High Electromigration Resistance Research Articles

Low-temperature hybrid bonding with high electromigration resistance scheme for application on heterogeneous integration

The novel low-temperature Cu/SiO2 hybrid bonding scheme using cluster-Ag passivation has been proposed in this study for the heterogeneous integration application. With the addition of cluster-Ag passivation layer, electromigration and reliability of hybrid bonding structure have been investigated for the first time, indicating negligible deterioration of electrical performance in the bonding structure after applying 107 A/cm2 current density for over 2000 h. The bonding mechanism of cluster-Ag passivation has been explored by the analysis of HR-TEM, and the difference between cluster-Ag passivation and thin-film Ag passivation has been discussed in detail. Based on an ultrathin cluster-Ag passivation layer, Cu-Cu bonding can be achieved at 70 ℃, showing high mechanical strength and good reliability. Therefore, the achievements reported in this paper realize Cu-Cu bonding and hybrid bonding with high quality and high reliability under ultra-low thermal budget, paving the way for the development of next-generation heterogeneous integration and three-dimensional integrated circuits (3DIC) technologies.

P‐5.4: The Improvement of Wet Stripping Residuals in Cu Process

Copper conductor has lower electrical resistivity (approximately) 2μ͉·;cm) and higher electromigration resistance than aluminum conductor, so it is applied to TFT to solve the undercharge of pixel electrode resulted from aluminum conductor. However there are filiform residuals generated through stripping in the copper process, and more serious than aluminum process. In this paper, we verified through a series of experiments that the quality improvement of clean water of stripping can improve the filiform residuals.

Thermal interface material with graphene enhanced sintered copper for high temperature power electronics

Sintered nano-copper is becoming a promising candidate as thermal interface material (TIM) for die attaching in high power electronics. It exhibits much higher thermal conductivity and operating temperature than conventional TIMs based on polymer and solder joints, and higher electromigration resistance and lower cost than sintered nano-silver TIM. However, the performance of existing sintered nano-copper is lower than expected because of high porosity resulted from poor sintering of copper particles with oxide shell. Here we demonstrate a method of improving the thermal conductivity of sintered copper by addition of graphene/Cu–Cu x O with controllable diameter of ∼163 nm. The measured thermal conductivity of the sintered composite TIM is enhanced by up to 123.5% compared with that of sintered pure copper. It can be understood as a result of the formation of graphene heat transfer network in sintered TIM. In addition, the C–O–Cu bonds formed at the interface between graphene and copper nanoparticles are critical for improving thermal performance as well as electrical and mechanical performance of the TIM. The developed TIM can be widely used in high power electronic packaging especially for high temperature applications, including IGBT, SiC and GaN power devices.

Progresses on Development of Manufacturing Methods of High Purity Tungsten Targets

Tungsten has many excellent properties such as high melting point, high electrical conductivity, high electromigration resistance, high electron emission coefficient, high thermal stability and so on. Because of these excellent properties, high purity tungsten targets have wide applications and development prospects in the integrated circuit (IC) industry. In this paper, some manufacturing methods of tungsten targets was summarized and analyzed. The high melting point of tungsten makes powder metallurgy (PM) be the manufacturing methods of tungsten targets. After preforming of the tungsten powders, some sintering and densification processes like atmosphere pressure sintering, Hot Pressing (HP), Hot Isostatic Pressing(HIP) have been carried out. The grain size and the density of the tungsten targets is different by different manufacturing methods.

Surface roughness analysis of Cu films deposited on Si substrates: A molecular dynamic analysis

Cu is a promising material to replace Al and Au in integrated circuits and microscale devices because of its low electrical resistivity, high electromigration resistance, and low cost. However, surface roughness affects the contact resistance of these devices, especially when the device is on a microscale or nanoscale. This paper focuses on surface roughness analysis of Cu films deposited on Si substrates by molecular dynamic simulation based on the mechanism of physical vapor deposition. The effects of film thickness, deposition temperature, deposition interval, and reflow temperature on the surface roughness of Cu films are studied in detail. The simulation results show that the surface roughness can be improved by appropriate adjustments of these parameters. They also provide a foundation for further work on the deposition of Cu films on Si substrates.

