Article

Comparative measurements on atomic layer deposited Al2O3 thin films using ex situ table top and mapping ellipsometry, as well as X-ray and VUV reflectometry

Authors:
If you want to read the PDF, try requesting it from the authors.

Abstract

In this study we compare the thicknesses and optical properties of atomic layer deposited (ALD) Al2O3 films measured using table top and mapping ellipsometry as well as X-ray and optical reflectometry. The thickness of the films is varied in the range of 1-50 nm. ALD samples are used as references with well-controlled composition and thickness, as well as with a good lateral homogeneity. The homogeneity is checked using mapping ellipsometry. Optical models of increasing complexity were developed to take into account both the top (surface roughness on the nanometer scale) and bottom interfaces (buried silicon oxide and interface roughness). The best ellipsometric model was the one using a single interface roughness layer. Since the techniques applied in this work do not measure in vacuum, organic surface contamination even in the sub-nanometer thickness range may cause an offset in the measured layer thicknesses that result in significant systematic errors. The amount of surface contamination is estimated by in situ reflectometry measurement during removal by UV radiation. Taking into account the surface contamination the total thicknesses determined by the different methods were consistent. The linearity of the total thickness with the number of atomic layer deposition cycles was good, with an offset of 1.5 nm, which is in good agreement with the sum of thicknesses of the interface layer, surface nanoroughness, and contamination layer.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... As proposed by Petrik et al. [10], roughness at the Si/Al 2 O 3 interface was taken into account in the ellipsometry data analysis. An interface layer was introduced as a Bruggeman effective medium composed of 50% Si substrate and 50% Al 2 O 3 layer (Model 2) to simulate the surface roughness. ...
... Table 1 summarizes the results obtained with Model 1 and Model 3. Note that although the fit accuracy is improved by the introduction of the sub-oxide interface for thin film, MSE increases for thicker Al 2 O 3 layers (500 and 1000 cycles). This phenomenon was already described by Petrik et al. [10] and may indicate a slight inhomogeneity in thickness or refractive index or other non-ideality of the layer that are not taken into account by these models. Based on ellipsometry studies of the Si/SiO 2 interfaces [12,14] and on the high value of the extracted refractive index (3.17), the composition of the SiO x interface layer could be roughly estimated to x b 0.4. ...
... However, even with a SiO x interface layer, an increase of the extracted Al 2 O 3 refractive index is observed for thickness below 10 nm. This effect is attributed to the larger influence of the interface for very thin films and the strong correlation between the refractive index and thickness in the optical models and measurements [7,10]. To confirm the influence of an interface layer between Si and Al 2 O 3 on the refractive index values, ellipsometry measurements were performed on Al 2 O 3 layers deposited on sapphire substrates where the presence of SiO x is excluded. ...
Article
Al2O3 thin films with thickness between 2 and 100 nm were synthetized at 250 °C by thermal atomic layer deposition on silicon substrates. Characterizations of as-deposited and annealed layers were carried out using ellipsometry, X-ray reflectivity, and X-ray photoelectron spectroscopy. A silicon-rich SiOx layer at the interface between Si and Al2O3 was introduced in the optical models to fit the experimental data. Surface passivation performances of Al2O3 layers deposited on n-type float-zone monocrystalline silicon were investigated as a function of thickness and post-deposition annealing conditions. Surface recombination velocity around 2 cm.s − 1 was measured after the activation of the negative charges at the Si/Al2O3 interface under optimized annealing at 400 °C for 10 min. The evolution of the interface layer and of the material properties with the thermal treatment was studied.
... Afin de s'approcher au mieux des mesures, plusieurs modèles optiques ont été considérés (Figure C Ce phénomène a déjà été décrit par Petrik et al. [Petrik13] et peut indiquer une légère hétérogénéité de l'indice de réfraction dans l'épaisseur ou d'autres non-idéalités de la couche qui ne sont pas prises en compte par ces modèles. Compte tenu des études des interfaces Si / SiO 2 par ellipsométrie [Aspnes79, Tomozeiu11] et de la valeur élevée de l'indice de réfraction (n SiOx ~ 3,17)qui permet d'ajuster nos résultats expérimentaux, la composition de la couche d'interface SiO x peut être estimée à x <0,4. ...
