| 1. |
- Nadhom, Hama, 1986-
(författare)
-
Area Selective Chemical Vapor Deposition of Metallic Films using Plasma Electrons as Reducing Agents
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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
|
|
| 2. |
- Nadhom, Hama, et al.
(författare)
-
Area selective deposition of iron films using temperature sensitive masking materials and plasma electrons as reducing agents
- 2021
-
Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : A V S AMER INST PHYSICS. - 0734-2101 .- 1520-8559. ; 39:4
-
Tidskriftsartikel (refereegranskat)abstract
- The potential of area-selective deposition (ASD) with a newly developed chemical vapor deposition (CVD) method, which utilizes plasma electrons as reducing agents for deposition of metal-containing films, is demonstrated using temperature sensitive polymer-based masking materials. The masking materials tested were polydimethylsiloxane, polymethylmethacrylate, polystyrene, parafilm, Kapton tape, Scotch tape, and office paper. The masking materials were all shown to prevent film growth on the masked area of the substrate without being affected by the film deposition process. X-ray photoelectron spectroscopy analysis confirms that the films deposited consist mainly of iron, whereas no film material is found on the masked areas after mask removal. Scanning electron microscopy analysis of films deposited with nonadhesive masking materials show that film growth extended for a small distance underneath the masking material, indicating that the CVD process with plasma electrons as reducing agents is not a line-of-sight deposition technique. The reported methodology introduces an inexpensive and straightforward approach for ASD that opens for exciting new possibilities for robust and less complex area-selective metal-on-metal deposition.
|
|
| 3. |
- Nadhom, Hama, 1986-, et al.
(författare)
-
Area Selective Deposition of Metals from the Electrical Resistivity of the Substrate
- 2021
-
Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 12:17, s. 4130-4133
-
Tidskriftsartikel (refereegranskat)abstract
- Area selective deposition (ASD) of films only on desired areas of the substrate opens for less complex fabrication of nanoscaled electronics. We show that a newly developed CVD method, where plasma electrons are used as the reducing agent in deposition of metallic thin films, is inherently area selective from the electrical resistivity of the substrate surface. When depositing iron with the new CVD method, no film is deposited on high-resistivity SiO2 surfaces whereas several hundred nanometers thick iron films are deposited on areas with low resistivity, obtained by adding a thin layer of silver on the SiO2 surface. On the basis of such a scheme, we show how to use the electric resistivity of the substrate surface as an extension of the ASD toolbox for metal-on-metal deposition.
|
|
| 4. |
- Nadhom, Hama, et al.
(författare)
-
Chemical vapor deposition of metallic films using plasma electrons as reducing agents
- 2020
-
Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : A V S AMER INST PHYSICS. - 0734-2101 .- 1520-8559. ; 38:3
-
Tidskriftsartikel (refereegranskat)abstract
- Metallic thin films are key components in electronic devices and catalytic applications. Deposition of a conformal metallic thin film requires using volatile precursor molecules in a chemical vapor deposition (CVD) process. The metal centers in such molecules typically have a positive valence, meaning that reduction of the metal centers is required on the film surface. Powerful molecular reducing agents for electropositive metals are scarce and hamper the exploration of CVD of electropositive metals. The authors present a new CVD method for depositing metallic films where free electrons in a plasma discharge are utilized to reduce the metal centers of chemisorbed precursor molecules. They demonstrate this method by depositing Fe, Co, and Ni from their corresponding metallocenes using electrons from an argon plasma as a reducing agent.
|
|
| 5. |
- Niiranen, Pentti, et al.
(författare)
-
Biased quartz crystal microbalance method for studies of chemical vapor deposition surface chemistry induced by plasma electrons
- 2023
-
Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 94:2
-
Tidskriftsartikel (refereegranskat)abstract
- A recently presented chemical vapor deposition (CVD) method involves using plasma electrons as reducing agents for deposition of metals. The plasma electrons are attracted to the substrate surface by a positive substrate bias. Here, we present how a standard quartz crystal microbalance (QCM) system can be modified to allow applying a DC bias to the QCM sensor to attract plasma electrons to it and thereby also enable in situ growth monitoring during the electron-assisted CVD method. We show initial results from mass gain evolution over time during deposition of iron films using the biased QCM and how the biased QCM can be used for process development and provide insight into the surface chemistry by time-resolving the CVD method. Post-deposition analyses of the QCM crystals by cross-section electron microscopy and high-resolution x-ray photoelectron spectroscopy show that the QCM crystals are coated by an iron-containing film and thus function as substrates in the CVD process. A comparison of the areal mass density given by the QCM crystal and the areal mass density from elastic recoil detection analysis and Rutherford backscattering spectrometry was done to verify the function of the QCM setup. Time-resolved CVD experiments show that this biased QCM method holds great promise as one of the tools for understanding the surface chemistry of the newly developed CVD method.
|
|
| 6. |
- Niiranen, Pentti, et al.
(författare)
-
Plasma electron characterization in electron chemical vapor deposition
- 2024
-
Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : A V S AMER INST PHYSICS. - 0734-2101 .- 1520-8559. ; 42:2
-
Tidskriftsartikel (refereegranskat)abstract
- Recently, a novel approach of depositing metallic films with chemical vapor deposition (CVD), using plasma electrons as reducing agents, has been presented and is herein referred to as e-CVD. By applying a positive substrate bias to the substrate holder, plasma electrons are drawn to the surface of the substrate, where the film growth occurs. In this work, we have characterized the electron flux at the substrate position in terms of energy and number density as well as the plasma potential and floating potential when maintaining an unbiased and a positively biased substrate. The measurements were performed using a modified radio frequency Sobolewski probe to overcome issues due to the coating of conventional electrostatic probes. The plasma was generated using a DC hollow cathode plasma discharge at various discharge powers and operated with and without precursor gas. The results show that the electron density is typically around 10(16) m(-3) and increases with plasma power. With a precursor, an increase in the substrate bias shows a trend of increasing electron density. The electron temperature does not change much without precursor gas and is found in the range of 0.3-1.1 eV. Introducing a precursor gas to the vacuum chamber shows an increase in the electron temperature to a range of 1-5 eV and with a trend of decreasing electron temperature as a function of discharge power. From the values of the plasma potential and the substrate bias potential, we were able to calculate the potential difference between the plasma and the substrate, giving us insight into what charge carriers are expected at the substrate under different process conditions.
|
|
