Comparison Excimer Laser – Solid State Laser Rainer ... - Coherent

Comparison Excimer Laser – Solid State Laser
Rainer Paetzel
Lambda Physik AG
Goettingen, Germany
Abstract
Both, excimer and solid state lasers can produce intense ultraviolet light pulses as they are
required in a wide range of applications in materials processing and micro-fabrication. Excimer
lasers offer maximum power and the shortest wavelength of all narrow-band UV sources. Their
spatial coherence is low, which allows efficient beam mixing and homogeneous illumination of
larger areas. They are the ideal sources for e.g. lithographic processes, TFT display annealing
and fiber Bragg grating writing.
Solid state lasers exhibit excellent beam quality and produce the highest repetition rates. They
require little maintenance and can emit IR, visible and UV wavelengths. Beam quality and
repetition rate allow unmatched resolution and yield in micromachining applications requiring
tight focusing such as the drilling of holes in a wide range of hard and soft materials. Diode
pumped solid state lasers clearly improve lifetime, maintenance costs, uptime and efficiency in
comparison with flash lamp pumped lasers. Furthermore, the shorter pulse duration of diode
pumped lasers reduce slag and other deposits that are difficult to remove. The different
wavelengths of excimer and solid state lasers also enables to selection of a UV source that
perfectly matches the requirements of a certain process or material.
Excimer Lasers
Excimer lasers are gas-based lasers that generate intense, short ultraviolet pulses directly
without resorting to complex wavelength conversion schemes. In the years since their
introduction in 1975, excimer lasers have undergone rapid technological advancement and have
become valuable tools for processing heat-sensitive materials and applications demanding the
highest resolution. Excimer lasers are well adapted to applications where the shorter wavelengths
are an advantage. Excimer lasers provide a wide range of processing power, with average range
of processing powers up to 300 W, repetition rates to 4 kHz, energies higher than 1 J and pulse
lengths from 10 to 250 ns. The most widely used wavelengths are 157, 193, 248, 308 or 351 nm.
The intense UV output and high flexibility in terms of output parameters make the excimer
attractive for a variety of processes. Applications cover a broad range of research, medical and
industrial disciplines. A decade of steady improvements in reliability, simplicity and operation

has changed the excimer from a scientific to a production tool. The excimer’s gain medium is
composed of inert and halide gases. Excimer is an abbreviated form of “excited dimer”, meaning
a diatomic excited molecule with the same atomic composition; e. g., Xe2. The name excimer
also is applied to molecules like XeCl.
Figure 1: Typical Near field beam profile of a 248 nm excimer laser
Figure 1 shows the typical beam profile of the excimer laser that is very similar for all
wavelengths. The large dimension shows a top-hat profile whereas the short dimension gives a
Gaussian intensity distribution. The main performance characteristics of the excimer and solid
state lasers are summarized in Table 1.
Table 1: Excimer performance characteristics
Characteristic Excimer Laser Photon Diode pumped
Energy ND:YAG
Wavelength
1.17 eV 1064 nm,
2.33 eV 532 nm,
351 nm,
3.53 eV 355 nm
308 nm,
4.02 eV
4-66 eV 266 nm
248 nm,
5.00 eV
193 nm,
6.42 eV
157 nm
7.90 eV
Energy per Pulse 10 .... 1000 mJ 0.1 ... 3 mJ
Repetition Rate 10 .... 4000 Hz 1 kHz ... 100 kHz
Power Range 1 ... 300 W 5 ... 60 W
Pulse Length, FWHM 10 ... 35 ns 15 ... 100 ns
Beam Quality Large cross section,
TEM00
mutimode
M 2 < 1.2
Spatial Coherence Low High
Max. Energy Density ... 200 J/cm 2 ... 2500 J/cm 2
Max. Peak Power Density ... 1000 MW/cm 2 ... 200 GW/cm 2

