ACCELERATION SENSOR AND ITS MANUFACTURE

Abstract: An acceleration sensor having a novel structure by which acceleration measurement of high precision and reliability can be implemented. A monocrystalline silicon substrate (1) is joined to a monocrystalline silicon substrate (8) through an SiO₂ film (9). The monocrystalline silicon substrate (1) is a thin film. A cantilever (13) is formed on the monocrystalline silicon substrate (1). The thickness of the cantilever (13) in the direction parallel to the surface is smaller than that in the depth direction of the monocrystalline silicon substrate (1). The cantilever is movable in the direction parallel to the surface of the substrate. The surface of the cantilever (13) and the surface of the monocrystalline silicon substrate (1) which faces the cantilever (13) are covered with an SiO₂ film (5) in order to prevent the electrodes of the capacitance type acceleration sensor from being short-circuited. A signal processing circuit (10) is formed on the monocrystalline silicon substrate (1) to process signals produced by the movement of the cantilever (13).

Description: TECHNICAL FIELD The present invention relates to an acceleration sensor, and more specifically, a semiconductor type acceleration sensor suitable for air-bag system, suspension control system, or the like, for automobiles. BACKGROUND TECHNIQUES In producing a semiconductor type acceleration sensor, the movable part thereof has hitherto been prepared in such a way that it penetrates a singlecrystal silicon wafer. Accordingly, a size large enough to penetrate through the thickness of a wafer is required, and therefore, it has been difficult to miniaturize the acceleration sensor. In addition, in order to incorporate this sensor into a package, there has been required a relaxation part such as a pedestal in order to release the stress caused by the difference in coefficients of thermal expansion or the like. However, in Nikkei Electronics, November 11, 1991 (No. 540), pp 223 to 231, there is illustrated an acceleration sensor produced by use of a surface micro-machining technique. That is, a thin polysilicon film is laminated on a silicon substrate, and thereafter, this polysilicon film is etched, whereby a beam movable parallel to the surface of the substrate is formed, so as to form a differential capacity type acceleration sensor. However, when a beam structure is formed by use of polysilicon, if a signal processing circuit is formed around the formed beam structure, the sensor characteristics become unstable. This is because the beam structure is formed of a polycrystalline and amorphous material, resulting in noticeable variation for every production lot. U.S. Patent No. 4,679,434 discloses an accelerometer in which a silicon substrate with an inertial mass movable by an acceleration is sandwiched by insulative layers 26 and 28. A change of the capacitor caused by approaching the inertial mass toward the insulative layers due to acceleration is detected to determine the acceleration. Accordingly, it is still desirable to form an acceleration sensor by surface micro-machining singlecrystal silicon. Under such circumstances, the purpose of the present invention is to provide an acceleration sensor having a novel structure, by which high precision and high reliability can be realized. In addition, another purpose of the present invention is to produce this acceleration sensor with a good yield rate during the production process thereof. DISCLOSURE OF THE INVENTION This purpose is achieved by an acceleration sensor as defined in claim 1 and by processes for producing same as defined in claim 4 to 11. The gist of a first aspect of the invention resides in an acceleration sensor, comprising a second singlecrystal silicon substrate bonded onto a first singlecrystal silicon substrate with an insulating film interposed therebetween the second singlecrystal silicon substrate being made of a thin film, a beam formed on at least either of said first and second singlecrystal silicon substrates and movable in a direction parallel to the surface thereof, and a signal-processing circuit formed on at least one of said first and second singlecrystal silicon substrates for performing processing of signals produced by a movement of said beam, caused by an acceleration. In addition, the gist of a second aspect of the invention resides in a process for producing an acceleration sensor, comprising; a first step of forming, on a main surface of a first singlecrystal silicon substrate, a groove with a predetermined depth for formation of a beam; a second step of forming, on the main surface of said first singlecrystal silicon substrate, a film of a polycrystaline silicon, an amorphous silicon or a mixture thereof so as to fill said groove with said silicon film, and smoothing (flattening) the surface of said silicon film; a third step of bonding the main surface of said first singlecrystal silicon substrate to a second singlecrystal silicon substrate with an insulating film formed thereon, said insulating film being interposed between said first and second singlecrystal silicon substrates; a fourth step of polishing the reverse side of said first singlecrystal silicon substrate to a predetermined degree, so as to make said first singlecrystal silicon substrate a thin film; and a fifth step of forming a signal-processing circuit on at least either of said first and second singlecrystal silicon substrates, and thereafter, removing by etching said silicon film of a polycrystal silicon, an amorphous silicon or a mixture thereof from said reverse side of said first singlecrystal silicon substrates, to form a beam. In addition, the gist of a third aspect of the invention resides in a process for producing an acceleration sensor, comprising; a first step of bonding a main surface of a first singlecrystal silicon substrate to a second singlecrystal silicon substrate with an insulating film formed thereon, said insulating film being interposed therebetween; a second step of polishing the reverse side of said first