Characterization of The Nucleation and Growth Process of CVD-W On TiN Substrates
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ABSTRACT The nucleation and growth characteristics of CVD-W on Ti/TiN barrier layers with SiH 4 and H 2 reduction chemistries are presented. In particular, the reaction between WF 6 (precursor used for depositing W) and the underlying Ti of the barrier stack was studied to better understand the chemistry of 'volcano' formation - a phenomena that causes severe defects in the deposited W film. Ti/TiN processing parameters and stack thicknesses were varied, along with the CVD-W deposition chemistry (gas flows, pressures, temperatures and times), to evaluate film properties and characteristics after SiH 4 passivation, nucleation and full W plug deposition. The analysis was augmented with cross-sectional Scanning Electron Microscopy (SEM) on short-loop testers and films were characterized using Rutherford Backscattering Spectroscopy (RBS), Secondary Ion Mass Spectroscopy (SIMS), X-ray Diffraction (XRD) and Auger Electron Spectroscopy (AES) techniques. Several processing conditions are recommended for 'volcano-free' and 'defect-free' CVD-W films.
INTRODUCTION Multilayered metallization schemes are used extensively in CMOS technologies for maintaining a low contact resistance and junction integrity. Due to the inherently poor step coverage properties of aluminium alloys (Al), other metallization technologies such as tungsten (W) are becoming popular in manufacturing environments. Tungsten provides a low specific contact resistance' and serves as a diffusion barrier to prevent Al spiking at junctions. The reduction of tungsten hexafluoride (WF 6) with hydrogen (H2) and silane (SiH 4) is as follows: 3H2 + WF 6 --, W + 6HF 3i54 + 2WF 6 -, 2W + 3SiF 4 + 6H 2
(1), (2)'
In general, WF 6 reduction with Sil4 results in a very fine and smooth surface structure, whereas that with H 2 chemistry produces rough film textures'. Most conventionally used barriers are alloys of tungsten/titanium (TiW) or titanium nitride (TiN)2 . Reaction (1) is known to provide conformal step coverage 3 . However, SiH 4 chemistry is usually used for nucleation on TiN substrates and to protect the underlying Ti from WF6 attack 4' 5. It has been reported that the reaction of WF 6 with H2 is dependent on both temperature and pressure 67' and the surface reaction is believed to be chemical reaction controlling step. Decomposition of WF 6 with SiH 4 on TiN substrates is mass transfer limited'. Deposition rate and incubation periods are dependent on the type of TiN films. Sputtered Ti/TiN bilayer stacks with a 20% to 30% step coverage seem to be adequate barriers. Barrier failures generally occur at the corners of the contacts where TiN step coverage 401
Mat. Res. Soc. Symp. Proc. Vol. 382 01995 Materials Research Society
is poor and cracking of TiN is often observed. Diffusion of SiH 4 and WF 6 through the TiN results in a vigorous reaction with the underlying Ti to cause defects in the W film deposition known as 'volcanoes'. At the window level, the dielectric is usually flowed or reflowed and Ti/TiN bilayer barrier is also subjected to a rapid thermal
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