat affected zone (HAZ) or melt.
Inspection of the radiative processes that occur according to the
present methods illustrates that not only is Bremsstrahlung
observed, but also ultrasonic transients as well. By virtue of
optical adjustment and changes in geometric focus location, the
extent and the "stop-start" characteristics of this photoacoustic
modification in the material can be controlled--even if multiple
layers with gaps comprise the target material. The character of the
edges so produced, is fundamentally distinct from those produced
using slower, ablative processes that do not depend upon uniform
modification of the physical and chemical properties of the
materials so exposed.
Singulation
[0145] In some embodiments, the aforementioned filament forming
methods and apparatus is employed for the singulation of
transparent materials. The laser induced index change created
during the aforementioned filamentation process may be employed to
produce a filament array for singulating parts with substantially
or effectively no weight loss in the material being singulated.
[0146] In one example implementation, a method of singulation may
include loading, aligning, mapping and irradiating the substrate
with one or more bursts of pulses of a laser beam having a
distributed focus, wherein the substrate is substantially
transparent to the laser beam, and wherein the one or more of
pulses have an energy and pulse duration selected to produce a
filament.
[0147] An array of filaments is formed by translating the material
substrate relative to the focused laser beam to irradiate the
substrate and produce an additional filament at one or more
additional locations. The filaments array thus defines an
internally scribed path for cleaving the substrate. These arrays
can exist in one or more dimensions around the line of translation,
and can be formed in straight or curved profiles, where an example
curved profile 395 is shown in FIG. 6. The filaments may be formed
such that they extend over a substantial fraction (for example,
more than approximately 15% of target material, and typically
greater than 50 .mu.m, or in some cases, greater than 1 mm or even
10 mm) of the target material.
[0148] In some embodiments, the methods disclosed herein involve
lateral translation of the focused laser beam to form an array of
closely positioned filament-induced modification tracks. This
filament array defines a pseudo-continuous curtain of modifications
inside the transparent medium without generating laser ablation
damage at either of the top or bottom surfaces (unless specifically
desired). This curtain renders the irradiated material highly
susceptible to cleaving when only very slight pressure (force) is
applied, or may spontaneously cleave under internal stress. The
cleaved facets may be devoid of ablation debris, show minimal or no
microcracks and microvents, and accurately follow the flexible
curvilinear or straight path marked internally by the laser with
only very small kerf width as defined by the self-focused beam
waist.
[0149] In some embodiments, for selected materials and processing
conditions, once a filamentation array is formed in the transparent
substrate, only small mechanical pressure may be required to cleave
the substrate into two parts on a surface shape that is precisely
defined by the internal laser-filamentation curtain. In certain
materials and especially in chemically strengthened glasses, the
separation event may be spontaneous, requiring no further steps to
affect singulation.
[0150] The user selectable process conditions may be chosen to vary
the time interval between exposure and separation from 0 seconds
(immediately separates upon exposure) to an infinite number of
seconds (requires a follow-on step of some kind to complete the
singulation process).
[0151] In some embodiments, multiple angle cuts may be employed to
create, for example, using 3 sequential cuts, a chamfered or
faceted edge of the material being singulated, greatly reducing
production time and costs. In one example implementation, this may
be performed as an X pass process, where X represents the number of
angled sides or edges.
[0152] In some embodiments, the substrate may be cleaved using one
or more of the following: additional laser processing steps,
physical heating, cooling, and/or mechanical means. It has been
found that when a filament array is formed according to the
aforementioned filament generation methods, the cleavage step
requires far less applied force and produced superior edge quality
relative to known approaches. Depending upon the nature of the
material, the process of scribing and separation (singulation) may
take place in a single step requiring no further exposure to any
forces or thermo-mechanical tension.
[0153] In some embodiments, the filament array may be formed such
that the filaments touch (e.g. are tangential to one another; e.g.
for filaments circular in the distribution about the beam center or
axis of incidence) or are separated by a user-selectable distance,
which can be variable. In some embodiments, the distance between
filaments forming the array is constant. In other examples, the
properties of the material may be such that improved singulation is
obtained for filament arrays that are formed with variable spacing
along the perimeter of the shape to be removed or cut. The suitable
filament spacing for efficient cleaving will thus generally be
determined by the characteristics of the material and the
requirements of the application, including the physical/electrical
properties of the singulated part. In addition to the varying beam
parameters such as pulse width, pulse-to-pulse separation within a
burst, burst repetition rate, wavelength, and pulse energy, the
polarization state may be varied by utilizing a polarization
rotation element (such as a waveplate) and varying the degree of
rotation from about 1 degree to about 80 degrees and from random to
linear to circular or to some mixture of the two during processing
as required by the desired end result. It is observed that edge
quality and therefore post-singulation edge strength can be
modulated by using this technique, as well as others.
[0154] As noted above, the laser source may include a burst pulse
laser, for example, a pulsed laser with a regenerative amplifier,
but other burst pulse systems may be employed. For example, in one
embodiment, a multi-pass amplifier equipped with a fast
electro-optic switch, either internally or externally mounted, may
be employed to deliver the beam to the substrate via computer
controlled steering (and optional focus mechanisms and polarization
control), and the substrate may be translated relative to the
focused laser beam with a constant velocity along the path of
exposure. In some embodiments, the tangential velocity may be
constant when forming curved portions of a filament array, such as
when forming filament arrays at corners, such that the array of
filaments so created is constant in its spatial irradiance, dose
and temporal characteristics. In other embodiments, computer
control may be employed to translate the beam relative to a fixed
substrate. In other embodiments, the computer control may be
employed to control the motion of both the beam and the
substrate.
[0155] For example, the translation rate employed to form the
filament array may be determined according to the velocity of a
simple linear stage supporting the substrate, or may be determined
according to the combination of the stage velocity and the beam
velocity, in the case of a scanning system based on telecentric or
non-telecentric final objectives, depending upon the desired
process.
[0156] The translation rate may be selected to produce
user-selectable filament spacing on a micron scale, depending, for
example, upon the desired characteristics (physical, optical,
electrical, chemical, thermal, etc.) of the materials thus
singulated. Accordingly, by varying one or more of the processing
parameters in real-time, filament arrays with locally controlled or
tailored properties may be formed--i.e. arrays of filaments where
the properties of the filaments vary spatially among different
regions of the material, thereby spatially modifying properties of
the material itself. This aspect of laser processing has not been
achievable using previously known laser singulation approaches and
systems. Indeed, the present method of forming arrays with
locally-controlled properties may be employed for a wide range of
applications. A non-limiting list of example properties that may be
locally controlled according to the present filamentation process
include electrical performance, light output and post-singulation
break strength.
[0157] Properties of the beam of laser pulses employed to form the
filament array may varied according to pre-selected, and computer
controlled, process parameters, in order to provide sufficient beam
intensity within the substrate to cause self-focusing of the laser
beam. For example, the laser beam may be controlled such that the
filament is formed at all points along the beam axis within the
material to be processed. In other words, the beam properties may
be controlled as to exceed a particular characteristic energy level
to create acoustic compression within the substrate(s), thus
rendering it (them) singulated or ready to be so, depending upon
the nature of the target materials.