singlecrystal silicon substrate to a predetermined degree, so as to make the first singlecrystal silicon substrate a thin film; a third step of forming a groove with a predetermined depth for formation of a beam; a fourth step of forming, on the reverse side of said first singlecrystal silicon substrate, a film of a polycrystal silicon, an amorphous silicon or a mixture thereof, so as to fill said groove with said silicon film, and smoothing the surface of said silicon film; and a fifth step of forming a signal-processing circuit on at least one of said first and second singlecrystal silicon substrates, and thereafter, removing by etching said film of polycrystal silicon, amorphous silicon or a mixture thereof from the reverse side of the first singlecrystal silicon substrate, to form a beam. According to the first aspect, when an acceleration is applied in a direction parallel to the surface of the bonded singlecrystal silicon substrates, the beam formed on the first or second singlecrystal silicon substrate moves. As this beam moves, signal processing is performed in the signal-processing circuit formed on the first or second singlecrystal silicon substrate. According to the second aspect, as a first step, a groove of a predetermined depth for formation of a beam is formed on the main surface of the first singlecrystal silicon substrate, and as a second step, a film of a polycrystalline silicon, an amorphous silicon or a mixture thereof is formed on the main surface of the first singlecrystal silicon substrate, whereby the groove is filled with the silicon film, and the surface of this silicon film is flattened. Subsequently, as a third step, the main surface of the first singlecrystal silicon substrate is bonded to a second singlecrystal silicon substrate having an insulating film formed thereon, said insulating film being interposed between the first and second singlecrystal substrates, and, as a fourth step, the reverse side of the first singlecrystal silicon substrate is polished to a predetermined degree, whereby the first singlecrystal silicon substrate is made into a thin film. Subsequently, as a fifth step, a signal-processing circuit is formed on the first or second singlecrystal silicon substrate, whereafter the polycrystalline, amorphous or mixed silicon film is removed by etching from the reverse side of the first singlecrystal silicon substrate, and a beam is formed. As a result, an acceleration sensor according to the first aspect of the invention is produced. According to the third aspect, as a first step, the main surface of a first singlecrystal silicon substrate is bonded to a second singlecrystal silicon substrate with an insulating film formed thereon, said insulating film being interposed between the first and second substrates, and as a second step, the reverse side of the first singlecrystal silicon substrate is polished to a predetermined degree, so that the first singlecrystal silicon substrate is made into a thin film. Subsequently, as a third step, a groove of a predetermined depth for formation of a beam is formed on the reverse side of the first singlecrystal silicon substrate, and as a fourth step, a film of polycrystalline silicon, an amorphous silicon or a mixture thereof is formed on the reverse side of the first singlecrystal silicon substrate, whereupon the groove is filled with the silicon film, and the surface of the silicon film is flattened. Subsequently, as a fifth step, a signal-processing circuit is formed on the first or second singlecrystal silicon substrate, whereafter the polycrystalline, amorphous or mixed silicon film is removed by etching from the reverse side of the first singlecrystal silicon substrate, and a beam is formed. As a result, an acceleration sensor according to the first aspect of the invention is produced. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plan of an acceleration sensor according to the first embodiment of the present invention, and Fig. 2 is a figure illustrating a section taken along the line A-A in Fig. 1. Figs. 3 to 10 are illustrations for explanation of the manufacturing process of the first embodiment, and, respectively, sectional view for each production stage. Fig. 11 is a plan illustrating an application example of the first embodiment, and Fig. 12 is a sectional view taken along the line B-B in Fig. 11. Figs. 13 to 21 are illustrations for explanation of the manufacturing process of the second embodiment, and, respectively, a sectional view for each production stage. Figs. 22 to 28 are illustrations for explanation of the manufacturing process of the third embodiment, and, respectively, a sectional view for each production stage. Figs. 29 to 31 are illustrations for explanation of the manufacturing process of the fourth embodiment, and, respectively, a sectional view for each production stage. Figs. 32 to 34 are illustrations for explanation of the manufacturing process examples to which the fourth embodiment is applied, and, respectively, a sectional view for each production stage. Fig. 35 is a plan illustrating an example of a sensor tip formed by use of an acceleration sensor according to the present invention. MOST PREFERRED EMBODIMENTS FOR WORKING THE INVENTION In the following, the embodiments of the present invention, will be explained with reference to the attached drawings. First Embodiment Fig. 1 is a view plan of an acceleration sensor produced by the first embodiment of the production process of the present invention, and Fig. 2 is a sectional view taken along the line A-A in Fig. 1. In addition, in the present embodiment, the sensor part and the signal-processing circuit are formed of the same singlecrystal silicon substrate. The present acceleration sensor is a capacity type acceleration sensor. As illustrated in Fig. 2, there is bonded onto a singlecrystal silicon substrate 8 a singlecrystal silicon substrate 1 through an SiO 2 film 9, and in the singlecrystal silicon substrate 1, there is formed a cantilever 13 by a trench 3 penetrating said substrate 1. As illustrated in Fig. 1, this cantilever 