F.L. Degertekin

Fabrication and characterization of surface micromachined capacitive ultrasonic immersion transducers

In this paper, several innovative steps used in fabricating surface micromachined capacitive ultrasonic immersion transducers are reported. The investigation is focused on major steps in the device fabrication processes necessary to optimize transducer performance. Such steps include membrane formation, vacuum sealing, and electrode metallization. Three transducer membrane structures are evaluated: a nitride membrane with an oxide sacrificial layer; a polysilicon membrane with an oxide sacrificial layer; and a nitride membrane with a polysilicon sacrificial layer. Three vacuum sealing mechanisms are compared, each of which requires a different degree of lithographic sophistication, uses a particular sealing mechanism, and results in a sealed cavity. Submicrometer via sealing requires sophisticated lithography but is amenable to LPCVD nitride, LTO, and other sealing procedures. Standard g-line lithography results in vies 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 material, is demonstrated. Finally, the impact of electrode metallization on the impedance, bandwidth, and efficiency of the transducers is discussed. The experiments in the paper are guided by theoretical analysis and computer simulations when applicable. The new process results in optimized devices which have a broad-band 50-/spl Omega/ real part impedance in the megahertz range. A transducer dynamic range in excess of 100 dB is achieved around 4.5 MHz. An untuned transducer exhibits more than 100% bandwidth when connected to electronics with 50-/spl Omega/ input impedance. In addition, beam pattern measurement shows the immersion devices behave like uniform piston transducers and are readily suitable for array applications. The fabrication techniques and results herein reported indicate that surface micromachined ultrasonic immersion transducers are an attractive alternative to piezoelectric transducers in immersion applications.

Characterization of one-dimensional capacitive micromachined ultrasonic immersion transducer arrays

We report on the characterization of 1D arrays of capacitive micromachined ultrasonic transducers (cMUT). A 275/spl times/5600 /spl mu/m 1D CMUT array element is experimentally characterized, and the results are found to be in agreement with theoretical predictions. As a receiver, the transducer has a 0.28-fm//spl radic/Hz displacement sensitivity, and, as a transmitter, it produces 5 kPa/V of output pressure at the transducer surface at 3 MHz with a DC bias of 35 V. The transducer has more than 100% fractional bandwidth around 3 MHz, which makes it suitable for ultrasound imaging. The radiation pattern of isolated single elements, as well as those of array elements are measured, and two major sources of acoustical crosstalk are identified. A weakly dispersive non-leaky interface wave (Stoneley wave) is observed to be propagating at the silicon substrate-fluid interface at a speed close to the speed of sound in the fluid. This wave causes internal reflections, spurious resonance, and radiation from the edges of the silicon substrate. The large lateral component of the particle velocity generated by the membranes at the edge of the cMUT array elements is found to be the source of this interface wave. Lowest order Lamb waves in the silicon substrate are also found to contribute to the crosstalk between elements. These waves are excited at the edges of individual vibrating membranes, where they are anchored to the substrate, and result in a narrowing of the beam profile of the array elements. Several methods, such as trench isolation and wafer thinning, are proposed and implemented to modify the acoustical cross coupling between array elements.

Low temperature fabrication of immersion capacitive micromachined ultrasonic transducers on silicon and dielectric substrates

A maximum processing temperature of 250/spl deg/C is used to fabricate capacitive micromachined ultrasonic transducers (CMUTs) on silicon and quartz substrates for immersion applications. Fabrication on silicon provides a means for electronics integration via post-complementary metal oxide semiconductor (CMOS) processing without sacrificing device performance. Fabrication on quartz reduces parasitic capacitance and allows the use of optical displacement detection methods for CMUTs. The simple, low-temperature process uses metals both as the sacrificial layer for improved dimensional control, and as the bottom electrode for good electrical conductivity and optical reflectivity. This, combined with local sealing of the vacuum cavity by plasma-enhanced chemical-vapor deposition of silicon nitride, provides excellent control of lateral and vertical dimensions of the CMUTs for optimal device performance. In this paper, the fabrication process is described in detail, including process recipes and material characterization results. The CMUTs fabricated for intravascular ultrasound (IVUS) imaging in the 10-20 MHz range and interdigital CMUTs for microfluidic applications in the 5-20 MHz range are presented as device examples. Intra-array and wafer-to-wafer process uniformity is evaluated via electrical impedance measurements on 64-element ring annular IVUS imaging arrays fabricated on silicon and quartz wafers. The resonance frequency in air and collapse voltage variations are measured to be within 1% and 5%, respectively, for both cases. Acoustic pressure and pulse echo measurements also have been performed on 128 /spl mu/m/spl times/32 /spl mu/m IVUS array elements in water, which reveal a performance suitable for forward-looking IVUS imaging at about 16 MHz.

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.

