High-speed and crack-free direct-writing of microchannels on glass by an IR femtosecond laser

Fabrication of high-resolution 3D structures with laser radiation on the surface of brittle materials has always been a challenging task. Even with femtosecond laser machining, micro-cracks and edge chipping occur. In order to evaluate processing modes optimal both in quality and productivity, we investigated high-speed (50 kHz) femtosecond laser processing of BK7 glass with the use of design of experiments and regression analysis. An automated inspection technique was developed to extract quality characteristics of test-objects. A regression model was obtained appropriate to fabricate microchannels with a predefined depth in the range of 1–30 µm with average accuracy of 5%. It was found that high quality machining modes are in the range of 0.91–2.27 µJ energy pulses, overlap of 53–62%, three and more number of passes. A material removal rate higher than 0.3 mm³/min was reached and microfluidic structures were formed based on data obtained.     

Melting and solidification in laser-irradiated HgCdTe

The results of the numerical analysis of the effects induced by pulsed Nd:YAG and ruby laser on Hg_0.8Cd_0.2Te are presented. The proposed model facilitates the planning of HgCdTe laser processing and the choice of the processing parameters such as: melt depth, melt duration of the surface layer and melt front velocity, as well as the irradiation parameters. The influence of the optical parameters and the temperature dependence of the HgCdTe thermal parameters on the results of laser irradiation are specially analyzed.     

脉冲激光与电化学复合的应力刻蚀加工质量研究

脉冲激光电化学复合加工可以有效去除激光辐照区域内的电解产物, 提高加工效率, 改善加工质量. 针对高性能金属材料的微细加工要求, 采用脉冲激光电化学复合的应力刻蚀加工方法对铝合金的刻蚀特性进行理论和试验研究. 通过比较激光直接刻蚀加工和激光电化学复合加工的特点, 应用扫描电子显微镜、光学轮廓仪等检测技术分析了刻蚀区域的形貌特征. 根据力学电化学原理, 探讨了金属材料微结构加工的应力去除机理. 通过加工试验, 研究了工艺参数和加工方式对加工质量的影响, 采用优化的工艺参数, 加工出了质量较好的微结构. 试验结果表明, 激光电化学复合的连续扫描加工稳定性好, 可以有效地降低表面粗糙度, 提高加工质量. 关键词： 激光电化学 应力刻蚀 加工质量 工艺参数     

Wood engraving by Q-switched diode-pumped frequency-doubled Nd:YAG green laser

Laser deep engraving is one of the most promising technologies to be used in wood carver operations. In this method, a laser beam is used to ablate a solid wood bulk, following predetermined patterns. The sculpture is obtained by repeating this process on each successive thin layer. Obviously, in order to achieve larger material removal rates, the process needs a controllable variation of the depth to carve a 3D (three dimensional) profiles.The degree of precision of the shape, the removal rate and the surface quality during the engraving process strictly depend on the materials properties, the laser source characteristics and the process parameters.The aim of this work is to investigate the influence of the process parameters on the material removal rates by engraving panels made of different types of wood using a Q-switched diode-pumped Nd:YAG green laser working with a wavelength λ=532 nm. The examined parameters were: the mean power that depends on the pulse frequency, the beam speed and the number of laser scansions, also called repetitions. The working parameters and the engraved depth were related and an energy-based model was proposed in order to predict the latter.Experimental results showed that the Q-switched diode-pumped frequency-doubled Nd:YAG green laser can be successfully used to machine different types of wood, obtaining decorative drawing and 3D engraved geometries without burning. However, an accurate selection of the wood types and the process parameters is necessary in order to obtain deep engraving without carbonization and a homogeneous carving.     

Excimer laser micromachining of aspheric microlenses with precise surface profile control and optimal focusing capability

This paper demonstrates the unique and exceptional capability of excimer laser micromachining in fabricating aspheric microlenses with precise surface profile control. A newly developed laser scanning method is introduced for machining refractive types of microlenses, which have pre-designed surface profiles aiming at minimizing the optical focal spot sizes. The machining accuracy and machined surface roughness are examined experimentally, and very good results are obtained. Optical testing on the fabricated aspheric microlenses shows significant improvement in focusing capability and the focal spot sizes are approaching optical diffraction limits. The proposed excimer laser micromachining method is flexible, versatile, and accurate, hence can be very useful and powerful in machining 3D microstructures of complex profiles and demanding profile accuracy.     

