Capacitive Micromachined Ultrasonic Transducer Having Adjustable Bending Angle, And Method For Manufacturing Same

Disclosed are a capacitive micromachined ultrasonic transducer having an adjustable bending angle and a method for manufacturing same. The ultrasonic transducer according to one embodiment may comprise: a substrate; a plurality of transducer elements spaced apart from each other and stacked on top of the substrate; flexible hinges which are positioned between the plurality of transducer elements and formed so as to pass through the substrate; a first polymer layer formed so as to cover the bottom of the substrate; and an actuator layer formed under the first polymer layer.

TECHNICAL FIELD

The following disclosure relates to a capacitive micromachined ultrasonic transducer having an adjustable bending angle and a method of manufacturing the same.

BACKGROUND ART

A capacitive micromachined ultrasonic transducer (CMUT) has a structure with a membrane positioned above a micromachined cavity and may be used to convert an acoustic signal into an electrical signal or an electrical signal into an acoustic signal.

CMUT is a micromachined device, and a two-dimensional (2D) transducer array may be more easily configured using a CMUT. Therefore, compared to other transducer arrays, a transducer array including a CMUT may include more transducers and provide a wider bandwidth.

Conventional ultrasonic transducers convert signals based on piezoelectricity, but a CMUT performs energy conversion based on a change in the capacitance of a cavity caused by vibrations of a membrane. In general, a CMUT is biased using a direct current (DC) voltage that determines an operating position of a device. When an alternating current (AC) signal is applied to electrodes of the biased CMUT, the membrane vibrates to generate an ultrasonic wave, such that the CMUT operates as an ultrasonic transmitter. On the other hand, when an ultrasonic wave is applied to the membrane of the biased CMUT, an electrical signal is generated as the capacitance of the CMUT changes, such that the CMUT operates as an ultrasonic receiver.

DISCLOSURE OF THE INVENTION

Technical Goals

An ultrasonic focusing technology that is reusable and does not require a complicated circuit is demanded.

According to an embodiment, it is possible to provide a technique for causing an ultrasonic beam to be focused by variably controlling the bending angle of an ultrasonic transducer.

However, the technical goals are not limited to those described above, and other technical goals may be present.

Technical Solutions

An ultrasonic transducer according to an embodiment may include a substrate, a plurality of transducer elements stacked on a top of the substrate to be spaced apart from each other, a flexible hinge positioned between the plurality of transducer elements and formed to pass through the substrate, a first polymer layer formed to cover a lower portion of the substrate, and an actuator layer formed on a bottom of the first polymer layer.

The flexible hinge may include a second polymer layer positioned over a separation space formed between adjacent transducer elements, and a liquid metal layer extending from a bottom of the second polymer layer and passing through the substrate.

The actuator layer may include an insulating layer formed on the bottom of the first polymer layer, a first electrode layer formed on a bottom of the insulating layer, a dielectric elastomer formed on a bottom of the first electrode layer, and a second electrode layer formed on a bottom of the dielectric elastomer.

The first polymer layer may include polydimethylsiloxane.

The second polymer layer may include polyimide.

The liquid metal layer may include a bismuth (Bi)-lead (Pb)-indium (In)-tin (Sn)-cadmium (Cd) fusible alloy.

The liquid metal layer may undergo a phase transition from solid to liquid based on heat generated by a voltage applied to the substrate.

The dielectric elastomer may bend by a voltage applied to the first electrode layer and the second electrode layer when a fusible alloy included in the flexible hinge is in a liquid state.

A method of manufacturing an ultrasonic transducer according to an embodiment may include forming a substrate, stacking a plurality of transducer elements on a top of the substrate to be spaced apart from each other, forming a flexible hinge between the plurality of transducer elements to pass through the substrate, forming a first polymer layer to cover a lower portion of the substrate, and forming an actuator layer on a bottom of the first polymer layer.

The forming of the flexible hinge may include forming a second polymer layer over a separation space formed between adjacent transducer elements, and forming a liquid metal layer extending from a bottom of the second polymer layer and passing through the substrate.

The forming of the second polymer layer may include stacking a polymeric material on the substrate and the plurality of transducer elements, and forming the second polymer layer by patterning the polymeric material.

