U.S. patent application number 14/178414 was filed with the patent office on 2014-06-12 for pulsed laser micro-deposition pattern formation.
This patent application is currently assigned to IMRA AMERICA, INC.. The applicant listed for this patent is IMRA AMERICA, INC.. Invention is credited to Yong Che, Zhendong Hu, BING LIU, Makoto Murakami, Jingzhou Xu.
Application Number | 20140161998 14/178414 |
Document ID | / |
Family ID | 42678530 |
Filed Date | 2014-06-12 |
United States Patent
Application |
20140161998 |
Kind Code |
A1 |
LIU; BING ; et al. |
June 12, 2014 |
PULSED LASER MICRO-DEPOSITION PATTERN FORMATION
Abstract
A method of forming patterns on transparent substrates using a
pulsed laser is disclosed. Various embodiments include an
ultrashort pulsed laser, a substrate that is transparent to the
laser wavelength, and a target plate. The laser beam is guided
through the transparent substrate and focused on the target
surface. The target material is ablated by the laser and is
deposited on the opposite substrate surface. A pattern, for example
a gray scale image, is formed by scanning the laser beam relative
to the target. Variations of the laser beam scan speed and scan
line density control the material deposition and change the optical
properties of the deposited patterns, creating a visual effect of
gray scale. In some embodiments patterns may be formed on a portion
of a microelectronic device during a fabrication process. In some
embodiments high repetition rate picoseconds and nanosecond sources
are configured to produce the patterns.
Inventors: |
LIU; BING; (Ann Arbor,
MI) ; Hu; Zhendong; (Ann Arbor, MI) ;
Murakami; Makoto; (Ann Arbor, MI) ; Xu; Jingzhou;
(Ann Arbor, MI) ; Che; Yong; (Ann Arbor,
MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
IMRA AMERICA, INC. |
Ann Arbor |
MI |
US |
|
|
Assignee: |
IMRA AMERICA, INC.
Ann Arbor
MI
|
Family ID: |
42678530 |
Appl. No.: |
14/178414 |
Filed: |
February 12, 2014 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12400438 |
Mar 9, 2009 |
8663754 |
|
|
14178414 |
|
|
|
|
Current U.S.
Class: |
428/29 ; 118/708;
427/597; 428/195.1 |
Current CPC
Class: |
B41M 5/267 20130101;
B41M 5/262 20130101; C23C 14/3435 20130101; Y10T 428/24802
20150115; G02B 26/101 20130101; Y10T 428/24917 20150115; C23C 14/28
20130101; B44F 1/10 20130101; C23C 20/04 20130101 |
Class at
Publication: |
428/29 ; 427/597;
118/708; 428/195.1 |
International
Class: |
C23C 20/04 20060101
C23C020/04; B44F 1/10 20060101 B44F001/10 |
Claims
1. A method of pulsed laser deposition to produce a pattern on a
medium, said medium being substantially transparent at a wavelength
of said pulsed laser, said method comprising: generating pulsed
laser beams from a pulsed laser source; transmitting said pulsed
laser beams through said medium; focusing said pulsed laser beams
onto a target, said target producing ejecta in response to an
interaction of said pulsed beams and said target; scanning said
laser beams relative to said medium and target; accumulating at
least a portion of said ejecta on said medium to form material
deposits on said medium; and varying at least one of a laser beam
scan speed and scan line density for controlling thickness of said
material deposits to vary an optical density of a region of said
medium, and to form a spatial pattern having varying optical
density, wherein said laser beam scan speed or said scan line
density is varied according the gray scale of the pattern to be
printed, the gray scale including at least three gray levels in a
digitized image of said pattern.
2. The method of claim 1, wherein said interaction causes
sufficiently high temperature and pressure to expel ejecta sideways
relative to said scan direction and to concentrate ejecta toward
the periphery of the path of said laser beams such that the
material deposits are formed with central low thickness portions
bounded by immediately adjacent, thicker outer portions, and
wherein said medium and said target are spaced apart by a gap, or
are in direct physical contact, and the thickness of said deposits
is controlled with a repetition rate and said scan speed of said
laser beams.
3. The method of claim 1, wherein said pulsed laser source provides
a repetition rate from about 100 kHz to 1 GHz, a laser pulse having
a pulse duration in the range from about 10 femtosecond up to 100
nanosecond, and a laser pulse energy in the range from about 100
nanoJoules (nJ) to about 100 microJoules (.mu.J).
4. The method of claim 1, wherein said medium comprises glass,
quartz, sapphire, or a polymer.
5. The method of claim 1, wherein said target comprises a
metal.
6. The method of claim 1, wherein said target comprises a
functional material for emitting light, said function comprising
one or more of phosphor luminescence and electro-luminescence.
