W. J. Xu

Numerical and experimental investigation of kerf depth effect on high-frequency phased array transducer.

BACKGROUND High-frequency ultrasonic transducer arrays are essential for high resolution imaging in clinical analysis and Non-Destructive Evaluation (NDE). However, the structure design and fabrication of the kerfed ultrasonic array is quite challenging when very high frequency (≥100MHz) is required. OBJECTIVE AND METHOD Here we investigate the effect of kerf depth on the performances of array transducers. A finite element tool, COMSOL, is employed to simulate the properties of acoustic field and to calculate the electrical properties of the arrays, including crosstalk effect and electrical impedance. Furthermore, Inductively Coupled Plasma (ICP) deep etching process is used to etch 36°/Y-cut lithium niobate (LiNbO(3)) crystals and the limitation of etching aspect ratio is studied. Several arrays with different profiles are realized under optimized processes. At last, arrays with a pitch of 25μm and 40μm are fabricated and characterized by a network analyzer. RESULTS Kerf depth plays an important role in the performance of the transducer array. The crosstalk is proportional to kerf depth. When kerf depth is more than 13μm, the array with crosstalk less than -20dB, which is acceptable for the real application, could provide a desired resolution. Compared to beam focusing, kerf depth exhibits more effect on the beam steering/focusing. The lateral pressure distribution is quantitatively summarized for four types of arrays with different kerf depth. The results of half-cut array are similar to those of the full-cut one in both cases of focusing and steering/focusing. The Full-Width-at-Half-Maximum (FWHM) is 55μm for the half-cut array, and is 42μm for the full-cut one. The 5-μm-cut array, suffering from severe undesired lobes, demonstrates similar behaviors with the no-cut one. ICP process is used to etch the 36°/Y-cut LiNbO(3) film. The aspect ratio of etching profile increases with the kerf width decreasing till it stops by forming a V-shaped groove, and the positive tapered profile angle ranges between 62° and 80°. If the mask selectivity does not limit the process in terms of achievable depth, the aspect ratio is limited to values around 1.3. The measurement shows the electrical impedance and crosstalk are consistent with the numerical calculation. CONCLUSION The numerical results indicate that half-cut array is a promising alternative for the fabrication of high-frequency ultrasonic linear arrays. In fact, the minimum pitch that could be obtained is around 25μm, equivalent to a pitch of 1.6λ, with a kerf depth of 16μm under the optimized ICP parameters.

Modelling and simulation of high-frequency (100 MHz) ultrasonic linear arrays based on single crystal LiNbO3

BACKGROUND High-frequency ultrasonic transducer arrays are essential for high resolution imaging in clinical analysis and Non-Destructive Evaluation (NDE). However, the fabrication of conventional backing-layer structure, which requires a pitch (distance between the centers of two adjacent elements) of half wavelength in medium, is really a great challenge. OBJECTIVE AND METHOD Here we present an alternative buffer-layer structure with a silicon lens for volumetric imaging. The requirement for the size of the pitch is less critical for this structure, making it possible to fabricate high-frequency (100MHz) ultrasonic linear array transducers. Using silicon substrate also makes it possible to integrate the arrays with IC (Integrated Circuit). To compare with the conventional backing-layer structure, a finite element tool, COMSOL, is employed to investigate the performances of acoustic beam focusing, the influence of pitch size for the buffer-layer configuration, and to calculate the electrical properties of the arrays, including crosstalk effect and electrical impedance. RESULTS For a 100MHz 10-element array of buffer-layer structure, the ultrasound beam in azimuth plane in water could be electronically focused to obtain a spatial resolution (a half-amplitude width) of 86μm at the focal depth. When decreasing from half wavelength in silicon (42μm) to half wavelength in water (7.5μm), the pitch sizes weakly affect the focal resolution. The lateral spatial resolution is increased by 4.65% when the pitch size decreases from 42μm to 7.5μm. The crosstalk between adjacent elements at the central frequency is, respectively, -95dB, -39.4dB, and -60.5dB for the 10-element buffer, 49-element buffer and 49-element backing arrays. Additionally, the electrical impedance magnitudes for each structure are, respectively, 4kΩ, 26.4kΩ, and 24.2kΩ, which is consistent with calculation results using Krimholtz, Leedom, and Matthaei (KLM) model. CONCLUSION These results show that the buffer-layer configuration is a promising alternative for the fabrication of high-frequency ultrasonic linear arrays dedicated to volumetric imaging.

A self-aligned mask-free fabrication process for high-frequency ZnO array transducer

High-frequency ultrasonic array transducers are essential for high resolution imaging in clinical analysis and Non Destructive Evaluation (NDE). However, the fabrication of piezoelectric array transducers is a great challenge due to the small features in elaborating piezoelectric array films. This paper describes a MEMS based self-aligned mask-free process for fabrication of ZnO linear array transducers of more than 100MHz. A four-step-rotation deposition approach is proposed and investigated, that improves the lateral growth in ZnO array deposition. The ratio of vertical to lateral growth is improved by 40% compared to one-step deposition method. The results prove that the reduction of the lateral growth helps to achieve full-kerfed ZnO array with smaller pitch.

