Micro Processing: Processing of Transparent Materials

Pulsed laser induced photo ablation of diamond
Vitaly Ivanovich Konov, Vitali Viktorovich Kononenko, Maxim Sergeevich Komlenok

Laser processing of diamond is governed by metastability of its crystal structure. When heated up to Tg~2000K diamond lattice tends to rearrange globally showing so called graphitization. Several regimes of short-pulsed laser graphitization of diamond are observed. If laser fluence exceeds a certain threshold Eg then multi-photon absorption in diamond can lead to material heating up to Tg and formation of permanent graphitic layer. After that pulse energy can be easily absorbed (absorbtion in graphite is many orders of magnitude higher than that of diamond). This results in graphitic layer heating and vaporization (ablation) at the sample surface. For E<Eg so called accumulation effect takes place: diamond graphitization (blackening) requires a number N pulses, N increasing for smaller E. This effect is explained as a two step process. Initially graphitic nano (micro) regions are formed. Diamond lattice rearrangement starts at tiny graphitic inclusions. Then thermal growth of graphitic regions size from pulse to pulse results in phase transformation of adjacent diamond layers. Further removal of thus formed graphitic layer can be realized either by its vaporization or chemical etching if irradiation takes place in reactive atmosphere, e.g. air. In the latter case, photoinduced phenomena can play an important role. This regime was called laser nanoablation and takes place at E<<Eg. Its first step is diamond ionization. Each act of ionization contributes to lattice rearrangement and activation of a number of top-layer carbon atoms which can react more easily with oxygen. This process is threshold free, that is pure photolytic. The number of exited and correspondingly oxidized carbon atoms grows with concentration of charge carriers. Diamond ablation rates, photoionization and surface graphitization in air were investigated for femtosecond (=800, 400 and 266 nm) and nanosecond (=193 nm) laser pulses. Depending on multiple-pulsed irradiation conditions ablation rates ranging from 10-7nm (reactive etching of exited surface carbon atoms) to 100 nm/pulse (laser graphitized surface vaporization) were observed at fluence 0,1<E<10 J/cm2. The examples of fine diamond micro and nanostructuring will be presented.


Nanosecond laser processing of diamonds
Jan-Patrick Hermani

Laser processing of synthetic diamonds for tooling application has been industrially established since the early 2000’s. Though there are many publications dealing with the interaction between laser radiation and diamond mate rials using scientific laser sources, there are no comprehensive studies available for achievable ablation rates using industrially wide spread nanosecond laser sources. This paper will focus on the investigation of laser ablation results for various polycrystalline diamond materials using a NIR laser source with maximum pulse energy of 1 mJ and tunable pulse width between 10 and 240 ns. Single pulse ablation results for different laser parameters show that the ablation depth for polycrystalline diamonds is limited by absorption of laser radiation in the plasma plume. The plasma plume is visualized by high speed photography and correlated to the ablation results.

Keywords: Laser ablation, diamond materials, short pulse, nanosecond, graphite


Laser Induced Micro-Dot Generation Inside Transparent Materials: A) Formation Dynamics, Refractive Character and Internal Stress
Alexandre Mermillod-Blondin, Arkadi Rosenfeld, David Ashkenasi

The formation dynamics of ultra-short laser-induced micro-dots inside the bulk of transparent materials was studied using time-resolved phase contrast microscopy. A random laser is used as a stroboscopic illumination source, enabling the acquisition of speckle-free, time-resolved phase-contrast images with a temporal resolution in the nanosecond range. The results demonstrate the onset and propagation of a heat front following the laser energy deposition. The heat-affected zone develops in a region that exceeds largely the footprint of the microdot. Based on the analysis of the thermal transients, it is possible to provide an estimate of the heat diffusion coefficient of the host substrate. Complementary results obtained with polarization microscopy reveal the appearance of a permanent stress field around the microdot. Interestingly, the amount of laser-induced permanent stress depends strongly on the number of pulses and on the laser polarization. We demonstrate that by using appropriate laser parameters, ultra-short laser-induced microdots are a suitable method for embedded direct part marking in stress-sensitive materials.

Keywords: ultra-short laser processing, micro-dots, time-resolved microscopy


Laser Induced Micro-Dot Generation Inside Transparent Materials: B) Process Implementation, Optimization and Utilization
David Ashkenasi, Manuela Schwagmeier, Alexandre Mermillod-Blondin, Arkadi Rosenfeld

Laser-induced micro-dots find their application in direct part marking, to address full life cycle traceability. In our studies on ultra-short laser interaction with transparent materials we address the possibility of generating internal markings with minimal stress. At present, we concentrate our effort in utilizing picosecond laser pulses at a wavelength of 532 nm. One important strategy followed in (single-shot) laser pulse micro-dot generation is using scanner systems with standard optics, e.g. F-Theta lens with 80 mm focal length, to ensure industrial-near implementation. Recent results show that the transient energy relaxation processes up to several 100 ns after laser excitation can strongly affect the host material over a region that exceeds the micro-dot size by several micrometers. We will present and discuss the processing strategies recently developed to optimize the size and appearance of internal micro dots, based on demonstrators and referring to the scientific results in material response obtained using time-resolved phase microscopy after femtosecond and picosecond laser pulse excitation.


