Contact for semiconductor device and method of manufacturing the same

Abstract: A semiconductor device including a MOS element (100) having a buried contact structure (20). The buried contact structure includes a first contact diffused region (22) formed by diffusion from a polycrystalline silicon layer and a second contact diffused region (24) formed by diffusion deeper than the first contact diffused region, so that a parasitic resistance of the MOS element can be reduced. In a composite element composed of the MOS element and a bipolar element, partly since the first contact diffused region and an emitter diffused region of the bipolar element can be formed simultaneously, and partly since the depth of connection of the emitter diffused region, with the parasitic resistance of the MOS element being reduced, it is possible to realize a high-speed operation.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: This invention relates to a semiconductor device, and more particularly to a semiconductor device including a MOS element, such as a semiconductor device (hereinafter called "Bi-MOS element") in which both a bipolar element and a MOS element are formed on a single semiconductor substrate or a MOS field effect transistor (hereinafter called "MOSFET") and a method of manufacturing such semiconductor device. 2. Description of the Related Art: As advances have recently been made in integrating elements in a large scale in MOS semiconductor IC devices, application of a buried contact structure is on the rise. In a typical buried contact structure in conventional MOSFETs, a gate electrode and a source electrode and/or a drain electrode (hereinafter called "source/drain electrode") are formed during the growth and process of one and the same polycrystalline silicon, and a contact with a source diffused region or a drain diffused region (hereinafter called "source/drain diffused region") is formed via a contact diffused region by impurity diffusion from the source/drain electrode. In response to the minute elements, an LDD (Lightly Doped Drain) structure as a measure against the hot electron problem was used in MOSFETS. The structure of this type semiconductor device is schematically shown in FIG. 2 of the accompanying drawings. In FIG. 2, MOSFET 100 is an n-channel MOS transistor having an LDD structure. This MOSFET 100 has a buried contact structure on a p-type silicon substrate 10. A contact diffused region 28 constituting this buried contact structure is formed of an n+ diffusion layer formed by impurity diffusion from a source/drain electrode 40 formed of an n + -type polycrystalline silicon layer. The source/drain electrode 40 is formed together with a wiring portion 44 while the layer of a gate electrode 30 is formed. The n + -type contact diffused region 28 is connected to one n + -type source/drain diffused region 42a via an n - -type offset region 52 which is located right under a side wall 50 contiguous to the source/drain electrode 40. In FIG. 2, reference numeral 12 designates a field oxide film; 14, a gate oxide film; and 18, a silicon layer hollow formed during etching the polycrystalline silicon layer. However, the semiconductor device of FIG. 2 has the following problems resulting from the buried contact structure: (1) If an impurity having a small diffusion constant is used as the impurity in the source/drain electrode 40 of the n + -type polycrystalline silicon, the depth of the n + -type contact diffused region 28 would be shallow. The depth of the n - -type offset region 52 to be connected to the n + -type contact diffused region 28 is originally shallow and the region 52 has a high resistance. Therefore, the resistance of the whole buried contact structure would increase to add a high parasitic resistance to the source/drain diffused region 42a of MOSFET, thus impairing the mutual conductance of the MOSFET and the characteristic of the on-current adapted to flow when the transistor is in on state. (2) During forming the n + -type gate electrode 30 and the n + -type source/drain electrode 40 by etching, the exposed portion of the p-type silicon substrate 10 also would be etched to form the silicon layer hollow 18. If the n - -type offset region 52 is formed by ion implantation, the impurity would hardly be doped to the peripheral portion of the silicon layer hollow 18 so that the impurity concentration of the n - -type offset region 52 to be formed tends to become lower than a predetermined value while the resistance of the same n - -type offset region 52 tends to become increased. As a result, the function of the LDD structure would work only inadequately to cause the above problem (1). This also results in a large irregularity of the above-mentioned characteristic due to the non-uniform processing, such as in the etching step. The foregoing problems are remarkable