... Cette étude précise que le carbone ne provient pas des groupes méthyl du précurseur TMA (Al(CH 3 ) 3 ) qui pourraient rester insérés dans la couche. Petrik et al. [Petrik13] mettent également en évidence une couche carbonée de surface pour des échantillons déposés par PEALD. Pour définir l'origine du carbone (volume ou surface), l'angle d'incidence a été réduit de 90 ° à 30 ° pour augmenter la proportion de surface sondée. ...
... Il a été mis en évidence que les origines des charges sont des défauts ponctuels (ionisés) [Robertson05,Stesmans05,McIntyre07,Liu10]. Des mesures de spectrométrie photo-électronique X (XPS) sur des couches de 5 nm d'épaisseur d'Al 2 O 3 sur silicium ont été présentées au Chapitre C. Malgré un rapport O/Al proche de 1,9, l'excédent d'oxygène détecté peut provenir des liaisons O-H ou Si-O à l'interface Si/Al 2 O 3 [Naumann12] ou encore des produits carbonés (CO x ) à la surface de l'échantillon [Petrik13]. De plus une couche adsorbée de H 2 O à la surface de l'échantillon en contact avec l'air ambiant peut également contribuer. ...
Thesis
La réduction des recombinaisons aux surfaces des cellules solaires est un enjeu fondamental pour l'industrie photovoltaïque. La passivation des défauts électriques en surface peut être obtenue par la formation de liaisons chimiques ou par l'apport de charges électriques capables de repousser un type de porteurs. Ces effets peuvent être obtenus grâce à des couches minces fonctionnalisées déposées sur les surfaces des matériaux qui constituent les cellules. Dans le cadre de cette thèse nous avons étudié la passivation de surface du silicium par des couches minces d’Al2O3 déposées par ALD. La caractérisation physique, optique, structurale et chimique des couches déposées a été réalisée. Une optimisation du procédé d’élaboration (nettoyage pré dépôt, paramètres de dépôt et de recuit) de couches d’alumine a été nécessaire pour répondre aux exigences de la réduction de recombinaisons de surface et obtenir des résultats de passivation optimisés. Enfin, différentes briques technologiques nécessaires à l’intégration de ces couches dans l’architecture d’une cellule solaire silicium ont été étudiées et développées.
... If few nanometers have to be measured in air, the contamination from the ambient is also a significant problem, which consists of hydrocarbons accumulated in a few weeks [61]. Extensive comparative investigations showed that the systematic deviation of some measured thin film parameters is primarily also coming from the contamination and the above mentioned aberrations [62][63][64], which results in a larger thickness and amount of material measured by ellipsometry in air than measured by other techniques such as vacuum ultraviolet reflectometry [63] or X-ray fluorescence [64]. When comparing many different techniques [62], sub-nanometer accuracy is a challenge not only because of a possible contamination, but also because of the different spot-sizes, the different interpretation of the interfaces -even a very small lateral inhomogeneity can cause differences if the spot size or the measurement location cannot be exactly the same in the different techniques. ...
... If few nanometers have to be measured in air, the contamination from the ambient is also a significant problem, which consists of hydrocarbons accumulated in a few weeks [61]. Extensive comparative investigations showed that the systematic deviation of some measured thin film parameters is primarily also coming from the contamination and the above mentioned aberrations [62][63][64], which results in a larger thickness and amount of material measured by ellipsometry in air than measured by other techniques such as vacuum ultraviolet reflectometry [63] or X-ray fluorescence [64]. When comparing many different techniques [62], sub-nanometer accuracy is a challenge not only because of a possible contamination, but also because of the different spot-sizes, the different interpretation of the interfaces -even a very small lateral inhomogeneity can cause differences if the spot size or the measurement location cannot be exactly the same in the different techniques. ...
