Interfacial Phase Formation Research Articles

Role of Ag in the formation of interfacial intermetallic phases in Sn-Zn soldering

This study explored the effect of Ag as the substrate or alloying element of solders on the interfacial reaction in Sn-Zn soldering. Results show that instead of Ag-Sn compounds, ζ-AgZn and γ-Ag5Zn8 form at the Sn-Zn/Ag interface. The addition of Ag in Sn-Zn solders leads to the precipitation of e-AgZn3 from the liquid solder on preformed interfacial intermetallics. The morphology of this additional AgZn3 is closely related to the solidification process of Ag-Zn intermetallics and the under intermetallic layer.

Temperature sintering effects on the magnetic, electrical and transport properties ofLa0.67Sr0.33MnO3/Nd0.67Sr0.33MnO3 composites

The microstructure, magnetic and electrical transport properties ofLa0.67Sr0.33MnO3/Nd0.67Sr0.33MnO3 composites are studied as a function of sintering temperature. At900 °C, the intergrain exchange couplings between the adjacent particles give rise to an increase inresistance. Raising the sintering temperature induces the formation of an interfacial phase near theLa0.67Sr0.33MnO3 and Nd0.67Sr0.33MnO3 grain boundaries as indicated in the XRD pattern and SEM image. At1100 °C, the formation of interfacial phases near the grain boundaries enhances the conductivity in thecomposite. Broad magnetoresistance across room temperature is observed in a composite sintered at1300 °C. This observation may be ascribed to the coexistence of multiphase in the composite whichleads to a decrease in resistance. The results suggest that sintering temperaturehas a prominent effect on the properties of the grain boundary, which plays animportant role in determining the electrical transport behaviour of the composites.

Contact metallization on strained-Si

Metal–semiconductor interfaces play a crucial role in electronic and optoelectronic devices. Strained-Si heterostructure devices are expected to play an important role due to its compatibility with Si processing technology. The interfacial reactions and chemical phase formation between nickel and ultrahigh vacuum chemical vapor deposited (UHVCVD) strained-Si layers on relaxed Si 0.75Ge 0.25 layers have been studied in the temperature range of 300–900 °C for forming low resistive and uniform silicide films for future generation strained-Si metal oxide semiconductor field-effect transistors (MOSFETs). The silicide films were characterized by scanning electron microscopy (SEM) and cross sectional transmission electron microscopy (XTEM). Smooth and uniform nickel monogermanosilicide films have been obtained for samples annealed around 450 °C. Ni/strained-Si Schottky barrier diodes (SBDs) have been fabricated and characterized in the temperature range of 150–300 K for the determination of barrier height ( φ b) and ideality factor ( n). SBDs having epitaxial strained-Si layer thickness of 200 and 520 Å have been modeled by incorporating an interfacial layer and associated series resistance to achieve a better agreement with the experimental data. Schottky barrier height (SBH) was found to decrease with a decrease in the measurement temperature while the ideality factor was found to increase. The interface states have been characterized using the capacitance–voltage ( C– V) characteristics of the diodes.

Interfacial phase formation in Cu–Mg alloy films on SiO2

We report the chemical changes and phase formation at the film-SiO2 interface during vacuum annealing of supersaturated Cu–10 at. % Mg alloy films sputter-deposited on SiO2. High-resolution transmission electron microscopy, energy dispersive x-ray spectroscopy and electron energy loss spectroscopy reveal that both Cu and Mg penetrate ∼20–40 nm into silica at ∼400 °C. At higher temperatures Mg reduces SiO2 leading to the crystallization of a 35-nm-thick continuous interfacial layer comprised of equiaxed grains of cubic MgO and small amounts (≲1 vol %) of monoclinic CuMgSi2O6. Manipulating oxide and silicate formation pathways could open up possibilities for the use of Cu alloys to obtain ultra-thin diffusion barriers at Cu-dielectric interfaces for future metallization structures in integrated circuits.

