Ultrasound devices

An ultrasound device is described. The ultrasound device may include a cavity, a membrane, and a sensing electrode. When an electrical signal is applied to the sensing electrode and a static bias is applied to the membrane, the membrane vibrates within the cavity and produces ultrasonic signals. The cavity, the membrane, and the sensing electrode may be considered a capacitive micromachined ultrasonic transducer (CMUT). The sensing electrode may be shaped as a ring, whereby the central portion of the sensing electrode is removed. Removal of the central portion of the sensing electrode may reduce the parasitic capacitance without substantially affecting the production of ultrasonic signals by the CMUT. This, in turn, can result in an increase in the signal-to-noise ratio (SNR) of the ultrasonic signals. The ultrasound device may further include a bond pad configured for wire bonding, and a trench electrically isolating the bond pad from the membrane.

FIELD

Generally, the aspects of the technology described herein relate to ultrasound devices.

BACKGROUND

Ultrasound devices may be used to perform diagnostic imaging and/or treatment, using sound waves with frequencies that are higher with respect to those audible to humans. Ultrasound imaging may be used to see internal soft tissue body structures, for example to find a source of disease or to exclude any pathology. When pulses of ultrasound are transmitted into tissue (e.g., by using a probe), sound waves are reflected off the tissue with different tissues reflecting varying degrees of sound. These reflected sound waves may then be recorded and displayed as an ultrasound image to the operator. The strength (amplitude) of the sound signal and the time it takes for the wave to travel through the body provide information used to produce the ultrasound image. Many different types of images can be formed using ultrasound devices, including real-time images. For example, images can be generated that show two-dimensional cross-sections of tissue, blood flow, motion of tissue over time, the location of blood, the presence of specific molecules, the stiffness of tissue, or the anatomy of a three-dimensional region.

BRIEF SUMMARY

Some embodiments relate to an ultrasound device comprising an ultrasonic transducer disposed on a complementary metal-oxide-semiconductor (CMOS) substrate and having an electrode wherein a central portion of the electrode is absent.

Some embodiments relate to an ultrasound device comprising a membrane, a first bond pad disposed with a trench that electrically isolates the first bond pad from the membrane, and a second bond pad that is not electrically isolated from the membrane.

Some embodiments relate to an ultrasound device comprising an ultrasonic transducer disposed on a complementary metal-oxide-semiconductor (CMOS) substrate and having an electrode wherein a central portion of the electrode is absent.

In some embodiments, the electrode is a first electrode, and wherein the ultrasonic transducer further comprises a membrane and a cavity disposed between the membrane and the first electrode.

In some embodiments, the ultrasound device further comprises a second electrode disposed in the central portion of the first electrode.

In some embodiments, the second electrode is electrically coupled to ground.

In some embodiments, the ultrasound device further comprises a metal extending beneath the cavity and disposed a distance away from the first electrode.

In some embodiments, the metal is electrically coupled to ground.

In some embodiments, the first electrode is circular, and the first electrode comprises slots extending along a radial direction of the first electrode.

In some embodiments, the ultrasound device further comprises a redistribution layer disposed beneath the first electrode, wherein the redistribution layer is electrically coupled to ground.

In some embodiments, a bottom surface of the membrane facing the cavity comprises aluminum oxide.

In some embodiments, a bottom surface of the membrane facing the cavity comprises hafnium oxide.

In some embodiments, the first electrode comprises titanium.

In some embodiments, the first electrode comprises tungsten.

In some embodiments, wherein the first electrode is formed with a damascene process.

In some embodiments, the first electrode is formed with a dual damascene process.

In some embodiments, the first electrode comprises a sea of vias.

Some embodiments relate to an ultrasound device comprising a membrane;

a first bond pad disposed with a trench that electrically isolates the first bond pad from the membrane; and a second bond pad that is not electrically isolated from the membrane.

In some embodiments, the ultrasound device further comprises a complementary metal-oxide-semiconductor (CMOS) chip disposed beneath the membrane; and a first metal extending from the second bond pad along a top surface of the ultrasound device to the membrane. The first metal is not electrically coupled to the CMOS chip.

