U.S. patent application number 16/715460 was filed with the patent office on 2020-04-16 for separation of transparent workpieces.
This patent application is currently assigned to SCHOTT AG. The applicant listed for this patent is SCHOTT AG. Invention is credited to Klaus Gerstner, Andreas Habeck, Georg Haselhorst, Andreas Ortner, Fabian Wagner.
Application Number | 20200115269 16/715460 |
Document ID | / |
Family ID | 50555693 |
Filed Date | 2020-04-16 |
United States Patent
Application |
20200115269 |
Kind Code |
A1 |
Ortner; Andreas ; et
al. |
April 16, 2020 |
SEPARATION OF TRANSPARENT WORKPIECES
Abstract
A method is provided for preparing transparent workpieces for
separation. The method includes generating aligned filament
formations extending transversely through the workpiece along an
intended breaking line using ultra-short laser pulses.
Inventors: |
Ortner; Andreas;
(Gau-Algesheim, DE) ; Habeck; Andreas; (Undenheim,
DE) ; Gerstner; Klaus; (Bischofsheim, DE) ;
Haselhorst; Georg; (Schmitten, DE) ; Wagner;
Fabian; (Mainz, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SCHOTT AG |
Mainz |
|
DE |
|
|
Assignee: |
SCHOTT AG
Mainz
DE
|
Family ID: |
50555693 |
Appl. No.: |
16/715460 |
Filed: |
December 16, 2019 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14711881 |
May 14, 2015 |
|
|
|
16715460 |
|
|
|
|
PCT/EP2013/073329 |
Nov 8, 2013 |
|
|
|
14711881 |
|
|
|
|
61726065 |
Nov 14, 2012 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B23K 2103/50 20180801;
B23K 26/364 20151001; B23K 2103/54 20180801; B23K 26/40 20130101;
B23K 26/123 20130101; B23K 26/14 20130101; C03B 33/0222 20130101;
B23K 2101/18 20180801; B23K 26/0624 20151001; B23K 26/38 20130101;
B23K 26/08 20130101; B23K 26/402 20130101; B28D 5/0011 20130101;
B23K 26/53 20151001; B23K 26/083 20130101; B23K 2103/52 20180801;
B23K 26/127 20130101 |
International
Class: |
C03B 33/02 20060101
C03B033/02; B23K 26/402 20060101 B23K026/402; B23K 26/14 20060101
B23K026/14; B23K 26/12 20060101 B23K026/12; B23K 26/08 20060101
B23K026/08; B23K 26/364 20060101 B23K026/364; B23K 26/0622 20060101
B23K026/0622; B23K 26/53 20060101 B23K026/53; B23K 26/40 20060101
B23K026/40; B28D 5/00 20060101 B28D005/00; B23K 26/38 20060101
B23K026/38 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 14, 2012 |
DE |
10 2012 110 971.0 |
Claims
1. A method for separating a workpiece by focused laser radiation,
comprising: exposing the workpiece to a first atmosphere including
protective gas; directing ultra-short pulsed laser radiation onto
the workpiece, the workpiece being transparent in a range of
wavelengths of the laser radiation to cause a filamentary material
modification in depth in the workpiece; moving the workpiece and/or
laser radiation with respect to one another to define a separation
area in the workpiece; exposing, after the laser irradiation, the
workpiece to a second atmosphere including a content of hydroxyl
(OH) ions that is higher than that of the protective gas
atmosphere; breaking the workpiece along the separation area
defined by the material modification.
2. The method as claimed in claim 1, wherein the workpiece
comprises toughened glass or glass ceramics.
