Stabilizing contact resistance of isotropically conductive adhesives on various metal surfaces by incorporating sacrificial anode materials

The contact resistance stability of isotropically conductive adhesives (ICAs) on non-noble metal surfaces under the 85°C/85% relative humidity (RH) aging test was investigated. Previously, we demonstrated that galvanic corrosion has been shown as the main mechanism of the unstable contact resistance of ICAs on non-noble metal surfaces. A sacrificial anode was introduced into the ICA joint for cathodic protection. Zinc, chromium, and magnesium were employed in the ICA formulations as sacrificial anode materials that have much lower electrode-potential values than the metal pad surface, such as tin or tin-based alloys. The effect of particle sizes and loading levels of sacrificial anode materials were studied. Chromium was not as effective in suppressing corrosion as magnesium or zinc because of its strong tendency to self-passivate. The corrosion potential of ICAs was reduced by half with the addition of zinc and magnesium into the ICA formulation. The addition of zinc and magnesium was very effective in controlling galvanic corrosion that takes place in the ICA joints, resulting in stabilized contact resistance of ICAs on Sn, SnPb, and SnAgCu surfaces during the 85°C/85% RH aging test.     

碱性抛光液中表面活性剂对铜钽的电偶腐蚀

在阻挡层化学机械抛光(CMP)过程中,阻挡层材料钽(Ta)易与铜(Cu)发生电偶腐蚀.针对这一问题,通过电化学分析方法研究了碱性抛光液中非离子表面活性剂对铜钽腐蚀电位的影响;通过CMP实验研究了非离子表面活性剂对铜钽去除速率的影响.结果表明,随着非离子表面活性剂体积分数增加至9％,铜钽的腐蚀电位均降低.最终确定最佳非离子表面活性剂的体积分数为6％.此时,在静态条件下,铜钽电极之间的电位差为1 mV;在动态条件下,铜钽电极之间的电位差为40 mV,可极大地减弱铜钽电偶腐蚀.同时,铜钽的去除速率分别为47 nm·min-1和39 nm·min-1,铜钽去除速率选择比满足阻挡层CMP要求.     

Effect of 1,2,4-triazole on galvanic corrosion between cobalt and copper in CMP based alkaline slurry

Cobalt has become a new type of barrier material with its unique advantages since the copper-interconnects in the great-large scale integrated circuits (GLSI) into 10 nm and below technical nodes,but cobalt and copper have severe galvanic corrosion during chemical-mechanical flattening.The effect of 1,2,4-triazole on Co/Cu galvanic corrosion in alkaline slurry and the control of rate selectivity of copper and cobalt were investigated in this work.The results of electrochemical experiments and polishing experiments had indicated that a certain concentration of 1,2,4-triazole could form a layer of insoluble and dense passive film on the surface of cobalt and copper,which reduced the corrosion potential difference between cobalt and copper.Meantime,the removal rate of cobalt and copper could be effectively controlled according to demand during the CMP process.When the study optimized slurry was composed of 0.5 wt％ colloidal silica,0.1％vol.hydrogen peroxide,0.05 wt％ FA/O,345 ppm 1,2,4-triazole,cobalt had higher corrosion potential than copper and the galvanic corrosion could be reduced effectively when the corrosion potential difference between them decreased to 1 mV and the galvanic corrosion current density reached 0.02 nA/cm2.Meanwhile,the removal rate of Co was 62.396 nrn/min,the removal rate of Cu was 47.328 nm/min,so that the removal rate ratio of cobalt and copper was 1.32:1,which was a good amendment to the dishing pits.The contact potential corrosion of Co/Cu was very weak,which could be better for meeting the requirements of the barrier CMP.     

