477 research outputs found
Ultra-Thin Integrated ALD Alumina Electron-transparent Windows for Nanoreactor Applications
No full textNanocatalysis has received considerable attention in the scientific community due to their superior reactivity compared to their macro-sized counterparts. MEMS nanoreactors allow scientists to view these reactions in-situ. This opens up doors to finally understanding the underlying mechanisms of the nanocatalyst's effectiveness, which may eventually improve our every day lives.This thesis focuses on the viewing port of the device -- the electron-transparent window (ETW) -- and how they can be improved for future generations. From scattering theory, it was determined that the best way to improve the imaging quality of current ETWs was to develop thinner ones. Two questions were then asked: can a thinner ETW be integrated into a nanoreactor process? If so, how will it affect the mechanical strength of the ETW?Aluminum oxide (alumina) was chosen specifically because of its deposition method, atomic layer deposition (ALD). The characteristics and material properties of ALD alumina were assessed and it was determined that they are suitable for ETW applications.Using ALD alumina presented a few challenges in its integration into nanoreactors, especially the use of vapor hydrogen fluoride. ALD alumina ETWs were able to be successfully integrated into a nanoreactor down to 5 nm thick. The 5 nm alumina ETWs are able to withstand a pressure difference at least 0.75 bar, however they were not able to survive inside a TEM as they disintegrated immediately under the electron beam.Alumina ETWs that are 10 nm were able to be imaged in a TEM. These windows were tested in a transmission electron microscope (TEM) and scanning electron microscope (SEM) to showcase the improved electron-transparency. The ultra-thin ALD alumina ETWs can improve the imaging quality of nanoreactors, due to their lower thickness compare to current nanoreactors. The successful release of 5 nm membranes may also be useful for other applications, such as sensors. <br/
Review Article: Recommended reading list of early publications on atomic layer deposition—Outcome of the “Virtual Project on the History of ALD”
No full textAtomic layer deposition (ALD), a gas-phase thin film deposition technique based on repeated,
self-terminating gas–solid reactions, has become the method of choice in semiconductor
manufacturing and many other technological areas for depositing thin conformal inorganic material
layers for various applications. ALD has been discovered and developed independently, at
least twice, under different names: atomic layer epitaxy (ALE) and molecular layering. ALE, dating
back to 1974 in Finland, has been commonly known as the origin of ALD, while work done
since the 1960s in the Soviet Union under the name “molecular layering” (and sometimes other
names) has remained much less known. The virtual project on the history of ALD (VPHA) is a
volunteer-based effort with open participation, set up to make the early days of ALD more transparent.
In VPHA, started in July 2013, the target is to list, read and comment on all early ALD
academic and patent literature up to 1986. VPHA has resulted in two essays and several presentations
at international conferences. This paper, based on a poster presentation at the 16th
International Conference on Atomic Layer Deposition in Dublin, Ireland, 2016, presents a recommended
reading list of early ALD publications, created collectively by the VPHA participants
through voting. The list contains 22 publications from Finland, Japan, Soviet Union, United
Kingdom, and United States. Up to now, a balanced overview regarding the early history of ALD
has been missing; the current list is an attempt to remedy this deficiency.peerReviewe
Interfacial band parameters of ultrathin ALD-Al<sub>2</sub>O<sub>3</sub>, ALD-HfO<sub>2</sub>, and PEALD-AlN/ALD-Al<sub>2</sub>O<sub>3</sub> on c-plane, Ga-face GaN through XPS measurements
No full textUltrathin oxides (UOs) and ultrathin nitrides (UNs) play a crucial role in forming lattice-mismatched semiconductor heterostructures that are fabricated by using semiconducting grafting approach. The grafting approach has shown its great potential to realize GaN-based heterojunction bipolar transistors by fulfilling the missing high-performance p-type nitrides with other p-type semiconductors. A handful of UO and UN dielectrics readily available by atomic layer deposition (ALD) satisfy the requirements of double-sided surface passivation and quantum tunneling for semiconductor grafting. Due to the states existing between the UO or UN conduction band and that of the GaN, the ALD deposited UO or UN layer can generate significant effects on the surface band-bending of GaN. Understanding the band parameters of the interface between UO or UN and c-plane Ga-face GaN can guide the selection of interfacial dielectrics for grafted GaN-based devices. In this study, we performed x-ray photoelectron spectroscopy measurements to obtain the band-bending properties on c-plane, Ga-face GaN samples coated by different ALD cycles of ultrathin-HfO2 or ultrathin AlN. The valence band spectra of GaN coated with ultrathin-ALD-Al2O3, ALD-HfO2, or PEALD-AlN/ALD-Al2O3 were further analyzed to calculate the valence and conduction band offsets between the ALD dielectrics and the Ga-face GaN under different thicknesses and post-deposition annealing conditions of the dielectrics. © 2022 Author(s)
