Laser Micromachining Processes Research Articles

Model for designing process strategies in ultrafast laser micromachining at high average powers

The great potential to scale the productivity of laser micromachining processes offered by the development of ultrafast lasers with average powers exceeding 1 kW comes at the cost of new physical and technical challenges. The large number of adjustable parameters, the different physical process limits, and the various possible optimization strategies complicate the design of suitable laser micromachining processes that enable high quality and high throughput.In this contribution, a comprehensive model is proposed that helps to identify the optimization strategies and process parameters required for the scaling of laser micromachining to high throughput at high average laser power without exceeding the process limits to ensure high surface quality.The model was experimentally verified on the example of laser micromachining of pockets in metals and silicon using an ultrafast laser with an average power of 1.01 kW, a pulse duration of 600 fs, different pulse energies, pulse bursts, and beam diameters. As a result, high material removal rates exceeding 120 mm3/min were achieved for silicon, stainless steel, copper, and aluminum, which exceed previously achieved removal rates by one to two orders of magnitude. The machined surfaces exhibit high quality as confirmed by the low measured roughness of about 1 µm.

In-Situ Depth Measurement of Laser Micromachining

Precision laser micromachining plays an important role in the biomedical, electronics, and material processing industries. During laser drilling, precision depth detection with micrometer-level resolution is required, particularly with blind-hole or heterogeneous structures. We present an optical detection system utilizing an optical confocal structure, experimentally confirmed to achieve a >95% accuracy for micron-diameter holes that are tens-of-microns deep. This system can be easily integrated into commercial laser micromachining processes, and can be employed in laser drilling and three-dimensional active-feedback laser printing.

Development and investigations on a hybrid tooling concept for coaxial and concurrent application of electrochemical and laser micromachining processes

In hybrid laser-electrochemical micromachining, both laser and electrochemical process energies act along the same machining axis and thus both influence material removal by their interaction. The traditional nozzle based laser assisted jet-ECM concepts require laser to be focused on workpiece surface in an electrochemical environment and are limited in aspect ratios as the nozzle stays above the workpiece surface. In this work, a hybrid tooling concept is proposed for a novel process scheme of precision hybrid laser-electrochemical micromachining. The tool serves the function of both an ECM electrode as well as a leaky-type multimode waveguide for the laser and delivers laser homogeneously together with the electrolyte on the workpiece surface without requiring laser to be focused on the workpiece surface. A precision prototype hybrid machine-tool is developed which employs short pulsed nano-second laser and micro-second pulsed voltage source for precision micromachining. For this system, ray tracing and detailed multiphysics electrochemical micromachining process simulations are carried out to demonstrate the applicability of this hybrid tooling concept and explain the shape evolution. Successful experimental realization of coaxial and concurrent application of electrochemical and laser processes is presented. Prototype tool electrodes are fabricated and experiments are carried out on an in-house developed prototype hybrid machine tool. The results reveal that the proposed hybrid tool is successfully capable of concentrating laser and electrochemical process energies simultaneously in the same machining zone. However, with the initial design of this hybrid tool, a maximum of 30–40% of laser power is available in the machining zone. Some suggestions for further research will be presented.

Ultrafast stamping by combination of synchronized galvanometer scanning with DOE’s or SLM

The up-scaling of laser micromachining processes with ultrashort pulses is limited due to heat accumulation and shielding effects. Multi beam scanning represents one of the strategies to overcome this drawback. It is in general realized by combining a diffractive beam splitter with a galvanometer scanner. A full synchronization with the laser repetition rate offers new possibilities with minimum thermal impact. We will demonstrate this by means of a multipulse-drilling on the fly process with a regular 5x5 spot pattern having a spot to spot spacing of 160µm. At a repetition rate of 100 kHz and an average power of 16 W we were able to drill more than 2’300 holes/s in a 10µm thick steel foil. We have further extended this technology with a special light modulator for different periodic spot patterns and more complex intensity distributions.

