Excimer Laser Micromachining Research Articles

A low-profile consolidated metastructure for multispectral signature management

This work pertains to the design, numerical investigation, and experimental demonstration of an optically transparent, lightweight, and conformable metastructure that exhibits multispectral signature management capabilities despite its extremely low-profile configuration. In comparison to the existing hierarchical approaches of designing multispectral stealth solutions, attention has been paid to accommodate the conflicting requirements of radar and infrared (IR) stealth using a single metasurface layer configuration, which required a few constraints to be incorporated during the design stage to ensure compatibility. This methodology promulgates the desired multispectral response with minimal manufacturing footprint and facilitates an efficient integration with the other existing countermeasure platforms. The resulting design exhibits a polarization-insensitive and incident angle stable broadband microwave absorption with at least 90% absorption ranging from 8.2 to 18.4 GHz. Concomitantly it also exhibits an averaged IR emissivity of 0.46 in the 8–14 µm long-wave IR regime, along with high optical transparency (71% transmission at 632.8 nm). Notably, the total thickness of the metastructure stands at 0.10 ( corresponds to the wavelength at lowest frequency). The metastructure has been fabricated with indium tin oxide coated polyethylene terephthalate sheets, on which the frequency selective pattern is machined using Excimer laser micromachining, and the performances are verified experimentally. Furthermore, a hybrid theoretical model has been developed that not only provides crucial insights into the operation of metastructure but also presents a methodical semi-analytical approach to design.

Wrapping of Curved Surfaces With Conformal Broadband Metamaterial Microwave Absorber

In this letter, three different wrapping methods i.e., circle having incisions (CI), pentagon and hexagon (P&H) and petals wrapping are proposed for covering of spherical shaped surfaces with flat microwave metamaterial absorber (MMA) without defects. To achieve the better performance, an extensive analytical study was carried for counting the number of unit cells and packing fraction in the aforementioned techniques. It was found that maximum number of unit cell can be wrapped over a spherical ball by using CI method. The maximum packing fraction was found to be in P&H method. Further, we studied variation in the number of unit cells and packing fraction for variation in the shapes (circle, square, and hexagon) and size (2–4 mm) of the unit cell. For experimental purpose, an indium tin oxide coated over polyethylene terephtalate (ITO/PET)-based conformal MMA was designed and then fabricated using excimer laser micromachining. The proposed conformal absorber gives broadband absorption from 6.21 to 15 GHz. Finally, the fabricated absorber was wrapped on a spherical ball and microwave response was measured. The measured microwave response was compared with the simulated response and found to be in good agreement with each other.

FPI-based fibre-optic sensor fabricated by excimer laser micromachining and fusion splicing HCF for multi-parameter measurements

An all-fibre splicing Fabry–Perot interferometer (FPI) sensor is successfully demonstrated for measurements of pressure, refractive index (RI) and temperature. The sensor structure is fabricated via fusing a tiny segment of hollow-core fibre (HCF) with single-mode fibre (SMF), and a micro-hole is micromachined on the HCF using a 193 nm excimer laser. Thus, the open micro-HCF can sense changes in air pressure and liquid RI. In contrast, the micro-SMF can only be used to measure temperature changes. The proposed sensor can be used for the measurements of air pressure, liquid RI and temperature. Experimental results show that the pressure resolution and liquid RI resolution of the sensor are 0.012 MPa and 10−5 RIU, respectively, and the temperature resolution is superior to be 0.08°C. The all-fibre sensor with the merits of small dimensions, easy fabrication and low cost can be used in harsh and small volume environments.

Study of excimer laser ablation of photoresist polymer in presence of hydrogen gas environment for micro-fluidic applications

Excimer laser micro-machining has been extensively explored to fabricate micro-patterns in the last few decades in the field of micro-fluidics, precision bearings, optics, micro-electronics etc in their previous work, it was observed by the authors that hydrogen gas has greater impact on the material removal rate and surface quality while ablating photo-resist polymer. In the present work, effect of hydrogen gas on the material removal rate has been mathematically modeled and simulated using finite element technique. In this simulation work, variation of surface temperature as well as ablation depth as a function of laser pulse energy have been analyzed. The simulation result showed an enhanced material removal rate in hydrogen environment due to laser assisted chemical reaction(LACR) involving nascent hydrogen and polymer. Further, a set of laser ablation experiments on polymer using 248 nm KrF Excimer laser has been conducted both with and without Hydrogen environment and under inert gas environment of argon, to validate the mathematical model. Results of the laser ablation experiments have shown the influence of hydrogen gas environment in enhancing both the material removal rate and the excellent edge definition, which is a novel outcome of the present research work and best suited process for the fabrication of micro-fluidic channels. The simulation results have been found in good agreement with the experimental data which supports technical soundness of the proposed model presented and discussed in the paper.