Long-term encapsulation of platinum metallization using a HfO2 ALD - PDMS bilayer for non-hermetic active implants
No full textIn this work, we investigate the insulating performance of an atomic layer deposited (ALD) HfO2 - polymer bilayer for platinum (Pt) metallization. As test vehicles, Pt interdigitated comb structures (IDC) were designed and fabricated on SiO2/Si substrates. The IDCs were first coated with a 100 nm thin HfO2 ALD layer. A group of samples was further encapsulated with a low-viscosity biocompatible polydimethylsiloxane (PDMS) which resulted in an HFO2-PDMS bilayer. All samples were soaked in phosphate buffered saline for 450 days at room temperature. Evaluation of the coatings included monthly optical inspection and electrochemical impedance spectrometry. For ALD-only coated IDC structures, impedance results right after submersion in saline indicated the presence of defects in the layer. Long-term impedance recordings showed a slight drop, indicating water ingress through the defects, further exposing the metal to saline. For the HfO2-PDMS encapsulated samples, on the other hand, stable impedance results were recorded over the duration of the soak study. This suggests the excellent properties of low-viscosity PDMS both in filling the defects of the ALD layer and in maintaining a long-term underwater adhesion to HfO2. The results from this investigation, therefore, propose a new encapsulation method based on HfO2-PDMS bilayer for long-term packaging of active implants incorporating Pt metallization.Bio-Electronic
Towards the production of core-shell nanoparticles with fluidized bed ALD
No full textReducing particle size or material structure size to nanometer scale can make the material properties, such as light absorption and electronic structure, change compared to the same materials at normal scale. This gives them properties that can make them suitable for the development of highly efficient and improved micro-electronics, sensor,medicine, batteries, catalysts and third generation solar cells. There are, however some challenges that need to be overcome in the development of nanoparticle-based devices. The first is protection of nanoparticles against corrosion and oxidation. This phenomenon is increased by the large surface area available for corrosion. Furthermore, when nanoparticles or nanostructures are used in electronic devices, lowresistive electrical contacts should be made between electrodes and nanoparticles. The second challenge is, thus how to make electrical contacts without compromising the material’s nanostructure. This thesis deals with the development of a synthesis process for core-shell nanoparticles, existing of a core that is coated with a thin layer of material that is able to provide protection, or electrical contacts. The first chapter describes the electrical contact between titanium nitride (TiN), which is a metallically conductive material that is used as contact material in electronic devices, and cadmium sulfide (CdS), a II-IV semiconductor that is used in (second generation) thin film solar cells and to increase light absorption in Grätzel-type solar cells. The experiments show that indeed we can make a Ohmic contact between TiN and CdS, meaning that the contact resistance between the two materials is low and that current is not blocked by the contact. The use of thin coatings as protective layers is investigated by coating thin CdS films,which can be used as photo-catalyst in solar hydrogen production cells, with thin, inert titanium dioxide (TiO2) to protect the CdS from corrosion under influence of solar radiation. The goal of this research was to deposit a TiO2 layer that was thick enough to provide full protection against corrosion, yet thin enough to enable electrons to be transferred between the CdS electrode and the electrolyte. The TiO2 coating was deposited with Atomic Layer Deposition (ALD), a technique used to deposit extremely thin layers of material by letting two precursors (A and B) react on the surface of a substrate to form product C. The first step in this process is chemisorption of precursor A. This chemisorption reaction is self-limiting and stops whenever the complete substrate is covered with a monolayer of component A. After completion of pulse A, precursor B is fed to the reactor and reacts with component A to form component C and prepare the surface of the substrate for a new pulse of component A. By repeating this cycle coatings can be made atomic layer by atomic layer. The experiments with TiO2-coated CdS films in photoelectrochemical hydrogen production cells show that, even though the samples are coated with protective TiO2 layers, the CdS remains sensitive to photocorrosion. The photocorrosion mechanism