U.S. patent application number 15/660009 was filed with the patent office on 2018-02-15 for edge chamfering methods.
The applicant listed for this patent is Corning Incorporated. Invention is credited to Sasha Marjanovic, Albert Roth Nieber, Garrett Andrew Piech, Helmut Schillinger, Sergio Tsuda, Robert Stephen Wagner.
Application Number | 20180044219 15/660009 |
Document ID | / |
Family ID | 53367275 |
Filed Date | 2018-02-15 |
United States Patent
Application |
20180044219 |
Kind Code |
A1 |
Marjanovic; Sasha ; et
al. |
February 15, 2018 |
EDGE CHAMFERING METHODS
Abstract
Processes of chamfering and/or beveling an edge of a glass or
other substrate of arbitrary shape using lasers are described
herein. Three general methods to produce chamfers on glass
substrates are disclosed. The first method involves cutting the
edge with the desired chamfer shape utilizing an ultra-short pulse
laser. Treatment with the ultra-short laser may be optionally
followed by a CO.sub.2 laser for fully automated separation. The
second method is based on thermal stress peeling of a sharp edge
corner, and it has been demonstrated to work with different
combination of an ultrashort pulse and/or CO.sub.2 lasers. A third
method relies on stresses induced by ion exchange to effect
separation of material along a fault line produced by an
ultra-short laser to form a chamfered edge of desired shape.
Inventors: |
Marjanovic; Sasha; (Painted
Post, NY) ; Nieber; Albert Roth; (Painted Post,
NY) ; Piech; Garrett Andrew; (Corning, NY) ;
Schillinger; Helmut; (Munchen, DE) ; Tsuda;
Sergio; (Horseheads, NY) ; Wagner; Robert
Stephen; (Corning, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Corning Incorporated |
Corning |
NY |
US |
|
|
Family ID: |
53367275 |
Appl. No.: |
15/660009 |
Filed: |
July 26, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14530410 |
Oct 31, 2014 |
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15660009 |
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61917213 |
Dec 17, 2013 |
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62022885 |
Jul 10, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y10T 428/24777 20150115;
B23K 26/0624 20151001; B23K 26/361 20151001; C03B 33/0222 20130101;
B23K 2103/50 20180801; C03B 33/082 20130101; C03B 33/091 20130101;
B23K 2103/54 20180801; C03C 21/002 20130101; B23K 26/362 20130101;
B23K 26/53 20151001; B23K 26/02 20130101; Y02P 40/57 20151101; B23K
26/037 20151001; B23K 26/0869 20130101; B24B 9/10 20130101; B23K
26/04 20130101; B23K 26/402 20130101; Y10T 428/15 20150115; B23K
26/083 20130101 |
International
Class: |
C03B 33/02 20060101
C03B033/02; C03B 33/09 20060101 C03B033/09; B23K 26/035 20140101
B23K026/035; B23K 26/0622 20140101 B23K026/0622; B23K 26/361
20140101 B23K026/361; C03B 33/08 20060101 C03B033/08; B23K 26/402
20140101 B23K026/402; B23K 26/362 20140101 B23K026/362; B23K 26/08
20140101 B23K026/08; B23K 26/04 20140101 B23K026/04; B23K 26/02
20140101 B23K026/02; C03C 21/00 20060101 C03C021/00; B23K 26/00
20140101 B23K026/00 |
Claims
1. A glass article including at least one chamfered edge having a
plurality of defect lines extending at least 250 .mu.m, the defect
lines each having a diameter less than or equal to about
5.mu.m.
2. The glass article of claim 1, wherein the glass article
comprises chemically strengthened glass.
3. The glass article of claim 1, wherein the glass article
comprises non-strengthened glass.
4. The glass article of claim 1, wherein the chamfered edge has an
Ra surface roughness less than about 0.5 .mu.m.
5. The glass article of claim 1, wherein the chamfered edge has
subsurface damage up to a depth less than or equal to about 75
.mu.m.
Description
[0001] This application is a divisional of U.S. patent application
Ser. No. 14/530,410 filed Oct. 31, 2014, which claims the benefit
of U.S. Provisional Application No. 61/917,213 filed on Dec. 17,
2013 as well as the benefit of U.S. Provisional Application No.
62/022,885 filed on Jul. 10, 2014 the entire disclosures of which
are incorporated herein by reference.
BACKGROUND
[0002] In all cases where glass panels are cut for applications in
architectural, automotive, consumer electronics, to mention a few
areas, there will be edges, which will very likely require
attention. There are as many different methods to cut and separate
glass as there are edge shapes. Glass can be cut mechanically (CNC
machining, abrasive waterjet, scribing and breaking, etc), using
electro-magnetic radiation (lasers, electrical discharges,
gyrotron, etc) and many other methods. The more traditional and
common methods (scribe and break or CNC machining) create edges
that are populated with different types and sizes of defects. It is
also common to find that the edges are not perfectly perpendicular
to the surfaces. In order to eliminate the defects and give the
edges a more even surface with improved strength, they are usually
ground. The grinding process involves abrasive removal of edge
material that can give it the desired finishing and also shape its
form (bull nosed, chamfered, pencil shape, etc). In order to allow
the grinding and polishing steps, it is necessary to cut parts that
are larger than the final desired dimensions.
[0003] While it is well known and understood that eliminating
defects will increase edge strength, there is not an agreement on
the impact that shape has on edge strength. The confusion occurs
mainly because it is well known that shape helps to increase damage
resistance to impact and handling of the edges. The fact is that
edge shape really does not determine edge strength as defined by
resistance to flexural (or bending) forces, but the defects size
and distribution do have a great impact. However, a shaped edge
does help to improve impact resistance by creating smaller cross
section and containing defects. For example, an edge with a
straight face that is perpendicular to both surfaces accumulates
stress at these right angled corners that will chip and break when
it is impacted by another object. Because of the accumulated
stress, the size of defects can be pretty big, which will diminish
the strength of that edge considerably. On the other hand, due to
its smoother shape, a rounded "bull-nosed" shaped edge will have
lower accumulated stress and smaller cross section which helps to
reduce the size and penetration of defects into the volume of the
edge. Therefore, after an impact, a shaped edge should have higher
"bending" strength than a flat edge.
[0004] For the reasons discussed above, it is often desirable to
have the edges shaped, as opposed to flat and perpendicular to the
surfaces. One important aspect of these mechanical cutting and edge
shaping methods is the degree of maintenance of the machines. Both
for cutting and grinding, old and worn down cutting heads or
grinding rolls can produce damage which can significantly affect
the strength of the edges, even if the naked eye cannot be see the
differences. Other issues with mechanical cutting and grinding
methods is that they are very labor intensive and require many
grinding and polishing steps until the final desired finish, which
generate a lot of debris and require cleaning steps to avoid
introduction of damages to the surfaces.
[0005] From process development and cost perspectives there are
many opportunities for improvement in cutting and chamfering edges
of glass substrates. It is of great interest to have a faster,
cleaner, cheaper, more repeatable and more reliable method of
creating shaped edges than what is currently practiced in the
market today. Among several alternative technologies, laser and
other thermal sources have been tried and demonstrated to create
shaped edges.
[0006] In general, ablative laser techniques tend to be slow due to
the low material removal rate and they also generate a lot of
debris and heat affected zones that lead to residual stress and
micro-cracks. For the same reason, melting and reshaping of the
edges are also plagued with a lot of deformation and accumulated
thermal stress that can peel that processed area. Finally, for the
thermal peeling or crack propagating techniques, one of the main
issues encountered is that the peeling is not continuous.
[0007] Subsurface damage, or the small microcracks and material
modification caused by any cutting process, is a concern for the
edge strength of glass or other brittle materials. Mechanical and
ablative laser processes are particularly problematic with regard
to subsurface damage. Edges cut with these processes typically
require a lot of post-cut grinding and polish to remove the
subsurface damage layer, thereby increasing edge strength to
performance level required for applications such as in consumer
electronics.
SUMMARY
[0008] According to embodiments described herein, processes of
chamfering and/or beveling an edge of a glass substrate of
arbitrary shape using lasers are presented. One embodiment involves
cutting the edge with the desired chamfer shape utilizing an
ultra-short pulse laser that may be optionally followed by a
CO.sub.2 laser for fully automated separation. Another embodiment
involves thermal stress peeling of a sharp edge corner with
different combination of an ultrashort pulse and/or CO.sub.2
lasers. Another embodiment includes cutting the glass substrate by
any cutting method, such as utilizing the ultra-short pulse laser,
followed by chamfering solely by the use of a CO.sub.2 laser.
[0009] In one embodiment, a method of laser processing a material
includes focusing a pulsed laser beam into a laser beam focal line
and directing the laser beam focal line into the material at a
first angle of incidence to the material, the laser beam focal line
generating an induced absorption within the material, the induced
absorption producing a defect line along the laser beam focal line
within the material. The method also includes translating the
material and the laser beam relative to each other, thereby forming
a plurality of defect lines along a first plane at the first angle
within the material, and directing the laser beam focal line into
the material at a second angle of incidence to the material, the
laser beam focal line generating an induced absorption within the
material, the induced absorption producing a defect line along the
laser beam focal line within the material. The method further
includes translating the material or the laser beam relative to one
another, thereby forming a plurality of defect lines along a second
plane at the second angle within the material, the second plane
intersecting the first plane.
[0010] According to another embodiment, a method of laser
processing a material includes focusing a pulsed laser beam into a
laser beam focal line, and forming a plurality of defect lines
along each of N planes within the material. The method also
includes directing the laser beam focal line into the material at a
corresponding angle of incidence to the material, the laser beam
focal line generating an induced absorption within the material,
the induced absorption producing a defect line along the laser beam
focal line within the material. The method further includes
translating the material and the laser beam relative to each other,
thereby forming the plurality of defect lines along the
corresponding plane of the N planes.
[0011] According to yet another embodiment, a method of laser
processing a workpiece includes focusing a pulsed laser beam into a
laser beam focal line directed into the workpiece at an angle of
incidence to the workpiece, the angle intersecting an edge of the
workpiece, the laser beam focal line generating an induced
absorption within the workpiece, and the induced absorption
producing a defect line along the laser beam focal line within the
workpiece. The method also includes translating the workpiece and
the laser beam relative to each other, thereby forming a plurality
of defect lines along a plane at the angle within the workpiece,
and separating the workpiece along the plane by applying an
ion-exchange process to the workpiece.
