Silicon Microsystems for Automotive Applications
Dr. Jiri Marek
Robert Bosch GmbH
Tübinger Straße 123
The integration of microelectronic and micromechanical devices into one system - the
microsystem technology - is rapidly gaining importance in automobiles. According to
market studies the content of electronics as well as of microsystems in automobiles is
increasing more than proportionally. The major driving forces are the environmental
requirements, safety and comfort. The microsystem technology contributes in these
areas due to the reduction of costs, weight and size as well as improved reliability and
functionality. Sensor systems for the measurement of manifold air intake pressure,
mass flow, acceleration for ABS and airbag and yaw rate will be discussed in detail.
In the nineties a new technology, the surface micromachining, is emerging. This
technology is being made available to small and medium size companies as well as to
universities and research institutes by a foundry-service-scheme sponsored by the
In the last decades the portion of electronics in automobiles is increasing steadily. A
study by Economist Intelligence Unit estimates the average portion of electronics in
automobiles to 300 US$ in the year 1980. The portion increased to 1200 US$ in the
year 1990; for the year 2000 the estimate is 2500 US$ . The world market for
automotive electronics will reach by the year 2000 more than 100 billion US$ per
The microsystem technology is defined as the integration of microelectronic,
micromechanical and microoptical components into one device. Due to the system
requirements microelectronical and micromechanical components are applied in
automotive systems. The application of microelectronical and micromechanical
devices in automobiles follows the consumer and industrial applications with and
delay of about 5 to 10 years. This delay is due to the large temperature range of -40 to
+85 °C or even +125 °C and the high reliability requirements. The application of
microelectronics and micromechanics is increasing steadily in automotive electronic
systems. Since the beginning of this decade even the monolithic integration of
microsystems is being applied in automobiles.
The driving force for microsystems in automobiles are as follows:
1.1 Environmental requirements
Due to the high motorization rate in the industrial and increasingly in the developing
nations the emission of each automobile has to be minimized. The optimization of the
combustion process in gazoline as well as in diesel engines is achieved by digital,
electronic motor management systems with fuel injection. In the last years the
additional requirement to reduce the emission of carbon dioxide (CO2) is gaining
importance. This reduction can be achieved only by diesel engines with direct
injection. This new diesel systems require increased electronical control.
The customers in the industrial nations ask for improved safety of the automobile. The
equipment rates of cars with anti-blocking systems (ABS) and with airbags is
increasing rapidly. In 1995 the introduction of the Vehicle Dynamic Control System
(VDC) improved the driving and safety performance of the car even further on.
The comfort electronics was limited to the car radio in the beginning. This area was
expanded by cassette decks and CD-players. Today the customer finds a wide variety
of systems: electronic speak control, navigation systems, electronic window and seat
positioning, electronic climate control.
The application of microsystems in the area of automobile electronics is dominated by
the following factors:
• Cost Reduction
Due to the high competition in the automotive industry this factor has high
priority. The miniaturization and the batch manufacturing reduces the
manufacturing cost for each unit even in spite of the high investment. The
integration of the sensor function and the electronics results in a cost advantage.
Due to the high development cost medium to high production volume is required.
The integration and the miniaturization reduces the amount of interfaces and
connections. The interconnections are the weak point in the harsh environmental
requirements in automobiles. Therefore the microsystems make higher reliability
• Weight and Size
Due to the miniaturization we can achieve a reduction of weight which is gaining
increasingly on importance. Since the complexity of electronic control units in car
is increasing steadily a reduction of size and therefore space on the printed circuit
board is of advantage.
The progress in microelectronics and integration results in an increased
functionality of new systems like self test, accuracy, detection of shorts and
interconnect problems, etc.
In this paper we will give a short history of microsystems in automobiles. Especially
the application of micromechanical devices will be shown. Several current devices
will be shown in more detail. The current trend of device development and process
technology will be discussed.
2 Beginning of Microsystems in Automobiles
The microsystems - mainly micromechanical devices - started the application in
automobiles in the beginning of the eighties. In this timeframe the volume
manufacturing of micromechanical pressure sensors started. The sensors contained a
discrete sensing chip which was completed to a sensing unit as a manufacturing part.
