ENGINEERS' INSIGHT

Second Generation MEMS Sensors Enable Impressive Performance for Automotive and Industrial IMUs

When Murata purchased Finnish MEMS specialist VTI Technologies and renamed it Murata Electronics Oy at the start of 2012, it gained the company’s unique 3D MEMS technology which has outstanding performance and reliability in harsh environments. VTI Technologies was already the leader in low-g accelerometers for automotive safety systems and medical cardiac rhythm management (CRM) systems, but continuing its development as Murata Electronics Oy has led to the technology finding more diverse applications. Industrial IMUs (inertial measurement units) are a key growth area as sensors begin to proliferate in factories and industrial machinery, as well as non-automotive transportation and other demanding applications.

One of the latest products to emerge is the SCC2000 series, which includes a 3-axis 2g or 6g accelerometer combined with either an X-axis or a Z-axis gyroscope plus an SPI-configurable 10 or 60Hz low pass filter. The availability of the Z-axis gyro on the same chip is particularly exciting as it means that a full 6 degrees of freedom can be implemented on one application PCB.

The accelerometer’s offset temperature drift is ±6mg for the 2g sensor and ±12mg for the 6g version. The gyro’s range is ±125°/s with a sensitivity of 50 LSB/°/s (±300°/s available on request), while its offset temperature drift is typically in the range ±0.5°/s. The X-axis gyro has a typical offset short-term bias stability of 1°/h, or 2°/h for the Z-axis product.

Other products include the SCA103T inclinometer with impressive measurement range (±30°) and resolution of 0.0013°, and the SCR1100 gyroscope which achieves class-leading bias instability of just <1°/h.

This is a performance level that up to now has only been available as part of more expensive module products (the SCC2000 series comes in a 24-pin SMD package). But how does Murata Electronics Oy achieve these performance levels while ensuring the devices can withstand temperature, shock and vibration in automotive and industrial applications? The answers are to be found by examining the company’s engineering efforts in its proprietary 3D MEMS device design, semiconductor processes and packaging methods to get the best possible results.

Device Design

Murata Electronics Oy’s 3D MEMS devices are based on mechanical movement or oscillation of conducting bodies inside the chip. As they move relative to each other, the structure’s capacitance changes, which is measured. The mechanical design of the MEMS devices plays a fundamental role in performance. In particular, gyros that aren’t well designed frequently suffer from spurious signals caused by vibration in the application. Murata’s robust single-axis 3D MEMS gyros are remarkably vibration-immune thanks to their innovative design. Two weights are connected with a fine spring so that they move in anti-phase. There are four sensing points, one at either end of each weight, and an algorithm can combine the four signals in a way that eliminates the effect of linear acceleration in any direction which could corrupt the required rotational acceleration signal. X- and Z-axis gyros require separate gyros, and the structures of both are patented by Murata.

Murata Electronics OY's 3D MEMS Structure

Features of Murata's 3D MEMS Technology

Precision Manufacturing

Precision manufacture is also part of the story. Like other MEMS makers, Murata Electronics Oy uses DRIE (deep reactive ion etching). They combine it with SOI (silicon on insulator) wafers which help the precision of the etch in the vertical direction. The SOI wafers also introduce an electrical isolation layer below the MEMS devices so they can be designed without regard to what’s below them while minimising any parasitic capacitances.

Murata is also the world leader in producing extremely precise capping wafers. Capping wafers are bonded to the surface of the 3D MEMS wafer (top and bottom) to protect the devices before further encapsulation, while maintaining a gap around the MEMS structures so the devices are free to move. Murata’s capping wafer technology uses a layer of glass, which is polished so it’s extremely flat; this can be used to create very small  (2.5µm) but extremely precise cavities for the MEMS to move in, which increases sensitivity.

Bonding the capping wafers to the 3D MEMS wafer is then done in another very tightly controlled process step. The MEMS and the capping wafer are joined using anodic bonding between the glass and the silicon, so the cavity is hermetically sealed, which contributes to the devices’ high reliability.

Packaging Technology

Electrical connection to the MEMS devices has to happen through the capping wafers. Murata was one of the first to use TSVs (through-silicon vias) to achieve this. Each TSV is effectively a hole in the wafer, filled with silicon to create a ‘wire’ to the MEMS device. The silicon in the holes is isolated from the bulk wafer with a layer of glass. The glass enables high insulation resistance and low parasitic capacitance between the TSVs, which enhances precision, stability and power consumption. And because the TSVs are formed by etching, they can be placed anywhere on the capping wafer (unlike with wire bonding, where the layout of the devices has to be such that a wire goes to the edge of the chip for connection).

After an initial probe test of the MEMS, an ASIC to do the required maths is assembled onto the bottom of the MEMS/capping wafer sandwich using flip-chip technology (only the known good sites are populated). Dicing follows, then final test of 100% of parts, followed by calibration of the sensors. The space around the ASIC is then filled with robust and reliable moulded plastic, which, again, helps it remain intact in the face of shock and vibration. This is totally different to how standard ICs are packaged and it does a great job of protecting the device while keeping its package nice and compact.

The result of the new proprietary 3D MEMS device designs, combined with precision processing and innovative packaging technology, is a series of devices that can withstand even the harshest industrial and automotive environments. Performance and reliability have been proven in the field, with high accuracy maintained in demanding applications such as high vibration and temperature environments. The parts also boast excellent mechanical shock endurance and good offset stability over temperature and time. 

WRITTEN BY

Adam Chidley

Adam joined Avnet Abacus in 2006 as a Sales Consultant, moving into Product Management 18 months later. As European Senior Product Manager, Adam is responsible for key supplier relationships and marketing strategy for Avnet Abacus’ passive business unit.

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