Microstructural Effects on Microwave Properties of Low-Temperature Cofired Ceramic Striplines: Experiments and Modeling

The effect of microstructure on microwave properties of low-temperature cofired ceramic (LTCC) stripline packages was studied with experiments and simulations. Microstructure was controlled experimentally by modifying organic compositions and heating procedures. Pores existing at the conductor/dielectric interface and inside the conductor line affected the microwave properties, such as the effective dielectric constant (ε_eff), the effective conductivity (σ_eff), and the unloaded quality factor ( Q _U) of the stripline packages. The models for simulation were introduced with a finite-element method using high-frequency simulation software (HFSS). Simulation results showed that the pores existing at the interface made Q _U increase by 7%–8% while [alt epsilon]_eff decrease by more than 10%. Pores inside the conductor line made σ_eff and Q _U decrease. The proposed models and simulation results will contribute to improving future designs and manufacturing of LTCC microwave applications.     

Processing and Electrical Characterization of a Varistor-Capacitor Cofired Multilayer Device

A comprehensive study of the materials selection, processing, cofiring, and electrical characterization of a multilayer varistor/capacitor (MLVC) device has been presented. The goal was to achieve component miniaturization and provide superior high-frequency and high-amplitude transient-voltage protection. A ZnO-based material and 0.75Pb-(Mg_1/3Nb_2/3)O₃-0.25Pb(Zn_1/3Nb_2/3)O₃ (PMN-PZN) were chosen as the respective varistor and capacitor components. Using a controlled-profile furnace, PMN-PZN with excess PbO resulted in an optimum cofiring with a complex varistor composition. The sintering study made it clear that the ability to cofire is dependent not only on matching sintering temperatures and final densities but also sintering rates. Composite-pellet studies assured the mechanical and electrical integrity of the device and indicated that interaction between the respective components was minimal. Prototype MLVC devices were fabricated using standard tape-casting techniques. Microstructural analysis of the MLVCs revealed solid ceramic/ceramic and ceramic/electrode interfaces with little evidence of interaction. Current-voltage and dielectric measurements both indicated good electrical properties that can be specifically tailored by changing the layer thickness.     

Role of Process Parameters on Microstructural and Electronic Properties of Rapid Thermally Grown MoS2 Thin Films on Silicon Substrates

Silicon - Miniaturization of the semiconducting materials propelled the discovery of low dimensional transition metal dichalcogenides (TMDC) thin films. In this work, MoS2 thin films have been...     

Characterization of the AIN–W Interface in a Cofired Multilayer AIN Substrate

The AIN–W Interfaces in a cofired multilayer AIN substrate were observed using an optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). Optical and SEM observations showed an intricate intricatelocking AIN-W grain structure at the interface. After the W pad was removed from the substrate with a NaOH etchant, the surface morphology of the W metal at the interface side was found to be very rough, with a small-grain microstructure compared with that at the free surface side. Electron microprobe analyses using SEM revealed that there was no diffusion of either W or Al at the interface at the order of a few micrometer's resolution. Bright-field images, dark-field images and selected area electron diffraction (SAD) patterns using TEM indicated there was no secondary phase between AIN and W. However, scanning transmission electron microscopy using an energy dispersive X-ray detector revealed that there was a 200-nm thick W diffusion layer from the interface into the AIN ceramics. It was concluded that the high adhesion strength between the W conductor and the AIN substrate (>20 MPa) was not due to any secondary phase but to mechanical interlocking of AIN and W during cofiring.     

Synthesis and Characterization of Crystallizable Anorthite-Based Glass for a Low-Temperature Cofired Ceramic Application

We report successful identification and preparation of a glass composition in the CaO–Al₂O₃–SiO₂ phase diagram with a judicious choice of fluxes that met all dielectric, electrical, and thermal property requirements for low-temperature cofired ceramic (LTCC) applications. The glass composition sintered at 900°C attains good density (2.45 g/cc) and does not precipitate any crystalline phase. However, when this glass powder is sintered at the same temperature in the presence of 30 vol% cordierite, crystallization of the anorthite phase is observed, which improves the properties of the composite for LTCC application.     

