Design and Functionality of Capacitive

Micromachined Ultrasonic Transducers for

Underwater Applications

Mason Valdisera and William Leisner

MEMS 1082

December 6th, 2018

Abstract

The capacitive micromachined ultrasonic transducer (CMUT) can be used in

underwater applications, mainly imaging. The CMUT is an alternative technology to

piezoelectric transducers, offering advantages such as wider bandwidths, ease of

fabricating large arrays, and the potential for integration with electronic circuits. The

transducer works by emitting an ultrasonic wave to penetrate murky water: increasing

image visibility, clarity, and range. There are a wide array of applications for this device

such as underwater observation, differentiating objects, and recreational activities. The

CMUT uses a combination of silicon, silicon dioxide, electrodes, and a gap (either

vacuum or air) to create the capacitive circuit.

CMUTs have been used in the biomedical field before, but new technologies that

seal the chamber allow this device to now be used underwater. The report will highlight

the principles behind the CMUT in underwater applications and the different

components involved that allow it to function properly. Also, the various ways to

fabricate a CMUT including sacrificial layer deposition and the wafer bonding method

will be detailed. Within each design and method there are advantages and drawbacks

that will be considered and expanded upon.

Introduction

CMUTs are an emerging transducer, and have recently been adapted for

underwater use, allowing scientists to explore their possibilities in a new field of

application, underwater imaging. Currently, piezoelectric micromachined ultrasonic

transducers (PMUTs) are the dominant ultrasonic transducer technology used in

underwater imaging and other applications [1]. They are based on lead zirconate

titanate (PZT). However, PZT based acoustic transducers have several drawbacks

such as low operational efficiency, narrow operating bandwidths, and difficulty in

processing two-dimensional arrays [1]. On the other hand, CMUTs have several

advantages over PMUTs. They have better matching with the acoustic impedance of

air and other fluids without needing a surface matching layer. They also have a large

immersion bandwidth and high electromechanical coefficient, which means an improved

image quality. Further, CMUTs have high directivity, a wide operating temperature

range, compatibility with integrated circuits, and low-cost manufacturing possibilities [2].

However, for CMUTs to truly replace PMUTs, CMUT transducer arrays need

improved bandwidth, sensitivity, directivity, and output pressure. These are all linked to

improved image quality. Improved ultrasonic transducers have the potential to benefit

fields such as sonar and high resolution imaging, non-destructive testing, and high

frequency medical imaging [3].

This paper will go over the history of CMUTs since their inception, highlighting

the changes and improvements in their design and manufacture. Then it will detail the

theory of CMUTs, giving an in-depth explanation of how they work. Then manufacturing

will be revisited with a look at how specific manufacturing methods work. Next, a

detailed look at the applications of CMUTs will be discussed.

History

Ultrasonic devices have been used for underwater imaging since the end of

World War I following the advent of the modern submarine. Ultrasonic devices have

also been used in medicine since the 1930s, most notably the use of ultrasound

imaging (sonography) to view a developing fetus was invented in the 1950s. Today,

ultrasonic devices continue to play a large role on submarines and surface craft sonar

systems.

The first capacitive ultrasonic transducer was developed in 1989 by Hohm and

Hess. In 1994, Haller and Khuri-Yakub developed the first CMUT using sacrificial layer

technology and with a sealed cavity, meaning it could be used for underwater

applications. In 2003, a wafer bonding technology to reduce process steps and improve

the quality of the product was developed. In 2006, Chang et al. developed a polymer

based CMUT using a sacrificial layer technique and fabricated on a polymer substrate,

allowing the CMUT to be flexible. CMUTs on very thin silicon also have bending

characteristics. In 2008, flexible CMUT arrays were fabricated by etching trenches into

a silicon wafer and filling them with polydimethylsiloxane (PDMS). Also in 2008, a

lamination technique was used to minimize residual stress to fabricate bonding

structures at low temperatures and pressures. In 2012, PDMS and bonding technology

were used to create a stretchable CMUT. In 2017, a low temperature roll-lamination

technique and silver nanowire electrodes were used to produce a transparent and

flexible CMUT [4].

