An on-machine error calibration method for a laser micromachining tool

In this paper, an on-machine error calibration method, covering error modeling and measurement, is proposed to evaluate and compensate the errors caused by the mechanical and optical system equipped in the micromachining center using the femtosecond laser. Through preliminary tests by dicing silicon wafer, it has revealed that the squareness, laser beam misalign and focal position offset, are the main causes to result in the inaccuracy of micromachining. Consequently, an error modeling method is proposed to evaluate the error distribution in the workspace, and hereafter a comprehensive error vector of the laser beam, combining the squareness errors of Z-axis with the laser beam misalign, is generated by the variable substitution method. Subsequently, an increment error model in the instant local coordinates is established to satisfy the requirement of the programming method commonly used in the laser machine tools. Furthermore, a series of holes and grooves are machined on the femtosecond laser micromachining center to validate the proposed approach and model. The machining dimensions including diameters, distances and angles, are measured on-machine to identify the squareness errors, laser beam misalign and focal position offset according to the proposed error model. Finally, the experimental results show that, comparing to the uncompensated tests, the machining accuracy has been significantly improved with the proposed method.     

Numerical study on thermal stress cutting of silicon wafers using two-line laser beams

We propose a method of cleaving silicon wafers using two-line laser beams. The base principle is separating the silicon wafer using crack propagation caused by laser-induced thermal stress. Specifically, this method uses two-line laser beams parallel to the cutting line such that the movements of the laser beam along the cutting line can be omitted, which is necessary when using a point beam. To demonstrate the proposed method, 3D numerical analysis of a heat transfer and thermo-elasticity model was performed. Crack propagation was evaluated by comparing the stress intensity factor (SIF) at the crack tip with the fracture toughness of silicon, where crack propagation is assumed begin when the SIF exceeds the fracture toughness. The influences of laser power, line beam width, and distance between two laser beams were also investigated. The simulation results showed that the proposed method is appropriate for cleaving silicon wafers without any thermal damage.

     

Robust parameter design and multi-objective optimization of laser beam cutting for aluminium alloy sheet

The application of laser beam for precise cutting of sheet metals, in general, and reflective sheet metals, like aluminium, in particular, has become of interest in the recent past. The optimum choice of the cutting parameters is essential for the economic and efficient cutting of difficult to cut materials with laser beams. In this paper, a robust design and quality optimization tool called the Taguchi methodology has been applied to find the optimal cutting parameters for cutting of a reflective sheet made of aluminium alloy with a Nd:YAG laser beam. All the steps of the Taguchi method, such as a selection of orthogonal array, computation of signal-to-noise ratio, decision of optimum setting of parameters, and the analysis of variance (ANOVA), have been done by a self-developed software called computer aided robust parameter design (CARPD). A considerable improvement in the kerf taper (KT) and material removal rate (MRR) has been found by using Taguchi method-based predicted results. Confirmatory experimental results have shown good agreement with predicted results. Further, the Taguchi quality loss function has also been used for multi-objective optimization of laser beam cutting of Al-alloy sheet. The results of multi-objective optimization are compared with the single-objective optimization and it has been found that the kerf taper was increased by 1.60% in multi-objective optimization while the MRR was same in both cases.     

High saturation solar light beam induced current scanning of solar cells

The response of the electrical parameters of photovoltaic cells under concentrated solar irradiance has been the subject of many studies performed in recent times. The high saturation conditions typically found in solar cells that are subjected to highly concentrated solar radiation may cause electrically active cell features to behave differently than under monochromatic laser illumination, normally used in light beam induced current (LBIC) investigations. A high concentration solar LBIC (S-LBIC) measurement system has been developed to perform localized cell characterization. The responses of silicon solar cells that were measured qualitatively include externally biased induced cell current at specific cell voltages, I(V), open circuit voltage, V(oc), and the average rate of change of the cell bias with the induced current, DeltaV/DeltaI(V), close to the zero bias region. These images show the relative scale of the parameters of a cell up to the penetration depth of the solar beam and can be obtained with relative ease, qualifying important electrical response features of the solar cell. The S-LBIC maps were also compared with maps that were similarly obtained using a high intensity He-Ne laser beam probe. This article reports on the techniques employed and initial results obtained.     

