Evaporated Al Films Research Articles

Nonvolatile and Voltage-Polarity-Independent Write-Once–Read-Many-Times Memory Feature of an Al/AlOₓ:N/n⁺-Si Device

An aluminum oxynitride (AlO_<i>x</i>:N) resistive memory with a large ON/OFF ratio of 10⁶ at a low read voltage of 0.5 V is proposed. The AlO_<i>x</i>:N resistive switching (RS) layer is fabricated using a two-stage thermal oxynitridation process of an evaporated Al film. The Al/AlO_<i>x</i>: N/n^&#x002B;-Si device shows write-once&#x2013;read-many-times (WORM) characteristics and the writing operation is voltage-polarity-independent. The resistance states can be repeatably read <inline-formula> <tex-math notation="LaTeX">$10^{4}\times $ </tex-math></inline-formula> and the data retention time is over 10⁴ s. A high read-disturb immunity is also observed for over 10⁴ s. Data retention and read-disturb characteristics measured at 85 &#x00B0;C predict a ten-year lifetime of the device. The carrier transport and the RS mechanisms are studied and illustrated.

Preparation of Al2O3 film by sol–gel method on thermally evaporated Al film

Sol–gel spin-coated Al2O3 film was deposited on quartz glass substrate with thermally evaporated Al film. Optical micrographs show that the Al film break forming perforated pinholes after the Al2O3 film was thermally processed. Pinholes on the film can be reduced by inserting a metal transition layer between the quartz glass substrate and the Al film but cannot be avoided after thermal process. Doping of scandium in evaporation aluminum cannot perfectly prevent the Al films from breaking and perforation. The breaking and perforation during the fabrication of the Al2O3 film can be dramatically reduced by heating the quartz glass substrate at 320 °C during thermal evaporation. The J–E curves show that the bottom Al electrode greatly affected the dielectric breakdown electric field.

Effect of Deposition Rate on Structure and Surface Morphology of Thin Evaporated Al Films on Dielectrics and Semiconductors

Aluminum (Al) films with thickness of 100 nm were grown on unheated glass, silicon and mica substrates by electron beam evaporation. The deposition rates were adjusted in the range between 0.1 nm/s and 2 nm/s, the pressure in the vac­uum chamber during deposition was lower than 1·10 –3 Pa. The structure and surface morphology of the as-deposited Al films were studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM imaging of the films showed that the mean grain size of thin Al films on all of the substrates increased from 20 nm – 30 nm to 50 nm – 70 nm with increase of the deposition rate. Quantitative AFM characterization showed that for all substrates the root mean square surface roughness increases monotonically with increasing the deposition rate from 0.1 nm/s to 2 nm/s. The observed effects of the deposition rate on the grain size and surface roughness are explained by the fundamental characteristics of the island growth mode, the influence of the background gases and the surface morphology of the bare substrates. DOI: http://dx.doi.org/10.5755/j01.ms.18.4.3088

Solution Processed Aluminum Paper for Flexible Electronics

As an alternative to vacuum deposition, preparation of highly conductive papers with aluminum (Al) features is successfully achieved by the solution process consisting of Al precursor ink (AlH(3){O(C(4)H(9))(2)}) and low temperature stamping process performed at 110 °C without any serious hydroxylation and oxidation problems. Al features formed on several kinds of paper substrates (calendar, magazine, and inkjet printing paper substrates) are less than ~60 nm thick, and their electrical conductivities were found to be as good as thermally evaporated Al film or even better (≤2 Ω/□). Strong adhesion of Al features to paper substrates and their excellent flexibility are also experimentally confirmed by TEM observation and mechanical tests, such as tape and bending tests. The solution processed Al features on paper substrates show different electrical and mechanical performance depending on the paper type, and inkjet printing paper is found to be the best substrate with high and stable electrical and mechanical properties. The Al conductive papers produced by the solution process may be applicable in disposal paper electronics.

Impact of firing on surface passivation of p-Si by SiO2/Al and SiO2/SiNx/Al stacks

Firing impacts on surface passivation provided by a SiO2 and SiO2/SiNx stack with evaporated Al films are studied by capacitance-based techniques on MIS capacitors. For devices with insulator layers consisting solely of as-deposited SiO2, the densities of either interface states (Dit) or fixed charges (Qfc) are hardly influenced by firing. Capping the SiO2 layer with a SiNx layer results in a shift of the peak activation energy of Dit toward the valence band (Ev) of Si. Firing this SiO2/SiNx stack leads to an increase of Qfc, a reduction of Dit, and a moderate shift of peak activation energy of Dit toward Ev. Co-firing with the Al film on top significantly reduces the Qfc, Dit, and Dit peak activation energy, which is resulting from the atomic hydrogen passivation. These results are of particular interest for the development of solar cells with rear surface passivation and local contacts.

