U.S. patent application number 14/607991 was filed with the patent office on 2015-05-21 for methods and systems for laser processing of coated substrates.
The applicant listed for this patent is IMRA America, Inc.. Invention is credited to Alan Y. Arai, Gyu Cheon Cho, Jingzhou Xu.
Application Number | 20150136744 14/607991 |
Document ID | / |
Family ID | 48780269 |
Filed Date | 2015-05-21 |
United States Patent
Application |
20150136744 |
Kind Code |
A1 |
Arai; Alan Y. ; et
al. |
May 21, 2015 |
METHODS AND SYSTEMS FOR LASER PROCESSING OF COATED SUBSTRATES
Abstract
Examples of methods and systems for laser processing of
materials are disclosed. Methods and systems for singulation of a
wafer comprising a coated substrate can utilize a laser outputting
light that has a wavelength that is transparent to the wafer
substrate but which may not be transparent to the coating layer(s).
Using techniques for managing fluence and focal condition of the
laser beam, the coating layer(s) and the substrate material can be
processed through ablation and internal modification, respectively.
The internal modification can result in die separation.
Inventors: |
Arai; Alan Y.; (Fremont,
CA) ; Cho; Gyu Cheon; (Ann Arbor, MI) ; Xu;
Jingzhou; (Ann Arbor, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
IMRA America, Inc. |
Ann Arbor |
MI |
US |
|
|
Family ID: |
48780269 |
Appl. No.: |
14/607991 |
Filed: |
January 28, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13714896 |
Dec 14, 2012 |
8969220 |
|
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14607991 |
|
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61586332 |
Jan 13, 2012 |
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Current U.S.
Class: |
219/121.68 ;
425/174.4 |
Current CPC
Class: |
B23K 2103/172 20180801;
H01L 21/67092 20130101; H01L 33/0095 20130101; B23K 26/40 20130101;
B23K 26/53 20151001; H01L 21/78 20130101; B23K 26/0006 20130101;
H01L 21/2686 20130101; B23K 2103/16 20180801; Y10S 438/94
20130101 |
Class at
Publication: |
219/121.68 ;
425/174.4 |
International
Class: |
B23K 26/40 20060101
B23K026/40; B23K 26/00 20060101 B23K026/00; H01L 21/67 20060101
H01L021/67 |
Claims
1. A system for processing a workpiece, the workpiece comprising a
substrate and a layer formed on the substrate, the layer having an
ablation threshold that is less than an ablation threshold of the
substrate, the system comprising: a laser source configured to
provide laser pulses having a pulse energy and a wavelength,
wherein the substrate is substantially transparent to the laser
pulses at the wavelength; a beam delivery system configured to
deliver the laser pulses to the workpiece; focusing optics; and a
controller configured to adjust fluence of the laser pulses and to
adjust the focusing optics to provide desired focal conditions of
the laser pulses at the layer and at a focal spot inside the
substrate such that: (1) intensity of the laser pulses in or near
the layer is at or above the ablation threshold of the layer and
below the ablation threshold of the substrate, and (2) intensity of
the laser pulses near a focal spot in the substrate is at or above
the ablation threshold of the substrate.
2. The system of claim 1, wherein the substrate is substantially
transparent to the laser pulses at the wavelength.
3. The system of claim 1, wherein said layer comprises a conductive
film.
4. The system of claim 3, wherein said conductive film comprises
ITO.
5. The system of claim 3, wherein said conductive film comprises
gold.
6. The system of claim 3, further comprising at least one
dielectric layer disposed between said conductive film and said
substrate, said at least one dielectric layer being substantially
transparent at the wavelength of said laser pulses.
7. The system of claim 6, wherein said at least one dielectric
layer comprises multiple dielectric layers.
8. The system of claim 1, wherein said layer comprises a low-k
dielectric.
9. The system of claim 1, wherein said system is configured such
that said processing with laser pulses having said intensity in or
near said layer cleanly removes said layer, said pulses having said
intensity in said substrate modifies an internal portion of said
substrate, and wherein the surface of said substrate is
substantially undamaged with said processing.
10. The system of claim 1, wherein said system is configured such
that at least a portion of said processing is carried out during
relative motion of said substrate and said laser pulses, and in a
single pass.
11. A laser-based system for processing a workpiece, the workpiece
comprising a substrate and a layer disposed on the substrate,
wherein the substrate is substantially transparent at a laser
wavelength, said system comprising: a laser source configured to
generate a focused laser processing beam having at least one pulse;
focusing optics to produce a beam diameter w.sub.0 at an internal
substrate position, and a beam diameter w.sub.1 at or near said
layer on said substrate, wherein w.sub.0<w.sub.1; and a
controller configured to control said focusing optics and said
laser source to remove at least a depthwise portion of said layer
and to controllably modify an internal portion of said substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. patent application
Ser. No. 13/714,896, filed Dec. 14, 2012, entitled "METHODS AND
SYSTEMS FOR LASER PROCESSING OF COATED SUBSTRATES," which claims
the benefit of priority under 35 U.S.C. .sctn.119(e) to U.S.