[0012] In still another embodiment, a method of laser processing a
material includes focusing a pulsed laser beam into a laser beam
focal line directed into the material, the laser beam focal line
generating an induced absorption within the material, and the
induced absorption producing a defect line along the laser beam
focal line within the material. The method also includes
translating the material and the laser beam relative to each other
along a contour, thereby forming a plurality of defect lines along
the contour within the material to trace a part to be separated,
and separating the part from the material. The method further
includes directing a focused infrared laser into the part along a
line adjacent an edge at a first surface of the part to peel a
first strip that defines a first chamfered edge, and directing the
focused infrared laser into the part along a line adjacent the edge
at a second surface of the part to peel a second strip that defines
a second chamfered edge.
[0013] The present disclosure extends to: [0014] A method of laser
processing comprising: [0015] focusing a pulsed laser beam into a
laser beam focal line; [0016] directing the laser beam focal line
into a material at a first angle of incidence to the material, the
laser beam focal line generating an induced absorption within the
material, the induced absorption producing a defect line along the
laser beam focal line within the material; [0017] translating the
material and the laser beam relative to each other, thereby forming
a plurality of defect lines along a first plane at the first angle
within the material; [0018] directing the laser beam focal line
into the material at a second angle of incidence to the material,
the laser beam focal line generating an induced absorption within
the material, the induced absorption producing a defect line along
the laser beam focal line within the material; and translating the
material or the laser beam relative to one another, thereby forming
a plurality of defect lines along a second plane at the second
angle within the material, the second plane intersecting the first
plane.
[0019] The present disclosure extends to:
[0020] A method of laser processing a material comprising: [0021]
focusing a pulsed laser beam into a laser beam focal line; [0022]
forming a plurality of defect lines along each of N planes within
the material, the forming plurality of defect lines including:
[0023] (a) directing the laser beam focal line into the material at
an angle of incidence to the material corresponding to one of the N
planes, the laser beam focal line generating an induced absorption
within the material, the induced absorption producing a defect line
along the laser beam focal line within the material; [0024] (b)
translating the material and the laser beam relative to each other,
thereby forming the plurality of defect lines along the one of the
N planes; and (c) repeating (a) and (b) for each of the N
planes.
[0025] The present disclosure extends to: [0026] A method of laser
processing a workpiece comprising: [0027] focusing a pulsed laser
beam into a laser beam focal line; [0028] directing the laser beam
focal line into the workpiece at an angle of incidence to the
workpiece, the angle of incidence intersecting an edge of the
workpiece, the laser beam focal line generating an induced
absorption within the workpiece, the induced absorption producing a
defect line along the laser beam focal line within the workpiece;
[0029] translating the workpiece and the laser beam relative to
each other, thereby forming a plurality of defect lines along a
plane at the angle within the workpiece; and separating the
workpiece along the plane by subjecting the workpiece to an
ion-exchange process.
[0030] The present disclosure extends to: [0031] A method of laser
processing a material comprising: [0032] focusing a pulsed laser
beam into a laser beam focal line; [0033] directing the laser beam
focal line into a material, the laser beam focal line generating an
induced absorption within the material, the induced absorption
producing a defect line along the laser beam focal line within the
material; [0034] translating the material and the laser beam
relative to each other along a contour, thereby forming a plurality
of defect lines along the contour within the material, the contour
tracing the perimeter of a part to be separated from the material;
[0035] separating the part from the material; [0036] directing a
focused infrared laser into the separated part along a line
adjacent an edge at a first surface of the part to peel a first
strip that defines a first chamfered edge of the separated part;
and directing the focused infrared laser into the separated part
along a line adjacent the edge at a second surface of the part to
peel a second strip that defines a second chamfered edge of the
separated part.
[0037] The present disclosure extends to: [0038] A glass article
including at least one chamfered edge having a plurality of defect
lines extending at least 250 .mu.m, the defect lines each having a
diameter less than or equal to about 5 .mu.m.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] The foregoing will be apparent from the following more
particular description of example embodiments of the disclosure, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating embodiments of the present disclosure.
[0040] FIGS. 1A-1C are illustrations of a fault line with equally
spaced defect lines of modified glass.
[0041] FIGS. 2A and 2B are illustrations of positioning of the
laser beam focal line, i.e., the processing of a material
transparent for the laser wavelength due to the induced absorption
along the focal line.
[0042] FIG. 3A is an illustration of an optical assembly for laser
drilling according.
[0043] FIGS. 3B-1-3B-4 are an illustration of various possibilities
to process the substrate by differently positioning the laser beam
focal line relative to the substrate.
[0044] FIG. 4 is an illustration of a second optical assembly for
laser drilling.
[0045] FIGS. 5A and 5B are illustrations of a third optical
assembly for laser drilling.
[0046] FIG. 6 is a schematic illustration of a fourth optical
assembly for laser drilling.
[0047] FIG. 7A is a flow chart of the various methods described in
the present application to form a more robust edge--creating
chamfers and sacrificial edges.
[0048] FIG. 7B illustrates a process of creating a chamfered edge
with defect lines.
[0049] FIG. 7C illustrates laser chamfering of glass edges using a
focused and angled ultrashort laser that generates defect lines
along pre-determined planes. Top shows an example using 3 defect
line planes compared to just two for the bottom images.
[0050] FIGS. 8A and 8B depict laser emission as a function of time
for a picosecond laser. Each emission is characterized by a pulse
"burst" which may contain one or more sub-pulses. Times
corresponding to pulse duration, separation between pulses, and
separation between bursts are illustrated.
[0051] FIG. 9 is an illustration of a thermal gradient created by
the focused laser that is highly absorbed by the glass. The
cracking line is between the strain and softening zones.
[0052] FIG. 10 illustrates edge chamfering by thermal peeling.
[0053] FIG. 11A is an illustration of edge chamfering process using
defect lines and then thermal peeling. First, the picosecond laser
is focused at an angle and a defect line is created on an angled
plane. Then a focused CO.sub.2 laser is scanned next to the defect
line, at a controlled lateral offset. A strip of glass is peeled
from that corner and forms a chamfer.
[0054] FIG. 11B illustrates, as shown in the side view of the edge,
that the strip of glass formed by the process shown in FIG. 11A
does not necessarily peel entirely along the defect line plane.
[0055] FIG. 12 is an illustration of edge chamfer changes with
peeling speed using only a focused CO.sub.2 laser. All other
CO.sub.2 laser parameters were kept the same.
[0056] FIG. 13 illustrates using defect lines which remain after
the cut part is released to serve as sacrificial regions, arresting
the propagation of cracks caused by impact to the edges of the
part.
[0057] FIG. 14A is an illustration of a cut part with internal
defect lines being placed into ion-exchange, which adds enough
stress to remove the perforated edges and form the desired edge
chamfer.
[0058] FIG. 14B is the use of ion exchange (IOX) to release
chamfered corners, similar to the illustration shown in FIG. 14A,
but with only two defect line planes.
[0059] FIG. 14C is an illustration of a chamfer with many angles
(more than 3 defect line planes).
DETAILED DESCRIPTION
[0060] A description of exemplary embodiments follows.
[0061] Embodiments described herein relate to processes of
chamfering and/or beveling an edge of a glass substrate and other
substantially transparent materials of arbitrary shape using
lasers. Within the context of the present disclosure, a material is
substantially transparent to the laser wavelength when the
absorption is less than about 10%, preferably less than about 1%
per mm of material depth at this wavelength. A first embodiment
involves cutting the edge with the desired chamfer shape utilizing
an ultra-short pulse laser that may be optionally followed by an
infrared (e.g., CO.sub.2) laser for fully automated separation. A
second embodiment involves thermal stress peeling of a sharp edge
corner with different combinations of an ultrashort pulse and/or
CO.sub.2 lasers. Another embodiment includes cutting the glass
substrate by any cutting method, such as utilizing the ultra-short
pulse laser, followed by chamfering solely by the use of a CO.sub.2
laser to work with different combinations of an ultrashort pulse
and/or CO.sub.2 lasers.
[0062] In the first method, the process fundamental step is to
create fault lines on intersecting planes that delineate the
desired edge shape and establish a path of least resistance for
crack propagation and hence separation and detachment of the shape
from its substrate matrix. This method essentially creates the
shaped edge while cutting the part out of the main substrate. The
laser separation method can be tuned and configured to enable
manual separation, partial separation, or self-separation of the
shaped edges out of the original substrate. The underlying
principle to generate these fault lines is described in detail
below and in U.S. Application No. 61/752,489 filed on Jan. 15,
2013, the entire contents of which are incorporated herein by
reference as if fully set forth herein.
[0063] In the first step, the object to be processed is irradiated
with an ultra-short pulsed laser beam that is condensed into a high
aspect ratio line focus that penetrates through the thickness of
the substrate. Within this volume of high energy density the
material is modified via nonlinear effects. It is important to note
that without this high optical intensity, nonlinear absorption is
not triggered. Below this intensity threshold, the material is
transparent to the laser radiation and remains in its original
state.
[0064] The selection of the laser source is predicated on the
ability to induce multi-photon absorption (MPA) in the transparent
material. MPA is the simultaneous absorption of multiple photons of
identical or different frequencies in order to excite a material
from a lower energy state (usually the ground state) to a higher
energy state (excited state). The excited state may be an excited
electronic state or an ionized state. The energy difference between
the higher and lower energy states of the material is equal to the
sum of the energies of the two or more photons. MPA is a nonlinear
process that is generally several orders of magnitude weaker than
linear absorption. It differs from linear absorption in that the
strength of MPA depends on the square or higher power of the light
intensity, thus making it a nonlinear optical process. At ordinary
light intensities, MPA is negligible. If the light intensity
(energy density) is extremely high, such as in the region of focus
of a laser source (particularly a pulsed laser source), MPA becomes
appreciable and leads to measurable effects in the material within
the region where the energy density of the light source is
sufficiently high. Within the focal region, the energy density may
be sufficiently high to result in ionization.
[0065] At the atomic level, the ionization of individual atoms has
discrete energy requirements. Several elements commonly used in
glass (e.g., Si, Na, K) have relatively low ionization energies
(.about.5 eV). Without the phenomenon of MPA, a wavelength of about
248 nm would be required to create linear ionization at .about.5
eV. With MPA, ionization or excitation between states separated in
energy by .about.5 eV can be accomplished with wavelengths longer
than 248 nm. For example, photons with a wavelength of 532 nm have
an energy of .about.2.33 eV, so two photons with wavelength 532 nm
can induce a transition between states separated in energy by
.about.4.66 eV in two-photon absorption (TPA), for example. Thus,
atoms and bonds can be selectively excited or ionized in the
regions of a material where the energy density of the laser beam is
sufficiently high to induce nonlinear TPA of a laser wavelength
having half the required excitation energy, for example.