The forming of the liquid metal layer may include forming a trench by etching the substrate positioned on the bottom of the second polymer layer, and forming the liquid metal layer by filling the trench with a liquid metal.

The forming of the actuator layer may include forming an insulating layer on the bottom of the first polymer layer, forming a first electrode layer on a bottom of the insulating layer, forming a dielectric elastomer on a bottom of the first electrode layer, and forming a second electrode layer on a bottom of the dielectric elastomer.

The first polymer layer may include polydimethylsiloxane.

The stacking of the polymeric material may include stacking polyimide through spin coating.

The liquid metal layer may include a Bi-Pb-In-Sn-Cd fusible alloy.

The liquid metal layer may undergo a phase transition from solid to liquid based on heat generated by a voltage applied to the substrate.

The dielectric elastomer may bend by a voltage applied to the first electrode layer and the second electrode layer when a fusible alloy included in the flexible hinge is in a liquid state.

An ultrasonic transducer system according to an embodiment may include the ultrasonic transducer of claim1, and a controller configured to control the ultrasonic transducer.

The controller may be further configured to control a flexible hinge included in the ultrasonic transducer and an actuator layer included in the ultrasonic transducer independently of driving the ultrasonic transducer.

BEST MODE FOR CARRYING OUT THE INVENTION

It should be noted that if it is described that one component is “connected”, “coupled”, or “joined” to another component, a third component may be “connected”, “coupled”, and “joined” between the first and second components, although the first component may be directly connected, coupled, or joined to the second component.

Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. When describing the embodiments with reference to the accompanying drawings, like reference numerals refer to like elements and a repeated description related thereto will be omitted.

FIG.1Ashows a cross-section of an ultrasonic transducer according to an embodiment, andFIG.1Bis a view of the ultrasonic transducer shown inFIG.1Afrom above.

A capacitive ultrasonic transducer100may generate an ultrasonic wave by converting electrical energy into mechanical energy. The capacitive ultrasonic transducer100may cause the generated ultrasonic beam to be focused. The ultrasonic transducer100may bend, and may deform reversibly, such as maintaining the bending shape or deforming back. The ultrasonic transducer100may cause the ultrasonic beam to be focused while maintaining the intensity of the ultrasonic beam.

The ultrasonic transducer100may include a substrate110, a plurality of transducer elements120, a flexible hinge130, a first polymer layer140, and an actuator layer150.

The substrate110may be a silicon substrate, and the transducer elements120or the like may be stacked thereon.

The plurality of transducer elements120may be stacked on the top of the substrate110to be spaced apart from each other. The plurality of transducer elements120may be driven simultaneously to form an ultrasonic beam with strong pressure, and may be driven separately to form various ultrasonic beams. Driving the plurality of transducer elements120will be described in detail with reference toFIG.3.

The flexible hinge130may be positioned between the plurality of transducer elements120and formed to pass through the substrate110. The flexible hinge130may contribute to the flexible and reversible deformation (e.g., bending) of the capacitive ultrasonic transducer100. The flexible hinge130may include a second polymer layer131and a liquid metal layer132.

The second polymer layer131may be positioned over a separation space formed between the transducer elements120. The second polymer layer131may be formed through spin coating of polyimide capable of withstanding the temperature at which a liquid metal included in the liquid metal layer132changes into a liquid. The second polymer layer131may contribute to the flexible deformation of the ultrasonic transducer100.

The liquid metal layer132may extend from the bottom of the second polymer layer131and pass through the substrate110. The liquid metal layer132may include a bismuth (Bi)-lead (Pb)-indium (In)-tin (Sn)-cadmium (Cd) fusible alloy. The Bi-Pb-In-Sn-Cd fusible alloy may be solid at room temperature and may be liquid at 47° C. or higher. The liquid metal layer132may undergo a phase transition from solid to liquid based on heat generated by a voltage applied to the substrate110. The liquid metal layer132may contribute to the reversible deformation of the ultrasonic transducer100.

The first polymer layer140may be formed to cover the lower portion of the substrate110. The first polymer layer140may be formed of polydimethylsiloxane, which is a flexible material, to prevent the liquid metal included in the liquid metal layer132from leaking out when it changes to liquid.

