Micromachining Using an Excimer (248nm) Laser

An excimer laser micromachining facility was developed to investigate laser-material interaction. It includes the COMPex 205i excimer laser (manufactured by Lambda Physik) using a krypton fluoride (KrF) (248nm wavelength) is the lasing medium, an 3-axis precision stage, and optical deliver system that includes an attenuator module, a homogenizer, field lens, and a doublet. The system was mounted on a vibration free isolated table. Multipulse, excimer laser micromachining studies ware conducted in air and under water on different workmaterials, including PMMA, borosilicate glass (Corning 0211glass), and silicon. Process parameters, such as pulse frequency, number of pulses, and pulse energy density were varied to optimum conditions for best finish and uniform geometry with minimal surface damage. The laser-ablated surfaces were examined using an optical microscope, MicroXAM laser interference microscope, and a scanning electron microscope (SEM). Threshold fluences (FTH) for ablation in air are 0.53, 2.19 and 2.44 J/cm2 for PMMA, borosilicate glass and silicon, respectively. Presence of thin film water during the ablation process facilitated in the removal of debris giving a clean finishing cut. Photothermal ablation was dominant when the fluence was below the threshold value and pure photochemical above this value. For the case of borosilicate glass, a hump or build-up of about 50 nm is found at low fluence.Mechanical & Aerospace Engineerin

Femtosecond pulsed laser ablation and patterning of 3C-SiC films on Si substrates for MEMS fabrication

Femtosecond pulsed laser (FPL) micromachining is a direct-writing technique in which an ultrashort pulse laser beam is focused to dimensions of a few microns inside or on the surface of the substrate and then moved around using a X-Y positioning table, thereby creating either features or patterns as required. It outperforms conventional micromachining technologies due to advantages such as precise resolution, minimal thermal or shock damage, and absence of discrimination among materials. 3C-SiC is a very important semiconductor in electronics and opto-electronics and more recently regarded as an optimal candidate for structural or coating applications in microelectromechanical systems (MEMS) used under harsh and high-temperature environments. However, it is a very difficult material to be machined or etched by mechanical or chemical methods.;In this work, fundamental studies on the interaction of femtosecond pulsed beam with 3C-SiC films were performed. The influence of laser parameters such as pulsed energy on the ablation and calculations of damage thresholds and ablation rates were determined. Based on these results, MEMS structures including micromotors, microturbine rotors, and lateral resonators were patterned with good quality and repeatability. Research demonstrates that FPL micromachining is capable of offering a unique solution to overcome the traditional barriers in SiC machining method, opening up opportunities for SiC materials to be used in industrial environment.;As a spinoff of femtosecond pulse micromachining, nanostructuring of 3C-SiC films on Si was observed. Nanoparticle surfaces were further studied in terms of formation conditions and characterizations of crystal structure and related properties. Incubation effects were identified and Coulomb explosion mechanism was proposed to be responsible for the generation of nanoparticles.;Results of research enhance our current understanding of ultrashort pulse-matter interactions and offer potential applications for SiC-MEMS

Laser Machining by short and ultrashort pulses, state of the art and new opportunities in the age of the photons

An overview is given of the applications of short and ultrashort lasers in material processing. Shorter pulses reduce heat-affected damage of the material and opens new ways for nanometer accuracy. Even forty years after the development of the laser there is a lot of effort in developing new and better performing lasers. The driving force is higher accuracy at reasonable cost, which is realised by compact systems delivering short laser pulses of high beam quality. Another trend is the shift towards shorter wavelengths, which are better absorbed by the material and which allows smaller feature sizes to be produced. Examples of new products, which became possible by this technique, are given. The trends in miniaturization as predicted by Moore and Taniguchi are expected to continue over the next decade too thanks to short and ultrashort laser machining techniques. After the age of steam and the age of electricity we have entered the age of photons now

Micromachining of Polyurethane (Pu) Polymer Using a Krf Excimer Laser (248nm)

