Tomotake Niizeki

Drop-on-Demand Laser Sintering With Silver Nanoparticles for Electronics Packaging

This paper proposes a “dry” laser-sintering method and discusses characteristics of a laser-sintered silver thin film on a polyimide or a copper substrate. This novel technology consists of the following processes: first, ink-jet printing of metal nanoparticles with dispersants and solvents for minute patterning; second, short preheating to remove organic substances in the ink; and finally, millisecond-order laser-beam irradiation under atmospheric conditions with the flow of argon gas for metallization. Regarding the wiring, visible lasers with high absorption on the ink develop rapid metallization and activate solvent evaporation, resulting in a rough surface with large pores. Interface adhesion is increased by the anchoring effect in the course of laser irradiation. In contrast, near-infrared lasers with low absorption heat the ink from the polyimide interface, yielding a dense, low-specific-resistance structure. Regarding pad formation on the copper leadframe without any surface pre-treatments, interdiffusion takes place at the Ag/Cu interface and increases adhesivity. The structural quality of the laser-sintered silver pad is almost the same as that of an electroplated one, so that no difference in good wire-bondability is obtained when the near-infrared continuous-wave laser is irradiated for a short time of a millisecond order per lead.

Laser sintering of Ag nanopaste film and its application to bond-pad formation

A novel coating technology by means of sintering the silver NanoPastereg by a laser beam has been proposed. The novelty lies in the use of a conventional NdYAG laser to metalize nanoparticles as an alternative to furnace sintering. Silver nanoparticles with 5 nm in average diameter dispersed in the the silver NanoPaste are successfully sintered on the Cu substrate by the proposed method. Laser sintering proceeds from the paste surface into the substrate, and the thickness of a peeling-free film is around 0.2 mum. Multi-step sintering by the repletion of spin coating and laser metallization enables us to make a thicker Ag pad of around 1 mum. No peeling of the laser-sintered Ag film from the substrate was observed. Its adhesive strength is higher than that of a furnace-sintered sample. The Ag film thus fabricated can be used as pads for wire bonding, being an alternative to electroplating.

Packaging of electronic modules through completely dry process

In order to establish technology of packaging electronic modules, we investigated conditions for laser sintering of Ag nanoparticles, and evaluated characteristics of the sintered film. First, we plotted minute patterns on a copper substrate by ink-jet printing, and then employed an NdYAG laser to metalize the nanopaste in a short time. The Ag nanoparticles (5 nm in average diameter) dispersed in organic solvents were solidified to form coarse agglomerates of about 0.05-0.5 mum with a pulsed laser, or about 0.05 mum by CW mode. We carried out a bend-peel test to find that no separation occurred at the interface between the sintered Ag film and the substrate. Adhesive strength of the laser-sintered pattern on the Cu substrate is higher or equal to than that obtained by furnace sintering. An SIM observation of FIBed cross-sections revealed that the laser-sintered film is as thin as less than 0.5 mum, and has a porous structure. As for wiring a polyimide substrate, the use of water-repellant is indispensable for ink-jet printing. Three-step laser sintering enables us to make Ag wires on the polyimide film at a low laser power, which leads to less thermal damage to the substrate.

Metal-ligand interactions in perturbed blue copper sites: a paramagnetic (1)H NMR study of Co(II)-pseudoazurin.

Abstract. Pseudoazurin is an electron transfer copper protein, a member of the cupredoxin family. The protein is frequently found in denitrifying bacteria, where it is the electron donor of nitrite reductase. The copper at the active site is coordinated to His40, Cys78, His81 and Met86 in a distorted tetragonal geometry. We have recorded and assigned the 1H NMR spectra of Co(II)-substituted pseudoazurin from Achromobactercycloclastes. The 1H NMR spectrum of Co(II)-pseudoazurin closely resembles that of Co(II)-rusticyanin, reflecting an altered conformation for the Met-Co(II)-Cys moiety in both proteins, compared to Co(II)-azurin, amicyanin and stellacyanin. The electron spin density onto the Sγ(Cys) is larger in Co(II)-pseudoazurin compared to Co(II)-rusticyanin. Instead, the Co(II)-Met interaction is similar in both derivatives. Hence, the different metal-ligand interactions might be independently modulated by the protein structure. The present work also shows that the electron spin density onto the Co(II)-Scys bond is sensibly smaller than the Cu(II)-Scys. Notwithstanding, NMR data on Co(II)-substituted blue copper proteins can be safely extrapolated to native Cu(II) proteins.

Influence of wavelength on laser sintering characteristics of Ag nanoparticles

The purpose of this study is to develop a dry process for minute wiring or interconnections without any wet process such as chemical etching and electroplating. The process is based on the use of metal nanoparticles and their metallization by laser sintering, which will bring a desktop manufacturing of electronic circuits in view. The present paper investigates the influence of wavelength on laser sintering characteristics of Ag nanoparticles, in which a CW Nd:YAG laser (1064 nm in wavelength), a CW laser diode (980 nm), a CW green laser (532 nm) and a CW Ar+ laser (488 nm) are employed. Sintering mechanism as well as adhesion to a polyimide substrate is extensively investigated by FIB observation and tensile-shear testing. As a result, the lasers with visible wavelength are preferable from the viewpoint of laser output and adhesion strength because of a high absorbance of light in the NanoPaste® containing Ag nanoparticles, a dispersant and solvents. On the other hand, the near-infrared lasers yield a denser sintered structure, which leads to lower specific resistance, although it is a couple of times high compared to furnace curing.

