Pure Aluminum Films Research Articles

Fatigue fracture mechanism of sputtered AlSi alloy film by pulsating-tension cyclic loading testing

This paper focuses on investigating the differences in fatigue fracture between sputtered aluminum silicon (AlSi) alloy and pure aluminum (Al) films by means of pulsating-tension cyclic loading tests. AlSi alloy film with 1.0 wt% (wt%) silicon (Si) and pure Al film are prepared at a substrate temperature of 450 ℃ and 260 ℃, respectively, by dc magnetron sputtering. After annealing those films at 490 ℃ under a hydrogen and nitrogen atmosphere for 1 hour, the tensile test specimens are prepared using semiconductor fabrication technologies. Quasi-static tensile tests measure the mean yield strength of 52 MPa and 33 MPa for AlSi and Al films, respectively, which are used as reference stress values for fatigue tests. During fatigue tests, a crack is introduced at the one-side edge of pure Al film specimen and propagates straightly to the other side, whereas a zig-zag crack is observed in AlSi alloy film. The crack propagation rate for AlSi film is around one-tenth of that for pure Al film. The mechanism of fatigue crack propagation is discussed based on element mapping results by energy dispersive X-ray analysis.

Electromagnetic Properties of Aluminum-Based Bilayers for Kinetic Inductance Detectors

The complex conductivity of a superconducting thin film is related to the quasiparticle density, which depends on the physical temperature and can also be modified by external pair breaking with photons and phonons. This relationship forms the underlying operating principle of Kinetic Inductance Detectors (KIDs), where the detection threshold is governed by the superconducting energy gap. We investigate the electromagnetic properties of thin-film aluminum that is proximitized with either a normal metal layer of copper or a superconducting layer with a lower $T_C$, such as iridium, in order to extend the operating range of KIDs. Using the Usadel equations along with the Nam expressions for complex conductivity, we calculate the density of states and the complex conductivity of the resulting bilayers to understand the dependence of the pair breaking threshold, surface impedance, and intrinsic quality factor of superconducting bilayers on the relative film thicknesses. The calculations and analyses provide theoretical insights in designing aluminum-based bilayer kinetic inductance detectors for detection of microwave photons and athermal phonons at the frequencies well below the pair breaking threshold of a pure aluminum film.

Morphological and Structural Characterization of Magnetron-Sputtered Aluminum and Aluminum-Boron Thin Films

In microelectronic mechanical systems applications, sputtered aluminum thin films may have large roughness, which promotes the optical degradation and electromigration. This challenge motivated the present research, where magnetron sputtering equipped by radio frequency allowed for preparing aluminum and aluminum-boron thin films. This study evaluated the effect of the sputtering power and the substrate type (silicon wafer and glass slides) on the deposited films. The film’s morphology and structure were characterized via an atomic force microscope and X-ray diffraction. Pure aluminum films’ topographic findings revealed a 25.23 nm average roughness with larger grain size and hillock formation. Conversely, the aluminum-boron films possessed a 3.41 nm average roughness, with smaller grains and hillocks suppression when higher sputtering power was used. The pure aluminum films’ structural analysis uncovered a material with low crystallinity, with (111) and (200) planes diffracting X-rays. On the other hand, aluminum-boron films displayed better crystallinity and a preferential (111) texture. Further characterization demonstrated how the sputtering power, the substrate material, and the studied targets affected the films’ morphological and structural. The improvements in morphological and structural aspects that were observed in the films that were obtained via the aluminum-boron target sputtering make this methodology an appealing alternative for metal films manufacturing.

Development of Large Array of Kinetic Inductance Detectors Using Commercial-Class External Foundries

This study focuses on kinetic inductance detectors (KIDs) fabricated in external foundries using 6 and 8-inch processes. These processes allow us to fabricate large arrays of KIDs in a scalable manner. The support of such a wide variety of astronomical and particle-physics applications requires large numbers of superconducting detectors. For example, the sensitivity of the cosmic microwave background measurements has exponentially improved over the past few decades. The most significant advantage of KIDs is scalability to a large array due to the intrinsic frequency multiplexing scheme. The first step of fabricating a large array of KIDs is designing test chips to check the performance of the 6 and 8-inch micro-electro-mechanical system processes. The KIDs are made of a single pure aluminum film and consist of coplanar waveguide quarter wavelength resonators with a feedline. Each wafer is filled with 32 chips, where the chip size is 20 × 20 mm2, with 48 resonators on each chip. The chips are evaluated using a dilution refrigerator at temperatures ranging from 100 to 500 mK. The quality factor of the resonators, temperature dependency of the resonance frequency, and yields are comparable with those of the KIDs fabricated in a dedicated clean room for superconducting detectors.