7. The method of claim 1, wherein said target comprises a material
for color printing.
8. The method of claim 1, wherein said laser beam scan speed is in
the range from about 1 mm/s to about 1 m/s, and utilized to produce
accumulated deposition of material.
9. The method of claim 1, wherein said medium is placed in contact
with said target, or at a distance up to about 5 millimeters from
said target.
10. The method of claim 1, wherein said target has a surface with a
roughness greater than an output wavelength of said laser
source.
11. The method of claim 1, wherein said pattern comprises a bitmap
image or a vector graphic.
12. The method of claim 1, wherein a grey scale image having a
discernible feature is obtainable with ambient or controlled
illumination of said pattern.
13. The method of claim 1, wherein said medium is disposed between
said source and said target, and said ejecta propagates reversely
to the laser incidence direction.
14. The method of claim 1, wherein said controlling comprises
scanning said pulsed laser beams, and varying said scan speed.
15. The method of claim 1, wherein controlling comprises scanning
said pulsed beams and varying the line density of said pulsed beam
scan.
16. The method of claim 1, wherein controlling comprises scanning
said pulsed beams in one or more of a raster or vector pattern over
said target.
17. A system for pulsed laser deposition to produce a pattern
having optical density on a medium, said medium being is
substantially transparent at a wavelength of said pulsed laser,
said system comprising: a high repetition rate laser source for
generating pulsed laser beams; a beam delivery system, comprising:
a focusing sub-system to transmit said pulsed laser beams through
said medium and to focus said pulsed laser beams onto a target;
said target producing ejecta in response to an interaction of said
pulsed beams and said target, accumulations of said ejecta forming
material deposits on said medium; and a controller coupled to said
source and said beam delivery system and controlling thickness of
said material deposits to form a spatial pattern having varying
optical density, by controlling scanning of said laser beams
relative to said medium and target such that at least one of a
laser beam scan speed and scan line density is varied, and wherein
said laser beam scan speed or said scan line density is varied
according to the gray scale of the pattern to be printed, the gray
scale including at least three gray levels in a digitized image of
said pattern.
18. The system of claim 17, wherein said delivery system comprises
a beam deflector, and said focusing sub-system comprises a scan
lens.
19. The system of claim 17, wherein said medium and said target are
spaced apart by a gap providing separation, and said gap is
configured to contain ambient air, flowing dry air or an inert
gas.
20. The system of claim 17, wherein said medium and said target are
in direct contact.
21. The system of claim 17, wherein the material deposits have
central low thickness portions bounded by immediately adjacent,
thicker outer portions, and wherein said medium and said target are
spaced apart by a gap, or are in direct physical contact, and
wherein said controller further controls a repetition rate of said
pulsed laser beams and controls thickness of said deposits with
said repetition rate and said scan speed of said laser beams.
22. A method of pulsed laser deposition to produce a pattern on a
medium, said medium being substantially transparent at a wavelength
of said pulsed laser, said method comprising: generating pulsed
laser beams from a pulsed laser source; transmitting said pulsed
laser beams through said medium; focusing said pulsed laser beams
onto a target, said target producing ejecta in response to an
interaction of said pulsed beams and said target; scanning said
laser beams relative to said medium and target; accumulating at
least a portion of said ejecta on said medium to form material
deposits on said medium, said deposits comprising a functional
material that is operable to emit radiation in response to a
stimulus; and varying at least one of a laser beam scan speed and
scan line density for controlling thickness of said material
deposits to vary an optical property of said material deposits, and
to form a spatial pattern having varying optical density, wherein
said laser beam scan speed or said scan line density is varied
according to a gray scale of the pattern to be printed, the gray
scale including at least three gray levels in a digitized image of
said pattern.
23. The method of claim 22, wherein an optical property of said
functional material comprises one or more of phosphorescence,
electro-luminescence, and selective light absorption and emission
for visual color effects.
24. The method of claim 22, wherein said stimulus comprises input
radiation.
25. A medium having a pattern formed thereon, said medium
comprising material deposits formed, over microscopic region(s),
with central low thickness portions bounded by immediately
adjacent, thicker outer portions, said material deposits of said
pattern providing for discernible images when viewed by the unaided
eye or at low magnification.
26. The medium of claim 25, wherein said pattern is viewed with
ambient illumination.
27. The medium of claim 25, wherein said pattern is viewed with
controlled illumination.
28. The medium of claim 25, wherein said pattern is formed with the
pulsed laser deposition method of claim 1.
29. The medium of claim 25, wherein said pattern is formed with the
pulsed laser deposition method of claim 22.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to U.S. Ser. No. 12/400,438,
entitled "Pulsed Laser Micro-Deposition Pattern Formation", filed
Mar. 9, 2009. U.S. Ser. No. 12/400,438 is hereby incorporated by
reference in its entirety.
