M.H. Badi

Fabricating capacitive micromachined ultrasonic transducers with wafer-bonding technology

Introduces a new method for fabricating capacitive micromachined ultrasonic transducers (CMUTs) that uses a wafer bonding technique. The transducer membrane and cavity are defined on an SOI (silicon-on-insulator) wafer and on a prime wafer, respectively. Then, using silicon direct bonding in a vacuum environment, the two wafers are bonded together to form a transducer. This new technique, capable of fabricating large CMUTs, offers advantages over the traditionally micromachined CMUTs. First, forming a vacuum-sealed cavity is relatively easy since the wafer bonding is performed in a vacuum chamber. Second, this process enables better control over the gap height, making it possible to fabricate very small gaps (less than 0.1 /spl mu/m). Third, since the membrane is made of single crystal silicon, it is possible to predict and control the mechanical properties of the membrane to within 5%. Finally, the number of process steps involved in making a CMUT has been reduced from 22 to 15, shortening the device turn-around time. All of these advantages provide repeatable fabrication of CMUTs featuring predictable center frequency, bandwidth, and collapse voltage.

Improved equivalent circuit and finite element method modeling of capacitive micromachined ultrasonic transducers

Equivalent circuit model has been widely used to predict the bandwidth of capacitive micromachined ultrasonic transducers (CMUTs). According to this model, the lower cutoff of the bandwidth is determined by the time constant of the parallel RC where R is dictated by the radiation and C is determined by the electrical capacitance of the transducer. The higher cutoff, on the other hand, is determined by the membranes anti-resonance. In the mechanical part of the model, the radiation impedance is simply added to the membrane impedance assuming that the membrane impedance does not change when it operates in the immersion medium. Therefore, the mass loading effect of the medium is neglected. Our finite element method calculations showed that the mass loading on the membrane impedance drastically lowers the membrane anti-resonance frequency degrading the bandwidth. In this paper, we present results of equivalent circuit modeling combined with finite element analysis. We constructed a 3D finite element model for one element of a 1D array. The element has 7 hexagonal membranes in the width dimension and it is assumed that the membranes are replicated in the length dimension infinitely by using symmetry boundary conditions. By combining membrane impedance with equivalent circuit model, we found that the center frequency of operation is 11 MHz and the bandwidth is 12.5 MHz close to the collapse voltage. We also investigated the effect of the DC bias on the center frequency. Decreasing the bias voltage increased the center frequency without affecting the bandwidth assuming the source impedance is zero.

Capacitive micromachined ultrasonic Lamb wave transducers using rectangular membranes

This paper details the theory, fabrication, and characterization of a new Lamb wave device. Built using capacitive micromachined ultrasonic transducers (CMUTs), the structure described uses rectangular membranes to excite and receive Lamb waves on a silicon substrate. An equivalent circuit model for the transducer is proposed that produces results, which match well with those observed by experiment. During the derivation of this model, emphasis is placed on the resistance presented to the transducer membranes by the Lamb wave modes. Finite element analysis performed in this effort shows that the dominant propagating mode in the device is the lowest order antisymmetric flexural wave (A/sub 0/). Furthermore, most of the power that couples into the Lamb wave is due to energy in the vibrating membrane that is transferred to the substrate through the supporting posts of the device. The manufacturing process of the structure, which relies solely on fundamental IC-fabrication techniques, is also discussed. The resulting device has an 18 /spl mu/m-thick substrate that is almost entirely made up of crystalline silicon and operates at a frequency of 2.1 MHz. The characterization of this device includes S-parameter and laser vibrometer measurements as well as delay-line transmission data. The insertion loss, as determined by both S-parameter and delay-line transmission measurements, is 20 dB at 2.1 MHz. When configured as a delay-line oscillator, the device functions well as a sensor with sensitivity to changes in the mass loading of its substrate.

Lamb wave devices using capacitive micromachined ultrasonic transducers

Lamb wave devices based on capacitive micromachined ultrasonic transducers (CMUTs) have been built on 500-μm-thick silicon wafers for frequencies in the vicinity of 1 MHz. CMUTs have been used to both excite and detect Lamb waves in the substrate. This configuration eliminates the need for piezoelectric materials, which are not compatible with the existing integrated circuit (IC) fabrication techniques, and allows easy integration of Lamb wave devices and electronics on the same wafer. Finite element analysis of the devices shows that the lowest order antisymmetric Lamb wave (A0) is the dominant mode in the substrate in this frequency range. This result is also confirmed by demonstration experiments.

