Provided is a capacitive micromachined ultrasonic transducer (CMUT) including a substrate, a top electrode provided on the substrate to be spaced apart from the substrate, a supporter made of an insulating material and coupled between the substrate and an edge of the top electrode to support and fix the edge of the top electrode and to define a gap between the substrate and the edge of the top electrode, and a plurality of nanoposts having both ends coupled and fixed to the substrate and the top electrode in the gap, and being compressible and stretchable in a longitudinal direction to at least vertically move the top electrode when power is applied to the top electrode.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2019-0011480, filed on Jan. 29, 2019, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The present invention relates to an ultrasonic device and, more particularly, to a capacitive micromachined ultrasonic transducer (CMUT).

2. Description of the Related Art

Ultrasonic transducers (or ultrasonic probes) refer to devices for converting an electrical signal into an ultrasonic signal or converting an ultrasonic signal into an electrical signal. Although piezoelectric micromachined ultrasonic transducers (PMUTs) for processing an ultrasonic signal by using a piezoelectric material have been widely used, currently, research is being conducted on capacitive micromachined ultrasonic transducers (CMUTs) capable of increasing an operating frequency range and a transducer bandwidth and of achieving integration through a semiconductor process.

However, the CMUTs may not easily have high transmission and reception sensitivity due to a small average displacement caused by a limited gap height between electrodes and a limited voltage. That is, in existing CMUTs, edges of moving cells arranged at a low density are all fixed and thus an average displacement is small due to large displacements only at a center portion and small displacements at an edge portion. Increasing of a gap height to increase the average displacement requires application of a high voltage and thus is not desirable. Furthermore, multi-frequency operation of CMUTs is required to increase applicability of the CMUTs.

RELATED ART DOCUMENT

Patent Document

SUMMARY

The present invention provides a capacitive micromachined ultrasonic transducer (CMUT) capable of increasing transmission and reception sensitivity by increasing an average displacement. The present invention also provides a CMUT capable of using multiple frequencies. However, the scope of the present invention is not limited thereto.

According to an aspect of the present invention, there is provided a capacitive micromachined ultrasonic transducer (CMUT) including a substrate, a top electrode provided on the substrate to be spaced apart from the substrate, a supporter made of an insulating material and coupled between the substrate and an edge of the top electrode to support and fix the edge of the top electrode and to define a gap between the substrate and the edge of the top electrode, and a plurality of nanoposts having both ends coupled and fixed to the substrate and the top electrode in the gap, and being compressible and stretchable in a longitudinal direction to at least vertically move the top electrode when power is applied to the top electrode.

Each of the plurality of nanoposts may include a lower reinforcement having a larger cross-sectional area compared to a body at a lower part of the nanopost in contact with the substrate, to increase coupling force to the substrate.

Each of the plurality of nanoposts may include an upper reinforcement having a larger cross-sectional area compared to a body at an upper part of the nanopost in contact with the top electrode, to increase coupling force to the top electrode.

Each of the plurality of nanoposts may include a body having a nano-diameter and extending in a longitudinal direction between the substrate and the top electrode, an upper reinforcement having a larger cross-sectional area compared to the body at an upper part of the nanopost in contact with the top electrode, to increase coupling force to the top electrode, and a lower reinforcement having a larger cross-sectional area compared to the body at a lower part of the nanopost in contact with the substrate, to increase coupling force to the substrate.

A cross-sectional area of the upper reinforcement may be gradually increased in a direction from the body toward the top electrode, and a cross-sectional area of the lower reinforcement may be gradually increased in a direction from the body toward the substrate.

Each of the plurality of nanoposts may include a multilayer structure of a plurality of different monocrystalline materials to adjust a ratio of stretchability and compressibility of the nanopost.

The plurality of monocrystalline materials may at least include a piezoelectric material capable of vibrating when an electrical signal is received.

The plurality of nanoposts may have a plurality of diameters, and a diameter of at least one first nanopost provided at a center portion of the top electrode may be greater than the diameter of at least one second nanopost provided at an edge portion of the top electrode.

A density of the plurality of nanoposts may be greater at a center portion compared to an edge portion of the top electrode.

