The nanoindentation characteristics of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript>, Cu<Subscript>3</Subscript>Sn,and Ni<Subscript>3</Subscript>Sn<Subscript>4</Subscript> intermetallic compounds in the solder bump

In general, formation and growth of intermetallic compounds (IMCs) play a major role in the reliability of the solder joint in electronics packaging and assembly. The formation of Cu-Sn or Ni-Sn IMCs have been observed at the interface of Sn-rich solders reacted with Cu or Ni substrates. In this study, a nanoindentation technique was employed to investigate nanohardness and reduced elastic moduli of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ IMCs in the solder joints. The Sn-3.5Ag and Sn-37Pb solder pastes were placed on a Cu/Ti/Si substrate and Ni foil then annealed at 240°C to fabricate solder joints. In Sn-3.5Ag joints, the magnitude of the hardness of the IMCs was in the order Ni₃Sn₄>Cu₆Sn₅>Cu₃Sn, and the elastic moduli of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ were 125 GPa, 136 GPa, and 142 GPa, respectively. In addition, the elastic modulus of the Cu₆Sn₅ IMC in the Sn-37Pb joint was similar to that for the bulk Cu₆Sn₅ specimen but less than that in the Sn-3.5Ag joint. This might be attributed to the strengthening effect of the dissolved Ag atoms in the Cu₆Sn₅ IMC to enhance the elastic modulus in the Sn-3.5Ag/Cu joint.     

Additive Occupancy in the Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript>-Based Intermetallic Compound Between Sn-3.5Ag Solder and Cu Studied Using a First-Principles Approach

A Cu₆Sn₅-based intermetallic compound containing a certain amount of Co or Ni is commonly formed at the interface between a Cu substrate and Sn-based solder. The Co or Ni additive is often found to occupy the Cu atom sublattice in the Cu₆Sn₅ crystal structure. In this paper, a first-principles approach based on density-functional theory is employed to explore the most favorable occupancy sites of Ni and Co dopants in the Cu₆Sn₅ crystal structure. It is found that, for up to 27.3 at.% concentration, both Ni and Co atoms tend to substitute for Cu in the Cu₆Sn₅-based structure and form more thermodynamically stable (Cu,Ni)₆Sn₅ and (Cu,Co)₆Sn₅ phases. In comparison, Ni is more effective than Co at stabilizing the Cu₆Sn₅ phase. At a lower concentration level (9.1 at.%), the Ni or Co atoms prefer to occupy the 4e Cu sublattice. At a higher concentration (27.3 at.%), the Ni atoms will likely be located on the 4e + 8f2 Cu sublattice. Analysis of density of states (DOS) and partial density of states (PDOS) indicates that hybridization between Ni-d (or Co-d) and Sn-p states plays a dominant role in structural stability. Compared with Cu₄Ni₂Sn₅, where Ni occupies the 8f2 Cu sublattice, Cu₄Co₂Sn₅ is less stable due to the lower amplitude of the Co-d PDOS peak and its position mismatch with the Sn-p PDOS peak.     

Growth Mechanism of a Ternary (Cu,Ni)<Subscript>6</Subscript>Sn<Subscript>5</Subscript> Compound at the Sn(Cu)/Ni(P) Interface

The growth mechanism of an interfacial (Cu,Ni)₆Sn₅ compound at the Sn(Cu) solder/Ni(P) interface under thermal aging has been studied in this work. The activation energy for the formation of the (Cu,Ni)₆Sn₅ compound for cases of Sn-3Cu/Ni(P), Sn-1.8Cu/Ni(P), and Sn-0.7Cu/Ni(P) was calculated to be 28.02 kJ/mol, 28.64 kJ/mol, and 29.97 kJ/mol, respectively. The obtained activation energy for the growth of the (Cu,Ni)₆Sn₅ compound layer was found to be close to the activation energy for Cu diffusion in Sn (33.02 kJ/mol). Therefore, the controlling step for formation of the ternary (Cu,Ni)₆Sn₅ layer could be Cu diffusion in the Sn(Cu) solder matrix.     

