Showing papers in "Journal of Electronic Materials in 2006"
TL;DR: In this article, the effects of a few common point defects, such as Zn interstitials, Zn vacancies, O vacancies, and complexes of each, are examined.
Abstract: ZnO is a wide-band-gap semiconductor material that is now being developed for many applications, including ultraviolet (UV) light-emitting diodes, UV photodetectors, transparent thin-film transistors, and gas sensors. It can be grown as boules, as thin films, or as nanostructures of many types and shapes. However, as with any useful semiconductor material, its electrical and optical properties are controlled by impurities and defects. Here, we consider various important donor-type impurities, such as H, Al, Ga, and In, and acceptor-type impurities, such as N, P, As, and Sb. We also examine the effects of a few common point defects, including Zn interstitials, Zn vacancies, O vacancies, and complexes of each. The main experimental techniques of interest here include temperature-dependent Hall-effect and low-temperature photoluminescence measurements, because they alone can provide donor and acceptor concentrations and donor energies. The important topic of p-type ZnO is also considered in some detail.
152 citations
TL;DR: In this paper, the growth stress in GaN on high-temperature (HT) AlN/Si(111) templates is compressive, although, due to relaxation, the stress we have measured is much lower than the theoretical value.
Abstract: In this work, we report on the growth by metalorganic vapor phase epitaxy (MOVPE) of GaN layers on AlN/Si(111) templates with step-graded AlGaN intermediate layers. First, we will discuss the optimization of the AlN/Si(111) templates and then we will discuss the incorporation of step-graded AlGaN intermediate layers. It is found that the growth stress in GaN on high-temperature (HT) AlN/Si(111) templates is compressive, although, due to relaxation, the stress we have measured is much lower than the theoretical value. In order to prevent the stress relaxation, step-graded AlGaN layers are introduced and a crack-free GaN epitaxial layer of thickness >1 µm is demonstrated. Under optimized growth conditions, the total layer stack, exceeding 2 µm in total, is kept under compressive stress, and the radius of the convex wafer bowing is as large as 119 m. The crystalline quality of the GaN layers is examined by high-resolution x-ray diffraction (HR-XRD), and the full-width-at-half maximums (FWHMs) of the x-ray rocking curve (0002) ω-scan and (−1015) ω-scan are 790 arc sec and 730 arc sec, respectively. It is found by cross-sectional transmission electron microscopy (TEM) that the step-graded AlGaN layers terminate or bend the dislocations at the interfaces.
141 citations
TL;DR: In this article, the volume difference between the various types of solder joints in electronic devices can be enormous, such as the difference between a 760-µm ball grid array solder joint and a 75 µm flip-chip solder joint is as high as 1000 times.
Abstract: The volume difference between the various types of solder joints in electronic devices can be enormous. For example, the volume difference between a 760-µm ball grid array solder joint and a 75-µm flip-chip solder joint is as high as 1000 times. Such a big difference in volume produces a pronounced solder volume effect. This volume effect on the soldering reactions between the Sn3AgxCu (x=0.4, 0.5, or 0.6 wt.%) solders and Ni was investigated. Three different sizes of solder spheres (300, 500, and 760 µm in diameter) were soldered onto Ni soldering pads. Both the Cu concentration and the solder volume had a strong effect on the type of the reaction products formed. In addition, (Cu,Ni)6Sn5 massively spalled from the interface under certain conditions, including smaller joints and those with lower Cu concentration. We attributed the massive spalling of (Cu,Ni)6Sn5 to the decrease of the available Cu in the solders. The results of this study suggest that Cu-rich SnAgCu solders can be used to prevent this massive spalling.
138 citations
TL;DR: In this article, the authors report first principles density functional calculations of the polarizations, piezoelectric stress constants, and elastic constants for the II-VI oxides MgO, ZnO, and CdO in the wurtzite structure.
Abstract: We report first-principles density functional calculations of the polarizations, piezoelectric stress constants, and elastic constants for the II-VI oxides MgO, ZnO, and CdO in the wurtzite structure. Using our pseudopotential self-interaction corrected implementation of density functional theory, we obtain polarization values of −0.060, −0.022, and −0.10 C/m2, and piezoelectric constants, e33 (e31) of 1.64 (−0.58), 1.34 (−0.57), and 1.67 (−0.48) C/m2 for structurally relaxed MgO (with its in-plane lattice parameter fixed to that calculated for ZnO), ZnO, and CdO, respectively. The large polarization gradients between the end-point compounds in the MgO-ZnO-CdO system augur well for the production of large internal fields in ZnO-based polarization field effect transistors.
