AlN films sputtered on iridium electrodes for bulk acoustic wave resonators

We report on the growth of highly c-axis-textured aluminum nitride (AlN) films sputtered on iridium layers for thin film bulk acoustic microresonator applications. Iridium films were evaporated on oxidized silicon covered with Ti or Mo/Ti seed layers to improve crystal quality and adhesion. The crystal quality of the AlN films as a function of the crystal structure of the iridium electrode and its pre-conditioning by Ar⁺ bombardment was analyzed by X-ray diffraction, atomic force microscopy and wet chemical etching. Solidly mounted resonators using a single low-impedance layer of silicon dioxide for acoustic isolation were fabricated for a preliminary assessment of the piezoelectric activity of AlN films and the influence of the AlN/Ir stack on the performance of the devices. The electromechanical coupling factor of the AlN films was derived by fitting the electrical response of the resonators to Mason's physical model. AlN films exhibited very high coupling factors (7.5%) barely dependent on the width of the rocking-curve around the AlN 00·2 reflection. The high acoustic impedance of iridium electrodes provided resonators with quality factors higher than those of similar resonators built on lighter electrodes, such as molybdenum.     

In-situ observations of stress-induced thin film failures

In this work, the failure modes of thin films under thermo-mechanical treatments were observed via in-situ white beam X-ray topography. The in-situ experiments were carried out using an experimental setup on Beamline 2-2 at the Stanford Synchrotron Radiation Laboratory. Magnetron sputtered polycrystalline thin films of Ta and CrN on Si substrates were selected for the present study due to their disparate states of intrinsic residual stresses: the Ta film was anisotropically compressive and the CrN film was isotropically tensile. Under a similar heating-cooling cycle in air, the two types of films exhibited distinct failure modes, which were observed in-situ and in a quasi-real-time fashion. The failures of the samples have been interpreted based on their distinctive growth stress states, superimposed on the additional stress development associated with different forms of thermal instabilities upon heating. These included the formation of oxide for the Ta/Si sample, which led to an increase in compressive stress, and a phase change for the CrN/Si sample, which caused the isotropic stress in the film to become increasingly tensile.     

Characteristics of post-annealed SrTiO3 thin films prepared by mirror-confinement-type ECR plasma sputtering

SrTiO₃ films were synthesized on Pt/Ti/SiO₂/Si multilayer substrates by mirror-confinement-type ECR plasma sputtering without substrate heating. All films were found to be well crystallized at a substrate temperature below 450 K. A low temperature post-annealing of the films by electromagnetic-wave radiation drastically improved the crystallographic and electric properties of Pt/SrTiO₃/Pt/Ti/SiO₂/Si capacitors. The crystallinity of the films indicated little variation by post-annealing, but irradiation of electromagnetic wave was confirmed to be effective for decreasing the post-annealing time and temperature. The electric properties of films annealed without Pt upper electrodes were better than those with them, and the film dielectric constant reached a value of 260, which is nearly equal to the bulk one, at an annealing temperature of 573 K.     

Electromigration testing of Ti: W/Al and Ti: W/Al-Cu film conductors

Electromigration-induced failures in metal film interconnections influence the reliability of integrated circuits. For shallow (< 1 μm) junction devices a barrier- metal interconnection system such as Ti: W/Al has been proposed to eliminate contact pitting due to silicon-aluminum reactions. The addition of copper to aluminum films is known to improve the electromigration resistance of aluminum film interconnections. Glass-passivated Ti: W/Al and Ti: W/Al-Cu (1.6 wt.% Cu) film conductors (9 μm wide, 1.14 mm long and 170 nm/800 nm thick) on oxidized silicon substrates were subjected to a current stress of 10⁶ A cm^-2 in the temperature range 150–270°C. Mean-time-to-failure data indicate an improvement of approximately a factor of two in electromigration resistance due to the addition of copper. This improvement is smaller than that reported by others. Life test data are consistent with activation energies of 0.61±0.05 and 0.71±0.03 eV for Ti: W/Al and Ti: W/Al-Cu film conductors respectively. Extrapolated mean times to failure are close to 24 and 100 a for Ti: W/Al and Ti: W/Al-Cu films respectively under a current stress of 5 × 10⁵ A cm^-2 at 55°C ambience. Projected failure rates at these operating conditions increase very rapidly with time and approach values of 9 × 10^-7 and 1 × 10^-11 h^-1 for Ti: W/Al and Ti: W/Al-Cu film conductors respectively at 100 000 h.     

