Note: Descriptions are shown in the official language in which they were submitted.
CA 02332154 2001-01-25
BURST-ULTRAFAST LASER MACHINING METHOD
FIELD OF THE INVENTION
The present invention relates generally to methods of laser processing
and modification of materials, and more particularly the present invention
relates
to laser processing and modification of a variety of materials using ultrafast
laser
pulses.
BACKGROUND OF THE INVENTION
Many efforts in the current generation of laser processing of materials can
be described as investigating new modalities in which the laser fluence may be
delivered to a workpiece, specifically the ways in which the pulse duration,
wavelength or pulse-shape give significant new control over the laser-material
interaction.
Various studies have shown that laser material processing in the
ultrashort-pulse regime (<100 picosecond) offers numerous advantages
compared with longer pulses, see for example SA. Kuper and M. Stuke, Appl.
Phys. B 44, 2045 (1987); S. Press and M. Stuke, Appi. Phys. Lett 67, 338
(1995); C. Momma et al., Optics Comm., 129, 134 (1996); C. Momma et al.,
Appl. Surf. Sci., 109/110, 15 (1997); D. von der Linde, K.Sokolowski-Tinten,
and
J. Bialkowski, Appl. Surf. Sci. 109/110, 1(1997); X.Liu, D.Du, and G. Mourou,
IEEE J. of Quantum Electron. 33, 1706 (1997) J. X. Zhao, B. Huttner, and A.
Menschig, SPIE Proc Vol. 3618, (1999); US Patent 5,361,275; U.S. Patent No.
5,656,186; U.S. Patent No. 5,720,894; U.S. Patent No. 6,090,507; U.S. Patent
No. 6,150,630; U.S. Patent No. 6,043,452; and patent publication WO 89/
08529. The first reported advantages in ultrafast laser processing by SA.
Kuper
and M. Stuke, Appl. Phys. B 44, 2045 (1987) and patent publication WO 89/
08529 emphasized improvements in surface morphology, absence of thermal
degradation, and reduced threshold fluence for polymers and inorganic non-
metallics such as teeth when using sub-picosecond ultraviolet lasers in
comparison with traditional nanosecond ultraviolet lasers. Ultrashort lasers
offer
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high intensity to micromachine, to modify and to process surfaces cleanly by
aggressively driving multi-photon, tunnel ionization, and electron-avalanche
processes, see J. Ihlemann, Appl Surf. Sci. 54 (1992) 193; D. Du, X. Liu, G.
Korn, J. Squier, and G. Mourou, Appl. Phys. Lett. 64 (1994) 3071; P. P.
Pronko,
S.K. Dutta, J. Squier, J.V. Rudd, D. Du, G. Mourou, Optics Comm. 114 (1995)
106; B.C. Stuart, M.D. Feit, S. Herman, A. M. Rubenchick, B.W. Shore, M.D
Perry, J. Opt. Soc. Am B 13 (1996) 459; and C.B. Schaffer, A. Brodeur, N.
Nishimura, and E. Mazur, SPIE 3616 (1999) 143.
Beyond the simple delivery of `raw' fluence, lasers offer the parameters of
intensity, wavelength, and pulse duration as factors which afford control over
essential aspects of material interaction. Particularly, ultrafast laser
interactions
have well-defined 'damage' thresholds offering improved precision in
processing
applications, including the fabrication of hole sizes that are smaller than
the
beam diameter, see United States Patent No. 5,656,186; X.Liu, D.Du, and G.
Mourou, IEEE J. of Quantum Electron. 33, 1706 (1997) and D. Du, X. Liu, G.
Korn, J. Squier, and G. Mourou, Appl. Phys. Lett. 64 3071 (1994). Much recent
literature has been devoted to ultrafast laser damage and processing of
transparent or wide-bandgap materials, see J. lhlemann, Appl Surf. Sci. 54
(1992) 193, D. Du, X. Liu, G. Korn, J. Squier, and G. Mourou, Appl. Phys.
Lett.
64 (1994) 3071. Nonlinear absorption mechanisms are key to coupling laser
energy into such non-absorbing media.
The thermal impact of picosecond and femtosecond laser interactions is
highly limited, confining laser energy dissipation to small optical
penetration
depths with minimal collateral damage. This precisely confined laser `heating'
minimizes the energy loss into the underlying bulk material, providing for an
efficient and controllable ablation process, see United States Patent No.
5,656,186; patent 5,720,894; patent 6,150,630; S.Preuss, A. Demchuk, and M.
Stuke, Appl. Phys. A, 61, 33 (1995); and T. Gotz and M. Stuke, Appl. Phys. A,
64, 539 (1997). Because the laser-matter interaction is so brief, there is a
shift in
the partition of absorbed energy. Relatively thin layers of near-solid density
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CA 02332154 2001-01-25
material are heated, during ultrafast-laser interaction, and this enhances
evaporative cooling: though the speed of expansion of the volume of heated
material is largely fixed by the temperature, the factor increase in volume of
a
thin layer is much greater. The volume of tenuous heated material more quickly
decouples thermally from the bulk, in the case of ultrafast laser-matter
interaction, and in this brief time less heat is transferred from the laser-
absorption zone to the underlying bulk material. A greater proportion of
absorbed
energy is carried away in the evaporated material than is the case for longer-
duration pulses.
Collectively, these ultrafast laser effects in small volumes minimize
thermal transport, mechanical shocks, cracks, charring, discolouration, and
surface melting in the nearby laser interaction zone. Ultrafast laser
machining
permits repair of ultrafine (sub-mircron) defects on photomasks, see United
States Patent No. 6,090,507. Such interactions also reduce pain during medical
procedures (see United States Patent No. 5,720,894) and enable the
microshaping of explosive materials without deflagration or detonation (see
United States Patent No. 6,150,630). The short duration further ensures that,
all
of the laser energy arrives at the surface before the development of a
significant
ablation plume and/or plasma; such efficient enei gy coupling is not available
with longer duration (>10's ps) laser pulses because of plasma reflection,
plasma
and plume scattering, and plume heating. Such ultrafast-processing features
are
highly attractive for the precise microprocessing of good heat conductors such
as
metals; at the same time, nonlinear absorption of these intense ultrafast
pulses
also reduces the ablation threshold for wide-bandgap or "transparent" optical
materials such as silica glasses.
Ultrafast lasers also offer the means to internally process transparent
glass. Microexplosions provide opportunities for 3-D optical storage (C.B.
Schaffer, A. Brodeur, N. Nishimura, and E. Mazur, SPIE 3616 (1999) 143) while
refractive index structures such as volume gratings and waveguides (K.M.
Davis,
K. Miura, N. Sugimoto, and K. Hirao, Opt. Lett. 21 (1996) 1729) have been
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formed, by the permanent alteration of the local index of refraction.
These prior studies and developments of ultrashort-laser processing of
materials have centered on ultrafast systems with pulse rates typically
operating
in the -1 Hz to 10,000 kHz regime. A high-repetition rate three-pulse laser
system is described by Opower in United States Patent No. 5,361,275 with pulse
separations of 0.5 to 5 ns (200 to 2000 MHz); each pulse is a different
wavelength, delivered such that a subsequent pulse arrives soon enough to
still
interact with the expanding plume of the previous pulse, thereby to benefit
from
more uniform heating of the plasma plume.
