Kilian Devloo-Casier

In situ monitoring of atomic layer deposition in nanoporous thin films using ellipsometric porosimetry

Ellipsometric porosimetry (EP) is a handy technique to characterize the porosity and pore size distribution of porous thin films with pore diameters in the range from below 1 nm up to 50 nm and for the characterization of porous low-k films especially. Atomic layer deposition (ALD) can be used to functionalize porous films and membranes, e.g., for the development of filtration and sensor devices and catalytic surfaces. In this work we report on the implementation of the EP technique onto an ALD reactor. This combination allowed us to employ EP for monitoring the modification of a porous thin film through ALD without removing the sample from the deposition setup. The potential of in situ EP for providing information about the effect of ALD coating on the accessible porosity, the pore radius distribution, the thickness, and mechanical properties of a porous film is demonstrated in the ALD of TiO(2) in a mesoporous silica film.

Air-stable short-wave infrared PbS colloidal quantum dot photoconductors passivated with Al2O3 atomic layer deposition

A PbS colloidal quantum dot photoconductor with Al2O3 atomic layer deposition (ALD) passivation for air-stable operation is presented. Two different types of inorganic ligands for the quantum dots, S2− and OH−, are investigated. PbS/S2− photoconductors with a cut-off wavelength up to 2.4 μm are obtained, and a responsivity up to 50 A/W at 1550 nm is reported. The corresponding specific detectivity is ∼3.4 × 108 Jones at 230 K. The 3-dB bandwidth of the PbS/S2− and PbS/OH− photodetectors is 40 Hz and 11 Hz, respectively.

In situ synchrotron based x-ray fluorescence and scattering measurements during atomic layer deposition : Initial growth of HfO2 on Si and Ge substrates

The initial growth of HfO2 was studied by means of synchrotron based in situ x-ray fluorescence (XRF) and grazing incidence small angle x-ray scattering (GISAXS). HfO2 was deposited by atomic layer deposition (ALD) using tetrakis(ethylmethylamino)hafnium and H2O on both oxidized and H-terminated Si and Ge surfaces. XRF quantifies the amount of deposited material during each ALD cycle and shows an inhibition period on H-terminated substrates. No inhibition period is observed on oxidized substrates. The evolution of film roughness was monitored using GISAXS. A correlation is found between the inhibition period and the onset of surface roughness.

In situ synchrotron based x-ray techniques as monitoring tools for atomic layer deposition

Atomic layer deposition (ALD) is a thin film deposition technique that has been studied with a variety of in situ techniques. By exploiting the high photon flux and energy tunability of synchrotron based x-rays, a variety of new in situ techniques become available. X-ray reflectivity, grazing incidence small angle x-ray scattering, x-ray diffraction, x-ray fluorescence, x-ray absorption spectroscopy, and x-ray photoelectron spectroscopy are reviewed as possible in situ techniques during ALD. All these techniques are especially sensitive to changes on the (sub-)nanometer scale, allowing a unique insight into different aspects of the ALD growth mechanisms.

Atomic Layer Deposition of TiO2 on Surface Modified Nanoporous Low-k Films

This paper explores the effects of different plasma treatments on low dielectric constant (low-k) materials and the consequences for the growth behavior of atomic layer deposition (ALD) on these modified substrates. An O2 and a He/H2 plasma treatment were performed on SiCOH low-k films to modify their chemical surface groups. Transmission FTIR and water contact angle (WCA) analysis showed that the O2 plasma changed the hydrophobic surface completely into a hydrophilic surface, while the He/H2 plasma changed it only partially. In a next step, in situ X-ray fluorescence (XRF), ellipsometric porosimetry (EP), and Rutherford backscattering spectroscopy (RBS) were used to characterize ALD growth of TiO2 on these substrates. The initial growth of TiO2 was found to be inhibited in the original low-k film containing only Si-CH3 surface groups, while immediate growth was observed in the hydrophilic O2 plasma treated film. The latter film was uniformly filled with TiO2 after 8 ALD cycles, while pore filling was delayed to 17 ALD cycles in the hydrophobic film. For the He/H2 plasma treated film, containing both Si-OH and Si-CH3 groups, the in situ XRF data showed that TiO2 could no longer be deposited in the He/H2 plasma treated film after 8 ALD cycles, while EP measurements revealed a remaining porosity. This can be explained by the faster deposition of TiO2 in the hydrophilic top part of the film than in the hydrophobic bulk which leaves the bulk porous, as confirmed by RBS depth profiling. The outcome of this research is not only of interest for the development of advanced interconnects in ULSI technology, but also demonstrates that ALD combined with RBS analysis is a handy approach to analyze the modifications induced by a plasma treatment on a nanoporous thin film.