Provisional Patent Application No. 61/586,332, filed Jan. 13, 2012,
entitled "METHODS AND SYSTEMS FOR LASER PROCESSING OF COATED
SUBSTRATES;" each of which is hereby incorporated by reference
herein in its entirety.
FIELD
[0002] This disclosure generally relates to processing materials
using pulsed lasers. Various implementations can be used for
cutting, scribing, dicing, singulating, or otherwise processing
materials such as semiconductor wafers, multilayer materials,
composite materials, and so forth.
BACKGROUND
[0003] Conventional methods for semiconductor wafer singulation use
diamond saw cutting or other mechanical dicing apparatus. In recent
years there has been a trend to reduce semiconductor wafer
thickness, which can lead to reduced package size and the ability
to stack silicon wafers to achieve higher integrated circuit
density and integration. The reduced thickness of wafers has led to
challenges for wafer singulation that uses saw cutting or
mechanical dicing, for example, chipping along die edges.
SUMMARY
[0004] In one aspect, the present disclosure provides a method for
using a processing laser in singulation of light emitting diode
(LED) or other semiconductor devices having a coating on a
substrate.
[0005] Embodiments of the present disclosure provide methods for
laser internal processing for die singulation when a coating layer
is present on the substrate.
[0006] In at least one embodiment, a first laser beam ablates the
coating layer. A clean ablation trench allows the laser beam to be
focused inside the substrate. A second laser beam, passing through
the ablation trench, modifies an internal portion of the substrate.
The internal modification facilitates subsequent die
separation.
[0007] In another aspect of the present disclosure, focal condition
and/or energy of a laser beam are managed so that both ablation of
coating layer(s) and modification of an internal portion of the
substrate occurs during a single processing pass in which the
substrate and laser pulses are moved relative to one another. Thus,
the coating and substrate processing can be carried out in
parallel, and essentially simultaneously when compared to a
multiple pass process. In some implementations the dies can be
separated with a one step processing pass.
[0008] In another aspect, a system for processing a workpiece is
provided. The workpiece comprises a substrate and a layer formed on
the substrate, and the layer has an ablation threshold that is less
than an ablation threshold of the substrate. The system comprises a
laser source configured to provide laser pulses having a pulse
energy and a wavelength, where the substrate is substantially
transparent to the laser pulses at the wavelength. The system also
comprises a beam delivery system configured to deliver the laser
pulses to the workpiece, focusing optics, and a controller. The
controller is configured to adjust fluence of the laser pulses and
to adjust the focusing optics to provide desired focal conditions
of the laser pulses at the layer and at a focal spot inside the
substrate such that: (1) intensity of the laser pulses in or near
the layer is at or above the ablation threshold of the layer and
below the ablation threshold of the substrate, and (2) intensity of
the laser pulses near a focal spot in the substrate is at or above
the ablation threshold of the substrate.
[0009] In another aspect, a method for processing a workpiece is
provided. The workpiece comprises a substrate and a layer formed on
the substrate, with the layer having an ablation threshold that is
less than an ablation threshold of the substrate. The method
comprises providing laser pulses having a pulse energy and a
wavelength, where the substrate is substantially transparent to the
laser pulses at the wavelength. The method also comprises
delivering the laser pulses to the workpiece, and adjusting fluence
and focal conditions of the laser pulses such that: (1) intensity
of the laser pulses in or near the layer is at or above the
ablation threshold of the layer and below the ablation threshold of
the substrate, and (2) intensity of the laser pulses near a focal
spot in the substratenother aspect, a laser-based method for
processing a workpiece is provided. The workpiece comprises a
substrate and a layer formed on the substrate, where the substrate
is substantially transparent at a laser wavelength. The method
comprises generating a focused laser processing beam having at
least one pulse, a beam diameter w0 at an internal substrate
position, and a beam diameter w1 at or near the layer on the
substrate, where w0<w1. The method also comprises removing at
least a depthwise portion of the layer, and controllably modifying
an internal portion of the substrate.
[0010] In another aspect, a laser-based system for processing a
workpiece is provided. The workpiece comprises a substrate and a
layer formed on the substrate, where the substrate is substantially
transparent at a laser wavelength. The system comprises a laser
source configured to generate a focused laser processing beam
having at least one pulse, and focusing optics to produce a beam
diameter w0 at an internal substrate position, and a beam diameter
w1 at or near the layer on the substrate, where w0<w1. The
system also comprises a controller configured to control the
focusing optics and the laser source to remove at least a depthwise
portion of the layer and to controllably modify an internal portion
of the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIGS. 1A and 1B schematically illustrate an example of laser
processing of a material workpiece and a corresponding result. FIG.
1A illustrates a focused laser processing beam having a diameter
w.sub.0 at an internal substrate position, and diameter w.sub.1 at
or near a coating on a substrate. FIG. 1B illustrates a processing
result in which a depthwise portion of the coating is removed over
diameter w.sub.1 and the substrate material is modified with a
relatively high intensity over diameter w.sub.0.
[0012] FIGS. 2A and 2B show example experimental results for
ablation of an indium tin oxide (ITO) layer on a glass substrate.
