Effect of Nitrogen Flow on the Growth of Nitrogen Ultrananocrystalline Diamond (N-UNCD) Films on Si/SiO2/HfO2 Substrate

Abstract

This paper describes initial R&D focused on growing ultrananocrystalline diamond films (N-UNCD) with nitrogen (N) atoms incorporated in grain boundaries chemically reacted with C atoms dangling bonds and providing electrons for electrical conductivity. The N-UNCD films are grown on a thin layer of hafnium dioxide (HfO2), to explore the integration of N-UNCD films with the main gate oxide in current CMOS devices. The HfO2 template layer was grown by atomic layer deposition (ALD) on top of a 300 nm layer of silicon dioxide (SiO2) on a silicon substrate. The N-UNCD films are grown using the hot filament chemical vapor deposition (HFCVD) technique. A mixture of Ar/CH4/H2/N2 gases pass through an array of filaments heated to ~2300 °C to crack the CH4 and N2 molecules into C, CHx (x=1,2,3) and N atoms. The radicals react at the surface to grow the N-UNCD. The N-UNCD film density, morphology, and presence of N atoms, which induce electrical conductivity (resistivity), appears to depend mainly on the N2 flow, thus density of N atoms arrival to the substrate surface in conjunction with the film growth temperature. In previous work it was fond that a carbide layer is form beneath the UNCD for both Si, tungsten (W) and Hf. However, when N is added to the gas flow it reduces the coverage of UNCD over the HfO2 layer but not over the SiO2 layer. Large N2 flows (10-20 standard cubic cm, sccm) result in N-UNCD films with globular non-connected structures, resulting in high resistivities (several MW-cm to open circuit). X-ray Photoelectron Spectroscopy (XPS) analysis revealed the presence of both hafnium carbide (HfC) and hafnium nitride (HfxNy) on the surface of N-UNCD films grown on HfO2. The formation of HfxNy may compete with C for surface binding sites on the HfO2, inhibiting the formation of a HfC layer. There are several candidates of hafnium nitride (Hf3N2, Hf3N4. and HfN). Since the starting substrate material is hafnium(IV) oxide, and there is no oxygen flow during the N-UNCD film growth, the hypothesis is that, with large N2 flow (10-20 sccm), the formation of hafnium(IV) nitride is dominant. Hafnium(IV) nitride has an orthorhombic unit cell while hafnium (IV) carbide has a cubic unit cell structure, similar to diamond. Thus, the presence of Hf3N4 may inhibit diamond growth via a mismatch of unit cells at the interface. On the other hand, a low flow of N2 (6 sccm) combined with low flows of H and Ar, produced, good dense, very low resistivity N-UNCD films.