Dynamic random access memory device and method for producing the same

Abstract: The semiconductor memory device employs a memory cell composed of a transfer gate transistor (T3) and a memory cell capacitor (C3). The transfer gate transistor (T3) has source and drain regions (14S and 14D), a gate electrode (13), and a bit line (BL32). The memory cell capacitor (C3) comprises a storage electrode (25a) of polysilicon, a dielectric film (26) of silicon dioxide, and an opposed electrode (27) of polysilicon. The storage electrode (25a) and the opposed electrode (27) are doped with an n+-type impurity such as As or P. A storage electrode contact hole (24) is formed in an insulation film by a self-alignment process in which the bit lines (BL32) function as a resist mask film, and the storage electrode contact hole (24) has a slit shape. By using the self-alignment process, it is unnecessary to carry out a positioning operation with respect to the storage electrode contact hole (24). The area occupied by the memory cell is also decreased.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to a dynamic random access memory device and a method for producing the same, and more particularly, to improvements in the layer structure of a dynamic random access memory device and a method for producing such an improved dynamic random access memory device. A dynamic random access memory (hereinafter simply referred to as a DRAM device) has a number of memory cells. Each memory cell is composed of a memory cell capacitor and a transfer gate transistor. Currently, there has been considerable activity in reduction of the size of a DRAM device. It is required particularly to reduce the size of a memory cell capacitor in order to reduce the size of DRAM devices. It is noted that the area of memory cell capacitors which occupies a semiconductor substrate decreases, capacitance of each memory cell capacitor decreases. The capacitance of the memory cell capacitor depends greatly on the size of a storage electrode of the memory cell, which is paired with an opposed electrode thereof. As the area of the storage capacitor which occupies the semiconductor substrate decreases, the capacitance of the memory cell capacitor decreases considerably. The reduction of the size of the memory cell capacitor, particularly the reduction of the size of the storage electrode causes the following problems in addition to the decrease in capacitance of the memory cell capacitor. First, soft errors due to irradiation of α-particles increase with a decrease of the capacitance of the memory cell capacitor. Secondly, although it is known to make the storage electrode thick so as to increase the side wall area thereof and thereby increase the entire surface area of the storage electrode, it is difficult to obtain a fine pattern because the aspect ratio with respect to bit lines and contact holes increases. Thirdly, neighbouring bit lines are located closer, or in other words, a separation distance between the neighbouring bit lines decreases. The closer the neighbouring bit lines are arranged, the greater the coupling capacitance therebetween. IEEE Journal of Solid-State Circuits 15, pages 661-666, discloses a dynamic random access memory device according to the preamble of accompanying claim 1. Three alternative types of memory cell are disclosed including a "top-capacitor-type" having bit lines shielded by capacitor electrodes, as well as an "intermediate-capacitor-type" and a "bottom-capacitor-type". A 16k-bit DRAM employing cells of the "bottom-capacitor-type" is discussed. EP-A-0 161 850 by the present applicant discloses a DRAM device having a folded bit line structure for improved noise immunity and employing a self-alignment process in which an opening to the substrate is formed between adjacent word lines. SUMMARY OF THE INVENTION It is therefore a general object of the present invention to provide a novel and useful random access memory device and a method for producing the same in which the aforementioned disadvantages are eliminated. A more specific object of the present invention is to provide a random access memory device having a storage electrode having an increased surface area. An increased surface area of the storage electrode can provide various advantages. Another object of the present invention is to provide a random access memory device in which neighbouring bit lines are electrically shielded very well. Another object of the present invention is to provide a method for producing a random access memory device having the above-mentioned advantages. A further object of the present invention is to provide a method for producing a random access memory device in which a contact hole with respect to the drain region and the storage electrode is formed by a self-alignment process. According to a first aspect of the present invention, there is provided a dynamic random access memory device in which a memory cell is formed by a storage capacitor and a transfer transistor, the transfer transistor electrically connecting the storage capacitor to a bit line in response to a select signal applied to a word line, the device comprising: a semiconductor substrate having a source region and a drain region for forming said transfer transistor; a first insulation film formed over a surface of the semiconductor substrate; a gate electrode formed over the first insulation film; a second insulation film formed so as to cover the gate electrode: a bit line formed over the second insulation film, the bit line being electrically connected with the source region through a first opening formed in the first and second insulation films; a third insulation film formed so as to cover the bit line; a storage electrode of said storage capacitor formed over the third insulation film and being electrically connected with the drain region through a second opening formed in the first, second and third insulation films; a dielectric film formed so as to surround the storage electrode; and an opposed electrode of said storage capacitor formed so as to cover the dielectric film; wherein said word