A digital beamformer for high-frequency annular arrays

Digital transmit and receive beamformers for a 45-MHz, 7-element annular array are described. The transmit beamformer produces 0- to 80-Vpp monocycle pulses with a timing error of less than +/-125 ps. Up to four adjustable transmit focal zones can be selected. The dynamic receive beamformer uses a variable frequency sampling technique in which the frequency of analog-to-digital conversion on each channel is adjusted as the signals are received. The variable frequency clock signals required to trigger analog-to-digital conversion are obtained using a pair of high-frequency field-programmable gate arrays and a precision quartz oscillator. The gate arrays are also used to sum the digitized signals. A maximum receive beamformer timing error of less than +/-900 ps was obtained on each channel. The performance of the combined transmit and receive beamformer was tested by imaging wire phantoms. Images of CD-1 mice were also generated. The system produced images with a dynamic range of 60 dB.     

Investigation of cross talk in Kerfless annular arrays for high-frequency imaging

The effect of electromechanical cross talk in high-frequency (> 30 MHz) kerfless annular arrays is investigated. Finite-element model predictions of the radiation patterns from arrays are compared to predictions from an ideal model without cross talk and with experimental measurements. High cross talk in the array causes element broadening and an increase in the amplitude of secondary lobes in the radiation pattern. However, an increase in the pulse ring-down time was not found. This can be attributed to the absence of lateral modes in the kerfless substrate. The level of the pedestal secondary lobes in the two-way radiation pattern increases linearly with the element path difference. The element broadening increases the effective element path difference, which increases the pedestal level for a kerfless annular array above the level for an ideal array. The broadening limits how close to an array one can image compared to the ideal case by reducing the contrast available in the image at small f-numbers. When the element broadening is taken into account by widening the electrode dimensions, the ideal radiation pattern agrees well with the finite-element model and experimental radiation patterns.     

Microfabrication of electrode patterns for high-frequency ultrasound transducer arrays

High-frequency ultrasound is needed for medical imaging with high spatial resolution. A key issue in the development of ultrasound imaging arrays to operate at high frequencies (?30 MHz) is the need for photolithographic patterning of array electrodes. To achieve this directly on 1-3 piezocomposite, the material requires not only planar, parallel, and smooth surfaces, but also an epoxy composite filler that is resistant to chemicals, heat, and vacuum. This paper reports, first, on the surface finishing of 1-3 piezocomposite materials by lapping and polishing. Excellent surface flatness has been obtained, with an average surface roughness of materials as low as 3 nm and step heights between ceramic/polymer of ～80 nm. Subsequently, high-frequency array elements were patterned directly on top of these surfaces using a photolithography process. A 30-MHz linear array electrode pattern with 50-μm element pitch has been patterned on the lapped and polished surface of a high-frequency 1-3 piezocomposite. Excellent electrode edge definition and electrical contact to the composite were obtained. The composite has been lapped to a final thickness of ～55 μm. Good adhesion of electrodes on the piezocomposite has been achieved and electrical impedance measurements have demonstrated their basic functionality. The array was then packaged, and acoustic pulse-echo measurements were performed. These results demonstrate that direct patterning of electrodes by photolithography on 1-3 piezocomposite is feasible for fabrication of high-frequency ultrasound arrays. Furthermore, this method is more conducive to mass production than other reported array fabrication techniques.     

Chirp-coded excitation imaging with a high-frequency ultrasound annular array

High-frequency ultrasound (HFU, > 15 MHz) is an effective means of obtaining fine-resolution images of biological tissues for applications such as opthalmologic, dermatologic, and small animal imaging. HFU has two inherent drawbacks. First, HFU images have a limited depth of field (DOF) because of the short wavelength and the low fixed F-number of conventional HFU transducers. Second, HFU can be used to image only a few millimeters deep into a tissue because attenuation increases with frequency. In this study, a five-element annular array was used in conjunction with a synthetic-focusing algorithm to extend the DOF. The annular array had an aperture of 10 mm, a focal length of 31 mm, and a center frequency of 17 MHz. To increase penetration depth, 8-micros, chirp-coded signals were designed, input into an arbitrary waveform generator, and used to excite each array element. After data acquisition, the received signals were linearly filtered to restore axial resolution and increase the SNR. To compare the chirpcoded imaging method with conventional impulse imaging in terms of resolution, a 25-microm diameter wire was scanned and the -6-dB axial and lateral resolutions were computed at depths ranging from 20.5 to 40.5 mm. The results demonstrated that chirp-coded excitation did not degrade axial or lateral resolution. A tissue-mimicking phantom containing 10-microm glass beads was scanned, and backscattered signals were analyzed to evaluate SNR and penetration depth. Finally, ex vivo ophthalmic images were formed and chirpcoded images showed features that were not visible in conventional impulse images.     