Nanomechanical properties of thin films manufactured via magnetron sputtering from pure aluminum and aluminum-boron targets

The present research focuses on the manufacturing of thin films using pure aluminum and aluminum-boron targets. The magnetron-sputtered films were produced at different discharge power levels on glass substrates and silicon wafers. Nanoindentation permitted characterizing the films’ hardness, elastic modulus, and adhesion strength, using the continuous stiffness measurement method and via a scratch test. In addition, this manuscript proposes a methodology to evaluate the film adhesion strength to the substrates upon a scratch test using a Berkovich tip. Nano-mechanical test results revealed that the film deposited from the aluminum-boron target had the larger elastic modulus, hardness, and adhesion strength than the pure aluminum films. Thus, the complementary characterization techniques allowed assessing how the material target, substrate type, and sputtering conditions influenced the mechanical behavior of the films.

Measurement of elastic precursor decay in pre-heated aluminum films under ultra-fast laser generated shocks

This article presents results from laser-driven shock compression experiments performed on pre-heated pure aluminum films at temperatures ranging from 23 to 400 °C. The samples were vapor deposited on the surface of a 500 μm thick sapphire substrate and mounted onto a custom holder with an integrated ring-heater to enable variable initial temperature conditions. A chirped pulse amplified laser was used to generate a pulse for both shocking the films and for probing the free surface velocity using Ultrafast Dynamic Ellipsometry. The particle velocity traces measured at the free surface clearly show elastic and plastic wave separation, which was used to estimate the decay of the elastic precursor amplitude over propagation distances ranging from 0.278 to 4.595 μm. Elastic precursors (which also correspond to dynamic material strength under uniaxial strain) of increasing amplitudes were observed with increasing initial sample temperatures for all propagation distances, which is consistent with expectations for aluminum in a deformation regime where phonon drag limits the mobility of dislocations. The experimental results show peak elastic amplitudes corresponding to axial stresses of over 7.5 GPa; estimates for plastic strain-rates in the samples are of the order 109/s. The measured elastic amplitudes at the micron length scales are compared with those at the millimeter length-scales using a two-parameter model and used to correlate the rate sensitivity of the dynamic strength at strain-rates ranging from 103 to 109/s and elevated temperature conditions. The overall trend, as inferred from the experimental data, indicates that the temperature-strengthening effect decreases with increasing plastic strain-rates.

Atomic force microscopy analysis of surface topography of pure thin aluminum films

Pure aluminium thin films were deposited on stainless and mild steel substrates through rf magnetron sputtering at rf powers of 150 and 200 W. Surface analysis of the films was undertaken using atomic force microscopy. The surface structure evolution, roughness and distribution were examined and discussed. Power spectral density, skewness and Kurtosis parameters were used to explain the nature and distribution of the surface structures on the thin aluminium films as reported from the line profile analyses. The result shows that the morphologies of the surface structures of Al thin films vary with power and substrate type. The coatings also exhibited higher roughness at the power of 200 W. A strong link exists between the atomic force microscopy (AFM) observations and scanning electron microscopy (SEM) analysis, which implies that AFM can be considerably used to study the microstructural evolution of thin films prepared by magnetron sputtering.

Structure, phase composition, and nanohardness of vacuum-annealed multilayer fullerite/aluminum films

The influence of the number of layers and thermal annealing on the structure, elemental and phase compositions, and nanohardness of multilayer fullerite/aluminum films has been studied by scanning electron microscopy, atomic force microscopy, X-ray diffraction, X-ray microanalysis, and nanoindentation. The results demonstrate that sequential growth of five aluminum layers and four fullerite layers, each 50 nm in thickness, on oxidized single-crystal silicon substrates leads to the formation of textured films, which retain 111 texture after vacuum annealing at 620 K (τ = 5 h). In the case of the growth of bilayer films of greater thickness, C60(200 nm)/Al(300 nm), the fullerite and aluminum have a polycrystalline structure with no growth texture. Thermal annealing of the bilayer films leads to the formation of a new phase, AlxC60. The materials studied here possess enhanced nanohardness compared to pure aluminum and fullerite films.

Tailoring the porous nanostructure of porous anodic alumina membrane with the impurity control

Porous Anodic Alumina (PAA) membranes have been fabricated from pure and doped aluminum films (Pure Al, Al–0.5wt.%Cu and Al–1wt.%Si) by the one-step anodization method at room temperature. Formation of the PAA has been performed by three different electrolytes, namely: sulfuric (1 M), oxalic (0.3 M) and phosphoric (0.75 M) acids. For this purpose, potentiostatic anodization in a two-electrode cell configuration has been carried out. High quality alumina with straight pores have been obtained starting from pure aluminum metal films. It is found that, different types of branched structures are formed by aluminum films doped with impurities such as Si and Cu. The relationship between the electrochemical current response and the obtained pore architectures are discussed. Oxide formation rates, barrier layer thickness, vertical and horizontal inter-pore distances are investigated. The present synthesized templates have a large surface area make it one of the mostpopular nanomaterial with applications across a gamut of areas including, energy storage, sensors and biosensors.