line crosses said bit line and extends between said source and drain regions, and said bit line is electrically connected with the source region at a first connection position of a first memory cell; characterised in that the storage electrode comprises an electrically conductive layer which is thicker than corresponding thicknesses of the layers of the word lines and bit lines; and in that the device has a folded bit line structure in which: said bit line is paired with and spaced from a further bit line which is electrically connected with a corresponding further source region at a second connection position of a second memory cell adjacent to said first memory cell; the first and second connection positions being located at respective, different locations spaced apart in the direction of extension of said bit line and said further bit line; and a first transfer transistor relating to the first connection position and a second transfer transistor relating to the second connection position being located so that two word lines extend in the space between the first and second connection positions. According to a second aspect of the present invention, there is provided a method for producing a dynamic random access memory device in which device a memory cell is formed by a storage capacitor and a transfer transistor, the transfer transistor electrically connecting the storage capacitor to a bit line in response to a select signal applied to a word line, the method comprising the steps of: (a) forming a word line including a gate electrode over a first insulation film formed over a semiconductor substrate, and forming source and drain regions in the semiconductor substrate so as to provide said transfer transistor, said word line extending between said source and drain regions; (b) forming a second insulation film so as to cover the gate electrode; (c) forming a first opening in the first and second insulation films; (d) forming a bit line over the second insulation film, crossing said word line and in electrical connection with the source region via the first opening at a first connection position, and forming a further bit line which is electrically connected with a corresponding further source region at a second connection position, the first and second connection positions being located at respective, different locations spaced apart in the direction of extension of said bit line and said further bit line; (e) forming a third insulation film so as to cover the bit line; (f) forming a second opening in the first, second and third insulation films; (g) forming a storage electrode of said storage capacitor so as to electrically connect to the drain region via the second opening and over the third insulation film, said storage electrode being thicker than corresponding thicknesses of the word lines and bit lines; (h) forming a dielectric film so as to surround the storage electrode; and (i) forming an opposed electrode of said storage capacitor so as to cover the dielectric film; whereby the device is formed in a folded bit line structure in which said source region is common to two adjacent memory cells; and whereby a first transfer transistor relating to the first connection position and a second transfer transistor relating to the second connection position are located so that two word lines extend in the space between the first and second connection positions. According to a modification of the second aspect, there is provided a method of forming a dynamic random access memory device including a plurality of memory cells, each memory cell having a storage capacitor and a transfer transistor which electrically connects the storage capacitor to an associated bit line in response to a select signal applied to the transfer transistor via an associated word line, said method comprising the steps of: (a) forming respective transistors of the plurality of memory cells so that each transfer transistor has a first impurity diffused region in common with an adjacent transfer transistor, spaced therefrom in a direction in which the associated bit lines extend, and a second impurity diffused region electrically connected to the storage electrode, a selected one of the first impurity diffused region and the associated bit line having a projecting portion for making an electrical connection between the first impurity diffused region and the associated bit line at a first connection position; said associated bit line being paired with a further bit line electrically connected to a corresponding first impurity diffused region at a second connection position; the first and second connection positions being located at respective, different positions spaced in said direction; and the transfer transistors relating to the first connecting positions and the transfer transistors relating to the second connecting position being located so that two respective word lines extend between the first and second connecting positions and crossing the bit lines; (b) forming a first insulating film covering the transfer transistors; (c) forming the associated bit lines over the first insulating film; (d) forming a second insulating film covering the associated bit lines; (e) forming storage electrodes over the second insulating film, said storage electrodes being formed thicker than corresponding thicknesses of the word lines and bit lines; and (f) forming an opposed electrode over the storage electrodes via dielectric films. Other objects, features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a circuit diagram of a memory cell of a DRAM device; FIG. 1B is a cross sectional view of a layer structure of a memory cell for a conventional DRAM device; FIG. 2A through 2C are views showing a first DRAM device embodying the present invention; FIGS. 3A through 3C are views showing a second DRAM device embodying the present invention; FIGS. 4A through 4C are views showing another embodiment of the present invention; FIGS. 5A through 5F are cross sections views taken along a line II A -II A shown in FIG. 2C, which are observed in different manufacturing steps