[0066] MPA can result in a local reconfiguration and separation of
the excited atoms or bonds from adjacent atoms or bonds. The
resulting modification in the bonding or configuration can result
in non-thermal ablation and removal of matter from the region of
the material in which MPA occurs. This removal of matter creates a
structural defect (e.g. a defect line, damage line, or
"perforation") that mechanically weakens the material and renders
it more susceptible to cracking or fracturing upon application of
mechanical or thermal stress. By controlling the placement of
perforations, a contour or path along which cracking occurs can be
precisely defined and precise micromachining of the material can be
accomplished. The contour defined by a series of perforations may
be regarded as a fault line and corresponds to a region of
structural weakness in the material. In one embodiment,
micromachining includes separation of a part from the material
processed by the laser, where the part has a precisely defined
shape or perimeter determined by a closed contour of perforations
formed through MPA effects induced by the laser. As used herein,
the term closed contour refers to a perforation path formed by the
laser line, where the path intersects with itself at some location.
An internal contour is a path formed where the resulting shape is
entirely surrounded by an outer portion of material.
[0067] The laser is ultrashort pulsed laser (pulse durations on the
order tens of picoseconds or shorter) and can be operated in pulse
mode or burst mode. In pulse mode, a series of nominally identical
single pulses is emitted from the laser and directed to the
workpiece. In pulse mode, the repetition rate of the laser is
determined by the spacing in time between the pulses. In burst
mode, bursts of pulses are emitted from the laser, where each burst
includes two or more pulses (of equal or different amplitude). In
burst mode, pulses within a burst are separated by a first time
interval (which defines a pulse repetition rate for the burst) and
the bursts are separated by a second time interval (which defines a
burst repetition rate), where the second time interval is typically
much longer than the first time interval. As used herein (whether
in the context of pulse mode or burst mode), time interval refers
to the time difference between corresponding parts of a pulse or
burst (e.g. leading edge-to-leading edge, peak-to-peak, or trailing
edge-to-trailing edge). Pulse and burst repetition rates are
controlled by the design of the laser and can typically be
adjusted, within limits, by adjusting operating conditions of the
laser. Typical pulse and burst repetition rates are in the kHz to
MHz range.
[0068] The laser pulse duration (in pulse mode or for pulses within
a burst in burst mode) may be 10.sup.-10 s or less, or 10.sup.-11 s
or less, or 10.sup.-12 s or less, or 10.sup.-13 s or less. In the
exemplary embodiments described herein, the laser pulse duration is
greater than 10.sup.-15.
[0069] The perforations may be spaced apart and precisely
positioned by controlling the velocity of a substrate or stack
relative to the laser through control of the motion of the laser
and/or the substrate or stack. As an example, in a thin transparent
substrate moving at 200 mm/sec exposed to a 100 kHz series of
pulses (or bursts of pulses), the individual pulses would be spaced
2 microns apart to create a series of perforations separated by 2
microns. This defect line (perforation) spacing is sufficiently
close to allow for mechanical or thermal separation along the
contour defined by the series of perforations.
[0070] FIGS. 1A-1C illustrate that a method to cut and separate a
substrate material (e.g., sapphire or glass) can be essentially
based on creating a fault line 110 formed of a plurality of
vertical defect lines 120 in the substrate material 130 with an
ultra-short pulsed laser 140. Depending on the material properties
(absorption, CTE, stress, composition, etc) and laser parameters
chosen for processing the material 130, the creation of a fault
line 110 alone can be enough to induce self-separation. In this
case, no secondary separation processes, such as tension/bending
forces, heating, or CO.sub.2 laser, are necessary. Distance between
adjacent defect lines 120 along the direction of the fault lines
110 can, for example, be in the range from 0.25 .mu.m to 50 .mu.m,
or in the range from 0.50 .mu.m to about 20 .mu.m, or in the range
from 0.50 .mu.m to about 15 .mu.m, or in the range from 0.50 .mu.m
to 10 .mu.m, or in the range from 0.50 .mu.m to 3.0 .mu.m or in the
range from 3.0 .mu.m to 10 .mu.m.
[0071] By scanning the laser over a particular path or contour, a
series of perforations is created (a few microns wide) that defines
the perimeter or shape of the part to be separated from the
substrate. The series of perforations may also be referred to
herein as a fault line. The particular laser method used (described
below) has the advantage that in a single pass, it creates highly
controlled perforation through the material, with extremely little
(<75 .mu.m, often <50 .mu.m) subsurface damage and debris
generation. This is in contrast to the typical use of spot-focused
laser to ablate material, where multiple passes are often necessary
to completely perforate the glass thickness, large amounts of
debris are formed from the ablation process, and more extensive
sub-surface damage (>100 .mu.m) and edge chipping occur. As used
herein, subsurface damage refers to the maximum size (e.g. length,
width, diameter) of structural imperfections in the perimeter
surface of the part separated from the substrate or material
subjected to laser processing in accordance with the present
disclosure. Since the structural imperfections extend from the
perimeter surface, subsurface damage may also be regarded as the
maximum depth from the perimeter surface in which damage from laser
processing in accordance with the present disclosure occurs. The
perimeter surface of the separated part may be referred to herein
as the edge or the edge surface of the separated part. The
structural imperfections may be cracks or voids and represent
points of mechanical weakness that promote fracture or failure of
the part separated from the substrate or material. By minimizing
the size of subsurface damage, the present method improves the
structural integrity and mechanical strength of separated
parts.
[0072] In some cases, the created fault line is not enough to
separate the part from the substrate spontaneously and a secondary
step may be necessary. If so desired, a second laser can be used to
create thermal stress to separate it. Separation can be achieved
after the creation of a fault line, for example, by application of
mechanical force or by using a thermal source (e.g. an infrared
laser, for example a CO.sub.2 laser) to create thermal stress and
force the part to separate from the substrate. Another option is to
have the CO.sub.2 laser only start the separation and then finish
the separation manually. The optional CO.sub.2 laser separation is
achieved, for example, with a defocused cw laser emitting at 10.6
.mu.m and with power adjusted by controlling its duty cycle. Focus
change (i.e., extent of defocusing up to and including focused spot
size) is used to vary the induced thermal stress by varying the
spot size. Defocused laser beams include those laser beams that
produce a spot size larger than a minimum, diffraction-limited spot
size on the order of the size of the laser wavelength. For example,
defocused spot sizes (1/e.sup.2 diameter) of about 2 to 12 mm, or 7
mm, 2 mm and 20 mm can be used for CO.sub.2 lasers, for example,
whose diffraction-limited spot size is much smaller given the
emission wavelength of 10.6 .mu.m. The power density of the cw
laser is controlled or selected to provide a relatively low
intensity beam, such that laser spot heats the surface of the
substrate material to create thermal stress without ablation and
without inducing formation of cracks that deviate substantially
from the plane containing the defect lines. The length of cracks
deviating from the defect lines is less than 20 .mu.m, or less than
5 .mu.m, or less than 1 .mu.m.
[0073] There are several methods to create the defect line. The
optical method of forming the line focus can take multiple forms,
using donut shaped laser beams and spherical lenses, axicon lenses,
diffractive elements, or other methods to form the linear region of
high intensity. The type of laser (picosecond, femtosecond, etc.)
and wavelength (IR, green, UV, etc.) can also be varied, as long as
sufficient optical intensities are reached in the region of focus
to create breakdown of the substrate material through nonlinear
optical effects. Substrate materials include glass, glass
laminates, glass composites, sapphire, glass-sapphire stacks, and
other materials that are substantially transparent to the
wavelength of the laser. A sapphire layer can be bonded onto a
glass substrate, for example. Glass substrates can include
high-performance glass such as Corning's Eagle X6.RTM., or
inexpensive glass such as soda-lime glass, for example.
[0074] In the present application, an ultra-short pulsed laser is
used to create a high aspect ratio vertical defect line in a
consistent, controllable and repeatable manner. The details of the
optical setup that enables the creation of this vertical defect
line are described below, and in U.S. Application No. 61/752,489
filed on Jan. 15, 2013, which is also referenced above, and the
entire contents of which are incorporated by reference as if fully
set forth herein. The essence of this concept is to use an axicon
lens element in an optical lens assembly to create a region of high
aspect ratio, taper-free microchannels using ultra-short
(picoseconds or femtosecond duration) Bessel beams. In other words,
the axicon condenses the laser beam into a high intensity region of
cylindrical shape and high aspect ratio (long length and small
diameter) in the substrate material. Due to the high intensity
created with the condensed laser beam, nonlinear interaction of the
electromagnetic field of the laser and the substrate material
occurs and the laser energy is transferred to the substrate to
effect formation of defects that become constituents of the fault
line. However, it is important to realize that in the areas of the
material where the laser energy intensity is not high (e.g.,
substrate surface, volume of substrate surrounding the central
convergence line), the material is transparent to the laser and
there is no mechanism for transferring energy from the laser to the
material. As a result, nothing happens to the substrate when the
laser intensity is below the nonlinear threshold.
[0075] As described above, it is possible to create microscopic
(e.g., <0.5 .mu.m and >100 nm in diameter or <2 .mu.m and
>100 nm in diameter) elongated defect lines (also referred to
herein as perforations or damage tracks) in a transparent material
using one or more high energy pulses or one or more bursts of high
energy pulses. The perforations represent regions of the substrate
material modified by the laser. The laser-induced modifications
disrupt the structure of the substrate material and constitute
sites of mechanical weakness. Structural disruptions include
compaction, melting, dislodging of material, rearrangements, and
bond scission. The perforations extend into the interior of the
substrate material and have a cross-sectional shape consistent with
the cross-sectional shape of the laser (generally circular). The
average diameter of the perforations may be in the range from 0.1
.mu.m to 50 .mu.m, or in the range from 1 .mu.m to 20 .mu.m, or in
the range from 2 .mu.m to 10 .mu.m, or in the range from 0.1 .mu.m
to 5 .mu.m. In some embodiments, the perforation is a "through
hole", which is a hole or an open channel that extends from the top
to the bottom of the substrate material. In some embodiments, the
perforation may not be a continuously open channel and may include
sections of solid material dislodged from the substrate material by
the laser. The dislodged material blocks or partially blocks the
space defined by the perforation. One or more open channels
(unblocked regions) may be dispersed between sections of dislodged
material. The diameter of the open channels is may be <1000 nm,
or <500 nm, or <400 nm, or <300 nm or in the range from 10
nm to 750 nm, or in the range from 100 nm to 500 nm. The disrupted
or modified area (e.g, compacted, melted, or otherwise changed) of
the material surrounding the holes in the embodiments disclosed
herein, preferably has diameter of <50 .mu.m (e.g, <10
.mu.m).
[0076] The individual perforations can be created at rates of
several hundred kilohertz (several hundred thousand perforations
per second, for example). Thus, with relative motion between the
laser source and the material these perforations can be placed
adjacent to one another (spatial separation varying from sub-micron
to several microns or even tens of microns as desired). This
spatial separation is selected in order to facilitate cutting.
