A doubly interdigitated grating gates structure was incorporated into a GaAs-based high-electron mobility transistor to configure a plasmon-resonant emitter. Two dimensional electrons are then periodically confined in 100nm regions. The devices exhibit a plasma-wave signature under 1.5μm cw laser illumination. Two devices with different geometries have been subjected to an impulsive laser at room temperature. The authors observed clear generation of terahertz radiation from both devices.
There is growing interest in Cu wire bonding for LSI interconnection due to cost savings and better electrical and mechanical properties. Cu bonding wires, in general, are severely limited in their use compared to Au wires; such as wire oxidation, lower bondability, forming gas of N 2 +5%H 2 , and lower reliability. It is difficult for conventional bare Cu wires to achieve the target of LSI application. A surface-enhanced Cu wire (EX1) has been developed. It is a Pd-coated Cu wire and has many advantages compared to bare Cu wires. Stitch strength was much better under fresh conditions and maintained without any deterioration after being stored in air for a prolonged period of time. EX1 had a lifetime of over 90 days in air, although it was 7 days for the bare Cu wire. Spherical balls were formed with pure N 2 (hydrogen-free), whereas the bare Cu produced off-center balls. Cost-effective and secure gas, pure N 2 was only available for EX1. The reliability for Cu wire bonding under conditions of high humidity was investigated in pressure cooker test (PCT). The lifetime for EX1 and the bare Cu was over 800 h and 250 h, respectively. Humidity reliability was significantly greater for EX1. Continuous cracking was formed at the bond interface for the bare Cu wire, although there was no cracking for EX1. Corrosion-induced deterioration would be the root cause of failure for bare Cu wires in PCT. EX1 improves the bond reliability by controlling diffusion at the bond interface. The excellent performance of Pd-coated Cu wire, EX1 is comparable with Au wires and suitable for LSI packaging.
In order to clarify the reliability on the mechanical and electrical properties of the bond between Au wire and Al thin pad, corrosion behavior of Au-Al intermetallic formed at the bond interface in the molding resin were investigated. The bonds were annealed at several elevated temperatures, T (423-573 K). Typical degradation was recognized by a decrease in bond strength as well as a remarkable increase in electrical resistance. The molding resin has great influence on the corrosion. Bi-phenyl (BP) epoxy resin was found to cause the degradation approximately six times faster than o-cresole novolac (OCN) epoxy. Activation energies of the bond failure were 1.5 eV (T>450 K) and 2.0 eV (T<450 K) in BP resin and 2.3 eV in OCN resin.The corroded part was revealed to be Au4Al intermetallic phase formed in the bond interface. The growth rate of corroded layer was proportional to annealing time t, which indicated the corrosion behavior was not diffusion-controlled. Activation energies of the growth rate of corroded layer in the BP and the OCN resin were 1.6 eV, 2.3 eV, respectively, which were very similar to those of the bond failure.The corrosion reaction of Au4Al and bromide produced minute Au precipitation (fcc) and Al oxide formation. The Al oxide was identified to be amorphous by EDS and electron diffraction.
Two-dimensional (2-D) electron plasma in a submicron channel of a high-electron mobility transistor (HEMT) is excited by interband photoexcitation, resulting in performing the photomixing function. The injected photoelectrons modulate the total 2-D electron density, affecting the plasma resonant properties. The modulation depth of the density of 2-D electrons by the photoelectrons deeply relates to the resonant intensity and f r . This effect was modeled analytically in the 2-D plasma hydrodynamic equation. In order to validate the analytical calculation, the plasma-wave resonance was experimentally observed for a 0.15-µm gate-length InGaP/InGaAs/GaAs pseudomorphic HEMT in the terahertz range. At the modulation depth of 30%, the resonance was clearly observed with a double peak (the peak at 1.9/5.8 THz corresponding to the fundamental/third harmonic resonance). The resonant frequencies slightly shifted downward and the intensity attenuated with decreasing the modulation depth. Observed resonant frequencies support the analytical calculation.
Highly-alloyed Au-Ag bonding wire could be effective in saving material costs. We investigated the effects of Ag alloying on ball formation, bond strength and bond reliability. Even with high Ag concentration (∼50at%), ball was formed spherically. Bond strength and ball deformability were good enough for IC's assembling when concentration of Ag was less than 30at%. Thermal reliability of bonds between Au-Ag wire and Al pad had the unique dependence of Ag concentration. Ball bonds of Au-14at%Ag yielded significant degradation through annealed. On the contrary bonds of Au-24at%Ag provided as good reliability as a commercial pure Au wire after annealed at 473K-1000h. The bond reliability has the connection with intermetallic growth as well as diffusion behavior at the bond interface. The growth of intermetallics was different from that of pure Au/Al bonds. Optimizing the Ag concentration in the wire was effective in improving the bond reliability.
Pd coated copper (PCC) wire and Au-Pd coated copper (APC) wire have been widely used in the field of LSI device. Recently, higher bond reliability at high temperature becomes increasingly important for on-vehicle devices. However, it has been reported that conventional PCC wire caused a bond failure at elevated temperatures. On the other hand, new-APC wire had higher reliability at higher temperature than conventional APC wire. New-APC wire has higher concentration of added element than conventional APC wire. In this paper, failure mechanism of conventional APC wire and improved mechanism of new-APC wire at high temperature were shown. New-APC wire is suitable for on-vehicle devices.
Diffusion across the Al2O3 film in the Au/Al2O3/Al film system (the Al film with 1 μm thickness was vapor-deposited on a SiO2/Si substrate and exposed to the atmosphere to form a natural oxide layer, then the Au film was deposited on it.) at temperatures between 25° and 500° has been studied by using Auger Electron Spectroscopy, X-ray Photoelectron Spectroscopy and electrical resistivity measurement. The temperature where Al is detected on the surface of the Au/Al2O3/Al system is 100°C higher than that on the surface of the regular Au/Al system without the Al2O3 film. The activation energies for the intermetallic layer growth of the Au/Al2O3/Al system and the Au/Al system are 110 and 72 kJ/mol, respectively. The Al2O3 film formed by the exposure in air (ca. 3.2 nm in thickness) acts as a barrier for diffusion in Au/Al. In addition, we observed the SEM image of cross section of the Au/Al2O3/Al system. The Au-Al intermetallic layer is formed in the Al layer in the initial stage of Au/Al2O3/Al diffusion by Au diffusion through the Al2O3 film into the Al layer.On the other hand, we studied the effect of annealing environment on the diffusion for the Au/Al2O3/Al system by using 18O as tracer for SIMS analysis. The Au-Al intermetallic layer grows in an island formation for the Au/Al2O3/Al system and when annealed in air, the number of islands decreases. Because, during heat treatment in air, the Al2O3 film is formed continuously by supply of O2 to the Al2O3 film through Au film.