3. An apparatus for separating glass or glass ceramics by focused
laser radiation, comprising: a workpiece chamber for accommodating
the glass or glass ceramics; a workpiece feeder that feed the glass
or glass ceramics into the workpiece chamber; an ultra-short pulsed
laser light source that generates a filamentary material
modification in depth in the glass or glass ceramics by laser
irradiation; a displacing device that moves the workpiece and/or
the laser light source relative to each another; wet steam feed
device that feeds a gas stream into the workpiece chamber; and a
separating device that separates the workpiece along a separation
line defined by the material modification.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 14/711,881 filed May 14, 2015, now allowed, which is a
continuation of International Application No. PCT/EP2013/073329
filed Nov. 8, 2013, which claims the benefit of German Application
No. 10 2012 110 971.0 filed Nov. 14, 2012 and claims the benefit of
U.S. Provisional Application 61/726,065 filed Nov. 14, 2012, the
entire contents of all of which are incorporated herein by
reference.
BACKGROUND
1. Field of the Disclosure
[0002] The present invention relates to the preparation for
separating workpieces and substrates using ultra-short pulsed laser
radiation. In particular toughened glass or glass ceramics are
contemplated as a workpiece material. The invention also relates to
the separation of workpieces.
2. Description of Related Art
[0003] From WO 2012/006736 A2 it is known that the Kerr effect can
be exploited to cause irreversible damages in glass in form of
filaments. By generating a linear array of such damages in glass it
is possible to separate transparent substrates. A filament is
formed by an ultra-short laser pulse. Due to the Kerr effect, the
laser beam experiences self-focusing in the interior of the glass
until the energy density at a point becomes so high that a plasma
is ignited. A plasma explosion is caused during which the glass
undergoes irreversible damage around this plasma generation
location. From there further radiation emanates which is subject to
self-focusing and ends up in a plasma explosion. This effect is
repeated several times, depending on the intensity. Energy
decreases along the entire thickness of the glass, so the first
plasma spots will have the highest energy and produce the greatest
damages. Furthermore, the plasma spots are round, which means that
emanating defects will occur randomly distributed in all
directions.
[0004] In glass that exhibits introduced stresses, for example,
chemically toughened glass, spontaneous self-breakage may occur
whereby the processing of especially comparatively large glass
sheets is considerably affected. As a result of breakage the
position of the glass sheet changes. Further exact processing is
impossible.
[0005] Patent document DE 102 13 044 B3 describes a method for
cutting or drilling material. Here, again, the nonlinear optical
effect occurring with high-intensity ultra-short laser pulses is
exploited to produce a filament due to the alternating focusing and
defocusing of the laser beam. Accordingly, a filament is a passage
of small diameter produced by a high-intensity laser light
beam.
[0006] Furthermore, document DE 10 2006 042 280 A1 describes a
method for processing transparent material using a laser.
Ultra-short laser pulses are used to generate both a surface groove
on the substrate and one or more laser-modified regions in the
volume of the material. The fracture ultimately leading to the
separation occurs at the superficial scribing trace and propagates
downwards across the substrate material. If the surface groove is
too flat, the fracture tends to migrate. A generation of breaking
edges with consistent high quality is not described.
[0007] DE 10 2007 028 042 B2 also discloses a method for laser
processing of transparent materials and describes a use of pulsed
laser radiation in the nanosecond range. The document mentions a
range of radiation intensity in which material changes occur
without plasma luminescence.
[0008] In summary, various processes have been known which allow to
modify regions in the volume of a material by means of ultra-short
pulsed laser radiation so as to provide one step of a separation
process. However, the separating and breaking which is required for
example for dicing substrates that have been modified in this
manner, has hitherto not been sufficiently accessible to industrial
processes. This problem is particularly acute with substrates
comprising toughened glass or glass ceramics, as these are prone to
uncontrolled breakage due to inherent stresses introduced by the
toughening, when processed with ultra-short pulsed laser
radiation.
[0009] However, for industrial application exact control is not
only required for the generation of a separation line in or on the
substrate, but also for the separating or breaking in order to
produce breaking edges of consistent high quality and to ensure
stability and safety of the process. This is very difficult
particularly in case of toughened glass, since the material
modifications caused by the laser irradiation can lead to an
uncontrolled occurrence and propagation of cracks, so that accurate
control of separation is very difficult.
