Tung Manh

Microfabricated 1-3 composite acoustic matching layers for 15 MHz transducers.

Medical ultrasound transducers require matching layers to couple energy from the piezoelectric ceramic into the tissue. Composites of type 0-3 are often used to obtain the desired acoustic impedances, but they introduce challenges at high frequencies, i.e. non-uniformity, attenuation, and dispersion. This paper presents novel acoustic matching layers made as silicon-polymer 1-3 composites, fabricated by deep reactive ion etch (DRIE). This fabrication method is well-established for high-volume production in the microtechnology industry. First estimates for the acoustic properties were found from the iso-strain theory, while the Finite Element Method (FEM) was employed for more accurate modeling. The composites were used as single matching layers in 15 MHz ultrasound transducers. Acoustic properties of the composite were estimated by fitting the electrical impedance measurements to the Mason model. Five composites were fabricated. All had period 16 μm, while the silicon width was varied to cover silicon volume fractions between 0.17 and 0.28. Silicon-on-Insulator (SOI) wafers were used to get a controlled etch stop against the buried oxide layer at a defined depth, resulting in composites with thickness 83 μm. A slight tapering of the silicon side walls was observed; their widths were 0.9 μm smaller at the bottom than at the top, corresponding to a tapering angle of 0.3°. Acoustic parameters estimated from electrical impedance measurements were lower than predicted from the iso-strain model, but fitted within 5% to FEM simulations. The deviation was explained by dispersion caused by the finite dimensions of the composite and by the tapered walls. Pulse-echo measurements on a transducer with silicon volume fraction 0.17 showed a two-way -6 dB relative bandwidth of 50%. The pulse-echo measurements agreed with predictions from the Mason model when using material parameter values estimated from electrical impedance measurements. The results show the feasibility of the fabrication method and the theoretical description. A next step would be to include these composites as one of several layers in an acoustic matching layer stack.

Microfabrication of stacks of acoustic matching layers for 15 MHz ultrasonic transducers.

This paper presents a novel method used to manufacture stacks of multiple matching layers for 15 MHz piezoelectric ultrasonic transducers, using fabrication technology derived from the MEMS industry. The acoustic matching layers were made on a silicon wafer substrate using micromachining techniques, i.e., lithography and etch, to design silicon and polymer layers with the desired acoustic properties. Two matching layer configurations were tested: a double layer structure consisting of a silicon-polymer composite and polymer and a triple layer structure consisting of silicon, composite, and polymer. The composite is a biphase material of silicon and polymer in 2-2 connectivity. The matching layers were manufactured by anisotropic wet etch of a (110)-oriented Silicon-on-Insulator wafer. The wafer was etched by KOH 40 wt%, to form 83 μm deep and 4.5mm long trenches that were subsequently filled with Spurrs epoxy, which has acoustic impedance 2.4 MRayl. This resulted in a stack of three layers: The silicon substrate, a silicon-polymer composite intermediate layer, and a polymer layer on the top. The stacks were bonded to PZT disks to form acoustic transducers and the acoustic performance of the fabricated transducers was tested in a pulse-echo setup, where center frequency, -6 dB relative bandwidth and insertion loss were measured. The transducer with two matching layers was measured to have a relative bandwidth of 70%, two-way insertion loss 18.4 dB and pulse length 196 ns. The transducers with three matching layers had fractional bandwidths from 90% to 93%, two-way insertion loss ranging from 18.3 to 25.4 dB, and pulse lengths 326 and 446 ns. The long pulse lengths of the transducers with three matching layers were attributed to ripple in the passband.

Au-Sn Solid-Liquid Interdiffusion (SLID) bonding for mating surfaces with high roughness

Au-Sn Solid-Liquid Interdiffusion (SLID) bonding of a standard piezoelectric material (PZT) to tungsten carbide (WC) has been investigated. Both materials have a bare bonding surface with an absolute roughness up to 1.5 μm. Bonded samples were characterized by means of acoustic coupling between the PZT and the WC, through electrical impedance measurements. Furthermore, the electrical impedance measurements were employed as a novel method for non-destructive characterization of voids in bond-lines. The results from the non-destructive impedance measurements were compared with the traditional cross-sectional microscopy to inspect voids in SLID bonds, showing good correspondence. Successful Au-Sn SLID bonding of a PZT to a WC is achieved at a temperature higher than 300 °C and a heating rate as high as 120 °C/minute. The bond-line consists of a layered structure Au / Au-Sn (ζ phase) / Au, in accordance with previous studies. Prior to the cross-sectional microscopy, the difference in the bond-lines is clearly observed by the non-destructive electrical impedance measurements. The present study has demonstrated the feasibility of adopting Au-Sn SLID bonding to samples with a high surface roughness.