The CMUT may further include a protrusion provided on the substrate to be spaced apart from the top electrode and to surround and be spaced apart from lower parts of the plurality of nanoposts, the substrate may be made of a conductive material to function as a bottom electrode, and the protrusion and the plurality of nanoposts may be formed by etching the substrate.

The CMUT may further include a bottom plate provided on the substrate in the gap to be spaced apart from the top electrode and to surround and be spaced apart from at least lower parts of the plurality of nanoposts, the substrate may be made of an insulating material, and the bottom plate may be made of a conductive material to function as a bottom electrode.

The top electrode may include a nanoplate coupled to the supporter and the plurality of nanoposts, and the CMUT may further include a top plate reinforcement on the nanoplate.

The top plate reinforcement may include a plurality of recesses or holes alternating with the plurality of nanoposts, and, when power is applied between the nanoplate and the bottom plate, on the whole, the nanoplate may operate at a first frequency by the plurality of nanoposts and parts of the nanoplate under the plurality of recesses or holes may operate at a second frequency.

According to another aspect of the present invention, there is provided a capacitive micromachined ultrasonic transducer (CMUT) including an insulating first substrate, a conductive second substrate provided on the first substrate, including a plurality of through holes, and functioning as a bottom electrode, a top electrode provided on the second substrate to be spaced apart from the second substrate, a supporter made of an insulating material and extending on the first substrate over the second substrate to define a gap between the first substrate and the top electrode and to support and fix an edge of the top electrode, and a plurality of nanoposts having both ends coupled and fixed to the first substrate and the top electrode though the plurality of through holes in the gap and being stretchable and compressible in a longitudinal direction to at least vertically move the top electrode when power is applied between the top electrode and the bottom electrode.

The top electrode may include a nanoplate, and the CMUT may further include a top plate reinforcement on the nanoplate.

The top plate reinforcement may include a plurality of recesses or holes alternating with the plurality of nanoposts, and, when power is applied between the nanoplate and the second substrate, on the whole, the nanoplate may operate at a first frequency by the plurality of nanoposts and parts of the nanoplate under the plurality of recesses or holes may operate at a second frequency different from the first frequency.

DETAILED DESCRIPTION

Hereinafter, the present invention will be described in detail by explaining embodiments of the invention with reference to the attached drawings.

FIG.1is a cross-sectional view of a capacitive micromachined ultrasonic transducer (CMUT)100according to an embodiment of the present invention.FIG.2is a plan view of the CMUT100ofFIG.1.

Referring toFIGS.1and2, the CMUT100may include a substrate105, a top electrode110, a supporter115, and a plurality of nanoposts120.

The substrate105may have conductivity and function as a counter electrode to the top electrode110, e.g., a bottom electrode. For example, the substrate105may include a semiconductor material, e.g., silicon, germanium, or silicon-germanium. The semiconductor material may be doped with n-type or p-type impurities to have conductivity. Furthermore, the substrate105may be provided by processing a semiconductor wafer to a certain thickness.

The top electrode110may be provided on the substrate105to be spaced apart from the substrate105. For example, the top electrode110may be supported by the supporter115to be spaced apart from the substrate105by a certain distance. The top electrode110may be provided as a conductive plate and function as a moving plate in the CMUT100. For example, the top electrode110may be provided as a conductive layer, e.g., a metalized layer or semiconductor layer, having a certain thickness. The semiconductor layer may be doped with n-type or p-type impurities to have conductivity.

The supporter115may be made of an insulating material and support and fix the edge of the top electrode110. For example, the supporter115may be provided in a loop structure wound along the edge of the top electrode110and be coupled to the substrate105and the edge of the top electrode110. As such, a gap125may be defined between the substrate105and the top electrode110. The gap125may be sealed from an external environment by the substrate105, the top electrode110, and the supporter115. For example, when the gap125is formed in a vacuum atmosphere, the gap125may be sealed and maintained in a vacuum state.

The nanoposts120may have both ends coupled and fixed to the substrate105and the top electrode110in the gap125. For example, the top electrode110may be bonded to the substrate105, which is provided as a semiconductor layer on another substrate and on which the nanoposts120and the supporter115are provided and the gap125is defined, and thus be coupled to the nanoposts120and the supporter115. As another example, the top electrode110may be provided on the nanoposts120and the supporter115and then the gap125may be formed by removing a sacrificial material in the gap125by using wet etching or the like.

