This application is based upon Japanese Patent Application No. 2000-387622 filed on Dec. 20, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor sensor, for example, an acceleration sensor or an angular velocity sensor that employs a structure movable in accordance with physical quantity applied thereto, and manufacturing method of the same.
2. Related Art
Recently, a semiconductor sensor that detects dynamic quantity such as acceleration or pressure can be provided by micro-machining process in which a laminated substrate such as a SOI (Silicon On Insulator) substrate is employed. FIG. 6 shows a schematic cross sectional view of a semiconductor dynamic quantity sensor 100. The semiconductor dynamic quantity sensor 100 has a laminated substrate 110 that is composed of a first semiconductor substrate 111 and a second semiconductor substrate 112 to support the first semiconductor substrate 111 with an oxide film 113 interposed therebetween. A structural portion is formed in the first semiconductor substrate 111.
The oxide film 113 and the second semiconductor substrate are partially removed at a region corresponding to the structure 101. As a result, the structure 101 forms a diaphragm shape. Incidentally, the structure 101 may form a one-end supporting beam structure (hereinafter, cantilevered beam structure) in accordance with a sensor structure.
The semiconductor dynamic quantity sensor 100 is formed with the following steps. First, the laminated substrate 110 is prepared. Then, a circuit or the like is formed on the first semiconductor substrate 111. After that, the second semiconductor substrate is partially removed by etching to form an opening 102. Moreover, the oxide film is removed at a portion corresponding to the opening 102 so as to form the structure 101 in the first substrate 111 on the opening 102.
It is easy to make the structure 101 having very fine and narrow gap by employing the oxide film 103 as a sacrificial layer in the laminated substrate 110, which is removed by etching in the process to form the minute structure 101, and to control thickness of the structure 101 when employing the laminated substrate.
When dynamic quantity is applied to the semiconductor dynamic quantity sensor 100, the structure having the diaphragm structure or the cantilevered beam structure is slightly deformed or displaced, so that the dynamic quantity can be detected. Recently, high accuracy in detection is required in the semiconductor dynamic quantity sensor 100.
It is, however, difficult to obtain the high accuracy in the semiconductor dynamic quantity sensor 100. FIG. 7 shows a schematic cross sectional view of a semiconductor dynamic quantity sensor 100 having a cantilevered beam structure. The structure 101 cambers to a direction opposite to the opening 102.
In the acceleration sensor, acceleration applied to the sensor is detected by detecting change in capacitance based on change in distance between a movable electrode and a fixed electrode as the cantilevered beam structure. Therefore, when the structure 101 cambers, it is difficult to face the movable electrode to the fixed electrode appropriately. As a result, it is difficult to detect the acceleration precisely.
This invention has been conceived in view of the background as described above and an object of the invention is to provide a semiconductor sensor capable of detecting dynamic quantity precisely.
The inventors in the present invention found a fact that the structure 101 formed in the first semiconductor substrate 111 cambers by a stress layer 114 formed on a surface (back surface) of substrate 111 that faces the opening 102.
It is supposed that the stress layer 114 is formed at a surface portion of the first substrate 111 adhering to the oxide film 113 by diffusion of oxygen when adhering the first substrate 111 to the second substrate 112 through the oxide film. The oxygen diffused in the first substrate is supposed to cause lattice stress.
This stress layer causes deterioration in detection in the pressure sensor as well as the acceleration sensor.
The inventors measured amount of camber in the structure 101 having the cantilevered beam as a sample of the semiconductor dynamic quantity sensor after removing the back surface of the structure 101. The sensor structure is shown in FIG. 7. Beam length L is 5.2 mm, thickness S of the first substrate 111 is 15 xcexcm. In this situation, the amount of camber xcex4 is measured when etching amount in the back surface of the structure 101 is changed. The amount of camber xcex4 is defined with distance between a surface of the first substrate 111 and a top of the cambered structure 101.
FIG. 5 shows a result of measurement of the camber. The amount of camber is reduced when the back surf ace of the structure 101 is etched by 0.2 xcexcm. Moreover, the amount of camber is reduced as the etching amount is increased. When the etching amount is 0.4 xcexcm or more, the amount of camber is reduced prominently as shown in FIG. 5.
After the first substrate 111 is adhered to the oxide film 113, the oxide film is removed and oxygen concentration is measured while the first substrate 111 is etched gradually from the back surface that adhered to the oxide film 113 to convince that the stress layer 114 is formed by the oxygen in the oxide film 113.
As a result, the oxygen concentration is highest at the back surface that adhered to the oxide film. Moreover, the oxygen concentration is reduced as the first substrate 111 is etched. Besides, dependency in the oxygen concentration with respect to the etching amount is similar to dependency in camber with respect to the etching amount shown in FIG. 5. Therefore, the stress layer 114 including oxygen in the first substrate 111 should be removed so that the camber is reduced sufficiently.
According to a first aspect of the invention, a semiconductor sensor has a structure formed in a first substrate that is formed on a second substrate with an oxide film interposed therebetween. The oxide film under the structure is removed. Besides, a part of the structure is removed at a portion that adheres to the oxide film before the oxide film is removed. The removed portion of the structure includes oxygen at high concentration in comparison with the bulk of the first substrate.
Preferably, a thickness of removed portion of the structure is 0.2 xcexcm or more to improve sensitivity of the sensor.
More preferably, the thickness of removed portion of the structure is 0.4 xcexcm or more.
Other features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings.