Estimation of the band gap of a series of new thermoelectric materials

Semiconductors - The band gap E g of a number of new thermoelectric materials, such as skutterudites, clathrates, Heusler phases, (Ge,Sn,Pb)(Te,Se)] m [(Bi,Sb)2(Te,Se)3] n (m, n = 0, 1, 2…)...     

Superplastic creep of low melting point solder joints

In prior work, we showed that eutectic Sn-Pb solder joints exhibit superplastic behavior after rapid solidification. Further examples of superplasticity in nominally air-cooled solder joints are reported in this study of three low-melting point alloys: 40In-40Sn-20Pb (wt. %), eutectic 52In-48Sn, and 43Sn-43Pb-14Bi, which were creep-tested in shear at 20°, 65°, and 90° C. The test results indicate that above 65° C, the indium-containing solders have stress exponents between 2.4 to 2.9, a possible overall shear strains of 500%, and an absence of primary creep; at 90° C, 43Sn-43Pb-14Bi solder has a stress exponent close to 2.3. Optical microstructures of the three solders are presented; they help to explain the superplastic behavior.     

Effects of Au and Pd Additions on Joint Strength, Electrical Resistivity, and Ion-Migration Tolerance in Low-Temperature Sintering Bonding Using Ag2O Paste

A new bonding process using an Ag₂O paste consisting of Ag₂O particles mixed with a triethylene glycol reducing agent has been proposed as an alternative joining approach for microsoldering in electronics assembly, which currently uses Pb-rich, high-temperature solders. Ag nanoparticles were formed at approximately 130°C to 160°C through a reduction process, sintered to one another immediately, and bonded to a metal substrate. An Au-coated Cu specimen was successfully bonded using the Ag₂O paste. The resulting joint exhibited superior strength compared with joints fabricated using conventional Pb-rich solders. To improve ion-migration tolerance, the Ag₂O paste was mixed with Au and Pd microparticles to form sintered Ag-Au and Ag-Pd layers, respectively. The additions of Au and Pd improved the ion-migration tolerance of the joint. Regarding the mechanical properties of the joints, addition of secondary Au and Pd both resulted in decreased joint strength. To match the joint strength of conventional Pb-10Sn solder, the mixing ratios of Au and Pd were estimated to be limited to 16?vol.% and 7?vol.%, respectively. The electrical resistivities of the sintered layers consisting of 16?vol.% Au and 7?vol.% Pd were lower than that of Pb-10Sn solder. Thus, the additive fractions of Au and Pd to the Ag₂O paste should be less than 16?vol.% and 7?vol.%, respectively, to avoid compromising the mechanical and electrical properties of the sintered layer relative to those of contemporary Pb-10Sn solder. Following the addition of Au and Pd to the paste, the ion-migration tolerances of the sintered layers were approximately 3 and 2 times higher than that of pure Ag, respectively. Thus, the addition of Au was found to improve the ion-migration tolerance of the sintered Ag layer more effectively and with less sacrifice of the mechanical and electrical properties of the sintered layer than the addition of Pd.     

A Study on the Breakdown Mechanism of an Electroless-Plated Ni(P) Diffusion Barrier for Cu/Sn/Cu 3D Interconnect Bonding Structures

This study examined the thermal stability of an electroless-plated Ni(P) barrier layer inserted between Sn and Cu in the bonding structure of Cu/Sn/Cu for three-dimensional (3D) interconnect applications. A combination of transmission electron microscopy (TEM) and scanning electron microscopy allowed us to fully characterize the bonding morphology of the Cu/Ni(P)/Sn/Ni(P)/Cu joints bonded at various temperatures. The barrier suppressed Cu and Sn interdiffusion very effectively up to 300°C; however, an interfacial reaction between Ni(P) and Sn led to gradual decomposition into Ni₃P and Ni₃Sn₄. Upon 350°C bonding, the interfacial reaction brought about complete disintegration of the barrier in local areas, which allowed unhindered interdiffusion between Cu and Sn.     

Fatigue crack growth behavior in 63Sn-37Pb and 95Pb-5Sn solder materials

Fatigue crack growth behavior of two typical solders 63Sn-37Pb and 95Pb-5Sn has been investigated under room temperature (20°C) and frequency of 10 Hz. Fatigue crack growth behavior of the 63Sn-37Pb specimen tested at stress ratios up to 0.5 and the 95Pb-5Sn specimen tested at stress ratios up to 0.7 were dominantly cyclic dependent. Intergranular crack growth was observed for the 63Sn-37Pb specimen tested at stress ratio of 0.7, which indicates that a crack grows under influence of creep. The difference of effect of stress ratio on crack growth behavior between two solders may results from the difference of homologous temperature. Fatigue crack growth mechanisms for solder materials are summarized as follows: A crack propagates in a transgranular manner at low homologous temperature and low stress ratio, while it propagates along eutectic grain boundaries at high homologous temperature and high stress ratio under influence of creep. At the intermediate homologous temperatures, a crack propagates in a transgranular manner with some region of intergranular fracture.     

Germanium-Rich SiGe Nanowires Formed Through Oxidation of Patterned SiGe FINs on Insulator

In this study, the authors report on the fabrication of Ge-rich SiGe nanowires (SGNWs) by oxidation of SiGe fins on insulator. Nanowires of different shapes and size are obtained by varying the initial fin shape, Ge content, oxidation process temperature, and oxidation time. Transmission electron microscopy observations revealed nanowires with rectangular, square, elliptical, circular, octagonal, and hexagonal cross-sections, with different Ge content. The elliptical, octagonal, and hexagonal facets are unique shapes formed with low-index faces belonging to (110) groups. These possess very high Ge content up to 95%, and were obtained in the samples oxidized from 850°C to 875°C. In␣addition, the in-plane strain in the fabricated SGNWs is evaluated using micro-Raman spectroscopy. The possible mechanism behind the formation and transformation of different nanowire shapes is discussed.     

Effect of aging on mechanical properties of high temperature Pb-rich solder joints

The effect of aging on the evolution of interfacial microstructure and mechanical properties of Pb-rich PbSnAg solder joints with different Sn content was investigated. Tensile samples were prepared by soldering two pieces of Ni or Cu strips resulting in joints with gap sizes of about 250 μm. Multi-layered structures composed of Ni coated Si-chips soldered onto Ni/Cu metallized ceramic substrates were used for fatigue testing. All samples were subjected to thermal aging at 250 °C up to 500 h. Microstructural investigations revealed that independent from the substrate material, increased Sn content of the solder alloy results in improvement of the tensile and fatigue properties of the joints and a higher growth rate of the interfacial intermetallic compound (IMC) layers. It was found that the thickness of the wettable substrate especially in the case of low-Sn alloys, also affects the interfacial properties of high temperature PbSnAg solder joints. In this case, a reduced Sn content in solder joints results in weakening of the interface during the reflow process and subsequent thermal aging. The dominant failure mode of the solder joints subjected to cyclic loading was delamination of the interfacial IMC layer from the substrate. Finite element simulations were conducted by using a strain rate and pressure dependent material model for PbSnAg solder in order to analyse the states of stress and strain during static and cyclic loading.     

Thermoelectric Characteristics of <Emphasis Type="Italic">p</Emphasis>-Type (Bi,Sb)<Subscript>2</Subscript>Te<Subscript>3</Subscript>/(Pb,Sn)Te Functional Gradient Materials with Variation of the Segment Ratio

The p-type (Bi,Sb)₂Te₃/(Pb,Sn)Te functional gradient materials (FGMs) were fabricated by hot-pressing mechanically alloyed (Bi_0.2Sb_0.8)₂Te₃ and 0.5 at.% Na₂Te-doped (Pb_0.7Sn_0.3)Te powders together at 500°C for 1 h in vacuum. Segment ratios of (Bi,Sb)₂Te₃ to (Pb,Sn)Te were varied as 3:1, 1.3:1, and 1:1.6. A reaction layer of about 350-μm thickness was formed at the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGM interface. Under temperature differences larger than 340°C applied across a specimen, superior figures of merit were predicted for the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGMs to those of (Bi_0.2Sb_0.8)₂Te₃ and (Pb_0.7Sn_0.3)Te. With a temperature difference of 320°C applied across a specimen, the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGMs with segment ratios of 3:1 and 1.3:1 exhibited the maximum output powers of 72.1 mW and 72.6 mW, respectively, larger than the 63.9 mW of (Bi_0.2Sb_0.8)₂Te₃ and the 26 mW of 0.5 at.% Na₂Te-doped (Pb_0.7Sn_0.3)Te.     

半导体锗纳米团簇和纳米层的生成与结构研究

我们在硅锗合金衬底上采用氧化等制膜方式生成零维和二维的纳米结构样品，用高精度椭偏仪(HPE)、卢瑟福背散射谱仪(RBS)和高分辨率扫描透射电子显微镜(HR-STEM)测量样品的纳米结构，并采用美国威思康新州立大学开发的Rump模拟软件对卢瑟福背散射谱(RBS)中的CHANNEL谱和RANDoM谱分别进行精细结构模拟，测量并计算出纳米氧化层与锗的纳米薄膜结构分布，并且反馈控制加工过程，优化硅锗半导体材料纳米结构样品的加工条件。我们测量出样品横断面锗纳米团簇和纳米层的PL发光谱。我们在硅锗合金的氧化层表面中首次发现纳米锗量子点组成的几个纳米厚的盖帽膜结构，我们首次提出的生成硅锗纳米结构的优化加工条件的氧化时间和氧化温度匹配公式的理论模型与实验结果拟合得很好。     

GeSb2Te4纳米薄膜的原位透射电子显微学研究

相变材料GeSbTe亚稳相中的局域结构长期以来一直是研究的热点,特别是亚稳相中Ge原子的分布对相变过程的进行具有重要作用.本文主要利用透射电子显微学,电子衍射技术和基于电子衍射技术的径向分布函数对不同时效温度时效过程中GeSb2Te4亚稳相的原子结构进行了深入研究.通过对径向分布函数的研究表明:250℃时效60min时GeSb2Te4亚稳相中几乎所有的Ge原子都位于四面体位置,局域形成类尖晶石结构;而时效温度为150℃、200℃下分别时效60min时发现没有或者几乎没有Ge原子位于四面体位置,时效后的结构以岩盐矿结构为主,部分晶化为六方结构.     

Characterization of eutectic Sn-Bi solder joints

This report presents experimental results on 58Bi-42Sn solder joints, optical and SEM microstructures of their matrix and of their interface with copper, solidification behavior studied by differential scanning calorimetry, wettability to copper, creep, and low cycle fatigue. These results are discussed in comparison with 60Sn-40Pb solder, and with three low temperature solders, 52In-48Sn, 43Sn-43Pb-14Bi, and 40In-40Sn-20Pb. The 58Bi-42Sn solder paste with RMA flux wets Cu matrix with a wetting angle of 35° and had a 15° C undercooling during solidification. The constitutive equation of the steady state shear strain rate, and the Coffin-Manson relation constants for the low cycle shear fatigue life at 65° C have been determined. The test results show that this solder has the best creep resistance but the poorest fatigue strength compared with the other four solders.     

Solderability of pre-tinned Cu sheet

The reliability and integrity of pre-tinned copper-clad printed circuit (PC) boards are serious concerns in the manufacture of electronic devices. The factors that influence the wetting during soldering of Cu are discussed. The results suggest that pre-tinning with a Pb-rich solder, such as 95Pb-5Sn, is preferred to pre-tinning with eutectic solder, since the latter can develop exposed intermetallics during aging that wet poorly. The results also confirm that the use of flux leads to carbon contamination in the solder.     

A Study on Sputtered Bi-Te Thermoelectric Films with Various Compositions: Microstructure Evolution and the Effects on Thermoelectric and Electrical Properties

This study examined the sensitive effects of composition on the microstructure evolution and thermoelectric properties of sputtered Bi-Te films. Bi-Te films of various Te compositions (49 at.% to 60 at.%) were grown by cosputtering deposition and annealed at 200°C for different durations. We examined the microstructure of the films using x-ray diffraction (XRD) and transmission electron microscopy (TEM), and measured the electronic transport and thermoelectric properties. As the Te composition of the films changed from 49 at.% to 60 at.%, the phase of the as-sputtered film changed from the rhombohedral BiTe-type phase to the metastable rock-salt phase, which eventually transformed to the Bi₂Te₃-type phase upon annealing, instigating microstructure evolution. This phase transformation profoundly influenced the electrical and thermoelectric properties of the films.     

