MEMS Resonator
In the past, MEMS devices were typically wire-bonded on a circuit board to separately packaged CMOS control chips. This approach is now being replaced by more advanced hybrid multi-chip and system-on-chip (SoC) solutions that combine MEMS and CMOS functions on a single substrate. For many applications, factors such as chip real estate, packaging & testing costs, or device sensitivity to parastic capacitances may drive the design toward a system on chip solution. SoC devices include both monolithic MEMS and IC integration (where the MEMS and IC structures are built on the same substrate) and heterogeneous MEMS and IC integration (in which the MEMS and IC structures are built on separate substrates and subsequently merged onto a single substrate).
SoC solutions are more complex than hybrid multi-chip integration schemes (where separate MEMS and CMOS devices are bonded together), due to the added process steps and more complex device structure. Development times (and reaching production yield) tend to be longer for SoC devices due to this added process complexity. Process modeling and virtual device fabrication are techniques that can mitigate this added complexity of developing SoC-based MEMS devices.
SEMulator3D® is a powerful 3D semiconductor and MEMS process modeling platform that is valuable in MEMS product development. SEMulator3D uses input design data and process flow information to create a virtual equivalent of a fabricated MEMS device. Most importantly, it can highlight the complex interactions between designs and integrated process flows, and identify process problems before chips are sent to fabrication.
In this example, we will highlight the value of process modeling in MEMS design by creating a virtual model of a single-substrate, integrated MEMS resonator. The design of this resonator is based upon information obtained from a publicly-available patent filing.
As stated in the patent, the integrated MEMS resonator is first created by partially forming the resonator at high temperature on the substrate. The surrounding circuitry is then formed in separate process steps on the wafer or die. Some remaining steps to form the resonator are subsequently performed on the substrate at a lower temperature.
Figure 1: SEM image of MEMS resonator from patent US20070072327A1
Figure 2: SEMulator3D model of a similar resonator with metal layers on die. Each electrical net is highlighted in a different color.
Figure 1 displays a SEM image of the MEMS resonator found in the patent application. The SEMulator3D model of the device (Figure 2) was created based upon an interpretation of the processing steps described in the patent document. Notice that the process steps included both the MEMS device and the electrical net, all on a single die.
Process model calibration can be completed by entering device process steps and other information into SEMulator3D. A 3D process simulation of the device (at each process step) can be quickly generated, providing an in-depth, visual understanding of the device at every fabrication step.
SEMulator3D can also be used to create large cross sectional and device layer images during process modeling. The resulting model can also be probed for metrology data, such as net connectivity and topography. In addition, layer geometry in SEMulator3D can be meshed and exported in a wide variety of formats.
SEMulator3D geometry can also be modeled with surface and volume meshes and exported to industry-standard field-solvers. SEMulator3D uses two sophisticated modeling methods: Voxel Modeling, a fast, robust digital approach, and Surface Evolution, an analog approach capable of modeling a wide range of physical process behavior with great accuracy. SEMulator3D is able to discretize the voxel model with mesh elements, to generate simulation-quality meshes. Both triangle surface and tetrahedral volume meshes can be exported from SEMulator3D to FEA modeling software such as CoventorWare. A meshed view of our sample resonator can be found in Figure 3.
Figure 3: View of meshed resonator in SEMulator3D
SEMuator3D is well-suited to the design of hybrid multi-chip and SoC MEMS devices. It does not distinguish between a two-die or integrated MEMS device, and can model the complex process sequences used in the most advanced MEMS technologies. In addition, SEMulator3D is able to generate highly accurate models of a MEMS device based upon the actual fabrication process, rather than the idealized geometry customarily used in traditional finite element analysis (FEA). The geometric fidelity of the SEMulator3D model greatly improves FEA simulation accuracy.
References:
- Microsystems & Nanoengineering, “Integrating MEMS and ICs”, Andreas C. Fischer, Fredrik Forsberg, Martin Lapisa, Simon J. Bleiker, Göran Stemme, Niclas Roxhed & Frank Niklaus, Volume 1, Article number: 15005 (2015)
- U.S. Patent 20070072327A1, Inventor: Jason W. Weigold, Analog Devices, Inc