[0010] The following issues are of concern: Cutting/Drilling using
filamentation: due to the process, formation of the filament occurs
inhomogeneously: due to the high initial energy density,
comparatively larger plasma volumes are ignited on the entry side
of the filament producing laser beam than at the subsequent plasma
spots deeper in the workpiece, i.e. the channel of damages in the
workpiece (corresponding to the filament formed) will taper. The
induced damages (microcracks) will thus be much stronger on the
entry side of the laser beam than on the exit side. Directional
strength tests four-point bending reveal a significant difference
in edge strength already with a glass thickness of 0.7 mm. Spatial
geometry of the plasma generation spots: The plasma generation
spots caused by self-focusing have a substantially spherically
symmetrical shape with spherically symmetrical energy distribution,
which causes direction-independent randomly distributed microcracks
around the plasma volume. As a result, cracks will even protrude
into the later breaking edge and have a strength-reducing effect.
Spontaneous breakage: During filamentation of brittle materials
with intrinsic stresses, uncontrolled spontaneous breakage of the
workpiece occurs during the process, resulting in an increased
rejection rate. Furthermore, spontaneous breakage causes a change
in the position of the workpiece, so that automated processing is
impeded or even made impossible.
SUMMARY
[0011] A major object of the invention is to improve the quality of
the edges produced.
[0012] The invention permits to improve the separation process for
hard and brittle materials.
[0013] Multifilamentation: In contrast to the prior art, the
preparation of the separation by cleaving the workpiece is
accomplished not only by generating a single tapering filament
formation, but by generating a series of a plurality of consecutive
filament formations. Each of these filament formations is
comparatively narrow and produces a significantly lower number of
microcracks transversely to the direction of the filament
formations as compared to the separation process with a single
filament formation that extends through the workpiece. Due to the
less pronounced tapering of each single filament formation
(=filament+microcracks) a better overall geometric accuracy of the
processing channel is achieved, in combination with a higher edge
strength of the breaking edge when the workpiece has been
separated.
[0014] The individual filament formations are generated by a
picosecond to femtosecond pulse train which is split and offset in
time and introduced into the workpiece starting from the exit side
of the laser beam. Larger cutting depths may be realized by
multi-pulse sequences at the same laser power, with a corresponding
reduction of the cutting speed.
[0015] The spatial geometric shape of the plasma may be influenced
by a special optic system. The laser radiation beam is generated
with an elongated cross-sectional shape, for example a
lancet-shaped, elliptic, or drop-shaped cross section. In this
manner, a controllable preferred direction of the damages/cracks
resulting from the plasma explosions is obtained.
[0016] In case the intended breaking line is curved or changes
direction, the laser radiation beam should be controlled in terms
of directional alignment of its cross-sectional shape so that the
longitudinal extension of the cross-sectional shape follows the
intended separation line of the workpiece.
[0017] The generation of filament formations under a protective
atmosphere is primarily an aspect of apparatus configuration, and
the manufacturing apparatus is adapted so that the atmosphere
surrounding the workpiece to be processed can be adjusted in a
predefined manner.
[0018] With a selectively adjusted atmosphere it is possible to
inhibit or prevent spontaneous breakage of the workpiece.
[0019] The invention also relates to a method for separating a
substrate by means of focused laser radiation, comprising the steps
of: exposing the substrate to a protective gas atmosphere;
directing ultra-short pulsed laser radiation onto the substrate,
the substrate being transparent in the wavelength range of the
laser radiation; generating a filamentary material modification in
depth in a predetermined volume of the substrate by the laser
irradiation; and breaking the substrate along the separation line
defined by the material modification.
[0020] There are two nonlinear optical effects that may be caused
by an ultra-short pulsed laser radiation in a transparent material,
which is the optical Kerr effect on the one hand, and on the other
the defocusing of the laser beam in a plasma bubble. Such effects
have already been known and will therefore be outlined only
briefly.
