METHOD FOR FABRICATION OF SEMICONDUCTOR DEVICE

Abstract: This invention is directed to prevent step breakage and short-circuit of wires resulting from steps of isolation trenches formed in an SOI substrate. An oxide film for a pad is formed on a main plane of an SOI layer formed on an insulating substrate and furthermore, a silicon nitride film and an SiO₂ film are sequentially formed. Thereafter, isolation trenches reaching the insulating substrate are formed by RIE using the SiO₂ film as the mask. An insulating film is then formed on the inner wall of the isolation grooves by thermal oxidation, and polycrystalline silicon is filled into the insolation trenches. `This polycrystalline silicon is etched back while control is made so that the upper end of polycrystalline silicon inside the isolation trenches is above the upper end of the silicon nitride film, and the excessive polycrystalline silicon deposited on the substrate surface is removed. Next, polycrystalline silicon inside the isolation trenches and the silicon nitride film are used as an etching stopper to etch and remove the SiO₂ film used as the mask at the time of the formation of the isolation trenches. Since this etching and removal of the SiO₂ film used as the mask is carried out after polycrystalline silicon is filled into the isolation trenches, the oxide film for isolating the substrates inside the SOI substrate is not etched when the mask is removed. When the masking SiO₂ film is etched and removed, polycrystalline silicon and the silicon nitride film inside the isolation trenches function as the etching stopper, and the oxide film for the pad as the lower layer and the insulating film formed on the inner wall of the trenches are prevented from being etched, and flatness in the trench portions is not lost.

Description: The present invention relates to a method for fabrication of a semiconductor device and particularly to a manufacturing method preferred for isolating elements from each other by forming a deep trench on a silicon substrate. BACKGROUND ART Up to now a method for isolating respective elements from each other by means of an insulator has been known as a element isolating method used in a monolithic semiconductor integrated circuit. In the Japanese patent Application JP-A- 61 059 852, for example, a semiconductor device manufacturing method of isolating elements from each other by forming an isolation trench on a bonded SOI. (silicon on insulator) substrate is disclosed. This method is the one which makes an SOI substrate by bonding a silicon substrate 42 with another silicon substrate 40 on which surface an insulating film 41 is formed as shown in Figure 36(A) through an insulating film 41 as shown in Figure 36(B), forms isolation trenches 43 reaching to the insulating film 41 in the SOI substrate from one main surface of this SOI substrate, then forms an insulating film 44 on the surface of the SOI substrate including inside walls of the isolation trench 43 by thermal oxidation or the like as shown in Figure 36(C), fills the isolation trenches 43 with polysilicon 45 as shown in Figure 36(D), removes extra parts of the insulating film 44 and polysilicon 45 which comes out from the isolation trenches 43, and electrically completely isolates respective element areas 46 from the substrate 40 or from each other by means of an insulator as shown Figure 36(E). As disclosed on page 66 of a reference paper "Ultra-Fast Silicon Bipolar Technology", for example, a method for forming an isolationtrench for isolating respective elements from each other after forming a field oxide film which is partially thick in thickness on the main surface of a silicon substrate is known as a method for forming an isolationtrench. The method will be described in the following. As shown in Figure 37, the method forms one by one in order a partially thick field oxide film 32, a silicon nitride film 33 and a silicon oxide film to be used as a mask on the main surface of a silicon substrate 31, forms an opening by selectively etching the field oxide film 32, the silicon nitride film 33 and the silicon oxide film 34 in an area where the field oxide film is thin in thickness, then forms an isolationtrench 35 by etching the silicon substrate 31 through the opening. And it etches off the silicon oxide film 34 used as a mask, forms an insulating film 36 on an inside wall of the isolation trench 35, then fills polysilicon 37 in the isolation trench 35. Further, it performs etching back of the polysilicon 37 deposited on the silicon nitride film 33 when filling the isolation trench with the polysilicon 37, etches off the silicon nitride film 33, and then forms a silicon oxide film 38 on the top of the polysilicon 37 in the isolation trench (see Figure 38). And by this manner, the method electrically divides completely the silicon substrate 31 into respective element areas by the isolation trench 35 and the insulating film 36. In a series of manufacturing treatmentes shown in Figure 36(A) to 36(E), up to now as described above, an oxide film is generally formed on the surface of an SOI substrate as a mask for forming an isolation trench on the SOI substrate. And the oxide film used as the mask for forming an isolation trench has been removed at once after forming the isolation trench. However as shown in Figure 36(E), in case of isolating an element area 46 by insulating film 41 from a substrate 40, an insulating film for isolating from the substrate is exposed inside the isolation trench immediately after forming the isolation trench. In this case, since the insulating film 41 inside the SOI substrate for isolating substrate and the oxide film for the mask have about equal etching rate, so that, if attempting to etch off the oxide film as the mask immediately after forming the isolation trench, the insulating film inside the substrate also is etched off at the same time. The present invention has been made by taking consideration of the above-mentioned actual facts. The first object of the invention is to provide a method for manufacturing a semiconductor device which makes it possible to prevent a local deterioration of dielectric strength in an isolation trench area by protecting the insulating film for isolating between the substrates which is exposed inside the isolation trenches from being etched in the course of the treatment. The reason why an isolation trench is formed in