Nikhil Apte

Non-contact thermoacoustic detection of embedded targets using airborne-capacitive micromachined ultrasonic transducers

A radio frequency (RF)/ultrasound hybrid imaging system using airborne capacitive micromachined ultrasonic transducers (CMUTs) is proposed for the remote detection of embedded objects in highly dispersive media (e.g., water, soil, and tissue). RF excitation provides permittivity contrast, and ultra-sensitive airborne-ultrasound detection measures thermoacoustic-generated acoustic waves that initiate at the boundaries of the embedded target, go through the medium-air interface, and finally reach the transducer. Vented wideband CMUTs interface to 0.18 μm CMOS low-noise amplifiers to provide displacement detection sensitivity of 1.3 pm at the transducer surface. The carefully designed vented CMUT structure provides a fractional bandwidth of 3.5% utilizing the squeeze-film damping of the air in the cavity.

Finite element analysis of CMUTs with pressurized cavities

We propose using CMUTs with pressurized cavities in environments with extreme pressure variations. By controlling the pressure inside the cavity, the pressure differential across the CMUT plate can be kept low, ensuring a stable operating point and preventing mechanical failure. In such CMUTs, a squeeze film is formed between the plate and the substrate, which provides additional damping as well as stiffening. The damping from the squeeze film helps increase the bandwidth of the CMUT. We present a new method for performing a finite element analysis for such structures using ANSYS. We fabricated a variety of vented CMUTs in the frequency range of ~100-200 kHz, which exhibited a quality factor of 25-30 in air at 1 atm pressure. Our finite element model successfully predicts the center frequency and quality factor for these devices.

Fabrication of CMUT Cells with Gold Center Mass for Higher Output Pressure

For decades, high intensity focused ultrasound (HIFU) transducers have been developed for minimally invasive and non‐invasive therapies. Capacitive micromachined ultrasonic transducer (CMUT) technology is a promising candidate for HIFU therapy as it allows the fabrication of arbitrary array geometries and is inherently magnetic resonance (MR) compatible. In this study we investigate a way to improve the output pressure of a single CMUT cell by a modification to the basic CMUT cell structure: adding a gold mass over the center of the top CMUT plate. Using the direct wafer bonding fabrication process we realized linear 1D CMUT arrays. On top of the 0.86 μm thick silicon plate, a 200‐nm thick aluminum layer and a 10‐nm thick titanium adhesion layer were deposited. A lift‐off technique was used to deposit a gold mass on top of the adhesion layer, at the center of each cell. The 1‐μm thick gold layer was deposited in multiple steps with intervening cool‐down periods to ensure low thermal‐induced stress between...

Bandwidth and sensitivity optimization in CMUTs for airborne applications

We previously proposed venting the cavities of CMUTs for using them in environments with extreme pressure variation. Such CMUTs have zero differential pressure across the plate at any ambient pressure, thus ensuring a stable operating point and preventing mechanical failure. The air in the cavity between the moving CMUT plate and the substrate forms a squeeze film which gives some stiffening and damping effect to the CMUT. The additional damping from the squeeze film helps to enhance the CMUTs bandwidth significantly. By properly selecting the size, number, and location of the venting holes, the squeeze film effect can be controlled and the sensitivity and bandwidth of the CMUT can be optimized. We developed a finite element model for simulating such CMUTs with vented cavities. Using this model, we designed a variety of CMUTs with varying sensitivity and bandwidth. These CMUTs were fabricated and characterized. The measurements closely match our finite element model results.

Singulation for imaging ring arrays of capacitive micromachined ultrasonic transducers

Singulation of MEMS is a critical step in the transition from wafer-level to die-level devices. As is the case for capacitive micromachined ultrasound transducer (CMUT) ring arrays, an ideal singulation must protect the fragile membranes from the processing environment while maintaining a ring array geometry. The singulation process presented in this paper involves bonding a trench-patterned CMUT wafer onto a support wafer, deep reactive ion etching (DRIE) of the trenches, separating the CMUT wafer from the support wafer and de-tethering the CMUT device from the CMUT wafer. The CMUT arrays fabricated and singulated in this process were ring-shaped arrays, with inner and outer diameters of 5 mm and 10 mm, respectively. The fabricated CMUT ring arrays demonstrate the ability of this method to successfully and safely singulate the ring arrays and is applicable to any arbitrary 2D shaped MEMS device with uspended microstructures, taking advantage of the inherent planar attributes of DRIE.

Non-contact thermoacoustic imaging of tissue with airborne ultrasound detection

Non-contact detection of abnormalities in tissue is performed with highly sensitive airborne capacitive micromachined ultrasonic transducers (CMUTs) detecting signals generated by the thermoacoustic effect at dielectric contrast interfaces. CMUTs with pressure sensitivities as low as 240 μPa are fabricated across a range of frequencies that can enable a system resolution sufficient for imaging. The detection limits of these CMUTs are assessed in the context of the end-to-end imaging system. Remote or non-contact imaging is of particular value in the medical imaging field for enabling portable and inexpensive imaging of various tissues.

Experimental evaluation of CMUTs with vented cavities under varying pressure

We propose venting the cavities of CMUTs for environments with extreme pressure variations. The CMUT have zero differential pressure across the plate at any ambient pressure, thus ensuring a stable operating point and preventing mechanical failure. The venting vias are etched through the substrate. We observe two resonances from the vented CMUTs - the mechanical resonance of the plate and an acoustic Helmholtz resonance associated with the cavity and the venting vias. We fabricated a variety of CMUTs varying the plate radius, thickness, gap height and via arrangement to study these two resonances. A pair of CMUTs was characterized in a pitch-catch setup under varying ambient pressure. The CMUTs were successfully able to transmit and receive ultrasound under an ambient pressure of up to 20 bar. As the pressure increases, the plate resonance dominated mode becomes weaker while the Helmholtz resonance dominated mode becomes stronger. The Helmholtz resonance dominated mode maintains its frequency and bandwidth under varying ambient pressure.

Large Area 1D CMUT Phased Arrays for Multi-Modality Ultrasound Imaging

We present the design and fabrication of 1D capacitive micromachined ultrasonic transducer (CMUT) arrays optimized for imaging using multiple modalities. A new variation of the fabrication process based on a thick buried oxide layer is used to build these CMUTs. In our process, the via connections for each cells electrode with the handle layer are made from the front side. This enables fabricating CMUT cells with smaller sizes, and higher resonant frequencies. Initial characterization results agree well with our design simulations.

Effect of fluid losses and acoustic resonances in CMUTs with vented cavities

We report on capacitive micromachined ultrasonic transducers (CMUTs) with vented cavities for varying their bandwidth and sensitivity in airborne applications. The devices are simulated using a finite element model which incorporates viscous and thermal fluid losses in the squeeze film and the vent holes, as well as acoustic radiation and acoustic resonance in the backing. Our model accurately predicts the behavior of such CMUTs. The model is also validated with measurements at elevated pressure.