Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
Figure 2: Package roadmap of automotive acceleration sensors
III. ACCELERATION SENSORS
The success story of MEMS acceleration sensors started
nearly 20 years ago with first high-g sensors for airbag
applications and continued with low-g sensors for ABS,
ESP, etc. Today over 90% of all new passenger cars are
sold with an airbag system with at least one
micromechanical acceleration sensor inside. These
impressing equipment rates are only possible due to a
continous and massive reduction of the costs of an airbag
system and therefore also of the integrated acceleration
sensor. This is achieved by strong improvements in the
micromechanical sensing elements, the ASIC and – last but
not least – the packaging. Fig. 2 shows the roadmap of
packages for airbag acceleration sensors at Bosch. The first
sensor, issued in the late 1970s, was a mechanical sensor
element in a metal can. In the mid 1980s the first
mechanical sensor followed with the ASIC integrated in the
same metal package. It was supplanted in 1996 by the first
generation of micromechanical acceleration sensors in a
PLCC28 package. The current generation of airbag
accelerometers, starting in 2010, uses a SOIC8 package.
This corresponds to a size reduction of more than 85% in 14
years.
The massive size reduction was achieved by several steps
in technology development. Due to design and process
progress the micromechanical sensor element could be
drastically reduced in size. The use of modern technologies
in IC processes led to a steady decrease of the ASIC size at
the same time – despite enhanced sensor performance and
higher self-test capabilities. With sophisticated state of the
art simulations – fed by the experience of several sensor
generations and of far more than 1 billion sensors produced
– key parameters of the package are optimized. The most
important of those are
• overall geometry (package height, length and width vs.
die size, symmetry, …)
• leadframe design (size, thickness, structure, …)
• die-attach (material parameters like E-modulus,
thickness,…)
• mold compound (CTEs, …)
• mold coverage (overall portion of mold compound vs.
Silicon content of the package)
The main hurdles for a more aggressive package size
reduction are the the capability for further processing and
the extreme environmental conditions automotive sensors
have to withstand.
Figure 3: Footprint of automotive and CE acceleration sensors
Figure 4: Crosssection and SEM picture of BMA220 (© Chipworks)
The consumer electronics (CE) industry has even higher
constraints regarding package size (footprint as well as
height). Bosch’s first acceleration sensor for CE in 2006
reduced the automotive package size to a 4 4 mm² QFNpackage
by half. Already one year later the size was further
reduced to 3 3 mm². At the beginning of 2010 Bosch
introduced the BMA220 - world’s first digital acceleration
sensor in a 2 2 mm² LGA package. Fig. 3 shows the
footprint development of automotive and CE sensors.
One major step towards the 2 2 cm² package was the
transition from side-by-side assembly to 3D stacked
assembly. With this 3D Integration approach the ASIC is
stacked on the micromechanical sensor element. Fig. 4
depicts insights into the construction of the BMA220.
IV. INERTIAL COMBI-SENSORS
Sooner or later the further size reduction will become
increasingly difficult. A new trend arises for sensors used in
systems with a standard combinations of different sensors.
An example are the inertial sensors used for vehicle
dynamics control systems like ESP®. A typical ESP system
needs the signals of a yaw rate sensor and an one or two
axial low-g acceleration sensor.
The first ESP systems were using a macro-mechanical
yaw rate sensor, which was based on a piezoelectrically
actuated, vibrating metal cylinder with piezo’s as sensing
element of the Coriolis force [1], for detection of the car´s
rotation along its vertical axis. In addition a mechanical
single-axis low-g accelerometer has been applied to detect
the vehicle´s dynamical state and for plausibilization of the
yaw rate signal.
2