an area where a field oxide film is thin in thickness as shown in Figure 37 is that deterioration of flatness of the surface of a substrate is prevented which is feared in case of forming the isolation trench in an area where the field oxide film is thick in thickness. In case of forming an isolation trench in an area where a field oxide film is thick, an end face of the field oxide film is largely exposed by the isolation trench, the field oxide film also is etched when etching off a silicon oxide film used as a mask, and a deep hollow is caused to deteriorate flatness of the substrate. Deterioration of flatness of the substrate causes such a problem as breaking and short-circuiting of a polysilicon wiring or aluminum wiring. Therefore, if an isolation trench is formed in an area where a field oxide film is thin in thickness, deterioration of flatness of the substrate does not come so much into problem even if the field oxide film is etched. On the other hand, in case of forming an isolation trench in an area where a field oxide film is thin after forming the field oxide film as in a former method shown in Figures 37 and 38, it is necessary to make a semiconductor device for example a transistor to be manufactured larger in size by allowing for some divergence in mask matching so as to surely prevent a trouble due to exposing the end face of a thick part of the field oxide film by an isolation trench. The second object of the invention is to prevent a semiconductor device from being made unnecessarily large in size without deteriorating flatness of the surface of a substrate even if forming an isolation trench in an area where a field oxide film is thick by taking consideration of the above-mentioned problems. An isolation trench as described above is generally formed by etching a silicon substrate through the R.I.E (reactive ion etching) process. The R.I.E process is a process in which a cathode fall voltage (bias voltage) is generated by applying a high frequency voltage to an electrode on which a silicon substrate is mounted and the silicon substrate is etched by having ions or radicals as active particles generated by being plasma discharged material gas of a fluoride series collide and react against the substrate. Since the R.I.E process being accompanied by a physical etching action is greatly aggressive to the silicon substrate, it makes crystal defects on the surface of inside walls of an isolation trench and the surface of the silicon substrate surrounding of the inside walls due to the etching damage to cause leakage of electric current. For the purpose of eliminating such crystal defects, up to now an inside wall of an isolation trench has been processed by means of a sacrifice oxidation process or C,D,E (chemical dry etching) process, as disclosed in Japanese patent Applications JP-A- 3 127 850 and JP-A- 3 129 854 for example. The sacrifice oxidation process is a process of eliminating crystal defects by etching off an oxide film after forming the oxide film reaching to a depth where crystal defects exist in inside walls of an isolation trench. And the C.D.E process is a process of etching off a part having crystal defects by only a chemical etching of less aggressivity by means of radicals of material gas activated by plasma discharge. Other prior art processes are to be found in the documents EP-A- 0 398 468 and JP-A- 58 190 040. DISCLOSURE OF INVENTION The invention is defined by claim 1. To attain the object of the invention, a method of the invention fills in an isolation trench with polysilicon, removes an extra part of the polysilicon protruded out from the isolation trench on the substrate, and then etches off an oxide film as a mask. Namely, the method of the invention comprises: a step of depositing one by one in order a first layer and a second layer on the main surface of an SOI layer set on an insulating substrate, a step of making an opening in the first and second layers to expose a specified part of the main surface of the SOI layer, a step of forming an isolation trench reaching to the insulating substrate by etching the SOI layer through the opening by making the second layer as a mask, a step of forming an insulating film on the surface of inside walls of the isolation trench, a step of filling in the isolation trench with a filler through the opening until the top of the filler comes to be higher in height than the first layer, and a step of removing the second layer by making the filler and the first layer as an etching stopper, as defined more precisely in claim 1. Therefore, when etching off the second layer used as a mask in forming the groove, the filler and the first layer prevent proceeding of etching to a layer lower than them, so that, for example, difference in level in a trench area caused by etching of a field oxide film and the like formed on the surface of a semiconductor substrate and the like does not appear. According to the invention, therefore, even if an isolation trench is formed in an area where a field oxide film is thick, flatness of the surface of a substrate is not deteriorated and unnecessary magnification in size of a semiconductor device up to now caused by expecting some divergence in mask matching can also be prevented. And when removing the second layer used as a mask, not only a field oxide film but also an insulating film formed on the surface of inside walls of the groove are not etched in the direction of depth. If the insulating film on the surface of the inside walls of the groove is etched, a sharp difference in level is formed on the surface of the substrate in an area of the trench brings a problem of breaking or short-circuiting of a polysilicon wiring or aluminum wiring on the surface of the substrate, but if the invention is applied, since difference in level does not appear on the trench area and the surface of the substrate can be made flat, it is made possible to manufacture a semiconductor device having no breaking or short-circuiting of a polysilicon wiring or aluminum wiring. BRIEF DESCRIPTION OF DRAWINGS Figures 1 to 13 are cross-sectional views of an SOI substrate for explaining in order manufacturing process of the SOI substrate in case of applying a manufacturing method of a first embodiment of the invention. Figures 14 to 17 are cross-sectional views of an SOI substrate for explaining in order manufacturing processes of the SOI substrate