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.     

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.     

SAW COM-parameter extraction in AlN/diamond layered structures

Highly c-axis oriented aluminum nitride (AlN) thin piezoelectric films have been grown on polycrystalline diamond substrates by pulsed direct current (DC) magnetron reactive sputter-deposition. The films were deposited at a substrate temperature below 50/spl deg/C (room temperature) and had a typical full width half maximum (FWHM) value of the rocking curve of the AlN-002-peak of 2.1 degrees. A variety of one-port surface acoustic wave (SAW) resonators have been designed and fabricated on top of the AlN films. The measurements indicate that various SAW modes are excited. The SAW phase velocities of up to 11.800 m/s have been measured. These results are in agreement with calculated dispersion curves of the AlN/diamond structure. Finally, the coupling of modes parameters have been extracted from S/sub 11/ measurements using curve fitting for the first SAW mode, which indicate an effective coupling K/sup 2/ of 0.91% and a Q factor of about 600 at a frequency of 1050 MHz.     

Magnetic recording configuration for densities beyond 1 Tb/in/sup 2/ and data rates beyond 1 Gb/s

A new magnetic recording system is evaluated that includes the single-pole head, a new medium design, and the soft underlayer of perpendicular recording. The proposed medium consists of perpendicular grains with anisotropy directions tilted optimally about 45/spl deg/ with respect to the perpendicular direction. Here, focus is on the tilt angle at 45/spl deg/ in the crosstrack direction, including a small but typical dispersion. The write pole consists of a tapered-neck single-pole head with a very small throat height that yields maximized write fields without increased edge track degradation. The advantages of tilted perpendicular recording are discussed using theoretical and numerical micromagnetic analyses. This design achieves a much higher signal-to-noise ratio (SNR) than conventional recording, because it is less sensitive to medium orientation distributions and, for the same thermal decay, can utilize media with much smaller grain sizes. The switching speed is much more rapid due to increased recording torque. Estimated recording limits for tilted perpendicular recording with a medium-jitter SNR of 17 dB are beyond densities of 1 Tb/in/sup 2/ and data rates of 1 Gb/s.     

Effect of deposition angle on fiber axis tilt in sputtered aluminum nitride and pure metal films

We show that the fiber axis orientation in sputtered aluminum nitride (AlN) responds strongly to deposition angle as compared with pure metal films. Fiber axis tilt was measured in films deposited at angles from 0° to 68° from the substrate normal. For pure metal films of Al and Ag, the strong (111) texture has a fiber axis tilt of < 10° from the substrate normal. For pure Nb films, the strong (110) texture also has a tilt of < 10°. In contrast, for films of the compound AlN, the distinct c-axis texture responds strongly to the deposition direction, with the fiber axis tilt almost following the deposition angle.     

Mode coupling in tilted planar waveguide gratings

Three different grating tilting formulas for predicting the optimum grating tilt angle for strong mode coupling in planar waveguide gratings are derived. The optimum tilt angles obtained by the ray-optics approach deviate ~1 degrees for transmissive mode coupling and ~1.5 degrees for reflective mode coupling from those computed by the coupled-mode approach. The coupled-mode analysis and ray-optics analysis of the tilted planar waveguide gratings show that the transmissive planar waveguide gratings should be tilted more than 84 degrees for strong TE(0)-to-TE(mu) mode coupling near 1550 nm.     

High-density perpendicular magnetic recording media of granular-type (FePt/MgO)/soft underlayer

Perpendicular magnetic recording media, composed of granular-type FePt-MgO films on Fe-Ta-C soft magnetic underlayer (SUL), have been fabricated on to 2.5-in glass disks. [001] textured FePt granular films with high-perpendicular magnetic anisotropy were obtained by annealing the FePt/MgO multilayer films. The FePt grain size, perpendicular coercivity, magnetic activation volume, and the exchange coupling between the FePt grains were found to be strongly dependent on the initial multilayer structures and the annealing conditions. The recording performance of the disks was evaluated by a spin-stand. The obtained results reveal a close correlation between the recording performance and magnetic properties. The thermal stability of the granular-type FePt media was studied using high-temperature magnetic force microscopy (MFM) technique, equipped with in situ sample heating, in the temperature range 25/spl deg/C-200/spl deg/C. The estimated signal decay at high temperature is ascribed to the temperature dependent magnetic anisotropy behavior.     

