An Integrated Resonant Accelerometer Microsystem for Automotive
*****Per Ohlckers, Reidar Holm, Henrik Jakobsen, Terje Kvisteroy, Gjermund Kittilsland, Martin
**** Nese, Svein M. Nilsen and Alain Ferber* SensoNor, Knudsrodvn.7, N-3191 Horten, Norway ** SINTEF Electronics and Cybernetics, P.O.Box 124, N-0314 Oslo, Norway
Future microsystems for the automotive market SUMMARY
have to meet demanding constraints with respect to
quality and fabrication costs, and consequently In the project “IRMA-EU” (IRMA: Integrated
leading to the need for an improved version of the Resonant Accelerometer Microsystem for
SA20 air bag sensor including features like Automotive Applications), a project sponsored by
calibration electronics and self test. In this project the European Commision under ESPRIT, SensoNor
the following alternative sensor principles for use in and project partners Autoliv and SINTEF are
automotive applications were evaluated in a developing a resonant structure two-chip
feasibility study with respect to the piezoresistive accelerometer silicon microsystem for automotive
principle: applications, the SA30: Crash Sensor for front
impacts, with range ? 50g.The project is focusing ? Capacitive element, bulk micromachined on the development of key process technologies, ? Capacitive element, surface micromachined product designs and manufacture of functional ? Resonating element, bulk or surface prototypes. The IRMA project is coordinated with micromachined other activities performed by the partners to cover all aspects of research and technology development Table 1: Parameter evaluation of accelerometer as well establishment of high volume production principles with respect to the piezoresistive capabilities needed for successful product principle. 0 is equal, + is more and - is less innovation of this new generation of crash sensors. attractive than the piezoresistive principle. The sensing principle is an acceleration sensitive resonant structure, with an ASIC for resonance Parameter Capacitive Capacitive Reso-control and signal conditioning. Prototypes in bulk surface nating silicon of both the sensor chip and the ASIC chip S/N-ratio - - + have confirmed the feasibility of the concept, and Accuracy - 0 + ramp up to high volume production has now started. Frequency 0 + + range Keywords: Crash sensor, resonant structure, silicon Power cons. + + + microsystem. Shock resist. - - ++ Sticking probl. - -- + DESIGN AND TECHNOLOGY Self test + + ++ EVALUATIONS EMI - - + Silicon area -- + + The existing SA20 air bag sensor from SensoNor Process compl. + - 0 is fabricated in a high production volume. This ASIC cost 0 +/- + sensor is based on a micromachined silicon beam Calibration 0 0 0 with a ceramic seismic mass, and the acceleration Dicing + - + induced stress is detected by a diffused Mounting 0 - 0 piezoresistive resistor bridge. Overload protection is Production cost - - + handled by oil damping. This sensor had a market Reliability 0 - + share of approximately 70 % in Europe in 1996 for Develop. cost + -- - automotive crash sensors for air bag safety systems, with more than 7 millions units shipped.
Table 1 gives the result of this evaluation, where The resonator is excited thermally by feeding a 0 indicates equal to the piezoresistive principle, + is current pulse into a diffused resistor located in one more and - is less attractive than the piezoresistive the beams, and the resonance frequency is detected principle. Please observe that this is a simplified by piezoresistors diffused into the other beam. and subjective presentation of the results of the evaluation process, and that the evaluation was The mass structure lateral dimensions are done several years ago. As can be seen from table 1, approximately 800 ?m by 200 ?m, and a thickness
the resonance principle was regarded superior in of approximately 20 ?m. The lateral dimensions of
almost all aspects regarded important in automotive the beams are approximately 150 ?m by 40 ?m, and
applications. Therefore, a resonating element was a thickness of approximately 3 ?m. chosen as sensor principle for the next generation of air bag sensor. Of special importance was the FABRICATION PROCESS following arguments: ? Inherent continuous self test function The resonator chip is based on a double-side implemented due to the resonating principle of polished silicon p-type, (100)-oriented substrate and operation. standard IC-processing to define the conduction ? A high ratio between the mechanical shock lines and diffused resistors. survival limit and the measurement range is obtained without mechanical overload Wet etching using electrochemical passivation at protection. a reverse biased p-n junction defining the thickness ? Low production cost. No need for oil-filling or of the plate as the dynamic system consisting of the evacuation of the resonator cavity. The higher beam elements and the mass structure. The order resonant mode to be used will have a Q-thickness is controlled within ? 0.2 µm.
factor in the range of 1000 at atmospheric pressure.
SA30 utilises a small single crystal silicon
resonator which shifts its frequency due to change in
acceleration. This concept for self-test has been
made possible due to the development of a new
sensing principle. The sensing element is made by
bulk micromachining in silicon to define the centre
mass structure suspended in two flexible beams,
Figure 2: SEM-picture of one of the beam thus forming the resonator structure.
elements after release by RIE.
