The article reviews recent developments of the thin film electro-acoustic (TEA) technology in view of the design and fabrication of micro-acoustic transducers for biosensor applications. The use of ...the TEA technology leads to transducer miniaturisation, compatibility with the IC technology, possibility for multiplexing, decrease in fabrication cost, reduction of consumables, mass fabrication, etc. Focus lies on the design, fabrication and evaluation of the transducer performance in liquid media as judged by electro-acoustic behaviour and ultimately by mass and viscosity resolution. The analysis draws the conclusion that the thickness excited quasi-shear thin film bulk acoustic resonator technology is far ahead in its development with regard to other alternative approaches in terms of both performance and level of maturity. Consequently, the main aspects of the quasi-shear thin film bulk acoustic resonator (FBAR) technology from film synthesis and fabrication through to performance evaluation and demonstration are reviewed in detail.
Microacoustic resonators made on suspended continuous membranes of LiNbO3 were recently shown to have very strong coupling and low losses at ≥5 GHz, suitable for high-performance filter design. ...Employing these simple resonator structures, the authors have designed, fabricated, and measured a 4.7 GHz bandpass ladder-type filter having 1 dB mid-band loss and 600 MHz bandwidth to address the 5G Band n79 requirements. The filter is fabricated on a monolithic substrate using standard i-line optical lithography and standard semiconductor processing methods for membrane release, starting with commercially available ion-sliced wafers having 400 nm thickness crystalline LiNbO3 layers. The filter is well-matched to a 50 Ω network and does not require external matching elements. Through accurate resonator engineering using our finite element method software filter design environment, the passband is spurious-free, and the filter provides better-than 30 dB rejection to the adjacent WiFi frequencies. This filter demonstrates the performance and scalable technology required for high-volume manufacturing of microacoustic filters >3.5 GHz.
A shear mode thin film bulk acoustic resonator (FBAR) operating in liquid media together with a microfluidic transport system is presented. The resonator has been fabricated utilizing a recently ...developed reactive sputter-deposition process for AlN thin films with inclined
c-axis relative to the surface normal with a mean tilt of around 30°. The resonator has a resonance frequency of around 1.2
GHz and a
Q value in water of around 150. Sensor operation in water and glycerol solutions is characterized. Theoretical analysis of the sensor operation under viscous load as well as of the sensitivity and stability in general is presented. The theoretical predictions are compared with experimental measurements. 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.
Thin film integrated circuits compatible resonant structures using the lowest order symmetric Lamb wave propagating in thin aluminum nitride (AlN) film membranes have been studied. The 2-mum thick, ...highly c-oriented AlN piezoelectric films have been grown on silicon by pulsed, direct-current magnetron reactive sputter deposition. The films were deposited at room temperature and had typical full-width, half-maximum value of the rocking curve of about 2 degrees. Thin film plate acoustic resonators were designed and micromachined using low resolution photolithography and deep silicon etching. Plate waves, having a 12-mum wavelength, were excited by means of both interdigital (IDT) and longitudinal wave transducers using lateral field excitation (LW-LFE), and reflected by periodical aluminum-strip gratings deposited on top of the membrane. The existence of a frequency stopband and strong grating reflectivity have been theoretically predicted and experimentally observed. One-port resonator designs having varying cavity lengths and transducer topology were fabricated and characterized. A quality factor exceeding 3000 has been demonstrated at frequencies of about 885 MHz. The IDT based film plate acoustic resonators (FPAR) technology proved to be preferable when lower costs and higher Qs are pursued. The LW-LFE-based FPAR technology offers higher excitation efficiency at costs comparable to that of the thin film bulk acoustic wave resonator (FBAR) technology
We recently introduced an in-liquid sensing concept based on surface acoustic resonance (SAR) in a lab-on-a-chip resonant device with one electrical port. The 185 MHz one-port SAR sensor has a ...sensitivity comparable to other surface acoustic wave (SAW) in-liquid sensors, while offering a high quality factor (Q) in water, low impedance, and fairly low susceptibility to viscous damping. In this work, we present significant design and performance enhancements of the original sensor presented in part I. A novel 'lateral energy confinement' (LEC) design is introduced, where the spatially varying reflectivity of the SAW reflectors enables strong SAW localization inside the sensing domain at resonance. An improvement in mass-sensitivity greater than 100% at resonance is achieved, while the measurement noise stays below 0.5 ppm. Sensing performance was evaluated through real-time measurements of the binding of 40 nm neutravidin-coated SiO2 nanoparticles to a biotin-labeled lipid bilayer. Two complementary sensing parameters are studied, the shift of resonance frequency and the shift of conductance magnitude at resonance.