Experimental analysis of galvanic corrosion of a thin metal film in a multilayer stack for MEMS application

Experimental analysis of galvanic corrosion of an aluminium (Al)–chromium (Cr)–gold (Au) multilayer stack is presented in this paper. The use of two or more stacks of different metal films is common for realisation of various microelectromechanical system (MEMS) devices. However, patterning of the multilayer metal films by lithographic and etching process is very critical due to galvanic corrosion. In a multilayer metal stack film, the knowledge of etch rate of the individual metal layers is very important for designing the process flow for the fabrication of micro-sensors. In the present study, galvanic corrosion characteristics of Al–Cr binary metal stack and Al–Cr–Au ternary metal stack in different etching solutions have been studied. The intermetallic contact area and the exposed metal area in the electrolyte solution were varied using an innovative process step involving silicon shadow mask technique and lithographic process. It is observed from the experimental results that for an intermetallic contact area to exposed metal area ratio of 2, etch rate of aluminium layer is increased by more than two times in aluminium etchant and 80% in Cr etchant as compared to the etch rate of the aluminium layer without intermetallics effect. The results obtained from this study have been applied for designing the fabrication flow and successful realisation of a MEMS piezoresistive accelerometer.     

Artificial Graphite Paper as a Corrosion-Resistant Current Collector for Long-Life Lithium Metal Batteries

The employment of ultra-thin lithium metal anode with high loading cathode is the key to realizing high-energy-density rechargeable lithium batteries. Ultra-thin lithium foils are routinely loaded on a copper substrate in batteries, however, the close contact of these two metals causes galvanic corrosion in the presence of electrolyte, which results in irreversible consumption of lithium and decomposition of electrolyte. Herein, a lightweight and highly conductive flexible graphite paper (GP) is applied to replace Cu foil as the current collector for lithium metal anode. It is demonstrated that the application of GP prevents galvanic corrosion and maintains intimate and steady contact between Li foil and GP current collector during cycling, thereby improving the electrochemical performance of the battery. A 1.08 Ah pouch cell assembled with Li@GP anode and LiNi_0.8Co_0.1Mn_0.1O₂ cathode exhibits a long lifetime of 240 cycles with a capacity retention of 91.6% under limited Li, high cathode loading and lean electrolyte conditions.     

Anticorrosive Copper Current Collector Passivated by Self-Assembled Porous Membrane for Highly Stable Lithium Metal Batteries

The regulation of lithium plating/stripping behavior is considered to be critical for next-generation safe and high-energy-density lithium metal batteries. Lithium deposition with maximum granular size and minimum microstructural tortuosity can significantly improve the lithium plating/stripping efficiency. Here, a self-assembled organosilane layer with nanopores is constructed on Cu current collector surface via a thiol-Cu reaction. In contrast to typical stacked-particle morphology with small grain size and high specific area in ether electrolyte, dough-like and lateral-growth lithium deposition can be plated on the modified Cu current collector due to the low surface energy of a lithiophilic Si O Si membrane. The planar and dense lithium deposition contributes to the stable implementation of up to near 500 cycles in full cells with high-loading LiFePO₄ cathode. Anticorrosion in rigorous Cl-ion containing solution can even be achieved due to the corrosive repellency of hydrophobic organosilane. A high Coulombic efficiency (97.12%) is remained after corroding for 300 min. Moreover, the irreversible capacity loss caused by galvanic corrosion, an ignored but crucial aspect, has been significantly suppressed due to the passivation of high-redox-potential Cu by organosilane coating.     

Mechanisms underlying the unstable contact resistance of conductiveadhesives

One critical obstacle of current conductive adhesives is their unstable contact resistance with nonnoble metal finished components during high temperature and humidity aging. It is commonly accepted that metal oxide formation at the interface between the conductive adhesive and the nonnoble metal surface is responsible for the contact resistance shift. Two different mechanisms, simple oxidation and galvanic corrosion, both can cause metal oxide formation, but no prior work has been conducted to confirm which mechanism is the dominant one. Therefore, this study is aimed at identifying the main mechanism for the metal oxide formation and the unstable contact resistance phenomenon of current conductive adhesives. A contact resistance test device, which consists of metal wire segments and conductive adhesive dots, is specially designed for this study. Adhesives and metal wires are carefully selected and experiments are systematically designed. Based on the results of this systematic study, galvanic corrosion has been identified as the underlying mechanism for the metal oxide formation and for the observed unstable contact resistance phenomenon of conductive adhesives     

An integratable dual metal gate CMOS process using an ultrathin aluminum nitride buffer layer

We propose and demonstrate a novel approach for dual metal gate CMOS process integration through the use of a very thin aluminum nitride (AlN/sub x/) buffer layer between metal and gate oxide. This buffer layer prevents the gate oxide from being exposed to a metal etching process which potentially causes oxide thinning and damage. Subsequent annealing consumes the very thin AlN/sub x/ layer and converts it into a new metal alloy film by reacting with gate metals, resulting in no increase in EOT due to this buffer layer. The work function of the original gate metal is also modified as a result of its reaction with AlN/sub x/, making this approach extremely attractive for engineering the work function for dual metal gate CMOS applications.     

