The uniqueness of this demonstrated device is the implementation of a fully differential sensing scheme with common-centroid wiring of the symmetrically partitioned sensing capacitor groups which also allows offset cancelation for large sensitivity. The motivation is to exploit some particular merits in MetalMUMPs technology, such as the excellent elastic and electrical properties of nickel, good process controllability, robust metal structure, low wiring parasitics and low cost potential. In this paper, a MetalMUMPs capacitive accelerometer employing in-plane sensing mechanism has been designed, fabricated and characterized. In these reported displacement sensing scheme, a half-bridge capacitive sensing circuit is constructed using sidewall capacitance formed by nickel electrodes. Although MetalMUMPs technology is widely used in actuators, due to constrains of its design rules, only few capacitive sensors have been attempted using this process. In the reported metal gyroscope, the larger proof mass results in higher Coriolis force and increases the sensitivity in microgyroscope design.

Compared to polysilicon sensing structures, the metal sensing elements, which normally have thicker structures, allow for larger proof mass, greater sensing capacitance, therefore smaller overall device size with comparable performance. Recently metal sensors such as capacitive gyroscopes have been explored using MetalMUMPs or other adapted processes. Various types of actuators fabricated using MetalMUMPs technology have been reported, of which most take great advantage of the thick nickel structural layer for large actuation force and displacement. Among the many MEMS foundry services, MUMPs provides relatively mature technologies for a variety of MEMS materials including metal. For process controllability and overall low cost of MEMS elements, foundry services are also preferable. Electroplated metal as sensing structural material has been attempted for robust sensors.

One of the current trends in physical sensor technologies is hybrid rather than monolithic integration for optimal sensing element and dedicated electronics, and overall low cost and better performance. The findings in the presented work will provide useful information for design of sensors and actuators such as accelerometers, gyroscopes and electrothermal actuators that are to be fabricated using MetalMUMPs technology. Temperature dependence of the sensing capacitance is also characterized. The sensitivity of the sensor is measured as 0.52 mV/g prior to off-chip amplification. Dynamic characterization of the sensor is performed using a vibration shaker with a high-end commercial calibrating accelerometer as reference. In device characterization, surface profile of the fabricated device is measured using a Veeco surface profilometer and mean and gradient residual stress in the nickel structure are calculated as approximately 94.7 MPa and в€’5.27 MPa/Ојm, respectively.

The MUMPs foundry fabrication process of the sensor allows for high yield, good process consistency and provides 20 Ојm structural thickness of the sensing element, which makes the capacitive sensing eligible. CoventorWare is used in structural and electrical design and simulation of the fully differential accelerometer. The unique structural configuration and common-centriod wiring of the metal electrodes enables a fully differential sensing scheme with robust metal sensing structures. This paper presents a fully differential single-axis accelerometer fabricated using the MetalMUMPs process.

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