Tuning the Energy Deposition of Ultrashort Pulses inside Transparent Materials for Laser Cutting Applications
Malte Kumkar, Klaus Bergner, Daniel Flamm, Daniel Grossmann, Myriam Kaiser, Jonas Kleiner, Stefan Nolte

Laser cutting of sheet like brittle materials, in particular glass and sapphire, is attracting attention for an increasing number of applications for the display industry, micro optics, micro electronics and others. Nonlinear absorp tion in the bulk of transparent materials enables tailored energy deposition for cleaving even along complex contours. The separation can be induced by transient effects, the separation plane defined by permanent modifications. We demonstrate examples of tuning the energy deposition by controlling the geometry, the density and accumulation of the energy absorbed in the bulk. The influence of pulse duration, pulse groups, repetition rate, feed rate and beam shaping on the absorption, the inscribed modification and the cutting results are presented. This enables developing different processing strategies for cutting applications covering a broad range of materials and requirements.

Keywords: Micro Processing, Micro Cutting; Processing of Transparent Materials


Laser-Induced Subsurface Modification of Silicon Wafers
Paul Christiaan Verburg, Lachlan Smillie, Gert-Willem Römer, Bianca Haberl, Jodie Bradby, Jim Williams, Bert Huis in ’t Veld

In addition to the processing of transparent dielectric materials, pulsed lasers can be used to produce subsurface modificati ons inside silicon by employing near- to mid-infrared light. An application of these modifications is laser-induced subsurface separation, which is a method to separate wafers into individual dies. We investigated the subsurface modification process using a combination of numerical simulations and experiments. Different wavelengths, pulse durations and pulse energies were tested. We found that subsurface melting of silicon followed by rapid resolidification is the primary material modification mechanism. Lattice defects and transformations to both amorphous silicon and pressure-induced high density silicon phases occurred as a result of the laser irradiation.

Silicon, subsurface, wafer dicing, laser-material interaction, electron microscopy.


Investigation on Bragg grating formation in a perfluorinated polymer optical fiber
Simon Kibben, Michael Koerdt, Frank Vollertsen

We present first results of ultraviolet laser induced fiber Bragg gratings (FBG) in a commercially available perfluorinated polymer optical fiber (POF) and their characterization results. FBG have gained interest in the past few years as the fiber is very thin and therefore easily can be integrated into composite structures and monitor the temperature and strain state. The type of polymer used in this study shows a much higher transparency compared to Polymethylmethacrylate (PMMA) based optical fibers. Up to now, only gratings in thin slabs of CYTOP ® (cyclic transparent optical polymer) were successfully detected. The gratings presented were inscribed using a krypton fluoride excimer laser and the well-known phase mask method. The polymer optical fiber (POF) type used in the investigations was GigaPOF -62SR from Chromis Fiberoptics, Inc. The fabricated gratings have a reflection maximum at 1450 nm and show many reflection peaks over a bandwidth of 10 nm. Prior to the inscription, an overcladding was removed using dichloromethane. The gratings were inscribed with 40 mJ/cm² pulse fluence and a total fluence of 2.5 to 5 kJ/cm². A pre-treatment of the POF in an evacuated steel tube followed by an oxygen atmosphere at 80°C, each for one day, and a temper step after the grating inscription show promising results for a much lower total fluence of 1 kJ/cm² and lower. This also means shorter grating inscription duration. The inscription time is crucial for a commercial use of this new type of POF-FBG in the field of structural health monitoring of composite structures.

Keywords: polymer optical fiber, fiber Bragg grating


New approach for laser processing of transparent materials
Michael Werner, Robin Zimny, Michael Grimm

The classical way of processing materials using laser ablation with galvanometer scanners or other optical elements limits the possible structures and geometries of features and parts. Holes without taper or other structures such as trenches with rectangular profile are hardly possible to machine.
In this paper 3D-Micromac AG presents a novel laser micromachining method for transparent materials using ultra short pulsed lasers. With this new method the described limits can be overcome. Structures like taper-free or negative taper walls walls can be achieved with very high aspect ratios. Typical dimensions of those structures are in range of a few 10 microns up to a few 100 microns. This type of structures are interesting as inkjet or other fluidic nozzles, for friction reducing surfaces or as casting moulds for polymer parts for life science and medical applications. Furthermore, laser machined micro parts like micro gears are presented. Such parts are of interest for micro drives, pumps for micro f luidics and other upcoming applications. In the presentation the concept of the process flow, latest results and limitations will be shown.


