Browsing by Author "Liu, Xuwen"

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Aluminum oxide/titanium dioxide nanolaminates grown by atomic layer deposition: Growth and mechanical properties
(2017-01-01) Ylivaara, Oili M E; Kilpi, Lauri; Liu, Xuwen; Sintonen, Sakari; Ali, Saima; Laitinen, Mikko; Julin, Jaakko; Haimi, Eero; Sajavaara, Timo; Lipsanen, Harri; Hannula, Simo-Pekka; Ronkainen, Helena; Puurunen, Riikka
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Atomic layer deposition (ALD) is based on self-limiting surface reactions. This and cyclic process enable the growth of conformal thin films with precise thickness control and sharp interfaces. A multilayered thin film, which is nanolaminate, can be grown using ALD with tuneable electrical and optical properties to be exploited, for example, in the microelectromechanical systems. In this work, the tunability of the residual stress, adhesion, and mechanical properties of the ALD nanolaminates composed of aluminum oxide (Al2O3) and titanium dioxide (TiO2) films on silicon were explored as a function of growth temperature (110-300 °C), film thickness (20-300 nm), bilayer thickness (0.1-100 nm), and TiO2 content (0%-100%). Al2O3 was grown from Me3Al and H2O, and TiO2 from TiCl4 and H2O. According to wafer curvature measurements, Al2O3/TiO2 nanolaminates were under tensile stress; bilayer thickness and growth temperature were the major parameters affecting the stress; the residual stress decreased with increasing bilayer thickness and ALD temperature. Hardness increased with increasing ALD temperature and decreased with increasing TiO2 fraction. Contact modulus remained approximately stable. The adhesion of the nanolaminate film was good on silicon.
Comparison of mechanical properties and composition of magnetron sputter and plasma enhanced atomic layer deposition aluminum nitride films
(2018-09-01) Sippola, Perttu; Pyymaki Perros, Alexander; Ylivaara, Oili M.E.; Ronkainen, Helena; Julin, Jaakko; Liu, Xuwen; Sajavaara, Timo; Etula, Jarkko; Lipsanen, Harri; Puurunen, Riikka L.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A comparative study of mechanical properties and elemental and structural composition was made for aluminum nitride thin films deposited with reactive magnetron sputtering and plasma enhanced atomic layer deposition (PEALD). The sputtered films were deposited on Si (100), Mo (110), and Al (111) oriented substrates to study the effect of substrate texture on film properties. For the PEALD trimethylaluminum-ammonia films, the effects of process parameters, such as temperature, bias voltage, and plasma gas (ammonia versus N2/H2), on the AlN properties were studied. All the AlN films had a nominal thickness of 100 nm. Time-of-flight elastic recoil detection analysis showed the sputtered films to have lower impurity concentration with an Al/N ratio of 0.95, while the Al/N ratio for the PEALD films was 0.81-0.90. The mass densities were ∼3.10 and ∼2.70 g/cm3 for sputtered and PEALD AlN, respectively. The sputtered films were found to have higher degrees of preferential crystallinity, whereas the PEALD films were more polycrystalline as determined by x-ray diffraction. Nanoindentation experiments showed the elastic modulus and hardness to be 250 and 22 GPa, respectively, for sputtered AlN on the (110) substrate, whereas with PEALD AlN, values of 180 and 19 GPa, respectively, were obtained. The sputtered films were under tensile residual stress (61-421 MPa), whereas the PEALD films had a residual stress ranging from tensile to compressive (846 to −47 MPa), and high plasma bias resulted in compressive films. The adhesion of both films was good on Si, although sputtered films showed more inconsistent critical load behavior. Also, the substrate underneath the sputtered AlN did not withstand high wear forces as with the PEALD AlN. The coefficient of friction was determined to be ∼0.2 for both AlN types, and their wear characteristics were almost identical.
Effect of tetrahedral amorphous carbon coating on the resistivity and wear of single-walled carbon nanotube network
(2016-05-14) Iyer, Ajai; Kaskela, Antti; Novikov, Serguei; Etula, Jarkko; Liu, Xuwen; Kauppinen, Esko I.; Koskinen, Jari
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Single walled carbon nanotube networks (SWCNTNs) were coated by tetrahedral amorphous carbon (ta-C) to improve the mechanical wear properties of the composite film. The ta-C deposition was performed by using pulsed filtered cathodic vacuum arc method resulting in the generation of C+ ions in the energy range of 40-60 eV which coalesce to form a ta-C film. The primary disadvantage of this process is a significant increase in the electrical resistance of the SWCNTN post coating. The increase in the SWCNTN resistance is attributed primarily to the intrinsic stress of the ta-C coating which affects the inter-bundle junction resistance between the SWCNTN bundles. E-beam evaporated carbon was deposited on the SWCNTNs prior to the ta-C deposition in order to protect the SWCNTN from the intrinsic stress of the ta-C film. The causes of changes in electrical resistance and the effect of evaporated carbon thickness on the changes in electrical resistance and mechanical wear properties have been studied.
