There has been a variety of sensors in which a semiconductor strain gauge is employed, the semiconductor strain gauge utilizing a piezo electric effect characterized in that a large resistance change is displayed when strain is applied to a semiconductor. For example, there has been a thin film pressure sensor constituted in such a manner that a diaphragm is made of a metal such as stainless steel and a strain gauge made of a semiconductor thin film such as silicone thin film or the like is formed on the diaphragm via an insulating film.
As shown in FIG. 1A which is a cross sectional view, the thin film pressure sensor is structured in such a manner that a sensor portion 5 is, via a silicon oxide (SiO.sub.2) film 2 serving as an insulating film, formed on the surface of a diaphragm 1 made of stainless steel. The sensor portion 5 comprises: a strain gauge 3 constituted by polycrystalline silicone layer patterns formed on the SiO.sub.2 film 2; and an electrode 4 constituted by aluminum layer patterns and for supplying power to the strain gauge 3, the sensor portion 5 being covered with a passivation film 6 made of a silicone nitride layer. As shown in FIG. 1B, the sensor portion 5 is constituted by four patterns R1 to R4 forming the strain gauge 3 and six circuit patterns E1 to E6 for forming the electrode 4, the electrode 4 serving to supply power to the strain gauge 3. As shown in FIG. 1C, the sensor portion 5 forms a bridge circuit when it is expressed as an equivalent circuit so that the pressure can be measured by detecting the change in voltage between the electrode circuit patterns E2 and E5 due to the change in the level of the resistance of the strain gauge 3 when pressure is applied to the thin film pressure sensor.
However, the semiconductor has a disadvantage in that its characteristics excessively depend upon temperature. However, it has an excellent repetitive reproductionality against temperature. The characteristics of this type raise the reliability of the device after the compensation has been performed.
In the case of a semiconductor thin film pressure sensor, the resistance of the strain gauge changes in accordance with temperature as well as in accordance with the specific resistance of the strain gauge and the resistance change due to pressure. Therefore, both the pressure sensitivity and the zero point in a bridge circuit formed by combining strain gauges are inevitably changed. Since accuracy of the pressure sensor depends upon the way of performing the temperature compensation, a variety of methods have been attempted.
FIG. 2 illustrates an equivalent circuit acting in accordance with a method of compensating the temperature change of the strain gauge by using a temperature compensating circuit with a pressure sensor arranged to be operated by a rated voltage, the temperature compensating circuit being formed by combining a transistor and resistors.
The temperature compensating circuit 7 comprises an NPN-type transistor Tr and resistors R5 and R6 connected between the emitter of the transistor Tr and the base of the same and between the collector of the transistor Tr and the base of the same, the emitter of the transistor Tr being connected to the contact between electrode circuit patterns E4 and E6. The collector of the transistor Tr is connected to the positive side of a power supply source Vin via a circuit pattern E8 (omitted from FIG. 1).
The temperature compensating circuit 7 of the type described above is, as shown in FIG. 3, disposed outside the thin pressure sensor. That is, a thin film pressure sensor 100 is included in a case 101, and is included, together With the case 101, in a case 102 for an external circuit. The case 102 accommodates a printed board 104 to which an amplifier 105 and a temperature-compensating device 6 constituting the temperature compensating circuit 7 are connected. The accommodating portion accommodating the printed board 104 is closed by a cover 103. The output from the thin film pressure sensor 100 is temperature compensated by a temperature-compensating device 106 on the printed board 104, and is amplified by the amplifier 105 before being transmitted to an external circuit (omitted from illustration).
The sensitivity of the thin film pressure sensor 100 is, as designated by a line 1 of FIG. 4A, linearly lowered in accordance with the rising of the temperature, where the term "sensitivity" means the rate of change between the level of the pressure received by the pressure sensor 100 and the resistance caused from the pressure, that is, the following relationship is held: ##EQU1## As is shown from this, the more the sensitivity rises, the more the accuracy improves.
