Deep-UV sensors based on SAW oscillators using low-temperature-grown AlN films on sapphires

High-quality epitaxial AlN films were deposited on sapphire substrates at low growth temperature using a helicon sputtering system. SAW filters fabricated on the AlN films exhibited excellent characteristics, with center frequency of 354.2 MHz, which corresponds to a phase velocity of 5667 m/s. An oscillator fabricated using AlN-based SAW devices is presented and applied to deep-UV light detection. A frequency downshift of about 43 KHz was observed when the surface of SAW device was illuminated by a UV source with dominant wavelength of around 200 nm. The results indicate the feasibility of developing remote sensors for deep-UV measurement using AlN-based SAW oscillators.     

Gas sensors based on MWCNTs-PVP composite films for 1,2-dichloroethane vapor detection

Multi-walled carbon nanotubes (MWCNTs) - polyvinylpyrrolidone (PVP) composite materials were applied as sensitive films of resistive gas sensors for the first time to detect 1,2-dichloroethane vapor. Four sensors containing different quantities of PVP were prepared. The results revealed that composite films got a larger sensing response compared with the MWCNTs single-layer film. In addition, sensitivity for 1,2-dichloroethane vapor was improved due to the addition of PVP. The repeatability and long-term stability of the composite film sensors were studied as well. Moreover, MWCNTs-PVP composite films had a good selectivity for 1,2-dichloroethane vapor.     

Detecting and evaluating the signals of wirelessly interrogational passive SAW resonator sensors

The wireless sensing signal of a passive surface acoustic wave (SAW) resonator sensor is the response of the SAW resonator in a passive circuit to wireless radio frequency interrogation. The response is produced only in the case that the interrogation covers the operational frequency band of the resonator. The wireless response is transient and can only be detectable in a proximity after switching off the interrogation. Due to the fact that, while used as a sensor, the resonant frequency of the resonator is related to and varying with the measurand, the interrogation to a passive SAW resonator sensor has to trace and follow the correspondent variation of the frequency band of the device. The energy evaluation of the response is applied to detect the availability of the sensing response and is used as a feedback argument to roughly localize the operational frequency range of the sensor. A modified frequency estimation is employed to estimate the sensing characteristic frequency in the transient wireless sensing signal with a low signal-to-noise ratio. The estimation is used to further adjust the interrogation frequency to follow the frequency variation of the sensor until the response becomes optimal. The evaluation of signal energy along with the statistical quantity of frequency estimation gives a reference for the confidence of the estimated frequency.     

Single-walled carbon nanotubes nanocomposite microacoustic organic vapor sensors

We have developed highly sensitive microacoustic vapor sensors based on surface acoustic waves (SAWs) configured as oscillators using a two-port resonator 315, 433 and 915 MHz device. A nanocomposite film of single-walled carbon nanotubes (SWCNTs) embedded in a cadmium arachidate (CdA) amphiphilic organic matrix was prepared by Langmuir–Blodgett technique with a different SWCNTs weight filler content onto SAW transducers as nanosensing interface for vapor detection, at room temperature. The structural properties and surface morphology of the nanocomposite have been examined by X-ray diffraction, transmission and scanning electron microscopy, respectively. The sensing properties of SWCNTs nanocomposite LB films consisting of tangled nanotubules have been also investigated by using Quartz Crystal Microbalance 10 MHz AT-cut quartz resonators. The measured acoustic sensing characteristics indicate that the room-temperature SAW sensitivity to polar and nonpolar tested organic molecules (ethanol, ethylacetate, toluene) of the SWCNTs-in-CdA nanocomposite increases with the filler content of SWCNTs incorporated in the nanocomposite; also the SWCNTs-in-CdA nanocomposite vapor sensitivity results significantly enhanced with respect to traditional organic molecular cavities materials with a linearity in the frequency change response for a given nanocomposite weight composition and a very low sub-ppm limit of detection.     

SAW filters based on parallel-connected coupled resonator filters with offset frequencies