FIG. 2A shows a top view of examples of scribing lines formed in
the ITO layer using different laser pulses energies (in nJ), which
are listed below the respective scribing line. FIG. 2B shows an
example depth profile measurement of the ITO layer removed with
laser pulse energies that do not substantially ablate the glass
surface.
[0013] FIGS. 3A and 3B show an experimental result of using a
single laser beam to process an ITO layer on the surface of a glass
substrate and the inside of the glass substrate in parallel. FIG.
3A shows a top view of the ITO layer ablated with laser pulses at
the threshold energy sufficient to remove the ITO layer while
keeping the glass surface substantially ablation free. FIG. 3B is a
side cross-sectional view of the glass substrate that shows the
processing that occurred near the focal point positioned inside the
glass sample using the same laser beam which ablated the ITO
layer.
[0014] FIGS. 4A and 4B show example results of single pass
processing in which a gold coating deposited on a sapphire
substrate was ablated and an internal scribe line was formed in the
substrate. The processing occurred with high spatial overlap
between adjacent laser pulses, and during relative translation of
the substrate and the laser pulses in the single pass.
[0015] FIG. 5 schematically illustrates examples of some components
that can be used for ultrashort laser processing. The example
processing system comprises a laser system that is operatively
coupled to a controller and scanning system.
DETAILED DESCRIPTION
Overview
[0016] Utilization of a laser to separate dies of semiconductor
devices, such as light emitting diodes (LEDs), from the original
wafer can provide advantages compared to conventional mechanical
dicing methods. Laser processing may be particularly advantageous
when the wafer thickness decreases, and the device distribution
becomes denser. Laser singulation methods can generally be
categorized with either external processing or internal processing.
In some external processing methods, laser processing (e.g.,
ablation) starts from the surface of the wafer. In some internal
processing methods, the inside of the wafer is modified, such as by
generating micro cracks, while leaving the surface relatively
untouched. Some such internal processing methods leave very little
or no cutting kerf, which allows a greater amount of the wafer to
be utilized for circuitry. Some internal processing methods may
generate little or no chipping on the surface; thus little or no
pre- or post-processing may be needed. Such internal processing may
increase or maximize the die strength. Additionally, since a large
part of the separation edge may be cleaved, some such methods may
provide superior optical performance. This can be beneficial for
optical devices such as high brightness LEDs.
[0017] Although internal laser processing has many potential
advantages, the material being processed generally should be
substantially transparent to the wavelength of the laser, so that
the laser energy can reach the inside of the wafer. However, some
semiconductor devices have one or more coating layers disposed on a
wafer substrate, as will be illustrated below. The coating layer
formed on the substrate need not be in direct contact with the
substrate, but may be separated from the substrate by one or more
films or layers (e.g., dielectric films) therebetween. These
coating layer(s) may be relatively opaque to the processing laser
light. In this case, internal laser processing typically is not
employed unless the relatively opaque coating is removed in
advance. In other implementations, the internal laser processing
may be applied from a side of the wafer without the opaque coating,
which may not be desirable in some cases, because of processing
convenience or other reasons.
Examples of Laser Processing Systems and Methods for Coated
Materials
[0018] FIGS. 1A and 1B schematically illustrate an example of laser
processing of a material workpiece. A processing laser beam can be
focused to process the workpiece. The processing laser beam may
include a pulsed laser, such as a fiber-based laser system
operating at repetition rates in the kHz-MHz range. The workpiece
can include a semiconductor wafer, a multilayer material, a
composite material, or any other material. The workpiece can
include a substrate and a coating. In some implementations, the
workpiece includes a semiconductor wafer having multiple devices,
for example high brightness LEDs, formed thereon. In some such
cases, the LED can have a sapphire substrate with a thickness of a
few 10 s to a few 100 s .mu.m. Gallium nitride (GaN) based LEDs can
be grown on top of the sapphire substrate. Although certain
examples and experiments are described for GaN LEDs including a
sapphire substrate, these examples and experiments are intended to
be illustrative and not limiting. The methods and systems described
herein can be used to laser process other workpieces and other
types of coated substrates. For example, a substrate may comprise
sapphire, glass, silicon, or silicon carbide. The substrate may
comprise a material that is substantially transparent to the
wavelength of the laser beam used for processing the workpiece. In
some implementations a substrate may be weakly absorbing at the
laser wavelength.
[0019] One singulation technique (that may be used for LED dies) is
to tightly focus a laser beam, which is transparent to the sapphire
substrate, inside the substrate. When the laser intensity is high
enough, it can locally modify the substrate material, e.g., by
making micro-cracks through the process of multi-photon absorption.
The laser beam can be scanned along desired cutting lines, and then
the wafer can be extended to separate the LED dies along the
cutting lines. Such techniques are utilized in commercially
available LED processing equipment.
[0020] To improve performance of the LED output, some semiconductor
designs are arranged with a coating, for example a metal thin film
layer on a surface of the sapphire substrate. The coating acts as a
mirror to increase light emission. Although the thickness of the
metal film may be only a few tens of nanometer to few tens of
microns, the metal film can be sufficiently opaque to block the
processing laser light from reaching inside of the substrate for
the internal processing. In addition to its use for LEDs, a coating
layer is common for a variety of other semiconductor devices. Some
such devices can be used for electronic purposes, such as to form
electrical contacts, and some devices can be used for optical
purposes, and still other devices can be used for thermal or
mechanical purposes. In many of these cases, the coating is removed
either prior to or at the same time as die separation. Typically a
multi-step-processing with multiple techniques is used to remove
the coating.
