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FIGURE 5-1 Block diagram of generic packaging requirements.
an impact motor that converts a mechanical resonance into a linear motion (e.g., Lee et al., 1993; Daneman et al., 1995)
a microvalve that converts electrical energy to thermopneumatic force (e.g., Zdeblick, 1996)
a microanemometer that converts fluid flow into an electrical signal (e.g., Tai, 1996)
a flexure beam that converts a mechanical motion to a light modulation (e.g., TI's DMD)
It is vital that the MEMS be designed to be insensitive to unwanted parameters first and then that the package be designed to admit the desired signal variables (e.g., fluid, thermal, inertial, or optical) with minimal distortion, while screening out unwanted variables. At the same time, reasonable cost, robustness for the intended application, and good manufacturing characteristics, including acceptable AP&T properties, must be achieved. These engineering problems must be solved in order for MEMS to compete successfully with alternative system designs.
The interfaces between a MEMS component and its operating environment are often troublesome and may present considerable design and manufacturing challenges. Signals admitted to the MEMS package may come from many sources, including electrical, thermal, inertial, fluid, chemical, and optical domains. Output can also vary greatly. Examples include electrical, optical, mechanical, chemical, hydraulic, and magnetic signals (Figure 5-2). Conduits into and out of the packaged MEMS for these signals must be designed to avoid distortion.
The difficulties of designing interfaces vary with the nature of the MEMS. Accelerometer interfacing is relatively easy because it requires only proper orientation and rigid mounting. Interfacing a pressure-sensing MEMS is usually harder because it requires resistance to corrosion from the working fluid and protection against excessive pressures. Chemical-sensor interfacing is inherently difficult, and chemical sensors often fail to provide good long-term stability. Interface considerations can be conceptually segregated into considerations of the chip package, the package environment, and the chip environment. Considerations for interfaces in the biomedical, optical, electric power, fluid, and mechanical domains are discussed below.
The long-term impact of MEMS in the biomedical field is likely to be very strong. One application that has already
FIGURE 5-2 Schematic diagram summarizing various input/output modalities for MEMS systems. Source: Adapted from a figure by H. Zappe, The IBM Almaden Research Center.