The effects of oxygen partial pressure on BST thin films deposited on multilayered bottom electrodes

Ba_0.5Sr_0.5TiO₃ (BST) thin films have been deposited by r.f. magnetron sputtering on silicon and platinum-coated silicon substrates with different buffer and barrier layers. BST films deposited on Si/SiO₂/SiN/Pt and Si/SiO₂/Ti/TiN/Pt multilayer bottom electrode have been used for the fabrication of capacitors. XRD and SEM studies were carried out for the films. It was found that the crystallinity of the BST thin film was dependent upon oxygen partial pressure in the sputtering gas. The role of multilayered bottom electrode on the electrical properties of Ba_0.5Sr_0.5TiO₃ films has been also investigated. The dielectric properties of BST films were measured. The results show that the films exhibit pure perovskite phase and their grain sizes are about 80–90 nm. The dielectric properties of the BST thin film on Si/SiO₂/Ti/TiN/Pt electrode was superior to that of the film grown on Si/SiO₂/SiN/Pt electrode.     

Oxidation of nanostructured Ti films produced by low energy cluster beam deposition: An X-ray Photoelectron Spectroscopy characterization

We used in-situ X-ray Photoelectron Spectroscopy (XPS) to study the oxidation process of a cluster-assembled metallic titanium film exposed to molecular oxygen at room temperature. The nanostructured film has been grown on a Si(111) substrate, in ultra high vacuum conditions, by coupling a supersonic cluster beam deposition system with an XPS experimental chamber. Our results show that upon in-situ oxygen exposure Ti^3 + is the first oxidation state observed, followed by Ti^4 +, whereas Ti^2 + is practically absent during the whole process. Our results compare well with the existing literature on Ti films produced using other techniques.     

Influences of Ti film thickness on electrochemical properties of Si/Ti/Cu film electrodes

Si and Si/Ti films were fabricated on a Cu current collector (substrate) using the DC sputtering system. The Ti film as a buffer layer was inserted between the Si film and the Cu current collector. Their structural and electrochemical properties were investigated with various Ti film thicknesses of 20-90 nm. The Si and Ti films deposited on a polycrystalline Cu substrate were amorphous. The Si/Ti/Cu film electrode exhibited better electrochemical properties than the Si/Cu electrode in terms of capacity, charge-discharge efficiency, and cycleability. In the Si/Ti/Cu electrode, the film electrode with a 55 nm Ti film thickness showed the best electrochemical properties: 367 microA h/cm2 initial capacity, 91% efficiency, and 50% capacity retention after 100 cycles. These good electrochemical properties are attributed to the enhanced adhesion between the Si and Ti films. Additionally, the modified surface morphology of Si film with a cluster structure could withstand the lateral volume change during the charge-discharge process.     

Influence of aluminum nitride interlayers on crystal orientation and piezoelectric property of aluminum nitride thin films prepared on titanium electrodes

Highly c-axis-oriented aluminum nitride (AlN) thin films have been prepared on titanium (Ti) bottom electrodes by using AlN interlayers. The AlN interlayers were deposited between Ti electrodes and silicon (Si) substrates, such as AlN/Ti/AlN/Si. The crystallinity and crystal orientation of the AlN films and Ti electrodes strongly depended on the thickness of the AlN interlayers. Although the sputtering conditions were the same, the X-ray diffraction intensity of AlN (0002) and Ti (0002) planes drastically increased, and the full-width at half-maximum (FWHM) of the X-ray rocking curves decreased from 5.1° to 2.6° and from 3.3° to 2.0°, respectively. Furthermore, the piezoelectric constant d₃₃ of the AlN films was significantly improved from − 0.2 to − 4.5 pC/N.     

