Martin Knaut

Atomic Layer Deposition for Coating of High Aspect Ratio TiO2 Nanotube Layers

We present an optimized approach for the deposition of Al2O3 (as a model secondary material) coating into high aspect ratio (≈180) anodic TiO2 nanotube layers using the atomic layer deposition (ALD) process. In order to study the influence of the diffusion of the Al2O3 precursors on the resulting coating thickness, ALD processes with different exposure times (i.e., 0.5, 2, 5, and 10 s) of the trimethylaluminum (TMA) precursor were performed. Uniform coating of the nanotube interiors was achieved with longer exposure times (5 and 10 s), as verified by detailed scanning electron microscopy analysis. Quartz crystal microbalance measurements were used to monitor the deposition process and its particular features due to the tube diameter gradient. Finally, theoretical calculations were performed to calculate the minimum precursor exposure time to attain uniform coating. Theoretical values on the diffusion regime matched with the experimental results and helped to obtain valuable information for further optimization of ALD coating processes. The presented approach provides a straightforward solution toward the development of many novel devices, based on a high surface area interface between TiO2 nanotubes and a secondary material (such as Al2O3).

Barrier performance optimization of atomic layer deposited diffusion barriers for organic light emitting diodes using x-ray reflectivity investigations

The importance of O3 pulse duration for encapsulation of organic light emitting diodes (OLEDs) with ultra thin inorganic atomic layer deposited Al2O3 layers is demonstrated for deposition temperatures of 50 °C. X-ray reflectivity (XRR) measurements show that O3 pulse durations longer than 15 s produce dense and thin Al2O3 layers. Correspondingly, black spot growth is not observed in OLEDs encapsulated with such layers during 91 days of aging under ambient conditions. This implies that XRR can be used as a tool for process optimization of OLED encapsulation layers leading to devices with long lifetimes.

Atomic layer deposited high-κ nanolaminates for silicon surface passivation

Nanolaminates comprising of TiO2 or HfO2 sublayers within an Al2O3 matrix are grown with atomic layer deposition. These nanolaminates provide an improved silicon surface passivation compared to conventional Al2O3 films. The physical properties of the nanolaminates can be described with a dynamic growth model that considers initial and steady-state growth rates for the involved metal oxides. This model links the cycle ratios of the different atomic layer deposition precursors to the thickness and the material concentrations of the nanolaminate, which are determined by means of spectroscopic ellipsometry. Effective carrier lifetime measurements show that Al2O3-TiO2 nanolaminates achieve values of up to 6.0 ms at a TiO2 concentration of 0.2%. In Al2O3-HfO2 nanolaminates, a maximum effective carrier lifetime of 5.5 ms is reached at 7% HfO2. Electrical measurements show that the TiO2 incorporation causes strong hysteresis effects, which are linked to the trapping of negative charges and result in an enhanced f...

In-situ real-time ellipsometric investigations during the atomic layer deposition of ruthenium: A process development from [(ethylcyclopentadienyl)(pyrrolyl)ruthenium] and molecular oxygen

In this work, a process for the thermal activated atomic layer deposition (ALD) of ruthenium from the organometallic heteroleptic precursor [(ethylcyclopentadienyl)(pyrrolyl)ruthenium] with molecular oxygen was developed and characterized. Silicon substrates were precleaned in hydrofluoric acid and preheated to a specific temperature before coating with ruthenium. The corresponding cycle-by-cycle growth was monitored throughout the entire ALD process time, utilizing an in-situ real-time spectroscopic ellipsometer. Transmission electron microscopy and atomic force microscopy were applied at a reference sample to generate an appropriate optical model for the translation of the ellipsometric spectra into Ru film thicknesses. Given a representative set of process parameters the cycle-by-cycle growth was studied in detail, obtaining information about incubation, nucleation, linear growth and delamination. In order to determine the ALD characteristic dependencies, the following process parameters were varied wh...

Hard x-ray nanofocusing by refractive lenses of constant thickness

In order to focus light or x rays, the thickness of a refractive lens is typically varied over its aperture. Here, we present a refractive x-ray lens made of lamellae of constant thickness, the refractive lamellar lens. Refractive power is created by a specific bending of the lamellae rather than by a concave lens profile. This very special design has the technological advantage that materials like sapphire or diamond can be used to make lenses by coating techniques. A first lens prototype focused x rays with a photon energy E = 15.25 keV to a lateral beam size of 164 nm × 296 nm full width at half maximum.

