DRAM device with upper and lower capacitor and production method

Abstract: A semiconductor memory device provided with capacitors formed above and below a cell transistor includes first and second transistors formed in a first level, a first storage electrode (200) connected to the first transistor and formed below the first level, and a second storage electrode (300) connected to the second transistor and formed above the first level. The first and second storage electrodes (200,300) are connected to each source (40,42) via a spacer (21,23) formed on the sidewalls of each source, and undercuts (400) are formed between the storage electrode and the transistor, to thereby obtain double or more cell capacitance, a stable cell transistor characteristic and reduced short-channel effects.

Description: The present invention relates to a semiconductor memory device and a method for manufacturing the same, and more particularly, to a DRAM memory device provided with capacitors formed above and below a cell transistor, and a method for manufacturing the same. Recently, there have been many studies to improve the structure of a stacked capacitor cell and a trench capacitor cell for DRAM devices of 256Mb and beyond. However, it is still very difficult, despite an extremely complicated manufacturing process, to ensure a sufficient cell capacitance in a DRAM cell having a 1.5V operation voltage and a 0.5 µ m 2 cell size. Further, for a layout having a sufficient alignment margin, either a cell having a feature size smaller than a current minimum feature size has to be formed, or a storage electrode whose height is larger than the current height has to be formed. The former is almost impossible due to a limit of a current photo-etching process, and the latter causes problems in manufacturing. In addition, as an impurity concentration of source and drain constituting a metal-oxide semiconductor (MOS) device becomes high, a junction leakage current gradually becomes larger, thereby causing data storage problems. FIGS. 1A-1E are section views showing a method for manufacturing a DRAM having a buried capacitor (see "A Buried Capacitor DRAM Cell with Bonded SOI for 256M and 1Gbit DRAMs" by Toshiyuki Nishihara et al., IDEM, 1992, pp803-806), for solving the above-described problems. Here, a trench for forming a cell separating insulation film is formed on a silicon substrate 500, and SiO 2 film is deposited thereon and etched, to thereby form a cell-separating insulation film 502. Subsequently, polysilicon/SiO 2 pillars 504 are formed in order to form a storage electrode connected to the substrate. Meanwhile, in a peripheral region, i.e., the region where the storage electrode is not formed, dummy patterns 505 are formed to compensate for the height of pillars 504 (FIG. 1A). Next, polysilicon is deposited all over the resultant structure on which the pillars and dummy patterns are formed. Then, the polysilicon is etched back, to thereby form a spacer 506 on the sidewalls of the pillars (FIG. 1B). After the peripheral region is covered with a photoresist pattern 508, the SiO 2 portion of each pillar is removed using HF solution. As a result, a 1.6 µ m-high storage electrode 510 is formed (FIG. 1C). A dielectric film 512 and a plate electrode 514 are then formed. Subsequently, a polysilicon buffering layer 516 is deposited and etched back, to thereby planarize the surface thereof. Then, a support wafer 518 is bonded to the buffering layer 51b (FIG. 1D). Lastly, the rear surface of silicon substrate 500 is polished, thereby leaving an active region 520 of 80nm between each cell-separating insulation film 502 (FIG. 1E). According to the above disclosure, a cell capacitor is formed below an active region and completely buried so that the degree of planarization of a word line and bit line formed in the subsequent process can be improved. In addition, the margin for forming a storage electrode is slightly increased. However, there are certain drawbacks to this method. First, for the case of a cylindrical storage electrode, the height of the storage electrode has to be above 1.5 µ m in order to ensure sufficient cell capacitance with the limited cell size. Second, the process becomes more difficult because a dummy pattern is needed. Third, since a contact hole for connecting a storage electrode to an active region and a contact hole for connecting a bit line to an active region are formed together in a limited active region, the contact hole size and channel length are reduced. As a result, a contact resistance increase and a short channel pattern are generated, which may cause a serious problem in an operation of a device. Fourth, if a storage electrode that repeats charge/-discharge is formed above a transistor, an operation characteristic of a transistor is unstable due to the charge/discharge. Therefore, it is desirable to form a storage electrode avoiding the region where a transistor is to be formed. As for the Nishihara et al. invention, when a folded bit line structure (where a bit line is formed lengthwise and a word line is formed crosswise of an active region) is adopted to stabilize an operation characteristic of a transistor, cell size at a word line direction becomes 0.3 µ m to 0.4 µ m. Thus, an area occupied by a cell capacitor becomes extremely small, which causes great difficulty in ensuring sufficient cell capacitance. Accordingly, to ensure