Capacitive micro-fabricated ultrasonic transducers for biometric applications

Major steps used in fabricating surface micromachined capacitive ultrasonic immersion transducers are investigated. Such steps include membrane formation and cavity sealing under vacuum. Three transducer membrane structures are evaluated: a nitride membrane with an LTO sacrificial layer; a polysilicon membrane with an LTO sacrificial layer; and a nitride membrane with a polysilicon sacrificial layer. The major differences in the three processes are the conductivity, dielectric constant and residual stress of the membrane. Three vacuum sealing mechanisms are compared, each of which requires a different degree of lithographic sophistication, and results in a sealed cavity. Submicron via sealing requires sophisticated lithography, but is amenable to LPCVD nitride, LTO and other sealing procedures. Standard g-line lithography results in vias which seal only with high sticking coefficient species, such as LTO. A novel etch channel structure, which results in lateral sealing and requires neither sophisticated lithography nor a particular sealing is demonstrated. The experiments in the paper are guided by theoretical analysis and computer simulations when applicable. The optimized process based on a nitride membrane with a polysilicon sacrificial layer results in devices which have a broad band 50 /spl Omega/ real part impedance in the megahertz range. In addition, a transducer dynamic range in excess of 100 dB is achieved with an untuned bandwidth of 50%. The fabrication techniques and results herein reported indicate that surface micromachined capacitive ultrasonic transducers are an alternative to piezoelectric transducers in immersion applications.

Electrostatic or capacitive ultrasonic transducers (EUTs or CUTs) were originally developed from the condenser transmitters and microphones with grooved and roughened backplates in the 1930s. Significant progress in theoretical and experimental research has been made since the introduction of micromachining techniques at the end of the 1980s. We have seen the advancement not only from the roughened ultrasonic transducers to the 2‐D array micromachined capacitive ultrasonic transducers (mCUTs), but also from the grooved ultrasonic transducers to the micromachined capacitive Lamb wave transducers (mCLWTs). Thus, the theoretical TDK model originally developed for grooved ultrasonic transducers [L.‐F. Ge, Proceedings 16th Intl. Conf. Acoustics/135th ASA Meeting, l08l–1082, 1998; and Sciences in China (A), (in English), 42, 1308–1315 (1999)] can be used for mCLWTs. It is most important for a reliable model to be able to predict the transducer’s natural frequencies, at which the lowest‐order antisymmetric Lamb wave (A0 wave) is excited. As an example using a typical transducer [M. H. Badi et al., IEEE Trans. Ultrason. Ferroelectr. Freq. Control 50(9), 1191–1203 (2003)], the fundamental frequency predicted using the TDK model is 2.18 MHz, which agrees well with the measured result of 2.1 MHz. [Work supported by NSFC (60374044).]

Since the proposing of capacitive micromachined ultrasonic transducer by Khuri Yakub group in 1994 that this kind of transducer occupying the advantages of wide bandwidth, impedance matching well with the propagation medium especially in fluid and air and high sensitivity, has shown a great potential for wide ranges of applications. This Letter reports kind of micro‐electro‐mechanical systems (MEMS) capacitive ultrasonic transducer with the novel cavities embedded in the device layer of silicon on insulator wafer bonded with a glass substrate. The optimum geometric dimensions are confirmed by both mechanical vibrating of the membrane and the electrical characteristics analysis. Finite‐element analysis is adopted to determine the operation mode. The safety and reliability of the proposed device is ensured by the obtained deflections and equivalent stress under operation/collapse voltage. The bottom electrodes of the proposed transducer are fabricated on the top surface of the glass substrate. The parallel parasitic capacitance is reduced, thus improving the electromechanical coupling coefficient. The test results show that the electromechanical coupling coefficient is 69.65%, which demonstrates that this proposed MEMS capacitive ultrasonic transducer structure can enhance the performance significantly.

Parametric equalization is often used to achieve a desired response from an audio transmitter, but is rarely applied to ultrasonic transducer systems. The ability of a broadband ultrasonic transmission and reception system to adapt its frequency and time domain response to changing acoustic conditions would be a distinct advantage in certain applications. Ultrasonic remote monitoring systems would benefit significantly from this ability, as signal levels could be minimized and consequentially the transmitter power consumption decreased. This work presents a real-time adaptive ultrasonic parametric equalizer using optimization driven Matlab code to control the coefficients of a switched capacitor filter network implemented in a Cypress PSOC (Programmable System On a Chip). In this work, adaptive parametric magnitude equalization of a through-transmission ultrasonic system using CUTs (Capacitive Ultrasonic Transducers) has been achieved in real time by tracking a desired SNR (signal to noise ratio) across the operational frequency spectrum. A Matlab general radial basis function (GRBF) artificial neural network (ANN) was developed to control the equalization filter coefficients based on the received frequency response data. The adaptive parametric equaliser adjusts the magnitude of the driving signal to maintain the desired SNR as closely as possible. The neural network was trained using PSO (Particle Swarm Optimization) back-propagation, based on a state space model of the system developed from frequency response data. The developed equalization circuitry, which is switched capacitor based and was fully implemented on the PSOC, is also described.

