Levent Degertekin

A resonant CMUT sensor for fluid applications

Fluid waves at the interface of CMUTs (capacitive micromachined ultrasound transducers) to the surrounding fluid are an often discussed and unwanted effect for medical imaging applications, as they cause ringing artifacts. A new approach for a surface wave sensor is presented which uses these dispersive surface waves for sensing fluid properties like mass density and viscosity. After a short introduction to the theory and our FEM model we will present first results showing the sensitivity of the sensor to the viscosity of different silicone oil samples and will discuss the results.

A tunable analog delay element for high-frequency dynamic beamforming

Implementing beamforming for high frequency arrays is challenging because of the accurate delay requirements at high frequencies. High frequency digital beamforming is not suitable for catheter based applications as a large number of cables is required between the array and the external beamformer. A possible solution is to perform analog beamforming on an integrated circuit adjacent or monolithically integrated to the imaging array. In this study, we introduce an improved voltage in voltage out low pass filter as an analog delay cell for high frequency dynamic beamformers. This circuit can generate three times more delay with a given bandwidth when compared to conventional low pass filters. Delay of the circuit is tunable and the gain of the cell is inherently very close to unity. The proposed delay cell operates single ended and therefore is more suitable for CMUT operation which generates single ended output. We designed a test beamformer for a 30MHz, equal area, annular array with 100% bandwidth using the proposed delay cell and the unit-delay focusing architecture. Required delays are implemented using a delay line made up of improved delay elements with tunable delays. To demonstrate functionality we designed and fabricated a custom front-end IC in a 0.5µm standard CMOS process. The IC chip consists of 8 transimpedance amplifiers, voltage-to-current converters, the analog dynamic beamformer, and two buffers. We present results of preliminary imaging experiments that demonstrate the focusing capability.

Thermal mechanical noise based characterization of CMUTs using monolithically integrated low noise receiver electronics

Monolithic integration of CMUTs and CMOS electronics minimizes interconnect parasitics which is essential for low noise receiver design. Ideally the system noise should be dominated by the CMUT thermal-mechanical noise, which is shaped by CMUT electrical impedance. In addition to improved signal to noise ratio (SNR), a thermal mechanical noise limited detection system can be helpful to reveal information on CMUT characteristics. A transimpedance amplifier is custom designed in 0.35µm CMOS to be monolithically integrated with a forward looking volumetric imaging CMUT array element. TIA design is optimized to minimize noise taking advantage of the minimized parasitics provided by CMUT-on-CMOS approach. Output noise spectrum of the TIA connected to the CMUT element in air with and without DC bias clearly shows that the noise spectrum has a peak at the CMUT resonance frequency when a DC bias is applied. This demonstrates that noise measurements of CMUT elements in air can be used to characterize uniformity and functionality of array elements without the need for any external electrical or acoustic inputs. Transducer noise dominated detection in immersion is demonstrated through output noise spectrum measurement of the integrated system in water tank. A noise figure of 1.76dB is obtained in the CMUT bandwidth. In addition, the relation between the CMUT impedance and noise spectrum can provide means for implementing other types of sensors using CMUTs.

Active control of grating interferometers for extended-range low-noise operation.

An active control method is proposed and demonstrated to improve grating-based laser interferometry to achieve low-noise (subpicometer resolution) and multiwavelength unambiguous range of operation simultaneously. The method modifies a recurrent calibration-based path stabilization algorithm to extract high-resolution and low-resolution data in parallel. This extended range recurrent calibration method is experimentally verified by implementing it on the micromachined scanning grating interferometer (microSGI).

Experimental study of dual-ring CMUT array optimization for forward-looking IVUS

Forward-looking (FL) catheters have guiding and volumetric imaging capacities which are highly desirable for IVUS applications. Large channel and firing counts have to be reduced to enable 3-D real-time imaging and simplify front-end electronics. Recently, we have proposed an optimization procedure for dual ring FL arrays which is based on finding an optimal coarray set using the simulated annealing algorithm. The presented algorithm is based on finding a predefined number of optimal firing set which results in elimination of redundant spatial frequencies in the coarray. In this study, we present the experimental demonstration of the proposed method with fabricated single chip CMUT on CMOS system based FL dual ring arrays. The dual ring CMUT arrays were monolithically fabricated on top of CMOS chips which have 25-V pulsers and low-noise transimpedance amplifiers for each transmit and receive array elements. The fabricated CMUT arrays have 56 transmit and 48 receive elements operating at 12 MHz with a 1.4 mm outer diameter. To test the imaging performance of the optimal reduced set, we obtained a 512-element coarray set from the full 2688-element set. In the experiment, we used a phantom of 100-μm aluminium wires immersed in oil tank. We have reconstructed both 2-D PSFs and B-scan images of wire targets. Experimental results demonstrate that the simulated annealing based optimal firing set achieves acceptable lateral and contrast resolution performances with 1/5 of the full set.

