2, Applied Materials, Santa Clara, California, United States
Transition metal disilicides which have tunable work-functions, are of significant interest as contact silicides. Moreover, selective deposition of silicides obviates the need for lithography onto complicated 3D structures on sub 10 nm 3D devices. Metallic tungsten (W) deposition on H-terminated Si in preference to OH-terminated SiO2 via a selective ALD process has been demonstrated using WF6 and SiH4 or Si2H6 by B. Kalanyan et al. In these processes, SiH4(g) or Si2H6(g) are considered a sacrificial reductant for W film by forming SiF4(g) and SiHF3 as byproducts: 2WF6(g) + 1.5Si2H6(g) à W(s) + SiHF3(g) + 3.5H2(g) + 2SiF4(g) + HF(g). Here, we demonstrated that sub-stoichiometric silicide, MoSix (x=0.4 – 1.1), can also be selectively deposited on a nanometer scale on H-terminated Si (001) but not on silicon oxide and silicon nitride using MoF6 and Si2H6.
X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical composition of MoSix at each experimental step. The growth rate of MoSix on Si was ~0.8 Å/cycle or higher thus even 5 selective ALD cycles are sufficient for deposition of contacts. By dosing extra Si2H6 pulses after 5 ALD cycles, more Si could be selectively inserted into the film. To confirm an in-situ selective deposition as well as the thickness of the film, MoSix was deposited on a sample patterned with SiO2 and Si3N4 and cross-section of the patterned sample was observed using tunneling electron microscope (TEM). From the TEM image, selectivity of MoSix on Si over SiO2 and Si3N4 was 100%. From in-situ STM, MoSix on Si had a conformal and atomically flat surface with root mean square (RMS) roughness of 2.8 Å. Post-annealing in a ultra-high vacuum at 500°C for 3 mins further decreased the RMS roughness to 1.7 Å. A 900°C spike anneal on the MoSix film under the 5% H2/N2 atmosphere showed RMS roughness of 4.75 Å measured by ex-situ AFM. A resistance of the 900°C annealed MoSix film was obtained using 4-probe measurement. The resistance of the film was determined to be ~500 µohm-cm using a sheet approximation model. The high selectivity achieved on patterned samples shows that this process should be applicable for selective contact silicide deposition on the nano scale.