Percolative approach for failure time prediction of thin film interconnects under high current stress

The present study deals with the use of a rapid and non-destructive technique based on percolation theory to predict failure times during the reliability analysis of thin film interconnects under high current stress. Al–Cu test structures were used for this purpose. Small populations of these structures of similar geometry were subjected to extremely high current density conditions (30.6 and 46.6 MA/cm²) and their corresponding failure times were noted. The critical exponent (μ_B) for the Al–Cu structures stressed at both the current densities was calculated to be 0.16. The value of the μ_B showed that the structures undergo biased percolation and have similar failure mechanisms (due to Joule heating) at both current densities. The calculated value of μ_B was used to predict the failure times of the fuses under each of the current stresses. The discrepancy between the experimental failure time and the predicted failure time was significantly low (<12%) in both cases thus expressing the strength of this prediction technique.     

Nonstoichiometry and defects in III–V compounds

The deviation from stoichiometry or nonstoichiometry characterises a homogeneity range of chemical compounds. It is determined as the difference in the nonmetal- to metal atoms ratio between a real A_nB_m+δ (δ><0) and stoichiometric A_nB_m composition. Nonstoichiometry creates defects which have an effect on all properties of a crystal. The classification of defect is given. The enthalpies and entropies of quasi-chemical reactions describing the defects composition of III–V compounds are presented and give the possibility to estimate the concentration of the major defects for the solubility limits of III–V compounds.     

Structural, electrical and optical properties of GZO/HfO2/GZO transparent MIM capacitors

Metal–insulator–metal (MIM) transparent capacitors were prepared by pulsed laser deposition (PLD) on glass substrates. The effect of the thickness of the dielectric layer and oxygen pressure on structural, electrical, and optical properties of these capacitors was investigated. Experimental results show that film thickness and oxygen pressure have no effect on the structural properties. It is also found that the optical properties of the HfO₂ thin films depend strongly on both the film thickness and oxygen pressure. The electrical properties of transparent capacitors were investigated at various thickness of the dielectric layer. The capacitor shows an overall high performance, such as a high dielectric constant of 28 and a low leakage current of 2.03×10⁻⁶ A/cm² at ±5 V. Transmittance above 70% was observed in visible region.     

Nanoscale structural characteristics and electron field emission properties of transition metal–fullerene compound TiC60 films

Transition-metal compound TiC₆₀ thin films were grown by co-deposition from two separated sources of fullerene C₆₀ powder and titanium. Study of structural properties of the films, by Raman spectroscopy, atomic force microscopy, and scanning tunneling spectroscopy reveals that the films have a deformed C₆₀ structure with certain amount of sp³ bonds and a rough surface with a large number of nanoclusters. z–V tunnelling spectroscopic measurements suggest that several charge transport mechanisms are involved in as the tip penetrates into the thin film. Conventional field electron emission (FEE) measurements show a high emission current density of 10 mA/cm² and a low turn-on field less than 8 V/μm, with the field enhancement factors being 659 and 1947 for low-field region and high-field region, respectively. By exploiting STM tunneling spectroscopy, local FEE on nanometer scale has also been characterized in comparison with the conventional FEE. The respective field enhancement factors are estimated to be 99–355 for a gap varying from 36 to 6 nm. The enhanced FEE of TiC₆₀ thin films can be ascribed to structural variation of C₆₀ in the films and the electrical conducting paths formed by titanium nanocrystallites embedded in C₆₀ matrix.     

Study of light-induced annealing effects in a-Si:H thin films

Light-induced metastability was examined in hydrogenated amorphous silicon thin films using a 500 W xenon lamp and a 10 mW HeCd laser. Positron beam annihilation spectroscopy (PAS) and fourier transform infrared spectroscopy (FTIR) were examined to investigate the effects of light on the structural properties of the films. The experimental results exhibited significant decrease in the S-parameter of the PAS, indicating marked reduction in the defect density of the films. The FTIR spectroscopy showed significant reduction in the transmission coefficient of IR radiation at frequencies corresponding to Si–H and Si–H₃ phonon modes, indicating that the observed annealing effects were due to light-induced formation of Si–H and Si–H₃ bonds. A second thermal annealing process conducted after the light exposure experiment resulted in a further substantial decrease in defect density for the sample exposed to HeCd laser. The experimental results are explained by a competing, light induced, dangling bond creation/annealing process, in which the incoming photons caused the annealing of dangling bonds, particularly those at around the voids. However, in the bulk region, the photons caused both the breaking of weak Si–Si bonds as well as the annealing of dangling bonds.     