sufficient cell capacitance, a more complicated process is needed or storage electrode height has to be increased. JP2106958 (D1) discloses a DRAM device having a memory cell with a stack capacitor and a trench capacitor arranged respectively above and below a transistor. JP63239969 discloses a memory cell having alternately arranged trench capacitors and stack capacitors as the capacitors for the memory cell. JP5243521 discloses a DRAM device having a first stack capacitor arranged below a transistor and a second stack capacitor arranged directly above the first capacitor and transistor. US-A-5055898 discloses a silicon-on-insulator memory cell having a semiconductor substrate and a plurality of trench capacitors formed at least partially within the substrate and dielectrically isolated from the substrate. Accordingly, it is a first object of the present invention to provide a semiconductor memory device having capacitors formed above and below a cell transistor, to overcome the drawbacks of the conventional art. It is a second object of the present invention to provide a semiconductor memory device having capacitors formed above and below a cell transistor, in which a cell capacitance of at least twice that of a conventional cell can be obtained. It is a third object of the present invention to provide a method for manufacturing the above semiconductor memory device. According to a first aspect of the present invention there is provided a semiconductor memory device provided with capacitors formed above and below a transistor cell, comprising: first and second transistors having silicon-on-insulator (SOI) structure formed in a first level; a lower storage stack electrode connected to said first transistor and formed below said first level; a first plate electrode associated with said lower storage stack electrode; an upper storage stack electrode connected to said second transistor and formed above said first level; and, a second plate electrode associated with said upper storage stack electrode, wherein said storage electrodes are each connected to a respective transistor, each transistor having a respective spacer formed on the sidewalls of the source of said respective transistor, the spacers being in electrical contact with the respective storage electrodes; and, in that said upper storage stack electrode and said lower storage stack electrode cross each other and partially overlap. Preferably, an undercut which is filled by said plate electrodes is formed between an uppermost surface of said lower storage stack electrode and said transistors and between a lowermost surface of said upper storage stack electrode and said transistors. In addition, the upper storage stack electrode is connected to the respective spacer via a connection pad, and bit lines connected to drains constituting the transistors are positioned between the active regions. Here, the bit lines are connected to the drains via connection pads. As another embodiment, bit lines connected to the drains of the transistors are placed between the active regions. In addition, bit lines are connected to the drains of the transistor via connection pads. According to a second aspect of the present invention, there is provided a method for manufacturing a semiconductor memory device according to the first aspect of the present invention, comprising the steps of: (a) forming an insulation film pattern on a first substrate, etching the first substrate by using the insulation film pattern as an etching mask, and forming first and second active regions which are separated from each other and protruded; (b) forming a spacer on the sidewalls of the active regions; (c) removing the part of spacer other than the part of the spacer formed on the sidewalls of the region where the source of a transistor is to be formed, to thereby form a first spacer contacting the first active region and a second spacer contacting the second active region; (d) depositing an insulating material over the entire resultant structure and etching back the insulating material to thereby fill only space between the active regions with the insulating material; (e) forming a first capacitor consisting of a first storage electrode connected to the first spacer, a first dielectric film and a first plate electrode on the resultant structure; (f) planarizing a surface of the first plate electrode; (g) forming an insulation film on the first plate electrode and bonding a second substrate with the resultant structure; (h) turning the first substrate upside down, etching back the first substrate such that the spacers and active regions can be exposed; (i) forming first and second transistors on the first and second active regions, respectively; and (j) forming a second capacitor consisting of a second storage electrode connected to the second spacer, a second dielectric film and a second plate electrode on the resultant structure. As an embodiment, a step for respectively forming a first connection pad connected to the second spacer and a second connection pad connected to the drain of a transistor, and a step for forming a bit line connected to the second connection pad and located between the active regions are included in the step (i). As another embodiment, a step of forming only a first connection pad connected to the second spacer is further included in the step (i). As another embodiment, a step of forming an