Ultrashort Pulse Laser Micromachining Research Articles

Influence of Oxide Formation Following Ultrashort Pulsed Laser Micromachining on Self‐Propagating Reactions in Free‐Standing Ni/Al Reactive Multilayer Foils

Reactive metallic multilayers, renowned for their exothermic self‐propagating reactions, show a distinct ability to achieve minimal thermal influence in processes such as welding and soldering, where minimizing the thermal impact on the material being joined is crucial. Ultrashort pulsed lasers offer precision micromachining with negligible thermal damage, making them ideal for processing such sensitive materials. However, the formation and redeposition of oxide phases is a commonly observed side effect, especially when structuring in ambient air. This study investigates the effects of oxide formed after femtosecond laser treatment on self‐propagating properties, including propagation velocity and microstructure of Ni/Al reactive multilayer foils. Samples with varied line spacings between laser‐cut trenches are created. Scanning electron microscopy, high‐speed camera videography, and image analysis are employed to analyze the microstructure and quantify velocities. Thermal simulations enhance the understanding of the oxide's role in self‐propagating dynamics. The findings suggest that even a small oxide layer significantly decelerates the self‐propagating reaction. The oxide functions as a thermal ballast by absorbing the thermal energy generated during the reaction, without actively participating in the reaction mechanism.

Polarization effects on Laser-Inscribed angled Micro-Structures

The polarization of the laser beam exhibits more substantial differences in laser micromachining as the angle of incidence deviates from zero. In the reported work, our focus was to explore the effects of circularly, p- and s-polarized laser on angled ultrashort pulse laser micromachining of micropillar arrays. The examination encompassed laser process factors, including angles of incidence, microstructure dimensions, and inter-pillar spacing. A comparison between the resulting structures demonstrated that p-polarized laser beam was the most efficient in material removal in angled laser micromachining, followed by circularly polarized laser. While the s-polarized beam exhibited the lowest ablation efficiency among the three. Such distinction is mainly attributed to the distinguishing reflectivity of the three states of polarization on tilted planes. The development of structural heights during ablation processes was examined, and potential defects in laser processing methodologies were interpreted. The dependency of structural heights on inter-pillar spacing was analyzed. This study bridges the gap between existing studies on angled ultrashort pulse laser machining and the influences of polarization on laser machining. The comparison between structures produced using laboratory-scale and industrial-scale laser systems also yielded pertinent recommendations for facilitating a smooth transition of angled laser micromachining from laboratory-scale research to industrial applications.

Ultrashort pulse laser micro drilling of silicon

Ultrashort pulse laser processing of materials induces negligible thermal effects on materials, thereby reducing the post-processing operations, which makes the process a “green manufacturing process”. In the present research work, micro holes are fabricated in silicon using ultrashort pulse laser micromachining with pulse energy of 40 µJ at high repetition rates from 100 to 500 kHz. The hole depths obtained are 16.7, 17.8, 28.1, 33.9, and 32.2 µm at repetition rates of 100, 200, 333, 400, and 500 kHz, respectively. An analytical model is used to estimate the surface temperature obtained as 2000, 2800, 3700, 4000, and 4500 K at the center of the micro holes at repetition rates of 100, 200, 333, 400, and 500 kHz respectively. It is observed that the saturation of surface temperature occurred after a certain number of pulses. Due to the saturation of temperature after a certain number of pulses, the increase in micro hole depth gets saturated. The present investigation is beneficial for producing micro holes in silicon which are highly demanding for MEMS, photonics, solar cells, and microfluidic applications.