13 has a structure such that an end side thereof is branched into two parts. The cantilever 13 can be moved in a direction parallel to the surface of the singlecrystal silicon substrate 1 (the arrow direction C in Fig. 1). In addition, in the singlecrystal silicon substrate 1, a signal-processing circuit 10 is formed and is electrically insulated from the cantilever 13 by a polysilicon film 6 and an SiO 2 film 5. In the following, the first embodiment of the production process of the present invention, which is suitable for the production of the aforesaid structure, will be explained with reference to Figs. 3 to 10. First, as illustrated in Fig. 3, an n-type (100) singlecrystal silicon substrate 1 with a resistance of 1 to 20Ω·cm is provided, and on the main surface thereof, there is formed an SiO 2 film 2 with a thickness of about 1 µm by thermal oxidation, and the SiO 2 film 2 is formed into a predetermined pattern by a photolithographic method. This pattern is a pattern exposing, on the main surface of the silicon substrate, an area to be formed as a groove separating at least an area to be formed as a movable electrode (cantilever) from the substrate, and in the present embodiment, this pattern is formed as a pattern exposing also the main surface of the silicon substrate in an area for transversely insulating and separating the signal-processing circuit. Subsequently, on the main surface side of the singlecrystal silicon substrate 1, there is formed a trench 3 having a perpendicular wall with a predetermined depth, e.g. ranging from about 0.2 to about 30 µm. In the present embodiment, explanation will be made with respect to the case where the wall thickness is about 3 µm. Subsequently, after the SiO 2 film 2 has been removed, as illustrated in Fig. 4, an n + diffused layer 4 formed using phosphorus, arsenic or the like is formed on the main surface of the singlecrystal silicon substrate 1, including the internal wall of the trench 3, and further, an SiO 2 film 5 with a thickness ranging from 0.1 to 1 µm is formed by thermal oxidation or the like. In this case, in order to prevent damage by etching, there may be conducted the so-called "sacrifice oxidation" in which SiO 2 is formed by thermal oxidation and removed before formation of the n + diffused layer 4. Subsequently, as illustrated in Fig. 5, a polysilicon film 6 is formed on the main surface of the singlecrystal silicon substrate 1, and the trench 3 is filled with the polysilicon film 6. In addition, in the case where an impurity is introduced into the polysilicon film 6 in order to use the polysilicon film 6 as a conductive path, forming a thin polysilicon layer and diffusing phosphorus or the like in a high concentration before formation of the polysilicon film 6, allows the impurity to be introduced into the polysilicon film 6. Next, as illustrated in Fig. 6, the surface of the polysilicon film 6 is mirror finished, so as to make the thickness of the polysilicon film 6 a predetermined value. Subsequently, boron ions or other impurity are implanted into the polysilicon film 6 to form p + diffused layers 7 in predetermined areas. On the other hand, as illustrated in Fig. 7, another (100) singlecrystal silicon substrate 8 is provided, and an SiO 2 film 9 with a thickness ranging from 0.1 to 1.0 µm is formed on the main surface of the silicon substrate 8. Subsequently, the singlecrystal silicon substrate 1 and the singlecrystal silicon substrate 8 are dipped into e.g. an aqueous mixed solution of aqueous hydrogen peroxide and sulfuric acid, so as to subject these substrates to a hydrophilicity-imparting treatment. Thereafter, these substrates are dried, and as illustrated in Fig. 8, the main surface of the singlecrystal silicon substrate 1 and that of the singlecrystal silicon substrate 8 are put together at room temperature, and these substrates are introduced into a furnace at 400 to 1100°C for a period of time ranging from 0.5 to 2 hours, so as to strongly bond them. Subsequently, as illustrated in Fig. 9, by use of an aqueous solution of an alkali type, e.g. an aqueous KOH solution or the like, the reverse side of the singlecrystal silicon substrate 1 is subjected to selective polishing. The polishing is continued until the SiO 2 film 5 is exposed. As a result, the thickness of the singlecrystal silicon substrate 1 reaches a value which is approximately determined by the depth of the trench, e.g. about 3 µm, so that the substrate 1 is made thin. Subsequently, as illustrated in Fig. 10, in a predetermined area of the singlecrystal silicon substrate 1, there is formed a signal-processing circuit 10 (IC circuit portion) by use of an ordinary CMOS process, bipolar process or the like. In addition, a diffused layer for connecting to wiring and a metallic electrode film composed of an aluminum film or the like are formed, and wiring at the sensor part, wiring at the circuit part, and connection of the sensor part to the circuit part are performed. In Fig. 1 and Fig. 10, a MOS transistor alone is illustrated as a part of the signal-processing circuit 10. Further, on the upper surface of the signal-processing circuit 10, there is formed, as a passivation film 11, a plasma SiN film (P-SiN) e.g. by a plasma CD method. Subsequently, at the sensor part side, windows 12 are opened at predetermined areas of the passivation film 11, and the polysilicon film 6 filled in the trench 3 is exposed from the surface. By this window opening procedure, the singlecrystal portions where a cantilever or fixed electrodes are to be formed are demarcated from the polycrystal portions embedded in the trench, on the surface of the substrate. Subsequently, as illustrated in Fig. 2, by use of a 20% solution of TMAH (tetramethylammonium hydroxide) (CH 3 ) 4 NOH, the polysilicon film 6 is removed by etching through the windows 12 of the passivation film 11, from the reverse side (the upper side in Fig. 2) of the singlecrystalline silicon substrate 1. In this case, the passivation film 11 (P-SiN), SiO 2 film 5, aluminum wiring layer and p + diffused layer (p + polysilicon film) 7 are hardly etched by the selective etching. Accordingly, the bonding of the singlecrystal silicon substrate to the lower singlecrystal silicon substrate 8 is secured through the p + diffused layer (p + polysilicon film) 7. In addition, if trenches 3 are preliminarily formed also in the wide area of the cantilever 13 in Fig. 1, and etching windows 48 are provided simultaneously with the formation of the windows 12, in such a way that the etching windows 48 communicate with said trenches 3, the polysilicon film 6 below the movable part (cantilever 13) of the sensor can be more securely removed by etching through this etching window 48 when the polysilicon film 6 is removed by etching. By the aforesaid procedures, the cantilever 13 is formed. In this case, the cantilever 13 becomes, as illustrated in Fig. 2, smaller in the thickness L2 in a direction parallel to the surface of the singlecrystal silicon substrate 1 than in the thickness L1 in a direction along the depth of the singlecrystal silicon substrate 1. In a capacity type acceleration sensor, the end portions (bifurcated portions) of the cantilever 13 are formed as movable electrodes, and as illustrated in Fig. 1, the parts of the singlecrystal silicon substrate 1, opposing the end portions of the cantilever 13, are formed as fixed electrodes 14, 15, 16, and 17, respectively. In addition, as illustrated in Fig. 1, fixed electrode 14 and fixed electrode 16 are derived through an aluminium wiring layer 18a, fixed electrode 15 and fixed electrode 17 are derived through an aluminium wiring layer 18b, and the cantilever 13 (movable electrode) is derived through an aluminium wiring layer 18c. These aluminium wiring layers 18a, 18b, and 18c are connected to the signal-processing circuit 10, and by this signal-processing circuit 10, signal processing is conducted as the cantilever (movable electrode) 13 is displaced owing to an acceleration. In addition, by the n + diffused layers 4 (see Fig. 2) disposed on the cantilevers 13 (movable electrodes) and fixed electrodes 14, 15, 16, and 17, the electric potential is maintained at a constant value. Although a capacity type acceleration sensor is made in the present embodiment, if a piezo resistance layer is formed at the surface of the root portion of the cantilever 13, a piezo resistance type acceleration sensor can be formed. If these two types of sensors are formed in a one and same substrate, the precision and reliability of the acceleration sensor can further be improved. In the acceleration sensor thus produced, the singlecrystal silicon substrate 1 is bonded through an SiO 2 film to the singlecrystal silicon substrate 8, so as to form an SOI structure. In addition, in the cantilever 13, its thickness L2 in a direction parallel to the surface of the singlecrystal silicon substrate 1 is smaller than the thickness L1 in a direction of the depth of the singlecrystal silicon substrate 1. Accordingly, the cantilever 13 becomes movable, on the surface of the singlecrystal silicon substrate 1, in a direction parallel to the surface, whereby an acceleration to a direction parallel to the substrate surface is detected. As mentioned in the foregoing, in the present embodiment, on the main surface of the singlecrystal silicon surface 1, there is formed a trench (groove) 3 of a predetermined depth for formation of the cantilever 13 (the first step), and the polysilicon film 6 is formed on the main surface of the singlecrystal silicon substrate 1, so as to fill the trench 3 with said polysilicon film 6, and the surface of the polysilicon film 6 is flattened (smoothed) (the second step). Subsequently, the main surface of the singlecrystal silicon substrate 1 is bonded to the singlecrystal silicon substrate 8 with an SiO 2 film 9 (insulating film) formed thereon, through said SiO 2 film 9 being interposed between the substrates 1 and 8 (the third step), whereafter the reverse side of the singlecrystal silicon substrate 1 is polished to a predetermined degree, so as to make the singlecrystallibe silicon substrate into a thin film (the fourth step). Subsequently, the signal-processing circuit 10 is formed on the surface of the singlecrystal silicon substrate, whereafter the polysilicon film 6 is removed by etching from the reverse side of the singlecrystal silicon substrate, so as to form the cantilever 13 (the fifth step). Accordingly, during the process for the formation of the signal-processing circuit 10, in the course of the wafer process, the trench 3 in the surface portion of the singlecrystal silicon substrate, is filled with the polysilicon film 6, whereby contamination of the IC elements, contamination of production equipment, and degradation or deterioration of electrical properties accompanied therewith can be prevented. That is, in the wafer process, by contriving to prevent the surface structures such as concave portions or penetration holes from appearing on the wafer surface in the heat treatment, photolithographic treatment and the like, in the course of the process, it is possible to prevent contamination and the like, and to thereby stably provide acceleration sensors of high precision. The thus produced acceleration sensor comprises the singlecrystal silicon substrate 1, which is bonded through an SiO 2 film (insulating film) to the singlecrystal silicon substrate 8, and which is made a thin film; the cantilever 13, which is formed on said singlecrystal silicon substrate 1 and which is movable in a direction parallel to the surface of the substrate; and the signal-processing circuit 10, which is also formed on the singlecrystal silicon substrate 1 and which performs signal processing as the cantilever 13 moves owing to an acceleration. When an acceleration is applied in a direction parallel to the surface of the singlecrystal silicon substrate 1, the cantilever formed on the singlecrystal silicon substrate 1 moves. As the cantilever 13 moves, signal-processing is performed by the signal-processing circuit 10 formed on the singlecrystal silicon substrate 1. In such a way as above, by