High-resolution ultrasound imaging of the anterior portion of the eye has been shown to provide important information for sizing of intraocular lens implants, diagnosis of pathological conditions, and creation of detailed maps of corneal topography to guide refractive surgery. Current ultrasound imaging systems rely on mechanical scanning of a single acoustic element over the surface of the eye to create the three-dimensional information needed by clinicians. This mechanical scanning process is time-consuming and subject to errors caused by eye movement during the scanning period. This paper describes development of linear ultrasound imaging arrays intended to increase the speed of image acquisition and reduce problems associated with ocular motion. The arrays consist of a linear arrangement of high-frequency transducer elements designed to operate in the 50 - 75 MHz frequency range. The arrays are produced using single-crystal lithium niobate piezoelectric material, thin film electrodes, and epoxy-based acoustic layers. The array elements have been used to image steel test structures and bovine cornea.

An acoustic microscopy system was designed to perform 2D imaging in the C-plane with a single-element transducer. The ultrasound transducer was fabricated by polishing bulk lithium niobate (LiNbO3) to the required thickness (approximately 60 or 45 μ) for the desired operating frequency (55 or 75 MHz). The polished LiNbO3 was attached to acoustic backing and matching layers. Finally, an epoxy lens was applied and the transducer mounted in a housing. The transducer was mounted in a 3D motorized positioning stage and operated by a high-frequency pulser/receiver. Received echoes were sampled with a 2 GHz ADC card and displayed on a PC using software developed in the Matlab environment. Transducer frequency and bandwidth were measured off a steel plate positioned at the focal length. A penny was scanned initially to confirm expected performance before acquiring data from liver (n=3) and spleen (n=3) specimens. For the first probe, the peak frequency was 54.05 MHz with a −6 dB bandwidth of 6.76 MHz. The axial and lateral resolutions were estimated to be 114 and 188 μm, respectively. For the second probe, the peak frequency was measured to 82 MHz with a −6 dB bandwidth of approximately 23 MHz. The axial and lateral resolutions were estimated to be around 33 and 81 μm, respectively. C-scans of the penny clearly showed detailed structures on front and back, while the capsule and the trabecular structures of the splenic tissues could easily be separated in different layers. In conclusion, an acoustic microscopy system operating at 55–75 MHz has been constructed and the feasibility of obtaining high-resolution images of tissue specimens demonstrated.

Creare is developing microfabrication techniques to manufacture low-cost, multi-dimensional ultrasonic transducer arrays with single- and multi-layer piezoelectric elements. The fabrication approach is scaleable for manufacturing arrays in the frequency range of 10-500 MHz in dense or sparse array designs. Our approach employs the following processes: (1) high-rate Physical Vapor Deposition (sputtering) of high-quality, piezoelectric films using reactive sputtering of a metallic target and (2) photolithography and masking to provide the interlayer electrodes. This transducer technology has the potential to provide new tools to the clinician which should enable higher resolution ultrasound imaging and miniaturization of ultrasound arrays for in vivo imaging.

We describe measurements of the electrical resistivity of micron-size crystallites of boron-doped diamond. Electron-beam lithography was employed for writing sample-specific contacts on small, well-faceted diamond crystals grown by chemical-vapor deposition on silicon substrates. After generating a three-dimensional computer model of the crystallite, a finite-element analysis was used to calculate the internal electrostatic potential distribution. Multiterminal resistance measurements, in conjunction with a computed geometrical factor, enabled the absolute resistivity to be determined. We find that the resistivities obtained from two different crystallites agree to better than 10%. The results are compared with transport measurements on a large-area homoepitaxial diamond film grown simultaneously with the crystallites. This method can be generalized to obtain electrical transport properties of other small, irregularly shaped samples.

We determine the resistivity of small micrometer-sized conductors with arbitrary shapes. It is shown that with n terminals attached to the sample, there are n(n-1)/2 independent measurements of the resistance that can be made; from which the sample resistivity and the contact resistances can be extracted. An image of the sample is digitized and a finite element analysis is used to determine the geometrical factors that arise from the nonuniform current flow and hence control the resistance measurements. It is shown that all the elements of the resistance matrix for the sample are generated from the diagonal elements alone for an Ohmic sample, which provide a useful check of the experimental resistance measurements. To illustrate this approach, micrometer-sized diamond crystallites with four terminals were used, and the sample resistivity and the contact resistances extracted. This technique is a practical example of an inverse problem.

This paper describes experimental studies of the growth morphology of Au/SiO 2 nanocomposites consisting of nanometer-sized metal particles imbedded in a metal oxide, formed by the sequential deposition of metal and metal-oxide layers by RF sputtering. The materials work is aimed at the development of novel resistive-type exhaust gas constituent sensors. Using digitized images from a transmission electron microscope and an automated particle counting routine, we have measured the particle size distributions for films with varying layer thicknesses and deposition conditions. We find strong dependence of the particle size and overall composite structure on the Au and SiO 2 layer thicknesses, in agreement with previous work.[1] We also identified four microstructure classes which can occur.

The interpretation of electronic transport in polycrystalline diamond has proven difficult as a result of the disorder arising from physical and chemical inhomogeneities. We describe experimental methods, based on electron beam lithography, for studying transport in single crystallites grown by CVD on Si substrates. The electrical contacting process utilizes the flexibility of the scanning electron microscope to write metallic contacts to individual crystallites with lateral dimensions of a few μm. A detailed analysis of the current distribution in the crystals allows us to extract the electronic conductivity from arbitrarily shaped, faceted crystals.

A novel technique for studying electron kinetics has been created by combining transverse electron focusing (TEF) with micro-contact resistance measurements. The technique provides a means to separate the contributions of bulk and surface electrons to magnetic field B singularities. We report observations of a phenomenon similar to the quantum Hall effect (QHE).

An electron-beam lithography technique for fabricating submicron point contacts to planar surfaces of bulk samples is described. We have demonstrated the technique by creating a linear array of point contacts, oriented along the bisectrix axis of a bismuth single crystal, which act as emitters and collectors in multiprobe transport measurements. In a transverse electron focusing geometry, we find the expected series of periodic voltage peaks as a function of applied magnetic field at low temperatures. The lithographically fabricated contacts offer advantages over conducting-needle probes in electrical integrity, thermal robustness, lack of damage to the contact site, ability to make multiple submicron contacts with ≤10 μm separations and ability to align the contacts precisely along crystallographic axes.