Effect of ENEPIG metallization for solid-state gold-gold diffusion bonds

Gold-gold (AuAu) diffusion bonding behavior of different tri-layer thicknesses of Electroless Ni/Electroless Pd/Immersion Au (ENEPIG) plating on a high-density system on a flex (SOF) package was examined. Plating thickness has a significant effect on surface roughness and void formation at the AuAu bonding interface, which exhibits degraded bond strength with an affected failure mode. It is seen that relatively smooth surface roughness (Ra < 100 nm) of thicker Ni(P) plating samples facilitates the shrinkage of voids and significantly increases bonding strength. Higher surface roughness in the low Ni(P) sample has a poor surface profile, which results in large lenticular shape voids and requires more energy to shrink by diffusion and a creep process. Enhancing bonding parameters constitutes an essential feature to compensate the physical and mechanical properties of ENEPIG plating. Based on this study, the authors recommend a suitable ENEPIG plating thickness for a high quality metallurgical bond, which passes different reliability tests.     

Modified Face-Down Bonding of Ridge-Waveguide Lasers Using Hard Solder

A modified face-down bonding technique of ridge-waveguide laser diodes (LDs) using 80Au20Sn solder has been performed. For ease of manufacturability, a bonding window with good bonding integrity and improved optical performance was determined. Metallographical investigation showed that the solder joint comprised of a layer of delta phase compound near the solder/heatsink interface, a layer of (Au,Ni)Sn intermetallic compound (IMC) at the solder/heatsink interface, and zeta' phase Au/Sn compound at the center of the solder joint. The delta phase shifted to the interfaces after reflow was postulated by its lower surface tension than zeta' phase Au/Sn compound. Good bonding integrity was observed with LD residues still adhering onto the bond pad after die shear testing. Scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) analyses of the fracture surface showed that the fracture occurred within the LD, at the GaAs/SiN interface. LDs bonded with this modified bonding process achieved an optical improvement of 2.5-3X compared to the unbonded LDs due to its good thermal management. These bonded LDs further exhibited good long-term reliability with no significant degradation in optical performance and no significant microstructure evolution in the solder joint after 500 thermal cycling test.     

Study of the Au/In Reaction for Transient Liquid-Phase Bonding and 3D Chip Stacking

The latest three-dimensional (3D) chip-stacking technology requires the repeated stacking of additional layers without remelting the joints that have been formed at lower levels of the stack. This can be achieved by transient liquid-phase (TLP) bonding whereby intermetallic joints can be formed at a lower temperature and withstand subsequent higher-temperature processes. In order to develop a robust low-temperature Au/In TLP bonding process during which all solder is transformed into intermetallic compounds, we studied the Au/In reaction at different temperatures. It was shown that the formation kinetics of intermetallic compounds is diffusion controlled, and that the activation energy of Au/In reaction is temperature dependent, being 0.46 eV and 0.23 eV for temperatures above and below 150°C, respectively. Moreover, a thin Ti layer between Au and In was found to be an effective diffusion barrier at low temperature, while it did not inhibit joint formation at elevated temperatures during flip-chip bonding. This allowed us to control the intermetallic formation during the distinct stages of the TLP bonding process. In addition, a minimal indium thickness of 0.5 μm is required in order to enable TLP bonding. Finally, Au/In TLP joints of ∅40 μm to 60 μm were successfully fabricated at 180°C with very small solder volume (1 μm thickness).     

Phase formation and diffusion soldering in Pt/In,Pd/In,and Zr/Sn thin-film systems

Potential candidates for thin-film diffusion soldering were investigated by analysis of phase formation and measurements of mechanical and thermal stability of thin-film bonds. Bilayers of Pt/In, Pd/In, and Zr/Sn of 500 nm/500 nm thickness were prepared by direct current magnetron sputtering followed by a 5 nm, thin protective Au layer. Single bilayer samples were heat-treated between 200°C and 500°C for 3–30 min and studied by x-ray diffraction (XRD) analysis. Some bilayers were bonded face-to-face between 300°C and 500°C for 3–30 min and sheared-off either in shear-strength measurements at room temperature or in remelting experiments up to 1,100°C. Phase formation in Pd/In and Pt/In thin films is much faster than in Zr/Sn thin films. An interaction of Au in addition to a questionable thermal stability of PtIn₂ complicated the reaction in Pt/In samples. A revised partial Pd-In phase diagram was constructed, correcting the compound ‘PdIn₃’ to Pd₃In₇. The Pt/In and Pd/In thin-film systems are very promising candidates for thin-film diffusion soldering.     