Inhibiting AuSn<Subscript>4</Subscript> Formation by Controlling the Interfacial Reaction in Solder Joints

Au was used in an electronic package to protect the conductor from oxidation. However, Au dissolved into solders and reacted with the Sn-rich phase to form AuSn₄ during soldering. After aging, Au diffused from AuSn₄ toward the solder/metallization interface. If Ni₃Sn₄ formed at the soldering interface, a layer of AuSn₄ was redeposited on Ni₃Sn₄. In contrast, Au diffused into Cu₆Sn₅-based intermetallic compounds (IMCs) to produce either (Cu,Au)₆Sn₅ or (Cu,Ni,Au)₆Sn₅, while Cu₆Sn₅ or (Cu,Ni)₆Sn₅ was formed at the soldering interface. Gibbs free energy evaluation revealed that both (Ni,Au)₃Sn₄ and (Cu,Au)₆Sn₅ were more thermodynamically stable than AuSn₄. The maximum amount of Au diffused in Ni₃Sn₄ was 4.6 at.%, while the maximum dissolution of Au in Cu₆Sn₅ was 24.3 at.% at 150°C. Thus, dissolution of Au in Ni₃Sn₄ was limited, and residual Au rereacted with Sn to produce the layer-type AuSn₄. If Cu₆Sn₅ formed at the interface, most of the Au in AuSn₄ diffused into Cu₆Sn₅. Consequently, AuSn₄ formation could be inhibited by controlling formation of Cu₆Sn₅ in solder/under-bump metallization (UBM) assemblies.     

Analysis of the redeposition of AuSn<Subscript>4</Subscript> on Ni/Au contact pads when using SnPbAg,SnAg, and SnAgCu solders

Interfacial reactions between SnPbAg, SnAg, and SnAgCu solders and Ni/Au surface finish on printed wiring board and especially the redeposition of AuSn₄ intermetallic compound have been investigated. The following major results were obtained. The first phase to form during soldering in the (SnPbAg)/Ni/Au and the (SnAg)/Ni/Au systems was Ni₃Sn₄. During the subsequent solid-state annealing, the redeposition of AuSn₄ as (Au,Ni)Sn₄ occurred in both systems. This was explained with the help of the concept of local equilibrium and the corresponding ternary phase diagrams. It was concluded that the stabilizing effect of Ni on the (Au,Ni)Sn₄ provided the driving force for the redeposition. Contrarily, when the solder alloy contained some Cu, the first intermetallic to form was (Cu,Ni,Au)₆Sn₅ and no redeposition of AuSn₄ was observed. Thus, a very small addition of Cu to the Sn-rich solder alloys changed the behavior of the interconnection system completely. This behavior was explained thermodynamically by using Cu-Ni-Sn and Au-Cu-Sn ternary phase diagrams. The growth kinetics of the interfacial reaction products in the three systems was observed to be somewhat different. The reasons for the observed differences are also discussed.     

Nanotribological Characteristics of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript>, Cu<Subscript>3</Subscript>Sn,and Ni<Subscript>3</Subscript>Sn<Subscript>4</Subscript> Intermetallic Compounds

Nanotribological characteristics, including the coefficient of friction, wear coefficient, and wear resistance, of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ intermetallic compounds developed by the annealing of Sn–Cu or Sn–Ni diffusion couples were investigated in this work. The scratch test conditions combined a constant normal load of 10 mN, 20 mN, or 30 mN and a scratch rate of 0.1 μm/s, 1 μm/s, or 10 μm/s. Experimental results indicated that, as the normal load increases, the pile-up grows taller and the scratch deepens, leading to a greater coefficient of friction and wear coefficient, and reduced wear resistance. Moreover, the scratch rate does not have a significant effect on the nanotribological characteristics except for those of Cu₆Sn₅ and Cu₃Sn under a normal load of 10 mN. Though the hardness of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ is similar, Ni₃Sn₄ appears to be more prone to wear damage.     

Formation of Intermetallic Compounds Between Liquid Sn and Various CuNi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Metallizations

