Femtosecond laser drilling of alumina ceramic substrates

In the paper, the result on femtosecond laser drilling of alumina ceramic substrate was reported. The effects of various laser parameters such as different focus position, traverse speed, drilling pattern, pausing time, etc. on the drilled hole quality in terms of surface finish, heat affected zone (HAZ), hole circularity, debris, microcracks were studied. The quality of laser-drilled holes on alumina ceramic substrates was evaluated with optical microscope, SEM/EDX, and X-ray μ-CT analysis. The optimum drilling conditions were identified. High-quality laser-drilled holes on alumina ceramic substrates were demonstrated. The developed process has potential application in manufacturing of alumina substrate based electronic devices.     

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/cm², which is much higher than that achievable by the femtosecond laser for the equivalent energy fluence. Ablation threshold was determined as 2 mJ/cm². 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.     

Drilling rate of five metals with picosecond laser pulses at 355, 532, and 1064 nm

Experimental results on picosecond laser processing of aluminum, nickel, stainless steel, molybdenum, and tungsten are described. Hole drilling is employed for comparative analysis of processing rates in an air environment. Drilling rates are measured over a wide range of laser fluences (0.05–20?J/cm²). Experiments with picosecond pulses at 355?nm are carried out for all five metals and in addition at 532?nm, and 1064?nm for nickel. A comparison of drilling rate with 6-ps and 6-ns pulses at 355?nm is performed. The dependence of drilling rate on laser fluence measured with picosecond pulses demonstrates two logarithmic regimes for all five metals. To determine the transition from one regime to another, a critical fluence is measured and correlated with the thermal properties of the metals. The logarithmic regime at high-fluence range with UV picosecond pulses is reported for the first time. The energy efficiency of material removal for the different regimes is evaluated. The results demonstrate that UV picosecond pulses can provide comparable quality and higher processing rate compared with literature data on ablation with near-IR femtosecond lasers. A significant contribution of two-photon absorption to the ablation process is suggested to explain high processing rate with powerful UV picosecond pulses.     

Investigation of a novel hybrid process of laser drilling assisted with jet electrochemical machining

Recast layer and spatter are two inherent defects commonly associated with holes produced with laser drilling. This paper reports a novel hybrid process of laser drilling assisted with jet electrochemical machining (JECM-LD) that aims to minimize such defects and improve the quality of laser-drilled holes. The process based on the application of a jet electrolyte, being aligned coaxially with the focused laser beam, on the workpiece surface during laser drilling. The effect of the jet electrolyte mainly is an electrochemical reaction with materials. The jet electrolyte also cools the workpiece and transports debris during the process. On the basis of a measurement of laser attenuation in electrolyte, an experimental apparatus system is made and JECM-LD experiments have been performed on 0.5-mm-thick 321S20 stainless steel with two lasers at wavelength of 1064 and 532 nm. It is shown that recast layer and spatter have been effectively reduced during the JECM-LD compared with laser drilling in ambient atmosphere conditions.     

A statistical approach to determine process parameter impact in Nd:YAG laser drilling of IN718 and Ti-6Al-4V sheets

The numerous unique advantages afforded by pulsed Nd:YAG laser systems have led to their increasing utility for producing high aspect ratio holes in a wide range of materials. Notwithstanding the growing industrial acceptance of the technique, the increasingly tighter geometrical tolerances and more stringent hole quality requirements of modern industrial components demand that “defects” such as taper, recast, spatter etc., in laser-drilled holes are minimized. Process parameters like pulse energy, pulse repetition rate, pulse duration, focal position, nozzle standoff, type of gas and gas pressure of the assist gas are known to significantly influence hole quality during laser drilling. The present study reports the use of Taguchi design of experiments technique to study the effects of the above process variables on the quality of the drilled holes and ascertain optimum processing conditions. Minimum taper in the drilled hole was considered as the desired target response. The entire study was conducted in three phases:(a) screening experiments, to identify process variables that critically influence taper in laser drilled holes, (b) Optimization experiments, to ascertain the set of parameters that would yield minimum taper and (c) validation trials, to assess the validity of the experimental procedures and results. Results indicate that laser drilling with focal position on the surface of the material being drilled and employing low level values of pulse duration and pulse energy represents the ideal conditions to achieve minimum taper in laser-drilled holes. Thorough assessment of results also reveals that the laser-drilling process, optimized considering taper in the drilled hole as the target response, leads to very significant improvements in respect of other hole quality attributes of interest such as spatter and recast as well.     