Numerical simulation and experimental study on laser micromachining of 304L stainless steel in ambient air

The evolution of surface topography of 304L stainless steel, induced by a fiber laser with a wavelength of 1070 nm and a pulse width of 10 μs in ambient air, is studied experimentally in the fluence range between roughly 30 J/cm2 and 118 J/cm2. A two-dimensional (2D) transient model is developed to simulate the laser micromachining process of 304L stainless steel, which incorporates the heat and mass transfer as well as fluid flow. An oxidation kinetics model is established to describe the mass inflow caused by high temperature oxidation of stainless steel in both solid and liquid phases, and the oxygen concentration on the surface of the molten liquid is treated as a function of temperature and then applied to the expression of surface tension coefficient. A modified level-set method is developed to trace the free surface and consider the mass transfer caused by oxidation and evaporation, and the possible effects on the free surface boundary containing surface tension, recoil pressure, Marangoni effect and the shear stress caused by metallic vapor flow are coupled in the model. Based on this model, the impacts of laser fluence, ambient air and pulse duration on both laser micromachining processes and the evolution of surface topography from the bump to the crater are numerically investigated. Comparing the experimental results and the numerical results, the validity of the model is verified, and the formation mechanisms of both various topographies and phenomena during laser micromachining are analyzed. The results show that with the increase of laser fluence the irradiated zone presents the “flat-topped” bump, the “M-shaped” bump and the “Gaussian-like” crater successively. Furthermore, the mass inflow has a great influence on the topography of both the “flat-topped” bump and the “M-shaped” bump. Even though the surface tension considering oxygen concentration plays an important role in the formation of both the “M-shaped” bump and the “Gaussian-like” crater, evaporation largely determines the surface topography of the “Gaussian-like” crater at high laser fluences.

Influence of pulse repetition rate on morphology and material removal rate of ultrafast laser ablated metallic surfaces

Ultrafast laser ablation is an efficient method for precise micro-machining. Thanks to the recent development of high repetition rate ultrafast lasers, high speed laser scanning of surfaces is more and more employed to generate micro-/nano- surface structures on metals for a vast variety of applications. Issues associated with these lasers are also identified in micro-machining practice. It is commonly believed that, due to possible shielding effects, the conditions of high fluences and high repetition rates compromise an efficient material removal. However, in this study, based on topography and differential weighing evaluations, we report that the material removal rate holds constant even in sub-MHz regime, up to about 20 J/cm2. The morphology of the post-irradiated surface is found to be determined not only by laser processing conditions but also by the material properties on the other hand. Two trends are experimentally identified in surface laser ablation of Ni, Cu, titanium alloy TA6V and stainless steel 316L: while the former two show a low roughness (Ra below 0.5 µm) at all irradiation conditions, the machining quality of the two later ones degrades rapidly with increasing fluences and repetition rates. In such a scenario, a rugged surface layer of 10–20 µm thickness is formed with the presence of numerous subsurface voids. Microstructural analysis is carried out in order to infer physical transition involved in the micro-machining process. Possible mechanisms accounting for the observation are discussed, especially those related to the electron-phonon coupling, plasma dwelling, and capillary waves. These insights pave the way for tailored, material dependent optimizations of ultrafast laser micro-machining processes.

Low-cost camera based laser power monitoring and stabilizing for micro-hole drilling

In this study, a laser power monitoring and stabilizing system for micro-hole drilling based on optical outputs of a CMOS sensor is presented. The correlation between the laser power and the average brightness of the beam spots on the images was investigated. The estimated laser power was used to build an on-line closed loop control of micro-hole drilling. Experimental results showed that the shortest response time of the laser monitoring system was only 30 ms, which was much faster than a thermopile power meter. The average measuring error was 3%, compared with thermopile power meter. In the PCB drilling experiments of 30 kHz pulse repetition frequency, the PID controller could compensate the power disturbances from 38.4% to 1.8%, and the aspect ratio from 20% to 5%. This study demonstrates the feasibility to develop a low-cost laser power monitoring and stabilizing system in laser micromachining processes.