The sensing unit was attached to a printed circuit board with the associated evaluation
and trimming circuit. Later on this system was changed to thick film technology for
the evaluation electronics. The calibration of the sensor was performed with a laser.
The first sensors were used for the measurement of the manifold air intake pressure
for electronic fuel injection. Even today this application is the largest portion of the
The applications of the pressure sensors were expanded: the measurement of
atmospheric pressure, the ABS hydraulic pressure and the airconditioning compressor
are the next systems. In the beginning of the nineties the first integrated pressure
sensors are being manufactured. The sensor consists of the sensing element as well as
the evaluation and compensation circuit on one single chip. The measurement of MAP
pressure was also the first application in this case.
Besides the pressure the acceleration is the important physical value to be measured in
the car. For airbag systems an acceleration sensor was developed in the eighties. A
bimorph piezoelectric element was used for signal generation. The generated electrical
charge is amplified by a hybrid electronic circuit. In this area the first
micromechanical solutions are appearing in the beginning of the nineties.
3.1 Pressure Sensors
e p ita x i e (n) p ie z o re s is t o rs
The first figure
e v al u at io n c i rc ui t
m em b ra n e shows a
section of an
[2, 3]. A standard
S i -s u bs t ra t e (p ) c a v ern e
bipolar process is
used for the
circuit on the
front side of the
p y rex g l a s s
m e ta l li z a ti o n
Figure 1: Schematic cross section of the integrated pressure sensor. process the
well as the interconnects are manufactured for the sensing element. After the bipolar
process the wafer is exposed with the structure of the cavity on the backside. This
process requires special equipment since the structure on the backside has to be
aligned to the frontside. The passivation in the exposed area is removed. Anisotropic
etching is used to remove the silicon in this area of the wafer.
The etching stops automatically at the interface of the buried layer which has been
manufactured in this area during the bipolar process. The silicon wafer is attached to a
plate of pyrex glass by anodic bonding. A pressure difference across the membrane
results in a mechanical deflection of the plate. The resulting mechanical strain has a
maximum at the middle of the edges of the membrane. Piezo-resistors were diffused
in this area during the bipolar process.
The four piezo-resistors are
connected to a wheatstone bridge.
This bridge delivers raw signal of
about 100 mV without
amplification. The evaluation and
trimming circuit as well as the
sensor membrane is shown on the
chip photograph (figure 2).
The evaluation circuit amplifies the
signal to a calibrated 5 V output.
The accuracy requirement for MAP
sensors is on the order of 1 %. The
manufacturing tolerances of
sensitivity, offset as well as the
temperature coefficients have to be
compensated. The signal
Figure 2: Chip photograph of the integrated pressure sensor.
conditioning for the sensor is
shown in figure 3.
The output of the
Sensor Signal Conditioning wheatstone bridge is
converted to a current
signal. At this node an
current as well as a
TCO-current is added.
PROGRAMMING The final amplifier
TRIMMING OFFSET, uses this current to
generate the 5 V output
pins signal. The sensitivity
LOGIC ... signal pins
is adjusted by changing
the amplification factor
Figure 3: Schematics of the electronic evaluation and trimming circuit of the
of this amplifier. The
pressure sensor. temperature coefficient
of sensitivity is
adjusted by supplying the wheatstone bridge with a temperature depended supply
voltage. The compensation is performed using an electronic trimming process. A
digital compensation data word is supplied to the logic circuit. This logic circuit
selects the different compensation paths. Each paths contains a compensation
thyristor. A binary weighted compensation current is short circuited to ground by the
thyristor or this current goes to the compensation node in the off-mode. The
compensation data can be changed and the output characteristic of the sensor is
modified. In the case of proper output characteristic the programming voltage is
increased and the data is stored in the thyristor by a method similar to zener zapping:
the thyristor will become permanently conductive.
This calibration method substitutes the capital intensive laser trimming. The electronic
compensation is performed at the end of the manufacturing process on the finished
device. The final inspection and trimming is combined into one manufacturing step
resulting in a cost reduction.