[0021] The Kerr effect refers to a change in the optical property
of a transparent material (transparent in the range of wavelengths
of the laser radiation) as a function of the applied or occurring
strength of the electric field. The laser radiation involves an
electric field in the transparent material whose strength depends
on the light intensity of the laser radiation. The electric field
causes a change in the optical characteristics of the irradiated
material, including an increase in refractive index. This in turn
leads to the self-focusing of the laser radiation.
[0022] Due to the self-focusing and the resulting reduction of the
irradiated cross-sectional area, power density per unit area and
hence radiation intensity strongly increases and may reach very
high values. As a result, the electric field is further intensified
leading to multiphoton ionization. Ionization means charge
separation in molecules or atoms and plasma formation at focusing
spots. Anyway damage is caused in the material of the substrate in
the focusing spots, which will also be referred to as a material
modification below and manifests itself as plasma bubbles.
[0023] In the region of the locally generated plasma bubble
defocusing of the laser beam occurs, which is again followed by a
next focusing of the laser beam. In this manner, a kind of pearl
string can be produced in the material of the substrate, which
consists of a series of multiple consecutively aligned focusing and
defocussing regions and which are referred to as filaments.
[0024] Due to the focusing and defocusing, the laser beam may be
caused to propagate into the material of the substrate so that the
material is subjected to a kind of perforation in depth. This
effect has been known from the processing of transparent materials
such as glass and is used, for example, to produce a kind of
perforation line in the substrate as an intended breaking line or
separation line, by moving the substrate relative to the laser
radiation.
[0025] With a higher laser power, a correspondingly deeper
perforation may be achieved, which facilitates separation in the
region of the perforation.
[0026] However, an application of deeper perforation poses problems
in conjunction with pre-stressed glass, that is to say a material
which exhibits increased inherent stress already in the starting
state. The elevated inherent stress may result in spontaneously
formation and propagation of cracks in the perforated regions, as
described in WO 2012/006736 A2, inter alia. In particular with a
comparatively high laser power, breakage of the substrate may occur
already during the processing. Therefore, industrial application is
considerably complicated because the separation process cannot be
reliably managed and controlled.
[0027] The inventors have found that the spontaneous cracking can
be reduced or even completely prevented if during laser irradiation
of the substrate, the workpiece is exposed to a specific
atmosphere. For example, cracking is significantly reduced or
delayed if a protective gas atmosphere is prevailing during the
laser irradiation, which is poor in hydroxyl (OH) ions or even free
of OH ions.
[0028] By applying a nitrogen atmosphere during the laser
irradiation, the period of time until spontaneous breaking cracks
occurred in the region of the perforation was sufficiently
extended.
[0029] In this way it is possible to conclude with the process of
laser irradiation of the substrate before spontaneous breaking
cracks occur in the substrate so that the substrate breaks.
Accordingly, during irradiation with ultra-short pulsed laser
radiation the substrate is exposed to an atmosphere that is low in
OH ions or free of OH ions. For example an appropriately gas-tight
chamber can be used for this purpose, in which the substrate is
accommodated during the laser irradiation.
[0030] Particularly good experiences were made with a protective
gas atmosphere including a water content of less than 0.2 vol %,
preferably less than 0.1 vol %.
[0031] Thus, the laser irradiation enables to generate a
filamentary material modification in depth similar to a perforation
in a predetermined volume of the substrate. The predetermined
volume of the substrate refers to the predefined separation area
along which the perforation is to be produced and which therefore
defines the later separation area or separation line on the surface
of the substrate.
[0032] Particularly advantageously, the substrate or the laser beam
may be moved relative to one another in order to enable the
perforation in the depth of the substrate along the separation line
in this way. Typically, the substrate may be moved, for example by
means of an X-Y axes adjustment assembly which allows
two-dimensional displacement of the substrate with a constant
distance to the laser radiation. This arrangement may additionally
be combined with a movable Z-axis arrangement in order to enable
adjustment of the distance between the substrate and the laser
beam.