if arsenic having a small diffusion constant is used as the impurity in the n + -type polycrystalline silicon layer (source/drain electrode 40), the n - -type offset region 52 and the n + -type source/drain diffused regions 42a, 42b. Further, for the same reason, the above problem (1) will be caused even in the case of MOSFET having a buried contact structure to which no LDD structure is added. Also in the case of a Bi-MOS element, when forming an n + -type emitter diffused region of the bipolar transistor, the same problem as the MOSFET would be caused due to the impurity diffusion of arsenic from the n + -type polycrystalline silicon layer. In order to realize a high-speed bipolar transistor, the depth of the n + -type emitter diffused region should be shallow. Therefore, when forming a contact diffused region of MOSFET simultaneously with the emitter diffused region, the depth of the contact diffused region would be shallow to cause the above problems (1) and (2). Consequently, if in a Bi-MOS element a parasitic resistance resulting from the buried contact structure is to be reduced, the depth of the diffused region should be deep. This would be an obstacle to a high-speed Bi-MOS element. A semiconductor device according to the prior art portion of claim 1 is disclosed in the document JP-A-60-219771. In this prior art a buried contact diffused structure is formed by only the first diffused region formed to overlap a drain/source region and having a diffusion depth larger than that of the source/drain diffused region. The layer between this first buried contact diffused region and the polycrystalline silicon layer is a high melting point metal silicide film formed on top of the substrate surface, rather than being a diffusion layer formed in the substrate. The purpose of this layer is to prevent a silicon layer hollow to occur as a result of a subsequent etching of the polycrystalline silicon layer into the required electrode pattern. A semiconductor device disclosed in the document GB-A-2 075 255 comprises a buried contact diffused structure formed by a single diffused region formed to overlap a drain/source region and having a diffusion depth smaller than that of the source/drain diffused region. The document JP-A-61-13668 discloses a semiconductor device having an MOS structure, wherein surface layers of source and drain regions, which are in contact with a source and drain slilicon electrode are more heavely doped than the rest of the source and drain regions. The document EP-A-0 200 603 discloses a semiconductor device having MOS and bipolar elements integrated on the same substrate. It is an object of the present invention to provide a semiconductor device which hardly receives influence from non-uniform processing such as by an etching step, is stable in its characteristics and has a low-resistance buried contact structure. Another object of the present invention is to provide a method of manufacturing such semiconductor device. These objects are achieved with a semiconductor device as claimed in claim 1 and a method as claimed in claims 9 and 10, respectively. Preferred embodiments of the invention are subject-matter of the dependent claims. With this semiconductor device, since the diffused region forming the buried contact structure of the MOS element includes the first contact diffused region formed by impurity diffusion from the polycrystalline silicon layer and the second contact diffused region formed by diffusion deeper than the first contact diffused region, it is possible to realize a semiconductor device which hardly receives influence of non-uniform processing such as in the etching step, is stable in characteristic, has a low-resistance buried contact structure and has a low-parastic resistance. Further, since the MOS element of this invention has a deep diffusion layer to increase the area of connection to the source/drain, it is suitable for use in, for example, a static random access memory (SRAM). As advances have recently been made in integrating elements, demands for shallow depth of the source/drain of a transistor are on the rise, for which the area of such connection must be reduced. Thus the amount of connection to the source/drain would be reduced so that the amount of connection of storage node in a SRAM using this MOS element will become inadequate. As a result, in SRAM, characteristics such as resistance against noise, leak current and a ray soft error. This problem would be remarkable particularly on the side of the drain. However, if an n-channel transistor in which the MOS element of the invention is incorporated is used, it is possible to increase the area of connection to the source/drain and hence the amount of connection, with the depth of the source/drain near the channel of the transistor being kept shallow, thereby eliminating the foregoing problems. In addition, since the depth of diffusion and the impurity concentration of the second contact diffused region of the buried contact structure can be set independently of the impurity concentration in the polycrystalline silicon layer, it is possible to improve the degree of freedom of device designing. If this MOS element is combined with a bipolar element, it is possible not only to achieve a high-speed operation of the bipolar element but also to realize a Bi-MOS element having a small parasitic resistance. The foregoing semiconductor devices can be manufactured in the same number of steps of production as conventional. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view schematically showing a MOS-type semiconductor device according to a first embodiment of this invention; FIG. 2 is a cross-sectional view schematically showing a conventional MOS-type semiconductor device; FIG. 3 is a cross-sectional view similar to FIG. 1, showing a modification according to the first embodiment; FIGS. 4A through 4G are cross-sectional views showing the manufacturing process of the semiconductor device of FIG. 1; FIG. 5 is a cross-sectional view schematically showing a Bi-MOS-type semiconductor device according to a second embodiment; FIGS. 6A through 6G are cross-sectional views schematically showing the manufacturing process of the semiconductor device of FIG. 5; FIG. 7 is a cross-sectional view schematically showing a MOS-type semiconductor device according to a third embodiment; and FIG. 8 is a cross-sectional view schematically showing a Bi-MOS-type semiconductor device according to the third embodiment. DETAILED DESCRIPTION Typical embodiments of this invention will now be described with reference to the accompanying drawings. Embodiment 1: FIG. 1 shows a semiconductor device schematically according co the first embodiment of this invention. Like reference numerals designate similar parts or elements throughout FIGS. 1 and 2. This semiconductor device comprises an n-channel MOSFET 100 having an LDD structure. In the MOSFET 100, a field oxide film 12 and a gate oxide film 14 are formed on a p-type silicon substrate 10. On the gate oxide film 14, a gate electrode 30 is formed, and a source/drain electrode 40 is formed at a position spaced from the gate electrode 30, and a wiring portion 44 is formed contiguously to the source/drain electrode 40. On the opposite sides of the gate electrode 30 and the end of the source/drain electrode 40, side walls 50 each in the form of a silicon oxide film is formed. Under each of the side walls 50, an n - -type offset region 52 constituting an LDD structure is formed. In the silicon substrate 10, n + -type source/drain diffused regions 42a, 42b are formed on the opposite sides of the gate electrode 30. In this embodiment, one n + -type source/drain diffused region 42a is connected with the source/drain electrode 40 by a buried contact structure 20. The buried contact structure 20 includes a first n + -type contact diffused region 22 formed shallowly on the surface of the silicon substrate 10, and a second n + -type contact diffused region 24 formed on the surface of the silicon substrate 10 more deeply than the first n + -type contact diffused region 22. The second n + -type contact diffused region 24 is formed by previous diffusion of phosphorous of a large diffusion constant as an impurity into the silicon substrate 10, with a part of the second n + -type contact diffused region 24 being connected to the source/drain diffused region 42a. The first n + -type contact diffused region 22 is formed by impurity diffusion of arsenic of a small diffusion constant from the source/drain electrode 40. The depth of the second n + -type contact diffused region 24 should be adequately large so that the contact resistance of the buried contact structure 20 can be reduced lower than a predetermined value. For example, the first n + -type contact diffused region 22 may have a depth of diffusion of 0.1 to 0.3 µm, and the second n + -type contact diffused region 24 may have a depth of diffusion of 0.5 to 0.8 µm. In the arrangement, the diffused regions forming the buried contact structure 20 include the first n + -type contact diffused region 22 formed by impurity diffusion of arsenic, and the second n + -type contact diffused region 24 formed by impurity diffusion of phosphorous deeper than the first contact diffused region 22. Because of this double-layer structure, it is possible to reduce the buried contact resistance, which would be a cause for the conventional problems, to an adequately small value, realizing a MOSFET having a small parasitic resistance. Further, having the second n + -type contact diffused region 24 deep in connection, the MOSFET 100 allows its connecting area to the n + -type source/drain diffused region 42a to increase so that with the depth