Interfacial reactions between liquid Sn and various Cu-Ni alloy metallizations as well as the subsequent phase transformations during the cooling were investigated with an emphasis on the microstructures of the reaction zones. It was found that the extent of the microstructurally complex reaction layer (during reflow at 240°C) does not depend linearly on the Ni content of the alloy metallization. On the contrary, when Cu is alloyed with Ni, the rate of thickness change of the total reaction layer first increases and reaches a maximum at a composition of about 10 at.% Ni. The reaction layer is composed of a relatively uniform continuous (Cu,Ni)₆Sn₅ reaction layer (a uniphase layer) next to the NiCu metallizations and is followed by the two-phase solidification structures between the single-phase layer and Sn matrix. The thickness of the two-phase layer, where the intermetallic tubes and fibers have grown from the continuous interfacial (Cu,Ni)₆Sn₅ layer, varies with the Ni-to-Cu ratio of the alloy metallization. In order to explain the formation mechanism of the reaction layers and their observed kinetics, the phase equilibria in the Sn-rich side of the SnCuNi system at 240°C were evaluated thermodynamically utilizing the available data, and the results of the Sn/Cu _x Ni_1−x diffusion couple experiments. With the help of the assessed data, one can also evaluate the minimum Cu content of Sn-(Ag)-Cu solder, at which (Ni,Cu)₃Sn₄ transforms into (Cu,Ni)₆Sn₅, as a function of temperature and the composition of the liquid solders.     

Thermoelectric Properties of Co<Subscript>4</Subscript>Sn<Subscript>6</Subscript>Se<Subscript>6</Subscript> Ternary Skutterudites

A ternary ordered variant of the skutterudite structure, the Co₄Sn₆Se₆ compound, was prepared. Polycrystalline samples were prepared by a modified ceramic method. The electrical conductivity, the Seebeck coefficient and the thermal conductivity were measured over a temperature range of 300–800 K. The undoped Co₄Sn₆Se₆ compound was of p-type electrical conductivity and had a band gap E _g of approximately 0.6 eV. The influence of transition metal (Ni and Ru) doping on the thermoelectric properties was studied. While the thermal conductivity was significantly lowered both for the undoped Co₄Sn₆Se₆ compound and for the doped compounds, as compared with the Co₄Sb₁₂ binary skutterudite, the calculated ZT values were improved only slightly.     

Physical Properties of Sn<Subscript>96.5</Subscript>Ag<Subscript>3.5</Subscript>-Based Solders with Additions of Bi,Ge, and In

The melting temperature, electrical resistivity, surface tension, and density of the (Sn_0.965Ag_0.035)_95.17Bi_4.83, (Sn_0.965Ag_0.035)_95.17Bi_4.73Ge_0.1, and (Sn_0.965Ag_0.035)₉₄-Bi₂In₄ alloys have been studied in comparison with the Sn₆₀Pb₄₀ and Sn_96.5Ag_3.5 binary alloys (all wt.%). The electrical conductivity of the solid alloys based on Sn_96.5Ag_3.5 is comparable to that of the Sn₆₀Pb₄₀ alloy. The wetting behavior on Cu and Ni surfaces has been investigated in a wide temperature interval. It is established that the addition of Bi to Sn_96.5Ag_3.5 decreases the surface tension and improves the wetting properties of the alloy. The addition of a small quantity of Ge to the Sn-Ag-Bi alloy did not improve the wetting behavior on either Cu or Ni surfaces. The wetting ability of the (Sn_0.965Ag_0.035)₉₄Bi₂In₄ alloy was slightly worse as compared with (Sn_0.965Ag_0.035)_95.17Bi_4.83.     

Morphological transition of interfacial Ni<Subscript>3</Subscript>Sn<Subscript>4</Subscript> grains at the Sn-3.5Ag/Ni joint

The transition in morphology of Ni₃Sn₄ grains that formed at the interface between liquid Sn-3.5Ag (numbers are in wt.% unless specified otherwise) solder and Ni substrate has been observed at 250–650°C. The morphological transition of Ni₃Sn₄ is due to the decrease of entropy of formation of the Ni₃Sn₄ phase and has been explained well by the change of Jackson’s parameter with temperature. According to the variation of solder joint strength with temperature, it decreased rapidly between 350°C and 450°C, where the thickness of the Ni₃Sn₄ intermetallic compound (IMC) layer was around 6.5 μm. However, the solder joint strength decreased slowly with an increase of soldering time without a significant drop, although the thickness of the IMC was larger than 6.5 μm. The notable drop of solder joint strength and the fracture mode transition with increase of soldering temperature appears to come from excessive lateral growth of IMC grains between 350°C and 450°C.     