in case of applying a manufacturing method of a second embodiment of the invention. Figure 18 is a diagram for showing a result of measuring density of defects after forming an isolation trench. Figures 19 to 31 are cross-sectional views of an SOI substrate for explaining in order manufacturing processes of the SOI substrate in case of applying a manufacturing method of a third embodiment of the invention. Figures 32 to 35 are cross-sectional views of an SOI substrate for explaining in order manufacturing processes of the SOI substrate in case of applying a manufacturing method of a fourth embodiment of the invention. Figures 36(A) to 36(E) are cross-sectional views of an SOI substrate for explaining in order former manufacturing processes of the SOI substrate. Figures 37 an 38 are cross-sectional views of a substrate for explaining a former treatment of forming a trench. BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the invention will be described in the following as referring to the drawings. [THE FIRST EMBODIMENT] The first embodiment of the invention applies a final polishing to one main surface of a first P - type single crystal silicon substrate 1 and then forms an insulating film 2 of specified thickness on it by thermal oxidation. And the embodiment bonds a second single crystal silicon substrate 3 having the finally-polished main surfaces performed mirror-like polishing to the insulating film 2 side of the surface of the first silicon substrate 1 under a sufficiently clean atmosphere and heats them to bond them in one so as to hold the insulating film 2 between the respective silicon substrates 1 and 3. In this manner, an SOI substrate is made which is composed by bonding the second substrate 3 through the insulating film 2 on the first silicon substrate 1 (see Figure 1). In Figure 1, a reference number 4 shows and N type high-density impurity (Sb) layer made by doping from the surface of the second N - type silicon substrate 3 before performing the bonding. And the embodiment forms a pad oxide film 8a on the surface of the second silicon substrate 3 side by thermal oxidation, deposits one by one in order on this surface an Si 3 N 4 film 9 as a first insulating layer and an SiO 2 film 10 as a second insulating film by a CVD method, and anneals the substrate at 1000°C to make the SiO 2 film 10 closely, and then deposits an unshown photoresist on it, processes it by means of a known photolithography process and R.I.E process using a gas of CF 4 or CHF 3 series as an etching gas, and makes an opening 11 by selectively etching the SiO 2 film 10, Si 3 N 4 film 9 and pad oxide film 8a up to reaching to the surface of the silicon substrate 3 using the photoresist deposited on the surface of the SiO 2 film 10 as a mask (see Figure 2). Figure 2 shows a state after stripping off the photoresist. Next, the embodiment makes an isolation trench 12 reaching up to the insulating film 2 by selectively etching the second silicon substrate 3 using the SiO 2 film as a mask by means of an R.I.E process using a gas of HBr series as an etching gas (see Figure 3). In this case, the thickness of the SiO 2 film 10 to be deposited has been determined in the previous step so that the isolation trench 12 successfully reaches to the insulating film 2 according by the etching rates of the SiO 2 film and the silicon substrate 3. Next, a C.D.E treatment is applied to the surface of the inside walls of the isolation trench 12. The C.D.E treatment is performed using an RF discharge type plasma etching system under a condition, for example, of material gases of CF 4 , O 2 and N 2 , a discharge frequency of 13.56 MHz, an etching rate of 150 nm/mn (1500 Å/min), and a distance from plasma to the wafer of 100 cm. In this manner, the inside wall of the isolation trench 12 is etched by about 150 nm (1500Å). Next, the inside walls of the isolation trench 12 processed in the C.D.E treatment are annealed. The annealing process is performed, for example, by heating for 30 minutes at 1000°C in an atmosphere of N 2 . Next, a sacrifice oxidation process may be applied to the annealed inside walls of the isolation trench 12. the sacrifice oxidation process is performed, for example, by forming a sacrifice oxide film of 50 nm (500Å) by means of a dry oxidation process at 1000°C and then removing this sacrifice oxide film by fluoric acid. And an insulating film 13 is formed on the inside walls of the isolation trench 12 by means of a wet thermal oxidation at 1050°C, for example, and then polysilicon 14 is deposited on it by means of an LP-CVD method. At this time, the polysilicon not only fills the isolation trench 12 but also deposits on the SiO 2 film 10 (see Figure 4). Next, an extra part of the polysilicon 14 deposited on the SiO 2 film 10 is subjected to etching back (the first time) by means of a dry etching method (see Figure 5). At this time, the etching process is stopped so that the top of the polysilicon 14 remaining in the isolation trench 12 come to be higher than the Si 3 N 4 film 9. Next, the SiO 2 film 10 is etched off by means of a wet etching method using a fluorine solution (see Figure 6). In this case, since the Si 3 N 4 film 9 and the polysilicon 14 left so that its top is higher than the Si 3 N 4 film 9 act as an etching stopper, the pad oxide film 8a and the insulating film 13 formed on the inside walls of the isolation trench 12 are not etched. Next, the part projecting beyond the Si 3 N 4 film 9 of the polysilicon 14 filled in the isolation trench 12 by the dry etching is etched back (the second time) (see Figure 7). In this case, it is desirable to control the top of the polysilicon 14 to come to be about 0.3 µm lower than the top of the pad oxide film 8a so that a thermal oxide film 15 may come to be on a same level with the surrounding pad oxide film 8a when having had the thermal oxide film 15 grow on the polysilicon 14 in the next step as it will describe later. Next, after the oxide film 15 has been grown by selectively thermally oxidizing the top of the polysilicon 14 filled in the isolation trench 12 through an opening of the Si 3 N 4 film 9 (see Figure 8), the Si 3 N 4 film 9 is etched off (see Figure 9). As clearly known from Figure 9, an area of the isolation trench 12 is kept flat without forming of difference in level. And a P well region 5, N well region 6 and deep N + region 7 are formed in the second silicon substrate 