of the n + -type source/drain diffused region 42a, 42b near the channel being kept shallow, adequate connection capacitance can be secured. Therefore the MOSFET 100 is suitable for use as a static random acess memory (SRAM). Since the depth of the second N + -TYPE contact diffused region 24 is set to be deeper than the depth of the first n + -type contact diffused region 22, it is possible to obtain a low-resistance buried contact structure stably with no influence from the non-uniform processing, such as irregular depth of etching of the silicon substrate 10. Further, since the depth of diffusion and the impurity concentration of the second n + -type contact diffused region 24 can be set independently of the impurity concentration in the polycrystalline silicon layer, it is possible to improve the degree of freedom of device designing. In this embodiment, the buried contact structure is formed only for one source/drain electrode. Alternatively, the buried contact structure may be formed for both the source and drain electrodes. FIGS. 4A through 4G shows the manufacturing process of the semiconductor device of FIG. 1. Like reference numerals designate similar parts or elements throughout FIGS. 1 and 4. (A) In the step shown in FIG. 4A, a field oxide film 12 and a gate oxide film 14 are formed on a p-type silicon substrate 10 by a usual method. Then, using a resist film R1 as a mask, ion implantation (indicated by arrows in FIG. 4A) of phosphorous is made in the region defining a buried contact structure 20, under such a condition that energy of implantation is 80 to 120 KeV and an amount of implantation is about 5×10 13 to 5×10 14 cm -2 . After removing the resist film R1, annealing is performed in the atmosphere of nitrogen at 900 to 1,000°C for 40 to 80 minutes. In this step, a second n + -type contact diffused region 24 is formed. (B) In the step shown in FIG. 4B, opening H1 is formed in the gate oxide film 14 of the region defining the buried contact structure 20, and then a polycrystalline silicon layer 16 having a thickness of about 2,000 to 5,000 ×10 -10 m is piled on the silicon substrate 10. Then, under such a condition that energy of implantation is 60 to 100 KeV and an amount of implantation is 5×10 15 to 1×10 16 cm -2 , ion implantation (indicated by arrows in the drawing) of arsenic is performed, and annealing is performed in the atmosphere of nitrogen at 900 to 1,000°C for 20 to 30 minutes. In this step, an n + -type contact diffused region 22 is formed by impurity diffusion of arsenic from the polycrystalline silicon layer 16. (C) In the step shown in FIG. 4C, the polycrystalline silicon layer 16 is etched by the usual photolithography and etching technology to form an n + -type gate electrode 30, an n + -type source/drain electrode 40 and a wiring portion 44 contiguous thereto. At that time, the exposed portion of the p-type silicon substrate 10 also is etched to form a hollow 18 on the substrate 10. (D) In the step shown in FIG. 4D, under such a condition that energy of implantation is 50 to 100 KeV and an amount of implantation is 1×10 13 to 3×10 13 cm -2 , ion implantation (indicated by arrows in the drawing) of phosphorous or arsenic is performed in order to form an n - -type offset region of an LDD structure. In this step, a diffused region 52a is formed on the surface of the silicon substrate 10. (E) In the step shown in FIG. 4E, a silicon oxide film 50a having a thickness of about 3,000 to 7,000 ×10 -10 m is piled up by chemical vapor deposition (CVD). (F) In the step shown in FIG. 4F, the silicon oxide film 50a is etched by anisotropic etching. In this step, a side wall 50 in the form of a silicon oxide film is formed on the end of each of the gate electrode 30 and the source/drain electrode 40. (G) In the step shown in FIG. 4G, under such a condition that energy of implantation is 50 to 100 KeV and an amount of implantation is 1×10 15 to 5×10 15 cm -2 , ion implantation (indicated by arrows in the drawing) of phosphorous or arsenic is performed, and annealing is performed in the atmosphere of nitrogen at 900 to 1,000°C for 20 to 60 minutes. In this step, the n + -type source diffused region 42a and the n + -type drain diffused region 42b are formed. Under the side wall 50, an n - -type offset region 52 constituted by a part of the diffused region 52a is formed to provide an LDD structure. By the foregoing series of steps, it is possible to obtain the MOSFET shown in FIG. 1. Another MOSFET 300 according to the first embodiment, as shown in FIG. 3, can have a buried contact structure to which no LDD structure is added. This MOSFET 300 is essentially identical in structure with the LDD-type MOSFET 100 of FIG. 1, except that there is no region corresponding to the n - -type offset region 52 of the MOSFET 100. Embodiment 2: FIG. 5 shows a semiconductor device according