Method for planarizing a semiconductor substrate surface

Abstract: The present invention is structured including a process of depositing a film (4) composed of BPSG or PSG on a substrate surface having unevenness, a process of depositing a SiO₂ film (5) or depositing a film (5) composed of BPSG or PSG having concentration lower than phosphorus concentration or boron concentration in the film or a SiO₂ film (5) on the film, (4) and a process of melting and fluidizing these films so as to flatten them by applying heat treatment.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for forming a film used for flattening a substrate surface having unevenness in a manufacturing process of a semiconductor device. Formation of a very fine pattern and a multilayer interconnection structure are demanded according as high densification and high integration of a semiconductor device in recent years, and a tendency that the unevenness of the substrate surface becomes still more conspicuous is being developed. However, as the unevenness of the substrate surface becomes more conspicuous, it becomes more difficult to form a highly precise fine pattern and disconnection of interconnection and the like are also liable to occur. Therefore, lowering of reliability of a device and lowering of yield of semiconductor devices are caused. Thus, techniques for flattening the substrate surface are required. In the early stage of flattening techniques, a PSG film obtained by adding phosphorus to a SiO 2 film has been used as a flattening film which is able to flatten a substrate surface at a comparatively low temperature. This PSG film has a nature that the flattening temperature is lowered by increasing an addition quantity of added phosphorus, but the film absorbs moisture and the film quality is deteriorated when the added quantity of phosphorus is increased, thus becoming unfit for practical use. In a PSG film having phosphorus concentration showing no deterioration of the film quality, the softening point is approximately 1,000°C at the most. Thereupon, a BPSG film obtained by adding both boron and phosphorus having a softening point lower than that of the PSG film has been developed as a flattening film. For example, a flattening temperature of approximately 850°C has been obtained by adding boron and phosphorus into a SiO 2 film at 3 to 4 mol%, respectively, and the foregoing is the main current of present techniques of flattening a VLSI as a low temperature process. EP-A-0 421 203 discloses two separate BPSG layers on a step substrate, said second layer being characterized as a moisture resistant layer. However, both layers are deposited by a CVD method under identical conditions. This results in identical concentrations of impurities in both layers. EP-A-0 360 992 teaches providing of a flat surface on a stepped substrate. To due so the document discloses a second, self-planarizing layer on a first silicon layer. Thus, mainly the second layer functions in planarizing this surface. The document is silent about relative impurity concentrations in the two layers. In another approach JP-A-56-108235 teaches a BPSG film formed by a CVD method on an silicon substrate. Within this single film the phosphorus concentration gradually decreases to wards the surface of the film, wherein SiO 2 accounts from more than 99%. In addition from patent abstract of Japan, vol. 3, no. 58, May 18, 1979, there is also known a method of depositing an SiO 2 film on a BPSG film and subsequently heating for densification of the SiO 2 film. The SiO 2 film covers the BPSG film to prevent an aluminium interconnection layer formed thereon from directly contacting the BPSG film. The document is, however, silent about fusing the BPSG film and the SiO 2 film into a monolayer and fluidizing the monolayer to flatten an uneven substrat surface. After heat treatment both films, which have about the same thickness, remain separate. Fig. 12 shows diagrams showing a process of flattening a BPSG film by means of reflow related to a conventional example. In Fig. 12(a), a SiO 2 film 23 is formed on a Si substrate 22, and a polysilicon film 24 formed in a pattern is formed on the SiO 2 film 23. Next, a BPSG film 25 is formed by a Chemical Vapor Deposition (CVD) method as shown in Fig. 12(b). Next, when heat treatment (reflow processing) is applied at 850°C for about one hour in nitrogen (N 2 ) atmosphere, the BPSG film 25 is softened, melted and fluidized. Thus, the substrate surface is flattened. Further, the film quality of the heat treated BPSG film is made fine, thus forming a film of good quality having no hygroscopic property. When the substrate surface is flattened as described above, it becomes possible to make the pattern of interconnector or film formed thereon fine, thus enabling to manufacture a highly reliable semiconductor device having little disconnection and short-circuit of interconnection. Hereupon, as refinement of a device is advanced, a low temperature, and consequently a still lower temperature of the flattening temperature (reflow temperature) becomes required, and it is demanded in