1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor acceleration sensor consisting of a supporting part, a beam part and a weight part made of semiconductors used for sensing an acceleration by detecting the deflection of the beam part accompanying the displacement of the weight part.
2. The Description of the Prior Art
A sensor for detecting an acceleration by finding the amount of deflection, due to the acceleration, of a beam part having a weight, from the change in the resistance value or the like has already been known widely, and miniaturization of the acceleration sensor through formation of the sensor using silicon as the semiconductor is being put into practice. As an example of such a semiconductor acceleration sensor, one may mention the device disclosed in a paper entitled "A Batch-Fabricated Silicon Accelerometer", IEEE Electron Devices, Vol. ED-26, No.12, p.1911, December 1979. According to this paper, an acceleration sensor consisting of a supporting part 31, a beam part 32 and a weight part 33 as shown in FIG. 1 can be formed by subjecting a single sheet of silicon substrate to anisotropic etching.
In such a sensor, the shape of the weight part and the beam part directly affects the sensitivity of the sensor, and in particular a high dimensional accuracy is demanded for the thickness of the beam part.
The fabrication process of a semiconductor acceleration sensor according to anisotropic etching is shown in FIGS. 2A to 2D as a first example of the prior art. In this method, first, using thermal oxide films 42 and 43 on a silicon substrate 41 as the etching masks (FIGS. 2A and 2B), the region between a supporting part 44 and a weight part 45 is etched from the rear surface by anisotropic etching (FIG. 2C), then a gap part 46 surrounding the periphery of the weight part is formed by giving the anisotropic etching also from the front surface (FIG. 2D).
As techniques of accomplishing a high precision working of the beam thickness, methods of using a silicon substrate consisting of two crystal layers with different conductivity types, and removing the crystal layer of one conductivity type and using the crystal layer of the other conductivity type as the beam part will be described as a second and third examples of the prior art in the following.
FIGS. 3A to 3F show the fabrication process of a second example of the prior art disclosed in Japanese Laid-Open Patent Publication No. 63-292071. According to this method, first, an n-type silicon substrate 52 is formed on a p-type silicon substrate 51, and a thermal oxide film 53 is formed on both surface of the laminated substrate (FIG. 3A). The thickness of the n-type silicon substrate 52 is the same thickness of a desired beam thickness using the thermal oxide film 53 as the mask, the n-type silicon substrate 52 and a part of the p-type silicon substrate 51 are removed in a V-shaped groove 54 to form a gap part (FIG. 3B). Next, a metallic film 55 for establishing electrical conduction with the n-type silicon substrate 52 is formed on the n-type silicon substrate (FIG. 3C). Next, the p-type silicon part between the supporting part 56 and the weight part 58 is removed by using an electrolytic etching, or electrochemical etching from the p-type silicon substrate 51 side (FIG. 3E). When the etching is reached to the metallic film 55, electrochemical etching is stopped automatically. Thus the thickness of the n-type silicon substrate 52 becomes the beam thickness.
In FIGS. 4A to 4G, the fabrication process of a third example of the prior art is shown. First, an n-type silicon substrate 62 is formed on a p-type silicon substrate 61, and a thermal oxide film 63 is formed on both sides of the laminated substrate (FIG. 4A). After removing the p-type silicon part in the region between the supporting part and the weight part by applying an electrochemical etching from the rear surface side using the thermal oxide film as the etching mask (FIG. 4C), the entire rear surface is covered with a protective film 64 of gold or the like (FIG. 4D), and a gap part is formed by carrying out an anisotropic etching from the front surface side (FIG. 4F). Finally, the sensor is completed by removing the gold protective film 64 (FIG. 4G).
Although several fabrication methods of the prior art semiconductor acceleration sensor have been proposed, these prior art fabrication methods have the following drawbacks. To begin with, in the first example of the prior art fabrication method, the thickness of the beam part varies with the temperature of etchant at the time of etching, the etching time and the stirring condition of the etchant. Furthermore, the etching takes a long time because of the large amount to be etched as much as 200 to 400 .mu.m, so that high accuracy control of the beam thickness is not easy to accomplish.
In the second example of the prior art, as shown in FIG. 3D, since the bottom portion of the V-shaped groove is dug into the p-type silicon substrate, the electrochemical etching in the step shown in FIG. 3E does not proceed beyond the bottom portion of the V-shaped groove. Thus a desired beam thickness cannot be achieved.
Furthermore, in the third example of the prior art, the covering property of the rear surface protective film becomes a matter of concern, where pin holes are generated also in the beam part due to the pin holes in the protective film, giving rise to a drawback that about 40% of units produced are defective.