Straightforward Laser Machining of Thin Glass
David Ashkenasi, Tristan Kaszemeikat, Norbert Mueller

The LMTB has been working very actively on the industrially related implementation of laser machining via micro-ablation of transparent substrates, such as float glass, borosilicate, soda-lime, quartz and sapphire. By selecting the right laser wavelength, machining can be initiated from the rear side of glass substrates. This very attractive method becomes difficult to control for glass thickness below 0.2 mm. For these thin glasses, the LMTB developed a short focal trepanning system type1.f18 for straightforward cutting and drilling. The machining can be characterized as micro milling, only using short or ultra-short laser pulses instead of a mechanical blade. This paper presents and discusses the system developments and laser processing strategies implemented at the LMTB laser application lab to optimize the cutting results for thin glass.

Keywords: laser micro processing; cutting; drilling; glass; quartz


3D weld seam characterization based on optical coherence tomography for laser-based thermal joining of thermoplastics to metals
Philippe Ackermann, Guilherme Mallmann, Robert Schmitt, Jean Pierre Bergmann, Klaus Schricker, Martin Stambke

Laser-based thermal joining of metals to plastics shows a great potential for functional constructions especially in terms of light weight design. Thereby, the process strategy and time-temperature regime affects the resulting joint zone; hence process monitoring is a central issue. State of the art monitoring and quality assurance systems for this technology are limited as no direct information about the joining zone itself is given. Within this paper, a new method for the quality assurance in metal-plastic hybrids is presented using optical coherence tomography (OCT). This approach enables the measurement of the joint geometry as well as bubbles, imperfections and the wetted bonding area.

Keywords: optical coherence tomography, metal-plastic joining, laser-based thermal joining, process monitoring and control


Laser controlled ion exchange process and its applications
Xinghua Li, Garner Sean

We report on progress made in laser controlled ion exchange processes. In this study, a scanning CO2 laser beam was used to heat a glass substrate and locally control an ion exchange process. The heat from the glass melted the required KNO3 ion source, and the exchange occurred only where the laser was locally incident on the substrate. This process does not require global heating of the glass substrate, and the ion exchange process can be varied across the object. A depth of layer of about 18 um was obtained after several minutes of laser processing. The ion exchange depth and relative ion concentrations were evaluated by SIMS analysis. This laser controlled ion exchange process could potentially be applied in processes for writing ion exchanged optical waveguides as well as locally strengthening glass substrates. Specific examples will be described for increased the strength of glass substrates near mechanically drilled holes by using laser controlled ion exchange processes. Potential applications of the technique to other areas of interest are discussed.


Fabrication of micropump device with mixing functionality in fused silica with ultrashort laser pulses
Valdemar Stankevic, Gediminas Raciukaitis

The microfluidic micro-pump device fabricated by femtosecond direct laser writing technique in combination with chemical etching is presented. The contour and lines scanning method was used to adjust the best etching rate of micro-pump structure inside bulk fused silica. The working principle of micro-pump is based on the Venturi tube model. The design modelling was performed using COMSOL Multiphysics. The flow dynamics inside micro-channels was investigated in order to achieve the suction effect in micro-pump. Depending on the used input flow speed, the liquid injection or pumping can be achieved. When the both inlets of the designed micro-pump were connected to liquids with different concentration of dyes, the micro-pump acted as a micro-mixer. Pumping and mixing capability of the device were characterized, depending on the input flow rate, and the results are presented.

Keywords: Microfluidic; femtosecond; transparent materials; chemical etching


Laser line etching technique using the nozzle-induced bubble jet impact for glass
Chwan-Huei Tsai, Di-Wen Chiue

The study proposes a new technique of laser line etching for glass. An Nd:YAG laser was applied to the backside of the glass which was partially submerged in water. A metal plate was placed below the glass substrate. Most of the laser energy is absorbed by the metal plate. The metal vaporized the water and generated a bubble jet. A rectangular nozzle whose inlet and outlet are narrow and rectangular shape was proposed to enhance the impact of the bubble jet. Material ablation occurred by softening and rupture from impact of the bubble jet. The parameters of nozzle geometry, laser power, and laser scanning speed were obtained. The proposed laser etching method was successfully demonstrated for etching a line-strip of 50-500 m in width on a glass surface. It was found that the bubble jet of the small width nozzle inlet was well confined and created a strong jet impact on the glass surface. The proposed technique can prevent thermal damage and has great potential as an improved solution for the micro-machining of glass.