[0158] In some embodiments, the filament arrays are formed by
filaments that are substantially symmetric about their longitudinal
axis (usually the incident axis of the laser beam). The length of
the filament may be controllable (for example, from approximately
10 .mu.m to over 10 mm) by changing the process parameters, such as
power, focusing characteristics and beam shape, which are
controlled machine parameters. Varying such parameters may result
in a change in the characteristics associated with the
photoacoustic modification created in the material.
[0159] An important distinction exists between chirped pulses and
the burst pulses employed in the methods of the present disclosure,
both in terms of how they are generated, but also in terms of the
energy characteristics of each, with the burst machining method
exhibiting much greater flexibility in processing, particularly
when coupled with a scanner and the appropriate focusing optics to
render telecentric behavior over a defined field size.
[0160] In one embodiment, a system is provided for auto-focusing of
the filament forming beam in real-time. For example, in some
implementations, the beam can be moved at high rates of speed using
galvanometers and/or acoustic optical deflectors to steer the beam
in a coordinated manner fully under computer control.
[0161] FIG. 7(a) presents an example configuration of a laser
processing system for forming filament arrays, including an
ultrafast laser 500 supplying a train of burst-mode pulses,
preferably with a pulsewidth less than 100 picoseconds, equipped
with a suitable collection of beam steering optics, such that the
beam or beams can be delivered to a multi-axis rotation and
translation stage including: a rotational stage in the XY plane
(theta, .theta.), a 3D XYZ translational stage, and an axis for
tipping the beam or the part relative to the X axis (gamma, y) in a
coordinated control architecture. In the example embodiment shown,
the beam is manipulated by conditioning optic 502 (e.g. a positive
or negative lens or combination of lenses capable of delivering a
weakly focused spot that can be further conditioned or
manipulated), beam sampling mirror 504, power meter 506, X-Y
scanner 505, final focusing lens 520, and servo-controlled stage
510 for positioning workpiece 522. Control and processing unit 550,
which is described in further detail below, is employed for the
control of the laser filamentation and/or singulation apparatus
embodiments disclosed herein. Filament position and depth may be
controlled (as illustrated in FIG. 1(f)) using an auto-focus
configuration (e.g. using a position-sensing device) that maintains
a constant working distance.
[0162] FIG. 7(b) provides an example implementation of control and
processing unit 550, which includes one or more processors 552 (for
example, a CPU/microprocessor), bus 554, memory 556, which may
include random access memory (RAM)and/or read only memory (ROM),
one or more optional internal storage devices 558 (e.g. a hard disk
drive, compact disk drive or internal flash memory), a power supply
560, one more optional communications interfaces 562, optional
external storage 564, an optional display 566, and various optional
input/output devices and/or interfaces 568 (e.g., a receiver, a
transmitter, a speaker, an imaging sensor, such as those used in a
digital still camera or digital video camera, an output port, a
user input device, such as a keyboard, a keypad, a mouse, a
position tracked stylus, a position tracked probe, a foot switch,
and/or a microphone for capturing speech commands). Control and
processing unit 550 is interfaced with one or more of laser system
500, laser scanning/position system 505, the positioning system for
the processed material 510, and one or more metrology devices or
systems 511, such as one or more metrology sensors or imaging
devices.
[0163] Although only one of each component is illustrated in FIG.
7(b), any number of each component can be included in the control
and processing unit 550. For example, a computer typically contains
a number of different data storage media. Furthermore, although bus
554 is depicted as a single connection between all of the
components, it will be appreciated that the bus 554 may represent
one or more circuits, devices or communication channels which link
two or more of the components. For example, in personal computers,
bus 554 often includes or is a motherboard.
[0164] In one embodiment, control and processing unit 550 may be,
or include, a general purpose computer or any other hardware
equivalents. Control and processing unit 550 may also be
implemented as one or more physical devices that are coupled to
processor 552 through one of more communications channels or
interfaces. For example, control and processing unit 550 can be
implemented using application specific integrated circuits (ASICs).
Alternatively, control and processing unit 550 can be implemented
as a combination of hardware and software, where the software is
loaded into the processor from the memory or over a network
connection.
[0165] Control and processing unit 550 may be programmed with a set
of instructions which when executed in the processor causes the
system to perform one or more methods described in the disclosure.
Control and processing unit 550 may include many more or less
components than those shown.
[0166] While some embodiments have been described in the context of
fully functioning computers and computer systems, those skilled in
the art will appreciate that various embodiments are capable of
being distributed as a program product in a variety of forms and
are capable of being applied regardless of the particular type of
machine or computer readable media used to actually effect the
distribution.
[0167] A computer readable medium can be used to store software and
data which when executed by a data processing system causes the
system to perform various methods. The executable software and data
can be stored in various places including for example ROM, volatile
RAM, non-volatile memory and/or cache. Portions of this software
and/or data can be stored in any one of these storage devices. In
general, a machine readable medium includes any mechanism that
provides (i.e., stores and/or transmits) information in a form
accessible by a machine (e.g., a computer, network device, personal
digital assistant, manufacturing tool, any device with a set of one
or more processors, etc.).
[0168] Examples of computer-readable media include but are not
limited to recordable and non-recordable type media such as
volatile and non-volatile memory devices, read only memory (ROM),
random access memory (RAM), flash memory devices, floppy and other
removable disks, magnetic disk storage media, optical storage media
(e.g., compact discs (CDs),digital versatile disks (DVDs), etc.),
among others. The instructions can be embodied in digital and
analog communication links for electrical, optical, acoustical or
other forms of propagated signals, such as carrier waves, infrared
signals, digital signals, and the like.
[0169] Some aspects of the present disclosure can be embodied, at
least in part, in software. That is, the techniques can be carried
out in a computer system or other data processing system in
response to its processor, such as a microprocessor, executing
sequences of instructions contained in a memory, such as ROM,
volatile RAM, non-volatile memory, cache, magnetic and optical
disks, or a remote storage device. Further, the instructions can be
downloaded into a computing device over a data network in a form of
compiled and linked version. Alternatively, the logic to perform
the processes as discussed above could be implemented in additional
computer and/or machine readable media, such as discrete hardware
components as large-scale integrated circuits (LSI's),
application-specific integrated circuits (ASIC's), or firmware such
as electrically erasable programmable read-only memory (EEPROM's)
and field-programmable gate arrays (FPGAs).
[0170] FIGS. 8(a) and 8(b) illustrate example embodiments showing
the ability to control multiple axes via the control of stage 605,
using (a) non-telecentric 600 and (b) telecentric 602 lenses. In
the case of a non-telecentric lens 600, angled filament paths can
be created by the natural distortion present in a
non-field-corrected lens. Rotation about the X (gamma) axis may be
performed to provide angled filament modified zones (612, 614)
within workpiece 610 using normally incident light. It is to be
understood that other optical configurations are possible.
[0171] FIG. 8(c) illustrates an alternative embodiment in which the
stage supporting the material being processed is rotated to produce
filaments that are angled relative to the material surface. This
embodiment is configured to present a tilted sample with respect to
the beam incidence angle for producing results similar to apparatus
embodiments employing a scan lens.