The authentication via accelerometer-based biometric gait recognition offers a user-friendly alternative to common authentication methods on smartphones. It has the great advantage that the authentication can be performed without user interaction. When the user is walking, his walk-pattern can be extracted from the accelerations measured using the integrated sensors of the smartphone. This pattern can be used for authentication. A study showed that users often deactivate the authentication methods of their mobile devices because they consume too much time. Because all steps necessary to perform biometric gait recognition can be executed in the background, no user interaction is necessary for the presented technique. Performing a continuous authentication while the user is walking, an up-to-date authentication result is available at any point in time. During log-in, no calculations are necessary anymore, hence there is no delay. Only in cases where the user is not walking, an alternative authentication method has to be used. This is a great benefit for the user because he has the advantages of a phone which is protected by authentication but without the disadvantages common methods impose. This high user-friendliness is likely to increase the number of smartphones for which the screen-lock is linked with an authentication. Therefore, a higher security of the data stored in smartphones can be achieved. A misuse of the stored information by an unauthorized user can be prevented. Due to the growing distribution of powerful smartphones, the number of available applications is increasing as well. These applications result in a growing amount of data stored on the devices, which make the protection of the device necessary. These data comprise e.g. addresses, appointments or GPS-information. Additionally, some applications, e.g. of e-mail-providers or social networks, require the user to authenticate himself. Often these credentials are stored by the user on the phone, such that it is not necessary to enter them each time. In case an unauthorized person has access to such a phone he can use these services without restrictions and therefore substantially harm the user. The objective of this thesis was to develop methods for accelerometer-based biometric gait recognition which achieve sufficient low error rates, as well as to demonstrate that their computational effort is low and allows for an execution on current smartphones. Because the basis of existing methods is the extraction of gait cycles (i.e. two steps) from the accelerometer data, a cycle-based method was developed and evaluated in a scenario test. This method uses raw data of the gait cycles as feature vectors and accomplishes the classification using distance functions. In addition, a further approach was selected, which does not need the time-costly and error-prone gait cycle extraction. Instead, it is using overlapping segments of a fixed time length. Several features are extracted from these segments and combined to feature vectors. Machine learning algorithms are used for classification. A benchmark of the approaches on a challenging database showed that these methods yield low equal error rates between 6% and 7% and are outperforming the cycle-based methods. These error rates were achieved under the realistic conditions that training and probe data are not collected on the same day. It was shown that five minutes of gait data are sufficient to thoroughly train the models. It should be regarded that the training data contain the different walking velocities at which the user should be recognized later on. To obtain low false rejection rates, the classification should be based on around three minutes walk data. Two of the developed methods were implemented on a smartphone. It was shown that both methods are able to perform the classification fast enough to allow for an authentication without delay for the user.

The possibility to perform an early and repeatable assessment of imaging performance is fundamental in the design and development process of new ultrasound (US) probes. Particularly, a more realistic analysis with application-specific imaging targets can be extremely valuable to assess the expected performance of US probes in their potential clinical field of application. The experimental protocol presented in this work was purposely designed to provide an application-specific assessment procedure for newly-developed US probe prototypes based on Capacitive Micromachined Ultrasonic Transducer (CMUT) technology in relation to brain imaging. The protocol combines the use of a bovine brain fixed in formalin as the imaging target, which ensures both realism and repeatability of the described procedures, and of neuronavigation techniques borrowed from neurosurgery. The US probe is in fact connected to a motion tracking system which acquires position data and enables the superposition of US images to reference Magnetic Resonance (MR) images of the brain. This provides a means for human experts to perform a visual qualitative assessment of the US probe imaging performance and to compare acquisitions made with different probes. Moreover, the protocol relies on the use of a complete and open research and development system for US image acquisition, i.e. the Ultrasound Advanced Open Platform (ULA-OP) scanner. The manuscript describes in detail the instruments and procedures involved in the protocol, in particular forthe calibration, image acquisition and registration of US and MR images. The obtained results prove the effectiveness of the overall protocol presented, which is entirely open (within the limits of the instrumentation involved), repeatable, and covers the entire set of acquisition and processing activities for US images.

