Thin Film Stacks Research Articles

Modeling crack propagation for advanced 4-point bending testing of metal–dielectric thin film stacks

For monitoring and improving mechanical properties of BEoL (back-end of line) interconnect structures in microprocessor technology, it is crucial to analyze their adhesion and crack propagation properties. In the present investigation, a camera assisted 4-point bending beam technique has been used to obtain fast and reliable adhesion measurements including locally resolved crack length information. To interpret the obtained crack propagation data, a finite-element modeling approach has been utilized. The combination of local measurement of the crack energy release rate and modeling enables to evaluate measurement curves for both symmetric and asymmetric crack propagation modes and to describe the crack propagation properties of the involved film stacks not attainable in such detail by conventional 4-point bending technique.

Exploitation of multiple incidences spectrometric measurements for thin film reverse engineering

In the present paper we determine the optical constants and thicknesses of multilayer thin film stacks, in the visible and near infrared ranges. These parameters are derived from the transmittance and reflectance spectra measured by a spectrophotometer, for several angles of incidence. Several examples are studied, from a simple single layer structure up to a 22-layer dielectric filter. We show that the use of a large number of incidence angles is an effective means of reducing the number of mathematical solutions and converging on the correct physical solution when the number of layers increases. More specifically, we provide an in-depth discussion of the approach used to extract the index and thickness of each layer, which is achieved by analysing the various mathematical solutions given by a global optimization procedure, based on as little as 6 and as many as 32 variable parameters. The results show that multiple incidences, lead to the true solution for a filter with a large number of layers. In the present study, a Clustering Global Optimization algorithm is used, and is shown to be efficient even for a high number of variable parameters. Our analysis allows the accuracy of the reverse engineering process to be estimated at approximately 1 nm for the thickness, and 2 10(-3) for the index of each layer in a 22-layer filter.

Residual stress of AlN films RF sputter deposited on Si(111) substrate

AlN films with (002) crystal orientation are widely used in various resonator-based applications. Low intrinsic stress is one of the important requirements for the thin film stacks employed in electroacoustic devices. In this paper, AlN films were deposited by reactive radio frequency (RF) sputtering with various conditions. The effects of N2 concentrations, sputtering pressure, substrate temperature, and RF power on the microstructure and stress of AlN films were investigated. The X-ray diffraction, atomic force microscopy, and field emission scanning electron microscopy were utilized to investigate the orientation and surface morphology of AlN films. A systematic study of the stress of AlN was done as a function of the deposition parameters. As the nitrogen concentration decreased, the sputtering pressure increased. The residual stress has a change tendency from compressive to tensile. The substrate temperature and RF power affect stress slightly. The N2 concentrations is the major fact that influences on stress of sputtering AlN films.

Electrical Properties of Thin-Film Capacitors Fabricated Using High Temperature Sputtered Modified Barium Titanate.

Simple thin-film capacitor stacks were fabricated from sputter-deposited doped barium titanate dielectric films with sputtered Pt and/or Ni electrodes and characterized electrically. Here, we report small signal, low frequency capacitance and parallel resistance data measured as a function of applied DC bias, polarization versus applied electric field strength and DC load/unload experiments. These capacitors exhibited significant leakage (in the range 8–210 μA/cm2) and dielectric loss. Measured breakdown strength for the sputtered doped barium titanate films was in the range 200 kV/cm −2 MV/cm. For all devices tested, we observed clear evidence for dielectric saturation at applied electric field strengths above 100 kV/cm: saturated polarization was in the range 8–15 μC/cm2. When cycled under DC conditions, the maximum energy density measured for any of the capacitors tested here was ~4.7 × 10−2 W-h/liter based on the volume of the dielectric material only. This corresponds to a specific energy of ~8 × 10−3 W-h/kg, again calculated on a dielectric-only basis. These results are compared to those reported by other authors and a simple theoretical treatment provided that quantifies the maximum energy that can be stored in these and similar devices as a function of dielectric strength and saturation polarization. Finally, a predictive model is developed to provide guidance on how to tailor the relative permittivities of high-k dielectrics in order to optimize their energy storage capacities.