Excimer laser machining of microvias in glass substrates for the manufacture of high density interconnects

Machining of microvias in 100?C50???m thick CMZ glass using an excimer laser (248?nm) was investigated. The effect of various laser process parameters: pulse energy, repetition rate, irradiation time were studied to optimise the microvia drilling process and a process window was identified. Through-hole drilling of 100???m diameter (entry hole) microvias was achieved at a fluence (energy density) as low as 2.3?J/cm² with an irradiation time of 30?C40?s at a repetition rate of 20?Hz, giving a taper angle between 22?C24^° relative to the vertical. However, by increasing the fluence to 4.5?J/cm², this reduced the machining time to 5?C10?s and taper angle to 14^°, giving an exit hole diameter of around 45?C50???m. With 50???m thick glass, it was possible to machine through-hole microvias with smaller entry hole diameters down to 40???m. Machined microvias were characterised to investigate debris, recast layer and microcrack formation. Debris and recast layer around the machined features was minimised by using a protective photoresist layer coating on the glass and through appropriate operating parameter selection. Microcracks along the sidewalls of the microvias could not be avoided, but their severity depended on the laser machining parameters used.     

Laser through-hole drilling and laser cutting in teflon

We report a novel technique for laser high-speed drilling and cutting in teflon films. The new laser drilling surpasses the conventional techniques in simplicity, throughput and spatial resolution. The laser cutting and drilling process consists of three simple steps. First, a thin absorbing layer (in this case 300 Å of gold) is deposited on the teflon to allow for laser absorption. Second, the drilling is performed by pulsed-laser irradiation at the rate of one hole per pulse. The irradiation process does not completely open the holes in which debris still remain. Third, the ultrasonic cleaning in water is used to remove the modified and weakly bound material inside the drilled holes, leaving behind 50 m diameter through holes in 25 m thick teflon sheets. The drilling process-window is well mapped. The cutting process is obtained by fast scanning the laser beam at laser powers above a threshold value. This new technique is desirable for packaging because of its drilling speed as high as 60 000 holes per minute, its fast cutting and its low laser equipment cost.     

Rapid micromachining of high aspect ratio holes in fused silica glass by high repetition rate picosecond laser

We present multiple methods of high aspect ratio hole drilling in fused silica glass, taking advantage of high power and high repetition rate picosecond lasers and flexible beam delivery methods to excise deep holes with minimal collateral damage. Combinations of static and synchronous scanning of laser focus were explored over a range of laser repetition rates and burst-train profiles that dramatically vary laser plume interaction dynamics, heat-affected zone, and heat accumulation physics. Chemically assisted etching of picosecond laser modification tracks are also presented as an extension from femtosecond laser writing of volume nanograting to form high aspect ratio (77) channels. Processing windows are identified for the various beam delivery methods that optimize the laser exposure over energy, wavelength, and repetition rate to reduce microcracking and deleterious heating effects. The results show the benefits of femtosecond laser interactions in glass extend into the picosecond domain, where the attributes of higher power further yield wide processing windows and significantly faster fabrication speed. High aspect ratio holes of 400 μm depth were formed over widely varying rates of 333 holes per second for mildly cracked holes in static-focal positioning through to one hole per second for low-damage and taper free holes in synchronous scanning.     

Microhole machining of silicon wafer in air and under deionized water by pulsed UV laser system

The study investigated the laser microhole drilling performance of polycrystalline silicon using the trepanning drilling method combined with the helix swing path with varying parameters, including laser pulse energy, pulse repetition frequency, and galvanometric scan speed. A pulsed ultraviolet laser system was used in an atmospheric condition and under deionized water. Moreover, the trepanning method was used to obtain a larger via diameter. The surface morphology, taper angle, and melted residual high were evaluated using a three-dimensional confocal laser scanning microscope and field emission scanning electron microscope. This method can produce larger holes and can be applied to crystalline silicon, multicrystalline silicon, thin-film silicon, and other materials for photovoltaic applications.     

Analysis of materials modifications caused by UV laser micro drilling of via holes in AlGaN/GaN transistors on SiC

Pulsed UV laser drilling can be applied to fabricate vertical electrical interconnects (vias) for AlGaN/GaN high electron mobility transistor devices on single-crystalline silicon carbide (SiC) substrate. Through-wafer micro holes with a diameter of 50-100 μm were formed in 400 μm thick bulk 4H-SiC by a frequency-tripled solid-state laser (355 nm) with a pulse width of ≤30 ns and a focal spot size of ∼15 μm. The impact of laser machining on the material system in the vicinity of micro holes was investigated by means of micro-Raman spectroscopy and transmission electron microscopy. After removing the loosely deposited debris by etching in buffered hydrofluoric acid, a layer of <4 μm resolidified material remains at the side walls of the holes. The thickness of the resolidified layer depends on the vertical distance to the hole entry as observed by scanning electron microscopy. Micro-Raman spectra indicate a change of internal strain due to laser drilling and evidence the formation of nanocrystalline silicon (Si). Microstructure analysis of the vias’ side walls using cross sectional TEM reveals altered degree of crystallinity in SiC. Layers of heavily disturbed SiC, and nanocrystalline Si are formed by laser irradiation. The layers are separated by 50-100 nm thick interface regions. No evidence of extended defects, micro cracking or crystal damage was found beneath the resolidified layer. The precision of UV laser micro ablation of SiC using nanosecond pulses is not limited by laser-induced extended crystal defects.     

