Harmonic cmut devices and fabrication methods

Description of equivalent WO 2005084267 A2

0001HARMONIC CMUT DEVICES AND FABRICATION METHODS CROSS REFERENCE TO RELATED APPLICATIONS AND PRIORITY CLAIMS This Application claims the benefit of United States Provisional Application Serial Number 60/548,192 filed on 27 February 2004. TECHNICAL FIELD The present invention relates generally to chip fabrication, and more particularly, to fabricating harmonic capacitive micromachined ultrasonic transducers ("cMUTs") and harmonic cMUT imaging arrays. BACKGROUND Capacitive micromachined ultrasonic transducers generally combine mechanical and electronic components in very small packages. The mechanical and electronic components operate together to transform mechanical energy into electrical energy and vice versa. Because cMUTs are typically very small and have both mechanical and electrical parts, they are commonly referred to as micro-electronic mechanical systems ("MEMS") devices. cMUTs, due to their miniscule size, can be used in numerous applications in many different technical fields, including medical device technology. One application for cMUTs within the medical device field is imaging soft tissue. Tissue harmonic imaging has become important in medical ultrasound imaging, because it provides unique information about the imaged tissue. In harmonic imaging, ultrasonic energy is transmitted from an imaging array to tissue at a center frequency (f₀) during transmission. This ultrasonic energy interacts with the tissue in a nonlinear fashion, especially at high amplitude levels, and ultrasound energy at higher harmonics of the input frequency, such as 2f₀, are generated. These harmonic signals are then received by the imaging array, and an image is formed. To have a good signal to noise ratio during harmonic imaging, ultrasonic transducers in the imaging array would preferably be sensitive around both the fundamental frequency f₀and the first harmonic frequency 2f₀. Conventional ultrasonic transducers are not capable of performing in such a manner. For example, piezoelectric transducers are not suitable for harmonic imaging applications because these transducers tend to be efficient only at a fundamental frequency (f₀) and its odd harmonics (3f₀, 5f₀, etc.). To compensate for the odd harmonic efficiencies of piezoelectric transducers, the transducer is typically damped and several matching layers are used to create a broad band (~ 90% fractional bandwidth) transducer. This approach, however, requires a trade-off between sensitivity and bandwidth, since significant energy is lost due to the backing and matching layers. Additionally, conventional piezoelectric transducers and fabrication methods do not enable device manufacturers to control or adjust the vibration harmonics of conventional piezoelectric transducers. Conventional cMUTs are also not generally configured for tissue harmonic imaging.

0002For example, conventional cMUTs are not adapted to and do not utilize the multiple vibration modes of a cMUT membrane. Rather, conventional cMUTs, like conventional piezoelectric transducers, have a substantially uniform circular or rectangular membrane that have only utilized the first vibration mode of the cMUT membrane. In addition, conventional cMUTs and fabrication methods do not provide cMUTs capable of having adjustable vibration modes or controllable vibration harmonics. Due to the design of conventional cMUT types, a 90% fractional bandwidth is usually desired to have a reasonable signal to noise ratio. This fractional bandwidth, however, precludes use of multiple vibration orders of a cMUT membrane for medical imaging applications. Specifically, conventional cMUT designs are not optimized to achieve higher sensitivity over a wide bandwidth or adapted to exploit multiple vibration modes of a cMUT membrane. Therefore, there is a need in the art for a cMUT fabrication method enabling fabrication of a cMUT with an enhanced membrane to increase and enhance cMUT device performance for tissue harmonic imaging applications. Additionally, there is a need in the art for fabricating cMUTs to utilize multiple vibration modes and multiple vibration harmonics of a membrane to increase and enhance cMUT device performance. Additionally, there is a need in the art for a cMUT device capable of receiving and transmitting ultrasonic energy using frequencies associated with different vibration modes for a cMUT membrane. Still yet, there is a need in the art for a cMUT device having a membrane with vibration modes that are harmonically related. It is to the provision of such cMUT fabrication and cMUT imaging array fabrication that the embodiments of present invention are primarily directed. BRIEF SUMMARY OF THE INVENTION The present invention comprises harmonic cMUT array transducer fabrication methods and systems. The present invention provides cMUTs for imaging applications having enhanced membranes and multiple-element electrodes for optimizing the transmission and receipt of ultrasonic energy or waves, which can be especially useful in medical imaging applications. The cMUTs of the present invention can have membranes with non-uniform mass distributions adapted to receive a predetermined frequency. The present invention also provides cMUTs having membranes that can be adapted to have vibration modes that are harmonically related. In addition, the present invention provides cMUTs having membranes capable of being fabricated such that the vibration harmonics of cMUT membranes can be adjusted to correspond with operational frequencies and associated harmonics. Still yet, the present invention provides cMUTs capable of being fabricated with electrodes located near multiple vibration mode peaks of cMUT membranes when the cMUT membranes are immersed in an imaging medium. The cMUTs can be fabricated on dielectric or transparent substrates, such as, but not limited to, silicon, quartz, or sapphire, to reduce device parasitic capacitance, thus improving electrical performance and enabling optical detection methods to be used. Additionally, cMUTs constructed according to a preferred embodiment of the present invention can be used in immersion applications such as intravascular catheters and ultrasound imaging. The present invention preferably comprises a cMUT including a membrane and a membrane frequency adjustor for adjusting a vibration mode of the membrane. The membrane frequency adjustor can adjust the membrane so that at least two vibration modes of the membrane are harmonically related. The membrane frequency adjustor can comprise the membrane having a non-uniform mass distribution along at least a portion of it length. The non-uniformity in mass can be provided by varying the thickness of the membrane, varying the density of the membrane, or for example, providing the membrane with a mass load proximate the membrane. The mass load can be a single mass source providing the mass non-uniformity along its length, or it can be a plurality of separate mass loads elements located in various places along the membrane. The cMUT can include a mass load being an electrode element of the cMUT. The mass load preferably is Gold. The plurality of mass load elements modifies the frequency response of the membrane. The membrane can have a plurality of vibration modes, and the membrane frequency adjustor can adapt the membrane so that the vibration modes of the membrane are harmonically related. The membrane can be adapted to vibrate at a fundamental frequency and the membrane