The nanoposts120may be spaced apart from each other at certain intervals in the gap125. The intervals between the nanoposts120may be set uniformly or non-uniformly according to the purpose thereof. For example, the nanoposts120may be appropriately provided considering supporting forces thereof to move the top electrode110in a flat state without being bent. Based on this structure, because only the edge of the top electrode110is fixed by the supporter115and the entirety of a remaining part of the top electrode110exposed by the gap125is vertically movable together with the nanoposts120, an average displacement may be greatly increased compared to existing technology.

As illustrated inFIG.3, the nanoposts120may be provided to be compressible and stretchable in a longitudinal direction to at least vertically move the top electrode110when power50is applied to the top electrode110. That is, the nanoposts120may serve as springs to vertically move the top electrode110while the edge of the top electrode110is being fixed. For example, the nanoposts120may be made of a semiconductor material. It is known that semiconductor single crystals exhibit a low stretchability and compressibility in a bulk structure but monocrystalline wires having a nanometer-level diameter may be compressed and stretched by about 20% corresponding to a theoretical limit.

The nanoposts120may be provided in various shapes at a nano level. For example, the nanoposts120may have a cylinder shape, an elliptical cylinder shape, or a polygonal prism shape (e.g., a triangular prism shape, a rectangular prism shape, or a pentagonal prism shape), or have a partially hollow shape of the above-mentioned shape. For example, when holes are provided in the nanoposts120like the latter example, lateral restraint thereof may be reduced and thus a higher stretchability and compressibility may be provided. For example, the nanoposts120may be formed by processing a semiconductor wafer by using a semiconductor process, e.g., a lithography process or an etching process.

For example, the nanoposts120may be made of a single monocrystalline semiconductor material. For example, the nanoposts120may be formed integrally with the substrate105by patterning the same material as the substrate105. As another example, the nanoposts120may include a multilayer structure of a plurality of different monocrystalline materials to adjust a ratio of stretchability and compressibility thereof. In this case, the monocrystalline materials may include, for example, silicon (Si), germanium (Ge), silicon carbide (SiC), and a piezoelectric material. The piezoelectric material is a material capable of vibrating when an electrical signal is received and of outputting an electrical signal when vibration is received, and may include, for example, lead magnesium niobate-lead titanate (PMN-PT), lead magnesium niobate-lead zirconate titanate (PMN-PZT), or zinc oxide (ZnO). For example, using at least one piezoelectric material, the nanoposts120may simultaneously utilize electrostatic force, and stretchability and compressibility due to the piezoelectric material and thus an operating frequency range of the CMUT100may be increased.

As illustrated inFIG.3, when the power50, e.g., radio-frequency (RF) power within a certain frequency range, is applied between the top electrode110and the substrate105, the nanoposts120may be compressed and stretched to vibrate by electrostatic force due to capacitive coupling between the two and thus ultrasonic waves may be transmitted. On the contrary, when ultrasonic waves reflected from an object are incident on the CMUT100from the outside, the nanoposts120may be compressed and stretched to move the top electrode110and to change a capacitance and thus ultrasonic vibration may be received.

In some embodiments, the substrate105may include a semiconductor wafer and an integrated circuit (IC) provided on the semiconductor wafer. The nanoposts120may be monolithically formed on the substrate105by using a semiconductor process. That is, the IC may be provided on the semiconductor wafer and the nanoposts120may be formed by performing thereon a semiconductor process such as a deposition process or an etching process. The top electrode110may be provided on the nanoposts120by using a bonding process.

Based on the CMUT100according to the current embodiment, unlike existing technology, because displacements are achievable using most of the area of the top electrode110, an average displacement may be increased and thus transmission and reception sensitivity of an ultrasonic signal may be greatly increased. Furthermore, a usable frequency range may be increased by providing the nanoposts120in a multilayer structure of different materials.

FIGS.4A to4Dare cross-sectional views of CMUTs100ato100daccording to other embodiments of the present invention. The CMUTs100ato100daccording to the current embodiments are partially modified from the above-described CMUT100ofFIGS.1to3and thus a repeated description thereof is not provided herein.