134 citations
TL;DR: In this paper, an inverse correlation between magnetization and electron density as controlled by Sn-doping was found in films doped with Mn during pulsed laser deposition (PLD), suggesting that carrier concentration alone alone cannot explain the magnetic properties of ZnO:Mn and factors such as crystalline quality and residual defects play a role.
Abstract: ZnO is a very promising material for spintronics applications, with many groups reporting room-temperature ferromagnetism in films doped with transition metals during growth or by ion implantation. In films doped with Mn during pulsed laser deposition (PLD), we find an inverse correlation between magnetization and electron density as controlled by Sn-doping. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn and factors such as crystalline quality and residual defects play a role. Plausible mechanisms for ferromagnetism include the bound magnetic polaron model or exchange that is mediated by carriers in a spin-split impurity band derived from extended donor orbitals. The progress in ZnO nanowires is also reviewed. The large surface area of nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for microlasers or memory arrays. Single ZnO nanowire depletion-mode metal-oxide semiconductor field effect transistors exhibit good saturation behavior, threshold voltage of ∼−3 V, and a maximum transconductance of 0.3 mS/mm. Under ultraviolet (UV) illumination, the drain-source current increased by approximately a factor of 5 and the maximum transconductance was ∼5 mS/mm. The channel mobility is estimated to be ∼3 cm2/Vss, comparable to that for thin film ZnO enhancement mode metal-oxide semiconductor field effect transistors (MOSFETs), and the on/off ratio was ∼25 in the dark and ∼125 under UV illumination. The Pt Schottky diodes exhibit excellent ideality factors of 1.1 at 25°C, very low reverse currents, and a strong photoresponse, with only a minor component with long decay times thought to originate from surface states. In the temperature range from 25°C to 150°C, the resistivity of nanorods treated in H2 at 400°C prior to measurement showed an activation energy of 0.089 eV and was insensitive to ambient used. By contrast, the conductivity of nanorods not treated in H2 was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors. Sensitive pH sensors using single ZnO nanowires have also been fabricated.
132 citations
TL;DR: In this article, an electron avalanche photodiode (EAPD) with near noiseless gain was proposed, which is characterized by a uniform, exponential, gain voltage characteristic that is consistent with a holeto-electron ionization coefficient ratio, k=αh/αe, of zero.
Abstract: Electron injection avalanche photodiodes in short-wave infrared (SWIR) to long-wave infrared (LWIR) HgCdTe show gain and excess noise properties indicative of a single ionizing carrier gain process. The result is an electron avalanche photodiode (EAPD) with “ideal” APD characteristics including near noiseless gain. This paper reports results obtained on long-, mid-, and short-wave cutoff infrared Hg1−xCdxTe EAPDs (10 µm, 5 µm, and 2.2 µm) that use a cylindrical “p-around-n” front side illuminated n+/n-/p geometry that favors electron injection into the gain region. These devices are characterized by a uniform, exponential, gain voltage characteristic that is consistent with a hole-to-electron ionization coefficient ratio, k=αh/αe, of zero. Gains of greater than 1,000 have been measured in MWIR EAPDS without any sign of avalanche breakdown. Excess noise measurements on midwave infrared (MWIR) and SWIR EAPDs show a gain independent excess noise factor at high gains that has a limiting value less than 2. At 77 K, 4.3-µm cutoff devices show excess noise factors of close to unity out to gains of 1,000. A noise equivalent input of 7.5 photons at a 10-ns pulsed signal gain of 964 measured on an MWIR APD at 77 K provides an indication of the capability of this new device. The excess noise factor at room temperature on SWIR EAPDs, while still consistent with the k=0 operation, approaches a gain independent limiting value of just under 2 because of electron-phonon interactions expected at room temperature. The k=0 operation is explained by the band structure of the HgCdTe. Monte Carlo modeling based on the band structure and scattering models for HgCdTe predict the measured gain and excess noise behavior.
132 citations
TL;DR: In this paper, the authors have grown GaN and AlGaN nanowires on Si (111) substrates with gassource molecular beam epitaxy (MBE), and the wires were well separated, 50-250 nm in diameter and grew to lengths ranging from 2 µm to 7 µm.