Self-assembly of ultrathin composite TiO2/polymer films

A wet layer-by-layer self-assembly of composite TiO₂/polymer films on Si and Al₂O₃/Al, substrates has been studied by AFM, STM, and ellipsometry techniques. The quality of the first adsorbed TiO₂ layer has been found to be the governing factor in multilayer film growth. The first layer consists of single particles and particle agglomerates 30–120 nm wide The surface coverage in the layer is determined by the chemical composition of the substrate surface and water pH in post-adsorption rinsing procedure. Well-packed TiO₂/polymer film completely covering the surface has been prepared in five adsorption cycle on Al₂O₃/Al substrate. The film remained crack-free after heat treatment at 300°C. I–V curves measurement reveals high resistivity (R∼10¹⁰ O in the voltage range from −2 to +3 V) of TiO₂/polymer films prepared in ten adsorption cycles.     

Inspection of intermediate stress-induced electronic traps in Si/Al2O3 system

The physical characteristics of device-grade thin silicon film at (1 0 0) grown on α-Al₂O₃ substrate using the chemical vapour deposition (CVD) technique has been studied in this paper. Its thickness, crystalline structure, elemental inter-diffusion in the interface region and the quality were characterized by X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), core level X-ray photoelectron spectroscopy (XPS) and nuclear resonance reaction ²⁷Al(p,γ)²⁸Si, respectively. The results of stoichiometric defect profile and individual silicon suboxide (such as SiO, and Si₂O₃ components with respect to the metallic Si element) formation in the intermediate region were observed. The deep traps located around E_c=0.26eV, in ∼500 nm thick n-type Si films, were attributed to the defects caused by the strain of the silicon lattice. Raman spectroscopy was used to evaluate the compressive stress in the Si film.     

Interface stress induced hardness enhancement and superelasticity in polytetrafluoroethylene/metal multilayer thin films

Polytetrafluoroethylene (PTFE)/Al, PTFE/Cu, and PTFE/Ti multilayer thin films have been deposited in order to investigate effects of interface energy on mechanical properties. PTFE, which has a low surface energy of 19.2 mJ/m², was used to introduce a large interface energy into multilayer thin films. PTFE thin film was deposited by rf magnetron sputtering using a PTFE sheet target. Al, Cu, and Ti were deposited by dc magnetron sputtering. The multilayer thin films were fabricated sequentially without breaking vacuum. Substrate used was aluminosilicate glass. The modulation period was changed from 6.7 to 200 nm. The total thickness was about 200 nm for all samples. The internal stress of metal layers changed from tensile to compressive and increased with decreasing modulation period for all of PTFE/Al, PTFE/Cu, and PTFE/Ti. Both hardness enhancement and superelasticity were observed in the results of nanoindentation measurements. The energy dissipated during nanoindentation process (one load and unload cycle) decreased with decreasing modulation period. The minimum value of the ratio of dissipated/loaded energy was < 40%, which is smaller than the values obtained for monolithic PTFE or metal films (about 73% for PTFE and 87% for Al, 72% for Cu, and 71% for Ti, respectively). This meant that the PTFE/metal nano-multilayer thin films became more elastic with decreasing modulation period. The tendency of change in the mechanical properties strongly correlated to internal stress. Mechanisms involved in anomalous behaviors in film hardness and elasticity were discussed based on the relationship to interface energy, interface stress, and internal stress, induced by multilayering of the films. It is concluded that a large compressive stress introduced in the thin films increased the energy needed to deform elastically or plastically the thin film during indentation, resulting in the increase in hardness and elasticity. The nanoindentation analysis of the multilayer thin films emphasized that in PTFE/metal multilayer thin films mechanical properties of the films depend on interface stress induced by the accumulated interface energy, being independent of bulk materials properties composing thin films, resulting in increase in hardness and elasticity.     

Characteristics of post-annealed SrTiO3 thin films prepared by mirror-confinement-type ECR plasma sputtering

SrTiO₃ films were synthesized on Pt/Ti/SiO₂/Si multilayer substrates by mirror-confinement-type ECR plasma sputtering without substrate heating. All films were found to be well crystallized at a substrate temperature below 450 K. A low temperature post-annealing of the films by electromagnetic-wave radiation drastically improved the crystallographic and electric properties of Pt/SrTiO₃/Pt/Ti/SiO₂/Si capacitors. The crystallinity of the films indicated little variation by post-annealing, but irradiation of electromagnetic wave was confirmed to be effective for decreasing the post-annealing time and temperature. The electric properties of films annealed without Pt upper electrodes were better than those with them, and the film dielectric constant reached a value of 260, which is nearly equal to thebulk one, at an annealing temperature of 573 K.     