to the second embodiment of this invention. In FIG. 5, a Bi-MOS element includes an n-p-n-type vertical bipolar transistor 200 of washed emitter structure formed in an n-type well 10a on a p-type silicon substrate 10, and an n-channel MOSFET 100 of the LDD structure formed in a p-type well 10b. MOSFET 100 of this embodiment is identical in construction with the MOSFET 100 of the first embodiment. Therefore, like reference numerals designate similar parts or elements throughout FIGS. 1 and 5; its detailed description is omitted here for clarity. The bipolar transistor 200 includes an n + -type emitter diffused region 62 formed in the n-type well 10a, a p-type base diffused region 64 formed under the n + -type emitter diffused region 62, and an n + -type collector diffused region 66 separated from the p-type base diffused region 64 via a field oxide film 12. An emitter electrode 60 is formed on the n+-type emitter diffused region 62. The n + -type emitter electrode 60 of the bipolar transistor 200, the n + -type gate electrode 30 of MOSFET 100, a source/drain electrode 40 and a wiring portion 44 connected to a buried contact region 20 are formed of the n + -type polycrystalline silicon layer having arsenic by the same layer forming process. A first n + -type contact diffused region 22 of a buried contact structure 20 of MOSFET 100, and the n + -type emitter diffused region 62 of the bipolar transistor 200 are simultaneously formed by impurity diffusion of arsenic from the n + -type polycrystalline silicon layers (40, 60), having substantially the same depth of diffusion and impurity concentration. A second n + -type contact diffused region 24 of the buried contact structure 20 of MOSFET 100 and the n + -type collector diffused region 66 for connecting the collector electrode of the bipolar transistor 200 are simultaneously formed by impurity diffusion of phosphorous, having substantially the same depth of diffusion and impurity concentration. In this embodiment, the first n + -type contact diffused region 22 of MOSFET 100 has a depth of diffusion of 0.1 to 0.3 µm, and the second n + -type contact diffused region 24 has a depth of diffusion of 0.5 to 0.8 µm. According to this embodiment, since the buried contact structure 20 of MOSFET 100 includes the first n + -type contact diffused region 22 formed by impurity diffusion and the second n + -type contact diffused region 24 formed by impurity diffusion deeper than the first contact diffused region 22, it is possible to reduce the resistance of the buried contact structure. Therefore it is possible to form on one substrate both MOSFET 100 having a small parasitic resistance and the high-speed bipolar transistor 200 with shallow diffusion. Since the second n + -type contact diffused region 24 of the buried contact structure 20 is made deep, a buried contact structure having a low resistance can be formed with no influence by non-uniform processing, such as irregular depth of etching on the polycrystalline silicon layer as described in connection with the first embodiment. It is possible to set the depth of diffusion and impurity concentration independently for the n + -type emitter diffused region 62 of the bipolar transistor 200 and the second n + -type contact diffused region 24 of MOSFET 100. Therefore, the characteristics of the buried contact structure 20 of MOSFET 100 can be independently set from the bipolar transistor 200, improving the degree of freedom of device designing. FIGS. 6A through 6G shows the manufacturing process of the semiconductor device of FIG. 5. (A) In the step shown in FIG. 6A, using the usual method, an n-type well 10a and a p-type well 10b are formed in the p-type silicon substrate 10, and a field oxide film 12 and an oxide silicon film 14a are formed on the silicon substrate 10. Then, under such a condition that energy of implantation is 80 to 120 KeV and an amount of implantation is 5×10 13 to 5×10 14 cm -2 , ion implantation (indicated by arrows in the drawing) of phosphorous are performed on the silicon substrate 10 via a resist film R2 as a mask having openings, and annealing is performed in the atmosphere of nitrogen at 900 to 1,000°C for 40 to 80 minutes. In this step, an n + -type collector diffused region 66 for connecting the collector electrode, and a second n + -type contact diffused region 24 are formed. (B) In the step of FIG. 6B, ion implantation of a p-type impurity such as boron is performed to form a base diffused region 64, whereupon a resist film R2 is removed. Then, the oxide silicon film 14a shown in Fig. 6A is formed with openings H2, H3 for forming a buried contact structure and an emitter diffused region, respectively. A polycrystalline silicon layer 16 is deposited on the substrate 10 to a thickness of about 2,000 to 5,000 ×10 -10 m by CVD. Under such a condition that energy of implantation