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.     

Experimental investigation on the impact of stress free temperature on the electromigration performance of copper dual damascene submicron interconnect

Electromigration experiments are conducted for submicron dual damascene copper lower level interconnect samples of different stress free temperatures. The electromigration life-time is found to be strongly depend on the stress state of the metallization and the stress gradient that exist due to thermal mismatch of various materials surrounding the copper metallization. It is found that by reducing the stress free temperature, electromigration lifetime can be improved. In order to explain the life-time behavior, an atomic flux divergence based coupled field finite element model is developed. The model predicts a reduction in the atomic flux divergence at the electromigration test condition due to the reduction in the stress free temperature as the key factor responsible for longer electromigration life-time observed experimentally.     

Characteristic of copper wire and transient analysis on wirebonding process

In the present paper, two major analyses are achieved. In the first, experimental procedures were accomplished to measure tensile mechanical properties of copper (Cu) wire (? = 1 mil) before/after electric flame-off (EFO). Characteristics of free air ball (FAB), heat affected zone (HAZ) and thermal stable zone (TSZ) in as-drawn wire have been carefully investigated by microhardness, self-design pull test fixture, nanoindentation and atomic force microscopy (AFM). A 2nd EFO real-time technique has been conducted to reduce the strength of Cu wire and increase the bonding region. Secondary, with the obtained experimental material data, a comprehensive finite element wirebonding model based on explicit time integration software ANSYS/LS-DYNA is developed to predict the overall strain/stress distributions on the aluminum (Al) bond pad. Finite element analysis (FEA) results demonstrate that plastic deformation on Al bond pad around smashed FAB can be reduced by increasing the surface roughness on FAB. A series of comprehensive parametric studies were conducted in this research.     

冷原子束在线检测系统

应用荧光法和飞行时间(TOF)法实现了冷原子束纵向速度谱、通量及原子能态分布的在线检测。设计用于荧光收集的光学系统和机械结构,实现了焦距可微调的即插即用式荧光检测装置,实现了冷原子束检测系统的集成性和检测结果的高信噪比(SNR)。利用LabView软件实现对光电倍增管(PMT)、飞行时间法检测时序及检测激光的扫频范围等的控制,可在线得到原子束性能参数,并对数据进行平滑处理。检测结果表明,检测系统检测信号的信噪比为57:1(在20ms内)。     

Modeling the effect of barrier thickness and low-k dielectric on circuit reliability using 3D model

The continuous scaling down of the device size and escalating circuit speed drives the requirement for EM-resistant Cu interconnect with diffusion barrier and the low-k dielectric. The study of barrier layer thickness and low-k dielectric effect in a complete 3D circuit is necessary as the actual physical implementation of an integrated circuit in a wafer is indeed 3D in nature. This paper investigates the effect of barrier layer thickness and low-k dielectric on the circuit reliability of a complete 3D circuit model. It was found that the maximum atomic flux divergence (AFD) value increases with decreasing barrier layer thickness, which implied a shorter EM lifetime with thinner barrier. Low-k dielectric will give a higher maximum AFD due to higher stress gradient, and thus a shorter EM lifetime.     

Effects of Moisture on the Switching Characteristics of Oxide‐Based,Gapless‐Type Atomic Switches

Resistive switching memories based on the formation and dissolution of a metal filament in a simple metal/oxide/metal structure are attractive because of their potential high scalability, low‐power consumption, and ease of operation. From the standpoint of the operation mechanism, these types of memory devices are referred to as gapless‐type atomic switches or electrochemical metallization cells. It is well known that oxide materials can absorb moisture from the ambient air, which causes shifts in the characteristics of metal‐oxide‐semiconductor devices. However, the role of ambient moisture on the operation of oxide‐based atomic switches has not yet been clarified. In this work, current–voltage measurements were performed as a function of ambient water vapor pressure and temperature to reveal the effect of moisture on the switching behavior of Cu/oxide/Pt atomic switches using different oxide materials. The main findings are: i) the ionization of Cu at the anode interface is likely to be attributed to chemical oxidation via residual water in the oxide layer, ii) Cu ions migrate along grain boundaries in the oxide layer, where a hydrogen‐bond network might be formed by moisture absorption, and iii) the stability of residual water has an impact on the ionization and migration processes and plays a major role in determining the operation voltages. These findings will be important in the microscopic understanding of the switching behavior of oxide‐based atomic switches and electrochemical metallization cells.     

Nernst effect in Tl-Ba-Ca-Cu-O superconducting films due to infrared laser heating

A Nernst effect has been observed in a high temperature superconductor for the first time. Irradiating superconducting Tl?Ba?Ca?Cu?O thin films by short pulses of a TEA-CO₂ laser, a photovoltaic signal is detected perpendicular to a magnetic field applied parallel to the film surface. The signal is attributed to magnetic flux line depinning and flux line transport driven by the laser induced temperature gradient. The results are described by thermal flux line activation leading to a calculated distribution of pinning energies from 100 K to 4000 K.     

