# Processing of Dielectric Optical Coatings by Nanosecond and Femtosecond UV Laser Ablation

Authors: J. Ihlemann; J. Békési; J.-H. Klein-Wiele; P. Simon
Journal: Laser Chemistry
 (2008)

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## Abstract

Microprocessing of dielectric optical coatings by UV laser ablation is demonstrated. Excimer laser ablation at deep UV wavelengths (248 nm, 193 nm) is used for the patterning of thin oxide films or layer stacks. The layer removal over extended areas as well as sub-μm-structuring is possible. The ablation of SiO2, Al2O3, HfO2, and Ta2O5 layers and layer systems has been investigated. Due to their optical, chemical, and thermal stability, these inorganic film materials are well suited for optical applications, even if UV-transparency is required. Transparent patterned films of SiO2 are produced by patterning a UV-absorbing precursor SiOx suboxide layer and oxidizing it afterwards to SiO2. In contrast to laser ablation of bulk material, in the case of thin films, the layer-layer or layer-substrate boundaries act as predetermined end points, so that precise depth control and a very smooth surface can be achieved. For large area ablation, nanosecond lasers are well suited; for patterning with submicron resolution, femtosecond excimer lasers are applied. Thus the fabrication of optical elements like dielectric masks, pixelated diffractive elements, and gratings can be accomplished.


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## Body

## 1. Introduction

Optical coatings have  a variety of applications. Especially design, fabrication,
and applications of dielectric optical interference coatings are subject  of
numerous studies [1]. For the classical use as mirror, beam splitter, or
antireflection coating, extended areas have to be homogeneously coated. Therefore,
the  production of dielectric coatings has been optimized to obtain such high quality coatings nearly
without defects on large substrates. But there are other applications, where
the coating is needed in locally well-defined areas, for example, masks or waveguides. Coating
technology is not well developed in this direction. Deposition through stencil masks
is possible but not with high spatial
resolution. In this case, the coatings have to be processed following the
deposition process in order to generate spatially well-defined patterns. Usually
lithographic processes are applied, but they have limited applicability,
because the required etching processes are complicated and not sufficiently
developed for all used materials.Laser ablation is a versatile and widespread method for microprocessing
of materials. Nearly any kind of technical materials like polymers, metals,
glass, ceramics, and composite materials has been investigated with respect to its response to intense
pulsed laser irradiation [2, 3]. In the field of high-precision
microfabrication, especially (UV-) excimer lasers are used due to their
inherent capability of producing high spatial resolution. Various applications
like drilling of micro holes or nozzles, marking of eye glasses, or stripping
of wires have been developed and transferred into industrial processes.But laser ablation is not only good for the treatment of bulk material, it
can be utilized for the patterning of thin films as well, for example, optical
coatings. In contrast to the treatment of bulk materials, where many laser
pulses are required to, for example, drill a material to a certain depth, in
the case of thin layers very few or even a single pulse is sufficient to
generate the desired ablation pattern. Regarding the capability of simultaneous
processing of large areas by mask projection, which is enabled by the flat top
beam profile of excimer lasers, there seems to be a huge potential of
patterning thin films by this method.Because under certain conditions the layer-layer or layer-substrate
boundaries act as predetermined breaking points, the ablated depth profile is
not directly correlated with the (sometimes inhomogeneous) beam profile.
Whereas in the case of bulk material ablation, spatial variations of the beam
intensity will lead to a correspondingly irregular surface profile, this will
not necessarily be the case for layer ablation, because there is a certain
process window concerning the fluence for complete layer ablation [4]. Thus
even with some spatial variations of the irradiation fluence, a very flat
surface can be achieved.Patterning of
optical layers is useful for the fabrication of optical components like
waveguides, coupling gratings, refractive, reflective, or diffractive
structures. In most cases, transparent films with high
transmission in the operation wavelength range have to be used. For
applications in the visible or infrared spectral ranges, often polymeric
materials can be applied, though their mechanical and thermal stability is
limited. For example, the laser patterning of layers for fabricating
(diffractive) optical elements has already been performed with polyimide films
[5]. For UV applications, the control of processing UV-transparent materials,
especially inorganic oxide materials, is necessary. Within the broad range of
methods for the microfabrication of optical elements, this paper treats the
structuring of (UV-) transparent inorganic oxide layers and layer stacks by laser
ablation. Special attention is paid to a combined process, where an absorbing
suboxide (SiO) layer is patterned and afterwards oxidized into a transparent
oxide (SiO2).


