Shear mode coupling and tilted grain growth of A1N thin films in BAW resonators.

Polycrystalline A1N thin films were deposited by RF reactive magnetron sputtering on Pt(111)/Ti electrode films. The substrates were tilted by an angle ranging from 40 degrees to 70 degrees with respect to the target normal. A low deposition temperature and a high sputter gas pressure were found ideal for tilted growth. The resulting grain tilt angle amounts to about half the substrate tilt angle. For coupling evaluation, 5 GHz solidly mounted resonator structures have been realized. The tilted grain A1N films exhibited a permittivity in the 9.5-10.5 range and loss tangent of 0.3%. Two shear modes as well as the longitudinal mode could be clearly identified. The coupling coefficient k2(eff) of the fundamental thickness shear mode (TS0) was found to be about 0.5%, which is compatible with a c-axis tilt of about 6 degrees.     

Characteristics of pure-shear mode BAW resonators consisting of (1120) textured ZnO films

Thickness pure-shear mode film bulk acoustic wave resonators (FBARs) made of (1120) textured ZnO films have been fabricated. We also have fabricated FBAR structure consisting of two layers of the (1120) textured ZnO film with opposite polarization directions. This FBAR structure operated in second overtone pure-shear mode and allowed shear-mode FBARs at higher frequency. The effective electromechanical coupling coefficients k2 of pure-shear mode FBAR and second overtone pure-shear mode FBAR in this study were found to be 3.3% and 0.8%, respectively. The temperature coefficient of frequency (TCF) of thickness extensional mode FBAR, pure-shear mode FBAR, and second overtone pure-shear mode FBAR were measured in the temperature range of 10-60 degrees C. TCF values of -63.1 ppm/degrees C, -34.7 ppm/degrees C, and -35.6 ppm/degrees C were found for the thickness extensional mode FBAR, the pure-shear mode FBAR, and the second overtone pure-shear mode FBAR, respectively. These results demonstrated that pure-shear mode ZnO FBARs have more stable temperature characteristics than the conventional thickness extensional mode ZnO FBARs.     

Synthesis of textured thin piezoelectric AlN films with a nonzero c-axis mean tilt for the fabrication of shear mode resonators

A method for the deposition of thin piezoelectric aluminum nitride (AlN) films with a nonzero c-axis mean tilt has been developed. The deposition is done in a standard reactive magnetron sputter deposition system without any hardware modifications. In essence, the method consists of a two-stage deposition process. The resulting film has a distinct tilted texture with the mean tilt of the c-axis varying roughly in the interval 28 to 32 degrees over the radius of the wafer excluding a small exclusion zone at the center of the latter. The mean tilt angle distribution over the wafer has a circular symmetry. A membrane-type shear mode thickness-excited thin film bulk acoustic resonator together with a micro-fluidic transport system has been subsequently fabricated using the two stage AlN deposition as well as standard bulk micro machining of Si. The resonator consisted of a 2-microm-thick AlN film with 200nm-thick Al top and bottom electrodes. The resonator was characterized with a network analyzer when operating in both air and water. The shear mode resonance frequency was about 1.6 GHz, the extracted device Q around 350, and the electromechanical coupling kt2 2% when the resonator was operated in air, whereas the latter two dropped down to 150 and 1.8%, respectively, when the resonator was operated in pure water.     

Experimental investigation of acoustic substrate losses in 1850-MHz thin film BAW resonators

After optimizing for electromechanical coupling coefficient K², the main performance improvement in the thin film bulk acoustic wave resonators and filters can be achieved by improving the Q value, i.e., minimizing the losses. In Braggreflector- based solidly mounted resonator technology, a significant improvement of Q has been achieved by optimizing the reflector not only for longitudinal wave, the intended operation mode, but also for shear waves. We have investigated the remaining acoustic radiation losses to the substrate in so-optimized 1850-MHz AlN resonators by removing the substrate underneath the resonators and comparing the devices with and without substrate by electrical characterization before and after the substrate removal. Several methods to extract Q-values of the resonators are compared. Changes caused by substrate removal are observed in resonator behavior, but no significant improvement in Q-values can be confirmed. Loss mechanisms other than substrate leakage are concluded to dominate the resonator Q-value. Difficulties of detecting small changes in the Q-values of the resonators are also discussed.     