Investigation of the aluminum oxide/Si(1 0 0) interface formed by chemical vapor deposition

Thin films of aluminum oxide were deposited using trimethylaluminum and oxygen. The deposition rate was found to decrease with increasing temperature. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the film/substrate interface. When dry O 2 was used during deposition, the film/substrate interface was free of any silicon dioxide or aluminum silicate phase. On annealing the as-deposited films in Ar, a layer of silicon dioxide film formed at the interface. XPS results indicated that the O/Al ratio in the as-deposited films was higher than that in stoichiometric Al 2O 3. However, the ratio was found to decrease in the annealed samples suggesting that excess oxygen present in the deposited films is responsible for the formation of interfacial silicon dioxide layer. Interfacial phase formation was observed in the as-deposited samples, when small amounts of ozone along with oxygen were used as the oxygen precursor.

Microstructural analysis of multilayered titanium aluminide sheets fabricated by hot rolling and heat treatment

This study is concerned with the microstructural analysis of multilayered or bulk Ti aluminide sheets fabricated by the self-propagating high-temperature synthesis (SHS) reaction using hot rolling and heat treatment. Multilayered Ti/Al sheets were prepared by stacking thin Ti and Al sheets alternately, and a good Ti/Al interfacial bonding was achieved after rolling at 500 °C. When these sheets were held at 1000 °C, spheroidal TiAl3 phases were formed by the SHS reaction at Ti/Al interfaces and inside Al layers. Microstructural analysis on the hot-rolled, multilayered Ti/TiAl3 sheets revealed that intermetallic phases such as TiAl2, TiAl, and Ti3Al were formed at Ti/TiAl3 interfaces due to interaction between Ti and TiAl3 and that pores formed in the TiAl3 layer were significantly reduced during hot rolling. When multilayered Ti/Ti aluminide sheets were heat treated at 1000 °C, Ti3Al, TiAl, and TiAl2 were grown as Ti and TiAl3 were consumed. As the heat treatment proceeded, TiAl grew further, eventually leading to the fabrication of multilayered sheets composed of Ti3Al and TiAl. Bulk Ti aluminide sheets, having a lamellar structure of Ti3Al and TiAl, instead of multilayered sheets, were also fabricated successfully by heat treatment at 1400 °C. This fabrication method of the bulk sheets had several advantages over the method by hot forging or rolling of conventional cast Ti aluminides. From these findings, an idea to fabricate multilayered or bulk Ti aluminide sheets by hot rolling and heat treatment is suggested as an economical and continuous fabrication method, and the formation and growth mechanisms of interfacial phases are elucidated in this study.

Thermal reaction of nickel and Si0.75Ge0.25 alloy

The interfacial reactions and chemical phase formation between nickel and ultrahigh vacuum chemical vapor deposited Si0.75Ge0.25 alloy have been studied within the temperature range of 300–900 °C for forming low resistive and uniform silicide films for future application in SiGe based metal–oxide–semiconductor field effect transistor devices. The silicided films were characterized by the x-ray diffraction, Auger electron spectroscopy, scanning electron microscopy, transmission electron microscopy, and micro-Raman microscopy techniques. Smooth and uniform nickel monogermanosilicide NiSi0.75Ge0.25 films have been observed for samples annealed at around 400–500 °C. For annealing temperatures of 500 °C and above, Ge-rich Si1−zGez grains where z>0.25 were found among Ge deficient Niy(SiwGe1−w)1−y grains where w<0.25 and the Niy(Si1−wGew)1−y phase is thermally stable up to an annealing temperature of 800 °C. We found that the Ni/SiGe reaction is mainly diffusion controlled with Ge and Ni as the dominant diffusing species compared to Si during the annealing process. In addition, Ge has been found to promote agglomeration especially above 700 °C, leading to an abrupt increase in the sheet resistance.

