Al-Cu Films Research Articles

Micromechanical Characterization of AlCu Films for MEMS Using Instrumented Indentation Method.

The micromechanical properties (i.e., hardness, elastic modulus, and stress-strain curve) of AlCu films were determined by an instrumented indentation test in this work. For three AlCu films with different thicknesses (i.e., 1 µm, 1.5 µm, and 2 µm), the same critical ratio (hmax/t) of 0.15 and relative indentation depth range of 0.15-0.5 existed, within which the elastic modulus (i.e., 59 GPa) and nanoindentation hardness (i.e., 0.75 GPa, 0.64 GPa and 0.63 GPa for 1 µm, 1.5 µm and 2 µm films) without pile-up and substrate influence can be determined. The yield strength (i.e., 0.754 GPa, 0.549 GPa and 0.471 GPa for 1 µm, 1.5 µm and 2 µm films) and hardening exponent (i.e., 0.073, 0.131 and 0.150 for 1 µm, 1.5 µm and 2 µm films) of Al-(4 wt.%)Cu films for MEMS were successfully reported for the first time using a nanoindentation reverse method. In dimensional analysis, the ideal representative strain εr was determined to be 0.038. The errors of residual depth hr between the simulations and the nanoindentation experiments was less than 5% when the stress-strain curve obtained by the nanoindentation reverse method was used for simulation.

Growth of Al-Cu Thin Films on LiNbO3 Substrates for Surface Acoustic Wave Devices Based on Combinatorial Radio Frequency Magnetron Sputtering

Al-Cu thin films were fabricated by RF magnetron sputtering from aluminum (Al) and copper (Cu) metal targets to improve the acoustic performance of SAW devices on LiNbO3 substrates. To optimize the electrode material for SAW devices, Al-Cu films with various compositions were fabricated and their electrical, mechanical, and acoustic properties were comprehensively evaluated. The Al-Cu films exhibited a gradual decrease in resistivity with increasing Al content. The double-electrode SAW devices composed of Al-Cu films demonstrated a resonant frequency of 70 MHz and an average insertion loss of −16.1 dB, which was significantly lower than that of devices made with traditional Au or Al electrodes. Additionally, the SAW devices showed an increase in the FWHM values of the resonant frequency and a decrease in the insertion loss as the Al content in the IDT electrode decreased. These findings indicate that improving the performance of SAW devices can be achieved by reducing the density of the IDT electrodes, rather than focusing solely on their electrical characteristics.

Laser induced breakdown spectroscopy for composition monitoring of graded Al[sbnd]Cu alloy surface

Laser-induced breakdown spectroscopy (LIBS) is a powerful method for non-invasive elemental composition monitoring which can be applied for processing graded metal alloys, provided that challenges with reliable and rapid spectral data processing are overcome to enable composition analysis in real-time. In this study, we describe an integrated experimental-computational methodology for analyzing LIBS spectra data obtained from graded composition metal alloy surfaces with a mean rate of composition analysis up to 15.7 Hz. The accuracy and speed of this method was assessed via analysis of a graded AlCu film prepared by magnetron sputtering representing a gradient alloy processed by additive manufacturing. Using a pulsed femtosecond laser and gated sCMOS detector, a series of LIBS measurements were taken across the composition gradient of an AlCu sputtered alloy. Energy Dispersive Spectroscopy (EDS) was used both to calibrate composition measurements and quantify the error of the LIBS-based rapid analysis method demonstrated in this paper, which yielded a mean percent error relative to the EDS data in the composition measurement along the gradient as low as 1.4 % when optimized for the cumulative number of probe laser pulses on a single measurement spot. The femtosecond probe pulse method described in this paper allows for the alloy composition to be accurately measured in hundredths of a second, potentially allowing for real-time composition monitoring in gradient alloy processing, including thin film sputtering, additive manufacturing, and other rapid processing technologies for graded metal alloys.