A comprehensive study of the long pulse Nd:YAG laser drilling of multi-layer carbon fibre composites

The results of an extensive experimental study of the free running Nd:YAG laser drilling of a multi-layer carbon fibre composite, where adjacent layers have differently orientated fibres, are reported. For holes drilled with the laser operating in fixed-Q mode at 1064 nm, parallel sections of blind holes illustrating discontinuities in the hole size along a given section direction will be shown to occur at the interface between adjacent layers. An explanation for this effect is proposed. Detailed single pulse drilling characteristics will be presented illustrating the exit hole diameter as a function of pulse energy and material thickness. These characteristics illustrate a ‘stable' drilling regime in which the exit hole diameters are least sensitive to changes in pulse energy or material thickness and a less ‘stable' regime in which they are more strongly dependent on these parameters. Drilling characteristics will be given for two different beam qualities, illustrating the greater drilling depth and reduced hole size achievable with an improved beam quality. Finally holes drilled through a 2 mm thick sample of material with multiple pulses are considered. Size distribution curves for entrance and exit holes will be presented. The total energy required (number of pulses × pulse energy) to drill through 2 mm thick material will be reported as a function of pulse energy in stationary air and argon atmospheres and in a partial vacuum, illustrating a threshold energy which is dependent upon the drilling atmosphere. The threshold energies will be discussed with reference to plasma formation and the reactivity of the drilling atmosphere.     

Analysis of machining characteristics in electrochemical etching using laser masking

Electrochemical etching using laser masking (EELM), which is a combination of laser beam irradiation for masking and electrochemical etching, allows the micro fabrication of stainless steel without photolithography technology. The EELM process can produce various micro patterns and multilayered structures. In this study, the machining characteristics of EELM were investigated. Changes in characteristics of recast layer formation and the protective effect of the recast layer according to the laser masking conditions and electrochemical etching conditions were investigated by field emission scanning electron microscopy (FE-SEM), focused ion beam (FIB) and X-ray photoelectron spectroscopy (XPS). The oxidized recast layer with a thickness of 500 nm was verified to yield a superior protective effect during electrochemical etching and good form accuracy. Finally, micro patterns and structures were fabricated by EELM.     

Ultra-short pulse laser deep drilling of C/SiC composites in air

Ultra-short pulse laser machining is an important finishing technology for high hardness materials. In this study, it demonstrated that the ultra-short pulse laser can be used to drill the film cooling holes and square holes in aero-engine turbine blades made of C/SiC composites. Both the edges and bottoms of the drilling holes are covered with small particles. The following factors have a great effect on drilling holes according to this work: (1) circular holes can be processed only at a relative small helical lines spacing. (2) With the increase of laser scanning speed, the depth of holes reduces while the diameter rarely changes. (3) Through the holes of high aspect ratio can be obtained via high processing power.     

Experimental study on the development of a micro-drilling cycle using ultrashort laser pulses

Microholes for the production of high precision devices were obtained by ultrashort pulsed laser machining of martensitic stainless steels. A micro-drilling cycle based on the sequence of a drilling through phase, an enlargement and finishing phase is proposed in order to solve the trade-off between process time and quality of the ablated surfaces without making use of complex design of experiments. The three phases were studied taking into account the evolution of the microhole shape as a function of the main process parameters (number of passes per phase, incidence angle and radius of the beam trajectory respect to the hole׳s axis).Experiments testified that the drilling strategy was able to produce cylindrical holes with diameter of 180±2 μm on a 350 μm thick plate in total absence of burrs and debris within a drilling time of 3.75 s. Repeatability tests showed a process capability of nearly 99%. SEM inspection of the inner surface of the microholes showed the presence of elongated and periodic ripples whose size and inclination can be controlled adjusting the incidence angle of the beam over the tapered surface before the ultimate finishing phase.     

Analysis of surface and material modifications caused by laser drilling of AlN ceramics

For the laser drilling of aluminum nitride ceramic the processing results and the effects related to pulsed irradiation were investigated. Images of the drilled surface revealed regular, cylindrically shaped holes of about 100 μm in diameter independently of the laser wavelength (1064/532/355 or 266 nm). The holes were surrounded by circular heat-affected zones of larger diameter. A comparison of the elemental compositions of the original material and the processed one indicated a decrease of the nitrogen concentration in the affected area. The spectral analysis of the ablated material composition revealed the presence of ions and neutrals in dependence on the laser intensity applied. It was found that at intensity values close to the ablation threshold the ejected material consisted mainly of neutrals, while doubling of the intensity resulted in appearance of single-ionized Al species, which were also observed together with Al clusters in the mass spectra of the UV-excited plasma. Their prevailing content was revealed for drilling at higher intensities around 15 GW/cm² at 532 nm. Results of model calculations indicated, in agreement with the experiment, that at the threshold the ceramic decomposes into gaseous nitrogen and solid Al particulates, while at a higher fluence the material particles vaporize and influence the quality of drilling.     