Abstract: We have grown GaN and AlGaN nanowires on Si (111) substrates with gassource molecular beam epitaxy (MBE). No metal catalysts were used. The nanowires displayed a number of interesting materials properties, including room-temperature luminescence intensity greater than that of free-standing HVPE-grown GaN, relaxed lattice parameters, and the tendency of nanowires dispersed in solvents to align in response to electric fields. The wires were well separated, 50–250 nm in diameter, and grew to lengths ranging from 2 µm to 7 µm. Transmission electron microscopy indicated that the wires were free of defects, unlike the surrounding matrix layer.
129 citations
TL;DR: In this paper, the physical behavior of the glass frit during heat treatment as well as the resulting Ag−Si contact interface structure was studied and it was found that a glass Frit that crystallizes fast during the firing cycle after etching the silicon nitride and Si emitter results in smaller Ag crystallite precipitation at the contact interface.
Abstract: The aim of this study is to understand the effect of the glass frit chemistry used in thick-film Ag pastes on the electrical performance of the silicon solar cell. The study focuses on the physical behavior of the glass frit during heat treatment as well as the resulting Ag−Si contact interface structure. We observe that the glass frit transition temperature (Tg) and softening characteristics play a critical role in the contact interface structure. The glass transition temperature also significantly influences the contact ohmicity of the thick-film metal grid. A high glass frit transition temperature generally results in thinner glass regions between the Ag bulk of the grid and the Si emitter. It was found that a glass frit (with high Tg) that crystallizes fast during the firing cycle after etching the silicon nitride and Si emitter results in smaller Ag crystallite precipitation at the contact interface. This results in smaller junction leakage current density (Jo2) and higher open-circuit voltage (Voc). Using high Tg pastes (with the appropriate Ag powder size), greater than 0.78 fill factors and >17.4% efficiency were achieved on 4 cm2 untextured single crystal Si solar cells with 100 Ω/sq emitters.
121 citations
TL;DR: In this article, a new abrasive-free planarization method for silicon carbide (SiC) wafers was proposed using the catalytic nature of platinum (Pt).
Abstract: A new abrasive-free planarization method for silicon carbide (SiC) wafers was proposed using the catalytic nature of platinum (Pt). We named it catalyst-referred etching (CARE). The setup equipped with a polishing pad made of Pt is almost the same as the lapping setup. However, CARE chemically removes SiC with an etching agent activated by a catalyst in contrast to mechanical removal by the lapping process. Hydrofluoric acid which is well known as an etchant of silicon dioxide (SiO2) that cannot etch SiC, was used as the source of the etching agent to SiC. The processed surfaces were observed by Nomarski differential interference contrast (NDIC) microscopy, atomic force microscopy (AFM), and optical interferometry. Those observations presented a marked reduction in surface roughness. Moreover, low-energy electron diffraction (LEED) images showed that a crystallographically well-ordered surface was realized.
111 citations
TL;DR: In this paper, the tensile properties of the joints between Bi-Ag solders and two metallic substrates, Cu and Ni, were investigated and it was shown that the formation of those Ni-Bi intermetallics had a strong influence on tensile strength and fracture morphology.
Abstract: This study investigated the interfacial morphology and tensile properties of the joints between Bi-Ag solders and two metallic substrates, Cu and Ni. Instead of forming intermetallic compounds, grooving occurred at the intersections of Cu grain boundaries with the Bi-Ag/Cu interface and thus provided mechanical bonding. As for the Ni substrate, cellular NiBi3 was observed at the interface and also, massive NiBi3 in the form of long blades emanating from the interface was located within the solder region. A thin NiBi layer formed through a solid-state reaction between NiBi3 and the solder. The formation of those Ni-Bi intermetallics had a strong influence on the tensile strength and fracture morphology of the Bi-Ag/Ni joints.
110 citations
TL;DR: In this article, an in-depth characterization of the interfacial reaction of Zn-added Sn-Ag-Cu solders on Cu and Au/Ni(P) surface finishes is reported.
