Semiconductor Memory Device With Stacked Capacitor Structure and the Manufacturing Method Thereof

1. Field of the Invention

The present invention relates to semiconductor memory devices, and, more particularly, to a dynamic random access memory having a stacked capacitor cell structure.

2. Description of the Related Art

With the increasing needs for high performance of digital systems, developments of semiconductor memory devices, particularly, random access memories, continue for higher integration density. With restriction on the size of a chip substrate, if many memory cells, each constituted by one transistor and one capacitor, are simply packed, the occupying area of each memory cell would decrease The reduction of the memory cell size decreases the area of the cell capacitor. This deteriorates the performance of accumulating data charges. For instance, reduction in electrode area of a capacitor decreases the maximum amount of charge accumulatable, thus resulting in malreading of memory contents and increasing the chance of causing data damage by radiation. This impairs the reliability of data accessing accordingly.

Recently, for a dynamic random access memory (hereinafter called "DRAM" according to the practice of the concerned technical field), it becomes popular to employ the "stacked capacitor cell" structure in order to pack a greater number of memory cells on a chip substrate with a limited size without reducing the reliability of data accessing. According to this technique, a capacitor is insulatively disposed above a cell transistor formed on the chip substrate. An insulative layer sandwiched between the transistor and capacitor has a contact hole formed therein. An electrode of the capacitor is electrically coupled via the hole to the diffusion layer of the underlying transistor, which serves as a source or drain thereof.

The integration of DRAMS is generally improved by effectively increasing the effective area of a cell capacitor electrode to thereby increase the capacitance of the capacitor. According to DRAMS with the aforementioned stacked capacitor cell structure, however, higher integration of the DRAMs employing such a technique cannot be expected for the following reasons To increase the effective area of the capacitor electrode without increasing the occupying area of each capacitor on the substrate, a recess should be formed in an insulative layer for electrically separating the transistor from capacitor and the lower electrode layer of the cell capacitor should be formed so as to be stuck to the inner wall of the recess. The higher the integration density of DRAMs, the smaller the area of one cell becomes and, naturally, the narrower the recess gets. With the present state-of-art technology, therefore, it is very difficult to effectively form, within the recess, a capacitor electrode layer, which is expected to be relatively thick to have a thickness greater than a predetermined value. If the electrode layer portions formed on the facing inner walls of the recess come in contact with each other, the entire effective area of the capacitor electrode cannot be increased.

It is therefore an object of this invention to provide a new and improved semiconductor memory device which has a high integration density and an excellent operation reliability, and a method of manufacturing the same.

In accordance with the above object, the present invention is addressed to a specific semiconductor memory device which has a memory cell with a stacked capacitor cell structure provided on a semiconductive substrate. This memory cell has a transistor and a capacitor. The cell transistor has source and drain layers formed in the substrate and a gate electrode insulatively disposed above the substrate. The cell capacitor is insulatively stacked on the transistor. An insulative layer for electrically separating the transistor from the capacitor is formed on the substrate, then, a first conductive layer is formed on this insulative layer. These layers are simultaneously subjected to an etching process, thereby defining a contact hole which is connected to one of the source and drain layers through those layers. A second conductive layer is formed on the first conductive layer so as to substantially uniformly cover the inner wall of the contact hole and the surface portion of one of the source and drain layers exposed through the contact hole. The second conductive layer has a recess corresponding to the contact hole. The first and second conductive layers are subjected to patterning to thereby constitute a lower electrode of the capacitor. After a capacitor insulative thin film is formed on this lower electrode layer, a third conductive layer is formed on the insulative thin film in such a way as to bury the recess of the second conductive layer. The third conductive layer constitutes an upper electrode of the cell capacitor, thus completing the capacitor.

