Smooth and Conformal TiO₂Thin-Film Formation Using Supercritical Fluid Deposition

Abstract

Conformal TiO2 having a smooth surface was deposited using a flow-type reactor for supercritical fluid deposition (SCFD). Ti(O-iPr)2(tmhd)2 was selected as the best candidate for SCFD from among three candidates because it exhibited the best results according to the following criteria: solubility in supercritical CO2 (scCO2), carbon impurities in the TiO2, and surface morphology of the deposited film. The growth rate increased with increasing temperature from 200 to 300°C. The activation energy in this temperature range was measured to be about 46.3 kJ/mol. Compared with conventional methods, such as chemical vapor deposition (CVD; activation energy of 85 kJ/mol, decomposition temperature above 450°C), TiO2-SCFD had a much lower activation energy owing to the solvent effect of scCO2. We also examined the growth-rate dependence on the precursor concentration and found first-order reaction kinetics such that the surface reaction rate constant (ks) was as low as 2.0 × 10−6 m/s at 300°C. This process resulted in good step coverage during film formation on trenches with an aspect ratio of 12.5.

A Review on 1‐D Nanomaterials: Scaling‐Up with Gas‐Phase Synthesis

“…Zhao et al [316] . deposited smooth conformal films of polycrystalline TiO 2 on substrates etched with high‐aspect ratio trenches using Ti(O‐iPr) 2 (tmhd) 2 precursor in scCO 2 at 200–350 °C and 100 bar.…”

“…Zhao et al [316] deposited smooth conformal films of polycrystalline TiO 2 on substrates etched with high-aspect ratio trenches using Ti(O-iPr) 2 (tmhd) 2 precursor in scCO 2 at 200-350 °C and 100 bar. Growth rates of such SCF-deposition are an order of magnitude faster than ALD, on a like-forlike basis.…”

“…Favourable processing performance depends on precursor type, as demonstrated in the study of Zhao et al [316] where three structurally-similar titanium candidates were considered, with Ti(O-iPr) 2 (tmhd) 2 exhibiting the best solubility in scCO 2 , and least carbon impurities in the resulting TiO 2 film. Growth rate is a critical factor in mass production of nanomaterialsexemplified by CNT synthesis.…”

Supercritical Fluid Atomic Layer Deposition: Base-Catalyzed Deposition of SiO2

“…[1][2][3][4] While ALD has been used mainly to grow films on flat or low surface area materials, there has been an increased emphasis to extend ALD to control film growth on powders and porous materials. [5][6][7][8] However, surface modification and ALD growth is hampered by the diffusion of reagents into the porous powders and by uncontrolled levels of polymerization of the ALD reagents in the water-filled pores. 9,10 Modification of silica substrates using supercritical CO 2 (sc-CO 2 ) as a solvent could overcome this limitation.…”

“…Atomic layer deposition (ALD) has found widespread interest as a film growth technique because of the demonstrated high level control in thickness and conformality of the resulting films. − While ALD has been used mainly to grow films on flat or low surface area materials, there has been an increased emphasis to extend ALD to control film growth on powders and porous materials. − However, surface modification and ALD growth are hampered by the diffusion of reagents into the porous powders and by uncontrolled levels of polymerization of the ALD reagents in the water-filled pores. , Modification of silica substrates using supercritical CO 2 (sc-CO 2 ) as a solvent could overcome this limitation. − Sc-CO 2 exhibits properties of high diffusivity and low surface tension and has facilitated delivery of ALD precursors to regions in silica powder not accessible by conventional vapor-based protocols. − Sc-CO 2 is also a desiccant for silica . While silica powders are used as a drying agent for most organic solvents, sc-CO 2 is, in effect, a drying agent for silica.…”