Abstract: The near-ternary eutectic Sn-Ag-Cu alloys have been identified as leading Pb-free solder candidates to replace Pb-bearing solders in microelectronic applications. However, recent investigations on the processing behavior and solder joints reliability assessment have revealed several potential reliability risk factors associated with the alloy system. The formation of large Ag3Sn plates in Sn-Ag-Cu joints, especially when solidified in a relatively slow cooling rate, is one issue of concern. The implications of large Ag3Sn plates on solder joint performance and several methods to control them have been discussed in previous studies. The minor Zn addition was found to be effective in reducing the amount of undercooling required for tin solidification and thereby to suppress the formation of large Ag3Sn plates. The Zn addition also caused the changes in the bulk microstructure as well as the interfacial reaction. In this paper, an in-depth characterization of the interfacial reaction of Zn-added Sn-Ag-Cu solders on Cu and Au/Ni(P) surface finishes is reported. The effects of a Zn addition on modification of the interfacial IMCs and their growth kinetics are also discussed.
TL;DR: In this paper, lead-free test assemblies were thermally cycled (−45°C/+125°C, 15min. dwell time, 750 cycles) or isothermally annealed (125°c, 500 h) before the standard drop test, and the average number of drops to failure increased when the thermal cycling was performed before the drop test (1,500 G deceleration, 0.5 ms half sine pulse).
Abstract: To simulate more realistically the effects of strains and stresses on the reliability of portable electronic products, lead-free test assemblies were thermally cycled (−45°C/+125°C, 15-min. dwell time, 750 cycles) or isothermally annealed (125°C, 500 h) before the standard drop test. The average number of drops to failure increased when the thermal cycling was performed before the drop test (1,500 G deceleration, 0.5 ms half-sine pulse). However, the difference was not statistically significant due to the large dispersion in the number of drops to failure of the assemblies drop tested after the thermal cycling. On the other hand, the average number of drops to failure decreased significantly when the isothermal annealing was carried out before the drop test. The failure analysis revealed four different failure modes: (1) cracking of the reaction layers on either side of the interconnections, (2) cracking of the bulk solder, (3) mixed mode of component-side intermetallic and bulk solder cracking, and (4) voidassisted cracking of the component-side Cu3Sn layer. The assemblies that were not thermally cycled or annealed exhibited only type (1) failure mode. The interconnections that were thermally cycled before the drop test failed by mode (2) or mode (3). The drop test reliability of the thermally cycled interconnections was found to depend on the extent of recrystallization generated during the thermal cycling. This also explains the observed wide dispersion in the number of drops to failure. On the other hand, the test boards that were isothermally annealed before the drop testing failed by mode (4).
TL;DR: In this paper, a ZrO2 nanoparticle strengthened lead-free Sn-3,5Ag-ZrO 2 solder was prepared by mechanically stirring ZrOs nanoparticles into the molten melt of eutectic Sn 3.5Ag alloy.
Abstract: A ZrO2 nanoparticle strengthened lead-free Sn-3,5Ag-ZrO2 solder was prepared by mechanically stirring ZrO2 nanoparticles into the molten melt of eutectic Sn-3.5Ag alloy. The influence of ZrO2 nanoparticles on the eutectic solidification process, in particular, the formation of Ag3Sn intermetallic compounds (IMCs) and the associated microstructure that forms and microhardness of Sn-3.5Ag solder, was systematically investigated. The addition of ZrO2 nanoparticles significantly refined the size of Ag3Sn IMCs due to the strong adsorption effect of the ZrO2 nanoparticles. The refined Ag3Sn IMCs increase the Vicker’s microhardness of the prepared Sn-3.5Ag-ZrO2 solder, which corresponds well with the prediction of the classic theory of dispersion strengthening.
TL;DR: In this paper, the effect of the addition of Ni to Sn-Cu solders on the formation of intermetallic compounds (IMCs) was investigated to reveal the effect that Ni addition to Sn 0.7Cu solder very effectively inhibits the formation and growth of the IMC between solder and substrate.