[0172] FIG. 9 illustrates the layout of an example laser system 700
suitable for part singulation. Laser 765 is a laser system capable
of delivering burst pulses, for example, with energies in the range
of approximately 1 uJ-50 mJ, at a repetition rate of up to
approximately 2.5 MHz.
[0173] Granite riser 702 is designed to be a reactive mass for
dampening mechanical vibrations, as is commonly used in industry.
This could be a bridge on which the optics above the stage can
translate along one axis, X or Y relative to the stage, and in
coordination with it. Granite base 704 provides a reactive mass
that may support any or all components of the system. In some
embodiments, handling apparatus 740 is vibrationally decoupled from
the system for stability reasons.
[0174] Z axis motor 710 is provided for translating the optics
(conditioning and focusing and scan optics if needed) in the Z axis
relative to the stage. This motion can be coordinated with the XY
stage and X or Y motion in the overhead granite bridge, and the XY
motion of the stage on the granite base, which holds the sample
material to be processed.
[0175] Stages 720 include, for example, XY and Theta stages with a
tilt axis, gamma ("yaw"). The motion of stages 720 is coordinated
by a control computing system, for example, to a create part shape
desired from a larger mother sheet. Metrology apparatus 730
provides post processing or preprocessing (or both) measurements,
for example, for mapping, sizing, and/or checking edges quality
post cut.
[0176] FIGS. 10(a)-(d) show the angled cut out approach for making
internal features with angled edges requiring no post singulation
processing to achieve the desired angular result. In FIGS.
10(a)-(c), the beam track is accomplished via rotation around the
theta axis 755 with a fixed incidence angle from the laser beam,
equal to the slope desired on the final part edge 765. This
non-limiting embodiment enables angled cutting and translation of
the rotary stage as an apparatus to support the creation of complex
cutouts via filament arrays.
[0177] FIG. 10(d) illustrates an example implementation of the
formation of a chamfered part 770 via processing with multiple
filament forming beams 775 at different angles. It is to be
understood that the beam and filament paths can be controlled to
form chamfered or bevel edges of various degrees. In the case of
concerted (parallel) formation, the beam can be split and directed
through optics to achieve multiple beam paths arriving at the
target exhibiting angles of incidence other than normal, along with
a normally incident beam, such that a three-face edge or chamfer is
created.
[0178] It is to be understood that chamfers can be created with two
or more faces, depending, for example, on the degree of splitting
tolerated by the process. Some example configurations are
illustrated in FIG. 10(e).
[0179] FIGS. 10(f) to 10(n) illustrate the processing of sodalime
glass using multiple cuts to obtain chamfered edges in a number of
different configurations. In FIGS. 10(f) and 10(h), sodalime glass
substrates with a thickness of 1.6 mm, a scan speed of 500 mm/sec,
and angles of incidence of 12 degrees are processed with two beams,
where one side is scribed, the glass substrate is flipped, and the
second side is again scribed. The corresponding post-cleaved
structures are shown in FIGS. 10(g) and 10(i), respectively. FIGS.
10(j)-10(l) show the edge view of the chamfered facet at multiple
zoom levels.
[0180] In some embodiments, as described below, the laser
processing system can be configured such that one laser (with beam
splitting optics) can perform both scribing steps
simultaneously.
[0181] FIGS. 10(m), (n) and (o) show the processing of a part with
three cuts to obtain an edge having an intermediate vertical edge
and two chamfered edges, using conditions similar to those
described above. In this case, the substrate was processed at one
side at an incident angle of 12 degrees; the substrate was flipped
and processed on the other side at an incident angle of 12 degrees,
and then the incident angle was changed to zero degrees for the
vertical processing step. As noted above, it will be understood
that these processing steps may be performed simultaneously using a
single laser with appropriate beam splitting, provided that the
laser has sufficient power. It has been found, for example, that a
laser with an average power of approximately 75 W is sufficient to
perform all processing steps simultaneously.
[0182] The aforementioned apparatus, with multi-axis rotational and
translational control, may be employed for the purpose of bringing
the beam on to the work piece(s) at variable focus positions,
non-normal angles of incidence and at variable, recipe controlled
positions to create curvilinear zones of filament arrays, for the
purpose of singulating the parts into component pieces, cutting out
closed-form shapes and creating products such as cover glass for
mobile devices with high (e.g. greater than approximately 30 MPa)
as-singulated break strength, which is presently not possible using
the techniques currently employed by the device manufacturers.
[0183] Those skilled in the art will recognize that all of these
axes are not required for all applications and that some
applications will benefit from having simpler system constructions.
Furthermore, it is understood that the apparatus shown is but one
example implementation of the embodiments of the present
disclosure, and that such embodiments may be varied, modified or
hybridized for a wide variety of substrates, applications and part
presentation schemes without departing from the scope of the
present disclosure.
[0184] FIG. 11(a) illustrates a schematic of an example rotary
processing system configuration 800 for use in high volume
manufacturing according to the methods disclosed herein. The
example system includes a rotary stage to transport parts to the
various stations to eliminate load and unload overhead, as well as
post-process metrology and pre-process mapping.
[0185] Mapping subsystem 805 is provided to determine, for example,
bow in sample, size, part orientation, or curvature over which
singulation is to be affected. Processing station 810 performs
singulation, texturing, drilling, etc., according to the methods
disclosed herein. Metrology station 815 performs measurements such
as measuring a part against a stored part profile or inspection of
edges. Load and unload station 820 is configured to bring parts
into and out of the processing station. Linear slide station 830
provides rapid part exchange with minimal moving parts and cost.
The stage rotates (as shown at 825) about the central axis of the
system, transporting parts from station to station.
[0186] FIG. 11(b) illustrates an example implementation of
processing stage 810, providing multi-substrate, multi-beam, and
multi-laser head capability. This enables multiplexing of the
process stage, such that multiple samples can be processed in
concert. In the example embodiment shown in the Figure, beams
emitted by lasers 832 and 834 are split and directed towards
respective X, Y, Z, 2 and 8 stages before being directed onto parts
that are positionable at four locations by handler shuttle 838.
[0187] FIGS. 11(c)-11(f) illustrate another example implementation
involving a dual laser beam system for the processing of four
wafers. As shown in FIG. 11(c), four wafers (1-4) are spaced in
four quadrants for processing, with a controllable gap 1100 between
wafers. Referring to FIG. 11(d), two incident and laterally spaced
(in the x-direction) burst laser beams are formed from a single
laser system using a movable beam delivery system including
beamsplitter 1105, mirror 1110, and lenses 1115. The beams are each
focused onto separate wafers, (e.g. wafers 1 and 3, or 2 and 4),
where the separate wafers are supported by a common support
1120.
[0188] As shown in FIG. 11(e), the beam delivery system is
translated relative to the wafers in the Y-direction in order to
scribe the wafers via filament processing according to the present
disclosure. After having completed a scribe along a given line in
the Y-direction, the relative position between the wafers and the
beam delivery system in the X-direction is changed, and the wafers
are once again scribed in the Y-direction. This process is repeated
to facilitate laser processing along all required lines in the
Y-direction. It will be appreciated that the speed of wafer
processing is more dependent on the speed of the Y-stage than that
of the X-stage. Accordingly, in some embodiments, the Y-stage may
be controlled by a motor having a higher speed than that of the
motor controlling the X-stage.
