MEMS Electromagnetic Modeling
Applications of micro-electromechanical systems (MEMS) in automobiles are fairly recent. The two most common examples of MEMS use in automobiles are in crash sensing for airbag deployment, and in manifold absolute pressure sensing. There are, however, several other areas where MEMS devices are expected to replace more traditional technologies within the next few years. MEMS devices/systems (e.g. sensors and actuators) have several vital advantages over more traditional technologies. Because of highly reliable batch processing techniques, large volumes of highly uniform devices can be produced at relatively low unit cost. Since MEMS have virtually no moving parts to wear out, they are extremely reliable and long lasting. With the advent of microprocessor compatibility imposed on many automotive sensor/actuator applications, silicon based MEMS sensors will have a very efficient interaction with the controlling microprocessors.
With increasing use of MEMS devices in automotive applications, modeling and simulation of these devices will become more and more important. Although CAE has a very important place in the development cycle of the automobile, the CAE needed for MEMS has some significant differences. Because of the much smaller dimensions of MEMS devices, forces that are normally neglected in macro-structural CAE cannot be neglected any more. Behavior of material in bulk form is quite different from that in thin film form. MEMS devices exhibit the interaction of mechanical, electrical, magnetic, thermal, and other physical phenomena, and therefore simulation of these devices has to be able to capture this multi-physical interaction. This paper will discuss all these important issues that need to be addressed in MEMS modeling and illustrate them through actual simulation cases of MEMS devices.
INTRODUCTION
MEMS is an acronym that stands for micro-electromechanical systems. MEMS contain components of size ranging from 1 micrometer to 1 millimeter. The core elements in MEMS generally consist of two main types of components: sensors and actuators. Actions of these devices are initiated by chemical, thermal, electrical, or magnetic means. Thus, two characteristics that separate MEMS systems from more traditional engineering systems are their size and the multi-physical governing principles.
Applications of MEMS devices can be found in a variety of industries such as biomedical, aerospace, instrumentation, automotive, etc. New developments and applications in the MEMS field are finding their way into many of these industries everyday. Good performance-cost ratios are responsible for the popularity of industrial MEMS devices in today's market. They are innately faster as speed usually scales with size. Most MEMS devices operate on very little power so a high level of power consumption is not an issue. These devices also reflect high levels of accuracy, reliability, and reproducibility. In the manufacturing of MEMS, Batch Microfabrication techniques have led to newly developed effects and products in the MEMS market all over the world.
In the automotive industry MEMS applications hold a lot of promise. Automotive components need to be produced in very large volumes not only from a demands point of view, but also from the necessity of recovering the initial investments. Operating lifetimes of up to 10 years along with very low unit prices are also required. These qualities are inherent in MEMS devices. Due to the progress made in batch manufacturing of MEMS, large volumes of highly uniform devices can be created at relatively low cost.
Also, since MEMS devices have very few or no moving parts, they don't wear out and as a result are very reliable.
Two areas where MEMS devices are currently being used in automobiles are in engine control and airbag deployment. Manifold absolute pressure sensors are used in engine control of many vehicles and silicon accelerometers are used to trigger airbags. Apart from these two MEMS devices that are on production vehicles today, there are many devices that are at various stages of development. Some of these will be used on production vehicles in the very near future. Application areas where MEMS devices may be seen in standard production include wheel speed sensing, yaw-rate sensing, active safety and steering, navigation, seatbelt pre-tensioning, road condition monitoring, etc.
Modeling and simulation of MEMS devices is an important step in the design, development and successful application of MEMS products. While modeling techniques for large devices and systems are well established and tested, modeling and simulation of microdevices pose some unique challenges. Thus, engineers trained in modeling traditional automotive components need to be aware of these vital differences when they try to model MEMS devices. In the next few sections, some of these challenges are highlighted and some of the commonly used modeling tools for MEMS work are described. In the end, two very commonly used MEMS devices are analyzed for their behavior in response to external input. The devices chosen are common components in many MEMS applications and are also useful for automotive MEMS devices.
MODELING AND SIMULATION OF MEMS
Modeling and simulation methods for MEMS can be broadly divided into three different levels of approximation. The lowest level, sometimes called the "Geometry level" (see Figure1), is closest to reality. At this level the physical phenomena are described through partial differential equations. Numerical solution of these equations is achieved through techniques such as the finite element method or the boundary element method. When done right, these methods yield very accurate results of residual stresses, heat distribution, distortions, natural frequencies, etc. Because these methods also require considerable computational resources, they are used only when detailed data about the MEMS structures are needed. At a higher level of abstraction, models used are called "network models." Examples of network models are circuit simulators or multi-body simulators.
Figure 1: Different levels of MEMS modeling
These can be used to represent all the MEMS subcomponents and are represented through sets of ordinary differential equations. The solutions of these equations provide information about the transient behavior of the system. At this level, the physical behavior of the system and the interrelation of its subcomponents are still well captured.
At the highest level of abstraction, MEMS models use block diagrams with signal inputs and outputs (similar in approach to signal processing and controls). The level to be used in a particular situation is determined by the kind of results one is expecting to obtain. For the rest of this paper we will talk most about the "Geometry level" modeling.