A systematic study of the mass sensitivity and its dependence on the material's properties and thicknesses in composite multilayer Thin Film Bulk Acoustic Resonators (FBAR) is presented. The Mason ...transmission line model has been employed in combination with the acoustic energy balance principle for the determination of the FBAR mass sensitivity. The results have been experimentally verified. Further, the mass sensitivity dependence on various parameters has been studied and correlated with wave reflection and interference within the composite structure in addition to the well-known dependence on resonator acoustic impedance and operation frequency. The mass sensitivity for both the fundamental and the second harmonic mode of operation has been studied in view of their practical relevance. In particular, sensitivity amplification induced by the presence of an on-top deposited low acoustic impedance layer has been identified for the first harmonic and its potential applicability discussed in terms of gas and in-liquid sensing. Optimized structures for both sensing applications are suggested by considering the overall sensor resolution defined by both the mass sensitivity and the FBAR performance.
Improved performance thin-film plate acoustic wave resonators (FPAR) using the lowest order symmetric Lamb wave (S0) propagating in highly textured AlN membranes have been previously demonstrated for ...the first time. In this work, an experimental study of the resonators' performance vs. a variety of design parameters is performed. Devices operating in the vicinity of the stopband center exhibiting a Q-value of up to 3000 at a frequency of around 875 MHz are demonstrated. Further, low-loss high-Q micromachined 2-port longitudinally coupled thin-film resonators using the S0 mode are demonstrated for the first time. For the analysis of the proposed structures, the coupling-of-modes (COM) approach is successfully employed. Initially, the COM model is used for the extraction of physical parameters from one-port FPAR measurements. Subsequently, using the COM model, a satisfactory agreement with the proposed experimental frequency characteristics of S0 2-port FPARs has been achieved, and possibilities for further improvements in the performance discussed. Finally, the frequency spectrum of the one-port devices has been studied and the excited plate modes at different frequencies identified and presented with their Q-factors and temperature coefficients of frequency (TCF).
A microwave bandpass filter with a large ratio between the output and the input impedance has been designed and fabricated. Consequently, it functions both as a voltage transformer and a bandpass ...filter, or transfilter for brevity. It represents a two‐port micro‐acoustic resonator employing Lamb waves in a thin piezoelectric AlN film grown onto a Si carrier substrate with a centre frequency of around 887 MHz. The transfilter has a transformer ratio of 10 and a voltage efficiency of over 80%. The component has a small size (< 0.5 mm2) and is shown to sustain power levels of 250 mW. It can be used in a variety of cases where both voltage amplification and frequency filtering are required. Examples include: charge pumps in RFID tags, energy scavenging, remotely triggered switches, wake‐up radios in wireless networks, stand‐by units in home electronics etc.
Two-port film plate acoustic resonators (FPAR) devices operating on the lowest order symmetric Lamb wave mode (S0) in C-oriented AlN membranes on Si were fabricated and tested for their power ...handling capabilities in a feedback-loop power oscillator circuit. The FPAR was operated at an incident power level of 24 dBm for several weeks without performance degradation. Its flicker noise constant was calculated from close-in phase noise data as αR=2.1×10^−36/Hz. The results indicate that IC-compatible S0 FPARs are well suited for integrated microwave oscillators with thermal noise floor (TNF) levels below −175 dBc/Hz.
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.
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