a micro-machining technique using singlecrystal silicon, an acceleration sensor is formed, by the novel structure of which high precision and high reliability can be realized. In addition, since the surface of the aforesaid cantilever 13 and the part of the singlecrystal silicon substrate 1, opposing said cantilever 13, are coated with the SiO 2 film (insulator) 5, electrode short-circuit in the capacity type acceleration sensor can previously be prevented. In addition, it suffices if at least either of the surface of the cantilever 13 or the part of the singlecrystal silicon substrate 1 opposing the cantilever 13 is coated with the SiO 2 film (insulator) 5. Further, as an application of the present embodiment, as illustrated in Figs. 11 and 12, the cantilever may be separated from the signal-processing circuit (IC circuit portion) 10 and an air bridge wiring is formed, in order to reduce parasitic capacity. In addition, the fixed electrodes 14, 15, 16, and 17 may be formed so as to have the same structure as above. This can be realized by forming a p + type polysilicon film 7 at the minimum position necessary for bonding the fixed electrodes to the lower substrate. In addition, although an aluminium wiring layer is used in the aforesaid embodiment, the wiring part may be formed by use of a polysilicon layer. Further, although two movable electrodes are formed at the end of the beam and simultaneously, four fixed electrodes 14, 15, 16, and 17 are formed, in the aforesaid embodiment, the movable and fixed electrodes may be formed like the teeth of a comb in order to further improve the sensitivity of the sensor. In addition, an oxide film may selectively be formed, instead of the formation of the p + polysilicon film 7. Second Embodiment Next, there will be made explanations about the production process in the second embodiment, laying stress on the points different from those in the first embodiment. In addition, in the second embodiment to be hereafter explained, explanations will be made by way of example of the case where a sensor having a structure according to the structure illustrated in Figs. 1 and 2 as explained in the aforesaid first embodiment, and there will be illustrated a sectional view corresponding to the A-A section of Fig. 1. In the aforesaid first embodiment, in order to form the cantilever 13, the p + diffused layer (p + polysilicon film) 7 is formed for the purpose of separating the cantilever portion from the singlecrystal silicon substrate at a predetermined distance, but in the present embodiment, a concave portion is formed before formation of a trench, for the purpose of separating the cantilever from the substrate at a predetermined distance. In Figs. 13 to 21, the production process is illustrated. First, as illustrated in Fig. 13, an n type (100) singlecrystal silicon substrate 20 is provided, and on the main surface of the provided singlecrystal silicon substrate 20, there is formed a concave portion 21 with a predetermined depth e.g. ranging from 0.1 to 5 µm. Subsequently, as illustrated in Fig. 14, on the main surface of the singlecrystal silicon substrate 20, there is formed an SiO 2 film 22, and a pattern is formed by a photolithographic means, in the same way as in the aforesaid first embodiment. Subsequently, on the main surface of the singlecrystal silicon substrate 20 including the bottom of the concave portion 21, there is formed a trench with a depth ranging from about 0.1 to about 30 µm (3 µm in the present embodiment) by dry etching or the like. Subsequently, as illustrated in Fig. 15, on the main surface of the singlecrystal silicon substrate 20 including the internal wall of the trench 23, there is formed an n + diffused layer 24, and an SiO 2 film 25 is formed by thermal oxidation. Thereafter, as illustrated in Fig. 16, a polysilicon film 26 is deposited in the trench 23 by the LPCVD method. Subsequently, as illustrated in Fig. 17, the surface of the polysilicon film 26 is polished by use of the SiO 2 film as an etching stopper, so as to smooth the surface. In the above case, although it is desirable that the surfaces of the polysilicon film 26 and the SiO 2 film 25 become smooth, even if the polysilicon film 26 is rather indented, so long as the surface of the SiO 2 film is made smooth, no inconvenience is caused in the subsequent wafer cementing. On the other hand, as illustrated in Fig. 18, another (100) singlecrystal silicon substrate 27 is provided, and, on the main surface of the substrate 27, there is formed an SiO 2 film with a thickness ranging from 0.1 to 1.0 µm by thermal oxidation of the substrate. Subsequently, singlecrystal silicon substrates 20 and 27 are dipped in e.g. a solution of an aqueous hydrogen per oxide and sulfuric acid, so as to subject them to a hydrophilicity-imparting treatment. Subsequently, the substrates are dried, and thereafter, the main surfaces of the two singlecrystal silicon substrates 20 and 27 are put together at room temperature, and introduced into a furnace at 400 to 1100°C for a period of time ranging from 0.5 and 2 hours, so as to strongly bond the two surfaces. Subsequently, as illustrated in Fig. 19, the reverse side of the singlecrystal silicon substrate 20 is subjected to selective polishing by use of an aqueous solution of alkali type, e.g. an aqueous KOH solution. The selective polishing is performed until the SiO 2 film 25 appears on the surface. As a result, the thickness of the singlecrystal silicon substrate 20 becomes e.g. about 3 µm, so as to be made a thin film. Subsequently, as illustrated in Fig. 20, a signal-processing circuit (IC circuit portion) 10 is formed through an ordinary CMOS process, bipolar process, or the like. Further, on the upper surface of the signal-processing circuit 10, there is formed, as a passivation film 11, a plasma SiN film (P-SiN film) by e.g. plasma CVD method. Subsequently, windows 12 are opened at predetermined areas of the passivation film 11, and the polysilicon film 20 is exposed to the surface at the sensor portion. Subsequently, as illustrated in Fig. 21, by