Interfacial reaction in bimetallic Sn/Cu thin films

Interfacial reaction in electroplated bimetallic Sn/Cu (the layer grown last is given first) thin films was studied by Auger depth profiling and X-ray diffraction measurements. Direct experimental evidence was found for the formation of intermetallic compounds in the SnCu interface, i.e. η'-Cu₆Sn₅ at room temperature and both η'-Cu₆Sn₅ and ε-Cu₃Sn at 150°C. The results of a quantitative analysis of the film composition and sputtering-induced effects are also discussed.     

Interfacial reaction and adhesion between SiC and thin sputtered nickel films

Thin sputtered nickel films grown on SiC were annealed in an Ar/4 vol % H2 atmosphere at temperatures between 550 to 1450 °C for various times. The reactivity and the reaction-product morphology were characterized using optical microscopy, surface profilometry, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The reaction with the formation of silicides and carbon was observed to first occur above 650 °C. Above 750 °C, as the reaction proceeded, the initially formed Ni3Si2 layer was converted to Ni2Si and carbon precipitates were observed within this zone. The thin nickel film reacted completely with SiC after annealing at 950 °C for 2 h. The thermodynamically stable Ni2Si is the only observed silicide in the reaction zone up to 1050 °C. Above 1250 °C, carbon precipitated preferentially on the outer surface of the reaction zone and crystallized as graphite. The relative adhesive strength of the reaction layers was qualitatively compared using the scratch test method. At temperatures between 850 to 1050 °C the relatively higher critical load values of 20–33 N for SiC/Ni couples are formed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Interfacial reaction and adhesion between SiC and thin sputtered cobalt films

Thin sputtered cobalt films on SiC were annealed in an Ar/4 vol% H₂ atmosphere at temperatures between 500 and 1450 °C for various times. The reaction process and the reaction-product morphology were characterized using optical microscopy, surface profilometry, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The relative adhesive strength between the film and substrate was determined by the scratch test method. Below 850 °C sputtered cobalt with a thickness of 2 m on SiC showed no detectable reaction products. Cobalt initially reacted with SiC at 850 °C producing Co₂Si and unreacted cobalt in the reaction zone. At 1050 °C the first-formed Co₂Si layer reacted to CoSi, and carbon precipitates were formed in the reaction zones. Sputtered thin cobalt layers reacted completely with SiC after annealing at 1050 °C for 2 h. Above 1250°C only CoSi was observed with carbon precipitates having an oriented structure in the reaction zone. Above 1450°C, a significant amount of graphitic carbon in the reaction zone was detected.     

Fabrication and microstructures of sequentially electroplated Au-rich, eutectic Au/Sn alloy solder

An Au-rich, eutectic Au/Sn alloy was fabricated by sequential electroplating of Au and Sn, and reflowing the as-deposited Au/Sn/Au triple-layer film at 320–350 °C. Microstructures and phase compositions for the as-deposited Au/Sn/Au triple-layer film and the reflowed Au-rich, eutectic Au/Sn alloys were studied. Two Si wafers, each with the Au-rich, eutectic Au/Sn alloy solder, were bonded together. For the deposited Au/Sn/Au triple-layer film, reaction between Au and Sn occurs at room temperature leading to the formation of AuSn and AuSn₄. After reflowing at 320 °C, two phases remain, AuSn and Au₅Sn, with the AuSn particles distributed randomly in the Au₅Sn matrix. There are also some micropores and microcracks in the reflowed alloy. If the annealing temperature is increased to 350 °C, the Au/Sn alloy is denser and contains fewer micropores. However, microcracks remain, forming preferentially along the Au₅Sn/AuSn interface. After reflowing at 320 °C under a pressure of 13 kPa, two Si wafers are joined using the Au-rich, eutectic Au/Sn alloy solder. The solder is in intimate contact with the Si wafers; however, there are some micropores within the solder. After reflowing at 350 °C, the bond is quite good, without microcracks or micropores at the Si wafer/solder interface or within the solder.     