[0033] The substrate material is optically transparent at least in
the range of wavelengths of the laser radiation, with an optical
transmittance in this range of at least 80%/m, preferably at least
85%/cm, and most preferably at least 90%/cm. Thus, the laser
radiation can penetrate into the material.
[0034] The substrate may comprise materials selected from a group
comprising glass, sapphire, and diamond. Surprisingly, the laser
irradiation according to the invention can even be employed for
toughened glass as known for display applications, for example, and
also with sheet glass. Furthermore surprisingly, glass ceramic
materials can also be processed.
[0035] The perforation in depth produced in the substrate along a
path extends along the separation line at which a separation of the
substrate is intended to occur. The separation line may be a
rectilinear line, but also a non-straight or curved line. For
example, the separation line may have very small radii to enable a
separation of the material similar to a drilled hole.
[0036] The perforation may extend from the surface of the substrate
vertically into the depth. However, it may likewise be produced at
a certain angle with respect to the surface, for example to produce
oblique separation edges at the substrate. In this case the laser
beam is not directed perpendicularly but rather at a predetermined
angle to the surface of the substrate.
[0037] Subsequently, separation of the material is accomplished
along the perforated separation line. For this purpose it is
necessary that the material modification which is generated by the
perforation and is equivalent to a damage of the material reaches a
particular extent in order to enable separation with a specific
surface quality of the separation edge and strength of the
separated substrate.
[0038] It has been found that the separation at the separation line
is well done when the perforation extends into a depth
corresponding to at least 40% of the material thickness of the
substrate, preferably at least 50%, and most preferably at least
55%. In this case the filaments should have a spacing to each other
ranging from about 200 .mu.m to 800 .mu.m. A filament may have a
cross section in a range from about 15 .mu.m to 250 .mu.m.
[0039] In this manner, a sufficiently extensive pre-damage of the
substrate material may be achieved along the separation line
resulting in good separability. When separating the material along
the separation line, separation edges are formed with a
surprisingly good quality of the so produced separation edges.
[0040] In the region of perforation, the separation edges typically
exhibit a pattern of adjacent parallel filaments, and in the
underlying region of the separation edge a rather conchoidal
fracture pattern. Roughness values obtained for the separation edge
were in a range of Ra<100 .mu.m.
[0041] The separation edges produced moreover exhibit high level
edge strength. This was determined by a 4-point bending test. The
average strength value achieved was at least 120 MPa in case of
toughened glass.
[0042] The laser source is selected according to the range of
wavelengths in which the substrate is transparent. The wavelength
range of the emitted radiation is within the transmission range of
the substrate.
[0043] The laser beam may be spatially focused with a Gaussian
intensity distribution in order to achieve a sufficiently high
intensity. The first focal point is located within the substrate,
that is to say in the substrate volume. When the laser pulse hits
this point in the volume of the substrate, a plasma may be
generated and hence a material modification may be produced there.
With the subsequent defocusing and further focusing effects, the
filamentary perforation comprising a plurality of focusing points
can be produced in the substrate volume.
[0044] The focusing spots in transparent glass, for example, often
have a spherical approximately symmetric shape. However, by means
of a special optic system it was possible to generate focusing
points of non-spherical spatial shape in toughened glass. For
example, elliptic, lancet-shaped, or drop-shaped focusing spots
were generated. Such shapes of the focusing spots promote a
formation of cracks in particular from one focusing spot to the
next due to a better propagation of the crack, thus also improving
the quality of the separation edge that can be achieved.
[0045] A suitable laser source according to the present invention
operates with a repetition rate between 10 kHz and 120 kHz,
preferably between 30 kHz and 110 kHz, and most preferably between
35 kHz and 105 kHz.
[0046] An appropriate pulse duration of a laser pulse is in a range
below 100 picoseconds, preferably less than 10 picoseconds, and
most preferably less than 1 picosecond. Particularly favorably, the
laser source is operated at a power in a range from about 7 to 12
watts.