[0077] Turning to FIGS. 2A and 2B, a method of laser drilling a
material includes focusing a pulsed laser beam 2 into a laser beam
focal line 2b, viewed along the beam propagation direction. Laser
beam focal line 2b can be created by several ways, for example,
Bessel beams, Airy beams, Weber beams and Mathieu beams (i.e,
non-diffractive beams), whose field profiles are typically given by
special functions that decay more slowly in the transverse
direction (i.e. direction of propagation) than the Gaussian
function. As shown in FIG. 3A, laser 3 (not shown) emits laser beam
2, at the beam incidence side of the optical assembly 6 referred to
as 2a, which is incident onto the optical assembly 6. The optical
assembly 6 turns the incident laser beam into a laser beam focal
line 2b on the output side over a defined expansion range along the
beam direction (length l of the focal line). The planar substrate 1
to be processed is positioned in the beam path after the optical
assembly overlapping at least partially the laser beam focal line
2b of laser beam 2. Reference 1a designates the surface of the
planar substrate facing the optical assembly 6 or the laser,
respectively, and reference 1b designates the reverse surface of
substrate 1 (the surface remote, or further away from, optical
assembly 6 or the laser). The substrate thickness (measured
perpendicularly to the planes 1a and 1b, i.e., to the substrate
plane) is labeled with d.
[0078] As FIG. 2A depicts, substrate 1 is aligned substantially
perpendicularly to the longitudinal beam axis and thus behind the
same focal line 2b produced by the optical assembly 6 (the
substrate is perpendicular to the drawing plane) and viewed along
the beam direction it is positioned relative to the focal line 2b
in such a way that the focal line 2b viewed in beam direction
starts before the surface 1a of the substrate and stops before the
surface 1b of the substrate, i.e. still within the substrate. In
the overlapping area of the laser beam focal line 2b with substrate
1, i.e. in the substrate material covered by focal line 2b, the
laser beam focal line 2b thus generates (in case of a suitable
laser intensity along the laser beam focal line 2b which intensity
is ensured due to the focusing of laser beam 2 on a section of
length l, i.e. a line focus of length l) a section 2c aligned with
the longitudinal beam direction, along which an induced nonlinear
absorption is generated in the substrate material. Such line focus
can be created by several ways, for example, Bessel beams, Airy
beams, Weber beams and Mathieu beams (i.e, non-diffractive beams),
whose field profiles are typically given by special functions that
decay more slowly in the transverse direction (i.e. direction of
propagation) than the Gaussian function. The induced nonlinear
absorption induces defect line formation in the substrate material
along section 2c. The defect line formation is not only local, but
extends over the entire length of section 2c of the induced
absorption. The length of section 2c (which corresponds to the
length of the overlapping of laser beam focal line 2b with
substrate 1) is labeled with reference L. The average diameter or
extent of the section of the induced absorption (or the sections in
the material of substrate 1 undergoing the defect line formation)
is labeled with reference D. The average extension D basically
corresponds to the average diameter 6 of the laser beam focal line
2b, that is, an average spot diameter in a range of between about
0.1 .mu.m and about 5 .mu.m.
[0079] As FIG. 2A shows, the substrate material (which is
transparent for the wavelength .lamda. of laser beam 2) is heated
due to the induced absorption along the focal line 2b. FIG. 2B
illustrates that the heated substrate material will eventually
expand so that a corresponding induced tension leads to micro-crack
formation, with the tension being the highest at surface 1a.
[0080] Representative optical assemblies 6, which can be applied to
generate the focal line 2b, as well as optical systems in which
these optical assemblies can be applied, are described below. All
assemblies or systems are based on the description above so that
identical references are used for identical components or features
or those which are equal in their function. Therefore only the
differences are described below.
[0081] To ensure high quality (regarding breaking strength,
geometric precision, roughness and avoidance of re-machining
requirements) of the surface of the separated part along which
separation occurs, the individual focal lines positioned on the
substrate surface along the line of separation (fault line) should
be generated using the optical assembly described below
(hereinafter, the optical assembly is alternatively also referred
to as laser optics). The roughness of the separated surface (the
perimeter surface of the separated part) results is determined
primarily by the spot size or the spot diameter of the focal line.
Roughness of a surface can be characterized, for example, by the Ra
surface roughness parameter defined by the ASME B46.1 standard. As
described in ASME B46.1, Ra is the arithmetic average of the
absolute values of the surface profile height deviations from the
mean line, recorded within the evaluation length. In alternative
terms, Ra is the average of a set of absolute height deviations of
individual features (peaks and valleys) of the surface relative to
the mean.
[0082] In order to achieve a small spot size of, for example, 0.5
.mu.m to 2 .mu.m for a given wavelength .lamda. of the laser 3 that
interacts with the material of substrate 1), certain requirements
must usually be imposed on the numerical aperture of laser optics
6. These requirements are met by laser optics 6 described below. In
order to achieve the required numerical aperture, the optics must,
on the one hand, dispose of the required opening for a given focal
length, according to the known Abbe formulae (N.A.=n sin (theta),
n: refractive index of the glass to be processes, theta: half the
aperture angle; and theta=arctan (D.sub.L/2f); D.sub.L: aperture
diameter, f: focal length). On the other hand, the laser beam must
illuminate the optics up to the required aperture, which is
typically achieved by means of beam widening using widening
telescopes between laser and focusing optics.
[0083] The spot size should not vary too strongly for the purpose
of a uniform interaction along the focal line. This can, for
example, be ensured (see the embodiment below) by illuminating the
focusing optics only in a small, circular area so that the beam
opening and thus the percentage of the numerical aperture only vary
slightly.
[0084] According to FIG. 3A (section perpendicular to the substrate
plane at the level of the central beam in the laser beam bundle of
laser radiation 2; here, too, laser beam 2 is incident
perpendicularly (before entering optical assembly 6) to the
substrate plane, i.e. angle .theta. is 0.degree. so that the focal
line 2b or the section of the induced absorption 2c is parallel to
the substrate normal), the laser radiation 2a emitted by laser 3 is
first directed onto a circular aperture 8 which is completely
opaque for the laser radiation used. Aperture 8 is oriented
perpendicular to the longitudinal beam axis and is centered on the
central beam of the depicted beam bundle 2a. The diameter of
aperture 8 is selected in such a way that the beam bundles near the
center of beam bundle 2a or the central beam (here labeled with
2aZ) hit the aperture and are completely absorbed by it. Only the
beams in the outer perimeter range of beam bundle 2a (marginal
rays, here labeled with 2aR) are not absorbed due to the reduced
aperture size compared to the beam diameter, but pass aperture 8
laterally and hit the marginal areas of the focusing optic elements
of the optical assembly 6, which is designed as a spherically cut,
bi-convex lens 7 in this embodiment.
[0085] Lens 7 centered on the central beam is deliberately designed
as a non-corrected, bi-convex focusing lens in the form of a
common, spherically cut lens. In this design embodiment, the
spherical aberration of such a lens is deliberately used. As an
alternative, aspheres or multi-lens systems deviating from ideally
corrected systems, which do not form an ideal focal point but a
distinct, elongated focal line of a defined length, can also be
used (i.e., lenses or systems which do not have a single focal
point). The zones of the lens thus focus along a focal line 2b,
subject to the distance from the lens center. The diameter of
aperture 8 across the beam direction is approximately 90% of the
diameter of the beam bundle (beam bundle diameter defined by the
extension to the decrease to 1/e.sup.2) and approximately 75% of
the diameter of the lens 7 of the optical assembly 6. The focal
line 2b of a not aberration-corrected spherical lens 7 generated by
blocking out the beam bundles in the center is thus used. FIG. 3A
shows the section in one plane through the central beam, the
complete three-dimensional bundle can be seen when the depicted
beams are rotated around the focal line 2b.
[0086] One potential disadvantage of this type of a focal line
formed by lens 7 and the system shown in FIG. 3A is that the
conditions (spot size, laser intensity) along the focal line, and
thus along the desired depth in the material, vary and that
therefore the desired type of interaction (no melting, induced
absorption, thermal-plastic deformation up to crack formation) may
possibly occur only in selected portions of the focal line. This
means in turn that possibly only a part of the incident laser light
is absorbed in the desired way. In this way, the efficiency of the
process (required average laser power for the desired separation
speed) is impaired on the one hand, and on the other hand the laser
light might be transmitted into undesired deeper places (parts or
layers adherent to the substrate or the substrate holding fixture)
and interact there in an undesirable way (heating, diffusion,
absorption, unwanted modification).
[0087] FIG. 3B-1-4 show (not only for the optical assembly in FIG.
3A, but basically also for any other applicable optical assembly 6)
that the laser beam focal line 2b can be positioned differently by
suitably positioning and/or aligning the optical assembly 6
relative to substrate 1 as well as by suitably selecting the
parameters of the optical assembly 6. As FIG. 3B-1 outlines, the
length l of the focal line 2b can be adjusted in such a way that it
exceeds the substrate thickness d (here by factor 2). If substrate
1 is placed (viewed in longitudinal beam direction) centrally to
focal line 2b, the section of induced absorption 2c is generated
over the entire substrate thickness. The laser beam focal line 2b
can have a length l in a range of between about 0.1 mm and about
100 mm or in a range of between about 0.1 mm and about 10 mm, or in
a range of between about 0.1 mm and about 1 mm, for example.
Various embodiments can be configured to have length 1 of about 0.1
mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.7 mm, 1 mm, 2 mm, 3 mm or 5
mm, for example.
[0088] In the case shown in FIG. 3B-2, a focal line 2b of length l
is generated which corresponds more or less to the substrate
extension d. As substrate 1 relative to line 2 is positioned in
such a way that line 2b starts in a point before, i.e. outside the
substrate, the length L of the section of induced absorption 2c
(which extends here from the substrate surface to a defined
substrate depth, but not to the reverse surface 1b) is smaller than
the length l of focal line 2b. FIG. 3B-3 shows the case in which
the substrate 1 (viewed along the beam direction) is positioned
above the starting point of focal line 2b so that, as in FIG. 3B-2,
the length l of line 2b is greater than the length L of the section
of induced absorption 2c in substrate 1. The focal line thus starts
within the substrate and extends over the reverse (remote) surface
1b to beyond the substrate. FIG. 3B-4 shows the case in which the
focal line length l is smaller than the substrate thickness d so
that--in the case of a central positioning of the substrate
relative to the focal line viewed in the direction of
incidence--the focal line starts near the surface 1a within the
substrate and ends near the surface 1b within the substrate
(1=0.75d).
[0089] It is particularly advantageous to realize the focal line
positioning in such a way that at least one surface 1a, 1b is
covered by the focal line, i.e. that the section of induced
absorption 2c starts at least on one surface. In this way it is
possible to achieve virtually ideal drilling or cutting avoiding
ablation, feathering and particulation at the surface.