for the first DRAM device; FIG. 6A through 6F are cross sectional views taken along a line II B -II B shown in FIG. 2C, which are observed in different manufacturing steps for the first DRAM device; FIGS. 7A through 7H are cross sectional views taken along a line III A -III A shown in FIG. 4C, which are observed in different manufacturing steps for the FIG. 4 embodiment of the present invention; FIGS. 8A through 8H are cross sectional views taken along a line III B -III B shown in FIG. 4C, which are observed in different manufacturing steps for the FIG. 4 embodiment; FIGS. 9A and 9B are cross sectional views for describing some problems which may be encountered in the first DRAM device; FIGS. 10A through 10C are views showing a modification of the first DRAM device; FIG. 11A through 11K are cross sectional views taken along a line XIII A -XIII A shown in FIG. 10C, which are observed at different manufacturing steps for the modification of FIGS. 10A through 10C; and FIGS. 12A through 12K are cross sectional views taken along a line XIII B -XIII B shown in FIG. 10C, which are observed at different manufacturing steps for the modification of FIGS. 10A through 10C. DETAILED DESCRIPTION Before describing the preferred embodiments of the present invention, a description is given of a memory cell for a conventional DRAM device with reference to FIGS. 1A and 1B. FIG. 1A is a circuit diagram of a memory cell for a DRAM device. As shown, a memory cell is composed of a transfer gate transistor T and a memory cell capacitor C. A source of the transfer gate transistor T is connected to 0a corresponding bit line BL, and a drain thereof is connected to a storage electrode 6. A gate of the transfer gate transistor T is connected to a corresponding word line WL. An opposed electrode 8 is connected to a constant power source. A dielectric film 7 is provided between the storage electrode 6 and the opposed electrode 8. FIG. 1B shows a cross section of a conventional layer structure which provides the memory cell of FIG. 1A. The memory cell comprises a p-type silicon substrate 1, a field insulation film 2, a gate insulation film 3, a drain region 4D, a source region 4S, and a silicon dioxide film 5. The gate electrode WL1 is concerned with the transfer gate transistor T, and the gate electrode WL2 is concerned with another transfer gate transistor (not shown). Gate electrodes WL1 and WL2 of polysilicon are formed on the gate insulation film 3 and the silicon dioxide film 5, respectively. The storage electrode 6 is formed so as to be in contact with the drain region 4D. The dielectric film 7 is formed so as to surround the storage electrode 6. The dielectric film 7 is covered with the opposed electrode 8. A passivation film 9 of phosphosilicate glass (PSG) is formed on the entire surface of the silicon substrate 1. The bit line BL which overlies the passivation film 9 is in contact with the source region 4S. In the conventional layer structure of FIG. 1B, it is particularly noted that the bit line BL has a vertical layer level higher than that of the layer of the memory cell capacitor C composed of the storage electrode 6, the dielectric film 7 and the opposed electrode 8. First and second DRAM devices embodying the present invention will now be described with reference to FIGS. 2A to 2C, and FIGS. 3A to 3C. FIGS. 2A through 2C show a first DRAM device embodying the present invention. FIG. 2C is a plan view of the device, showing that it has a folded bit line structure. FIG. 2A is a cross sectional view taken along the line II A -II A shown in FIG. 2C, and FIG. 2B is a cross sectional view taken along the line II B -II B . Referring to FIGS. 2A through 2C, a field insulation film 12 is formed on a main surface of a silicon substrate 11. A gate insulation layer 13 is formed on the surface of the silicon substrate 11. Gate electrodes WL3 and WL4 of polysilicon films are formed on the gate insulation layer 13 and the field insulation film 12, respectively. The gate electrodes WL3 and WL4 function as word lines of the transfer gate transistor T1. Then, impurity diffusion layers 14S and 14D are formed by implanting arsenic ions (As + ) into the silicon substrate 11, wherein the gate electrodes WL3 and WL4 are used as a mask. The impurity diffusion layers 14S and 14D function as the source and drain of a transfer gate transistor T1, respectively. An interlayer insulation film 15 of silicon dioxide (SiO 2 ) is formed so as to cover the gate electrodes WL3 and WL4. Bit lines BL1 of polycide or polysilicon which contains n + -type impurity ions such as arsenic ions (As + ) are formed so as to make contact with the source region 14S through a bit line contact hole 16, and overlie the interlayer insulation film 15. An insulation film 18 of silicon dioxide film is formed on the interlayer insulation film 15 so as to cover the bit lines BL1. The transfer gate transistor T1 is composed of the drain and source regions 14D and 14S, the gate electrode WL3 and the bit line BL1. A storage electrode 20a is formed on the silicon dioxide film 18 and is in contact with the drain region 14D through a contact hole 20A. The storage electrode 20a is made of polysilicon, and is doped with n + -type impurity ions such as arsenic (As + ). The storage electrode 20a is covered with a dielectric film 21 of silicon dioxide, which is formed by thermal oxidation with respect to the storage electrode 20a. An opposed electrode 22, which is paired with the storage electrode 20a, is formed so as to surround the dielectric film 21. The opposed electrode 22 is made of polysilicon containing n + -type impurity ions such as phosphorus. A memory cell capacitor or storage capacitor C1 is composed of the storage electrode 20a, the dielectric film 21 and the opposed electrode 22. It is particularly noted that each bit line BL1 has a layer level lower than that of the storage electrode 20a. Therefore, it is unnecessary to form bit lines on an insulation film which covers the memory cell capacitor. Additionally, it becomes possible to increase the storage electrode 20a because there is no bit line