Referring toFIG.4A, each of nanoposts120amay include a body1201and a lower reinforcement1202. For example, the body1201may be a structure having a nano-diameter and extending in a longitudinal direction between the substrate105and the top electrode110, and the lower reinforcement1202may be provided with a larger cross-sectional area compared to the body1201at a lower part of the nanopost120ain contact with the substrate105, to increase coupling force to the substrate105. Based on this structure, when the substrate105is etched to form the nanoposts120a, weakening of lower parts of the nanoposts120aand separation of the nanoposts120afrom a lower layer, e.g., the substrate105, may be prevented.

Referring toFIG.4B, each of nanoposts120bmay include the body1201, the lower reinforcement1202, and an upper reinforcement1203. For example, the lower reinforcement1202may be provided with a larger cross-sectional area compared to the body1201at a lower part of the nanopost120bin contact with the substrate105, to increase coupling force to the substrate105, and the upper reinforcement1203may be provided with a larger cross-sectional area compared to the body1201at an upper part of the nanopost120bin contact with the top electrode110, to increase coupling force to the top electrode110. As such, the nanoposts120bmay be firmly coupled to the top electrode110and the substrate105on and under the nanoposts120b, and thus separation of the nanoposts120bfrom upper and lower layers may be prevented even when the nanoposts120bare repeatedly compressed and stretched.

Referring toFIG.4C, each of nanoposts120cmay include the body1201, a lower reinforcement1202c, and an upper reinforcement1203c. A cross-sectional area of the lower reinforcement1202cmay be gradually increased in a linear manner in a direction from the body1201toward the substrate105, and a cross-sectional area of the upper reinforcement1203cmay be gradually increased in a linear manner in a direction from the body1201toward the top electrode110. As such, the body1201may be smoothly connected to the lower reinforcement1202cand the upper reinforcement1203c. This structure may be formed using dry etching.

Referring toFIG.4D, each of nanoposts120dmay include the body1201, a lower reinforcement1202d, and an upper reinforcement1203d. A cross-sectional area of the lower reinforcement1202dmay be gradually increased in a curvilinear manner in a direction from the body1201toward the substrate105, and a cross-sectional area of the upper reinforcement1203dmay be gradually increased in a curvilinear manner in a direction from the body1201toward the top electrode110. As such, the body1201may be smoothly connected to the lower reinforcement1202dand the upper reinforcement1203d. This structure may be formed using wet etching.

FIG.5is a cross-sectional view of a CMUT100eaccording to another embodiment of the present invention, andFIG.6is a plan view of the CMUT100eofFIG.5. The CMUT100eaccording to the current embodiment is modified from the CMUT100ofFIGS.1to3and thus a repeated description thereof is not provided herein.

Referring toFIGS.5and6, in the CMUT100e, the nanoposts120may have a plurality of diameters. For example, the nanoposts120may have different diameters at center and edge portions of the substrate105or the top electrode110. For example, a diameter of at least one first nanopost120-1,120provided at the center portion of the top electrode110may be greater than the diameter of at least one second nanopost120-2,120provided at the edge portion of the top electrode110. Furthermore, the diameter of the nanoposts120may be reduced gradually or stepwise from the center portion toward the edge portion of the top electrode110.

Because the top electrode110is supported and fixed by the supporter115at the edge thereof and has a lower supporting force at the center portion thereof, rigidity of the first nanopost120-1,120provided at the center portion may be increased by increasing the diameter thereof.

In the CMUT100eaccording to the current embodiment, the nanoposts120may be further modified to have the structures of the nanoposts120ato120dofFIGS.4A to4D.

FIG.7is a cross-sectional view of a CMUT100faccording to another embodiment of the present invention, andFIG.8is a plan view of the CMUT100fofFIG.7. The CMUT100faccording to the current embodiment is modified from the CMUT100ofFIGS.1to3and thus a repeated description thereof is not provided herein.

Referring toFIGS.7and8, in the CMUT100f, a density of the nanoposts120may be greater at a center portion compared to an edge portion of the substrate105or the top electrode110. For example, a density of third nanoposts120-3,120provided at the center portion of the top electrode110may be greater than the density of fourth nanoposts120-4,120provided at the edge portion of the top electrode110. Furthermore, the density of the nanoposts120may be reduced gradually or stepwise from the center portion toward the edge portion of the top electrode110.