is investigated with electrochemical measurements in which the photocurrent over time can be described with a model that strongly resembles the Johnson-Mehl-Avrami model for phase transitions in solids. Analysis of the experimental results with the model shows that the corrosion starts in small defects in the TiO2 coating and that the corrosion spreads mostly in lateral directions. The next step in the research was to deposit coatings on individual nanoparticles with a fluidized bed ALD reactor (FB-ALD) that was specially developed for this purpose. In this reactor, the nanoparticles are agitated by a constant flow of inert carrier gas. The ALD precursors, tetrakis-dimethylaminotitanium (TDMAT) and water, are added to the carrier gas and hence brought into contact with the nanoparticles and layer-by-layer form a TiO2 shell on the particles. In the design of the reactor that, is used for loose nanoparticles, care has been taken to make the reactor both safe and versatile in operation. Furthermore, the possibility of extensive monitoring of the reactor is provided. With this reactor, silica (SiO2) nanoparticles are coated with 1.6 nm TiO2 layers. The growth rate is 0.32 Å per ALD cycle and independent of precursor pulse time and exposure. Electron microscope analysis (TEM) tells us that particles have a core-shell structure in which the SiO2 core is coated by a homogenous TiO2 layer. To show that this deposition technique can also be used to deposit conductive coatings on nanoparticles, SiO2 nanoparticles have been coated with conductive TiN layers. In this case TDMAT and ammonia (NH3) were used as precursors. The growth rate of TiN showed the saturation plateau that is typical for ALD growth but depended on the amount of ALD cycles: more cycles led to a lower growth rate. This decline in growth rate can be attributed to the formation of reaction by-products that can adsorb on the particle surface and hence block the adsorption of precursor molecules. The TiN-coated nanoparticles did show good electrical conductivity, with a resistance that depended strongly on the deposition conditions. The results of this research are a step towards the use of FB-ALD in the synthesis of core-shell nanoparticles, with batteries and nanostructured third generation solar cells as the most promising applications. Future research should focus on technological challenges of the FB-ALD technique itself and, on a fundamental level, on optimization of the core-shell structure. The fundamental questions relate to the electronic structure of the nanoparticles: the behavior of nanostructured materials can be fundamentally different from behavior of "normal materials”. Fundamental studies, based on calculations and simulations of electronic structure, can determine the ideal core-shell material combination for each application. The particles can be synthesized in a fluidized bed ALD reactor. In the further development of the FB-ALD technique, safety, with respect to the processing of loose nanoparticles, will be the most important aspect to be looked at, especially when dealing with (nano-) toxic materials and materials that are easily oxidized in air. This is mostly important for loading fresh particles and in loading of processed particles. The importance of a proper loading and unloading procedure has been demonstrated with the TiN-coated particles that spontaneously ignited when they came in contact with air. Another, more practical challenge is controlling nanoparticle agglomeration and maintaining a stable, homogeneously fluidized particle bed at large scale. Several techniques are available, besides the vibrating fluidized bed described in this thesis, to break agglomeraties and create an homogeneous fluidized bed at lab scale. Scale-up of these techniques should be investigated. Furthermore, the static head (pressure drop) of large scale fluidized beds is often higher than the desired absolute operating pressure of the ALD reaction. The influence of the relatively large pressure drop over the bed on fluidization homogeneity should be thoroughly investigated. Despite the technological challenges that come with scaling up of the technique, FB-ALD is a promising technique for the production of core-shell nanoparticles. The flexibility in materials selection, both for the core and the shell, and the homogeneity and quality of the coatings will provide a large advantage over other techniques.Materials for Energy Conversion and StorageApplied Science
The Use of Corrected Occlusal X-Ray for Determination of ALD : A Comparative Study
No full textIn this study arch length discrepancy or ALD was studied in 50 untreated patients. The measurements obtained from Hilgers\u27 corrected occlusal x-ray using his Occlusal Paralleling Instrument or O.P.I, were compared with those derived by four conventional methods, - Eye balling, digital caliper determination, conventional mandibular x-ray, and the Rocky Mountain Data Systems computer analysis (31 cases). When using the OPI, results give a higher negative ALD as it takes into consideration the space required to level a curve of Spee.