frequency adjustor can adjust the membrane to vibrate at a frequency substantially equal to twice the fundamental frequency. The present invention can further comprising a method of controlling vibration modes of a cMUT including the steps of providing a membrane, determining a target vibration frequency of the membrane, and altering the mass distribution of the membrane along at least a portion of the length of the membrane to induce the target vibration frequency of the membrane. In a preferred embodiment, the target vibration frequency of the membrane is substantially twice a fundamental frequency of the membrane. The step of altering the mass distribution of the membrane along at least a portion of the length of the membrane can comprise providing a membrane having a varying thickness along at least a portion of the length of the membrane, or providing a membrane having a varying density along at least a portion of the length of the membrane. Preferably, the membrane has a first vibration mode and a second vibration mode that is approximately twice the frequency of the first vibration mode, the membrane being adapted to transmit ultrasonic energy at the first vibration mode and receive ultrasonic energy at the second vibration mode. A method of fabricating a cMUT according to a preferred embodiment of the present invention comprises the steps of providing a membrane and configuring the membrane to have a non-uniform mass distribution to receive energy at a predetermined frequency. The step of configuring the membrane to have a non-uniform mass distribution can include providing a plurality of mass loads proximate the membrane. A further step of adapting the membrane to transmit ultrasonic energy at a first vibration mode and receive ultrasonic energy at a second vibration mode, wherein the second vibration mode is approximately twice the frequency of the first vibration mode, can be provided. Additionally, the membrane can be adapted so that the vibration modes of the membrane are haπnonically related, and a further step of positioning an electrode element proximate a vibration mode of the membrane can be added. A preferred embodiment of the present invention comprises a membrane and a mass load proximate the membrane. The mass load can adapt the membrane to receive energy at a predetermined frequency. In addition, a plurality of mass loads can be disposed on the membrane so that the membrane has a non-uniform mass distribution along at least a portion of its length. The mass load can be part of, proximate, or positioned along the membrane. The mass load can be of different materials than the membrane. The membrane can be formed to have regions of different thickness using the mass load to distribute the mass of the membrane so that the membrane's vibration modes are harmonically related. Alternatively, a portion of the non-uniform mass distribution of the membrane can be formed by patterning the membrane to have regions of varying thickness. The harmonic cMUT can also comprise a cavity defined by the membrane, a first electrode proximate the membrane, and a second electrode proximate a substrate. The cavity can be disposed between the first electrode and second electrode. The first electrode and the second electrode can be configured to have multiple elements. In another preferred embodiment, a method to fabricate a cMUT can comprise providing a membrane proximate a substrate and configuring the membrane to have a non- uniform mass distribution along at least a portion of its length. A method to fabricate a cMUT can also comprise providing a sacrificial layer proximate the first conductive layer, providing a first membrane layer proximate the sacrificial layer, providing a second membrane layer proximate the second conductive layer, and removing the sacrificial layer.

0003The first and second membrane layers can form the membrane. A cMUT fabrication method can also comprise shifting the frequency and shape of a vibration mode of the membrane and adapting the membrane to operate in a receive state to receive ultrasonic energy and a transmission state to transmit ultrasonic energy. In yet another preferred embodiment, a method to control a harmonic cMUT can comprise determining a vibration mode of the membrane and positioning one or more mass loads on the membrane to induce a membrane vibration mode corresponding to a predetermined frequency. The harmonic cMUT can have a top electrode proximate a membrane, a bottom electrode proximate a substrate, and a cavity between the membrane and the bottom electrode. A method to control a harmonic cMUT can also include positioning a first electrode element to correspond with a vibration mode of the membrane. The first electrode element can be a part of a top electrode and/or a bottom electrode. A predetermined frequency can be substantially twice a fundamental frequency of a membrane.

0004A membrane can have a first vibration mode and a second vibration mode that is approximately twice the frequency of the first vibration mode. The membrane can be adapted to transmit ultrasonic energy at a first vibration mode and receive ultrasonic energy at a second vibration mode. These and other features as well as advantages, which characterize the various preferred embodiments of present invention, will be apparent from a reading of the following detailed description and a review of the associated drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a cross-sectional view of a harmonic cMUT in accordance with a preferred embodiment of the present invention. FIG. 2 illustrates a sample pulse-echo frequency spectrum of a harmonic cMUT in accordance with a preferred embodiment of the present invention. FIG. 3 illustrates a fabrication process utilized to fabricate a harmonic cMUT in accordance with a preferred embodiment of the present invention. FIG. 4 illustrates a logical flow diagram depicting a fabrication process utilized to fabricate a harmonic cMUT in accordance with a preferred embodiment of the present invention. FIG. 5 illustrates a cMUT imaging array system comprising multiple harmonic cMUTs formed in a ring-annular array in accordance with a preferred embodiment of the present invention. FIG. 6 illustrates a cMUT imaging array system comprising multiple harmonic cMUTs formed in a side-looking array in accordance with a preferred embodiment of the present invention. FIG. 7 is a diagram illustrating a graph illustrating the calculated average velocity as a function of frequency over the surface of the cMUTs illustrated in FIG. 7. FIG. 8 is a graph illustrating the calculated peak velocity amplitude as a function of frequency over the surface of the cMUT membrane illustrated in Fig. 1. FIG. 9A is a diagram illustrating a vibration profile for the cMUT membrane illustrated in FIG. 1 at approximately 0.8MHz. FIG. 9B is a diagram illustrating a magnitude of the vibration profile for the cMUT membrane illustrated in FIG. 1 at approximately 8 MHz FIG. 9C is a diagram illustrating a phase of the vibration profile for the cMUT membrane illustrated in FIG. 1 at approximately at 8 MHz. FIG. 10A is a diagram illustrating a cross section of a cMUT membrane vibrating at its third mode. FIG. 10B is a diagram illustrating a cross section of a mass loads positioned along a cMUT membrane. FIG. 11 is a diagram illustrating a comparison of an average velocity for the cMUT membrane illustrated in FIG. 1 being loaded and unloaded with mass loads. FIG. 12 is a diagram of a sample calculated average velocity corresponding to transmit and receive electrode elements for a harmonic CMUT. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS cMUTs have been developed as an alternative to piezoelectric ultrasonic transducers, particularly for micro-scale and array applications. cMUTs are typically surface micromachined and can be fabricated into one or two-dimensional arrays and customized for specific applications. cMUTs can have performance comparable to piezoelectric transducers in terms of bandwidth and dynamic range, but are generally significantly smaller. A cMUT typically incorporates a top electrode disposed within a membrane suspended above a conductive substrate or a bottom electrode proximate or coupled to a substrate. An adhesion layer or other layer can optionally be disposed between the substrate and the bottom electrode. The membrane can have elastic properties enabling it to fluctuate in response to stimuli. For example, stimuli may include, but are not limited to, external forces exerting pressure on the membrane and electrostatic forces applied through cMUT electrodes. cMUTs are often used to transmit and receive acoustic waves. To transmit an acoustic wave, an AC signal and a large DC bias voltage are applied to a cMUT electrode disposed within a cMUT membrane. Alternatively, the voltages can be applied to the bottom electrode. The DC voltage can pull down the membrane to a position where transduction is efficient and the cMUT device response can be linearized. The AC voltage can set the membrane into motion at a desired frequency to generate an acoustic wave in a surrounding medium, such as gases or fluids. To receive an acoustic wave, a capacitance change can be measured between cMUT electrodes when an impinging acoustic wave sets a cMUT membrane into motion. The present invention provides cMUTs comprising an enhanced membrane to control the vibration harmonics of a cMUT. A cMUT membrane according to the present invention can have a non-uniform mass distribution along the length of the membrane. The membrane can have, for example, a substantially uniform thickness, but have variations in densities providing the mass distribution profile. Alternatively, the mass distribution can be provided by varying the thickness of the membrane. If the membrane is fashioned from a single material have a substantially uniform thickness and density, mass loads can also be utilized. Controlling the mass distribution along the membrane enables the vibration harmonics of a cMUT membrane to be controlled. As an example, multiple mass loads can be proximate, a part of, or positioned along a membrane to aid in shifting or adjusting membrane vibration modes. A cMUT membrane having a non-uniform mass distribution can enhance the transmission and reception of ultrasonic energy, such as ultrasonic waves. A cMUT membrane having a non-uniform mass distribution and a plurality of electrodes corresponding with vibration modes of a cMUT membrane can enhance the transmission and reception of ultrasonic energy, such as ultrasonic waves at desired, but separate, frequency ranges during transmission and reception. In addition, a cMUT having an enhanced membrane according to the present invention can utilize a fundamental operating frequency of a cMUT membrane and harmonic frequencies of the fundamental operating frequency to transmit and receive ultrasonic signals. Exemplary equipment for fabricating cMUTs according to the present invention can include, but are not limited to, a PECVD system, a dry etching system, a metal sputtering system, a wet bench, and photolithography equipment. cMUTs fabricated according to the present invention generally include materials deposited and patterned on a substrate in a build-up process. The present invention can utilize low-temperature PECVD processes for depositing various silicon nitride layers at approximately 250 degrees Celsius, which is preferably the maximum process temperature when a metal sacrificial layer is used.

0005Alternatively, the present invention according to other preferred embodiments can utilize an amorphous silicon sacrificial layer deposited as a sacrificial layer at approximately 300 degrees Celsius. Referring now the drawings, in which like numerals represent like elements, preferred embodiments of the present invention are herein described. FIG. 1 illustrates a cross-sectional view of a harmonic cMUT 100 in accordance with a preferred embodiment of the present invention. The cMUT 100 generally comprises various components proximate a substrate 105. These components can comprise a substrate 105, a bottom electrode 110, a cavity 150, a membrane 115, a first top electrode element 130A, a second top electrode element 130B, and a third top electrode element 130C. The cMUT 100 can also comprise mass loads 155, 160, which will be understood shown exaggerated in the figures, and not to scale. The mass loads 155, 160 can be proximate, disposed on, or positioned along the membrane 115, and can be separate from, or integral with, the membrane 115. As will be discussed in further detail below with reference to FIGs. 5 and 6, a plurality of cMUTs 100 can be used in a cMUT imaging array. The substrate 105 can be formed of silicon and can contain signal generation and reception circuits. The substrate 105 can also comprise materials enabling optical detection methods to be utilized, preferably transparent. The substrate 105 can comprise an integrated circuit 165 at least partially embedded in the substrate 105 to enable the cMUT 100 to transmit and receive ultrasonic energy or acoustical waves. In alternative embodiments the integrated circuit 165 can be located on another substrate (not shown) proximate the substrate 105. The integrated circuit 165 can be adapted to generate and receive electrical and optical signals. The integrated circuit 165 can also be adapted to provide signals to an image processor 170. For example, the integrated circuit 165 can be coupled to the image processor 170. The integrated circuit 165 can contain both signal generation and reception circuitry or separate integrated generation and reception circuits can be utilized. The image processor 170 can be adapted to process signals received or sensed by the integrated circuit 165 and create an image from electrical and optical signals. The bottom electrode 110 can be deposited and patterned onto the substrate 105. In an alternative embodiment, an adhesive layer (not shown) can be disposed between the substrate 105 and the bottom electrode 110. An adhesion layer can be used to sufficiently bond the bottom electrode 110 to the substrate 105. The adhesion layer can be formed of Chromium, or many other materials capable of bonding the bottom electiode 110 to the substrate 105. The bottom electrode 110 is preferably fabricated from a conductive material, such as Gold or Aluminum. The bottom electrode 110 can also be patterned into multiple, separate electrode elements (not shown). Multiple electrode elements of the bottom electrode 110 can be similar to the top electrode elements 130A, 130B, 130C. The multiple elements of the bottom electiode 110 can be isolated from each other with an isolation layer deposited on the multiple elements of the bottom electiode 110, although upon later fabrication, some of the electrode elements can be electrically coupled. An isolation layer can also be utilized to protect the bottom electiode 110 from other materials used to form the cMUT 100. The membrane 115 preferably has elastic characteristics enabling it to