[0047] With such a laser radiation, very good results were obtained
for the generation of separation lines on toughened glass and
perforated in depth.
[0048] An increase of the laser power may lead to a greater
tendency to spontaneous formation of cracks, in particular in
toughened glass, whereas in non-toughened glass it is possible to
achieve a more dense perforation which in turn improves
separability.
[0049] The invention further relates to an apparatus for separating
a substrate, in particular toughened glass or glass ceramic, by
means of focused laser radiation, comprising: a gas-tight chamber
for accommodating the substrate; an ultra-short pulsed laser light
source; means for displacing the substrate and/or the laser light
source relative to each other, while: the substrate is exposed to a
protective gas atmosphere; ultra-short pulsed laser radiation is
directed onto the substrate, which substrate is transparent in the
wavelength range of the laser radiation; by the laser irradiation,
a filamentary material modification is generated in a predetermined
volume and extending into the depth of the substrate; separation is
accomplished along the separation line defined by the material
modification.
[0050] The invention further relates to an article of toughened
glass or glass ceramic that has been processed at least on one side
by a method according to the invention.
[0051] In a modification of the method for separating a substrate,
in particular toughened glass or glass ceramic, by means of focused
laser radiation, the substrate is exposed to a second atmosphere
following the perforation of the substrate along the later
separation line. This second atmosphere is different from the
protective gas atmosphere which accordingly represents the first
atmosphere, in its content of hydroxyl (OH) ions. The second
atmosphere has a higher content of OH ions than the first
atmosphere.
[0052] The inventors have found that an increased content of OH
ions can promote the separation or cleaving of the workpiece along
the perforated separation line. By exposition to an atmosphere
enriched in OH ions, such as wet steam, a formation of cracks may
be promoted and thus controlled. In this way, the process step of
cleaving for accomplishing material separation can be selectively
influenced, so that easy industrial applicability is provided. In
particular it is possible to prevent spontaneous breaking cracks
from occurring.
[0053] Particularly good experiences have been made with a second
atmosphere having an OH ions content of at least 1.4 vol %,
preferably at least 2 vol %.
[0054] Therefore, particularly advantageously, the chamber for
accommodating the substrate may be gas-tight so that a first
atmosphere poor in OH ions is easily established. Furthermore
particularly advantageously, this chamber may as well be adapted
for exposing the substrate to an OH ion enriched atmosphere. But it
is likewise possible that the apparatus comprises two separate
chambers which are adapted accordingly for applying the first or
the second atmosphere.
[0055] Further details of the invention will become apparent from
the description of the illustrated exemplary embodiments and the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] Exemplary embodiments of the invention will now be described
with reference to the drawings, wherein:
[0057] FIG. 1 shows a laser processing device while processing a
workpiece on a workpiece table;
[0058] FIG. 2 shows an enlarged detail of FIG. 1;
[0059] FIG. 3 shows an elliptical beam cross section of the laser
processing device;
[0060] FIG. 4 shows lancet-shaped beam cross sections; and
[0061] FIG. 5 shows drop-shaped beam cross sections.
DETAILED DESCRIPTION
[0062] FIG. 1 shows a laser processing device 1 above a workpiece 2
resting on a workpiece table 3. The laser processing device
comprises an ultra-short pulsed laser 10 and a focusing optic
system 11 to provide a focused radiation beam 12 having a focal
point 13 near the upper surface of the workpiece 2. A cut line or
breaking line 20 is indicated on workpiece 2, along which line the
workpiece is to be separated or cleaved. Provisions are made so
that the focus 13 can be displaced along this line 20, which is
facilitated by adjusting the table in the two coordinate directions
21, 22. Very small adjustment increments are used.
[0063] The ultra-short pulsed laser 10 is able to deliver laser
pulse trains in two or more successive periods. The wavelength of
the radiation is chosen so as to be in a range for which the
workpiece 2 is transparent. The energy of the laser pulses is
dimensioned so that in each case a respective line-shaped damage
formation 14 is formed transversely to the surface of workpiece 2.
