Silicon-Germanium MEMS Accelerometer
Virtual fabrication with SEMulator3D enables visualization of complex MEMS structures and process flows. This example demonstrates the capabilities of SEMulator3D for process modeling, using a capacitive MEMS accelerometer fabricated with imec’s poly-SiGe MEMS technology. Monolithic integration using the imec post-CMOS MEMS process results in fewer unwanted parasitics, better performance and smaller form factor for the accelerometer. However, the integration scheme has implications for process development, manufacturing and yield.
A single-axis accelerometer model was developed in SEMulator3D. Isometric and cross sectional views are shown in Figure 1 (below). SEMulator3D considers the 2D geometry dimensions, materials used and process flow information when automatically building the model. It takes just minutes to build the 3D model, which is extraordinarily fast given the level of structural detail. Users can rotate, zoom, pan, scale and slice the model in a 3D visualization mode. Subsequently, they can verify and optimize the design as well as the complete 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 to form a differential capacitive sensing mechanism. The motion of the proof mass (due to input acceleration or inertia) changes the sensed capacitance, which is reflected in the output electrical signal. 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, Figure 1 above) are formed to provide additional probing and testing capabilities.
SEMulator3D can construct highly predictive and accurate 3D process models that reflect the complex interactions between designs and integrated process flows. The 3D process model is built using a series of unit process steps (some requiring masks) to produce a highly accurate “virtual” 3D structure. In Figures 2 and 3, the single-axis accelerometer model built in SEMulator3D is compared to an actual SEM photo of a similar device. The actual SEM photomicrographs (courtesy of Katholieke Universiteit Leuven) can be seen on the right, with the equivalent SEMulator3D model on the left.
|Figure 2: SEMulator3D model view of accelerometer||Figure 3: SEM picture of similar device|
|SEM image and device layout are courtesy of L.Wen and R.Puers, Katholieke Universiteit Leuven ESAT-MICAS, Leuven Belgium.|
Details about the interconnects between the MEMS and IC devices can be visualized by carefully adjusting and positioning a cross-sectional cut in SEMulator3D. A cross-sectional view in SEMulator3D (Figure 4) displays the interface between the different layers. In this example, a metal connection is shown through the SiOx isolation and SiC protection layers. The SiGe electrical line from the MEMS accelerometer is visibly connected to the CMOS top metal (Al) line in the SEMulator3D cross-sectional view.
A 3D model view in SEMulator3D (Figure 5) highlights the shock protector, along with the release holes in the center of the image (left side of the device).
|Figure 4: Scaled (5X in Z-dir) zoom-in view of SEMulator3D model captures interconnections between MEMS and IC devices.||Figure 5: Modeled view of shock protector and release holes on the proof mass|
Figure 6: Animation of predictive 3D process model for MEMS accelerometer
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.