## 2. Methods and Materials

### 2.1. Laser Systems

Excimer
lasers are optimally suited for a number of tasks concerning ablation of
optical coatings. The emitted UV-light is readily absorbed by most of the
relevant coating materials. However, due to the  specific band edge of each
material, different laser wavelengths are required.TiO2, for example, s already absorbed in the near UV (308 nm). Efficient absorption of Ta2O5 is obtained at 248 nm, and for HfO2 an even shorter wavelength of 193 nm is required. Excimer
lasers emit powerful pulses with a flat top beam profile enabling large area
processing, for example, by mask projection. Even the pulse duration of 10 or
20 nanosecond seems to be quite optimal for the ablation of optical layers. The
thermal diffusion length L of around 50 to 500 nm which is related to the laser
pulse length τ by L∼τ1/2 and characterizes the heat-affected zone (HAZ) is
short enough to minimize lateral damage, but sufficiently long to provide heat
flow within the typical layer thickness, which promotes the liftoff of a
complete layer with a single pulse [6, 7]. However, when structure details in
the submicron range have to be fabricated, for example, gratings with a period
of several 100 nm, this thermal diffusion length is too large, so that shorter
laser pulses are required. To combine the capability of  high optical  resolution with that of a small
heat-affected zone, a UV ultrafast laser system is applied.
This comprises a Ti:Sa front-end
laser, a frequency tripling unit to convert the wavelength of the ultrashort
pulses into the UV spectral range, and a specially designed KrF amplifier to boost
up the energy of the pulses to the several mJ range. The  current laser
arrangement (Figure 1) uses a commercial Ti:Sa front-end system delivering
pulses of 150 femtoseconds duration at a wavelength of 745 nm. After
frequency tripling, seed pulses are obtained for the KrF amplifier module which
is the key component of the system. This module is a modified version of a
Lambda Physik NovaLine laser. In a
three-pass amplification scheme, the pulses are amplified up to an energy of
30 mJ at repetition rates exceeding 350 Hz, resulting in an average power
of 10 W at 248 nm [8]. The results on UV-femtosecond laser processing shown in Section 3 have been obtained using a predecessor system with 500 femtoseconds
pulse duration.Figure 1
Layout of the UV femtosecond hybrid laser system.


### 2.2. Beam Delivery

Excimer
laser processing is usually carried out in a mask projection configuration. A
mask consisting of transparent and opaque (or reflective) areas is illuminated
by the flat top laser beam. Using a lens or a complex imaging system, the mask
is projected on the surface of the work piece, so that the pattern of the mask
is reproduced (usually demagnified) in this plane leading to material ablation
in the irradiated regions at sufficiently high fluence.Mask projection is known to be a well-suited irradiation
strategy for simultaneous treatment of extended sample areas. On the other
hand, multiple beam interference has shown to be very effective in creating a
great variety of periodic nanostructures [9–11]. But also a combination
of these two techniques allows the fabrication of well-defined, versatile
periodic nanostructures over large sample areas [12, 13]. To create a linear
grating pattern, for example, a transmission grating is placed in the mask position,
and this mask plane is imaged onto the sample surface using a lens or a
microscope objective. By using a suitable arrangement of apertures, distinct
diffraction orders can be transmitted while others are blocked. This way the
simplest case of two beam interference can be realized, even with laser beams
of limited spatial and temporal coherence.


