Masahiro Koizumi

Reliability of thick Al wire bonds in IGBT modules for traction motor drives

Reliability enhancement of thick Al wire bonds during thermal fatigue test has been investigated from a metallurgical viewpoint. Al wire bonds degrade with the increase of crack length during thermal fatigue tests with high /spl Delta/T/sub j/ due to the tensile stress generated by the thermal expansion coefficient mismatch between Al wires and Si. It is also found that cracks propagate along the small grain boundaries of Al wires at the bonding interface. It is predicted that the Al wire bonds may not degrade due to thermal fatigue if /spl Delta/T/sub j/ is controlled below 40 K, i.e., keeping it within the actual temperature fluctuation range in IGBT modules for traction motor drives. The reliability of Al wire bonds can be enhanced by increasing the grain size of the Al wire at the bonding interface. The high temperature bonding is considered to be a good candidate for enhancing the reliability of Al wire bonds.

Investigation of the Reliability of Copper Ball Bonds to Aluminum Electrodes

The reliability of Cu/Al bonds was compared with that of Au/Al bonds to establish reliable copper ball bonding for ICs. Diffusion-controlled intermetallic compound phases of CuAl and. CuAl 2 were identified in. the interfaces of the Cu/Al bonds after isothermal aging by micro-X-ray diffraction. In addition, diffusion-controlled compound phases of Au 4 Al, Au 5 Al 2 , Au 2 Al, AuAl, and. AuAl 2 were identified in the. interfaces of Au/Al bonds. The activation energies for the formations of the compounds are 1.26 eV for Cu/Al and 1.0 eV for Au/AI bonds, respectively. The ball bonding strengths after aging of both systems are lowered as the compound layer thickncss is increased. The critical thickness of the compound layer corresponding to the degradation of bonds is about .2 µm for CU/Al and 7 µm for Au/Al bonds, respectively. The estimated degradation times for Cu/Al and Au/Al bonds at 150°C are 4 x 104h and 1 x 104h, respectively. Cu/Al bonds are little affected by resin, while Au/Al bonds are strongly influenced by resin. It was suggested that Cu/Al bonds possess higher reliability than Au/Al bonds. The reliability tests of resin-molded ICs using copper ball bonding have given results which are at least as satisfactory as those from conventional gold wire

Influence of ball-forming conditions on the hardness of copper balls

Effect of ball‐forming conditions on the microstructure and the hardness of balls has been investigated in order to obtain soft balls suitable for a reliable ball‐bonding process using copper wire. Copper balls formed in a room‐temperature shield gas are found to be harder by 6 Hv than fully annealed copper wires of the same purity. This is caused by many dislocation loops which are generated in balls due to inclusion of gaseous impurities from the atmosphere and due to rapid solidification. On the other hand, copper balls formed in a shield gas heated above 175 °C are found to be softer by the amount of 4.5 Hv than balls formed in a room‐temperature shield gas. This is due to disappearance of dislocation loops caused by the reduction of solidification rate and by elimination of gaseous impurities from the balls during solidification. Impurities, especially oxygen, are found to have a strong influence on the hardening of copper balls. From these results, we conclude that copper balls formed in a shield ga...

Polarization and wavelength insensitive MQW electroabsorption optical gates for WDM switching systems

We propose using MQW electroabsorption (EA) modulators as optical gates in wavelength-division-multiplexing (WDM) switching systems. A fabricated MQW-EA gate with integrated waveguides showed a high extinction ratio (>30 dB), a low polarization-dependent loss (0.3 dB), and a low wavelength-dependent loss (1.1 dB) within the gain band (1545-1560 nm) of erbium doped fiber amplifiers (EDFAs). Ultra-high-speed (<40 ps) switching of a WDM signal was demonstrated.

High-reliability interconnections for ULSI using Al-Si-Pd-Nd/Mo layered films

An Al-Si-Pd-Nb alloy and a bilayered interconnection using this alloy with molybdenum have been investigated for ULSI interconnections. The electromigration lifetime of Al-Si-0.3 wt.% Pd-0.4 wt.% Nb was 5 times better as compared to Al-Si-0.5 wt.% Cu. In addition, layering this alloy with low-resistivity molybdenum improved the electromigration resistance considerably as compared to Al-Si-Cu layered with a high-resistivity metal, i.e., TiW. PdO was thought to be formed on the Al-Si-0.3 wt.% Pd-0.4 wt.% Nb alloys surface. The corrosion resistance of this alloy is much better than that of Al-Si-Cu because of this PdO. The ease in patterning the alloy at submicrometer linewidths (to 0.5 mu m) is quite satisfactory. The Al-Si-Pd-Nb/Mo layered system is therefore thought to be promising for future interconnection applications requiring durability against high current densities. >