[0090] FIG. 4 depicts another applicable optical assembly 6. The
basic construction follows the one described in FIG. 3A so that
only the differences are described below. The depicted optical
assembly is based the use of optics with a non-spherical free
surface in order to generate the focal line 2b, which is shaped in
such a way that a focal line of defined length 1 is formed. For
this purpose, aspheres can be used as optic elements of the optical
assembly 6. In FIG. 4, for example, a so-called conical prism, also
often referred to as axicon, is used. An axicon is a special,
conically cut lens which forms a spot source on a line along the
optical axis (or transforms a laser beam into a ring). The layout
of such an axicon is principally known to one skilled in the art;
the cone angle in the example is 10.degree.. The apex of the axicon
labeled here with reference 9 is directed towards the incidence
direction and centered on the beam center. As the focal line 2b of
the axicon 9 already starts in its interior, substrate 1 (here
aligned perpendicularly to the main beam axis) can be positioned in
the beam path directly behind axicon 9. As FIG. 4 shows, it is also
possible to shift substrate 1 along the beam direction due to the
optical characteristics of the axicon while remaining within the
range of focal line 2b. The section of the induced absorption 2c in
the material of substrate 1 therefore extends over the entire
substrate depth d.
[0091] However, the depicted layout is subject to the following
restrictions: Since the region of focal line 2b formed by axicon 9
begins with axicon 9, a significant part of the laser energy is not
focused into the section of induced absorption 2c of focal line 2b,
which is located within the material, in the situation where there
is a separation between axicon 9 and the material to be processed.
Furthermore, length l of focal line 2b is related to the beam
diameter for the refraction indices and cone angles of axicon 9.
This is why, in case of relatively thin materials (several
millimeters), the total focal line is much longer than the
thickness of the substrate, having the effect that the laser energy
is again not specifically focused into the material.
[0092] For this reason, it may be desirable to use an optical
assembly 6 which includes both an axicon and a focusing lens. FIG.
5A depicts such an optical assembly 6 in which a first optical
element (viewed along the beam direction) with a non-spherical free
surface designed to form a laser beam focal line 2b is positioned
in the beam path of laser 3. In the case shown in FIG. 5A, this
first optical element is an axicon 10 with a cone angle of
5.degree., which is positioned perpendicularly to the beam
direction and centered on laser beam 3. The apex of the axicon is
oriented towards the beam direction. A second, focusing optical
element, here the plano-convex lens 11 (the curvature of which is
oriented towards the axicon), is positioned in beam direction at a
distance Z1 from the axicon 10. The distance Z1, in this case
approximately 300 mm, is selected in such a way that the laser
radiation formed by axicon 10 circularly incident on the outer
radial portion of lens 11. Lens 11 focuses the circular radiation
on the output side at a distance Z2, in this case approximately 20
mm from lens 11, on a focal line 2b of a defined length, in this
case 1.5 mm. The effective focal length of lens 11 is 25 mm in this
embodiment. The circular transformation of the laser beam by axicon
10 is labeled with the reference SR.
[0093] FIG. 5B depicts the formation of the focal line 2b or the
induced absorption 2c in the material of substrate 1 according to
FIG. 5A in detail. The optical characteristics of both elements 10,
11 as well as their positioning is selected in such a way that the
extension 1 of the focal line 2b in beam direction is exactly
identical with the thickness d of substrate 1. Consequently, an
exact positioning of substrate 1 along the beam direction is
required in order to position the focal line 2b exactly between the
two surfaces 1a and 1b of substrate 1, as shown in FIG. 5B.
[0094] It is therefore advantageous if the focal line is formed at
a certain distance from the laser optics, and if the greater part
of the laser radiation is focused up to a desired end of the focal
line. As described, this can be achieved by illuminating a
primarily focusing element 11 (lens) only circularly (annularly) on
a required zone, which, on the one hand, serves to realize the
required numerical aperture and thus the required spot size, on the
other hand, however, the circle of diffusion diminishes in
intensity after the required focal line 2b over a very short
distance in the center of the spot, as a basically circular spot is
formed. In this way, the defect line formation is stopped within a
short distance in the required substrate depth. A combination of
axicon 10 and focusing lens 11 meets this requirement. The axicon
acts in two different ways: due to the axicon 10, a usually round
laser spot is sent to the focusing lens 11 in the form of a ring,
and the asphericity of axicon 10 has the effect that a focal line
is formed beyond the focal plane of the lens instead of at a focal
point in the focal plane. The length l of focal line 2b can be
adjusted via the beam diameter on the axicon. The numerical
aperture along the focal line, on the other hand, can be adjusted
via the distance Z1 (axicon-lens separation) and via the cone angle
of the axicon. In this way, the entire laser energy can be
concentrated in the focal line.
[0095] If the defect line formation is supposed to continue to the
back side of the substrate, the circular (annular) illumination
still has the advantage that (1) the laser power is used optimally
in the sense that most of the laser light remains concentrated in
the required length of the focal line, and (2) it is possible to
achieve a uniform spot size along the focal line--and thus a
uniform separation process along the focal line--due to the
circularly illuminated zone in conjunction with the desired
aberration set by means of the other optical functions.
[0096] Instead of the plano-convex lens depicted in FIG. 5A, it is
also possible to use a focusing meniscus lens or another higher
corrected focusing lens (asphere, multi-lens system).
[0097] In order to generate very short focal lines 2b using the
combination of an axicon and a lens depicted in FIG. 5A, it would
be necessary to select a very small beam diameter of the laser beam
incident on the axicon. This has the practical disadvantage that
the centering of the beam onto the apex of the axicon must be very
precise and that therefore the result is very sensitive to
directional variations of the laser (beam drift stability).
Furthermore, a tightly collimated laser beam is very divergent,
i.e. due to the light deflection the beam bundle becomes blurred
over short distances.
[0098] Turning to FIG. 6, both effects can be avoided by inserting
another lens, a collimating lens 12 in the optical assembly 6. The
additional positive lens 12 serves to adjust the circular
illumination of focusing lens 11 very tightly. The focal length f
of collimating lens 12 is selected in such a way that the desired
circle diameter dr results from distance Z1a from the axicon to the
collimating lens 12, which is equal to f'. The desired width br of
the ring can be adjusted via the distance Z1b (collimating lens 12
to focusing lens 11). As a matter of pure geometry, the small width
of the circular illumination leads to a short focal line. A minimum
can be achieved at distance f'.
[0099] The optical assembly 6 depicted in FIG. 6 is thus based on
the one depicted in FIG. 5A so that only the differences are
described below. The collimating lens 12, here also designed as a
plano-convex lens (with its curvature towards the beam direction)
is additionally placed centrally in the beam path between axicon 10
(with its apex towards the beam direction), on the one side, and
the plano-convex lens 11, on the other side. The distance of
collimating lens 12 from axicon 10 is referred to as Z1a, the
distance of focusing lens 11 from collimating lens 12 as Z1b, and
the distance of the focal line 2b from the focusing lens 11 as Z2
(always viewed in beam direction). As shown in FIG. 6, the circular
radiation SR formed by axicon 10, which is incident divergently and
under the circle diameter dr on the collimating lens 12, is
adjusted to the required circle width br along the distance Z1b for
an at least approximately constant circle diameter dr at the
focusing lens 11. In the case shown, a very short focal line 2b is
intended to be generated so that the circle width br of
approximately 4 mm at lens 12 is reduced to approximately 0.5 mm at
lens 11 due to the focusing properties of lens 12 (circle diameter
dr is 22 mm in the example).
[0100] In the depicted example, it is possible to achieve a length
of the focal line 1 of less than 0.5 mm using a typical laser beam
diameter of 2 mm, a focusing lens 11 with a focal length f=25 mm,
and a collimating lens with a focal length f'=150 mm, and choosing
Z1a=Z1b=140 mm and Z2=15 mm.
[0101] Once the fault lines are created, separation can occur via:
1) manual or mechanical stress on or around the fault line; the
stress or pressure should create tension that pulls both sides of
the fault line apart to break the areas that are still bonded
together; 2) using a heat source to create a thermal stress zone
around the fault line to put the defect line in tension and induce
partial or total separation along the fault line; and 3) using an
ion exchange process to introduce stress in the region around the
fault line. Additionally, the use of the picosecond laser process
on either non-chamfered edges or incompletely chamfered edges, but
that have "sacrificial" regions that control damage caused by edge
impact is described below.
[0102] The second method takes advantage of an existing edge to
create a chamfer by applying a focused (typically CO.sub.2) laser
very close to the intersection between the surfaces of the edge and
substrate. The laser beam must be highly absorbed by the substrate
material to create a temperature gradient that spans the interval
extending from the material's melting temperature down to its
strain point. This thermal gradient generates a stress profile that
results in separation or peeling of a very thin strip of the
material. The thin strip of material curls and peels off from the
bulk of the material and has dimensions determined by the depth of
the region defined between the strain and softening zones. This
method can be combined with the previous method to peel the thin
strip of material off at planes dictated by the fault lines. In
this embodiment, the thermal gradient is established in the
vicinity of the fault line. The combination of thermal gradient and
fault line can yield better control of the chamfer edge shape and
surface texture than would otherwise be possible by using purely
thermal means.
[0103] FIG. 7A gives an overview of the processes described in the
present application.
[0104] One method relies on induced nonlinear absorption to create
fault lines as described hereinabove for forming the desired shapes
of parts and edges using a short-pulse laser. The process relies on
the material transparency to the laser wavelength in the linear
regime (low laser intensity), which provides high surface and edge
quality with reduced subsurface damage created by the area of high
intensity around the laser focus. One of the key enablers of this
process is the high aspect ratio of the defect line created by the
ultra-short pulsed laser. It allows creation of a fault line with
long and deep defect line that can extend from the top to the
bottom surfaces of the material to be cut and chamfered. In
principle, each defect line (perforation) can be created by a
single pulse and if desired, additional pulses can be used to
increase the extension of the affected area (depth and width).
[0105] Using the same principle illustrated in FIGS. 1A-1C to
separate a glass substrate with flat edges, the process to produce
chamfered edges can be modified as illustrated in FIG. 7B. To
separate and form a chamfered edge, three separate planes of defect
lines that intersect and define the boundaries of the desired edge
shape can be formed in the material. Different shapes can be
created by using just two intersecting defect line planes as
illustrated in FIG. 7C, but the interior flat part of the edge may
need to be broken or separated without any defect lines (e.g.
through mechanical or thermal means).
Laser and Optical System
[0106] For the purpose of cutting glass or other transparent
brittle materials, a process was developed that uses a 1064 nm
picosecond laser in combination with line-focus beam forming optics
to create defect lines in substrates. A sample Corning.RTM.