Microstructural characterization of pulsed electrodeposited Au/Sn alloy thin films

Based on previous work that identified an electrodeposited composite, multi-layer structure as a viable method of producing eutectic Au/Sn alloys for solder applications, a study of individual phase formation was undertaken. The AuSn phase, because of its higher deposition current (>2.0 mA cm⁻²), has a much faster deposition rate than Au₅Sn, which is deposited at <1.0 mA cm⁻². AuSn formation is growth controlled, while Au₅Sn formation is nucleation controlled. The AuSn forms a continuous layer within 60 s with a grain size of 50–75 nm. Because of the high deposition current, the dominant formation mechanism is two-dimensional nucleation, resulting in a relatively rough surface finish. Au₅Sn, on the other hand, forms a continuous layer within 600 s with an average grain size of 200 nm. Because of the significantly lower deposition current, the dominant formation mechanism is lateral spreading instead of two-dimensional nucleation. The result is a very smooth finish on the deposit surface.     

Interfacial reaction and elemental redistribution in Sn3.0Ag0.5Cu-xPd/immersion Au/electroless Ni solder joints after aging

Sn3.0Ag0.5Cu solder doped with 0, 100, and 500 ppm Pd was reflowed with electroless Ni/immersion Au substrate. As Pd concentration increased in the solder, formation and growth of (Cu,Ni)₆Sn₅ were suppressed. After thermal aging, Cu₄Ni₂Sn₅ and Cu₅NiSn₅ were observed at interface of Sn3.0Ag0.5Cu-xPd/Au/Ni systems. As compared to Cu₄Ni₂Sn₅, more Pd dissolved in Cu₅NiSn₅. In addition, Pd doping enhanced the growth of Cu₄Ni₂Sn₅ and slowed the formation of Cu₅NiSn₅, which would stabilize the intermetallic compound. Based on quantitative analysis by field emission electron probe microanalyzer, the correlation between Pd doping and elemental redistribution in solder joints was probed and discussed. This study described a possible mechanism of the formation of different intermetallic compounds in Pd-doped lead-free solder.     

Interfacial reactions between palladium thin films and InP

Palladium appears to be an important component in ohmic contact metallizations to III–V semiconductors. Very little is known about its interaction with InP. Consequently, the reaction between a thin layer of Pd (100 nm) and an InP substrate has been studied at annealing temperatures ranging from 250–450 °C for up to 30 sec, i.e., typical annealing conditions encountered during contact fabrication. Palladium reacts readily with InP, initially forming an amorphous ternary phase, which transforms to crystalline Pd₂InP on annealing. Pd₂InP has an ordered cubic structure, with a lattice parameter of 0.830 nm, and grows epitaxially on InP. Microtwins, 2–3 atomic layers thick, have been identified in the ternary phase and these form along the (110) and ( 10) planes.     

Study on the solid state reaction between bilayered Pd/Au films and silicon substrates

Bilayers of pure palladium and gold films were evaporated alternatively on (1 0 0) and (1 1 1) monocrystalline silicon substrates. After annealing, in a vacuum furnace from 100 to 650 °C during 30 min, the growth sequence of the Pd₂Si and PdSi phases that evolved as the result of the diffusion reaction was examined by means of Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), whereas the surface morphology was investigated by scanning electron microscopy (SEM) technique. The effect of the intermediate gold layer is investigated in order to test its effectiveness as barrier for Cu and Si atoms interdiffusion and its influence on the morphology of the formed palladium silicides. The effect of substrate orientation on the palladium silicides growth and formation was also explored.     

Interfacial reaction and properties of Sn0.3Ag0.7Cu containing nano-TiN solder joints

Ultra-low silver Sn0.3Ag0.7Cu (SAC0307) solder is arousing widespread attention because of its low cost. In this paper, the morphology of interfacial intermetallic compounds, microstructure, melting point, wettability and mechanical property of SAC0307 containing nano-TiN solders were investigated using scanning electricity microscope, transmission electron microscopy, micro-joints strength tester and differential scanning calorimetry. Results show that the addition of trace nano-TiN into SAC0307 solder can restrict the growth behavior of interfacial IMC and refine the microstructure of the solder joints. When 0.2 wt% nano-TiN particles were added, the interfacial thickness of SAC0307 solder joint dropped from 2.1 to 1.92 μm. Moreover, the wettability and mechanical property of SAC0307 solder joints were also significantly enhanced, but it has little influence on the melting characteristics of the solder.

     

Interfacial reaction during fabricating of full Cu3Sn joints in microelectronic packaging

Two copper substrates electroplated with Sn, both consisted of Cu/Sn?+?Sn/Cu structures, but they were bonded over different times in order to investigate the interfacial reaction. The growth morphologies of Cu₆Sn₅ and Cu₃Sn were analysed, respectively. The growth mechanisms for Cu₆Sn₅ and Cu₃Sn were investigated. The results show that the growth of Cu₆Sn₅ is primarily controlled by grain boundary-diffusion. However, the growth Cu₃Sn is controlled by the reaction of Cu-Cu₆Sn₅ at the beginning of the reaction, and then controlled by volume-diffusion as the thickness of the Cu₃Sn layer increases.     