recent years to obtain the flattening temperature at 800°C or below. With respect to a BPSG film (SiO 2 -B 2 O 3 -P 2 O 5 ), the softening temperature can also be lowered by increasing the concentration of B(B 2 O 3 ) and P(P 2 O 5 ) in the film similarly to the case of the PSG film. Fig. 8(b) shows experimental data showing the relationship between the total quantity of impurities (P 2 O 5 +B 2 O 3 ) contained in a BPSG film and a reflow angle α at the reflow temperature of 900°C (parameter is boron concentration), and a tendency that the reflow angle α becomes smaller as the total quantity of impurities (P 2 O 5 +B 2 O 3 ) is increased, i.e., a tendency of being flattened is shown as shown in Fig. 8(b). Besides, Fig. 8(a) is a diagram for explaining the definition of the reflow angle. However, as the impurity quantity increases, hygroscopic property of the film becomes higher, and phenomena such as precipitation (B 2 O 3 or boric acid, etc.), moisture absorption (generation of H 3 PO 4 ) and clouding on the surface (devitrification) become liable to be present, which cause an obstacle in the next process. Namely, an unstable process is caused, and there are many problems in point of the reliability of the device, too. Fig. 9 and Fig. 10 are diagrams for explaining the foregoing, and show the relationship between phosphorus concentration/boron concentration and film quality stability of the BPSG film. The axis of abscissa represents boron concentration (mol %), the axis of ordinate represents phosphorus concentration (mol %), and the characteristic curve shows a boundary whether the film quality is stable or not. Namely, the instability of the film quality is increased when going in the right upper direction with the curve as the boundary, and the film quality is further stabilized when going in the left under direction. The characteristic curve was determined by whether the film quality of the BPSG film was deteriorated or not within one week after the BPSG film was deposited. As described, the film quality is deteriorated in general as the phosphorus concentration/the boron concentration is increased. Besides, the substrate temperature when the BPSG film is deposited is used as the parameter in Fig. 9, and the ozone concentration when the BPSG film is deposited is used as the parameter in Fig. 10. Further, Fig. 11 shows experimental data (parameters are boron concentration and phosphorus concentration) showing the relationship between the reflow temperature and the reflow angle α, and shows a tendency of being flattened as the reflow temperature is increased as shown in Fig. 11 (F.S. Becker, D. Pawlik, H. Schafer and G. Standigl J. Vac. Technol. B4(3), 1986, pp. 732-744). It has been in such a dilemma that, when the phosphorus concentration/the boron concentration is increased aiming at a low temperature, the film quality is deteriorated thereby to affect the device, and, when the phosphorus concentration/the boron concentration is decreased, reflow at a high temperature is compelled as described above. A limit is placed to 850°C at present for the reflow temperature of the BPSG film at concentration showing no deterioration of the film quality. It is an object of the present invention to provide a method of manufacturing a semiconductor device which is able to lower a reflow temperature by utilizing a BPSG film or a PSG film having high impurity concentration as a flattening film. Further, it is another object of the present invention to provide a method of manufacturing a semiconductor device which is able to arrange so that a BPSG film or a PSG film of high impurity concentration including a drawback that the film quality is deteriorated by moisture absorption does not absorb moisture. SUMMARY OF THE INVENTION According to claim 1 of the invention there is provided a method for manufacturing a semiconductor device comprising: depositing a first layer made of an insulating film selecting from the group consisting of a BPSG and a PSG film on an uneven substrate surface: depositing on said first layer a second layer being thinner in thickness than said first layer, and being made of an insulating film selecting from the group consisting of a non-doped SiO 2 film, a BPSG and a PSG film containing lower impurity concentration than said first layer; and heating to move impurities from said first layer to said second layer so as to fuse both of said first and said second layer into a single layer containing a uniform concentration of impurities in the entire thickness thereof, and to fluidize the single layer so as to flatten the uneven substrate surface. Preferably said second layer is 500 Å or thinner in thickness. Preferably the extent of unevenness on the substrate surface is reduced in advance by forming a film having self-flattening property before the process of depositing the first layer. In the next place, the principle of the present invention will be described with reference to Fig. 1. Fig. 1(a) shows a deposited state of a film on a substrate I before reflow treatment. Namely, a BPSG film II having a film thickness of 8,000Å (10Å = 1nm) is formed on the substrate I, and the boron concentration and the phosphorus concentration (boron concentration : 4 mol%; phosphorus concentration : 4 mol%) showing the flattening temperature at 800°C are contained in the film II. Further, a SiO 2 film III having a film thickness of 300Å is formed on the BPSG film II as a protective film of the BPSG film II. In Fig. 1, the axis of abscissas represents the film thickness, and the axis of ordinates represents the impurity concentration in the BPSG film II. Now, it is important to form the SiO 2 film III on the BPSG film II while maintaining the state that the BPSG film II is not exposed to the atmosphere. Accordingly, for example, the SiO 2 film III may be grown continuously in a same furnace without taking the substrate I outside after forming the BPSG film II, or may be grown in a different furnace in case it is not exposed to the atmosphere by load lock and the like. Since the BPSG film II is covered by the SiO 2 film III in either case, the film quality thereof is prevented from being deteriorated by absorbing moisture in the atmosphere. Besides, the BPSG film II does not touch the atmosphere directly and absorb moisture after the BPSG film II is covered by the SiO 2 film III. Therefore, there is no problem even if wafers are exposed to the atmosphere while the wafers are being moved for next reflow processing. Typically, the moisture resistant film is formed after reflow treatment, as tought e.g. in Patent Abstracts of Japan, vol. 9, No 81, July 26, 1985. Fig. 1(b) shows a state of a film on the substrate I after heat treatment (reflow processing) at 800°C for 30 minutes after the process shown in Fig. 1(a). When heat treatment is applied as described above, B 2 O 3 , P 2 O 5 and the like in the BPSG film II are taken into the SiO 2 film III and vitrified. Thus, a new BPSG film VI is formed, and the BPSG film VI is molten and fluidized thereby to be flattened. Further, the BPSG film quality becomes fine so as to show less hygroscopic property by heat treatment at a high temperature of 800°C at the same time, thus making it possible to form a stable BPSG film having no precipitation and devitrification. Since the reflow temperature of the BPSG film VI covered by the SiO 2 film III having a thin film thickness is at almost the same reflow temperature of the BPSG film VI which is not covered by the SiO 2 film III as described above, it becomes possible to flatten the substrate surface at a low temperature without deteriorating the film quality. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows diagrams for explaining the operation of the present invention; Fig. 2 shows explanatory diagrams (1) for explaining a method of manufacturing a semiconductor device according to the first embodiment of the present invention; Fig. 3 is an explanatory diagram (2) for explaining a method of manufacturing a semiconductor device according to the first embodiment of the present invention; Fig. 4 shows explanatory diagrams (1) for explaining a method of manufacturing a semiconductor device according to the second embodiment of the present invention; Fig. 5 shows explanatory diagrams (2) for explaining a method of manufacturing a semiconductor device according to the second embodiment of the present invention; Fig. 6 is a schematic diagram of a manufacturing apparatus applied with a method of an embodiment of the present invention; Fig. 7 is a diagram showing the relationship between the flow rate of TMB/TMOP and the impurity concentration in the BPSG film; Fig. 8 shows Diagrams showing the relationship between the total impurity quantity in a BPSG film and a reflow angle α; Fig. 9 is a diagram (1) showing the relationship between phosphorus concentration/boron concentration and film quality stability of a BPSG film; Fig. 10 is a diagram (2) showing the relationship between phosphorus concentration/boron concentration and film quality stability of a BPSG film; Fig. 11 is a diagram showing the relationship between a reflow temperature and a reflow angle α; and Fig. 12 shows explanatory diagrams of a conventional example. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the next place, embodiments of the present invention will be described with reference to the drawings. Fig. 6 is a schematic block diagram of a semiconductor manufacturing apparatus used in a method of manufacturing a semiconductor device according to an embodiment of the present invention. In Fig. 6, 11a to 11d represent flow meters (MFC), 12a to 12h represent valves, 13 represents an ozonizer for changing oxygen (O 2 ) into ozone (O 3 ), 14 represents a Tetraethylorthosilicate (TEOS) solution set at a temperature of 40°C to 65°C, 15 represents a Trimethylphosphate (TMOP) solution set at approximately 50°C to 60°C, and 16 represents a Trimethylborate (TMB) solution set at approximately 5°C to 50°C. Further, 17 represents a chamber, 18 represents a heater, 19 represents a gas outflow head, 20a and 20b represent wafers which become the objects of film formation, and 21 represents a gas exhaust port. Fig. 7 shows results of experiment