Gorilla.RTM. Glass code 2320 substrate with 0.7 mm thickness was
positioned so that it was within the line-focus. With a line-focus
of .about.1 mm extension, and a picosecond laser that produces
output power of >30 W at a repetition rate of 200 kHz
(.about.150 .mu.J/pulse), then the optical intensities in the line
region can easily be high enough to create non-linear absorption in
the material. A region of damaged, ablated, vaporized, or otherwise
modified material is created that approximately follows the linear
region of high intensity.
[0107] Note that the typical operation of such a picosecond laser
creates a "burst" of pulses. Each "burst" may contain multiple
sub-pulses of very short duration (.about.10 psec). Each sub-pulse
is separated in time by approximately 20 nsec (50 MHz), with the
time often governed by the laser cavity design. The time between
each "burst" will be much longer, often .about.5 .mu.sec, for a
laser repetition rate of .about.200 kHz. The exact timings, pulse
durations, and repetition rates can vary depending on the laser
design. But short pulses (<15 psec) of high intensity have been
shown to work well with this technique.
[0108] More specifically, as illustrated in FIGS. 8A and 8B,
according to selected embodiments described herein, the picosecond
laser creates a "burst" 500 of pulses 500A, sometimes also called a
"burst pulse". Bursting is a type of laser operation where the
emission of pulses is not in a uniform and steady stream but rather
in tight clusters of pulses [See reference]. Each "burst" 500 may
contain multiple pulses 500A (such as 2 pulses, 3 pulses, 4 pulses,
5 pulses, 10, 15, 20, or more) of very short duration T.sub.d up to
100 psec (for example, 0.1 psec, 5 psec, 10 psec, 15 psec, 18psec,
20 psec, 22 psec, 25 psec, 30 psec, 50 psec, 75 psec, or
therebetween). The pulse duration is generally in a range from
about 1 psec to about 1000 psec, or in a range from about 1 psec to
about 100 psec, or in a range from about 2 psec to about 50 psec,
or in a range from about 5 psec to about 20 psec. These individual
pulses 500A within a single burst 500 can also be termed
"sub-pulses," which simply denotes the fact that they occur within
a single burst of pulses. The energy or intensity of each laser
pulse 500A within the burst may not be equal to that of other
pulses within the burst, and the intensity distribution of the
multiple pulses within a burst 500 may follow an exponential decay
in time governed by the laser design. Preferably, each pulse 500A
within the burst 500 of the exemplary embodiments described herein
are separated in time from the subsequent pulse in the burst by a
duration T.sub.p from 1 nsec to 50 nsec (e.g. 10-50 nsec, or 10-40
nsec, or 10-30 nsec, with the time often governed by the laser
cavity design. For a given laser, the time separation T.sub.p
between each pulses (pulse-to-pulse separation) within a burst 500
is relatively uniform (.+-.10%). For example, in some embodiments,
each pulse is separated in time from the subsequent pulse by
approximately 20 nsec (50 MHz pulse repetition frequency). For
example, for a laser that produces pulse-to-pulse separation
T.sub.p of about 20 nsec, the pulse-to-pulse separation T.sub.p
within a burst is maintained within about .+-.10%, or is about
.+-.2 nsec. The time between each "burst" (i.e., time separation
T.sub.b between bursts) will be much longer (e.g.,
0.25.ltoreq.T.sub.b.ltoreq.1000 microseconds, for example 1-10
microseconds, or 3-8 microseconds,) For example in some of the
exemplary embodiments of the laser described herein it is around 5
microseconds for a laser repetition rate or frequency of about 200
kHz. The laser repetition rate is also referred to as burst
repetition frequency or burst repetition rate herein, and is
defined as the time between the first pulse in a burst to the first
pulse in the subsequent burst. In other embodiments, the burst
repetition frequency is in a range of between about 1 kHz and about
4 MHz, or in a range between about 1 kHz and about 2 MHz, or in a
range of between about 1 kHz and about 650 kHz, or in a range of
between about 10 kHz and about 650 kHz. The time T.sub.b between
the first pulse in each burst to the first pulse in the subsequent
burst may be 0.25 microsecond (4 MHz burst repetition rate) to 1000
microseconds (1 kHz burst repetition rate), for example 0.5
microseconds (2 MHz burst repetition rate) to 40 microseconds (25
kHz burst repetition rate), or 2 microseconds (500 kHz burst
repetition rate) to 20 microseconds (50 kHz burst repetition rate).
The exact timings, pulse durations, and repetition rates can vary
depending on the laser design and user-controllable operating
parameters. Short pulses (T.sub.d<20 psec and preferably
T.sub.d.ltoreq.15 psec) of high intensity have been shown to work
well.
[0109] The required energy to modify the material can be described
in terms of the burst energy--the energy contained within a burst
(each burst 500 contains a series of pulses 500A), or in terms of
the energy contained within a single laser pulse (many of which may
comprise a burst). For these applications, the energy per burst
(per millimeter of the material to be cut) can be from 10-2500
.mu.J, or from 20-1500 .mu.J, or from 25-750 .mu.J, or from 40-2500
.mu.J, or from 100-1500 .mu.J, or from 200-1250 .mu.J, or from
250-1500 .mu.J, or from 250-750 .mu.J. The energy of an individual
pulse within the burst will be less, and the exact individual laser
pulse energy will depend on the number of pulses 500A within the
burst 500 and the rate of decay (e.g, exponential decay rate) of
the laser pulses with time as shown in FIGS. 8A and 8B. For
example, for a constant energy/burst, if a pulse burst contains 10
individual laser pulses 500A, then each individual laser pulse 500A
will contain less energy than if the same burst pulse 500 had only
2 individual laser pulses.
[0110] The use of lasers capable of generating such pulse bursts is
advantageous for cutting or modifying transparent materials, for
example glass. In contrast with the use of single pulses spaced
apart in time by the repetition rate of a single-pulsed laser, the
use of a burst pulse sequence that spreads the laser energy over a
rapid sequence of pulses within burst 500 allows access to larger
timescales of high intensity interaction with the material than is
possible with single-pulse lasers. While a single-pulse can be
expanded in time, conservation of energy dictates that as this is
done, the intensity within the pulse must drop as roughly one over
the pulse width. Hence if a 10 psec single pulse is expanded to a
10 nsec pulse, the intensity drops by roughly three orders of
magnitude. Such a reduction can reduce the optical intensity to the
point where non-linear absorption is no longer significant and the
light-material interaction is no longer strong enough to allow for
cutting. In contrast, with a burst pulse laser, the intensity
during each pulse or sub-pulse 500A within the burst 500 can remain
very high--for example three pulses 500A with pulse duration
T.sub.d 10 psec that are spaced apart in time by a separation
T.sub.p of approximately 10 nsec still allows the intensity within
each pulse to be approximately three times higher than that of a
single 10 psec pulse, while the laser is allowed to interact with
the material over a timescale that is three orders of magnitude
larger. This adjustment of multiple pulses 500A within a burst thus
allows manipulation of timescale of the laser-material interaction
in ways that can facilitate greater or lesser light interaction
with a pre-existing plasma plume, greater or lesser light-material
interaction with atoms and molecules that have been pre-excited by
an initial or previous laser pulse, and greater or lesser heating
effects within the material that can promote the controlled growth
of defect lines (perforations). The amount of burst energy required
to modify the material will depend on the substrate material
composition and the length of the line focus used to interact with
the substrate. The longer the interaction region, the more the
energy is spread out, and the higher the burst energy that will be
required.)
[0111] A defect line or a hole is formed in the material when a
single burst of pulses strikes essentially the same location on the
glass. That is, multiple laser pulses within a single burst can
produce a single defect line or a hole location in the glass. Of
course, if the glass is translated (for example by a constantly
moving stage) or the beam is moved relative to the glass, the
individual pulses within the burst cannot be at exactly the same
spatial location on the glass. However, they are well within 1
.mu.m of one another-i.e., they strike the glass at essentially the
same location. For example, they may strike the glass at a spacing
sp where 0<sp.ltoreq.500 nm from one another. For example, when
a glass location is hit with a burst of 20 pulses the individual
pulses within the burst strike the glass within 250 nm of each
other. Thus, in some embodiments 1 nm<sp<250 nm. In in some
embodiments 1 nm<sp<100 nm.
Hole or Damage Track Formation
[0112] If the substrate has sufficient stress (e.g. with ion
exchanged glass), then the part will spontaneously separate along
the fault line traced out by the laser process. However, if there
is not a lot of stress inherent to the substrate, then the
picosecond laser will simply form defect lines in the substrate.
These defect lines may take the form of holes with interior
dimensions (diameters) .about.0.5-1.5 .mu.m.
[0113] The holes or defect lines may or may not perforate the
entire thickness of the material, and may or may not be a
continuous opening throughout the depth of the material. FIG. 1C
shows an example of such tracks perforating the entire thickness of
a piece of 700 .mu.m thick unstrengthened Gorilla.RTM. Glass
substrate. The perforations or damage tracks are observed through
the side of a cleaved edge. The tracks through the material are not
necessarily through holes--there may be regions of glass that plug
the holes, but they are generally small in size.
[0114] Note that upon separation at the fault line, fracture occurs
along the defect lines to provide a part or edge having a surface
with features derived from the defect lines. Before separation, the
defect lines are generally cylindrical in shape. Upon separation,
the defect lines fracture and remnants of the defect lines are
evident in the contours of the surface of the separated part or
edge. In an ideal model, the defect lines are cleaved in half upon
separation so that the surface of the separated part or edge
includes serrations corresponding to half-cylinders. In practice,
separation may deviate from an ideal model and the serrations of
the surface may be an arbitrary fraction of the shape of the
original defect line. Irrespective of the particular form, features
of the separated surface will be referred to as defect lines to
indicate the origin of their existence.
[0115] The lateral spacing (pitch) between the defect lines is
determined by the pulse rate of the laser as the substrate is
translated underneath the focused laser beam. Only a single
picosecond laser pulse or burst is necessary to form an entire
hole, although multiple pulses or bursts may be used if desired. To
form defect lines at different pitches, the laser can be triggered
to fire at longer or shorter intervals. For cutting operations, the
laser triggering generally is synchronized with the stage driven
motion of the part beneath the beam, so laser pulses are triggered
at a fixed interval, such as every 1 .mu.m, or every 5 .mu.m. The
exact spacing between adjacent defect lines is determined by the
material properties that facilitate crack propagation from
perforation to perforation, given the stress level in the
substrate. Instead of cutting a substrate, it is also possible to
use the same method to only perforate the material. In this case,
the defect lines may be separated by larger spacings (e.g. 5 .mu.m
pitch or greater).
