Area Selective Chemical Vapor Deposition of Metallic Films using Plasma Electrons as Reducing Agents

Abstract: Metallic films are used to improve optical, chemical, mechanical, magnetic, and electrical properties and are therefore of high importance in many applications, from electronics and catalysis, environmental protection and health, to wearable and flexible electronic materials. Many of these applications, however, require that the metal films are deposited uniformly on topographically complex surfaces and structures. Some form of chemical vapor deposition (CVD) where the deposition is governed by the surface chemistry is needed for uniform film deposition on topographically complex surfaces. Furthermore, area selective deposition (ASD) has gained large considerations lately, where films deposited only on specified areas of the substrate, and not on others, simplifies the processing significantly and opens the way for less complex fabrication of, for instance, nanoscaled electronics. ASD occurs when the surface chemical reactions are disabled on selected areas of the substrate. Since the metal centers in CVD precursor molecules typically have a positive valence, a reductive surface chemistry is required to form a metallic film. This is usually done by using a second precursor, i.e., a molecular reducing agent. The negative standard reduction potential of the first-row transition metals (Ti, V, Cr, Mn, Fe, Co, and Ni) means that CVD of these metals requires either very high temperatures or very powerful molecular reducing agents. This thesis describes a new low temperature CVD method for depositing metallic films where instead free electrons in a plasma discharge are utilized to reduce the metal centers of chemisorbed precursor molecules. By applying a positive bias voltage to the substrate holder, the plasma electrons are attracted to the substrate for electron-precursor interactions. This was demonstrated by successfully depositing iron, cobalt, and nickel films from their corresponding metallocene precursors. The electrical resistivity of the substrate and the polarity of the substrate bias were shown to play an important role in depositing metallic films with this CVD approach. The experimental results show that films deposited, with +40 V bias voltage, on silver substrates contain substantially higher metal concentration compare to films deposited on silicon substrates. Deposition on electrically insulating silicon dioxide substrates however yielded no detectable amount of metal atoms on the substrate surface. This indicates that electron current through the substrate is essential to grow metal films in this CVD process. The effect of the electrical resistivity of the substrate was studied for ASD. The new CVD method is shown to be inherently area selective from the surface electrical resistivity by depositing iron from ferrocene on silicon dioxide substrate partially coated with silver. No, or very small, detectable amount of metal atoms could be found on areas with high resistivity, whereas several hundred nm thick iron films are deposited on areas with low resistivity. The only heating of the substrate emanates from the electric current from the plasma through the substrate holder, resulting in a slight heating to 35–50 °C, depending on the substrate bias voltage. This was regarded as the deposition temperature. Such low deposition temperature was exploited to achieve ASD by a masking approach with different temperature sensitive materials such as polydimethylsiloxane (PDMS), polymethylmethacrylate (PMMA), polystyrene (PS), Parafilm, Kapton tape, Scotch tape, and office paper. These materials were used to mask area of the substrate in the new CVD method as demonstrated by depositing iron from ferrocene on partially masked silver substrates.   All initial experiments rendered only a phenomenological understanding of the new CVD process. Therefore, a quartz crystal microbalance (QCM) system was modified, by the addition of a positive bias voltage, and used to further understand the chemical and physical processes controlling the deposition process. The results show that differences in film deposition with different deposition parameters, such as plasma power and bias voltage, can be observed using the new QCM approach where the QCM crystal indeed works as a substrate in our new CVD process.   In summary, a new CVD concept has been developed for metallic thin films. This method uses the free plasma electrons as reducing agents and can also be utilized for ASD of metal thin films. This