The actuator layer150may be formed on the bottom of the first polymer layer140. The actuator layer150may control the bending angle of the ultrasonic transducer100, thereby contributing to causing the ultrasonic beam to be focused while maintaining the intensity of the ultrasonic beam. The actuator layer150may include an insulating layer151, a first electrode layer152-1, a second electrode layer152-2, and a dielectric elastomer153.

The insulating layer151may be formed on the bottom of the first polymer layer140. The insulating layer151may include a silicon elastomer so that the actuator layer150may maintain a state of being electrically insulated from the transducer elements120.

The first electrode layer152-1may be positioned on the top of the dielectric elastomer153, and the second electrode layer152-2may be positioned on the bottom of the dielectric elastomer153. The first electrode layer152-1and the second electrode layer152-2may serve as an upper electrode and a lower electrode of the actuator150, respectively. The first electrode layer152-1and the second electrode layer152-2may be formed of carbon powder.

The dielectric elastomer153may be formed on the bottom of the first electrode layer152-1. The dielectric elastomer153may be formed of an acrylic elastomer that bends when a high voltage is applied thereto. When the fusible alloy included in the flexible hinge130is in a liquid state, the dielectric elastomer153may bend by a voltage applied to the first electrode layer152-1and the second electrode layer152-2.

The capacitive ultrasonic transducer100may bend through the flexible hinge130, the first polymer layer140, and the actuator layer150. Specifically, the capacitive ultrasonic transducer100may perform a reversible deformation, such as maintaining the shape or deforming, using a phase transition of the liquid metal included in the flexible hinge130. In addition, the ultrasonic transducer100may control the bending angle through the dielectric elastomer153included in the actuator layer150, thereby causing the ultrasonic beam to be focused while maintaining the intensity of the ultrasonic beam. Hereinafter, the operation of controlling the bending angle of the ultrasonic transducer100will be described in detail.

FIG.2is a view illustrating an operation of controlling a bending angle of the ultrasonic transducer shown inFIG.1A.

Referring toFIG.2, the bending angle of the capacitive ultrasonic transducer100may be controlled by applying voltages (e.g., Va and Vb).

When a direct current (DC) voltage Va is applied to both ends of the substrate110of the capacitive ultrasonic transducer100, heat may be generated in the silicon substrate110and the liquid metal layer132. The fusible alloy included in the liquid metal layer132may undergo a phase transition from solid to liquid as the voltage Va increases. The Bi-Pb-In-Sn-Cd fusible alloy included in the liquid metal layer132may have a characteristic of being solid at room temperature and undergoing a phase transition to liquid at 47° C. or higher.

After the phase transition of the Bi-Pb-In-Sn-Cd fusible alloy to liquid, a DC voltage Vb may be applied to the upper electrode152-1and the lower electrode152-2of the actuator150. When the voltage Vb is applied to the upper electrode152-1and the lower electrode152-2, the top surface of the dielectric elastomer153(e.g., the surface in contact with the electrode152-1) may receive a compressive force, and the bottom surface of the dielectric elastomer153(e.g., the surface in contact with the electrode152-2) may receive a tensile force. As different forces are applied respectively to the top surface and the bottom surface of the dielectric elastomer153, the dielectric elastomer153may bend, and the ultrasonic transducer100may also bend. The magnitude of the voltage Vb may correspond to the bending angle of the ultrasonic transducer100, and as the magnitude of the voltage Vb increases, the bending angle of the ultrasonic transducer100may increase.

When it is desired to maintain the bending angle of the ultrasonic transducer100, the temperature of the fusible alloy may be lowered by eliminating the DC voltage Va applied to both ends of the substrate110. When the temperature of the fusible alloy falls below 47° C., the fusible alloy may undergo a phase transition from liquid to solid and harden. Therefore, the ultrasonic transducer100may harden in the bending state, and the bending angle of the ultrasonic transducer100may be maintained even when the DC voltage Vb applied to the upper electrode152-1and the lower electrode152-2of the actuator150is eliminated.