Atomistic mechanism of AlCu thin film alloy growth on trenched Si substrate

Molecular dynamics simulation was employed to study the growth mechanisms of an AlCu thin film on a trenched Si(001) substrate. The atomic interactions between Al, Cu, and Si atoms were described by using the Modified Embedded Atom Method (MEAM). This investigation focused on examining the effect of both incident energy and trench heights on the structure, and the morphology of deposited AlCu thin film. The results reveal that the growth mode follows an island mode, and the AlCu film surface exhibits both non-flat and non-uniform texture. The increase in the trench height leads to a more pronounced change in surface morphology and roughness. However, a higher incident energy can significantly reduce the surface roughness due to the mobility of deposited atoms. In addition, a mixing region was observed at the interface between the film and substrate. It is revealed that increasing the incident energy leads to deeper penetration of Al and Cu atoms into the substrate. More importantly, Al atoms penetrate deeper than the Cu atoms. On the other hand, deposition on a trench with 15 monoatomic layers leads to the formation of voids in the film. However, these voids disappear with higher incident energy. The microstructural evolution during deposition film was also analyzed based on the radial distribution function. The results of this function indicate that changes in trench height and the energy of incident adatoms do not significantly influence the structure of the film.

Study of Cu Content Reference Materials Used in AlCu Films for X-ray Energy/Wavelength-Dispersive Spectrometer Calibration

Study of Cu Content Reference Materials Used in AlCu Films for X-ray Energy/Wavelength-Dispersive Spectrometer Calibration

Self-assembled binary immiscible Cu-transition metal multilayers prepared by co-sputtering deposition

A series of Cu-X (X = W, Mo, Nb, Cr, V, Al) films with similar Cu content (about 50 at.%) were prepared by dual-target co-deposition technique. By using this method, we found that the multilayered structure can be detected in Cu-W, Cu-Mo, Cu-Nb, Cu-V and Cu-Cr films, but not in Cu-Al films. Except for Cu-Al system, the multilayered structures are made up of Cu-rich and X-rich sublayers rather than conventional pure Cu and X layers. Based on our previous work, we further proved that the confocal targets, rotation of sample stage and the existence of diffusion barrier elements are the essential conditions for obtaining the multilayered structure within the films. The formation mechanism of the multilayered structure by co-deposition was discussed. Initially, the multilayered structure is formed by co-sputtering deposition with the sample stage rotation. And the multilayered structure can remain stable and become more prominent with the uphill diffusion of Cu in immiscible systems. However, in miscible systems, because of the interdiffusion and forming intermetallic compounds between Cu and X (such as Al), the initial multilayered structure can be interrupted and disordered. In addition, the morphology of multilayered structure formation also depends on the diffusion barrier abilities of the elements.

Samarium influence on current induced atomic displacement in Aluminium and Copper combinatorial thin film alloys

Combinatorial thin film deposition from vapor phase combined with photolithography was involved in preparation of bidimensional wires for electromigration testing along Al-Cu, Al-Sm and Cu-Sm thin film libraries. Samarium was chosen as alloying element due to its larger atomic radius, directly impacting the crystallographic properties of the conducting alloy and decreasing the total grain boundary surface through amorphisation. As reference, pure Al and Cu wires were additionally deposited and studied. Empirical estimation of the experimental errors obtained during electromigration testing was performed by high throughput measurements and a value around 2% was found for pure Al and Cu metals. The resistance against electromigration of test wires was increased by electrochemical passivation (leading to void pinning), while the empirically evaluated experimental errors increased to 30%. Electrical screening of Al and Cu-based thin film combinatorial libraries was performed at extremely high current densities (MA cm−2). In the Al-Cu system, Al–14 at.% Cu was identified as the alloy able to withstand the highest power density of 2.10 MW cm−2 before failure. The Al-Sm and Cu-Sm thin film libraries showed compositional regions of total or partial amorphization likely impacting the electromigration resistance of the alloys. Electrical screening indicated that Al-6 at.% Sm is the alloy withstanding the highest power density before failure with a value of 2.48 MW cm−2. The highest current densities before electrical failure in conducting lines were measured on pure Al at 1.39 MA cm−2, with a value of maximum power density below 2.62 MW cm−2. Careful discrimination between these two variables is necessary before choosing one of the studied materials as ideal for specific applications.

