Silicon-Germanium MEMS Accelerometer
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Virtual fabrication with SEMulator3D enables visualization of complex MEMS structures and process flows. The example demonstrated here is a capacitive MEMS accelerometer fabricated with IMEC’s poly-SiGe MEMS technology. Monolithic integration via this post-CMOS MEMS process results in fewer unwanted parasitics, better performance and smaller form factor for the accelerometer. However, this type of integration scheme also has implications on the process integration, manufacturing and yield.
The single-axis accelerometer model generated by SEMulator3D is shown below with different views. SEMulator3D takes into account the 2D geometry dimensions, materials and process flow information when automatically building the model. The build takes just minutes, which is extraordinarily fast given the level of structural detail. Users can rotate, zoom, pan, scale and slice the model to visualize in 3D and eventually verify and optimize the design as well as process flow.
|Figure 1: Isometric and cross sectional views of SEMulator3D accelerometer model|
The accelerometer has a center proof mass suspended by four folding beams fixed on anchors. There are two groups of stationary comb fingers on each side of the proof mass. These four groups of comb fingers can be connected and routed such that a differential capacitive sensing mechanism is applied to relate the motion of the proof mass to the output electrical signal due to input acceleration or inertia. Numerous side fingers connected to the center proof mass collectively act as a sensing electrode and provide output signal to the CMOS interface circuitry. In addition, two sets of comb finger structures are formed on both ends of the proof mass for the general purpose of self-testing.
To protect the device from structural damages in harsh conditions such as shock events, four shock protectors are specifically designed to limit the movement range of the proof mass with respect to the substrate. All of the above elements are fabricated out of the mechanical poly-SiGe layer. Five electrical pads (left side in device image above) are formed to provide additional probing and testing capabilities.
|SEM image and device layout are courtesy of L.Wen and R.Puers, Katholieke Universiteit Leuven ESAT-MICAS, Leuven Belgium.|
Details of interconnects between MEMS and IC devices are captured by carefully adjusting and positioning a cross-sectional cut through the interface between different layers. The process enables metal connection through the SiOx isolation and SiC protection layers, and the dimension of each W connection line is 0.5um*0.5um*1um (depth). The SiGe electrical line from the MEMS accelerometer is connected to the CMOS top metal (Al) line underneath.
SEMulator3D also models the shock protector and release holes on the top left corner of the device.
L. Wen, K. Wouters, L. Haspeslagh, A. Witvrouw, and R. Puers, “A Comb Based In-Plane SiGe Capacitive Accelerometer for Above-IC Integration,” MicroMechanics Europe’10, Enschede, Netherlands, Sep. 26-29, 2010.