Atomic layer deposition of anatase TiO2 on porous electrodes for dye-sensitized solar cells

Thin TiO2 films were grown by atomic layer deposition on planar and porous substrates and characterized by Raman spectroscopy, x-ray diffraction, high resolution transmission electron microscopy, and spectroscopic ellipsometry. The growth conditions of anatase TiO2 are investigated, motivated by the application in dye-sensitized solar cells, where best results are achieved with electrodes based on anatase TiO2. To enforce an anatase TiO2 growth on substrates stimulating rutile growth, a symmetry breaking ultra thin buffer layer of five cycles Al2O3 was introduced. With this buffer layer anatase TiO2 deposition was demonstrated on planar rutile TiO2 substrates. However, it was found that the necessity of the buffer layer depends on the substrate structure. On porous TiO2 electrodes containing a mixture of anatase and rutile TiO2 domains, a direct anatase TiO2 growth was possible even without a buffer layer.

Experimental and simulation approach for process optimization of atomic layer deposited thin films in high aspect ratio 3D structures

The authors present a new method to determine film thicknesses and sticking coefficients (SC) of precursor molecules for atomic layer deposition (ALD) in high aspect ratio three dimensional (3D) geometries as they appear in microelectromechanical system manufacturing. The method combines a specifically designed experimental test structure with the theoretical predictions from a novel 3D Monte Carlo process simulation for large structures. The authors exemplify our method using Al2O3 and SiO2 ALD processes. SCs for trimethylaluminium and bis-diethyl aminosilane (BDEAS) are extracted. The SC for BDEAS is determined for the first time.

Monitoring atomic layer deposition processes in situ and in real-time by spectroscopic ellipsometry

Depositing ultra-thin metallic films with an accurate control of the film properties (like film thickness, surface roughness and electrical properties) both in the initial and in the progressed film growth regime is a critical challenge. Monitoring atomic layer deposition processes by spectroscopic ellipsometry allows film thickness control in the sub-nanometer range. In addition, ellipsometry serves as a powerful technique for process analysis as it covers many relevant issues, like the evaluation of substrate temperatures as well as the quantification of film properties during the entire ALD process (i. e. in all relevant growth regimes).

In situ XPS investigation of the chemical surface composition during the ALD of ultra-thin aluminum oxide films

Atomic layer deposition (ALD) is the most advanced technique for the fabrication of ultra-thin conformal films. To yield high quality films, the knowledge of chemical reactions and interactions between the substrate surface and the precursors is becoming increasingly important, especially within the very first ALD cycles. In this work, the ALD process of aluminum oxide with trimethylaluminum (TMA) and water is studied by using X-ray photoelectron spectroscopy (XPS) without vacuum break. This allows the investigation of the initial gaseous-solid-reactions, i. e. the chemisorption mechanism of the precursor molecules, with sub-monolayer resolution. The results show the ligand exchange during the ALD reactions and the dependence of the growth mode on the presence of hydroxyl groups and oxygen as adsorption sites on the substrate surface.

Al 2 O 3 -TiO 2 Nanolaminates for Conductive Silicon Surface Passivation

Al₂O₃-TiO₂ nanolaminates are very attractive candidates for future conductive passivation layers because they are purely based on dielectric materials, which allow a simple integration in the state-of-the-art manufacturing process. In this study, Al₂O₃-TiO₂ double and multilayers are grown by atomic layer deposition and systematically investigated. The nanolaminates feature good silicon surface passivation and moderate electrical conductivity. The best performance is found for a double-layer stack consisting of a 5 nm Al₂O₃ interface layer and a 15 nm TiO₂ capping layer after postdeposition annealing in N₂ or forming gas. With this stack, a surface recombination velocity of 15 cm/s and a contact resistance of 20 Ω·cm² are achieved. In Al₂O₃-TiO₂ nanolaminates, the electrical transport is strongly influenced by the interaction of TiO₂ and Al₂O₃ during layer growth. Raman measurements reveal that high conductivity correlates with a phase transition of TiO₂ from amorphous to anatase.