High-pulse energy Excimer lasers for precise material ablation
Ralph Delmdahl*, Burkhard Fechner
Lambda Physik AG, Hans-Boeckler-Str. 12, D-37079 Goettingen, Germany
ABSTRACT
Pulsed excimer lasers are the strongest and most efficient laser sources in the ultraviolet spectral region. Record short
wavelengths from 351 nm down to 157 nm as well as record high 1200 mJ pulse energy as available for the 248 nm
excimer lasers are commercially provided for numerous laser material ablation approaches. Virtually no material is able
to withstand the high photon energies ranging from 3.5 to 7.9 eV emitted by excimer lasers. As a result of the irradiation
of material with high energy excimer laser photons at sufficient fluence immediate bond breaking due to electronic
excitation is induced. In combination with short -term laser material interaction of only 10 to 30 ns excimer pulse
duration, material ablation proceeds via fast vaporization and consecutive ejection of material with only negligible
dissipation of heat transfer to the surrounding zone. The effect is an inherently precise and clean ablation quality.
Keywords: Excimer laser, ultraviolet, micromachining, ablation, Pulsed Laser Deposition, PLD
1. INTRODUCTION
Ongoing progress in material research and laser processing industry is fueled to a large extent by the development of
capable excimer lasers providing the required pulse energy and output stability. Among the most promising techniques
generating almost unlimited functionality to material surfaces is excimer laser based Pulsed Laser Deposition (PLD).
With this powerful and versatile deposition method, multi-component target materials can be ablated and deposited onto
a substrate to form functional layers with tailored unprecedented physical properties. Monitoring of growth parameters
such as thickness and surface roughness is mostly performed in-situ via electron diffraction or other diagnostic tools.
High pulse energy excimer lasers with photon energies as high as 7.9 eV lend maximum flexibility to both precise
micromachining of even hard and optically transparent substrates such as quartz and to PLD in particular since virtually
every target material is amenable to excimer laser ablation and its subsequent stoichiometric transfer to a substrate.
Spectral properties as well as recent technical advances in high -pulse energy excimer lasers designed for efficient
material ablation are elucidated in this article. The high -pulse energy excimer laser series LPX Pro and COMPex Pro
were redesigned to provide cost-efficient, stable, high pulse energy excimer lasers which fully meet the needs of todays
advanced micromachining and thin film manufacturing.
2. RECENT EXCIMER LASER TUBE ADVANCES
Excimer lasers used for precise material ablation must meet high standards in regards to performance and output
characteristics. For reproducible results of high -quality the excimer laser must maintain stable performance over a long
period and in all operation cycles in order to increase productivity for various ablation applications. In the following
paragraphs recent technical advances in high pulse energy excimer lasers COMPex Pro and LPX Pro for Pulsed Laser
Deposition and laser material processing and the resulting output energy characteristics and beam parameters are
discussed.
2.1 Ceramic preionization
Based on the proven metal-ceramic technology NovaTube®, the preionization concept of both the high -duty cycle LPX
Pro series and the medium duty-cycle COMPex Pro series has been optimized in order to obtain highest pulse energies in
combination with homogeneous discharge conditions. The newly employed and Lambda Physik patented smooth
ceramic preionization scheme uniquely combines the efficiency of a discharge driven preionization source, such as
former spark preionization designs with soft and homogeneous volume preionization as provided by e.g. corona
preionization which is far less efficient and thus only viable for low pulse energy excimer lasers. This is achieved by
surface guiding the initially restricted sparks in a smooth and even manner along an appropriately shaped inert ceramic
(see Fig. 1) yielding largely extended and homogeneous volume preionization while sputtering from the metal
counterelectrode is avoided.
Figure 1: Left: Former spark preionization scheme with restricted spark volumes and restricted preionization areas. Right: Principle of
surface guided smooth ceramic preionization yielding even illumination along the main discharge.
2.2 Optimization of electrostatic gas purification system
In order to extend the hands-off operation time of both the LPX Pro series and the COMPex Pro series, the internal
electrostatic gas purification system has been adapted in design in a way that high voltage capability and therefore
electrostatic dust removing effect has been increased. Laser gas contaminants are efficiently filtered out by careful
optimization of the gas flow via the adapted electrostatic filter elements. With the sophisticated gas purification system
inside the LPX Pro and COMPex Pro series lasers, the windows performance remains to a large extent unaffected during
long-term operation even at multi-hundred millijoules of laser pulse energy (see Fig. 2).