FIELD OF THE INVENTION
[0002] This invention is related to pulsed laser deposition, and
the formation of patterned materials therewith.
BACKGROUND OF THE INVENTION
[0003] When using a pulsed laser for patterned material deposition,
methods fall into two general categories: laser-induced forward
transfer (LIFT) and laser-induced backward transfer (LIBT). The
ablated material is transferred to the receiving substrate in the
same direction with LIFT, or in a reverse direction relative to the
incident laser with LIBT. In LIFT, a target film needs to be
deposited on a laser-transparent supporting substrate. The
receiving substrate is placed facing the target film. The laser
beam, incident from the uncoated side of the target supporting
substrate, causes ablation in the target film. The ablated material
is transferred forwardly in the same direction as the laser, and to
the receiving substrate. In a LIBT setup, the geometry is reversed.
The laser is guided through the laser-transparent receiving
substrate first and focused on the target. The target can be a
plate made of the desired target material. Upon ablation, the
ablated material is transferred backwardly, in a reverse direction
to the incident laser beam, and deposited on the receiving
substrate.
[0004] Several LIFT methods are disclosed in, for example, U.S.
Pat. Nos. 4,752,455 and 6,159,832 issued to Mayer, U.S. Pat. No.
4,987,006 issued to Williams et al., U.S. Pat. Nos. 6,177,151 and
6,766,764 issued to Chrisey et al. A few LIBT methods are described
in U.S. Pat. No. 5,173,441 issued to Yu et al, Japan patent
2005-79245 issued to Hanada et al., and US patent application
2007/0243328 to Liu et al.
[0005] Laser-induced-plasma assisted ablation has been used for
color marking of metal targets, as disclosed by Hanata et al,
"Colour marking of transparent materials by
laser-induced-plasma-assisted ablation (LIPAA)", Journal of
Physics: Conference Series 59 (2007), 687-690. Various lasers were
tested, and produced various picosecond, nanosecond, and
femtosecond outputs, with a maximum repetition rate of 10 KHz. For
this RGB process it was concluded that a conventional nanosecond
pulse width has great potential for high-quality and cost effective
marking in the laser-marking industry.
[0006] An object of the above methods is precise and patterned
deposition of materials. If applied to printing, these methods are
binary and would provide an on/off effect or a visually black/white
effect. In order to print a bitmap image over a large gray scale
range, two requirements need to be satisfied: (i) sufficient number
of gray scale levels and (ii) a practically acceptable speed of
printing.
[0007] A recent international patent application, WO 2008/091898 by
Shah et al., assigned to the assignee of the present application,
discloses a method of ultrashort pulsed laser printing of images on
solid surfaces. This method is based on surface texturing induced
by ultrashort pulsed laser interaction with solid surfaces. In a
range of laser fluence and exposure time (average power per unit
area), several types of surface textures can be produced after
laser irradiation, including ripples, pillars, pores and many types
of random micro-protrusions. A controlled arrangement of these
textures produces a visual effect of gray scale by scattering,
trapping, and absorbing light. This method does not involve
material transfer from a target to a substrate.
[0008] LIFT, LIBT, and LIPAA systems have utilized Nd:YAG,
Ti:Sapphire at a 1 kHz repetition rate, and up to about 10 KHz with
NdYVO.sub.4 based systems. Forming patterns or images at high
resolution on a macroscopic scale could take up to a thousand
minutes as a result of the low repetition rates, limiting the
application of these methods. Moreover, as set forth above, many
systems are limited to production of binary patterns.
SUMMARY OF THE INVENTION
[0009] An objective of one or more embodiments is precise
deposition of materials on transparent substrates, with both the
location and thickness under control. The substrate may be a glass,
or other suitable medium.
[0010] At least one embodiment provides a LIBT method for forming a
pattern on a transparent medium at a high speed.
[0011] In various embodiments the location and thickness of
deposited material is controlled to vary the optical density of a
region of the medium such that a gray scale image is obtainable
with illumination of the medium. By way of example, the location
and thickness of deposited material is controlled over microscopic
regions of the medium, and associated variations in reflectance
over the medium create a visual effect of gray scale, and a
discernible image when viewed with the un-aided eye, or at low
magnification. Either ambient or controlled illumination may be
utilized.
[0012] In various embodiments a receiving substrate is placed
adjacent and opposite to the target plate. A laser beam is guided
through the receiving substrate and is focused on the target such
that the material is ablated and transferred backwardly to the
receiving substrate.