[0021] Various embodiments described herein can eliminate a need
for the multiple processing steps while maintaining or exceeding
the functionality of singulation systems. In the example shown in
FIG. 1A, the workpiece includes a portion of a GaN LED wafer. The
wafer includes a sapphire substrate 140 that (optionally) includes
a distributed Bragg reflector (DBR) 130 for reflecting visible
wavelengths (WL) of light. A thin film metallic reflector 160 is
formed or coated on a surface of the sapphire substrate. The
metallic reflector 160 can include one or more layers of metal
film. The metallic reflector 160 can (optionally) include one or
more layers of dielectric. The example wafer also includes one or
more GaN emitters 150 for the LED. The GaN emitters 150 are
disposed on a side of the sapphire substrate 140 opposite to the
side of the substrate that is coated with the metallic reflector
160. Focusing optics 120, e.g., a focal lens, can be used to adjust
the focal properties of a laser beam 110 incident on the coated
surface 160 of the substrate. For example, the laser source and
focusing optics 120 and the workpiece can be relatively positioned
so that the laser beam 110 is incident on the coated surface 160 of
the substrate 140 before the laser beam 110 interacts with the
substrate 140 (e.g., the coated surface can be disposed on an
optical path between the substrate 140 and the focusing optics
120).
[0022] By way of example, w.sub.1 represents a laser beam diameter
at or near the surface of metallic reflector 160, and w.sub.0
represents the laser beam diameter position within substrate 140,
which corresponds to a beam waist in this example. The laser beam
may have a more tight focus 110b in the interior of the substrate
140 than the focus 110a on the surface of the substrate (e.g., at
the position of the coated surface 160). Thus, the intensity is
higher over the diameter w.sub.0 than the intensity over the
diameter w.sub.1. A beam delivery system (not shown) can be used to
deliver the laser beam 110 from a laser source (e.g., a pulsed
laser) to the workpiece. The beam delivery system can include a
scanning system that relatively positions the laser beam and the
workpiece. In various embodiments, the workpiece can be positioned
relative to a fixed laser system, or the laser beam can be
positioned relative to a fixed workpiece, or both the workpiece and
the laser beam can be positioned relative to each other to achieve
a desired scan.
[0023] In many cases of a workpiece including a substrate and one
or more coating layers, the coating layer(s) and the substrate have
different ablation thresholds. For example, the ablation threshold
of a sapphire substrate is approximately 1.5 J/cm.sup.2, when being
processed using ultrafast laser pulses with a central wavelength
around 1 .mu.m and sufficient pulse overlap. The ablation threshold
of a thin film metal coating may be about 1 to 2 orders of
magnitude lower under the same processing conditions (e.g.,
ablation thresholds of about 0.01 to about 0.1 J/cm.sup.2). In
various implementations, the ablation threshold of the substrate is
greater than the ablation threshold of the coating by a factor that
can be 2, 5, 10, 15, 25, 50, 75, 100, 150, 200, 250, 1000, or
more.
[0024] In certain implementations, the disclosed systems and
methods can utilize this difference in ablation thresholds to apply
the same laser beam for processing both the coating layer(s) and
the substrate. The coating layer(s) can, but need not be, formed
from a different material than the substrate. Focusing of the
processing laser beam can be managed so that the laser intensity on
the coating layer is different from the laser intensity near the
internal processing point. For example, the laser beam can be
relatively tightly focused 110b at the internal processing point
(e.g., the focal spot of the laser beam) while the laser beam is
more loosely focused (or relatively unfocused) 110a on the coating
layer, as shown in FIG. 1A. The laser fluence can be adjusted so
that the laser intensity on the coating layer is higher than the
ablation threshold of the coating layer but still lower than the
ablation threshold of the substrate. Therefore, the processing
laser can remove the coating layer while not ablating the substrate
surface. The laser intensity at an internal portion of the
substrate (e.g., at the focus 110b) can be higher than the ablation
threshold of the substrate, and the processing laser can internally
modify the substrate (e.g., at or nearby the position of the focus
110b). Accordingly, in the example shown in FIG. 1A, the laser beam
can ablate the coating layer and modify the internal portion of the
substrate substantially simultaneously.
[0025] FIG. 1B schematically illustrates a result of processing the
workpiece with one or more laser beams. An ablated portion of
metallic layer 160 at the substrate surface is shown. As will be
discussed below, by reducing heat affected zones near the processed
part of the metallic film, the substrate surface can remain
sufficiently transmissive such that the laser beam can pass through
the surface regions to reach the interior of the substrate for
internal processing and material modification to create an internal
feature 190 at or near focal position 110b. For example, DBR 130
may be transmissive at near infrared (IR) laser wavelengths and the
incident laser beam, with diameter w.sub.1, below an ablation
threshold for modification of DBR materials. In some
implementations all or a portion of the DBR dielectric layers in
the beam path may be removed and the internal feature 190
formed.
