Rasim Guldiken

Annular-ring CMUT arrays for forward-looking IVUS: transducer characterization and imaging

In this study, a 64-element, 1.15-mm diameter annular-ring capacitive micromachined ultrasonic transducer (CMUT) array was characterized and used for forward-looking intravascular ultrasound (IVUS) imaging tests. The array was manufactured using low-temperature processes suitable for CMOS electronics integration on a single chip. The measured radiation pattern of a 43 /spl times/ 140-/spl mu/m array element depicts a 40/spl deg/ view angle for forward-looking imaging around a 15-MHz center frequency in agreement with theoretical models. Pulse-echo measurements show a -10-dB fractional bandwidth of 104% around 17 MHz for wire targets 2.5 mm away from the array in vegetable oil. For imaging and SNR measurements, RF A-scan data sets from various targets were collected using an interconnect scheme forming a 32-element array configuration. An experimental point spread function was obtained and compared with simulated and theoretical array responses, showing good agreement. Therefore, this study demonstrates that annular-ring CMUT arrays fabricated with CMOS-compatible processes are capable of forward-looking IVUS imaging, and the developed modeling tools can be used to design improved IVUS imaging arrays.

A new atomic force microscope probe with force sensing integrated readout and active tip

We introduce a novel probe structure for the atomic force microscope. The probe has a sharp tip placed on a micromachined membrane with an integrated displacement sensor, a diffraction-based optical interferometer. We use this probe in a microscope to directly measure the transient interaction forces between the probe tip and the sample when operating in a dynamic mode. We form images related to viscoelasticity and adhesion of the samples by recording salient features of individual tap signals. We also produce tapping mode images of sample topography an order of magnitude faster than current probe microscopes using an integrated electrostatic actuator to move the probe tip. We envision a broad range of applications for this device that range from life sciences to microelectronics.

Experimental and Numerical Investigation of Nanoparticle Removal Using Acoustic Streaming and the Effect of Time

The removal of nanoparticles is becoming increasingly challenging as the minimum linewidth continues to decrease in semicon-ductor manufacturing. In this paper, the removal of nanoparticles from flat substrates using acoustic streaming is investigated. Baresilicon wafers and masks with a 4 nm silicon cap layer are cleaned. The silicon-cap films are used in extreme ultraviolet masks toprotect Mo–Si reflective multilayers. The removal of 63 nm polystyrene latex PSL particles from these substrates is conductedusing single-wafer megasonic cleaning. The results show higher than 99% removal of PSL nanoparticles. The results also showthat dilute SC1 provides faster removal of particles, which is also verified by the analytical analysis. Particle removal from the4 nm Si-cap substrate is slightly more difficult as compared to bare silicon wafers. The experimental results show that the removalof nanoparticles takes a relatively long removal time. Numerical simulations showed that the long time is due to particleoscillatory motion and redeposition, and that this phenomenon is not observed in the removal of sub- m or larger size particles.© 2006 The Electrochemical Society. DOI: 10.1149/1.2217287 All rights reserved.Manuscript submitted January 9, 2006; revised manuscript received April 24, 2006. Available electronically July 19, 2006.

CMUTS with dual electrode structure for improved transmit and receive performance

We report on the use of a dual electrode structure to improve the performance of capacitive micromachined ultrasonic transducers (CMUTs.). In the dual electrode structure, separate transmit and receive electrodes are embedded either in the same CMUT membrane or in the dielectric substrate beneath the same membrane. This configuration has several advantages. First, the maximum pressure amplitude during transmit mode can be significantly increased by locating the transmit electrodes near the edges of the membrane. In this case, leveraged bending increases the maximum displacement without collapsing the membrane. Second, since the DC bias of the electrodes can be controlled independently, the device capacitance can be increased during the receive time by biasing both receive and transmit electrodes. Additionally, separate electronics can be used for transmit and receive without the need for a switch. We have fabricated a 0.3/spl times/0.65 mm/sup 2/ CMUT element consisting of 20 /spl mu/m wide, 100 /spl mu/m long and 0.9 /spl mu/m thick rectangular silicon nitride membranes. The transmit electrodes are 4 /spl mu/m wide and located at the edges of the membrane, whereas the 8 /spl mu/m wide receive electrode is at the center. With this non-optimized structure, we measured a 6.8 dB increase in maximum output pressure at 9 MHz with side electrode excitation. We also demonstrated the possibility of using dual electrodes for simultaneous excitation and detection of several CMUT membrane vibration modes.

Sheathless Size-Based Acoustic Particle Separation

Particle separation is of great interest in many biological and biomedical applications. Flow-based methods have been used to sort particles and cells. However, the main challenge with flow based particle separation systems is the need for a sheath flow for successful operation. Existence of the sheath liquid dilutes the analyte, necessitates precise flow control between sample and sheath flow, requires a complicated design to create sheath flow and separation efficiency depends on the sheath liquid composition. In this paper, we present a microfluidic platform for sheathless particle separation using standing surface acoustic waves. In this platform, particles are first lined up at the center of the channel without introducing any external sheath flow. The particles are then entered into the second stage where particles are driven towards the off-center pressure nodes for size based separation. The larger particles are exposed to more lateral displacement in the channel due to the acoustic force differences. Consequently, different-size particles are separated into multiple collection outlets. The prominent feature of the present microfluidic platform is that the device does not require the use of the sheath flow for positioning and aligning of particles. Instead, the sheathless flow focusing and separation are integrated within a single microfluidic device and accomplished simultaneously. In this paper, we demonstrated two different particle size-resolution separations; (1) 3 μm and 10 μm and (2) 3 μm and 5 μm. Also, the effects of the input power, the flow rate, and particle concentration on the separation efficiency were investigated. These technologies have potential to impact broadly various areas including the essential microfluidic components for lab-on-a-chip system and integrated biological and biomedical applications.

