Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common substrate (typically a silicon substrate) through micro fabrication technology. MEMS promises to bring together silicon-based microelectronics with micromachining technology, making possible the realization of complete systems-on-a-chip. The prospect of making microsensors and micro actuators using well-developed and highly efficient semiconductor fabrication technology is a powerful argument for MEMS.
A MEMS device generally has two major components integrated together. The first component is the electronics which are fabricated using usual integrated circuit (IC) processes (e.g., CMOS, Bipolar, or BICMOS processes), while the second component is the micromechanical parts which are fabricated using compatible micromachining processes. Micromachining refers to the fabrication process to make microscopic structures using selective adding tools (deposition, bonding, injection etc.) and subtracting tools (chemical etching, plasma etching, laser ablation, ion-milling). When combined with patterning tools, the available adding tools and subtracting tools make micromachining a powerful method to create exciting new micro devices.
One example of MEMS devices is micromachined ultrasonic transducers (MUT). An ultrasound transducer performs a chain of energy transformation to realize its function of a transducer. In its receiving mode, the acoustic energy of ultrasound waves propagating in a medium where the transducer is placed is transformed to mechanical energy of a movable part (conventionally a vibrating membrane) in the transducer. The motion of the movable part is then transformed to a detectable electromagnetic (usually electrical) signal. In its transmitter mode, the reverse chain of energy transformation takes place.
Various types of ultrasonic transducers have been developed for transmitting and receiving ultrasound waves. Ultrasonic transducers can operate in a variety of media including liquids, solids and gas. These transducers are commonly used for medical imaging for diagnostics and therapy, biochemical imaging, non-destructive evaluation of materials, sonar, communication, proximity sensors, gas flow measurements, in-situ process monitoring, acoustic microscopy, underwater sensing and imaging, and many others. In addition to discrete ultrasound transducers, ultrasound transducer arrays containing multiple transducers have been also developed. For example, two-dimensional arrays of ultrasound transducers are developed for imaging applications.
Compared to the widely used piezoelectric (PZT) ultrasound transducer, the MUT has advantages in device fabrication method, bandwidth and operation temperature. For example, making arrays of conventional PZT transducers involves dicing and connecting individual piezoelectric elements. This process is fraught with difficulties and high expenses, not to mention the large input impedance mismatch problem presented by such elements to transmit/receiving electronics. In comparison, the micromachining techniques used in fabricating MUTs are much more capable in making such arrays. In terms of performance, the MUT demonstrates a dynamic performance comparable to that of PZT transducers. For these reasons, the MUT is becoming an attractive alternative to the piezoelectric (PZT) ultrasound transducers.
Among the several types of MUTs, the capacitive micromachined ultrasonic transducer (cMUT), which uses electrostatic transducers, is widely used. Other MUTs using piezoelectric (pMUT) and magnetic (mMUT) transducers are also adopted.
FIG. 1 shows a cross-sectional view of a basic structure of a prior art cMUT having multiple cells. Four cells are shown. The cMUT is built on a substrate 10 and has a parallel plate capacitor consisting of a rigid bottom electrode 12 and a top electrode 14 residing on or within a flexible membrane 16 that is used to transmit or receive an acoustic wave in the adjacent medium. The flexible membrane 16 in each cell is supported by the insulation wall or posts 18. In practice, a cMUT is made of many cells all connected in parallel. A DC bias voltage is applied between the electrodes 12 and 14 to deflect the membrane 16 to an optimal position for cMUT operation, usually with the goal of maximizing sensitivity and bandwidth. During transmission an AC signal is applied to the transducer. The alternating electrostatic force between the top electrode and the bottom electrode actuates the membrane 16 in order to deliver acoustic energy into the medium (not shown) surrounding the cMUT. During reception the impinging acoustic wave vibrates the membrane 16, thus altering the capacitance between the two electrodes. An electronic circuit detects this capacitance change. Alternatively the membrane 16 can be actuated and the displacement of the membranes detected using piezoelectric (pMUT) and magnetic (mMUT) transducers.
Methods of fabrication for making a cMUT shown in FIG. 1 have been developed. Exemplary methods are disclosed in U.S. Pat. Nos. 6,632,178 and 6, 958,255.
There are drawbacks in the cMUTs of the prior art structures and methods. Many of these drawbacks relate to the fact that the cMUTs are made of many individual cells and the cMUT membranes are clamped or fixed on their edges. Examples of the drawbacks Are listed below.
(1) The average displacement of the membranes is small because of the clamped edges. As a result both the device transmission and reception performance are poor.
(2) Surface areas occupied by the clamped areas (e.g., edges) and the walls or posts are non-active, and this reduces the device fill factor and the overall efficiency.
(3) Anchor areas introduce a parasitic capacitance which decreases the device sensitivity.
(4) The anchor pattern within the surface of the cMUT element may cause ultrasonic wave interference which limits the device bandwidth.
(5) The non-uniform displacement of the membrane may disturb the ultrasonic wave pattern. For example, the non-uniform displacement may affect the ultrasonic beam pattern emitted from the transducer surface and also cause acoustic cross coupling through the transducer surface.
(6) The resonant frequencies of individual cells in the same cMUT element may be different between each other because of the process variation. This causes phase differences of the membrane motion among different cells in the same cMUT element during operation. As a result, the sum of the average displacement of the cMUT element may degrade dramatically. This problem degrades the device performance especially when the cMUT works in a high quality factor (Q-factor) condition, for example in air.
(7) The acoustic energy can couple into the transducer substrate through supporting walls and cause undesired effects such as acoustic cross coupling between the cMUT elements. An effort to reduce the cross-coupling through the substrate by introducing materials with desired acoustic properties may require occupation of extra space between elements.
The above problems also exist in the pMUT and mMUT of the prior art since they have a similar structure as the cMUT as shown in FIG. 1.
Another method of fabrication for making a cMUT device having a compliant support structure built on the substrate to support the membrane is disclosed in the U.S. Pat. No. 7,030,536. Compared to the conventional cMUT structure shown in FIG. 1, the structure disclosed in U.S. Pat. No. 7,030,536 uses a compliant support structure in place of the conventional insulation wall 18 for fastening perimeter ends of the membrane 16. Because a relatively complex compliant support structure takes the place of the simple and narrow insulation wall 18 in FIG. 1 to become the supporting peripherals, there would be a heightened challenge to make the inactive areas occupied by these peripheral support structures according to that design. That patent further suggests making supplemental electrodes on the compliant support structures to reduce the inactive areas occupied by the compliant support structures. There is however no indication that such a design would solve the above identified problems, or whether it even works at all.
In general, MUTs described above belong to a type of MEMS devices that have a movable mechanical part for energy transformation. Fabrication of such a movable part and its integration with the other aspects of MEMS fabrication poses a challenge. Due to the importance of these MEMS devices such as MUTs, it is desirable to improve the technology in terms of performance, functionality, and manufacturability.