Article
Understanding interface processes has been gaining crucial importance in many applications of biology, chemistry, and physics. The boundaries of those disciplines had been quickly vanishing in the last decade, as metrologies and the knowledge gained based on their use improved and increased rapidly. Optical techniques such as microscopy, waveguide sensing, or ellipsometry are significant and widely used means of studying solid‐liquid interfaces because the applicability of ions, electrons, or X‐ray radiation is strongly limited for this purpose due to the high absorption in aqueous ambient. Light does not only provide access to the interface making the measurement possible, but utilizing the phase information and the large amount of spectroscopic data, the ellipsometric characterization is also highly sensitive and robust. This article focuses on ellipsometry of biomaterials in the visible wavelength range. The authors discuss the main challenges of measuring thickness and optical properties of ultra‐thin films such as biomolecules. The authors give an overview on different kinds of flow cells from conventional through internal reflection to combined methods. They emphasize that surface nanostructures and evaluation strategies are also crucial parts of in situ bioellipsometry and summarize some of the recent trends showing examples mainly from their research.
... However, even with a SiO x interface layer, an increase of the extracted refractive index is observed with decreasing thickness below 10 nm. This can be due to the larger influence of the interface [6] for very thin films and the strong correlation between the refractive index and thickness in the optical models. By plotting the layer thickness as a function of the number of ALD cycles (Fig. 3) we obtain (slope of the curve, The thickness and refractive index uniformity was measured on an 8 inches wafer coated with a 100 nm-thick Al 2 O 3 film. ...
... This temperature is compatible with firing of the contacts that is normally done around 800°C for a few seconds. Jakschin et al. [6] have reported a crystallization temperature of 900°C for a 5 nm Al 2 O 3 film deposited by thermal ALD. They compared samples of different thicknesses and noted that decreasing thickness leads to a higher crystallization temperature, so our results are in agreement with their observations. ...
Article
Full-text available
Thermal Atomic Layer Deposition was used to deposit Al2O3 layers with thickness ranging from 2 to 100nm for surface passivation of silicon solar cells. Various characterization techniques were used to evaluate the chemical, physical and optical properties of the layers and interfaces. Minority carrier lifetime around 2ms was measured for an optimal thickness of 15nm for as-deposited layers on high resistivity n-type silicon substrate. An annealing step at 400°Cincreases lifetime up to 5.7ms for the same structure.
... This corresponds to a refractive index at λ = 632.8 nm of n = 1.874 at 350°C, n = 1.886 at 375°C and n = 1.898 at 400°C, considerably higher than that of α-Al 2 O 3 (n = 1.765 [30] and n = 1.772 [31] at λ = 632.8 nm), but lower than that of t-ZrO 2 (n = 2.192 [32] and n = 1.976 [33] at λ = 632.8 ...
Article
To reveal the fundamental role of structural order of the parent alloy on the oxidation mechanism, thermal oxidation of amorphous and crystalline Al–Zr alloys with identical compositions were investigated in detail. An (Al0.68Zr0.32)O1.66 amorphous oxide emerged on the crystalline Al2Zr alloy, whereas an (Al0.33Zr0.67)O1.83 amorphous oxide formed on the respective amorphous Al68at.%Zr32at.% alloy under the same conditions. Oxidation kinetics was fast and linear for the crystalline Al2Zr alloy but slow and parabolic for the amorphous Al68at.%Zr32at.% alloys. The underlying mechanisms of such striking differences were disclosed by molecular dynamics simulations. The findings thus offer new prospects in the field of surface engineering.