Electrochemical Impedance Characterization of Porous Aluminum Oxide after Different Stages of Two-Step Anodization

In this paper, porous anodic aluminum oxide (AAO) membrane was prepared with highly pure aluminum film in 0.3M oxalic acid under dc voltage of 40V at about 8°C by two-step anodization. The morphology of the specimens processed in different stages of two-step anodization was observed with a scanning electron microscope (SEM). The electrochemical behavior of the anodized aluminum after different stages of two-step anodization was studied by electrochemical impedance spectroscopy (EIS). Different equivalent circuits were established to fit EIS experiment data, based on morphologies and structures of AAO during processing. The electrical elements in the equivalent circuits were used to explain experimental parameters.

Kirkendall void formation during room-temperature air-oxidation of thin aluminium and aluminium – lithium alloy films

The effect of room-temperature (~20°C) air-oxidation on void formation in sputter-deposited thin films of aluminum and its alloys was investigated using a transmission electron microscope. It was found that after air-oxidation, only lithium-bearing aluminum alloy films exhibited a high (~4 × 1016 cm−3) density of small (~2 nm) voids, whereas pure aluminum or lithium-free aluminum alloy films did not contain any voids. In lithium-bearing aluminum alloy films, both aluminum and lithium atoms migrate to the surfaces to form their surface oxide during room-temperature ageing after film deposition. In the course of the atom migration, excess vacancies are generated as a result of the large diffusivity difference existing between aluminum and lithium atoms (DLiinAl ≫ DAl) in the alloy matrix. The agglomeration of these excess vacancies led to the formation of so-called Kirkendall voids inside the alloy. Thus the presence of both aluminum and lithium in the alloys was a key factor for generating these Kirkendall voids in the films.

Characterization of sputtered Al-B-Si thin films produced with composite targets for device applications

AbstractSputtered films were deposited over glass and silicon (Si) substrates by using homemade centrifuged aluminum (Al) targets containing boron (B) AlB2 and AlB12 particles as their constituents. Additional films were produced with an as-received aluminum target for comparison purposes. The composite targets were mounted in the magnetron sputtering system working at varying discharge powers, ranging from 200 to 450 W, to produce films with the smallest surface roughness. This roughness analysis showed that films deposited from the composite targets (fabricated by centrifugal casting) possessed lower surface roughness than the pure aluminum films if they were deposited over silicon substrates. Also, preliminary studies of film structure and mechanical properties revealed that the films produced with the composite targets had smaller grain size, dominant compression stresses, higher disorder in the crystalline structure, and higher hardness and elastic modulus, when compared with the films produced with the pure aluminum target.

Laser damage mechanism of porous Al 2O 3 films prepared by a two-step anodization method

Pure aluminum films were deposited on a B270 glass by electron beam evaporation technique. These aluminum films, which were used as anode, were put in sulfuric acid and oxalic acid to prepare porous alumina films using a two-step anodization method. The microstructure and laser damage characteristics of the alumina films were then tested. Results show that the microstructure of the alumina films formed in sulfuric acid and oxalic acid were vertical (cylindrical) pores with different diameters. The laser-damaged spot of the porous films was formed by innumerable small damaged pits with no mutual influence. Films prepared in different acids reveal different damage characteristics and reflect different mechanisms.

Laser-induced damage characteristic of porous alumina optical films

Pure aluminum films were deposited on B270 glass by electron beam evaporation technique. The aluminum films, which were used as anode, were placed in phosphoric acid to prepare porous alumina films using a two-step anodization method. They were subsequently annealed in high temperature to improve transmission. The microstructure, transmitted spectrum and laser damage characteristics of the alumina films were then tested. Their microstructure formed in phosphoric acid was retiform. The transmissivity can be increased efficiently if the annealing temperature is adequately high. The laser-damaged spots of the porous films were formed by innumerable small damaged pits with no mutual effect. In contrast, the laser-damaged spots of the compact films were formed by several larger damaged pits which acted together toward expansion. The damage process of the porous films comprised heat ablation and stress fracture.