Capacitive micromachined ultrasonic transducers ~CMUTs!, introduced about a decade ago, have been shown to be a good alternative to conventional piezoelectric transducers in various aspects, such as sensitivity, transduction efficiency, and bandwidth. In this paper, we discuss the principles of capacitive transducer operation that underlie these aspects. Many of the key features of capacitive ultrasonic transducers are enabled with micromachining technology. Micromachining allows us to miniaturize device dimensions and produce capacitive transducers that perform comparably to their piezoelectric counterparts. The fabrication process is described briefly, and the performance of the CMUT transducers is evaluated by demonstrating characterization results. It is shown that the transduction efficiency as defined by the electromechanical coupling coefficient can be close to unity with proper device design and operating voltage. It is also shown that CMUTs provide large bandwidth ~123% fractional bandwidth! in immersion applications which translate into high temporal and axial resolution. Finally, the feasibility of using CMUTs is demonstrated by showing imaging examples in air and in immersion.

Single-plane ultrasound (US) probes are commonly used to initiate ultrasound-guided peripheral intravenous (USGPIV) access for patients with difficult intravenous access (DIVA) in the emergency department (ED), although their use is not yet standardized across all EDs. This pilot project introduced the use of a biplane US probe for USGPIV access at a Level 1 Trauma Center ED. Following the training of 10 registered nurses (RNs) in the application of biplane US probes, the project aimed to decrease reliance on ED providers (e.g., NPs, PAs, and physicians) for USGPIV insertions in DIVA patients, allowing providers to focus on other critical tasks and improving patient care efficiency. Primary outcomes measured included the average number of attempts required, progression to critical care access devices, ED patient flow times, and RN utilization rates of the biplane US probe post-training. Results showed that the biplane method improved outcomes compared to the single-plane method, with a 23.8% decrease in the average number of attempts required to obtain access, an 80% decrease in subsequent critical care vascular access placement, and an increase in the rate of both single-plane and biplane US probe use after training compared to baseline. While ED flow rate did not improve, this pilot project demonstrated that biplane US probes for USGPIV access in the ED setting improve patient outcomes.

Objective To evaluate the predictive performance of diaphragm thickening fraction (DTF) assessed by ultrasound in the feasibility of weaning from mechanical ventilation in patients with chronic obstructive pulmonary disease (COPD). Methods Forty-three patients with COPD were enrolled for prospective study.All patients were ventilated mechanically for more than 48 hours and were expected to be weaned when they met clinical criteria in the intensive care unit from February 2015 to August 2015.Patients received a spontaneous breathing trial under pressure support for 1 h. At the end of spontaneous breathing trial, the right hemi-diaphragm was visualized in the zone of apposition using a 6-13 MHz linear ultrasound probe. Diaphragm thickness was recorded at end-inspiration(DTei) and end-expiration(DTee), and the DTF was calculated as percentage from the following formula: (DTei - DTee) / DTee. Also the rapid shallow breathing index (RSBI) was calculated. Patients meeting weaning criteria were extubated. Weaning successfully was defined as spontaneous breathing for >48 h without any form of ventilation support. Results Twenty-five patients were weaned successfully and failure of weaning was found in 18 patients. A significant differences in DTF(39.66±13.22)%vs.(23.84±8.85)%, P<0.05 and RSBI(62.74±26.05)vs.(98.89±35.44), P<0.05 were observed between patients with successful weaning and patients with failure. The sensitivity and specificity of DTF≥30 % for successful weaning were 84% and 83.88 %, respectively. The area under the receiver operating characteristic curve was 0.872(95 % CI: 0.759-0.985)for DTF. By comparison, when RSBI was ≤105, there was a sensitivity of 92 %, and a specificity of 38.89 % for determining successful weaning. The area under the receiver operating characteristic curve was 0.804(95 % CI: 0.669-0.940)for RSBI. Conclusions This study shows that in a cohort of COPD patients, the assessment of DTF using diaphragm ultrasound may be useful to predict success weaning or failure during spontaneous breathing trial. Key words: Ultrasound; Diaphragm; Chronic obstructive pulmonary disease; Mechanical ventilation; Weaning; Bedside; Critical ill; Predictive value

In the bioelectronic medicine field, vagus nerve stimulation (VNS) is a promising technique that is expected to treat numerous inflammatory conditions, in addition to the currently FDA approved treatment for epilepsy, depression and obesity [1]. However, current VNS techniques are still limited in the spatial resolution that they can achieve, which limits its therapeutic effect and induces side effects such as coughing, headache and throat pain. In our prior work, we presented a curved ultrasound (US) transducer array with a diameter of 2 mm and with 112 miniature US transducer elements, small enough to be wrapped around the vagus nerve for precise ultrasound nerve stimulation [2]. Due to the curved alignment of the US transducers with 48 of the elements simultaneously excited, the emitted US was naturally focused at the center of the curvature. Building on this work, we employ a beam steering technique to move the focal spot to arbitrary locations within the focal plane of the transducer array. The beam steering was controlled through an in-house built US driver system and was visualized using a pulsed laser schlieren system. The propagation of the US pulse in water was imaged and recorded. This method was found to be a rapid and effective means of visualizing the US propagation.