[0189] FIG. 11(e) illustrates two example scribe lines in the
Y-direction, namely first scribed lines 1130 and mid-wafer scribed
lines 1140. Also indicated in the Figure are acceleration region
1150, where the relative speed between the beam delivery system and
the wafers increases prior to scribing, and deceleration region
1160, where the relative speed between the beam delivery system and
the wafers decreases after scribing.
[0190] The laser pulses may be blocked or attenuated when the laser
beam is not positioned over a scribe line on a wafer. Two example
laser power time dependent profiles are shown in FIG. 11(f), where
temporal profile (i) corresponds to first scribe line 1130 in FIG.
11(d), while where temporal profile (ii) corresponds to mid-wafer
scribe line 1140 in FIG. 11(d).
[0191] After all scribe lines in the Y-direction have been formed,
the wafers are then rotated by 90 degrees relative to the beam
delivery system, and the process is repeated to scribe all required
lines in the X-direction, as shown in FIG. 11(f).
[0192] In some embodiments, the polarization of the incident laser
pulses may be horizontal, vertical, or circular, as described in
further detail below. For example, it has been found for some
materials that employing horizontal polarization during scribing
result in improved scribing efficiency.
[0193] FIG. 12 illustrates the processing of a transparent
substrate to produce a part 840 with complex edge, internal cutout
features 842 and rounded corners. As shown in the Figure, the
corners 844 may be fixed or may vary in radius. The Figure also
demonstrates the ability to form arbitrary curvilinear arrays of
filaments as well for the applications of closed form shapes and
internal features.
[0194] In another embodiment, the final lens can be of large clear
aperture, for example, approximately 50 mm, which may be employed
to generate an approximately 25 mm.times.25 mm field up to 100
mm.times.100 mm, and field uncorrected, such that angular
distortions create angles of incidence other than normal, and such
that the singulated product has angled or chamfered surfaces
immediately after processing.
[0195] In some embodiments, there may exist a certain degree of
programmability in the burst pulses as they are created, relative
to the rising or falling energy level of each sub-pulse comprising
the burst. In other words, a degree of control is given the user by
choosing the rising or falling waveform profile of the burst
envelope of pulses. This pulse energy profile modulation, allows
the user to determine the rate of heat formation and therefore the
rate at which the material undergoes acoustic compression. Such a
method therefore allows for acoustic compression control using a
burst pulse profile modulated by pulse to pulse control.
[0196] As noted below, the present singulation methods may be
employed for achieving a higher yield process than conventional
approaches, since, in some embodiments, singulation may be achieved
in a single process step and at higher bend strength with
conventionally singulated parts.
[0197] The energy characteristics of the burst pulses, focused in a
spatially distributed manner, enable the apparatus to deliver a
material-compressing wave of substantially uniform density over an
extended length in the material, for single layer or multi-layer
materials, provided that sufficient energy exists in the beam after
each layer or surface has been traversed. As further described
below, in example embodiments involving a multi-material layered
stack with gaps formed by air, gases, vacuum, or other materials
with substantially different indices of refraction (e.g. complex
and/or real) between some or all of the intervening layers,
multi-layer filamentation can occur, again, provided that
sufficient energy and focusing conditions are employed. As noted
above, the spacing of the filament arrays can be varied by changing
the relative rate of translation between the beam and the work.
[0198] As noted above, the filamentation modification methods
disclosed above enable rapid and low-damage singulation, dicing,
scribing, cleaving, cutting, and facet treatment of transparent
materials. In some embodiments, filament-based singulation may be
performed on flat or curved materials, and thus may be employed in
numerous manufacturing applications. The method generally applies
to any transparent medium in which a filament may form via a burst
of ultrafast laser pulses. The apparatus provided according to the
embodiments disclosed below may provide a means for coordinated
beam motion in multiple axes, for example, extending over and
around curved surfaces, and with optional auto-focusing elements
for programmable control (e.g. according to a pre-selected recipe)
of the end product's characteristics, such as bend strength, edge
roughness, electrical or optical efficiency, cost of production,
and as-processed characteristics, such as edge shape and
texture.
[0199] In some example implementations, for glass materials
(including, for example, aluminosilicates, sodium silicates, doped
dielectric oxides, and similar compounds or stoichiometries),
singulation methods disclosed herein may be employed for dicing or
cleaving of liquid crystal display (LCD), flat panel display (FPD),
organic display (OLED), glass plates, multilayer thin glass plates,
autoglass, tubing, display cover glasses, protective windows,
safety glass, laminated structures, architectural glass, electro
chromic and otherwise, biochips, optical sensors, planar lightwave
circuits, optical fibers, laboratory, industrial and household
glassware, and art work.
[0200] For semiconductor materials (such as silicon, III-V, and
other semiconductor materials, particularly, those in thin wafer
form), singulation methods disclosed herein may be employed for the
processing of microelectronic chips, memory chips, sensor chips,
light emitting diodes (LED), laser diodes (LD), vertical cavity
surface emitting laser (VCSEL) and other optoelectronic
devices.
[0201] In other example implementations, the filament methods
disclosed herein may be employed for the dicing, cutting, drilling
or scribing of transparent ceramics, polymers, transparent
conductors (i.e. ITO), wide bandgap glasses and crystals (such as
crystal quartz, diamond, sapphire).
[0202] The methods disclosed herein may also be extended to
composite materials and assemblies in which at least one material
component is transparent to the laser wavelength to facilitate such
filamentation processing. Non-limiting examples include silica on
silicon, silicon on glass, metal-coated glass panel display,
printed circuit boards, microelectronic chips, electro chromic
displays, mirrors, glasses, windows or transparent plates, optical
circuits, multi-layer FPD or LCD, biochips, microfluidic devices,
sensors, actuators, MEMs, micro Total Analysis Systems (.mu.TAS),
and multi-layered polymer packaging.
[0203] The filament's radial symmetry relative to the beam axis
renders the material particular easy to cleave by follow-on methods
such as, but not limited to, other laser processes, heating,
cooling, gas jets, and other means of singulating the parts in a
touch-free method, in order to provide a high break strength.
[0204] In one embodiment, another laser exposure, may be employed
to trace the filament array line created by the first exposure.
This can be accomplished with the laser employed for filament
formation, or another more economical laser. In this way full
singulation is affected on glass parts for which natural
self-cleaving is either not desired or unrealized by the techniques
wrought herein, due to thickness or material properties. The
additional laser exposure may be pulsed or CW. The power level of
the additional laser exposure may be approximately 10 W or more.
The wavelength of the additional laser exposure may be longer than
532 nm. The relative translation speed of the additional laser
exposure may be approximately 500 mm/s. The additional laser
exposure may be delivered using static or dynamic (scanning)
optics.
[0205] Products and materials formed according to the methods
disclosed herein may exhibit unique electrical and light producing
properties by virtue of process conditions and how the filaments
have been made within them. For example, stronger parts (glass or
sapphire) may be formed that exhibit resistance to failure
(mechanical or electrical) and very low, but programmable edge
roughness. The resistance to failure also extends to devices
fabricated upon or within these parts thus singulated.