Optical calibration for nanocalorimeter measurements

We describe a method for calibration of nanocalorimeters from 573 K to 873 K, using resistive heating and optical pyrometry for temperature measurement. A platinum strip suspended on a silicon nitride membrane serves as both heater and temperature sensor. The calibration described here, relating resistance to temperature, enables subsequent temperature measurement. Measurements of the emissivity of as-deposited and annealed platinum thin films were also performed as a function of wavelength and temperature; these measurements are needed to correct the temperature recorded using the pyrometer. The calibration was validated by measurement of the melting point of a pure aluminum film; the melting point established with a Gaussian fit to the d T/d t data agreed within 0.1% (0.7 K) at 933.5 K. The melting point established from the minimum in the d T/d t plot agreed within 0.6% (5.5 K) at 933.5 K. Finite element modeling was used to characterize the temperature distribution.

ChemInform Abstract: Chemical Vapor Deposition Precursor Chemistry. Part 2. Formation of Pure Aluminum, Alumina, and Aluminum Boride Thin Films from Boron- Containing Precursor Compounds by Chemical Vapor Deposition.

ChemInform Abstract: Chemical Vapor Deposition Precursor Chemistry. Part 2. Formation of Pure Aluminum, Alumina, and Aluminum Boride Thin Films from Boron- Containing Precursor Compounds by Chemical Vapor Deposition.

Effect of annealing on morphology, optical reflectivity, and stress state of Al–(0.19–0.53) wt.% Sc thin films prepared by magnetron sputtering

Degradation caused by formation of hillocks is a problem when traditional aluminum thin film acts as the reflector of the novel automobile lamp in which light emitted diode (LED) serves as the light source. This degradation in lamp performance is attributed to the annealing effect caused by cyclic change of temperature. Thin Al–0.19–0.53 wt.% Sc alloy films prepared by magnetron sputtering are potential candidates for substituting pure aluminum films as the reflector due to their excellent optical reflectivity and corrosion resistance. The annealing effect on the morphology of Al–Sc thin films arisen from thermal cycle during the service of auto lamps is of interest. The films were heated from room temperature to 450 °C and cooled down to examine their surface morphology. Hillocks on the pure Al film could be dramatically reduced in Al–0.19–0.48 wt.% Sc films and their formation was completely inhibited in Al–0.53 wt.% Sc. Examination through scanning electron micrographs (FE-SEM) reveals that Al–0.53 wt.% Sc comprises nano grains that are much finer than those in other films. According to stress analysis on the film, we proposed a mechanism for interpreting the formation of hillocks on the Al film and their inhibition on Al-Sc films. Al–0.53 wt.% Sc thin film is suggested to be a candidate for reflectors in a LED auto lamp.

Effect of Capping Layer on Hillock Formation in Thin Al Films

The effect of capping layers on hillock formation has been studied in pure aluminum films. For this purpose, different capping materials (Ti, Mo, SiO2) of various thicknesses (from 200 A to 1000 A) were deposited on an Al/glass substrate. The density, diameter and height of hillocks were analyzed after annealing for 400 min at 280 °C. As the thickness of the capping layer increases, the hillock density, diameter and height decreases. The total volume of hillocks per unit area of the film decreases while both the thickness and the biaxial modulus of the capping layers increases. Using a spherical cap model of the bulge test, a simple equation which can predict the total volume of hillocks per unit area of the film is suggested. The comparison shows excellent agreement between the modeled and experimental profiles.

The influence of proton irradiation on the microstructure of pure Al films

In this work, we investigated the microstructures of pure aluminum films irradiated with 160 keV proton by varying the irradiation dose. The vacancy clusters induced by proton irradiation in Al films are characterized by TEM, and the density and size distribution are determined. In the irradiation dose range of our studies, the density and size of vacancy clusters and the dislocation density increase with increasing irradiation dose. Our experimental results show that the dislocation density and size of vacancy clusters increase as the irradiation dose increases from 1×1011 to 4×1011/mm2. When the irradiation dose is as high as 6×1011/mm2, the number density of dislocation loops decreases, while the size of dislocation loop increases obviously. The vacancies tend to form larger clusters, accompanied by the formation of banded structures in the irradiated sample at higher irradiation doses. We found that there were small crystal orientation differences among the boundaries of banded structure. This banded structure is closely related to the degradation of mechanical and physical performance of irradiated material.

A model for hillock growth in Al thin films controlled by plastic deformation

The dependence of total hillock volume (per unit area) on film thickness and annealing temperature has been studied in pure aluminum films. The total hillock volume (per unit area) increases linearly with both the film thickness and annealing temperature. Other characteristics involve a critical temperature and a critical thickness for hillock formation. It is shown that these phenomena can be explained by assuming that hillock growth is controlled by plastic deformation in the surrounding film. A simple analytical model is developed to account for these observations.