Micro Electro Mechanical Systems—comprising sensors and actuators—augment the computational ability of microelectronics to perceive(i.e., sensing) and control (i.e., actuation) our world. MEMS sensors and actuators have been commercialized in a wide range of applications since the 1970’s, including pressure measurements, motion sensing, optical projection, and fluid flow control to name a few. To do their work, sensors and actuators require a transduction mechanism, e.g., realized by a mechanical element in physical sensors. Generally stated, sensor transduction mechanisms convert nonelectrical parameters to electrical ones in a calibrated way, while the reverse is the case for actuators.
Compared to electronic chips, packaging of MEMS devices is more challenging. The goal in the former is to seal the chip from the environment. However, MEMS devices have to be protected from the environment, while also exposed to it for perception (i.e., sensing) and control (i.e., actuation). These two requirements are often contradictory.
Most generally, the MEMS chip and its package have to be co-designed to ensure that the device will meet the expected function, form and cost. Table 1 outlines the evolution of MEMS technology, in the context of which, the packaging technology has evolved and continues so. As volume has grown and price eroded, packaging has moved from a low-volume, custom to a high-volume, standardized domain.
Table 1: Evolution of MEMS technology
Packaging and test are ~30% of the cost of the sensor in consumer applications. The trend is to have the package add minimally to the cost and size of the MEMS chip. The package does not just accommodate the MEMS sensors, but often also the accompanying application specific integrated circuit. A consumer-volume pricing of ~$1 for a single sensor solution is a good rule of thumb, though it can be much less in cases like microphones or more in cases of multichip packaged, multi-sensor solutions like nine-axis motion sensors.
The emergence of Internet of Things (IoT) will indeed force further evolution of MEMS packaging. Flex may become the newly added materials technology and printing the additional fabrication technology, while volume grows toward trillions. Together, they will create the need for new packaging techniques. A new paradigm is likely, one in which the chips are fabricated self-packaged, i.e., integrating the chip and package into a seamless design and fabrication process. In fact, we have been consistently moving toward this new paradigm as MEMS packaging has evolved to support the evolution outlined in Table 1.
Wearables are an important step in the evolution of IoT and a new application domain for MEMS sensors. Wearables have evolved into products that leverage thin, flexible printed circuit boards for better form factor. These conformal form factors can still use consumer MEMS devices, though pressure will continue to build for further reduction of sensor package height and footprint to allow better product look and feel, as well as pack more functionality. A corollary to this point is the likely trend that multi-sensor solutions will be packed into multi-chip packages.
Smart watches are moving rapidly to integrate new sensors for monitoring additional vitals. For example, current smart watches are integrating sensors for heart rate and blood oxygen. For now, the sensors are packaged into the watch’s housing, but as the housing disappears into a flexible, conformal wearable, the sensor packaging approach must evolve as well.
Everything else being equal, wearables will likely be the key IoT driver of MEMS packaging than other applications, e.g., industrial, agricultural, environment, food, etc. These other applications do not seem to present similar packaging miniaturization pressures. Nevertheless, volume growth toward trillions will necessitate new material and process technologies to reduce sensor cost substantially. Without self-packaging (i.e., fabricating the sensor and its package in an integrated process), the packaging and test component of the sensor cost would be a barrier.
Finally, conformality is enabler of many new applications, for example smart labels. Printing is a high-throughput, low-cost process. The combination of flexible substrates and printed electronics and sensors will perhaps provide a new pathway for realizing the trillion-sensor volume driven by IoT. The packaging requirements and strategies to support these developments are yet to be developed.