F. Levent Degertekin

Integrated optical interferometric detection method for micromachined capacitive acoustic transducers

An integrated optical interferometric detection method for micromachined capacitive acoustic transducers is reported. The back electrode of the capacitive transducer on a transparent substrate is shaped as an optical diffraction grating and the displacement of the transducer membrane is determined with interferometric resolution by measuring the intensity of the reflected diffraction orders. By applying voltage to deflect the membrane electrostatically, the detection sensitivity is kept at the optimum level and transmission signals are generated. Initial experiments on devices microfabricated on quartz substrates show a minimum detectable displacement of 2×10−4 A/√Hz around 250 kHz and low frequency detection capability for microphone applications. Ultrasonic pulse-echo experiments in air at 750 kHz are also demonstrated using both a HeNe laser and a 850 nm vertical cavity surface emitting laser as the light source.

Micromachined two-dimensional array piezoelectrically actuated transducers

This letter presents micromachined two-dimensional array flextensional transducers that can be used to generate sound in air or water. Individual array elements consist of a thin piezoelectric ring and a thin, fully supported, circular membrane. We report on an optimum design for an individual array element based on finite element modeling. We manufacture the transducer in two-dimensional arrays using planar silicon micromachining and demonstrate ultrasound transmission in air at 2.85 MHz. Such an array could be combined with on-board driving and an addressing circuitry for different applications.

IN SITU ACOUSTIC TEMPERATURE TOMOGRAPHY OF SEMICONDUCTOR WAFERS

Spatial temperature distribution in semiconductor wafers during rapid thermal processing is obtained by means of acoustic tomography. Ultrasonic Lamb waves are excited in the wafer by acoustic transducers bonded to spring‐loaded quartz support pins located at the wafer periphery. The Lamb wave time of flight in the wafer is used to measure the average wafer temperature with ±0.5 °C accuracy for a S/N ratio exceeding 55 dB. Spatial temperature information is gathered by electronic switching of the transmitter and receiver transducers. Tomographic reconstruction techniques are then used to calculate the temperature distribution from the measurements with different pixel maps. Using eight transducers, the thermal image of a 10 cm, (100) silicon wafer is obtained with 2×2 cm2 pixel resolution. Thermal image rates of 2 images/s are achieved by the system enabling real‐time control of wafer temperature distribution during rapid thermal processing.

Single-chip CMUT-on-CMOS front-end system for real-time volumetric IVUS and ICE imaging

Intravascular ultrasound (IVUS) and intracardiac echography (ICE) catheters with real-time volumetric ultrasound imaging capability can provide unique benefits to many interventional procedures used in the diagnosis and treatment of coronary and structural heart diseases. Integration of capacitive micromachined ultrasonic transducer (CMUT) arrays with front-end electronics in single-chip configuration allows for implementation of such catheter probes with reduced interconnect complexity, miniaturization, and high mechanical flexibility. We implemented a single-chip forward-looking (FL) ultrasound imaging system by fabricating a 1.4-mm-diameter dual-ring CMUT array using CMUT-on-CMOS technology on a front-end IC implemented in 0.35-μm CMOS process. The dual-ring array has 56 transmit elements and 48 receive elements on two separate concentric annular rings. The IC incorporates a 25-V pulser for each transmitter and a low-noise capacitive transimpedance amplifier (TIA) for each receiver, along with digital control and smart power management. The final shape of the silicon chip is a 1.5-mm-diameter donut with a 430-μm center hole for a guide wire. The overall front-end system requires only 13 external connections and provides 4 parallel RF outputs while consuming an average power of 20 mW. We measured RF A-scans from the integrated single- chip array which show full functionality at 20.1 MHz with 43% fractional bandwidth. We also tested and demonstrated the image quality of the system on a wire phantom and an ex vivo chicken heart sample. The measured axial and lateral point resolutions are 92 μm and 251 μm, respectively. We successfully acquired volumetric imaging data from the ex vivo chicken heart at 60 frames per second without any signal averaging. These demonstrative results indicate that single-chip CMUT-on-CMOS systems have the potential to produce realtime volumetric images with image quality and speed suitable for catheter-based clinical applications.

Contact stiffness of finite size subsurface defects for atomic force microscopy: Three-dimensional finite element modeling and experimental verification

We describe a three-dimensional (3D) finite element analysis model of the contact between an atomic force microscopy (AFM) tip and a substrate with finite size subsurface structures. The model can simulate the contact stiffness measured by a scanning AFM tip on the surface of a sample with buried nanoscale structures. In addition to the analytical verification and convergence analysis, we present the results of an experimental verification study. For this purpose, we use an atomic force acoustic microscopy setup and special silicon samples with well defined subsurface cavities fabricated by focused ion beam techniques. The 3D model is also used for parametric analysis of subsurface defect detection, and imaging simulations are performed for practical applications such as AFM imaging of electromigration defects.

Single mode Lamb wave excitation in thin plates by Hertzian contacts

We present novel techniques to selectively excite the lowest order symmetric (So) and antisymmetric (Ao) Lamb wave modes in thin solid plates. Hertzian contacts are formed between the plates and the end of specially designed quartz rods which guide extensional waves generated by PZT‐5H transducers bonded at their other end. Mode selectivity is achieved by applying shear and/or longitudinal traction at the edge or the surface of the plates according to the results of a two‐dimensional normal mode theory. In aluminum plates, mode selectivity is measured as a function of frequency for different traction forces. With normal forces, Ao mode selectivity of more than 46 dB is obtained for fd<0.4 MHz mm. With antisymmetric shear traction at the edge of the plate, a selectivity exceeding 55 dB is achieved for single mode So operation.