[0026] The focal properties of the laser beam can be managed to
control the intensity ratio between the surface and the focal spot
inside the substrate, so that the intensity at the focal spot
inside the substrate is greater than the substrate ablation
threshold. Thus, internal processing of the substrate material can
be performed. In some implementations, a controller can be used to
manage or adjust the properties of the laser beam pulses, the focal
conditions of the laser beam pulses, the relative positioning of
the workpiece and the laser beam pulses, and so forth. In some
embodiments a dynamic focusing mechanism may be utilized to adjust
the focal position, for example with relative translation 170 of
optical components and/or substrate 140, for example. In some
embodiments a controllable telescope (not shown) may be utilized to
adjust the laser beam size before the focusing lens. Changing the
beam size can change the ratio between the fluence at the focal
point, w.sub.0, and the fluence at or near a layer. Such an
arrangement may be used to optimize the processing conditions for
different substrate thicknesses (e.g., focus depth) and film
ablation thresholds. Such arrangements, and other techniques and
systems for controlling laser parameters, are disclosed in U.S.
Pat. No. 7,486,705, assigned to the assignee of the present
application, and may be used with various embodiments disclosed
herein. U.S. Pat. No. 7,486,705 is hereby incorporated by reference
herein in its entirety for all it discloses. Commercially available
dynamic focusing arrangements utilizing motorized lens systems may
be used in some implementations, and may be used in combination
with beam scanning mechanisms.
[0027] Table I lists examples of the processing laser pulse energy
and focal conditions such that the laser intensity is approximately
at the ablation threshold of both the substrate and the coating
layer. In Table I, examples of the thin film ablation threshold
being 3 times and 100 times weaker than the substrate ablation
threshold are shown, and two different focal depths from the
surface of the substrate are shown.
TABLE-US-00001 TABLE I Examples of laser pulse energy and focal
condition under different processing conditions Thin film ablation
threshold (relative to ablation threshold of substrate) 1/3 1/3
1/100 1/100 Distance of focal spot from 50 200 50 200 substrate
surface (.mu.m) Focal spot radius (.mu.m) 2.6 5.2 0.98 1.96 Laser
pulse energy (.mu.J) 0.32 1.27 0.045 0.18 Focal lens numerical
aperture 0.13 0.06 0.34 0.17 (NA)
[0028] The example laser energy and focal conditions summarized in
Table I are for example cases when the laser intensity at the thin
film coating and at the substrate are at the ablation threshold. In
some singulation applications, it may be desired that the laser
intensity be some level above the ablation threshold (at the
coating and/or in the substrate), or the energy loss may need to be
considered. In those cases, the focusing of processing laser beam
and/or the laser pulse energy may be adjusted accordingly relative
to the examples shown in Table I.
[0029] In various implementations, various types of pulsed lasers,
for example, nanosecond, picosecond, or femtosecond pulsed lasers,
can be used for processing coated substrates. Ultrashort pulsed
lasers (e.g., with pulse widths from tens of femtoseconds to about
a picosecond) may be desirable in many processing applications. For
example, an ultrashort pulsed laser may provide sufficiently high
peak laser intensity for multi-photon internal processing.
Ultrashort laser pulses are generally able to process wide
varieties of materials as compare to continuous wave (cw) or longer
pulsed lasers. Further, ultrashort laser pulses may process the
workpiece material such that there are reduced or minimal heat
affected zones (HAZs). Reducing the amount of HAZ can be
advantageous to remove the coating layer(s) while still keeping the
substrate surface sufficiently optically smooth to allow the
processing laser beam to propagate through the surface for internal
processing.
[0030] Another commonly used coating on semiconductor wafers
includes indium tin oxide (ITO). FIGS. 2A and 2B show example
experimental results for ablation of an ITO layer on a glass
substrate. The ITO layer had a thickness of about 120-150 nm. The
laser was an ultrafast fiber laser with a central wavelength of
1.045 .mu.m, generating pulses with pulse widths of about 0.4 ps,
and at a repetition rate of 100 kHz. With management of the laser
pulse energy, the experimental results demonstrate that the ITO
layer can be removed while still leaving the glass surface
substantially unmodified. FIG. 2A shows a top view of examples of
scribing lines formed in the ITO layer using different laser pulses
energies, which are listed below the respective scribing line. FIG.
2A shows that the widths of the scribing lines increase as the
laser pulse energy increases. An inset showing a 2.5 .mu.m long bar
is provided for comparison. This example illustrates damage of the
glass surface that occurred at a pulse energy of 310 nJ. The indium
tin oxide (ITO) layer was partially removed with 50 nJ pulses. A
clear trench on the ITO film was achieved with other laser energies
above 50 nJ, and without damage to the glass surface (for pulse
energies below about 310 nJ).
[0031] FIG. 2B shows an example depth profile measurement of the
ITO layer removed with optimized laser pulse energies. A
substantially flat bottom of the ablation trench demonstrates that
the ITO was substantially removed while the glass surface was
substantially undamaged and ablation free. The bottom of the depth
profile is substantially flat to less than about 4 nm. The width of
the scribed line, at the upper surface of the remaining ITO layer,
is about 8 .mu.m in this example. The width of the scribed line, at
the glass surface, is about 4 .mu.m in this example.