Texture-enhanced Al-Cu electrodes on ultrathin Ti buffer layers for high-power durable 2.6 GHz SAW filters

Achieving high resistance to acoustomigration and electromigration in the electrodes used in high-power and high-frequency surface acoustic wave (SAW) filters is important to mobile communications development. In this study, the effects of the Ti buffer layers on the textures and acoustomigration and electromigration resistances of the Al-Cu electrodes were studied comprehensively. The results demonstrate that both power durability and electromigration lifetime are positively correlated with the Al-Cu electrode texture quality. Ultrathin (∼2 nm) Ti can lead to the strongest Al-Cu (111) textured electrodes, with a full width at half maximum of the rocking curve of 2.09°. This represents a remarkable enhancement of the power durability of high-frequency 2.6 GHz SAW filters from 29 dBm to 35 dBm. It also produces lifetime almost 7 times longer than those of electrodes without Ti buffer layers in electromigration tests. X-ray diffraction and transmission electron microscopy analyses revealed that these improved acoustomigration and electromigration resistances can be attributed primarily to the reductions in overall and large-angle grain boundaries in the highly Al-Cu (111) textured electrodes. Furthermore, the growth mechanism of highly Al-Cu texture films is discussed in terms of surface-interface energy balance.

A New Microfabrication Method for Ion-Trap Chips That Reduces Exposure of Dielectric Surfaces to Trapped Ions

Accumulated electrostatic charges on the dielectric surfaces of ion traps are known to induce stray fields, leading to ion micromotions. In typical microfabricated ion-trap chips, metal electrodes are electrically isolated using thick dielectric pillars, which can accumulate stray charges on their sidewalls. This letter presents a new microfabrication method for ion-trap chips that reduces the exposure of dielectric surfaces to trapped ions. The dielectric pillars are fabricated with large T-shaped overhangs, and the sidewalls and top surfaces are coated with AlCu (1%) films. The bottom sides of the overhang parts provide electrical isolation. To prevent oxidation of the AlCu (1%) films, the electrode surfaces are coated with an additional Au film. The fabricated chips were implemented to trap 174Yb+ ions, and the laser-induced stray fields were measured. The results indicated that the trap chip fabricated by the newly developed method generates significantly smaller stray fields as compared with previous chips. [2017-0233]

Enhanced electrical conductivity and reliability for flexible copper thin-film electrode by introducing aluminum buffer layer

Great demands for flexible conducting electrodes (FCEs) are promoted considering the rapid development of flexible electronics. Conventional thin-film FCE, however, has limits of low conductivity, poor mechanical reliability and requires complicated fabrication process due to the use of nanowires and nanoparticles. In this work, aluminum-copper bilayer FCEs were fabricated via facile thermal evaporation. Aluminum buffer layer is introduced to improve copper crystallinity and reduce internal thermal stress in the electrode films, resulting in enhanced electrical conductivity and reliability. The preparation condition and thickness of buffer layer (Al) has been optimized. The lowest electrical resistivity of 3.05μΩ·cm was achieved for Al(0.86μm)-Cu bilayer FCE, which is comparable to that of the bulk material (1.77μΩ·cm). Additionally, the Al-Cu films exhibited excellent adhesion and flexibility under a bending radius of larger than 6mm (ε=1.82%) over 1000cycles. And nanoindenter test was also conducted to describe the elastic characteristics of the flexible electrode and explain the mechanism of crack evolution. Meanwhile, superior reliability with negligible resistance changes was observed for Al-Cu FCEs after thermal shock tests.

Surface-driven, one-step chemical vapor deposition of γ-Al4Cu9 complex metallic alloy film

The present paper is a paradigm for the one-step formation of complex intermetallic coatings by chemical vapor deposition. It genuinely addresses the challenge of depositing an intermetallic coating with comparable contents of Cu and Al. Depending on processing conditions, a pure γ-Al4Cu9 and multi-phase Al-Cu films are grown with wetting properties of the former being similar to its bulk counterpart. The deposition process and its parametric investigation are detailed. Two metalorganic precursors are used taking into account their transport and chemical properties, and deposition temperature ranges. On line and ex situ characterizations enlighten the competition which occurs at the growing surface between molecular fragments, and which limits growth rates. Notably, introducing a partial pressure of hydrogen gas during deposition reduces Al growth rate from dimethylethylamine alane (DMEAA), by displacing the hydrogen desorption equilibrium. This Al partial growth rate decrease is not sufficient to achieve a Cu/Al atomic ratio that is high enough for the formation of intermetallics with close Al and Cu compositions. A fivefold increase of the flux of the gaseous copper(I) cyclopentadienyl triethylphosphine CpCuPEt3, whereas the DMEAA flux remains constant, results in the targeted Al/Cu atomic ratio equal to 44/56. Nevertheless, the global growth rate is rendered extremely low by the deposition inhibition caused by a massive phosphine adsorption (-PEt3). Despite these limitations, the results pave the way towards the conformal coating of complex surface geometries by such intermetallic compounds.

