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Introduction The AE801 sensor element is designed to be a general purpose and easy to use tool in the measurement of mechanical properties such as position, pressure, force and acceleration. This note is written as additional information to the data sheet for customers starting to use the elements in building their own transducers or measuring set-ups. The note gives more detailed information about the elements and the principle being used. The basic difference between the AE801 sensor element and most other
semiconductor strain-gauges is that it is pre-assembled on a header, making it
easier to make electrical connections, and easier to assemble. The AE801 is a half Wheatstone bridge sensor element containing two
active resistors one on each side of the silicon die. Being silicon strain gauge, the
element has the advantage of high sensitivity and good long-term stability. The
AE801
sensor element is well proven after being used in transducer products for many years
covering applications within medical, military, industrial and automotive instrumentation.
Description of the Sensor Element The sensor element consists of two main parts, the silicon cantilever beam itself and the header to which the beam is mounted. The beam is made of single crystal N-type silicon and has one ion-implanted P-type resistor on each side. The surface of the beam is passivated with thermally grown silicon dioxide. An alloying technique has been developed to obtain a creep free mounting of the beam for joining the four pins of the header to contact areas on the beam. Thus, the four pins of the header serve both as a mechanical mount as well as electrical connections for the resistors. Mechanical Strength Single crystal silicon is nearly an ideal elastic material that can be bent almost to the breaking point without any hysteresis effects. Hysteresis in the sensor element output signal is therefore very low and does usually not contribute to significant signal error. As part of the quality assurance in the sensor element production, each element are being bent in both directions to an output signal of 30 mV corresponding to movement of the tip of the beam of about 60 - 80 um. When used, the sensor element should not be bent more than corresponding to this output signal.The sensor element is in principle a miniature position sensor with low hysteresis, good linearity and relatively high spring constant. It can be used in basically two different ways. When the sensor element is used as an accelerometer or as a balance, the applied load can act directly on the beam. In this case the silicon beam acts both as position sensor and spring element. In the other case the beam is used as a position pick-up sensor for other spring elements such as diaphragms and bourdon-tubes. When the beam is used for measuring the position of a spring element, it is important that the beam is rigidly mounted so that no unwanted movement between the beam and the spring mount can take place. Because of the small movement needed to give the beam the full-scale deflection, unwanted mechanical distortions must be kept to fractions of a micron to obtain reliable results. This again makes it necessary to use small, compact and rigid designs. In all designs where the sensor element is to be used precaution must be taken that the strain in the beam does not exceed the value of breakage because of overload. When designing overload protection, one must bear in mind that the allowable deflection is in the order of 60 - 80 um. When handling the sensor elements one must make sure that the beam is not overstressed. In order to avoid too large forces being transferred to the beam, one should preferably handle the sensor element by holding only one of the leads and as far out on the leads as possible. When handling the element this way, it can be inserted through small openings and it will not break even if the beam touches the wall. The same precaution should be taken when removing and inserting the elements into the glass-tubes used for storage and transport. The header can be assembled by pressing into a conical hole or by gluing the header body to a cylindrical hole The resistors are designed to be as robust as possible against surface contamination on the sensor element die, but should be handled to avoid contact with acids, bases, salty solutions and other corrosive media. High cleanliness should be used in all assembly operations and in use. The elements should not be used in electrical conducting media. Touching the silicon beam by naked fingers should be avoided because contamination from fingers (sodium and potassium) can lead to long-term drift of resistance values and leakage currents, in particular at high temperature. If it is necessary to touch the silicon by hand, clean plastic or cotton gloves or fingertip protections should be used. Fingerprints may be rinsed away in electronic grade hot trichlorethylene followed by rinsing in acetone and isopropyl alcohol.
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