In some embodiments, the ultrasound device further comprises second metal disposed between the first metal and the CMOS chip.

In some embodiments, the ultrasound device further comprises a first electrode and a cavity disposed between the membrane and the first electrode, wherein a central portion of the first electrode is absent.

In some embodiments, the ultrasound device further comprises a second electrode disposed in the central portion of the first electrode.

DETAILED DESCRIPTION

Aspects of the present application relate to ultrasound devices having membranes for producing ultrasonic signals, and in which center portions of the sensing electrodes106are absent. A sensing electrode is an electrode positioned near a membrane to cause and/or detect vibrations of the membrane.

Applicant has appreciated that, in some ultrasound devices, the electric field existing between a membrane and a sensing electrode may be sufficient to cause a portion of the membrane to contact a portion the sensing electrode. The portion of the membrane that contacts the portion of the sensing electrode may be a central portion of the membrane, and the portion of the sensing electrode on which the portion of the membrane collapses may be a central portion of the sensing electrode. When such a contact occurs, the central portion of the membrane may not contribute to production of ultrasonic signals. Therefore, any signal applied to the central portion of the sensing electrode does not contribute to production of ultrasonic signals. A parasitic capacitance may nonetheless exist between the center portion of the sensing electrode and the center portion of the membrane, thus negatively affecting the sensitivity with which the ultrasound device detects ultrasound signals.

According to some aspects of the present application, the aforementioned problem may be addressed by providing ultrasound devices in which the center portion of a sensing electrode is absent. For example, a sensing electrode106may be shaped as a ring with a circular portion of the center being absent. Ultrasound devices in which the center portion of a sensing electrode is absent may exhibit a reduced parasitic capacitance, without substantially affecting the production of ultrasonic signals. This, in turn, can result in an increase in the sensitivity of the ultrasound device.

According to other aspects of the present application, ultrasound devices of the types described herein may be electrically interfaced with other electronic devices using wire bonding. Thus, some ultrasound devices include bond pads on which wires are bonded. A first bond pad may serve as the point of access to electronic circuitry designed to control the operations of the ultrasound device. A second bond pad may serve as the point of access to a membrane. To ensure proper operations, the first bond pad should be electrically isolated from the membrane.

Applicant has appreciated, however, that conductors are used for electrically connecting the first bond pad to the electronic circuitry, and that the presence of such conductors could inadvertently short any voltage applied at the first bond pad to the membrane. To limit the risk that the membrane be shorted to the first bond pad, in some embodiments, the ultrasound device includes a trench formed between the first bond pad and the membrane to ensure electrical isolation. In some embodiments, the trench may surround the first bond pad.

It should be appreciated that the embodiments described herein may be implemented in any of numerous ways. Examples of specific implementations are provided below for illustrative purposes only. It should be appreciated that these embodiments and the features/capabilities provided may be used individually, all together, or in any combination of two or more, as aspects of the technology described herein are not limited in this respect.

FIG.1illustrates an example cross-section of an ultrasound device100in accordance with certain embodiments described herein. The ultrasound device includes a cavity102, a membrane104, and a sensing electrode106. When an electrical signal is applied to the sensing electrode106and a static bias is applied to the membrane104, the membrane104may vibrate within the cavity102and produce ultrasonic signals. The cavity102, the membrane104, and the sensing electrode106may be considered a capacitive micromachined ultrasonic transducer (CMUT). Electrical contact to the membrane104may be made through contacts132, as will be described further below, in order to supply a bias voltage to the membrane104. The ultrasound device100includes oxide134that may insulate various portions of the ultrasound device100, or be deposited for specific purposes during fabrication of the ultrasound device100. While the oxide134is shown as one continuous portion, the oxide134may be formed during different steps in different portions of the ultrasound device100using different processes. For example, the cavity102may be etched in oxide formed using high-density plasma (HDP) chemical vapor deposition (CVD), while other oxide in the ultrasound device100may be thermal oxide. The top surface of the ultrasound device100may be passivated with passivation120(e.g., silicon oxide and silicon nitride).

