A physical quantity sensor such as an acceleration sensor and a yaw rate sensor detects physical quantity based on capacitance change.
The acceleration sensor for detecting acceleration of an mobile object such as a vehicle is disclosed in JP-A-H06-88837 corresponding to U.S. Pat. No. 5,561,248. The sensor includes a pair of electrodes, which are spaced apart from each other, so that the sensor detects the acceleration according to capacitance change between the electrodes.
The sensor is made of SOI substrate having a N conductive type and a low resistivity and made of silicon. The resistivity of the substrate is 10 m [Ω·cm], which is equal to 0.01 [Ω·cm]. The SOI substrate provides an upper substrate. An electrode for retrieving a wiring is disposed on an upper side of the SOI substrate. The electrode is made of, for example, conductive alloy such as gold and titanium. Thus, the upper side of the substrate is connected to a circuit for detecting capacitance via a lead wire. JP-A-2004-311951 teaches that the substrate may be made of silicon wafer having a P conductive type impurity such as boron.
However, the present inventors study that stress may be concentrated at an interface between a semiconductor layer and an insulation film when the electrode for detecting a capacitance is formed in the semiconductor layer via the insulation film having a doped N conductive type impurity. Thus, characteristics of the device may be reduced.
In the device in JP-A-H06-88837, since the N conductive type substrate is used, resistivity range of the device having no diffusion layer becomes narrow. Thus, it is required to improve the characteristics of the device without the diffusion layer and to improve mechanical reliability of the device by reducing the stress concentration at the interface between the semiconductor layer and the insulation film.