Finite element method and normal mode modeling of capacitive micromachined SAW and Lamb wave transducers

Surface wave and Lamb wave devices without piezoelectricity are the latest breakthrough applications of the capacitive micromachined ultrasonic transducers (CMUTs). CMUTs were introduced for airborne and immersion applications. However, experiments showed that those devices couple energy not only to the medium but also to the substrate they are built on. By placing the CMUTs on a substrate in an interdigitated configuration, it is possible to couple energy to Lamb wave or Rayleigh wave modes with very high efficiency without a need for any piezoelectric material. In this study, we calculate the acoustic field distribution in a silicon substrate as well as the acoustic impedance of the CMUT membrane, which includes the power coupled to the substrate. We apply the normal mode theory to find the distribution of the acoustic power among different Lamb wave modes. For low frequency (1 MHz) devices, we find that the lowest order antisymmetric (A/sub 0/) mode Lamb wave is the dominant mode in the substrate, and 95% of the power propagates through this mode. For high frequency devices (100 MHz), interdigital CMUTs excite Rayleigh waves with efficiencies comparable to piezoelectric surface acoustic wave (SAW) devices.

A first experimental verification of micromachined capacitive Lamb wave transducers

Capacitive Micromachined Ultrasonic Transducers (cMUTs) are generally used to transmit and receive ultrasound in both air and water. These devices can be made on silicon and manufactured using standard CMOS processing techniques. When cMUTs are used in this way, significant effort is made to minimize energy loss into the substrate. If this loss is instead exploited so that the devices are optimized to couple energy into the silicon bulk, Lamb waves and Rayleigh waves are generated with high efficiency. These waves can then be detected using a similar device structure. With this method it is possible to fabricate Lamb wave devices on silicon using conventional integrated circuit processing techniques. This paper discusses the manufacturing and characterization of the first of these devices: a 1 MHz Lamb wave transducer that is fundamentally based on cMUT technology. The characterization of this device demonstrates that the energy coupled into the substrate results in a Lamb Wave where the lowest order anti-symmetric mode (A/sub 0/) is dominant. The insertion loss of this device in air is 43.06 dB.

Improved modeling and fabrication techniques for capacitive micromachined ultrasonic Lamb wave transducers

This paper discusses improvements in the theoretical and experimental framework of capacitive micromachined ultrasonic Lamb wave transducers. Theoretically, a new method for the analysis of these Lamb wave devices is proposed using the electro-mechanical capabilities of ANSYS, a commercial finite element package. The model used in these simulations has been verified by comparing its predictions when configured as a clamped transducer to those predicted by the standard equivalent circuit model; the input impedances obtained using the two methods agree to within 1%. This method performs harmonic and transient analyses to predict device performance metrics in the presence of electrostatic forces. Experimentally, this paper introduces a new manufacturing process for the fabrication of Lamb wave devices using CMUT wafer bonding technology. Devices have successfully been built using this technique and preliminary results show significantly improved uniformity in membrane to membrane performance and successful Lamb wave propagation between transducers. The membranes fabricated are 1 cm long and 60 /spl mu/m wide have an insertion loss of 16.2 dB near 2.6 MHz.

Lamb wave devices based on capacitive micromachined ultrasonic transducers

This paper describes the theory, design, and realization of a new type of Ultrasonic Lamb Wave Transducer. The excitation mechanism of this device is unlike any other as it relies on the Capacitive Micromachined Ultrasonic Transducer (CMUT). Built using fundamental integrated circuit techniques, this device has an insertion loss of 20 dB at an operating frequency of 2.1 MHz. The dominant propagating mode in the device is that of the lowest order antisymmetric flexural wave (A/sub 0/). The substrate upon which the device rests is 18 /spl mu/m thick and is almost entirely made up of crystalline silicon. When configured as a delay line oscillator, the transducer functions well as a sensor to changes in environmental conditions.

Capacitive micromachined interdigital transducers

Capacitive Micromachined Ultrasonic Transducers (CMUTs) have become very popular for medical imaging and sonar applications during the last decade. They can compete with piezoelectric transducers in terms of efficiency and bandwidth. Recently, it has been noticed that these devices couple energy into silicon wafer in addition to the immersion medium. By placing the CMUTs on a substrate in an interdigitated configuration, Lamb wave or Rayleigh wave modes can be excited with very high efficiency without a need for any piezoelectric material. In this study, the acoustic power coupled into the silicon substrate as well as the acoustic impedance of the CMUT membrane are calculated by using finite element analysis. Normal mode theory is used to find the distribution of the acoustic power among different Lamb wave modes. For low frequency (1 MHz) devices, the lowest order antisymmetric (A0) mode Lamb wave is the dominant mode in the substrate. For high frequency devices (100 MHz), interdigital CMUTs excite Rayle...