### 2.3. Irradiation Configurations

In
principle, there are two different methods to induce ablative removal of a film
from a substrate. Either the beam is directed head-on towards the film leading
to “front-side ablation,” or the beam hits the film from the other side after passing through
the substrate “rear-side ablation” (Figure2). Rear-side ablation is possible, if the substrate is
sufficiently transparent at the laser wavelength. For standard excimer laser
wavelengths, this is the case, for example, for UV-grade fused silica.Figure 2
Front-side and rear-side laser ablation of coatings.In
the case of front-side ablation, depending on laser fluence, film thickness,
and absorption properties of film and substrate, the whole film or part of it
can be ablated. If the film consists of a layer stack, it is even possible to
ablate layer after layer by successive laser pulses [14]. In the case of rear-side
irradiation with sufficient fluence, the whole film or layer stack is ablated
with a single-laser pulse. The advantage of this method is that due to the
forward transfer of the material in the direction of the laser beam, there is
no possibility for the interaction of the laser radiation with already ablated
fragments. In the case of front-side ablation, this interaction can lead to
considerable debris formation around the ablated area, if no countermeasures
are applied. Furthermore the required laser fluence for complete ablation is
much higher and the edge quality is lower for front-side ablation.


### 2.4. Materials and Material Transformation

The
materials of the coatings studied are mainly transparent metal oxides likeAl2O3,
HfO2, Ta2O5, Nb2O5 with
absorption edges somewhere in the UV. As a sufficiently high absorption is a
prerequisite for precise ablation, special care has to be taken when selecting
the laser wavelength. The applied laser wavelength should be below the
absorption edge of the material. Ta2O5, Nb2O5 are transparent  above about 300 nm, therefore, for ablation, a shorter wavelength, for example, 248 nm is required. HfO2 is
transparent down to about 220 nm, so that 193 nm should be used for
ablation. Al2O3 and SiO2 are transparent even
at 193 nm, so the use of a 157 nm radiation is necessary. An alternative
approach is the following: instead of an SiO2-coating, a silicon
monoxide (SiO) or at least a substoichiometric silicon oxide (SiOx,
x<2) is deposited. These materials are UV-absorbing and can therefore be easily
processed using 193 nm or 248 nm excimer lasers. After patterning, the remaining
SiOx is oxidised to SiO2 by heating it in air to obtain
the desired functionality of the patterned coating (Figure 3). The ability of
UV laser radiation at 248 nm to cause ablation of SiOx films
has already been demonstrated long time ago [15].Figure 3
Processing scheme for patterning of SiO-coating with
subsequent oxidation toSiO2.


## 2.1. Laser Systems

Excimer
lasers are optimally suited for a number of tasks concerning ablation of
optical coatings. The emitted UV-light is readily absorbed by most of the
relevant coating materials. However, due to the  specific band edge of each
material, different laser wavelengths are required.TiO2, for example, s already absorbed in the near UV (308 nm). Efficient absorption of Ta2O5 is obtained at 248 nm, and for HfO2 an even shorter wavelength of 193 nm is required. Excimer
lasers emit powerful pulses with a flat top beam profile enabling large area
processing, for example, by mask projection. Even the pulse duration of 10 or
20 nanosecond seems to be quite optimal for the ablation of optical layers. The
thermal diffusion length L of around 50 to 500 nm which is related to the laser
pulse length τ by L∼τ1/2 and characterizes the heat-affected zone (HAZ) is
short enough to minimize lateral damage, but sufficiently long to provide heat
flow within the typical layer thickness, which promotes the liftoff of a
complete layer with a single pulse [6, 7]. However, when structure details in
the submicron range have to be fabricated, for example, gratings with a period
of several 100 nm, this thermal diffusion length is too large, so that shorter
laser pulses are required. To combine the capability of  high optical  resolution with that of a small
heat-affected zone, a UV ultrafast laser system is applied.
This comprises a Ti:Sa front-end
laser, a frequency tripling unit to convert the wavelength of the ultrashort
pulses into the UV spectral range, and a specially designed KrF amplifier to boost
up the energy of the pulses to the several mJ range. The  current laser
arrangement (Figure 1) uses a commercial Ti:Sa front-end system delivering
pulses of 150 femtoseconds duration at a wavelength of 745 nm. After
frequency tripling, seed pulses are obtained for the KrF amplifier module which
is the key component of the system. This module is a modified version of a
Lambda Physik NovaLine laser. In a
three-pass amplification scheme, the pulses are amplified up to an energy of
30 mJ at repetition rates exceeding 350 Hz, resulting in an average power
of 10 W at 248 nm [8]. The results on UV-femtosecond laser processing shown in Section 3 have been obtained using a predecessor system with 500 femtoseconds
pulse duration.Figure 1
Layout of the UV femtosecond hybrid laser system.


