双次曝光积分效应实现杂质浓度分布均匀化

激光诱导扩散中,当入射激光光强为高斯分布甚至均匀分布时，微小扩散区的温度分布不均匀。由于扩散系数是温度的函数，必将导致扩散后杂质浓度分布的均匀性较差，无法制作出高性能的p-n结。提出采用多次激光诱导扩散的积分效应来实现杂质浓度分布的均匀化整形。对于InP衬底的CO2激光诱导Zn扩散,利用温度闭环测控系统测得的基片表面热斑温度场分布，分析计算了两次激光诱导扩散重叠区域的浓度分布积分效应。在此基础上模拟计算出，用双次曝光积分效应做杂质浓度分布的均匀化整形时，基片上两次激光照射位置的最佳间隔为20 μm。这为改进激光诱导扩散工艺，用多次曝光实现面均匀的杂质浓度分布奠定了理论基础。     

双次曝光积分效应实现杂质浓度分布均匀化

 激光诱导扩散中,当入射激光光强为高斯分布甚至均匀分布时，微小扩散区的温度分布不均匀。由于扩散系数是温度的函数，必将导致扩散后杂质浓度分布的均匀性较差，无法制作出高性能的p-n结。提出采用多次激光诱导扩散的积分效应来实现杂质浓度分布的均匀化整形。对于InP衬底的CO_{2激光诱导Zn扩散,利用温度闭环测控系统测得的基片表面热斑温度场分布，分析计算了两次激光诱导扩散重叠区域的浓度分布积分效应。在此基础上模拟计算出，用双次曝光积分效应做杂质浓度分布的均匀化整形时，基片上两次激光照射位置的最佳间隔为20 μm。这为改进激光诱导扩散工艺，用多次曝光实现面均匀的杂质浓度分布奠定了理论基础。}     

10.6μm激光诱导扩散中热致破坏的抑制

在半导体激光诱导扩散实验中,用连续波CO2 10．6μm激光聚焦后照射基片表面。为实现局部区域的选择扩散,激光光斑半径仅数十微米。要使曝光区温度达到扩散实验要求,必须使曝光区功率密度很高。另一方面,Si、InP等半导体材料对10．6μm波长激光的吸收系数随温度的升高而增大,这导致实验时容易产生热致破坏,损伤基片。在分析热致破坏的产生机理后,提出了在聚焦激光束照射下,曝光区温度的数值计算方法。计算结果表明,在半导体基片初始温度为室温时,以恒定功率的激光束照射基片,曝光区温度不能稳定在扩散试验需要的温度范围。在此基础上,提出了预热基片及对曝光区温度进行实时控制等抑制热致破坏的方法,有效地克服了这一困难。这对于用激光微细加工制作出高性能的单片光电集成电路(OEICs)器件有重要意义。     

纳秒脉冲激光诱导CCD探测器铝层金属液-固相变时间特性研究

从理论仿真计算方面开展了脉冲激光诱导CCD探测器铝层金属温升变化的液-固相变时间特性研究。通过傅里叶热传导方程计算仿真了纳秒激光诱导CCD探测器铝层金属材料的温升曲线,获得了铝层金属材料液-固相变起始时刻和液-固相变时间长度随激光脉冲峰值功率和激光入射角度的变化规律。理论计算结果表明,随着入射激光脉冲峰值功率增加,激光诱导CCD探测器铝层表面的最高温度逐渐升高,铝层材料的液-固相变起始时刻往后延迟,且液-固相变时间长度增加;随激光入射角度的增大,铝层表面的最高温度逐渐降低,液-固相变起始时刻不断前移,而液-固相变时间长度逐渐变短。研究结果表明,激光脉冲峰值功率密度和激光入射角对激光诱导CCD探测器的液-固相变时间特性有重要影响,对揭示纳秒激光诱导CCD探测器的热损伤机制有重要的理论意义。     