insulation layer over the entire resultant structure is included in the step (d) and step (i). Here, the insulation layer is removed after forming storage electrodes is completed. According to a semiconductor memory device of the present invention and a method for manufacturing the same, cell capacitors connected with transistors are formed above and below an active region, to thereby increase an area occupied by a capacitor per unit cell in a chip. As a result, the cell capacitance is increased. Specific embodiments of the present invention are described below, by way of example, with reference to the attached drawings in which: FIG. 1A to FIG. 1E are section views showing a method for manufacturing a DRAM having a buried capacitor, based on a conventional method; FIGS. 2A-2C are section views showing a DRAM having capacitors formed above and below a cell transistor and which is manufactured according to a first embodiment of the present invention; FIGS. 3A-3G are layout diagrams showing a method for manufacturing the DRAM of FIGS. 2A-2C, according to the manufacturing process sequence thereof; FIGS. 4A-4G are section views taken along line IV-IV of FIGS. 3A-3G, respectively; FIGS. 5A-5G are section views taken along line V-V of FIGS. 3A-3G, respectively; FIGS. 6A-6G are section views taken along line VI-VI of FIGS. 3A-3G, respectively; FIG. 7 is layout showing a method for manufacturing a DRAM having capacitors formed above and below a cell transistor, according to a second embodiment of the present invention; FIG. 8 is layout showing a method for manufacturing a DRAM having capacitors formed above and below a cell transistor, according to a third embodiment of the present invention; FIG. 9 is layout showing a method for manufacturing a DRAM having capacitors formed above and below a cell transistor, according to a fourth embodiment of the present invention; and FIGS. 10A-10C are section views taken along lines AA, BB and CC of FIG. 9, respectively. FIG. 2A to FIG. 2C are section views showing a DRAM having capacitors formed above and below a cell transistor and which is manufactured according to embodiment 1 of the present invention. The DRAM device manufacturing according to embodiment 1 of the present invention includes a first transistor formed in a first active region 37 and comprises a first source 40, a first drain 41 and a first gate electrode 45; a second transistor formed in a second active region 38 formed at a material level same as first active region 37 and which consists of a second source 42, a second drain 43 and a second gate electrode 46; a first spacer 21 connected to the sidewalls of first source 40; a second spacer 23 connected to the sidewalls of second source 42; a first active insulation film pattern 13 formed under the first active region 37; a second active insulation film pattern 15 formed under the second active region 38; a first storage electrode 200 connected to first spacer 21 and formed under the active insulation film patterns 13 and 15; a first dielectric film 210 formed on the surface of the first storage electrode 200; a first plate electrode 220 with planarized bottom surface formed on the first dielectric film 210; an insulating film 34 formed at the bottom surface of first plate electrode 220; a second substrate 36 bonded with a bottom surface of the insulating film 34; a first pad 53 connected to first drain 41 and formed above the transistor; a second pad 54 connected to second drain 43 and formed at the same level as the first pad 53; a third pad 55 connected to second source 42 and formed at the same level as the first pad 53; a first bit line 62 connected to first pad 53 and formed above first pad 53; a second bit line 63 connected to second pad 54 and formed at the same level as the first bit line 62; a second storage electrode 300 connected to third pad 55 and formed above bit lines 62 and 63; a second dielectric film 310 formed on the surface of the second storage electrode 300; and a second plate electrode 320 formed on the second dielectric film 310. Here, reference numeral 44 denotes a gate insulation film, reference numeral 52 denotes an insulation film for insulating a gate electrode from other conductive layer, reference numeral 22 denotes an isolation region for insulating between active regions 37 and 38, reference numerals 58 and 64 each denote an interlayer insulating film, reference numerals 24 and 66 each denote an etching blocking film, and reference numeral 70 denotes a device protection film. Referring to FIG. 2C, cell capacitors are formed respectively above and below the first and second transistors which are formed at the same material layer. The capacitors formed above and below the transistor partially overlap. Referring to FIG. 2A and FIG. 2B, storage electrodes 200 and 300 of each cell are connected to transistors of each cell via a spacer 21 and 23 formed on the source sidewalls, and an undercut 400 is formed at the bottommost surface of each storage electrode. A memory device having an upper- and lower-level capacitor of the present invention has advantages as follows. First, a cell capacitor is formed above and below a transistor, thereby obtaining twice the cell capacitor area of a conventional DRAM. As a result, a cell capacitance can be easily increased. Second, a storage electrode is connected to a source via a spacer formed on the sidewalls of the source. Therefore, a