METHOD OF PRODUCING BIPOLAR SEMICONDUCTOR DEVICES

Abstract: A method of producing semiconductor devices that is adapted to forming bipolar transistors that exhibit small change in characteristics maintaining high yields. According to a conventional method based upon the self-alignment intended to increase the degree of integration, complex manufacturing steps were involved resulting in the change in the emitter area and making it difficult to form bipolar transistors with uniform characteristics. According to this invention in which an emitter hole (14) is formed utilizing lithography, the emitter is formed through simplified steps maintaining good dimensional precision. Lithography inevit­ ably involves a change in position where the emitter is formed which, however, affects the transistor characteristics very little. Therefore, the invention makes it very easy to form bipolar transistors having uniform characteristics com­ pared with the method in which the emitter is formed in a self-aligned manner.

Description: Technical Field: This invention relates generally to a method of producing a semiconductor device, and more particularly to a method of easily producing a semiconductor device which is suitable for producing a bipolar semiconductor device having a high packing density. Background Art: Many methods of producing bipolar type semiconductor devices have been proposed to this date in order to improve the operating speed and packing density of the devices, as is well known in the art. For instance, a method has been proposed which forms a base contact and an emitter by self-alignment so as to improve the packing density (T. Sakai et al, JJAP, vol. 20-1, 155, 1981). Since the base contact and the emitter can be formed by self-alignment as described above, this method has a characterizing feature in that a bipolar transistor having an extremely small occupying area but having high performance can be obtained without using fine lithography. However, this method is not without drawbacks, in that the production process is complicated, and bipolar transistors having uniform characteristics cannot be formed easily because the emitter area is likely to vary. Therefore, a method which can be practised more simply and more easily has been desired. JP-A-59-105363 discloses a method of producing a bipolar semiconductor device which includes method steps similar to steps (a) to (c) and (g) to (k) defined in claim 1. In the prior art, the base region is, however, formed at a later stage. Furthermore, in the known process, after depositing a polycrystalline silicon film on a substrate and forming a hole in the film to define the area where the semiconductor device is to be formed, a second hole for contacting the base region is produced by steps which require the alignment of a mask with respect to the first hole and further involve the filling of the second hole with polysilicon and a subsequent polishing of the silicon area. It is an object of the invention to provide a method of producing a bipolar semiconductor device, which eliminates the problems encountered in the prior art methods described above and which permits producing bipolar semiconductor devices having a high packing density with a high yield. This object is accomplished by the method set forth in claim 1. In contrast to the method known from JP-A-59-105363, the process of the invention employs steps which, in themselves are common in the production of semiconductor devices, without requiring new alignment between two subsequent masks and without relying on a polishing step. In a preferred embodiment of the invention, the following lithography technique is employed. A film whose solubility changes when it is irradiated with light, an electron beam or x-rays (i.e., resist film) is coated on the entire surface of a material to be worked such as a semiconductor substrate, an insulating film, a metal film or a polycrystalline silicon film, and the light, electron beam or X-rays are then radiated selectively to desired portions of the resist film so as to change the solubility of the irradiated portions. If the resist is of a negative type, the solubility of the irradiated portions drops, and if it is of a positive type, the solubility increases. The portions of the resist film having a high solubility are removed by development, and the exposed portions of the material to be worked are etched. In this manner, the desired portions of the material to be worked can be selectively etched and a desired pattern can be formed. The method which radiates light to change the solubility of the resist film is referred to as "photolithography". Similarly, the methods which radiate the electron beam and X-rays are referred to as "electron beam lithography' and "X-ray lithography", respectively. Brief Description of Drawings: Fig. 1 is a process diagram showing one method of producing a bipolar semiconductor device; Figs. 2, 3 and 4 are sectional view, plan view and diagram useful for explaining the positional deviation of an emitter, respectively; Fig. 5 is a process diagram showing an embodiment of the present invention; and Fig. 