JPH0329372A - Manufacturing method of semiconductor pressure sensor - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention manufactures a semiconductor pressure sensor in which a strain cage is provided in a silicon pressure receiving diaphragm formed by digging a recess into semiconductor silicon. Regarding the method for [Prior Art] As is well known, semiconductor pressure sensors are developed using semiconductor technology because a silicon strain cage with a very high cage factor can be fabricated on a pressure-receiving diaphragm made of single-crystal silicon, which is an ideal highly elastic material. It has the advantage of being able to obtain a compact and highly sensitive sensor using a sensor, but in order to mass-produce sensors with uniform sensitivity, it is necessary to precisely drill recesses in the silicon wafer and make the dimensions, especially the thickness, of the pressure-receiving diaphragm uniform. For this reason, various efforts have been made to date. In the first conventional technique, an epitaxial layer having a different resistivity from that of the silicon substrate is formed, a strain cage is formed on the surface of the epitaxial layer, and then an acid layer is formed from the back side of the Go card. Chemically etch the recesses using a mixed aqueous solution such as nitric acid. The etching rate of this solution is selective depending on the resistivity of silicon, and as the etching progresses to the epitaxial layer, the etching rate drops rapidly. The thickness of the pressure receiving diaphragm can be made the same. In the second conventional technology, a p-type epitaxial layer is formed on an n-type substrate, a strain cage is similarly formed on the epitaxial layer side, and the strain cage is removed from the back side of the substrate by plasma etching or the like. First, a pilot hole is drilled several tens of meters before the interface with the taxial layer, and then the pilot hole is enlarged by electrolytic etching using an acid solution as an electrolyte and the n-type substrate as an anode to finish the recess. The process automatically stops when the p-type epitaxial layer is reached while anodizing the dozens of substrates left with this finishing digging, so the thickness of the pressure-receiving diaphragm can be determined more accurately than in the first conventional technique. You can arrange the . In the third conventional technique, the recess is dug only by plasma etching, and the etching conditions are accurately controlled electrically to control the depth of the trench and make the thickness of the pressure receiving diaphragm uniform. Therefore, there is no particular need to increase the size of the epitaxy medium layer on the substrate, and a strain cage with good detection characteristics can be fabricated by using single crystal silicon cut at a desired angle as the substrate. [Problem to be solved by the invention] All of the above conventional techniques were created as a result of ingenuity, and while each has the advantages described above, they all have the following problems. The first conventional technique generates a lot of heat and bubbles due to chemical reactions during etching.
As the digging progresses, the dissipation of heat and bubbles becomes insufficient, resulting in irregularities in the depth and shape of the recess compared to other conventional techniques. Therefore, its practical application is limited to digging large and shallow depressions. In the second conventional technique, the electrolytic etching automatically stops at the pn junction between the substrate and the epitaxial layer, so the thickness of the pressure receiving diaphragm can be controlled by the thickness of the epitaxial layer, but the electrolytic etching speed is slow and the process is slow. Since drilling is required, the process is divided into two steps, and irregularities may occur due to over-drilling or under-drilling or reverse leakage current of the pn junction. In addition, a porous polycrystalline silicon film tends to grow on the bottom of the recess, making it difficult to remove and easily falling off during use. In the third conventional technique, the uniformity of the digging depth of the recess is not as good as in the second conventional technique. To solve this problem, it is advantageous to make the plasma etching conditions isotropic, but under these conditions, the bottom of the recess is likely to be rounded, and the pressure receiving diaphragm is thicker at the center and periphery. This results in significantly different irregular shapes and causes variations in sensitivity. For this reason,
In practice, it is advantageous to make the trench shallower and thus make the wafer thinner, but this creates a problem that makes handling during the wafer process extremely difficult. In this way, all of the conventional technologies have their advantages and disadvantages, and it is currently difficult to find a definitive solution. However, in mass production of semiconductor pressure sensors, it is important to uniformly control the thickness of the pressure receiving diaphragm and therefore the sensitivity of the sensor, as described above. Most important, and secondly, it is desirable to be able to dig into the recess as easily as possible. Based on such circumstances, an object of the present invention is to provide a method for manufacturing a semiconductor pressure sensor that allows the thickness of a pressure receiving diaphragm to be easily controlled with high precision. [Means for Solving the Problems] According to the method of the present invention, a silicon substrate and a silicon plate are bonded to each other between a thin insulating film deposited on one polished surface and the other polished surface. A wafer is prepared by polishing the silicon plate to a predetermined thickness as described above, and a strain cage is created on the silicon plate side surface of the wafer. This is accomplished by digging out a recess from the hole using a plasma etching method until it reaches the insulating film, and forming the pressure-receiving diaphragm of the pressure sensor using the portion of temporary silicon that corresponds to this recess. Note that the simplest insulating film in the above structure is a silicon oxide film, and its thickness is preferably 0.5 to 2 μm. Although this insulating film may be provided on either the silicon substrate or the silicon plate before bonding, it is preferable to provide it on the former in order to completely seal the pressure sensor. It is also desirable to mirror polish the surface on which this insulating film is to be provided and the surface to be bonded to it. The silicon substrate and the silicon plate are preferably bonded to each other through the second insulating film by electrostatically adhering them together under heating, and the heating temperature at this time is 350 to 400'C.