Excimer laser micromachining of indium tin oxide for fabrication of optically transparent metamaterial absorbers

Excimer laser micromachining has been extensively studied to machine Indium Tin oxide which is uniformly deposited over PET to realize transparent broadband metamaterial absorbers. Various micromachining parameters such as fluence and spot overlap are considered and optimized to ablate the ITO film with controlled damage to the PET substrate for obtaining desired roughness to achieve uniform optical transmittance. A correlation has been developed between the generated roughness and optical transmittance of the machined area, so that any desired value of the optical transmittance can be achieved. Parameters for machining were selected in such a way that optical transmittance between machined and non-machined area are closely matched. Two different structures are demonstrated over a large area of about 243 mm2. To reduce the machining time of such a large area, an algorithm has been developed to determine the size of the masks for machining complex structures of any shape. Further, a thermal model is presented for better understanding of the ablation phenomenon. We conclude that the machining of ITO on PET substrates occurs via delamination of the ITO from the PET surface rather than a melt vaporization of the ITO.

Biomimetic extracellular matrix coatings improve the chronic biocompatibility of microfabricated subdural microelectrode arrays.

Intracranial electrodes are a vital component of implantable neurodevices, both for acute diagnostics and chronic treatment with open and closed-loop neuromodulation. Their performance is hampered by acute implantation trauma and chronic inflammation in response to implanted materials and mechanical mismatch between stiff synthetic electrodes and pulsating, natural soft host neural tissue. Flexible electronics based on thin polymer films patterned with microscale conductive features can help alleviate the mechanically induced trauma; however, this strategy alone does not mitigate inflammation at the device-tissue interface. In this study, we propose a biomimetic approach that integrates microscale extracellular matrix (ECM) coatings on microfabricated flexible subdural microelectrodes. Taking advantage of a high-throughput process employing micro-transfer molding and excimer laser micromachining, we fabricate multi-channel subdural microelectrodes primarily composed of ECM protein material and demonstrate that the electrochemical and mechanical properties match those of standard, uncoated controls. In vivo ECoG recordings in rodent brain confirm that the ECM microelectrode coatings and the protein interface do not alter signal fidelity. Astrogliotic, foreign body reaction to ECM coated devices is reduced, compared to uncoated controls, at 7 and 30 days, after subdural implantation in rat somatosensory cortex. We propose microfabricated, flexible, biomimetic electrodes as a new strategy to reduce inflammation at the device-tissue interface and improve the long-term stability of implantable subdural electrodes.

Microfabricated intracortical extracellular matrix-microelectrodes for improving neural interfaces

Intracortical neural microelectrodes, which can directly interface with local neural microcircuits with high spatial and temporal resolution, are critical for neuroscience research, emerging clinical applications, and brain computer interfaces (BCI). However, clinical applications of these devices remain limited mostly by their inability to mitigate inflammatory reactions and support dense neuronal survival at their interfaces. Herein we report the development of microelectrodes primarily composed of extracellular matrix (ECM) proteins, which act as a bio-compatible and an electrochemical interface between the microelectrodes and physiological solution. These ECM-microelectrodes are batch fabricated using a novel combination of micro-transfer-molding and excimer laser micromachining to exhibit final dimensions comparable to those of commercial silicon-based microelectrodes. These are further integrated with a removable insertion stent which aids in intracortical implantation. Results from electrochemical models and in vivo recordings from the rat’s cortex indicate that ECM encapsulations have no significant effect on the electrochemical impedance characteristics of ECM-microelectrodes at neurologically relevant frequencies. ECM-microelectrodes are found to support a dense layer of neuronal somata and neurites on the electrode surface with high neuronal viability and exhibited markedly diminished neuroinflammation and glial scarring in early chronic experiments in rats.

Enhancement of Optical Fiber Coupling Based on Collimated and Focused Laser Diode Beam with Micro Aspherical Plano-Convex Lens Fabricated by Excimer Laser

ABSTRACTIn this article, we have proposed and demonstrated experimentally the double plano-convex micro-aspherical lens scheme to amplify an edge-emitting laser diode (LD) to optical fiber coupler. Both microlenses have two different profiles along their x- and the y-axes direction. The optical simulation software Zemax is used to design and optimize the proposed microlens profile and then fabricate by an excimer laser micromachining system. Focal spot size of LD around 9 µm is achieved experimentally. Light coupling efficiency between LD and optical fiber, using this double plano-convex microlens, is 27.57% and 44.23% for single-mode and multi-mode fiber, respectively.