[0206] As shown in FIG. 13(a) and (b), the degree of overlap of the
filaments (or the discrete spacing of filaments) is user and recipe
selectable, such that the edge roughness (850, 852) of the parts
can be controlled on the micron scale. Such control over the edge
roughness may be useful where the device performance is to be
affected or controlled by singulation conditions. Accordingly, the
presently disclosed methods and systems may be capable of producing
new materials geometries and/or parts, thereby opening new avenues
for manufacturing alternatives in segments such as the consumer
product, aerospace, automotive and architectural segments.
Filamentation for Singulated Parts with High Break Strength
[0207] The preceding embodiments may be employed to produce
substrates, with strong, damage free edges with break strengths
exceeding those achievable via other laser processing methods. Such
substrates may be used in a variety of applications such as, tablet
PCs, handheld devices, mirrors, glass plates, semiconductors, film
stacks, display lens arrays, electro-focusing arrays, electro
chromic assemblies, displays, LCD and FPDs exhibiting chamfered,
beveled, or bull nose edges.
[0208] For example, it has been found that substrates processed
according to the methods disclosed above, and subsequently
singulated, may exhibit a break strength greater than 50 MPa. FIGS.
14(a)-(c) illustrate the break strength testing protocol as
described in ASTMC158 for determining the as processed break
strength of the materials thus singulated. FIGS. 14(a) and (b) show
two example break strength measurement configurations, while FIG.
13(c) shows an example Weibull plot for determining the
characteristic strength. The example method of reporting shown is
the Weibull plot, which is designed to convey the statistical
outcome of the material under test and to predict when and under
what conditions it could fail.
[0209] In some embodiments, the methods disclosed herein may be
employed to provide edge quality that is sufficiently high to
support break strengths in excess of 100 MPa. For instance,
as-singulated break test data for materials singulated according to
the methods disclosed herein has demonstrated break strengths as
high as 300 MPa in non-chemically strengthened glass. The break
strength of the material and any product thus created can be
positively influenced by judicious choice of process conditions. It
is noted that while break strengths in excess of 100 MPa is
desirable, such high break strengths have been unachievable using
other methods without further processing.
Apparatus for Complex Spline Processing
[0210] In some embodiments, a system for forming a filament within
a substrate according to the methods disclosed above may include a
rotary stage and an automated gimbal mounted final objective (gamma
axis, .gamma.), coupled with coordinated Z position control, for
rendering complex spline parts. Such an embodiment supports the
creation of high bend strength parts at high yield and without need
for further refinement or post processing.
[0211] FIG. 15(a) and (b) illustrate the processing of samples with
a complex spline surface 900, from which parts may be cut of
arbitrary shape with normal or non-normal beam incidence across the
entire perimeter of the part shape as dictated by the desired
characteristics of the part thus singulated (e.g. strength,
conductivity, electrical efficiency of devices therein/thereon,
etch resistance or efficacy, etc.). Coordinated motion in the theta
and gamma axes with appropriate translation in the XY plane coupled
with auto focus for constant objective lens spacing, can be
employed to generate parts with user-selectable (over a reasonable
range) properties depending upon the application of the part and
its required/desired performance envelope. The optics (FIG. 15(a))
and/or the part being processed may be translated and/or rotated to
achieve this capability. FIGS. 15(b) and (c) illustrate the
translation and/or rotation of the part being processed via a stage
905. FIG. 15(d) provides an example implementation of such an
embodiment, showing a glass part processed via filament formation
to exhibit a rounded edge.
Processing of Multiple Layers
[0212] In other embodiments, multi-level filaments can be produced
across several layers of glasses separated by transparent gas or
other transparent materials, or in multiple layers of different
transparent materials. The substrate may include two or more
layers, wherein a location of a beam focus of the focused laser
beam is selected to generate filament arrays within at least one of
the two or more layers.
[0213] For example, the multilayer substrate may comprise
multi-layer flat panel display glass, such as a liquid crystal
display (LCD), flat panel display (FPD), and organic light emitting
display (OLED). The substrate may also be selected from the group
consisting of autoglass, tubing, windows, biochips, optical
sensors, planar lightwave circuits, optical fibers, drinking glass
ware, art glass, silicon, III-V semiconductors, microelectronic
chips, memory chips, sensor chips, electro-optical lenses, flat
displays, handheld computing devices requiring strong cover
materials, light emitting diodes (LED), laser diodes (LD), and
vertical cavity surface emitting laser (VCSEL).
[0214] Alternatively, the location of a beam focus of the focused
laser beam may be first selected to generate filament arrays within
a first layer of the two or more layers, and the method may further
comprise the steps of: positioning a second beam focus to create an
index change within a second layer of the two or more layers;
irradiating the second layer and translating the substrate to
produce a second array defining a second internally (internal to
the stack, not the individual layers) scribed path for cleaving the
substrate. The substrate may be irradiated from an opposite side
relative to when irradiating the first layer. The substrate may be
further illuminated from top and bottom or from multiple angles of
incidence, either in concerted or subsequent process steps.
Furthermore, prior to irradiating the second layer, a position of
the second beam focus may be laterally translated relative a
position of the beam focus when irradiating the first layer. A
second focused laser beam may be used to irradiate the second
layer. This beam can be supplied by the system from a single source
or a second source. Multiple beams operating in concert can
therefore process multiple substrates in parallel.
[0215] FIG. 16(a) illustrates how substrates having multiple stacks
of materials 935 (optionally with gaps 940 having larger or smaller
refractive index n) can be singulated by the formation of filament
zones 930 within, at an arbitrary angle of incidence by a tipped
part or tipped beam 925--tipped meaning non-normal incidence--or
both, to render singulation of complex stacks. In addition,
conditions may be chosen to affect ablation at intermediate and
terminal interfaces of the part and its components. This is
primarily adjusted by controlling the onset of filament formation,
typically a set distance from the final objective lens, to coincide
with the z position of the target layer wherein filament formation
is desired. By adjusting the z height of the part or optic, a high
degree of control can be afforded the user in determining where the
filament first forms.
[0216] FIG. 16(b) shows an example implementation of such an
embodiment, in which a triple layer laminated glass substrate
having a thickness of 2.1 mm was processed via filament formation
in a single pass at a speed of 0.5 m/s. FIG. 16(c) shows an
electron microscope image, post-cleavage, of a filament-processed
multi-layer device including two air gaps and, an intermediate
adhesive layer.
[0217] In some embodiments, by controlling the laser exposure to
only form filaments in the solid transparent layers, one can avoid
ablation and debris generation on each of the surfaces in the
single or multi-layer plates. This offers significant advantages in
manufacturing, for example, where thick glasses or delicate
multilayer transparent plates must be cleaved with smooth and crack
free facets.
[0218] For example, FIG. 16(d) shows microscope images illustrating
an example implementation of the cleavage of a laminated liquid
crystal display substrate using a hybrid processing method, in
which the top surface is processed via a V-groove, and the bottom
surface is processed via filament formation.