... This "steep interface" assumption seems obvious when considering the atomic layer deposition principles. Nevertheless, since other physical and chemical phenomena occur during the ALD process, it seems crucial to investigate the ALD film layer structure in more detail [30]. This paper proposes, firstly, to investigate the interfacial properties between the silicon substrate and PE-ALD alumina thin films deposited from Al(CH3)3/O2 precursors, and, secondly, to study the structural, optical, mechanical and dielectric properties of the grown layers, focusing on the impact of a post-growth rapid thermal anneal step (in the 500°C -1100°C temperature range) on their evolution. ...
... For determination of the thickness of the amorphous SiO 2 (a:SiO 2 ) the interface was modeled as an effective medium composition of Al 2 O 3 , SiO 2 and Si 18 . Figure 2 shows that both the interface thickness, a, and the volume fraction of a:SiO 2 , b, increase in the irradiated spot. ...
Article
Full-text available
Al2O3 (5 nm)/Si (bulk) sample was subjected to irradiation of 5 keV electrons at room temperature, in a vacuum chamber (pressure 1 × 10-9 mbar) and formation of amorphous SiO2 around the interface was observed. The oxygen for the silicon dioxide growth was provided by the electron bombardment induced bond breaking in Al2O3 and the subsequent production of neutral and/or charged oxygen. The amorphous SiO2 rich layer has grown into the Al2O3 layer showing that oxygen as well as silicon transport occurred during irradiation at room temperature. We propose that both transports are mediated by local electric field and charged and/or uncharged defects created by the electron irradiation. The direct modification of metal oxide/silicon interface by electron-beam irradiation is a promising method of accomplishing direct write electron-beam lithography at buried interfaces.
... If no a priori information is available about the depth profile, the strong wavelength dependence of the penetration depth of light in silicon † at standard wavelengths (250-800 nm) can be utilized for making a "depth scan" when systematically changing the wavelength range used for the fit. 17 The capabilities of ellipsometry for measuring ultra-thin surface 18,19 or even embedded layers 20 have also been revealed for numerous materials and layer structures. Using spectroscopic optical characterizations, not only layer thicknesses and multi-layer structures but also material properties like elemental composition, 4, 21 phase (even in situ 22, 23 ) or crystallinity 3, 24 can be determined (see Fig. 1.1 of Ref. 13). ...
Article
Full-text available
Optical techniques have been intensively developed for many decades in terms of both experimental and modeling capabilities. In spectroscopy and scatterometry material structures can be measured and modeled from the atomic (binding configurations, electronic band structure) through nanometer (nanocrystals, long range order) to micron scales (photonic structures, gratings, critical dimension measurements). Using optical techniques, atomic scale structures, morphology, crystallinity, doping and a range of other properties that can be related to the changes of the electronic band structure can most sensitively be measured for materials having interband transition energies in the optical photon energy range. This will be demonstrated by different models for the dielectric function of ZnO, a key material in optoelectronics and in numerous other fields. Using polarimetry such as spectroscopic ellipsometry, sub-nanometer precision has long been revealed for the thickness of optical quality layers. The lateral resolution of spectroscopic ellipsometry is limited (> 50 μm) by the use of incoherent light sources, but using single-wavelength imaging ellipsometry, a sub-micron lateral resolution can be reached. In case of sub-wavelength structures, the morphology (of e.g. porous or nanocrystalline materials) can be characterized using the effective medium theory. For structure sizes comparable to the wavelength, scatterometry is applied in a broad versatility of configurations from specular to angle resolved, from coherent to incoherent, from monochromatic to spectroscopic, from reectometric to polarimetric. In this work, we also present an application of coherent Fourier scatterometry for the characterization of periodic lateral structures.