## 2.2. Beam Delivery

Excimer
laser processing is usually carried out in a mask projection configuration. A
mask consisting of transparent and opaque (or reflective) areas is illuminated
by the flat top laser beam. Using a lens or a complex imaging system, the mask
is projected on the surface of the work piece, so that the pattern of the mask
is reproduced (usually demagnified) in this plane leading to material ablation
in the irradiated regions at sufficiently high fluence.Mask projection is known to be a well-suited irradiation
strategy for simultaneous treatment of extended sample areas. On the other
hand, multiple beam interference has shown to be very effective in creating a
great variety of periodic nanostructures [9–11]. But also a combination
of these two techniques allows the fabrication of well-defined, versatile
periodic nanostructures over large sample areas [12, 13]. To create a linear
grating pattern, for example, a transmission grating is placed in the mask position,
and this mask plane is imaged onto the sample surface using a lens or a
microscope objective. By using a suitable arrangement of apertures, distinct
diffraction orders can be transmitted while others are blocked. This way the
simplest case of two beam interference can be realized, even with laser beams
of limited spatial and temporal coherence.


## 2.3. Irradiation Configurations

In
principle, there are two different methods to induce ablative removal of a film
from a substrate. Either the beam is directed head-on towards the film leading
to “front-side ablation,” or the beam hits the film from the other side after passing through
the substrate “rear-side ablation” (Figure2). Rear-side ablation is possible, if the substrate is
sufficiently transparent at the laser wavelength. For standard excimer laser
wavelengths, this is the case, for example, for UV-grade fused silica.Figure 2
Front-side and rear-side laser ablation of coatings.In
the case of front-side ablation, depending on laser fluence, film thickness,
and absorption properties of film and substrate, the whole film or part of it
can be ablated. If the film consists of a layer stack, it is even possible to
ablate layer after layer by successive laser pulses [14]. In the case of rear-side
irradiation with sufficient fluence, the whole film or layer stack is ablated
with a single-laser pulse. The advantage of this method is that due to the
forward transfer of the material in the direction of the laser beam, there is
no possibility for the interaction of the laser radiation with already ablated
fragments. In the case of front-side ablation, this interaction can lead to
considerable debris formation around the ablated area, if no countermeasures
are applied. Furthermore the required laser fluence for complete ablation is
much higher and the edge quality is lower for front-side ablation.


## 2.4. Materials and Material Transformation

The
materials of the coatings studied are mainly transparent metal oxides likeAl2O3,
HfO2, Ta2O5, Nb2O5 with
absorption edges somewhere in the UV. As a sufficiently high absorption is a
prerequisite for precise ablation, special care has to be taken when selecting
the laser wavelength. The applied laser wavelength should be below the
absorption edge of the material. Ta2O5, Nb2O5 are transparent  above about 300 nm, therefore, for ablation, a shorter wavelength, for example, 248 nm is required. HfO2 is
transparent down to about 220 nm, so that 193 nm should be used for
ablation. Al2O3 and SiO2 are transparent even
at 193 nm, so the use of a 157 nm radiation is necessary. An alternative
approach is the following: instead of an SiO2-coating, a silicon
monoxide (SiO) or at least a substoichiometric silicon oxide (SiOx,
x<2) is deposited. These materials are UV-absorbing and can therefore be easily
processed using 193 nm or 248 nm excimer lasers. After patterning, the remaining
SiOx is oxidised to SiO2 by heating it in air to obtain
the desired functionality of the patterned coating (Figure 3). The ability of
UV laser radiation at 248 nm to cause ablation of SiOx films
has already been demonstrated long time ago [15].Figure 3
Processing scheme for patterning of SiO-coating with
subsequent oxidation toSiO2.