Local structural modifications of the HfO2 layer in the Al2O3 capped high‐k dielectric films as probed by EXAFS

AbstractThe information on the subtle local structural modifications around the Hf atom in HfO2 high‐k dielectric stacks has been demonstrated to be crucial in determining the resulting electronic properties of the complementary metal oxide (CMOS) devices. In this work, using extended X‐ray‐absorption fine structure (EXAFS) spectroscopy, the local structural characterization of Hf‐based advanced gate stacks thin films were investigated. The thin film stacks used in this project are Al2O3 capped HfO2 thin films deposited on silicon substrates. EXAFS simulations and fits were applied to the data in order to extract crucial structural modifications on these films upon postdeposition annealing (PDA). The local crystal symmetry and coordination around the Hf atom were investigated under various annealing conditions. Specifically, in Hf‐based dielectric thin films on silicon substrates capped with Al2O3 layers, the local crystal symmetry, and coordination around the Hf atom were investigated under various annealing conditions. The questions addressed are the effects of various annealing mechanisms to the diffusion mechanisms in the cap layers and modifications on the emerging local structures around the Hf atom. The diffusion of Al into HfO2 films were monitored through the EXAFS simulations. The non‐linear least‐squares fitting to the EXAFS data revealed that the PDA in NH3 (or N2) ambient after HfO2 deposition had prevented Al diffusion to the HfO2 layers.

Study on optimization of YBCO thin film stack for compact NMR magnets

A compact high temperature superconducting (HTS) magnet, which consists of a stack of 500 HTS thin film annuli, was constructed and tested. Each thin film annulus, manufactured by the AMSC using the Rolling Assisted Bi-axially Textured Substrate (RABiTS) method, has a square cross-section of 40mm×40mm with a thickness of 80m. It has a 25-mm center hole created by machining. This paper reports a study on the anisotropic Jc issue due to the rolling procedure of the Ni substrate direction and its impact on field homogeneity. Also, three different stacking methods with rotation angles of 22.5°, 90°, and 0° against the rolling direction have been tested to study their impacts on strength, spatial homogeneity, and temporal stability of trapped fields. Finally, the 500-annulus magnet was tested at 21K under a cryogen-free environment using a GM cryocooler. The spatial field homogeneity and temporal stability were measured at 21K and compared with those obtained in a bath of liquid nitrogen at 77K.

Study of tri-layer antireflection coatings prepared by sol–gel method

Antireflective coatings (ARCs) on tri-layer thin film stacks were studied in this paper. Silica sols have been prepared by acid-catalyzed or base-catalyzed hydrolysis and condensation reactions of tetraethyl orthosilicate. Antireflective nanometric SiO2/TiO2 films are formed on both sides of the glass substrates by combining the sol–gel method and the dip-coating technique. Seen from the transmittance spectra of different films, a maximum light transmittance of 99.9% was obtained at the band of 300–800 nm. Scanning electron microscope (SEM) and atomic force microscopy (AFM) confirm the well-covered surface morphology. By the SEM observations we can see that the films are full of coverage on glass surface and containing no voids or cracks. The image root mean square roughness of the two types of ARCs provided by the AFM is 1.21 and 3.04 nm, respectively. Furthermore, a surface profiler was used to determine the thickness of each layer in the obtained multi-layer coating system.

Polycrystalline Silicon Thin Film Formed By Multiple Pulsed Rapid Thermal Annealing – Intrinsic a-Si Film Thickness Effect

ABSTRACTThe polycrystalline n+/intrinsic silicon thin film stacks with various original intrinsic amorphous silicon layer thicknesses were formed using the multiple pulsed rapid thermal annealing process with the Ni-induced crystallization mechanism. The thick polycrystalline silicon stack was prepared by repeated steps of 1) amorphous silicon thin film deposition, 2) solution oxidation, 3) dehydrogenation, 4) pulsed rapid thermal annealing, and 5) oxide stripping. The poly-Si film properties, such as the grain size, orientation, and volume fraction of the crystalline phase, were related to the original intrinsic silicon film thickness and the total thermal budget. This process is effective in preparing the high volume fraction polycrystalline silicon thin film, which is important for low-cost thin-film solar cells, electronic and optoelectronic devices.

Design of ultrabroadband double-chirped mirror pairs for ultrafast lasers

According to the requirements for design and manufacture, we choose double-chirped method for designing the initial structure of the chirped coatings, and analyze the dependencs of reflectivity and group delay dispersion on the parameters of coatings such as the double-chirped modulation factor and the the proportion of double-chirped layers on the total thin film stack. Further more, we establish their optimal values. Based on the analysis, a chirped mirror pair, whose reflectivity is more than 99.5% and average group delay dispersion is -40 fs2 with a residual ripple of less than 20 fs2 in a wavelength range of 600 nm-1050 nm, is demonstrated, which is to be used in the Ti: Sapphire femtosecond laser system for dispersion compensation.

