Reactive Phase Formation Research Articles

Phase stability, phase transformation, and reactive phase formation in electronic materials

The development of electronic products has dramatically changed the life of human beings in the last decade, and its amazing infl uence continues. Phase stability, phase transformation, and reactive phase formation in electronic materials are crucial to the properties and reliability of electronic products. The introduction of new materials brings new properties and new opportunities, and new challenges come, as well. The copper metallization in integrated circuits and lead-free solders in packaging are among the most important new electronic materials. Three articles have been invited to address these important issues. The fi rst article is “The Physical Phase Stability, Phase Transformation, and Reactive Phase Formation in Electronic Materials

Sulphurisation of gallium-containing thin-film precursors analysed in-situ

It has been demonstrated that rapid thermal sulphurisation of sputtered Cu/In precursor layers is suitable for industrial production of thin-film photovoltaic modules. The process is relatively straightforward and the underlying fundamental aspects, such as phase formation sequence and reaction rates, have been studied intensively. Using lab-scale preparation technology, incorporation of gallium is known to improve transport properties of the absorber and to enable the fabrication of wide-gap cells. In this work we have used energy dispersive in-situ X-ray diffraction to study the sulphurisation of sputtered Cu:Ga/In precursors. It is the basis for the future development of an industrially feasible production of Cu(In,Ga)S 2 films. Precursor stacking sequence and sulphur partial pressure in relation to precursor temperature have been varied. In many cases, in particular when establishing sulphur partial pressure already at low precursor temperature, we observe a severe reduction of reaction rates after going from pure Cu to Cu:Ga in the precursor. In consequence, single phase films cannot be prepared within the feasible ranges of time and temperature. Adhesion failure and at least intermediate formation of CuIn 5S 8 are other problems frequently encountered. In spite of these problems, promising pathways to single phase Cu(In,Ga)S 2 films prepared from sputtered Cu:Ga/In precursors have now been identified.

Reactive Diffusion in Cu-Nb-Sn Internal Tin Superconducting Wires for the ITER Project

The reactive diffusion and phase formation sequences in two types of ‘internal tin’ superconducting wires designed for the ITER project, which investigates the production of electricity by means of nuclear fusion, have been studied during heat treatments both in situ, using electrical resistometry [1] and ex situ, using optical and scanning electron microscopy, energy dispersive X-ray spectrometry (EDS) and X-ray Micro-Tomography (XMT). XMT reveals long pores in the longitudinal direction which may result in tin deficiency thereat and hence local off-stoichiometric Nb3Sn. Microscopy suggests there are incomplete conversion of elemental tin to copper-tin intermetallics before ramping above the tin melting temperature, nonuniform distribution of tin before formation of Nb3Sn, and filament movement and bridging, stacking cracks and unreacted niobium at the end of the heat treatment. FEGSEM shows a fine microstructure which nevertheless could still be improved.

Studies of crystal phase formations in high-strength lithium disilicate glass–ceramics

The objective of the study was to analyze the nucleation, primary phase formation and solid state reaction to form lithium disilicate glass–ceramics derived from the SiO 2–Li 2O–Al 2O 3–K 2O–ZrO 2–P 2O 5 system. The concentration of P 2O 5 was increased from zero up to 3.2 wt%. Thermal analysis, scanning electron microscopy and X-ray diffraction were used to characterize the microstructure formation, the nucleation process and the solid state reaction of the crystal phase precipitation in the glass–ceramics. Additives of P 2O 5 allowed the control of bulk crystallization. Nucleation was catalyzed by nano-scaled Li 3PO 4 phases, visualized by HR-SEM. Li 3PO 4 reacts most probably as the heterogeneous catalyst, acting by epitaxy, of both Li 2SiO 3 and Li 2Si 2O 5 crystals. Based on the discussion of the main results, the authors deduced a four-step reaction mechanism. This mechanism demonstrated that after nucleation of lithium metasilicate and lithium disilicate, the latter phase grows as agglomerated nanocrystals, but remained in a relative small amount. By contrast, lithium metasilicate grows rapidly and decomposes at 780–820 °C with the result of a drastic increase of lithium disilicate phase. This was a result of a solid state reaction with the SiO 2-rich glassy phase. In a parallel reaction, cristobalite was formed as a preliminary phase. The final product of a glass–ceramic with 3.2 wt% P 2O 5 shows a highly crystalline interlocking microstructure demonstrating a high-strength of 726 ± 63 MPa and translucency.