Deposition of a-C:N films and evaluation of their robustness in electrochemical applications

A multilayer a-C:N film electrode deposition process has been developed using the filtered cathodic vacuum arc (FCVA) system based on the highly conductive silicon wafer with a Ti interlayer for ohmic contacts. Its robustness has been evaluated under the practical electrochemical conditions and shows that it has no pin-hole and no breaking point happened when voltages are applied on it. Extremely large errors will arise in the electrochemical characterizing a-C:N film electrodes (hydrogen and oxygen evolutions and oxygen reduction) when there is a pin-hole or a broken point in the films. And the error caused by the non-ohmic contact amplifies at a high potential range. It is expected that non-robust a-C:N film electrodes and non-ohmic contacts mislead the electrochemical characterizing on a-C:N films.     

Electrical properties of AlNx/Al/Mo composite film prepared for use in thin-film transistors

In the present study, AlN_x/Al/Mo composite films with various thicknesses of AlN_x and Al layers were prepared to replace commercial AlNd/Mo composite film as the gate metal of the two metal layers (namely the gate metal and the source-drain metal) in thin-film transistor (TFT) specimens. The prerequisite for the TFT device is that no hillock is formed. The electrical properties of the AlN_x/Al/Mo TFT device rival those of the AlNd/Mo TFT device. One of eight kinds of AlN_x/Al/Mo composite films (0.05 µm/0.2 µm/0.07 µm) without hillocks was compared with the AlNd/Mo (0.25 µm/0.07 µm) composite film. The line width after development and strip inspections, the I_g (gate leakage current)-V_g (gate voltage) curve, the coating film resistance to electricity, the contact resistance between the indium tin oxide (ITO) film and the metal film, the I_d-V_g curve, and the critical dimension loss (CD loss) were compared. The experimental results indicate that the metal line widths for these two composite films are similar. The coating film resistance, the contact resistance between the ITO film and the metal film, and the I_d-V_g curve for the AlN_x/Al/Mo TFT device were similar to those for the AlNd/Mo TFT device. The CD loss shown in the AlN_x/Al/Mo TFT device was lower than that for the AlNd/Mo TFT device.     

Microstructural changes of Al/amorphous SiC layered films subjected to heating

Layered specimens composed of aluminium and a-SiC films were prepared at room temperature by r.f. magnetron sputtering. a-SiC/Al/a-SiC triple-layered films were heated in the trans mission electron microscope (TEM), and thein-situ microstructural changes were observed continuously. In order to estimate the interfacial reaction process kinematically, electrical resistance measurements were conducted on Al/a-SiC double-layered films heated isothermally at 573, 598 and 623 K. Thein-situ TEM observations showed that no pronounced interfacial reactions occurred up to about 600 K, that silicon precipitates were formed and grew around 673 K, and that Al₄C₃ compounds were produced during heating at temperatures above about 753 K. The electrical resistance of Al/a-SiC double-layered film heated isothermally increased gradually with heating time, and this increase was closely related to the volume fraction of silicon precipitates in the aluminium film. From the kinematical analyses of the results of electrical resistance measurements, an activation energy of the interfacial reaction process of aluminium with a-SiC films in the temperature range 573 to 623 K was found to be about 2.6 eV, being close to the bond energy of Si-C.     

Photoluminescence of erbium-oxygen-codoped silicon multilayers prepared by molecular beam epitaxy

Erbium-oxygen-codoped silicon multilayer film, which consists of alternate erbium-oxygen-codoped silicon layers and oxygen-doped silicon layers, was synthesized by molecular beam epitaxy. Er³⁺-related luminescence from erbium-oxygen-codoped multilayer film is stronger than that of erbium-oxygen-codoped monolayer film and shows a weak temperature quenching behaviour. These improvements may be explained by the effect of O-doped Si layers in the multilayer film. The O-doped Si layers have a wide gap which prohibits the energy backtransfer effectively. At the same time, photogenerated carriers from silicon nanocrystals in O-doped Si layers could transfer the energy to neighboring Er³⁺ ions in Er-O-codoped Si layers by a tunneling process, thus enhancing the Er³⁺-related luminescence.     

