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					   Silicon Microsystems for Automotive Applications

                                      Dr. Jiri Marek

                                   Robert Bosch GmbH
                                   Tübinger Straße 123
                                    72765 Reutlingen
                                        Germany



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
European Union.

1 Introduction

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$ [1]. The world market for
automotive electronics will reach by the year 2000 more than 100 billion US$ per
year.

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.


1.2 Safety
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.


1.3 Comfort
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.

•   Reliability
    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
    level                                                                  possible.

•   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.

•   Function
    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
market.

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 Applications


    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
                                                                                         schematical cross
                                                                                         section of an
                                                                                         integrated
                                                                                         pressure sensor
                                                                                         [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
                                                                                         manufacturing of
                                                                                         the     evaluation
                                                                                         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
                                                                                         wafer.      During
                                                                                         the         bipolar
Figure 1: Schematic cross section of the integrated pressure sensor.                     process         the
                                                                                         piezo-resistors as
      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
    TCS     SENSOR
        T
                                             A
                                                                              signal. At this node an
                               U/I
                                                   UA
                                                                              offset    compensating
                                                                              current as well as a
                                                                              TCO-current is added.
    PROGRAMMING                                                               The final amplifier
       VOLTAGE
                      TRIMMING           OFFSET,                              uses this current to
                      CIRCUIT               TCO
                                         T
                                                                              generate the 5 V output
                                                      trimming
                                                      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
      applications.


                                                            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 [4].


                                                                     3.2 Massflow Sensors
                                                                     Besides              the
                                                                     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



                                                                                       ∆ T
      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.
                                                                           3.3 Accelerome
                                                                           ters
                                                                           Silicon      bulk
                                                                           micromachining
                                                                           is used mainly
                                                                           for            the
                                                                           manufacturing of
                                                                           highly sensitive
                                                                           acceleration
                                                                           sensors with full
                                                                           scale of one to
                                                                           two g. In this
Figure 7: Micromechanical mass flow sensor with hybrid evaluation circuit. segment
                                                                           capacitive pickup
    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).

                                                                          The         largest
                                                                          segment         for
                                                                          accelerometers
                                                                          are the airbag
                                                                          systems.       The
                                                                          first acceleration
                                                                          sensors         for
                                                                          airbag systems
                                                                          were produced
                                                                          using
                                                                          piezoelectric
                                                                          sensing elements
                                                                          in the middle of
                                                                          the eighties.
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 [5].

                                                          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
    manufactured.
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 [6].




 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.

                             discrete sensors
                             (pressure, acceleration)      application extension


                                                    integration IC/sensor


                                                                         integration µC/sensor


                                                    complex sensors (mass flow, yaw rate)

                                                                         chemical sensors


                                                            passive structures


                                                                             actuators



                   1980      1985          1990           1995           2000         2005

           Figure 15: Trends of microsystem devices in automotive systems.


                                             bulk micromachining



                                                         surface micromachining


                                                                             additive technology



                                                                 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.

                USER
                                                             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
  Re-Design




               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
                                                                       small          and
                                                                       medium
                                                                       companies access
                                                                       to     a      new
                                                                       technology which
                                                                       is otherwise only
                                                                       accessable      to
                                                                       very few large
                                                                       companies. After
                                                                       the     prototype
                                                                       manufacturing
                                                                       the customer can
                                                                       enter    into    a
                                                                       supply agreement
                                                                       in order to cover
Figure 18: Typical structures in surface micromachining                the        volume
technology.                                                            manufacturing.
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
made possible.



Literature

[1]   The Electronics Revolution in the Motor Industry.
      London: The EIU, 1994

[2]   Kress, H.-J.; Häckel, K.; Schatz, O.; Muchow, J.:
      Integrated Silicon Pressure Sensor with On-Chip Compensation of Temperature
      Effects Using Programmable Thyristors.
      Microsystems, 1994

[3]   Kress, H.-J.; Marek, J.; Mast, M.; Schatz, O.; Muchow, J.:
      Integrated Silicon Pressure Sensor for Automotive Application with Electronic
      Trimming.
      SAE Paper Nr. 950533, 1995

[4]   Arand, D.; Marek, J.; Weiblen, K.; Lipphardt, U.:
      Integrated Barometric Pressure Sensor with SMD Packaging.
      SAE Technical Paper Series 960756, 1996

[5]   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

[6]   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

				
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