Monitoring
[0219] In one embodiment, an example apparatus for performing the
aforementioned methods includes vision and alignment capability,
utilizing variable wavelengths, which may be user selectable for
contrast, for edge location, wafer mapping and metrology both pre-
and post-laser processing. In one example implementation, standard
machine vision components for image acquisition and analysis are
sufficient to accomplish this task and coupling this with an
alignment algorithm provides the needed level of control. A voice
coil or similar implement may be provided and used to drive the
optic or part in the z direction, and linear motors may be used for
XY positioning. The motors may be equipped with 0.1-10 .mu.m
precision and accuracy in the encoder signals.
[0220] In the case of LED wafers, for example, the wafers are not
flat. At the loading station, the wafers can be picked up and then
pre-mapped to calculate sample curvature or other distortions
induced by dicing tape, DAF (die attach film) or mounting. The
pre-mapping is typically completed by optical scanning of a beam or
a light source across the work piece, where the reflected light is
interrogated to measure the distance between the work and the
camera. For example, such an embodiment could be implemented as a
confocal system. A confocal or similar fast auto focus mechanism
may be sufficient to provide accurate street location and die
corner edge location relative to LED wafer notch or flat. The z
position would be selected to coincide with the desired focus
position, plus or minus the wafer curvature offset, as a function
of XY and theta positions. This pre-mapped data could then be
loaded into the system control computer, and used to drive the
real-time auto-focus system in Z with a servo coordinated signal.
Such a system could enable autofocus control of approximately +/-50
nm in the position of the geometric focus relative to the surface
of the part or stage, while translating the sample relative to the
optical beam at high linear speeds (such as up to 1.5 m/s).
[0221] In one example implementation, this functionality could be
accomplished with a force frame-metrology frame configuration,
where the reactive forces are damped into non-coupled machine
components rendering the optical frame unperturbed from the induced
vibrations.
[0222] In some embodiments, the vision system is able to measure
the characteristics and/or dimensions of the filaments to track
part file fidelity, producing an alert, report, or other
notification when a measurement is not within a prescribed range.
As noted above, the system may be equipped with a vision system
that tracks the filament formation in cross-section.
[0223] In some embodiments, electronics capable of fast feedback
(e.g. sampling rates >1 KHz) may be employed to measure one or
more process parameters associated with filament formation in
real-time. One example implementation is shown in FIG. 17(a). Any
or all of the monitoring components may be configured (translated
or otherwise varied in position of angle) to track the location of
filament formation. For example, with example implementation shown
in FIG. 17(a), the two cameras (imaging devices) and one detector
are arranged to measure and monitor the size, depth and spacing of
the filaments. Camera or detector 950 is positioned to detect
optical radiation emitted during the filament formation process,
where signals associated with the detected optical radiation can be
processed for metrology. Camera 952 is positioned to monitor the
depth and/or size of the filaments 960 as they are formed, through
an edge of the transparent material being processed. Detector 954
(which may be an imaging device or a camera) is positioned to
measure the width of the filament array (FIG. 17(b) illustrates an
example image of filament array 960 obtained by this camera).
[0224] Any of the measured properties or parameters may be provided
to a control and processing system 970 in order to verify process
quality and/or to provide feedback measures for actively
controlling the process. For example, the measured properties or
parameters may be compared with pre-determined values stored in the
control system. The control and processing system 970 may be a
computer or computing device equipped with a processor that is
programmed to control the filament formation process (see FIG.
7(b)).
[0225] In one example implementation, the output from one or more
imaging devices is processed to identify the filament end point and
position in order to provide feedback to a z-servo for monitoring
of the depth of the filament. Such an embodiment may be beneficial
when it is desirable to ensure that the filament stops inside a
material (e.g. LED dicing). As shown in FIG. 19, additional imaging
devices may be included for providing metrology data associated
with the shape and/or position fidelity.
[0226] As shown in FIGS. 17(c) and 17(d), a light source 970, such
as an infrared light source, may be employed to generate an optical
sampling beam that passes through one or more filaments and is
subsequently detected by a detector/camera 975. FIG. 19(c)
illustrates an example of in-plane monitoring embodiment, while
FIG. 19(d) illustrates an example of an angled out-of-plane
monitoring embodiment.
[0227] The preceding embodiments may be particularly useful for
example applications involving the singulation of LED wafers with
DBR (distributed Bragg reflector) and GaN structures. There is
substantial advantage in being able to create a cleavage plane
within a substrate and control the depth to which this feature
persists. In the case of LEDs with GaN layers, the GaN is disturbed
by the presence of ablative processes at the interface between
substrate and GaN adhesion layers. The present method enables not
only a way to control this depth to within approximately <10
.mu.m of the z position, but also the ability to rapidly diverge
the laser beam after the filament formation event. This means the
material immediately below the filament zone is less affected
(optically, mechanically, thermally and vibrationally) than a
conventional laser process. The filament produces more gentle
internal effects by being a very short duration process with high
divergence at the filament terminus.
[0228] As noted above, in order to further enhance the metrology
and monitoring capability of the laser system, white light emitted
with forming the filaments (generated by non-linear processes) can
be monitored for intensity and/or spectral changes indicating
changes in the substrate. In general, any one or more of the size,
position, pattern fidelity and depth of the filament, as well as
chemical and physical properties of the target material, can be
monitored and optionally employed for active feedback loops for
controlling the system.
[0229] In some embodiments, any one of the vision, alignment and
metrology systems can be located above and/or below the target
substrate and can be utilized to direct the focusing optics via
servo controlled actuators to a predetermined position relative to
the target layer in the target stack. This target layer can be one
or more layers thick and can be transparent or opaque to the
incident laser wavelength, provided that at least one layer is
sufficiently transparent to support the creation of filaments.
Ablation and Aligning Semiconductor Devices via Filament
Formation
[0230] In other example implementations, the methods and apparatus
disclosed above may be employed to produce ablative markings in the
first incident layers (the layer that first encounters the laser
radiation) deposited upon a substantially transparent substrate, in
order to aid in the subsequent relative location and alignment of
the substrate during further processing, such as for assisting in
the relative positioning of devices created on or within the
substrate during the processing of a semiconductor device.
[0231] For example, it is well known that the opaque layers of an
LED wafer can be difficult to align during processing. Past efforts
have employed infrared cameras to locate the alignment marks or
fiducials and then register these to the laser coordinate system.
New wafer morphologies make this much more difficult to achieve, as
the metal layers are becoming sufficiently thick to block the
infrared signal and thus prevent alignment.
[0232] Accordingly, in selected embodiments, in applications in
which conventional vision configurations are not practical due to
the presence of thick metal layers (with a thickness such that IR
viewing is ineffective), the preceding embodiments may be employed
to provide a means of locating the structures of interest, such as
streets between neighboring dice on a semiconductor wafer that are
to be singulated.
[0233] Accordingly, in order to produce alignment markings through
metal layers residing within or on a transparent substrate (such as
a laser die), the following example embodiments may be employed. In
particular, the processes described herein may be adapted to
produce ablation through a metal layer (instead of the formation of
a filament, or in addition to the formation of a filament), by
changing the laser power and increasing the number of pulses in a
burst. In some embodiments, a metal may be ablated according to the
aforementioned filament processing methods and conditions with
metal layers less than approximately 50 .mu.m thick on transparent
substrates. It has been found that the first few pulses in the
burst ablate the metal, and then the pulses in the burst proceed to
form a filament.
