Segregation of Cu in Cu(Ti) alloys during nitridation in NH3

Cu(Ti 27 at.%) alloys on SiO2 were reacted in NH3 for 30 min over the temperature range 400-700 degrees C. Rutherford backscattering spectrometry in conjunction with high resolution transmission electron microscopy were utilized to investigate reaction products. At 400-450 degrees C, Ti is observed to segregate to the free surface to react with NH3, forming an Ti oxynitride layer. Above 500 degrees C, Ti segregates to both the free surface and to the alloy/SiO2 interface, leaving relatively-pure Cu layer. Reaction between Ti and SiO2 results in a TiO/Ti5Si3 bilayer structure. By use of high spatial resolution energy dispersive X-ray spectroscopy, the presence of a Cu-containing layer at the TiO/Ti5Si3 interface is observed. This layer may also contain Ti, Si and/or O. We propose a mechanism for Cu segregation to this interface which requires Cu diffusion across TiO and subsequent dissociation of Ti5Si3. Thermodynamic calculations support this mechanism.