Optics Lifetime of COMPex PRO 205 @ 50 Hz, 248 nm
900
800
700
600
500
Newfill performed at each data point
400
300
200
100
0
0
33
57
87
103
121
Million Pulses
Figure 2: Long-term optics performance of COMPex Pro 205 at 50 Hz and a wavelength of 248 nm at an initial pulse energy of 890
mJ. Only 12 % windows transmission loss is observed over 100 million pulses operation at constant maximum discharge voltage.
2.3 External resonator design
With high reproducibility of the ablation results in mind the resonator optics of the LPX Pro excimer laser series have
been innovatively designed such that they are completely decoupled from the laser cavity. As a result, pressure changes
as induced by e.g. gas fill procedures or temperature fluctuations inducing mechanical tension are completely avoided to
exert any kind of physical stress on the resonator adjustment. While the cavity is sealed with non-resonating tube
windows, the coated and hence more sensitive optics forming the resonator are entirely decoupled from laser gas and
tube pressure or tube temperature effects. To obtain longest optics and coatings lifetime the external resonator design is
purgeable with inert gas during laser operation. The decoupled, external resonator design (see Fig. 3) yields unmatched
beam pointing stability for the LPX Pro way below 100 µrad along both beam axis directions.
Figure 3: Principle of the LPX Pro external resonator design. The resonator is decoupled from the discharge chamber preventing
thermal or mechanical stress to affect the resonator optics alignment stability.
3. LASER OUTPUT IMPROVEMENTS
3.1 Pulse-to-Pulse Stability
The smooth ceramic preionization uniquely combines high pulse energies of multi-hundred millijoules with extremely
good pulse-to-pulse energy stability. A typical energy stability measurement for a COMPex Pro laser operated at high
pulse energy in burst sequences at a wavelength of 248 nm and with 1 Hz repetition rate is shown in Figure 4.
1000
900
Pulse Energy [mJ]
800
700
600
500
400
300
200
100
0
10 min
Energy Stability: 0.4 %
(1sigma)
E = 881 mJ; E
= 892 mJ
Time [a.u.]
Figure 4: Energy stability (1 sigma, %) obtained for the COMPex PRO when operated at 248 nm, at constant high voltage and with a
repetition rate of 1 Hz in bursts of 10 shots with burst intervals of 10 minutes each representing typical PLD operation conditions.
3.2 Beam Homogeneity
Excimer lasers have typically a rectangular beam profile, where the long axis exhibits a top head and the short axis has a
gaussian like shape. The highly homogeneous spatial distribution of the COMPex Pro beam profile obtained from a
single shot exposure 70 cm behind the laser exit at 400 mJ pulse energy is shown in Figure 5, The short axis crosssection showing a near-gaussian distribution, the long axis the flat -top distribution. Due to the efficient and smooth
ceramic preionization scheme in COMPex and LPX lasers this high homogeneity is maintained at high pulse energy
operation.
Figure 5: Near-field spatial energy distribution of the COMPex PRO laser series obtained with smooth ceramic preionization at a
wavelength of 248 nm taken at 30 kV. High spatial homogeneity and clean profile edges with no artefact structure are obtained.
3.3 Beam Pointing Stability
Reproducible micromachining results require a laser beam with high directional stability. Most stable beam performance
is possible when the laser resonator optics are decoupled from the discharge chamber i.e. from thermal and mechanical
stress. As a consequence, the industrial grade LPX Pro laser series employes purgeable, external resonator optics for
rock-solid pointing stability.
To determine the laser beam stability the beam pointing stability is measured. The beam pointing describes the angular
movement of the beam and is given as the center-of-gravity distribution of the far field beam profiles measured in µrad.
The typical beam pointing stability of the LPX Pro is way below < 100 µrad sigma. Figure 6 shows a beam pointing
stability of the LXP external resonator of 18 µrad sigma along the short axis and 21 µrad sigma in the long axis
direction.
Figure 6: The excellent beam pointing stability of 28 µrad (1 sigma) clearly shows the superior stability of the external resonator.
3.4 Stabilized Energy Operation
The performance of an excimer laser is dependent on laser gas, the optics, and the laser tube lifetime. For sensitive
applications it is important to keep laser parameters such as output energy, energy deviation, and beam properties
constant during the process. The basic approach to stabilize the output energy of an excimer laser is to increase the high
voltage between the discharge electrodes as soon as the laser pulse energy starts to decline during long term operation.
The reason for decreasing energy is the slowly decreasing halogen concentration in the laser tube even in well passivated
systems. Since the beam geometry can be slightly affected as a result of increasing high voltage, on the fly gas actions
namely, halogen injections have been introduced for excimer laser systems. Lambda Physik has optimized the halogen
injections in a way that only very small quantities of halogen (typ. < 5 mbars) are injected whenever a threshold high
voltage level is reached. The injections keep the beam properties within the specified range during the laser operation.
