D. D. Perovic

Materials chemistry for low-k materials

The microelectronics industry is constantly trying to reinvent itself, to find new technological solutions to keep pace with the trend of increasing device densities in ultra-large-scale integrated (ULSI) circuits. Integral in this development has been the replacement of the conventional Al/SiO2 metal and dielectric materials in multilevel interconnect structures. Higher-conductivity Cu has now successfully replaced Al interconnects, but there is still a need for new low dielectric constant (k) materials, as an interlayer dielectric.

Luminescence origins in molecular beam epitaxial Si1−xGex

Interstitial‐type features smaller than ∼1.5 nm and in areal densities up to 7×108 cm−2 have been identified as the origin of a broad photoluminescence (PL) band from thick, fully strained layers of Si1−xGex alloys grown by molecular beam epitaxy. The strong PL band was predominant when the alloy layer thickness was greater than 4–10 nm, depending on x and the growth temperature. Thinner alloy layers exhibited phonon‐resolved transitions originating from shallow dopant bound excitons, similar to bulk material but shifted in energy due to strain and hole quantum confinement.

Vacuum-assisted aerosol deposition of a low-dielectric-constant periodic mesoporous organosilica film.

Adv. Mater. 2010, 22, 99–102 2010 WILEY-VCH Verlag Gm Since the conceptual and experimental breakthrough in 1992 on a soft-templating method to synthesize periodic mesoporous silica materials, their synthesis has evolved to the level of being able to adroitly control composition, pore size, mesopore architecture, and morphology. A significant advance in controlling the composition of periodic mesoporous silica materials is the successful integration of bridging organic groups into the siliceous pore wall to create periodic mesoporous organosilicas (PMOs). The enabling power of PMOs to fuse organic synthesis and inorganic materials chemistry has created opportunities for application in numerous areas, encompassing catalysis, separation, drug delivery, photonics, and nanoelectronics. Most PMOs synthesized to date are powders but many technologically relevant applications, such as sensors, membranes, displays, optical coatings, and microelectromechanical systems, require thin-film morphology because of the ease of materials property characterization and device fabrication on thin films. A quintessential example of the need for thin-film PMOs is their application as a lowdielectric-constant (low-k) insulating material on microelectronic chips in the semiconductor industry. The synthesis of PMO thin films has been achieved primarily through evaporation-induced self-assembly liquid-phase delivery techniques, such as spin-coating and dip-coating. However, the best-controlled and preferred techniques for fabricating insulating dielectrics in the semiconductor industry are vaporphase delivery techniques, such as chemical vapor deposition. An aerosol-deposition strategy for fabricating mesostructured thin films combines the advantages of vapor-phase delivery and evaporation-induced self-assembly and, at the same time, circumvents the difficulty of vaporizing low-volatility templates. Here, we report a vacuum-assisted aerosol deposition evaporation-induced self-assembly (VAAD-EISA) approach for fabricating highly ordered PMO thin films. The aerosol is generated by atomizing a solution of PMO precursors, surfactant, acid catalyst, and water in a chosen organic solvent. During aerosol transport, vacuum assists the evaporation of the solvent to induce self-assembly of themesostructure inside sub-micrometer aerosol droplets at room temperature. The aerosol is then forced through a specially designed converging nozzle to facilitate its deposition on a spinning substrate to form thin films with uniform surface coverage. After aging at room temperature, the as-synthesized films are heated under a nitrogen flow to remove templates. The structural, dielectric, and mechanical characterization of the thin films are subsequently carried out, and the results are reported herein. Figure 1 shows the VAAD system. It consists of an atomizer (TSI Model 3076), a transport tube, a deposition chamber with a spinning stage and a vacuum system. To achieve thin-film deposition, a de Laval nozzle design is used in order to accelerate the aerosol flow at the nozzle exit. A higher aerosol flow rate and a shorter nozzle-to-substrate distance (usually 0.5–1mm) significantly increase the deposition rate and decrease the deposition time from a few minutes to 10–20 seconds. The solution preparation is based on the previous spin-coating procedure with an increased amount of ethanol so as to extend the window of optimum deposition time.

Exciton luminescence in Si1−xGex/Si heterostructures grown by molecular beam epitaxy

Coherent Si1−xGex alloys and multilayers synthesized by molecular beam epitaxy (MBE) on Si(100) substrates have been characterized by low‐temperature photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM). Phonon‐resolved transitions originating from excitons bound to shallow impurities were observed in addition to a broad band of intense luminescence. The broad PL band was predominant when the alloy layer thickness was greater than 40–100 A, depending on x and the strain energy density. The strength of the broad PL band was correlated with the areal density (up to ∼109 cm−2) of strain perturbations (local lattice dilation ∼15 A in diameter) observed in plan‐view TEM. Thinner alloy layers exhibited phonon‐resolved PL spectra, similar to bulk material, but shifted in energy due to strain and hole quantum confinement. Photoluminescence excitation spectroscopy, external quantum efficiency, time‐resolved PL decay, together with the power and temperature dependence of luminescence inten...