Sensing physical fluid properties with CMUT arrays

Fluid waves at the interface of CMUTs (Capacitive Micromachined Ultrasound Transducers) to the surrounding fluid are an often discussed and unwanted effect for medical imaging applications, as they cause ringing artifacts. A new approach for a surface wave sensor is presented which uses these dispersive surface waves for sensing fluid properties like mass density and viscosity. After a short introduction to the theory and our FEM model we will present first results. A mixture of water and glycerin was used to investigate the sensitivity of the sensor to dynamic viscosity.

Diffraction based optical MEMS microphones and accelerometers with active electrostatic force feedback

Diffraction‐based optical displacement detection method and its use in low noise micromachined microphones have been shown earlier. [Hall et al., J. Acoust. Soc. Am. 118, 3000‐3009 (2005), Garcia et al., J. Acoust. Soc. Am. 121, 3155 (2007)]. In these devices, the integrated electrostatic port of the sensor is uncoupled from the integrated optical sensing. This structure enables one to use this port for sensitivity tuning, self characterization, and active control to adjust the device dynamics. Given that the displacement noise of integrated optical sensor is below the thermal‐mechanical noise of the mechanical structure, one can implement force feedback methods such as active Q‐control, or adjust device stiffness without adding substantial noise to the system. We implemented micromachined optical microphones and accelerometers with integrated optoelectronics integrated in a 1.5mm3 volume. We present experimental results on force feedback Q‐control of low noise omnidirectional, and biomimetic directional ...

Quantitative Subsurface Imaging by Acoustic AFM Techniques

We review the modeling techniques developed for analyzing the effects of 2-D and 3-D subsurface structures on the stiffness measurements by acoustic AFM. Starting from the analytical Hertzian model, we describe important parameters such as penetration depth and subsurface resolution for acoustic AFM imaging. These definitions point to the need for analytical–numerical models based on mechanical surface impedance method and finite element modeling of arbitrary 2-D and 3-D structures buried under the surface. By using the 2-D and 3-D models, the dependence of penetration depth and subsurface resolution on material properties, subsurface structure geometry, and imaging parameters are investigated. It has been shown that high contrast between subsurface structure and substrate increases the detectability of the structure and the visible depth of the structure depends highly on the contact radius. Soft subsurface structures or voids can be detected with appropriate tip radius and force even if they are as deep as 450 nm. However, the sensitivity is higher while detecting stiff structures under thin soft layers. These results can be extrapolated for different applications using the presented guidelines.

A biologically inspired silicon differential microphone with active Q control and optical sensing

A MEMS differential microphone is described in which the diaphragm design is inspired by the mechanics of directional hearing in the fly Ormia ochracea. The 1 mm by 3 mm diaphragm is designed to rotate about a central pivot in response to sound pressure gradients. The diaphragm is designed to have its dominant resonance mode within the audible frequency range and to have as little viscous damping as possible (to minimize the effects of thermal noise). The motion of the diaphragm is detected using an optical sensing scheme that includes a semiconductor laser (VCSEL), photodetectors, a mirror, and a diffraction grating. To minimize the adverse effects of the light damping on the response, an active feedback system is implemented to achieve active Q control. This uses the output of the optical detection scheme to drive the diaphragm through a capacitive actuator. The microphone and optoelectronics are packaged into an assembly that can be incorporated into a mock behind-the-ear hearing aid. The microphone is...

Electronic enhancement of the directional response of a micromachined biomimetic optical microphone.

A silicon micromachined biomimetic optical microphone has been recently demonstrated to have directional response and low noise [J. Acoust. Soc. Am. 125(4), 2013–2026 (2009)]. In this microphone, the motion of one end of the pivoting rectangular diaphragm is detected using an integrated optical interferometer and each end can be actuated using separate electrostatic actuators. This configuration enables one to selectively enhance either the directional, anti‐symmetric rocking mode or the omni‐directional symmetrical second vibrational mode of the diaphragm in two separate active feedback loops. A circuit model with two electrical ports to actuate each side of the diaphragm, and two acoustic ports to drive each mode has been developed to illustrate the electronic method. The model includes the effect of the air medium and the backside cavity through mechanical impedances which are verified through measurements. With the two‐sided active feedback scheme, the model predicts significant improvements in the di...