Formation and Growth of Intermetallic Compound Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> at Early Stages in Lead-Free Soldering

In this work, the early stages of the formation and growth of the intermetallic compound Cu₆Sn₅ during soldering reactions between a Cu substrate and liquid Sn are examined through phase-field simulations. The liquid Sn-based solder (L phase) and the copper substrate (α phase) are considered to be under metastable equilibrium conditions that eventually lead to nucleation of the Cu₆Sn₅ intermetallic compound (IMC) (η phase) at the solid/liquid interface. Nucleation is incorporated into the model through a classical treatment considering that individual nucleation events follow a Poisson distribution function. The driving forces for the nucleation and phase transformations are obtained by coupling the phase-field simulations to CALPHAD models. In the phase-field simulations, physical properties such as liquid surface as well as IMC interfacial energies are treated parametrically to probe the behavior of the system under various growth conditions. The simulations are compared with previous works and are shown to have good (qualitative) agreement with recent detailed studies on the early stages of the interaction between Cu and liquid Sn.     

Cross-Interaction Study of Cu/Sn/Pd and Ni/Sn/Pd Sandwich Solder Joint Structures

Cross-interactions between Cu/Sn/Pd and Ni/Sn/Pd sandwich structures were investigated in this work. For the Cu/Sn/Pd case, the growth behavior and morphology of the interfacial (Pd,Cu)Sn₄ compound layer was very similar to that of the single Pd/Sn interfacial reaction. This indicates that the growth of the (Pd,Cu)Sn₄ layer at the Sn/Pd interface would not be affected by the opposite Cu/Sn interfacial reaction. We can conclude that there is no cross-interaction effect between the two interfacial reactions in the Cu/Sn/Pd sandwich structure. For the Ni/Sn/Pd case, we observed that: (1) after 300 s of reflow time, the (Pd,Ni)Sn₄ compound heterogeneously nucleated on the Ni₃Sn₄ compound layer at the Sn/Ni interface; (2) the growth of the interfacial PdSn₄ compound layer was greatly suppressed by the formation of the (Pd,Ni)Sn₄ compound at the Sn/Ni interface. We believe that this suppression of PdSn₄ growth is caused by heterogeneous nucleation of the (Pd,Ni)Sn₄ compound in the Ni₃Sn₄ compound layer, which decreases the free energy of the entire sandwich reaction system. The difference in the chemical potential of Pd in the PdSn₄ phase at the Pd/Sn interface and in the (Pd,Ni)Sn₄ phase at the Sn/Ni interface is the driving force for the Pd atomic flux across the molten Sn. The diffusion of Ni into the ternary (Pd,Ni)Sn₄ compound layer controls the Pd atomic flux across the molten Sn and the growth of the ternary (Pd,Ni)Sn₄ compound at the Sn/Ni interface.     

Dissolution and Interfacial Reactions of (Cu,Ni)6Sn5 Intermetallic Compound in Molten Sn-Cu-Ni Solders

(Cu,Ni)₆Sn₅ is an important intermetallic compound (IMC) in lead-free Sn-Ag-Cu solder joints on Ni substrate. The formation, growth, and microstructural evolution of (Cu,Ni)₆Sn₅ are closely correlated with the concentrations of Cu and Ni in the solder. This study reports the interfacial behaviors of (Cu,Ni)₆Sn₅ IMC (Sn-31 at.%Cu-24 at.%Ni) with various Sn-Cu, Sn-Ni, and Sn-Cu-Ni solders at 250°C. The (Cu,Ni)₆Sn₅ substrate remained intact for Sn-0.7 wt.%Cu solder. When the Cu concentration was decreased to 0.3 wt.%, (Cu,Ni)₆Sn₅ significantly dissolved into the molten solder. Moreover, (Cu,Ni)₆Sn₅ dissolution and (Ni,Cu)₃Sn₄ formation occurred simultaneously for the Sn-0.1 wt.%Ni solder. In Sn-0.5 wt.%Cu-0.2 wt.%Ni solder, many tiny (Cu,Ni)₆Sn₅ particulates were formed and dispersed in the solder matrix, while in Sn-0.3 wt.%Cu-0.2 wt.%Ni a lot of (Ni,Cu)₃Sn₄ grains were produced. Based on the local equilibrium hypothesis, these results are further discussed based on the liquid–(Cu, Ni)₆Sn₅–(Ni,Cu)₃Sn₄ tie-triangle, and the liquid apex is suggested to be very close to Sn-0.4 wt.%Cu-0.2 wt.%Ni.     

