David A. Hutt

Blue photoluminescence and local structure of Si nanostructures embedded in SiO2 matrices

Strong and stable blue photoluminescence (PL), visible to the naked eye under 0.4 μW of 300 nm and 2.7 μW of 370 nm excitation, has been observed for samples of Si clusters embedded in SiO2 matrices, prepared by rf co‐sputtering followed by N2 annealing at 800 °C. Si K‐edge extended x‐ray absorption fine structure (EXAFS) and near‐edge x‐ray absorption fine structure (NEXAFS) strongly suggest the existence of Si nanoclusters with crystalline cores in the efficient emitting material. The PL excitation dependence is explained by an increase in the conduction band density of states deep in the band, and the formation of a band tail.

Electroless nickel bumping of aluminum bondpads. I. Surface pretreatment and activation

Electroless nickel bumping of aluminum (Al) bond-pads followed by solder paste printing is seen as one of the lowest cost routes for the bumping of wafers prior to flip-chip assembly. However, the electroless nickel bumping of Al bondpads is not straightforward and a number of activation steps are necessary to enable the nickel deposit to form a strong, electrically conductive bond with the Al. For the electroless nickel coating of mechanical components made of aluminum, a zincate activation process has been used for many years; however, extension of these techniques to semiconductor wafers requires careful control over these pretreatments to avoid damage to the very thin bondpads. This paper reports a number of experiments designed to characterize the activation of Al bondpads to electroless nickel plating, focusing on the effects of solution exposure time and bondpad composition. In addition, the results are discussed in the context of other studies presented in the literature to provide an understanding of the mechanism of the zincate activation process applied to Al bondpads.

Rates of attachment of fibroblasts to self-assembled monolayers formed by the adsorption of alkylthiols onto gold surfaces

Self-assembled monolayers (SAMs) of alkylthiols on gold surfaces have been prepared and characterised by contact angle goniometry and X-ray photoelectron spectroscopy. Murine 3T3 fibroblasts were cultured on single-component monolayers of adsorbates with carboxylic acid and methyl termini, and the rates of cellular attachment measured over 90 minutes. Cell attachment was found to be approximately twice as rapid for carboxylic acid-terminated monolayers as for methyl-terminated monolayers. After 24 hours, substantially greater numbers of 3T3 cells were found to be attached to the hydrophilic monolayers than were attached to the hydrophobic ones. Murine 3T3 cells were also cultured on patterned SAMs which were prepared using UV photochemistry and characterised using scanning electron microscopy. The cells attached exclusively to the acid-functionalised areas of the patterned surface, leaving the methyl-functionalised regions bare. The patterns also strongly influenced the morphology of the attached cells.

Application of adhesives in MEMS and MOEMS assembly: a review

This paper presents a review of the recent literature on the use of adhesives in MEMS packaging applications. The aim of this review has been to establish the current applications of adhesives in MEMS and MOEMS assembly and to investigate the limitations and future requirements of these materials. The review has shown that while there is a wealth of information available on the packaging of MEMS devices, there is very limited detail available within the public domain regarding the specific uses of adhesives and in particular exactly which products are in use. The paper begins with an overview of the uses of adhesives in MEMS packaging, subdivided into sections on structural adhesives, adhesives for optical applications and other applications. The paper then describes methods for adhesive dispensing and issues with adhesive use which affect the reliability of the package. The reliability of MEMS devices assembled using adhesives is a challenging issue, being more than a simple combination of electrical, mechanical and material reliability. Many failure modes in MEMS devices can be attributed to the adhesives used in the assembly; for example, thermal expansion mismatches can cause stress in the die attach, while outgassing from epoxies can cause failure of sealed devices and contamination of optical surfaces.

The Evolution of Pd ∕ Sn Catalytic Surfaces in Electroless Copper Deposition

This paper describes the different catalytic surfaces of Pd/Sn formed before electroless copper deposition onto a glass substrate. In this study, silanization of the glass surfaces with (3-aminopropyl) trimethoxysilane was used to provide a surface-coupled layer of functional molecules to assist in the adsorption of Pd/Sn catalyst and the subsequent copper deposition. The composition and microstructure of the modified glass surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry. These showed that catalytic Pd/Sn structures on the surface changed with increasing immersion time in the catalyst bath. The core-level XPS spectrum of Pd indicated that metallic Pd(0) became more significant in the catalyst layer than Pd(II) with the increasing immersion time. A model of the adsorption process is proposed to explain these changes. It was observed that too high a quantity of Pd(0) does not always improve the adhesion of the Cu deposits in the electroless process.