Theory

A CMUT is a capacitor cell that consists of a thin moveable plate suspended over

a vacuum gap. A metal coating on the thin plate or the thin plate itself acts as the top

electrode for the capacitor. An underlying conductive substrate acts as the bottom

electrode. By applying a DC voltage, the two electrodes are attracted to one another,

but they are restricted by a restoring force caused by the stiffness of the plate. If AC

voltage is used to drive the capacitor it generates ultrasound. If the capacitor is

subjected to an ultrasonic pressure, a current is induced due to the capacitive change of

the device.

The goal of an electrostatic transducer is to operate at a high electrical field. The

ability of the CMUT to produce this high electrical field (> 108 V/cm) makes it a

competitive device compared to other electrostatic transducers. By operating with a

high electrical field, the device must be handled with extreme care in order to maintain

its accuracy and function. The frequency of the CMUT is determined mainly by the size,

shape, and material properties of the thin plate. Often times an array of thin plates with

different shapes and sizes are put in parallel to allow the device to operate at various

frequencies.

The transducer can be covered with a thin layer of elastic polymer to provide

electrical insulation for the device. The CMUT has a very large bandwidth which gives it

a distinct advantage over piezoelectric devices which have smaller bandwidths. The

broad bandwidth is due to the fact that the mechanical impedance of the plate is

significantly lower than the loading impedance. The lower cutoff frequency is

determined by the spring constant of the device. As the spring constant increases, the

bandwidth of the CMUT decreases. Selecting the correct rod, with a specific spring

constant, that will support the device, but also maintain the broad bandwidth must be

analyzed when constructing the CMUT. The higher cutoff frequency is determined by

the mass loading of the immersion medium [5].

When used in underwater imaging, CMUTs are paired, with one group

transmitting and the other group receiving. A transducer operating in transmitting mode

requires a high cavity height to obtain high output pressure, while a transducer working

in receiving mode requires only a low cavity height to attain high sensitivity [1]. When

the size of the top electrode is 40% to 50% of the size of the membrane, the transducer

can have a large bandwidth [1]. One way to test a CMUT for its output pressure and

bandwidth is by immersion in a tank with the setup shown in Figure 1.

Figure 1: Schematic of test setup to determine CMUT bandwidth and output pressure [1].

The pressure recorded at the hydrophone at different distances can be

measured, and compared to the theoretical pressure that should be recorded at that

distance with a given output pressure from the CMUT. Then the Hydrophone is

replaced by another CMUT, and the distance can be measured by taking the time it

takes for the receiver CMUT to pick up the transmitted signal sent by the transmitting

CMUT. This time can be multiplied by the speed of sound in the water, giving the

distance.

Figure 2: Transmission and Reception of CMUT [6]

The major difference between the CMUT that transmits and the CMUT that

receives is that the receiver is a sensor and the transmitter is an actuator. In the

receiver, the ultrasonic wave causes the beam to vibrate which induces a capacitance

change. The beam vibration causes the capacitance change because the gap between

the two plates fluctuate as the beam vibrates. Equation 1 shows the relationship

between the capacitance, C, and the gap thickness, t.

𝐶 =𝜖𝐴

𝑡 (1)

The beam vibration causes t to change which would then change the

capacitance value. The change in capacitance is then used to create a change in

voltage across the device. Equation 2 shows the relationship between voltage and

capacitance.

𝑄 = 𝐶𝑉 (2)

The charge remains constant across the capacitor, so changing the capacitance

changes the voltage. In the transmitter, the voltage is not used as an output, rather the

driving input signal. The AC voltage applied to the beam causes the capacitance to

change using equation 2. With a change in capacitance, using equation 1, the gap

thickness must change. The constantly changing gap thickness causes the beam to

vibrate. If the input voltage is at a high enough frequency, the beam will vibrate at a

frequency high enough to induce an ultrasonic wave.

When the transmitter emits an ultrasonic wave, if the wave comes in contact with

an object it will bounce back toward the CMUT. The bounced back wave will then be in

contact with the receiver. The time from when the transmitter emits the ultrasonic wave

to when the voltage across the receiver changes is recorded to calculate the distance

away that an object is.

For a CMUT transducer used in underwater imaging, important parameters to

determine are the axial (∆x) and lateral (∆y) resolutions. These are the minimum

distances that can be distinguished between two echo sources. The ∆x and ∆y

resolutions are determined using Equations 3 and 4 respectively. For Equation 3, n is

the number of scanning lines, λ is the wavelength and nλ is the pulse width. For

Equation 4, D is the aperture size of the probe and S is the distance between the focus

and the CMUT [1].

