The Surface Morphology of Titanium Nitride / Copper Bilayers Annealed at High Temperatures
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ABSTRACT Bilayers of 40 nm titanium nitride and 500 nm copper were sputter deposited on oxidized silicon substrates. The films were annealed for one hour at temperatures from 4000 C to 8000 C in either vacuum or Ar/H, ambients. Neither x-ray diffraction or Rutherford backscattering spectrometry indicated any interaction between the titanium nitride and copper films. While no differences in surface morphology were detected at annealing temperatures of less than 5000 C, above this temperature a strong dependence on annealing ambient was found. In the vacuum annealed samples, surface grooves formed at the grain boundaries of the copper film. These grooves became progressively larger as the annealing temperature was increased, eventually leading to void formation. Only small grain boundary grooves were present in the Ar/H, annealed films regardless of annealing temperature. The driving force for the formation of the grooves is believed to be a difference between the grain boundary and surface energies in the copper films. It is speculated that in Ar/H, anneals this driving force is reduced because hydrogen segregates to the copper grain boundaries and lowers the grain boundary energy. The formation of grooves was found to retard copper grain growth and texturing. Resistivity of the copper film was also found to increase if the void density was sufficiently high.
INTRODUCTION
The implementation of copper interconnections in multilevel metallization schemes requires the development of a stable diffusion barrier to prevent copper from diffusing through the SiO, dielectric layers during high temperature processes. Titanium nitride is widely used in current aluminum and tungsten based interconnections as an adhesion promoter, diffusion barrier and anti-reflective coating. Since deposition equipment is also widely available, it is a leading candidate for use in first generation copper technologies. Several investigators have reported on the effectiveness of titanium nitride as a diffusion barrier for copper'- 34 . However, extended anneals above 5000 C were not performed, and the surface morphology of the copper film was not investigated. The deposition technique for creating the titanium nitride films also varied widely.
EXPERIMENTAL In this study, 40 nm films of stoichiometric titanium nitride were reactively sputtered on oxidized silicon wafers in a Varian 3190 sputtering system. The argon partial pressure was 4.2 mTorr, while that of the nitrogen was 0.9 mTorr. The power applied to the titanium target was 1800 W. The wafers were held at 400' C with an applied substrate bias of -200 V. Prior to the 451 Mat. Res. Soc. Symp. Proc. Vol. 318. @1994 Materials Research Society
deposition of copper, the titanium nitride films were annealed in a N/0 2 ambient for 20 minutes at 450' C. At temperatures below 10000 C, diffusion through titanium nitride occurs almost exclusively through high diffusivity paths such as grain boundaries and dislocations4'5 . This anneal was designed to form titanium oxide in these paths, thereby dr
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