By internal modification of transparent glasses and crystals with ultrafast laser radiation and subsequent wet-chemical etching (here named SLE = selective laser-induced etching), very precise 3D structures have been produced, especially in quartz glass (fused silica), for more than a decade.
By the combination of a high precision three-axis system to move the glass sample and a fast 3D beam steering system to move the laser focus, the SLE process is now suitable to produce more complex structures in a shorter time [1]. We have programmed a printer driver for commercial CAD software and the flexible machine software enabling automated production of complex 3D glass parts with the LightFab 3D Printer. Some examples of 3D precision glass parts e.g. for lab-on-a-chip applications (cell-sorting microfluidics), electronics (glass via and connectors), semiconductor (quartz chucks), optics and precision mechanics are presented.
The SLE process is very scalable for high throughput since a faster writing speed results in higher selectivity and thus larger precision of the resulting structures. Thus SLE is a process which is suitable for mass production of 3D structures in glasses. Some examples of rapidly produced structures using our high speed beam deflection modules are demonstrated, which are the basis of our special machines enabling mass-production.
Since the LightFab 3D Printer includes an ultrafast laser with programmable pulse duration and variable repetition rates also other in-volume processes than SLE like 2-photon-polymerization, direct writing of waveguides in glasses, internal glass welding can be done in the same machine. As example of process chains the combination of SLE with glass welding by ultrafast laser radiation will be shown.
[1] J. Gottmann, M. Hermans, N. Repiev, J. Ortmann (2017) Selective Laser-Induced Etching of 3D Precision Quartz Glass Components for Microfluidic Applications—Up-Scaling of Complexity and Speed, Micromachines 8, pp. 110-120
By modification of transparent glasses and crystals with ultrafast laser radiation and subsequent wet-chemical etching (here named SLE = selective laser-induced etching), precise 3D structures have been produced, especially in quartz glass (fused silica), for more than a decade.
By the combination of a high precision three-axis system to move the glass sample and a fast 3D beam steering system to move the laser focus, the SLE process is now suitable to produce more complex structures in a shorter time. We have programmed a printer driver for commercial CAD software and flexible machine software enabling the automated production of complex 3D glass parts with the LightFab 3D Printer. New examples of 3D precision glass parts e.g. for lab-on-a-chip applications (cell-sorting microfluidics), electronics (glass via and connectors), semiconductor (quartz chucks), optics and precision mechanics are presented.
The SLE process is very scalable for high throughput since a faster writing speed results in higher selectivity and thus larger precision of the resulting structures. Thus SLE is a process which is suitable for mass production of 3D structures in glasses. Some examples of rapidly produced structures using our high speed beam deflection modules are demonstrated, which are the basis of our special machines enabling mass-production. Therefore, 3D printing of glasses is not only a niche technology for prototypes anymore.
Using an in-house developed micro scanner three-dimensional micro components and micro fluidic devices in fused
silica are realized using the ISLE process (in-volume selective laser-induced etching). With the micro scanner system the
potential of high average power femtosecond lasers (P > 100 W) is exploited by the fabrication of components with
micrometer precision at scan speeds of several meters per second. A commercially available galvanometer scanner is
combined with an acousto-optical and/or electro-optical beam deflector and translation stages. For focusing laser
radiation high numerical aperture microscope objectives (NA > 0.3) are used generating a focal volume of a few cubic micrometers. After laser exposure the materials are chemically wet etched in aqueous solution. The laser-exposed
material is etched whereas the unexposed material remains nearly unchanged. Using the described technique called ISLE the fabrication of three-dimensional micro components, micro holes, cuts and channels is possible with high average power femtosecond lasers resulting in a reduced processing time for exposure. By developing the high speed micro scanner up-scaling of the ISLE process is demonstrated. The fabricated components made out of glass can be applied in various markets like biological and medical diagnostics as well as in micro mechanics.
In-situ observation of the in-volume modification of glasses by focused ultra-short pulsed laser radiation with an
interferometer microscope allows for the spatially resolved measurement of the transient optical path difference (OPD)
in the surrounding of the laser-induced modification. By the relation of refractive index and temperature an estimation of
temperature during modification process is possible. The absorption of the laser radiation is measured and is, together
with the estimation of processing temperature during modification, a first step towards a process model for the induced
modifications of the transparent material.
