The present invention relates to pressure sensors, and specifically to an electric capacitance pressure sensor and manufacturing method thereof.
JP-A-2000-214035 discloses a conventional electric capacitance pressure sensor, a cross sectional view of which is shown in FIG. 15. The pressure sensor includes a substrate 10, a capacitance change portion 800 that changes a capacitance value thereof based on an applied pressure, and a reference portion 900 that minimally changes capacitance value thereof.
The capacitance change portion 800 has a first lower electrode 830 formed on a side of the substrate 10, a first movable electrode (first upper electrode) 840 that forms a diaphragm, a cavity portion 861 formed between the first lower electrode 830 and the first movable electrode 840, and an opening portion 11. The first movable electrode 840 is formed on the substrate 10 so as to be separated from the first lower electrode 830. The cavity portion 861 is formed by etching a sacrificial layer that is formed between the first lower electrode 830 and the first movable electrode 840. The opening portion 11 is formed by etching the substrate 10 from the other side thereof and communicates with the cavity portion 861.
The reference portion 900 also has a second lower electrode 930 formed on the surface of the substrate 10, and a second upper electrode 940 formed on the second lower electrode 930 through an isolation layer 920 to face the second lower layer 930.
In the pressure sensor, the movable electrode 840 deforms based on a pressure difference (P1xe2x88x92P2) between a pressure P1 applied to the movable electrode 840 on the side of the cavity portion 861 and a pressure P2 applied to the movable electrode 840 on the side of an opposite side thereof. Therefore, a change in electrical is capacitance formed by the lower electrode 830 and the movable electrode 840 is detected based on a difference between outputs of the capacitance change portion 800 and the reference portion 900.
In the pressure sensor shown in FIG. 15, the cavity portion 861 is formed by etching the sacrificial layer by pouring etching liquid through the opening portion 11, after forming the opening portion 11 by etching the substrate 10 and the lower electrode 830 formed in the substrate 10. Therefore, a surface of the lower electrode 830 on which the etching liquid is poured is removed. Accordingly, an area of the lower electrode 830 decreases and the electrical capacitance also decreases. As a result, the sensitivity of the pressure sensor decreases.
Further, the output characteristics of the pressure sensor are shown in FIG. 16. FIG. 16 shows the capacitance value change xcex94C1 of the capacitance change portion 800 and the capacitance value change xcex94C2 of the reference portion 900 when the pressure P1 is fixed and the pressure P2 is increased. As shown in FIG. 16, the capacitance value xcex94C2 is fixed even if the pressure P1 is changed, while the capacitance value xcex94C1 is changed when the pressure C1 is changed because the gap between the movable electrode 840 and the first lower electrode 830 decreases.
However, characteristics of an output difference (xcex94C1xe2x88x92xcex94C2) between the capacitance value xcex94C1 and the capacitance value xcex94C2 to the applied pressure P1, P2 produce concave graphical results because deformation characteristics of the movable electrode 840 relative to the applied pressure P1, P2 are nonlinear. Accordingly, accuracy of the pressure sensor decreases.
It is therefore an object of the present invention to provide an electrical pressure sensor and manufacturing method thereof that is capable of obviating the above problem.
It is another object of the present invention to provide an electrical pressure sensor manufacturing method that is capable of minimizing decreases in sensitivity of the sensor.
It is another object of the present invention to provide an electrical pressure sensor that is capable of obtaining high accuracy.
In an electrical capacitance pressure sensor of the present invention, a lower electrode is formed on a first surface of a substrate. The substrate has an opening portion that penetrates the substrate from the first surface to the second surface. Also, the lower electrode has at least one window portion that penetrates the lower electrode from the side of the substrate to the side of a cavity portion and communicates the cavity portion with the opening portion of the substrate.
According to the pressure sensor of the present invention, the at least one window portion can be formed in the lower electrode through the opening portion of the substrate. Therefore, a sacrificial layer formed between the lower electrode and a movable electrode is removed through the at least one window portion and the opening portion. In this case, because it is unnecessary to remove a part of the lower electrode, a decrease in the electrode area is minimized. Accordingly, it is possible to provide an electrical capacitance pressure sensor that is capable of controlling the decrease in sensitivity thereof.
The pressure sensor alternatively has a cover member formed on the lower electrode on the side of the opening portion of the substrate. The cover member covers the at least one window portion to separate the cavity portion from an outside thereof and to form a reference pressure chamber by the cavity portion.
Accordingly, the cavity portion is separated from an outside of the pressure sensor and forms a reference pressure chamber. Therefore, an absolute pressure sensor can be constructed.
In this case, the at least one window portion is preferably formed diagonally with respect to a perpendicular direction of the lower electrode. At least one large diameter member having a diameter larger than that of the window portion can alternatively cover the window portion. For example, the at least one large diameter member may be a ball member or a circular cylinder member.
Also, in an electrical capacitance pressure sensor of the present invention, an upper electrode is formed on the movable electrode and is separated by a predetermined distance from the movable electrode. A second cavity portion is formed between the upper electrode and the movable electrode to form a second electrical capacitance that is to be changed to a second capacitance value thereof when the movable electrode deforms based on a pressure difference between both sides thereof. The upper electrode has at least one second window portion that penetrates the upper electrode from the side of the second cavity portion to the outside thereof to communicate the second cavity portion to the outside.
The pressure sensor detects relative pressure between pressures of the first cavity portion and the second cavity portion. The at least one second window portion is formed in the upper electrode as well as at least one first window portion in the lower electrode. Therefore, a second sacrificial layer formed between the movable electrode and the upper electrode is removed through the at least one second window portion. Accordingly, it is possible to provide an electrical capacitance pressure sensor that is capable of minimizing a decrease in sensitivity thereof. The output characteristics of the pressure sensor have good linearity.