Thomas P. Niesen

Review: Deposition of ceramic thin films at low temperatures from aqueous solutions

Many techniques for the synthesis of ceramic thin films from aqueous solutions at low temperatures (25–100°C) have been reported. This paper reviews non-electrochemical, non-hydrothermal, low-temperature aqueous deposition routes, with an emphasis on oxide materials for electronic applications. Originally used for sulfide and selenide thin films, such techniques have also been applied to oxides since the 1970s. Films of single oxides (e.g., transition metal oxides, In2O3, SiO2, SnO2) and multicomponent films (doped ZnO, Cd2SnO4, ZrTiO4, ZrO2-Y2O3, Li-Co-O spinel, ferrites, perovskites) have been produced. The maximum thicknesses of the films obtained have ranged from 100 to 1000 nm, and deposition rates have ranged from 2 to 20,000 nm/h. Compared to vapor-deposition techniques, liquid-deposition routes offer lower capital equipment costs, lower processing temperatures, and flexibility in the choice of substrates with respect to topography and thermal stability. Compared to sol-gel techniques, the routes reviewed here offer lower processing temperatures, lower shrinkage, and (being based on aqueous precursors) lower costs and the potential for reduced environmental impact. This review emphasizes the influence of solution chemistry and process design on the microstructures and growth rates of the films. The current understanding of the mechanisms of film formation is presented, and the advantages and limitations of these techniques are discussed.

Review: deposition of ceramic thin films at low temperatures from aqueous solutions

Many techniques for the synthesis of ceramic thin films from aqueous solutions at low temperatures (25–100°C) have been reported. This paper reviews non-electrochemical, non-hydrothermal, low-temperature aqueous deposition routes, with an emphasis on oxide materials for electronic applications. Originally used for sulfide and selenide thin films, such techniques have also been applied to oxides since the 1970s. Films of single oxides (e.g., transition metal oxides, In2O3, SiO2, SnO2) and multicomponent films (doped ZnO, Cd2SnO4, ZrTiO4, ZrO2-Y2O3, Li-Co-O spinel, ferrites, perovskites) have been produced. The maximum thicknesses of the films obtained have ranged from 100 to 1000 nm, and deposition rates have ranged from 2 to 20,000 nm/h. Compared to vapor-deposition techniques, liquid-deposition routes offer lower capital equipment costs, lower processing temperatures, and flexibility in the choice of substrates with respect to topography and thermal stability. Compared to sol-gel techniques, the routes reviewed here offer lower processing temperatures, lower shrinkage, and (being based on aqueous precursors) lower costs and the potential for reduced environmental impact. This review emphasizes the influence of solution chemistry and process design on the microstructures and growth rates of the films. The current understanding of the mechanisms of film formation is presented, and the advantages and limitations of these techniques are discussed.

An X-ray reflectivity study of solution-deposited ZrO2 thin films on SAMs: growth, interface properties, and thermal densification

Thin films of ZrO 2 were deposited from aqueous solution on Si(100) substrates precovered by functionalized alkyltrichlorosilane self-assembled monolayers (SAMs). The interface structure, thermal stability, and densification of these films in the temperature range from room temperature to 750 °C in vacuum were measured using in situ x-ray reflectivity. The growth rate is a nonlinear function of time in solution, with a pronounced nonuniformity during the first 30 min. The as-deposited films exhibit about 3-nm roughness and a density below that of bulk ZrO 2 . Measurements in vacuum reveal decreasing film thickness, increasing film density, and decreasing roughness upon annealing up to 750 °C. The densification saturates at the highest measured temperatures, presumably following evaporation of residual contaminants from the aqueous synthesis procedure. Above 200 °C the SAM/ZrO 2 interface began to deteriorate, possibly due to interdiffusion. The ZrO 2 film structure obtained at the highest annealing temperatures persisted upon cooling to room temperature, and there was no visible evidence of stress-induced microstructural changes, such as peeling or cracking.

Deposition of titania thin films from aqueous solution by a continuous flow technique

Organic self assembled monolayer (SAMs) on p-type silicon (001) single crystal wafers were used as substrates for the formation of TiO2 films from aqueous solution. It was previously shown that this organic modification allows the formation of homogeneous thin films of TiO2 below 100 °C under static conditions. The formation of a titanium complex in the presence of H2O2 is used to avoid the otherwise spontaneous precipitation of titania from the aqueous solution. In the present paper, the synthesis of TiO2 thin films is realized by a continuous flow method (cfm). In this method the silicon substrate is placed in a cylindrical reaction chamber, through which the solution is pumped with a constant flow rate. SEM, TEM, ellipsometry and XPS measurements illustrate that this technique allows deposition of thicker films than are obtained using a static deposition method, while achieving similar homogeneity. The films are crystalline and a uniform surface topography can be achieved.

Synthesis and Characterization of Titania and Vanadia Thin Films at Organic Self-Assembled Monolayers

Functionalized self-assembled monolayers (SAMs) on single-crystal Si wafers have been used as substrates for the deposition of titania and vanadia thin films. The formation of a titanium chelate was used to stabilize an otherwise spontaneously precipitating aqueous titanium solution. Uniform titania films have been synthesized from Ti(O 2 ) 2+ in aqueous HCl solutions at 80°C on sulfonated SAMs. Vanadium oxide hydrate films, V 2 O 5 ·0.7 H 2 O, have been directly formed from aqueous vanadate solutions on NH 2 -terminated SAMs at 45°C. In the as-deposited films, water molecules were intercalated between the vanadium oxide layers. Subsequent annealing at 350°C in air led to nanocrystalline V 2 O 5 .