This invention and its objects and advantages will become more apparent from the following detailed description of preferred embodiments

In the detailed description of preferred embodiments of the present invention presented below, reference is made to the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating, in enlargement, the plane structure of the essential portion of a DRAM according to one preferred embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating the cross-sectional structure of the DRAM of FIG. 1 along the line II--II;

FIGS. 3A to 3F are cross-sectional views illustrating the essential structures attained by main processes of a manufacturing method embodying the present invention to produce the DRAM shown in FIGS. 1 and 2;

FIGS. 4A to 4D are cross-sectional views illustrating the essential structures attained by the main processes of a DRAM manufacturing method according to another embodiment of the present invention; and

FIGS. 5A to 5G are cross-sectional views illustrating the essential structures attained by the main processes of a DRAM manufacturing method according to a further embodiment of the present invention.

Referring to FIGS. 1 and 2, a DRAM according to one preferred embodiment of the present invention is generally designated by reference numeral "10." The DRAM 10 has a silicon substrate 12 with a P type conductivity. As shown in FIG. 2, diffusion layers 14 and 16 with an N type conductivity are formed on that top surface of the substrate 12 which is surrounded by a device separating insulative layer 18 to thereby define one memory cell. These diffusion layers 14 and 16 respectively serve as the source and drain of a metal oxide semiconductor field effect transistor (MOSFET) Q of one memory cell. A gate oxide film 20 is formed on the substrate 12, and a polycrystalline silicon layer 22a serving as the gate of the MOSFET Q is deposited on the film 20. This gate electrode 22a is self-aligned with the source and drain layers 14 and 16. A polycrystalline silicon layer 22b serves as the gate electrode of a neighboring MOSFET.

The MOSFET Q is covered with an insulative layer 24, which may be a SiO.sub.2 layer. The insulative layer 24 has openings 26 and 28 on the source and drain layers 14 and 16 of the MOSFET Q. The opening 26 serves as a capacitor groove and a contact hole while the opening 28 serves as a contact hole. A first conductive layer 30 is deposited around an opening 26a of the insulative layer 24. This layer 30, which may be a polycrystalline silicon layer, has an opening 26b formed continuous with the source contact hole 26a. The openings 26a and 26b may be considered substantially as one contact hole because they are simultaneously formed through one etching process, as will be described later. A second conductive layer 34 is formed in such a way as to uniformly cover (or be stuck to) the side walls and bottom of the opening or hole 26 and to be stacked on the first conductive layer 30. This second conductive layer may be a thin polycrystalline silicon layer. As illustrated in FIG. 1, the first and second polycrystalline silicon layers 30 and 34 overlap each other and serve as the lower electrode of a cell capacitor C.

An insulative layer 36 is formed on the lower capacitor electrode 30, 34 so as to have a uniform thickness. This insulative layer 36 is so thin that it holds the recess shape on the source contact hole 26 with a good fidelity. The insulative layer thin film 36 extends to peripheral edge portions of the lower electrode. A third conductive layer 38, relatively thick, is formed so as to cover the insulative thin film 36. The third conductive layer 38, which may also be a polycrystalline silicon layer, buries the interior of the capacitor groove 26, so that the surface of the layer 38 in the groove faces the layer 34 of the lower capacitor electrode stacked structure, at an increased area, through the thin capacitor insulative thin film 36. The cell capacitor C is completed by such a stacked structure. With this structure, the insulative thin film 36 serves as a capacitor insulative layer, and the polycrystalline silicon layer 38 as the upper capacitor electrode.

The capacitor structure is covered with a thick insulative layer 40 having a contact hole 28b formed continuous with the aforementioned drain contact hole 28a. The openings 28a and 28b are substantially a single contact hole. A thin elongated conductive layer 42 shown in FIG. 1 is deposited on the insulative layer 40, and is in electric contact with the drain layer 16 via the drain contact hole 28.

The method for producing the above stacked DRAM cell structure will now be described referring to FIGS. 3A through 3F.