Fabrication of silicon-polymer composite acoustic matching layers for high frequency transducers

We have used micromachining methods taken from the MEMS industry to fabricate acoustic matching layers for high frequency transducers. The matching layers are made as silicon-polymer composites, with acoustic impedance determined by the ratio between the two materials. Two different fabrication methods were used, anisotropic wet etch and deep reactiveion etch (DRIE), and the composites were fabricated as both type 1–3 and 2–2 connectivity. These methods were used to fabricate structures suitable for a 15 MHz transducer. The resulting 2–2 composite has silicon width 4.5 µm and polymer width 18 µm, whereas the 1–3 composite has 7 µm wide posts and 9 µm spacing between posts. Both composites have 20% volume fraction of silicon, giving acoustic impedance 7 Mrayl. The acoustic behavior of the stack was investigated by FEM simulations, with emphasis on the composite layer. The simulations of pulse shape and bandwidth show that the stack should perform well as acoustic matching layers for high frequency transducers.

Dual frequency hybrid ultrasonic transducers - design and simulations

This paper studies a design concept of a dual frequency ultrasonic transducer, in which we use different technologies for each band. A hybrid structure consisting of a piezoelectric stack used for the lower frequency (LF) band and a Capacitive Micromachined Ultrasound Transducer (CMUT) array placed on the top surface used for the higher frequency (HF) band is demonstrated by Finite Element Method (FEM) simulations. Simulated results show that LF structure introduces ripples into the HF. Simulations also show that using an RTV lens on top of the device is sufficient to damp the vibrations from the CMUTs interfering to the LF band. An array to test the LF band of the design was fabricated and measured electrical impedances show a good correspondence to simulations. The work shows a potential of combining piezoelectric and MUTs technologies in ultrasound transducer design for use in medical applications.

Au-Sn Solid-Liquid Interdiffusion (SLID) bonding for piezoelectric ultrasonic transducers

This paper presents a bonding technique based on gold-tin (Au-Sn) intermetallics to bond an active layer to a backing material in a piezoelectric ultrasonic transducer stack. The bonding process was performed at 310°C. The temperature ramping rate was found to play an important role to the bond quality. Three different bonding temperature profiles P1, P2 and P3 corresponding to a ramping rate 120°C/min., 45°C/min. and 20°C/min., respectively, were investigated. The bonded samples were characterized by electrical impedance measurements in air and cross-sectional microscopy inspection. The most reproducible bondlines were found to be with the heating rate 120°C/min. The resulted bondlines show a layered structure of Au/Au-Sn intermetallic/Au with a corresponding average thickness of 5 μm/17 μm/5 μm, respectively. Based upon energy dispersive spectroscopy (EDS) analysis, the obtained Au-Sn intermetallic compounds are found in good agreement with those from literature. A 64-element one dimensional array formed by Au-Sn SLID method was fabricated and electrical impedance across elements was measured. The results show a good uniformity across elements in the array, and good accordance to Finite Element simulations from COMSOL. This shows the feasibility of using SLID bonding technology to assembly stacks of piezoelectric ultrasonic transducers.

Modeling of micromachined silicon-polymer 2-2 composite matching layers for 15MHz ultrasound transducers.