For MAP applications the sensing element is soldered on a TO8-type header (figure
4). The chip is wire bonded to the pins. The cap of the header is welded under
vacuum. The reference vacuum of the sensor is enclosed under the cap. The pressure
to be measured is guided by a small pipe to the backside of the chip. Due to the high
media requirements this elaborate mounting method has to be used for MAP
The integrated pressure chip can
be modified for different
applications. The mounting and
packaging can be also varied. For
the measurement of barometric
pressure for diesel motor
management systems the chip is
mounted on a ceramic substrate. In
this case the pyrex plate does not
contain a hole in the backside. The
reference vacuum is enclosed
between the sensor chip and the
pyrex. This device can be soldered
directly on to the printed circuit
board by a surface mount
Figure 4: Integrated manifold air intake pressure sensor. technique. A plastic cap is used for
mechanical protection (figure 5).
In the case of an application inside
the electronic control unit the requirements for media resistance are not as high.
Therefore the pressure can be applied on to the frontside of the chip .
3.2 Massflow Sensors
measurements of the
MAP pressure the
condition of the engine
can be deduced from the
measurement of the air
mass going into the
combustion chamber. In
the US and in Europe the
air mass flow sensors
found wide application.
Figure 5: Integrated barometric pressure sensor in SMD package.
M e a s u rin g P rin c ip le
T em p eratu re p rof ile
w ith ou A tröm u
oh n e At n sir F lown g
mithAA ir tröm u ng
w it n s F low
F trö m ire c tio n
S lo w du n g sric h tu n g
⇒ M em b ran e or elem ent
S en s orelem en t
T1 H e a tin g zo n e T2
S h eet m etal
T räg erb lec h
A u sw e rtu n god e r m e mra e ra tud if ife e re n e ∆ T = T 2 - T 1
E v a lu a tio f te T p e p tu re rd f f re n c z
⇒ K e n n c te ristic c n g e b h ä n g ig
C h a ralin ie ric h tuu rvsa d e p e n d e n t o n f lo w d ire c tio n
Figure 6: Principle of measurement funtion of the micromechanical mass flow sensor.
The figure 6 shows the functional principle of a micromechanical air flow sensor. A
thin membrane has been etched out of the silicon wafer in order to achieve a good
thermal isolation of the sensing elements. In the middle of the membrane a heating
element is deposited. A heating current increases the temperature in the middle of the
membrane. Without air flow the thermal profile is symmetrical to both sides of the
heating element. To the left and to the right of the heating element two temperature
sensors are located. An air flow from the left side decreases the temperature on this
side of the membrane; the temperature sensor T1 detects a lower temperature. The
measurement of the temperature difference between the left and the right temperature
sensors is a direct indicator of the air flow over the chip. The thermal mass of the
device is very small. This sensor exhibits small response times; even pulsation of air
inside the manifold can be detected.
The sensor chip is attached to a metall frame. The evaluation IC is situated on a small
hybrid circuit (figure 7). The sensor chip is connected with wire bonds to this
evaluation circuit. Due to the low power consumption power drivers are not needed.
is used mainly
sensors with full
scale of one to
two g. In this
Figure 7: Micromechanical mass flow sensor with hybrid evaluation circuit. segment
is of advantage. Figure 8 shows such an acceleration sensor. A seismic mass
suspended by two beams is etched out of the silicon wafer. This movable structure is
bonded between an upper and lower wafer. An acceleration perpendicular to the wafer
surface moves the mass out of the center position. This displacement is detected by
the top and bottom capacitance between the wafers. The capacitive half bridge is used
for signal pickup. The manufacturing processes for this device are very different from
IC manufacturing. Therefore the integration of this sensing chip onto ICs is not
possible. The sensors manufactured with this technology are applied mainly for ABS,
suspension control and vehicle dynamics control (VDC).
are the airbag
in the middle of
Figure 8: Capacitive acceleration sensor in bulk micromachining.
Acceleration sensors in this technology are still being produced in large volume.