contact hole in the insulation film which covers the memory cell capacitor. As a result, the storage electrode 20a thicker than that of the storage electrode 6 shown in FIG. 1B can be formed. It is noted that the thicker storage electrode 20a increases the capacitance of the memory cell capacitor. In the case where each bit line has a layer level higher than that of the storage electrode as in the case of the conventional DRAM device of FIG. 1B, there is a limit on the film thickness of the storage electrode 6. This is because the presence of a thick storage electrode in the conventional layer structure makes it difficult to make contacts between the source regions 3S and the bit lines BL. It is further noted that the adjacent bit lines BL1 of the first device are electrically shielded by the storage electrode 20a and opposed electrode 22 interposed therebetween. Although the storage electrode 20a is normally in a floating state, there exists the strong capacitive coupling between the storage electrode 20a and the opposed electrode 22. As a result, the storage electrode 20a can function as an electrical shield member with respect to the neighbouring bit lines BL1. With the above structure, it becomes possible to reduce noise introduced on the bit lines BL1. A description will now be given of a second DRAM device embodying the invention, again having a folded bit line structure, by referring to FIGS. 3A to 3C. Here, those parts which are the same as those in the previous figures are given the same reference numerals. FIGS. 3A and 3B are cross sectional views taken along the lines III A -III A and III B -III B of FIG. 3C, respectively. This DRAM device is composed of a transfer gate transistor T2 and a memory cell capacitor C2, as in the case of the first DRAM device. The second DRAM device is different from the first in position of bit line contact holes. As shown in FIG. 2C, each bit line contact hole 16 and the corresponding storage electrode contact hole 20A are aligned in the direction of extension of the bit lines BL1. Particularly, the bit line contact hole 16 is positioned in the center of a distance between the neighbouring bit lines BL1. On the other hand, as shown in FIG. 3C, a bit line contact hole 16a with respect to a bit line BL22 and the corresponding storage electrode contact hole 20A of the transfer gate transistor T2, are not arranged into a line. The bit line contact hole 16a of the bit line BL22 is positioned closer to the bit line BL22 than a bit line BL23. Similarly, the bit line contact hole 16a of a bit line BL21 is positioned closer to the bit line BL21 than the bit line BL22. The cross section of the contact hole 16a of the bit line BL22 does not appear in the cross section of FIG. 3A. That is, the cross section of FIG. 3A differs from the cross section of FIG. 2A. With the aforementioned arrangement of the bit line contact holes 16a, it becomes possible to improve insulation properties between the neighbouring bit lines such as the bit lines BL21 and BL22 and to achieve easy patterning. The source region 14S is also different in shape from the source region 14S of the first DRAM device. Other portions of the second DRAM device are the same as those of the first DRAM device. A description is given of another embodiment of the present invention with reference to FIGS. 4A through 4C. FIGS. 4A and 4B are cross sectional views taken along the lines IV A -IV A and IV B -IV B of FIG. 4C, respectively. The embodiment is composed of a transfer gate transistor T3 and a memory cell capacitor C3. The transfer gate transistor T3 comprises the source and drain regions 14S and 14D, the gate electrode 13 and a bit line BL32. The memory cell capacitor C3 comprises a storage electrode 25a of polysilicon, a dielectric film 26 of silicon dioxide, and an opposed electrode 27 of polysilicon. The storage electrode 25a and the opposed electrode 27 are doped with n + -type impurity such as arsenic (As) and phosphorus (P), respectively. This embodiment has an essential feature in which it has a storage electrode contact hole 24 which is formed in an insulation film by a self-alignment process in which bit lines BL31, BL32 and BL33 function as a resist mask film. As shown in FIG. 4C, the storage electrode contact hole 24 has a slit shape. By the self-alignment process, it is unnecessary to carry out a positioning operation with respect to the storage electrode contact holes 24. Additionally, a decreased area of the memory cell which occupies the silicon substrate 11, is obtainable. A description is given of a method for producing the first DRAM device described above (FIG. 2) with reference to FIGS. 5A through 5F, and FIGS. 6A through 6F. FIGS. 5A through 5F show cross sectional views along the line II A -II A shown in FIG. 2C observed at different manufacturing steps, and FIGS. 6A through 6F show cross sectional views along the line II B -II B shown in FIG. 2C observed at different manufacturing steps. Referring to FIGS. 5A and 6A, the field insulation film 12 is formed by a selective thermal oxidation process such as the LOCOS process in which an oxidation-resistant mask film is used. After removing the oxidation-resistant mask film, the gate insulation film 13 is formed to a thickness of about 200 [Å] (10 Å = 1 nm) by a thermal oxidation process. Then, a polysilicon film is deposited to a thickness of approximately 2000 Å by chemical vapor deposition (CVD). Thereafter, the polysilicon film is patterned so as to form the gate electrodes WL3 and WL4. The patterning process is carried out by forming a resist mask film by the conventional photolithography technology, and then etching the polysilicon film by reactive ion etching (RIE) in which an etching gas of CCl 4 /O 2 is used. Thereafter, impurity ions such as arsenic ions (As + ) are implanted into the silicon substrate by self-alignment in which the gate electrodes WL3 and WL4 are used as a mask film. Thereby, the source and drain regions 14S and 14D are formed in the silicon substrate 11. Referring to FIGS. 5B and 6B, the interlayer insulation film 15 of silicon dioxide is