Laser Sintering of Silver Nanoparticles for Electronic Use

The silver NanoPaste® having silver nanoparticles with 5 nm in average diameter, coated either on a polyimide substrate or a copper one, is successfully sintered with CW lasers. A rapid metallization from the paste surface with the visible laser makes the evaporation of solvent and dispersant difficult, resulting in an insufficient sintering with large pores. In contrast, the near-infrared laser with a little absorption in the paste heats the substrate first, and then develops metallization up to the paste surface, so that an easy evaporation makes the structure denser. No peeling was observed at the interface of the laser-sintered Ag film and the substrates. Its adhesive strength is nearly equal to that of the furnace-sintered sample.

Temperature soak reliability of laser-sintered Ag pads for wire bonding

A small amount of Ag nanoparticles (5 nm in average diameter) was supplied to the tips of Cu lead-pins by ink-jet printing, and then metallized by laser irradiation to form circular wire-bonding pads between 2 and 3 µm thick and φ150 µm. A continuous-wave Nd:YAG laser (1064 nm in wavelength and 150 W in maximum power output) was employed in an argon atmosphere. A φ25 µm gold wire was bonded between the adjacent pads using an ultrasonic wire-bonder. The wire-bonded parts were placed in an atmospheric electric furnace for temperature soak tests in dry conditions at 150°C for up to 1000 h. Metallographic changes in the junctions were investigated by SEM, FIB, and EDX, while mechanical strength was examined by pull tests. No metallographic changes were observed in the 500 h specimen, whereas a slight solid-phase diffusion took place at both Cu/Ag and Ag/Au interfaces in the 1000 h specimen. The Ag pad formed on the Cu lead, above 2 µm in thickness, also prevented the degradation of pull strength; an average of 8.8 cN was maintained at both soak times. No separation from the first (ball) and second (wedge) bonds occurred, but the wire fractured close to the hook of the tester.

Size effect of Ag nanoparticles on laser sintering and wire bondability

The present research aims to develop a dry process for forming wire-bonding pads on a Cu leadframe, where the dry process is defined as the use of the paste with metal nanoparticles and solvents and its laser sintering under air or argon atmosphere. In this report, the influence of Ag particle size on laser sintering and wire bondability is extensively investigated. Two types of Ag nanoparticles paste, φ 5 nm or φ 100 nm in average diameter, were coated on a Cu leadframe. Then, a focused near-infrared CW laser once scans the coated lead to make the paste metalized. Wire bondability between an Au wire and the Ag pad is examined by a pull test. Mechanisms of sintering as well as adhesion to the Cu substrate are discussed. Finally, these characteristics are compared with those of an Ag electroplated pad. As a result, the laser-sintered functional film using the φ5-nm-particle paste yields a crystallized structure and wire bondability similar to those of electroplating, whereas a porous structure with lower wire bondability tends to be produced by the φ100-nm-particle paste.

High-speed laser plating on Cu leadframe using Ag nanoparticles

The present paper proposes high-speed laser plating for forming wire-bonding pads on a Cu leadframe using Ag nanoparticles. Various aspects of the proposed method have been investigated, including the shape and surface roughness of the lead, experimental set-up, multistep printing, laser-plating parameters, quality of the sintered film with FIB-SIM and LSM, and wire bondability between the Ag pad and an Au wire with a pull test. Experimental results with Ag nanoparticles were compared with those of furnace curing and electroplating. As a result, superiority of the laser plating process has been confirmed from the viewpoints of sintering time (millisecond order per lead), material consumption (picoliter order), the necessity of pre- and post-processing, wire bondability, thermal damage to the pad and substrate, and environmental protection. In particular, novelty lies in the implementation of drop-on-demand laser plating on the specially designed round lead.

UV resonance Raman and NMR spectroscopic studies on the pH dependent metal ion release from pseudoazurin

Abstract UV resonance Raman (UVRR) and 1 H NMR spectra are measured for native Cu(I)– and Cu(II)–pseudoazurin, its apo-protein, and a few metal-substituted derivatives. The pH titration experiments of 1 H NMR enabled us to determine the p K a * values of three His residues (His6, His40, and His81). The UVRR band characteristic of a metal coordinated histidyl imidazole was observed at 1385 cm −1 for Cu(II)–pseudoazurin in D 2 O but not for Cu(I)–pseudoazurin. This frequency is consistent with the N δ coordination of His. For the Cu(I)–pseudoazurin a characteristic band of histidyl imidazolium was detected at 1408 cm −1 at acidic pH. This is assigned to protonated His81, which is deligated from Cu(I) at low pH values. The imidazolium Raman band at 1408 cm −1 was also detected in the UVRR spectrum of apo-pseudoazurin at pH* 3.9, and the acidification was accompanied by a significant change in the X-Pro bands at 1467 cm −1 . Pseudoazurin substituted with Zn 2+ and Cd 2+ gave the characteristic Raman bands of the metal coordinated imidazole at 1388 and 1384 cm −1 , respectively, at neutral pH, but its intensity diminished upon lowering the pH, and instead the imidazolium band at 1408 cm −1 grew. The X-Pro bands of the pseudoazurins substituted with Zn 2+ and Cd 2+ exhibited the pH dependence very similar to that observed for apo-pseudoazurin. These findings indicate that the Zn 2+ and Cd 2+ ions are released from the active site at acidic pH and it is accompanied by a change in hydrogen bonding state of Pro80. This behavior is clearly absent in both the Cu(I) and Cu(II) proteins, meaning that pseudoazurin discriminates between copper and the other metal ions.