Neural Implants Containing a Resorbable Chitosan Matrix

Summary We have developed a novel type of neural electrode array for future brain-machine interfaces (BMIs) or neural prosthesis requiring high resolution recording and stimulation on the cortex. The microelectrode arrays developed comprise of a thin film stack of platinum and polyimide or Parylene C insulation, which is finely microstructured for increased flexibility. The electrode stack is temporarily reinforced with a matrix of porous chitosan, which resorbs after implantation. This design ensures low invasiveness and mechanical irritation during prolonged use, while the chitosan matrix ensures the implant is stiff enough for practical handling during the implantation procedure and is resorbed afterwards. Furthermore, it adds haemostatic and antiseptic properties to the implant. The design and fabrication process are presented. In vitro and long-term in vivo test results are shown, showing the suitability for the application targeted.

Misfit strain dependence of ferroelectric and piezoelectric properties of clamped (001) epitaxial Pb(Zr0.52,Ti0.48)O3 thin films

A study on the effects of the residual strain in Pb(Zr0.52Ti0.48)O3 (PZT) thin films on the ferroelectric and piezoelectric properties is presented. Epitaxial (001)-oriented PZT thin film capacitors are sandwiched between SrRuO3 electrodes. The thin film stacks are grown on different substrate-buffer-layer combinations by pulsed laser deposition. Compressive or tensile strain caused by the difference in thermal expansion of the PZT film and substrate influences the ferroelectric and piezoelectric properties. All the PZT stacks show ferroelectric and piezoelectric behavior that is consistent with the theoretical model for strained thin films in the ferroelectric r-phase. We conclude that clamped (001) oriented Pb(Zr0.52Ti0.48)O3 thin films strained by the substrate always show rotation of the polarization vector.

Enhancements in specimen preparation of Cu(In,Ga)(S,Se) 2 thin films

When producing slices from Cu(In,Ga)(S,Se) 2 thin films for solar cells by use of a focused ion beam (FIB), agglomerates form on the Cu(In,Ga)(S,Se) 2 surfaces, which deteriorate substantially the imaging and analysis in scanning electron microscopy. Similar problems are also experienced when depth-profiling Cu(In,Ga)(S,Se) 2 thin films by means of glow-discharge or secondary ion mass spectrometry. The present work shows that the agglomerates are composed of (mainly) Cu, and that their formation may be impeded considerably by either cooling of the sample or by use of reactive gases during the ion-beam sputtering. The introduction of XeF 2 during FIB slicing resulted in excellent images, in which the microstructures of most layers in the Cu(In,Ga)(S,Se) 2 thin film stack are visible, including the microstructure of the 20 nm thin MoSe 2 layer. Acquisition of high-quality two-dimensional and also three-dimensional electron backscatter diffraction data was possible. The present work gives a basis for enhanced SEM imaging and analysis not only in the case of Cu(In,Ga)(S,Se) 2 thin films but also when dealing with further material systems exhibiting similar formations of agglomerates.

Rapid Sol–Gel Fabrication of High-Quality Thin-Film Stacks on Planar and Curved Substrates

A versatile and rapid sol–gel technique for the fabrication of high quality one-dimensional photonic bandgap materials was developed. Silica/titania multilayer materials are fabricated by a sol–gel chemistry route combined with dip-coating onto planar or curved substrates. A shock-cooling step immediately following the thin-film heat-treatment process is introduced. This step was found crucial in the prevention of film crack formation—especially in silica/titania alternating stack materials with a high number of layers. The versatility of this sol–gel method is demonstrated by the fabrication of various Bragg stack-type materials with fine-tuned optical properties by tailoring the number and sequence of alternating layers, the film thickness and the effective refractive index of the deposited thin films. Measured optical properties show good agreement with theoretical simulations, confirming the high quality of these sol–gel fabricated optical materials.

Semi-transparent small molecule organic solar cells with laminated free-standing carbon nanotube top electrodes

We have developed semi-transparent n–i–p organic solar cells (OSC) with free-standing multi-wall carbon nanotube (f-CNT) sheets as transparent top electrodes. By a simple and damage-free room temperature orthogonal liquid solution assisted self-laminating process, f-CNT top electrodes are successfully deposited on top of ZnPc:C 60 bulk heterojunction small molecule OSCs. The cells show high fill factors above 58% as well as efficiencies up to 1.5% and greater long-term stability compared to the device having a metal electrode. For the given cell structure with f-CNT semi-transparent electrodes, the influence of an optical spacer on light absorption is studied by a systematic variation of the hole transport layer thickness, supported by optical simulations. The results strongly indicate that OSCs with f-CNT top electrodes and optimized thin film stack are highly promising for semi-transparent OSCs, which can be used in tandem devices, in tinted smart windows, and similar applications by a simple and damage-free process in roll-to-roll configuration that can be scaled to large area manufacturing.