基于激光诱导击穿光谱的波长校准法研究

基于线列光探测器件的光谱仪具有多光谱通道探测的优点，以及其在物质光谱分析中担当的重要角色。分析了基于线列光探测器件光谱仪的波长准确度问题，提出利用激光诱导击穿原子光谱丰富波长校准特征波长谱线的思想。实验选用光谱波长范围为200nm～600nm的凹面光栅光谱仪、2048元焦平面的CCD器件和低压汞灯，分别对铜、铝、镁、钙、硅等5种样品进行了激光诱导击穿光谱试验；利用选取的10条激光诱导原子光谱波长数据并结合低压汞灯的10条特征谱线对光栅光谱仪重新进行波长校准，获取的试验数据有力地证明了这种思想的可行性。     

ns激光辐照硅光电探测器的损伤阈值

以He-Ne激光作为参考光,采用反射光能量法,测量了ns激光辐照硅光电探测器的损伤阈值,并测量了不同功率密度的强激光辐照下探测器对参考光的反射率.实验结果表明,ns激光辐照硅光电探测器的损伤阈值为4.1×106W/cm2,在探测器被强激光损伤的初期阶段,探测器对参考光的反射率下降很快,继续增加入射激光的能量,探测器对参考光的反射率下降趋于平缓.     

强激光辐照下光电探测器响应性能的研究

 响应度R是反映探测器性能的一项重要指标,当探测器被强激光损伤后,光电探测器的响应度将发生改变。设计了一套实时测量探测器响应度的装置,用能量逐渐增加的Nd∶YAG激光辐照PIN光电探测器,获得了探测器响应度与入射强激光功率密度的变化关系。从实验数据可知,探测器被功率密度低于7.6×105W/cm2的激光辐照后不会发生损伤,探测器对532nm参考光的响应度不变;当激光的功率密度超过1.27×106W/cm2时,激光辐照后,探测器对532nm参考光的响应度开始下降,当探测器被功率密度为6.01×106W/cm2的激光辐照后,响应度迅速下降,PN结遭到破坏是探测器响应度下降的根本原因,扫描电镜的结果与我们的分析相一致。     

用于软X光能谱测量的Ross滤片对的设计

 描述了一种测量激光等离子体发射的软X光能谱强度的宽带Ross滤片谱仪。该谱仪具有多个能道,每道由一对Ross滤片及X光探测器组成。这些能道的灵敏带没有间隔地覆盖了整个感兴趣的能区。给出了使用Al阴级X光二极管作为探测器的最佳9能道谱仪(0.1~1.5keV),并且计算了X光截面数据和滤片厚度不准确、以及滤片中光子散射等因素对探测精度的影响。     

基于OPO脉冲激发的血糖光声检测影响因素研究

构建了一套基于侧向模式的光学参数振荡(optical parameter oscillator,OPO)脉冲激光激发-超声传感的血糖光声检测系统,并且将激发波长、激光能量、探测频率和温度等对血糖光声检测的影响,在系统结构上进行了有效融合。然后,以不同浓度的葡萄糖溶液作为测试样品,实验研究了几个因素(激发波长、激光能量、探测频率和温度等)对葡萄糖光声检测和浓度预测的影响。实验得到了不同浓度葡萄糖溶液在不同影响因素条件下的葡萄糖实时光声信号和光声峰峰值,同时,为了得到不同因素对葡萄糖光声值和浓度预测的影响规律,通过线性回归算法建立了葡萄糖光声峰峰值与各因素变化,以及不同因素下光声峰峰值与浓度梯度的映射关系模型。实验和模型预测结果表明：相比1 200和1 300 nm波长而言,1 064 nm波长下的葡萄糖光声峰峰值与浓度梯度模型预测效果最好,模型相关系数达到0.986;葡萄糖光声峰峰值随着脉冲激光输出能量的增大呈线性增大趋势,且葡萄糖浓度预测准确度随着激光输出能量增大有所提高;根据不同探测频率的超声探测器捕获的葡萄糖光声峰峰值与浓度梯度建模效果来看,响应频率为1 MHz的超声探测器相比2.5和10 MHz的超声探测器,在葡萄糖光声信号探测上具有更好的探测效果;最后,实验发现,葡萄糖光声峰峰值随着温度和浓度的增大均呈线性增大趋势,且浓度预测误差随着温度的升高逐渐增大。     

用于高功率密度光束控制的光寻址光阀研制

为解决光寻址液晶光阀在高功率密度光束控制领域的应用限制,介绍一种可用于高功率密度激光系统的光寻址液晶光阀,该光阀开关比不低于140∶1,可在高于2300 W/cm²的连续激光系统中正常工作。同时,所研制的光阀可在高重频吉瓦(GW)级功率密度的fs脉冲激光系统中正常工作,在该系统最大功率密度激光作用下,光阀未见明显温度变化,该脉冲激光系统最大平均功率密度超过300 W/cm²。     

In-situ electrical characterisation of a photodiode during nano-structuring with a focussed ion beam