Facile Preparation of Self-Reducible Cu Nanoparticle Paste for Low Temperature Cu-Cu Bonding

Cu nanoparticle (NP) paste is considered the next-generation die-attach material because of its cost-effectiveness and high conductivity and electro-migration resistance. However, the spontaneous oxidation of Cu NPs severely restrains the incorporation of Cu NP paste into practical applications. Herein, a novel self-reducible Cu NP paste was prepared and demonstrated for low-temperature Cu-Cu bonding. The Cu NP paste was composed of 62 wt.% ultra-small Cu NPs (6.5 nm) and 38 wt.% organic components [isopropanolamine (IPA) stabilizer and ethylene glycol]. The reducing and sintering mechanisms of Cu NP paste were proposed, and the effects of sintering temperature on the mechanical properties and microstructure evolutions of Cu-Cu joints were systematically investigated. Consequently, the reducibility of the IPA stabilizer was beneficial for eliminating the surface oxides and enhancing the sinterability of Cu NPs, and the robust and high-strength Cu-Cu joints (> 20 MPa) were achieved at low temperature of 200°C in Ar atmosphere. Furthermore, the microstructure observations reveal that the robust bonding is attributed to the remarkable metallurgical interconnection between the substrates and sintered Cu NP layer.

Nanoindentation Study of Single-Crystalline and (101)-Oriented Nanotwinned Cu

Copper (Cu) is the interconnect material of choice in the integrated circuit (IC) devices and printed circuit boards (PCBs) due to its low electrical resistivity, high electromigration resistance, and excellent mechanical, chemical, and thermodynamical characteristics. In this study, the relationship between the Cu mechanical properties [e.g., hardness (H), Young's modulus (E), and stiffness (S)] and its crystallographic microstructure, including single-crystalline Cu and highly (101)-oriented nanotwinned Cu (nt-Cu), were characterized via nanoindentation, electron backscatter diffraction (EBSD), focused ion beam (FIB), and transmission electron microscopy (TEM). H, E, and S depended on the crystallographic orientation of single-crystalline and nt-Cu. A theoretical calculation based on the Schmid's law was made to rationalize the dependence of the Cu mechanical characteristics on the crystallographic orientation. The information associated with this orientation-dependent behavior is of great importance to the IC and PCB communities, particularly in developing the ultrafine Cu interconnect technique.

Self-Reducible Cu Nanoparticles for Conductive Inks

Copper nanoparticles (Cu NPs) are a potential material for conductive inks because of their low price, high conductivity, and high electromigration resistance. However, Cu NPs are prone to be oxidized in air, which is lethal to conductivity. In this work, we report on the self-reduction of Cu NPs which are synthesized by the thermal decomposition of copper formate (Cuf) using oleylamine (OAM) as the complexing ligand and stabilizing agent. Although the particle surface partially oxidized to Cu2O in air, the cuprous oxide could be reduced to copper during sintering, because of the release of H2 through the decomposition of OAM adsorbed on Cu NPs. This self-reduction ability without the help of additional reduction agent makes the Cu NPs a promising material for conductive inks. The feasibility to formulate conductive ink with the prepared Cu NPs is demonstrated.

Electrical Conductivity in Electrodeposited Cu-Ge(O) Alloy Films

Integrated circuits currently use mainly copper as the interconnect material; unfortunately the ongoing miniaturization currently requires materials with higher electromigration resistance and possibly improved conductivity. In this context we report on the structure, microstructure and electrical properties of a series of Cu-Ge(O) alloy films, electrodeposited from an alkaline tartrate electrolyte. The composition of the films varies between zero and 20 at% Ge, with a significant incorporation of oxygen. Film morphology is dense and uniform, with Cu-Ge films exhibiting smaller apparent grain size (∼50 nm) with respect to Cu films grown from a similar electrolyte. Solid solutions and phase mixtures of a solid solution with the intermetallic are observed with increasing Ge fraction; the presence of intermetallic phases is confirmed by TEM imaging and diffraction. The resistivity of 50 nm thick films follows the published trend, with a slight increase of the value upon solid solution formation and a minimum in correspondence of the intermetallic composition. Thicker films (∼1 um) on the other hand show a different trend, with resistivity increasing with Ge and O at%; in this case the resistivity is probably dominated by the oxygen incorporation.