Sputtered FeCoN soft magnetic thin films with high resistivity

Fe/sub 70/Co/sub 30/N thin films with thickness from 20 to 1100 /spl Aring/ were prepared by radio- frequency reactive sputtering in an N/sub 2/--Ar mixture. The FeCoN films prepared in a low nitrogen flow rate percentage (<6%) and sputtering pressure (<8 mTorr) have a high B/sub s/ of about 24.0 kG, but a moderate hard-axis coercivity H/sub ch/ of 5-30 Oe. With further increase in N/sub 2/ percentage or sputtering pressure, films become significantly softer, with H/sub ch/ of about 0.1-0.6 Oe, and have a higher resistivity of up to about 160 /spl mu//spl Omega//spl middot/cm. The change in the magnetic properties with nitrogen flow rate percentage and sputtering pressure can be attributed to the formation of an ultrafine grain size nanocrystalline FeCoN thin film as observed by high-resolution transmission electron microscope. The soft properties of FeCoN films with nano-sized crystallites remain stable even after being annealed at 270/spl deg/C.     

Film morphology modification in ion-assisted glancing angle deposition

Porous structured films grown with the glancing angle deposition technique have been widely studied for thin film optical device applications. We report the use of ion assistance to modify the structural and optical properties of porous silicon dioxide and titanium dioxide columnar thin films grown at deposition angles of 70° and 85°. Optical characterization studies show that tilted columnar structures will undergo an increase in tilt angle and film density with increasing ion dose. These two trends contrast with unassisted films where film density and column tilt angle are primarily controlled by the deposition angle. Thus, a regime of film structures simultaneously exhibiting high film density and large column tilt angle is enabled by incorporating an ion-assisted process. The phisweep substrate motion algorithm for minimizing columnar anisotropy used in conjunction with ion-assisted deposition provides additional control over film morphology and expands the utility of this modified fabrication process.     

Obliquely sputtered TbFe giant magnetostrictive films with in-plane anisotropy

We have found that in-plane magnetostriction characteristics at low fields can be greatly improved by an oblique sputtering technique. We report a study of deposition of in-plane anisotropic TbFe giant magnetostrictive films by dc magnetron oblique sputtering, including the influences of deposition angle on TbFe film magnetic and magnetostrictive performances. The in-plane magnetization of TbFe films at 1600 kA/m is drastically increased with a change of deposition angles from 90/spl deg/ to 15/spl deg/. Magnetic domain structures explored by magnetic force microscopy indicate that the easy magnetization directions of the films can be gradually changed from perpendicular to the film plane at sufficiently shallow deposition angles. The in-plane magnetostrictive coefficients /spl lambda/ at 16 kA/m also can be increased by decreasing the deposition angles from 90/spl deg/ to 15/spl deg/. The significant variation in the in-plane magnetic and magnetostrictive performances can be explained by the decrease of perpendicular anisotropy of TbFe films.     

Synthesis and bulk acoustic wave properties on the dual mode frequency shift of solidly mounted resonators

This study focused on the fabrication and the theoretical analysis of solidly mounted resonators (SMR) concerning dual-mode frequency responses and their frequency shift of bulk acoustic wave (BAW) resonance. For this device fabrication, RF/DC magnetron sputtering and photolithography were employed to constitute the required multilayer structure. For the theoretical analysis, the dualmode frequency shift was characterized by the Sauerbrey's formula, and a modified formula was carried out following the trend for the large frequency shift. In the fabrication of the SMR device, Mo/SiO2 was chosen to construct the Bragg reflector as the high/low acoustic impedance materials, respectively, and aluminum nitride (AlN) was used as a piezoelectric layer. To investigate the characteristics of BAW on the dual-mode frequency shift, the c-axis tilted angle of AlN was altered as well as the various mass loading on the SMR. Based on the experimental results, the dual-resonance frequencies showed a nonlinear decreasing trend with a linear increase of the mass loading. Therefore, a modified formula was carried out. Furthermore, the ratio of the longitudinal-resonant frequency to the shear-resonant frequency remained at a range around 1.76 despite the various c-axis tilted angles of AlN and gradual mass loading on the SMR. The electromechanical coupling coefficient, k2(eff), of the shear resonance rose with the increase of the c-axis tilted angle of AlN.     