For electrostatic adsorption, it is preferable to apply a DC voltage of about 400 to soov between the two. The bonding strength resulting from this is sufficiently high that if the silicon substrate and silicon plate are forcibly removed after bonding, the bond will not break at the bonded surface, but breakage will occur within the silicon. The strain cage to be provided on the surface of the prepared wafer on the silicon plate side is easiest to create by diffusion and is desirable in terms of obtaining a high cage factor, and the crystal plane of the silicon plate is selected so as to be suitable for this purpose. It is desirable that Plasma etching from the silicon substrate side surface of the wafer is preferably performed under anisotropic etching conditions in order to reduce the required chip area by engraving the recesses with side profiles that are approximately perpendicular to the surface. Under these conditions, the digging depth tends to be uneven, but by over-etching, the digging can be stopped within the insulating film. [Operation] The electrolytic etching method described above is advantageous in stopping the digging of the recess at the pn junction surface and making the thickness of the pressure-receiving diaphragm uniform, but it has the disadvantage of slow etching speed.In contrast, the plasma etching method The problem with conventional methods is that although the etching speed is fast, it is not very advantageous for digging recesses to a uniform depth, and each method has its advantages and disadvantages. The present invention focuses on the point that if an insulating film is formed inside the wafer in advance, the digging in four places by plasma etching can be stopped at the surface of this insulating film, similar to electrolytic etching. By adopting the structure described in the previous section for the manufacturing method of a semiconductor pressure sensor, it is possible to combine the advantages of electrolytic etching in which the thickness of the pressure-receiving diaphragm can be controlled with high precision and the advantages of plasma etching in which the etching speed is high. , most of the drawbacks of both methods were eliminated, and the problems were solved, making it possible to mass-produce highly sensitive and well-equipped semiconductor pressure sensors in a practical process. [Example] Hereinafter, an example of the method for manufacturing a semiconductor pressure sensor according to the present invention will be explained for each main process with reference to FIG. FIG. 1(a) shows a silicon substrate 1 and a silicon plate 2 that constitute a wafer 1G shown in FIG. 1(C) for fabricating a large number of small semiconductor pressure sensors. As can be seen from the semiconductor pressure sensor 20 in the fully-functioning state in FIG. 6 (6), silicon-based
is for the part that digs into the recess 22 on the back side of the pressure receiving diaphragm 2l, and since the support wall 23 around the recess 22 serves to support the pressure receiving diaphragm 21, it is necessary to match the coefficient of thermal expansion, but it is a mechanical part so to speak. Therefore, it can be constructed with any silicon material other than single crystal silicon. Its thickness is recess 22
It is selected according to the depth required for
It is assumed to be 0.4. The silicon plate 2 is for the pressure-receiving diaphragm 2l, so it is desirable to use single crystal silicon with high elasticity.The final required thickness is several tens of thousands, but if it is too thin, it will be inconvenient to handle when it is a single unit, so it is usually not used. 200
It is said to have a thickness of ~300.4 mm. In this example, since the strain cage incorporated in the pressure receiving diaphragm is of a diffusion type, it is assumed that n-type single crystal silicon having a (110) plane passing through the silicon [2] is used. As a preparatory step for bonding these silicon substrates 1 and 2 through an insulating film in the single state shown in FIG. 1(a), in this example, mirror polishing is applied to one side of the silicon substrate 1. On the polished surface 1a, a silicon oxide film is applied as an anti-green film 3, and one side of the silicon plate 2 is mirror polished and placed as a polished surface 2a. The insulating film 3 may be a normal thermal oxide film or a steam oxide film, and its thickness is preferably 0.5 to 2-2. Incidentally, at the same time as this insulating film 3 is deposited, the lower surface of the silicon Go plate 1 is covered with the same oxide film 4. In the next IVA (b) bonding process, silicon substrate 1 and silicon plate 2 are stacked and heated to 350 to 400 degrees Celsius, and a DC voltage E of 400 to 400 volts is applied to create a dense electrostatic charge between them. By adsorbing them, they are joined together and made into one body. Note that this bonding is also possible by ordinary high-temperature heating under pressure, but when a very thin insulating film is interposed on the bonding surface as in the present invention, this can be used to bond the silicon substrate 1.