[0013] Another objective is laser printing of images, including but
not limited to artistic or photographic images, on transparent
substrates. More particularly, with a high repetition rate
ultrashort pulsed laser, both a visual effect of gray scale and a
fast printing speed can be achieved.
[0014] In various embodiments the gray scales are produced by
varying material deposition such that the light transmission and
reflection of the printed patterns is varied depending on the
thickness of the deposits. The thickness may be continuously
controlled with control of laser parameters. A high repetition rate
laser is utilized such that the target under the laser irradiation
can receive a variable number of laser pulses over a focused spot
diameter.
[0015] In various embodiments the amount of deposition is varied in
two ways during printing: (i) varying the laser beam scan speed
while maintaining a constant scan line density, (ii) varying the
laser scan line density while maintaining a constant beam scan
speed. The first way provides for printing bitmap images of art,
photographs, and the like. The second way provides for printing
vector graphics such as text patterns and simple geometric
figures.
[0016] Various embodiments provide fast printing speed. For
example, in an embodiment with a laser repetition rate of 1 MHz, an
image of 2.times.2 square inch is printed in 20 sec to 1 min. With
other lasers having 1 kHz repetition rate, such a printing would
take up to a thousand minutes.
[0017] In various embodiments PLD pattern formation may be carried
out in air, and without a vacuum chamber. In some embodiments
vacuum or some other control of atmosphere may be utilized, for
example gas flow of dry air.
[0018] The target materials can be metals, for example, steel,
aluminum, or copper. Steel will provide a brownish color to the
printed image. Dielectric materials, including but not limited to
silicon and carbon can also be used.
[0019] Another objective is to print patterns with a functional
target material. Such a material provides special functions in
addition to modifying light transmission or reflection. In at least
one embodiment, a target made of phosphor materials is used such
that the printed image is nearly invisible under room or sun light
illumination, and only under special illumination with UV light,
the image becomes visible.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 schematically illustrates an arrangement for pulsed
laser micro-deposition pattern formation.
[0021] FIG. 2 schematically illustrates further details of an
arrangement for pulse laser micro-deposition of materials to
provide patterns of varying optical density.
[0022] FIGS. 3A and 3B show an example illustrating two optical
microscope images of printed patterns.
[0023] FIGS. 4A and 4B show an example illustrating two images
printed on 2-inch glass wafers with a steel plate as the
target.
[0024] FIGS. 5A and 5B show an example illustrating a text printed
on a 1.times.1 square inch glass wafer using a target made of a
phosphor material.
DETAILED DESCRIPTION OF THE INVENTION
[0025] FIG. 1 schematically illustrates a portion of an arrangement
for pulsed laser micro-deposition pattern formation, and provides
an illustration of laser-material interaction. In this example the
laser beam 1 is directed through a transparent medium 2, for
example a glass substrate, and is focused on the target 3. Ablation
removes materials at the focal point 4 and causes a small crater
(not to scale). In this example the ablated material, particularly
the ejecta 5, transfers backwardly, generally propagating in a
direction opposite to the laser incident direction. Material is
deposited on the substrate, for example the surface that is facing
the target.
[0026] In at least one embodiment the substrate is positioned near
the target so that a small gap remains between the target and the
receiving substrate 2. The gap width can be adjusted by inserting a
spacer of different thickness between the substrate and the target.
A small gap width is preferred, for example less than about 10
micrometers, to provide high image resolution. In practice, the
substrate may be placed in direct contact with the target. In such
case, judging from the interference fringes often appearing between
a smooth target surface and the substrate, the gap width is around
1 micrometer. In some embodiments the medium and target are spaced
apart by a gap that provides separation The gap may be filled with
ambient air, or with flowing dry air. In some embodiment the gap
may be filled with an inert gas, for example nitrogen or argon.
Physical parameters, for example pressure, within the gap may be
controlled.
[0027] The interference fringes between the substrate and the
target have high contrast when the target surface is smooth and
shiny. These fringes can degrade the quality of printing by
modulating the laser fluence. One way to avoid interference effects
is to use a rough target surface, for example a granular surface,
to randomize the reflection off the target. In general, with a
roughness greater than the laser wavelength the interference effect
can be reduced to negligible levels.
[0028] Referring again to FIG. 1, deposited material is heavily
concentrated near the periphery of the beam scan line (not shown).
A double line 6 is formed on the receiving substrate. This is
because the high temperature and high pressure under direct laser
irradiation expels the ablated material sideways. Therefore, a
localized region of the medium affected by the laser interaction
has a one-dimensional thickness profile characterizable by a
central portion of low thickness bounded by thicker portions having
controlled material deposition. By way of example, a resulting
total line width of the deposition is about 2-3 times of the laser
focal spot size. With a laser spot size of 20 micrometer in
diameter, the deposited line width is about 40-60 micrometer, yet
sufficient for relatively high resolution image printing in various
embodiments.
