A resonant accelerometer is a device that measures linear acceleration based upon shifts in the resonant frequency of the oscillator. A micromachining technique makes it possible to miniaturize traditional electromechanical sensors, including resonant accelerometers; this possibly ensures the following advantages: low cost, low power consumption, small size, and high shock tolerance. Thus far, however, the performance of MEMS resonant accelerometers is not adequate for strategic and precision navigation applications. Therefore, MEMS resonant accelerometers have been widely studied to improve their performance, and an increase in the scale factor has become the priority in many of these research efforts.
There are two basic approaches to obtaining a higher scale factor: the adoption of narrower and longer oscillating beams that generate greater frequency shift from the same inertial force, and micro-leverages that amplify an inertial force under the same acceleration. Both approaches enable an accelerometer to be endowed with a higher scale factor even in a device of limited size, but they also entail greater complexity in the manufacturing process, as well as a decrease in structural stability. Our resonant accelerometer with a high scale factor maintains its structural stability and robustness. A reliable fabrication process that realized the proposed accelerometer is presented, along with its testing results.
A convective accelerometer uses gas molecules inside the cavity instead of the solid proof mass and other simple layouts of heaters and sensing electrodes. Since there is no moving component, the fabrication process to realize the convective accelerometer is much simpler than others with the moving proof mass. Also, the convective accelerometer provides less troublesome mechanism against mechanical failure.
Various implementations of the convective accelerometer were reported since the first concept of operation of the device was introduced. Since the operation principle of the convective accelerometer is based on the free convection heat transfer, many researches have been focused on the geometry of the package (including the cavity) and sensors as well as the properties of gas medium inside the package to optimize their devices. However, it is also an important factor to design a heater layout to improve the performance of a convective accelerometer. Moreover, the development of an effective fabrication process is also a crucial factor to provide a good stability and high reproducibility as well as having a high yield of the device.
Liquid metal accelerometer
Various types of accelerometers based on microelectromechanical systems (MEMS) have been developed to measure acceleration, velocity, displacement and position in moving systems. Because of their small size, low cost, and low power consumption, they have numerous applications. However, to measure the input acceleration, they require complex signal processing steps, including amplification, filtering and conversion. Furthermore, many of these accelerometers use a proof-mass that is suspended by fixed beams that are complicated to fabricate, and which may also develop mechanical fatigue after long-term use.
To avoid these problems, we propose a MEMS digital accelerometer (MDA) that uses a liquid metal (e.g., mercury) droplet and a microstructured channel etched in photosensitive glass. To simplify device fabrication and to prevent mechanical fatigue, the MDA employs a microscale liquid metal droplet as a proof-mass and measures acceleration using the movement of the liquid metal droplet in the device channel.
To simplify signal processing, an array of electrodes is used to detect the position of the liquid metal droplet; this position is used to calculate the input acceleration. An etching characterization of the photosensitive glass is conducted to determine the optimum etching conditions for channel fabrication. The formation of the microstructured surface during the etching process for the channel enhances the non-wetting behavior of the liquid metal droplet in the channel; as a result the liquid metal droplet can move easily. The effect of the non-wetting behavior can be verified by measuring the sliding angle and the corresponding contact angle hysteresis (CAH).
Liquid metal inertia switch
Many different inertial switches have been developed to detect acceleration in various applications. An inertial system can use the acceleration to trigger a safety mechanism to prevent damage from a sudden impact during operation of the inertial system when the applied acceleration exceeds the threshold level of the inertial switch. Conventionally, to detect acceleration, most inertial switches use a solid-type microscale inertial mass and beams that require a complex fabrication processes, such as multiple lithographic patterning and etching steps. Moreover, the switching mechanism is based on solid-to-solid contact, which can cause reliability problems, such as signal bouncing and contact wear during the switching motion, because the resistance of the contact surface increases with degradation of the contact surface.
Unlike conventional inertial switches, our device made use of the advantageous properties of a liquid-metal (e.g., mercury) droplet. Initial work of our device was reported to verify the device concept and the advantageous properties of the liquid metal droplet such as non-wetting behavior, an effective inertial-proof mass in microscale, and stable electrical contact. Moreover, a configured channel shape with a selectively modified surface was used to control the movement of the liquid metal droplet. The modified channel surface contains microstructures that facilitate movement of the liquid metal droplet by reducing the CAH of the liquid metal droplet inside the channel.
The device concept, its fabrication, and systematically performed experiments for developing the novel inertial switch are reported. Also, the experimental results, device performance, and the characteristic of the selectively modified (i.e., microstructured) surface in our device are discussed.
Liquid metal RF switch
We propose a new type of liquid metal switch which is simpler, more stable and has higher isolation than existing liquid metal switches, and thus improves overall switching reliability and durability. We use a coplanar waveguide (CPW) which consists of one strip line and two grounds as the electrode; fabricating it is simple because only one side of the substrate needs to be patterned.
We use an anisotropically etched silicon (Si) chamber covered by a flexible membrane to confine the droplet, and exploit its high surface tension and electrical properties to cause switching. Initially a signal passes through a CPW line (i.e. the switch is on). To switch signals, pneumatic pressure on the membrane reduces the gap between the bottom surface of the liquid metal droplet and the CPW line; when the gap becomes less than a critical distance, the signal is blocked (i.e. the switch is off). Because of the pressure, the contact area between the liquid metal droplet and CPW is larger and gives higher isolation due to increasing impedance mismatching than that when the droplet merely lays on the CPW. We used a commercial simulation tool to confirm the feasibility of this concept, then fabricated a prototype switch and measured its switching performances.