Figure 7 shows this positive effect yielding more than 100 million laser pulses non-stop hands-off operation at 400 mJ
stabilized using a repetition rate of 200 Hz. Approximately 10 halogen injections of about 5 mbar each are performed on
the fly during these laser runs. Apparently, the automated halogen injections have no noticable effect on the energy
output stability but drastically enhance the hands-off operation time.
Long-term run at 400 mJ stabilized energy, 200 Hz, 248 nm
450
Energy [mJ]
400
350
300
250
200
150
100
50
0
12:00
10
30
40
60
70
80
90
020
0
050
0
0
19:2010 02:4 20 10:0030 17:240 00:4 50 08:0060 15:2 70 22:4 80 06:0090
Million Pulses
Pulses
Million
Time
Figure 7: Hands-off non-stop run at stabilized energy of 400 mJ over more than 100 million laser pulses at a wavelength of 248 nm
achieved with the advanced LPX Pro laser design. The energy stability was < 2 % (1 sigma) over the entire run.
4. OVERVIEW OF LASER PARAMETERS
Table 1 gives a concise overview of the main 248 nm output parameters of the COMPex Pro and LPX Pro excimer lasers
(individual numbers dependent on laser series model).
Wavelength
Max. pulse energy
Max. Average power
Max. repetition rate
Energy stability (1 sigma)
Beam size (FWHM, v x h, typ.)
Divergence (1/e², v x h, typ.)
Beam pointing stability
Pulse Length (typical, FWHM)
COMPex Pro
248 nm
700 mJ
30 W
100 Hz
<1%
23 x 11 mm²
3 x 1 mrad²
< 200 µrad
22 ns
LPX Pro
248 nm
1200 mJ
80 W
200 Hz
<1%
10 x 25 mm²
1 x 3 mrad²
< 100 µrad
24 ns
Table 1: Key parameters of energy output and laser beam performance of COMPex Pro and LPX Pro excimer laser series for a laser
wavelength of 248 nm.
5. HIGH-PULSE ENERGY EXCIMER APPLICATIONS
5.1 Pulsed Laser Deposition
With proper focusing conditions the fluence of the excimer laser beam is sufficiently intense to vaporize any hard and
transparent target material lending maximum flexibility in terms of the material spectrum which is to be ablat ed1. On
account of the unique lateral resolution of 2 µm achievable with short-wavelength excimer laser based ablation systems
as well as of the high depth resolution reaching down to 0.1 µm, excimer lasers are extensively used in high-precision
marking, surface treatment, micro patterning and micromachining to name but a few. Due to the unique spectral
properties of excimer lasers, composites and alloys can be evenly ablated without fractionation of the different
constituents. Excimer lasers are hence the first-choice ablation sources to be employed in creating thin films by means of
the pulsed laser deposition technique (PLD). In this particularly straightforward method a pulsed excimer laser beam
focused on the target leads to rapid evaporation of the target material. The vaporized material recoils perpendicularly to
the target surface in a highly directed so-called plasma plume consisting of excited and ionized species. The plume
particulates evolve at high-speed toward the substrate which is typically located at some centimeters distance where they
deposit and grow forming a thin film.
The PLD method is straightforward and only a few parameters including pressure, energy density and pulse repetition
rate of the excimer laser need to be controlled during the process of thin film creation. The targets used in PLD are small
compared with the large size required for other sputtering techniques. Multi-layered films of different materials are
easily produced by sequential ablation of assorted targets on a rotat ing disk. By adapting the number of pulses, accurate
control of film thickness down to atomic monolayer is possible. With the short wavelength excimer laser light the
stoichiometry of the target like the crystal structure of the target can be retained. A recent overview over latest PLD
developments has been given by Ashfold et al.2.
5.2 Micromachining
Excimer lasers represent the short edge in terms of commercially available laser wavelengths and, thus, deliver the
necessary optical resolution as well as the required photon energies to precisely and efficiently structure even transparent
and hard-to-machine substrates. In today’s industrial manufacturing the majority of excimer laser systems used at a high
level of production maturity make use of the classical mask projection technique3. This method is ideally suited to
excimer lasers due to their spatially extended rectangular beam profile of ca. 2 x 1 cm 2 allowing to cover a relatively
large substrate area, and thus parallel, repeated structuring has been well-established. Perhaps, next to microlithographic
chip fabrication, most prominent are the areas of aircraft cable marking and as well ink-jet printer nozzle drilling where
excimer lasers working at high pulse energy of several hundred millijoules pulse energy and repetition rates of up to 300
Hz are capable to drill some 100 accurate nozzle holes within a second4. A recent successful feasibility approach based
on micro-mirror array technology it has been demonstrated to combine both parallelity and unlimited pattern flexibility
in excimer laser marking and also ablation5.