By generating a strong electrostatic adsorption force between the silicon plate 2 and the silicon plate 2, it is possible to bond at low temperatures, reducing the residual thermal distortion in the silicon plate 2 and reducing the deflection when pressure is applied to the pressure receiving diaphragm 21. can be kept constant and the variation in detection sensitivity can be reduced. Additionally, as mentioned above, it is possible to form a bond stronger than that of silicon. Subsequently, FIG. 1(C) shows a finishing process for preparing a wafer 10, in which the surface 2b of the silicon plate 2 is ground and polished to accurately adjust its thickness to a desired value, and in this embodiment, a strain cage is formed by diffusion. Finish it to a mirror finish so that you can create it. In the method of the present invention, the thickness of the pressure-receiving diaphragm 2l can be substantially determined by the finished thickness of the silicon plate 2, and its value is of course selected depending on the pressure to be detected, but is usually about several dozen. FIG. 1(B) shows the process of assembling the strain cage 6, and as shown in the partially enlarged view on the right side of the figure, in this embodiment, a normal process oxidation Wlt5 is used as a mask from the surface of the n-type silicon plate 2. The shaped layer 6 is diffused to form a strain cage. This strain cage 6 is usually provided in four pieces, and is bridge-connected by a connection M7 made of aluminum or the like that makes conductive contact with both ends of the strain cage 6 through a window formed in the process oxide film 5. The top of the bonding film 7 is covered with a protection layer 118 made of silicon nitride or the like as usual. 1st
Figures (e) and (f) show the drilling process of the recess 22 by plasma etching, with Figure 1 (e) showing the process in progress and Figure 1 (f) showing the completed state. For this plasma etching, high-frequency plasma is used as usual, and the reactive gas is, for example, SP- mixed with 10 to 30% oxygen, and the etching rate for the silicon base It and the green film 3 is set to 2. The difference is about an order of magnitude. Furthermore, in order to dig the recess 22 into a shape having side surfaces 22a with almost no slope as shown in the figure, it is desirable to select gas pressure, high frequency power, etc. to provide anisotropic etching conditions. During the plasma etching, a recess 22 is dug through the window using a mask 9 made of Al ξ or a copper film attached to the back surface of the silicon substrate 1 of the wafer 1G. By making the etching anisotropic, the side surface 22a of the recess tends to be slightly pot-shaped, but as shown in the figure, it has a shape that is generally not inclined, and the side surface 22a of the recess has a tendency to become slightly pot-shaped. The so-called etch amount is about A. However, with this anisotropic etching, it is difficult to avoid slight variations in the digging depth, and as shown in FIG.
In other recesses, the etching residue 1b tends to appear on the silicon substrate due to insufficient digging. FIG. 1(f) shows the completed state of digging after over-etching to eliminate all of the remaining portion 1b. In this way, in the method of the present invention, even if the depth of excavation caused by plasma etching is considerably uneven, over-etching is always applied and the excavation is stopped within the green-free film 3, so that all the recesses 22 can be completely etched. The depth of digging can be made the same. This stopping of digging is of course due to the difference in etching speed between the silicon-based Fil and the insulating film 3 by more than two orders of magnitude, and conversely, in the method of the present invention, digging stops within the insulating film during over-etching. Select the thickness of the insulating film so that the As mentioned above, the depth of the recess 22 is usually less than 500 mm, and the irregularity in the depth of excavation is less than 50 tsa of lO% even under anisotropic etching conditions. ! If the etching rate of G3 is 1/100 that of silicon, the thickness should be 0.54 or more. Furthermore, if the insulating film 3 has this thickness, a sufficient voltage can be applied between the silicon substrate 1 and the silicon temporary 2 in the process shown in FIG. ．． However, in practice, it is desirable to allow a slight margin for the thickness of the insulating film 3, and based on experience, in the present invention, it is appropriate to set the insulating film thickness to 0.5 to 2 m as described above. be. The wafer 1 in which the recesses 22 have been dug in this way
0 is isolated into the semiconductor pressure sensor 20 in the guising process shown in FIG. 1(6) after the mask 9 is removed. The semiconductor pressure sensor 20 manufactured as described above is a small one having, for example, a 3 square thin chimney with a 2 square diameter recess 22 drilled therein, and the lower surface of the support wall 23 is used as the mounting surface. The pressure-receiving diaphragm 2l is used with the detected pressure and reference pressure applied to the upper and lower surfaces of the pressure-receiving diaphragm 2l, respectively, and the detection output is taken out from a bridge connection circuit of a plurality of strain cages 6 built into the pressure-receiving diaphragm 2l. In this semiconductor pressure sensor, since the thickness of the pressure receiving diaphragm 2l is determined by the thickness of the silicon plate 2 in the wafer 