3 side made into an SOI layer by means of a known photolithography process and impurity diffusion process (see Figure 10). After this, a field oxide film 8 is formed on the surface of the second silicon substrate 3 side by means of LOCOS (local oxidation of silicon) method (see Figure 11). The LOCOS method is a process which forms again an Si 3 N 4 film as an oxidation deterring film on a specified part of a substrate and then forms a thick field oxide film 8 by oxidizing a part where the Si 3 N 4 film is not formed by means of thermal oxidation or the like. Figure 11 shows a state after removing the Si 3 N 4 film by H 3 PO 4 after performing oxidation by means of the LOCOS method. Next, the process of the embodiment forms a thin gate film after removing a pad oxide film 8a, forms a polysilicon wiring (gate electrode) 16 by applying LP-CVD method, photolithography process and etching method, and forms a P + diffusion layer 17 and N + diffusion layer 18 by means of selective doping (see Figure 12). While this period, since an etched depth of the field oxide film 8 is about 0.2 µm, flatness of the isolation trench 12 area is not deteriorated. After this, the process deposits an interlayer insulating film 19 of PSG, BPSG or the like, makes a contact hole in a necessary part, and forms an aluminum wiring 20 and a passivation film 21 made of a nitride film or the like by means of plasma CVD method, and thus manufactures a Bi-CMOS semiconductor device in which a CMOS transistor and a bipolar transistor are composited (see Figure 13). In this manner, a manufacturing method of the invention prevents proceeding of etching to the pad oxide film 8a or an insulating film 13 and the like which exist under the Si 3 N 4 film 9 and the polysilicon 14 when etching off the SiO 2 film 10 in the parts of isolation trench 12 area. Therefore, since flatness of the isolation trench 12 area can be obtained without making difference in level, such a problem as breaking or short-circuiting of a polysilicon wiring 16 or aluminum wiring 20 is not caused. And since the embodiment performs C.D.E treatment and annealing process after forming an isolation trench, it can prevent crystal defects which have been inevitably generated on the inside walls of the isolation trench and its surrounding surface of the silicon substrate when forming the isolation trench. This will be described in detail in the following. In the above-mentioned embodiment, after an insulating film 13 was formed on the inside walls of the isolation trench 12, the insulating film 13, SiO 2 film 8a and Si 3 N 4 film 9 were etched off, crystal defects were revealed by means of a Secco etching process, and the surface of the second silicon substrate 3 was observed by means of an optical microscope. And density of defects was computed by counting defects observed within a square of which one side is 200 µm. The result is shown in Figure 18. For comparison, density of defects was examined in the same manner also for ones which were processed neither C.D.E treatment nor annealing process, only an annealing process, an annealing process and a sacrifice oxidation process, only a C.D.E treatment, a C.D.E treatment and a sacrifice oxidation process, and a C.D.E treatment and an annealing process. The result also is shown in Figure 18. In Figure 18, a curve plotted by points + shows a result of observing the middle part of the surface of the second silicon substrate 3, a curve plotted by □ shows a result of observing the upper part of the surface of the second silicon substrate 3, and a curve plotted by ▵ shows a result of observing the lower part of the surface of the second silicon substrate 3. As the result, in the substrates which were processed by means of neither C.D.E treatment nor annealing process and by means of only one of them, reduction in density of defects was not seen, but crystal defects were observed in the vicinity of the isolation trench 12 on the surface of the second silicon substrate 3. Namely, it has proved that a former method performing only a sacrifice oxidation process and a C.D.E treatment cannot completely prevent crystal defects and cannot fully solve a trouble caused by crystal defects. On the other hand, crystal defects could be prevented in both substrates which were processed respectively by means of a C.D.E treatment and an annealing process and by means of a C.D.E treatment, an annealing process and a sacrifice oxidation process. As the result, it can be confirmed that it is possible to prevent crystal defects by applying at least a C.D.E treatment and an annealing process. Figure 18 shows that the density of defects of them is 10 4 defects/cm 2 , but this value is brought from the measurement limit and actually any crystal defects have not been observed. The above-mentioned measurement of density of defects was made by observing only the surface of the second silicon substrate 3, but as a result of observing a cross section of the second silicon substrate 3, the method of the embodiment could prevent crystal defects in the inside walls of the isolation trench also. In this way, damaged layers generated on an inside wall of a groove and on the surface of a silicon substrate in the vicinity of the groove when forming the groove are fully removed by process C.D.E treatment to the inside wall of the groove in the silicon substrate. And a damaged layer which has not completely been removed by the C.D.E process and another damaged layer newly generated by the C.D.E treatment are recovered by annealing the inside wall of the groove. In this manner, it is possible to eliminate crystal defects which have been inevitably generated on an inside wall of a groove and on the surface of a silicon substrate in the vicinity of the groove when forming the trench and to prevent a trouble of a current leakage caused by crystal defects. A condition of the C.D.E treatment is not particularly limited, but it is desirable to be a condition for making it possible to completely etch off a damaged layer generated when forming a groove, and furthermore which can be suppressed to the utmost a new damaged layer from being generated by the C.D.E process. This C.D.E treatment etches off the substrate to a depth of twice to 5 times of the damaged layer generated when forming the groove. And a condition of the annealing is not particularly limited so long as it can recover a damaged layer which