Representing an example for applications requiring optically transparent substrates, figure 8 (left) shows a cylindrical
lens structure machined in quartz at an excimer laser wavelength of 157 nm. The energy density on the quartz substrate
was 2 J/cm² yielding an ablation rate of 0.15 µm/pulse. Figure 8 (right) depicts a fresnel lens in glass obtained by
adaptive beam size control using a wavelength of 193 nm. The precise fabrication of such miniaturized optical
components plays a vital role in the development of integrated optical systems for communication purposes.
Figure 8: Cylindrical lens machined in quartz using 157 nm (left). Fresnel lens structure obtained in glass by adaptive
beam size control at a wavelength of 193 nm (right).
Chemically inert polymer-based array platforms are important for various biomedical applications such as highthroughput screening for fast drug discovery. Corresponding biological microsystems (BioMEMS) can act e.g. as
microtiter plates with a size of far less than that of a chip card with the advantages of considerably speeding up time for
parallel analysis and reducing the required probe volumes. High-volume fabrication of these micro-devices requires a
"zero-defect" master template which can then be used for mass production of disposable microstructure devices via
consecutive electroforming and injection moulding production steps. This process is depicted in figure 9 where a sample
of polymer polymethylmetacrylate (PMMA) has been accurately structured with excimer laser light of the wavelength
193 nm (left) and an inverse nickel replica (middle) obtained from this initial structure has been used for creating the
final micro-device made of polyoxymethylene (POM) by injection-moulding (right).
Figure 9: Grid microstructure of 200 µm depth and 20 µm wall thickness; left: grid structure obtained in polymer PMMA at a
wavelength of 193 nm; middle: inverse nickel replica; right: injection-moulded final micro-device made of polymer POM.
Lab-on-chip-technology pursues the idea that a high number of specific reactions will be performed simultaneously and
detected by an automated system. This may help to speed up medical diagnosis or the tracing of substances in the
environment. Principal functions of such devices comprise delivery of test substances, mixing with agents, incubation,
exposing to a sensing area and disposal of reactants. These devices are intended for high volume production of
disposable functional devices where again mass fabrication methods like injection molding or hot embossing will be
employed. In the development phase of such devices, excimer laser machining of polymer materials can deliver
prototype devices for design tests in a short time and at reasonable costs. The mask projection technique is perfectly
suited to machine the required micro-fluidic structures. Extended patterns consisting of tapered channels or junctions of
separate channels can be machined in a constant depth with high surface quality as shown in figure 10 where a microfluidic structure has been machined in polycarbonate at a wavelength of 248 nm. Due to the parallel nature of excimer
laser machining patterning of such structures usually takes a few seconds.
Figure 10: Micro-fluidic structures, the basis of a lab-on-chip-technology, machined into polycarbonate using a wavelength of 248 nm.
6. CONCLUSION
The advanced high-pulse energy excimer laser series LPX Pro and COMPex Pro have been described in this paper.
Based on proven technologies applied for industrial and lithography excimer laser sources, stable and cost-efficient highpulse energy lasers fitted to advanced micromachining and thin film manufacturing demands have been realized. The
COMPex Pro and LPX Pro laser series deliver stabilized pulse energies up to 700 and 1200 mJ, respectively, with
exceptional output stability and beam homogeneity over many 10 million pulses hands-off operation shown for the
preferred ablation wavelength 248 nm.
REFERENCES
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3.
4.
5.
G. Spiecker, R. F. Delmdahl, Laser Magazin, Vol. 6 , 10 (2002)
M. N. R. Ashfold, F. Claeyssens, G. M. Fuge, S. J. Henley, Chem. Soc. Rev., Vol. 33, 23 (2003)
R. Delmdahl, Laser Focus World, Optoelectronics Supplement, Vol. 7, S3 (2002)
H. Endert, M. Kauf, R. Pätzel, Laser Opto, Vol. 31, 46 (1999)
T. Kuntze, M. Panzner, U. Klotzbach, E. Beyer, SPIE Proc. 4th Int. Symp. on Laser Precision Microfabrication, Munich (2003)
*rdelmdahl@lambdaphysik.com; phone +49 551 6938-397; fax +49 551 6869-1