6 is a sectional view showing another embodiment of the present invention. Example 1 Referring to Figs. 1(a) through 1(f), a collector buried layer 2 having a conductivity type opposite to that of a silicon substrate 1 is formed by known methods on the surface of the silicon substrate 1 as shown in Fig. 1(a), and a 1 µm-thick silicon epitaxial layer 3 of a conductivity opposite to that of the silicon substrate 1 is then formed on the substrate surface. An SiO₂ film 4 is formed by thermal oxidation of the surface of the silicon epitaxial layer 3, and an Si₃N₄ film 5 is deposited thereon by the known CVD technique. Unnecessary portions are selectively removed by photo-etching, thereby forming isolated patterns consisting of the laminated Si₃N₄/SiO₂ films 4, 5. The surface of the exposed portion of the epitaxial layer 3 is removed in a thickness ranging from 0.3 to 0.5 µm by etching, and thermal oxidation treatment is then carried out using the Si₃N₄ film 5 described above as a mask, forming an about 1 µm thick isolation oxide film 6 as shown in Fig. 1(b). The remaining Si₃N₄ film 5 and SiO₂ film 4 are thereafter removed. After an impurity doped region 7 for an external collector is formed by selectively doping an impurity having the same conductivity type as that of the collector buried layer 2, a 30 to 60 nm-thick silicon dioxide (SiO₂) film 8 is formed again by thermal oxidation of the surface of the epitaxial layer 3, and ion implantation is further effected using a resist film 9 as a mask to form a base 10 having the conductivity type opposite to that of the epitaxial layer 3. After the resist film 9 is removed, an Si₃N₄ film 11 is formed on the entire surface as shown in Fig. 1(c), and desired portions are selectively removed by photo-etching to form a contact hole 12 for external base. In this case, it is possible to employ ordinary photolithography technique using ultraviolet rays, but if the desired portions of the Si₃N₄ film 11 are selectively removed by electron beam lithography, an extremely fine contact hole 12 which is below about 0.5 µm wide can be formed. Next, a polycrystalline silicon film 13 is formed as shown in Fig. 1(d), and an impurity having the same conductivity type as that of the base 10 is doped. Then, the portion at which the emitter is to be formed is selectively etched and removed and an emitter hole 14 is formed. Incidentally, the portions which are not necessary for the external base among the polycrystalline silicon film 13 are simultaneously etched and removed in this case. If the known electron beam lithography is used in the same way as in the case of the contact hole for the external base, an extremely fine emitter hole which is below about 0.5 µm wide can be formed. That is, after a resist film is formed on the entire surface, desired portions are irradiated and then developed, and the exposed polycrystalline silicon film is removed. It is evident that ordinary photolithography can be employed in this treatment. Next, as shown in Fig. 1(e), the surface of the polycrystalline silicon film 13 is oxidized to form an SiO₂ film 15. The exposed portions of the Si₃N₄ film and SiO₂ film described above are then directionally etched by reactive ion etching using a resist film 16 having a hole slightly larger than the hole 14, thereby forming an emitter contact hole 17. As shown in Fig. 1(f), a polycrystalline silicon film 18 for forming an emitter, to which large quantities of an impurity having the conductivity type opposite to that of the base is doped, is formed inside the emitter hole 17, and is then heated in order to diffuse the impurity in the polycrystalline silicon film 18 into the base region 10, and thereby to form an emitter 19. In this case, the impurity which is contained in the external base 13 and has the same conductivity as that of the base 10 is simultaneously diffused into the epitaxial layer 3, forming a graft base layer 20 having a high impurity concentration. Therefore, electric connection between the intrinsic base region 10 and the polycrystalline silicon film (external base) can be established reliably. Finally, a base electrode 21, an emitter electrode 22 and a collector electrode 23 are formed to complete a bipolar transistor. The variance of the emitter dimension of the bipolar transistors produced in this manner is about 5%. The variance of the emitter dimension in the conventional bipolar transistors produced by use of self-alignment not only for the base contact but also for the emitter is about 30%. Therefore, the above method can remarkably improve the accuracy of the emitter formation and can stabilize the transistor characteristics. Yield can be approximately doubled. The hole for forming the emitter is formed by lithography as described above. Therefore, an emitter having a high dimensional accuracy can be formed by a simple process, but positional deviation occurs inevitably at the time of mask alignment. Figure 2 shows the structure of the transistor when the emitter contact hole is formed where the mask is misaligned to the left in relation to the contact hole for the external base, and the emitter 19 