Structure, properties, and thermal stability of nanocrystallite Fe-Ti-N soft magnetic films

We deposited Fe-Ti-N magnetic films with a high sputtering power of 7 W/cm/sup 2/. When the composition of the films was in the range of Fe-Ti(3.9 at.%)-N(8.8 at.%) to Fe-Ti(3.3 at.%)-N(13.5 at.%), the films were composed of /spl alpha/' and Ti/sub 2/N precipitates. With the addition of nitrogen, 4/spl pi/M/sub s/ became higher than that of pure iron, reaching a maximum of 23.8 kG. At the same time, H/sub c/ was reduced to a minimum of 1.12 Oe. The best films can meet the needs of the recording head in dual-element giant magnetoresistive/inductive heads, yielding high storage density (10 Gb/in/sup 2/). The incorporation of N in /spl alpha/-Fe brought about the /spl alpha/' phase with its higher saturation magnetization. Ti additions inhibited the equilibrium decomposition /spl alpha/'/spl rarr//spl alpha/+/spl gamma/'. Because H/sub C//sup D//spl prop/D/sup 6/, where D is average grain diameter, grain size control is very important. The nitrogen induces severe distortion of the /spl alpha/' lattice, which can cause the grains to break into pieces and reduce the grain size. High sputtering power also led to the formation of fine grains, with diameter in the order of 14 nm. Probably Ti/sub 2/N is preferentially precipitated on the grain boundary, pinning the grain boundary and stabilizing the grain size during high-temperature heat treatment. The temperature limit for stability of the structure and its associated low coercivity was not less than 520/spl deg/C.     

Synthesis and surface acoustic wave properties of AlN films deposited on LiNbO3 substrates

The c-axis-oriented aluminum nitride (AlN) films were deposited on z-cut lithium niobate (LiNbO₃) substrates by reactive RF magnetron sputtering. The crystalline orientation of the AlN film determined by x-ray diffraction (XRD) was found to be dependent on the deposition conditions such as substrate temperature, N₂ concentration, and sputtering pressure. Highly c-axis-oriented AlN films to fabricate the AlN/LiNbO₃-based surface acoustic wave (SAW) devices were obtained under a sputtering pressure of 3.5 mTorr, N₂ concentration of 60%, RF power of 165 W, and substrate temperature of 400°C. A dense pebble-like surface texture of c-axis-oriented AlN film was obtained by scanning electron microscopy (SEM). The phase velocity and the electromechanical coupling coefficient (K²) of SAW were measured to be about 4200 m/s and 1.5%, respectively. The temperature coefficient of frequency (TCF) of SAW was calculated to be about -66 ppm/°C     

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.     

Temperature stable Hall effect sensors

Magnetic field sensors are needed for high-accuracy position, angle, force, strain, torque, and current flow measurements. Molecular beam epitaxy was used to grow tellurium-doped indium-gallium antimonide thin films. Hall effect sensors made from these films have been studied for their magnetic sensitivity and thermal stability. For a range of alloy composition near In/sub 0.8/Ga/sub 0.2/Sb and n-type doping levels near 2/spl times/10/sup 17/ cm/sup -3/, high magnetic sensitivity from -40/spl deg/C to +200/spl deg/C was found with a resolution of better than /spl plusmn/0.5% over the entire temperature range.     

Sputtering of ZnO:Al films from dual tube targets with tilted magnetrons

Aluminum doped zinc oxide (ZnO:Al) films were deposited by mid-frequency sputtering rotating tube targets at high discharge powers in a double cathode system. The magnetrons located inside the tube targets were tilted by ± 30°, leading to different racetrack orientations. Deposition rate and electrical properties of statically deposited films were investigated. Different properties of ZnO:Al films show lateral variations corresponding to the racetrack positions, which shift according to the tilt angles of double magnetrons. The highest average static deposition rate and the corresponding dynamic value were up to 360 nm/min and 111 nm m/min, respectively, for magnetrons tilted towards the center of the cathodes. The material properties of the ZnO:Al film prepared in dynamic mode were found to behave like the superpositions of properties of static films at different positions. Upon wet chemical etching in diluted hydrochloric acid (HCl), the surfaces of sputtered ZnO:Al films became rough, and three typical surface structures were observed and identified on statically deposited ZnO:Al films. The related plasma physics, growth and chemical etching mechanisms were discussed.     

