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Fast femtosecond laser ablation for efficient cutting of sintered alumina
substrates
Reece N. Oosterbeek1,2,3, Thomas Ward1,3, Simon Ashforth1,3,4, Owen Bodley1,2,3, Andrew Rodda5,
M. Cather Simpson1,2,3,4*
1 The Photon Factory, The University of Auckland, New Zealand
2 School of Chemical Sciences, The University of Auckland, New Zealand
3 The MacDiarmid Institute for Advanced Materials and Nanotechnology and The Dodd Walls Centre
for Quantum and Photonic Technologies, New Zealand
4 Department of Physics, The University of Auckland, New Zealand
5 Aeroqual Ltd., Auckland, New Zealand
* Corresponding author email: c.simpson@auckland.ac.nz
Abstract
Fast, accurate cutting of technical ceramics is a significant technological challenge because of these
materials’ typical high mechanical strength and thermal resistance. Femtosecond pulsed lasers offer
significant promise for meeting this challenge. Femtosecond pulses can machine nearly any material
with small kerf and little to no collateral damage to the surrounding material. The main drawback to
femtosecond laser machining of ceramics is slow processing speed. In this work we report on the
improvement of femtosecond laser cutting of sintered alumina substrates through optimisation of laser
processing parameters. The femtosecond laser ablation thresholds for sintered alumina were measured
using the diagonal scan method. Incubation effects were found to fit a defect accumulation model, with
Fth,1 = 6.0 J/cm2 (±0.3) and Fth,∞ = 2.5 J/cm2 (±0.2). The focal length and depth, laser power, number of
passes, and material translation speed were optimised for ablation speed and high quality. Optimal
conditions of 500 mW power, 100 mm focal length, 2000 µm/s material translation speed, with 14
passes, produced complete cutting of the alumina substrate at an overall processing speed of 143 µm/s
– more than 4 times faster than the maximum reported overall processing speed previously achieved by
Wang et al. [1]. This process significantly increases processing speeds of alumina substrates, thereby
reducing costs, making femtosecond laser machining a more viable option for industrial users.
Keywords: femtosecond laser cutting, ablation threshold, alumina substrate, d-scan
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1. Introduction
Technical ceramics have a wide range of applications, from bio-implants, chemical resistant parts,
thermal barriers, wear resistant coatings, and electronics. The success of these materials in such
application lies in their favourable properties, including high dielectric strength (8 kV/mm), excellent
thermal stability (Tm = 2032oC) and high thermal conductivity (25 W/m.K) [2, 3]. Alumina-based
technical ceramics in particular are widely used in devices as diverse as wear resistant mechanical parts,
electrical insulators, and high power radio frequency circuits [1, 3].
Production of devices from manufactured alumina substrates necessarily involves cutting the material.
Several methods currently exist for this process, including diamond saw cutting and both CO2 and
excimer (nanosecond) laser machining, however these have a number of drawbacks. Mechanical cutting
using a diamond saw has well known disadvantages: wear of expensive tools, linear only geometries,
and frequent breakage of the brittle alumina workpiece. CO2 laser machining is currently the preferred
method; it is highly flexible and gives high throughput. Unfortunately, the thermal nature of the
machining process leads to significant heat affected zones (HAZ) around the cut, with common cracking
and spattering destructive effects [1, 3-5]. Excimer (nanosecond) laser machining also has been
attempted. The resultant cuts were of quite poor quality, with debris and cracking seen in the
surrounding HAZ [6].
Ultrafast laser micromachining (using femtosecond laser pulses) is an emerging technology with the
potential to provide a solution to many of these issues. Ultrashort pulse durations deliver extremely high
peak power, resulting in multiphoton absorption followed and non-linear ionisation processes [7-12].
This mechanism opens up new possibilities for machining of materials that is not dependent on the
single photon absorption properties of the material. The main benefit of ultrafast laser machining lies
in the strongly non-thermal nature of the ultrafast machining process. The pulse durations are smaller
than the thermal diffusion time (electron – phonon coupling timescale). Hence femtosecond laser
micromachining can generate extremely clean, precise and complex cut features with minimal HAZ [7-
12].