Soldering-induced Cu diffusion and intermetallic compound formation between Ni/Cu under bump metallization and SnPb flip-chip solder bumps

Nickel-based under bump metallization (UBM) has been widely used as a diffusion barrier to prevent the rapid reaction between the Cu conductor and Sn-based solders. In this study, joints with and without solder after heat treatments were employed to evaluate the diffusion behavior of Cu in the 63Sn-37Pb/Ni/Cu/Ti/Si₃N₄/Si multilayer structure. The atomic flux of Cu diffused through Ni was evaluated from the concentration profiles of Cu in solder joints. During reflow, the atomic flux of Cu was on the order of 10¹⁵–10¹⁶ atoms/cm²s. However, in the assembly without solder, no Cu was detected on the surface of Ni even after ten cycles of reflow. The diffusion behavior of Cu during heat treatments was studied, and the soldering-process-induced Cu diffusion through Ni metallization was characterized. In addition, the effect of Cu content in the solder near the solder/intermetallic compound (IMC) interface on interfacial reactions between the solder and the Ni/Cu UBM was also discussed. It is evident that the (Cu,Ni)₆Sn₅ IMC might form as the concentration of Cu in the Sn-Cu-Ni alloy exceeds 0.6 wt.%.     

Modeling of interfacial sliding and film crawling in back-end structures of microelectronic devices

The fine-scale of interconnect structures in the back-end of modern microelectronic devices makes them susceptible to unusual, scale-sensitive deformation phenomena during processing or service because of internal stresses induced by thermal expansion mismatch between adjoining materials. During thermo-mechanical cycling associated with processing or service, dimensional changes may occur in Cu interconnect lines embedded in a low-K dielectric (LKD) due to plasticity/creep, strain incompatibilities may arise between Cu and LKD due to diffusionally accommodated interfacial sliding, and Cu lines may crawl or migrate via plastic deformation and interfacial sliding under far-field shear stresses imposed by the package. Although small, these effects can have a pronounced effect on component reliability. This paper presents shear-lag based modeling approaches to simulate out-of-plane (OOP) strain incompatibilities which arise within a single-layer Cu-LKD back-end structure (BES) during back-end processing, and in-plane (IP) deformation and migration of Cu interconnects within the BES after the chip is attached to a flip-chip package. Both models incorporate a previously developed constitutive interfacial sliding law, and help rationalize experimentally observed interfacial strain incompatibilities within Cu-LKD BES.     

Studies of the light output properties for a GaN based blue LED using an electro-optical simulation method

In this study, an electro-optical simulation method is developed to predict the light intensity distribution and luminous flux of an in-house fabricated GaN based blue LED chip. The entire modeling process links an electrical simulation with ANSYS and optical simulation with LightTools, by assuming a proportional relation between the distributed current density and light emission energy on the multiple quantum well (MQW) layer. Experimental results show that the proposed simulation method can give a good prediction on the light intensity distribution for a semi-packaged GaN based blue LED chip. Further analysis on the simulation results reveals that an increase of at most 8% of the luminous flux can be achieved when the current density is controlled to evenly distribute on the MQW layer whereas the chip structure and electro pattern remains the same.     

Electro- and thermomigration induced Cu3Sn and Cu6Sn5 formation in SnAg3.0Cu0.5 bumps

Two important trends in the microelectronics business are the development of three dimensional packaging solutions which increase the number electronics components on the same area, and the application of VLSI electronics under harsh environment conditions. Both trends lead to a growing importance of intermetallic compound (IMC) formation in Sn based solder joints. Due to miniaturization a growing part of the solder joint volume is transformed into IMCs and finally the reflow process becomes a transient liquid phase soldering (TLPS) process. For harsh environment applications TLPS enables the transformation of low melting Sn contacts into high melting IMC joints. In both cases a model for the prediction of migration-induced IMC formation is required for the fabrication of IMC joints.For the general prediction of the migration induced IMC formation the related material parameters are needed. Against this background the Cu₃Sn and Cu₆Sn₅ formation was observed during temperature storage tests on Amkor® Package-on-Package packages (12 × 12 mm) with SnAg_3.0Cu_0.5 ball grid arrays. A mathematical model was developed to calculate the average mass flux of Sn and Cu during the stress tests. Based on the mass flux values the activation energies and diffusion constants for Cu and Sn in Cu₃Sn and Cu₆Sn₅ were determined. Afterwards the temperature storage was combined with an AC and a DC current load to investigate thermo- and electromigration-related phenomena. Based on the IMC formation speed during the AC and the DC tests the heat of transport Q^* and the effective charge of the moving ion Z^* were calculated. An interpretation of the material parameters is given in consideration of the high defect density in Cu₃Sn and Cu₆Sn₅.     