Article
In the early 1960s, scientists achieved the breakthroughs in the fields of solid surfaces and artificial layered structures. The advancement of surface science has been supported by the advent of ultra-high vacuum technologies, newly discovered and established scanning probe microscopy with atomic resolution, as well as some other advanced surface-sensitive spectroscopy and microscopy. On the other hand, it has been well recognized that a number of functions are related to the structures of the interfaces, which are the thin planes connecting different materials, most likely by layering thin films. Despite the scientific significance, so far, research on such buried layers and interfaces has been limited, because the probing depth of almost all existing sophisticated analytical methods is limited to the top surface. The present article describes the recent progress in the nanometer scale analysis of buried layers and interfaces, particularly by using X-rays and neutrons. The methods are essentially promising to non-destructively probe such buried structures in thin films. The latest scientific research has been reviewed, and includes applications to bio-chemical, organic, electronic, magnetic, spintronic, self-organizing and complicated systems as well as buried liquid-liquid and solid-liquid interfaces. Some emerging analytical techniques and instruments, which provide new attractive features such as imaging and real time analysis, are also discussed.
Article
Full-text available
The thickness, refractive index, density, and interface properties of thin thermal oxides on both Si- and C-terminated 4H-SiC faces were investigated by ellipsometry using optical models of increasing complexity. We used different parametrizations of the dielectric function, a transition layer, and also investigated the multisample approach. The thickness of the transition layer increases with decreasing oxide thickness below the layer thickness of about 30 nm, it correlates with the surface roughness measured by atomic force microscopy, and it was found to be significantly larger for the C-terminated than that for the Si-terminated face. For oxide layer thicknesses larger than 30 nm, the refractive index of the bulk oxide layer is the same as that of thermal SiO2 on Si. We found an apparent decrease in mass density (as well as optical density) with decreasing oxide thickness using a combination of ellipsometry and backscattering spectrometry, which can be explained by the surface roughness, depending on the layer thickness revealed by atomic force microscopy.
Article
Full-text available
In the VUV spectrum we see a significant decrease in reflection due to organic contamination on the surface of mirrors. To study VUV mirrors it is requisite to have calibration standards. Such standards are useless as calibration tools if the surface has organic contamination. For our standard, we use a thermally oxidized silicon wafer with a 27 nm oxide overlayer. We found that silicon wafer samples capped with native oxide acquire 0.1 to 0.2 nm of organic contamination within two hours of being cleaned with stored in closed, but nonvacuum, conditions. After a week there is an additional 0.2 to 0.5 nm deposition after which no further significant deposition is measured up to 90 days. We place the samples in air within one cm of a xenon excimer lamp that radiates 7.2 eV photons which remove half of the remaining contamination every minute. Five minutes exposure is sufficient to clean both fresh and stored samples. Data are determined using spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS). Additionally this paper addresses the need to ensure that these characterization tools are not a source of organic contamination. We determined that the antechamber of our XPS was contaminating samples at a rate of 0.6 nm/30 min as they waited for transfer to the analysis chamber. This contamination was virtually eliminated by attaching an oxygen radical source (ORS) device (Evactron® C De-Contaminator RF Plasma Cleaning System) directly to the antechamber.
Article
Full-text available
Optical constant spectra for silicon and thermally grown silicon dioxide have been simultaneously determined using variable angle of incidence spectroscopic ellipsometry from 0.75 to 6.5 eV. Spectroscopic ellipsometric data sets acquired at multiple angles of incidence from seven samples with oxide thicknesses from 2 to 350 nm were analyzed using a self-contained multi-sample technique to obtain Kramers–Kronig consistent optical constant spectra. The investigation used a systematic approach utilizing optical models of increasing complexity in order to investigate the need for fitting the thermal SiO2 optical constants and including an interface layer between the silicon and SiO2 in modeling the data. A detailed study was made of parameter correlation effects involving the optical constants used for the interface layer. The resulting thermal silicon dioxide optical constants were shown to be independent of the precise substrate model used, and were found to be approximately 0.4% higher in index than published values for bulk glasseous SiO2. The resulting silicon optical constants are comparable to previous ellipsometric measurements in the regions of overlap, and are in agreement with long wavelength prism measurements and transmission measurements near the band gap. © 1998 American Institute of Physics.