showing the relationship between flow rate of TMB/flow rate of TMOP and impurity concentration contained in a BPSG film when the BPSG film is deposited using the manufacturing apparatus of a semiconductor device shown in Fig. 6. In Fig. 7, the axis of abscissas represents the flow rate of TMB (SLM; litter/min.) and the axis of ordinates shows the flow rate of TMOP (SLM; litter/min.), and numeric values shown at the ends of respective curves (the solid line shows B 2 O 3 and the broken line shows P 2 O 5 ) show the concentration in the film. (1) The first embodiment of the present invention Fig. 2 shows process drawings for explaining a method for manufacturing a semiconductor device according to the first embodiment of the present invention. A method for manufacturing a semiconductor device according to an embodiment of the present invention will be described with respect to the apparatus for manufacturing a semiconductor device shown in Fig. 6 and with reference to depositing conditions of the BPSG film shown in Fig. 7. First, a polysilicon film 3 having a film thickness of 1 µm is formed on a SiO 2 film 2 formed on a Si substrate I, and patterning is applied to the polysilicon film. Next, the Si substrate I (wafers 20a and 20b in Fig. 6) is housed in a chamber 17 shown in Fig. 6, TEOS gas, TMOP gas, TMB gas and O 3 gas are let flow by opening valves 12a to 12h, and flow meters lla to lld are regulated with reference to depositing conditions shown in Fig. 7. Thus, a BPSG film 4 having boron concentration at 4 mol% and phosphorus concentration at 4 mol% is deposited in a thickness of 8,000Å on a polysilicon film 3 at the substrate temperature of 400°C by a CVD method of TEOS-TMOP-TMB-O 3 system (Fig. 2(b)). Then, the impurity gas (TMB gas, TMOP gas) is stopped to be supplied while holding the wafers in the chamber 17, and a SiO 2 film 5 having a thickness of 500Å is deposited on the BPSG film 4 at the substrate temperature of 400°C. This SiO 2 film 5 functions as a capping film for protecting the SiO 2 film 4 against moisture. Next, the wafers are taken out of the chamber 17 and moved to another high temperature processing furnace (not shown). The wafers may come in touch with the atmosphere at this time. Then, reflow processing is performed at 800°C and for 30 minutes in N 2 atmosphere. With this, as shown in Fig. 3(d), P 2 0 5 , B 2 O 3 and the like in the BPSG film 4 also move into the SiO 2 film 5 and are vitrified, and lead to a single layer BPSG film 6 having a uniform impurity concentration. Thus, the substrate surface is flattened. When the substrate surface is flattened in such a manner, it becomes possible to refine the interconnection pattern and the film pattern formed in and after next process, and to aim at higher integration of the semiconductor device. The film quality of the BPSG film 6 is made fine by heat treatment to generate reflow at the same time. Therefore, even if the film is exposed to the atmosphere in a process thereafter, the BPSG film 6 absorbs moisture and the film quality will never be deteriorated. Besides, the reflow temperature at this time is almost the same as the temperature at which the BPSG film reflows independently, and is hardly influenced by the existence of the SiO 2 film 5. Namely, since reflow at a low temperature is possible similarly to the case of the BPSG film alone, it is possible to realize a process at a low temperature. This is considered that the reflow temperature becomes almost the same as the case of the original BPSG film 4 because of the fact that P 2 O 5 and B 2 O 3 in the BPSG film 4 also move into SiO 2 film 5 by reflow processing and are taken therein so that a new BPSG film 6 is formed, and the concentration of P 2 O 5 and B 2 O 3 in this new BPSG film 6 is at an almost the same value of the concentration of P 2 O 5 and B 2 O 3 in the original BPSG film 4 since the film thickness of the SiO 2 film 5 is thin as compared with the film thickness of the BPSG film 4. Such a fact has been confirmed by the analysis of impurity distribution in the BPSG film 6 after reflow. Further, it has been ascertained through experiment that there is almost no difference in the configuration (flatness) of the film after reflow irrespective of the existence of the SiO 2 film 5 having a thickness of 500Å. As described above, according to a method of manufacturing in the first embodiment of the present invention, the film quality is not deteriorated even if a BPSG film of high impurity concentration is used, and the substrate surface can be flattened by having it reflow at a low temperature. Thus, such a method will go far toward lowering the temperature of a semiconductor process and refinement of a semiconductor device. (2) The second embodiment of the present invention Fig.4 shows process diagrams for explaining a method for manufacturing a semiconductor device according to the second embodiment of the present invention. First, as shown in Fig. 4(a), a polysilicon film 3 having a film thickness of 1 µm is formed on a SiO 2 film 2 formed on a Si substrate I, and patterning is applied to the