contact hole area formed to connect the source of the cell transistor with a storage electrode is not needed in an active region. As a result, a corresponding channel length can be further ensured. Third, an undercut is formed at the bottommost surface of each storage electrode. Therefore, a plate electrode fixed at a regular voltage, instead of a storage electrode which is repeatedly charged and discharged faces a cell transistor. As a result, cell transistor characteristics can be prevented from destabilization caused by the charge/discharge action of the storage electrode. Next, a method for forming a memory device of embodiment 1 of the present invention will be explained with reference to FIGS. 3A-3G, 4A-4G, 5A-5G and 6A-6G. Here, FIGS. 3A-3G are layouts showing a method for manufacturing the DRAM having an upper- and lower-level capacitor of the embodiment 1 according to manufacturing process sequence, and FIGS. 4A-4G, 5A-5G and 6A-6G are section views taken along lines IV-IV, V-V and VI-VI of FIGS. 3A-3G, respectively. Referring to FIGS. 3A, 4A, 5A and 6A, a process sequence for forming a first active insulation film pattern 13, a second active insulation film pattern 15, a first spacer 18 and a second spacer 19 includes the steps of (a) forming sequentially a first insulation film 12 and a second insulation film 14 onto first substrate 10, (b) employing mask patterns 100 and 102 (marked by a dotted line) for forming first and second active regions, and performing a photo-etching process where the first and second insulation films 12 and 14 are used as etching objectives, to thereby form first and second active insulation film patterns 13 and 15 consisting of first and second insulation films 12 and 14, (c) performing an etching by employing the active insulation film patterns as an etching mask, to thereby form a trench 17 onto a first substrate, (d) forming a thermal oxide film 16 on the exposed surface of the first substrate, (e) forming a first material layer over the entire resultant structure and anisotropically etching the first material layer, to thereby form a dummy spacer consisting of the first material layer on the sidewalls of the active insulation film pattern 13 and 15 and trench 17, (f) forming an etch blocking film pattern 20 for leaving only a dummy spacer contacting the region where the source of a transistor is to be formed, by employing mask patterns 104 and 106 (marked as a solid line) for forming first and second dummy spacers 18 and 19, and (g) employing etch blocking film pattern 20 as an etching mask and performing an etching process where the dummy spacer is used as the etching objective, to thereby leave a first dummy spacer 18 (marked by hashed lines in mask pattern 104) on the sidewalls of first active insulation film pattern 13 and a second dummy spacer 19 (marked as hashed lines in mask pattern 106) on the sidewalls of second active insulation film pattern 15. Here, first insulation film 12 is formed by depositing an insulation material, for example, high-temperature oxide (HTO), to a thickness of about 2,000Å (10Å=1nm). Second insulation film 14 is formed by depositing an insulation material, for example, silicon nitride (SiN), to a thickness of about 500Å. The depth of trench 17 is approximately 2,000Å. Thermal oxide film 16 is formed to a thickness of about 200Å. The first material layer consists of undoped polysilicon. The dummy spacer is formed to a thickness of about 500Å. Etch blocking film pattern 20 may consist of a photosensitive film. In addition, the anisotropic etching performed in step (e) employs an etching method such as chemical dry etching (CDE) or plasma etching. Besides the shape shown in FIG. 3A, mask patterns 104 and 106 can be alternatively patterned, for example, as mask patterns 170 and 172 of FIG. 7 or mask pattern 180 of FIG. 8. According to mask patterns 104 and 106 of FIG. 3A, first and second dummy spacers are formed only on one surface of first and second active insulation film patterns. However, according to the mask pattern shown in FIG. 7 and FIG. 8, the first and second insulation spacers can be formed on two or three surfaces. As a result, the contact area of the storage electrode can be broadened. Thus, a contact resistance between a storage electrode and a source is decreased and a memory cell characteristic can be enhanced. An undoped polysilicon is employed as the material constituting the first material layer. If a conductive material, for example, an impurity-doped polysilicon, is employed, the impurity doped in the polysilicon is diffused into a first substrate 10. Thus, the impurity becomes doped into the region where a source and a drain are not to be formed, which causes a problem. A material having an excellent etching selectivity to a material constituting the first insulation film and a device separating film (described in a later process) can be employed instead of an undoped polysilicon. FIGS. 3B, 4B, 5B and 6B show the step of forming a device separating film 22, material layers for forming a first storage electrode and a contact hole 31 for connecting a first storage electrode to a first spacer 21. The process sequence includes the steps of: (a) removing the etching preventing film pattern (reference numeral 20 of FIG. 4A); (b) forming a third insulation film over the entire resultant structure; (c) etching back the third insulation film