Excimer laser micromachining has generated considerable research interest leading to numerous commercial applications in the last decade. Polyurethane (PU) polymer, due to its biocompatibility, weather resistance, and favorable physical properties, such as good flex-life, temperature resistance, electrical insulation and tear resistance finds a number of applications in medical implants, protective coatings, and as a prototype material for structural components in MEMS devices. An Excimer laser (wavelength = 248 nm, FWHM = 25 ns) is employed in this research work for micromachining of polyurethane (PU) polymer and pattern design for some potential MEMS applications. The main objective of this research is to establish a fundamental understanding of ablation mechanism in polyurethane (PU) polymer. The effect of various operating parameters, such as fluence per unit area, energy per pulse, number of pulses, repetition rates, and environment on the resulting geometries and ablation behavior are investigated. Micromachining is conducted in air and under water environments with variation in mask sizes and pattern geometries. Microgears (up to 360 μm diameter) are etched on the surface of polyurethane with several similar geometries used in MEMS devices, such as microfluidic channels, and microcircuits. It was observed that, for air environment, the ablation rate is 0.18 μm/pulse, and for underwater environment, the ablation rate is 0.07 μm/pulse (underwater ablation threshold: 0.10 J/cm2). The relationship was developed between wall taper angles behavior of the ablated regions with process parameters, which concluded low taper angles (~32°) for in air as compared to high taper angles (~65°) with underwater micromachining. The relationship between mask size and resulting seam quality, seam width and ablation depth for pattern generation was developed and analyzed. The experimental results for air and under water micromachining demonstrate that the ablation mechanism differs in polymers depending upon fluence (J/cm2), repetition rate (Hz), and working environment (in air or underwater). A combination of photo-thermal and photo-chemical ablation mechanism was attributed in the material removal process for polyurethane (PU) polymer. However, to be able to successfully and effectively produce MEMS devices, further research into the micromachining of polymers is required.Mechanical & Aerospace Engineerin

Ultrafast laser deposition and microfabrication of thin films for MEMS structures

This thesis is organized into a chapter on literature review, three papers and a chapter on conclusions. The first paper deals with the femtosecond pulsed laser deposition and fabrication of micromotor on Teflon thin films along with a comparison of the traditional excimer deposition. The second paper deals with the femtosecond pulsed laser deposition and fabrication of microgripper on 3C-SiC along with a comparison of films deposited by excimer laser and atmospheric chemical vapor deposition methods. The third paper deals with the details of fabrication of a microgripper on 3C-SiC thin film

Micromachining and Surface Build-up on Borosilicate Glass Using Excimer Laser

Micromachining in borosilicate glass (because of its properties) is advancing rapidly for numerous nanotechnology applications. The main feature this glass requires is ability to be shaped for MEMS components. Since other machining techniques result in microcracks, laser micromachining is used. Material build-up and material removal result when an excimer laser beam impinges a borosilicate glass surface. Their features are studied by investigating various parameters such as input energy, media, size of mask and number of loops using optical microscope and laser interference microscope. Good material built-up, on the order of 25 to 35 nm was obtained for 1 mm mask at low energies when tests were conducted in air, distilled water, and methanol. It is also found that borosilicate glass results in minimal number of cracks when micromachining under polymer is done compared to micromachining in air, or under water.Mechanical & Aerospace Engineerin

Micromachining of Borosilicate Glass and Laser Induced Backside Wet Etching of Quartz Using an Excimer Laser (248 Nm)

Borosilicate glass is widely used in optical communications, optoelectronics as well as biomedical technologies and microelectromechanical systems (MEMS). Laser micromachining is an alternative approach for machining of glass. Further, UV transparent materials, such as quartz, and sapphire are materials of importance in optical and optoelectronics because of their outstanding properties, such as transparency in a wide wavelength range and strong damage resistance for laser irradiation. Laser induced backside wet etching (LIBWE) is a novel one step method for machining transparent materials, such as quartz, fused silica, and sapphire. In this investigation, micromachining of borosilicate glass and quartz is conducted using a short pulse (FWHM=25ns) KrF Excimer Laser (248nm wavelength) that generates laser energy in the range of 100-600 mJ. The machined surfaces were examined using conventional optical and laser interference microscopes. The impact of changing major operating parameters, such as pulse fluence and different media on the resulting geometries is studied. Simple as well as complex geometries, such as microfluidic channels, inductors, part geometries used in medical applications, and RF circuits were machined.Mechanical & Aerospace Engineerin

The characteristics and feasibility of an in-line debris control technique for KrF excimer laser ablative micromachining

To observe KrF excimer laser ablation through thin liquid film of de-ionized (DI) water and the effects thereof on debris control, equipment was designed to contain a small control volume that could be supplied with a fixed flow velocity thin film of DI water to immerse a bisphenol A polycarbonate workpiece. Using the same equipment comparison with ablation in ambient air was possible. The positional debris deposition of samples machined in ambient air was found to show modal tendency reliant on the feature shape machined and according to species size. This is proposed to be due to the interaction of multiple shockwaves at the extent of ablation plumes generated at geometry specific locations in the feature. Debris was deposited where the shockwaves collide. Ablating under a flowing thin film of DI water showed potential to modify the end position and typical size of the debris produced, as well as increased homogeneity of deposition density. Compared with a sample machined in ambient air, the use of immersion has reduced the range of debris deposition by 17% and the deposition within the boundary of the ablation plume has a comparatively even population density. Unlike samples machined in ambient air, outside the ablation plume extents positional control of deposited debris by thin film flowing DI water immersion was evidenced by rippled flow line patterns, indicating the action of transport by fluid flow. A typical increase in debris size by an order of magnitude when using DI water as an immersing liquid was measured, a result that is in line with a colloidal interaction response... cont'd