Figure 9a shows a new design for an airbag piezoelectric acceleration sensor. The
piezoceramic is mounted perpendiculary onto a thick film hybrid using a new
mounting technology. An acceleration perpendicular to the hybrid plate bends the
piezoceramic and generates electrical charge. The electrical charge is amplified with a
high impendance amplifier. The calibration of sensitivity is performed also in this IC.
Due to the moisture-sensitivity of the piezoceramic a hermetic metall package is used
(Figure 9b). The new mounting process for the piezoceramic makes the direct
soldering of this package into the printed circuit board possible. No additional
mounting components are necessary.
Figure 9b: Hermetic metall package.
In the beginning of the nineties the surface
Figure 9a: Piezoelectric airbag acceleration sensor
micromachining technology was used for
with direct mounting in printed circuit boards.
the first time for commercial devices: the
airbag sensors. In contrast to bulk micromachining the surface micromachining uses
layers on the surface of the wafer. Polycrystalline silicon layers are desposited using
vapor phase deposition onto the silicon wafer. Out of this layer the movable structures
are etched out. This etching has to be performed with high structural precision and has
to yield perpendicular walls. After the structuring of the polysilicon the underlaying
silicon oxide layer is etched away. This so called sacrifical layer was deposited
previously between the wafer and the polysilicon. Since the sacrifical layer is
removed, the polysilicon structure can move freely.
Figure 10: Functional principle of an acceleration sensor in surface micromachining.
Figure 10 shows the functional principle of a surface micromachined acceleration
sensor. The seismic mass is suspended at the four corners by springs. The fingers to
both sides of the seismic mass as well as fingers attached to the wafer surface form a
capacitance for signal pick up. This capacitance is changed during acceleration. For
particle and handling protection a silicon cap is attached to the front of the wafer .
Figure 11 shows the acceleration
sensor as well as the associated
evaluation circuit inside a standard
plastic PLCC-package. Due to the
polysilicon thickness of 10 µm
large working capacitances can be
realized. The device shows only
very small sensitivity to
Figure 11: Surface micromachined acceleration sensor in mechanical stresses during plastic
PLCC28 package. packaging. Therefore, standard
plastic packages can be used for
the acceleration sensor. The small sensing element makes highly integrated satellite
airbag sensors possible. The Peripheral Acceleration Sensor (PAS) is shown in figure
12. The sensing element is mounted onto a hybrid substrate. The electrical wire bonds
go directly to the evaluation circuit. This circuit contains also the peripheral functions
for the PAS. A standard microcontroller is the third component on the hybrid
substrate. Very compact, highly integrated peripheral acceleration sensor is
Figure 12: Peripheral Acceleration Sensor for side airbag.
3.4 Yaw Rate Sensors
A precision yaw rate sensor can be realized using a combination of surface and bulk
micromachining. Figure 13 shows the principle function for this device. Two plates
have been etched out of the silicon wafer using bulk micromachining. Each of the
plates is suspended by four folded beams at each of the edges. Aluminium wires are
deposited on these springs and plates. An oscillating electrical current in combination
with a magnetic field drives the two plates at the resonant frequency. On top of the
oscillating masses two acceleration sensors using surface micromachining have been
manufactured. A rotation along the axis perpendicular to the wafer produces a coriolis
force onto the oscillating mass; the acceleration sensors are deflected from their center
position. The difference between the acceleration sensors measures the yaw rate along
the perpendicular axes. The complete device is shown in figure 14. A highly complex
evaluation and trimming circuit is necessary for the yaw rate sensor resulting in a
highly accurate signal .
Figure 13: Functional principle of the yaw rate sensor.
4 Development Trends
The field application of micromachining
started in the beginning of the eighties with the
manufacturing of discrete sensors using bulk
micromachining. Two main segments were
selected: the measurement of the MAP
pressure and the realization of airbag sensors.
These two segments covered the majority of
this technology in the automotive field. Now
the applications of the descrete sensors is
being diversified (figure 15 + 16). Pressure
Figure 14: High precision yaw rate sensor in sensors for the measurement of ABS-pressure,
surface bulk micromachining. oil-pressure, air conditioning compressor
pressure, fuel tank pressure and others are
realized. In the beginning of the nineties the first integrated microsystems are coming
to the market: the first integrated pressure sensors are entering mass production. These
sensors are also based on bulk micromachining.