While ultrafast lasers offer exciting prospects for processing materials, at
present undesirable effects exist and processing =vvindows are poorly defined.
Effects requiring more control in laser processing and modification of
materials
includes, for example, incubation (defect generation) effects that change
etching
rates, self-focusing and clouding effects, `gentle' and 'strong' ablation
phases
developing with increasing number of pulses, pre-pulse or pedestal effects,
poor
morphology,: periodic surface structures, melt, debris, surface swelling,
shock-
induced microcracking, slow processing rates and saturation of hole depth in
via/hole formation.
It is advantageous to provide a method of laser processing of materials
that addresses the aforementioned difficulties present in present processing
methods.
SUMMARY OF THE INVENTION
The present invention provides a method of processing and/or modifying
materials based on high repetition-rate (continuous or pulsetrain-burst)
application of ultrafast laser pulses to materials. The high-repetition rate
provides
a new control over laser interactions by defining the arrival time of
subsequent
laser pulse(s), for example: to be after the timescale of plasma-plume
expansion
and dissipation, but before thermal and other relaxation processes in the
material have fully evolved. In one embodiment, the present invention provides
a
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CA 02332154 2009-02-17
novel method of controlling the delivery of laser fluence to a material during
laser
processing that reduces unwanted damage in the material.
In one aspect of the invention there is provided a method of laser induced
modification of a material, comprising applying at least one burst of laser
pulses
contained in a pulse train to a material, the at least one burst of laser
pulses having a
high repetition rate > 100 kHz such that the laser pulses have a time
separation
between individual laser pulses in a range appropriate to exploit the
persistence of a
selected transient effect arising from the interaction of a previous pulse in
the pulse
train with the material, said laser pulses having a pulse width of less than
about 10
picoseconds, and collectively having fluence above a threshold value for
modification
of said material.
The invention may also provide a method of laser material processing,
comprising providing a material to be processed and applying laser pulses to a
target zone on the material, the laser pulses having a time separation between
individual laser pulses sufficiently long to permit hydrodynamic expansion of
a
plume and/or plasma so that a next subsequent laser pulse is not substantially
reflected, scattered and/or absorbed by the plume and/or plasma, and the laser
pulses having a time separation between laser pulses sufficiently short so
that a
thermal and/or other relaxation process (for example, mechanical, stresses,
melt
phases, metastable or long-lived states, transient species, shock waves,
discoloration, deformation, absorption spectrum, fluorescence spectrum,
chemical structure) in the target zone presents heated material or material
alternated from the relaxed state to successive laser pulse(s).
The laser pulses may be applied at rates above 100 kHz, wherein thermal
transport does not completely dissipate the heat deposited and/or transported
in
or near the processing volume by each laser pulse, or wherein other relaxation
processes have not fully dissipated in or near the processing volume of each
laser pulse. A region of warmed material is therefore preserved, and presented
to each subsequent laser pulse.
This thermal component and other relaxing processes offer a new
5
CA 02332154 2009-02-17
modality for controlling ultrafast-laser processing. By adjusting the pulse-to-
pulse separation (inverse of repetition rate), the temperature rise, and the
extent
of the residually heated zone is controlled. In another embodiment, a
subsequent laser pulse can be presented at a critical time in the evolution of
material properties in or nearby the laser interaction zone to alter the
subsequent
laser interactions for a controlled change and/or improvement in the laser
process. For material heating, subsequent laser interactions offer several
advantages and opportunities that are not available for material processing at
lower repetition rate, as for example, when the sample interaction has relaxed
to
close to the substrate temperature. An increased temperature dramatically
alters
the materials properties in a manner that can positively affect the ultrafast
interaction, and control subsequent events such as shock development, defect
formation, annealing, surface morphology, debris formation, plume evolution,
material removal rates, and geometry of excisions. The combination of high-
repetition rate with ultrafast laser pulses provides added control and new
avenues in material processing that have not been described before.
BRIEF DESCRIPTION OF THE DRAWINGS
The following is a description, by way of example only, of the method of
laser processing of materials in accordance with the present invention,
reference
being had to the accompanying drawings, in which:
Figure 1 a shows an atomic force microscopy (AFM) image of a micro-hole
in fused silica, drilled by a single 1.2 ps laser pulse with a peak fluence of
9.1
J/cm2 using a Prior Art method;
Figure 1 b shows a depth profile corresponding to the hole shown in Figure
1 a;
Figure 1 c shows an atomic force microscopy (AFM) image of a micro-hole
in fused silica, drilled by a single 1.2 ps laser pulse with a peak fluence of
38
J/cm2 (bottom);
Figure 1 d shows a depth profile corresponding to the hole shown in Figure
1 c;
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CA 02332154 2009-02-17
Figure 2 shows a plot of excised hole depth as a function of accumulated
laser fluence (i.e., number of laser pulses) for single-pulse fluences of 9.6
and 31
J/cm2 using a Prior Art method;
Figure 3 shows a plot of etching depth per pulse in fused silica as a
logarithmic function of laser fluence using a Prior Art method;
Figure 4a shows a series of optical microscope photographs of fused
silica ablated by 1.2 ps Nd:glass pulses at 140 J/cm2, from left to right,
holes
were drilled by one, two, three, four, and five pulses using Prior Art
methods;
Figure 4b shows two SEM photographs at two different maginifications of
fused silica showing features of the shock-induced microcracks, the holes were
ablated with four pulses at 93 J/cmz fluence (-0.06 Hz) using a Prior Art
method;
Figure 5 shows an SEM angle view of hole excised in BK7 glass by a
mode-locked pulse train consisting of -250 single 1.2 ps laser pulses with a
pulse-to-pulse separation of 7.5 ns;
Figure 6 shows etch depth in BK7 glass plotted as a logarithmic function
of the total burst laser energy;
Figure 7 shows the number of shots (pulse-trains) to drill through
aluminum foils of 12.5 pm, 25 pm and 100 pm thickness as a function of the
pulse-train fluence;
Figure 8 shows etch rates per pulse-train burst as a function of the burst
fluence for various foil thicknesses of aluminum;
Figure 9 shows the etch rate per individual picosecond pulse as a function
of the single pulse fluence for 12.5 pm thick aluminum foil, the squares for
individual pulses within the pulse-train and the circles for an isolated
single
pulse;
Figure 10a shows an SEM photograph of a hole drilled through 200 pm
thick aluminum foil (laser-irradiated surface) with one pulse-train burst at
3.16
kJ/cm2 fluence;
Figure 10b shows an SEM photograph of a hole drilled through 200 pm
thick aluminum foil (rear surface) with one pulse-train burst at 3.16 kJ/cm2
7
CA 02332154 2001-01-25
fluence;
Figure 11 a shows an SEM photograph of laser irradiated surfaces
comparing two holes drilled through a 50 1um thick aluminum foil: left hole:
fluence 480 J/cm2, three bursts followed by one `cleaning' shot; right hole:
fluence 5.36 kJ/cm2, one shot plus one `cleaning' shot;
Figure 11 b are SEM photographs of the holes of Figure 11 a but taken
from the rear surface of the holes with the left (right) hole corresponding to
the
right (left) hole in Figure 11a;
Figure 12 shows a plot of observed hole-sizes machined through 100 ,um
thick aluminum foil, as a function of fluence; and
Figure 13 shows a plot of observed etch-depths into a 150,um, foil,
compared to calculated vaporization depth.