Design and fabrication of a 40-MHz annular array transducer

This paper investigates the feasibility of fabricating a five-ring, focused annular array transducer operating at 40 MHz. The active piezoelectric material of the transducer was a 9-microm thick polyvinylidene fluoride (PVDF) film. One side of the PVDF was metallized with gold and forms the ground plane of the transducer. The array pattern of the transducer and electrical traces to each annulus were formed on a copper-clad polyimide film. The PVDF and polyimide were bonded with a thin layer of epoxy, pressed into a spherically curved shape, then back filled with epoxy. A five-ring transducer with equal area elements and 100-microm kerfs between annuli was fabricated and tested. The transducer had a total aperture of 6 mm and a geometric focus of 12 mm. The pulse/echo response from a quartz plate located at the geometric focus, two-way insertion loss (IL), complex impedance, electrical crosstalk, and lateral beamwidth all were measured for each annulus. The complex impedance data from each element were used to perform electrical matching, and the measurements were repeated. After impedance matching; fc approximately equal to 36 MHz and -6-dB bandwidths ranged from 31 to 39%. The ILs for the matched annuli ranged from -28 to -38 dB.     

Design and parallel fabrication of wire-grid polarization arrays for polarization-resolved imaging at 1.55 microm

Polarization-resolved imaging can provide information about the composition and topography of the environment that is invisible to the eye. We demonstrate a practical method to fabricate arrays of small, orthogonal wire-grid polarizers (WGPs) that can be matched to individual detector pixels, and we present design curves that relate the structure to the polarization extinction ratio obtained. The photonic area lithographically mapped (PALM) method uses multiple-exposure conventional and holographic lithography to create subwavelength patterns easily aligned to conventional mask features. WGPs with polarization extinction ratios of approximately 10 at a 1.55 microm wavelength were fabricated, and square centimeter areas of square micrometer size WGP arrays suitable for polarization-resolved imaging on glass were realized.     

Binary code design for high-frequency ultrasound

This paper proposes an approach to designing binary codes suitable for high-frequency applications of coded excitation in medical ultrasound. For a high-frequency ultrasound system, transmitting well-designed binary codes with a low sampling ratio (i.e., the bit rate divided by the transducer center frequency) is a practical way to improve the signal-to-noise ratio (SNR) because the challenge of implementing arbitrary-waveform generators for transmitting nonbinary codes increases with the frequency and the switching speed of square-wave pulsers are limited. One conventional approach designs codes using a base sequence that modulates wideband sequences up to the transducer passband. Because a major portion of codes is excluded as a candidate, codes designed using this approach typically need long compression filters for restoring the axial resolution, and they do not improve the SNR efficiently. In contrast, the approach proposed here searches all the codes that match the transducer passband; hence, the resultant codes exhibit better performance. The technique was tested using a bit rate of 50 MHz and a sampling ratio of 2. For a transducer with an ideal Gaussian frequency response with a center frequency of 25 MHz and a -6 dB bandwidth of 15 MHz, the SNR for the same side-lobe extent was 1 to 6 dB higher for the codes designed using the proposed approach compared with those designed using the conventional approach. When a real transducer response with a center frequency of 26.4 MHz and a one-way -6 dB bandwidth of 20.7 MHz was considered, the codes designed using the proposed approach were superior by 0.5 to 5 dB. Therefore, our approach is better than the conventional approach for designing binary codes for high-frequency ultrasound, with the results indicating that the moderate bit rate of 50 MHz will suffice when the ultrasonic center frequency is 25 MHz.     

Assembly fabrication of oligonucleotide arrays

In this paper, a simple, reliable and flexible method for fabricating oligonucleotide arrays integrating in in-situ synthesis with a spotting technique is described. In this approach, different oligonucleotide sequences were synthesized on coded modification glass slides using combinatorial chemistry and a mature phosphoramidite chemistry protocol. The slides were then sliced into smaller pieces. Finally, an oligonucleotide array was fabricated by arbitrarily assembling the different coded pieces onto another solid support. A 5 x 5 array including four different sequences of the P16 gene and a control (blank) was successfully assembled. The results indicated that the hybridization fluorescence intensities from the same sequences located at different places on the array were homogeneous and uniform. Background fluorescence was much lower. The fluorescence intensity ratio of a matched sequence to a one-base mismatched sequence, a two-base mismatched sequence and a three-base mismatched sequence was 0.499, 0.236 and 0.04, respectively. The results for the same sequence at different spots in the chip were reproducible with the relative standard deviation ranging from 6.64% to 10.2% (n = 5). This method has the advantages of high probe-density of in-situ synthesis, and off-chip flexibility. Moreover, it does not need any immobilization process to bond oligonucleotides on the substrate.     