Laser (a pulsed Nd:YAG) cladding of AZ91D magnesium alloy with Al and Al2O3 powders

The laser surface cladding of an AZ91D magnesium alloy with Al and Al₂O₃ powders was investigated using a pulsed Nd:YAG laser. The optimum ratio of Al to Al₂O₃ and the suitable range of laser processing parameters were identified. The resulting microstructure in the modified surface layer was examined and the wear resistance property was evaluated. The results show that the wear resistance of the laser treated samples was much superior to that of the untreated samples.     

Energy mechanism during machining process by high-power continuous CO2 laser

The high-power continuous CO₂ laser (4 KW) can provide an energy capable of causing melting or even, with special treatment of surface, vaporizing an XC42 - iron sample. During the laser-metal interaction, the energetic machining mechanism takes place according to the following assumptions: Laser energy absorbed by metal is maximal for a p-polarization. The melting front precedes the laser beam. The beam interacts with a preheated surface whose temperature is near the melting point. In such conditions one finds that mean average absorptive power (A), calculated through Maxwell's equations at fusion temperature, is around 25%, which enables us to calculate the laser energy absorbed by the metal. The available thermal models provide a lot of information concerning thermal diffusion but are unable to describe the physical process of the groove. Hence practical information required for industrial applications cannot be obtained. So in this work we have established a model able to calculate the characteristic parameters of the groove (or cut) as a function of laser energy and beam impact diameter (D). This model is based on writing down the balance of exchanged heat during the time of laser-material interaction ((t). This new procedure makes it possible to determine the machining parameters (laser power P, impact diameter D, and machining speed V) which one has to use during the machining process in order to implement an optimum groove (or cut) with predetermined characteristics (width L_s, and groove depth P_r).     

Experimental approach to the laser machining of PMMA substrates for the fabrication of microfluidic devices

This paper deals with CO₂ laser machining of a suitable amorphous polymer (PMMA) as a flexible technique for the rapid fabrication of miniaturized structures such as microfluidic devices.A model to estimate the main dimensions (depth and width) of the grooves produced by the laser on PMMA is presented, taking into account the influence of the main process parameters (incident power, scanning speed and spot diameter). This theoretical model allows to control the engraving process showing that laser could represent a valid alternative for the production of microchannels. PMMA single-use devices are found to be easier to manufacture with respect to the conventional glass or silicon products.In a second step, IR laser vaporization is adopted for the removal of a single layer of PMMA. This is achieved using multiple overlapping sequences of straight grooves with different scanning directions. The proposed technique showed that the removal depth varied proportionally with the number of layers machined, while surface roughness is influenced by the grooves spacing and the orientation of the scanning direction between successive layers.A method for thermally bonding the PMMA sheets, constituting the 3D structure of the chip, is also presented. The combination of high temperatures and low bonding pressures makes it possible to generate a bulk junction enabling good performances in terms of sealing characteristics.     

Fast femtosecond laser ablation for efficient cutting of sintered alumina substrates

Fast, accurate cutting of technical ceramics is a significant technological challenge because of these materials' typical high mechanical strength and thermal resistance. Femtosecond pulsed lasers offer significant promise for meeting this challenge. Femtosecond pulses can machine nearly any material with small kerf and little to no collateral damage to the surrounding material. The main drawback to femtosecond laser machining of ceramics is slow processing speed. In this work we report on the improvement of femtosecond laser cutting of sintered alumina substrates through optimisation of laser processing parameters. The femtosecond laser ablation thresholds for sintered alumina were measured using the diagonal scan method. Incubation effects were found to fit a defect accumulation model, with F_th,1=6.0 J/cm² (±0.3) and F_th,∞=2.5 J/cm² (±0.2). The focal length and depth, laser power, number of passes, and material translation speed were optimised for ablation speed and high quality. Optimal conditions of 500 mW power, 100 mm focal length, 2000 µm/s material translation speed, with 14 passes, produced complete cutting of the alumina substrate at an overall processing speed of 143 µm/s – more than 4 times faster than the maximum reported overall processing speed previously achieved by Wang et al. [1]. This process significantly increases processing speeds of alumina substrates, thereby reducing costs, making femtosecond laser machining a more viable option for industrial users.     