AuCl4 ions-assisted fabrication of bimetallic {Poly(ethylenimine)-Ag/Au} multilayer polyelectrolyte film and application in electrocatalysis

Bimetallic {Poly(ethylenimine) (PEI)-Ag/Au} multilayer film was in situ simultaneously fabricated by alternating immersions of a substrate in PEI-Ag⁺ and AuCl₄⁻ solutions followed by chemical reduction with NaBH₄ solution. In the process, the AuCl₄⁻ ions not only play an important role of a reaction reagent, but also served as an assembly reagent. Au, Ag nanoparticles (NPs) were observed with a spherical morphology and well-dispersed in the composite multilayer film, and the size of Au NPs in the bimetallic {PEI-Ag/Au} multilayer film was smaller than that of the single Au NPs formed in {PEI/Au} multilayer films. It was also very interesting to observe that this bimetallic {PEI-Ag/Au} multilayer film exhibited more efficient electrocatalytic activity for the oxidation of ascorbic acid than the multilayer film containing only single Au or Ag NPs. These results indicated that this bimetallic composite multilayer film may be potentially applied in electrochemical biosensors.     

Electrochemical behavior of p-Si/TiC in aqueous HF

The electrochemical behavior of p-Si in HF solution by modification of the silicon electrolyte interface is studied. For this purpose, the samples were coated with titanium carbide (TiC) deposited by RF pulverization of titanium under methane/argon atmosphere. The current-potential characteristics of TiC-coated electrodes in 5% HF are similar to the ones obtained with silicon. The scanning electron microscopy observation of the Si/CH_x surface structures shows that the coating is stable in HF environment but it can be destroyed upon flowing of an anodic current. The results presented suggest that a TiC film layer can be used as a potential tool for low-thickness masking and patterning. In addition, a promising class of optical filters is introduced, based on a p-Si/porous silicon/TiC.     

Preparation of titanium-dioxide films by heating titanium/silicon-dioxide structures on silicon in oxygen

 When titanium/silicon-dioxide (Ti/SiO₂) structures prepared by depositing titanium (Ti) on thermally oxidized silicon in vacuum were heated to temperatures of 800–1000°C in flowing oxygen gas, silicon surfaces were covered with a mixture films containing preferentially (110)-oriented Ti0₂ instead of the SiO₂ films. The thickness of the mixture films could be determined by that of the deposited Ti films. Titanium silicide grew only in the region near between the grown mixture film and the silicon substrate. The dielectric constants of the grown mixture films increased exponentially with increasing oxidation temperature and increased slowly with increasing the Ti film thickness, while the breakdown field strength increased slowly with increasing oxidation temperature and increased exponentially with increasing the Ti film thickness. The oxide films prepared at 1000°C had dielectric constants of (15–25)ɛ_o resistivities of 10¹⁰–10¹¹ Ω cm, and breakdown field strengths of about 10⁶ V/cm. Received: 10 February 1998 / Accepted: 10 March 1998     

RF sputtered piezoelectric zinc oxide thin film for transducer applications

This paper demonstrates the substrate dependency of the c-axis zinc oxide growth in radio-frequency sputtering system. Different deposition conditions were designed to study the influences of Si, SiO₂/Si, Au/Ti/Si, and Au/Ti/SiO₂/Si substrates on the piezoelectric and crystalline qualities of the ZnO thin films. Experimental results showed that the multilayer of Au/Ti/SiO₂/Si-coated silicon substrate provided a surface that facilitated the growth of ZnO thin film with the most preferred crystalline orientation. The 1.5 μm-thick thermally grown amorphous silicon dioxide layer effectively masked the crystalline surface of the silicon substrate, thus allowing the depositions of high-quality 20 nm-thick titanium adhesion layer followed by 150 nm-thick of gold thin film. The gold-coated surface allowed deposition of highly columnar ZnO polycrystalline structures. It was also demonstrated that by lowering the deposition rate at the start of sputtering by lowering RF power to less than one-third of the targeted RF power, a fine ZnO seed layer could be created for subsequent higher-rate deposition. This two-step deposition method resulted in substantially enhanced ZnO film quality compared to single-step approach. The influence of stress relaxation by annealing was also investigated and was found to be effective in releasing most of the residual stress in this layered structure.