On the other hand, the potential drop across the transistor Tr for compensating the temperature becomes smaller in accordance with the rising of the temperature. Therefore, the voltage applied to the sensor portion 5 via the transistor Tr is, as designated by a line m of FIG. 4B, raised if the supplied voltage is made constant. Therefore, in a pressure sensor of a type having the above-described temperature-compensating circuit 7, the degree of deterioration in the sensitivity of the sensor portion due to the temperature rise is compensated by the rising of the voltage applied to the sensor portion 5. As a result, even if temperature has been raised, constant sensitivity level can be maintained as designated by a line n of FIG. 4A. As described above, the thin film pressure sensor 100 including the temperature compensating device 106 exhibits an excellent reliability since its sensitivity is not changed due to temperature.
In a pressure sensor having the above-described temperature compensating circuit 7, the rate of change in the potential drop of the transistor Tr, which is being used, due to temperature and the rate of change in sensitivity deterioration of the strain gauge 3 due to temperature do not always coincide with each other. However, since the temperature-dependency of the potential drop of the transistor Tr can be freely varied by changing the resistance ratio of the two resistor devices R5 and R6 connected to the transistor Tr, the resistance ratio of the resistor devices R5 and R6 is changed so as to coincide the change rate of the potential drop of the transistor Tr with the change rate of the sensitivity deterioration of the strain gauge 3. Thus, the sensitivity compensation of the thin film pressure sensor 100 against temperature is accurately performed.
On the other hand, in order to compensate the zero point against the dispersion between the strain gauges, a method has been employed in which a resistor is inserted in series with any side of the bride circuit, that is any of the patterns R1 to R4 of the strain gauge 3. In Japanese Patent Laid Open No. 53-22385, a technology has been disclosed in which a semiconductor gauge resistor is formed by diffusing impurities in a portion which is deformed by the pressure of a diaphragm type silicone mono-crystal and which thereby generates strain. Furthermore, a zero-point compensating semiconductor resistor is formed in a portion which is not deformed by the pressure of the silicone mono-crystal by diffusing impurities of the same density as that of the semiconductor gauge resistor. The thus formed gauge resistor and the zero-point compensating semiconductor resistor are connected to each other by an aluminum electrode.
However, even if the zero-point compensating resistor is formed by a semiconductor, the resistors R5 and R6 of the temperature compensating circuit 7 for compensating the sensitivity must be soldered to the printed board 104 together with the transistor Tr. Hitherto, the transistor Tr and the two resistor devices have been soldered to the printed board 104 or the electrode of the pressure sensor has been connected to the printed board 104. Since above--described elements are very small in size, the above-described connections have been very difficult to establish. In particular, since the conventional thin film pressure sensor has been manufactured by soldering the two resistor devices to the printed board 104 thereof, the number of elements and the manufacturing processes increase inevitably. As a result, a factor deteriorating the manufacturing efficiency and a factor deteriorating the yield due to the defective elements or imperfect contact of elements are excessively increased.
Hitherto, a sensor module consisting of the diaphragm and the sensor portion 5 is assembled to a pressure transducer or the other devices to be measured, a holding surface 1a (holding position Q) at which the sensor module is held and a pressure receiving surface ib are positioned at substantially the same surface (see FIG. 5). Therefore, when pressure is applied to the pressure receiving surface 1b of the conventional thin film pressure sensor 100, the holding surface 2 is deformed, causing the holding position to be deviated. As a result, the pressure receiving portion receives a bad influence. That is, in the conventional thin film pressure sensor 100, when the holding position on the holding surface 1a is radially shifted as q1, q2, q3 and q4 as shown in FIG. 5, the distribution of stress or on the surface of the diaphragm changes considerably as shown in the upper portion of FIG. 5. Therefore, when the holding position, which has been at a position shown in FIG. 6A when no pressure had been applied, is shifted to the periphery portion as shown in FIG. 6B due to the deformation of the diaphragm 1 and a holding member 110 by the pressure applied, the stress distribution in the pressure receiving portion of the diaphragm 1 is changed. As a result, a problem arises in that the linearity of the output characteristics of the pressure sensor with respect to the pressure receives a bad influence, and the accuracy is thereby deteriorated.