Microstructural Characterization of Reactively Sintered Mullites

Mullite ceramics have been produced by reaction sintering of powders prepared using pseudoboehmite–colloidal silica and aluminum sulfate–colloidal silica mixtures. The microstructural development of these mullites was studied by a number of transmission electron microscopy based techniques including diffuse dark field, Fresnel fringe defocus imaging, and high-resolution transmission electron microscopy. This characterization procedure showed that mullite ceramics free from glassy phases at triple junctions and grain boundaries could be produced from both mixtures using suitable sintering temperatures and alumina/silica ratios. The wetting of grain boundaries by glass, occurring in the mullite ceramics from either incomplete reaction between alumina and silica components or release of silica from the mullite structure with increasing temperature, was found to depend on the prior thermal history of the ceramics.     

Microstructural Characterization of Graded-Index Antireflective Films

The miscrostructure of graded-index antirefection (GIAR) films was characterized using transmission and scanning electron microscopy, small-angle X-ray scattering, and replication. Transmission electron microscopy and X-ray scattering results agree for the entire coarsening history of the phase-separated substrate borosilicate glass. The complexity of the replication process and the metallization used for scanning electron microscopy caused the effective resolution limits for GIAR films to be much larger than the inherent resolution limits for each instrumentation.     

Microstructural Characterization of Small-Particle Plasma Spray Coatings

The microstructure of a small-particle plasma spray (SPPS) aluminum oxide coating sprayed onto a prepared mild steel substrate has been characterized using a variety of microscopic techniques as part of a process optimization study. The coating was highly conforming to the substrate as evidenced in high-resolution transmission electron microscopy, although some voids were present at the interface. The layered splat microstructure, characteristic of conventional plasma-sprayed coatings but smaller in size, was discerned in partially thinned samples in a focused-ion-beam scanning electron microscope (FIB-SEM). Microcracks and microporosity, generally less than 0.5 µm in size, was also seen between splats using transmission electron microscopy. Cubic alumina of the crystallographic form gamma-Al₂O₃ was identified by electron diffraction.     

Microstructural Characterization of High-Thermal-Conductivity Aluminum Nitride Ceramic

An aluminum nitride (AlN) ceramic with a thermal conductivity value of 272 W·(m·K)⁻¹, which is as high as the experimentally measured thermal conductivity of an AlN single crystal, was successfully fabricated by firing at 1900°C with a sintering aid of 1 mol% Y₂O₃ under a reducing N₂ atmosphere for 100 h. Oxygen concentrations were determined to be 0.02 and 0.03 mass% in the grains and in the grain-boundary phases, respectively. Neither stacking fault in the grains nor crystalline phase in the grain-boundary regions was found by transmission electron microscopy. An amorphous phase possessing yttrium and oxygen elements was detected between the grains as thin films with a thickness of <1 nm. Because the amount of grain-boundary phase was small, the high-thermal conductivity of the ceramic was attributable to the low oxygen concentration in the AlN grains.     

Mechanical Characterization of Diesel Particulate Filter Substrates

Test procedures for the mechanical evaluation of diesel particulate filter (DPF) substrate materials have been developed and applied for the characterization of porous cordierite under ambient conditions. Specifically the double-torsion test method was employed to characterize fracture toughness and slow crack behavior while resonant ultrasound spectroscopy (RUS) was used to determine the elastic properties of the substrate walls. A dry grinding procedure was developed to fabricate test specimens for these tests. The fracture behavior of porous cordierite was related to the pore structure inside the filter wall. Implications of the test results on the mechanical reliability of DPFs are discussed.     