Dual-Phase-Lag Heat Conduction in Finite Slabs With Arbitrary Time-Dependent Boundary Heat Flux

Applying a constant or transient heat flux on a plane slab is a common technique in microelectronics technology and material processing, including laser patterning, micromachining, and laser surface treatment processes. Although Fourier's law is typically very precise for evaluating temperatures in solids, a number of experimental observations suggest the existence of non-Fourier transient conduction in these applications. Since the dual-phase-lag (DPL) model of heat conduction can be compatible with the hypothesis of local equilibrium thermodynamics (as shown here), the effects of temperature gradient relaxation time on the non-Fourier hyperbolic conduction in a finite slab subjected to an arbitrary time-dependent surface heat flux is examined by this model. The combination of diffusion- and wave-like features in heat conduction process is properly monitored by the DPL model for two types of heat flow regimes, namely, gradient precedence and flux precedence. The results indicate considerable deviations between the predictions of these regimes.

UV 레이저 미세 가공공정에서의 물 액적 렌즈 효과에 관한 연구

Recently UV laser micromachining processes is widely introduced to meet the needs of advanced components of IT, BT and ET industries. Due to the characteristics of non-contact and high-speed laser processing, UV laser micromachining is applied to manufacture very thin substrate such as polymer, metals and composite. These minimum line width obtained by UV laser micromachining is generally determined from laser wavelength, optical lens and its numerical aperture. In this paper we will show the lens effect of water droplet on the surface of workpiece to reduce the line width when UV laser light is irradiated and focused through the water droplet. Because of the refraction effect generated by the semi-spherical or spherical shape of water droplet, we can find smaller line width. And water droplet could not only protect thermal deformation, but also carry away burr around micro dent. Firstly fundamental theory of minimum line width was derived from relationship between the geometry of water droplet and laser light trace, and then experimental and simulation results will be finally compared to verify the effectiveness of water droplet lens effect of UV laser micromachining process.

Capillary wicking of liquid lithium on laser textured surfaces for plasma facing components

Liquid lithium has been proposed as a coating for plasma facing components (PFC) in fusion reactors in order to reduce recycling and protect the PFCs from high heat loads. Lithium can be introduced onto the surface of the PFCs through a passive capillary wicking mechanism. Surfaces capable of wicking molten lithium to support passive cooling are traditionally produced using a porous material which is added to the surface of a structural material. Laser micro-machining or texturing processes, on the other hand, allow functional surfaces to be applied directly onto existing structural materials. Both 316L stainless steel and titanium–zirconium–molybdenum (TZM) alloys were textured using a Nd:VO4 laser to produce a pattern of open channels with depths between 40μm and 260μm. Screening tests using an isopropanol solution were used to evaluate the ease of wicking for different texturing geometries, and molten lithium wicking experiments were then performed with the samples placed at 45° and 90° orientations. In order to quantitatively measure the progress of the wicking front, a time of flight (TOF) technique, which measures the progress of a temperature gradient using pre-placed thermocouples, was developed to track the progress of the lithium wicking front. The laser textured surfaces on both the 316L and TZM alloys displayed the ability to fully wick both isopropanol and liquid lithium and outperformed commercially available porous materials in the wicking of an isopropanol solution. A comparison of the different texturing geometries on 316L stainless steel samples shows that deeper grooves in the wicking direction combined with shallow grooves, of approximately half the depth, in the cross-channel direction improve the wicking rate and uniformity, and achieve full wetting of the sample surface.