Design and fabrication of diffractive microlens arrays with continuous relief for parallel laser direct writing

Diffractive microlens arrays with continuous relief are designed, fabricated, and characterized by using Fermat's principle to create an array of spots on the photoresist-coated surface of a substrate for parallel laser direct writing. Experimental results indicate that a diffraction efficiency of 71.4% and a spot size of 1.97 microm (FWHM) can be achieved at normal incidence and a writing laser wavelength of 441.6 nm with an array of F/4 fabricated on fused silica, and the developed array can be used to improve the utilization ratio of writing laser energy.     

Beam steering with segmented annular arrays

Two-dimensional (2-D) arrays of squared matrix have maximum periodicity in their main directions; consequently, they require half wavelength (lambda/2), interelement spacing to avoid grating lobes. This condition gives rise to well-known problems derived from the huge number of array elements and from their small size. In contrast, 2-D arrays with curvilinear configuration produce lower grating lobes and, therefore, allow the element size to be increased beyond lambda/2. Using larger elements, these arrays have the advantage of reducing the number of elements and of increasing the signal-to-noise ratio (SNR). In this paper, the beamforming properties of segmented annular phased arrays are theoretically analyzed and compared with the equivalent squared matrix array. In the first part, point-like elements are considered in order to facilitate the field analysis with respect to the array structure. Afterward, the effect of the element size on the steered beam properties also is presented. In the examples, it is shown that the segmented annular array has notably lower grating lobes than the equivalent squared matrix array and that it is possible to design segmented annular arrays with interelement distance higher than lambda whose beam characteristics are perfectly valid for volumetric imaging applications.     

Design and fabrication of linear pixel arrays for organic photovoltaic modules to achieve scalable power output

We describe a design and fabrication method to enable simpler manufacturing of more efficient organic solar cell modules using a modified flat panel deposition technique. Many mini-cell pixels are individually connected to each other in parallel forming a macro-scale solar cell array. The pixel size of each array is optimized through experimentation to maximize the efficiency of the whole array. We demonstrate that integrated organic solar cell modules with a scalable current output can be fabricated in this fashion and can also be connected in series to generate a scalable voltage output.     

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 highfrequency 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.     

Template-based fabrication of nanowire-nanotube hybrid arrays

The fabrication and structure characterization of ordered nanowire-nanotube hybrid arrays embedded in porous anodic aluminum oxide (AAO) membranes are reported. Arrays of TiO(2) nanotubes were first deposited into the pores of AAO membranes by a sol-gel technique. Co?nanowires were then electrochemically deposited into the TiO(2) nanotubes to form the nanowire-nanotube hybrid arrays. Scanning electron microscopy and transmission electron microscopy measurements showed a high nanowire filling factor and a clean interface between the Co nanowire and the TiO(2) nanotube. Application of these hybrids to the fabrication of ordered nanowire arrays with highly controllable geometric parameters is discussed.     

Single-step fabrication of refractive microlens arrays

Arrays of submillimeter microlenses are made from droplets of UV-curable optical adhesive dispensed from a pressurized syringe under computer control. Measurements of the focal length uniformity, the minimum focused spot size, and the spherical aberration are presented. An excellent lens diameter and focal length uniformity are achieved over 100 element arrays.     

Polymer-assisted fabrication of gold nanoring arrays

In this paper,we report a new strategy for the fabrication of gold nanoring arrays via colloidal lithography and polymer-assisted self-assembly of gold nanoparticles (Au NPs).First,multi-segmented polymer nanorod arrays were fabricated via colloidal lithography.They were then used as templates for Au NP adsorption,which resulted in nanoparticles on the poly(4-vinyl pyridine) (P4VP) segments.Continuous gold nanorings were formed after electroless deposition of gold.The diameter,quantity,and spacing of the gold nanorings could be tuned.Three dimensional coaxial gold nanorings with varying diameters could be fabricated on a polymer nanorod by modifying the etch parameters.The nanorings exhibited optical plasmonic resonances at theoretically predicted wavelengths.In addition,the polymer-assisted gold nanorings were released from the substrate to generate a high yield of free-standing nanorings.This simple,versatile method was also used to prepare nanorings from other metals such as palladium.     