[0029] FIG. 2 further illustrates an example of an arrangement for
pulsed laser micro-deposition of materials to provide patterns of
varying optical density. In this example, laser 7 is preferably a
high repetition rate pulsed laser. A pulse selector, for example an
acousto-optic or electro-optic modulator (not shown), may be
connected to controller 12 and used to select pulses for delivery
to the target. A beam scanner 8 is used, under computer control, to
form patterns. The scanner is controllable to produce varying scan
speeds. Scanner 8 may comprise two scanning minors 9 and 10, and a
focal lens 11, for example an f-theta lens to provide a flat beam
scan plane. A telecentric optical system may be utilized in some
embodiments. The scanner receives computerized control signals from
the controller 12. A commercially available beam scanner system can
be used, such as various products available from SCANLAB America
Inc., which includes a scan head, a controller with a computer
interface, and a user software to load images and edit texts and
geometric figures.
[0030] In various embodiments other scan mechanisms may be
utilized, alone or in any suitable combination, to form
pre-determined spatial patterns having varying optical density. For
example, acousto-optic deflectors, polygons, rotating prisms, and
the like may provide for further increase in scan speeds. Some
embodiments may include a combination of fast and slow deflection
mechanisms to control deposition while maintaining high scan speed.
For example, a first scanning mechanism may scan at a fixed rate in
a first direction, and a second scanner at a second rate in a
direction opposite the first.
[0031] Various scan patterns may be generated, including trepanned
or dithered patterns. Such mechanisms have been proposed and
utilized in laser marking, drilling, and micromachining, and may
also be configured for pulsed laser micro-deposition pattern
formation.
[0032] High repetition rate ultrashort lasers provide some benefits
for PLD pattern formation. Compared with nanosecond pulsed laser
ablation, ultrashort pulsed laser ablation requires less pulse
energy to reach ablation threshold. The available ultrahigh peak
power with an ultrashort pulse duration contributes to the low
threshold. For example, a femtosecond pulse of a few micro-Joule
has a higher peak power than a conventional nanosecond pulse of a
few milli-Joule. Moreover, a reduced heat-affected zone (HAZ) at
the focal spot significantly increases the energy efficiency for
ablation.
[0033] IMRA America Inc., the assignee of the present application,
disclosed several fiber-based chirped pulse amplification systems
which have a high repetition rate above 1 MHz, an ultrashort pulse
duration from 500 femtosecond to a few picoseconds, and a high
average power of more than 10 watts. Various fiber configurations
are available commercially, as set forth below.
[0034] With a high laser repetition rate, for example in the range
of 100 kHz to above 1 MHz, the target receives multiple laser
pulses in a short time interval before the beam moves away from a
localized focal region. For example, with 1 MHz repetition rate, a
beam scan speed of 1 m/s, and a spot size of 20 micrometer in
diameter, the number of overlapping laser pulses is about 20,
corresponding to about 95% overlap between adjacent spots. Multiple
laser pulses with a close time separation between pulses, for
example 1 microsecond or less, may produce physical effects to be
considered for image formation. For example, (i) accumulation of
deposition and (ii) accumulation of heat and pressure in the air
gap are of consideration. With a variable laser beam scan speed,
the first effect produces different light transmission and
reflection due to different deposit thickness, which is preferably
controlled in a continuous manner. The variation in thickness and
associated changes in transmission and/or reflection creates a
visual effect of gray scale. The second effect relates to the
observation that the deposits are concentrated near the periphery
of the laser beam path, as illustrated in FIG. 1, and will be
further illustrated with example images in FIG. 3.
[0035] A high repetition rate pulsed laser is also needed for high
printing speed. Conventional solid state lasers such as Q-switched
lasers and ultrashort laser systems based on regenerative
amplifiers provide typical repetition rates from 10 Hz to tens of
kHz. Although about 20 sec to 1 min is required to print a
2.times.2 square inch image with 1 MHz repetition rate,
approximately one thousand minutes are needed with a repetition
rate of 1 kHz to have the same spatial overlap between pulses.
[0036] Various embodiments may utilize a fiber-based high
repetition rate ultrashort pulsed laser, for example a model FCPA
.mu.Jewel made by IMRA America Inc. The laser has a repetition rate
from 100 kHz up to 5 MHz, a pulse duration of 500 fs to 10 ps, and
a pulse energy up to 20 micro-Joule. With a focused beam spot of
20-30 micrometers in diameter, this laser can ablate many metals,
dielectrics, and semiconductor materials.