Fabrication of Micro-Cantilevers Using RF Magnetron Sputtered Silicon Carbide Films

In the present work, silicon carbide (SiC) films were deposited by RF magnetron sputtering process on Si (100) substrates for micro-cantilever fabrication. The films were deposited without external substrate heating using a ceramic SiC target at 10 mTorr sputtering pressure, 200 W power and 50 mm target-to-substrate spacing. X-ray diffraction pattern shows that the films were amorphous in nature. In order to investigate the chemical inertness, the SiC coated Si substrates were dipped in buffered HF (BHF) at room temperature and in 40% KOH solution at 80 °C for varying length of time. Atomic force microscope was used to investigate surface roughness and morphology of the films before and after chemical processing. Micro-cantilever beams of the SiC film were fabricated by a single mask process. The SiC film was patterned using reactive ion etching (RIE) in SF6-O2 plasma. Thermally evaporated Al film was used as a mask during RIE process. This process also resulted in the formation of convex corners which were exploited for anisotropic etching of Si under the SiC film. The SiC cantilever beams were released by anisotropic etching of Si in KOH at 80 °C without using additional masking material. Scanning electron microscopy was used to observe the fabricated SiC micro-cantilever beams. The morphology of the SiC film after prolonged exposure to KOH was observed to be similar to that of the as-deposited film. The RF magnetron sputtered SiC films were found to be highly inert in KOH and BHF solutions. Due to difficulty in micromachining of bulk SiC material and its high cost, the RF sputtered SiC films on Si can provide a low cost structural material in MEMS.

In situ TEM Straining of Nanograined Al Films Strengthened with Al2O3 Nanoparticles

AbstractGrowing interest in nanomaterials has raised many questions regarding the operating mechanisms active during the deformation and failure of nanoscale materials. To address this, a simple, effective in situ TEM straining technique was developed that provides direct detailed observations of the active deformation mechanisms at a length scale relevant to most nanomaterials. The capabilities of this new straining structure are highlighted with initial results in pulsed laser deposited (PLD) Al-Al2O3 thin films of uniform thickness. The Al-Al2O3 system was chosen for investigation, as the grain size can be tailored via deposition and annealing conditions and the active mechanisms in the binary system can be compared to previous studies in PLD Ni and evaporated Al films. PLD Al-Al2O3 free-standing films of various oxide concentrations and different thermal histories were produced and characterized in terms of average grain and particle sizes. Preliminary in situ TEM straining experiments show intergranular failure for films with 5 vol% Al2O3. Further work is in progress to explore and understand the active deformation and failure mechanisms, as well as the dependence of mechanisms on processing routes.

Characterization of micro-contact properties using a novel micromachined apparatus

This study demonstrated a micromachined testing device consisting of two chips to characterize the micro-contact properties, such as the contact force, the contact resistance and the contact surface roughness, of metal thin films. The testing device was designed to remove the parasitic resistance during the measurement. In addition, the two chips of the testing device can be disassembled and re-assembled; the interfacial properties can be quantified at different contact cycles. Thus, both the qualitative and quantitative relationships between the interfacial properties and the contact resistance can then be investigated. To demonstrate the feasibility of the present apparatus, the contact characteristics of evaporated Al films were characterized at different contact forces and cycles. Potential applications of the present apparatus included the performance enhancement for RF-MEMS switches, micro-connectors and micro-probes.

Adsorption and deposition of anthraquinone-2-carboxylic acid on alumina studied by inelastic electron tunneling spectroscopy, infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy

The adsorption state of anthraquinone-2-carboxylic acid (AQ-2-COOH) deposited from acetone solutions (0.01–1.00 mg/ml) on native oxide surfaces of Al films was characterized by inelastic electron tunneling spectroscopy, infrared reflection absorption spectroscopy, and X-ray photoelectron spectroscopy. The oxide was prepared on evaporated Al films at room temperature in an oxygen-dc glow discharge. The morphology of the deposited AQ-2-COOH on the oxide surfaces was observed and analyzed by atomic force microscopy. These surface analyses showed that AQ-2-COOH is adsorbed predominantly as a uniform nanometer-scale film of carboxylate anions on the oxide surfaces deposited from solutions with concentrations lower than or equal to 0.02 mg/ml. It was found that AQ-2-COOH is adsorbed as both a uniform film of anions and as micron-sized particles of neutral molecules with heights of a few tens of nanometers when AQ-2-COOH is deposited from solutions with concentrations higher than 0.02 mg/ml. A comparison of the results obtained by these surface analytical techniques clearly shows the features and advantages of these analytical techniques.

