Shufang Luo

Low-Temperature Sintering of Nanoscale Silver Paste for Attaching Large-Area

A low-temperature sintering technique enabled by a nanoscale silver paste has been developed for attaching large-area (>100 mm2) semiconductor chips. This development addresses the need of power device or module manufacturers who face the challenge of replacing lead-based or lead-free solders for high-temperature applications. The solder-reflow technique for attaching large chips in power electronics poses serious concern on reliability at higher junction temperatures above 125°C. Unlike the soldering process that relies on melting and solidification of solder alloys, the low-temperature sintering technique forms the joints by solid-state atomic diffusion at processing temperatures below 275°C with the sintered joints having the melting temperature of silver at 961°C. Recently, we showed that a nanoscale silver paste could be used to bond small chips at temperatures similar to soldering temperatures without any externally applied pressure. In this paper, we extend the use of the nanomaterial to attach large chips by introducing a low pressure up to 5 MPa during the densification stage. Attachment of large chips to substrates with silver, gold, and copper metallization is demonstrated. Analyses of the sintered joints by scanning acoustic imaging and electron microscopy showed that the attachment layer had a uniform microstructure with micrometer-sized porosity with the potential for high reliability under high-temperature applications.

({>}100~{\rm mm}^{2})

The low-temperature joining of semiconductor chips by sintering of silver paste is emerging as an alternative lead-free solution for power electronics devices and modules working in a high-temperature environment. A promising die-attachment material that would enable the rapid implementation of the sintering process is nanoscale silver paste, which can be sintered at temperatures below 300°C without an external pressure. In this paper, we report our findings on the silver migration in sintered nanosilver electrode-pair patterns on an alumina substrate. The electrode pairs were biased at an electric field ranging from 10 to 100 V/mm and at a temperature between 250°C and 400°C in dry air. The leakage currents across the electrodes were measured as the silver patterns were tested in an oven. Silver dendrites formed across the electrode gap were observed under an optical microscope and analyzed using scanning electron microscopy and energy dispersive spectroscopy (EDS). The silver migration was found in the samples tested at 400°C, 350°C, 300°C, and 250°C. The measurements on the leakage current versus time were characterized by an initial incubation period, called “lifetime,” followed by a sharp rise as the silver dendrites were shorting the electrodes. A simple phenomenological model was derived to account for the observed dependence of lifetime on the electric field and temperature. The EDS mappings revealed the significant presence of oxygen on the positive electrode but the complete absence on the negative electrode. A mechanism involving the oxidation of silver and the dissociation of silver oxide at the anode was suggested. We suggest that the migration of a sintered nanosilver die attachment can be prevented in high-temperature applications through packaging or encapsulation to reduce the partial pressure of oxygen.

Chips

This research is aimed to develop a nanomicelle delivery system in order to enhance the solubility and stability of camptothecin (CPT) in aqueous media. In this case, alpha,beta-poly[(N-carboxybutyl)-L-aspartamide] (PBAsp)-CPT was conjugated by the esterification between PBAsp and 20-OH of CPT, and hence used to fabricate nanomicelles with a particle size between the pore size of blood capillary in normal tissue and that in tumor tissue. It was worthy of note that the drug-loaded system of PBAsp-CPT nanomicelle improved the solubility and stability of CPT in aqueous media. However, with an increase of the CPT loading in PBAsp-CPT, the solubility sharply decreased. Meanwhile, the sizes of PBAsp-CPT nanomicelles showed a tendency of increase. Moreover, the drug release of PBAsp-CPT nanomicelles displayed a linear sustaining profile, and hence resulted in the essential decrease of cytotoxicity to L929 cell line. The assembled nanomicelles based on the PBAsp-CPT conjugates showed a great potential as polymer prodrug of tumor therapy, and the controlled nano-scale might achieve the passive tumor targeting.

Migration of Sintered Nanosilver Die-Attach Material on Alumina Substrate Between 250

To design peptide-targeted iron oxide as magnetic resonance imaging (MRI) contrast agents, amino-functionalized magnetic nanogels were prepared by using N-(2-aminoethyl) methacrylamide hydrochloride (AEM x HCl) as monomer via new photochemical approach. Their chemical structure and composition were characterized by Fourier transform infrared spectra (FTIR) and thermogravimetric analyses (TGA). The core-shell structure of magnetic nanogels was confirmed by high-resolution transmission electron microscopy (HRTEM). The good storage stability, high magnetic content (88.7%), high saturation magnetizations and superparamagnetic behavior suggested their great potentials as MRI contrast agents, which were confirmed by their measurements of r(2) and coronal image of the crossing of mouse kidney.