We study the fabrication and power conversion efficiency of GaAs photodiodes, which have been nano-structured and covered with colloidal quantum dots. A focussed ion beam is used to etch vertical channels into the photodiodes and the detrimental effects of this treatment are characterised in-situ during the fabrication process. A novel experimental configuration allows the electrical characterization of the photodiodes under laser illumination during the nano-fabrication process and reveals the gradual decrease of the photodiodes’ shunt resistance with increasing laterally revealed surface along the etched channels. This is interpreted as evidence for leakage currents through redeposited material and surface states on the lateral channel surface. After the fabrication step the channels are filled with colloidal quantum dots, which upon absorption of light transfer electronic excitations to the photodiode via resonance energy transfer. It is found that after the addition of quantum dots the nano-structured photodiodes show larger enhancements of the energy conversion efficiency under simulated solar irradiance than the pristine photodiodes. Nevertheless, the device degradation induced by the ion beam treatment itself cannot be compensated for.     

Study on the structural characteristics of HgCdTe photodiodes using laser beam-induced current

The structural characteristics of typical n⁺-on-p HgCdTe photodiodes have been studied by laser beam-induced current (LBIC). The dependence of LBIC on laser wavelength, junction depth and localized leakage has been presented. The diffusion length of minority carrier of p-type region is extracted by the exponential decay fitting of the curve of LBIC. It is found that the peak magnitude of LBIC and junction depth approximate to a linear relationship for practical values of device fabrication. The diffusion length monotonously increases with the junction depth. A notable shift of LBIC profile is observed when localized leakage exists. This provides a useful explain for LBIC applying to characterize the structure and process uniformity of HgCdTe infrared detectors.     

On the delamination and crack formation in a thin wall fabricated using laser solid freeform fabrication process: An experimental–numerical investigation

Parts fabricated using laser solid freeform fabrication (LSFF) are subject to thermal stresses due to the layer-by-layer material deposition and the temperature distribution characteristic throughout the process domain. The thermal stress patterns and intensity contribute significantly to potential delamination and crack formation. In this paper, the temperature distribution and stress field induced during the multilayer LSFF process, and their correlation with delamination and crack formation are studied. This is performed by a numerical and experimental investigation in the fabrication of a thin wall of 304L stainless steel. For time-dependent predictions on the locations of maximum temperatures and thermal stresses and their patterns, a three-dimensional (3D) transient finite element model is employed to simulate the process, including the geometry of the deposited materials as well as coupled temperature and stress distributions across the process domain. The experimental results are used to verify the numerical results as well as to investigate the correlation between the numerical results and micro-crack formations across the fabricated parts. The experiments are conducted with the same process parameters used in the numerical analyses using a 1 kW Nd:YAG pulsed laser. The trend of numerical and experimental results reveals that by preheating the substrate prior to the fabrication process, it is possible to substantially reduce the micro-cracks formed across the part. To demonstrate the feasibility of preheating on the reduction of micro-cracks, several simulations and experiments are performed in which a crack-free result is obtained when the substrate is preheated to 800 K. For this case, 22% reduction in thermal stresses is obtained throughout the process domain.     

Fabrication of a random convex-lens-shaped microstructure using a laser ablation

A convex-lens-shaped microstructure with a diameter of 50 μm on a metallic mold substrate was fabricated in this paper. A laser ablation process, in which the laser beam was focused and irradiated on the metallic mold substrate in order to remove a part of the substrate, was used for that. The convex-lens-shaped microstructure has not been reported in any studies of microstructure using the laser ablation process. It was proposed that the unbalanced ablation and re-adherence of the melted particles was the processing mechanism of the convex lens shape. The convex-lens-shaped microstructure fabricated in this study is smaller than the focused spot. It was expected that the same convex-lens-shaped microstructure can be fabricated even if the focused spot size is increased, so long as the fluence of the laser can be maintained. Therefore, the method proposed in this paper will improve the low processing speed, which has been the problem of a laser ablation process. The fabricated convex-lens-shaped microstructure on the metallic substrate can be used as the mold for the micro lens.     

Direct laser printing for high efficiency silicon solar cells fabrication

Silicon solar cells still require cost reduction and improved efficiency to become more competitive. New architectures can provide a significant increase in efficiency, but today most of the approaches need additional fabrication steps. In this context, laser processing offers a unique way to replace technological steps like photolithography that is not compatible with the requirements of the photovoltaic industry. In particular laser induced thermal effects can be used to activate or re-organise dopants at the silicon surface to design new emitter geometries. In this paper dopant diffusion using a nanosecond UV laser on phosphorous-doped silicon emitters is studied. The presence of a phosphosilicate glass underneath a silicon nitride layer leads to a local decrease of the emitter sheet resistance from 100 Ω/sq to 20 Ω/sq. Laser induced damage, phosphorus diffusion profile and electrical shunt are assessed in the perspective of selective emitter silicon solar cells fabrication compatible with electrochemical metal contacts deposition.     