Computer simulation of normal grain growth in polycrystalline thin films

A modified Monte Carlo method is proposed to simulate the two-dimensional normal grain growth in polycrystalline thin films. With the newly modified method, not only the simulation efficiency is improved but also the simulated time exponent of grain growth attained n = 0.49 ± 0.01, which is very close to the theoretical value for steady grain growth n = 0.5. Simulation of the complete process of normal grain growth including the steady state is made possible by means of the present method. The grain size distribution in the simulated thin films was found to vary continuously and slowly with time, the gamma and the Hillert functions may be two of the expression forms during its transition, and the latter corresponds to quasi steady grain growth. The so called self-similarity of the grain size distribution during the normal grain growth in two-dimensions is also discussed according to the simulation results.     

Combined atomistic and mesoscale simulation of grain growth in nanocrystalline thin films

We have combined molecular-dynamics (MD) simulations with mesoscale simulations to elucidate the mechanism and kinetics of grain growth in nanocrystalline palladium with a columnar grain structure. The conventional picture of grain growth assumes that the process is governed by curvature-driven grain-boundary (GB) migration. Our MD simulations demonstrate that, at least in a nanocrystalline material, grain growth can also be triggered by the coordinated rotations of neighboring grains so as to eliminate the common GB between them. Such rotation–coalescence events result in the formation of highly elongated, unstable grains which then grow via the GB migration mechanism. These insights can be incorporated into mesoscale simulations in which, instead of the atoms, the objects that evolve in space and time are discretized GBs, grain junctions and the grain orientations, with a time scale controlled by that associated with grain rotation and GB migration and with a length scale given by the grain size. These mesoscale simulations, with physical insight and input materials parameters obtained by MD simulation, enable the investigation of the topology and long-time grain-growth behavior in a physically more realistic manner than via mesoscale simulations alone.     

Fiber mode coupling in transmissive and reflective tilted fiber gratings

Whereas core-mode reflection and core-mode-to-radiation-mode coupling in tilted fiber Bragg gratings is well understood, as is coupling between a core mode and higher-order core and cladding modes in untilted gratings, here we analyze in detail the coupling among core modes and cladding modes in reflective and transmissive tilted fiber gratings. We show that strong coupling between an LP(01) core mode and the exact (1m) cladding modes occurs in a transmissive tilted grating for nearly any tilt angle except angles close to 90 degrees , whereas the LP(01)-to-(lm) cladding mode coupling (l not equal 1) is appreciable only for tilt angles just below 90 degrees (~88 degrees ). In a reflective grating, strong coupling between the LP(01) core mode and the exact (1m) cladding modes occurs only for angles less than ~5 degrees , whereas coupling to (1m) cladding modes for m > 1 occurs only for angles greater than ~5 degrees . Coupling among bound core modes exhibits a similar behavior, except that in general the coupling is stronger. Experimentally we show coupling to both higher-order bound core modes and cladding modes in a transmissive tilted grating at visible and near-infrared wavelengths.     

