无铅焊料的研究（4）——电迁移效应

电迁移是金属原子沿着电流方向的移动。阐述了无铅焊料中电迁移的物理特性,由于焊点的特殊几何形状,电流拥挤效应将发生在焊点与导线的接点处;电迁移效应导致无铅焊料中金属间化合物(IMC)的生成与溶解,以及焊点下的金属化层(UBM)的溶解和消耗,使原子发生迁移并会产生孔洞,造成焊点破坏,缩短了焊点平均失效时间(MTTF),从而带来可靠性问题。     

无铅焊料的电迁移效应

电迁移是金属原子沿着电流方向的移动。本文阐述了无铅焊料中电迁移的物理特性,由于焊点的特殊几何形状,电流拥挤效应将发生在焊点与导线的接点处;电迁移效应导致无铅焊料中金属间化合物（IMC）的生成与溶解,以及焊点下的金属化层（UBM）的溶解和消耗,使原子发生迁移并会产生孔洞,造成焊点破坏,缩短了焊点平均失效时间（MTTF）,从而带来可靠性问题。     

Sn-Ag-Cu无铅钎料互连焊点的电迁移研究进展

电迁移问题作为影响焊点可靠性的关键问题之一,容易导致焊点出现裂纹、丘凸和空洞等焊接缺陷.其失效机制有电流拥挤效应、焦耳热效应、极化效应和金属间化合物失效等.聚焦Sn-Ag-Cu系无铅钎料焊点的电迁移问题,介绍了这一领域电迁移的失效机制、影响因素和防止措施的研究现状,并展望了今后的研究发展趋势.     

铜互连线内电流拥挤效应的影响

提出了一种新的测试结构(S结构),通过实验、理论推导和有限元分析,研究了铜与TaN扩散阻挡层界面的电流拥挤效应对电迁移致质量输运特性的影响.实验和有限元分析表明,铜互连线内由于电流拥挤效应的存在,在用户温度下沿特定通道输运的局部原子通量显著增大,而焦耳热所产生的温度梯度对原子通量和通量散度增大的影响则相对有限.     

电子封装微互连中的电迁移

 随着电子产品不断向微型化和多功能化发展,电子封装微互连中的电迁移问题日益突出,已成为影响产品可靠性和耐久性的重要因素.本文在回顾铝、铜及其合金互连引线中电迁移问题的基础上,对目前微电子封装领域广泛采用的倒装芯片互连焊点结构中电迁移问题的几个方面进行了阐述和评价,其中包括电流拥挤效应、焦耳热效应、极化效应、金属间化合物、多种负载交替或耦合作用下的电迁移以及电迁移寿命预测等.     

高压大功率器件小电流控制技术研究

随着高压大功率双极功率集成电路在各种电源管理、功率驱动等领域中的应用日益广泛,产品的可靠性设计和控制显得尤其重要.介绍了高压大功率器件小电流的控制研究,说明了小电流形成的原理,及其对产品可靠性的影响;对各类曲线进行了详细分析,阐明了小电流的测试和控制技术,以及小电流对高压大功率双极型集成电路可靠性的重要性.     

深亚微米SRAM电流感应电路的分析和优化*

本文对随机掺杂浮动效应下传统的电流感应电路的可靠性做了定量的分析。主要考虑了晶体管尺寸、控制信号的下降时间和特定晶体管的阈值电压三方面对电流感应电路可靠性的影响。在这个基础上,我们做了最终的优化来提高电流灵敏放大器的可靠性。在90纳米工艺下,仿真结果显示最终优化后的电流感应电路的失败率能够比优化前减少百分之八十,而延时只是稍微增加一点。     

LED电极结构优化设计与仿真计算

LED电极结构极大地影响着LED芯片的电流扩展能力,优化电极结构,能够缓解电流拥挤现象.讨论了正装LED结构和倒装LED结构的电流分布模型,并通过SimuLED软件研究了电极结构对LED电流扩展能力的影响.仿真结果表明:采用插指型电板结构极大提高了正装LED的电流扩展能力,电极下方插入电流阻挡层(CBL)后改变了芯片的电流分布状况,有利于光效的提升;而倒装LED的通孔式双层金属电极结构利用两层金属的互联作用,使n电极能够在整个芯片范围内均匀分布,进一步提高了电流扩展性能.     