DETAILED DESCRIPTION OF THE INVENTION
Prior Art Method Of Low Repetition Ultrafast Processing Of Glass
In this section, results of 1.2-ps laser ablation of fused silica and BK7 at
repetition rates of 1 Hz or less are described as a reference to compare with
the
attributes of burst machining forming the present 1'nvention described in the
next
section. Fused silica and BK7 are highly transparent at the 1.05 ,um laser
wavelength and yield similar micromachining results. Surface morphology of
microholes formed by single laser-pulses are shown in the AFM photographs in
Figures 1 a and 1 c. Fluences of 9.1 and 38 J/cm2 each produced moderately
smooth holes of -2.0,um diameter (FWHM). Surface-profile traces, shown in
Figures 1 b(corresponding to Figure 1 a) and Figure 1 d(corresponding to
Figure
1 c)-, reveal hole depths of 100 and 360 nm, respectively.
A small ring structure is observable in the higher-fluence hole, a feature
also reported by D. Ashkenasi, H. Varel, A. Rosenfeld, F. Noack, and E.E.B.
Campbell, Nucl. Instr. & Meth. in Phys. Res. B 122, 359 (1997) for 3.2-ps
ablation of fused silica. The excised surface contour was found to crudely
follow
the laser beam profile, with small-scale surface roughness of 10% (rms) of
the
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CA 02332154 2001-01-25
hole depth. This 10% surface roughness was a general observation for the
`gentle' ablation phase, noted here for fluences, F, less than -44 J/cm2. Even
when several pulses were applied to the same area, surface roughness typically
increased in absolute terms, but remained limited to 10% of the final hole
depth.
Figure 2 shows the progress of hole depth with the number of laser
pulses, N, for fluence values of 9.6 and 38 J/cm2. (Accumulated fluence was
used for the abscissa to better account for the 10% variations of the laser
energy). For 9.6 J/cm2 [38 J/cm2], the depth increases linearly with N, or
accumulated fluence, to an apparent peak value of 2.7,um [2.2 kcm] after 14
[6]
pulses. The nominal plateau apparent for higher pulse-number is simply an
artifact of the AFM tip, which cannot probe larger aspect-ratio holes, those
deeper than their -2 m diameter. It is anticipated that the hole depth will
in fact
rise linearly with number of laser pulses through and beyond this plateau
region
until incubation processes raise the density of defects or color centers to a
critical value. At fluences beyond this critical value, the `gentle' ablation
process
is expected to give way to 'strong' ablation, a distinct regime wherein etch
rates
(depth per pulse) can be increased more than 10 fold, see A.C. Tam, J.L.
Brand,
D.C. Cheng, and W. Zapka, Appl. Phys. Lett. 55, 2045 (1989). For fused silica,
there have been reported etch rates of 550 nm per pulse when 100's of pulses
at
1.3 ps duration were applied at 12 J/cm2 fluence, see H. Varel, D. Ashkenasi,
A.
Rosenfeld, R. Herrmann, F. Noack, E.E.B. Cariipbell, Appl. Phys. A 62, 293
(1996). Their rate is triple this 180 nm/pulse rate for the same fluence, but
with
N < 10. For reasons given below, the transition to strong ablation with
increasing
N was not studied here.
The etch-depth data in Figure 2 show that material removal was initiated
with the first laser pulse (N = 1), for both 9.6 and 38 J/cm2 fluences.
Incubation
effects developing at fluences before the onset of ablation were not studied
in
the present work although such effects are already anticipated below our
single-
pulse ablation threshold of -5.5 J/cm2. Kautek et al. reported the need for 7
9
CA 02332154 2001-01-25
incubation pulses before low-fluence (1 J/cm2) ablation of barium-aluminum
borosilicate glass could proceed with 50 fs laser pulses, see W. Kautek, J.
Kruger, M. Lenzner, S. Sartania, C. Spielmann, and F. Krausz, Appl. Phys.
Lett.
69, 3146 (1996). Such incubation processes are generally undesirable for
practical applications, since they impair control of etching rates.
Single-pulse etch rates were collected from the slopes of data in graphs
like Figure 2 and plotted in Figure 3 as a function of single-pulse fluence.
Two
regimes, gentle and strong ablation, are identified. Representation of the
data
(solid lines) by (9/aeõ) log (F/Fih) provide values for threshold fluence and
the
effective absorption coefficient in each regime. The etch-rate data follow a
logarithmic fluence-dependence from an extrapolated ablation threshold of 5.5
J/cmZ to -44 J/cm2, the onset of strong ablation. This fluence window (5.5 to
45
J/cm2) defines the gentle-ablation processing window for controllable etching
of
smooth features in fused silica. Thin layers, -100 nm deep or less, could be
accurately excised with appropriate choice of fluence. The logarithmic fluence
dependence, normally associated with single-photon absorption mechanisms, is
surprising here, considering the nonlinear mechanisms that are understood to
drive absorption in this transparent material. Kautek et al. have also
reported a
logarithmic fluence dependence for 20-fs to 3-ps ablation of barium aluminum
borosilicate glass (W. Kautek, J. Kruger, M. Lenzner, S. Sartania, C.
Spielmann,
and F. Krausz, Appl. Phys. Lett. 69, 3146 (1996)). For 1.2-ps ablation of
fused
silica, the slope of the solid curve in Figure 3 (for F < 44 J/cm2) provides
an
effective penetration depth of 1/ae,f = 235 nm, a value commensurate with the
-100-nm layer-by-layer resolution cited above. The 5.5 J/cm2 threshold fluence
is
in accord with the damage threshold of 5 1 J/cmz reported by Varel et al. for
1.0-
ps ablation of fused silica, see H. Varel, D. Ashkenasi, A. Rosenfeld, R.
Herrmann, F. Noack, E.E.B. Campbell, Appl. Phys. A 62, 293 (1996). This group
also report in a later paper, etch rates of -200 nrr,/pulse with 3.2-ps pulses
at 10
J/cm2 fluence, see D. Ashkenasi, H. Varel, A. Rosenfeld, F. Noack, and E.E.B.
Campbell, Nucl. Instr. & Meth. in Phys. Res. B 122, 359 (1997). This etch rate
is
CA 02332154 2001-01-25
only slightly larger than our 180 nm/pulse value from Figure 3 for 1.2-ps
pulses.