polysilicon film. Next, as shown in Fig. 4(b), a SiO 2 film having self-flattening property is formed in a thickness of 2,000Å on the polysilicon film 3 by an atmospheric CVD method. Here, the film having self-flattening property means a film having a tendency showing flow configuration by itself during film formation, which can be formed by TEOS/O 3 atmospheric CVD method for instance. According to this method, a film configuration similar to that of a film applied with high temperature reflow processing at a film formation temperature of approximately 400°C. Then, in a similar manner as the embodiment 1, the Si substrate I (wafers 20a and 20b) are housed in the chamber 17 shown in Fig. 6, TEOS gas, TMOP gas, TMB gas and O 3 gas are let flow by opening valves 12a to 12h, and flow meters 11a to 11d are regulated with reference to depositing conditions shown in Fig. 7. In such a manner, a BPSG film 8 having boron concentration at 6 mol% and phosphorus concentration at 5 mol% is deposited on the polysilicon film 3 in the thickness of 7,000Å at the substrate temperature of 400°C by a CVD method of TEOS-TMOP-TMB-O 3 system (Fig. 4(c)). Then, the impurity gas (TMB gas, TMOP gas) is stopped to be supplied while keeping the wafers in the chamber 17, and a SiO 2 film 9 having a thickness of 200Å is deposited on a BPSG film 4 at the substrate temperature of 400°C (Fig. 5(d)). Next, the wafers are taken out of the chamber 17, and moved to another high temperature processing furnace (not shown). Then, reflow processing is performed at 780°C for 30 minutes in N 2 atmosphere. With this, phosphorus and boron in the BPSG film 8 diffuse into a SiO 2 film 9 and a SiO 2 film 7 so as to form a BPSG film 10 which reflows, and thus the substrate surface is flattened as shown in Fig. 5(e). At the same time, the film quality of the heat treated BPSG film 6 is made fine by reflow. Thus, even the film is exposed to the atmosphere in a process thereafter, the BPSG film 6 absorbs moisture and the film quality will never be deteriorated. As described above, according to a manufacturing method in the second embodiment of the present invention, unevenness of the substrate surface is reduced in advance by the SiO 2 film 7 having self-flattening property. Thus, the flattening property can be improved further in addition to the effect shown in the first embodiment of the present invention. Besides, a BPSG film has been used as a flattening film for the purpose of description in respective embodiments, but it is a matter of course that the present invention may be applied by using a PSG film in place of the BPSG film. Further, a SiO 2 film has been used as a film (a capping film) for covering and protecting the BPSG film, but it is apparent that a BPSG film or a PSG film of low concentration having little hygroscopic property may be used in place of the SiO 2 film. Furthermore, it is also possible to have the BPSG film reflow at a temperature lower than the reflow temperature described in the embodiments such as a low temperature at 750°C or below by increasing the impurity concentration of the BPSG film.

Claims: A method of manufacturing a semiconductor device comprising: depositing on an uneven surface of a substrate (1) a first layer (4, 8) made of an insulating film selected from the group consisting of a BPSG and a PSG film; depositing on said first layer (4, 8) a second moisture-resistant layer (5, 9) which is thinner in thickness than said first layer (4, 8), and is made of an insulating film selected from the group consisting of a non-doped SiO 2 film and an SiO 2 film doped with impurities having a lower impurity concentration than said first layer (4, 8); and heating to move impurities from said first layer (4, 8) to said second layer (5, 9) so as to fuse both of said first and said second layer (4, 8; 5, 9) into a single layer (6) having a uniform impurity concentration in the entire thickness thereof, and to fluidize the single layer (6) so as to flatten the uneven substrate surface. A method of manufacturing a semiconductor device according to Claim 1, wherein said second layer (5, 9) is 500 Å or thinner in thickness. A method of manufacturing a semiconductor device according to Claim 1 or 2, wherein the SiO 2 film (5, 9) doped with impurities is a PSG film or a BPSG film. A method of manufacturing a semiconductor device according to one of claims 1 to 3, wherein the extent of unevenness on the surface of the substrate (I) is reduced in advance by forming on said substrate surface a film (2, 3) having self-flattening properties before the process of depositing the first layer (4, 8).

Inventors: MAEDA KAZUO, MATSUI BUNYA, NISHIMOTO YUKO, MAEDA, KAZUO, MATSUI, BUNYA, NISHIMOTO, YUKO, MAEDA, KAZUO C/O SEMICONDUCTOR PROCESS LABORATORY, MATSUI, BUNYA C/O SEMICONDUCTOR PROCESS LABORATORY, NISHIMOTO, YUKO C/O SEMICONDUCTOR PROCESS LAB. CO.
Applicants: ALCAN TECH CO INC, CANON SALES CO INC, SEMICONDUCTOR PROCESS LAB CO, ALCAN- TECH CO., INC., CANON SALES CO., INC., SEMICONDUCTOR PROCESS LABORATORY CO., LTD.
Application year: 1992
Main classification: H01L21
IPC codes: H01L21