Abstract: The interfacial reaction between Sn-0.7mass%Cu-(Ni) solders and a Cu substrate was investigated to reveal the effect of the addition of Ni to Sn-Cu solder on the formation of intermetallic compounds (IMCs). Sn-0.7Cu-xNi solders (x=0, 0.05, 0.1, 0.2 mass%) were prepared. For the reflow process, specimens were heated in a radiation furnace at 523 K for 60 sec, 300 sec, and 720 sec to estimate the interfacial reaction between the molten solder and Cu substrate. Then, for the aging process, some specimens were heat-treated in an oil bath at 423 K for 168 h and 504 h. The cross sections of soldered specimens were observed to measure the dissolution thickness of the Cu substrate and the thickness of the IMC and to investigate the microstructures of IMC. The results showed that, just after the reflow process, the dissolution thickness of the Cu substrate increased with the increase of Ni content in the Sn-0.7Cu-xNi solder and the thickness of the IMC between the solder and Cu substrate was the minimum in the Sn-0.7Cu-0.05Ni solder. After the aging process, the IMC grew with the increase of aging time. In the case of 0.05% Ni, the IMC thickness was the thinnest regardless of aging time. It is clear that 0.05% Ni addition to Sn-0.7Cu solder very effectively inhibits the formation and growth of the IMC between solder and Cu substrate. Electron probe microanalysis of the IMC showed that the IMC layer in the Sn-0.7Cu-Ni solder contained Ni, and the IMC was expressed as (Cu1−y
,Ni
y
)6Sn5.
TL;DR: In this paper, the shear ductility after extensive aging of Sn-Ag-Cu (SAC) solders has been improved radically by Co or Fe modifications, and several SAC+X candidates (X=Mn, Ni, Ge, Ti, Si, Cr, and Zn) now have been tested.
Abstract: Addressing the potential for drop impact failure of Pb-free interconnects, the shear ductility after extensive aging of Sn-Ag-Cu (SAC) solders has been improved radically by Co or Fe modifications. Several other SAC+X candidates (X=Mn, Ni, Ge, Ti, Si, Cr, and Zn) now have been tested. Solder joint microstructures and shear strength results show that new SAC+X alloys also suppress void formation and coalescence at the Cu (substrate)/Cu3Sn interface (and embrittlement) after aging at 150°C for up to 1,000 h. Microprobe measurements of 1,000 h aged samples suggest that Cu substitution by X is usually accentuated in the intermetallic layers, consistent with X=Co and Fe results.
TL;DR: In this article, a model for the photoconductive response based on rate equations is presented providing an accurate fit to measured photoconductivity data, suggesting the presence of hole trap levels in the energy range of 0.6 eV to 1.0 eV relative to the valence bandedge.
Abstract: A strong photoconductive response is observed for ZnO epilayers in the presence of both above bandgap and below bandgap photoexcitation. Photoexcitation for energies larger than the bandgap results in a photoconductive response with fast and slow time constants on the order of nanoseconds and larger than milliseconds, respectively. The fast and slow time constants are attributed to minority carrier recombination and slow escape of holes from traps, respectively. Photoexcitation in the visible spectral region, below the bandgap energy, results in slow rise and fall time constants on the order of minutes and hours. A model for the photoconductive response based on rate equations is presented providing an accurate fit to measured photoconductivity data. The rate equation model suggests the presence of hole trap levels in the energy range of 0.6 eV to 1.0 eV relative to the valence bandedge. The passivation of the ZnO surface with SiO2 shows significantly reduced photoconductive transient decay time constants, suggesting a significant reduction of deep surface defects on the ZnO material.
TL;DR: In this paper, the effect of rare earth (RE) elements on the microstructure and solderability of a near-eutectic Sn-3.5 wt.% Ag-0.7 Cu-xRE solder alloys was investigated.
Abstract: The near-eutectic Sn-3.5 wt.% Ag-0.7 wt.% Cu (Sn-3.5Ag-0.7Cu) alloy was doped with rare earth (RE) elements of primarily Ce and La of 0.05–0.25 wt.% to form Sn-3.5Ag-0.7Cu-xRE solder alloys. The aim of this research was to investigate the effect of the addition of RE elements on the microstructure and solderability of this alloy. Sn-3.5Ag-0.7Cu-xRE solders were soldered on copper coupons. The thickness of the intermetallic layer (IML) formed between the solder and Cu substrate just after soldering, as well as after thermal aging at 170°C up to 1000 h, was investigated. It was found that, due to the addition of the RE elements, the size of the Sn grains was reduced. In particular, the addition of 0.1wt.%RE to the Sn-3.5Ag-0.7Cu solder improved the wetting behavior. Besides, the IML growth during thermal aging was inhibited.
TL;DR: In this paper, the growth kinetics of tin-free tin-base solders with three different substrates (Cu, Ni, and Fe-42Ni) were investigated.