(pressure, acceleration) application extension
complex sensors (mass flow, yaw rate)
1980 1985 1990 1995 2000 2005
Figure 15: Trends of microsystem devices in automotive systems.
thin film technology on silicon
chemicaly sensitive materials
1980 1985 1990 1995 2000 2005
Figure 16: Technology trends for automotive microsystems.
Several years later the first integrated acceleration sensors are being introduced. For
this sensor surface micromachining is commercialized for the first time. Further steps
of integration of sensor function and evaluation circuits is expected. The next step of
integration would be the integration of sensing element and the evaluation circuit onto
a microcontroller. The advancement of microelectronics and integration techniques
makes the realization of very complexe systems possible. The high integration
complexity will be achieved only for few devices. These devices are dominated by the
advantages of the integration:
• reduction of interconnections,
• increased functionality,
• reduction of costs.
The integration has also several disadvantages:
• increased process complexity,
• longer development cycles,
• reduction of yields, sometimes even a cost increase,
• higher tooling costs,
• lower flexibility.
In some sensor areas hybrid-integrated sensors will remain:
• sensors with small manufacturing volume,
• complex sensors with large chip area and high process complexity,
• sensors with processing steps very different from IC technology.
The decision for monolithic or hybrid-integration sensors is very important for the
development cycle. Therefore, this decision is of strategic importance.
Besides sensors for the measurement of physical values chemical sensors based on
micromachining will be introduced at the end of this decade. Due to the low
compatibility of these processes with IC manufacturing discrete realization method
will be prefered. The application of such sensors in automobiles is dominated by the
requirements on selectivity and long term stability. The prove of this performance will
decide about the application in automotive systems.
Integration of microcontrollers and sensorfunction will be feasible by the year 2000.
However, the development aspects listed above will decide about the volume
production of such systems. The low flexibility and the very high production volume
in one segment are a big disadvantage.
5 Foundry-Service Surface Micromachining
As shown in figure 14 surface micromachining is a new technology for microsystems.
Robert Bosch GmbH decided to offer this new technology also to external customers
and universities as well as research institutes. This project is being supported by the
European Community within Framework IV.
D es ig n R u le s
S up po rt
U se r D e sig n
M u lti-P ro jec t W afer
S e r or C hip
U sensD e sig n
Ev alua tio n
P ro d uc tio n
Figure 17: Foundry service for devices in surface micromachining technology.
Projects within this foundry-service are shown in figure 17. Bosch published in June
1996 design rules for the surface micromachining process. The customer designs
structures according to the design rules. Designs of different customers are combined
to a multi-chip-wafer and are processed during one run. After the processing the chips
are sawn and delivered to the different customers. Standard packages are also offered
within the project. Figure 18 shows typical structures of these devices.
This project gives
to a new
is otherwise only
very few large
the customer can
enter into a
in order to cover
Figure 18: Typical structures in surface micromachining the volume
Since the process is used for a variety of devices and customers the large volume
results in a very well controlled process. Manufacturing of smaller volumes is being
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Integrated Silicon Pressure Sensor with On-Chip Compensation of Temperature
Effects Using Programmable Thyristors.
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Integrated Silicon Pressure Sensor for Automotive Application with Electronic
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 Arand, D.; Marek, J.; Weiblen, K.; Lipphardt, U.:
Integrated Barometric Pressure Sensor with SMD Packaging.
SAE Technical Paper Series 960756, 1996
 Offenberg, M.; Münzel, H.; Schubert, D.; Schatz, O.; Lärmer, F.; Müller, E.;
Maihöfer, B.; Marek, J.:
Acceleration Sensor in Surface Micromachining for Airbag Applications with
High Signal/Noise Ratio.
SAE Technical Paper Series, 960758
 Lutz, M.; Golderer, W.; Gerstenmeier, J.; Marek, J.; Maihöfer, B.; Mahler, S.;
Münzel, H.; Bischof, U.:
A Precision Yaw Rate Sensor in Silicon Micromachining.
To be published in Proceedings of Transducers ’97 Conference, 1997