Laser ablation of polymer waveguide and embedded mirror for optically-enabled printed circuit boards (OEPCB)

Due to their inherent BW capacity, optical interconnect (OI) offers a means of replacement to BW limited copper as bottlenecks begin to appear within the various interconnect levels of electronics systems. Low-cost optically enabled printed circuit boards are a key milestone on many electronics roadmaps, e.g. iNEMI. Current OI solutions found in industry are based upon optical fibres and are capable of providing a suitable platform for inter-board applications especially on the backplane. However, to allow component assembly onto high BW interconnects, an integral requirement for intra-board applications, optically enabled printed circuit boards containing waveguides are essential. Major barriers to the deployment of optical printed circuit boards include the compatibility of the technique, the cost of acquiring OI and the optical power budget. The purpose of this PhD research programme is to explore suitable techniques to address these barriers, primarily by means of laser material processing using UV and IR source lasers namely 248 nm KrF Excimer, 355 nm UV Nd:YAG and 10.6 µm IR CO2. The use of these three main lasers, the trio of which dominates most PCB production assembly, provides underpinning drive for the deployment of this technology into the industry at a very low cost without the need for any additional system or system modification. It further provides trade-offs among the suitable candidates in terms of processing speed, cost and quality of waveguides that could be achieved. This thesis presents the context of the research and the underlying governing science, i.e. theoretical analysis, involving laser-matter interactions. Experimental investigation of thermal (or pyrolitic) and bond-breaking (or photolytic) nature of laser ablation was studied in relation to each of the chosen lasers with regression analysis used to explain the experimental results. Optimal parameters necessary for achieving minimum Heat Affected Zone (HAZ) and surface/wall roughness were explored, both of which are key to achieving low loss waveguides. While photochemical dominance – a function of wavelength and pulse duration – is desired in laser ablation of photopolymers, the author has been able to find out that photothermallyprocessed materials, for example at 10.6 µm, can also provide desirable waveguides. Although there are literature information detailing the effect of certain parameters such as fluence, pulse repetition rate, pulse duration and wavelength among others, in relation to the etch rate of different materials, the machining of new materials requires new data to be obtained. In fact various models are available to try to explain the laser-matter interaction in a mathematical way, but these cannot be taken universally as they are deficient to general applications. For this reason, experimental optimisation appears to be the logical way forward at this stage of the research and thus requiring material-system characterisation to be conducted for each case thereby forming an integral achievement of this research. In this work, laser ablation of a single-layer optical polymer (Truemode™) multimode waveguides were successfully demonstrated using the aforementioned chosen lasers, thus providing opportunities for rapid deployment of OI to the PCB manufacturing industry. Truemode™ was chosen as it provides a very low absorption loss value < 0.04 dB/cm at 850 nm datacom wavelength used for VSR interconnections – a key to optical power budget – and its compatibility with current PCB fabrication processes. A wet-Truemode™ formulation was used which required that optical polymer layer on an FR4 substrate be formed using spin coating and then UV-cured in a nitrogen oxygen-free chamber. Layer thickness, chiefly influenced by spinning speed and duration, was studied in order to meet the optical layer thickness requirement for multimode (typically > 9 µm) waveguides. Two alternative polymers, namely polysiloxane-based photopolymer (OE4140 and OE 4141) from Dow Corning and PMMA, were sparingly utilized at some point in the research, mainly during laser machining using UV Nd:YAG and CO2 lasers. While Excimer laser was widely considered for polymer waveguide due to its high quality potential, the successful fabrication at 10.6 µm IR and 355 nm UV wavelengths and at relatively low propagation loss at datacom wavelength of 850 nm (estimated to be < 1.5 dB/cm) were unprecedented. The author considered further reduction in the optical loss by looking at the effect of fluence, power, pulse repetition rate, speed and optical density on the achievable propagation but found no direct relationship between these parameters; it is therefore concluded that process optimisation is the best practice. In addition, a novel in-plane 45-degree coupling mirror fabrication using Excimer laser ablation was demonstrated for the first time, which was considered to be vital for communication between chips (or other suitable components) at board-level