fluctuate relative to the substrate 105. In a preferred embodiment, the membrane 115 comprises silicon nitride and is formed from multiple membrane layers. For example, the membrane 115 can be formed from a first membrane layer and a second membrane layer. In addition, the membrane 130 can have side areas 116, 117, and a center area 118. As shown, the center area 118 can be generally located equally between the side areas 116, 117. The membrane 115 can also define a cavity 150. The cavity 150 can be generally disposed between the bottom electrode 110 and the membrane 115. The cavity 150 can be formed by removing or etching a sacrificial layer generally disposed between the bottom electrode 110 and the membrane 115. In embodiments using an isolation layer, the cavity would be generally disposed between the isolation layer and the membrane 115. The cavity 150 provides a chamber enabling the membrane 115 to fluctuate in response to stimuli, such as external pressure or electrostatic forces. In a preferred embodiment, the membrane 115 comprises multiple electrode elements 130A, 130B, 130C disposed within the membrane 115. Alternatively, a single electrode or electrode element can be disposed within the membrane 115. Two or more of the multiple electrode elements 130A, 130B, 130C can be electrically coupled forming an electrode element pair. Preferably, side electrode elements 130A, 130C are formed nearer the sides 116, 117 of the membrane 115, and center electrode element 130B is formed nearer the center area 118 of the membrane 115. The electrode elements 130A, 130B, 130C can be fabricated using a conductive material, such as Gold or Aluminum. The side electrode elements 130A and 130C can be electrically coupled, and isolated from the center electrode element 130B, to form an electrode element pair. The electrode elements 130A, 130B, 130C can be formed from the same conductive material and patterned to have predetermined locations and varying geometrical configurations within the membrane 115. The side electrode element pair 130A, 130C can have a width less than the center electiode 130B, and at least a portion of the pair 130A, 130C can be placed at approximately the same distance from the substrate 105 as the center electrode element 130B. In alternative embodiments, additional electrode elements can be formed within the membrane 115 at varying distances from the substrate 105. The electrode elements 130A, 130B, 130C can be adapted to transmit and receive ultrasonic energy, such as ultrasonic acoustical waves. The side electrode elements 130A, 130C can be provided with a first signal from a first voltage source 175 (Vi) and the center electrode 130B can be provided with a second signal from a second voltage source 180 (V₂). The side electrode elements 130A, 130C can be electrically coupled so that voltage or signal supplied to one of the electiode elements 130A, 130C will be provided to the other of the electrode elements 130A, 130C. These signals can be voltages, such as DC bias voltages and AC signals. The side electrode elements 130A, 130C can be adapted to shape the membrane 115 to form a relatively large gap for transmitting ultrasonic waves. It is desirable to use a gap size that during tiansmission allows for greater tiansmission pressure. Further, the side electrode elements 130A, 130C can be adapted to shape the membrane 115 to form a relatively small gap for receiving ultrasonic waves. It is desirable to use a reduced gap size for reception that allows for greater sensitivity of the cMUT 100. Both the center electrode element 130B and the side electrode element elements 130A, 130C can receive and transmit ultrasonic energy, such as ultrasonic waves. The cMUT 100 can be optimized for transmitting and receiving ultrasonic energy by altering the shape of the membrane 115. The electiode elements 130A, 130B, 130C can be provided with varying bias voltages and signals from voltage sources 175, 180 (Vi, V₂) to alter the shape of the membrane 115. Additionally, by providing the various voltages and signals, the cMUT 100 can operate in two states: a tiansmission state and a reception state. For example, during a receiving state, the side electrode elements 130A, 130C can be provided a DC bias voltage from the first voltage source 175 (Vi) to optimize the shape of the membrane 115 for receiving an acoustic ultrasonic wave. In a preferred embodiment of the present invention, the membrane 115 has a non- uniform mass distribution along its length. The membrane 115 has a varying mass distribution across its length, which variation can be a result of one or more of the following: varying thickness, density, material composition, and other membrane characteristics along the length of the membrane. In a preferred embodiment, mass loads 155, 160 are deposited and patterned onto the membrane 115 providing the membrane 115 to have a non-uniform mass distribution. Alternatively, the membrane 115 can be patterned to have a non-uniform mass distribution such that certain points along the length of the membrane 115 have varying masses via thickness and/or density variations. The mass loads 155, 160 are preferably formed of dense, malleable materials, including, but not limited to, Gold. Many other dense, malleable materials can be used to form the mass loads 155, 160. Gold is desirable because it is a dense, soft material, and thus does not significantly interfere with membrane vibration due to the membrane's stiffness. In a preferred embodiment of the present invention, the mass loads 155, 160 have a thickness of approximately one micro-meter and have a width of approximately two micro-meters. The size and shape of the mass loads 155, 160 can be modified to achieved desired results. The mass loads 155, 160 can be proximate the sides 116, 117, respectively. More than two mass loads 155, 160 can also be utilized in other embodiments. The mass loads 155, 160 can be used to control or adjust the vibrations and fluctuations of the membrane 115. For example, the mass loads 155, 160 can be placed or positioned to correspond with peak vibration regions of a particular vibration mode of the membrane 115. The membrane 115, due to its elastic characteristics, can vibrate at various frequencies and can also have multiple vibration modes. For example, the membrane 115 can have a first order vibration mode as well as other higher order vibration modes (e.g., second order, third order, etc.). Adjusting the vibration modes of the membrane 115 can result in improved cMUT 100 performance. For example, shifting the vibration modes of the membrane 115 to occur at the operational frequencies and harmonics of the operational frequencies utilized by the cMUT 100 enables the membrane 115 to resonate at these frequencies when used, resulting in efficient transmission and reception of ultrasonic energy. With a combination of signals applied to and received from the voltage sources 175, 180, the transmission of ultrasonic energy can be minimized at a predetermined frequency and the received signals can be maximized at that particular frequency. Modifying the mass distribution of the membrane 115 can aid in shifting vibration modes of the membrane 115 to desired locations in the frequency spectrum for the cMUT 100. For example, the membrane 115 can be mass loaded such that it receives a predetermined frequency. The predetermined frequency can be a harmonic frequency, such as a first harmonic frequency, of a signal transmitted by the cMUT 100. FIG. 2 illustrates a sample pulse-echo