deposited to a thickness of approximately 1000 Å so as to cover the gate electrodes WL3 and WL4. Then the interlayer insulation film 15 is etched by an anisotropic etching process such as RIE so as to form the opening (bit line contact hole) 16. In the RIE process, an etching gas of CHF 3 + H 2 is used. Referring to FIGS. 5C and 6C, an impurity-doped polysilicon film having a thickness of approximately 1000 Å is deposited on the entire surface of the silicon substrate 11 by low-pressure chemical vapor deposition. Then deposited polysilicon film is patterned by anisotropic etching such as RIE so as to form the bit lines (BL1) 17. A polycide film can be substituted with the polysilicon film. In this case, a polysilicon film is deposited, and then a tungsten silicide film is deposited on the polysilicon film. Thereafter, the polysilicon film and the tungsten silicide film are patterned by the conventional photolithography technology. The tungsten silicide film can be substituted with a refractory metal film of molybdenum silicide, tantalum silicide, or titanium silicide. Referring to FIGS. 5D and 6D, the insulation film 18 of silicon dioxide or silicon nitride is deposited by chemical vapor deposition so as to cover the bit lines 17. After providing a mask film (not shown) on the insulation film 18, the insulation film 18, the interlayer insulation film 15 and the gate insulation film 13 are etched so as to form an opening 19. The opening 19 functions as the storage electrode contact hole 20A. Referring to FIGS. 5E and 6E, a polysilicon film 20 is deposited by chemical vapor deposition on the entire surface of the silicon substrate 11, and is then patterned by anisotropic etching such as RIE so as to form the storage electrodes 20a. The polysilicon film 20 is approximately 4000 Å thick. An etching gas of CCl 4 /O 2 is used in the anisotropic etching process. Referring to FIGS. 5F and 6F, the dielectric film 21 having a thickness of approximately 100 Å is formed by thermal oxidation with respect to the storage electrodes 20a. Then, as shown in FIGS. 2A and 2B, the opposed electrodes 22 are deposited so as to cover the dielectric films 21. The manufacturing process for the second DRAM device (FIG. 3) is the same as that for the first device except for the size of the source region 14S and the mask film used for forming the opening 16a in the gate insulation film 13 and the interlayer insulation film 15. Therefore, a description of the manufacturing process for the second device is omitted. A description is given of a method for producing the FIG. 4 embodiment of the invention with reference to FIGS. 7A through 7H, and FIGS. 8A through 8H. The steps of FIGS. 7A and 8A, and FIGS. 7B and 8B are the same as those of FIG. 5A and 6A, and FIGS. 5B and 6B. Therefore, a description of the steps of FIGS. 7A, 8A, 7B and 8B is omitted. Referring to FIGS. 7C and 8C, a polycide film 17 and a silicon dioxide film 18 are grown by low-pressure chemical vapor deposition. The polycide film 17 can be substituted with a polysilicon film containing n + -type impurity ions. Referring to FIGS. 7D and 8D, after providing a resist mask film (not shown), the polycide film 17 is patterned by anisotropic etching such as RIE so as to form the bit lines such as BL32 and BL33. At this step, it is preferable that portions of the silicon dioxide film 18 on the patterned polycide films 17 are left. The presence of the silicon dioxide film 18 on the patterned polycide film 17 is necessary to form the storage electrode contact holes 24 by self-alignment, as described later. Referring to FIGS. 7E and 8E, the silicon dioxide film 23 having a thickness of approximately 1000 Å is deposited on the entire surface of the silicon substrate 11 by chemical vapor deposition. Referring to FIGS. 7F and 8F, the opening or storage electrode contact hole 24 is formed by anisotropic etching such as RIE so as to penetrate the silicon dioxide film 23, the interlayer insulation film 15 and the gate insulation film 13. The opening 24 can be formed by self-alignment in which the bit lines BL32 and BL33 are used as a mask film. It is noted that as shown in FIG. 8F, the upper surface of each polycide film 17 is covered with the silicon dioxide film 18, and side walls of each polycide film 17 and side walls of each the silicon dioxide film 18 are covered with the silicon dioxide film 23. The presence of the silicon dioxide film 18 on the upper surface of the polycide film 17 can prevent shorting between the polycide film 17 and a polysilicon film 25 deposited in the subsequent manufacturing step. In a case where the silicon dioxide film 18 is completely removed at the step of FIGS. 7D and 8D, the upper and/or side surfaces of the polycide film 17 may be partially exposed after the step of FIGS. 7F and 8F. For the sake of simplicity, a group of the insulation films covering the polycide film 17 is indicated by a reference numeral 23 in other figures. It is noted that an alignment margin or tolerance is not needed at the time of forming the opening 24 by the self-alignment process. On the other hand, in the aforementioned method for producing the first DRAM device, some alignment margin is necessary to prevent the bit lines BL1 from being exposed after the step of FIGS. 5D and 6D. Referring to FIGS. 7G and 8G, the polysilicon film 25 is deposited on the entire surface of the silicon substrate 11 by low-pressure chemical vapor deposition. Thereafter, a resist mask film (not shown) is deposited, and then the polysilicon film 25 is patterned by anisotropic etching such as RIE. Each patterned polysilicon film 25 provides the storage electrodes 25a. Then, as shown in FIGS. 7H and 8H, the dielectric film 26 of silicon dioxide is formed so as to cover the surface of each storage electrode 25. Finally, the polysilicon film 27 is deposited so as to cover the dielectric film 26 as shown in FIGS. 4A and 4B. A description is given of a modification of the first DRAM device with reference to FIGS. 10A through 10C. In FIGS. 10A through 10C, those parts which are the same as those in FIGS. 2A through 2C, are