The main drawback to ultrafast laser machining is the slow processing speed – speeds of up to 33 µm/s
have been reported for cutting through alumina substrates [1], whereas speeds up to 152 mm/s have
been achieved using CO2 lasers [13]. Therefore, increasing the efficiency and processing speed of
ultrafast laser micromachining is the single most important advance that will increase the uptake of this
powerful technique in industry.
In this work we examined the impact of a variety of optical variables upon overall processing speed and
quality. We tuned the focal length, focal depth, power, linear translation rate and number of passes, in
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order to determine an optimal set of conditions for cutting of alumina substrates. Priority was given to
maximum overall processing speed while maintaining acceptable cut quality.
This study is part of a larger project that aims to understand the complex interactions between
femtosecond laser pulses and dielectric materials, and then to exploit that knowledge to improve
femtosecond laser micromachining.
2. Materials and Methods
The alumina substrates used in these experiments were commercial sintered alumina tiles (Coorstek
Inc., USA), with a thickness of 250 µm. The laser used was a commercial Ti:Sapphire amplified
femtosecond laser (Mantis and Legend Elite, Coherent Inc., USA), with a maximum average power of
3.5 W. This system produces 800 nm wavelength pulses with duration of 110 fs and repetition rate of 1
kHz, with a Gaussian spatial profile. For most studies, the laser repetition rate was 1 kHz. For ablation
threshold tests at low pulse overlap values, the laser repetition rate was reduced using a mechanical
shutter and Pockels cell pulse picker (Model 5046ER, FastPulse Technology Inc., USA). The beam was
directed to a micromachining stage consisting of XYZ translation stages for sample movement. For d-
scan ablation tests, a purpose-built micromachining stage consisting of XYZ translation stages
(Thorlabs Inc., USA) capable of simultaneous XYZ movement was used. For all other micromachining
experiments, a more user-friendly commercial micromachining stage (IX-100C, JP Sercel Associates
Inc., USA) was used. The laser beam was focused through plano-convex lenses of varying focal length,
from 50 – 300 mm, and laser power was adjusted using a variable attenuator based on a half waveplate
and polarising beamsplitter (Watt Pilot, UAB Altechna, Lithuania).
Machined alumina substrates were analysed using a combination of scanning electron microscopy
(SEM) (Jeol JCM-6000 Neoscope, Coherent USA), stylus profilometry (Dektak XT, Bruker USA) and
optical profilometry (Contour GT-K, Bruker USA).
Ablation threshold measurements (minimum pulse fluence required to cause ablation) were performed
using the diagonal scan (or d-scan) technique originally developed by Samad and Viera [14]. This
method involves a sample placed in the beam path of a focused Gaussian laser beam, located above the
focal point. The sample is then translated along the y and z axes simultaneously (i.e. the directions
perpendicular and parallel to the optical axis) so that it passes through the focal point (Fig. 1). The
machined feature has a characteristic “two lobe” shape. By assuming a Gaussian beam profile, the
ablation threshold can be calculated from the maximum radius of the ablation feature ρmax using Eq. 1
(1)
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in which Fth is the ablation threshold in J/cm2, E0 is the pulse energy in J, ρmax is in cm. The method
does not require measurement of beam parameters such as beam waist, position, or focal length, making
it a faster and easier method than techniques such as diameter regression [15].
Incubation effects, variation in the ablation threshold for different numbers of incident laser pulses
striking the sample, also were investigated using this method. The number of overlapping pulses can be
calculated from measurement of the ρmax dimension, as well as knowledge of the laser repetition rate f
and material translation speed (MTS) in cm/s [16-18].
(2)
D-scan ablation threshold tests were carried out at a range of different material translation speeds (10 –
500 µm/s) and two different repetition rates (10 Hz and 1 kHz) to determine the incubation behaviour
of the ablation threshold of the material. No difference in the laser-material interaction is expected for
the different repetition rates used, because even at the maximum repetition rate used (1 kHz), the pulse
separation (1 ms) is sufficient time for any plasma or heat to dissipate – the only lasting effect expected
between pulses is permanent or quasi-permanent structural changes. This is confirmed by multiple
previous experiments [19-22].
Ablation tests were also carried out to determine the optimal conditions for cutting of alumina wafers.