Resistance switching of thin AlOx and Cu-doped-AlOx films

Thin copper-doped aluminum oxide (Cu-doped-AlOx) and AlOx films of about 5nm thick were generated by oxidizing the surfaces of Al and Al-5wt.% Cu (Al-Cu) films in an oxygen plasma. According to X-ray photoelectron spectroscopy analyses, these two oxide films were found to be deficient in oxygen and had gradient concentrations of Al and O. The oxide films were employed as resistor layers sandwiched between an Al top electrode and an Al or Al-Cu bottom electrode to form resistive memory devices. The devices demonstrated unipolar resistance switching between high resistance state and low resistance state (LRS), and their resistance ratios measured at +0.2V were around 105. Furthermore, their current–voltage characteristics showed ohmic conduction with the resistance increasing with temperature, in LRS. Conductive filaments were thought to form inside the AlOx film and the Cu-doped AlOx film, causing resistive switching. The resistive memory device using the AlOx film had unstable switching behaviors during cyclic testing, whereas the device using the Cu-doped AlOx film demonstrated stable resistance switching during 100cycles of testing. The presence of the Cu ingredient in the AlOx film is likely to facilitate the formation and rupture of conductive filaments and induced stable resistance switching.

Fabrication and Electrochemical Performance of Interconnected Silicon Nanowires Synthesized from AlCu Catalyst

Highly interconnected silicon nanowires (SiNW) were prepared by a vapor–liquid–solid process using a multilayered AlCu film as the catalyst. The resulting SiNWs had a branched and interconnected structure where the AlCu catalyst was incorporated into the SiNWs. The electrochemical lithiation of the SiNWs was investigated through both deep and shallow cycling. The first cycle Coulombic efficiency for lithiation/delithiation was 93.6% which increased to 99.96% upon cycling. The interconnected nanowire structures assisted in retaining a high Coulombic efficiency upon cycling by limiting the loss of active material during nanowire fracture and providing shorter/multiple electrical paths to the substrate. The performance for repeated shallow discharges, as encountered in a microbattery configuration for wireless sensors, was evaluated.

Molecular dynamics simulation of uniaxial deformation of thin Cu film and Al-Cu heterostructure

The paper discusses the effect of free surfaces and interfaces on mechanical characteristics of a thin Cu film and an Al-Cu film heterostructure. It is shown that the interface of the cyclically deformed Al-Cu heterostructure evolves from a chessboard distribution with a period dependent on the degree of inconsistency of the material lattice constants to anew more energy-beneficial distribution of hillocks and dimples with spatial scales dependent on the film thickness and specimen dimensions.

Effects of metal stacks and patterned metal profiles on the electromigration characteristics in super-thin AlCu interconnects for sub-0.13 μm technology

The electromigration (EM) characteristics of super-thin aluminum-copper alloy (AlCu) interconnects for sub-0.13μm complementary metal-oxide-semiconductor logic technology were investigated by varying the AlCu underlayers and etched sidewall profile of AlCu wire. Super-thin AlCu wire with a Ti/TiN underlayer and a smooth etched sidewall profile was confirmed to have the best EM resistance in terms of the mean-time-to-failure (MTTF) and the failure distribution. The Ti/TiN underlayer is believed to lead to a longer MTTF by dramatically reducing the effective current density at the super-thin AlCu film in the entire of EM test line with a smaller formation of TiAl3 at the TiN/AlCu interface. The smooth etched sidewall profile is considered to induce a steeper EM failure distribution by removing the early EM failure at the rough sidewall of aluminum. In terms of the location of EM-induced voids, the super-thin AlCu film inhibits the formation of EM-induced voids directly under the tungsten via by insignificant current crowding at the via–metal line corner.

ChemInform Abstract: Corrosion Susceptibility Testing of Al-Cu and Al-Cu-Si Films.

