Surface and Interface Diffusion Processes in Nanoscale Thin Films
Anirudhan Chandrasekaran is a PhD student in the research group XUV Optics. Supervisor is prof.dr. F. Bijkerk and co-supervisor is dr.ir. R.W.E van de Kruijs from the Faculty of Science & Technology.
This thesis describes advanced experimental research on the physical and chemical properties of bilayer and multilayer ultra-thin films. Atomic scale processes that occur at the interfaces of thin film stacks are known to dramatically influence their functional properties. This includes layer roughness and intermixing occurring during the film deposition process, as well as interdiffusion of layer materials and interlayer compound formation during usage of the thin-films, for example, exposure of an optically reflective multilayer stack to a high-power light source and electromigration due to current flow in integrated circuit devices. Currently, there is a critical demand for a broadly applicable material-selection-guide to design atomically sharp and stable layer stacks for various thin-film devices. This thesis has given an important onset for that. It is focused on understanding the surface and interface diffusion mechanisms involved during: (i) near room temperature layer growth, and (ii) low temperature annealing of transition metal (TM) and Si based layered systems. The thesis presents a novel scaling law for the effective interface width between two layers, which can be used for the selection of suitable layer materials to achieve well-controlled interfaces in a thin-film stack.
The intermixing and segregation processes during room temperature deposition were studied using high-sensitivity low-energy ion scattering (HS-LEIS). A film-on-substrate bilayer architecture was used for the layer growth studies. Using in vacuo LEIS, it was possible to obtain a “growth profile” by measuring the change in surface atomic composition as a function of sub-nanometer increase in the as-deposited film thickness. Key layer growth and interface properties such as the effective interface width, the layer closure thickness and the surface segregation of several TM and Si based bilayer systems were obtained from their corresponding LEIS growth profiles using a logistic-function-based interface atomic composition model.
A semi-empirical surface atomic exchange model was developed to describe the relation between the effective layer interface width and layer material properties such as surface energy and crystal structure. The model parameters were extracted by fitting the experimental effective interface width data of several TM-on-TM layered structures obtained from their corresponding LEIS growth profiles. The effective interface width could be described as an exponential function of the surface-energy difference between film and substrate atoms, with a subtrend based on the crystal structure of the layers. The model serves as a scaling law to predict the effective interface width and the layer closing thickness of TM-on-TM layered systems. In addition to the intermixing process, segregation of film or substrate atoms during growth was observed in some of the investigated TM-on-TM systems. The segregation phenomenon was attributed to the strain energy induced by the large atomic size between the film and substrate atoms (e.g. Ru-Sc), and in some cases, it could be attributed to the low activation energy for self-diffusion (e.g. Cu). The investigated TM-on-TM systems were categorized into four types based on the observed intermixing and segregation characteristics, and a general rule was proposed to predict the possible growth type of TM-on-TM systems based on the substrate-film atomic radii difference (rs – rf), the enthalpy of mixing, and the surface-energy difference between substrate and film atoms.
The applicability of the proposed general rules of growth and interface characteristics for TM-on-TM systems was also studied for TM-on-Si systems. In all TM-on-Si (TM: Zr, Ta, Mo, Ru, Ir, Pt) systems investigated in this work, segregation of Si atoms during growth was observed. The effective interface width of all investigated TM-on-Si systems, except Zr and Ta on Si, followed the interface width scaling law obtained from the TM-on-TM studies. For TM-on-Si systems with a large atomic size difference (rs - rf < – 0.22 Å), as in the case of Zr-on-Si and Ta-on-Si systems having large film atoms, a competing segregation mechanism of TM and Si atoms in the early stages of growth resulted in a reduced effective interface width when compared to the value predicted by the scaling law.
The interdiffusion of atoms during low temperature annealing (200ºC) was studied using high resolution cross-sectional transmission electron microscopy (XTEM). A multilayer architecture was used for thermal diffusion studies to limit the influence of surface oxidation and damage from XTEM sample preparation. In Nb/Si systems, Nb was found to undergo two microstructural transitions within the first 4 nm of as-deposited Nb on a Si layer: (i) an amorphous-to-crystalline transition with a strong texture around 2.1 nm, and (ii) transition to a polycrystalline growth around 3.3 nm. In order to study the effect of the Nb microstructure on interdiffusion via interfaces during low temperature annealing, three Nb/Si multilayers were deposited: (i) with 2 nm amorphous Nb layers, 3 nm strongly textured Nb layers, and 4 nm polycrystalline Nb layers with a reduced degree of texture. Nb/Si multilayers with amorphous and strongly textured Nb layers show better thermal stability during low temperature annealing (200ºC) when compared to a Nb/Si multilayer with polycrystalline Nb layers, because of the limited grain boundary pathways for diffusion in amorphous and textured Nb layers.
In the Zr/Si multilayer system, low temperature annealing (200ºC) resulted in complete solid state amorphization of the polycrystalline Zr layer and the formation of a thick amorphous ZrSi layer. The Zr/Si multilayers exhibited a stronger degree of interdiffusion and a solid state amorphization at 200ºC when compared to Nb/Si multilayers studied in this thesis and other Metal/Si structures reported in literature. The strong interdiffusion via the Zr-Si interfaces was explained by the large strain at the interfaces caused by the large atomic size difference (~ 0.42 Å) between Zr and Si atoms when compared to most other Metal/Si systems. In fact, Ce/Si multilayers, with a Ce-Si atomic size difference of ~ 0.64 Å, were shown to exhibit interdiffusion even near room temperature in the first few days after deposition.
The following conclusions are derived from this thesis work:
(1) For near room temperature growth of TM and Si based systems:
- Intermixing between two layers during growth is a consequence of surface atomic exchange processes that are activated by: (i) ballistic collision between film and substrate atoms during deposition, and (ii) surface energy minimization, where the lowest surface energy atom tends to move toward the surface.
- The surface energy difference and preferred crystal structure (BCC, HCP, FCC) of the layer materials determine the extent of the intermixing and the effective interface width between the layers. In general, the magnitude of intermixing will be higher if the surface free energy of the substrate atoms is lower than the film atoms. Also, BCC TMs tend to show less intermixing when compared to HCP and FCC TMs because of the higher bond strength of BCC atoms.
- Floating segregation or surface segregation of atoms can occur in some systems in addition to the intermixing process. The strength of segregation depends on the surface energy difference, atomic size difference, and the enthalpy of mixing between the film and substrate atoms. Floating segregation can also be observed in the case of film or substrate atoms with low activation energy for self-diffusion (e.g. Cu), however, atomic size difference is the dominating factor. Si tends to exhibit floating segregation behavior during TM-on-Si growth.
(2) For low temperature annealing of TM-Si systems
- Intermixing between layer materials during low temperature annealing (< 300ºC) is a consequence of bulk diffusion of Si atoms facilitated by the grain boundaries in TM layers.
- In TM-Si layered systems, amorphous and strongly textured TMs with limited number of grain boundary pathways for diffusion show enhanced stability during low temperature annealing when compared to polycrystalline TM layers with more grain boundaries.
- A large atomic size difference between TM and Si atoms (e.g. Zr-Si) will lead to an increased strain energy at the interfaces. This will enhance the interdiffusion process, resulting in solid state amorphization and complete intermixing of few nanometer thick layers during low temperature annealing. In fact, spontaneous intermixing can occur even at room temperature over a period of time as in the case of Ce-Si with ~ 0.64 Å atomic radii difference.