These tests varied the focal length and depth, laser power, number of passes, and MTS. At this point it
is useful to clarify the definitions of two different parameters; the MTS is the speed at which the sample
is translated under the focal point during a single pass, and the overall processing speed (OPS) is the
speed at which a finished, multipass cut is made – i.e. OPS = MTS ÷ number of passes.
3. Results and Discussion
3.1 Ablation Threshold
In Fig. 2 we see example surface plots of ablation profiles machined into the sintered alumina sample;
the degree of pulse overlap was set by the material translation speed and laser repetition rate, and was
greater for feature A than for feature B. The characteristic “two lobe” profile can be seen, however there
is clear asymmetry around the centre. The right side of the feature was machined with the focal point
above the sample; therefore we attribute this asymmetry to distortion of the beam caused by non-linear
effects at the focal point (as these experiments were carried out under ambient conditions). This
phenomenon is consistent with that seen by Samad et al. [23]. Because of this distortion, the maximum
damage radius (ρmax) was measured from the left lobe only, where the distortions arising from non-
linear effects are not observed as the sample surface is located above the focal point.
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These ablation threshold results (Fig. 3) show a significant decrease in the ablation threshold as the
number of overlapping pulses increases, a phenomenon known as an incubation effect. The trend can
be attributed to the accumulation of defects with successive pulses interacting with the material. It can
be described by Eq. 3 below [24]:
(3)
in which Fth,N is the ablation threshold for N pulses, Fth,∞ is the ablation threshold for ∞ pulses, Fth,1 is
the ablation threshold for one pulse, N is the number of pulses, and k is a constant. Fitting this equation,
we obtain the following ablation characteristics: Fth,1 = 6.0 J/cm2 (±0.3), Fth,∞ = 2.5 J/cm2 (±0.2), k =
0.03 (±0.01), with R2 = 0.96. The incubation behaviour measured here fits well to the model proposed
by Ashkenasi et al. [24] for dielectric materials, indicating that sintered alumina displays incubation
effects due to the accumulation of Frenkel-type defects.
The single pulse ablation threshold measured here is very similar to that seen by Kim et al. for alumina
ceramics (5.62 J/cm2, with λ = 785 nm, τp = 184 fs) [25]. At greater overlapping pulse numbers,
however, discrepancies noted can be attributed to the larger range of pulse numbers surveyed here; Kim
et al. conducted experiments for 1, 5, 10 and 100 pulses, whereas here we investigate 20 different values
of pulse superposition between 1.6 and 7500.
3.2 Focal length and focal depth
The effect of focal length upon the ablation depth was investigated by machining lines at an MTS of 20
µm/s, using a range of power settings and focal lengths. By increasing the focal length we increase both
the Rayleigh range and the effective beam waist [26]. A positive relationship between focal length and
machined depth was observed (Fig. 4), though the majority of the increase occurs between 50 and 100
mm focal lengths. We hypothesize that for focal lengths greater than 100 mm, the positive effect of the
increased Rayleigh range is negated by the reduced pulse fluence that is a result of the increased beam
waist. Alignment of the focal point of the laser to the sample surface became increasingly difficult at
longer focal lengths. This practical consideration would have an adverse effect in industrial settings
where ease of setup is important. In addition, the larger beam waists for longer focal lengths would lead
to increased kerf, also not optimal for industrial applications. As a result of the factors discussed here,
the optimal focal length for laser ablation of alumina ceramic was determined to be 100 mm – long
enough to result in significantly deeper ablation, but not so long as to make alignment difficult without
a corresponding improvement in machined depth.
To characterise the effect of focal depth upon laser machining characteristics, we defined the focal depth
as the depth into the surface of the material at which the beam is focused. Jiang et al. showed that
moving the focal point into a sample of stainless steel increased the machining depth per pulse [27].
We report similar behaviour for laser micromachining of bone [28]. The dependence of micromachined
feature depth upon focal point depth shows a peak, particularly apparent at higher incident laser powers
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(Fig. 5). At an MTS of 40 m/s and a focal length of 100 mm, the maximum material removal occurred
for a focal depth below the sample surface of 125 m.