粗糙面下方金属目标复合电磁散射的快速算法

为快速有效计算粗糙面下金属目标的复合电磁散射,提出了一种基于前后向迭代算法(FBM)和共轭梯度(CG)法的快速互耦迭代算(CCIA).首先建立目标与粗糙面的耦合积分方程组,并采用矩量法将其离散为矩阵方程.其次针对得到的耦合积分方程,用FBM求解粗糙面表面电流分布,用CG法求解目标表面电流分布,目标和粗糙面的相互作用通过更新两方程的激励项完成.最后,计算了高斯粗糙面下方无限长金属圆柱目标的复合电磁散射系数,当目标尺寸趋于零或目标深度趋于无穷时的结果与单独介质粗糙面相一致,验证了该数值方法的正确性;同时,讨论了不同粗糙面情况下该方法的收敛性,并分析了不同粗糙面媒质、目标尺寸和目标位置对双站散射系数的影响.     

Migration induced material transport in Cu–Sn IMC and SnAgCu microbumps

Microbumps consisting of intermetallic compounds like Cu₆Sn₅ or Cu₃Sn have a longer lifetime during electromigration tests than SnAgCu microbumps. To explain the difference in behavior of Cu–Sn IMCs and SnAgCu during stress test the migration induced mass flux was calculated for Cu₃Sn and Cu₆Sn₅. The results were compared to the mass flux in SnAgCu 305. Furthermore average effective charge values for Cu₃Sn and Cu₆Sn₅ were approximated by comparing the separated movement of Cu and Sn with three different models for an averaged mass flux in the IMCs.     

Development of lead-free wave soldering process

Lead-free wave soldering was studied in this work using a 95.5Sn/3.8Ag/0.7Cu alloy. A process DOE was developed, with three variables (solder bath temperature, conveyor speed, and soldering atmosphere), using a dual wave system. Four no-clean flux systems, including alcohol- and water-based types, were included in the evaluation. A specially designed "Lead-Free Solder Test Vehicle", which has various types of components, was used in the experiments. Both organic solderability preservative (OSP) and electroless nickel/immersion gold (Ni/Au, or ENIG) surface finishes were studied. Soldering performance (bridging, wetting and hole filling) was used as the responses for the DOE. In addition, dross formation was measured at different solder bath temperatures and atmospheres. Dross formation with Sn/Ag/Cu bath was compared to that with eutectic Sn/Pb bath. Regarding the connector-type component, a pad design giving the best soldering performance was evaluated based on the DOE results. Finally, a confirmation run with the optimum flux and process parameters was carried out using the Sn/Ag/Cu solder, and a comparative run was made with the Sn/Pb solder alloy and a no-clean flux used in production. The soldering results between the two runs indicate that with optimum flux and process parameters, it is possible to achieve acceptable process performance with the Sn/Ag/Cu alloy.     

Formation of the low-resistivity compound Cu<Subscript>3</Subscript>Ge by low-temperature treatment in an atomic hydrogen flux

The systematic features of the formation of the low-resistivity compound Cu₃Ge by low-temperature treatment of a Cu/Ge two-layer system in an atomic hydrogen flux are studied. The Cu/Ge two-layer system is deposited onto an i-GaAs substrate. Treatment of the Cu/Ge/i-GaAs system, in which the layer thicknesses are, correspondingly, 122 and 78 nm, in atomic hydrogen with a flux density of 10¹⁵ at cm² s^–1 for 2.5–10 min at room temperature induces the interdiffusion of Cu and Ge, with the formation of a polycrystalline film containing the stoichiometric Cu₃Ge phase. The film consists of vertically oriented grains 100–150 nm in size and exhibits a minimum resistivity of 4.5 µΩ cm. Variations in the time of treatment of the Cu/Ge/i-GaAs samples in atomic hydrogen affect the Cu and Ge depth distribution, the phase composition of the films, and their resistivity. Experimental observation of the synthesis of the Cu₃Ge compound at room temperature suggests that treatment in atomic hydrogen has a stimulating effect on both the diffusion of Cu and Ge and the chemical reaction of Cu₃Ge-compound formation. These processes can be activated by the energy released upon the recombination of hydrogen atoms adsorbed at the surface of the Cu/Ge/i-GaAs sample.     