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.     

Development of compatible wet-clean stripper for integration of CoWP metal cap in Cu/low-k interconnects

Implementation of CoWP metal caps into Cu/low-k integration schemes requires a wet stripper that not only gives efficient cleaning but also has good compatibility to CoWP and low-k dielectrics. This paper describes a novel non-fluoride CoWP compatible stripper, developed based on a systematic study of the effect of stripper components, i.e. solvent, corrosion inhibitor, and stripper pH. Electrochemical methods were used to characterize galvanic corrosion of the CoWP/Cu couple and to estimate CoWP etch rate. Our studies showed that a traditional fluoride stripper caused severe damage to CoWP capping layer. The new stripper achieved a good balance between cleaning efficiency and compatibility to CoWP and low-k dielectrics, and demonstrated significant advantages in electrical properties over the traditional fluoride stripper.     

Adhesion improvement of thermoplastic isotropically conductive adhesive

Generally, isotropically conductive adhesive formulations include epoxy resin as the polymeric matrix. Although epoxy has superior adhesion capability, its drawbacks include the tendency to absorb moisture and lack of reworkability (thermosetting polymer). In this study, a thermoplastic polymer with low moisture absorption (0.28 wt%), called polyarylene ether (PAE2), is used in isotropically conductive adhesive (ICA) formulation. Previous research work by Lu et al. showed that the moisture absorbed into epoxy caused galvanic corrosion, which result in the formation of metal oxide . By using a polymer with low moisture absorption, the amount of water present in ICA will be small, and the corrosion rate and formation of metal oxide can be reduced. However, previous measurements of contact resistance stability of PAE2-based ICA showed that they are not stable on all surface finishes. It was determined that for thermoplastic-based ICA, poor adhesion was the main mechanism for unstable contact resistance. Two methods of adhesion improvement will be evaluated in this work. The first is to use coupling agents and the second is to blend the thermoplastic with epoxy. Both methods showed promise in improving the contact resistance stability of polyarylene ether based ICA.     

阵列基板栅极制程的Cu腐蚀研究

在大尺寸液晶显示器的薄膜晶体管(Thin Film Transistor,简称TFT)TFT工艺技术中,Cu正逐步取代Al作为电极材料。与Al电极制程相比,在进行栅极(Gate)制程时Cu容易发生腐蚀,这会降低产品良率。本文结合ADS(Advanced Super Demension Switch)显示模式下0+4掩膜板(mask)技术的Gate刻蚀制程和1+4掩膜版技术Gate光刻胶(Photo Resist,简称PR)剥离制程的Cu腐蚀现象进行分析,结合实验验证,确定Cu腐蚀原因,最终提出改善方案。实验结果表明:0+4mask技术的Gate制程中,ITO刻蚀液所含的HNO3会使MoNb/Cu结构电极的Cu发生电化学腐蚀;将电极结构更改为单Cu层则可以避免电化学腐蚀。在1+4mask技术的PR剥离(Strip)制程中,基板经历的剥离时间长或进行多次剥离或在剥离设备中停留,均会引起Cu腐蚀;增加剥离区间与水区间空气帘(Air Curtain)吹气量、增加TFT基板在过渡区间(H2O与剥离液接触的区间)的传输速度,管控剥离液使用时间等措施可以缓解Cu腐蚀。     

Characterization of copper precipitates on aluminum copper bond pads formed after plasma clean and de-ionized water exposure

Semiconductor bond pads made from aluminum and small percentages of copper is susceptible to galvanic corrosion. In galvanic corrosion, the cathode (copper precipitate) is usually protected by the aluminum oxide that covers the surface of aluminum which acts as the anode. However, when the aluminum oxide thickness is reduced by plasma cleaning, the precipitates can be exposed. When exposed precipitates come in contact with de-ionized water, galvanic corrosion takes place. Therefore, though plasma cleaning in general is supposed to improve semiconductor bond pad surface in preparation for package level interconnection, adding the plasma clean step just before a process with de-ionized water can cause bond pad corrosion through the galvanic reaction between the exposed precipitate (cathode) and the surrounding aluminum (anode). This paper aims to investigate the mechanism of corrosion and characterize corroded bond pads by using wire bond ball shear method.     