The chlorine-induced corrosion susceptibility of Al-Cu films has been measured as a function of the weight percent of Cu. The adverse impact of Cu on the corrosion susceptibility of unsintered films is clearly evident at 0.3% and increases sharply between 1.0 and 2.0% with the latter being six times more corrosive than the former. Adding 1.0% Si to the Al-Cu(2%) alloys tends to lower the corrosion susceptibility of unannealed films by a factor of nine. Annealing (30 min at 450°C) the copper containing films also has a beneficial effect on corrosion susceptibility

Impact of Al in Cu alloy interconnects on electro and stress migration reliabilities

The reliability of Cu interconnects was successfully improved by applying a CuAl alloy seed. However, the effect of additive Al on the reliability is not fully understood. In order to reveal the reliability improvement mechanism, Cu films using CuAl alloy seed were investigated in detail. As stress induced voiding (SIV) as well as electromigration is caused by migration of vacancies and/or Cu atoms, the measured activation energy value of electromigration using CuAl indicates that the fast diffusion paths are Cu grain boundaries. The analysis using high lateral resolution scanning type secondary ion mass spectrometry (nano-SIMS) clarifies that additive Al in ECP-Cu film is mainly localized at grain boundaries. Furthermore, positron annihilation was used to probe vacancy-type defects in Cu films. The CuAl films before recrystallization contain larger and higher density vacancy-type defects. Whereas, the recrystallized CuAl films after annealing above 250 °C contain smaller and lower density defects. Furthermore, CuAl films with annealing above 350 °C contain less Al inside the grains. These results represent that Al atoms in Cu films with annealing above 350 °C are exhausted from inside grains to the grain boundaries, and the spewed Al atoms existing at Cu grain boundary effectively prevents the diffusion of Cu and/or vacancies.

Diameter Dependent Optical Constants of Gold Meso-particles Electrodeposited on Aluminum Films Containing Copper

Electrodeposition of Au meso-particles on anodized and chemically etched Al-Cu films deposited on Si wafers proceeds by instantaneous nucleation and with no diffusion limitations. Both of these phenomena favor the formation of relatively mono-dispersed Au particles. The mean particle diameter varies a function of electrodeposition time in the range of 40-80 nm. Optical constants of Au meso-particles are determined by spectroscopic ellipsometry. A two layer optical model is suggested to determine both extinction coefficients and refractive indexes of Au meso-particles as a function of the mean particle diameter. The absorption peak, associated with surface plasmons, is modeled with two Lorentz oscillators. The maximum of extinction coefficient shifts from 610 to 675 nm as the mean particle diameter increases from 42 to 74 nm.

Width of Anodization Mask Required to Preserve a Metallic Phase during Porous-Type Anodization of Aluminum-Copper Films

Patterned arrays with alternating regions of the metallic phase and porous aluminum oxide are fabricated by using a dense layer of barrier aluminum oxide as a mask for porous-type anodization of 99.5% Al and 0.5% Cu films. The pore curvature at the interface between the metallic phase and porous aluminum oxide is investigated as a function of anodization voltage. The degree of anisotropy, which is defined as a ratio between vertical and lateral propagation of pores, increases slightly as anodization voltage increases. The pore curvature compromises the mask transfer during anodization due to the lateral pore propagation under the anodization mask. For miniaturization of regions composed of Al-Cu, it is important to establish the width of anodization mask required to preserve the metallic phase between two regions of porous aluminum oxide. Our results show that a wide anodization mask is necessary during porous-type anodization of thick Al-Cu films at a chosen set of anodization conditions (40 V, , 5°C).

Electrochemical Micromachining

Porous-type anodization of patterned ∼10 μm thick Al-Cu films results in microstructures with alternating regions of Al-Cu and porous Al 2 O 3 . The volumetric expansion of porous Al 2 O 3 is minimized by increasing the anodization solution temperature. The fidelity of the mask transfer (10 μm wide) is compromised by the lateral pore propagation under the anodization mask. The Al-Cu pillars formed after complete dissolution of porous Al 2 O 3 have a trapezoidal shape. The height of pillars is twice (8 μm) the distance corresponding to the lateral pore propagation (4 μm). In contrast to wet chemical etching, porous-type anodization of patterned Al-Cu films is not isotropic.