Enhanced Thermoelectric Performance and Service Stability of Cu2Se Via Tailoring Chemical Compositions at Multiple Atomic Positions

Liquid‐like thermoelectric (TE) materials have the advantages of ultrahigh performance, low cost, and environment friendly, but their stability is greatly limited by the possible Cu/Ag deposition under a large current and/or temperature gradient. The pratical application based on liquid‐like TE materials requires both a high TE figure of merit (zT) for high energy conversion efficiency and large critical voltage for good stability, but they are very difficult to be simultaneously achieved in one material. In this work, both the zT and critical voltage are simultaneously optimized in Cu₂Se via tailoring chemical compositions at multiple atomic positions, i.e., introducing Cu deficiency at the Cu‐sites to lower Cu ion chemical potential and alloying sulfur at the Se‐sites to reduce carrier concentrations. A maximum zT of 2.0 at 1000 K has been successfully achieved for Cu_1.96Se_0.8S_0.2, about a 30% improvement over that for Cu₂Se. More importantly, Cu_1.96Se_0.8S_0.2 demonstrates a much higher critical voltage than Cu₂Se, yielding a greatly enhanced service stability under the conditions with/without a temperature gradient. An Ni/Mo/Cu_1.96Se_0.8S_0.2 TE unileg is successfully fabricated with a stable power output even after 400 thermal cycles between 473 and 873 K. This study greatly accelerates the real application of Cu₂Se‐based liquid‐like materials.     

Stabilizing an optoelectronic microwave oscillator with photonic filters

This paper compares methods of active stabilization of an optoelectronic microwave oscillator (OEO) based on insertion of a source of optical group delay into an OEO loop. The performance of an OEO stabilized with either a high-Q optical cavity or an atomic cell is analyzed. We show that the elements play a role of narrow-band microwave filters improving an OEO stability. An atomic cell also allows for locking the oscillation frequency to particular atomic clock transitions. This reports a proof-of-principle experiment on an OEO stabilization using the effect of electromagnetically induced transparency in a hot rubidium atomic vapor cell.     

Cu5Zn8 Growth Reversion in Cu/Sn-8Zn-3Bi/Cu During Discontinuous Electromigration

A Cu/Sn-8Zn-3Bi/Cu structure was used to investigate the intermetallic compound (IMC) growth behavior during discontinuous electromigration under current density of 10⁴?A/cm² at 70°C. Cu₅Zn₈ IMC formed at both the anode and the cathode interfaces, and the thickness increased with the stressing time. With prolonging the current stressing time, a bulged Cu₅Zn₈ layer was squeezed out between the former Cu₅Zn₈ layer and Cu substrate in the samples to relax the excess compressive stress. Additionally, due to the back stress gradient built up by the Sn diffusion, the Zn atomic flux reacted with Cu to form Cu₅Zn₈ at the cathode side when the power was turned off. Finally, the total IMC thickness of the anode and the cathode under discontinuous current stressing showed a ??reversion?? in the 69?h and 310?h samples.     

Study of Electromigration-Induced Failures on Cu Pillar Bumps Joined to OSP and ENEPIG Substrates

This work studies electromigration (EM)-induced failures on Cu pillar bumps joined to organic solderability preservative (OSP) on Cu substrates (OSP–bumps) and electroless Ni(P)/electroless Pd/immersion Au (ENEPIG) under bump metallurgy (UBM) on Cu substrates (ENEPIG–bumps). Two failure modes (Cu pad consumption and gap formation) were found with OSP–bumps, but only one failure mode (gap formation) was found with ENEPIG–bumps. The main interfacial compound layer was the Cu₆Sn₅ compound, which suffered significant EM-induced dissolution, eventually resulting in severe Cu pad consumption at the cathode side for OSP–bumps. A (Cu,Ni)₆Sn₅ layer with strong resistance to EM-induced dissolution exists at the joint interface when a nickel barrier layer is incorporated at the cathode side (Ni or ENEPIG), and these imbalanced atomic fluxes result in the voids and gap formation. OSP–bumps showed better lifetime results than ENEPIG–bumps for several current stressing conditions. The inverse Cu atomic flux (J _Cu,chem) which diffuses from the Cu pad to cathode side retards the formation of voids. The driving force for J _Cu,chem comes from the difference in chemical potential between the (Cu,Ni)₆Sn₅ and Cu₆Sn₅ phases.     

焊点尺寸对微焊点拉伸断裂强度的影响

采用基于动态力学分析仪(DMA)的精密拉伸试验与ANSYS有限元数值模拟方法研究了“铜引线/Sn-3.0Ag-0.5Cu钎料/铜引线”三明治结构微焊点(直径均为200 μm,高度为75～225 μm)的拉伸断裂行为.结果表明:微焊点直径不变而高度为225,175,125和75 μm时,其拉伸断裂强度分别为79.8,82...