Note again that the rates in Figure 3 are only valid where the number of laser
pulses is small. The onset of a strong ablation phase after 10's or 100's of
laser
pulses explains the 2 or 3-fold faster etch rates reported for ablation of
deep
channels in fused silica in D. Ashkenasi, H. Varel, A. Rosenfeld, M. Whamer-
and E.E.B. Campbell, Appl. Phys. A 65, 367 (1997).
At higher fluence, F> 44 J/cm2 in Figure 3, etch rates abruptly rise to
values 2- or 3-fold faster than by simple extrapolation of the gentle ablation
data.
This enhanced rate is related to the incubation phenomenon described
previously where now a single pulse provides sufficient fluence to fully
incubate
the underlying glass material. The effective penetration depth rises to 9/aeõ
=
780 nm, supporting rapid etch rates of up to 2 m per pulse at a fluence of -
150
J/cm2. Such rapid etch rates are attractive for many applications, however,
this
strong-ablation regime provides less control over etch depth as evidenced by
the
wider scatter of data points in Figure 3. A further disadvantage is the
development of microcracks following 2 or 3 ablation pulses at high fluence as
discussed below.
While these low-repetition-rate ultrafast laser observations appear
promising for controllable etching of optical materials, detrimental effects
are
noted. Most significant is the development of shock-induced microcracks, and
shearing and flaking of surrounding surfaces following a small number of
moderate-intensity pulses. Figures 4a and 4b shows the rapid development of
shock-induced microcracks forming around the perimeter of laser-ablated holes.
Figure 4a shows a series of optical microscope photographs of fused silica
ablated by 1.2 ps Nd:glass pulses at 140 J/cm2, from left to right; holes were
drilled by one, two, three, four, and five pulses. Figure 4b shows two SEM
photographs at two different magnifications of fused silica showing features
of
the shock-induced microcracks, the holes were ablated with four pulses at 93-
J/cm2 fluence (-0.06 Hz). Shock-induced microcracks developed quickly, by the
third pulse, for this large fluence.
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CA 02332154 2001-01-25
At the 140 J/cm2 fluence, microcracks and surface swelling (noted by AFM
and SEM) developed very quickly by the third laser pulse thus posing a
significant limitation to precise shaping of smooth optical surfaces. At lower
fluence, microcracking developed more slowly. Over the 5.5 to 170 J/cmZ
fluence-window studied here, these undesirable surface features appeared
consistently after an onset number of laser pulses, N, that approximately
followed
N, = 1.7 + 80/F (F in J/cm2).
Because these N, values are small, peaking at -25 near the threshold fluence
for the gentle ablation region, there was no practical reason for extending
studies
to integrate large numbers of pulses (N > 60). Therefore, the transition from
gentle ablation to strong ablation with increasing N was not observed here,
preceded by the early development of microcracks, the main limitation to
smooth
surface-structuriiig of fused silica. D. Ashkenasi, H. Varel, A. Rosenfeld, M.
Whamer and E.E.B. Campbell, Appl. Phys. A 65, 367 (1997) also reported the
formation of microcracks around deep (-1 mm) channels etched in fused silica
by hundreds of laser pulses of 100-fs to 30-ps duration. Their study showed a
favorable trend of reduced microcracking with decreasing pulse duration.
Combination of N, in Equation 1 with the per-pulse-etch rates in Figure 2
provides a coarse guide to the maximum ablation depth one can attain without
deleterious microcracking or surface swelling phenomena. Structures up to -1
gm deep with 10% rms surface roughness are shown here to be possible,
establishing a practical but very restricted processing window for ultrafast-
laser
micromachining of fused silica and related transparent materials.
Burst Ultrafast Laser Processing of Materials
The method of high-repetition-rate ultrafast laser processing of materials
in accordance with the present invention will be exemplified with two
illustrative
non-restrictive examples. Detailed examples are provided here for two classes
of
materials including brittle transparent glass and ductile metal aluminum. It
will be
12
CA 02332154 2009-02-17
understood that the principles demonstrated herein are extensible to a large
range of material classes for broad application in ultrafast-laser material
processing.
Laser Systems
For applications directed at material processing, ultrafast-laser systems
presently available typically combine a mode-locked oscillator with an
amplifier
that raises the single-pulse energy to levels suitable for material
modification.
While such oscillators provide high repetition rates (-100MHz continuous or in
bursts), practical considerations in the amplifier power have precluded the
amplification of every oscillator pulse. Only a small number of the oscillator
pulses are amplified in ultrafast laser systems currently employed in material
processing research and development (typically at rates of several Hz to -1
kHz,
and much less than 100 kHz). For these considerations, thermal diffusion
between laser pulses at such low rates enjoys sufficient time to transport
away
most or all of any thermal energy deposited by the laser into the volume
immediately surrounding the processing region, and to provide near-complete
relaxation of other transient physical, chemical, or other changes brought on
the
by each laser pulse. Such heat transport cools the sample surface to that of
the
underlying bulk material before the arrival of the next ultrafast laser pulse;
other
physical and chemical properties and material parameters relax also to values
similar to the underlying bulk material . Under these conditions, ultrashort
laser
pulses interact with materials that have mostly relaxed to the state of the
underlying bulk material.
The present invention makes use of lasers in the high-repetition-rate
ultrafast-laser processing of materials. The results described here are not
particular to the laser system described below, but share a common physical
process/interaction with all ultrafast laser systems operating at high
repetition
rate (>100 kHz).
A feedback-controlled Nd:glass oscillator (X= 1054 nm) operating at 1 Hz
repetition rate provided a near flat-topped train of 430 mode-locked pulses
(quasi
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cw) with pulse-to-pulse separation of 7.5 ns, see R.S. Marjoribanks, F.W.
Budnik, L. Zhao, G. Kulcsar, M. Stanier, & J. Mihaychuk, Optics Lett. 18, 361
(1993). A single high-contrast pulse of 1.2 ps duration was selected by an
external Pockels cell and amplified 13-fold in a four-pass geometry at -0.06
Hz
(limited by flashlamp pumping). The -3 uJ pulses were focussed by
interchangeable aspherical lenses (AR-coated BK7; f= 15.4, 11.0, 4.5 and 3.1
mm) to near diffraction-limited spot sizes of 3.2, 2.0, 1.0, and 0.8 m (1/e2)
diameter, respectively.
The test samples (UV-grade fused silica Corning 7940; BK7 glass,
aluminum foil) were mounted on a precision x-y-z stage. Focussing was
monitored by image-relaying the retro-reflected beam from the focal spot, with
magnification, onto a CCD camera. On-target fluence was varied over the range
2 to 170 J/cm2 by adjusting the amplifier gain, using neutral density filters,
and
employing different focal-length lenses. Excisions were made using between 1
and -100 pulses, of various fluence values. All samples were irradiated in
air,
and transverse nitrogen gas flow was at times used to reduce the accumulation
of ablation debris. Self-focusing effects in air were not seen, at this pulse
duration, peak power, and with the short focal-length lenses used. Laser
focussing conditions also did not produce bulk discoloration or damage effects
in
glass regions beneath the excised holes (as evidenced by optical microscopy).