Unit-delay focusing architecture and integrated-circuit implementation for high-frequency ultrasound

High-frequency ultrasound (above 10 MHz) has been used successfully in many medical applications, including eye, skin, gastrointestinal, intravascular, and Doppler flow imaging. Most of these applications use single-element transducers, thereby imposing a tradeoff between resolution and depth of field. Fabrication difficulties and the need for high-speed electronic beamformers have prevented widespread use of arrays at high frequencies. In this paper, a unit-delay focusing architecture suitable for use with high-frequency ultrasound annular arrays is described. It uses a collection of identical, active delay cells that may be simultaneously varied to accomplish focusing. Results are presented for an analog integrated circuit intended for use with a five-element, 50-MHz planar annular array. Focusing is possible over an axial range for which the ratio of maximum to minimum f-number is 2.1. Unit-delay architectures also are described for curved annular arrays and linear arrays.     

Bessel and conical beams and approximation with annular arrays

The Bessel beam is one of the relatively new limited-diffraction beams that have been discovered. It is compared with the conical transducer, which also gives an approximate limited-diffraction solution to the wave equation. The conical transducer's field deviates from the predicted field in the nearfield, where it is wider. Therefore, the Bessel beam is better for use in a hybrid system where a limited-diffraction beam is used for transmission and a dynamically focused beam for reception. The limited-diffraction Bessel beam of order zero can be excited on an annular transducer with equal-area division of elements and with a fixed prefocus, i.e., conventional transducers used in commercial medical imaging equipment. The element division implies that the scaling parameter must be chosen to contain the first lobe of the Bessel function in the first element. In addition, the prefocus must be such that the array is steerable to infinite depth with minor loss. Even when the Bessel beam yields a larger depth of field than that of an unfocused transducer, it has the advantage of a narrower beam. Simulated examples are shown where the approximate Bessel beam compares favorably with a spherically focused beam with a fixed focus, an unfocused beam, and a conical transducer.     

Modeling and optimization of high-frequency ultrasound transducers

Obtaining an accurate transducer model for a high-frequency transducer can be troublesome using traditional models, such as the KLM model, since it is often difficult to measure precisely the piezoelectric, dielectric, and mechanical properties of the transducer. This paper describes an alternative method of modeling transducers using network theory. The network theory model for a transducer is determined from a measurement of the transducer impedance in water and the pulse-echo response of the system for a given electrical source and load. A discussion of how this model can be used to optimize the design of an electrical matching circuit is given. This method is illustrated by designing a two-element transmission line matching circuit for a miniature 53 MHz transducer. Excellent agreement between the network model prediction and the experimental response is obtained     

Performance and characterization of new micromachined high-frequency linear arrays

A new approach for fabricating high frequency (> 20 MHz) linear array transducers, based on laser micromachining, has been developed. A 30 MHz, 64-element, 74-microm pitch, linear array design is presented. The performance of the device is demonstrated by comparing electrical and acoustic measurements with analytical, equivalent circuit, and finite-element analysis (FEA) simulations. All FEA results for array performance have been generated using one global set of material parameters. Each fabricated array has been integrated onto a flex circuit for ease of handling, and the flex has been integrated onto a custom printed circuit board test card for ease of testing. For a fully assembled array, with an acoustic lens, the center frequency was 28.7 MHz with a one-way -3 dB and -6 dB bandwidth of 59% and 83%, respectively, and a -20 dB pulse width of -99 ns. The per-element peak acoustic power, for a +/- 30 V single cycle pulse, measured at the 10 mm focal length of the lens was 590 kPa with a -6 dB directivity span of about 30 degrees. The worst-case total cross talk of the combined array and flex assembly is for nearest neighboring elements and was measured to have an average level -40 dB across the -6 dB bandwidth of the device. Any significant deviation from simulation can be explained through limitations in apparatus calibration and in device packaging.     

Facile fabrication of Si nanowire arrays for solar cell application

Large-area Si nanowire arrays have been fabricated on phosphorus doped Si surface by a facile silver-catalyzed chemical etching process. The solar cell incorporated with Si nanowire arrays shows a power conversion efficiency of 6.69% with an open circuit voltage of 558 mV and a short circuit current density of 25.13 mA/cm2 under AM 1.5 G illumination without using any extra antireflection layer and surface passivation technique. The high power conversion efficiency of Si nanowires based-solar cell is attributed to the low reflectance loss of Si nanowire arrays for incident sunlight. Optimization of electrical contact and phosphorus diffusion process will be critical to improve the performance of Si nanowires-based solar cell in the future.