is 60 to 100 KeV and an amount of implantation is 5×10 15 to 1×10 16 cm -2 , ion implantation (indicated by arrows in the drawing) of arsenic is performed, and annealing is performed in the atmosphere of nitrogen at 900 to 1,000 C for 20 to 30 minutes. In this step, an n + -type emitter diffused region 62 and a first n + -type contact diffused region 22 are formed by arsenic diffusion from a polycrystalline silicon layer 16. (C) In the step shown in FIG. 6C, the polycrystalline silicon layer 16 is photo-etched to form an n + -type emitter electrode 60, an n + -type gate electrode 30, an n + -type source/drain electrode 40 to be connected to the buried contact structure, and a wiring portion 44. At this time, an exposed portion of the p-type silicon substrate 10 is also etched to form a hollow 18 on the substrate 10. (D) In the step shown in FIG. 6D, a resist film R3 is formed on a bipolar transistor forming region. Then, under such a condition that energy of implantation is 50 to 100 KeV and an amount of implantation is 1×10 13 to 3×10 13 cm -3 , ion implantation (indicated by arrows in the drawing) of arsenic or phosphorous is performed to form an offset region for the LDD structure on a MOSFET forming region. In this step, an n - -type diffused region 52a is formed in the MOSFET forming region on the surface of the substrate 10. (E) In the step shown in FIG. 6E, after removing the resist film R3, a silicon oxide film 50a having a thickness of 3,000 to 7,000 ×10 -10 m is piled up by CVD. (F) In the step shown in FIG. 6F, the oxide silicon film 50a undergoes the anisotropic etching to form silicon oxide side walls 50 on the end of each of the gate electrode 30 and n + -type source/drain electrode 40 and on the opposite ends of the n + -type emitter electrode 60. (G) In the step shown in FIG. 6G, a resist film R4 is formed on a bipolar transistor forming region. Then, under such a condition that energy of implantation is 50 to 100 KeV and an amount of implantation is 1×10 15 to 5×10 15 cm -2 , ion implantation (indicated by arrows in the drawing) of arsenic or phosphorous is performed, and annealing is performed in the atmosphere of nitrogen at 900 to 1,000°C for 20 to 60 minutes. In this step, an n + -type source diffused region 42a and an n + -type drain diffused region 42b are formed for MOSFET 100. The offset region 52 constituted by part of the n - -type diffused region 52a is formed under the side wall 50 to provide an LDD structure. In the second embodiment, like the first embodiment, in MOSFET it is also possible to obtain a buried contact structure to which no LDD structure is added. Both the n + -type emitter diffused region 62 and the first n + -type contact diffused region 22 can be formed by the same first step, and the n + -type collector diffused region 66 and the second contact diffused region 24 are formed by the same second step, thus simplifying the manufacturing procass of a semiconductor device. Embodiment 3: FIG. 7 shows a modification of the MOSFET of the first embodiment. MOSFET 400 of this embodiment is essentially identical in construction with the MOSFET 100 of FIG. 1, except that there is a cover portion 46. The cover portion 46 is formed by extending the end of the source/drain electrode 40 of the polycrystalline silicon layer, covering the gate oxide film 14. The cover portion 46 prevents the silicon layer of the first embodiment from being formed with a hollow 18, since the silicon substrate 10 is not exposed during etching. Therefore, no leak current is caused on the diffused layer due to the hollow 18. For this purpose, a masking film is formed on the silicon layer 16 at a cover portion forming region during etching (FIG. 4C). FIG. 8 shows a Bi-MOS element similar to the Bi-MOS element of the second embodiment, to which the structure of MOSFET 400 is used. The Bi-MOS element of this embodiment is identical with that of FIG. 5, except that the cover portion 46 is formed. In addition to the above-identified advantages as attained by the first and second embodiments, MOSFET and Bi-MOS elements of this embodiment can offer an advantage that the cover portion 46 serves to assist in preventing the leak current in the diffused layer which leak current might be caused by etching. Therefore, it is possible not only to prevent the leak current in the buried contact structure but also to prevent the subthreashold characterics of MOSFET from being deteriorated. Although certain preferred embodiments have been shown and described, it should be understood that many changes and modifications may be made therein without departing from the scope of the appended claims. For example, this invention is also applicable to p-n-p bipolar elements and/or p-channel MOS elements as well as the n-channel MOSFET as a MOS element and n-p-n bipolar transistor as the bipolar element.