The concept of coupled resonators is applied to synthesize surface acoustic wave filters. Employing two parallel-connected filter tracks, with a frequency shift imposed between them, a wide passband with low insertion loss together with well-controlled rejections is achieved. The operation of the two-track device is based on the mutual interaction of the individual transfer functions for the pair of tracks. Each track serves to contribute a part of the passband, enabling a wide band. Outside of the passband, the signals passing through the two channels may cancel each other, thus facilitating efficient control over the rejections. However, obtaining rejection stopbands at just the predetermined frequencies requires precise values for the materials parameters and a reliable fabrication process. Prototype devices fabricated with this approach are demonstrated both on quartz and, for the first time, on 42 degrees-LiTaO3. Results for two-track devices having either two or three transducers per track and operating either single-ended or with a balanced output are presented. The devices are designed employing the coupling-of-modes model and transmission-matrix approach, and the separate tracks are optimized simultaneously and independently. The center frequencies are 868 MHz and 1960 MHz. On quartz, a minimum insertion loss of 4 dB and a passband width of 0.23% are achieved at 868 MHz. On 42 degrees-LiTaO3, the corresponding figures of merit are 1.3 dB for minimum insertion loss and 4.1% bandwidth at 1960 MHz. The filters on 42 degrees-LiTaO3 also have remarkably flat passbands.     

Chemical sensors based on nano-polymer films

A procedure is described for formation of a nano-structured organic-organic interface with increased conductivity at the interface of two polymer films. It is established that conductivity is determined by the type and degree of external action. It is shown that on the basis of this interface it is possible to create chemical sensors, and in fact sensors for the relative humidity of air, ethanol vapor pressure, and pH readings. Translated from Izmeritel’naya Tekhnika, No. 4, pp. 62–64, April, 2009.     

Sezawa mode SAW pressure sensors based on ZnO/Si structure

Surface acoustic wave (SAW) devices have been shown to be suitable for many sensor applications. One of these applications is pressure sensor. In this study we investigate the performance of SAW pressure sensors formed with ZnO/Si(001) structure. The pressure sensitivities of Rayleigh mode as well as the Sezawa mode are studied as a function of normalized thickness (kh = 2pihZnO/lambda). The experimental results show an opposite strain effect in the ZnO layer and Si substrate. A theoretical approach based on the perturbation method has been developed for the evaluation of pressure sensitivity in the Sezawa mode. Experimental and theoretical results obtained for the ZnO/Si SAW sensor prepared with kh = 1.18 are in good agreement. For kh < or = 1.2, the ZnO contribution to the sensor sensitivity can be neglected in the Sezawa mode in which ZnO acts mainly as an electromechanical conversion layer.     

Viscoelastic properties of polymer films on surface acoustic wave organophosphorous vapor sensors

This work investigates the viscoelastic properties of the fluoropolyol (FPOL) polymer on the surface acoustic wave (SAW) organophosphorous vapor sensors. A complex shear modulus is used to express different polymer types (glassy, glassy-rubbery, and rubbery). The different polymer types leads to different propagating properties of SAW, such as attenuation change and velocity shift. Calculation results indicate that the glassy-rubbery film exhibits the highest sensitivity for detecting organophosphorous vapor. The thicker the glassy and glassy-rubbery film implies a higher sensitivity. Moreover, the SAW vapor sensor based on the rubbery film represents the response of acoustically thick layers which has a peak in attenuation with an increasing vapor adsorption. The selectivity factor between DMMP (10 ppm) and H₂O (40%RH) is so low that the selectivity of FPOL film towards water is ineffecient. However, the selectivity factor between ethanol (10 ppm) and DMMP (10 ppm) is as high as 2512, thus confirming that the selectivity of FPOL film towards ethanol is good. Therefore, a precise and dry humidity control in the sensors system with FPOL coating is required.     

Recognition of organic solvents molecules by simultaneous detection using SAW oscillator sensors and optical fiber devices coated by Langmuir-Blodgett cadmium arachidate films

Surface acoustic waves (SAW) 433 and 315 MHz, two-port resonator-based oscillators coated with a Langmuir-Blodgett (LB) thin layer of chemosensitive cadmium arachidate (CdA) provide highly sensitive chemical acoustic sensors for detection and monitoring of organic vapors, at room temperature. LB CdA film-coated silica optical fibers (SOF) have been successfully fabricated and studied for organic solvents molecules sensing applications. The sensing performance of both types of acoustic and optical transducers has been compared for detecting six molecular species. Simultaneous measurements of frequency changes (delta f) and optoelectronic signal changes (deltaV) of the LB CdA film assembled onto SAW sensors and SOF devices have been realized for organic vapors recognition purposes. Six molecular species such as ethanol, methanol, isopropanol, ethylacetate, acetone, and toluene have been identified and recognized by a specific index (deltaf/deltaV), which can be considered a characteristic property of the chemosensitive material. The discrimination of the six molecular species examined also has been obtained by chemical patterns using a couple of specific index (deltaf433/deltaV; deltaf315/deltaV) measured by combining SAW 433 or 315 MHz oscillators and SOF sensing devices. Transient responses, calibration curves, intertransducer relationships, and chemical patterns are presented and discussed.     