High-transparency Ni/Au bilayer contacts to n-type GaN

A unique metallization scheme has been developed for obtaining both Schottky and low-resistance Ohmic contacts to n-GaN. It has been demonstrated that the same metallization can be used to make both Schottky and Ohmic contacts to n-GaN using a Ni/Au bilayer composite with Ni in contact to GaN. Using this metallization, contacts with a specific contact resistivity, ρs, as low as 6.9×10−6 Ω cm2 for a doping level of 5.0×1017 cm−3 was obtained after annealing the sample for 10 s at 800 °C in a rapid thermal annealer. The presence of only (111)Au and (111)Ni peaks in the x-ray diffraction (XRD) pattern of as-deposited samples indicates that both metals participate to form epitaxial or highly textured layers on the basal GaN plane. When the contact layer is annealed, Au and Ni react with GaN creating interfacial phases. Both XRD and transmission electron microscopy confirm that Ni3Ga and Ni2Ga3 intermetallic phases together with Au and Ni based face-centered-cubic solid solutions, are formed during annealing. The high optical transmission achieved (in the range of 400–700 nm) through this contact after annealing suggests that it is, indeed, very useful for electro-optic device applications. The contacts also demonstrate exceptional thermal stability. Experimental data suggest that the formation of interfacial phases with a low work function is responsible for the low contact resistance of the system. The Ni–Au layer forms a robust composite enabling the contacts to have high-temperature applications. Unlike the Ni/Au Ohmic contact, the Ni/Au Schottky contact to n-GaN has a relatively large barrier height. Improved material quality and Schottky contact technology are needed to improve upon the reverse breakdown voltage.

Formation of interfacial phase and its effects on the magnetic and transport properties of the La0.82Ca0.18MnO3/La0.18Ca0.82MnO3 composite

Formation of interfacial phase in the La0.82Ca0.18MnO3/La0.18Ca0.82MnO3 composite and associated effects have been experimentally studied. Interfacial phase appears when the composite is sintered at temperatures above 950°C, and its composition experiences a considerable change during the sintering according to the x-ray diffraction analysis. Compared with magnetization, the transport property is relatively insensitive to the phase constitution of the composite. The appearance of the third phase and the resulted extra grain boundaries produce only a weak enhancement of magnetoresistance at the percolation point (~26% in volume fraction) of the new phase.

Formation of interfacial phases in the epitaxial growth of Sb on Si(111)-7 ¥ 7 reconstructed surface

Abstract Understanding the evolution of the Sb/Si(111) interface is of great interest in the formation of devices of nanodimensions. We have undertaken in situ (∼10-11 torr) studies of Sb adsorption (at room temperature) and its desorption on the 7 X 7 reconstructed Si(111) surface, by complementary techniques such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and electron energy loss spectroscopy (EELS). For room-temperature (RT) Sb adsorption, the overlayer grows in the Frank van der Merwe mode, forming an interface state of δ(7 X 7) in the submonolayer Sb coverage regime. Adsorption of 1.0 monolayer (ML) Sb at RT shows an abrupt shift of 0.8 eV in the peak position of the Sb 3d5/2 transition owing to band-bending caused by a metallic (7 X 7) to a semiconducting (1 X 1) surface phase transformation. Changes observed in full width at half-maximum (fwhm) and Sb 3d3/2 and 3d5/2 branching ratio are discussed. Thermal annealing experiments provide evidence for agglomeration of Sb islands, before the multilayer and monolayer desorption. During this desorption process, we have observed two novel surface phases of (5 X 5) at 0.4 ML and (5√3 X 5√3­R30°) at 0.2 ML, stable at higher temperatures.

Structural and electrical characterisation of titanium and nickel silicide contacts on silicon carbide

The interfacial reaction and phase formation as a function of the annealing temperature (600–1000°C) and time were investigated on both titanium and nickel thin films evaporated on n-type 6H-SiC (0001) substrate. The study was carried out employing a combination of Rutherford backscattering spectrometry, X-ray diffraction, transmission electron microscopy and sheet resistance measurements. A correlation has been found between the annealing process and the electrical measurements on transmission line method (TLM) structures and Schottky diodes. In the case of titanium, the formation of different phases in the analysed temperature ranges significantly changes the electrical properties. In fact, while a double layer of TiC and Ti 5Si 3 was formed at 900 and 950°C, the ternary phase Ti 3SiC 2 was observed only at 1000°C. With this high temperature process the specific contact resistance decreases from 10 −4 to 6.7×10 −5 Ωcm 2. In the case of nickel silicide the only phase that has been observed between 600 and 950°C was the Ni 2Si. The carbon of the consumed silicon carbide layer has been dissolved in the silicide film, during the reaction, forming carbon precipitates. The specific contact resistance reaches the lowest value (3.6×10 −5 Ωcm 2) after the highest temperature anneal. The Ni 2Si/SiC Schottky diodes show almost ideal characteristics ( n=1.07) and a barrier height of ∼1.3 eV. From the electrical characterisation, a non-uniform Schottky barrier height seems to be formed.