Air-coupled nondestructive evaluation using micromachined ultrasonic transducers

Nondestructive evaluation techniques which use conventional piezoelectric transducers typically require liquid coupling fluids to improve the impedance mismatch between piezoelectric materials and air. Air-coupled ultrasonic systems can eliminate this requirement if the dynamic range of the system is large enough such that the losses at the air-solid interfaces are tolerable. Capacitive micromachined ultrasonic transducers (cMUTs) have been shown to have more than 100 dB dynamic range when used in bistatic transmission mode. This dynamic range, along with the ability to transmit ultrasound efficiently into air, makes cMUTs ideally suited for air-coupled nondestructive evaluation applications. These transducers can be used either in through transmission experiments at normal incidence to the sample or to excite and detect guided waves in aluminum and composite plates. In this paper, we present results of a pitch-catch transmission system using cMUTs that achieves a dynamic range in excess of 100 dB. The pair of transducers is modeled with an equivalent electrical circuit which predicts the transmission systems insertion loss and dynamic range. We also demonstrate the feasibility of Lamb wave defect detection for one-sided nondestructive evaluation applications. A pair of cMUTs excites and detects the so mode in a 1.2 mm-thick aluminum plate with a received signal-to-noise ratio of 28 dB without signal averaging.

Physical methods for intracellular delivery: practical aspects from laboratory use to industrial-scale processing.

Effective intracellular delivery is a significant impediment to research and therapeutic applications at all processing scales. Physical delivery methods have long demonstrated the ability to deliver cargo molecules directly to the cytoplasm or nucleus, and the mechanisms underlying the most common approaches (microinjection, electroporation, and sonoporation) have been extensively investigated. In this review, we discuss established approaches, as well as emerging techniques (magnetofection, optoinjection, and combined modalities). In addition to operating principles and implementation strategies, we address applicability and limitations of various in vitro, ex vivo, and in vivo platforms. Importantly, we perform critical assessments regarding (1) treatment efficacy with diverse cell types and delivered cargo molecules, (2) suitability to different processing scales (from single cell to large populations), (3) suitability for automation/integration with existing workflows, and (4) multiplexing potential and flexibility/adaptability to enable rapid changeover between treatments of varied cell types. Existing techniques typically fall short in one or more of these criteria; however, introduction of micro-/nanotechnology concepts, as well as synergistic coupling of complementary method(s), can improve performance and applicability of a particular approach, overcoming barriers to practical implementation. For this reason, we emphasize these strategies in examining recent advances in development of delivery systems.

Micromachined microphones with diffraction-based optical displacement detection

Micromachined microphones with diffraction-based optical displacement detection are introduced. The approach enables interferometric displacement detection sensitivity in a system that can be optoelectronically integrated with a multichip module into mm3 volumes without beamsplitters, focusing optics, or critical alignment problems. Prototype devices fabricated using Sandia National Laboratories’ silicon based SwIFT-Lite™ process are presented and characterized in detail. Integrated electrostatic actuation capabilities of the microphone diaphragm are used to perform dynamic characterization in vacuum and air environments to study the acoustic impedances in an equivalent circuit model of the device. The characterization results are used to predict the thermal mechanical noise spectrum, which is in excellent agreement with measurements performed in an anechoic test chamber. An A weighted displacement noise of 2.4×10−2A measured from individual prototype 2100μm×2100μm diaphragms demonstrates the potential fo...

Monolithic CMUT-on-CMOS integration for intravascular ultrasound applications

One of the most important promises of capacitive micromachined ultrasonic transducer (CMUT) technology is integration with electronics. This approach is required to minimize the parasitic capacitances in the receive mode, especially in catheter-based volumetric imaging arrays, for which the elements must be small. Furthermore, optimization of the available silicon area and minimized number of connections occurs when the CMUTs are fabricated directly above the associated electronics. Here, we describe successful fabrication and performance evaluation of CMUT arrays for intravascular imaging on custom-designed CMOS receiver electronics from a commercial IC foundry. The CMUT-on-CMOS process starts with surface isolation and mechanical planarization of the CMOS electronics to reduce topography. The rest of the CMUT fabrication is achieved by modifying a low-temperature micromachining process through the addition of a single mask and developing a dry etching step to produce sloped sidewalls for simple and reliable CMUT-to-CMOS interconnection. This CMUT-to-CMOS interconnect method reduced the parasitic capacitance by a factor of 200 when compared with a standard wire-bonding method. Characterization experiments indicate that the CMUT-on-CMOS elements are uniform in frequency response and are similar to CMUTs simultaneously fabricated on standard silicon wafers without electronics integration. Ex- periments on a 1.6-mm-diameter dual-ring CMUT array with a center frequency of 15 MHz show that both the CMUTs and the integrated CMOS electronics are fully functional. The SNR measurements indicate that the performance is adequate for imaging chronic total occlusions located 1 cm from the CMUT array.