Applicability of X-ray fluorescence spectroscopy as method to determine thickness and composition of stacks of metal thin films: A comparison with imaging and profilometry

In this work the applicability of X-ray fluorescence spectroscopy (XRF) for fast, accurate and non-destructive determination of the thickness of a variety of single-layer and multi-layer metal thin films deposited on glass and silicon is investigated. Data obtained with XRF is compared with information from profilometry and images from scanning electron microscopy (SEM). Whereas thickness determinations based on profilometry and cross-sectional SEM-imaging have restrictions with respect to thickness of metal stacks or hardness of the metals, XRF has no such limitations. Moreover, XRF can discriminate between sublayers in a multi-layer film, and can also be utilized for compositional analysis and density estimations. Good agreement between thickness data obtained with XRF, profilometry and SEM-images is found, under the justifiable assumption that the density of sputter-deposited and evaporated thin films is ca. 5% below that of bulk metals. Similar XRF-results are found for non-patterned areas (64 mm 2 metal) as well as lithographically patterned areas containing a series of small metal lines (total metal surface ca. 8 mm 2). As a consequence, it is concluded that XRF is a versatile technique for analysis, verification, control or evaluation of the thickness, density or (elemental) composition of thin metal film line-patterns, during their fabrication as well as prior or post to applications.

Multilayer thin-film inspection through measurements of reflection coefficients

A vibration-insensitive interferometer is described to measure the thickness, refraction index and surface profile of thin-film stack at normal incidence. By satisfying the continuous boundary conditions of electric and magnetic fields at interfaces in a multilayer film stack, the reflection coefficient phase of the thin-film stack can be distinguished from the phase of spatial path difference, thus thickness and refraction index can be extracted. The experiment results showed that the measurement precision is significantly increased after the phase analysis was added into the reflectance analysis.

Electric field induced changes in the coercivity of a thin-film ferromagnet

Data are presented which indicate a modification of magnetic anisotropy in the MgO/CoFeB/Pd and MgO/CoFeB/Pt systems, using electric fields of order 500 MV m−1 (0.5 V nm−1) applied across a thermally grown SiO2 as a gate dielectric. The effect is most prominent at low temperature (12 K) and is manifested as a small change in coercivity. The sign of the effect depends on the choice of both capping layer and annealing temperature. The results suggest that both interfaces play a role in the appearance of perpendicular magnetic anisotropy in these thin-film stacks, and not just the interface with MgO.

Simplified design of thin-film polarizing beam splitter using embedded symmetric trilayer stack

An analytically tractable design procedure is presented for a polarizing beam splitter (PBS) that uses frustrated total internal reflection and optical tunneling by a symmetric LHL trilayer thin-film stack embedded in a high-index prism. Considerable simplification arises when the refractive index of the high-index center layer H matches the refractive index of the prism and its thickness is quarter-wave. This leads to a cube design in which zero reflection for the p polarization is achieved at a 45° angle of incidence independent of the thicknesses of the identical symmetric low-index tunnel layers L and L. Arbitrarily high reflectance for the s polarization is obtained at subwavelength thicknesses of the tunnel layers. This is illustrated by an IR Si-cube PBS that uses an embedded ZnS-Si-ZnS trilayer stack.

A surface micromachined thermopile detector array with an interference-based absorber

A thermo-electric (TE) infrared detector array composed of 23 thermopiles, each with 5 thermocouples on a suspended beam of 650 × 36 µm2 dimensions, has been fabricated in a CMOS-compatible MEMS process. The array is used for realization of an IR micro-spectrometer in the 1–5 µm spectral range. Interference filter-based IR absorbers using titanium/aluminum layers with a silicon carbide cavity layer have been designed, fabricated and validated. These thin film stacks are more suitable for the subsequent processes as compared to conventional techniques. The silicon carbide layer is also used for device protection. The TE detector with an interference filter-based absorber features a sensitivity of 294 V W−1 in the 2.15 µm wavelength range and a thermal time constant of 4.85 ms in vacuum.

Implementation of the CMOS MEMS Condenser Microphone with Corrugated Metal Diaphragm and Silicon Back-Plate

This study reports a CMOS-MEMS condenser microphone implemented using the standard thin film stacking of 0.35 μm UMC CMOS 3.3/5.0 V logic process, and followed by post-CMOS micromachining steps without introducing any special materials. The corrugated diaphragm for the microphone is designed and implemented using the metal layer to reduce the influence of thin film residual stresses. Moreover, a silicon substrate is employed to increase the stiffness of the back-plate. Measurements show the sensitivity of microphone is −42 ± 3 dBV/Pa at 1 kHz (the reference sound-level is 94 dB) under 6 V pumping voltage, the frequency response is 100 Hz–10 kHz, and the S/N ratio >55 dB. It also has low power consumption of less than 200 μA, and low distortion of less than 1% (referred to 100 dB).