Influence of microsupersonic gas jets on nanosecond laser percussion drilling

This paper reports on etching rates and hole quality for nanosecond laser percussion drilling of 200-μm thick 316L stainless steel performed with micro supersonic gas jets. The assist-gas jets were produced using nozzles of 200, 300 and 500 μm nominal throat diameters. Air and oxygen were used separately for the process gas in the drilling trials and the drilling performance was compared to drilling in ambient conditions. The highest etch rate of 1.2 μm per pulse was obtained in the ambient atmosphere condition, but this was reduced by about 50% with assist-air jets from the 200 μm nozzle. Increasing the jet diameter and/or using oxygen assist gas also decreased the etching rate and increased the hole diameter. The 200 μm nozzle using air-assist jets produced the least amount of recast and gave the best compromise for etching rate. A combination of plasma shielding and different gas dynamic conditions inside the holes and at the surface are correlated to the observations of different drilling rates and hole characteristics.     

The influence of pre-melting in laser drilling with temporally modulated pulse

Laser drilling by temporally modulated pulse is a promising technique and has many advantages compared with normal pulse drilling. In this work, the effect of modulated pulse comprising pre-heating front and sharp trail was mainly studied. The function of the former was to pre-melt the radiated material, and the latter was to expel the liquid melt from the molten pool, thus to form a blind hole. While the trail subpulse was kept constant, the difference in the pre-heating subpulse parameter could cause a considerable influence on the hole quality and drilling efficiency. The depth and volume of the molten pool were proportional to the pre-heating energy, and inversely proportional to the pre-heating duration. With pre-heating subpulses of proper parameters, the sharp trail subpulse was very effective in expelling the melt liquid, leaving only a small quantity of melt to re-solidify as the recast layer, which was observably thinner compared with the holes drilled using the normal pulse mode. In the pre-melting process, the directional melt flow and heat conduction were found to be the reasons why the deep melting phenomenon had occurred.     

Laser etching of fused silica using an adsorbed toluene layer

A new method for laser etching of transparent materials with a low etch rate and a very good surface quality is demonstrated. It is based on the pulsed UV-laser backside irradiation of a transparent material that is covered with an adsorbed toluene layer. This layer absorbs the laser radiation causing the etching of the solid. The threshold fluence for etching of fused silica amounts to 0.7 J/cm². The constant etch rate of about 1.3 nm/pulse that has been observed in a fluence interval from 2 to 5 J/cm² is evidence of a saturated process. The limited thickness of the adsorbed layer causes the low etch rates and the rate saturation. The etched surface structures have well defined edges and low surface roughness values of down to 0.4 nm rms. PACS 81.65.Cf; 81.05.Kf; 79.20.Ds; 61.80.Ba; 42.55.Lt     

Band-pass filters for THz spectral range fabricated by laser ablation

The terahertz resonant metal-mesh filters were fabricated using the laser direct writing technique. UV picosecond laser was employed to cut matrixes of cross-shaped holes in stainless steel foil and molybdenum layer deposited on polyimide substrate. Different laser processing strategies were developed: holes were cut through in the metal foil and the molybdenum film was removed from the polyimide by laser ablation. Band-pass filters with a different center frequency were designed and fabricated. The regular shape, smoothness of edges and sharpness of corners of the cross-shaped holes in the metal were the main attributes for quality assessment for the laser ablation process. Spectral characteristics of the filters, determined by the mesh period, cross-arm length, and its width, were investigated by terahertz time-domain spectroscopy and conventional space-domain Fourier transform spectroscopy. Experimental data were supported by three-dimensional finite-difference time-domain simulations.     

A novel high-peak power double AO Q-switches pulse Nd:YAG laser for drilling

This paper reports on the characterization and analysis of a novel high power with double acousto-optical (AO) Q-switch pulse laser. It is shown that two AO Q-switches, in which acousto-fields are perpendicular to each other, switch-loss is nearly three times larger than one AO Q-switch, one time larger than the two AO Q-switches in which acousto-field are parallel. The laser pulse bursts, with 5–50 kHz repetition rate of the burst, typically 200 ns duration, 400 kW the peak power, 5 mm mrad beam parameter product, are obtained. Using the laser for drilling, the perfect drilling results are given to a thinner recast layer.