Johan Bjurström

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 piezo-electric 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 de-position as well as standard bulk micro machining of Si. The resonator consisted of a 2-mum-thick AlN film with 200-nm-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 kt 2 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

Lateral-field-excited thin-film Lamb wave resonator

The basic principles and technology for the development of lateral-field-excited Lamb acoustic wave resonators on sputter-deposited c-oriented thin aluminum nitride films are presented. The experimental results demonstrate that Lamb waves can be successfully used as an alternative to high-velocity surface acoustic waves.

Temperature compensation of liquid FBAR sensors

In this work we demonstrate a practically complete temperature compensation of the second harmonic shear mode in a composite Al/AlN/Al/SiO2 thin film bulk acoustic resonator (FBAR) in the temperature range 25 °C–95 °C. The main advantages of this mode are its higher Q value in liquids as well as its higher frequency and hence higher resolution for sensor applications. For comparative reasons the non-compensated fundamental shear mode is also included in these studies. Both modes have been characterized when operated both in air and in pure water. Properties such as Q value, electromechanical coupling, dissipation and sensitivity are studied. An almost complete temperature compensation of the second harmonic shear mode was observed for an oxide thickness of 1.22 µm for an FBAR consisting of 2 µm thick AlN and 200 nm thick Al electrodes. Thus, the measured temperature coefficient of frequency (TCF) in air for the non-compensated fundamental shear mode (1.25 GHz) varied between −31 and −36 ppm °C−1 over the above temperature range while that of the compensated second harmonic shear mode (1.32 GHz) varied between +2 ppm °C−1 and −2 ppm °C−1 over the same temperature interval. When operated in pure water the former type shows a Q value and coupling coefficient, k2t, around 180 and 2%, respectively, whereas for the second harmonic these are 230 and 1.4%, respectively.

Dependence of the electromechanical coupling on the degree of orientation of c-textured thin AlN films

Highly c-oriented thin aluminum nitride (AlN) films have been grown at room temperature with reactive sputter deposition. Membrane film bulk (FBAR) thickness excited resonators have been subsequently fabricated by bulk micro machining of silicon (Si). The resonators were then electrically characterized with a network analyzer in a one-port configuration. Subsequently, the coupling coefficients and the Q factors of both the longitudinal and the shear mode were extracted from fitting the measured admittance with that of the equivalent circuit model at the resonance frequencies. The goal of this work is to study the variation of the electromechanical coupling and the quality factor of the resonators, for both the longitudinal and the shear modes as a function of the degree of film texture. It is observed that the films exhibit a mean tilt of the c-axis relative the surface normal. This tilt is found to depend on both the film texture and the distance from the wafer radius. It is also demonstrated that the textured films exhibit a behavior of the electromechanical coupling effectively identical to that of a single crystalline material of equivalent tilt. Thus, it is shown that the electromechanical coupling for the longitudinal mode decreases from 8% to 4%, and that for the shear mode increases from 0% up to 3% by varying the full width half maximum (FWHM) of the [002] rocking curve in the interval from 2/spl deg/ to 10/spl deg/.

Synthesis of c-axis-oriented AlN thin films on high-conducting layers: Al, Mo, Ti, TiN, and Ni

Thin piezoelectric polycrystalline films such as AlN, ZnO, etc., are of great interest for the fabrication of thin film bulk/surface acoustic resonators (TFBARs or TFSARs). It is well-known that the degree of c-axis orientation of the thin films correlates directly with the electromechanical coupling. However, the degree of c-axis orientation of the piezoelectric film is, in turn, influenced by other parameters such as the structure of the substrate material, the matter of whether the c-axis is up or down (polarity), and the growth parameters used. The correlation of these three aspects with the electromechanical coupling of the AlN-thin films, is studied here. Thin AlN films, prepared in a magnetron sputtering system, have been deposited onto thin Al, Mo, Ni, Ti, and TiN films. Such thin high-conducting layers are used to form the bottom electrode of TFBAR devices as well as to define a short-circuiting plane in TFSAR devices. In both cases, they serve as a substrate for the growth of the piezoelectric film. It has been found that the degree of orientation and the surface roughness of the bottom metal layer significantly affects the texture of the AlN films, and hence its electroacoustic properties. For this reason, the surface morphology and texture of the metal layers and their influence on the growth of AlN on them has been systematically studied. Finally, FBARs with both Al and Ti electrodes have been fabricated and evaluated electroacoustically.