Microstructural Characterization of Commercial Hot-Pressed Boron Carbide Ceramics

Two commercial hot-pressed boron carbide ceramics were investigated by transmission electron microscopy. Atomic-scale observations suggest that the grain boundaries of the two materials are free of grain-boundary films. Two triple-junction phases were found and characterized to be rhombohedral Fe₂B₁₀₃ and orthorhombic Ti₃B₄. In addition, intra-granular precipitates, AlN, Mo₂(C, B) and graphite, were identified and found to have coherent relationships with the boron carbide matrix. Micron-scale inclusions were also observed and most of them were determined to be graphite. The formation mechanisms of the secondary phases and their possible influence on mechanical properties are also discussed.     

Thermally Induced Failure of Copper-Bonded Alumina Substrates for Electronic Packaging

Although copper-bonded alumina substrate has been used for some power electronic devices, its use has been rather limited because of lack of information on the thermally induced failure of the substrate. The phenomenology of thermal cracking was experimentally studied in this work. Interface debonding occurred at the side edge of copper chips and grew inwardly along the interface. Subsequently, the interface crack deviated into the alumina after propagating up to approximately 200 μm. The curving of the interface crack was caused by a mixed Mode I and Mode II loading near the edge. The cracking was widespread, whereby small cracks coalesced with one another to form a long, shallow crack along the perimeter of the copper chips. The cracking sensitivity was influenced by such design factors as edge sharpness, nickel plating, and copper backing. Annealing the substrate above 700 K before testing promoted the cracking considerably. The substrate cracking is discussed in terms of stress conditions near the edge.     

Microstructural Characterization of Ferroelectric Thin Films in Transverse Section

A technique has been developed for the TEM examination of ferroelectric thin films in transverse section. Some preliminary results are reported for three different thin-film/substrate systems. The microstructures of thin films of lead scandium tantalate deposited onto sapphire and MgO, and lead titanate deposited onto AIN, have been examined, with particular attention being paid to the quality of the thin-film/substrate interfaces and to the changes in the nature of the microstructures of the thin films as a function of distance from their substrates. It is demonstrated that the technique successfully produces adequate electron transparent regions for the characterization of the thin-film/substrate interface of all the samples examined and that it is possible to prepare transverse sections of ferroelectric thin films routinely.     

Microstructural Characterization of Alumina and Silicon Carbide Slip-Cast Cakes

The effect of solids loading, particle-size distribution, and suspension viscosity on the resultant microstructure of slipcast monolithic ceramics prepared from aqueous suspensions of alumina and silicon carbide was studied. Unimodal alumina suspensions (average particle size = 0.6 μm) were prepared at 35, 37, and 42 vol%. Silicon carbide suspensions (average particle size = 0.7 μm) were produced with different quantities of dispersant at 37 vol%. Similarly, aqueous alumina suspensions of 42 and 50 vol% were produced with a bimodal particle-size distribution. The slip-cast microstructures were characterized by mercury porosimetry and small-angle neutron scattering, which provided pore size (distribution), pore fraction, and pore morphology. Essentially, the combination of these techniques deciphered packing differences obtained in the cake microstructures. For the alumina cakes produced from the 35,37, and 42 vol% suspensions, the individual characterization techniques, mercury intrusion, and the neutron scattering measurements showed that the cake microstructures were similar in pore size and quantity. However, comparison of the techniques and their assumptions showed differences in the pore shape. Mercury porosimetry and neutron scattering showed bimodal porosity for the cake produced from a mixture of 85% 6-μm particles and 15% 0.6-μm particles. Pore volume fraction and pore size increases were correlated with increased viscosity in the silicon carbide suspensions. In addition, the silicon carbide cake microstructures were measured, and homogeneity was evaluated as a function of position in the cast.     