The ultrasound device100includes a complementary metal-oxide-semiconductor (CMOS) chip126. The CMOS chip126may include silicon (in which semiconductor devices may be formed) and oxide on the bottom surface as passivation. The CMOS chip126may include integrated circuitry (not shown inFIG.1) for controlling operation of the ultrasound device100. For example, the integrated circuitry may provide electrical signals to the sensing electrode106and receive and process electrical signals from the sensing electrode106. The CMOS chip126includes metal128that may serve as interconnect for routing electricity in the CMOS chip126. The metal128may include two or more vertically stacked layers of metal wiring, with vias connecting the different layers. The CMOS chip126further includes a redistribution layer (RDL)108. The RDL108may include mainly aluminum (with a small portion of copper and/or silicon), or copper. The RDL108may be used to redistribute signals in the CMOS chip126. For example, inFIG.1, the RDL108may electrically contact the sensing electrode106through vias130, and route signals to the sensing electrode106from the metal128of the CMOS chip126. The RDL108may include two or more vertically stacked layers connected with vias. In the ultrasound device100, the RDL108may also be used to shield the sensing electrode106, as will be described further below.

In some embodiments, the membrane104may include silicon and a layer of oxide on its bottom surface. In some embodiments, the oxide may be thermal oxide (e.g., formed with wet oxidation, dry oxidation, or a dry-wet-dry cycle) while in other embodiments the oxide on the bottom surface of the membrane may be aluminum oxide or hafnium oxide (e.g., formed with atomic layer deposition (ALD)). In some embodiments, the sensing electrode106may include titanium, titanium nitride, and/or tungsten. In embodiments in which the sensing electrode106includes tungsten, the vias130may also include tungsten, and the sensing electrode106and the vias130may be formed with a dual damascene process. In some embodiments, the sensing electrode106may be formed from a sea (a large plurality) of tungsten vias formed with a single damascene process. In some embodiments, the vias130and/or the sensing electrode106can include copper.

FIG.2illustrates an example side view of the cavity102, the membrane104, and the sensing electrode106in accordance with certain embodiments described herein.FIG.2illustrates the membrane104in a mode in which the electric field between the membrane104and the sensing electrode106is sufficient to cause a portion of the membrane104to contact a portion the sensing electrode106. This may be considered a collapse mode, and the voltage between the membrane104and the sensing electrode106required to cause the membrane104to enter collapse mode may be considered the collapse voltage. The portion of the membrane104that contacts the portion of the sensing electrode106in collapse mode may be a central portion of the membrane104, and the portion of the sensing electrode106on which the portion of the membrane104collapses may be a central portion of the sensing electrode106. In collapse mode, the central portion of the membrane104does not vibrate in response to the electrical signal applied to the sensing electrode106. Therefore, in collapse mode, the central portion of the membrane104does not contribute to production of ultrasonic signals, and any signal applied to the central portion of the sensing electrode106does not contribute to production of ultrasonic signals. However, a parasitic capacitance may exist between the sensing electrode106and the membrane104.

With repeated collapsing of the membrane104onto the sensing electrode106during the lifetime of the ultrasound device100, charge may accumulate on the membrane104, the bottom of which is an insulator (e.g., oxide). This charging of the membrane104may counteract the voltage applied between the membrane104and the sensing electrode106. More charging may occur on the central portion of the membrane104compared with other portions of the membrane104, as the central portion of the membrane104may contact the sensing electrode106during collapse most often.

FIG.3illustrates an example top view of the cavity102and the sensing electrode106in accordance with certain embodiments described herein. The sensing electrode106may be considered ring-shaped in that a circular portion of the center portion of the sensing electrode106is absent. As described above, in collapse mode, the central portion of the sensing electrode106may not contribute to production of ultrasonic signals. However, the central portion of the sensing electrode106may contribute to parasitic capacitance. In a ring-shaped sensing electrode106, the removal of the central portion of the sensing electrode106may reduce the parasitic capacitance without substantially affecting the production of ultrasonic signals by the CMUT in collapse mode. This, in turn, can result in an increase in the signal-to-noise ratio (SNR) of the ultrasonic signals. In some embodiments, the outer diameter of the sensing electrode106may be approximately 55-115 microns, approximately 70-100 microns, or approximately 85 microns. In some embodiments, the inner diameter of the sensing electrode106may be approximately 40-50 microns.