Intermetallic compound formation and morphology evolution in the 95Pb5Sn flip-chip solder joint with Ti/Cu/Ni under bump metallization during reflow soldering

Intermetallic compound formation and morphology evolution in the 95Pb5Sn flip-chip solder joint with the Ti/Cu/Ni under bump metallization (UBM) during 350°C reflow for durations ranging from 50 sec to 1440 min were investigated. A thin intermetallic layer of only 0.4 μm thickness was formed at the 95Pb5Sn/UBM interface after reflow for 5 min. When the reflow was extended to 20 min, the intermetallic layer grew thicker and the phase identification revealed the intermetallic layer comprised two phases, (Ni,Cu)₃Sn₂ and (Ni,Cu)₃Sn₄. The detection of the Cu content in the intermetallic compounds indicated that the Cu atoms had diffused through the Ni layer and took part in the intermetallic compound formation. With increasing reflow time, the (Ni,Cu)₃Sn₄ phase grew at a faster rate than that of the (Ni,Cu)₃Sn₂ phase. Meanwhile, irregular growth of the (Ni,Cu)₃Sn₄ phase was observed and voids formed at the (Ni,Cu)₃Sn₂/Ni interface. After reflow for 60 min, the (Ni,Cu)₃Sn₂ phase disappeared and the (Ni,Cu)₃Sn₄ phase spalled off the NI layer in the form of a continuous layer. The gap between the (Ni,Cu)₃Sn₄ layer and the Ni layer was filled with lead. A possible mechanism for the growth, disappearance, and spalling of the intermetallic compounds at the 95Pb5Sn/UBM interface was proposed.     

Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> Morphology Transition and Its Effect on Mechanical Properties of Eutectic Sn-Ag Solder Joints

The morphologies of Cu₆Sn₅ grains formed at the interface between Sn-3.5Ag (wt.% unless otherwise specified) and Cu substrates were studied in this work. Reflow experiments were performed for 60 s at peak temperatures of 513 K, 533 K, 543 K, and 553 K. Two morphologies of interfacial Cu₆Sn₅ grains were observed in wetting reactions: prism type, above 543 K, and scallop type, below 533 K. During aging, the two morphologies gradually transitioned to layer type. These three morphologies could be transformed into each other as long as the corresponding condition changed. The morphology transition of Cu₆Sn₅ in the wetting reaction was explained by the change in Jackson’s parameter with temperature. In addition, the effect of the Cu content in molten solder on interfacial Cu₆Sn₅ grains was examined. Significant differences in shear strength were observed for solder joints with different interfacial Cu₆Sn₅ morphologies in the case of a lower shear height. Joint strength is discussed in terms of the microstructure of the solder matrix and the morphology of interfacial Cu₆Sn₅ grains.     

Interfacial reaction between multicomponent lead-free solders and Ag,Cu, Ni,and Pd substrates

The interfacial reaction between two prototype multicomponent lead-free solders, Sn-3.4Ag-1Bi-0.7Cu-4In and Sn-3.4Ag-3Bi-0.7Cu-4In (mass%), and Ag, Cu, Ni, and Pd substrates are studied at 250°C and 150°C. The microstructural characterization of the solder bumps is carried out by scanning electron microscopy (SEM) coupled with energy dispersive x-ray analysis. Ambient temperature, isotropic elastic properties (bulk, shear, and Young’s moduli and Poisson’s ratio) of these solders along with eutectic Sn-Ag, Sn-Bi, and Sn-Zn solders are measured. The isotropic elastic moduli of multicomponent solders are very similar to the eutectic Sn-Ag solder. The measured solubility of the base metal in liquid solders at 250°C agrees very well with the solubility limits reported in assessed Sn-X (X=Ag, Cu, Ni, Pd) phase diagrams. The measured contact angles were generally less than 15° on Cu and Pd substrates, while they were between 25° and 30° on Ag and Ni substrates. The observed intermediate phases in Ag/solder couples were Ag₃Sn after reflow at 250°C and Ag₃Sn and ζ (Ag-Sn) after solid-state aging at 150°C. In Cu/solder and Ni/solder couples, the interfacial phases were Cu₆Sn₅ and (Cu,Ni)₆Sn₅, respectively. In Pd/solder couples, only PdSn₄ after 60-sec reflow, while both PdSn₄ and PdSn₃ after 300-sec reflow, were observed.     