## 3. Results and Discussion

### 3.1. Single Layer Coatings

Figure4 shows the results of rear-side ablation of a single silicon
monoxide layer. Clean ablation with sharp edges and nearly without redeposition
of debris is observed. The ablation depth as a function of the laser fluence for
a 285 nm thick SiO-layer on fused silica is displayed in Figure 5. Within a
fluence range of about 200 to 600 mJ/cm2 the complete
layer is ablated with a single-laser pulse (the ablation depth is constant and corresponds
to the film thickness). Above this process window, at about 1 J/cm2 the fused silica
substrate is ablated, too. These observations can be explained by treating the
layer-substrate boundary as predetermined end points, so that precise depth
control and a very smooth surface can be achieved, even if the laser beam is
somewhat inhomogeneous within the limits given by the process window. This
behavior is similar to that observed in early work on ablating thin metal films
with an excimer laser [16], which showed good potential resolution with liftoff
through heat flow to the interface.175 nm SiO on fused silica, rear-side ablation, 193 nm, 20 nanoseconds, 540 mJ/cm2, 1 pulse.
(a)(b)Figure 5
Ablation depth versus fluence of 193 nm-single pulse rear side ablation of a 285 nm thick SiO layer on fused silica. In the range from 200 to 600 mJ/cm2 (marked grey),
the complete layer is ablated; at fluences above 600 mJ/cm2, in addition
material from the substrate is ablated.


### 3.2. Diffractive Phase Elements

As
an example for the patterning of a transparent film on a transparent substrate,
the fabrication of a binary diffractive phase element (DPE) is demonstrated.
DPEs are very attractive for beam shaping because of their basically lossless
operation. They can be implemented by a (pixelated) surface profile on a
transparent optical material and can be applied for beam homogenization, beam
splitting or efficient mask illumination. A binary phase element for operation
at a UV-wavelengthλ can be made by ablation patterning a UV-transparent
oxide film with a refractive index n and a thickness of  D=λ/[2(n−1)]. Pixel sizes of about 10×10μm2 are convenient
for fabrication and application. The ablation process can be performed pixel by
pixel according to the calculated DPE design or by creating the whole structure
at the same time using a mask. For applications of the DPE in the near UV, Ta2O5 is an adequate material. Ta2O5 absorbs at 248 nm
sufficiently, to be patterned by ablation, but is transparent at 308 nm.
This means that a DPE for use at 308 nm can be fabricated using
248 nm [17].For
fabricating a phase element to operate in the deep UV, for example, at 193 nm, a film material that is transparent at this wavelength is
necessary, for example,SiO2. In this case, the above mentioned two-step
process is applied [18]. First, an absorbing SiO layer is patterned by laser
ablation, then the patterned film is oxidised
to UV-transparent SiO2. Figure 6 shows a DPE made by this method.Figure 6
SiO2-diffractive phase element made by single-pulse rear-side ablation of SiO at 248 nm, 25 nanoseconds, 350 mJ/cm2, and subsequent oxidation.


### 3.3. Multilayer Coatings

A typical application for the
patterning of multilayers for optical applications is the fabrication of dielectric
optical masks. Such multilayer stacks, for example, of alternatingHfO2-
(high refractive index) and SiO2-layers (low refractive index) can
be ablated by an ArF-excimer laser, because HfO2 is absorbing at
193 nm. Although the thickness of the film is more than 1 μm, under
certain conditions sub-μm edge definition is achieved in the case of rear side
ablation.If both materials of the dielectric layer stack are
transparent at 193 nm, the ablation of these systems has to be performed
either at even shorter wavelengths (Vacuum-UV) [19], or with an absorbing
subsidiary layer. Thus dielectric mirrors with high reflectivity at 193 nm
consisting of a stack of alternating SiO2- and Al2O3-layers
were patterned by depositing a 193 nm absorbing HfO2- or SiO-layer
between substrate and HR-stack and ablating in a rear side configuration (Figure 7) [20].Figure 7
Al2O3/SiO2-multilayer
stack with underlying SiO-absorber layer on fused silica, rear side ablation,
193 nm, 20 nanoseconds, 500 mJ/cm2, 1 pulse.Dielectric
masks fabricated by this method can be applied for high intensity laser
applications, where metal masks (Cr on quartz) would be easily damaged. It is
even possible to fabricate grey-level masks by ablating only a defined number
of single layers instead of the whole stack [14]. As this process works only by
front-side ablation, the edge definition of the ablated structures is limited.
To achieve high-precision edges even in the case of front-side ablation, the
UV-femtosecond laser system was applied. Figure 8 shows such an ablation edge,
where the multilayer structure of the stack can be clearly resolved.Figure 8
HfO2/SiO2-multilayer
stack on fused silica, front-side ablation, 248 nm, 500 femtoseconds,
multipulse irradiation.