Electrical impedance and radiation modes Determination for LiNbO3 MEMS ultrasonic array transducer using KLM and FEM modelling

In MEMS realization of high frequency ultrasonic array transducer, electrical impedance matching may become difficult for the reason of the miniaturization of the piezo-array elements. In this study, the electrical impedance of the array elements is estimated first by using KLM model, and then calculated by using finite elements method (FEM), in the case of a LiNbO3 based 30MHz array transducer. The impedance frequency behavior is related both to the eigen-resonance modes and the electrode driven modes with the free mechanical boundary condition of the element and to the coupling of these modes to the array structure when the element being integrated as the transducer. The array transducer properties and performances such as lateral modes, crosstalk, sensibility and bandwidth, field radiation, focusing/steering are also simulated and analyzed.

Micromachined high-frequency ZnO ultrasonic linear arrays

ZnO thin film has been widely applied in high-frequency acoustic microscopy using single element transducer. It needs mechanical scanning which is much more time-consuming than electronic scanning with transducer array. However, one of the challenges in the implementation of ZnO transducer array is the patterning of small scale features in the array elements. In this paper, a controllable wet-chemical etching method is investigated to fabricate high-frequency ZnO-based ultrasonic transducer arrays. The wet-chemical etchant is NH4Cl aqueous solution with a concentration of 10 wt% and the etching rate is 53 nm/min at room temperature. A ZnO array is achieved with a small ratio (0.25) of lateral etching to vertical etching. Finite element method is employed to calculate acoustic field, electrical impedance and crosstalk of the transducer. The characteristics of the transducer are measured and compared to the theoretical predictions. This etching method provides a possibility to acquire a pitch of a λ in a 300 MHz array. It indicates that the proposed wet etching is promising in the fabrication of high-frequency ZnO transducer arrays.

Study of rough surface to decrease reverberation noise in ultrasonic imaging

Rough back surface is investigated to decrease the reverberation noise in ultrasonic imaging. Silicon crystal is selected as the backing substrate of the ultrasonic transducer because rough structure is convenient to be fabricated on silicon substrate using microfabrication technologies. Different dimensions of rough boundaries are designed and simulated to scatter the undesired waves based on finite element method modeling. Transient analysis indicates that a rough surface whose dimension (including depth and width) is around 1.0 λ should be considered to scatter a majority of incident waves.

A LiNbO3 ultrasonic phased array transducer of more than 100 MHz

High-frequency ultrasonic transducer arrays are essential for high resolution imaging in clinical analysis and Non-Destructive Evaluation (NDE). However, the structure design and fabrication of the kerfed ultrasonic array is quite challenging when very high frequency (≥ 100 MHz) is required. Inductively Coupled Plasma (ICP) deep etching process is used to etch 36°/Y-cut lithium niobate (LiNbO3) crystals. Furthermore, a finite element tool, COMSOL, is employed to calculate the electrical properties of the arrays, including crosstalk effect and electrical impedance. At last, arrays with a pitch of 40 μm are fabricated and characterized by a network analyzer. The measured results agree well with the theoretical predictions.

Fabrication and characterization of half-kerfed LiNbO 3 -based high-frequency (>100MHz) ultrasonic array transducers

The effect of kerf depth is investigated on the performances of array transducers. A finite element tool, COMSOL, is employed to simulate the properties of acoustic field and to calculate the electrical properties of the arrays, including crosstalk effect and electrical impedance. Furthermore, Inductively Coupled Plasma (ICP) deep etching process is used to etch 36°/Y-cut lithium niobate (LiNbO3) crystals and the limitation of etching aspect ratio is studied. Several arrays with different profiles are realized under optimized processes. At last, arrays with different pitches are fabricated and characterized by a network analyzer.

A novel method to fabricate full-kerfed highfrequency (>100 MHz) ultrasonic array transducers

High-frequency ultrasonic transducer arrays are essential for efficient imaging in clinical analysis and nondestructive evaluation (NDE). However, the fabrication of piezoelectric transducers is really a great challenge due to the small features in piezoelectric films. This paper describes a novel technique to fabricate thick-film ZnO ultrasonic array transducers. Piezoelectric elements are formed by sputtering thick-film ZnO onto etched features of a silicon substrate so that the difficult etching process for ZnO films is avoided by etching silicon. This process is simple and efficient. A 13-μm-pitch ZnO sandwich array is achieved with a thickness of 8 μm for 300 MHz. Finite element method is employed to calculate its electrical properties, including electrical impedance and crosstalk. The array is characterized by a network analyzer. The measured results are in good agreement with the theoretical predictions.

Study of enhanced biosensors based on 2-D sandwiched plasmon photonic crystals

A novel sandwich structure of photonic crystal with periodic square array of hexagon holes was fabricated. The infrared optical properties in reflectance of the devices were tested. A sharp peak of surface plasmon resonance was observed at the incident angles of 30°. By the comparison of the results between non-sandwich structure and sandwich structure, the latter has higher sensitivity. This relative sensitivity of the optical biosensor has been enhanced by almost 300%.