CHARACTERIZATION OF INTEGRATED-CIRCUIT ALUMINUM BONDING PADS BY NANOINDENTATION AND SCANNING FORCE MICROSCOPY

The mechanical properties of thin films and surface modified layers ca n be measured by a variety of different techniques, with nanoindentati on being one of the most recent developments in this growing field. It is particularly suited to the characterization of bonding pads used t o connect individual microcircuits and chips. Such pads are commonly m ade of aluminium and have a thickness of approximately 1 mu m. By usin g a depth-sensing indentation method it is possible to obtain quantita tive values for the hardness and modulus, and thus gain better insight into the response of a bonding pad material to controlled deformation at such small scales. The nano hardness tester (NHT) is a recently de veloped instrument using an already established method where an indent er tip with a known geometry is driven into a specific site of the mat erial to be tested, by applying an increasing normal load. When reachi ng a preset maximum value, the normal load is reduced until partial or complete relaxation occurs. At each stage of the experiment the posit ion of the indenter relative to the sample surface is precisely monito red with a differential capacitive sensor, giving a load/displacement curve characteristic of the sample material. By consequently measuring the topography of the residual indent using a surface imaging techniq ue (e.g., scanning force microscopy) and combining this information wi th the indentation data, it is possible to gain a fuller understanding of the material response, as well as additional information on surfac e roughness and frictional properties, the latter being of particular importance in bonding applications. Experimental results are presented for a variety of integrated circuit bonding pads, these confirming th e ability of the NHT to becoming a common test in quality assurance an d process development of thin films and coatings for the microelectron ics industry. (C) 1998 Elsevier Science S.A.