Electrodeposition Though Polyelectrolyte Layers: Control of Interfacial Reaction, Deposition Rate, and Morphology of Deposited Thin Films

Demand for the development of deposition strategies of well-defined metal thin films with good chemical and physical characteristics has been rapidly growing in the field of various electronics applications. For example, copper electro- or electroless deposition has been widely utilized and investigated in microelectronics industry due to its low electrical resistivity and high electromigration resistance, as compared with aluminum. The conventional plating bath for electro- or electrolless copper deposition involves various chemical reagents such as deposition promoter (or inhibiter), brightening and smoothing agent, etc. These additives, typically organic molecules, sometimes cause mechanical problems due to inclusion of such additives during deposition process, and the discharge from the deposition bath can be a serious environmental issue. Therefore, the development of a facile process that enables additive-free deposition of metallic thin films as well as low environmental toxicity is an important challenge for the fabrication of microelectronic elements. In order to sustain the demand for generating multichip packaging systems for future electronic devices, it would be exceedingly useful to develop novel electrochemical deposition strategy with high-throughput capability that would allow stability of electrochemical reaction at higher deposition rate from plating bath without organic additives. In this context, direct metal deposition processes though ion conducting films have been investigated, such as the fabrication of metallic patterns on polymer substrates using ion-doped precursor films. A variety of direct deposition techniques have been introduced by several groups, including ours,1,2) based on photoinduced chemistry, thermal treatment, and selective chemical reduction for polyimide and other functional polymers. We have also reported a chemical metallization strategy that utilizes electrochemical constructive lithography with post lift-off process, which enables the fabrication of metal patterns on the polyimide substrate.3) The use of ion-doped precursors has triggered the development of new concept for electrochemical deposition. Herein we report for the first time that metallic thin films can be deposited through polyelectrolyte thin layers placed on the cathode electrode by the diffusion of metallic ions from the interior of polyelectrolyte, which is essentially different to conventional solution-phase electrodeposition processes (Fig. 1). By taking advantage of the cation-exchangeable nature of polyelectrolyte layers used for creation of solid interface, we demonstrate in this study how it is possible to merge the electrochemical deposition at cathode-polyelectrolyte interface and diffusion of cations through polyelectrolytes for development of novel solid electrodeposition (SED) process. As a proof of concept study, we herein present the successful electrochemical deposition of copper and nickel thin films at the interface between an electrode and ion-doped polyelectrolyte layers from the baths containing only metal salts (without any additives). The effect of cation concentration, temperature, and electrochemical deposition conditions on the deposition rate, growth process and morphology of the films has been investigated. Several electrochemical, microscopic and quantitative analysis revealed that the metallic films have been successfully deposited at the cathode-polyelectrolyte interface through ion exchange reaction in the polyelectrolyte layers, the deposition rate of which are determined by ion exchange rate of cations. This strategy offers an opportunity to translate solution-phase electrochemical deposition into a high-throughput, cost-effective, environmentally-friendly process for the fabrication of metal thin films on substrates. 1) Matsumura, Y. Enomoto, T. Tsuruoka, K. Akamatsu, H. Nawafune, Langmuir, 26, 12448-12454 (2010). 2) S. Ikeda, H. Yanagimoto, K. Akamatsu, H. Nawafune, Adv. Funct. Mater., 17, 889-897 (2007). 3) K. Akamatsu, Y. Fukumoto, T. Taniyama, T. Tsuruoka, H. Yanagimoto, H. Nawafune, Langmuir, 27, 11761-11766 (2011). Figure 1

Potassium Sodium Niobate-Based Lead-Free Piezoelectric Multilayer Ceramics Co-Fired with Nickel Electrodes.

Although lead-free piezoelectric ceramics have been extensively studied, many problems must still be overcome before they are suitable for practical use. One of the main problems is fabricating a multilayer structure, and one solution attracting growing interest is the use of lead-free multilayer piezoelectric ceramics. The paper reviews work that has been done by the authors on lead-free alkali niobate-based multilayer piezoelectric ceramics co-fired with nickel inner electrodes. Nickel inner electrodes have many advantages, such as high electromigration resistance, high interfacial strength with ceramics, and greater cost effectiveness than silver palladium inner electrodes. However, widely used lead zirconate titanate-based ceramics cannot be co-fired with nickel inner electrodes, and silver palladium inner electrodes are usually used for lead zirconate titanate-based piezoelectric ceramics. A possible alternative is lead-free ceramics co-fired with nickel inner electrodes. We have thus been developing lead-free alkali niobate-based multilayer ceramics co-fired with nickel inner electrodes. The normalized electric-field-induced thickness strain (Smax/Emax) of a representative alkali niobate-based multilayer ceramic structure with nickel inner electrodes was 360 pm/V, where Smax denotes the maximum strain and Emax denotes the maximum electric field. This value is about half that for the lead zirconate titanate-based ceramics that are widely used. However, a comparable value can be obtained by stacking more ceramic layers with smaller thicknesses. In the paper, the compositional design and process used to co-fire lead-free ceramics with nickel inner electrodes are introduced, and their piezoelectric properties and reliabilities are shown. Recent advances are introduced, and future development is discussed.