𝛥𝑥 =

𝑛𝜆

2

(3)

𝛥𝑦 =

1.22𝜆

𝐷 𝑆

(4)

Increasing the operating frequency (reducing wavelength) and reducing scanning lines

decreases (improves) the axial resolution. At a specific focus, increasing aperture size

and reducing wavelength improve the lateral resolution.

Manufacturing

The first CMUTs were fabricated using conventional machining tools. A rough

metal surface was the back plate of the capacitor and the top electrode consisted of a

metallized Mylar membrane [7]. Micromachining replaced conventional machining to

create cavities in silicon, but metallized dielectric films were still used as the membrane

material, preventing vacuum sealing. In 1994, the first fully micromachined CMUT was

created using the sacrificial release method.

Sacrificial release methods vary, but all follow essentially the same principles.

Sacrificial release is a type of surface micromachining. In surface micromachining,

microstructures are built via deposition and etching of structural layers over a substrate,

or the base material of the structure, which is usually made of silicon. In the fabrication

process, sacrificial layers (typically oxides) are layers that are applied to allow the

deposition of structural layers (typically poly-silicon), but are then removed to leave

behind just the structural material, creating desired cavities and voids. Surface

micromaching is illustrated in Figure 3.

Figure 3: Illustration of surface micromachining [8].

This is not the same as bulk micromachining, where a substrate is selectively

etched either chemically or physically to create miniaturized mechanical components.

There are low and high temperature sacrificial release methods, with lower

temperatures allowing the CMUT to be manufactured on the same silicon wafer as other

electronics, allowing for integration with electronics. For the process to be done on

electronic bearing wafers, the highest temperature at any point in the process cannot

exceed 400°C [7]. A high temperature process developed at Stanford University is an

example of a popular sacrificial release method.

The process begins with a silicon wafer that has been doped so as to be highly

conductive on its surface. This forms the back electrode of the CMUT. Next, the wafer

is then deposited with an etch-stop layer of silicon nitride (Si3N4) at 785°C with low

pressure chemical vapor deposition (LPCVD). Then the sacrificial-layer depositioning

and patterning is done in two steps so that the channels used by potassium hydroxide

(KOH) to remove the sacrificial layer are thinner than the cavity thickness. First, a layer

of LPCVD poly-silicon is deposited and photolithography is used to define regions of

reduced channel height. Then a dry etch removes all of the poly-silicon in the defined

regions, stopping at the Si3N4 layer. Next, another layer of poly-silicon is deposited.

The thickness of this layer determines the thickness of the etch channels while the

thickness of both layers combined determines the initial cavity height. These

depositions are often done at lower temperatures because this allows for better

thickness control [7]. Then, another photolithography and dry etch are done to define

the cavity and membrane shape. The membrane shape is a hexagon or circle because

it is easy to model. It is important to note that the shape and size of the membrane are

important design parameters that determine the frequency response of the CMUT

element [7]. Next, a critical membrane layer is deposited. It is an etch-stop layer of

Si3N4 that is deposited by LPCVD at 785°C, similar to the first one deposited on the

wafer. After this, another lithography and dry etch step creates small holes in the etch-

stop layer. These holes are located over the etch channels, and are to allow the KOH

to pass bellow the critical membrane to etch out the cavity beneath. Then the wafer is

immersed in KOH for anywhere from a few hours to a few days depending on the

temperature, which drives the poly-silicon etch rate of KOH.

Once the etching is complete, the holes are sealed by a layer of Si3N4 in another

step of LPCVD. Not only does this seal the holes, but since the process occurs at

extremely low pressures, the cavities are considered to be vacuum sealed [7]. This is

the essential step in creating a CMUT that can be used in immersion applications since

the sealed cavity will maintain its vacuum, not filling with whatever fluid the CMUT is

immersed in. The rest of the process creates electrical connections from the bond pads

to the top and bottom electrodes. First, a lithography and etch step creates connections

to the ground plane through the Si3N4 layers. Then aluminum is sputtered over the

wafer and patterned using lithography and a wet etch. The final step is an optional

annealing step that improves the ohmic contact to ground of the CMUT but also

introduces high levels of stress to the component. This step is done or left out

depending on the possible benefits or drawbacks in its application [7]. Figure 4 shows

the final fabricated CMUTs on a silicon wafer, using the sacrificial layer method, and a

diagram of what the individual elements look like.

