Gunilla Wingqvist

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

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.

A micromachined thermally compensated thin film Lamb wave resonator for frequency control and sensing applications

Micromachined thin film plate acoustic wave resonators (FPARs) utilizing the lowest order symmetric Lamb wave (S0) propagating in highly textured 2 µm thick aluminium nitride (AlN) membranes have been successfully demonstrated (Yantchev and Katardjiev 2007 IEEE Trans. Ultrason. Ferroelectr. Freq. Control 54 87–95). The proposed devices have a SAW-based design and exhibit Q factors of up to 3000 at a frequency around 900 MHz as well as design flexibility with respect to the required motional resistance. However, a notable drawback of the proposed devices is the non-zero temperature coefficient of frequency (TCF) which lies in the range −20 ppm K−1 to −25 ppm K−1. Thus, despite the promising features demonstrated, further device optimization is required. In this work temperature compensation of thin AlN film Lamb wave resonators is studied and experimentally demonstrated. Temperature compensation while retaining at the same time the device electromechanical coupling is experimentally demonstrated. The zero TCF Lamb wave resonators are fabricated onto composite AlN/SiO2 membranes. Q factors of around 1400 have been measured at a frequency of around 755 MHz. Finally, the impact of technological issues on the device performance is discussed in view of improving the device performance.

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

On the applicability of high frequency acoustic shear mode biosensing in view of thickness limitations set by the film resonance

The IC-compatible thin film bulk acoustic resonator (FBAR) technology has made it possible to move the thickness excited shear mode sensing of biological layers into a new sensing regime using substantially higher operation frequencies than the conventionally used quartz crystal microbalance (QCM). The limitations of the linear range set by the film resonance using viscoelastic protein films are here for the first time addressed specifically for FBARs operating at 700 MHz up to 1.5 GHz. Two types of protein multilayer sensing were employed; one utilizing alternating layers of streptavidin and biotinated BSA and the other using stepwise cross-linking of fibrinogen with EDC/NHS activation of its carboxyl groups. In both cases the number of protein layers within the linear regime is well above the number of protein layers usually used in biosensor applications, further verifying the applicability of the FBAR as a biosensor. Theoretical calculations are also presented using well established physical models to illustrate the expected behavior of the FBAR sensor, in view of both the frequency and the dissipation shifts.

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

New Materials for chemical and biosensors

ABSTRACT Wide band gap materials such as SiC, AlN, GaN, ZnO, and diamond have excellent properties such as high operation temperature when used as field effect devices and a high resonating frequency of the substrate materials used in piezoelectric resonator devices. Integration of FET and resonating sensors on the same chip enables powerful miniaturized devices, which can deliver increased information about a gas mixture or complex liquid. Examples of sensor devices based on different wide band gap materials will be given.

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

Temperature compensation of thin AlN film resonators utilizing the lowest order symmetric lamb mode

Micromachined thin film plate acoustic wave resonators (FPAR) utilizing the lowest order symmetric Lamb wave (S0) propagating in highly textured 2 mum thick Aluminum Nitride (AlN) membranes have been successfully demonstrated. However, a notable drawback of the proposed devices is their non-zero temperature coefficient of frequency (TCF) which lies in the range -20 ppm/K to -25 ppm/K. In this work temperature compensation of thin AlN film Lamb wave resonators is studied and demonstrated. Temperature compensation, while retaining at the same time the device electromechanical coupling, is experimentally demonstrated. The zero TCF Lamb wave resonators are fabricated onto composite AlN/SiO2 membranes. Q factors of around 1400 have been measured at a frequency of around 755 MHz.

Shear mode A1N 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 30 are successfully grown. Using the above process, a biosensor consisting of a shear mode thin film bulk acoustic resonator (TFBAR) and a microfluidic transport system was fabricated. The biosensor operation in water, glycerol and albumin was characterised. The resonator had a resonance frequency of around 1.2 GHz and a Q value in water of around 150. 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. Keywords-component; thickness mode resonator; quasishear polarased acoustic wave; FBAR; AlN; tilted films; biosensor;