Experiment and Prediction of Ablation Depth in Excimer Laser Micromachining of Optical Polymer Waveguides

Extending the data transfer rates through dense interconnections at inter‐ and intraboard levels is a well‐established technique especially in consumer electronics at the expense of more cross talk, electromagnetic interference (EMI), and power dissipation. Optical transmission using optical fibre is practically immune to the aforementioned factors. Among the manufacturing methods, UV laser ablation using an excimer laser has been repeatedly demonstrated as a suitable technique to fabricate multimode polymer waveguides. However, the main challenge is to precisely control and predict the topology of the waveguides without the need for extensive characterisation which is both time consuming and costly. In this paper, the authors present experimental results of investigation to relate the fluence, scanning speed, number of shots, and passes at varying pulse repetition rate with the depth of ablation of an acrylate‐based photopolymer. The depth of ablation essentially affects total internal reflection and insertion loss, and these must be kept at minimum for a successful optical interconnection on printed circuit boards. The results are then used to predict depth of ablation for this material by means of adaptive neurofuzzy inference system (ANFIS) modelling. The predicted results, with a correlation of 0.9993, show good agreement with the experimental values. This finding will be useful in better predictions along with resource optimisation and ultimately helps in reducing cost of polymer waveguide fabrication.

Maskless beam pen lithography based on integrated microlens array and spatial-filter array

A beam pen lithography system is proposed for ultraviolet (UV) patterning of complicated patterns in a maskless manner. The system consists of a single UV light-emitting diode (LED), a double-sided microlens/spatial-filter array (MLSFA), and a motorized x-y stage. The double-sided MLSFA contains a pinhole array sandwiched by two microlens arrays. The microlens arrays are fabricated by an excimer laser micromachining system to achieve accurate and optimized lens profiles obtained from optical simulation. Techniques have been developed in the fabrication processes to guarantee good alignment between the pinhole array and the two microlens arrays. Finally, arrayed UV spots are formed with feature size around 4 to 5 μm. Through mechanical movement of stage and intensity control on the UV LED, a number of complicated patterns are experimentally demonstrated using this type of beam pen lithography. It offers a simple, inexpensive, and portable choice on maskless UV patterning with great potential for future applications.

A ball valve micro-pump based on axially symmetrical nozzle fabricated by excimer laser micromachining technology

This paper presents a new type of ball valve micro-pumps with much simplified structure design and excellent performance. The key element in this micro-pump is a pair of ball valves implemented by confining a micro-ball within a nozzle. The nozzles are axially symmetrical with a specifically designed profile and are directly fabricated using excimer laser micromachining approaches. The ball valves are integrated with a flow chamber, an electromagnetically actuated PDMS membrane, and a coil. When passing an alternative current (AC) through the coil, the flow chamber is actuated to pump fluid flowing along one direction which is controlled by the ball valves. The performance of the proposed micro-pump is evaluated using an AC voltage in the range of 2-6 V and a frequency of 10-70 Hz. Experimental results show that a maximum flow rate of 389 μL/min and a maximum back pressure of 423 mm-H 2 O are achieved. It offers a low-cost, simple, and effective solution for a wide range of microfluidic pumping applications.

Optimization of femtosecond laser cutting of biodegradable polymer for medical devices manufacturing