The burst mode was provided by a waveplate which passed the full
oscillator train of several hundred pulses for high-repetition-rate machining
at
133 MHz. The -3 ,us long pulse train was amplified and focussed onto glass or
metal surfaces as described above, accumulating a total fluence of -40 kJ/cm2
in
a diameter of -2 ,um. The burst duration was varied from -250 to 430 pulses.
The resulting ultra-high repetition rate (133 MHz) pulsetrain had a nearly
flat
waveform, with a risetime of about 100 ns and a falltime of about 500 ns. With
the four-pass amplifier optional, pulse-train energies of 0.05 to 2 mJ were
available for all studies.
This particular beam focusing and alignment arrangement is only one
14
CA 02332154 2001-01-25
approach amongst many for delivering ultrafast pulsetrain `bursts' to a
sample,
and is not a pre-condition for applicability of the present invention. The
advantages of high-repetition rate ultrashort laser processing can be accessed
by any beam-shaping and optical delivery system that brings an appropriate
fluence dose to the sample.
By employing various electrooptic devices, and alternative cavity and/or
amplifier designs, tuning of the single-pulse duration, pulse-to-pulse
separation,
number of pulses per pulsetrain burse (up to cw operation), and the temporal
profile of the burst envelope becomes available to optimize and control the
laser-
material interaction and subsequent processes in the sample material.
Other embodiments of `burst' type ultrashort lasers are possible. The
Coherent (USA) Model MIRA Optima 900F provides 76 MHz continuous output
of -100 fs pulses at -1 W power, yielding a pulse energy of 13 nJ which is
sufficient under tight focus to modify certain materials. The Coherent (USA)
Model Reg A 9000 offers microjoule energy in -100 fs pulses at repetition
rates
adjustable up to 300 kHz. IMRA (USA) has developed but not commercialized
laser systems offering higher rates (>1 MHz) and similar pulse energies. High
Q
Lasers (Germany) offers a Nd:Van laser producing 100 nJ pulses of 7 ps
duration at 10 W power and -100 nJ energy per pulse. These and other
commercial laser systems could exploit the benefits of the present invention.
As used herein, the term "Interaction geometry" means those aspects of
the initial geometry (e.g. dimensional proportions, relationship to unheated
material) of the laser-heated material that affect the evolution of the heated
material. With subtle exceptions, the rate of volume expansion defines the
rate
of dissipation of the plasma plume. A plasma having dimensions x by y by z
will
have a volume V = x*y*z. The evolution of the plasma in each dimension
therefore determines the evolution of the volume.
There are several cases to consider.
i) Focussing onto the surface of a material. Expanding freely in each
dimension, the plasma may grow equally in each direction in absolute
dimension.
CA 02332154 2009-02-17
For those dimensions which are already large, this will be a tiny relative
change;
for those dimensions which are tiny, this will be an enormous relative change.
Therefore the relative rate of volume change depends on the aspect ratio
(width
to thickness) of the heated zone:
a) large focal spot: a large flat layer, thin in 'z' -- there is little
relative
change in x and y, though z may quickly double; volume expands proportional to
elapsed time;
b) line focus: a thin layer long in one dimension and tiny in the other two
will expand cylindrically: there is little relative change in x, though y and
z may
quickly double; volume expands proportional to square of elapsed time;
c) point focus: a thin layer tiny in x and y - will expand spherically: x, y
and z may each quickly double; volume expands proportional to cube of elapsed
time.
ii) Focussing within a channel, deep hole or via. The material geometry
around the plasma plume restricts expansion to one dimension, down the
channel. Volume expands proportional to time.
iii) Focussing within bulk of material. The material geometry around the
plasma plume inhibits expansion in all directions. The characteristic time for
dissipation, a determinative factor for Ot in this invention, is here
determined e.g.
by the physics of shock formation, cavitation, and thermal and radiative
dissipation. This may be a substantially longer time than previously, as for
example in transparent biological tissues, or it may be comparable to, or
shorter
than, hydrodynamic times of free-expansion.
The present invention will now be illustrated with the following non-limiting
examples.
EXAMPLE 1
Burst Ultrafast Processing Of Glass
The full oscillator pulse train comprising between -250 and 400 pulses of
1.2-ps duration, at 5 to 150 J/cm2 fluence each, was applied to fused silica
and
BK7 surfaces. The pulse train could be flat-topped, or shaped to improve
control
16
CA 02332154 2001-01-25
of the laser interaction with the material. The SEM photo in Figure 5 shows a
high aspect-ratio via, or through-hole, formed as a result of this single
pulse-train
burst. A smooth symmetric hole of -10 ,um diameter was excised to 15 ,um depth
(determined with optical microscopy). The -15 ,um deep hole has smooth walls
and shows no evidence of fractures, cracks, or collateral damage. Only a small
mount of ejected melt has solidified on the entrance hole perimeter. The
entrance hole diameter of -14 /cm exceeds the focused laser beam diameter of
-1.8 microns. The burst energy was 1.48 mJ and the total (integrated) fluence
was -49 kJ/cm'.
In comparison, the low repetition rate (1-Hz) result described in the
previous section showed that microcracks formed after only 3 pulses for the
same single-pulse fluence. Cumulative heating effects associated with the 133-
MHz pulse-repetition rate in the pulse train are believed to improve the
ductility of
the surrounding glass, thereby mitigating the shock-induced microcracking in
regions immediately surrounding the hole perimeter. Such low-grade heating
effects also support an annealing effect, by which stresses incorporated into
the
material by thermal cycling are relieved. Heat incompletely dissipated on a
nanosecond time-scale may account, in part, for the enlargement of hole
diameters to 8-10 um which is -5X larger than the diameter of the focused
laser
beam. For fluences well above threshold, ablation also extends appreciably
into
the weaker edges of the Gaussian beam. In the operation of the feedback-
controlled mode-locked oscillator, the first dozen pulses at the leading edges
of
the train are also somewhat longer-duration (-10 ps); possibly this may also
have an effect.
Etch depths excised in BK7 are plotted in Figure 6 as a function of total
energy in the 250-pulse envelope. A single train of -250 pulses of 1.2 ps
duration each with a pulse-to-pulse separation of 7.5 ns was applied. Single
bursts were applied to one surface of a 90 prism and etched depths were
measure from the adjacent surface with an optical microscope. The data are
coarsely represented by a logarithmic energy dependence with an extrapolated
17
CA 02332154 2001-01-25
ablation threshold of - 15 ,uJ energy or 500 J/cm2 fluence and an effective
absorption coefficient of 2900 cm-' (from the inverse slope). The latter value
yields an effective optical penetration depth of 3.4,um for the full train of
pulses,
a value 4-fold [15-fold] larger than the corresponding value for strong
[gentie]
single-pulse ablation regime in Figure 3 for fused silica.