硅锗合金氧化后生成的锗纳米结构的特性研究

我们将SiGe合金在干氧吹气环境下以不同的温度和不同的时值进行氧化处理，用卢摄福散射仪RBS和高精度椭偏仪HP-ESM测量样品，获得10～80nm厚的硅氧化层和1nm厚的富锗层．新发现快速氧化生成的氧化膜表面有1～2nm厚的锗层．分析了锗纳米结构对应的PL发光谱，注意到锗纳米层对应的541nm波长的尖锐的发光峰和不同尺寸的锗原子团对应的从550～720nm波长的发光带．从量子受限模型和局域密度泛函计算出发。合理地解释了实验的结果．     

Joule Heating Enhanced Phase Coarsening in Sn37Pb and Sn3.5Ag0.5Cu Solder Joints during Current Stressing

This paper investigated the effect of Joule heating on the phase coarsening in Sn37Pb and Sn3.5Ag0.5Cu ball grid array (BGA) solder joints stressed at −5°C and 125°C with a 6.0 × 10² A/cm² electric current. The phase growth under current stressing was also compared with those under aging at 125°C. It was found that the current stressing produced a substantial Joule heating in the solder joints and conductive traces. Hence, the solder joints underwent a considerable temperature rise by 30–35°C when stressed at −5°C and 125°C in this study. Coarsening of Pb-rich and Ag-rich phases was confirmed to be accelerated by the current stressing as a result of enhanced diffusion at elevated temperature and atomic stimulation due to numerous collisions between electrons and atoms. Different controlling kinetics were suggested for the cases stressed or aged at different temperatures.     

Fundamental Study of the Intermixing of 95Pb-5Sn High-Lead Solder Bumps and 37Pb-63Sn Pre-Solder on Chip-Carrier Substrates

This study investigated the intermixing of 95Pb-5Sn solder bumps and 37Pb-63Sn pre-solder in flip-chip solder joints. The reaction conditions included multiple reflows (up to ten) at 240°C, whereby previously solder-coated parts are joined by heating without using additional solder. We found that the molten pre-solder had an irregular shape similar to a calyx (i.e., a cup-like structure) wrapped around a high-lead solder bump. The height to which the molten pre-solder ascended along the solid high-lead solder bump increased with the number of reflows. The molten pre-solder was able to reach the under bump metallurgy (UBM)/95Pb-5Sn interface after three to five reflows. The molten pre-solder at the UBM/95Pb-5Sn interface generated two important phenomena: (1) the molten solder dewetted (i.e., flowed away from the soldered surface) along the UBM/95Pb-5Sn interface, particularly when the number of reflows was high, and (2) the molten pre-solder transported Cu␣atoms to the UBM/95Pb-5Sn interface, which in turn caused the Ni-Sn compounds at the chip-side interface to change into (Cu_0.6Ni_0.4)₆Sn₅.     

Experimental determination of tie-lines in the Hg-Cd-Te system

Starting with powdered Hg_1−xCd_xTe, several tie-lines at 500 and 560°C were established using an energy dispersive spectrometer on a scanning electron microscope for the quantitative analysis. After holding at 500 or 560°C for time periods based upon the powder size and the published interdiffusion constant, then water quenching to room temperature, the primary grains were found to be uniform in composition and covered with a 5-6 μ layer of HgTe or low x Hg_1−xCd_xTe. The primary grain and overall compositions establish directions for tie-lines that are in good agreement with published experimental and theoreti-cal results.     