Thin-film reactions during diffusion soldering of Cu/Ti/Si and Au/Cu/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with Sn interlayers

The multilayer thin-film systems of Cu/Ti/Si and Au/Cu/Al₂O₃ were diffusion-soldered at temperatures between 250°C and 400°C by inserting a Sn thin-film interlayer. Experimental results showed that a double layer of intermetallic compounds (IMCs) η-(Cu_0.99Au_0.01)₆Sn₅/δ-(Au_0.87Cu_0.13)Sn was formed at the interface. Kinetics analyses revealed that the growth of intermetallics was diffusion-controlled. The activation energies as calculated from Arrhenius plots of the growth rate constants for (Cu_0.99Au_0.01)₆Sn₅ and (Au_0.87Cu_0.13)Sn are 16.9 kJ/mol and 53.7 kJ/mol, respectively. Finally, a satisfactory tensile strength of 132 kg/cm² could be attained under the bonding condition of 300°C for 20 min.     

Interfacial Reactions between Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Alloy Underbump Metallurgy and Sn-Ag-<Emphasis Type="Italic">z</Emphasis>Cu Solders

A reaction study of Cu _x Ni _y alloy (x = 0.2–0.95) under bump metallization (UBM) with Sn-Ag-zCu solder (z = 0–0.7) was conducted. Formation and separation of intermetallic compounds (IMCs), effect of Cu addition to the Cu _x Ni _y alloy and the solders, and compatibility of reaction products with currently available phase diagrams are extensively investigated. The increase of Cu content both in the Cu _x Ni _y alloy and in the solder promoted IMC growth and Cu _x Ni _y consumption; though, with regard to solder composition, the reverse trend was true of the solder reactions in the literature. The liquid + Cu₆Sn₅ area in the Sn-rich corner needs to be larger compared to the currently available Cu-Ni-Sn ternary phase diagram, and the maximum simultaneous soluble point of Ni and Cu in Sn needs also to be moved to the Ni-Sn side (e.g., Sn-0.6Cu-0.3Ni).     

Superior suppression of gate current leakage in Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Si<Subscript>3</Subscript>N<Subscript>4</Subscript> bilayer-based AlGaN/GaN insulated gate heterostructure field-effect transistors

AlGaN/GaN-based metal-insulator-semiconductor heterostructure field-effect transistors (MIS-HFETs) with Al₂O₃/Si₃N₄ bilayer as insulator have been investigated in detail, and compared with the conventional HFET and Si₃N₄-based MIS-HFET devices. Al₂O₃/Si₃N₄ bilayer-based MIS-HFETs exhibited much lower gate current leakage than conventional HFET and Si₃N₄-based MIS devices under reverse gate bias, and leakage as low as 1×10⁻¹¹ A/mm at −15 V has been achieved in Al₂O₃/Si₃N₄-based MIS devices. By using ultrathin Al₂O₃/Si₃N₄ bilayer, very high maximum transconductance of more than 180 mS/mm with ultra-low gate leakage has been obtained in the MIS-HFET device with gate length of 1.5 μm, a reduction less than 5% in maximum transconductance compared with the conventional HFET device. This value was much smaller than the more than 30% reduction in the Si₃N₄-based MIS device, due to the employment of ultra-thin bilayer with large dielectric constant and the large conduction band offset between Al₂O₃ and nitrides. This work demonstrates that Al₂O₃/Si₃N₄ bilayer insulator is a superior candidate for nitrides-based MIS-HFET devices.     

Reaction Mechanism and Mechanical Properties of the Flip-Chip Sn-3.0Ag-0.5Cu Solder Bump with Cu/Ni-<Emphasis Type="Italic">x</Emphasis>Cu/Ti Underbump Metallization After Various Reflows

Ni underbump metallization (UBM) has been widely used as the diffusion barrier between solder and Cu pads. To retard the fast dissolution rate of Ni UBM, Cu was added into Ni thin films. The Ni-Cu UBM can provide extra Cu to the solders to maintain the Cu₆Sn₅ intermetallic compound (IMC) at the interface, which can thus significantly decrease the Ni dissolution rate. In this study, the Cu content of the sputtered Cu/Ni-xCu/Ti UBM was varied from 0 wt.% to 20 wt.%. Sn-3Ag-0.5Cu solder was reflowed with Cu/Ni-Cu/Ti UBM one, three, and five times. Reflow and cooling conditions altered the morphology of the IMCs formed at the interface. The amount of (Cu,Ni)₆Sn₅ increased with increasing Cu content in the Ni-Cu film. The Cu concentration of the intermetallic compound was strongly dependent on the composition of the Ni-Cu films. The results of this study suggest that Cu-rich Ni-xCu UBM can be used to suppress interfacial spalling and improve shear strength and pull strength of solder joints.