Abstract
This paper describes a high accuracy displacement sensor using deffused type semiconductor strain gages. This sensor consits of a canti-lever made from iron-nickel-cobalt alloy with low thermal expansion coefficient along with high elasticity, and two silicon pellets having respectively two p type resistors diffused on the (110) plane with the [111] direction. These pellets are bonded on both surfacs of the canti-lever with eutectic alloy solder, respectively, so that the pellet's [111] direction is parallel to the direction of stress.
Four strain gages on both surfaces of the canti-lever form a wheatstone bridge and the strain caused by the displacement of the cantilever tip is transduced into a voltage through this strain gage bridge.
Non-linearity, sensitivity and temperature effect of piezoresistance are affected by such factors as the plane and direction of silicon crystal, surface concenttation of impurities, relative directions between strain and current, and maximum strains applied.
Experiments using silicon canti-levers have proved that the (110) plane and [111] direction are suitable for crystal, and also that a suraface concentration of 2×1018/cm3 is optimum for impurities. It is further shown that the smallest nonlinearity of ±0.2% is obtained with a tensile stress of 300MPa applied on the longitudinal gage.
Displacement sensors satisfying the conditions mentioned above are manufactured and their characteristics are cofinrmed; the non-linearity is less than ±0.1% under the applied strain of 700με and a zero point shift below ±0.2% is obtained under the respeated strains of 108 times.
The temperature coefficient of semiconductor non-linearly relates to the temperature coefficients of piezo-resistor and piezoresistance coefficient, respectively. It is confirmed that temperature effects on the zero point and the span are less than ±0.5% respectively over the wide temperature range of 230-390K using a non-linear compeasntion procedure combining thick film thermistors with diffused type semiconductor resistors.
