Deposition of Ni-Based Thin Films in Supercritical Carbon Dioxide Solution Via Flow-Type Reaction System
Authors: Sudiyarmanto Sudiyarmanto , Kondoh Eiichi 
Published: December 2020

Ni-based films have been extensively studied owing to their potential application in several fields such as metal coatings 1, microelectronic devices 2, magnetic devices 3, and catalysis 4. One of the techniques to deposit Ni thin films is supercritical fluid chemical deposition (SFCD) that offers the potential for the capability of high conformal step coverages and high-aspect-ratio features at materials dimensions 5-6. This technique includes the dissolution of precursor in supercritical fluids, adsorption of precursor and reducing agent onto the substrate surface, the reaction between adsorbed precursor and a reducing agent to its metal or metal oxide form on the substrate surface, and desorption of hydrogenated ligands from the substrate into the supercritical fluids phase. These process steps typically proceed under supercritical carbon dioxide (scCO2) solution due to its relatively moderate critical temperature (31.1°C) and pressure (7.38 MPa) 7 that has high molecular density, high diffusivity, low viscosity and zero surface tension 8.
There are two basic type reaction systems for the SFCD technique namely batch-type and flow-type systems 9. In a typical batch-type system, a substrate, a fixed amount of precursor, CO2, and H2 gas are together charged in a closed reactor. The temperature and pressure could ramp up/down because of there is no inlet or outlet in this system. Consequently, the unreacted precursor or the reaction by-product accumulates in the reactor which may make the final product in poor film quality. To overcome this problem, we present here the deposition of Ni thin films in scCO2 solution from Ni(hfac)2
·3H2O precursor through hydrogen reduction with a variety of independent variables via a flow-type reaction system. In this system, a precursor, CO2, and H2 are continuously supplied at a fixed flow rate, concentration, and pressure into a tubular flow reactor. Furthermore, we also employed the deposition of Ni-Sn alloy thin films with various compositions by flow-type SFCD reaction system, and then their activity was evaluated.
The raw materials used in this study were Si(100) wafers having a TiN/SiO2 layer on top, Ni(hfac)2.3H2O, and (i-C3H7)4Sn as Ni and Sn precursor sources respectively, liquid CO2, hydrogen gas, and acetone. Liquid CO2 was cooled and pressurized above a CO2 critical pressure. H2 gas with a certain pressure was added to the scCO2 solution using a gas mixing unit. The precursor solution was added to scCO2 using a high-pressure pump. The substrate was placed inside a tubular flow reactor. And the pressure of this system was adjusted by a back-pressure regulator. All the process parameters of Ni thin film deposition are consisting of precursor concentration, hydrogen concentration, and growth temperature.
Ni thin films were successfully deposited onto TiN/SiO2 film by the SFCD technique. Fig. 1 shows the XRD patterns of Ni films deposited at various precursor concentrations and temperatures. This shows that the Ni thin films were highly (111) structure-oriented. For the XRD patterns of Ni-Sn alloy thin films, the Ni(111) and Ni(200) peaks after added an Sn ratio was slightly shifted and became low intensity that might indicate the formation of the Ni-Sn alloys in a wide composition range. Based on the precursor concentration dependence, the curve trend lines in this study indicated Langmuir-Hinshelwood-type adsorption that the Ni growth rate found to be a zero-order reaction. At temperature dependence, the increase in H2 concentration resulted in the Ni growth rate increase, indicating the activation energy decreased with increasing H2 concentration.

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Figure 1