An interrogation unit for passive wireless SAW sensors based on fourier transform

The application of surface acoustic wave (SAW) resonators as sensor elements for different physical parameters such as temperature, pressure, and force has been well-known for several years. The energy storage in the SAW and the direct conversion from physical parameter to a parameter of the wave, such as frequency or phase, enables the construction of a passive sensor that can be interrogated wireless. This paper presents a temperature-measurement system based on passive wireless SAW sensors. The principle of SAW sensors and SAW sensor interrogation is discussed briefly. A new measurement device developed for analyzing the sensor signals is introduced. Compared to former interrogation units that detect resonance frequency of the SAW resonator by comparing amplitudes of sensor response signals related to different stimulating frequencies, the new equipment is able to measure the resonance frequency directly by calculating a Fourier transformation of the resonator response signal. Measurement results of an experimental setup and field tests are presented and discussed.     

Conductive polymer films as ultrasensitive chemical sensors for hydrazine and monomethylhydrazine vapor

Thin films of the electrically conductive polymer poly(3-hexylthiophene) were investigated as ultrasensitive chemical sensors for hydrazine and monomethylhydrazine vapor. The threshold limit value for these highly toxic species, which are used extensively as rocket fuels, has recently been lowered to 10 ppb for 8-h exposure, necessitating the development of instrumentation with improved sensitivity. The present study describes the fabrication, calibration, and testing of simple, rugged, polymer-based sensors for detection of hydrazines in both ambient and vacuum environments. For reasonable choices of film thickness, initial resistance, and integration time, it is demonstrated that concentrations in the 0.1-100 ppb range can be monitored with an accuracy of ±20%. The sensor can be utilized for both dosimetric and real-time detection. Reproducible fabrication was achieved using standard spin-coating techniques. The polymer sensors exhibit good specificity to hydrazines in the presence of NH(3), amines, and ambient H(2)O and have a shelf-life of several years when stored in cold, dry conditions.     

Scalable arrays of chemical vapor sensors based on DNA-decorated graphene

Arrays of chemical vapor sensors based on graphene field effect transistors functionalized with single-stranded DNA have been demonstrated. Standard photolithographic processing was adapted for use on large-area graphene by including a metal protection layer, which protected the graphene from contamination and enabled fabrication of high quality field-effect transistors （GFETs）. Processed graphene devices had hole mobilities of 1,640 ± 250 cm2.V-1.s-1 and Dirac voltages of 15 ± 10 V under ambient conditions. Atomic force microscopy was used to verify that the graphene surface remained uncontaminated and therefore suitable for controlled chemical functionalization. Single-stranded DNA was chosen as the functionalization layer due to its affinity to a wide range of target molecules and π-π stacking interaction with graphene, which led to minimal degradation of device characteristics. The resulting sensor arrays showed analyte- and DNA sequence-dependent responses down to parts-per-billion concentrations. DNA/GFET sensors were able to differentiate among chemically similar analytes, including a series of carboxylic acids, and structural isomers of carboxylic acids and pinene. Evidence for the important role of electrostatic chemical gating was provided by the observation of understandable differences in the sensor response to two compounds that differed only by the replacement of a （deprotonating） hydroxyl group by a neutral methyl group. Finally, target analytes were detected without loss of sensitivity in a large background of a chemically similar, volatile compound. These results motivate further development of the DNA/graphene sensor family for use in an electronic olfaction system.     

The detection properties of ammonia SAW gas sensors based on L-glutamic acid hydrochloride

This study has investigated an improved surface acoustic wave (SAW) ammonia gas sensor based on L-glutamic acid hydrochloride. It presents an excellent reversibility, sensitivity, and repeatability to ammonia. The frequency shift versus ammonia concentration above 40 degrees C was a monotonic function, and the limit of detection of the sensor at 50 degrees C was 80 ppb.     

Analysis of viscosity sensitivity for liquid property detection applications based on SAW sensors

An investigation of viscosity sensitivity for liquid property detection applications based on the ZnO/SiO₂/Si layered structure Love mode surface acoustic wave (SAW) sensors is presented. One of our interests in this paper is to optimize the SAW viscosity sensor under the condition of temperature stability by considering the relations among electromechanical coupling coefficient, viscosity sensitivity and temperature coefficient of delay (TCD). Some important results have been obtained by solving the system of coupled electromechanical field equations and Navier–Stokes equation. It is found that the electromechanical coupling coefficient and viscosity sensitivity can be further improved by adjusting the thickness of SiO₂ thin film and a zero TCD device also can be obtained by introducing a SiO₂ thin film with proper thickness. We try to obtain a device which possesses the viscosity sensitivity as high as possible and has zero TCD. Another interest of this paper is to improve the traditional viscosity sensitivity expression by considering the coupling effect between the liquid viscosity and density. It is shown that the coupling effect cannot be neglected from the numerical results. This modification could make the obtained viscosity more accurate. This analysis is meaningful for the manufactures and applications of the ZnO/SiO₂/Si structure Love wave sensor for liquid property detection.     