use of a 20% solution of TMAH (tetramethylammonium hydroxide) (CH 3 ) 4 NOH, the polysilicon film 26 is removed by etching from the reverse side of the singlecrystal silicon substrate 20 through the windows 12 on the passivation film 11. In the above case, the passivation film 11 (P-SiN), SiO 2 film, and aluminium wiring layer are hardly etched by the selective etching. As a result, a cantilever 13 is formed. Also by the present embodiment, there is obtained the same effect as in the aforesaid first embodiment. Third Embodiment Next, there will be made explanations about the production process in the third embodiment, laying stress on the differential points between the first and third embodiments. Although, in the aforesaid first and second embodiments, the trench is filled with polysilicon before the bonding of the wafers, in the present embodiment, the trench is filled with polysilicon after the bonding of wafers, and in the final stage, the thus filled polysilicon is removed, so as to produce an acceleration sensor. In Figs. 22 to 28, the production process is illustrated. First, as illustrated in Fig. 22, an n-type (100) singlecrystal silicon substrate 30 is provided, and on the main surface of the provided singlecrystal silicon substrate 30, there is formed a concave portion 31 in a depth ranging from 0.1 to 5 µm, in the same way as in the aforesaid second embodiment. On the other hand, as illustrated in Fig. 23, a singlecrystal silicon substrate 32 is provided, and an SiO 2 film is formed by thermal oxidation on the main surface of the singlecrystal silicon substrate 32. Thereafter, the main surface of the singlecrystal silicon substrate 30 is bonded to the main surface of the single crystal silicon substrate 32. Subsequently, as illustrated in Fig. 24, the reverse side of the singlecrystal silicon substrate 30 is subjected to mirror polishing to a predetermined thickness (0.1 to 30 µm). Thereafter, as illustrated in Fig. 25, there is formed an SiO 2 film 34 to a thickness ranging from 0.1 to 2µm, following which the SiO 2 film is subjected to patterning, and a trench 35 is formed by etching. Thereby, a cantilever 13 and a transversal insulatedly separated area of the processing circuit portion are formed. Next, by thermal diffusion or the like, there is introduced an N type impurity of arsenic or phosphorus in a high concentration, and a highly concentrated n + layer 36 is formed in the silicon area which is not covered with SiO 2 films 33 and 34. Subsequently, as illustrated in Fig. 26, a thermal oxidation film is formed on the side wall of the trench 35 and the like, whereafter a polysilicon film 37 is formed on the surface of the singlecrystal silicon substrate 30, and the trench 35 is filled with the polysilicon film 37. Thereafter, as illustrated in Fig. 27, the surface of the polysilicon film 37 is selectively polished and smoothed until the SiO 2 film 34 appears on the surface. Further, as illustrated in Fig. 28, a signal-processing circuit 10 is formed, and finally, the polysilicon film 37 is removed by etching from the reverse side (upper surface side) of the singlecrystal silicon substrate 30, so as to again separate the cantilever 13 from the substrate to allow it to move. As described above, in the present third embodiment, the main surface of the singlecrystal silicon substrate 30 is bonded to the singlecrystal silicon substrate 32 with the SiO 2 film (insulating film) 33 formed thereon, through said SiO 2 film 33 being interposed between the substrates 30 and 32 (first step), and the reverse side of the singlecrystal silicon substrate is polished to a predetermined degree, so as to make the singlecrystal silicon substrate 30 a thin film (second step). Subsequently, on the reverse side of the singlecrystal silicon substrate 30, there is formed a trench (groove) 35 with a predetermined depth for formation of a cantilever 13 (third step), and the trench 35 is filled with the polysilicon film 37, and the surface of the polysilicon film 37 is smoothed (fourth step). Subsequently, a signal-processing circuit is formed on the singlecrystal silicon substrate 30, whereafter the polysilicon film 37 is removed, by etching, from the reverse side of the singlecrystal silicon substrate 30, so as to form a cantilever 13 (fifth step). Therefore, in the process of formation of the signal-processing circuit 10 in the wafer, the trench 35 is filled with the polysilicon film 37 in the upper surface part of the singlecrystal silicon substrate 30, whereby contamination of the IC elements, contamination of the production apparatus, and degradation or deterioration of electrical properties attended therewith can be prevented. That is, by contriving not to make a surface structure such as a concave portion or a perforation of the like appear on the wafer surface in the heat treatment, the photolithographical treatment or the like in the course of the wafer process, contamination and the like can be prevented, so as to stabilize the wafer process, and a stable supply of acceleration sensors, of high precision, can be produced. Fourth Embodiment In the following, there will be made explanations about the production process of the fourth embodiment, laying stress on the differences between the present embodiment and the third embodiment. The present embodiment is intended to produce an acceleration sensor at a lower cost than the production process of the third embodiment. In Figs. 29 to 31, the production process is illustrated. First, as illustrated in Fig. 29, an SiO 2 film with a thickness ranging from 0.1 to 2 µm is formed on the main surface of a singlecrystal silicon substrate 40, and a singlecrystal silicon substrate 42 is bonded to the substrate 40 so as to interpose the SiO 2 film 41 between the substrates 40 and 42. Subsequently, as illustrated in Fig. 30, the upper surface of the singlecrystal silicon substrate 42 is polished so as to make the thickness of the singlecrystal silicon substrate a predetermined value. That is, the thickness of the singlecrystal silicon substrate 42 is reduced