R2R-Technologies for the Production of OPV

As research and development for organic electronic devices shifts into high gear the need for larger-than-lab production equipment becomes evident. As experts in laser micro machining, 3D-Micromac started the development of laser based roll-to-roll patterning processes for organic electronic devices six years ago.Working hand in hand with leading research partners, 3DMicromac strives to offer complete modular production lines including not only the laser patterning step but also deposition processes such as coating and printing, as well as required processes for thermal treatment, packaging and separation steps.For the coating step the so-called “slot-die coating” technology is proposed as an easy, precise and effective way for applying solution-based materials to a substrate. Depending on the ratio of solid parts, such as metal nano scaled particles or conjugated polymers, dry layers of several microns down to 30nm can be deposited with a very homogenous distribution. Typical values for the relative thickness variation of the wet film range from 10% down to 1% in cross direction and machine direction making the slot-die coating technology a suitable fit for post process steps with high demands on the layers homogeneity.In order to achieve the best possible efficiency of the OPV follow-up process steps like drying and annealing are of utmost importance. 3D-Micromacs solution for these thermal treatments is the integration of micro-wave and hot air into a single drying and annealing system. In contrary to pure classic hot air ovens the integration of microwave technology allows for a very efficient energy transport into the material which reduces the operational costs and increases the maximum throughput of the oven. However, as the microwave radiation has to be able to interact with the material, the use is commonly known to be limited to a very small range of materials. The major and most obvious use of microwave technology therefore is the drying of water based thin films (comparable to consumer electronic microwaves), but the technology is also able to introduce energy into electrical conductive materials and thin films with a surface resistivity of down to 7Ω/□. This feature enables the use for the production of OPV based on substrates with TCO layers. In device layouts the energy can be introduced into the TCO material and the generated heat is then dissipated into the surrounding material and can be used for not only drying but also annealing steps. The required control of the energy distribution in the oven is realized through a special design of the dryer chamber and magnetrons. With this approach the overall size and energy demand of the unit compared to a dryer using solely hot air can be reduced significantly.The patterning step for the functional surface covering layer can be realized by laser micromachining processes applying ultrashort pulsed laser sources. These laser sources provide laser pulses with pulse durations well below 10ps which is necessary in order to minimize negative effects such as particle generation bulging and heat affected zones making them feasible for thin film applications. Furthermore different wavelengths and optical setups for the beam guidance can be chosen depending on the material system and the layout of the OPV.The combination of such a variety of processes into one modular production line, 3D-Micromac's microFLEX, which not only meets the requirements for a controlled environment but also allows for a later change of the process sequence or an easy extension of existing capabilities is unique in the current landscape of available equipment and will be presented in the talk.

Measurement of femtosecond laser-induced damage and ablation thresholds in dielectrics

The paper is focused on the importance of accurate determination of surface damage/ablation threshold of a dielectric material irradiated with femtosecond laser pulses. We show that different damage characterization techniques and data treatment procedures from a single experiment provide complementary physical results characterizing laser–matter interaction. We thus compare and discuss two regression techniques, well adapted to the measurement of laser ablation threshold, and a statistical approach giving the laser damage threshold and further information concerning the deterministic character of femtosecond damage. These two measurements are crucial for laser micromachining processes and high peak-power laser technology in general.

Laser-induced damage threshold of sapphire in nanosecond, picosecond and femtosecond regimes

The surface laser-induced damage threshold fluence of sapphire is determined under various experimental conditions concerning the material irradiation (femtosecond, picosecond and nanosecond temporal regimes) and preparation (surface state). The results are of interest for optimising laser micromachining processes and for robust operation of high-peak power femtosecond Ti:sapphire laser chains.

The reduction of the threshold fluence at laser-induced backside wet etching due to a liquid-mediated photochemical mechanism

The determination of materials erosion thresholds is of key importance for laser micromachining processes themselves and their applications. At laser-induced backside wet etching (LIBWE) of fused silica with organic liquid toluene employing an excimer laser (λ = 248 nm, τp = 25 ns), the threshold fluence is reduced from 1.10 J cm−2 to 0.22 J cm−2 with an increasing number of pulses from a single pulse to an almost infinite quantity, respectively. A similar effect is observed with an increasing laser spot size. Here, the threshold fluence decreases from 0.52 to 0.30 J cm−2 with the growing size of the laser spot from 400 to 4225 500 µm2. As the origin of the incubation effect, a laser-induced liquid-mediated process of surface modification of the fused silica sample is discussed. A first incubation model for LIBWE is developed and applied to the experimental data that comprise an enhancement of the solid–liquid interface absorbance by a layer of decomposition products from photochemical fragmentation of the liquid.