Independent tuning of size and coverage of supported Pt nanoparticles using atomic layer deposition

Synthetic methods that allow for the controlled design of well-defined Pt nanoparticles are highly desirable for fundamental catalysis research. In this work, we propose a strategy that allows precise and independent control of the Pt particle size and coverage. Our approach exploits the versatility of the atomic layer deposition (ALD) technique by combining two ALD processes for Pt using different reactants. The particle areal density is controlled by tailoring the number of ALD cycles using trimethyl(methylcyclopentadienyl)platinum and oxygen, while subsequent growth using the same Pt precursor in combination with nitrogen plasma allows for tuning of the particle size at the atomic level. The excellent control over the particle morphology is clearly demonstrated by means of in situ and ex situ X-ray fluorescence and grazing incidence small angle X-ray scattering experiments, providing information about the Pt loading, average particle dimensions, and mean center-to-center particle distance.The performance of supported nanoparticle catalysts is closely related to their size, shape and interparticle distance. Here, the authors introduce an atomic layer deposition-based strategy to independently tune the size and coverage of platinum nanoparticles with atomic-level precision.

Low temperature plasma-enhanced atomic layer deposition of thin vanadium nitride layers for copper diffusion barriers

Thin vanadium nitride (VN) layers were grown by atomic layer deposition using tetrakis(ethylmethylamino)vanadium and NH3 plasma at deposition temperatures between 70 °C and 150 °C on silicon substrates and polymer foil. X-ray photoelectron spectroscopy revealed a composition close to stoichiometric VN, while x-ray diffraction showed the δ-VN crystal structure. The resistivity was as low as 200 μΩ cm for the as deposited films and further reduced to 143 μΩ cm and 93 μΩ cm by annealing in N2 and H2/He/N2, respectively. A 5 nm VN layer proved to be effective as a diffusion barrier for copper up to a temperature of 720 °C.

Controllable nitrogen doping in as deposited TiO2 film and its effect on post deposition annealing

In order to narrow the band gap of TiO2, nitrogen doping by combining thermal atomic layer deposition (TALD) of TiO2 and plasma enhanced atomic layer deposition (PEALD) of TiN has been implemented. By altering the ratio between TALD TiO2 and PEALD TiN, the as synthesized TiOxNy films showed different band gaps (from 1.91 eV to 3.14 eV). In situ x-ray diffraction characterization showed that the crystallization behavior of these films changed after nitrogen doping. After annealing in helium, nitrogen doped TiO2 films crystallized into rutile phase while for the samples annealed in air a preferential growth of the anatase TiO2 along (001) orientation was observed. Photocatalytic tests of the degradation of stearic acid were done to evaluate the effect of N doping on the photocatalytic activity.

Improved thermal stability and retention properties of Cu–Te based CBRAM by Ge alloying

In this work we investigate the influence of Ge as an alloying element in Cu–Te based thin films for application as a cation supply layer in Conductive Bridge Random Access Memory (CBRAM). The thermal stability of the alloys and their functionality as a copper supply layer in CBRAM are investigated. The thermal stability is studied by means of in situ X-ray diffraction, which reveals information on phase separation, phase transformations and melting of the material. We demonstrate that addition of Ge to Cu0.6Te0.4 inhibits crystallization up to 300 °C. However, phase separation occurs upon crystallization, which might result in device to device variability when this occurs in memory devices. This is solved by using Cu2GeTe3 that forms a single phase upon crystallization. The most promising alloys are implemented in 580 μm diameter dot Pt/CuxTeyGe1−x−y/Al2O3/Si CBRAM cells. Their functionality is verified by DC cycling and the influence of Ge is studied by comparing the switching to binary Cu0.6Te0.4 based memory cells. The retention of the programmed memory states is measured at 85 °C. Functional CBRAM is demonstrated, and improved filament stability and retention properties are observed for the Ge containing cells compared to Cu0.6Te0.4. We mainly attribute this to the Ge–Te bonds that are formed in the supply layer. This lowers the tendency for Cu–Te formation which results in a lower driving force for the Cu to go back to the supply layer, and hence contributing to a more stable filament. The formation of Ge–Te bonds was confirmed by XPS measurements.

Synchrotron based in situ characterization during atomic layer deposition

Synchrotron based x-ray fluorescence (XRF) and grazing incidence small angle scattering (GISAXS) are demonstrated to be excellent in situ methods for monitoring atomic layer deposition (ALD) processes. XRF allows to identify and to quantify the amount of material deposited, whereas GISAXS is a powerful technique for monitoring nanoscale morphology. Three case studies are discussed where these in situ techniques are used to investigate specific aspects of ALD processes that are of relevance for applications in micro-electronics: the initial growth of gate oxides, the initial nucleation during metal ALD processes, and the penetration of ALD deposited materials into nanoporous low-k oxides.