so as to leave the third insulation film only inside the trench, to thereby form device separating film 22; (d) removing the first and second dummy spacers exposed by step (c); (e) removing the thermal oxide film (reference numeral 16 of FIG. 4A); (f) depositing a first conductive material layer over the entire resultant structure and performing an etch back where a surface of device separating film 22 is used as an end point, to thereby form first and second spacers 21 and 23 consisting of the first conductive material layer; (g) sequentially forming a fourth insulation film 24, a fifth insulation film 26, a second conductive material layer 28 and a sixth insulation film 30 over the entire resultant structure; (h) performing an etching process where the fourth, fifth, sixth insulation films 24, 26 and 30 and the second conductive material layer 28 are used as an etching objective and a mask pattern 110 is employed, to thereby form a contact hole 31 for connecting the first storage electrode to the first space 21; and (i) forming a third conductive material layer 32 over the entire resultant structure. Here, as the third insulation film, an oxide film formed by a chemical vapor deposition (CVD) method is employed. The etch back process adopting a chemical mechanical polishing (CMP) method is performed by using the second insulation film that constitutes the active insulation film patterns 13 and 15 as an end point. The step (d) is performed by a chemical dry etching (CDE) method or plasma etching method. The first conductive material layer that constitutes the spacers 21 and 23 is formed by depositing a conductive material, for example, an impurity-doped polysilicon. Fourth insulation film 24 is formed by depositing an insulation material, for example, silicon nitride (SiN), to a thickness of about 200Å. Fifth insulation film 26 is formed by depositing an oxide film to a thickness of about 1,000Å by employing a CVD method. Second conductive material layer 28 is formed by depositing a conductive material, for example, an impurity-doped polysilicon, to a thickness of about 3,000Å. Sixth insulation film 30 is formed by depositing an oxide film to a thickness of about 1,000Å by employing a CVD method. In addition, third conductive material layer 32 is formed by depositing a conductive material, for example, an impurity-doped polysilicon, to a thickness of about 3,000Å to 5,000Å. Fifth insulation film 26 is formed for forming an undercut (to be explained later) at the bottommost surface of a storage electrode, and fourth insulation film 24 is formed to prevent a lower material layer (for example, the device separating film 22 or active insulation film pattern 13 and 15) from being damaged during the process of forming the undercut. Third conductive material layer 32 is connected to first spacer 21 via the contact hole. Referring to FIGS. 3C, 4C, 5C and 6C, a process sequence for forming first storage electrode 200 includes the steps of (a) forming a photo-sensitive film pattern 33 for forming a first storage electrode on the resultant structure by employing mask pattern 120 and (b) performing an etching employing photo-sensitive film pattern 33 as an etching mask, to thereby form first storage electrode 200. Here, photo-sensitive film pattern 33 is defined by cell units, centering around a source (to be formed on a first substrate connected to the first spacer 21) of a first transistor. In addition, the second conductive layer (reference numeral 28 of FIG. 4A) and the third conductive layer (reference numeral 32 of FIG. 4B) are etched using photo-sensitive film 33 as an etching mask, and whole fifth insulation film (reference numeral 26 of FIG. 4A) and sixth insulation film (reference numeral 30 of FIG. 4A) are etched in the etching process of step (b). Referring to FIG. 4C and FIG. 6C, an undercut (uc) is formed at the bottommost surface of first storage electrode 200 by eliminating the fifth insulation film. Here, fourth insulation film 24 prevents a lower material layer from being damaged during the elimination of the fifth insulation film. Referring to FIGS. 3D, 4D, 5D and 6D, a process sequence for forming first capacitors 200, 210 and 220, a second substrate 36, first transistors 40, 41 and 45 and second transistors 42, 43 and 46 includes the steps of: (a) eliminating the photosensitive film pattern (reference numeral 33 of FIG. 4C) and forming a first dielectric film 210 over the entire first storage electrode 200; (b) forming a fourth conductive material layer over the entire resultant structure and etching back the resultant structure, to thereby form a first plate electrode 220 having a planarized surface; (c) forming a seventh insulation film 34 over the entire resultant structure; (d) bonding a second substrate 36 onto the resultant structure; (e) turning the resultant structure upside down so that second substrate 36 is oriented towards the bottom; (f) performing an etching back using a surface of device separating film 22 as an end point and first substrate 10 as an etching objective and respectively forming first and second active regions 37 and 38 consisting of first substrate on first and second active insulation film patterns 13 and 15; (g) forming a gate oxide film 44 on surfaces of the first and second active regions 37 and 38; (h) forming a fifth conductive material layer and an eighth insulation film 48 over the entire