[0116] The laser power and lens focal length (which determines the
focal line length and hence power density) are particularly
important to ensure full penetration of the glass and low surface
and sub-surface damage.
[0117] In general, the higher the available laser power, the faster
the material can be cut with the above process. The process(s)
disclosed herein can cut glass at a cutting speed of 0.25 msec, or
faster. A cut speed (or cutting speed) is the rate the laser beam
moves relative to the surface of the substrate material (e.g.,
glass) while creating multiple defect lines holes. High cut speeds,
such as, for example 400 mm/sec, 500 mm/sec, 750 mm/sec, 1 msec,
1.2 msec, 1.5 msec, or 2 msec, or even 3.4 msec to 4 msec are often
desired in order to minimize capital investment for manufacturing,
and to optimize equipment utilization rate. The laser power is
equal to the burst energy multiplied by the burst repetition
frequency (rate) of the laser. In general, to cut glass materials
at high cutting speeds, the defect lines are typically spaced apart
by 1-25 .mu.m, in some embodiments the spacing is preferably 3
.mu.m or larger--for example 3-12 .mu.m, or for example 5-10
.mu.m.
[0118] For example, to achieve a linear cutting speed of 300
mm/sec, 3 .mu.m hole pitch corresponds to a pulse burst laser with
at least 100 kHz burst repetition rate. For a 600 mm/sec cutting
speed, a 3 .mu.m pitch corresponds to a burst-pulsed laser with at
least 200 kHz burst repetition rate. A pulse burst laser that
produces at least 40 .mu.J/burst at 200 kHz, and cuts at a 600 mm/s
cutting speed needs to have a laser power of at least 8 Watts.
Higher cut speeds require accordingly higher laser powers.
[0119] For example, a 0.4 msec cut speed at 3 .mu.m pitch and 40
.mu.J/burst would require at least a 5 W laser, a 0.5 msec cut
speed at 3 .mu.m pitch and 40 .mu.J/burst would require at least a
6 W laser. Thus, preferably the laser power of the pulse burst ps
laser is 6 W or higher, more preferably at least 8 W or higher, and
even more preferably at least 10 W or higher. For example, in order
to achieve a 0.4 msec cut speed at 4 .mu.m pitch (defect line
spacing, or damage tracks spacing) and 100 .mu.J/burst, one would
require at least a 10 W laser, and to achieve a 0.5 msec cut speed
at 4 .mu.m pitch and 100 .mu.J/burst, one would require at least a
12 W laser. For example, a to achieve a cut speed of 1 m/sec at 3
.mu.m pitch and 40 .mu.J/burst, one would require at least a 13 W
laser. Also, for example, 1 m/sec cut speed at 4 .mu.m pitch and
400 .mu.J/burst would require at least a 100 W laser.
[0120] The optimal pitch between defect lines (damage tracks) and
the exact burst energy is material dependent and can be determined
empirically. However, it should be noted that raising the laser
pulse energy or making the damage tracks at a closer pitch are not
conditions that always make the substrate material separate better
or with improved edge quality. A pitch that is too small (for
example <0.1 micron, or in some exemplary embodiments <1
.mu.m, or in other embodiments <2 .mu.m) between defect lines
(damage tracks) can sometimes inhibit the formation of nearby
subsequent defect lines (damage tracks), and often can inhibit the
separation of the material around the perforated contour. An
increase in unwanted micro cracking within the glass may also
result if the pitch is too small. A pitch that is too long (e.g.
>50 .mu.m, and in some glasses >25 .mu.m or even >20
.mu.m) may result in "uncontrolled microcracking"--i.e., where
instead of propagating from defect line to defect line along the
intended contour, the microcracks propagate along a different path,
and cause the glass to crack in a different (undesirable) direction
away from the intended contour. This may ultimately lower the
strength of the separated part since the residual microcracks
constitute flaws that weaken the glass. A burst energy for forming
defect lines that is too high (e.g., >2500 .mu.J/burst, and in
some embodiments >500 .mu.J/burst) can cause "healing" or
re-melting of previously formed defect lines, which may inhibit
separation of the glass. Accordingly, it is preferred that the
burst energy be <2500 .mu.J/burst, for example, <500
.mu.J/burst. Also, using a burst energy that is too high can cause
formation of microcracks that are extremely large and create
structural imperfections that can reduce the edge strength of the
part after separation. A burst energy that is too low (e.g. <40
.mu.J/burst) may result in no appreciable formation of defect lines
within the glass, and hence may necessitate especially high
separation force or result in a complete inability to separate
along the perforated contour.
[0121] Typical exemplary cutting rates (speeds) enabled by this
process are, for example, 0.25 m/sec and higher. In some
embodiments, the cutting rates are at least 300 mm/sec. In some
embodiments, the cutting rates are at least 400 mm/sec, for
example, 500 mm/sec to 2000 mm/sec, or higher. In some embodiments
the picosecond (ps) laser utilizes pulse bursts to produce defect
lines with periodicity between 0.5 .mu.m and 13 .mu.m, e.g. between
0.5 and 3 .mu.m. In some embodiments, the pulsed laser has laser
power of 10 W-100 W and the material and/or the laser beam are
translated relative to one another at a rate of at least 0.25
m/sec; for example, at the rate of 0.25 m/sec to 0.35 m/sec, or 0.4
m/sec to 5 m/sec. Preferably, each pulse burst of the pulsed laser
beam has an average laser energy measured at the workpiece greater
than 40 .mu.J per burst per mm thickness of workpiece. Preferably,
each pulse burst of the pulsed laser beam has an average laser
energy measured at the workpiece greater of less than 2500 .mu.J
per burst per mm thickness of workpiece, and preferably lass than
about 2000 .mu.J per burst per mm thickness of workpiece, and in
some embodiments less than 1500 .mu.J per burst per mm thickness of
workpiece; for example, not more than 500 .mu.J per burst per mm
thickness of workpiece.
[0122] We discovered that much higher (5 to 10 times higher)
volumetric pulse energy density (.mu.J/.mu.m.sup.3) is required for
perforating alkaline earth boroaluminosilicate glasses with low or
no alkali content. This can be achieved, for example, by utilizing
pulse burst lasers, preferably with at least 2 pulses per burst and
providing volumetric energy densities within the alkaline earth
boroaluminosilicate glasses (with low or no alkali) of about
0.050/.mu.J/.mu.m.sup.3 or higher, e.g., at least 0.1
.mu.J/.mu.m.sup.3, for example 0.1-0.5 .mu.J/.mu.m.sup.3.
[0123] Accordingly, it is preferable that the laser produces pulse
bursts with at least 2 pulses per burst. For example, in some
embodiments the pulsed laser has a power of 10 W-150 W (e.g., 10
W-100 W) and produces pulse bursts with at least 2 pulses per burst
(e.g., 2-25 pulses per burst). In some embodiments the pulsed laser
has a power of 25 W-60 W, and produces pulse bursts with at least
2-25 pulses per burst, and periodicity or distance between the
adjacent defect lines produced by the laser bursts is 2-10 .mu.m.
In some embodiments, the pulsed laser has a power of 10 W-100 W,
produces pulse bursts with at least 2 pulses per burst, and the
workpiece and the laser beam are translated relative to one another
at a rate of at least 0.25 m/sec. In some embodiments the workpiece
and/or the laser beam are translated relative to one another at a
rate of at least 0.4 m/sec.
[0124] For example, for cutting 0.7 mm thick non-ion exchanged
Corning code 2319 or code 2320 Gorilla.RTM. glass, it is observed
that pitches of 3-7 .mu.m can work well, with pulse burst energies
of about 150-250 .mu.J/burst, and burst pulse numbers that range
from 2-15, and preferably with pitches of 3-5 .mu.m and burst pulse
numbers (number of pulses per burst) of 2-5.
[0125] At 1 m/sec cut speeds, the cutting of Eagle XG.RTM. glass
typically requires utilization of laser powers of 15-84 W, with
30-45 W often being sufficient. In general, across a variety of
glass and other transparent materials, applicants discovered that
laser powers between 10 W and 100 W are preferred to achieve
cutting speeds from 0.2-1 m/sec, with laser powers of 25-60 W being
sufficient (or optimum) for many glasses. For cutting speeds of 0.4
msec to 5 msec, laser powers should preferably be 10 W-150 W, with
burst energy of 40-750 .mu.J/burst, 2-25 bursts per pulse
(depending on the material that is cut), and defect line separation
(pitch) of 3 to 15 .mu.m, or 3-10 .mu.m. The use of picosecond
pulse burst lasers would be preferable for these cutting speeds
because they generate high power and the required number of pulses
per burst. Thus, according to some exemplary embodiments, the
pulsed laser produces 10 W-100 W of power, for example 25 W to 60
W, and produces pulse bursts at least 2-25 pulses per burst and the
distance between the defect lines is 2-15 .mu.m; and the laser beam
and/or workpiece are translated relative to one another at a rate
of at least 0.25 msec, in some embodiments at least 0.4 msec, for
example 0.5 msec to 5 msec, or faster.
Cutting and Separating Chamfered Edges
Chamfer Method 1
[0126] Different conditions were found that allow the separation of
chamfered edges using unstrengthened Gorilla.RTM. Glass,
specifically Corning code 2320. The first method is to use the
picosecond laser to create defect lines to form a fault line
consistent with the desired shape (in this case a chamfered edge).
After this step, mechanical separation can be accomplished by using
a breaking plier, manually bending the part, or any method that
creates tension that initiates and propagates the separation along
the fault line. To create chamfered edges with defect lines in 700
.mu.m thick unstrengthened Gorilla.RTM. Glass and mechanically
separate the parts, the best results were found for the following
optics and laser parameters:
[0127] Picosecond Laser (1064 nm)
[0128] Input beam diameter to axicon lens .about.2 mm
[0129] Axicon angle=10 degrees
[0130] Initial collimating lens focal length=125 mm
[0131] Final objective lens focal length=40 mm
[0132] Focus set to be at Z=0.7 mm (i.e. line focus set to be
centered with regard to the glass thickness)
[0133] Laser power at 100% of full power (.about.40 Watts)
[0134] Burst repetition rate of the laser=200 kHz.
[0135] Energy per burst=200 .mu.J (40 W/200 kHz)
[0136] Pitch=5 .mu.m
[0137] 3 pulses/burst
[0138] Single pass per defect line
[0139] An alternative method of achieving separation is to use a
relatively defocused CO.sub.2 laser beam (.about.2 mm spot
diameter) that follows the picosecond laser step after the
picosecond laser has finished tracing the desired contour. The
thermal stress induced by the CO.sub.2 laser is enough to initiate
and propagate the separation or shaping of the edge along the
desired contour. For this case, the best results were found for the
following optics and laser parameters:
[0140] Picosecond Laser (1064 nm)
[0141] Input beam diameter to axicon lens .about.2 mm
[0142] Axicon angle=10 degrees
[0143] Initial collimating lens focal length=125 mm
[0144] Final objective lens focal length=40 mm
[0145] Focus set to be at Z=0.7 mm (i.e. line focus set to be
centered with regard to the glass thickness)
[0146] Laser power at 75% of full power (.about.30 Watts)
[0147] Burst repetition rate of the laser=200 kHz.