Soft Imprinting: Creating Highly Ordered Porous Anodic Alumina Templates on Substrates for Nanofabrication

AbstractWe demonstrate a “soft‐imprinting” method for the fabrication of highly ordered porous anodic alumina (HOPAA) templates on different substrates (such as Si, glass slides, and flexible polyimide films) over large areas (&gt; 1.5 cm2). In this process, Ar plasma etching is employed to soft imprint an evaporated Al film on the substrates using a free‐standing HOPAA template as a mask, thus creating ordered nanoindentations on the Al surface. The ordered nanoindentations in turn guide the subsequent anodization of Al to generate HOPAA templates on the substrates (HOPAA–substrates), which inherit the pattern of the free‐standing HOPAA mask. This soft‐imprinting technique is also applicable to the fabrication of HOPAA on flexible polymer films. To demonstrate the potential uses of the HOPAA–substrates in nanofabrication, highly ordered Au nanowire arrays are fabricated on a Si substrate and TiO2 nanotube arrays are prepared on a glass substrate via solution‐ and vapor‐based fabrication processes, respectively.

Selective GaN Epitaxy on Si(111) Substrates Using Porous Aluminum Oxide Buffer Layers

Epitaxial GaN layers were grown by metallorganic vapor phase epitaxy on Si(111) substrates by using aluminum oxide (AlOx) buffer layers. The AlOx was obtained by anodic oxidation of an evaporated Al film. High-resolution X-ray diffraction and room temperature photoluminescence measurements were used to investigate the grown GaN layers. Epi-growth of GaN on the AlOx stripes was shown to be possible, while no growth took place on the neighboring Si surface, hereby demonstrating perfect selectivity.

Microstructure and Electronic Properties of Al/Zr/LiNbO&lt;sub&gt;3&lt;/sub&gt; Multilayers

To obtain both high power durability and fine-dimensional control in high-frequency surface acoustic wave devices, a highly oriented (111) texture was developed in electronic beam evaporated Al films on Zr underlayer. In this paper, the effects of Zr underlayer on the microstructure and resistivity of Al/Zr/LiNbO3 films were investigated. The films show an extremely smooth surface. The optimum annealing temperature is 200 °C to obtain low resistivity. For Al films with Zr underlayer, reactive ion etching with gases containing BCl3 can be more easily performed than that for Al films with Cu underlayer.

Temperature dependence of the dielectric response of anodized Al–Al2O3–metal capacitors

The temperature dependence of capacitance, CM, and conductance, GM, of Al–Al2O3–metal capacitors with Cu, Ag, and Au electrodes has been measured between 100 and 340 K at seven frequencies between 10 kHz and 1 MHz. Al2O3 films between 15 and 64 nm thick were formed by anodizing evaporated Al films in borate-glycol or borate-H2O electrolyte. The interface capacitance at the Al2O3–metal interface, CI, which is in series with the capacitance CD due to the Al2O3 dielectric, is determined from plots of 1/CM versus insulator thickness. CI is not fixed for a given metal–insulator interface but depends on the vacuum system used to deposit the metal electrode. CI is nearly temperature independent. When CI is taken into account the dielectric constant of Al2O3 determined from capacitance measurements is ∼8.3 at 295 K. The dielectric constant does not depend on anodizing electrolyte, insulator thickness, metal electrode, deposition conditions for the metal electrode or measurement frequency. By contrast, GM of Al–Al2O3–metal capacitors depends on both the deposition conditions of the metal and on the metal. For Al–Al2O3–Cu capacitors, GM is larger for capacitors with large values of 1/CI that result when Cu is evaporated in an oil-pumped vacuum system. For Al–Al2O3–Ag capacitors, GM does not depend on the Ag deposition conditions.