Chemical Etching and Patterning of Copper, Silver, and Gold Films at Low Temperatures

Copper, silver, and gold share unique properties that offer numerous application possibilities. For instance, low resistivity and high electromigration resistance make them attractive as interconnect layers in integrated circuits and microelectronic devices, while the optical properties of their nano-scale structures allow fabrication of plasmonic devices. Etching or patterning techniques are required in order to fabricate nano-structures, devices, and circuits. Although liquid or vapor phase techniques can be applied, plasma or glow discharge methods are typically invoked due to the need to generate anisotropic nanometer pattern sizes. Group 11 metals (Cu, Ag, and Au) form few volatile compounds at temperatures below 150°C, which limits the approaches that can be used to perform etching/patterning. Halogenated plasmas are widely used to etch metal layers; however, the low volatility of Cu, Ag, and Au halides precludes low temperature processes. Group 11 metals form hydrides readily in plasmas containing hydrogen species. Despite the thermodynamic instability of these metal hydrides, they appear to form at low (below room) temperature and can be desorbed from the etching surface by ion- or photon-assisted processes. Similarly, methylated metal etch products can be generated with hydrocarbon etching plasmas. As a result, etch rates above 12 ± 1 nm/min can be achieved, even when polymer forming plasmas (e.g., methane or other hydrocarbons) are used for patterning. These simple subtractive plasma etching approaches offer significant advantages relative to other vapor phase or liquid techniques in the manufacture of nano-scale devices and circuits.

Fast phase transformation due to electromigration of 18 μm microbumps in three-dimensional integrated-circuit integration

The electromigration behavior was investigated in 18-μm SnAg microbumps at 150°C. The monitored resistance increase abruptly soon after the current stressing of 4.6×104A/cm2 was applied but the resistance rose much slower after a certain period of time. The formation of the high-resistivity Ni3Sn4 intermetallic compounds (IMCs) was responsible for the abrupt resistance increase in the beginning of the current stressing. The whole solder joint was transformed into an IMC joint. Because IMC has a higher electromigration resistance than the solder, a much slower increase in resistance was resulted after the solder joint was transformed into IMCs.

Significant enhancement in electromigration resistance and texture of aluminum films using an ultrathin titanium underlayer

Obtaining electrodes with high electromigration resistance, which are used in high-frequency surface acoustic wave devices, is imperative for the rapidly developing mobile communications industry. The effect of a Ti underlayer on the electromigration resistance and texture of Al films was studied comprehensively. It was found that crystalline orientation, crystalline size and electromigration resistance varied significantly with the thickness of the Ti underlayer. A highly (111) oriented Al film with a full width at half maximum value of the rocking curve of 1.7° and a crystalline grain with a bamboo-like structure was successively deposited on an ultrathin titanium underlayer (∼1nm). The median time to failure of Al interconnects with the ultrathin Ti underlayer was enhanced by one order of magnitude higher than those without the Ti underlayer. The influence of the Ti underlayer on the texture of Al films was investigated using a combination of X-ray diffraction and transmission electron microscopy techniques. The results revealed that the ultrathin discontinuous Ti underlayer facilitated the Al (111) texture growth. The mechanism of texturing was discussed in terms of surface-interface energy balance along with high resolution transmission electron microscopy analysis.

Copper Metallization for Current Very Large Scale Integration

As silicon technology scaling progresses to the 32 nm node or even further, the design on the propagation of electromagnetic signals becomes increasingly appealing due to their unyielding constraints on interconnect delay. Because of its high conductivity and electromigration resistance, Cu is now the interconnect materials in current VLSI. To ensure the signal propagation via the Cu interconnects upon the increasingly reduction in the interconnect width, related issues on Cu interconnects, such as electron scattering at surfaces and grain boundaries, electromigration failure and surface oxidation, still need to be further understood and addressed. Besides this, the performance of low-k dielectrics and reliable barrier structures, which are also much important among the device parts, are required to be further improved to minimize the signal delay and to prevent penetration of different materials, respectively. On the basis of the paper published at Recent Patent on Nanotechnology 2007; 1: 193-209, this review will focus on recent patents and some studies on Cu metallization including Cu interconnect wires, low-k dielectrics and related barrier materials as well as manufacturing techniques in VLSI, which are one of the most essential concerns in microelectronic industry and decide further development of VLSI. This review will benefit for the design of the Cu metallization in the current VLSI.