In FIG. 3A, there is shown the substrate 12 of P type silicon. The insulative layer 18 constituted by a device separating oxide film is deposited on the substrate 12 by, for example, selective oxidization method. The exposed substrate surface region is a device forming region where forming a DRAM memory cell is permitted.

Subsequently, the insulative layer 20 of 12-25 nanometer thickness, serving as a gate oxide film, is formed through thermal oxidization on the substrate surface. The polycrystalline silicon film 22 is deposited on the layer 20. These layers 20 and 22 are subjected to patterning by simultaneous etching, thereby providing the gate electrodes 22a and 22b as shown in FIG. 3b. Then, an N type impurity is doped in the substrate 12 by, for example, ion implantation with gate electrodes 22a, 22b used as masks, and N type diffusion layers 14 and 16 are formed therein. These layers 14, 16 serve as the source and drain of the MOSFET Q (cell transistor). Through the above process, the MOS transistor Q of the memory cell is completed. It should be clear from FIG. 3B that the source and drain layers 14, 16 are substantially self-aligned with the gate electrode 22a. As clearly shown in FIG. 1, the gate electrode 22 continuously extends along an array of memory cells of DRAM in one direction and serves as a word line.

Then, as shown in FIG. 3C, the silicon oxide layer 24 is formed on the entire surface of the resultant structure by a CVD method. The silicon oxide layer 24 serves as an insulative layer to insulatively separate the cell MOS transistor Q from the overlying cell capacitor C. (This layer 24 is generally called "layer-insulation layer.") The first conductive layer 30 is deposited on the entire surface of this insulative layer 24. This conductive layer 30 may be a polycrystalline silicon layer of 300-nanometer thickness, for example The first conductive layer 30 will serve as the lower electrode of the cell capacitor C.

Thereafter, known etching is carried out and the contact hole 26 is formed in the layers 24, 30. This contact hole 26 penetrates the layers 24, 30 and reaches the top surface of the substrate 12 where the source 14 is formed. The resultant structure is then subjected to noble hydrofluoric acid treatment, thereby depositing the second conductive layer 34 on the entire surface of the layer 30. The conductive layer 34 may be a polycrystalline silicon layer of 50-nanometer thickness, for example The second conductive layer 34 is so thin that, as illustrated, it is uniformly stuck to the side walls and bottom of the contact hole 26. The second conductive layer 34 will also serve as the lower electrode of the cell capacitor C. Then, arsenic is doped in the substrate 12 by ion implantation under the implanting conditions that can realize about the same range of thickness as that of the polycrystalline silicon layer 34, for example, at an accelerating voltage of 60 kilo electron volts and with a dose of 1.times.10.sup.16 /cm.sup.2.

The first and second polycrystalline silicon layers 30, 34 are then subjected to patterning to thereby provide the lower capacitor electrode whose two edges partially overlap the electrodes 22a, 22b, as shown in FIG. 3E. The plane shape of the resultant structure is clearly illustrated in FIG. 1. The stacked structure of the lower capacitor electrode is designated by reference numeral "50" in FIG. 3E.

Subsequently, the thin insulative layer 36 serving as the capacitor insulation film is formed on the lower capacitor electrode 50. This insulative layer 36 may be a thermal oxide film, or, more preferably, a stacked layer of Si.sub.3 N.sub.4 and SiO.sub.2 films. The third conductive layer 38 is deposited on this layer 36, thus completing the cell capacitor C. The conductive layer 38 may be a polycrystalline silicon layer of 300-nanometer thickness, for example. This third conductive layer 38 serves as the upper electrode of the cell capacitor C.

After the silicon oxide layer 40 (see FIG. 2) is deposited by the CVD method on the entire surface of the resultant structure, the contact hole 28 is formed for the drain 16 of the MOS transistor Q. A noble hydrofluoric acid treatment is carried out to form the conductive layer 42 which is directly coupled to the drain 16 through the contact hole 28. The layer 42 is then subjected to patterning to provide a bit line intersecting the aforementioned word line layers 22a, 22b at right angles. Through the above process, a DRAM having the stacked capacitor cell structure as shown in FIGS. 1 and 2 is completed.