多电流路径抑制片上电感电流拥挤效应

从电磁理论出发解释了片上电感电流分配的原因,得出小横界面积金属的趋肤效应弱,大的金属线宽和相邻金属间距比的偶耦合临近效应小的结论.采用4层金属的0.35μm标准CMOS工艺制造片上电感,将差分电感的单电流路径,分成多电流路径并联,在保持了电感的电学对称性的前提下,抑制了电感的电流拥挤效应,电感的最大品质因数提高了40%,同时降低了其自谐振频率.     

多电流路径抑制片上电感电流拥挤效应

从电磁理论出发解释了片上电感电流分配的原因,得出小横界面积金属的趋肤效应弱,大的金属线宽和相邻金属间距比的偶耦合临近效应小的结论.采用4层金属的0.35μm标准CMOS工艺制造片上电感,将差分电感的单电流路径,分成多电流路径并联,在保持了电感的电学对称性的前提下,抑制了电感的电流拥挤效应,电感的最大品质因数提高了40%,同时降低了其自谐振频率.     

Reliability of AlGaInP light emitting diodes with an ITO current spreading layer

Three aging experiments were performed for AlGalnP light emitting diodes (LED) with or without indium tin oxide (ITO), which is used as a current spreading layer. It was found that the voltage of the LED with an ITO film increased at a high current stressing, while there was little change for that of the LED without the ITO. The results of the LEDs with different thicknesses of the ITO film show that the LED with a thicker ITO has a higher reliability. The main reason for the voltage increase of the LED with the ITO film might be the current crowding in the ITO film around the P-type electrode.     

ITO作为电流扩展层的AlGaInP 发光二极管可靠性研究

Three aging experiments were performed for AlGaInP light emitting diodes（LED） with or without indium tin oxide（ITO）,which is used as a current spreading layer.It was found that the voltage of the LED with an ITO film increased at a high current stressing,while there was little change for that of the LED without the ITO.The results of the LEDs with different thicknesses of the ITO film show that the LED with a thicker ITO has a higher reliability.The main reason for the voltage increase of the LED with the ITO film might be the current crowding in the ITO film around the P-type electrode.     

A study of thermal processes in high-power InGaN/GaN flip-chip LEDs by IR thermal imaging microscopy

Results of an experimental study of temperature fields generated in high-power AlGaInN heterostructure flip-chip light-emitting diodes (LEDs) via their self-heating at high working currents are presented. The method of IR thermal imaging microscopy employed in the study enables a direct measurement of the temperature distribution over the p-n junction area with a high resolution of ∼3 μm at an absolute measurement error of ∼2 K. It is shown that large temperature gradients may arise in high-power LEDs at high excitation levels as a result of current crowding. This effect should be taken into account when designing lightemitting chips and estimating the admissible operation modes. The method of IR thermal imaging microscopy can also reveal microscopic defects giving rise to current leakage channels and impairing device reliability.     

Titanium nitride as spreading layers for AlGaInP visible LEDs

Thin titanium nitride (TiN) films with low sheet resistance and high transparency were deposited on AlGaInP light-emitting diodes (LEDs) to improve light extraction from the LED surface. Comparison test devices were fabricated both with and without TiN spreading layers. Results show LED current crowding at high current is reduced for devices with TiN current spreading film, improving external efficiency. It is confirmed that TiN films are feasible as current spreading layers of AlGaInP LEDs.     

Improved performance of lateral GaN-based light emitting diodes with novel buried CBL structure in ITO film and reflective electrodes

In this letter, lateral GaN-based Light Emitting Diodes (LEDs) with a SiO₂ current blocking layer (CBL) buried in the indium tin oxide (ITO) film and highly reflective metal materials have been proposed. Compared with the conventional CBL structure which was inserted between ITO film and p-type GaN, simulation results showed that LEDs with a buried CBL in the ITO film effectively facilitated current spreading under the CBL. We demonstrated that buried CBL was beneficial for suppressing current crowding (CC) effect around the edge of CBL and may facilitate higher LED efficiency. Furthermore, experimental results showed that LEDs with the buried structure we proposed showed lower working voltage and higher light output power (LOP) compared with those with conventional CBL structure. These results further confirmed that the buried CBL scheme was effective to reduce current crowding (CC) effect. In addition, highly reflective metal materials of Cr/Al/Pt/Au were employed to reduce light absorption and achieve high light extraction efficiency.     