Pursuit of Future Interconnect Technology with Aligned Carbon Nanotube Arrays [Nanopackaging

As the feature size of interconnect technology continues to scale down to nanometer sizes, the state-of-the-art Cu interconnect is expected to run into its physical limit in the near future. Carrier scattering at surfaces and grain boundaries leads to a dramatic increase of Cu resistivity at the nanoscale, resulting increasing resistance capacitance signal delay. The Cu interconnect is becoming more and more vulnerable to the electromigration (EM) caused by the increasing current density. More conductive and robust materials are required for future integrated circuit interconnects to meet these technology development challenges. Among a variety of potential materials, the metallic carbon nanotube (CNT) has emerged as a promising candidate for next-generation interconnect conductor because of its excellent electrical conductivity and high EM resistance.

Recent Developments in Carbon Nanotube Sorting and Selective Growth

AbstractDue to their high carrier mobilities, electromigration resistance, and tailorable optical properties, carbon nanotubes are promising candidates for high-performance electronic and optoelectronic applications. However, traditional synthetic methods have lacked control over the structure and properties of carbon nanotubes. This polydispersity problem has confounded efforts to take carbon nanotubes from the research laboratory to the marketplace, especially for electronic and optoelectronic applications, where reliable and reproducible performance is paramount. In recent years, the research community has devoted significant effort to this issue, leading to substantial advances in the preparation of monodisperse carbon nanotube materials. This article highlights the most recent and promising developments from two perspectives: post-synthetic sorting and selective growth of carbon nanotubes of predetermined physical and electronic structure. These complementary approaches have yielded improved uniformity in carbon nanotube materials, resulting in impressive advances in carbon nanotube electronic and optoelectronic technology.

Effects of Cr dopant on the microstructure and electromigration performance of Cu interconnects

Cu and Cu(Cr) alloy films were deposited on SiO 2 substrates by magnetron sputtering. The microstructure and electromigration performance of films were investigated. A small amount of Cr refines the Cu grains and improves the surface morphology of Cu films. After annealing at 450 °C, in contrast to the Cu film with large lateral grown grains, the Cu(Cr) alloy film exhibits fine columnar grains with a 〈1 1 1〉 preferred orientation. Most of Cr in the annealed Cu(Cr) film has segregated at the film surface and the film/substrate interface. The grain boundary grooving at the film/substrate interface is completely prohibited for Cu(Cr) films. As a result, the electromigration lifetimes of annealed Cu(Cr) lines are 10–100 times longer than those of annealed Cu lines. The final resistivity of the annealed Cu(Cr) film is 2.55 μΩ cm which is close to that of the annealed Cu film. With the improved surface morphology and high electromigration resistance, the dilute Cu(Cr) alloy film can be a viable interconnect material or a seed layer in the Cu-damascene technology.

Prevention of electromigration-induced Cu pad dissolution by using a high electromigration-resistance ternary Cu–Ni–Sn layer

Abstract Electromigration-induced Cu dissolution occurred at serious levels on the Cu pad under 10 4 A/cm 2 current-stressing for 20 h at 160 °C. A high electromigration-resistance (Cu, Ni) 6 Sn 5 layer, which converted from a thin Ni layer on the Cu pad, effectively prevented the Cu pad from dissolving into the pure Sn bump under 10 4 A/cm 2 current-stressing for 30 h at 160 °C.

Cu Wettability and Diffusion Barrier Property of Ru Thin Film for Cu Metallization

The main issue of Cu metallization is the electromigration of Cu through the interface between Cu and the barrier or capping layer. To improve electromigration resistance at the Cu and barrier metal interface, insertion of a glue layer which enhances the adhesion of Cu onto the under layer may be effective. The wettability of Cu on Ru and Ta glue layers was evaluated as the index of Cu adhesion strength onto glue layers. The wetting angle of Cu (43°) on a Ru substrate was three times lower than that of Cu (123°) on a Ta substrate after annealing. Lower wetting angle of Cu on a Ru substrate indicates a good adhesion property between Cu and Ru and may imply a high electromigration resistance. The better Cu wettability of Ru compared to Ta can be explained by the concept of lattice misfit. A Ru(002) plane has lower lattice misfit, which suggests lower interface energy, and enhanced the adhesion of Cu onto Ru. However, the Ru film showed poor Cu diffusion barrier properties, which suggests Ru should be used as a glue layer in combination with another barrier layer.