Ultrashort Pulse Laser Micromachining of Silicon: Effect of Repetition Rate and Assessment of Surface Integrity of Microchannels

Ultrashort Pulse Laser Micromachining of Silicon: Effect of Repetition Rate and Assessment of Surface Integrity of Microchannels

Accelerating ultrashort pulse laser micromachining process comprehensive optimization using a machine learning cycle design strategy integrated with a physical model

Accelerating ultrashort pulse laser micromachining process comprehensive optimization using a machine learning cycle design strategy integrated with a physical model

Focal zone engineering with hollow spatially variable waveplates applicable in laser micromachining

The ultrashort pulsed laser micromachining of transparent and opaque materials has become a mature technology providing microscale accuracy. While the ultrashort laser sources are being constantly developed with growing average power or more complex pulse temporal control, studies in the spatial domain can also lead to improving the performance i.e., higher process speed, better fabrication quality, etc. By purposely transforming the Gaussian beam of the laser output to a custom beam shape we are able to improve speed and quality in many laser micromachining tasks, where micro-welding of various materials is one of them. In this work, we apply custom hollow spatially variable waveplates based on nanograting technology (manufactured by Workshop of Photonics) that help to form curved or flat intensity distribution beams. Using numerical modeling and experimental research we will demonstrate beam transformations that could have an impact on enhancing the micro-welding process in terms of reduced stress or better heat dissipation.

Heat accumulation temperature measurement in ultrashort pulse laser micromachining

Ultrashort pulse laser micromachining is affected by the heat accumulation resulting from the residual heat from previous laser pulses on the sample surface. Up to now, most of the works analysed the accumulation by numerical modelling. The present work focussed on development and application for the first time of a measurement system of heat accumulation temperature directly during the processes in nanosecond and microsecond time ranges. The measurement system was based on the infrared radiometry and contained liquid nitrogen cooled fast HgCdTe photodetector and paraboloid mirrors. Micromachining of grooves was done using a 14 W picosecond laser with different pulse energies, repetition frequencies and scanning speeds. Calibration of the measurement system was done in order to obtain temperatures from the measured signal. The calibration was not straightforward due to very small laser spot (25 μm), small signal and changing of the size of the heated area for low scanning speeds. Obtained heat accumulation temperature ranged from 300°C to 2600°C for scanning speeds from 8 m/s to 0.07 m/s and pulse energies from 0.1 µJ to 100 µJ. According to the scanning electron microscope (SEM) images, the material was already partially melted (small droplets on boarders) for low scanning speeds. Surface roughness and ablation rate were determined by 3D confocal laser microscope. Good correlation was found between the roughness and the heat accumulation temperature, thus confirming the validity of calibration. Measured heat accumulation temperature was surprisingly the highest for the most efficient ablation parameters and at the same time low surface roughness was achieved.

Investigation of an interlaced laser beam scanning method for ultrashort pulse laser micromachining applications

Abstract This article investigates picosecond and sub-picosecond laser micromachining of Borofloat®33 glass and provides clear evidence that a simple modification of the laser beam scanning strategy can lead to significant improvement of machining efficiency and hence process throughput. Besides studying the impact of the fundamental laser machining parameters, such as laser fluence, pulse overlap, pulse repetition frequency (PRF), pulse duration and laser spot diameter, on the machined depth, surface roughness and material removal rate (MRR), it also compares the machining results for two different laser beam scanning strategies, called here sequential” method (SM) and “interlaced” method (IM). By changing the scanning strategy from SM to IM, the MRR can be significantly increased because IM allows high-quality machining of the glass at higher PRF values. The experimental results show that this simple, cost-free modification allows the MRR value to be increased by more than 4 times, i.e. from 0.12 mm3/s to 0.53 mm3/s. Moreover, by using a Phantom V2512 high-speed camera, the picosecond laser micromachining process using both SM and IM was filmed. The videos show that SM leads to the accumulation of glass particles within the laser-machined area, whereas in IM the glass material is removed layer by layer which leads to the generation of “cleaner” and deeper areas. The mechanisms associated with these machining improvements are discussed.