Refractive index modifications are fabricated in the volume of rare-earth-doped glass materials namely Er- and Pr-doped
ZBLAN (a fluoride glass consisting of ZrF4, BaF2, LaF3, AlF3, NaF), an Er-doped nano-crystalline glass-ceramic and
Yb- and Er-doped phosphate glass IOG. Femtosecond laser radiation (τ=500fs, λ=1045nm, f=0.1-5MHz) from an Ybfiber
laser is focused with a microscope objective in the volume of the glass materials and scanned below the surface
with different scan velocities and pulse energies. Non-linear absorption processes like multiphoton- and avalanche
absorption lead to localized density changes and the formation of color centers. The refractive index change is localized
to the focal volume of the laser radiation and therefore, a precise control of the modified volume is possible. The width
of the written structures is analyzed by transmission light microscopy and additionally with the quantitative phase
microscopy (QPm) software to determine the refractive index distribution perpendicular to a waveguide. Structures
larger than 50μm in width are generated at high repetition rates due to heat accumulation effects. In addition, the
fabricated waveguides are investigated by far-field measurements of the guided light to determine their numerical
apertures. Using interference microscopy the refractive index distribution of waveguide cross-sections in phosphate glass
IOG is determined. Several regions with an alternating refractive index change are observed whose size depend on the
applied pulse energies and scan velocities.
Sub wavelength ripples (spacing < λ/4) perpendicular to the polarisation of the laser radiation are obtained by scanning a
tightly focused beam (~1μm) of femtosecond laser radiation from a Ti:Sapphire laser (τ =100fs, λ =800nm & 400nm,
f=1kHz) and from a Yb:glass fiber laser (τ =400fs, λ =1045nm, f=0.1-5MHz) over the surface of various materials like
amorphous Nd:Gd3Ga5O12 films 1 μm in thickness on YAG substrates, diamond, polytetrafluoroethylene, LiF, MgF2,
ZBLAN, Al2O3, LiNbO3, SiO2, Si, Cu and Au. The ripple patterns extend coherently over many overlapping laser pulses
and scanning tracks. Investigated are the dependence of the ripple spacing Λ on the material, the lateral distance of the
laser pulses, the N.A. of the focussing optics, the repetition rate and the applied wavelength. The ripples are
characterised using electron microscopy. Some possible models for the origin of the ripple growth are discussed and
conditions under which these phenomena occur are contained. New results concerning the scaling of the production
process using a high repetition rate laser and a fast translation stage are demonstrated. Potential applications are
presented and consequences for precise nano- and microstructuring using ultra short pulsed lasers are discussed.
In recent years new generations of precision lasers have been demonstrated and are increasingly available on an industrial level. For example high beam quality and diffraction limited Fiber lasers, Slab lasers, Disk lasers and still Rod lasers are used very successfully.
This paper focuses on - ns and μs drilling of shaped holes by helical drilling1 - drilling of extreme aspect ratios in dielectrics/glass by ns-slab lasers2 - nm-size periodic structuring of polymers by interferometric approaches - ablation by ns- and ps-pulses for metal moulds - generation of waveguide structures in glass by fs-pulses.3 On the laboratory scale a next generation of diffraction limited short pulse lasers is at the horizon.4 In particular, ps-lasers at multi-hundred watts of average power with repetition rates of several MHz,2fs-lasers at 400W2 average power and green, frequency doubled lasers at 200W are under construction. At the short end of pulses, attosecond lasers have been demonstrated and themselves shall open a new domain of interaction of light and matter5.
Sub wavelength ripples (spacing < λ/4) perpendicular to the polarisation of the laser radiation are obtained by scanning
a tightly focused beam (~1μm) of femtosecond laser radiation from a Ti:Sapphire laser (τ=100fs, λ=800nm & 400nm,
f=1kHz) and from a Yb:glass fiber laser (τ=400fs, λ=1045nm, f=0.1-5MHz) over the surface of various materials like amorphous Nd:Gd3Ga5O12 films 1 μm in thickness on YAG substrates, diamond, polytetrafluoroethylene, LiF, MgF2,
ZBLAN, Al2O3, LiNbO3, SiO2, Si, Cu and Au. The ripple patterns extend coherently over many overlapping laser pulses and scanning tracks. Investigated are the dependence of the ripple spacing Λ on the material, the lateral distance of the laser pulses, the N.A. of the focussing optics, the repetition rate and the applied wavelength. The ripples are characterised using electron microscopy. Some possible models for the origin of the ripple growth are discussed. New results concerning the scaling of the production process using a high repetition rate laser and a fast translation stage are demonstrated. The cross-sections of the ripples are investigated using electron microscopy. A very large aspect ration of ~10 is observed for the periodical nanostructures in fused silica. Using in-volume selective laser etching (ISLE) of sapphire results in deep hollow nanoplanes ~200 nm in width and up to 1 mm in length. Microchannels have been produced using in-volume selective laser etching with a scanning speed of 1 mm/s.