Multiple pulsetrain bursts were applied to fused silica at 1-Hz repetition
rate, resulting in a slight increase (-20%) in hole diameter and a moderate
advance in etch depth to several 10's of microns. Deeper holes are anticipated
with modification of the laser parameters and the focusing geometry. Hole
depth
saturated quickly for 100-J/cm2 single-pulse fluence. Saturation of hole depth
when drilling deep channels in fused silica with femtosecond and picosecond
laser pulses at 10-1000 Hz repetition rate has been previously reported, see
D. Ashkenasi, H. Varel, A. Rosenfeld, M. Whamer and E.E.B. Campbell, Appl.
Phys. A 65, 367 (1997). However, unlike these lower repetition rate results,
the
application of many bursts in the method disclosed herein has not led to the
formation of microcracks, fractures, or swelling for any samples in the
present
work, an unanticipated and important advantage for shaping smooth surface
structures, especially high aspect ratio holes and blind vias.
High repetition-rate multi-pulse ablation disclosed herein is clearly a
promising new option for controlling the micromachining quality of brittle
materials. The 7.5-ns pulse-to-pulse separation used herein is sufficiently
short
to reduce the material cooling between laser pulses, thereby permitting the
presentation of a heated and more ductile glass to succeeding laser pulses in
the small processing volume. During the 7.5-ns interval between pulses, the
thermal diffusion scale length, (4DT )1'2, is -0.17 ,uni in glass, a value
significantly
comparable with the effective optical penetration depth of 0.25 ,um in the
single-
pulse gentle ablation regime (Figure 3). Since the plume will carry not all
absorbed laser energy away, retention of this dissipated energy within a scale
length comparable with the laser penetration dep+h ensures that subsequent
laser pulses interact with a thermally modified glass while minimizing the
heat-
18
CA 02332154 2001-01-25
affected zone. An important additional consideration of the pulse-to-pulse
separation is to provide sufficient time for hydrodynamic expansion and
dispersion of the laser-produced plume and plasma, reducing or eliminating
obscuration of subsequent laser pulses. This is an important benefit that
retains
the advantages of ultrafast-laser material processing (i.e., laser dissipation
in the
bulk material) while also offering control of the heat retained in the nearby
laser-
interaction volume of the material. The pulse-to-pulse separation becomes an
important new optimization parameter, controlling the amount of laser-
generated
heat retained in the sample (higher temperature when reduced) and the amount
of laser energy lost to absorption and scattering in an incompletely
dissipated
plume (less loss when increased). This control and these general advantages
are available to brittle materials in general, and include but not limited to
glasses,
crystals, ceramics, tooth enamel, bone, and composite materials for a wide
range of applications.
EXAMPLE 2
Burst Ultrafast Processing Of Aluminum
High-repetition rate burst machining was applied to aluminum
(Goodfellow, 99%) foils of thicknesses of 12.5, 25, 50, 100 and 200 ,um.
Samples were mounted free-standing to preclude effects of heat conduction into
any substrate. A photodiode was placed directly behind the foil to signal the
laser burnthrough of the foil. On-target laser energy was controlled by
neutral
density filters and amplifier gain. Ablated surfaces were examined by scanning
electron microscopy (SEM), atomic-force microscopy (AFM), and optical
microscopy.
The number of laser pulses necessary to drill through 12,5 m, 25 m and
100 m foils are plotted in Figure 7, as a function of pulse-train fluence.
The
fluence values are divided by 250 to obtain the single 1.2 ps pulse fluence. A
large fluence range of 80 to 9000 J/cm2 was examined. Qualitatively, the
ablation
behaves as expected: the minimum number of bursts required to drill through a
19
CA 02332154 2009-03-17
foil increases with increasing foil thickness and decreases with increasing
fluence.
For each thickness there was a threshold fluence below which the target could
not
be pierced by even a hundred shots, even though this fluence was itself well
above
the damage threshold at the surface. This is understood to be related to a
reduction of etch rate with depth. In the measurements disclosed herein this
piercing threshold increases with foil thickness, from -120 J/cm2 for 12.5 pm
foils to
600 J/cm2 for 100 pm foils. This -5-fold difference in threshold is
attributed, in part,
to distributed absorption of laser energy along the length of increasingly
deep
channels. The inventors have observed that the coherence-degradation effect of
imperfect waveguiding in the multimode-sized channels also reduces laser
intensity
at the hole-bottom. Such losses raise the material-removal threshold fluence
as
increasingly deeper channels are bored out by the laser. Beyond a maximum
fluence of -200, 300, and 7000 J/cm2 for 12.5, 25, and 100 pm foils,
respectively,
single bursts will cut through the foil. Except for single-shot piercing data,
pulse-to-
pulse energy fluctuations of -30% lead to a scatter of data points especially
near
the through-hole fluence threshold.
Burnthrough etch-rate data are presented in Figure 8 for each of the foil
thicknesses tested. Etch depths were interpreted from graphs such as Figure 7,
identifying the minimum fluence necessary to reproducibly punch through the
foil for
a given number of pulses, then plotting against that fluence the average etch
rate
per pulse (from the foil thickness and number of pulses to pierce). All foils
except
the 200 pm thickness could be consistently drilled through with a single
pulsetrain
burst for the present laser configuration. The etch-depth data are strongly
dependent on foil thickness.
Figure 9 compares the average value of single-pulse etch rates in various
foil thickness when applied as isolated pulses (circles) at < 1 Hz repetition
rate and
as part of a burst train at 133 MHz repetititon rate (squares). Clearly
evident is the
need for much larger single-pulse fluence, greater than 10 J/cm2, to
eventually cut
through the foils compared with fluences of only several 100 mJ/cm2 when the
pulses are part of a high-frequency pulse train. Burst-mode machining offers a
CA 02332154 2001-01-25
new means for rapid etching through metallic materials.
Figure 10 shows SEM photographs of the front (10a) and the back (10b)
surfaces of the 200 um foil that was drilled with one burst at 3.16 kJ/cm2.
The
hole perimeter is relatively clean at both surfaces with only a thin (-3 gm)
wide
rim of melt splattered around the entrance hole. No optimization effort was
made to minimize this splatter. The entrance hole diameter of 30 Atm reduces
to
7.5 Aim at the backside, yielding a 7:1 aspect-ratio hole with tapered sides
at -3
on either side of target normal. The aspect ratio could be adjusted with
changes
to the laser fluence and focussing conditions.
Figure 11 illustrates the influence of the laser fluence on the hole
diameter. The left hole in Figure 11a was drilled with three pulse-train
bursts,
each at a fluence of 480 J/cmZ, followed by a fourth shot of the same fluence
to
trim the hole of any melt/flow irregularities inside. The entrance hole
diameter is
-6 ,um which corresponds closely to the 5.6 rn laser-beam diameter. A
comparatively large amount of re-solidified material is also seen to surround
the
hole perimeter. The laser fluence is only -50% above the minimum fluence
required to produce a through-hole for this case (240 J/cm2) and melt
processes
appear to reduce the hole quality. Note that this laser fluence is
approximately
an order of magnitude above the surface damage threshold. A SEM photo of the
rear side of this through-hole is shown on the right in Figure 11 b.