Efficient and damage-free ultrashort pulsed laser cutting of polymer intraocular lens implants

Ophthalmic intraocular lenses are conventionally machined by diamond tools. A promising alternative approach is contour cutting by ultrashort pulsed laser micromachining. To improve process knowledge, a parametric study of picosecond and femtosecond laser machining of medical grade hydrophilic copolymers and PMMA is carried out. Material removal rates and machining quality with respect to main process parameters are determined. Reasons for chipping and formation of heat affected zones are identified and an optimized process strategy is derived. By choosing a defined pulse overlap, heat accumulation is kept minimal while increasing absorptivity through incubation avoids chipping.

Increasing the Specific Removal Rate for Ultra Short Pulsed Laser-Micromachining by Using Pulse Bursts

Increasing the Specific Removal Rate for Ultra Short Pulsed Laser-Micromachining by Using Pulse Bursts

Ablation threshold dependence on incident wavelength during ultrashort pulsed laser ablation

Ultrashort pulse laser micromachining is an advanced materials processing technique that allows cold of almost any material. This is especially of interest in the semiconductor industry, where mechanical cutting of wafers generates large amounts of waste - if laser micromachining could be applied here, there is the potential for huge increases in the efficiency and flexibility of semiconductor manufacturing. The biggest barrier to industrial application of this technology is the cutting speed, however by tailoring the pulse properties, we hypothesise that machining speed can be increased greatly. The commonly used metric for evaluation of laser micromachining is the ablation threshold - the energy density required to cause material ablation. In this study the effect of incident laser wavelength on the ablation threshold for materials of interest for microfabrication (e.g., silicon) was investigated. This was achieved using a Ti:Sapphire pumped optical parametric amplifier (TOPAS-C) producing femtosecond pulses (τ = 110 fs, repetition rate = 1 kHz) with wavelengths ranging from 400 nm to 1200 nm using the D-Scan technique. Future work will employ advanced beam shaping technology to tailor pulses in both the spatial and temporal domains to further improve machining efficiency.

The Next Generation of Ultra‐short Pulsed Laser Micro Processing

AbstractUltra‐short pulsed (USP) laser processes for industrial applications put high demands on the machine concept: The need for high flexibility in the machine configuration, constant machining results over long process cycles and efficient laser processes to reduce processing costs require a holistic approach in the design of a laser micro machine. For the development of a new laser machine for USP laser micromachining, the joint venture Micar combines expert competences in the fields of laser system technology, sheet metal system design and manufacturing and simulation of mechanical and fluid systems. The result is a new machine concept based on a mineral casted machine base for high flexibility in the machine structure and application.

Ultrashort-laser-pulse machining characteristics of aluminum nitride and aluminum oxide

This paper analytically investigates the ablation characteristics of aluminum nitride and aluminum oxide irradiated by an ultrashort pulse laser. Aluminum oxide or aluminum nitride has been one of the main ceramic packaging materials used for microelectronics packaging. However, a more extensive use of these ceramics has been limited due to their high inherent hardness and brittleness. An ultrashort pulsed laser micromachining could be a viable technology for these ceramics. The hyperbolic thermal model instead of heat transfer equation based on Fourier’s law is employed to study the ablation rate of an ultrashort laser pulse machining for aluminum nitride and aluminum oxide. The ablation depth per pulse predicted by this study agrees with the available experimental data. The delay of temperature rise is presented for the case of laser pulse duration shorter than thermal diffusion relaxation time. This study also demonstrates the reasons why high temperature is obtained for an ultrashort laser pulse machining of materials. The distinctions of properties such as thermal diffusivity, optical penetration depth, thermal expansion coefficient and reflectivity result in the differences of the ablated depth per pulse between aluminum nitride and aluminum oxide.