resultant structure and performing an etching employing mask patterns 130 and 132, to thereby form a first gate electrode 45 traversing the first active region 37 and a second gate electrode 46 traversing the second active region 38; (i) performing an impurity doping process to the entire resultant structure and forming a first source 40 and a first drain 41 constituting the first transistor and a second source 42 and a second drain 43 constituting the second transistor; and (j) forming a ninth insulation film 50 over the entire resultant structure. Here, first plate electrode 220 is formed by depositing a conductive material, for example, an impurity-doped polysilicon, to a thickness of about 2,000Å. An oxide film is employed as a material for constituting seventh insulation film 34. The etch-back process performed in the step (e) adopts a polishing method, for example, CMP. The fifth conductive material layer constituting first and second gate electrodes 45 and 46 is made up of a conductive material, for example, an impurity-doped polysilicon. First source 40 and first drain 41 are self-aligned to first gate electrode 45. Second source 42 and second drain 43 are self-aligned to second gate electrode 46 and their bottom surfaces contact surfaces of active insulation film patterns 13 and 15. Eighth and ninth insulation films 48 and 50 consist of a pure oxide film where an impurity is not doped. Referring to FIGS. 3E, 4E, 5E and 6E, a process sequence for forming a first pad 53, a second pad 54, a third pad 55, a first contact hole 56 and a second contact hole 57 includes the steps of: (a) performing an anisotropic etching process on ninth insulation film (reference numeral 50 of FIG. 4D) to thereby form a protection film 52 for insulating the first and second gate electrodes 45 and 46 from other conductive layer and form a contact hole for connecting a first bit line, a second bit line and a second storage electrode to first drain 41, second drain 43 and second source 42, respectively; (b) forming a sixth conductive material layer over the entire resultant structure, and performing a photo-etching process employing the sixth conductive material layer as an etching objective and employing mask pattern 140, 142 and 144, to thereby form first pad 53 connected with first drain 41, second pad 54 connected with second drain 43 and third pad 55 connected with second source 42; (c) forming a tenth insulation film 58 over the entire resultant structure and planarizing the result; and (d) performing a photo-etching process employing the tenth insulation film as an etching objective and by employing mask patterns 146 and 148, to thereby form a first contact hole 56 for partially exposing first pad 53 and a second contact hole 57 for partially exposing second pad 54. Here, the sixth conductive material layer that constitutes the pads 53, 54 and 55 is formed by depositing an impurity-doped polysilicon to a thickness of about 1,000Å. Tenth insulation film 58 is formed by depositing an insulation material, for example, oxide film, to a thickness of about 2,000Å to 3,000Å by employing a CVD method. The pads 53, 54 and 55 are formed in order to reduce a contact failure by means of lowering the aspect ratio (contact hole height to width) of contact holes 56 and 57 when connecting the transistor to bit lines and a second storage electrode which is to be formed in the sequent process, respectively. Referring to FIGS. 3F, 4F, 5F and 6F, a process sequence for forming a first bit line 62 and a second bit line 63 includes the steps of: (a) forming a seventh conductive material layer over the entire resultant structure where the first and second contact holes 56 and 57 are formed: and (b) performing an etching process employing the seventh conductive material layer as an etching objective and employing mask patterns 150 and 152, to thereby form first bit line 62 connected to first drain 41 via first pad 53 and second bit line 63 connected to second drain 43 via second pad 54. Here, the seventh conductive material layer is formed by a conductive material, for example, an impurity-doped polysilicon. First and second bit lines 62 and 63 are formed between the active regions 37 and 38 (FIG. 3F). Referring to FIGS. 3G, 4G, 5G and 6G, a process sequence for forming second capacitors 300, 310 and 320 includes the steps of: (a) forming an eleventh insulation film 64 having a planarized surface over the entire resultant structure where first and second bit lines 62 and 63 are formed; (b) sequentially forming twelfth insulation film 66, thirteenth insulation film, an eighth conductive material layer and a fourteenth insulation film on eleventh insulation film 64; (c) performing an etching process employing a material layer formed on third pad 55 as an etching objective and employing a mask pattern 160, to thereby form a third contact hole 68 for exposing the third pad 55; (d) forming a ninth conductive material layer over the entire resultant structure and performing an etching process using a material layer formed on twelfth insulation film 66 as an etching objective and employing a mask pattern 162, to thereby form a second storage electrode 300; (e) forming a second dielectric film 310 on a surface of the second storage electrode 300; (f) forming a tenth conductive material layer over the entire resultant structure so as to form a second plate electrode 320 and planarizing