镀镍铜粉填充型电磁屏蔽硅橡胶性能研究

研究了镀镍铜粉填充型电磁屏蔽硅橡胶的力学性能、电性能、电磁屏蔽效能及耐电化腐蚀性能。结果表明,在保证其良好的力学性能前提下,镀镍铜粉填充型硅橡胶在30 MHz～18 GHz的频段范围内具有良好的屏蔽效能,并且可提供极好的耐电化腐蚀性能。在高温高湿环境中具有良好的电性能和屏蔽效能稳定性。通过与镀银铜粉填充型的硅橡胶相比较,发现材料体积电阻率的大小和屏蔽性能的优劣没有直接的相关关系。     

Case study: Root cause of fluorine detection during TiN ARC layer corrosion of AlSiCu metal lines

The present study examines the cause of fluorine detection during the corrosion of the TiN antireflection coat (ARC) layer of AlSiCu metal lines. When a crack is generated in the tetraethyl orthosilicate (TEOS) oxide or spin-on-glass (SOG) film of an LSI device, the corrosion of the TiN ARC layer (TiO_xN_y-oxidation) may occur due to residual moisture inside the device. In this case, concentrated fluorine is detected around the corroded TiN ARC layer by energy-dispersive X-ray spectroscopy (EDX) analysis. Fluorine concentration was correlated with the degree of corrosion on the TiN ARC layer, suggesting the contribution of fluorine to the corrosion of this layer. When a wider distribution of fluorine concentrations was evaluated, however, the concentration of fluorine and the degree of corrosion on the TiN ARC layer did not match; instead, a higher concentration of fluorine was observed near the crack of the TEOS oxide film. The corroded TiN ARC layer of the sample was then removed, and the Al line of the underlying layer was observed. Etching was observed on the Al line surface where a high concentration of fluorine was detected. More specifically, EDX analysis detected that fluorine reacted with the Al line in the underlying layer after diffusion through the TiO_xN_y film, causing decreased film density due to the corrosion of the TiN ARC layer.     

An Alternative Route Towards Metal–Polymer Hybrid Materials Prepared by Vapor‐Phase Processing

Transition metals incorporated into polymers lead to unusual or improved physical properties that significantly differ from those of purely organic polymers. A simple and practicable incorporation of diverse transition metals into any available polymer would make an important contribution to overcome some of the synthetic difficulties of metal‐polymer hybrid materials. Here, it is demonstrated that atomic layer deposition (ALD) can be a promising means to resolve some of those difficulties. It is found that even polytetrafluoroethylene (PTFE) with its great physical and chemical stability can be easily transformed into a transition metal–PTFE hybrid material simply by applying a metal‐oxide ALD process to PTFE. Upon metal incorporation into the PTFE, the molecular structure as well as mechanical properties (tensile behavior) of PTFE were observed to significantly change. For a better understanding of the changes to the material, experimental investigations using Raman spectroscopy, attenuated‐total‐reflection Fourier‐transform infrared spectroscopy, wide‐angle X‐ray diffraction, and energy‐dispersive X‐ray analysis were performed. In addition, with density functional theory calculations, potential bonding states of the incorporated metal into PTFE were modeled and predicted. The ALD‐based vapor‐phase approach for metal incorporation into a polymer could bring about rapid progress in the research area of metal–polymer hybrid materials.     