Effect of high-frequency orbital and vertical oscillations of the laser focus position on the quality of the cut surface in a thick plate by laser beam machining

In laser beam machining with oxygen gas, striations are formed on the cut surface due to the cyclic oxidization reaction, which is affected by the cutting conditions and the thermal properties of the workpiece. The formation of striations causes an increase in surface roughness. In order to reduce the surface roughness, we propose controlling the formation of striations by utilizing multi-DOF oscillation of the focus position of the laser. In this paper, we construct a laser machining system in which positioning control of the focus position is achieved by driving the focal lens. The effect of orbital oscillation of the laser focus position parallel to the top surface of the workpiece and vertical oscillation perpendicular to this surface on the formation of striations was evaluated by performing laser cutting tests on thick mild steel plates. The experimental results show that control of the formation of striations can be realized by choosing the appropriate oscillation conditions. The surface roughness with oscillation was less than half that without oscillation.     

Finite element analysis of laser inert gas cutting on Inconel 718

Inconel 718 has high strength, which makes it difficult to cut using conventional cutting methods. In the present study, the laser inert gas cutting of Inconel 718 was simulated by finite element analysis software ANSYS. Finite element method was used to predict thermal stress and kerf width formation during the laser cutting process. ANSYS Parameter Design Language was used to model the Gaussian-distributed heat flux from the laser beam acting on the workpiece. The removal of melted material during laser cutting to form the kerf width was modeled by employing the element death methodology in ANSYS. In addition, laser cutting was simulated at continuous wave (CW) and the effects of laser power and cutting speed on kerf width were investigated. A series of experiments were carried out to verify the predictions. The temperature fields on the workpiece were measured using thermocouples. The kerf width size was measured using a profile projector, whereas the metallurgical and morphological changes at the cutting edge were examined using scanning electron microscopy. A good correlation was found between the simulation and experimental results.     

A study on soldering characteristics of laser tabbing system for crystalline silicon photovoltaic module production

The most significant task in the solar cell industry today is to minimize the cost of solar cell development, thereby establishing grid parity early. One way to achieve this goal is to reduce the thickness of silicon solar cell, which would subsequently result in reduction in raw silicon material costs. The most commonly used tabbing process in solar cell production today is the heating bar process. This process utilizes electric heating bars to heat the ribbons on solar cells. In this study, a laser tabbing machine was developed to overcome the problems of tabbing process of heating bar technique for a thin crystalline silicon solar cell. An electric test and peeling test were executed on soldering ribbon on solar cells. The results indicate that the bonding force of ribbon does not affect electrical output of the solar cell. The electric power of soldered solar cell was decreased around 5% in output as compared to the original unsoldered solar cell. The electric powers of the laser soldered module and the heating-bar soldered module were very close. The decline in efficiency of both modules was about 1.13%. As a result of this study, it was confirmed that the laser tabbing system developed in this research can be applied in module manufacturing processes.     

Design and implementation of a system for laser assisted milling of advanced materials

Laser assisted machining is an effective method to machine advanced materials with the added benefits of longer tool life and increased material removal rates. While extensive studies have investigated the machining properties for laser assisted milling(LAML), few attempts have been made to extend LAML to machining parts with complex geometric features. A methodology for continuous path machining for LAML is developed by integration of a rotary and movable table into an ordinary milling machine with a laser beam system. The machining strategy and processing path are investigated to determine alignment of the machining path with the laser spot. In order to keep the material removal temperatures above the softening temperature of silicon nitride, the transformation is coordinated and the temperature interpolated, establishing a transient thermal model. The temperatures of the laser center and cutting zone are also carefully controlled to achieve optimal machining results and avoid thermal damage. These experiments indicate that the system results in no surface damage as well as good surface roughness, validating the application of this machining strategy and thermal model in the development of a new LAML system for continuous path processing of silicon nitride. The proposed approach can be easily applied in LAML system to achieve continuous processing and improve efficiency in laser assisted machining.     

高精度激光光束准直系统设计

为了进一步提高激光光束用作直线基准的精度,建立了基于反射镜平动式光束稳定器以及两点式光束漂移分离法的高精度激光光束准直系统。首先,对系统中基于反射镜平动的光束稳定器进行研究,对其光束偏转原理以及影响因素进行分析,并对两点式光束漂移分离方法进行介绍。然后,对光束偏转单元的分辨力以及偏转范围、所使用的压电陶瓷非线性和迟滞特性、以及光束偏转单元的频率响应特性进行实验测试。最后,对该激光准直系统的激光光束准直精度进行测试。实验结果表明本文提出的光束偏转单元在120μrad范围内的光束偏转分辨力可以达到5 nrad,频率响应特性高于2 kHz;最终激光准直系统的准直精度在二维方向上分别达到1.9×10^-8 rad和2.1×10^-8 rad,相对于现有技术约提高了3倍,满足激光光束用作高精度直线基准的需求。     