With the central portion of the sensing electrode106absent, a larger collapse voltage may be needed for the CMUT to enter collapse mode than if the central portion of the sensing electrode106were not absent. A larger collapse voltage may result in a larger electric field existing between the membrane104and the sensing electrode106, which may in turn result in greater sensitivity of the CMUT to received ultrasonic signals.

Additionally, the central portion of the sensing electrode106may be the portion of the sensing electrode106that causes the most charging on the membrane104. Because the central portion of the sensing electrode106is absent, this charging may be reduced. The outer regions of the membrane104may not be charged as much as the central portion of the membrane104would if the central portion of the sensing electrode106were present, as the outer regions of the membrane104may not contact the ring-shaped sensing electrode106as often during collapse.

FIG.4illustrates an example top view of the cavity102and the sensing electrode106with an additional electrode410included in the empty central portion of the sensing electrode106in accordance with certain embodiments described herein. The sensing electrode106may be similar to the sensing electrode106ofFIG.3. The electrode410may be coupled to ground. The electrode410may contribute to a bypass capacitance between the membrane104and ground. Noise from the metal128(e.g., metal128carrying voltage for supplying power) may couple to metal112. The noise may be reduced, at least in part, by the presence of the RDL108. The bypass capacitance between the membrane104and ground may further reduce the noise on metal112coupled from metal128. In some embodiments, the electrode410may contribute to a lower collapse voltage compared with the collapse voltage of the embodiment shown inFIG.3. This may be because, in some embodiments, the ground electrode410may contribute to attracting the membrane104towards the bottom of the cavity102. Higher electric fields in the cavity102may contribute to greater charging of the membrane104(e.g., the oxide at the bottom surface of the membrane104) due to repeated collapsing, and therefore it may be helpful to reduce the voltage required to achieve collapse in order to reduce charging.

FIG.5illustrates an example top view of the cavity102and the sensing electrode106in accordance with certain embodiments described herein. The sensing electrode106includes slots514(i.e., empty space). The slots514may be required by design rules for manufacturing the ultrasound device100. The slots514extend along the radial direction of the sensing electrode106. As the membrane104contacts the sensing electrode106during collapse, the membrane104may first contact a small portion of the center of the sensing electrode106, then a larger portion of the center of the sensing electrode106, etc. In other words, the membrane104may progressively contact the sensing electrode106in a radial direction. Because the slots514extend along the radial direction of the sensing electrode106, a small change in contact of the membrane104with the sensing electrode106may correspond to a small change in contact of the membrane104with one or more of the slots514. If the slots514did not extend along the radial direction, it could be possible for a small change in contact of the membrane104with the sensing electrode106to correspond to a large change in contact of the membrane104with one or more of the slots514, which could cause a disturbance in the performance of the CMUT. Arranging the slots along the radial direction of the sensing electrode106may reduce this disturbance. It should be appreciated that slots514may be used in either of the embodiments shown inFIGS.3and4.

Referring back toFIG.1, the ultrasound device100includes an oxide layer136at the bottom of the cavity102. The oxide layer136may include, for example, aluminum oxide or hafnium oxide. The oxide layer136may be formed by atomic layer deposition (ALD). Because the membrane104includes oxide on its bottom surface, during collapse, the oxide of the membrane104may contact oxide at the bottom of the cavity102. Because an insulator contacts another insulator, charging of the membrane104may be reduced. In some embodiments, the oxide on the bottom surface of the membrane may be thermal oxide (e.g., formed with wet oxidation, dry oxidation, or a dry-wet-dry cycle) while in other embodiments the oxide on the bottom surface of the membrane may be aluminum oxide or hafnium oxide (e.g., formed with ALD).