has not completely removed by a C.D.E treatment and another damaged layer newly generated by a C.D.E process, but it can be done, for example, in a condition of heating for 10 to 30 minutes at 1000 to 1100°C in an atmosphere of inactive N 2 . In the above-mentioned first embodiment, a first etching-back treatment of the polysilicon 14 is performed by a dry etching process, but it may be performed by polishing. [THE SECOND EMBODIMENT] A second embodiment using a polysilicon film 9' instead of an Si 3 N 4 film 9 in the above-mentioned first embodiment is described in the following. The present embodiment gets an SOI substrate shown in Figure 1 as mentioned above, forms a pad oxide film 8a on the substrate as described above, deposits in order a polysilicon film 9' by means of LP-CVD method and an SiO 2 film 10 by means of a CVD method, and anneals the substrate at 1000°C to make the SiO 2 film 10 closely in the same manner as shown in Figure 2. After this, the embodiment deposits a photoresist on the substrate, makes a photoresist pattern by means of photolithography treatment, makes an opening 11 in the SiO 2 film 10, the polysilicon film 9' and the pad oxide film 8a by means of R.I.E treatment using a gas of CF 4 or CHF 3 series as an etching gas, and deposits a Si 3 N 4 film 22 on the surface of the substrate (see Figure 14). and the embodiments processes the substrate by means of an anisotropic R.I.E treatment and leaves the Si 3 N 4 film 22 only on the side wall of the opening 11 (see Figure 15). The Si 3 N 4 film 22 prevents the polysilicon film 9' exposed inside the opening 11 from being oxidized at the same time as when an insulating film 13 is formed on an inside wall of the isolation trench 12 by means of thermal oxidation in the following step. Next, the embodiment selectively etches the second silicon substrate 3 using the SiO 2 film as a mask by means of an R.I.E process using a gas of HBr series as an etching gas to make an isolation trench 12 reaching up to the insulating film 2, and processes the inside walls of the isolation trench 12 by means of a C.D.E treatment and an annealing process in order, as described above. And an insulating film 13 is formed by thermally oxidizing the inside walls of the isolation trench 12 and then the Si 3 N 4 film 22 covering the wall of the opening 11 is removed by a H 3 PO 4 solution (see Figure 16). As described above, in case of forming the insulating film 13, the polysilicon film 9' is not oxidized, since it is covered by the Si 3 N 4 film 22 in the opening 11. If the polysilicon film 9' is oxidized at this time, the oxidized part of the polysilicon film 9' also comes to be etched by etchant at the same time as when etching off the SiO 2 film 10 in the following step, and this causes difference in level in the isolation trench 12 area. Next, in the same manner as the step shown in Figure 4, after depositing the polysilicon 14 (see Figure 17), by passing through the same steps as shown in Figure 5 to 13, a Bi-CMOS semiconductor device is manufactured. In the present embodiment, since the polysilicon film 9' and the polysilicon 14 filled in the isolation trench 12 act as an etching stopper when removing the SiO 2 film 10, it is prevented that the pad oxide film 8a under the polysilicon film 9' and the insulating film 13 are etched at the same time. And as described above, since there is not an oxidized part also in the polysilicon film 9', etching does not proceed to a lower layer under there. Furthermore, the present second embodiment can remove the polysilicon film 9' at the same time as when applying the second etching-back process to the polysilicon film 14. [THE THIRD EMBODIMENT] The above-mentioned first embodiment forms a pad silicon oxide film of uniform thickness, forms an isolation trench after deposition of an Si 3 N 4 film and an SiO 2 film by means of a CVD method, performs in order forming an insulating film on the inside wall of the isolation trench, filling the isolation trench with polysilicon, etching back an extra part of the polysilicon, and etching off the SiO 2 film using the Si 3 N 4 film and the polysilicon as an etching stopper, performs patterning of the Si 3 N 4 film or redepositing of the Si 3 N 4 film, and then forms a field oxide film by processing the pad silicon oxide film by means of an LOCOS process, but the field oxide film 8 may be formed by means of an LOCOS process in advance. An example of it is shown in the third embodiment. The third embodiment of the invention applies a mirror-like polishing to one main surface of a first P - type single crystal silicon substrate 1 and then forms an insulating film 2 of specified thickness on it by means of thermal oxidation. And the third embodiment bonds a second single crystal silicon substrate 3 having a mirror-like polished main surfaces to the insulating film 2 side of the surface of the first silicon substrate 1 and heats them in a sufficiently clean atmosphere to bond them in one so as to hold the insulating film 2 between the respective silicon substrates 1 and 3. In this manner, an SOI substrate is made which is composed by bonding the second silicon substrate 3 through the insulating film 2 to the first silicon substrate 1 (see Figure 19). In Figure 19, a reference number 4 shows an N type high-density impurity (Sb) layer made by doping from the surface of the second N - type silicon substrate 3 before bonding of the two substrates. And the embodiment forms a P well region 5, N well region 6 and deep N + region 7 on the second silicon substrate 3 side made into an SOI layer by means of a series of oxidation, photolithography and impurity diffusion process (see Figure 20). While this period, an oxide film on the surface of the second silicon substrate 3 can be freely grown and removed. After this, a field oxide film 8 is formed on the surface of the second silicon substrate 3 side by means of an LOCOS method (see Figure 21). Figure 21 shows a state after removing an Si 3 N 4 film with H 3 PO 4 after performing oxidation by means of an LOCOS process. And the third embodiment deposits again in order an Si 3 N 4 film 9 as a first insulating layer and an SiO 2 film 10 as a second insulating layer on the surface of the substrate by means of a CVD process, and anneals them at 1000°C to make the SiO 2 film 10 closely. Next, the embodiment deposits an unshown photoresist and in an area where the field