Liquid-immersion laser micromachining of GaN grown on sapphire

Liquid-immersion nanosecond-pulsed laser micromachining is introduced as an efficient way for device isolation and rapid prototyping on GaN-on-sapphire wafer. Using deionized water as an ambient medium for laser micromachining, smooth trenches that are free from redeposition can be formed in the GaN layer. Coupled with the large difference between the ablation thresholds and ultraviolet absorption coefficients of GaN and sapphire, the GaN/sapphire interface can be left undamaged after the ablation process. This technique overcomes the limitation of heat accumulation in nanosecond-pulse regime, and offers a cost-effective alternative to ultrashort-pulse laser micromachining. In this report, the advantages offered by liquid immersion are elucidated in terms of improved heat conduction, increased plasma-induced recoil pressure due to water confinement, weakened plasma shielding effect in water, and the collapse of cavitation bubbles. Simulation results show that the reduced fluctuation of temperature profile over time in water could be correlated with the reduced redeposition of Ga from thermal decomposition at the trench sidewalls.

Picosecond pulsed laser ablation and micromachining of 4H-SiC wafers

Ultra-short pulsed laser ablation and micromachining of n-type, 4H-SiC wafer was performed using a 1552 nm wavelength, 2 ps pulse, 5 μJ pulse energy erbium-doped fiber laser with an objective of rapid etching of diaphragms for pressure sensors. Ablation rate, studied as a function of energy fluence, reached a maximum of 20 nm per pulse at 10 mJ/cm 2, which is much higher than that achievable by the femtosecond laser for the equivalent energy fluence. Ablation threshold was determined as 2 mJ/cm 2. Scanning electron microscope images supported the Coulomb explosion (CE) mechanism by revealing very fine particulates, smooth surfaces and absence of thermal effects including melt layer formation. It is hypothesized that defect-activated absorption and multiphoton absorption mechanisms gave rise to a charge density in the surface layers required for CE and enabled material expulsion in the form of nanoparticles. Trenches and holes micromachined by the picosecond laser exhibited clean and smooth edges and non-thermal ablation mode for pulse repetition rates less than 250 kHz. However carbonaceous material and recast layer were noted in the machined region when the pulse repetition rate was increased 500 kHz that could be attributed to the interaction between air plasma and micro/nanoparticles. A comparison with femtosecond pulsed lasers shows the promise that picosecond lasers are more efficient and cost effective tools for creating sensor diaphragms and via holes in 4H-SiC.

Ultrashort pulse laser micromachined microchannels and their application in an optical switch

The capability of direct writing makes ultrashort pulse laser significant in the microfabrication of MEMS devices based on polymer and glass. In particular, nanosecond and femtosecond lasers are able to transfer the adequate energy in femtosecond intervals for the removal of the materials. Because of its advantages, just like the small feature size, smooth finishing surface, flexible structuring and the minimum thermal effect, ultrashort pulse lasers have become a convincing technique with the high peak power. This paper presents the femtosecond laser machining results of the polycarbonate, aluminosilicate glasses and nanosecond laser machining of aluminosilicate glasses. The microchannels with the critical micron-scale dimensions and the sub-micron scale surface roughness were achieved by the optimized operating parameters of the laser. The major influence factors such as cutting speed, power energy, and power stability were analyzed to obtain the optimized parameters for the fabrication of the microchannels for a bubble switch. The ultrashort pulse laser micromachining was applied in the prototype of a bubble optical switch. By miniaturization of the structure of the microchannel, the switch speed can be promisingly improved.

Laser ablation and micromachining with ultrashort laser pulses

The mechanisms of ultrashort-pulse laser ablation of materials are discussed, and the differences to that of long laser pulses are emphasized. Ultrashort laser pulses offer both high laser intensity and precise laser-induced breakdown threshold with reduced laser fluence. The ablation of materials with ultrashort pulses has a very limited heat-affected volume. The advantages of ultrashort laser pulses are applied in precision micromachining of various materials. Some femtosecond laser pulse micromachining results, including comparison with long pulses, are presented. Ultrashort-pulse laser micromachining may have a wide range of applications where micrometer and submicrometer feature sizes are required.