Effects of fourth alloying additive on microstructures and tensile properties of Sn–Ag–Cu alloy and joints with Cu

The effects of the fourth elements, i.e., Fe, Ni, Co, Mn and Ti, on microstructural features, undercooling characteristics, and monotonic tensile properties of Sn–3 wt.%Ag–0.5 wt.%Cu lead-free solder were investigated. All quaternary alloys basically form third intermetallic compounds in addition to fine Ag₃Sn and Cu₆Sn₅ and exhibit improved solder structure. The precipitates of Sn–3Ag–0.5Cu (–0.1 wt.%X; X=Ni, Ti and Mn) alloy are very fine comparing with the other alloys. The effective elements for suppressing undercooling in solidification are Ti, Mn, Co and Ni. All quaternary bulk alloys exhibit similar or slightly larger tensile strengths; especially Mn and Ni can improve elongation without degrading strength. The interfacial phases of Sn–3Ag–0.5Cu (–0.1 wt.%X; X=Fe, Mn and Ti)/Cu joints are typical Cu₆Sn₅ scallops. Sn–3Ag–0.5Cu (–0.1 wt.%X; X=Ni and Co)/Cu joints form very fine Sn–Cu–Ni and Sn–Cu–Co scallops at interface. The Cu/Sn–3Ag–0.5Cu–0.1Ni/Cu joint exhibits improved tensile strength prior to thermal aging at 125 and 150 °C. The fracture surface of Cu/Sn–3Ag–0.5Cu/Cu joint exhibits mixture of ductile and brittle fractures, while Cu/Sn–3Ag–0.5Cu (–0.1X; X=Ni and Co)/Cu joints exhibit only brittle fracture at interface. The Sn–3Ag–0.5Cu–0.1Ni alloy is more reliable solder alloy with improved properties for all tests in the present work.     

Effect of displacement rate on ball shear properties for Sn–37Pb and Sn–3.5Ag BGA solder joints during isothermal aging

The interfacial reactions and ball shear properties of ball grid array (BGA) solder joints aged at 170 °C for up to 21 days were investigated with different displacement rates. Two different kinds of solders, Sn–37Pb and Sn–3.5Ag (all wt.%), and an electroplated Ni/Au BGA substrate were employed in this work. A continuous Ni₃Sn₄ intermetallic compound (IMC) layer was formed at the interfaces between both the Sn–37Pb and Sn–3.5Ag solders and the substrate during reflow. After aging, two different reaction layers, consisting of (Au_xNi_1−x)Sn₄ IMC and Pb-rich phase, were additionally observed between the Sn–37Pb solder and the Ni₃Sn₄ IMC layer. The thicknesses of these interfacial reaction layers increased with increasing aging time. After reflow, all the fractures occurred inside the bulk solder. The fracture location of the Sn–37Pb solder joints was shifted toward the solder/Ni interface with increasing aging time and displacement rate, whereas the fracture of the Sn–3.5Ag solder joints mainly occurred inside the bulk solder, irrespective of the aging time and displacement rate. Consequently, the shear properties of the Sn–37Pb solder joints significantly decreased with increasing aging time, whereas those of the Sn–3.5Ag solder joints slightly decreased. The tendency toward brittle fracture of the Sn–37Pb solder joints was intensified with increasing displacement rate. The shear properties of the ductile solder joints increased with increasing displacement rate, while the displacement until fracture, deformation energy and displacement rate sensitivity of the brittle solder joints significantly decreased with increasing displacement rate.     

Porous-Silicon Formation in HF–HNO3–H2O Etchants

The chemical formation of porous silicon in HF–HNO₃–H₂O etchants is studied experimentally. A technique is devised to determine the ranges in which the proportions of the acids should be varied in order to change from etch polishing to pore formation. The structure and properties of porous layers are examined in relation to the proportion of etchant ingredients.     

High-Permittivity-Insulator EEPROM Cell Using Al2O3 or ZrO2

A theoretical investigation is carried out into memory cells based on a polysilicon–oxide–nitride–oxide–silicon structure in which a high-permittivity dielectric is used instead of SiO₂ as the gate insulator. The dielectric is taken to be Al₂O₃ or ZrO₂. Write/erase (W/E) cycles are simulated numerically. It is shown for the first time that changing to a high-permittivity insulator reduces the unwanted carrier injection from the gate region and allows one to employ lower and/or shorter W/E pulses; specifically, the W/E time can be decreased from 1 ms to 10 s. It is concluded that high-permittivity insulators might be useful in carrier-trapping EEPROMs and RAMs.     