According to the above embodiment of the present invention, the lower electrode 50 of each cell capacitor C is constituted by the polycrystalline silicon layer 30, subjected to a process for simultaneously forming the layer-insulation layer 24 and contact hole 26, and the thin polycrystalline silicon layer 34 formed to be effectively fitted in the hole 26 after formation of the hole 26. With such an arrangement, even if the area of one cell region is reduced due to high integration of a DRAM, the effective area of the capacitor electrode can be increased to thereby increase the capacitance of the capacitor. For instance, as compared with the conventional capacitor structure provided under the same producing conditions, the capacitor structure of the present invention can easily increase the capacitance 1.3 to 1.4 times as great as the conventional type.

In addition, since the first polycrystalline silicon layer 30 serving as part of the lower electrode 50 of each cell capacitor C is formed prior to forming the contact hole 26 for the drain, this layer 30 serves as an etching-resistive protection layer of the underlying silicon oxide layer 24 in the noble hydrofluoric acid treatment performed after the formation of the contact hole 26. In the noble hydrofluoric acid treatment, therefore, the silicon oxide layer 24 can be effectively prevented, by the coverage of the polycrystalline silicon layer 30, from being undesirably etched. This can surely eliminate insulation breakdown that would otherwise generate electrical short-circuit between gate electrodes 22a and 22b through silicon oxide layer 24. This can significantly contribute to improvement of the operational reliability of DRAMs.

A modification of the above DRAM production method will be described below referring to FIGS. 4A through 4D, in which the same or like reference numerals are used to specify those identical or corresponding to the elements shown in FIGS. 3A to 3F, thus omitting their otherwise redundant detailed description only for descriptive simplification.

FIG. 4A illustrates the structure immediately before the process shown in FIG. 3C. In other words, it illustrates the cross section of the resultant structure immediately after only the silicon oxide layer 24 is deposited by the CVD method on the gate electrodes 22a, 22b. This silicon oxide layer 24 is then etched for approximately one minute by a buffer hydrofluoric acid solution. Through this process, the surface portion of the silicon oxide layer 24 is slightly etched out, making the top surface smoother.

In the subsequent step, a thin insulative layer 60 is deposited on the silicon oxide layer 24 by the CVD method, as shown in FIG. 4B. This insulative layer 60 may be a Si.sub.3 N.sub.4 film of 20-nanometer thickness, for example. The first conductive layer 30 serving as the lower electrode of the cell capacitor C is then formed on the Si.sub.3 N.sub.4 film 60.

The subsequent manufacturing processes are basically similar to the fabrication procedures described above referring to FIGS. 3D through 3F. Though redundant in a way, these processes will be briefly described below for the purpose of memory refreshment. The layers 24, 60 and 30 are simultaneously subjected to etching process to form the contact hole 26 which penetrates these layers and reaches the source layer 14 of the cell MOS transistor Q, as shown in FIG. 4C. The second conductive layer 34 is formed on the resultant structure. The layers 30, 34 are then subjected to patterning to thereby define the stacked structure 50 of the capacitor lower electrode shown in FIG. 3E. Then, the capacitor upper electrode layer 38 is formed, thus completing the capacitor C. After the insulative layer 40 is deposited on the resultant structure, the layers 24, 60 and 40 are simultaneously subjected to etching so as to form the contact hole 28 which penetrate these layers and reaches the drain layer 16 of the cell MOS transistor Q, as shown in FIG. 4D.

In the DRAM produced by the above fabrication method, it is also possible to eliminate the insulation breakdown that would otherwise generate short-circuit between the lower capacitor electrode structure 50 and gate electrode 22 through silicon oxide layer 24 in each memory cell of the DRAM. This can significantly contribute to improvement of the operational reliability of DRAMs because the Si.sub.3 N.sub.4 film 60 is formed prior to forming the contact hole 26 and serves as an etching-resistive protection layer of the underlying silicon oxide layer 24 in the noble hydrofluoric acid treatment performed after the formation of the contact hole 26.