3.3 Power and speed
Power and speed configurations were optimised by machining lines with the laser focused on the surface
of the alumina sample (i.e. focal depth = 0), with an f = 100 mm focal length lens. Figure 6 shows that
the ablation depth decreases quickly as the MTS is increased. This is to be expected, as a higher MTS
results in fewer consecutive pulses overlapping at a given position.
Note the machined depth was not significantly different for the 500, 700 and 850 mW power settings,
but when the incident power was decreased to 250 mW or below, the depth was reduced significantly
at low MTS. For this reason, 500 mW was chosen as the optimal power setting for micromachining
alumina with 800 nm, 110 fs pulses. , as this results in a large amount of material removal, without
some of the increase in cut width observed at higher powers.
No significant difference is seen between different power settings at higher MTS – this can be attributed
to the low number of overlapping pulses at high MTS. Increased power is expected to result in slightly
deeper machining per pulse at high MTS; however the low number of overlapping pulses will make the
total difference negligible.
3.4 Optimised method
The series of optimisation experiments reported here have established the optimal focal length (100
mm) and power (500 mW) for micromachining sintered alumina. Moving the focal point into the sample
also increases machining depth per pulse; the optimal point for this parameter will depend upon MTS
and the number of passes. In this next section, we report upon investigation of the ablation as a function
of these latter two parameters: the MTS and the number of passes. We achieved this by cutting through
alumina substrates using a range of MTS settings, focal depths and number of passes.
Focal depths were calculated based on the wafer thickness (250 µm) and the number of passes, assuming
that each pass removes an equal amount of material, with the laser focused at the centre of this region.
For example for 5 passes, the laser was focused 25, 75, 125, 175, and 225 µm into the sample, whereas
for 3 passes the laser was focused 42, 125 and 208 µm into the sample. MTS settings of 500, 1000,
1500, and 2000 µm/s were trialled, to provide a broad range of settings, while avoiding the reduced cut
depth at higher MTS and the reduced quality observed at lower MTS due to heat build-up (thermal
effects are not typically present in femtosecond laser micromachining, but can be induced when using
high power and very long exposure times).
From these results (Fig. 7), we see the range of conditions that have resulted in successful complete
cutting of the alumina substrates. The optimal conditions for ablation (using 110 fs, 800 nm laser pulses)
were found to be either 2000 µm/s with 14 passes (i.e. focussing 8.9 µm into the sample to begin with,
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and stepping up 17.9 µm after each pass), or 1000 µm/s with 7 passes (i.e. focussing 17.9 µm into the
sample to begin with, and stepping up 35.7 µm after each pass), at a power of 500 mW and focal length
of 100 mm as previously determined. These conditions allowed complete cutting of the alumina
substrate at an OPS of 143 µm/s – more than 4 times faster than the maximum previously reported
processing speed (for femtosecond laser cutting of 250 µm thick alumina substrates) achieved by Wang
et al. [1].
Other conditions led to partial cutting: 1000 µm/s with 6 passes, 1500 µm/s with 9 passes, or 2000 µm/s
with 12 passes, at a power of 500 mW and focal length of 100 mm. Under these conditions cutting was
not complete, but the workpiece was able to be split accurately along the machined cut with very little
force. The results suggest however, that cut quality may be reduced, due to the remaining material that
must be snapped. These conditions allow scribing and snapping with an OPS of 167 µm/s, which could
be valuable in cases where speed is of greater importance than cut quality.
It is interesting to note where the onset of cutting (and partial cutting) occurs in terms of both MTS and
OPS (Fig. 7). We see that complete cutting consistently occurs at an OPS of slightly less than 150 µm/s,
while partial cutting (although this is a more subjective measure) consistently occurs at 167 µm/s. This
is very useful finding, and indicates that the maximum OPS for complete or partial cutting is
independent of the combination of MTS and number of passes used.
To investigate the cut quality produced by these two conditions, alumina substrates that were
micromachined using the two optimal conditions described here were analysed using SEM (Fig. 8). The
edges that result from micromachining under both conditions are characteristic of femtosecond laser
machining. No cracking or charring was visible. The top edges (incident to laser) of both cuts are of
high quality, with little redeposited material seen. Some differences are seen on the bottom edges, where
the 2000 µm/s, 14 pass sample has been cut through cleanly, while on the 1000 µm/s, 6 pass sample
some residual material can be seen protruding from cut face (Fig. 8 B and F). This has potential to cause
issues in some applications so these conditions should only be used when processing speed is of
paramount importance.