Measurements were made by the author and 3 faculty members using the above methods. Statistical analysis of the means derived by each procedure and by each researcher were compared in all possible combinations. Reliability of the measurements made by the author was tested by remeasuring randomly selected cases. To 12 of the most difficult cases for caliper measurements were added 4 randomly chosen cases, and these 16 subjects were compared to OPI determinations.
The OPI method was the only one to show significant differences between all combinations of procedures. The mean OPI ALD was the highest of any method. In this study the corrected occlusal x-ray was more consistent between measurements made by different individuals than was the caliper method.
Despite the progress orthodontics has made, there still is not a standardized method for measuring ALD. The O.P.I. helps to standardize the occlusal x-ray. This corrected occlusal x-ray was shown to be a reliable and consistent instrument with which to determine ALD
Ex Situ In Vacuo and In Situ Studies on Mechanisms of ALD Processes
No full textThis master's thesis consists of two parts related to atomic layer deposition (ALD) processes: a literature survey of so-called ex situ in vacuo analysis methods used in investigations of the ALD chemistry and a summary of the work performed by the author using in situ methods.
The first part of the thesis is divided into four sections. In the first two sections ALD as a thin film deposition method is introduced, and in situ and ex situ in vacuo publications related to ALD are summarized. The third section is a general overview of ex situ in vacuo analysis methods, and the final section a literature review covering publications where ex situ in vacuo techniques have been employed in studying ALD processes, with a strong emphasis on analysis methods which are based on the use of x-rays.
The second part of the thesis consists of in situ quartz crystal microbalance and quadrupole mass spectrometry studies of the V(NEtMe)4/D2O, V(NEtMe)4/O3, Mg(thd)2/TiF4 and Cu2(CH3COO)4/D2O ALD processes. The experimental apparatus and related theory are given a brief overview, followed by a presentation and discussion of the results.Tämä pro gradu –tutkielma koostuu kahdesta osasta: atomikerroskasvatus(ALD)prosessien tutkimisessa käytettyjen niin sanottujen ex situ in vacuo –analyysimenetelmien kirjallisuuskatsauksesta, sekä kirjoittajan itse tekemistä in situ –tutkimuksista.
Tutkielman ensimmäinen osa on jaettu neljään osioon. Näistä kahdessa ensimmäisessä esitellään ALD ohutkalvojen kasvatusmenetelmänä ja tiivistetään ALD-prosessien ex situ in vacuo –tutkimukset. Kolmas osio on yleiskatsaus erilaisista ex situ in vacuo –menetelmistä ja viimeinen osio kirjallisuuskatsaus, joka kattaa sellaiset julkaisut, missä ex situ in vacuo –menetelmiä on käytetty ALD-prosessien tutkimisessa. Katsauksessa painotutaan erityisesti röntgensäteitä käyttäviin analyysimenetelmiin.