Solid-state interfacial reactions at the solder joints employing Au/Pd/Ni and Au/Ni as the surface finish metallizations

A tri-layer of nickel/palladium/gold (Au/Pd/Ni) is a promising candidate to replace the conventional Au/Ni bi-layer as the surface finish metallization for lead-free packaging. A surface finish metallization (Au/Pd/Ni or Au/Ni) and a Sn layer are sequentially deposited on a Cu substrate and then are subjected to thermal aging at 150 and 200 °C to investigate the interfacial reactions in the stacking multilayer structure made by low-temperature solid-state bonding. Because of the absence of the reflow process, the Pd and Au layers do not dissolve in the Sn matrix but remain at the interface and participate in the interfacial reaction to form the (Pd,Ni,Au)Sn₄ and (Au,Ni)Sn₄ phases at the Au/Pd/Ni- and Au/Ni-based interfaces, respectively. Though the Pd layer was only 0.4 μm, its resulting (Pd,Ni,Au)Sn₄ phase is much thicker than the (Au,Ni)Sn₄ phase. These two intermetallic compounds exhibit very different microstructural evolution which significantly affects the interfacial microstructures and growth rate of other intermetallic compound formed at the same interfaces.     

Crystallographic Properties of Ge/Si Heterojunctions Fabricated by Wet Wafer Bonding

Ge/Si heterojunctions formed by wet wafer bonding were observed using transmission electron microscopy and energy-dispersive x-ray spectroscopy. For the samples annealed at 880°C, there was a transition layer at the heterointerface with modified regions in the Si and Ge extending 20 nm to 30 nm from the interface. In these modified regions, crystal defects were observed, and a large amount of Ge was detected on the Si side of the junction. For the samples annealed at 250°C or 350°C, the transition layers had an amorphous-like structure with a thickness of about 10 nm. No modified layer or enlargement of lattice spacing was observed.     

Au/Zn/Au/p-In P欧姆接触的界面研究

研究了离子束溅射制备的Au/Zn/Au/p-InP欧姆接触的界面特性.在480℃退火15s比接触电阻达到最小,为1.4×1 0-5 Ω·cm 2.利用俄歇电子能谱(AES)和X射线光电子能谱(XPS)研究了接触界面的冶金性质.实验结果表明,在室温下InP中的In就可以扩散到接触的表面,退火后可与Au形成合金.退火后,Zn的扩散可以在p-InP表面形成重掺杂层,从而降低接触势垒高度,减小势垒宽度,有助于欧姆接触的形成;在接触与p-InP的界面产生一个P聚集区,同时Au与InP反应生成Au2P3,其P的2p3/2电子的结合能约为129.2 eV.     

The formation and growth of intermetallic compounds and shear strength at Sn–Zn solder/Au–Ni–Cu interfaces

The microstructures and shear strength of the interface between Sn–Zn lead-free solders and Au/Ni/Cu interface under thermal aging conditions was investigated. The intermetallic compounds (IMCs) at the interface between Sn–Zn solders and Au/Ni/Cu interface were analyzed by field emission scanning electron microscopy and transmission electron microscopy. The results showed the decrease in the shear strength of the interface with aging time and temperature. The solder ball with highly activated flux had about 8.2% increased shear strength than that with BGA/CSP flux. Imperfect wetting and many voids were observed in the fracture surface of the latter flux. The decreased shear strength was influenced by IMC growth and Zn grain coarsening. In the solder layer, Zn reacted with Au and then was transformed to the β-AuZn compound. Although AuZn grew first, three diffusion layers of γ-Ni₅Zn₂₁ compounds were formed after aging for 600 h at 150 °C. The layers divided by Ni₅Zn₂₁ (1), (2), and (3) were formed with the thickness of 0.7 μm, 4 μm, and 2 μm, respectively.     

Effect of Pd Surface Roughness on the Bonding Process and High Temperature Reliability of Au Ball Bonds

A 0.3-μm-thick electrolytic Pd layer was plated on 1 μm of electroless Ni on 1 mm-thick polished and roughened Cu substrates with roughness values (R _a) of 0.08 μm and 0.5 μm, respectively. The rough substrates were produced with sand-blasting. Au wire bonding on the Ni/Pd surface was optimized, and the electrical reliability was investigated under a high temperature storage test (HTST) during 800 h at 250°C by measuring the ball bond contact resistance, R _c. The average value of R _c of optimized ball bonds on the rough substrate was 1.96 mΩ which was about 40.0% higher than that on the smooth substrate. The initial bondability increased for the rougher surface, so that only half of the original ultrasonic level was required, but the reliability was not affected by surface roughness. For both substrate types, HTST caused bond healing, reducing the average R _c by about 21% and 27%, respectively. Au diffusion into the Pd layer was observed in scanning transmission electron microscopy/ energy dispersive spectroscopy (STEM–EDS) line-scan analysis after HTST. It is considered that diffusion of Au or interdiffusion between Au and Pd can provide chemically strong bonding during HTST. This is supported by the R _c decrease measured as the aging time increased. Cu migration was indicated in the STEM–EDS analysis, but its effect on reliability can be ignored. Au and Pd tend to form a complete solid solution at the interface and can provide reliable interconnection for high temperature (250°C) applications.     