oxide film 8 is thick in thickness the embodiment makes an opening 11 reaching to the surface of the silicon substrate 3 by selectively etching the SiO 2 film 10, Si 3 N 4 film 9 and field oxide film 8 using the photoresist as a mask by means of a known photolithography process and R.I.E process using a gas of CF 4 or CHF 3 series as an etching gas (see Figure 22). Figure 22 shows a state after stripping off the photoresist. Next, the embodiment makes an isolation trench 12 reaching to the insulating film 2 by selectively etching the second silicon substrate 3 using the SiO 2 film 10 as a mask by means of an RIE process using a gas of HBr series as an etching gas film 2 (see Figure 23). In this case, the thickness of the SiO 2 film 10 to be deposited in the previous step has been determined so that the isolation trench 12 may successfully reach to the insulating film 2 by the etching rates of the SiO 2 film 10 and the silicon substrate 3. Next, a C.D.E treatment is applied to the surface of the inside walls of the isolation trench 12. This C.D.E treatment is performed by means of an RF discharge type plasma etching apparatus under a condition, for example, of material gases of CF 4 , O 2 and N 2 , a discharge frequency of 13.56 MHz, an etching rate of 150 nm/mn (1500 Å/min), and a distance from the plasma to the wafer of 100 cm. In this manner, the inside walls of the isolation trench 12 are etched by about 150 nm (1500 Å). Next, the inside walls of the isolation trench 12 in the C.D.E treatment are annealed. This annealing process is performed, for example, by heating for 30 minutes at 1000°C in an atmosphere of N 2 . Next, a sacrifice oxidation process may be applied to the annealed inside walls of the isolation trench 12. In this sacrifice oxidation process, for example, after forming a sacrifice oxide film of 50 nm (500 Å) by means of a dry oxidation process at 1000°C, then this sacrifice oxide film is removed by fluoric acid. And an insulating film 13 is formed on the inside walls of the isolation trench 12 by means of a wet thermal oxidation at 1050°C, for example, and then polysilicon 14 is deposited on it by means of an LP-CVD method. At this time, the polysilicon not only fills the isolation trench 12 but also deposits on the SiO 2 film 10 (see Figure 24). Next, the polysilicon 14 deposited on the SiO 2 film 10 by dry etching process is etched back (the first time) (see Figure 25). At this time, the etching process is stopped so that the top of the polysilicon 14 remaining in the isolation trench 12 may come to be higher than the Si 3 N 4 film 9. Next, the SiO 2 film 10 is etched off by means of a wet etching process using a fluorine solution (see Figure 26). In this case, since the Si 3 N 4 film 9 and the polysilicon 14 left so that its top may be came to the portion higher than the Si 3 N 4 film 9 acts as an etching stopper, the field oxide film 8 under the Si 3 N 4 film 9 and the insulating film 13 formed on the inside walls of the isolation trench 12 are not etched. Next, the part of the polysilicon 14 filled in the isolation trench 12 which projects beyond the Si 3 N 4 film 9 is etched back (the second time) (see Figure 27). In this case, it is desirable to control the top of the polysilicon 14 to come to be 0.3 µm lower than the top of the field oxide film 8 so that a thermal oxide film 15 may come to be on a same level with the surrounding field oxide film 8 when growing the thermal oxide film 15 which will be described later on the polysilicon 14 in the next step. Next, after the oxide film 15 has been grown by thermally oxidizing the top of the polysilicon 14 filled in the isolation trench 12 by the Si 3 N 4 film 9 (see Figure 28), the Si 3 N 4 film 9 is etched off (see Figure 29). As clearly known from Figure 29, an area of the isolation trench 12 is kept flat without making difference in level. And the third embodiment forms a thin gate oxide film after removing a pad oxide film 8a, then forms a polysilicon wiring (gate electrode) 16 by means of an LP-CVD method, photolithography process and etching process, and forms a P + diffusion layer 17 and N + diffusion layer 18 by means of selective doping (see Figure 30). While this period, since an etching depth of the field oxide film 8 is about 0.2 µm, and flatness of the isolation trench 12 area is not deteriorated. After this, the present embodiment deposits an interlayer insulating film 19 of PSG, BPSG or the like, makes a contact hole in a necessary part, and forms an aluminum wiring 20 and a passivation film 21 made of a nitride film or the like by means of a plasma CVD method, and thus manufactures a Bi-CMOS semiconductor device in which a CMOS transistor and a bipolar transistor are composited (see Figure 31). In this manner, a manufacturing method of the invention prevents the proceeding of etching for an oxide film such as a field oxide film 8 or an insulating film 13 and the like which exist under the Si 3 N 4 film 9 and the polysilicon 14 when etching off the SiO 2 film 10 in the isolation trench 12 area. Therefore, since flatness of the isolation trench 12 area can be obtained without making difference in level, such a trouble as breaking or short-circuiting of a polysilicon wiring 16 or aluminum wiring 20 is not caused. And in the above-mentioned former method shown in Figures 37 and 38, since an isolation trench 35 is formed in an area where a field oxide film 32 is thin in thickness, a edge part B of the silicon substrate 31 is existed around the top of the isolation trench 35, so the etched-back top of the polysilicon 37 comes to be lower than the edge part B. Therefore, a vertical bird's beak A comes to be formed in a corner when forming an oxide film 38 on the top of the polysilicon 37 and as a result a problem has been brought that crystal defects are liable to be generated by stress concentration to the edge part B of the silicon substrate 31. In the present embodiment, since an isolation trench is formed in an area where a field oxide film 8 is thick in thickness and the top of the polysilicon 14 which has performed the second etching-back process comes to be higher than the top of the second silicon substrate 3 (see Figure 27), crystal defects are not generated when oxidizing the polysilicon 14 without stress concentration to the second silicon substrate 3 caused by a vertical bird's beak as formed in a former method which forms an isolation