When it is desired to change the bending angle of the ultrasonic transducer100, the intensity of the voltage Vb applied to the upper electrode152-1and the lower electrode152-2of the actuator150may be adjusted in a state where the voltage Va is applied to both ends of the substrate110, whereby the bending angle of the ultrasonic transducer100may be changed.

When it is desired to eliminate the bending angle of the ultrasonic transducer100(e.g., when it is desired to return to the flat state), the voltage Va may be applied again to both ends of the substrate110to cause a phase transition of the fusible alloy to liquid, rather than applying the voltage Vb to the upper electrode152-1and the lower electrode152-2of the actuator150, whereby the bending angle of the ultrasonic transducer100may be eliminated. When the voltage Va is eliminated thereafter, the fusible alloy may undergo a phase transition to solid, such that the state where the bending angle of the ultrasonic transducer100is eliminated may be maintained.

FIG.3is a simplified block diagram of an example of an ultrasonic transducer system according to an embodiment, andFIG.4is a diagram illustrating the ultrasonic transducer system shown inFIG.3.

An ultrasonic transducer system10may include an ultrasonic transducer (the ultrasonic transducer100shown inFIG.1), a bias T200, and a controller300. The ultrasonic transducer system10may generate an ultrasonic wave by driving the ultrasonic transducer100through the bias T200and control the bending angle of the ultrasonic transducer100through the controller300to cause the ultrasonic wave to be focused.

The ultrasonic transducer100may generate an ultrasonic wave as a DC voltage VDCor an AC voltage VACis applied thereto, and may cause the ultrasonic wave to be focused according to the bending angle.

The DC voltage VDCmay be applied to an upper electrode121and a lower electrode (e.g., the substrate110) of a transducer element (the transducer element120shown inFIG.1) to vibrate a membrane122. Compared to the AC voltage VAC, the DC voltage VDCmay cause the membrane122to move further toward the lower electrode110, thereby achieving a higher sound wave efficiency.

The AC voltage VACmay generate an ultrasonic wave in the form of a sine wave or square wave, and may be applied in a unipolar or bipolar form. The AC voltage VACmay be applied corresponding to the unique vibration frequency of the transducer element120, thereby vibrating the membrane122and generating a sound wave.

The Bias T200may simultaneously apply the DC voltage VDCor the AC voltage VACfor generating an ultrasonic wave to the plurality of transducer elements120included in the ultrasonic transducer100. The bias T200may include a capacitor and a resistor or a capacitor and an inductor.

The controller300may control the bending angle of the ultrasonic transducer100to cause an ultrasonic beam generated by the ultrasonic transducer100to be focused. The controller300may independently control the flexible hinge130included in the ultrasonic transducer100and the actuator layer150included in the ultrasonic transducer100. That is, the controller300may control the bending angle of the ultrasonic transducer independently of driving the ultrasonic transducer100.

The ultrasonic transducer system10may obtain ultrasonic beams having various focusing shapes using the ultrasonic transducer100that variably bends at various angles, and may cause an ultrasonic beam with a stronger intensity to be focused on a very small local area by bending the ultrasonic transducer100more.

FIG.5is a diagram illustrating a radius of curvature (ROC) according to a bending angle of the ultrasonic transducer shown inFIG.1A.

Referring toFIG.5, eight transducer elements may be included in an ultrasonic transducer, the distance between transducer elements may be 500 micrometers (um), and a membrane of an ultrasonic transducer may have a thickness of 0.5 nanometers (nm) and vibrate at 5 megahertz (MHz). In addition, an ultrasonic medium may be water, and the size of the medium may be 10 millimeters (mm) wide and 300 mm long.

When the bending angle of the ultrasonic transducer doubles, the radius of curvature (ROC) of the ultrasonic beam may halve. For example, when the bending angle of the ultrasonic transducer is 0°, the ROC of the ultrasonic beam may be infinite, such that the ultrasonic beam may reach the end of the medium. When the bending angle of the ultrasonic transducer is 5.625°, the ROC of the ultrasonic beam may be 76 mm. When the bending angle of the ultrasonic transducer is 11.25°, the ROC of the ultrasonic beam may be 38 mm. When the bending angle of the ultrasonic transducer is 45°, the ROC of the ultrasonic beam may be 9 mm.

