Microengineering Aerospace Systems
Henry Helvajian, editor
Chapter 1: Introduction to MEMS (cont.)
M. Mehregany and S. Roy
1.5 Trends in MEMS Technology
MEMS technology is extending and increasing the ability to both perceive and control the environment by merging the capabilities of sensors and actuators with information systems. Future MEMS applications will be driven by processes that enable greater functionality through higher levels of electronic-mechanical integration and greater numbers of mechanical components working either alone or together to enable a complex action. These process developments, in turn, will be paced by investments in the development of new materials, device and systems design, fabrication techniques, packaging/assembly methods, and test and characterization tools.
1.5.1 Design and Simulation
MEMS is more demanding of design aids than microelectronics production. Most industrial designs of physical sensors today are based on detailed finite-element modeling of the mechanical microstructures using software available for conventional mechanics.
MEMS requires new drawing and layout tools to generate the patterns that will be used to add or remove material during processing. In addition, MEMS requires a number of different modeling tools, including simulators for mechanical deformation, electrostatic fields, mechanical forces, electromagnetic fields, material properties, and electronic devices. MEMS also needs the connective algorithms to reconcile and blend results from all the different simulators.
As devices become more complex and multiple simulators are involved, the complexity of both the simulations and the coupling increases considerably. Traditional modeling techniques become impractical and may even fail. Radically new approaches to modeling and simulation for the many physical effects and different MEMS functions have to be developed.
1.5.2 Materials Issues
An extensive, well-documented materials database that meets the requirements of MEMS development is essential for continuing progress in the field. Many of the new material property simulators will need new models and data to relate process parameters to material properties relevant for MEMS design.
The accuracy of the existing microelectronic device simulators is built on historic and huge amounts of material and device measurements, coupled to carefully controlled process conditions. By knowing the relationship between processing conditions and the resulting material parameters, microelectronics manufacturers can control material properties, and hence, device yields. Circuit designers are typically interested in those material properties that relate to the electronic function of the devices, such as doping levels and dielectric constants.
The material needs of the MEMS field are well recognized but are at a preliminary stage. In addition to single-crystal Si, polysilicon, Si3N4, and SiO2, other materials are being explored for MEMS. Interesting examples include SiC, shape memory alloy (SMA) metals, permalloy, and high-temperature superconductive materials. All these materials possess certain unique properties that, when combined with MEMS technology, make them attractive for certain applications.
A thorough understanding of the material properties of existing MEMS materials is just as important as the development of new materials for MEMS. There are very few reliable measurements of material properties (for example, modulus, residual stress, or reflectivity) relevant to the production of MEMS. The goal of studying the material properties in MEMS, and in thin films generally, is to develop models that relate process parameters to the film microstructure, as well as to the corresponding mechanical, electrical, optical, and thermal properties. Chapter 3 elaborates on the material properties and the required tests to enable a valid database.
1.5.3 Integration with Microelectronics
Future MEMS products will demand yet higher levels of electrical-mechanical integration and more intimate interaction with the physical world. The full potential of MEMS technology will only be realized when microelectronics is merged with the electromechanical components. Integrated microelectronics provides the intelligence to MEMS and allows closed-loop feedback systems, localized signal conditioning, and control of massively parallel actuator arrays.27
Although MEMS fabrication uses many of the materials and processes of semiconductor fabrication, there are important distinctions between the two technologies. The most significant distinctions are in the process recipes (the number, sequence, and type of deposition, removal, and patterning steps used to fabricate devices) and in the end stages of production (bonding of wafers, freeing of parts designed to move, packaging, and test). The fundamental challenge of using semiconductor processes for MEMS fabrication is not so much in the type of processes and materials used, but more in the way those processes and materials are used.
MEMS will need the development of operating conditions on standard semiconductor equipment suited and optimized to the requirements of MEMS. For other processing steps unique to MEMS, the development of new manufacturing equipment and associated processes will be required. Table 1.3 lists some of the specialized process equipment that is required to enhance the manufacturability of MEMS.
Table 1.3. Examples of Process Equipment Specific to MEMS
| Fabrication Technology | Process Equipment |
|---|---|
| Surface micromachining | Release and drying systems to realize free-standing microstructures |
| Bulk micromachining | Dry etching systems to produce deep, 2D free-form geometries with vertical sidewalls in substrates Anisotropic wet etching systems with protection for wafer front sides during etching Bonding and aligning systems to join wafers and perform photolithography on the stacked substrates |
| Micromolding | Batch-plating systems to create metal molds in LIGA process Plastic injection molding systems to create components from metal molds |