Copper wire bonding package decapsulation using the anodic protection method

We show that Cu wire connected at the GND terminal tends to corrode more than other terminals after fuming HNO₃ chemical etching. The electrochemical anodic protection method was found effective in preventing the corrosion of Cu wire in all terminals. Because the potential of Cu wires in mixed acid was maintained in the passive region, the anodic corrosion reaction of the GND terminal was prevented, therefore, the Cu wire disconnection drastically improved. Moreover, we found differences in the degree of corrosion of Cu wires depending on the metal masking tape used. Cu wires tended to easily disconnect when Al masking tape was used instead of Cu masking tape. The current flow from the Cu wire to the Al masking tape decreased the potential of Cu wire in mixed acid and moved it in the more active corrosion region. When the connection of the device and metal masking tape was nonelectrical, the corrosion of the Cu wire was minimized. Finally, we conclude that the anodic protection method is more effective than the nonelectrical connection method in controlling the corrosion of Cu wires in fuming HNO₃-rich acid solution. In fuming HNO₃-rich acid solution, the Cu wire disconnection rate improved from 90% to 4%.     

A novel sacrificial gate stack process for suppression of boronpenetration in p-MOSFET with shallow BF2-implantedsource/drain extension

A novel process flow employing a sacrificial tetraethyl orthosilicate/polycrystalline silicon (TEOS/poly-Si) gate stack is proposed for fabricating fluorine-enhanced-boron-penetration-free p-channel metal oxide semiconductor field effect transistors (p-MOSFET's) with shallow BF₂-implanted source/drain (S/D) extension. With the presence of the sacrificial TEOS/poly-Si gate stack as the mask during the shallow BF₂ implant, the incorporated fluorine atoms are trapped in the sacrificial TEOS top layer and can be subsequently removed. The new process thus offers a unique opportunity of achieving an ultra shallow S/D extension characteristic of the BF₂ shallow implant, while not suffering from any fluorine-enhanced boron penetration normally accompanying the BF₂ implant. Excellent transistor performance with improved gate oxide integrity has been successfully demonstrated on p-MOSFET's fabricated with the new process flow     

Chemical and mechanical adhesion mechanisms of sputter-depositedmetal on epoxy dielectric for high density interconnect printed circuitboards

Strong chemical reactions between metal and polymer substrates significantly enhance adhesion of the metal to the polymer. This study investigated the adhesion of three types of thin film metals, including Cu, NiCr, and Cr, to a fully epoxy-based polymer. Before depositing these thin film metals, the epoxy surface was treated with either an Ar or O₂ plasma etch. It was found that NiCr and Cr produced higher peel strengths than Cu, but NiCr and Cr did not produce different peel strengths than each other. It was also found that O₂ plasma etch produced significantly higher peel strengths than Ar plasma etch for Cu and Cr, but not for NiCr. An XPS (X-ray photoelectron spectroscopy) study was performed to investigate the reactivities and possible chemical adhesion mechanisms of the metal thin films with the epoxy. It was determined that Cr reacted more strongly than Ni in forming metal oxide at the metal-epoxy interface. Cu was not seen to react strongly in forming oxide with the epoxy. Thermodynamic information supported the relative amounts of oxides found by XPS. Thermodynamic information also suggested that O₂ plasma etch did not produce significantly higher adhesion than Ar plasma etch on the NiCr samples due to the large Ni component of the NiCr thin film. An AFM (atomic force microscopy) study was performed to investigate possible mechanical adhesion mechanisms. Implications of the AFM results were that the main adhesion mechanism for all samples was chemical and that the Cu oxide that was available on the Cu samples was beyond the detection limits of the XPS equipment     

Excimer Laser Projection Photoablation Patterning of Metal Thin Films for Fabrication of Microelectronic Devices and Displays

A nonlithographic process is demonstrated for patterning Al, Cr, Cu, Ni, Ti, and W thin films, which are widely used in microelectronic and display fabrication. A projection photoablation process using 248-nm-deep ultraviolet radiation from a KrF excimer laser was used to pattern a polyimide film coated on a SiN layer deposited on glass. The photoablation-patterned polyimide film was used as a sacrificial layer in a lift-off patterning process for the metal films, which resulted in clean metal patterns with fine line-edge definition being fabricated after lift-off. This process provides a simpler and more economical patterning technique compared to conventional lithography methods, eliminating the developing and etching steps.