[0037] Operation at higher repetition rates is possible. U.S.
provisional application U.S. 61/120,022, entitled "Highly
Rare-Earth-Doped Optical Fibers for Fiber Lasers and Amplifiers" to
Dong et al., is incorporated herein by reference. Various examples
disclosed in the '022 application include highly rare earth doped
gain fibers having pump light absorption of up to about 5000 dB/m,
and gain per unit length in the range of 0.5-5 dB/cm. Various
dopant concentrations reduce Yb clustering thereby providing for
high pump absorption, large gain, with low photodarkening. Such
rare-earth doped fibers provide for construction of short cavity
length fiber lasers, and for generation of high energy ultrashort
pulses at a repetition rate exceeding 1 GHz. With availability of a
GHz fiber source having increased pulse energy, an improved figure
of merit can be obtained based on various combinations of pulse
width, energy, spot size, and average power, and preferably with
the use of an all-fiber system.
[0038] In various embodiments a repetition rate may be increased
with a combination of beam splitter and optical delay lines.
[0039] FIGS. 3 and 4 illustrate the visual effect of gray scale.
FIG. 3 shows two microscopic portions with different gray levels
taken from the printed artistic image shown in FIG. 4(a). In FIG.
3(a), the two bright lines are made by a fast beam scan with a scan
speed of 8-10 m/s. Referring back to FIG. 1, the deposited material
6 corresponds to the region corresponds to the region between the
bright lines in FIG. 3a. As explained above, the whiteness of the
scan lines is due to the high temperature and high pressure under
the direct illumination of the laser beam, which forces the ejecta
sideways. In FIG. 3(a), because of the fast scan speed, relatively
few deposits remain, particularly on the top of the image.
[0040] FIG. 3(b) shows three scan lines made with a slow scan speed
of 0.2 m/s on average, where much thicker deposits formed between
the lines, resulting in an overall visual effect of darkness. The
white scan lines are also present. Therefore, different beam scan
speeds control material deposition between the scan lines and
produce the gray scale variation. In this way, a famous artistic
image is printed and shown in FIG. 4(a). A nickel coin is placed
beside the glass wafer to indicate the dimension.
[0041] In the above example, the number of gray levels is
determined by the minimum increment of the beam scan speed and the
maximum scan speed, assuming a linear dependence of deposit
thickness with beam scan speed. For example, with a maximum scan
speed of 10 m/s and an increment of speed of 1 mm/s, the increment
of the amount of deposit is sufficiently small to produce a
visually continuous gray scale.
[0042] FIG. 4(b) shows images of three identical text patterns with
different gray scales. In this example the patterns were formed by
controlling scan line density while maintaining a constant beam
scan speed of 5 m/s. For the three images from the top to the
bottom, the scan line densities are 4, 8, and 12 lines per
millimeter, respectively. This is an example of printing vector
graphics. This pattern formation technique is also suitable for
filling simple geometric shapes. By varying the scan line density,
the available number of gray levels can exceeds 10. In various
embodiments an optical density (O.D.) of at least 1 unit (10:1) may
be provided, and up to about 3-4 units.
[0043] In various embodiments patterns are formed using materials
providing functions other than changing the reflection or
transmission of light. One example is shown in FIG. 5, where text
is printed on a 1.times.1 square inch glass wafer with a target
made of a phosphor material. This material is a white powder under
room or sun light, but with UV illumination, the material emits
orange luminescence. Using this material as the target, the printed
text in FIG. 5(a) is barely visible under room light. In FIG. 5(b),
with UV illumination, the text becomes brightly luminescent. This
demonstrates that the light emitting property of the original
target material is preserved during printing.
[0044] In the above implementation, special physical and chemical
functions of the target material are preserved, such as
phosphorescence or fluorescence properties. Related physical and
chemical properties of the original material are not destroyed by
laser ablation, although the material is disintegrated with laser
irradiation. Ultrashort pulses provide such benefits.
[0045] Without subscribing to any particular theory, the process of
pulsed laser ablation can generally be separated into several
stages, including (i) light absorption, (ii) heating and phase
transition, and (iii) plasma expansion. The final material
deposition strongly depends on laser parameters including pulse
duration, pulse energy, wavelength, and repetition rate, and also
on the types of target materials, for example metals or
dielectrics. Among these factors, pulse duration is the first to
consider and compare between a conventional nanosecond pulsed laser
and an ultrashort pulsed laser, because of the large difference of
several orders of magnitude.
[0046] With a nanosecond pulsed laser such as a Q-switched Nd:YAG,
Nd:YLF, or Nd:YVO.sub.4 laser, the pulse duration is longer than
the time scale of energy exchange between electrons and ions in a
solid. The time scale is typically a few tens of picoseconds. The
nanosecond laser pulse thermally heats the solid and results in
thermal evaporation and ionization, and a plasma is formed by the
laser. The tail of the laser pulse can also further heat up the
plasma, resulting in a nearly completely atomized and highly
ionized vapor plume, except for a few large liquid droplets. In the
presence of the ambient air, a strong chemical reaction, e.g.,
oxidation, is expected during ablation, which will change the
physical and chemical properties of the ablated material.