The short wavelength of the excimer provides high optical resolution as required in microlithography
for chip dimensions below 0.25 µm. On the other hand the photon energy of the UV
wavelength is very high, even higher than the bond energy of several molecules that lead to a
specific interaction. Only excimer lasers emit UV wavelength with acceptable efficiency and
without additional frequency conversion.
Solid State Laser
From the large range of solid state lasers in this paper, the diode pumped Nd:YAG laser will
be considered in more detail. The advantage of diode pumping of solid state lasers is that the
pump wavelength can be matched with the absorption characteristic of the laser medium. As a
result a high efficiency is reached with unmatched beam quality. The beam quality allows for
efficient frequency doubling and tripling using non-linear crystals. In such arrangements the
fundamental wavelengths at 1064 nm, the visible 532 nm and the 355 nm UV wavelength can be
generated with high efficiency and stability.
Figure 2: Typical beam profile of a diode pumped solid state laser (GATOR)
Diode pumped solid state lasers emit a so called “diffraction limited” beam quality (M 2 < 1.2)
which provides the smallest possible size for micro-features.
D ≈ (4 x λ x (f/D) x M2) / π
Table 2 provides an overview of typical spot sizes and peak power densities for F numbers
(f/D) of 10-20 as obtained by Lambda Physik’s DPSS Laser “PowerGator”:
Table 2: DPSSL laser spot sizes and peak power densities
Wavelength Spot Size Peak Intensity
1064 nm 15-30 µm > 63 GW/cm 2
532 nm 10-15 µm > 56 GW/cm 2
355 nm 5-10 µm > 170 GW/cm 2

Excimer Laser Applications
Excimer lasers are used in a variety of industrial applications that take advantage of the broad
spectrum of output specifications. By far, the largest industrial application of excimer laser is
found in micro-lithography. In this application the wavelengths of the excimer laser 248 nm,
193 nm and 157 nm are a precondition to achieve the optical resolution required to print pattern
of 180 nm down to 70 nm. The laser light is used to expose the photo-resist on the wafer. The
scanners illumination system, which today must provide various illumination settings such as
annular or quadrupole, is well adapted to the excimer laser beam. The excimer’s low spatial
coherence helps to avoid speckle interference phenomena in the image. The refractive imaging
lens of the scanner demands a very narrow output spectrum of less than 0.5 pm, FWHM in order
to avoid chromatic aberrations. Another prominent application, which is dominated by the
excimer laser, is the use for annealing of TFT flat panel displays. In this process the laser is used
to transfer amorphous silicon into a poly-crystalline silicon which is then used to build up
transistors and driver circuits. Only excimers deliver the wavelength (308 nm) and the required
output power (300 W) to cost efficiently process the substrates. High power XeCl excimer lasers
are also applied for the surface treatment of metal alloys like cast iron as used in cylinder liners
of combustion engines. The ablation process using 308 nm wavelengths leads to opening of
graphite spherulites which then act as an oil reservoir and greatly reduce wear and oil
consumption.
Micromachining with Excimer Laser
Micromachining is a large area in which both lasers, excimer and Nd:YAG are employed in
various applications such as drilling, scribing, cutting and structuring. In general both lasers use
different strategies to machine the sample. This difference can be described using the example of
hole drilling.
Excimer laser drilling utilizes a mask that is imaged onto the work-piece with some demagnification.
The field size is determined by the energy density required for the ablation and
the available energy of the excimer laser. In many material processing tasks a single pattern has
to be repeated many times. Using masks with patterned arrays the large cross section of excimer
beams can be utilized conveniently. Processing of most polymers with excimer radiation is
usually performed at energy densities of about 1 J/cm 2 . Using 248 nm (KrF) or 308 nm (XeCl)
leads to an ablation rate of about 1 µm per pulse. If small holes such as 20 – 50 µm have to be
drilled, this implies that only a minute fraction of the laser beam is really needed. Therefore, a
useful technique for a more economical use of the laser beam is the drilling of many holes
simultaneously. The simplest set-up is shown in Figure 3.

Figure 3: Simple set-up for hole drilling with excimer laser
A mask of 0.1 mm stainless steel or metal coated quartz contains the pattern on an area of
4 x 12 mm. This area is small enough to be within the homogeneous part of the beam cross
section. The field lens collimates the beam to ensure that only the center part of the imaging lens
is used to minimize aberrations. A reduction ratio of three serves to increase the energy density
on the work piece to about 1 J/cm 2 . In the above set-up only the homogeneous portion of the
beam is utilized which corresponds to about 50% of the laser energy. Depending on the pattern,
hole size and density, a certain portion of the beam contributes to the ablation process. To
achieve optimal results, illumination and imaging optics have to be adapted precisely to the
processing task. In order to increase the utilization of the excimer beam more optical elements
must be added which homogenize the raw beam and illuminate the exact mask pattern. The
principle of a high efficient homogenizer built by MicroLas ® is shown in Figure 4.
Figure 4: Principle of Beam Homogenizer using crossed arrays of cylindrical lenses