misaligned to the left. The condition in which a problem such as a leakage current does not occur at the base-emitter junction is expressed by the following formula (1) as shown in Fig. 2: c ≳ a + b (1) Where a is the distance from the contact hole for the external base and the end portion of the graft base 20, b is the distance between the emitter contact hole and the end portion of the emitter 19 and c is the distance between the contact hole for the external base and the emitter contact hole. When a = 0.1 µm and b = 0.2 µm, for example, the problem described above does not occur if c is at least 0.3 µm. Assuming that the mask alignment error occurring this time is 0.2 µm, there is no posibility that graft base region 20 will come into contact with the emitter 19 if c is at least 0.5 µm. Another problem resulting from the position deviation at the time of mask alignment is the increase of base resistance. For instance, if the position deviation of the emitter occurs in a transistor having a planar structure shown in Fig. 3, the base resistance fluctuates as shown in Fig. 4. In Fig. 3, reference numeral 24 represents an isolation pattern, 25 is a pattern for a contact hole for external base, 26 is an emitter hole pattern and 27 is a base contact pattern. The dimension of the emitter hole pattern 26 is 0.7 µm x 2.0 µm. If the alignment error at the time of mask alignment occurs in a direction represented by an arrow a , it is confirmed that the change of the base resistance resulting from the alignment error is within the range +5% to -10% as can be seen clearly from Fig. 4. Mask alignment accuracy of 0.2 µm can be attained with photolithography using light, but if electron beam lithography is used, the alignment error can be further reduced and the change of the base resistance can be further reduced. If the distance between the contact hole for external base and the emitter contact hole satisfies the formula (1) above, the occurrence of abnormalities such as leakage current can be prevented and even if the mask alignment error occurs to some extent, the effect upon the transistor characteristics is extremely small, so that a satisfactory bipolar transistor can be formed. Example 2 In Example 1 described above, both the contact hole for the external base and the emitter contact hole are formed by lithography using a mask, but the contact hole for the external base can be formed without using the fine lithographic technique. First of all, as shown in Fig. 5(a), an SiO₂ film 29 is formed on a silicon substrate 28, and ion implantation is then performed using a photoresist film as a mask to form an intrinsic base region 30. An Si₃N₄ film 31 and a polycrystalline silicon film 32 are sequentially deposited, and an impurity having the same conductivity as that of the base 30 (boron, for example, if the base is of the P type) is doped into the polycrystalline silicon film 32. Thereafter, the polycrystalline silicon film 32 on the base is selectively etched and removed. Next, thermal oxidation is carried out to form an SiO₂ film 33 on the surface of the polycrystalline silicon film 32 as shown in Fig. 5(b), and a non-doped polycrystalline silicon film 34 to which no impurity is substantially doped is further formed on the SiO₂ film 33. The non-doped polycrystalline silicon film 34 is then removed in such a manner as to leave a region which is greater by 0.5 to 1.0 µm than the open portion. After the SiO₂ film 33 is etched and removed using a buffered HF solution, the exposed portion of the Si₃N₄ film 31 below the eaves of the non-doped polycrystalline silicon film 34 is etched and removed using hot phosphoric acid. The non-doped polycrystalline silicon film 34 is selectively removed as shown in Fig 5(d) using an etching solution for etching the non-doped polycrystalline silicon (hydrazine, for example, when the doped impurity is Boron) without etching the polycrystalline silicon to which the impurity is doped, and then the exposed portion of the SiO₂ film 29 is removed. There is thus formed a contact hole for external base which is from 0.2 to 0.4 µm wide. A polycrystalline silicon film 35 is formed on the entire surface as shown in Fig. 5(e), and an impurity having the same conductivity type as the base 30 is doped. Desired portions of the polycrystalline silicon film 35 are selectively removed by known photoetching technique, and an emitter hole 36 is formed. The subsequent steps are the same as those of Example 1. As shown in Fig. 5(f), a polycrystalline silicon film 18 to which large quantities of an impurity having a conductivity type opposite to that of the base 30 is doped is formed in such a manner as to cover at least the emitter hole 36, and heating is then performed to form an emitter 19 and a graft base 20. A protective film and wirings are thereafter formed by a known method to complete a bipolar transistor. It is confirmed that this example, too, can provide the same transistor characteristics as those of Example 1. Example 3 This example aims to isolate the bipolar transistors, which are formed in accordance with Example 1 or 2, by use of a groove having a U-shaped