Conductance measurements on a leaky SAW harmonic one-port resonator

Conductance measurements are reported on a leaky SAW (LSAW) harmonic one-port resonator on a 64 degrees Y-X LiNbO(3) substrate. This employed a short three-finger IDT for fundamental and second harmonic operation together with long reflection gratings. Conductances were measured with and without the end gratings. From an analysis of the measurements, it was deduced that, for optimum second harmonic performance, the grating stop-band frequency should be higher than the IDT unperturbed center frequency. This result is in contrast to fundamental frequency resonator designs in which the end grating stop-band frequency is placed below the IDT center frequency for optimum performance.     

SAW synchronous multimode resonator with gold electrodes on quartz

A surface acoustic wave synchronous multimode resonator with gold electrodes on ST-cut quartz was designed, fabricated, and measured. At a frequency of about 194.3 MHz, an insertion loss of about 14 dB, and loaded and unloaded quality factors of 11,500 and 14,500, respectively, were obtained.     

Imprinted laminate wafer-level packaging for SAW ID-tags and SAW delay line sensors

We have developed a wafer-level packaging solution for surface acoustic wave devices using imprinted dry film resist (DFR). The packaging process involves the preparation of an imprinted dry film resist that is aligned and laminated to the device wafer and requires one additional lithography step to define the package outline. Two commercial dry film solutions, SU-8 and TMMF, have been evaluated. Compared with traditional ceramic packages, no detectable RF parasitics are introduced by this packaging process. At the same time, the miniature package dimensions allow for wafer-level probing. The packaging process has the great advantage that the cavity formation does not require any sacrificial layer and no liquids, and therefore prevents contamination or stiction of the packaged device. This non-hermetic packaging process is ideal for passive antenna modules using polymer technology for low-cost SAW identification (ID)-tags or lidding in low-temperature cofired ceramic (LTCC) antenna substrates for high-performance wireless sensors. This technique is also applicable to SAW filters and duplexers for module integration in cellular phones using flip-chip mounting and hermetic overcoating.     

Micromachined piezoelectric force sensors based on PZT thin films

A micromachined lead zirconate titanate (PZT) force sensor for scanning force microscope (SFM) is conceptualized by its piezoelectricity. The fabrication procedure is interpreted, and mechanical characteristics of the micromachined PZT force sensors with various lengths are studied in this paper. A compact SFM is constructed by using the piezoelectric PZT sensor. A very clear image is taken by this SFM. The current study of the micromachined PZT force sensor can be considered as a breakthrough of design of SFM as well as a good example of integrated piezoelectric microdevices     

A SAW resonator filter using longitudinal and transverse modes

A SAW resonator filter using not only transverse but also longitudinal modes is presented. The design principle permits to construct four-pole filters without cascading or parallel connection. The input and output transducers arranged side by side have equal construction characterized by unsymmetrical withdrawing weighting. Neighboring disturbing longitudinal modes are suppressed due to weighting determined by an iterative procedure. The successful application of the principle to a device is demonstrated by experimental measurements     

Orthogonal frequency coding for SAW tagging and sensors

Surface acoustic wave (SAW)-based sensors can offer wireless, passive operation in numerous environments, and various device embodiments are used for retrieval of the sensed data information. Single sensor systems typically can use a single carrier frequency and a simple device embodiment because tagging is not required. In a multisensor environment, it is necessary to both identify the sensor and retrieve the sensed information. This paper presents the concept of orthogonal frequency coding (OFC) for applications to SAW sensor technology. The OFC offers all advantages inherent to spread spectrum communications, including enhanced processing gain and lower interrogation power spectral density (PSD). It is shown that the time ambiguity in the OFC compressed pulse is significantly reduced as compared with a single frequency tag having the same code length, and additional coding can be added using a pseudo-noise (PN) sequence. The OFC approach is general and should be applicable to many differing SAW sensors for temperature, pressure, liquid, gases, etc. Device embodiments are shown, and a potential transceiver is described. Measured device results are presented and compared with coupling of modes (COM) model predictions to demonstrate performance. Devices then are used in computer simulations of the proposed transceiver design, and the results of an OFC sensor system are discussed.