That is, as the ultrasonic transducer bends more, the ROC of the ultrasonic beam may decrease more, and the ultrasonic beam may be focused on a more minute area.

FIG.6illustrates graphs of −3 dB distances according to the bending angle of the ultrasonic transducer shown inFIG.1A, andFIG.7is a table illustrating the −3 dB distances according to the bending angle of the ultrasonic transducer shown inFIG.1A.

Referring toFIGS.6and7, ultrasonic focal lengths, −3 dB axial distances, and −3 dB lateral distances according to bending angles of an ultrasonic transducer including eight transducer elements may be compared.

As the bending angle of the ultrasonic transducer increases, the ultrasonic focal length, the −3 dB axial distance, and the −3 dB lateral distance may decrease. For example, when the bending angle of the ultrasonic transducer is 0°, the focal length may be 75.54 mm, the −3 dB axial distance may be 394.97 mm, and the −3 dB lateral distance may be 7.42 mm. When the bending angle of the ultrasonic transducer is 5.625°, the focal length may be 45.53 mm, the −3 dB axial distance may be 138.04 mm, and the −3 dB lateral distance may be 2.20 mm. When the bending angle of the ultrasonic transducer is 45°, the focal length may be 9.36 mm, the −3 dB axial distance may be 3.94 mm, and the −3 dB lateral distance may be 0.39 mm.

As the bending angle of the ultrasonic transducer increases, the ultrasonic focal length, the −3 dB axial distance, and the −3 dB lateral distance may decrease exponentially. For example, when the bending angle of the ultrasonic transducer is 45°, the ultrasonic focal length may decrease to ⅛, the −3 dB axial distance may decrease to 1/100, and the −3 dB lateral distance may decrease to 1/19 compared to those when the bending angle of the ultrasonic transducer is 0°.

That is, as the ultrasonic transducer bends more, the ultrasonic focal length, the −3 dB axial distance, and the −3 dB lateral distance may decrease more, and the ultrasonic beam may be focused on a more minute area.

FIG.8illustrates a graph of a maximum pressure ratio according to the bending angle of the ultrasonic transducer shown inFIG.1A, andFIG.9is a table illustrating the maximum pressure ratio according to the bending angle of the ultrasonic transducer shown inFIG.1A.

Referring toFIGS.8and9, maximum pressure ratios (P/P0) according to bending angles of an ultrasonic transducer including eight transducer elements may be compared.

The maximum pressure ratio (P/P0) may be calculated through the ratio of the maximum pressure P of a bending ultrasonic transducer to the maximum pressure P0of a flat ultrasonic transducer.

As the bending angle of the ultrasonic transducer increases, the maximum pressure ratio may increase. For example, when the bending angle of the ultrasonic transducer is 0°, the maximum pressure ratio may be 1. When the bending angle of the ultrasonic transducer is 11.25°, the maximum pressure ratio may be 1.88. When the bending angle of the ultrasonic transducer is 45°, the maximum pressure ratio may be 3.6.

That is, as the ultrasonic transducer bends more, the maximum pressure ratio may increase more, and the ultrasonic beam may be focused.

Hereinafter, a pitch-catch ultrasonic transducer system will be described in detail.

FIG.10is a simplified block diagram of another example of an ultrasonic transducer system according to an embodiment.

An ultrasonic transducer system20may transmit or receive an ultrasonic wave. The ultrasonic transducer system20may connect biases T (e.g., a bias T1to a bias Tn) respectively to a plurality of transducer elements (e.g., a first transducer element to an n-th transducer element) and separately drive the transducer elements. For example, the ultrasonic transducer system20may use the first transducer element as an ultrasonic transmission module and use the n-th transducer element as an ultrasonic reception module. Further, the ultrasonic transducer system20may separately drive the second transducer element and the third ultrasonic element to cause various types of ultrasonic beams to be focused.

The ultrasonic transducer system20may apply corresponding AC voltages (e.g., VAC1to VACn) to the respective transducer elements (e.g., the first transducer element to the n-th transducer element), or may apply the same DC voltage VDCto the transducer elements.