[0047] With an ultrashort pulsed laser having a pulse duration in
the range of several hundred femtoseconds to a few tens of
picoseconds, and with a laser fluence within a range near the
ablation threshold, the ablated material can disintegrate into
small particles. Such particles may be in the nanometer range, as
reported in references No. 1-6 listed below. Several original
physical and chemical properties are maintained, such as
crystallinity, chemical composition, and alloy composition, as
reported in references No. 1-3. Thus, functional properties may be
retained. Some examples of functional properties are
phosphorescence, electro-luminescence, and selective light
absorption and emission for visual color effects. As illustrated in
the example of FIG. 5, such properties may be exploited for PLD
based microdeposition and pattern formation.
[0048] Many possibilities exist for high-repetition rate sources
suitable for PLD pattern formation. Ultrashort pulses and various
configurations disclosed above provide for precise and repeatable
material removal. However, in various embodiments a high repetition
rate picosecond or nanosecond source may be utilized. It is known
that the effective repetition rate of q-switched sources may be
increased by splitting and recombining outputs and/or combining
multiple laser outputs. For example, a q-switched laser may have a
base repetition rate of 70 KHz that is increased to well over 100
KHz with the multiple lasers and/or splitting and combining.
Moreover, semiconductor laser diodes may produce picosecond or
nanosecond pulses, and the diodes can be modulated at very high
repetition rates, at least tens of MHz. An output of the diode may
be amplified with a fiber amplifier to increase the energy level of
picosecond or nanosecond pulses to the range of microjoules, for
example. Pulse selectors may be used to gate pulses for
amplification and delivery to the target. Many possibilities
exist.
[0049] In various embodiments a metal target will be ablated, and
various laser parameters may be pre-selected to control speed and
resolution. By way of example, pulsed laser micro-deposition
pattern formation may be carried out with pulse widths less than
100 ns, and preferably below 10 ps, at a repetition rate of at
least about 100 kHz and much higher. Pulse energy below about 20
.mu.J provides a fluence of at least about 2.8 J/cm.sup.2 in a
focused spot diameter of about 30 .mu.m, and suitable for forming
various patterns. The fluence is substantially greater than an
ablation threshold of many metals. Smaller spot diameters may be
utilized. For a given fluence, the required energy decreases with
spot area, providing for a potential increase in repetition rate
for a given average power, but increased time for scanning. In
various embodiments material deposition may be carried out with
fluence near the ablation threshold of a metal target.
[0050] Thus the inventors have described methods, systems, and a
materials for pulsed laser micro-deposition and pattern
formation.
[0051] At least one embodiment includes a method of pulsed laser
deposition to produce a pattern on a medium, the medium being
substantially transparent at a wavelength of the pulsed laser. The
method includes generating pulsed laser beams from a pulsed laser
source, and focusing the pulsed laser beams onto a target. The
target produces ejecta in response to an interaction of the pulsed
beams and the target. The method includes accumulating at least a
portion of the ejecta on the medium to form material deposits on
the medium. The method includes controlling thickness of the
material deposits to vary an optical density of a region of the
medium, and to form a spatial pattern having varying optical
density.
[0052] In various embodiments:
[0053] the method includes transmitting the pulsed laser beams
through the medium; scanning the laser beams relative to the medium
and target; and varying at least one of a laser beam scan speed and
scan line density to control the thickness.
[0054] at least a portion of the pattern is characterizable with a
one-dimensional thickness profile having a central portion of lower
thickness than an immediately adjacent surrounding portion, the
thickness of surrounding portion being controlled to vary the
optical density. [0055] a pulsed laser source has a repetition rate
from about 100 kHz to 100 MHz, and up to about 1 GHz. [0056] a
laser pulse has a pulse duration in the range of about from about
10 femtosecond up to about 100 nanosecond. [0057] a laser pulse
energy is in the range of about 100 nano-Joule to about 100
micro-Joule. [0058] a medium comprises glass, quartz, sapphire,
plastic sheets, or a polymer. [0059] a target comprises a metal,
and the metal may comprise steel, aluminum, copper, gold, silver
and/or platinum. [0060] a target comprises a non-metal, and the
non-metal may comprise carbon, silicon, and/or organics materials
such as a polymer. [0061] a target comprises a functional material
for emitting light, the function comprising one or more of phosphor
luminescence and electro-luminescence. [0062] a target comprises a
material for color printing. [0063] a target comprises a structure
made of a target material. [0064] a target material is a metal, and
the metal may comprise a precious metal. [0065] a target material
comprises a dielectric, and the dielectric may comprise a mineral
and/or metal oxide. [0066] a laser beam scan speed is varied
according the gray scale of the pattern to be printed. [0067] a
laser beam scan speed of 1 mm/s-1 m/s is used to produce
accumulated deposition of material. [0068] a laser beam scan line
density is in a range of 1-100 lines per millimeter. [0069] a
medium is placed in contact with the target, placed within 100
micrometers to the target, or placed within 5 mm to the target.