Tutkielman toinen osa koostuu seuraavien ALD prosessien in situ kvartsikidevaaka ja kvadrupolimassaspektrometritutkimuksista: V(NEtMe)4/D2O, V(NEtMe)4/O3, Mg(thd)2/TiF4 ja Cu2(CH3COO)4/D2O. Aluksi annetaan yleiskatsaus käytetystä laitteistosta ja siihen liittyvästä teoriasta, minkä jälkeen esitellään tutkimusten tulokset
Effect of Different ALD Al2O3 Oxidants on the Surface Passivation of Black Silicon
No full textAbstractWe study how different oxidants in atomic layer deposition of aluminium oxide (ALD Al2O3) affect the surface passivation of black silicon. Here we show that processes using ozone cause higher fixed charge but surprisingly lead to lower lifetimes in black silicon samples as compared to water-based samples. In planar samples however, the best surface passivation is reached with O3-based processes. In case of water as oxidant, the planar wafers suffer from severe blistering and poorer surface passivation, while this seems to be the best process for black silicon. To find a reason for the lifetime differences we also study different Al2O3 stacks where both H2O and O3 are used as oxidants. In conclusion, surface texture seems to affect the optimal oxidant in the ALD process
Comparison of the physical, chemical and electrical properties of ALD Al2O3 on c- and m-plane GaN
No full textThis study compares the physical, chemical and electrical properties of Al[subscript 2]O[subscript 3] thin films deposited on gallium polar c- and nonpolar m -plane GaN substrates by atomic layer deposition (ALD). Correlations were sought between the film's structure, composition, and electrical properties. The thickness of the Al[subscript 2]O[subscript 3] films was 19.2 nm as determined from a Si witness sample by spectroscopic ellipsometry. The gate dielectric was slightly aluminum-rich (Al:O=1:1.3) as measured from X-ray photoelectron spectroscopy (XPS) depth profile, and the oxide-semiconductor interface carbon concentration was lower on c -plane GaN. The oxide's surface morphology was similar on both substrates, but was smoothest on c -plane GaN as determined by atomic force microscopy (AFM). Circular capacitors (50-300 μm diameter) with Ni/Au (20/100 nm) metal contacts on top of the oxide were created by standard photolithography and e-beam evaporation methods to form metal-oxide-semiconductor capacitors (MOSCAPs). The alumina deposited on c -plane GaN showed less hysteresis (0.15 V) than on m -plane GaN (0.24 V) in capacitance-voltage (CV) characteristics, consistent with its better quality of this dielectric as evidenced by negligible carbon contamination and smooth oxide surface. These results demonstrate the promising potential of ALD Al[subscript 2]O[subscript 3] on c -plane GaN, but further optimization of ALD is required to realize the best properties of Al[subscript 2]O[subscript 3] on m -plane GaN
Learnings from an Open Science Effort: Virtual Project on the History of ALD
No full textThis work summarizes learnings from an Open Science effort “Virtual project on the History of ALD” (VPHA), started in 2013 to clarify the early history of atomic layer deposition (ALD). ALD is a multi-tool of nanotechnology and has been e.g. enabler of the continuation of Moore’s law of transistor scaling. ALD has been developed historically through two independent routes: atomic layer epitaxy (ALE) and molecular layering (ML). Especially the details on ML have remained little known to a broader audience. In this contribution, learnings in VPHA are seen from the viewpoint of its voluntary coordinator (the author self) related to historical details of ALD as well as from an organizational viewpoint and some other viewpoints. Selected details related to ALD’s history not fully accurately described in three earlier review articles are pointed out. The work made in VPHA has resulted in journal articles, presentations and an exhibition, and VPHA has in part provided the foundation for granting the 2018 Millennium Technology Prize to Dr. Tuomo Suntola. At the time of writing this contribution, in July 2018, VPHA is still on-going, and more volunteers are welcome to join the effort
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