Shear mode AlN thin film electroacoustic resonator for biosensor applications

A thin film thickness excited shear acoustic wave resonator is presented. Utilizing a newly developed reactive sputtering process AlN thin films with inclined c-axis relative to the surface normal with a mean tilt of around 30deg are successfully grown. Using the above process, a biosensor consisting of a shear mode thin film bulk acoustic resonator (FBAR) and a microfluidic transport system was fabricated. The biosensor operation in water, glycerol and albumin was characterized. The resonator had a resonance frequency of around 1.2 GHz and a Q value in water of around 150. Results concerning the stability and resolution are also presented. The results demonstrate clearly the potential of FBAR biosensors for the fabrication of highly sensitive low cost biosensors, bioanalytical tools as well as for liquid sensing in general

Synthesis of textured thin piezoelectric A1N films with a nonzero c-axis mean tilt for the fabrication of shear mode resonators

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

Suppression of spurious lateral modes in thickness-excited FBAR resonators

Thin film bulk acoustic wave resonators (FBAR) utilize thickness-excited modes in which the resonant frequency is determined by the thickness of the structure and the wave velocity of the mode used. Unfortunately, other resonant modes also may be excited in the device. Some of these correspond to low-frequency, laterally-excited modes arid, although a relatively small amount of the total energy is absorbed by these modes, their harmonics may produce an undesirable response around the fundamental resonance frequency of the desired thickness mode. This work explores various ways of suppressing the spurious effects caused by lateral-excited modes by studying their dependence of the electrode geometry. The origin of the lateral-excited modes is discussed in detail, and the results from a number of different electrode geometries are compared. A new elliptical electrode shape for suppression of spurious modes is developed and demonstrated.

Synthesis of c-tilted AlN films with a good tilt and thickness uniformity

This communication describes a method for the deposition of thin piezoelectric AlN films with an inclined c-axis relative to the surface normal. Further, the tilt over the wafer is sufficiently uniform and exhibits a planar symmetry as well as good thickness uniformity. Careful control of both the nucleation and growth stages is needed to obtain tilted films with excellent quality. Thus in the nucleation state, it is argued that two independent mechanisms, namely seed layer texture and/or surface roughness, are mainly responsible for the subsequent titled growth. To achieve the latter, however, a certain directionality of the deposition flux is also necessary. The directionality of the deposition flux is achieved through the use of an array of linear magnetrons tilted under a certain angle with respect to the substrate normal.

3I-5 Design and Fabrication of Temperature Compensated Liquid FBAR Sensors

In this work we demonstrate a practically complete temperature compensation of the second harmonic shear mode in composite AlN/SiO2 FBARs in the temperature range 25degC to 95degC. The main advantages of this mode over the fundamental mode are its higher Q value in liquids as well as its higher frequency and hence higher resolution for sensor applications. For comparative reasons the non-compensated fundamental shear mode is also included in these studies. Both modes have been characterized when operated both in air and in pure water. Properties such Q value, electromechanical coupling, dissipation and sensitivity are studied both theoretically and experimentally. An almost full temperature compensation of the second harmonic shear mode was observed for an oxide thickness of 1.22 mum and a typical 2 mum thick AlN resonator with 200 nm thick Al electrodes. Thus, the measured TCF in air for the non-compensated fundamental shear mode (1.25 GHz) varied between -31 and -36 ppm/ degC over the above temperature range while that of the compensated second harmonic shear mode (1.32 GHz) varied between + 2 ppm/ degC and -2 ppm/ degC over the same temperature interval