Advances make MEMS sensors easier to integrate

-February 19, 2016

The integration of microeletromechanical systems (MEMS) with classic CMOS processes has long been a challenge, with only incremental gains over the years. However, now that MEMS sensors are foundational to the IoT, there is a renewed sense of urgency around solving the MEMS integration conundrum.

Also, as wearables and the healthcare industries now turn to ultra-low-power devices with a smaller footprint, this only increases the pressure for sensor vendors to create smarter MEMS products that address the cost, footprint, and power issues facing the IoT industry at large.

To help with this, the MEMS industry is tackling the integration challenges head on at the package, wafer, and die levels, respectively, starting with multiple MEMS sensors that are now being integrated into either smart miniaturized modules or in system-in-package (SiP) devices.

SiP integration

Some companies are now looking into packaging bare-die MEMS sensors onto SiP devices. For now, however, more modest SiP solutions for MEMS parts are available, and they offer sensors like accelerometers and the interface chip housed in the same package.

These basic SiPs are mostly found in automotive safety applications. NXP’s MPXY8300 is a SiP solution for tire pressure monitoring sensors (TPMS) that incorporates an accelerometer, pressure sensor, 8-bit MCU, and RF transmitter.

However, medical and wearable devices are pushing for smaller and more efficient SiPs that support aggressive form factors and low-power operations. Take ON Semiconductor, for instance, which recently unveiled its Struix SiP with advanced die stacking for portable medical devices like glucose monitors (Figure 1).

Figure 1 ON Semiconductor’s Struix SiP for precision sensing packages an analog front end (AFE) along with an ARM Cortex-M3 low-power 32-bit processor.

Smart modules

MEMS sensor modules kick in as a good option at the point where integrated SiP devices hit physical barriers (Figure 2). Some MEMS companies are pushing the envelope for heterogeneous integration in a single module that incorporates ICs, MEMS, and passives using advanced packaging.

Figure 2 Fairchild's motion tracking module FMT1000 for industrial apps includes accelerometers, gyroscopes, magnetometers, 10-ppm crystal, and an MCU.

The smart modules are raising the bar for package level integration by offering stable interface and software support (drivers, middleware, etc.) at one hand and lowering the size and power consumption at the other. They feature advanced sensor fusion algorithms, dynamic accuracy, and standard industry interfaces such as I2C and UART.

3D stacking

It's a more radical solution that employs techniques like 3D wafer level packaging (Figure 3). The 3D stacking of MEMS and ICs in a miniaturized sensor node could significantly reduce the footprint and power consumption in wearable and medical devices.

Figure 3 A high-level view of 3D stacking of MEMS and CMOS devices.

Here, the MEMS sensor and CMOS chip are stacked on top of each other using flexible interconnect and techniques like through silicon vias (TSVs). The outsourced assembly and test (OSAT) industry is a major driving force behind 3D stacking.

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