Microfabrication in free-standing gallium nitride using UV laser micromachining

Gallium nitride (GaN) and related alloys are important semiconductor materials for fabricating novel photonic devices such as ultraviolet (UV) light-emitting diodes (LEDs) and vertical cavity surface-emitting lasers (VCSELs). Recent technical advances have made free-standing GaN substrates available and affordable. However, these materials are strongly resistant to wet chemical etching and also, low etch rates restrict the use of dry etching. Thus, to develop alternative high-resolution processing for these materials is increasingly important. In this paper, we report the fabrication of microstructures in free-standing GaN using pulsed UV lasers. An effective method was first developed to remove the re-deposited materials due to the laser machining. In order to achieve controllable machining and high resolution in GaN, machining parameters were carefully optimised. Under the optimised conditions, precision features such as holes (through holes, blind or tapered holes) on a tens of micrometer length scale have been machined. To fabricate micro-trenches in GaN with vertical sidewalls and a flat bottom, different process strategies of laser machining were tested and optimised. Using this technique, we have successfully fabricated high-quality micro-trenches in free-standing GaN with various widths and depths. The approach combining UV laser micromachining and other processes is also discussed. Our results demonstrate that the pulsed UV laser is a powerful tool for fabricating precision microstructures and devices in gallium nitride.

Experimental study on integration of laser-based additive/subtractive processes for meso/micro solid freeform fabrication

Solid freeform fabrication has attracted considerable attention lately because of its ability to build a 3D structure with a complex and arbitrary shape. This work presents initial studies to adapt this technology for the fabrication of meso-/micro-3D structures. A pulsed Nd:YAG laser was used for laser microdeposition (additive) and micromachining (subtractive) processes. An ultrasonic-based micropowder feeding system was developed to generate precise patterns of micropowders on a substrate without any pre-processing. Laser microdeposition of copper and stainless steel micropowders was accomplished. The characterization of micromachining was performed on stainless steel and copper plates with a laser beam of wavelengths of 355 nm and 266 nm. The integration of laser microdeposition and micromachining processes improved the resolution and edge quality of the meso-/micropatterns.

Micro Scale Laser Shock Processing of Metallic Components

Laser shock processing of copper using focused laser beam size about ten microns is investigated for its feasibility and capability to impart desirable residual stress distributions into the target material in order to improve the fatigue life of the material. Shock pressure and strain/stress are properly modeled to reflect the micro scale involved, and the high strain rate and ultrahigh pressure involved. Numerical solutions of the model are experimentally validated in terms of the geometry of the shock-generated plastic deformation on target material surfaces as well as the average in-depth strains under various conditions. The residual stress distributions can be further influenced by shocking at different locations with certain spacing. The potential of applying the technique to micro components, such as micro gears fabricated using MEMS is demonstrated. The investigation also lays groundwork for possible combination of the micro scale laser shock processing with laser micromachining processes to offset the undesirable residual stress often induced by such machining processes.

Sputtered coatings for microfluidic applications

Magnetron sputter-deposited features and coatings are finding a broad range of uses in microfluidic devices being developed at the Pacific Northwest National Laboratory. Such features are routinely incorporated into multilayer laminated microfluidic components where specific functionality is required, and where other methods for producing these features have been deemed unacceptable. Applications include electrochemical sensors, heaters and temperature probes, electrical leads and insulation layers, piezoelectric actuators and transducers, and chemical modification of surfaces. Small features, such as those required for the production of microsensor electrodes or miniature resistive heaters on microfluidic chips, were patterned using standard lithographic methods, or with masks produced by laser micromachining processes. Thin-film piezoelectric materials such as aluminum nitride have been deposited at low temperatures for use with temperature sensitive materials. Use of the coating technology and its application in the fabrication of specific microfluidic devices, including a groundwater sensor, miniature piezoelectric ultrasonic transducers and actuators, a polymerase chain reaction thermal cycler, and a microchannel flow diagnostic device, are discussed. Technical issues associated with these coatings, such as adhesion, chemical resistance, and surface defects are also addressed.

Combination of excimer laser micromachining and replication processes suited for large scale production

Results are reported obtained by a process sequence comprising excimer laser ablation of microstructures, inverse replication by an electroplating process and second inverse replication by injection moulding (“Laser-LIGA”). Thus, the original microstructures, made from PMMA and novolak layers applying a commercial 193 nm excimer laser micromachining work-station and showing structural variety and aspect ratios up to 10, were replicated with high structural fidelity by a low cost process suited for large scale production.