Effects of Zn-Containing Flux on Sn-3.5Ag Soldering with an Electroless Ni-P/Au Surface Finish: Microstructure and Wettability

The microstructure resulting from Sn-3.5Ag soldering on an electroless Ni-P/Au pad using flux containing Zn(II) stearate was investigated. The content of zinc compound in the flux was 0 wt.% (Z-0), 20 wt.% (Z-20) or 50 wt.% (Z-50). A study of the interfacial microstructure revealed that both Z-20 and Z-50 fluxes yielded a thinner P-rich layer at the interface than did the Z-0 flux. In addition, compared with the bulky Ni–Sn intermetallics of the Z-0 joint interface, refined interfacial intermetallic compounds (IMCs) were observed when using Zn-containing fluxes, Z-20 and Z-50. Based on qualitative analyses of both Z-20 and Z-50 joint interfaces, it was presumed that their intermetallic layers would consist of Ni, Zn, and Sn. Additionally, the Ni content in the IMC layer of the Z-50 joint was lower than that of the Z-20 joint. Electron probe microanalysis (EPMA) of the initial Z-50 joint interface revealed Zn in the interfacial reaction layer, suggesting that Zn participated in the reaction between solder and the surface finish at an early stage of soldering. Consequently, the supply of Zn from the flux diminished Ni diffusion into the molten solder during heating. This effect may have caused a thin P-rich layer to form at the joint interface.     

Nitrogen bonding configurations near the oxynitride/silicon interface after oxynitridation in N2O ambient of a thin SiO2 gate

The identification of the bonding environments and their progressive modifications upon reaching the oxynitride/silicon interface, in a SiO₂/SiO_xN_y/Si structure, have been investigated by means of X-ray photoemission spectroscopy (XPS). The SiO₂ film was grown at 850 °C by means of a mixed dry-steam process, followed by a 60 min, 950 °C furnace oxynitridation in N₂O gas. A depth profile analysis was carried out by a progressive chemical etching procedure, reaching a residual oxide thickness of about 1.2 nm. XPS analysis of the Si 2p and N 1s photoelectron peaks pointed out that the chemistry of the oxynitride layer is a rather complex one. Four different nitrogen bonding environments were envisaged. Both the overall nitrogen content, which rises up to 2.5%, and its bonding configurations are progressively changing while moving towards the silicon interface.     

Interfacial Reactions Between Pb-Free Solders and In/Ni/Cu Multilayer Substrates

This study investigates the interfacial reactions between Sn-3.0wt.% Ag-0.5wt.%Cu (SAC) and Sn-0.7wt.%Cu (SC) on In/Ni/Cu multilayer substrates using the solid–liquid interdiffusion bonding technique. Samples were reflowed first at 160°C, 180°C, and 200°C for various periods, and then aged at 100°C for 100 h to 500 h. The scalloped Cu₆Sn₅ phase was formed at the SAC/In/Ni/Cu and SC/In/Ni/Cu interfaces. When the reflowing temperatures were 160°C and 180°C, a ternary Ni-In-Sn intermetallic compound (IMC) was formed when the samples were further aged at 100°C. This ternary Ni-In-Sn IMC could be the binary Ni₃Sn₄ phase with extensive Cu and In solubilities, or the ternary Sn-In-Ni compound with Cu solubility, or even a quaternary compound. As the reflow temperature was increased to 200°C, only one Cu₆Sn₅ phase was formed at the solder/substrate interface with the heat treatment at 100°C for 500 h. Mechanical test results indicated that the formation of the Ni-In-Sn ternary IMC weakened the mechanical strength of the solder joints. Furthermore, the solid–liquid interdiffusion (SLID) technique in this work effectively reduced the reflow temperature.     