Front-end receiver electronics for high-frequency monolithic CMUT-on-CMOS imaging arrays

This paper describes the design of CMOS receiver electronics for monolithic integration with capacitive micromachined ultrasonic transducer (CMUT) arrays for high-frequency intravascular ultrasound imaging. A custom 8-inch (20-cm) wafer is fabricated in a 0.35-μm two-poly, four-metal CMOS process and then CMUT arrays are built on top of the application specific integrated circuits (ASICs) on the wafer. We discuss advantages of the single-chip CMUT-on-CMOS approach in terms of receive sensitivity and SNR. Low-noise and high-gain design of a transimpedance amplifier (TIA) optimized for a forward-looking volumetric-imaging CMUT array element is discussed as a challenging design example. Amplifier gain, bandwidth, dynamic range, and power consumption trade-offs are discussed in detail. With minimized parasitics provided by the CMUT-on-CMOS approach, the optimized TIA design achieves a 90 fA/√Hz input-referred current noise, which is less than the thermal-mechanical noise of the CMUT element. We show successful system operation with a pulseecho measurement. Transducer-noise-dominated detection in immersion is also demonstrated through output noise spectrum measurement of the integrated system at different CMUT bias voltages. A noise figure of 1.8 dB is obtained in the designed CMUT bandwidth of 10 to 20 MHz.

Droplet formation and ejection from a micromachined ultrasonic droplet generator: Visualization and scaling

Visualization and scaling of drop-on-demand and continuous-jet fluid atomization of water are presented to elucidate the fluid physics of the ejection process and characterize the modes of operation of a novel micromachined ultrasonic droplet generator. The device comprises a fluid reservoir that is formed between a bulk ceramic piezoelectric transducer and an array of liquid horn structures wet etched into (100) silicon. At resonance, the transducer generates a standing ultrasonic pressure wave within the cavity and the wave is focused at the tip of the nozzle by the horn structure. Device operation has been demonstrated by water droplet ejection from 5to10μm orifices at multiple resonant frequencies between 1 and 5MHz. The intimate interactions between focused ultrasonic pressure waves and capillary waves formed at the liquid–air interface located at the nozzle tip are found to govern the ejection dynamics, leading to different ejection modalities ranging from individual droplets to continuous jet. Spec...

Thin film characterization by atomic force microscopy at ultrasonic frequencies

We present a technique in which atomic force microscopy at ultrasonic frequencies is used to determine the thickness of thin films. In this technique, the resonance frequency of a flexural mode of an atomic force microscope cantilever is used to determine the tip-sample contact stiffness. This allows the film thickness to be determined, provided that the tip and sample elastic moduli and radii of curvature are known. We report experimental results for thin metal and polymer films deposited on silicon substrates and compare them with the predictions of a theoretical model.

Fabrication and characterization of a micromachined acoustic sensor with integrated optical readout

Implementation and characterization of a micromachined acoustic sensor with integrated optoelectronic readout is described. The mechanical part of the sensor is surface micromachined on a quartz substrate and consists of an aluminum membrane which is electrostatically actuated by a back electrode shaped in the form of a diffraction grating. Optical detection is performed by measuring the reflected diffraction orders when the grating is illuminated through the quartz substrate. This scheme provides interferometric displacement detection sensitivity as well as a compact optical interconnect to a custom designed silicon photodetector array fabricated with n-well CMOS technology. The array also contains optical apertures formed by post-CMOS deep reactive ion etching for backside illumination. A compact hybrid packaged sensor array is formed by bonding the silicon photodetector array to the quartz substrate, resulting in an integrated acoustic sensor volume of 2.5 mm/sup 3/. Experimental characterization has been performed on integrated sensors with 200-mm-diameter, 1-mm-thick aluminum membranes. The results show a minimum detectable membrane displacement of 2.08/spl times/10/sup -4/ /spl Aring///spl radic/Hz at 20 kHz and 1.35/spl times/10/sup -4/ /spl Aring///spl radic/Hz at 100 kHz with 61-mW laser power detected on the integrated photodetector. Operation of the device with a pulsed vertical cavity surface emitting laser as the light source and differential detection of diffraction orders for noise reduction are demonstrated to show the potential for low-power, low-cost micromachined acoustic sensors.

Contact stiffness of layered materials for ultrasonic atomic force microscopy

A method to calculate the contact stiffness between a layered material and an ultrasonic atomic force microscope (UAFM) tip is proposed. The radiation impedance method is used to determine the ratio of the applied force to the average displacement within the contact area. This information is used in an iterative algorithm based on Hertzian theory to obtain the contact stiffness. The algorithm converges into a couple of iterations and does not suffer from numerical convergence difficulties as does finite element analysis (FEA). In the ultrasonic frequency range, comparisons with Hertzian theory and FEA show the validity of the results in a quasistatic case. Definitions of the minimum detectable layer thickness and the penetration depth of the UAFM are given and evaluated for several thin film–substrate pairs. These results also show the potential of the method for modeling defects and power loss due to radiation in layered materials.