[0032] As shown in the above examples, when the laser pulse energy
is sufficiently low, the laser beam only partially removes the ITO
layer. At threshold pulse energy, the entire depthwise portion of
the ITO layer was removed, while the underlying glass surface was
not damaged. Laser pulse energies above the threshold ablate the
glass surface while also removing the ITO layer. Because a
threshold can depend on laser pulse parameters, for example beam
focus and pulse overlap, it is to be understood that the threshold
may vary from a nominal value. For example, the results of FIG. 2A
were obtained with twice the spot-to-spot overlap relative to that
of FIG. 2B. The damage threshold for glass was slightly different
in the two experiments. It is known that the damage threshold for
glass is not linearly proportional to the pulse overlap. In various
implementations for substrate processing such variable conditions
may be included for determination of a process window, and for
corresponding adjustments to compensate for variations in materials
or laser processing conditions.
[0033] FIGS. 3A and 3B show an experimental result using a single
laser beam to process an ITO layer on the surface of a glass
substrate while processing the inside of the glass substrate. The
laser beam was focused 0.2 mm below the glass surface with a beam
waist of about 2.5 .mu.m. The beam spot diameter on the ITO layer
was about 36 .mu.m. The laser intensity at the focal spot inside
the glass was about 50 times larger than the laser intensity on the
ITO layer. In this example the laser pulse energy incident on the
sample was 3 .mu.J, the pulse width 0.4 ps, and the pulse
repetition rate was 100 kHz. As discussed above, by increasing the
laser pulse energy, the ITO layer removal varies from insufficient
ablation to remove the layer (e.g., at laser pulse energies below
threshold), to optimized ablation to remove the layer without
damaging the glass surface (e.g., at laser pulse energies at or
just above the threshold), to over ablation which removes the ITO
layer but also at least partially ablates the glass surface (e.g.,
at laser pulse energies substantially above the threshold).
[0034] FIG. 3A shows a top view of the ITO layer ablated with laser
pulses at the threshold energy sufficient to remove the ITO layer
while keeping the glass surface substantially ablation free. The
ITO layer was removed with a trench width of 22 .mu.m. FIG. 3B is a
side cross-sectional view of the glass substrate that shows the
processing that occurred around the focal spot positioned inside
the glass sample using the same laser beam which ablated the ITO
layer. The experiment demonstrates that, by using desired focal
conditions, the disclosed systems and methods can remove a coating
layer on a substrate while internally processing the substrate
using the same laser beam during the same processing pass.
[0035] FIGS. 4A and 4B show example results of single pass
processing in which a gold coating deposited on a sapphire
substrate was ablated and an internal scribe line was formed in the
substrate. The processing occurred with high spatial overlap
between adjacent pulses, and during relative translation of the
substrate with respect to the laser pulses in the single pass. The
laser system was the same as for the example experiments described
with reference to FIGS. 2A, 2B, 3A, and 3B. Each beam was focused
such that the beam waist, w.sub.0, was formed about 100 .mu.m below
the substrate surface. The substrate comprised a c-plane orientated
sapphire wafer, having a 400 .mu.m thickness. Both the front and
back surfaces of the wafer were polished. A gold thin film,
approximately 100 nm thick, was deposited on half of polished
sapphire wafer. The processing laser beam was scanned relative to
the coated/uncoated boundary. In this example high spot overlap of
about 99% or greater was used.
[0036] FIG. 4A shows a high magnification (50.times.) microscope
image of portions of the substrate, taken with bright field
illumination. The left side of the image shows bare wafer 401 and
the right side shows a portion 403 of the gold coated surface. The
gold coating was cleanly removed along a scribe line as illustrated
in the image portion 405 of the narrow depth of field image. No
damage of sapphire surface was observed in this experiment.
[0037] FIG. 4B is a transmission image of the same portion of the
sample, where microscope focus was adjusted toward an internal
region of the substrate rather than on its surface. A uniform
internal scribe line 411 across the gold coating boundary to the
bare wafer was observed. The experiment result indicates removal of
gold coating and internal processing of sapphire substrate. FIG. 4B
shows that the internal processing of the sapphire wafer occurred
in both the uncoated 401 and gold coated 403 portions of the
substrate. The inset illustrates a transmission image of the
defocused scribe line 415 through the removed region of gold
coating with focus on the surface of the substrate. In effect, the
removed gold forms an aperture through which the scribe line is
observable with microscopic viewing. The uniform transmission of
light in the region where gold was removed indicates clean gold
coating removal and no damage on the sapphire surface, which allows
the processing laser pulses to transmit through this removal area.
Notably, the internal scribing line is barely affected by the gold
coating on substrate surface.
[0038] Beam focus and laser fluence are some of the factors to
consider for scribing. As shown in the above examples, if laser
intensity on the gold coating is too high, then the processing beam
can damage the sapphire surface while removing the gold coating. On
the other hand, if the laser intensity is too low, then the power
loss on gold film removal will result in a weaker scribing line
under the gold coating as compared to under a bare sapphire wafer.