Kinetics of Solid-State Reactive Diffusion in the (Pd-Ni)/Sn System

The growth of compounds during energization heating at the interconnection between a Sn-based solder and a multilayer Pd/Ni/Cu conductor may be inhibited by the alloying of Pd with Ni. To examine such influence of Ni on the compound growth, the kinetics of solid-state reactive diffusion in the (Pd-Ni)/Sn system was experimentally determined in the present study. Experiments were conducted using Sn/(Pd-Ni)/Sn diffusion couples with Ni mol fractions of y = 0.257, 0.505, and 0.746 which were prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed in the temperature range of 433 K to 473 K for various times up to 771 h. During annealing, different compounds are formed as rather uniform layers at the interface in the diffusion couple. In all the annealed diffusion couples, (Pd,Ni)Sn₄ was observed clearly. Furthermore, (Pd,Ni)Sn₃ and (Pd,Ni)Sn₂ were recognized for y = 0.257, and Ni₃Sn₄ was discerned for y = 0.746. However, no other compounds except (Pd,Ni)Sn₄ were detected for y = 0.505. The total thickness of the compound layers is proportional to a power function of the annealing time. The exponent of the power function is rather close to 0.5 for y = 0.257 and 0.505 but smaller than 0.5 for y = 0.746. Thus, volume diffusion is the rate-controlling process of the compound growth for y = 0.257 and 0.505, but boundary diffusion contributes to the rate-controlling process for y = 0.746. At the experimental annealing times, the overall growth rate of the compound layers is insensitive to y at y < 0.5 but decreases monotonically with increasing value of y at y > 0.5. Consequently, the compound growth is actually decelerated by the addition of Ni into Pd with y > 0.5 in the multilayer Pd/Ni/Cu conductor.     

Mechanical reliability of Sn-rich Au–Sn/Ni flip chip solder joints fabricated by sequential electroplating method

In this study, we evaluated the mechanical reliability of Sn-rich, Au–Sn/Ni flip chip solder bumps by using a sequential electroplating method with Sn and Au. After reflowing, the average diameter of the solder bump was approximately 80 μm and only a (Ni,Au)₃Sn₄ intermetallic compound (IMC) layer was formed at the interface. Due to the preferential consumption of Sn atoms within the solder matrix during aging, the solder matrix was transformed sequentially in the following order: β-Sn and η-phase, η-phase, and η-phase and ε-phase. In the bump shear test, the shear force was not significantly changed despite aging at 150 °C for 1000 h and most of the fractures occurred at the interfaces. The interfacial fracture was significantly related to the formation of brittle IMCs at the interface. The Sn-rich, Au–Sn/Ni flip chip joint was mechanically much weaker than the Au-rich, Au–Sn/Ni flip chip joint. The study results demonstrated that the combination of Sn-rich, Au–Sn solder and Ni under bump metallization (UBM) is not a viable option for the replacement of the conventional, Au-rich, Au–20Sn solder.     

Effect of plasma treatments on interface adhesion between SiOCH ultra-low-k film and SiCN etch stop layer

The effect of different plasma treatments on the interfacial bonding configurations and adhesion strengths between porous SiOCH ultra-low-dielectric-constant film and SiCN etch stop layer have been investigated in this study. From X-ray photoelectron spectroscopic analyses, interlayer regions of about 10 nm thick with complicated mixing bonds were found at SiOCH/SiCN interfaces. With plasma treatments, especially H₂/NH₃ two-step plasma, a carbon-depletion region of about 30 nm thick with more Si-O related bonds of high binding energy formed at the interface. Furthermore, the adhesion strengths of the SiOCH/SiCN interfaces were measured by nanoscratch and microscratch tests. For the untreated interface, the adhesion energy was obtained as about 0.22 and 0.44 J/m² by nanoscratch and microscratch tests, respectively. After plasma treatments, especially the H₂/NH₃ treatment, the interfacial adhesion energy was effectively improved to 0.41 and 0.89 J/m² because more Si-O bonds of high binding energy formed at the interfaces.     

Thermal stability of back side metallization multilayer for power device application

Metallization multilayers on the back side of a power device were focused in this study. Si wafers coated with high melting point metals were exposed at 300 °C for 300 h to investigate diffusion condition of the metallization layer. We developed and examined the thermal stability of die bonding material (Au paste) including sub–micrometer–sized Au particles. Auger electron spectroscopy was applied to observe the atomic composition of the multilayers in depth direction after the high temperature aging. Surface morphology was observed using optical microscope and scanning electron microscope. While atomic composition on Ti/Au changed drastically after the high temperature aging, other multilayers maintained their metallization composition. However, the surface morphology was slightly changed on Ti/Ru/Au, W/Au, and Ta/Au. Bond strength on the Ti/Pt/Au kept over 40 MPa with unified bonding layer after exposing at 300 °C for 1000 h.     