^{\circ}\hbox{C}

Gold nanoshells have shown a great potential for use as agents in a wide variety of biomedical applications, and some of which require the delivery of large numbers of gold nanoshells onto or into the cells. Here, we develop a ready method to enhance the cellular uptake of gold nanoshells by modifying with meso-2,3-dimercaptosuccinic acid (DMSA). The quantifiable technique of inductively coupled plasma atomic emissions spectroscopy (ICP-AES) and transmission electron microscopy (TEM) were used to investigate the cellular uptake of unmodified and DMSA-modified gold nanoshells. Three cell lines (RAW 264.7, A549, and BEL-7402) were involved and the results indicated that the cellular uptake of the DMSA-modified gold nanoshells was obviously enhanced versus the unmodified gold nanoshells. The reason possibly lies in the nonspecific adsorption of serum protein on the DMSA-modified gold nanoshells (DMSA-GNs), which consequently enhanced the cellular uptake. As a continued effort, in vitro experiments with endocytic inhibitors suggested the DMSA-GNs internalized into cells via receptor-mediated endocytosis (RME) pathway. This study has provided a valuable insight into the effects of surface modification on cellular uptake of nanoparticles.

and 400

The aim of research is to develop and optimize delivery system for cis-dichlorodiammine platinum(II) (CDDP) based on polymer-metal complex nanomicelles with controllable particle size in order to achieve the passive tumor targeting. In particular, graft copolymers, mPEG-g-alpha,beta-poly [(N-amino acidyl)-DL-aspartamide] (mPEG-g-PAAsp) were synthesized by the ring-opening reaction of polysuccinimide with mPEG-NH(2) (M(w): 2000 and 5000 Da), and then with l-aspartic acid and l-glutamic acid, respectively. mPEG-g-PAAsp-CDDP complex nanomicelles were fabricated from mPEG-g-PAAsp and CDDP. The formation of mPEG-g-PAAsp-CDDP nanomicelles was confirmed by fluorescence spectrophotoscopy, electrical conductivity and particle size measurements. It was found that all the nanomicelles showed spherical shapes with clear core-shell structures and narrow size distributions. Their sizes ranged from 80 to 160 nm, suggesting of their passive targeting potential to tumor tissue. With the increase of the molecular weight of mPEG, the sizes of mPEG-g-PAAsp-CDDP micelles showed a tendency to increase. mPEG-g-PAAsp-CDDP nanomicelles showed linear gradual drug release profiles in 40 h, suggestion of their sustained drug release behaviors. Compared with CDDP, mPEG-g-PAAsp-CDDP micelles showed essential decreased cytotoxicity to Bel-7402 cell line.

^{ \circ}\hbox{C}

Low-temperature silver sintering technology is emerging as a lead-free die-attach solution to significantly improve the heat dissipation and reliability packaging of power devices and modules die attached by solder alloys. With the recent introduction of nanosilver materials, which dramatically simplify the bonding process by lowering the required pressure down to a few megapascals, the silver sintering die-attach solution is poised for wide use in the manufacture of electronic products. In this paper, the thermomechanical reliability of sintered-silver joints was studied in comparison with soldered joints of two lead-free solders, SN100C and SAC305. Die-attach samples were fabricated by bonding 10 mm × 10 mm silicon mechanical chips to silver-metalized copper blocks and direct-bond copper (DBC) substrates according to the respective heating profiles of a nanosilver paste and the two solders. The die-attach samples were thermally cycled between -40 °C and +125 °C. The bonding reliability was evaluated by measuring the bending curvatures of the cycled samples and examining the cross sections of the samples under an electron microscope. Bending of the bonded structures, which is the result of mismatched coefficients of thermal expansion between silicon and copper or DBC, offered a nondestructive method for monitoring the integrity of the bond line. The bending curvatures of all of the die-attach samples decreased rapidly after they were thermally cycled. Most of the drop in curvature can be attributed to stress relaxation in the bonding materials without bond-line cracking. However, in the samples on copper blocks, after 800 cycles, the curvatures of the soldered samples decreased to near 0 μm-1, but those of the silver-sintered samples still had about 30% of the original curvatures. Scanning electron microscopy images showed that the joints of the soldered samples with near 0 μm-1 curvature had been cracked almost all the way through, but the joints of the sintered samples were still intact. These results demonstrate that the sintered-silver joints are more reliable than the soldered joints.