Nanotechnology-Enabled Sensors

166 Chapter 4: Nano Fabrication and Patterning Techniques
Thermal CVD relies on high substrate temperatures and catalytic reactions
between precursors and substrate surfaces to achieve the dissociation
of input gases with binding energies in the 1 to 3 eV range. As a consequence,
thermal CVD deposition rates are relatively low (100 to
1000 Å/min). 46
A hot-wall CVD reactor set-up for the deposition of titanium diboride
(TiB2) is shown in Fig. 4.24. In the process, the titanium tetrachloride
(TiCl4) precursor is first produced in a pre-reactor, in which chlorine (Cl2)
gas is reacted with the titanium (Ti) metal to give TiCl4 gas:
Ti + 2Cl2 → TiCl4. (5.5)
In the CVD process, TiCl4, H2, and BCl3 gases react at the surface of the
heated substrate to form TiB2 thin films. The byproduct from this reaction
is HCl gas, which is removed through an exhaust. The chemical reaction of
this CVD process can be summarized as:
TiCl4 + 2BCl3 + 5H2 → TiB2 + 10HCl. (5.6)
Fig. 4.24 The CVD setup for deposition of titanium diboride.