4. Conclusions
The femtosecond laser ablation threshold of sintered alumina and its associated incubation effects were
measured using the d-scan method. The ablation behaviour was found to follow the model proposed by
Ashkenasi et al. [24], with Fth,1 = 6.0 J/cm2 (±0.3), Fth,∞ = 2.5 J/cm2 (±0.2), and k = 0.03 (±0.01), with
R2 = 0.96.
The effects of focal length and depth, laser power, number of passes, and material translation speed
upon the machining of sintered alumina were investigated in order to find an optimal condition for fast,
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high quality cutting. The optimal conditions were found to be machining at a power of 500 mW, focal
length of 100 mm, material translation speed of 2000 µm/s, with 14 passes (or 1000 µm/s with 7 passes
for lower quality), with the focal depth increased after each pass. These conditions allowed complete
cutting of the alumina substrate at an overall processing speed of 143 µm/s –more than 4 times faster
than the maximum reported processing speed (for femtosecond laser cutting of 250 µm thick alumina
substrates) previously achieved by Wang et al. [1].
Other sets of conditions were also found to be useful: 1000 µm/s with 6 passes, 1500 µm/s with 9 passes,
or 2000 µm/s with 12 passes, at a power of 500 mW and focal length of 100 mm, with the focal depth
increased after each pass. Complete cutting was not achieved under these conditions; however the
workpiece could be easily split along the machined cut with minimal force, allowing faster processing,
with slightly reduced quality owing to residual material protruding from the cut face.
These results indicate that femtosecond laser micromachining of sintered alumina can be industrially
relevant, if optimal laser conditions are chosen.
5. Acknowledgements
This work was supported by the New Zealand Ministry of Business, Innovation and Employment Grant
UOAX1202.
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Figure 1 D-scan method for measurement of femtosecond pulsed laser ablation thresholds, where the sample is
translated simultaneously along y and z, across and through the laser focal point.
Figure 2 Optical profiler surface plots of example d-scan ablation profiles machined into sintered alumina at
different MTSs (800 nm, 110 fs 10 Hz): MTS = 10 µm/s (A) and MTS = 50 µm/s (B).
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Figure 3 Ablation threshold as a function of pulse superposition (semi-logarithmic scale) for femtosecond pulsed
laser (800 nm, 110 fs) ablation on alumina, fitted to the defect accumulation model proposed by Ashkenasi et al. [24], with R2
= 0.96. Measured data is shown as black diamonds, while the red line denotes the fitted model.
Figure 4 The effect of laser focal length on machined depth in alumina, for different laser powers, at an MTS of 20
µm/s (800 nm, 110 fs, 1 kHz).
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Figure 5 The effect of laser focal depth on machined depth in alumina, for different laser powers, at an MTS of 40
µm/s (f = 100 mm, 800 nm, 110 fs, 1 kHz). Solid lines are straight line fits with break points at a focal depth of 125 µm, where
the maximum was observed.
Figure 6 The effect of MTS on machined depth in alumina, for different laser powers (f = 100 mm, 800 nm, 110 fs,
1 kHz).
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Figure 7 Parameter space for cutting alumina wafers, showing the net effect of MTS, number of passes, and focal
depth settings, using the optimal settings (500 mW, f = 100 mm) for our laser (800 nm, 110 fs). Dotted lines represent lines of
constant OPS (overall processing speed OPS = MTS / number of passes. “Partial” indicates where the substrate was not
completely severed under these conditions, however it split along the cut line with minimal force applied.
Figure 8 SEM images of alumina substrates cut under different conditions. Images A, C and E show the cut
produced for 14 passes at MTS = 2000 µm/s (OPS = 143 µm/s), while images B, D, and F show the cut produced for 6 passes
at MTS = 1000 µm/s (OPS = 167 µm/s). Images A and B show a complete view of the cut face, while images C and D show
the top edge (incident to laser), and images E and F show the bottom edge.