Strong ultraviolet and violet photoluminescence from Si-based anodic porous alumina films

We have investigated photoluminescence (PL) from Si-based anodic porous alumina films formed by real-time controlled anodization of electron-beam evaporated Al films. As-anodized samples show three strong PL bands at 295, 340, and 395 nm. These bands blueshift and their intensities decrease after the samples are annealed. When the annealing temperature increases to 1000 °C, the blueshift becomes specially pronounced and meanwhile the structures of the films develop toward crystalline Al2O3. Based on discussions on the thermal annealing behaviors of the PL and PL excitation spectra, we suggest that optical transitions in oxygen-related defects, F+ (oxygen vacancy with one electron) centers, are responsible for the observed ultraviolet and violet PL.

Interface states at the anodized Al2O3-metal interface

The capacitance of Al–Al2O3-metal capacitors with Al2O3 of different thicknesses, formed by anodizing evaporated Al films in ethylene glycol-based or H2O-based electrolytes to different voltages, has been measured between 10 kHz and 1 MHz for ten metal electrodes. The linear dependence of inverse capacitance on anodizing voltage, and thus on Al2O3 thickness, shows that there is an interface capacitance in series with the bulk capacitance due to the dielectric properties of anodized Al2O3. The interface capacitance, CI, is not a characteristic of the Al2O3-metal combination; it depends on the vacuum deposition conditions for the metal electrodes. CI is associated with interface states at the Al2O3-metal interface, not with field penetration into metal electrodes.

Simultaneous real-time study of initial hillocking and changes in overall stress in evaporated Al films during heating

Optical quality Al films were evaporated by an electron beam onto Si wafers in an ultrahigh vacuum system. The as-deposited samples were radiative heated at a rate of 3 °C/s in air environment. During heating, measurements of initial hillocking and changes in overall film stress were performed simultaneously and in real time as a function of time and temperature with a specially designed optical instrument. The physical principles of this instrument are based on laser beam deflection caused by film stress induced wafer bending and partial integrated light scattering from surface roughening. The experimental results show how the initial hillocking is accompanied by changes in the overall stress and yield a very good correlation between the onset of hillock formation and the maximum change in overall stress.

Synthesis of plasma-polymerized hexamethyldisiloxane (HMDSO) films by microwave discharge

The microwave-plasma assisted deposition of hexamethyldisiloxane [HMDSO—(CH 3) 3SiOSi(CH 3) 3] plasma polymer (PP) films for corrosion protection of Al sheet metal surfaces was investigated. Ar, O 2 and H 2 carrier gases at flow rates up to 150 sccm and microwave power densities up to 5 W/cm 2 were used. The process pressure was varied between 1.5 and 3.7mbar. A mass spectrometer was used to monitor the exhaust gas composition and the monomer decomposition. FTIR and XPS measurements were used to characterize the highly cross-linked PP-HMDSO layers. The deposited films exhibited corrosion protection properties similar to those observed for specially prepared evaporated Al films with a chemical resistance to a 1.0mol/l NaOH solution. Also, the adhesion of the coating to the substrate was good. The polymer layers (thickness ∼1 µm) were found to be quite chemically homogenous. Static contact angles <35^ measured with respect to water characterize the wetting properties.

The effects of oxygen exposure on the electrical conductance characteristics of high frequency oxygen plasma anodized Al films

This paper discusses the dependence of the electrical conductance characteristic of high frequency oxygen plasma anodized Al film on the oxygen exposure during the anodization process. The results showed that under the same anodization conditions, the surface resistance of the oxide film is 6.56~ 0.11 Ω □ (where Ω/□ denotes the unit of sheet resistance) when the vacuum evaporated Al film is thick (750 ~ 800 nm) and the resistance is 1256~ 4.81 Ω □ when the Al film is thin (37~ 82 nm). The results also showed that the surface oxide layer in the aluminum oxide film is γ-Al 2O 3 with a lattice constant a o = 7.9124 A ̊ . At a constant temperature, the electrical conductance characteristic of the plasma anodized Al film depends on the oxygen exposure during the anodization process.