[0234] In some embodiments and applications, it may be beneficial
and/or desirable to process material having a metal layer such that
the metal is locally ablated without forming a filament adjacent to
the metal layer. This may be preferable, for example, in
applications in which the presence of the filament would damage a
semiconductor layer (e.g. a GaN layer of an LED wafer) in the
immediate vicinity of the metal layer. This may be achieved by
avoiding the required energy density for filament formation, while
maintaining sufficient energy to cleanly ablate the metal. For
example, it has been found that a laser power of approximately 5 W,
and with 20 pulses in a burst at 1064 nm, is suitable for ablating
thin metal layers without forming a filament.
[0235] It will be understood that the preceding embodiments may be
adapted to perform ablation of any metal layer at any surface on or
within the transparent material. For example, the metal layer may
reside on an external surface of the transparent material, or at an
internal surface within the transparent material. Furthermore, in
some embodiments, two or more metal layers may be ablated, in
parallel or in series, according to the preceding embodiments.
[0236] In one example embodiment, illustrated in FIGS. 18(a)-(c), a
semiconductor wafer is having an array of devices formed thereon,
and having a metallization layer beneath active device layers (e.g.
on a bottom surface of the wafer), is processed according to such a
method. Substrate/wafer 1000 is shown from a top view in FIG.
18(a), illustrating the array of devices. As shown in FIG. 18(b),
the bottom surface of the device includes at least one metal layer
1020. Alignment marks are made ablatively through the metal
layer(s), based on irradiation from the back surface of the
substrate using a lower power burst train 1010, as described above.
These alignments marks are spatially registered relative to a
reference frame. The alignment/fiducial marks 1030 may subsequently
utilized when processing the sample from above with a burst train
1040 suitable for filament formation (for example, the sample may
be flipped over and processing may be conducted utilizing the
ablative marks as fiducial marks). Accordingly, the present methods
may be advantageous in avoiding limitations imposed by advanced LED
substrates containing thicker metal layers that serve as heat sinks
and/or reflectors.
[0237] FIG. 18(d) is an overhead image of a LED wafer processed
according to this method, in which burst laser pulses were employed
to process all layers, including the metal layer (low power
marking), the DBR layer, the PSS layer, and the sapphire and GaN
layers.
[0238] The flexibility of this approach is apparent in the ability
of the user to employ filament formation in transparent materials
through metalizations on both the semiconductor (e.g. GaN) side and
the reflector side of the LED device wafer. The scribing can be
applied without damage to the surrounding devices or delamination
of the semiconductor layer (GaN) from the sapphire substrate. The
process can be applied to both sides to effect singulation from
either side, depending upon the limitations inherent in the device
design or its presentation.
[0239] The processing according to the present embodiment may be
performed without causing damage to the dicing tape, regardless of
which direction is employed during singulation. In particular, the
dicing tape may be spared damage because of the very large
divergence angle of the beam and its lower power after passing
through the substrate. This is shown, for example, in FIGS. 18(e)
and (f), which show the post-processed substrates with intact
dicing tape. FIGS. 18(g) and (h) show the processed substrates
after the removal of the dicing tape. It is noted that the minor
residual tape marking can be removed, for example, with a cotton
swab or other suitable cleaning instrument.
[0240] The presently contemplated embodiments would, in some
example implementations, employ conventional optical cameras for
alignment, dispensing with the need for expensive vision
systems.
[0241] Unlike other processing methods, the rapid dissipation of
the filament, whereby the laser beam experiences high divergence
after the filament formation is quenched, that leads to application
of this laser process to an LED processing station equipped with an
auto focus mechanism with a high degree of z precision, for example
-0.01 .mu.m to singulate the dice without inducing damage to GaN or
DBR (distributed Bragg reflector) layers. It is to be understood
that the processing of specific layers can be selected by
modulating the beam power and choosing the appropriate optical
focusing conditions to produce external pre and/or post material
foci (beam waists), without the formation of an external plasma
channel, for "dumping" unwanted power.
[0242] The flexibility in this approach also allows for the
creation of alignment marks in any incident surface within the
target material or stack that is accessible from either side of the
device, thus enabling fully-aligned singulation of even the most
advanced LED stacks now appearing in development laboratories,
where the layers and their composition render traditional alignment
and singulation techniques completely incompatible and
ineffective.
[0243] The alignment marks may be created using filament and/or
ablative techniques--ablative techniques for marks "on" a surface
and filaments techniques for marks desired "within" a material,
depending upon the materials present near the surface being marked
or the material in which the mark is being made. Metal on
dielectric, for instance will render both types of marks available
for inspection and location by the vision system.
EXAMPLES
[0244] The following examples are presented to enable those skilled
in the art to understand and to practice the present disclosure.
They should not be considered as a limitation on the scope of the
embodiments provided herein, but merely as being illustrative and
representative thereof.
Example 1: Singulation of Glass Samples via Laser Filamentation
[0245] To demonstrate some of the embodiments disclosed above,
glass samples were processed on a laser system equipped with a 50 W
ps laser operating at high rep rate (>400 kHz) to facilitate
very rapid scans of the laser beam across the target, with stages
moving at a rate of approximately 500 mm/s-1000 mm/s, where 0.7 mm
thick Gorilla glass had been mounted. The laser, operating at the
fundamental, 1064 nm with a pulse width less than 25 ps, was set to
operate in burst mode with 20 sub-pulses in a burst.
[0246] Both rectilinear cuts and curvilinear shapes have been
achieved at high speed with very good edge quality and high bend
strength. For example, samples of Gorilla thus processed have shown
>110 MPa as-cut bend strength. FIG. 19 shows a micrograph of the
facet edge after formation of the modified zone (the so-called
scribe step) and the cleave step (singulation).
[0247] The roughness shown is less than 10 .mu.m RMS over a
substantial portion of the surface. FIG. 20 shows post-singulation
surface roughness measurements of an example substrate in
orthogonal directions.
[0248] In another example, radius corner and faceted edge parts
have been created in sapphire having a thickness of 0.4 mm, with
roughness even better roughness with measured values as low as
approximately 200 nm RMS, as shown in FIG. 21.
[0249] The stage motion of the system was coordinated with the
laser pulse and trigger signals such that the relative motion laser
and the part is synchronized so that the part is never waiting for
the laser or stages to catch up. The velocity of the beam relative
to the material was thus maintained at a constant value, even when
producing filament arrays on curved portions, such as around the
corners. This was achieved by controlling the relative motion of
the beam and the material based on data from a spline file (the
spline file was read into the computer from the Adobe illustrator
file). The constant velocity maintains a constant relative spacing
between filaments, thereby producing consistent filament formation
and interface quality after singulation at all locations. It is to
be understood that this embodiment may be applied to any method of
laser processing, and is not limited to the aforementioned
embodiments involving processing via laser filamentation.
[0250] The burst characteristics were empirical selected based on
the materials to be tested. It was empirically found that improved
singulation results were obtained when the filament length extended
more than approximately 10% of the substrate thickness. This was
found to be especially true for thick, soft glass and for
substrates containing sensitive electrical devices, such as LED
wafers. Softer glasses, like borosilicate and soda lime, may
benefit from much longer filaments, maybe up to 75% of the sample
thickness in order to produce cleavage with consistent and high
material quality, including edge roughness with minimal
chipping.