Abstract: Soldering with the lead-free tin-base alloys requires substantially higher temperatures (∼235–250°C) than those (213–223°C) required for the current tin-lead solders, and the rates for intermetallic compound (IMC) growth and substrate dissolution are known to be significantly greater for these alloys. In this study, the IMC growth kinetics for Sn-3.7Ag, Sn-0.7Cu, and Sn-3.8Ag-0.7Cu solders on Cu substrates and for Sn-3.8Ag-0.7Cu solder with three different substrates (Cu, Ni, and Fe-42Ni) are investigated. For all three solders on Cu, a thick scalloped layer of η phase (Cu6Sn5) and a thin layer of e phase (Cu3Sn) were observed to form, with the growth of the layers being fastest for the Sn-3.8Ag-0.7Cu alloy and slowest for the Sn-3.7Ag alloy. For the Sn-3.8Ag-0.7Cu solder on Ni, only a relatively uniform thick layer of η phase (Cu,Ni)6Sn5 growing faster than that on the Cu substrate was found to form. IMC growth in both cases appears to be controlled by grain-boundary diffusion through the IMC layer. For the Fe-42Ni substrate with the Sn-3.8Ag-0.7Cu, only a very thin layer of (Fe,Ni)Sn2 was observed to develop.
TL;DR: In this article, varying weight percentages of multiwalled carbon nanotubes were successfully incorporated into 95.8Sn-3.5Ag-0.7Cu solder to synthesize novel lead-free composite solders.
Abstract: In this study, varying weight percentages of multiwalled carbon nanotubes were successfully incorporated into 95.8Sn-3.5Ag-0.7Cu solder to synthesize novel lead-free composite solders. The composite solders were synthesized using a powder metallurgy route consisting of blending, compaction, sintering, and extrusion. The extruded materials were then characterized for their physical, thermal, and mechanical properties. With the addition of increasing weight percentage of carbon nanotubes, the composite solders experienced a corresponding decrease in density values and an improvement in wetting properties. The melting temperatures of the composite solders were found to be unchanged with additions of carbon nanotubes. However, improvements in the mechanical properties, in terms of microhardness and tensile properties, were observed with increasing weight percentages of carbon nanotubes.
TL;DR: In this article, the defect levels in CdTe were studied using thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) techniques.
Abstract: We have studied the defect levels in as grown and post growth processed cadmium telluride (CdTe) using thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) techniques. We have extracted the thermal energy (Eth) and trapping cross section (σth) for the defect levels using the initial rise and variable heating rate methods. We have identified 10 different defect levels in the crystals. Thermal ionization energy values obtained experimentally were compared to theoretical values of the transition-energy levels of intrinsic and extrinsic defects and defect complexes in CdTe determined by first-principles band-structure calculations. On the basis of this comparison, we have associated the observed ionization levels with various native defects and impurity complexes.
TL;DR: In this paper, a model was developed based on classical microslip theory to explain the general phenomena observed in the evolution of bond footprints left on the substrate, and it was shown that relative motion exists at the bonding interface as peripheral microslips at lower powers, transitioning into gross sliding at higher powers.
Abstract: The effects of the process parameters of ultrasonic power and normal bonding force on bond formation at ambient temperatures have been investigated with scanning electron microscopy (SEM) and energy-dispersive x-ray (EDX) analysis. A model was developed based on classical microslip theory1 to explain the general phenomena observed in the evolution of bond footprints left on the substrate. Modifications to the model are made due to the inherent differences in geometry between ball-bonding and wedge-bonding. Classical microslip theory describes circular contacts undergoing elastic deformation. It is shown in this work that a similar microslip phenomenon occurs for elliptical wire-to-flat contacts with plastically deformed wire. It is shown that relative motion exists at the bonding interface as peripheral microslip at lower powers, transitioning into gross sliding at higher powers. With increased normal bonding forces, the transition point into gross sliding occurs at higher ultrasonic bonding powers. These results indicate that the bonding mechanisms in aluminum wire wedge-bonding are very similar to those of gold ball-bonding, both on copper substrate. In ultrasonic wedge-bonding onto copper substrates, the ultrasonic energy is essential in forming bonding by creating relative interfacial motion, which removes the surface oxides.