frequency spectrum of a harmonic cMUT 100 in accordance with a preferred embodiment of the present invention. As shown, a frequency response 205 for the harmonic cMUT 100 has a first peak 210 and a second peak 220. The first peak 210 can coincide with a transmit frequency range 215 substantially centered around an operational frequency (f₀). The second peak 220 can coincide with a receive frequency range 225 substantially centered around a second harmonic frequency of the operational frequency (2*f₀). The membrane 115 of the cMUT 100 can be adjusted so that the frequency of the first vibration order is centered around the operational frequency (f₀) and the second vibration order is centered around the second harmonic frequency of the operational frequency (2*f₀). Such a configuration enables the vibration modes of the membrane 115 to be harmonically related such that the peaks of the vibration modes correspond to the operational frequency and harmonics of the operational frequency. The membrane 115 of the cMUT 100 can be enhanced to have a frequency response as shown in FIG. 2. The membrane can be adapted to transmit and receive ultrasonic energy at a desired operational frequency and the second harmonic of the operational frequency. The present invention can also be used to enhance a cMUT membrane to operate at multiple vibration modes corresponding to a cMUT membrane. For example, the membrane 115 could be adjusted by locating mass loads in certain locations on the membrane 115 to aid in moving a third vibration mode of the membrane 115. The third vibration mode of the membrane 115 could be moved or adjusted to correspond with a third harmonic frequency (3*f₀) to improve transmitted and received signals at the third harmonic frequency range. In addition to shifting vibration modes to correspond with certain harmonic frequencies, broad bandwiths can be created around the harmonic frequencies by shifting the vibration modes, thus increasing the transmitted and receiving ranges of the membrane 115. FIG. 3 illustrates a fabrication process utilized to fabricate a harmonic cMUT in accordance with a preferred embodiment of the present invention. Typically, the fabrication process is a build-up process that involves depositing various layers of materials on a substrate, and patterning the various layers in predetermined configurations to fabricate a cMUT 100 on the substrate 105. In a preferred embodiment of the present invention, a photoresist such as Shipley S- 1813 is used to lithographically define various layers of a cMUT. Such a photoresist material does not require the use of the conventional high temperatures for patterning vias and material layers. Alternatively, many other photoresist or lithographic materials can be used. A first step in the present fabrication process provides a bottom electiode 110 on a substrate 105. The substrate 105 can comprise dielectric materials, such as silicon, quartz, glass, or sapphire. In some embodiments, the substrate 105 contains integrated electionics, and the integrated electronics can be separated for transmitting and receiving signals. Alternatively, a second substrate (not shown) located proximate the substrate 105 containing suitable signal tiansmission and detection electronics can be used. A conductive material, such as conductive metals, can form the bottom electrode 110. The bottom electiode 110 can also be formed by doping a silicon substrate 105 or by depositing and patterning a conductive material layer, such as metal, on the substrate 105. Yet, with a doped silicon bottom electrode 110, all non-moving parts of a top electrode can increase parasitic capacitance, thus degrading device performance and prohibiting optical detection techniques for most of the optical spectrum. To overcome these disadvantages, a patterned bottom electiode 110 can be used. As shown in FIG. 3(a), the bottom electiode 110 can be patterned to have a different length than the substrate 105. By patterning the bottom electrode 110, device parasitic capacitance can be significantly reduced. The bottom electiode 110 can be patterned into multiple electiode elements, and the multiple electrode elements can be located at varying distances from the substrate 105.

0006Aluminum, chromium, and gold are exemplary metals that can be used to form the bottom electiode 110. In one preferred embodiment of the present invention, the bottom electrode 110 has a thickness of approximately 1500 Angstroms, and after deposition, can be patterned as a diffraction grading, or to have various lengths. In a next step, an isolation layer 315 is deposited. The isolation layer 315 can isolate portions of or the entire bottom electrode 110 from other layers placed on the bottom electrode 110. The isolation layer 315 can be silicon nitride, and preferably has a thickness of approximately 1500 Angstroms. A Unaxis 790 PECVD system can be used to deposit the isolation layer 315 at approximately 250 degrees Celsius in accordance with a preferred embodiment. The isolation layer 315 can aid in protecting the bottom electrode 110 or the substrate 105 from etchants used during cMUT fabrication. Once deposited onto the bottom electrode layer 110, the isolation layer 315 can be patterned to a predetermined thickness. In an alternative preferred embodiment, an isolation layer 315 is not utilized. After the isolation layer 315 is deposited, a sacrificial layer 320 is deposited onto the isolation layer 315. The sacrificial layer 320 is preferably only a temporary layer, and is etched away during fabrication to form a cavity 150 in the cMUT 100. When an isolation layer 315 is not used, the sacrificial layer 320 can be deposited directly on the bottom electrode 110. The sacrificial layer 320 is used to hold a space while additional layers are deposited during cMUT fabrication. The sacrificial layer 320 can be formed with amorphous silicon that can be deposited using a Unaxis 790 PECVD system at approximately 300 degrees Celsius and patterned with a reactive ion etch ("RIE"). Sputtered metal can also be used to form the sacrificial layer 320. The sacrificial layer 320 can be patterned into different sections, various lengths, and different thicknesses to provide varying geometrical configurations for a resulting cavity or via. A first membrane layer 325 is then deposited onto the sacrificial layer 320, as shown in FIG. 3(b). For example, the first membrane layer 325 can be deposited using a Unaxis 790 PECVD system. The first membrane layer 325 can be a layer of silicon nitride or amorphous silicon, and can be patterned to have a thickness of approximately 6000 Angstroms. The thickness of the first membrane layer 325 can vary depending on the particular implementation. Depositing the first membrane layer 325 over the sacrificial layer 320 aids in forming a vibrating membrane 115. After patterning the first membrane layer 325, a second conductive layer 330 can be deposited onto the first membrane layer 325 as illustrated in FIG. 3(c). The second conductive layer 330 can form the top electrode(s) of a cMUT. The second conductive layer 130 can be patterned into different electrode elements 130A, 130B, 130C that can be isolated from each other. The electrodes 130A, 130B, 130C can be placed at varying distances from the substrate 105. One or more of the electrode elements 130A, 130B, 130C can be electrically coupled forming an electrode element pair. For example, the side electrode elements 130A, 130C can be coupled together, forming an electiode element pair.