Fluxless Soldering of Copper Substrates Using Self-Assembled Monolayers for Preservation

Self-assembled monolayers (SAMs) of octadecanethiol have been used as a preservative to enable fluxless soldering of copper substrates. The SAMs were deposited onto copper surfaces that had previously been etched to remove any existing oxide layers and wetting balance testing under an inert atmosphere was used to measure their fluxless solderability as a function of storage time and environment. It was found that SAM coated samples could be stored in air at room temperature for a short period of time before they became unwettable as a result of oxidation, however this storage time could be greatly increased by keeping them at low temperature. X-ray photoelectron spectroscopy was used to monitor the level of oxidation within the materials, which was found to correlate with the fluxless solderability. The SAM is thought to present a barrier to the penetration of oxygen to the copper surface, but at the same time can be readily displaced by heat and molten solder

Structural investigation of ordered Sb adsorption phases on Ag(111) using coaxial impact collision ion scattering spectroscopy

Abstract The ordered overlayer structures formed following the adsorption of Sb on Ag(111) have been studied using coaxial impact collision ion scattering spectroscopy (CAICISS). At coverages up to several atomic layers there is evidence for layer-by-layer growth at room temperature, although the individual layers show no long-range order. Subsequent annealing gives rise to two ordered phases, a (√3 × √3)R30°Sb and a (2√3 × 2√3)R30°Sb. Neon ion CAICISS data have been used to distinguish between overlayer and substitutional structural models for the (√3 × √3)R30°Sb phase, and pure substitutional and mixed substitutional/overlayer models for the (2√3 × 2√3)R30°Sb phase. Despite the complexity of multiple scattering contributions, these data clearly favour the substitutional adsorption site for the (√3 × √3)R30°Sb phase giving a surface alloy of the form Ag2Sb. For the (2√3 × 2√3)R30°Sb phase, the data are best described by a model involving an ordered p(2 × 2)-Sb overlayer (periodicity relative to the substrate) superimposed on the (√3 × √3)R30°Sb surface alloy.

Static secondary ion mass spectrometry studies of self-assembled monolayers: electron beam degradation of alkanethiols on gold

Static secondary ion mass spectrometry (SIMS) has been used to investigate the degradation of self-assembled monolayers (SAMs) of alkanethiols subjected to bombardment by keV electrons. Because of its remarkable structural specificity, SIMS revealed significant structural modifications to the SAMs following irradiation. Both positive and negative ion spectra exhibited dramatic changes after exposure of SAMs to electron bombardment. In the positive ion spectra, peaks were observed between m/z 100 and 200 that were attributed to polycyclic aromatic ions with masses greater than the adsorbate molecule. These species are the result of interchain crosslinking initiated by electron impact. In the negative ion spectra, gold-molecular clusters disappeared after only small doses of electrons, attributed to the rapid oxidation of thiolates to disulfides. After doses as high as 3×10 17 electrons cm –2 , there were still significant levels of sulfur at the surface along with graphitised carbonaceous material. It was concluded that keV electron impact leads to only slow removal of material from the SAM. These data illustrate the power of SIMS for probing surface reactions in SAMs.

Glass as a Substrate for High Density Electrical Interconnect

The high volume production of substrates able to support high density interconnection is becoming increasingly difficult as the pitch of devices continues to decrease. An important issue is the alignment of microvias and pads which is made particularly challenging by the dimensional instability of traditional organic substrates. In this work, glass has been investigated as an alternative substrate material. Glass offers potential advantages including: electrical insulation, dimensional stability, thermal expansion similar to Si and optical transparency. This paper examines manufacturing processes for the preparation of substrates built up from 50 to 100 ¿m thick glass sheets. Excimer laser machining was used to form microvias and tracks in the glass which were metallised using electroless plating methods. Using a photoresist layer during the laser machining process helped to reduce debris on the glass surface and could be used to direct the subsequent deposition of electroless copper or nickel such that circuit patterns could be created. Lamination of the thin glass sheets was investigated using direct glass to glass bonding and through the use of an intermediate layer. A method for the combination of these techniques to create multilayer substrates is proposed.

Integrated optical and electronic interconnect PCB manufacturing research

Purpose – The purpose of this paper is to provide an overview of the research in a project aimed at developing manufacturing techniques for integrated optical and electronic interconnect printed circuit boards (OPCB) including the motivation for this research, the progress, the achievements and the interactions between the partners.Design/methodology/approach – Several polymer waveguide fabrication methods were developed including direct laser write, laser ablation and inkjet printing. Polymer formulations were developed to suit the fabrication methods. Computer‐aided design (CAD) tools were developed and waveguide layout design rules were established. The CAD tools were used to lay out a complex backplane interconnect pattern to meet practical demanding specifications for use in a system demonstrator.Findings – Novel polymer formulations for polyacrylate enable faster writing times for laser direct write fabrication. Control of the fabrication parameters enables inkjet printing of polysiloxane waveguides...