Phase Diagrams of Pb-Free Solders and their Related Materials Systems

Replacing Pb-Sn with Pb-free solders is one of the most important issues in the electronic industry. Melting, dissolution, solidification and interfacial reactions are encountered in the soldering processes. Phase diagrams contain equilibrium phase information and are important for the understanding and prediction of phase transformation and reactive phase formation at the solder joints. This study reviews the available phase diagrams of the promising Pb-free solders, and their related materials systems. The solders are Sn-Ag, Sn-Cu, Sn-Ag-Cu, Sn-Zn, Sn-Bi, Sn-In and Sn-Sb. The materials systems are the solders with the Ag, Au, Cu, Ni substrates, such as Sn-Ag-Au, Sn-Ag-Ni, Sn-Cu-Au, and Sn-Cu-Ni ternary systems. For the Pb-free solders and their related ternary and quaternary systems, preliminary phase equilibria information is available; however, complete and reliable phase diagrams over the entire compositional and temperature ranges of soldering interests are lacking.

The evolution of structure induced by intensive milling in the system 2Bi2O3 · 3TiO2

Concurrent mechanochemical treatment of either a stoichiometric 2Bi2O3·3TiO2 powder mixture or (pulverized) Bi4Ti3O12 compound was performed in a planetary ball mill. Relevant structural parameters: crystallite size, amount of amorphous phase and the transformed fraction (as a result of chemical reaction between Bi2O3 and TiO2) of the powders milled for various milling times and intensities were derived from X-ray powder diffraction data. The obtained structural parameters were used to follow the kinetics of the reduction of crystallite size, amorphous phase formation and chemical reaction. In the powder mixture, during the early stage of mechanochemical treatment the Bi2(CO3)O2 phase was found as an intermediate product which transformed into the highly amorphized Bi4Ti3O12 phase as the milling progressed. On the other hand, mechanochemical treatment of Bi4Ti3O12 powder induced a gradual deformation of the crystal lattice and destruction of the perovskite-type structure. However, in both cases, after a certain milling time, a very disordered, amorphous/nanocrystalline structure was obtained. It was demonstrated that in the steady state, the amorphous/nanocrystalline phase ratio depends on the milling conditions. Higher milling intensities induce ‘nanocrystallization’ of the amorphous phase, i.e., precipitation of crystallites with an average size below 20nm. A kinetic model involving a nanocrystalline↔amorphous reaction, in which the forward and reverse reaction were first-order was postulated and used to analyze the formation of an amorphous phase.

The Effects of Ca and Mn Excess Co-Doping in CMR Manganites Solid Solution Structures

A comprehensive study of CMR manganite related phases, particularly those that develop under the effects of Ca and Mn-excess co-doping is undertaken. The relationships between phase composition, processing and observed crystalline structure are investigated for co-precipitated powders of composition La1-xCaxMn1+zO3+δ, thermally treated at 800 and 1000 °C in air for Ca contents of x=0.10 and x=0.15 and Mn content from z=0 up to z=0.88. A relevant structural transition from orthorhombic to rhombohedric symmetry is observed as a function of Mn excess near 25% A site vacancies. This solid solution is in equilibrium with the conjugated phases, either LaMn7O12 at 800°C, or Mn3O4 at 1000°C, respectively. Results suggest that at the polymorphic phase transition the solid solution is stable enough to slow down Mn diffusion into the perovskite cell. Ca-doping introduces stability in the perovskite structure and broadens the domain for phase formation reaction. The solid solution limit of the Ca-manganite solid solution is set above 50% Mn excess.