Hereinafter, a method of manufacturing the ultrasonic transducer described above will be described in detail.

FIGS.11A to11Dare views illustrating a method of manufacturing the ultrasonic transducer shown inFIG.1A.

In operation1005, an oxide film1123-1may be formed on a silicon wafer1110. The oxide film1123-1may be formed by furnace equipment that is heat treatment equipment.

In operation1010, a patterned oxide film1123-2may be formed on a membrane layer1122. The patterned oxide film1123-2may be patterned to correspond to a cell design of a transducer element. The patterned oxide film1123-2may be formed on a silicon-on-insulator (SOI) wafer1101or may be formed on the silicon wafer1110.

In operation1015, the silicon wafer1110and the SOI wafer1101may be joined through bonding. For example, the SOI wafer1101may be joined on the silicon wafer1110in an inverted state. The oxide film1123including the cell design may be formed by joining the oxide film1123-1and the patterned oxide film1123-2by joining the silicon wafer1110and the SOI wafer1101.

In operation1020, a photoresist layer1102may be formed on the top of the SOI wafer1101and a patterned photoresist layer1103may be formed on the bottom of the silicon wafer1110.

In operation1025, an insulating layer formed on the bottom of the silicon wafer1110may be etched based on the patterned photoresist layer1103, and the silicon wafer1110may be etched in an anisotropic form based on the etched insulating layer. A trench1104and a substrate1110-1may be formed by etching the silicon wafer1110using tetramethylammonium hydroxide (TMAH).

In operation1030, all the layers (e.g., the SOI wafer1101) positioned on the top of the membrane layer1122may be removed through a mechanical method and a chemical method.

In operation1035, a plurality of membranes1122-1may be formed by patterning the membrane layer1122. The membrane1122-1may vibrate to generate an ultrasonic wave.

In operation1040, a first electrode1121may be formed on the top of the membrane1122-1. The first electrode1121may be formed through a vacuum thin film deposition process using sputter equipment or evaporator equipment. The first electrode1121may include chrome and gold.

In operation1045, a polymer layer1131may be formed by applying polyimide thereto through a spin coating method.

In operation1050, a second polymer layer1131-1may be formed by patterning the polymer layer1131.

In operation1055, a trench1104-1may be formed by etching (e.g., wet etching or dry etching) the substrate1110-1and the insulating layer1123in a radial direction of the trench1104.

In operation1060, a liquid metal layer1132may be formed by filling the trench1104-1with a liquid metal (e.g., a Bi-Pb-In-Sn-Cd fusible alloy).

In operation1065, a first polymer layer1140may be formed on a lower portion of the substrate1110-1. The first polymer layer1140may be formed of polydimethylsiloxane, which is a flexible material, to prevent the fusible alloy of the liquid metal layer1132from leaking out when it changes to liquid.

In operation1070, an actuator layer1150may be formed on the bottom of the first polymer layer1140. An insulating layer1151may be formed on the bottom of the first polymer layer1140and may be formed of a silicone elastomer material. A first electrode layer1152-1may be formed on the bottom of the insulating layer1151and may be formed of carbon powder. A dielectric elastomer1153may be formed on the bottom of the first electrode layer1152-1and may be formed through an acrylic elastomer. A second electrode layer1152-2may be formed on the bottom of the dielectric elastomer1153and may be formed of carbon powder.

The ultrasonic transducer100manufactured by the ultrasonic transducer manufacturing method described above may have a structure of the flexible hinge130, the polymer layer140, and the actuator layer150and thus bend. Specifically, the ultrasonic transducer100may perform a reversible deformation, such as maintaining the shape or deforming, using a phase transition of the liquid metal (e.g., the Bi-Pb-In-Sn-Cd fusible alloy) included in the flexible hinge130. In addition, the ultrasonic transducer100may control the bending angle of the ultrasonic transducer100through the dielectric elastomer153included in the actuator layer150, thereby causing the ultrasonic beam to be focused while maintaining the intensity of the ultrasonic beam.

The above-described devices may be configured to act as one or more software modules in order to perform the operations of the above-described embodiments, or vice versa.