trench in an area where a field oxide film 8 is thin in thickness (see Figures 37 and 38). Therefore, leakage of current caused by crystal defects can be prevented. Furthermore, in opposition to the former method which forms an isolation trench in an area where a field oxide film 8 is thin in thickness, the present embodiment does not need to make a semiconductor larger in size for the allowance for some divergence in mask matching, so it can make the semiconductor device smaller in size. In this manner, the embodiment makes it possible to form an isolation trench in an area where a field oxide film is thick in thickness without deteriorating flatness of a silicon substrate. Therefore, since it is not necessary to allow for some divergence in mask matching and occurrence of crystal defects of a silicon substrate is also deterred, a semiconductor device can be manufactured which does not have breaking or short-circuiting of a polysilicon wiring and aluminum wiring and is not made unnecessarily large in size. And since an isolation trench is formed after forming a field oxide film, it is thought that occurrence of crystal defects around the isolation trench can be deterred. Namely, if a field oxide film is formed after forming an isolation trench, it is feared that the volume expands when forming the field oxide film and crystal defects are generated by stress concentration to the boundary between a silicon substrate and an isolation trench, but the present embodiment does not have such a fear. Furthermore, the present embodiment also applies a C.D.E treatment and annealing process to an inside wall of the isolation trench 12. Thanks to this, the embodiment makes it possible to fully or completely remove by means of a C.D.E treatment a damaged layer generated on the inside wall of the isolation trench 12 when forming the isolation trench 12, to remove by means of the following annealing process the damaged layer which has not completely been removed by the C.D.E treatment and another damaged layer newly generated by the C.D.E process, and elimination of crystal defects on the inside walls and the like of the isolation trench 12 becomes possible. In the above-mentioned third embodiment, the first process of etching back the polysilicon 14 is performed by a dry etching process, but it may be done by a polishing process. [THE FOURTH EMBODIMENT] The fourth embodiment using a polysilicon film 9' instead of an Si 3 N 4 film 9 used in the third embodiment is described in the following. After processing a substrate in the steps shown in Figures 19 to 21, the present embodiment deposits in order a polysilicon film 9' by means of an LP-CVD method and an SiO 2 film 10 by means of a CVD method and anneals the substrate at 1000°C in the same way as the process shown in Figure 22 to make the SiO 2 film 10 closely. After this, the present embodiment deposits a photoresist, forms a photoresist pattern by means of a photolithography process, makes an opening 11 in the SiO 2 film 10, polysilicon film 9' and field oxide film 8 by means of an R.I.E process using a gas of CF 4 or CHF 3 based as an etching gas, and deposits an Si 3 N 4 film 22 on the surface of the substrate (see Figure 32). And the embodiment leaves the Si 3 N 4 film 22 only on a side wall of the opening 11 by means of an anisotropic R.I.E process (see Figure 33). The Si 3 N 4 film 22 prevents the polysilicon film 9' exposed in the opening 11 from being oxidized at the same time as when forming an insulating film 13 on an inside wall of the isolation trench 12 by thermal oxidation in the following step. Next, the embodiment selectively etches the second silicon substrate 3 using the SiO 2 film 10 as a mask by means of an R.I.E process using a gas of HBr series as an etching gas to form the isolation trench 12 reaching to the insulating film 2, applies in order a C.D.E treatment and an annealing process on the inside wall of the isolation trench 12 as described above, and forms the insulating film 13 by means of thermal oxidation, and then removes the Si 3 N 4 film 22 covering the surface of the wall of the opening 11 by means of H 3 PO 4 solution (see Figure 34). As described above, in case of forming the insulating film 13, the polysilicon film 9' is not oxidized, since it is covered by the Si 3 N 4 film 22 in the opening 11. If the polysilicon film 9' is oxidized at this time, the oxidized part of the polysilicon film 9' also comes to be etched by an etchant at the same time as when etching off the SiO 2 film 10 in the following step, and this causes difference in level in the isolation trench area. Next, in the same manner as the step shown in Figure 24, after depositing the polysilicon 14 (see Figure 35), by passing through the same steps as shown in Figures 25 to 31, a Bi-CMOS semiconductor device shown in Figure 31 is manufactured. In the present embodiment, since the polysilicon film 9' and the polysilicon 14 filled in the isolation trench 12 act as an etching stopper when etching off the SiO 2 film, it is prevent that the field oxide film 8 and the insulating film 13 under the polysilicon film 9' are etched at the same time. And as described above, since there is not an oxidized part also in the polysilicon film 9', the etching does not proceed to a lower layer from there. Furthermore, the present fourth embodiment can remove the polysilicon film 9' at the same time as when applying the second etching-back process to the polysilicon film 14. In the above-mentioned various embodiments, as an oxide film to be used as a mask when forming an isolation trench, an SiO 2 film is formed by means of a CVD method, but PSG (phospho silicate glass) film also may be formed instead of the SiO 2 film. INDUSTRIAL APPLICABILITY According to a manufacturing method of the invention described above, an insulating film in a groove, a field oxide film around the groove, or the like can be prevented from being etched at the same time as when etching off a film used as a mask when forming the groove. Therefore, such disadvantages as local reduction of dielectric strength in a groove area, deterioration of flatness of the surface of a substrate in the groove area, and the like are not brought about and a semiconductor substrate having a trench with high reliable in a wiring layer can be supplied, and the invention is very effective in manufacturing an SOI substrate having an isolation trench.