The right hole in Figure 11 a was drilled with a single burst at a fluence of
-5.36 kJ/cm2, followed by an additional shot to clean out the drilled hole of
debris. The 11-fold higher fluence produced a larger hole diameter of -30 m,
about 5 times the laser focal spot size (FWHM) and producing a hole 25 times
the area. The rear side of the hole, shown on the left in Figure 10b, is also
much
larger (-20 ym) in diameter than the lower-fluence example. Assuming linear
absorption, the increase in hole diameter is commensurate with the increase in
laser power: for a Gaussian profile, the intensity of the laser has near the
same
value at the hole-edge in each case. This argues for a local threshold effect,
such as the specific energy in the target material passing that amount needed
for
21
CA 02332154 2001-01-25
melting. Thermal transport could also play a role in increasing the hole size.
Figure 12 shows how the diameter d of the hole produced in a 100 m-thick
aluminum foil by pulsetrain-burst machining increases with fluence F. The
fitted
line shows a power-law dependence of d= 12 (F)0 43. Since the radius of a
point
at constant fluence increases proportionally to (IogF)0 5, this different
observed
functional dependence illustrates that nonlocal issues, such as heat
transport,
are significant in determining hole size. Figures 10, 11 and 12 together
demonstrate control over hole diameter, morphology, hole smoothness and
quality, and aspect ratio with laser fluence in burst laser machining of metal
foils.
The absence of substantial melt debris, especially for fluences lOx to
100x above the surface-damage threshold, demonstrates that long-pulse physics
dominated by melt-phase material ejection is not taking place here. However,
during such a microsecond pulse-train, heat will have diffused into the
material
surrounding the laser spot in a manner similar to that described above for the
glass studies. A simple consideration of the thermal diffusion length, (4DT
)'12
provides a heat scale-length of a fraction of a micron for the 7.5 ns interval
between picosecond pulses in the train, and 28 ,um over the whole 2,us pulse-
burst. While this scale-length can be misleading as a rule-of-thumb in
assessing
the hole diameter, it demonstrates a compact scaie length over which heated
material is presented to each ultrafast pulse within the train. At or below
100 kHz
repetition rates, much of the heat retained by the sample will have diffused
into
the underlying bulk material, lowering the sample surface temperature to that
of
the bulk. The transitionai pulsetrain repetition-rate for such cooling will
vary with
the material's optical and thermal properties, the beam diameter, and the
energy
delivered to the sample by an individual pulse. At 133 MHz, the ultrafast
interaction takes place within a heated zone of the material left behind by
preceding pulses. In this way, deeper channels can be excised because of a
reduced ablation threshold, improving the energy efficiency of the material
removal. Through-holes can excised in thick foils with single bursts, greatly
improving machining time over that provided by traditional sub-MHz lasers. The
22
CA 02332154 2001-01-25
thermal component also affords control over the diameter and aspect ratio of
the
hole. These benefits are in addition to those normally associated with
ultrafast
laser processing, a constitute a part of the present invention.
The thermal physics of pulse-train burst interaction is therefore
intermediate between that of single long-duration and ultrafast pulses. It
appears that there are some advantages of heating or annealing surrounding
material without the gross melting characteristic of longer-duration pulses.
Likely
this is because ultrafast laser pulses have the advantage of evaporative
cooling,
over a hydrodynamic timescale of the expanding plume, as the locally heated
material vaporizes and expands away from the solid, decoupling from it
thermally. In this case, much of the heat impulse of an ultrafast laser pulse
is
carried away with the plume/plasma, producing etching more similar to material
sublimation than is possible for quasi-cw machining. As a result, the
characteristic heating time is limited to the timescale of hydrodynamic
expansion
of the thin heated layer (and evaporative cooling). For this reason, it
appears
that the etched hole-size is fairly closely linked to the local specific
energy
deposition by the laser, as it compares to the specific energy of
vaporization, and
less by lateral thermal transport.
This is supported by the results of Figure 13. Figure 13 shows a plot of
average observed etch rates for cutting through a 150 /im foil, compared to
putative vaporization depth. The solid line marks the deepest holes possible,
by
thermodynamic arguments, if 10% of the incident laser energy were invested in
vaporizing the material directly underlying the focal spot, i.e., without
considering
any lateral-transport effects. Vaporization of aluniinum will take place with
an
energy investment of -36 kJ/cm3 (inciuding therrnal capacity, heat of fusion,
and
heat of vaporization). Setting aside the laser absorption efficiency, this
value can
be used to determine the maximum depth attainable by evaporative ablation, D,
as a function of fluence: D = F / (3.6 J/cm' '. Since all the data of Figure
13
fall below this curve, we see that mode-locked laser bursts nominally can
provide
more than adequate fluence to vaporize aluminum to the observed depths. Even
23
CA 02332154 2001-01-25
with 10% absorption, this construct overestimates the depth: using the
observed
(larger) hole-diameters, instead of the laser spot-size, would bring this
available-
energy vaporization depth closer to quantitative agreement with the data, as
would smaller absorption fractions.
From this picture, the 1.2-ps laser-matter interactions appear to drive a
vaporization-phase ablation process with the commensurate advantages of
ablation carrying the heat away with the evaporated material, leading to
little
melt-debris and an improved feature-size resolution that is not available with
nanosecond or microsecond interactions. A vaporization process was similarly
inferred by Zhu et al. for single-pulse femtosecond ablation of aluminum, see
X.
Zhu, D.M. Villeneuve, A.Y. Naumov, S. Nikumb, P. Corkum, Experimental study
of drilling sub-10 micron holes in tin metal foils with femtosecond laser
pulses,
Appl. Surf. Sci. 152, 138-148 (1999).
Summarizing for aluminum, it has been observed that drilling of smooth
and relatively clean, high aspect-ratio through-holes in foil thicknesses up
to 200
m with single microsecond bursts provided a faster process than possible with
current kHz repetition-rate systems. As a hybrid way of delivering laser-
fluence to
target, these mode-locked bursts exploit an excellent combination of quasi-cw
heating effects to support rapid etching rates and ultrafast-laser
interactions for
clean ejection of material. Plume-absorption effects are also mitigated to the
degree that the 7.5-ns pulse-to-pulse separation supports hydrodynamic
expansion of ablation vapor/plasma from the surface. In this way, large aspect-
ratio holes could be formed in thick metal foils with a single burst.
Ultrafast laser interactions and thermal diffusion are similar for metals and
semiconductors so that the inventors anticipate the general advantages of the
present burst-ultrafast processing of aluminum are therefore extensible to
this
broad and general class of materials. The results of laser machining of glass
entails the other extreme of material properties (brittle and high melt/vapour
temperatures), demonstrating that burst-ultrafast lasers offer a wide spectrum
of
applications and advantages in laser processing and laser modification of
24
CA 02332154 2001-01-25
materials having widely diverse properties.