Thermal Ta2O5––alternative to SiO2 for storage capacitor application

Tantalum pentoxide thin layers (10–100 nm) obtained by thermal oxidation of rf sputtered Ta films on Si have been investigated with respect of their dielectric, structural and electric properties. It is established that stoichiometric Ta₂O₅ detected at the surface of the layers is reduced to tantalum suboxides in their depth. The oxide parameters are discussed in terms of a presence of an unavoidable ultrathin SiO₂ between Si and Ta₂O₅ and bond defects in both the oxide and the interface transition region. Conditions which guarantee obtaining high quality tantalum oxide with a dielectric constant of 32–35 and a leakage current less than 10⁻⁷–10⁻⁸ A/cm² at 1.5 V (SiO₂ equivalent thickness of 2.5–3 nm) are established. These specifications make the layers obtained suitable alternative to SiO₂ for high density DRAMs application.     

Impact of the electron, proton and helium irradiation on the forward I–V characteristics of high-power P–i–N diode

2.5 kV/100 A high-power P–i–N diode was electron, proton and helium irradiated in a wide range of irradiation doses with irradiation energies in the MeV range. The resulting forward I–V curves were registered in the temperature range 30–125 °C to investigate the magnitude of the crossing point current of the I–V curves––I_XING. I_XING was found to decrease with increasing irradiation dose and to disappear at high doses for all three irradiation treatments with exception of ion irradiated diodes with defect peaks placed deeply into the anode region. Using a simple model based on the thermal and injection dependence of the carrier lifetime, the explanation of this effect is presented with the support of the non-isothermal 2-D device simulation of helium irradiated devices.     

Electrical characterization of the hafnium oxide prepared by direct sputtering of Hf in oxygen with rapid thermal annealing

Electrical characterization of the hafnium oxide (HfO₂) gate dielectric films prepared by Hf sputtering in oxygen was conducted. By measuring the current–voltage (I–V) characteristics at temperature ranging from 300 to 500 K, several abnormalities in the I–V characteristics are recorded. For temperatures below 400 K, the current–voltage characteristics in high field region can be plotted with the Fowler–Nordheim law but a stronger temperature dependence was observed. Large flatband voltage shifts in the Al/HfO₂/Si capacitor were observed. The capacitance–voltage characteristics and flatband shifts are found to depend strongly on the post-deposition annealing temperature and duration. To study the reliability against high electric field, constant voltage stressing on the samples was conducted. We found that the trap energy levels are shallow and the oxide traps can be readily filled and detrapped at a low bias voltage.     

Noise and transport characterisation of tantalum capacitors

A low frequency noise and charge carrier transport mechanisms were investigated on tantalum capacitors made by various producers. The model of Ta–Ta₂O₅–MnO₂ MIS structure was used to give physical interpretation of I–V characteristics in normal and reverse modes. The noise in time and frequency domain was examined and noise sources were identified. We evaluated correlation between leakage current and noise spectral density and discussed corresponding quality and reliability indicators.     

Evaluation of MOS structures processed on 4H–SiC layers grown by PVT epitaxy

MOS capacitors have been fabricated on 4H–SiC epilayers grown by physical vapor transport (PVT) epitaxy. The properties were compared with those on similar structures based on chemical vapor deposition (CVD) layers. Capacitance–voltage (C–V) and conductance measurements (G–V) were performed in the frequency range of 1 kHz to 1 MHz and also at temperatures up to 475 K. Detailed investigations of the PVT structures indicate a stable behaviour of the interface traps from room temperature up to 475 K. The amount of positive oxide charge Q_O is 6.83 × 10⁹ cm⁻² at room temperature and decreases with temperature increase. This suggests that the processed devices are temperature stable. The density of interface states D_it obtained by Nicollian–Brews conductance method is lower in the structure based on the PVT grown sample.     

Effect of microwave radiation on the properties of Ta2O5–Si microstructures

The paper presents results of the effect of microwave irradiation at room temperature on the properties of thin layers of tantalum pentoxide deposited on Si by rf sputtering. Electrical characterization is performed in conjunction with Auger electron spectroscopy and atomic force microscopy. Among exposure times used (1; 5; 10 s), treatment of about 5 s shows the best promise as an annealing step––an improvement of number of parameters of the system Ta₂O₅–Si is established (dielectric constant and surface morphology; stoichiometry and microstructure of both the bulk oxide and the interfacial transition region; electrical characteristics in terms of oxide charge density, leakage current and breakdown fields). At the same time the microwave irradiation is not accompanied by crystalization effects in Ta₂O₅ and/or additional oxidation of Si substrate. It is concluded that the short-time microwave irradiation can be used as an annealing process for Ta₂O₅–Si microstructures and it has a potential to replace the high-temperature annealing processes for high-k insulators.     