indicated by the same reference numerals. The modification of the first DRAM device has an essential feature in which side wall films are provided so as to surround side surfaces of the gate electrodes WL3 and WL4 and the bit lines BL1. In FIGS. 9A and 9B which show the cross section of the aforementioned first DRAM device, film portions surrounded by circles are not flat, and have some differences in vertical layer level. In other words, the film portions surrounded by the circles are convex and recess portions. The presence of some convex and recess portions makes it difficult to suitably form a film thereon. Additionally, it is inevitable that the film formed on the convex and recess portions becomes thin. Additionally, a film portion under an overhang portion may remain without being etched. As a result, insulation properties of the film are degraded. The modification of the first DRAM device can overcome the above disadvantages. Referring to FIGS. 10A through 10C, side wall films 48a of silicon dioxide or silicon nitride are provided so as to surround vertical side walls of the gate electrodes WL3 and WL4. The cross section of each side wall film 48a has a slope which connects the upper surface of the word line WL3, WL4 and the upper surface of the field insulation film 12 or the gate insulation film 13. The side wall films 48a contribute to smoothing the upper surface of the insulation film 15, providing easy etching with respect to films formed on the insulation film 15, and improving the insulation properties of the insulation film 15. Side wall films 49a of silicon dioxide or silicon nitride are provided so as to surround vertical side walls of the bit lines BL1. The cross section of each side wall film 49a has a slope which connects the upper surface of the bit line BL1 and the upper surface of the interlayer insulation film 15. The side wall films 49a contribute to improving the insulation properties of the interlayer insulation film 18, providing easy etching with respect to films on the insulation film 18, and smoothing the upper surface of the interlayer insulation film 18. A description is given of a method for producing the modified device of FIG. 10A through 10C with reference to FIGS. 11A through 11K, and FIGS. 12A through 12K. The step of FIGS. 11A and 12A is the same as that shown in FIGS. 5A and 6A. Therefore, a description with respect to FIGS. 11A and 12A is omitted. Referring to FIGS. 11B and 12B, an insulation film 48 of silicon dioxide is formed on the entire surface of the silicon substrate 11 by chemical vapor deposition. Thereafter, the insulation film 48 is dry-etched by anisotropic etching such as RIE. Thereby, as shown in FIGS. 11C and 12C, film portions around the side walls of the gate electrodes WL3 and WL4 are remained without being removed. Such film portions are the side wall films 48a. In the etching process of the silicon dioxide film 48, an etching gas of CHF 3 + H 2 is used. Referring to FIGS. 11D and 12D, the interlayer insulation film 15 of silicon dioxide having a thickness of about 1000 Å is deposited on the entire surface of the silicon substrate 11. Then a resist mask film (not shown) is placed on the silicon substrate 11, and the insulation film 15 is etched by anisotropic etching such as RIE so as to form the opening 16 of the bit line contact hole. Referring to FIGS. 11E and 12E, the polysilicon film containing n + -type impurity ions such as arsenic ions (As + ) is deposited to a thickness of approximately 1000 [Å] by low-pressure chemical vapor deposition. Subsequently, a tungsten silicide film is deposited to a thickness of approximately 1000 Å by chemical vapor deposition. Then the polysilicon and tungsten silicide films are subjected to the patterning process based on RIE. Thereby, the patterned polysilicon and tungsten silicide films 17 of the bit lines BL1 are formed. Referring to FIGS. 11F and 12F, a silicon dioxide film 49 is deposited to a thickness of about 2000 Å by chemical vapor deposition. Thereafter, the silicon dioxide film 49 is subjected to anisotropic etching such as RIE in which CHF 3 + H 2 is used. Thereby, as shown in FIGS. 11G and 12G, film portions 49a around the side walls of the bit line BL1 are left without being removed. Such film portions 49a are side wall films around the bit lines BL1. Referring to FIGS. 11H and 12H, the insulation film 18 of silicon dioxide or silicon nitride is deposited on the entire surface of the silicon substrate 11 by chemical vapor deposition. The insulation film 18 is approximately 1000 Å . Referring to FIGS. 11I and 12I, by using a resist mask film (not shown), the insulation film 18 and the interlayer insulation film 15 are selectively etched so as to form the opening 19 through which a surface of the drain region 11D is exposed. The opening 19 functions as the storage electrode contact hole. Referring to FIGS. 11J and 12J, the polysilicon film 20 containing n + -type impurity ions is deposited, and is then patterned by anisotropic etching such as RIE in which a resist mask film (not shown) is used. The patterned polysilicon film 20 provides storage electrodes 20a. Referring to FIGS. 11K and 12K, the dielectric film 21 is formed by thermal oxidation so as to surround the surface of each storage electrode 20a. Finally, the opposed electrode 22 is formed so as to cover the dielectric films 21. According to the modification of the first DRAM device, the electrical insulation between the gate electrodes WL3 and WL4 and the storage electrode 20a and between the bit lines BL1 and the storage electrode 20a, can be greatly improved. Additionally, upper surfaces of the insulation films such as the interlayer insulation film 15 and the insulation film 18 are made flat. As a result, easy etching for obtaining bit lines BL1 and storage electrodes 20a is obtainable. The aforementioned side wall films are also applicable to the FIG. 4 embodiment of the invention. The present invention is not limited to the aforementioned embodiments, and variations and modifications may be made without departing from the scope of the present invention.