It consists of crossed cylindrical lenses. The incident laser beam is cut into segments by a lens
array and superimposed in the plane of integration. The more light segments are superimposed,
the better the homogeneity. With such an arrangement about 90% of the laser energy are
transferred into a homogeneous illumination field.
The ink jet nozzle drilling into polyimide or KAPTON is one prominent example for the hole
drilling by excimer laser. Utilizing 248 nm or 308 nm with energies of up to 1000 mJ allows cost
effective drilling of complete nozzle plates in one process step as used for the ink jet printer
industry.
Figure 5: Nozzle plate of Ink-Jet-Printer produced by excimer laser drilling
Other applications that also require a large and fixed pattern density are the production of
multi-chip-modules, pcb and flex-pcb. Depending on the process, 308 nm or 248 nm
wavelengths are utilized to ablate the insulator material such as Polyimide or Epoxy. Choosing a
suitable energy density of i.e. 250 mJ/cm 2 enables to “drilling” through the insulation layer
without affecting the underlying copper layer. The low energy density that is used for the
ablation using 248 nm or 308 nm leads to a large process area and as a result low operating cost
per work-piece.
The treatment of hard materials such as Al2O3, ZrO3, Sapphire and Si3N4 ceramics is also
achieved by 248 nm excimer laser with high precision. With energy densities of more than
25 J/cm 2 suitable flank angles and surface roughness are achieved for the production of so called
spinnerets. Spinnerets are used as tiny nozzles for chemical fiber production. The processing is
carried out by means of the “projection technique” typical for excimer laser micromachining.
With typical ablation rates of less than 0.1 µm per pulses structuring of a 200 µm ceramic plate is
achieved in less than 20 seconds with 100 Hz repetition rate of the excimer laser. Using a similar
set-up the excimer is also suitable for a variety of other applications such as electronics, sensors
and medical technologies where these ceramic substrates are structured with high precision down
to 10 µm.
As shown in Table 1 the photon energy of the excimer wavelength is very high. Using
193 nm or 157 nm wavelength gives an advantage for the machining of glass, quartz and PTFE,
which are transparent at longer wavelength. PTFE is a mechanically, electrically and chemically
stable material used in various applications from the life science to electronics industries. For the
micromachining of PTFE longer wavelengths require high fluence and thermally overload the

material. Applying 157 nm leads to very clean results with no evidence of debris or thermal
damage.
Figure 6: PTFE processed at 157 nm
Figure 7: Fused silica processed at 157 nm
Figure 6 shows blind holes machined into PTFE by the 157 nm excimer laser using 1.4 J/cm 2
energy density in a contact mask technique. The edges are well defined and the bottom surface is
very smooth. Similar advantageous results are observed for other wide band-gap materials such
as fused silica, which is fully transparent even at 193 nm wavelength. A processing result of
fused silica is shown in Figure 7. Clean blind holes have been achieved without thermal damage
or micro-cracking of the substrate.
Micromachining with Nd:YAG
Laser micromachining is emerging as a viable manufacturing solution due to the development
of industrial grade diode pumped, short-pulse, frequency-multiplied solid-state lasers. Lamppumped
Nd:YAG lasers with pulse length of several tenths milliseconds are well-established
tools for drilling of metal alloys and composites. However, if high precision is required, laser
drilling with milliseconds pulses could not fulfill the requirements up to now. The major problem
is the melt produced when using long pulsed lasers. Irregular and incomplete melt expulsion
affects the shape of the hole, produces recast layers and may even partially close a hole during
the drilling process that was open initially.
The radiation in short pulse, diode pumped solid-state lasers (DPSSL) results in beam and
pulse characteristics that make these lasers a versatile tool for precision engineering. With a
gaussian power distribution and a beam quality factor of M 2 = 1.1, the laser beam can be used for
nearly diffraction limited resolution. A pulse peak power in the gigawatt range can be achieved
on the sample. The excellent beam quality also ensures efficient frequency conversion. Metals
are ideally processed at wavelengths of 1064 nm and 532 nm, while ceramics, glasses, and
polymers are processed with minimal thermal effects by applying UV-laser radiation.