Claims: Method for manufacturing a semiconductor device, comprising the steps of: [a] depositing in order a first layer (9, 9') and a second layer (10) on a main surface of a semiconductor layer (3, 4) on an insulating substrate (2), said semiconductor layer (3, 4) on said insulating layer (2) forming an SOI substrate; [b] forming an opening (11) in said first layer (9, 9') and said second layer (10) exposing a portion of the main surface of said semiconductor layer (3); [c] forming a groove (12) constituting an isolation trench (12) reaching through to said insulating substrate (2) by etching said semiconductor layer (3, 4) through said opening (11) using said second layer (10) as a mask; [d] oxidizing the portion of said semiconductor layer (3, 4) exposed as walls of said groove (12) without oxidizing said first layer (9, 9'), thereby forming an insulating film (13) on said walls; [e] filling said groove (12) having said oxidized semiconductor walls with a filler (14) through the opening until a top surface of said filler (14) becomes higher than the top surface of said first layer (9, 9'), said filler contacting said first layer (9, 9') at the opening; and [f] removing said second layer (10) using said filler (14) having a top surface higher than the top surface of said first layer (9, 9') and said first layer (9, 9') as an etching stopper, thereby preventing said insulating film (13) located below the contact portion between said first layer (9, 9') and said filler (14) from being etched during the removal of said second layer (10). Method for manufacturing a semiconductor device according to claim 1, comprising a further step, after the step of removing said second layer, of selectively removing said first layer (9; 9'). Method for manufacturing a semiconductor device according to claim 1 or 2, comprising further steps, after the step of forming a groove (12) by etching said semiconductor substrate, of: processing an inside wall of said groove (12) by means of a chemical dry etching process; and annealing the inside wall of said groove processed by the chemical dry etching process. Method for manufacturing a semiconductor device according to claim 3, wherein the annealing process is a process of heating said semiconductor substrate at about 1,000 to 1,100°C in an atmosphere of inactive gas. Method for manufacturing a semiconductor device according to claim 4, wherein the inactive gas is nitrogen. Method for manufacturing a semiconductor device according to any one of claims 1 through 5, wherein the step of filling said groove includes steps of: filling said groove (12) with said filler (14) through the opening; and exposing said second layer (10) by etching back a portion of said filler deposited on the surface of said second layer until a top surface of said filler becomes lower than said second layer in height but higher than said first layer in height, whereby said filler contacts said first layer (9; 9') at the opening. Method for manufacturing a semiconductor device according to any one of claims 1 through 6, comprising a further step, after the removal of said second layer (10), of oxidizing a surface (15) of said filler (14) using said first layer (9) as a mask. Method for manufacturing a semiconductor device according to any one of claims 1 through 7, wherein said first layer (9) is a silicon nitride film, said second layer (10) is a silicon oxide film, and said insulating film (13) is a silicon oxide film. Method for manufacturing a semiconductor device according to any one of claims 1 through 6, further comprising a step, after the step of forming the opening (11) in said first and second layers but before the step of forming said groove (12), of covering an end face of said first layer (9') exposed inside the opening (11) with an oxidation-resisting film (22). Method for manufacturing a semiconductor device according to claim 9, comprising a further step, after the step of forming said insulating film (13) on the inside wall of said groove, of removing said oxidation-resisting film (22). Method for manufacturing a semiconductor device according to claim 9 or 10, wherein said first layer (9) is a polycrystalline silicon film, said second layer (10) is a silicon oxide film, said insulating film (13) is a silicon oxide film, and said oxidation-resisting film (22) is a silicon nitride film. Method for manufacturing a semiconductor device according to any one of claims 1 through 11, comprising a further step, before the step of depositing said first and second layers on the main surface of said semiconductor substrate, of forming a silicon oxide film (8, 8a) on the main surface of said semiconductor substrate (3) so as to have a thick portion (8) at a region corresponding to a field region of said semiconductor device, and wherein the step of forming the opening in said first and second layers includes a step of forming the opening (11) in the thick portion (8) of said silicon oxide film (8, 8a) through to a surface of said semiconductor substrate (3). Method for manufacturing a semiconductor device according to any one of claims 1 through 12, wherein said filler (14) is of polycrystalline silicon.

Inventors: KOMURA ATSUSHI, MIURA SHOJI, SAKAKIBARA TOSHIO, SUGISAKA TAKAYUKI, KOMURA, ATSUSHI, MIURA, SHOJI, SAKAKIBARA, TOSHIO, SUGISAKA, TAKAYUKI
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1992
Main classification: H01L21
IPC codes: H01L21