Effect of thermal aging on the interfacial structure of SnAgCu solder joints on Cu

Interfacial structure plays a great role in solder joint reliability. In solder joints on Cu, not only is Kirkendall voiding at the solder/Cu interface a concern, but also the growth of interfacial Cu–Sn intermetallic compounds (IMCs). In this work, evolution of microstructure in the interfacial region was studied after thermal aging at 100–150 °C for up to 1000 h. Special effort was made during sample preparation to reveal details of the interfacial structure. Thickness of the interfacial phases was digitally measured and the activation energy was deduced for the growth of Cu₃Sn. Kirkendall voids formed at the Cu/Cu₃Sn interface as well as within the Cu₃Sn layer. The thickness of Cu₃Sn significantly increased with aging time, but that of Cu₆Sn₅ changed a little. The interfacial Cu₃Sn layer was found growing at the expense of Cu₆Sn₅. Evolution of the interfacial structure during thermal aging is discussed.     

Structural and electronic properties of zirconia phases: A FP-LAPW investigations

The structural and electronic properties of ZrO₂ polymorphs were investigated using density functional theory (DFT). The Kohn–Sham equations were solved by applying the full-potential linearized augmented plane wave (FP-LAPW) method. We used the generalized gradient approximation (GGA) in the Perdew–Wang formalism to the exchange and correlation energy functional. The ground state properties such as lattice parameter, transition pressures, bulk modulus and its pressure derivative as well as the structural phase stability were calculated. The results were compared with previous calculations and experimental data when available. The FP-LAPW method correctly orders the zero temperature energies of all zirconia polymorphs. We have also studied the effect of distortion from the cubic to the tetragonal structure on the basis of charge density calculations. On the other hand, band structure and density of states (DOS), which allow us to discuss the features of orbital mixing, are also given. Our results suggest that the cotunnite structure should be better than the other zirconia phases as gate dielectric material.     

Effects of oxygen partial pressure on structural and electrical characteristics of HfAlO high-k gate dielectric grown on strained SiGe by pulsed-laser deposition

Dependence of oxygen partial pressures on structural and electrical characteristics of HfAlO (Hf:Al=1:1) high-k gate dielectric ultra-thin films grown on the compressively strained Si₈₃Ge₁₇ by pulsed-laser deposition were investigated. The microstructure and the interfacial structure of the HfAlO thin films grown under different oxygen partial pressures were studied by transmission electron microscopy, and the their electrical properties were characterized by capacitance–voltage (C−V) and conductance–voltage measurements. Dependence of interfacial layer thickness and C–V characteristics of the HfAlO films on the growth of oxygen pressure was revealed. With an optimized oxygen partial pressure, an HfAlO film with an effective dielectric constant of 16 and a low interface state density of 2.1×10¹⁰ cm⁻² eV⁻¹ was obtained.     

Lateral base design rules for optimized low-frequency noise of differentially grown SiGe heterojunction bipolar transistors

The low-frequency noise dependence on lateral design parameters was investigated for SiGe heterojunction bipolar transistors fabricated by differential epitaxy. The low-frequency noise was found to vary substantially as a function of the extrinsic base design. The dominant noise sources were located either at the interface between the polycrystalline and epitaxial Si/SiGe base, in the epitaxial Si/SiGe base link region, in the base–emitter depletion region, or at the thin SiO₂ interface layer between the polysilicon and monosilicon emitter. Boron was found to passivate interfacial traps, acting as low-frequency noise sources. Generation–recombination noise with a strong dependence on the lateral electrical field was observed for some of the designs.     

Electromigration performance of Al–Si–Cu filled vias with titanium glue layer

Electromigration performance of vias filled with high temperature (480°C) sputtered Al alloys on Ti glue layers was investigated in comparison with W-stud vias. Electromigration lifetime and failure mode are quite different according to via structures and kinds of Al alloys used. Electromigration lifetime of W-stud via chain and Al–Cu filled via chain depends on the via to via distances, while that of Al–Si–Cu filled via chain does not depend on the via to via distances. Failure mode observations revealed that voids were formed only at a few locations in the test structure in Al–Si–Cu filled via chain while voids were formed at every via in W-stud via chains and Al–Cu filled via chains. It is supposed that Al moves through the Al–Si–Cu filled vias during electromigration test in spite of the existence of the Ti glue layer at the via bottom. The Al transportation, however, was prohibited at W-stud vias and Al–Cu filled vias. Glue Ti deposited at via bottom was converted to Al–Ti–Si alloy in Al–Si–Cu filled vias, while Al₃Ti alloy was formed at Al–Cu filled via bottom. It is speculated that Al transportation occurs through via bottom Al–Ti–Si alloy layer during electromigration test in the case of Al–Si–Cu filled vias.