Laser radiation is used both for the deposition of the laser active thin films and for the micro structuring to define wave guiding structures for the fabrication of waveguide lasers. Thin films of Er:ZBLAN (a glass consisting of ZrF4, BaF2, LaF3, AlF3, NaF, ErF3) for green upconversion lasers (545 nm), Nd:YAG (Y3Al5O12) and Nd:GGG (Gd3Ga5O12) for infrared lasers (1064 nm) are produced. Manufacturing of the laser active waveguides by micro-structuring is done using fs laser ablation of the deposited films. The structural and optical properties of the films and the damping losses of the structured waveguides are determined in view of the design and the fabrication of compact and efficient diode pumped waveguide lasers. The resulting waveguides are polished, provided with resonator mirrors, pumped using diode lasers and characterized.
Laser operation of a ridge waveguide structure grown by pulsed laser deposition and structured by fs laser ablation is demonstrated. A 1 &mgr;m thick, 100 &mgr;m wide and 3 mm long structured waveguide consisting of amorphous neodymium doped Gd3Ga5O12 has shown laser activity at 1.068 &mgr;m when pumped by a diode laser at 808 nm.
Sub-wavelength ripples are obtained by scanning a tightly focused beam (~1μm) of femtosecond laser radiation (λ=266, 400 and 800 nm, τp=100 fs) over the surface of various dielectrics, metals and silicon. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarisation. Investigated are the dependence of the ripple spacing on the lateral and temporal spacing of successive pulses, the direction of polarisation and the material. The evolution of the ripples is investigated by applying pulse bursts with N=1 to N=20 pulses. The conditions under which these phenomena occur are specified, and some possible mechanisms of ripple growth are discussed. The ripples have been continued not only in one dimension but also in two dimensions on the surface by several scans with an offset, resulting in the formation of an extended grating.
Sub-wavelength (1/4*λ-3/4*λ) laser induced periodic surface structures are generated by irradiation of either bulk fused silica and silicon or Er:BaTiO3 thin films by scanning a tightly focused beam (Θ = 1 μm) of femtosecond laser radiation (λ = 800 nm, tp = 100 fs) on the surface. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarization of the laser radiation. The dependence of the ripple spacing on the spacing of successive pulses, the direction of polarization and the properties of the material is investigated. The evolution of the ripples is investigated by applying pulse bursts with 1 - 20 pulses. The development conditions of the stuctures are specified and possible mechanisms of ripple growth are discussed.
The dynamics of the laser induced plasma during pulsed laser deposition of BaTiO3 thin films is studied theoretically and experimentally and related to the resulting film properties. The expansion of the laser induced plasma is modelled taking inelastic collisions between ablated particles and processing gas particles into account. The predictions of the model are in agreement with data from high speed photography of the plasma emission. Pulsed laser deposition with KrF excimer laser radiation (wavelength 248nm, pulse duration 20 ns) is used to grow dense, transparent, amorphous, poly-crystalline and single crystalline erbium doped BaTiO3 thin films for photonic applications. Visible emission due to up-conversion luminescence (wavelength 528 nm and 548 nm) under excitation with diode laser radiation at a wavelength of 970-985 nm is investigated as a function of the erbium concentration of 1-46 mol % and structural film properties. The dielectric films are micro machined to form optical wave guiding structures using Ti:sapphire laser radiation (wavelength 810 nm, pulse duration 63-150 fs) by scanning the focussed laser beam relatively to the sample.
Laser radiation is used both for the deposition of dielectric Er:BaTiO3 thin films and for material removal to generate wave guiding structures for photonic applications. Pulsed laser deposition with KrF excimer laser radiation (wavelength 248 nm, pulsed duration 20 ns) is used to grow dense, transparent amorphous or crystalline erbium doped BaTiO3 thin films. Visible emission due to up-conversion luminescence (wavelength 528 nm and 548 nm) under excitation with diode laser radiation at a wavelength of 975 nm is investigated as a function of the erbium concentration and structural film properties. The dielectric films are micro machined to form optical wave guiding structures using Nd:YAG laser radiation (wavelength 532 nm, pulsed duration 40 ps) and Ti:sapphire laser radiation (wavelength 810 nm, pulse duration 63 - 150 fs) by scanning the focused laser beam relatively to the sample.