Figure 4: An example of a wafer with CMUTs manufacture on it using the sacrificial layer method. The blow up diagram with the largest zoom shows what the individual CMUT elements look like. Created

using images from [1].

A more recent method of manufacturing the CMUT is through the wafer bonding

method. The sacrificial layer manufacturing had high levels of complexity that

developed the need for a different method. The wafer bonding provides that increase in

simplicity that allows an increase in manufacturing rate and decrease in manufacturing

cost. The wafer bonding method also improves on control and repeatability issues that

plagued the sacrificial layer method. The wafer bonding method begins with the simple

wafer bonding, but then additional processes such as the LOCOS, thick-buried-oxide,

and piston-CMUT can be performed to improve flaws that exist by simple wafer bonding

[5].

The simple wafer bonding method begins with two wafers, a high quality silicon

wafer and a silicon-on-insulator wafer. The cavity is created on the high quality silicon

wafer by etching thermal oxide down onto the substrate. The insulation layer is grown

by inducing a second thermal oxidation [5]. The two silicon wafers are then placed

together in vacuum. The wafers are subjected to high temperatures (around 1000°C) to

form strong covalent bonds. The handle and the etched thermal oxide layer are

removed from the silicon-on-insulator wafer leaving a thin, single layer silicon crystal

suspended over the cavity. The electrical connections between the substrates are

created through opening in the oxide layer. Deep trenches are etched into the silicon

wafer to define silicon pillars that serve to establish interconnection between different

elements within the CMUT. Because the suspended layer is a single crystal, the

thickness, uniformity, and mechanical properties of the plate are well controlled. Also,

the gap height of the CMUT is well controlled [5]. These provide distinct advantages

over the sacrificial layer technique. There are also drawbacks to this method. The

minimum gap height remains limited by the thickness of the initial oxide layer. The

interconnection struggle is vulnerable to shorts that can occur when the device

becomes contaminated. The contamination can occur when handling the device,

storing the device, or packaging the device [5].

The LOCOS process includes an extended insulation layer in the post area to

address the low breakdown voltage issue associated with the simple wafer bonding

method. The extended layer is formed through local oxidation of silicon (LOCOS). This

process maintains the precise gap control from the simple wafer bonding [5].

The thick-buried-oxide process seeks to further reduce the dielectric breakdown

by isolating the bottom electrode to the region where the electric field needs to be high.

This removes a high electric field in unnecessary areas. This method works by applying

a thick buried oxide layer on the silicon-on-insulator wafer. The arrangement allows for

each cell to have its own connection with the electrode [5].

The final improvements can be made to the CMUT by improving the structure of

the device. One such improvement involves forming the central mass by etching the

device layer of an SOI layer or by plating metal on top of the plate structure [5]. The

added mass shows improved performance and a wider bandwidth. Another

improvement involves adding long, narrow posts to provide the spring constant for the

device. Both additions cause a more flexible design space and separate the mass and

spring components [5].

Figure 5: Diagrams of all the Manufacturing Techniques. (a) Sacrificial Layer Method, (b) Simple Wafer Bonding, (c) LOCOS Process, (d) Thick-Buried-Oxide Process, (e) Adding Mass to Plate, (f) Lengthening

and Narrowing of the Posts.

In 2017, Pang et al. developed a transparent, flexible CMUT using a roll-

lamination technique [4]. In roll-lamination, there are two important steps. The first, is

to prepare vibrating membranes on a PET release layer. The second step is to

laminate these membranes onto a sidewall. The temperatures and pressures are

careful controlled to ensure that the vibrating membrane bonds to the sidewall. The

vibrating membrane is the CMUT and the sidewall provides structural support. Silver

nanowires were used as the electrodes since they are transparent to the human eye.

The completed transparent, flexible CMUT is shown in Figure 6.

Figure 6: A transparent, flexible CMUT fabricated using a roll-lamination method [4].