[0251] These results illustrate that the nature of the filament can
be readily manipulated by varying the pulsed nature of the laser
exposure. In other words, in addition to the parameters of energy,
wavelength, and beam focusing conditions (i.e. numerical aperture,
focal position in sample), pulse parameters can be tailored to
obtain a desired filament profile. In particular, number of pulses
in a pulse burst and the delay time between successive pulses can
be varied to control the form of the filaments produced. As noted
above, in one embodiment, filaments are produced by providing a
burst of pulses for generating each filament, where each burst
comprises a series of pulses provided with a relative delay that is
less than the timescale for the relaxation of all the material
modification dynamics.
[0252] In the industrial application of single sheet glass
scribing, flat panel glass scribing, silicon and/or sapphire wafer
scribing, there is a demand for higher scribing speeds using laser
systems with proven reliability. To demonstrate such an embodiment,
experiments were performed using a high repetition rate commercial
ultrafast laser system having a pulse duration in the picosecond
range.
[0253] In some experimental investigations, mobile phone glass
displays and tablet cover glasses have been singulated according to
the methods disclosed herein. Eagle 2000 or variable thickness and
Gorilla glass pre and post ion exchange have been singulated from
large mother sheets and smaller phone-sized units, with great
flexibility and speed. The inclusion of faceted edges and complex
spline shapes represents a substantial extension of the state of
the art in brittle material singulation. Use of the regenerative
amplifier-based platform has produced the best results to date, in
both femtosecond and picosecond pulse regimes.
[0254] In some experiments, the polarization of the incident beam
was modified, in addition to its temporal and spatial
characteristics. Manipulation of these parameters has generated a
parameter space which drives machine design in subsequent
production-ready systems. For example, polarization delivers the
process flexibility as a servo'd and coordinated polarizer can be
rotated to improve or optimize angle cutting through substantially
thick substrates. For example, such control over the polarization
state of the beam may be useful for creating beveled glass parts,
with interior and exterior lines. Parts that require rounded
corners and or rotation of the laser beam delivery system
(translation at least if not rotation) may be processed according
to such a method, as the processing of such parts involves a beam
incidence angle that changes as the part and laser move relative to
one another, which in turn affects the filament formation efficacy.
Accordingly, the polarization state of the incident beam relative
to the surface of the material may be controlled as the angle of
incidence of the beam is varied during processing. This may be
achieved, for example, using an automated beam delivery system (as
described herein) in which the polarization state is controlled
during processing in addition to, and in association with, the beam
position and orientation.
[0255] Example parts have been generated across a wide range of
glass thickness ranging from 0.3 to 3.2 mm or even broader ranges
of thicknesses, from 0.1 to 8 mm. Glass parts were generated with
translation speeds of 500 mm/s and greater. Sapphire materials were
generated with translation velocities of 500 mm/s. The speed of
formation of modified zones takes advantage of the machine's
characteristic high speed stages and the ability of the parts to
change direction quickly, but in a smooth and consistent manner,
thereby producing edges that are faithful reproductions of the part
files. Electrochromic windows are a suitable example application
for the present system. An example of such an embodiment is the
processing of aerospace glass. Singulated parts exhibit curves and
precise edges, and are effectively ready for assembly, immediately
post singulation.
[0256] The flexibility of the present approach is highlighted by
the wide array of parts that can be fabricated, each with a
different part presentation scheme. Part of this flexibility stems
from the use of an adjustable optical train capable of quickly
moving the focus and the spatial distribution of the beam in
response to process needs. The production of substantially or
effectively lossless singulation with low roughness edges, coupled
with the flexibility demonstrated herein, offers commercial
opportunities for this technology in display and general brittle
materials singulation markets where high yield, high strength parts
are required with less than 30 .mu.m RMS edge roughness parts are
required at high speed, and lower cost of ownership than any
competitive technology.
Example 2: Formation of 6 mm Long Filaments in Glass Substrates
[0257] In one example implementation, the laser beam may include a
burst of pulses having a pulse duration less than approximately 500
ps, which is collimated and focused to a spot outside of the target
(for example, with a waist greater than approximately 1 .mu.m and
less than approximately 100 .mu.m. Without intending to be limited
by theory, and as noted above, it is believed that the non-linear
interactions that result in filament formation cause a series of
acoustic compressions within the material. These acoustic
compressions are understood to be substantially symmetric about the
beam axis. The longitudinal length of this zone is determined by a
number of pulse and beam parameters, including the position of the
focus, the laser power and the pulse energy, as described
above.
[0258] For example, using a 50 W laser with a burst train of pulses
each having a pulsewidth of approximately 10 ps, with a 2 MHz rep
rate, for example, filaments with a length in excess of 10 mm can
be created within glass materials. Such filaments can be formed
such that they are not divergent, are continuous, and exhibit a
substantially constant diameter from the top surface of the
material to the bottom surface of the material.
[0259] In particular, such structures have been observed to possess
a small and narrow diameter (e.g. approximately 3 .mu.m) tube
beginning on the top surface and continuing in a smooth and uniform
way (with an interior RMS surface roughness less than approximately
10 .mu.m), out the bottom of the target layer or stack, such that
the exit diameter is also on the order of approximately 3 .mu.m (in
the present example). Such filaments have controllable properties,
both in terms of their properties and the effects on the material
in which they are formed. One example of a parameter for
controlling the filaments is the speed of translation of the beam
across the work (or the translation of the work relative to the
beam).
[0260] One important differentiator between the present methods and
all previously known methods is the rate at which these filaments
and therefore scribe/cleave/dicing arrays can be created. In the
present example, 6 mm filaments can be created at approximately 600
mm/s. This morphology, rate and post scribe material integrity is
unprecedented in the history of laser processing.
Example 3: Filament Formation Using 1064 nm Pulsed Laser
[0261] In one example implementation of the methods, apparatus and
systems disclosed herein, a laser configured to output bursts of
picosecond pulses as described is admitted to an optical train with
a collimator and steering optics, optionally a scanner with a field
corrected region capable of delivering at user selectable angles, a
beam with optics designed to induce aberrated wavefronts which can
be focused via negative or positive lenses such that the
interaction zone exceeds the depth of the target layer to be
scribed. In one example implementation, bursts of picosecond pulses
emitted at 5 MHz from a 50 W 1064 nm laser is focused by a series
of lenses to create a 5 .mu.m spot at focus outside the material
using a doublet or triplet of lenses where the ratio, W, of focal
lengths lies between -20 and +20 (L1.sub.fl/L2.sub.fl=W), depending
upon the target substrate and the intended final result (full cut,
scribe and break, etc.) as the length of the interaction zone will
determine the characteristics of the parts as processed. As noted
above, in some embodiments, a ratio of lens focal lengths--of up to
approximately -300 to 300 may be employed.
[0262] The specific embodiments described above have been shown by
way of example, and it should be understood that these embodiments
might be susceptible to various modifications and alternative
forms. It should be further understood that the claims are not
intended to be limited to the particular forms disclosed, but
rather to cover all modifications, equivalents, and alternatives
falling within the spirit and scope of this disclosure.
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