The subject matter disclosed herein relates generally to transducers, and more particularly to acoustic stacks and methods for manufacturing acoustic stacks.
Ultrasound technology is used in a variety of fields, including the field of medical imaging. Ultrasound scanning within the healthcare field typically involves the use a transducer, which emits high-frequency sound waves from a probe, through a gel on a patient's body, and into a patient's body. The transducer then collects sounds that “bounce back” or echo from a target of interest, where a computer may convert the sound waves into an image.
Ultrasound systems typically include ultrasound scanning devices (e.g., an ultrasound transducer, all or a portion of which may be housed within a probe) that perform various ultrasound scans to produce images of a body or other volume. The scanning devices include acoustic elements or components that transmit and receive ultrasound signals, which may be arranged in an array. Ultrasound transducers convert electrical signals into ultrasonic energy, which may then be converted into electrical signals. The ultrasound signals received by the acoustic elements are used to generate an image of the body or other volume. For example, the received ultrasound signals may be used to generate an image of internal tissues of a patient, such as, but not limited to, an image of a patient's heart, to guide a procedure, such as a biopsy, or to diagnose a variety of conditions, such as tumors or blockage in blood vessels.
A typical ultrasound transducer uses one or more matching layers to couple the acoustic energy produced in a piezoelectric to a patient or other volume. The matching layers may lie above the transducer and in proximity to the patient or volume being examined. Acoustic coupling is achieved layer-by-layer. The relatively high acoustic impedance of a piezoelectric material in a transducer as compared to a body is spanned by intervening impedances of the matching layers. A first matching layer of a first impedance may be used, where the first matching layer is the first layer encountered by the sound path from the transducer to the body. If additional matching layers are used, a progressively lower impedance is used through each successive layer. The topmost layer, therefore has a higher impedance than a body or volume, but where the impedance is lowered using more than one layer, a smoother impedance transition is achieved in acoustically coupling the ultrasound generated at the piezoelectric to the body or volume and in coupling the ultrasound returning from the body to the piezoelectric.
Matching layers are typically stiff enough that the layers for each element of the array must be separated from each other mechanically or through some other means to keep each element acoustically independent of the others. This can be achieved by saw cuts that penetrate the two matching layers and the piezoelectric material.
A matching layer may be comprised of a single row or multiple rows of elements formed from ceramics, graphite composites, polyurethane and other materials. In a 2D transducer, for example, a sound wave oscillates at a certain frequency, and the sound wave and oscillation frequency can be associated with one another. In some 2D transducers, since element impedance is lower, the impedance of the matching layers should also be lower. 2D transducers are currently built with two or more matching layers.
A piezoelectric transducer of an ultrasound probe uses electric fields provided by the piezoelectric. Electrodes then detect the fields produced by the piezoelectric, where the electrodes are attached to at least two faces of the piezoelectric. A voltage is applied between the electrodes requiring electrical connections to be made to the electrodes. Each element of the transducer may receive a different electrical input. Elements may be attached perpendicularly to the sound path or to a common ground on top of or under the array. The matching layer may serve as a ground plane or to a separate ground plane.
At least some known ultrasound systems include electronics that transmit and/or receive beamforming operations on the ultrasound signals. These electronics may include one or more integrated circuits. Such beamforming electronics are electrically and mechanically connected to the acoustic elements of the ultrasound transducer for performing the beamforming operations. The electrical and mechanical connection between the beamforming electronics and the acoustic elements may be a direct connection or may be provided through an interposer that extends between the acoustic elements and the beamforming electronics.
The manufacture of some acoustic sensor arrays, for example ultrasound transducers, require lamination or gluing of net shaped components or electrical components via ohmic contact to achieve certain bond lines, such as between 0 to 3 μm. Subsequently, the pre-diced configurations of an acoustic stack may be diced into high aspect ratio pixels to produce acoustic elements or transducer elements.
Traditional acoustic sensor array processing is costly because of its high cycle time, equipment requirements, equipment costs, and high scrap rates where assemblies or material cannot be used, repaired, or restored, and must be discarded. Currently available adhesives, for example 3M™ Scotch-Weld™ Epoxy Adhesive DP460 (hereinafter, “DP460”), 3M, Maplewood, Minn., USA, or other high viscosity adhesives are a main driver in the acoustic sensor array production costs.
A typical curing cycle may range from 3 to 24 hours at a low temperature for high viscosity adhesives. As a result of the long curing cycle, the current adhesives require the use of specific tooling, fixturing, and equipment designed to precisely align and hold the various components. At the same time, pressure must be applied to the acoustic array components for a long period of time to facilitate the extensive curing times required. These long, low temperature cure cycles allow the traditional aid the cure kinetics and allow the adhesives to flow as heat is applied. These flow profiles can cause portions of the interface to be missed or have too little adhesive leading to starved bondlines, while excess adhesive may be found in other portions. Starved bondlines can produce weakened areas or joints, cause poor adhesion, and result in disbanding of stack layers while excess adhesive results in poor capacitance, open joints, loop gain failures, and sensitivity failures.