Applications

The CMUT covers a wide range of applications, and improved designs have

created new possibilities in the applications that it serves. Many applications include

imaging, as the CMUT is able to resolve clear images in small or low-visibility locations.

The CMUT was initially designed for air coupled applications. However, it began to shift

towards medical and underwater applications.

The CMUT device works well for the medical industry because it is able to take

images of moving internal structures. The CMUT equipment is cost-effective and

portable, which makes the device even more appealing to the medical industry. For

these reasons, CMUT imaging is used in cardiac structures, the vascular system, the

fetus and uterus, abdominal organs such as the liver, the kidneys and gall bladder, and

the eye [5].

In 1994, Khuri-Yakub led his team to create the first CMUT with a sealed cavity

[1]. The CMUT will scan across a specified distance underwater and any objects that

are within this range will be displayed. The imaging device utilizes the two CMUT

devices, one transmitter and one receiver, as stated in the theory section [1]. The two

transducers move laterally to scan a certain range of distances. Figure 7 shows the

result of using a CMUT underwater to detect a target object. From the image, the target

is clearly displayed by the white markings. The axial and lateral distance plots give the

spatial location of the target relative to the CMUT. Although the edges of the image

remain blurred, the CMUT undoubtedly identifies the target while submerged in water

[1]. When the water is too murky for regular cameras and imaging to work, the CMUT

can provide this plot to accurately display what objects are nearby and where they are

located. The distances measured compared to the CMUT when tested showed roughly

1 cm in error [1].

Figure 7: Plot showing detection and position of object underwater [1].

There are broad uses for underwater imaging including the commercial,

scientific, and military industries [3]. The commercial industry would use the imaging to

identify where fish or any other catch are located. The CMUT imaging will improve the

efficiency of fishing, especially in areas with murky water. The scientific application of

the CMUT imaging includes studying underwater habitats and ship wreck locations.

Coral reef destruction has been a topic of concern, and with the CMUT imaging, the

size and status of a coral reef can be monitored. Additionally, the CMUT can be used to

study a shipwreck. Often the locations of sunken ships are dark, with extremely low

visibility. Using the CMUT imaging, the location of wrecked ships can be found [3].

Finding the ships allows for them to be studied and explain why the ship was wrecked.

The other application of the CMUT imaging would be for military purposes [3]. The

imaging can be used to detect enemy submarines. Knowing the location of an enemy

vessel gives the CMUT user a tactical advantage.

Figure 8: Different applications of underwater imaging [3].

Conclusion

In conclusion, the CMUT utilizes a capacitor cell with a thin movable plate to emit

ultrasonic waves for various imaging applications. The CMUT has distinct advantages

over piezoelectric counterparts such as cost, manufacturability, and range of

applications. The CMUT has become increasingly popular in underwater imaging with

the breakthrough of sealing the chamber of the capacitive device. Objects that lie in

murky water can be clearly identified using the CMUT. There are various reasons to

need the clear underwater imaging such as studying marine biology, naval use, and

even for recreational activity. With continual developments in manufacturing, the CMUT

has increased its broad frequency bandwidth, sensitivity, and flexibility. While the

CMUT succeeds in identifying underwater objects, it must continue to undergo testing

and analysis to further improve the clarity of the images themselves. This can be done

my modifying the 2-D arrays in the CMUT. Already a leader in underwater applications,

further improvements will make the CMUT the best option for underwater imaging.

References

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Underwater Imaging Applications.” www.mdpi.com/journal/sensors.

[2] Zhang, Rui et al. “Design and performance analysis of capacitive micromachined

ultrasonic transducer (CMUT) array for underwater imaging.” Microsyst Technol, 16

November, 2015.

[3] Oralkan, Ömer et al. “Underwater Acoustic Imaging using Capacitive Micromachined

Ultrasonic Transducer Arrays.” https://web.stanford.edu/group/khuri-

yakub/publications/02_Oralkan_02.pdf

[4] Pang, Da-Chen. Chang, Cheng-Min. “Development of a Novel Transparent Flexible

Capacitive Micromachined Ultrasonic Transducer.” Department of Mechanical

Engineering, National Kaohsiung University of Applied Sciences, 20 June, 2017.

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Transducers for Medical Imaging and Therapy.” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3158704/

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