Dual-electrode CMUT with non-uniform membranes for high electromechanical coupling coefficient and high bandwidth operation

In this paper, we report measurement results on dual-electrode CMUT demonstrating electromechanical coupling coefficient (k2) of 0.82 at 90% of collapse voltage as well as 136% 3 dB one-way fractional bandwidth at the transducer surface around the design frequency of 8 MHz. These results are within 5% of the predictions of the finite element simulations. The large bandwidth is achieved mainly by utilizing a non-uniform membrane, introducing center mass to the design, whereas the dual-electrode structure provides high coupling coefficient in a large dc bias range without collapsing the membrane. In addition, the non-uniform membrane structure improves the transmit sensitivity of the dual-electrode CMUT by about 2dB as compared with a dual electrode CMUT with uniform membrane.

CMUTs with dual electrode structure for improved transmit and receive performance

In this paper, we introduce capacitive micro-machined ultrasonic transducers (CMUTs) with electrically isolated multiple electrodes embedded in the same silicon nitride CMUT membrane. Some of the advantages of this structure are demonstrated using a dual-electrode CMUT with separate transmit and receive electrodes as an example. By locating the transmit electrodes near the edges of a rectangular CMUT membrane, the stable displacement range, hence the maximum pressure amplitude during transmit mode is increased without collapsing the membrane when operated within static collapse voltage range. In the receive mode, the center receive electrode is brought closer to the substrate by biasing the side electrodes, and a higher electromechanical transformer ratio is obtained at low direct current (DC) bias. Therefore, dual-electrode CMUT has an effectively larger gap as compared to conventional CMUT during transmit, and it has an effectively smaller gap during receive. Demonstrative experiments are performed on dual-electrode CMUTs with rectangular membranes with different side and center electrode sizes for transmit and receive measurements. By using the two 4-/spl mu/m wide side electrodes and an 8-/spl mu/m wide center electrode on a 20-/spl mu/m wide membrane, a 6.8 dB increase in maximum output pressure is obtained with side electrode excitation as compared to conventional center electrode. Similarly, the receive performance improvement was demonstrated while reducing the DC bias requirements. Simple finite-element and equivalent circuit-based models were developed to successfully model the behavior of dual-electrode CMUTs. Simulations show that, with simple modifications, more than 10 dB overall sensitivity improvement is feasible with dual-electrode CMUTs with rectangular membranes.

Single chip CMUT arrays with integrated CMOS electronics: Fabrication process development and experimental results

One of the most important promises of capacitive micromachined ultrasonic transducer (CMUT) technology is integration with electronics. This approach maximizes transducer sensitivity by minimizing parasitic capacitances and ultimately improves the signal to noise ratio. Additionally, due to physical size limitations required for catheter based imaging devices, optimization of area occurs when the CMUTs are fabricated directly above the associated electronics. Here, we describe successful fabrication of CMUTs on custom designed CMOS electronics from a commercial IC foundry. This is achieved by modifying a low temperature micromachining process by adding one additional mask and developing a sloped wall dry etching step on isolation oxide layer for metal interconnect. Experiments show that both the CMUT and the integrated CMOS electronics are functional, resulting combination generating pulse-echo response suitable for ultrasound imaging.

Experimental and Analytical Study of Submicrometer Particle Removal from Deep Trenches

Particle removal from patterned wafers and trenches presents a tremendous challenge in semiconductor manufacturing. In this paper, the removal of 0.3 and 0.8 μm polystyrene latex (PSL) particles from high-aspect-ratio 500 μm deep trenches is investigated. An experimental, analytical, and computational study of the removal of submicrometer particles at different depths inside the trench is presented. Red fluorescent polystyrene latex (PSL) particles were used to verify particle removal. The particles are counted using scanning fluorescent microscopy. A single-wafer megasonic tank is used for the particle removal. The results show that once a particle is removed from the walls or the bottom of a trench, the vortices and circulation zones keep the particles in the trench for a few minutes before eventually moving the particle out of the trench. The experimental results show that the time required for complete removal of particles from the bottom of the trench takes a much longer time than particles on the surface. This has been also verified and explained by physical modeling of the cleaning process. The removal efficiency and cleaning time are reported at different trench depths.

Wideband frequency tunable liquid metal monopole antenna

A frequency tunable liquid metal monopole antenna is introduced. The antenna is formed inside a microfluidic channel embedded within Polydimethylsiloxane (PDMS) substrate. The bottom side of the channel is sealed with 25μm thick liquid crystal polymer (LCP) layer in order to interface the antenna with a conventional microstrip feed line using the capacitive coupling mechanism. The physical length of the antenna is dynamically changed by using pumps to accomplish tunability over a wide frequency range. The concept is demonstrated through the design and experimentation of a 2.9:1 tunable monopole antenna that can operate between 1.7GHz and 4.9GHz.