Si Micromachined Pinhole and Surrounding Photodiode for Incident Beam Position Sensing and Automatic Alignment

The spatial filter of a Si micromachined pinhole integrated with photodiodes is fabricated. The photodiode cells placed around the center pinhole can detect the relative position between the incident beam spot and the pinhole. The position signal obtained from a two or four cell type photodiode and position sensitive detector show the sensitivity even when the beam spot is near the center pinhole. Combined with the feedback control of the shape memory alloy actuator based on the obtained position signal, the pinhole is aligned automatically.     

飞秒激光微构造硅光电二极管的光电响应

在一定条件SF6气体氛围中,硅可在飞秒激光辐照区产生m量级的尖峰结构。针对不同尖峰高度的微构造硅,在不同温度下退火,采用电子蒸发的方法在正反面分别镀上铝电极,制备出了飞秒激光微构造光电二极管,并测试了其光电响应。实验结果表明:飞秒激光微构造光电二极管的响应随微构造硅光电二极管的尖峰高度和退火温度的不同而不同。尖峰高度为3～4 m的样品在973 K温度退火30 min后,响应度可达0.55 A/W。即使在1100 nm波长处,这种新型的硅光电二极管的响应仍可高达0.4 A/W。     

Fabrication and verification for the small-form-factor holographic optical pickup

In this investigation, a small-form-factor (SFF) pickup head with a holographic optical element (HOE) is fabricated. The system employs a finite-conjugate object lens to focus a light beam. A holographic optical element is used to simplify the optical configuration. It provides a better means of alignment of fabrication and reduces the size of system relative to reflective light route. Micro prisms are fastened with HOE and a silicon substrate, laser diode and photodiode are integrated into the optical system. The pickup head system based on discrete components and a flip chip bonder with highly accurate alignment was to integrate it. The micro holographic optical pickup head is fabricated and tested. Experimental results including the spot diameter and the focusing error signal (FES), demonstrate that the optical system is a feasible.     

A sensor for adenosine triphosphate fabricated by laser-induced forward transfer of luciferase onto a poly(dimethylsiloxane) microchip

Laser-induced forward transfer (LIFT) of the enzyme luciferase was explored as a potential technique to be used in the fabrication of a microchip adenosine triphosphate (ATP) sensor. Poly(dimethylsiloxane) (PDMS) was selected as the substrate for deposition of the luciferase. In comparison with other solid substrates, such as glass and polystyrene, it was found that the flexibility of PDMS made it a superior substrate for the immobilization of micro-spots of luciferase. LIFT of luciferase onto a PDMS substrate using a 355 nm laser was successfully carried out, while the bioactivity of the enzyme was maintained. Yellow luminescence ascribed to luciferase was observed from a transferred spot on the PDMS chip from the enzymatic reaction between luciferin and ATP. A microchip ATP sensor was also fabricated by attaching a small photodiode to the PDMS chip. On the basis of the fabricated microchip, the Michaelis-Menten relation between the luminescence intensity from the spot, and the ATP concentration was confirmed. The potential for fabricating biosensors using a combination of the LIFT technique with a PDMS substrate was shown to be very good.     

Impact of an extended source in laser ablation using pulsed digital holographic interferometry and modelling

Pulsed digital holographic interferometry has been used to study the effect of the laser spot diameter on the shock wave generated in the ablation process of an Nd:YAG laser pulse on a Zn target under atmospheric pressure. For different laser spot diameters and time delays, the propagation of the expanding vapour and of the shock wave were recorded by intensity maps calculated using the recorded digital holograms. From the latter, the phase maps, the refractive index and the density field can be derived. A model was developed that approaches the density distribution, in particular the ellipsoidal expansion characteristics. The induced shock wave has an ellipsoid shape that approaches a sphere for decreasing spot diameter. The ellipsoidal shock waves have almost the same centre offset towards the laser beam and the same aspect ratio for different time steps. The model facilitates the derivation of the particle velocity field. The method provides valuable quantitative results that are discussed, in particular in comparison with the simpler point source explosion theory.