Such lower laser intensity can cause gold coating removal without
internal scribing.
[0039] In the experiments described with reference to FIGS. 2A-4B,
the capability of removing conductive films (e.g., ITO and gold in
these examples), and modifying an internal portion of the substrate
was demonstrated for single pass operation. The intensity at the
beam waist, w.sub.0, inside the transparent substrate, was
sufficiently high to modify the substrate material, yet the
intensity over the defocused diameter, w.sub.1, at the substrate
surface, was sufficient to cleanly remove a layer of metal film.
Such an arrangement may simplify processing and increase
throughput, because, the internal and surface processing can be
performed in a single pass with the same set of laser parameters
(e.g., beam focus and fluence). However, it is to be understood
that in some embodiments a group of laser pulses generated at a
high repetition rate may be utilized, effectively resulting in a
further increase in overlap. Moreover, in some embodiments the
method and systems disclosed herein may be utilized in combination
with processing arrangements in which multiple depthwise focused
beams are simultaneously or sequentially applied to the target
material with multiple passes. For example, methods and systems
described in U.S. Patent Publication No. 2010/0025387, which is
owned by the assignee of the present application, may be used with
embodiments disclosed herein. The '387 Publication teaches, among
other things, that a single pass of a focused beam of ultrashort
laser pulses may be used to create a surface groove in a
transparent material and at least one modified region within the
bulk of the material. The surface groove and at least one modified
region are each formed by interaction the focused beam with the
material. U.S. Patent Publication No. 2010/0025387 is hereby
incorporated by reference herein in its entirety for all it
discloses. Also, two or more laser passes can be used in some
implementations.
[0040] FIG. 5 schematically illustrates examples of some components
that can be used for ultrashort laser processing. The processing
system can comprise a laser system 504 that is operatively coupled
to a controller and scanning system 506. In some embodiments, the
laser system 504 is configured to output laser pulses that comprise
one or more ultrashort pulses (USP). In some implementations the
USP laser comprises a fiber-based chirped pulse amplification
system that generates femtosecond (fs) pulses with energy exceeding
1 .mu.J, at a repetition rate of 100 KHz or greater. Such fiber
based systems are commercially available from IMRA America, Inc
(Ann Arbor, Mich.). In various embodiments the USP system will
provide for adjustment of certain pulse parameters over a
substantial range.
[0041] In the embodiment illustrated in FIG. 5, the scanning system
506 includes two beam deflectors 508, for example galvanometric
scanning mirrors, capable of two-dimensional scanning. In other
embodiments, a different number and/or type of scanning mirrors may
be used. In some embodiments, the scanning may be one-dimensional.
The scanning system 506 may also include focusing optics 510 such
as, for example, an integrated F-theta lens capable of producing a
substantially flat field of view at the target substrate. An
optional motion system 520 may position the substrate in one-,
two-, or three-dimensions, and may further provide mechanism(s) for
rotation of the substrate (e.g.: roll, pitch, and yaw). In some
embodiments the substrate may be processed during motion of the
stage. An optional fume extractor 514 may be included to capture
any debris.
[0042] In some embodiments a laser pulse width may be in the range
from femtoseconds (fs) to tens of picoseconds (ps), and most
preferably up to a few hundred fs. Pulse energy may be in the range
from about 10 nJ to 100 nJ, and up to a few hundred .mu.J. Pulse
repetition rates may be in the kHz to MHz range, and preferably
from about 100 kHz to about 1 MHz. A spot diameter, w.sub.0,
corresponding to a beam waist position inside the substrate, may be
in the range from a few .mu.m to a few tens of .mu.m. A spot
diameter, w.sub.i, corresponding to a diameter at or near the
surface of the substrate, may be in the range from tens of microns
to a few hundred microns. Spatial overlap between pulses may vary
with scan speed and pulse rate, and may be greater than about 70%,
greater than about 80%, greater than about 90%, or greater than
about 99%. The examples of nominal parameters listed above are not
necessarily independent. For example, the fluence can depend on
both the spot size and pulse energy, and the ablation rate in turn
can depend on the fluence and pulse overlap. A laser wavelength may
be in the near IR range, e.g., from about 700 nm up to a few
microns. In the above examples, layer(s) included ITO and/or gold
conductive layers. Other suitable layer materials may be utilized
in various implementations, and may include conductor(s),
semiconductor(s), or dielectric(s). For example, some integrated
circuits are fabricated with low-k dielectric layer(s) deposited on
silicon carbide (SiC). In some implementations, the low-k
dielectric was removed, and an internal portion of the SiC was
modified, in a single pass.