Article
Full-text available
Nanometre thin high-k hafnium oxide (HfO2) or Hf silicate layers combined with a sub-nm SiO2 layers have become Si compatible gate dielectrics. Medium energy ion scattering (MEIS) analysis has been applied to a range of such MOCVD grown HfO2/SiO2 and HfSiOx(60%Hf)/SiO2 gate oxide films of thickness between 1 and 2 nm on top of Si(100), before and after decoupled plasma nitridation (DPN). MEIS in combination with energy spectrum simulation provides quantitative layer information with sub-nm resolution on these layer structures and their atomic composition that is in excellent agreement with a) the as grown layer parameters and b) results obtained from techniques, such as SE, XPS, XRF and XTEM. MEIS analysis of a metal gate, high-k TiN/Al2O3/HfO2/SiO2/Si stack shows the interdiffusion, after thermal treatment, of Hf and Al from the caplayer, which was inserted to modify the metal gate work function.
Article
Ever progressive miniaturization of integrated circuits and breakthroughs in knowledge of biological macromolecules deriving from DNA and protein surface research are propelling ellipsometry, a measurement technique based on phase and amplitude changes in polarized light, to greater popularity in a widening array of applications. Ellipsometry, without contact and non-damaging to samples, is an ideal measurement technique to determine optical and physical properties of materials at the nano scale.
Article
Our aim was to make possible to use spectroscopic ellipsometry for mapping purposes during one measuring cycle (minimum one rotation period of polarizer or analyzer) on many sample points. Our new technique uses non-collimated (non-parallel, mostly diffuse) illumination with an angle of incidence sensitive pinhole camera detector system and it works as an unusual kind of imaging ellipsometry. Adding multicolour supplemets, it provides spectral (a few wavelengths on a 2D image or a full spectrum along a line) information from rapid measurements of many points on a large (several dm2) area. This technique can be expanded by upscaling the geometry (upscaling the dimensions of the instrument, and characteristic imaging parameters such as focal lengths, distances, etc.). The lateral resolution is limited by the minimum resolved-angle determined by the detector system, mainly by the diameter of the pinhole. (The diameter of the pinhole is a compromise between the light intensity and the lateral resolution.) Small-aperture (25 mm diameter) polarizers are incorporated into both the polarization state generator (PSG) and polarization state detection (PSD) components of the instrument.
  • P Petrik
  • E Szilagyi
  • G Battistig
  • M Fried
  • G Dobrik
  • L P Biro
P. Petrik, E. Szilagyi, G. Battistig, M. Fried, G. Dobrik, L. P. Biro, J. Appl. Phys. 106 (2009) 123506.
College of Engineering, A. Science, Handbook of Thin Film Materials, College of Engineering and Applied Science
  • T Lichtenstein
  • U Rochester
T. Lichtenstein, U. of Rochester. College of Engineering, A. Science, Handbook of Thin Film Materials, College of Engineering and Applied Science, University of Rochester, 1979.
  • J A Van Den
  • M A Berg
  • A Reading
  • M Parisini
  • B Kolbe
  • S Beckhoff
  • M Ladas
  • P Fried
  • P Petrik
  • T Bailey
  • T Noakes
  • S D Conrad
  • Gendt
J. A. Van Den Berg, M. A. Reading, A. Parisini, M. Kolbe, B. Beckhoff, S. Ladas, M. Fried, P. Petrik, P. Bailey, T. Noakes, T. Conrad, S. D. Gendt, ECS Transactions 25 (2009) 349.
  • A E Pap
  • P Petrik
  • B Pecz
  • G Battistig
  • I Barsony
A. E. Pap, P. Petrik, B. Pecz, G. Battistig, I. Barsony, Z. Szekrenyes, K. Kamaras, Z. Schay, Z. Nenyei, IEEE Proceedings 978-1-4244-1959-0 (2008) 219.
  • R J Archer
R. J. Archer, J. Electrochem. Soc. 104 (1957) 621.
  • E Strein
  • D Allred
E. Strein, D. Allred, Thin Solid Films 517 (2008) 1011.