The deposition of different hard ceramics coatings as Al2O3, ZrO2, c-BN and DLC thin films by pulsed laser deposition (PLD) has been of increasing interest as alternative process compared to the latest progress in CVD and PVD deposition. For instance, in pulsed laser deposition, the properties of the resulting thin films are influenced by the composition, ionization state, density, kinetic and excitation energies of the particles of the vapor/plasma. In order to deposit hard ceramics with different properties and applications, various substrates as Pt/Ti/Si multilayer, glass (fused silica), steel, polymethylmethacrylate (PMMA), polycarbonate (PC), Si(100) and Si(111) are used. These thin films are deposited either by excimer laser radiation ((lambda) equals 248 nm) or by CO2 laser radiation ((lambda) equals 10.6 micrometers ). To characterize the structural, optical and mechanical properties of the hard ceramics thin films, different techniques as Raman spectroscopy, ellipsometry, FTIR spectroscopy and nanoindentation are used.
Sintered targets of ZrO2, Al2O3 and BaTiO2 are ablated by KrF excimer laser radiation (lambda) equals 248 nm, (tau) equals 25 ns. The processing gas atmosphere consists of O2 at typical pressures of 10-3-0.5 mbar. The films are deposited on a Pt/Ti/Si multilayer substrate. The investigations concentrate on the influence of the kinetic energy of the ablated particles on the crystal structure and morphology of the forming films. The kinetic energy is described as a function of the processing gas pressure, the target-to-substrate distance and the fluence of the laser radiation on the target. The compaction of the zirconia and alumina thin films is achieved by particles impinging on the growing surface with kinetic energies above 30 eV. To deposit ferroelectric BaTiO3 thin films at low substrate temperatures the mean kinetic energy of the Ba particles have to be < 30 eV to prevent displacements beneath the film surface. Analytical techniques used for the structural characterization of the films are X-ray diffraction, micro- Raman spectroscopy and electron microscopy. Dielectric and ferroelectric properties of the films are determined by impedance measurements. The thickness and the complex refractive index are determined by fitting a model for the film geometry to the measured ellipsometry data.
Raman spectroscopy is employed for structural characterization of BaTiO3 ferroelectric thin films, deposited by a hybrid DC-field enhanced pulsed laser deposition process. Pulsed excimer laser radiation (KrF, (lambda) equals 248 nm, (tau) equals 25 ns) is used for material removal from a sintered BaTiO3-target in an O2 processing gas atmosphere (pressure p(O2) approximately equals 1 * 10-1 mbar) with subsequent deposition on a substrate. Additional energy is supplied to the laser- induced plasma via a system of two concentric ring electrodes lying on different electric potentials (difference (Delta) V up to several hundred V), leading to further activation of the plasma and ignition of a DC- discharge in the processing gas atmosphere. Micro-Raman spectroscopy is performed with Ar+ laser radiation ((lambda) equals 488 nm), using a microscope unit to achieve a high spatial resolution in the range of 1 micrometers . The Raman spectra of the BaTiO2 films show peaks typical for the tetragonal/cubic Perovskite structure. Polarization- dependent measurements reveal a mean c-axis orientation normal to the substrate surface, regardless whether a DC- field is applied or not. Using low DC-bias voltages ((Delta) V equals 50 V) allows lowering the substrate temperature without affecting the crystal quality of the films, as determined from the full width at half maximum of the Raman peaks, which is a measure for the crystal quality. High DC-bias voltages ((Delta) V equals 700 V), however, lead to amorphous films. The dielectric constant of the BaTiO3 films is strongly correlated to the crystal quality.
Pulsed laser deposition with KrF-excimer laser radiation ((lambda) equals 248 nm, (tau) equals 25 ns) is used to grow thin films of diamond-like carbon (DLC) on Si substrates by material removal from a graphite target in vacuum (10-5 - 10-4 mbar) or in processing gas atmospheres (He, Ne, Ar, Kr, p approximately equals 10-2 mbar). Additional ion bombardment is performed by extracting inert gas ions from a low pressure rf gas discharge which is ignited in the deposition chamber. Raman spectroscopy reveals that the sp3-content of the DLC films depends on the fluence of the laser radiation on the target, the substrate temperature and processing gas pressure. Especially the applied laser fluence and the inert gas pressure strongly influence the energy of the film-forming particles and therefore the relative amounts of sp2 and sp3 bonds in the films. Using additional ion bombardment from the low pressure gas discharge leads to more nanoclustered films in the case of higher ion masses due to a more effective defect creation. However, the films are predominantly sp2-bonded, caused by substrate heating due to dissipation of electric energy at the substrate electrode.
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