0007Preferably, the formed electrode pair 130A, 130C is isolated from the center electrode element 130B. The electrode element pair 130A, 130C can be formed from conductive metals such as Aluminum, Chromium, Gold, or combinations thereof. In an exemplary embodiment, the electrode element pair 130A, 130C comprises Aluminum having a thickness of approximately 1200 Angstroms and Chromium having a thickness of approximately 300 Angstroms. Aluminum provides good electrical conductivity, and Chromium can aid in smoothing any oxidation formed on the Aluminum during deposition. Additionally, the electrode element pair 130A, 130C can comprise the same conductive material or a different conductive material than the first conductive layer 110. In a next step, a second membrane layer 335 is deposited over the electrode elements 130A, 130B, 130C as illustrated in FIG. 3(d). The second membrane layer 335 increases the thickness of the cMUT membrane 115 at this point in fabrication (formed by the first and second membrane layers 325, 335), and can serve to protect the second conductive layer 330 from etchants used during cMUT fabrication. The second membrane layer 335 can also aid in isolating the first electiode element 130A from the second electrode element 130B. The second membrane layer can be approximately 6000 Angstroms thick. In some embodiments, the second membrane layer 335 is adjusted using deposition and patterning techniques so that the second membrane layer 335 has an optimal geometiical configuration. Preferably, once the second membrane layer 335 is adjusted according to a predetermined geometric configuration, the sacrificial layer 320 is etched away, leaving a cavity 150 as shown in FIG. 3(f). The first and second membrane layers 325, 335 can form the membrane 115. The membrane 115 can fluctuate or resonate in response to stimuli, such as external pressures and electrostatic forces. In addition, the membrane 115 can have multiple vibration modes due to its elastic characteristics. The location of these vibration modes can be helpful in designing and fabricating a cMUT according to the present invention. For example, the first and second conductive layers 310, 330 can be patterned into electrodes or electiode elements proximate the vibration modes of the composite membrane. Such electiode and electrode element placement can enable efficient reception and transmission of ultrasonic energy. In addition, the location of vibration modes for the membrane 115 can be adjusted and controlled by changing the mass distribution of the membrane 115. To enable etchants to reach the sacrificial layer 320, apertures 340, 345 can be etched through the first and second membrane layers 325, 335 using an RIE process. As shown in FIG. 3(e), access passages to the sacrificial layer 320 can be formed at apertures 340, 345 by etching away the first and second membrane layers 325, 335. When an amorphous silicon sacrificial layer 320 is used, one must be aware of the selectivity of the etch process to silicon. If the etching process has low selectivity, one can easily etch through the sacrificial layer 320, the isolation layer 315, and down to the substrate 105. If this occurs, the etchant can attack the substrate 305 and can destroy a cMUT device. When the bottom electrode 110 is formed from a metal that is resistant to the etchant used with the sacrificial layer, the metal layer can act as an etch retardant and protect the substrate 105. Those skilled in the art will be familiar with various etchants and capable of matching the etchants to the materials being etched. After the sacrificial layer 320 is etched, the cavity 350 can be sealed with seals 342, 347, as shown in FIG. 5(f). The cavity 350 can be formed between the isolation layer 315 and the membrane layers 325, 335. The cavity 350 can also be disposed between the bottom electrode 110 and the first membrane layer 325. The cavity 350 can be formed to have a predetermined height in accordance with some preferred embodiments of the present invention. The cavity 350 enables the cMUT membrane 115, formed by the first and second membrane layers 325, 335, to fluctuate and resonate in response to stimuli. After the cavity 350 is formed by etching the sacrificial layer 320, the cavity 350 can be vacuum sealed by depositing a sealing layer (not shown) on the second membrane layer 335. Those skilled in the art will be familiar with various methods for setting a pressure in the cavity 350 and then sealing it to form a vacuum seal. The sealing layer is typically a layer of silicon nitride, having a thickness greater than the height of the cavity 350. In an exemplary embodiment, the sealing layer has a thickness of approximately 4500 Angstroms, and the height of the cavity 350 is approximately 1500 Angstroms. In alternative embodiments, the second membrane layer 335 is sealed using a local sealing technique or sealed under predetermined pressurized conditions. Sealing the second membrane layer 335 can adapt the cMUT for immersion applications. After depositing the sealing layer, the thickness of the cMUT membrane 115 can be adjusted by etching back the sealing layer since the cMUT membrane 115 may be too thick to resonate at a desired frequency. A dry etching process, such as RIE, can be used to etch the sealing layer. In a next step, the non-uniform mass distribution of the membrane of the cMUT can be accomplished by depositing multiple mass loads 155, 160 onto the second membrane layer 335. Multiple mass loads 155, 160 can be placed at various places on the second membrane layer 335. The location of the multiple mass loads 155, 160 on the second membrane layer 335 can correspond to vibration modes of the membrane 115 formed by the first and second membrane layers 325, 335. The multiple mass loads 155, 160 can also be used to shift or adjust the vibration modes of the membrane formed by the first and second membrane layers 325, 335 to certain predetermined areas. This feature of the present invention enables a specific vibration mode of interest to be selectively controlled. These predetermined areas can be located near the electrode elements 130A, 130B, 130C so that the electrode elements 130A, 130B, 130C can be used to transmit and receive ultrasonic acoustical waves. In an alternative embodiment, the second membrane layer 335 can be patterned to have regions of different thickness to form a membrane having a non-uniform mass distribution. A final step in the present cMUT fabrication process prepares the cMUT for electrical connectivity. Specifically, RIE etching can be used to etch through the isolation layer 315 on the bottom electrode 110, and the second membrane layer 335 on the electrode elements 130A, 130B, 130C making them accessible for connections. Additional bond pads can be formed and connected to the electrodes. Bond pads