Surface roughness analysis and improvement of PMMA-based microfluidic chip chambers by CO2 laser cutting

For the microfluidic chip, the surface roughness of the chamber sidewall is an important parameter in estimating its quality. In this work, the chambers of the polymethyl methacrylate (PMMA)-based microfluidic chip were fabricated by CO₂ laser cutting, and then the surface roughness of the sections cut using different laser parameters and ambient temperature was studied by a non-contact 3D surface profiler. Our observation shows that the surface roughness results primarily from the residues on the laser-cut edge, which are produced by the bubbles bursting. To reduce the surface roughness of the cut section, a new approach is proposed, that is preheating the PMMA sheet to a suitable ambient temperature during laser processing. The results indicate that at a preheat temperature of 70-90 °C, the surface roughness resulting from the cut would be reduced. In our experiment, the best result was that the arithmetical mean roughness is R_a = 100.86 nm when the PMMA sheet was heated to 85 °C.     

飞秒激光直写PMMA制备微流道的工艺技术研究

针对目前PMMA微流道加工质量差和效率低的问题, 对飞秒激光直写PMMA制备微流道的工艺技术进行了研究。通过实验分析了不同激光参数对微流道的宽度、深度、粗糙度、微流道两侧堆积物火山口高度的影响及变化规律。实验结果表明, 当激光扫描速度为20 mm/s时, 激光功率为0.5 W时, 微流道粗糙度较低且变化幅度不明显; 激光能量从0.5 W增加到0.75 W时, 微流道的宽度、深度与激光能量呈线性关系增加; 激光功率大于0.5 W时, 随着激光功率以及加工次数的增加, 微流道宽度、深度、粗糙度以及堆积物火山口的高度逐渐增加。经过计算得出, PMMA的烧蚀阈值为0.357 J/cm2。通过优化工艺参数, 制备出粗糙度较低、表面光滑、深度为16 μm的微流道芯片。     

Laser micro-processing of amorphous and partially crystalline Cu45Zr48Al7 alloy

This paper presents a microstructural study of laser micro-processed high-purity Cu₄₅Zr₄₈Al₇ alloys prepared by arc melting and Cu-mould casting. Microprocessing of the Cu₄₅Zr₄₈Al₇ alloy was performed using a Rofin DC-015 diffusion-cooled CO₂ slab laser system with 10.6-μm wavelength. The laser was defocused to a spot size of 0.2 mm on the sample surface. The laser parameters were set to give 300- and 350-W peak power, 30% duty cycle and a 3000-Hz laser pulse repetition frequency (PRF). About 100-micrometer-wide channels were scribed on the surfaces of disk-shaped amorphous and partially crystalline samples at traverse speeds of 500 and 5000 mm/min. These channels were analysed using scanning electron microscopy (SEM) and 2D stylus profilometry. The metallographic study and profile of these processed regions are discussed in terms of the applied laser processing parameters. The SEM micrographs showed that striation marks developed at the edge and inside these regions as a result of the laser processing. The results from this work showed that microscale features can be produced on the surface of amorphous Cu–Zr–Al alloys by CO₂ laser processing.     

Simulation of KrF laser ablation of Al2O3-TiC

Previous work by the authors on micromachining of Al₂O₃-TiC ceramics using excimer laser radiation revealed that a columnar surface topography forms under certain experimental conditions. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations show that the columns develop from small globules of TiC, which appear at the surface of the material during the first laser pulses. To understand the mechanism of formation of these globules, a 2D finite element ablation model was developed and used to simulate the time evolution of the temperature field and of the surface topography when a sample of Al₂O₃-TiC composite is treated with KrF laser radiation. Application of the model showed that the surface temperature of TiC rises much faster than that of Al₂O₃, but since TiC has a very high boiling temperature, its vaporization is significant only for a short time. By contrast, the surface temperature of Al₂O₃ rises above its boiling temperature for a much longer period, leading to a greater ablation depth than TiC. As a result, a small TiC globule stands above the Al₂O₃ surface. The results of the model are compared with experimental measurements performed by AFM. After three pulses, the height of the globules predicted by the model is about 340 nm, in good agreement with the height measured experimentally, about 400 nm.     

