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 (Part 1 of this feature looked at challenges in the design of automotive safety sensors.)

The micromechanical acceleration sensor is designed with an STMicroelectronics silicon technology called ThELMA^TM, which stands for Thick Epitaxial Layer for Micro actuators and Accelerometers. ThELMA is a surface micromachining process based on a 0.8 μm technology.

Surface micromachining, as opposed to bulk micromachining, is an additive process requiring the building up of various layers of materials that are selectively left behind or removed by subsequent processing [4]. Hereafter is a quick comparison among several technologies used in the planar silicon process.

The MEMS sensor is made of two wafers, bonded together: One with the sensing structure, the second with the main function of protecting the tiny mechanical formation during the injection molding process. While historically the bonding between cap and sensor was made with glue made of glass-based material, STMicroelectronics is pioneering metallic bonding, which is the use of an intermediate metallic layer as the bonding layer between the two silicon wafers.

MEMS sensors in satellite accelerometers
In airbags systems, the sensors located in the car body might be subjected to shocks that range from 25 to 500g. Covering this extremely wide range with a single structure poses some technical issues related to the rigidity of the structure versus its sensitivity. STMicroelectronics MEMS are of capacitive type: they convert an acceleration signal into a capacitive variation, which is processed by an interface IC to give useful output voltage information proportional to the acceleration seen at the input.

We have therefore defined two different single-axis sensors to cover the entire required range:

Hereafter is the layout of the structures, sensitive respectively along the z (orthogonal) axis and along the in-plane (x or y) axes:

Many generations of mechanical sensor elements have lead to optimized structures for out-of-plane sensing (above, left) and in-plane sensing (above, right).

ASICs
To address the different requirements, the ASICs use different technologies. A central ECU based sensor is typically built using CMOS technologies. The satellite sensors require high voltage bus interfaces and thus are using specialized BCD technologies. The block diagrams of the PSI5 ASIC (immediately below) and of the DSI ASIC (below, bottom) using those high-voltage silicon technologies are hereafter indicated:

In both ASICs, we can identify four common blocks:

• Analog front end to the sensor
• Analog-to-digital converter
• Memory
• Sub-blocks implementing the communication protocols

Since both PSI5 and DSI require sustaining voltage levels in the range of 16V and 40V respectively, the challenge for silicon manufacturers is the integration of low voltage processing, memory, and the high-voltage portion needed for the communication interface.

Outlook
Airbag systems will further benefit from integration: Acceleration sensors and bus interfaces will merge; in the ECU, building blocks like bus interface and squib drivers will merge. Even power management functions can go into such a bus/squib/power IC. This reduced number of ICs in the ECU improves space, energy consumption, cost, and last but not least, the quality of the system.

As airbags are mandatory systems, and electronic stability control (ESC) will become mandatory soon, it can be assumed that the industry will merge airbag and ESC controls into one ECU. Then we will have several accelerometers (low g and high g), gyros (yaw and roll), and temperature sensors in the ECU. The complexity of such an ECU will increase; hence there will be a significant potential for savings if integration continues.

Sensors could come in IMUs (inertial measurement units), also called clusters, and no longer as single sensors. Furthermore, we will see advanced driver assistant systems (ADAS) like adaptive cruise control (ACC) and CMOS camera clusters interacting with the airbag/ESC system to go one further step towards accident free driving.

References

[4] B.Vigna, More than Moore: micro-machined products enable new applications and open new markets, IEEE 2005