enable external electrical connections to be made to the top and bottom electrodes 110, 130 with wire bonding. In some embodiments, gold can be deposited and patterned on the bond pads to improve the reliability of the wire bonds. In an alternative embodiment of the present invention, the sacrificial layer 320 can be etched after depositing the first membrane layer 325. This alternative embodiment invests little time in the cMUT 100 before performing the step of etching the sacrificial layer 320 and releasing the membrane 115 formed by the membrane layers 325, 335. Since the top electiode 130 has not yet been deposited, there is no risk that pinholes in the second membrane layer 335 could allow the top electiode 330 to be destroyed by etchants. FIG. 4 illustrates a logical flow diagram depicting a preferred method to fabricate a • harmonic cMUT 100 in accordance with a preferred embodiment of the present invention. The first step involves providing a substrate 105 (405). The substrate 105 can be of various constructions, including opaque, translucent, or transparent. For example, the substrate 150 can be, but is not limited to, silicon, glass, or sapphire. Next, an isolation layer can deposited onto the substrate 105, and patterned to have a predetermined thickness (410). The isolation layer is optional, and may not be utilized in some embodiments. An adhesive layer can also be used in some embodiments ensuring that an isolation layer bonds to a substrate 105, or the bottom electiode 110 can adequately bond to the substrate 105. After the isolation layer is patterned, a first conductive layer 110 is deposited onto the isolation layer, and patterned into a predetermined configuration (415). Alternatively, a doped surface of a substrate 105, such as a doped silicon substrate surface, can form the first conductive layer 110. The first conductive layer 110 preferably forms a bottom electrode 110 for a cMUT 100 on a substrate 105. The first conductive layer 110 can be patterned to form multiple electrode elements. At least two of the multiple electiode elements can be coupled together to form an electrode element pair. Once the first conductive layer 110 is patterned into a predetermined configuration, a sacrificial layer 320 is deposited onto the first conductive layer 110 (420). The sacrificial layer 320 can be patterned by selective deposition and patterning techniques so that it has a predetermined thickness. Then, a first membrane layer 325 can be deposited onto the sacrificial layer 320 (425). The deposited first membrane layer 325 is then patterned to have a predetermined thickness, and a second conductive layer 130 is then deposited onto the first membrane layer 325 (430). The second conductive layer 130 preferably forms a top electrode 130 for a cMUT 100. The second conductive layer 130 can be patterned to form multiple electrode elements 130A, 130B, 130C. At least two of the multiple electrode elements 130A, 130B, 130C can be coupled together to form an electrode element pair. After the second conductive layer 130 is patterned into a predetermined configuration, a second membrane layer 335 is deposited onto the patterned second conductive layer 130 (435). The second membrane layer 335 can also be patterned to have an optimal geometric configuration. The first and second membrane layers 325, 335 can encapsulate the second conductive layer 130, enabling it to move relative to the first conductive layer 110 due to elastic characteristics of the first and second membrane layers 325, 335. After the second membrane layer 335 is patterned, the sacrificial layer 320 is etched away, forming a cavity 150 between the first and second conductive layers 110, 130 (435). The cavity 150 formed below the first and second membrane layers 325, 335 provides space for the resonating first and second membrane layers 325, 335 to move relative to the substrate 105. In a next step, the second membrane layer 335 is sealed by depositing a sealing layer onto the second membrane layer 335 (435). In a final step (440), a mass load can be formed on the second membrane layer 335. Multiple mass loads can also be formed on the second membrane layer 335, and they can be placed at point on the second membrane layer 335 corresponding to vibration modes of a membrane 115 formed by the first and second membrane layers 325, 335. The mass loads are preferably formed of dense, malleable materials, such as Gold. The mass loads can aid in changing the mass distribution of the membrane layer 115 so that the membrane layer 115 has regions of varying thickness. In an alternative embodiment, the membrane layer 115 can be patterned to have regions of varying thickness or densities. The embodiments of the present invention can also be utilized to form a cMUT array for a cMUT imaging system. Those skilled in the art will recognize that the cMUT imaging arrays illustrated in FIGs. 5 and 6 are only exemplary, and that other imaging arrays are achievable in accordance with the embodiments of the present invention. FIG. 5 illustrates a cMUT imaging array device formed in a ring-annular array on a substrate. As shown, the device 500 includes a substrate 505 and cMUT arrays 510, 515. The substrate 505 is preferably disc-shaped, and the device 500 may be utilized as a forward looking cMUT imaging array. Although the device 500 is illustrated with two cMUT arrays 510, 515, other embodiments can have one or more cMUT arrays. If one cMUT array is utilized, it can be placed near the outer periphery of the substrate 505. If multiple cMUT arrays are utilized, they can be formed concentrically so that the circular-shaped cMUT arrays have a common center point. Some embodiments can also utilize cMUT arrays having different geometrical configurations in accordance with some embodiments of the present invention. FIG. 6 illustrates a cMUT imaging array system formed in a side-looking array on a substrate. As shown, the device 600 includes a substrate 605, and cMUT arrays 610, 615. The substrate 605 can be cylindrically-shaped, and the cMUT arrays can be coupled to the outer surface of the substrate 605. The cMUT arrays 610, 615 can comprise cMUT devices arranged in an interdigital fashion and used for a side-looking cMUT imaging array. Some embodiments of device 600 can include one or multiple cMUT imaging arrays 610, 615 in spaced apart relation on the outer surface of the cylindrically-shaped substrate 600. The present invention also contemplates analyzing a cMUT 100 or cMUT array to determine the location of the vibration modes of a cMUT membrane and to deteπnine the position of mass loads to adjust the vibration modes of a cMUT membrane. For convenience, the components of the cMUT discussed below are with reference to FIG. 7. The description of particular functions of the components, or specific arrangement and sizes of