Some studies on temperature profiles in AISI 304 stainless steel sheet during laser beam welding using FE simulation

The investigation of transient temperature profiles of a weld joint produced by the laser welding process is presented. A three-dimensional finite element model is developed using a commercial finite element code ANSYS in order to obtain the behavior of temperature field and molten pool shape during the welding process. A three-dimensional conical Gaussian heat source is employed as a heat source model for performing a non-linear transient thermal analysis. The temperature-dependent material properties of AISI 304 stainless steel sheet are taken into account, which has a great influence on the temperature fields indicated by the simulation results. The effect of latent heat and the convective and radiative boundary conditions are also included in the model. A series of laser welds are performed using a 2-kW continuous wave Nd:YAG laser welding system. The experimental trials are conducted by varying the laser input parameters namely beam power, welding speed, and beam incident angle to validate the model. The results show that there is a good agreement between the finite element simulation and the experimental observations.     

Femtosecond laser-induced simultaneous surface texturing and crystallization of a-Si:H thin film: morphology study

Hydrogenated amorphous silicon (a-Si:H) thin films have been considered for use in solar cell applications because of their significantly reduced cost compared with crystalline bulk silicon; however, their overall efficiency and stability are less than that of their bulk crystalline counterparts. Limited work has been performed on solving the efficiency and stability issues of a-Si:H simultaneously. Surface texturing and crystallization on a-Si:H thin film can be achieved through one-step femtosecond laser processing, which can potentially alleviate the disadvantages of a-Si:H in solar cell applications. In this study, submicrometer conical and pillar-shaped spikes are fabricated by irradiating a-Si:H thin films deposited on glass substrates with hundreds of 800 nm-wavelength, 130 fs-duration laser pulses in air, and water environments, respectively. The formation mechanisms for the surface spikes are discussed, and the differences in the surface feature characteristics are also presented and explained within the context of the different processing environments. The effect of laser processing on light absorption and crystallinity will be studied later.     

Parameter optimization of non-vertical laser cutting

In some cases, in order to avoid interference during 3D laser cutting of thin metal a laser head could not be kept vertical to the surface of a work piece. In such situations, the cutting quality depends not only on “typical” cutting parameters but also on the slant angle of the laser head. Traditionally, many tests had to be done in order to obtain the best cutting results. In this paper, an experimental design is employed to reduce the number of tests and an artificial neural network (ANN) is set up to describe quantitatively the relationship between cutting quality and cutting parameters in the non-vertical laser cutting situation. A quality point system is used to evaluate the cutting result of the thin sheet quantitatively. Testing of this novel method shows that the calculated “quality point” using ANN is quite closely in accord with the actual cutting result. The ANN is very successful for optimizing parameters, predicting cutting results and deducing new cutting information.     

高功率激光热效应对合束系统的影响

提出利用镀膜合束的方法对三路光束进行合束用于高功率红外激光合束系统设计。考虑系统中关键元件使用的红外材料ZnSe易受热效应影响,采用光机热耦合分析方法,研究了在温度边界条件固定时,各波段激光所产生的耦合热效应对各路激光波前畸变的影响,同时定性分析了系统中存在的激光偏置热效应。研究结果显示,系统中各波段的激光波前畸变均方根值(RMS)均满足设计要求(各波段波前畸变小于λ/8);激光偏置造成的波面高频成分增大了长波激光波前畸变量,但高频成分对系统波前畸变影响依然满足要求;轴向温差可在35s达到平衡,对光束波前造成主要影响的是各块镜片的面型畸变。根据分析结果搭建了实验平台,利用系统中短波400 W激光进行实验,采集了该条件下的面型并与仿真结果进行了对比,实验结果验证了该分析方法计算结果的准确性。     

分体式激光扩束系统平行度测量装置的设计

为精确测量强激光发射系统中高功率激光经分体式扩束系统后光束的传输方向,设计了一种新型分体式扩束系统输出光平行度测量装置.根据高功率激光分体式扩束系统及红外激光的特点,该装置采用高分辨率红外CCD作为监测成像设备.采用轻质高刚度的优质铝合金对装置的机械结构进行了设计,切换部件搭载在高精度线性位移平台上.基于高分辨率CCD和精密线性位移平台,该装置可较好地完成动态和静态测量.测试结果表明,该平行度测量装置工作稳定、可靠,测量精度优于2.0″;装置设计合理,实用,可为扩束系统的装调及应用提供可靠依据.     

