in Standard CMOS
Member of SWEETS
Forcesensor – Demonstrator
Principle of function Practical realization
The forcesensor is realized in a standard As the sensor also exhibits magnetic
CMOS semiconductor technology as an sensitivity two sensor elements are used.
Fraunhofer Institute for n-doped well with two additional con- These are oriented at an angle of 90°
Integrated Circuits IIS tacts at the sides. The measurement is with respect to each other. A homo-
based on the piezoresistive effect. genous magnetic field generates the
Prof. Dr.-Ing. Heinz Gerhäuser Mechanical stress applied to the well same voltage in both sensor elements.
changes the resistance ot the well Subtracting the two signals eliminates
Am Wolfsmantel 33 anisotropically. To balance the drive the influence of this field. Since the
91058 Erlangen, Germany
force onto the carriers a lateral electric low-resistance direction is rotated by
Phone: +49 (0) 91 31/77 6-0 field must build up by accumulation of 90° in one sensor channel compared to
Fax: +49 (0) 91 31/77 6-9 99 charge carriers at the sides of the well. the other, the part of the signal which
firstname.lastname@example.org This can be measured as lateral voltage is created by the force effect has diffe-
drop across the well. The angle between rent sign in the two sensor elements.
Contact: the main current flow and the direction Subtracting the two signals therefore
Microsystems Technology of least resistance is very important. results in an addition of the force
Phone: +49 (0) 91 31/7 76-4 01
Fax: +49 (0) 91 31/7 76-4 99
The highest signal is achieved at an effects of the two sensor elements.
email@example.com angle of 45° while it becomes zero at
www.iis.fraunhofer.de/asic/analog an angle of 0° or 90°.
03-2003 / IIS / ritter
Fabrication properties of the substrate. The sensor
element itself shows an offset in the
The devices are manufactured using a mV-range, which changes by mounting
commercially available standard semi- the sensor onto a substrate. If the offset
conductor technology (CMOS). This value is too high for the application the
ensures high reproducibility and espe- sensor can be calibrated after mounting.
cially at high quantities cost-effective
production without additional steps. Pressure sensing
Since the technology has been develo- Mounting techniques
ped for electrical circuits it is of course The sensor can also be used for the
possible to add components for signal The easiest way to mount the force measurement of pressure in fluids or
conditioning and postprocessing on the sensor is to attach it into a ceramic gases when the pressure is applied to
same semiconductor chip.These include package with an adhesive and connect the system in such a way that it
amplifiers, filters, analog-to-digital it electrically by wire-bonding. deforms the chip.
converters, digital signal processors, This can be done as simple as in the
serial or parallel interfaces, etc. For figure above, where the ASIC is
many applications designs can be exposed to the pressure through a hole
derived from standard cell libraries in the middle of the PCB.
available at Fraunhofer IIS.
The constructional elements the sensor
is mounted on and the adhesives must
Main features not be elastic materials, otherwise the
system will show hysteresis effects.
The features of the sensor mainly
depend on the substrate the chip is
mounted on and slightly on the Applications
adhesive used. For the measurements
Sensor in a ceramic package
in the figure below the sensor was – Intelligent single-chip force sensors
mounted in a ceramic package. It shows with optimal thermal coupling to an
a high linearity and low hysteresis. In Another possibility is to adhere the chip on-chip temperature sensor for
this case the sensitivity was 0.13 V/Nm to a printed circuit board or a multi- temperature compensation
and the resolution was ± 1.5 . 10-3 Nm chip-module. This module then can be – Stress recording of staticcomponents
(± 1%). attached to the loaded constructional – Detection of maximum and
The measurement range extends from element by screwing or similiar overload forces
a few mNm up to very high forces techniques. The highest accuracy can – Online arbor-force detection at
limited only by the mechanical be achieved by mounting the sensor vehicles with central analysis
chip directly onto the device to be – Automatic limiting of mechanical
– High linearity and low hysteresis
– Cost-effective standard CMOS
– On-chip integration of analog and
digital signal processing
Output voltage vs. torsion Sensor on the device – High reproducibility