Development and Microstructural Characterization of High-Thermal-Conductivity Aluminum Nitride Ceramics

The effect of several additives, such as CaC₂, CaO, Y₂O₃, and C, on thermal conductivity of hot-pressed AlN ceramics was investigated. The addition of CaC₂ reductant was found to be useful for achieving high thermal conductivity of 180 W/(m·K) at room temperature. The characterization of AlN ceramics with CaC₂ additive was performed by chemical analysis of Ca, C, and O and microstructural analysis using transmission electron microscopes equipped with an energy-dispersive X-ray analyzer and an electron energy loss spectrometer. The major influence on high thermal conductivity is the disappearance of a thermal barrier caused by oxygen impurities at the grain boundary.     

Characterization of Material for Low-Temperature Sintered Multilayer Ceramic Substrates

The sintering temperature of multilayer ceramic substrates must decrease to 1000° or below to avoid melting the conductors (Pd-Ag, Au, or Cu) during sintering. In this study, SiO₂, CaO, B₂O₃, and MgO were used as additives to Al₂O₃ to decrease the firing temperature by liquid-phase sintering. Compositions with 18.0 and 22.5 wt% B₂O₃ were sintered at around 1000° in an air atmosphere to yield dense ceramics with good properties: relative dielectric contant between 6 to 7 (1 MHz), tan δ≤× 3 × 10⁻⁴ (1 MHz), insulating resistivity > 10¹⁴ω cm, coefficient of thermal expansion ∼ 7.0 × 10⁻⁶/°, and thermal conductivity ∼ 4.1 W/(m · K).     

Microstructural Characterization of Synroc C and E by Electron Microscopy

High-resolution electron microscopy, analytical electron microscopy, and scanning electron microscopy were used to examine the microstructure and chemistry of the titania-based nuclear waste ceramics Synroc C and Synroc E. The mineral assemblages of these formulations are dependent upon the level of radwaste incorporation. The principal phases identified are zirconolite, zirkelite, pyrochlore, polymignyte, perovskite, hollandite, magnetoplumbite, and intermetallic alloys. Thin intergranular films and "glassy" triple points were found to exist between different phases. In Synroc formulations containing a large excess of TiO₂ the radwaste-containing minerals are embedded in an inert, continuous rutile matrix; this microencapsulation operates at the level of 10 to 20 nm. Backscattered electron images suggest that the distribution of radwaste ions may not be completely homogeneous throughout the ceramic.     

Synthesis and Microstructural Characterization of Unsupported Nanocrystalline Zirconia Thin Films

A polymeric precursor spin-coating technique is illustrated in which yttrium-stabilized zirconia (YSZ) thin films are produced on Si, Al₂O₃, and NaCl at temperatures less than 350°C. High-resolution transmission electron microscopy (HRTEM) examinations show that the YSZ films are nanocrystalline (grain size of less than 5 nm), fully dense, and have a stabilized cubic fluorite structure. Using the polymeric precursor spin coating method, unsupported nanocrystalline thin films of YSZ with thicknesses ranging from 30 to 1000 nm are prepared by transferring the films from a host substrate to metallic TEM grids with unsupported areas exceeding 1 mm².     

Microstructural Characterization of SiC–SiC Joint by Raman Spectroscopy

Analysis of the SiC–SiC joint and its brazing mixture has been performed using a Raman spectroscopy microprobe technique. A careful mapping of the sample clearly shows the spatial distribution of the chemical species close to and within the joint. A different distribution of the 4H and 6H α-SiC polytypes, grown during the brazing process, was observed inside the joint. Furthermore, identification of the bands related to the Nowotny phase was also possible.     

Microstructural Characterization of Structural Ceramics Using Image Processing and Analysis

Digital imaging and analysis methods are applied to the quantitative study of microstructural changes which occur during hot-pressing of yttria-doped silicon nitride. Effects of processing changes upon the grain growth and microstructural anisotropy are described. Relationships between grain cross-sectional area and mechanical properties are established. It was found that, over the range of processing conditions used, increases in grain size correlated strongly with an increase in fracture toughness. The grain size distribution broadened significantly with hot-press time, resulting in reduced flexural strength. Although no significant change in the mean grain shape factor was observed, the variance in shape factor decreased as the hot-press time was extended.