Crystallographic tilt in GaN-on-Si (111) heterostructures grown by metal–organic chemical vapor deposition

We report on the studies of crystallographic tilt induced by miscut of the Si (111) substrate in GaN-on-Si (111) heterostructures grown by metal–organic chemical vapor deposition. By employing high-resolution X-ray diffraction, we found that the onset of crystallographic tilt occurred at the interface between the AlN nucleation layer and the Si (111) substrate. The orientation of the GaN overlayer always follows that of the AlN nucleation layer irrespective of its quality and miscut of the substrates. The resultant GaN [0002] is tilted toward GaN (11?20) and (10?10) atomic planes for the miscuts of Si (111) toward Si [1?10] and [11?2], respectively. In both cases, the misorientation of GaN (0002), i.e., the tilt of GaN [0002] from the surface normal direction, is in the same direction of the miscut of Si (111). The misorientation angle of the GaN epilayer is generally smaller than the miscut angle of the substrate. However, the crystallographic tilt, i.e., the angle formed between GaN [0002] and Si [111], is always much larger than the Nagai tilt. These observations are attributable to misfit dislocations that are anisotropically generated at the AlN/Si (111) interface. This mechanism is discussed based on recent microscopic observations of in-plane misfit dislocations at the interface near the atomic step edges.     

Influence of the seed layer on structural and electro-acoustic properties of sputter-deposited AlN resonators

The growth of high-quality AlN films has been studied by reactive sputtering onto Mo electrodes with Ni, Ti, and TiW seed layers and subsequent integration into thin film bulk acoustic wave resonators. The crystalline structure and morphology of the Mo and c-axis oriented AlN films were found to vary strongly with seed layer material and thickness. The smoothest Mo electrodes were obtained on thin Ti films. Reactive sputtering of AlN on top of these optimized electrodes resulted in a dense columnar grain structure with a well-aligned (002) crystal orientation and good electro-acoustic properties, including an effective coupling coefficient of 6.89% and quality factor above 1000.     

Intrinsic stability of ferroelectric and piezoelectric properties of epitaxial PbZr0.45Ti0.55O3 thin films on silicon in relation to grain tilt

Abstract

Piezoelectric thin films of PbZr_0.45Ti_0.55O₃ were grown on Si substrates in four different ways, resulting in different crystalline structures, as determined by x-ray analysis. The crystalline structures were different in the spread in tilt angle and the in-plane alignment of the crystal planes between different grains. It is found that the deviations of the ferroelectric polarization loop from that of the ideal rectangular loop (reduction of the remanent polarization with respect to the saturation polarization, dielectric constant of the film, slanting of the loop, coercive field value) all scale with the average tilt angle. A model is derived based on the assumption that the tilted grain boundaries between grains affect the film properties locally. This model describes the observed trends. The effective piezoelectric coefficient d_33,eff shows also a weak dependence on the average tilt angle for films grown in a single layer, whereas it is strongly reduced for the films deposited in multiple layers. The least affected properties are obtained for the most epitaxial films, i.e. grown on a SrTiO₃ epitaxial seed layer, by pulsed laser deposition. These films are intrinsically stable and do not require poling to acquire these stable properties.     

Migration of specific planar grain boundaries in bicrystals: application of magnetic fields and mechanical stresses

Recent research on the dynamics of planar grain boundaries is reviewed. Novel measuring techniques developed for in situ observation and recording of magnetically and stress driven grain boundary migration are presented. The results of migration measurements obtained on bismuth, zinc and aluminum bicrystals are addressed. The experiments revealed that the inclination of a 〈112〉 tilt boundary in Bi has a very strong influence on its mobility. The migration of planar tilt grain boundaries with different misorientation angles was measured in situ in bicrystals of high purity zinc. The results proved that there is a pronounced misorientation dependence of grain boundary mobility in the investigated angular range. The shear stress induced migration of planar symmetric 〈100〉 tilt boundaries in aluminum bicrystals was observed to be accompanied by a lateral translation of the adjacent grains. The coupling between boundary motion and shearing is not confined to low angle and some low Σ high angle boundaries, but occurs also for non-coincidence high angle 〈100〉 tilt boundaries. It has been found that also for stress induced grain boundary motion there is a misorientation dependence of the migration activation parameters. Lower values of the activation enthalpy and the pre-exponential mobility factor can be associated with boundaries with tilt angles close to low Σ CSL orientation relationships.