Investigation of the surface generation mechanism of mechanical polishing engineering ceramics using discrete element method

Machining technology about ceramics has been developed very fast over recent years due to the growing industrial demand of higher machining accuracy and better surface quality of ceramic elements, while the nature of hard and brittle ceramics makes it difficult to acquire damage-free and ultra-smooth surface. Ceramic bulk can be treated as an assemblage of discrete particles bonded together randomly as the micro-structure of ceramics consists of crystal particles and pores, and the inter-granular fracture of the ceramics can be naturally represented by the separation of particles due to breakage of bonds. Discrete element method (DEM) provides a promising approach for constructing an effective model to describe the tool–workpiece interaction and can serve as a predicting simulation tool in analyzing the complicated surface generation mechanism and is employed in this research to simulate the mechanical polishing process of ceramics and surface integrity. In this work, a densely packed particle assembly system of the polycrystalline Si₃N₄ has been generated using bonded-particle model to represent the ceramic workpiece numerically. The simulation results justify that the common critical depth of cut cannot be used as the effective parameters for evaluating brittle to ductile transformation in ceramic polishing process. Therefore, a generalized criterion of defining the range of ductile regime machining has been developed based on the numerical results. Furthermore, different distribution of pressure chain is observed with different depth of cut which ought to have intense relationship with special structure of ceramics. This study also justified the advantage of DEM model in its capability of revealing the mechanical behaviors of ceramics at micro-scale.     

Effects of laser operating parameters on metals micromachining with ultrafast lasers

180 femtoseconds (1 kHz) and 10 picoseconds (1-50 kHz) ultrafast laser micro-structuring of the metals Ti alloy, Al and Cu have been studied for the purpose of industrial application. The effects of some key laser operating parameters were investigated. The evolution of surface morphology revealed that laser pulses overlap in a range around the spatial FWHM can help to achieve optimal residual surface roughness. While observed ablation rate (unit: μm³ per pulse) changed dramatically with repetition rate due to the combined effects of plasma absorption, residual thermal energy and phase transition, higher throughput can be achieved with higher repetition rate. This study also indicated that residual surface roughness is almost independent of repetition rate at 10 ps temporal pulse length. The ablation depth is approximately proportional to the number of overscan; however, machining accuracy deteriorates, especially for femtosecond laser processing and metals with low thermal conductivity and short electron-phonon coupling time.     

Process mapping of laser surface modification of AISI 316L stainless steel for biomedical applications

A 1.5-kW CO₂ laser in pulsed mode at 3 kHz was used to investigate the effects of varied laser process parameters and resulting morphology of AISI 316L stainless steel. Irradiance and residence time were varied between 7.9 to 23.6 MW/cm² and 50 to 167 μs, respectively. A strong correlation between irradiance, residence time, depth of processing and roughness of processed steel was established. The high depth of altered microstructure and increased roughness were linked to higher levels of both irradiance and residence times. Energy fluence and surface temperature models were used to predict levels of melting occurring on the surface through the analysis of roughness and depth of the region processed. Microstructural images captured by the SEM revealed significant grain structure changes at higher irradiances, but due to increased residence times, limited to the laser in use, the hardness values were not improved.     

UV laser ablation of alumina ring faces for mechanical seal applications

Al₂O₃ seal ring faces were treated by KrF excimer laser irradiation. Surface characteristics induced by laser irradiation depend upon laser fluence, the number of laser pulses, the frequency and duration of the laser pulses, the rotation rate of the ring, and the processing atmosphere. Microstructural analyses of the surface and cross section of the laser-processed seal faces showed that, at low fluence (1.8 J/cm²), the surface is covered by scale due to the melting/resolidification processes. At high fluence (7.5 J/cm²), there is no continuous scaling on the surfaces. Material is removed by decomposition/vaporisation and the ablation depth is linearly dependent on the number of pulses; on the surface, a network of microcracks forms. The evolution of surface morphology and roughness is discussed with reference to composition, the microstructure and physical and optical properties of Al₂O₃, and laser processing parameters.     