cross-section. The cross-sectional structure is shown in Fig. 6. In other words, this example packs SiO₂ films 36, 39, an Si₃N₄ film 37 and a polycrystalline silicon film 38 into the groove formed in the device isolation region so as to isolate the devices (transistors) from each other. This example is particularly effective for reducing the necessary area and parasitic capacitance. In the examples given above, polycrystalline silicon is used for the external base, but silicides of various metals such as W, Mo, Ni, and the like, can be used, and the base resistance can be further reduced by so doing. This holds true also of polycrystalline silicon used for the emitter diffusion, and good results can be obtained by use of silicides of various metals. In either case, extremely satisfactory results can be obtained if heating is carried out while a polycrystalline silicon layer is interposed between the semiconductor substrate and the silicide layer. The emitter can of course be formed by ion implantation in place of thermal diffusion. Industrial Applicability: The present invention can easily and simply form an emitter having a high dimensional accuracy, and can therefor obtain a far higher yield than the conventional production methods which form the base contact and the emitter by the self-alignment method. Since the emitter is formed by ordinary lithographic technique, a positional error at the time of mask alignment occurs inevitably, but the effect resulting from the positional error upon the transistor characteristics is negligible, and bipolar transistors having high performance can be produced stably and with high producibility.

Claims: A method of producing a bipolar semiconductor device, comprising the following sequence of steps: (a) forming a first silicon dioxide film (29) on the surface of a semiconductor substrate (28), and then doping an impurity of a first conductivity type into a desired portion of the surface region of said semiconductor substrate to form a base region (30); (b) forming a silicon nitride film (31) on the entire surface of said first silicon dioxide film (29), and depositing thereon a polycrystalline silicon film (32) into which an impurity of said first conductivity type is doped and which has a first hole over said base region; (c) oxidising the surface of the first polycrystalline silicon film (32) to form a second silicon dioxide film (33), (d) forming an undoped second polycrystalline silicon film (34) on the entire surface of the structure and then removing an undesired portion thereof to leave the second polycrystalline silicon film (34) in the first hole and on a portion of the second silicon dioxide film (33) surrounding the first hole, (e) removing the second silicon dioxide film (33) by etching, (f) removing the exposed portion of the silicon nitride film (31), the second polycrystalline film (34) and the exposed portion of the first silicon dioxide film (29) by etching, thereby forming a second hole on said base region (30) at a position where an external base region is to be formed in contact with said base region (30), (g) forming a third polycrystalline silicon film (35) doped with impurities of the first conductivity type on the entire surface of the structure and contacting the base region (30) through the second hole, (h) forming a resist film on the entire surface of the third polycrystalline silicon film (35) and selectively increasing the solubility of a desired portion of the resist film over said base region (30) at a position where an emitter region (19) is to be formed in said base region, (i) removing the high-solubility portion of the resist film by development and removing the exposed portion of the third polycrystalline silicon film (35), (j) oxidising (15) the surface of the third polycrystalline silicon film (35), and (k) removing the exposed portions of said silicon nitride and first silicon dioxide films (31, 29) to form a third hole (36) on said base region (30), and doping an impurity of a second conductivity type opposite to said first conductivity type into a portion of the surface region of the base region (30) to form an emitter region (19). The method of claim 1, wherein a metal silicide film is formed on the entire surface of the structure surface in place of the third polycrystalline silicon film (35) in step (g). The method of claim 1 or 2, wherein the increase of solubility of the resist film in step (h) is effected by irradiation with light or with an electron beam or with X-rays. The method of any one of claims 1 to 3, wherein the emitter region (19) is formed in step (k) by doping from a polycrystalline silicon film (18) or a metal silicide film. The method of claim 4, wherein a graft base region (20) is formed together with said emitter region (19) in step (k). The method of any one of claims 1 to 3, wherein the emitter (19) is formed in step (k) by ion implantation.

Inventors: HASEGAWA, NORIO, NISHIZAWA, HIROTAKA, OKAZAKI, SHINJI, SAGARA, KAZUHIKO, TAKAKURA, TOSHIHIKO, TAMAKI, YOICHI
Applicants: HITACHI, LTD.
Application year: 1985
Main classification: H01L29
IPC codes: H01L21, H01L29