### 3.4. High-Resolution Patterning

Patterning a thin film with high (sub-μm) resolution is desirable, for
instance, for the fabrication of optical gratings. To achieve the required
optical resolution at high-fluence levels, a mask projection setup using a
reflective objective is suitable. The laser pulse duration has to be limited, so
that thermal diffusion of the energy coupled into the film does not lead to
blurring of the submicron pattern. For metal films, pulse durations as short as
50 ps already lead to diminished structure resolution [21]. For polymer films
with their low-thermal diffusivity, comparatively long nanosecond pulses can be
applied, but metal oxide films exhibit rather high-thermal diffusivities, so
that short pulses are required. Then it is possible to adjust the depth of the
pattern in the film by the number of pulses, so that the film can be ablated
partly or completely down to the substrate (front side ablation). Figure 9
shows a grating with 500 nm period made in a Ta2O5-film
with the described ultrashort pulse excimer laser. Such gratings may by applied
for coupling light into planar waveguide, or for the so-called grating waveguide structures, which are
used, for example, for biosensors based on fluorescence detection [22–24]. Also SiO
layers can be patterned with high resolution using this UV-fs laser system (Figure 10).Figure 9
150 nmTa2O5 on glass, front
side ablation, 248 nm, 500 femtoseconds, 300 mJ/cm2, 2 pulses.175 nm SiO on fused silica, front side ablation at
248 nm, 500 femtoseconds.
(a)(b)


## 3.1. Single Layer Coatings

Figure4 shows the results of rear-side ablation of a single silicon
monoxide layer. Clean ablation with sharp edges and nearly without redeposition
of debris is observed. The ablation depth as a function of the laser fluence for
a 285 nm thick SiO-layer on fused silica is displayed in Figure 5. Within a
fluence range of about 200 to 600 mJ/cm2 the complete
layer is ablated with a single-laser pulse (the ablation depth is constant and corresponds
to the film thickness). Above this process window, at about 1 J/cm2 the fused silica
substrate is ablated, too. These observations can be explained by treating the
layer-substrate boundary as predetermined end points, so that precise depth
control and a very smooth surface can be achieved, even if the laser beam is
somewhat inhomogeneous within the limits given by the process window. This
behavior is similar to that observed in early work on ablating thin metal films
with an excimer laser [16], which showed good potential resolution with liftoff
through heat flow to the interface.175 nm SiO on fused silica, rear-side ablation, 193 nm, 20 nanoseconds, 540 mJ/cm2, 1 pulse.
(a)(b)Figure 5
Ablation depth versus fluence of 193 nm-single pulse rear side ablation of a 285 nm thick SiO layer on fused silica. In the range from 200 to 600 mJ/cm2 (marked grey),
the complete layer is ablated; at fluences above 600 mJ/cm2, in addition
material from the substrate is ablated.


## 3.2. Diffractive Phase Elements

As
an example for the patterning of a transparent film on a transparent substrate,
the fabrication of a binary diffractive phase element (DPE) is demonstrated.
DPEs are very attractive for beam shaping because of their basically lossless
operation. They can be implemented by a (pixelated) surface profile on a
transparent optical material and can be applied for beam homogenization, beam
splitting or efficient mask illumination. A binary phase element for operation
at a UV-wavelengthλ can be made by ablation patterning a UV-transparent
oxide film with a refractive index n and a thickness of  D=λ/[2(n−1)]. Pixel sizes of about 10×10μm2 are convenient
for fabrication and application. The ablation process can be performed pixel by
pixel according to the calculated DPE design or by creating the whole structure
at the same time using a mask. For applications of the DPE in the near UV, Ta2O5 is an adequate material. Ta2O5 absorbs at 248 nm
sufficiently, to be patterned by ablation, but is transparent at 308 nm.
This means that a DPE for use at 308 nm can be fabricated using
248 nm [17].For
fabricating a phase element to operate in the deep UV, for example, at 193 nm, a film material that is transparent at this wavelength is
necessary, for example,SiO2. In this case, the above mentioned two-step
process is applied [18]. First, an absorbing SiO layer is patterned by laser
ablation, then the patterned film is oxidised
to UV-transparent SiO2. Figure 6 shows a DPE made by this method.Figure 6
SiO2-diffractive phase element made by single-pulse rear-side ablation of SiO at 248 nm, 25 nanoseconds, 350 mJ/cm2, and subsequent oxidation.