Interfacial reactions between liquid Sn3.5Ag0.5Cu solders and Ag substrates

The morphology and growth kinetics of intermetallic compounds (IMCs) formed during the soldering reactions between Sn3.5Ag0.5Cu and Ag substrates at various temperatures ranging from 250 to 350 °C were investigated. The interfacial microstructure was quantified with scanning electron microscopy (SEM) for each processing condition. Experimental results show that the thickness of the scallop-shaped Ag₃Sn IMCs layer increased with increasing soldering time and temperature. Furthermore, Cu₆Sn₅ particle precipitates were observed in the Ag₃Sn IMCs layer around and thus suppressing the Ag₃Sn IMCs layer growth. Furthermore, the large Cu₆Sn₅ IMCs tend to appear in the vicinity of interfacial wicker-Ag₃Sn IMCs. Kinetics analyses showed that growth of the Ag₃Sn intermetallic compound was diffusion controlled. The activation energies for the growth of Ag₃Sn IMCs are calculated to be 66.7 kJ/mol.     

Effect of Ni on reactive diffusion between Au and Sn at solid-state temperatures

Influence of Ni on the kinetics of the reactive diffusion between Au and Sn was experimentally studied at solid-state temperatures. Binary Sn–Ni alloys with Ni concentrations of 1, 3 and 5 mass% were used to prepare sandwich (Sn–Ni)/Au/(Sn–Ni) diffusion couples by a diffusion bonding technique. The diffusion couples were isothermally annealed at temperatures of T = 433, 453 and 473 K for various times in an oil bath with silicone oil. After annealing, AuNiSn₈, AuSn₄, AuSn₂ and AuSn compound layers were observed to form at the (Sn–Ni)/Au interface in the diffusion couple. The total thickness l of the compound layers monotonically increases with increasing annealing time t according to the equation l = k(t/t₀)ⁿ, where t₀ is unit time, 1 s. The exponent takes values between n = 0.29 and 0.37 under the present annealing conditions. Such values of n < 0.5 indicate that the grain boundary diffusion contributes to the rate-controlling process and the grain growth occurs at certain rates. The higher the Ni concentration of the Sn–Ni alloy is, the faster the overall growth of the compound layers occurs. This means that Ni is an accelerator for the reactive diffusion between Au and Sn at solid-state temperatures. The acceleration effect of Ni becomes more remarkable at higher annealing temperatures. Such influence of Ni on the kinetics is mainly attributed to the dependencies of the growth rate of the AuNiSn₈ layer on the composition of the Sn–Ni alloy and the annealing temperature.     

Interfacial void structure of Au/Sn/Al metalizations on GAAlAs light-emitting diodes

Electron microscopy was used to study the cause of an erratic adhesion problem which occurred between Au/Sn/Al metallizations and Ga-Al-As during the fabrication of light-emitting diodes. A cross-sectional study revealed that the metallization layer was actually separated from much of the Ga-Al-As surface by a complex void structure which was a potential source for poor adhesion. Rapid grain boundary diffusion of tin into gold at room temperature was found to be the origin of the observed void structure. Furthermore, the occurrence of an interfacial crack, a major feature of the void structure, was attributed to interdiffusion-induced high tensile stresses which caused the separation of the metallization layer from the Ga-Al-As substrate.     

Kinetics of reactive diffusion between Au and Sn during annealing at solid-state temperatures

The reactive diffusion between Au and Sn was experimentally studied at solid-state temperatures using Sn/Au/Sn diffusion couples prepared by a diffusion bonding technique. The diffusion couples were annealed at temperatures of T = 393 and 473 K for various times in an oil bath with silicone oil. After annealing, compound layers composed of AuSn₄, AuSn₂ and AuSn were recognized to form at the Au/Sn interface. The thickness of the AuSn₄ layer is about six and four times greater than those of the AuSn₂ and AuSn layers at T = 393 and 473 K, respectively. The ratio of the thicknesses of the compound layers is kept constant independently of the annealing time. The total thickness l of the compound layers is described as a function of the annealing time t by the equation l = k(t/t₀)ⁿ, where t₀ is unit time, 1 s. The exponent n is nearly equal to 1/2 at T = 393 K but takes a value between 1/4 and 1/2 at T = 473 K. Such an intermediate value of n at T = 473 K indicates that the grain boundary diffusion contributes to the reactive diffusion and the grain growth occurs at certain rates. As the annealing temperature decreases, the contribution of the grain boundary diffusion should become more remarkable, but the grain growth will slow down. Consequently, n becomes close to 1/2 at T = 393 K. According to the constancy of the ratio of the thicknesses, it is concluded that the same rate-controlling process works in the AuSn₄, AuSn₂ and AuSn layers at a constant annealing temperature.     

