Study Of The Reliability And Related Issues Of Wire Bonding For High Temperature Applications

In this thesis, Pd wire and new bonding pad metallizations: Pd/Ti, Au/Ti film, on different substrates: Si, SiO2/Si and A12O3, were chosen as new wire bonding material-systems to replace the conventionally used Au/Al system for high temperature application. The high temperature (>=300 ) reliability and related issues were studied by the in-situ resistivity measurement, mechanical strength tests, and some micro-analysis tools (e.g., SEM, AES, SIMS and Rutherford Backscattering). The main results are summarized as follows:1. The resistivity and mechanical strength change of Pd wire annealed at about 300 under different ambient were studied. The results showed that the mechanical and electronic properties of Pd wire at high temperature were quite stable and suitable to be used as bonding wire under high temperature environments.2. The interdependency of flowing N2 and resistivity change of Pd wire at hightemperatures (about 300 ) was also studied. The results showed that the N atoms would diffuse into the Pd wire and cause the resistivity to increase. The relation of resistivity change vs. time can be fitted well by a bulk-diffusion model with infinite-long cylinder geometry, the derived apparent diffusion coefficient vs. temperature matches the Arrhenius plot well, and the apparent activation energy for this process was derived to be 1.3ev. The tensile test results showed that the mechanical properties of Pd wire annealed in flowing N2 is better than those in air. This result overthrows thetraditionally accepted conclusion: nitrogen is insoluble in Pd below 1400 .3. High temperature reliability of multilayer Au/Ti and Pd/Ti films used as bonding pads was also studied. Results are as below:(1) Strong diffusion of Ti atoms in Au film and an obvious increase of the in-situ resistivity will occur when the film is annealed at 300 . The resistivity change can be fitted well by using a one-dimension infinite-long plane bulk-diffusion model. When the Au film is thicker, the oxidation of Ti atoms will not accelerate the diffusion of Ti atoms in Au film when annealed at 300 in air, but retard the diffusion process. So the diffusion speed of Ti atoms in Au film is faster in flowing Ar than that in air, and this result overthrows the conclusion of some references that the oxidation of Ti atoms will speed the diffusion of Ti atoms in Au film.(2) The resistivity of the Pd/Ti multiplayer film changes very slightly when annealed at 300 showing that the intermetallic film between the Pd/Ti film formed by interdiffusion will retard the further interdiffusion of Pd and Ti atoms. The reason why the resistivity changes more (about 3%) in air than in flowing Ar may due to the oxidation of Ti.(3) When the Pd/Ti film annealed in flowing N2, like the case in Pd wire, the N atoms will diffuse into the Pd film and cause the resistivity increasing, also the diffused N atoms may enhanced the diffusion of Ti atoms in Pd film. This result is the first time verifying the unverified inference that the inactive ambient N2 will diffuse into the noble metal Pd and enhance the diffusion of Ti atoms in Pd.Also, the resistivity change of Pd/Ti film is less than that of Au/Ti film may show that the high temperature reliability of Pd/Ti film is better than that of Au/Ti film.4. The bonding parameters of Pd wire on selected pads were optimized bymonitoring the pull strength and bond width. The results showed that good bondability could be achieved by using the optimized parameters. And the pull test results were quite uniform, with little scattering, which is essential for a high assembly yield. After high temperature aging, the bond strength will decrease, but was still high enough to meet the reliability requirement.5. The failure mechanism of Pd wire bonded on these selected pads annealed at even higher temperature was studied. Results showed:(1) The interdiffusion between different metal layers and the substrate plays a predominate role for the reliability at higher temperatu...