## 3.3. Multilayer Coatings

A typical application for the
patterning of multilayers for optical applications is the fabrication of dielectric
optical masks. Such multilayer stacks, for example, of alternatingHfO2-
(high refractive index) and SiO2-layers (low refractive index) can
be ablated by an ArF-excimer laser, because HfO2 is absorbing at
193 nm. Although the thickness of the film is more than 1 μm, under
certain conditions sub-μm edge definition is achieved in the case of rear side
ablation.If both materials of the dielectric layer stack are
transparent at 193 nm, the ablation of these systems has to be performed
either at even shorter wavelengths (Vacuum-UV) [19], or with an absorbing
subsidiary layer. Thus dielectric mirrors with high reflectivity at 193 nm
consisting of a stack of alternating SiO2- and Al2O3-layers
were patterned by depositing a 193 nm absorbing HfO2- or SiO-layer
between substrate and HR-stack and ablating in a rear side configuration (Figure 7) [20].Figure 7
Al2O3/SiO2-multilayer
stack with underlying SiO-absorber layer on fused silica, rear side ablation,
193 nm, 20 nanoseconds, 500 mJ/cm2, 1 pulse.Dielectric
masks fabricated by this method can be applied for high intensity laser
applications, where metal masks (Cr on quartz) would be easily damaged. It is
even possible to fabricate grey-level masks by ablating only a defined number
of single layers instead of the whole stack [14]. As this process works only by
front-side ablation, the edge definition of the ablated structures is limited.
To achieve high-precision edges even in the case of front-side ablation, the
UV-femtosecond laser system was applied. Figure 8 shows such an ablation edge,
where the multilayer structure of the stack can be clearly resolved.Figure 8
HfO2/SiO2-multilayer
stack on fused silica, front-side ablation, 248 nm, 500 femtoseconds,
multipulse irradiation.


## 3.4. High-Resolution Patterning

Patterning a thin film with high (sub-μm) resolution is desirable, for
instance, for the fabrication of optical gratings. To achieve the required
optical resolution at high-fluence levels, a mask projection setup using a
reflective objective is suitable. The laser pulse duration has to be limited, so
that thermal diffusion of the energy coupled into the film does not lead to
blurring of the submicron pattern. For metal films, pulse durations as short as
50 ps already lead to diminished structure resolution [21]. For polymer films
with their low-thermal diffusivity, comparatively long nanosecond pulses can be
applied, but metal oxide films exhibit rather high-thermal diffusivities, so
that short pulses are required. Then it is possible to adjust the depth of the
pattern in the film by the number of pulses, so that the film can be ablated
partly or completely down to the substrate (front side ablation). Figure 9
shows a grating with 500 nm period made in a Ta2O5-film
with the described ultrashort pulse excimer laser. Such gratings may by applied
for coupling light into planar waveguide, or for the so-called grating waveguide structures, which are
used, for example, for biosensors based on fluorescence detection [22–24]. Also SiO
layers can be patterned with high resolution using this UV-fs laser system (Figure 10).Figure 9
150 nmTa2O5 on glass, front
side ablation, 248 nm, 500 femtoseconds, 300 mJ/cm2, 2 pulses.175 nm SiO on fused silica, front side ablation at
248 nm, 500 femtoseconds.
(a)(b)


## 4. Summary

UV-transparent inorganic oxide layers are patterned with excimer laser
radiation with high precision. Complete layer stacks or only parts of them are
ablated depending on fluence and irradiation conditions. To achieve high spatial
resolution, a UV-fs-laser system is applied. Nonabsorbing layers are patterned
by laser processing of an absorbing substoichiometric “precursor” layer and
subsequent oxidation. Thus, dielectric masks, pixelated diffractive phase
elements, and submicron gratings are micro fabricated.



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*Source: 623872-2008-10-16.xml*