This paper describes the experimental parameters involved in the femtosecond laser micromachining of biodegradable poly(L-lactide) which is frequently used in biomedical applications such as vascular stents or scaffolds. We investigated the influence of laser pulse energy, scanning strategy and number of overscans on the laser cutting throughput. The process parameters that enable reducing of a heat affected zone were determined. As a result, the optimal scanning strategy was determined in order to obtain high aspect ratio trenches in 380 ?m thick biodegradable polymer sheet. Full Text: PDF ReferencesW. Jia et al. "Effects of high-repetition-rate femtosecond laser micromachining on the physical and chemical properties of polylactide (PLA)", Opt. Express 23, 21 (2015). CrossRef F. Hendricks, R. Patel, and V.V. Matylistsky, "Micromachining of bio-absorbable stents with ultra-short pulse lasers", Proc. SPIE 9355, 935502 (2015). CrossRef W.Y. Yeong et al. "Annealing of Biodegradable Polymer Induced by Femtosecond Laser Micromachining", Adv. Eng. Mater. 4, 12 (2010). CrossRef K. Stolberg et al. "IR and green femtosecond laser machining of heat sensitive materials for medical devices at micrometer scale", Proc. SPIE 8968, 89680E (2014). CrossRef F. Hendricks et al. "High aspect ratio microstructuring of transparent dielectrics using femtosecond laser pulses: method for optimization of the machining throughput", Appl. Phys. A 117, 1 (2014). CrossRef A. Antonczak et al. "Degradation of poly(l-lactide) under CO2 laser treatment above the ablation threshold", Polym. Deg. Stab. 109, 97-105 (2014) CrossRef B. Stepak et al. "The influence of ArF excimer laser micromachining on physicochemical properties of bioresorbable poly(L-lactide)", Proc SPIE 9736, 97361T (2016). CrossRef

Design and fabrication of a biconvex aspherical microlens for maximizing fiber coupling efficiency with an ultraviolet laser diode

This paper introduces a biconvex microlens to maximize the light coupling between an edge emitting laser diode (LD) and an optical fiber. Both convex surfaces have different profiles along their x- and y-axis so that the elliptical light beam from an LD can be fed into the core of a fiber. The optimized lens profiles are first simulated by the ray-tracing software Zemax and then fabricated by an excimer laser micromachining system. Focal spot size around 10μm is achieved experimentally. Light coupling efficiency using this single biconvex microlens is 31.8% and 47.6% for single-mode and multi-mode fiber, respectively.

Dynamic modelling and experimental study of asymmetric optothermal microactuator

This paper reports the dynamic modelling and experimental study of an asymmetric optothermal microactuator (OTMA). According to the principle of thermal flux, a theoretical model for instantaneous temperature distribution of an expansion arm is established and the expression of expansion increment is derived. Dynamic expansion properties of the arm under laser pulse irradiation are theoretically analyzed indicating that both of the maximum expansion and expansion amplitude decrease with the pulse frequency increasing. Experiments have been further carried out on an OTMA fabricated by using an excimer laser micromachining system. It is shown that the OTMA deflects periodically with the same frequency of laser pulse irradiation. Experimental results also prove that both OTMA’s maximum deflection and deflection amplitude (related to maximum expansion and expansion amplitude of the arm) decrease as frequency increases, matching with the theoretical model quite well. Even though the OTMA's deflection decrease at higher frequency, it is still capable of generating 8.2μm maximum deflection and 4.2μm deflection amplitude under 17Hz/2mW laser pulse irradiation. This work improves the potential applications of optothermal microactuators in micro-opto-electro-mechanical system (MOEMS) and micro/nano-technology fields.

Excimer laser micromachining of oblique microchannels on thin metal films using square laser spot

Excimer laser micromachining of thin metal films with a sacrificial polymer coating is a novel technique that produces features with smooth edges. Using this technique, oblique microchannels are fabricated by workpiece dragging and using a square laser spot, where the axis of traverse of the workpiece is not parallel to the edges of the square laser spot. The microchannels have serrated edges that are particular to the shape of the mask producing the spot. The edge roughness of the channels, machined with a square laser spot of side 100 μm, is found to be most affected by the fluence–spot overlap interaction, and the channel width by spot-overlap and the angle of tilt of the traversed path. Polymer coated metal films underwent close to ideal machining, aided by the clamping action of the polymer layer. Through this technique of machining post polymer coating, the edge roughnesses of the microchannels have been curtailed to less than 10 μm, and channel widths to 150 μm. This technique may be used in fabrication of oblique and circular patterns using excimer laser micromachining with rectangular and square laser spots.

Etch Performance of KRF Excimer Laser Micromachining Characterization on Silicon Material

Excimer laser micromachining enables us to overcome the conventional lithography-based microfabrication limitations and simplify the process of creating three dimensional (3D) microstructures.The objective of this study is to investigate the relation between the number of laser pulses, number of laser passes through the channel of ablation site and their etch performance. Parameters such as frequency, fluence and velocity were retained as constants. In this paper, we present a parametric characterization study on silicon using KrF excimer laser micromachining. From the result, the etch rate change were recorded as the two major laser parameters (Number of laser pulses and number of laser passes) were varied. Both parameters were showing declination profile however from comparing both graphs, it showed that etch rate dropped more steeply when varied number of laser passes rather than number of pulses.

