FEATURE MEMS MANUFACTURING MEMs devices: A microscopic view
Carey Robertson a Director of Product Marketing at Mentor Graphics Corporation, explores MEMS technology and manufacturing in the microscale and the latest processes to achieve this
M
icro-electro-mechanical systems (MEMS) are fascinating devices,
even more so to engineers. MEMS devices are micro-scaled mechanisms designed for two major applications: • Sensing—changes in sound, motion, pressure, and temperature. Sensing can include not just physical movement, but also vibration, acoustic waves, fluid waves, light waves, heat, and air pressure.
• Actuating—conversion and management of light projection/reception, radio frequency signal processing, and fluid management. Actuation can include detection, filtering, conversion, and modulation. When you tilt your mobile phone sideways, and the screen repositions itself, that’s because a MEMS device recognises the movement from vertical to horizontal. MEMS functionality has been incorporated across a wide range of industrial and consumer markets, such as automobile safety and control systems, smartphones, tablets, video game controllers, drug delivery, microphones, gas and chemical sensors, and much, much more. These markets are continuing to grow as new innovations and applications emerge (Figure 1). In place of traditional integrated circuit
(IC) components like transistors, resistors, and diodes, MEMS mechanisms contain highly calibrated physical structures such as springs, screws, gears, and plates (Figure 2). MEMS devices also use different types of processing. To achieve the physical dimensions, techniques such as surface machining, micro-machining, bulk machining, and moulding are required. Historically, MEMS devices have been
manufactured using a variety of highly- specialised processes that are customised for each device type. This wide assortment of MEMS fabrication processes results in higher market production costs and a longer time to market schedule. As the market moves towards the integration of MEMS devices into high-volume CMOS IC manufacturing, the MEMS technology faces the challenge of shortening development time, reducing costs, and meeting increasingly complex performance and reliability requirements.
18 SPRING 2015 | MICROMATTERS
Incorporating MEMS design directly into the CMOS design flow offers the best chance to achieve these goals. What is needed to make this integration happen? Component library. A MEMS component
library contains proven physical parametrised primitives that are fabrication-ready. Designers can select primitives and define parameters (length, radius, thickness, etc.) to instantiate a component in the layout. The library is also used to supply these material properties to subsequent simulations.
DESIGN TOOLS An effective MEMS design tool must provide a graphical user interface to enable designers to initiate instantiation, create new blocks, connect components, and add application-specific structures and modifications. This same tool should also provide a layout view to display the MEMs design and generate 3D views. Process simulation tool. This
Figure 1:
The MEMS market is predicted to grow substantially over the next few years (image courtesy of Yole Développement).
the effect of the process on the final physical geometry of the MEMS device. However, since they are based on actual physical models, they are often very time-consuming. Productivity tools. This category
includes automatic layout generation tools that take abstract MEMS system descriptions as input and generate detailed multi-component physical layouts, as well as macro model generators, which convert detailed physical verification results into compact models for analysis and system-level verification. Linear, iterative design flows can
“When you tilt your
mobile phone sideways, and the screen repositions itself, that’s because a
tool models the results of the actual process flow based upon process settings and physical simulation of the process, such as diffusion, growth or etching, which is unique to the target foundry and manufacturing line. It is similar in nature to IC lithography simulation tools. Designers use this tool to conduct
MEMS device recognises the movement from vertical to horizontal ...”
quickly become a time sink for the MEMS design team. Structured design flows that can efficiently exchange information between the schematics, process flows, layout, 3D finite element analysis (FEA) and boundary element analysis (BEA), and package analysis can allow the MEMS designer to exchange information with process, design, electronics, packaging, integration, and software engineers. Verification of MEMS devices is quite different from traditional integrated circuit design verification. The IC world typically uses layout vs. schematic (LVS) and design rule checking (DRC) techniques for physical verification. While DRC can be used in for MEMS devices, support for curves, beziers, and all-angle geometries are needed. Fortunately, by using advanced physical verification technologies like equation-based DRC, rules can be written that accurately describe real
design errors by translating the curves into a gridded structure like GDSII. LVS is a different paradigm, due to the
simulations using multi-physics simulators, which conduct simultaneous simulations of the device in different domains (e.g., mechanical and electrical). They can also refine device behavioural models, and identify and correct design errors before test fabrication. These simulations help designers understand
inherent 3D nature of the design. The first action of LVS is to “extract” the layout and develop a device and connectivity model of the entire design. This step requires the LVS tool to recognise the complex 3D structures that exist in the design, and differentiate those from the structures, wires, etc. that connect the devices to each other, to electrical sensors, and to the outside
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