ALD/ALE 2021 Session EM8: Nanolaminates

The ZrO₂-Co₃O₄ 5-layer nanolaminates were grown by atomic layer deposition at 300 ℃ from ZrCl₄ and Co(acac)₃ as metal precursors and O₃ as the oxygen precursor. The Fe₂O₃-BiOCl two-layer thin-film structures were deposited from FeCl₃, BiCl₃ and H₂O at 375 ℃.

The results showed that the multilayers of both studies were crystallized in the as-deposited state. In the ZrO₂-Co₃O₄ 5-layer nanolaminates, the cubic and monoclinic ZrO₂ phases and cubic Co₃O₄ phase were present. Orthorhombic ε-Fe₂O₃ and tetragonal BiOCl phases were detected in the case of Fe₂O₃-BiOCl two-layer thin-film structures. In Table I, selected multilayer structures, their total thicknesses and measured coercivity values are presented. Magnetic hysteresis loops showed nonlinear and saturative magnetization and measurable coercivity, indicating ferromagnetic-like behaviour at room temperature. The highest coercivity value measured was 9757 Oe (776.4 kA/m), which characterized one particular Fe₂O₃-BiOCl two-layer thin-film structure (Table I). SEM images of this type of multilayer structure are displayed in Figure 1. In the case of the ZrO₂-Co₃O₄ nanolaminates, the coercivity values were 21 and 32 Oe (1.7 to 2.5 kA/m) (Table I). All five layers of the nanolaminates were clearly distinguishable (Figure 2).

[1] H. Seemen, K. Kukli, T. Jõgiaas, P. Ritslaid, J. Link, R. Stern, S. Dueñas, H. Castán, A. Tamm, Properties of atomic layer deposited iron oxide and bismuth oxide chloride structures, J. Alloys Compd., 846 (2020) 156099.

Atomic Layer Deposition (ALD) of ternary TiSi_xN leads to nanocomposites of metallic TiN atomically mixed with insulating Si₃N₄. Formulating TiSi_xN films with various Ti:Si ratios lead to the emergence of a temperature regime where resistivity is independent of thermal drift, denoted as near-zero temperature coefficient of resistivity (nz-TCR). Further, the ease with which nanocomposites of TiSi_xN can be deposited using ALD offer precise tunability in Ti:Si ratio, thickness, mass density, crystallinity and electrical properties.

Recently, our group explored TiSi_xN films deposited using a Eugenus® 300 mm commercial QXP mini-batch system by modulating the ratio of Ti and Si precursors with NH₃ as a co-reactant.Si-content was varied from 0 at % (pure TiN) to 24.2 at % Si while maintaining thickness ~ 140 nm. The X-ray reflectivity and grazing incidence X-ray diffraction measurements showed a reduction in film density and transition from nano-crystalline to pure amorphous phase with increase in Si-fraction. Spectroscopic ellipsometry revealed the optical constants, composition, and electrical resistivities and were supported by X-ray photoelectron spectroscopy and electrical measurements. Room-temperature resistivity measurements show an increase in film resistivity with increasing at % Si. Temperature-dependent Van der Pauw measurements found a nz-TCR of -23 ppm K^-1 in the temperature range of 298 K – 398 K and at 3.4 at % Si content.

We have now discovered that an at % Si = 3.0% induces a nz-TCR of -5.7 ppm K^-1 from 80 K – 420 K – one of the best reported nz-TCR values for ALD thin films. Fine tuning the at % Si in TiSi_xN films, possible only via ALD, significantly elongated the temperature window of nz-TCR behavior. Mapping the local conductivity of individual grains through conductive atomic force microscopy (c-AFM) indicated higher resistance at the grain boundaries. The local composition at the grain boundaries may play a major role in determining the nz-TCR behavior of TiSi_xN films. In addition, variable temperature Hall effect measurements were performed to provide deeper insights into the nz-TCR mechanism, decoupling carrier concentration from carrier mobility effects while determining film resistivity.

Compared to other nz-TCR films, which are deposited using physical vapor deposition techniques, ALD based nz-TCR films presents a unique synthesis platform for interconnect technology in topologically complex, 3D devices, circuits and sensors that undergo large temperature variation during operation but need to maintain stability in their electrical characteristics.