Consideration of the Actual Current-Spreading Length of GaN-Based Light-Emitting Diodes for High-Efficiency Design

Based on the proposed experimental method, the current spreading length of GaN-based light-emitting diodes (LEDs) was measured and analyzed for practical device design. In this study, Thompson's and Guo's models, which are categorized according to vertical series resistance (in particular, p-type contact resistance), were used to extract device parameters. It was shown that the measured current spreading length strongly depends on the injected current density. For LEDs fabricated with low-resistance p-type contacts, this behavior could be explained in terms of the accelerated current crowding with higher current densities occurring as a result of the reduced voltage drop across the junction, which is in good agreement with Thompson's relation. However, for LEDs fabricated with high-resistance p-contacts, unlike Guo's prediction, the measured current spreading length also showed a strong dependence on the injected current density. This was attributed to thermal heating at the p-contact, resulting in the reduction of the voltage drop across the p-contact and so junction voltage, which is also in agreement with Thompson's model. Based on the measured parameters and the design rule, efficient p-type reflectors, namely, hybrid reflectors were designed. Compared with conventional ones, LEDs fabricated with the hybrid reflectors exhibited better output power at a reasonable forward voltage, indicating that the proposed method is effective in understanding the actual current spreading and hence the practical design of high-efficiency LEDs.     

阵列化互连LED模组寿命分布的蒙特卡洛模拟

利用蒙特卡洛方法对阵列化互连LED模组的可靠寿命进行了模拟,假设分档后的大功率白光LED的正向电压符合正态分布,额定电流下的寿命符合对数正态分布,且寿命和电流、温度的关系符合Eying模型,研究了n×n(6≤n≤12)LED阵列的寿命分布.模拟结果表明,对于n×n LED阵列,寿命随LED数目的增加没有下降反而略有增加...     

Experimental study of electroluminescence and temperature distribution in high-power AlGaInN LEDs & LED arrays

Comprehensive analysis of current spreading, temperature distribution, and near-field electroluminescence of high-power “face-up” AlInGaN LEDs and LED Arrays was performed by combination of different experimental methods. Measurement results of thermal impedance and temperature distribution (mapping) of powerful light-emitting diode assemblies are described. A thermal resistance characterization consists in investigations of transient processes of temperature-sensitive parameter under heating by step-form or harmonically pulse-width modulated direct current and analysis using a thermal equivalent circuit (the Cauer and Foster models). By the involved method, thermal resistances of internal elements of the LEDs are determined. At the same time high resolution mapping of EL and thermal radiation was obtained by optical microscope and infrared images technique. It has been established correlation of the thermal resistance with a change in the current distribution (current crowding) at high excitation levels.     

Enhancement of (In,Ga)N light-emitting diode performance by laser liftoff and transfer from sapphire to silicon

(In,Ga)N light-emitting diodes (LEDs) fabricated on a sapphire growth substrate were successfully integrated onto Si substrates by a double-transfer technique using excimer laser liftoff and Pd-In transient-liquid-phase bonding. This transfer method resulted in a bonded LED heterostructure with the same orientation (p-side up) as the heterostructure before transfer from the sapphire growth substrate. Such a layer transfer approach enables a top and backside contact metallization scheme that reduces device series resistance, current crowding, and top electrode coverage area. Enhancement of the performance of the transferred LEDs was found in terms of the threshold voltage (at 20 mA) and the electroluminescence output from the front surface.     

A review on the humidity reliability of high power white light LEDs

High power white LEDs are replacing current lighting sources, not only for indoor usage, but also for outdoor and harsher environmental applications. This calls for higher reliability with respect to electrical, thermal as well as humidity. In this work, a comprehensive review on the study of humidity reliability of high power LEDs is provided, and the humidity induced degradation mechanisms in packaged high power white LEDs and their failure sites are described. The failure degradation mechanisms are divided into three groups, namely the package level, chip level and interconnect level degradations. Modeling of the moisture degradation is also described, and new test designed for the humidity study is also introduced. The inability of current acceleration model to extrapolate accelerated test results to normal operating conditions for high power LEDs is shown, and this provides a new challenge for the estimation of the lifetime of high power LEDs under normal applications, along with other challenges that need to be addressed.