Claims: A dynamic random access memory device in which a memory cell is formed by a storage capacitor and a transfer transistor, the transfer transistor electrically connecting the storage capacitor to a bit line in response to a select signal applied to a word line, the device comprising: a semiconductor substrate (11) having a source region and a drain region (14S, 14D) for forming said transfer transistor; a first insulation film (13) formed over a surface of the semiconductor substrate; a gate electrode (WL3) formed over the first insulation film; a second insulation film (15) formed so as to cover the gate electrode: a bit line (BL1, BL22, BL32) formed over the second insulation film, the bit line being electrically connected with the source region (14S) through a first opening (16, 16a) formed in the first and second insulation films; a third insulation film (18, 23) formed so as to cover the bit line; a storage electrode of said storage capacitor (20a, 25a) formed over the third insulation film and being electrically connected with the drain region (14D) through a second opening (20A, 24) formed in the first, second and third insulation films; a dielectric film (21, 26) formed so as to surround the storage electrode; and an opposed electrode (22, 27) of said storage capacitor formed so as to cover the dielectric film; wherein said word line (WL3) crosses said bit line and extends between said source and drain regions (14S,14D), and said bit line (BL1, BL22, BL32) is electrically connected with the source region (14S) at a first connection position of a first memory cell; characterised in that the storage electrode comprises an electrically conductive layer which is thicker than corresponding thicknesses of the layers of the word lines and bit lines; and in that the device has a folded bit line structure in which: said bit line is paired with and spaced from a further bit line which is electrically connected with a corresponding further source region at a second connection position of a second memory cell adjacent to said first memory cell; the first and second connection positions being located at respective, different locations spaced apart in the direction of extension of said bit line and said further bit line; and a first transfer transistor relating to the first connection position and a second transfer transistor relating to the second connection position being located so that two word lines extend in the space between the first and second connection positions. A dynamic random access memory device as claimed in claim 1, wherein said bit line (BL1, BL22, BL32) is formed on the second insulation film (15) and in contact with the source region (14S). A dynamic random access memory device as claimed in claim 1 or 2, wherein said storage electrode (20a, 25a) is formed on the third insulation film (18, 23) and in contact with the drain region (14D). A dynamic random access memory device as claimed in claim 1, 2 or 3, wherein the second opening (20A, 24) is a contact hole formed by self-alignment in which the bit line (BL1, BL22, BL32) is used as a mask film. A dynamic random access memory device as claimed in any preceding claim, wherein the first opening (16, 16a) and the second opening (20A, 24) are aligned in the direction of extension of the bit line (BL1, BL22, BL32). A dynamic random access memory device as claimed in any preceding claim, wherein a distance between the bit line and the first opening (16, 16a) is smaller than a distance between the bit line (BL1, BL22, BL32) and the second opening (20A, 24). A dynamic random access memory device as claimed in any preceding claim, wherein the bit line (BL1, BL22, BL32) comprises a polycide film composed of a polysilicon film, and a refractory metal film formed on said polysilicon film. A dynamic random access memory device as claimed in any preceding claim, wherein an insulation side wall film (49a) is formed so as to surround a side wall of the bit line (BL1, BL22, BL32). A dynamic random access memory device as claimed in any preceding claim, wherein an insulation side wall film (48a) is formed so as to surround a side wall of the gate electrode (WL3). A dynamic random access memory device as claimed in claim 1, wherein the electrical connections between the bit line and the source region, and between the further bit line and the further source region, are provided by means of a projecting portion of the respective source region. A dynamic random access memory device as claimed in claim 1, wherein the electrical connections between the bit line and the source region, and between the further bit line and the further source region, are provided by means of a projecting portion of the respective bit line. A dynamic random access memory device as claimed in any preceding claim, wherein the word lines have bent portions in areas in which the transfer transistors are formed. A dynamic random access memory device as claimed in any preceding claim, wherein the storage electrode comprises electrically conductive portions which are spaced apart from each other in a vertical direction and which extend laterally. A dynamic random access memory device as claimed in any preceding claim, further comprising a field insulating film on which the word lines are provided, wherein the storage electrode extends above and overlaps an adjacent word line. A method for producing a dynamic random access memory device in which device a memory cell is formed by a storage capacitor and a transfer transistor, the transfer transistor electrically connecting the storage capacitor to a bit line in response to a select signal