Water Repellent Periodic Mesoporous Organosilicas

This paper demonstrates for the first time thermally induced gradual hydrophobization, monitored quantitatively by ellipsometric porosimetry, of four prototypical periodic mesoporous organosilicas (PMOs) that are tailored through materials chemistry for use as low-dielectric-constant (low k) materials in microprocessors. Theoretical aspects of this quantification are briefly discussed. A comparison of structural, mechanical, dielectric, and hydrophobic properties of ethane, methane, ethene, and 3-ring PMOs is made. Particularly, ethane, methane, and 3-ring PMOs show impressive water repellency at post-treatment temperatures as low as 350 °C, with corresponding Youngs modulus values greater than 10 GPa and k values smaller than 2, a figure of merit that satisfies the technological requirements of future generation microchips.

Mapping electrically active dopant profiles by field‐emission scanning electron microscopy

Secondary electron (SE) image contrast from p‐type silicon has been studied using field‐emission scanning electron microscopy (FE‐SEM). Cross‐sectional FE‐SEM images of boron‐doped silicon heterostructures have been compared with atomic concentration and free carrier profiles measured by secondary‐ion mass spectroscopy and electrochemical capacitance‐voltage profiling, respectively. FE‐SEM image contrast due to dopants has been shown to be electronic in origin. Since electrically active dopant species contribute solely to SE image contrast, FE‐SEM can be effectively used to map electrically active dopant profiles in two dimensions with a sensitivity as low as 1016 cm−3.

Bone mimetics : a composite of hydroxyapatite and calcium dodecylphosphate lamellar phase

The synthesis of composites based upon mineral and organic constituents is of importance for the development of materials for biomedical applications, such as bone replacement, augmentation and repair. Herein we describe a biologically inspired inorganic materials chemistry approach to bone mimetics. The synthetic strategy is based upon the surfactant-templated cooperative assembly of a composite that is composed of a calcium dodecylphosphate lamellar phase (CaDDP) and a calcium hydroxyphosphate (CaP) mineral phase. The measured properties of the chemically formed composite suggest that it is distinct from simple physical mixtures of the CaDDP and CaP components. A key difference is the generation of biologically important hydrogenphosphate located at the interface between the CaDDP and CaP phases only in the chemically formed composite. The synthesis of the composite is considered to involve the synergistic interaction of CaDDP and CaP, possibly aided by interfacial complementarity of charge and geometry. CaDDP is chemically stable and non-toxic, rendering the composite potentially useful for biomedical applications.

Growth and band gap of strained 〈110〉 Si1−xGex layers on silicon substrates by chemical vapor deposition

We report chemical vapor deposition growth of strained Si1−xGex alloy layers on 〈110〉 Si substrates. Compared to the same growth conditions on 〈100〉 substrates, a slightly lower Ge composition and a much lower growth rate was observed. From photoluminescence measurements, the band gap of these films for 0.16≤x≤0.43 is evaluated and compared to theory. Finally, a surprisingly large ‘‘no‐phonon’’ replica line strength ratio was observed as compared with that observed in 〈100〉 layers.

Some aspects of nucleation and growth in Pb free Sn-Ag-Cu solder

Abstract To investigate the minimum superheat necessary to solder components on printed circuit boards successfully using 95.5 wt-%Sn, 3.8 wt-%Ag, 0.7 wt-%Cu solder, experiments were carried out using separate solder balls of the type used in ball grid arrays. Significant differences in microstructure were observed depending on the peak temperature reached in the liquid on melting. On cooling, substantial undercooling was often observed, with values up to 18 K. Under some freezing conditions, the primary phase formed was Ag3Sn, while under other conditions the primary phase was Sn. The amount and type of eutectic microstructure formed was observed to vary with freezing conditions. The types of microstructure formed are illustrated. Nucleation phenomena and their effect on subsequent growth are discussed.

Effect of cooling rate on microstructure of Ag–Cu–Sn solder alloys

Abstract The effect of cooling rate on the formation of Sn dendrites during freezing was investigated for Pb free solder alloys having compositions near the Sn rich ternary eutectic of the Ag–Cu–Sn system. For Ag and Cu concentrations less than eutectic, the volume fraction of Sn dendrites tended to increase with cooling rate, while for Ag and Cu concentrations greater than eutectic a decrease was observed. For an alloy having the composition 3.8 wt-%Ag, 0.7 wt-%Cu and 95.5 wt-%Sn, the volume fraction of Sn dendrites was found to vary from ∼5 vol.-% at a cooling rate of 1 K s−1 to 65 vol.-% at 100 K s&minus1, resulting in a corresponding enrichment of Ag and Cu in the eutectic regions, in the form of increased volume fractions of Ag3Sn and Cu6Sn5. The surface contour of frozen samples was observed to depend on the scale of interdendritic shrinkage during freezing, and was especially smooth for eutectic freezing.