Characterization of laser processing of single‐crystal natural diamonds using micro‐Raman spectroscopic investigations

The application of lasers for processing diamond has revolutionized the diamond industry and its applications in microelectronics, microelectromechanical system (MEMS) and microoptoelectromechanical system (MOEMS) technologies. The process quality can be evaluated using spectroscopic techniques. In the present investigation, four different types of Q‐switched solid‐state lasers (with different beam parameters), namely, a lamp‐pumped Nd:YAG laser operating at 1064 nm, a lamp‐pumped Nd:YAG laser operating at second harmonically generated 532 nm, a diode‐pumped Nd:YVO₄ laser operating at 1064 nm and a diode‐pumped Nd:YAG laser operating at 1064 nm, have been employed for the processing of a single‐crystal, gem‐quality, natural diamond. The main objective behind the selection of these lasers with different beam parameters was to study the effect of wavelength, pulse width, pulse energy, peak power and beam quality factor (M² factor) on various aspects of processing (such as microcracking, material loss and cut surface quality) and their relative merits and demerits. The overall weight loss of the diamond and formation of microcracks during processing have been studied for the above four cases. The characteristics of the graphite formed during processing, elemental analysis, surface morphology of the cut surface and process dynamics have been studied using micro‐Raman spectroscopy and scanning electron microscopy (SEM). We observed that laser cutting of single‐crystal diamonds used for industrial applications can be accomplished without microcracking or surface distortion using Q‐switched Nd:YAG lasers. This allows direct processing without extensive postgrinding and polishing stages. Very efficient diamond processing is possible using diode‐pumped lasers, which results in the lowest possible breakage rate, good accuracy, good surface finish and low weight loss. From the micro‐Raman and SEM studies, it is concluded that the surface quality obtained is superior when diode‐pumped Nd:YVO₄ laser is used, owing to its extremely high peak power. The maximum graphite content is observed while processing with lamp‐pumped Nd:YAG laser at 532 nm. An overall comparison of all the laser sources leads to the conclusion that diode‐pumped Nd:YAG laser is a superior option for the efficient processing of natural diamond crystals. Copyright © 2006 John Wiley & Sons, Ltd.     

Picosecond laser ablation of nickel-based superalloy C263

Picosecond laser (10.4 ps, 1064 nm) ablation of the nickel-based superalloy C263 is investigated at different pulse repetition rates (5, 10, 20, and 50 kHz). The two ablation regimes corresponding to ablation dominated by the optical penetration depth at low fluences and of the electron thermal diffusion length at high fluences are clearly identified from the change of the surface morphology of single pulse ablated craters (dimples) with fluence. The two corresponding thresholds were measured as F _th(D1)¹=0.68±0.02 J/cm² and F _th(D2)¹=2.64±0.27 J/cm² from data of the crater diameters D _1,2 versus peak fluence. The surface morphology of macroscopic areas processed with a scanning laser beam at different fluences is characterised by ripples at low fluences. As the fluence increases, randomly distributed areas among the ripples are formed which appear featureless due to melting and joining of the ripples while at high fluences the whole irradiated surface becomes grainy due to melting, splashing of the melt and subsequent resolidification. The throughput of ablation becomes maximal when machining at high pulse repetition rates and with a relatively low fluence, while at the same time the surface roughness is kept low.     

Crackless linear through-wafer etching of Pyrex glass using <Emphasis Type="Italic">liquid-assisted</Emphasis> CO<Subscript>2</Subscript> laser processing

Pyrex glass etching is an important technology for the microfluid application to lab-on-a-chip devices, but suffers from very low etching rate and mask-requiring process in conventional HF/BOE wet or plasma dry etching as well as thermal induced crack surface by CO₂ laser processing. In this paper, we applied the liquid-assisted laser processing (LALP) method for linear through-wafer deep etching of Pyrex glass without mask materials to obtain a crackless surface at very fast etching rates up to 25 μm/s for a 20 mm long trench. The effect of laser scanning rate and water depth on the etching of the 500 μm thick Pyrex glass immersed in liquid water was investigated. The smooth surface without cracks can be achieved together with the much reduced height of bulge via an appropriate parameter control. A mechanism of thermal stress reduction in water and shear-force-enhanced debris removal is discussed. The quality improvement of glass etching using LALP is due to the cooling effect of the water to reduce the temperature gradient for a crackless surface and natural convection during etching to carry away the debris for diminishing bulge formation. An erratum to this article can be found at