TL;DR: In this article, the microstructure and mechanical properties of La-containing Sn-3.9Ag-0.7Cu alloys with small amounts of La (up to 0.5 wt.%) were reported.
Abstract: Sn-rich solders have been shown to have superior mechanical properties when compared to the Pb-Sn system. Much work remains to be done in developing these materials for electronic packaging. In this paper, we report on the microstructure and mechanical properties of La-containing Sn-3.9Ag-0.7Cu alloys. The addition of small amounts of La (up to 0.5 wt.%) to Sn-Ag-Cu refined the microstructure by decreasing the length and spacing of the Sn dendrites and decreased the thickness of the Cu6Sn5 intermetallic layer at the Cu/solder interface. As a result of the change in the microstructure, Sn-Ag-Cu alloys with La additions exhibited a small decrease in ultimate shear strength but significantly higher elongations compared with Sn-Ag-Cu. The influence of LaSn3 intermetallics on microstructural refinement and damage evolution in these novel solders is discussed. Our results have profound implications for improving the mechanical shock resistance of Pb-free solders.
TL;DR: In this paper, the transport properties of single GaN and InN nanowires grown by thermal catalytic chemical vapor deposition were measured as a function of temperature, annealing condition (for GaN) and length/square of radius ratio (for InN).
Abstract: The transport properties of single GaN and InN nanowires grown by thermal catalytic chemical vapor deposition were measured as a function of temperature, annealing condition (for GaN) and length/square of radius ratio (for InN). The as-grown GaN nanowires were insulating and exhibited n-type conductivity (n ≈ 2×1017 cm−3, mobility of 30 cm2/V s) after annealing at 700°C. A simple fabrication process for GaN nanowire field-effect transistors on Si substrates was employed to measure the temperature dependence of resistance. The transport was dominated by tunneling in these annealed nanowires. InN nanowires showed resistivity on the order of 4×10−4 Ω cm and the specific contact resistivity for unalloyed Pd/Ti/Pt/Au ohmic contacts was near 1.09×10−7 Ω cm2. For In N nanowires with diameters <100 nm, the total resistance did not increase linearly with length/square of radius ratio but decreased exponentially, presumably due to more pronounced surface effect. The temperature dependence of resistance showed a positive temperature coefficient and a functional form characteristic of metallic conduction in the InN nanowires.
TL;DR: In this paper, a low-temperature hermetic wafer level packaging scheme for the RF-MEMS devices is presented, where the Au-Sn multilayer metallization with a square loop of 70 µm in width is performed.
Abstract: Development of packaging is one of the critical issues toward realizing commercialization of radio-frequency-microelectromechanical system (RF-MEMS) devices. The RF-MEMS package should be designed to have small size, hermetic protection, good RF performance, and high reliability. In addition, packaging should be conducted at sufficiently low temperature. In this paper, a low-temperature hermetic wafer level packaging scheme for the RF-MEMS devices is presented. For hermetic sealing, Au-Sn eutectic bonding technology at temperatures below 300°C is used. Au-Sn multilayer metallization with a square loop of 70 µm in width is performed. The electrical feed-through is achieved by the vertical through-hole via filling with electroplated Cu. The size of the MEMS package is 1 mm × 1 mm × 700 µm. The shear strength and hermeticity of the package satisfies the requirements of MIL-STD-883F. Any organic gases or contamination are not observed inside the package. The total insertion loss for the packaging is 0.075 dB at 2 GHz. Furthermore, the robustness of the package is demonstrated by observing no performance degradation and physical damage of the package after several reliability tests.
TL;DR: In this article, the electrical characteristics of both n-and p-type GaN metal-oxide semiconductor capacitors utilizing plasma-enhanced CVD-SiO2 as the gate dielectric were measured.
Abstract: The electrical characteristics of both n- and p-type GaN metal-oxide semiconductor (MOS) capacitors utilizing plasma-enhanced CVD-SiO2 as the gate dielectric were measured. Both capacitance and conductance techniques were used to obtain the MOS properties (such as interface state density). Devices annealed at 1000°C/30 min. in N2 yielded an interface state density of 3.8×1010 cm−2 eV−1 at 0.19 eV from the conduction band edge, and it decreased to 1.1×1010 cm−2 eV−1 deeper into the band gap. A total fixed oxide charge density of 8×1012 q cm−2 near the valence band was estimated. Unlike the symmetric interface state density distribution in Si, an asymmetric interface state density distribution with lower density near the conduction band and higher density near the valence band was determined.