Effect of Ti on the interfacial reaction between Sn and Cu

The effect of Ti on the solid state reactions between Sn and Cu has been investigated in this work. Based on the experimental results the following statements about the effect of Ti can be made: Firstly, the presence of Ti does not have measurable effect on the thickness of either Cu₆Sn₅ or Cu₃Sn during solid state annealing. However, the unevenness of both Cu₆Sn₅ and Cu₃Sn layers is increased by the addition of Ti. Secondly, there is no marked solubility of Ti to either Cu₆Sn₅ or Cu₃Sn. Rather Ti reacts with Sn to form large Ti₂Sn₃ platelets inside the solder matrix. These findings were subsequently rationalized with the help of the assessed Cu–Sn–Ti phase diagram. By utilizing this phase diagram information, the absence of any marked effects of Ti on the growth of Cu–Sn intermetallic compound (IMC) formation was rationalized. As there is a very low solubility of Ti to SnAg solder and to Cu–Sn IMC’s, Ti cannot change activities of components in the solder nor influence the stability of the IMC layers. Hence, these results throw significant doubts over the concept of trying to influence the Cu–Sn IMC layer thickness or quality by Ti alloying.     

Interfacial reaction issues for lead-free electronic solders

The interfacial reactions between Sn-based solders and two common substrate materials, Cu and Ni, are the focuses of this paper. The reactions between Sn-based solders and Cu have been studied for several decades, and currently there are still many un-resolved issues. The reactions between Sn-based solders and Ni are equally challenging. Recent studies further pointed out that Cu and Ni interacted strongly when they were both present in the same solder joint. While this cross-interaction introduces complications, it offers opportunities for designing better solder joints. In this study, the Ni effect on the reactions between solders and Cu is discussed first. The presence of Ni can in fact reduce the growth rate of Cu₃Sn. Excessive Cu₃Sn growth can lead to the formation of Kirkendall voids, which is a leading factor responsible for poor drop test performance. The Cu effect on the reactions between solders and Ni is then covered in detail. The knowledge gained from the Cu and Ni effects is applied to explain the recently discovered intermetallic massive spalling, a process that can severely weaken a solder joint. It is pointed out that the massive spalling was caused by the shifting of the equilibrium phase as more and more Cu was extracted out of the solder by the growing intermetallic. Lastly, the problems and opportunities brought on by the cross-interaction of Cu and Ni across a solder joint is presented.     

光纤定位激光钎焊键合过程中钎料与镀层的界面反应

光电子封装中,光导纤维的定位键合是一项关键技术,并且焊点界面处的显微组织对于焊点的可靠性有重要影响.本文选用80Au20Sn和52In48Sn钎料实现了激光钎焊条件下的光纤键合,采用扫描电子显微镜及能谱分析的方法对于两种钎料分别与硅片上的Au/Ti镀层和光纤上的Au/Ni镀层反应形成的界面微观组织形态及形成规律进行了分析.结果表明:对于80Au20Sn钎料,除了共晶组织ζ相+δ相,在AuSn/Au/Ti镀层界面形成了大量枝状的先共晶ζ相,在AuSn/Au/Ni镀层界面形成了针状的(Au,Ni)3Sn2;对于52In48Sn钎料,在InSn/Au/Ti镀层界面形成了连续层状的Au(In,Sn)2,随着输入能量的增加,其逐渐转变为不连续的块状化合物AuIn2,在熔融钎料流的作用下部分AuIn2脱离界面进入钎料中,在InSn/Au/Ni镀层界面形成了一层极薄的Au(In,Sn)2.     

Growth mechanism and kinetics of Cu3Sn in the interfacial reaction between liquid Sn and diversely oriented Cu substrates

Journal of Materials Science: Materials in Electronics - In this study, the effects of reflow temperature, reflow time, and substrates (polycrystalline and (001/110/111) monocrystalline Cu...