Microstructural evidence on electrical properties of Ta/Ti/Al and Ti/Ta/Al ohmic contacts to n-AlGaN/GaN

Electrical properties and microstructures of Ta/Ti/Al and Ti/Ta/Al contacts to n-AlGaN/GaN heterostructure field-effect transistor structures were investigated using the transmission line method and transmission electron microscopy. The specific resistivity (5.3×10−7 Ω cm2) of Ta/Ti/Al contacts is much lower than that (5.1×10−4 Ω cm2) of Ti/Ta/Al contacts, for the same heterostructure and similar metallization. The contact resistivity was found to depend on the thickness of the AlGaN layer, interfacial phase, and interface roughness. The formation of interfacial phases by solid-state reactions with the metal layer during annealing appears to be essential for ohmic behavior on n-III-nitrides suggesting a tunneling contact mechanism.

Structural characterisation of titanium silicon carbide reaction

Interfacial reaction and phase formation as a function of annealing temperature (900, 950 and 1000°C) and times were investigated on titanium thin films evaporated on n-type 6H-SiC (0001) substrate. The study was carried out employing a combination of Rutherford backscattering spectrometry, X-ray diffraction and sheet resistance measurements. At 900°C a double layer of TiC and Ti 5Si 3 phases was formed and it was found to be stable up to 7 h of annealing. The ternary phase Ti 3SiC 2 appeared at 950°C with a small fraction, after an annealing for 45 min. At 1000°C there is a strong presence of this phase in conjunction with the Ti 5Si 3 phase.

Structural and Electrical Characterisation of Nickel Silicides Contacts on Silicon Carbide

ABSTRACTThe interfacial reaction and phase formation as a function of the annealing temperature (600÷1000°C) and times were investigated on nickel thin films evaporated on n type 6H-SiC (0001) substrate. The study was carried out employing a combination of Rutherford Backscattering Spectrometry, X-Ray Diffraction, Transmission Electron Microscopy and sheet resistance measurements. Also several TLM structures and Schottky diodes were fabricated with the same processes and a correlation has been found between the annealing process and the electrical measurements. The only nickel silicide phase that has been observed between 600 and 950 °C was the Ni2Si. The carbon of the consumed silicon carbide layer has been dissolved in the silicide film, during the reaction, forming carbon precipitates. The Ni2Si/SiC Schottky diodes show an almost ideal characteristics (n=1.07) and a barrier height of about 1.3 eV. From the electrical characterisation a non uniform Schottky barrier height seems to be formed.

Electrical conductivity and thermal expansion coefficient of internal phases occurring in multilayer ceramic structures

The electrical conductivity and thermal expansion coefficients of interfacial phases, PdPbO 2, PdBi 2O 4, Pd(Pb), and Pd 3Pb, have been studied. The interfacial reactions form low conductivity products, which result in discontinuity of the internal electrode. Besides, due to the volume change associated with the interfacial phase formation and wide variation in thermal expansions, physical defects may occur at the electrode–ceramic interfaces during processing.

Interfacial oxide and carbide phases in the deposition of diamond films on beryllium metal

We have investigated beryllium metal as a substrate for the microwave plasma chemical vapor deposition of diamond films. The dependence of oxide and carbide interfacial phase formation with temperature and their influence on diamond nucleation and growth behavior were studied by thin film X-ray diffraction. Although a native oxide (BeO with the hexagonal wurtzite structure) remains for the range of substrate temperatures studied (700–800°C), the formation of a carbide (Be 2 C with the cubic antifluorite structure) is found only above a critical substrate temperature of approximately 750°C. Without the formation of Be 2 C, the diamond growth rate is low and a significant amorphous carbon component is observed. Just above the critical temperature, films exhibited high growth rates with high phase-purity diamond. Thick films (>30 μm) grown above the critical temperature were observed to fracture completely within the Be 2 C layer, suggesting this to be the weak structural link.