Claims: A semiconductor device having an MOS element (100; 300; 400) formed on a main surface of a semiconductor substrate (10), the device comprising: source and drain diffused regions (42a, 42b) formed in the substrate (10), a gate electrode (30) formed on said substrate (10) via a gate oxide film (14), a polycrystalline silicon layer (40) formed on the substrate (10), a first buried contact diffused region (24), and a layer (22) provided between the first buried contact diffused region (24) and the polycrystalline silicon layer (40) wherein one of the source and drain diffused regions (42a, 42b) is connected to the polycrystalline silicon layer (40) via the first buried contact diffused region (24) and said layer (22), and the first buried contact diffused region (24) is formed deeper than said one of the source and drain diffused regions (42a, 42b), characterized in that said layer (22) between said first buried contact diffused region (24) and the polycrystalline silicon layer (40) consists of a second contact diffused region (22), wherein that edge of said second contact diffused region (22) that faces said gate electrode (30) is flush with the corresponding edge of said polycrystalline silicon layer (40). The device of claim 1 further comprising a bipolar element (200) formed on said main surface of the substrate, an emitter electrode (60) of the bipolar element (200) and a source/drain electrode (40) of the MOS element (100; 400) being formed of said polycrystalline silicon layer and an emitter diffused region (62) of the bipolar element being formed by diffusion from said emitter electrode (60). The device of claim 2, wherein said semiconductor substrate (10) is a silicon substrate, said MOS element (100; 400) is an MOS transistor formed in a well (10b), and said bipolar element (200) is a bipolar transistor formed in a well (10a). The device of claim 1 or 2, wherein said second contact diffused region (22) has a depth of diffusion of 0.1 to 0.3 µm and said first contact diffused region (24) has a depth of diffusion of 0.5 to 0.8 µm. The device of claim 1 or 2, wherein said second contact diffused region (22) is formed by doping arsenic having a small diffusion constant and said first contact diffused region (24) is formed by doping phosphorous having a large diffusion constant. The device according to claim 1 or 2, wherein said polycrystalline silicon layer (40) connected to said second buried contact diffused region (22) has a cover portion (46) formed to extend over said gate oxide film (14) so as to cover a gap between said gate oxide film (14) and said polycrystalline silicon layer (40). The device of claim 1 or 2, wherein offset regions (52) are formed in both, a region between said one of said source and drain diffused regions (42a, 42b) and the projection of an edge of said gate electrode (30) facing said one of said source and drain diffused regions, and a connecting portion between said first and second contact diffused regions (24, 22) so as to provide an LDD structure. The device of claim 1 or 2, wherein said second contact diffused region (22) has a higher concentration of impurities than the source and drain diffused regions (42a, 42b). A method of manufacturing a semiconductor device as defined in claim 1, comprising the steps of: (a) forming the first contact diffused region (24) by forming a field oxide film (12) and the gate oxide film (14) on said substrate (10), and then doping an impurity of a large diffusion constant to a buried contact diffused structure forming region, (b) forming the second contact diffused region (22) by forming an opening in the gate oxide film (14), forming the polycrystalline silicon layer (40) on the substrate (10), doping an impurity of a small diffusion constant to the polycrystalline silicon layer (40), and then diffusing the first contact diffused region (24) from the polycrystalline silicon layer (40) via the opening, (c) etching the polycrystalline silicon layer to form the gate electrode (30) and a source/drain electrode (40), said etching being performed such that that edge of the source/drain electrode (40) that faces the gate electrode (30) is flush with the corresponding edge of said second contact diffused region (22), (d) doping an impurity to form a source/drain diffused region, such that one of the source and drain diffused regions (42a, 42b) is connected to the polycrystalline silicon layer (40) via the first buried contact diffused region (24) and the second contact diffused region (22), wherein these steps are performed in this order. A method of manufacturing the semiconductor device as defined in claim 2, comprising the steps of: (a) forming the first contact diffused region (24) and a collector diffused region (66) by forming a well (10b, 10a) and then a field oxide film (12) and the gate oxide film (14a, 14) on the substrate (10) and then doping an impurity of a large diffusion constant to a buried contact diffused structure forming region and a collector diffused region forming region; (b) forming a base diffused region (64) in a bipolar element forming region while keeping an MOS element forming region masked, then forming openings in the gate oxide film corresponding to the buried contact diffused structure forming region and an emitter diffused region forming region, forming a polycrystalline silicon layer (16) on the substrate, with the MOS element forming region unmasked, and then doping an impurity of a small diffusion constant from the polycrystalline silicon layer via the openings to form said second contact diffused region (22) and an emitter diffused region (62); (c) etching the polycrystalline silicon layer to form the gate electrode (30), and a source/drain electrode (40) of the MOS element and the emitter electrode (60) of the bipolar element, said etching being performed such that that edge of the source/drain electrode (40) that faces the gate electrode (30) is flush with the corresponding edge of said second contact diffused region (22), and (d) doping an impurity, with the bipolar element forming region masked, to form a source/drain diffused region (42a, 42b), such that one of the source and drain diffused regions (42a, 42b) is connected to the polycrystalline silicon layer (40) via the first buried contact diffused region (24) and the second contact diffused region (22), wherein these steps are performed in this order.

Inventors: FURUHATA TOMOYUKI, FURUHATA, TOMOYUKI, Furuhata, Tomoyuki, c/o Seiko Epson Corporation
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1991
Main classification: H01L29
IPC codes: H01L27, H01L21, H01L23, H01L29