To move the top electrode110in a flat state without being bent, a low supporting force of the third nanoposts120-3,120provided at the center portion may be increased by increasing the density thereof.

In the CMUT100faccording to the current embodiment, the nanoposts120may be further modified to have the structures of the nanoposts120ato120dofFIGS.4A to4D.

FIG.9is a cross-sectional view of a CMUT100gaccording to another embodiment of the present invention, andFIG.10is a cross-sectional view showing operation of the CMUT100gofFIG.9. The CMUT100faccording to the current embodiment is modified from the CMUT100ofFIGS.1to3and thus a repeated description thereof is not provided herein.

Referring toFIGS.9and10, the CMUT100gmay further include a protrusion130on the substrate105. For example, the protrusion130may be provided on the substrate105to be spaced apart from the top electrode110and to surround at least lower parts of the nanoposts120. For example, the protrusion130and the nanoposts120may be made of the same material as the substrate105and be formed by, for example, etching a single substrate105.

Based on this structure, a capacitance value may be adjusted by maintaining a height of the nanoposts120to be equal to that in the CMUT100ofFIG.1, i.e., by constantly maintaining a height of the gap125, and by reducing a distance between the top electrode110and the protrusion130functioning as a part of a bottom electrode. In this structure, when the power50is applied between the top electrode110and the substrate105, capacitive coupling may occur between the top electrode110and the substrate105and between the top electrode110and the protrusion130, and the nanoposts120may be compressed to move the top electrode110.

In the CMUT100gaccording to the current embodiment, the nanoposts120may be further modified to have the structures of the nanoposts120ato120dofFIGS.4A to4Dor to have the diameters or densities of the nanoposts120ofFIGS.5to8.

FIG.11is a cross-sectional view of a CMUT100haccording to another embodiment of the present invention, andFIG.12is a cross-sectional view showing operation of the CMUT100hofFIG.11. The CMUT100haccording to the current embodiment is partially modified from the CMUT100ofFIGS.1to3and the CMUT100gofFIGS.9and10and thus a repeated description thereof is not provided herein.

Referring toFIGS.11and12, a substrate105amay be made of an insulating material and a bottom plate135may be provided on the substrate105a. The bottom plate135may be provided on the substrate105ain the gap125to be spaced apart from a top electrode110aand to surround and be spaced apart from at least lower parts of the nanoposts120. For example, the bottom plate135may be made of a conductive material to function as a bottom electrode, and include a plurality of through holes137. The bottom plate135may be made of, for example, a semiconductor material and be formed by, for example, etching a semiconductor wafer. For example, the top electrode110amay be provided as a nanoplate having a nano-thickness and, in the current embodiment, the top electrode110amay also be called a nanoplate110a.

Based on the CMUT100h, a parasitic capacitance may be reduced by separating the nanoposts120from the bottom electrode, i.e., the bottom plate135. Furthermore, based on this structure, a length of the nanoposts120, i.e., a height of the gap125, may be adjusted independently of the bottom plate135, and the gap125between the bottom plate135and the top electrode110amay be adjusted independently of the height of the nanoposts120.

Considering functions and forming processes, the substrate105amay also be called an insulating first substrate and the bottom plate135may also be called a conductive second substrate. In this regard, the second substrate may be provided on the first substrate to function as the bottom electrode. The top electrode110amay be provided on the second substrate to be spaced apart from the second substrate. The supporter115may extend on the first substrate over the second substrate to define the gap125between the first substrate and the top electrode110aand to support and fix the edge of the top electrode110a. The nanoposts120may have both ends coupled and fixed to the first substrate and the top electrode110athough the plurality of through holes137in the gap125, and be stretchable and compressible in a longitudinal direction to at least vertically move the top electrode110awhen power is applied between the top electrode110aand the bottom electrode.

In the CMUT100haccording to the current embodiment, the nanoposts120may be further modified to have the structures of the nanoposts120ato120dofFIGS.4A to4Dor to have the diameters or densities of the nanoposts120ofFIGS.5to8.