Another embodiment of the present invention will be described below referring to FIGS. 5A through 5G, in which the same or like reference numerals are also used to specify those identical or corresponding to the elements shown in FIGS. 3 and 4, thus omitting their otherwise redundant detailed description only for descriptive simplification.

The structure shown in FIG. 5A in cross section substantially corresponds to the one shown in FIG. 3C. The insulative layer 24 and polycrystalline silicon layer 30, which will serve as part of the lower electrode of the cell capacitor, are subjected to etching to thereby form a contact hole 70 as shown in FIG. 5B. The N type diffusion layer 16 serving as a drain is partially exposed at the top surface through this contact hole 70.

The etching process for forming the contact hole 70 does not end at the time the N type diffusion layer 16 is exposed, but it continues the bottom 72 of the contact hole 70 gets deeper, penetrating the layer 16, to erode even the substrate 12. The depth of the hole 70 at the exposed surface portion may be approximately 1 micrometer, for example. Those layer portions of the diffusion layer 16 which are separated by the contact hole 70 are designated by reference numerals "16a" and "16b" in FIG. 5C.

Subsequently, as shown in FIG. 5D, the polycrystalline silicon layer 34 as the second conductive layer is deposited on the entire surface of the layer 30 after noble hydrofluoric acid treatment in such a way as to be stuck to the side walls and bottom of the deep hole 70. Then, arsenic is doped in the entire region of substrate 12 by ion implantation After the substrate is subjected to heat treatment, an N type diffusion layer 74, which contacts the diffusion layer portions 16a, 16b and substantially evenly surrounds the bottom 72 of the contact hole 70, is formed relatively shallow in the substrate 12. These layers 16a, 16b and 74 thereafter become one integral diffusion layer.

In the subsequent step, the layers 30, 34 are subjected to patterning to provide the stacked structure of the lower capacitor electrode, as shown in FIG. 5E. Then, as shown in FIG. 5F, the insulative thin film 36 serving as the capacitor insulative layer is formed on the resultant structure, and the third polycrystalline silicon layer 38 serving as the upper capacitor electrode is formed on the thin film 36. Through the above process, the stacked capacitor C is completed. The subsequent manufacturing processes are basically similar to the fabrication procedures described previously: after formation of the insulative layer 40 that entirely covers the resultant structure, a contact hole 76 is defined by the insulative layers 20, 24, 40, as shown in FIG. 5G. After executing noble hydrofluoric acid treatment, the conductive layer 42 is formed, which is directly coupled to the diffusion layer 14 via the contact hole 76. The layer 42 is subjected to patterning to provide a bit line intersecting the aforementioned word line layers 22a, 22b at the right angles. At this stage, the DRAM having the stacked capacitor cell structure according to the third embodiment of the present invention is completed.

Since the opening 70 serving as capacitor groove is formed deep so as to bore even the surface portion of the substrate 12, the above arrangement can produce an additional effect such that the effective capacitor electrode area can be increased without increasing the capacity-occupying area. The formation of the recess at the substrate surface contributes to reducing the electric contact between the N type diffusion layer 74 and the underlying electrode of the capacitor C. Particularly, since the N type diffusion layer 74 formed on the substrate 12 to surround this recess is formed by solid phase diffusion from the polycrystalline silicon layer which serves as the lower capacitor electrode, the depth and/or the impurity concentration of the diffusion layer 74 can be well controlled to form this diffusion layer at the proper shallowness. This can suppress or prevent deterioration of the transistor characteristic and a current leak between adjoining memory cells.

Although the invention has been described with reference to specific embodiments, it shall be understood by those skilled in the art that numerous modifications may be made that are within the spirit and scope of the invention