Self-limiting low-temperature growth of crystalline AlN thin films by plasma-enhanced atomic layer deposition

We report on the self-limiting growth and characterization of aluminum nitride (AlN) thin films. AlN films were deposited by plasma-enhanced atomic layer deposition on various substrates using trimethylaluminum (TMA) and ammonia (NH₃). At 185 °C, deposition rate saturated for TMA and NH₃ doses starting from 0.05 and 40 s, respectively. Saturative surface reactions between TMA and NH₃ resulted in a constant growth rate of ~ 0.86 Å/cycle from 100 to 200 °C. Within this temperature range, film thickness increased linearly with the number of deposition cycles. At higher temperatures (≥ 225 °C) deposition rate increased with temperature. Chemical composition and bonding states of the films deposited at 185 °C were investigated by X-ray photoelectron spectroscopy. High resolution Al 2p and N 1s spectra confirmed the presence of AlN with peaks located at 73.02 and 396.07 eV, respectively. Films deposited at 185 °C were polycrystalline with a hexagonal wurtzite structure regardless of the substrate selection as determined by grazing incidence X-ray diffraction. High-resolution transmission electron microscopy images of the AlN thin films deposited on Si (100) and glass substrates revealed a microstructure consisting of nanometer sized crystallites. Films exhibited an optical band edge at ~ 5.8 eV and an optical transmittance of > 95% in the visible region of the spectrum.     

BAW temperature sensitivity and coupling in langanite

One of the new materials belonging to the trigonal class 32, to which quartz belongs, is langanite (LGN, La3Ga5.5Nb0.5O14). High-quality LGN single crystals are now available, and, although similar in composition and structure to langasite (LGS, La3Ga5SiO14), LGN has smaller thermal expansion coefficients and comparable piezoelectric constants to LGS. These are desirable material properties for both SAW and BAW applications that require low frequency dependence on temperature. This paper examines in detail the LGN characteristics: phase velocity, temperature coefficient of frequency (TCF), electromechanical coupling coefficient, and power flow angle for both singly and doubly rotated plate cuts. Contour plots of these characteristics are constructed, revealing orientation regions where zero TCF and high coupling exist and suggesting potentially interesting cuts for practical BAW device design. Temperature compensated cut regions with coupling coefficients as high as 0.16 are predicted, which is twice the value for AT-cut quartz, along with a temperature compensated cut with cubic behavior around room temperature for one of the sets of material constants used. With such desirable properties, LGN is a promising candidate material for BAW applications requiring low temperature sensitivity with superior bandwidth characteristics due to its values of coupling coefficient larger than quartz. Several other orientations with low TCF and high coupling are also identified.     

AlN薄膜的KrF准分子脉冲激光沉积

研究了采用脉冲激光沉积(PLD)技术在Si(100)衬底上AlN薄膜的制备及其性质。结果表明,衬底温度从室温到800℃的范围内,所得到的AlN薄膜为(002)择优取向的纤锌矿结构。随着衬底温度的升高,AlN薄膜从纳米晶结构转为多晶结构,同时表面微粗糙度上升。AlN晶粒呈柱状生长机制。     

Observation and analysis of self-organized surface grain structures in silica films under nonepitaxial growth mode

Silica films under present reactive electron beam deposition conditions have depicted a novel self-organized surface grain structures when probed through atomic force microscopy, 2D fast Fourier transform and glancing incidence X-ray diffraction techniques. The formation of such ordered surface grain structures is observed to be strongly correlated to the nucleation and growth process of the silica films. However, the nature of the substrate (amorphous or crystalline) and multilayer geometries have influenced the shapes, sizes and abundances in the grain structures and the ordering. The strain mediation of such ordered structures when buried under polycrystalline layers like Gd₂O₃ have shown to influence both the grain size as well as roughness. A variety of grain structure evolutions and morphological changes in silica layers were noticed in different multilayer geometries. It is, hence, inferred that by appropriately using combinations of these materials, it is possible to have a control over the multilayer morphology and grain structures, which is a very relevant factor in developing precision ultraviolet laser coatings.     

The critical layer number of Stranski-Krastanov growth mode epitaxial growth for bcc metallic thin films

Based on the classical elastic theory and a thermodynamic model for surface energy, the critical layer number n_c of Stranski-Krastanov growth mode epitaxial growth for bcc metallic thin films is calculated. n_c is determined by the consideration that the sum of the surface energy of a film and the lattice mismatch elastic energy between a substrate and the film is equal to the surface energy of the substrate. When the film layer number n is larger than n_c, a flat growth of the film on the substrate will transform to an island growth. Our predictions on several metallic films are in agreement with experimental results.     