Modelling of the transient thermal field in laser surface treatment test

In order to develop adequate predictive models of the laser heat treating process in the recent past, the laser surface treatment (LST) test was proposed to relate the obtained microstructure to the known thermal cycle undergone by the material. One of the flat end surfaces of a cylindrical steel rod is exposed to a laser beam. As a consequence the material undergoes different thermal cycles and different microstructure transformations due to the heating effect of the laser beam that it propagates inside the material. The paper presents a thermal model that can be used in the LST test to predict the thermal field inside the rod exposed to the laser beam. The model describes the thermal field generated by a uniform and time-dependent heat source, and also accounts for the convection losses that are due to gas impinging on the exposed surface. The model was verified by comparing the obtained thermal field to direct measurements by thermocouples placed along the rod and to the thermal field obtained with a numerical simulation. Results were found to be in good agreement with the experimental measurements and the numerical results.     

水辅助激光加工技术的实验研究

介绍水辅助激光打孔和切割实验研究。研究表明 ,用毫秒级YAG脉冲激光对不锈钢和Al2 O3 陶瓷加工时 ,熔屑易从加工区排出 ,有助于提高加工的表面质量 ;加工单晶硅时 ,加工表面易产生微裂纹 ,使加工质量变差。激光通过水层时 ,有能量损失 ,水层深度越深 ,能量损失越大。     

激光清洗硅片表面Al2O3颗粒的试验和理论分析

以KrF准分子激光器为激光源,对目前工业上常用的硅片研磨抛光液的主要成分Al₂O₃颗粒进行激光清洗的试验和理论分析。建立一维热传导模型,利用有限元分析软件MSC.MarC模拟硅片表面的温度随激光作用时间和能量密度的分布。通过理论计算,量化了颗粒所受到的清洗力以及其与硅片表面之间的粘附力,理论预测出1 μm Al₂O₃颗粒的激光清洗阈值为60 mJ/cm²。在理论分析的指导下,利用248 nm、30 ns的KrF准分子激光进行单因素试验,研究激光能量密度、脉冲个数、激光束入射角度对激光干法清洗效率的影响,并且实验验证了清洗模型以及场增强效应对激光清洗结果的影响。     

Investigation of damage generation process by stress waves during femtosecond laser drilling of SiC

Femtosecond laser processing has garnered significant attention as a method for micromachining silicon carbide (SiC), which is expected to be used in next-generation power semiconductors. However, a significant amount of damage is generated around the processing area during the femtosecond laser processing of SiC. In this study, high-speed phenomena during the femtosecond laser drilling of SiC are observed with high temporal and spatial resolutions by combining pump-probe imaging and a high-speed camera. In addition, a stress wave propagation simulation based on experimental results is conducted. Based on the experimental and simulation results, the mechanism of damage generation by the stress wave generated during material removal is elucidated. The damage generation mechanism elucidated in this study will facilitate the development of damage suppression methods and the expansion of industrial applications of SiC.     

硅片的激光吸收系数修正与弯曲试验

考虑热物性能参数随温度变化的因素,以硅为对象进行激光弯曲模拟和试验,借助APDL语言编写激光弯曲成形的仿真程序,对单脉冲作用过程进行模拟,以得到单点脉冲周期内的温度分布;并采用NiCr/NiSi合金薄膜热电偶对单脉冲作用过程中的温度分布进行测量,对比上述的温度模拟与测量结果,修正硅材料的激光综合吸收系数。采用有限元分析软件实现了硅片的脉冲激光弯曲成形的仿真和模拟,并对多次连续扫描的模拟结果与硅片弯曲试验结果进行对比,验证了仿真程序的有效性,为硅片的激光弯曲成形提供了理论与试验依据。     

厚硅片的高速激光切片研究

基于激光切片原理的分析,给出了厚硅片的高速激光切片方法,采用平凸腔补偿工作物质的热透镜效应,利用Nd∶YAG棒本身的自孔径选模作用,获得了光束质量因子M²等于4.19的50 W 1.064 μm激光输出。选取合适的扩束倍数、重复频率和出气孔直径,当切割0.75 mm厚的硅片时,切片速度达400 mm/min;当切割两层叠放的0.75 mm厚的硅片时,切片速度达到100 mm/min。切片的切口光滑,切缝较窄,重复精度高,切片质量好,达到用传统方法难以达到的切片效果。