Organic doped/undoped interface based diode structure: Distinct mechanisms underlying forward and reverse bias

Organic semiconductor diodes fabricated using doped/undoped (high-low) homojunction has the potential of providing controlled and high quality current density-voltage (J-V) characteristics based on majority carrier transport. We study mechanisms of transport underlying such characteristics both in forward and reverse bias regimes of typical doped/undoped homojunction organic diodes fabricated using 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA). We study the J-V characteristics over a wide temperature range (200–300 K), and by varying the intrinsic layer thickness between 10 and 100 nm. The forward bias current, before entering into the space charge limited regime, is exponential over several orders of magnitude with the slope being temperature independent for all intrinsic layer thickness down to 10 nm. The reverse bias characteristics, on the other hand, are highly sensitive to the thickness of the intrinsic layer. While the forward bias is controlled by tunneling at the homojunction interface, the reverse bias is controlled by the interface of cathode (aluminum in this case) and the intrinsic layer. We show that the reverse current is due to Fowler-Nordheim tunneling across a barrier height, which is temperature independent but is sensitive to the layer thickness of the intrinsic layer. The origin of the thickness dependence of barrier height (0.45–0.72 eV for 10–20 nm) is attributed to the change in background carrier concentration in the intrinsic layer due to diffusion of carriers from the highly doped side. The results clearly show that the diffusion length of the majority carriers is approximately 1 nm and is comparable to the nearest neighbor jump distance.     

Study of Ni-Si thin-film interfacial reactions by coupling differential scanning calorimetry measurements and transmission electron microscopy analyses

In this paper, the solid-state interactions between a 500 nm thick Ni layer and a Si wafer are studied for temperatures up to 500 °C by coupling Differential Scanning Calorimetry (DSC) and Transmission Electron Microscopy (TEM). The phase transformation temperatures determined by DSC are about 250, 300, 350 and 410 °C. Dedicated samples were prepared to identify phase transformations occurring during heating up to these temperatures. TEM analyses show that the reaction product always consists of a continuous layer so that the nature of phase(s) formed at the interface can be determined. The reaction layer thickness is about 25, 50 and 150 nm for samples heated to 250, 300 and 350 °C, respectively. Moreover, from TEM diffraction patterns, it is shown that, for such a thick layer of Ni deposited on Si substrate, the first phase forming at the Ni/Si interface is the metastable Ni₃Si compound.     

Electron injection barrier and energy-level alignment at the Au/PDI8-CN2 interface via current–voltage measurements and ballistic emission microscopy

We probe electron transport across the Au/organic interface based on oriented thin films of the high-performance n-type perylene diimide semiconductor PDI8-CN₂. To this purpose, we prepared organic-on-inorganic Schottky diodes, with Au directly evaporated onto PDI8-CN₂ grown on n-Si. Temperature-dependent current–voltage characteristics and complementary ballistic electron emission microscopy studies reveal that rectification at the Au/PDI8-CN₂ interface is controlled by a spatially inhomogeneous injection barrier, that varies on a length scale of tens of nanometers according to a Gaussian distribution with mean value ∼0.94 eV and standard deviation ∼100 meV. The former gradually shifts to ∼1.04 eV on increasing PDI8-CN₂ thickness from 5 nm to 50 nm. Experimental evidences and general arguments further allow to establish the energetics at the Au/PDI8-CN₂ interface. Our work indicates injection-limited current flow in PDI8-CN₂-based devices with evaporated Au electrodes. Furthermore, it suggests chemical reactivity of PDI8-CN₂ with both Au and Si, driven by the lateral isocyano groups.     

Effects of alloying elements on microstructure and thermal aging properties of Au bonding wire

We studied the effects of alloying elements Cu and Ni on the microstructure and the thermal aging properties of Au bonding wire. The thermal aging properties of samples bonded with an Al pad, and annealed at 200 °C for durations ranging from 0 to 300 h was investigated using mechanical tests. Both of the alloyed specimens showed higher thermal aging properties than the 4 N Au-bonded specimen, as investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), an electron probe micro analyzer (EPMA) and transmission electron microscopy (TEM). The Cu-alloyed Au bonding wire formed, at the Au-Al interface, a discontinuous Cu-rich layer that was considered to have delayed the growth of the Au-Al intermetallic compound (IMC). Meanwhile, at the Au-Al interface, the Ni-alloyed Au bonding wire formed secondary phase particles that were believed to have improved the bonding strength by interrupting micro-crack propagation.