USE OF AUGER AND OTHER SURFACE ANALYSIS TOOLS TO IDENTIFY FAILURE MECHANISMS IN THIN FILMS USED IN TAB AND OTHER DEVICE PACKAGING
Author: Ronald S. Nowicki Company: Integrated Processing Date Published: 2/2/1999
Pan Pacific Symposium
Abstract: There has been an assumption that thin and thick film metallizations remain functionally and metallurgically stable after extended storage at room temperature. Some examples of these are Chrome/Gold [1-3], Molybdenum/Gold , Titanium-Tungsten/Gold [5-7], Titanium Nitride/Gold [8-10] and Chrome/Copper/Chrome  for thin film devices, and electroless-plated Nickel+Gold  for other device types. Since these metallizations conform to the normal time and Arrhenius temperature dependencies involved in diffusion phenomena, it will be shown that such systems can yield device failures due to interdiffusion after only a few years exposure to room temperature. With elevated temperature exposure, the failure rates will be accelerated. For example, the Titanium-Tungsten/Gold metallization on aluminum, frequently employed in Tape Automated Bonding (TAB) packages can yield bonding and adhesion failures in only three years at room temperature due to grain boundary interdiffusion. Such interdiffusion has been found to exhibit energy-of-activation values as low as 0.6 eV . Temperature excursions above room temperature were found to accelerate the diffusion, wherein the “barrier metal" can diffuse to the surface of the gold resulting in low strength bonds or poor wetability/solderability, or adhesion failures of the plated gold "bumps". Chrome/Gold can behave similarly. In this paper, it will be shown that these metallizations can be rendered more robust by the introduction of impurities such as oxygen and nitrogen, which are thought to reside preferentially in the grain boundaries of these films, thus blocking the diffusion pathways. In the case of electroless nickel on aluminum, reduction in the phosphorus content in the nickel can result in diffusion of the aluminum to the nickel surface, where the aluminum can oxidize to yield poor solderability of the nickel. Similarly, Kovar, or Beam lead" parts which have been tin or gold plated can exhibit poor solderability when such interdiffusion occurs. The use of surface mounted parts (SMT) having such metallization combinations can also yield adhesion failures or poor solderability due to such interdiffusion. The limitations of typical investigative protocol, i.e. SEM/EDAX, employed by many companies in "failure analysis," will be presented, followed by a review of the more sensitive and definitive analytical techniques currently available.