Low‐Temperature, Single Step, Reactive Sintering of Lead Magnesium Niobate Using Mg(OH)2‐Coated Nb2O5 Powders

A low‐temperature, single step, reactive sintering method for Pb(Mg1/3Nb2/3)O3 (PMN) and PMN–PbTiO3 (PMN–PT) processing was developed based on the coating of Mg(OH)2 on Nb2O5. This method simplified the processing of PMN and PMN–PT to a single step of heat‐treatment and decreased the sintering temperature to 1000°C. It was found that the pyrochlore phase formation reaction at 500°C reduced the particle size to 130 nm. The overlap of the pyrochlor‐perovskite phase transformation between 700° and 900°C and the densification process between 800° and 1000°C improved the sintering process. These two factors were the major reasons of the low temperature sintering.

Self-aligned silicides for Ohmic contacts in complementary metal–oxide–semiconductor technology: TiSi2, CoSi2, and NiSi

Metal silicides continue to play an indispensable role during the remarkable development of microelectronics. Along with several other technological innovations, the implementation of the self-aligned silicide technology paved the way for a rapid and successful miniaturization of device dimensions for metal–oxide–semiconductor field-effect transistors (MOSFETs) in pace with the Moore’s law. The use of silicides has also evolved from creating reliable contacts for diodes, to generating high-conductivity current paths for local wiring, and lately to forming low-resistivity electrical contacts for MOSFETs. With respect to the choice of silicides for complementary metal–oxide–semiconductor (CMOS) technology, a convergence has become clear with the self-alignment technology using only a limited number of silicides, namely TiSi2, CoSi2, and NiSi. The present work discusses the advantages and limitations of TiSi2, CoSi2, and NiSi using the development trend of CMOS technology as a measure. Specifically, the reactive diffusion and phase formation of these silicides in the three terminals of a MOSFET, i.e., gate, source, and drain, are analyzed. This work ends with a brief discussion about future trends of metal silicides in micro/nanoelectronics with reference to potential material aspects and device structures outlined in the International Technology Roadmap for Semiconductors.

In situ HRTEM observations of the formation of reaction phases from liquid Ag–Cu–Tialloy and SiC, Si and C substrates

Processes of reaction between Ag–Cu–Ti molten alloy and silicon, carbonand SiC substrates were directly observed by in situ heating high-resolutionelectron microscopy. It was found that TiC grew in a layer by layer modeat the spreading front, and that a double layer consisting of an outerTi5Si3 and an inner TiC layers was formed by the reaction between the SiC substrate and themolten alloy. The morphology of the spreading front was also found to dependon the chemical composition of the dissociated substrate in the molten alloy.

Wire-Bond Void Formation During High Temperature Aging

Voids formed in Au-Al intermetallic phases degrade the long-term reliability of gold wire bonds to aluminum pads. In this study, a series of microstructural studies were performed to evaluate void formation in wire bonds. Voids are classified as initial, annular or minute. Probe marks and Al pad contamination are the main causes of initial voids that block alloy diffusion and slow down intermetallic growth. Annular voids are caused by the ultrasonic squeeze effect of thermosonic wire bonding. These bonding gaps may become pathways for halide species that corrode and degrade wire bonds. Minute voids are formed during the Au/sub 4/Al phase. The two Au/sub 4/Al phase textures in these voids may be due to different Au/sub 4/Al phase formation reactions or be related to grain boundary effects on the surface layer of the Au ball.

The Kirkendall effect in multiphase diffusion

Recently, the presence of two Kirkendall planes has been discovered in solid-state diffusion couples in various binary metallic systems in our laboratory, and in this work even three Kirkendall marker planes were found in a couple Ti/TiAl 3. The bifurcation and trifurcation of the Kirkendall plane can quantitatively be described using the classical diffusion theory in terms of Kirkendall velocity construction. The position of a Kirkendall plane is revealed in the reaction zone not only by inert markers, but also by a different crystal morphology on either side of the plane. A physico-chemical approach is developed which elucidates the role of the Kirkendall effect in the morphogenesis of interdiffusion systems. The occurrence of one or more Kirkendall planes, characterized by morphology changes in the reaction layers, turns out to be related to different nucleation sites of the product grains. The model is demonstrated using the experimental results in the Co–Si system. It is shown that the predictions using the physico-chemical approach are in good agreement with the experimentally found positions of the Kirkendall plane(s). The presence or absence of inert markers at the Kirkendall planes provides insight into the initial stages of reactive phase formation.