The ultrasound device100includes metal112. The metal112extends below the cavity102, and may include the same material as the sensing electrode106. As shown inFIG.6, in some embodiments the ultrasound device100is formed by bonding two substrates together. The top substrate602includes the membrane104and the bottom substrate604includes the cavity102, the sensing electrode106, and the metal112. Bonding occurs at bonding interfaces606between the top substrate602and the bottom substrate604. As can be seen, bonding does not occur at the cavity102. The metal112extends along the length of the bonding interface606in the bottom substrate604. This may contribute to the structures below the bonding interface606in the bottom substrate604being substantially homogenous along the length of the bonding interface606, which may in turn contribute to the top surface of the bottom substrate604being substantially homogenous along the bonding interface606. This homogeneity may help to ensure homogeneity of the bonding between the two substrates. (While the RDL108, the metal128, and other structures further below the bonding interface606may not be homogenous along the bonding interface606, these structures may contribute less to homogeneity of the top surface of the bottom substrate604than structures closer to the top of the bottom substrate604). Gaps between the metal112and the sensing electrode106, which could potentially cause inhomogeneity of the top surface of the bottom substrate604, are below the cavity102where no bonding occurs. The metal112may be connected to ground in the CMOS chip126through the vias130, the RDL108, and the metal128. The metal112may further serve as bypass capacitance between top membrane104and ground. In some embodiments, the gap between the sensing electrode106and the metal112may be approximately 4-6 microns, such as approximately 5 microns. In some embodiments, the metal112may extend approximately 6-10 microns, such as approximately 8 microns, beneath the cavity102.

Referring back toFIG.1, the bond pad114may be used for electrically coupling (e.g., using wire bonding) the ultrasound device100to a package (e.g., a package for mounting on a printed circuit board). Voltage at the bond pad114may be transmitted to the RDL108through the via116. The via116includes metal118connecting the bond pad114to the RDL108and the passivation120over the metal118. The metal118could contact the membrane104as the metal118passes from the bond pad114to the RDL108. The presence of metal118could short any voltage applied at the bond pad114to the membrane104. This may not be desirable if the voltage applied at the bond pad114is not intended to be applied to the membrane104(e.g., if the voltage is analog or digital voltage for powering or controlling certain circuits in the CMOS chip126). To limit the risk that membrane104be shorted to bond pad114, the ultrasound device100includes a trench122that extends around the bond pad114(or otherwise between bond pad114and membrane104) that electrically isolates the membrane104from the metal118. (The full extension of the trench122around the bond pad114is not visible inFIG.1.) Due to the electrical isolation of the membrane104from the metal118of the via116, a voltage applied at the bond pad114may not short to the membrane104. (The metal118does contact a portion124that is formed from the same layer as the membrane104, but this portion124does not overlay a cavity102and therefore any shorting of the bond pad114to the portion124may not substantially affect ultrasonic performance of the ultrasound device100.)

FIG.7illustrates an example cross-section of an ultrasound device100in accordance with certain embodiments described herein. The cross-section ofFIG.7may be along a different plane of the ultrasound device100than the cross-section ofFIG.1.FIG.7shows a bond pad714that may be used for electrically coupling (e.g., using wire bonding) the ultrasound device100to a package (e.g., a package for mounting on a printed circuit board). In particular, the bond pad714is electrically connected to the membrane104. Voltage for biasing the membrane104may be applied at the bond pad714. A trench is therefore not needed around the bond pad714to isolate the bond pad714from the membrane104, as opposed to the trench122which may be needed to isolate the bond pad114(at which other voltages that should not short to the membrane104may be applied) from the membrane104. Furthermore, as seen inFIG.7, a metal718connects the bond pad714to the membrane104, with the metal718extending across the top of the ultrasound device100to the electrical contact132, as opposed to the metal118, which extended down to the RDL108. It may be helpful for the metal718not to extend down to the RDL108of the CMOS chip126because the voltage applied to the membrane104may be a high voltage (e.g., 80V-100V) and certain materials of the CMOS chip126may not perform properly when exposed to such high voltages. The metal118avoids this high voltage from extending into the CMOS chip126.