TL;DR: In this article, the authors measured the thermal conductivities of bulk ZnO samples under different conditions using scanning thermal microscopy and showed that surface carrier concentration as well as surface morphology affected thermal conductivity.
Abstract: Thermal conductivities (κ) of melt-grown bulk ZnO samples thermally treated under different conditions were measured using scanning thermal microscopy. Samples annealed in air at 1050°C for 3 h and treated with N-plasma at 750°C for 1 min. exhibited κ=1.35±0.08 W/cm-K and κ=1.47±0.08 W/cm-K, respectively. These are the highest values reported for ZnO. Atomic force microscopy (AFM) and conductive-AFM measurements revealed that surface carrier concentration as well as surface morphology affected the thermal conductivity.
TL;DR: In this article, a flip-chip solder joint failed due to local melting of a PbSn eutectic solder, due to a sequence of events induced by the microstructure changes in the flip chip solder joint, where the formation of a depression in the current-crowding region induced the local electrical resistance to increase.
Abstract: A new electromigration failure mechanism in flip-chip solder joints is reported. The solder joints failed by local melting of a PbSn eutectic solder. Local melting occurred due to a sequence of events induced by the microstructure changes in the flip-chip solder joint. The formation of a depression in the current-crowding region of a solder joint induced the local electrical resistance to increase. The rising local resistance resulted in a larger Joule heating, which, in turn, raised the local temperature. When the local temperature rose above the eutectic temperature of the PbSn solder, the solder joint melted and consequently failed. The results of this study suggest that a dynamic, coupled simulation that takes into account the microstructure evolution, current density distribution, and temperature distribution may be needed to fully solve this problem.
TL;DR: In this paper, the effects of different process parameters on the quality of blanket copper wafer bonding are reviewed and summarized, and an overall view of copper Wafer bonding for different bonding parameters, including pressure, temperature, duration, clean techniques and anneal option, can be established.
Abstract: A reliable copper wafer bonding process condition, which provides strong bonding at low bonding temperature with a short bonding duration and does not affect the device structure, is desirable for future three-dimensional (3-D) integration applications. In this review paper, the effects of different process parameters on the quality of blanket copper wafer bonding are reviewed and summarized. An overall view of copper wafer bonding for different bonding parameters, including pressure, temperature, duration, clean techniques, and anneal option, can be established. To achieve excellent copper wafer bonding results, 400°C bonding for 30 min. followed by 30 min. nitrogen anneal or 350°C bonding for 30 min. followed by 60 min. anneal bonding is necessary. In addition, by meeting the process requirements of future integrated circuit (IC) processes, the best bonding condition for 3-D integration can be determined.
TL;DR: In this paper, a self-assembled localized microscopic electrochemical cell model was proposed to describe the self-organized Si nanowires growth and a diffusion-limited aggregation process was used to explain the formation of the silver dendritic nanostructures.
Abstract: The intergrowth mechanism of silicon nanowires and silver dendrites formed by electroless metal deposition has been investigated by scanning electron microscopy. A self-assembled localized microscopic electrochemical cell model can adequately describe the self-organized Si nanowires growth. Using these in situ prepared Si nanowire arrays as templates, a diffusion-limited aggregation process is proposed to explain the formation of the silver dendritic nanostructures.
TL;DR: In this article, the effects of alloying elements, such as lanthanum, titanium, aluminum, and chromium, on oxidation resistance, wetting properties, and tensile properties of eutectic Sn-Zn solder are described.
Abstract: Sn-Zn alloys have been considered for use as lead-free solders. Their poor wetting and oxidation resistance properties are the main obstacles that prevent them from becoming commercially viable solders. The effects of alloying elements, such as lanthanum, titanium, aluminum, and chromium, on oxidation resistance, wetting properties, and tensile properties of eutectic Sn-Zn solder are described herein. Results show the addition of alloying Ti, Al, and Cr can improve the oxidation resistance of Sn-9Zn solder. La, Ti, and Cr do not have much effect on the wetting properties, whereas Al worsens the wetting. Differential scanning calorimetry investigations reveal the solidus temperature of these solders to be ∼200°C. Addition of Cr improves ductility while maintaining tensile strength.