Metal Silicides in CMOS Technology: Past, Present, and Future Trends

Metal silicides have played an indispensable role in the raped development of microelectronics since PtSi was first used to improve the rectifying characteristics of diodes in early 1960s. This work first provides a brief historical overview of the many different silicides, and, correspondingly, the different processing methodologies used in the past.With regard to the present use of silicides in CMOS technologies a convergence becomes clear with the self aligned technology using only a limited number of silicides, namely, TiSi2, CoSi 2, and NiSi. A section on fundamental aspects is included to cover thermodynamics and kinetics, which are essential for understanding the silicide formation processes. The advantage and disadvantages of TiSi2, CoSi 2, and NiSi are analyzed with the development trend of CMOS technologies as a measure. Specifically, the reactive diffusion and phase formation of these silicades in the three terminals of a metal-oxides-semiconductor device, that is, gate, source, and drain are scrutinized. The review ends with an extended discussion of about future trends of metal-silicides in micro/nanoelectronics, with reference to the potential material aspects and device structures outlined in the International Technologies Roadmap for Semiconductors.

Bulk Shape Memory NiTi with Refined Grain Size Synthesized by Mechanical Alloying

Shape memory alloys (SMAs) constitute a unique class of materials that undergo a reversible phase transformation allowing the material to display dramatic stress-induced and temperature-induced deformations which are recoverable. Nickel Titanium (NiTi) SMA has be synthesized with refined grain size by a mechanical alloying process. The SMA is produced through a cold rolling fabrication strategy that yields nanocrystalline product structures. By starting with a stacked sandwich array of individual alloy components and subjecting this sample arrangement to repeated rolling and folding, a metal-metal multilayer composite is created. After the layer thickness is reduced to the nanoscale by multiple cold rolling and folding passes, an alloying and intermediate phase formation reaction is initiated. X-ray diffraction profiles confirm that the primary phase present is intermetallic NiTi phase, which is the phase that displays shape memory behavior. The presence of superelastic behavior and the shape memory effect in the product material is established by mechanical testing. At this refined grain size, the hysteresis stress observed is exceptionally small.

Microstructure evolution during solid state reactions of Nb/Al multilayers

The microstructure produced during investigations of reactive formation of NbAl 3 from Nb/Al multilayer films was found to be in general agreement with that employed in the two-stage kinetic model of Coffey et al. [ Appl. Phys. Lett. 55 (1989) 852]. Upon completion of the reaction, however, recrystallization and rapid grain coarsening led to significant changes in film microstructure and deviations from the predictions of the model. In an effort to clarify the picture of reactive phase formation emerging from these studies, several complementary analyses were employed to characterize the reaction. First, thermal data obtained from constant-heating-rate differential scanning calorimetry experiments were used to determine the annealing conditions for ex situ cross sectional transmission electron microscopy studies of the evolving film microstructure. Next, high-resolution chemical mapping was employed to track the penetration of the product phase into the Al grain boundaries. It was found, for example, that the NbAl 3 layer was 8–10 nm thick when the first stage of reaction was completed. It was also found that during stage two, the NbAl 3 layer consisted of columnar, somewhat faceted grains and that substantial recrystallization occurred after the termination of the reaction. These observations suggested refinements of the structure type classification scheme for reacted films proposed by Barmak et al. [ J. Electron. Mater. 26 (1997) 1009]. Lastly, the impact of bilayer thickness on the product microstructure was investigated using a novel sample architecture in which several bilayers with different periods comprise one multilayer. Investigations of this multi-period, multilayer sample correlated with those on single-period multilayers, indicating that the reaction reached completion at lower temperatures as the bilayer thickness was reduced.