Galvanic corrosion of structural non-stoichiometric silicon nitride thin films and its implications on reliability of microelectromechanical devices
(2015) Broas, Mikael; Liu, Xuwen; Ge, Yanling; Mattila, Toni Tuomas; Paulasto-Kröckel, Mervi
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Hardness, elastic modulus, and wear resistance of hafnium oxide-based films grown by atomic layer deposition
(2016-09-01) Berdova, Maria; Liu, Xuwen; Wiemer, Claudia; Lamperti, Alessio; Tallarida, Grazia; Cianci, Elena; Fanciulli, Marco; Franssila, Sami
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The investigation of mechanical properties of atomic layer deposition HfO2 films is important for implementing these layers in microdevices. The mechanical properties of films change as a function of composition and structure, which accordingly vary with deposition temperature and post-annealing. This work describes elastic modulus, hardness, and wear resistance of as-grown and annealed HfO2. From nanoindentation measurements, the elastic modulus and hardness remained relatively stable in the range of 163-165 GPa and 8.3-9.7 GPa as a function of deposition temperature. The annealing of HfO2 caused significant increase in hardness up to 14.4 GPa due to film crystallization and densification. The structural change also caused increase in the elastic modulus up to 197 GPa. Wear resistance did not change as a function of deposition temperature, but improved upon annealing.
Mechanical and optical properties of as-grown and thermally annealed titanium dioxide from titanium tetrachloride and water by atomic layer deposition
(2021-08-31) Ylivaara, Oili M.E.; Langner, Andreas; Liu, Xuwen; Schneider, Dieter; Julin, Jaakko; Arstila, Kai; Sintonen, Sakari; Ali, Saima; Lipsanen, Harri; Sajavaara, Timo; Hannula, Simo-Pekka; Puurunen, Riikka L.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The use of thin-films made by atomic layer deposition (ALD) is increasing in the field of optical sensing. ALD TiO2 has been widely characterized for its physical and optical properties, but systematic information about the influence of thermal history to optical and mechanical properties of the film is lacking. Optical applications require planar surface and tunability of the refractive index and residual stress. In addition, mechanical properties such as elastic modulus and film hardness influence the performance of the layer, especially, when optics is integrated with microelectromechanical systems. In this work, optical properties, density, elemental analysis, residual stress, elastic modulus and hardness of as-grown ALD TiO2 thin films on silicon were studied at temperature range from 80 to 350 °C and influence of post-ALD thermal annealing was studied on films annealed up to 900 °C. ALD TiO2 films were under tensile stress in the scale of hundreds of MPa. The stress depended both on the ALD temperature and film thickness in a complex way, and onset of crystallization increased the residual stress. Films grown at 110 and 300 °C were able to withstand post-ALD annealing at 420 °C without major change in residual stress, refractive index or extinction coefficient. Elastic modulus and hardness increased upon crystallization with increasing ALD temperature. The results presented here help to improve the design of the optical devices by choosing films with desired optical properties, and further help to design the post-ALD thermal budget so that films maintain their desired features.
Nanodiamond embedded ta-C composite film by pulsed filtered vacuum arc deposition from a single target
(2016-11) Iyer, Ajai; Etula, Jarkko; Ge, Yanling; Liu, Xuwen; Koskinen, Jari
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Detonation Nanodiamonds (DNDs) are known to have sp3 core, sp2 shell, small size (few nm) and are gaining importance as multi-functional nanoparticles. Diverse methods have been used to form composites, containing DNDs embedded in conductive and dielectric matrices for various applications. Here we show a method, wherein DND-ta-C composite film, consisting of DNDs embedded in ta-C matrix have been co-deposited from the same cathode by pulsed filtered cathodic vacuum arc (p-FCVA) method. Transmission Electron Microscope (TEM) analysis of these films revel the presence of DNDs embedded in the matrix of amorphous carbon. Raman spectroscopy indicates that the presence of DNDs does not adversely affect the sp3 content of DND-ta-C composite film compared to ta-C film of same thickness. Nanoindentation and nanowear tests indicate that DND-ta-C composite films possess improved mechanical properties in comparison to ta-C films of similar thickness.