FIG.13is a cross-sectional view of a CMUT100iaccording to another embodiment of the present invention, andFIG.14is a cross-sectional view showing operation of the CMUT100iofFIG.13. The CMUT100iaccording to the current embodiment is partially modified from the CMUT100hofFIGS.11and12and thus a repeated description thereof is not provided herein.

Referring toFIGS.13and14, a top plate reinforcement140may be added onto the nanoplate110afunctioning as a top electrode. The top plate reinforcement140may structurally reinforce the nanoplate110ato prevent the nanoplate110afrom being bent while being moved by the nanoposts120. Based on this structure, the nanoplate110amay vertically move in a flat state while being supported and fixed by the supporter115at the edge thereof and being structurally reinforced by the top plate reinforcement140.

FIG.15is a cross-sectional view of a CMUT100jaccording to another embodiment of the present invention, andFIG.16is a plan view of the CMUT100jofFIG.15. The CMUT100jaccording to the current embodiment is partially modified from the CMUT100iofFIGS.13and14and thus a repeated description thereof is not provided herein.

Referring toFIGS.15and16, in the CMUT100j, a top plate reinforcement140amay include a plurality of holes142. The holes142may alternate with the nanoposts120in a plan view of the top plate reinforcement140a. Based on this structure, parts of the nanoplate110aexposed by the holes142may vibrate separately from spring motion of the nanoposts120. The holes142may be provided in various shapes, e.g., a circular shape, an elliptical shape, a polygonal shape, or a hollow shape.

As illustrated inFIG.17, when power of a first frequency is applied between the nanoplate110aand the bottom plate135, on the whole, the nanoplate110amay operate at the first frequency by the nanoposts120. That is, the nanoposts120may be compressed and stretched and thus the nanoplate110acoupled to the nanoposts120may vertically move together with the top plate reinforcement140a. Otherwise, when power of a second frequency is applied between the nanoplate110aand the bottom plate135, the parts of the nanoplate110aexposed by the holes142may operate at the second frequency. The first frequency and the second frequency are basically different in that different parts move, but may also be designed to be the same.

As illustrated inFIG.18, in a second frequency band, the parts of the nanoplate110aexposed by the holes142may independently operate at the second frequency without operation of the nanoposts120independently of motion of the nanoposts120.

Therefore, the CMUT100jmay operate at multiple frequencies such as the first frequency and the second frequency. Furthermore, wideband-frequency operation corresponding to a sum of the first frequency and the second frequency may be implemented by adjusting an interval of the first frequency and the second frequency. Besides, operation at two or more frequencies may be implemented by changing the structure and shape of the top plate reinforcement140a. Although existing medical imaging technology uses a plurality of CMUTs of different operating frequency ranges because different body parts have different operating frequencies, the CMUT100jaccording to the current embodiment may operate at multiple frequencies in a wideband and thus may image various body parts by using one or a small number of CMUTs100jby setting an operating frequency required by a circuit.

FIG.19is a cross-sectional view of a CMUT100kaccording to another embodiment of the present invention. The CMUT100kaccording to the current embodiment is partially modified from the CMUT100jofFIGS.15to18and thus a repeated description thereof is not provided herein.

Referring toFIG.19, in the CMUT100k, a top plate reinforcement140bmay include a plurality of recesses144. The recesses144may alternate with the nanoposts120in a plan view of the top plate reinforcement140b. The recesses144may be formed to a depth equal to or greater than a certain depth in such a manner that the bottom of the nanoplate110aunder the recesses144has a thickness equal to or less than a certain thickness to allow motion of parts of the nanoplate110aunder the recesses144. Based on this structure, the parts of the nanoplate110aunder the recesses144may vibrate separately from spring motion of the nanoposts120. The recesses144may be provided in various shapes, e.g., a circular shape, an elliptical shape, a polygonal shape, or a hollow shape.

According to the afore-described embodiments of the present invention, in a CMUT, transmission and reception sensitivity may be increased by increasing an average displacement between electrodes. Furthermore, the CMUT according to some embodiments of the present invention may operate at multiple frequencies and thus capture medical images by using one or a small number of CMUTs without changing CMUTs for different body parts. However, the scope of the present invention is not limited to the above-described effects.