the components, however, are not intended to limit the scope of FIG. 7 and are provided only for example, and not limitation. An approach to analyze a cMUT is to simulate the motion of a cMUT membrane in a fluid, such as water. For example, a finite element analysis tool, such as the ANSYS™ tool, can been used to simulate the motion of a cMUT membrane. In a preferred embodiment of the present invention, the membrane can have a width of approximately 40μm and a thickness of approximately 0.6μm. Alternatively, other dimensions can be used. Since the membrane can be long and rectangular, 1-D analysis can be used. Other simulations can use other dimensional analysis parameters, such as 2-D or 3-D. To simulate electrostatic actuation of the cMUT a uniform pressure of lkPa (kilo- Pascal) can be applied to the membrane. A resulting vibration profile of the membrane can then be calculated. FIG. 7 shows an average velocity 700 over the membrane as a function of frequency. As can be seen, the spectrum 705 is relatively flat in the 2-30MHz range with the exception of nulls 710, 715 at approximately 8MHz and approximately 24MHz. To further understand the vibration profile of the membrane, the maximum velocity over the membrane can be calculated and plotted, as illustrated in FIG. 8. As shown in FIG. 8, the velocity of the membrane can have five peaks 805A, 805B, 805C, 805D, 805E. The local peak velocities of the membrane can be more than an order of magnitude larger than the average velocity. When the membrane displacement profile is plotted around the frequencies where the peaks occur, the nulls in the average velocity occur at frequencies where the membrane moves close to its third and fifth resonances. FIGs. 9A-C illustrate the vibration profiles over the membrane at 0.8MHz and 8MHz. These frequencies correspond to the first and third vibration modes of the membrane. Although the cMUT does not generate any considerable pressure output around 8MHz, the membrane locally vibrates with large amplitude in response to an applied pressure. Therefore, by placing localized electrodes over the parts of the membrane where a particular mode has peak velocity, large output signals can be generated around a certain frequency range. Furthermore, by selectively displacing the location of the particular vibration mode one can determine where the enhanced response would occur. The present invention also contemplates utilizing the higher order vibration modes for cMUT design by selectively controlling the frequency of a particular membrane vibration mode of interest. For example, this can be accomplished by disposing mass loads on the membrane at predetermined locations. The mass distribution of a membrane can be altered by depositing and patterning mass loads on a uniform membrane, resulting in a membrane with a non-uniform mass distribution. The third vibration mode, for example, is targeted and the mass loads are concentrated on the regions of the membrane having peak strain energy (i.e. peaks). The mass loads are preferably Gold due to its high density and low stiffness. The Gold can be configured to have a thickness of approximately one micro-meter and a width of approximately two micro-meters. The mass loads can be positioned at the peak displacement locations 1015, 1020 as shown in FIG. 10A-B. As shown in FIGs. 10A-B, by positioning the mass loads at peak displacement locations 1015, 1020 the third vibration mode frequency can be shifted from approximately 8 MHz (see 1105) to approximately 6.5 Mhz (see 1110). The shifting of a third vibration mode frequency for the membrane can occur without significantly affecting the surrounding vibration modes of the membrane, such as the second and fourth vibration modes. As an example of the mass loading approach discussed above, the membrane can be designed to reduce a null occurring at approximately 8 MHz in a cMUT spectrum, as shown in FIG. 11. The membrane can be loaded with different mass loads positioned to correspond with a third vibration mode. The membrane can have a width and thickness of approximately one micro-meter, and the mass loads can have a thickness of approximately one micro-meter and a width of approximately two micro-meters. As shown in FIG. 11, positioning the mass loads along the membrane adjusts the average velocity of the membrane. FIG. 11 shows a reduction on the null 1110 occurring at approximately 8 MHz. Thus, by enhancing the shape of the membrane, the frequency response of the membrane can be optimized. As further illustiated by FIG. 11, the mass loading does not greatly affect the average velocity of the membrane for most of the spectrum, which evinces that the mass loading of the membrane does not reduce the overall efficiency of the cMUT. The resulting frequency spectrum of the cMUT can be further shaped by continuously positioning additional mass loads along the membrane. A preferred application utilizing cMUTs with high order vibration mode control as contemplated by the present invention is harmonic imaging. Since mass loads can be used to change the location of peaks in a cMUT's frequency spectrum, signals received at desired frequency ranges can be improved. In addition, by patterning cMUT electrodes into multiple elements, as discussed above, vibrations local to the multiple elements can be selectively detected. For example, a cMUT having a dual electrode element structure having side electrode elements with a width of approximately 10 micro-meters and a center electiode element of approximately 15 micro-meters can be used to selectively detect vibrations occurring at different vibration modes. FIG. 12 shows an estimated transmit and receive spectra of a harmonic cMUT. Both center and side electrode elements can be used in transmitting ultiasonic energy, and only side electrode elements can be used to receive ultrasonic energy. As FIG. 12 illustrates, a harmonic cMUT can have a wideband transmit spectrum 1300 suitable for transmitting a fundamental frequency of approximately 4 MHz. In addition, the spectrum of the received signal 1310, which shows that the harmonic signals around 8MHz, is amplified relative to the transmitted spectrum by nearly 15dB. Since harmonic signals are subject to more attenuation, the present invention provides improved cMUT design with enhanced receive and transmit frequency spectrums. While the various embodiments of this invention have been described in detail with particular reference to exemplary embodiments, those skilled in the art will understand that variations and modifications can be effected within the scope of the invention as defined in the appended claims. Accordingly, the scope of the various embodiments of the present invention should not be limited to the above discussed embodiments, and should only be defined by the following claims and all applicable equivalents.