applied to a word line, the method comprising the steps of: (a) forming a word line including a gate electrode (WL3) over a first insulation film (13) formed over a semiconductor substrate, and forming source and drain regions (14S, 14D) in the semiconductor substrate so as to provide said transfer transistor, said word line extending between said source and drain regions; (b) forming a second insulation film (15) so as to cover the gate electrode; (c) forming a first opening (16, 16a) in the first and second insulation films (13, 15); (d) forming a bit line (BL1, BL22, BL32) over the second insulation film, crossing said word line and in electrical connection with the source region via the first opening at a first connection position, and forming a further bit line which is electrically connected with a corresponding further source region at a second connection position, the first and second connection positions being located at respective, different locations spaced apart in the direction of extension of said bit line and said further bit line; (e) forming a third insulation film (18, 23) so as to cover the bit line; (f) forming a second opening (20A, 24) in the first, second and third insulation films (13, 15, 18, 23); (g) forming a storage electrode (20a, 25a) of said storage capacitor so as to electrically connect to the drain region via the second opening and over the third insulation film, said storage electrode being thicker than corresponding thicknesses of the word lines and bit lines; (h) forming a dielectric film (21, 26) so as to surround the storage electrode; and (i) forming an opposed electrode (22, 27) of said storage capacitor so as to cover the dielectric film; whereby the device is formed in a folded bit line structure in which said source region (14S) is common to two adjacent memory cells; and whereby a first transfer transistor relating to the first connection position and a second transfer transistor relating to the second connection position are located so that two word lines extend in the space between the first and second connection positions. A method as claimed in claim 15, wherein step (d) comprises forming said bit line over the second insulation film in contact with the source region (14S). A method as claimed in claim 15 or 16, wherein step (g) comprises forming said storage electrode (25a) over the third insulation film in contact with the drain region (14D). A method as claimed in claim 16 or 17, wherein step (f) comprises forming said second opening as a contact hole by self-alignment, using the bit line (BL1, BL22, BL32) as a mask film. A method as claimed in any of claims 16 to 18, further comprising the steps of forming a fourth insulation film (48) so as to cover the gate electrode (WL3), and patterning the fourth insulation film by an anisotropic etching process so that a portion (48a) of the fourth insulation film around the gate electrode is left and functions as a side wall film. A method as claimed in any of claims 16 to 19, further comprising the steps of forming a fifth insulation film (49) so as to cover the bit line, and patterning the fifth insulation film by an anisotropic etching process so that a portion (49a) of the fifth insulation film around the bit line is left and functions as a side wall film. A method of forming a dynamic random access memory device including a plurality of memory cells, each memory cell having a storage capacitor and a transfer transistor which electrically connects the storage capacitor to an associated bit line in response to a select signal applied to the transfer transistor via an associated word line, said method comprising the steps of: (a) forming respective transistors of the plurality of memory cells so that each transfer transistor has a first impurity diffused region in common with an adjacent transfer transistor, spaced therefrom in a direction in which the associated bit lines extend, and a second impurity diffused region electrically connected to the storage electrode, a selected one of the first impurity diffused region and the associated bit line having a projecting portion for making an electrical connection between the first impurity diffused region and the associated bit line at a first connection position; said associated bit line being paired with a further bit line electrically connected to a corresponding first impurity diffused region at a second connection position; the first and second connection positions being located at respective, different positions spaced in said direction; and the transfer transistors relating to the first connecting positions and the transfer transistors relating to the second connecting position being located so that two respective word lines extend between the first and second connecting positions and crossing the bit lines; (b) forming a first insulating film covering the transfer transistors; (c) forming the associated bit lines over the first insulating film; (d) forming a second insulating film covering the associated bit lines; (e) forming storage electrodes over the second insulating film, said storage electrodes being formed thicker than corresponding thicknesses of the word lines and bit lines; and (f) forming an opposed electrode over the storage electrodes via dielectric films. The method as claimed in claim 21, wherein the step (e) of forming the storage electrodes further comprises forming a plurality of electrically conductive films.

Inventors: EMA TAIJI, EMA, TAIJI, EMA, TAIJI, C/O FUJITSU LIMITED
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1988
Main classification: H01L27
IPC codes: H01L21, H01L27