the surface of second plate electrode 320; and (g) forming a fifteenth insulation film 70 over the entire resultant structure. Here, eleventh insulation film 64 is formed by depositing an insulation material, for example, oxide film, to a thickness of about 2,000Å to 3,000Å. Twelfth insulation film 66 is formed by depositing silicon nitride (SiN) to a thickness of about 300Å to 500Å. The thirteenth and fourteenth insulation films are formed by depositing an insulation material, for example, oxide film, by a CVD method. The eighth and ninth conductive material layers are formed by depositing a conductive material, for example, an impurity-doped polysilicon. The tenth conductive material layer is formed by depositing a conductive material, for example, an impurity-doped polysilicon, to a thickness of about 2,000Å. Fifteenth insulation film 70 is formed by an insulation material, for example, an oxide film. Referring to FIG. 5G, second storage electrode 300 is connected to second source 42 via third pad 55, and an undercut 400 is formed at the bottommost surface of the second storage electrode 300, as in the case of the first storage electrode 200. Here, twelfth insulation film 66 prevents an underlying material layer beneath the thirteenth insulation film from being damaged by the eliminating process of the thirteenth insulation film. Referring to FIG. 3G, mask pattern 162 for forming a second storage electrode and partially overlaps the mask pattern (reference numeral 120 of FIG. 3C) for forming the first storage electrode. Embodiment 1 of the present invention has distinct advantages. First, cell capacitors are formed respectively above and below the cell transistor. Thus, cell capacitances can be obtained which at least twice that of a conventional method which forms a cell capacitance only above or below a cell transistor. Second, since a storage electrode and source are connected by employing a spacer formed on the sidewalls of the source, a gate electrode covers more of an active region than in a conventional method that connects a storage electrode with the source via a contact hole. As a result, transistor short-channel effects are reduced. Third, since an undercut is formed at the bottommost surface of each storage electrode, it is not a storage electrode which undergoes repeated charging and discharging but a plate electrode at a fixed voltage that faces a cell transistor. Thus, cell transistor characteristics can be prevented from destabilization by charge/discharge action of a storage electrode. FIG. 7 and FIG. 8 are layouts showing a method for manufacturing a DRAM having a buried capacitor corresponding to embodiments 2 and 3 of the present invention. As shown in FIG. 3A, first and second spacers may be formed not only on one surface of an active insulation film pattern and but also on two (hatched portion of FIG. 7) or three surfaces (hashed portion of FIG. 8). In FIG. 7 and FIG. 8, the same reference numerals as those of FIG. 3A denote the same mask patterns. According to embodiments 2 and 3, since area of first and second spacers connected to first and second storage electrodes can be extended, a contact characteristic between first and second storage electrodes and first and second sources is improved compared with embodiment 1. FIG. 9 is a layout showing a method for manufacturing a DRAM having a buried capacitor of embodiment 4 of the present invention. The method connects first and second bit lines directly to the drain of a transistor without using a pad. Here, reference numeral 190 denotes a mask pattern for forming a contact hole for connecting a third pad with a second source, and reference numeral 192 denotes a mask pattern for forming a third pad. Here, the same reference numerals as those of FIGS. 3E-3G denote the same mask patterns. FIGS. 10A-10C are section views, taken along lines AA, BB and CC of FIG. 9, respectively. Referring to FIGS. 10A-10C, after the process of FIGs. 3D, 4D, 5D and 6D for forming ninth insulation film 50, the process sequence includes the steps of: (a) performing an etching process employing a mask pattern 190 for forming a contact hole for connecting a third pad with a second source and using the ninth insulation film around second source 42 as an etching objective, to thereby expose the second source; (b) forming a sixth conductive material layer over the entire resultant structure and performing an etching process using the sixth conductive material layer as an etching objective employing a mask pattern 192 for forming a third pad , to thereby form a third pad 55 for connecting the second storage electrode 300 with second source 42; and (c) eliminating the material deposited on first and second drains 41 and 43, to thereby form a contact hole for connecting the first and second bit lines 62 and 63 to the first and second drains 41b and 43, respectively. The subsequent process steps are the same as that of embodiment 1. Here, like components of FIGS. 10A-10C are denoted by the same reference numerals as those of FIGS. 3A-3G to 6A-6G. According to a semiconductor memory device of the present invention and a method for manufacturing the same, at least twice the cell capacitance of a conventional method can be obtained. In addition, cell transistor characteristics are stabilized and short-channel effects can be reduced. As a result, a next-generation DRAM chip can be manufactured.