KeywordsLaser ablation, Laser etching, Bubble jet, Glass;


Rear Side Processing of Soda-Lime Glass Using DPSS Nanosecond Laser
Paulius Gecys, Juozas Dudutis, Gediminas Raciukaitis

Common drawbacks of the front-side direct laser ablation are laser-induced thermal effects and interaction between the laser beam and ablation products. These aspects consequently lead to the lower processing efficiency, lower quality and the need for post-processing. When the processing is initiated from the rear side of transparent samples, the ablated material is ejected through the formed channel in the opposite direction. Therefore, the incident radiation is not scattered and the laser fluency inside can be kept constant. Tightly focused nanosecond laser pulses are absorbed in the bulk of glass via avalanche ionization, build up thermal stresses and induce formation of micro-cracks in the substrate. By setting a proper laser pulses arrangement in the volume of glass, it is possible to control the material cracking and form a preferred structure. Using this processing approach, less energy is wasted for the material melting and evaporation as the material is removed in particle form. In our research, we investigated the free-shape cutting of thick soda-lime glass sheets initiating the process from the back-side of samples. Cuts were formed by using diode-pumped solid-state Nd:YVO4 nanosecond laser (Baltic HP, 10-30 ns, 100 kHz, from Ekspla). The laser beam was positioned in the XY plane by galvanometric scanners (from ScanLab) and focused by the f-theta lens. In our work, we proposed the wobble mode laser beam scanning technique combined with vertical sample movement. This approach enabled us to achieve taper-less geometry of laser cuts. The wall surface was controlled by adjusting laser fluency and laser pulses overlap. The nanosecond laser rear-side processing was proved to be a fast and highly efficient method for forming complex cuts in the soda-lime glass.

Keywords: soda-lime glass; glass processing; cutting.


Glass Processing with High Power Q-Switch CO2 Laser Radiation
Sebastian Heidrich, Christian Weingarten, Edgar Willenborg, Reinhart Poprawe

Recent results of glass processing with a prototype high power Q-switch CO2 laser source with a maximum output power of PLmax,cw 190 W are presented. For this, several glass materials (fused silica, BK7, S-TIH6, S-FPL53) are investigated and results regarding the achieved ablation rate and the resulting roughness are compared amongst each other. Moreover, an analysis of the chemical composition of the ablated surface is conducted and the relevance of these results for several industrial applications is discussed.

Micro Processing; Ablation; Processing of Transparent Materials


Precise structuring by 2-photon absorption in positive photoresist materials
Gordon Zyla, Andreas Aumann, Sarah Isabelle Ksouri, Evgeny Gurevich, Andreas Ostendorf

Two-photon polymerization, based on two-photon absorption, is a convenient direct laser writing process to fabricate maskless structures in micro- and nanometer range with submicrometer resolution. Negative photoresists are used in combination with this technique, however, utilization of positive resists with two-photon absorption is very innovative. Due to less shrinkage and economic manufacturing, positive photoresists have many advantages. Possible applications of this technique are the production of micro-electro-mechanical systems (MEMS) or micro-opto-electro-mechanical systems (MOEMS). In this paper, two-photon absorption of positive photoresist is discussed to be a potential basis for LIGA-process (lithography, electroplating, and molding) by two-photon absorption of positive photoresist is discussed. The maskless ultra short pulse illumination by femtosecond laser pulses with 780nm wavelength for generation of microstructures in positive photoresist is demonstrated. To assure high quality structure surfaces, the influence of processing parameters on the pattern width are studied. Therefore, the influence of average power of femtosecond laser pulses, as well as the influence of the scan velocity on the resolution for 3D-grid structure is experimental tested. These studies enable fabrication of arbitrary 3D-structures with high resolution and high aspect ratio. By utilizing a commercial positive photoresist, a minimum linewidth in submicron order with essential rise of the aspect ratio has been accomplished. Furthermore, the test of a precise structuring of complex geometry fabrication is presented with an elastomeric molding. Based on these results, the application possibility for LIGA-process is demonstrated.

Keywords: positive photoresist, two-photon nanolithography, LIGA


Laser Power Modulation to Minimize the Electrical Resistance of Auminum-Copper Welds
Florian Fetzer, Michael Jarwitz, Rudolf Weber, Thomas Graf

Power modulated overlap-welding of Cu-OF to Al99.5 was investigated with the aim of minimizing the welds electrical resistance. In-situ X-ray videography was used to measure the intrusion depth of copper into the aluminum sheet with acquisition rates of up to 5 kHz. Temperatures in the weld pool were measured synchronously and a concurrence of both signals is found. It is shown that laser power modulation at frequencies from 100 Hz to 400 Hz can increase the process stability, whereas its influence on the electrical resistance of the joints is negligible. Beside, an influence of the top material on this resistance was found and we claim the aluminum-top configuration to be superior.

Keywords: Laser welding, diagnostics, dissimilar metals, electrical resistivity ;