Silicon-polymer composites fabricated by micromachining technology offer attractive properties for use as matching layers in high frequency ultrasound transducers. Understanding of the acoustic behavior of such composites is essential for using them as one of the layers in a multilayered transducer structure. This paper presents analytical and finite element models of the acoustic properties of silicon-polymer composites in 2-2 connectivity. Analytical calculations based on partial wave solutions are applied to identify the resonance modes and estimate effective acoustic material properties. Finite Element Method (FEM) simulations were used to investigate the interaction between the composite and the surrounding load medium, either a fluid or a solid, with emphasis on the acoustic impedance of the composite. Composites with lateral periods of 20, 40 and 80μm were fabricated and used as acoustic matching layers for air-backed transducers operating at 15MHz. These composites were characterized acoustically, and the results were compared with analytical calculations. The analytical model shows that at low to medium silicon volume fraction, the first lateral resonance in the silicon-polymer 2-2 composite is defined by the composite period, and this lateral resonant frequency is at least 1.2 times higher than that of a piezo-composite with the same polymer filler. FEM simulations showed that the effective acoustic impedance of the silicon-polymer composite varies with frequency, and that it also depends on the load material, especially whether this is a fluid or a solid. The estimated longitudinal sound velocities of the 20 and 40μm period composites match the results from analytical calculations within 2.7% and 2.6%, respectively. The effective acoustic impedances of the 20 and 40μm period composites were found to be 10% and 26% lower than the values from the analytical calculations. This difference is explained by the shear stiffness in the solid, which tends to even out the surface displacements of the composites.

Modeling and characterization of a silicon-epoxy 2-2 composite material

This paper presents modeling and characterization of a 2-2 silicon-epoxy composite used as matching layer for high frequency transducers. The composite was fabricated using Deep Reactive Ion Etching (DRIE), common in the MEMS industry, to form deep trenches into a silicon wafer, and fill them with epoxy resin. This composite was used as acoustic matching layer in an air-backed 15 MHz transducer and characterized by electrical impedance measurements in air. The effective acoustic properties of the composite, i.e., speed of sound, acoustic impedance and mechanical loss tangent, were deduced from the measured electrical impedances. The estimated parameters were compared with results from analytical and FEM models. The models show that the first lateral resonance in the silicon-epoxy 2-2 composite is primarily defined by the composite period, not by the epoxy kerf, and no switching between the two lowest modes is seen near the “interaction zone” in the dispersion curves, where the two lowest branches are close to each other. Higher loss was also observed in coarser composite structures, probably due to dispersion. The simulation results were verified by pulse-echo measurements on two transducers with the composite matching layer period 20 μm and 40 μm, which are just below and just above the interaction zone. The measurements show good agreement with the theoretical calculations.

1–3 microfabricated composite acoustic matching layers for high frequency transducers

This paper presents the fabrication and characterization of a 1-3 silicon-polymer composite matching layer made by Deep Reactive Ion Etch (DRIE) method. A well-defined composite layer thickness of 83 μm was obtained by using Silicon-on-Insulator (SOI) wafers as substrate. The resulting composite has 7 μm size posts and 9 μm spacing between posts. A slight tapering of the posts was observed after the DRIE process, causing the posts to be narrower in the bottom than at the top. The composite was used as acoustic matching layer in an air-backed 15 MHz transducer and characterized by electrical impedance measurements in air. The effective acoustic properties of the composite, speed of sound and acoustic impedance, deduced from measured results, were found to be lower than those predicted from iso-strain model. This deviation can be explained by tapering of the trench walls and the dispersion caused by the finite dimensions of the bi-phase material, an explanation that was verified by FEM simulations.

A new design for a high frequency broadband ultrasound transducer using micromachined Silicon

We present a new design for a small ultrasound transducer, using thick-film PZT on Silicon to obtain high frequency and large bandwidth. The transducer is intended for use in high resolution medical imaging systems, e.g. intravascular catheters. Air is used as backing material for the PZT film, and the ultrasound pulses are transmitted through the silicon substrate, The silicon substrate is micromachined to build three acoustic matching layers, to optimize the acoustic coupling between the active PZT layer and the tissue, and achieve a large bandwidth. We present the fabrication steps for manufacturing such a transducer, together with 1D analytical calculations and 3D FEM simulations of its performance. Our calculations show that the proposed design will have a 3 dB bandwidth ranging from 37 MHz to 102 MHz, or 94% bandwidth at center frequency 70 MHz. FEM model simulations of the structure support the analytical calculations, showing good agreement between analytical and simulation results.