A Solid-Dielectric Capacitive Pressure Transducer

Monitoring corrosion is a vital step to prevent catastrophic structural failure. With accurate corrosion rate data, the risk of failure can be mitigated by performing routine maintenance on components that have become compromised. When corrosion occurs in an inaccessible area such as the inside of a pipe or back side of an underground storage tank, however, conventional monitoring techniques such as visual inspection are inadequate or impossible. Non-destructive evaluation (NDE) techniques must be used to determine the condition of the structure to maintain its integrity. Ultrasonic (UT) Thickness Sensors were investigated for corrosion monitoring use. The chosen sensors had a resolution of a limit of 25.4μm. A system was designed to utilize Electrochemical Machining (ECM) to simulate a corrosion at various rates in a localized region (approximately 1 cm in diameter) while its thickness was monitored by a permanently attached UT sensor. Multiple techniques such as a 3D profilometer, and depth-gauge were used to validate the measurement of the permanently attached UT sensor. Multiple ECM experiments were conducted using NaCl electrolyte solution at 1, 10, and 100 g/L concentrations. Occasionally, variations in pit formation were observed and likely caused by the orientation of the counter electrode to the steel surface. The smallest and most consistent surface roughness was obtained using the 10 g/L concentration. Larger variations in surface roughness were observed at 1 and 100 g/L NaCl concentrations leading to the selection of 10 g/L solution for experiments. The ECM system rendered reproducible machining rates showing that the UT sensors were capable of monitoring very low corrosion rates averaged to 3μm per day. Discrepancies up to a few mm were sometimes measured between the UT sensor, 3D profilometer and depth-gage values. The discrepancies were attributed to non-uniform corrosion in the vicinity of the permanently attached UT sensor. The sensor therefore provided readings of a specific localized spot of the corroded region, or the profile of the corroded region interfered with the UT signals. The accuracy of UT sensor readings was also studied for various geometrical pit profiles. Certain defect geometries made it impossible for the UT sensor to record accurate measurements. This was also observed on naturally corroded steel plates. Therefore, UT sensors may sometimes provide inaccurate or no reading over certain corrosion profiles and should therefore be used with prudence.

The effects of magnetic resonance imaging (MRI)-compatible ultrasound (US) probes on MRI monitoring were evaluated. It was found that MRI-compatible US probes with backing material containing 100 ppm ferrite, did not disturb MR monitoring except within a few mm radius from the US probe's position. MRI temperature monitoring of an excised swine liver irradiated with a high-intensity focused US beam from an MRI-compatible therapeutic transducer with US image guidance was then performed, and the potential usefulness of such a therapeutic system in minimally invasive therapy was demonstrated.

Objective To develop Z-score reference ranges for mitral valve-tricuspid valve distance (MTD) in normal fetuses from the measurements of femur length (FL),biparietal diameter (BPD) or gestational age (GA) using fetal echocardiography.Methods A retrospective cross-sectional study of 1216singleton normal fetuses was performed.The gestation age ranged from 20 to 40 weeks.Non-cardiac fetal biometric parameters included FL and BPD and GA based on menstrual age.MTD was measured in a standard apical four-chamber view.Normal Z-score ranges were developed for MTD using GA,BPD and FL as independent variables.These were accomplished by using first standard regression analysis and then weighted regression of absolute residual values for each parameter in order to adjust for inconstant variance.Results Linear regression model was the best description of the data in each case and correlations between MTD and independent variables (FL,BPD,GA) were excellent.The linear regression equations between MTD and FL,BPD,GA were:Y =0.04629 × FL-0.02868 (r =0.827) ; Y =0.04018 × BPD-0.08293(r =0.767) ; Y =0.0098 × GA-0.07075 (r =0.721),respectively.Heteroscedasticity of standard deviation(SD) with increasing independent variables also could be modeled with a simple linear regression.Linear regression equations between SD of MTD and FL,BPD,GA were:Y =0.0149 × FL-0.0095 (r =0.9947),Y =0.0126 × BPD-0.02793 (r =0.9931),Y =0.00305 × GA-0.021 (r =0.991),respectively.According to these equations,MTD Z-score =(the actual measurement of MTD-predicted MTD)/prediction SD.Conclusions Normal reference ranges and Z-scores for MTD have been provided.These normative data may be useful tools for assessment of fetal MTD,especially has potential applications in Ebstein anomaly or other malformations involved atrioventricular valves. Key words: Ultrasonography, prenatal; Fetal development; Z-score; Biometry; Mitral valve-tricuspid valve distance

Air-coupled generation and detection of ultrasonic bulk waves in metals using micromachined capacitance transducers