[0043] Various embodiments and examples of this disclosure
illustrate how a single laser beam with a pre-determined fluence
may remove a portion of a layer disposed on a substrate and modify
an internal portion of the substrate, and do so nearly
simultaneously, and without a requirement for intervening depthwise
adjustment of a focused laser beam position. In certain
implementations a group of time separated ultrafast laser pulses
with pre-determined spatial overlap between the pulses may be
utilized, and without a requirement for intervening depthwise
adjustment of a focused laser beam position. For example,
pulse-to-pulse overlap, produced by the combination of translation
speed and laser pulse repetition rate, can be selected so that
there is sufficient overlap, wherein the first pulses ablate and
remove the top layer without damaging the substrate surface and the
subsequent pulses, impinging on the same area of the substrate,
focused below the surface, generate the subsurface material
modification. Many other variations are possible. For example,
processing is not restricted to a single substrate and single layer
disposed thereon. In at least one embodiment a transparent medium
which, for example, may include glass or thin films, may be
disposed between the focusing optics 120 and the layer(s) and
substrate. In such arrangements the optical system can be adjusted,
or otherwise modified, to provide proper fluence at depthwise
location(s) to be processed.
[0044] The example experiments, experimental data, tables, graphs,
plots, photographs, figures, and processing and/or operating
parameters (e.g., values and/or ranges) described herein are
intended to be illustrative of some possible operating conditions
of the disclosed systems and methods and are not intended to limit
the scope of the operating conditions for other embodiments of the
methods and systems disclosed herein. Additionally, the
experiments, experimental data, calculated data, tables, graphs,
plots, photographs, figures, and other data disclosed herein
demonstrate various regimes in which embodiments of the disclosed
systems and methods may operate effectively to produce one or more
desired results. Such operating regimes and desired results are not
limited solely to specific values of operating parameters,
conditions, or results shown, for example, in a table, graph, plot,
figure, or photograph, but also include suitable ranges including
or spanning these specific values. Accordingly, the values
disclosed herein include the range of values between any of the
values listed or shown in the tables, graphs, plots, figures,
photographs, etc. Additionally, the values disclosed herein include
the range of values above or below any of the values listed or
shown in the tables, graphs, plots, figures, photographs, etc. as
might be demonstrated by other values listed or shown in the
tables, graphs, plots, figures, photographs, etc. Also, although
the data disclosed herein may establish one or more effective
operating ranges and/or one or more desired results for certain
embodiments, it is to be understood that not every embodiment need
be operable in each such operating range or need produce each such
desired result. Further, other embodiments of the disclosed systems
and methods may operate in other operating regimes and/or produce
other results than shown and described with reference to the
example experiments, experimental data, tables, graphs, plots,
photographs, figures, and other data herein.
[0045] Other systems, setups, and parameters may be used in other
implementations, which may provide the same or different results.
Many variations are possible and are contemplated within the scope
of this disclosure. Films, layers, components, features,
structures, and/or elements may be added, removed, combined, or
rearranged. Additionally, process or method steps may be added,
removed, or reordered. No single feature or step, or group of
features or steps, is indispensable or required for each
embodiment.
[0046] For purposes of this disclosure, certain aspects,
advantages, and novel features are described herein. It is to be
understood that not necessarily all such advantages may be achieved
in accordance with any particular embodiment. Thus, for example,
those skilled in the art will recognize that the systems and
methods may be embodied or carried out in a manner that achieves
one advantage or group of advantages as taught herein without
necessarily achieving other advantages as may be taught or
suggested herein. Furthermore, embodiments may include several
novel features, no single one of which is solely responsible for
the embodiment's desirable attributes or which is essential to
practicing the systems and methods described herein. Additionally,
in any method or process disclosed herein, the acts or operations
of the method or process may be performed in any suitable sequence
and are not necessarily limited to any particular disclosed
sequence.
[0047] Conditional language used herein, such as, among others,
"can," "could," "might," "may," "e.g.," and the like, unless
specifically stated otherwise, or otherwise understood within the
context as used, is generally intended to convey that certain
embodiments include, while other embodiments do not include,
certain features, elements and/or steps. Thus, such conditional
language is not generally intended to imply that features, elements
and/or steps are in any way required for one or more embodiments or
that one or more embodiments necessarily include logic for
deciding, with or without author input or prompting, whether these
features, elements and/or steps are included or are to be performed
in any particular embodiment. The terms "comprising," "including,"
"having," and the like are synonymous and are used inclusively, in
an open-ended fashion, and do not exclude additional elements,
features, acts, operations, and so forth. Also, the term "or" is
used in its inclusive sense (and not in its exclusive sense) so
that when used, for example, to connect a list of elements, the
term "or" means one, some, or all of the elements in the list. In
addition, the articles "a" and "an" as used in this application and
the appended claims are to be construed to mean "one or more" or
"at least one" unless specified otherwise.
[0048] While certain embodiments of the inventions disclosed herein
have been described, these embodiments have been presented by way
of example only, and are not intended to limit the scope of the
inventions disclosed herein. No single feature of group of features
is necessary or indispensable for each embodiment. Reference
throughout this disclosure to "some embodiments," "an embodiment,"
or the like, means that a particular feature, structure, step,
process, or characteristic described in connection with the
embodiment is included in at least one embodiment. Thus,
appearances of the phrases "in some embodiments," "in an
embodiment," or the like, throughout this disclosure are not
necessarily all referring to the same embodiment and may refer to
one or more of the same or different embodiments. Indeed, the novel
methods and systems described herein may be embodied in a variety
of other forms; furthermore, various omissions, substitutions,
equivalents, and changes in the form of the methods and systems
described herein may be made without departing from the spirit of
the inventions disclosed herein.
* * * * *