Abnormal grain growth in thin films not caused by decreased energy of their free surface

We modeled the grain growth process in thin films on substrate. The only driving force for grain growth was a decrease in the total grain boundary energy; possible difference in the energies of the free surface of neighboring grains was not taken into account. The main features of capillarity-driven microstructure evolution qualitatively agree with those observed experimentally. This leads to the conclusion that abnormal grain growth in thin films can develop without assistance of decreased energy of the free film surface.     

Characterization of electromechanical coupling coefficients of piezoelectric films using composite resonators

By analyzing the resonance frequency spectrum of a composite resonator consisting of a piezoelectric ceramic film deposited on a substrate plate, the thickness extensional mode electromechanical coupling coefficient of the film, k(t)(2), can be directly calculated from the effective coupling factor values, k(eff )(2), for two special modes of the resonator. The effects of the mechanical loss in the piezoelectric films on the measurement are investigated by numerical simulation, and some guidelines for improving the accuracy of the k(t)(2) measurement are reported.     

Effect of fractal-matrix resonators on the properties of thin copper films

The effect of fractal-matrix resonators of AIRES type, situated outside the discharge zone, on the properties of thin (submicron) copper films obtained using magnetron ion sputtering is studied. Using the proposed technique, it is possible to obtain copper films of submicron thickness with local regions representing self-organized fractal structures.     

Preparation of AlN thin films by nitridation of Al-coated Si substrate

AlN thin films have been grown on Al-coated Si(100) and Si(111) substrates by using nitridation in high-purity nitrogen ambient, where the Al layer was previously deposited on Si by ultra-high vacuum (UHV) electron beam evaporation. The temperature of nitridation was found to play an important role in the formation of AlN films. XRD results showed AlN films formed by nitridation at 1000°C for 30 min exhibited good crystallinity with the preferred orientation of (002) for both Si(111) and Si(100) cases. Other analysis techniques, like Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy have been used to evidence the formation and purity of the AlN films. Scanning electron microscope observations of the films revealed a closely-packed granular texture.     

Optical properties of pulse laser deposited AlN thin films on silicon

Aluminium-nitride thin films were deposited on silicon Si(111) substrate with pulsed laser deposition in a Riber LDM-32 system. The optical properties of the films were studied by means of optical spectroscopy with an incoherent light source mainly covering the visible range. It is demonstrated that, in comparison with an aluminium mirror, under certain deposition conditions, the film may behave as a metallic thin film as far as the optical reflection is concerned. In this case, there is an enhanced plasmonic reflection peak in the optical spectrum and the peak may be modified according to the degree of the phase transition. The microscopic structures as well as the surface topographies of the films were also studied with X-ray photoelectron spectroscopy and scanning electron microscopy. It turns out that the density and the size of the microscopic domains in the film determine whether the film remains dielectric or becomes metallic. The diamagnetic effect in the enhanced plasma increases in the process when the sample is smoothed out with the optimized nitrogen gas pressure. The nitrogen pressure is thus identified as the most influential deposition condition to the phase transition.     

Lattice and grain boundary diffusion of copper in thin aluminum films

The lattice D₁ and grain boundary δD_b diffusivities of Cu in Al thin films at 130–185°C are calculated from measurements employing Auger electron spectroscopy and Ar ion beam etching. The calculated values are D₁ = 0.065 cm² s^-1 × exp(-122 kJ/RT) and δD_b = 4.5 × 10^-9 cm³s^-1 exp(-97.4 kJ/RT). The D₁ value is 3–5 times larger at 130–185°C than that predicted by an extrapolation of radioactive tracer measurements of large grain bulk specimens at 433–652°C. The higher value measured here is attributed to the higher density of subgrain defect structures in the thin film.