Microstructure and diffusion in Nb/NbSn 2 metal bonding structure
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This communication reports on the microstructure and interdiffusion observed in a new NbSn2/Nb metallization structure on Si02, previously reported [H. S. Chen et al., Appl. Phys. Lett. 66, 2191 (1995)l. The as-deposited NbSn2 layer was found to be amorphous. After heating, the NbSnz becomes polycrystalline and heavily diffused with Au from the Au-Sn solder. The Nb layer remains pure and intact after heating. The microstructure, compositions, and phases of the Au-Sn solder layer are also presented.
A recent paper' reported on a Nb-based metallization for possible use as a bonding structure for semiconductor devices and integrated circuits. In particular, using Au-Sn solder with a NbSn2/Nb metallization was shown to have potential for bonding integrated circuit dies to submounts. This previous study showed that the Nb layer has excellent adhesion to submounts (Si02 on Si and CVD diamond), and that is a perfect diffusion barrier to Au and Sn. The advantages of this simple and economical bonding structure were shown by comparing it to existing more complex metallization schemes. This communication gives the results of a more extensive study of the microstructure and interdiffusion in the bonding structure A U ~Sn29/Sn/NbSn2/Nb. I The metallization films were deposited by dc magnetron sputtering, consisting of three sputtering targets and an ion-beam gun. The substrates were cleaned by ion-beam sputtering prior to deposition. The as-deposited I metallization structure studied was as follows: A U ~SnZ9 (3.0 ,um)/Sn (500 A)/NbSn;? (1000 A)/Nb (1000 A) on Si02/Si. Heating of the films was done in a differential scanning calorimeter under pure nitrogen. The samples were heated at a scan rate of 20 "C/min to 320 "C, held at that temperature for 1 min, then cooled at 10 "C/min to 200 "C. A designated heating time of 1 + 1 min means the sample was heated a second time to 320 "C to determine if any new reactions had taken place. Heating for short times simulates the nonequilibrium situation used in the die-submount bonding process. Some reflown samples were also studied. The reflow process is where two samples are placed solder-to-solder and heated as above to bond the samples together, as would be done in the bonding of a die to a submount. Also studied was a solid-state reaction sample where the heating was done at 265 "C for 20 h. The determination of the microstructure and phases present was done by transmission electron microscopy and diffraction (TEM and TED), convergent beam electron diffraction (CBED), scanning electron 2988
J. Mater. Res., Vol. 10, No. 12, Dec 1995
http://journals.cambridge.org
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microscopy (SEM), and x-ray diffraction (XRD). To determine the interdiffusing species and composition of phases, elemental microprobe analysis was done by x-ray energy dispersive analysis (XEDA). In our previous work' it was shown that the Nb layer bonds strongly to S O 2 . High magnification images showed a sharp interface between the Nb and S O 2 . In the present study, it was found, by using XED
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