It will be understood that the method of the present invention may be used
for processing in the bulk of the material and is not restricted to processing
the
surface of the material. In this case the process involves applying laser
pulses to
a target zone within the body of the material, the laser pulses having a time
separation between individual laser pulses sufficiently long to permit
acoustic
and thermal shock to the material to spread and/or dissipate so that a next
subsequent laser pulse is not substantially reflected, scattered and/or
absorbed
by the temporarily altered material properties. The laser pulses have a time
separation between pulses sufficiently short so that, in one embodiment of the
invention, a thermal component in the target zone presents heated material to
successive ultrafast laser pulses in the burst, control of which residual or
accumulated heat serves the purpose of preventing or mitigating against the
deleterious effects of material stresses in the material due to acoustic or
thermal
shock, while optimizing the useful range of such effects.
The method disclosed herein defines a new way of controlling the delivery
of laser fluence to optimize performance by utilizing the attributes of
ultrafast
laser interactions with advantages of long pulse heating or long-pulse
modification of the material properties. The heat-induced stresses caused by
thermal cycling by known machining approaches, with repetition rates up to
multi-kHz, include 'bound' stresses, normally caused by laser-heating. If the
material cools down between pulses, stresses are locked into glasses and
ceramics. Then the third pulse may cause brittle fracture. The advantages of
the
present invention include specifically that the high repetition rate avoids
thermal
cycling (calculate by thermal diffusion times, roughly); also that the low-
grade
residual heat anneals thermal stresses pre-existing or accumulating in the
material. Specific to the present method: picosecond and femtosecond pulses
leave only a small residual of heat, suitable to this desired effect, because
of the
evaporative cooling effect described above. Thus the method disclosed herein
of
delivering fluence has this special advantage to the material processing not
CA 02332154 2001-01-25
available if the repetition rates are low or if the pulses are not ultrafast.
Thus, in accordance with the present invention, applying high frequency
bursts (2, 3, 4, ..., pulses, through to continuous high repetition rate
pulsetrains)
at frequencies of 100 kHz to 100's of MHz provides control over the thermal
physics and relaxation processes not available with low-pulse (<100 kHz) rate
laser systems because thermal transport and relaxation of other properties
removes dissipated laser energy not carried away by the plume. The thermal
heat and/or modified material extend is intermediate between long pulse
interactions and single-ultrafast (<100 kHz) laser interactions. The process
is
widely applicable to all classes of materials and of general advantage to, but
not
limited to, the following processes: machining, micromachining, cutting,
surface
structuring, surface texturing, rapid protyping, annealing, shock treatment,
refractive index profiling, laser-induced breakdown spectroscopy, via
formation,
surface cleaning, pulsed-laser deposition, medical procedures.
The pulse-to-pulse separation (the inverse of repetition rate) is a key
control parameter of this novel method that provides several significant
advantages. The sufficiently long separation between laser pulses permits
hydrodynamic expansion of the plume and/or plasma to avoid laser shielding
effects; the subsequent laser pulse is not reflected/absorbed by the plume and
all or most of the laser energy strikes the dense target material, for high
efficiency energy-coupling. The degree of plume/plasma shielding is controlled
by the pulse-to-pulse separation (amongst other parameters such as hole depth
or temporal profile of the burst envelope).
Sufficiently short separation retains a thermal component in the target
material that presents heated/modified materiai to successive ultrafast laser
pulses in the burst. This thermal component is key to numerous attributes
(described below) that are not available with low repetition rate (<100 kHz)
ultrafast lasers. The degree of thermal component (surface temperature) is
controlled by the pulse-to-pulse duration (amongst other parameters such as
fluence, laser spot size). The thermal component can modify permanently the
26
CA 02332154 2001-01-25
material properties, providing refractive index changes in optical materials,
densificiation or swelling of materials, visible marks.
The thermal component can anneal the surrounding material improving
the overall quality of the laser process. The annealing process `heals' a
material
in certain situations that, for example, eliminates incubation processes that
change the absorption and other material properties, and make lower-rep rate
laser processing less predictable. Surface swelling can also be avoided and
precise rates of processing become available.
The thermal component raises the temperature in the surrounding
processing volume, changing the state of the material to one possibly more
conducive to the laser process. In brittle materials, the higher temperature
confers material ductility, preventing and/or reducing the initiation of
microcracks
or other defects that can propagate by laser induced shock and other
processing
mechanisms. This brittle-to-ductile transition is particularly attractive in
processing brittle materials such as glasses, wide bandgap materials,
semiconductors, ceramics, layered materials, and composites, providing the
means for forming excisions without surface cracks, sheared flakes, collateral
damage, defect formation and the like.
The thermal component reduces the required laser fluence (per pulse)
with several advantages including more energy efficient material removal,
excision of high aspect ratio holes, deeper holes are possible and the process
is
attractive for good conductors such as metals or semiconductors. The burst
mode permits the excision of through holes in a single pulse burst in foils
and
thicker (-1 mm) metal plates, thereby providing faster processing than lower-
repetition-rate-applications, as well as low dwell-time advantages.
The present method reduces laser-induced shocking during processing,
reducing the potential for damage (microcracking, exfoliation, shearing,
delamination) since most laser energy is carried away in plume (ultrafast
laser
advantage), the small remaining thermal part is controllable by the pulse
separation; this avoids gross melting that is characteristic of long pulse
27
CA 02332154 2001-01-25
(nanosecond or longer) laser processing while retaining the advantages of
ultrafast laser processing.
Since ultrafast laser machining frequently supports material vaporization,
the high repetition rate (>100 kHz) permits a controlled cooling phase of
remaining material that prevents the formation of a melt phase normally
present
with long pulse or cw laser interactions; this provides better-quality
excisions
with less debris and splattered melt.
The short pulse-to-pulse duration of the present invention further offers
laser interactions with temporarily modified material where complete
relaxation of
physical and chemical changes brought on by previous pulses have not fully
relaxed.
Therefore, the pulse-to-pulse separation is a new control `knob' to be
tuned to an optimal value (typically less than 1000 ns) that depends on
parameters including the laser fluence, wavelength, material properties, beam
area and layout geometry. The key is to time the pulse separation to optimal
conditions such that the surrounding material temperature, phase (shorter
increases temperature), physical or chemical properties offer a controlled
laser
interaction, while permitting enough time for the plume/plasma to expand and
open a transparent path to the sample surface for the next pulse. The volume
of
heated region is controllable, and small, and does not have to damage
remaining
material thereby minimizing collateral damage. There are, however,
circumstances in which this same control could be used to advantage in the
opposite way, by controlling pulse-separation times so as to deliberately
provide
a heated plasma plume which benefits processing, e.g., by excluding or
reacting
with the ambient atmosphere around the workpiece, by producing a bath of
electromagnetic radiation to the workpiece, or by removing adsorbed
contaminants on the workpiece surface through plasma bombardment.
The foregoing description of the preferred embodiments of the invention
has been presented to illustrate the principles of the invention and not to
limit the
invention to the particular embodiment illustrated. It is intended that the
scope of
28
CA 02332154 2001-01-25
the invention be defined by all of the embodiments encompassed within the
following claims and their equivalents.
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J. lhlemann, Excimer Laser ablation of fused silica, Appl Surf. Sci. 54 193-
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High-contrast terawatt chirped-pulse-amplification laser that uses a 1-ps
Nd:glass oscillator, Optics Lett. 18, 361 (1993).
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