By displacing the focusing optic system 11 along the intended
breaking line 20, a series of line-shaped damage formations 14 is
generated in the workpiece 2, which virtually define the intended
breaking face. The invention relates to the generation of this
series of line-shaped damage formations 14 along the line 20.
[0064] FIG. 2 schematically shows a damage formation 14 extending
transversely through the workpiece 2. In the illustrated exemplary
embodiment, the damage formation 14 comprises three filament
formations 4, 5, and 6 aligned along a straight line. Each of these
filament formations is generated by an ultra-short pulsed laser
pulse train. In case of filament formation 4, a plasma spot 41 is
generated due to self-focusing of the laser beam 12, at which
plasma spot the material of the workpiece transitions into the
plasma phase, which is accompanied by emitted radiation 42 which
due to self-focusing leads to a further plasma spot 43, and the
process continues until the energy of the laser pulse is exhausted.
A virtual plasma explosion takes place at plasma generation spots
41, 43, 45, due to thermal expansion, which causes cracks forming
mainly along a gap that is created into the interior of the
workpiece, which is desirable, but also transversely to this
channel, as indicated by cracks 46. These transverse cracks 46 are
undesirable and are intended to be kept as small as possible with
the invention.
[0065] For this purpose, the damage formation 14 is created in
several stages. This is achieved by emitting the laser pulses in
two or more successive periods. The energy of the laser pulses
during one period is chosen to be so small that only a few small
plasma explosion spots are produced during one emission period. In
this way, a formation of detrimental lateral cracks 46 is
significantly reduced. Along filament formation 4 gaps and cleaving
cracks are formed which predefine the later fracture in the
workpiece.
[0066] In a second laser pulse period, filament formation 5 is
generated similarly as filament formation 4. Plasma bubbles 51, 53,
55 and defocusing-focusing spots 52, 54 are produced; similarly as
with plasma bubbles 61, 63 and defocusing-focusing spots 62 during
the third laser pulse period. The greater depth approach is
successful due to the previously formed gaps and cleaving cracks in
the direction of the breaking point which virtually presents a
guiding channel for the second and subsequent laser pulse periods.
The number of successive laser pulse periods is determined
according to the thickness of workpiece 2.
[0067] To promote a formation of gaps in the direction of breaking
face 20 it is useful to choose cross-sectional shapes of the laser
beam which are elongated or have a larger dimension in the intended
breaking direction. Such cross-sectional shapes are illustrated in
FIGS. 3, 4, and 5. The elliptical cross-sectional shape may be
obtained based on an originally circular cross-sectional shape of
the laser beam by combining cylindrical lenses. The lancet-like
shape of FIG. 4 and the drop shape of FIG. 5 of the beam cross
section can be obtained by special lenses. These shapes promote
cleaving cracks and gaps in the direction of the intended breaking
line 20.
[0068] If, as illustrated, the breaking line is desired to be
curved, the laser radiation beam has to be controlled accordingly
so that the larger cross-sectional dimension is continuously
aligned along the intended breaking direction.
[0069] When processing brittle material, there is a risk that the
workpiece spontaneously breaks when the damage formations along the
intended breaking line 20 have not yet been all completed. In order
to minimize or completely avoid this risk, the processing is
performed in a "neutral" atmosphere, such as under nitrogen. In
this manner, prepared workpieces are obtained which are prepared
for being separated or cleaved. The final separation or cleaving is
then performed by subjecting the workpiece to a mechanical tension,
and under water vapor or in another atmosphere containing hydroxyl
(OH) groups.
[0070] It should be noted that the finer the fracture pattern in
the separation plane is desired to be obtained, the smaller the
spacings are chosen between the locations of damage formations 14
along line 20. The spacings are of the same order of magnitude as
the diameter of the damage formations.
* * * * *