[0148] 3 pulses/burst
[0149] Energy per burst =150 .mu.J (30 W/200 kHz)
[0150] Pitch=5 .mu.m
[0151] Single pass
[0152] CO.sub.2 Laser
[0153] Laser is a 200 W full power laser
[0154] Laser translation speed: 10 m/min
[0155] Laser power=100%
[0156] Pulse duration 17 .mu.s
[0157] Laser modulation frequency 20 kHz
[0158] Laser duty cycle=17/50 .mu.s=34% duty (about 68 Watt
output).
[0159] Laser beam defocus (relative to the incident surface of the
glass)=20 mm
Chamfer Method 2
Method 2A
[0160] A second chamfering method takes advantage of an existing
edge to create a chamfer by applying a highly-focused CO.sub.2
laser very close to the intersection between the surfaces of the
edge and substrate. In contrast to the CO.sub.2 laser conditions
described above, in this case the size of the focused CO.sub.2 beam
at the substrate surface is .about.100 .mu.m diameter, which allows
the beam to heat the glass locally to much higher temperatures than
the defocused beam described in Method 1. The laser must be highly
absorbed by the substrate material to create an intense thermal
gradient that spans the temperature range from the material's
melting temperature down to the material's strain point. The
thermal gradient generates a stress profile that induces separation
or peeling of a very thin strip of the material that curls and
peels off from the bulk of the material. The dimensions of the thin
strip are determined by the depth of the region in the material
having temperatures between the strain and softening points.
[0161] This method can be combined with the previous method to peel
off at the planes dictated by the fault lines. In other words, a
picosecond laser can be employed as described hereinabove to form a
fault line having a shape consistent with the desired shape or
contour of the edge and a thermal gradient can be established in
and around the fault line to prompt release of the thin strip of
material. In this embodiment, the fault lines produced by the
picosecond laser guide the direction of curling or peeling of the
thin strip of material and finer control of the shape or contour of
the edge may be achieved.
[0162] As illustrated in FIG. 9, the second method relies on the
absorption by the substrate of the laser wavelength (e.g., a
CO.sub.2 at 10.6 .mu.m). Absorption of the laser by the material
leads to the establishment of a thermal gradient that encompasses
temperatures that extend from at least the strain point of the
material to at least the softening point of the material. As shown
in FIG. 10, a strip of glass separates from the bulk of the
substrate to form a curled peel when such a thermal gradient is
created. When the laser is tightly focused near the edge (e.g.
within <100 .mu.m from the edge) as shown in FIG. 9, a strip of
curled glass is peeled from the right angle corner and forms a
chamfer that is generally concave as shown in FIG. 10. To chamfer
both corners, the sample can be flipped over and the process can be
repeated on the second corner. As shown in FIG. 10, the defect
lines of the flat portion of the edge show a texture consistent
with that shown in FIG. 1C for a flat edge formed by through-hole
perforations. FIG. 12 shows that by changing the chamfering speed
(defined as the CO.sub.2 beam scan speed), it is possible to change
the characteristics of the chamfered edges: chamfer angle, width of
the flat face (A) or height/width (B/C). By changing the CO.sub.2
laser scan speed, the rate of laser energy deposition onto the
material varies and the characteristics of the thermal gradient
(e.g. spatial extent, temperature range) are changed. By moving the
laser faster, the fault line becomes shallower and the strip of
material that peels becomes narrower and shallower. The chamfering
speed was varied from 3 m/min to 10 m/min in the examples shown in
FIG. 12. The CO.sub.2 laser had a peak power of 200 W and was set
to a repetition rate of 30 kHz with a pulse width of 2.9 .mu.s,
which created a CO.sub.2 output power governed by the 9% duty cycle
of .about.18 W.
[0163] CO.sub.2 Laser Conditions for Peel
[0164] Laser is a 200 W full power laser
[0165] Laser translation speed: 3 m/min (50 mm/s)
[0166] Laser power=100%
[0167] Pulse duration 2.9 .mu.s
[0168] Laser modulation frequency 30 kHz
[0169] Laser duty cycle=2.9/33 .mu.s=9% duty (about 18 W
output).
[0170] Laser beam defocus=0.7 mm
Method 2B
[0171] In this example, the picosecond perforation portion of
Chamfer Method 1 was combined with the thermal peeling of Chamfer
Method 2A to create a controlled peeling with separation guided by
the defect line planes. As shown in FIGS. 11A and 11B, peeling of
the right angle corners occurs. Peeling and detachment may not,
however, occur entirely along the defect plane because the thermal
gradient in the softening zone provides a secondary driving force
that may influence the path of detachment. Depending on the
relative position between the defect plane and the cracking line
defined by the thermal gradient, separation may occur to a greater
or lesser extent along the fault line. FIG. 11B illustrates an
example in which a portion of the peeling path deviates from the
path defined by the defect lines. The deviation is most pronounced
along the flat portion of the edge. It should be possible, however,
to separate the corner at the defect line plane with the proper
combination of defect line characteristics and proper heating with
the CO.sub.2 laser.
Sacrificial Edges
[0172] Even if the peeled glass does not entirely follow the defect
line plane, the presence of the residual defect line inside the
glass can be beneficial because it may arrest the propagation of
cracks that form when the edge is impacted. In this case, the
residual interior defect line planes can be used to serve as damage
arrest locations, in effect creating a "sacrificial" edge part of
the region of substrate material that is on the surface side of the
residual interior defect lines. In fact, creation of sacrificial
edges that include a residual interior defect line on the interior
side of the separated edge (or a set of residual interior defect
lines that intersect to form a more complex interior bevel inside
of the true edge), may be a method of improving the reliability of
the chamfered part without the need for a physical chamfer feature
on the outside edge of the part and without the mechanical grinding
and polishing needed to create that feature. Some options for this
type of sacrificial edge are shown in FIG. 13. Since the picosecond
laser process described above creates each defect line in a single
pass and at speeds of up to 1 m/s, it is very easy and
cost-effective to create extra "damage stop" lines. When subjected
to stress, for example an impacting force, the glass will separate
along the sacrificial edge and prevent cracks from the impact from
propagating into the interior of the part, thus leaving the balance
of the part intact.
Chamfer Method 3
[0173] Finally, separation of the outside glass edge pieces formed
by the defect lines need not be done by application of the CO.sub.2
laser or application of mechanical force. In many instances, the
glass part separated from a glass substrate is sent for chemical
strengthening in an ion exchange process. Ion exchange itself can
create enough stress to prompt peeling or separation at the chamfer
regions or corners of the part. The introduction of new ions into
the glass surface can create enough stress to cause the outside
corner pieces to peel or separate. In addition, the high
temperature salt bath used in the ion exchange process can provide
thermal stress sufficient to induce peeling or separation along the
fault line to provide a chamfered or otherwise shaped edge. In
either case, the ultimate result is an edge that more closely
follows the interior defect lines to form the desired chamfer shape
see FIG. 14).
[0174] Additionally or alternatively, etching of the part in an
acid solution (e.g., a solution of 1.5 M HF and 0.9 M
H.sub.2SO.sub.4) can create enough stress to cause the outside
corner pieces to peel or separate.
[0175] The chambering methods described herein can also be applied
to Corning.RTM. Eagle XG.RTM. (with the exception of the methods
including ion exchange) glass as described in application entitled
Laser Cutting of Display Glass Compositions(U.S. Provisional Patent
Application Ser. No. 62/023471).
[0176] The methods described above provide the following benefits
that may translate to enhanced laser processing capabilities and
cost savings and thus lower cost manufacturing. In the current
embodiment, the cutting and chamfering processes offer:
[0177] Chamfering or fully cutting parts with chamfered edges: the
disclosed method is capable of completely separating/cutting
Gorilla.RTM. Glass and other types of transparent glasses
(strengthened or unstrengthened) in a clean and controlled fashion.
Full separation and/or edge chamfering were demonstrated using
several methods. With Chamfer Method 1, the part is cut to size or
separated from glass matrix with a chamfered edge and, in
principle, no further post processing is required. With Chamfer
Method 2, the part is already cut to size with pre-existing flat
edges and the laser is used to chamfer the edges.
[0178] Reduced subsurface defects: with Chamfer Method 1, due to
the ultra-short pulse interaction between laser and material, there
is little thermal interaction and thus a minimal heat affected zone
that can result in undesirable stress and micro-cracking. In
addition, the optics that condenses the laser beam into the glass
creates defect lines that are typically 2 to 5 microns diameter on
the surface of the part. After separation, the subsurface damage
can be as low as <30 .mu.m. This has great impact on the edge
strength of the part and reduces the need to further grind and
polish the edges, as these subsurface damages can grow and evolve
into micro-cracks when the part is submitted to tensile stress and
weaken the strength of the edge.
[0179] Process cleanliness: Chamfer Method 1 is capable of
chamfering glass in a clean and controlled fashion. It is very
problematic to use conventional ablative processes for chamfering
because they generate a lot of debris. Such ablation-generated
debris is problematic, because it can be hard to remove even with
various cleaning and washing protocols. Any adhered particulates
can cause defects for later processes where the glass is coated or
metalized to create thin film transistors, etc. The characteristics
of the laser pulses and the induced interactions with the material
of the disclosed method avoid this issue because they occur in a
very short time scale and the material transparency to the laser
radiation minimizes the induced thermal effects. Since the defect
line is created within the object, the presence of debris and
adhered particles during the cutting step is virtually eliminated.
If there are any particulates resulting from the created defect
line, they are well contained until the part is separated.
Elimination of Process Steps
[0180] The process to fabricate glass plates from the incoming
glass panel to the final size and shape involves several steps that
encompass cutting the panel, cutting to size, finishing and edge
shaping, thinning the parts down to their target thickness,
polishing, and even chemically strengthening in some cases.
Elimination of any of these steps will improve manufacturing cost
in terms of process time and capital expense. The presented method
may reduce the number of steps by, for example: [0181] Reduced
debris and edge defects generation--potential elimination of
washing and drying stations [0182] Cutting the sample directly to
its final size with shaped edges, shape and thickness--reducing or
eliminating need for mechanical finishing lines and a huge
non-value added cost associated with them.
[0183] The relevant teachings of all patents, published
applications and references cited herein are incorporated by
reference in their entirety.
[0184] While exemplary embodiments have been described herein, it
will be understood by those skilled in the art that various changes
in form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
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