Claims: A semiconductor memory device provided with capacitors formed above and below a transistor cell, comprising: first and second transistors having silicon-on-insulator (SOI) structure formed in a first level; a lower storage stack electrode (200) connected to said first transistor and formed below said first level; a first plate electrode (220) associated with said lower storage stack electrode (200) and facing said first level; an upper storage stack electrode (300) connected to said second transistor and formed above said first level; and, a second plate electrode (320) associated with said upper storage stack electrode (300) and facing said first level, wherein said storage electrodes (200,300) are each connected to a respective transistor, each transistor having a respective spacer (21,23) formed on the sidewalls of the source (40,42) of said respective transistor, the spacers being in electrical contact with the respective storage electrodes; and, in that, in a plan perspective, said upper storage stack electrode (300) and said lower storage stack electrode (200) cross each other and partially overlap. A semiconductor memory device according to claim 1, wherein an undercut (400) which is filled by said plate electrodes is formed between an uppermost surface of said lower storage stack electrode (200) and said transistors and between a lowermost surface of said upper storage stack electrode (300) and said transistors. A semiconductor memory device according to claim 2, wherein said upper storage stack electrode (300) is connected to said spacer (23) via a connection pad (55). A semiconductor memory device according to claim 3, wherein bit lines (62,63) connected to the drains (41,43) of said transistors are placed between said active regions (37,38). A semiconductor memory device according to claim 4, wherein said bit lines (62,63) are connected to the drains (41,43) of said transistor via connection pads (53,54). A method for manufacturing a semiconductor memory device according to any preceding claim, comprising the steps of: (a) forming an insulation film pattern on a first substrate, etching said first substrate by using said insulation film pattern as an etching mask, and forming first and second active regions which are separated from each other and protruded; (b) forming a spacer on the sidewalls of said active regions; (c) removing the part of said spacer other than the part of said spacer formed on the sidewalls of the region where the source of a transistor is to be formed, to thereby form a first spacer contacting said first active region and a second spacer contacting said second active region; (d) depositing an insulating material over the entire resultant structure and etching back said insulating material, to thereby fill only the space between said active regions with said insulating material; (e) forming a first capacitor consisting of a first storage electrode connected to said first spacer, a first dielectric film and a first plate electrode on the resultant structure; (f) planarizing a surface of said first plate electrode; (g) forming an insulation film on said first plate electrode and bonding a second substrate with the resultant structure; (h) turning said first substrate upside down, etching back said first substrate such that said spacers (21,23) and active regions can be exposed; (i) forming first and second transistors on said first and second active regions, respectively; and (j) forming a second capacitor consisting of a second storage electrode connected to said second spacer, a second dielectric film and a second plate electrode on the resultant structure. A method according to claim 6, further comprising the step of respectively forming first and second connection pads connected to said second spacer and the drain of a transistor, in said step (i). A method according to claim 7, further comprising the step of forming a bit line connected to said second connection pad and placed between said active regions, in said step of forming first and second connection pads. A method according to claim 6, further comprising the step of forming a first connection pad connected to said second spacer, in said step (i). A method according to claim 6, further comprising the step of forming an insulation layer over the entire resultant structure, in each of said steps (d) and (i), said insulation layers being eliminated after formation of storage electrodes.

Inventors: LEE JOO-YOUNG, LEE, JOO-YOUNG
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1995
Main classification: H01L27
IPC codes: H01L21, H01L27, H01L29