Microfeature edge quality enhancement in excimer laser micromachining of metal films by coating with a sacrificial polymer layer

A novel technique for enhanced excimer laser micromachining of metallic thin films by first coating the metal film with a thin polymer film is presented. The sacrificial polymer film acts as a protective and a clamping layer, preventing the metal film from undergoing cracking and damage during the laser ablation. The machined patterns are characterized regarding their quality in terms of edge roughness, lateral overcut and boundary integrity in proximity machining. Significant improvement in these aspects is observed when the machining is carried out on metal films coated with thin polymer films. Details of the effects of the fluence and spot overlap on the micromachined patterns are investigated. The technique allows sharp machining of micropatterns on thin metal films, over length scales ranging from hundreds of micrometers down to a single micrometer, thereby proving to be the only technique that can be used to laser micromachine thin films at the length scale of a single micrometer. This technique is expected to be useful for large scale patterning of metallic films, particularly for plasmonic applications and infrared/terahertz metamaterials.

Flexible metamaterial absorbers with multi-band infrared response

A flexible metamaterial with a tri-layer metal-dielectric-metal structure is fabricated by combining Excimer laser micromachining of a polyimide sheet and oblique angle physical vapour deposition methods. Excimer laser micromachining is used to generate an array of micro-disks on the flexible polymer sheet followed by physical vapour deposition at normal incidence to produce continuous metal and dielectric layers while oblique angle deposition of metal vapour is used to finally form discrete oblate ellipsoids on top of the micro-disks. The fabricated metamaterial shows multi-band metamaterial absorption exceeding 90% simultaneously over infrared bands centred at 3 µm, 5 µm, and 13.85 µm. The multi-band absorption arises due to multipole resonances of the disk structure and is accurately modelled by electromagnetic simulation as well. A theoretical model of a perfect absorber as an array of optimally impedance matched antennas is also presented.

ArF Excimer Laser Micromachining of MEMS Materials: Characterization and Applications

Excimer laser ablation is a versatile technique that can be used for a variety of different materials. Excimer laser ablation overcomes limitations of conventional two-dimensional (2D) microfabrication techniques and facilitates three-dimensional (3D) micromanufacturing. Previously, we have reported a characterization study on 248 nm KrF excimer laser micromachining. This paper extends the study to 193 nm ArF excimer laser micromachining on five representative micro-electro-mechanical systems (MEMS) materials (Si, soda-lime glass, SU-8, polydimethylsiloxane (PDMS), and polyimide). Relations between laser parameters (fluence, frequency and number of laser pulses) and etch performances (etch rates, aspect ratio, and surface quality) were investigated. Etch rate per shot was proportional to laser fluence but inversely proportional to number of laser pulses. Laser frequency did not show a notable impact on etch rates. Aspect ratio was also proportional to laser fluence and number of laser pulses but was not affected by laser frequency. Materials absorbance spectrum was found to have important influence on etch rates. Thermal modeling was conducted as well using numerical simulation to investigate how the photothermal ablation mechanism affects the etching results. Thermal properties of material, primarily thermal conductivity, were proved to have significant influence on etching results. Physical deformation in laser machined sites was also investigated using scanning electron microscopy (SEM) imaging. Element composition of redeposited materials around ablation site was analyzed using energy dispersive x-ray spectroscopy (EDXS) analysis. Combined with our previous report on KrF excimer laser micromachining, this comprehensive characterization study provides guidelines to identify optimized laser ablation parameters for desired microscale structures on MEMS materials. In order to demonstrate the 3D microfabrication capability of ArF excimer laser, cutting and local removal of insulation for a novel floating braided neural probe made of polyimide and nichrome was conducted successfully using the optimized laser ablation parameters obtained in the current study.

Simulations and experiments on excimer laser micromachining of metal and polymer

Excimer laser at 248 nm has been used to micromachine holes and channels on stainless steel and polymethyl methacrylate using mask projection methods. The machining is numerically simulated considering the laser beam as a heat source only, thermal diffusion, and melt vaporization. The depth and width of the machined features at different pulse energies and number of pulses are measured by optical profilometry. Both the depth and the aspect ratio (depth-to-width) are found to increase with increasing number of pulses and pulse energies. The depth predicted by the simulations based on the thermal ablation model is closer to the experimental observations for steel as compared with that for the polymer. This allows for the quantification of the contribution of thermal ablation processes in the excimer laser machining of metals and polymers. High-energy pulses are found to have higher ablation efficiency in case of metal and lower ablation efficiency in case of polymer.