FIG.8illustrates an example top view of the ultrasound device100in accordance with certain embodiments described herein.FIG.8shows the membrane104, which extends across the top surface of the ultrasound device100, the bond pads714which provide bias voltage to the membrane104, the metal718, the bond pads114which provide voltages that should not short to the membrane104(e.g., analog or digital voltages for powering or controlling certain circuits in the CMOS chip126), and the trenches122. The metal718extends from the bond pads714and forms a network that distributes the bias voltage from the bond pads714to the membrane104. The trenches122isolate the bond pads114from the membrane104. It should be appreciated that the passivation120and other structures may overlay the membrane104but are not shown inFIG.8. In some embodiments, ultrasound device100includes a trench712around the periphery of a die. The trench712may separate contiguous dies from one another, and may facilitate dicing. For example, trench712may provide a channel in which a die-saw blade is inserted and actuated during dicing. Use of trench712may reduce the likelihood that the dies are damaged during dicing.

FIG.9illustrates an example cross-section of an ultrasound device100in accordance with certain embodiments described herein. The cross-section ofFIG.9may be along a different plane of the ultrasound device100than the cross-section ofFIG.1. InFIG.9, the RDL108below the sensing electrode106is not electrically coupled to the sensing electrode106. The RDL108may be electrically coupled to ground in the CMOS chip126through the metal128. There may be power lines within the metal128that run under the sensing electrode106. Noise on these power lines may couple to the sensing electrode106. The RDL108below the sensing electrode106that is electrically coupled to ground may help to shield the sensing electrode106from this noise. In some embodiments, one layer of the RDL108may be used to electrically couple the sensing electrode106to the CMOS chip126while another layer of the RDL108may be electrically coupled to ground in the CMOS chip126and be used to shield the sensing electrode106.

In some embodiments, the vias130may be planarized (e.g., using chemical mechanical planarization (CMP)) prior for forming the sensing electrode106on the vias130. The sensing electrode106may then be planarized (e.g., using CMP). The oxide in which the cavity102is formed may then be deposited (e.g., using HDP-CVD) on the planarized sensing electrode106. By virtue of the planarization of the sensing electrode106and vias130, the oxide in which the cavity102is formed may not require planarization after deposition. This may be helpful, because the thickness of the deposited oxide in which the cavity102is formed may dictate the depth of the cavity102, and it may be desirable to tightly control this depth. Planarization of the oxide could reduce the depth of the cavity102from the desired depth and/or loosen control over the depth of the cavity102. Avoiding planarization of the oxide may help to maintain control over the depth of the cavity102. In some embodiments, the vias130may be formed without planarization, the sensing electrode106may be formed on the vias130, and the sensing electrode106may then be planarized. Planarization may ensure that the roughness of the oxide in which the cavity102is formed is less than 5 angstroms. It should be appreciated that the same steps used for forming the sensing electrode106may be used for forming the metal112.

Aspects of the present application may provide one or more benefits, some of which have been previously described. Now described is a non-limiting example of such benefits. It should be appreciated that not all aspects and embodiments necessarily provide all of the benefits now described. Further, it should be appreciated that aspects of the present application may provide additional benefits to the one now described.

Aspects of the present application provide ultrasound devices with sensing electrodes shaped to reduce undesired parasitic capacitances without substantially affecting the production of ultrasonic signals. In this way, the signal-to-noise ratio associate with the detection signal is increased, and so is the ultrasound device's ability to form images.

As used herein, reference to a numerical value being between two endpoints should be understood to encompass the situation in which the numerical value can assume either of the endpoints. For example, stating that a characteristic has a value between A and B, or between approximately A and B, should be understood to mean that the indicated range is inclusive of the endpoints A and B unless otherwise noted.

Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be object of this disclosure. Accordingly, the foregoing description and drawings are by way of example only.