MEMS CAD
There are a lot of discussions related to MEMS modeling and simulation in the industry, and one of the reasons is because it involves many diverse problems with varying spatial and temporal scale. Moreover, simulating MEMS devices as a whole gets complicated especially in a new design. The design might involve unfamiliar physics, such as lumped models and continuum approximations and might be difficult to handle as one piece. There are several commercial tools available for modeling the behavior of MEMS devices at the geometry level. In these tools, analysis methods such as the Finite Element, Boundary Element or the coupled Finite and Boundary element method are usually used. CAD tools nowadays give the user the option of choosing the standard MEMS fabrication process involved, such as Bulk Micromachining, Surface Micromachining, LIGA, etc. With this information, the standard process template is retrieved and along with the planar geometry information from the mask layout editor – a three dimensional structure is created. Simulation of any problem is now possible, such as thermomechanical, electrostatic, and completely coupled thermo-electromechanical analysis. In the next few paragraphs some of the CAD tools are discussed. The list considered here is by no means complete. There are other tools which we are not able to mention here.
For many decades IntelliSense Software Corporation provided design and development services as well as software tools to MEMS developers. Intellisuite, a product of IntelliSense, is a dedicated software for a total MEMS solution. With MEMaterial, AnisE, IntelliMask, and IntelliFAB, it consists of a whole suite of process tools comprising of material databases, process characterization and optimization tools, anisotropic etch modeling, layout, and process design. The analysis modules include thermoelectromechanical, Packaging, PiezoMEMS, ElectroMagnetic & RF MEMS, and BioMEM & Microfluids. But the underlying FEA solver is ABAQUS, linking to IntelliSense Software's in-house code for coupled analysis. By incorporating process templates, thin-film material engineering, mask layout, and device analysis within a single tool, IntelliSuite enables MEMS engineers to optimize devices prior to fabrication, reducing prototype development time and cutting manufacturing costs.
MEMSCAP provides MEMS Pro that enables designers to create designs and couple them with electronic systems that drive the parts. ANSYS Multiphysics allows for coupled energy domain modeling and simulation with applications in microfluidics technology, high-frequencyelectromagnetics, and electrostatic-structural coupling. ABAQUS Piezo allows the modeling of piezoelectric behavior in MEMS devices. Since the fields that need to be addressed in MEMS modeling are as diverse as the devices that operate on their principles, MEMS modeling in different industries is yet to be standardized.
Important issues in MEMS modeling
As has been mentioned before, two critical aspects of MEMS are their small length scales and their multi-physics nature. As a result, in modeling MEMS devices one needs to be careful about issues that don't receive any attention in the traditional approaches of modeling continuum material.
MEMS devices function by converting one or more physical or chemical conditions into electrical signals (in case of sensors) or vice versa (in case of actuators). For example, a pressure sensor may convert the change in pressure into an output voltage and micropumps convert a voltage into output pressure. Modeling of these devices, say for example a cantilever plate or pressure membrane, may involve electrostatic, electromagnetic, and piezoelectric phenomena to name a few. These characteristics are often coupled with static or dynamic mechanical response of the micro-structure .
In MEMS modeling of electrostatically actuated devices such as beams, diaphragms, comb-drives, and nano-tweezers [13], one has to be wary of proper calibration [14] of the device according to the pull-in voltage. The maximum deflection achieved before the onset of the instability is referred to as the pull-in distance. Because this distance is so small, this value is a limiting factor in the design of all systems of this kind.
In modeling micro fluidic devices such as micro pumps or micro valves, one needs to account for both compressible and incompressible fluid dynamics in the micron size-domain. Compressible fluid in that regime may no longer be considered continuum flow. Therefore, simulation algorithms for viscous flow and/or low-pressure damping are needed to assist the design of these devices.
Most commercial Finite element codes are developed using the concepts of continuum mechanics and are tested for bulk material (properties). They cannot be directly applied for MEMS devices. Many MEMS devices are electroactive or magnetoactive in nature. For example, mechanical responses of piezoelectric materials are a result of applied voltage. These types of behavior are not modeled within most general-purpose commercial software packages. MEMS materials are rarely a continuum, because they often consist of layers of thin films that are bonded to dissimilar materials. Thin-film material databases are very important to any MEMS designer. They give people ready access to material properties such as Young's Modulus and dielectric constants. These differ from properties of bulk materials and tend to vary significantly as a function of machine settings from which they are being fabricated. Sometimes faulty designs and structural or particle misbehavior may take place because MEMS structures are more sensitive to faulty material properties than bulk structures. Even with a correct model simulation, results received may be severely inaccurate upon the use of incorrect material data.
Microfabrication processes play an important role in the behavior of MEMS devices. In many cases fabrication related device feature affects the performance quite significantly. Fabrication may induce residual stresses and/or other effects on MEMS devices that cannot be ignored (like bulk materials) due to the size of such devices. That is why, in modeling the behavior of these devices, the effects of the fabrication process through a process model needs to be considered as well. MEMS fabrication can be of different types such as micromachining, MUMPs (Multi-User MEMS processes), etc. Micromachining is the bulk anisotropic etching of crystalline silicon. The angle between crystalline planes or the etching angle is naturally 54.74 degrees. It is a combination of fabrication processes that are used for example to create MEMS devices and Lab-on-chip systems. Today, complex systems such as micromirrors, comb-drives, cantilever arrays, microgears, and microfluidic flow sensors are all created using a few of these basic techniques. For resistors, diodes, and transistors, dopants are introduced to form electrically active regions. Bulk micromachining has been the standard for producing capacitive and piezoresistive pressure sensors. MUMPs is a program that provides MEMS fabrication processes to the industry. PolyMUMPs is a three-layer polysilicon surface micromachining process. MetalMUMPs is an electroplated nickel process, and SOIMUMPs is a silicon-on-insulator micromachining process.
Cesar Augusto Suarez
CI 17394384
CAF
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