On the Mechanical and Corrosion Properties of TiN/654SMO Metal Matrix Composites
(1996) Liu, Xuwen
Helsinki University of Technology | Master's thesis
On the reliability of nanoindentation hardness of Al2O3 films grown on Si-wafer by atomic layer deposition
(2014) Liu, Xuwen; Haimi, Eero; Hannula, Simo-Pekka; Ylivaara, Oili M. E.; Puurunen, Riikka L.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Single Walled Carbon Nanotube network - Tetrahedral Amorphous Carbon composite film
(2015) Iyer, Ajai; Kaskela, Antti; Johansson, Leena-Sisko; Liu, Xuwen; Kauppinen, Esko I.; Koskinen, Jari
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A study on the erosion and erosion-oxidation of metal matrix composites
(2003-10-31) Liu, Xuwen
Doctoral dissertation (monograph)
The potential of metal matrix composites (MMCs) as new generation erosion and erosioncorrosion resistant materials is investigated. As a type of wear attack, erosion involves destructive impact of free-moving particles on a solid. The high-strain-rate and hence adiabatic deformation conditions that the target surface experiences during erosion are often the reasons behind the poor correlation of erosion rate with static mechanical properties of the targets, imparting difficuties on erosion study. On the other hand, a good correlation exists between erosion rate and the thermophysical properties of the target. This, however, suggests limited possibilities of enhancing erosion resistance on a monolithic alloy. A literature study at the beginning elaborates the complex of erosion as a physical phenomenon, and lies the theoretical base for the experiment. In the experiment, nickel-chromium superalloy- and heat resistant steel-based composites are manufactured through a powder metallurgy route. Ceramic particles are added at different volume fractions as the reinforcements. Efforts are made to optimise the powder mixing and powder consolidation procedures to ensure a homogeneous microstructure and full densification of the final composites. Two types of erosion tester are used in the study: a centrifugal erosion tester is for the erosion tests at room temperature, and a gas-blast erosion tester is for the erosion-oxidation tests at high temperature. With the gas-blast tester, both the temperature and oxygen concentration is controlled to simulate real application situations. The discussion is focused on the tests under a 60° angle of impact. Optical and SEM (scanning electron microscopy) observations are carried out on the eroded surfaces to assess the damage pattern and general behaviour of the studying composites. Meanwhile, on cross-sectioned specimens the bonding and material dissolution at the matrix/reinforcement interfacial regions and the deformation situation at the sub-surfaces are examined by EDS (energy dispersion spectroscopy) and microhardness measurements to reveal the erosion mechanisms of the composites. The experimental results suggest that the strain localisation mode is valid in describing erosion process of a ductile metal, but may need modifications when applying to MMCs. The erosion response of a MMC system is determined by not only the internal parameters of the composites, but also external factors involving all parameters of erosion. In addition, this study provides useful guidelines to the selection of composite systems having high erosion resistance for industrial applications and to the future study as well.
Tribological properties of thin films made by atomic layer deposition sliding against silicon
(2018-01-01) Kilpi, Lauri; Ylivaara, Oili M.E.; Vaajoki, Antti; Liu, Xuwen; Rontu, Ville; Sintonen, Sakari; Haimi, Eero; Malm, Jari; Bosund, Markus; Tuominen, Marko; Sajavaara, Timo; Lipsanen, Harri; Hannula, Simo Pekka; Puurunen, Riikka L.; Ronkainen, Helena
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Interfacial phenomena, such as adhesion, friction, and wear, can dominate the performance and reliability of microelectromechanical (MEMS) devices. Here, thin films made by atomic layer deposition (ALD) were tested for their tribological properties. Tribological tests were carried out with silicon counterpart sliding against ALD thin films in order to simulate the contacts occurring in the MEMS devices. The counterpart was sliding in a linear reciprocating motion against the ALD films with the total sliding distances of 5 and 20 m. Al2O3 and TiO2 coatings with different deposition temperatures were investigated in addition to Al2O3-TiO2-nanolaminate, TiN, NbN, TiAlCN, a-C:H [diamondlike carbon (DLC)] coatings, and uncoated Si. The formation of the tribolayer in the contact area was the dominating phenomenon for friction and wear performance. Hardness, elastic modulus, and crystallinity of the materials were also investigated. The nitride coatings had the most favorable friction and wear performance of the ALD coatings, yet lower friction coefficient was measured with DLC a-C:H coating. These results help us to take steps toward improved coating solutions in, e.g., MEMS applications.