ChemInform Abstract: Reactive Phase Formation in Binary and Ternary Silicide Systems

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Ohmic contacts on p-GaN (Part I):: investigation of different contact metals and their thermal treatment

The changes in contact resistivity and I– V curve linearity after annealing of Pd, Ni/Au, Cr/Au, Co/Au, Pt and Au contacts on p-GaN after tempering below and above 500°C have been investigated. CTLM-layered structures on p-GaN were thermally stressed and electrically analyzed. The I– V curve linearity as a measure of ohmic behavior has been derived from the correlation coefficient of the I– V curves. Below 500°C the Pd, Ni/Au, Cr/Au and Co/Au contacts become ohmic after annealing. The best final values of the specific contact resistance ρ c after annealing increase in the sequence Ni/Au, Pd, Co/Au, Cr/Au, Au and Pt. After annealing above 500°C the contact resistance increases for all contact materials and the linearity decreases. A competition between the formation of reaction phases at the interface and the decomposition of the p-GaN epi-layer is a probable reason for this behavior.

Reaction synthesis of Ni-Al-based particle composite coatings

Electrodeposited metal matrix/metal particle composite (EMMC) coatings were produced with a nickel matrix and aluminum particles. By optimizing the process parameters, coatings were deposited with 20 vol pct aluminum particles. Coating morphology and composition were characterized using light optical microscopy (LOM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Differential thermal analysis (DTA) was employed to study reactive phase formation. The effect of heat treatment on coating phase formation was studied in the temperature range 415 °C to 1000 °C. Long-time exposure at low temperature results in the formation of several intermetallic phases at the Ni matrix/Al particle interfaces and concentrically around the original Al particles. Upon heating to the 500 °C to 600 °C range, the aluminum particles react with the nickel matrix to form NiAl islands within the Ni matrix. When exposed to higher temperatures (600 °C to 1000 °C), diffusional reaction between NiAl and nickel produces (γ′)Ni3Al. The final equilibrium microstructure consists of blocks of (γ′)Ni3Al in a γ(Ni) solid solution matrix, with small pores also present. Pore formation is explained based on local density changes during intermetallic phase formation, and microstructural development is discussed with reference to reaction synthesis of bulk nickel aluminides.

Kinetic Analysis of PbTiO3 Phase Formation Via Diffusion Couple Method

ABSTRACTA detail kinetic analysis was performed on the PbTiO3 phase formation reaction occurring via PbO-TiO2 diffusion couple experiments. The diffusion model of Pb2+ and O2− ions was used to derive the kinetics equation of PbTiO3 phase formation reaction. From the parabolic kinetics between the thickness of PbTiO3 layer and time, the reaction rate constant for PbTiO3 formation was determined to be a function of the average diffusion coefficient of Pb2+ ions. By substituting the diffusion couple experimental results into the present kinetics equation, average diffusion coefficient values of Pb2+ ions in PbTiO3 layer were calculated according to temperature values. The activation energy value for the diffusion of Pb2+ ions was estimated for PbTiO3 formation using Arrhenius plot and the equation for the diffusion coefficient of Pb2+ ions was determined.

Reaction-sintering of intermetallic alloys of the Ni–Al–Mo system

A powder metallurgy route has been used for producing binary and ternary alloys of the Ni–Al–Mo system. Elemental powder mixtures were compacted and, then, sintered in a dilatometer. In this way the dimensional changes involved with thermally induced transformations could be followed during continuous heating runs up to the sintering temperatures. Sintering was assisted by the formation of a liquid phase, promoted by the heat output coming from the intermetallic phase formation reactions. The amount of liquid phase and the efficiency of sintering was highly dependent on the heating rate. A threshold value for optimal densification was identified for some compositions. The effect of other processing parameters, such as pre-sintering compaction pressure and sintering atmosphere has been considered too. The characterisation of the final products was mainly based on X-ray diffraction analyses. The microstructural parameters and the phase composition of the sintered materials were evaluated. On the basis of these results it is possible to draw some conclusions concerning the main phenomena occurring during the sintering process.