10, slight irregularities in the thickness of the insulation 11R3 may occur due to over-etching when digging the recess 22. The thickness of the pressure-receiving diaphragm can also be controlled with high accuracy of ±2 to 3-3. Furthermore, in this embodiment, as mentioned above, the silicon go temporary 1 and the silicon plate 2 are bonded at a relatively low temperature, so there is little residual thermal strain within the pressure receiving diaphragm 2l, and therefore the bridge detection output of the strain cage 6 is It has good linearity with respect to detected pressure, and the variation in pressure detection sensitivity level can be controlled to be small. Furthermore, in this embodiment, since the insulating film 3 is a thermal oxide film or the like on the silicon substrate 1, the airtightness of the interface between it and the supporting wall 23 is very high, and the pressure sensor can be operated without fear of the detected pressure leaking to the reference pressure side. The detection sensitivity can be maintained stably during long-term use. Further, by employing anisotropic plasma etching in this embodiment, the recess 22 can be dug with an almost ideal side shape without any inclination, and the chip size of the pressure sensor can be reduced to the minimum. It should be noted that the cross-sectional shape of the semiconductor pressure sensor 20 shown in the figure is approximately twice as wide in the vertical direction as in the horizontal direction for convenience of illustration. The present invention is not limited to the embodiments described above, but can be implemented in various ways. For example, the type of insulating film. The target to which the insulating film is attached before bonding. Bonding method of silicon substrate and silicon plate, plasma etching conditions. The types of strain cages, etc. are best described in the examples, but they are merely examples, and all or part of the above-mentioned effects can be obtained by making appropriate modifications as necessary. [Effects of the Invention] As explained above, in the present invention, a silicon substrate and a silicon plate are bonded to each other between a thin insulating film deposited on one polished surface and the other polished surface, and then the silicon plate is bonded. First, a wafer for fabricating a semiconductor pressure sensor is prepared by polishing it to a predetermined thickness.A strain cage is fabricated on the silicon plate side surface of this wafer, and this strain cage is fabricated from the silicon substrate side surface of the wafer. By using a plasma etching method, a recess is dug in the area corresponding to the area where the wafer is inserted until it reaches the wafer height, and the silicon plate corresponding to the recess is used as a pressure receiving diaphragm that receives the pressure to be detected. For all semiconductor pressure sensors manufactured, the digging of the recess is always stopped within the insulating film.
By aligning the pressure-receiving diaphragm to the thickness of the silicon plate in the wafer within an error of ±2 to 31, it is possible to mass-produce semiconductor pressure sensors with much more constant detection sensitivity than conventional methods. moreover,
The following effects can be achieved by the present invention and its advantageous embodiments. (6) Plasma etching has a fast etching speed, and deep recesses can be dug in a short time in a single process, so semiconductor pressure sensors of any design can be mass-produced at low production costs. (b) By preparing the wafer using a low-temperature bonding method using electrostatic adsorption, residual thermal distortion within the pressure receiving diaphragm can be reduced, the pressure sensitivity level of the semiconductor pressure sensor can be made uniform, and the bridge detection output characteristic can be straightened. You can improve your sexuality. (C) When using a diffusion type strain cage, by using single-crystal silicon with the most suitable crystal plane for the silicon plate for the pressure-receiving diaphragm, it is possible to create a strain cage with a high cage factor and therefore high detection sensitivity. (b) By digging the recess using anisotropic plasma etching, the recess can be made into an ideal shape and the semiconductor pressure sensor can be made smaller. (e) By providing an insulating film on the silicon substrate side before bonding, a pressure sensor with very good sealing performance and highly reliable detection characteristics can be provided.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing each main process of the best embodiment of the method for manufacturing a semiconductor pressure sensor according to the present invention. In the figure,

Claims (1)

[Claims] A step of preparing a wafer by bonding a silicon substrate and a silicon plate to each other between a thin insulating film deposited on one polished surface and the other polished surface, and then polishing the silicon plate to a predetermined thickness; A process of creating a strain cage on the surface of the wafer on the silicon plate side, and digging a recess from the silicon substrate side surface of the wafer in the area corresponding to the area where the strain cage is created until it reaches the insulating film using plasma etching method. 1. A method for manufacturing a semiconductor pressure sensor, characterized in that the portion of the silicon plate corresponding to the recess is used as a pressure receiving diaphragm that receives the pressure to be detected.