Figure 8: Process strategies to achieve high accuracy in laser drilling
Different strategies have been developed for the Nd:YAG based laser drilling, which are
chosen depending on the laser energy, hole diameter and required accuracy. Traditionally single
pulse or percussion drilling has been applied. To achieve higher surface accuracy the trepanning
method has been developed which is well suited for shorter pulses as emitted by the DPSS lasers.
Helical drilling uses a multitude of ablation steps to enhance drilling accuracy. The helical
drilling reaches the breakthrough after many turns of a spiral, describing the path of the ablation
front. By variation of focal position, energy and helix radius geometrically complex holes can be
achieved. In all of the methods the main benefits of the DPSSL are utilized which are beam
quality, short pulse, wavelength from IR to UV and highest peak power.
S. Govorkov 1 using stainless steel foil with different thickness has studied the influence of
the laser wavelength on the drilling process using the PowerGator of Lambda Physik. The result
is that the machining of high aspect ratio holes (1:10) in thick foils (0.870 mm) produces the
highest average ablation rate at 355 nm, which is about 1 µm per pulse compared to only 0.1 µm
per pulse at 1064 nm. The effect is attributed to the laser plasma effect inside high aspect ratio
holes and shows that a shorter wavelength provides a larger process window and less collateral
damage at the hole entrance.
DPSS Lasers have a pulse length of between 15 ns and 50 ns typically. The pulse length is
determined by the design of the DPSS laser. The side-pumping set-up achieves short pulse length
as desired for micromachining. In this set-up the pump light from the diodes is coupled to the
laser gain medium of substantial length and it is suitable for virtually unlimited scaling up of the
output power. Side pumping facilitates generation of short laser pulses that are pre-requisite for
efficient harmonic generation.
Utilizing the different wavelength, high peak power and short pulses; DPSS lasers are an ideal
tool to micro-machine diverse materials such as diamond, alumina ceramics, stainless steel and

some carbon steels. The following picture shows an example of stainless steel cut by DPSS laser
using 355 nm wavelength and 15 ns pulse length.
Figure 9: 304 SS tubing, 0.5 mm OD, 0.25 mm ID, cut by Gator UV (355 nm (~ 56 µm lines,
84 µm spaces) (50X)
Stainless steel or NiTi alloy are used in the fabrication of stents. Such stents are widely used
in life science to stabilize or increase the diameter of narrowed arteries. Stents are produced in
sizes of 0.4 mm to 25 mm diameter. Laser machining is the preferred method of making the finedetail
cuts to produce the flexible and elastic structure of the stent. Today flash-lamp pumped
ND:YAG are used for this application; however to reach the targeted cut width of below 25 µm
the DPSS laser shows great advantage due to its shorter pulse and higher quality beam.
DPSS lasers such as Lambda Physik’s GATOR or POWERGATOR deliver high peak
power density that has been shown to be ideal for high quality processing of semiconductor
materials such as silicon, GaAs, ceramics and other wafer materials. The micromachining of
0.47 mm thick silicon is shown in Figure 10.
Figure 10: Micromachining of 0.47 mm thick silicon with 355 nm DPSS laser

Fabrication of micro features like via holes, slots, grooves, notches and cutouts down to
5 micrometers are produced by DPSS laser. This performance enables the efficient use of DPSS
lasers in the manufacturing of multi-chip modules, chip scale packages, ball grid arrays, microelectro-mechanical
systems, and telecommunication components.
Conclusion
A variety of applications are established for excimer lasers and Nd:YAG lasers. For
micromachining both lasers show good results for the feature size of 5 µm to 100 µm range.
Choosing UV wavelength, short pulses and suitable process conditions minimizes the recast of
material and microcracking.
Only excimer lasers deliver high UV power and economically can reach short wavelength such
as 248 nm, 193 nm and 157 nm. The beam characteristics make the excimer the ideal laser
source for large area machining with high resolution at moderate energy density.
The ND:YAG laser applications are approaching higher resolutions as well by utilizing the
benefits of DPSS laser technology such as short pulses, UV wavelength and high repetition rate
operation.
The excellent beam quality of the DPSS lasers with a M 2 value of < 1.2 leads to extremely
high peak power densities that are a precondition for the machining of hard materials such as
metal and allows precise drilling with aspect ratios up to 50:1.
References
1.
2.
Govorkov, S.V.; Scaggs, M.; Theoharidis, H. High Resolution Microfabrication of Hard
Materials with Diode-Pumped Solid State (DPSS) UV Laser, Proc. ICALEO 2001,
Oct. 15-18, 2001, Jacksonville, FL, paper M603.
Abeln, T.; Radtke, J.; Dausinger, F. High Precision drilling with short-pulsed solid-state
lasers. Proc. ICALEO`99; November 15-17, 1999, Orlando, FL. Laser Institute of America,
Orlando, FL, 1999; pp. 195-203.
Biography
Rainer Pätzel, born in 1955, has studied biomedical engineering and received his diploma at
the Fachhochschule in Giessen, Germany. He is working for Lambda Physik AG in Goettingen
for 20 years. Presently Rainer is the leader of the product management team which covers the
excimer lasers and DPSS lasers for medical, scientific and industrial applications.