The beam and mass elements is released by RIE-
etching in Cl-plasma after wet etching. This process
is optimised to avoid etching and contamination on
the substrate backside degrading the anodic
bonding process between the silicon substrate and
the glass substrate. A SEM-picture of the released
spring element is given in figure 3, and by
inspection we have not observed contamination or
etching effects on the back side of the spring
elements after the release process. Figure 1: Picture of resonating element after anodic bonding. Anodic bonding is used to seal the silicon die between two glass plates to obtain a mechanically
rigid structure, and a controlled and stable
atmosphere for the resonating structure. The at all times, even during a crash without interrupts resonator chip after anodic bonding is given in and loss of information. figure 1. A special patented process is developed to
minimise electrical effects from the glass substrates
on the diffused conductors in the anodic bonding
area (feed through).
ASIC AND INTEGRATION
The resonator sensing element is a part of a closed loop feedback system including an ASIC as Figure 4: The electrictrical schematic for the shown in figure 7. The ASIC is developed with the SA30 Crash Sensor. following two main functions: ? Actuate the micromachined silicon structure to RESULTS AND DISCUSSIONS vibrate at a selected resonance frequency ? Sense the acceleration sensitive resonance The thickness of the mass structure and the frequency spring elements are measured after wet etching by FTIR spectroscopy based upon destructive and The ASIC and the resonator chip will be constructive interference in the plane parallel mounted into a miniature moulded plastic package structure layers. An algorithm is developed from for surface mount assembly. Chip-and-wire basic interference theory ?1,2? to extract the technology will be used to interconnect the two structure thickness from one FTIR transmission -1 given by; chips as well as from chip pads to the plated spectrum with resolution 1 cm terminals on the lead frame. This is made possible
d = m / (2?n???) due to the anodic bonding forming a closed cavity
of the silicon resonator. The outline of the fully
where m is number of observed periods, n is integrated SA30 airbag sensor is given in figure 3
refractive index of the medium, and ?? is ?3?.
bandwidth of the number of periods involved.
The accuracy of the thickness measurement is
estimated to ? 1 %. Histogram of thickness for the
spring elements and the mass structure is given in
figure 5 and 6 respectively.
Standard deviation in the structure thickness
given in the histograms are 4 % for the beam
element and 2 % for the mass element. The main Figure 3: SA30 Crash Sensor fully reason for these structure variations are process integrated in a surface mount transfer moulded variations for the spring element definition between epoxy package. different prototype batches. Structure thickness
after wet etching within one batch, i.e. no process The output signal can be either PWM (Pulse-variations, have typical standard deviation of Width-Modulated) for innovative system designs, approximately 1 %. with respect to noise, EMI or A/D conversion, or analog (ratiometric) for traditional interfacing. A
threshold signal is available for designers to
improve the system reliability and performance.
Due to the intrinsic continuous self-test of the
sensor, monitoring of a status signal is all that is
needed to check the reliability of the sensor signal.
This solution makes it possible to check the sensor
The main future action of the project group will 16be to further characterise and evaluate the 12prototypes to improve performance within the given
8Waferscost restrictions. The resulting pilot production
prototypes of the SA30 Crash Sensor will then be 4implemented into air bag systems for automotive 0applications. Thereafter, we will initiate and ramp 3,03,23,43,63,84,0up production to several millions per year Thickness [microns]
production volumes of the SA30 microsystems and
the associated air bag systems. Figure 5: Histogram of the beam element thickness after wet etching. In addition, the technology platform developed
will be used as input to the development of a new
pressure sensor microsystem technology.
The technology development will also in this
case be organised by SensoNor as a concerted
action with many contributors, of which SINTEF
will be among the main collaborators.
The SA30 Crash Sensor prototypes development
in the IRMA project has successfully demonstrated the feasibility of chosen design for a crash sensing Figure 6: Histogram of the mass element thickness microsystem, using an acceleration sensitive after wet etching. resonant structure, with an ASIC for resonance control and signal conditioning. The developed One important concern regarding the anodic technology platform will be used in the future to bonding process is contamination from glass in the develop new silicon pressure sensing microsystems. cavity causing electrical defects. Measurements on MOS-transistors results in a threshold voltage for ACKNOWLEDGEMENT majority carrier inversion of the silicon surface in the range 6-6.5 V. The breakdown voltages is The development work for the SA 30 Crash measured in the range 13-15 V reverse bias for both Sensor is supported by the ESPRIT Project - the excitation bridge and the detection bridge, and Mumber 2176 „IRMA-EU” (IRMA: Integrated the leakage currents are in the range 10-100 pA at Resonant Accelerometer Microsystem for 5V reverse bias. Therefore, electrical measurements Automotive Applications) indicate negligible effects from the glass materials on the electrical characteristics for components in REFERENCES the resonator cavity. 1. E. Hetcht, Scaum: “Theory and Problems of Optics”, McGraw-Hill, 1975. Q-factor higher than 1000 is obtained at 2. A. Ferber and K. Aamold: “FTIR Based atmospheric pressure. The resonance frequency for Thickness Measurements of Silicon Sensors” the micromachined device is 400 to 700 kHz. The SINTEF-report, Jan. 1996. sensitivity is in the range of 6-7 Hz/g. 3. “Data sheet: SA30 Crash Sensor” SensoNor asa, P.O.Box 196, N-3192 Horten, Norway.