Structure,Surface Morphology,and Optical and Electronic Properties of Annealed SnS Thin Films Obtained by CBD

SnS thin films were initially coated onto Pyrex substrates by the chemical bath deposition (CBD) method and annealed at various temperatures ranging from 200°C to 600°C for 30 min in nitrogen gas. X-ray diffraction (XRD) analysis revealed that a structural transition from face-centered cubic to orthorhombic occurs when the annealing temperature is over 500°C. The surface morphology of all thin layers was investigated by means of scanning electron microscopy and atomic force microscopy. The elemental composition of Sn and S, as measured by energy dispersive spectroscopy, is near the stoichiometric ratio. Optical properties studied by means of transmission and reflection measurements show an increase in the absorption coefficient with increasing annealing temperatures. The band gap energy is close to 1.5 eV, which corresponds to the optimum for photovoltaic applications. Last, the thermally stimulated current measurements show that the electrically active traps located in the band gap disappear after annealing at 500°C. These results suggest that, once again, annealing as a post-deposition treatment may be useful for improving the physical properties of the SnS layers included in photovoltaic applications. Moreover, the thermo-stimulated current method may be of practical relevance to explore the electronic properties of more conventional industrial methods, such as sputtering and chemical vapor deposition.     

Solid Liquid Interdiffusion Bonding of (Pb,Sn)Te Thermoelectric Modules with Cu Electrodes Using a Thin-Film Sn Interlayer

A (Pb, Sn)Te thermoelectric element plated with a Ni barrier layer and a Ag reaction layer has been joined with a Cu electrode coated with Ag and Sn thin films using a solid–liquid interdiffusion bonding method. This method allows the interfacial reaction between Ag and Sn such that Ag₃Sn intermetallic compounds form at low temperature and are stable at high temperature. In this study, the bonding strength was about 6.6 MPa, and the specimens fractured along the interface between the (Pb, Sn)Te thermoelectric element and the Ni barrier layer. Pre-electroplating a film of Sn with a thickness of about 1 μm on the thermoelectric element and pre-heating at 250°C for 3 min ensures the adhesion between the thermoelectric material and the Ni barrier layer. The bonding strength is thus increased to a maximal value of 12.2 MPa, and most of the fractures occur inside the thermoelectric material. During the bonding process, not only the Ag₃Sn intermetallics but also Cu₆Sn₅ forms at the Ag₃Sn/Cu interface, which transforms into Cu₃Sn with increases in the bonding temperature or bonding time.     

Low carrier concentration (PbSn)Te by molecular beam epitaxy

pb_0.77.77SN_0.23Te was grown by evaporation of the pure elements or of the binary compounds from Knudsen cell sources at l0^-10 torr. The growth chamber was provided with ion bombardment, LEED, Auger spectroscopy and quartz crystal deposition moni-toring. A Sn-wetted heating block was used for close control of substrate temperature. On chemomechanically-polished BaF₂ (100) substrates, monocrystalline films smooth to tens of Angstroms were obtained at 1 micrometer/hour growth rate over a substrate temperature range from 300 to 420°C; above A20°C, re-evaporation occurred. In growth from the elements, excess Pb remains as droplets on the film surface, but most excess Te re-evaporates. The remaining Te produces Pb vacancies which generate p-carriers; conversely, excess Pb produces n-carriers. A quartz crystal deposition rate monitor operat-ing at the growth temperature was used to tune impinging (Te/ Pb+Sn) ratio by observing the discontinuity in rate vs. (Te/ Pb+Sn) which occurred at the stoichiometric ratio. This technique yielded films with as-grown carrier concentrations in the high l0¹⁷ p/cm³ range at 77K. Closer stoichiometry control was obtained by growing from PbTe and SnTe sources with the use of a separate Pb source for reduction of inher-ent Te excess. For Pb_0.77Sn_0.23Te, The metal-rich single-phase boundary was found to cross the stoichiometric composition at 395 + 5°, so that only p-type material was obtainable above this temperature. By using 0.014 atomic fraction excess Pb, it was possible to constrain the film composition to this boundary and thus obtain close control over carrier concen-tration by appropriate adjustment of substrate temperature. Carrier concentrations of 2×l0¹⁷/cc at 77K were obtained in this manner for both n- and p-type material. Excess Pb coalesced into droplets spaced sparsely enough on the sur-face so as not to interfere with device fabrication. Hall mobilities were about 1×10⁴ cm² /v⋅sec at 77K. Attempts to reduce carrier concentration below 2×10¹⁷ cm^-3 resulted in anomalously low Hall voltages, suggesting existence of p and n domains in the films. The origins of this inhomogeneity are not clear at present.