High-reliability interconnection formation by a two-step switching bias sputtering process

Abstract A two-step switching bias sputtering method which can considerably improve both step coverage and electromigration resistance compared with that attainable by conventional d.c. and d.c. bias sputtering has been developed for the formation of reliable interconnections for LSIs. One-step switching bias sputtering consists of alternating operations of d.c. and d.c. bias sputtering. This method features two-step bias, which consists of deep bias followed by shallow bias sputtering. The first bias enhances step coverage of walls by resputtering Al films at the base of the holes. The second bias removes the contaminated Al layer using Ar. Al films of good quality and electromigration resistance values comparable with those attained by conventional d.c. sputtering can be formed by reducing the cyclic number of alternating operations of two-step switching bias sputtering from 18 to 1. The step coverage of Al films at small contact holes by this method is three times higher than that obtained by conventional d.c. sputtering.

An Embrittlement Mechanism of Impact Fracture of Sn–Ag–Cu Solder Joints on BGA Using Electroplated Ni/Au Surface Finishes

The impact toughness evaluation and fracture mechanism analysis in board level of Sn-3 mass%Ag-0.5 mass%Cu solder joints of ball grid arrays (BGAs) using electrolysis Ni/Au plating were performed. The cause of impact toughness degradation of BGA solder ball joints is the segregation of impurities to the (Cu, Ni)6Sn5 intermetallic compound grain boundary formed in the solder joints. The impurities, consisting of Cl and organic matters, are taken in the Ni plating film at the time of Ni plating. The organic matter impurities come primarily from the solder mask of the BGA interposer substrates. To improve the impact toughness of the Sn-3 mass%Ag-0.5 mass%Cu solder joint of the BGA, it is necessary to lower the concentration of these impurities. This, in turn, places importance on solder mask material selection (to minimize Ni plating bath contamination) as well as contamination prevention and plating bath sanitization.

Investigation on enhancement of copper ball bonds during thermal cycle testing

Copper ball bond failures of resin molded ICs during thermal cycle test are found to be caused by crack propagation along coarse grain boundaries in the roots of balls. An investigation of the critical grain size leading to the wire break in the roots of ball bonds during thermal cycle and of the ball-forming conditions to obtain a grain size of less than the critical value is discussed. Grain size less than 10 mu m in the roots of balls is essential to enhance the reliability of ball bonds. The appropriate grain size is obtained if the optimum bonding condition is employed. >

Investigation of Aluminum Ball Bonding Mechanism

The effects of various factors, e.g., ball hardness, amount of ball deformation, thickness, and surface cleanliness of the aluminum electrode, which can influence bonding strength were investigated. Metallurgical bonding between balls and electrodes occurs by rupturing the oxide film existing on the aluminum electrodes and proceeds from outside, inwards to the center of the bonds. The bonding strength is found to be a linear function of the true bonded area between balls and electrodes. The factors increasing the true bonded area are a high ball hardness, a large amount of deformation, a thick aluminum electrode, and a clean electrode surface. In addition, a high strength ball bonding comparable to gold can be realized by using aluminum alloy wires containing magnesium.

The Embrittlement Mechanism and Improvement of Impact Strength for Lead-Free Solder Joints in BGA Packages Using Electrolytic Ni/Au Plating

Impact strength evaluation and fracture mechanism analysis in board level of Sn–3mass%Ag–0.5mass%Cu solder joints of ball grid arrays (BGA) using electrolytic Ni/Au plating were performed. The cause of impact strength degradation of BGA solder ball joints is the existence of low density defects, which contain organic materials, in the (Cu,Ni)6Sn5 intermetallic compound grain boundary formed in the solder joints. These organic materials are taken in by the nickel plating film at the time of nickel plating. To improve the impact strength of the Sn–3mass%Ag–0.5mass%Cu solder joint of the BGA, it is necessary to lower the concentration of these organic materials. The contamination prevention and nickel plating bath sanitization, solder mask material selection (to minimize nickel plating bath contamination) and higher current density of nickel plating are effective to keep a lower concentration of organic materials in nickel plating film.