to e.g. about 3 µm. Thereafter, a highly concentrated n + diffused layer 43 is formed on the upper surface of the singlecrystal silicon substrate 42, and further, an SiO 2 film 44 is formed thereon. It suffices if this highly concentrated n + diffused layer 43 is formed corresponding to the parts of the sensor which will be formed as movable electrodes and fixed electrodes. Subsequently, as illustrated in Fig. 31, patterning is applied to an SiO 2 film, as in the third embodiment, and a trench 45 is formed on the singlecrystal silicon substrate 42. Thereafter, the SiO2 film 41 below this trench 45 is partly removed by etching with an aqueous hydrofluoric acid solution. In this case, a part of SiO 2 film 41, below the part to be formed as a cantilever 13, is completely removed, and another part of the SiO 2 film 41, below the part to be formed as solid electrodes and signal-processing circuit portion, is left. Because the lower portion of the part is to be formed as the cantilever 13 it is narrower than with the other part. In addition, in the above case, the highly concentrated n + diffused layer 43 is separated into the cantilever 13 and fixed electrodes. Subsequently, through the same processes as illustrated in Figs. 26 to 28, there is produced a capacity type acceleration sensor. In the following, there will be explained application examples of the fourth embodiment with reference to Figs. 32 to 34. First, as illustrated in Fig. 32, an SiO 2 film with a thickness ranging from 0.1 to 2 µm is formed on the main surface of the singlecrystal silicon substrate 40, and a concave portion 47 with a depth ranging from 0.1 to 3 µm is formed at the area of the main surface of the singlecrystal silicon substrate 42, on which area a cantilever is to be formed. Subsequently, the main surface of the singlecrystal silicon substrate 42 is bonded to the main surface of the singlecrystal silicon substrate 40, the SiO 2 film 41 being put therebetween. Further, as illustrated in Fig. 33, the upper surface of the singlecrystal silicon substrate 42 is polished so as to give the singlecrystal silicon substrate 42 a predetermined thickness. That is, the thickness of the singlecrystal silicon substrate 42 is made thin to a value of about 3 µm. Subsequently, the aforesaid highly concentrated diffused layer 43 is formed on the upper surface of the singlecrystal silicon substrate 42, and an SiO 2 film 44 is formed thereon. Subsequently, as illustrated in Fig. 34, onto the singlecrystal silicon substrate 42, there are formed trenches 45 extending to the concave portions 47, and cantilevers 13 are formed. Thereafter, a capacity type acceleration sensor is produced through the same processes as illustrated in Figs. 26 to 28. By conducting the aforesaid procedures, electrical insulation can be made more securely as compared with the case where the SiO 2 film 41 is partly removed by etching. In addition, the mechanical strength of the sensor can be improved. In addition, the present invention is not restricted to the aforesaid embodiments, and is applicable to a twin-lever spring or a polylever spring embodiment, in addition to a cantilever spring embodiment. In addition, as illustrated in Fig. 35, it is possible to form two acceleration sensors 13a and 13b onto a singlecrystal silicon substrate 50, for detecting the acceleration in the direction X by the acceleration sensor 13a, and the acceleration in the direction Y by the acceleration sensor 13b. Further, it is possible to form an acceleration sensor capable of detecting the acceleration in a direction surface perpendicular to these X and Y direction acceleration sensors 13a and 13b, on the same substrate, so as to detect an acceleration in three-dimensions. In addition, when the present acceleration sensor is used as a capacity type acceleration sensor, it is possible to further stabilize the properties by forming the present acceleration sensor into a so-called "servo type" sensor (with a closed-loop circuit). In addition, in the aforesaid embodiments, the trenches (grooves) 3, 23, and 35 are filled with polysilicon films 6, 26, and 37, respectively, but there may be used a film of polysilicon, amorphous silicon or a mixed silicon containing polysilicon and amorphous silicon. In addition, in the aforesaid embodiments, a sensor portion and a signal-processing portion are formed in the singlecrystal silicon substrate to be formed as the upper side, but the present acceleration sensor is not restricted to such a structure, and it is possible to utilize also a singlecrystal silicon substrate formed as a base, and form a sensor portion and a signal-processing portion on the lower substrate. As described in detail in the foregoing, according to the present invention, high precision and high reliability can be realized by formatting a novel structure. In addition, when a signal-processing circuit is provided on the same chip as a movable beam, since there is produced neither a hollow part nor a groove during the production process, it is possible to make the processing stable. In addition, it is at the final step that the movable beam is made movable with respect to the substrate, whereby, in the case of the movable beam being bonded to the lower substrate to be formed as a pedestal or in the case of a circuit being formed, or the like, minute beams can be prevented from being broken, and the yield thereof can be made higher. In addition, since it is a micro-machining technique which determines the shape of the present acceleration sensor, the present acceleration sensor can be produced with high precision. [Industrial Availability] As described in the foregoing, the present invention is useful for the production of a semi-conductor acceleration sensor having minute movable parts, and the present acceleration sensor is suitable as an acceleration sensor to be used for air bag system, suspension control system and the like of automobiles. In addition, the present invention can be applied to a capacity type acceleration sensor for detecting acceleration in multiple directions.