Thickness dependence of refractive index for anodic aluminium oxide films

Anodic oxide films on aluminium have been employed in a variety of devices such as solar cell [1], sensors [2] and thin film transistor liquid crystal display (TFT/LCD) [3]. It was reported [3, 4] that anodic aluminium oxide film is a solution to depress hillock formation. Thin film transistors with Al gates and anodic Al2O3 ‡ Si3N4 double layer gate insulators have been successfully fabricated in an 10.4inch diagonal multicolour LCD display panel [3]. Consequently, much progress has been made in understanding the structure [5, 6], composition [7– 10] and electrical properties [6, 7] of anodic aluminium oxide. In previous work it has been shown that an anodic oxide layer formed on pure Al without any prior heat treatment exhibits better dielectric properties, uniformity, and stability than oxide layers formed on Al pre-annealed at 410 8C [6]. Si or Cu doping of Al film was also found to reduce the quality of the anodic oxide layer [5]. For device application, the control of anodic film thickness and uniformity is very important. There are many methods to evaluate the thickness of an anodic oxide film, such as coulometry [11], transmission electron microscopy (TEM), ellipsometry [12], impedance [11], spectrophotometry [13], photoluminescence [14] etc. Coulometry is most convenient for anodic oxide film thickness determination, the thickness being calculated from the charge consumption based on Faraday’s law. The calculated thickness, however, is subjected to uncertainties due to non-ideal current efficiency, roughness of the electrode, film non-stoichiometry, and error in the presumed film density. TEM offers a direct and absolute measurement of film thickness, but is time consuming and destructive. Ellipsometry can determine not only the thickness but also the refractive index of the film [12, 15]. For a top surface layer on a substrate with given optical properties the evaluation of both thickness and refractive index requires the numerical solution of two complex simultaneous equations, and the accuracy of the thickness results depends sensitively on the calculated refractive index [12]. If, however, either the thickness or the refractive index of the top film is known precisely then the numerical solution of the other quantity can be greatly simplified and its accuracy significantly increased. The purpose of this study is to measure the thickness of anodic Al2O3 films precisely by cross-sectional TEM techniques and then use the thickness value to fit the ellipsometry data for the unique solution of refractive index as a function of film thickness. This calculation is made possible by the assumption that the extinction index of the Al2O3 film is zero, or in other words, the film is non-absorbing. Dell’oca [16] has carefully studied anodic Al2O3 films formed on evaporated Al films ellipsometrically and showed that the non-absorbing model fits his experimental data the best. He estimated the extinction index of anodic Al2O3 film to be 0.002. The error in the calculated refractive index is about 0.01 if the extinction index is 0.002 instead of zero. The effect of absorption may be neglected since it affects the refractive index by less than 0.6%. We hope that the refractive index data obtained in the present study can be used in device applications for quick and precise thickness measurement by ellipsometry. Sample preparation is similar to that described previously [5–6]. A 300 nm thick pure Al film was sputtered by DC magnetron on BPSG(borophosphosilicate glass)/SiO2/Si substrates. The substrates were 4-inch diameter, p-type, k1 0 0l Si wafers. Wafers were anodized after the metal deposition without any heat treatment, and the anodization was conducted at room temperature in an AGW electrolyte [6] (AGW electrolyte is a mixture of 3% aqueous solution of taitaric acid and propylene glycol at a volume ratio of 2:8). The wafers were anodized, one at at time, at constant current mode (current density ˆ 0.4 mA/cm2) initially until reaching 100 V, then the anodizing was automatically switched to constant voltage mode until a preset time was reached. To monitor the anodic oxide growth, specimens were anodized for different durations varying from 3.5 to 30 min. TEM samples were prepared for film thickness measurement by ion milling in the usual fashion [17] and examined with a Philips CM20 microscope operating at 200 kV. Using the thickness data obtained from the TEM analysis the refractive index of the anodic oxide film was evaluated by ellipsometry. A Rudolph Research Auto EL-ILL ellipsometer using an He–Ne laser at a wavelength of 632.8 nm was employed for ellipsometry with an incident angle of 708. The refractive index (n) and extinction coefficient (k) for the substrate aluminium are presumed to be 1.3 and 6.5, respectively [16]. Fig. 1a, b, c and d show the morphology of Al2O3

Deformation mechanisms of Al films on oxidized Si wafers

The mechanism for plastic deformation of 0.5 μm thick, 0.5 μm grain-size evaporated Al films on oxidized Si wafers has been studied using wafer curvature measurements over a temperature range from room temperature to 500 °C. Extensive evidence for both morphology changes and plastic deformation was obtained. Transmission electron microscopy confirmed the occurrence of grain growth, and stress changes attributed to recrystallization were observed. Deformation under tension could be explained by dislocation glide according to the kinetics observed in bulk Al at the same temperatures, stresses, and grain sizes. The kinetics of deformation under compression were investigated at 400 °C and were completely different from those under tension. This is either due to a difference in the deformation mechanism or to the occurrence of work softening.