[0070] an optical density corresponds to at least three gray levels
in a digitized image of the pattern. [0071] a target has a surface
with a roughness greater than the wavelength of the laser. [0072] a
pattern comprises a bitmap image and/or a vector graphic. [0073] a
grey scale image having a discernible feature is obtainable with
ambient or controlled illumination of the pattern.
[0074] a medium is disposed between the source and the target, and
ejecta propagates in reverse to the laser direction.
[0075] controlling comprises scanning the pulsed beams and varying
the scan speed.
[0076] controlling comprises scanning the pulsed beams and varying
the line density of the scan.
[0077] a medium is positioned relative to the target in such a way
to control the spatial resolution of the pattern.
[0078] controlling comprises scanning the pulsed beams in one or
more of a raster and vector pattern over the target.
[0079] at least one pulse width is in the range of about 100 fs to
about 10 ps.
[0080] At least one embodiment includes a system for pulsed laser
deposition to produce a pattern having optical density on a medium,
the medium being substantially transparent at a wavelength of the
pulsed laser. The system includes a high-repetition rate laser
source for generating pulsed laser beams, and a beam delivery
system. The beam delivery system includes a focusing sub-system to
focus the pulsed laser beams onto a target, the target producing
ejecta in response to an interaction of the pulsed beams and the
target. At least a portion of the ejecta are accumulated on the
medium and form material deposits on the medium. A controller is
coupled to the source and the beam delivery system for controlling
thickness of material deposits to vary an optical density of a
region of the medium. A spatial pattern having varying optical
density is formed.
[0081] In various embodiments:
[0082] a delivery system comprises a beam deflector, and the
focusing sub-system comprises a scan lens.
[0083] a controller is configured to vary at least one of a laser
beam scan speed and scan line density to control thickness.
[0084] the medium and target are spaced apart by a gap that
provides separation, and the gap may be filled with ambient air, or
with flowing dry air, or an inert gas, for example nitrogen or
argon. Physical parameters, for example pressure, within the gap
may be controlled.
[0085] At least one embodiment produces a product, including a
medium having a pattern formed thereon. The pattern is formed with
a pulsed laser deposition method described above. In various
embodiments a pattern corresponds to a gray scale image having at
least three detectable gray levels in a digitized image.
[0086] At least one embodiment includes a method of pulsed laser
deposition to produce a pattern on a medium, the medium being
substantially transparent at a wavelength of the pulsed laser. The
method includes generating pulsed laser beams from a pulsed laser
source, and focusing the pulsed laser beams onto a target. The
target produces ejecta in response to an interaction of the pulsed
beams and the target. The method includes accumulating at least a
portion of the ejecta on the medium to form material deposits on
the medium. The deposited material comprises a functional material
that is operable to emit radiation in response to a stimulus. The
method includes controlling thickness of the material deposits to
vary an optical property of the material deposits.
[0087] In various embodiments an optical property of the functional
material comprises one or more of phosphorescence,
electro-luminescence, and selective light absorption and emission
for visual color effects. The stimulus may comprise radiation, for
example short wavelength radiation that causes fluorescence
excitation.
[0088] Thus, while only certain embodiments have been specifically
described herein, it will be apparent that numerous modifications
may be made thereto without departing from the spirit and scope of
the invention. Further, acronyms are used merely to enhance the
readability of the specification and claims. It should be noted
that these acronyms are not intended to lessen the generality of
the terms used and they should not be construed to restrict the
scope of the claims to the embodiments described therein.
PUBLICATIONS REFERENCED
[0089] 1. B. Liu et al., Appl. Phys. Lett. 90, 044103 (2007).
[0090] 2. B. Liu et al., Proc. SPIE 6460, 646014 (2007). [0091] 3.
B. Liu et al., Laser Focus World, 43, 74 (2007). [0092] 4. S.
Eliezer et al., Phys. Rev. B 69, 144119 (2004). [0093] 5. S.
Amoruso et al., Phys. Rev. B 71, 033406 (2005). [0094] 6. T. E.
Itina et al., Proc. of SPIE, 6458, pp 64581U-1 (2007).
* * * * *