Claims: An acceleration sensor, characterized in that it comprises: a second singlecrystal silicon substrate (8; 27; 32; 40), which is bonded onto a first singlecrystal silicon substrate (1; 20; 30; 42), an insulating film (9; 28; 33; 41) being interposed between the substrates; a movable beam (13), formed on at least one of said first or second singlecrystal silicon substrates, and which is movable in a direction (C) parallel to the surface of the substrate; and a signal-processing circuit (10), formed on at least one of the said first or second singlecrystal silicon substrates for performing processing of signals produced by a movement of said beam caused by acceleration. An acceleration sensor as claimed in claim 1, wherein said beam (13) is smaller in thickness in a direction parallel to the surface of said singlecrystal silicon substrates (1, 8; 20, 27; 30, 32; 40, 42) as compared to the thickness in a direction of the depth of said singlecrystal silicon substrates. An acceleration sensor as claimed in claim 1, wherein at least one of the surface of said beam (13) and the surface of the second singlecrystal silicon substrate (8; 27; 32; 40) opposing said beam is covered with an insulator (5, 9; 25, 28; 33, 34). A process for producing an acceleration sensor, comprising: a first step of forming, on a main surface of a first singlecrystal silicon substrate (1; 20), a groove of a predetermined depth at an area surrounding a movable beam (13) to be formed; a second step of depositing, on the main surface of said first singlecrystal silicon substrate (1; 20), a film (6; 26) of a polycrystalline silicon, amorphous silicon or a mixture thereof, so as to fill said groove with said silicon film, and smoothing the surface of said silicon film; a third step of bonding the main surface of said first singlecrystal silicon substrate (1; 20) to a second singlecrystal silicon substrate (8; 27) with an insulating film (9; 28) interposed therebetween; a fourth step of polishing the surface of said first singlecrystal silicon substrate (1; 20) on the reverse side, which is the side opposite to the main surface thereof, to a predetermined degree, so as to make the first singlecrystal silicon substrate a thin film; and a fifth step of forming a signal processing circuit (10) on at least either of said first and second singlecrystal silicon substrates (1, 8; 20, 27), and thereafter, removing by etching said silicon film (6; 26) of polycrystalline silicon, amorphous silicon or a mixture thereof from said reverse side of said first singlecrystal silicon substrate (1; 20), to form said beam (13). A process for producing an acceleration sensor as claimed in claim 4, wherein, in said first step, said predetermined depth is set according to the thickness of said beam (13) in the vertical direction thereof. A process for producing an acceleration sensor as claimed in claim 4, wherein, in said third step, said insulating film (9; 28) is formed on the surface of said second singlecrystal silicon substrate (8; 27). A process for producing an acceleration sensor as claimed in claim 4, wherein, in said second step, there remains said flat silicon film (6) on the main surface of said first singlecrystal silicon substrate (1). A process for producing an acceleration sensor as claimed in claim 7, wherein, prior the said third step, there is added a step of making said silicon film (6) remaining on the main surface of said singlecrystal silicon substrate (1) to be resistant to the etching in said fifth step, except for the silicon film (6) positioned at an area surrounding said movable beam (13). A process for producing an acceleration sensor as claimed in claim 4, wherein said first step additionally includes a step of forming a concave portion (21) on the main surface of said first singlecrystal silicon substrate (20), at the position where said beam (13) is to be formed. A process for producing an acceleration sensor as claimed in claim 4, wherein, prior to said fifth step, there is added a step of forming a semi-conductor element (10) on said first (1; 20) or second (8; 27) singlecrystal silicon substrate. A process for producing an acceleration sensor, comprising: a first step of boding the main surface of a first singlecrystal silicon substrate (30; 42) to a second singlecrystal silicon substrate (32; 40) with an insulating film (33; 41) interposed therebetween; a second step of polishing the surface of said first singlecrystal silicon substrate (30; 42) on the reverse side, which is the side opposite to the main surface thereof, to a predetermined degree, so as to make the first singlecrystal silicon substrate a thin film; a third step of forming a groove in the first singlecrystal substrate (30; 42), the groove extending from the reverse side of the substrate (30; 42) to said insulating film (33; 41) in an area surrounding a movable beam (13) to be formed; a fourth step of depositing, from the reverse side of the said first singlecrystal silicon substrate (30; 42), a film (37) of polycrystalline silicon, amorphous silicon, or a mixture thereof, so as to fill said groove with said silicon film, and smoothing the surface of said silicon film; and a fifth step of forming a signal processing circuit (10) on at least one of said first and second singlecrystal silicon substrates (30, 32; 40, 42), and thereafter removing by etching said film (37) of polycrystalline silicon, amorphous silicon, or a mixture thereof from the reverse side of the first singlecrystal silicon substrate (30; 42), to form said beam (13). A process for producing an acceleration sensor as claimed in claim 11, wherein, in said first step, said insulating film (33; 41) is formed on the surface of said second singlecrystal silicon substrate (33, 40). A process for producing an acceleration sensor as claimed in claim 11, wherein said fifth step includes a step of removing said insulating film (41) at a position corresponding to the position where said beam (13) is formed. A process for producing an acceleration sensor as claimed in claim 11, wherein said first step includes a step of forming, prior to said bonding, a concave portion (47) in an area of the main surface of said first singlecrystal silicon substrate (42), corresponding to the position where said beam (13) is to be formed.

Inventors: FUJII TETSUO, FUJII, TETSUO
Applicants: NIPPON DENSO CO, NIPPONDENSO CO., LTD.
Application year: 1993
Main classification: G01P15
IPC codes: H01L29, G01P15, B81B3