In Situ Observation of the Interfacial Phase Formation at Si Melt/Silica Glass Interface

In situ observation has been carried out to investigate the interfacial phase of “brownish rings” forming at the Si melt/silica glass interface during Czochralski (CZ) silicon crystal growth. It is found that some small brownish dots appeared at the interface immediately after Si melting, and gradually grew laterally. The growth rate of the interfacial phase increases with increasing temperature, and it only nominally depends on the silica dissolution rate when the dissolution rate is lower than a critical value. The formation mechanism of the brownish rings has been investigated and two formation mechanisms are proposed. (1) Cristobalite forms in the central region of the interfacial phase. (2) The periphery of the interfacial phase is thicker than the central region, and only part of the periphery remains after passing through the triple junction where the dissolution rate is very high.

On the role of brittle interfacial phases on the mechanical properties of carbon fibre reinforced Al-based matrix composites

Abstract High-resolution microfractography performed in combination with transmission electron microscopy shows that the mechanical behaviour of carbon fibre reinforced Al-based matrix composites is related to the presence of brittle interfacial phases. Attaining the required specifications in the unreinforced part of cast aluminium aeronautical components locally reinforced with a carbon preform necessitates the use of a highly alloyed Al-based matrix. Moreover, the precipitate hardening heat treatment appropriate to the unreinforced alloy must be applied to the whole component. The two composites M40J/A357 and M40J/Cpyr/A357 (pyrolytic carbon coated fibres), either as-cast or after the usual T6 heat treatment of the A357 alloy (16 h solution treatment at 540°C followed by water quenching and 6 h ageing at 160°C) have been investigated. The formation of Si and Al4C3 interfacial brittle phases resulting from both the dendritic solidification and the solution treatment leads to the premature failure of the composite.

Simultaneous Determination of Chemical Diffusion and Surface Exchange Coefficients of Oxygen by the Potential Step Technique

Oxygen diffusion is treated in a dense electronically conducting cobaltate pellet blocked ionically on one surface, electronically on the other, and sealed on its cylindrical periphery. A procedure is developed for extracting the chemical diffusion and surface exchange coefficients for oxygen by use of the asymptotic equations derived for the current response to a potential step at short and long times. It is shown that, while the formation of interfacial phases by reaction between the sample and the electrolyte may affect the surface exchange coefficient, the chemical diffusion coefficient data determined by the present approach are independent of such interfacial phenomena. The consistency of data obtained from several specimens with varying thickness and manner of interfacing with the electrolyte validates the diffusion model and the method used for data analysis. An oxygen permeation cell is also developed in this work as a modification of the diffusion cell. The new cell allows monitoring of the permeation rate by electrochemical means. The steady‐state permeation data obtained by the permeation cell are consistent with the chemical‐diffusion and surface‐exchange coefficients measured by the blocked diffusion cell as long as the assumptions of the related theoretical models are satisfied. This is a further validation of the diffusion model and the related methodology developed here for obtaining the necessary data for characterizing oxygen exchange and transport in such materials. © 1999 The Electrochemical Society. All rights reserved. ©2001 American Geophysical Union

W and WSix Ohmic contacts on p- and n-type GaN

W and WSi Ohmic contacts on both p- and n-type GaN have been annealed at temperatures from 300 to 1000 °C. There is minimal reaction (⩽100 Å broadening of the metal/GaN interface) even at 1000 °C. Specific contact resistances in the 10−5 Ω cm2 range are obtained for WSix on Si-implanted GaN with a peak doping concentration of ∼5×1020 cm−3, after annealing at 950 °C. On p-GaN, leaky Schottky diode behavior is observed for W, WSix and Ni/Au contacts at room temperature, but true Ohmic characteristics are obtained at 250–300 °C, where the specific contact resistances are, typically, in the 10−2 Ω cm2 range. The best contacts for W and WSix are obtained after 700 °C annealing for periods of 30–120 s. The formation of β-W2N interfacial phases appear to be important in determining the contact quality.