PART III - Contamination of Aluminum Bonds in Integrated Circuits

Designers of semiconductor devices have been strivi,ng to resolve problems associated with Au-A1 alloys in bonded in.tercomzeclions. One approach now being- used is that of waintaining a physical seyav-atioz between the two metals in bond areas. This is accolrzplished by alunzincnz-plating a bonding area on the tips oJ the kovar leads and using alcminurn wires to join the senzicondictor device to the leads. The portion of the kovar lead which is on the externul side of the sealed package is gold-plated to provide an oxide-free surface for soldering or welding. A discoloration condition originally thought to be sinilar to purple plague, occuving in the yluled uluninur bonding area after package sealing, has been investigated to determine its efiects ipm bond integrity. Electron-micro-probe analysis determined that no1 only gold, but lead, zinc, and silicon were also present in the discolored area. A series of samples conlaining' conkrolled umonts of these inzpitrities weve prepared and subjected to a sil.zuluted sealing process. The investigations swcued that, of the contawiinants, only zinc toas detrinenlul to Lhe bond integily. The discoloration condition itself was found not to be detrimental to the bond integrity. DESIGNERS of semiconductor devices have been striving to resolve problems associated with Au-A1 alloys in bonded interconnections. One approach now being used is that of maintaining a physical separation between the two metals in bond areas. This is accomplished by aluminum plating a bonding area on the tips of the kovar leads and using aluminum wires to join the semiconductor device to the kovar leads. The portion of the kovar lead which is on the external side of the sealed package is gold-plated to provide an oxide-free surface for soldering or welding. Contamination as evidenced by discoloration of the aluminum-plated area was observed in a number of integrated circuits undergoing examination for defect characteristics which cause electrical failures.' This paper contains the results of an investigation to determine the nature of this discoloration, its cause, and its effect upon the integrity of the interconnection bond. I) THE NATURE AND EXTENT OF ALUMINUM-BOND CONTAMINATION The initial hypothesis in the investigation was that the discoloration was caused by reaction of the aluminum film with some unknown contaminants during the sealing of the hermetically sealed integrated-circuit flat package. The package is a rectangular ceramic container sealed with glass which surrounds the kovar leads as well as joining the top to the bottom. The seal is made hermetic by heating and cooling the package to devitrify the glass. In the case of the packages under investigation, the hermetic sealing had been accomplished with dry air as internal atmosphere. The apparent effect of contaminations as observed by microscopic examination was the formation of surface oxides having variations in color encompassing the whole spectrum of visible light. The contamination appeared to be related to one of the more notorious examples of these colorations, the so called purple plague.' In addition to purple plague, Fig. 1 shows the tarnish in the luster of the aluminized surface in the bond area which had been observed in many of the integrated circuits. To identify the contaminant in the bond area electron-probe microanalysis techniques were used.3 Fig. 2 shows the result of this analysis. The contaminants identified were gold, aluminum, zinc, lead, silicon, and cobalt. Fig. 2(a) is a back-scatter display of the area under study. The back-scattered electrons provide a general indication of the distribution of elements in the specimen surface. Elements with higher atomic number scatter more electrons back from the surface and are seen as light areas in the picture. The sample current, Fig. 2(b), is the amount of current conducted by the specimen as a result of electron-beam striking it and is an indication of element distribution. The Sample current is the reverse of back-scatter and complements it. Other pictures in Fig. 2 are produced by characteristic X-rays generated by the elements, allowing the isolation of the element of interest. The isolated element appears white and all other elements are dark. In this manner a comparative study provides a correlation between different surface areas and the elements which are in these areas. The area covered by the gold film, Fig. 2(c), shows that the boundary between the gold film and the kovar is not sharp as expected and that some sort of diffusion has taken place. Fig. 2(c) shows that some gold particles have been carried to the bond area and are in the proximity of the bonded wire in spite of the presence of a physical barrier in the form of the un-